Natural rubber thermoplastic elastomer

Yamauchi, K., Akasaka, S., Hasegawa, H., Koizumi, S., Deepraserlkul, C., Laokijcharoen, P. et al. (2005) Structural study of natural rubber thermoplastic elastomers and their composites with carbon black by smaH-angle neutron scattering and transmission electron microscopy. Composites Part A Applied Science and Manufacturing, 36, 423—429. 

In this part, we will discuss AFM images and nanomechanical data obtained in smdies of natural and synthetic rubbers, thermoplastic elastomers (TPE), and their vulcanized counterparts— thermoplastic vulcanizates (TPV). 

Elastomers can be divided into two general categories, natural rubber and synthetic rubbers. Synthetic elastomers in turn are either termed general purpose rubbers (GPR) or special purpose rubbers. Natural rubber is generally obtained from southeast Asia or Africa. Synthetic rubbers are produced from monomers obtained from the cracking and refining of petroleum. The most common monomers are styrene, butadiene, isoprene, isobutylene, ethylene, propylene, and acrylonitrile. There are monomers for specialty elastomers which include acrylics, chlorosulfonated polyethylene, chlorinated polyethylene, epichlorohy-drin, ethylene-acrylic, ethylene-octene rubber, ethylene-propylene rubber, flu-oroelastomers, polynorbornene, polysulfides, sihcone rubber, thermoplastic elastomers, urethanes, and ethylene-vinyl acetate. 

In addition to the somewhat sophisticated triblock thermoplastic elastomers described above, mention should be made of another group of thermoplastic diene rubbers. These are physical blends of polypropylene with a diene rubber such as natural rubber. These may be considered as being an extension to the concept of thermoplastic polyolefin rubbers discussed in Section 11.9.1 and although extensive experimental work has been carried out with these materials they do not yet appear to have established themselves commercially. 

Oil resistance demands polar (non-hydrocarbon) polymers, particularly in the hard phase. If the soft phase is non-polar but the haid phase polar, then swelling but not dissolution will occur (rather akin to that occurring with vulcanised natural rubber or SBR). If, however, the hard phase is not resistant to a particular solvent or oil, then the useful physical properties of a thermoplastic elastomer will be lost. As with all plastics and rubbers, the chemical resistant will depend on the chemical groups present, as discussed in Section 5.4. 

If polypropylene is too hard for the purpose envisaged, then the user should consider, progressively, polyethylene, ethylene-vinyl acetate and plasticised PVC. If more rubberiness is required, then a vulcanising rubber such as natural rubber or SBR or a thermoplastic polyolefin elastomer may be considered. If the material requires to be rubbery and oil and/or heat resistant, vulcanising rubbers such as the polychloroprenes, nitrile rubbers, acrylic rubbers or hydrin rubbers or a thermoplastic elastomer such as a thermoplastic polyester elastomer, thermoplastic polyurethane elastomer or thermoplastic polyamide elastomer may be considered. Where it is important that the elastomer remain rubbery at very low temperatures, then NR, SBR, BR or TPO rubbers may be considered where oil resistance is not a consideration. If, however, oil resistance is important, a polypropylene oxide or hydrin rubber may be preferred. Where a wide temperature service range is paramount, a silicone rubber may be indicated. The selection of rubbery materials has been dealt with by the author elsewhere. ... 

Plastics, such as PE, PP, polystyrene (PS), polyester, and nylon, etc., and elastomers such as natural rubber, EPDM, butyl rubber, NR, and styrene butadiene rubber (SBR), etc., are usually used as blend components in making thermoplastic elastomers. Such blends have certain advantages over the other type of TPEs. The desired properties are achieved by suitable elasto-mers/plastic selection and their proportion in the blend. 

Although rubber originally meant a natural thermoset material obtained from a rubber tree, with the development of plastics it identifies a thermoset elastomer (TSE) or thermoplastic elastomer (TPE) material. Different properties identify the elastomers such as strength and stiffness, abrasion resistance, solvent resistance, shock and... 

