Production of Polymer Blends

Traditional ZN catalysts are typically complex heterogeneous systems, consisting of multiple active sites each of which produces polymers and copolymers with different structure (e.g., tacticity, molecular weight, composition). The result is the production of polymer blends. Controlling blend composition through modification of the heterogeneous catalyst surface was challenging and dominated R D in this area for decades. 

In non-reactive blending, a two- (or multi-)phase mixture is formed when the immiscible polymers are physically mixed with each other. The minor phase, rich in B, is dispersed as droplets into a major phase rich in A. Apart from low interfadal tension, high shear rates and similar viscosities of both polymers are important for the size of the dipersed phase and therefore for the product quality. The reactive route follows the synthesis of a minor component via polymerization into a major component that acts as a host polymer. An alternative route for reactive blending is in situ formation of block co-polymers during the mixing process to decrease the interfacial tension. An extruder is the most commonly applied apparatus for the continuous production of polymer blends. 

Most Critical Step in the Production of Polymer Blends - Melting/Mixing I 243... 

In the previous section we discussed dispersive mixing in solid-liquid systems. Another important type of dispersive mixing is in liquid-liquid systems. This occurs when we mix incompatible or partially incompatible polymer melts. The production of polymer blends is very important in the polymer industry as a result, liquid-liquid dispersive mixing has received significant attention over the last two or more decades. 

Another area of current research deals with combining the earlier work on production of polymer blends in the elastic melt extruder and the observation of the maximm in melt elasticity as a fimction of composition as fo md in the orthogonal rheometer studies on blends. It would be expected that as the melt elasticity increased the forces required for melt orientation would also increase. This has been confirmed in current studies of melt tension in fiber spinning experiments. The important point is that the composition of the blend can be used to control the molecular orientation. 

Ternary Blends. Discussion of polymer blends is typically limited to those containing only two different components. Of course, inclusion of additional components may be useful in formulating commercial products. The recent Hterature describes the theoretical treatment and experimental studies of the phase behavior of ternary blends (10,21). The most commonly studied ternary mixtures are those where two of the binary pairs are miscible, but the third pair is not. There are limited regions where such ternary mixtures exhibit one phase. A few cases have been examined where all three binary pairs are miscible however, theoretically this does not always ensure homogeneous ternary mixtures (10,21). 

Whilst the volume production of completely new polymers which have achieved commercial viability in recent years has been small, the development of polymer blends has been highly significant. Of these the most important involve a glassy... 

In polymer blends, or mixtures, the primary question is whether one of the components segregates preferentially to the surface. One of the reasons this is of interest is because most commercial polymers contain more than one component and a surface segregation of one of the components from a miscible mixture during, for example, extrusion of the material, could affect the surface finish of the product. Because polymer blends are generally dense liquids, from the previous discussion it is clear that packing effects are expected to dominate the surface properties. 

S. Patachia, Blends based on poly(vinyl alcohol) and the products based on this polymer , in Handbook of Polymer blends and composites , C. Vasile and A.K. Kulshreshtha (eds.), Chap. 8, RAPRA Technology LTD., England, Chap.8. 2003. p. 288-365. 

Even the outstanding mechanical properties of natural products are based on the principle of polymer blends, like, for example, wood, which is composed in a complicated way of cellulose and lignin. 

EOS models were derived for polymer blends that gave the first evidence of the severe pressure - dependence of the phase behaviour of such blends [41,42], First, experimental data under pressure were presented for the mixture of poly(ethyl acetate) and polyfvinylidene fluoride) [9], and later for in several other systems [27,43,44,45], However, the direction of the shift in cloud-point temperature with pressure proved to be system-dependent. In addition, the phase behaviour of mixtures containing random copolymers strongly depends on the exact chemical composition of both copolymers. In the production of reactor blends or copolymers a small variation of the reactor feed or process variables, such as temperature and pressure, may lead to demixing of the copolymer solution (or the blend) in the reactor. Fig. 9.7-1 shows some data collected in a laser-light-scattering autoclave on the blend PMMA/SAN [46],... 

Previous sections have dealt with some of the fundamental issues of the technology of polymer blends, and it should be quite clear that there are many important questions which remain unanswered. Despite this lack of fundamental guidance, there has been a strong effort to develop commercially attractive products from polymer blends, and a considerable number of these products are on the market today. In this section, we will give a brief overview of the status of this commercial practice however, it will be useful to first give some rationale for this commercial interest in the concept of polymer blending. 

Control of this property is possible by controlling the structure of the polymer chain. Monomers with bulky side groups restrict chain mobility and thus raise the glass transition temperature. The composition of copolymers and the ratio of polymer blends often are determined by the desired glass transition temperature of the final product. 

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