Cross-Linking of Papers Based on Thermomechanical Pulp Fibers by

Article pubs.acs.org/IECR

Cross-Linking of Papers Based on Thermomechanical Pulp Fibers by Polycarboxylic Acids: Influence on the Wet Breaking Length Houssein Awada,* Daniel Montplaisir, and Claude Daneault Centre de Recherche sur les Matériaux Lignocellulosiques (CRML), Université du Québec à Trois-Rivières, 3351, Boulevard des Forges, Trois-Rivières (Québec), G9A 5H7, Canada ABSTRACT: The use of two polycarboxylic acids (PCAs), such as 1,2,3,4-butanetetracarboxylic acid (BTCA) and citric acid (CA), was employed to esterify handsheets prepared from thermomechanical pulp (TMP) fibers. Sodium hypophosphite was used as a catalyst. The Fourier transform infrared (FTIR) spectrometry technique was utilized to verify the ability of these PCAs to form ester functions between the fibers. The modifications of both the temperature and the mass amount of the PCA were investigated. Finally, the wet breaking length of the prefabricated and cured handsheets was measured and an improvement was observed in both cases. The raise of the mass amount increased the wet breaking length for the two cases. On the other hand, a comparison between the two PCAs was carried out. For the same mass amount, BTCA was more effective to cross-link the TMP fibers at high temperature while CA showed slightly better results at low temperature.

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INTRODUCTION

Figures 2 and 3 show the mechanism of the esterification reaction using BTCA and CA in the presence of SHP as a catalyst.9,10 The kinetics of this kind of reaction have been recently reported.11 According to the literature, BTCA is more effective than CA and further cross-links the cellulose fibers. The difference is due to the number of the adjacent carboxylic functions on the molecule. The presence of four carboxylic functions in the case of the BTCA enhances its ability to cross-link the fibers through at least two ester functions. While in the case of the CA, if the first ester function is formed between the medium carboxylic function and the alcohol function of the fibers, the remaining carboxylic functions will not be able to form the five-membered cyclic anhydride intermediate. In this case one can only observe an increasing of the carboxylic function on the surface fibers. On the other hand, reaction between the five-membered cyclic anhydride and the alcohol of the CA is also possible during the curing step leading to a decrease of the cross-linked fibers. Also, reaction between two molecules of CA is possible during the curing due to the presence of the alcohol function. In the two cases, BTCA and CA, the reaction depends on the probability of the presence of the alcohol function next to the five-membered cyclic anhydride. To the best knowledge of the authors, there has not been a study about the esterification of papers based on thermomechanical pulp (TMP). These fibers, used mainly to elaborate newspapers and carton materials, lead to high yield and significant energy reductions compared to kraft fibers. Nevertheless, the surface chemistry of these fibers is different from that of the kraft fibers dominated by the presence of the celluloses. In fact, the surface of such fibers contains mainly lignin, cellulose, and hemicellulose. The mechanical properties of paper are

Polycarboxylic acid (PCA) has been used as a nonformaldehyde durable press finishing agent to replace traditional N-methylol reagents such as dimethyloldihydroxylethyleneurea (DMDHEU).1−5 PCA cross-links cotton fibers through ester linkages. At high temperature, the esterification occurrs mainly in two steps. The first one is the formation of a five-membered cyclic anhydride intermediate by the dehydration of two adjacent carboxyl groups. The second one is the reaction between the alcohol function of the cellulose and the anhydride intermediate to form an ester linkage. It was demonstrated that sodium hypophosphite (SHP) is the most effective catalyst of the esterification reaction. The SHP can accelerates both the formation of the cyclic anhydride intermediate and the reaction between the anhydride intermediate and the alcohol function of the fibers.6 PCA has also been used, as a cross-linking agent, for paper based on kraft fibers. It is well-established that the employment of such agents have increased the wet strength properties of the papers after treatment.7 Studies on the mechanism and the different experimental conditions have been reported in the literature.8 1,2,3,4-Butanetetracarboxylic acid (BTCA) and citric acid (CA) (Figure 1) are commonly used PCAs in the literature to cross-link the cellulose fibers.

