- Email: [email protected]
Ballistic composites – protecting the protectors
Reinforced Plastics Volume 00, Number 00 October 2015
www.reinforcedplastics.com
FEATURE
Ballistic composites – protecting the protectors George Marsh It seems scarcely credible that thin polymer fibers, bound together in resin, can stop projectiles ranging from a hand-gun bullet to a high-power rifle round, but they can and have done so, saving many lives in the process. Composite body armor protects a wide range of civilians from security guards to police officers and from bailiffs to VIPs. But of course, it is military forces who are the leading user group. Traditional solutions for 20th Century military armor, based chiefly on steel and ceramic plates, were really too heavy for soldiers, indeed for many vehicles as well. Composites have increasingly proved to be the answer, being much lighter for the same stopping power and more pliable. Certain polymer composites show, when appropriately engineered, remarkable energy dissipation properties, being able to absorb the kinetic energy from bullets and other high-speed projectiles before these can harm their human targets. They can also protect against knives. Various mechanisms account for this. Pushing fibers aside against their stiffness and the hold exerted by the composite they are part of, absorbs energy and the greater the number of fibers encountered, the stronger is the effect. Still more energy is absorbed as fibers become stretched during contact with projectiles, elongation-before-break being an important variable for armor designers. A third mechanism is that of delamination, whereby energy is absorbed in parting fibers from their resin containment medium. Yet another mechanism occurs in woven fabrics where the woven intersections slow down the shock waves propagated along the fibers from the impact point, absorbing energy as they do so. This does, however, increase the strain within the material and when this exceeds what the material can tolerate, penetration can occur. Hence whether to use wovens or non-wovens is a matter for careful consideration by protection designers (Figs. 1–3). According to ballistic specialists with Swiss-headquartered composites firm Gurit AG, stopping a bullet has three distinct stages. First is the blunting of the projectile so that its penetrating power is E-mail address: [email protected].
degraded. Second is a slowing phase and third is the catching of the round so that it is retained within the protective garment. Laminates are designed to maximize the effectiveness of these stages. Outer layers that provide controlled delamination on impact are effective in deforming the tip of a projectile, thereby blunting it. Certain non-polymer composites, for instance those that are ceramic based, may be harder than polymer composites and therefore more effective in this phase. Even so, reinforced plastics often provide the best balance of weight, anti-ballistic performance and cost. Underlying composite layers subsequently absorb kinetic energy progressively as more and more fibers are displaced and
FIG. 1
Flack jackets based on Kevlar or other proprietary aramid can be worn under normal clothing by both men and women. Image licensed by Shutterstock.
0034-3617/ß 2015 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.repl.2015.10.003
1 Please cite this article in press as: G. Marsh, Reinf. Plast. (2015), http://dx.doi.org/10.1016/j.repl.2015.10.003
REPL-451; No of Pages 4 FEATURE
Reinforced Plastics Volume 00, Number 00 October 2015
pockets incorporated in the garment. Such inserts are typical of protection that is rifle ballistic rated since fabrics alone cannot normally stop a rifle round traveling at 800 m/s whereas they can prevent penetration by a hand gun bullet traveling at half that speed. Although body armor designed to defeat rifle fire is inevitably more rigid than fabric-only garments, keeping the armor inserts separated ensures an adequate level of garment flexibility for most purposes.
Nylon start
FEATURE
FIG. 2
Kevlar helmet with camouflage cover and protective goggles. Image licensed by Shutterstock.
