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Preparation and tribological studies of stearic acid-modified biopolymer coating
Progress in Organic Coatings xxx (xxxx) xxxx
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Preparation and tribological studies of stearic acid-modified biopolymer coating Shih-Chen Shi*, Yao-Qing Peng Department of Mechanical Engineering, National Cheng Kung University (NCKU), Tainan, Taiwan
A R T I C LE I N FO
A B S T R A C T
Keywords: Friction-reducing coatings Wear mechanism Solid lubricant additives Self-lubricating composites Biopolymer Tribology
Adding fatty acids to cellulose derivatives can effectively improve the hydrophobicity and surface energy of materials and decrease the microscale coefficient of friction. In this research, stearic acid (SA) and the hydroxypropyl methylcellulose (HPMC) solution were mixed to prepare HPMC/SA composite films. The SA molecules formed crystals distributed within the HPMC film, thus increasing the surface roughness and surface wettability. The intermolecular interaction between HPMC and SA causes the formation of micelles. SEM images of the wear marks indicate that SA debris generated during wear as a third body. Third-body layer provide load capacity during wear and reduces direct contact between the grinding object and the HPMC coating, reducing friction coefficient and wear effectively. It is considered the dominate wear mechanism of HPMC/SA composite under macroscopic wear.
1. Introduction Hydroxypropyl methylcellulose (HPMC) exhibits a film-forming property and high pH stability [1] without biological toxicity [2] and causes no effects on the development of aquatic organisms at 0.5 wt.% [3]. HPMC can form transparent films with excellent mechanical properties after solvent evaporation and is one of the main materials used in capsule shells [4] and edible coatings [5]. Several studies shown that HPMC exhibits excellent tribological properties. P. A. Simmons found that HPMC molecules can be adsorbed onto the surfaces of several types of gel lens and form a thick and durable liquid layer, thus providing moisturizing and lubricating effects and further reducing users’ discomfort [6]. S. C. Shi prepared HPMC coatings with controlled film thicknesses for wear tests [7]. HPMC coating, as sacrificial layers, was adhered onto chrome steel balls and formed a transfer layer, thus avoiding direct contact between the metal and substrate and reducing the wear and friction coefficients [7,8]. T. F. Huang demonstrated the self-healing properties of HPMC in the condition of directly replenish HPMC solution on damaged coating, or in the environment with high temperature and high humidity [9,10]. J. Y. Wu prepared HPMC/MoS2 composite films. MoS2 and HPMC formed a transfer layer that provided a self-lubricating effect, thus improving the lubrication and wear resistance capacity of the HPMC film and enabling the films to accommodate high loads and high sliding velocities [11–13].
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Considering these previous studies, the HPMC film has the potential to become a substitute for artificial polymers and packaging materials [14,15]. However, as a cellulose derivative, the hydrophilicity characteristic of the HPMC film may deteriorate its mechanical properties and even its frictional properties in applications where condensed water is expected to precipitate on the surface, which is a substantial limitation [16–18]. To address this limitation, stearic acid (SA) is a saturated fatty acid comprising an 18-carbon-long chain on the hydrophobic end and a carboxyl on the hydrophilic end [19]. SA is amphipathic and active at an interface. SA has been used by several research teams to improve the susceptibility of hydrophilic materials to moisture. Recently, L. Zhang immersed a cellulose film in a SA solution and hot-pressed the sample such that the SA on the surface formed flaky crystals after being dried; the resulting cellulose/SA composite film had a super-hydrophobic surface that reduced the water absorption under high-humidity environment and successfully slowed the deterioration of the mechanical properties [20]. N. Garoff used fatty acids with different carbon numbers to form a self-assembled monolayer (SAM) coating on the cellulose surface; their results demonstrated that the SAM coating formed by saturated fatty acids with more than 16 carbons, including SA, could effectively improve surface hydrophobicity and reduce the microscale coefficient of friction [21]. Several research reports have demonstrated that HPMC/SA composites formed by adding SA molecules to aqueous HPMC solutions can
Corresponding author. E-mail address: [email protected] (S.-C. Shi).
https://doi.org/10.1016/j.porgcoat.2019.105304 Received 12 April 2019; Received in revised form 28 August 2019; Accepted 1 September 2019 0300-9440/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Shih-Chen Shi and Yao-Qing Peng, Progress in Organic Coatings, https://doi.org/10.1016/j.porgcoat.2019.105304
Progress in Organic Coatings xxx (xxxx) xxxx
S.-C. Shi and Y.-Q. Peng
Table 1 Parameters of HPMC and SA solutions. Solution
Pure HPMC SA/ethanol (2.8 mM) SA/ethanol (5.3 mM) SA/ethanol (10.5 mM) SA/ethanol (21.1 mM) SA/ethanol (42.2 mM)
Solute
Solvent
HPMC powder (g)
SA powder (g)
Deionized water (ml)
Ethanol (ml)
3 0 0 0 0 0
0 0.08 0.15 0.3 0.6 1.2
100 0 0 0 0 0
0 100 100 100 100 100
2. Materials and methods
effectively improve the hydrophobicity of bulk material, surface hydrophobicity and surface energy of the HPMC film, enhancing the film’s anti-moisture properties on macroscopic and microscopic scales and strengthening the film’s tribology capacity [22,23]. R.D. Hagenmaier prepared HPMC/SA composite films for water vapor permeability tests using a solvent evaporation method in which SA molecules were added at different percentages to the HPMC solution; the composite film containing 45 wt.% SA had the lowest volume of water vapor permeation, i.e., the best moisture insulation capacity [22]. In addition, the surface of the film formed by the HPMC/SA solution with a high content of SA was covered by SA crystals that had more significant effects on the moisture resistance capacity than the SA molecules distributed within the HPMC film [22]. A. Jiménez added fatty acid molecules with different carbon numbers and saturation levels to aqueous HPMC solutions to prepare films following methods similar to those used by R. D. Hagenmaier group. A. Jiménez showed that the size of colloids formed by the SA molecules in the aqueous solution is associated with the microstructure organization of composite films after evaporation and may also influence the surface roughness, transparency, mechanical properties, and water vapor permeation [23]. A. Fahs prepared films by mixing aqueous HPMC and SA/ethanol solutions at a concentration of 0–1 wt.%, much lower than the concentration of Hagenmaier’s group used; based on contact angle, atomic force microscopy, ball-on-disk, and other results, the migration of SA molecules to the HPMC surface substantially reduced the surface roughness, capillary acting force, and surface energy, and decreased the microscale and macroscale coefficient of friction to 0.1 [24]. A. Fahs studied the tribological properties of HPMC/SA composite films, investigating only the surface energy effects via a macroscale wear test with a short wear distance of 100 mm, probably to avoid significant effects on the coefficient of friction due to increased wear mark depth. However, in addition to the surface energy, macroscale frictional properties are associated with surface roughness, mechanical properties, abrasive particle characteristics, and third body effects among the frictional interfaces during the wear process, as well as the generation and development of the transfer layers and tribo-chemical reactions induced by friction [25,26]. Surface condition including surface roughness, surface energy, water vapor permeability (WVP) and surface structure of the film and the formation of the three-body layer, are important factors that affecting the tribology properties of coating. Therefore, for further understanding the tribological beahvior and wear mechanism of the coating, it is necessary to prepare a composite coating which conduct a longer distance wear test. In this research, HPMC/SA composite coatings were prepared. Surface morphology was observed by SEM and 3D profiling; surface hydrophobicity was measured with a contact angle meter; and surface roughness was measured with a stylus roughness meter. A ball-on-disk wear test was conducted with a chrome steel ball as the abrasive component, and the wear marks were analyzed, with the expectation that the state of the current contact area during wear and frictional mechanisms could be inferred based on the results, thus providing a reference for other researchers interested in strengthening the frictional properties of environmentally-friendly materials using SA.
