The results of the thermal analysis that was carried out by means of differential scanning calorimetry (DSC) are presented in Table 3. Overall, there were increased effect of both the content of the CaCO3 filler and its healing on the melting temperature (T*m*) and the crystallization temperature (T*c*). However, a decrease in the energy required for the fusion of the crystalline parts was noted when the content of the filler in the composite increased. The melting enthalpy and crystallinity % (X*c*) of the CaCO3 containing composites with DAP (0.06 %wt.) were a little higher than the composites without DAP. Yet, the values of heat of fusion remain lower than that of homopolymer PP or LDPE. It can therefore be noted that the filler alters the crystalline phase of the polymer.

to PP-LDPE surface. The decrease of yield stress is likely due to the depending between inorganic fillers and the PP matrix at large deformations. In particular, a higher drop in tensile strength at yield is observed for PP-LDPE/CaCO3, possibly due to the splitting of aggregated particles as well as depending between CaCO3 particles and the PP matrix. In Table 2, with/without peroxide it is clearly shown that elongation at break values of the composites decreases with increasing ratio of CaCO3. On the other hand, while comparing samples with CaCO3 to each other the highest mechanical properties were observed in PC6. The reason for this is that adding DAP effect LDPE by crosslinking and PP by degradation. Shore D values of the composites with/without DAP increase in some degree with increasing amount of CaCO3. As mentioned above, the reasons for increase in shore d values of the composites are addition of DAP effect to LDPE by crosslinking, PP by degradation, and dispersion of CaCO3 in PP and LDPE. Table 2 shows that, increasing amounts of CaCO3 resulted a decrease on heat sealing strength values at 145 °C and 150 °C independent of DAP. Therefore, the decrease of heat sealing strength is owing to the depending between in organic filler and the blends matrix of deformations. PP-LDPE/CaCO3 composites still gave better heat sealing strength results than homopolymer PP and LDPE. In previous studies, we have examined heat sealing strength of homopolymer PP and LDPE (Sirin & Balcan, 2010; Şirin, 2008) Same as in mechanical properties values, heat sealing strength values showed similar trend in composite PC6 for

Figure 1 (a-g) shows the scanning electron microscopy (SEM) micrographs of composites reinforced with different amounts of CaCO3 (0, 5, 10, and 20 %wt.) and DAP (0, 0.06 %wt.). From these micrographs, it is clear that CaCO3 fillers were dispersed well in PC4, PC6 and PC7 matrices with DAP of composites. In these matrices, a homogeneous dispersion of reinforcing particles can also be observed. Moreover, the fillers remained intact within the matrix. This indicates that good bonding existed between the CaCO3 particles and matrix. In contrast, the agglomeration of fillers can be observed for the composites containing CaCO3 particles and dialkylperoxide (DAP). In homogeneous dispersion of fillers can cause a loss in the mechanical strength of the composites considerably. At higher loading levels however, CaCO3 will agglomerate and remain

The results of the thermal analysis that was carried out by means of differential scanning calorimetry (DSC) are presented in Table 3. Overall, there were increased effect of both the content of the CaCO3 filler and its healing on the melting temperature (T*m*) and the crystallization temperature (T*c*). However, a decrease in the energy required for the fusion of the crystalline parts was noted when the content of the filler in the composite increased. The melting enthalpy and crystallinity % (X*c*) of the CaCO3 containing composites with DAP (0.06 %wt.) were a little higher than the composites without DAP. Yet, the values of heat of fusion remain lower than that of homopolymer PP or LDPE. It can therefore be noted that

best results.

**3.2 Morphology observation**

confined in the polymer matrix.

**3.3 Thermal analysis of composites**

the filler alters the crystalline phase of the polymer.

Fig. 1. SEM micrograph of blend and composites (**a**) PC0 (**b**) PC1 (**c**) PC2 (**d**) PC3 (**e**) PC4 (**f**) PC5 (**g**) PC6 (**h**) PC7

The Influence of Filler Component on Mechanical Properties and

PC1, PC2, and PC3)

PC5, PC6 and PC7)

Thermal Analysis of PP-LDPE and PP-LDPE/DAP Ternary Composites 353

Fig. 2. TG and DTA curves of composites containing the different amounts of CaCO3 (PC0,

TG curves for all the compounds exhibits one stage decomposition and a similar characterizations. The thermal decomposition of composites occurs between 403 and 500 ºC. The thermal stabilities of composites increased usually with increasing CaCO3 content. PC6 has the highest thermal stability among the polymer blends. It is also shown that the rate curve related to compound shifts to a higher temperature. Also, three endothermic thermal effects at different temperature in DTA profiles correspond to the melting and the decomposition of composites. In DTA curves, the first two peaks are two melting peaks.

Fig. 3. TG and DTA curves of composites containing the different amounts of CaCO3 (PC4,

Table 3. Thermal properties of PP-LDPE/CaCO3 composites

Thermal behavior of the composites was studied with a thermogravimetric analyzer under a protective nitrogen atmosphere. The temperature was scanned from 30 ºC to 700 ºC at a heating rate of 10 ºC.min-1. Figure 2 and 3 shows the typical thermogravimetric curves for composites with/without DAP and different filler contents are presented. 5 % and 20 % weight loss temperatures (T*-5 %* and T*-20 %*) and maximum weight loss temperature (bWmax.T), derived from the derivative weight loss and differential thermal analysis curves are tabulated in Table 4.

Table 4. Thermal decomposition values of all the compounds (aThe onset temperature, bMaximum Weight Temperature)

LDPE PP LDPE PP LDPE PP

PC0 102.00 2.90 162.10 91.00 95.00 3.00 118.40 88.90 1.00 43.50 PC1 102.60 2.90 162.90 83.10 95.50 1.40 118.90 90.40 1.00 39.80 PC2 102.80 2.30 162.30 73.20 96.80 1.20 119.30 76.00 0.80 35.00 PC3 103.40 2.20 162.60 70.30 99.40 1.10 120.10 66.30 0.70 33.60 PC4 102.50 3.10 162.30 85.50 94.20 2.90 118.50 87.00 1.10 40.90 PC5 103.20 2.40 164.90 81.70 95.30 2.00 119.00 80.40 0.80 34.20 PC6 106.30 2.80 165.10 86.40 99.20 2.80 120.70 86.90 0.70 39.10 PC7 104.70 1.10 164.60 62.00 97.40 1.30 120.40 76.90 0.40 29.70

Thermal behavior of the composites was studied with a thermogravimetric analyzer under a protective nitrogen atmosphere. The temperature was scanned from 30 ºC to 700 ºC at a heating rate of 10 ºC.min-1. Figure 2 and 3 shows the typical thermogravimetric curves for composites with/without DAP and different filler contents are presented. 5 % and 20 % weight loss temperatures (T*-5 %* and T*-20 %*) and maximum weight loss temperature (bWmax.T), derived from the derivative weight loss and differential thermal analysis curves are

TG DTA

PC0 403 454 406 438 0.5 105.2, 162.2, 457 PC1 402 453 406 438 3 105.8, 162.6, 458 PC2 405 454 407 440 9 106.6, 163.2, 457 PC3 405 455 407 440 18 106.5, 163.5, 459 PC4 407 456 409 443 6 107.2, 164.4, 459 PC5 409 457 413 444 11 107.9, 165.7, 459 PC6 414 461 415 444 11 108.1, 167.3, 462 PC7 414 459 409 444 26 108.7, 164.5, 460

Table 4. Thermal decomposition values of all the compounds (aThe onset temperature,

weight loss/

(20 %) residual Endo

(5 %)

∆H*c*/ J.g-1

T*c*/ ºC

∆H*c*/ J.g-1

X*c*/ %

X*c*/ %

Melting ( from second heating scans) Crystallization ( from second cooling scans)

T*c*/ ºC

Sample Code

T*m*/ ºC

tabulated in Table 4.

∆H*f*/ J.g-1

T*m*/ ºC

Table 3. Thermal properties of PP-LDPE/CaCO3 composites

Compounds aTon bWmax.T weight loss/

bMaximum Weight Temperature)

∆H*f*/ J.g-1

Fig. 2. TG and DTA curves of composites containing the different amounts of CaCO3 (PC0, PC1, PC2, and PC3)

TG curves for all the compounds exhibits one stage decomposition and a similar characterizations. The thermal decomposition of composites occurs between 403 and 500 ºC. The thermal stabilities of composites increased usually with increasing CaCO3 content. PC6 has the highest thermal stability among the polymer blends. It is also shown that the rate curve related to compound shifts to a higher temperature. Also, three endothermic thermal effects at different temperature in DTA profiles correspond to the melting and the decomposition of composites. In DTA curves, the first two peaks are two melting peaks.

Fig. 3. TG and DTA curves of composites containing the different amounts of CaCO3 (PC4, PC5, PC6 and PC7)

The Influence of Filler Component on Mechanical Properties and

ISSN:0924- 0136

ISSN: 0032-3861

0924-0136

pp.489-500, ISSN: 1525-6111.

vol.58, pp.235– 240, ISSN:0167-577X

*Technology*, vol.21, pp.250–255, ISSN: 1099-1581.

their properties, PhD.Thesis, Ege University, Izmir, Turkey.

*Polymer Science*, vol.107, No.6, pp.3609-3614, ISSN:1097-4628

*Polymer Science,* vol.106. No.3, pp.1932-1938. ISSN:1097-4628

0266-3538

Thermal Analysis of PP-LDPE and PP-LDPE/DAP Ternary Composites 355

Gonzalez, J.; Albano, C.; Ichazo, M. & Diaz, B.(2002), Effects of coupling agents on

CaCO3. *European Polymer Journal*, 38(12), 2465-2475. ISSN: 0014-3057 Jancar, J. & Dibenedetto, A.T.(1995). Failure mechanics in ternary composites of

composites *Journal of Applied Polymer Science*, 48(7), 1271-1278

properties. *Polymer,* vol.33, pp.4961-4967. ISSN: 0032-3861

toughness*. Journal of Materials Science*, 30(9), 2438-2445, ISSN:1573-4803. Kim, B.K.; Kim, M.S. & Kim, K.J.(1993). Viscosity effect in polyolefin ternary blends and

Kolarik, J. & Jancar, J. (1992). Ternary composites of polypropylene/elastomer/calcium

Liang, J.Z.(2007), Flow and mechanical properties of polypropylene-low density

Ma,C.G.; Mai, Y.L.; Rong, M.Z.; Ruan, W.H. & Zhang M.Q. (2007). Phase structure and

Mishra, S.; Perumal, G.B. & Naik, J.B. (1997). Studies on Mechanical Properties of Polyvinyl

Premphet, K. & Horanont, P.(2000). Polymer, Phase structure of ternary polypropylene

Rai, U.S. & Singh, R.K. (2003). Synthesis and mechanical characterization of polymer-matrix

Shonaike, G.O. & Advani, S.G.(2003). Advanced Polymeric Materials Structure Property Relationships, CRC Pres, pp. 463-478, ISBN 1-58716-047-1, New York, USA Sirin, K. & Balcan, M.(2010). Mechanical properties and thermal analysis of low-density

Şirin, K. (2008). Preparation of polymer blends and their composites, and determination of

Tang, C.Y. & Liang, J.Z.(2003), A study of the melt flow behaviour of ABS/CaCO3

Wang, W.Y.; Wang, G.Q.; Zeng, X.F.; Shao, L. & Chen, J.F. (2008). Preparation and properties

Wang, W.Y.; Zeng, X.F.; Wang, G.Q. & Chen J.F.(2007). Preparation and Characterization of

mechanical and morphological behavior of the PP/HDPE blend with two different

polypropylene with inorganic fillers and elastomer inclusions: Part II Fracture

carbonate: effect of functionalized components on phase structure and mechanical

polyethylene blends. *Journal of Material Processing Technology,* 66, 158-164,

mechanical properties of ternary polypropylene/elastomer/nano-CaCO3 composites, *Composites Science and Technology*, vol.67, No.14, pp.2997-3005, ISSN:

Chloride Composites. *Polymer-Plastics Technology and Engineering*, vol.36, No.4,

/elastomer /filler composites: effect of elastomer polarity, *Polymer*, 41, 9283–9290,

composites containing calcium carbonate/white cement filler. *Materials Letters,*

polyethylene + polypropylene blends with dialkyl peroxide. *Polymer Advanced*

composites, *Journal of Materials Processing Technology*, vol.138, pp.408–410. ISSN:

of nano-CaCO3/acrylonitrile-butadiene-styrene composites. *Journal of Applied*

Calcium Carbonate/ Low-Density-Polyethylene Nanocomposites. *Journal of Applied*

This is another proof showing that the components are incompatible. But CaCO3 dispersed completely in the blends and it didn't result another peak. As observed, melting peaks were between 105-108 ºC (LDPE) and 162-167 ºC (PP), decomposition temperatures related to the maximum weight loss (Wmax.T) were between 454-459 ºC. In the case of composites, although PP and LDPE decomposition completely at 500 °C, according to the amount of CaCO3 5, 10, or 20 % amount of mass, remained due to the reason that CaCO3's decomposition temperature is between 850-900°C.

In conclusion, polypropylene-Low Density Polyethylene blends with/without DAP and containing different amount CaCO3 filler component was prepared by melting-blend with a single-screw extruder. The effects of CaCO3 filler component on mechanical and thermal properties of prepared composites were investigated. Addition of CaCO3 particles to the polymer matrix with DAP significantly increased MFI values. It was observed that mechanical properties (tensile strength at break, at yield, elongation at break values and heat sealing strength values) of the composite PC6 showed high peaks. With addition of CaCO3, while mechanical properties of the composites were decreasing shore D values showed increasing. Heat sealing strength at 150 ºC increased by increasing amount of CaCO3 particles in the polymer matrix with DAP. SEM images showed that CaCO3 particles were well-dispersed in the polymer matrix with DAP. The observation of TG/DTG/DTA curves revealed that the thermal stabilities of composites increased usually by increasing amounts of CaCO3 and the blends are incompatible.

#### **4. Acknowledgements**

This study was carried out in Petkim Petrochemical Holding A.Ş., Turkey. In addition, it was supported by the research funds of Celal Bayar University (Project No: FEF-2006/085).

