CHANGES OF MACROMOLECULAR PROPERTIES ...

Recycling

CHANGES OF MACROMOLECULAR PROPERTIES OF RECYCLED PULP FIBRES František Kačík Iveta Čabalová Danica Kačíková Technical University in Zvolen, Faculty of Wood Sciences and Technology, Zvolen, Slovakia ABSTRACT The production of pulps and paper has a variety of negative influence on the environment. There are some technologies which can moderate the negative impacts and they have also a positive economical effect (e.g. paper recycling). It causes the paper alterations, which results in the degradation of their physical and mechanical properties. The recycling causes a chemical, thermal, biological and mechanical destruction, or their combination. The aim of this work was to investigate changes of macromolecular weight distribution (MWD), polydispersity (PD) and degree of polymerization (DP) of recycled hardwood pulp fibres. These attributes were measured by size exclusion chromatography (SEC) of cellulose tricarbanilates (CTC). From the experimental results can be concluded that the changes of investigated properties are not linear throughout the recycling process. Generally, decrease of DP values in consequence of glycosidic bonds splitting was observed. Moderate increase in some stages of recycling is due to cellulose cross-linking, amorphous part of cellulose degradation, as well as various states of fibres at various recycling stages. Keywords: pulp recycling, cellulose, tricarbanilates, size exclusion chromatography, hardwood pulps INTRODUCTION The production of pulp and paper has negative effects on the environment (raw materials, energy and water consumption; solid waste, waste water and gas pollutions production). However, there are several technologies able to diminish the negative impact on the environment and they also have a positive economic effect. One of these processes is a recycling, what on the side of the process inputs saves natural resources and reduces the harmful substances at the side of the manufacturing outputs. Paper recycling saves the natural wood raw stock, decreases the operation and capital costs to paper unit, decrease water consumption and last but not least this paper processing gives rise to the environment preservation (e.g. 1 t of waste paper can replace approx. 2.5 m3 of wood). The consumption of recovered paper has been in continuous growth during the past decades. According to the Confederation of European Paper Industries (CEPI), the use of recovered paper was almost even with the use of virgin fibers in 2005. This development has been boosted by technological progress and 807

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the good price competitiveness of recycled fibers, but also by environmental awareness – at both the producer and consumer ends – and regulation that has influenced the demand for recovered paper. The European paper industry suffered a very difficult year in 2009 during which the industry encountered more down-time and capacity closures as a result of the weakened global economy. Recovered paper utilisation in Europe decreased in 2009, but exports of recovered paper to countries outside CEPI continued to rise, especially to Asian markets (96.3%). However, recycling rate expressed as “volume of paper recycling/volume of paper consumption” resulted in a record high 72.2% recycling rate after having reached 66.7% the year before [1-5]. The paper recycling, simplified, means the repeated defibring, grinding and drying, when there are altered the mechanical properties of the secondary stock, the chemical properties of fibres, the polymerisation degree of pulp polysaccharidic components, mainly of cellulose, their supramolecular structure, the morphological structure of fibres, range and level of interfibres bonds e.g.. The cause of above mentioned alterations is the fibres ageing at the paper recycling and manufacturing, mainly the drying process. At the repeat use of the secondary fibres, it need deliberate the paper properties alter due to the fibers deterioration during the recycling, when many alteration are irreversible. The alteration depth depends on the cycle’s number and way to the fibres use. The main problem is the decrease of the secondary pulp mechanical properties with the continuing recycling, mainly the paper strength [6-10]. Recycled pulps and papers have different properties in the comparison to virgin ones. The worse properties of the recycled fibres in comparison with the primary fibres can be caused by hornification but also by the decrease of the hydrophilic properties of the fibres surface during the drying due to the redistribution or migration of resin and fat acids to the surface [11-12]. The paper recycling causes its alterations, which results in the degradation of their physical and mechanical properties. The recycling causes chemical, thermal, biological and mechanical paper destruction, or their combination [13-14]. A lot of important properties of pulps and papers depend on the degree of polymerisation (DP) and molecular weight distribution (MWD). Due to pulp recycling cellulose DP decrease and the mechanical and optical properties deteriorate [15-17]. The aim of this work is to investigate changes of macromolecular weight distribution (MWD), polydispersity (PD) and degree of polymerization (DP) of recycled hardwood pulp fibres. EXPERIMENTAL Laboratory experimental conditions of pulp fibers recycling have been simulated to the real paper processing. Virgin pulp fibers as a starting material were used. Fibers from kraft softwoods bleached pulp were 8-times recycled. Drying temperatures of prepared paper sheets were set at 80 and 120 °C, respectively. The degree of polymerization (DP) and molecular weight distribution (MWD) of both the control and recycled samples were obtained by size exclusion chromatography (SEC) of their tricarbanilates (CTC – cellulose tricarbanilates). Carbanilation procedure was carried out as described previously [18]. The GPC of the tricarbanilated samples was carried out on PLgel MIXED B column (Polymer Laboratories). THF was used as an eluent and the data were detected by diode array detector at 240 nm [18]. The system was calibrated with 808

