Morphology and Adhesive Force of Natural Rubber Latex Films by

Chapter 17

Morphology and Adhesive Force of Natural Rubber Latex Films by Atomic Force Microscopy 1,2

C. C. Ho and M . C. Khew

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Department of Chemistry, University of Malaya, 50603 Kuala Lumpur, Malaysia Current address: Omnigrace (Thailand) Ltd., Tambon Banpru, Hatyai, Songkhla 90250, Thailand

Downloaded by UNIV OF BATH on October 30, 2014 | http://pubs.acs.org Publication Date: November 6, 2001 | doi: 10.1021/bk-2002-0801.ch017

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Commercial natural rubber latex concentrate is used almost exclusively in the production of rubber-dipped goods, the bulk of which is in the form of gloves. When a 'former' is dipped into the latex, a latex film is formed after the water evaporates and the particles come into contact and fuse together. Any factors that hinder good film formation of the latex would adversely affect the film morphology and hence its application as a barrier material. In view of its low glass transition temperature, natural rubber (NR) latex film is soft and tacky and cannot be used as a glove in its native form. Vulcanization and chemical modification by chlorination of the film surface are additional process steps in glove manufacturing that enable N R latex film to become finally donnable. Basically these processes reduce the surface friction and improve the lubricity of the film surface. A clear understanding of the nature of the surface of the latex film in relation to its performance is pivotal in ensuring product quality. The adhesive force of various N R latex films such as powdered, vulcanized, chlorinated, and polymer-coated was determined from the force-distance curves using atomic force microscopy (AFM)

© 2002 American Chemical Society

In Polymer Colloids; Daniels, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

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and compared with those of synthetic polymers and commercial glove samples. Distinctive adhesive forces between the A F M probe tip and these materials allow them to be distinguished from one another. Results indicate that the magnitude of the pull-off force is strongly influenced by surface inhomogeneity and topography of the film. The behavior of the pull-off forces agrees with interpretation based on current understanding of this phenomenon. The technique is well-suited for assessing the structure-performance relation of gloves.

Natural rubber (NR) latex, like its counterpart synthetic latex, is a stable colloidal dispersion containing dispersed latex particles in an aqueous phase. Fresh N R latex from the Hevea brasilensis tree contains about 4-5% of nonrubber, made up by proteins (1.0%), carbohydrates (1.0%) and lipids (1.6%) (7). The fresh latex, containing only 30 - 35 % rubber, putrefies within hours after leaving the tree and auto-coagulates. Technically-specified commercial latex concentrate, at a solids content of ca. 60% rubber, is produced from the fresh latex by preservation and centrifugation. The latex particles in commercial N R latex concentrate are stabilized by an adsorbed layer of mainly long-chain fatty acid soaps (the hydrolysis products of phospholipids), polypeptides, and proteins (2) . Commercial ammoniated N R latex concentrate finds extensive use in the production of rubber-dipped goods, mainly in the manufacture of medical examination gloves. The necessary condition of such application is the formation of a continuous film with the appropriate mechanical strength. A n inherent difference between the synthetic and N R latexes is that N R latex contains a host of non-rubbers that originate from the tree as mentioned above. The bulk of these non-rubber materials are removed by centrifugation during the concentration process in the production of commercial latex concentrates. Some residue plant proteins and lipid materials are, however, tenaciously bound to the rubber particles and become incorporated into the film when the latex dries. Thus, in the manufacturing of N R rubber gloves, it is necessary to extract the latex film by leaching it with water at an elevated temperature to remove the remaining residue proteins and processing chemicals (3) . The actual amount of residual non-rubbers remaining after leaching really depends on the vigor of the leaching process employed. The evaporation of water during drying of the film has aided the migration of these non-rubbers to the latex film surface. Our recent work (4) on the visualization of the latex film surface by atomic force microscopy (AFM) has shown very clearly the exudation and accumulation of these materials at the film surface during the film formation



