Heat Treatment of Green Wood Accomplishes Crystal Hardening
There is considerable interest in high-strength cellulose nanocrystals (CNC) that can be used as a reinforcing phase in cellulose–plastic composites. Normally, these are produced by treatment of wood pulp, but when the treatment is carried out on wood, a very low yield of CNC is obtained. Although the repeating character of the crystal unit cell can be observed using X-ray methods, Raman methods reveal considerable disorder within the crystals or apparent crystals of green wood. These apparent crystals are permeable to acid and do not resist acid hydrolysis, the chemical method that successfully isolates crystals from other forms of cellulose. The chemical and thermal treatment in kraft pulping increases the internal hydrogen bonding in the crystals, resulting in less disorder and a chemically robust crystal. Forest Service research demonstrates heat treatment of green wood accomplishes the same crystal "hardening" and allows CNC to be obtained without the need for pulping and bleaching.