| The general procedures used to process prepolymers apply to both polyether and polyester types; however, differences do exist in regard to temperatures employed.
Storage
The prepolymer should be stored in its sealed container at 50 to 90°F (10 to 32°C) and protected from water. Partially used containers should be purged with dry nitrogen before they are resealed.
Shelf Life
When stored under proper conditions, the prepolymer will have a shelf life of two years.
Melt Time
Before using a polyurethane prepolymer, it is usually necessary to heat the product in its container in order to reduce the viscosity sufficiently to allow for pouring or pumping.
A circulating hot air oven is recommended, although commercially available drum warmers can be satisfactory if temperature control is sufficiently maintained to prevent localized overheating. Rotating the drum while heating will ensure the even distribution of heat and will reduce the time required to melt the product. For melting guidelines, see Hand Batching.
Heat Stability
Polyurethane prepolymers are heat sensitive and can be damaged by excessive exposure to high temperatures, causing a loss in NCO and an increase in viscosity. To avoid this, the total heat exposure before processing should be less than 7 days at 160ºF (70ºC) for polyester types, and 7 days at 140°F (60°C) for polyether types. The prepolymer should not be held at the processing temperature (212°F, or 100°C) for more than 8 hours |
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