Evaluation of Frictional Influence and Thermal Analysis of Flexible Paper‐Based Materials Used in the Vertical Form–Fill–Seal Process: An Experimental Study

This study evaluated the frictional behaviour of flexible polymer‐coated and dispersion‐coated materials used in vertical form–fill–seal (VFFS) machines. A laboratory‐scale friction measurement device was developed to investigate the relationship between different surface topographies, roughness and frictional properties. Furthermore, surface roughness, contact angle and surface energy of the material were analysed and a microscopic analysis was performed to further understand the effect of material properties on friction. Thermal camera analysis was performed to measure the temperature changes within the material during the film‐transport stage of the VFFS machine. The friction measurement results indicated that the sliding direction did not affect the static and kinetic coefficients of friction. The significant influence of friction on the surface topography of the plate (forming shoulder and tube) was particularly evident. Some materials experienced the stick–slip phenomenon during sliding, depending on the surface type. Teflon tape, which is commonly used in the forming tubes of VFFS machines, exhibited the lowest coefficient of friction. Thermal camera analysis revealed that polymer‐coated paper materials generated more heat than dispersion‐coated papers during VFFS trials and thermoplastic films generated the least heat. No clear relationship between the surface roughness of the paper, fibre orientation, surface energy and friction coefficient was noted. Furthermore, paper‐based materials exhibited a higher coefficient of friction, which is possibly related to the dispersion components of surface energy. The findings of this study provide additional knowledge for designing the forming tube, shoulder geometry and surface plate to minimize the occurrence of defects, such as wrinkles, in paper‐based packages.


| Introduction
Paper-and petroleum-based polymer flexible packaging materials are commonly used for a variety of food applications [1].In several converting machines, such as vertical form-fill-seal (VFFS) machines [2], thermoplastic materials are commonly used because they provide excellent formability and runnability compared with coated paper-based materials [3].With the growing requirement for sustainable materials by governments worldwide, paper-based materials are gaining popularity as alternatives in the packaging industry [4].These materials provide several advantages such as recyclability, biodegradability, good mechanical strength, robustness and good printability [5].In alignment with environmental awareness, the European Union's Green Deal and Circular Economy Action Plan-the European Packaging Waste Directive (94/62/62EC)-encourage the reuse and recycling of packaging materials and at least 85% of paper and paperboard packaging materials to be reused and recycled by 2030 [6].
Friction of several paper and paperboard packaging materials has been a major subject of research.It is a critical factor in packaging technology, particularly in processes such as press forming, three-dimensional (3D) forming, deep drawing, FFS method and other packaging methods.The coefficient of friction determines how easily the material glides over the packaging machine [7] and is directly influenced by the material formulation of special slip additives.The addition of slip additives decreases the coefficient of friction owing to the increased migration of amides from the bulk matrix to the surface [8].The demand for high-quality prints of coated paper is growing.After printing the material, the migration of printing particles affects friction or slippage when packed by the packaging machine [9].The frictional properties of paper materials are critical parameters for finishing, conversion and end-use operations [10].Various studies have assessed the friction of paper, which is highly dependent on fibre alignment, handling processes and environmental conditions [11].Previous studies have shown that the coefficient of friction between the paper and the metal surface is influenced by various operating parameters, including the applied pressure, speed of conversion and surface roughness of the metal [12].The most significant factor is surface roughness, which is caused by adhesion and abrasion [13].Furthermore, different theories have been drawn from the existing literature regarding the relationship between paper friction and surface roughness.For example, calendaring is performed to enhance surface smoothness; however, the results are mixed and contradictory.Some studies observed an increase in friction that was attributed to higher adhesive forces from a broader contact area between the surfaces [14], whereas other studies found no significant impact on friction [15].
The coefficient of friction is sensitive to environmental factors, including relative humidity [12,16].Inoue et al. stated that increasing the moisture content of the paper increases friction [16].Fellers et al. combined all previous findings for further investigation.They showed that an increase in friction owing to high moisture content is primarily because of the change in adhesive forces or surface energy [17].However, the friction mechanism of paper is complex [16].Kawashima et al. compared the friction of paper under three atmospheric conditions: dry, moderate and humid.They found that the coefficient of friction of paper significantly increased at higher humidity by approximately 10 times between moderate and humid atmospheres [10].
Extensive research and development on paper-based materials have been conducted to increase their market potential and meet the growing demand.Previous studies have predominantly focused on the frictional forces of paperboards in forming processes such as deep drawing [18] and press forming [19].Some of this research includes the ability to improve the influence of friction and formability of paperboards [20,21].The properties of paper-based materials are mechanically and chemically improved by alternating the fibres and fibre network structures [18].Vishtal and Retulainen defined paper formability as a complex mechanical property that influences the performance of a material during the paperboard forming process.This property depends on four main factors: paper-to-metal friction, elongation, compressive strain and compressive strength [18].Increasing the tool or mould temperature improves formability, leading to more flexible and high-quality deep-drawn paperboards [22] because heat weakens the inter-fibre bonds [23].
The coefficient of friction of packaging materials affects the efficacy and potential issues encountered during packaging operations.For VFFS machine applications, the coefficient of friction of the flexible paper material plays an important role as the material travels through the rolls over the forming shoulder and bends over a 90° direction to enter the forming tube.The formability of paper influences the runnability, web handling and visual appearance of paper-based materials, such as wrinkling or, in severe cases, tearing [24].These issues are associated with high friction between the material and the contact surface.To date, little attention has been paid to the frictional effects of paper materials used in VFFS machines.Furthermore, the paper-based materials, which are becoming increasingly popular because of environmental considerations, often present unique challenges in VFFS machine convertibility.
Therefore, this study aimed to extend the findings of Merabtene et al. [24] by investigating the relationship between different surface topographies, roughness and frictional behaviours of flexible paper-based materials used in VFFS machines.In this study, a new frictional testing device developed at the Laboratory of Packaging Technology, Lappeenranta-Lahti University of Technology LUT, was used to measure the static and kinetic coefficients of friction.In the friction measurement device, a horizontal plane technique that meets the requirements to replicate the conditions encountered during the forming process in VFFS machines was employed.The influence of varying the surface topography and roughness on the static friction (SF) and kinetic friction (KF) of paper-based materials to achieve optimal sliding surface performance was examined.An optical microscope was used to examine the surface structure of paper-based materials.A thermal camera was used to measure the contact temperature of various steps of the forming process.The findings of this study offer insights that can guide VFFS packaging manufacturers to refine the design of forming tubes and shoulder geometries to minimize the defects observed in paper-based materials.