stretched. Composites used in body armor, where structural strength is less of a requirement than in vehicle protection, generally have very high fiber content, up to 80% or more, and specialized polymer fibers are used. Even in armor that has a ceramic outer face, this is generally backed by composite laminate to meet the slowing and catching requirements, including of penetrative fragments that are expelled from improvised explosive devices (IEDs) and fragmentation rounds (such fragments are known as spall). IEDs deliver blast, fragments and fire so that combined protection against these is desirable. Materials that mitigate these effects as well as being environmentally tolerant – to moisture, heat, etc. – are in demand. Another required attribute is that they should be able to resist not just single hits but also repeated shots, as from a machine gun. Body armor is typically based on woven and non-woven fabrics that are flexible enough to provide wearers with freedom of movement while still possessing high anti-ballistic properties. Many garments incorporate extra protection in strategic areas such as over the heart; this can take the form of plates of composite, ceramic, metal or hybrid material inserted into containment
An early example of the transition from metals to composites for personnel protection was the British Army’s GS Mk 6 combat helmet, issued during the 1960s. This had an outer shell of ballistic nylon impregnated with a 50:50 mix of phenoformaldehyde and polyvinyl butyral (PVB) resins, this matrix accounting for 20% of the composite’s weight. The shell comprised 23 layers of plainweave 290 g/m2 nylon, weighing 1 kg for the medium size of helmet. An inner impact absorbing layer of high-density polyethylene (HDPE) foam brought this weight up to 1.3 kg. Fiberglass, and even combined nylon and fiberglass, were also tried during those early years, providing some benefits over nylon. During the 1970s, the US Army was to adopt para-aramid fibers for head protection, albeit in resins similar to those used by the British. PVB-based resins were readily Beta-staged into a prepreg that could then be hot-molded to the tight complex curvatures required for combat helmets. Aramids, the best known of which are DuPont’s Kevlar and Twaron from Teijin (the Netherlands and Japan), subsequently spread from helmets into body protection. The Kevlar ‘flak jacket’, for instance, became almost a generic, initially for ballistic vests that protect against fragments from shells, grenades and other munitions, but also later for garments that are substantially bullet-proof. Para-aramid fibers have a rigid rod-like molecular structure that provides high-tensile strength, high elongation-to-break and good damage tolerance. They are also inherently non-flammable. Today they tend to be embedded in more contemporary matrices, typically epoxy or phenolic. The resulting composites are heavily fiber dominated. Like other specialists in ballistic fibers, DuPont Protection Technologies has worked hard to optimize its base material
FIG. 3
From police officers to soldiers, Dyneema Force Multiplier Technology is protecting personnel around the world. 2 Please cite this article in press as: G. Marsh, Reinf. Plast. (2015), http://dx.doi.org/10.1016/j.repl.2015.10.003
REPL-451; No of Pages 4
for ballistic use. Refined molecular structures in products like Kevlar XP and Kevlar M5 combine high strength with superior thermal and flame resistance. According to global business director Jeroen B. Jacobs, Kevlar is, weight for weight, five times stronger than steel and, in its various forms optimized for different environments and user groups, has saved thousands of lives. Despite the dominance of Kevlar and other aramids, glass still survives as an armor fiber. There is a place for the higher-performance fibers, such as S-glass used in preference to the more general E-glass. According to Gurit, this raises ballistic tolerance by 10–20%. Although this company’s recently launched PF700 ballistic prepreg is mainly intended as a vehicle armor rather than for personnel protection applications, it is worthy of mention here because it exemplifies once again the sheer versatility of glass fiber and its unsuspected utility in meeting tough anti-ballistic requirements. PF700 uses the fibers in combination with phenolic resin, the latter being preferred over epoxy due to its flame resistance, even though epoxy may be stronger. Carbon and boron have been experimented with but the brittle nature of these fibers has restricted their use in anti-ballistic applications.
Later fibers Latterly there has been penetration of the market by high performing fibers based on ultra high-density polyethylene (UHDPE). These include Dyneema from DSM Dyneema, part of Dutch concern Royal DSM, and Spectra from Honeywell in the United States. These are claimed to be 15 or more times stronger than steel and up to 40% stronger than aramid on a weight for weight basis. Spectra fibers have a tensile modulus of 900–1500 g per denier, compared with less than 500 g/d for steel or glass. Both fiber brands have been used in formulations designed to protect against bullets fired from, inter alia, powerful AK47 assault rifles. On the downside for UHDPE fibers are a lower thermal tolerance, limited drapability and high cost. Moreover, it is difficult to weave these stiff fibers, hence ballistic vests made with them tend not to be woven but to utilize parallel spun fibers instead. Nevertheless, Honeywell says that the high resistance to water, chemicals and ultra-violet light that its Spectra fibers possess, along with their remarkable strength, are making them a material of choice for personnel protection. The company’s proprietary Spectra Shield is a prepreg laminate comprising U/D fiber tape impregnated with thermoplastic rubber-based resin film. (Some helmets utilize thermoset resins, used to contribute the structural strength needed when soldiers sit on their helmets!) Alternating 08 and 908 plies are press-consolidated to form a flexible laminate material that can be sold to customers in rolls. Honeywell Shield is used in expanded small arms protective inserts (ESAPI), each of these comprising a ceramic strike plate backed by a Shield spall liner. Shield was recently selected by Reed Composite Solutions for the AMUR body armor plate inserts it produces for law enforcement and military armor vests. Meanwhile, Korean body armor manufacturer Dae-Sung is using Spectra Shield II material in combat helmets that weigh a fifth less than their predecessors. Spectra Shield and Gold Shield (the latter is a hybrid that also incorporates aramid fibers) ballistic materials are the basis for a new generation of stronger, lighter combat helmets under evaluation for the US Army.
