Elastomers melt processable rubbers

Wallace J.G., Single phase melt processible rubber. Handbook of Thermoplastic Elastomer (Walker B.M. and Rader C.P., eds.). Van Nostrand Reinhold, New York, 1988, 141. 

EA-MPR Elastomer alloy melt processable rubber Alcryn EA-TPV Elastomer alloy thermoplastic vulcanizate Lomod Santoprene... 

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). 

A comparatively new group of materials— thermoplastic elastomers or thermoplastic rubbers —combines the ease of processing of thermoplastics with qualities of traditional vulcanized rubbers, especially elasticity. Because of convenience in processing there is much interest too in blends of plastics with elastomers, which may be modified by the inclusion of filler or glass fibre. As an example, a rubber-like material that can be processed as a thermoplastic can be made by blending and melt-mixing an ethylene-propylene rubber with polypropylene. The use of such blends may be helpful when there are needs to reclaim and re-process material, and in order to obtain products with qualities intermediate between those of the main components of the blends. 

Blends of butadiene-acrylonitrile copolymer rubber (nitrile rubber or NBR) and PVC are among the oldest known examples of commercial elastomer/ thermoplastic blends. The shortage of natural rubber during World War II stimulated research in the USA on the compounding and modification of synthetic polymers to produce rubber-like materials. An outcome of this research was the commercial introduction of NBR/PVC blends by B.F. Goodrich in 1947 under the trade name of Geon Polyblends [Pittenger and Cohan, 1947]. The blend showed improved ozone resistance and melt processability compared to the nitrile rubber (Table 15.12). 

Alcryn A trade name for DuPont s fanuly of melt-processable elastomers and rubbers. 

The EP thermoplastic elastomers are distinguished from the crossUnked analogues, which are not thermoplastics since reforming is impossible. A very important thermoplastic elastomer is comprised of a blend of an EP copolymer with an ethylene-propylene-diene (EPDM) terpolymer. This latter material is, of course, a crosslinkable thermoset however, these materials can be processed as thermoplastics if the crosslinkable component is present at low enough concentration to be present as an isolated phase. Melt-processing causes the formation of chemical bonds within the isolated rubber phase, a process called dynamic vulcanization. A commercial example of this type of material is Santoprene [4] manufactured by Advanced Elastomer Systems. Other blends of noncrosslinkable TPEs with crosslinkable materials are used commercially. These materials are classified as elastomer blends and are the subject of Chapter 12. 

In contrast to the rigid TPOs described above, low-modulus/flexible grades of TPO blends are also produced commercially. In flexible TPOs, the rubber content can be as high as 60 %, and in some cases, the dispersed rubber may also be partially cross-linked during the mixing without losing the thermoplastic character of the matrix. However, the latter type of dynamically vulcanized elastomeric alloys or thermoplastic vulcanizate rubbers (TPVs) are considered as a separate class of elastomeric materials and hence will be discussed under elastomer blends. On the other hand, the soft TPO blends discussed here contain a low-modulus olefln copolymer elastomer as the major component with some polypropylene added to impart melt processability. 

Dynamically vulcanized, elastomeric thermoplastic alloys or TPVs display properties as good as or even better than the block copolymers, viz., a high degree of rubber elasticity yet good melt processability. The main advantages of the thermoplastic vulcanizate elastomer blends over the uncured thermoplastic/elasto-mer blends are... 

Thermoplastic polyurethane elastomers are polymers that bridge the gap between rubbers and plastics. They can be used in a wide range of properties, from hard rubbers to soft engineering thermoplastics as they are elastic and melt-processable. They can be processed on extrusion as well as injection, blow and compression molding equipment. They can be vacuum-formed or solution-coated and are suited for a wide variety of fabrication methodologies. They provide a considerable number of physical property combinations high resilience, good compression set, resistance to abrasions, tears, impacts, weather, and even hydrocarbons. Such materials... 

The appearance of several new types of rubbers called thermoplastic elastomers was apparently a most exciting development in recent years. These materials have properties of conventional rubbers such as high strength, and they are readily fabricated by various melt-processible techniques, which are characteristic of thermoplastic materials because they do not have any chemical cross-links in the... 

Polyurethane rubbers have now been used as seal materials for some time on account of their unique ability to combine resistance to swelling in oil with high strengths and high stiffnesses. Their ability, in some classes, to be processed as thermoplastics, is also considered useful as manufacture of the seal can then be automated and hence quality is more reproducible. A limitation in their use has been that they depend upon physical types of crosslinking for their elastic and strength properties and when certain specific temperatures are reached these crosslinks rapidly weaken and the polyurethane elastomer melts and fails. At present most rubber seals are made from vulcanized covalently crosslinked rubbers where crosslinks are based on sulphur or carbon, and these do not melt at elevated temperatures, but instead decompose. 

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