The influence of environmental humidity on the PU foaming process is mainly reflected in bubble stability, reaction rate and product quality. Specific impacts and responses are as follows:
Effects of Environmental Humidity on the PU Foaming Process
1.Reduced bubble stability
In high-humidity environments, moisture in the air sticks to the surface of the feedstock, causing gases from the decomposition of foaming agents to react with moisture to form carbon dioxide, creating irregular bubbles. These bubbles are prone to rupture or merge during curing, resulting in uneven cell structure, bubble collapse or surface defects. For example, if humidity is not controlled during summer construction, needle holes or air pockets may appear on the surface of PU gaskets.
Abnormal reaction rate
2. Excessive humidity: the addition of moisture will accelerate the decomposition of the blowing agent, the reaction rate of the blowing agent is too fast, the cell membrane wall coagulates too early, it is difficult to form pore structure, improve the closing rate.
Low humidity: Lack of moisture in raw materials can lead to incomplete decomposition of foaming agents, reduced gas production, reduced cell density, and even a shortage of raw materials.
Product quality deterioration
Humidity fluctuations will directly affect the density, elasticity and breathability of PU foam. For example, polyurethane sponge produced in high-humidity environments may lose elasticity due to closed spaces, while in low-humidity environments the density of the sponge may not be up to standard, affecting its performance.
3. Countermeasures
Controlling ambient humidity
Production workshop: Stabilize humidity by 50%-60% through dehumidifiers or air conditioners to avoid humidity exceeding 75% or below 40%. For example, during summer construction, windows and doors should be closed to prevent excessive humidity during rainy days. In winter, it requires to be heated and dehumidified to prevent the raw material from absorbing moisture.
Storage of raw materials: Ensure that raw materials are stored in airtight containers to avoid exposure to humid air. The moisture content of the raw material needs to be tested before use. If above standard, it should be dried.
Optimize process parameters
Adjust the amount of foaming agent: Increase or decrease the proportion of foaming agent according to humidity. Reduce the amount of foaming agent when humidity is high and increase the amount of foaming agent when humidity is low to balance gas production.
4. Optimization of catalyst combination: Open catalysts (such as triethylenediamine derivatives) were selected in combination with conventional catalysts at a ratio of 1:3-1: 5 to facilitate cell membrane wall rupture and reduce the number of closed cells.
Control raw material temperature: keep the raw material temperature stable at 20-25 ℃, prevent from reaction rate too fast because of high temperature, or increase the viscosity of raw material because of low temperature, and stir unevenly.
Improvement of equipment and mold design
Mold exhaust optimization: reasonable air holes and exhaust ports are set up in the die to ensure smooth gas discharge and prevent bubble rupture due to pressure accumulation. For example, preheating molds match the reaction temperature to reduce local pressure variations.
Stir and pour control: Increase stirring speed to 2000-3000r/min and stirring time to 10-15 seconds to ensure adequate mixing of A and B materials. Control the pouring speed to avoid turbulence damaging the battery structure.
The post-processing process is complete.
Mechanical perforation: For sponges with a high density of enclosed cells, acupuncture (50 to100 needles per square centimetre, 80 to 90 per cent of the thickness of the sponge) or rolling pressure (0.3 to0.5MPa, repeated 2-3 times) is used to facilitate cell connectivity.
Wash and dry: After curing, wash the sponge in warm water at 40-50°C to remove any remaining raw material. Then dry at 80-100°C for 2-3 hours to ensure the inside of the battery dries.