The basic raw material used in the manufacture of natural rubber latex exam gloves is latex concentrate which is produced from Hevea Brasiliensis natural latex using a steel tapping knife. Tapping is done at early dawn and the latex, collected in cups, is harvested several hours later and preserved with ammonia to stop it from premature coagulation. Because of its high water and non-rubber contents, about 70%, the latex is concentrated and purified by centrifugation to a 60% strength latex concentrate and stabilised using lauric soap for long term storage.
The major producers of this atex is Thailand, Indonesia and Malaysia. Before the latex can be used for manufacturing gloves it must be mixed with a recipe of processing chemicals which include sulfur, zinc oxide, accelerators, pigments, stabilisers, dewebbing agent and antioxidant. These are mixed with the latex and allowed to mature over a period of 24-36 hours for the mix to cure. During this period the sulfur will become mixed with the rubber particles to yield a latex compound ready for dipping.
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The hand molds (or formers) suspended on a continuous moving chain are first dipped into a slurry of alcium nitrate solution and calcium carbonate. The nitrate is a latex coagulant while the carbonate is a former release agent. After the nitrate and carbonate has partially dried out, the molds are next dipped into the latex compound. At this stage the nitrate immediately coagulate a layer of latex over the mold. Because a thin layer of carbonate separates the film from the mold, it help both in removing the gloves from the molds as well as in preventing the gloves from sticking together.
The freshly molded gloves in gel form are next leached in hot water to remove residual water- oluble protein and chemicals. The leaching process has became mandatory as a result of concern with allergic reactions associated with latex proteins. This step is essential to reduce residual extractable latex protein and the effectiveness is based on the amount of clean hot water used and dwell time. Beading brushes are run over the cuff ends to create a zone called beads which assist the users in donning the gloves.
The next stage involves drying and curing the wet gloves inside a long and deep oven heated either electrically, by natural gas or hot air. Curing, or vulcanization, converts the gloves into an elastic state by causing the chemicals added during compounding to react with the rubber molecules in the latex. This is the most critical step as without curing the gloves will not be elastic as well as they will tear easily. Vulcanization is a chemical process, discovered by Charles Goodyear, by which the physical properties of natural or synthetic rubber are improved.
It consists principally of heating rubber with sulfur and other substances, such as accelerators and activators. The sulfur does not simply dissolve or disperse in the rubber, but rather combines chemically, mostly in the form of cross-links (bridges) between rubber chain molecules and the sulfur atoms. On the other hand, accelerators and activators act as catalysts to initiate the process as well as boost the reaction at room temperatures. Cross-linking is analogous to net building using strings that are knotted together. The sulphur atoms reside in the nodes that link the rubber molecule chains.
Just as a net provides strength and a network to catch fishes, ross-linked rubber in latex provides the base upon which gloves are moulded. vulcanlzea ruDDer nas nlgner tenslle strengtn ana reslstance to swelling ana abrasion, and is elastic over a greater range of temperatures. In other words, natural rubber without undergoing vulcanization will eventually become too brittle in cold weather or become sticky in hot weather. Upon exiting the oven the gloves are dipped once again into hot water to perform a dry film post-leaching wash to extract residual proteins and chemicals that bloomed to the glove surface.
It has been established that a combination of wet and dry leaching has been very effective in removing residual proteins and chemicals and has significantly reduced the number of cases of protein allergies and sensitization. The leached gloves are then dipped into a slurry of corn starch and dried. This layer of corn starch assist in glove removal and donning. The final step of glove manufacturing involves stripping of the finished gloves from the molds using pneumatic air Jets. The now bare molds are then given a horough chemical wash and rinsing before entering the next dipping cycle.
Typically a mold will be dipped 4-5 times in an hour and once a week they will be given a thorough scrub to remove built-up stains and residues. Periodically the molds are visually examined for defects and replaced. Stripped gloves are hot-air tumbled to even them out as well as remove detachable powder. They are batched in bins, tagged and sampled for inspection. Operators examine the samples for visual defects, measure their weights and dimensions and check them for air leaks. Batches hat pass the inspections are routed for direct packing.
Those that fail are bagged for rework and retest. Passed gloves are also tested for physical properties. These are tensile strength and elongation at break before and after ageing to ensure they can withstand prolong storage and handling stress. All packed gloves are water tested for compliance to the maximum allowable leak failures called AQL. Batches that fail will be reworked or downgraded. Only gloves that meet the in-house specifications will be allowed to be shipped to customers warehouses.