Polymer coating for dipped goods


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I could catch up with some interesting technical papers published by some leading Rubber Technologists across the World.
Here they are edited for yr ready ref.

Developments in Polymer Coatings for Dipped Goods
Bill Howe
President, PolyTech Synergies LLC
Canal Fulton, Ohio USA

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Polymer coating for dipped goods

  1. 1. 1 Dear Linked in Members I could catch up with some interesting technical papers published by some leading Rubber Technologists across the World. Here they are edited for yr ready ref. Developments in Polymer Coatings for Dipped Goods By Bill Howe President, PolyTech Synergies LLC Canal Fulton, Ohio USA (Latex glove donning requires lubricious surfaces other than powders.)
  2. 2. 2 (Polymer coatings are applied on-line in properly designed dip tanks) (The catheter industry has routinely used silicone lubricious coatings for patient comfort) The use of corn starch, calcium carbonate, and talc as slip agents for dipped products has seen better days for those manufacturing these processing products. Historically, these products of choice dominated latex processing as an anti-tack and donning application agent. The
  3. 3. 3 manufacturing of products using powders was generally quite simple to control, and easy to apply during the manufacturing process. A simple controlled dosage of powder in a water or alcohol slurry tank normally produced acceptable results, followed by de-powdering through a tumbling process. Products such as gloves, condoms, finger cots, breathing bags, medical balloons, probe covers, and toy balloons all use this approach routinely during the manufacturing process. However, with the advent of heightened alerts of latex allergies in the 1990’s, it was determined that proteins could be spread by residual airborne powder from using latex gloves, which in turn jeopardized latex sensitized personnel who could potentially react to latex allergens. This led to the growth of powderfree latex products, which was primarily achieved through chlorination of the finished product. Powders were still employed in the manufacturing process for purposes of anti-tack of work-in-process inventory. The finished latex products were then transferred to a chlorination room where separate processing equipment enabled the product to be subjected to chlorine, which in turn removed the processing powders, and altered the surface of the film, allowing for improved donnability and/or lubricity of the product. However, there were many disadvantages to chlorination as a choice to achieve the feature of powderfree donnable products, such as: • Controlling levels of chlorination during batch processing proved challenging, yielding batches with different donnability characteristics, frustrating manufacturers and end users.
  4. 4. 4 • Thin film products such as medical gloves and finger cots, required extensive downstream handling, adding to manufacturing cost. These products required a two cycle chlorination process – one for each side of the product, with a manual turning operation in between. This was necessary because the thin film characteristics did not allow the chorine batch to penetrate the inside of the product successfully. Exceptions to this problem are household gloves, and other thicker film products which allowed the chlorine batch to properly penetrate the inner layer using one cycle only. • Chlorination tends to degrade and weaken latex film properties. At times, the chlorination-associated glove degradation results in poor glove donning. Also, chlorinated gloves tend to adhere to each other as part of the packaging process, especially exam gloves and finger cots, which are packed in bulk boxes or poly- bags. • Those attempting to reduce downstream manufacturing handling cost by applying the chlorine on-line during the dipping process were introduced to other challenges of harrowing proportions. It was often difficult to find space within existing dip lines to achieve this process. Furthermore, without absolutely impeccable ventilation control of chlorine fumes from the on line tanks, dip lines were subjected to extensive corrosiveness from the process. Those companies attempting to maintain a clean process found the exercise futile, as particles were generated from damaged machine structure and form carrier components.
  5. 5. 5 • The buying marketplace generally did not warm up to the yellow appearance of the dipped product, especially for surgical gloves. The color “white” was king and perceived to be a more pristine, reliable product. In the 1980’s, a patent was issued for Biogel, a hydrogel product that continues in use today. This coating was developed by then London International Group, primarily for surgical gloves. This ingenious product led to what today is a wave of choices for powderfree dipped products through slip coatings. Enough history – let’s get on to today’s choices for anti-tack and donning coatings. Key RequirementsKey RequirementsKey RequirementsKey Requirements to Considerto Considerto Considerto Consider Allegiance Healthcare (Cardinal Health), in its informative on-line bulletin on “Powder Coatings for Powderfree Medical Gloves”, offers some basic advice on preparing for the use of coating technologies for latex products. The polymer design must consider the following requirements:1 1. It must adhere to the underlying rubber latex substrate and offer durability and good donning characteristics. 2. It must be resistant to chlorination and the vigorous post-forming processing steps that can include rinsing, extraction, and drying. 3. It should not degrade after sterilization.
  6. 6. 6 In Bill Williams’ (Research and Development Manager – Best Manufacturing Co.) article from an earlier Rubber Asia article on “The Science of Donning Coatings”, he states that donning coatings actually serve two purposes. In addition to the obvious use discussed, which is as a slip coating for donning latex products, he suggests that a second purpose is to provide additional barrier properties. A properly applied barrier film in some cases may reduce the affects of type 1 and type 4 allergic reactions. Agents such as anti-microbial agents, Aloe Vera and vitamin E can be incorporated into the coating for improved benefit to the end user. This is an excellent example of how differentiated the latex glove business has become since the turn of the millennium. In today’s highly differentiated product world of latex medical and industrial products, the development of donning coatings can in be applied to many substrate materials including natural rubber, nitrile, polychloroprene, polybutadiene, polyvinylchloride, polyurethane, polyisoprene, styrene diblock and triblock copolymers, and other synthetic elastomers. The application of donning or slip coatings on latex dipped goods is primarily for one time or disposable use. Most coatings tend to be hydrophilic, which reacts with the human skin, and degrades with repeated use. Therefore the use of donning coatings with products that are worn repeatedly, such as industrial gloves, is not a normal occurrence.