FEATURE
Dyneema is a gel-spun multiple-filament UHDPE fiber that combines high strength, high modulus in the fiber direction and resistance to most chemicals. Gel spinning involves melting the PE polymer but also dissolving it in a solvent so that molecular chains become realigned, the resulting high alignment enhancing fiber strength. The solvent is then extracted. Low diameter fibers are produced in a range of strengths and densities to suit different ballistic applications. Another polymer fiber, polyphenylene-benzobisoxiazole (PBO) that emerged in the 1990s, notably under the trade name Zylon (Toyoba, Japan), has 1.6 times the tensile strength of aramid, but has a high cost. An all-PBO ballistic vest would probably be very expensive but the fact that such a garment could be made much thinner for equivalent performance could be a considerable draw if costs could be reduced. A US company started using PBO in protection for police officers in 1998, but five years later withdrew the product after an officer died despite having been wearing a PBO vest when shot. Degradation in properties over time proved to be the issue.
Ballistic optimization There are so many variables ballistic garment designers must juggle – material amount, fiber volume fraction, fiber type and diameter, woven/non-woven, fabric geometry, fiber sizing, ply count, fiber/resin bond strength, resin type and process parameters, and so on – that optimization can be a considerable challenge, requiring careful experimentation and modeling. Designers optimize their products in different ways according to the types of fiber they are dealing with. For example, Spectra and Dyneema are difficult to wet out and so lend themselves to material forms that are highly fiber dominated, as in ‘starved’ laminates. On impact, much energy is absorbed through fiber elongation. A glass composite, on the other hand, with its strong resin-fiber bonds, will tend to dissipate impact energy through delamination, so tailoring interfacial bond strength can influence the amount of energy that can be absorbed in the process. It is well known that fabric woven with a large number of densely packed, fine-denier fibers is a better impact absorber than a thicker, coarser fabric. The way material is laid up can also have a strong influence on energy dissipation. 0–908 cross-ply forms, for instance, enable energy to be spread orthogonally whereas a component that is long and narrow might benefit from a unidirectional lay-up. Stacking para-aramid material in layers, and perhaps stitching them together with z direction fibers, enables a projectile’s kinetic energy to be distributed volumetrically, giving further scope for harmless dissipation. Sometimes a material manufacturer can provide benefits ancillary to effective optimization. For example, Teijin Aramid BV, producer of high-performance Twaron aramid fiber, maximized the ‘multiple fine fibers effect’ in its Laminated Fabric Technology (LFT), which utilizes woven microfiber yarns sandwiched between thermoplastic film layers. This results in a very light protective material that is used in soft body armor. Interestingly, Teijin says its ballistic materials are recyclable and the company buys discarded material back for conversion into other forms such as pulp for use as an asbestos replacement. This, along with guaranteed certificated demilitarization, means that used items can be turned into a revenue stream. 3
Please cite this article in press as: G. Marsh, Reinf. Plast. (2015), http://dx.doi.org/10.1016/j.repl.2015.10.003
FEATURE
Reinforced Plastics Volume 00, Number 00 October 2015
REPL-451; No of Pages 4 FEATURE
Then there are novel and emergent forms, some of which differ from established formats. Certain material hybrids, such as combined steel and polymer fibers or ceramic/polymer fibers, embedded in resin, are non-conventional composites that expand the available
Reinforced Plastics Volume 00, Number 00 October 2015
materials portfolio. Future possibilities include the use of nano particles, which experimenters report can harden ballistic composites while, further ahead still, composites that incorporate graphene may prove able to augment that vital ‘protection for the protectors‘.
