Structural failure can occur in FRP materials when:

#### *3.3.3. Material requirements*

The matrix must also meet certain requirements in order to first be suitable for the FRP proc‐ ess and ensure a successful reinforcement of it. The matrix must be able to properly saturate, and bond with the fibres within a suitable curing period. The matrix should preferably bond chemically with the fibre reinforcement for maximum adhesion. The matrix must also com‐ pletely envelope the fibres to protect them from cuts and notches that would reduce their strength, and to transfer forces to the fibres. The fibres must also be kept separate from each other so that if failure occurs it is localized as much as possible, and if failure occurs the ma‐ trix must also debond from the fibre for similar reasons. Finally the matrix should be of a plastic that remains chemically and physically stable during and after reinforcement and molding processes. To be suitable for reinforcement material fibre additives must increase the tensile strength and modulus of elasticity of the matrix and meet the following condi‐ tions; fibres must exceed critical fibre content; the strength and rigidity of fibres itself must exceed the strength and rigidity of the matrix alone; and there must be optimum bonding between fibres and matrix.

## **3.4. Glass fibre material**

*3.3.1. Advantages and limitations*

28 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

*3.3.2. Failure modes*

FRP allows the alignment of the glass fibres of thermoplastics to suit specific design programs. Specifying the orientation of reinforcing fibres can increase the strength and resistance to deformation of the polymer. Glass reinforced polymers are strongest and most resistive to deforming forces when the polymers fibres are parallel to the force be‐ ing exerted, and are weakest when the fibres are perpendicular. Thus this ability is at once both an advantage and a limitation depending on the context of use. Weak spots of perpendicular fibres can be used for natural hinges and connections, but can also lead to material failure when production processes fail to properly orient the fibres par‐ allel to expected forces. When forces are exerted perpendicular to the orientation of fi‐ bres the strength and elasticity of the polymer is less than the matrix alone. In cast resin components made of glass reinforced polymers such as UP and EP, the orientation of fi‐ bres can be oriented in two-dimensional and three-dimensional weaves. This means that when forces are possibly perpendicular to one orientation, they are parallel to another

**•** Tensile forces stretch the matrix more than the fibres, causing the material to shear at the

**•** Tensile forces near the end of the fibres exceed the tolerances of the matrix, separating the

**•** Tensile forces can also exceed the tolerances of the fibres causing the fibres themselves to

The matrix must also meet certain requirements in order to first be suitable for the FRP proc‐ ess and ensure a successful reinforcement of it. The matrix must be able to properly saturate, and bond with the fibres within a suitable curing period. The matrix should preferably bond chemically with the fibre reinforcement for maximum adhesion. The matrix must also com‐ pletely envelope the fibres to protect them from cuts and notches that would reduce their strength, and to transfer forces to the fibres. The fibres must also be kept separate from each other so that if failure occurs it is localized as much as possible, and if failure occurs the ma‐ trix must also debond from the fibre for similar reasons. Finally the matrix should be of a plastic that remains chemically and physically stable during and after reinforcement and molding processes. To be suitable for reinforcement material fibre additives must increase the tensile strength and modulus of elasticity of the matrix and meet the following condi‐ tions; fibres must exceed critical fibre content; the strength and rigidity of fibres itself must exceed the strength and rigidity of the matrix alone; and there must be optimum bonding

orientation; this eliminates the potential for weak spots in the polymer.

Structural failure can occur in FRP materials when:

interface between matrix and fibres.

fracture leading to material failure [29].

fibres from the matrix.

*3.3.3. Material requirements*

between fibres and matrix.

FRPs use textile glass fibres; textile fibres are different from other forms of glass fibres used for insulating applications. Textile glass fibres begin as varying combinations of SiO2, Al2O3, B2O3, CaO, or MgO in powder form. These mixtures are then heated through a direct melt process to temperatures around 1300 degrees Celsius, after which dies are used to extrude filaments of glass fibre in diameter ranging from 9 to 17 μm. These filaments are then wound into larger threads and spun onto bobbins for transportation and further processing. Glass fibre is by far the most popular means to reinforce plastic and thus enjoys a wealth of production processes, some of which are applicable to aramid and carbon fibres as well ow‐ ing to their shared fibrous qualities. Roving is a process where filaments are spun into larger diameter threads. These threads are then commonly used for woven reinforcing glass fabrics and mats, and in spray applications. Fibre fabrics are web-form fabric reinforcing material that has both warped and weft directions. Fibre mats are web-form non-woven mats of glass fibres. Mats are manufactured in cut dimensions with chopped fibres, or in continuous mats using continuous fibres. Chopped fibre glass is used in processes where lengths of glass threads are cut between 3 and 26 mm, threads are then used in plastics most commonly in‐ tended for moulding processes. Glass fibre short strands are short 0.2–0.3 mm strands of glass fibres that are used to reinforce thermoplastics most commonly for injection moulding.

#### **3.5. Aramid fibre material process**

Aramid fibres are most commonly known Kevlar, Nomex and Technora. Aramids are gener‐ ally prepared by the reaction between an amine group and a carboxylic acid halide group (aramid); commonly this occurs when an aromatic polyamide is spun from a liquid concen‐ tration of sulfuric acid into a crystallized fibre. Fibres are then spun into larger threads in order to weave into large ropes or woven fabrics (Aramid) [29]. Aramid fibres are manufac‐ tured with varying grades to base on varying qualities for strength and rigidity, so that the material can be somewhat tailored to specific design needs concerns, such as cutting the tough material during manufacture.

#### **3.6. FRP, applications**

Fibre-reinforced plastics are best suited for any design program that demands weight sav‐ ings, precision engineering, finite tolerances, and the simplification of parts in both produc‐ tion and operation. A molded polymer artifact is cheaper, faster, and easier to manufacture than cast aluminum or steel artifact, and maintains similar and sometimes better tolerances and material strengths. The Mitsubishi Lancer Evolution IV also used FRP for its spoiler ma‐ terial [30-32].

#### *3.6.1. Carbon fibre reinforced polymers*

Rudder of commercial airplane

**•** Advantages over a traditional rudder made from sheet aluminum are:

## *3.6.2. Structural applications of FRP*

FRP can be applied to strengthen the beams, columns and slabs in buildings. It is possible to increase strength of these structural members even after these have been severely damaged due to loading conditions. For strengthening beams, two techniques are adopted. First one is to paste FRP plates to the bottom (generally the tension face) of a beam. This increases the strength of beam, deflection capacity of beam and stiffness (load required to make unit de‐ flection). Alternatively, FRP strips can be pasted in 'U' shape around the sides and bottom of a beam, resulting in higher shear resistance. Columns in building can be wrapped with FRP for achieving higher strength. This is called wrapping of columns. The technique works by restraining the lateral expansion of the column. Slabs may be strengthened by pasting FRP strips at their bottom (tension face). This will result in better performance, since the tensile resistance of slabs is supplemented by the tensile strength of FRP. In the case of beams and slabs, the effectiveness of FRP strengthening depends on the performance of the resin chos‐ en for bonding [32].

#### *3.6.3. Glass fibre reinforced polymers*

Engine intake manifolds are made from glass fibre reinforced PA 66.

#### *3.6.4. Design considerations*

FRP is used in designs that require a measure of strength or modulus of elasticity those nonreinforced plastics and other material choices are either ill suited for mechanically or eco‐ nomically. This means that the primary design consideration for using FRP is to ensure that the material is used economically and in a manner that takes advantage of its structural en‐ hancements specifically. This is however not always the case, the orientation of fibres also creates a material weakness perpendicular to the fibres. Thus the use of fibre reinforcement and their orientation affects the strength, rigidity, and elasticity of a final form and hence the operation of the final product itself. Orienting the direction of fibres either, unidirectional, 2 dimensionally, or 3-dimensionally during production affects the degree of strength, flexibili‐ ty, and elasticity of the final product. Fibres oriented in the direction of forces display greater resistance to distortion from these forces and vice versa, thus areas of a product that must withstand forces will be reinforced with fibres in the same direction, and areas that re‐ quire flexibility, such as natural hinges, will use fibres in a perpendicular direction to forces. Using more dimensions avoids this either or scenario and creates objects that seek to avoid any specific weak points due to the unidirectional orientation of fibres. The properties of strength, flexibility and elasticity can also be magnified or diminished through the geomet‐ ric shape and design of the final product. These include such design consideration such as ensuring proper wall thickness and creating multifunctional geometric shapes that can be molding as single pieces, creating shapes that have more material and structural integrity by reducing joints, connections, and hardware [30].

#### *3.6.5. Disposal and recycling concerns*

**•** 25% reduction in weight

30 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

costs of flying the airplane.

*3.6.2. Structural applications of FRP*

en for bonding [32].

safety.

*3.6.4. Design considerations*

*3.6.3. Glass fibre reinforced polymers*

**•** Up to a 60% reduction in weight

**•** Improved surface quality and aerodynamics

**•** Advantages over stamped aluminum are:

Engine intake manifolds are made from glass fibre reinforced PA 66.

**•** Advantages this has over cast aluminum manifolds are:

simplifying the production and operation of the design.

**•** 95% reduction in components by combining parts and forms into simpler molded parts.

**•** Overall reduction in production and operational costs, economy of parts results in lower production costs and the weight savings create fuel savings that lower the operational

FRP can be applied to strengthen the beams, columns and slabs in buildings. It is possible to increase strength of these structural members even after these have been severely damaged due to loading conditions. For strengthening beams, two techniques are adopted. First one is to paste FRP plates to the bottom (generally the tension face) of a beam. This increases the strength of beam, deflection capacity of beam and stiffness (load required to make unit de‐ flection). Alternatively, FRP strips can be pasted in 'U' shape around the sides and bottom of a beam, resulting in higher shear resistance. Columns in building can be wrapped with FRP for achieving higher strength. This is called wrapping of columns. The technique works by restraining the lateral expansion of the column. Slabs may be strengthened by pasting FRP strips at their bottom (tension face). This will result in better performance, since the tensile resistance of slabs is supplemented by the tensile strength of FRP. In the case of beams and slabs, the effectiveness of FRP strengthening depends on the performance of the resin chos‐

**•** Reduction in components by combining parts and forms into simpler molded shapes. Au‐ tomotive gas and clutch pedals made from glass fibre reinforced PA 66 (DWP 12-13)

**•** Pedals can be molded as single units combining both pedals and mechanical linkages

**•** Fibres can be oriented to reinforce against specific stresses, increasing the durability and

FRP is used in designs that require a measure of strength or modulus of elasticity those nonreinforced plastics and other material choices are either ill suited for mechanically or eco‐

As a subset of plastic FR plastics are liable to a number of the issues and concerns in plastic waste disposal and recycling. Plastics pose a particular challenge in recycling processes be‐ cause they are derived from polymers and monomers that often cannot be separated and re‐ turned to their virgin states, for this reason not all plastics can be recycled for re-use, in fact some estimates claim only 20% to 30% of plastics can be material recycled at all. Fibre rein‐ forced plastics and their matrices share these disposal and environmental concerns. In addi‐ tion to these concerns, the fact that the fibres themselves are difficult to remove from the matrix and preserve for re-use means FRP amplify these challenges. FRP are inherently diffi‐ cult to separate into base a material that is into fibre and matrix, and the Fibre-reinforced plastic matrix into separate usable plastic, polymers, and monomers. These are all concerns for environmentally informed design today, but plastics often offer savings in energy and economic savings in comparison to other materials, also with the advent of new more envi‐ ronmentally friendly matrices such as bioplastics and UV-degradable plastics, FRP will simi‐ larly gain environmental sensitivity [29].

## **4. Mechanical properties measurements**

#### **4.1. Strength**

Strength is a mechanical property that you should be able to relate to, but you might not know exactly what we mean by the word "strong" when are talking about polymers. First, there is more than one kind of strength. There is tensile strength. A polymer has tensile strength if it is strong when one pulls on it. Tensile strength is important for a material that is going to be stretched or under tension. Fibers need good tensile strength.

Then there is compressional strength. A polymer sample has compressional strength if it is strong when one tries to compress it. Concrete is an example of a material with good com‐ pressional strength. Anything that has to support weight from underneath has to have good compressional strength [32]. There is also flexural strength. A polymer sample has flexural strength if it is strong when one tries to bend it.

