Foaming agents,  chemical foaming agents can be added to the plastic lumber  as a master batches.. they produce a fine, uniform cellular structure, reduce the density of the extruded board, and increase line speed.

Lubricants, Much as the name implies are used to reduce friction of extruded part and to improve the flow characteristics thereby permitting the die or mold to be filled with greater ease and increasing production.

Color pigments,  These are naturally used to control the color of the board however UV resistance and  durability are of equal importance. The type and amount of pigment controls these factors.  Color pigment are usually added as masterbatches and these can be ordered already color matched to the desired color.  

UV stabilizers, UV light stabilizers are used frequently to protect the Plastic lumber from the long-term degradation effects from sunlight.  Different UV stabilizers are utilized depending upon the substrate, intended functional life, and sensitivity to UV degradation.