FEATURE 4 Please cite this article in press as: G. Marsh, Reinf. Plast. (2015), http://dx.doi.org/10.1016/j.repl.2015.10.003
www.reinforcedplastics.com
FEATURE
Ballistic composites – protecting the protectors George Marsh It seems scarcely credible that thin polymer fibers, bound together in resin, can stop projectiles ranging from a hand-gun bullet to a high-power rifle round, but they can and have done so, saving many lives in the process. Composite body armor protects a wide range of civilians from security guards to police officers and from bailiffs to VIPs. But of course, it is military forces who are the leading user group. Traditional solutions for 20th Century military armor, based chiefly on steel and ceramic plates, were really too heavy for soldiers, indeed for many vehicles as well. Composites have increasingly proved to be the answer, being much lighter for the same stopping power and more pliable. Certain polymer composites show, when appropriately engineered, remarkable energy dissipation properties, being able to absorb the kinetic energy from bullets and other high-speed projectiles before these can harm their human targets. They can also protect against knives. Various mechanisms account for this. Pushing fibers aside against their stiffness and the hold exerted by the composite they are part of, absorbs energy and the greater the number of fibers encountered, the stronger is the effect. Still more energy is absorbed as fibers become stretched during contact with projectiles, elongation-before-break being an important variable for armor designers. A third mechanism is that of delamination, whereby energy is absorbed in parting fibers from their resin containment medium. Yet another mechanism occurs in woven fabrics where the woven intersections slow down the shock waves propagated along the fibers from the impact point, absorbing energy as they do so. This does, however, increase the strain within the material and when this exceeds what the material can tolerate, penetration can occur. Hence whether to use wovens or non-wovens is a matter for careful consideration by protection designers (Figs. 1–3). According to ballistic specialists with Swiss-headquartered composites firm Gurit AG, stopping a bullet has three distinct stages. First is the blunting of the projectile so that its penetrating power is E-mail address: [email protected].
degraded. Second is a slowing phase and third is the catching of the round so that it is retained within the protective garment. Laminates are designed to maximize the effectiveness of these stages. Outer layers that provide controlled delamination on impact are effective in deforming the tip of a projectile, thereby blunting it. Certain non-polymer composites, for instance those that are ceramic based, may be harder than polymer composites and therefore more effective in this phase. Even so, reinforced plastics often provide the best balance of weight, anti-ballistic performance and cost. Underlying composite layers subsequently absorb kinetic energy progressively as more and more fibers are displaced and
FIG. 1
Flack jackets based on Kevlar or other proprietary aramid can be worn under normal clothing by both men and women. Image licensed by Shutterstock.
0034-3617/ß 2015 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.repl.2015.10.003
1 Please cite this article in press as: G. Marsh, Reinf. Plast. (2015), http://dx.doi.org/10.1016/j.repl.2015.10.003
REPL-451; No of Pages 4 FEATURE
Reinforced Plastics Volume 00, Number 00 October 2015
pockets incorporated in the garment. Such inserts are typical of protection that is rifle ballistic rated since fabrics alone cannot normally stop a rifle round traveling at 800 m/s whereas they can prevent penetration by a hand gun bullet traveling at half that speed. Although body armor designed to defeat rifle fire is inevitably more rigid than fabric-only garments, keeping the armor inserts separated ensures an adequate level of garment flexibility for most purposes.
Nylon start
FEATURE
FIG. 2
Kevlar helmet with camouflage cover and protective goggles. Image licensed by Shutterstock.
