Those lubricants characterized by polar functional groups tend to be more compatible with PVC resins than their nonpolar counterparts. Increased compatibility yields an internal lubricating effect. The quantity of lubricant used will also have a significant effect on its function. For example, if an internal lubricant is added in a quantity exceeding the limit of its solubility in the PVC resin, the insoluble portion will act on the surface of the resin, promoting an external lubricating effect. Lubricants used in rigid PVC are classified according to their lubricating action, i.e., whether they lubricate internally or externally.
Some internal lubricants are esters of fatty acids, fatty alcohols, and metal salts of fatty acids.
Some external lubricants are nonoxidized and oxidized polyethylene polymers, paraffin oils, and paraffin waxes.