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Continuously extruded micro-textured polypropylene films.


Numerous applications of polymeric films require low sliding friction surfaces, such as high-speed packaging equipment and low-friction tape wraps [1-4]. Conventional methods of reducing the sliding friction of polymeric surfaces include: (a) external coating of liquid lubricants such as silicone oil or powdered solid lubricants such starch or talc, and (b) internal fluoropolymer additives [5-8]. Friction reducing chemicals that are applied to film surface or ones that bloom to the surface can possibly contaminate products and add processing complexity.

Prior literature studies have established that the frictional force, F, developed between two sliding surfaces has a dual, molecular-mechanical nature [9], Hence, frictional force depends on the contact bonding shear stress, t0, arising from the adhesive forces (molecular interactions), and on the contact area, [A.sub.t], between the two surfaces [2, 9]: F = [[tau].sub.0][A.sub.t]. Thus, the fabrication of regular micro-textures that reduce contact area ([A.sub.t]) of the film surface can lead to a reduction in COF. Previous studies have reported that by creating micro-sized features in metallic and polymeric surfaces, the tribological characteristics and hydrophobicity can be modified [10-17]. This can help avoid the use of external lubricants, such as silicone oils and fluorinated compounds, that must be avoided in environmentally sensitive applications.

For various materials, micro-textural features have been typically created by means of batch techniques such as mold casting using lithographically made micro-molds [14, 18], plasma processes [19], and chemical/ion beam etching or laser texturing [11]. These processes can be expensive and slow for bulk production of polyolefin films. However, the processing of micro-textured films by continuous melt-extrusion of polymers, a scalable technique, has not been systematically reported in the literature studies. Therefore, the objectives of this study were to (i) assess the microstructure of isotactic polypropylene (i-PP) films continuously extruded through a micro-patterned die, and (ii) measure the effect of the surface micro-texture on the sliding coefficient of friction (COF) characteristics of the films.



The polymer used throughout this study was PP Dow INSPiRE[TN] 114, a film grade i-PP. It has a melting point of 164[degrees]C and density of 0.9 g/[cm.sup.3]. The metallic sleds for friction measurements were made of 304 grade stainless steel (SS) and polished with 400-grit sandpaper prior to testing. To compare the performance of the textured films with that of standard lubricated films, silicone oil (Molykote[R] 316) from Dow Coming was applied at about 1 wt% as a thin coating on the films.

Micro-Textured Die and Processing

The extrusion die used to produce the film was made of 4140 steel. This material was chosen for its toughness, machinability, and anti-corrosion properties. The dimensions of these cavities in the die were ~100 [micro]m, i.e., below the range of conventional machining techniques. So, the surfaces were created using micromachining methods, but on the meso scale rather than the macro scale [20]. Figure 2a displays a micrograph of the film extrusion die: the pattern consisted of straight sides and a semicircular bottom. The micro-patterned die had a nominal cross-sectional area of 20 [mm.sup.2]. The die was 35-mm wide and had a 10-mm deep land.

The extrusion equipment was a cast-film line consisting of a 25-mm diameter (L/D = 20) single screw extruder feeding to a constant flow rate gear pump (Zenith HBP Series). The films were produced at a constant throughput of 1 [cm.sup.3]/min and at a constant temperature of about 220[degrees]C as monitored at the die. An air-knife was used to cool the melt right after the die exit. Two take-up speeds, 100 and 1000 mm/min, were used that corresponded to draw-down ratios (DDR) of approximately 2 and 20, respectively.

Microstructural Characterization

To obtain sharp cross-sections, the films were thoroughly cooled in liquid [N.sub.2] and cut with special scissors (Kevlar[R] cutter grade). The resulting profile was analyzed by scanning electron microscopy (SEM, Hitachi S-4800 Field Emission Scanning Electron Microscope). The films were platinum-coated prior to the profile inspection. For each cross-section and lateral surface of the film, at least three spots were imaged. All micro-dimensions of die and films were assessed and measured by image analysis software (ImagePro[R]) from at least 10 different SEM micrographs.

