Thermoplastic Elastomers (TPE): Bridging the Gap Between Rubber and Plastics （1/3）
Thermoplastic elastomers (TPE) combine the elasticity of rubber with the processability of plastics, making them an essential part of modern materials science. Their versatility allows them to be widely used in various fields such as automotive, medical, electronics, and daily life products. This blog series will be divided into three articles, guiding you to realize the historical development, material properties, and practical applications of TPE.
The first article will depict the discovery history of natural rubber first, then describe the development of elastomers, and show how TPE overcame the limitations of traditional rubbers. The second article will describe deeply the unique properties, processing techniques, and various applications of TPE. The last article will show the innovative TPE applications through some practical case studies. Stay tuned!
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 1
From Natural Rubber to Thermoplastic Elastomers
By Dr. Hong-Bing Tsai
Preface
Elastomers refer to the materials with elasticity, usually polymers with rubber-like elasticity. The well-known rubber is natural rubber. Natural rubber was first obtained by treating the latex collected from rubber trees. The main component of natural rubber is polyisoprene, which is a sticky linear polymer. Goodyear discovered that the rigidity and elasticity of natural rubber were enhanced greatly after vulcanization, and more practical applications could be achieved. Vulcanization of rubber is a cross-linking reaction that forms chemical bonds. Therefore, vulcanized rubber is irreversible and cannot be directly recycled and reprocessed.
Traditional processing methods of vulcanized rubber are usually thermosetting ones. After shaping and vulcanization, it is an irreversible chemical cross-linking reaction. The processing is cumbersome and the finished product is not easy to recycle due to chemical cross-linking. If the rubber is designed to be physically cross-linked, it can be processed using thermoplastic methods to become thermoplastic rubber, generally known as thermoplastic elastomer (TPE).
Natural rubber
Natural rubber was first obtained by treating the latex collected from rubber trees (Hevea brasiliensis). By the way, oak is the common name for plants of the genus Quercus in the family Fagaceae. The wood used for oak barrels is mainly American white oak and French oak. The rubber tree belongs to the Euphorbiaceae family, a different family of flowering plants from the oak tree. The latex collected from rubber trees contains natural rubber, the main component of which is polyisoprene.
People in Central and South America have been collecting natural rubber since ancient times. The Mesoamerican ballgame was played with a ball made of natural rubber as early as 1600 BCE. Natural rubber was also used as straps for fastening wooden handles to stone or metal tools and in padding for handles. The Maya also used natural rubber to make shoes. Natural rubber was introduced to Europe in the late 15th century. British scientist J. Priestley (1733-1804) discovered that pencil mark could be erased using a piece of natural rubber, so the English name of rubber is named as rubber. However, natural rubber without crosslinking becomes hard in winter and emits an unpleasant odor in summer. C. Goodyear, an American, attempted to overcome the shortcomings of natural rubber through experiments. In 1839, C. Goodyear discovered by chance that natural rubber mixed with sulfur had improved heat resistance, rigidity and elasticity after heating. The vulcanization of natural rubber is a cross-linking reaction and has become a standard procedure in the rubber industry. The Goodyear Tire & Rubber Company was founded in 1898 and its name was in honor of C. Goodyear.
The appearance of rubber tires also promoted the development of the automobile industry. The development of tires was closely related to natural rubber. In 1845, Scotsman R. W. Thomson had a new idea to put inflatable objects on the spokes of a carriage to absorb shock. Perhaps this is the earliest idea for tires. However, there was a lack of suitable materials at that time, and after several practical applications, it was not successful. Natural rubber could be used in tires after C. Goodyear discovered the method of rubber vulcanization. In 1887, British inventor J. B. Dunlop saw his son riding a tricycle in a stone-covered yard and wondered how to make the wheels softer to absorb shocks, so he invented the inflatable rubber tubes put on the wooden spokes. This is a prototype of a pneumatic tire. In 1892, the French Michelin company invented a pneumatic rubber bicycle tire that could be quickly disassembled. Later, Michelin also invented easily replaceable automobile rims and radial tires arranged in a radial shape. Michelin is one of the largest tire manufacturers in the world today. In 1910, the American Goodrich Tire Company added carbon black to rubber to increase friction and durability, and the tires changed from white to black. In 1937, Goodrich used cheaper synthetic rubber. In 1946, Goodrich invented the tubeless tire, also known as the high-speed tire.
