Polyamides, Polycarbonates and Polyacetals

This section describes the chemical recycling, via chemolysis, of certain condensation polymers which, although being produced in significantly lower amounts than polyesters and polyurethanes, are used in important applications, and so also contribute to the plastic waste stream. 

Polyamides are obtained either by the condensation of a dicarboxylic acid and an alkylene diamine or by the head-to-tail condensation between an amino carboxylic acid or the corresponding lactam. Polyamides may have aliphatic or aromatic chain backbones. Aliphatic polyamides (nylon) have the most important commercial applications, mainly in the manufacture of fibres. Nylon-6 and nylon-6,6 account for around 85% of all nylon currently used. Nylon-6 is derived from the polymerization of e-caprolactam, whereas nylon-6,6 is obtained by the condensation of hexamethylene diamine and adipic acid. 

At present, the principal method of polyamide chemical recycling is hydrolysis, which can be carried out under neutral, acidic or basic conditions. Several commercial processes are already operating with this technology.2 

Acid hydrolysis by contact with superheated steam has been mainly applied 

In the case of nylon-6,6, basic hydrolysis is the preferred treatment. Depolymerization occurs by reaction with sodium hydroxide, hexamethylene diamine and sodium adipate being the initially formed products. In a second step, the latter is converted into adipic acid via acidification with HC1. 

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