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|[ Article ]|
|Journal of Korea Technical Association of the Pulp and Paper Industry - Vol. 54, No. 6, pp. 116-124|
|Abbreviation: J. Korea TAPPI|
|ISSN: 0253-3200 (Print)|
|Print publication date 31 Dec 2022|
|Received 04 Oct 2022 Revised 21 Dec 2022 Accepted 23 Dec 2022|
|The Effect of Calendering Treatment on the Friction Coefficient of Recycled Linerboard from Aseptic Carton|
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: firstname.lastname@example.org (Address: Division of Wood & Paper Science, College of Forest and Environmental Sciences, Kangwon National University, Chuncheon, 24341, Republic of Korea)|
Recently, when a paper mill in Thailand produced linerboard by mixing recycled OCC pulp with recycled pulp from aseptic cartons, the coefficient of friction decreased rapidly as the mixing ratio of pulp from aseptic cartons increased. The study was carried out to investigate the main causes of reduced coefficient of friction of recycled linerboard made by blending aseptic cartons and OCC. Three types of stock from the actual site were used for analysis: aseptic carton, old corrugated containers (OCC) and 50:50 blended stock. Handsheets were made from these three stocks by using a square handsheet former and the effect of machine calendering treatment on the handsheet was analyzed to investigate the relationship between the coefficient of friction and the surface roughness of the handsheet. Regardless of the type of pulp, calendering reduced the surface roughness of paper and the coefficient of friction as well. However, when comparing papers of different types of pulp, simply having a lower surface roughness than other papers did not mean that the coefficient of friction is lower. In the case of paper made from OCC mixed with aseptic carton pulp, the surface friction was sharply lowered by calendering treatment. It was judged that calendering exacerbated the problem of a sharp decrease in coefficient of friction according to the increase of mixing ratio of aseptic carton pulp. The relationship between friction and actual contact area of handsheets was discussed.
|Keywords: Coefficient of friction, roughness, calendering, aseptic carton, OCC
The global packaging industry continues to grow as demand is driven by factors such as pandemic, e-commerce growth and the changing dietary habits of the resident citizens. The widespread usage of these packaging materials leads to large amount of packaging waste affecting the environment and ultimately pointing to the importance of sustainable packaging. Paper packaging, for example, is environmentally friendly. Among the different types of paper packaging, recycling aseptic cartons can prove to be one of the best way to reduce waste and pollution.
There are several important physical and mechanical properties that must be considered in the production of recycled paper products, such as coefficient of friction, compression strength, burst strength and so on. In particular, it is very important to control the frictional properties of paper and paperboard in the manufacture of a paper based flexible packaging material. Common issues caused by paper and paperboard stacks slipping during CNC (computer numerical control) cutting or boxing are defects in the final product, which ultimately increases production cost. In addition, slipping of stacked products during transportation and storage can damage the products and even cause personal injury. Slippage of paper-based products due to their low coefficient of friction can cause delays in production, increased production costs, and damage to packaged products.
Friction is essential in the manufacture and end-use of various recycled paper products, including the recycled linerboard produced from aseptic cartons. More specifically, higher friction improves the stability of corrugated containers and their ability to prevent slipping during transportation and storage. It is common practice in the papermaking industry that the coefficient of friction of the liners used for the outer surface of corrugated containers is significantly higher than that of other base papers.
In spite of the importance of friction, commercial paper sheets undergo calendering, an operation that compresses the paper beyond the elastic threshold between two hard or soft rolls. This operation reduces the roughness of the paper and generally increases the gloss of the surface. The cross machine direction length is only very slightly affected by calendering, the most noticeable effects being the general decrease of roughness that in turn leads to the reduction possibility of friction coefficient.
Recently, Thai paper mill A, when producing linerboard by blending recycled pulp from aseptic cartons in recycled OCC pulp, faced the problem of a sharp decrease in the coefficient of friction as the blending ratio of pulp from aseptic cartons increased. For this reason, there was a limitation in increasing the blending ratio of aseptic carton recycled pulp and this problem tended to be exacerbated by the machine calendering treatment. Therefore, the following investigation has been carried out to study the effect of calendering treatment on the friction coefficient of recycled linerboard from aseptic carton.
