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Emerald Group Publishing Limited
Copyright © 1998, MCB UP Limited
Keywords Coatings, Plastics, USA
Eighty percent of coatings (interior and exterior) sold in the USA are manufactured using an aqueous base. Plastics are inert to water. This single property has dramatically accelerated the use of plastics in the packaging of coating products. All other economically viable materials react to water and require the addition of a coating material to protect them from the corrosive or softening effects of water. For all the benefits plastics offer, the packaging of paint and coatings in plastics can present numerous challenges to both the manufacturer of containers and to the filler/producer. Solvent-based coatings are typically not compatible with plastics, including polyethylenes. While aqueous-based paint and coatings may appear to be innocuous ladings for a plastic container, all additives must be identified and tested as non-reactive or low-reactive. For example, while PE is chemically resistant to hydrochloric acid, lye and other products that are obviously dangerous and would appear difficult to package, surfactants and wetting agents that are an integral and necessary component of coatings can indeed be insidious in affecting packaging performance.
Wetting agents, which allow a product to flow and coat a surface are particularly aggressive to PE. These surfactants attach PE via a phenomenon known as environmental stress cracking (ESC). Surfactants and wetting agents will microscopically attack PE, with the potential of ultimately producing failures in both pails and lids. To avoid such problems one must choose a PE that is tailored to accommodate aqueous-based solutions. Not all PEs are the same. When choosing a PE for paints and coatings, it is necessary to choose a plastic that has desirable environmental stress crack resistance (ESCR) properties. Resins with higher levels of short chain branching have proven to be effective. Alternatively, or in conjunction with lower density resins, a higher molecular weight resin is effective. These products have a high level of ESCR.
A commonly misperceived notion is that a diluted surfactant is less aggressive. However, the opposite is generally the case. The industry test to determine the ESCR properties of moulded plastic pails consists of filling pails with a 10 percent surfactant/90 percent water solution and then subjecting the pails to a topload at elevated temperatures. The standard surfactant is Igepal C0-630. The 10 percent Igepal solution is significantly more aggressive to PE than 100 percent Igepal.
This is somewhat analogous to a eutectic antifreeze solution for your car: a 50/50 mixture of antifreeze and water is more effective than using 100 percent ethylene glycol in preventing freezing and boilover. Paints and coatings vary widely in their chemical makeup. What may appear to be a benign formulation may indeed be excessively aggressive to polyethylene.