Here in the UK, we commonly refer to to this versatile material as Upvc, but to give it its proper term, it is actually "pvc-U."

First discovered in Germany by the chemist Eugen Baumann as (PVC), it did not contain the missing ingredient (U) that gives this material it's rigidity, which makes the Upvc door we know of today. 

PVC (Poly viynl chloride) in this form is a realitively flexible material which is no good for the door industry. In simple terms, and as there is no plasticisers in the material at this stage to make it rigid, the materail is in effect flexible.

To overcome the flexibility to create what we now refer to as Upvc, an "unplaticiser" is added to the formula to create "U" in "pvc-U".