Study On Toughening Modification Of Poly(Lactic Acid)

Research and development of biodegradable materials with super properties and wide applications have been an important trend worldwide. Poly(lactic acid) (PLA), as a biodegradable plastic with high strength, modulus and excellent transparency, has attracted great attention in recent years. Many researches focusing on the modification of the low toughness, thermal stability and melt strength of PLA have been reported. However, the toughening modification of PLA based on the environmentally friendly materials mainly results in the increase in the elongation at break, nor the notched impact strength, and the relationships among the morphology, mechanical properties and thermal properties of the blends need further investigation. In this paper, the toughness of PLA was improved through melt blending with different environmentlly friendly toughening modifiers. The relationship between the morphology and properties, the synergetic modification effects between components and the toughening mechanism were systemically investigated. The thermal stability of PLA composites was also discussed. The main conclusions are listed as follows.The effects of dicumyl peroxide (DCP) on the mechanical properties, phase morphology and thermal behavior of PLA/poly(butylene succinate) (PBS) blends were investigated and the toughening mechanism was discussed. The elongation at break of PLA increased from 4 % to 250 % after the addition of 20 % PBS, while the notched Izod impact strength was only 37 J/m. Further addition of 0.05⁰.2 phr DCP could significantly increase the notched Izod impact strength and transparency of the blends. The PLA/PBS (80/20) blend with 0.1 phr DCP had the impact strength of 300 J/m and the light transmittance of 81 %. The morphological and thermal analyses showed that the addition of DCP effectively reduced the domain size and crystallinity of PBS component and increased the interfacial adhesion between PLA and PBS. Under impact testing, the"amorphous"PBS domains could act as rubbery particles in rubber-toughened polymers, thus the notched impact strength of the blends was significantly increased.The morphology, mechanical properties, and thermal behavior of PLA plasticized with Di(isononyl) cyclohexane-1,2-dicarboxylate (DINCH) were investigated and the factors influencing the durability of plasticized PLA were discussed. DINCH had limited compatibility with PLA. Phase separation occurred when the DINCH content was above 5 phr, in which DINCH formed spherical structure and the glass transition temperature (Tg) of PLA remained at 50 oC. The notched Izod impact strength and elongation at break of the blends gradually increased with increasing DINCH content. The elongation at break, notched Izod impact strength and shore D hardness of the PLA/DINCH (100/20) blend were 200 %, 93 J/m and 76, respectively. The increase of the toughness can be explained based on the rubber-toughened polymers mechanism. Compared with PLA plasticized with tributyl citrate ester (TBC), PLA plasticized with DINCH had better durability against water immersion and physical aging. The analysis revealed that the higher Tg of the plasticized PLA and the lower solubility of the plasticizer in the medium, the better the durability.Further addition of organoclay (DK2) into the PLA/DINCH (100/20) blend could significantly improve the notched impact strength, which depends on the dispersion and location of the clay layers. DINCH could help the dispersion of the organoclay and make it completely exfoliated in PLA/DINCH/organoclay composites. The dispersed DINCH domain acted as a stress concentrator for craze initiation and the clay platelets at the PLA-DINCH interface acted as barriers to prevent craze propagation and void coalescence, leading to a sharp brittle-ductile impact transition and super-toughness of the composites. The PLA/DINCH/DK2 composite with high notched impact strength (386 J/m) and good transparency (80 %) was prepared after the addition of 5 phr DK2. The isothermal crystallization test showed that the half crystallization time of PLA significantly reduced after the incorporation of DINCH and DK2, which resulted from the facts that liquid DINCH facilitated the mobility of PLA chains and DK2 acted as the heterogeneous nucleating agent for PLA.Finally, PLA was blended with poly(butylene succinate-co-adipate) (PBSA) at a fixed weight ratio of 70/30, and the effects of polyhedral oligomeric silsesquioxanes (POSS) or silica (SiO₂) on the properties of the blend were investigated. Octavinyl POSS (vPOSS) had physical interactions with PLA/PBSA molecules, and the crystalline vPOSS aggregates were poorly dispersed in the matrix. For epoxycyclohexyl POSS (ePOSS), the epoxy groups of ePOSS could react with the hydroxyl and carboxyl groups of PLA/PBSA, which results in the increase of the viscosity and better dispersion of ePOSS. Polarized optical microscopy analysis revealed that the two types of POSS could act as heterogeneous nucleating agents for PLA. The thermal stability of the PLA/PBSA blend was improved by adding the two types of POSS, and the stabilization became higher after the addition of ePOSS, which should be attributed to the reduced hydroxyl and carboxyl end groups and the limited molecular chain mobility resulted from the reactions.PBSA and SiO₂ have a synergistic effect on toughening PLA, which depends on the tpye and content of SiO₂. The impact strength, flexural strength and modulus of PLA/PBSA blends increased after the addition of hydrophobic SiO₂ without decreasing the elongation at break, and the elongation at break monotonically decreased with increasing hydrophilic SiO₂ content. Melt elasticity and viscosity of the PLA/PBSA blend increased with the addition of SiO₂. The hydrophilic SiO₂ was mainly encapsulated by the dispersed PBSA phase, while the hydrophobic SiO₂ was more uniformly dispersed and mainly located in the PLA matrix, which was desirable for the optimum reinforcement of the PLA/PBSA blend. TGA results showed the addition of the two types of SiO₂ increased the initial decomposition temperature and activation energy, consequently retarding the thermal degradation of PLA/PBSA. The retardation of degradation is prominent with the addition of hydrophobic SiO₂.