Science: For Rainy Days

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One way to make a raincoat is to use a solid sheet of rubber or plastic that water cannot penetrate. Another and newer way is to cover the fibers of ordinary cloth with a substance that water does not "wet." Many substances have this property, e.g., the oil that waterproofs a duck's feathers. But most of them are unsatisfactory; they wear off or are easily removed by cleaning processes. The current house organ of the Dow Chemical Co. tells about waterproofing agents made of silicones: organic compounds with atoms of silicon built into their molecules.

When a drop of water hits unprotected fibers such as cotton or rayon, its molecules are attracted more strongly by the fibers than they are by one another. So the drop breaks up and its water molecules spread over (i.e., wet) the fibers. But silicone has less attraction for water than the water molecules have for one another. When fibers are covered with a film of silicone, raindrops do not wet them. The water remains in drop form, like globules of mercury, and does not penetrate the cloth.

The silicone treatment, says Dow, does not change the appearance or feel of the cloth perceptibly. When it gets dirty, a treated fabric can be dry-cleaned or laundered repeatedly without losing its waterproofness.