By Mitch Patterson and Jerry Thiel
|Shakeout of 4-in. valve body leaving only white core sand (above) and assembly of a 16-in. valve body with 250-lb bio-urethane core (below) at McWane-Clow Valve, in Oskaloosa, IA.|
The Center for Advanced Bio-based Binders (CABB), a project funded by the U.S. Department of Energy and conducted by the University of Northern Iowa Metal Casting Center (UNI/MCC; www.mcc.uni.edu) has focused on developing binders and processes that use bio-renewable materials to lower emissions, volatile organic compounds (VOCs), and hazardous air pollutants (HAPs) in foundries. UNI continues to push forward in making the foundry more efficient with economical use of renewable resources. Currently, the university has developed two new patent pending metal casting bio-urethane sand binders.
Phenolic urethane’s fast-cure and high-strength qualities have made it the world’s most widely used foundry binder. Phenolic urethane binders work when a polyfunctional alcohol (part 1) reacts with a polyisocyanate (part 2) in the presence of a tertiary amine (catalyst) to create a urethane. In no-bake applications the catalyst for the reaction is a liquid amine; cold-box processes use atomized amines such as DMEA, DMIPA, or TEA in a nitrogen carrier gas. In phenolic urethane chemistry, the part 1 resin is based on phenol and formaldehyde, both considered HAPs. The system requires thinning the resins with solvents for proper sand grain coating. Solvents are traditionally comprised of aromatic compounds, although dibasic ester and fatty acid methyl ester (biodiesel) solvents have been included to improve properties while lowering emissions and HAPs.
UNI has developed bio-urethanes focusing on the replacement of the part 1 resin. The first new bio-urethane is based on the saccharides of corn syrup. Saccharides are a family of carbohydrates that range from small molecules, sugars, to very large carbohydrates, such as cellulose and starches. Saccharides are considered polyfunctional alcohols as they contain many alcohol groups that can react with the isocyanate resin. The low cost, availability, and environmental friendliness of corn syrup and many other saccharide sources make it a suitable, bio-based part 1 replacement. Additionally, saccharides are compatible with reactive solvents that are low in HAPs and VOCs.
The second bio-urethane is based on the humic acids of lignite. Lignite or brown coal fits between coal and peat. Lignite contains humic acids derived from decayed biomolecules. The humic acids contain many alcohol groups available for a reaction with the isocyanate resin. When in a suspension of reactive solvent, lignite also can reinforce the additional polymer to increase core strength. Lignite is readily available and mined in areas all over the world, making it a suitable bio-based part 1. The lignite resins also are compatible with low HAP and VOC solvents.
|Bio-urethane cold-box cores (above) and as-cast aluminum ATV engine heads (below) at Progress Castings, New Hampton, IA.|
UNI’s bio-urethanes serve as a direct replacement of phenolic urethane. The binders employ an isocyanate resin (part 2) and an amine catalyst very similar to phenolic urethane no bake and cold box binders, so current standard foundry equipment can be used. With identical processes and mechanisms as phenolic urethane, the bio-urethane binder systems can be integrated into existing foundry operations with ease.
Traditional phenolic urethane binders tend to soften as the temperature increases: at 200oC, more than half of the room-temperature strength is lost. This influences the most import property of a binder, casting quality. Unlike phenolic urethane, tests in the bio-urethane binders have shown a lack of thermal softening. Testing has shown that the UNI bio-urethane binders exhibit less core distortion than conventional phenolic urethane binders. Both thermogravimetric analysis (TGA) and elevated temperature tensile testing have revealed that the lignite binder retains properties longer than phenolic urethane. However, the binder quickly and completely degrades around 250oC. The high temperature strength retention is followed by a quick degradation period for improved dimensional accuracy, with excellent shakeout properties.
The thermal profile of the saccharide urethane also yields unique casting properties. This binder is similar to the lignite binder as it too experiences a quick degradation period. Unlike the lignite urethane, the saccharide urethane retains additional strength after the casting has been poured. Caramelization of the sugar is thought to be the cause of the increased retained strength.
Replacing the phenolic resin in the part 1 has several environmental advantages. The base materials are less dependent on petrochemicals and show less market price fluctuation. When making molds, the characteristic phenolic urethane smell produced by the part 1 is eliminated, and the workplace contains fewer odors, VOCs, and HAPs during mold and coremaking. When compared to a traditional phenolic urethane no-bake, both bio-urethanes generate over 50% reductions in overall emissions of HAPs during pouring, cooling, and shakeout.
|Production (above) and bio-urethane core (below) for a grey iron harvester discharge elbow, for John Deere Foundry, Waterloo IA, at the University of Northern Iowa Metal Casting Center, Cedar Falls, IA.|
Mold gas emissions are a serious concern of operating foundries, and replacing the conventional part 1 with bio-urethane showed a reduction in excess of 80% in phenol and formaldehyde emissions. Foundries can keep the reaction speed and flexibility advantages of a urethane binder while drastically reducing emissions.
The new bio-urethanes have been successful at producing aluminum, brass, gray iron, ductile iron, and steel castings. The binders have been rigorously tested in both no-bake and cold-box applications at the UNI foundry, as well as in several foundry trials.
The first foundry trials were run at McWane-Clow Valve in Oskaloosa, IA. Castings poured included a 4-in. ductile iron valve body with a 35-lb. bio-urethane no-bake (BUNB) core and a 16-in. gray iron valve body with a 250-lb BUNB core. Both castings showed improved casting quality and required no shakeout: white core sand fell to the ground.
Next, the bio-urethane binders were tested at Progress Castings in New Hampton, IA. There, the castings poured included an aluminum ATV engine head with a lignite based bio-urethane cold box (BUCB) core. Cores were cured using DMIPA gas with a cycle time of 40 seconds. Testing of the finished products showed excellent casting results 15 minutes after cores were produced. It was notable that the BUCB cores did not show the core stain usually associated with the conventional urethane cores.
Foundry trials were conducted at John Deere Waterloo foundry in Waterloo, IA. Three castings were chosen to test the binders in medium to large sized castings. A grey iron axle housing, harvester discharge elbow, and 4000 lbs grey iron slag pot. Casting results included excellent shakeout and surface finish without the need for core coatings. All of the castings exhibited casting surface finishes comparable to conventional urethane binders.
|Pouring (above) and final castings (below) of CF8M stainless steel pumps and impellers using bio-urethanes at Viking Engineered Cast Products, Cedar Falls, IA.|
The bio-urethane binders were next tested at Viking Engineered Cast Products in Cedar Falls, IA. Cores were produced for both pump casings and impellers produced from CF8M stainless steel. Viking reported that castings poured using the bio-urethane cores exhibited less hot tearing and crack than conventional ester cured phenolic cores normally used. Possible carbon pickup was evaluated but not detected in any of the castings poured.
The new bio-urethanes were developed by UNI to be foundry-ready. The improved shakeout of the two binders without sacrificing casting quality is ideal for many foundry applications. This means less time and energy for shakeout and less shakeout damage. It’s also expected that the foundry will also see a safer mold and core room environment with improved emissions on the production floor. The binders decrease our dependence on petrochemicals by using materials grown or mined all over the world. Thanks to strong industry support, the funding for the CABB project has been extended and UNI will continue to develop innovative binders and processes.
Mitch Patterson is the project manager and Jerry Thiel is the director at the University of Northern Iowa’s Metal Casting Center, which coordinates academic research capabilities and resources for private-sector, industrial applications. Contact the MCC at Tel. 319-273-6894.