Latex collected from rubber trees is an emulsion containing natural rubber and various plant compositions. Natural rubber latex is itself useful. Early raincoats, today's latex gloves, latex clothing and latex mattresses are typical examples. Natural rubber latex is usually concentrated and coagulated into crude natural rubber, then it is suitable for the tire manufacturing process. The processing and shaping method of crude natural rubber is obviously different from those of other materials. The general process is kneading, mixing, embryo-making, and then processing, shaping and vulcanization to form rubber products. Kneading and mixing are usually carried out using a Banbury mixer or rollers. The procedures can be considered as the tailor-made method for natural rubber. Various rubber additives are added to be to mixed at this stage. Important additives include vulcanizers, vulcanization accelerators, antioxidants, softeners, fillers, reinforcements, colorants and processing aids. Therefore, the formulation of rubber products is quite important and is often considered as an art. For example, carbon black is an advantageous additive for rubber used in tires. Carbon black can increase the friction, wear resistance and durability of tires, so it has the effects of reinforcement, anti-oxidation and enhancement of weather resistance. Furthermore, it is also an effective colorant. As the so called words, "the black color hides all the ugliness", carbon black is the perfect example here. The kneading and mixing temperature should not be too high to avoid vulcanization. Therefore, kneading and mixing step usually form an embryo material, which is then processed and shaped, and finally vulcanized at 150-220°C to obtain the final product.
With the progress of modern chemistry, people understood that the main component of natural rubber is polyisoprene, and people gradually understood how to synthesize some polymers. Then, the so-called “synthetic rubber” began to enter the rubber industry. Important synthetic rubbers include styrene-butadiene rubber (SBR), polybutadiene rubber (BR), acrylonitrile-butadiene rubber (NBR), synthetic polyisoprene rubber (IR; isoprene rubber) and ethylene-propylene-diene rubber (EPDM).
Today, the major applications of natural rubber are tires and rubber tubes. The second largest category is consumer goods and industrial products. Traditional processing methods of vulcanized rubber are usually thermosetting methods. After shaping and vulcanization, it is an irreversible chemical cross-linking reaction. The processing is cumbersome and the finished product is not easy to recycle due to chemical cross-linking. If the rubber is designed to be physically cross-linked, it can be processed using thermoplastic methods to become thermoplastic rubber, generally known as thermoplastic elastomer (TPE).
Structure of thermoplastic elastomers
How does elasticity come about? This is a topic of dynamic mechanics. When an object or material is subjected to an external force, it deforms. Usually, the force applied per unit area is called stress (σ), and the ratio of deformation caused by it (for example, the change in length divided by the original length) is called strain (ε). The ratio of stress to strain is the elastic modulus (E).
E = σ/ε
The modulus of elasticity, or modulus simply, is similar to the spring constant of a spring. For polymer materials, temperature has a significant effect on the modulus, as shown in Figure 1(a). For amorphous polymers, they are in the glassy state below the glass transition temperature (Tg), and behave hard and brittle material with high modulus. As the temperature increases, the modulus of glassy amorphous polymers decreases slightly below Tg. When the temperature rises to near Tg, the free volume increases, the motion space of the molecules increases, and the material is easier to be deformed, so the modulus of the polymer decreases greatly. Tg is a secondary transition and is a temperature range. When the temperature is higher than Tg, the molecular chains of the amorphous polymer will begin to move locally, and the material will become soft and flexible like rubber. This is the rubbery state. At higher temperatures, amorphous polymers become fluid and can flow. For semi-crystalline polymers, as shown in Figure 1(a), the modulus decreases near Tg in a less extent than the amorphous polymers. Above Tg, since the crystalline phase restricts the movement of molecular chains, its modulus decreases slowly with increasing temperature. However, near the melting point (Tm), the modulus of the polymer drops significantly. Above the melting point, the polymer becomes a liquid and is usually flowable.
(a)
(b)
Figure 1. (a) Effect of temperature on modulus of polymers; (b) Effect of cross-linking.
