Degradation during processing

The conditions during melt processing are extreme compared to those during the subsequent product life. Melt temperatures are high, as is the diffusion coefficient for oxygen, and mechanical stresses can be applied. Processing conditions and additives should be chosen so that the chemical 

Temperature at which half the mass of a polymer is lost on heating in a vacuum vs. the bond dissociation energy (from Kelen, T, Polymer Degradation, Van Nostrand Rheinhold, 1983). 

Polypropylene is more susceptible to melt degradation than polyethylene, because of the presence of more reactive tertiary hydrogen atoms (attached to the carbon atom that is bonded to three other C atoms). At a temperature of 270 °C in injection moulding, tertiary alkyl free radicals R are generated thermally. If oxygen is present, a rapid reaction (R + 02- ROO ) produces a peroxide radical, which reacts further to form hydroperoxides (ROO -h RH ROOH-h R ). When the dissolved oxygen is used up, there is a greater chance of the chain scission reaction 

Hindered phenol stabilisers scavenge alkyl radicals and prevent chain scission. An example is butylated hydroxytoluene (BHT) 

Melt degradation is used commercially to narrow the molecular weight distribution, from a starting value of Mw/Mn = 5, to make it more suitable for fibre spinning and blow moulding. Random chain scission at tertiary C—H bonds eventually produces a most probable molecular weight distribution with M /Mn = 2. In practice, the distribution, produced by deliberately degrading the polypropylene with added peroxides, is broader, but 

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ACS polymers, developed primarily in Japan, are grafts of acrylonitrile and styrene onto elastomeric chlorinated polyethylene. Although the polymer has good weathering properties it is somewhat susceptible to thermal degradation during processing and to date these polymers have been of limited interest. 

Evaluates recent advances in analytical methodology for detecting both natural and synthetic colorants, their quality, quantity, and degradation during processing and storage... 

Increased resistance to degradation during processing or apphcation... 

Working with a solution is needed for polymers which above their melting point would degrade (example aromatic polyamide fibres such as Kevlar and Twaron). For fibres the removal of the solvent is not too problematic. In e.g. injection moulding applications solvents caimot be used here thermotropic LCP s have to be used. Since these would degrade during processing, they are diluted by copolymerisation (example poly-hydroxy-benzoic acid - co - PETP)... 

It may be anticipated that the antioxidant consumption during use leads to a shorter remaining life-time and that it is necessary to add more. Recycled plastics suffer from the consequences of degradation during processing and first-life application. This leads to the introduction of new functional groups, which in particular for oxidisable polymers such as PE, enhance the sensitivity of the recyclate to thermal- and photo-degradation. With the formation of new pro-oxidative moieties, a substantial part of the stabilisers are simultaneously consumed [14]. 

It should also be noted that maple syrup contains high molecular weight polysaccharides (80) which probably are also partially degraded during processing and thus can serve as sources for additional flavor compounds. 

Ethanox 376 is a stabilizer that provides heat stability by preventing thermo-oxidative degradation during processing and service life. It provides compatibility with resins and extraction resistance. It can be applied in polyolefins, such as polyethylene, polypropylene, polybutene-1 and other polymers such as engineering plastics, styrenes, polyurethanes, saturated and unsaturated elastomers, styrenics, rubber modified styrenics, segmented block copolymers, and PVC. 

With the analysis embodied in Fig. 12, the formation of carbon fiber reinforced Si3N4 matrix materials by directed metal oxidation of molten silicon can also be described. To avoid fiber degradation during processing, the fiber should, ideally, be thermodynamically stable with respect to... 

While solution crystallization is the main focus of this book, there are applications which are special cases of the CrystaUization methods described in earher chapters. For example, a process requiring sterile crystallization is a special example of an antisolvent addition and is presented in Example 11-1. In this particular example, it is necessary to produce fine particles in order to meet drug dissolution time specifications in the final single-dose vials. Furthermore, the material is thermally unstable. Therefore, special precautions have to be taken to minimize any degradation during processing. 

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