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Materials Used in Armor Products

Aramid
Polyethylene
Dyneema®

In the construction of composite when using a ceramic strike face of Alumina, Boron Carbide or Silicon Carbide, a backing material is required.

The reinforcing composite technology most commonly used for rifle armor is Aramid (Kevlar), Twaron, or Polyethylene (UHMWPE). In some cases, Fiberglass S-2 can be used where cost is the driving factor.

From Wikipedia, the free encyclopedia:

Aramid

Kevlar (poly-paraphenylene terephthalamide) is the DuPont Company’s brand name for a synthetic material constructed of para-aramid fibers that the company claims is five times stronger than the same weight of steel, while being lightweight, flexible and comfortable. It is also very heat resistant and decomposes above 400 °C without melting. It was invented by Stephanie Kwolek of DuPont from research into high performance polymers, and patented by her in 1966 and first marketed in 1971. Kevlar is a registered trademark of E.I. du Pont de Nemours and Company.

Originally intended to replace the steel belts in tires, it is probably the most well known name in soft armor (bulletproof vests). It is also used in extreme sports equipment, high-tension drumhead applications, animal handling protection, compositeaircraft construction, fire suits, yacht sails, and as an asbestos replacement.

When this polymer is spun in the same way that a spider spins a web, the resulting commercial para-aramid fiber has tremendous strength, and is heat and cut resistant. Para-aramid fibers do not rust or corrode, and their strength is unaffected by immersion in water. When woven together, they form a good material for mooring lines and other underwater objects. However, unless specially waterproofed, para-aramid fiber’s ability to stop bullets and other projectiles is degraded when wet.

Kevlar is a type of aramid that consists of long polymeric chains with a parallel orientation. Kevlar derives its strength from inter-molecular hydrogen bonds and aromatic stacking interactions between aromatic groups in neighboring strands. These interactions are much stronger than the van der Waals interaction found in other synthetic polymers and fibers like Dyneema. The presence of salts and certain other impurities, especially calcium, would interfere with the strand interactions and has to be avoided in the production process. Kevlar consists of relatively rigid molecules,which form a planar sheet-like structure similar to silk protein.

Polyparaphenylene Terephthalamide Intermolecular Hydrogen Bonding

These properties result in its high mechanical strength and its remarkable heat resistance. Because it is highly unsaturated, i.e. the ratio of carbon to hydrogen atoms is quite high, it has a low flammability. Kevlar molecules have polar groups accessible for hydrogen bonding. Water that enters the interior of the fiber can take the place of bonding between molecules and reduce the material's strength, while the available groups at the surface lead to good wetting properties. This is important for bonding the fibers to other types of polymer, forming a fibre reinforced plastic. This same property also makes the fibers feel more natural and "sticky" compared to nonpolar polymers like polyethylene.

In structural applications, Kevlar fibers can be bonded to one another or to other materials to form a composite.

Kevlar's main weaknesses are that it decomposes under alkaline conditions or when exposed to chlorine. While it can have a great tensile strength, sometimes in excess of 4.0 GPa, like all fibers it tends to buckle in compression.

Twaron is the Teijin Company’s brand name for a synthetic material constructed of para-aramid fibers.

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PE. POLYETHYLENE
(Ultra High Molecular Weight Polyethylene)

15 times stronger than steel and up to 40% stronger than Kevlar.

Manufacturer: DSM, Brandname: Dyneema®
Manufacturer: Honeywell, Brandname: Spectra®

Ultra high molecular weight polyethylene (UHMWPE), also known as high modulus polyethylene (HMPE) or high performance polyethylene (HPPE), is a thermoplastic made from oil. It has extremely long chains, with molecular weight numbering in the millions, usually between 3.1 and 5.67 million. The high molecular weight results a very good packing of the chains into the crystal structure. This results in a very tough material, with the highest impact strength of any thermoplastic presently made. It is highly resistant to corrosive chemicals, with exception of oxidizing acids. It has extremely low moisture absorption, very low coefficient of friction, is self lubricating and is highly resistant to abrasion (10 times more resistant to abrasion than Carbon Steel). Its coefficient of friction is significantly better than nylon and acetal, and is comparable to teflon, but UHMWPE has better abrasion resistance than teflon. It is odorless, tasteless, and nontoxic.

UHMWPE finds use in high modulus fibers (for example, Spectra or Dyneema) for bulletproof vests. Due to its low friction and wear resistance it is used in industrial impact, wear, and sliding applications in both normal and corrosive environment. It is also used in orthopaedicimplants (artificial hips, knees).

