ABS

TRAC

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27J-FOR Journal of Science & Technology for Forest Products and Processes: VOL.1, NO.1, 2011

TRADITIONAL AREA CONTRIBUTIONS

There has been very little success in trying to provide a method that increases the wet-web strength (WWS) of paper, even though it deter-mines to a large extent the runnability of a papermaking machine. 1-ethyl-3-[3-(dimethylaminopropyl)] carbodiimide (EDC) has been shown to promote the crosslinking of soluble polysaccharides containing carboxyl groups [1-3]. 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.

ALVARO TEJADO*, MIRO ANTAL AND THEO G.M. VAN DE VEN

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

There has been very little past success in trying to provide a method that increases the wet-web strength (WWS) of paper. Most products claimed to be effective, like chitosan [4-5] or cationic starches containing aldehyde groups [6], failed in mill trials and the search for such agents continues. The most promising recent ap-proach came from Pelton and co-workers [7-9] 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 never-dried cellulose surfac-es. Nevertheless, these new products have to date only been tested on films made of regenerated cellulose, where the cellulose-cellulose 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 calci-um carbonate (PCC) is the most common-ly used, has traditionally led to a drop in

both WWS and dry strength. Although this is true for the dry strength, as the filler par-ticles unavoidably interfere with the forma-tion of inter-fibre hydrogen (H) bonding, we recently showed this is not necessarily the case in WWS [10-11]. Instead, if the fillers posses the correct size distribution, the WWS could be even slightly increased [10]. Such an increase, ascribed to a cor-responding increase in fibre roughness [11], is very minor and the production of filler-containing paper products, especially those highly-loaded, would benefit if the

WWS could be enhanced by other means.We recently demonstrated [12] that a well-known bioconjugation reaction, name- ly the 1-ethyl-3-[3-(dimethylaminopropyl)] carbodiimide (EDC)-assisted reaction of carboxyl and amine groups [3, 13], can be used to crosslink fibres in a never-dried sheet. By using such chemistry we obtained increases as high as 500% and 100% in the WWS of papers made fro-munbeaten and beaten hardwood Kraft pulp respectively, after pre-treating the fibres with carboxymethyl cellulose and

INTRODUCTION

THEO G.M. VAN DE VEN Pulp & Paper Research Centre, Department of Chemistry, McGill University3420 University st., Montreal, Qc,Canada, H3A 2A7

MIRO ANTAL Pulp & Paper Research Centre, Department of Chemistry, McGill University3420 University st., Montreal, Qc,Canada, H3A 2A7

ALVAJO TEJADO Pulp & Paper Research Centre, Department ofChemistry, McGill University3420 University st.,Montreal, Qc,Canada, H3A 2A7* Contact: alvaro.tejado@gmail.com

J-FOR Journal of Science & Technology for Forest Products and Processes: VOL.1, NO.1, 201128

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. 1. 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.

MATERIALS AND METHODSFPInnovations supplied never-dried bleached hardwood Kraft pulp. The beat-en pulp (bHKP) was obtained by subject-ing 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 Free-ness tester (Tappi Test Method T227 om-99, 2001), was 250 mL CSF (Canadian Standard Freeness). Domtar Inc provided PCC. The nominal diameter and specific surface area were respectively 1.3 μm and 10.1 m2/g. Carboxymethyl cellulose (CMC) sodium salt with degree of substi-tution 0.7 was obtained from Hercules Inc. (type CMC-7MT). EDC with ≥97% pu-rity and ADH ≥98% pure were obtained from Fluka and Sigma respectively. All

chemicals were used as received. Scheme 1 shows the chemical structures of CMC, EDC and ADH and the reaction route.

Crosslinking experiments were con-ducted as follows: 100 g of pulp suspen-sion at 2% consistency (2 g oven dried pulp) were first treated with CMC to obtain CMC-coated fibres, following the general recipe of Laine et al. [14]. After purifica-tion of the product by filtration and re-dispersion 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 1N HCl. The precise recipe used for each experiment is shown in Table 1.

Treated pulp suspensions were used to prepare paper sheets using a standard British handsheet maker following TAPPI T 205 sp-95, except for different basis weights. Two kinds of sheets were pre-pared: 100 g/m2 sheets to measure the WWS of unloaded and moderately PCC-

Fig. 1 - Effect of beating, crosslinking and beating plus crosslink-ing on the WWS of hardwood fibres for three solids contents (after [12]).

Scheme 1 - Chemical structures of CMC, EDC and ADH, and cross-linking reaction scheme.

TABLE 1 Experimental conditions used

CMC attaching Crosslinking + Formation Loading

NameCMC

(mg/g)CaCl2(M)

T-t(oC-h)

EDC(mmol/g)

ADH(mmol/g)

pHA B

PCC(g/g)

bHKP-1bHKP-2bHKP-3bHKP-4bHKP-5bHKP-6bHKP-7

- - - - --

6.66.6

6.66.6

6.66.66.6

-

-

1.21.24.84.8

4040

40

40

-

-

0.050.05

0.05

0.05

-

-

-

-

-

-

-

-

80-280-2

80-2

80-2

1.31.3

1.3

1.3

8.6

8.6

8.6

4.511

1

1

-

-

-

Note: Dosages of reactants (CMC, EDC and ADH) and filler (PCC) refer to the amounts origi-nally added and are expressed per gram of dried pulp. T-t stand for reaction temperature (oC) and time (h), respectively. pH A refers to the pH at which the mixture containing CMC-coated 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-1 although a specific one was prepared for each set of experiments.

