OVERVIEW
With the increasing concern for allergic reactions from natural rubber latex (NRL), the search for suitable alternatives has been expanding. This is especially true for medical devices in general and medical gloves (exam and surgical) in particular. These gloves are the so-called "unsupported" gloves - simply meaning that the polymer (rubber) film is not deposited on a fabric or any other supporting material. Before discussing the properties, benefits and potential concerns of polychloroprene latex, I will provide a brief overview of latex in general and the dipping process involved for glove manufacture.
LATEX IN GENERAL
A latex, by definition, is a homogeneous colloidal dispersion in water of polymeric particles,1 mostly 100-300 mu in size.2 These polymers can be either natural or synthetic. Because of the submicron size of the polymer particles, they offer a large surface area to be covered. These particles are stabilized with the use of surface active agents such as anionic soaps and nonionic surfactants. Without the presence of these surface active materials, the polymer particles would coagulate and separate out of the aqueous medium as a solid mass. Overall latex properties are the combination of the polymer characteristics and the colloidal properties. Chemically speaking, the primary high molecular weight polymer in NRL is poly(isoprene). This polymer by itself, like other common commercial synthetic polymers, is not very chemically reactive under normal conditions and so would be innocuous to the human body. However, the stabilizing soap system in the NRL serum contains several natural proteins (proteins are biopolymers) and some of these proteins in residual amount or in entangled state with poly(isoprene) may cause allergic reactions. In the production of commercial synthetic latex, no biopolymers are used and hence they are free of allergenic proteins.
GLOVE PRODUCTION
The basic process for producing unsupported gloves by dipping process, irrespective of the latex used, is fundamentally the same.3,4 A glove former is first cleaned and then dipped in an aqueous inorganic salt solution (a coagulant). The former is then immersed into a latex compound that has been treated with vulcanizing chemicals. At this point, the colloidal equilibrium of the latex is disturbed by the electrical charges of the salt ions and as a result the solid polymer is separated from the latex. The polymer deposits as a rubbery film on the surface of the former and subsequently heated (vulcanized). The purpose of vulcanization is to chemically link-up the individual polymer chains into a stronger polymer network. This is also known as crosslinking. The gloves are then removed (stripped) from the former. Vulcanized gloves (polymer films) exhibit the characteristic properties of the polymer present in the latex used.
POLYCHLOROPRENE
Polychloroprene5 or chloroprene rubber (CR) is a polymer produced from the monomer (single building unit), called chloroprene. The systematic organic name of chloroprene is: 2-chlorobutadiene. Commercially, it is also known as baypren or neoprene. 2-chlorobutadiene is structurally very close to the monomer units of NRL, isoprene - 2-methylbutadine.
CR-latex is manufactured by a direct emulsion polymerization process. A mixture of liquid monomeric chloroprene and aqueous soap system forms an emulsion. The polymerization process is a typical free radical addition chemistry - where the individual monomer molecules add to each other to form a long chain molecule. The polymerization is initiated by a water-soluble, free radical initiator, and a wide range of polymerization temperatures can be chosen. Molecular modifier levels and conversion levels are determined to control the molecular weight and gel content. The solid content of the latex increases with the progress of the polymerization and once the desired level of solid is reached, the polymerization is terminated with the use of an inhibitor. The unreacted monomer is removed by steam distillation. A separate creaming step may also be employed to produce a desired higher level of solid.
Most of the commercial grades of CR is homopolymer - meaning no other monomer is used. However, small amounts of other monomers can be introduced in the polymerization step. Choice of the polymerization temperature, molecular modifier level, etc. dictates some of the polymer properties like its molecular weight, gel content and crystallinity. These in turn would control the final properties of the gloves.
Vulcanization of CR can be manipulated to get the optimum tensile strength, elongation and modulus. These properties guide the overall usefulness of the gloves. The vulcanization chemistry of CR is similar to NR. Both of these polymers have a double bond left in the polymer backbone, and these double bonds can be forced to form a network through the use of vulcanizing chemicals and heat.
Polychloroprene rubber (CR) gloves are very supple and come fairly close to NRL gloves in terms of feel, comfort and physical properties. The chemical structure of CR comes closest to that of NR because of the similarity in chemical structure of the constituting monomers: isoprene (for NRL) and chloroprene (for CR). Since polychloroprene is mostly a homopolymer (meaning it has no other building block other than chloroprene), the polymer backbone is very similar to that of natural rubber polymer. But because of the presence of chlorine in the molecule, CR gloves offer better chemical resistance compared to NR. It also has comparable water resistance and provides acceptable puncture and tear resistance. CR surgical gloves can be formulated to have superior wet donability. The combination of these properties: wet donability, comfort, tear and puncture resistance makes polychloroprene a good alternative choice for surgical gloves. Because of the thickness of surgical gloves, no other synthetic polymer comes closer to natural rubber gloves in terms of overall properties, protection and comfort. Being a synthetic product, it is also free from any natural allergens that may be associated with NRL.
From a manufacturers' point of view, production of CR surgical gloves is relatively problem free. Elasticity of the thicker film provides for ease of stripping and lower rejection rate. Chlorination may still be needed to produce powder-free gloves. Polychloroprene films are also compatible to selected polymers for lamination as well. This could be a thin film of polyurethane, for example, to provide better donning characteristics. If the polymers are not compatible for lamination, separation of two layers of different polymers would take place.
CR has more self-adhesion characteristics compared to NR or nitrile. This tacky property can be reduced significantly by choosing a higher state of vulcanization along with post-vulcanization chlorination treatment. Once a tack-free film is obtained, polychloroprene exam gloves offer the same supple, comfortable wearing characteristics provided by the thicker surgical gloves. The self-adhesion properties of polychloroprene latex can be successfully utilized in forming adhesive layers where solvent-free, NRL-free substrates are required.
Another promising possibility is latex blending. However, mixing of different types of latex can potentially create a number of compatibility problems including a lack of compatibility in the colloidal state as well as that of the polymers. It is possible to blend polychloroprene latex with other synthetic latexes, like nitrile, but this must be done with care in order to preserve the latex stability both before and after mixing with the vulcanizing materials. Since CR is a relatively soft polymer, blending with a harder polymer can expand the latitude of the polymer physical properties. Any ingredient that influences the pH, colloidal stability or the polymer interpenetrating network would influence the efficiency of the latex blend as well as the glove properties. Use of two different polymers with similar vulcanizing chemistry opens the possibilities of cross-linking of two different polymers. These aspects of cross-blending are not well studied, but offer some exciting potentials. Polychloroprene presents the desirable properties to be a component of the blends needed for glove industry.
REFERENCES
- Polymer Colloids II, Fitch, R.M., Ed, Plenum, New York, 1980.
- Ruan, J.J., Polymer Latexes, ACS Symposium series #492, 289, (1991), Daniels, E.S., et.al.(editors)
- Gelbert, C.H. and Berkheimer, H.E., ACS Rubber Div Education Symposium #18. (1987).
- General references are compiled in "Latex Dipping," ACS Rubber Div Library bibliography no. 15.
- Matner, M, Application of latices for the production of dipped goods, The Plastic & Rubber Institute Conference: "Emulsion Polymers," June 1982 (London).
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