NANO LETTERS
AFM Characterization of Surface Segregated Erucamide and Behenamide in Linear Low Density Polyethylene Film
2002 Vol. 2, No. 1 9-12
Marı´a X. Ramı´rez,† Douglas E. Hirt,*,† and Laura L. Wright‡ Department of Chemical Engineering and Center for AdVanced Engineering Fibers and Films, Clemson UniVersity, Clemson, South Carolina 29634-0909, and Department of Chemistry, Furman UniVersity, GreenVille, South Carolina 29613 Received July 31, 2001; Revised Manuscript Received September 14, 2001
ABSTRACT Coefficient of friction (COF) measurements and AFM of LLDPE films containing erucamide and behenamide indicated that COF reduction is not necessarily dependent on additive coverage of the film surface. A film containing 1010 ppm erucamide yielded a kinetic COF of 0.2 without complete surface coverage, whereas a 1080 ppm behenamide film exhibited a saturated surface and higher COF. AFM showed distinct differences in crystal formation between the surface-segregated additives, with erucamide exhibiting a plate-like structure.
Polyolefin films typically possess surfaces that are tacky and exhibit a high coefficient of friction (COF). To reduce the COF and make the films easier to handle, additives are blended into the polymer prior to extrusion.1-8 Once the film solidifies, the additives migrate to the surface over time and modify the surface properties. Commonly used additives include erucamide and behenamide, which may be added in tandem to serve specific functions. These two additives are primary fatty amides with 22 carbons and differ by a double bond at the C-13 position in erucamide. Erucamide is considered an effective slip agent because it reduces a film’s sliding friction, and therefore COF, for both film-on-film and film-on-metal contact. The latter case is extremely important since commercial film travels over metal surfaces in high-speed packaging applications. Reportedly,5 behenamide promotes favorable antiblocking properties so that films can be pulled apart, which is particularly valuable for blown film (other common antiblocking agents include silica and talc). Behenamide can also lower the COF, although not as effectively as erucamide.2,4 Figure 1 shows data for film-on-metal kinetic COF (COFk) of linear low-density polyethylene (LLDPE) films as a function of erucamide and behenamide concentration (in this paper we report COFk, corresponding to continuous sliding of film on metal, as opposed to static COF, which corresponds to the initiation of sliding). It is well known that the COFk decreases with increased additive loading. In the case * Corresponding author. Tel: (864) 656-0822; Fax: (864) 656-0784; E-mail:
[email protected]. † Clemson University. ‡ Furman University. 10.1021/nl015591l CCC: $22.00 Published on Web 11/27/2001
© 2002 American Chemical Society
Figure 1. Kinetic COF (COFk) values at equilibrium for erucamide (KE) and standard (KB) and refined behenamide (KB/R) film samples.
of erucamide, there is a critical loading at which the COFk curve plateaus at a value of about 0.2. It is also evident that, for equivalent bulk loadings, the COFk for films containing only behenamide is greater than that for erucamide. This research focused on the COFk-lowering capability of these two amides. In particular, we were interested in characterizing the nanoscale structure of surface-segregated amides and determining whether a film surface had to be saturated with additive to yield a low COFk. The characterization was performed using atomic force microscopy (AFM) to qualitatively examine the structure of erucamide and behenamide crystals on LLDPE films surfaces. Films for this study were made by preblending each amide with LLDPE (DOWLEX 2045 from Dow Chemical Co.) into a master batch, which was then mixed with amide-free
Figure 2. Height profile (a) and phase (b) images of a neat film sample. The contrast in (a) covers height variations in the 0-50 nm range in a 1.5-µm scanned area.
LLDPE pellets and extruded in a cast-film process to a nominal thickness of 50 microns. An 18-mm, twin-screw Leistritz extruder was used (L/D ) 40, 100 rpm) with a flexlip die. The temperature was 208 °C at the feed section and increased to 235 °C at the die. The erucamide (Kemamide E Ultra) was obtained from Witco CK. The bulk loadings of erucamide in the LLDPE film ranged from 40 to 5000 ppm. Two versions of behenamide were used, namely, standard (Kemamide B) and refined (Kemamide B/R) from Witco CK. The bulk loadings of behenamide in the LLDPE film ranged from 500 to 2400 ppm. After extrusion, all films were immediately stored in a freezer to minimize additive migration. The data in Figure 1 were obtained from film samples that were allowed to achieve equilibrium. Specimens were cut from the film rolls and aged at room temperature (∼25 °C) for at least 7 days, which has been shown to be more than enough time for an equilibrium bulk-to-surface partitioning to be established at a film thickness of 50 microns.9 Once the films were aged, they were tested for film-on-metal COF according to ASTM standard D 1984-9310 and scanned with AFM to investigate the corresponding surface structure. Equivalent film samples were subjected to microwave extraction to determine the actual additive loadings9,11 so that Figure 1 could be produced. The AFM survey of the film surface was performed using a Digital Instruments NanoScope IIIa Multimode system with an E scanner operated at ambient conditions. The surface analyses were conducted using Phase Imaging12 in TappingMode13 with TESP tips. This technique can be used to map variations in surface properties of soft, easily damaged samples, providing real-space visualization at the nanometer scale. As a result, high resolution topographic and phase images can be obtained simultaneously. Three different positions on each sample were scanned to ensure that the images obtained were representative of the surface structure; only one location was then examined in detail. In all of the subsequent figures, a height profile is provided on the left and a phase image on the right. As shown in Figure 2, an LLDPE film without additive was examined with AFM to establish the surface characteristics of a neat film surface. In the phase image in Figure 2b, there are distinct light and dark regions. The predominant 10
Figure 3. Height profile (a) and phase (b) images of an LLDPE film with a 5000 ppm erucamide loading. The contrast in (a) covers height variations in the 0-200 nm range in a 5-µm scanned area.