Silicon substrate (Summit-Tech, Hsinchu, Taiwan) with surface roughness (Ra) of 30 nm was cut into 16.5 × 16.5 mm2, and sequentially washed with methyl alcohol, acetone, isopropanol, and deionized water to remove contaminants on the substrate, thus avoiding a decrease in adhesive force between the coating film and the substrate. HPMC powder (PHARMACOAT® 606, Shin-Etsu Chemical Co., Ltd, Tokyo, Japan) and SA powder (99% purity, Pharma-Up Enterprise Co., Ltd., Taipei, Taiwan) were used in the experiment. 2.1. Film formulation HPMC solution was prepared by adding 3 g of HPMC powder into the heated deionized water (100 mL, 80 °C), then stirred slowly until powder completely dissolved. SA solution was prepared by adding 0.08–1.2 g SA powder into 95% ethanol (100 mL) as shown in Table 1. 10 mL of SA/ethanol solution was added to 100 mL of HPMC solution and stirred for 10 min. The generation of micelles caused the HPMC/SA blending solution become white and turbid. 300 μL of the blended solution was pipetted and applied onto the silicon substrate. Sample was placed in a controlled environment chamber (DE-60, Denyng Instruments, New Taipei City, Taiwan) at 30℃and RH40% and kept for 6 h. All test samples were stored in a dry cabinet (AD-88S, Eureka Auto Dry Box, Taoyuan, Taiwan) at 25℃ and RH30% until the experiment started. 2.2. SEM imaging SEM (XL-40FEG, Philips, Amsterdam, Netherlands & SU-5000, Hitachi, Tokyo, Japan) imaging was performed to observe the surface and the silicon profile in the vicinity of the wear marks on the HPMC/ SA composite films. Because both HPMC and SA are non-conductive, all samples were plated with 10 nm gold coating. 2.3. Attenuated total reflectance (ATR)-Fourier transform infrared spectroscopy (FTIR) analysis FTIR (Thermo Nicolet NEXUS 470, Golden Valley, MN, USA) in ATR mode was used with an incident angle larger than the critical angle to allow total reflection of the infrared ray to occur continuously between the sample surface and the crystals. The light was attenuated by multiple reflections, and the attenuated spectrum was analyzed to investigate the structural vibration behavior of the material’s surface molecules and obtain the frequency spectrum. The absorption signal peak was utilized to determine the presence of various functional groups and bonds on the surface and further identify the surface distribution of the HPMC and SA constituents of the composite film. Each test sample was measured three times. 2.4. 3D profiler measurements A 3D profiler (VK9700, Keyence, Osaka, Japan) was used to 2
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Fig. 1. SEM images of HPMC/SA composites: (a) B-2.8 mM, (b) B-5.3 mM, (c) B-21.1 mM, and (d) B-42.2 mM (5000x magnification).
measure the surface profile, wear scar profile, and film thickness of the HPMC/SA composite films. Half of the coating on the silicon was peeled off. Subsequently, the 3D profiler was employed to scan the boundary between the two area. After repeating 5 times, the average value of the film thickness was obtained. From the cross section of the resulting profile, the height difference beyond the area that was plastically deformed under stress was measured to obtain the local film thickness. For each sample, three different sites were measured, and the mean value was considered as the overall film thickness.
water vapor permeability experiment for the prepared films. Films were cut into discs with a diameter slightly larger than the diameter of the cup. Distilled water (10 ml) was dispensed into the cup, then it was covered with the film and sealed with the ring. The cup was weighed with their contents and placed inside the environment chamber provided with inside temperature of 30oC and ± 40 RH. Cups were weighed in every 8 h to get minimum 4 data to plot. WVP of the film was calculated using following equations.
2.5. Surface roughness (Ra) measurements
WVP = ((WVTR × y))/((p1−p2))
Water vapor transmission rate (WVTR)= weight loss per time / film area (1) (2)
Where, y: is mean film thickness; p1: is water vapor partial pressure underside of the film. p2: is water vapor partial pressure at the upper side of the film.
The 3D profiler could have been used to measure the surface roughness; however, the optical measuring method would have produced errors due to the transparency of the pure HPMC films and the B2.8-mM HPMC/SA composite films. 3D profiler was only used to observe the general profile and not to measure the surface roughness. Stylus profilometer (SE-300, Kosaka Laboratory, Ltd, Tokyo, Japan) was used to measure the 1d surface roughness. An appropriate cutoff value was selected according to ISO Standard 4288:1996 [27]. Each sample was tested three times. The mean value and standard deviation were compared.
2.8. Ball-on-disk wear tests The ball-on-disk (POD-FM406-10NT, Fu Li Fong Precision Machine, Kaohsiung, Taiwan) was used for the wear tests. An AISI52100 chrome steel ball with a diameter of 6.35 mm served as the upper test component, and the HPMC/SA composite films were placed beneath the steel ball. Relative sliding and wear were allowed to occur between the two components in contact. A sensor was used to detect the lateral force during each run, and the data were analyzed to obtain the coefficient of friction corresponding to the wear distance. The test was conducted in two groups. One group was subjected to short-distance wear tests, with a fixed wear distance of 1 m, sliding linear velocity of 0.01 m/s, and varying loads of 2 N, 5 N and 8 N. The effects of different loads on the frictional mechanism of the composite films with different SA contents were investigated. The second group was subjected to long-distance wear tests, with a fixed load of 2 N, sliding linear velocity of 0.01 m/s, and wear distance of 1, 11, and 30 m. Changes in the frictional mechanism with increasing depth of the wear marks were investigated. All samples were tested three times to verify the laboratory results.
2.6. Water contact angle measurements A contact angle meter (FTA-1000B, First Ten Angstroms, Portsmouth, UK) was used to measure the water contact angle (WCA). 4 μL of distilled water was dripped onto the sample surface. Each measurement was performed at 10 s after the initial drip, and hysteresis was not considered. Each sample was tested at least three times, and the mean and standard deviation were computed to analyze the surface hydrophilicity and hydrophobicity of the HPMC/SA composite films. 2.7. Water vapor permeability Water vapor permeability of the films was measured using the ASTM 1290-93. Glass cup with a diameter of 6.5 cm and a depth of 3.5 cm with a small platform at top to seal the films, being used to do 3
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Fig. 2. (a) ATR-FTIR analysis results from pure HPMC, HPMC/SA composite films (2.8 mM and 21.1 mM), and pure SA (21.1 mM), (b) Film thickness measurement results for pure HPMC and HPMC/SA composite films.
