What are Elastomers?

Elastomers generally refer to materials that can return to their original shape after removing external forces, but materials with elasticity are not necessarily elastomers. Elastomers only deform significantly under weak stress. After stress relaxation, they can quickly recover to the original state and size of polymer materials ^[1] . Thermoplastic elastomers are an important component of elastomers. The two concepts should not be confused.

Chinese name: Elastomer

Foreign name: elastomer

Elastomer elastomer and rubber

Compared with rubber, the more focused meaning of elastomer is a concept of physics and materials science, which has a wider scope. In addition to rubber, the elastomer also includes many polymer materials with very different elastic deformation and diversified macromolecular chain crosslinking methods. ^[1] It can be seen that rubber is the most representative type of elastomer. In this sense, common rubbers include styrene-butadiene rubber, butadiene rubber, isoprene rubber, ethylene-propylene rubber, butyl rubber, neoprene, and nitrile rubber, etc. are all elastomers.

Due to some historical reasons, when talking about elastomers in China in the early days, it usually refers to thermoplastic elastomers, and does not include the meaning of rubber, which has also affected some specific technical exchanges. ^[2] With the development of elastomer synthesis technology and the improvement of performance, the concepts of rubber and elastomer have become increasingly common. The two have traditionally been called synonyms and often substitute each other. ^[1]

Elastomer classification

According to whether the elastomer can be plasticized, it can be divided into two categories: thermoset elastomers and thermoplastic elastomers ^[3] . Thermoset elastomer, which is the traditional meaning of rubber (Rubber), thermoplastic elastomer (Thermoplastic elastomer), abbreviated as TPE, began to be increasingly commercialized in the 1990s. This classification also shows that the two types of elastomers are processed in two different ways: rubber is processed with thermoset equipment, and TPE is processed with thermoplastic equipment.

Traditionally, humans refer to hot solid elastomers, that is, traditional rubbers, such as styrene rubber, such as styrene-butadiene rubber, cis-butadiene rubber, and silicone rubber. Rarely, styrene-butadiene elastomer, cis-butadiene elastomer or silicone elastomer is called. As for the thermoplastic elastomer, it is customarily called a certain kind of elastomer, such as polyurethane elastomer, SBS elastomer and POE elastomer. This is one of the reasons that most people confuse the concept of elastomer and rubber.

Elastomer thermoplastic elastomer

Thermoplastic elastomer (thermoplastic elastomer, TPE) is defined as a polymer material that exhibits rubber elasticity at normal temperature and can be plasticized at high temperature. Therefore, these polymers have certain characteristics of both thermoplastic rubber and thermoplastics. The basic structural feature of the thermoplastic elastomer polymer chain is that it is connected in series or grafted with some plastic segments (hard segments) and rubber segments (soft segments) with different chemical compositions at the same time. The force between the hard segments is sufficient to condense into micro-domains (such as vitrified micro-domains or crystalline micro-domains) to form a physical "crosslink" between the molecules. The soft segment is a high-end segment with a large self-rotation capability.

Thermoplastic elastomers are an important component of elastomers. The following are common:

Elastomer styrenic thermoplastic elastomer

Styrenic block copolymer thermoplastic elastomer is the earliest researched thermoplastic elastomer, mainly including SBS, hydrogenated SBS (SEBS), SIS, and hydrogenated SIS, etc. It is currently the world's largest and fastest growing thermoplastic elastomer body. From an application perspective, styrenic thermoplastic elastomers are most interesting in that their properties at room temperature are similar to vulcanized rubbers. In addition, their elastic modulus is exceptionally high and does not change with relative molecular mass. With its high strength, softness, rubber elasticity and small permanent deformation, styrenic thermoplastic elastomers are used in shoemaking, plastic modification, asphalt modification, waterproof coatings, liquid sealing materials, wires, cables, automotive parts, Medical device parts, household appliances, office automation and adhesives have a wide range of applications. The biggest problem of SBS and SIS is that it is not heat resistant, and the operating temperature generally does not exceed 80 ° C. At the same time, its elongation, weather resistance, oil resistance, abrasion resistance, etc. can not be compared with rubber. The modified hydrogenated SBS (SEBS) and hydrogenated SIS have much higher performance in practical applications than ordinary linear and star SBS, and the use temperature can reach 130 ° C, especially with excellent ozone resistance, oxidation resistance, UV and weather resistance, comparable to ethylene-propylene rubber in non-dynamic applications. ^[4]

Elastomer polyurethane thermoplastic elastomer

Polyurethane-based thermoplastic elastomers (TPU) are generally polymerized by addition of a long-chain polyol (polyether or polyester) with an average molecular weight of 600 to 4000, and a chain extender and polyisocyanate with a relative molecular weight of 61 to 400. Linear polymer materials. The long-chain polyol (polyether or polyester) in the TPU macromolecule main chain constitutes a soft segment, which mainly controls its low temperature performance, solvent resistance and weather resistance, while the chain extender and polyisocyanate constitute a hard segment. Because the ratio of the hard and soft sections can be adjusted within a wide range, the obtained thermoplastic polyurethane can be either a soft elastomer or a brittle high-modulus plastic, or it can be made into a film or fiber. It is a TPE The only species that can do it.