stretched. Composites used in body armor, where structural strength is less of a requirement than in vehicle protection, generally have very high fiber content, up to 80% or more, and specialized polymer fibers are used. Even in armor that has a ceramic outer face, this is generally backed by composite laminate to meet the slowing and catching requirements, including of penetrative fragments that are expelled from improvised explosive devices (IEDs) and fragmentation rounds (such fragments are known as spall). IEDs deliver blast, fragments and fire so that combined protection against these is desirable. Materials that mitigate these effects as well as being environmentally tolerant – to moisture, heat, etc. – are in demand. Another required attribute is that they should be able to resist not just single hits but also repeated shots, as from a machine gun. Body armor is typically based on woven and non-woven fabrics that are flexible enough to provide wearers with freedom of movement while still possessing high anti-ballistic properties. Many garments incorporate extra protection in strategic areas such as over the heart; this can take the form of plates of composite, ceramic, metal or hybrid material inserted into containment
An early example of the transition from metals to composites for personnel protection was the British Army’s GS Mk 6 combat helmet, issued during the 1960s. This had an outer shell of ballistic nylon impregnated with a 50:50 mix of phenoformaldehyde and polyvinyl butyral (PVB) resins, this matrix accounting for 20% of the composite’s weight. The shell comprised 23 layers of plainweave 290 g/m2 nylon, weighing 1 kg for the medium size of helmet. An inner impact absorbing layer of high-density polyethylene (HDPE) foam brought this weight up to 1.3 kg. Fiberglass, and even combined nylon and fiberglass, were also tried during those early years, providing some benefits over nylon. During the 1970s, the US Army was to adopt para-aramid fibers for head protection, albeit in resins similar to those used by the British. PVB-based resins were readily Beta-staged into a prepreg that could then be hot-molded to the tight complex curvatures required for combat helmets. Aramids, the best known of which are DuPont’s Kevlar and Twaron from Teijin (the Netherlands and Japan), subsequently spread from helmets into body protection. The Kevlar ‘flak jacket’, for instance, became almost a generic, initially for ballistic vests that protect against fragments from shells, grenades and other munitions, but also later for garments that are substantially bullet-proof. Para-aramid fibers have a rigid rod-like molecular structure that provides high-tensile strength, high elongation-to-break and good damage tolerance. They are also inherently non-flammable. Today they tend to be embedded in more contemporary matrices, typically epoxy or phenolic. The resulting composites are heavily fiber dominated. Like other specialists in ballistic fibers, DuPont Protection Technologies has worked hard to optimize its base material
FIG. 3
From police officers to soldiers, Dyneema Force Multiplier Technology is protecting personnel around the world. 2 Please cite this article in press as: G. Marsh, Reinf. Plast. (2015), http://dx.doi.org/10.1016/j.repl.2015.10.003
REPL-451; No of Pages 4
for ballistic use. Refined molecular structures in products like Kevlar XP and Kevlar M5 combine high strength with superior thermal and flame resistance. According to global business director Jeroen B. Jacobs, Kevlar is, weight for weight, five times stronger than steel and, in its various forms optimized for different environments and user groups, has saved thousands of lives. Despite the dominance of Kevlar and other aramids, glass still survives as an armor fiber. There is a place for the higher-performance fibers, such as S-glass used in preference to the more general E-glass. According to Gurit, this raises ballistic tolerance by 10–20%. Although this company’s recently launched PF700 ballistic prepreg is mainly intended as a vehicle armor rather than for personnel protection applications, it is worthy of mention here because it exemplifies once again the sheer versatility of glass fiber and its unsuspected utility in meeting tough anti-ballistic requirements. PF700 uses the fibers in combination with phenolic resin, the latter being preferred over epoxy due to its flame resistance, even though epoxy may be stronger. Carbon and boron have been experimented with but the brittle nature of these fibers has restricted their use in anti-ballistic applications.
Later fibers Latterly there has been penetration of the market by high performing fibers based on ultra high-density polyethylene (UHDPE). These include Dyneema from DSM Dyneema, part of Dutch concern Royal DSM, and Spectra from Honeywell in the United States. These are claimed to be 15 or more times stronger than steel and up to 40% stronger than aramid on a weight for weight basis. Spectra fibers have a tensile modulus of 900–1500 g per denier, compared with less than 500 g/d for steel or glass. Both fiber brands have been used in formulations designed to protect against bullets fired from, inter alia, powerful AK47 assault rifles. On the downside for UHDPE fibers are a lower thermal tolerance, limited drapability and high cost. Moreover, it is difficult to weave these stiff fibers, hence ballistic vests made with them tend not to be woven but to utilize parallel spun fibers instead. Nevertheless, Honeywell says that the high resistance to water, chemicals and ultra-violet light that its Spectra fibers possess, along with their remarkable strength, are making them a material of choice for personnel protection. The company’s proprietary Spectra Shield is a prepreg laminate comprising U/D fiber tape impregnated with thermoplastic rubber-based resin film. (Some helmets utilize thermoset resins, used to contribute the structural strength needed when soldiers sit on their helmets!) Alternating 08 and 908 plies are press-consolidated to form a flexible laminate material that can be sold to customers in rolls. Honeywell Shield is used in expanded small arms protective inserts (ESAPI), each of these comprising a ceramic strike plate backed by a Shield spall liner. Shield was recently selected by Reed Composite Solutions for the AMUR body armor plate inserts it produces for law enforcement and military armor vests. Meanwhile, Korean body armor manufacturer Dae-Sung is using Spectra Shield II material in combat helmets that weigh a fifth less than their predecessors. Spectra Shield and Gold Shield (the latter is a hybrid that also incorporates aramid fibers) ballistic materials are the basis for a new generation of stronger, lighter combat helmets under evaluation for the US Army.