| Materials
In this study, one thermoplastic reference material, OPP+PE Ref. film, was primarily used as a reference material which exhibits a total thickness of 53 μm, divided into two distinct layers: 18 μm layer of oriented polypropylene (OPP) and 35 μm layer composed of ink, adhesive and polyethylene (PE).The OPP+PE Ref. film was used to evaluate and compare the performance with four paper-based materials, one polymer coated paper and three dispersion-coated papers (Table 1).The physical properties of papers were measured according to ISO 536:1995 for grammage and ISO 534:2005 for thickness.All paper-based materials were stored and tested at a constant humidity chamber at standard climate (23°C and 50% relative humidity).The moisture content of the material was measured using the Adams Equipment PMB 53 Moisture Analyser (Oxford, CT, USA).
The Prego+PE coated paper is a 75 gsm polymer coated paper (UPM-Kymmene Oyj, Lappeenranta).The Disp. coated paper 65 is a dispersion-coated paper which is heat sealable with 65 gsm dispersion-coated paper grade with material thickness of 66 μm (UPM-Kymmene Oyj, Lappeenranta).It consisted of 60 gsm of base paper and 5 μm of dispersion coating comprising a mixture of coating pigment and binder.The heat-sealability is achieved through the thermoplastic (thermosoftening) nature of the whole barrier structure.
The CHP BAR A and CHP BAR B (CH-Polymers Oy, Kouvola) are additional dispersion coated papers which were coated using the SUTCO coating line (VTT Technical Research Centre of Finland, Espoo).The coating speed was 10 m/min using the metered rod coating method with wet coating thickness of about 28 μm.The Prego 55 gsm paper substrate (UPM-Kymmene Oyj, Lappeenranta) was used for CHP BAR A and CHP BAR B tests.All three dispersion-coated papers are mainly differentiated by coating recipes, grammages and thicknesses.

