The polyurethane urea elastomer (PUU), or poly (urethane urea) elastomer, is a flexible polymeric material that is composed of linkages made out of

polyurethane Polyurethane (; often abbreviated PUR and PU) is a class of polymers composed of organic chemistry, organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethane term ...

and

polyurea Polyurea is a type of elastomer that is derived from the reaction product of an isocyanate component and an amine component. The isocyanate can be aromatic or aliphatic in nature. It can be monomer, polymer, or any variant reaction of isocyanate ...

compounds. Due to its hyperelastic properties, it is capable of bouncing back high-speed ballistic projectiles as if the material had “hardened” upon impact. PUUs were developed by researchers from the U.S. Army Research Laboratory (ARL) and the Army’s Institute for Soldier Nanotechnology at the Massachusetts Institute of Technology (MIT) to potentially replace

polyethylene Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging (plastic bags, plastic films, geomembranes and containers including bott ...

materials in

body armor Body armour, personal armour (also spelled ''armor''), armoured suit (''armored'') or coat of armour, among others, is armour for human body, a person's body: protective clothing or close-fitting hands-free shields designed to absorb or deflect ...

and other protective gear, such as combat helmets, face shields, and

ballistic vests Ballistics may refer to: Science * Ballistics, the science that deals with the motion, behavior, and effects of projectiles ** Forensic ballistics, the science of analyzing firearm usage in crimes ** Internal ballistics, the study of the proce ...

Composition

In general, PUUs are composed of both hard and soft segments that each play a role in the material’s physical properties. The soft segments consist of two types of chemical compounds, long-chain

polyols In organic chemistry, a polyol is an organic compound containing multiple hydroxyl groups (). The term "polyol" can have slightly different meanings depending on whether it is used in food science or polymer chemistry. Polyols containing two, thre ...

and diisocyanates, that react and connect together with urethane linkages. On the other hand, the short-chain

diamines A diamine is an amine with two amino groups. Diamines are used as monomers to prepare polyamides, polyimides, and polyureas. The term ''diamine'' refers mostly to Primary (chemistry), primary diamines, as those are the most reactive. In terms of ...

react with the diisocyanates to form the hard segments that are held together with urea linkages. The mechanical properties of the PUU largely depend on the specific diisocyanates, long-chain polyols, and short-chain diamines in play, because how these components interact determines how well the soft and hard segments of the elastomers both

crystallize Crystallization is a process that leads to solids with highly organized atoms or molecules, i.e. a crystal. The ordered nature of a crystalline solid can be contrasted with amorphous solids in which atoms or molecules lack regular organization ...

and undergo microphase separation. As a result, variations in this molecular arrangement of chemical compounds have been shown to greatly affect the elastomer’s morphology and the macroscopic, mechanical properties that it exhibits.

Hyperelastic behavior

In 2017, researchers from the Army Research Laboratory and MIT reported that PUUs are capable of demonstrating hyperelastic properties, meaning that the material becomes extremely hardened upon being deformed within a very short time. As a result, the material may withstand ballistic impacts at exceptionally high speeds. For the study, the researchers investigated the performance of different PUU variants where 4,4’-dicyclohexylmethane diisocyanate ( HMDI) was chosen as the diisocyanate compound, diethyltoluenediamine (DETA) was chosen as the short-chain diamine compound, and poly(tetramethyleneoxide) (PTMO) was chosen as the long-chain polyol compound. Despite consisting of the same chemical compounds with the same stoichiometric ratio of 2:1:1 of DMI

ETA Eta ( ; uppercase , lowercase ; ''ē̂ta'' or ''ita'' ) is the seventh letter of the Greek alphabet, representing the close front unrounded vowel, . Originally denoting the voiceless glottal fricative, , in most dialects of Ancient Greek, it ...

TMO the samples differed regarding the

molecular weight A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...

of their respective PTMO component, namely , , and , for the soft segments of the elastomers. Each of the three samples were subjected to a laser-induced projectile impact test (LIPIT), which tested the dynamic response of the material by using a pulsed laser to shoot it with microparticles made of

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...

at speeds ranging from . The researchers found that the sample with the PTMO was the most rigid variant with the particle exhibiting a shallow penetration of about upon impact despite travelling at before rebounding at . In contrast, the sample with the PTMO displayed a deeper penetration of about , but had a slower particle rebound of , making it the most rubber-like among the PUU samples. The strain-rates associated with these impacts were on the order of 2.0 x 10^8/s for the former and 8.1 x 10^7/s for the latter. However, all three PUU variants demonstrated rebound capabilities with no signs of post-mortem damage after impact from the microparticles. In contrast, when the LIPIT was performed on a ductile, glassy

polycarbonate Polycarbonates (PC) are a group of thermoplastic polymers containing carbonate ester, carbonate groups in their chemical structures. Polycarbonates used in engineering are strong, toughness, tough materials, and some grades are optically transp ...

at similar speeds to that of the PTMO PUU variant, the polycarbonate displayed predominant deformation upon impact, despite its high

fracture toughness In materials science, fracture toughness is the critical stress intensity factor of a sharp Fracture, crack where propagation of the crack suddenly becomes rapid and unlimited. It is a material property that quantifies its ability to resist crac ...

and ballistic strength. According to the researchers, the effectiveness of the PUUs may come from how the molecules “resonate” similar to chain-mail upon impact with each oscillations at specific frequencies dissipate the absorbed energy. In comparison, the polycarbonate lacked the broad range of relaxation times, a characteristic that reflects how efficiently the molecules in the polymer chains respond to an external impulse, that PUUs are known to have. As a result, the researchers concluded that even the most rubber-like variant of the PUU, specifically the PTMO sample, demonstrated greater robustness and dynamic stiffening than the glassy polycarbonate. ARL researchers have stated that the PUU’s primary benefit comes not from its extra strength but its fabric-like flexibility, which demonstrates its potential as a replacement material for the rigid ceramic and metal plates generally found in military battle armor. However, as of 2018, the PUU is still under development in the testing phase.

References

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External links

PU Polyurethane Open Belts
Military technology Polyurethanes Plastics Elastomers Body armor