Coefficient of Friction Measurements

Figure 1 displays (a) a photograph of the experimental set-up and (b) a schematic representation of the coefficient of friction, [mu], measurements. Tests were performed according to ASTM D-1894 (Method of Assembly C), "Standard Test Method for Static and Kinetic Coefficients of Friction (COF) of Plastic Film and Sheeting" with the following exceptions: (a) crosshead speed of 50 mm/min was used (due to limitations of the equipment), when ASTM specifies 150 [+ or -] 30 mm/min, and (b) film width of ~30 mm was used, when ASTM requires 127 mm. Consequently, a 19-mm wide (63.5-mm long and 6.3-mm thick) SS sled was used, instead of the 63.5-mm wide ASTM sled. In this manner, the edge-bead regions on films were avoided during the COF measurements. Steel-on-steel control experiments were carried out using 63.5-, 44.5-, 38-, 25-, and 13-mm wide sleds. The COF measurements were performed using four replicates for each film type and configuration. All experiments were performed at approximately 25[degrees]C and 50% RH using an ATS 900 Universal tester. The films were adhered to the supporting base by using double-sided tape to avoid bumps and irregularities. Acetone was used to remove any oily residues that might be present on the films surfaces (possibly from handling).

Due to the longitudinal texture of the films, two orientations of the films were studied: machine-direction (MD) and transverse-direction (TD). For film-on-film COF measurements, a piece of film was stuck to the sled surface and slid on top of the other film mounted on the base. Three configurations were investigated: MD-on-MD, MD-on-TD and TD-on-TD. Performance of non-textured (NT) film on textured films was also analyzed for the MD and TD configurations of the textured films.

The COF measurements on lubricated films were conducted by coating a thin layer of silicone oil with a clean paper (kim wipe) and letting it dry for about 5 min for solvent release following a procedure recommended by the manufacturer (Dow Corning Molykote[R] 316). The amount of silicone oil reamaining on the films (after solvent evaporation) was determined by gravimetric analysis at about 1 wt%. Thus, metal-on-film and film-on-film experiments were carried out in the MD and compared with the performance of the textured films. Tensile tests were conducted using the ASTM D638 technique at ambient conditions (~25[degrees]C and 50% RH). ASTM Type V specimens were die-cut (25-mm long) for the machine-direction testing. In the TD, only a gage length of 13 mm could be obtained due to the limited sample width. An ATS Universal 900 tensile tester at a cross-head speed of 50 mm/min was used to test four replicates per film type (n = 4).



Figure 2b displays a representative SEM micrograph of the cross-section of a film obtained from the micro-patterned die (displayed in Fig. 2a). It is evident that surface micro-textured films could be successfully extruded. The stability of the micro-texturing process was assessed by carrying out an extrusion study over 10 h of continuous extrusion. After such period of time, the pressure drop across the micro-patterned die was unchanged (~4 MPa), and no gel formation was observed visually. Also, the film extrudate profile/shape did not vary, confirming the feasibility of the micro-textured film extrusion.

Detailed dimensions of the films and die are displayed in Table 1. The die micro-features have a height of 200 [+ or -] 9 [micro]m, which resulted in film micro-texture heights of 70 [+ or -] 3 and 17 [+ or -] 2 [micro]m for DDR of 2 and 20, respectively. These are about 3 and 12 times shorter than the height of die micro-feature. The thickness of the die, as measured excluding the micro-features, was about 400 [micro]m. The corresponding thickness of the films was 264 [+ or -] 36 and 23 [+ or -] 2 [micro]m for the DDRs of 2 and 20. Figure 2 also shows that the rectangular-semicircular die patterned is not identically retained in the extrudate. This difference is a consequence of the viscoelastic nature of polymeric fluids, which results in well-known phenomena such as "die-swell" [21].

Coefficient of Friction (COF)

Metal on Metal (Control Experiments). A set of control experiments was first carried out on SS sled on SS substrate for different sleds widths. The ASTM recommended sled width (63.5 mm) displayed no significant differences in the kinetic COF ([COF.sub.k]) value with respect to those obtained for other sled widths (0.24 [+ or -] 0.04 vs. 0.26 [+ or -] 0.05). Also, a consistent average value of 0.34 [+ or -] 0.04 was experimentally measured for the static COF ([COF.sub.s]) for the different sleds. The consistent values result from the fact that all sleds applied a constant surface contact pressure (i.e., sled weight per contact area) of about 500 Pa. Rusinek & Molenda [22] reported a [COF.sub.k] for SS-on-SS ranging from 0.17 to 0.23 obtained by the shear test method. Budzynski et al. [23], using a ball-on-disk tribotester, measured a friction coefficient between 0.2 and 0.6 for different nitrogen-implanted AISI 316L grades. Similarly, other studies have reported values of COF ranging from 0.2 to 0.65 for different SS grades and different testing methodologies [24-26], Thus, the COF for the steel grade used in the current study, measured at 0.24 [+ or -] 0.04, is consistent with the values reported in the literature studies.

