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Effect of recycling on paper properties
Erik Rusdi Sutjipto, Kecheng Li, Saisanee Pongpattanasuegsa and Mousa M. Nazhad

ABSTRACT
To study the changes in fibre properties in the process of recycling, different chemical pulps (i.e., hardwood and softwood market pulps as well as laboratory pulps) and CTMP pulp were recycled three times, and some of their physical properties were determined and analyzed.

It was observed that the recycling effect on chemical pulps (Kraft pulps) is similar regardless to the wood species. The rate of strength loss was more pronounced for the laboratory pulps compared to the market pulp. In general, the rate of tensile strength loss was always twice that of the density regardless to wood species. Recycling CTMP suggested that the gain in density was almost negligible compared to gain in tensile strength. Recycled chemical pulps showed a very high bending stiffness compared to virgin pulp. Bending stiffness of CTMP pulp also benefited from recycling.

It was inferred that recycled pulp could be a better choice for some grades of paper and board, a point which has been overlooked by papermakers.

INTRODUCTION
Recycling is not a new technology. It has become a commercial proposition since Matthias Koops established the Neckinger mill, in 1826, which produced white paper from printed waste paper. However, there were very few investigations into the effect of recycling on sheet properties until late 1960's. From then until the late 1970's, a considerable amount of work was carried out to identify the effects of recycling on pulp properties and the cause of these effects [1-2]. In the late 1980's and early 1990's, recycling issues have emerged stronger than before due to the higher cost of landfills in developed countries and an evolution in human awareness. The findings of the early 70's on recycling effects have since been confirmed, although attempts to trace the cause of these effects are still not resolved [3]. Recycling has been thought to reduce the fibre swelling capability, and thus the flexibility of fibres. The restricted swelling of recycled fibres has been ascribed to hornification, which has been introduced as a main cause of poor quality of recycled paper [4]. Since 1950's, fibre flexibility among the papermakers has been recognized as a main source of paper strength [5]. Therefore, it is not surprising to see that, for over half a century, papermakers have supported and rationalized hornification as a main source of tensile loss due to drying, even though it has never been fully understood.

The hornification hypotheses conflicted with fibre surface deactivation due to recycling [6]. Surface deactivation addresses loss in the fibre surface bonding agent. The effect of surface bonding compared with fibre swelling on strength properties of recycled fibres has been recently examined [7]. It has been reported that the surface treatment is more effective than fibre swelling if the strength properties of the recycled fibres are considered. It has also been recently reported that loss in fibre flexibility due to recycling is, at the most , 1/10th of the loss in fibre to fibre bonding strength [8]. Despite these findings, the recycling community is still uncertain on the role that fibre surface could play in recycling. 

This work addresses the issue of the effect of recycling on fibre properties in two parts: The first part looks at recycling effect on paper properties, with an assumption that the behavior of different raw materials in recycling may unlock some clues in the puzzle of recycling effect on fibre properties. Although, the literature is abundant [1-4, 6-8] in the field of recycling effects, most of the works only focus on softwood Kraft pulp. Any literature on the recycling effect on different pulps is rare in the literature, which created the need for this systematic look at the recycling effect on fibre properties using different pulps. 

The second part of this work, which examines the surface properties of recycled fibres, is not yet complete and is thus not included in this report.

EXPERIMENT
Different Kraft pulps (unbeaten, beaten, market or laboratory pulp, softwood or hardwood pulp) and CTMP pulp were examined for recycling effect. The pulps prior to processing were soaked overnight in soft water, after which the pulp was disintegrated according to SCAN C18:65 for chemical pulp and SCAN M2:64 for mechanical pulp. Refining was done using PFI mill. The target freeness was set at 25 SR. The disintegrated pulps were made into handsheets and tested according to SCANC26:76.

Each pulp was recycled 3 times, and coded as C0 (virgin pulp), C1 (cycle 1), C2 (cycle 2), and C3 (cycle 3). The strength and optical properties of the pulps were measured and compared.

RESULTS AND DISCUSSION

Softwood (SW) market Kraft pulp (unbeaten)
The pulp was dried pine with 89% dry matter content. The pulp was soaked in soft water overnight, then examined for recycling effect without having any beating history.

The effect of recycling on the pulp is recorded in Table 1 and demonstrated with Figure 1. As the figure suggests, the strength properties (tensile strength, tensile energy absorption (TEA) and tear strength) are increased at the first cycle or the first and second cycles, then decreased sharply. This is due to the fact that a considerable percentage of market pulp contains deformed fibres. Kraft pulp experiences mechanical treatment at the end of cooking at the homogenisation stage. Both of these treatments generate deformed (i.e., kinked, curled, dislocated or microcompressed) fibres. Paper made of deformed fibres is weaker in tensile strength as well as tensile stiffness. This is due to the fact that the deformed fibres have poor surface accessibility for bonding as well as poor potential for segment activation. Segment activation is an important parameter in development of paper tensile strength [1].