29J-FOR Journal of Science & Technology for Forest Products and Processes: VOL.1, NO.1, 2011

TRADITIONAL AREA CONTRIBUTIONS

loaded paper, and 25 g/m2 to test highly PCC-loaded 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 “pre-cut” 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 con-trol curve was made for each system, re-peating the exact procedure (e.g. re-filling and re-draining the handsheet machine).

The loading of PCC was carried out through a wet-loading process, according to a method already tested and proved to be valid in a previous work [15]. A similar procedure was also successfully tested on pilot machine trials [16]. Briefly, but im-mediately after the formation of a regu-lar sheet of paper, the handsheet maker was carefully refilled with water (bottom-up) and a previously homogenized PCC suspension was added. A second drain-ing step produced the final loaded pa-per. The sheet of consistency ~10% was neither dried nor pressed before loading the PCC, allowing the distribution of the filler throughout the highly-swollen fibre network. Prior to refilling with water, the weak paper was protected by gently placing

a soft polyurethane screen (125x500 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 consoli-dation, while the new one allowed a higher PCC loading. The determination of PCC was finally carried out by ashing the sam-ples in a closed furnace at 525±25°C, ac-cording to TAPPI standard T 211 om-93.

RESULTS AND DISCUSSIONAs discussed in a previous work [12], the proposed crosslinking route is found to be extremely well adapted to papermak-ing 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 al-lows the ADH molecules to create inter-fibre bridges, resulting in a spectacular increase in WWS. In this sense a slightly acidic pH is desirable, as there is no re-action 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 particu-lar chemistry [3]) has no positive effect on

the WWS, whereas the same procedure at pH 6.6 gives rise to a very remarkable increase, as high as 70% at 40% solids.

It is well known that PCC dissolves at pH below 8 but such a process is very slow under neutral or slightly acidic condi-tions. That means the pH behaviour ref-erenced 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 14% of PCC causes a considerable reduction in paper strength. As discussed in a previ-ous study [10], such a decrease must be as-cribed to the presence of large aggregates in the PCC suspension rather than an in-herent effect of the fillers. This is likely, as storage often leads to aggregation and because the filler was used without any pre-treatment 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 PCC-loaded sample and the control in-creases 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/ADH-crosslinked bHKP paper (basis weight=100 g/m2). bHKP-1 ( ), bHKP-2 ( ) and bHKP-3 ( ).

Fig. 3 - Effect of moderate PCC loading on the WWS of bHKP paper with and with-out CMC/EDC/ADH crosslinking (basis weight=100 g/m2). bHKP-1 ( ), bHKP-4 ( ) and bHKP-5 ( ).

Fig. 4 - Effect of high PCC loading on the WWS of bHKP paper with and without CMC/EDC/ADH crosslinking (basis weight=25 g/m2). bHKP-1 ( ), bHKP-6 ( ) and bHKP-7 ( ).

2330 J-FOR Journal of Science & Technology for Forest Products and Processes: VOL.1, NO.1, 2011

distribution throughout the sheet thick-ness.

As can be seen in the graph, when the sheet of paper is formed after the fi-bres have been subjected to the discussed chemical treatment, the benefit over the non-treated paper is clear: the crosslinked sheet is stronger than the non-crosslinked one regardless the degree of dryness. Re-markably, 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 de-rived from a decreased inter-fibre contact area. It is also worth noting that the ab-solute increase in strength brought by this procedure (when compared to the non-crosslinked loaded one) is almost constant up to 45% solids. This behaviour, (noted in Fig. 2, and described in detail for non-loaded unbeaten fibres in a previous work [12]), 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 comple-tion 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 con-cluded from Fig. 4. Once again the pres-ence of PCC weakens the paper already at low solids content, suggesting aggregation of the particles before being loaded. How-ever, in this case the large amount of filler (37%) inhibits almost completely the for-mation of H bonds and as a result there is very little increase in strength upon dry-ing. Such plateau-like curve at high PCC content is consistent with previous ob-servations for similar systems [11]. This situation implies that no new fibre-fibre contact surface is being created as the pa-per gets drier, due to the presence of the filler particles. Similarly to Fig. 3, crosslink-ing the fibres generates a stronger paper

along the whole consistency range even with slightly higher PCC content (40%) 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 forma-tion of inter-fibre H bonding and leads to a plateau-shaped curve. Very interest-ingly, in this situation the absolute in-crease 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 com-plete occurrence of the reaction imme-diately after the formation of the sheet. This ability differentiates this reaction mechanism from nearly all the routes re-ported 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 extrap-olation to an industrial scale would require additional experiments. Also, the proposed crosslinking chemistry is likely too expen-sive for practical applications. However the paper proves the concept that crosslinking leads to a very significantly improvement in WWS, warranting the investigation on al-ternative less expensive crosslinking agents.

CONCLUSIONSEDC-assisted crosslinking of CMC-coat-ed 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 con-veniently at slightly acidic pH, which in turn allows its application in PCC loaded papers. This always results in a stronger pa-per compared to the loaded and un-cross-linked ones, and the increases in strength are higher than 22% and 65%, respectively, for moderate and highly loaded papers at 40% solids content. Interestingly, no wa-ter removal is needed for the effectiveness of the reaction that is shown to be already complete at the lowest consistencies test-ed. The ability to promote covalent bond-ing in very wet conditions, combined with the outstanding improvement in paper

strength, makes EDC an excellent WWS agent. ACKNOWLEDGEMENTSThe authors would like to thank NSERC and FPInnovations for the funding of an IRC in “Colloid and Papermaking Chemistry”. Dr. Meng Ran Wu and Xiao-jun Liu are also acknowledged for work-ing in the early stages of the project.

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