Chain-extension And Stabilization Of Phosphites As Chain Extenders In Polylactide

Polylactide(PLA) is one of environmentally friendly biodegradable plastics.PLA has excellent mechanical properties and thermal performances compared to polypropylene, polyethylene and polystyrene,so it will have a good marketing prospect.However its thermal stability and crystallization capacity are poor,which hinder the further development and application.Increasing the molecular weight through chain extension is an effective way to improve the thermal stability.Phosphites can react with hydroxyl and carboxyl group of PLA simutaniously,so they are efficient chain extenders.Although lots of phosphites were synthesized,there is few kinds to be used as chain extenders and the effect of different structure on the chain extending reaction has been little investigated.So it is very vital to study the structure effect of phosphites on the chain extension in PLA in order to briden the application of phosphites in PLA.Firstly, five different phosphites, which were respectively named as triphenyl phosphite (marked as TPP), tris(2,4-di-tert-butylphenyl) phosphite(marked as 168),2,2’-methylene-bis (4,6-di-t-butylphenyl)phosphite(marked as M46TBPP),2-ethylhexyl diphenyl phosphite (marked as PTC),bis(2-ethylhexyl) phenyl phosphite(marked as PDOP),were used as chain extenders to improve the molecular weight and stability of PLA in the thesis. The molecular weights, complex viscosities and modulus of virgin PLA and PLAs stabilized by the phosphites were characterized by gel permeation chromatography and rheometer in order to study the structure effect of phosphite on the chain extension. The results showed that the structure has an important influence on the chain extension in PLA. Bigger substitute around the P element and alkyl group is not benefit to chain extension. When there is a bigger substitute around P-O bound, the steric hindrance will prevent the contact of PLA and phosphites, so the chain extension will be weak. TPP was found a good chain extender due to the smallest steric hindrance around P-O bound, so it can increase the stabilization of PLA in the melt processing.Then,research,which was about the chain extension of TPP in PLA,showed that the molecular weight of PLA stabilized by TPP was increased from 90,462 g/mol to 251,676g/mol, when 2wt%TPP was added into PLA, the reaction temperature was 180℃ and the reaction time was 15min.At the same time, the complex viscosity, storage modulus and loss modulus were enhanced obviously compared to PLA-Virgin. Furthermore, research on the oxidation stability of PLA-TPP (PLA was stabilized by TPP) indicated that the addition of TPP increased the oxidation onset temperature of PLA from 251.4℃ to 278.0℃, because TPP made degradation activation energy of PLA increase from129 kJ/mol to147 kJ/mol.In the end, the author studied the effect of chain extension TPP on the crystallization of PLA. The DSC results indicated that the cold-crystallization temperature decreased from 115.4℃ to 102.8℃ and the degree of crystallinity increased from 30.2% to 34.9% when 2wt% TPP was added into PLA, and that PLA-TPP had more perfect crystal compared to PLA-Virgin.Because PLA-TPP had some plasticization, so it made PLA-TPP with good moving ability and crystallization ability. In addition, the POM results showed that the crystals of PLA-Virgin and PLA-TPP were all spherulite. When PLA-TPP reached the same crystal size with PLA-Virgin, the crystallization time was only one half of that of PLA-Virgin. In a word, PLA-TPP crystallized easily.