PHB/Bentonite Compounds. Effect of Clay Modification and Thermal Aging on Properties

Poly(3-hydroxybutyrate) (PHB) was compounded with three different Bentonite clays: natural, purified by ultrasound/sonicated and organically modified with hexadecyltrimethylammonium bromide. PHB/Bentonite masterbatches with 30% clay were prepared in a laboratory internal mixer and letdown with pure matrix to 1% and 3% w/w clay. Test samples were injection molded and characterized by x-ray diffraction (XRD), differential scanning calorimetry (DSC), infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). Increase in Bentonite hydrophobic character was evinced by FTIR for organoclays. XRD of composites showed increase in clay interlayer distance and peak broadening, suggesting formation of intercalated nanocomposites. DSC showed increase in crystallinity and crystallization rate for compounds, especially for PHB/organoclay formulations. Thermal aging was conducted by exposing specimens at 115ºC for up to 120 hours, and mechanical properties were measured according to ASTM standards. Elastic modulus increased and impact strength decreased with time and clay content; clay purification had little effect on the tensile properties. Tensile strength of thermal aged samples showed little variation, except for the organoclay nanocomposites, for which it significantly decreased with exposure time. SEM images displayed a whitened honeycomb structure and detachment of PHB/Bentonite layers which may be connected to cold crystallization and degradation processes taking place during thermal aging.

聚(3-羟基丁酸酯)（Poly(3-hydroxybutyrate)，PHB）与三种不同的膨润土粘土共混制备得到复合材料：天然膨润土、经超声纯化的膨润土，以及以十六烷基三甲基溴化铵进行有机改性的膨润土。首先在实验室密炼机中制备膨润土质量分数为30%的PHB/膨润土母粒，随后采用纯PHB基体将其稀释至膨润土质量分数为1%和3%。将所得试样注塑成型后，分别通过X射线衍射（X-ray diffraction，XRD）、差示扫描量热法（differential scanning calorimetry，DSC）、傅里叶变换红外光谱（Fourier Transform Infrared Spectroscopy，FTIR）以及扫描电子显微镜（scanning electron microscopy，SEM）进行表征。傅里叶变换红外光谱结果证实，有机改性膨润土的疏水性能得到显著增强。复合材料的X射线衍射结果显示，膨润土层间距增大且衍射峰宽化，表明成功形成了插层型纳米复合材料。差示扫描量热法测试表明，共混物的结晶度与结晶速率均有所提升，其中尤以PHB/有机膨润土体系最为显著。热老化测试将试样置于115℃环境中保温长达120小时，力学性能测试依据美国材料与试验协会（ASTM）标准开展。结果显示，弹性模量随老化时间与膨润土含量的增加而升高，冲击强度则随之降低；膨润土的超声纯化处理对拉伸性能影响极小。经热老化处理的试样拉伸强度基本无明显变化，但有机膨润土基纳米复合材料的拉伸强度却随老化时间的延长显著下降。扫描电子显微镜图像显示，试样表面出现发白的蜂窝状结构，且PHB与膨润土层发生剥离，该现象可能与热老化过程中发生的冷结晶与降解过程相关。