| Surface Characterization
The purpose of this study was to investigate the frictional characteristics of paper-based materials using the ASTM D-1894 standard.This experiment was performed to evaluate the interaction of the material and measure the static and kinetic coefficients of friction using various testing surface plates.Sterile surgical gloves were worn during experiments to prevent contamination.The plates were sterilized with acetone before each series of tests to eliminate residual contaminants.Prior to testing, the materials were cut to dimensions of 63.50 × 63.50 mm.The testing condition was maintained at a constant climate of 23°C and 50% relative humidity.

| Optical Characterization
To facilitate the evaluation and comparison of friction measurements, several physical and surface properties of paperbased materials were measured.Initially, the surface of the paper-based material was examined using the Wild M400 macroscope (MM50-M4 DSLR Camera System, Heerbrugg, Switzerland) using a lens with 32 × magnification and a digital camera software ToupView 3.7.Second, the cross sections of the thermoplastic reference and paper-based materials were visually inspected using a scanning electron microscope (SEM) (Hitachi SU3500, Tokyo, Japan) to assess the surface topology.The cross-sectional areas were cast in an acrylic resin (Struers ClaroCit) for micrographic analysis.The microscope variable pressure setting and micrographs were obtained using backscatter electron imaging in the compositional mode.The parameters set for this process included an acceleration voltage, pressure and working distance of 15 kV, 30 Pa and 10 mm, respectively.This approach was selected to enhance the visibility and contrast between the different layers of the samples.
The surface roughness of the materials was measured using a high-speed surface roughness tester, the Parker Print-Surf (PPS) apparatus (Messmer Instruments Ltd., Gravesend, Kent, United Kingdom), in accordance with the ISO 8791-4:2021 standard [25].The Attension Theta optical tensiometer (Biolin Scientific, Gothenburg, Sweden) was used to measure the contact angles on the paper side (back side) and barrier side (coated side).The solvents used in this measurement were ethylene glycol, di-iodomethane and deionized water with drop volumes of 3, 1 and 3 μL, respectively.The measurement time was 10 s, and the values were recorded 1 s after adding the drop onto the surface.The surface energy ( ) of the solid (s) was estimated using the van Oss acid-base methodology, represented in Equation [1], based on the contact angles ( ) of two polar liquids and one nonpolar liquid (li) and the following components:Lifschitz-van der Waals (LW ) forces, electron acceptor (+) and electron donor (−) [26].The technical data of the material properties of paper-based materials are summarized in Table 1.

| Coefficient of Friction Measurements
The experimental setup consisted of a customized horizontal table as the testing apparatus with a data acquisition program (1) for performing precise coefficient of friction measurements, as shown in Figure 1.The experimental materials were attached to a 200-g sled and allowed to rest on the table.A Nylon monofilament was used in the testing apparatus to pull the sled at a constant speed of 150 mm/min using an electrical drive motor load sensor (LRF400, FUTEK, Advanced Sensor Technology, INC., Irvine, USA) with an accuracy of ±0.05%.SF was automatically recorded the moment when the sled was moved.As the sled transitioned to a constant speed, the force stabilized and reached the kinetic friction phase.KF was measured over a length of 50 mm, and the average coefficient of friction was recorded.The coefficient of friction was calculated for each side of the material (back and coated sides) and surface topography combination along the machine direction (MD) and cross direction (CD).Ten consecutive measurements were performed for each material parameter to ensure statistically significant results.
The horizontal testing plates used in this experiment were the polished AISI 304 stainless steel, pattern-rolled 5WL AISI 304 stainless steel, Teflon tape and 3D-printed polyethylene terephthalate glycol (PETG) plates, as shown in Figure 2. The polished AISI 304 stainless steel plate was ground and polished with 1000 grit to achieve a smooth finish.The pattern-rolled 5WL AISI 304 stainless steel plate was embossed in a specific type to achieve a distinctive wave-like design pattern.The height/depth and distance of the embossed patterns were 211 μm and 217 μm, respectively.The Teflon taped plate was used to impart nonstick properties to the materials.The 3D-printed PETG plate was printed using the Original Prusa i3 MK3 3D printer (Prusa Research by Josef Prusa, Prague, Czech Republic).The device utilizes the fused deposition modelling (FDM) technique to extrude PETG thermoplastic filaments, which were heated to their melting points and then deposited layer-by-layer to build a three-dimensional object.The surface roughness was measured using the Keyence VR-3200 (Keyence Corporation of America, Illinois, USA) 3D-profilometry device.The characteristics of the plates are listed in Table 2.

