Page 29 - Curriculum Visions Dynamic Book

See Vol. 7: Fibres to fnd out more about waterproof material.
for example, out of a material with very small gaps between the threads. To further resist water and add
to the waterproofing effect, the material can be treated with a substance that repels water, such as silicone. Materials such as silicone are hydrophobic, that is, they repel molecules of water at the molecular level. But they cannot bridge large gaps and only act on surfaces, so a close weave such as on umbrellas coupled with a surface treatment of silicone produces the best results.
The same effects that help with waterproofing can make it very difficult to wet a material thoroughly, for example, a garment that needs washing. To get over this problem, the surface tension of the water needs to be reduced. This is one of the properties of a detergent, for it contains a chemical called a wetting agent that mixes with the water and lowers the attraction of water molecules to each other. Wetting agents of this kind are called surfactants.
(Below) Layers of tissue designed to absorb water. This paper has been treated with
a chemical surfactant that reduces the surface tension of water and so allows water to flow into the pores of the tissue even more easily.
(Above) Drops of water on the surface of an umbrella.
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