Received: Revised: Accepted: Published:

Figure 1. Structure of the of 1,2,3,4-butanetetracarboxylic acid (BTCA) and citric acid (CA) used in this research. © 2014 American Chemical Society

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January 8, 2014 February 21, 2014 February 24, 2014 February 24, 2014 dx.doi.org/10.1021/ie500101n | Ind. Eng. Chem. Res. 2014, 53, 4312−4317

Industrial & Engineering Chemistry Research

Article

Figure 2. Esterification of cellulose fibers by BTCA in the presence of SHP.

directly linked to the capability of its fibers to establish inter- and intrafiber hydrogen bonds. These bonds are dependent on the presence of the alcohol functions on the surface of the fibers. The high number of these functions raises the numbers of the hydrogen bonds between the fibers and therefore increases their mechanical properties. Nevertheless, the presence of lignin decreases these functions and, further, decreases the mechanical properties of the paper. According to the literature, the presence of the lignin decreases the ratio of O/C. The TMP fibers have a ratio from 0.31 to 0.40.12,13 For the pure cellulose, the theoretical value is 0.83, since we have five oxygen atoms for six carbon atoms.14 These mechanical properties can be can be improved either by chemical modification15−17 or by physical adsorption of a cationic component.18 Herein we focused on the chemical cross-linking of the handsheets based on the esterification reaction. Mainly, BTCA and CA have been used to cross-link the TMP cellulose fibers together in the presence of SHP as catalysts. The influence of the temperature, during the curing, and the mass amount of either BTCA or CA on the esterfication of the fibers, has been investigated. The wet breaking length properties of handsheets have been measured after each treatment. A comparison between the results obtained by both the BTCA and the CA is reviewed.

(BTCA), was purchased from Sigma-Aldrich (purity = 99%). Anhydrous citric acid (CA) was acquired from Jungbunzlauer (purity = 99%). Sodium hypophoshite monohydrate (NaH2PO2· H2O) was supplied by American Chemicals LTD (purity > 99%). All of these products were used as received.

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METHODS Handsheet Process. Handsheets of 60 g/m2 were prepared in a British sheet-mold according to the PAPTAC Standard Testing Methods. All handsheets were pressed respectively for 5 min on one side and 2 min on the other side with 345 kPa pressure. Then, the handsheets were stocked overnight in a conditioning room at 23 °C and 50% of relative humidity (according to the standard TAPPI conditioning), before characterization or esterification. Handsheet Esterification. The esterification reactions, using the BTCA or the CA as cross-linking agents, take place in solid-state systems. The process used in both cases is the same. First, the handsheet is treated in a solution containing the crosslinking agent and the catalyst (each solution had equal amounts of the cross-linking agent and of the catalyst) for 30 s. Second, the handsheet is pressed between two blotting papers, in order to remove the solution from the surface, then dried at 80 °C, and further cured at a given temperature for 5 min. The temperature and the mass amounts (wt %) of the cross-linking agents were modified. Table 1 represents the different specific conditions used in this study in the case of BTCA or CA. The handsheets treated and cured were washed in water to remove the untreated

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MATERIALS A sample of unbleached TMP was taken after the twin-wire press at a Kruger mill in Trois-Rivières (Québec). Pulp consistency was approximately 32−38%. 1,2,3,4-Butanetetracarboxylic acid 4313

dx.doi.org/10.1021/ie500101n | Ind. Eng. Chem. Res. 2014, 53, 4312−4317


Article

Figure 3. Different possibilities of the CA reaction with the cellulose fibers during the esterification in the presence of SHP. X indicates that the reaction is stopped at this level because the formation of the intermediate is not allowed.

carbonyl groups in the celluloses fibers: ester and carboxyl. The peak at the wavelength 1730 cm−1 represents the ester carbonyl group (which confirms the covalent bond between the cellulose and PCA), as well as the carboxyl carbonyl group of both the TMP fibers and the un-cross-linked PCA. The intensity of the peak at this wavelength number does not, therefore, present the real degree of esterification. To separate between the ester and carboxylic function, the samples is first treated in a 0.1 M NaOH solution for 5 min at room temperature. With this treatment, carboxyl groups are converted into carboxylate, which absorb at 1590 cm−1. Then the samples are dried at 80 °C for 5 min. The handsheets used for the measurement (before and after esterification) are ground to form a powder to improve sample uniformity. A 2 mg portion of the grounded papers and 100 mg crystalline KBr were ground together using an alumina mortar and pestle and pressed to form discs. The discs were scanned over the range 4000−400 cm−1 with a total of 16 scans at a resolution of ±4 cm−1. Handsheet Characterization. Wet Breaking Length Measurement. Two strips (15 mm wide × 150 mm length) from each handsheet are prepared. In total 10 specimens are tested for each handsheet set. The strips were immersed in water for 5 min and blotted between two blotting papers. Then, the wet breaking length was measured using an Instron 4201 according to the TAPPI T494os70 method.

