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The onset of thermal degradation dictates the maximum temperature at which a polymer can be used. It is an important limitation in how the polymer is manufactured and processed. For instance, polymers become less viscous at higher temperatures which makes injection moulding easier and faster, but thermal degradation places a ceiling temperature ...
Polymers are composed of long molecular chains which form irregular, entangled coils in the melt. Some polymers retain such a disordered structure upon freezing and readily convert into amorphous solids. In other polymers, the chains rearrange upon freezing and form partly ordered regions with a typical size of the order 1 micrometer. [3]
Ceiling temperature is a measure of the tendency of a polymer to revert to its constituent monomers. When a polymer is at its ceiling temperature, the rate of polymerization and depolymerization of the polymer are equal. Generally, the ceiling temperature of a given polymer is correlated to the steric hindrance of the polymer’s monomers.
Polymer degradation is the reduction in the physical properties of a polymer, such as strength, caused by changes in its chemical composition.Polymers and particularly plastics are subject to degradation at all stages of their product life cycle, including during their initial processing, use, disposal into the environment and recycling. [1]
The thermal characteristics of polymers are fundamentally different from those of most solid materials. Solid materials typically have one melting point, the T m, above which the material loses internal molecular ordering and becomes a liquid. Polymers have both a melting temperature T m and a glass transition temperature T g.
The higher its value, the slower the conduction of heat produced by viscous dissipation and hence the larger the temperature rise. [2] [3] In, for example, a screw extruder, the energy supplied to the polymer melt comes primarily from two sources: viscous heat generated by shear between elements of the flowing liquid moving at different velocities;
Melting zone (also called the transition or compression zone): most of the polymer is melted in this section, and the channel depth gets progressively smaller. Metering zone (also called the melt conveying zone): this zone melts the last particles and mixes to a uniform temperature and composition.
Unlike classical polymer melts, whose flow properties are largely dependent on friction between monomers, vitrimers become a viscoelastic fluid because of exchange reactions at high temperatures as well as monomer friction. [11] These two processes have different activation energies, resulting in a wide