| VFFS Trials and Convertibility
The VFFS GKS-Compack CP350 Plus machine (GKS Packaging, Eindhoven, Netherlands) was used in this experiment.The interaction between flexible materials and critical components of the VFFS process, such as the forming tube and shoulder, was measured using the FLIR A8201sc thermal camera (Täby, Sweden).It consists of a high-resolution sensor of 1024 × 1024 pixels with a sensitivity below 20 mK and is equipped with a 25-mm lens to accurately capture the temperature effects at various stages of the VFFS process.The thermal images were analysed using FLIR Research IR analysis software.The findings are essential for understanding the frictional issues that influence the runnability and formation of defects, such as wrinkling, in paper-based materials as well as provide solutions for VFFS packaging manufacturers to improve material runnability, refine the design of the forming tube, and shoulder geometries to minimize the defects observed in paper-based materials.

| Friction Characterization
The coefficient of friction is a critical parameter that affects the formability [18] and runnability [24] of a material.The study identified that there is no clear correlation between the plate's surface roughness and the coefficient of friction as it highly depends on the topography of the plate (forming shoulder or tube).This chapter discusses the frictional behaviour of packaging materials, measured along the MD and CD, on different surfaces.
The results revealed a linear relationship between the measured friction and sliding direction (MD and CD), with approximately ±6% variance in frictional values.The sliding direction had no significant effect on the coefficient of friction.In VFFS machines, flexible materials always flow along the MD.However, minimal sliding of the material can occur in the CD as it travels over the shoulder.For this purpose, all friction results discussed hereafter are primarily focused on the MD.
Several variables influence the static and kinetic coefficients of friction.The corresponding bar graphs in Figure 3 show the calculated coefficients of friction as an average of 10 repetitions.
The study of frictional forces across different surface topographies revealed variations in the behaviours of SF and KF.The findings indicated that SF was higher than KF.This phenomenon is attributed to the presence of microscopic asperities on the surfaces of materials that tend to be interlocked.The sliding material requires a higher force to overcome and initiate movement, resulting in static friction.Once the material reaches the true maximum SF, the transition starts and transforms into KF [27].