  7. 7. 7 Polymer Coating ChoicesPolymer Coating ChoicesPolymer Coating ChoicesPolymer Coating Choices The development of polymer coatings for latex products continues to expand. Listed below are those most commonly employed as of this writing. 1. Polyurethane This type of polymer coating continues to be the most popular choice for donning coatings on latex products. They have excellent biocompatibility. They are normally applied to the latex layer as part of the on-line dipping process. The adhesion of the polyurethane to the glove substrate has of late been enhanced by the formulation’s ability to be applied to the wet gel state of the latex during processing. During the most recent International Latex Conference, Mr. Fung Bor Chen of Medline Industries reported in his presentation and paper that this capability of cross linking is achieved through inclusion of hydroxyl and carboxyl functionalities in the polymer composition.2 This crosslinking improves blocking and increases stain resistance. He reports that a system pH of 9 – 10 and a concentration of 0.1 to 1.0 phr of cross-linking agent will enhance results. Some recent advances have combined polyurethane with other materials to achieve desired results. One PU dispersion is a linear dihydroxy polyester reacting with diisocyanatetodiphynylmethane. This emulsion is a copolymer of ethoxy ethyl methacrylate and methoxyl methacrylate. A blend of 70 parts of PU dispersion with 30 parts of the acrylate emulsion is used to coat the glove. Polyurethane dispersions
  8. 8. 8 are typically diluted with water to 5-10% solids and applied to the surface of the glove by dipping. Another recent innovation, patented by Tony Yeh of Allegiance Healthcare in March, 2002, suggests the achievement of powderfree gloves with a silicone impregnated cross-linked polyurethane inner coating. 3 This formulation suggests the aqueous coating composition employ the following general recipe: 100 parts PU; 11.1 parts silicone; 1.0 parts surfactant; 2.2 parts cross-linking agent. Surfactants are added to give stability to the compound and improve the wetting of the film. It is critically important not to use an inordinate amount of surfactant, as too much leads to oven efficiency issues and can cause excessive foaming in the dip tank. In his patent, Mr. Yeh suggests the following process for making a powder-free elastomeric article having a coating comprised of a crosslinked polyurethane impregnated with silicone on an internal surface of the article. The steps include: (a) dipping a former into a coagulant dispersion to deposit a coagulant layer onto the former; (b) drying the former with the deposited coagulant layer; (c) dipping the former with the deposited coagulant layer into an elastomer to produce a second layer comprising coagulated elastomer thereon; (d) dipping the second layer of coagulated elastomer into a powder-free dispersion comprised of polyurethane dispersion, wetting agent, silicone emulsion, crosslinker, stablizers and water to form a coating on the second layer; (e) curing the layers and the coating on the former; (f) stripping the article
  9. 9. 9 from the former; (g) treating the article to remove powder; and (h) drying the article. 4 2. Acrylics Thermoplastic or cross-linkable acrylics are employed as a powderfree solution for latex dipped goods. It is reported that the capability of cross-linking is achieved through inclusion of hydroxyl and carboxyl. A hydroxyl is cross-linked using amino resins and isocyanates, whereas a carboxyl is cross-linked through carbodiimides, azridines, and epoxies. Acrylic coatings are based on acrylate polymers with elastic properties. Their tensile strength and elongation are comparatively low to that of natural rubber. This differentiation in properties may cause the acrylic coating to crack and possibly delaminate or separate from the natural or synthetic rubber. From a manufacturing viewpoint, some acrylic coated devices are difficult to strip from the mould when hot, which may produce challenges for processes being retrofitted from earlier latex equipment designs. 3. Hydrogels The initial hydrogel employed in actual production was Biogel, which was a coating applied to surgical gloves. This coating dominated the slip coating industry in the 1980’s and early 1990’s. Hydgrogel coatings are used routinely on other medical products such as heart balloons,
  10. 10. 10 contact lenses, and catheters. The absorbed water in the hydrogel coating forms a water film on the substrate surface. The key challenge is to achieve high reliability of bond of the hydrogel to the substrate surface. It is reported that some treat the substrate layer of rubber with acid or other adhesive before applying the hydrogel to the product. This has obvious disadvantages by making the manufacturing process complex and inconsistent. In many cases, if an adhesion is poor, then the hydrogel coating will delaminate and flake, generating harmful particulate during use. 4. Silicone Past practice in latex processing has led to the application of silicone to finished dry film products. For many years, latex breathing bags were treated with silicone in downstream tumble dryers, which in turn provided a sheen or appealing cosmetic appearance to the end product. For other medical devices, such as Foley catheters, silicone coatings have been routinely used to improve the surface lubricity and patient comfort during use. Using silicone coatings for catheters rather than chlorination, again produces a more pristine looking product, given its white appearance as opposed to the less attractive yellowed chlorinated catheter. Some gloves today are treated with silicone as a donning agent, but in general are not as popular as other donning polymers in that they typically fail to provide adequate damp and wet donning capability.