Figure 4. Height profile (a) and phase (b) images of an LLDPE film with a 1010 ppm erucamide loading at equilibrium. The contrast in (a) covers height variations in the 0-200 nm range in a 5-µm scanned area.
dark regions correspond to the polymer’s softer, amorphous portion while the light regions correspond to the surface lamellae. These results match published AFM images obtained for melt-crystallized LLDPE and commercial LLDPE film.14 The width of the lamellar structures varies in the 10-20 nm range. Once the neat film was imaged, the next goal was to evaluate the surface coverage of erucamide-containing films and its relationship to COFk. For this study we chose to examine a 5000-ppm film15 and a 1010-ppm film, both of which have a kinetic COFk ∼ 0.2. The results for the 5000ppm film are shown in Figure 3, and from the phase image it is evident that the surface was completely covered with crystalline erucamide. By contrast, the 1010-ppm film in Figure 4 was not completely covered such that the polymer’s lamellae and amorphous regions can be clearly observed in the gaps between the erucamide crystals. Thus, complete coverage of the film surface with erucamide is not necessary to obtain the low COFk plateau value. A film sample containing 1080 ppm Kemamide B (standard behenamide) with a COFk of 0.48 was then imaged. As shown in Figure 5, the surface was completely covered with behenamide crystals, with no evidence of lamellae appearing from the underlying LLDPE. The noteworthy feature here is that the film surface can be saturated with behenamide (Figure 5) or erucamide (Figure 3), yet there is a significant difference in the COFk of those surfaces. A Nano Lett., Vol. 2, No. 1, 2002
surface shown in Figure 3. Based on these results, it is interesting to speculate about the mechanism of COFk reduction for erucamide vs behenamide. If we accept the hardness analogy described above, then the unsaturated erucamide would be softer than saturated behenamide. Coupled with the images in Figure 6, it is reasonable to hypothesize that larger, softer, plate-like crystals may smear more readily and provide less physical resistance to sliding motion, thereby reducing the COFk.16
Figure 5. Height profile (a) and phase (b) images of an LLDPE film with a 1080 ppm standard behenamide loading at equilibrium. The contrast in (a) covers height variations in the 0-100 nm range in a 5-µm scanned area.
Acknowledgment. We gratefully acknowledge the financial support of this research from the Cryovac Division of Sealed Air Corp. This work also made use of ERC shared facilities supported by the National Science Foundation under Award Number EEC-9731680. References
Figure 6. (a) Phase image of erucamide crystals at the surface of a 1010-ppm KE/LLDPE film (same film as in Figure 4) (1.5-µm scanned area). (b) Phase image of behenamide crystals at the surface of a 1080-ppm KB/LLDPE film (2-µm scanned area).