#### **5. References**

This is another proof showing that the components are incompatible. But CaCO3 dispersed completely in the blends and it didn't result another peak. As observed, melting peaks were between 105-108 ºC (LDPE) and 162-167 ºC (PP), decomposition temperatures related to the maximum weight loss (Wmax.T) were between 454-459 ºC. In the case of composites, although PP and LDPE decomposition completely at 500 °C, according to the amount of CaCO3 5, 10, or 20 % amount of mass, remained due to the reason that CaCO3's

In conclusion, polypropylene-Low Density Polyethylene blends with/without DAP and containing different amount CaCO3 filler component was prepared by melting-blend with a single-screw extruder. The effects of CaCO3 filler component on mechanical and thermal properties of prepared composites were investigated. Addition of CaCO3 particles to the polymer matrix with DAP significantly increased MFI values. It was observed that mechanical properties (tensile strength at break, at yield, elongation at break values and heat sealing strength values) of the composite PC6 showed high peaks. With addition of CaCO3, while mechanical properties of the composites were decreasing shore D values showed increasing. Heat sealing strength at 150 ºC increased by increasing amount of CaCO3 particles in the polymer matrix with DAP. SEM images showed that CaCO3 particles were well-dispersed in the polymer matrix with DAP. The observation of TG/DTG/DTA curves revealed that the thermal stabilities of composites increased usually by increasing

This study was carried out in Petkim Petrochemical Holding A.Ş., Turkey. In addition, it was supported by the research funds of Celal Bayar University (Project No: FEF-

Ayae, S. & Takashi K. (2004). Calcium carbonate/polymer composites: polymorph control for aragonite, *Composite Interfaces*, vol.11, No.4, pp.287-295(9), ISSN: 1568-5543 Brandrup, J. & Imergut, E.H.(2003). Polymer Handbook Second Edition, John Wiley and

Braun, D.; Richter, S.; Hellmann, G.P. & Ratzsch, M. (1998). Peroxy-initiated chain

Chen, N.; Wan, C.Y.; Zhang, Y. & Zhang, Y.X. (2004). Effect of nano-CaCO3 on mechanical

El-Sabbagh, A.; Steuernagel, L. & Ziegmann, G.(2009). Processing and modeling of the

degradation, crosslinking, and grafting in PP-PE blends, *Journal of Applied Polymer*

properties of PVC and PVC/Blendex blend. *Polymer Testing*, vol.23, No.2, pp.169-

mechanical behavior of natural fiber thermoplastic composite: Flax/polypropylene.

Sons, 4th Edition V-(13–27). ISBN: 978-0-471-47936-9

*Science*, vol.68, No.12, pp.2019-2028. ISSN:1097-4628

*Polymer Composites*, vol.30, pp.510-519, ISSN:1548-0569

decomposition temperature is between 850-900°C.

amounts of CaCO3 and the blends are incompatible.

**4. Acknowledgements**

194, ISSN: 0142-9418

2006/085).

**5. References**

**19**

*Iran*

**The Effects of Adding Nano-Calcium Carbonate**

The performance characteristics of plastic injection molded parts are depended upon the compositions of processing raw materials, parts and molds design specifications as well as the processing conditions. By appropriate incorporation of additives into a polymer matrix, it is possible to achieve the desired characteristics in the molded parts. Polypropylene (PP) is a semi-crystalline thermoplastic and is widely used for general applications. PP possesses the advantages of processing ease, very resistant to moisture absorption, and good chemical resistance to solvents. However, its applications as an engineering thermoplastic are limited due to its high shrinkage rate and relatively poor impact resistance at room or low

Recent developments in fillers and reinforcements technology have made it possible to enhance the properties and applications of PP. In formulating different compositions of materials, it is essential to bring into account the cost effectiveness, adding value, ease of processing and wide range of applications. Types, shapes, concentrations and dimensional conditions of fillers and reinforcements may directly affect the processing ease, production consistency, molding cycle time, parts dimensional conditions, mechanical, thermal as well as tribological properties. Platelike and layered particles may lead to poor impact strength, and the impact resistance is in reverse proportion to particle size (DeBoest, 1988; Mohd Ishak et al., 2008). These fillers may act either as sites of stress concentration or micro cracks initiator. Micron-sized spherical fillers such as CaCO3 have marginal influence on the impact resistance (DeBoest, 1988). While nano-sized CaCO3 may act as a nucleating agent (Avella et al., 2006) and impact modifier (K. Yang & Q. Yang, 2006) in a polymer matrix. The presence of nano-CaCO3 may possibly facilitate the mobilization of macromolecular chains and improve the ability of matrix polymer to adapt to deformation and hence to increase the ductility and impact strength of composites. The nanoparticles may also initiate micro-void formations which locally deform the matrix surrounding the particles and initiate mass plastic deformation and, in consequence, increase the toughness and impact energy (Kemal

**1. Introduction**

temperatures (Lam et al., 2009).

**Particles on the Mechanical and Shrinkage**

**Characteristics and Molding Process**

Karim Shelesh-Nezhad, Hamed Orang and Mahdi Motallebi

**Consistency of PP/nano-CaCO3**

**Nanocomposites**

*University of Tabriz*

## **The Effects of Adding Nano-Calcium Carbonate Particles on the Mechanical and Shrinkage Characteristics and Molding Process Consistency of PP/nano-CaCO3 Nanocomposites**

Karim Shelesh-Nezhad, Hamed Orang and Mahdi Motallebi *University of Tabriz Iran*

#### **1. Introduction**

356 Polypropylene

Zhang, L.; Wang, Z.H.; Huang, R.; Li, L.B. & Zhang, XY. (2002). PP/elastomer/calcium

Zweifel, H. (2001).Plastic additives Handbook, Ch14, Crosslinking and Controlled Degradation of Polyolefin, ISBN: 3- pp.723, 446-19579-3, Switzerland

ISSN:1573-4803

carbonate composites: effect of elastomer and calcium carbonate contents on the deformation and impact behavior. *Journal of Materials Science,* vol.37, pp.2615-2621,

> The performance characteristics of plastic injection molded parts are depended upon the compositions of processing raw materials, parts and molds design specifications as well as the processing conditions. By appropriate incorporation of additives into a polymer matrix, it is possible to achieve the desired characteristics in the molded parts. Polypropylene (PP) is a semi-crystalline thermoplastic and is widely used for general applications. PP possesses the advantages of processing ease, very resistant to moisture absorption, and good chemical resistance to solvents. However, its applications as an engineering thermoplastic are limited due to its high shrinkage rate and relatively poor impact resistance at room or low temperatures (Lam et al., 2009).

> Recent developments in fillers and reinforcements technology have made it possible to enhance the properties and applications of PP. In formulating different compositions of materials, it is essential to bring into account the cost effectiveness, adding value, ease of processing and wide range of applications. Types, shapes, concentrations and dimensional conditions of fillers and reinforcements may directly affect the processing ease, production consistency, molding cycle time, parts dimensional conditions, mechanical, thermal as well as tribological properties. Platelike and layered particles may lead to poor impact strength, and the impact resistance is in reverse proportion to particle size (DeBoest, 1988; Mohd Ishak et al., 2008). These fillers may act either as sites of stress concentration or micro cracks initiator. Micron-sized spherical fillers such as CaCO3 have marginal influence on the impact resistance (DeBoest, 1988). While nano-sized CaCO3 may act as a nucleating agent (Avella et al., 2006) and impact modifier (K. Yang & Q. Yang, 2006) in a polymer matrix. The presence of nano-CaCO3 may possibly facilitate the mobilization of macromolecular chains and improve the ability of matrix polymer to adapt to deformation and hence to increase the ductility and impact strength of composites. The nanoparticles may also initiate micro-void formations which locally deform the matrix surrounding the particles and initiate mass plastic deformation and, in consequence, increase the toughness and impact energy (Kemal

The Effects of Adding Nano-Calcium Carbonate Particles on the Mechanical and

(wt%)

Table 1. List of various compounds used in the experimentations

channels and in a manner to produce samples free of defects.

Molding Parameter Unit Value for tensile

Nozzle Temperature °C 180 180 Mold Temperature °C 40 40 Injection Pressure MPa 57 89 Injection Speed cm3/s 68 68 Holding Time s 8 15 Holding Pressure MPa 32 63 Screw feeding speed rpm 108 108

Table 2. The values of molding parameters settings for standard tensile, flexural and impact

The uniformity of injection molding process of different compounds was characterized by melt pressure measurement inside the mold cavity. A piezo-electric sensor (9221 AA0.6, Kistler, Switzerland) incorporated inside the tensile mold cavity and located behind and in contact to the ejector pin to measure the force exerted to the pin by melt pressure during the cyclic molding process against the time. The influence of nano-CaCO3 inclusion in the PP

Material PP

the die and a screw speed of 400 rpm.

specimens

**2.2 Characterization**

**2.2.1 Molding process consistency**

Shrinkage Characteristics and Molding Process Consistency of PP/nano-CaCO3 Nanocomposites 359

PP 100 0 0 P5 94 5 1 P10 88 10 2 P15 82 15 3

The neat PP and PP-g-MAH were dried in a vacuum oven at 80°C for 6 hours. Melt extrusion technique was applied to produce different compositions of PP/nano-CaCO3 by a ZSK-25 (Coperion Werner Pfliederer-Germany) co-rotating twin-screw extruder (D= 25 mm, *L*/*D* = 40) with a barrel temperature profile ranging from 160°C near the hopper to 200°C at

Molded samples utilized throughout the experimentations comprised standard tensile (ASTM D-638), flexural (ASTM D-790) and impact (ASTM D-256) specimens. An advanced microprocessor control injection molding machine (Poolad-110/380) with clamping force capacity of 110 tones and shot size capacity of 268 grams was employed to produce corresponding samples. The reciprocating screw diameter was equal to 45 millimeters and the ratio of screw length to diameter was 20. The values of molding parameters settings for standard tensile, flexural and impact specimens are given in table 2. The values of different settings were obtained on the basis of dimensional conditions of molds cavities and feed

specimens

nano-CaCO3 (wt%)

PP-g-MAH (wt%)

> Value for Flexural and impact specimens

et al., 2009). Stiffness or Young's modulus can be readily improved by adding either microor nano-particles since rigid inorganic particles generally have a much higher stiffness than polymer matrices. However, strength strongly depends on the stress transfer between the particles and the matrix. For well-bonded particles, the applied stress can be effectively transferred to the particles from the matrix; this clearly improves the strength (Fu et al., 2008).

Due to its non-polar chemical structure, PP interacts poorly with the typically polar fillers such as CaCO3, and optimum dispersion is normally difficult to achieve. Compatibilisers are frequently used to improve the interfacial adhesion between CaCO3 and PP, in order to gain the envisaged enhancement in mechanical properties (Fuad et al., 2010). Bi-functional molecules such as maleic-anhydride grafted PP (PP-g-MAH) are commonly used as compatibilisers for PP and CaCO3 (Roberts & Constable, 2003).

The crystallographic morphology of PP matrix can be noticeably altered by the presence of nano-CaCO3 because of its nucleating effect (Lin et al., 2011; Zhang et al., 2004). The coated CaCO3 nanoparticles affect the crystallization of PP in two ways: by serving as heterogeneous nucleation sites and also by reducing the spherulitic growth rate due to block the diffusion of polymer chains. Heterogeneous nucleation is, however, the dominating step controlling the crystallization rate of the PP/CaCO3 nanocomposite blends (Lin et al., 2011).

Besides mechanical properties, thermal contraction and uniformity of molded samples may also be influenced by different concentrations of the nano-CaCO3 in PP matrix. The objective of this research is to determine mechanical performances, shrinkage behavior and at the mean time the injection molding consistency of PP filled with nano-CaCO3 particles. In this research, PP/nano-CaCO3 polymer nanocomposites of different compositions were prepared by using a twin-screw extruder. PP-g-MAH compatibiliser was applied to improve the interfacial interaction between nano-CaCO3 and PP, and to extend the dispersion of nanoparticles in polymer matrix. An injection molding machine was employed to produce the standard specimens. The melt pressure inside the mold cavity was measured during the injection molding process of PP-CaCO3 nanocomposites to assess the production consistency. Dimensional conditions of different samples were characterized in order to determine the effect of nano-CaCO3 inclusion on the shrinkage rates. Morphology was observed and tensile, flexural and impact properties were examined to ascertain the influence of nano-CaCO3 on the mechanical performances.

## **2. Experimental procedures**

#### **2.1 Material used, compounding and sample preparation**

Polypropylene (PP500P, SABIC) has melt flow rate of 3.1 (2.16 kg at 230 °C) and density of 905 kg/m3 was used as matrix resin. Nano-sized synthetic ultrafine surface treated precipitated calcium carbonate (Socal 312, Solvay, France) with mean particle diameter of 70 nm used as filler phase. PP-g-MAH compatibiliser (Priex 20097, Solvay, France) with a maleic anhydride content of 0.05 wt % and MFI of 15 (2.16 kg at 230 °C) was employed to promote the interfacial interaction between nano-CaCO3 and PP, and to extend the dispersion of nanoparticles in polymer matrix. Compounds used as processing materials are listed in the table 1.

Table 1. List of various compounds used in the experimentations

The neat PP and PP-g-MAH were dried in a vacuum oven at 80°C for 6 hours. Melt extrusion technique was applied to produce different compositions of PP/nano-CaCO3 by a ZSK-25 (Coperion Werner Pfliederer-Germany) co-rotating twin-screw extruder (D= 25 mm, *L*/*D* = 40) with a barrel temperature profile ranging from 160°C near the hopper to 200°C at the die and a screw speed of 400 rpm.

Molded samples utilized throughout the experimentations comprised standard tensile (ASTM D-638), flexural (ASTM D-790) and impact (ASTM D-256) specimens. An advanced microprocessor control injection molding machine (Poolad-110/380) with clamping force capacity of 110 tones and shot size capacity of 268 grams was employed to produce corresponding samples. The reciprocating screw diameter was equal to 45 millimeters and the ratio of screw length to diameter was 20. The values of molding parameters settings for standard tensile, flexural and impact specimens are given in table 2. The values of different settings were obtained on the basis of dimensional conditions of molds cavities and feed channels and in a manner to produce samples free of defects.

