Resin-coated sand consists of base sand whose surface is coated with thermoplastic phenolic resin and the curing agent hexamethylenetetramine. Condensation during molding and decomposition during pouring release ammonia, while phenol and aldehyde are sources of air pollution.
These gases consist mainly of ammonia. China's labor protection legislation stipulates that ammonia concentrations in molding areas should be below 30 mg/m3, but in practice they often exceed the limit by 8–10 times, reaching 280 mg/m3. This irritates workers' throats, worsens the environment and affects health. Harmful gases spread tens of meters around the area, causing pollution and becoming a major obstacle to the development of shell-mold and shell-core casting.
In the late 1980s, patents began to appear for alternatives to traditional thermoplastic phenolic resins [7], specifically the use of high-molecular-weight bisphenol A epoxy resins, with molecular weights of 850–3,800, in shell-mold casting. Their softening points increase with molecular weight. Coating trials using epoxy resin with a softening point of 85–95°C and an average molecular weight of 1,400, designated E-12 in China's new grading system, were successful.
This resin is a yellow-to-amber solid, added at 3% of the sand mass. The curing agent is liquid phenolic resin, used at 3%–6% of the resin mass. Phenolic resin contains groups that can react with epoxy and hydroxyl groups in epoxy resin. Under certain conditions, such as heating, the two can crosslink and cure, producing ammonia-free resin-coated sand.
Resin-coated sand can be prepared by cold or hot methods. In cold coating, the resin is dissolved in acetone to form a viscous solution at a concentration of 70%–80%. First mix the curing agent evenly with the sand, then add the viscous solution, mix until the material becomes loose, and screen it to obtain the finished sand.
For hot coating, preheat the sand to slightly above the resin's melting point, at 95–105°C, so that the resin coats the sand grains. Spray with water to cool it. Near the end, spray an aqueous curing-agent solution and continue cooling until the sand is discharged at 50°C.
At a curing temperature of 180°C, specimens cured for 2 min had a hot tensile strength of 0.16 MPa/1% (specific strength), more than 50% below the 0.25 MPa specific strength of phenolic resin-coated sand. However, the cold specific strength was 2.0–2.15 MPa, no lower than that of phenolic resin. Improving hot strength requires a higher curing rate and a more suitable curing agent.
Pyrolysis mass analysis of the products generated during molding and pouring with the new coated sand found environmental pollutants including phenol <10×10-6, xylene <100×10-6 and cresol <10×10-6. The new coated sand sells in the United States for USD 165 per ton, less than phenolic resin-coated sand, but its use is limited and its performance needs further improvement [8].