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Journal of Korea Technical Association of the Pulp and Paper Industry - Vol. 54 , No. 6
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Journal of Korea Technical Association of the Pulp and Paper Industry - Vol. 54, No. 6, pp. 135-144
Abbreviation: J. Korea TAPPI
ISSN: 0253-3200 (Print)
Print publication date 31 Dec 2022
Received 04 Oct 2022 Revised 23 Dec 2022 Accepted 26 Dec 2022
DOI: https://doi.org/10.7584/JKTAPPI.2022.12.54.6.135

The Effect of Fines on the Change of the Friction Coefficient of Recycled Linerboard during Calendering Treatment
Sek Bih Fang1 ; Jeong Yong Ryu2, †
1Division of Wood & Paper Science, College of Forest and Environmental Sciences, Kangwon National University, Graduate Student
2Division of Wood & Paper Science, College of Forest and Environmental Sciences, Kangwon National University, Professor

Correspondence to : †E-mail: jyryu@kangwon.ac.kr (Address: Division of Wood & Paper Science, College of Forest and Environmental Sciences, Kangwon National University, Chuncheon, 24341, Republic of Korea)

Abstract

When linerboard is manufactured by blending recycled OCC pulp with recycled pulp from aseptic carton, the results reported that factors other than surface roughness are related to the coefficient of friction. Calendering exacerbated the problem of a sharp decrease in coefficient of friction according to the mixing of aseptic carton pulp. Therefore, we tried to understand the aforementioned changes in the coefficient of friction by evaluating the effect of the surface residue of fines derived from aseptic carton on the sliding angle and surface roughness of aseptic carton-based recycled paper. Handsheets were made from recycled pulp extracted from aseptic cartons and then coated with fines slurries at 3% and 5% concentrations. It was confirmed that the higher the proportion of recycled pulp fines used in handsheet coating, the lower is the coefficient of friction of the handsheets. Coating the handsheets with a large amount of recycled pulp fines reduced friction after calendering. This study suggested that influence factors such as changes in actual contact area are essential to be investigated in controlling the friction coefficient of recycled paper.

Keywords: Coefficient of friction, roughness, fines, aseptic carton, OCC
1. Introduction

In a previous study,[1] the relationship between the roughness and the slide angle after calendering was investigated by comparing handsheets made only from aseptic cartons and handsheets made from a 50/50 mixture of aseptic cartons and OCC. Analysis of the changes in roughness and slide angle of the handsheets showed that when the handsheets were made from a 50/50 mixture of aseptic carton and OCC, the decrease in slide angle due to the calendering treatment was greater than the reduction in roughness.

It is presumed that the previously mentioned results [1] were obtained because the calendering treatment buffered the change in the coefficient of friction as it increased the actual contact area while reducing the roughness of the aseptic carton handsheet. The incorporation of OCC into the aseptic carton increased the fines fraction of recycled paper, and so the paper surface was covered with fines to make it flat, and only low-height bumps were remained.

Fines on paper surface lowered the roughness but bumps were responsible for maintaining the contact between two sliding papers and governs friction. Low roughness was further reduced after calendering for handsheets made from a 50/50 mixture of aseptic carton and OCC. In this case, it is presumed that a slipping phenomenon occurred in which the slide angle of the handsheet was significantly lowered despite the increase of the actual contact area.

There are actually a number of factors that can influence a linerboard to have a low coefficient of friction, including the surface roughness, the influence of extractives, etc. Bowden and Taylor[2] explored the theory that the coefficient of friction that reflects the amount of dry sliding friction is caused by shearing at junctions formed at regions of contact, known as the real area of contact, between the sliding surfaces. The real area of contact does not necessarily depend on the average surface roughness, indeed, it is depending on the type of roughness and the materials’ properties. To elaborate, for certain textures and materials, making a surface smoother reduces friction. But elsewhere, rougher surfaces can actually have less friction.

