PET Bottles are formed by molding plastic into "preforms", which are
heated and blown into molds (usually at the bottling plant). Here are
some preforms:
Layers
Some bottles are mono-layer, and some are multi-layer (bottles made from
recycled material are usually multi-layer). This affects the way they
fail in water rockets - the new symmetrical splice that folk are using
prevents the most common failure when using asymetrical splices, which
is caused by shear forces between the layers.
Porous
PET is somewhat porous, and is etched severely by acid-cure silicones.
Bottling plants have a quality program aimed at minimising the amount of
material which goes into the bottles while still maintaining an adequate
shelf-life (the fizz is gradually lost through the plastic).
Crystallising PET
PET plastic which is warned to the optimum temperature of about 94 degrees
Centigrade can crystallise. The plastic goes opaque, becomes more
brittle, and is stronger.