Table 1. Different Cross-Linking Solutions and Experimental Conditions Used in This Study amount of BTCA or CA on pulp (wt %)

curing temperature

0 (control handsheet) 1 5 10 20 5 5

180 °C 180 °C 180 °C 180 °C 180 °C 150 °C 200 °C

acid and the catalyst, then pressed respectively for 5 min on one side and 2 min on the other side with 345 kPa pressure, and, finally, stocked overnight in a conditioning room at 23 °C and 50% of relative humidity, before characterization. For each case presented in the Table 1, five TMP paper sheets are treated and analyzed. Control handsheets are prepared; in this case the samples are treated in water using the same experimental condition employed during the cross-linking reaction (temperature and time). The pH of all the solution was adjusted to 2.4. Fourier Transform Infrared (FTIR) Spectrometry. Absorbance infrared spectra were recorded using a PerkinElmer 2000 Fourier Transform Infrared (FTIR) spectrometer. When the cellulose fibers are treated with the PCA (BTCA or CA), and SHP as the catalyst, depending on the experimental conditions, an ester bond should be formed between these fibers and PCA. After the finished reaction of cross-linking there are two types of 4314



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Article

RESULTS AND DISCUSSION FTIR Analysis. The FTIR spectra of the untreated TMP fibers and of the TMP fibers treated with a 0.1 M NaOH solution for 5 min are presented in the Figure 4.

treatment with a 0.1 M NaOH solution, we can clearly observe a decreasing of the carbonyl peaks (spectra d, e, and f). This decreasing comes with an appearance of a carboxylate peak at 1590 cm−1. The remaining peaks at 1730 cm−1 indicate that the ester bond has been formed. The intensity of both peaks (1730 and 1590 cm−1) was increased with the temperature. These results showed that the increasing of the temperature (from 150 to 200 °C) improved the formation of the ester function. We can also increase the carboxylic function on the surfaces fibers. On the other hands, these results confirmed that the ester carbonyl peak of cured papers could be separated from the overlapping carboxyl peak. In the same manner, the FTIR spectra of the TMP fibers treated with a 5% CA/5% SHP and cured at different temperature [(a) 150, (b) 180, and (c) 200 °C] are presented in the Figure 6. The FTIR spectra for the same fibers after treatment with a 0.1 M NaOH solution are also presented on the Figure 5d−f.

Figure 4. FTIR spectra of (a) the untreated TMP fibers and of (b) the TMP fibers treated with a 0.1 M NaOH solution for 5 min.

Spectrum a in Figure 4 (a typical spectrum of the TMP fibers) shows a characteristic peak at 1730 cm−1 corresponding to the carbonyl stretching vibration with strong intensity. After the treatment with a 0.1 M NaOH solution for 5 min (spectrum b), we should observe a disappearance of this peak and an appearance of a peak at 1590 cm−1. These results show that the peak at 1730 cm−1 is due to the presence of carboxylic acid groups on the fibers.19 The fibers used in this study have 42 ± 3 mmol/kg carboxylic functions.13 Moreover, this results shows that all of the carboxyl groups in the sample could be converted to carboxylate ones. Finally, from this experience, we can conclude that the fibers do not have any ester function on the surface able to be detected by FTIR. Temperature Influence. The FTIR spectra of the TMP fibers treated with a 5% BTCA/5% SHP and cured at different temperature [(a) 150, (b) 180, and (c) 200 °C] are presented in the Figure 5. The FTIR spectra for the same fibers after treatment with a 0.1 M NaOH solution are also presented on the Figure 5d−f. It can be seen, from the spectra that, when the temperature was increased, the intensity of the carbonyl peaks (1730 cm−1) increased (spectra a, b, and c). The raise of these peaks can be related to either the ester or carboxylic functions. After the

Figure 6. FTIR spectra of the papers treated with 5% CA/5% SHP and cured during 5 min: (a) 150, (b) 180, and (c) 200 °C. After treatment by a 0.1 M NaOH solution: (d) 150, (e) 180, and (f) 200 °C.

