Surface modification of silica with a hydrophilic polymer and its influence on reinforcement of natural rubber latex

Abstract

Surface modification of silica particles by means of macromolecules is a novel technology and surface modified particles are used in diverse applications; amongst them reinforcement of polymer materials is distinguished. However, reinforcement of natural rubber latex (NRL) is still in research stage. In the present study precipitated silica (Ultrasil VN3) is modified with a hydrophilic polymer of which the hydrophilic groups interact with surface bound hydroxyl groups of silica and then the modified silica is used to reinforce NRL. 
The hydrophilic polymer was synthesized in the laboratory by solution polymerisation technique. Viscosity average molecular weight of the polymer was determined by using Ubbelohde viscometer.  Surface modification of precipitated silica particles was carried out with the synthesised polymer separately in two mediums; aqueous and non aqueous. FTIR spectroscopy of the treated silica was performed to confirm the surface modification. Aqueous dispersions of unmodified/modified silica were prepared prior to compounding and the colloidal stability of the dispersions was observed at different pH levels. The dispersions at neutral pH were incorporated in different loadings namely 5, 10, 15 and 20 phr, to un-compounded and compounded NRL and thin latex films from them were produced by casting. The distribution of silica particles in rubber matrix was examined through microstructural studies and contribution of surface modification towards enhancement of mechanical properties was investigated by measuring tensile and tear properties of unmodified (UM) and modified filler added NRL cast films.
Optimum stability of the aqueous dispersions of modified silica was conveyed at pH close to seven.  Uniform distribution of modified silica particles within rubber matrix was seen in films cast from modified silica incorporated NRL whereas a settled layer of filler was seen in UM filler added films. Influence of surface modification upon the reinforcement of NRL was confirmed by comparing the values of tensile and tear strengths of UM and modified filler added vulcanized films. Tear strength of all the films first increased, passed a maximum and then decreased with the increase of filler loadings. The maximum values of both modified filler added films were nearly 50% higher than that of UM filler in the range of 5-10 phr of filler loadings. The tensile strength of modified filler added films also showed a maximum at 5 phr of filler addition whilst that of UM filler added films was slightly increased with the concentration of filler.  However, the modified filler added films showed nearly 35% increase at 5 phr compared that with the highest value of UM filler added films at 20 phr. It is suggested that uniform distribution of filler particles and the improvement in mechanical properties envisage the better compatibility of modified silica particles with NRL via interfacial interactions and enhanced reinforcement.