The Tg of natural rubber is -70°C. It will crystallize at temperatures below 10°C, so it is in a rubbery state at room temperature. The molecular weight of natural rubber (100,000-1,000,000) is very high, so the entanglement of its molecular chains let it exhibit a wide temperature range of rubbery state. The entanglement of molecular chains is similar to the case of tangled yarns, and makes it difficult for rubber molecules to flow and also increases the modulus. The vulcanization of natural rubber directly binds the molecules together through chemical bonds. This chemical cross-linking also increases the modulus of natural rubber. Figure 1(b) depicts the effect of cross-linking on the modulus of rubber-like polymers. In the glassy state, the cross-linking increases the modulus only slightly. In the rubbery state, the elastic modulus increases significantly with increasing cross-linking density. So, on a sensory scale, vulcanization obviously increases the elasticity of natural rubber. In addition, vulcanization also greatly restricts high temperature flowability, so vulcanized rubber can exhibit better temperature resistance.
Thermoplastic elastomers can be considered as systems of rubbery molecules that are physically cross-linked. At processing temperatures, the physical cross-links are destroyed, so that the polymer can flow. However, the physical cross-links restore after cooling, and the polymer becomes elastic material again, so that the polymer is easy to flow and can be processed. However, after cooling back to room temperature, the physical cross-links restored and the material becomes elastic again.
How to form physical cross-links? One is by ionic bond, as shown in Figure 2(a). The ionic bonds of the rubbery molecules with ionic groups were destroyed at high temperatures, so that the polymer is easy to flow and can be processed. After cooling to room temperature, the aggregation of ionic groups results cross-linking effects. Therefore, ionomers can be used to design thermoplastic elastomers. The second method is to restrict or reinforce the rubber phase with a fusible hard phase, as shown in Figure 2(b). If the rubbery phase is the continuous phase, the dispersed glassy or crystalline phase acts like reinforcing particles that restrict the motion of rubber segments and exhibit a cross-linking effect. At processing temperature, the hard phase becomes flowable and melt processable. There are several structures that can form this mechanism. Graft copolymers are the first type. By grafting glassy or crystalline polymers onto rubbery molecules, it is possible to form thermoplastic elastomers. Block copolymers are the second type. Segmental block copolymers with alternating hard and soft segments, such as TPU, can easily form thermoplastic elastomers. The tri-block copolymers such as styrene-butadiene-styrene copolymer (SBS) form a typical thermoplastic elastomer structure. The two outer hard segments form a dispersed phase, which fastens the rubbery continuous phase formed by the middle segment. The third type is the blend of the copolymer rubber and the hard polymer. Some segments of the copolymer rubber molecules are compatible with the hard polymer. and the interaction with the hard dispersed phase causes physically cross-linking of the rubber phase. The third is a special structure, as shown in Figure 2(c). A typical example is vulcanized EPDM particles dispersed in polypropylene. Imagine polyethylene (PE) foam, where air bubbles are dispersed in PE. Although PE is a plastic material, PE foam is quite elastic. Since polypropylene and EPDM have good compatibility, the interface between the particles and the polypropylene continuous phase will not form weakness. Therefore, this structure has excellent elasticity. The vulcanized rubber particles are coated with thermoplastic polypropylene and the system behaves the elasticity of rubber. At high temperatures, polypropylene melts and can flow and can be processed, so this system is also called thermoplastic vulcanizates (TPV).
(a)
(b)
(c)
Figure 2. Physical cross-linking: (a) ionic polymer model; (b) block copolymer model; (c) TPV model.
Commercialization of Thermoplastic Elastomers
Thermoplastic elastomers have some advantages as compared to thermosetting rubbers. First, the processing methods are similar to those of ordinary plastics, which are simple and fast, and the overall processing cost is low. Second, scraps or waste materials as well as discarded finished products can be recycled. Third, the precision of molded product components is high, making quality control easier. Of course, their disadvantages are also obvious, for example, the heat resistance is often not as good as thermosetting rubbers.
thermoplastic polyurethanes (TPU)
Perhaps, the first successful commercial thermoplastic elastomer was thermoplastic polyurethanes (TPU).
