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Effect of recycling on paper properties
Table 1, Recycling effect on softwood market pulp (unbeaten)
TrI: Tear Index; BS: Bending stiffness; TEA: Tensile energy absorption; TSI: Tensile stiffness index; TeI: Tensile index; Br: Brightness; Op: Opacity; LSC: Light scattering coefficient; Lw: length weighted average fibre length
The marginal increase in density may well be attributed to the straightened fibres as well. Bending stiffness increases continuously even though the deformed fibres suffer from low elastic modulus. The elastic modulus increases with removal of fibre deformation . Loss in the scattering coefficient is minor, indicating that the fibre to fibre contact is slightly benefited from deformation removal (Table 1).
Softwood (SW) market Kraft pulp (beaten)
The pulp was dried pine with 89% dry matter content. The pulp was soaked in soft water overnight, then beaten to 3750 revolutions using PFI mill. The target drainage resistance was set to be 25 SR. Then the pulp was examined for recycling effect.
Figure 2 and Table 2 show the effect of recycling on market softwood Kraft pulp. Paper properties of commercial softwood pulp were changed due to recycling. Tensile strength, TEA and density were reduced, but tear, bending stiffness and scattering coefficient were increased. The drop of tensile compared to density follows the ratio of 20:10. The higher rate in tensile loss indicates that surface bonding potential of fibres (i.e., specific bond strength, SBS) are also lost due to recycling, although its share could not be deduced from these observations.
Table 2, Effect of recycling on market softwood pulp (beaten)
Paper density is sensitive to fibre flexibility , the flexible fibres conform well on the plane of paper, and by doing so they form a denser paper. Flexible fibres contribute in enhancing the relative bonded area (RBA). Tensile strength is sensitive to bonding strength (SBS) as well as the relative bonded area (RBA), which originates from the surface properties of fibres and the fibre flexibility, assuming that the other fibre or paper properties (fibre length, fibre strength, formation) remain intact. The aforementioned premises highlight the fact that recycling causes the loss of both fibre flexibility as well as specific bond strength (i.e., surface bonding potential of fibres).
TEA is also lost, indicating the loss of tensile strength as well as the breaking strain. The handsheets were dried in a laboratory cylinder dryer, which does not completely restrain the fibres from drying shrinkage. Therefore, the loss in TEA may well be attributed to loss in bonding as well as breaking strain. Scattering coefficient was almost doubled due to recycling (the ratio was about 40:20) indicating loss in surface bonding as well as fibre flexibility. Increase in tear indicates loss in fibre to fibre bonding, due to loss in surface bonding potential of fibres or flexibility. Neither of these observations quantifies surface bonding loss compared to flexibility loss due to recycling, but they all confirm that the recycling deteriorates surface bonding potential of fibres as well as their flexibility.
Bending stiffness (BS) is the product of elastic modulus (E) and cube of thickness (t), (BS = E × t3/12 ). The elastic modulus decreases with recycling (See the trend in tensile stiffness, Table 2). Tensile stiffness is the product of elastic modulus with the specimen thickness (TS = E × t). Thus, the decrease in tensile stiffness is due to decrease in elastic modulus, otherwise the thickness is increasing (Table 2). The continuous increase in bending stiffness should be attributed to the gain in the paper thickness (i.e., loss in density), which is in turn due to a loss in fibre flexibility rather than surface deactivation. However, loss in fibre flexibility could also happen because of loss in bonding agents of fibre. For instance, loss in hemicelluloses component of fibre wall results in poor fibre swelling, thus fibre stiffness.
An increase of about 80% (Table 2) in bending stiffness due to recycling indicates the value of recycled fibres for the products where the bending stiffness is their critical properties. Bending stiffness is very important for runnability in paper machine, press room and converting machines.
Laboratory softwood (SW) pulp
The raw material was pine wood chips with 86% dry matter content. The pulp was oxygen delignified and bleached up to 88% ISO. The pulp was soaked in soft water overnight, then beaten to 3650 revolutions using PFI mill. The target drainage resistance was set at 25 SR.
As Figure 3 suggests, the behavior of the laboratory softwood pulp is almost the same as the commercial softwood pulp in the process of recycling. Density, tensile strength and TEA decreased, but tear strength and bending stiffness increased.
Figure 3, Properties of Lab scale softwood pulp
The decrease in tensile is also nearly twice the density (40/20). The loss in strength properties are almost doubled for the laboratory pulp. This is due to the fact that the market pulp is already dried once, even though it did not go through a complete papermaking cycle. Increase in scattering coefficient is also higher than the softwood market pulp (Table 3). In addition, the laboratory pulp is free from many damages coming from the commercial processes, probably a reason for its different behavior compared to market pulp. This observation also highlights the value of recycled chemical pulp for the products in need of higher bending stiffness (Table 3).
Hardwood (HW) laboratory pulp
The raw material was birch wood chips with 93% dry matter content. The pulp was oxygen delignifed and bleached up to 88% ISO, then soaked overnight in soft water. The pulp was refined to 2650 revolutions using PFI mill. The drainage target was set at 25 SR.
