Down to Earth: Scratch-resistant glass
This week Stuart Higgins brings a crystal clear scientific invention down to earth...
Stuart- A clean supply of water is critical if you want to send humans to space. In the 1970s, researchers at NASA were trying to develop a water filter for space missions using a process called reverse osmosis. Contaminated water is forced under pressure through the filter which contains pores so small that only the pure water can pass through.
At the time filters were made by coating different types of plastics onto a surface and allowing them to form a thin skin with tiny holes in. However, making these filters was difficult because factors such as how warm or humid it was in the day could affect the properties of the manufactured filter.
So the NASA team came up with a new way of making them instead; they used plasma polymerisation. They paced an ordinary piece of filter paper, with it’s larger pores, inside a glass jar, pumped out the air, and pumped in a small amount of either nitrogen or argon plus the chemical ingredients for making a plastic.
By applying a high frequency electric field between two metal plates across the jar, they generated an electrical plasma - atoms of gas that had their electrons ripped away by the electric field. This plasma interacted with the plastic making chemicals causing them to deposit and link together on the surface of the filter paper. This layer of plastic contained the small pores needed to make a good filter.
The benefit of their approach was that it was highly controlled and allowed consistent and strongly bound layers of plastic to form. While this solved their original problem, the engineers quickly realised they could use the same trick with different chemicals to create other coatings.
In 1972, the US government introduced new rules that meant that the lenses used in a pair of glasses needed to be shatter resistant to protect the user from injury. Plastic lenses were a cheap and easy way to manufacture alternative glass but were very easy to scratch in everyday life.
The NASA engineers applied their plasma technique to coating the plastic lenses with a hard material based on organosilanes. Organosilanes are materials that contain both carbon and silicon and they can be made to form strong bonds to both surfaces and to themselves. This meant that the coated plastic became tougher and more resistant to scratches allowing the widespread adoption of plastic lenses.
So that’s how engineers working on water filters for space missions used the same technology to develop a scratch resistant coatings for plastic lenses improving the safety and usefulness of glasses around the world.