Titanate Coupling Agents Improve the Surface Modification of Calcium Carbonate
Most of the fillers used in composites are natural or synthetic inorganic fillers, which are polar and water-insoluble. When they are dispersed in organic polymer resins of very low polarity, the difference in polarity between the two often results in poor compatibility; direct or excessive filling can easily reduce the mechanical properties of the material. It is therefore of practical significance to modify the surface of inorganic fillers by physical or chemical means so that the filler surface becomes non-polar, improving its compatibility with the resin and thereby enhancing the mechanical properties of the composite article.
In this experiment the surface modification of the filler was carried out by the dry method of surface chemical modification. A titanate coupling agent was made into an ethanol solution in a 1:1 ratio (by weight), dripped into dried ground calcium carbonate with stirring, and the time was controlled at 40–60 minutes so that the treating agent fully coated the filler surface. The treated filler was then placed in an oven and kept at a constant temperature of about 40°C for 3 hours.
Without strong interaction with the matrix, the modulus of polymers containing rigid particulate fillers also increases somewhat. The decline in the mechanical properties of the composite system is caused by the polarity difference between the filler and the resin. In a filled system the filler is the dispersed phase and is in fact divided within the continuous phase formed by the matrix resin. Even if the filler particles bond well with the matrix resin without bubbles or voids, the area of matrix resin on a loaded cross-section is necessarily smaller than in a pure resin or fibre system. Because the interaction between filler and matrix resin is weak, when an external force is applied the matrix resin is pulled away from the particle surface, so that the strength of the filled system is lower than that of the unfilled system.
Titanate coupling agents contain two types of functional groups. At one end is a short-chain alkoxy group that hydrolyses readily: when the surface of the calcium carbonate has combined with the monomolecular-layer bound water, it attaches to the surface of the calcium carbonate filler, making the filler surface non-polar. At the other end, the long-chain alkoxy group of the coupling agent reacts chemically with the UP resin. In effect, the titanate coupling agent plays a “bridging” role, improving the interfacial bonding between filler and resin. According to the chemical bonding theory, interfacial bonding achieved through the formation of chemical bonds can in theory give very high bonding strength. Test results on specimens show that the strength and modulus of UP resin castings and GFRP filled with the titanate-coupling-agent-modified filler are improved to varying degrees.
The addition of calcium carbonate significantly reduces the tensile and flexural strength of UP resin castings and GFRP, and the effect becomes more marked as the filler loading increases. In contrast, when the calcium carbonate is modified with the titanate coupling agent, the tensile and flexural strengths of the filled UP resin castings and GFRP are significantly higher than those of the unmodified filled system, demonstrating a good surface-modification effect on calcium carbonate.
