Mooney viscosity, that was not observed with a short chain substituent [76].

ammonium methylsulfate) in poly(epichlorohydrine) [78].

performing vulcanization reactions at lower temperatures.

sulphur accelerating anionic species were proposed [89].

**9. Barrier properties of RCN**

matrix was apolar rubber such as NBR [79].

**8. Vulcanization of RCN**

Platelets with a high aspect ratio substantially reduce the diffusion of penetrating molecules in polymer matrices. The tortuous path model, represented in Figure 8, is generally accepted to explain this result.

**Figure 8.** Tortuous path for molecules in a matrix containing platelets

Various continuum models were developed and applied to polymer clay nanocomposites, assuming in most cases a random dispersion of plates, parallel to each other and perpendicular to the direction of molecules diffusion, as summarized in refs. [90] and [91]. According to these models, reduction of permeability is enhanced by the increase of the platelet aspect ratio. These models are able to interpret the reduction of permeability in a polymer matrix, without necessarily having the presence of nano-platelets. This means that a nano-effect should not be invoked. It was commented [91] that permeating molecules have a sub-nano level and their permeation is slowed down by platelets, whatever is their size.

### **9.2. Improvement of barrier properties of rubber matrices thanks to the use of clays**

Better barrier properties of a rubber matrix, thanks to clay addition, were demonstrated for many different rubbers. In NR as the matrix, the barrier properties were enhanced at low clay (Mt) content, preparing the nanocomposite through the emulsion blending: 1, 2 and 3 phr of clay led respectively to more than 35% and to about 45% and 50% reduction of oxygen permeability [92]. 3 phr of OC (Mt/ didodecyl methyl amine) gave a 50% reduction of the oxygen permeability and a 40% reduction of toluene absorption at 20 oC [93]. 5 and 10 phr of OC led, respectively, to about 10% and 15% reduction of the oxygen permeability, and, at 15% OC, to a 30% reduction of toluene absorption at 30oC [94]. In BR as the matrix, 5 phr OC (Mt/dimethyl ditallow-ammonium) dispersed from solution blending, led to a reduction of about 80% of water vapour permeability, whereas 10 phr OC gave about 20% reduction of toluene uptake at equilibrium [95]. In NBR as the

matrix, 1 phr of OC (Mt/dimethyl ditallow-ammonium) dispersed from solution blending brought to about 80% decrease of water vapour permeability [96]. Better barrier properties were observed by increasing the AN content of NBR: the relative nitrogen permeability was reduced by 11.5, 10.4, and 9.0% for 42NBR, 35NBR, and 26NBR (the figures indicate the wt% of AN), respectively, with 10 phr OC (Mt/dimethyl dialkyl (C14– C18) ammonium) [97].

A clay with a high aspect ratio such as rectorite was found to give better barrier than carbon black N330 [38].

## **10. Mechanical properties of RCN**

### **10.1. The origin of reinforcement: nano-structured and nano-fillers**

Particulate fillers are used to reinforce traditional rubber compounds: they are carbon black and silica and are made by spherical individual particles of few tents of nanometers as the diameter, fused together to form aggregates extending up to few hundreds of nanometers. These fillers are called nano-structured as the aggregates can not be separated into individual particles by thermomechanical mixing.

Theories on the origin of reinforcement [98], developed on carbon black based compounds, led to present the modulus of a filled rubber compound as due to the sum of different contributions, as it shown in Figure 8.

**Figure 9.** Contributions to the modulus of a rubber compound filled with a particulate filler

Contributions that do not depend on the strain amplitude are due to the polymer network (entanglements, physical and chemical crosslinks), hydrodynamic effects (related to the filler volume fraction and implying the strain amplification mechanism and thus the enhancement of the modulus), immobilization of rubber on filler particles (that transform a highly viscous liquid in a solid). A contribution to the modulus that strongly depends on the strain amplitude is due to the so called filler network: filler particles are joined together either directly or through polymer layers.

104 Advanced Elastomers – Technology, Properties and Applications

**10. Mechanical properties of RCN**

individual particles by thermomechanical mixing.

contributions, as it shown in Figure 8.

C18) ammonium) [97].

black N330 [38].

matrix, 1 phr of OC (Mt/dimethyl ditallow-ammonium) dispersed from solution blending brought to about 80% decrease of water vapour permeability [96]. Better barrier properties were observed by increasing the AN content of NBR: the relative nitrogen permeability was reduced by 11.5, 10.4, and 9.0% for 42NBR, 35NBR, and 26NBR (the figures indicate the wt% of AN), respectively, with 10 phr OC (Mt/dimethyl dialkyl (C14–

A clay with a high aspect ratio such as rectorite was found to give better barrier than carbon

Particulate fillers are used to reinforce traditional rubber compounds: they are carbon black and silica and are made by spherical individual particles of few tents of nanometers as the diameter, fused together to form aggregates extending up to few hundreds of nanometers. These fillers are called nano-structured as the aggregates can not be separated into

Theories on the origin of reinforcement [98], developed on carbon black based compounds, led to present the modulus of a filled rubber compound as due to the sum of different

**Figure 9.** Contributions to the modulus of a rubber compound filled with a particulate filler

**10.1. The origin of reinforcement: nano-structured and nano-fillers**

The Guth Equation is used to correlate the compound initial modulus with the filler volume fraction (equation 1 as follows).

$$\mathbf{E} = \text{Em}\left(1 + 0.67\,\text{f}\phi + 1.62\,\text{f}^2\,\phi^2\right) \tag{3}$$

This equation holds up to a threshold of filler content, known as the percolation threshold, at which a continuous network is established in the rubber matrix and thus accounts for the sum of the three contributions commented above and shown in Figure 8 that do not depend on the strain amplitude.

In equation 1, appears the *shape factor f,* that was already commented in Paragraph 2. The *shape factor f* was introduced in the Guth model in order to account for the fact that particle aggregation (clustering) has a significant impact on stiffness at high volume fractions (higher than 0.15). These clusters are formed by the filler aggregates and the impact of the *f*  value on the quadratic term of the Guth equation makes them much important for the total value of the modulus.

In the case of a clay, as it was shown in Figure 1b, the *shape factor f* is given by the ratio between the longest lateral side of the layer and the height of clay stacks, the highest *f* value being obtained in the case of a single layer. Therefore, the exfoliation of a clay not only favours a better clay dispersion but also improves the modulus of the compound. To take into account the lower contribution to the modulus of a platelet like filler, a modulus reduction factor of about 0.7 was determined [99] by fitting experimental data.

An important contribution to the reinforcement is given by the so called "in rubber structure": fillers are able to accomodate polymer chains in the voids of their structure. This mechanism is well known in the case of nanostructured fillers such as carbon black and silica. In the case of clays, the immobilization of polymer chains would require their intercalation in the interlayer space, phenomenon that is however, as explained in paragraph 6.1, still a matter of debate.

OC give an important contribution to the polymer network. In fact, as discussed in paragraph 8, they promote a higher crosslinking density, thanks to the cation-thiolate interaction. Clay layers and aggregates could be represented as framed in a cage formed by short sulphur bonds, that prevents their slip on the polymer chains, phenomenon that occurs in uncrosslinked samples, as commented in paragraph 7.

In the same paragraph, it was commented that a low clay concentration is needed for achieving the percolation threshold in a hydrocarbon rubber matrix. This conclusion was

drawn also by determining the dependence of the Young modulus (obtained from stress-strain curves) on the OC concentration, in a SBR matrix (with Nanomer I.42E from Nanocor as OC) [100] or in an isoprene rubber matrix (OC was Mt/2HT), either synthetic [67] or naturally occurring [101]. The excess of the Young modulus was plotted versus the filler fraction [102], determining the percolation threshold, that was: 2.7 vol % in SBR [100], 2.9 vol % in IR [67] and 4 vol % in NR [101]. The excess modulus scales with a power law with an exponent between 1.8 and 2.5 above the percolation threshold, lower with respect to the one typical of carbon black (about 4). The structural difference of the filler is proposed to justify this difference. As mentioned above, the filler network is responsible for the contribution to the modulus that depends on the strain amplitude. Thanks to the easy formation of a filler network, clays promote a remarkable non linearity of the dynamicmechanical behaviour of rubber nanocomposites, a phenomenon known as Payne effect [103]. Data that the show the remarkable Payne effect of RCN, increasing with the clay content, were reported for NR [104] and IR [67] as the rubber matrices. A strong reduction of the dynamic modulus was observed also by increasing the temperature [50]. The 2-D melting of the paraffinic chains substituents of the compensating ion [105] was commented as a possible explanation [66, 101].

### **10.2. The improvement of rubber mechanical properties thanks to the use of clays**

Many data are available in the literature to show that both pristine clays and OC bring about the improvement of the mechanical properties of a rubber matrix, preparing the nanocomposites from latex, melt and solution blending. Some examples are reported as follows.

Pristine Mt was dispersed in a NR latex, observing an increase of modulus for the obtained nanocomposite up to 30 phr of clay, in the presence of worse ultimate properties [41], as well as higher 300% stress, shore A hardness, tensile strength and tear strength, when Mt was at 20 phr level [29]. In a SBR matrix as well, tensile and tear strengths were improved by adding a pristine Mt via emulsion blending, up to 20 phr [29], without any improvement for a further addition [35].

OC were typically used at lower levels. In NR as the matrix, 5 phr of Mt modified by tetraoctyl phosphonium bromide led to a 3-fold increase of tensile strength [81] and best ultimate properties were obtained with 10 phr of OC (Mt/octadecyltrimethyl ammonium) [106]. In BR as the matrix, best mechanical properties were obtained with a Mt having dimethyl dihydrogenated tallow quaternary ammonium as the compensating cation [107], an ammonium cation largely diffused in RCN field. In a SBR matrix, mechanical properties improved up to 10 phr of OC (Mt/octadecyltrimethylamine, Nanomer I.28E) [87] (Mousa and Karger-Kocsis, 2001), decreasing for a further addition. In a nanocomposite based on maleic anhydride modified EPM (EPM-MA, with 0.42 wt% of maleic anhydride) about 5% of OC (Mt/octadecyl amine) gave a modulus three times higher than that of EPM-MA. Modulus increased and elongation at break decreased with clay content [108]. Reinforcement was obtained from OC also in rubber blends. 2 phr of OC (Mt/octadecyltrimethylamine) in an NR/ENR blend (10 phr ENR50) gave an increase of both strain and stress at break values [109] (Teh et al., 2004).

It was commented in Paragraph 7 the importance of sulphur based vulcanization to prevent the OC slip on the polymer chains. OC (Mt modified with hydrogenated tallow ammonium) was reported to promote a remarkable reinforcement also of a peroxide cured rubber matrix. However, in this case, the rubber was NBR, with acrylonitrile content as high as 50% [110]. It was also shown that higher values of tensile strength and better tear resistance were obtained, with a worse elongation at break, by increasing the AN content, with 10 phr of OC (Mt/dimethyl dialkyl (C14–C18) ammonium) [111].

