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Understand the interaction between cellulose and phosphate in papermaking process

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  • Save American Journal of M aterials Science 2013, 3(1): 19-23 DOI: 10.5923/j.materials.20130301.03 Understanding Interactions between Cellulose and Phosphate Esters in Papermaking Elhadji Babacar Ly1,2,*, François Brouillette1 1Lignocellulosic M aterials Research Center (CRM L), Université du Québec à Trois-Rivières, Trois-Rivières, G9A 5H7, Canada 2Département de Chimie, Université de Ziguinchor, Ziguinchor, BP523, Senegal Abstract Linting is a major problem affecting printing processes. It is characterised by the detachment of fibres fro m paper surface and their accu mulat ion on press blankets. Recently, the addition of blends of phosphate esters to the pulp suspension prior to sheet formation has been identified as an effect ive way of reducing the lint ing propensity of paper. The objective of this study was to understand how phosphate esters are retained on cellulosic substrates and how they can contribute to the potential reduction of the linting o f paper. Techniques like XPS, M S and conductometric titrat ion were used to give an exp lanation of this phenomenon. The results tend to confirm the presence of phosphorus on cellulosic substrates and provide evidence of phosphate ester-cellulose linkages. This study allows us to elucidate part of the retention mechanis m of phosphate esters in paper and explain their lint reduction ability. Keywords Cellu lose, Phosphate Esters, Linting, Papermaking 1. Introduction Nowadays, chemical products are common ly added to the wet pulp to imp rove paper properties[1-5]. A mong these many chemicals, phosphate esters have been shown to reduce the linting propensity of paper during the printing process.However, this effect was mainly demonstrated by industrial experience[2-4]. The exact mechanism by which phosphate esters prevents linting is still under investigation. It has been proposed that the surfactant nature of phosphate esters made then act as “internal” release agents preventing the partial detachment of fibres during sheet pressing and drying thus reducing the complete detachment of fibres in the printing process (linting)[5]. This paper is an attempt to determine if phosphate esters are retained in the sheet during formation on the papermachine and, if it is the case, by what kind of interaction with the fibres. Ho wever, it was shown that the efficiency of these mo lecules depend greatly on variables such as product solubility, alky l chain length and type, physicochemical parameters of the pulp suspension (pH, calciu m ion concentration) and adsorption on fibres. Therefore, the aim of this study was to investigate the occurrence of phosphate esters action in lint reduction applications by trying to find interactions between cellulose fibres and phosphate esters. * Corresponding author: (Elhadji Babacar Ly) Published online at Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved 2. Experimental In this investigation, cellulose powder (CP, 20 µm, Aldrich) and bleached kraft pulp (KP, Kruger Wayagamack Inc., Trois-Rivières Canada) were suspended in solutions of phosphate esters in standard papermaking conditions. Phosphate esters consisted in a comme rcia l blend of C8 to C12 mono- and di- esters. Its exact component was already determined[6]. 2.1. Methods Figure 1. Possible cellulose-Phosphate ester reaction products KP fibres or CP were added to solutions of phosphate esters (6, 12 and 24 g/ L). Cellulosic substrate suspensions (5% w/w) were kept under stirring for 6h at room temperature. After the reaction, KP fibres and CP were isolated by centrifugation at 2 500 rp m for 10 min, then dried for 24h at 105°C and stored under a dry at mosphere. CP samples identified as A061, A 121 and A241 are samples treated with 6, 12 and 24g/L of phosphate esters respectively. The same nomenclature was used for KP fibres (K061, K121 and K241). Recovered solutions after centrifugation are identified A062 (for fibre suspension A061) and so on. The initial phosphate esters solution is identified as Z. Possible reaction products that are obtained after this treatment are 20 Elhadji Babacar Ly et al.: Understanding Interactions between Cellulose and Phosphate Esters in Papermaking presented in Figure 1. 