Plastic flow terms Links

Originally, vulcanization implied heating natural rubber with sulfur, but the term is now also employed for curing polymers. When sulfur is employed, sulfide and disulfide cross-links form between polymer chains. This provides sufficient rigidity to prevent plastic flow. Plastic flow is a process in which coiled polymers slip past each other under an external deforming force when the force is released, the polymer chains do not completely return to their original positions. 

The techniques of forming thermoplastics are obviously influenced by the fact that the molecules of thermoplastics do not cross-link on heating and therefore can be maintained in a softened state while being made to flow under pressure into a new shape. The properties of each thermoplastic, however, sometimes limit the nature of the possible processes in which it will be involved. Some, for example, will be flexible enough to be ejected from a mould having undercuts in it others will be too brittle to withstand such a treatment. It is therefore preferable to know about each plastics in terms of properties and applications in order to appreciate which forming techniques may be most suited to it. 

In practical terms, the use of poly (vinyl chloride) in the manufacture of plastic raincoats provides a good illustration of the effect of crystallite cross-linking. The polymer is plasticized, nntil is below ambient, to make the material flexible, and one might expect that if the coat was hung on a hook (i.e., subjected to a tensile load), it would evenmally flow onto the floor after prolonged tension. This is not so the material behaves as though it was a chemically cross-linked elastomer because it contains a sufficient number of crystallites to act as restraining points and prevent flow. 

First of all, let s start with the problem of polymer flow. This also happens to be an important practical issue, because, during shaping and processing, polymers must undergo fluid flow (the "plastic" state). Long chains (which are not cross-linked) may slide and flow like any other liquid—when in solution or in a state that allows motion (above Tg, in the case of an amorphous polymer, or above T for a crystalline polymer). Compared to simple liquids, polymers are very different and have extremely high viscosity and a special flow characteristic, which is termed "non-Newtonian." It is therefore appro-... 

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