The stability of wood resin colloids in paper manufacture

Within the pulp and paper industry, the recycling of water to reduce water consumption leads to accumulation of colloidal material in the process water and greater risk of deposition. Resinous materials, arising from the wood extractives released during papermaking, form colloidal particles in solution that agglomerate and deposit onto the different surfaces within the mill. The formation of the resinous deposits, known as pitch, can be detrimental to the paper quality, process control and efficiency. A major factor in the colloidal stability of these substances is the presence of natural polymers originating from the wood and salts that accumulate in the process water as a result of increased system closure. To date, most work has looked at the stability of northern hemisphere woods. The work of this thesis explores the stability and formation of southern hemisphere Pinus radiata wood resins through the application of novel techniques. It investigates the factors that affect the stability of wood extractive colloids under varying conditions of ionic strength, ionic valency, shear, temperatures, pH, and mixtures of cations and wood polymers released from Pinus radiata thermomechanical pulp. Electron paramagnetic resonance (EPR) was used to study model wood extractive colloids. Nitroxides were chosen as EPR probes to gain a greater understanding of the different interior vs. surrounding parts of the colloidal droplet in order to assess current proposed models of the structure of the wood extractive colloid. Additionally, salt was added to solution in order to understand the macroscopic environmental interactions that the colloid experiences. A revised model for the colloid structure has been proposed to better explain the behaviour of the wood extractive colloids. Through the use of the photometric dispersion analyser (PDA), the coagulation kinetics for wood resins were determined. The coagulation kinetics allowed the stability factor (W) for the addition of various concentrations of salt to be determined with variation in a number of supernatant physiochemical factors (salt type, valency, temperature, pH, shear and simultaneous addition of multiple salts). Coagulation of a colloidal wood extractive solution by a single salt was found to follow the Schultz-Hardy rule, with the critical coagulation concentration (CCC) for a salt strongly influenced by salt valency (z). Addition of trivalent salts indicates that the affinity of aluminium and iron salts for the colloidal wood resin surface is greater than their affinity for hydrating water molecules. Changes in temperature and pH of an aqueous colloid suspension were observed to affect the concentration of salt required to destabilise the colloid, as expected from DLVO theory. An increase of the supernatant’s pH resulted in an increase in the CCC for calcium. An increase in temperature resulted in an increased CCC for all salts tested. The degree of variation in CCC with temperature was found to be valency dependant. The stability of the colloidal wood resins was found to be highly dependent on the shear within the system. This aspect had not been reported before, particularly the effect of shear and metal ion valency. Increased shear within the system was found to decrease the CCC. The effect was found to be dependent on both the salt type and the valency of the metal ion. A change to the DLVO theory has been postulated to explain this. It is proposed that hydrodynamic forces need to be included in the relationship between the CCC and the counter ion charge: \(CCC\) `α (Ωz)^(-6τ)` or \(CCC\) `α [z^(-6τ) + Ω]`; where `Ω=f(G)` and `τ = g(G)`. The addition of multiple salts simultaneously to solution (essential for industrial colloids) was explored. The interaction between multiple salts and the colloids resulted in complex behaviour, both in destabilisation and restabilisation of the colloid. A nonlinear decrease in the CCC was found. Restabilisation of the colloids was observed to occur at high salt concentrations when two salts of differing valency were added. This restabilisation is thought to occur as a result of charge reversal mechanism by which metal cations are adsorbed onto the colloid surface. The coagulation kinetics for the addition of water-soluble wood polymers to the wood resins dispersions was determined in the same manner as the addition of salt through the use of the PDA. The interaction between the wood resin and the water-soluble wood polymer showed complex behaviour with two stages of destabilisation of the wood extractive colloids, separated by an apparently stable region. The behaviour was typical of aggregation by synthetic polymers: first, polymer bridging at low polymer additions caused colloid destabilisation with subsequent steric stabilisation of the colloids at medium concentration of the polymer, and then depletion flocculation was followed finally by depletion stabilisation at higher polymer concentrations. The deposition rate of colloidal wood resin onto hydrophobic and hydrophilic model surfaces was measured at the solid-liquid interface by impinging jet microscopy (IJM) and the effect of cation specificity in solution on deposition was quantified. On both model surfaces, the wood resin deposition was slightly faster with calcium ions than with magnesium at the same salt concentration (800 mg/L). The rate of colloidal wood resin deposition on hydrophobic surfaces was far greater (up to a 2.5 times) than on hydrophilic surfaces for both salts. Film thinning, or spreading of the wood resin particles, occurred on the hydrophobic surfaces with calcium and to a lesser extent with magnesium salt. The formation of oil films has not been previously reported.