Mechanical mixing, polymer blends

Systems with concentrations between the two spinodal points will be unstable and decompose spontaneously. Polymer mixtures with overall concentrations between the binodal and the spinodal, will be metastable, and decompose following a nucleation-and-growth mechanism. For polymer blends with concentrations outside the two binodals, the system is mixed in a thermodynamically stable single phase. 

There are three methods of making polymer blends mechanical blending, solution mixing, and chemical synthesis. This chapter will focus only on the mechanical blending of polymers. 

Polymaric plasticizars can ba mada by (1) Internal plasticization whoroby a monomor is copolymorizod with on which tends to yield soft polymers by itself (2) Mechanical mixing of a polymerizable monomer with a polymer, followed by polymerization (3) Mechanical blending of two compatible polymers. In many cases It Is necessary to combine the polymeric plasticizer with a liquid plasticizer because the compatibility of polymers with each other is generally limited. From the industrial polymeric plasticizers, especially polyesters of low degree of polymerization and several copolymers of butadiene with acrylonitrile, acrylic add esters and fumaric add esters were studied. These polymeric plasticizers are characterized by good compatibility and improved cold resistance of the final product. 

Texture is important in polymer processing because (a) laminar and even chaotic distributive mixing inevitably lead to it, (b) many products are visually examined for lack of texture or for a certain desired texture, and (c) mechanical properties of blends depend on the texture of the mixture. 

Over the last decade, the poor economics of new polymer and copolymer production and the need for new materials whose performance/ cost ratios can be closely matched to specific applications have forced polymer researchers to seriously consider purely physical polymer blend systems. This approach has been comparatively slow to develop, however, because most physical blends of different high molecular weight polymers prove to be immiscible. That is, when mixed together, the blend components are likely to separate into phases containing predominantly their own kind. This characteristic, combined with the often low physical attraction forces across the immiscible phase boundaries, usually causes immiscible blend systems to have poorer mechanical properties than could be achieved by the copolymerization route. Despite this difficulty a number of physical blend systems have been commercialized, and some of these are discussed in a later section. Also, the level of technical activity in the physical blend area remains high, as indicated by the number of reviews published recently (1-10). 

Converting polymers to almost 35,000 plastics includes mechanical mixing/blending one or more polymers with additives, fillers, and/or reinforcement. They do not normally depend on chemical bonds, but do often require special compatibilizers. Mechanical compounding is extensively used (Chapter 5). 

For economic reasons we achieved the same quality blends by mechanically mixing commercial polystyrene with the block polymers. Good quality products were obtained from two-roll mills, the Brabender plastograph, the Banbury mixer, and continuous high intensity mixers. 

Three main ways exist for making homogeneous blends of polymers mechanical mixing, mixing in a common solvent, and in situ polymerisation. Mixing in a common solvent has been the commonest method in academic studies though this method, including the recovery of the blend by solvent evaporation or precipitation of the polymers in a non-solvent, would have very limited industrial application. 

It seems possible that complete homogeneity may be close to impossible to achieve by mechanical mixing of two high molecular weight polymers. It will however remain to be of the greatest practical importance in the preparation of commercial blends. 

Polymers are frequently mixed or blended in order to optimize some combination of thermal, mechanical, and other properties without the work and expense of designing and synthesizing new polymers with the desired properties. Unusual combinations of... 

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