| Coefficient of Friction Results
The coefficients of friction of two different AISI 304 stainless steel surfaces were compared.The materials that slid over the polished AISI 304 stainless steel surface had a higher coefficient of friction than that slid over the pattern-rolled 5WL AISI 304 stainless steel surface.The increased pattern of the 5WL metal sheet significantly reduced the SF and KF of the OPP+PE Ref.
film by 35% and 25%, respectively.With paper-based materials, particularly on coated sides, SF and KF were reduced by approximately 10% and 20%, respectively.As summarized in Table 2, although the pattern-rolled 5WL sheet had a similar surface roughness to the polished flat surface plate, we conclude that the surface topography of the pattern-rolled 5WL sheet significantly reduced the coefficient of friction.The real contact area between the material and the plate plays a significant role in the frictional behaviour of paper [28].Therefore, the observed reduction in the coefficient of friction with increasing surface roughness is directly associated with a decrease in the actual contact area with the material [29], particularly in this specific case study.
The materials sliding on the 3D-printed PETG plate had the highest coefficients of friction, except for the OPP+PE Ref.
film.From the surface profilometry measurements, we observed that the 3D-printed PETG plate had an uneven waviness and nonhomogeneous surface roughness.The printed PETG material was 265-765 μm in height and 220-660 μm in width.The measured average surface roughness (R a ) and peak roughness (R z ) were 83 and 566 μm, respectively.These irregularities were caused by the layer-by-layer manufacturing systems used for FDM [30].This significantly affected the frictional properties of the paper-based materials (back side), resulting in periodic fluctuations in the peaks or stick-slip phenomena during the measurements, as shown in Figure 4 (left).
There are two possibilities for the occurrence of the stick-slip phenomena.Lenske et al. reported that periodic fluctuations are the result of the tribocharging of surfaces in contact with    the sliding surface [20].Kawashima et al. [28] observed similar peaks in the transition from SF to KF across multiple test repetitions.Furthermore, Baytekin et al. [32] suggested that surfaces exhibit a random distribution of charged regions at the nanoscale level, which enhances the fluctuations in the coefficient of friction upon repeated measurements.Lenske et al. [20] suggested that cleaning with acetone may accidentally induce triboelectric charges.However, this is likely not possible because acetone was used to eliminate any residual contaminants.The major cause of fluctuations in the kinetic coefficient of friction is possibly microstructural irregularities in the fibre textures.The SEM cross-sectional views, as shown in Figure 4 (right), of the Disp.coated paper 65 and OPP+PE Ref. film were compared.Based on the cross-sectional images, the paper-based material was found to have some degree of nonuniform surface roughness and topology owing to the fibre arrangement, whereas the OPP+PE Ref. film exhibited a smoother and more homogeneous OPP+PE layer.The examined polymer-laminated material exhibited a low surface roughness.This smoothness is crucial because it helps the material slide easily through the VFFS machine and glide over the forming shoulder for efficient production.
The interaction between the irregular fibrous texture of the paper and the PETG plate increased the mechanical interlocking mechanism and enhanced the frictional force and shear.
Although the tribological behaviour of PETG possesses antifrictional features compared with other printing materials, its surface roughness was found to be influenced by the printing temperature [30].Batista et al. [30] recommended using a higher printing temperature to minimize the irregular surface roughness.However, only the coated sides of Prego+PE-coated paper and Disp.coated paper 65 resulted in minimal fluctuations and less stickiness during KF measurements.
As shown in Figure 3, the Teflon tape surface exhibited a significantly lower coefficient of friction than the polished AISI 304 stainless steel, pattern-rolled 5WL AISI 304 stainless steel, and 3D-printed PETG surfaces.Teflon is frequently used in nonstick applications because of its low surface energy [33].In general, Teflon has a homogenous surface roughness and weak local cohesion in the shear direction.This weak internal interface allows the material to slip, leading to low friction despite the large contact area [34].Teflon tape tended to reduce the SF and KF of the OPP+PE Ref. film by 25% and 40%, respectively.The coefficient of friction of paper-based materials on the paper side did not have a major frictional influence.The most significant decrease in friction was observed on the coated side of CHP BAR A, wherein the static coefficient of friction was reduced by less than half.

| Paper Material Convertibility in VFFS
Several challenges were found to be associated with the coated side of CHP BAR A. As the material slid on the surface of the polished AISI 304 stainless steel, a noticeable zigzag motion was observed as the material transitioned to KF, as shown in Figure 5 (left).The maximum SF was 1.2 and the kinetic coefficient of friction ranged from 0.4-1.Owing to these variations, it is difficult to accurately measure KF.It is likely that the CHP BAR A-coated side experienced stick-slip phenomena during the friction measurements.Stick-slip causes the material to alternate between sticking and slipping on a sliding surface.This results in unpredictable speeds and a combination of SF and KF.The sled may stop temporarily during each cycle.This phenomenon was observed in this study because, unlike CHP BAR B, CHP BAR A does not possess anti-blocking properties that influence the surface roughness of the coating.
Typical VFFS machines are equipped with a Teflon tape on the forming tube to resist friction and ensure smooth material flow.The friction grip mechanism of the transport belt requires an optimal friction coefficient to drag the material between the belts/material and the material/tube through the forming tube for an effective VFFS machine operation.During the experimental VFFS trials in this study, CHP BAR A exhibited exceptional stickiness compared with other materials.This stickiness resulted in generation of excessive friction and heat at the interface between the coating layer of the material and the Teflon tape on the forming tube, creating a glue-like effect.It is also possible that the observed stickiness may be attributed to the stick-slip phenomena or alterations in the KF resulting from high shear forces, as observed from friction measurements.These factors potentially modified the surface structure of the CHP BAR A coating layer, causing it to seal or adhere to the Teflon tape and limit the movement of the material through the forming tube, as shown in Figure 5 (right).
The effects of the applied contact pressure, sliding speed during contact, coefficient of friction, and thermal conductivity of the material are interconnected and influence the heat generation due to friction [35].The tribological properties of PE typically consist of poor wear and friction owing to factors such as a high coefficient of friction and low thermal conductivity [36].
The polymer coating layers in OPP+PE Ref. film and Prego+PE coated paper exhibits viscoelastic behaviour and can undergo temperature increase resulting from friction [37].This increase in temperature creates a temperature difference within the PE, which can change the material's structure and how it behaves.
In principle, as the PE coating layer tends to approach its plastic deformation, it can affect its tribological performance and increase the coefficient of friction as evidenced by the laboratory friction measurements of OPP+PE Ref. film and Prego+PE coated paper as compared to Disp.coated paper 65.
A thermal camera was used to analyse and compare the temperature changes in the OPP+PE Ref. film, Prego+PE-coated paper, and Disp.coated paper 65 as they were dragged by the transport belt through the forming tube, as shown in Figure 6.
The initial temperatures of the materials were approximately 23 ± 1°C.The experimental observations suggest that the paperbased materials retained more heat than the thermoplastic film because of the insulating properties of the paper substrate.
The thermal analysis indicates that the Prego+PE-coated paper and Disp.coated paper 65 showed significant temperature increases of up to 52°C and 41°C, respectively.As the material slid on the Teflon tape surface, as shown in Figure 3, the Disp.coated paper 65 had a higher kinetic coefficient of friction than the Prego+PE-coated paper.This suggests that the Prego+PE coating layer caused higher heat generation and plastic deformation, thus generating more heat and increasing the temperature owing to shear stress.The coefficient of friction of the OPP+PE Ref. film exhibited higher KF owing to the possible high shear stress, which could be because the thermal properties of the OPP+PE composite allowed heat dissipation, minimized heat generation and improved wear resistance.

