The Recycling of Thermoset Materials into Thermoplastic Composites
Thermoset process scrap costs companies millions of dollars annually. Specific thermoplastics could benefit from the addition of recycled thermoset material. The incorporation of thermoset regrind into thermoplastic material would provide a viable alternative for the thermoset scrap that is currently sent to the landfills.
Theory and Background
When a thermoset part has been cross-linked, that material either has to be used in the application it was intended for, or thrown out. In process scrap, generally shear sand/saw grindings and other waste, is shipped to the landfill for disposal. The lack of recycling in the thermoset industry results in higher final prices for the consumer. Developing a practical method to recycle thermoset materials would create a commercially feasible composite material. The addition of the regrind would reduce the requirement for expensive virgin material, decreasing the composite material’s price. This would provide the thermoset and thermoplastic industries with substantial savings.
Thermoset regrind, from Haysite Re-Inforced plastics, polypropylene co-polymer from ExxonMobil, and Silane Q1-6106 from Dow Corning were used to create the new composite. At Haysite, the regrind is a by-product of the sanding of large laminate sheets of fiberglass-reinforced polyester. Each sheet is sanded to final size, with the waste dust being sent to the landfill. The dust used in this study was taken from Haysite's EMD product line. This regrind consists of approximately 30% glass fibers, 31.5% calcium carbonate filler, 20% polyester resins, and the final 18.5% of the mixture is comprised of organic catalysts, pigments, and trace amounts of highly concentrated chemicals. The silane acts as a coupling agent between the polypropylene and the regrind on a molecular level. The silicon and oxygen molecules, present in the silane, form strong ionic bonds between the glass particles in the regrind. This part of the silane-coupling agent also forms secondary force bonds with the chains of the propylene polymer. Van der wal forces may create bond strength between the glass fibers and the polypropylene as well.
The polyester resin portion of the reground material also utilizes van der wal forces. These forces weakly bond the resin molecules to the virgin propylene.
The resin and glass fiber portion of the regrind is best if bonded to the polypropylene directly. The silane does not provide any bonding forces between the calcium carbonate and propylene molecules. Calcium carbonate is also hygroscopic which may necessitate material drying if properties are affected by the water absorption.
Dow Corning's Silane Q1-6106 is a high viscosity liquid, and compares best to a waxy gel. The addition of the silane provide significant wetting between the materials, which causes clumping. Multiple, less expensive materials can reproduce the wetting characteristic of the silane, however, the molecular bonding benefits are not duplicable with these other materials.
Originally, it was proposed that the material be compounded through extrusion and palletizing with subsequent injection molding. A regrind level of 20% and 30% was selected to ensure the financial benefits would outweigh the costs associated with silane and the compounding of the composite material. Also, a less expensive alternative was desirable as the costs of the overall process would rise and detract from the cost savings provided by the use of the thermoset regrind. When it became evident that the material could be directly injection molded, this alternative was selected over the extrusion operation. Overall, the procedure followed was clean, efficient, and could be used to produce large quantities of the composite material in a short period of time.
Mixing was preformed to the following guidelines. A five-gallon bucket was lined with two garbage bags, and filled with 4.35 kilograms of the virgin polypropylene. Next, the resin was wetted with 0.17 kilograms of silane. The mixture was then tumbled to avoid clumping in a specific area and to prevent poor distribution of the silane. The second wetting also consisted of the 0.17 kg of silane and a second tumbling. The regrind was then integrated into this mixture by splitting the amount in half, and adding the regrind in two batches of 0.99 kg each. After each addition of regrind the material was tumbled.
To produce the 20% loaded parts, half of the 30% regrind loaded parts were reground into normal pellet size and mixed with