Effect of Mannanase and Lipase on the properties of colloidal wood extractives and their interaction with mechanical pulp fines

The effects of enzymatic treatments of dissolved and colloidalsubstances (DCS) released from thermomechanical pulp (TMP) have beeninvestigated. A model dispersion of DCS was made by leaching several portionsofTMP in distilled water and separating the fibrous material. Some samples wereenriched in colloidal particles by removing dissolved substances usingultra-filtration. The DCS, which had been subjected to different enzymatictreatments, were added in a fixed quantity to TMP fines that had been madecationic, and were subsequently used to form handsheets. All DCS additionsincreased the content of lipophilic extractives in the sheets. Lipase gave acomplete hydrolysation of triglycerides into free fatty acids. The untreatedDCSgave no significant decrease in tensile strength, because of the relativelysmall addition. A treatment of the DCS with Lipase gave a higher extractivescontent and a tensile strength on the same level as the reference. A Mannanasetreatment gave a decrease in strength compared with the reference at the sameamount of extractives in the sheet. A combined treatment with Mannanase andLipase gave a more pronounced decrease in tensile strength. Two possiblereasonsfor the differences in strength at a given amount of extractives weresuggested:(i) the destabilisation of the colloidal wood extractivesdue to the Mannanase could affect the distribution of the colloid in the sheet,making it more detrimental to sheet strength compared with the stable colloid.This would account for the observation that Lipase did not affect sheetstrengthas such, but the combination with Mannanase gave the lowest tensilestrength; (ii) the decomposition of galactoglucomannans in aqueoussolution would diminish their positive effect on tensile strength and/or affectthe adsorption of the colloid. A reflectometry technique was used to quantifythe adsorption of the differently treated DCS onto a model surface of thecationic fines. Colloidal wood extractives were identified on the surfacesafteradsorption using staining and light microscopy. No variations in adsorbedamounts were found that could explain the differences in sheet strength, whichindirectly suggests that the distribution of the colloid over the surface wasaffecting the ability of a strong bonded joint to be formed between two suchsurfaces.