Influence of thermo-mechanical and chemo-thermo-mechanical pulping on the properties of oak fibres

Thermo-mechanical pulps (TMP) and chemo-thermo-mechanical pulps (CTMP) were prepared from chips of oak wood with more than 70% heartwood. The fibres retrieved by both processes were separated by sieve analysis into different fibre fractions [≤ 0.1 mm (fine fraction) and > 1 mm (coarse fraction)]. Both fibre fractions were analysed for their extract content. The pH-value and buffering capacity of cold and hot water extracts, the formaldehyde release as well as the emission of volatile organic acids (formic and acetic acid) were also determined for the different fibre fractions. Further, the monosaccharides in the extractives before and after hydrolysis using trifluoroacetic acid (TFA) were assessed. The results make abundantly clear that the fine fraction (≤ 0.1 mm) contains a higher amount of water-soluble extracts than the coarse fraction (> 1 mm). After hydrolysis with TFA the amount of monosaccharides in the extractives, in particular the xylose content, increased tremendously, indicating that after pulping a part of the liberated carbohydrates is still in the oligomeric state. With increasing fineness of the fibres, the formaldehyde release, as determined by the flask method (EN 717-3), soars to higher values. The formaldehyde release of CTMP fibres was much lower than that of the TMP fibres. In contrast, the emission of acetic and formic acid from the CTMP was much higher than that from TMP. This may be due to the saponification of acetyl groups in the hemicelluloses of oak wood under the alkaline environment of the CTMP-process. The results explain in-depth why removing the water-soluble carbohydrates through washing the fibres negatively impacts their bondability by UF-resins, as established in previous publications.