We can clearly observe the same results obtained for the CA meaning that the latter are also able to cross-link the TMP fibers and that the esterification depends on the temperature. For a temperature of 150 °C, it should be noted that the proportion of the peak intensities for the fibers treated with the CA is slightly higher than theirs treated with the BTCA. On the other hand, when the temperature of 180 and 200 °C were employed, the peak intensities of both the ester and of the carboxylate functions, in the case of the BTCA, were slightly larger than theirs observed for the CA. In fact, the esterification of the fibers started at lower temperature for the CA. Indeed the CA has melting point (153 °C) lower than that of the BTCA (195 °C). The difference of the melting point influences on the formation of the five-membered cyclic anhydride intermediate and consequently favorite the formation of the ester formation at low temperature in the case of the CA.20 On the other hand, the structure of the CA (trifunctional acid) and of the BTCA (tetrafunctional acid) should explain the difference at high temperature. Experimentally, when CA esterifies fibers through the central carbon, it is not possible to the CA to form another ester function. In this case, an increase of the carboxylic function on the fibers surfaces is only observed. In the case of the BTCA, the probability to form a second ester function depends only on the presence of alcohol function on the fibers surfaces. Another disadvantage of the CA is the possibility to have an intermolecular esterification between

Figure 5. FTIR spectra of the papers treated with 5% BTCA/5% SHP and cured during 5 min: (a) 150, (b) 180, and (c) 200 °C. After treatment by a 0.1 M NaOH solution: (d) 150, (e) 180, and (f) 200 °C. 4315



Article

the cyclic anhydrides intermediate of a CA molecule and the alcohol function coming from another CA molecule. Influence of the CA or BTCA Mass Amount. The mass amount of both the BTCA and the CA (1, 5, 10, and 20%) was modified and the samples were cured at 180 °C during 5 min in the presence of the SHP (the mass amount of the SHP was equal to those of the BTCA or the CA). The FTIR spectra of the TMP fibers treated with the BTCA and the CA and treated with a 0.1 M NaOH solution are respectively presented in Figures 7 and 8.

Figure 9. Variation of the wet breaking length (presented as a percentage comparing to the controls handsheets) of the handsheets treated with a 5% of CA (or BTCA), in the presence of the SHP (5%) as a function of the temperature. The wet breaking length of the control handsheets was 300 ± 10 m.

measurements, the handsheets were treated with 5% of CA (of 5% of BTCA) and 5% of SHP. One can observe a relation between the temperature applied during the curing and the wet breaking length. A clear raise of the latter was observed when the temperature was increased. Moreover, a difference between the handsheets treated by CA and BTCA was observed. At low temperature (150 °C), the wet breaking the handsheets treated by the CA seems to be more effective than those treated by the BTCA. This result can be related to the low value of the melting point of the CA comparing to the BTCA melting point. At high temperature (180 and 200 °C), the results, obtained when the BTCA was used, were better. At 180 °C, we started to be close to the melting point of the BTCA. On the other hand, a meaningful raise was observed when the temperature was increased from 180 to 200 °C. Since the temperatures were higher than the melting point of both the CA and the BTCA, the amount of the ester functions depends on the structure of the CA (trifunctional acid) and of the BTCA (tetrafunctional acid). Therefore, the latter leads to a larger number of ester functions. These results are in good agreement with those obtained by the FTIR. Indeed, the raise of the ester function between the fibers increased the wet breaking length of the handsheets. These results are not surprising while the presence of the water decreases the influence of the hydrogen bonds; the physical properties of the handsheets are governed by the chemical bonds (i.e., the ester bonds) between the fibers. The influence of the mass amount (from 1 to 20%) of both CA and BTCA, during the esterification, on the wet breaking length of the handsheets was also investigated. Figure 10 shows the variation of these percentages with the different of the mass amount. For all of these measurements, the handsheets were treated with a temperature of 180 °C. One can observe a relation between the amounts of the PCA applied during the curing and the wet breaking length. The dependence of these amounts is clearer in the case of the BTCA. These results are in good agreement with their observed in FTIR. In the case of the CA even that we observed a significant enhancement of the ester function by FTIR, the increase of the wet strength properties was not in the same manner as that of the BTCA. This difference can be attributed to two reasons. The first one can depend on the ability of the CA to cross-link the fibers. This ability is related to the position that the first ester function is formed after the formation of the five-membered cyclic anhydride intermediate. The second reason should depend on