  11. 11. 11 5. Nitrile The newest innovation in donning coatings, especially in use with natural rubber products, is nitrile latex. Nitrile is a synthetic latex comprised of acrylonitrile, butadiene, and carboxylic acid. An advanatage of using nitrile is that it has excellent chemical resistant properties. Its benefits include good tensile strength, adequate elasticity, and adequate adhesion to natural rubber during the manufacturing process. Cardinal Health introduced this concept with their Protegrity® surgical glove, a triple dipped product. The outer layer is natural rubber latex. The inner layer is reportedly a blend of natural rubber latex and nitrile, with the donning side comprised of 100% nitrile polymer. Cardinal’s product literature makes performance claims of tensile strength range from 3919 to 4064 psi and ultimate elongation in excess of 900%.
  12. 12. 12 Coating CharacteristicsCoating CharacteristicsCoating CharacteristicsCoating Characteristics The following table offers some general guidelines to the use and performance of slip coatings. HydrogelHydrogelHydrogelHydrogel PUPUPUPU AcrylicAcrylicAcrylicAcrylic SilicSilicSilicSiliconeoneoneone NitrileNitrileNitrileNitrile TensileTensileTensileTensile strengthstrengthstrengthstrength Low Medium - High Low Low High Adhesion toAdhesion toAdhesion toAdhesion to NRNRNRNR Low Medium Medium Low Medium ElongationElongationElongationElongation Low High Low Medium High LubricityLubricityLubricityLubricity High Medium Medium High High PuncturePuncturePuncturePuncture resistanceresistanceresistanceresistance Low Medium - High Medium Low High AbrasionAbrasionAbrasionAbrasion resistanresistanresistanresistancececece Low Medium Medium Low Medium Oil resistanceOil resistanceOil resistanceOil resistance Medium High High Low High HydrationHydrationHydrationHydration (lower is(lower is(lower is(lower is better)better)better)better) High Medium Low Low Low Integrity afterIntegrity afterIntegrity afterIntegrity after stretchingstretchingstretchingstretching Low Medium - High Low Low High
  13. 13. 13 (Table made courtesy of Cardinal Health; Billmeyer, 1984; Pauly, 1989) Manufacturing ConsiderationsManufacturing ConsiderationsManufacturing ConsiderationsManufacturing Considerations It is imperative that polymer coatings be applied uniformly on the latex substrate surface for optimal performance. Since a polymer coating is a mixture of water and polymer, it has a higher surface energy than dry latex films. Therefore, coatings tend to perform better when applied to a latex substrate in wet gel state, and especially when the latex substrate is dipped using lower solids content. The higher water content tends to benefit the application of polymer coating when in a wet gel. The quality of water employed in the slip coating solution contributes to performance as well. Water with high iron content and rich in calcium can lead to an instable polymer. Deionized water may be helpful to performance, but one must be careful as bacteria can breed more easily, and typically is lower in pH. Careful use of FDA approved bactericides can overcome these issues. The application of the water based polymer is performed as part of the on line dip system. The latex substrate should be leached as a wet gel prior to applying the coating. Leaching time should not be less than 1.5 minutes, and preferably more. After drying the film to remove surface water resultant from the leach system, the coating application can then ensue. One should allow the polymer to drip for a short time (12 seconds), before camming to help distribute the polymer coating for
  14. 14. 14 uniformity purposes. The product is then ready for final vulcanization. On-line, post oven leaching follows the final cure process. For the coagulant bath, some continue to use calcium carbonate powders to achieve proper release from the mold. For example, Mr. Yeh reports in his patent for PU and silicone coating, that the coag bath is comprised of calcium nitrate, calcium carbonate powders, wetting agents and water (or alcohol for alcohol based coagulant dispersion). However, others have made successful strides to eliminate calcium carbonate from coag solutions to achieve a true powderfree product. Products such as Coag Plus Hydrogel and Coag Plus Dispersion, by Crusader Chemical in the USA, are employed with water or alcohol accompanied by calcium nitrate. No calcium carbonate is employed using this process. The Coag Plus products have been prepared with proper defoamers and surfactants, to enable the manufacturer to simply add Coag Plus to warm water, followed by the addition of calcium nitrate. Products such as this have been designed to promote leveling, wetting, and latex pick up, as well. Unicote® is a product offering of National Starch and Chemical which also claims to offer solutions for both coagulant and polymer coating application. Simple Tests of Coating EffectivenessSimple Tests of Coating EffectivenessSimple Tests of Coating EffectivenessSimple Tests of Coating Effectiveness
  15. 15. 15 At the International Latex Conference in 2004, Mr. Fung presented a basic method for screen testing of polymer coated films, as follows: 1. Cytotoxicity – take a cup of water, placing the glove in the cup with water about ¾ full. Rub the glove under water for 2 minutes. Remove the glove and observe bubbles formed. The more bubbles that are formed, the more likelihood that the glove will cause an allergic reaction to those sensitized. 2. Dry Donning – Hold glove in one hand at the cuff area. With the other hand, twist the glove with two fingers, feeling for the ease of movement for one surface with the other. 3. Wet Donning - Fill the glove with water, and allow to set for a few seconds. Pour out the water. Hold the glove in one hand at the cuff area. With the other hand, twist the glove with two fingers, feeling for the ease of movement for one surface with the other. 4. Adhesion – Stretch the glove while rubbing the coating repeatedly with thumb. Observe the degree of coating that flakes off the glove to the thumb. 5. Block – Press two gloves together. Evaluate adhesion by observing the ease of separation between the two when pulled apart. With many choices available today for polymer coatings, it is apparent that the dipped goods industry has been rewarded because of its due diligence towards reducing latex protein exposure, and providing improved product offerings.
