High Prepolymer Viscosity

High prepolymer viscosity can lead to problems in processing and final product quality.

Potential Causes

Overheated Prepolymer
Because prepolymers are generally heated for processing, it is possible to degrade them by applying too much heat. Normally, the isocyanate (NCO) groups within a prepolymer react with a curative to form an elastomer. When the prepolymer has been overheated, however, these NCO groups react with each other instead, leading 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.

Low Processing Temperature
The viscosity of any liquid is a function of temperature; the higher the temperature, the lower the viscosity. 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 under-heated because they require higher processing temperatures than do polyether prepolymers of similar hardness.

Use of Inappropriate Prepolymer Type
Polyether prepolymers are typically lower in viscosity than polyester prepolymers. Similarly, different prepolymer production methods yield products with different viscosity characteristics.

Solutions

Adhere to Melting Guidelines
To avoid overheating a prepolymer, adhere to the melting time and temperature guidelines provided by the prepolymer supplier. Although prepolymers degrade at different rates upon heating, minimizing heat exposure is always recommended. To monitor this, keep a log of the prepolymer heat history. Use of a band heater is not recommended because heating is too localized. Also, if possible, the prepolymer drum should be rolled while being heated.

Check the Prepolymer Temperature with a Thermocouple
To avoid low processing temperatures, check the temperature of the prepolymer with a reliable thermocouple to ensure that the preheat oven is working properly. Do not assume that the prepolymer is at the same temperature as the setting of the oven. There may be localized hot or cold spots in the oven, or the prepolymer may not have been in the oven long enough.

Choose the Most Appropriate Prepolymer Type
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. Airthane prepolymers also provide longer pot life. Discuss your requirements with your supplier to determine the best prepolymer to meet your needs.