Statistical modeling and response surface optimization on natural weathering of wood–plastic composites with calcium carbonate filler

An experimental mixture design was used to determine the optimum composition of wood–plastic composites (WPCs), manufactured from recycled polypropylene (rPP), rubberwood flour (RWF), calcium carbonate (CC), maleic anhydride-grafted polypropylene (MAPP), ultraviolet (UV) stabilizer, and lubricant (Lub). The composite samples were prepared by extrusion and compression molding. The effects of varying compositions were evaluated using analysis of variance (ANOVA) and response surface methodology (RSM). The results showed that all of the factors significantly (p < 0.05) affected on the properties of the composites. The mechanical properties decreased after natural weathering for periods of 30 and 90 days and immersion in water for similar periods. Increasing the CC content increased the flexural strength, modulus, and hardness, but decreased the water absorption and thickness swelling. Natural weathering sharply changed the lightness and discoloration; whereas, the proportions of rPP and RWF in the composites caused smaller changes. The optimal formulation was determined with regard to the overall properties and consisted of 52.1 wt% rPP, 35.9 wt% RWF, 6.9 wt% CC, 3.9 wt% MAPP, 0.2 wt% UV stabilizer, and 1.0 wt% Lub. The desirability of the overall properties was 0.767, suggesting that the model was able to predict the response to an adequate extent of 76.7%. The experimental results were found to be in a good agreement with the predicted results from RSM within a 5% error.