Handbook of elastomers , A.K. Bhowmick and H.L. Stephens Marcel Dekker (1988) Series Plastics Engineering, Volume 19 ISBN 0824778006. This handbook systematically addresses the manufacturing techniques, properties, processing, and applications of rubbers and rubber-like materials. The Handbook of Elastomers provides authoritative information on natural rubbers, synthetic rubbers, liquid rubbers, powdered rubbers, rubber blends, thermoplastic elastomers, and rubber-based composites— offering solutions to many practical problems encountered with rubber materials. 

Table 17 provides a list of various polysiloxane-poly(aryl ether) copolymers investigated. Depending on the type, nature and the level of the hard blocks incorporated, physical, thermal and mechanical properties of these materials can be varied over a very wide range from that of thermoplastic elastomers to rubber modified engineering thermoplastics. Resultant copolymers are processable by solution techniques and in some cases by melt processing 22,244). 

Chattopadhyay S., Chaki T.K., and Bhowmick A.K., New thermoplastic elastomers from poly(ethyle-neoctene) (engage), poly(ethylene-vinyl acetate) and low-density polyethylene by electron beam technology structural characterization and mechanical properties. Rubber Chem. TechnoL, 74, 815, 2001. Roy Choudhury N. and Dutta N.K., Thermoplastic elastomeric natural rubber-polypropylene blends with reference to interaction between the components. Advances in Polymer Blends and Alloys Technology, Vol. 5 (K. Finlayson, ed.), Technomic Publishers, Pensylvania, 1994, 161. 

Roy Choudhury N., De P.P., and Bhowmick A.K., Thermoplastic elastomeric natural rubber-polyolefin blends. Thermoplastic Elastomers from Rubber Plastic Blend (De S.K. and Bhowmick A.K., eds.), Ellis Horwood, London, 1990, 11. 

Akhtar, S. Morphology and Physical Properties of Thin Films of Thermoplastic Elastomers from Blends of Natural Ruhher and Polyethylene, Rubber Chem. Technol. 61, 599-583, 1988. 

Al-Malaika, S. and Amir, E.J., Thermoplastic elastomers Part III—Ageing and mechanical properties of natural rubber-reclaimed rubber/polypropylene systems and their role as solid phase dispersants in polypropylene/polyethylene blends, Polym. Degrad. Stab., 26, 31, 1989. 

Ismail, H. and Suryadiansyah, S., Thermoplastic elastomers based on polypropylene/natural rubber and polypropylene/recycle rubber blends. Polymer Test., 21, 389, 2002. 

Solvent Resistance. One of the distinct advantages of a crystalline thermoplastic elastomer over an amorphous one should be its superior solvent resistance, since the latter types are generally soluble. Table III shows the swelling behavior of the H2-BIB triblocks in toluene at 25°C. It can be seen that the maximum swelling obtained was in the case of the H2-BIB-34, which had the lowest end-block content. Furthermore, the equilibrium swelling ratio of 3-26 obtained for this polymer is considerably less than the value of 5 or 6 generally exhibited by a well-vulcanized natural rubber. 

The nonterminating nature of living anionic polymerization allows the synthesis of block copolymers,480,481 which are useful thermoplastic elastomers. They have many properties of rubber (softness, flexibility, resilience) but in contrast to rubber can be processed as thermoplastics 482,483 Block copolymers can be manufactured by polymerizing a mixture of two monomers or by using sequential polymerization. 

Natural rubber is a polymer of isoprene- most often cis-l,4-polyiso-prene - with a molecular weight of 100,000 to 1,000,000. Typically, a few percent of other materials, such as proteins, fatty acids, resins and inorganic materials is found in natural rubber. Polyisoprene is also created synthetically, producing what is sometimes referred to as "synthetic natural rubber". Owing to the presence of a double bond in each and every repeat unit, natural rubber is sensitive to ozone cracking. Some natural rubber sources called gutta percha are composed of trans-1,4-poly isoprene, a structural isomer which has similar, but not identical properties. Natural rubber is an elastomer and a thermoplastic. However, it should be noted that as the rubber is vulcanized it will turn into a thermoset. Most rubber in everyday use is vulcanized to a point where it shares properties of both, i.e., if it is heated and cooled, it is degraded but not destroyed. 