Metal on Films (Non-lubricated). The COF of the as-produced films at a draw-down ratio of 2 are summarized in Table 2. For SS sled sliding over NT films, the static and kinetic COF were measured to be 0.369 [+ or -] 0.036 and 0.340 [+ or -] 0.024, respectively. These values are consistent with a value of 0.30 reported for the [COF.sub.k] of polypropylene by Yamaguchi [2],

A combination of the SS sled sliding on the textured films in the MD displayed [COF.sub.k] and [COF.sub.k] values of 0.247 [+ or -] 0.028 and 0.245 [+ or -] 0.003, respectively. Thus, as compared with the COF of the NT films, the COF of micro-textured (MD) films was about 30% lower. In the TD of the film, the [COF.sub.s] and [COF.sub.k] values were 0.309 [+ or -] 0.099 and 0.287 [+ or -] 0.033, respectively. Therefore, relative to the COF of the NT films, the [COF.sub.k] of the micro-textured (TD) films was reduced by about 16%.

Although the MD and TD configurations have the same nominal contact area with the sled, the COF values in the transverse-direction were slightly higher. This can be explained in terms of the level of mechanical interlocking between the film micro-texture and the edge/surface roughness of the metal sled. Figure 3a,b displays schematic representations of the interlocking likely observed in the film MD & TD directions, respectively. In the MD orientation, the only interlocking occurs between the very small area of the texture and the edge of the sled, whereas for the TD configuration, the entire width of the sled can interlock with the transversely aligned texture. This makes the TD COF higher than the MD COF. Moreover, the local stiffness or resonating frequency of a micro-textural "rib" depends on height and width of the rib, the material stiffness and hysteresis (viscoelastic properties), and the thickness or stiffness of the underlying base layer [11, 27]. Thus, a wave or vibration propagating along a rib is quite different from one propagating across several ribs, in which more energy is dissipated. A combination of these phenomena contributes to the higher COF observed in TD configuration.

Metal on Lubricated Films. As described earlier, a commercial silicone oil grade was used to coat various films to simulate typical lubricated applications. The [COF.sub.k] and [COF.sub.k] values of the SS sled sliding on the lubricated NT-L films, also displayed in Table 2, were 0.392 [+ or -] 0.020 and 0.149 [+ or -]0.016, respectively. As compared with these NT-L films, the textured but non-lubricated films displayed a [COF.sub.s] value that was about 40% smaller, but the [COF.sub.k] value was not lowered below that of NT-L (it remained about 40% higher). The SS sled was also tested over lubricated textured film in the machine-direction, and the [COF.sub.s] and [COF.sub.k] were measured at 0.334 [+ or -] 0.020 and 0.136 [+ or -] 0.017, respectively. Thus, when compared with NT-L films, the lubricated textured films displayed COF values that were 10-15% lower.

Film-on-Film (Non-lubricated). The experimental COF values for films sliding on films are also displayed in Table 2. As a base case, the NT films sliding on NT films were found to possess [COF.sub.s] and [COF.sub.k] values of 0.197 [+ or -] 0.053 and 0.193 [+ or -] 0.016, respectively. For NT films sliding on textured film in the MD direction, the [COF.sub.s] and [COF.sub.k] values were 0.121 [+ or -] 0.003 and 0.113 [+ or -] 0.004, respectively. Both COF values were approximately 40% less than that for NT-NT combination. The NT films sliding over the textured films in the TD direction led to a [COF.sub.k] value of 0.149 [+ or -] 0.015, which is about 25% lower than the NT-NT counterpart.

