Changes in Pot Life or Gel Time
All prepolymer/curative combinations have a specific pot life and gel time. Pot life is generally defined as the amount of time available after mixing to work with the system before it becomes too thick to pour. Gel time is typically defined as the time it takes for the system to cease exhibiting any liquid-like characteristics. Generally, the higher the percent NCO of the prepolymer, the shorter the pot life and gel time. Pot life and gel time are also both influenced by temperature.
Potential Causes
Degraded Prepolymer
A degraded prepolymer will usually exhibit a reduction in percent NCO. This reduction will shorten the apparent pot life and possibly lengthen the demold time.
Degraded Curative
Some curatives, such as Mboca, can be degraded with exposure to too much heat. When Mboca begins to decompose, a significantly faster reaction with the prepolymer results.
Improper Catalyst Addition
Catalysts are often added to decrease the pot life and gel time of a system to achieve increased productivity. Typical catalysts used for this purpose are amine, acid or tin based. (See Recommended Catalyst Use Levels for a list of recommended catalysts and their use levels.) Catalysts are added at very small use levels, and it can be very difficult to monitor an exact amount. In addition, since they are used at such low levels, a small amount of catalyst will last a long time. Some catalysts, especially tin-based products, can become less active over time and can lose their effectiveness.
Incorrect Temperatures
Higher reactant temperatures will cause an increased reactivity rate between the prepolymer and curative, thereby reducing the system pot life and gel time. The converse is also true. With smaller parts, mold temperatures that are significantly different from the mix temperature can cause reactivity problems by quickly adding or removing too much heat from the system.
Solutions
Minimize the System Heat History Before Processing
To eliminate degradation of the prepolymer and/or curative due to excessive exposure to heat, follow the supplier's processing temperature guidelines for each product.
Check the Catalyst Addition
Catalysts, if they are used, can have a significant effect on the reactivity of the system. Some catalysts, especially tin-based products, can become less active over time. If reduced catalyst activity is suspected, a new lot of catalyst should be tried. To assist in monitoring catalyst addition amounts, consider mixing the catalyst in a curative or plasticizer, such as Benzoflex.
Try a New Lot of Catalyst
Monitor Batch Temperatures Closely
Changes in batch temperature and mold temperature will affect reactivity. Monitor these temperatures carefully. Also, do not rely solely on oven temperature gauges—always have a secondary method to determine temperatures.
Organic Acids
Azealic Acid
Oxalic Acid |
Used with diamines |
0.3–0.6 parts |
Tertiary Amines
Dabco 33-LV® catalyst
Dabco® 25-S catalyst |
Used with diamines or diols |
0.3–0.6 parts |
Tins
Dabco T-12 catalyst
Dabco T131 catalyst |
Used with diols |
0.1–0.5 parts | |
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