FEATURE
Dyneema is a gel-spun multiple-filament UHDPE fiber that combines high strength, high modulus in the fiber direction and resistance to most chemicals. Gel spinning involves melting the PE polymer but also dissolving it in a solvent so that molecular chains become realigned, the resulting high alignment enhancing fiber strength. The solvent is then extracted. Low diameter fibers are produced in a range of strengths and densities to suit different ballistic applications. Another polymer fiber, polyphenylene-benzobisoxiazole (PBO) that emerged in the 1990s, notably under the trade name Zylon (Toyoba, Japan), has 1.6 times the tensile strength of aramid, but has a high cost. An all-PBO ballistic vest would probably be very expensive but the fact that such a garment could be made much thinner for equivalent performance could be a considerable draw if costs could be reduced. A US company started using PBO in protection for police officers in 1998, but five years later withdrew the product after an officer died despite having been wearing a PBO vest when shot. Degradation in properties over time proved to be the issue.
Ballistic optimization There are so many variables ballistic garment designers must juggle – material amount, fiber volume fraction, fiber type and diameter, woven/non-woven, fabric geometry, fiber sizing, ply count, fiber/resin bond strength, resin type and process parameters, and so on – that optimization can be a considerable challenge, requiring careful experimentation and modeling. Designers optimize their products in different ways according to the types of fiber they are dealing with. For example, Spectra and Dyneema are difficult to wet out and so lend themselves to material forms that are highly fiber dominated, as in ‘starved’ laminates. On impact, much energy is absorbed through fiber elongation. A glass composite, on the other hand, with its strong resin-fiber bonds, will tend to dissipate impact energy through delamination, so tailoring interfacial bond strength can influence the amount of energy that can be absorbed in the process. It is well known that fabric woven with a large number of densely packed, fine-denier fibers is a better impact absorber than a thicker, coarser fabric. The way material is laid up can also have a strong influence on energy dissipation. 0–908 cross-ply forms, for instance, enable energy to be spread orthogonally whereas a component that is long and narrow might benefit from a unidirectional lay-up. Stacking para-aramid material in layers, and perhaps stitching them together with z direction fibers, enables a projectile’s kinetic energy to be distributed volumetrically, giving further scope for harmless dissipation. Sometimes a material manufacturer can provide benefits ancillary to effective optimization. For example, Teijin Aramid BV, producer of high-performance Twaron aramid fiber, maximized the ‘multiple fine fibers effect’ in its Laminated Fabric Technology (LFT), which utilizes woven microfiber yarns sandwiched between thermoplastic film layers. This results in a very light protective material that is used in soft body armor. Interestingly, Teijin says its ballistic materials are recyclable and the company buys discarded material back for conversion into other forms such as pulp for use as an asbestos replacement. This, along with guaranteed certificated demilitarization, means that used items can be turned into a revenue stream. 3
Please cite this article in press as: G. Marsh, Reinf. Plast. (2015), http://dx.doi.org/10.1016/j.repl.2015.10.003
FEATURE
Reinforced Plastics Volume 00, Number 00 October 2015
REPL-451; No of Pages 4 FEATURE
Then there are novel and emergent forms, some of which differ from established formats. Certain material hybrids, such as combined steel and polymer fibers or ceramic/polymer fibers, embedded in resin, are non-conventional composites that expand the available
Reinforced Plastics Volume 00, Number 00 October 2015
materials portfolio. Future possibilities include the use of nano particles, which experimenters report can harden ballistic composites while, further ahead still, composites that incorporate graphene may prove able to augment that vital ‘protection for the protectors‘.
FEATURE 4 Please cite this article in press as: G. Marsh, Reinf. Plast. (2015), http://dx.doi.org/10.1016/j.repl.2015.10.003