Polyurethane Manufacturing Operations

Reducing Waste in Polyurethane Manufacturing Operations

Process Background

Tandem Products manufactures products made of plastic-coated metal, and molded polyurethane elastomer. Its polyurethane products are used in a variety of applications, including the agricultural and materials-handling industries.

Tandem uses six machines in the manufacturing process, with each machine producing a different product line. In four of the machines, a prepolymer (a combination of isocyanate and a polyol) and a curative agent (1,4-butanediol) are combined in the mixing head to form the final urethane product, which is then poured into a mold and allowed to cure. The other two machines were not included in this project.

Three types of waste are generated during the production process: start-up, flash (or “squeeze-out”) and flush (or “blow-down”).

  1. Start-up waste is generated when the prepolymer and curative are not in a proper ratio when combined at the beginning of production. This initial mixture does not exhibit the desired characteristics, and must be discarded as waste. During the twelve months before the intern project, Tandem disposed of over 35,000 pounds of urethane start-up waste.
  2. Flash or “squeeze-out” waste occurs when uncured urethane overflows or leaks from the mold and cures. It must then be removed after the part is released from the mold. Over 55,000 pounds of flash waste were disposed of from mid-1992 to mid-1993.
  3. Flush or “blow-down” waste is generated at the end of a production run when the machines are shut down. Within seconds to a few minutes after shut-down (depending on the urethane formulation), the mixing head is flushed with a dibasic ester (DBE) solvent to prevent the polymer from gelling and the equipment from seizing-up. Over 14,000 pounds of flush waste were generated from mid-1992 to mid-1993, which contained spent DBE, unreacted material and solid urethane.

Incentives for Change

Tandem Products’ incentives to reduce or eliminate urethane waste were due to:

  • Disposal Costs: From mid-1992 to mid-1993, Tandem spent an estimated $180,000 to dispose of the three wastes listed above. Tandem anticipates that these costs will increase in the future.
  • Disposal Restrictions. Although Tandem’s urethane waste and spent DBE are both currently considered nonhazardous waste, Tandem is concerned about possible future disposal restrictions.

Intern Activities

During the summer of 1993, the MnTAP intern identified and evaluated the following options for reducing or eliminating start-up, flash and flush wastes.

1. Start-up Waste

Opportunities Investigated. The intern focused on researching options for eliminating start-up waste by reusing it. One option was to pour the waste material into “start-up” molds, which were used to form miscellaneous parts such as shipping blocks or welding plugs. These parts would be produced until the machine operated at a steady state, and the flow of prepolymer and curative was balanced at the proper ratio.

Another reuse option investigated by the intern was to consider using a three-way valve on each machine to recirculate the prepolymer and the curative between the mixing head and the respective holding tanks. The pneumatically controlled three-way valve allowed materials to flow in their lines in the correct proportion and combine in the mixing head almost instantly at the proper ratio.

Problems. One problem at the beginning of the project was due to operator resistance to change. This required additional operator training to ensure that start-up waste would be reused in the start-up molds.

Another problem encountered was that the reused start-up material caused minor cosmetic defects on some miscellaneous parts. However, since these parts were not required to meet strict quality specifications, approximately 90 percent were graded as satisfactory. The rejected parts were sent to the landfill. Production costs that were not recovered from inferior parts included: labor, mold preparation, and parts polishing.

2. Flash Waste

Opportunities Investigated. Flash waste is the most significant waste at Tandem because of the volume generated and the cost of disposal. The intern researched four options for reducing flash waste.

The first option investigated whether flash waste could be reduced or eliminated. After determining the sources of waste, three design or production modifications were examined, including: 1) using a better gasket on the mold to eliminate the leaking problems; 2) redesigning the mold so that material would not leak out of air hole funnels; and 3) preventing overfilling of the molds.

The second option focused on recycling the flash waste. The intern listed the flash waste with three local materials exchange programs, including: Businesses Allied to Recycle Through Exchange and Reuse (B.A.R.T.E.R.); MnTAP’s materials exchange program; and the Southeast Minnesota Recycler’s Exchange (SEMREX). Materials exchange programs match companies that have virgin or reusable material with companies that can use those materials, thereby eliminating the need for disposal.

The third option examined reusing finely ground flash waste as a filler material in producing large urethane products.