### **10.3. Clays in a rubber matrix for lighter weight of the compound**

106 Advanced Elastomers – Technology, Properties and Applications

as a possible explanation [66, 101].

**clays**

follows.

a further addition [35].

drawn also by determining the dependence of the Young modulus (obtained from stress-strain curves) on the OC concentration, in a SBR matrix (with Nanomer I.42E from Nanocor as OC) [100] or in an isoprene rubber matrix (OC was Mt/2HT), either synthetic [67] or naturally occurring [101]. The excess of the Young modulus was plotted versus the filler fraction [102], determining the percolation threshold, that was: 2.7 vol % in SBR [100], 2.9 vol % in IR [67] and 4 vol % in NR [101]. The excess modulus scales with a power law with an exponent between 1.8 and 2.5 above the percolation threshold, lower with respect to the one typical of carbon black (about 4). The structural difference of the filler is proposed to justify this difference. As mentioned above, the filler network is responsible for the contribution to the modulus that depends on the strain amplitude. Thanks to the easy formation of a filler network, clays promote a remarkable non linearity of the dynamicmechanical behaviour of rubber nanocomposites, a phenomenon known as Payne effect [103]. Data that the show the remarkable Payne effect of RCN, increasing with the clay content, were reported for NR [104] and IR [67] as the rubber matrices. A strong reduction of the dynamic modulus was observed also by increasing the temperature [50]. The 2-D melting of the paraffinic chains substituents of the compensating ion [105] was commented

**10.2. The improvement of rubber mechanical properties thanks to the use of**

Many data are available in the literature to show that both pristine clays and OC bring about the improvement of the mechanical properties of a rubber matrix, preparing the nanocomposites from latex, melt and solution blending. Some examples are reported as

Pristine Mt was dispersed in a NR latex, observing an increase of modulus for the obtained nanocomposite up to 30 phr of clay, in the presence of worse ultimate properties [41], as well as higher 300% stress, shore A hardness, tensile strength and tear strength, when Mt was at 20 phr level [29]. In a SBR matrix as well, tensile and tear strengths were improved by adding a pristine Mt via emulsion blending, up to 20 phr [29], without any improvement for

OC were typically used at lower levels. In NR as the matrix, 5 phr of Mt modified by tetraoctyl phosphonium bromide led to a 3-fold increase of tensile strength [81] and best ultimate properties were obtained with 10 phr of OC (Mt/octadecyltrimethyl ammonium) [106]. In BR as the matrix, best mechanical properties were obtained with a Mt having dimethyl dihydrogenated tallow quaternary ammonium as the compensating cation [107], an ammonium cation largely diffused in RCN field. In a SBR matrix, mechanical properties improved up to 10 phr of OC (Mt/octadecyltrimethylamine, Nanomer I.28E) [87] (Mousa and Karger-Kocsis, 2001), decreasing for a further addition. In a nanocomposite based on maleic anhydride modified EPM (EPM-MA, with 0.42 wt% of maleic anhydride) about 5% of OC (Mt/octadecyl amine) gave a modulus three times higher than that of EPM-MA. Modulus increased and elongation at break decreased with clay content [108]. The most important application of rubber compounds is in tires. Lighter rubber compounds are thus pursued, in the light of their remarkable impact on the environment. Direct comparisons were reported between OC and CB in promoting the mechanical reinforcement of rubber compounds.

In NR as the matrix, a comparable mechanical reinforcement was obtained with 10 phr OC (Mt/octadecylamine) and 40 phr CB, with almost twice elongation at break, a lower compression set and similar abrasion loss with OC [112]. With the same OC, a much lower heat build up was obtained with respect to 50 phr of CB [113].

In BR as the matrix, better mechanical performance was obtained with 3 phr OC (Mt/dimethyl dihydrogenated tallow quaternary ammonium) compared with 10 phr of CB [114]. In NBR as the matrix, a comparable tensile strength was obtained with either 10 phr OC or with 40 phr CB [115]. In EPM-MA, 5% of OC (Mt/octadecyl amine) gave the same reinforcement as 30 wt% CB [108].

### **10.4. Synergistics effects between clays and nanostructured fillers**

An increasing number of studies is becoming available in the literature on RCN based on clays and a nanostructured filler such as silica and, in particular, CB. A large scale application of RCN could reasonably imply the use of hybrid filler systems, with a minor amount of OC added to a major part of a traditional filler. In SBR as the matrix, a hybrid sepiolite-silica filler system was adopted [116]. Hybrid OC/CB filler systems were used in the following matrices: SBR [117-119], brominated poly(isobutylene-co-paramethylstyrene) rubber [117], IR [50, 57, 120], NR [67, 121] NR/SBR blend [122], chlorobutyl rubber [123], ENR [124], EPDM [125] (Malas and Das, 2012). In some of these works [50, 57, 120-122], CB was used at a level typical of commercial applications (higher than 50 phr). A remarkable enhancement of material dynamic-mechanical properties was reported, thanks to the use of the hybrid filler system. The most important finding from these study was the synergism developed by the two fillers [118, 120, 121]. In particular, it was shown [126] that initial modulus values obtained with the hybrid CB-OMt filler system were much higher than

those calculated through the simple addition of the two initial moduli of composites with only CB or only OC. Figure 9, taken from ref. [127], demonstrates the synergistic effect between OC and CB. The dashed line indicates the initial modulus calculated by simply adding the initial moduli of composites with only CB and only OC, whereas points refer to experimental data taken from composites containing the hybrid filler system.

**Figure 10.** Initial modulus values () of IR based composites containing the hybrid OC/CB filler system. The dashed line refers to values predicted through the addition of initial moduli values of composites with only CB and only OC.

### **11. Applications of RCN**

In this paragraph, applications of RCN are examined, moving from products already available on the commercial scale. The most meaningful patents were taken as the reference literature for the considered applications. RCN applications, diffused on the commercial scale, well documented and advertised, are essentially in two fields: tires and sport balls.

#### **11.1. Applications of RCN for tyres**

The most abundant application for rubbers is in tire compounds. NR is the most diffused rubber, with almost 11 million ton in 2010 [128] and SBR is the most produced synthetic rubber, with about 6 million ton. The application of both these rubbers is for about 75% in tire compounds. The world demand for tires is rising, at a pace of about 4.7 per year through 2015, achieving a number of produced tires of about 3.3 billion units. In the same period, the tire market is projected to increase, in terms of value, of about 6.5 percent annually, achieving a total value level of \$220 billion 129]. It is thus evident that the application of RCN in tire compounds can lead to a real commercial diffusion as well as to an important economic impact.

A tire can be defined as a toroidal high performance composite behaving as a flexible membrane, able to contain gases under pressure and with the following capabilities: load carrying, cushioning, road handling. Figure 9 reports the structure of a tire, with the name of constitutive parts.

**Figure 11.** Structure of a tire

108 Advanced Elastomers – Technology, Properties and Applications

with only CB and only OC.

economic impact.

**11. Applications of RCN**

**11.1. Applications of RCN for tyres**

those calculated through the simple addition of the two initial moduli of composites with only CB or only OC. Figure 9, taken from ref. [127], demonstrates the synergistic effect between OC and CB. The dashed line indicates the initial modulus calculated by simply adding the initial moduli of composites with only CB and only OC, whereas points refer to

**Figure 10.** Initial modulus values () of IR based composites containing the hybrid OC/CB filler system. The dashed line refers to values predicted through the addition of initial moduli values of composites

In this paragraph, applications of RCN are examined, moving from products already available on the commercial scale. The most meaningful patents were taken as the reference literature for the considered applications. RCN applications, diffused on the commercial scale, well documented and advertised, are essentially in two fields: tires and sport balls.

The most abundant application for rubbers is in tire compounds. NR is the most diffused rubber, with almost 11 million ton in 2010 [128] and SBR is the most produced synthetic rubber, with about 6 million ton. The application of both these rubbers is for about 75% in tire compounds. The world demand for tires is rising, at a pace of about 4.7 per year through 2015, achieving a number of produced tires of about 3.3 billion units. In the same period, the tire market is projected to increase, in terms of value, of about 6.5 percent annually, achieving a total value level of \$220 billion 129]. It is thus evident that the application of RCN in tire compounds can lead to a real commercial diffusion as well as to an important

experimental data taken from composites containing the hybrid filler system.

RCN have been so far essentially applied in three parts of a tire: tread, base (i.e. a compound immediately before the tread in contact with the road), liner. Table 4 explains the position in a tire of such compounds and clarifies their role.

RCN in tire compounds are used to achieve the following properties and performances: reduced weight, reduced energy dissipation, enhanced air retention and extension of the balance of the so called magic triangle performances for a tire tread: rolling resistance, traction, wear.

*RCN in tire treads.* By examining the patent literature [130-132], it appears that clays were used in tire tread compounds aiming at longer life of the compound and lower fuel consumption without negatively affecting the tread grip on the road at low temperature. Clays were used with or without an ammonium cation modifier, in the former case in the presence of a large amount of silica. Compound properties claimed in patents are: better ultimate properties, lower abrasion and an hysteresis that remains analogous at low temperature and is lower at medium – high temperature.

*RCN in base compounds*. OC were used in base compounds of tires commonly referred to as "HP" (High Performance) or "UHP" (Ultra high performance) tires, belonging to classes "V" or "Z", designed to experience extreme driving conditions, as they achieve maximum speeds in a range from 210 to 240 km/h and higher than 240km/h, respectively. In this type

of tires, a low thickness base compound is used to favour performances such as tread block stability, road grip, steering stability, cornering stability and ride comfort. This base compound should have high tensile and dynamic-mechanical properties. OC were reported in the patent literature to give a remarkable improvement of the compound dynamic mechanical properties [133-136] and Pirelli Tire launched in 2007 P Zero tires for the HP and UHP segments, with a base compound containing an OC. In particular, it was reported that OC was able to give a much more isotropic behavior (i.e. an equal performances in longitudinal and lateral directions), with respect to traditional reinforcing fibers, such as the aramide ones.

**Table 4.** Tyre compounds explored for RCN application

*RCN in innerliner.* Most research efforts were dedicated to develop OC based innerliner compounds, with the aim to exploit the barrier property provided by clay platelets, trying in particular to achieve a high clay dispersion. In order to have the best barrier, the following technical solutions were adopted: use of clays with a high aspect ratio, control of the clay organization (stacks of exfoliated layers), use of reactive rubbers to promote the clay exfoliation, use of coating layers with a high impermeability.

A kaolin [137] and a mica [138] with high aspect ratio (at least 50) were used in butyl and in butyl, butadiene and natural rubber, respectively.

Stacks of OC were used in BR [139], in BIMS [140] and in SBR [141]. With an OC content as low as 9 phr, the oxygen transmission was reduced to about one fourth [139].

