A ballistic vest or bullet-proof vest is an item of armour that absorbs the impact from gun-fired projectiles and explosive fragments fired at the torso. Soft vests made from layers of tightly-woven fibres protect wearers from projectiles fired from handguns, shotguns, and shrapnel from explosives such as hand grenades. When metal or ceramic plates are used with a soft vest, it can also protect wearers from shots fired from rifles. Soft vests are commonly worn by police forces, private citizens and private security guards, and hard-plate reinforced vests are mainly worn by combat soldiers in the armies of various nations as well as armed response police forces.
Modern body armour may combine a ballistic vest with other items of protective clothing, such as a helmet. Vests intended for police and military use may also include ballistic shoulder armour for more protection and leg armour for protection against grenade blasts.
Vests may be augmented with metal (steel or titanium), ceramic or polyethylene plates that provide extra protection to vital areas. These hard armour plates have proven effective against all handgun bullets and a range of rifles. These "tactical body armour" vests have become standard in military use, as soft body armour vests are ineffective against most military rifle rounds. The CRISAT NATO (Collaborative Research Into Small Arms Technology-North Atlantic Treaty Organization) standard for body armour specifies the use of titanium backing. This titanium plate may be removable or sewn in.
A vest does not protect the wearer by deflecting bullets. Instead, the layers of material catch the bullet and spread its force over a larger portion of the body, absorbing energy more quickly and hopefully bringing it to a stop before it can penetrate into the body. This tends to deform the bullet, further reducing its ability to penetrate. While a vest can prevent bullet wounds, the wearer still absorbs the bullet's energy, which can cause blunt force trauma. The majority of users experience only bruising, but impacts can still cause severe internal injuries.
Most vests offer little protection against arrows, ice picks, stabbing knife blows, bullets with their points sharpened or armour-piercing rounds. As the force is concentrated in a relatively small area with bladed weapons and armour-piercing rounds, they can push through the weave of most bullet-resistant fabrics. Specially-designed vests which protect against bladed weapons and sharp objects are often used in vests for prison guards and other law enforcement officers. Some materials like Dyneema offer considerable protection against bladed weapons and slash attacks.
Ballistic vests may provide little or no protection against rifle ammunition or even against handgun ammunition fired from a pistol-calibre carbine. The exception is the common .22 LR ammunition, which can usually be stopped by these vests even when fired from a rifle. These vests are usually protective against handgun ammunition fired from handguns of all calibres, depending on the armour level of the vest.
The oldest bullet-resistant fabric vests were made from silk and they resembled medieval padded jacks, which used 18 to 30 layers of cloth to protect wearers from arrow penetration. In 1881 Dr. George Emery Goodfellow of Arizona witnessed a gunfight between two people. When he examined one man who had been shot through his breast pocket, he found that the bullet had been slowed by the layers of the man's silk handkerchief. Dr. Goodfellow documented various other cases of silk fabric protecting people from gunshot wounds, including a noted case where a man's bandanna saved his carotid artery from being severed.
Casimir Zeglen of Chicago, Illinois used Goodfellow's findings to develop a bullet-proof vest made of silk fabric at the end of the 1800s. Zeglen's expensive vests could stop the relatively slow rounds from black powder handguns. The vests cost $800 USD each in 1914, which is equivalent to about $15,000 in 2005 dollars. On June 28, 1914, Franz Ferdinand, Archduke of Austria, heir to the Austro-Hungarian throne was wearing a silk bullet-proof vest. However, the vest did not protect him, because he was shot in the neck above the vest with a .32 ACP bullet fired by Gavrilo Princip using a handgun.
World War I German Infantry-Panzer, 1918
During World War I, the United States developed several types of body armour, including the chrome nickel steel Brewster Body Shield, which consisted of a breastplate and a headpiece and could withstand Lewis Gun bullets at 2,700 ft/s (820 m/s), but was clumsy and heavy at 40 pounds (18 kg). Another type of body armour was designed in February 1918 by the Metropolitan Museum of Art. This breastplate was based on armour of the 1400s, weighed 27 pounds (12 kg), and was considered too noisy and restricting. A scaled waistcoat of overlapping steel scales fixed to a leather lining was also designed; this armour weighed 11 pounds (5 kg), fit close to the body, and was considered more comfortable.