The fourth option, incineration, was determined to be an option of last resort if no other options for reusing or recycling the flash waste were available. Incineration was also an expensive disposal option, costing an estimated $324 per 55-gallon drum.

Problems. Redesigning the molds to eliminate flashing was expensive and time consuming; Tandem plans to pursue this option when feasible.

Attempts to grind flash waste to a powder were unsuccessful using conventional grinding methods. The resulting flash pieces were too large (¼-inch) to pass through the mixing head and process lines. In addition, if the mold-release compound was not removed from parts before they were reused, the ground pieces would not adhere to the prepolymer when mixed.

3. Flush Waste

Opportunities Investigated. Two options were investigated by the intern that focused on reducing flush waste, and filtering and reusing the DBE cleaning solvent.

First, an attempt was made on two process machines to reduce flush waste generated at the end of production. To accomplish this, pressurized air (air blow-down) was used to force residual material out of the mixing head and the lines, thereby reducing the amount of DBE required to flush the system.

Second, an effort was made to filter out the solid urethane from the DBE so that the solvent could be reused. Filtering equipment was made from materials on hand in the plant and consisted of: a container, a stir bar, and a 12-inch-square aluminum screen with a 1/16-inch mesh. Flush waste was collected from machines at the end of each day and filtered through screens into a five-gallon pail. A small amount of degelling agent was then added and mixed in before the container was sealed.

Problems. The air used to test the blow-out process contained enough moisture to cause a reaction with the residual prepolymer. This resulted in the mixture solidifying in the mixing head.

Problems with recycling DBE occurred when production colors changed. The recycled DBE would become contaminated with a different colored dye, rendering it unusable. In addition, the pots containing DBE were of different sizes in each machine, which added to the cost of pot liners used to prevent cross-color contamination of parts.


1. Start-up Waste

  • Based on testing start-up molds on one machine, up to 100 percent of the start-up waste could be reused (approximately 4,800 pounds of urethane waste). This would result in an annual cost savings of $6,900 per machine in reduced costs for raw materials and waste disposal.
  • A pneumatically driven recirculation system was built and installed on one of the processing machines at a cost of about $1,800. This option has the potential to eliminate start-up waste because the prepolymer and the curative are at the proper ratio immediately upon mixing. This could reduce waste disposal costs by $19,000 annually. Another benefit of this recirculation system is that it is easier to operate because only one control is used to activate the prepolymer, curative and dye lines.

2. Flash Waste

  • Although the urethane waste material was listed with three materials exchange programs, there was no response from potential recipients of the material at the time of this printing. Tandem will continue to list material through these programs.
  • Because the conventional grinding methods could not grind the waste into a usable powder form, cryogenic grinding may be a promising alternate method to reduce the urethane waste into a powder form. However, the feasibility of this process was not investigated.

3. Flush Waste

  • A blow-down system was installed on two process machines to force residuals out of the mixing head and lines. Nitrogen was used, until dry air could be supplied, to prevent the prepolymer from solidifying in the mixing head. Using the blow-down process to remove residual material decreased by 50 to 75 percent the amount of DBE solvent required for cleaning-out the mixing head and the lines. The blow-down systems also remove most of the contaminants from the DBE, making it easier to filter and recycle.
  • Tandem will be standardizing DBE pot sizes to allow for easy exchange of pot liners. This will prevent color contamination of parts when using recycled DBE.
  • Recycled DBE can be reused up to four times, and then shipped out in 55-gallon drums for recovery by distillation or disposal. By recycling DBE, Tandem is saving approximately $4,400 annually in reduced DBE purchase costs. In 1993, Tandem recycled approximately 7,300 pounds of DBE.


As a result of the intern project, Tandem estimates that it will reduce waste by approximately 42,000 pounds and save over $23,000 annually. This total includes 35,000 pounds of start-up waste and approximately 7,000 pounds of flush waste.

The results of the project also show that waste polyurethane materials can often be reused in the manufacturing process, and can frequently be reduced or eliminated. Options for recycling the flash waste materials that cannot be reused or reduced will take additional effort and time to change the materials into a usable form, or to establish outlets.

This project was conducted in 1993 by MnTAP intern Yi Lin, a chemical engineering student at the University of Minnesota.