**2. Polypropylene and aramid**

Polypropylene (PP), one of the most widely used polymers all over the world, has now become the commodity polymer par excellence. One of the most important properties placing it above others is a versatility which allows it to be modified and designed for specific applications, along with an excellent balance of physical, chemical and mechanical properties. Due to the rheological and thermal behavior of molten materials, the materials based on this resin offer an extended period of processability, from injection molding to blown film extrusion. Whichever is the case, thermoplastics offer greater advantages over thermosets given the dexterity with which these materials can be molded, simply by the application of pressure, temperature and a short consolidation period 1.

All these properties conferring such versatility to PP are highly dependent on the degree of crystallinity, which in turn depends on the stereoregularity of the polymeric chain, a term known as tacticity. PP can present three types of tacticity; isotactic, syndiotactic and atactic,

Advantages of Low Energy Adhesion PP for Ballistics 195

lines. During production, the material can be obtained in two forms; the first is non-woven fiber, on continuous spools, while the second format is by means of a fabric which can be bidimensional or tridimensional, in which there are subcategories depending on the format.

Synthetic fiber fabrics are formed by fibers placed lengthwise in two perpendicular directions: where one direction is the warp and the other the weft. The warp usually marks the direction parallel to the fabric production line. Differences between different types of fabric depend on the way the weft is intercalated with the warp, some fabric configurations are shown in Fig. 2. Fig. 2a shows a plain weave configuration, Fig. 2b a satin weave configuration and Fig. 2c a twill weave configuration; these are the most common fabric types used on the market 2. Each configuration has a preferential application depending on the requirements. The woven configuration is most commonly used in ballistics, since it presents the highest factor of impenetrability in comparison with other fabric configurations. The impenetrability factor of a fabric is a measurement of weave

Fig. 2. Configurations of fabrics commonly used in composite materials, a) plain weave, b)

One of the most commonly used fibers in composite materials and ballistics is aramid; these fibers were discovered in the early 60's and were put on the market by DuPont in the early 70's under the trademark of Kevlar®, they are well known for their mechanical properties and are currently one of the three most important fibers for applications in composite materials together with fiberglass and carbon fiber. The first structure was conceived by DuPont (Fig. 3) and has since, undergone changes which have improved its properties according to the required applications, this is also the case for Teijin Company with their

n

Fig. 3. Chemical structure of Poly(p-phenylene tereftalamide) or PPTA

This work focuses on a particular study of a bi-dimensional aramid fabric.

a) c) b)

impenetrability as a function of the coverage factor 3.

satin weave and c) twill weave

fiber trademark Twaron® 4.

where each term represents the balance between three-dimensionally ordered regions and regions with no discernable order (amorphous regions); polymers with high degrees of crystallinity are denominated isotactic and the absence of any degree of crystallinity is known as atactic, while syndiotactic is an intermediate degree of crystallinity 1. Crystallinity has a very marked effect on mechanical properties, since polymers with high percentages of crystallinity (between 60% and 70%) present increased resistance to traction at the yield point, as well as an increase in rigidity and resistance to flexure; however, they also present a reduction in tenacity and resistance to impact. In contrast, polymers with low percentages of crystallinity show low resistance to traction and flexure but high tenacity and good resistance to impact, as well as excellent transparency, making them very appropriate for use in the production of transparent films with high refraction rates up to 1.49 1.

With the manipulation of tacticity, it is possible to generate a type of polypropylene with an increased capacity to resist impact; copolymers in blocks, which are part of a new family of elastomers, namely thermoplastic elastomers. This material has the properties of an elastomer, the main difference being that the polymeric chains are not joined by covalent links but by secondary links, in other words, they are elastomers which are not crosslinked. This polypropylene, also called PP-impact (Fig. 1), is composed of polymeric chains with isotactic and atactic blocks, which create highly ordered and amorphous areas. These highly-ordered areas function as covalent links, the main difference being that they can be separated only with temperature.

Fig. 1. Block copolymer, a) elastomeric PP, and b) PP chain by blocks

Another way to improve the mechanical properties of PP is by adding a second stage, which may be continuous or discontinuous fibers, thereby forming a composite fiber-reinforced material. This second stage is usually characterized by higher mechanical, thermal, electrical and chemical properties in comparison with those observed in PP bulk, which are subsequently transformed when combine together in a composite. From a mechanical point of view, resistance increases in response to a more optimal distribution of loads in the material, resulting in a better weight–resistance relationship, a property known as specific resistance; defined as the resistance of material per unit of weight, which is greatly appreciated in the transport industry since weight reduction means increased efficiency in any vehicle.