During the late 1920s through the early 1930s, gunmen from criminal gangs in the United States began wearing less-expensive vests made from thick layers of cotton padding and cloth. These early vests could absorb the impact of handgun rounds such as .22, .25, S&W .32 Long, S&W .32, .380 ACP, and .45 ACP travelling at slower speeds of up to approximately 1000 ft/s (300 m/s). To overcome these vests, law enforcement agents such as the FBI began using the new .357 Magnum cartridge.
World War II
In the early stages of World War II, the United States designed body armour for infantrymen, but most models were too heavy and mobility-restricting. These armour vests were often incompatible with existing equipment as well. The military diverted its research efforts to developing "flak jackets" for aircraft crews. These flak jackets were made of nylon fabric and capable of stopping flak and shrapnel, but not bullets.
The Japanese army produced a few types of infantry body armour during World War II, but they did not see much use. Near the middle of 1944, development of infantry body armour in the United States restarted. Several vests were produced for the US military, including but not limited to the T34, the T39, the T62E1, and the M12.
Sn-42 Body Armour
There were several models of body armour in the Red Army, called SN-38, SN-39, SN-40, SN-40A, and SN-42 (The native Cyrillic abbreviation for the vest was СН, the Cyrillic letters Es and En.) "Stalynoi Nagrudnik" is Russian for "steel breastplate", and the number denotes the design year. All were combat tested, but only the SN-42 was put in production. It consisted of two pressed steel plates that protected the front torso and groin. The plates were 2 mm and weighed 3.5 kg. This armour was supplied to SHISBr (assault engineers) and to Tankodesantniki (infantry that rode on tanks) of some tank brigades. The SN armour protected wearers from the German MP-40 9 mm bullet at around 100-125 meters, which made it useful in urban battles (Stalingrad). However, the SN's weight made it impractical for infantry on foot in an open outdoor setting, and the 7.92x57mm cartridges fired by the Mauser Karabiner 98k and MG42 easily penetrated it.
During the Korean War several new vests were produced for the United States military, including the M-1951 (Chriss Body, 2002), "a vast improvement on weight, but the armour failed to stop bullets and fragments very successfully" (Military, 2004). The Vietnam war era vests were simply various combinations of the nylon and were still not capable of stopping Rifle rounds.
In 1969, American Body Armour was founded and began to produce a patented combination of quilted nylon faced with multiple steel plates. This armour configuration was marketed to American law enforcement agencies by the Smith & Wesson gun company under the trade name "Barrier Vest". The "Barrier Vest" was the first police vest to gain wide use during high threat police operations.
In the mid-1970s, the DuPont Corporation introduced (Kevlar) synthetic fibre, when woven into a fabric and layered. Immediately Kevlar was incorporated into a National Institute of Justice (NIJ) evaluation program to provide lightweight, concealable body armour to a test pool of American law enforcement officers to ascertain if everyday concealable wearing was possible. Lester Shubin, a program manager at the NIJ, managed this law enforcement feasibility study within a few selected large police agencies, and quickly determined that Kevlar body armour could be comfortably worn by police daily, and would save lives.
In 1975 Richard A. Armellino, the founder of American Body Armour marketed an all Kevlar vest called the K-15, comprised of 15 layers of Kevlar that also included a 5" X 8" ballistic steel "Shok Plate" positioned vertically over the heart and was issued U.S Patent #3,971,072 for this ballistic vest innovation. Similarly sized and positioned "trauma plates" are still used today on the front ballistic panels of most concealable vests, reducing blunt trauma and increasing ballistic protection in the centre-mass heart/sternum area.
In 1976, Richard Davis, founder of Second Chance Body Armor designed this company's first all-Kevlar vest, named the Model Y. The lightweight, concealable vest industry was launched and a new form of daily protection for the modern police officer was quickly adapted. By the mid to late 1980s, an estimated 1/3 to 1/2 of police patrol officers wore concealable vests daily. By the year 2006, more than 2,000 documented police vest "saves" were recorded, validating the success and efficiency of lightweight concealable body armour as a standard piece of everyday police equipment.
