Today, vinyl siding is manufactured by co-extrusion. Two layers of PVC are laid down in a continuous extrusion process; the top layer is weatherable and durable material, which comprises up to 25% of the siding thickness. This capstock can include about 10% titanium dioxide, depending on the color, which is a pigment and provides resistance to breakdown from UV light. Vinyl siding that is exposed to the sun will begin to fade over time. However, the fade rate is slower with vinyl than most other claddings. Most manufacturers offer 50 year warranties that their products will not fade much over that period of time. In the past darker colors tended to fade more than lighter ones, but advancements in technology and materials can mean this is no longer the case.
The lower layer, known as substrate, is typically about 15% ground limestone (which is largely calcium carbonate). The limestone reduces cost, and also balances the titanium dioxide, keeping both extrusion streams equally fluid during manufacturing. A small quantity of tin mercaptan or butadiene is added as a stabilizer to chemically tie up any hydrochloric acid that is released into the PVC material as the siding ages. Lubricants are also added to aid in the manufacturing process.