Q&A About Polyurethanes: Raw Materials Storage Issues
How should polyurethane and polychloroprene raw materials be stored to optimize their shelf life?
I am an adhesives manufacturer and, as business is increasing, I have a need for a larger warehouse for the polyurethane and polychloroprene raw materials used. How should these raw materials be stored to optimize their shelf life?
Time and temperature are the two primary factors that influence product quality. Storage in a warehouse at a constant temperature of 25°C would be best for most products, although this isn’t always practical. In any case, some controls must be in place to prevent materials from experiencing the coldest temperatures of winter or the extreme heat that can occur in a warehouse in the summer. The storage requirements for raw materials differ based on their chemistry and delivery form. There may be a great deal of overlap in the recommended storage temperature for different materials, but there can be negative consequences if the products experience extreme temperatures.
High-molecular-weight polyurethane adhesive raw materials can be supplied as solid pellets or polyurethane dispersions. Polyurethane pellets are used for solvent-based adhesives. The adhesive films produced from these products are often designed to soften at temperatures between 45-60°C, so it is important that the storage temperatures don’t approach these temperatures to prevent clumping of the pellets and the resultant handling issues.
Exposure of polyurethane dispersions to excessively low or high temperatures can significantly influence the performance of these products. The typical recommended storage temperature range for polyurethane dispersions is 5-25°C. The changes that occur upon exposure to low temperatures range from an increase in viscosity to complete coagulation of the polymer. The coagulated polymer cannot be dispersed upon stirring. Some selected dispersions on the market contain solvents and provide limited freeze/thaw performance.
If a dispersion is subjected to elevated temperatures for an extended period of time, performance can be reduced. However, this change it is not as apparent as with cold temperature exposures. The molecular weight of the polymer backbone can see a reduction, which often leads to a drop in initial adhesive strength or a decrease in elevated temperature performance. Changes of this type are more prevalent with dispersions with a polyester backbone and carboxylate rather than sulfonate stabilization. End use application testing is required to determine if a critical degree of polymer degradation has occurred.
Similar temperature controls must also be followed for polychloroprene products. High-molecular-weight polychloroprene polymers are supplied as chips for use in solvent-based applications or as an anionic dispersion. The recommended storage temperature range is 10-25°C for both chip and dispersion grades. As with the polyurethanes pellets, the chips will agglomerate if they are stored at high temperatures. This would be most apparent for bags at the bottom of a pallet.
Polychloroprene dispersions can coagulate upon exposure to low temperatures. Performance will be reduced after storage for an extended period of time (> 6 months) at around 25°C or after a shorter exposure at elevated temperatures. A drop in pH from about 13 down to 11 or so can be an indication that polymer changes have occurred. This could lead to a decrease in adhesive performance or formulation instability. Application testing must be carried out to determine fitness for use. The discussion has focused on storage conditions in your warehouse, but temperatures must also be monitored as the adhesive is transported to your customer.