Next, the textured films sliding on textured films were evaluated (also reported in Table 2). The textured films in the machine-direction sliding on the textured films in the machine-direction (MD-MD) led to [COF.sub.s] and [COF.sub.k] values of 0.163 [+ or -]0.011 and 0.161 [+ or -] 0.013, respectively (about 17% less than that of the NT-NT combination). For the cross configuration of textures (MD-TD), the [COF.sub.s] and [COF.sub.k] values were 0.154 [+ or -] 0.016 and 0.153 [+ or -] 0.008, respectively. Thus, a reduction of 21% in COFk was observed for the cross configuration with respect to the NT-NT configuration.

The textured transverse-direction sliding on the transverse-direction (TD-TD) was also assessed (reported in Table 2). This configuration provided much larger [COF.sub.s] and [COF.sub.k] values of 0.449 [+ or -] 0.104 and 0.302 [+ or -] 0.034, respectively. These are 128% and 57% higher than that of the NT films. This increased resistance is a consequence of the mechanical interlocking of microgrooves in the TD-TD configuration.

Film on Lubricated Film. Finally, the tribological response of various lubricated films (DDR of 2) is also displayed in Table 2. The NT film sliding on lubricated NT-L film displayed a [COF.sub.s] value of 0.334 [+ or -] 0.042, whereas for the non-lubricated textured counterpart, it was only 0.163 [+ or -] 0.011. This means that the micro-texture (without lubrication) led to a smaller initial resistance to motion relative to that provided by lubrication, which is advantageous for micro-textured films. The [COF.sub.k] value for NT films sliding on NT-L films was only 0.099 [+ or -] 0.012, about 40% lower than that for non-lubricated NT-NT combination.

For textured films sliding over lubricated textured films in the machine-direction, a [COF.sub.k] value as low as 0.063 [+ or -] 0.006 was obtained. However, a [COF.sub.s] value of 0.426 [+ or -] 0.031 was measured, which is 2.5 times greater than that of the similar non-lubricated combination. Similarly, when sliding NT films over lubricated textured films, values of 0.418 [+ or -] 0.072 and 0.061 [+ or -] 0.004 were found for [COF.sub.s] and [COF.sub.k], respectively. Thus, silicone oil reduced the [COF.sub.k], but increased the [COF.sub.s]. This is consistent with a stochastic frictional phenomenon, the "stick-slip behavior," reported in prior literature studies [28-31]. The stick-slip motion of certain polymer films is a result of the deformation behavior of the semi-macroscopic contact regions [29], and is sequentially formed by a periodic static lapse in which elastic energy is stored, followed by a kinetic lapse in which it is released. The stick-slip process has also been attributed to the formation and rupture of adhesive bonds [30]. Consistent with prior literatures results, the lubricated i-PP films exhibited a higher static COF and a lower kinetic COF than those of their non-lubricated counterparts. Therefore, it is evident that the combination of non-lubricated NT films sliding over non-lubricated MD textured films ([COF.sub.k] = 0.113 [+ or -] 0.004) performed as well as the lubricated NT-L films. In some orientations, the textured films without the use of any externally applied lubricant have lower COF as compared with the lubricated NT films.

Effect of Microtexture Size (DDR = 20)

Finally, to assess the effect of smaller micro-texture size on the COF, the films produced using the higher draw-down ratio (DDR ~ 20) were investigated. Figure 4 displays SEM micrographs of the cross-section and the lateral surface of such films, including the surface roughness and pitch, i.e., peak-to-peak distance of the micro-texture. Various ratios of the micro-features of the textured films (for both DDR) are also provided in Table 1. As the DDR increased 10-fold, the thickness of the films decreased by a factor of ten, whereas the height and width of the micro-features decreased by a factor of only about 4. The pitch of the micro-texture decreased about 10% in the transverse-direction. Note that the film dimensions satisfy the overall material balance as the DDR was increased. However, due to the cooling air used during the drawing process, a more rapid solidification of the micro-texture occurred, in contrast to the height of micro-features that did not display the same proportional reduction.

Figure 5 presents a comparison of the COF values of the films produced at a DDR of 2 as compared with those produced at the higher DDR of 20 in MD. For the metal-on-film COF experiments, the micro-textured films (DDR = 20) displayed slightly lower values, but not significantly different, when compared with their NT counterparts. Thus, the smaller micro-textural features obtained at the higher draw-down did not significantly reduce the metal-on-film COF.

