Amino-Modified Halloysite Nanotubes to Reduce Polymer Degradation and Improve the Performance of Mechanically Recycled Poly(lactic acid)

From an environmental point of view, mechanical recycling is, in general, a good end-of-life option for poly(lactic acid) (PLA), one of the most important biobased polymers. However, the degradation of PLA during the service life and, especially, during the mechanical recycling process, leads to a decrease in the properties of PLA, thus reducing the applications of the recycled plastic. The main aim of this work was to study the addition of small amounts of halloysite nanotubes, during the recycling step, as the basis of a cost-effective method for improving the properties of the recycled PLA. Raw halloysite was modified with an aminosilane, and 2% by weight of both raw and modified halloysite were melt compounded with PLA previously subjected to accelerated ageing. The addition of the nanotubes led to recycled materials with improved properties because halloysite reduces the degradation of PLA by blocking the carboxyl groups, generated during the ageing and washing steps, which catalyze the degradation during the recycling process. This effect was more intense in the silanized nanotubes, because the carboxyl groups were effectively blocked by acid–base interactions with the amino groups of the chemical modification. The properties of the recycled plastic with only 2 wt% of silanized halloysite were very close to those of the virgin plastic.
The authors would like to thank the Centro Nacional de Microscopía Electrónica of the Universidad Complutense de Madrid (Spain), for the collaboration in the TEM measurements. This work was supported by MINECO-Spain (Grant Number CTM2017-88989-P) and Universidad Politécnica de Madrid (project UPM RP 160543006).