All surfaces of solid bodies are rough on an atomic scale. Roughness always exists in micro or macro scale. When two surfaces come into contact, the actual contact area composed of interacting asperities is usually a very small fraction of the apparent contact area. The reason being that the surface roughness causes contact to occur only at discrete spots, sometimes referred to as junctions. In particular, contact between solid surfaces is discontinuous and actual contact area is the summation of contact area of each asperities that is dependent on the surface texture, on the material properties and on the interfacial loading conditions.[3-9]

Contact between the two surfaces will initially occur only at a few points to support the load upon loading. As the normal load increases, a larger number of asperities on the two surfaces come into contact, and existing contact areas grow to support the load.[3-9]

Contact between rough surfaces with fractal properties can be established by small-lengthscale asperities, sufficiently small to produce sizable static and sliding friction. As higher surface slopes result greater normal forces at contact points, surfaces of greater fractality unsurprisingly exhibit a general tendency towards higher resistance to shear and a larger macroscopically observed coefficient of friction.[10]

According to an article published by The Hebrew University of Jerusalem, “while frictional motion is often thought of as the motion of two bodies against each other, separated by a perfectly smooth plane, in fact, due to the microscopic roughness of sliding surfaces, all of the contact between sliding bodies takes place in only a tiny area”. Thus, only a sparsely spaced microscopic “bumps” are responsible for maintaining the contact between two sliding bodies. It is the behavior of these bumps which governs friction, and in this study, particularly the recycled pulp fines.

To test the validity of the above hypothesis, fines were isolated from aseptic cartons and collected for addition to wet-end originally. However, there was difficulty in retention of fines during the handsheet making process of aseptic carton stock. Therefore, instead of wet-end addition, a surface coating with fines was applied to the handsheet made from the aseptic carton.

Finally, in this study, the relationship between the coefficient of friction and the amount of fines exist on the paper surface was being investigate by observing the effect of fines on the change of coefficient of friction of handsheet before and after calendering.

2. Experimental Design
2.1 Materials and methods

The complete research procedures for this study is being in Fig. 1.


Fig. 1. 
Schematic drawing of the experimental procedure.

2.1.1 Pulping conditions

In preparatory work, 1 kg of aseptic cartons (Tetra Pak based on unbleached kraft pulp) were torn into approximately 50 mm×50 mm squares. Then, they were pulped by the lab-scale low consistency pulper at a consistency of 5.0% for 15 minutes.

2.1.2 Segregation and collection of clean fibers and recycled pulp fines

A Somerville screening was then applied with recirculating wash water to obtain fibers without fines and to collect fines. Aseptic carton pulp samples were screened by Somerville screen for 15~20 minutes each time, and the total amount of the screened pulp was approximately 5 kg. It was then being hyperwashed with fresh water until when only fines-free pulp remained on a 200 mesh screen. Clarified water was decanted off. In addition, centrifugation was performed with the settled filtrate to produce more condensed pulp fines. The fines consistency ranged from 3.0% and 5.0%.

2.1.3 Handsheet molding

All handsheets in this study were made from the fines-free pulp by using the square handsheet former, with the grammage of 120 g/m2. Calendering treatment was then applied to the handsheets to create an optimum pre-coating condition. In this experiment, a Beloit Wheeler Model 753 Laboratory Calendar was used to calender the sheet at the low calendar temperature of approximately 20°C and a calender pressure of 100 kN/m. The calender speed was kept constant at 1.90 m/min. After calendering, the handsheets were coated with water, 3% and 5% slurry of recycled pulp fines respectively. A lab scale rod coater was used to coat the fines slurry on the handsheet. The reason why plain water was coated instead of fines slurry is to take into account the changed roughness and friction coefficient of the paper surface due to moisture.

2.1.4 Handsheet testing procedure

Non-destructive measurements included grammage, thickness, roughness and formation. The Bendtsen Tester was used to measure the roughness of handsheets. Whereas the slide angle friction was measured by the Inclined Plane Coefficient of Friction Tester. After handsheet testing, calendering treatment was carried out again. The physical properties of calendered handsheets, particularly thickness, roughness and slide angle friction were tested. Apart from that, scanning electron microscopy (SEM) was carried out for the surface coating sample materials.

2.1.5 Coefficient of static friction (slide angle)

In this study, the coefficient of friction tester used conforms to the international standard of TAPPI T815 which is in regards to the coefficient of static friction (slide angle) of packaging and packaging materials (including shipping sack papers, corrugated and solid fiberboard) (inclined plane method). This method determines the coefficient of static friction of most packaging materials by measuring the angle at which one test surface begins to slide against another as the slope increases at a constant and prescribed rate. The test is frequently referred to as slide angle measurement. The coefficient of friction is numerically equivalent to the tangent of that angle.