241 process. Some of the lipids are known to be natural accelerators and natural antioxidants. They are beneficial to the final products. However, rubber proteins may cause an allergic reaction in certain sensitized users. The actual amount of residual proteins on the surface after leaching may be minute (non-rubbers in the original commercial latex concentrate are ca 1.0 % (5)), and therefore difficult to quantify as such, but their effect on the quality of the final product is significant and cannot be easily ignored. Various leaching steps are thus essential stages in glove manufacturing to ensure that the glove meets thé stringent standards set by the regulatory body for medical devices. For example, the lowest acceptable water-extractable residue proteins as determined by the current A S T M D5712 test method is limited to 50 μg/g of glove (6). Reports and excellent monographs on the properties of these N R proteins and lipids are available (7-/0). The effect of these substances on the colloid stability of the N R latex has been reported by us previously (2). Apart from the presence of non-rubbers, one other difference between synthetic and N R latex is that the particle size of the N R latex varies widely whereas, the synthetic latex particles are more monodisperse and the particle size is also smaller. When these water-based latexes are spread on a substrate, a latex film is formed after the water evaporates and the particles come into contact. A mechanically continuous homogeneous film will result via flattening of the latex particles concurrently with inter-particle diffusion of polymer chains if the film is at a temperature above the glass transition temperature (T ) of the latex particles (77,72). In the case of NR, since the T (-63.6 °C) is low, a continuous latex film is formed which is soft and tacky. This renders N R latex films useless and unsuitable in its native form as a barrier material in glove manufacturing. In practice, vulcanization of the latex increases the hardness as well as the tensile strength of the latex film. In particular, pre-vulcanization of N R latex in combination with leaching of the film was found to result in an increase in the modulus and tensile strength of the film (75). This was attributed to an increase in integration of the film following the removal of the non-rubbers from the original latex particle surface. The A F M results on the morphology of prevulcanized latex film surfaces lend further support to the occurrence of inter particle diffusion of polymer chains during gradual coalescence of the filmforming process (14). In addition, chemical modification by chlorination of the film surface would increase both the surface hardness and roughness. Both of these modifications result in reduced surface tackiness and hence better donning of the glove. Recent results (75) on solution chlorination of N R latex films have confirmed that chlorination increased the hydrophilicity and reduced the surface tackiness of the film. Chlorination was accompanied by cracking and hardening of the film surface. Cross-linking reactions brought about by the chlorine on the rubber molecules also contributed to a hardening of the film surface. This results in a lowering of the adhesive force and surface friction of the latex film against g

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242 another surface. This clearly demonstrates that chemical modification of the film surface, or more drastically, the modification of the latex itself, to meet performance requirements is usually required of N R latex films. While natural rubber is well suited for the tire tread and footwear industries in view of its high surface coefficient of friction, this property is a distinct disadvantage for its use as sheeting, liners, and wipers. Similarly, the necessary application requirement of a glove is good donnability where a low surface tack and sliding friction of the glove surface is essential. This is related to the lubricity of the film surface against another surface, in the case of gloves, the human skin. A common method used to reduce surface friction and tack is by the use of solid lubricant such as talc or corn starch. This solution, as employed in powdered gloves, is at best semi-permanent, as the lubricating particles are easily removed from the surface during service. This is deemed unsatisfactory, in particular, for a surgeon's glove which has tight regulatory requirements. Chemical modification of bulk rubber vulcanizates such as wiper blades and belting to reduce surface tack using aqueous chlorination have been reported by Noakes (76). Results of simple friction tests on halogenated rubber sheets by acidified hypochlorite solution were given by Roberts and Brackley (17-19) and Romberg (20). Basically, chlorination roughens and hardens the rubber surface, and thus reduces the effective areas of contact between the surfaces, leading to a decrease in frictional resistance of the rubber surface. The chlorination process is simple but beset with poor reproducibility and accompanied by possible deterioration in physical properties, in particular, for rubber gloves which are thinner and more flexible compared to other rubber vulcanizates. In addition, major concerns are health and safety hazards associated with the process. Any improvement resulting in a safer and more environmentally-friendly production process would be most beneficial to the glove industry. A n entirely different approach in surface friction reduction is by coating the glove surface with a low friction polymeric material. For example, surgeon's gloves coated with a proprietary material known as biogel are available in the market. This new approach depends on the availability of suitable materials that can adhere well to the latex film surface without flaking during service. Various commercial polymeric materials of the polyurethane type are currently being investigated also, with a view to eventually replace chlorination. Surface chlorination studies (16-19) on bulk rubber vulcanizates in the literature were confined mainly to physical property changes such as the surface coefficient of friction. The surface morphology and extent of surface chlorination had not been fully established. Recently, a systematic study (75) of the effect of aqueous chlorination on surface morphology and compositional changes of thin rubber latex films confirms the simultaneous occurrence of surface hardening and roughening. Chlorinated and oxygenated species were found on the film surface following chemical reactions of the double bonds of


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the rubber molecules with chlorine and the hydrophilicity of the chemicallymodified surface improved after chlorination. The thickness of the chlorinated layer on the film surface was less than 10 μπι. We have now extended the investigation to include the correlation between the extent of chlorination with the adhesive force of the film surface as measured by A F M . It is hoped that such information together with similar measurement on some commercial glove samples could shed some light on the relationship between film morphology and the performance of rubber latex gloves.

