Fabrication And Properties Of Composites Based On Biodegradable Poly(Lactic Acid)

Poly (lactic acid) (PLA) is a biodegradable polymer, which is prepared from renewable plant resources by chemical synthesis methods. It will be decomposed into CO₂ and H₂O after using, so it is a representative green polymer. Nontoxic and nonirritant PLA has excellent biodegradability, biocompatibility and mechanical properties, and it can be processed by using traditional methods, therefore, PLA replacing some general-purpose oil-based plastics have become an inevitable trend. Because the shortcomings from its strength, brittleness, permeability resistance, heat resistance limit the scope of its applications, modifying PLA has become one of critical researches.In this paper, we gave an overview of the latest research progress of PLA. With the focus on the modification of PLA, we used silkworm silk fiber (Silk), chicken feather fiber (CFF), carbon nanotubes (CNTs) and nanodiamond (ND) to reinforce PLA, and investigated the effect of four reinforcements on the properties of PLA. The main research contents are as follows:(1) To reinforce PLA and expand the application areas of silkworm silk fiber (Silk), the first time we used silkworm silk fiber as reinforcement for PLA, and fabricated completely biodegradable PLA/Silk biocomposites by using melting compound method which is a common technology in industry. By means of Mechanical tensile test, Dynamic mechanical analysis, Thermogravimetric analysis, Differential scanning calorimetry, Thermal mechanical analysis and Enzymatic degradation test, the structure, mechanical and thermal properties and enzymatic degradation behavior of the biocomposites were studied. When 5 wt% silk fiber with 5 mm length is added into PLA matrix, PLA/Silk biocomposites has the best mechanical tensile properties. Among the rubbery plateau of PLA, the addition of silk fiber will significantly enhance the storage modulus of PLA; the glass transition temperature (T_g) of all the PLA/Silk biocomposites is slightly lower than that of PLA due to the plasticization effect of silk fiber. Silk fiber acts as nucleating agent for the crystallization of PLA, and will slightly decrease the melting point temperature (T_m) of PLA. The presence of silk fiber will reduce the thermal stability of PLA, but the thermal dimensional stability of PLA can be somewhat improved. The enzymatic degradation of PLA/Silk biocomposites occurs not only on their surface, but also into the internal; the water absorption ratio of PLA can be enhanced by the addition of silk fiber, and thus the enzymatic degradation of PLA will be accelerated.(2) To reinforce PLA and effectively use chicken feather fiber (CFF), the first time we used CFF as reinforcement for PLA, and prepared completely biodegradable PLA/CFF biocomposites by using melting compound method. The structure, mechanical and thermal properties and enzymatic degradation behavior of the biocomposites were studied. When 5 wt% down feather fiber from semiplume of chicken is added into PLA matrix, PLA/CFF biocomposites has the best mechanical tensile properties. In the glassy and rubbery region of PLA, the addition of CFF will significantly enhance the storage modulus of PLA; when the CFF content is 10 wt%, the T_g value of PLA will be increased somewhat. CFF acts as nucleating agent for the crystallization of PLA, and the T_m value of PLA will be slightly increased as the CFF content is 2-8 wt%. The addition of CFF will reduce the thermal stability and the dimensional stability of PLA. The enzymatic degradation of PLA/CFF biocomposites occurs not only on their surface, but also into the internal; the water absorption ratio of PLA can be enhanced by the addition of CFF, and thus the enzymatic degradation of PLA will be accelerated. However, CFF will hinder the enzymatic degradation of PLA, when the CFF content is over 8 wt%.(3) The first time we used nanodiamond (ND) as reinforcement for PLA, and fabricated PLA/ND nanocomposites by using melting compound method. The effect of ND on the structure, mechanical and thermal properties and enzymatic degradation behavior of PLA was studied. Due to homogeneous dispersion of ND nanoclusters and good adhesion between ND and PLA matrix, PLA/ND nanocomposites has the best mechanical tensile properties, when 0.5 wt% ND is added into PLA matrix. In the glassy and rubbery region of PLA, the addition of ND will significantly enhance the storage modulus of PLA. ND could act as nucleating agent for the crystallization of PLA, and the T_m value of PLA will be decreased with the increase of ND content as the ND content is over 0.5 wt%. The presence of ND will improve the thermal stability and the dimensional stability of PLA. The enzymatic degradation of PLA/ND nanocomposites occurs not only on their surface, but also into the internal; the water absorption ratio of PLA can be enhanced by the addition of ND, and thus the enzymatic degradation of PLA will be accelerated.(4) The PLA/carbon nanotubes (CNTs) nanocomposites were fabricated by using melting compound method. The first time we investigated the effect of purified CNTs (pCNTs), hydroxy CNTs (hCNTs) and carboxylic CNTs (cCNTs) on the structure, mechanical and thermal properties and enzymatic degradation behavior of PLA. The best mechanical tensile properties of PLA/ CNTs nanocomposites will be given by the addition of cCNTs, followed by hCNTs and pCNTs, which is ascribed to the different dispersion of three CNTs in the PLA matrix and the different interaction between three CNTs and the PLA matrix; the best dispersion and strongest interaction will result in generating the best reinforcing effectiveness. All the CNTs will significantly enhance the storage modulus of PLA from 85 to 150℃, and cCNTs and hCNTs can more significantly decrease the damping of PLA and increase the T_g value of PLA. CNTs could act as nucleating agent for the crystallization of PLA, and the T_m value of PLA will be slightly decreased by the addition of CNTs. The presence of CNTs will improve the thermal stability and the dimensional stability of PLA; pCNTs shows better performance for improving the thermal stability of PLA, and the effectiveness of cCNTs and hCNTs for improving the dimensional stability of PLA is better than that of pCNTs. The enzymatic degradation of PLA will be accelerated somewhat, when the pCNTs content is lower than 1 wt%, however, pCNTs will hinder the enzymatic degradation of PLA, as the pCNTs content is over 1 wt%; compared with pCNTs, cCNTs and hCNTs can more easily hinder the enzymatic degradation of PLA.