Table 2. The values of molding parameters settings for standard tensile, flexural and impact specimens

### **2.2 Characterization**

358 Polypropylene

et al., 2009). Stiffness or Young's modulus can be readily improved by adding either microor nano-particles since rigid inorganic particles generally have a much higher stiffness than polymer matrices. However, strength strongly depends on the stress transfer between the particles and the matrix. For well-bonded particles, the applied stress can be effectively transferred to the particles from the matrix; this clearly improves the strength (Fu et al.,

Due to its non-polar chemical structure, PP interacts poorly with the typically polar fillers such as CaCO3, and optimum dispersion is normally difficult to achieve. Compatibilisers are frequently used to improve the interfacial adhesion between CaCO3 and PP, in order to gain the envisaged enhancement in mechanical properties (Fuad et al., 2010). Bi-functional molecules such as maleic-anhydride grafted PP (PP-g-MAH) are commonly used as

The crystallographic morphology of PP matrix can be noticeably altered by the presence of nano-CaCO3 because of its nucleating effect (Lin et al., 2011; Zhang et al., 2004). The coated CaCO3 nanoparticles affect the crystallization of PP in two ways: by serving as heterogeneous nucleation sites and also by reducing the spherulitic growth rate due to block the diffusion of polymer chains. Heterogeneous nucleation is, however, the dominating step controlling the crystallization rate of the PP/CaCO3 nanocomposite blends (Lin et al., 2011). Besides mechanical properties, thermal contraction and uniformity of molded samples may also be influenced by different concentrations of the nano-CaCO3 in PP matrix. The objective of this research is to determine mechanical performances, shrinkage behavior and at the mean time the injection molding consistency of PP filled with nano-CaCO3 particles. In this research, PP/nano-CaCO3 polymer nanocomposites of different compositions were prepared by using a twin-screw extruder. PP-g-MAH compatibiliser was applied to improve the interfacial interaction between nano-CaCO3 and PP, and to extend the dispersion of nanoparticles in polymer matrix. An injection molding machine was employed to produce the standard specimens. The melt pressure inside the mold cavity was measured during the injection molding process of PP-CaCO3 nanocomposites to assess the production consistency. Dimensional conditions of different samples were characterized in order to determine the effect of nano-CaCO3 inclusion on the shrinkage rates. Morphology was observed and tensile, flexural and impact properties were examined to ascertain the

Polypropylene (PP500P, SABIC) has melt flow rate of 3.1 (2.16 kg at 230 °C) and density of 905 kg/m3 was used as matrix resin. Nano-sized synthetic ultrafine surface treated precipitated calcium carbonate (Socal 312, Solvay, France) with mean particle diameter of 70 nm used as filler phase. PP-g-MAH compatibiliser (Priex 20097, Solvay, France) with a maleic anhydride content of 0.05 wt % and MFI of 15 (2.16 kg at 230 °C) was employed to promote the interfacial interaction between nano-CaCO3 and PP, and to extend the dispersion of nanoparticles in polymer matrix. Compounds used as processing materials are

compatibilisers for PP and CaCO3 (Roberts & Constable, 2003).

influence of nano-CaCO3 on the mechanical performances.

**2.1 Material used, compounding and sample preparation**

**2. Experimental procedures**

listed in the table 1.

2008).

#### **2.2.1 Molding process consistency**

The uniformity of injection molding process of different compounds was characterized by melt pressure measurement inside the mold cavity. A piezo-electric sensor (9221 AA0.6, Kistler, Switzerland) incorporated inside the tensile mold cavity and located behind and in contact to the ejector pin to measure the force exerted to the pin by melt pressure during the cyclic molding process against the time. The influence of nano-CaCO3 inclusion in the PP

The Effects of Adding Nano-Calcium Carbonate Particles on the Mechanical and

SEM observations, the samples were made conductive by gold sputtering.

**2.2.4 SEM**

**3. Results and discussions**

respect to the nano-CaCO3 contents.

Fig. 2. Melt flow rate versus nano-CaCO3 contents

reduction of energy consumption of molding process.

compounds are indicated in table 4 and figure 3.

**3.2 Injection molding uniformity**

process

**3.1 Melt flow rate**

Shrinkage Characteristics and Molding Process Consistency of PP/nano-CaCO3 Nanocomposites 361

A scanning electron microscope (Tescan VegaII) was employed to observe the dispersion of nano-CaCO3 particles in PP matrix in the fracture sections of impact specimens. Prior to

Figure 2, indicates the melt flow rates of different compounds of PP/nano-CaCO3 with

Inclusion of nano-CaCO3 considerably raised the melt flow rate. The presence of nano-CaCO3 in the molten PP has a rolling effect which facilitates sliding of melt on the cylinder wall of MFI tester (Jiang & Huang, 2008). According to Xie et al., spherical nanoparticles serve as ball bearings, reducing the interlayer interaction of melts (2004). The increment of MFI rate can facilitate the injection molding of thin-walled parts and can lead to the

The results of melt pressure measurement during the molding cycles for different

Material Integral (N-s) Max. Force (N)

Table 4. The magnitudes of control parameters and their deviations during the molding

PP 4953.29 133.30 843.28 8.75 P5 4923.87 138.26 820.49 11.20 P10 4894.25 50.45 826.49 7.49 P15 4468.54 52.53 800.05 4.46

Ave. Value Std Dev. Ave. Value Std Dev.

matrix resin on the sameness of pressure-time profiles and hence the variation of molding process was investigated. Two variables encompassing the maximum force and the forcetime integral were chosen as the control parameters to assess the consistency of injection molding process during the molding cycles. For each compound, 10 samples were examined and the mean values of control parameters were considered.

## **2.2.2 Dimensional conditions**

Standard tensile specimens (ASTM D-638) of PP with different contents of nano-CaCO3 were molded and employed to analyze the dimensional conditions and shrinkage rates. Figure 1, indicates the reference dimensions and their locations for shrinkage estimation of corresponding molded parts. The reference dimensions of matching cavity including cavity length, width and depth were equal to the 165.83, 19.10 and 3.20 millimeters respectively. The molded parts reference dimensions including parts length, width and thickness were measured with 0.01 mm accuracy after passing two weeks of molding. The differences of cavity dimensions and molded parts dimensions were considered as shrinkage values. For each compound, 5 samples were tested and the mean values were taken into account.

Fig. 1. Reference dimensions and their locations for shrinkage assessment of part length (L), part width (W) and part thickness (H)

#### **2.2.3 Mechanical properties**

Tensile, flexural and impact tests were performed based on the ASTM D-638, ASTM D-790 and ASTM D-256 standards respectively in order to analyze the effect of adding nano-CaCO3 particles on the mechanical performances of PP/CaCO3 nanocomposites. For each compound, 5 samples were tested and the mean values were considered. The specifications of testing equipment and conditions are presented in table 3.

Table 3. The specifications of mechanical properties testing equipment and conditions

## **2.2.4 SEM**

360 Polypropylene

matrix resin on the sameness of pressure-time profiles and hence the variation of molding process was investigated. Two variables encompassing the maximum force and the forcetime integral were chosen as the control parameters to assess the consistency of injection molding process during the molding cycles. For each compound, 10 samples were examined

Standard tensile specimens (ASTM D-638) of PP with different contents of nano-CaCO3 were molded and employed to analyze the dimensional conditions and shrinkage rates. Figure 1, indicates the reference dimensions and their locations for shrinkage estimation of corresponding molded parts. The reference dimensions of matching cavity including cavity length, width and depth were equal to the 165.83, 19.10 and 3.20 millimeters respectively. The molded parts reference dimensions including parts length, width and thickness were measured with 0.01 mm accuracy after passing two weeks of molding. The differences of cavity dimensions and molded parts dimensions were considered as shrinkage values. For

each compound, 5 samples were tested and the mean values were taken into account.

Fig. 1. Reference dimensions and their locations for shrinkage assessment of part length (L),

Tensile, flexural and impact tests were performed based on the ASTM D-638, ASTM D-790 and ASTM D-256 standards respectively in order to analyze the effect of adding nano-CaCO3 particles on the mechanical performances of PP/CaCO3 nanocomposites. For each compound, 5 samples were tested and the mean values were considered. The specifications

Tensile test

Speed range: 1 to 200 mm/min Adjusted crosshead speed: 50 mm/min Impact resistance test

Flexural strength test

Load range: 3000 kg Speed range: 10 to 200 mm/min

End point: 7.00 mm Adjusted speed: 10 mm/min Table 3. The specifications of mechanical properties testing equipment and conditions

Manufacturer: Zwick/Roll Model: TIFR010THA50

Manufacturer: Gotech Model: GT-7045-I Capacity: 100 kg-cm Test type: Izod-type

Manufacturer: Gotech Model: GT-7010A2

Extension range: 10.00 mm Span: 30.00 mm

Load range: 0.5 g to 3000 kg Adjusted load sensitivity: 0.5 g

and the mean values of control parameters were considered.

of testing equipment and conditions are presented in table 3.

**2.2.2 Dimensional conditions**

part width (W) and part thickness (H)

**2.2.3 Mechanical properties**

A scanning electron microscope (Tescan VegaII) was employed to observe the dispersion of nano-CaCO3 particles in PP matrix in the fracture sections of impact specimens. Prior to SEM observations, the samples were made conductive by gold sputtering.

## **3. Results and discussions**

## **3.1 Melt flow rate**

Figure 2, indicates the melt flow rates of different compounds of PP/nano-CaCO3 with respect to the nano-CaCO3 contents.

Fig. 2. Melt flow rate versus nano-CaCO3 contents

Inclusion of nano-CaCO3 considerably raised the melt flow rate. The presence of nano-CaCO3 in the molten PP has a rolling effect which facilitates sliding of melt on the cylinder wall of MFI tester (Jiang & Huang, 2008). According to Xie et al., spherical nanoparticles serve as ball bearings, reducing the interlayer interaction of melts (2004). The increment of MFI rate can facilitate the injection molding of thin-walled parts and can lead to the reduction of energy consumption of molding process.

## **3.2 Injection molding uniformity**

The results of melt pressure measurement during the molding cycles for different compounds are indicated in table 4 and figure 3.

Table 4. The magnitudes of control parameters and their deviations during the molding process

The Effects of Adding Nano-Calcium Carbonate Particles on the Mechanical and

tendency to agglomerate is observed at 15 wt% loading of nano-CaCO3.

growth of PP Matrix.

**3.4 Morphology**

CaCO3

**3.5 Mechanical properties**

and Figures 5 and 6.

Shrinkage Characteristics and Molding Process Consistency of PP/nano-CaCO3 Nanocomposites 363

The neat PP possessed the highest shrinkage rate. The inclusion of nano-CaCO3 reduced the thermal contraction because of its filling effect and its lower thermal contraction. The molded samples with concentration of 5 wt% nano-CaCO3 possessed the lowest shrinkage along the flow and across the flow directions. Addition of higher values of nano-CaCO3 (10, 15 wt%) to the PP, elevated the shrinkage rates along and across the flow directions. Nano-CaCO3 has nucleating effect and can lead to the increment of crystallization rate in the PP matrix (Chan et al., 2002; K. Yang & Q. Yang, 2006). Addition of higher values of nano-CaCO3 (10, 15 wt%) to the PP, slightly increased the tendency of non-isotropic thermal contraction along and across the flow directions. This can possibly be related to increment of PP crystallinity as a result of higher loadings of nano-CaCO3, and non-isotropic crystal

Figure 4, indicates the presence and distribution of nano-CaCO3 (5 and 15 wt%) in the PP matrix in the cross sections of impact test samples. Higher concentration of nano-CaCO3 particles is observed in the PP matrix with higher content, i.e. 15wt%, of nano-CaCO3. Due to the surface modification of nano-CaCO3 and application of PP-g-MAH compatibiliser, relatively good dispersions of nanofillers were achieved. According to figure 4-b, more

Fig. 4. Morphologies of PP/nano-CaCO3 at two loadings of (a) 5 and (b) 15 wt% of nano-

The tensile tests results of various compounds of PP/nano-CaCO3 are indicated in Table 6

Fig. 3. Force-time traces of different compounds

As the nano-CaCO3 loading was increased, the maximum force and the force-time integral were decreased. This may be attributed to the influence of nano-CaCO3 on melt fluidity and crystallization behavior of PP matrix. By incorporating higher loading of nano-CaCO3 (10 and 15 wt%), the deviation of molding process was significantly declined. By adding 15 wt% of nano-CaCO3, the maximum force standard deviation dropped from 8.75 to 4.46 (N) and the force-time integral standard deviation declined from 133.30 to 52.53 (N-s). According to the figure 3, by increasing the nano-CaCO3 content in the PP matrix, a change in the slope on the right side of force-time profile is observed. This may possibly be related to the increase of PP crystallinity and, in consequence, higher thermal contraction of melt during the molding process, as a result of nano-CaCO3 presence.

#### **3.3 Shrinkage behavior**

Table 5, indicates the shrinkage rates along the flow, across the flow, and along the thickness of molded parts with respect to the amount of nano-CaCO3 content.

Table 5. Effect of nano-CaCO3 concentration on the shrinkage of PP/nano-CaCO3

The neat PP possessed the highest shrinkage rate. The inclusion of nano-CaCO3 reduced the thermal contraction because of its filling effect and its lower thermal contraction. The molded samples with concentration of 5 wt% nano-CaCO3 possessed the lowest shrinkage along the flow and across the flow directions. Addition of higher values of nano-CaCO3 (10, 15 wt%) to the PP, elevated the shrinkage rates along and across the flow directions. Nano-CaCO3 has nucleating effect and can lead to the increment of crystallization rate in the PP matrix (Chan et al., 2002; K. Yang & Q. Yang, 2006). Addition of higher values of nano-CaCO3 (10, 15 wt%) to the PP, slightly increased the tendency of non-isotropic thermal contraction along and across the flow directions. This can possibly be related to increment of PP crystallinity as a result of higher loadings of nano-CaCO3, and non-isotropic crystal growth of PP Matrix.

## **3.4 Morphology**

362 Polypropylene

As the nano-CaCO3 loading was increased, the maximum force and the force-time integral were decreased. This may be attributed to the influence of nano-CaCO3 on melt fluidity and crystallization behavior of PP matrix. By incorporating higher loading of nano-CaCO3 (10 and 15 wt%), the deviation of molding process was significantly declined. By adding 15 wt% of nano-CaCO3, the maximum force standard deviation dropped from 8.75 to 4.46 (N) and the force-time integral standard deviation declined from 133.30 to 52.53 (N-s). According to the figure 3, by increasing the nano-CaCO3 content in the PP matrix, a change in the slope on the right side of force-time profile is observed. This may possibly be related to the increase of PP crystallinity and, in consequence, higher thermal contraction of melt during

Table 5, indicates the shrinkage rates along the flow, across the flow, and along the thickness

0 1.64 1.74 5.00 5 1.42 1.50 4.93 10 1.51 1.65 4.87 15 1.53 1.64 4.62

Table 5. Effect of nano-CaCO3 concentration on the shrinkage of PP/nano-CaCO3

Ave. Shrinkage across the flow (%)

Ave. Shrinkage along the flow thickness (%)

Fig. 3. Force-time traces of different compounds

the molding process, as a result of nano-CaCO3 presence.

of molded parts with respect to the amount of nano-CaCO3 content.

Ave. Shrinkage along the flow (%)

**3.3 Shrinkage behavior**

nano-CaCO3 (wt%)

Figure 4, indicates the presence and distribution of nano-CaCO3 (5 and 15 wt%) in the PP matrix in the cross sections of impact test samples. Higher concentration of nano-CaCO3 particles is observed in the PP matrix with higher content, i.e. 15wt%, of nano-CaCO3. Due to the surface modification of nano-CaCO3 and application of PP-g-MAH compatibiliser, relatively good dispersions of nanofillers were achieved. According to figure 4-b, more tendency to agglomerate is observed at 15 wt% loading of nano-CaCO3.

Fig. 4. Morphologies of PP/nano-CaCO3 at two loadings of (a) 5 and (b) 15 wt% of nano-CaCO3

### **3.5 Mechanical properties**

The tensile tests results of various compounds of PP/nano-CaCO3 are indicated in Table 6 and Figures 5 and 6.

The Effects of Adding Nano-Calcium Carbonate Particles on the Mechanical and

compounds.