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polystyrene standards with weight-average MW in the range 500-6,035,000 (Polymer Laboratories, Tosoh). A universal calibration for determination of molecular weights was used with the constants K = 2.01 x 10-3, α = 0.92 [19]. All GPC results represent the mean of two different samples, and each CTC was chromatographed twice (total of four runs for each sample). The values of DP were calculated by dividing the molecular weight by the monomer equivalent weight of cellulose tricarbanilate (DP = M/519). RESULTS AND DISCUSION In the processes both of recycling and drying the degradation of cellulose chains occurs. The decrease of mechanical properties is due to diminishing of DP value. The extent of these changes depends mainly at number of recycling cycles. Numerous authors [20-22] have dealt with relationships between a decrease in cellulose DP and loss of paper mechanical properties at various conditions (influence of temperature, humidity, acidity, etc.). Results of size exclusion chromatography (SEC) are presented in Tabs 1-2. Tab. 1 Results of Mw, Mn, Mz, Mz+1, PD and DP of pulp samples before and after recycling dried at the temperature 80 °C number of recycling

Mn

virgin pulp

24,842

0

Mw

Mz

Mz+1

PD

DP

179,700 464,026 747,610

7.23

1,109

17,896

157,029 477,331 765,375

8.77

969

1

28,144

184,183 487,103 781,329

6.54

1,137

2

18,479

132,315 408,397 644,249

7.16

817

3

21,801

174,503 461,813 733,687

8.00

1,077

4

29,368

184,651 481,595 779,354

6.29

1,140

5

26,798

204,862 507,539 797,347

7.64

1,265

6

31,032

186,659 468,599 747,502

6.02

1,152

7

25,284

181,852 465,899 751,319

7.19

1,123

8

29,506

181,742 471,832 765,683

6.16

1,122

Note: Mn = number average molecular weight (MW), Mw =weight-average MW, Mz = z average MW, Mz+1 = z+1 average MW, DP = degree of polymerisation, PD (polydispersity) = Mw/Mn

The results in Tabs 1-2 show that cellulose DP values decrease at pulp recycling. This drop is due to cleavage of glycoside bonds in cellulose and shortening of its chains. However, this process is not linear and at some recycling pulps the DP value tends to increase. This effect is probably in consequence of different fibres states at various

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recycling stages – increased active surface as well as fibres shortened cause diminishing of cellulose DP values. Tab. 2 Results of Mw, Mn, Mz, Mz+1, PD and DP of pulp samples before and after recycling dried at the temperature 120 °C number of recycling