The clay exfoliation was promoted either by dispersing clays in the emulsion were polymerization of monomers such as isoprene and styrene was performed [142], or by performing the exchange reaction with a cationic polymer latex [143],or by mixing the clay (in a BIMS matrix) in the presence of a tertiary amine. In this latter case, with only 3 phr of clay, a reduction of 20% permeability was detected [144].

An amine terminated oligomer (e.g. a butadiene-acrylonitrile copolymer) was used as the reactive polymer to prepare a 25/75 clay/SBR blend and the oxygen transmission was reduced to less than one half [145]. A clay was mixed with a aqueous dispersion of a dienebased elastomer, with one or more functional groups such as acid or an anhydride [146].

OC was melt blended with poly(isobutylene-co-p-alkylstyrene) and poly(isobutylene-coisoprene) elastomers and at a concentration of 10 phr the permeability was reduced to one half [147]. OC was melt blended with an halogenated copolymer of isobutylene and pmethyl-styrene and, at an OC level of 5 phr, the air retention was improved up to 30% [148]. Poly(isobutylene-co-N,N-dimethylvinylbenzylamine) copolymer was blended with an OC and an improvement of impermeability up to 70% was reported [149].

Barrier coating mixtures were prepared by mixing an elastomer (butyl rubber) an exfoliated clay with a high aspect ratio (preferably a vermiculite) and a surfactant, adopting a polymer/clay ratio from 1:1 to 20:1 and obtaining a permeability reduction of about 25%. This technology found a large commercial success in tennis ball and it will be discussed in the following paragraph [150].

### **11.2. Applications of RCN for sport balls**

As mentioned above, a clay based barrier coating technology was developed [151-154], and applied to sport balls, with the aim to drastically improve the pressure retention without negatively affecting properties such as the bounce, the feel and the reproducibility of performances. A relatively thin coating layer (about 10-30 micron) was deposed, made by an elastomeric nanocomposite prepared by combining aqueous dispersions of a vermiculite (having a aspect ratio up to 1000) and butyl rubber. In a laboratory study [90] (Takahashi et al., 2006) a coating layer containing vermiculite (20 - 30 wt%) in butyl rubber led to a reduction of the diffusion coefficients by two orders of magnitude and of gas permeability by 20–30 fold. This technology was developed by InMat company and Air D-Fense products were commercialized.

### **Author details**

110 Advanced Elastomers – Technology, Properties and Applications

Base (tread cushion) Below the tread.

**Table 4.** Tyre compounds explored for RCN application

butyl, butadiene and natural rubber, respectively.

clay, a reduction of 20% permeability was detected [144].

exfoliation, use of coating layers with a high impermeability.

aramide ones.

Liner or innerliner

of impermeable rubber.

Thin layer

of tires, a low thickness base compound is used to favour performances such as tread block stability, road grip, steering stability, cornering stability and ride comfort. This base compound should have high tensile and dynamic-mechanical properties. OC were reported in the patent literature to give a remarkable improvement of the compound dynamic mechanical properties [133-136] and Pirelli Tire launched in 2007 P Zero tires for the HP and UHP segments, with a base compound containing an OC. In particular, it was reported that OC was able to give a much more isotropic behavior (i.e. an equal performances in longitudinal and lateral directions), with respect to traditional reinforcing fibers, such as the

Tread In contact with road Dry, wet, ice and snow traction.

*RCN in innerliner.* Most research efforts were dedicated to develop OC based innerliner compounds, with the aim to exploit the barrier property provided by clay platelets, trying in particular to achieve a high clay dispersion. In order to have the best barrier, the following technical solutions were adopted: use of clays with a high aspect ratio, control of the clay organization (stacks of exfoliated layers), use of reactive rubbers to promote the clay

A kaolin [137] and a mica [138] with high aspect ratio (at least 50) were used in butyl and in

Stacks of OC were used in BR [139], in BIMS [140] and in SBR [141]. With an OC content as

The clay exfoliation was promoted either by dispersing clays in the emulsion were polymerization of monomers such as isoprene and styrene was performed [142], or by performing the exchange reaction with a cationic polymer latex [143],or by mixing the clay (in a BIMS matrix) in the presence of a tertiary amine. In this latter case, with only 3 phr of

low as 9 phr, the oxygen transmission was reduced to about one fourth [139].

Under the carcass Impermeability to air and

Stability at high speed. Abrasion

Low hysteresis. Good adhesion.

resistance. Protection of compounds below.

Good fatigue, tear and durability. Compatibility

moisture. Prevention of degradation of tire structure (due to air and moisture). Good flex fatigue, crack, long

term aging resistance.

Tyre compound Position in the tyre Performances

Between tread and nylon 0°

Maurizio Galimberti *Politecnico di Milano, Dipartimento di Chimica, Materiali e Ingegneria Chimica G. Natta, Milano, Italia*

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112 Advanced Elastomers – Technology, Properties and Applications

Ltd, Shawbury, Shrewsbury, Shropshire UK

Science. 50: 962–1079

Mater. Eng. 291: 1449–1476

Rapra Publishing, UK, 344.

Group, London (UK).

593.

818-828.

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	- [153] Feeney C.A., Farrell M., Tannert K., Goldberg H.A., Lu M., Grah M.D. (2001) US Patent 6,232,389. to InMat and Michelin Recherche et technique
	- [154] Feeney C.A., Farrell M., Tannert K., Goldberg H.A., Lu M., Grah M.D. (2000) US Patent 6,087,016 to InMat and Michelin Recherche et technique

**Chapter 5**

## **Pigment and Dye Modified Fillers as Elastomeric Additives**

Anna Marzec and Marian Zaborski

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/50735

### **1. Introduction**

120 Advanced Elastomers – Technology, Properties and Applications

elastomer nanocomposites. US 5,807,629 to Exxon Mobil

[141] Ishida, K., Fujiki, K., 2003. WO 03/087214 A1 to Bridgestone

nanocomposites. US 2005/0027058 to Exxonmobil.

nanocomposite. WO 04/005387 to Exxonmobil.

Michelin and Herberts Gmbh.

7,078453 to InMat Inc

7,119,138 to InMat Inc

thereof. EP 1321489 A1 to Goodyear Tyre and Rubber Company.

US 2003/0144401 A1 to Goodyear Tyre and Rubber Company

containing same. WO 058874 A1 to Bridgestone Corporation.

6,232,389. to InMat and Michelin Recherche et technique

6,087,016 to InMat and Michelin Recherche et technique

Rubber Ind.

Engineering Company.

[138] Miyazaki T. (2006) Rubber composition for inner liner. EP 1726620 A1 to Sumitomo

[139] Elspass W.C., Peiffer G.D., Kresge E.N., Wright P.J., Wang H.C. (1998) Tactoidal

[140] Elspass C.W., Peiffer D.G. (2000) Nanocomposite materials formed from inorganic layered materials dispersed in a polymer matrix. US 6,034,164 to Exxon Research and

[142] Elspass C.W., Peiffer D.G., Kresge E.N., Hsieh D.T., Chludzinski J.J., Liang K.S., (1999) Nanocomposite materials. US 5,883,173 to Exxon Research and Engineering Company [143] Parker D.K., Larson B.K.F., Yang X. (2003) Preparation and use of a nanocomposite of elastomer and exfoliated clay platelets formed in situ within an elastomer host and articles of manufacture, including tires, having at least one component comprised

[144] Dias A.J., Tsou A. H., Chung D.Y.L. Weng W. (2005). Low permeability

[145] Kresge E.N., Lohse D.J. (1997) Tire inner-liners comprising a solid rubber and a complex of a reactive rubber and layered silicate clay. US 5,665,183 to Exxonmobil [146] Ajbani M., Geiser J.F., Parker D.K. (2003) Nanocomposite of elastomer and dispersion therein of intercalated clay prepared in an aqueous medium with funcional elastomer.

[147] Gong C., Dias A.J., Tsou A. H., Poole B.J., Karp K.R. (2004) Functionalized Elastomer

[150] Feeney C.A., Balzer, R.J. (1998) Barrier coating of an elastomer and a dispersed layered filler in a liquid carrier and coated compositions, particularly tires. WO 98/56598 to

[151] Feeney C. A., Goldberg,H. A., Farrell M., Karim D. P., Oree K. R. (2006) US Patent

[152] Feeney C. A., Goldberg H. A., Farrell M., Karim D. P., Oree K. R. (2006). US Patent

[153] Feeney C.A., Farrell M., Tannert K., Goldberg H.A., Lu M., Grah M.D. (2001) US Patent

[154] Feeney C.A., Farrell M., Tannert K., Goldberg H.A., Lu M., Grah M.D. (2000) US Patent

[148] Grah M.D. (2004) Tire with improved inner liner. US 2004/0194863 A1 to Michelin [149] Wang X., Fudemoto H., Hall J., Araki S., Hogan T., Foltz V., Sadhukhan P., Bohom, G.G.A. (2004) Method for clay exfoliation, compositions therefore, and modified rubber Pigments are a dynamically developinged area of chemical technology. This interesting scientific concept involves preparation of hybrid pigments by the formation of a stable dye sphere surrounding an inorganic substrate grain, such as silica, titanium dioxide, or aluminosilicates. The concept of combining properties of organic and inorganic pigments has been employed for a long time. At the end of the 19th century, this concept was applied to the production of a pigment lake where freshly precipitated aluminium hydroxide was employed as the support. These lakes were used for the production of graphic dyes due to their averaged properties. Silica, zinc oxide, carbon black, and titanium dioxide may be employed in rubber compounds as pigments and fillers. Fillers boast a wide range of industrial applications, such as in the pharmaceutical and textile industries, and are important as additives for paints and varnishes. Fillers are relatively inexpensive, solid substances that are added in fairly large volumes to polymers to adjust the volume, weight, cost, surface, colour, expansion coefficient, conductivity, permeability, and mechanical properties. They can be roughly divided into inactive or extender fillers and active or functional or reinforcing fillers. One of the most popular fillers is synthetic silica, which is used in the polymer industry as an active filler (Liu et al., 2008, Wang et al., 2009). The rubber industry utilises fine silicas, which have a large specific surface leading to favourable effects on the strength of the composites. One important processing challenge is that silica surfaces are hydrophilic in nature, which impedes their dispergation in polymer media. Nearly all of the silica forms tend to agglomerate, which is an undesirable behaviour from the point of view of utilising their functional properties in industrial applications. On-going research on this topic focuses on altering the characteristics of the filler surfaces through some type of modification (Ciesielczyk et al., 2007; Ciesielczyk et al., 2011; Klapiszewska et al., 2003; Kohno et al., 2011; Ladewig et al., 2012; Kishore et al., 2012; Natinee et al., 2011 ). For example, industry has developed hydrophobic silicas that easily dispergate in polymers. The

