Modification of filler-matrix interactions by adsorption of surfactants and its effect on the strength of high-density polyethylene/chalk composites

—The interactions between cationic and anionic surfactants and natural chalk were studied by means of adsorption and zeta potential measurements. Using these experimental results, possible orientations of adsorbed surfactant layers on chalk were deduced and the adsorption free energy was estimated. Chalk modified with stearic acid was extracted with different solvents in order to distinguish the individual bond types between stearic acid and chalk. The investigation of the influence of the surfactant constitution and concentration on selected mechanical properties (impact strength, tensile strength, flexural stress) of high-density polyethylene/ chalk composites showed that the efficiency of the modification with surfactants correlated with their ability to adsorb on the filler surface. The decrease in tensile strength and flexural stress caused by surfactants can be described in the same way as the action of surfactants in the textile washing process. The effect is directly proportional to the adsorbed molar amount of the surfactant. The experimental results also allow the effect of adsorbed water on the strength of filled or reinforced polymers to be described. If the bonding energy of the modifier on the filler is of the same order of magnitude as the interaction energy between the filler and the matrix, the modifier decreases the tensile strength. On the contrary, the impact strength is controlled by factors that cannot be deduced primarily from the interaction energy. Different mechanisms are discussed, among them the possibility of a 'zeta correlation'.