CaCO3

Shrinkage Characteristics and Molding Process Consistency of PP/nano-CaCO3 Nanocomposites 365

Figure 7, Compares the stress-strain characteristics of median specimens of different

Fig. 7. The influence of adding nano-CaCO3 on the stress-strain behavior of PP/nano-

sites of nano particles in the PP matrix as presented in figure 4-b.

Figure 8 depicts the results of flexural tests.

PP matrix. Figure 9 shows the impact tests results.

The results of tensile tests revealed that the inclusion of nano-CaCO3 slightly increased modulus and decreased tensile strength and significantly increased the elongation at break. Addition of rigid particles to a polymer matrix can easily improve the modulus since the rigidity of inorganic fillers is generally much higher than that of organic polymers (Fu et al., 2008). The reduction of tensile strength may be attributed to the weakly bonded nanoparticles which promote matrix yielding (Kemal et al., 2009). According to Zhang et al., when CaCO3 particles were introduced, small and imperfect spherulites formed. The reduction of spherulite size and the disappearance of sharp interfaces among spherulites favored the increase of elongation at break for the PP/CaCO3 composites (2004). Xie et al. reported that the increases in elongation at break can be attributed to ellipsoidal voids formation in the matrix surrounding the particles, allowing ductile pull out (2004). At high fraction of nano-CaCO3 (i.e.15 wt%), the elongation at break was declined. This result is consistent with the morphology of corresponding compound which shows agglomeration

The incorporation of nano-CaCO3 led to the elevation of flexural strength in PP/nano-CaCO3 compounds. This may also be related to the nucleating effect of nano-CaCO3 in the

Addition of nano-CaCO3 elevated the impact strength significantly. This may be explained by the fact that the presence of nano-CaCO3 in the PP matrix lead to a more uniform distribution of impact energy. Additionally, the presence of nano-CaCO3 may influence the crystallization behavior by reducing the spherulites size and subsequently alter the impact strength. Kemal et al. reported that the raise of toughness and impact energy may be attributed to enhanced micro-void formations initiated by nanoparticles, which locally deform the matrix surrounding the particles and initiate mass plastic deformation (2009).

Table 6. The results of tensile tests

Fig. 5. Tensile strength of PP/nano-CaCO3 against nano-CaCO3 content

Fig. 6. Elongation of PP/nano-CaCO3 against nano-CaCO3 content

Figure 7, Compares the stress-strain characteristics of median specimens of different compounds.

Fig. 7. The influence of adding nano-CaCO3 on the stress-strain behavior of PP/nano-CaCO3

The results of tensile tests revealed that the inclusion of nano-CaCO3 slightly increased modulus and decreased tensile strength and significantly increased the elongation at break. Addition of rigid particles to a polymer matrix can easily improve the modulus since the rigidity of inorganic fillers is generally much higher than that of organic polymers (Fu et al., 2008). The reduction of tensile strength may be attributed to the weakly bonded nanoparticles which promote matrix yielding (Kemal et al., 2009). According to Zhang et al., when CaCO3 particles were introduced, small and imperfect spherulites formed. The reduction of spherulite size and the disappearance of sharp interfaces among spherulites favored the increase of elongation at break for the PP/CaCO3 composites (2004). Xie et al. reported that the increases in elongation at break can be attributed to ellipsoidal voids formation in the matrix surrounding the particles, allowing ductile pull out (2004). At high fraction of nano-CaCO3 (i.e.15 wt%), the elongation at break was declined. This result is consistent with the morphology of corresponding compound which shows agglomeration sites of nano particles in the PP matrix as presented in figure 4-b.

Figure 8 depicts the results of flexural tests.

364 Polypropylene

Ave. Tensile Strength (Mpa)

Fig. 5. Tensile strength of PP/nano-CaCO3 against nano-CaCO3 content

Fig. 6. Elongation of PP/nano-CaCO3 against nano-CaCO3 content

PP 960.17 36.04 19.81 75.03 P5 971.65 35.67 11.62 122.88 P10 983.94 34.64 6.81 161.29 P15 990.41 33.80 7.58 118.87

Ave. Elastic

modulus (Mpa) Material

Table 6. The results of tensile tests

Ave. Strength at break (Mpa) Ave. Elongation at break (%)

> The incorporation of nano-CaCO3 led to the elevation of flexural strength in PP/nano-CaCO3 compounds. This may also be related to the nucleating effect of nano-CaCO3 in the PP matrix. Figure 9 shows the impact tests results.

> Addition of nano-CaCO3 elevated the impact strength significantly. This may be explained by the fact that the presence of nano-CaCO3 in the PP matrix lead to a more uniform distribution of impact energy. Additionally, the presence of nano-CaCO3 may influence the crystallization behavior by reducing the spherulites size and subsequently alter the impact strength. Kemal et al. reported that the raise of toughness and impact energy may be attributed to enhanced micro-void formations initiated by nanoparticles, which locally deform the matrix surrounding the particles and initiate mass plastic deformation (2009).

The Effects of Adding Nano-Calcium Carbonate Particles on the Mechanical and

*European Polymer Journal*, Vol. 42, pp. (1548–1557)

nanocomposites. *Polymer*, Vol. 43, pp. (2981-2992)

**5. References**

Vol. 4, pp. (611–620)

Vol. 43, No. 15, pp. (5305–5312)

A, Vol. 501, pp. (87–93)

Salerno (Italy), June 2008

354, pp. (570–576)

(805-809)

Carbonate. *Polymer*, Vol. 50, pp. (4066-4079)

played important roles in determining the performances of PP/nano-CaCO3.

Shrinkage Characteristics and Molding Process Consistency of PP/nano-CaCO3 Nanocomposites 367

high as 107%. The nano-sized feature, shape and dispersion conditions of nano-CaCO3,

Avella, M.; Cosco, S., Di Lorenzo, M.L., Di Pace, E. & Errico, M.E. Gentile G. (2006).

Chan, C.M.; Wu, J.S., Lee, J. X. & Chung, Y. K. (2002). Polypropylen/calcium carbonate

Fuad, M.Y.A.; Hanim, H., Zarina, R., Mohd Ishak, Z.A. & Hassan, A. (2010).

Jiang, G. & Huang, H.X. (2008). Online shear viscosity and microstructure of PP/nano-

Kemal, I.; Whittle, A., Burford, R., Vodenitcharova, T. & Hoffman, M. (2009). Toughening of

Lam, T. D.; Hoang, T. V., Quang, D. T. & Kim, J. S. (2009). Effect of nanosized and

Lin, Y.; Chen, H., Chan, C.M. & Wu, J. (2011). Nucleating effect of calcium stearate coated

Mohd Ishak, Z.A.; Kusmono, Chow, W.S., Takeichi, T. & Rochmadi (2008). Effect of

Roberts, D. & Constable, R.C. (2003). Chemical coupling agents for filled and grafted

Xie, X.L.; Liu, Q.X., Li, R.K.Y., Zhou, X.P., Zhang, Q.X., Yu, Z.Z. & Mai, Y.W. (2004).

Yang, K. & Yang Q. (2006). Morphology and mechanical properties of

*Composites,* Karian, H. G., pp. (28–68), Marcel Decker, New York

by In Situ Polymerization. *Polymer*, Vol. 45, pp. (6665-6673)

DeBoest, J. F. (1988). Reinforced polypropylenes, In: *Engineering Plastics*, pp. (192-193) Fu, S.Y.; Feng, X.Q., Lauke, B. & Mai, Y. W. (2008). Effects of particle size, particle/matrix

polymer composites. *Composites*: Part B, Vol. 39, pp. (933–961)

Nucleation activity of nanosized CaCO3 on crystallization of isotactic polypropylene, in dependence on crystal modification, particle shape, and coating.

interface adhesion and particle loading on mechanical properties of particulate–

Polypropylene/calcium carbonate nanocomposites – effects of processing techniques and maleated polypropylene compatibiliser. *eXPRESS Polymer Letters*,

CaCO3 composites produced by different mixing types. *Journal of Materials Sci*ence,

Unmodified Polyvinylchloride through the Addition of Nanoparticulate Calcium

surface-modified precipitated calcium carbonate on properties of CaCO3/polypropylene nanocomposites. *Materials Science and Engineering*: Part

CaCO3 nanoparticles on polypropylene. *Journal of Colloid and Interface Science*, Vol.

Organoclay Modification on the Mechanical, Morphology, and Thermal Propertiese of Injection Molded Polyamide6/Polypropylene/ Montmorillonite Nanocomposites*. Proceedings of the Polymer Processing Society 24th Annual Meeting*,

polypropylene composites, In: *Handbook of Polypropylene and Polypropylene*

Rheological and Mechanical Properties of PVC/CaCO3 Nanocomposites Prepared

polypropylene/calcium carbonate nanocomposites. *Materials Letters*, Vol. 60, pp.

Fig. 8. Flexural strength of PP/nano-CaCO3 versus nano-CaCO3 content

Fig. 9. Impact strength of PP/nano-CaCO3 versus nano-CaCO3 content

#### **4. Conclusion**

In this study the influences of nano-CaCO3 on the production consistency, shrinkage and melt flow rates, as well as mechanical properties of PP/nano-CaCO3 nanocomposites were experimentally investigated. PP-g-MAH compatibiliser with a maleic anhydride content of 0.05 wt% was employed to improve the interfacial adhesion between nano-CaCO3 and PP and to extend dispersion of nanoparticles in polymer matrix. Inclusion of nano-CaCO3 raised the melt flow rate as high as 77%. By incorporating higher loading of nano-CaCO3 (10 and 15 wt%), the deviation of molding process was significantly declined. The molded samples with concentration of 5 wt% nano-CaCO3 possessed the lowest shrinkage along the flow and across the flow. The results of tensile tests revealed that the inclusion of nano-CaCO3 slightly increased modulus and decreased tensile strength and significantly increased the elongation at break. At high fraction of nano-CaCO3 (i.e.15 wt%), the elongation at break was declined. Addition of nano-CaCO3 elevated the impact strength as high as 107%. The nano-sized feature, shape and dispersion conditions of nano-CaCO3, played important roles in determining the performances of PP/nano-CaCO3.

## **5. References**

366 Polypropylene

Fig. 8. Flexural strength of PP/nano-CaCO3 versus nano-CaCO3 content

Fig. 9. Impact strength of PP/nano-CaCO3 versus nano-CaCO3 content

In this study the influences of nano-CaCO3 on the production consistency, shrinkage and melt flow rates, as well as mechanical properties of PP/nano-CaCO3 nanocomposites were experimentally investigated. PP-g-MAH compatibiliser with a maleic anhydride content of 0.05 wt% was employed to improve the interfacial adhesion between nano-CaCO3 and PP and to extend dispersion of nanoparticles in polymer matrix. Inclusion of nano-CaCO3 raised the melt flow rate as high as 77%. By incorporating higher loading of nano-CaCO3 (10 and 15 wt%), the deviation of molding process was significantly declined. The molded samples with concentration of 5 wt% nano-CaCO3 possessed the lowest shrinkage along the flow and across the flow. The results of tensile tests revealed that the inclusion of nano-CaCO3 slightly increased modulus and decreased tensile strength and significantly increased the elongation at break. At high fraction of nano-CaCO3 (i.e.15 wt%), the elongation at break was declined. Addition of nano-CaCO3 elevated the impact strength as

**4. Conclusion**

**20**

*UK*

Jia Ma and Ton Peijs

*Queen Mary University of London*

**Preparation of Polypropylene Nanocomposites**

In recent years polypropylene (PP) nanocomposites have attracted great interest in academia. The attractiveness of this new class of material lies in the large improvements in both mechanical and thermal properties, as well as in gas barrier and fire resistance (Grunes et al., 2003). Nanofillers, being additives of nanometre scale, are dispersed in PP matrix, offering multifunctional and high-performance polymer characteristics beyond those

Many methods have been employed for preparing PP nanocomposites (Wu & Lerner, 1993; Andrews et al., 2002; Zhao et al., 2003; Garcia-Leiner and Lesser, 2004). Among them, supercritical CO2 technology has been intensively studied in recent years. The main advantages of this technology are that CO2 is a 'green' non-combustible and non-toxic solvent and the addition of scCO2 into the polymer can cause plasticization, which brings the opportunity to lower polymer viscosity, leading to improved dispersion, and to process the polymer at lower temperatures (Behles & Desimone, 2001; Cansell et al., 2003). Furthermore, CO2 can act as a transport medium, which facilitates the diffusion of monomers, initiators and molecules into a polymer matrix (Kikic & Vecchione, 2003). The aim of this chapter is to provide a review of the use of supercritical CO2 technology in the preparation of PP nanocomposites. In this chapter, the different nanofillers used, the processing methods based on using supercritical CO2 technology and the resulting properties of the nanocomposites will be discussed in detail. The progress and challenges on

The term 'nanofillers' is vague and no precise definition exists. Nanofillers are understood, in essence, to be additives in solid form, which differ from the polymer matrix in terms of their composition and structure. Nanofillers are of the order of 100 nm or less in at least one

Nanofillers are often added to enhance one or more of the properties of polymers. Inactive fillers or extenders raise the quantity and lower the cost price, while active fillers bring about targeted improvements in certain mechanical or physical properties. Common nanofillers include calcium carbonate, ceramic nanofillers, carbon black, carbon nanotubes (CNTs), carbon

**1. Introduction**

**2. Nanofillers**

dimension.

possessed by traditional filled materials.

various aspects of PP nanocomposites will also be discussed.

**Using Supercritical Technology**

Zhang, Q.X.; Yu, Z.Z., Xie, X.L. & Mai, Y.W. (2004). Crystallization and Impact Energy of Polypropylene/CaCO3 Nanocomposites with Nonionic Modifier. *Polymer*, Vol. 45, pp. (5985-5994)

## **Preparation of Polypropylene Nanocomposites Using Supercritical Technology**

Jia Ma and Ton Peijs *Queen Mary University of London UK*

## **1. Introduction**

368 Polypropylene

Zhang, Q.X.; Yu, Z.Z., Xie, X.L. & Mai, Y.W. (2004). Crystallization and Impact Energy of

pp. (5985-5994)

Polypropylene/CaCO3 Nanocomposites with Nonionic Modifier. *Polymer*, Vol. 45,

In recent years polypropylene (PP) nanocomposites have attracted great interest in academia. The attractiveness of this new class of material lies in the large improvements in both mechanical and thermal properties, as well as in gas barrier and fire resistance (Grunes et al., 2003). Nanofillers, being additives of nanometre scale, are dispersed in PP matrix, offering multifunctional and high-performance polymer characteristics beyond those possessed by traditional filled materials.

