Influence of excessive filler coating on the tensile properties of LDPE-calcium carbonate composites

Calcium carbonate fillers are usually coated with stearic acid to reduce their surface energy and improve their dispersion in polymers. Commercial products are often over-coated and contain an excess of surfactant. It was found that stearic acid linearly increases the modulus and yield stress of LDPE but reduces its tensile strength, yield strain, and ultimate elongation. The influence of surfactant excess on the tensile properties of low-density polyethylene (LDPE)-CaCO3 composites was investigated. Compounds of LDPE and optimally coated filler or with excess surfactant were prepared and their properties compared. CaCO3 increased the stiffness and yield stress of the polymer but reduced all its other tensile properties. Over-coating the filler did not lead to linear accumulation of the effects of filler and stearic acid on the polymer matrix. In fact, surfactant excess amplifies the reinforcing effect on the stiffness but reduces all other mechanical properties of the composite. Calcium stearate, which is sometimes used as acid scavenger, lubricant or processing aid, has the same effect on the polymer properties as stearic acid, but to a smaller extent. It is concluded that it is most advantageous to coat the filler with the optimal amount of surfactant necessary to cover its surface with an organic monolayer unless the influence of excessive coating is required for a certain application. Care must also be taken in interpreting some of the published results, where the quality of the filler coating was not investigated. (C) 2003 Elsevier Ltd. All rights reserved.