3. Results and discussion
controlled, which represents the stability of the process, and because the thickness of the film is sufficient, the substrate effect can be eliminated in the subsequent discussion of the coefficient of friction and wear mechanism. Fig. 3(a)–(f) demonstrated the surface profiles measured by the 3D profiler, and Fig. 3(g) presents the 1d surface roughness measured by the stylus profilometer. The surface roughness (Ra) of HPMC/SA composite films increased from 87 nm in pure HPMC to the micron scale with increasing SA content. This trend was similar to the experimental results obtained by A. Jiménez et al. [23,24]. The WCA test results from HPMC/SA composite films are shown in Fig. 4. The black circles represent the pure HPMC film, and the red triangles represent the HPMC/SA composite films. With increasing SA content, the WCA of the HPMC/SA composite films exhibited a decreasing trend. The result of the WCA test may reflect the wettability of the material surface. Smaller WCA indicate greater affinity between the drops and the material surface, suggesting increased surface wettability. On the contrary, larger angles between the material surface and the water drops indicate that the material does not easily retain the liquid, resulting in poorer wettability [34]. According to K. Kubiak, the WCA for the surface of a given material will also change with the different surface roughness [35]. Results obtained from the SEM images (Fig. 1) and the surface chemical components. The SA crystals of the composite films were encapsulated in the base material, leaving the material surface covered by HPMC. Thus, the water contact angle of the HPMC/SA composite films decreased with the increasing SA content, because of the surface geometry. The water contact angle decreased with increasing surface roughness. According to the Wenzel model, increased surface roughness would be expected to increase the equivalent contact area between the liquid drops and the material surface, thus decreasing the water contact angle [35]. In the previous research by A. Fahs., the SA molecules within the HPMC/SA composite film diffused to the film surface and formed a separation phase, thus improving the surface hydrophobicity and decreasing the surface energy [24]. However, the experimental results from this study showed that the SA molecules formed crystals and were encapsulated in the HPMC rather than diffusing to the surface. The associated effects on the surface roughness decreased the water contact angle. According to the research by A. Jarray, the long-chain HPMC molecules could become entangled with the SA, thus impeding the contact between the SA molecules and the water molecules, restricting the free motion of the SA molecules, and changing the interactions among the molecules in the solution [36].
3.1. Film characterization Fig. 1 presents the SEM images of the HPMC/SA composite films. The SA formed crystals that were encapsulated in the HPMC base material. With increasing SA concentrations in the solution, the number of crystals and size intervals marginally increased. Comparing the measured surface profiles and the 1D surface roughness, increased surface roughness may be influenced by the SA crystals close to the HPMC surface: a larger quantity of SA distributed close to the film surface would be expected to result in increased surface roughness. Fig. 2a presents the results of the ATR-FTIR analysis. The black line indicates the measurement results from the pure HPMC film. The two characteristic peaks with the strongest signals at 947 and 1059 cm−1 originated from the stretching vibration of the bonds between the pyranose rings and the ethereal CeOCe groups linking the pyranose rings in the HPMC structure. The broad band at 3450 cm−1 was caused by the stretching vibration of hydroxy and intermolecular H-bonding [28,29]. The cellulose and its derivatives enabled the material to contain much moisture from the environment because of these components’ high hydrophilicity. Increased numbers of hydroxyl groups and vibration modes caused the peaks at 1650 and 3450 cm−1 to shift and the band width to increase [30,31]. The dark blue line represents the absorption spectrum of pure SA. Several peaks between approximately 1150 and 1350 cm−1 corresponded both to the out-of-plane bending (wagging) of CH2 and the simultaneous formation of SA dimers with cis and trans structures. The peaks at 1701 and 945 cm−1 originated from carboxylic acid groups: the former was caused by the C]O stretching vibration, and the latter was caused by out-of-plane bending of OH. In addition, these results confirm that the SA molecules formed the most stable C-type crystals [32]. With increasing amounts of SA, the surface signals from B-2.8 mM (red signal) and B-21.1 mM (light blue signal) films were the same as that of the pure HPMC film, indicating that the low-depth ATR-FTIR could detect only the surface HPMC signal because the SA molecules were encapsulated in the HPMC base matrix. The measured film thicknesses are shown in Fig. 2b. The thicknesses of almost all the coating films were approximately 30 μm. The thickness increased slightly with increasing amounts of SA additive. This result was attributed to the higher solute concentration in a solution of the same volume. According to a report by K. Holmberg, during wear tests of coating films, the coating film thicknesses of different samples should be controlled to exclude the effects of the film thickness on the coefficient of friction [33]. Fig. 2b shows that the film thickness is precisely 4
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Fig. 3. Surface 3D profiles of: (a) pure HPMC, (b) 2.8 mM, (c) 5.3 mM, (d) 10.5 mM, (e) 21.1 mM, (f) 42.2 mM, and (g) surface roughness of pure HPMC and HPMC/ SA composite films.
Fig. 4. Water contact angles of HPMC and HPMC/SA composite films.
Fig. 5. Water vapor permeability of HPMC and HPMC/SA composite films.
3.2. Water vapor permeability
unsaturated fatty acids. SA are saturated fatty acids type and hydrophobic in nature. Water vapor transmission rate depends on the hydrophobicity and melting point of added fatty acids. These results are consistent with previous work for reduction of WVP. S. Kamper found that the water vapor transmission rate of blending film of
The WVP for the HPMC 606 and SA is depicted in above Fig. 5. It was decreasing gradually as the SA concentration increased, after some level it increase gradually. It was because that the saturated fatty acids were more effective in reducing WVP of the resulted films than 5
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Fig. 6. Comparison of (a) wear depth (b) average coefficient of friction of pure HPMC and HPMC/SA composite films under 2, 5, and 8 N at a wear distance of 1 m.
hydroxypropyl methylcellulose (HPMC) 606 with stearic acid was decreasing gradually as there was increase in stearic acid concentration [37]. Increase in WVP at 21.1 wt.% was due to formation of granules structure in the HPMC matrix and it leads to formation of cracks or defect inside the matrix.