Many methods have been employed for preparing PP nanocomposites (Wu & Lerner, 1993; Andrews et al., 2002; Zhao et al., 2003; Garcia-Leiner and Lesser, 2004). Among them, supercritical CO2 technology has been intensively studied in recent years. The main advantages of this technology are that CO2 is a 'green' non-combustible and non-toxic solvent and the addition of scCO2 into the polymer can cause plasticization, which brings the opportunity to lower polymer viscosity, leading to improved dispersion, and to process the polymer at lower temperatures (Behles & Desimone, 2001; Cansell et al., 2003). Furthermore, CO2 can act as a transport medium, which facilitates the diffusion of monomers, initiators and molecules into a polymer matrix (Kikic & Vecchione, 2003). The aim of this chapter is to provide a review of the use of supercritical CO2 technology in the preparation of PP nanocomposites. In this chapter, the different nanofillers used, the processing methods based on using supercritical CO2 technology and the resulting properties of the nanocomposites will be discussed in detail. The progress and challenges on various aspects of PP nanocomposites will also be discussed.

## **2. Nanofillers**

The term 'nanofillers' is vague and no precise definition exists. Nanofillers are understood, in essence, to be additives in solid form, which differ from the polymer matrix in terms of their composition and structure. Nanofillers are of the order of 100 nm or less in at least one dimension.

Nanofillers are often added to enhance one or more of the properties of polymers. Inactive fillers or extenders raise the quantity and lower the cost price, while active fillers bring about targeted improvements in certain mechanical or physical properties. Common nanofillers include calcium carbonate, ceramic nanofillers, carbon black, carbon nanotubes (CNTs), carbon

Preparation of Polypropylene Nanocomposites Using Supercritical Technology 371

into plastics can potentially provide structural materials with a dramatic increase in both stiffness and strength. Extensive studies have been carried out producing strong polymer/CNT composites (Coleman et al., 2006; Moniruzzaman & Winey, 2006; Ahir, 2005, Wang et al., 2007). The effective use of CNTs as reinforcements still presents some major difficulties (Wang et al., 2007). The key challenge still remains in breaking down bundles of aggregated CNTs and reaching a fine dispersion in the selected polymer matrix. Much work needs to be done to optimise the conditions required for the potential dispersion of nanotubes, especially at higher CNT loadings, as well as a good interfacial interaction

CNTs have extremely low electrical resistance and their electric-current-carrying capacity is 1000 times higher than copper wires (Thostenson et al., 2001). Devices have been developed using CNTs such as field-emission displays (Fan et al., 1999). CNTs have also been widely used to produce conductive polymer composites (CPCs). It has been reported that the percolation threshold in CPC can be as low as 0.0025 wt% in the case of low viscosity thermosetting resins (Bryning et al., 2005). In thermoplastics the percolation threshold is typically around 1 wt%, but can be significantly lower using latex technology (Lu et al., 2008) and is highly dependent on processing history (Zhang et al., 2009; Deng et al., 2009a

Clay is a natural, earthy, fine-grained material and is the main constituent of the sedimentary rocks in marine sediments and in soils. Clay minerals belong to the family of phyllosilicates (or layered silicate) and have particles less than 2 µm in size as stated in ISO 14688 (Moore & Reynolds, 1997). There are three main groups of clays: kaolinite, montmorillonite-smectite and halloysite, and most clay are a mixture of these different types. The shape of clay minerals is of a distinctive character. Montmorillonite, which has irregular flakes, is the most commonly used nanoclay in polymer/clay nanocomposites (Fig. 2a). Recently, sepiolite, a fibrous shaped mineral, has gained increasing attention as nanofiller reinforcement (Y.P. Zheng &Y. Zheng, 2006). Sepiolite is a needle-like shaped nanoclay composed with elemental particles of lengths of 0.2-4 μm, widths of 10-30 nm and thicknesses of 5-10 nm (Fig. 2b). Sepiolite can have a surface area as high as 200-300 m2.g-1 and normally it is found stuck together as bundles of fibres which can form microagglomerates. Sepiolite is a hydrous magnesium silicate with

Compared with layered montmorillonite, the morphology of fibre-like sepiolite provides a lower specific surface area and smaller contact surface between the nanoclay particles. Therefore, polymer chains have a better chance not only of interacting with the external surface of the sepiolite, but also of penetrating into the structure, which facilitates a more even dispersion of the clays in the polymer matrix. Sepiolite provides a pseudoplastic and thixotropic behaviour which make it a valuable material in multiple applications to improve the processability, application or handling of the final product. Sepiolite has been successfully used to reinforce different polymers, such as poly (hydroxyethyl acrylate),

epoxy, PP, etc (Bokobza et al., 2004; Y.P. Zheng &Y. Zheng, 2006; Bilotti et al., 2008).

[Si12O30Mg8(OH)4(H2O)4·8H2O] as the unit cell formula.

(Moniruzzaman & Winey, 2006).

and 2009b).

**2.2 Nanoclays**

nanofibres, cellulose nanowhiskers, nanoclays, gold particles, kaolin, mica, silica, silver nanoparticles, titanium dioxide, etc. Because of their impressive intrinsic mechanical properties, nanoscale dimensions and high aspect-ratio, nanofillers such as CNTs or nanoclays are among the most promising due to the fact that small amounts (less than 5 %) of them can provide the resulting nanocomposite material with significant property improvements.

### **2.1 Carbon Nanotubes (CNTs)**

CNTs are considered to be ideal candidates for a wide range of applications in materials science because of their exceptional mechanical, thermal, and electronic properties (Baughman & Zakhidov, 2002). Carbon nanotubes exist as two types of structures: singlewall carbon nanotubes (SWNTs) and multiwall carbon nanotubes (MWNTs). Fig. 1 shows the schematic pictures of different types of carbon nanotubes. SWNT can be considered as graphene sheet rolled cylinders of covalently bonded carbon atoms with very high aspect ratios of 1000 or more. MWNTs consist of a number of graphene cylinders concentrically nested like rings in a tree trunk with an interlayer distance of ~0.34 nm.

Fig. 1. A schematic illustration of a single-walled carbon nanotube and a multi-walled carbon nanotube (Dresselhaus et al., 2003).

The carbon-carbon covalent bonds in graphite and carbon nanotubes are considered to be one of the strongest atomic bonds in nature. The mechanical properties of CNTs have been extensively studied, both experimentally and theoretically (Yakobson et al., 1996; Lu, 1997). CNTs possess tensile moduli and strengths as high as 1 TPa and 150 GPa respectively. Especially in terms of strength CNTs are exceptional as this value is more than an order of magnitude higher than for high strength carbon fibres. Their density can be as low as 1.3 g.cm-3, which is lower than commercial carbon fibres at 1.8-1.9 g.cm-3. CNTs are thermally stable at up to 2800 oC in vacuum. Their thermal conductivity is about twice as high as that of diamond, while their electric-current-carrying capacity is 1000 times higher than that of copper wires (Thostenson et al., 2001).

Since the first CNTs and polymer composites were made in 1994 (Ajayan et al., 1994), large amounts of work have been done on polymer/CNT composites. Incorporating nanotubes into plastics can potentially provide structural materials with a dramatic increase in both stiffness and strength. Extensive studies have been carried out producing strong polymer/CNT composites (Coleman et al., 2006; Moniruzzaman & Winey, 2006; Ahir, 2005, Wang et al., 2007). The effective use of CNTs as reinforcements still presents some major difficulties (Wang et al., 2007). The key challenge still remains in breaking down bundles of aggregated CNTs and reaching a fine dispersion in the selected polymer matrix. Much work needs to be done to optimise the conditions required for the potential dispersion of nanotubes, especially at higher CNT loadings, as well as a good interfacial interaction (Moniruzzaman & Winey, 2006).

CNTs have extremely low electrical resistance and their electric-current-carrying capacity is 1000 times higher than copper wires (Thostenson et al., 2001). Devices have been developed using CNTs such as field-emission displays (Fan et al., 1999). CNTs have also been widely used to produce conductive polymer composites (CPCs). It has been reported that the percolation threshold in CPC can be as low as 0.0025 wt% in the case of low viscosity thermosetting resins (Bryning et al., 2005). In thermoplastics the percolation threshold is typically around 1 wt%, but can be significantly lower using latex technology (Lu et al., 2008) and is highly dependent on processing history (Zhang et al., 2009; Deng et al., 2009a and 2009b).

#### **2.2 Nanoclays**

370 Polypropylene

nanofibres, cellulose nanowhiskers, nanoclays, gold particles, kaolin, mica, silica, silver nanoparticles, titanium dioxide, etc. Because of their impressive intrinsic mechanical properties, nanoscale dimensions and high aspect-ratio, nanofillers such as CNTs or nanoclays are among the most promising due to the fact that small amounts (less than 5 %) of them can

CNTs are considered to be ideal candidates for a wide range of applications in materials science because of their exceptional mechanical, thermal, and electronic properties (Baughman & Zakhidov, 2002). Carbon nanotubes exist as two types of structures: singlewall carbon nanotubes (SWNTs) and multiwall carbon nanotubes (MWNTs). Fig. 1 shows the schematic pictures of different types of carbon nanotubes. SWNT can be considered as graphene sheet rolled cylinders of covalently bonded carbon atoms with very high aspect ratios of 1000 or more. MWNTs consist of a number of graphene cylinders concentrically nested like rings in a tree trunk with an interlayer distance of ~0.34 nm.

provide the resulting nanocomposite material with significant property improvements.

Fig. 1. A schematic illustration of a single-walled carbon nanotube and a multi-walled

The carbon-carbon covalent bonds in graphite and carbon nanotubes are considered to be one of the strongest atomic bonds in nature. The mechanical properties of CNTs have been extensively studied, both experimentally and theoretically (Yakobson et al., 1996; Lu, 1997). CNTs possess tensile moduli and strengths as high as 1 TPa and 150 GPa respectively. Especially in terms of strength CNTs are exceptional as this value is more than an order of magnitude higher than for high strength carbon fibres. Their density can be as low as 1.3 g.cm-3, which is lower than commercial carbon fibres at 1.8-1.9 g.cm-3. CNTs are thermally stable at up to 2800 oC in vacuum. Their thermal conductivity is about twice as high as that of diamond, while their electric-current-carrying capacity is 1000 times higher than that of

Since the first CNTs and polymer composites were made in 1994 (Ajayan et al., 1994), large amounts of work have been done on polymer/CNT composites. Incorporating nanotubes

**2.1 Carbon Nanotubes (CNTs)**

carbon nanotube (Dresselhaus et al., 2003).

copper wires (Thostenson et al., 2001).

Clay is a natural, earthy, fine-grained material and is the main constituent of the sedimentary rocks in marine sediments and in soils. Clay minerals belong to the family of phyllosilicates (or layered silicate) and have particles less than 2 µm in size as stated in ISO 14688 (Moore & Reynolds, 1997). There are three main groups of clays: kaolinite, montmorillonite-smectite and halloysite, and most clay are a mixture of these different types. The shape of clay minerals is of a distinctive character. Montmorillonite, which has irregular flakes, is the most commonly used nanoclay in polymer/clay nanocomposites (Fig. 2a). Recently, sepiolite, a fibrous shaped mineral, has gained increasing attention as nanofiller reinforcement (Y.P. Zheng &Y. Zheng, 2006). Sepiolite is a needle-like shaped nanoclay composed with elemental particles of lengths of 0.2-4 μm, widths of 10-30 nm and thicknesses of 5-10 nm (Fig. 2b). Sepiolite can have a surface area as high as 200-300 m2.g-1 and normally it is found stuck together as bundles of fibres which can form microagglomerates. Sepiolite is a hydrous magnesium silicate with [Si12O30Mg8(OH)4(H2O)4·8H2O] as the unit cell formula.

Compared with layered montmorillonite, the morphology of fibre-like sepiolite provides a lower specific surface area and smaller contact surface between the nanoclay particles. Therefore, polymer chains have a better chance not only of interacting with the external surface of the sepiolite, but also of penetrating into the structure, which facilitates a more even dispersion of the clays in the polymer matrix. Sepiolite provides a pseudoplastic and thixotropic behaviour which make it a valuable material in multiple applications to improve the processability, application or handling of the final product. Sepiolite has been successfully used to reinforce different polymers, such as poly (hydroxyethyl acrylate), epoxy, PP, etc (Bokobza et al., 2004; Y.P. Zheng &Y. Zheng, 2006; Bilotti et al., 2008).

Preparation of Polypropylene Nanocomposites Using Supercritical Technology 373

Solution intercalation has been known for over a century and has proved to be one of the most successful methods for incorporating nanofillers into polymers. Nanocomposites with water-soluble polymers such as poly (ethylene oxide) and poly (vinyl alcohol) and organic solvent-soluble polymers such as HDPE, have been successfully prepared via this method (Wu & Lerner, 1993; Ogata, et al., 1997; Joen et al., 1998; Wang et al., TP, 2007). In terms of PP nanocomposites, the poor solubility of PP in most organic solvents has severely limited the use of this method. Although PP nanocomposites have been reported being produced using xylene, tetrhydronaphthalene and decalin as solvents (Chang et al., 2005; Grady et al., 2002), elevated temperatures are necessary for the evaporation of these high-boiling solvents. Also, their application on an industrial scale is still hindered by the involvement of

In-situ polymerisation has been intensively investigated in recent years. The advantage of this process is that the polymer chain can be grafted onto the nanofillers on a molecular scale. This gives excellent dispersion and the potential for good interfacial strength between the nanofillers and the polymer matrix. A relatively good dispersion can be maintained even with high nanotube loading in the matrix. Successful investigations have been reported in the literature from different groups on PP nanocomposites (Ma et al., 2001; Zhao et al., 2003; Koval'chuk et.al, 2008; Funk & Kaminsky, 2007). PP/ MMT nanocomposites were prepared by in-situ polymerization using a Ziegler-Natta catalyst (Zhao et al., 2003). PP/ MWNT nanocomposites have been fabricated by Funck and Kaminsky (2007) using a metallocene/ methylaluminoxane (MAO) catalyst. However, the molecular weight of the polymer is often

Melt compounding is the most common method used to create thermoplastic polymer nanocomposites because it is a cost-effective technology for polyolefin-based systems and is compatible with current industrial practices, such as extrusion, injection moulding, etc (Andrews et al., 2002). However, melt compounding, especially in the case of PP matrices, is generally less effective at dispersing nanofillers such as CNTs and clays, and is limited to low nanofiller loadings due to the high viscosity of the composite systems caused by the addition of nanofillers (Andrews et al., 2002). Moreover, the high shear rates and high temperatures utilised can also cause thermal instability of the molten polymers (Potschke et al., 2003). One approach is using the masterbatch process, where a pre-mixed highly loaded

Much effort has been expended to facilitate the achievement of good dispersion of nanofillers and efficient stress transfer. The de-agglomeration of CNTs is necessary for dispersing individual nanotubes before mixing them with the polymer matrix. The ultrasonication process is the most common method, but severe sonication may make the tubes shorter (Saito et al., 2002, Inam et al., TP, J. Comp Materials, 2011). Milling and grinding is considered to be a cheap and fast method for industrial processes although it is the most destructive method for CNTs (Pierard et al., 2001). Non-covalent functionalizations, such as surfactants, are often utilised to overcome carbon nanotube entanglements resulting from Van der Waals forces (Bonduel et al., 2005). Covalent functionalization of CNTs helps to break up the CNT bundles and improves the polymer / CNT interfacial adhesion (Liu et al., 2005). Polymer wrapping has been proposed as an alternative method to achieve a good dispersion of the CNTs without destroying their electrical properties (Star et al., 2001). Dissociated CNTs were produced by grafting polymer chains directly onto the CNTs to achieve a homogeneous polymer coating on the CNT

significantly lower with a wide distribution by comparison with other methods.

nanofiller composite is diluted with a fresh polymer melt (Prashantha et al., 2009).

large quantities of organic solvent.