The most common reinforcement used in composite materials is organic fibers, consisting of thousands of filaments with diameters from 5 to 15 µm, obtained from fabric production

where each term represents the balance between three-dimensionally ordered regions and regions with no discernable order (amorphous regions); polymers with high degrees of crystallinity are denominated isotactic and the absence of any degree of crystallinity is known as atactic, while syndiotactic is an intermediate degree of crystallinity 1. Crystallinity has a very marked effect on mechanical properties, since polymers with high percentages of crystallinity (between 60% and 70%) present increased resistance to traction at the yield point, as well as an increase in rigidity and resistance to flexure; however, they also present a reduction in tenacity and resistance to impact. In contrast, polymers with low percentages of crystallinity show low resistance to traction and flexure but high tenacity and good resistance to impact, as well as excellent transparency, making them very appropriate for

With the manipulation of tacticity, it is possible to generate a type of polypropylene with an increased capacity to resist impact; copolymers in blocks, which are part of a new family of elastomers, namely thermoplastic elastomers. This material has the properties of an elastomer, the main difference being that the polymeric chains are not joined by covalent links but by secondary links, in other words, they are elastomers which are not crosslinked. This polypropylene, also called PP-impact (Fig. 1), is composed of polymeric chains with isotactic and atactic blocks, which create highly ordered and amorphous areas. These highly-ordered areas function as covalent links, the main difference being that they can be

> Crystalline regions

use in the production of transparent films with high refraction rates up to 1.49 1.

Fig. 1. Block copolymer, a) elastomeric PP, and b) PP chain by blocks

a) b)

Another way to improve the mechanical properties of PP is by adding a second stage, which may be continuous or discontinuous fibers, thereby forming a composite fiber-reinforced material. This second stage is usually characterized by higher mechanical, thermal, electrical and chemical properties in comparison with those observed in PP bulk, which are subsequently transformed when combine together in a composite. From a mechanical point of view, resistance increases in response to a more optimal distribution of loads in the material, resulting in a better weight–resistance relationship, a property known as specific resistance; defined as the resistance of material per unit of weight, which is greatly appreciated in the

Amorphous regions

Isotactic block

Atactic block

The most common reinforcement used in composite materials is organic fibers, consisting of thousands of filaments with diameters from 5 to 15 µm, obtained from fabric production

transport industry since weight reduction means increased efficiency in any vehicle.

separated only with temperature.

Amorphous regions

> Crystalline regions

lines. During production, the material can be obtained in two forms; the first is non-woven fiber, on continuous spools, while the second format is by means of a fabric which can be bidimensional or tridimensional, in which there are subcategories depending on the format. This work focuses on a particular study of a bi-dimensional aramid fabric.

Synthetic fiber fabrics are formed by fibers placed lengthwise in two perpendicular directions: where one direction is the warp and the other the weft. The warp usually marks the direction parallel to the fabric production line. Differences between different types of fabric depend on the way the weft is intercalated with the warp, some fabric configurations are shown in Fig. 2. Fig. 2a shows a plain weave configuration, Fig. 2b a satin weave configuration and Fig. 2c a twill weave configuration; these are the most common fabric types used on the market 2. Each configuration has a preferential application depending on the requirements. The woven configuration is most commonly used in ballistics, since it presents the highest factor of impenetrability in comparison with other fabric configurations. The impenetrability factor of a fabric is a measurement of weave impenetrability as a function of the coverage factor 3.

Fig. 2. Configurations of fabrics commonly used in composite materials, a) plain weave, b) satin weave and c) twill weave

One of the most commonly used fibers in composite materials and ballistics is aramid; these fibers were discovered in the early 60's and were put on the market by DuPont in the early 70's under the trademark of Kevlar®, they are well known for their mechanical properties and are currently one of the three most important fibers for applications in composite materials together with fiberglass and carbon fiber. The first structure was conceived by DuPont (Fig. 3) and has since, undergone changes which have improved its properties according to the required applications, this is also the case for Teijin Company with their fiber trademark Twaron® 4.

