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Air and water tightness

Airtight zippers were first developed by NASA for making high-altitude pressure suits and later space suits, capable of retaining air pressure inside the suit in the vacuum of space.

The airtight zipper is built like a standard toothed zipper, but with a waterproof sheeting (which is made of fabric-reinforced polyethylene and is bonded to the rest of the suit) wrapped around the outside of each row of zipper teeth. When the zipper is closed, the two facing sides of the plastic sheeting are squeezed tightly against one another (between the C-shaped clips) both above and below the zipper teeth, forming a double seal.

This double-mated surface is good at retaining both vacuum and pressure, but the fit must be very tight to press the surfaces together firmly. Consequently, these zippers are typically very stiff when zipped shut and have minimal flex or stretch. They are hard to open and close because the zipper anvil must bend apart teeth that are being held under tension. They can also be derailed, causing damage to the sealing surfaces, if the teeth are misaligned while straining to pull the zipper shut.

These zippers are very common where airtight or watertight seals are needed, such as on scuba diving dry suits, ocean survival suits, and hazmat suits.

A less common water-resistant zipper is similar in construction to a standard toothed zipper, but includes a molded plastic ridge seal similar to the mating surfaces on a Ziploc bag. Such a zipper is easier to open and close than a clipped version, and the slider has a gap above the zipper teeth for separating the ridge seal. This seal is structurally weak against internal pressure, and can be separated by pressure within the sealed container pushing outward on the ridges, which simply flex and spread apart, potentially allowing air or liquid entry. Ridge-sealed zippers are sometimes used on lower-cost surface dry suits.