© 2012 Marzec and Zaborski, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

production of these silicas involves a silica silanisation reaction with alkoxysilane containing functional groups (Jesionowski et al., 2001, Krysztawkiewicz et al., 2000; Elham et al., 2012; Kister et al., 2012; Jianzhong et al., 2011; Wunpen et al., 2011; Jung-Woo et al., 2011). Over the past decade, numerous studies have employed a dye to modify the surface of silica to obtain hydrophobic silica surfaces and visually appealing end product colours (Ansarifar et al., 2004; Al Dwayya et al., 2012). The presence of amino groups in these supports enables the formation of ionic bonds with acid and direct dyes as well as the formation of covalent bonds with reactive dyes (Donia et al., 2009). Dyes, which were applied for this purpose, belonged to the textile class. Although they were combined with silica, they could not fulfill the requirements of modern pigments, such as light-resistance or resistance to solvents and to high temperature. High temperature during polymer processing can damage or destroy the dye causing changes in the shade or loss of colour. The present study was directed at obtaining composite pigments of indigothiazine and silicas with different surface areas. Indigothiazine consists of two thiazine residues conjugated to each other by a double bond. Because their cyclic carbonamide groups , thiazine can form hydrogen bond to silanol groups present on the silica surface. The thiazine pigments exhibited good heat stability, insolubility and exposure durability. An alternative method of producing coloured fillers is via a mechanical approach in which a coloured organic system is mechanically applied to the surface of a non-organic support. This method is a simple and cheap method of producing an organic-inorganic pigment encasing silica in the coloured chromophore with the additional benefit of eliminating the undesirable solvents from the process. The anti-ageing properties of indigothiazine were confirmed by oxidation-reduction potential measurements. The composites were produced by mechanically applying a coloured organic system (indigothiazine pigment) to a non-organic support (silica). The prepared pigment composites were characterised by the following properties: agglomerate size distribution, zeta potential, and specific surface area of the silica fillers before and after application of indigothiazine. The composite pigments were evaluated for their functional properties and subsequently employed as fillers in an acrylonitrile butadiene rubber (NBR). The mechanical properties as well as the spectrophotometric and DSC data of the vulcanisates produced with the composite pigment fillers were also studied before and after being aged under UV radiation.

### **2. Colourants in polymer composites**

#### **2.1. Fillers modification**

Compared to inorganic pigments, organic pigments exhibit greater vividness, higher colour intensity and higher staining potential. Their drawbacks include greater sensitivity to the action of chemical compounds, temperature and solvents (Binkowski et at., 2000). Several studies were conducted to obtain organic pigments that were permanently attached to a silica core to improve the resistance of the pigments to chemical and thermal treatments. Numerous studies have shown that the reaction mechanism that occurs on the surface of inorganic substances is extremely difficult to identify. For example, the reaction that occurs on the surface of silica with fluorescein results in the formation of covalent bonds between the reagents. The interactions may involve the formation of hydrogen bonds as well as van der Waals or electrostatic interactions (Jesionowski 2003; Wu et al., 1997). Silica is one of the most popular fillers employed as a pigment carrier. Silicas are extensively modifiable materials. Their modification results in products that have new functional groups on their surface and are capable of interacting with various organic compounds. For example, modification with aminosilane provided functional amino groups that can react with the carbonyl groups of aldehydes, ketones or esters. The aminosilane-modified silicas find multiple applications as polymer fillers in industry. Recently, they have been used with increasing frequency as coupling agents in pigment or organic systems. Until recently, studies of organic pigments involved modification of their basic properties, such as stability or intensity of the colours. However, in many cases, the pigments must satisfy additional requirements (e.g., well defined particle diameters). Pigments with silica cores can satisfy these requirements (Krysztafkiewicz et al., 2003). Bińkowski (Bińkowski et al., 2000) proposed a simple procedure for obtaining coloured silicas, which involves attaching the dye to a modified silica surface. The procedure is possible due to the availability of a wide range of commercial dyes. The resulting pigment exhibits a structure in which the silanol groups on the surface of the silica are chemically coupled to an organic dye (e.g., azo dye) through an aminosilane coupling agent. They represented the pigment structure using the following formula: S-(O-Si-R1-Z-D)n where R1 is the alkyl group, Z is the amine bridge fragment , D is the dye, n is a number not less than 1 and -Si-R1-Z- represents a group originating from the silane coupling agent. In this work, they employed precipitated silica, 3-aminopropylotriethoxysilane (U-13), 2-(aminoethyl)-3-aminopropylotriethoxysilane (U-15) and a dye obtained from Boruta-Kolor, C.I. Direct Red 81. The modifications resulted in uniform pigments. A subsequent report by the authors (Jesionowski et al., 2011) showed the adsorption of C.I. Mordant Red 3 on the surface of silica that was both unmodified and modified with *N*-2-(aminoethyl)-3-aminopropyltrimethoxysilane. They proposed two mechanisms for the dye addition to the silane modified silica surface including via a hydrogen bond between the hydroxyl group of C.I. Mordant Red 3 and the amine groups of the silane coupling agents and/or via electrostatic interaction between the dissociated anion of the dye and the cation appearing on the silica surface modified with silane. Sudam (Sudam et al., 2003) described the adsorption behaviour of selected styryl pyridinium dyes on silica gel. The adsorbents were styryl pyridinium dyes with a monochromatic group containing an alkyl chain at the pyridinium nitrogen and a bischromatic group containing methylene groups bridging at the pyridinium nitrogen atoms. They observed that the bischromophoric dyes anchor on the silica surface in a flat-on position in which the methylene units also come in contact with the silica surface. Andrzejewska (Andrzejewska et al., 2004) performed studies to obtain pigments by adsorption of organic dyes on the modified surface of a titanium dioxide pigment system. The titanium oxide surface was modified with silane coupling agents, such as 3-aminopropyltriethoxysilane (U-13), *N*-2- (aminoethyl)-3-aminopropyltriethoxysilane (U-15D), in various solvents (methanol, toluene, acetone and methanol-water mixture). C.I. Acid Orange 7 and C.I Reactive Blue 19 were employed to prepare the pigments. The organic dyes were deposited on the surface of the modified titanium white. The modification of the titanium white with both silanes (U-13 and U-15D) boosted the efficiency of adsorption of C.I. Acid Orange 7 dye on the surface. The improved adsorption efficiency of C.I Reactive Blue 19 on the titanium was obtained

122 Advanced Elastomers – Technology, Properties and Applications

radiation.

**2. Colourants in polymer composites**

**2.1.  Over the past decade, numerous studies have employed a dye to modify the surface of silica to obtain hydrophobic silica surfaces and visually appealing end product colours (Ansarifar et al., 2004; Al Dwayya et al., 2012). The presence of amino groups in these supports enables the formation of ionic bonds with acid and direct dyes as well as the formation of covalent bonds with reactive dyes (Donia et al., 2009). Dyes, which were applied for this purpose, belonged to the textile class. Although they were combined with silica, they could not fulfill the requirements of modern pigments, such as light-resistance or resistance to solvents and to high temperature. High temperature during polymer processing can damage or destroy the dye causing changes in the shade or loss of colour. The present study was directed at obtaining composite pigments of indigothiazine and silicas with different surface areas. Indigothiazine consists of two thiazine residues conjugated to each other by a double bond. Because their cyclic carbonamide groups , thiazine can form hydrogen bond to silanol groups present on the silica surface. The thiazine pigments exhibited good heat stability, insolubility and exposure durability. An alternative method of producing coloured fillers is via a mechanical approach in which a coloured organic system is mechanically applied to the surface of a non-organic support. This method is a simple and cheap method of producing an organic-inorganic pigment encasing silica in the coloured chromophore with the additional benefit of eliminating the undesirable solvents from the process. The anti-ageing properties of indigothiazine were confirmed by oxidation-reduction potential measurements. The composites were produced by mechanically applying a coloured organic system (indigothiazine pigment) to a non-organic support (silica). The prepared pigment composites were characterised by the following properties: agglomerate size distribution, zeta potential, and specific surface area of the silica fillers before and after application of indigothiazine. The composite pigments were evaluated for their functional properties and subsequently employed as fillers in an acrylonitrile butadiene rubber (NBR). The mechanical properties as well as the spectrophotometric and DSC data of the vulcanisates produced with the composite pigment fillers were also studied before and after being aged under UV

Compared to inorganic pigments, organic pigments exhibit greater vividness, higher colour intensity and higher staining potential. Their drawbacks include greater sensitivity to the action of chemical compounds, temperature and solvents (Binkowski et at., 2000). Several studies were conducted to obtain organic pigments that were permanently attached to a silica core to improve the resistance of the pigments to chemical and thermal treatments. Numerous studies have shown that the reaction mechanism that occurs on the surface of inorganic substances is extremely difficult to identify. For example, the reaction that occurs on the surface of silica with fluorescein results in the formation of covalent bonds between

following modification with silane U-13. For the titanium modified with U-15D, the efficiency of C.I. Reactive Blue 19 dye adsorption decreased slightly. A subsequent report (Raha et al., 2012) proposed a new type of inorganic carrier, Na-montmorilllonite (MMT). If the carrier is a smectite clay or layered silicate, the most effective pathway found involved using the ion exchange process. For that reaction, the dye must exist in a cationic form, which is the case for basic or cationic dyes. Example for that intercalated products of Rhodamine dyes with hectorite, montmorilllonite and other smectite silicates. The synthesis of the water-soluble cationic dyes was performed using two azo dyes (i.e., Solvent Red 24 and Solvent Yellow 14) and one disperse dye (i.e., Red 60) intercalated with Na+-MMT to create nano-structured pigments. The dye-intercalated montmorillonite was successfully obtained. The modification of the dyes into their respective cationic species was confirmed by NMR study, and XRD indicated that these nanopigments had a nanostructured morphology. The authors also observed that the use of the nanopigments in PP reduced the migration of the dye, which results from the intercalation /adsorption of the dye within the clay-based pigments. The most common methods of modifications are presented in Table 1.

**Table 1.** Common methods for the preparation of pigments.

#### **2.2. Organic and inorganic pigments**

124 Advanced Elastomers – Technology, Properties and Applications

**Type of supporter**

Sodiumaluminium silicate

following modification with silane U-13. For the titanium modified with U-15D, the efficiency of C.I. Reactive Blue 19 dye adsorption decreased slightly. A subsequent report (Raha et al., 2012) proposed a new type of inorganic carrier, Na-montmorilllonite (MMT). If the carrier is a smectite clay or layered silicate, the most effective pathway found involved using the ion exchange process. For that reaction, the dye must exist in a cationic form, which is the case for basic or cationic dyes. Example for that intercalated products of Rhodamine dyes with hectorite, montmorilllonite and other smectite silicates. The synthesis of the water-soluble cationic dyes was performed using two azo dyes (i.e., Solvent Red 24 and Solvent Yellow 14) and one disperse dye (i.e., Red 60) intercalated with Na+-MMT to create nano-structured pigments. The dye-intercalated montmorillonite was successfully obtained. The modification of the dyes into their respective cationic species was confirmed by NMR study, and XRD indicated that these nanopigments had a nanostructured morphology. The authors also observed that the use of the nanopigments in PP reduced the migration of the dye, which results from the intercalation /adsorption of the dye within the clay-based pigments. The most common methods of modifications are presented in Table 1.

