Natural rubber (NR) latex is predominantly used in the production of surgical and examination gloves. This is attributed to the superior processing behaviour of natural rubber latex and high physical strength of gloves made from natural rubber latex. The production of latex gloves involves a process of coagulant dipping, in a continuous chain dipping line. During the coagulant dipping operation, clean porcelain formers are first dipped into a coagulant solution, usually calcium nitrate and then into a natural rubber latex compound. The next stage involves the bead rolling of the gloves by rotating brushes after a short drying. This followed by leaching of the wet gel on the former in hot water. Corn-starch is then applied to the outer surface of the formers via a slurry dip before the gloves are subjected to a drying and vulcanisation process in an oven.  Subsequently, the dried gloves are manuallly stripped from the porcelain formers, tested and packed.

Malaysia is currently the leading manufacturer of latex medical gloves, exporting some 11.7 billion pieces in 1992 with a total value of RM1.36 billion. Latex glove manufacturing is therefore an important sector of Malaysian natural rubber industry. However, the incidence of latex allergy associated with water-extractable protein in gloves, of a Type 1 hyper-sensitivity which can be potentially fatal, has posed a serious challenge to the latex glove manufacturers^1,2. This has resulted in various proposed actions by Food and Drug Administration (FDA) of U.S.A. such as the mandatory requirement of labelling all NR medical devices and the extension of biocompatability tests required currently for surgical gloves to examination gloves as well. The American Society of Testing and Materials (ASTM) has formed a task group to study test methods for measuring extractable protein in latex medical devices. Indeed, a regulation for latex medical device may be introduced as early as 1995 by the FDA of U.S.A. There is therefore an urgent need to manufacture gloves of low extractable protein content to meet these furture requirements. Several reasons have been given for the desire to produce latex medical gloves with low protein contents. These are as follows: 

• Prevention of gloves of excessively high protein contents from getting into the market and thus sensitising further individuals
• Gloves of low protein contents are likely to give low allergic response⁵
• To ensure that medical devices are safe for the users.

Several methods have been suggested to effectively reduce the water extractable proteins from NR latex gloves⁶ . These include:

• Use of low-protein latex
• Proper leaching of latex products during production
• Chlorination of latex products.

This paper examines the use of a low protein latex in the form of re-centrifuged prevulcanised NR latex and suitable leaching operations required to produce latex gloves of low protein contents. In addition, the allergic response of some resulting latex gloves is investigated by a skin-prick test on latex sensitive patients^7,8 .

Currently, intensified effort made at the Rubber Research Institute of Malaysia (RRIM) emphasises on finding practical measures to reduce to the minimum the extractable protein contents in latex examination gloves. Such measures include chlorination, the optimisation of the leaching operation, and the use of a recentrifuged prevulcanised latex or a low protein latex (LOPROL) in the production of latex examination gloves.

Leaching is the process of removal of hydrophillic materials from latex dipped products by washing them in water⁹ . It is an essential process in the production of latex dipped products. The removal of excess calcium nitrate and water-soluble non-rubbers such as proteins and added compounding ingredients results in improvement of physical properties such as tensile strength and film clarity, prevention of surface 'blooms' and reduction in water absorption of latex dipped products. The effectiveness of the leaching process is critical in the determination of the overall quality of gloves produced.

Therefore are basically two methods of leaching viz. Wet gel leaching and dry-film leaching. The wet gel leaching invloves the washing of the 'wet gel' i.e. gelled deposit on former, prior to drying and vulcanisation. Wet gel leaching is usually carried out on-line. In contrast, dry-film leaching consists of the washing of the dried, vulcanised latex product after removal from the former and is an 'offline' process. Where complete removal of hydrophillic materials is required, dry-film leaching for an extended period of 16 - 48 h, depending on the type of products made, is the recommended practice.

In the production of latex examination gloves, wet gel leaching is ofen carried out for a period of several minutes, usually 1-10 min in a continuous chain dipping line, the actual leaching time is very much dependent upon the design of the dipping unit. Leaching is by far the simplest and practical method to reduce the extractable protein in latex gloves. However, the need to incorporate the dry-film leaching operation in addition to wet gel leaching has largely been ignored in the current process of production of latex examination gloves. This is rather unfortunate as it has been previously established that a substantial amount of water-soluble protein is generated upon drying and vulcanisation of dipped products and that proteins are drawn towards the surface away from the former during this stage, giving rise to asymmetry of extractable protein distribution^11,12. Any form of leaching or washing, including the slurry dip, after drying is therefore expected to further remove the extractable proteins.