Some of the conditions used in rubber test methods may need modifying for application to thermoplastic elastomers because of their intrinsic thermoplastic nature. If the temperatures generally used in ageing and compression set tests on thermosetting rubbers were applied to thermoplastic materials they could appear to perform extremely badly. Whether this was significant would depend on the service temperature. Data sheets need to be checked as those for thermoplastic elastomers may have used much lower temperatures that would be found for conventional rubbers, and it is only too easy to get a misleading impression of performance. 

Other thermoplastic elastomer combinations, in which the elastomer phase may or may not be cross-linked, include blends of polypropylene with nitrile (30,31), butyl (33), and natural (34) rubbers, blends of PVC with nitrile mbber (35,36), and blends of halogenated polyolefins with ethylene interpolymers (29). Collectively, thermoplastic elastomers of this type are referred to herein as hard polymer/elastomer combinations. Some of the more important examples of the various types are shown in Table 3. 

Thermoplastic tri-block copolymers are interesting since they possess novel properties different from those of the homo- or copolymers. The thermoplastic elastomers have many of the physical properties of rubbers, i.e., softness, resilience, and flexibility. The unique properties of this kind of copolymer are due to the microphase separation of the hard crystalline domains dispersed in a continuous amorphous matrix (Fig. 6). Such phase morphology provides a physical network of flexible chains cross-linked by crystalline microdomains. The advantages over natural vulcanized rubbers are that thermoplastic elastomers are readily soluble in an appropriate solvent and can be processed as thermoplastics [109],... 

Thermoplastic elastomers (TPE s) are characterized by the exceptional property that, without vulcanization, they behave as cross-linked rubbers. They are block-copolymers, in which blocks of the same nature assemble in hard domains, acting as cross-links between the rubbery parts of the chain. These hard domains lose their function when they reach their softening temperature, so that the material can then be processed as a thermoplast. One of the oldest member of the family of TPE s is SBS (styrene-butadiene-styrene block copolymer), but several other TPE s have been developed, i.a. on the basis of polyesters, polyurethanes and polyolefins. In their properties these polymers cover a broad range between conventional rubbers and soft thermoplastics. 

As a result of its saturated polymer backbone, EPDM is more resistant to oxygen, ozone, UV and heat than the low-cost commodity polydiene rubbers, such as natural rubber (NR), polybutadiene rubber (BR) and styrene-butadiene rubber (SBR). Therefore, the main use of EPD(M) is in outdoor applications, such as automotive sealing systems, window seals and roof sheeting, and in under-the-hood applications, such as coolant hoses. The main drawback of EPDM is its poor resistance to swelling in apolar fluids such as oil, making it inferior to high-performance elastomers, such as fluoro, acrylate and silicone elastomers in that respect. Over the last decade thermoplastic vulcanisates, produced via dynamic vulcanisation of blends of polypropylene (PP) and EPDM, have been commercialised, combining thermoplastic processability with rubber elasticity [8, 9]. 

They have all the rubberiness of the ethylene/propylene (EPR) rubber matrix, and the crystalline polypropylene (PP) domains hold them together. As saturated elastomers, they have natural resistance to oxygen and ozone aging. They are the second largest class of thermoplastic elastomers, 25 percent of the total market, used mainly in mechanical rubber parts. 

Compression molding is an old and common method of molding thermoset (TS). It now processes TS plastics as well as other plastics such as thermoplastics (TP), elastomers (TS and TP), and natural rubbers (TS). By this method, plastic raw materials are converted into finished products by simply compressing them into the desired shapes... 

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