In the previous article, we discussed the development of elastomers and how TPE has overcome the limitations of traditional rubber. In this article, we will dive deeper into the properties of TPE, its processing technologies, and its wide range of applications!
Practical Topics in Thermoplastic Elastomers
1. From Natural Rubber to Thermoplastic Elastomers
2. Properties, Processing, and Applications of Thermoplastic Elastomers
3. Some Practical Applications of Thermoplastic Elastomers
Author: Dr. Hong-Bing Tsai
Ph.D., Chemical Engineering, National Tsing Hua University
Professor, Department of Chemical and Materials Engineering, National Ilan University
Technology Director, AR Display Co., Ltd.
*Translated by Ching-Yuan Wu
Practical Topics in Thermoplastic Elastomers 2
Properties, Processing, and Applications of Thermoplastic Elastomers
Properties of TPU
TPUs exhibit the broadest properties among commonly used TPEs, so the applications are also the widest. The main reason is that the raw material components are diversified, and the properties can be adjusted by composition.
Generally, the TPUs are segmental copolymers. The soft segments are derived from polyether polyols or polyester polyols. The major polyether polyols used include the polyether polyols based on polypropylene oxide and poly (tetramythylene ether) glycol (PTMEG). The major polyester polyols used are those made by polymerization of ethylene glycol or 1,4-butanediol with adipic acid. The structure and molecular weight of the polyols influence the properties of the formed PU significantly. For example, TPU made from higher molecular weight PTMEG has better low temperature resistance due to its low Tg. Sometimes, commercial TPUs are divided into two categories: polyester type and polyether type, referring to the type of soft segments. Diisocyanates react with short-chain diols to form hard segments. The main diisocyanates used are toluene diisocyanate (TDI) and 4,4’-diphenylmethane diisocyanate (MDI). Commonly used short-chain diols are ethylene glycol and 1,4-butanediol. In the manufacturing process of some TPUs, the prepolymer with terminal isocyanate groups is first synthesized from the polyol and the diisocyanate, then the prepolymer, the diisocyanate and a short-chain diol react to form the TPU. Therefore, the short-chain diol is also called a chain extender.
TPU is one kind of block copolymers, but unlike SBS, in which three blocks are linked one by one, the distribution of the soft segments in the whole molecule is random. Therefore, this structure is called a segmental copolymer. In other words, TPU is influenced by composition and structure to more extent. The main factor affecting the properties of TPU is the content of soft segments. Generally, as the content of soft segments increases, the hardness, modulus, and strength of TPU decrease, but the elongation increases. An increase in elongation means the increase in so-called elasticity. Therefore, when choosing TPU (or rubber), the first thing to consider is usually hardness. The polyols used that form the soft segments also affect the properties of TPU. The type and structure of polyols have some influence on strength, wear resistance, chemical resistance, low temperature resistance, hydrolysis resistance, and aging resistance. The molecular weight of the polyol used in the manufacture significantly affects the properties of TPU. As the molecular weight of the polyol increases, the elongation of TPU increases, the hardness decreases, and sometimes the melting point of the hard segments increases, thereby the heat resistance increases. The structure and composition of the hard segments also significantly affect the properties of TPU. For example, the hard segments formed by MDI and 1,4-butanediol give TPU higher strength, melting point, and heat resistance. The increase in the melting point also affects the processability.
Table 1 shows the properties of some TPU(Isothane) products. It can be seen from Table 1 that the physical properties of some grades of TPU are close to those of common engineering plastics. For example, Isothane 8101 with a Shore D hardness of 80 has a tensile strength of more than 70 MPa, while the tensile strength of common engineering plastics is in the range of 60-70 MPa. In other words, TPU has a wide range of properties and uses, and many molding technologies are available for TPU processing.
Table 1. Properties of some Isothane TPUs
/%E8%8B%B1%E6%96%87/1.jpg?7403)
/%E8%8B%B1%E6%96%87/%E6%8F%92%E5%9C%961.jpg?9600)
Processing of TPU and some TPEs
Injection molding is the most commonly used processing method. In other words, the original intention of developing TPE was to allow rubber to be injection molded. Basically, TPU and some TPEs are easy to process by injection molding. Typical injection molding conditions are shown in Table 2.