There are other kinds of strength we could talk about. A sample torsional strength if it is strong when one tries to twist it. Then there is impact strength. A sample has impact strength if it is strong when one hits it sharply and suddenly, as with a hammer.

To measure the tensile strength of a polymer sample, we take the sample and we try to stretch. We usually stretch it with a machine for these studies. This machine simply has clamps on each end of the sample, then, when you turn it on it stretches the sample. While it is stretching the sample, it measures the amount of force (F) that it is exerting. When we know the force being exerted on the sample, we then divide that number by the cross-sec‐ tional area (A) of our sample. The answer is the stress that our sample is experiencing. Then, using our machine, we continue to increase the amount of force, and stress naturally, on the sample until it breaks. The stress needed to break the sample is the tensile strength of the material. Likewise, one can imagine similar tests for compressional or flexural strength. In all cases, the strength is the stress needed to break the sample. Since tensile stress is the force placed on the sample divided by the cross-sectional area of the sample, tensile stress, and tensile strength as well, are both measured in units of force divided by units of area, usually N/cm2. Stress and strength can also be measured in megapascals (MPa) or gigapascals (GPa). It is easy to convert between the different units, because 1 MPa = 100 N/cm2 , 1 GPa = 100,000 N/cm2 , and of course 1 GPa = 1,000 MPa. Other times, stress and strength are meas‐ ured in the old English units of pounds per square inch, or psi. If you ever have to convert psi to N/cm2 , the conversion factor is 1 N/cm2 = 1.45 psi.

#### **4.2. Elongation**

But there is more to understanding a polymer's mechanical properties than merely knowing how strong it is. All strength tells us is how much stress is needed to break something. It doesn't tell us anything about what happens to our sample while we're trying to break it. That's where it pays to study the elongation behavior of a polymer sample. Elongation is a type of deformation. Deformation is simply a change in shape that anything undergoes un‐ der stress. When we're talking about tensile stress, the sample deforms by stretching, be‐ coming longer. We call this elongation, of course. Usually we talk about percent elongation, which is just the length the polymer sample is after it is stretched (L), divided by the original length of the sample (L0), and then multiplied by 100.

There are a number of things we measure related to elongation. Which is most important depends on the type of material one is studying. Two important things we measure are ulti‐ mate elongation and elastic elongation. Ultimate elongation is important for any kind of ma‐ terial. It is nothing more than the amount you can stretch the sample before it breaks. Elastic elongation is the percent elongation you can reach without permanently deforming your sample. That is, how much can you stretch it, and still have the sample snap back to its orig‐ inal length once you release the stress on it. This is important if your material is an elasto‐ mer. Elastomers have to be able to stretch a long distance and still bounce back. Most of them can stretch from 500 to 1000 % elongation and return to their original lengths without any trouble [32].

## **4.3. Modulus**

strength if it is strong when one pulls on it. Tensile strength is important for a material that

Then there is compressional strength. A polymer sample has compressional strength if it is strong when one tries to compress it. Concrete is an example of a material with good com‐ pressional strength. Anything that has to support weight from underneath has to have good compressional strength [32]. There is also flexural strength. A polymer sample has flexural

There are other kinds of strength we could talk about. A sample torsional strength if it is strong when one tries to twist it. Then there is impact strength. A sample has impact

To measure the tensile strength of a polymer sample, we take the sample and we try to stretch. We usually stretch it with a machine for these studies. This machine simply has clamps on each end of the sample, then, when you turn it on it stretches the sample. While it is stretching the sample, it measures the amount of force (F) that it is exerting. When we know the force being exerted on the sample, we then divide that number by the cross-sec‐ tional area (A) of our sample. The answer is the stress that our sample is experiencing. Then, using our machine, we continue to increase the amount of force, and stress naturally, on the sample until it breaks. The stress needed to break the sample is the tensile strength of the material. Likewise, one can imagine similar tests for compressional or flexural strength. In all cases, the strength is the stress needed to break the sample. Since tensile stress is the force placed on the sample divided by the cross-sectional area of the sample, tensile stress, and tensile strength as well, are both measured in units of force divided by units of area, usually N/cm2. Stress and strength can also be measured in megapascals (MPa) or gigapascals

strength if it is strong when one hits it sharply and suddenly, as with a hammer.

(GPa). It is easy to convert between the different units, because 1 MPa = 100 N/cm2

, the conversion factor is 1 N/cm2

length of the sample (L0), and then multiplied by 100.

ured in the old English units of pounds per square inch, or psi. If you ever have to convert

But there is more to understanding a polymer's mechanical properties than merely knowing how strong it is. All strength tells us is how much stress is needed to break something. It doesn't tell us anything about what happens to our sample while we're trying to break it. That's where it pays to study the elongation behavior of a polymer sample. Elongation is a type of deformation. Deformation is simply a change in shape that anything undergoes un‐ der stress. When we're talking about tensile stress, the sample deforms by stretching, be‐ coming longer. We call this elongation, of course. Usually we talk about percent elongation, which is just the length the polymer sample is after it is stretched (L), divided by the original

There are a number of things we measure related to elongation. Which is most important depends on the type of material one is studying. Two important things we measure are ulti‐ mate elongation and elastic elongation. Ultimate elongation is important for any kind of ma‐

, and of course 1 GPa = 1,000 MPa. Other times, stress and strength are meas‐

= 1.45 psi.

, 1 GPa =

is going to be stretched or under tension. Fibers need good tensile strength.

strength if it is strong when one tries to bend it.

32 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

100,000 N/cm2

psi to N/cm2

**4.2. Elongation**

In the elastomers are need show the high elastic elongation. But for some other types of materials, like plastics, it usually they not stretch or deform so easily. If we want to know how well a material resists deformation, we measure something called modulus. To measure tensile modulus, we do the same thing as we did to measure strength and ultimate elongation. This time we measure the stress we're exerting on the material, just like we did when we were measuring tensile strength. First, is slowly increasing the amount of stress, and then we measure the elongation the sample undergoes at each stress level. We keep doing this until the sample breaks. This plot is called a stressstrain curve. (Strain is any kind of deformation, including elongation. Elongation is the word we use if we're talking specifically about tensile strain.) The height of the curve when the sample breaks is the tensile strength, of course, and the tensile modulus is the slope of this plot. If the slope is steep, the sample has a high tensile modulus, which means it resists deformation. If the slope is gentle, then the sample has a low tensile modulus, which means it is easily deformed. There are times when the stress-strain curve is not nice and straight, like we saw above. The slope isn't constant as stress in‐ creases. The slope, that is the modulus, is changing with stress. In a case like this we usually, the initial slope change as the modulus change [32].

In general, fibers have the highest tensile moduli, and elastomers have the lowest, and plas‐ tics have tensile moduli somewhere in between fibers and elastomers.

Modulus is measured by calculating stress and dividing by elongation, and would be meas‐ ured in units of stress divided by units of elongation. But since elongation is dimensionless, it has no units by which we can divide. So modulus is expressed in the same units as strength, such as N/cm2 .

Intrinsic deformation is defined as the materials' true stress-strain response during ho‐ mogeneous deformation. Since generally strain localization phenomena occur (like neck‐ ing, shear banding, crazing and cracking), the measurement of the intrinsic materials' response requires a special experimental set-up, such as a video-controlled tensile or a uniaxial compression test. Although details of the intrinsic response differ per material, a general representation of the intrinsic deformation of polymers can be recognized [33], see Figure 1.

**Figure 1.** Schematic representation of the intrinsic deformation behavior of a polymer material [33].

#### **4.4. Toughness**

That plot of stress versus strain can give us another very valuable piece of information. If one measures the area underneath the stress-strain curve (figure 2), colored red in the graph below, the number you get is something we call toughness.

**Figure 2.** Plot of stress in function of strain.

Toughness is really a measure of the energy a sample can absorb before it breaks. Think about it, if the height of the triangle in the plot is strength, and the base of the triangle is strain, then the area is proportional to strength strain. Since strength is proportional to the force needed to break the sample, and strain is measured in units of distance (the distance the sample is stretched), then strength strain is proportional is force times distance, and as we remember from physics, force times distance is energy.

From a physics point of view the strength, is that strength tells how much force is needed to break a sample, and toughness tells how much energy is needed to break a sample. But that does not really tell you what the practical differences are. What is important knows that just because a material is strong, it isn't necessarily going to be tough as well [34-35].

**Figure 3.** Plot of stress in function of strain, strong and tough concepts.

**Figure 1.** Schematic representation of the intrinsic deformation behavior of a polymer material [33].

below, the number you get is something we call toughness.

34 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

we remember from physics, force times distance is energy.

That plot of stress versus strain can give us another very valuable piece of information. If one measures the area underneath the stress-strain curve (figure 2), colored red in the graph

Toughness is really a measure of the energy a sample can absorb before it breaks. Think about it, if the height of the triangle in the plot is strength, and the base of the triangle is strain, then the area is proportional to strength strain. Since strength is proportional to the force needed to break the sample, and strain is measured in units of distance (the distance the sample is stretched), then strength strain is proportional is force times distance, and as

**4.4. Toughness**

**Figure 2.** Plot of stress in function of strain.

The gray plot is the stress-strain curve for a sample that is strong, but not tough (figure 3). As you can see, it takes a lot of force to break this sample. Likewise, this sample ca not stretch very much before it breaks. A material like this which is strong, but can not deform very much before it breaks is called brittle [36].

The gray plot is a stress-strain curve for a sample that is both strong and tough. This materi‐ al is not as strong as the sample in the gray plot, but the area underneath its curve is a lot larger than the area under the gray sample's curve. So it can absorb a lot more energy than the gray-black sample plot.

The gray-black sample elongates a lot more before breaking than the gray sample does. You see, deformation allows a sample to dissipate energy. If a sample can't deform, the energy won't be dissipated, and will cause the sample to break [37].

In real life, we usually want materials to be tough and strong. Ideally, it would be nice to have a material that would not bend or break, but this is the real world. The grayblack sample has a much higher modulus than the red sample. While it is good for ma‐ terials in a lot of applications to have high moduli and resist deformation, in the real world it is a lot better for a material to bend than to break, and if bending, stretching or deforming in some other way prevents the material from breaking, all the better. So when we design new polymers, or new composites, we often sacrifice a little bit of strength in order to make the material tougher.

## **4.5. Mechanical properties of real polymers**

The rigid plastics such as polystyrene, poly(methyl methacrylate or polycarbonate can with‐ stand a good deal of stress, but they won't withstand much elongation before breaking. There is not much area under the stress-strain curve at all. So we say that materials like this are strong, but not very tough. Also, the slope of the plot is very steep, which means that it takes a lot of force to deform a rigid plastic. So it is easy to see that rigid plastics have high moduli. In short, rigid plastics tend to be strong, at resist deformation, but they tend not to be very tough, that is, they are brittle.

Flexible plastics like polyethylene and polypropylene are different from rigid plastics in that they don not resist deformation as well, but they tend not to break. The ability to deform is what keeps them from breaking. Initial modulus is high, that is it will resist deformation for awhile, but if enough stress is put on a flexible plastic, it will eventually deform. If you try to stretch it a plastic bag, it will be very hard at first, but once you have stretched it far enough it will give way and stretch easily. The bottom line is that flexible plastics may not be as strong as rigid ones, but they are a lot tougher.

It is possible to alter the stress-strain behavior of a plastic with additives called plasticizers. A plasticizer is a small molecule that makes plastics more flexible. For example, without plasticizers, poly(vinyl chloride), or PVC for short, is a rigid plastic. Rigid unplasticized PVC is used for water pipes. But with plasticizers, PVC can be made flexible enough to use to make things like hoses.

Fibers like KevlarTM, carbon fiber and nylon tend to have stress-strain curves like the aquacolored plot in the graph above. Like the rigid plastics, they are more strong than tough, and do not deform very much under tensile stress. But when strength is what you need, fibers have plenty of it. They are much stronger than plastics, even the rigid ones, and some poly‐ meric fibers, like KevlarTM, carbon fiber and ultra-high molecular weight polyethylene have better tensile strength than steel.