3.3. Tribology performance analysis Fig. 6 presents the wear depth and the average coefficient of friction of the HPMC/SA composite films under loading of 2, 5, 8 N with wear distance of 1 m, respectively. The pure HPMC film and the B-2.8 mM composite film ruptured and became detached from the substrate under loads of 5 and 8 N with a wear distance shorter than 1 m. Thus, these two films could not be measured and are indicated with red “x” marks. When the SA content is low, the SA distributed on the surface of the coating will be less uniform, and the wear will be more severe and the larger fluctuation. When the amount of SA added exceeds 21.1 mM, the amount of SA distributed on the surface of the coating has reached saturation and leads to the stabilized in wear depth and COF. Interestingly, the wear depth and coefficient of friction of films with the same SA content under a load of 5 or 8 N marginally decreased compared with those characteristics under a load of 2 N. The wear marks on the pure HPMC under a load of 2 N at a wear distance of 1 m were approximately 1 μm deeper than those on any of the HPMC/SA composite films. According to the research by M. Sedlaček, dry friction would result in a small coefficient of friction, and only a long run-in period can guarantee a stable coefficient of friction [38]. Therefore, the HPMC composite films with a high level of roughness were inferred to have a small contact area, low coefficient of friction, and reduced wear depth in the case of a low load and short wear distance when the rough peaks on the surface were only partially removed. According to the third body theory proposed by M. Godet, velocity accommodation modes such as elastic deformation, normal breaking/ rupture, shearing, rolling, etc., would be expected to occur in the SA debris acting as the third body during wear process under stress. The SA debris could bear loads, adjust the velocity difference of the abrasive component, and impede direct contact with the abrasive component, thus decreasing the coefficient of friction and wear [39,40]. Thus, the HPMC/SA composite films with a large amount of SA distributed in the wear marks showed better frictional properties than the films with a small amount of SA. As a result, the coefficient of friction of a film with a large amount of SA under a load of 5 or 8 N might be slightly lower than that under a load of 2 N, if all other parameters are the same. Fig. 7 presents the coefficient of friction of the HPMC/SA composite films under a load of 2 N with a wear distance of 30 m. The pure HPMC
Fig. 7. Coefficient of friction comparison between pure HPMC and HPMC/SA composite films.
and B-2.8 mM films ruptured and became detached from the silicon substrate when they were abraded at distances of less than 6 m. The coefficient of friction of the B-5.3 mM film increased marginally when the B-5.3 mM film was abraded at a distance of 21 m, and local ruptures were inferred to occur at that distance. The coefficient of friction did not increase continuously when the wear distance was between 21 m and 30 m. Soft HPMC and SA wear debris residue may remain at the film rupture site and could still provide some lubricating effects. The B10.5 mM, B-21.1 mM, and B-42.2 mM films had smaller coefficients of friction and longer film life compared with the pure HPMC and the composite films with low SA content. The films with higher SA contents maintained stable coefficients of friction even when the wear distance reached 30 m. 3.4. Wear mechanism of HPMC/SA composites in macro-scale As shown in Fig. 8, the wear marks of the B-21.1 mM film under a load of 2 N were observed at distances of 1, 11, and 30 m. The morphology of the wear marks at a wear distance of 1 m was the same as that mentioned in the above paragraphs and in the initial stage of wear. The rough peaks of HPMC were ground flat to produce relatively smooth wear marks. The morphology of the wear marks at a wear distance of 11 m were similar to the morphology of the wear marks at a wear distance of 1 m under loads of 5 and 8 N. A great deal of deformed SA debris was observed under a high-power SEM. The SA debris of the wear marks at a wear distance of 30 m were more fragmented and smaller than the debris associated with wear marks at a wear distance 6
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Fig. 8. Wear marks on B-21.1 mM under a load of 2 N at a wear distance of: (a) 1 m, 100x magnification; (b) 1 m, 3000x magnification; (c)11 m, 100x magnification; (d) 11 m, 3000x magnification; (e) 30 m, 100x magnification; and (f) 30 m, 3000x magnification.
roughness from the nanoscale of pure HPMC to the micrometer scale. In addition, the thickness of the HPMC/SA composite film is approximately 30 μm. 2 The water contact angle of the HPMC/SA composite film decreases with increased SA content under the effects of surface roughness, and HPMC remains the main surface component. 3 According to the result of the wear test, the HPMC/SA composite films have larger coefficients of friction and longer anti-wear abilities than the pure HPMC film, indicating that the addition of SA can effectively improve the macroscale frictional properties of HPMC. 4 Based on the wear depths under different loads (2, 5, and 8 N) with a wear distance of 1 m, and the wear mark morphology under a load of 2 N with different wear distances (1, 11, and 30 m), the rough peaks of the HPMC surface are gradually removed upon contact with the metal abrasive in the initial wear stage of the HPMC/SA composite films. With increased wear distance, the chance of contact between the SA crystals distributed within the HPMC base material and the abrasive component increases. These crystals act as a third body and provide lubrication, stabilizing the wear after debris deformation under stress.
of 11 m. This result may be due to the repeated wear of debris by the abrasive component during the test. Based on those results, we suggested a wear mechanism of the HPMC/SA composite films containing a large amount of SA additive. In the initial stage of wear, the rough peaks of the material surface are in contact with the metal ball and are gradually being removed. With increased wear depth, the chance of contact between the SA crystals distributed in the HPMC base material and the abrasive component increases. The presence of sufficient SA debris to act as a third body stabilizes the coefficient of friction and substantially decreases wear. 4. Conclusions This research investigated the wear mechanism by which SA additives in HPMC/SA composite films affect the frictional properties of the material. We mixed SA/ethanol solution and pure HPMC aqueous solution to form HPMC/SA composite films using the evaporation solvent method and applied the films to silicon substrates. Based on the experimental results, including ATR-FTIR analysis, surface geometry, water contact angle, and macroscale frictional properties, etc., the following conclusions were drawn.
Acknowledgments
1 SA molecules form crystals, and the crystals are distributed within the composite film. The quantity of distributed crystals increases significantly with increased SA concentration. The increased quantity of SA crystals close to the film surface increases the surface
The authors gratefully acknowledge the financial support for this project from the Ministry of Science and Technology in Taiwan (MOST 106-2221-E-006-092-MY3). The authors also thank the Center for 7
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Micro/Nano Science and Technology and Instrumentation, National Cheng Kung University (NCKU), for technical support.