Kevlar soft armour had its shortcomings because if "large fragments or high velocity bullets hit the vest, the energy could cause life-threatening, blunt trauma injuries" in selected, vital areas (Military, 2004). So the Ranger Body Armor was developed for the American military in 1994. Although it was the second modern US body armour that was able to stop rifle calibre rounds and still be light enough to be worn by infantry soldiers in the field, it still had its flaws: "it was still heavier than the concurrently issued PASGT (Personal Armor System for Ground Troops) anti-fragmentation armour worn by regular infantry and ... did not have the same degree of ballistic protection around the neck and shoulders" (Military, 2004). The format of Ranger Body Armor (and more recent body armour issued to US special operations units) highlights the tradeoffs between force protection and mobility that modern body armour forces organizations to address.
The newer armour issued by the United States military to large numbers of troops is known as Interceptor Multi-Threat Body Armor System. The Kevlar Interceptor vest is intended mainly to provide shrapnel protection, but is rated for threats up to and including 9mm submachine gun fire. Small Arms Protective Insert (SAPI) plates, made of ceramic materials, are worn front and back and protect the vital organs from threats up to and including 7.62x51mm NATO rifle rounds.
Since the 1970s, several new fibers and construction methods for bullet-proof fabric have been developed besides woven Kevlar, such as DSM's Dyneema, Honeywell's GoldFlex and Spectra, Teijin Twaron's Twaron, Pinnacle Armor's Dragon Skin, and Toyobo's Zylon (now controversial, as new studies report that it degrades rapidly, leaving wearers with significantly less protection than expected). These newer materials are advertised as being lighter, thinner and more resistant than Kevlar, although they are much more expensive.
Both the Underwriters Laboratories (UL Standard 752) and the United States National Institute of Justice (NIJ Standard 0101.04) have specific performance standards for bullet resistant vests used by law enforcement. The US NIJ rates vests on the following scale against penetration and also blunt trauma protection (deformation) (Table from NIJ Standard 0101.04):
Bomb disposal officers often wear heavy armour designed to protect against most effects of a moderate sized explosion, such as bombs encountered in terror threats. Full head helmet, covering the face and some degree of protection for limbs is mandatory in addition to very strong armour for the torso. An insert to protect the spine is usually applied to the back, in case an explosion blasts the wearer. Visibility and mobility of the wearer may be severely limited.
In terms of Kevlar, a IIA vest has around sixteen layers and a IIIA vest around thirty layers.
German standards allow for bullet impact depression of 20 millimetres on the mannequin's wax body under the vest; US standards allow for more than twice that (44 millimetres), which can be potentially lethal.
In addition, there are vests available for police dogs which offer a measure of protection for the animals.
An Aramid vest's material must not get wet, because it will lose its protective capability until dry again, or in some cases be permanently degraded (water acts as a lubricant, helping the bullet slip through between the fibres; it may also weaken the structure of the fibre by breaking hydrogen bonds, see Kevlar for details). Most bullet-proof vests have panels in sealed enclosures, but waterproofing is usually not perfect. Dyneema and Spectra based vests do not have the same difficulties with water.
In recent years advances in material science have opened the door to the old idea of a literal "bullet-proof vest" that will be able to stop handgun and rifle bullets without the assistance of heavy and cumbersome extra metal or ceramic plating. Current soft body armour can stop most handgun rounds. Plates are currently needed to stop rifle rounds and unique handgun rounds such as 7.62x25. Research aims to develop artificial spider silk which could be super strong, yet light and flexible. Other research has been done to harness nanotechnology to help create super strong materials that could be used in future bullet-proof vests.
Currently, there are two methods by which nanomaterials are being implemented into body armour production. The first is based on nanoparticles within the suit that become rigid enough to protect the wearer as soon as a pressure threshold is surpassed, which the impact of a bullet would register. These nano-infused suits are significantly lighter than alternative forms of body armour because of the properties that govern them.The second was introduced in 2005 by American company ApNano. They developed a material that was always rigid, and announced that this nanocomposite based on Tungsten Disulfide was able to withstand shocks generated by a steel projectile travelling at velocities of up to 1.5 km/second. The material was also reportedly able to withstand shock pressures generated by the impacts of up to 250 tons per square centimetre. During the tests, the material proved to be so strong that after the impact the samples remained essentially unmarred. Additionally, a recent study in France tested the material under isostatic pressure and found it to be stable up to at least 350 tons/cm². As of mid-2006, spider silk bullet-proof vests and nano-based armours are being developed for potential market release
Body armour is legal in most countries. One exception is Australia, where body armour has been prohibited for some time. This ban may have its origins in the late 19th century, when the iconic Australian outlaw and folk hero Ned Kelly used home-made armour with mixed results. While the steel armour worn by Kelly defeated the soft lead, low velocity bullets fired by police Martini-Henry rifles, it greatly restricted his movement.
