|Rice Straw for Paper Making
and Dissolving-Grade Pulps 83
Project Leader and Principal UC Investigators
D. L. Brink, Forest Products Laboratory, University of California, Berkeley
S. H. Zeronian, Division of Textiles and Clothing, University of California, Davis
The primary purpose of this project is to develop commercial uses for rice straw fiber and its chemical constituents. The two specific uses investigated were paper making and dissolving-grade pulps which have many industrial uses.
A substantial effort was made to select the preferred integrated process for the production of dissolving-grade pulp from rice straw.
Production of an acceptable pulp requires the reduction of ash and silica as well as lignin, hemicelluloses, and extraneous materials, while still preserving the structure of the cellulose. The system used requires (1) removal of dirt, sand, silica enriched leaf fragments, and other foreign matter by mechanical separation, (2) acid pretreatment, (3) pulping by the soda process to remove lignin, some of the hemicelluloses, and much of the silica in rice straw, (4) bleaching to remove residual lignin, part of remaining hemicelluloses, and to decrease silica and ash, and (5) alkaline refining of bleached pulp.
The original objectives of producing a pulp from rice straw having dissolving-grade properties has been achieved. However, an unexpected color component of rice straw has been found which is resistant to the processing methods used. This material is present in less than 0.6 percent of the particular dissolving pulp analyzed. The material has been detected in all bleaching experiments and is resistant in part to all pulping-bleaching combinations studied. It is responsible for serious yellowing of bleached pulps and must be evaluated relative to qualitative and quantitative aspects that would affect utilization.
Three pulps were selected for evaluation as paper making grades. Although results are preliminary, the unbleached paper making grades of rice straw pulps were unusually high in tensile strength but only moderate in tear and bursting factors.
Significant differences were found between the properties of rice straw cellulose, wood cellulose, and cotton cellulose. The cellulose crystallites are longer in rice straw pulps than in wood pulps, but they are not as long as in cotton cellulose. Also, the degree of crystallinity of rice straw pulps appears to be less than that of wood cellulose. Low pulp crystallinity can be useful when a cellulose derivative is to be manufactured from a pulp. In addition, rice straw cellulose absorbs 40 percent more moisture than cotton cellulose at the same relative humidity and temperature, indicating that rice straw cellulose may be useful in high absorbancy products. Further research and development is required to confirm this.
The ash content of wood dissolving pulps ranges between 0.1 and 0.5 percent. The ash content of rice straw dissolving pulps is on the high side of this range. It appears that by modifying pulping procedures, it should be possible to meet the ash specifications set for dissolving celluloses.
The technical feasibility of making paper and dissolving grade pulps from rice straw has been proven, although some problems remain. An economic assessment can be made based on the specifications of the process derived from these studies. This was not done during 1983 because of manpower limitations.
At present, and for some years to come, the United States can be supplied with paper from wood pulp in paper mills strategically located near pulpwood sources. Paper made from rice straw would need to be economically competitive with current sources.