EFFECT OF EDC/ADH FIBRE CROSSLINKING ON THE WET-WEB STRENGTH OF BHKP WITH AND WITHOUT PCC LOADING

Transcription

1 EFFECT OF EDC/ADH FIBRE CROSSLINKING ON THE WETWEB STRENGTH OF BHKP WITH AND WITHOUT PCC LOADING ALVARO TEJADO*, MIRO ANTAL AND THEO G.M. VAN DE VEN ABSTRACT There has been very little success in trying to provide a method that increases the wetweb strength (WWS) of paper, even though it determines to a large extent the runnability of a papermaking machine. ethyl3[3(dimethylaminopropyl)] carbodiimide (EDC) has been shown to promote the crosslinking of soluble polysaccharides containing carboxyl groups [3]. In this work we use beaten hardwood Kraft pulp to show the enormous increase in WWS that can be obtained by crosslinking the fibres with EDC/adipic dihydrazide (ADH) after attachment of carboxymethyl cellulose to the fibres. Further, we show that such and increase in strength is very beneficial for highly precipitated calcium carbonate (PCC) loaded papers. Results prove the ability of EDC to promote covalent bonding in wet conditions makes it an excellent WWS agent. INTRODUCTION There has been very little past success in trying to provide a method that increases the wetweb strength (WWS) of paper. Most products claimed to be effective, like chitosan [45] or cationic starches containing aldehyde groups [6], failed in mill trials and the search for such agents continues. The most promising recent approach came from Pelton and coworkers [79] with the synthesis of phenylboronic acid derivatives of polyvinylamine. These copolymers instantaneously form strong bonds with wet cellulose, resulting in an enormous increase in the delamination force between neverdried cellulose surfaces. Nevertheless, these new products have to date only been tested on films made of regenerated cellulose, where the cellulosecellulose contact area is ideally large and still needs to be demonstrated on actual papermaking pulps. For the same reason, there is no data available to confirm how the presence of fillers would affect such properties. The addition of fillers to the paper furnish, among which precipitated calcium carbonate (PCC) is the most commonly used, has traditionally led to a drop in both WWS and dry strength. Although this is true for the dry strength, as the filler particles unavoidably interfere with the formation of interfibre hydrogen (H) bonding, we recently showed this is not necessarily the case in WWS [0]. Instead, if the fillers posses the correct size distribution, the WWS could be even slightly increased [0]. Such an increase, ascribed to a corresponding increase in fibre roughness [], is very minor and the production of fillercontaining paper products, especially those highlyloaded, would benefit if the ALVAJO TEJADO * Contact: alvaro.tejado@gmail.com MIRO ANTAL WWS could be enhanced by other means. We recently demonstrated [2] that a wellknown bioconjugation reaction, namely the ethyl3[3(dimethylaminopropyl)] carbodiimide (EDC)assisted reaction of carboxyl and amine groups [3, 3], can be used to crosslink fibres in a neverdried sheet. By using such chemistry we obtained increases as high as 500% and 00% in the WWS of papers made fromunbeaten and beaten hardwood Kraft pulp respectively, after pretreating the fibres with carboxymethyl cellulose and THEO G.M. VAN DE VEN JFOR Journal of Science & Technology for Forest Products and Processes: VOL., NO., 20 27

2 Fig. Effect of beating, crosslinking and beating plus crosslinking on the WWS of hardwood fibres for three solids contents (after [2]). Scheme Chemical structures of CMC, EDC and ADH, and crosslinking reaction scheme. using adipic dihydrazide as the crosslinker. Furthermore, experiments showed the crosslinking reaction does not counteract the beating effect, as has typically occurred in the past for other crosslinking routes. It alternatively instead complements it, as is graphically shown in Fig.. In this paper we use highly beaten hardwood Kraft pulp (bhkp) to show the significant increase in WWS that is obtained by crosslinking the fibres with that chemistry, and how such treatment also has a very positive impact on the WWS of PCC loaded paper. chemicals were used as received. Scheme shows the chemical structures of CMC, EDC and ADH and the reaction route. Crosslinking experiments were conducted as follows: 00 g of pulp suspension at 2% consistency (2 g oven dried pulp) were first treated with CMC to obtain CMCcoated fibres, following the general recipe of Laine et al. [4]. After