## **3. Polypropylene nanocomposites**

For decades we have been dealing with polymer microcomposites, where the length scale of the fillers is in micrometres. In the case of polymer nanocomposites, nanofillers, being additives of nanometre scale, are dispersed in a polymer matrix, offering multifunctional, high-performance polymer characteristics beyond those possessed by traditional filled polymeric materials. Improvements in physical and mechanical properties have been well documented in the literature (Dubois & Alexandre, 2006; Thostenson et al., 2001; Koo, 2006). Apart from mechanical enhancements, the value of polymer nanocomposites comes from providing value-added properties not present in the polymer matrix, without sacrificing the inherent processability and mechanical properties of the matrix. The multifunctional features consist of improved thermal, fire, and moisture resistance, decreased permeability, charge dissipation, and chemical resistance (Moniruzzaman & Winey, 2006; Bokobza et al., 2004; Koo, 2006). Because of PP's good balance between properties and cost and its wide usage in industry, PP nanocomposites have been extensively studied in recent years (Deng et al., 2009, 2010; Bilotti et al., 2008; Andrews et al., 2002; Liu & Wu, 2001).

The properties of nanocomposites can be greatly affected by the dispersion of the nanofillers in the polymer matrix (Ray & Okamoto, 2003). Generally, the better the dispersion, the better is the properties of the final nanocomposite. However, nanofillers are, in essence, agglomerates due to their high surface energy, and it is very difficult to disperse them in most polymers. Due to the lack of polar groups in PP, many efforts have been attempted to improve the dispersion of inorganic fillers such as clay and CNTs into a PP matrix for the preparation of effective PP nanocomposites and enhanced mechanical properties. The achievement of well-dispersed nanofillers is the most investigated research topic worldwide (Coleman et al., 2006; Koo, 2006; Ray & Okamoto, 2000; Lu et al., 2008).

#### **4. Preparation methods**

#### **4.1 Traditional processing methods**

Polymer nanocomposites are generally prepared by three methods: solution intercalation, in-situ polymerisation or melt compounding.

(a) (b)

Fig. 2. The structure of (a) montmorillonite (Beermann & Brockamp, 2005) and (b) sepiolite

For decades we have been dealing with polymer microcomposites, where the length scale of the fillers is in micrometres. In the case of polymer nanocomposites, nanofillers, being additives of nanometre scale, are dispersed in a polymer matrix, offering multifunctional, high-performance polymer characteristics beyond those possessed by traditional filled polymeric materials. Improvements in physical and mechanical properties have been well documented in the literature (Dubois & Alexandre, 2006; Thostenson et al., 2001; Koo, 2006). Apart from mechanical enhancements, the value of polymer nanocomposites comes from providing value-added properties not present in the polymer matrix, without sacrificing the inherent processability and mechanical properties of the matrix. The multifunctional features consist of improved thermal, fire, and moisture resistance, decreased permeability, charge dissipation, and chemical resistance (Moniruzzaman & Winey, 2006; Bokobza et al., 2004; Koo, 2006). Because of PP's good balance between properties and cost and its wide usage in industry, PP nanocomposites have been extensively studied in recent years (Deng

The properties of nanocomposites can be greatly affected by the dispersion of the nanofillers in the polymer matrix (Ray & Okamoto, 2003). Generally, the better the dispersion, the better is the properties of the final nanocomposite. However, nanofillers are, in essence, agglomerates due to their high surface energy, and it is very difficult to disperse them in most polymers. Due to the lack of polar groups in PP, many efforts have been attempted to improve the dispersion of inorganic fillers such as clay and CNTs into a PP matrix for the preparation of effective PP nanocomposites and enhanced mechanical properties. The achievement of well-dispersed nanofillers is the most investigated research topic worldwide

Polymer nanocomposites are generally prepared by three methods: solution intercalation,

et al., 2009, 2010; Bilotti et al., 2008; Andrews et al., 2002; Liu & Wu, 2001).

(Coleman et al., 2006; Koo, 2006; Ray & Okamoto, 2000; Lu et al., 2008).

(Bilotti et al., 2008)

**4. Preparation methods**

**4.1 Traditional processing methods**

in-situ polymerisation or melt compounding.

**3. Polypropylene nanocomposites**

Solution intercalation has been known for over a century and has proved to be one of the most successful methods for incorporating nanofillers into polymers. Nanocomposites with water-soluble polymers such as poly (ethylene oxide) and poly (vinyl alcohol) and organic solvent-soluble polymers such as HDPE, have been successfully prepared via this method (Wu & Lerner, 1993; Ogata, et al., 1997; Joen et al., 1998; Wang et al., TP, 2007). In terms of PP nanocomposites, the poor solubility of PP in most organic solvents has severely limited the use of this method. Although PP nanocomposites have been reported being produced using xylene, tetrhydronaphthalene and decalin as solvents (Chang et al., 2005; Grady et al., 2002), elevated temperatures are necessary for the evaporation of these high-boiling solvents. Also, their application on an industrial scale is still hindered by the involvement of large quantities of organic solvent.

In-situ polymerisation has been intensively investigated in recent years. The advantage of this process is that the polymer chain can be grafted onto the nanofillers on a molecular scale. This gives excellent dispersion and the potential for good interfacial strength between the nanofillers and the polymer matrix. A relatively good dispersion can be maintained even with high nanotube loading in the matrix. Successful investigations have been reported in the literature from different groups on PP nanocomposites (Ma et al., 2001; Zhao et al., 2003; Koval'chuk et.al, 2008; Funk & Kaminsky, 2007). PP/ MMT nanocomposites were prepared by in-situ polymerization using a Ziegler-Natta catalyst (Zhao et al., 2003). PP/ MWNT nanocomposites have been fabricated by Funck and Kaminsky (2007) using a metallocene/ methylaluminoxane (MAO) catalyst. However, the molecular weight of the polymer is often significantly lower with a wide distribution by comparison with other methods.

Melt compounding is the most common method used to create thermoplastic polymer nanocomposites because it is a cost-effective technology for polyolefin-based systems and is compatible with current industrial practices, such as extrusion, injection moulding, etc (Andrews et al., 2002). However, melt compounding, especially in the case of PP matrices, is generally less effective at dispersing nanofillers such as CNTs and clays, and is limited to low nanofiller loadings due to the high viscosity of the composite systems caused by the addition of nanofillers (Andrews et al., 2002). Moreover, the high shear rates and high temperatures utilised can also cause thermal instability of the molten polymers (Potschke et al., 2003). One approach is using the masterbatch process, where a pre-mixed highly loaded nanofiller composite is diluted with a fresh polymer melt (Prashantha et al., 2009).

Much effort has been expended to facilitate the achievement of good dispersion of nanofillers and efficient stress transfer. The de-agglomeration of CNTs is necessary for dispersing individual nanotubes before mixing them with the polymer matrix. The ultrasonication process is the most common method, but severe sonication may make the tubes shorter (Saito et al., 2002, Inam et al., TP, J. Comp Materials, 2011). Milling and grinding is considered to be a cheap and fast method for industrial processes although it is the most destructive method for CNTs (Pierard et al., 2001). Non-covalent functionalizations, such as surfactants, are often utilised to overcome carbon nanotube entanglements resulting from Van der Waals forces (Bonduel et al., 2005). Covalent functionalization of CNTs helps to break up the CNT bundles and improves the polymer / CNT interfacial adhesion (Liu et al., 2005). Polymer wrapping has been proposed as an alternative method to achieve a good dispersion of the CNTs without destroying their electrical properties (Star et al., 2001). Dissociated CNTs were produced by grafting polymer chains directly onto the CNTs to achieve a homogeneous polymer coating on the CNT

Preparation of Polypropylene Nanocomposites Using Supercritical Technology 375

Supercritical fluids are unique solvents with a wide range of interesting properties. Supercritical fluids have high diffusivities, similar to gas which allows them to effuse through solids, while also having liquid-like densities that allow them to act as effective solvents for many compounds. In addition, small changes in pressure and temperature greatly affect the density of a supercritical fluid and therefore many properties of SCF can be 'fine-tuned'. The unique properties of supercritical fluids allow them to be widely exploited in materials processing. The most promising developments are the processing of fine powders, core-shell particles, the processing and / or impregnation of aerogels, foams,

Among all the supercritical fluids, the use of scCO2 is the most desirable for polymer processing because of its environmental compatibility as well as the following properties:

CO2 has relatively easily accessible critical points of 31.06 °C and 1070 psi (7.38 MPa)

The density of scCO2 is easily tunable (Fig. 4). Therefore, the solvent strength and

CO2 is non-combustible and non-toxic in contrast to most of the organic solvents

It is also easily available because it occurs naturally as well as being the by-product of

As CO2 is a gas at ambient temperatures and pressures it can be easily removed, leaving

surface modifications and the processing of polymers. (Cansell et al., 2003)

many industrial processes; therefore CO2 is relatively inexpensive

no solvent residues in the processed material (Behles & Desimone, 2001)

Fig. 4. The density of CO2 as a function of pressure for a range of temperatures

ScCO2 has found widespread application in industrial processes, including the extraction of metals or organic material, the decaffeination of green coffee beans, dry cleaning and degreasing, nano- and micro-particle formation, impregnation and dyeing, the processing and synthesis of polymers and composites, tissue engineering scaffolds, drug delivery, etc (Quirk et al., 2004; Hyde et al., 2001). The important fields most relevant to this Chapter are in the scCO2 processing of polymers and composites which will be discussed in greater

(Hyatt, 1984)

detail below.

processes can be easily controlled

suitable for supercritical applications

surface (Peeterbroeck et al., 2007; Dubois & Alexandre, 2006). High-density polyethylene has been used to coat MWNTs when producing PP nanocomposites and composites in other polymer matrices such as ethylene-vinyl acetate, polycarbonate and polyamide (Deng et al., 2010; Star et al., 2001; Peeterbroeck et al., 2007; Dubois & Alexandre, 2006; Pötschke et al., 2008). Compared with the direct incorporation of CNTs in polymer melts, enhanced dispersion and improved properties have been reported.

There are two ways to modify the surface of hydrophilic clays in order to improve their dispersion in the polymer matrix. The first one is to modify the surface with cationic surfactants to make the silicate surface organophilic via ion exchange reactions (Lan & Pinnavaia, 1994; Shi et al., 1996). The second method is based on grafting polymeric molecules through covalent bonding to the hydroxyl groups existing on the particles. However, for polyolefin polymers such as PP and PE, they are non-polar and incompatible with silicate surfaces even after modifying them with non-polar long alkyl groups. Therefore, a compatibilizer is often needed to facilitate the interaction between the polymer and the clays (Hasegawa et al., 2000; Garcia-Lopez et al., 2003). Compatibilizers are usually polar functional oligomers providing both a hydrophobic part (which can be easily mixed with a polymer) and a hydrophilic part (which is compatible with clay). Maleic anhydride grafted polypropylene (PP-g-MA) is a commonly used compatibilizer to aid the dispersion of clay or CNTs in PP matrix (Bilotti et al, 2008 and 2010).

#### **4.2 The supercritical fluid technique**

In recent years, supercritical technology, especially supercritical carbon dioxide (scCO2), has been widely applied in the processing of polymer nanocomposites. A supercritical fluid is defined as "any substance, the temperature and pressure of which are higher than their critical values, and which has a density close to, or higher than, its critical density" (Darr & Poliakoff, 1999). Fig. 3 shows a schematic representation of the density and organization of molecules of a pure fluid in solid state, gas state, liquid state and the supercritical domain. No phase separation occurs for any substance at pressures or temperatures above its critical values. In other words, the critical point represents the highest temperature and pressure at which gas and liquid can coexist in equilibrium.

Fig. 3. A schematic representation of the microscopic behaviour of a pure fluid in the P-T plane phase diagram (Cansell et al., 2003)

surface (Peeterbroeck et al., 2007; Dubois & Alexandre, 2006). High-density polyethylene has been used to coat MWNTs when producing PP nanocomposites and composites in other polymer matrices such as ethylene-vinyl acetate, polycarbonate and polyamide (Deng et al., 2010; Star et al., 2001; Peeterbroeck et al., 2007; Dubois & Alexandre, 2006; Pötschke et al., 2008). Compared with the direct incorporation of CNTs in polymer melts, enhanced

There are two ways to modify the surface of hydrophilic clays in order to improve their dispersion in the polymer matrix. The first one is to modify the surface with cationic surfactants to make the silicate surface organophilic via ion exchange reactions (Lan & Pinnavaia, 1994; Shi et al., 1996). The second method is based on grafting polymeric molecules through covalent bonding to the hydroxyl groups existing on the particles. However, for polyolefin polymers such as PP and PE, they are non-polar and incompatible with silicate surfaces even after modifying them with non-polar long alkyl groups. Therefore, a compatibilizer is often needed to facilitate the interaction between the polymer and the clays (Hasegawa et al., 2000; Garcia-Lopez et al., 2003). Compatibilizers are usually polar functional oligomers providing both a hydrophobic part (which can be easily mixed with a polymer) and a hydrophilic part (which is compatible with clay). Maleic anhydride grafted polypropylene (PP-g-MA) is a commonly used compatibilizer to aid the dispersion

In recent years, supercritical technology, especially supercritical carbon dioxide (scCO2), has been widely applied in the processing of polymer nanocomposites. A supercritical fluid is defined as "any substance, the temperature and pressure of which are higher than their critical values, and which has a density close to, or higher than, its critical density" (Darr & Poliakoff, 1999). Fig. 3 shows a schematic representation of the density and organization of molecules of a pure fluid in solid state, gas state, liquid state and the supercritical domain. No phase separation occurs for any substance at pressures or temperatures above its critical values. In other words, the critical point represents the highest temperature and pressure at

Fig. 3. A schematic representation of the microscopic behaviour of a pure fluid in the P-T

dispersion and improved properties have been reported.

of clay or CNTs in PP matrix (Bilotti et al, 2008 and 2010).

