Polypropylene hydrolysis, resistance

Polyesters make tough and wear-resistant urethanes. The one major drawback is the hydrolysis at the ester grouping. The hydrolysis can either be acid or alkali promoted. In more neutral conditions, the major breakdown product is normally adipic acid that catalyses further attack. The normal approach is to use carbodiimides to block further breakdown. Polyols made using polypropylene carbonate produce polyurethanes with polyester characteristics but with enhanced hydrolysis resistance. 

Polypropylene resists hydrolysis well but is naturally sensitive to light and UV. It must be protected by addition of anti-UV and other protective agents or by 2-3% of a suitable carbon black. 

The most common material used is cellophane, which is a cellulose film, which acts as a membrane and is capable of resisting zinc penetration. The cycle life of cells utilizing this material is severely limited due to the hydrolysis of the cellophane in alkaline solution. Various methods have been tried to stabilize cellulose materials, such as chemical treatment and radiation grafting to other polymers, but none have, as of now proved economically feasible. The most successful zinc migration barrier material yet developed for the nickel—zinc battery is Celgard microporous polypropylene film. It is inherently hydrophobic so it is typically treated with a wetting agent for aqueous applications. 

Two elastomers are made based on statistical copolymers of ethylene and propylene EPDM, in which the diene portion, D, serves as a cross-linking site, and its non-cross-linking counterpart, EPM. Since these materials have few reactive sites, they are relatively impervious to oxidation or hydrolysis. Frequently they are blended with either polyamides or polypropylene to form impact-resistant plastics see Section 13.8. 

Polypropylene glycol polyol represents a typical polyether polyol. With the ether linkage, the polyurethanes made from this are generally more flexible at lower temperatures. Also, polyurethanes based on this type of polyol backbone are more resistant to hydrolysis attack by water (especially in alkaline solution) than polyurethanes with backbones of polyester. 

PHB has a melting temperature (Tm) of 180"C, a glass transition temperature (Tg) of 5 "C and a high molecular weight. It is naturally not crystalline, and is converted in a more crystalline form during the extraction process. Research has been undertaken to avoid this transformation step that causes a decrease in the mechanical properties. The properties of PHB are similar to those of polypropylene, except for its biodegradability. It is also more rigid, more brittle and denser than PP. It resists oxidation but presents low chemical resistance. PHB is insoluble in water and relatively resistant to hydrolysis, the opposite of most biopolymers. 

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