Poly(lactic acid) (PLA) and northern bleached softwood kraft (NBSK) or black spruce medium density
fiberboard (MDF) fibers were melt compounded using a co-rotating twin screw extruder and subsequently
microcellular injection molded. Poly(ethlylene glycol) (PEG) was used as a lubricant. The microcellular
structure, thermal properties, and crystallization behaviors were characterized using scanning
electron microscopy, thermogravimetric analysis, differential scanning calorimetry, and wide angle
X-ray diffraction. Results show that cellulosic fibers, acting as crystal nucleating agents, increased
the crystallization temperature and the crystallinity and decreased the crystallization half time. The
dissolved N2, the shear stress, and biaxial stretching during foaming also enhanced the crystallinity of
PLA. Compared to PLA/PEG, a finer and more uniform cell structure was achieved in the cellulosic
fiber composite foams. The improved foam morphology was attributed to the cell nucleating effects of
the fibers and the increased melt strength by the addition of cellulosic fibers and by the gas- and
fiber- induced crystallization