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properties of starch/biodegradable polyester blends, J. Polym. Sci. Part B: Polym. Phys. 39 (2001) 920–930. M.L. Free, Introduction to Surfactants, Surfactants in Tribology Vol. 1 CRC Press, 2008. M. He, M. Xu, L. Zhang, Controllable stearic acid crystal induced high hydrophobicity on cellulose film surface, ACS Appl. Mater. Interfaces 5 (2013) 585–591. N. Garoff, S. Zauscher, The influence of fatty acids and humidity on friction and adhesion of hydrophilic polymer surfaces, Langmuir 18 (2002) 6921–6927. R.D. Hagenmaier, P.E. Shaw, Moisture permeability of edible films made with fatty acid and hydroxypropyl methyl cellulose, J. Agric. Food Chem. 38 (1990) 1799–1803. A. Jiménez, M. Fabra, P. Talens, A. Chiralt, Effect of lipid self-association on the microstructure and physical properties of hydroxypropyl-methylcellulose edible films containing fatty acids, Carbohydr. Polym. 82 (2010) 585–593. A. Fahs, M. Brogly, S. Bistac, M. Schmitt, Hydroxypropyl methylcellulose (HPMC) formulated films: relevance to adhesion and friction surface properties, Carbohydr. Polym. 80 (2010) 105–114. B. Briscoe, Wear of polymers: an essay on fundamental aspects, Tribol. Int. 14 (1981) 231–243. N. Myshkin, M. Petrokovets, A. Kovalev, Tribology of polymers: adhesion, friction, wear, and mass-transfer, Tribol. Int. 38 (2006) 910–921. R. Leach, H. Haitjema, Bandwidth characteristics and comparisons of surface texture measuring instruments, Meas. Sci. Technol. 21 (2010) 032001. W. Jablonska, The Use of ATR-FTIR to Probe the Release Mechanism From Hydrophilic Matrices, Sheffield Hallam University, 2011. S. Sahoo, C.K. Chakraborti, S.C. Mishra, S. Naik, Analytical characterization of a gelling biodegradable polymer, Drug Invention Today (2011) DOI. G. Velazquez, A. Herrera-Gómez, M. Martı́n-Polo, Identification of bound water through infrared spectroscopy in methylcellulose, J. Food Eng. 59 (2003) 79–84. C. Liang, R. Marchessault, Infrared spectra of crystalline polysaccharides. II. Native celluloses in the region from 640 to 1700 cm− 1, J. Polym. Sci. 39 (1959) 269–278. F. Kimura, J. Umemura, T. Takenaka, FTIR-ATR studies on Langmuir-Blodgett films of stearic acid with 1-9 monolayers, Langmuir 2 (1986) 96–101. K. Holmberg, A. Mathews, Coatings tribology: a concept, critical aspects and future directions, Thin Solid Films 253 (1994) 173–178. C. Piao, J.E. Winandy, T.F. Shupe, From hydrophilicity to hydrophobicity: a critical review: part I. Wettability and surface behavior, Wood and Fiber Science 42 (2010) 490–510. K. Kubiak, M. Wilson, T. Mathia, P. Carval, Wettability versus roughness of engineering surfaces, Wear 271 (2011) 523–528. A. Jarray, V. Gerbaud, M. Hemati, Stearic acid crystals stabilization in aqueous polymeric dispersions, Chem. Eng. Res. Des. 110 (2016) 220–232. S. Kamper, O. Fennema, Water vapor permeability of an edible, fatty acid, bilayer film, J. Food Sci. 49 (1984) 1482–1485. M. Sedlaček, B. Podgornik, J. Vižintin, Influence of surface preparation on roughness parameters, friction and wear, Wear 266 (2009) 482–487. Y. Berthier, M. Godet, M. Brendle, Velocity accommodation in friction, Tribol. Trans. 32 (1989) 490–496. M. Godet, Third-bodies in tribology, Wear 136 (1990) 29–45.
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Preparation and tribological studies of stearic acid-modified biopolymer coating Shih-Chen Shi*, Yao-Qing Peng Department of Mechanical Engineering, National Cheng Kung University (NCKU), Tainan, Taiwan
A R T I C LE I N FO
A B S T R A C T
Keywords: Friction-reducing coatings Wear mechanism Solid lubricant additives Self-lubricating composites Biopolymer Tribology
Adding fatty acids to cellulose derivatives can effectively improve the hydrophobicity and surface energy of materials and decrease the microscale coefficient of friction. In this research, stearic acid (SA) and the hydroxypropyl methylcellulose (HPMC) solution were mixed to prepare HPMC/SA composite films. The SA molecules formed crystals distributed within the HPMC film, thus increasing the surface roughness and surface wettability. The intermolecular interaction between HPMC and SA causes the formation of micelles. SEM images of the wear marks indicate that SA debris generated during wear as a third body. Third-body layer provide load capacity during wear and reduces direct contact between the grinding object and the HPMC coating, reducing friction coefficient and wear effectively. It is considered the dominate wear mechanism of HPMC/SA composite under macroscopic wear.
1. Introduction Hydroxypropyl methylcellulose (HPMC) exhibits a film-forming property and high pH stability [1] without biological toxicity [2] and causes no effects on the development of aquatic organisms at 0.5 wt.% [3]. HPMC can form transparent films with excellent mechanical properties after solvent evaporation and is one of the main materials used in capsule shells [4] and edible coatings [5]. Several studies shown that HPMC exhibits excellent tribological properties. P. A. Simmons found that HPMC molecules can be adsorbed onto the surfaces of several types of gel lens and form a thick and durable liquid layer, thus providing moisturizing and lubricating effects and further reducing users’ discomfort [6]. S. C. Shi prepared HPMC coatings with controlled film thicknesses for wear tests [7]. HPMC coating, as sacrificial layers, was adhered onto chrome steel balls and formed a transfer layer, thus avoiding direct contact between the metal and substrate and reducing the wear and friction coefficients [7,8]. T. F. Huang demonstrated the self-healing properties of HPMC in the condition of directly replenish HPMC solution on damaged coating, or in the environment with high temperature and high humidity [9,10]. J. Y. Wu prepared HPMC/MoS2 composite films. MoS2 and HPMC formed a transfer layer that provided a self-lubricating effect, thus improving the lubrication and wear resistance capacity of the HPMC film and enabling the films to accommodate high loads and high sliding velocities [11–13].
⁎
Considering these previous studies, the HPMC film has the potential to become a substitute for artificial polymers and packaging materials [14,15]. However, as a cellulose derivative, the hydrophilicity characteristic of the HPMC film may deteriorate its mechanical properties and even its frictional properties in applications where condensed water is expected to precipitate on the surface, which is a substantial limitation [16–18]. To address this limitation, stearic acid (SA) is a saturated fatty acid comprising an 18-carbon-long chain on the hydrophobic end and a carboxyl on the hydrophilic end [19]. SA is amphipathic and active at an interface. SA has been used by several research teams to improve the susceptibility of hydrophilic materials to moisture. Recently, L. Zhang immersed a cellulose film in a SA solution and hot-pressed the sample such that the SA on the surface formed flaky crystals after being dried; the resulting cellulose/SA composite film had a super-hydrophobic surface that reduced the water absorption under high-humidity environment and successfully slowed the deterioration of the mechanical properties [20]. N. Garoff used fatty acids with different carbon numbers to form a self-assembled monolayer (SAM) coating on the cellulose surface; their results demonstrated that the SAM coating formed by saturated fatty acids with more than 16 carbons, including SA, could effectively improve surface hydrophobicity and reduce the microscale coefficient of friction [21]. Several research reports have demonstrated that HPMC/SA composites formed by adding SA molecules to aqueous HPMC solutions can
Corresponding author. E-mail address: [email protected] (S.-C. Shi).
