A Little More Love for Latex

There are more than 40,000 types of commercial products that are made from natural rubber latex (NRL), an extract of the Pará rubber tree (Hevea brasiliensis). Valued for its desirable properties, NRL is used in numerous products in the medical industry, including latex gloves, adhesive bandages, catheters, breather bags, and condoms. However, out of more than 200 proteins contained within NRL, 13 are known to be allergens. The American Latex Allergy Association estimates that 3% of the general population and 17% of healthcare workers exhibit some form of latex allergy, thus hindering the use of such products.

Fortunately, a solution exists for removing the antigenic proteins that can cause such immune responses. It involves the addition of aluminum hydroxide, Al(OH)₃, a well-known protein-binding chemical, to NRL while it is still in liquid form. This compound acts as a binding agent to the latex and produces protein complexes that can be removed using existing industry practices. The result is an ultra-low-protein variant of NRL that retains all of the advantages of the material while reducing the exposure of individuals to the allergenic proteins. How is modified NRL made? What makes it superior to traditional untreated NRL? And what advantages does it offer healthcare workers and patients in terms of specific medical applications? These are the questions this article attempts to address.

Manufacturing a Low-Protein Latex

The complex properties of NRL—as well as its availability, ease of production, and performance—allow its use in a broad range of medical applications. In addition, its proven “green” biodegradability often makes it an appropriate choice in an increasingly environmentally conscious society.

Manufacturers have utilized low-protein latexes, combined with improved leaching processes, in an attempt to offer consumers safer latex products. The Association of Operating Room Nurses (AORN) has even defined safer latex as latex with less than 50 µg/dm² for total extractable protein and less than 10 µg/dm² of antigenic protein. Latex products that meet this standard are referred to as “Latex Safe” by AORN. A relatively new form of modified NRL not only contains significantly fewer allergenic and total proteins than previous versions but also has additional benefits that enable manufacturers to offer better products with the modified NRL as a cost-effective ingredient. Each end-product discussed in this article has its own unique compounding additive requirements as determined by its respective manufacturer. The benefits of removing specific nonrubber impurities naturally found in NRL include the production of a more stable, cleaner latex that may require fewer compounding additives such as whiteners and stabilizers during production.

Unlike most synthetic alternatives, Al(OH)₃–treated NRL uses green chemistry to modify pure latex. The modified NRL derived from the Hevea brasiliensis rubber tree remains 100% natural. As proof, note that bacteria and fungi are capable of degrading NRL;^1,2 one elegant experiment has demonstrated that latex balloons degrade equally, if not faster than, oak leaves.³ In contrast, many synthetic alternatives to latex, such as PVC vinyl, styrene, nitrile, choloroprene, and polyurethane, which are made from petrochemical derivatives, are neither biodegradable nor compostable. Incineration of these synthetic products can liberate toxins and carcinogens, such as dioxin, cyanide, vinyl chlorides, and hydrogen chloride.⁴ Unlike such synthetic alternatives, the modified NRL has minimal impact on the environment.

The treatment process for modified NRL removes specific nonrubber impurities from NRL through the directed application of Al(OH)₃. A commonly used absorbent, emulsifier, ion exchanger, and antacid, Al(OH)₃ is commonly used in the process of water purification. It forms a jellylike structure suspending any unwanted materials in water, including bacteria.⁵ Using traditional latex processing methods, a slurry of Al(OH)₃ can be strategically added to the pure latex. Protein and other nonrubber impurities from the pure latex emulsion bind to insoluble Al(OH)₃, with some of the nonrubber impurities adsorbed to the reactive surface of the Al(OH)₃ crystals.⁶  

With this processing step integrated into the manufacturing stage, there is no added expense of capital equipment. Reacted Al(OH)₃ complexes are removed by a combination of filtration and centrifugation. The remaining rubber particles retain the surrounding lipid layer, which, during subsequent maturation, improves the mechanical stability of the latex as higher fatty acids are formed. Notably, scientists have observed that this process yields products that exhibit greater clarity and significantly reduced odor when compared with untreated NRL, in addition to fewer antigenic proteins, without sacrificing the properties that give NRL its advantages over synthetic alternatives. Prior industry efforts have produced low-antigenic-source latex through the treatment of raw latex with enzymes with little commercial success.⁷ Results from field evaluations suggest that modified NRL offers improved performance characteristics that scientists believe to be a result of the nonrubber impurity removal process, and one that is not part of the enzymatic treatment. These benefits include improved overall stability, color, gel time, pickup (i.e., weight increment or thickness), and air retention.

Advantages over Traditional Untreated NRL

The most prominent products made from NRL are latex-dipped goods that encompass nearly 50% of latex production, e.g., gloves, condoms, balloons, breather bags, and tubing.⁸ Other items made from latex include foam products (mattresses, pillows, and cushions), adhesives (pressure-sensitive and cold-seal applications and footwear and bandages), and elastic thread (socks, hosiery, and undergarments). Modified NRL can transform those industries that rely on natural rubber latices. The reduction in certain nonrubber constituents in the source material contributes to its greater stability, and therefore longer shelf life, compared with standard NRL. Customer observations reflect the clean appearance and lack of odor in the expansive list of products made from modified NRL.

