On the modification of TMP by acetylation and the mechanical strength of the acetylated pulp sheet.

In order to modify the mechanical properties of high-yield pulps, thermomechanical pulp (TMP) and the other pulps were acetylated by a acetic acid/acetic anhydride/sulfuric acid mixture under the swelling condition (method A) or by a toluene/acetic anhydride/perchloric acid mixture (method B), and then the physical properties and sheet strengths of acetylated pulps were measured.The water retention value (WRV) of acetylated TMP did not depend solely on the degree of acetyl substitution. TMP acetylated by method A produced sample having high WRV up to 402%, while acetylation of TMP by method B gave pulp with less WRV than the value of the original pulp (108%).The total heat of wetting (THW) of acetylated TMP also was determined calorimetrically. The change in THW was markedly affected by the methods of acetylation. The THW of acetylated TMP having lower acetyl contents up to 20%, which was prepared by method A, was almost same with the value of original TMP. But, the THW of acetylated TMP with higher acetyl content and all acetylated TMP prepared by method B decreased sharply with an increase in acetyl content.The sheet strengths, such as, tensile strength, tear factor and burst factor of sheet from TMP acetylated by method A showed maxima at 20% of acetyl content, while these strengths of the pulp sheets acetylated by method B decreased gradually with an increase in acetyl content.From the change in morphological structure of acetylated TMP, it is indicated that swelling occurs only in TMP acetylated by method A and that interfiber contact area increases in the sheets from the TMP.