TPU has excellent abrasion resistance, oil resistance and cold resistance, and has sufficient resistance to oxygen, ozone and radiation. At the same time, it has high tensile strength and elongation at break as an elastomer, and it also has compression. Excellent performance such as small permanent deformation and large bearing capacity. TPU has been widely used in many areas of the national economy, such as the shoe industry, medical and health, clothing fabrics, and national defense products, but its disadvantages are poor aging resistance, low wet surface friction coefficient, and easy slipping. In addition, TPU has strong polarity. During the processing, when the shearing effect is strong, the internal heat easily causes degradation, and its melt viscosity has a strong temperature dependence. Small temperature changes can cause sharp changes in its viscosity. Therefore, the processing temperature range is narrow, coupled with the high cost and high price, further limiting the promotion and application of TPU. ^[4]

Elastomer polyolefin thermoplastic elastomer

Polyolefin-based thermoplastic elastomers (TPO) mainly include three types of block copolymers, graft copolymers and blends. Among them, polyolefin thermoplastic elastomers ethylene-octene copolymers (POE) and The thermoplastic vulcanizates prepared by the dynamic vulcanization method are two main types of polyolefin-based thermoplastic elastomers.

1. Compared with conventional traditional Ziegler-Natta catalysts, metallocene polyolefin elastomer ethylene-octene copolymer metallocene catalysts have an ideal single active center, so they can precisely control the relative molecular mass distribution, comonomer content and their main properties. Distribution and crystalline structure on the chain. The synthesized polymer is a high stereoregular polymer with a narrow relative molecular mass distribution, which can accurately control the physical and mechanical properties and processing properties of the polymer. Polyolefin thermoplastic elastomer ethylene-octene copolymer (POE) synthesized with metallocene catalyst has a narrow molecular weight and short branched chain distribution on the one hand, so it has excellent physical and mechanical properties (high elasticity, high strength, high elongation) Ratio) and good low temperature performance, and because its molecular chain is saturated, it contains relatively few tertiary carbon atoms, so it has excellent heat resistance and UV resistance. POE is better than EVA in thermal stability, optical properties and resistance to dry cracking, weathering and aging resistance is better than SBS, embrittlement temperature is lower than -76 , still has good toughness and ductility at low temperature, POE shear Good, good for high-speed extrusion and molding, little or no plasticizer, and long service life. POE can be cross-linked by peroxide, silane and radiation methods. The physical and mechanical properties, chemical resistance and ozone resistance of the material after cross-linking are close to those of EPDM; heat resistance and UV resistance are better than EPDM and EPM, so POE It is more suitable for outdoor use, and POE has less thermal compression set than EPDM. As a modifier, POE can modify both rubber and plastic. Due to the low processing temperature of POE, it is easier to mix with non-polar rubber, especially EPR, EPDM, NR, SBR and BR. The biggest application of POE is still on plastic products. The notched impact strength of PP is modified by POE; the elastomer after grafting POE with maleic anhydride is used to modify PA6, which can reduce the material's hygroscopicity and greatly improve the impact strength. ^[4]

2. Thermoplastic vulcanizate The thermoplastic elastomer produced by the dynamic vulcanization method is called thermoplastic vulcanizate (TPV). TPV is a special type of thermoplastic elastomer (TPE). Unlike elastomeric block copolymers, TPV is generated by the synergy of elastomer-thermoplastic polymer blends, and has better properties than simple blends. nature. The key technology for the preparation of thermoplastic vulcanizates is the dynamic vulcanization technology. One of the advances in this technology is the use of low-cost existing processing methods to prepare new products by blending existing polymers. Compared with the traditional process of producing new materials with high capital investment intensity, this process can also meet the environmental protection requirements for large-scale polymerization units. Other advantages of TPV technology over block copolymers as a source of thermoplastic elastomers are: high upper-limit use temperature, resistance to hydrocarbon media, and low compression set. ^[4]

Elastomer polyamide thermoplastic elastomer

Polyamide thermoplastic elastomer (TPEA) is composed of high melting point crystalline polyamide hardness and non-crystalline polyester or polyether soft satin ^[1] . According to the raw materials required for the synthesis of polyamide thermoplastic elastomers, the synthesis methods can be divided into dibasic acid method and isocyanate method. Using the diacid method, T PAE is prepared from a carboxyl-terminated aliphatic polyamide block and a hydroxyl-terminated polyether diol through an esterification reaction. The isocyanate method uses semi-aromatic amide as the hard segment, and aliphatic polyester, polyether or polycarbonate as the soft segment. Using the isocyanate method, the semi-aromatic amide hard segment is obtained by the reaction of an aromatic diisocyanate and a dicarboxylic acid, instead of the traditional diamine and dicarboxylic acid polymerization, ring-lactam ring-opening polymerization, or a diamine. It can be prepared by reaction with dibasic acid chloride. In contrast, the former avoids the problems of lower aromatic diamine activity and the difficulty in obtaining and reacting aromatic cyclolactam monomers with the release of corrosive hydrogen chloride. ^[5]