Fig. 3. Chemical structure of Poly(p-phenylene tereftalamide) or PPTA

Advantages of Low Energy Adhesion PP for Ballistics 197

a) b)

Fig. 4. Aramid samples impacted at different velocities, a) with polypropylene matrix and b)

One particularly important parameter in this type of materials, and in composites in general, is the role played by fiber-matrix interfacial adhesion within mechanical resistance; this parameter which has become extremely important in the design of composite materials now represents a third entity in these materials. The following can give us an idea of the importance of this parameter; it is estimated that in 1 kg of PP with a CaC3 load at 50% weight fraction, with a nominal particle size of 5 µm, can give an interfacial adhesion area

The interface is defined as a bi-dimensional surface which divides two phases or components in a system; this is characterized by an abrupt change in properties and chemical composition. This surface does not possess a physical property in itself, since it only exists mathematically (see Fig. 5). The interphase, in contrast with the interface, is a layer in three dimensions surrounding the fiber with properties that are different from those of the fiber and the matrix (Fig. 5); this third entity is commonly used to improve load transfer from the matrix to the fiber, given the incapacity of the interface to carry out this task. Both interface and interphase fulfill the same purpose, to try to achieve load transfer as efficiently as possible from the matrix to the fiber; therefore, this parameter must be

Since load transfer is carried out via the interface, it is of vital importance to understand this parameter as it has considerable influence on the physical constants of the material, such as the elastic modulus, the Poisson ratio and tenacity of fracture. Due to the complexity of the

Fiber

Matrix

without polypropylene matrix

Interface

Interphase

between materials equal to three football fields 10.

controlled in order to determine the behavior of the material.

Fig. 5. Constituents commonly found in a fiber-matrix composite

The word aramid comes from the hybrid aromatic polyamide, where the main difference between the polyamides resides in the fact that 85% of the amide groups are linked to two aromatic rings 5. Aramid fibers are produced by extracting an acid solution with an appropriate precursor (a polycondensation produced between terephthaloyl chloride and pphenylenediamine) through a plate with small perforations. During this process, the aramid molecules acquire a high degree of orientation along the fiber, resulting in excellent tensile properties 6.

Due to the excellent specific resistance of this product, its most important application is in personal protection where its capacity to absorb impact energy from a projectile is extraordinary. The types of aramid most commonly used in ballistics are Kevlar 29, Kevlar 49, Kevlar 129 and Kevlar KM2, with important applications, such as the famous PASGT (Personal Armor System for Ground Troops) which have been the bulletproof vests used by US military from the 80's up to 2005, based on Kevlar 29. More recently, the bulletproof vests used during the intervention of troops in Iraq and Afghanistan were made from Kevlar KM2, an improved aramid for ballistics 4.

The ballistic resistance of armor vests made with highly flexible polymeric fibers is based on an energy absorption mechanism in which the impact load is transferred to a network of fibers (fabric) which are in contact with the projectile, making penetration resistance highly related to exclusive parameters of the fabric 7, thus, ballistic resistance is mainly dependent on the interaction between fabric nodes. It is necessary, therefore, to protect the fibers from environments that might degrade their properties. The fabric must be isolated as much as possible from humidity which is particularly harmful as it can affect the ballistic resistance of the fabric, either by degrading the fibers, lubricating them in excess or facilitating separation of the threads at the moment of contact with the projectile.

Friction between the nodes of armor plating promotes better load transfer between fiber bundles as it restricts their mobility. This is further increased by the polymeric matrix, thereby conferring the degree of armor fabric to the composite material (armor-grade composite). Restricting the movement of fibers increases interaction between them and generates other failure mechanisms (interlaminar and intralaminar delamination), which contribute to the process of energy absorption 8; however, the degree of movement restriction should not reach the point where it might generate fragility. Fig. 4 shows the difference between armor fabric with a polymeric matrix (Fig. 4a) and without a polymeric matrix (Fig. 4b). This figure clearly shows how the matrix limits the movement of the fibers, thereby making more of them interact with each other during an impact at medium speed (300 m/s approximately).

Armor-grade composite materials present several disadvantages, such as the sensitivity of their resistance to manufacturing procedures, the high cost of production processes, the sensitivity and difficulty in locating damage generated at low impact and, most importantly, the difficulty in modeling their mechanical behavior due to the number of parameters involved deriving from the large number of failure mechanisms that intervene in the energy absorption process during a ballistic event. Another disadvantage is the viscoelastic nature of polymeric materials which make them highly dependent on impact velocity 9.