Figure 4 shows the effect of recycling on laboratory hardwood pulp. The trend is similar to the market or laboratory softwood pulp. The rate of change in paper properties are also the same as the softwood laboratory pulp, although it was not expected. If it is assumed that the hardwood fibres are stiff and less susceptible to flexibility loss, then it is plausible to assume that the major change in fibre properties is due to surface bonding loss (loss in SBS) than the loss in fibre flexibility. TEA, tensile strength and density are reduced due to recycling, but tear strength and bending stiffness increased. The increase in tensile strength is twice the density (ratio of 20:10) at the first cycle, but the ratio increases two-fold at the second cycle (Fig. 4). Figure 4 also shows that the bending stiffness benefits from recycling even though it is less pronounced compared to the softwood pulp.
Figure 4, Strength properties of Lab. hardwood pulp
Table 4, Recycling effect on hardwood Lab. pulp
Scattering coefficient also shows almost the same gain as of laboratory softwood pulp due to recycling (Table 4).
Chemithermomechanical pulp (CTMP)
Market CTMP pulp made of spruce was soaked in soft water overnight, then disintegrated according to the Scandinavian standards. The pulp without any further treatment was subjected to recycling process.
Figure 5 and Table 5 show the effect of recycling on strength and optical properties of market CTMP pulp.
Figure 5 shows that increase in tensile due to recycling is 25% at the first cycle, but the increase in density due to recycling after the third cycle is less than 4%. Thus, the loss in tensile cannot be attributed to the loss in fibre flexibility. The work of Alanko  on TMP pulp also did not show any correlation between apparent density and tensile strength. The apparent density increase was significant only for the third cycle, but the gain in tensile was statistically significant for all the cases. The data of Howard and Bichard  suggests an opposite trend; as the authors reported a positive correlation between density and tensile strength.
Decrease in scattering coefficient is also high (i.e., about 12% at first cycle, even though the corresponding gain in density does not exceed 4% after the third cycle (Table 5). Loss in bending stiffness is significant (i.e., about 10% at second cycle – Figure 5). Although loss in thickness is minor, it yet reflects itself in bending stiffness. A slight decrease in tear strength at first cycle could not be justified.
Bending stiffness and tear strength show the same trend in the processing of recycling (See Figs. 1-5). This observation indicates that both properties are sensitive to the same fibre attributes.
1.This work indicated that the market pulp (unbeaten chemical) behavior in the process of recycling - to some extent - was different from the laboratory pulp (chemical).
2.The rate of tensile strength loss was two-fold higher than the loss in density for chemical pulp.
3.Corresponding gain in density compared to tensile strength for CTMP pulp was marginal.
4.The strength loss due to recycling initiates from bonding loss although the share of specific bond strength (i.e., surface bonding potential) compared to the relative bonded area (RBA) could not be deduced.
5.Recycled fibres are very valuable raw materials for the end-products in need of higher bending stiffness.
6.In recycling process a similar trend between bending stiffness and tear strength was observed.
1 Nazhad, M. M. ,Recycled fibre quality – A review', Journal of industrial and engineering chemistry, Korean Journal, 11(3): 314 (2005)
2 Nazhad, M. M. and Paszner, L. 'Fundamentals of Strength Loss in Recycled Paper,' Tappi, 77(9): 171(1994)
3 Howard, R.C., and Bichard, W.J., "The basic effect of recycling on pulp properties", JPPS, 18(4): J151 (1992)
4 Scallan, A.M. and Tydeman, A.C., Swelling and elasticity of the cell walls of pulp fibres, JPPSc, 18(5): J188 (1992)
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6 Eastwood, F.G. and Clarke, B., 'Handsheet and pilot machine recycling degradation mechanism', in Trans. BPBIF, Symp. Fibre water interactions in papermaking, V:II , London, 835( 1978).
7 Phoung, D. T. and Nazhad, M. M., 'Effect of swelling or surface agents on strength of old corrugated containers (OCC),' 60th Appita annual conference and exhibition, Melbourne, Australia, 3-5 April (2006)
8 Gurnagul, N., Ju, S. and Page, D. H., Fibre-fibre bond strength of once-dried pulps, JPPS, 27(3): 88 (2001).
9 Page D.H and Tydeman P.A. A new theory of shrinkage, structure and properties of paper. In The formation and structure of paper. Trans. 2nd Fund. Res. Symp. (ed. F.Bolam), BPBMA, London, 397 (1962).
10 Alanko, K., Recyclability of thermomechanical pulp fibres, Master Thesis, Department of Forest Products Technology, Helsinki University of Technology, 51 (1993).
Erik Rusdi Sutjipto: Production Manager, Advance Agro Co. Ltd., Bangkok, Thailand
Kecheng Li: Associate Professor, Pulp and Paper Center, Chemical engineering Dept., Unb, Canada.
Saisanee Pongpattanasuegsa: Lecturer, Engineering Department, Industerial Engineering, University of Rangsit, Thailand
Mousa M. Nazhad: Corresponding author: Associate Professor, Pulp and Paper Technology, Asian institute of Technology, Bangkok, Thailand, Email: email@example.com, firstname.lastname@example.org
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