  16. 16. 16
  17. 17. 17 About the Author Bill Howe is President of PolyTech Synergies, a firm specializing in engineering and marketing services for dip molded and dip coated polymer products. Contact Mr. Howe at w.howe@sbcglobal.net or sales@polytechsynergies.com. 1 “Research & Technology Supporting Your Decisions”,www.allegiance.net, p.1. 2 Fung Bor Chen, “Overview of Powder-Free Technology and Materials”, 2004 International Latex Conference – Akron, Ohio. 3 Y.S.T. Yeh, “Powder-free gloves with a silicone impregnated cross-linked polyurethane inner coating and method of making same, “US Patent Application No. 20020029402, March 14, 2002. 4 Y.S.T. Yeh, “Powder-free gloves with a silicone impregnated cross-linked polyurethane inner coating and method of making same, “US Patent Application No. 20020029402, March 14, 2002. mportant Facts about Latex and Latex Gloves Natural Rubber Latex Gloves - Made from a Renewable Resource Natural rubber (NR) late gloves are natural products. They are derived from NR latex obtained from the Hevea Brasiliensis tree when the bark is tapped (Figure 1). This is unlike all synthetic gloves, wh are made from petrol chemicals.
  18. 18. 18 Figure 1. Natural rubber latex collected in a cup after skillful tapping of the bark o Hevea Brasiliensis tree. Raw NR Latex This is a milky fluid comprising 25%-40% of rubber hydrocarbon in the form of particles suspended in an aqueous serum together with a few percent of other non-rubber substa such as proteins, lipids, carbohydrates, sugars, some metals, fatty acids, and other substa known as the non-rubber fraction. The remaining major component is water. NR Latex Concentrate Latex collected from the tree after tapping is concentrated generally by centrifugation, to remove much of the aqueous components. The concentrated latex with about 60% dry ru content (or drc) is then usually preserved with ammonia to combat bacterial growth. The resulting latex concentrate becomes the starting material for all natural rubber latex prod whether by dipping (for gloves, balloons, condoms, catheters, baby soothers, rubber tubi toys and dental dams) or other processes such as foaming (for latex foam to sponge), or extrusion (for latex thread, more commonly known as "elastic"). Steps in the Manufacturing Process The manufacture of most NR latex gloves follow roughly the same sequence. However, m manufacturers include processing steps that reduce the level of protein in their gloves. Th typical glove manufacturing process is as follows:
  19. 19. 19 The salient features of the above manufacturing processes include the following: Dipping: Liquid latex concentrate is mixed with various compounding chemicals and is introduced into one of the tanks in the processing line. Clean, dry formers in the shape o hands are immersed first in a coagulant and then in the latex mix for appropriate dwell to give the desired latex film thickness. The coagulant is applied to facilitate the depositio layer of latex on the formers. Wet-gel leaching and beading: The thin latex film on each former is partially dried and leached briefly in clean water to remove the water-soluble materials. Beading also is introduced at this stage to give each glove a rolled bead or rim at the open end. Drying and curing: The gloves are then dried and vulcanized. Drying and vulcanization o curing of the gloves are usually done in hot-air ovens, initially at lower temperatures of 8 C, and then at higher temperatures of 100º-140º C where necessary.
  20. 20. 20 Post-cure leaching or dry-film leaching: The cured gloves are immersed in clean water ta remove more water-soluble substances, particularly proteins on the surface of the gloves Powdered gloves: The leached gloves are dipped into cornstarch powder slurry to pick up coat of lubricant that makes them easier to don. They are then further dried. Glove stripping: This is the final operation on the production line - removal of gloves from formers. This is often carried out manually, frequently with the assistance of compressed but an automatic stripping system is becoming more common. Powder-free gloves: Latex gloves with very little or no powder lubricant can be prepared either (i) chlorination or (ii) polymer coating. While chlorination oxidizes the outer rubbe surface to eliminate tackiness and reduce the residual soluble protein content, polymer coating involves replacing powder with a suitable lubricating coat on the glove surface. B processes can be carried out on-line, without the powder-coating step, or off-line by was first the finished powdered gloves, then subjecting them to the chlorination or polymer-c treatment. Removing Glove Proteins Protein Status in Latex When subjected to ultracentrifugation at approximately 59,000 gmax, latex can be separ into three main fractions: (i) top rubber hydrocarbon fraction, (ii) the ambient serum (kno C-serum) in which all rubber particles are suspended, and (iii) the denser bottom non-ru particle fraction, particularly lutoids, which contain yet another serum (known as B-serum • Yip E. & Cacioli P, The manufacture of gloves from natural rubber latex, J. Allergy C Immunol., 2002; 110: S3-14 Like all plant materials, Hevea latex contains proteins. Of the approximately 1% of total proteins present in the latex system, about one-quarter are found on rubber particle surf (i), the remaining three-quarters are in the non-rubber phase [fractions (ii) and (iii)] of the and they are mostly water soluble (Figure 3).
