Thermoplastic Elastomers (TPE)

Properties

Thermoplastic elastomers (TPE), sometimes referred to as thermoplastic rubbers (TPR), are either blends of two or more polymers or special types of block copolymers. These elastomers combine the performance benefits of rubber with the easy processability of thermoplastics, but are more versatile than either material. The majority of thermoplastic elastomers are block-copolymers based on elastic and rigid blocks. TPEs are usually diblock copolymers (AB) with one restraining and one flexible block or they are triblock copolymers (ABA) with a hard segment at each polymer end of the elastomer. Other types include branched and star-shaped (A-B)_n types and combinations of these types. The elastic blocks have typically a much higher molecular weight than the hard and rigid blocks. The latter self-agglomerate to glassy or crystalline restraining domains that function as physical crosslinks which prevent plastic deformation but allow for elastic deformation of the amorphous flexible blocks. Both the hard and soft domains contribute to the mechanical and physical properties of the thermoplastic elastomer. The elastic block should have a high molecular weight and all of the other characteristics required of an ordinary rubber whereas the hard block(s) should have a low molecular weight to minimize their negative impact on elasticity. When these materials are cooled from the melt, the hard blocks self-assemble into small crystalline or glassy domains (physical crosslinks) that resist creep and viscous flow under load whereas the elastic portion of the polymer remains amorphous and soft.

TPEs can be formulated to exhibit a wide range of physical and mechanical properties unachievable by a rubber or plastic alone. The hard domains provide plastic properties such as high-temperature performance, thermoplastic processability, as well as good tensile and tear strength whereas the rubbery phase provides the elastomeric properties such as low-temperature performance, durometer hardness (resistance to indentation), flexibility as well as compression and tension set. Like ordinary thermoplastics, TPEs can be processed on conventional, high-volume injection molding and extrusion equipment. The lack of chemical crosslinks in TPEs, however, results in some major limitations; since TPEs melt or soften when the temperature reaches the melting or softening point of the hard phase, they are unsuitable for applications requiring even short-term exposure to temperatures above their melting/softening point. In other words, the reinforcing action of the hard phase vanishes above its softening point and the TPE behaves like a viscous liquid. Furthermore, at high extensions and when under stress for longer times, TPEs undergo permanent deformation due to viscous flow. This deformation is known as creep. Chemical cross-linking, on the other hand, suppresses viscous flow. However, cross-linked elastomers cannot be molten and thus cannot be reprocessed and recycling is much more challenging. The absence of crosslinks also results in much more swelling when the TPO is immersed in a non-polar solvent or exposed to solvent vapor.

TPEs can be classified by their chemical composition. The five commercially most important TPEs are styrenic block copolymers (SBCs), thermoplastic olefins (TPOs), thermoplastic polyurethanes (TPUs), copolyesters (COPE), and copolyamides (COPA). These block copolymers are often blended with other polymers, and with oils, fillers and other additives, which allows for a versatile modification of product properties.

Styrenic Block Copolymers (SBCs) are copolymers of polystyrene (hard blocks) and a low-molecular-weight diene (soft elastomeric block) such as polybutadiene, polyisoprene or poly(ethylene–butylene). The latter provides improved heat resistance. The two most common SBCs are triblock copolymers of styrene and butadiene (SBS) and styrene and isoprene (SIS). In many regards, SBCs have properties that are similar to those of vulcanized butadiene rubber but can be easily extruded and molded using conventional thermoplastic processing equipment. However, SBCs are less resilient than chemically crosslinked (vulcanized) butadiene rubber and thus do not recover as efficiently from deformation as vulcanized diene elastomers.

Thermoplastic polyurethane elastomers (TPUs) are low priced thermoplastic elastomers. The hard and soft segments are formed by the reaction of a diisocyanate with a polyester or polyether diol (chain extender). These transparent elastomers have high tensile and impact strength, good elasticity and good low temperature flexibility. They are also highly resistant to abrasion, ozone, aliphatic solvents, and petroleum based fuels and oils. The properties of TPUs can be varried over a wide range by the choice of the soft elastic midblock (ether or ester polyol), and rigid endblocks, i.e. the type of isocyanate that forms the restraining urethane blocks.

Polyamide block copolymers (COPAs) are polyester-amide, polyether-esteramide, or polyether-amide block copolymers. The polyamide portion forms the hard (thermoplastic) domains, while the polyester, polyether-ester or polyether blocks form the soft elastomeric matrix. COPAs have a relative high service temperature when compared to other types of TPEs due to the strong hydrogen-bonding amide domains which results in low creep, good heat aging, and improved solvent resistance. They retain operative properties up to about 150 - 175°C depending on the amide block, midblock, and composition.

Copolyester elastomers (COPE), also known as polyether-ester block copolymers (TPC-ET), are composed of a hard crystalline polyester and a long-chain soft amorphous polyether block. This type of TPE possesses excellent (low temperature) flexibility and flex fatigue along with good resistance to weathering, chemicals, wear and heat which are far better than those of conventional rubber. The properties can be adjusted over a wide range by altering the ratio and type of hard to soft segments. COPEs combine the excellent properties of high-performance elastomers with those of polyester plastics.

Thermoplastic polyolefin blends (TPOs) typically consists of ethylene propylene rubber (EPR) or ethylene propylene diene (EPDM) as the elastomeric component and polypropylene as the thermoplastic component. Other polymers sometimes used in TPO blends include low-cost olefins such as LDPE, HDPE, and LLDPE and copolymers of ethylene and other monomers like ethylene vinyl acetate (EVA), ethylene-ethylacrylate (EEA), and ethylene-methyl-acrylate (EMA). TPOs are low-cost elastomers but also have one of the lowest performance among thermoplastic elastomers. They are typically used when the maximum service temperature is low and good fluid resistance is not required and when a high level of creep and set is acceptable.