Slow curing of polyurethane foam (gel reaction lag) is usually caused by imbalance in raw material formulation, improper process conditions or external environmental interference, which needs to be analyzed from the perspective of chemical reaction kinetics and thermodynamics. The following are the specific reasons and corresponding solutions:

                              Defective raw material formulation

Insufficient isocyanate (NCO) content

Reasons: The isocyanate index (NCO/OH) in the formulation is too low, and the active-NCO group is insufficient to complete the cross-linking with polyols/water in time.

Improper storage of isocyanate raw materials (e.g., moisture absorption) and partial - NCO groups react with water to form urea, resulting in a decrease in effective content.

Performances: The foam is soft, slow to rebound, or even cannot be formed, and the strength is insufficient after maturation.

Insufficient or improper ratio of gel catalysts

Reasons: The addition of gel-type catalysts (such as organotins, such as dibutyltin dilaurate) is insufficient to effectively accelerate the crosslinking reaction between isocyanates and polyols.

The proportion of foaming catalysts (such as triethylenediamine) is too high, which preferentially promotes the foaming reaction, but the gel reaction lags behind, resulting in slow curing and easy rupture of the pore wall.

Performances: The bubble expands normally during the foaming process, but the foam does not harden, and even collapses or flow.

Polyol properties do not match

Reasons: The molecular weight of polyols is too large or the function is too low, the hydroxyl group activity is insufficient, and the reaction rate with isocyanates is slow.

Polyols contain a large amount of secondary hydroxyl groups (e.g., PO-based polyethers) and are less reactive than primary hydroxyl groups (e.g., EO-terminated polyethers).

The viscosity of the system is too high (e.g., too much high molecular weight polyol) to hinder molecular diffusion and delay cross-linking.

Abnormal water content or foaming agent interference

Reasons: Excessive use of chemical blowing agent (water) and large consumption - NCO generates urea bonds (less exothermic and low crosslinking efficiency), resulting in lag in gel reaction.

Physical blowing agents (such as cyclopentane and HFC-245fa) volatilize and absorb endothermic, greatly reducing the system temperature and inhibiting the reaction rate.

                          The process conditions are out of control

The temperature is too low

Reasons: The temperature of the raw material (polyols, isocyanates) is lower than the process requirements (usually controlled at 20~40°C), resulting in insufficient activation energy of the reaction.

If the ambient temperature is too low (e.g. winter production), especially in open foaming scenarios without heating devices, heat is lost quickly.

Performances: Slow foam initiation, curing time is extended several times, and even surface stickiness and internal uncured.

Inadequate mixing or insufficient pressure

Reasons: too low mixing speed or insufficient mixing time (e.g. when foaming by hand), raw materials are not evenly dispersed, local - insufficient contact between the NCO and the hydroxyl group.

Insufficient pressure in high-pressure foaming equipment (e.g. below 10MPa) leads to poor material impact mixing effect, especially in high-viscosity systems.

Performances: The foam appears to be local soft lumps, the density is uneven, and there are still areas with insufficient elasticity after maturation.

Foaming speed is imbalanced with gel speed

Reason: The foaming reaction rate is much faster than that of gel reaction, and the bubble wall is not cured in time when the bubble expands, resulting in foam collapse or curing delay.

For example, the fast foaming formula (high water content strong foaming catalyst) is not equipped with enough gel catalyst, and the bubbles are "long but not hard".

                                External interference factors

Impurities or inhibitors are present

Reasons: The raw materials contain acidic substances (such as phosphates in flame retardants, unneutralized carboxylic acids), neutralizing alkaline catalysts (such as triethylene diamine), reducing catalytic efficiency.

The system is mixed with water, alcohols, amines, and other active impurities, which consume isocyanates or compete for reaction sites.

Metal ions (e.g., Cu², Fe³) may inhibit certain catalyst activity.

Surfactant overdose

Reason: Excessive use of silicone stabilizers forms a strong interface film, which hinders the contact between isocyanates and polyol molecules and delays cross-linking.

Performance: The foam bubbles are fine but cure slowly, and the surface is smooth but the inside is soft.

 Filler or additive influence

Reasons: The amount of inert fillers (such as calcium carbonate and talc) is too high, which dilutes the concentration of reactive groups and reduces the crosslinking efficiency.

Some additives (such as certain antioxidants and UV absorbers) react with catalysts and weaken catalytic activity.

                          Resolution strategies and troubleshooting steps

Formulation optimization

Adjust the catalyst: increase the proportion of gel catalysts (e.g., increase the amount of organotin by 0.1~0.5 phr), or replace the high-efficiency catalyst (e.g., stannous caprylate instead of dibutyltin dilaurate).

Reduce the amount of foaming catalyst or use a delayed foaming catalyst (e.g., partially encapsulated triethylenediamine) to synchronize the foaming with the gel reaction.

Calibrate isocyanate index: Increase the NCO index to 1.05~1.15 (according to the recipe) to ensure that the crosslinking group is sufficient and the NCO content of the raw material can be detected by titration.

Optimize polyols: switch to low-molecular-weight, high-hydroxyl polyols (e.g., polyether with a hydroxyl number of 400~600 mgKOH/g), or increase the primary hydroxyl content (e.g., EO end-capping ratio).

Process regulation

Heating treatment: Preheat the raw materials to 25~35°C, and maintain the ambient temperature at 18~25°C (heating workshop or equipment is required in winter).

For large pieces of foam, it can be accelerated and cured by a maturation oven (40~60°C).

Improve mixing results: increase the mixing speed (e.g. high-pressure foaming machine pressure to 12~15MPa) or extend the manual mixing time to emulsify evenly.

Reduce system viscosity (e.g., add 1~3% solvent or low-viscosity polyol) to improve flowability.

Impurity control and raw material testing

The polyolic acid value should be measured (< 0.1 mgKOH/g to avoid excessive acidic substances).

Ensure that the isocyanate does not absorb moisture and deteriorate (store in a dry environment, seal in time after opening);

Reduce the amount of filler or switch to a surface treatment filler (e.g., coupling agent modified calcium carbonate) to reduce interference with the reaction.

Dynamic balance test

The matching of foam initiation speed and curing time is observed through foaming tests (such as rising curve tests): normal foam should be preliminarily cured (non-sticky) within 1~3 minutes after the peak of initiation (foam rises to the maximum volume);

If it does not cure within 5 minutes of initiation, focus on the gel catalyst and NCO content.

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