United States law restricts possession of body armour for convicted violent felons. Many US states also have penalties for possession or use of body armour by felons. In February of 1999, the late Russell Jones a.k.a. "Ol' Dirty Bastard" was arrested in California for possession of body armour by a convicted felon. In other states, such as Kentucky, they do not prohibit possession, but deny probation or parole for a person convicted of certain violent crimes while wearing body armour and carrying a deadly weapon.
Canadian legislation makes it legal to wear and to purchase body armour such as ballistic vests. However, there are current proposals to the legislation to make it illegal to wear such body armour during the commission of a criminal offence.
In medical terminology, blunt trauma, blunt injury, non-penetrating trauma or blunt force trauma refers to a type of physical trauma caused to a body part, either by impact, injury or physical attack; the latter usually being referred to as blunt force trauma. The term itself is used to refer to the precursory trauma, from which there is further development of more specific types of trauma, such as contusions, abrasions, lacerations, and/or bone fracturing.
Abdominal Trauma (BAT)
Blunt abdominal trauma is often referred to as the most common type of trauma, representing around 50 to 75 percent of blunt trauma. The majority of BAT is often attributed to car-to-car collisions, in which rapid deceleration often propels the driver forwards into the steering wheel or dashboard, causing contusions in less serious cases or rupturing of internal organs due to briefly increased intraluminal pressure in more serious cases where speed or forward force is greater.
Abdominal trauma caused by deceleration and impact shows a similar effect to trauma to any other part of the body; namely the rupturing or damage of free and relatively fixed objects, a classic example of such an injury would be a hepatic tear along the ligamentum teres followed with injuries to the renal arteries.
As with most trauma, blunt abdominal trauma is often the case of further injury, depending upon the severity of the accident. In the majority of cases, the liver and spleen (see Blunt splenic trauma) are most severely affected, followed by damage to the small intestine. Recent studies utilizing CT scanning have suggested that hepatic and other concomitant injuries may develop from blunt abdominal trauma.
In rare cases, BAT has been attributed to several medical techniques such as the heimlich manoeuvre, attempts at cardiopulmonary resuscitation, and manual thrusts to a clear an airway. Although these are rare causes of blunt abdominal trauma, it is often thought that they are caused by applying unnecessary pressure when administering such techniques.
Although blunt trauma is a condition in itself, the main emphasis on the diagnosis of blunt trauma is to ascertain the cause of the accident, any further injury and its correlation with the medical, dietary, and physiological history of the patient gathered from various sources, such as family and friends, or previous physicians, in order to establish the most swift path to recovery. This method is given the mnemonic "SITEMAP";
Usually, in the case of examination, areas such as the head or those linked with the respiratory system have a higher priority, and are examined before the abdomen, so as to administer, if necessary, medical treatments which will immediately limit the amount of progressive damage which could be caused from such injuries. The amount of time spent on diagnosing abdominal injury should be minimal, and expedited by using relatively quick methods of determining the extent of such injury, such as by identifying free intra-abdominal fluid through diagnostic peritoneal lavage (DPL) before recommending a laparotomy if the situation requires one.
Whenever any blunt trauma is sustained to the body, it is normal to ensure first that there is no bleeding, internal or back injury, or breathing problems before administering any type of rehabilitative care to the patient. In cases of car accidents, or where a patient has had some form of accelerated impact, the likelihood is that there will be progressive damage to internal organs, as well as the fracturing of bones, both of which are dealt with by splinting fractures and controlling external hemorrhaging. Most cases require IV therapy along with other methods of stabilisation such as securing the airway or providing a respirator.
