Degradation injection moulding

Another event which may occur is hydrolysis. This is a chemical reaction between the plastic and water. It occurs extremely slowly at room temperature but can be significant at moulding temperatures. Hydrolysis causes degradation, reduction in properties (such as impact strength) and it is irreversible. Table 4.3 indicates the sensitivity of plastics to moisture. Note that generally extrusion requires a lower moisture content than injection moulding to produce good quality products. 

In summary, therefore, processing, and in particular injection moulding, can introduce limited chemical degradation, local polymer orientation, orientation of short fibre reinforcements, internal stresses, warpage, shrinkage and defects such as weld lines and voids. 

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)... 

Gomes, M. E., Ribeiro, A. S., Malafaya, R. B., Reis, R. L., Cunha, A. M. (2001). A new approach based on injection moulding to produce biodegradable starch based polymeric scaffolds Morphology, mechanical, and degradation behaviour. Biomaterials., 22, 883-889. 

Fig.36. Variation in electrical conductivity (o) with molecular weight for polyethylene composites filled with 4% by volume carbon black, demonstrating the effects of orientation (I), degradation (II) and flow-induced segregation of carbon black aggregates (III). ( ) injection moulded (O) compression moulded (unoriented) [181]...

The dithiocarbamates have the pentacoordinate binuclear structure (44). The diamyl- and diethyl-dithiocarbamate complexes have been found to inhibit the hardening of asphalt, but the effect appears too weak to be useful.127 The latter complex is an effective antioxidant for polyethylene,128 polypropylene,129 polystyrene,130 poly(methyl methacrylate)130 and an isoprene-styrene copolymer.131 The di-n-butyldithiocarbamate complex is important in the vulcanization and injection moulding of rubber,132 as a stabilizer against photolytic and thermal degradation. 

It is known (Friedrich Fakirov, 1985) that injection moulding of PET which has not been carefully dried results in a serious degradation of its mechanical properties profile due to hydrolitic degradation of the molecular weight, leading to an increase... 

Styrene copolymer. The di-n-butyldithiocarbamate complex is important in the vulcanization and injection moulding of rubber, as a stabilizer against photolytic and thermal degradation. 

Keywords Block copolymers, Interface reactive injection moulding, Polyamide degradation, Polyolefine polyamide graft copolymers, PTFE polyamide materials... 

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... 

---