Preventing Strength Loss of Unbleached Kraft Fibers

Abstract

The purpose of this study was to understand the mechanism of paper strength loss that occurs when paper made from chemical pulps is recycled. It is found that due to drying, unbleached kraft pine fibers lost cellulose viscosity, water retention value, fiber flexibility and accessible surface area. Handsheets made from dried fibers had lower paper strength and lower apparent density compared to corresponding primary handsheets made from never-dried fibers. With the increase in drying temperature of virgin fibers, the above properties of dried fibers and recycled handsheets experienced larger effects.

It was hypothesized that adding certain chemicals to virgin fibers before drying could prevent strength loss upon recycling. Results showed that relatively low molecular weight additives (such as sucrose and glucose) appeared to interfere with the mechanism of pore closure during drying and improved the strength of recycled paper. Higher molecular weight chemicals added to never dried virgin fibers (such as starch) also increased the strength of the recycled paper but this was attributed to residual chemical being retained on the fiber surface during recycling. Although the effect of adding certain chemicals to virgin fibers before drying could significantly prevent strength loss in recycled paper, it was determined that improvements of recycled paper strength due to refining were of much larger magnitude.

It is found that recycled handsheets had lower paper strength compared with virgin handsheets at all pH values considered during paper formation within the range of pH 3 to pH 8. There was no significant effect of pH on paper strength within this range.

The fiber flexibility tests showed that the method is useful to determine the flexibility of individual fibers. In the case of sugar treatment, treated fibers showed higher flexibility compared to untreated fibers after drying, and glucose was found to have larger effect than sucrose. With respect to papermaking conditions, fibers were more flexible under alkaline conditions than fibers under acidic conditions, but fibers became less flexible with increasing salt concentration and hardness.