Spider silk, also known as gossamer, is a fibre spun by spiders. Spider silk is a remarkably strong material. Its tensile strength is comparable to that of high-grade steel — according to Nature, spider dragline silk has a tensile strength of roughly 1.3 GPa, while one source lists a tensile strength for one form of steel at 1.65 GPa. However, spider silk is much less dense than steel; its tensile strength to density ratio is roughly five times higher than that of steel (i.e. it is five times as strong as steel of the same density — as strong as Aramid filaments, such as Twaron or Kevlar.) In fact, a strand of spider silk long enough to circle the earth would weigh less than 16 ounces (450 g).
A female specimen of Argiope appensa wraps her prey in silk.
Spiders normally use their silk to make structures, either for protection for their offspring, or for predation on other creatures. They can also suspend themselves using their silk, normally for the same reasons.
Many small spiders use silk threads for ballooning. They extrude several threads into the air and let themselves become carried away with upward winds. Although most rides will end a few meters later, it seems to be a common way for spiders to invade islands. Many sailors have reported that spiders have been caught in their ship's sails, even when far from land.
A garden spider spinning its web.
Structure of spider silk. Inside a typical fibre, one finds crystalline regions separated by amorphous linkages. The crystals are beta-sheets that have assembled together.
Spider silk is also especially ductile, able to stretch up to 40% of its length without breaking. This gives it a very high toughness (or work to fracture), which "equals that of commercial polyaramid (aromatic nylon) filaments, which themselves are benchmarks of modern polymer fibre technology."
The notion that spider silk is stronger than any industrial fibre is a common misconception as whilst some may be stronger, none are tougher (total energy to break). Numerous artificial fibres are similar or stronger, notably aramids like Kevlar and carbon fibre materials (see tensile strength for common comparisons). Nonetheless, there is much interest in duplicating the silk process artificially, since spiders use renewable materials as input and operate at room temperature, low pressures and using water as a solvent. Spider silk can be harvested in large scale quantities if one has proper harvesting equipment. One can also make near-indestructible spider silk threads by weaving the fine threads into thicker and more durable weaves in the same fashion as other industrial threads.
Spider silk is composed of complex protein molecules. This, coupled with the isolation relating from the spider's predatory nature, has made the study and replication of the substance quite challenging. Because of the repetitive nature of the DNA encoding the silk protein, it is difficult to determine its sequence and to date, silk-producing sequences have only been decoded for fourteen species of spider. In 2005, independent researchers in the University of Wyoming (Tian and Lewis), University of the Pacific (Hu and Vierra), the University of California at Riverside (Garb and Hayashi) and Shinshu University (Zhao and Nakagaki) have uncovered the molecular structure of the gene for the protein that various female spider species use to make their silken egg cases.
Although different species of spider, and different types of silk, have different protein sequences, a general trend in spider silk structure is a sequence of amino acids (usually alternating glycine and alanine, or alanine alone) that self-assemble into a beta sheet conformation. These "Ala rich" blocks are separated by segments of amino acids with bulky side-groups. The beta sheets stack to form crystals, whereas the other segments form amorphous domains. It is the interplay between the hard crystalline segments, and the elastic semi amorphous regions, that gives spider silk its extraordinary properties.
The unspun silk dope is pulled through silk glands, resulting in a transition from stored gel to final solid fibre. Many species of spider have different glands for different jobs, such as housing and web construction, defence, capturing and detaining prey, mobility and in extreme cases even as food. Thus, different specialized silks have evolved with material properties optimized for their intended use.
The gland's visible, or external, part is termed the spinneret. Depending on the species, spiders will have anything from two to eight spinnerets, usually in pairs. The beginning of the gland is rich in thiol and tyrosine groups. After this beginning process, the ampulla acts as a storage sac for the newly created fibres. From there, the spinning duct effectively removes water from the fibre and through fine channels also assists in its formation. Lipid secretions take place just at the end of the distal limb of the duct, and proceeds to the valve. The valve is believed to assist in rejoining broken fibres, acting much in the way of a helical pump.
Various compounds other than protein are used to enhance the fibre's properties. Pyrrolidine has hygroscopic properties and helps to keep the thread moist. It occurs in especially high concentration in glue threads. Potassium hydrogen phosphate releases protons in aqueous solution, resulting in a pH of about 4, making the silk acidic and thus protecting it from fungus and bacteria that would otherwise digest the protein. Potassium nitrate is believed to prevent the protein from denaturating in the acidic milieu.