**4.2 The supercritical fluid technique**

which gas and liquid can coexist in equilibrium.

plane phase diagram (Cansell et al., 2003)

Supercritical fluids are unique solvents with a wide range of interesting properties. Supercritical fluids have high diffusivities, similar to gas which allows them to effuse through solids, while also having liquid-like densities that allow them to act as effective solvents for many compounds. In addition, small changes in pressure and temperature greatly affect the density of a supercritical fluid and therefore many properties of SCF can be 'fine-tuned'. The unique properties of supercritical fluids allow them to be widely exploited in materials processing. The most promising developments are the processing of fine powders, core-shell particles, the processing and / or impregnation of aerogels, foams, surface modifications and the processing of polymers. (Cansell et al., 2003)

Among all the supercritical fluids, the use of scCO2 is the most desirable for polymer processing because of its environmental compatibility as well as the following properties:

Fig. 4. The density of CO2 as a function of pressure for a range of temperatures

ScCO2 has found widespread application in industrial processes, including the extraction of metals or organic material, the decaffeination of green coffee beans, dry cleaning and degreasing, nano- and micro-particle formation, impregnation and dyeing, the processing and synthesis of polymers and composites, tissue engineering scaffolds, drug delivery, etc (Quirk et al., 2004; Hyde et al., 2001). The important fields most relevant to this Chapter are in the scCO2 processing of polymers and composites which will be discussed in greater detail below.

Preparation of Polypropylene Nanocomposites Using Supercritical Technology 377

Garcia-Leiner and Lesser (2004) have studied the use of scCO2 in the processing of a variety of polymers (s-PS, FEP, and PTFE) using a modified single-screw extruder with a CO2 injection (Fig. 5). The present of CO2 significantly enhanced the processability of the polymer–CO2 system by plasticization effect and the hydrostatic contribution. This presented an effective alternative to process intractable or high melt viscosity polymers.

Fig. 5. Modified extrusion system for CO2-assisted polymer processing (Garcia-Leiner &

agents, and nanoparticles (Kikic & Vecchione, 2003).

A wide range of opportunities have opened up for scCO2 that have an impact on the plastics industry (Tomasko et al., 2003). These include usages for extraction, foaming and impregnation. Extraction occurs merely by removing the soluble extractant material, such as any unreacted monomer, while leaving the insoluble substrate. Foaming occurs when rapid decompression forms gaseous CO2 inside the polymer, leaving the polymer in the form of a porous (micro) material. This is very important in supporting the growth of blood vessels and collagen fibres in the matrix of biodegradable polymers or when the final product is intended to be used as a catalyst. As for impregnation, CO2 can act as a transport medium facilitating the diffusion of monomers, initiators and molecules to impregnate a polymer, while the CO2 can be cleanly removed afterwards. A better dispersion of the molecules has been provided within the polymer matrix. Substances impregnated into polymers have included dyes, fragrances, drugs for controlled release, anti-microbial and anti-fungal

The effective dispersion of the fillers in the polymer matrix and the improvement of polymer-filler interactions are two key challenges in the field of polymer nanocomposites. The development of polymer processing technologies in scCO2 has enabled the synthesis of very complex polymer nanocomposites. Zerda et al. (2003) developed poly (methyl methacrylate) / montmorillonite composites via in-situ polymerization. ScCO2 was used as the reaction medium to distribute homogeneously the monomer and the initiator and allowing polymerization under lower viscosity conditions. The modulus of the nanocomposites increased 50% with 40 wt % of clay content. A significant improvement in physical properties has also been reported by Green et al. (2000) for scCO2 processed poly

Lesser, 2004)

ScCO2 processing, as one of the new and cleaner processing methods for polymer nanocomposites, has recently received increasing attention. The solubility of polymers in scCO2 is poor for many high molecular weight polymers. Table 1 gives a representative sample of the available literature data for the solubility of CO2 in a variety of polymers. The low solubility is a result of the lower density of scCO2 and the weak interaction between the CO2 molecules and the non-polar groups of many polymers (Cansell et al., 2003). However, even for polymers which are not soluble in scCO2, the CO2 is still able to permeate resulting in substantial and sometimes dramatic changes in the properties of these polymers. The permeation of scCO2 into a polymer causes it to swell. Aided by their zero surface tension, the addition of scCO2 into the polymer phase gives the chains a greater mobility. The CO2 molecules act as lubricants, which reduces the chain-chain interactions as it increases the interchain distance and free volume of the polymer. This is called plasticization. The physical properties of the polymer are changed dramatically, including the depression of the glass transition temperature (*Tg*), the lowering of interfacial tension and a reduction of the viscosity of the polymer melt. ScCO2 may increase the crystallinity of the polymers because the polymer chains are given more freedom to align themselves into a more favourable order.

aMethod: GM-M, gravimetric method (microbalance); GM-D, gravimetric method (desorption); BM, barometric method. Units: SCC/cm3, cm3(STP)/cm3 of polymer; g/100g, g of CO2/100g of polymer.

Table 1. The solubility of CO2 in polymers (Tomasko et al., 2003)

ScCO2 processing, as one of the new and cleaner processing methods for polymer nanocomposites, has recently received increasing attention. The solubility of polymers in scCO2 is poor for many high molecular weight polymers. Table 1 gives a representative sample of the available literature data for the solubility of CO2 in a variety of polymers. The low solubility is a result of the lower density of scCO2 and the weak interaction between the CO2 molecules and the non-polar groups of many polymers (Cansell et al., 2003). However, even for polymers which are not soluble in scCO2, the CO2 is still able to permeate resulting in substantial and sometimes dramatic changes in the properties of these polymers. The permeation of scCO2 into a polymer causes it to swell. Aided by their zero surface tension, the addition of scCO2 into the polymer phase gives the chains a greater mobility. The CO2 molecules act as lubricants, which reduces the chain-chain interactions as it increases the interchain distance and free volume of the polymer. This is called plasticization. The physical properties of the polymer are changed dramatically, including the depression of the glass transition temperature (*Tg*), the lowering of interfacial tension and a reduction of the viscosity of the polymer melt. ScCO2 may increase the crystallinity of the polymers because the polymer

chains are given more freedom to align themselves into a more favourable order.

**[atm]**

**poly(methyl methacrylate)** GM-M 50 65 46 SCC/cm3

**high-impact polystyrene** GM-D 204 70 0.5 wt.%

**poly(ethylene terephthalate)** GM-D 136 40 1.5 wt.% **poly(vinyl chloride)** GM-D 68 25 8 g/100g

**poly(vinyl acetate)** GM-D 54.4 25 29 g/100g **low density polyethylene** GM-D 68 40 0.2 wt.% **high density polyethylene** GM-D 68 40 0.1 wt.% **polypropylene** GM-D 68 40 0.1 wt.%

**Nylon 66** GM-D 68 40 1.8 wt.% **polyurethane** BM 136 40 2.2 wt.% **Teflon** GM-D 68 40 0.0 wt.%

aMethod: GM-M, gravimetric method (microbalance); GM-D, gravimetric method (desorption); BM, barometric method. Units: SCC/cm3, cm3(STP)/cm3 of polymer; g/100g, g of CO2/100g of polymer.

Table 1. The solubility of CO2 in polymers (Tomasko et al., 2003)

**polystyrene** GM-M 13.2 25 14.5 SCC/cm3

**polycarbonate** GM-M 13.2 25 24 SCC/cm3

**Temp.**

GM-D 204 70 10.5 wt.%

GM-D 68 25 13 g/100g

GM-D 136 40 0.1 wt.%

GM-D 204 25 0.1 wt.% BM 73 160 1.59 g/100g BM 61.2 200 1.09 g/100g

**[°C] Solubility**

**Polymer Methoda Pressure**

Garcia-Leiner and Lesser (2004) have studied the use of scCO2 in the processing of a variety of polymers (s-PS, FEP, and PTFE) using a modified single-screw extruder with a CO2 injection (Fig. 5). The present of CO2 significantly enhanced the processability of the polymer–CO2 system by plasticization effect and the hydrostatic contribution. This presented an effective alternative to process intractable or high melt viscosity polymers.

Fig. 5. Modified extrusion system for CO2-assisted polymer processing (Garcia-Leiner & Lesser, 2004)

A wide range of opportunities have opened up for scCO2 that have an impact on the plastics industry (Tomasko et al., 2003). These include usages for extraction, foaming and impregnation. Extraction occurs merely by removing the soluble extractant material, such as any unreacted monomer, while leaving the insoluble substrate. Foaming occurs when rapid decompression forms gaseous CO2 inside the polymer, leaving the polymer in the form of a porous (micro) material. This is very important in supporting the growth of blood vessels and collagen fibres in the matrix of biodegradable polymers or when the final product is intended to be used as a catalyst. As for impregnation, CO2 can act as a transport medium facilitating the diffusion of monomers, initiators and molecules to impregnate a polymer, while the CO2 can be cleanly removed afterwards. A better dispersion of the molecules has been provided within the polymer matrix. Substances impregnated into polymers have included dyes, fragrances, drugs for controlled release, anti-microbial and anti-fungal agents, and nanoparticles (Kikic & Vecchione, 2003).

The effective dispersion of the fillers in the polymer matrix and the improvement of polymer-filler interactions are two key challenges in the field of polymer nanocomposites. The development of polymer processing technologies in scCO2 has enabled the synthesis of very complex polymer nanocomposites. Zerda et al. (2003) developed poly (methyl methacrylate) / montmorillonite composites via in-situ polymerization. ScCO2 was used as the reaction medium to distribute homogeneously the monomer and the initiator and allowing polymerization under lower viscosity conditions. The modulus of the nanocomposites increased 50% with 40 wt % of clay content. A significant improvement in physical properties has also been reported by Green et al. (2000) for scCO2 processed poly

Preparation of Polypropylene Nanocomposites Using Supercritical Technology 379

(2010) studied the use of scCO2 to assist melt compounding of PP nanocomposites. The nanocomposites are prepared in an autoclave. The autoclave was filled with liquid CO2 and held at 2175 psi and 200 °C under stirring for 30 min. In their work, pristine MWNTs without HDPE coating were observed to exhibit good dispersion in the PP matrix under scCO2 assisted mixing (Fig. 7c). This indicates the great efficiency of scCO2 being a

Fig. 7. SEM images of freeze-fractured samples of PP nanocomposites with 0.5 wt.% MWNT loading (a) melt-compounded MWNT (b) melt-compounded PP/coated MWNT (Deng et

In Zhao & Huang' (2008) work, a better dispersion of clays in PP matrix was also confirmed with scCO2 assisted mixing. Fig. 8 shows that the sample prepared with 2.5% CO2 has more

Ma et al. (2007) also reported a good dispersion of sepiolite clays in PP matrix with scCO2 assisted mixing. Without the use of any compatibilizer, the dispersion of sepiolite in the PP matrix improved significantly in the scCO2 assisted mixing compared to melt mixing. This indicates that compatibilizers such as PP-g-MA are not needed to achieve good dispersions in the case of scCO2 assisted processing of polyolefins. This is of particular relevance because the role of compatibilizers, such as PP-g-MA, is generally regarded as essential for the creation of well dispersed nanoclays composites based on polyolefins using traditional

al., 2010) (c) scCO2 PP/MWNT (Ma et al., 2010)

(c)

(a) (b)

melt compounding methods.

uniform distributed clay and aggregations of clay are much thinner.

processing lubricant between the polymer chains enhancing the polymer diffusion.

(methyl methacrylate) / silicate nanocomposites, in which scCO2 acted as both a plasticizer for the polymer matrix and a carrier for the monomer. Polystyrene / clay has also been produced via in-situ polymerization by Li et al. (2006), where styrene monomer and initiator were directly intercalated into organomontmorillonite (OMMT) with the aid of sCCO2.

For PP nanocomposites, PP will become plasticized when treated with CO2. Varma-Nair et al. (2003) have suggested that 1% CO2 could be dissolved in PP at 50 °C at 180 psi. Both decreases in *Tm* and *Tc* of PP in presence of CO2 have been reported in the literature (Varma-Nair et al., 2003; Garcia-Leiner & Lesser, 2003). Studies in PP have indicated improvements in the dispersion of nanofillers in the matrix via melt compounding in scCO2 (Garcia-Leiner & Lesser, 2003, Ma et al, 2007, Zhao & Huang, 2008). Zhao & Huang (2008) improved a twinscrew extruder with the aid of scCO2 for the preparation of PP / clay nanocomposites (see Fig. 6). A CO2 injection system was connected to the extruder and several kneading and reverse conveying elements were added to prevent CO2 from leaking. This continuous extrusion process of PP nanocomposites using scCO2 has set good examples for scCO2 usage in an industrial environment.

Fig. 6. ScCO2 used in the extrusion process for the preparation of PP/ clay composites (Zhao & Huang, 2008)

## **5. Morphology and properties of PP nanocomposites**

The preparation methods have crucial impact on the dispersion of the nanofillers and the final properties of the nanocomposites. In this section, the morphology of the PP nanocomposites and various nanocomposite properties will be discussed.

#### **5.1 Morphology**

Polymer-grafted CNTs were used in the study of Yang et al. (2008). They reported that PP-g-MWNTs were dispersed individually in the PP matrix at 1.5 wt%, but difficulty was found at higher CNT contents (Yang et al., 2008). Deng et al. (2010) reported that large bundles of MWNTs were present in their PP/MWNT samples using traditional melt compounding methods and better dispersions of MWNTs were observed for the HDPE coated MWNTs samples. The scanning electron microscopy (SEM) images of fracture surfaces of the nanocomposites are presented in Fig. 7. The HDPE coats the MWNTs and reduces the tendency of the nanotubes to aggregate and is partly miscible with the PP matrix. Ma et al.

(methyl methacrylate) / silicate nanocomposites, in which scCO2 acted as both a plasticizer for the polymer matrix and a carrier for the monomer. Polystyrene / clay has also been produced via in-situ polymerization by Li et al. (2006), where styrene monomer and initiator were directly intercalated into organomontmorillonite (OMMT) with the aid of sCCO2.

For PP nanocomposites, PP will become plasticized when treated with CO2. Varma-Nair et al. (2003) have suggested that 1% CO2 could be dissolved in PP at 50 °C at 180 psi. Both decreases in *Tm* and *Tc* of PP in presence of CO2 have been reported in the literature (Varma-Nair et al., 2003; Garcia-Leiner & Lesser, 2003). Studies in PP have indicated improvements in the dispersion of nanofillers in the matrix via melt compounding in scCO2 (Garcia-Leiner & Lesser, 2003, Ma et al, 2007, Zhao & Huang, 2008). Zhao & Huang (2008) improved a twinscrew extruder with the aid of scCO2 for the preparation of PP / clay nanocomposites (see Fig. 6). A CO2 injection system was connected to the extruder and several kneading and reverse conveying elements were added to prevent CO2 from leaking. This continuous extrusion process of PP nanocomposites using scCO2 has set good examples for scCO2 usage

Fig. 6. ScCO2 used in the extrusion process for the preparation of PP/ clay composites (Zhao

The preparation methods have crucial impact on the dispersion of the nanofillers and the final properties of the nanocomposites. In this section, the morphology of the PP

Polymer-grafted CNTs were used in the study of Yang et al. (2008). They reported that PP-g-MWNTs were dispersed individually in the PP matrix at 1.5 wt%, but difficulty was found at higher CNT contents (Yang et al., 2008). Deng et al. (2010) reported that large bundles of MWNTs were present in their PP/MWNT samples using traditional melt compounding methods and better dispersions of MWNTs were observed for the HDPE coated MWNTs samples. The scanning electron microscopy (SEM) images of fracture surfaces of the nanocomposites are presented in Fig. 7. The HDPE coats the MWNTs and reduces the tendency of the nanotubes to aggregate and is partly miscible with the PP matrix. Ma et al.

