Matuana Laurent M, Faruk Omar, Diaz Carlos A
Department of Forestry, Michigan State University, East Lansing, Michigan 48824, USA.
Bioresour Technol. 2009 Dec;100(23):5947-54. doi: 10.1016/j.biortech.2009.06.063. Epub 2009 Jul 16.
Poly(lactic acid) (PLA) was foamed with an endothermic chemical foaming agent (CFA) through an extrusion process. The effects of polymer melt flow index, CFA content, and processing speed on the cellular structures, void fraction, and cell-population density of foamed PLA were investigated. The apparent melt viscosity of PLA was measured to understand the effect of melt index on the cell morphology of foamed PLA samples. The void fraction was strongly dependent on the PLA melt index. It increased with increasing melt index, reaching a maximum value, after which it decreased. Melt index showed no significant effect on the cell-population density of foamed samples within the narrow range studied. A gas containment limit was observed in PLA foamed with CFA. Both the void fraction and cell-population density increased with an initial increase in CFA content, reached a maximum value, and then decreased as CFA content continued to increase. The processing speed also affected the morphology of PLA foams. The void fraction reached a maximum value as the extruder's screw speed increased to 40 rpm and a further increase in the processing speed tended to reduce the void fraction of foamed samples. By contrast, cell-population density increased one order of magnitude by increasing the screw speed from 20 to 120 rpm. The experimental results indicate that a homogeneous and finer cellular morphology could be successfully achieved in PLA foamed in an extrusion process with a proper combination of polymer melt flow index, CFA content, and processing speed.
聚乳酸（PLA）通过挤出工艺与吸热型化学发泡剂（CFA）一起发泡。研究了聚合物熔体流动指数、CFA含量和加工速度对发泡PLA的泡孔结构、孔隙率和泡孔密度的影响。测量了PLA的表观熔体粘度，以了解熔体指数对发泡PLA样品泡孔形态的影响。孔隙率强烈依赖于PLA熔体指数。它随着熔体指数的增加而增加，达到最大值后又下降。在所研究的窄范围内，熔体指数对发泡样品的泡孔密度没有显著影响。在用CFA发泡的PLA中观察到了气体容纳极限。孔隙率和泡孔密度都随着CFA含量的初始增加而增加，达到最大值，然后随着CFA含量的继续增加而下降。加工速度也影响PLA泡沫的形态。随着挤出机螺杆速度增加到40 rpm，孔隙率达到最大值，加工速度的进一步提高往往会降低发泡样品的孔隙率。相比之下，通过将螺杆速度从20 rpm提高到120 rpm，泡孔密度增加了一个数量级。实验结果表明，通过聚合物熔体流动指数、CFA含量和加工速度的适当组合，在挤出过程中发泡的PLA中可以成功实现均匀且更精细的泡孔形态。