Elastomers like polyisoprene, polybutadiene and polyisobutylene have completely different mechanical behavior from the other types of materials. Take a look at the pink plot in the graph above. Elastomers have very low moduli. You can see this from the very gentle slope of the pink plot, but you probably knew this already. You already knew that it is easy to stretch or bend a piece of rubber [34]. If elastomers did not have low moduli, they would not be very good elastomers.

But it takes more than just low modulus to make a polymer an elastomer. Being easily stretched is not much use unless the material can bounce back to its original size and shape once the stress is released. Rubber bands would be useless if they just stretched and did not bounce back. Of course, elastomers do bounce back, and that is what makes them so amaz‐ ing. They have not just high elongation, but high reversible elongation.

## **4.6. Tensile properties**

**4.5. Mechanical properties**

36 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

be very tough, that is, they are brittle.

strong as rigid ones, but they are a lot tougher.

to make things like hoses.

better tensile strength than steel.

be very good elastomers.

The rigid plastics such as polystyrene, poly(methyl methacrylate or polycarbonate can with‐ stand a good deal of stress, but they won't withstand much elongation before breaking. There is not much area under the stress-strain curve at all. So we say that materials like this are strong, but not very tough. Also, the slope of the plot is very steep, which means that it takes a lot of force to deform a rigid plastic. So it is easy to see that rigid plastics have high moduli. In short, rigid plastics tend to be strong, at resist deformation, but they tend not to

Flexible plastics like polyethylene and polypropylene are different from rigid plastics in that they don not resist deformation as well, but they tend not to break. The ability to deform is what keeps them from breaking. Initial modulus is high, that is it will resist deformation for awhile, but if enough stress is put on a flexible plastic, it will eventually deform. If you try to stretch it a plastic bag, it will be very hard at first, but once you have stretched it far enough it will give way and stretch easily. The bottom line is that flexible plastics may not be as

It is possible to alter the stress-strain behavior of a plastic with additives called plasticizers. A plasticizer is a small molecule that makes plastics more flexible. For example, without plasticizers, poly(vinyl chloride), or PVC for short, is a rigid plastic. Rigid unplasticized PVC is used for water pipes. But with plasticizers, PVC can be made flexible enough to use

Fibers like KevlarTM, carbon fiber and nylon tend to have stress-strain curves like the aquacolored plot in the graph above. Like the rigid plastics, they are more strong than tough, and do not deform very much under tensile stress. But when strength is what you need, fibers have plenty of it. They are much stronger than plastics, even the rigid ones, and some poly‐ meric fibers, like KevlarTM, carbon fiber and ultra-high molecular weight polyethylene have

Elastomers like polyisoprene, polybutadiene and polyisobutylene have completely different mechanical behavior from the other types of materials. Take a look at the pink plot in the graph above. Elastomers have very low moduli. You can see this from the very gentle slope of the pink plot, but you probably knew this already. You already knew that it is easy to stretch or bend a piece of rubber [34]. If elastomers did not have low moduli, they would not

But it takes more than just low modulus to make a polymer an elastomer. Being easily stretched is not much use unless the material can bounce back to its original size and shape once the stress is released. Rubber bands would be useless if they just stretched and did not bounce back. Of course, elastomers do bounce back, and that is what makes them so amaz‐

ing. They have not just high elongation, but high reversible elongation.

**of real polymers**

The discussion of which types of polymers have which mechanical properties has focused mostly on tensile properties. When we look at other properties, like compressional proper‐ ties or flexural properties things can be completely different. For example, fibers have very high tensile strength and good flexural strength as well, but they usually have terrible com‐ pressional strength. They also only have good tensile strength in the direction of the fibers.

Some polymers are tough, while others are strong, and how one often has to make trade-offs when designing new materials; the design may have to sacrifice strength for toughness, but sometimes we can combine two polymers with different properties to get a new material with some of the properties of both. There are three main ways of doing this, and they are copolymerization, blending, and making composite materials.

The copolymer that combines the properties of two materials is spandex. It is a copolymer containing blocks of elastomeric polyoxyethylene and blocks of a rigid fiber-forming polyur‐ ethane. The result is a fiber that stretches. Spandex is used to make stretchy clothing like bi‐ cycle pants.

High-impact polystyrene, or HIPS for short, is an immiscible blend that combines the prop‐ erties of two polymers, styrene and polybutadiene. Polystyrene is a rigid plastic. When mixed with polybutadiene, an elastomer, it forms a phase-separated mixture which has the strength of polystyrene along with toughness supplied by the polybutadiene. For this rea‐ son, HIPS is far less brittle than regular polystyrene [38].

In the case of a composite material, we are usually using a fiber to reinforce a thermoset. Thermosets are crosslinked materials whose stress-strain behavior is often similar to plas‐ tics. The fiber increases the tensile strength of the composite, while the thermoset gives it compressional strength and toughness.

## **5. Conclusions**

This brief review of FRP has summarized the very broad range of unusual functionalities that these products bring (Polymers, Aramids, Composites, Carbon FRP, and Glass-FRP). While the chemistry plays an important role in defining the scope of applications for which these materials are suited, it is equally important that the final parts are designed to maxi‐ mize the value of the inherent properties of these materials. Clearly exemplify the constant trade-off between functionality and processability that is an ongoing challenge with these advanced materials. The functionality that allows these materials to perform under extreme conditions has to be balanced against processability that allows them to be economically shaped into useful forms. The ability of a polymer material to deform is determined by the mobility of its molecules, characterized by specific molecular motions and relaxation mecha‐ nisms, which are accelerated by temperature and stress. Since these relaxation mechanisms are material specific and depend on the molecular structure, they are used here to establish the desired link with the intrinsic deformation behavior.

## **Acknowledgement**

The author would like to offer a special thanks to Universidad Nacional de San Luis, to In‐ stituto de Física Aplicada, and to Consejo Nacional de Investigaciones Científicas y Técnicas for being generously support used in this research works.

## **Author details**

Martin Alberto Masuelli

Address all correspondence to<EMAIL_ADDRESS>
Instituto de Física Aplicada, CONICET. Cátedra de Química Física II, Área de Química Físi‐ ca. Facultad de Química, Bioquímica y Farmacia. Universidad Nacional de San Luis. Chaca‐ buco 917 (CP: 5700), San Luis, Argentina

## **References**

[10] Giancaspro James, Papakonstantinou Christos, Balaguru P. Mechanical behavior of fire-resistant biocomposite. Composites: Part B 40 (2009) 206-211.

**Acknowledgement**

**Author details**

**References**

ier 2002.

ty. Springer 2012.

International (1993) 48-53.

Martin Alberto Masuelli

The author would like to offer a special thanks to Universidad Nacional de San Luis, to In‐ stituto de Física Aplicada, and to Consejo Nacional de Investigaciones Científicas y Técnicas

Instituto de Física Aplicada, CONICET. Cátedra de Química Física II, Área de Química Físi‐ ca. Facultad de Química, Bioquímica y Farmacia. Universidad Nacional de San Luis. Chaca‐

[2] Hinton M.J., Soden P.D., Kaddour A.S. Failure Criteria in Fibre-Reinforced-Polymer

[3] Tong L., Mouritz A.P., Bannister M. 3D Fibre Reinforced Polymer Composites. Elsev‐

[4] Ravi Jain, Luke lee. Fiber Reinforced Polymer (FRP) Composites for Infrastructure Applications. Focusing on Innovation, Technology Implementation and Sustainabili‐

[5] Kimg Hwee TAN. Fibre Reinforced Polymer. Reinforcement for Concrete Structures. Proceedings of the Sixth International Symposium on FRP Concrete Structures, vol‐

[6] Erki M.A., and Rizkalla S.H. FRP Reinforcement for Concrete Structures. Concrete

[7] Han E.H. Meijer, Govaert Leon E. Mechanical performance of polymer systems: The relation between structure and properties. Prog. Polym. Sci. 30 (2005) 915-938.

[9] Entsiklopediia polimerov, vols. 1–3. Moscow, 1972-77. The Great Soviet Encyclope‐

for being generously support used in this research works.

38 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

Address all correspondence to<EMAIL_ADDRESS>
[1] http://www.matter.org.uk/matscicdrom/manual/co.html.

ume 1-2 (FRPRCS-6). World Scientific 2003.

[8] McGraw-Hill Science & Technology Encyclopedia

dia, 3rd Edition (1970-1979). 2010 The Gale Group, Inc.

Composites: The World-Wide Failure Exercise. Elsevier 2004.

buco 917 (CP: 5700), San Luis, Argentina

## **Chapter 2**

## **Natural Fibre Bio-Composites Incorporating Poly(Lactic Acid)**

Eustathios Petinakis, Long Yu, George Simon and Katherine Dean

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/52253

## **1. Introduction**

[28] José M. García, Félix C. García, Felipe Serna, José L. de la Peña. High-performance

[29] Smallman, R.E., and Bishop R.J.. Modern Physical Metallurgy and Materials Engi‐

[30] Erhard, Gunter. Designing with Plastics. Trans. Martin Thompson. Munich: Hanser

[31] Rosato Donald V., Rosato Dominick V., Murphy John. Reinforced Plastics Handbook.

[32] Composite moulding (http:/ / web. archive. org/ web/<PHONE_NUMBER>0801/ http:/ / www.

[33] Klompen, Edwin T.J. Mechanical properties of solid polymers. Constitutive model‐ ling of long and short term behaviour. Eindhoven: Technische Universiteit Eind‐

[35] Odian, George. Principles of Polymerization. 3rd ed., J. Wiley, New York, 1991.

Engineering. Enciclopedia of Life Support Systems – UNESCO, 2011.

[36] Jang, B.Z.; Advanced Polymer Composites: Principles and Applications, ASM Inter‐

[37] Bhowmick, Anil. Mechanical Properties of Polymers. Volume 1, Material Sciences

[38] Hashim M.H.M., Sam A.R.M., Hussin M.W. The Future of External Application of Fi‐ bre Reinforced Polymer in Civil Infrastructure for Tropical Climates Region. Interna‐ tional Journal of Mechanical and Materials Engineering (IJMME), 6, 2 (2011) 147-159.

aromatic polyamides. Progress in Polymer Science 35 (2010) 623-686.

neering. 6th ed. Oxford. Butterworth-Heinemann, 1999.

quatrocomposites. com/ comp101proc. htm).

40 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

[34] FRP Library (http://www.compositepedia.com)

national, Materials Park, OH, 1994.

Publishers, 2006.

hoven, 2005.

Elsevier; 2004; page 586

In recent years increasing awareness in relation to the worlds petrochemical resources no longer being finite, the rising cost of oil and concerns surrounding climate change and the necessity for reducing our carbon footprint, are resulting in a renewed demand and expe‐ diation of development of polymeric materials that are produced from sustainable and eco‐ logically sound raw material feedstocks that are not petrochemically derived and are generally more abundant. Poly(lactic acid) (PLA), being a compostable synthetic polymer produced using monomer feedstock derived from corn starch, satisfies many of the environ‐ mental impact criteria required for an acceptable replacement for oil-derived plastics [1]. PLA exhibits mechanical properties that make it useful for a wide range of applications, but mainly in applications that do not require high performance including plastic bags, packag‐ ing for food, disposable cutlery and cups, slow release membranes for drug delivery and liq‐ uid barrier layers in disposable nappies [2]. However, the wider uptake of PLA is restricted by performance deficiencies, such as its relatively poor impact properties which arise from its inherent brittleness, but also the limited supply and higher cost of PLA compared with commodity polymers such as polyethylene and polypropylene [3].

Polymer composite materials often possess mechanical and physical properties that make them better suited for a wide range of applications than the individual composite compo‐ nents. The use of natural fibers to produce polymer composites having improved mechani‐ cal and impact performance is well-documented, and is of particular interest for enhancing the properties of biodegradable polymeric materials such as PLA [4-6]. The benefits of using natural fibers compared with other potential reinforcing agents, such as glass fibers, talc, or carbon fibers, for improving the performance of biodegradable polymers include the reten‐

© 2013 Petinakis et al.; licensee InTech. This is an open access article 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. © 2013 Petinakis et al.; licensee InTech. This is a paper 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.

tion of the biodegradability of the composite, but generally also exhibit lower density, supe‐ rior performance, and lower cost due to the abundance of many natural fibres. An example of such a product is wood-flour, a bio-waste product produced from the preparation of tim‐ ber for the building and other related industries.