https://doi.org/10.1016/j.porgcoat.2019.105304 Received 12 April 2019; Received in revised form 28 August 2019; Accepted 1 September 2019 0300-9440/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Shih-Chen Shi and Yao-Qing Peng, Progress in Organic Coatings, https://doi.org/10.1016/j.porgcoat.2019.105304
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Table 1 Parameters of HPMC and SA solutions. Solution
Pure HPMC SA/ethanol (2.8 mM) SA/ethanol (5.3 mM) SA/ethanol (10.5 mM) SA/ethanol (21.1 mM) SA/ethanol (42.2 mM)
Solute
Solvent
HPMC powder (g)
SA powder (g)
Deionized water (ml)
Ethanol (ml)
3 0 0 0 0 0
0 0.08 0.15 0.3 0.6 1.2
100 0 0 0 0 0
0 100 100 100 100 100
2. Materials and methods
effectively improve the hydrophobicity of bulk material, surface hydrophobicity and surface energy of the HPMC film, enhancing the film’s anti-moisture properties on macroscopic and microscopic scales and strengthening the film’s tribology capacity [22,23]. R.D. Hagenmaier prepared HPMC/SA composite films for water vapor permeability tests using a solvent evaporation method in which SA molecules were added at different percentages to the HPMC solution; the composite film containing 45 wt.% SA had the lowest volume of water vapor permeation, i.e., the best moisture insulation capacity [22]. In addition, the surface of the film formed by the HPMC/SA solution with a high content of SA was covered by SA crystals that had more significant effects on the moisture resistance capacity than the SA molecules distributed within the HPMC film [22]. A. Jiménez added fatty acid molecules with different carbon numbers and saturation levels to aqueous HPMC solutions to prepare films following methods similar to those used by R. D. Hagenmaier group. A. Jiménez showed that the size of colloids formed by the SA molecules in the aqueous solution is associated with the microstructure organization of composite films after evaporation and may also influence the surface roughness, transparency, mechanical properties, and water vapor permeation [23]. A. Fahs prepared films by mixing aqueous HPMC and SA/ethanol solutions at a concentration of 0–1 wt.%, much lower than the concentration of Hagenmaier’s group used; based on contact angle, atomic force microscopy, ball-on-disk, and other results, the migration of SA molecules to the HPMC surface substantially reduced the surface roughness, capillary acting force, and surface energy, and decreased the microscale and macroscale coefficient of friction to 0.1 [24]. A. Fahs studied the tribological properties of HPMC/SA composite films, investigating only the surface energy effects via a macroscale wear test with a short wear distance of 100 mm, probably to avoid significant effects on the coefficient of friction due to increased wear mark depth. However, in addition to the surface energy, macroscale frictional properties are associated with surface roughness, mechanical properties, abrasive particle characteristics, and third body effects among the frictional interfaces during the wear process, as well as the generation and development of the transfer layers and tribo-chemical reactions induced by friction [25,26]. Surface condition including surface roughness, surface energy, water vapor permeability (WVP) and surface structure of the film and the formation of the three-body layer, are important factors that affecting the tribology properties of coating. Therefore, for further understanding the tribological beahvior and wear mechanism of the coating, it is necessary to prepare a composite coating which conduct a longer distance wear test. In this research, HPMC/SA composite coatings were prepared. Surface morphology was observed by SEM and 3D profiling; surface hydrophobicity was measured with a contact angle meter; and surface roughness was measured with a stylus roughness meter. A ball-on-disk wear test was conducted with a chrome steel ball as the abrasive component, and the wear marks were analyzed, with the expectation that the state of the current contact area during wear and frictional mechanisms could be inferred based on the results, thus providing a reference for other researchers interested in strengthening the frictional properties of environmentally-friendly materials using SA.
Silicon substrate (Summit-Tech, Hsinchu, Taiwan) with surface roughness (Ra) of 30 nm was cut into 16.5 × 16.5 mm2, and sequentially washed with methyl alcohol, acetone, isopropanol, and deionized water to remove contaminants on the substrate, thus avoiding a decrease in adhesive force between the coating film and the substrate. HPMC powder (PHARMACOAT® 606, Shin-Etsu Chemical Co., Ltd, Tokyo, Japan) and SA powder (99% purity, Pharma-Up Enterprise Co., Ltd., Taipei, Taiwan) were used in the experiment. 2.1. Film formulation HPMC solution was prepared by adding 3 g of HPMC powder into the heated deionized water (100 mL, 80 °C), then stirred slowly until powder completely dissolved. SA solution was prepared by adding 0.08–1.2 g SA powder into 95% ethanol (100 mL) as shown in Table 1. 10 mL of SA/ethanol solution was added to 100 mL of HPMC solution and stirred for 10 min. The generation of micelles caused the HPMC/SA blending solution become white and turbid. 300 μL of the blended solution was pipetted and applied onto the silicon substrate. Sample was placed in a controlled environment chamber (DE-60, Denyng Instruments, New Taipei City, Taiwan) at 30℃and RH40% and kept for 6 h. All test samples were stored in a dry cabinet (AD-88S, Eureka Auto Dry Box, Taoyuan, Taiwan) at 25℃ and RH30% until the experiment started. 2.2. SEM imaging SEM (XL-40FEG, Philips, Amsterdam, Netherlands & SU-5000, Hitachi, Tokyo, Japan) imaging was performed to observe the surface and the silicon profile in the vicinity of the wear marks on the HPMC/ SA composite films. Because both HPMC and SA are non-conductive, all samples were plated with 10 nm gold coating. 2.3. Attenuated total reflectance (ATR)-Fourier transform infrared spectroscopy (FTIR) analysis FTIR (Thermo Nicolet NEXUS 470, Golden Valley, MN, USA) in ATR mode was used with an incident angle larger than the critical angle to allow total reflection of the infrared ray to occur continuously between the sample surface and the crystals. The light was attenuated by multiple reflections, and the attenuated spectrum was analyzed to investigate the structural vibration behavior of the material’s surface molecules and obtain the frequency spectrum. The absorption signal peak was utilized to determine the presence of various functional groups and bonds on the surface and further identify the surface distribution of the HPMC and SA constituents of the composite film. Each test sample was measured three times. 2.4. 3D profiler measurements A 3D profiler (VK9700, Keyence, Osaka, Japan) was used to 2
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Fig. 1. SEM images of HPMC/SA composites: (a) B-2.8 mM, (b) B-5.3 mM, (c) B-21.1 mM, and (d) B-42.2 mM (5000x magnification).
measure the surface profile, wear scar profile, and film thickness of the HPMC/SA composite films. Half of the coating on the silicon was peeled off. Subsequently, the 3D profiler was employed to scan the boundary between the two area. After repeating 5 times, the average value of the film thickness was obtained. From the cross section of the resulting profile, the height difference beyond the area that was plastically deformed under stress was measured to obtain the local film thickness. For each sample, three different sites were measured, and the mean value was considered as the overall film thickness.
water vapor permeability experiment for the prepared films. Films were cut into discs with a diameter slightly larger than the diameter of the cup. Distilled water (10 ml) was dispensed into the cup, then it was covered with the film and sealed with the ring. The cup was weighed with their contents and placed inside the environment chamber provided with inside temperature of 30oC and ± 40 RH. Cups were weighed in every 8 h to get minimum 4 data to plot. WVP of the film was calculated using following equations.
2.5. Surface roughness (Ra) measurements
WVP = ((WVTR × y))/((p1−p2))
Water vapor transmission rate (WVTR)= weight loss per time / film area (1) (2)
Where, y: is mean film thickness; p1: is water vapor partial pressure underside of the film. p2: is water vapor partial pressure at the upper side of the film.
The 3D profiler could have been used to measure the surface roughness; however, the optical measuring method would have produced errors due to the transparency of the pure HPMC films and the B2.8-mM HPMC/SA composite films. 3D profiler was only used to observe the general profile and not to measure the surface roughness. Stylus profilometer (SE-300, Kosaka Laboratory, Ltd, Tokyo, Japan) was used to measure the 1d surface roughness. An appropriate cutoff value was selected according to ISO Standard 4288:1996 [27]. Each sample was tested three times. The mean value and standard deviation were compared.
