High prepolymer viscosity can lead to problems in processing and final product quality. Incorrect viscosity can arise from a number of sources, including overheated prepolymer, low prepolymer processing temperature and use of an inappropriate prepolymer type.

Overheated Prepolymer
Because prepolymers are generally heated for processing, it's possible to degrade them by applying too much heat. When this happens, the isocyanate (NCO) groups that normally react with the curative to form an elastomer react with each other instead. This reaction leads to an increase in prepolymer viscosity. In severe cases, the prepolymer can actually gel to a solid mass. If overheated prepolymer is used in processing, soft parts will result—even if the prepolymer has not degraded to the point of gelation.

To avoid this, adhere to the melting time and temperature guidelines provided by your prepolymer supplier. Although prepolymers degrade at different rates upon heating, minimizing heat exposure is always recommended. To monitor this, we suggest keeping a log of the prepolymer heat history. The use of band heaters is not recommended because heating is too localized. Also, if possible, the prepolymer drum should be rolled while being heated.

Low Processing Temperature
All prepolymers have recommended processing temperatures which differ from one product to another. When the prepolymer is at too low a temperature, it will be difficult or impossible to process. Polyester prepolymers are especially likely to be underheated because they require higher processing temperatures than do polyether prepolymers of similar hardness.

To avoid low processing temperatures, check the temperature of the prepolymer with a reliable thermocouple to ensure that the preheat oven is working properly. It's often assumed incorrectly that a prepolymer heating in an oven is the same temperature as the oven; however, there may be localized hot or cold spots.

Inappropriate Prepolymer Type
Polyether prepolymers are typically lower in viscosity than polyester prepolymers. Similarly, different prepolymer production methods yield products with different viscosity characteristics. To obtain lower viscosity, choose polyether prepolymers over polyester prepolymers, if the application allows. Airthane® prepolymers have lower viscosities than conventionally made prepolymers with similar percent NCOs. They also provide longer pot life. Discuss your requirements with your supplier to determine the best prepolymer to meet your needs.

For further information about other common elastomer processing and performance problems, go to our on-line Trouble Shooting Guide, accessible from the navigation at the left of this page.