2.1.6 Water drop contact angle

The PG-3 Pocket Goniometer is not only used to measure the static contact angle, but also the volume and spread of a drop of liquid on the surface over time. This dynamic absorption test (DAT) is applied to test the liquid absorbency of handsheets in this study. DAT can be used to determine the degree of sizing or water hold out of paper with time. The Pocket Goniometer conforms to the international standard of TAPPI T 458 that is to measure the surface wettability of paper (angle of contact method). This physical phenomenon appears at the interface between a liquid droplet and a substrate surface. The contact angle test is commonly used to understand how a liquid and a substrate surface interact with each other. PG-3 used in this study is an automated instrument for measurements of static and dynamic contact angles similar to the model PG-2. This model has an integrated micro-pump, which can deliver accurate droplets in 0.5 steps.

3. Results and Discussion
3.1 Roughness difference

As described above, it was assumed that the fines of the aseptic carton affects the friction coefficient of the recycled linerboard, and in particular, it was assumed that the calendering treatment caused a large change in the sliding angle of the paper surface covered with the fines. Surface coating treatment was applied to handsheets made from recycled fiber stock to investigate the influence of fines proportion present at the handsheet surface on the coefficient of friction of handsheets. The surface of the handsheet was coated with fines dispersions of 3% and 5% concentrations, while water coating was performed in the control experiment.

Fig. 2 show the changes in roughness of handsheet surface before and after calendering. There were significant reduction by calendering in surface roughness of all surface coated handsheets. In this regards, the handsheets coated with water achieved the smallest reduction in roughness by less than 47%, from 1273.33 ml/min to 663.11 ml/min, followed by the reduction of less than 56% for the handsheet coated with 3% of fines. And the greatest reduction in surface roughness of handsheet coated with 5% of fines was approximately 60%, from 1366.17 ml/min to 545.97 ml/min.


Fig. 2. 
Changes in the roughness of the surface-coated handsheets before and after calendering.

3.2 Slide angle difference

Fig. 3 shows the change in slide angle according to fines-coating before and after calendering. The slide angle of the handsheet was increased with a 5% fines dispersion coating, but calendering lowered the slide angle by more than 36%, from 39.8° to 25.28°. The slide angle of the handsheet coated with 3% fines dispersion was reduced by more than 31% from 37.7° to 25.91° by calendering. By comparison, the water-coated handsheet had the smallest reduction of less than 11% from 27.8° to 24.88° due to calendering.


Fig. 3. 
Change of slide angle of surface-coated handsheet before and after calendering.

3.3 Analysis of surface roughness versus slide angle

Analysis of surface roughness versus slide angle of handsheets before and after calendering showed that the 5% fines dispersion coated handsheet with the highest roughness before calendering experienced the greatest reduction in slide angle after calendering. It is inferred that the higher the fines concentration of the handsheet surface coating, the greater the slide angle decrease after calendering. From these results (Fig. 4), it can be confirmed that the bumps formed by the fines that can maintain the surface friction force is flattened by the calendering process, and the friction coefficient sharply decreases, resulting in a slippery state.


Fig. 4. 
Change of surface roughness versus slide angle of surface-coated handsheets before and after calendering.

3.4 Water drop contact angle of surface coated handsheets

As shown in Fig. 5 below, there was almost no difference in the static contact angles of the water droplet in equilibrium between the handsheets. It can be concluded that the fines coating does not change the chemical properties and surface energy of the handsheet.


Fig. 5. 
Changes in the contact angle of water droplets according to the surface coating treatment of the handsheet.

3.5 Scanning electron microscope (SEM)

SEM analysis was used to observe the surface morphology of these handsheets to compare the changes that occur on the surface of the handsheets before and after calendering. It appears that there were bumps on the handsheets surface coated with fines prior to calendering. However, these bumps were reduced as the handsheet surface became smooth after calendering and the fines originated from aseptic cartons were compressed. Changes in the surface roughness and actual contact area of handsheets before and after calendering could be observed through the SEM image and better understand through the diagram in Fig. 6.


Fig. 6. 
SEM images of surface coated handsheets before and after calendering.

3.6 Surfaces under elastic deformation

In this study, some changes take place to the texture and the surface material properties of handsheet once a sliding surface has run-in for a while, for instance coating and calendering operation. The asperities by the recycled pulp fines thus begin to support the load elastically.