Table 2. Typical injection molding conditions for TPE
/%E8%8B%B1%E6%96%87/2.jpg?6615)
*Styrenic block copolymer
Although TPEs are easy to be processed by injection molding, there are some key points that favor the competitiveness of TPE products. When selecting an injection molding machine and mold designing, the shot volume per mold should be 25-80% of the maximum shot capacity. If this ratio can be increased to 40-80%, the utilization efficiency of the equipment can be considered appropriate. Mold design influences the performance of the final products significantly. Vents are sometimes necessary and should be distributed reasonably. Typical sizes of vents are 3-10 mm wide and 5-20 μm in gap. Runner design is often ignored. Sometimes, runners affect the flow of the molten polymer. Generally, the more uniform the shape of the runner is, the result is the better. Therefore, a circular runner is most appropriate, as shown in Figure 1. Since TPE is a material with very low rigidity, it is sometimes difficult to produce high-precision parts. For example, when different cavities in a mold have different filling pressures during injection, the size of the finished products may be different. Therefore, when designing a mold, runner balance is sometimes very important. The conceptual diagram of runner balance design is shown in Figure 2. Generally, the space utilization is considered in the design, as shown in Figure 2(a), but it is difficult to achieve the balance of the runner. Perhaps after multiple mold modifications, the balance of the runner can be achieved, as shown in Figure 2(b). The balance of runner is first considered in mold design, as shown in Figure 2(c), this is the key point of precision parts. The design of the sprue is usually short and round, preferably. The sprue is usually designed with an appropriate cold slug well.
/%E8%8B%B1%E6%96%87/eng1.jpg?6089)
Figure 1. Runner shape and the performance
/%E8%8B%B1%E6%96%87/eng2.png?4281)
Figure 2. Conceptual diagram of runner balance design
Extrusion is also a commonly used processing technology for TPU and TPEs. As with plastics, compounding and granulation are usually done using an extruder. Sometimes, in order to make the compositions uniform, a twin-screw extruder can be effectively used to achieve this. If the extruder is equipped with a suitable die head, it can be used to manufacture semi-finished products such as films, sheets, tubes, profiles, wire coatings, and elastic fibers. Coextrusion can be used to produce multi-color or multi-layer products. For many extrusion products, die design is a key technology.
Other processing techniques that may be used include blow molding, calendering, compression molding, casting, etc.
/%E8%8B%B1%E6%96%87/%E6%8F%92%E5%9C%962.jpg?844)
Applications of TPU
TPUs possess a wide range of properties and hardness, and are widely used. Initially, one of the applications PU in Bayer was fiber. Today, one of the important applications of TPU is elastic fiber. The derived textiles, such as cloth and elastic fabrics, have been widely used in tights, socks, sportswear, gloves, etc. The elasticity and excellent wear resistance of TPU make it widely used in shoes and various sports equipment. TPU pipe fittings are also widely used in daily necessities, stationery, groceries, medical equipment and water pipes. TPU film is also used in textiles, building materials and furniture, agriculture, automobiles, and electronic and electrical industries. Some wires are coated with TPU. It can be said that TPUs have been widely used in automotive parts and electronic and electrical parts due to the wide range of properties.
/%E8%8B%B1%E6%96%87/%E6%8F%92%E5%9C%963.jpg?1024)
Styrenic Block Copolymers
Styrenic thermoplastic elastomers are a series of block copolymers with polystyrene forming the hard phase. This type of thermoplastic elastomer is sometimes abbreviated as TPS (styrenic thermoplastic elastomers). The most typical one is SBS. The main factors affecting the properties of SBS is the polystyrene hard phase content, the length of the polybutadiene soft segment and the SBS molecular weight. The greater the amount of styrene used during synthesis, the higher the SBS hardness and the lower the elongation. The higher the molecular weight of the polybutadiene soft segment, the better the elasticity and extensibility, and sometimes the tensile strength is improved. As the molecular weight of SBS increases, the melt viscosity and solution viscosity will increase significantly.
The properties of some SBS are very similar to vulcanized natural rubber. Maybe many people have had this experience: shoe soles are more suitable to be made of rubber (vulcanized natural rubber). Then, can't we use SBS, which is easy to process, to replace vulcanized natural rubber? That’s right, one of the main applications of SBS is shoe materials, among which shoe soles are the most widely used. Another use of SBS is to replace vulcanized rubber. Toys, daily necessities, leisure products, automotive parts, and electronic and electrical parts are some typical examples. SBS molecules can easily penetrate into the substrate under pressure to form adhesion, so it is suitable for pressure-sensitive adhesives. For example, many sticky notes are coated with SBS pressure-sensitive adhesive. Various adhesives are also the main applications of SBS. In the early days, it was discovered that after asphalt was mixed with SBS, its properties such as low temperature resistance were significantly improved. Therefore, SBS has also become a modifier for many materials (including plastics). For example, SBS is an excellent impact modifier for polystyrene (PS).