Peasants in the southern Carpathian Mountains used to cut up tubes built by Atypus and cover wounds with the inner lining. It reportedly facilitated healing, and even connected with the skin. This is believed to be due to antiseptic properties of spider silk (which is made of protein) Some fishermen in the indo-pacific ocean use the web of Nephila to catch small fish. Spider silk, normally that of the golden orb spider, is occasionally harvested and spun into usable textiles. Due to the difficulty of the process, the resulting fabric is invariably extremely expensive, and is generally utilized in medical applications.
At one time, it was common to use spider silk as a thread for crosshairs in telescopes, microscopes and similar optical instruments.
Artificial spider silk
Spider silk's properties have made it the target of industrial research efforts. It is not generally considered possible to use spiders themselves to produce industrially useful quantities of spider silk, due to the difficulties of managing large quantities of small spiders (although it was tried with Nephila silk). Compared with silkworms, spiders are aggressive and will eat one another, making it inadvisable to keep many spiders together in the same space. Other efforts have involved extracting the spider silk gene and using other organisms to produce the required amount of spider silk. In 2000, Nexia, a Canadian biotechnology company, was successful in producing spider silk protein in transgenic goats. These goats carried the gene for spider silk protein, and the milk produced by the goats contained significant quantities of the protein. Attempts to spin the protein into a fibre similar to natural spider silk failed, however. The spider's highly sophisticated spinneret is instrumental in organizing the silk proteins into strong domains. Specifically, the spinneret creates a gradient of protein concentration, pH, and pressure, which drive the protein solution through liquid crystalline phase transitions, ultimately generating the required silk structure (which is a mixture of crystalline and amorphous biopolymer regions). Replicating these complex conditions in lab environment has proved difficult. Nexia attempted to press the protein solution through small extrusion holes in order to simulate the behaviour of the spinneret, but this was insufficient to properly organize the fibres. Ultimately, Nexia was forced to abandon research on artificial spider silk, despite having successfully created the silk protein in genetically modified organisms. Extrusion of protein fibres in an aqueous environment is known as 'wet-spinning'. This process has so far produced silk fibres of diameters ranging from 10-60 μm, compared to diameters of 2.5-4 μm seen in natural spider silk.
Nanocomposites are materials that are created by introducing nanoparticulates (often referred to as filler) into a macroscopic sample material (often referred to as the matrix). This is part of the growing field of nanotechnology. After adding nanoparticulates to the matrix material, the resulting nanocomposite may exhibit drastically enhanced properties. For example, adding carbon nanotubes tends to drastically add to the electrical and thermal conductivity. Other kinds of nanoparticulates may result in enhanced optical properties, dielectric properties or mechanical properties such as stiffness and strength. In general, the nanosubstance is dispersed into the matrix during processing. The percentage by weight (called mass fraction) of the nanoparticulates introduced is able to remain very low (on the order of 0.5% to 5%) due to the incredibly high surface area to volume ratio of nanoparticulates. Much research is going into developing more efficient combinations of matrix and filler materials and into better controlling the production process.
Advantages of Nanosized Additions
The Nanocomposites 2000 conference has revealed clearly the property advantages that nanomaterial additives can provide in comparison to both their conventional filler counterparts and base polymer. Properties which have been shown to undergo substantial improvements include:
· Mechanical properties e.g. strength, modulus and dimensional stability
· Decreased permeability to gases, water and hydrocarbons
· Thermal stability and heat distortion temperature
· Flame retardancy and reduced smoke emissions
· Chemical resistance
· Surface appearance
· Electrical conductivity
· Optical clarity in comparison to conventionally filled polymers
Aramid fibres are a class of heat-resistant and strong synthetic fibres. They are used in aerospace and military applications, for ballistic rated body armour fabric, and as an asbestos substitute. The name is a shortened form of "aromatic polyamide". They are fibres in which the chain molecules are highly oriented along the fibre axis, so the strength of the chemical bond can be exploited.
Aromatic polyamides were first introduced in commercial applications in the early 1960s, with a meta-aramid fibre produced by DuPont under the trade name Nomex. This fibre, which has the hand of normal textile apparel fibres, is characterised by its excellent resistance to heat, as it neither melts nor ignites in normal levels of oxygen. It is used extensively in the production of protective apparel, air filtration, thermal and electrical insulation as well as a substitute for asbestos. Meta-aramid is also produced in the Netherlands and Japan by Teijin under the trade name Teijinconex, in China by Yantai under the trade name New Star and a variant of meta-aramid in France by Kermel under the trade name Kermel.