  21. 21. 21 Figure 3. Freshly collected Hevea Brasiliensis latex separated into its three main fractions ultra centrifugation at 59,000 gmax. When processed into latex concentrate, considerable amounts of the soluble proteins are removed. Further conversion of the latex concentrate into gloves removes more of these proteins through the leaching and washing steps. Therefore, the remaining levels of solub proteins - or the residual extractable proteins implicated in allergic reactions - are marke low. Depending on which manufacturing process is used, the level of residual extractabl protein can vary widely. • Dalrymple S.J. and Audley G.B. "Allergenic proteins in dipped products: Factors influencing extractable protein levels," Rubber Developments, 1992; 45(2/3):51 • Yunginger J.W., Jones R.T., Fransway A.F., et al. "Extractable proteins in disposable medical gloves and other rubber products," J. Allergy Clin. Immunol. 1994; 93: 836 • Ng K.P, Yip E., Mok K.L. "Production of Natural Rubber Latex Gloves with Low Extra Protein Content: Some Practical Recommendations," J. nat. Rubber Research, 1994; 95.
  22. 22. 22 Latex Allergens Not all proteins in the residual extractable fraction cause the allergic reaction. Although t 13 proteins (mostly soluble) in raw Hevea latex have been reported to be possible allerge defined by their display of IgE antibody binding activities, it is unlikely that all of them w be present in the finished products after processing. • Alenius H., Turjanmaa K. and Palosuo T. "Natural rubber latex allergy," Occup. Envi Med, 2002; 59: 419-424; • Yeang H.Y. "Natural rubber latex allergens: New developments," Curr Opin Allergy C Immunol. 2004; 4: 99-104), Cross Reactivity “Close structural similarities between any two allergens from divergent sources can prod similar allergic reactions in sensitive patients, and is termed cross-reactivity or cross-sensitization. Ingestion of some foods could produce allergic symptoms in patients sensitive to latex proteins due to the presence of these common or cross-reactive protein allergens, such as in the case of avocado, banana, chestnut and kiwi. However, it is impo to note that not all food-allergic individuals are sensitive to latex, and not all patients wit allergies will have problems with these foods. A study of binding patterns of IgE antibodies from the blood sera of individuals, who wer latex allergic but who had reactions to fruits, supported this. The study findings also show that multiple bindings could occur between latex serum proteins and IgE from many wh reacted to extracts of fruits but not to latex gloves. On the other hand, more specific and f bindings to latex protein were observed by those who were skin tested positive to latex g extracts.” • Hasma H., Shahnaz M., Yip E., Azizsah M., Mok K.L. and Nasuruddin B.A. "Binding Patterns of IgE Antibodies in Sera of Rubber Tappers to Fresh Hevea Latex Serum Proteins," J. Rubber Research, 1998; 1(3): 146-153
  23. 23. 23 Allergenicity of Latex Gloves The allergenic potential of latex gloves can be measured in-vivo by skin-prick testing (SPT latex-allergic subjects, or in-vitro by specific IgE antibody-inhibition immunoassays. The method is known to be more specific and more sensitive than the IgE binding technique However, all of these methods are relatively sophisticated, and require further improvem and they are also expensive to perform. The presently preferred method is the quantification of total proteins using the modified micro-assay, which is technically easy and possible to standardize as well as cost effectiv However, the test is not allergen specific. Nevertheless, significant correlations between residual total extractable protein content and the allergen levels of extracts of NR latex gl based on both serological IgE specific inhibition immunoassays, and the SPT testing, hav been established. Latex gloves with high residual extractable protein contents are associa with positive SPT or high allergen contents. Latex gloves with very low residual extractab proteins, on the other hand, tend to have very low or negligible SPT reactions by latex sen subjects.