The slurry dip of corn-starch is often being carried out prior to the drying and vulcanisation stage during the production of latex glove. This has been found to be inadequate in reducing the extractable protein content in the latex glove. In fact, it has been observed that a slurry dip immediately after drying gives much more effective reduction of the extractable protein content in latex gloves. A comparison of extractable protein values between slurry dips before and after drying clearly demonstrates this (Table 1). The slurry dip has been carried out for a period of 10s and under agitation. However, it should be borne in mind that while a substantial amount of extractable protein of latex gloves can be effectively removed when the slurry dip is performed after drying, there is a danger of build-up of proteins with time in the slurry tank during the continuous production of gloves. A soluble protein level of as high as 1 mg/ml has been noted in corn-starch slurry which has been tipped up daily for a period of several weeks. In such cases there is a possibility of re-deposition of the soluble proteins onto the gloves, particulary when the concentration has reached a certain high level. Therefore while it is advantageous to slurry dip after drying, the protein level in the tank should bot be allowed to build-up.

In order to effectively remove the extractable protein during the production of latex examination glove from a prevulcanised latex, it has been found that a combination of wet-gel leaching and drying film leaching is most desirable (Table 2). In addition, the dry-film leaching can be carried out either 'on-line' or 'off-line'. The 'off-line' leaching treatment appears to be more effective, as both surfaces of the gloves are invloved (Figure 1).

The application of a direct water spray to wash off the water-soluble proteins from the dried gloves is an alternative method to an on-line dry-film leaching for the production of NR latex gloves of low extractable proteins from a prevulcanised latex. It is observed that a spraying time of about 30 s could reduce the EP of gloves to below 0.1 mg/g for a normal prevulcanised NR latex (Table 3). 

A substantial amount of water -soluble protein can be generated during the compounding of latex and upon heating of a NR latex compound. As such, by a process of recentrifugation of a diluted (with water) prevulcanised NR lated, it should be possible to remove a large amount of water-soluble protein in the prevulcanised NR latex. Indeed, a re-centrifuged prevulcanised NR latex is found to contain a significantly lower extractable protein content when compared to a normal prevulcanised latex (Table 4).

The best result, in terms of reduction of the extractable protein contents of latex gloves to the minimum, can be achieved by the use of a re-centrifuged prevulcanised NR latex together with a combined protocol of wet gel and dry-film leaching (Table 5) or a combined protocol of wet gel leaching and direct on-line water spraying (Table 6) . A final low extractable protein content of 0.024 - 0.026 mg/g can be obtained.

The allergic response of gloves made from prevulcanised NR latices has been studied by a skin-prick test performed on 10-13 latex-sensitive patients¹³. Except for gloves not subjected to wet gel leaching and have relatively higher extractable protein contents, all samples tested clinically indicate low allergic responses, particularly those obtained from the normal prevulcanised latex after a wet gel leach of 5 min and a dry-film leaching of 10 min and the re-centrifuged prevulcanised latex after a wet gel leach of 2 min and a water spray of 30 s. In fact 100% negative responses are shown by the patients tested with these two glove samples (Table 7). For comparison, a commercial glove with a high extractable protein content and giving 100% positive response in included as a positive control.

The physical properties of latex gloves, made from prevulcanised and re-centrifuged prevulcanised latices, with different extractable protein contents have been studied. In general, the unaged and aged properties of the latex gloves are found to be satisfactory (Table 8).

CONCLUSION

Our findings show that it is possible to produce NR latex gloves with low extractable protein contents and low allergic responses from a prevulcanised latex by mainly leaching. To achieve this the following applications are recommended:

• A suitable combination of wet gel and dry-film leaching operations, or
• A combination of wet gel leaching and a direct water spraying operation immediately after drying and/or
• The use of a re-centrifuged prevulcanised latex.

In each case a final post-drying slurry dip is essential.

In accordance with our findings, a wet-gel leaching of 2-5 min at 50°C coupled with a short dry-film leaching of about 30 s is sufficiently adequate for the production of gloves with low extractable proteins and low allergic response from a natural rubber prevulcanised latex. The most effective method of reduction of extractable protein contents in NR latex gloves is found to be the use of a re-centrifuged prevulcanised latex in combination of a protocol of wet gel leaching and dry-film leaching or a direct water spraying application. A guide of leaching protocol required for the production of NR latex gloves with low extractable protein contents is summarised in Table 9. Some modifications to these conditions may be necessary for continuous commercial production of latex gloves in view of the different designs for glove dipping units and variations in latex compound formulations and preparations.

All low protein gloves tested indicate low allergic response when tested clinically on latex hypersensitive persons.

In general, the physical properties of the low-protein gloves are found to meet the requirements of ASTM D3578-91 for NR latex examination gloves.

The authors wish to thank the Director of the Rubber Research Institute of Malaysia for permission to present this paper. Our thanks are also extended to Dr.P.F.Lai, Head of Latex Technology Division, for his valuable comments and encouragement. We would like to thank also Dr. K. Turjanmaa of Tampere University, Finland for kindly carrying out the skin-prick tests. The technical assistance rendered by En. Alias and En. Ng Cheong Sing of RRIM is greatly appreciated.

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