Individual Fiber Strength of Wood Pulp

Abstract

The individual fiber strength were studied on sulfite pulp and kraft pulp prepared separately from pine early- and late-woods by varing cooking time. Pulp yield varied from 67% to 42% according to cooking time. The permanganate number of pulps, lateral order distribution and DP of cellulose were determined, and their effects on the strength of pulp were discussed.The fiber strength of sulfite pulp and kraft pulp decrease with decreasing yield (sulfite pulp : 32-14 g/fiber, kraft pulp : 34-21 g/fiber). Comparing the fiber strength of these two pulps at the same level of yield, kraft pulp gives a considerably high strength. Neither DP nor lateral order distribution of cellulose can account for the cause of the higher fiber strength of kraft pulp. Leopold (1961) reported that xylan in wood fiber is located between fibrils, and contributes to the fiber strength by binding the fibrils. The fiber strength of kraft pulp rich in xylan, therefore, is expected to drop by the alkali extraction mo-re remarkably than sulfite pulp. However, on extraction of hemicellulose, the fiber strength of sulfite pulp decreased similarly as kraft pulp. In general, the degree of decrease in fiber strength diminishes as lowering pulp yield.Forgacs (1961) pointed out that the fiber would be broken at “node”, i.e. local defects in sulfite fiber. Microscopical observation, however, revealed that the node exists in kraft pulp as well as in sulfite pulp (ref. to photo 3), and in many cases fiber was broken at this position on tensile test. Therefore, individual fiber strength shows the strength of locally damaged portion of wood fiber. In order to clarify the difference of fiber strength between sulfite pulp and kraft pulp, it is necessary to know the physical properties of the damaged portion.The decrease in fiber strength on alkali extraction seems to be attributed to the expansion of this defect by the concentration of stress resulted from strong swelling. This assumption was ascertained by the results obtained from the determination of fiber strength of nitrated pulps.