2.2. Physicochemical Characterisati on Several analytical techniques like mass spectrometry (MS), conductometric analysis and X-ray photoelectron spectroscopy (XPS) were used to characterise treated fibres and recovered solutions. The composition of recovered solutions was determined by MICROMASS spectrometer (QUATROMICRO model). Samples were injected by syringe and ionized by the electrospray method (ESI). Conductometric titrat ions were performed with a THERMO ORION conductometer (model 150). The t itration of acidic groups required a pre-treat ment of dried fibres. Therefore, 6g of dried fib res were treated with 200 mL of 0.1N HCl for 45 min, filtered and washed 5 times with 100 mL of demineralised water. This step was repeated once again. The titration was performed by suspending, 3g of pre-treated fibres in 400 mL of 0.001N NaCl and 5 ml of 0.100N HCl. The suspension was held at room temperature and stirred under nitrogen gas flow to minimise carbon dio xide interference. A 0.0100N NaOH solution was added at a rate of 0.1 mL/ min with continuous stirring[7-9]. XPS experiments were recorded on a KRATOS ULTRA ELECTRON SPECTROM ETER (Kratos Analytical) using a monochromat ic Al Kα X-ray source (15 kV, 5 mA). The samples were p reviously dried for a week and then placed in an ultrahigh vacuum chamber (10-8 mbar) and collected data were analysed by CASA XPS software. The C-H signal was used as a reference peak at 284.9 eV. A co mplete description of the XPS characterisation method can be found in the literature[10]. 6h of t reatment, two phases were obtained. The reduction of the quantity of monohydrogen phosphate in recovered solutions tends to indicate that hydrogen phosphates are adsorbed onto cellulose substrates. The LC-M S results show that there are hydrogen phosphates adsorbed onto cellulosic substrates.After6 hours of treat ment,two phases were obtained (liquid and solid).Since hydrogen phosphates were not found in the liquid phase, they should all be in the solid phase (fib res). But in light of these results, some reflect ions must be taken into account. Probably some dihydrogen phosphate may be adsorbed on the fibres without chemica l bonds. Table 1. Quant itat ive analysis of LC-MS spectra Z A061 K061 A121 M %R mC8 209 9.3 1.8 2.2 0 mC10 237 6.3 0.7 1.1 0 diC8 321 25.2 28.1 27.4 33.2 diC9a 349 22.3 22.8 23.5 25.3 diC10 377 13.2 14.9 13.6 15.1 diC1 a 1 401 11.9 16.0 20.1 13.9 diC12 429 5.0 9.9 9.6 5.6 diC1 a 3 457 4.2 4.4 2.6 5.1 485 2.6 1.4 0.0 1.8 K121 0 0 28.9 29.2 16.3 15.1 5.1 3.3 2.1 3.2. Fi bres Characterization Results of conductometric titrat ion of acid ic groups for CP and KP fib res are presented in Table 2. Titration curves of treated fibres exhib it a p lateau and a shift of the equivalence point (Figure 2). 3. Results and Discussion 3.1. Recovered S olution Anal ysis In agreement with Daemen results[6], the presence of different hydrogeno-phosphate (mono- and di-ester) was observed in the original ester blend (Z) (Tab le 1). After 6h of contact with cellu losic substrates and filtrat ion, monohydrogen phosphates, identified mC8 and mC10, were not detected in the recovered solution indicating a reaction with cellu lose fibres or an adsorption on their surface (Tab le 1). Moreover, M S results also showed that all phosphate esters in the mixture d id not react equally with fibres. This is quite normal, because monohydrogen phosphates are supposed to have a higher reactivity than dihydrogen. In the original ester mixture, the relative amounts of mC8 and mC10 are 9.3% and 6.3% respectively. After 6h of reaction with cellu lose, at least 76% and 100% of monohydrogen phosphate reacted with 6g/ L and 12g/ L of mixtu re solution. We can also observe in Table 1 that the proportion of monohydrogen phosphate decreased with increasing concentration of ester solution. Similar results were obtained for the two studied cellulosic substrates. After Figure 2. Conductometric curves of unmodified and modified fibres These are evidence of the presence of new acid functions in fibres. Since adsorbed phosphates are removed by the pre-treat ment, the reactivity of monohydrogen phosphates, which was showed by MS analysis, is confirmed. The increased acidity can only be caused by the reaction of monohydrogen phosphates with a single cellulose hydroxyl group (see Figure 1.a). Co mpounds (b) and (c) in Figure 1 do not contribute to the acidity. Fibre titrat ion analyses (Table 2) also show an increase in acid function concentration. This fibres acidity increases American Journal of M aterials Science 2013, 3(1): 19-23 21 with esters concentration, which is consistent with MS results on ester adsorption. But for the same ester concentration, CP is more react ive than KP fibres. Table 2. Titration of acidic group of untreated and treated fibres Cellulosic subst rat es Cellulosepowder (CP) Kraft pulp (KP) Samples A A061 A121 