Types and application fields of thermoplastic elastomers

Elastomers can be divided into thermosetting elastomers and thermoplastic elastomers according to whether they can be plasticized. Thermosetting elastomers are rubbers in the traditional sense. Thermoplastic Elastomers (TPEs) have been increasingly commercialized since the 1990s. These two types of elastomers are processed in two different ways: rubber is processed with thermoset equipment, and TPE is processed with thermoplastic equipment.

TPE is defined as a polymer material that exhibits rubber elasticity at room temperature and can be plasticized at high temperature. These polymers combine certain characteristics of thermoplastic rubbers and thermoplastics. The basic structural feature of the TPE polymer chain is that it simultaneously connects or grafts some plastic segments (hard segments) and rubber segments (soft segments) with different chemical compositions.

The types of TPE are as follows: styrene thermoplastic elastomer, polyurethane thermoplastic elastomer, polyolefin thermoplastic elastomer, polyamide thermoplastic elastomer.

01. Styrenic thermoplastic elastomer

Styrenic block copolymer thermoplastic elastomers are the earliest researched thermoplastic elastomers, mainly including SBS, hydrogenated SBS (SEBS), SIS and hydrogenated SIS, etc. It is currently the world's largest and fastest-growing thermoplastic elastomer. From an application point of view, styrenic thermoplastic elastomers are most interesting in that their performance at room temperature is similar to that of vulcanized rubber, and their elastic modulus is exceptionally high and does not vary with relative molecular mass. With its high strength, softness, rubber elasticity, and small permanent deformation, styrene thermoplastic elastomers are widely used in the footwear industry, plastic modification, asphalt modification, waterproof coatings, liquid sealing materials, wires, cables, etc.

02. Polyurethane thermoplastic elastomer

Polyurethane thermoplastic elastomer (TPU) is generally composed of long-chain polyols (polyether or polyester) with an average molecular weight of 600-4000, chain extenders with a relative molecular weight of 61-400, and polyisocyanate addition-polymerized linear polymer materials. The long-chain polyol (polyether or polyester) in the TPU macromolecule main chain constitutes the soft segment, which mainly controls its low temperature performance, solvent resistance and weather resistance, while the chain extender and polyisocyanate constitute the hard segment. Since the ratio of hard and soft segments can be adjusted in a wide range, the obtained TPU can be either a soft elastomer or a brittle high-modulus plastic.

TPU has been widely used in many fields of the national economy, such as footwear industry, medical and health care, clothing fabrics and defense supplies, but its disadvantages are poor aging resistance, low wet surface friction coefficient, and easy slippage.

03. Polyolefin thermoplastic elastomer

Polyolefin thermoplastic elastomers (TPO) mainly include three types of block copolymers, graft copolymers and blends, among which, polyolefin thermoplastic elastomer ethylene-octene copolymer (POE) synthesized by metallocene catalyst and thermoplastic dynamic vulcanizate (TPV) prepared by dynamic vulcanization are two main polyolefin thermoplastic elastomers.

POE

POE refers to a polyolefin-based elastomer obtained by copolymerizing ethylene-α-olefin, such as ethylene-octene copolymer (EOC), ethylene-butene copolymer (EBC), and ethylene-hexene copolymer.

The so-called POE mainly refers to ethylene-octene copolymer elastomer, and the mass fraction of octene is more than 20%. The particularity of the molecular structure of POE endows it with excellent mechanical properties, rheological properties and aging resistance. It can be used not only as rubber, but also as thermoplastic elastomer, and as impact modifier and toughening agent for plastics. It has good low temperature toughness and high cost performance. It is gradually replacing ethylene-propylene rubber in many occasions and is widely used in plastic modification.

The molecular structure of POE is similar to EPDM, so POE also has excellent properties such as aging resistance, ozone resistance, and chemical resistance. By cross-linking POE, the heat-resistant temperature of the material is increased, the permanent deformation is reduced, and the main mechanical properties such as tensile strength and tear strength are greatly improved.

The main cross-linking methods of POE are: Electron beam or γ-ray radiation cross-linking method, peroxide cross-linking method and silane cross-linking method, in addition, POE can also use other cross-linking methods such as photo-cross-linking and salt cross-linking. Commonly used peroxide crosslinking agents are: Dicumyl peroxide DCP, 2,5-dimethyl-2,5-bis (tert-butylperoxy) hexane (bis-2,5), Bis (tert-butylperoxyisopropyl) benzene BIPB, and benzoyl peroxide (BPO), etc.

TPV

Thermoplastic elastomers obtained by dynamic vulcanization are called thermoplastic vulcanizates (TPVs). TPVs are a special type of TPE that, unlike block copolymers which are elastic, are produced from the synergy of elastomer/thermoplastic polymer blends, with better properties than simple blends.

Elastomers commonly used in the preparation of TPV are ethylene-propylene-diene rubber (EPDM). EPDM is a terpolymer of ethylene, propylene and non-conjugated diene. The diene double bond is reactive. Under the action of a vulcanizing agent, EPDM with a high degree of cross-linking can be obtained. EPDM has good heat resistance, weather resistance and mechanical properties, but poor hardness and processability, which limit its development. The key technology for preparing TPV is dynamic vulcanization technology. One of the advancements in this technology is to use low-cost existing processing methods to prepare new products by blending existing polymers. For example: nylon/EPDM type TPV, PP/EPDM type TPV, nylon/nitrile rubber type TPV, etc.

04. Polyamide thermoplastic elastomer (TPAE)

TPAE consists of high melting point crystalline hard segment (polyamide) and amorphous soft segment (polyester or polyether). Its properties depend on the type of hard segment and the length of both blocks. Due to the existence of the hard segment polyamide, TPAE has excellent toughness, chemical resistance, abrasion resistance and noise reduction. By selecting and controlling the block class, its mechanical, thermal and chemical properties can be varied over a wide range. According to the raw materials required for the synthesis of TPAE, its synthesis methods can be divided into dibasic acid method and isocyanate method. Using the diacid method, TPAE is prepared by esterification of carboxyl-terminated aliphatic polyamide blocks with hydroxyl-terminated polyether diols.

The isocyanate method uses semi-aromatic amide as the hard segment and aliphatic polyester, polyether or polycarbonate as the soft segment.

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