In addition to having a variety of foam applications, NRL plays a role in the adhesives market. NRL was the first polymer used to produce pressure-sensitive adhesives. NRL sticks to itself, making it ideal for cold-seal adhesives. NRL has several key physical properties that are advantageous when used in pressure-sensitive and contact adhesive formulations. For example, its low glass transition temperature, –70°C (versus –40°C for polychloroprene), and low surface energy enable NRL to effectively flow evenly over surfaces when in liquid form. Additionally, the proteins in NRL can cause an allergic reaction, manifested as sensitized skin. This sensitization may be mitigated with the use of a modified NRL with no detectable carryover of the antigenic proteins that can cause the allergic responses into the end product.⁹

Significantly, the modified NRL, having an ultralow protein content of 50 µg/g versus 1000 µg/g for untreated NRL as measured in cohesive bandages, has exceptionally high shear stability compared with traditional untreated NRL. Furthermore, gel times can be customized using additives to suit particular application needs. Manufacturers noted positive processing characteristics of the modified NRL in both spray and roll coating applications.

Another advantage to the use of modified NRL is the decreased amount of water and energy required for its manufacture. Within the latex-dipped goods industry, manufacturers have demonstrated increased efficiency by reducing processes such as excessive washing and leaching, which are typically used to remove compounding chemicals and reduce protein levels. This reduction can significantly lower water and energy consumption, and simultaneously reduce the presence of harmful leachates such as zinc and other potential contaminants in waste water. The overall environmental impact is minimized, resulting in increased production cost savings. Modified NRL is slightly more expensive than traditional untreated NRL, with a 25% premium on average. It is priced comparably to, and in many cases less expensively than nitrile, chloroprene, polyisoprene, and other synthetic alternative materials, and provides significant cost value when compared with other synthetic latices.

Raw, natural latex is a liquid. When dried and cured, the film dries semitransparent yellow. Manufacturers can add whitening agents, such as titanium dioxide or calcium carbonate, to express whiteness in the finished product or to provide a white background for which color pigments can be used. Although it is a common alternative, the use of titanium dioxide can be more expensive. Because the modified NRL is characteristically white in appearance, its use reduces the amount and cost of whitening agents.

Collectively, these results suggest that manufacturers can achieve savings in energy and material costs when using modified NRL. Further, decreased pigmenting is an added cost benefit for such dipped good industries as breather bags and rubber tubings. Additionally, the use of modified NRL decreases the costs of compounding chemicals as well as rinsing and leaching for gloves and foam. It also decreases the costs of spraying for adhesives as well as those of compounding and pigmenting chemicals, dipping, and rinsing and leaching for balloons. 

Manufacturers interested in advances in latex technology may find modified NRL intriguing. Natural products that minimize the environmental effect while maximizing economic, health, and safety benefits are critical to the sustainability of the latex industry. This need is addressed by commercializing the process of modifying NRL while enhancing its attributes and performance. Preliminary studies of the process of using Al(OH)₃ indicate that it may eliminate up to 95% of proteins and other nonrubber composition in latex. If confirmed by further research, use of Al(OH)₃ would allow manufacturers to offer notably safer pigmented products. Additional benefits worth gauging in further detail include modified NRL’s gas and air retention and drying time.   

Preliminary studies indicate that foam made with modified latex may emit significantly less odor and be much whiter in appearance compared with foam made from standard latex. An increased stability of modified latex would give foam manufacturers the ability to increase their use of a natural product whose environmental advantages would tie into a growing global initiative for green products. Latex-dipped goods manufacturers, who traditionally are users of large amounts of water and energy required to achieve acceptable protein levels, would benefit from using modified latex, if it is confirmed that the latex modification process results in an ultra-low-protein product. Additionally, the modified latex compound requires fewer additives such as stabilizers and whitening agents, both of which are traditionally used in the latex modification process in preparation for end-product manufacturing. 

The use of modified latex represents an interesting potential alternative for manufacturers that are currently using traditional untreated latex across a broad scope of industries, including medical manufacturing industries. If the material in fact possesses the performance benefits attributed to it, it would offer a unique value proposition to manufacturers, allowing them to capitalize on its green advantages while addressing health and safety concerns. Production cost-saving opportunities using modified latex could make this a sensible material of choice for future generations. Balancing material acquisition and production costs, manufacturers could quantify the true cost savings of Al(OH)₃–treated NRL.

Surgical and Examination Gloves

The rise of the AIDS epidemic in the 1980s highlighted the widespread use of latex gloves to protect against infection. But for many healthcare professionals, the increased exposure to latex led to allergic reactions. Symptoms ranged from watery and itchy eyes to red and irritated skin to trouble breathing and even life-threatening anaphylaxis. Healthcare professionals developed dangerous latex allergies that, in some cases, limited or ended their care-providing careers. Latex gloves were also negatively perceived because of the powder associated with the gloves that left residue on users’ hands and caused skin irritation.

