In the polyurethane spraying construction process, the long curing time after foaming is one of the common technical problems. The curing process not only affects the construction rhythm, but also has an adverse effect on the strength, adhesion and overall insulation effect of the foam. This article will conduct a comprehensive analysis from the aspects of ambient temperature , ratio accuracy , raw material quality and construction operation , and propose practical solutions.
1. Insufficient ambient temperature
The polyurethane foam curing process relies on the chemical reaction between isocyanate (ISO) and polyol (POL), and the reaction rate is extremely sensitive to ambient temperature. When the ambient temperature is below 15°C , the reaction rate is significantly reduced, resulting in slow foam expansion, prolonged surface drying and actual drying time, and possible shrinkage, shedding and other problems.
2. Ratio error or insufficient curing agent ratio
When using low-pressure or high-pressure spray equipment, the mixing ratio of POL and ISO must be precisely controlled at 1:1 (volume ratio) . Any deviation, especially insufficient isocyanate dosage, will significantly reduce the cross-linking density, making it difficult for the foam to fully cure.
In addition, some formulas add co-catalysts (such as triethylenediamine) to accelerate the reaction. If the catalyst ratio is too low, the curing reaction will be delayed.
It is recommended to calibrate the metering system of the spray equipment regularly and use a pressure balance automatic control system to keep the two-component flow rate consistent.
3. Raw material quality fluctuations
Poor quality raw materials or improper storage (such as polyols absorbing moisture and deteriorating, isocyanates turning yellow and crystallizing) will also reduce the efficiency of the foaming reaction. The polyurethane system has high requirements for the purity and activity of raw materials, especially the moisture content: the moisture content of POL should be less than 0.1% , otherwise it will release excessive CO₂ gas and slow down the overall reaction speed.
Use qualified raw materials and ensure that the raw materials are stored in a dry and sealed state. Nitrogen purging or desiccant should be used for pretreatment before construction.
4. Improper construction operation
During the spraying process, the following improper operations can also lead to slow curing:
- Spraying thick and wet or too cold (such as the surface temperature of the metal plate is lower than 10°C) will affect bonding and reaction;
- The spray gun does not mix too much (>25mm/time), the heat in the middle layer is difficult to release, and the reaction is incomplete;
- The base material is too sufficient , and the A/B materials are not evenly mixed to enter the reaction stage.
The recommended spraying thickness is no more than 20mm per layer, and the spraying should be carried out in layers. The substrate should be preheated before spraying to ensure that the temperature is above 15°C.
V. Summary and Suggestions
Source of the problem | Performance | Suggested Actions |
Ambient temperature is too low | Long dry time | Control the temperature at 20~30°C and heat the equipment if necessary |
Inaccurate ratio | Uneven foaming, delayed foaming | Calibrate equipment, use flow monitoring system |
Raw material deterioration | Slow or failed curing | Use desiccant to dehydrate and avoid moisture absorption of raw materials |
Spraying thickness is too large | Uncured between layers | Spray in layers, each time controlled at 15~20mm |
Insufficient mixing | Local drawing and bubbling | Clean the spray gun regularly and check the dynamic mixing effect |
Conclusion
As an efficient and energy-saving insulation method, polyurethane spraying is extremely critical to the control of “curing time”. The construction party should start from the whole process of environment, equipment, raw materials and technology, and optimize the system. Choosing mature spraying equipment and reliable raw material system is the guarantee to ensure rapid curing and project quality.
If you need selection advice or technical support, please contact us to obtain the overall solution of polyurethane spraying.