Effect of metal ions and wood pitch on paper properties

In this study, the effect of colloidal wood pitch and metal ions on paper properties was determined. The pitch emulsions used were made from model components, and consisted of fatty acids, resin acids and triglycerides. The metal ions tested were calcium, magnesium and aluminum. Thermomechanical pulp (TMP) of Norway spruce was obtained from a Finnish pulp mill. The TMP was disintegrated and agitated at 60°C and pH 11 for one hour, to de-acetylate released hemicelluloses. The pH of the pulp was then adjusted to 8. A mixture of pitch components in acetone were dispersed into distilled water during agitation. TMP suspensions were mixed with pitch emulsions and metal salts, and diluted to a consistency of 0.1%. Papers were made at pH 8 on a laboratory sheet former; the target grammage was 80 g/m ². Three papers were freeze-dried directly after formation, and me rest were air-dried at room temperature. The pitch content was determined by gas chromatography after soxhlet-extraction with acetone/water/acetic acid. The tensile index and the contact angle were also determined. The content of pitch that could be extracted from the papers depended largely on the drying procedure of the paper. Extraction and analysis by gas chromatography of freeze-dried papers gave a much higher pitch content compared to air-dried papers. Additions of metal salts increased the content of pitch in me papers. More pitch was retained in the papers when Ca ²⁺ was added compared to when Mg ²⁺ was added. This implied that aggregates of pitch and calcium were tackier or larger than aggregates of pitch and magnesium. More resin acids were retained in the papers at 5 mM of Ca ²⁺ and Mg ²⁺ than at 10 mM. A possible explanation for this is that the resin acids did remain water-soluble at high concentration of metal ions or formed aggregates with low affinity to TMP. The tensile index decreased slightly when 5-10 mM Ca ²⁺ or 10 mM Mg ²⁺ was added to the furnish prior to papermaking. The contact angle was affected by the different metal ions but no clear trend was seen as a function of pitch content.