purification of the product by filtration and redispersion in distilled water twice, various amounts of EDC and ADH were added and this mixture reacted overnight at room temperature under magnetic stirring. In one case the ph was previously adjusted to 4.5 using N HCl. The precise recipe used for each experiment is shown in Table. Treated pulp suspensions were used to prepare paper sheets using a standard British handsheet maker following TAPPI T 205 sp95, except for different basis weights. Two kinds of sheets were prepared: 00 g/m 2 sheets to measure the WWS of unloaded and moderately PCC MATERIALS AND METHODS FPInnovations supplied neverdried bleached hardwood Kraft pulp. The beaten pulp (bhkp) was obtained by subjecting the previous to a refining process in a PFI mill, after which most of the fines were removed by flotation. The extent of fibrillation, characterized through a Freeness tester (Tappi Test Method T227 om 99, 200), was 250 ml CSF (Canadian Standard Freeness). Domtar Inc provided PCC. The nominal diameter and specific surface area were respectively μm and 0. m 2 /g. Carboxymethyl cellulose (CMC) sodium salt with degree of substitution 0.7 was obtained from Hercules Inc. (type CMC7MT). EDC with 97% purity and ADH 98% pure were obtained from Fluka and Sigma respectively. All Name bhkp bhkp2 bhkp3 bhkp4 bhkp5 bhkp6 bhkp7 CMC (mg/g) TABLE Experimental conditions used CMC attaching Crosslinking + Formation Loading CaCl 2 (M) Tt ( o Ch) EDC (mmol/g) ADH (mmol/g) ph A B PCC (g/g) Note: Dosages of reactants (CMC, EDC and ADH) and filler (PCC) refer to the amounts originally added and are expressed per gram of dried pulp. Tt stand for reaction temperature ( o C) and time (h), respectively. ph A refers to the ph at which the mixture containing CMCcoated fibres, EDC and ADH is allowed to react overnight, while ph B is the ph at which the sheet is made. All controls are grouped under bhkp although a specific one was prepared for each set of experiments. 28 JFOR Journal of Science & Technology for Forest Products and Processes: VOL., NO., 20

3 loaded paper, and 25 g/m 2 to test highly PCCloaded paper, both expressed as gram of pulp per square meter of paper. A metal template mould, with openings to accommodate eight 25 mm wide and 55 mm long paper strips, was placed over the metal screen of the Standard British Handsheet Machine. After pressing each precut sheet between two Teflon plates at 350 kpa for 5.5 minutes, the paper strips were dried until the desired degree and used to measure the WWS in a TMI LabMaster tensile machine. A new control curve was made for each system, repeating the exact procedure (e.g. refilling and redraining the handsheet machine). The loading of PCC was carried out through a wetloading process, according to a method already tested and proved to be valid in a previous work [5]. A similar procedure was also successfully tested on pilot machine trials [6]. Briefly, but immediately after the formation of a regular sheet of paper, the handsheet maker was carefully refilled with water (bottomup) and a previously homogenized PCC suspension was added. A second draining step produced the final loaded paper. The sheet of consistency ~0% was neither dried nor pressed before loading the PCC, allowing the distribution of the filler throughout the highlyswollen fibre network. Prior to refilling with water, the weak paper was protected by gently placing a soft polyurethane screen (25x500 μm pores, from Johnson Screens) on top of it. Comparing the quality of the resulting sheets with those obtained by means of the traditional procedure initially verified the validity of the method. Both methods demonstrated equally good sheet consolidation, while the new one allowed a higher PCC loading. The determination of PCC was finally carried out by ashing the samples in a closed furnace at 525±25 C, according to TAPPI standard T 2 om93. RESULTS AND DISCUSSION As discussed in a previous work [2], the proposed crosslinking route is found to be extremely well adapted to papermaking conditions. It permits the physical adsorption of the chemicals onto the fibres in a stirred suspension but, if the ph is controlled, the reaction occurs only after the paper sheet is formed. This allows the ADH molecules to create interfibre bridges, resulting in a spectacular increase in WWS. In this sense a slightly acidic ph is desirable, as there is no reaction under alkaline conditions. If the ph is too low the reaction takes place rapidly on the surface of