Overview of Thermoplastics Elastomer
Thermoplastic elastomers are a unique class of materials that combine the key processing and recycling properties of thermoplastics with many of the physical properties of thermoset rubbers such as elasticity, low compression set and high flexibility. In many aspects one can describe TPE as two phase materials consisting of a rubbery elastomeric component and thermoplastic rigid component. The elastomeric phase determines rubberlike properties such as elasticity, softness, flexibility, compression set and minimum service temperature. The rigid phase determines thermoplastic properties such as hardness, processability, recyclability and maximum service temperature.
The first commercial TPE’s were thermoplastic polyurethanes (TPE_U) developed in Bayer laboratories. Over the years TPE product family and application areas grew significantly. Even today TPE’s are one of the fastest growing polymer markets. We can classify thermoplastic elastomers into two main groups: Block copolymer based TPE’s and Elastomer/Hard Polymer Blends. Block copolymer can further be divided into two subdivisions: Copolymers with amorphous rigid segments (styrenic block copolymers TPE_S) and copolymers with crystalline rigid segments (polyurethanes TPE_U, copolyesters TPE_E, copolyamides TPE_A). Elastomer/Hard Polymer blends also are further divided into blends where the elastomeric phase is not vulcanized TPE_O and blends where elastomeric phase is dynamically vulcanized during mixing TPE_V. The structure of each group is summarized in Figure 1 (Classification)
The general properties of thermoplastic elastomers are summarized below:
• Retention of elastomeric properties at a broad range of service temperature
• Wide range of hardness
• Fatigue resistance
• Chemical resistance
• Abrasion Resistance
• Weathering and ozone resistance
• Ease of processing
• Part economics
Enplast is currently producing TPE_V’s under the trade name Enflex®; TPE_O and TPE_S under the trade name Ensoft®. In the following the characteristics of these three product ranges are described.
Styrenic Block Copolymers (TPE_S)
Styrenic block copolymers are based on A-B-A type block structure where A is the hard phase and B is the elastomer. Styrenic block copolymers that are practical interest for TPE applications are SBS and SEBS. In these copolymers the elastomeric B phase is the main constituent and the morphology of the polymer is as presented in Figure 1. This morphology is formed by microscopic scale phase separation of the polystyrene end block and polydiene/ethylene-butylene mid-block, due to their chemical incompatibility . The separated hard polystyrene domains, embedded in the continuous elastomeric matrix, act as physical cross-links. During processing polystyrene domains soften and the material becomes fluid making material thermoplastic. In general SBS and SEBS copolymers are not used alone but as compounds in combination with a thermoplastic (such as PP or PS) a plasticizer oil and fillers. This allows the compounder to formulate a product range with a wide spectrum of properties tailored to the specific application. The unsaturated (butadiene) mid block of SBS makes its compounds more sensitive to the attack of Ozone, UV and limits their upper service temperature. As such their applications in technical items are limited and are mainly used in footwear. SEBS, produced through hydrogenation of the unsaturated butadiene mid block, has a stable structure that can resist weathering and withstand higher temperatures. Compounds based on SEBS can be used in technical items and demanding applications (appliances, automotive, pharmaceutical, profiles, seals…).
Elastomer/Hard Polymer Blends
Another way of getting TPE’s is by mechanically blending an elastomer and a thermoplastic material. If the elastomeric phase is uncured the two component forms interdispersed (co-continuous) blend morphology. (Figure 1) If the elastomeric phase is cured during mechanical blending (a process called dynamic vulcanization) then an island-matrix type morphology is obtained (Figure1). The most common uncured blends are thermoplastic polyolefin elastomers (TPO). These are blends of copolymer polypropylene (PPC) and ethylene-propylene-diene (EPDM) or ethylene-propylene (EPM) rubber. Due to absence of physical or chemical cross-links the elastic recovery and compression set of TPO’s are limited and they are used mainly in low cost technical applications requiring toughness and low temperature flexibility (automotive). Thermoplastic vulcanizates (TPV) that are made through dynamic vulcanization of PP/EPDM blends are the other important commercial product range. In these blends obtaining a good degree of cure and a finely dispersed rubber phase is crucial. In TPV’s the rubber phase is cross-linked as a result rubberlike properties (such as elastic recovery and compression set), high temperature properties and oil/solvent resistance of the material is much better. This allows TPV’s to be used in technical applications where traditionally vulcanized rubber was used (automotive, appliances, building, profiles, seals…).
The properties of all major commercial TPE families along with our product range is summarized in the below table.