Spreading and adhesion of lipophilic extractives on surfaces in paper machines

Contact angles, adhesion and spreading kinetics of liquid wood extractives on surfaces of materials occurring in paper machines were investigated. The surfaces included wood components, polymers used for pitch control (polyethylene oxide, PDADMAC, sodium polystyrene sulfonate) and paper machine surface materials (polyamide, fluoropolymer, stainless steel, TiO2, SiO2). Adhesion of extractives in solution to the hydrophobic fluoropolymer and polyamide was much stronger than to hydrophilic substrates. The lowest values of adhesion detected were those to cellulose and sodium polystyrene sulphonate. Swelling of adsorbed PDADMAC due to increase in salt concentration lowered the adhesion of extractives. Adhesion of wood extractives to PEO in aqueous solution was similar above and below its cloud point, indicating that the hydrophobic ethylene group determines the adhesion to PEO. Measurements of wetting kinetics in solution on all surfaces indicated a pre-wetting period (0.1-several seconds) after application of the liquid. After this, initial spreading was rapid but stable contact angles were reached only after several minutes. The observation suggest that in practise drops of wood resin do not have time to reach the equilibrium contact angle (=full adhesion) due to the fast time-scales of process operations. Adhesion was also determined with a Micro Adhesion Measurement Apparatus based on JKR theory. The results correlated well with those achieved by the contact angle method. It was concluded that liquid extractives should tend to accumulate on wet hydrophobic paper machine parts and hydrophobic spots on paper web. To avoid formation of deposits, wet surfaces in paper machines should be rendered as hydrophilic as possible. Drastic lowering of the surface energy of dry surfaces (e.g. by coating with fluoropolymer) will lower the tendency to form deposits.