**The type of coupling agent Type of dye Reference**

Yellow 1, Acid Blue 25, Guinea Green B

bischromatic styryl piridinium dyes

C.I. Reactive Blue 19, C.I. Acid Red 18

C.I. Acid Red 18, C.I. Acid Violet 1

C.I. Reactive Blue 19, C.I. Acid Green 16, C.I. Acid red 18, C.I. Acid Violet 1, C.I. Direct Red 81

C.I. Acid Orange 7, C.I. Reactive Blue 19 Wu et al., 1997

Parida et al., 1997

C.I. Direct Red 81 Binkowski et al.,

C.I. Reactive Blue 19 Jesionowski et

C.I. Acid Red 18 Jesionowski et

2000

2001

al., 2002

al., 2003

Jesionowski 2003

Krysztafkiewicz et al., 2003

Jesionowski et al., 2004

Andrzejewska et

al., 2004

Silica **-** D&C Red 6, Acid

Silica - Monochromatic and

(aminoethyl)-3-aminopropyltriethoxysilane

(aminoethyl)-3-aminopropyltriethoxysilane,

Vinyltris(β-methoxyethoxy)silane; γ-methacryloxypropyltrimethoxysilane

aminopropyltrimethoxysilane

aminopropyltrimethoxysilane

aminopropyltrimethoxysilane

3-ureidopropyltriethoxysilane

aminopropyltrimethoxysilane

aminopropyltrimethoxysilane

Silica 3-aminopropyltriethoxysilane, *N*-2-

Silica 3-aminopropyltriethoxysilane, *N*-2-

Titanium dioxide 3-aminopropylotriethoxysilane, *N*-2- (aminoethyl)-3-

Silica *N*-2-(aminoethyl)-3-

Silica *N*-2-(aminoethyl)-3-

Silica *N*-2-(aminoethyl)-3-

Silica *N*-2-(aminoethyl)-3-

Colourants used in polymers are pigments or dyes (Drobny, 2007). Dyes are organic compounds that are soluble in the polymers forming a molecular solution. They produce bright, intense colours and are transparent and easy to disperse and process. In general, pigments are insoluble in polymers. They produce opacity or translucence in the final product. Pigments can be inorganic or organic compounds and are available in a variety of forms, such as dry powders, colour concentrates and liquids. Pigments and dyes produce colour in a resin due to the selective absorption of visible light with wavelengths ranging from ~380 (violet) to 760 nm (red). Because the dyes are in solution, the colour is produced only from light absorption, and the material is transparent. The colour shade is dependent on the particle size of the pigment. Ultramarine blue pigments are nonreflective due to their refractive index, which is similar to that of the polymer. Pigments must be adequately dispersed in the polymer for optimum scattering. Specks and uneven colouration can result from incomplete dispersion. If agglomerates are present, they may adversely affect the mechanical properties (i.e., tensile strength, impact strength, and flex fatigue) of the resulting product. The pigments should be compatible with the polymer because poor compatibility may result in part failure. High temperature during processing can damage or destroy the pigment resulting in shade variation or colour loss. The thermal sensitivity is related to both temperature and duration of exposure. Long cycles during injection moulding and rotational moulding can have a more adverse effect than high-speed extrusion. Some pigments can act as nucleating agents altering the mechanical properties and improving the clarity of the polymer. The most common inorganic pigments include oxides, sulphides, hydroxides, chromates, and other complexes based on metals, such as cadmium, zinc, titanium, lead, and molybdenum. In general, they are more thermally stable than organic pigments and are more opaque and resistant to migration, chemicals, and fading. They can cause wear on the processing equipment (e.g., extrusion machine screws and barrels). The use of heavy metal compounds (e.g., cadmium) has been restricted due to toxicity issues. The most widely used white pigment is titanium oxide (rutile),which is used either alone or in combination with other colourants to control the opacity and produce pastel shades. Other white pigments include zinc oxide, zinc sulphide, and lead carbonate (lead white).The most widely used black pigment is carbon black, which is essentially composed of pure carbon. When combined with white pigments, it produces various shades of grey depending on the particle size and tinting strength of the carbon black grade. Iron oxide, Fe3O4, is another black pigment that has a lower thermal stability and tinting strength. A variety of inorganic compounds are used as pigments to colour polymers. Drobny (Drobny, 2007) classified the most common types of pigments. The yellow pigments include chrome yellow, chrome-titanium yellow, iron oxides, and lead chromates. The orange pigments include molybdate orange and cadmium orange. The brown pigments include iron oxide or a combination of chrome/iron oxides. The red pigments include iron oxide and cadmium sulphide/selenide. The blue pigments include ultramarine (aluminosilicate with sodium ion and ionic sulfur groups) and mixed metal oxides that are primarily based on cobalt aluminate. The green pigments include chrome oxide and cobalt-based mixed oxides. In general, the organic pigments are brighter, stronger, and more transparent than inorganic pigments but are not as light resistant. They may be partially soluble in many polymers but exhibit a much greater tendency to migrate. The largest group of organic pigments are the azo pigments, which contain one or more azo chromophoric groups and form yellow, orange, and red pigments. Monoazo pigments, which have only one chromophore, exhibit low thermal and light stability and have a tendency to bleed. In addition, the monoazo pigments are not typically employed in plastics. Polyazo pigments, which have more than one chromophore, do not tend to bleed and have better thermal stability and excellent chemical stability. Nonazo pigments have a variety of structures including polycyclic and metal complexes. Phtalocyanine blues and greens, most of which are complexed with copper, are highly stable to light, heat, and chemicals and form highly transparent, intense colours with a high tinting strength. Other organic pigments include quinacridones (red, violet, orange), dioxazines (violet), isoindolines (yellow, orange, red), perylenes, flavanthrones, and anthraquinones. In addition, thiazine pigments are suitable for almost all organic pigment applications (Smith, 2002). In low concentrations, thiazine pigments provide high value-in-use due to their heat stability (up to 290 -300 0 C) in polyolefins enabling warpage-/deformation-free HDPE mouldings, high opacity coupled with very high colour saturation, ease of incorporation (dispersion, flow), chemical inertness, high insolubility in nearly all solvents, high performance in the intended applications (processability and durability), ability to form supramolecules with synergistic values, potential environmental compatibility, and cheap cost.

### **3. Experimental**

126 Advanced Elastomers – Technology, Properties and Applications

refractive index, which is similar to that of the polymer. Pigments must be adequately dispersed in the polymer for optimum scattering. Specks and uneven colouration can result from incomplete dispersion. If agglomerates are present, they may adversely affect the mechanical properties (i.e., tensile strength, impact strength, and flex fatigue) of the resulting product. The pigments should be compatible with the polymer because poor compatibility may result in part failure. High temperature during processing can damage or destroy the pigment resulting in shade variation or colour loss. The thermal sensitivity is related to both temperature and duration of exposure. Long cycles during injection moulding and rotational moulding can have a more adverse effect than high-speed extrusion. Some pigments can act as nucleating agents altering the mechanical properties and improving the clarity of the polymer. The most common inorganic pigments include oxides, sulphides, hydroxides, chromates, and other complexes based on metals, such as cadmium, zinc, titanium, lead, and molybdenum. In general, they are more thermally stable than organic pigments and are more opaque and resistant to migration, chemicals, and fading. They can cause wear on the processing equipment (e.g., extrusion machine screws and barrels). The use of heavy metal compounds (e.g., cadmium) has been restricted due to toxicity issues. The most widely used white pigment is titanium oxide (rutile),which is used either alone or in combination with other colourants to control the opacity and produce pastel shades. Other white pigments include zinc oxide, zinc sulphide, and lead carbonate (lead white).The most widely used black pigment is carbon black, which is essentially composed of pure carbon. When combined with white pigments, it produces various shades of grey depending on the particle size and tinting strength of the carbon black grade. Iron oxide, Fe3O4, is another black pigment that has a lower thermal stability and tinting strength. A variety of inorganic compounds are used as pigments to colour polymers. Drobny (Drobny, 2007) classified the most common types of pigments. The yellow pigments include chrome yellow, chrome-titanium yellow, iron oxides, and lead chromates. The orange pigments include molybdate orange and cadmium orange. The brown pigments include iron oxide or a combination of chrome/iron oxides. The red pigments include iron oxide and cadmium sulphide/selenide. The blue pigments include ultramarine (aluminosilicate with sodium ion and ionic sulfur groups) and mixed metal oxides that are primarily based on cobalt aluminate. The green pigments include chrome oxide and cobalt-based mixed oxides. In general, the organic pigments are brighter, stronger, and more transparent than inorganic pigments but are not as light resistant. They may be partially soluble in many polymers but exhibit a much greater tendency to migrate. The largest group of organic pigments are the azo pigments, which contain one or more azo chromophoric groups and form yellow, orange, and red pigments. Monoazo pigments, which have only one chromophore, exhibit low thermal and light stability and have a tendency to bleed. In addition, the monoazo pigments are not typically employed in plastics. Polyazo pigments, which have more than one chromophore, do not tend to bleed and have better thermal stability and excellent chemical stability. Nonazo pigments have a variety of structures including polycyclic and metal complexes. Phtalocyanine blues and greens, most of which are complexed with

#### **3.1. Characterization and preparation pigment and pigment composites**

The silicas used as the support medium of composite pigments in these studies are as follows: Aerosil 380 (Degussa S.A.) – a hydrophilic fumed silica with a specific surface area of 340 - 380 m2/g; Zeosil 175 (Rhodia) – a precipitated silica with a specific surface area of 175 m2/g; Silica Gel 60 (Merck) – a silica with a specific surface area of 470 – 540 m2/g. The indigothiazine pigment was used as the coloured chromophore. Below 180 ºC, the *cis* form of this pigment is yellow in colour and transforms into the highly durable *trans* form characterised by the red colour upon heating (Figure 1). The latter form is highly resistant to organic solvents, high temperatures, and light. The *cis* form of indigothiazine was obtained by according to the description in the patent Bansi Lal (Bansi Lal et al., 2002).

*cis*- indigothiazine *trans*- indigothiazine

**Figure 1.** Isomeric conversion of *cis*- to *trans*–indigothiazine – ([2,2']-bi(1,4-benzothiazynylidene)-3,3'- (4H, 4'H)-dione.

#### **3.2. Characterisation and preparation pigment and pigment composites**

The composite pigments were prepared by mechanically applying *cis*-indigothiazine on silica surfaces and subjecting them 180 ºC for 2 h to induce their transformation into the highly durable *trans* form. The mechanical application of the *cis* form on silica was accomplished by means of a high-speed blender (Warning Commercial) at ~14,000 rpm. The quantity of the pigment was measured relative to the weight of silica, and the modification required 15 min (Fig. 2).