PLA/natural fibre composites containing less than 30%w/w fiber have been shown to have increased tensile modulus and reduced tensile strength compared with PLA, and this has been attributed to factors that include the weak interfacial interaction between the hydro‐ phobic PLA matrix and the hydrophilic cellulose fibers, and lack of fiber dispersion due to a high degree of fiber agglomeration. Various methods of modifying the surface of the cellulo‐ sic fibers have been explored in an effort to improve the interaction that occurs at the inter‐ face between the PLA matrix and the fibers, including esterification [7], acetylation [8], and cyanoethylation [9]. It is apparent that the stronger the molecular interactions that occur at the interface between cellulose fibers and polymer matrix, the resulting interfacial adhesion is stronger and the optimal stress transfer efficiency. The use of coupling agents or compati‐ bilisers have proven to be a much more efficient means of improving interfacial interactions between polymer matrices and cellulose fibers. The strongest adhesion can be achieved when covalent bonds are formed at the interface between cellulose fibers and coupling agent as well as molecular entanglement between coupling agent and the polymer itself [10]. Cou‐ pling agents also have the benefits of improving fibre dispersion, since they can also induce better flow of the molten polymer during processing, improve melt elasticity and melt strength in the resulting polymer composites [11].

The nature of the interface/interphase in polymer composites incorporating natural fibers is still not well understood, since many of the chemical reagents used in surface modification of natural fibres and/or coupling agents used do not form covalent bonds, but in most cases are rarely created [12]. Therefore, the aims of this book chapter research will attempt to ad‐ dress this issue and provide a fundamental understanding of the surface and adhesion properties of polymer composite systems incorporating natural fibres. The authors of this book chapter will demonstrate such understanding thru their research into PLA based bio‐ composites incorporating wood-flour as the natural fiber/filler. It has been demonstrated in the preliminary research that the best path forward for developing polymer composites with enhanced physical properties is by modifying the surface of wood-flour particles to induce chemical bonding at the interface and to enhance compatibilisation with the PLA matrix.

## **2. Poly(lactic acid) bio-composites based on natural fibres**

## **2.1. Structure and properties of natural fibres**

Natural fibres can be classified into five major types: bast, leaf, seed, fruit and wood, de‐ pending upon the source. In order to develop polymer composites from natural resources it is important to understand the microstructure and chemical composition of natural fibres. Natural fibres comprise of three principal components; cellulose, lignin and hemicellulose. These three hydroxyl- containing natural polymers are distributed throughout the cell wall. Cellulose and hemicellulose are polysaccharides. Cellulose is a highly crystalline polymer with a regular structure, which comprises of thousands of anhydroglucose units with a DP (degree of polymerisation) around 10,000 [13]. Cellulose is the major component which is re‐ sponsible for the inherent strength and stability of the natural fibre. Hemicellulose is a short‐ er branched polymer composed of various five- and six- carbon ring sugars. The molecular weight is much lower than cellulose but still hemicellulose still contributes to the structure of natural fibres. Lignin is an amorphous, cross-linked polymer network, which consists of an irregular array of variously bonded hydroxy-and methoxy- substituted phenyl propane units. The chemical structure of lignin depends on the source of the wood. Lignin is not as polar as cellulose and the major function of lignin is to function as a chemical adhesive be‐ tween cellulose fibers.

**Figure 1.** Classification of natural fibres (courtesy of Mohanty et al [14])

tion of the biodegradability of the composite, but generally also exhibit lower density, supe‐ rior performance, and lower cost due to the abundance of many natural fibres. An example of such a product is wood-flour, a bio-waste product produced from the preparation of tim‐

PLA/natural fibre composites containing less than 30%w/w fiber have been shown to have increased tensile modulus and reduced tensile strength compared with PLA, and this has been attributed to factors that include the weak interfacial interaction between the hydro‐ phobic PLA matrix and the hydrophilic cellulose fibers, and lack of fiber dispersion due to a high degree of fiber agglomeration. Various methods of modifying the surface of the cellulo‐ sic fibers have been explored in an effort to improve the interaction that occurs at the inter‐ face between the PLA matrix and the fibers, including esterification [7], acetylation [8], and cyanoethylation [9]. It is apparent that the stronger the molecular interactions that occur at the interface between cellulose fibers and polymer matrix, the resulting interfacial adhesion is stronger and the optimal stress transfer efficiency. The use of coupling agents or compati‐ bilisers have proven to be a much more efficient means of improving interfacial interactions between polymer matrices and cellulose fibers. The strongest adhesion can be achieved when covalent bonds are formed at the interface between cellulose fibers and coupling agent as well as molecular entanglement between coupling agent and the polymer itself [10]. Cou‐ pling agents also have the benefits of improving fibre dispersion, since they can also induce better flow of the molten polymer during processing, improve melt elasticity and melt

The nature of the interface/interphase in polymer composites incorporating natural fibers is still not well understood, since many of the chemical reagents used in surface modification of natural fibres and/or coupling agents used do not form covalent bonds, but in most cases are rarely created [12]. Therefore, the aims of this book chapter research will attempt to ad‐ dress this issue and provide a fundamental understanding of the surface and adhesion properties of polymer composite systems incorporating natural fibres. The authors of this book chapter will demonstrate such understanding thru their research into PLA based bio‐ composites incorporating wood-flour as the natural fiber/filler. It has been demonstrated in the preliminary research that the best path forward for developing polymer composites with enhanced physical properties is by modifying the surface of wood-flour particles to induce chemical bonding at the interface and to enhance compatibilisation with the PLA matrix.

Natural fibres can be classified into five major types: bast, leaf, seed, fruit and wood, de‐ pending upon the source. In order to develop polymer composites from natural resources it is important to understand the microstructure and chemical composition of natural fibres. Natural fibres comprise of three principal components; cellulose, lignin and hemicellulose. These three hydroxyl- containing natural polymers are distributed throughout the cell wall.

**2. Poly(lactic acid) bio-composites based on natural fibres**

ber for the building and other related industries.

42 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

strength in the resulting polymer composites [11].

**2.1. Structure and properties of natural fibres**

Natural fibres also consist of varying amounts pectin, wax and other low molecular weight compounds or extractives. Extractives are described as non-structural components in wood, which are composed of extra cellular and low molecular weight compounds. There are three types of lipophilic extractive compounds: terpenes (and terpenoids), aliphatics (fatty acids and their esters) and phenolic compounds [15].  Terponoids include turpentine and resin acids. Phenolic compounds include tannins, flavnoids, lignans, stilbines and tropolones. Extrac‐ tives can diffuse to the surface of natural fibres during drying, which can influence the de‐ gree of adhesion. This is an important factor to consider during the processing of polymer composites incorporating lignocellulosic fillers, since extractives may influence the degree of interfacial adhesion between polymer matrix and lignocellulosic filler. Extractives can be typically removed by solvent extraction or steam distillation or even water treatment if com‐ pounds are water soluble. Steam distillation can be used to remove the volatile terpenes, whereas solvent extraction can remove resin acids, fatty alcohols, fatty acids and waxes.

#### **2.2. Structure and properties of PLA**

Polymers from renewable resources can be classified into three major groups: natural poly‐ mers such as starch and cellulose; synthetic polymers from natural monomers such as poly‐ lactic acid (PLA); polymers from microbial fermentation such as polyhydroxybutyrate (PHB). Polylactic acid is one of the most promising biodegradable polymers, which can be derived from natural feedstocks such as corn starch but can also be derived from rice, pota‐ toes, sugar beet and other agricultural waste. Intially, PLA synthesis involves conversion of the raw material feedstock into dextrose, which then undergoes conversion into lactic acid or lactide via a fermentation process in the presence of a catalyst. The lactide undergoes fur‐ ther processing in order to purify the monomer and this is followed by conversion of the purified monomer in into a polymeric form of PLA through polymerisation in the presence of a suitable catalyst [16]. Polylactic Acid (PLA) can be processed by conventional methods such as injection moulding, blow moulding, extrusion and film forming operations, since PLA has a Tg of 55-65o C and a melting temperature between 150-175o C. The mechanical properties of PLA are similar or in most cases are superior too petrochemical polymers, such as polypropylene. Therefore, PLA has attracted great interest as a commodity polymer which is capable of replacing petrochemical polymers like polypropylene and polyethylene, particularly in the area of single use packaging applications. However, PLA exhibits low toughness due to its brittle nature, but also the molecular weight in comparison to conven‐ tional polymers, is much lower. In order to overcome the brittle nature of PLA it is useful to incorporate natural fillers into the polymer matrix. It has already been stated that incorpora‐ tion of natural fillers into polymer matrices can optimise mechanical properties but from an economical viewpoint, natural fillers can make the composites more cost competitive due to their high abundance and lower cost.

**Figure 2.** Structure of Poly(lactic acid)

#### **2.3. Natural fibres used in PLA based bio-composites**

The major factors that can influence the development of polymer composites using natural fibers are listed as follows [17]:

**2.2. Structure and properties of PLA**

44 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

PLA has a Tg of 55-65o

HO

their high abundance and lower cost.

CH3

**Figure 2.** Structure of Poly(lactic acid)

fibers are listed as follows [17]:

O

O CH3

The major factors that can influence the development of polymer composites using natural

**2.3. Natural fibres used in PLA based bio-composites**

Polymers from renewable resources can be classified into three major groups: natural poly‐ mers such as starch and cellulose; synthetic polymers from natural monomers such as poly‐ lactic acid (PLA); polymers from microbial fermentation such as polyhydroxybutyrate (PHB). Polylactic acid is one of the most promising biodegradable polymers, which can be derived from natural feedstocks such as corn starch but can also be derived from rice, pota‐ toes, sugar beet and other agricultural waste. Intially, PLA synthesis involves conversion of the raw material feedstock into dextrose, which then undergoes conversion into lactic acid or lactide via a fermentation process in the presence of a catalyst. The lactide undergoes fur‐ ther processing in order to purify the monomer and this is followed by conversion of the purified monomer in into a polymeric form of PLA through polymerisation in the presence of a suitable catalyst [16]. Polylactic Acid (PLA) can be processed by conventional methods such as injection moulding, blow moulding, extrusion and film forming operations, since

C and a melting temperature between 150-175o

O

n

O CH3

properties of PLA are similar or in most cases are superior too petrochemical polymers, such as polypropylene. Therefore, PLA has attracted great interest as a commodity polymer which is capable of replacing petrochemical polymers like polypropylene and polyethylene, particularly in the area of single use packaging applications. However, PLA exhibits low toughness due to its brittle nature, but also the molecular weight in comparison to conven‐ tional polymers, is much lower. In order to overcome the brittle nature of PLA it is useful to incorporate natural fillers into the polymer matrix. It has already been stated that incorpora‐ tion of natural fillers into polymer matrices can optimise mechanical properties but from an economical viewpoint, natural fillers can make the composites more cost competitive due to

C. The mechanical

OH

O

Oksman et. al. incorporated cellulose fibres as reinforcement in PLA [18]. Due to the brittle nature of PLA, triacetin was used as a plasticizer for the matrix as well as PLA/flax compo‐ sites in order to improve the impact properties. Plasticizers can be used during processing in order to lower the viscosity of the matrix polymer, which can then facilitate better fiber dis‐ persion within the matrix polymer. Fiber dispersion is a critical factor to be considered dur‐ ing the development of biodegradable natural fibre composites. Shibata et. al. evaluated the use of short abaca fibres in the development of biocomposites using biodegradable polyest‐ ers. In this study it was shown that strength and modulus increase with decreasing fibre di‐ ameter for both untreated and treated abaca fibre[19].