Based on earlier research by Monsanto and Bayer, a fibre - para-aramid - with much higher tenacity and elastic modulus was also developed in the 1960s-1970s by DuPont and Akzo Nobel, both profiting from their knowledge of rayon, polyester and nylon processing.
Much work was done by Stephanie Kwolek in 1961 while working at DuPont, and that company was the first to introduce a para-aramid called Kevlar in 1973. A similar fibre called Twaron with roughly the same chemical structure was introduced by Akzo in 1978. Due to earlier patents on the production process, Akzo and DuPont had a patent war in the 1980s. Twaron is currently owned by the Teijin company (see Production).
The Federal Trade Commission definition for aramid fibre is:
A manufactured fibre in which the fibre-forming substance is a long-chain synthetic polyamide in which at least 85% of the amide linkages, (-CO-NH-) are attached directly to two aromatic rings.
World capacity of para-aramid production is estimated at about 41.000 tons/yr in 2002 and increases each year by 5-10%. In 2007 this means a total production capacity of around 55.000 tons/yr.
Aramids are generally prepared by the reaction between an amine group and a carboxylic acid halide group. Simple AB homopolymers may look like:
nNH2-Ar-COCl → -(NH-Ar-CO)n- + nHCl
The most well-known aramids (Nomex, Kevlar and Twaron) are AABB polymers. Nomex or Teijinconex contain predominantly the meta-linkage and are poly-metaphenylene isophtalamides (MPIA). Kevlar and Twaron are both p-phenylene terephtalamides (PPTA), the simplest form of the AABB para-polyaramide. PPTA is a product of p-phenylene diamine (PPD) and terephtaloyl dichloride (TDC or TCl). Production of PPTA relies on a co-solvent with an ionic component (calcium chloride (CaCl2)) to occupy the hydrogen bonds of the amide groups, and an organic component (N-methyl pyrrolidone (NMP)) to dissolve the aromatic polymer. Prior to the invention of this process by Leo Vollbracht, who worked at the Dutch chemical firm Akzo, no practical means of dissolving the polymer was known. The use of this system led to a patent war between Akzo and DuPont.
After production of the polymer, the aramid fibre is produced by spinning the solved polymer to a solid fibre from a liquid chemical blend. Polymer solvent for spinning PPTA is generally 100% (water free) sulphuric acid (H2SO4).
Other types of aramids
Beside meta-aramids like Nomex, other variations belong to the aramid fibre range. These are mainly of the copolyamide type, best known under the brand name Technora, as developed by Teijin and introduced in 1976. The manufacturing process of Technora reacts PPD and 3,4'-diaminodiphenylether (3,4'-ODA) with terephtaloyl chloride (TCl). This relatively simple process uses only one amide solvent and therefore spinning can be done directly after the polymer production.
Aramid fibre characteristics
Aramids share a high degree of orientation with other fibres such as Ultra high molecular weight polyethylene, a characteristic which dominates their properties.
Ultra high molecular weight polyethylene (UHMWPE), also known as high modulus polyethylene (HMPE) or high performance polyethylene (HPPE), is a thermoplastic. It has extremely long chains, with molecular weight numbering in the millions, usually between 2 and 6 million. The longer chain serves to transfer load more effectively to the polymer backbone by strengthening intermolecular interactions. 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 (15 times more resistant to abrasion than carbon steel). Its coefficient of friction is significantly lower than that of nylon and acetal, and is comparable to teflon, but UHMWPE has better abrasion resistance than teflon. It is odourless, tasteless, and non-toxic.
Polymerisation of UHMWPE was commercialised in the 1950s by Ruhrchemie AG, which changed names over the years; today UHMWPE powder materials are produced by Ticona. UHMWPE is available commercially either as consolidated forms, such as sheets or rods, and as fibres. UHMWPE powder may also be directly moulded into the final shape of a product. Because of its resistance to wear and impact, UHMWPE continues to find increasing industrial applications, including the automotive and bottling sectors, for example. Since the 1960s, UHMWPE has also been the material of choice for total joint arthroplasty in orthopaedic and spine implants .
