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Coagulation dip coating of rubber latex

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The coagulation dip coating process

Thin rubber films for making balloons, latex gloves or condoms are prepared by the coagulation dip-coating process. The process starts with preparing a latex rubber dispersion. Latex, a natural polymer derived from the sap of rubber trees, is mixed with various additives to create a homogeneous dispersion of rubber particles in water. The particles must be stabilized by negative charges, typically available on carboxylic acid groups at high pH. Synthetic latex can also be applied and can be made via emulsion polymerization like the well-known XNBR latex that is used for nitrile rubber gloves. The carboxylic acid groups are polymerized in the rubber polymer by using acid monomers like maleic acid or acrylic acid. Polybutadiene latex is also prepared by emulsion polymerization and here the negative groups stem from the rosin or tallow fatty acid that is used during the emulsion polymerization to stabilize the particles. Finally, latex can be made by direct mechanical emulsification of synthetic rubber solutions like polyisoprene using anionic emulsifiers like rosin acid.

During the dipping process a mold or form of the desired shape is dipped into the latex solution. The mold is often made of metal, glass, or ceramic and is pre-treated to prevent the latex from sticking to it. Prior to immersion in the latex solution, the mold undergoes a coating process with a coagulant, typically calcium nitrate, along with optional additives such as calcium carbonate (CaCO3) particles and wetting agents. Within the latex dispersion, the coagulant dissolves, prompting the rubber particles to coagulate into a particle gel. This gel impedes further diffusion of the coagulant within the latex dispersion, and the duration of immersion (dwell time) is a critical parameter influencing the thickness of the wet gel. Additionally, the concentration and size of the latex particles serve as crucial factors in this process. It's important to note that this technology diverges from conventional dip coating methods. Unlike the conventional approach, where film formation occurs during withdrawal from the solution, here, the film forms within the latex itself. This distinction allows for lower viscosity and faster operation, enhancing efficiency.

After the mold is dipped into the latex solution, it is then slowly withdrawn. As the mold is removed from the solution, the latex film adheres to its surface. As the latex-covered mold is exposed to air, the solvent in the latex solution begins to evaporate. As the solvent evaporates, the latex undergoes a process called coalescence, the fusion of the rubber particles transform the gelled latex into a solid continuous rubber matrix. The coagulation process may be aided by the addition of additives and by controlling temperature and humidity conditions.

The latex film on the mold is dried to remove any remaining solvent. The drying process may involve air drying or the application of heat. Additionally, the rubber film may undergo curing, a process in which it is subjected to heat and chemicals to further strengthen and stabilize its structure. After the rubber film has dried and cured sufficiently, it is carefully removed from the mold.

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