| Optical Characterization
The relationship between the measured surface properties, as presented in Table 1, and the coefficient of friction results, as shown in Figure 3, was not clear or coherent.The surface roughness of the material did not show any direct correlation with friction.As the coefficients of friction between the MD and CD were very similar, the fibre and coating alignments could have a more significant influence on the results than the MD [17].
Figure 7 shows macrographs of paper-based materials, showing the paper and coated surfaces before the friction measurements.Small fractions of pigments and holes (pores) were observed on the paper side of the material.The dispersion-coated paper materials had several pores on the coated side, possibly because of their low coating thickness.The measured surface roughness, calculated surface energy, and fibre orientation had no influence on the friction measurements on the paper side, which contradicts some previous findings.
Regarding the coated side of the paper, several microstructural differences in terms of pigments, shapes, concentrations, antiblocking additives, and binders used were noted, which had the greatest influence on the coefficient of friction [9].The surface roughness (PPS method) of the coated side did not exhibit any clear relationship with the measured coefficient of friction.Previous studies have discussed the use of minerals such as calcium carbonate [38,39] and kaolin [40] to enhance the material properties of coated papers.Rättö et al. reported that the microstructure of the coated paper was the most influential factor affecting the friction properties.They demonstrated that the addition of kaolin and calcium carbonate affected the coefficient of friction.Significant variations in the friction properties were observed varying average diameters and shapes of calcium carbonate particles, which influenced the microstructure [11].Similarly, Chen et al. [39] discussed the alteration in the mechanical properties and microstructure of red clay by varying the concentration of calcium carbonate particles.The addition of calcium carbonate to the red clay increased the shear strength, internal friction angle, porosity and particle size distribution within the microstructure.Therefore, it is likely that such connections exist between paperbased materials.These findings support the evidence that the Prego+PE-coated paper has a higher surface temperature because it is dragged by the transport belt in the forming tube.
In general, Prego+PE-coated paper, had a relatively smooth coating layer and the lowest surface roughness of 0.7 μm, as presented in Table 1.The surface roughness of the dispersion-coated paper varied only slightly.Compared with Disp.coated paper 65, the coated sides of CHP BAR A and B had slightly lower surface roughness.However, because CHP BAR A had no anti-slip agent or possible pigments, it had the highest SF, as discussed above.It is argued that the total surface energy of the coating layer influences the coefficient of friction [17]; however, this was not the case because the coated papers had similar surface wetting or surface energy.This indicates that the frictional behaviour of the coated papers is influenced by the microstructural properties rather than the surface energy itself.However, the study found that the dispersion components of the surface energy appeared to correlate with the kinetic coefficient of friction, even if the total surface energy did not.
Figure 8 illustrates the summary of these findings, indicating that, the higher the coefficient of friction, the higher the  dispersion components surface energy.The specific cause of this phenomenon is unclear as the frictional behaviour depends on many factors, such as surface roughness of the plate, material's surface roughness and coating composition, etc.The correlation typically occurs between the adhesion and polar component of the surface energy rather than the dispersive component [41,42].Based on our studies, the dispersion component may play an important role as it seems to correlate the frictional behaviour, rather than the total surface energy.Moreover, it seems that the observed phenomena cannot be completely explained with the existing research and further investigation is needed to understand the impacts of dispersion components on the friction coefficient.