A241 K K061 K121 K241 [Est er] g/L 0 6 12 24 0 6 12 24s Acidity mmol/kg* 15.4 25.9 38 68.3 28.3 33.4 48 52 * millimol of acid per kg of fibres These results tend to confirm the presence of phosphate esters in cellulose fibres. The lint reduction in papermaking is probably related to their presence. Since phosphate esters are retained in the sheet, they can still have a release effect on the printing press. However, it was not possible to measure the quantity actually adsorbed on cellulosic substrates. XPS spectroscopy was also used to characterise cellu losic substrate surfaces (Figure 3, Figure 4, and Tab le 3). Full XPS (Figure 3) spectra of CP (a) and KP (b) fibres before treatment show only the presence of carbon C1s(285 eV) and o xygen O1s (532 eV) peaks[10-16]. After treat ment, two new peaks at 132 eV and 191 eV are observed corresponding to P2p and P2s respectively[10, 17-19]. Figure 3. Full XPS of cellulose (CP and KP) before and after treatment 22 Elhadji Babacar Ly et al.: Understanding Interactions between Cellulose and Phosphate Esters in Papermaking Table 3 presents the XPS results for the surface composition of treated and untreated fibres. It can also be noted from Tab le 2 that samples treated with the 12g/ L ester solution (A121 and K121) have more phosphorus than those treated with the 24g/L solution. This may seem amb iguous in co mparison with conductometric results, but it is due to the fact that XPS results take into account only the contribution of surface atoms. Moreover the treatment of fibres was done in water which is a swelling mediu m, it cannot be excluded that diffusion of ester and reaction with internal hydro xyl groups occurred. These different results (MS, XPS and conductometric) show the presence of phosphorus in cellulosic substrates. The phosphorus content that was obtained was not high enough to determine the degree of substitution of cellulose. According to MS analysis, the linkage between fibres and phosphate is done only through cellulose hydroxyl groups and ester function without alcohol liberation. We propose that phosphate esters are adsorbed on the cellu lose fibres by hydrogen linkage. A fter washing and drying at 105°C, water mo lecules are released and a small nu mber of CellO-P lin kages are formed. Table 3. Fibre surface composition (%) by XPS Energy (eV) A A121 A241 K K121 K241 P2p/P2s 132/191 eV 0 1.22 1.10 0 2.61 1.43 C1s 285 eV 58.14 63.38 63.47 67.73 64.94 64.49 O1s 532 eV 41.86 35.40 35.43 32.27 32.45 34.08 The o xygen spectra have been deconvoluted by using CASA XPS Software (mu lti Gaussian fit) calibrated at 532eV. The deconvoluted O1s peak of untreated cellu lose fibres show peaks assigned to O-C/O-H (O2) and O=C (O1) bonds at 532 and 530.9 eV respectively[10, 20-23]. After treatment of fibres, XPS analysis of fibres reveals a new oxygen peak (O3) at 533.6 assigned to O-P[16, 22] was observed (Figure 4). 4. Conclusions Results presented above show the presence of phosphate esters on the two studied cellulosic substrates. The preference for monoester overdiester was also noted. This study makes it possible to confirm interactions between cellu lose fibres and phosphate esters. Itwas demonstrated that phosphorus could be introduced on fibre surfaces by adding phosphate esters to a fibre suspension. Itwas also revealedthatchemical bonds were formed between fibres and the anti-lint compound. O-P bonds were observed on the fibre surface after mu lt iple washing of the treated fibres. So we can conclude that phosphate esters are linked by che mical bonding and the CellO-P bond is a part of O-P bonds. ACKNOWLEDGEMENTS This research was financially supported the Natural Sciences and Engineering Research Council of Canada (NSERC). The authors would like to thank M. Traoré (undergraduate student) and A. Lejeune and M. Paquin for their technical assistance. REFERENCES [1] P. M angin, A critical review of effect of printing parameters on the linting propensity of paper. Journal of Pulp and Paper Science, 1991. 17(5), 156 -163. Figure 4. O1s deconvolution of unmodified (a) and modified (b) fibres [2] F. Brouillette , D. M orneau, and C.S.H. AG, Additive for reducing paper linting and dusting, European Patent Office, Concerning O=P bond, many suggestions are proposed. 2006, EP1670988 (A1) Some authors[20-22] positioned it at lo wer energ ies than the O-P bond, between 530.9 and 531.5 eV, and identified it with O=C bond in the same zone. Others[24, 25] suggest that the [3] F. Brouillette et al., Evaluation of new lint reduction additives in wood-containing paper manufacturing. 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