Wollerdorfer et. al. investigated the influence of plant fibres such as flax, jute, ramie, oil palm fibres and fibres made from regenerated cellulose on the mechanical properties of bio‐ degradable polymers. The so-called biocomposites produced by embedding natural fibres, e.g. flax, hemp, ramie, etc. into a biopolymeric matrix made of derivatives from cellulose, starch, lactic acid, etc., new fibre reinforced materials were developed [20]. Huda et. al. eval‐ uated the use of recycled cellulose in the development of "green" composites using PLA as the matrix and recycled cellulose from newsprint. The physico-mechanical properties of the composites as well as the morphological properties were investigated as a function of vary‐ ing amounts of recycled cellulose [21]. Bax et al [22] investigated the impact of cordenka and flax fibres on the impact and tensile properties. The study showed that PLA composites with cordenka fibres with a maximum fibre loading of 30% show promise as alternative biocom‐ posites for industrial applications due to optimisation in impact properties. However, both biocomposites showed evidence of poor interfacial adhesion between the PLA matrix and the cordenka and flax fibres, respectively.

Mathew et. al. conducted a study towards developing PLA based high performance nanocomposites using microcrystalline cellulose as reinforcement. The study was concerned with achieving the best possible outcome for dispersion of the MCC within PLA during process‐ ing. Comparisons were also made with using wood flour and wood pulp as an alternative reinforcement for PLA [23]. Tzerki et. al. investigated the usefulness of lignocellulosic waste flours derived from spruce, olive husks and paper flours as potential reinforcements for the preparation of cost-effective bio-composites using PLA as the matrix [24]. Petinakis et al studied the effect of wood-flour content on the mechanical properties and fracture behav‐ iour of PLA/wood-flour composites. The results indicated that enhancements in tensile modulus could be achieved, but the interfacial adhesion was poor [25]. Therefore, it can be seen that incorporation of lignocellulosic materials into biodegradable polymer matrices, such as PLA, has the affect of improving mechanical properties, such as tensile modulus. But the strength and toughness of these bio-composites are not necessarily improved. This can be attributed to several reasons, such as the hydrophilic nature of natural fillers, com‐ patibility with the hydrophobic polymer matrix can be problematical. In addition to the poor interaction between the phases, the hydrophilic nature of natural fibres leads to a ten‐ dency for fibres to mingle or form agglomerations, which can generally result in low impact properties, especially at high fibre loadings.

In order to overcome these shortcomings a variety of chemical and physical treatments can be utilised to improve fibre-matrix adhesion in biodegradable polymer composites as well as improve dispersion of natural fibres within biopolymer matrices. There are many articles in the public domain that have reported the use of coupling agents and compatibilisers for improving fibre-matrix interfacial adhesion in polymer composite systems incorporating a polyolefinic matrix, such as polypropylene and polyethylene.

## **3. Strategies for improving interfacial adhesion in PLA/natural fibre composites**

Surface modification of natural fillers can be classified into two major types; chemical and physical methods. Surface modification is a critical processing step in the development of biopolymer composites, since natural fillers tend to be highly hydrophilic in nature and in order to improve the compatibilsation with the hydrophobic polymer matrix this level of processing is required. The use of surface modification techniques can facilitate fibre disper‐ sion within polymer matrix as well as improve the fibre-matrix interaction [26]. Some of the techniques that have been previously reported in the literature for improving fibre-matrix adhesion include: treatment of fibres by bleaching, acetylation, esterification, grafting of monomers and the use of bi-functional molecules [27]. The use of coupling agents and com‐ patibilisers has also been widely reported in the development of conventional polymer com‐ posites. Coupling agents include silanes, isocyanates, zirconates, titanates and chitosan [28]. One of the most widely reported compatibilisers in the literature has been the use of func‐ tional polyolefins such as maleated polypropylene (MAPP) [29-34]. More recently, Xu et al synthesised a novel graft copolymer, polylactide-graft-glycidyl methacrylate (PLA-GMA), which was produced by grafting glycidyl methacrylate onto the PLA chain via free radi‐ cal polymerisation, which was then used to produce biocomposites using PLA and bam‐ boo flour [35]. All techniques have proven successful in improving the fibre-matrix interactions, which have resulted in polymer composites with greatly improved mechani‐ cal properties.

## **3.1. Chemical techniques**

### *3.1.1. Alkaline treatment*

modulus could be achieved, but the interfacial adhesion was poor [25]. Therefore, it can be seen that incorporation of lignocellulosic materials into biodegradable polymer matrices, such as PLA, has the affect of improving mechanical properties, such as tensile modulus. But the strength and toughness of these bio-composites are not necessarily improved. This can be attributed to several reasons, such as the hydrophilic nature of natural fillers, com‐ patibility with the hydrophobic polymer matrix can be problematical. In addition to the poor interaction between the phases, the hydrophilic nature of natural fibres leads to a ten‐ dency for fibres to mingle or form agglomerations, which can generally result in low impact

In order to overcome these shortcomings a variety of chemical and physical treatments can be utilised to improve fibre-matrix adhesion in biodegradable polymer composites as well as improve dispersion of natural fibres within biopolymer matrices. There are many articles in the public domain that have reported the use of coupling agents and compatibilisers for improving fibre-matrix interfacial adhesion in polymer composite systems incorporating a

**3. Strategies for improving interfacial adhesion in PLA/natural fibre**

Surface modification of natural fillers can be classified into two major types; chemical and physical methods. Surface modification is a critical processing step in the development of biopolymer composites, since natural fillers tend to be highly hydrophilic in nature and in order to improve the compatibilsation with the hydrophobic polymer matrix this level of processing is required. The use of surface modification techniques can facilitate fibre disper‐ sion within polymer matrix as well as improve the fibre-matrix interaction [26]. Some of the techniques that have been previously reported in the literature for improving fibre-matrix adhesion include: treatment of fibres by bleaching, acetylation, esterification, grafting of monomers and the use of bi-functional molecules [27]. The use of coupling agents and com‐ patibilisers has also been widely reported in the development of conventional polymer com‐ posites. Coupling agents include silanes, isocyanates, zirconates, titanates and chitosan [28]. One of the most widely reported compatibilisers in the literature has been the use of func‐ tional polyolefins such as maleated polypropylene (MAPP) [29-34]. More recently, Xu et al synthesised a novel graft copolymer, polylactide-graft-glycidyl methacrylate (PLA-GMA), which was produced by grafting glycidyl methacrylate onto the PLA chain via free radi‐ cal polymerisation, which was then used to produce biocomposites using PLA and bam‐ boo flour [35]. All techniques have proven successful in improving the fibre-matrix interactions, which have resulted in polymer composites with greatly improved mechani‐

properties, especially at high fibre loadings.

46 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

**composites**

cal properties.

polyolefinic matrix, such as polypropylene and polyethylene.

Alkaline treatment is one of the most widely used chemical treatments for natural fibres for use in natural fibre composites. The effect of alkaline treatment on natural fibres is it dis‐ rupts the incidence of hydrogen bonding in the network structure, giving rise to additional sites for mechanical interlocking, hence promoting surface roughness and increasing matrix/ fibre interpenetration at the interface. During alkaline treatment of lignocellulosic materials, the alkaline treatment removes a degree of the lignin, wax and oils which are present, from the external surface of the fibre cell wall, as well as chain scission of the polymer backbone resulting in short length crystallites. The treatment exposes the hydroxyl groups in the cellu‐ lose component to the alkoxide.

In alkaline treatment, wood fibres/flour is immersed in a solution of sodium hydroxide for a given period of time. Beg et. al. studied the effect of the pre-treatment of radiate pine fibre with NaOH and coupling with MAPP in wood fibre reinforced polypropylene composites. It was found that fibre pre-treatment with NaOH resulted in an improvement in the stiffness of the composites (at 60% fibre loading) as a function NaOH concentration, however at the same time, a decrease was observed in the strength of the composite [36]. The reason for a reduction in the tensile strength was attributed to a weakening of the cohesive strength of the fibre, as a result of alkali treatment. The use of alkali treatment in conjunction with MAPP was found to improve the fibre/matrix adhesion. However, it seems that only small concentrations of NaOH can be used to treat fibres, otherwise the cohesive strength can be compromised. Ichazo et. al also studied the addition of alkaline treated wood flour in poly‐ propylene/wood flour composites. It was shown that alkaline treatment only improved fibre dispersion within the polypropylene matrix but not the fibre-matrix adhesion. This was at‐ tributed to a greater concentration of hydroxyl groups present, which increased the hydro‐ philic nature of the composites. As a result, no significant improvement was observed in the mechanical properties of the composites and a reduction in the impact properties [37]. From previous studies it is shown that the optimal treatment conditions for alkalization must be investigated further in order to improve mechanical properties. Care must be taken in select‐ ing the appropriate concentration, treatment time and temperature, since at certain condi‐ tions the tensile properties are severely compromised. Islam et al studied the effect of alkali treatment on hemp fibres, which were utilised to produce PLA biocomposites incorporating hemp fibres. This study showed that crystallinity in PLA was increased due to the nuclea‐ tion of hemp fibres following alkaline treatment. The degree of crystallinity had a positive impact on the mechanical and impact performance of the resulting composites with alkaline treated hemp fibres as opposed to the composites without treated hemp fibres.

#### *3.1.2. Silane treatment*

Silane coupling agents have been used traditionally in the past in the development of con‐ ventional polymer composites reinforced with glass fibres. Silane is a class of silicon hydride with a chemical formula SiH4. Silane coupling agents have the potential to reduce the inci‐ dence of hydroxyl groups in the fibre-matrix interface. In the presence of moisture, hydro‐ lysable alkoxy groups result in the formation of silanols. Silanols react with hydroxyl groups of the fibre, forming a stable, covalently bonded structure with the cell wall. As a result, the hydrocarbon chains provided by the reaction of the silane produce a cross-linked network due to covalent bonding between fibre and polymer matrix. This results in a hydrophobic surface in the fibre, which in turn increases the compatibility with the polymer matrix. As mentioned earlier silane coupling agents have been effective for the treatment of glass fibres for the reinforcement of polypropylene. Silane coupling agents have also been found to be useful for the pre-treatment of natural fibres in the development of polymer composites. Wu et. al. demonstrated that wood fibre/polypropylene composites containing fibres pre-treated with a vinyl-tri methoxy silane significantly improved the tensile properties. It was discov‐ ered that the significant improvement in tensile properties was directly related to a strong interfacial bond caused by the acid/water condition used in the fibre pre-treatment [38].

In a study by Bengtsson et al. the use of silane technology in crosslinking polyethylenewood flour composites was investigated [39]. Composites of polyethylene with wood-flour were reacted in-situ with silanes using a twin screw extruder. The composites showed im‐ provements in toughness and creep properties and the likely explanation for this improve‐ ment was that part of the silane was grafted onto polyethylene and wood, which resulted in a cross-linked network structure in the polymer with chemical bonds occurring at the sur‐ face of wood. X-ray microanalysis showed that most of the silane was found within close proximity to the wood-flour. It is known that silanes can interact with cellulose through ei‐ ther free radical or condensation reaction but also through covalent bonding by the reaction of silanol groups and free hydroxyl groups at the surface of wood, however the exact mech‐ anism could not be ascertained. In a study by González et al, focused on the development of PLA based composites incorporating untreated and silane treated sisal and kraft cellulose fibres [40]. The tensile properties of the resulting composites did not present any major stat‐ istical difference between composites with untreated cellulose fibres and silane treated cellu‐ lose fibres, which suggested that silane treatment of the cellulose fibres did not contribute to further optimisation in the reinforcing affect of the cellulose fibres. The analysis of the high resolution C1s spectra (XPS) indicates that for C1 (C-C, C-H), the percentage of lignin in the intreated sisal fibres was higher, in comparison with kraft fibres. But after modification with silanes, the C1 signal decreases for sisal fibres, which shows that attempted grafting with the silane has resulted in removal of lignin and exposed further cellulose. The higher C1 signal reported for kraft fibres suggested some grafting with silane as a result of the contribution from the alkyl chain of the attached silanol, but no further characterisation was provided to support grafting of silanes to kraft fibres.