The word aramid comes from the hybrid aromatic polyamide, where the main difference between the polyamides resides in the fact that 85% of the amide groups are linked to two aromatic rings 5. Aramid fibers are produced by extracting an acid solution with an appropriate precursor (a polycondensation produced between terephthaloyl chloride and pphenylenediamine) through a plate with small perforations. During this process, the aramid molecules acquire a high degree of orientation along the fiber, resulting in excellent tensile

Due to the excellent specific resistance of this product, its most important application is in personal protection where its capacity to absorb impact energy from a projectile is extraordinary. The types of aramid most commonly used in ballistics are Kevlar 29, Kevlar 49, Kevlar 129 and Kevlar KM2, with important applications, such as the famous PASGT (Personal Armor System for Ground Troops) which have been the bulletproof vests used by US military from the 80's up to 2005, based on Kevlar 29. More recently, the bulletproof vests used during the intervention of troops in Iraq and Afghanistan were made from

The ballistic resistance of armor vests made with highly flexible polymeric fibers is based on an energy absorption mechanism in which the impact load is transferred to a network of fibers (fabric) which are in contact with the projectile, making penetration resistance highly related to exclusive parameters of the fabric 7, thus, ballistic resistance is mainly dependent on the interaction between fabric nodes. It is necessary, therefore, to protect the fibers from environments that might degrade their properties. The fabric must be isolated as much as possible from humidity which is particularly harmful as it can affect the ballistic resistance of the fabric, either by degrading the fibers, lubricating them in excess or facilitating

Friction between the nodes of armor plating promotes better load transfer between fiber bundles as it restricts their mobility. This is further increased by the polymeric matrix, thereby conferring the degree of armor fabric to the composite material (armor-grade composite). Restricting the movement of fibers increases interaction between them and generates other failure mechanisms (interlaminar and intralaminar delamination), which contribute to the process of energy absorption 8; however, the degree of movement restriction should not reach the point where it might generate fragility. Fig. 4 shows the difference between armor fabric with a polymeric matrix (Fig. 4a) and without a polymeric matrix (Fig. 4b). This figure clearly shows how the matrix limits the movement of the fibers, thereby making more of them interact with each other during an impact at medium speed

Armor-grade composite materials present several disadvantages, such as the sensitivity of their resistance to manufacturing procedures, the high cost of production processes, the sensitivity and difficulty in locating damage generated at low impact and, most importantly, the difficulty in modeling their mechanical behavior due to the number of parameters involved deriving from the large number of failure mechanisms that intervene in the energy absorption process during a ballistic event. Another disadvantage is the viscoelastic nature of polymeric materials which make them highly dependent on impact

properties 6.

Kevlar KM2, an improved aramid for ballistics 4.

(300 m/s approximately).

velocity 9.

separation of the threads at the moment of contact with the projectile.

Fig. 4. Aramid samples impacted at different velocities, a) with polypropylene matrix and b) without polypropylene matrix

One particularly important parameter in this type of materials, and in composites in general, is the role played by fiber-matrix interfacial adhesion within mechanical resistance; this parameter which has become extremely important in the design of composite materials now represents a third entity in these materials. The following can give us an idea of the importance of this parameter; it is estimated that in 1 kg of PP with a CaC3 load at 50% weight fraction, with a nominal particle size of 5 µm, can give an interfacial adhesion area between materials equal to three football fields 10.

The interface is defined as a bi-dimensional surface which divides two phases or components in a system; this is characterized by an abrupt change in properties and chemical composition. This surface does not possess a physical property in itself, since it only exists mathematically (see Fig. 5). The interphase, in contrast with the interface, is a layer in three dimensions surrounding the fiber with properties that are different from those of the fiber and the matrix (Fig. 5); this third entity is commonly used to improve load transfer from the matrix to the fiber, given the incapacity of the interface to carry out this task. Both interface and interphase fulfill the same purpose, to try to achieve load transfer as efficiently as possible from the matrix to the fiber; therefore, this parameter must be controlled in order to determine the behavior of the material.