Greenwood and Williamson developed the model by which surface asperities support the load elastically and further extended our understanding of sliding friction.[7] For pure elastic deformation at the asperities, the sliding friction does have a dependence on the surface texture since the real area of contact depends on the nature of the texture.

If the asperities in this study, particularly the amount of fines have small size, assuming elastic deformation, the real area of contact will be small and thus the sliding friction reduced. If, however the amount of fines as asperities have large size, the real area of contact will be larger and thus the friction will be higher.

3.7 Effect of calendering to the surface texture and actual contact area between handsheets

Paper is a complex composite material that its structure and surface texture greatly influence its friction coefficient.[12,13] It is being passed between several pairs of heated rollers during commercial calendering process. The handsheet surface and structural transformations occurring in the nip of the calendering are mostly permanent.[14] As a result of the compaction, the thickness of the handsheet decreases and the gloss increases. The study by Lepoutre and Means revealed that heat influences the microroughness while the pressure influences the macro-roughness.[15]

In this work, it was found out that calendering treatment not only reduces the surface roughness of handsheet, but also bring changes to the actual contact area of handsheet during sliding and stacking. The changes of surface roughness and actual contact area for surface coated handsheets before and after calendering are shown in Fig. 7. When the actual contact area of the handsheet during sliding and stacking is large enough, it gave greater influence to the coefficient of friction of the handsheet than on the surface roughness of handsheets. The friction coefficient of handsheets is closely related to the actual contact area of the handsheets.


Fig. 7. 
Changes of surface roughness and actual contact area before and after calendering.

  • • When the surface roughness of the handsheet is high, the actual contact area of the handsheet is small. Hence, despite the surface roughness, a low coefficient of friction of the handsheet was observed on the handsheets coated with water.
  • • Conversely, the lower the surface roughness of the handsheet, the larger the real contact area of the handsheet. Despite the low surface roughness, a high coefficient of friction of the handsheet was observed on the handsheets coated with both 3% and 5% of recycled pulp fines before calendering.
  • • Calendering treatment to the handsheets has smoothened the handsheets surface and thus bringing a great reduction in surface roughness of handsheets after calendering. The actual friction area of the handsheet surface changed accordingly by the reduction of surface roughness of the coated handsheets. The microscopic bumps of handsheets coated with fines was eventually diminished by calendering treatment and maximize the contact area between paper. In general, a smooth surface has less friction because there is less resistance present than a rough surface. This is because smooth surfaces have the least amount of friction due to fewer irregularities that stop the moving object whereas rough surfaces have more irregularities.
Conclusions
  • 1. There was a relationship between the level of coefficient of friction of recycled pulp fiber and the proportion of the fines present on the surface of the paper. The higher the proportion of fines present on the paper surface, the higher is the coefficient of friction of paper.
  • 2. Though the paper friction changed accordingly with the surface roughness reduction after calendering, the actual contact area of the paper surface during sliding and stacking has a greater influence than surface roughness towards the difference in paper friction.
  • 3. The higher the surface roughness of handsheets, the smaller is the actual contact angle, and hence the coefficient of friction of paper is small. In contrast, the lower surface roughness of handsheets leads to a larger actual contact area which in turn makes the paper have high coefficient of friction.
  • 4. Calendering with the purpose to smoothen the paper surface has greatly reduced the surface roughness of paper. Though vanish of microscopic bumps on the paper surface maximizes the real contact area between paper, the paper surface becomes smooth and the least amount of friction is to resist the movement of moving object. Therefore, the slide angle of handsheets coated with 3% and 5% of recycled pulp fines were greatly reduced after calendering, indicates a low coefficient of friction of paper.
  • 5. The actual contact area was proven to have greater influence on the changes of COF of recycled handsheet made from aseptic carton compared to surface roughness factor. When the pulp of recycled sterile cartons is mixed with linerboard manufacturing stock, the fines derived from the pigmented coating of the aseptic carton act as bumps that increase the coefficient of friction of the linerboard. It is interpreted that the friction coefficient dropped sharply because the bumps on the linerboard were flattened by the calendering. Therefore, it is interpreted that the coefficient of friction is lowered when aseptic carton recycled pulp is applied to the manufacture of recycled linerboard.
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