Siding is made in two layers. The top layer and the bottom layer are made from different materials, though they are primarily PVC, or Polyvinyl Chloride.
These two layers of PVC are extruded together. Extrusion is a process used to make any material that has a cross-section profile – like a dual-layered piece of siding. In extrusion, the material is pushed through a die of the cross-section.
In the case of vinyl siding, it is a continuous co-extrusion process.
The top layer is about one-quarter of the thickness of the siding. This layer is partly comprised of titanium dioxide – about 10% — and the amount of titanium dioxide is dependent upon how light or dark the color of siding is. This top layer is sometimes called the “capstock.”
What About the Lower Layer?
The lower layer, the other three-quarters of the thickness, is the substrate. This lower layer is usually around 15% ground limestone, which is mainly calcium carbonate. Using limestone makes the lower layer less expensive to produce. The limestone also balances the titanium dioxide of the top layer, and helps keep both layers fluid during the whole manufacturing process.
Other materials are added during the extrusion process, including lubricants and small amounts of tin mercaptan. The tin mercaptan helps stabilize the vinyl siding as it ages; it bonds with any hydrochloric acid that the siding may release into the PVC as it ages.
The vinyl siding is produced primarily from PVC resin. This is a different form of PVC than what we generally see in PVC plumbing pipes or automotive parts.