Polypropylene/chemically treated coir composites: optimizing coir delignification conditions using central composite design

The effect of alkaline treatment of coir on polypropylene/coir composite properties was investigated, using a two-factor central composite design where NaOH concentration and reaction time were varied. Alkaline treatment favored delignification, and at certain conditions, increased composite stiffness without loss in tensile strength. Increase in cellulose concentration likely increases the number of free hydroxyls, which is less favorable to composite mechanical properties. Treatments with 2 and 9% NaOH for 5 h resulted in cellulose concentration of about 50 and 70%, respectively. Treatments with high concentrations and for long periods are disadvantageous to composite properties, resulting in lower tensile strength likely due to the increased number of free hydroxyls as well as to possible breaks in cellulose structure. A desirability function, used to optimize the delignification process, revealed the highest tensile modulus and strength (3.5 GPa and 31 MPa, respectively) could be obtained at 5.75% NaOH and reaction time of 3 h.