During paper drying, fibres shrink laterally creating drying stresses in the network. These stresses straighten slack segments enabling them to enhance the network load carrying capacity [9]. The recycling process, which consists of re-slushing, soaking overnight, disintegration, handsheet formation, pressing and drying may straighten the fibres, thus helping to develop the paper strength. 

Tensile strength and tensile energy absorption both increased due to recycling of unbeaten market pulp (Figure 1). Increase in tensile strength is due to fibre straightening. The major contribution to TEA is probably a result of fibre straightening as well. Tear strength has also benefited from the straightening of the fibres. 

The strength properties deteriorated with the progress of recycling due to loss in surface bonding potential of fibres (Fig. 1). This observation highlights the fact that the increase in tensile strength of the unbeaten pulp is due to its deformed fibres, however the trend is reversed if the deformation is removed. This is an important finding, in regard to recycling behavior of chemical pulps. This observation suggests that the recycling trend for unbeaten chemical market pulp is the same as beaten market pulp if the fibre deformation is removed prior to recycling (Figure 1). Increase in density is marginal (about 4% at second cycle), and remains unchanged with further recycling.  
recyc_fig1
Figure 1, Strength properties of unbeaten softwood market pulp

Fibre or paper properties

Number of cycles

Values

0

1

2

3

Density (kg/m2)

483

494

502

503

TrI (mN. m2/g)

16

19

18

17

BS (mN)

40

41

43

45

TEA (Joule/m2)

38

43

45

42

TS (N.m/kg)

3

3

4

3

Breakload (N)

23

24

28

29

TeI (N.m/g)

25

29

31

31

Br (%ISO)

86

85

86

85

Op (%ISO)

77

76

76

79

LSC (m2/kg)

41

39

38

39

Lw (mm)

2.3

2.21

2.32

2.34


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 [6]. 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.

recyc_fig2
Figure 2, Effect of recycling on market softwood pulp (beaten)

Fibre or paper properties

Number of cycles

Values due to recycling

0

1

2

3

Density (kg/m2)

638

590

583

573

TrI (mN. m2/g)

8

11

10

12

BS.(mN)

30

36

45

55

TEA (Joule/m2)

154

128

136

128

TSI (N.m/kg)

7

6

5

5

TeI (N.m/g)

95

78

79

75

Br (%ISO)

83

85

85

84

Op (%ISO)

59

69

70

72

LSC (m2/kg)

20

28

28

30

Lw (mm)

2.13

2.26

2.26

2.38


Table 2, Effect of recycling on market softwood pulp (beaten)


Paper density is sensitive to fibre flexibility [1], 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.

recyc_fig3
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).


Fibre and paper properties

Number of cycles

Values

0

1

2

3

Density (kg/m2)

662

574

558

514

TrI(mN. m2/g)

9

12

12

15

BS (mN)

30

40

44

52

TEA (joule/m2)

185

127

105

88

TSI (N.M/Kg)

6

5

5

4

TeI (N.M/g)

95

71

64

55

Br (%ISO)

83

86

86

87

Op (%ISO)

61

70

71

73

LSC (m2/kg)

19

28

29

31

Lw (mm)

1.99

2.09

2.06

2.11

Table 3, Recycling effect on laboratory softwood pulp

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.

recyc_fig4
Figure 4, Strength properties of Lab. hardwood pulp

Fibre or paper properties

Number of cycles

Values

0

1

2

3

Density (kg/m2)

707

642

643

633

TrI (mN. m2/g)

6

7

8

8

BS (mN)

35

41

45

46

TEA (Joule/m2)

216

128

83

81

TSI (N.m/Kg)

7

6

5

5

TeI (N.m/g)

97

72

54

50

Br (%ISO)

85

87

87

87

Op (%ISO)

60

70

72

73

LSC (m2/kg)

19

30

32

33

Lw (mm)

 

0.88

0.89

0.9


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.

recyc_fig5
Figure 5, Strength properties of market CTMP

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 [10] 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 [3] suggests an opposite trend; as the authors reported a positive correlation between density and tensile strength.

Fibre and paper properties

Number of cycles

Values

0

1

2

3

Density (kg/m2)

362

374

370

372

TrI (mN. m2/g)

7

6

6

6

BS (mN)

61

59

59

61

TeI (N.M/g)

29

37

38

37

Br (%ISO)

64

63

61

62

Op (%ISO)

87

84

85

87

LSC (m2/kg)

43

38

37

40

Lw (mm)

1.49

1.92

1.89

1.92

Table 5, Recycling effect on market CTMP pulp

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. 

CONCLUSIONS
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. 

REFERENCES
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)
5 Emertone, H. W., Fundamentals of beating process, Marshal press, 84 (1957).
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).

CONTACT DETAILS
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: mousanazhad@ait.ac.th, mousanazhad@yahoo.com

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