It must be stressed that NRL gloves are known for their strength, thinness, and tactile sensitivity. As such, they have been, and still are, widely used—particularly in healthcare settings where effective barrier protection is of great importance against viral transmission and infectious diseases. With the exception of vinyl or PVC gloves, which have been shown to provide lesser barrier protection, NRL gloves are generally less expensive than many synthetic alternatives, such as polyisoprene, polychloroprene, and often nitrile.

As outlined above, there appears to be a need for a safer, modified NRL in the surgical and examination glove market that contains less than 50 µg/dm² of total protein considered “Latex Safe” by AORN, but that retains the barrier protection and tactile sensitivity of latex. Both exam and surgical gloves in manufacturer trials contained significantly fewer proteins than untreated control gloves. These results indicate that glove manufacturers using the modified NRL as their raw feedstock could adhere to ASTM glove protein compliance levels with only “pre-leaching” with Al(OH)₃–modified NRL. While mitigating immune responses that can result from the antigenic proteins, such gloves would preserve the durability, comfort, fit, tactile sensitivity, and high resistance to puncture and tear for which NRL is known. One major U.S. healthcare company has received 510(k) clearance for an exam glove made from modified NRL, and others are expected to follow suit. 

Adhesive Bandages, Catheters, and Condoms

Latex gloves represent just one of several potential markets for modified NRL within the healthcare arena. A second notable potential application is adhesive bandages. Many cohesive medical bandages use NRL as an adhesive; however, if the standard NRL is not modified, it can once again pose a potential risk of provoking a sensitive skin reaction. Bandages produced with a modified NRL–formulated adhesive have in preliminary tests shown a 95% reduction in the protein content than bandages made with adhesive containing standard NRL. A combination of greatly reduced protein, improved processing, and effective adhesion could make the modified NRL a prominent candidate for the next generation of NRL adhesives.    

 
Certain catheters, including urinary catheters, use NRL. All such products that routinely directly or indirectly come into contact with mucosa, nonintact skin, and internal bodily spaces (including catheters) may trigger complications. Therefore, they represent another potential target for modified NRL manufacturers. Anesthesiologists who handle breather bags made of latex may also experience allergic symptoms and thus this equipment is also a likely market.
Finally, some individuals who suffer from latex allergies have exhibited irritation associated with traditional untreated latex condoms. A major healthcare company has recently introduced a new condom made with modified NRL that, according to some tests, retains all the positive qualities of the traditional untreated latex variety, including barrier protection, tensile strength, tactile sensitivity, elasticity, comfort, and fit, and includes the only label claim cleared by FDA of less than 2 µg/dm² of antigenic protein. This in fact is the first modified NRL product available to consumers.

Conclusion

The development of modified NRL has the potential to pave the way for a new era in the use of latex products in the healthcare arena. As production of this material is ramped up in coming years, and if its safety is independently analyzed and confirmed, it is reasonable to expect that hospitals and other facilities will find it a welcome alternative to traditional untreated NRL as it already meets AORN’s “Latex Safe” standard. Both patients and their caregivers who are prone to latex allergies may find a new class of products at their disposal that can aid in the healing process while mitigating the immune responses that could result from traditional untreated NRL proteins.   

References

1.    A Ikram, “Environmental Degradation of Natural Rubber Gloves,” Malaysian Rubber Board, Monograph No. 2 (1999): 1–20.
2.    DK Burchette, “A Study of the Effect of Balloon Releases on the Environment,” National Association of Balloon Artists (1989); available from Internet: www.hi-float.com/biodegradeablility.pdf.
3.    “Balloon Facts,” The Balloon Council; available from Internet: www.balloonhq.com/BalloonCouncil/facts.html.
4.    MD Moriss, “Health Considerations of Synthetic Alternative to Natural Rubber Latex,” Journal of Natural Rubber Research 9, no. 2 (1994): 121.
5.    HF Holtzclaw, WR Robinson, and WH Nebergall, General Chemistry, 7th edition (Washington D.C.: Heath and Co., 1984): 884–885.
6.    M Swanson et al., “Vytex Natural Rubber Latex: An Innovative Source Material for Natural Rubber Products,” available from Internet: www.vytex.com/technicalpapers.aspx?id1=2&id2=2&pageid=M19.
7.    S Nakade et al., “Highly Purified Natural Rubber. Preparation and Characteristics of Gloves and Condoms,” Journal of Natural Rubber Research 12, no. 1 (1997): 33–42.
8.    L Casarett and C Klassen, Casarett and Doull’s Toxicology: The Basic Science of Poisons, 6th edition (New York: McGraw-Hill, 2001): 540.
9.    K Cornish et al., “ASTM D1076 Category 4 Latex and Quantifying Guayule (NRG) and Hevea (NR) Latex Protein,” in Proceedings of the International Latex Conference (Charlotte, NC: Rubber & Plastics News, 2006), 246–261.

William R. Doyle is president and CEO of Vystar Corp. (Duluth, GA).