  24. 24. 24 Figure 4. Total extractable protein content (as measured by modified Lowry assay) of la gloves and percentage negative skin prick test response shown by latex sensitive subject Yip E., Turjanmaa K., Ng K.P. and Mok K.L. "Residual extractable proteins and allergenic natural rubber products," J. nat. Rubber Research, 1994; 9: 79-86;) Figure 5. Total extractable protein content and allergen level of 46 lots of latex gloves determined by modified Lowry test and IgE latex specific ELISA-inhibition respectively Yip E., Palosuo T., Alenius H., and Turjanmaa K. "Correlations between total extractable pr and allergen levels of natural rubber latex gloves," J. nat. Rubber Research, 1997; 12: 120
  25. 25. 25 This method, although non-allergen specific, offers a technically easy and standardizable method that is very helpful in product developments and improvements. • Yip E., Turjanmaa K., Ng K.P. and Mok K.L. "Allergic Responses and Levels of Extractab Proteins in NR Latex Gloves and Dry Rubber Products," J. nat. Rubber Research, 1994; 86; • Yip E., Palosuo T., Alenius H., and Turjanmaa K. "Correlation Between Total Extractable Proteins and Allergen Levels of Natural Rubber Latex Gloves," J. nat. Rubber Research, 12: 120-130; • Palosuo T., Makinen-Kiljunen S., Alenius H., Reunala T., Yip E. and Turjanmaa K. "Measurement of natural rubber latex allergen levels in medical gloves by allergen-sp IgE-ELISA-inhibition, RAST inhibition, and skin prick test," Allergy, 1998; 53: 59-67; • Beezhold D., Pugh B., Liss G. and Sussman G. "Correlation of protein levels with skin p test reactions in patients allergic to latex," J. Allergy Clin. Immunol., 1998; 98: 1097-11 • Yip E. and Sussman G. L. "Allergenicity of latex gloves with reference to protein sensiti individuals in a Canadian population," J. nat. Rubber Research, 2000; 3: 129-141. Protein Reduction - Product Improvement Residual extractable protein content of gloves can now be reduced from as high as 1,000 µg/g of gloves to a low of less than 50 µg/g using improved manufacturing technologies, include: o Use of low-protein latex concentrates o Proper leaching protocols o Chemical or enzymatic deproteinization o Chlorination o Polymer-coating
  26. 26. 26 AUTOMATIC STRIPPING OF GLOVES IN A HIGH VOLUME PRODUCTION ENVIRONMENT - A TECHNICAL PRESENTATION AT MARGMA GLOVE CONFERENCE; K-L, MALAYSIA; 1999 – By: By: William L. Howe President PolyTech Synergies LLC 8751 Mardel Ave. NW Canal Fulton, OH 44614 USA (P) 330 854 6715 1.0 INTRODUCTION Probably no innovation in the 1990's for automation in the glove manufacturing sector has impacted productivity of manufacturing plants like that of technology for automatic stripping of gloves. The purpose of this paper is to inform and prepare the reader for the following; 1. Identification of the critical evaluation factors before investing in technology for automatic stripping. 2. he general techniques employed today for successful automatic stripping of gloves. 3. ealistic expectations for commissioning and performance of the technology. 4. To spur creative thinking and planning for related downstream automation, connected to production machine auto strip devices. In general, this paper will address techniques for unsupported thin gauge gloves, with brief mention of techniques for household gloves and supported industrial work gloves. 2.0 TECHNIQUES
  27. 27. 27 The writer and his company have identified and/or designed for five (5) different techniques for automatic stripping. These techniques can also be considered for dipped products other than gloves, such as bags, condoms, catheters, balloons, etc. Selection technique for each application will depend upon many factors, which will be identified and briefly described in Section 3.0 of this presentation. 2.1 Pressure Pad / Rotating Brushes This technique is used primarily for a "straight-off " stripping of a dipped product. For the glove industry, one will find this technique utilized for stripping of supported industrial work gloves. Typically, a pair of cushioned pads (to protect ceramic formers) would encase the glove former as it passes, using a two axis motion (squeeze and drop), which through pressure, will enable the glove to loosen and free from the mold, for deposition onto a conveyor or tote bin. A similar technique uses a pair of rotating brushes, which encase the former. The brush technique typically involves the use of one axis motion only (brush rotation) to accomplish the strip. However, a second axis can be added to accommodate different size formers entering the same brush system. This second axis is often accomplished with the use of compressed air or mechanical spring, to enable the brush to adjust to the differing former diameters. The rotating brush technique is seldom employed with glove stripping, and is more conducive to " straight-off " stripping of symmetrical dipped products, such as condoms and toy balloons (See Figure 2. 1. 1). 2.2 Water Jet Though not a popular choice for glove stripping, the use of water jets can enable gloves to be automatically removed from molds, if the "straight- off" method is desired. The primary disadvantage of this approach is that the glove becomes wet, which necessitates more attention to glove collection and downstream drying.