Fig. 5. Constituents commonly found in a fiber-matrix composite

Since load transfer is carried out via the interface, it is of vital importance to understand this parameter as it has considerable influence on the physical constants of the material, such as the elastic modulus, the Poisson ratio and tenacity of fracture. Due to the complexity of the

Advantages of Low Energy Adhesion PP for Ballistics 199

nature controlling ballistic events. The V50 is determined using the average velocity of six impacts, three which have totally perforated the armored plate and three which have partially perforated it with an interval not greater than 60 m/s between the six impact velocities. Other very important parameters determined in this type of tests are the

> 6 i i=1 50

The relationship between impact velocity and absorbed energy registers the amount of energy absorbed by the material at the moment of impact by a projectile at velocities equal or superior to its ballistic limit. The energy absorbed by the material (*Eabs*) is obtained based on the velocity at which the projectile impacts the sample and the velocity at which it exits at the back; these velocities are substituted in Equation 2, where *m* represents the mass of the projectile, ܸ the velocity at which the projectile impacts the sample and *Vres* the velocity at which the projectile exits the back of the material. The velocity at which the material is impacted is obtained with a Chrony chronograph which is capable of registering speeds between 10 and 2 134 m/s with a precision of 99.5%, conferring reliability to the readings.

V =

V

2 2 2 2 *imp res abs*

Ballistic gelatine is widely used in criminalistics due to the similarity of this material to the human body during high velocity impact. Due to the behavior presented by this material in response to a high velocity impact, it is possible to calculate the residual velocity with which the projectile impacted the material based on the depth of penetration. The velocity is obtained by characterizing the material during direct frontal impacts, where it is possible to generate an equation relating penetration length of the projectile with the velocity on entering the material. In some studies, such as the one carried out by Jorma Jussila 12 a more

Once the relationship between impact velocity and absorbed energy is obtained, a phenomenon quite particular to this impact regimen emerges; a fall in energy absorption at velocities slightly higher than V50 with a subsequent recovery in absorption levels. A logical deduction could be that when a material presents a particular absorption of energy at perforation threshold during high velocity impacts with a 1.11 g projectile, for example 100 Joules, when this projectile impacts at 120 J one might assume that the material will absorb its corresponding part (100 J) and will allow passage of the projectile with a residual energy of 20 J. However, in reality, this does not happen. With projectile impacts at velocities slightly higher than the ballistic limit, what we find is a noticeable reduction in the capacity of the material to absorb energy. A study carried out by Paul Wambua *et al.* 13, shows a composite of natural fiber with a PP matrix, where it can be observe this phenomenon at velocities close to 250 m/s. Fig. 6 shows the curve obtained with this

*mV mV <sup>E</sup>* (2)

(1)

6

relationship between impact velocity, absorbed energy and trauma depth.

Residual velocity was obtained with the aid of a ballistic gelatine.

detailed procedure for this methodology is presented.

particular behavior.

mathematical analysis required for this parameter, no studies were carried out until the last decade; in those studies the only indications of this value are reported with techniques such as pull-out test, which has resulted in considerable controversy due to the lack of a standard which can establish a specific methodology.

The methods for determining the interfacial shear stress can be direct or indirect. Direct methods are those which are analyzed from a micromechanics perspective, where a small representative sample of the unit is used. Some methods we can mention are fiber pull-out, fragmentation, single fiber micro-indentation and single fiber compression; however, due to the close relationship this parameter has with the mechanical properties of the composite, there are also indirect methods to determine interfacial shear stress, which are analyzed from a micromechanical perspective and where the behavior of the whole unit is analyzed in order to determine the levels of interfacial adhesion in the composite. These methods include the variable curvature method, slice compression, ball compression and bundle pullout. There are also some methods which are analyzed at a macromechanical level and by conventional tests; these are able to relate values of tension, compression or flexure at three or four points with interfacial adhesion values in the composite 11.

As it has already established, interfacial adhesion plays an important role in the properties of a material; however, what role does it play in high impact properties? Studies focusing on improved interfacial adhesion in composite materials at tension, compression and shear abound in the literature; however, in high impact it does not appear to be particularly sought after. This is demonstrated in a study carried out in Rohchoon Park, where an aramid/vynilester composite material is characterized at high impact. The material received a superficial treatment to improve adhesion and it was possible to observe a reduction in the ballistic limit when this property is increased. This is precisely where PP can play a very particular role. In addition to all the properties found in PP, it also presents poor interfacial adhesion with practically any material due to its incapacity to generate covalent links. The aim of this work, therefore, is to demonstrate how a polymer with poor interfacial adhesion can be used in applications with high levels of energy absorption by taking advantage of precisely its inert character to dissipate the energy through other mechanisms which are more efficient at high impact, such as back cone formation and load transmission from primary threads to secondary threads.
