双向拉伸工艺对PLA薄膜结晶、力学、阻隔及光学性能的影响

Effect of Biaxial Stretching Process on Crystallization,Mechanical,Barrier and Optical Properties of PLA Films

研究不同拉伸工艺对双向拉伸聚乳酸(BOPLA)薄膜的晶体结构及宏观性能的影响。采用挤出流延工艺制备PLA预制膜,在双向拉伸试验机上制备了不同拉伸工艺下的BOPLA薄膜,研究了薄膜的结晶、力学、阻隔及光学性能。结果表明,随着拉伸比的增大,拉伸诱导PLA薄膜结晶,相对结晶度提高至37.84%,较1×1(未拉伸)的BOPLA薄膜提高了近3倍,玻璃化转变温度(Tg)提高至65.32℃。沿纵向(MD)、横向(TD)的抗拉强度得到提升,分别达到84.47, 93.44 MPa,而断后伸长率在拉伸比为2×2时达到最大值,分别为39.81%、42.29%。拉伸温度的升高,导致分子链热运动加快,链段向晶核扩散和堆砌速度提高,结晶速率加快,从而相对结晶度提高,提升至48.99%,且热诱导结晶促进了更加完善的晶型(α晶型)的生成。拉伸温度升高有利于提高双向拉伸PLA薄膜的氧气阻隔性能,130℃时氧气透过系数下降至8.61×10^(-16)cm^(3)·cm/(cm^(2)·s·Pa),相比80℃拉伸温度下降了59.7%,但抗拉强度、断后伸长率都呈下降趋势。这是因为快速的链松弛会促使分子链解取向,限制应变诱导结晶,破坏分子链的缠结网络。拉伸速率的增加,增强了非晶链的取向和结晶。当拉伸速率为600%/s时,薄膜沿MD方向的断后伸长率得到大幅提升,能提升至37.62%。

In order to study the effects of different tensile processes on the crystal structure and macroscopic properties of biaxially oriented polylactic acid(BOPLA)films,BOPLA films were prepared by extrusion casting process,and the stretching test was carried out by using a film biaxial stretching testing machine to change the stretching process.The crystalline,mechanical,barrier and optical properties of the films were investigated.The results show that with the increase of the stretching ratio,the PLA film crystallization induced by tensile increased to 37.84%,which was nearly three times higher than that of the 1×1(unstretched)BOPLA film,and the glass transition temperature(T_(g))increased to 65.32℃.The longitudinal(MD)and transverse(TD)tensile strengths increased to 84.47,93.44 MPa,respectively,while the maximum elongation at break was 39.81%and 42.29%at 2×2 stretching ratio,respectively.With the increase in stretching temperature,the thermal movement of molecular chain accelerated,the diffusion rate of chain segment to the crystal nucleus and the rate of regular stacking increased,and the crystallization rate accelerated and the relative crystallinity increased up to 48.99%.Moreover,heat-induced crystallization promoted the formation of more complete crystalline forms(αcrystalline forms).The oxygen barrier property of PLA film was improved by increasing the stretching temperature.The oxygen barrier coefficient decreased to 8.61×10^(-16) cm^(3)·cm/(cm^(2)·s·Pa)at 130℃,which was 59.7%lower than that at 80℃,but the tensile strength and elongation at break showed a decreasing trend,since rapid chain relaxation would promote the molecular chain disorientation,limit the strain induced crystallization,and destroy the molecular chain entanglement network.With the increase in the stretching rate,the orientation and crystallization of the amorphous chain were enhanced,and the elongation at break along the MD direction of the film was greatly increased to 37.62%only at 600%/s.