Floating polyethylene spheres

Floating polyethylene spheres are capable of reducing volatile emissions by up to 90%. Polyethylene spheres are compatible with a broad range of compounds including inorganic acids and bases and most aromatic and aliphatic organic compounds.16... 

Ping-Pong Balls. The use of hollow spheres of materials such as polyethylene or polypropylene, or other plastics that are spread over the surface of a liquid to reduce the rate of vaporization, has been studied. The rationale behind this technique is to cover the confined pool of liquid with lightweight balls that will float on the surface of the liquid and thus prevent vaporization. 

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