Preparation of TPEs

Although blending is an easy method for the preparation of TPEs, most of the TPE blends are immiscible. Very often the resulting materials exhibit poor mechanic properties due to the poor adhesion between the phases. Over the years different techniques have been developed to alleviate this problem. One way is to alter the blending technique so that the interfacial area between the component phases can be increased. By the proper selection of the processing technique either a co-continuous or... 

Unlike conventional rubbers, TPEs are processed on thermoplastic machinery. Preparation requires no separate vulcanization stage, is easily done by internal mixers or extrusion and so productivity is high. The various methods of preparation of TPEs are discussed below. 

The use of living cationic polymerization in the preparation of TPEs was reviewed by Kennedy [45] in relation to graft and block copolymers, but the application of cationic polymerization to TPEs began before the arrival of the... 

The contribution of the difunctional oligomers to the preparation of TPEs should enjoy an important development their use is not limited to classical chain polymerization and polycondensation since they can be also applied to less common processes, such as the metathesis. A functional diene, for instance an a,ua-divinyl aliphatic or aromatic ester, is polyadditioned with an a,u)-divinyl-(soft-oligomer) in the presence of a metathesis catalyst, e.g., a ruthenium derivative [64,65]. 

The preparation of TPEs is closely related to the control of their structure and morphology, all the more that they are structurally complex systems, requiring accurate, efficient, and rapid analytical techniques. The characterization techniques are applied to the TPEs and their precursors. During the last thirty years, they enjoyed a fantastic development some of them appeared during the last decade. In the following, they are grouped into analytical branches, but it is essential to keep in mind that most of them are associated with some others. 

The sequential reactions are rapidly developing and their contribution to the preparation of TPEs, particularly of block copolymers, will drastically increase they were analyzed by Marbchal [39]. The sequences must not be limited to chain polymerization and polycondensation, and chemical modification will play an important role not only in grafting. 

TPEs from blends of rubber and plastics constitute an important category of TPEs. These can be prepared either by the melt mixing of plastics and rubbers in an internal mixer or by solvent casting from a suitable solvent. The commonly used plastics and rubbers include polypropylene (PP), polyethylene (PE), polystyrene (PS), nylon, ethylene propylene diene monomer rubber (EPDM), natural rubber (NR), butyl rubber, nitrile rubber, etc. TPEs from blends of rubbers and plastics have certain typical advantages over the other TPEs. In this case, the required properties can easily be achieved by the proper selection of rubbers and plastics and by the proper change in their ratios. The overall performance of the resultant TPEs can be improved by changing the phase structure and crystallinity of plastics and also by the proper incorporation of suitable fillers, crosslinkers, and interfacial agents. 

De Sarkar et al. [52] have reported a series of new TPEs from the blends of hydrogenated SBR and PE. These binary blends are prepared by melt mixing of the components in an internal mixer, such as Brabender Plasticorder. The tensile strength, elongation at break, modulus, set, and hysteresis loss of such TPEs are comparable to conventional rubbers and are excellent. At intermediate blend ratio, the set values show similarity to those typical of TPEs (Table 5.5). 

Ionic polymers are a special class of polymeric materials having a hydrocarbon backbone containing pendant acid groups. These are then neutralized partially or fully to form salts. lonomeric TPEs are a class of ionic polymers in which properties of vulcanized rubber are combined with the ease of processing of thermoplastics. These polymers contain up to 10 mol% of ionic group. These ionomeric TPEs are typically prepared by copolymerization of a functionalized monomer with an olefinic unsamrated monomer or direct functionalization of a preformed polymer [68-71]. The methods of preparation of various ionomeric TPEs are discussed below. 

Polyurethane-based FTPEs are produced by reacting fluorinated polyether diols with aromatic disocyanates. The resulting block copolymers contain fluorinated polyether soft segments.68 Another possible method of preparation of fluorinated TPE is dynamic vulcanization. Examples are a blend of a perfluoroplastic and a perfluoroelastomer containing curing sites or a combination of VDF-based fluo-roelastomers and thermoplastics, such as polyamides, polybutylene terephtalate, and polyphenylene sulhde.69 70... 