Structure and properties

Structure of UHMWPE, with n greater than 100,000

UHMWPE is a type of olefin and, despite relatively weak Van der Waals bonds between its molecules, derives ample strength from the length of each individual molecule. It is made up of extremely long chains of polyethylene, which all align in the same direction. Each chain is bonded to the others with so many Van der Waals bonds that the whole can support great tensile loads.

When formed to fibers, the polymer chains can attain a parallel orientation greater than 95% and a level of crystallinity of up to 85%. In contrast, Kevlar derives its strength from strong bonding between relatively short molecules.

The weak bonding between olefin molecules allows local thermal excitations to disrupt the crystalline order of a given chain piece-by-piece, giving it much poorer heat resistance than other high-strength fibers. Its melting point is around 144 or 152 degrees Celsius, and according to DSM, it is not advisable to use UHMWPE fibers at temperatures exceeding 80 to 100°C for long periods of time. It becomes brittle at temperatures below -150°C.

The simple structure of the molecule also gives rise to surface and chemical properties that are rare in high-performance polymers. For example, the polar groups in most polymers easily bond to water. Because olefins have no such groups, UHMWPE does not absorb water readily, but it also does not get wet easily, which makes bonding it to other polymers difficult. For the same reasons, skin does not interact with it strongly, making the UHMWPE fiber surface feel slippery. Similarly, aromatic polymers are often susceptible to aromatic solvents due to aromatic stacking interactions, an effect aliphatic polymers like Dyneema are also immune to. Since Dyneema does not contain chemical groups (such as esters, amides or hydroxylic groups) that are susceptible to attack from aggressive agents, it is very resistant to water, moisture, most chemicals, UV radiation, and micro-organisms.

Under tensile load, UHMWPE will deform continually as long as the stress is present - an effect called creep.

Production

UHMWPE is synthesized from monomers of ethylene, which are bonded together to form what is called ultra high molecular weight polyethylene (or UHMWPE). These are molecules of polyethylene which are several orders of magnitude longer than familiar, high density polyethylene due to a synthesis process based on metallocenecatalysts. HDPE molecules generally have between 700 and 1,800 monomer units per molecule, while UHMWPE molecules tend to have 100,000 to 250,000 monomers each. The material is also known as high modulus polyethylene (HMPE) or high performance polyethylene (HPPE). The polymers are aligned randomly when they are produced. To make fibers like Dyneema, they are dissolved and drawn into fibers as the solvent evaporates, causing the polymer chains to orient in the direction of the fiber.

The production of UHMWPE demands relatively little energy and uses no aggressive chemicals. The product can easily be recycled, so environmental pollution from product and process is minimal.

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Dyneema® (UHMWPE)

Dyneema® is a synthetic fiber based on UHMWPE, 15 times stronger than steel and up to 40% stronger than Kevlar. It is usually used in bulletproof vests, bow strings, climbing equipment and high performance sails in yachting. Dyneema® was invented by DSM in 1979. It has been in commercial production since 1990 at a plant in Heerlen, the Netherlands. In the Far East, DSM has a cooperation agreement with Toyobo Co. for commercial production in Japan. In the United States, DSM has granted a license to Allied Signal. Spectra is the brandname of the chemically identical product developed independently by Allied Signal (now Honeywell) in the USA. Though the production details will undoubtedly be different, the result is comparable to Dyneema. In this article, we will refer to both Dyneema and Spectra as Dyneema. Dyneema® is a registered trademark of Royal DSM N.V. (The Netherlands).

Dyneema fibers derive their strength from the extreme length of each individual molecule. The fibre can attain a parallel orientation greater than 95% and a level of crystallinity of up to 85%. In contrast, Kevlar derives its strength from strong bonding between relatively short molecules.

Its melting point is around 144 or 152 degrees Celsius, and according to DSM, it is not advisable to use Dyneema at temperatures exceeding 80 to 100 °C for long periods of time. It becomes brittle at temperatures below −150 °C. This contrasts strongly with other high-performance fibers, which tend to be quite heat-resistant.

The fibers feel slippery, similar to polypropylene and other hydrophobic fibers. Most people do not like the way Dyneema feels; for this reason, it is not often used in fabric. The slipperiness also makes such fibers less suitable for use in fibre reinforced plastics.

Another problem, in some applications, is that Dyneema will creep, meaning it will deform when placed under any long term stress. Like other olefins, it is very resistant to water, moisture, most chemicals, UV radiation, and microorganisms.

To make Dyneema fibers, a solution of UHMWPE is drawn into fibers as the solvent evaporates, causing the polymer chains to orient in the direction of the fiber.

Recently developed additions to the US Military's Interceptor body armor, designed to offer arm and leg protection, are said to utilize a form of Dyneema fabric.

It is also used in snowboards, often in combination with carbon fiber, reinforcing the fiberglasscomposite material, adding stiffness and improving its flex characteristics.

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