Hydrolysis Resistance of Polylactic Acid Enhanced by Combined Use of Monocarbodiimide and Aromatic Polycarbodiimide

We have found that combined use of a monocarbodiimide (mono-CDI: bis-2,6-diisopropylphenylcarbodiimide) and an aromatic polycarbodiimide [aro-pCDI: poly (2,4,6-triisopropylphenylene-1,3-diisocyanate)] improves the hydrolysis resistance of polylactic acid (PLA). The hydrolysis time was evaluated by measuring molecular weights of PLA in the PLA compounds including carbodiimides under high temperature and humidity (80 degrees C, 95%RH). When using representative carbodiimides including mono-CDI, aro-pCDI, and an aliphatic polycarbodiimide [ali-pCDI: poly(1,1'-dicyclo hexylmethane-4,4'-diisocyanate)], the hydrolysis time was not satisfactorily enhanced. On the other hand, when using mono-CDI and aro-pCDI at the same time, the hydrolysis time was improved remarkably; the molecular weight of PLA was kept for 600 h, while the original PLA hydrolyzed up in to 50 h. This is mainly due to effective binding of aro-pCDI with hydrolysis products of PLA, which resulted from capturing carboxyl end groups of PLA by mono-CDI.