Removing Natural Latex Proteins From Dipped Rubber Goods With Fumed Silica Additives



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Originally presented at a meeting of the Rubber Division, American Chemical Society Orlando, Florida

September 21-24, 1999


Published by SMTL: February, 2000 
Version: 1.0



Natural proteins are the main reason for Type I latex irritation suffered by sensitive people. About 10% of medical field employees dealing with dipped rubber products suffer from this allergy. This problem has surfaced in the last several years with the increase in production of rubber gloves for protection from infectious diseases.

Although natural latex is the best product available for price and performance, bad reputation and litigation potential has caused customers to shy away from it and look for more expensive and mostly inferior substitutes. Obviously many efforts were made to dispose off the proteins, however protein traces may still remain and cause irritation in sensitive people.

This paper suggests a way to eliminate the proteins with a simple, elegant and inexpensive method using fumed silica additives. The fumed silica attaches itself to the rubber particle and substitutes the proteins. The proteins are then easily removed. All this can be performed on line, eliminating chlorination and/or extensive washing and handling of the products off line.


The problem of proteins in natural rubber

Type I latex allergies can be caused by proteins in natural latex which occur in gloves, condoms, balloons and many other natural latex rubber articles. This problem is well publicized in the media and described in the literature [1] The mechanism by which the proteins in latex products are conducted to people might vary. The most common method of transport is by direct contact. Proteins in the latex migrate to the surface and penetrate the through the skin. A large number (estimated in the order of magnitude of 10% in the USA) of healthcare workers suffer from this problem and many have to quit their careers.

Total and specific analysis of proteins in latex goods.

The total quantity of latex proteins in a gloves are evaluated by extracting the rubber with an aqueous solution and measuring according to ASTM D5712. Analytical information on the Internet demonstrated that powdered latex gloves were found to contain high levels of protein, with between one and eight hundred micrograms of protein per gram of rubber. Certainly gloves contain less proteins today, but protein cleaning is an important ongoing issue.

The reporting level for ASTM D5712 is 50 ppm. Some laboratories claim that they can measure lower levels of proteins. For example Guthrie Research Institute, Sayre, PA claims detection level of 28 micrograms proteins per gram of rubber.

The latex protein analysis, ASTM D5712, might not represent a true picture of the latex irritation problem. Some of the proteins are harmful some are not. A general estimate is that only about 20% of the total protein evaluated by the ASTM test are harmful. ASTM D5712 might be misleading and can cause unfortunate conclusions. For example, someone can develop a process for latex protein cleaning, submit the clean latex to ASTM D5712 test and find that the protein level was reduces from 100 ppm to 20 ppm. At that level the protein are below detection, as far as ASTM D5712 is considered. It then could be concluded that the latex has been cleaned. However the 20 ppm proteins left, might happen to be the harmful proteins and the latex status as far as allergen protein cleaning did not change at all.

At least two specific tests were developed to further evaluate the harmful proteins, one is the LEAP test by the Guthrie Research institute [2] The LEAP test has a detection level of 0.2 ppm, about 100 times better than ASTM D5712. The other is the RAST test [3] [4] The detection level of the RAST test is better or equal to that of the LEAP test but it gives only relative allergen units per ml. Allergen units can range from 10.000 units per ml for an extract from raw latex film and 1 unit per ml extracted from vinyl glove film negative control or latex with undetectable level of allergenic proteins. The LEAP and RAST tests are very sophisticated biochemically and are good tools for evaluation of the harmful residues of proteins in latex goods. Generally I have found correlation between the two tests.

A previous paper on cleaning latex films with fumed silica [5] presents protein results derived by ASTM D5712. This paper presents further results derived by the LEAP and RUST tests.

Powder free latex gloves and other methods of reducing protein content

Latex gloves are the most commonly used latex articles and obviously the ones with the most problems. There is a major distinction between powdered and powder free gloves. Historically latex gloves are powdered because of two reasons. One is to protect the cured latex from sticking to itself in the manufacturing process, the other is to provide internal lubricity while donning the gloves. It is well known that proteins can transfer from the latex to the powder. While donning and removing powdered gloves, the protein coated particles become air borne and can contaminate large volumes. Glove manufacturers try to avoid powder by creating a powder free or powder reduced glove. In most cases the powder is removed by washing with water which further reduce proteins content in the latex itself by extraction. Other methods for protein treatment include passing the latex through special ion exchange resins [6] or altering the protein structure with protease enzymes [7]

Protein reduction by chlorination and polymer coating of latex gloves

Instead of introducing a powder cover to the cured rubber, some gloves are chlorinated to form a lubricious, non-tacky smooth surface [8] It is believed that chlorination or oxidation of the surface does also create a kind of protective skin that limits migration of residual proteins from the body of the latex to the outer layer. Another method of limiting protein migration is by coating the natural latex surface with a polymer barrier which also provides smoothness. However, these methods are good for the coated surface only, since proteins can migrate through the other side. Two sided coating is possible but expensive.

Extensive washing and film integrity

As stated previously, powder free latex gloves are produced by washing the powder away with water. Such powder free films might have a different kind of a problem. It is suggested by R. Thompson et al [9] that there is a relationship between hydration and compromised barriers in some surgical gloves. It is reported that

surgical gloves, long assumed effective barriers against blood-born pathogens, may not be effective as originally thought and, further, improperly manufactured gloves may cause a serious threat to healthcare workers and patients

It is thought that proteins in the body of the glove are being washed out and replaced by water molecules, creating diffusible channels through which small diameter species, like the HIV virus, could pass.