| Conclusion
The coefficient of friction of packaging materials is one of the most influential factors affecting the performance of VFFS machines, particularly when flexible paper-based materials are used.The study finds that the sliding direction, either in the MD or CD, had no major influence on the coefficient of friction during the friction measurements.In general, an increase in the plate's surface roughness does not necessarily lead to a higher coefficient of friction.However, the surface topography of the plates was found to be the most influential factor affecting the coefficient of friction.
The coefficients of friction of different AISI 304 stainless steel plates were compared.The surface topography of the patternrolled 5WL significantly reduced the coefficient of friction than that of the polished plate.This was directly associated with a reduction in the actual contact area with the material during sliding.Furthermore, the paper materials sliding on the 3D-printed PETG plate had the highest coefficient of friction, except for the OPP+PE Ref. film.The 3D-printed PETG plate had an uneven surface roughness.The irregularities observed on the plate's surface were attributed to the FDM manufacturing process.The surface exhibited periodic fluctuations in the coefficient of friction, causing the stick-slip phenomenon that was potentially influenced by either nanoscale tribocharging or microstructural irregularities of the fibre layers in contact.
The materials sliding on the surface of the Teflon tape had the lowest coefficient of friction compared with that those sliding on the surfaces of other materials.Among the different materials used, the coated side of CHP BAR A exhibited the most significant reduction in the coefficient of friction.However, challenges were observed when the material was subjected to actual VFFS test trials, leading to excessive stick-slip phenomena and a gluelike effect on the forming tube.This exceptional stickiness was owing to the lack of anti-blocking properties and the generation of excessive friction and heat at the interface between the coating layer of the material and the surface of the Teflon tape on the forming tube.Thermal camera analysis proved that paper-based materials tend to generate more heat than thermoplastic films owing to factors such as high contact pressure, sliding speed and friction coefficient.It is also possible that the thermal properties of the OPP+PE Ref. film allowed the material to dissipate more heat than paper-based materials.
There was no clear correlation between the surface roughness of materials, fibre orientation, calculated total surface energy, and coefficient of friction.However, there exists a possible correlation between the dispersion components of the surface energy and kinetic friction.Therefore, we conclude that the findings related to the frictional behaviour of paper-based materials on different surface plates provide valuable insights for the development of VFFS machine tools, such as the refinement of forming tubes and shoulder geometries, to improve the runnability and minimize the defects observed in paper-based materials.

FIGURE 1 |
FIGURE 1 | Schematic representation of the friction measurement testing device.

FIGURE 2 |
FIGURE 2 | Customized horizontal testing plate with various surface topographies.

FIGURE 3 |
FIGURE 3 | Average static and kinetic coefficients of friction of materials sliding on different surface plates along the machine direction.(A), (B), (C) and (D) show the coefficients of friction of materials sliding on the surfaces of polished AISI 304 stainless steel, pattern-rolled 5WL AISI 304 stainless steel, 3D-printed PETG and Teflon tape, respectively.

FIGURE 5 |
FIGURE 5 | Static and kinetic coefficients of friction of CHP BAR A sliding on the polished AISI 304 stainless steel and Teflon tape surfaces along the machine direction (left).The CHP BAR A stuck on the forming tube of VFFS GKS-Compack CP350 Plus (right).

FIGURE 6 |
FIGURE 6 | Comparative thermal analysis of the OPP+PE reference film, Prego+PE-coated paper and Disp.coated paper 65 during VFFS machine operation to evaluate the heat generation and friction in the forming tube.

FIGURE 7 |
FIGURE 7 | Comparative macroscopic analysis of the paper and coated sides for the paper-based materials before friction measurements.

FIGURE 8 |
FIGURE 8 | Relationship between kinetic coefficient of friction and dispersion components on various testing plates.

TABLE 1 |
Technical data of paper-based materials.

TABLE 2 |
Measured surface roughness for the horizontal testing plates.