Three types of stocks from mill A used in the analysis were ultra-high temperature processing aseptic cartons, old corrugated containers (OCC) and a 50:50 blend of the two stocks. Fiber analysis of these three types of stocks on site was carried out to identify the properties difference of fiber by using the optical analyzer (Fiber Tester Plus, L&W). The three stocks on site were repulped by using the low consistency pulper. After pulping, handsheets were made from these three stocks on a square handsheet former, with a grammage of 120 g/m2.
Nondestructive measurements included grammage, thickness, roughness and formation. The Bendtsen Tester (L&W) was used to measure the roughness of handsheets. Whereas the slide angle friction was measured by the Inclined Plane Friction Coefficient Tester (Frank-PTI). After handsheet testing, calendering treatment was carried out on all the handsheets. The physical properties of calendered handsheets, particularly thickness, roughness and slide angle were tested. In addition, uncalendered and calendered sample specimens were observed by scanning electron microscopy (SEM).The brief research procedures of this part of study is being shown in Fig. 1.
Generally, calenders are used as pre-calenders prior to coating or sizing, whereas coated papers are calendered to provide a smooth, glossy finish. 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.
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 equal to the tangent of that 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.
Fiber material characteristics included fiber length, width, fines content and coarseness. The fiber analysis results of aseptic carton, OCC and 50:50 blended stock showed that aseptic carton had the longest fiber length of 1.321 mm and the highest fiber width of 25.35 μm. The coarseness of aseptic carton, 139.35 μg/m was the highest among the three samples as well. On the other hand, OCC had the shortest fiber length of 0.76mm, the lowest fiber width of 17.35 μm and the lowest coarseness of 74.05 μg/m (Figs. 2, 3, 4). Concerning, fibers of higher coarseness are stiff and difficult to collapse which lead to poor bonding due to less bonding area and lower strength. This results in high surface roughness of the handsheet made from aseptic carton as shown in Fig. 5.
However, if we infer that the ash content (KSMISO 2144 Paper, board and pulps－Determination of residue(ash) on ignition at 900℃) of aseptic carton is 7.75%, which is higher than 3.42% of OCC, it can be confirmed that the fiber used in this study was extracted from an aseptic carton in which polyethylene (PE) was laminated after coating inorganic pigment on base paper. The fiber analysis result in Fig. 5 showed that 50:50 blended stock had the highest fines of 24.2% and ash of 8.62%, followed by aseptic carton with 18.5% of fines and 7.75% of ash. Finally, OCC had the lowest fines of 15.6% and an ash content of 3.42%.
The reason being the 50:50, a blended pulp of aseptic carton and OCC, was not the stock collected from the thickened machine chest, but the headbox stock diluted with short circulating silo white water. These analysis results (Fig. 5) forecast the surface irregularities of handsheet made from 50:50 blended stock due to presence of bumps formed by fines. According to the fiber analysis results, aseptic carton seems to be a mixture of long fibers and fines, while OCC is judged to be a stock mainly composed of short fibers.
Surface roughness and slide angle were analyzed after handsheet molding with 3 types of stocks from mill A. Analysis of handsheets made with aseptic carton, OCC, and 50:50 blended stocks before and after calendering showed that aseptic carton handsheet had the greatest roughness but lower slide angle. Although the aseptic carton stock is composed of long fibers and the surface roughness of the handsheet is superior to that of the OCC handsheet, which is a short fiber, the slide angle of the aseptic carton handsheet was lower than that of the OCC (Fig. 6). These results confirm that when comparing papers of different types of pulp, simply having a lower surface roughness than that of other papers does not mean that the coefficient of friction is low.