Experimental Thin latex films (about 36 μπι thick) of ammoniated latex concentrate were prepared by dipping a newly-cleaved mica surface into the latex and the deposited gel was allowed to dry at room temperature for at least an hour before observation. N R latex films (0.3 mm thick) were surface chlorinated in acidified aqueous hypochlorite solution as described previously (75). These films were prepared in the absence of any vulcanizing agents. Thin films prepared from prevulcanized latex after maturation for 8 days were post-vulcanized according to reported procedures (14). The morphology of the latex films deposited on the mica surface was observed in a Leica S440 scanning electron microscope (SEM) after sputter-coating with a very thin layer of gold. Images were acquired at a probe current of 500 ρ A and voltage of 10 k V . Commercially-available powdered and chemically-treated N R latex gloves were used as received. A small test piece of the cut glove was held unstretched with the donning side facing upward on the A F M sample holder by double-sided tape. For comparison, latex films of polystyrene (PS) and poly(methyl methacrylate) latexes ( P M M A ) were also investigated (27). The spring constant of a 200 μπι long and 36 μπι wide V-shaped S i N cantilever was measured according to method developed by Cleveland et al. (22). Atomized tungsten spheres with a wide size distribution were used as the test masses (23). The sphere was positioned exactly near the end of the cantilever on the same side of the integrated tip. Resonant frequencies were measured from the split-photodiode response on a Nanoscope III A F M (Digital Instruments). The size of the attached sphere was determined using the S E M . Force-distance measurements were performed in ambient air (~ 28 °C) using a scanning probe microscope (Nanoscope III Multimode A F M , Digital Instruments). A single S i N cantilever was used to acquire the force-distance curves of all the samples within a given series to ensure that the spring constant and tip radius are the same between samples. On the average, about 50 forcedistance curves acquired at different locations over a 5 μπι χ 5 μηι scan area on 3

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244 the sample surface were recorded and the average pull-off force for the polymer determined.


Results and Discussion Since its development in the 1980s (24), A F M has seen tremendous expansion and extension of its operation to cover diverse areas of applications. Perhaps the most notable extension of A F M capabilities so far is the realization that lateral forces between the A F M probe and the sample could be measured too. Thus, in addition to topographical imaging, the scanning force microscope (SFM) can measure the very small force between the probe and the sample surface to provide unique localized chemical and nanomechanical information about the sample surface. Using the force-distance curve, the surface force, adhesion, friction and elasticity could be measured directly which are dependent on the probe, contact surface and the surrounding medium (25). Microscopic adhesion and friction affect a wide variety of surface phenomena and the S F M offers a new tool to study these important parameters. This technique is particularly suited for polymer systems because of its ability to characterize the surface structure, morphology, and surface properties from nano to millimeter scales. For example, the adhesive forces between a series of polymer film surfaces and chemically well-defined S F M probe tips have been investigated and found to depend strongly on the chemical nature of both probe and sample surfaces (26). The link between surface chemical nature, friction, and adhesion at the nanoscale was established using S F M by Frisbie et al. (27) who demonstrated different adhesion between surfaces (probe and substrate) coated with molecules with different end groups. The adhesive forces of these groups are in the same order as the corresponding frietional forces. In another study (28), it was found that the frietional force showed a clear correlation with the surface energy. Both adhesion and friction are larger on high surface energy materials. Overney et al. (29,30) showed that high adhesion is always associated with high friction and a topographically higher and stiffer substrate yields lower friction. These results translate concepts proven valid on macroscopic experiments to the nanoscale. Softer surface components will give rise to a larger area of contact for an equivalent load which in turn leads to a higher frietional force. Examples of the force-distance curves for NR, chlorinated NR, vulcanized NR, P M M A and PS latex films under ambient air conditions using a S i N cantilever are shown in Figure 1 and Figure 2. The pull-off force (F) is calculated using the equation: F = kAz where k is the spring constant and Az the cantilever displacement. The different shapes of the force-distance curves for the various polymers demonstrate clearly that the tip-sample interaction is strongly 3


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Figure 1. Force-distance curves between the unmodified S13N4 probe-tip and (a) untreated, (b) vulcanized, and(c) chlorinated NR latex films. Continued on next page.



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Figure 1. Continued



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Figure 2. Force-distance curves between the unmodified Si N4 probe-tip and (a) PMMA and (b) PS latex films. 3


