Important Facts about Latex and Latex Gloves

Natural Rubber Latex Gloves - Made from a Renewable Resource Natural rubber (NR) latex 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, which are made from petrol chemicals.
  Figure 1. Natural rubber latex collected in a cup after skillful tapping of the bark of a 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 substances such as proteins, lipids, carbohydrates, sugars, some metals, fatty acids, and other substances, 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 rubber 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 products, whether by dipping (for gloves, balloons, condoms, catheters, baby soothers, rubber tubing, 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, many manufacturers include processing steps that reduce the level of protein in their gloves. The typical glove manufacturing process is as follows:
manufactring diagram
 
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 of hands are immersed first in a coagulant and then in the latex mix for appropriate dwell time to give the desired latex film thickness. The coagulant is applied to facilitate the deposition of a 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 or curing of the gloves are usually done in hot-air ovens, initially at lower temperatures of 80º-90º C, and then at higher temperatures of 100º-140º C where necessary.
Post-cure leaching or dry-film leaching: The cured gloves are immersed in clean water tanks to 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 a 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 the formers. This is often carried out manually, frequently with the assistance of compressed air, but an automatic stripping system is becoming more common.
Powder-free gloves: Latex gloves with very little or no powder lubricant can be prepared by either (i) chlorination or (ii) polymer coating. While chlorination oxidizes the outer rubber 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. Both processes can be carried out on-line, without the powder-coating step, or off-line by washing first the finished powdered gloves, then subjecting them to the chlorination or polymer-coating treatment.
Removing Glove Proteins
Protein Status in Latex
When subjected to ultracentrifugation at approximately 59,000 gmax, latex can be separated into three main fractions: (i) top rubber hydrocarbon fraction, (ii) the ambient serum (known as C-serum) in which all rubber particles are suspended, and (iii) the denser bottom non-rubber particle fraction, particularly lutoids, which contain yet another serum (known as B-serum).
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 surfaces (i), the remaining three-quarters are in the non-rubber phase [fractions (ii) and (iii)] of the latex, and they are mostly water soluble (Figure 3).
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Figure 3. Freshly collected Hevea Brasiliensis latex separated into its three main fractions upon 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 soluble proteins - or the residual extractable proteins implicated in allergic reactions - are markedly low. Depending on which manufacturing process is used, the level of residual extractable protein can vary widely.
 
Latex Allergens
Not all proteins in the residual extractable fraction cause the allergic reaction. Although to date 13 proteins (mostly soluble) in raw Hevea latex have been reported to be possible allergens as defined by their display of IgE antibody binding activities, it is unlikely that all of them would be present in the finished products after processing.

Cross Reactivity
“Close structural similarities between any two allergens from divergent sources can produce 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 important to note that not all food-allergic individuals are sensitive to latex, and not all patients with latex allergies will have problems with these foods.

A study of binding patterns of IgE antibodies from the blood sera of individuals, who were not latex allergic but who had reactions to fruits, supported this. The study findings also showed that multiple bindings could occur between latex serum proteins and IgE from many who reacted to extracts of fruits but not to latex gloves. On the other hand, more specific and fewer bindings to latex protein were observed by those who were skin tested positive to latex glove extracts.”
 
Allergenicity of Latex Gloves
The allergenic potential of latex gloves can be measured in-vivo by skin-prick testing (SPT) on latex-allergic subjects, or in-vitro by specific IgE antibody-inhibition immunoassays. The SPT method is known to be more specific and more sensitive than the IgE binding techniques. However, all of these methods are relatively sophisticated, and require further improvements, and they are also expensive to perform.

The presently preferred method is the quantification of total proteins using the modified Lowry micro-assay, which is technically easy and possible to standardize as well as cost effective. 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 gloves based on both serological IgE specific inhibition immunoassays, and the SPT testing, have been established. Latex gloves with high residual extractable protein contents are associated with positive SPT or high allergen contents. Latex gloves with very low residual extractable proteins, on the other hand, tend to have very low or negligible SPT reactions by latex sensitive subjects.
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Figure 4. Total extractable protein content (as measured by modified Lowry assay) of latex gloves and percentage negative skin prick test response shown by latex sensitive subjects. (Ref: Yip E., Turjanmaa K., Ng K.P. and Mok K.L. "Residual extractable proteins and allergenicity of natural rubber products," J. nat. Rubber Research, 1994; 9: 79-86;)
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Figure 5. Total extractable protein content and allergen level of 46 lots of latex gloves, as determined by modified Lowry test and IgE latex specific ELISA-inhibition respectively. (Ref: Yip E., Palosuo T., Alenius H., and Turjanmaa K. "Correlations between total extractable proteins and allergen levels of natural rubber latex gloves," J. nat. Rubber Research, 1997; 12: 120-130)
 
This method, although non-allergen specific, offers a technically easy and standardizable method that is very helpful in product developments and improvements.
 
Protein Reduction - Product Improvement
Residual extractable protein content of gloves can now be reduced from as high as 1,000-2,000 µg/g of gloves to a low of less than 50 µg/g using improved manufacturing technologies, which include:
  • Use of low-protein latex concentrates
  • Proper leaching protocols
  • Chemical or enzymatic deproteinization
  • Chlorination
  • Polymer-coating
 
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