It is significant that the surface roughness and slide angle of handsheets reduced after calendering treatment. In other words, regardless of the type of pulp, calendering lowers the surface roughness of the paper and reduces the coefficient of friction. After calendering, there were reduction of more than 45%, 57% and 51% in the surface roughness of handsheets made from aseptic carton, OCC and 50:50 blend stock respectively. Handsheets made from aseptic carton were calendered to reduce surface roughness, which was adjusted similarly to OCC and a 50:50 blend stock of both pulps without calendering. In this case, the slide angle of the aseptic carton handsheet was lower by 29% than that of the OCC handsheet without calendering.
The 50:50 handsheet achieved the greatest reduction in slide angle, with calendering lowering the slide angle by more than 34%, from 28.4° to 18.7°. This means that in the case of paper with OCC mixed with aseptic carton, the surface friction is sharply lowered by calendering treatment.
Although the surface roughness of the handsheet made with 100% OCC and 50:50 blended stock are similar, the difference in slide angle between the two handsheets is 8.6°. Despite the fact that the surface roughness is similar, considering that the slide angle of OCC is 27.3° and the slide angle of the 50:50 mixtures is 18.7°, which is 31.5% of the difference in angle, it is estimated that there are other factors affecting the slide angle besides the roughness of the surface. Therefore, when Thai Paper Mill A mixes recycled OCC pulp and pulp from aseptic carton to produce linerboard, it is judged that calendering exacerbate the problem of a sharp decrease in coefficient of friction as the mixing ratio of aseptic carton pulp increases.
Generally, the force of friction comes from the roughness or other surface characteristics of materials that come into contact. Friction depends in part on the roughness of the contact surfaces, and a greater force is required to move the two surfaces past each other if they are rough than if they were smooth. But in this study, it is estimated that there are another factors affecting the slide angle besides the roughness of the surface.
In the study of surface topography, actual surfaces are far from ideally smooth, and exhibit more or less roughness. Contact between two solids is generally discrete, because of surface roughness that is, it occurs at areas of individual point contacts.
All surfaces are rough on a microscopic scale, and when the two rough surfaces are in contact the real area is very small compared to the apparent area of the contact (Fig. 7). When load presses two rough surfaces together, only some peaks of the surfaces will be in contact, thus, these peaks often carry very high loads. This effect was analyzed by Greenwood and Wu in 1966.
If both surfaces are hard as shown in Fig. 8, only a small percentage of the apparent contact area is in real contact. The concept of this real area of contact is closely related with roughness and surface structure.
When the surface A and surface B come near each other, the crest in surface B get close enough to contact with the crest of surface A. The more crests that are contacting between the two surfaces, the greater the total frictional force. It is presumed that greater contact area means greater frictional force.
From Fig. 6, the roughness and slide angle of both aseptic carton and OCC decrease in parallel proportion. To analyze the changes in roughness and slide angle before and after calendering, Fig. 9 showed the SEM analysis of handsheets before and after calendering. The comparison between handsheets surface before and after calendering determined the change in real contact area of aseptic carton and OCC handsheets before and after calendaring. The friction coefficient of both aseptic carton and OCC handsheets does not change significantly by calendering due to the increase in the real contact area, even if the surface smoothness is improved.
In Fig. 6, handsheet made from 50:50 blend stock showed the similar coefficient of friction with the recycled linerboard made from aseptic carton before calendering. The reason being that there is unevenness of the handsheet surface made from 50:50 blend stock due to presence of microscopic bumps on the surface and thus showed high slide angle with lower roughness.
As regards, calendering smooths the handsheet surface irregularities (bumps) and increases the total real contact area. It is therefore presumed that the slide angle of handsheet dropped sharply after calendering because those bumps formed by fines on the surface were being diminished by calendering. In results, the smoothness of handsheet was being improved significantly after smoothing of handsheet surface irregularities and the slide angle that reflect the coefficient of friction decreased rapidly. Hence, further study is essential to prove the validity of hypothesis made on the relationship between friction and actual contact area of these handsheets in particular the recycled linerboard from aseptic carton.
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