UHMWPE fibres, commercialised in the late 1970s by the Dutch chemicals company DSM, are widely used in ballistic protection, defence applications, and increasingly in medical devices as well.
Structure and properties
Structure of UHMWPE, with n greater than 100,000
UHMWPE is a type of polyolefin 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 fibres, 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 fibres. Its melting point is around 144 to 152 degrees Celsius, and according to DSM, it is not advisable to use UHMWPE fibres 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 fibre 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.
To anneal UHMWPE the material should be heated to 135 °C to 138 °C in an oven or a liquid bath of silicone oil or glycerine. The material must then be cooled down at a rate of 5 °C / hour to at least 65 °C. Finally the material should be wrapped in an insulating blanket for 24 hours to bring to room temperature.
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 metallocene catalysts. 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.
Finished UHMWPE is produced by 4 major methods: compression moulding, ram extrusion, gel spinning, and sintering. The leading manufacturers of each process UHMWPE in different ways:
Dyneema fibres are made using a DSM patented (1979) method called gel spinning. A precisely heated gel of UHMWPE is processed by an extruder through a spinneret. The extruded is drawn through the air and then cooled in a water bath. The end result is a fibre with a high degree of molecular orientation, and therefore exceptional tensile strength. Gel spinning depends on isolating individual chain molecules in the solvent so that intermolecular entanglements are minimal. Entanglements make chain orientation more difficult, and lower the strength of the final product.
Production capacity of DSM Dyneema is about 5000-6000 tons/yr by 2008 as the 10th production line will be finished in 2008. 5 lines are located in Heerlen, The Netherlands and 5 in Greenville, North Carolina (USA).
Dyneema is a registered trademark of Royal DSM N.V. (The Netherlands). 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 a production facility in Greenville, North Carolina which is the largest production facility in the United States for UHMWPE fibre.
Honeywell developed a chemically identical product, which is sold under the brand name Spectra. Though the production details are somewhat different, the resulting materials are comparable in properties.
Dyneema and Spectra are gel spun through a spinneret to form oriented-strand synthetic fibres of UHMWPE which have yield strengths as high as 2.4 GPa and density as low as 0.97 kg/l (for Dyneema SK75). This gives a strength/weight ratio as much as 15 times stronger than steel and up to 40% stronger than Aramid.
UHMWPE fibres are used in bullet-proof vests, bow strings, climbing equipment, fishing line, spear lines for spear guns, high performance sails, suspension lines on sport parachutes, rigging in yachting, kites and kites lines for kites sports. Spectra is also used as a high-end Wakeboard line.
For body armour, the fibres are generally aligned and bonded into sheets, which are then layered at various angles to give the resulting composite material strength in all directions. Recently developed additions to the US Military's Interceptor body armour, designed to offer arm and leg protection, are said to utilise a form of Spectra or Dyneema fabric.
Equipment used for climbing includes cord and webbing made of combinations of Dyneema and nylon yarns. Dyneema "Slings", multi-purpose sewn loops of webbing, have gained popularity for their low weight and bulk, though unlike their nylon counterparts they exhibit very low elasticity, making them unsuitable for limiting forces in a fall. Also, low elasticity translates to low toughness. Dyneema's very high lubricity leads to poor knot holding ability, and has led to the recommendation to use the triple fisherman's knot rather than the traditional double fisherman's knot in 6mm Dyneema core cord to avoid a particular failure mechanism of the double fisherman's, where first the sheath fails at the knot, then the core slips through.
It is also used in both skis, and snowboards often in combination with carbon fibre, reinforcing the fibreglass composite material, adding stiffness and improving its flex characteristics. The UHMWPE is often used as the base layer, that contacts the snow and is structured with abrasives to absorb and retain wax.
High-performance ropes for sailing and parasailing are made of Dyneema as well. Dyneema is the preferred material for sport kite lines for two main reasons. First the low stretch means that control inputs to the kite are transferred quickly and secondly the low friction allows the kite to remain controllable up to about ten twists in the line.
Dyneema was used for the 30-kilometre space tether in the failed ESA/Russian Young Engineers' Satellite 2 of September, 2007.
The extremely low friction coefficient of UHMWPE makes it a common top sheet for boxes in terrain parks.