For film-on-film sliding behavior, the films produced at a DDR of 20 showed [COF.sub.s] and [COF.sub.k] values of 0.314 [+ or -] 0.044 and 0.251 [+ or -] 0.014, respectively. Higher COF values were found for textured films than for the NT films. This counter-intuitive effect may be a consequence of the higher draw-down producing a rougher and non-uniform micro-texture, which can be observed in the SEM micrographs of Fig. 4d. The rougher micro-texture can lead to a greater extent of mechanical interlocking.

Also, as explained earlier, friction not only depends on the contact area, but also on the energy dissipated by deformational characteristic of the surfaces in relative motion. This adds another level of complexity whereby the deformation or buckling of the small texture generates waves of detachment that dissipate higher amount of energy [32, 33],

Figure 6 displays schematics of a metal sled moving on the textured films with two different textural sizes. In Fig. 6a, micro-texture sizes much larger than the sled roughness display minimum interlocking and less deformation. In contrast, small micro-texture sizes illustrated in Fig. 6b are more susceptible to bending and interlocking with the metal sled. Thus, micro-texture features with a high level of rigidity (to avoid texture deformation/ buckling) and with textural dimensions large enough to minimize mechanical interlocking result in lower COF values. With reference to Table 1, a reduction in the micro-features height of about four times led to a different frictional regime in the films.

Tensile Properties

Finally, to determine whether the micro-texture led to any adverse effect on the strength or ductility of the films, tensile tests were performed on the films. NT films displayed a tensile strength of 46 [+ or -] 5 MPa and 42 [+ or -] 11 MPa in the MD and TD, respectively. The textured films had tensile strength values of 45 [+ or -] 7 MPa and 35 [+ or -] 15 MPa, respectively. Strain-to-failure values were 645 [+ or -] 72% and 653 [+ or -] 168%, respectively, in the MD and TDs of the NT films. The textured films displayed strain-to-failure values of 635 [+ or -] 132% and 508 [+ or -] 78%, which were not significantly different from the respective values for the NT films. Thus, the micro-texture did not lead to any significant deterioration of the film properties.


Experiments conducted with SS sled sliding on i-PP continuously extruded micro-textured films displayed a significant reduction in COF values of about 30% in the MD and of 15% in the TD as compared with those of the SS sled sliding on NT films. The smaller reduction of TD COF relative to MD COF (15% vs. 30%) is consistent with the greater mechanical interlocking possible between the leading edge of the sled and transverse texture. As compared with the performance of i-PP lubricated NT-L films (with 1 wt% silicone oil), the micro-textured films displayed a [COF.sub.s] value that was 38% smaller (in the MD). Thus, when SS sled slides on the micro-textured films, less initial resistance to the relative motion of the surfaces than that provided by the lubricated NT films is observed. Tribological experiments performed with a combination of NT films sliding on micro-textured films oriented in the MD displayed a greater COF reduction of up to 40% in [COF.sub.k] (0.113 [+ or -] 0.004), close to that of Teflon[R]. Thus, the reduction in COF was found to be largely a consequence of the reduction of surface contact area provided by the micro-texture.

Friction measurements carried out for micro-textured films sliding on micro-textured films showed different behavior depending on the relative orientation of the films. When compared with the [COF.sub.k] of NT-NT combination (0.193 [+ or -] 0.016), a reduction of about 20% in [COF.sub.k] was found for the cross orientation of micro-textured (MD-TD) due to reduced contact area. In contrast, an increase of about 60% was observed for the [COF.sub.k] measured in the TD-TD configuration due to mechanical interlocking of the textures. The experiments with textured (MD) films sliding on textured (MD) films and those with NT films on textured (MD) films displayed [COF.sub.s] values that were 50% and 64% lower than those of their lubricated NT counterparts, respectively. These results establish that continuously extruded micro-textured polypropylene films can be effectively used in environmentally sensitive applications where the use of traditional lubricants must be avoided.