**5. Morphology and properties of PP nanocomposites**

nanocomposites and various nanocomposite properties will be discussed.

in an industrial environment.

& Huang, 2008)

**5.1 Morphology**

(2010) studied the use of scCO2 to assist melt compounding of PP nanocomposites. The nanocomposites are prepared in an autoclave. The autoclave was filled with liquid CO2 and held at 2175 psi and 200 °C under stirring for 30 min. In their work, pristine MWNTs without HDPE coating were observed to exhibit good dispersion in the PP matrix under scCO2 assisted mixing (Fig. 7c). This indicates the great efficiency of scCO2 being a processing lubricant between the polymer chains enhancing the polymer diffusion.

Fig. 7. SEM images of freeze-fractured samples of PP nanocomposites with 0.5 wt.% MWNT loading (a) melt-compounded MWNT (b) melt-compounded PP/coated MWNT (Deng et al., 2010) (c) scCO2 PP/MWNT (Ma et al., 2010)

In Zhao & Huang' (2008) work, a better dispersion of clays in PP matrix was also confirmed with scCO2 assisted mixing. Fig. 8 shows that the sample prepared with 2.5% CO2 has more uniform distributed clay and aggregations of clay are much thinner.

Ma et al. (2007) also reported a good dispersion of sepiolite clays in PP matrix with scCO2 assisted mixing. Without the use of any compatibilizer, the dispersion of sepiolite in the PP matrix improved significantly in the scCO2 assisted mixing compared to melt mixing. This indicates that compatibilizers such as PP-g-MA are not needed to achieve good dispersions in the case of scCO2 assisted processing of polyolefins. This is of particular relevance because the role of compatibilizers, such as PP-g-MA, is generally regarded as essential for the creation of well dispersed nanoclays composites based on polyolefins using traditional melt compounding methods.

Preparation of Polypropylene Nanocomposites Using Supercritical Technology 381

The elongation at break of the nanocomposites is often decreased. The increase of nanofiller content compromises the ductile nature of the PP matrix. Yielding followed by drawing still occurred in the 1 wt% sepiolite samples; while 2.5 wt% and 5 wt% sepiolite samples show

The effect on the modulus and yield stress of the scCO2 processed PP nanocomposites was studied for the addition of MWNTs and sepiolite clays (Ma et al., 2007 and 2010). The results

33

(a)

Fig. 10. The Young's modulus and yield stress of scCO2 processed and melt-compounded PP nanocomposites (a) PP / MWNT nanocomposites (b) PP / sepiolite nanocomposites. The label, melt.mPP/cMWNT, in the figure represents melt-compounded and masterbatch based HDPE-coated MWNT PP nanocomposites (Ma et al., 2007; Ma et al., 2010)

(b)

Yield stress (MPa)

0.0 0.2 0.4 0.6 0.8 1.0 MWNT content [wt.%]

012345 Clay content (wt.%)

scCO2 PP/MWNT scCO2.m PP/MWNT melt.m PP/cMWNT

scCO2 PP/sepiolite melt PP/sepiolite melt PP/PP-g-MA/sepiolite

34

35

Yield Stress [MPa]

36

37

were compared with traditional melt compounded nanocomposites.

scCO2 PP/MWNT scCO2.m PP/MWNT melt.m PP/cMWNT

0.0 0.2 0.4 0.6 0.8 1.0 MWNT content [wt.%]

012345 Clay content (wt.%)

scCO2 PP/sepiolite melt PP/sepiolite melt PP/PP-g-MA/sepiolite

brittle fractures at low strains (Fig. 9).

1.2

1

1.2

1.4

1.6

Young's modulus (GPa)

1.8

2

2.2

1.4

1.6

Young's Modulus [GPa]

1.8

Fig. 8. TEM images of PP/clay nanocomposites prepared (a) without and (b) with 2.5% CO2 (Zhao & Huang, 2008)

#### **5.2 Mechanical properties**

The mechanical properties including the tensile strength and Young's modulus of the PP nanocomposites can be significantly enhanced by the incorporation of nanofillers. Andrews et al. (2002) fabricated PP / MWNT composites by a shear mixer and found a modulus increase from 1.0 GPa to 2.4 GPa with a relatively high nanotube content of 12.5 wt%. However, this was at the expense of a reduction in yield stress from 30 MPa to 18 MPa. Similarly, Wang and Sheng (2005) found that the modulus of the PP nanocomposites increased from 788 MPa to 908 MPa by the addition of 7 wt% Attapulgite clay, but at the expense of a reduction in strength from 32 MPa to 26 MPa. Deng et al. (2010) investigated the effects of a HDPE coating onto MWNTs on the mechanical properties of PP / MWNT composites produced by melt compounding. A property increase from 1.42 GPa to 1.79 GPa for Young's modulus and from 34.5 MPa to 37.9 MPa for tensile strength, was achieved at relatively low loadings (0.5 wt%) of coated MWNTs.

Fig. 9. Stress-strain curves of scCO2 PP / sepiolite nanocomposites with different clay contents (Ma et al., 2007)

Fig. 8. TEM images of PP/clay nanocomposites prepared (a) without and (b) with 2.5% CO2

The mechanical properties including the tensile strength and Young's modulus of the PP nanocomposites can be significantly enhanced by the incorporation of nanofillers. Andrews et al. (2002) fabricated PP / MWNT composites by a shear mixer and found a modulus increase from 1.0 GPa to 2.4 GPa with a relatively high nanotube content of 12.5 wt%. However, this was at the expense of a reduction in yield stress from 30 MPa to 18 MPa. Similarly, Wang and Sheng (2005) found that the modulus of the PP nanocomposites increased from 788 MPa to 908 MPa by the addition of 7 wt% Attapulgite clay, but at the expense of a reduction in strength from 32 MPa to 26 MPa. Deng et al. (2010) investigated the effects of a HDPE coating onto MWNTs on the mechanical properties of PP / MWNT composites produced by melt compounding. A property increase from 1.42 GPa to 1.79 GPa for Young's modulus and from 34.5 MPa to 37.9 MPa for tensile strength, was achieved at

Fig. 9. Stress-strain curves of scCO2 PP / sepiolite nanocomposites with different clay

(Zhao & Huang, 2008)

**5.2 Mechanical properties**

contents (Ma et al., 2007)

relatively low loadings (0.5 wt%) of coated MWNTs.

(a) (b)

The elongation at break of the nanocomposites is often decreased. The increase of nanofiller content compromises the ductile nature of the PP matrix. Yielding followed by drawing still occurred in the 1 wt% sepiolite samples; while 2.5 wt% and 5 wt% sepiolite samples show brittle fractures at low strains (Fig. 9).

The effect on the modulus and yield stress of the scCO2 processed PP nanocomposites was studied for the addition of MWNTs and sepiolite clays (Ma et al., 2007 and 2010). The results were compared with traditional melt compounded nanocomposites.

Fig. 10. The Young's modulus and yield stress of scCO2 processed and melt-compounded PP nanocomposites (a) PP / MWNT nanocomposites (b) PP / sepiolite nanocomposites. The label, melt.mPP/cMWNT, in the figure represents melt-compounded and masterbatch based HDPE-coated MWNT PP nanocomposites (Ma et al., 2007; Ma et al., 2010)

Preparation of Polypropylene Nanocomposites Using Supercritical Technology 383

crystalline morphology of the polymer matrix can have pronounced effects on the mechanical properties of nanocomposites (Bhattacharyya et al., 2003; Wang et al., 2007). Hence, the mechanical reinforcing effects observed are a combination of the modification of the PP matrix

The effect of nanofillers on the melting temperature (*Tm*) of PP is not clear. Yang et al. (2008) and Bikiaris et al. (2008) have observed an increase in *Tm*. Lee et al. (2008) has reported a decrease in *Tm* of PP. Other authors reported that *Tm* remains unchanged (Seo et al., 2005; Jin

Fig. 11. The TGA of PP / MWNT nanocomposites processed using scCO2 assisted mixing, showing a slightly retarded thermal degradation of nanocomposites by the presence of

The thermal stability of PP is also slightly improved by the addition of the nanofillers.  PP / MWNT nanocomposites showed a higher onset decomposition temperature (*T*onset). The Reason is that the CNTs hinder the flux of degradation products and improves the heat dissipation within the composites (Huxtable et al., 2003). Seo & Park (2004) have observed a *T*onset increase from 278 C to 352 C at the addition of 5 wt% of MWNTs. The effect of the clay surface modification on the thermal stability of PP has been studied by Tartaglione et

PP can crystallize in three crystalline modifications: monoclinic (*α*), hexagonal (*β*), and orthorhombic (*γ*) (Bruckner et al., 1991). These phases can be examined by XRD. The XRD patterns for all the PP nanocomposites shows peaks corresponding to PP α-phase at 2*θ* = 14, 17, 18.5, 21.5, 21.9 and 25.4 ° for six major reflections: (110), (040), (130), (111), (041) and (060) plane, respectively. Most studies showed that only *α* phase of PP was present in the PP nanocomposites and no other forms of PP crystallites were detected. This indicates that the addition of sepiolite does not affect the crystal modification of the final PP molecules

al. on the melt-compounded PP / sepiolite composites (Tartaglione et al., 2008).

through increased crystallinity as well as true reinforcing effects from CNT fillers.

et al., 2009).

MWNTs (Ma et al., 2010)

**5.3.2 Microstructure of PP**

Increases of both properties were observed in all systems with the nanofiller content as expected. The melt-compounded and masterbatch based PP/coated MWNT composites show a continuous modulus increase up to 1.7 GPa by the addition of 1.0 wt% CNT loading (Fig. 10a). This is as a result of the de-aggregation of the MWNT bundles by the HDPE coating (Ma et al., 2010). ScCO2 processed nanocomposites with pristine MWNTs also showed a competitive increase in Young's modulus. Furthermore, the use of masterbatch in the scCO2 assisted mixing method did not show much improvement in terms of Young's modulus. Similarly, increases in yield stress were observed for scCO2 PP / MWNT samples, with a significant 6 % increase at 0.1 wt% CNT loading followed by roughly constant values across the rest of the CNT loading range up to 1 wt% (Fig. 10a). The degree of reinforcement of pristine MWNTs is as good as for melt-compounded PP/coated MWNT composites. This indicates that scCO2 assisted mixing achieves a better dispersion of CNTs without the need for a HDPE coating. The enhanced mechanical properties are also benefited from the relatively low shear forces involved in scCO2 assisted mixing, which causes less nanotube breakage and damage.

In Ma et al.' (2007) work about PP/sepiolite nanocomposites, the Young's modulus of melt compounded PP nanocomposites (no PP-g-MA used as compatibilizer) shows a reduction at 5 wt% sepiolite loading (Fig. 10b). This corresponds to the aggregation of clay at higher loadings with the absence of PP-g-MA. This is in contrast to the analogous material with PPg-MA, which shows a 68 % increase in modulus from 1 to 5 wt% clay content. The modulus for scCO2 processed PP nanocomposites without PP-g-MA shows a steady increase in modulus (by up to 56 % for 5 wt% sepiolite). In terms of yield stress, scCO2 processed PP nanocomposite without PP-g-MA shown the highest increase in yield stress of up to 23 % for 5 wt% sepiolite, which was benefited from the good dispersion of the sepiolite and also the benefit of preserving the clay fibre length when using scCO2 assisted mixing.

### **5.3 Characterization of PP**

#### **5.3.1 Thermal behaviour**

Nanofillers increase the crystallization temperature (*Tc)* of PP. It has been observed for both CNT and clay nanocomposites (Ma et al., 2007; Bilotti et al., 2008; Lee et al., 2008). The nanofillers act as nucleating agents enabling PP to crystallize at a higher temperature during the cooling process. This increase in *Tc* is dependent on the nanofiller content, which suggests that higher loadings of nanofiller can provide higher nucleating efficiency. It has been reported that SWNTs are more effective nucleating agents than MWNTs, which provided larger increase in *Tc* (Miltner et al., 2008)*.* Furthermore, pristine MWNTs also give a stronger nucleation effect than HDPE coated MWNTs, which suggests that the surface of the nucleating agents plays an important role in the crystallization process (Ma et al., 2010). It was also found that a more pronounced nucleating effect was observed for very low CNT loadings. Although nucleating agents significantly increase the number of nucleation sites, above certain loadings the introduction of more CNTs may hinder chain mobility and retard crystal growth.

Apart from the *Tc,* the crystallinity of PP has also been increased by the addition of nanofillers (Bilotti et al., 2008). The Young's modulus of the composites has been reported to increase with polymer matrix crystallinity in the work of Coleman et al. (Coleman et al., 2004). Similar results were also shown in other studies that changes in the crystallinity and the

Increases of both properties were observed in all systems with the nanofiller content as expected. The melt-compounded and masterbatch based PP/coated MWNT composites show a continuous modulus increase up to 1.7 GPa by the addition of 1.0 wt% CNT loading (Fig. 10a). This is as a result of the de-aggregation of the MWNT bundles by the HDPE coating (Ma et al., 2010). ScCO2 processed nanocomposites with pristine MWNTs also showed a competitive increase in Young's modulus. Furthermore, the use of masterbatch in the scCO2 assisted mixing method did not show much improvement in terms of Young's modulus. Similarly, increases in yield stress were observed for scCO2 PP / MWNT samples, with a significant 6 % increase at 0.1 wt% CNT loading followed by roughly constant values across the rest of the CNT loading range up to 1 wt% (Fig. 10a). The degree of reinforcement of pristine MWNTs is as good as for melt-compounded PP/coated MWNT composites. This indicates that scCO2 assisted mixing achieves a better dispersion of CNTs without the need for a HDPE coating. The enhanced mechanical properties are also benefited from the relatively low shear forces involved in scCO2 assisted mixing, which causes less nanotube

In Ma et al.' (2007) work about PP/sepiolite nanocomposites, the Young's modulus of melt compounded PP nanocomposites (no PP-g-MA used as compatibilizer) shows a reduction at 5 wt% sepiolite loading (Fig. 10b). This corresponds to the aggregation of clay at higher loadings with the absence of PP-g-MA. This is in contrast to the analogous material with PPg-MA, which shows a 68 % increase in modulus from 1 to 5 wt% clay content. The modulus for scCO2 processed PP nanocomposites without PP-g-MA shows a steady increase in modulus (by up to 56 % for 5 wt% sepiolite). In terms of yield stress, scCO2 processed PP nanocomposite without PP-g-MA shown the highest increase in yield stress of up to 23 % for 5 wt% sepiolite, which was benefited from the good dispersion of the sepiolite and also