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The stability of wood resin colloids in paper manufacture

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Lee, R (2012) The stability of wood resin colloids in paper manufacture. PhD thesis, University of Tasmania.

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Abstract

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  (z) -6τ or CCC  [z -6τ + ]; where
=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.

Item Type: Thesis (PhD)
Keywords: colloid, coagulation, stability, salt, hemicellulose deposition
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Date Deposited: 27 Aug 2012 01:34
Last Modified: 11 Mar 2016 05:53
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