#### *3.1.3. Esterification of natural fibres*

This section reviews research into the modification of wood constituents with organic acid anhydrides. Anhydrides can be classified into two major groups: non-cyclic anhydrides (i.e. Acetic) and cyclic anhydrides (i.e. Maleic). Of the non-cyclic anhydrides, Acetylation with Acetic Anhydride is the most widely reported [41-43]. The reaction involves the conversion of a hydroxyl group into an ester group by the chemical affiliation of the carboxylic group of the anhydride with the free hydroxyl groups in cellulose. Reactions involving non-cyclic an‐ hydrides are quite cumbersome as there are several steps involved during the treatment. These reactions also require the use of strong bases or catalysts to facilitate the reaction. Al‐ though the use of non-cyclic anhydrides can generally lead to good yields a large proportion of the treated cellulose can contain free anhydride, which cannot be easily removed from the treated cellulose. Generally, the modified cellulose may comprise of a distinct odour, which suggests the presence of free anhydride. The other drawback of the use of non-cyclic anhy‐ drides is the formation of acid by-products, which are generally present in the modified cel‐ lulose. Pyridine, a catalyst used in the reaction, acts by swelling the wood and extracting lignin to expose the cellular structure of the cellulose. This facilitates the exposure of the free hydroxyl groups in cellulose to the anhydride. However, due to the aggressive nature of pyridine, it can also degrade and weaken the structure of the cell wall, which may not allow efficient modification. The effect of esterification on natural fibres is it imparts hydrophobic‐ ity, which makes them more compatible with the polymer matrix.

Tserki et. al. investigated the reinforcing effect of lignocellulosic fibres, incorporating flax, hemp and wood, on the mechanical properties of Bionolle, an aliphatic polyester [44]. The use of acetic anhydride treatment of the fibres was proven not to be as effective for improv‐ ing the matrix tensile strength, compared with other techniques such as compatibilisation; however it did reduce the water absorption of the fibres. Lower tensile strengths were re‐ ported for composites reinforced with wood fibre, compared with flax and hemp. This may be attributed to the nature of the fibres, since flax and hemp are fibrous, whereas wood fibre is more flake like in nature with an irregular size and shape. The type and nature of lignocel‐ lulose fibres (chemical composition and structure) is of paramount importance in the devel‐ opment of polymer composites. It is shown that different fibres behave differently after various treatments. On the other hand, reactions of cellulose with cyclic anhydrides have al‐ so been performed [45]. Reactions involving cyclic anhydrides generally do not result in the formation of by-products and reactions can be performed with milder solvents, which don't interfere with the cell wall structure of cellulose. In order to facilitate reactions of wood flour with cyclic anhydrides it is important that the wood flour be pre-treated. Pre-treatment re‐ quires immersion of the wood flour in a suitable solvent, such as NaOH. This process is oth‐ erwise known as Mercerization, which is thought to optimise fiber-surface characteristics, by removing natural impurities such as pectin, waxy substances and natural oils. It is widely reported that the wood alone does not readily react with esterifying agents, since the hy‐ droxyl groups required for reaction are usually masked by the presence of these natural im‐ purities.

#### *3.1.4. Isocyanate treatment*

dence of hydroxyl groups in the fibre-matrix interface. In the presence of moisture, hydro‐ lysable alkoxy groups result in the formation of silanols. Silanols react with hydroxyl groups of the fibre, forming a stable, covalently bonded structure with the cell wall. As a result, the hydrocarbon chains provided by the reaction of the silane produce a cross-linked network due to covalent bonding between fibre and polymer matrix. This results in a hydrophobic surface in the fibre, which in turn increases the compatibility with the polymer matrix. As mentioned earlier silane coupling agents have been effective for the treatment of glass fibres for the reinforcement of polypropylene. Silane coupling agents have also been found to be useful for the pre-treatment of natural fibres in the development of polymer composites. Wu et. al. demonstrated that wood fibre/polypropylene composites containing fibres pre-treated with a vinyl-tri methoxy silane significantly improved the tensile properties. It was discov‐ ered that the significant improvement in tensile properties was directly related to a strong interfacial bond caused by the acid/water condition used in the fibre pre-treatment [38].

48 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

In a study by Bengtsson et al. the use of silane technology in crosslinking polyethylenewood flour composites was investigated [39]. Composites of polyethylene with wood-flour were reacted in-situ with silanes using a twin screw extruder. The composites showed im‐ provements in toughness and creep properties and the likely explanation for this improve‐ ment was that part of the silane was grafted onto polyethylene and wood, which resulted in a cross-linked network structure in the polymer with chemical bonds occurring at the sur‐ face of wood. X-ray microanalysis showed that most of the silane was found within close proximity to the wood-flour. It is known that silanes can interact with cellulose through ei‐ ther free radical or condensation reaction but also through covalent bonding by the reaction of silanol groups and free hydroxyl groups at the surface of wood, however the exact mech‐ anism could not be ascertained. In a study by González et al, focused on the development of PLA based composites incorporating untreated and silane treated sisal and kraft cellulose fibres [40]. The tensile properties of the resulting composites did not present any major stat‐ istical difference between composites with untreated cellulose fibres and silane treated cellu‐ lose fibres, which suggested that silane treatment of the cellulose fibres did not contribute to further optimisation in the reinforcing affect of the cellulose fibres. The analysis of the high resolution C1s spectra (XPS) indicates that for C1 (C-C, C-H), the percentage of lignin in the intreated sisal fibres was higher, in comparison with kraft fibres. But after modification with silanes, the C1 signal decreases for sisal fibres, which shows that attempted grafting with the silane has resulted in removal of lignin and exposed further cellulose. The higher C1 signal reported for kraft fibres suggested some grafting with silane as a result of the contribution from the alkyl chain of the attached silanol, but no further characterisation was provided to

This section reviews research into the modification of wood constituents with organic acid anhydrides. Anhydrides can be classified into two major groups: non-cyclic anhydrides (i.e. Acetic) and cyclic anhydrides (i.e. Maleic). Of the non-cyclic anhydrides, Acetylation with Acetic Anhydride is the most widely reported [41-43]. The reaction involves the conversion

support grafting of silanes to kraft fibres.

*3.1.3. Esterification of natural fibres*

Isocyanates are compounds containing the isocyanate functional group –N=C=O, which is highly reactive with hydroxyl groups in lignocellulose materials. The general reaction for cellulose with an isocyanate coupling agent is shown Equation 1:

**Equation 1.** Possible reaction mechanism of MDI with wood-flour

R can represent any chemical group, such as alkyl or phenyl. Pickering et al studied the ef‐ fects of Poly[methylene(polyphenyl isocyanate) and maleated coupling agents on New Zea‐ land radiata pine fibre-polypropylene composites. A modest improvement in strength (4%) was reported with the addition of isocyanate to the polymer matrix over the matrix alone. When the radiata pine was treated with isocyanate and added to matrix, the strength im‐ proved by 11.5% over untreated radiata pine, and the modulus exhibited a significant im‐ provement of 77% [46]. It appears that lignin content in wood fibres plays a significant role in relation to the ability of certain functional groups to interact with the cellulose compo‐ nent. The modest gains in tensile strength with the isocyanate can be attributed to the great‐ er percentage of lignin in radiata pine.

X-ray mapping using Electron Probe Microanalysis presents a useful technique for evaluat‐ ing the extent of cross-linking with MDI in biopolymer composites. Analysis of polished cross-sections was performed on unmodified wood-flour composite and the composites with MDI-mediated wood-flour. The aim of this was to detect the presence of nitrogen in the composites, which would indicate the extent of cross-linking in the modified PLA/woodflour composites with MDI. The micrographs with the X-ray mapping of micro-composite with unmodified wood-flour composite and (b) micro-composite with MDI-modified woodflour are shown in Figure 3. The nitrogen in the composite is depicted in the micrograph by the regions colored in green. The composite with unmodified wood-flour (Figure 3(a)) shows some nitrogen but this was expected since wood in its native form comprises tracea‐ ble amounts (<0.75%) of nitrogen. Figure 3(b) depicts the composite with MDI-modified wood-flour and shows a greater concentration of nitrogen, presumably associated with MDI, in close proximity to the particle and the fibre lumen (cells) of the wood-flour particles and some concentrated areas at the interface. Similar observations were also reported by Bengtsson et al., which demonstrated the X-ray mapping of silicon from the silane used to modify wood-flour for polyethylene composites[47]. This suggests some reaction of the MDI-modified wood-flour with the PLA matrix creating a cross-linked structure with chem‐ ical bonds joining MDI-modified wood-flour with the PLA matrix. This provides further evidence of the improvement in the mechanical properties as a result of an improvement in the interfacial interaction between PLA and the wood-flour particles. MDI appears to be also spread throughout the PLA matrix suggesting that part of it remains un-reacted within the host polymer.

**Equation 1.** Possible reaction mechanism of MDI with wood-flour

50 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

er percentage of lignin in radiata pine.

R can represent any chemical group, such as alkyl or phenyl. Pickering et al studied the ef‐ fects of Poly[methylene(polyphenyl isocyanate) and maleated coupling agents on New Zea‐ land radiata pine fibre-polypropylene composites. A modest improvement in strength (4%) was reported with the addition of isocyanate to the polymer matrix over the matrix alone. When the radiata pine was treated with isocyanate and added to matrix, the strength im‐ proved by 11.5% over untreated radiata pine, and the modulus exhibited a significant im‐ provement of 77% [46]. It appears that lignin content in wood fibres plays a significant role in relation to the ability of certain functional groups to interact with the cellulose compo‐ nent. The modest gains in tensile strength with the isocyanate can be attributed to the great‐

X-ray mapping using Electron Probe Microanalysis presents a useful technique for evaluat‐ ing the extent of cross-linking with MDI in biopolymer composites. Analysis of polished cross-sections was performed on unmodified wood-flour composite and the composites with MDI-mediated wood-flour. The aim of this was to detect the presence of nitrogen in the composites, which would indicate the extent of cross-linking in the modified PLA/woodflour composites with MDI. The micrographs with the X-ray mapping of micro-composite with unmodified wood-flour composite and (b) micro-composite with MDI-modified woodflour are shown in Figure 3. The nitrogen in the composite is depicted in the micrograph by the regions colored in green. The composite with unmodified wood-flour (Figure 3(a)) shows some nitrogen but this was expected since wood in its native form comprises tracea‐ ble amounts (<0.75%) of nitrogen. Figure 3(b) depicts the composite with MDI-modified wood-flour and shows a greater concentration of nitrogen, presumably associated with MDI, in close proximity to the particle and the fibre lumen (cells) of the wood-flour particles and some concentrated areas at the interface. Similar observations were also reported by Bengtsson et al., which demonstrated the X-ray mapping of silicon from the silane used to modify wood-flour for polyethylene composites[47]. This suggests some reaction of the MDI-modified wood-flour with the PLA matrix creating a cross-linked structure with chem‐

**Figure 3.** Electron Probe Microanalysis of PLA/wood-flour micro-composites containing (a) unmodified wood-flour (b) MDI-mediated wood-flour (wood-flour content = 30% w/w)

Ecotoxicity is an important factor to consider when developing polymer composites from re‐ newable resources. Isocyanate compounds, such as MDI, may not be regarded as a viable treatment method for natural fibres. Isocyanates upon decomposition in water can result in the formation of diamines. The decomposition products, such as 4, 4'-methylenedianiline and 2,4-diaminotoluene are suspected to be cancer causing agents and may also cause hepa‐ titis in humans [48]. An alternative isocyanate that has been reported in the literature is ly‐ sine-based di-isocyanate. Lee et al reported the use of LDI as a coupling agent in the development of biodegradable polymers produced from poly lactic acid/bamboo fibre and poly(butylene succinate)/bamboo fibre. LDI is based upon Lysine, a naturally occurring ami‐ no acid with two amino groups and one carboxylic group. LDI can react with hydroxyl groups in cellulose, forming an isocyanate bridge, which can then readily react with the car‐ boxylic and hydroxyl groups of the matrix polymer. MDI has previously been reported in the compatibilization of PLA and starch blends [49]. Wang discovered that blends of PLA with 45% wheat starch and 0.5% MDI resulted in composites with the highest tensile strength. It was also shown that moisture absorption increased as a function of increasing starch content. Water absorption can influence the mechanical properties of the composite. The moisture in the composite can react with MDI, which can effect interfacial interaction between starch mediated MDI with the PLA matrix by reducing the tensile strength or hav‐ ing a limited improvement. The reaction of moisture with MDI has also been reported in an‐ other paper [50] by Yu et al. It was interesting to note that the highest strength was achieved at 45%. This can be attributed to two major reasons: the level of water in the blend can aid processing of the PLA, whereby the water behaves as a plasticiser, and secondly, the viscosi‐ ty of the PLA at this level of water content maybe just sufficient to allow optimum disper‐ sion of the starch particles within the PLA matrix. However, in order to utilise these materials in commercial applications such as for short term packaging these materials would require water proofing on the surface in order to prevent the rapid degradation.