  28. 28. 28 The writer has specified the use of water jet systems for back-up stripping, when requested. In this format, the actual automatic stripping is first conducted by "dry" means in the main stripping station. If gloves are missed (which is a normal occurrence), the water jet(s), located in the former washing station, would eject gloves from the mold onto a screen inside a containment tank. Normally, the gloves removed using this back up technique, are considered as Scrap. The use of water stripping is more common for use with symmetrical products such as condoms and toy balloons. 2.3 100% Compressed Air If removing unsupported gloves via a "straight-off" technique, the most reliable method is by compressed air, which typically requires significant volume and pressure to accomplish the strip. If the manufacturer is chlorinating both glove sides downstream, this method can be employed successfully. Otherwise, the texture of the mold is transferred over to the inside of the glove, which is typically not the preferred result. Furthermore, the outside of the glove when used, would represent the side of the glove having seen most effects of the protein wash station. This means that the inside of the glove, which is next to the user's skin, would be the side not seeing the effects of the protein wash. The use of "straight-off" technique via air has the advantage of being a low cost in capital investment. However, operational costs are typically considered as high. 2.4 Combination Air/Mechanical The technique patented by the writer's company, combines the use of compressed air and mechanical grasp, which has been designed for reversing the glove during the pick to minimize downstream product handling. An artist rendering of the concept can be seen in Figure 2.4. 1. The technique is most commonly employed with continuous chain lines. A key consideration is that of line speed and the ability to synchronize the
  29. 29. 29 apparatus with the movement of the line. The system shown accomplishes that with mechanical gearing in conjunction with the conveyor chain. A second method for synchronization would be to accomplish this electronically by communicating pulses to the conveyor drive motor. This technique (first proven in production in the late 1980's), uses a three (3) step approach as follows; • First, engage a set of fingers to "hold" the glove at the middle finger area of the former. • During the engagement of the mechanical finger, a blast of air is enacted at the cuff area (effective for both beaded and non-beaded gloves) so that the film begins to move down the mold. The "holding" device prevents the fingers from inflating, which in conjunction with the air blast, allows the cuff area to reverse on itself, with the cuff area surrounding the mechanical fingers. • Thirdly, the mechanical "grasp" fingers cam away from the former, leaving the glove cuff free to be removed with a final set of rotating brushes into a vacuum delivery system or moving conveyor. This effectively fully reverses the glove. This technique has been considered an effective approach in stripping natural rubber latex gloves. I believe it fair and accurate to say that this specific technology has had limited success in conjunction with other polymers, such as nitrile and neoprene. Investment capital cost and operational cost for this technology are considered in the moderate range. 2.5 Full Mechanical Pick Technique Common sense would inform us that the best motion to simulate for automatic stripping would be that of the human motion. This technique involves the automation simulation of that thought. An example of such a technique can be seen in Figures 2.5. 1. The device normally employs the use of 3 axes for both batch machine and chain machine applications.
  30. 30. 30 The key to its success is that of accurately engaging the "finger grasp mechanism" to the bead on the glove or inside the cuff of the glove. This accuracy is often accomplished by a roll down brush followed by mechanical finger engagement in the cuff area, followed by a roll up brush back over the fingers. The two mechanical fingers are now positioned between the glove film and the former. After this first step is accomplished, the mechanism can be moved vertically in a downward stroke, to effectively reverse the glove and remove it from the mold. This technique has more universal appeal to different types of polymers, including natural rubber latex, nitrile, neoprene, and PVC. However, the primary disadvantage to this technology is that capital acquisition cost is typically high. Furthermore, on going maintenance costs make for moderate to high operational cost as well. This technique is adaptable to both batch dipping systems or chain dipping systems. 3.0 CRITICAL FACTORS FOR SUCCESS The following factors must be evaluated before advancing into the design and implementation phase of automatic stripping. A brief commentary on each factor will assist the reader in evaluation of his or her own factory situation. 3.1 Type of Machine Two primary types of dipping units are employed in production manufacturing - batch and continuous chain. The technique used for automatic stripping will differ in accordance with the general overall type or machine employed. General access into tile former rack or individual former is a consideration and must be evaluated. A batch machine often employs the use of pallets measuring 1.5 by 3.0 meters, containing a dense former pack. The key to consider automatic stripping in this environment is accessibility to all formers. The best condition for batch machines are those whereby individual former
  31. 31. 31 "strips" (containing several formers) separate from the pallet, which enables free and clear access for the automation. The key consideration for chain lines is that of former orientation and chain speed. For nonrotating former chain lines, orientation of the molds are already accomplished, making for an ideal auto stripping condition. However, rotating former lines, which are the most common type used in Asian factories, require the adaptation of a former orientation system when entering into .the automatic device. This can be accomplished with a "carrier spoke" or "D" cam device (machined flat surface on a round bar), both of which contact and slide across a frictionless surface to stabilize the mold. 3.2 Type of Glove and Sizing In general, supported gloves utilize the "straight-off" techniques and unsupported gloves necessitate the "glove reversal" techniques available. However, there are some exceptions to unsupported gloves, which can mandate "straight-off" approach. In general, ambidextrous gloves are easier to strip than hand specific ones, considering the reversal technique. The protruding thumb on a surgical glove former can make for a stubborn strip, unless employing the proper technique. Another consideration for hand specific gloves is that of straight finger versus curved finger design. The most challenging combination would be that of a curved finger surgical-mold, produced on a continuous chain conveyor. It can be accomplished but generally by using the full mechanical pick technique described in Section 2.5, which can involve a sizable investment in capital. 3.3 Type of Polymer The type of polymer employed will also greatly sway the selection technique. A general order of automatic stripping complexity by glove polymer list for unsupported gloves, in the opinion of the writer, is as follows, listed from easiest to hardest; Easiest 1 Natural rubber latex
  32. 32. 32 2 PVC 3 Nitrile 4 Neoprene 5 Styrene butadeine 6 Silicone Hardest 7 Polyurethane 3.4 Former Shape and Texture Mention has already been made for the consideration of ambidextrous gloves (formers) versus hand specific gloves (formers). However, another key to auto stripping successfully, lies in the former shape and texture. For natural rubber exam gloves, a more "tapered" mold shank from cuff to wrist area, functions better for certain strip techniques, particularly the combination air and mechanical approach. The other key area of the mold is that of the thumb orientation or protrusion. A more gradual 1, sloping" thumb allows the glove to work its way over the mold more easily, versus a sharp bend at this area. A lesser consideration, at least for natural rubber products, is that of glove texture. In general, all former surfaces can adapt well for unsupported natural rubber glove former textures employed, including unglazed, spray bisque, and glazed. However, for synthetic polymers such as silicone and polyurethane, a glazed former surface will perform more consistently for removal techniques, both manual and automatic. 3.5 Glove Sizing Management This factor may not affect many of the participants of this conference. In general, most current chain lines in Asia are dedicated to one glove size only. This is the most simplistic condition under which to address automatic glove stripping. Some of these machines (rotating form - over and under chain), may employ one size former on one line side, with another size former on the other machine side. This also represents a favorable condition.