248 dependent on the nature of the sample polymer material. A high pull-off force is associated with the N R film surface (165 + 2 nN), a low T , soft and tacky polymer at ambient temperature, in contrast to the rather low pull-off force (27 + 5 nN) observed for the hard P M M A latex film with a T of 126 °C. The pull-off force for the PS film at 50 ± 8 nN, is intermediate between those for the N R and P M M A films. The T for PS is 105 °C. The pull-off force for the N R latex film decreased immediately upon chlorination, from 165 + 2 nN for the control to ~ 54 + 9 nN for the N R latex film chlorinated at a chlorine dosage of 250 ppm (see Figure 3). The pull-off force after the initial drop remained essentially constant when the chlorine dosage was further increased to 2000 ppm. There is a close correlation of the pull-off forces with chlorination as compared with those of the water contact angle on a chlorinated N R latex film surface reported by us previously (15). Both exhibit a steep drop initially followed by a constant value with further chlorination. The hydrophilicity of the film surface increased with the extent of chlorination (15). There is a corresponding increase in the surface mean roughness with an increase in the extent of chlorination (Figure 3). This has given rise to a wide scattering of the pull-off forces of the chlorinated N R films (see histograms in Figure 4), especially at high chlorine dosages (from 750 to 2000 ppm) associated with an increase in surface mean roughness at high chlorine dosage as a result of surface cracking and hence roughening. In comparison, the spread of the pull-off forces for the control sample of unchlorinated N R film was narrow and centred around 160 nN. As pointed out previously, chlorination causes hardening and roughening of the film surface and these have resulted in a lowering of the pull-off force of the chlorinated N R . Roberts and Blackley (18,19) also found that the reduction in the areas of contact of the chlorinated surface with another surface and surface hardening are factors that contribute to a reduction in surface friction. The above observation is further confirmed by a slight decrease in the pulloff force of vulcanized N R latex film shown in Figure 5b. The pull-off forces of the vulcanized N R film spread over a range, from 110 to 170 nN, compared with those for the un vulcanized NR. Most of these are centered around 160 nN which still bears a strong resemblance to N R characteristics. This value is certainly still much greater than those of the P M M A and PS with a much higher T . It is obvious that sulfur vulcanization hardens the rubber but is insufficient to completely eliminate its tackiness. Thus, vulcanized rubber has to be further treated, as in the case of the rubber gloves, by powdering with cornstarch or chemically by chlorination or polymer coating, to become donnable. The vulcanized N R film surface is for the most part essentially smooth and structureless except for some small shallow depressions as shown in Figure 6a. g

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Figure 3. Variation of adhesive forces (pull-offforces) (·) and surface mean roughness ( A), R& ofNR latexfilmsas a function of chlorine dosage.


250 Mareanukroh et al. (31) showed that the force-distance curves were sensitive to the cross-link density of a styrene-butadiene elastomer. The penetration distance of the surface by the A F M tip decreased with an increase in the cross link density of the rubber and the maximum force varied from 500 nN to 700 nN over the cross-link density range of 4 X 10" mol cm" to 5 x 10" mol cm' . The experimental values of force per square of distance penetrated vs. cross-link density were in close agreement with those calculated from the cross-link density data and the half-angle of the conical probe tip. The reduction in the pull-off forces for the powdered N R rubber glove surface as compared to untreated N R latex film is marginal, but the spread of these forces is rather wide (Figure 5c). Two populations of pull-off forces are discernible, one centered around 175 nN corresponding closely to that for N R film and another at slightly lower value. S E M reveals that the film surface was covered by polydisperse corn starch particles (Figure 6b). The substrate surface of the N R film appears smooth and featureless. It would appear that the two populations of pull-off forces are due to N R and the starch particles on the surface, respectively. In comparison, the reduction in adhesive force for the above surfaces is much smaller than those of the commercial N R glove surfaces as shown in Figure 5d-5f Commercial glove samples from three different manufacturers were examined: sample A is a powder-free polymer-coated surgeon's glove, sample Β a powder-free polymer-coated examination glove, and sample C a powder-free polymer-rinsed examination glove. Different extents of reduction of the pull-off forces compared with N R latex films are evident: a polymer-coated glove (sample A ) appeared to be the most effective in reducing the surface adhesive force with a pull-off force of only 49 + 17 nN. The spread of the pulloff forces for this sample was also the smallest among the three commercial gloves (Figure 5d). The magnitude of the pull-off forces was in the same range as those by chlorination and polystyrene. S E M reveals cracks on the donning surface of glove sample A , resembling the effect of chlorination (Figure 6c). This glove seems to have been completely coated by the polymer. The polymer layer cracks in most areas and appears rather rough. The pull-off forces for the donning surface of sample Β (Figure 5e) spread over a very wide range of 45 to 135 nN. In addition, the donning surface (Figure 6d) appears very flaky and a lot more uneven compared to that of sample A . On the other hand, glove sample C gives pull-off forces in the range of 50-150 nN and is bimodal (Figure 5f). The morphology of the surface of glove sample C (Figure 6e) is relatively even with patches of flat and smooth regions, some of which are rather big, which points to a rather poor coating with exposed N R substrate. Again, the non-uniformity in the surface coverage and the uneven topography cause the wide scattering of the pull-off forces of these commercial gloves. It would appear that the magnitude and the spread of the pull-off forces are strongly dependent on the nature of the


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Morphology and Adhesive Force of Natural Rubber Latex Films by

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