#### **3.2. Physical techniques**

Physical methods [51-54] reported in the literature are the use of corona or plasma treat‐ ers for modifying cellulose fibres for conventional polymers. In recent years the use of plas‐ ma for treatment of natural fibres has gained more prominence as this provides a more "greener" alternative for the treatment of natural fibres for the development of polymer composites, but is of particular interest to polymer composites incorporating biopolymer matrices, since this technique provides further credence to the whole idea of "green materi‐ als. Sustainability and end of life after use are important considerations to make when de‐ veloping polymer composites from renewable resources is the toxicity and environmental impact of using various chemical or physical methods for improving the properties of these materials. Some chemical techniques may be toxic, e.g. isocyanates are carcinogenic, and therefore, the use of such agents may not be feasible for the development of polymer com‐ posites from renewable resources. Physical methods involving plasma treatments have the ability to change the surface properties of natural fibres by formation of free radical spe‐ cies (ions, electrons) on the surfaces of natural fibres [55]. During plasma treatment, surfa‐ ces of materials are bombarded with a stream of high energy particles within the stream of plasma. Properties such as wettability, surface chemistry and surface roughness of materi‐ al surfaces can be altered without the need for employing solvents or other hazardous sub‐ stances. Alternative surface chemistries can be produced with plasmas, by altering the carrier gas and depositing different reactive species on the surfaces of natural fibres [56]. This can then be further exploited by grafting monomeric and/or polymeric molecules on to the reactive natural fibre surface, which can then facilitate compatibilisation with the pol‐ ymer matrix.

#### **3.3. Toughening mechanisms in PLA/wood-flour composites**

Physical modification of PLA can be achieved with the incorporation of softer polymer seg‐ ments, which can attach to the polymer backbone. An example of an impact modification of PLA was performed with the addition of Poly (ethylene) acrylic acid (PEAA). The ef‐ fect of impact modification can be observed in the load-deflection curves depicted in Fig‐ ure 4. The load-deflection curve for PLA is almost linear and displays a rapid decrease in load once the peak load is reached, which is indicative of the well-known low resistance of PLA to crack propagation and its susceptibility to brittle fracture, with the smooth impact fracture surface of PLA (Figure 5) being typical of brittle failure. The load-deflection curve for the PLA/wood-flour micro-composite containing 20%w/w wood-flour shown in Figure 4(b) displays an increased peak load compared with PLA, and the less rapid decrease in the load after peak load is reached is further evidence for effective stress transfer from the PLA matrix to the wood-flour particles. The load-deflection result for the PLA/wood-flour microcomposite containing MDI shown in Figure 4(c) indicates that the addition of MDI leads to a higher peak load compared with the equivalent micro-composite with no added MDI (Fig‐ ure 4(b)), and the shape of the load-deflection curve is consistent with typical elastic-plas‐ tic deformation dominated by unstable crack growth. The increase in the peak load and width of the load-deflection profile, shown in Figure 4(d) indicates extensive plastic defor‐ mation of the PLA/wood-flour micro-composite containing PEAA. This increase in plastic deformation is attributed primarily to the increase in the rubbery nature of the blended PLA/PEAA matrix compared with PLA alone, resulting in more efficient dissipation of the energy associated with crack initiation and propagation [57].

at 45%. This can be attributed to two major reasons: the level of water in the blend can aid processing of the PLA, whereby the water behaves as a plasticiser, and secondly, the viscosi‐ ty of the PLA at this level of water content maybe just sufficient to allow optimum disper‐ sion of the starch particles within the PLA matrix. However, in order to utilise these materials in commercial applications such as for short term packaging these materials would require water proofing on the surface in order to prevent the rapid degradation.

Physical methods [51-54] reported in the literature are the use of corona or plasma treat‐ ers for modifying cellulose fibres for conventional polymers. In recent years the use of plas‐ ma for treatment of natural fibres has gained more prominence as this provides a more "greener" alternative for the treatment of natural fibres for the development of polymer composites, but is of particular interest to polymer composites incorporating biopolymer matrices, since this technique provides further credence to the whole idea of "green materi‐ als. Sustainability and end of life after use are important considerations to make when de‐ veloping polymer composites from renewable resources is the toxicity and environmental impact of using various chemical or physical methods for improving the properties of these materials. Some chemical techniques may be toxic, e.g. isocyanates are carcinogenic, and therefore, the use of such agents may not be feasible for the development of polymer com‐ posites from renewable resources. Physical methods involving plasma treatments have the ability to change the surface properties of natural fibres by formation of free radical spe‐ cies (ions, electrons) on the surfaces of natural fibres [55]. During plasma treatment, surfa‐ ces of materials are bombarded with a stream of high energy particles within the stream of plasma. Properties such as wettability, surface chemistry and surface roughness of materi‐ al surfaces can be altered without the need for employing solvents or other hazardous sub‐ stances. Alternative surface chemistries can be produced with plasmas, by altering the carrier gas and depositing different reactive species on the surfaces of natural fibres [56]. This can then be further exploited by grafting monomeric and/or polymeric molecules on to the reactive natural fibre surface, which can then facilitate compatibilisation with the pol‐

Physical modification of PLA can be achieved with the incorporation of softer polymer seg‐ ments, which can attach to the polymer backbone. An example of an impact modification of PLA was performed with the addition of Poly (ethylene) acrylic acid (PEAA). The ef‐ fect of impact modification can be observed in the load-deflection curves depicted in Fig‐ ure 4. The load-deflection curve for PLA is almost linear and displays a rapid decrease in load once the peak load is reached, which is indicative of the well-known low resistance of PLA to crack propagation and its susceptibility to brittle fracture, with the smooth impact fracture surface of PLA (Figure 5) being typical of brittle failure. The load-deflection curve

**3.2. Physical techniques**

52 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

ymer matrix.

**3.3. Toughening mechanisms in PLA/wood-flour composites**

**Figure 4.** Load-deflection curves for a) PLA, (b) PLA/wood-flour, (c) PLA/wood-flour containing MDI, (d) PLA/woodflour containing PEAA wood-flour content=20%w/w

**Figure 5.** SEM micrograph of impact fracture surface of PLA for (a) Pure matrix and (b) blended with 3% w/w PEAA

#### **4. Conclusions**

Increasing concerns and awareness of the environment and the impact of human activity on the environment is currently the driving force for developing materials that are sustainable, more ecologically sound and are purely produced from renewable resources. PLA based composite materials have been widely recognised as a noble replacement to more conven‐ tional polymer composites derived from petrochemical feedstocks, such as polyolefins. PLA based composites are biodegradable and have high mechanical performance compared with conventional polymer composites. However, the wider uptake of PLA is restricted by per‐ formance deficiencies, such as its relatively poor impact properties which arise from its in‐ herent brittleness, but also the limited supply and higher cost of PLA compared with commodity polymers such as polyethylene and polypropylene. It is expected that in the coming years PLA based materials will become more competitively priced as the demand increases and supply of abundant feedstock material becomes more widely available. The use of natural fibres presence a useful technique for developing PLA based composites, which are low-cost, biodegradable and can have properties that can be tailored for their spe‐ cific application. PLA/natural fibre composites have been shown to have increased tensile modulus and reduced tensile strength compared with PLA, and this has been attributed to factors that include the weak interfacial interaction between the hydrophobic PLA matrix and the hydrophilic natural fibers, and lack of fiber dispersion due to a high degree of fiber agglomeration. Various methods of modifying the surface of the cellulosic fibers have been explored in an effort to improve the interaction that occurs at the interface between the PLA matrix and natural fibres. Better development of processing technologies and improvements in natural fibre treatments will facilitate the production of PLA based composites with opti‐ mum mechanical and physical performance but also generate high cost competiveness and greater acceptance of these materials in the market place.

## **Author details**

Eustathios Petinakis1,2\*, Long Yu1 , George Simon2 and Katherine Dean1

\*Address all correspondence to<EMAIL_ADDRESS>
1 CSIRO, Materials Science and Engineering, Melbourne, Australia

2 Department of Materials Engineering, Monash University, Melbourne, Australia

## **References**

**Figure 5.** SEM micrograph of impact fracture surface of PLA for (a) Pure matrix and (b) blended with 3% w/w PEAA

54 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

Increasing concerns and awareness of the environment and the impact of human activity on the environment is currently the driving force for developing materials that are sustainable, more ecologically sound and are purely produced from renewable resources. PLA based composite materials have been widely recognised as a noble replacement to more conven‐ tional polymer composites derived from petrochemical feedstocks, such as polyolefins. PLA based composites are biodegradable and have high mechanical performance compared with conventional polymer composites. However, the wider uptake of PLA is restricted by per‐ formance deficiencies, such as its relatively poor impact properties which arise from its in‐ herent brittleness, but also the limited supply and higher cost of PLA compared with commodity polymers such as polyethylene and polypropylene. It is expected that in the coming years PLA based materials will become more competitively priced as the demand increases and supply of abundant feedstock material becomes more widely available. The use of natural fibres presence a useful technique for developing PLA based composites, which are low-cost, biodegradable and can have properties that can be tailored for their spe‐ cific application. PLA/natural fibre composites have been shown to have increased tensile modulus and reduced tensile strength compared with PLA, and this has been attributed to factors that include the weak interfacial interaction between the hydrophobic PLA matrix and the hydrophilic natural fibers, and lack of fiber dispersion due to a high degree of fiber agglomeration. Various methods of modifying the surface of the cellulosic fibers have been explored in an effort to improve the interaction that occurs at the interface between the PLA matrix and natural fibres. Better development of processing technologies and improvements in natural fibre treatments will facilitate the production of PLA based composites with opti‐

**4. Conclusions**

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**Applications in Concrete Repair with FRP**

## **The Use of Fiber Reinforced Plastic for The Repair and Strengthening of Existing Reinforced Concrete Structural Elements Damaged by Earthquakes**

George C. Manos and Kostas V. Katakalos

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/51326

## **1. Introduction**

During the last fifty years various parts of the world have been subjected to a number of damaging earthquakes. Greece is one of the countries where such damaging earthquakes oc‐ cur quite frequently ([1]). Some of these earthquakes, not necessarily the most intense, oc‐ curred near urban areas and thus subjected various types of structures to significant earthquake forces leading to damage ([2]). For some of these earthquakes, ground motion acceleration recordings were obtained at distances relatively close to the area of intense shaking, thus providing valuable information for correlating the observed damage with this ground motion recording and its characteristics. Moreover, following the most damaging of these earthquakes, studies were initiated that led to the revision of the provisions of Seismic Codes [3]. The damaged structures included old structural formations, sometimes older than one hundred years, which were not designed for seismic forces. They usually belong to cultural heritage and are under various forms of conservation status that does not allow all types of retrofitting but only retrofitting materials and techniques that are compatible with the existing materials; moreover, the applied retrofitting in these cases must also be reversi‐ ble so that it can be easily removed in case it demonstrates undesirable effects with time.  The vast majority of these structures are multistory reinforced concrete (R/C) buildings. There are other types of struc‐ tures apart from (R/C) buildings, such as structures forming the infrastructure or industrial facilities which can also develop earthquake damage. However, this chapter will be devoted to the usual R/C residential multi-story buildings, the earthquake damage of their structural elements and their strengthening.

© 2013 Manos and Katakalos; licensee InTech. This is an open access article 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. © 2013 Manos and Katakalos; licensee InTech. This is a paper 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.

#### **1.1. Damage observations for contemporary R/C structures**

These structures are usually designed and built according to the provisions of a Seismic Code ([3]). The cause of damage may be due to:

The code provisions underestimating the severity of the shaking and thus underestimating the seismic demand posed upon the various structural elements, their connections and the foundation of the whole structure.

The code provisions together with the specification of the materials resulting in such strengths that are below the seismic demands placed upon the structural elements.

