Friday, November 12, 2010

Keeping the soda in the bottle

kw: acronyms, materials science

I chanced upon a new acronym this morning: WVTR, which means Water Vapor Transmission Rate. Controlling this material property is the key to using plastic for drink bottles and for other uses where water transmission must be minimized. The standard unit is g/m2/day. For high density polyethylene (HDPE), the kind used for a one-gallon plastic milk jug, the WVTR is about 6. Since the surface area of a gallon jug is about 1/6 square meter, a gallon of milk will lose a gram of water daily. This is acceptable for milk jugs, because you have to use the milk in a week or so or it goes bad anyway.

The situation is different for soda and beer bottles. The permeation must be a lot lower because these products are pressurized by CO2, but fortunately this molecule is larger than the H2O molecule. Still, a soda bottle has five layers of plastic and coatings to get the WVTR down by a factor of 100 or so. Since 1/100 is 10-2, this is called E-2 performance.

The application I was researching is encapsulants for electronic materials. Pure silicon, for example, the "active material" in most electronic parts, is very water sensitive, so if you don't keep it bone-dry, it will react, corrode and soon fail. I found that companies are presently striving to develop material combinations that achieve WVTR ratings of E-4 to E-6 (that is, from 100 down to 1 µg/m2/day). All plastics are a little porous, a little permeable, so attaining such low permeability is quite an achievement.

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