Crystal and pore structure of wheat straw cellulose fiber during recycling

The changes in crystal structure and pore size of wheat straw fiber after repeated recycling were studied by means of X-ray diffraction, Fourier transform infrared spectrophotometry, and transmission electron microscopy. The results showed that in unbleached wheat straw cellulose crystallinity increased and the water retention value decreased with increasing rounds of recycling. After five rounds, the crystallinity increased by 14.6% compared with fiber never used for papermaking. The width of the crystallite in a 002 lattice plane (L-002) increased after each round of recycling, which indicated co-crystallization during the recycling process. The pore-size distribution of wheat straw fiber consisted of numerous micropores and mesopores but few macropores. The pore volumes of pulp micropores and mesopores decreased after two rounds of recycling, but subsequent rounds scarcely affected the pore-volume distribution. The average pore size and specific surface area of the fiber decreased after recycling. In addition, after recycling and rewetting, the fiber lumen became irreversibly collapsed and distorted, with numerous pleats that changed the shapes and size of the pores.