Thermoplastic Elastomer Mixing

Considerable work has also been conducted to try to find thermoplastic elastomers that can be used to simplify processing by enabling dry blending and melt casting instead of the conventional mixing and curing process (see Elastomers, synthetic). 

Urethanes are processed as mbber-like elastomers, cast systems, or thermoplastic elastomers. The elastomer form is mixed and processed on conventional mbber mills and internal mixers, and can be compression, transfer, or injection molded. The Hquid prepolymers are cast using automatic metered casting machines, and the thermoplastic peUets are processed like aU thermoplastic materials on traditional plastic equipment. The unique property of the urethanes is ultrahigh abrasion resistance in moderately high Shore A (75—95) durometers. In addition, tear, tensUe, and resistance to many oUs is very high. The main deficiencies of the urethanes are their resistance to heat over 100°C and that shear and sliding abrasion tend to make the polymers soft and gummy. 

An entirely new concept was iatroduced iato mbber technology with the idea of "castable" elastomers, ie, the use of Hquid, low molecular-weight polymers that could be linked together (chain-extended) and cross-linked iato mbbery networks. This was an appealing idea because it avoided the use of heavy machinery to masticate and mix a high viscosity mbber prior to mol ding and vulcanization. In this development three types of polymers have played a dominant role, ie, polyurethanes, polysulftdes, and thermoplastic elastomers. 

Considerable amounts of EPM and EPDM are also used in blends with thermoplastics, eg, as impact modifier in quantities up to ca 25% wt/wt for polyamides, polystyrenes, and particularly polypropylene. The latter products are used in many exterior automotive appHcations such as bumpers and body panels. In blends with polypropylene, wherein the EPDM component may be increased to become the larger portion, a thermoplastic elastomer is obtained, provided the EPDM phase is vulcanked during the mixing with polypropylene (dynamic vulcani2ation) to suppress the flow of the EPDM phase and give the end product sufficient set. 

Thermoplastic elastomers that are hard polymer/elastomer combinations are often not truly synthesized. Instead, the two polymers that form the hard and soft phases are intimately mixed on high shear equipment. 

The processing technologies for elastomeric blends, thermoplastic elastomer-based on mechanical mixing, and elastomer-plastic vulcanizates are distinctly different. Depending on the type and nature of blend, size, and their final application, a wide range of processing equipment is now in use both industrially as well as in laboratory scale preparation. 

Mixing of Vulcanisable Rubbers and Thermoplastic Elastomers, RR. Wood. 

Figure 14.13 2D deuterium exchange spectra of a thermoplastic elastomer (tBu-dU4A) [80]. Experimental results at 27 °C with mixing times of (a) tm=l ms, (b) tm=500 ms and (c) tm=3 s indicating the presence of slow 180° phenyl flips... 

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. 

Finally, it should be pointed out that hydrosilylation crosslinking during melt mixing is so efficient that many thermoplastic elastomers where a sec-... 

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. 

In addition to the relative ratio of the monomers, the arrangement of the units in the chain is important. This arrangement is referred to as the copolymer sequence distribution. In the previous discussion, the assumption was made that the comonomer units were well mixed in the polymer chain. If this is not the case, parts of the chain can reflect properties of the corresponding homopolymer. It is thus possible to produce polymers that have significantly different properties in different parts of the polymer chain. A most dramatic example of this can be found in styrene-butadiene-styrene or styrene-isoprene-styrene thermoplastic elastomers. The properties of these unique materials will be discussed in the section Thermoplastic Elastomers. ... 

Copoly(ester ester)s belong to the family of thermoplastic elastomers (TPEs) and consist in general of thermo-reversible hard and elastic soft domains [11]. The copoly(ester ester) used here consists of 60% poly(butylene terephthalate), 35% poly(butylene adipate) and 5% 4,4 -methylenebis(phenyl isocyanate), and shows domain sizes of about 20 nm [12]. The material possesses a rubber plateau between the glass transition temperature of the mixed amorphous PBA/PBT phase (the PBT phase is semi-crystalline) at about -30°C and the melting point of the PBT at about 220°C. Due to the vulnerability of the amorphous PBA/PBT soft domains towards water attack [13] the PBT/PBA copoly(ester ester) is used here to study the existence of ESC of a chemical rather than a physical nature. For the sake of clarity it should be emphasized that no additives have been used in the copoly(ester ester) described here. 

The segmented block copoly(ether ester)s, based on poly(butylene terephthalate) (PBT) and on polyoxytetramethylene (POTM) are typical examples for this class of thermoplastic elastomers . They can be described as random copolyesters of tereph-thalic acid with 1,4-butanediol and a-hydro-hard phase whereas the remaining ester segments mix with the ether segments and build up the soft matrix. 

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