The mid-soft block of SIS is polyisoprene. In fact, polyisoprene exhibits better elasticity than polybutadiene, but the cost is usually higher. The characteristics of SIS are similar to those of SBS, and the influence of composition and structure on its properties is roughly the same as that of SBS. If we have to compare, the elasticity and extensibility of SIS are better than those of SBS. Some properties of SIS are shown in Table 3. The applications of SIS are similar to those of SBS. One main application of SIS is hot melt adhesive, which shoes good adhesion properties. In addition, SIS exhibits excellent vibration energy absorption properties when used as a plastic modifier. Blends of SIS with some plastics such as PS, PP or EVA can be used as vibration-proof materials.
Table 3. Properties of some SIS
/%E8%8B%B1%E6%96%87/3.jpg?8596)
The mid-block of SBS and SIS possesses unsaturated double bonds and exhibits poor chemical stability. Hydrogenation of the unsaturated double bonds in the mid-block let the chemical stability be greatly improved. Obtained is the second generation of styrenic thermoplastic elastomers, mainly SEBS and SEPS. Table 4 lists the properties of some SEPS. The effects of composition and structure on the properties of SEBS and SEPS are similar to those of SBS and SIS. Generally, SEBS and SEPS are stronger than SBS and exhibits better heat resistance and weather resistance. The uses of SEBS and SEPS are similar to those of SBS, but due to their high cost, they are usually used in higher-end finished products. Rubber products used outdoors are typical examples. Toys, daily necessities, leisure products, automotive parts, electronic and electrical parts, etc. are application examples. Plastic modifiers, adhesives and sealants are also important uses. Chemical stability makes SEBS suitable for medical and food applications, and some grades of SEBS are FDA certified. Some grades of SEBS possess very high molecular weight and their solution viscosity is extremely high. SEBS has excellent compatibility with mineral oil, paraffin oil and some organic solvents, so SEBS absorbed suitable amount of paraffin oil can show excellent elasticity. Therefore, some SEBS compounds can be used in ultra-soft yet strong tool handles and grips. In fact, this mixture of high molecular weight SEBS and paraffin oil can be easily cast into toys and ultra-high elongation products.
Table 4. Properties of some SEPS
/%E8%8B%B1%E6%96%87/4.jpg?3768)
/%E8%8B%B1%E6%96%87/%E6%8F%92%E5%9C%964.jpg?2255)
/%E8%8B%B1%E6%96%87/%E6%8F%92%E5%9C%965.jpg?1361)
Ionomers
The more famous ionomer elastomer is DuPont's Surlyn. The properties of some Surlin ionomers are shown in Figure 5. Generally, as the ionic group content increases, the rigidity or modulus of the ionomer increases, the strength increases, but the elongation decreases. Surlyn is equivalent to low-density polyethylene (LDPE) cross-linked by ionic groups, so it has higher strength. The introduction of ionic groups increases the oil resistance and improves the adhesion to many substrates. Another feature is the good transparency, and it is also known as transparent polyethylene. Other features include excellent low temperature toughness, wear resistance and chemical resistance.
Typical applications of Surlyn ionomers include cosmetics, consumer products and sports equipment. The cosmetics field includes perfume bottle caps, cosmetic containers, etc. The consumer products include various handles, toys, pet chews, ice buckets and floor coverings. The sports equipment field includes golf ball shells, surfboards, snowboard surfaces, ski boots, skate boots, hockey helmets, shoe heels and protective vests. Other fields include buoys, outdoor safety lighting, glass product surface coatings, pipe screw protection covers, fluorescent lamp surface protection layers, etc.
Table 5. Properties of some Surlyn ionomers
/%E8%8B%B1%E6%96%87/5.jpg?6843)
/%E8%8B%B1%E6%96%87/%E6%8F%92%E5%9C%966.jpg?7483)
TPO
Thermoplastic polyolefin elastomers (TPO) derived from the blend of PP and EPDM are the TPE easiest to be made. The characteristics are good heat resistance and excellent weather resistance. Generally, as the PP content increases, the hardness and strength increase, and the elongation decreases. The properties of some TPO are shown in Table 6. The main application of TPO is automotive parts. TPO possesses good electrical properties and can be used in household appliance components and wires and cables. TPO also has applications in building materials, footwear and tool handles.