2.8. Ball-on-disk wear tests The ball-on-disk (POD-FM406-10NT, Fu Li Fong Precision Machine, Kaohsiung, Taiwan) was used for the wear tests. An AISI52100 chrome steel ball with a diameter of 6.35 mm served as the upper test component, and the HPMC/SA composite films were placed beneath the steel ball. Relative sliding and wear were allowed to occur between the two components in contact. A sensor was used to detect the lateral force during each run, and the data were analyzed to obtain the coefficient of friction corresponding to the wear distance. The test was conducted in two groups. One group was subjected to short-distance wear tests, with a fixed wear distance of 1 m, sliding linear velocity of 0.01 m/s, and varying loads of 2 N, 5 N and 8 N. The effects of different loads on the frictional mechanism of the composite films with different SA contents were investigated. The second group was subjected to long-distance wear tests, with a fixed load of 2 N, sliding linear velocity of 0.01 m/s, and wear distance of 1, 11, and 30 m. Changes in the frictional mechanism with increasing depth of the wear marks were investigated. All samples were tested three times to verify the laboratory results.
2.6. Water contact angle measurements A contact angle meter (FTA-1000B, First Ten Angstroms, Portsmouth, UK) was used to measure the water contact angle (WCA). 4 μL of distilled water was dripped onto the sample surface. Each measurement was performed at 10 s after the initial drip, and hysteresis was not considered. Each sample was tested at least three times, and the mean and standard deviation were computed to analyze the surface hydrophilicity and hydrophobicity of the HPMC/SA composite films. 2.7. Water vapor permeability Water vapor permeability of the films was measured using the ASTM 1290-93. Glass cup with a diameter of 6.5 cm and a depth of 3.5 cm with a small platform at top to seal the films, being used to do 3
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Fig. 2. (a) ATR-FTIR analysis results from pure HPMC, HPMC/SA composite films (2.8 mM and 21.1 mM), and pure SA (21.1 mM), (b) Film thickness measurement results for pure HPMC and HPMC/SA composite films.
3. Results and discussion
controlled, which represents the stability of the process, and because the thickness of the film is sufficient, the substrate effect can be eliminated in the subsequent discussion of the coefficient of friction and wear mechanism. Fig. 3(a)–(f) demonstrated the surface profiles measured by the 3D profiler, and Fig. 3(g) presents the 1d surface roughness measured by the stylus profilometer. The surface roughness (Ra) of HPMC/SA composite films increased from 87 nm in pure HPMC to the micron scale with increasing SA content. This trend was similar to the experimental results obtained by A. Jiménez et al. [23,24]. The WCA test results from HPMC/SA composite films are shown in Fig. 4. The black circles represent the pure HPMC film, and the red triangles represent the HPMC/SA composite films. With increasing SA content, the WCA of the HPMC/SA composite films exhibited a decreasing trend. The result of the WCA test may reflect the wettability of the material surface. Smaller WCA indicate greater affinity between the drops and the material surface, suggesting increased surface wettability. On the contrary, larger angles between the material surface and the water drops indicate that the material does not easily retain the liquid, resulting in poorer wettability [34]. According to K. Kubiak, the WCA for the surface of a given material will also change with the different surface roughness [35]. Results obtained from the SEM images (Fig. 1) and the surface chemical components. The SA crystals of the composite films were encapsulated in the base material, leaving the material surface covered by HPMC. Thus, the water contact angle of the HPMC/SA composite films decreased with the increasing SA content, because of the surface geometry. The water contact angle decreased with increasing surface roughness. According to the Wenzel model, increased surface roughness would be expected to increase the equivalent contact area between the liquid drops and the material surface, thus decreasing the water contact angle [35]. In the previous research by A. Fahs., the SA molecules within the HPMC/SA composite film diffused to the film surface and formed a separation phase, thus improving the surface hydrophobicity and decreasing the surface energy [24]. However, the experimental results from this study showed that the SA molecules formed crystals and were encapsulated in the HPMC rather than diffusing to the surface. The associated effects on the surface roughness decreased the water contact angle. According to the research by A. Jarray, the long-chain HPMC molecules could become entangled with the SA, thus impeding the contact between the SA molecules and the water molecules, restricting the free motion of the SA molecules, and changing the interactions among the molecules in the solution [36].
3.1. Film characterization Fig. 1 presents the SEM images of the HPMC/SA composite films. The SA formed crystals that were encapsulated in the HPMC base material. With increasing SA concentrations in the solution, the number of crystals and size intervals marginally increased. Comparing the measured surface profiles and the 1D surface roughness, increased surface roughness may be influenced by the SA crystals close to the HPMC surface: a larger quantity of SA distributed close to the film surface would be expected to result in increased surface roughness. Fig. 2a presents the results of the ATR-FTIR analysis. The black line indicates the measurement results from the pure HPMC film. The two characteristic peaks with the strongest signals at 947 and 1059 cm−1 originated from the stretching vibration of the bonds between the pyranose rings and the ethereal CeOCe groups linking the pyranose rings in the HPMC structure. The broad band at 3450 cm−1 was caused by the stretching vibration of hydroxy and intermolecular H-bonding [28,29]. The cellulose and its derivatives enabled the material to contain much moisture from the environment because of these components’ high hydrophilicity. Increased numbers of hydroxyl groups and vibration modes caused the peaks at 1650 and 3450 cm−1 to shift and the band width to increase [30,31]. The dark blue line represents the absorption spectrum of pure SA. Several peaks between approximately 1150 and 1350 cm−1 corresponded both to the out-of-plane bending (wagging) of CH2 and the simultaneous formation of SA dimers with cis and trans structures. The peaks at 1701 and 945 cm−1 originated from carboxylic acid groups: the former was caused by the C]O stretching vibration, and the latter was caused by out-of-plane bending of OH. In addition, these results confirm that the SA molecules formed the most stable C-type crystals [32]. With increasing amounts of SA, the surface signals from B-2.8 mM (red signal) and B-21.1 mM (light blue signal) films were the same as that of the pure HPMC film, indicating that the low-depth ATR-FTIR could detect only the surface HPMC signal because the SA molecules were encapsulated in the HPMC base matrix. The measured film thicknesses are shown in Fig. 2b. The thicknesses of almost all the coating films were approximately 30 μm. The thickness increased slightly with increasing amounts of SA additive. This result was attributed to the higher solute concentration in a solution of the same volume. According to a report by K. Holmberg, during wear tests of coating films, the coating film thicknesses of different samples should be controlled to exclude the effects of the film thickness on the coefficient of friction [33]. Fig. 2b shows that the film thickness is precisely 4
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Fig. 3. Surface 3D profiles of: (a) pure HPMC, (b) 2.8 mM, (c) 5.3 mM, (d) 10.5 mM, (e) 21.1 mM, (f) 42.2 mM, and (g) surface roughness of pure HPMC and HPMC/ SA composite films.
Fig. 4. Water contact angles of HPMC and HPMC/SA composite films.
Fig. 5. Water vapor permeability of HPMC and HPMC/SA composite films.
