天然纖維具有低價格及可生物分解性等優點，使天然纖維複合材廣受應用，然而，其中天然纖維複合材料的密度通常高於其高分子基材，因此限制了其用途，此缺點可以藉由發泡改善，本研究經由單螺桿押出機對稻殼/聚丙烯天然纖維複合材料進行押出發泡，使用的發泡劑為超臨界二氧化碳，並探討偶合劑、螺桿轉速、CO2含量和模頭溫度對於泡孔結構的影響。由於稻殼在加工過程中會釋放出水氣及有機揮發物，並惡化泡孔型態，因此藉由TGA模擬稻殼的熱降解程度。實驗結果發現添加偶合劑聚丙烯馬來酸酐(PP-g-MA)後，泡孔型態明顯改善，並且降低複合泡材的密度及孔徑，而未含有偶合劑的複合材，其泡材密度對於螺桿轉速與CO2含量具有相關性，但加入偶合劑後其相關性卻消失，此外，泡材密度會隨著模頭溫度下降而降低，而在本實驗裡最低的泡材密度可降至0.6 g/cm3，然而當溫度低於160°C之後，押出物會開始發生明顯的熔體斷裂情形。另外，當模頭溫度降至147°C時，可以觀察到泡孔型態呈現像花束般結構，推估是因為高分子已開始結晶造成如此的結構。

The biodegradability and low cost of natural fibers make them attractive for composite applications. However, the density of natural fibers composites (NFCs) is higher than that of polymers, which limits the application of NFCs. This drawback could be reduced by foaming. In this study, a polypropylene (PP)/rice husk NFC was extrusion foamed by a single-screw extruder using CO2 as the blowing agent. The effects of coupling agent, screw rotation speed, CO2 content, and die temperature on the cell structure were studied. Because moisture and volatile organic compounds from rice husks could be released during the extrusion process and deteriorate the cell structure, the thermal degradation behavior of rice husks was simulated using thermogravimetric analysis (TGA). The experiment results showed that the addition of a coupling agent improved the cell morphology and reduced the density and cell size of the composite. Before the coupling agent was added, there was a clear relation between the foam density and both the screw speed and the CO2 loading level. However, the relation disappeared after the coupling agent was added. The foam density decreased with the decreasing die temperature. The lowest foam density achieved in this experiment was 0.6 g/cm3. Melt fracture phenomena were observed when the die temperature was below 160°C. In addition, a bouquet-like cell structure was observed at low die temperature of 147°C, which may result from the PP crystallization.

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