In 1937, O. Bayer of IG Farben in Germany used isocyanate and polyether diol or polyester diol to make polyurethanes, this opened the world of PU. In 1942, Germany built a pilot plant for PU, using Igamid U as its trade name. Initial applications were fibers and soft foams. In 1954, Monsanto of the United States and Bayer of Germany jointly established Mobay Chemical Company, and began to use toluene diisocyanate (TDI) and polyester polyols to produce soft polyurethane foam in the United States. These PU resins are also used to produce rigid foams, adhesives and elastomers. Initially, PU elastomers were often processed and molded by casting. In the 1950s, Goodrich and Mobay in the United States introduced TPU. TPU is usually a segmental block copolymer obtained by reacting polyether or polyester polyol (polyol) as the soft segments, a chain extender and a diisocyanate. The properties of PU are wide-ranging and can be adjusted by varying the composition. Therefore, PU is widely used, from plastics, rubber, fiber, coatings to adhesives. PU and PVC (polyvinyl chloride) are two polymer materials that everyone likes to use, because it seems that everything can be made by them. That is to say, TPU is only one type of PU material. Due to the wide range of properties and applications of TPU, many factories have been set up to produce it around the world.
Styrene-based Block Copolymers
In the 1960s, Dutch Shell Chemical Company introduced styrene-based block copolymers into the market. Using living anionic polymerization, a block of polystyrene is first synthesized, then a block of polybutadiene or polyisoprene is polymerized, and finally a block of polystyrene is formed. The tri-block copolymer is thus formed. The first generation of tri-block copolymers was introduced by Shell under the trade name of Kraton. Among them, the tri-block copolymers with polybutadiene middle block are SBS, and the those with polyisoprene middle block are SIS. Due to the poor chemical stability of polybutadiene or polyisoprene block, in 1972, Shell introduced the second-generation styrene-based block copolymers, which were more stable SEBS, wherein the middle block was an ethylene-butylene copolymer. In 1968, Phillips of the United States introduced 3-4 arm radial styrene-based block copolymers. In 1990, Japan's Kuraray commercialized the production of hydrogenated SIS, also known as SEPS, and used Septon as the trade name. Styrene-based block copolymers possess unique properties and applications, and some manufacturing plants have been established in Taiwan and mainland China.
Ionomers
In the 1960s, DuPont of the United States introduced ionomer elastomers, which were neutralized ethylene-methacrylic acid copolymers and were marketed under the trade name of Surlyn. This ionomers can be regarded as polyethylene with ionic groups introduced, which exhibit higher strength.
Thermoplastic Polyolefin Elastomers (TPO)
In 1972, Uniroyl of the United States introduced thermoplastic polyolefin elastomers (TPO), which were a mixture of PP and EPDM. Since PP and EPDM are both materials with high chemical stability and are popular, TPO products have been launched around the world. In 1974, Monsanto of the United States introduced special blends of PP and vulcanized EPDM, known as TPV (Thermoplastic Vulcanizate), under the trade name of Santoprene. TPV exhibits excellent elasticity and chemical stability, and some companies also joined the development of TPV successively.
Thermoplastic Polyester Elastomer (TPEE)
In 1972, DuPont introduced polyester-type segmental block copolymers, namely thermoplastic polyester elastomers (TPEE), and used Hytrel as the trade name. This is a type of block copolyetherester whose properties can be adjusted by varying the composition. TPEE combines the elasticity of rubber, the strength and heat resistance of engineering plastics, and the excellent processing properties of thermoplastics. Therefore, TPEE recently followed a developing trend towards engineering applications such as automotive parts.
Thermoplastic Polyamide Elastomer (TPAE)
In 1981, Atochem of France introduced polyamide type block copolymers under the trade name of PEBAX. The soft segment of this thermoplastic polyamide elastomer (TPAE) is polyether, and the hard segment is polyamide 11, that is, nylon 11. The introduction of polyamide 11 makes it the lightest thermoplastic elastomer in density. Engineering applications and sports equipment are the developing trends of TPAE.
The second article will describe the unique properties, processing techniques, and various applications of TPE. (to be continued…)
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