3.2. Water vapor permeability
unsaturated fatty acids. SA are saturated fatty acids type and hydrophobic in nature. Water vapor transmission rate depends on the hydrophobicity and melting point of added fatty acids. These results are consistent with previous work for reduction of WVP. S. Kamper found that the water vapor transmission rate of blending film of
The WVP for the HPMC 606 and SA is depicted in above Fig. 5. It was decreasing gradually as the SA concentration increased, after some level it increase gradually. It was because that the saturated fatty acids were more effective in reducing WVP of the resulted films than 5
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Fig. 6. Comparison of (a) wear depth (b) average coefficient of friction of pure HPMC and HPMC/SA composite films under 2, 5, and 8 N at a wear distance of 1 m.
hydroxypropyl methylcellulose (HPMC) 606 with stearic acid was decreasing gradually as there was increase in stearic acid concentration [37]. Increase in WVP at 21.1 wt.% was due to formation of granules structure in the HPMC matrix and it leads to formation of cracks or defect inside the matrix.
3.3. Tribology performance analysis Fig. 6 presents the wear depth and the average coefficient of friction of the HPMC/SA composite films under loading of 2, 5, 8 N with wear distance of 1 m, respectively. The pure HPMC film and the B-2.8 mM composite film ruptured and became detached from the substrate under loads of 5 and 8 N with a wear distance shorter than 1 m. Thus, these two films could not be measured and are indicated with red “x” marks. When the SA content is low, the SA distributed on the surface of the coating will be less uniform, and the wear will be more severe and the larger fluctuation. When the amount of SA added exceeds 21.1 mM, the amount of SA distributed on the surface of the coating has reached saturation and leads to the stabilized in wear depth and COF. Interestingly, the wear depth and coefficient of friction of films with the same SA content under a load of 5 or 8 N marginally decreased compared with those characteristics under a load of 2 N. The wear marks on the pure HPMC under a load of 2 N at a wear distance of 1 m were approximately 1 μm deeper than those on any of the HPMC/SA composite films. According to the research by M. Sedlaček, dry friction would result in a small coefficient of friction, and only a long run-in period can guarantee a stable coefficient of friction [38]. Therefore, the HPMC composite films with a high level of roughness were inferred to have a small contact area, low coefficient of friction, and reduced wear depth in the case of a low load and short wear distance when the rough peaks on the surface were only partially removed. According to the third body theory proposed by M. Godet, velocity accommodation modes such as elastic deformation, normal breaking/ rupture, shearing, rolling, etc., would be expected to occur in the SA debris acting as the third body during wear process under stress. The SA debris could bear loads, adjust the velocity difference of the abrasive component, and impede direct contact with the abrasive component, thus decreasing the coefficient of friction and wear [39,40]. Thus, the HPMC/SA composite films with a large amount of SA distributed in the wear marks showed better frictional properties than the films with a small amount of SA. As a result, the coefficient of friction of a film with a large amount of SA under a load of 5 or 8 N might be slightly lower than that under a load of 2 N, if all other parameters are the same. Fig. 7 presents the coefficient of friction of the HPMC/SA composite films under a load of 2 N with a wear distance of 30 m. The pure HPMC
Fig. 7. Coefficient of friction comparison between pure HPMC and HPMC/SA composite films.
and B-2.8 mM films ruptured and became detached from the silicon substrate when they were abraded at distances of less than 6 m. The coefficient of friction of the B-5.3 mM film increased marginally when the B-5.3 mM film was abraded at a distance of 21 m, and local ruptures were inferred to occur at that distance. The coefficient of friction did not increase continuously when the wear distance was between 21 m and 30 m. Soft HPMC and SA wear debris residue may remain at the film rupture site and could still provide some lubricating effects. The B10.5 mM, B-21.1 mM, and B-42.2 mM films had smaller coefficients of friction and longer film life compared with the pure HPMC and the composite films with low SA content. The films with higher SA contents maintained stable coefficients of friction even when the wear distance reached 30 m. 3.4. Wear mechanism of HPMC/SA composites in macro-scale As shown in Fig. 8, the wear marks of the B-21.1 mM film under a load of 2 N were observed at distances of 1, 11, and 30 m. The morphology of the wear marks at a wear distance of 1 m was the same as that mentioned in the above paragraphs and in the initial stage of wear. The rough peaks of HPMC were ground flat to produce relatively smooth wear marks. The morphology of the wear marks at a wear distance of 11 m were similar to the morphology of the wear marks at a wear distance of 1 m under loads of 5 and 8 N. A great deal of deformed SA debris was observed under a high-power SEM. The SA debris of the wear marks at a wear distance of 30 m were more fragmented and smaller than the debris associated with wear marks at a wear distance 6
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Fig. 8. Wear marks on B-21.1 mM under a load of 2 N at a wear distance of: (a) 1 m, 100x magnification; (b) 1 m, 3000x magnification; (c)11 m, 100x magnification; (d) 11 m, 3000x magnification; (e) 30 m, 100x magnification; and (f) 30 m, 3000x magnification.
roughness from the nanoscale of pure HPMC to the micrometer scale. In addition, the thickness of the HPMC/SA composite film is approximately 30 μm. 2 The water contact angle of the HPMC/SA composite film decreases with increased SA content under the effects of surface roughness, and HPMC remains the main surface component. 3 According to the result of the wear test, the HPMC/SA composite films have larger coefficients of friction and longer anti-wear abilities than the pure HPMC film, indicating that the addition of SA can effectively improve the macroscale frictional properties of HPMC. 4 Based on the wear depths under different loads (2, 5, and 8 N) with a wear distance of 1 m, and the wear mark morphology under a load of 2 N with different wear distances (1, 11, and 30 m), the rough peaks of the HPMC surface are gradually removed upon contact with the metal abrasive in the initial wear stage of the HPMC/SA composite films. With increased wear distance, the chance of contact between the SA crystals distributed within the HPMC base material and the abrasive component increases. These crystals act as a third body and provide lubrication, stabilizing the wear after debris deformation under stress.
of 11 m. This result may be due to the repeated wear of debris by the abrasive component during the test. Based on those results, we suggested a wear mechanism of the HPMC/SA composite films containing a large amount of SA additive. In the initial stage of wear, the rough peaks of the material surface are in contact with the metal ball and are gradually being removed. With increased wear depth, the chance of contact between the SA crystals distributed in the HPMC base material and the abrasive component increases. The presence of sufficient SA debris to act as a third body stabilizes the coefficient of friction and substantially decreases wear. 4. Conclusions This research investigated the wear mechanism by which SA additives in HPMC/SA composite films affect the frictional properties of the material. We mixed SA/ethanol solution and pure HPMC aqueous solution to form HPMC/SA composite films using the evaporation solvent method and applied the films to silicon substrates. Based on the experimental results, including ATR-FTIR analysis, surface geometry, water contact angle, and macroscale frictional properties, etc., the following conclusions were drawn.
Acknowledgments
1 SA molecules form crystals, and the crystals are distributed within the composite film. The quantity of distributed crystals increases significantly with increased SA concentration. The increased quantity of SA crystals close to the film surface increases the surface
The authors gratefully acknowledge the financial support for this project from the Ministry of Science and Technology in Taiwan (MOST 106-2221-E-006-092-MY3). The authors also thank the Center for 7
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Micro/Nano Science and Technology and Instrumentation, National Cheng Kung University (NCKU), for technical support.
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