the segregated fibres still in suspension and there is no improvement in strength. This ph effect can be seen in Fig. 2. As shown carrying out the reaction at ph 4.5 (according to the literature, the optimal for this particular chemistry [3]) has no positive effect on the WWS, whereas the same procedure at ph gives rise to a very remarkable increase, as high as 70% at % solids. It is well known that PCC dissolves at ph below 8 but such a process is very slow under neutral or slightly acidic conditions. That means the ph behaviour referenced above also allows the use of this chemistry with PCC loaded paper, as will be shown next. Figure 3 shows the effect a moderate concentration of PCC has on the WWS of paper. Initially, the presence of 4% of PCC causes a considerable reduction in paper strength. As discussed in a previous study [0], such a decrease must be ascribed to the presence of large aggregates in the PCC suspension rather than an inherent effect of the fillers. This is likely, as storage often leads to aggregation and because the filler was used without any pretreatment to ensure the dispersing of the individual particles (such as intensive shearing and/or addition of stabilizing polymers) following the most common industrial practice. The gap between the PCCloaded sample and the control increases with the solids content, indicating that the presence of PCC also interferes with the formation of H bonding. This observation provides indirect evidence of the presence of the filler particles between fibres, and thus, of their penetration and Fig. 2 Effect of ph on the WWS of CMC/ EDC/ADHcrosslinked bhkp paper (basis weight=00 g/m 2 ). bhkp ( ), bhkp2 ( ) and bhkp3 ( ). Fig. 3 Effect of moderate PCC loading on the WWS of bhkp paper with and without CMC/EDC/ADH crosslinking (basis weight=00 g/m 2 ). bhkp ( ), bhkp4 ( ) and bhkp5 ( ). Fig. 4 Effect of high PCC loading on the WWS of bhkp paper with and without CMC/ EDC/ADH crosslinking (basis weight=25 g/ m 2 ). bhkp ( ), bhkp6 ( ) and bhkp7 ( ). JFOR Journal of Science & Technology for Forest Products and Processes: VOL., NO., 20 29

4 distribution throughout the sheet thickness. As can be seen in the graph, when the sheet of paper is formed after the fibres have been subjected to the discussed chemical treatment, the benefit over the nontreated paper is clear: the crosslinked sheet is stronger than the noncrosslinked one regardless the degree of dryness. Remarkably, all the strength loss caused by the presence of PCC is entirely recovered at low solids contents (up to ~35%), even at a slightly higher PCC concentration. Crosslinking the fibres, however, does not prevent the interference of the PCC particles with the H bond formation derived from a decreased interfibre contact area. It is also worth noting that the absolute increase in strength brought by this procedure (when compared to the noncrosslinked loaded one) is almost constant up to 45% solids. This behaviour, (noted in Fig. 2, and described in detail for nonloaded unbeaten fibres in a previous work [2]), shows that the crosslinking reaction does not depend on the amount of water (i.e. does not require any sort of drying process), but instead comes to completion already at the lowest consistencies. Such behaviour is a consequence of the extraordinary ability of EDC to promote covalent bonding in very wet conditions. The same arguments are valid in the case of highly loaded paper, as can be concluded from Fig. 4. Once again the presence of PCC weakens the paper already at low solids content, suggesting aggregation of the particles before being loaded. However, in this case the large amount of filler (37%) inhibits almost completely the formation of H bonds and as a result there is very little increase in strength upon drying. Such plateaulike curve at high PCC content is consistent with previous observations for similar systems []. This situation implies that no new fibrefibre contact surface is being created as the paper gets drier, due to the presence of the filler particles. Similarly to Fig. 3, crosslinking the fibres generates a stronger paper along the whole consistency range even with slightly higher PCC content (%) that is able to overcome the original loss in strength at low solids. Mimicking the behaviour of the previous curve, the abundance of PCC impedes the formation of interfibre H bonding and leads to a plateaushaped curve. Very interestingly, in this situation the absolute increase in strength due to the crosslinking is