**Figure 2.** Production process scheme of composite pigments (Copyright Lipińska at al.2011).

To assess the mechanism and kinetics of the electrochemical oxidation of the compounds under investigation, cyclic voltammetry (CV) and differential pulse (DPV) methods were employed on an Autolab analytical unit (EcoChemie, Holland). A three-electrode system was employed for the measurements. Platinum was used as the anode and the auxiliary electrode. The potential of the tested electrode was measured versus a ferricinium/ferrocene reference electrode (Fc+/Fc) where the standard potential is defined as zero and independent of the solvent used. Prior to the measurements, all of the solutions were deoxygenated with argon. During the measurements, an argon atmosphere was maintained over the solution. The effect of the scan rate on the electrooxidation of indigothiazine in an anhydrous medium was investigated. The differential scanning calorimeter (DSC) measurements of *cis–* and *trans–*indigothiazine and vulcanisates were performed on a DSC1 calorimeter (Mettler Toledo) with a heating rate of 10 0C/min. The morphology of the indigothiazine particles and the dispersion in the elastomer matrix containing either unmodified silica or the composite pigments was estimated using scanning electron microscopy with a LEO 1530 SEM microscope. The NBR vulcanisates were broken down in liquid nitrogen, and the fracture surfaces of the vulcanisate were examined. Prior to the measurements, the samples were coated with carbon. The prepared pigment composites were characterised to evaluate the agglomerate size distribution in the water medium using dynamic light scattering (Zetasizer Nano S90, Malvern) for a dispersion concentration of 0.2 g/l (the powders were pre-treated with ultrasound for 0.5 h). The specific surface area of the silica fillers before and after application of indigothiazine was determined. The measurements were performed using the Brunauer-Emmet-Teller (BET) nitrogen adsorption method (GEMINI 2360 V2.01). The zeta potential of the silicas and of pigment composite were evaluated (ZETASIZER 2000, the water dispersion with concentration 2 g/l)). The powders were pre-treated with ultrasound for 1 h.

#### **3.3. Preparation and vulcanization of rubber compounds**

The composite pigments were used as the rubber mixture fillers. The rubber mixture composition was made up of the following: NBR (Perbunan 28-45F containing 28 wt % acrylonitryle groups from Lanxess, 100 phr); sulphur crosslinking system (See Table 2) and optional: composite pigments or silica (30 phr).

aphr = parts per hundred of rubber by weight

**Table 2.** Rubber formulations.

128 Advanced Elastomers – Technology, Properties and Applications

required 15 min (Fig. 2).

highly durable *trans* form. The mechanical application of the *cis* form on silica was accomplished by means of a high-speed blender (Warning Commercial) at ~14,000 rpm. The quantity of the pigment was measured relative to the weight of silica, and the modification

**Figure 2.** Production process scheme of composite pigments (Copyright Lipińska at al.2011).

To assess the mechanism and kinetics of the electrochemical oxidation of the compounds under investigation, cyclic voltammetry (CV) and differential pulse (DPV) methods were employed on an Autolab analytical unit (EcoChemie, Holland). A three-electrode system was employed for the measurements. Platinum was used as the anode and the auxiliary The mixture was prepared in a laboratory rolling mill (Roll dimensions: D = 200 mm, L = 450), and the vulcanisation process was carried out in a hydraulic press (160 0C) in line with the curing time specifications (See Table 2). The vulcanisation kinetics was measured with evaluated using a Monsanto vulcameter with an oscillating rotor (ZACH Metalchem) in

compliance with the ISO-4317standard. The tests performed on the vulcanizates are summarised as follows: the mechanical properties were characterised using a Zwick 1435 strength testing machine and according to ISO-37. The crosslink density of vulcanizates was determined by equilibrium swelling in toluene, based on the Flory-Rehner (Flory et al., 1943) equation using the Huggins parameter of elastomer–solvent interaction µ = 0.381 + 0.671Vr [eq. (1)].

$$\upsilon\_{\varepsilon} = -\frac{\ln(1 - V\_r)V\_r + \mu V\_r^2}{\frac{1}{V\_0(V\_r^{\frac{\pi}{3}} - \frac{V\_r}{2})}} \tag{1}$$

where: �� - crosslink density, Vr - volume fraction of elastomer in swollen gel, Vo - molar volume of solvent [mol/cm3].

spectrophotometric data were taken using a KONICA–MINOLTA CM–3600 spectrophotometer under computer control with spectra analysis software (Colour Data Software CM – S 100 in Spectra Magic) according to the following standards: CIE No. 15; PN-EN ISO 105- J01; ASTM E1 164; and DIN 5033 Tell.7, resistance to ageing under UV radiation was tested by exposing the vulcanizates to a UV source (Atlas UV200) for a total of 5 days, equivalent to the ageing by approximately 3 months under normal conditions. The test timeline was as follows:day segment: UV power 0.7 W/m2; temperature 60 ºC; duration 8 h, night segment: UV power 0.0 W/m2; temperature 50 ºC; duration 4 h. The vulcanizates subjected to UV ageing were tested for tensile strength. Spectrophotometric measurements were performed before and after UV irradiation. For each vulcanizate being tested, the ageing coefficient, K, was computed using Equation 2:

$$\mathbf{K} = \frac{(\text{TS} \cdot \text{Eb})\_{\text{after aggregation}}}{(\text{TS} \cdot \text{Eb})\_{\text{before aginging}}} \tag{2}$$

where: K – ageing coefficients; TS – tensile strength; Eb – relative elongation.

#### **4. Results**

The electrode reactions characterising the electrochemical oxidation of indigothiazine at the platinum electrode were studied by cyclic pulse voltammetry. The half-wave potential of the peak in the cyclic voltammogram is characteristic of each subsequent step in the investigated electrode reaction. Selected cyclic voltammograms recorded in a solution consisting of the indigothiazine and the supporting electrolyte are presented in Fig. 3. The cyclic voltammograms recorded for the indigothiazine solution exhibit two peaks, which involve at least three electrode steps of the pigment electrooxidation in the potential range prior to the potential at which electrolyte decomposition is initiated. The supporting electrolyte (in *N*-methyl-*2*-pyrrolidone) has no characteristic peaks except for charging the electrical double layer. The electrooxidation potentials were determined fallowing: the first peak - 0.71 V, second peak - 0.82 V, third peak - 1.01 V. In addition, the electrode reactions of the examined connection are becoming more and more irreparable. Polarisation towards the negative potentials of the reduction reaction was not observed (Fig. 4). The recorded voltammograms, under the linear diffusion of the first electrooxidation step, were employed to determine the half-wave potential (E1/2). Based on the results provided in Table 3, indigothiazine, E1/2 = 0.36 V, was easily oxidised. The energy of the highest filled molecular orbital (EHOMO) was estimated to determine the ease of electron backdonation (potential of the ionisation) and the enthalpy (ΔH). The EHOMO energy (-7.65 eV) and enthalpy (22.83) confirmed the good antioxidant properties and stability.

**Table 3.** Cyclic voltammetry and molecular orbital parameters for the studied compound

130 Advanced Elastomers – Technology, Properties and Applications

ageing coefficient, K, was computed using Equation 2:

0.671Vr [eq. (1)].

**4. Results**

volume of solvent [mol/cm3].

compliance with the ISO-4317standard. The tests performed on the vulcanizates are summarised as follows: the mechanical properties were characterised using a Zwick 1435 strength testing machine and according to ISO-37. The crosslink density of vulcanizates was determined by equilibrium swelling in toluene, based on the Flory-Rehner (Flory et al., 1943) equation using the Huggins parameter of elastomer–solvent interaction µ = 0.381 +

�� = − �������)������

where: �� - crosslink density, Vr - volume fraction of elastomer in swollen gel, Vo - molar

spectrophotometric data were taken using a KONICA–MINOLTA CM–3600 spectrophotometer under computer control with spectra analysis software (Colour Data Software CM – S 100 in Spectra Magic) according to the following standards: CIE No. 15; PN-EN ISO 105- J01; ASTM E1 164; and DIN 5033 Tell.7, resistance to ageing under UV radiation was tested by exposing the vulcanizates to a UV source (Atlas UV200) for a total of 5 days, equivalent to the ageing by approximately 3 months under normal conditions. The test timeline was as follows:day segment: UV power 0.7 W/m2; temperature 60 ºC; duration 8 h, night segment: UV power 0.0 W/m2; temperature 50 ºC; duration 4 h. The vulcanizates subjected to UV ageing were tested for tensile strength. Spectrophotometric measurements were performed before and after UV irradiation. For each vulcanizate being tested, the

> K = ������)����� ������ ������)������ ������

The electrode reactions characterising the electrochemical oxidation of indigothiazine at the platinum electrode were studied by cyclic pulse voltammetry. The half-wave potential of the peak in the cyclic voltammogram is characteristic of each subsequent step in the investigated electrode reaction. Selected cyclic voltammograms recorded in a solution consisting of the indigothiazine and the supporting electrolyte are presented in Fig. 3. The cyclic voltammograms recorded for the indigothiazine solution exhibit two peaks, which involve at least three electrode steps of the pigment electrooxidation in the potential range prior to the potential at which electrolyte decomposition is initiated. The supporting electrolyte (in *N*-methyl-*2*-pyrrolidone) has no characteristic peaks except for charging the electrical double layer. The electrooxidation potentials were determined fallowing: the first peak - 0.71 V, second peak - 0.82 V, third peak - 1.01 V. In addition, the electrode reactions of the examined connection are becoming more and more irreparable. Polarisation towards the negative potentials of the reduction reaction was not observed (Fig. 4). The recorded voltammograms, under the linear diffusion of the first electrooxidation step, were employed to determine the half-wave potential (E1/2). Based on the

where: K – ageing coefficients; TS – tensile strength; Eb – relative elongation.

����� � � � �� � ) �

(1)

(2)

**Figure 3.** Voltammogram of indigothiazine oxidation on a Pt electrode; c = 1 mmol/dm3 in 0,1 mol/dm3 in *N*-methyl-*2*-pyrrolidone at various polarisation speeds.

**Figure 4.** Voltammogram of indigothiazine for negative potential on the Pt electrode; c = 1 mmol/dm3 in 0,1 mol/dm3 in *N*-methyl-*2*-pyrrolidone.

The thermal strength of *cis –* and *trans–* indigothiazine (Fig.6) was measured applying the DSC method (DSC 1 Mettler Toledo). The resulting diagrams show the melting bands of the pigment's crystalline phase at around 440° C (Fig. 5). The exothermic peak on the *cis–* indigothiazine curve is in the regime where the *cis*- to *trans-* conversion took place most intensively.