It is clear that the most important aspect of C -symmetric zirconocenes is the wide variability of their isospecificity, which has allowed the preparation of novel PP materials. This avenue has been opened by Chien and Rausch, who reported the preparation of thermoplastic—elastomeric polypro-pene (TPE-PP) with CpI-S-anti and its more active isomer The Zr analogue of Ci-... 

Similarly the disadvantages of PC are the stress cracking and chemical sensitivity. Stress cracking can be treated as a part of impact properties and a simple solution may thus be addition of ABS or ASA. On the other hand, to improve the solvent resistance—a property that is particularly important in automobile applications—a semicrystalline polymer may be added. From Table 4.37, it is apparent that TPEs (e.g., PBT, PET) could provide that property, but they also lack warp resistance and impact strength. Hence an ideal blend for automobile application based on PC and TPEs should be impact modified with, for example, an acrylic latex copolymer. A schematic of preparation of this type of toughened blend introduced by GEC-Europe in 1979 under the tradename Xenoy is shown in Figure 4.41. 

TPOs (olefinic blends) comprise a lower-performance, lower-cost class of TPEs (Fig. 4.39). Their performance and properties are generally inferior to those of thermoset rubbers. Yef they are suitable for uses where (1) the maximum service temperature is modest (below 80°C), (2) nonpolar flnid resistance is not needed, and (3) a high level of creep and set can be tolerated. Thns, TPOs are marketed more on the basis of cost rather than performance, competing directly with the lower-cost general-purpose rubbers (NR, SBR, and the hke). TPOs are associated with the traditional practice of rnbber componnd-ing and mixing. They can be prepared by the same techniques and equipment as for thermoset mbber however, they need to be processed at temperatures above the of the thermoplastic hard phase. The amounts of elastomer, rigid thermoplastic, plasticizer, and other ingredients can be varied to achieve specific properties in much the same manner as with rnbber componnds. 

Further evidence for surface activation of active uranium by alcohols was seen in the reaction of active uranium with benzophenone in the presence of methanol. When the methanol was added along with the benzophenone to freshly prepared active uranium, TPE production could be observed even at room temperature (3 h, 13%). After 70 h at 50°C, 38% TPE was seen. In the absence of methanol, only negligible amounts of TPE can be formed even at 50°C. 

In addition to aromatic solvents, we have seen reactivity of active uranium prepared in 1-decene and TMEDA. The yields of TPE and TPA resulting from preparation and reaction of active uranium with benzophenone in 1-decene are only slightly lower than those in aromatic solvents, presumably due to solubility considerations. There was little difference in the proportion of TPA to TPE. It appears that decene does not serve as either a hydride source (allylic hydrogens) or a hydride sink (via hydrogenation of the double bond). The preparation and reaction of active uranium with benzophenone in TMEDA gave yields of TPE comparable to other aromatic solvents at that temperature ( 20%). Negligible amounts of TPA were seen, however. The presence of a large excess of a basic solvent could serve to reduce the amount of metal hydrides present. 

When Szwarc et al. discovered [15,16], or rediscovered [17,18], the anionic living polymerization, a completely different preparation of these elastomers was proposed the study of TPEs passed from infancy to maturity. These authors used sodium metal naphthalene diinitiators to prepare poly (styrene-l>-isoprene-6-styrene), which was probably the first TPE with a perfectly defined structure. However, this copolymer could not be commercialized, as most of the poly-isoprene units were -3,4-, with poor elastomeric properties. It is only when the polymerization was initiated by alkyllithium that poly(styrene-l>-isoprene- -styrene) and poly (styrene-butadiene- -styrene) were obtained with the classical TPE properties very high tensile strength and elongation at break, very rapid elastic recovery, and no chemical crosslinking. Bailey et al [19] announced the existence of these materials in 1966 and Holden et al [20] published the corresponding theory in 1967 and extended it to other block copolymers. 

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