The isocyante's contact with the polyol (and any water) starts the reaction. In very simple terms, the faster you expose the chemicals to each other, the quicker the reaction proceeds. A mechanical mixer will often whip the ingredients together quickly vs slow hand mixing.

A catalyst can be used to help start the reaction and drive it to completion. Failure to mix well may cause poor quality and incomplete reaction. Heat of reaction is high exotherm which helps reaction speed if mixing is done well. Speed of mixing should be fast enough that mixing is nearly finished before reaction reaches CT/GT.

@Monireh Alizadeh: the power input (i.e. energy of mixing per unit time) can be much higher in high pressure mixing.  This is important because in thermoset reactions, there is always the competition between the reaction exotherm generating heat vs. the surrounding "heat sink" (the metal mold, air, etc.) taking out heat from your reaction mixture.  So the faster the reaction exotherm can be kick-started (i.e. from a higher mechanical energy of mixing) and generated,the faster the temperature will rise, much faster in some cases than the heat sink can pull away heat.   Thus, the cream time and gel time will also be lower (i.e. faster creaming/gelling).

@Kaoru Aou: thank you for your answer. I think in exothermic reaction, the faster the heat of reaction is removed, the better the reaction will be progressed; counter wise, for endothermic reaction, the faster the heat of reaction is added, the better the reaction will be progressed; then in this exothermic reaction, if high pressure mixing make the reaction starting much faster than the heat sink can pull away heat, dosen't it result in increasing gel/cream time?

@Monireh Alizadeh: I think there's some confusion here.  It's not about exo vs.  endothermic.  It's about more vs. less heat sink effect.   (also, to answer directly, exothermic reaction with more heat sink will lead to slower reaction than less heat sink)

The information provided by you is incomplete.

In addition, I have great doubt whether you can compare the two, in my opinion, very different mixing methods. In fact, I do not know if the second method (reaction injection molding) you used can be called agitation.

It is also worth noting that the smaller the value of CT / GT can attest to the lower value of CT but also higher value GT.

Moreover, lack of information about:

thermal conditions of the process (adiabatic ?)

time of the dispersion of one substrate in the other, (I suppose, however, that it is difficult to distinguish between a homogenous mixture and a cream.)

CT and GT values for first and second methods,

pressure in second method,

temperature before mixing and after obtaining the cream,

viscosity at the beginning of geling etc.

Currently, I can only speculate on relationship obtained by you.

The most important factor in my opinion is temperature.

It affects not only the rate of a chemical reaction but as well as viscosity. CG depends primarily on the temperature. The viscosity of the mixture may affect the gelation. Mixing, beyond the preliminary stage (the dispersio of one phase in the other) has a primary effect on the heat exchange and as a result temperature equalization.

Mass transport may affect processing only at the start of mixing. Further, the process is controlled by the very fast chemical reaction.

Under adiabatic conditions temperature rise. This has an impact on the growth rate of the reaction. In this case, the viscosity is affected by temperature increase (decrease in viscosity) and the progress of the reaction (viscosity increase) 

At the end, I do not understand the idea formulated in the first part of sentence “I think in exothermic reaction, the faster the heat of reaction is removed...............” This makes sense (removing reaction heat) in the case of a reversible reaction, the threat of evaporation of the mixture components, the possibility of ignition of the reactants, etc. But there is no sense in the case of the irreversible reaction.

@M Grzesik: on the argument made in the final paragraph of your recent post: the argument makes sense if the reactions are allowed to go to completion (complete depletion (let's say >99.9% as a pragmatic definition) of the limiting reagent).  In most cases, this will not happen.  In thermosetting systems such as polyurethanes,  there are two reasons this can happen - most commonly, 1) chemical gelation through crosslinking, so that reactions become mass diffusion limited; 2) vitrification (i.e. glass transition temperature of the evolving thermoset system approaches the system temperature to the point where reactions become mass diffusion limited).  It is point #2 that causes heat sink effects to push the system to the regime of mass diffusion limitation.  That is, heat sinks will prevent the system temperature from rising as quickly as the glass transition temperature of the system.

Kaoru Aou: My statement referred to the first part of the sentence on any exothermic chemical reaction, formulated by Monireh Alizadeh ("I think in exothermic reaction, the faster the heat of reaction is removed, the better the reaction will be progressed........"). It does not relate, however, to a chemical reactions with the accompanying transport phenomena and to a chemical reactions with the accompanying phase transitions.

When it comes to your opinion, I agree with it. It does not apply, however, to a situation to which I referred up. 

Hello Monireh,

I also think there is not enough information to give you a proper answer. The difference you can notice arises in the early stages of the reaction and therefore the initial conditions have a major effect.

In particular, you'll need to tell what was amount of material that was hand mixed vs machine mixed and what was the procedure to measure CT and GT in each case.

Could you please give more details?