In doing formulation, sometimes we have got the required gelling time, but not the cream time or vice-versa. Is there any simple way to tune in the catalyst side in order to get both cream time and gelling time as requested?
The simple answer is by reducing the catalysis of the PU system. Catalysts accelerate or enhance the chemical reaction by reducing the reaction times (including the cream time). Thus, reducing the catalyst concentration will increase the cream time. At the same time it is tricky, because reducing the catalysis of the PU system will lead to longer demolding times, less productivity, and eventually, if the catalyst package is not well balanced, to foam collapse. So, the amplified answer is: The polyurethane reaction basically is composed of two chemical reactions: isocyanate + water = CO2 (gas reaction, leading to the expansion of the foam), and isocyanate + polyol = (poly)uretane (gel reaction, leading to the crosslinking, or solidification, of the foam). The catalyst used to accelerate the PU reaction can be more effective in the gas reaction (gas balanced catlysts) or in the gel reaction (gel balanced catalysts). The cream time is the time when the liquid PU starts to expand, due to the gas reaction. Then, it comes the crosslinking of the foam (which speed can be measured by the so called string time) and the finish of the expansion (measured by the so called rise time). Some time after the rise time, depending on the processing conditions, and the type of PU formulation, the foam is finished, and the mold can be opened, in the so called demolding time. Depending on the catalyst package used in the formulation, the reaction profile will be faster at the begining, with a highly catalylsed gas reaction, and a short cream time, or faster at the end, with a highly catalysed gel reaction, and a longer cream time (but a shorter rise time). Reduging only the gas catalyst will lead to a slower gas reaction, and a longer cream time. That’s still a simple answer, because in the real world we need to consider some difficulties: •The coexistence of two parallel chemical reactions is delicate. If the gas and gel reactions are not balanced, the structure of the foam may change, and lead to higher densities, different cell structure, or even collapses. •The catalysts used in PU reactions are not totally gas or gel catalysts. All reaction times are affected by the percentage of the catalyst. For example, increasing the concentration of a strong gel catalyst will lead to a reduction of rise time (and demolding time), but also to a reduction of the cream time. On the contrary, the reduction of a gas catalyst will lead to a longer cream time, but also eventually to a longer rise time. The main conclusion is: defining the PU reaction profile is usually a question of finding the correct compromise. Many people come with the same question, requesting an increase of the cream time, which will give them the possibility to foam bigger parts, or more complicated geometries of mold, but then they realise that this will lead to longer demolding times (and less productivity), so at the end we need to find together a compromise.
NOTE: This was suggested by Servet Casabona at somewhere else, for requester's help I paste it here
a little amount of volatile organic acid depending to foaming temperature could solve your problem for example formic acid when temperature is around 60 decrease creame time but slowly evaporate and gelation and the molding remain intact
as prof Barikani said