constant throughout the whole solids content range analyzed, thus extending the behaviour commented before and giving definitive evidence of the complete occurrence of the reaction immediately after the formation of the sheet. This ability differentiates this reaction mechanism from nearly all the routes reported in the literature that require certain drying process in order to be efficient. Results shown were obtained with handsheets made at a lab scale and extrapolation to an industrial scale would require additional experiments. Also, the proposed crosslinking chemistry is likely too expensive for practical applications. However the paper proves the concept that crosslinking leads to a very significantly improvement in WWS, warranting the investigation on alternative less expensive crosslinking agents. CONCLUSIONS EDCassisted crosslinking of CMCcoated hardwood Kraft fibres with ADH leads to extraordinary increases of the WWS of papers made from such fibres. Such a mechanism is shown to proceed very conveniently at slightly acidic ph, which in turn allows its application in PCC loaded papers. This always results in a stronger paper compared to the loaded and uncrosslinked ones, and the increases in strength are higher than 22% and 65%, respectively, for moderate and highly loaded papers at % solids content. Interestingly, no water removal is needed for the effectiveness of the reaction that is shown to be already complete at the lowest consistencies tested. The ability to promote covalent bonding in very wet conditions, combined with the outstanding improvement in paper strength, makes EDC an excellent WWS agent. ACKNOWLEDGEMENTS The authors would like to thank NSERC and FPInnovations for the funding of an IRC in Colloid and Papermaking Chemistry. Dr. Meng Ran Wu and Xiaojun Liu are also acknowledged for working in the early stages of the project. REFERENCES Tomihata, K., and Ikada, Y., Crosslinking of hyaluronic acid with watersoluble carbodiimide Journal of Biomedical Material Research, 37(2):24325 (997). 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5 boronic acid derivatives of polyvinylamine to wet cellulose: A colloidal probe microscopy investigation Langmuir, 25(2): (2009). de Oliveira, M.H., Tejado, A., and van de Ven, T.G.M., Effects of fillers on the wet web strength of paper, Nordic Pulp and Paper Research Journal, 24(2):447 (2009). Tejado, A., and van de Ven, T.G.M., Why does paper get stronger as it dries? Materials Today, 3(9):4249 (200). Tejado, A., Antal, M., Liu, X., and van de Ven, T.G.M., Wet crosslinking of cellulose fibers via a bioconjugation reaction Industrial & Engineering Chemistry Research, (submitted) (200). Nakajima, N., and Ikada, Y., Mechanism of amide formation by carbodiimide for bioconjugation in aqueousmedia Bioconjugate Chemistry, 6():2330 (995). Laine, J., Lindstrom, T., Nordmark, G.G., and Risinger, G., Studies on topochemical modification of cellulosic fibres part. Chemical conditions for the attachment of carboxymethyl cellulose onto fibres Nordic Pulp and Paper Research Journal, 5(5): (2000). Alince, B., Vanerek, A., and van de Ven, T.G.M., Clay particle deposition in a fibre web: An alternative way of filling paper? Journal of Science, 28(9):3532 (2002). van de Ven, T.G.M., Vanerek, A., and Garnier, G., Filling wet paper with the use of a secondary headbox Industrial & Engineering Chemistry Research, 43(9): (2004). WHY JOIN PAPTAC s TECHNICAL COMMUNITIES Sharing information on specific topics & challenges facing the Canadian pulp and paper industry. Accessing an exclusive Canadian technical pulp and paper network. Continuing to learn from your peers, identifying and developping new problemsolving solutions. Being aware of the latest technological advancements and innovations. Greater value derived from participating in PAPTAC events (PaperWeek, PACWEST, conferences, webinars, etc.). The technical communities have been formed among PAPTAC members who are interested in a particular aspect of the industry. Members discuss practical and scientific developments in their area of specialty. They work on projects to learn more about the industry, and then disseminate this information to the industry through technical presentations and reports. For more information regarding PAPTAC's technical communities or to join as a member of a community (PAPTAC membership required), contact PAPTAC ( / tech@paptac.ca). PAPTAC NEWS JFOR Journal of Science & Technology for Forest Products and Processes: VOL., NO., 20 3