Experimental Results and Discussion

Low protein allergen analysis in latex compounded with fumed silica

Compounding of fumed silica dispersion for dipped natural latex application resulting in tensile and tear enhancement was described before [10] . In this study, two different brands of fumed silica were used, one from Cabot another from Wacker. It was found that the addition of fumed silica to latex compounds helps in manufacturing of cleaner latex goods with lower protein extractables. An additional advantage is enhancing latex viscosity but only with the Wacker product.

Protein values evaluated by Guthrie Research ASTM D5712 test are below 28 ppm per gram of rubber, their lowest detectable level. A non-silica rubber blank has 105 micrograms of proteins per gram of rubber, see Table I.

Further analysis by the LEAP method also at Guthrie Research, resulted in clean films having protein values below the detection level, 0.2 ppm, for this method. Latex processed without silica, does have protein residues, see Table II Table I and II report results with silica at a level of 1 to 5 W %.

Table III shows results with silica at a lower level of 0.3 W %. Here again the presence of silica greatly diminished the level of latex allergenic proteins measured by the LEAP method.

Latex films prepared with and without fumed silica were also analyzed by the RAST method at Johns Hopkins University School of Medicine, Reference Laboratory of Dermatology and Clinical Immunology. Two different sources of blood serum were used. Protein levels are below 1.0, the lowest level of detection, as clean as a vinyl film negative control. Latex processed without silica has protein residues, see Table IV

Mechanisms of protein cleaning by fumed silica - displacement or attachment.

The mechanism by which dispersed fumed silica reduces proteins content in the latex is not known. According to the literature [8] when the latex is in the tree, the particles are presumed to possess an adsorbed layer of proteins - lipid complex. As a consequence of the addition of ammonia for preserving the latex concentrate, the lipid materials are believed to hydrolyze slowly, releasing fatty acids. The proteins would stay adsorbed on the latex particles throughout the compounding process. It is suggested that the silica attaches itself to the surface of the latex particle substituting the protein molecules and displacing them, see Figure 1.

[Silica displacement diagram]Figure 1: Mechanism of Protein Displacement by Silica.

After displacement, the protein become mobile and easier to dispose off. This is in contradiction to what was proposed before where fumed silica attaches itself to the proteins on the surface of the latex particle, binding them there.

Summary and Conclusions

Cost and process advantage of latex dipped with fumed silica.

Presently, latex goods with relatively low protein content are being manufactured by various complicated methods. The silica dipping process is simple, short and inexpensive. Washing is minimal and since the latex is clean, there is no need for costly off line post treatments. The silica also serves as a thickening agent but contrary to conventional thickening agents, silica helps to enhance tensile not reduce it. Other advantages are an increase in tear strength, stability and transport properties. The only drawback is the price of this kind of silica which is just under $10 per dry pound. However if this is a concern, lower silica concentrations can be used with acceptable results.

Table I - Residual Protein in Dipped Latex With and Without Cabot Silica, Analyzed by ASTM D 5712 test, Guthrie Research Institute, Sayre, PA

Sample #
Guthrie Research 
Test #
Fumed Silica (W %) Proteins Extracted, 
ASTM D 5712 (ppm)
017401 - 1 9335 0 105
109607 - 2 9814 1.0 < 28
109607 - 3 9815 1.0 < 28
109607 - 1 9813 2.5 < 28
168901 - 1 9364 5.0 < 28

Table II. Residual Allergenic Protein in Dipped Latex With and Without Cabot Silica, Analyzed by the LEAP test, Guthrie Research Institute, Sayre, PA

Sample #
Guthrie Research 
Test #
Fumed Silica (W %) Allergenic Proteins 
149603-3 12253 0 26.8
149603-3 12253b 0 16.6
149603-3 12253c 0 15.3
109607-2 11652 1.0 < 0.2
109607-2 11652b 1.0 < 0.2
109607-2 11652c 1.0 < 0.2

Table III. Residual Allergenic Protein in Dipped Latex With and Without Wacker Silica, Analyzed by the LEAP test, Guthrie Research Institute, Sayre, PA

Sample #
Guthrie Research 
Test #
Fumed Silica (W %) Allergenic Proteins 
019906-04 16639 0 20.9
099906-04 16629 0.3 3.1

Table IV. Residual Allergenic Protein in Dipped Latex With and Without Cabot Silica, Analyzed by the RAST Inhibition test Method, The Johns Hopkins University Medical School, Baltimore, Maryland, Dr. Robert G. Hamilton, January 26, 1998

Serum tested, 
Brand A or B
Latex Allergen
Powdered Examination Glove , Positive Control A
Natural Latex No-Silica A
Natural Latex With 1.0 W % Cabot Silica A
Vinyl glove Negative Control A


1 - Tomazic, V.J., T.J. Withrow, R.J.Hamilton, Journal of Allergy and Clinical Immunology, 96, 635, 1995.

2 - Beezhold, D. Leap latex elisa for antigenic proteins, Volume 61, No. 2, The Guthrie Journal 1992.

3 - Gleich, G.D., J. B. Larson, R. T. Jones, H. Bear, J. Allergy and Clinical Immunology, 58, 158, 1974.

4 - Hamilton, R.G., N. F. Adkinson, Journal of Allergy and Clinical Immunology, 98, 872, 1996.

5 - Amdur, S., Polymer News, 23, 31, 1998

6 - Beezhold, D., US patent 5,563,241

7 - Perrella, F.W., Paper presented at the International Latex Conference, Akron, Ohio, July 22, 1998.

8 - Natural Rubber Science and Technology, A. D. Roberts, Oxford University Press 1988, Page 68 and Page 136.

9 - Thompson, R. et al, Journal of Allergy and Clinical Immunology, part 3, abstract 575, page 326, 1996.

10 - Amdur, S. et al, US patent 5,458,588.