Mn

virgin pulp

24,830

0

Mw

Mz

Mz+1

PD

DP

179,621 463,820 747,278

7.23

1,109

23,902

184,274 476,079 759,467

7.71

1,137

1

30,708

179,656 439,858 675,738

5.85

1,109

2

29,109

177,864 508,630 797,836

6.11

1,098

3

28,432

178,233 454,702 744,952

6.27

1,100

4

27,081

168,940 455,719 778,102

6.24

1,043

5

22,220

164,665 446,508 750,280

7.41

1,017

6

27,523

164,725 448,291 780,239

5.98

1,017

7

23,168

156,902 421,448 722,672

6.77

969

8

26,298

155,408 422,865 742,544

5.91

960

Note: Mn = number average molecular weight (MW), Mw =weight-average MW, Mz = z average MW, Mz+1 = z+1 average MW, DP = degree of polymerisation, PD (polydispersity) = Mw/Mn

Kato and Kameron [23] in their review paper have been summarised results several authors. From these results can be concluded that at thermal treatment the cleavage of cellulose chains take place in dependency of temperature, humidity and time of treatment. In the process of hardwood pulp recycling also crystallinity increase from 80.9% to 83.7% due to fact that amorphous cellulose hydrolyses faster than crystalline cellulose. Recycling of softwood pulp leads to increase of DP value from 1042 (1st recycling) to 1133(6th recycling) and polydispersity (PD) from 4.88 to 5.18, however authors do not present other values [12]. During the recycling some concurrent reactions proceed that affects contrary to DP reducing. Data in Tab 1-2 are consistent with the results by Geffertová et al. [10] dealing with recycled pulps breaking length and tear index of recycled hardwood pulps.

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Fig. 1 SEC chromatograms of cellulose from recycled pulps dried at 80 °C

Fig. 2 SEC chromatograms of cellulose from recycled pulps dried at 120 °C Size exclusion chromatograms (SEC) of cellulose tricarbanilates at all recycling stages are presented in Fig 1-2 (only selected curves are shown for clarity). Peaks have bimodal characters; the high molecular peak belongs to cellulose (DP approx. 900), the 811

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low molecular one belongs to hemicelluloses (DP approx. 100). From SEC chromatograms can be concluded that at pulp recycling the glycosidic bonds cleavages take place. The recycling also causes some shortening of the long cellulose chains and their shift to shorter fraction. Changes also occur in low molecular weight fractions due to different speed of hemicellulose degradation.

CONCLUSIONS The aim of this work was to investigate changes of macromolecular weight distribution (MWD), polydispersity (PD) and degree of polymerization (DP) of recycled hardwood pulp fibres. These attributes were measured by size exclusion chromatography (SEC) of cellulose tricarbanilates (CTC). From the experimental results can be concluded that the changes of investigated properties are not linear throughout the recycling process. Generally, decrease of DP values in consequence of glycosidic bonds splitting was observed. Moderate increase in some stages of recycling is due to cellulose cross-linking, amorphous part of cellulose degradation, as well as various states of fibres at various recycling stages. ACKNOWLEDGEMENT This research was supported by the Slovak Scientific Grant Agency (VEGA) under the contract No. 1/0490/09. REFERENCES [1] Hujala, M., Puumalainen, K., Tuppura, A. & Toppinen, A. Trends in the Use of Recovered Fiber – Role of Institutional and Market Factors. Progress in Paper Recycling, Vol. 19, No. 2, 2010, pp. 3-11, ISSN 1061-1452 [2] CEPI (Confederation of European Paper Industries). Special Recycling 2005 Statistics - European Paper Industry Hits New Record in Recycling. 27.02.2011, Available from: http://www.erpa.info/images/Special_Recycling_2005_statistics.pdf [3] European Declaration on Paper Recycling 2006 – 2010, Monitoring Report 2009 27.02. 2011, Available from: http://www.erpa.info/images/monitoring_report_2009.pdf [4] Huhtala, A., & Samakovlis, E. Does International Harmonization of Environmental Policy Instruments Make Economic Sense? Environmental and Resource Economics, 21(3), pp. 261-286, ISSN 0924-6460 [5] CEPI (Confederation of European Paper Industrie). Annual Statistic 2009. 27.02.2011, Available from: http://www.erpa.info/download/CEPI_annual_statistics%202009.pdf [6] Khantayanuwong, S., Toshiharu, E. & Fumihiko, O. Effect of Fiber Hornification in Recycling on Bonding Potential at Interfiber Crossings: Confocal Laser-scanning Microscopy (CLSM). Japan Tappi Journal 56(2), 2002, pp. 239-245, ISSN 0022-815X [7] Jahan, M.S. Changes of paper properties of nonwood pulp on recycling. Tappi Journal 2(7), 2003, pp. 9-12, ISSN 0734-1415

