Improvement of melt stability and degradation efficiency of poly (lactic acid) by using phosphite

Concurrent improvement of melt processing stability and degradation efficiency of poly(lactic acid) (PLA) is still a challenge for the industry. This article presents the use of phosphites: tris(nonylphenyl) phosphite (TNPP) and tris (2,4-di-tert-butylphenyl) phosphite (TDBP), to control the thermal stabilization, mechanical performance, and hydrolytic degradation ability of the compressed PLA films. The hydrolysis process is followed as a function of time at 45, 60, and 75°C. During melt extrusion, both phosphites function as a processing aid, besides acting as a chain extender stabilizing the PLA molecular weight. The phosphite structure plays a crucial role over crystallinity and water absorption, in controlling the hydrolytic degradation of PLA. The application of TNPP significantly catalyzes the hydrolysis of PLA, which is the initial step of the biodegradation process. The optimum amount of TNPP for best hydrolytic degradation efficiency and thermal stabilization of PLA is 0.5 wt%. The excessive TNPP loadings cause a drastic drop in PLA molecular weight and, as a consequence, a reduction of flexural strength. The reactions between PLA and phosphite molecules are discussed.