Figure 7. FTIR spectra of the handsheets treated at 180 °C during 5 min with (a) 1% BTCA/1% SHP, (b) 5% BTCA/5% SHP, (c) 10% BTCA/ 10% SHP, and (d) 20% BTCA/20% SHP. All of these samples were treated by a 0.1 M NaOH solution.

Figure 8. FTIR spectra of the handsheets treated at 180 °C during 5 min with (a) 1% CA/1% SHP, (b) 5% CA/5% SHP, (c) 10% CA/10% SHP, and (d) 20% CA/20% SHP. All of these samples were treated by a 0.1 M NaOH solution.

One can observe that the intensities of the peaks increased when the amount of the BTCA or of the CA are increased. On the other hand, it should be noted that the peaks intensities for the fibers treated with the BTCA is higher than that treated with the CA with the same mass amount. This difference is due to the less efficient of the CA comparing to the BTCA. Wet Breaking Length Measurement of the Handsheets. The effect of the temperature, during the esterification, on the wet breaking length of the handsheets was investigated. The values (average of three measurements on the three handsheets set) are presented as a percentage to the control handsheets. The wet breaking length of the control handsheets (average of three measurements on three handsheets set) was 300 ± 10 m. Figure 9 shows the variation of these percentages with the variation of the cured temperature. For all of these 4316



Article

(2) Yang, C. Q. Effect of pH on Nonformaldehyde Durable Press Finishing of Cotton Fabric: FT-IR Spectroscopy S1tudy Part I: Ester Crosslinking. Textile Res. J. 1993, 63 (7), 420−430. (3) Yang, C. Q.; Wang, X.; Kang, I.-S. Ester Crosslinking of Cotton Fabric by Polymeric Carboxylic Acids and Citric Acid. Textile Res. J. 1997, 67, 334−342. (4) Yang, C. Q.; Chen, D.; Guan, J.; He, Q. Cross-Linking Cotton Cellulose by the Combination of Maleic Acid and Sodium Hypophosphite. 1. Fabric Wrinkle Resistance. Ind. Eng. Chem. Res. 2010, 49, 8325−8332. (5) Yang, C. Q.; He, Q.; Voncina, B. Cross-Linking Cotton Cellulose by the Combination of Maleic Acid and Sodium Hypophosphite. 2. Fabric Fire Performance. Ind. Eng. Chem. Res. 2011, 50, 5889−5897. (6) Yang, C. Q. Infrared Spectroscopy Studies of the Effects of the Catalyst on the Ester Cross-Linking of Cellulose by Poly(carboxylic acids). J. Appl. Polym. Sci. 1993, 50, 2047−2053. (7) Caulfield, D. F. Ester crosslinking to improve wet performance of paper using multifunctional caboxylic acids, butanetetracarboxylic and citric acid. Tappi J. 1994, 77 (No. 3), 205−2012. (8) Yang, C. Q.; Xu, Y.; Wang, D. FT-IR Spectroscopy Study of the Polycarboxylic Acids Used for Paper Wet Strength Improvement. Ind. Eng. Chem. Res. 1996, 35, 4037−4042. (9) Zhou, Y. J.; Luner, P.; Caluwe, P. Mechanism of Crosslinking of Papers with Polyfunctional Carboxylic Acids. J. Appl. Polym. Sci. 1995, 58, 1523−1534. (10) Peng, H.; Yang, C. Q.; Wang, X.; Wang, S. The Combination of Itaconic Acid and Sodium Hypophosphite as a New Cross-Linking System for Cotton. Ind. Eng. Chem. Res. 2012, 51, 11301−11311. (11) Lund, K.; Brelid, H. Kinetics of Cross-Linking Softwood Kraft Pulp with 1,2,3,4-Butanetetracarboxylic Acid. Ind. Eng. Chem. Res. 2013, 52, 11502−11509. (12) Toth, A.; Cernakova, L.; Cernak, M.; Kunovska, K. Surface analysis of groundwood paper treated by diffuse coplanar surface barrier discharge (DCSBD) type atmospheric plasma in air and in nitrogen. Holzforschung. 2007, 61 (54), 528−531. (13) Awada, H.; Monplaisir, D.; Daneault, C. Growth of polyelectrolyte on lignocellulosic fibres: Study by z-potential, FTIR and XPS. BioResources 2012, 7 (2), 2090−2104. (14) Montplaisir, D.; Daneault, C.; Chabot, B. Surface composition of grafted thermomechanical pulp through XPS measurement. BioResources. 2008, 3 (4), 1118−1129. (15) Sang, Y.; Xiao, H. Preparation and application of cationic cellulose fibres modified by in situ grafting of cationic PVA. Colloids Surf. A: Physicochem. Eng. Aspects 2009, 335, 121−127. (16) Montplaisir, D.; Chabot, B.; Daneault, C. Cationisation of thermomechanical pulp fibers. Part 1: Grafting reaction optimization. Pulp Paper Canada 2006, 107 (10), 29−32. (17) Montplaisir, D.; Chabot, B.; Daneault, C. Cationisation of thermomechanical pulp fibres. Part 2: Influence on strength and retention. Pulp Paper Canada 2006, 107 (11), 39−42. (18) Gandini, A.; Pasquini, D. The impact of cellulose fibre surface modification on some physico-chemical properties of the ensuing papers. Ind. Crops Prod. 2012, 35, 15−21. (19) Lasseuguette, E. Grafting onto microfibrils of native cellulose. Cellulose 2008, 15, 571−580. (20) Yang, C. Q.; Wang, X. Formation of Five-Membered Cyclic Anhydride Intermediates by Polycarboxylic Acids: Thermal Analysis and Fourier Transform Infrared Spectroscopy. J. Appl. Polym. Sci. 1998, 70, 2711−2718.