similar COFk difference has been observed previously7 for the 18 carbon primary amides, oleamide (unsaturated) and stearamide (saturated). In that study, oleamide was able to provide a much lower COFk than stearamide, which was attributed to the basic shear and deformation properties of the amides. It was hypothesized that a softer additive at the film surface would deform and smear to enhance lubrication. In support of that hypothesis, microhardness measurements showed stearamide to be 1 order of magnitude harder than oleamide. Therefore, analogous to the 18 carbon amide pair, a similar difference in hardness and its effect on lubricating properties may be expected for erucamide and behenamide. As another mode of comparison between erucamide and behenamide, AFM was again used to obtain a closer image of the Kemamide E and Kemamide B crystals. To observe isolated behenamide crystals, a 1080-ppm Kemamide B film sample was removed from the freezer and examined with minimal aging time (about 3 min at RT for sample preparation). An image of erucamide crystals was obtained by zooming in on a region in Figure 4. The resulting phase images are shown in Figure 6. Although not exactly at the same scale, it is clearly evident that erucamide crystals are much larger than the behenamide crystals. Additionally, erucamide crystals show a layered or plate-like formation, while behenamide crystals show a ‘bumpy’ structure. Platelike erucamide crystals are also noticeable for the saturated Nano Lett., Vol. 2, No. 1, 2002
(1) Thompson, K. I. Coefficient of friction testing and factors that affect the frictional behavior of polyethylene films. Tappi J. 1988, 71 (Sept), 157-161. (2) Swanson, C. L.; Burg, D. A.; Kleiman, R. Meadowfoam Monoenoic Fatty Acid Amides as Slip and Antiblock Agents in Polyolefin Film. J. Appl. Polym. Sci. 1993, 49(9), 1619-1624. (3) Rinker, J. W. Surface Properties of Blown Low Density Polyethylene Films. In Paper Synthetics Conference, Proceedings of the Technical Association of the Pulp and Paper Industry, Atlanta, GA, Sept. 2426, 1979; pp 129-136. (4) Maltby, A.; Marquis, R. E. Slip Additives for Film Extrusion. In Polymers, Laminations and Coatings Conference, Proceedings of the Technical Association of the Pulp and Paper Industry, Atlanta, GA, 1996; pp 25-45. (5) Coupland, K.; Maltby, A. Modification of the Surface Properties of Polyethylene Plastomer Films by the Use of Additives. J. Plast. Film Sheet. 1997, 13(2), 142-149. (6) Wooster, J. J.; Simmons, B. E. Optimizing COF, Seal Performance, and Optical Properties of Polyolefin Plastomers. In Polymers, Laminations and Coatings Conference, Proceedings of the Technical Association of the Pulp and Paper Industry, Atlanta, GA, 1995; pp 553-560. (7) Briscoe, B. J.; Mustafaev, V.; Tabor, D. Lubrication of Polythene by Oleamide and Stearamide. Wear 1972, 19, 399-414. (8) Molnar, N. M. Erucamide. J. Am. Oil Chem. Soc. 1974, 51(3), 8487. (9) Rawls, A. S. M.S. Thesis, Clemson University, Clemson, SC, 1997. (10) Ramı´rez, M. X.; Hirt, D. E.; Roberts, W. P.; Havens, M. R.; Miranda, N. R. COF of LLDPE Films as a Function of Erucamide Surface Concentration. ANTEC 2000 [CD-ROM]; Technomic Publishing Company: Orlando, FL, 2000; Paper 634, p 2873 ff. (11) Ramı´rez, M. X.; Hirt, D. E.; Miranda, N. R. COF of LLDPE Film at Low Loadings of Erucamide. ANTEC 2001 [CD-ROM]; Technomic Publishing Company: Dallas, TX, 2001; Paper 587, p 2654 ff. (12) Babcock, K. L.; Prater, C. B. Phase Imaging: Beyond Topography; Application Notes AN11; Digital Instruments: Santa Barbara, CA (http://www.di.com/AppNotes/Phase/PhaseMain.html). (13) Prater, C. B.; Maivald, P. G.; Kjoller, K. J.; Heaton, M. G. TappingMode Imaging: Applications and Technology; Application Notes AN04; Digital Instruments: Santa Barbara, CA (http:// www.di.com/AppNotes/TapMode/TapModeMain.html). (14) Magonov, S.; Godovsky, Y. Atomic Force Microscopy, Part 8: Visualization of granular nanostructure in crystalline polymers. Am. Lab. 1999, 31(8), 52 ff. (15) This data point corresponds to a film sample that was immersed in ethyl ether for 5 s and then allowed to age at room temperature for 2 days before testing. Even though this sample does not exactly match a 5000 ppm erucamide loading at equilibrium, only a small fraction of erucamide is removed from the film with a 5 s wash so the total bulk loading is still close to 5000 ppm. Most importantly, this film sample provides a loading much greater than 1010 ppm to confirm where the COFk plateaus. (16) As quantified by AFM, the root-mean square (rms) roughnesses for the surfaces shown in Figures 2-6 were in the range of 5-10 nm. As a specific example, the rms roughnesses for the surfaces in Figures 11
6a and 6b were 6 and 7 nm, respectively. The similarity between these roughness values is surprising considering the differences in surface structure (large, flat crystals vs smaller, ‘bumpy’ crystals). While roughness can certainly influence COFk, the rms roughness may not be a good descriptor unless the surface structure is similar. For example, if Figure 6a had shown large bumps and Figure 6b small bumps, the rms roughnesses would have been different and
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possibly provided the opportunity to correlate roughness with COFk, but that was not the case. The specimens represented in Figure 6 exhibited significantly different COFks, suggesting that the additive characteristics (crystal structure and deformability) strongly affected the frictional behavior.
NL015591L
Nano Lett., Vol. 2, No. 1, 2002