**Figure 5.** DSC curves of indigothiazine pigment – *cis* and *trans* conformation.

**Figure 6.** SEM image of trans-indigothiazine pigment .

The fumed silica, Aerosil 380, used in the tests is characterised by the high numerical percentage (99.67 %) of aggregates in the range of 116-330 nm in size; however, these aggregates account for only 28.83 % of the filler's volume in the water medium. The outstanding fraction (71.17 %) consists of agglomerates whose sizes are in the range of 1056- 3000 nm. The application of indigothiazine pigment on the surface of Aerosil 380 silica reduced the average particle size and fragmented agglomerates, effectively producing the composite pigments with sizes ranging from 82-131 nm (100 %). For the precipitated silica, Zeosil 175, the application of pigment helped produce the composite pigments with 99.92 % of the aggregates in the size range from 116-262 nm and the rest the size range of 2379-3000 nm. Although the modification of Zeosil 175 reduced the average particle size and increased the particles uniformity, the particles still formed agglomerates in the order of a few µm in size. For Silica Gel 60, applying indigothiazine only slightly increased the particles tendency to agglomerate; that is, the aggregates size increased from 104-147 nm (unmodified) to 116- 208 nm (modified). The results are summarized in Table 4.

132 Advanced Elastomers – Technology, Properties and Applications

**Figure 5.** DSC curves of indigothiazine pigment – *cis* and *trans* conformation.

**Figure 6.** SEM image of trans-indigothiazine pigment .

intensively.

The thermal strength of *cis –* and *trans–* indigothiazine (Fig.6) was measured applying the DSC method (DSC 1 Mettler Toledo). The resulting diagrams show the melting bands of the pigment's crystalline phase at around 440° C (Fig. 5). The exothermic peak on the *cis–* indigothiazine curve is in the regime where the *cis*- to *trans-* conversion took place most

**Table 4.** Aggregates size and specific surface area of the silicas and composite pigments.

The specific surface area was measured using the BET nitrogen adsorption method before and after applying the indigothiazine pigment. Each composite pigment was characterised by reduced specific surface area as compared with the unmodified silicas. The decrease in the specific surface proceeded to a larger extent in the case of Aerosil 380 and Silica Gel 60 with a large specific surface.

Figures 7-9 shows the change in zeta potential as a function of pH for both unmodified silicas and composite pigments. The largest change in the surface charge was caused by the application of indigothiazine to Zeosil 175 silica. The difference in zeta potential between the composite and unmodified silica Zeosil 175 was approximately 30 mV (pH 4), indicating that the modification reduced the acidic characteristics of the Zeosil 175 surface, which is a favourable result for its use as a polymer composite filler. For the A 380/ indigothiazine pigment, the zeta potential was slightly higher than for the unmodified Aerosil 380. For Silica Gel 60 and the Silica Gel 60/ indigothiazine composite, the two zeta potential curves overlapped one another, indicating that the application of the pigment induced practically no change to the surface charge in this filler.

**Figure 7.** Zeta potential diagram for the composite pigments in which the supports was Aerosil 380.

**Figure 8.** Zeta potential diagram for the composite pigments in which the support was Zeosil 175.

134 Advanced Elastomers – Technology, Properties and Applications

Zeosil 175

Zeosil 175/indigothiazine

Aerosil 380/indigothiazine

0

10

**Figure 7.** Zeta potential diagram for the composite pigments in which the supports was Aerosil 380.

0 2 4 6 810 12

0 2 4 6 810 12

pH

pH

**Figure 8.** Zeta potential diagram for the composite pigments in which the support was Zeosil 175.

**Figure 9.** Zeta potential diagram for the composite pigments in which the support was Silica Gel 60.

The effects of silicas and the composite pigments on the properties of rubber mixtures were evaluated with respect to the curing kinetics and tensile strength of the vulcanizates (Table 5). Rheometric tests showed that adding composite pigments to the rubber mixture did not significantly affect the vulcanisation time, t09, torque increase, Δ*M*, and the initial viscosity values (*M*min) in comparison to the mixtures containing unmodified silicas. The only exception was the Zeosil 175/ indigothiazine composite, which showed a lower torque increase than the original Zeosil 175. Tg values from DSC curves for vulcanizates filled modified and unmodified silicas have similar glass transitios temperature in range -28 0C – 30 0C (Table 5) The vulcanisation process produced red-coloured vulcanizates, which were then subjected to tensile strength tests. The results showed that the application of the composite pigments had a similar effect on the mechanical properties of the vulcanizates as did the application of the unmodified fillers, as indicated by the tensile strength TS (Table 5) and the Scanning Electron Microscope SEM observations. The SEM images revealed agglomerates in the vulcanizates filled with unmodified silicas (Figs. 11, 13, 15) and composite pigments (Figs. 12, 14, 16). Even though the modification of the test fillers reduced the average size of agglomerates in the water medium, it did not significantly affect the dispersion level of silicas in the elastomer. The vulcanizates containing modified silicas had also similar values of curing density, module values at 100% stress and elongations at break EB in comparison to composites filled unmodified silicas.

**Figure 10.** Ageing coefficient value (K) of vulcanizates filled with composte pigments and unmodified silicas.

Next, the functional properties of the vulcanizates after undergoing the 120 h of UV ageing were re-examined. After ageing, all samples exhibited lower TS and EB values and a higher curing density; however, the vulcanizates containing either the A 380/ indigothiazine, Z 175/ indigothiazine and S60/ indigothiazine composites exhibited less UV degradation. Based on the K values (Fig.10), it is possible to conclude that the pigment protects against ageing most effectively when applied to the A380 and Z175 silica supports. The closer the K value is to unity, the less ageing is considered to have taken place. Colorimetric tests of the aged vulcanizates were also performed using the non-aged samples as benchmarks (Table 6). The colorimetric measurements were taken in the CIE-Lab space that expressed the colours in the Cartesian L-a-b coordinate system of the brightness L, the red-greenness a, and the yellow-blueness b. The L value ranges from 0 (black) to 100 (white), and all shades of grey are expressible by the values in between. The colorimetric data reveal that the UV radiation not only degrades the functional properties but also changes the colour of the vulcanizates (Table 6). The parameter dE\*ab depicts the degree of change in colour as a result of ageing. All the vulcanizates containing composite pigments exhibited a lesser degree of change in colour than those containing unmodified silica. The vulcanizates with either the A 380/ indigothiazine composite or the Silica Gel 60/ indigothiazine composite exhibited three times less the degree of change in colour than those with ordinary silicas.

silicas.

**Figure 10.** Ageing coefficient value (K) of vulcanizates filled with composte pigments and unmodified

NBR/A380

azine NBR/Z175

azine NBR/S60

**K**

NBR/A380/indigothi

NBR/Z175/indigothi

0 0.2 0.4 0.6 0.8 1

Next, the functional properties of the vulcanizates after undergoing the 120 h of UV ageing were re-examined. After ageing, all samples exhibited lower TS and EB values and a higher curing density; however, the vulcanizates containing either the A 380/ indigothiazine, Z 175/ indigothiazine and S60/ indigothiazine composites exhibited less UV degradation. Based on the K values (Fig.10), it is possible to conclude that the pigment protects against ageing most effectively when applied to the A380 and Z175 silica supports. The closer the K value is to unity, the less ageing is considered to have taken place. Colorimetric tests of the aged vulcanizates were also performed using the non-aged samples as benchmarks (Table 6). The colorimetric measurements were taken in the CIE-Lab space that expressed the colours in the Cartesian L-a-b coordinate system of the brightness L, the red-greenness a, and the yellow-blueness b. The L value ranges from 0 (black) to 100 (white), and all shades of grey are expressible by the values in between. The colorimetric data reveal that the UV radiation not only degrades the functional properties but also changes the colour of the vulcanizates (Table 6). The parameter dE\*ab depicts the degree of change in colour as a result of ageing. All the vulcanizates containing composite pigments exhibited a lesser degree of change in colour than those containing unmodified silica. The vulcanizates with either the A 380/ indigothiazine composite or the Silica Gel 60/ indigothiazine composite exhibited three times less the degree of change in colour than those with ordinary silicas.

*M*min. – minimum torque [dNm], Δ*M* – torque increase [dNm], *t09* – vulcanisation time [min]**,** *SE100* – stress at 100 % elongation [MPa], *TS* – tensile strength [MPa], *EB* – relative elongation at break [%], *ν<sup>T</sup>* – curing density of vulcanizates calculated using the Flory Rehner equation [mol/cm3]

**Table 5.** Effects of composite pigments and unmodified silicas on selected properties of NBR.

L\* – brightness parameter, a\* – redness – greenness reading, b\* – yellowness – blueness reading, dl\*, da\*, db\* – the difference in colour between trial and standard readings; dE\*ab – vector of overall errors in colour and colour darkness;

**Table 6.** Colorimetric data for the vulcanizates containing unmodified silicas or composite pigment.

**Figure 11.** SEM image of NBR vulcanizate filled with Aerosil 380.

**Figure 12.** SEM image of NBR vulcanizate filled with A 380/indigothiazine.

**Figure 13.** SEM image of NBR vulcanizate filled with Zeosil 175.

**Figure 14.** SEM image of NBR vulcanizate filled with Z 175/indigothiazine.

**Figure 11.** SEM image of NBR vulcanizate filled with Aerosil 380.

**Figure 12.** SEM image of NBR vulcanizate filled with A 380/indigothiazine.

**Figure 13.** SEM image of NBR vulcanizate filled with Zeosil 175.

**Figure 15.** SEM image of NBR vulcanizate filled with Silica Gel 60.

**Figure 16.** SEM image of NBR vulcanizate filled with S60/indigothiazine.

### **5. Conclusions**

Indigothiazine exhibited antioxidant properties, which allows them to be successfully employed as anti-ageing substances. Red-colour composite pigments were produced by modifying silica Aerosil 380, Zeosil 175, and Gel 60. The modification method employed in the present work eliminated harmful solvents employed in the conventional process. The newly developed method is much simpler, cheaper, and easier to use on an industrial scale compared to the conventional process, which requires a solvent. The silica modification process using the blender reduced the specific surface area of the tested fillers. For the composites with Aerosil 380, Zeosil 175 and Silica Gel 60, this process reduced the agglomerate sizes. For the samples with Aerosil 380 and Zeosil 175, the zeta potential also increased, which indicated that their surface became less polarised and their acceptor characteristic was partially lost. The addition of the composite pigments to the polymer mixtures did not significantly change their curing kinetics or processing properties. The vulcanisation of NBR in the presence of the modified silicas resulted in vulcanisates with a vivid red colour were produced.  The strength tests after UV ageing of the vulcanisates showed that the A 380/ indigothiazine composite and the Zeosil 175/ indigothiazine composite possess antiageing properties and have a favourable impact on the plastic's mechanical properties and colour stability. The present work led to the successful production of vulcanisates with enhanced ageing resistance.

### **Author details**

Anna Marzec and Marian Zaborski *Institute of Polymer and Dye Technology, Technical University of Lodz, Poland*

#### **6. References**

Bansi Lal, K. & Bruno, P. (2002). Production of thiazine-indigo pigments. 6339084, United States.

