Thermoplastic elastomers recycling

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

Thermoplastics are more suitable for recycling than elastomers or thermosetting polymers. Thermoplastics can be heated above their melting temperatures and then recast into new shapes. Elastomers and thermosets, on the other hand, have extensive cross-linking networks that must be destroyed and then reformed in the process of recycling. Processes that destroy cross-linking, however, generally break down the polymer beyond the point at which it can be easily reconstituted. 

Health and Safety Issues. Polyesterether elastomers derived from dimethyl terephthalate, butanediol, and Ptmeg are not hazardous according to the published Materials Safety Data Sheets (MSDS) for this elastomer. Polymers of a similar structure containing isophthalic acid are also not considered hazardous. For other copolymer elastomers, the MSDS put out by suppliers should be consulted by potential users before evaluation. One environmental advantage of thermoplastic elastomers of this type is that they are melt-reprocessible and thus scrap and off-specification material and even obselete parts can be easily recycled. Up to 25% by weight of recycled material can be incorporated (see Recycling, plastics). 

Rather than worry about whether a copolymer of two different monomers is heterogeneous or not, why not just make homopolymers of each and mix them together (make a polymer blend) The answer is easy Most polymers do not like other polymers, and, on mixing, each will tend to segregate into a phase that excludes any other polymers (will phase separate). These different phases tend not to stick to each other very well, resulting in a polymer sample with poor physical properties. We will discover the practical consequences of this in Chapter 7 when we discuss the properties of certain block copolymers called thermoplastic elastomers and in Chapter 9 when we discuss the recycling of plastics. 

The market of PP/EPDM blends has grown dramatically because of its recycling abihty and processability by conventional thermoplastic processing equipment. The unique characteristics of thermoplastic elastomer made it an attractive alternative to conventional elastomers in a variety of markets. Liu et al. showed from the experimental blends (53) that materials cost reduction of between 30% to 50% is possible in comparison to commercial products if one applies the PP/EPDM blends to the construction of a basketball court, a tennis court, and a roller hockey rink, which were estimated around 7000, 14,000, and 40,000, respectively. The cost comparison took into account the percentage of rubber or PP used in experimental blend, the exponential factor for a scale-up process and the overall surface area of the specific applications. Among many possible application of this blend two readily feasible applications are roofing and flooring. 

Another application of the injection system consists of recycling old and waste rubber. The reactive mixing of crumb rubber with a melted polymer represents an innovative way of recycling these waste rubbers [6]. Some thermoplastic elastomer may contain up to 60% recycled rubber derived from used vehicle tires, and thus through the reactive injection system the rubber scrap is revulcanized [7]. 

Styrene-butadiene block copolymers belong to a new class of polymers called thermoplastic elastomers (TPE). Products made from these polymers have properties similar to those of vulcanized mbbers, but they are made from equipment used for fabricating thermoplastic polymers. Vulcanization is a slow and energy-intensive thermosetting process. In contrast, the processing of thermoplastic elastomers is rapid and involves cooling the melt into a rubberlike solid. In addition, like true thermoplastics, scrap from TPE can be recycled. 

PROPERTIES OF SPECIAL INTEREST In general, thermoplastic elastomers (TPE) provide the mechanical properties of rubber in combination with the processing characteristics of plastics recyclable Kraton D s are the lowest cost TPE (> 0.85 Ib ). 

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