Chemical changes in wood induced by thermo-mechanical (TMP) and chemo-thermo-mechanical (CTMP) pulping and their impact on board properties

Thermo- and chemo-thermo-mechanical pulping induce different chemical changes in wood leading to the formation of different water soluble compounds like mono- and oligosaccharides, monobasic acids such as formic and acetic acid. Moreover, simple reactive compounds like formaldehyde and furfural are also formed. The paper focuses on the influence of removing water soluble compounds from thermo-mechanical and chemo-thermo-mechanical pulps on some bonding relevant chemical properties of fibres such as pH-value, buffering capacity towards alkali and consequently on the properties of the boards made therefrom. The results of comprehensive investigation reveal that removal of water soluble compounds from thermo-mechanical pulps made from beech wood increase their pH-value, decrease their alkaline buffering capacity and their formaldehyde emission. Moreover, it impairs the internal bond strength of boards made by using acid curing resins like UF- and MUF-resins. On the contrary, washing out the fibres made by thermo-mechanical pulping enhance their bondability towards alkaline curing binders such as phenol-formaldehyde (PF) and tannin-formaldehyde (TF) resins. The formaldehyde release as well as the release of volatile organic acids from the boards decreases enormously due to removal of water soluble compounds from thermo-mechanical pulps used for PF- and TF-boards. Chemo-thermo-mechanical pulping (CTMP) deacetylates wood to a much higher extent than thermo-mechanical pulping. The CTMP-process leads to fibres of higher wettability and accessibility towards bonding by PF-, TF-resins and PMDI binders. Therefore, CTMP-pulps are predestined for making moisture resistant boards. Nevertheless, washing out chemo-thermo-mechanical pulps also improves their bonding behaviour and leads to boards with much higher mechanical strength and lower formaldehyde emission than that of boards made from unwashed fibres. This applies for boards made from beech and pine wood.