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|You are here: Home > Technique > Processes > Scientific report of the LGP2 > Paper physics > Wet end chemistry||Update: July 21, 2011|
|Scientific report of the LGP2 (2006-2009)|
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|Researchers of the LGP2
Jérôme Da Silva Mourao, Elisa Zeno, Evelyne Mauret
The investigation conducted in this area deal with the use of additives at the wet end on the paper machine. These chemicals, which are added to the fibrous suspension at different stages of processing, can be divided into two main categories: process aids and performance chemicals. The most important process aids are the retention and dewatering agents that improve the retention of fine elements (cellulosic fines and mineral fillers) in the fibrous mat during the sheet formation. As they modify the flocculation level of the pulp suspension, they increase the pulp drainability and consequently the speed of the machine. The second category of additives is used to give the paper specific end-use properties. Dry or wet strengthening of paper or liquid penetration can be, for example, controlled. Thus, the modification of fibre surface is investigated in order to obtain paper exhibiting specific properties (PhD Thesis of O. Paquet; Post-doctoral works of M. Krouit and N. Tabary). Studies carried out also aim to better understand the main phenomena involved in the adsorption of polyelectrolytes like cationic starch or polyamide-amine-epichlorohydrine (PhD. Thesis of E. Siqueira), the effects of conventional retention agents on the pulp flocculation as well as those of disturbing substances on the efficiency of wet end chemicals. In this context, the main objective of the PhD thesis of J. Da Silva Mourao, carried out in the framework of a partnership (CIFRE) with the Centre Technique du Papier (CTP), is to study how internal sizing can be impacted by contaminants.
Hydrophobisation treatment or internal sizing is conducted to limit the penetration of water, or other liquids, into papers, mainly for printing and writing or converting purposes. The chemicals, introduced into the pulp suspension before the sheet formation, lower the surface energy of the fibres. AKD (Alkyl Ketene Dimer) and ASA (Alkenyl Succinic Anhydride) are largely used: they allow producing highly sized papers and can be processed in neutral or alkaline medium, thus making possible the use of calcium carbonate. It is well known that sizing can be highly affected in fine elements, by water circuits closure and by pulp grade. This problem is often associated with dissolved and colloidal substances (DCS) which can be leached by the fibres themselves. The main objective of this work is then dedicated to the effect of some wood extractives and calcium ions on the efficiency of AKD and ASA. For this purpose, two different sets of experiments were conducted. The first one consisted of removing some extractives from a thermomecanical pulp (TMP) and testing the sizing performance of AKD or ASA on extracted TMP. The extractions were conducted with different solvents - hexane (H), acetone (A) and water (W). Selective removing of extractive was therefore expected. The second set of experiments was performed on a bleached kraft pulp (BKP) in which two kinds of commercial model wood components were added i.e. dehydroabietic acid (DH) and xylan or galactomannans. Sizing level was evaluated by a specific test (Cobb test) assessing the quantity of water absorbed by the paper in given experimental conditions.
Sizing performance of AKD was evaluated on handsheets made of extracted or unextracted TMP. Figure 1 illustrates the obtained results by plotting the Cobb values as a function of the AKD dosage for H-, (H+A)-extracted and unextracted pulps. For comparison, Cobb values of BKP handsheets are also reported on the same figure. As already described in the literature, TMP is more difficult to size than BKP. Successive extractions performed on TMP increased the sizing efficiency of AKD. So, Hand (H+A)-extractions allow recovering a better sizing level, the two steps extraction procedure (H+A) giving rise to the highest gain of sizing. Similar trends are observed with A- and (A+W)- extracted pulps.
|Figure 1 - Effect of extraction on sizing response of thermomechanical pulp (TMP);
H-TMP: pulp extracted with hexane;
(H+A)-TMP: pulp extracted with hexane and acetone;
BKP: bleached kraft pulp (reported for comparison).
Two classes of extractives (dehydroabietic acid, as a representative of lipophilic extractives and xylan and galatomannan, corresponding to water soluble hemicelluloses) were added to a bleached kraft pulp. In addition, we explored the impact of calcium ions introduced alone or in conjunction with wood model components. Calcium ions lead to a decrease of the sizing efficiency [Figure 2] and the effect is more pronounced from 500 mg/L. Figure 3 illustrates the impact of DH alone on AKD sizing: from 30 mg/L DH introduced in the pulp suspension, the Cobb values dramatically increase. Two mechanisms may be evoked to interpret these results. Either colloidal aggregates of dehydroabietic acid dispersed in the pulp suspension impairs the retention of AKD or, after precipitation on the fibre surface, DH negatively impacts AKD migration / reaction steps.
|Figure 2 - Effect of addition of calcium ions
on sizing efficiency of a bleached kraft pulp (BKP)
|Figure 3 - Effect of addition of dehydroabietic acid (DH)
on sizing efficiency of a bleached kraft pulp (BKP)
One interesting feature is reported on Figure 4, which illustrates the coupled effect of DH and calcium ions added at 150 mg/L. For its lowest dosages, the negative impact of DH was totally prevented, whereas for the highest level (from 30 mg/L), the negative effect was greatly reduced by calcium ions. In this case, first added calcium ions are believed to react with DH and to help precipitating it onto the fibre surface in the form of Ca-soap, acting therefore as coagulants. DH is no more available to affect retention of AKD. Of course, an optimum dosage in calcium ions should exist, which may explain why AKD sizing is not entirely recovered for high additions in DH. It is interesting to note that similar trends were obtained when hemicelluloses are added to the pulp suspension.
|Figure 4 - Coupled effect of addition of dehydroabietic acid (DH)
and calcium ions (150 mg/L)
on sizing efficiency of a bleached kraft pulp (BKP).
In this project, similar investigations were conducted with ASA and experiments were done to measure retained and reacted AKD or ASA, in order to better understand the involved mechanisms. The effects of contaminants on the stability of the AKD dispersions or ASA emulsions were also studied. Promising results were obtained, thus allowing the use of sizing agents being optimized in industrial conditions.
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