  33. 33. 33 However, larger volume machines (the writer's company has designed machines with volumes up to 60,000 pieces per hour) typically contain several gloves sizes on the same system. Therefore, two further considerations must be given to this condition; 1. The technique employed must be able to adapt to different former sizes coming through the system. 2. After, the automatic strip is. accomplished, size sortation must be considered, which can be accomplished manually by a single operator, or by additional automatic means. 3.6 Polymer Formulation One word of caution to anyone considering implementation of automatic stripping technology to their plant - be prepared to aller your latex and coag formulations, if necessary. The writer is not qualified to comment on specifies of formulation adjustment. However, we have more often than not, seen our customer base require some modification to their formulation to avoid glove tearing (if using compressed air source as part of the technique) and ease of release from the mold. The amount of mold release in the coag may require adjustment. 3.7 Current Level of Formulation Reliability and Equipment Reliability This is key - key - key. The writer cannot emphasize enough the importance of consistent glove production in a manual stripping environment, before investing in automatic technology. Inconsistent formulation management in film properties from day to day, machine to machine, etc. can allow the auto strip technology to work some days, and falter on other days. For example, if the level of calcium carbonate in the coag fluctuates, auto strip effectiveness can plummet. On the machine side, one important performance statistic is that of "good beads (rolled cuffs)". If the bead roller unit on the machine misses beads from time to time, you can expect the auto strip device to do the same. If the system oven performance fluctuates thus causing the general state of curing to decrease, auto stripping performance will
  34. 34. 34 suffer. In general, auto stripping works best with a more highly cured glove. You should consider yourself a candidate for automatic stripping technology only if your day to day machine and formulation performance is consistent and reliable. 4.0 IMPLEMENTATION OF TECHNOLOGY The candidate for automatic stripping technology, after determining that they meet all prerequisites for institution of the automation, must be prepared to exercise patience during implementation. Initially, the first phase of the evaluation, which is proper identification of technology, should occur by an on site study on the part of the automation provider. After thorough assessment of the application and other factors, expect a design phase to ensue, even in the event the automation provider has already supplied technology to other firms. As insinuated throughout this paper, every plant can differ in machine conveyance, type, speed, and especially formulation. The state of glove cure at the strip station is crucial for reliable performance. After this phase, the automation equipment is fabricated and assembled for installation at the user's plant. Installation of the technology normally would require from 3 to 7 days to complete. The commissioning phase of the technology is the area whereby the user needs to exercise patience. Several adjustments to the technology are typically necessary. As previously mentioned, it may by imperative for the formulation to change to assist reliable take off of the glove. Itemized below for the reader's review is a representation of a typical schedule for an automation program; Phase One Technology Identification 2 to 6 weeks Phase Two Engineering Design 3 to 9 weeks Phase Three Build Equipment 8 to 10 weeks
  35. 35. 35 Phase Four Install Equipment 2 to 3 weeks Phase Five Commission and Debug to 12 weeks TOTAL PROGRAM 20 to 40 weeks 5.0 I A CHALLENGE FROM THE AUTHOR The writer encourages the reader to not limit the prospects of their plant to automatic stripping alone. Without question, the implementation of auto strip technology will make for significant productivity improvements in your operation. However, my challenging question to the reader is this - why limit your thinking to that of glove removal only? Today, in the USA, few glove manufacturing plants remain due to many reasons, the primary one being competitive forces and productivity improvements over the last 15 years in Asian operations. The plants that remain active and successful in the USA, in general, are successful for one reason - they have nearly removed all plant labor from the manufacturing process. This means they not only strip gloves automatically in a reliable fashion, but after stripping automatically sort gloves, and automatically convey them to the packing room, automatically moving them through any tumble drying necessary, and automatically counting and orienting the gloves, into an automatic packing operation. The writer has not only seen the advent of this technology, but has been intimate with it in concept and design. My challenge to you is to have this vision for your factory. This is not"drawing board fluff" -it is reality for the 20th century and beyond.