Table 6. Properties of some TPO
/%E8%8B%B1%E6%96%87/6.jpg?2223)
TPV
The special blends of PP and vulcanized EPDM (TPV) which experiencing the so-called dynamic vulcanization during manufacture exhibit completely different morphology. This system is the vulcanized rubber particles that were coated with the PP continuous phase. The results are excellent heat resistance, very good compression set and excellent solvent resistance. Generally, as the PP content increases or the degree of cross-linking increases, the hardness and strength of TPV tend to increase. Table 7 lists some properties of TPV. The main application of TPV is automotive parts. Among them, various connector sleeves and air pipes are typical uses. The temperature resistance and light weight make TPV be an excellent material for hot and cold air delivery hoses, suitable for blowers and compressors. Santoprene can also be used in building materials. A typical example is the elastic airtight strips for high-rise buildings. Others include food and medical parts, electronic and electrical parts, mechanical parts and sports equipment.
Table 7. Properties of some TPV
/%E8%8B%B1%E6%96%87/7.jpg?8530)
/%E8%8B%B1%E6%96%87/%E6%8F%92%E5%9C%967.jpg?9280)
/%E8%8B%B1%E6%96%87/%E6%8F%92%E5%9C%968.jpg?3830)
TPEE
In general, TPEE is the condensation polymer of terephthalic acid, 1,4-butanediol and poly(tetramethylene ether)glycol (PTMEG). TPEE, like TPU, is a segmental copolymer. After polymerization, PTMEG forms polyether soft segments and poly(butylene terephthalate) (PBT) forms hard segments. The effects of composition and structure on the properties of TPEE are similar to those of TPU. As the content of polyether soft links increases, the hardness, modulus, strength of TPEE and the melting point of PBT segments decrease, but the elongation increases. The properties of some TPEE are shown in Table 8.
TPEE is a strong material with excellent properties. It shows excellent fatigue resistance and excellent heat resistance, and is sometimes considered an engineering material. The main application field of TPEE is automotive parts. Typical application examples are various connectors and sleeves, cables, bumpers, body side strips and tire inserts. Mechanical parts and electronic and electrical parts are also important uses of TPEE. Other applications include footwear, food and medical parts.
Table 8. Properties of some TPEE
/%E8%8B%B1%E6%96%87/8.jpg?662)
/%E8%8B%B1%E6%96%87/%E6%8F%92%E5%9C%969.jpg?8068)
TPAE
TPAE is also a segmental copolymer. The soft segments of TPAE are polyether, and the hard segments can be nylon 12, nylon 11 or nylon 6. Obviously, the properties of TPAE are greatly affected by composition and structure, and the trend is similar to that of TPU. The specific gravity of nylon 12 is only 1.02, which is lower than that of PU and PBT. Therefore, TPAE based on nylon 12 has a lower specific gravity as compared to TPU and TPEE, and is known as "lightweight". The properties of some nylon 12 based TPAE are shown in Table 9.
TPAE possesses strong and toughness properties, exhibits excellent flexibility, good heat resistance and oil resistance, and excellent resilience, but the price is high. Engineering applications are the development trend. Among engineering TPEs, TPAE is known to have the advantages of "ultra-lightweight, high resilience and low temperature resistance". The main applications of TPAE include sporting goods, automotive parts, industrial parts, and electronic and electrical parts. TPAE exhibits an excellent processing and molding feature: over molding. This is great news for the design of certain sports shoes and sports equipment.
Table 9. Properties of some nylon 12-based TPAE
/%E8%8B%B1%E6%96%87/9.jpg?6942)
/%E8%8B%B1%E6%96%87/%E6%8F%92%E5%9C%9610.jpg?9603)
The third article will show some practical applications of thermoplastic elastomers. (to be continued…)
References
1. Wikipedia: https://zh.wikipedia.org/wiki
2. Baidu Baike: https://www.baidu.com
3. M. Morton, Rd., Rubber Technology, 2nd Ed., Van Nostrand Reinhold Company, Newyork, 1973.
4. N. R. Legge, G. Holden, H. E. Schroeder, Ed., Thermoplastic Elastomers: A Comprehensive Review, Hanser Publishers, New York, 1987.
5. G. Oertel, Ed., Polyurethane Handbook, Hanser Publishers, Munich, 1985.
6. K. Koei (L. S. Hsieh, transl.), Technology Hand Book of Thermoplastic Elastomers, Polymer Industry Magazine, Taipei, 2003.
7. C. C. Tsai, Ed., Modern Chemical Process, Industries and New Materials, New Wu Ching Developmental Publishing Co., Ltd., Taipei, 2014.
