Essentially all truck bed liners are thermoformed from high density polyethylene (HDPE) sheets. Other materials such as ABS, synthetic rubber plastic, and other thermoplastic polymers are sometimes used. The thermoplastic sheets are heated to a thermoforming temperature in a thermoforming oven and are deformed by ambient air pressure after application of vacuum on one side in a thermoforming mold to produce the desired product. Many products other than truck bed liners are also manufactured by this process. A disadvantage of the HDPE material however is its low coefficient of friction. HDPE surfaces become even more slippery when wet.
U.S. Pat. No. 4,693,507 (Dresen et al.) describes a truck bed liner with antislip surface properties. Increased coefficient of friction on the liner surface is achieved by applying a continuous film or layer of elastomeric material over the HDPE sheet. The thickness of the integral elastomeric layer or film is typically in the range of 25 mils-30 mils (625.mu.-750.mu.). The elastomeric film is preferably coextruded over the HDPE sheet which is typically 180 mils-250 mils (4.5 mm-6.2 mm) in thickness. Dresen et al. further state that the elastomeric film layer can alternatively be applied to the HDPE sheet by lamination, by adhesive, or by heat application.
According to the Dresen et al. scheme, an increased frictional force is therefore achieved primarily by the "plowing effect" or embedding effect of harder cargo pieces pressing into the softer continuous elastomer film layer. This antislip plowing effect is attributable to the continuous film or layer of the softer elastomer. A disadvantage of the Dresen et al. scheme however, is that the frictional force attributable to the integral elastomeric layer is reduced, and the layer also becomes slippery, when it is wet. Another disadvantage of the Dresen et al. method is the increase in cost of adding and applying the integral and continuous film layer of elastomeric material by coextrusion, lamination, adhesion, or heat application. The elastomer film of Dresen et al. generally extends across the entire substrate plastic sheet.