Accurate characterization of fiber and void volume fractions of natural fiber composites by pyrolysis in a nitrogen atmosphere

This paper proposes a simple and accurate technique to quantify both fiber and void volume fractions of natural fiber composites. This technique involves the separate pyrolysis of natural fibers, polymer resin, and natural fiber/polymer composite in a pure nitrogen atmosphere. The remained fractional residuals of these materials are used for the determination of fiber and matrix weight fractions. Consequently, the void and fiber volume fractions of the natural fiber composite are calculated by having weight fractions, as well as the density of natural fiber, epoxy resin, and composite sample. To validate this new method, a number of silk/epoxy composites at various levels of void content were manufactured. Specific results are given for the random mat silk/epoxy laminates made by wet lay-up vacuum bagging (WLVB) process, as well as plain weave silk/epoxy laminates fabricated by the vacuum assisted resin transfer molding (VARTM) process. Using the proposed method, the void volume fractions of the random mat and plain weave silk composites are calculated to be 9.5% and 1.7%, respectively. The fiber volume fraction of random mat and plain weave laminates are also determined to be 18.7% and 46.3%, respectively. The representative scanning electron microscopy (SEM) images of those laminates are also presented to visually compare the void and fiber contents of the parts. The results demonstrate that the proposed method enables an accurate determination of the fiber and void contents in silk/epoxy laminates and can be extended to composites containing other types of natural fibers.