140 Advanced Elastomers – Technology, Properties and Applications

Indigothiazine exhibited antioxidant properties, which allows them to be successfully employed as anti-ageing substances. Red-colour composite pigments were produced by modifying silica Aerosil 380, Zeosil 175, and Gel 60. The modification method employed in the present work eliminated harmful solvents employed in the conventional process. The newly developed method is much simpler, cheaper, and easier to use on an industrial scale compared to the conventional process, which requires a solvent. The silica modification process using the blender reduced the specific surface area of the tested fillers. For the composites with Aerosil 380, Zeosil 175 and Silica Gel 60, this process reduced the agglomerate sizes. For the samples with Aerosil 380 and Zeosil 175, the zeta potential also increased, which indicated that their surface became less polarised and their acceptor characteristic was partially lost. The addition of the composite pigments to the polymer mixtures did not significantly change their curing kinetics or processing properties. The vulcanisation of NBR in the presence of the modified silicas resulted in vulcanisates with a vivid red colour were produced.  The strength tests after UV ageing of the vulcanisates showed that the A 380/ indigothiazine composite and the Zeosil 175/ indigothiazine composite possess antiageing properties and have a favourable impact on the plastic's mechanical properties and colour stability. The present work led to the successful production of vulcanisates with

Al Dwayyan, A.S., Qaid, M.H., Khan, M.A. & Al Salhi M.S. (2012). Structural and spectral investigations of Rhodamine (Rh6G) dye-silica core–shell nanoparticles.

Andrzejewska, A., Krysztafkiewicz, A. & Jesionowski, T. (2004). Adsorption of organic

Anbia, M. & Salehi, S. (2012). Removal of acid dyes from aqueous media by adsorption

Ansarifar, A., Azharb, A., Ibrahima, N., Shiaha, S.F. & Lawtonc, J.M.D. (2005). The use of a silanised silica filler to reinforce and cross link natural rubber. *Int. J. Adhes.*

dyes on the aminosilane modified TiO2 surface. *Dyes Pigm.,* 62, 121-130.

onto amino-functionalized nanoporous silica SBA-3 *Dyes Pigm*., 94, 1-9.

**5. Conclusions**

enhanced ageing resistance.

*Technical University of Lodz,*

Anna Marzec and Marian Zaborski *Institute of Polymer and Dye Technology,*

*Opt. Mater.*, 34, 761–768.

*Adhes.,* 25, 77–86.

**Author details**

**6. References**

*Poland*

	- Jesionowski, T. (2005). Characterisation of pigments obtained by adsorption of C.I. Basic Blue 9 and C.I. Acid Orange 52 dyes onto silica particles precipitated via the emulsion route. *Dyes Pigm.*, 67, 81-92.
	- Jesionowski, T., Przybylska, A., Kurc, B. & Ciesielczyk, F. (2011). The preparation of pigment composites by adsorption of C.I. Mordant Red 11 and 9-aminoacridine on both unmodified and aminosilane-grafted silica supports. *Dyes Pigm*., 88, 116- 124.
	- Kishore, P.N.R. & Jeevanandam, P. (2012). A novel thermal decomposition approach for the synthesis of silica-iron oxide core–shell nanoparticles. *J. Alloy.Comp.,* 52, 51– 62.
	- Kister, O. & Roessner F. (2012). Synthesis and characterization of mesoporous and amorphous silica modified with silica-organo-sulfogroups*. J. Porous Mat*.,19, 119- 131.
	- Klapiszewska, B., Krysztafkiewicz, A. & Jesionowski, T. (2003). Highly dispersed green silicate and oxide pigments precipitated from model systems of postgalvanic waste. *Environ. Sci. Technol.,* 37, 4811-4818.
	- Kohno, Y., Senga, M., Shibata, M., Yoda, K., Matsushima, R., Tomita, Y., Maeda, Y. & Kobayashi, K. (2011). Stabilization of flavylium dye by incorporation into Fecontaining mesoporous silicate. *Rev. Mineral. Geochem*., 141, 77–80.
	- Krysztafkiewicz, A., Jesionowski, T. & Binkowski, S. (2000). Precipitated silicas modified with 3-aminopropyltriethoxysilane. *Colloids Surf.,* 173, 73–84.
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	- Krysztafkiewicz, A., Binkowski, S. & Wysocka, I. (2003). Pigments on amorphous silica carriers. *Powder Technol.*, 132, 190– 195.
	- Ladewig, K., Seifert, A., Hahn, H., Hietschold, M., Moszner, N., Burtscherd P. & Spange, S. (2012). apara-Nitroaniline- functionalized chromophoric organic– inorganic hybrid. *J. Mater. Chem*., 22, 3839-3844.
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emulsion route. *Dyes Pigm.*, 67, 81-92.

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silica surface. *Appl. Surf. Sci.*, 199, 31–39.

carriers. *Powder Technol.*, 132, 190– 195.

inorganic hybrid. *J. Mater. Chem*., 22, 3839-3844.

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Jesionowski, T., Przybylska, A., Kurc, B. & Ciesielczyk, F. (2011). The preparation of pigment composites by adsorption of C.I. Mordant Red 11 and 9-aminoacridine on both unmodified and aminosilane-grafted silica supports. *Dyes Pigm*., 88, 116-

Kishore, P.N.R. & Jeevanandam, P. (2012). A novel thermal decomposition approach for the synthesis of silica-iron oxide core–shell nanoparticles. *J. Alloy.Comp.,* 52, 51–

Kister, O. & Roessner F. (2012). Synthesis and characterization of mesoporous and amorphous silica modified with silica-organo-sulfogroups*. J. Porous Mat*.,19, 119-

Klapiszewska, B., Krysztafkiewicz, A. & Jesionowski, T. (2003). Highly dispersed green silicate and oxide pigments precipitated from model systems of postgalvanic waste.

Kohno, Y., Senga, M., Shibata, M., Yoda, K., Matsushima, R., Tomita, Y., Maeda, Y. & Kobayashi, K. (2011). Stabilization of flavylium dye by incorporation into Fe-

Krysztafkiewicz, A., Jesionowski, T. & Binkowski, S. (2000). Precipitated silicas

Krysztafkiewicz, A., Binkowski, S. & Jesionowski, T. (2002). Adsorption of dyes on a

Krysztafkiewicz, A., Binkowski, S. & Wysocka, I. (2003). Pigments on amorphous silica

Ladewig, K., Seifert, A., Hahn, H., Hietschold, M., Moszner, N., Burtscherd P. & Spange, S. (2012). apara-Nitroaniline- functionalized chromophoric organic–

Liu, X. & Zhao, S. (2008). Study on structure and properties of SSBR/SiO2 co-coagulated rubber and SSBR filled with nanosilica composites*. J. Appl. Polym. Sci.,*109, 3900-

Lipińska, M., Sokołowska, J., Boruszczak, Z. & Zaborski, M. (2011).Synthesis and use of composite silica/trans-benzothiazine indigo pigment in elastomeric systems. *Przem.*

Marzec, A., Lipińska, M., Sokołowska, J. & Zaborski M. (2010). Pigment-modified silicas

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containing mesoporous silicate. *Rev. Mineral. Geochem*., 141, 77–80.

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	- Jung-Woo, P., Jun, P. Y. & Chul-Ho, J. (2011). Post-grafting of silica surfaces with prefunctionalized organosilanes: new synthetic equivalents of conventional trialkoxysilanes. *Chem Commun*., 47, 4860-4871.

## **Smart Elastomers**

144 Advanced Elastomers – Technology, Properties and Applications

trialkoxysilanes. *Chem Commun*., 47, 4860-4871.

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**Chapter 6**

## **Microstructure and Properties of Magnetorheological Elastomers**

Anna Boczkowska and Stefan Awietjan

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/50430

### **1. Introduction**

Magnetorheological elastomers (MREs) belong to the new group of the functional materials called "smart". Although smart materials are known since long time, their intensive development started in the end of the XXth century. The term *smart materials, intelligent materials* or less frequently used *adaptive materials* or *multifunctional materials,* was introduced in the eighties of the twentieth century, when some materials, which were included in the group were already known. Till today there is no accepted universal definition of smart material, it is also not included in the encyclopedia devoted to these materials, published in 2002 [1, 2].

The term smart material generally refers to material which changes its properties under the influence of various external stimuli [3]. However, it seems that this approach is too simplistic. Much more precise description of smart material is given by Takagi in his work [4]. According to him, smart material is capable of reacting to external stimuli by changing its material properties for the desired way and effectively responds to these stimuli. Such material should therefore be some kind of sensor, processor, and actuator. These attributes should be looped, and the effect of changing the properties of the material should be done in real time.

Among smart materials, magnetorheological materials (MR) are an important group. They are a class of materials with rheological properties rapidly varied by the application of a magnetic field (Fig. 1). The change in their properties is in the proportion to the magnitude of the magnetic field applied and is immediately reversible. The Bingham plastic model is often used to model the behaviour of MR materials [1].

Research focussing on magnetorheological materials was initiated by Jacob Rabinow at the National Bureau of Standards, now the National Institute for Science and Technology, in the

© 2012 Boczkowska and Awietjan, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

United States in the 1940s and early 1950s. His early works on magnetorheological fluids led to a host of devices and products based on dry magnetic powders, for example the magnetic, powder brake. In addition to MR fluids, the MR materials also contain magnetic field responsive gels [5], foams, powders, and elastomers. The comparison of the most typical magnetorheological materials is shown in Table 1.

**Figure 1.** Magnetorheological material A- before, B – and after the application of an external magnetic field.

**Table 1.** Composition and properties of typical magnetorheological materials [6].

The interest in magnetorheological elastomers (MREs) has recently increased because of their prospects for application in various smart systems. They are still much less known than magnetorheological fluids. The MREs are smart materials, analogues of the magnetorheological fluids (MRFs), in which the fluid component is replaced by a crosslinked material like rubber or silicone. They consist of micron sized magnetically permeable particles in non-magnetic matrix. In a similar way, as in the case of MRFs, the particles tend to align themselves in the direction of the magnetic field, but after curing of the matrix such microstructure is fixed. Magnetorheological materials change their rheological properties under the influence of an external magnetic field. Their rheological properties can be changed continuously, rapidly and reversibly by changing an external magnetic field. This behaviour is described by the magnetorheological effect.

In this chapter the overview on magnetorheological elastomers is given, as well as basic knowledge about rheology and magneto-rhelogy. Also the effect of the amount, size and orientation of the particles on the microstructure and properties of MREs is discussed, as well as the influence of magnetic field on the compressive characteristics and rheological properties of developed MREs.

## **2. Magnetorheological elastomers**

The MREs consist of magnetically permeable particles (such as iron or other ferromagnetic particles) added to a viscoelastic polymeric material prior to crosslinking. The MREs contain ferromagnetic particles having sizes from few to few hundreds of μm. Pure iron has the highest saturation magnetization of known elements and it has also high permeability and low remanent magnetization, providing high, short-term inter-particle attraction. It is known from available literature that the amount and the shape of the particles, as well as the type of the polymer matrix influence the MREs properties.