Figure 10. Variation of the wet breaking length (presented as a percentage comparing to the controls handsheets) of the handsheets treated with at 180 °C with different amount of BTCA (or CA). The amount of the SHP was the same to those of the CA or the BTCA. The wet breaking length of the control handsheets was 300 ± 10 m.

the presence of the alcohol function on the CA molecule. After the formation of the five-membered cyclic anhydride intermediate with a first CA molecule (or on a molecule already related by an ester function on a cellulose fiber), ester function with the free alcohol function of a second molecule of the CA can be formed. These two possibilities contribute to the enhancement of the ester functions without a raise of the cross-linked fibers.

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CONCLUSION Handsheets based on TMP fibers are cross-linked using either the BTCA or the CA in the presence of the SHP as a catalyst. The FTIR analysis showed the ability of both of them to form ester bonds between the TMP fibers. The intensities of these bonds were related to the temperature during the curing and to the mass amount of both the BTCA and of the CA. The wet breaking lengths were measured and showed an improvement with the raise of both the temperature or of the amount (either BTCA or CA). A difference between the BTCA and the CA were observed. While, at low temperature, for the same amount, the CA was more effective; the raise of the temperature leads to an increase of the wet strength properties when the BTCA was employed. The raise of the mass amount showed more improvement if the wet performance of the handsheets in the case of BTCA. The presence of one more carboxylic acid in the case of BTCA and the presence of the alcohol function in the case of CA explain the difference between the two polycarboxylic acids. In the two cases, we can increase the carboxylate function in the surface of the cross-linked fibers. This function increased the functionalities of the handsheets and can be used to do further chemical modifications.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], houssein_awada@hotmail. com. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Welch, C. M. Tetracarboxylic Acids as Formaldehyde-Free Durable Press Finishing Agents: Part I: Catalyst, Additive, and Durability Studies. Textile Res. J. 1988, 58, 480−496. 4317


Cross-Linking of Papers Based on Thermomechanical Pulp Fibers by

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