COF    Coefficients of friction

DDR    Draw-down ratios

MD     Machine-direction

NT     Non-textured

i-PP   Isotactic polypropylene

SEM    Scanning electron microscopy

SS     Stainless steel

TD     Transverse-direction


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Byron S. Villacorta, (1) Sarah Hulseman, (2) Andrew H. Cannon, (2) Ralph Hulseman, (2) Amod A. Ogale (1)

(1) Chemical Engineering and Center for Advanced Engineering Fibers and Films (CAEFF), Clemson University, Clemson, South Carolina 29634

(2) Hoowaki LLC, Pendleton, South Carolina 29670

Correspondence to: Prof. Amod Ogale (e-mail: Contract grant sponsor: National Science Foundation; contract grant numbers: EEC-1128481, EEC-9731680. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

DOI 10.1002/pen.23762

Published online in Wiley Online Library (

TABLE 1. Dimensions of the micro-features of the
micro-patterned die and micro-textured films.

                            height (a)       Thickness (b)

Dimensional measurements ([micro]m)

  Micro-patterned die     200 [+ or -] 9    400 [+ or -] 12
  Film A (DDR of 2)        70 [+ or -] 3    264 [+ or -] 36
  Film B (DDR of 20)       17 [+ or -] 2     23 [+ or -] 2

Ratios of die and film dimensions

  Die/Film A ratio              ~3                ~1.5
  Die/Film B ratio              ~12               ~16
  Film A/ Film B ratio          rv4               ~10

                               Micro-             Micro-
                              feature            feature
                             pitch (c)          width (d)

Dimensional measurements ([micro]m)

  Micro-patterned die     265 [+ or -] 10    140 [+ or -] 15
  Film A (DDR of 2)        249 [+ or -] 3     119 [+ or -] 6
  Film B (DDR of 20)       236 [+ or -] 3     33 [+ or -] 3

Ratios of die and film dimensions

  Die/Film A ratio             ~ 1.06              ~1.2
  Die/Film B ratio             ~ 1.12              ~4.7
  Film A/ Film B ratio         ~ 1.05               ~4

(a) Form the film surface.

(b) Without the texture.

(c) Horizontal peak-to-peak.

(d) At the texture base.

TABLE 2. Static and kinetic coefficients of friction for the
different films sliding on the studied substrates (DDR of 2).


Substrate                    SS Sled                   NT

   SS (a)             0.291 [+ or -] 0.040
   NT (b)             0.369 [+ or -] 0.036    0 197 [+ or -] 0.053
   MD (c)             0.247 [+ or -] 0.028    0.121 [+ or -] 0.003
   TD (d)             0.309 [+ or -] 0.099    0.166 [+ or -] 0.005
   NT Lubricated      0.392 [+ or -] 0.020    0.334 [+ or -] 0.042
   Text-lubricated    0.334 [+ or -] 0.020    0.418 [+ or -] 0.072


Substrate                    SS Sled                   NT

   SS                 0.240 [+ or -] 0.040
   NT                 0.340 [+ or -] 0.024    0.193 [+ or -] 0.016
   MD                 0.245 [+ or -] 0.003    0.113 [+ or -] 0.004
   TD                 0.287 [+ or -] 0.033    0.149 [+ or -] 0.015
   NT Lubricated      0.149 [+ or -] 0.016    0.099 [+ or -] 0.012
   Text-lubricated    0.136 [+ or -] 0.017    0.061 [+ or -] 0.004


Substrate                      MD                      TD

   SS (a)
   NT (b)
   MD (c)             0.163 [+ or -] 0.011
   TD (d)             0.154 [+ or -] 0.016    0.449 [+ or -] 0.104
   NT Lubricated                -
   Text-lubricated    0.426 [+ or -] 0.031


Substrate                      MD                      TD

   MD                 0.161 [+ or -] 0.013
   TD                 0.153 [+ or -] 0.008    0.302 [+ or -] 0.034
   NT Lubricated                -
   Text-lubricated    0.063 [+ or -] 0.006

(a) Stainless steel.

(b) Non-textured.

(c) Machine-direction.

(d) Transverse-direction.

Error bars represent 95% confidence intervals.
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Author:Villacorta, Byron S.; Hulseman, Sarah; Cannon, Andrew H.; Hulseman, Ralph; Ogale, Amod A.
Publication:Polymer Engineering and Science
Article Type:Report
Date:Sep 1, 2014
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