The detailing and the realization of the design during construction or alteration during the life time of the building resulting again in such strengths for the various structural elements that are below the corresponding seismic demands placed upon them.

In all cases the appearance of structural damage results from the above fundamental causes, either from one of them or from their combination; this is expressed by the following in‐ equality (Eq. 1) between the strength and the demand put upon the various members of the structural system, whereby the demand is signified as Sd and the strength as Rd. Damage is expected to occur when this inequality is not satisfied [4].

$$\mathbf{S}\_{\mathbf{d}} \le \mathbf{R}\_{\mathbf{d}} \tag{1}$$

A serious task, after a strong earthquake affects an urban area, is to appraise the severity of the structural damage for a large number of buildings. This is usually done in Greece by en‐ gineers specially appointed for this task by the Government. The screening process com‐ mences a few days after the natural disaster, provided that the seismic sequence has subsided. There are certain guidelines published by the Hellenic Organization of Earth‐ quake Planning and Protection to facilitate the appointed engineers in this damage screen‐ ing process [5]. In general terms the first round of inspection must lead to the classification of each building to one of the three main categories. The first damage category is that there is no structural damage and the mainly non-structural damage does not pose any danger; so these buildings can be reused immediately. The second damage category is that there is nonstructural damage as well as some structural damage; the latter, although contained, may have led to a considerable decrease of the bearing seismic capacity of the damaged structur‐ al elements and the structure as a whole. There may be need for temporary shoring and the removal of dangerously damaged non-structural elements. These buildings will be subjected to a second round of inspection after these countermeasures have been accomplished. Their reuse will be decided after the second round of screening. The third category includes build‐ ings with relatively extensive damage to its structural elements (slabs, beams, columns or shear walls) [5], [6]; the damage is relatively widespread in terms of story level. Permanent deformations of the structural elements are evident in the form of concrete cracking and crushing in certain critical areas of the structural elements indicating that these areas have been overstressed and inequality 1 has ceased to be valid. There is serious consideration that this bearing seismic capacity reduction may lead to partial collapse. There may be urgent need for temporary shoring and the removal of dangerously damaged parts in order to avoid, if possible, partial collapse (figure 1b). These buildings will also be subjected to a sec‐ ond round of inspection. If it is decided that they pose a public threat, due to the possibility of partial or total collapse, they should be demolished (figure 1a); otherwise, a special design should devise a feasible scheme for their repair and strengthening. The following sections will address the task of repair and strengthening of structural elements that have been sub‐ jected to such earthquake damage to their structural system and have been classified as be‐ longing to the second or third seismic damage categories after the second round of screening process.

**1.1. Damage observations for contemporary R/C structures**

64 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

Code ([3]). The cause of damage may be due to:

foundation of the whole structure.

These structures are usually designed and built according to the provisions of a Seismic

The code provisions underestimating the severity of the shaking and thus underestimating the seismic demand posed upon the various structural elements, their connections and the

The code provisions together with the specification of the materials resulting in such

The detailing and the realization of the design during construction or alteration during the life time of the building resulting again in such strengths for the various structural elements

In all cases the appearance of structural damage results from the above fundamental causes, either from one of them or from their combination; this is expressed by the following in‐ equality (Eq. 1) between the strength and the demand put upon the various members of the structural system, whereby the demand is signified as Sd and the strength as Rd. Damage is

A serious task, after a strong earthquake affects an urban area, is to appraise the severity of the structural damage for a large number of buildings. This is usually done in Greece by en‐ gineers specially appointed for this task by the Government. The screening process com‐ mences a few days after the natural disaster, provided that the seismic sequence has subsided. There are certain guidelines published by the Hellenic Organization of Earth‐ quake Planning and Protection to facilitate the appointed engineers in this damage screen‐ ing process [5]. In general terms the first round of inspection must lead to the classification of each building to one of the three main categories. The first damage category is that there is no structural damage and the mainly non-structural damage does not pose any danger; so these buildings can be reused immediately. The second damage category is that there is nonstructural damage as well as some structural damage; the latter, although contained, may have led to a considerable decrease of the bearing seismic capacity of the damaged structur‐ al elements and the structure as a whole. There may be need for temporary shoring and the removal of dangerously damaged non-structural elements. These buildings will be subjected to a second round of inspection after these countermeasures have been accomplished. Their reuse will be decided after the second round of screening. The third category includes build‐ ings with relatively extensive damage to its structural elements (slabs, beams, columns or shear walls) [5], [6]; the damage is relatively widespread in terms of story level. Permanent deformations of the structural elements are evident in the form of concrete cracking and crushing in certain critical areas of the structural elements indicating that these areas have been overstressed and inequality 1 has ceased to be valid. There is serious consideration that this bearing seismic capacity reduction may lead to partial collapse. There may be urgent

Sd < Rd (1)

strengths that are below the seismic demands placed upon the structural elements.

that are below the corresponding seismic demands placed upon them.

expected to occur when this inequality is not satisfied [4].

**Figure 1.** a The old building of a pharmaceutical company that collapsed during the Athens 1999 earthquake togeth‐ er with an upgraded building next to it that survived unscathed [2]. b. Temporary shoring of 4-story building (Hardas) damaged during the Pyrgos 1993 earthquake [2].

#### **1.2. Structural damage description at the level of the structural element**

It is usual to describe the structural damage at the level of each reinforced concrete structur‐ al member, e.g. slab, beam, column and shear wall, always having in mind inequality 1 and the fact that the axial (N), bending (M) and shear (Q) force demands in each one of these structural members from the combination of dead and live loads plus the earthquake forces are of a particular nature. Thus, for the slabs the demands are mainly flexural whereas for the beams the demands are flexural and shear. For the columns the demands are flexural and shear with the presence of considerable axial forces whereas for the shear walls the de‐ mands are flexural and shear with the presence of a relatively lower level of axial forces than that of the columns. Apart from the structural elements themselves, one should also consider critical areas of their connections (joints) as well as the foundation. Both, the struc‐ tural connections and the foundation are very critical areas that require special considera‐ tion in both identifying the nature of the structural damage as well as proposing countermeasures. This presentation does not deal with either of these critical areas. The main flexural structural damage in slabs and beams develops in the areas of maximum bending moments. For the beams they usually develop near the joints with the columns and shear walls (figure 2a) where large bending moments are expected to develop from the seis‐ mic forces. Similarly, at the ends of the beams are the areas of large shear forces from the combination of earthquake forces with the dead and live loads; these will cause the appear‐ ance of shear damage in the form of diagonal cracks (figure 2a). The presence of large bend‐ ing moments mainly from the seismic load together with large axial forces will cause the formation of flexural damage at the top and the toe of columns (see figure 2b) whereas the presence of shear forces from the seismic loads together with axial forces will lead to the for‐ mation of shear damage at the columns (see figure 2c). The presence of large shear forces from seismic loads together with relatively low level axial forces will lead to the develop‐ ment of shear damage in the shear walls (see figure 3a) whereas the presence of short col‐ umns will lead to the development of large shear forces from seismic loads and the development of shear damage as shown in figure 3b.

**Figure 2.** a Typical damage of beam at the joint with the nearby column (6th story building, Aharnes, Athens earth‐ quake 1995), [2]. b Typical damage of the toe of a column at ground floor (two-story building, Nea Kifisia,, Athens earthquake 1995), [2]. c Typical shear damage at the columns of a "soft story" (4-story Metamorfosi building, Athens earthquake 1995), [2].

#### **1.3. The strategy for the repair and strengthening scheme**

In the previous discussion the emphasis in the description of damage was on attributing the various forms of structural damage to the nature of the demand (either flexural or shear with or without axial forces). However, as already stressed by inequality 1, structural dam‐ age is due to the fact that the said demand was not met by the existing strength. The strategy for retrofitting damaged structural systems, or structural systems that can be demonstrated by analytical methods prior to a strong earthquake to be prone to potential damage in the future, must be based on either somehow lowering the demands or increasing the corre‐ sponding strengths or both. In an effort to lower the demands, one can try to decrease the masses mobilized by the earthquake vibrations. This can be accomplished either by remov‐ ing unnecessary mass or by changing the structural system in such a way that the resulting dynamic system combined with the design spectrum leads to a decrease in the earthquake loads (e.g. seismic isolation). Lowering the demands in the way described before is not al‐ ways feasible. Thus, the retrofitting scheme is usually based on increasing the strengths of the structural members. In doing so, one must be aware that it is advisable to increase the deformability of the structural members thus increasing the ability of the structural system to dissipate the seismic energy through plastic deformations that are designed to develop at predetermined locations ([7], [8] [9], [10]). The location where these plastic deformations oc‐ cur should be such that it does not lead to unstable structural formations. Sometimes, a par‐ tial objective of the strengthening scheme is to increase the stiffness to a moderate degree, especially for structural systems that can develop excessive torsional response. However, frequently a considerable increase in the stiffness of the strengthened structural system re‐ sults as an indirect consequence of the adopted scheme whereas its main objectives were to increase the strength and deformability of the structural elements and of the structure as a whole. This increase in stiffness results in larger demands due to the dynamics of the strengthened structure, as it corresponds to higher amplitudes of design spectral accelera‐ tions than the un-strengthened structural system. This is usually the case when traditional strengthening schemes are employed utilizing reinforced concrete jacketing of the structural members (columns and beams) or the addition of shear walls by encasing reinforced con‐ crete elements within the bays of the corresponding R/C frames [6]. The strengthening schemes that can be employed in order to increase the flexural or shear capacity of columns, beams or shear walls utilizing fiber reinforcing plastics (FRP) usually made of carbon, glass or steel do not result in this undesirable increase in stiffness. Moreover, due to their external application, they usually require less interference with or breaking of the volume of existing R/C structural elements. Finally, such strengthening schemes become effective in a much shorter time than traditional strengthening schemes ([11], [12], [13], [14], [15], [16]).

ance of shear damage in the form of diagonal cracks (figure 2a). The presence of large bend‐ ing moments mainly from the seismic load together with large axial forces will cause the formation of flexural damage at the top and the toe of columns (see figure 2b) whereas the presence of shear forces from the seismic loads together with axial forces will lead to the for‐ mation of shear damage at the columns (see figure 2c). The presence of large shear forces from seismic loads together with relatively low level axial forces will lead to the develop‐ ment of shear damage in the shear walls (see figure 3a) whereas the presence of short col‐ umns will lead to the development of large shear forces from seismic loads and the

**Figure 2.** a Typical damage of beam at the joint with the nearby column (6th story building, Aharnes, Athens earth‐ quake 1995), [2]. b Typical damage of the toe of a column at ground floor (two-story building, Nea Kifisia,, Athens earthquake 1995), [2]. c Typical shear damage at the columns of a "soft story" (4-story Metamorfosi building, Athens

In the previous discussion the emphasis in the description of damage was on attributing the various forms of structural damage to the nature of the demand (either flexural or shear with or without axial forces). However, as already stressed by inequality 1, structural dam‐ age is due to the fact that the said demand was not met by the existing strength. The strategy for retrofitting damaged structural systems, or structural systems that can be demonstrated by analytical methods prior to a strong earthquake to be prone to potential damage in the future, must be based on either somehow lowering the demands or increasing the corre‐ sponding strengths or both. In an effort to lower the demands, one can try to decrease the masses mobilized by the earthquake vibrations. This can be accomplished either by remov‐ ing unnecessary mass or by changing the structural system in such a way that the resulting dynamic system combined with the design spectrum leads to a decrease in the earthquake loads (e.g. seismic isolation). Lowering the demands in the way described before is not al‐ ways feasible. Thus, the retrofitting scheme is usually based on increasing the strengths of the structural members. In doing so, one must be aware that it is advisable to increase the deformability of the structural members thus increasing the ability of the structural system to dissipate the seismic energy through plastic deformations that are designed to develop at predetermined locations ([7], [8] [9], [10]). The location where these plastic deformations oc‐ cur should be such that it does not lead to unstable structural formations. Sometimes, a par‐ tial objective of the strengthening scheme is to increase the stiffness to a moderate degree,

development of shear damage as shown in figure 3b.

66 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

**1.3. The strategy for the repair and strengthening scheme**

earthquake 1995), [2].