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[8] Hubbe, M.A., Venditti, R.A. & Rojas, O.J. What happens to cellulosis fibers during papermaking and recycling? A review. BioResources 2(4), 2007, pp. 739-788, ISSN 1930-2126 [9] Garg, M. & Singh, S.P. (2006). Reason of strength loss in recycled pulp. Appita Journal 59(4), pp. 274-279, ISSN 1038-6807 [10] Geffertová, J., Geffert, A. & Čabalová, I. Hardwood sulphate pulp in the recycling process. Acta Facultatis Xylologiae Zvolen, L (1), 2008, pp. 73 – 81, ISSN 1336 – 3824 [11] Nazhad, M. M. & Paszner, L. Fundamentals of Strength Loss in Recycled Paper, Tappi, 77(9), 1994, pp. 171-179, ISSN 0039-8241 [12] Nazhad, M. M. Recycled fibre quality – A review. Journal of Industrial and Engineering Chemistry, Vol.11, No.3, 314-329, 2005 [13] Milichovský, M. Recirkulace a recyklace – výzva současnosti. Papír a celulóza, 1994, 49 (2), pp. 29 – 34, ISSN 0031-1421 [14] Čabalová, I., Kačík, F. & Sivák, J. Changes of molecular weight distribution of cellulose during pulp recycling. Acta Facultatis Xylologiae Zvolen 51(1), 2009, pp. 1117, ISSN 1336-3824 [15] Dupont, A. L. & Mortha G. (2004). Comparative evaluation of size-exclusion chromatography and viscometry for the characterisation of cellulose. J. Chromatogr. A. 1026(1-2), 2004, pp. 129-141, ISSN 0021-9673 [16] Kučerová, V. & Halajová, L. Evaluation of changes of the recycled pulps by method the gel permeation chromatography. Acta Facultatis Xylologiae Zvolen, 51(2), 2009, pp. 87-92, ISSN 1336-3824 [17] Čabalová, I., Kačík, F. & Sivák, J. The changes of polymerization degree of softwood fibers by recycling and ageing process. Acta Facultatis Xylologiae Zvolen 53(1), 2011, pp. 61-64, ISSN 1336-3824 [18] Kačík, F., Kačíková, D., Jablonský, M. & Katuščák, S. Cellulose degradation in newsprint paper ageing. Polymer Degradation and Stability, 94, 2009, pp. 1509–1514, ISSN 0141-3910 [19] Valtasaari, L. & Saarela, K. Determination of chain lenght distribution of cellulose by gel permeation chromatography using the tricarbanilate derivate. Paper och Tra – Paperi ja Puu, 1975, 1, pp. 5-10, ISSN 0031-1243 [20] X. Zou, T. Uesaka, & N. Gurnagul: Prediction of paper permanence by accelerated aging I. Kinetics analysis of the aging process. Cellulose 3, 1996, p. 243-267. [21] Hill, D.J.T., Le, T. T., Darveniza, M. & Saha, T. A study of degradation of cellulosic insulation materials in a power transformer. Part 1: Molecular weight study of cellulose insulation paper. Polymer Degradation and Stability, 48, 1995, p. 79-87 [22] Hill, D.J.T., Le, T. T., Darveniza, M. & Saha, T. A study of degradation of cellulosic insulation materials in a power transformer. Part 2: Tensile strength of cellulose insulation paper. Polymer Degradation and Stability, 49, 1995, p. 429-435.

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[23] Kato, K.L. & Cameron, R.E. A review of the relationship between thermallyaccelerated ageing of paper and hornification. Cellulose 6, 1999, pp. 23-40, ISSN 09690239

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