Cold-cured high rebound polyurethane foam is an excellent cushioning material known for its advantages such as good resilience, flame retardancy, and low cost. However, in the actual production process of high rebound foam, various defects are often encountered, including foam shrinkage, local hollowing, collapse of foam, residual odors, surface imperfections with pores, and poor performance in wet-heat aging. In recent years, the author has explored solutions to these practical issues arising during production.
1.Foam Shrinkage
A common and challenging issue in actual production is foam shrinkage. The main causes of shrinkage are attributed to both the tooling molds and raw materials, and these two factors are interrelated.
- 1.1 Tooling Molds
Inadequate mold sealing can lead to material leakage, preventing the foam from achieving the designed density and causing foam shrinkage. Simultaneously, the foam products near the corresponding mold closure line may exhibit a phenomenon known as hard edges during shrinkage. This can be addressed by improving the mold sealing or appropriately increasing the mold clamping force.
- 1.2 Raw Materials
If the elasticity of the bubble film wall is significant during the foaming process, and when a large amount of gas is generated causing volume expansion, the foam pores will expand without rupturing, resulting in a high proportion of closed cells. When the foam cools, the pressure inside the bubbles decreases, leading to foam shrinkage and deformation. The author suggests four main solutions to address this closed-cell issue:
(1) Adjust the catalyst dosage to control foam pore size and open-cell ratio. Typically, amine catalysts catalyze the reaction between isocyanate and water (foaming reaction), while tertiary amine or organotin catalysts catalyze the reaction between isocyanate and polyol (gelation reaction). Excessive promotion of the gelation catalyst leads to premature gelation, and the bubble film wall becomes tough, making it less prone to rupture and forming closed cells. To control foam pore size and open-cell ratio, reduce the dosage of gelation catalyst appropriately to slow down molecular chain growth, reduce bubble film wall elasticity during the peak gas evolution, and decrease the closed-cell ratio.
(2) Closed-cell formation is also related to the degree of polymerization and branching of polyether polyols. Higher-functionality polyethers form a network structure more quickly during the NCO/OH reaction, resulting in larger bubble film wall elasticity and increased closed-cell ratio. Lowering the average functionality of polyethers helps reduce foam closed-cell ratio.
(3) Excessive use of foam stabilizers can lead to overly stable pores that do not open, causing shrinkage. Therefore, the foam stabilizer dosage in production should be appropriate.
(4) High isocyanate index may exacerbate foam closed-cell formation and shrinkage. Control the isocyanate index during production.
2.Local Hollowing and Collapse of Foam
During the production of high rebound polyurethane foam, local hollowing and collapse of the foam may occur due to two main reasons.
- 2.1 Imbalance in Gelation and Foaming Reaction Rates
In the foaming process, during the final stage of substantial gas generation, the viscosity of the bubble film wall is high, but the elasticity is poor. Under increasing gas conditions, the film wall cannot withstand stretching, leading to bubble rupture and gas escape, i.e., the formation of open cells. If, during the abundant gas generation, the foam film wall ruptures and the pore channels and skeleton lack sufficient strength to prevent this rupture, the rupture will further spread, causing the entire foam to collapse. If the rupture stops in a small part, it will result in local hollowing or cracking. To address this issue, increasing the gelation catalyst or reducing the foaming catalyst dosage can improve the balance between gelation and foaming reactions. This strengthens the bubble film wall during significant gas generation, appropriately reduces gas generation, and reduces or improves the occurrence of local hollowing or collapse.
- 2.2 Insufficient Foam Stabilizer Dosage
Organosilicon foam stabilizer is essential in the polyurethane foam production process, reducing the surface tension of various raw material components, stabilizing the foaming process, and ensuring uniform and fine pores. If the foam stabilizer dosage is insufficient, foam pore stability is poor, premature opening occurs, leading to collapse or local hollowing.
Appropriate foam stabilizer can coordinate the timing of opening pores. The opening of pores is a crucial process in the high rebound foam foaming process; otherwise, closed-cell shrinkage may occur. However, pore opening must occur when the foaming reaction and gelation reaction are essentially complete and in balance. That is, when the foam reaches its peak and the foam strength can support its weight. Otherwise, foam collapse or hollowing may occur.
3.Residual Odors in Foam
Residual odors in foam may come from three sources.
(1) Excess isocyanate can result in residual toluene diisocyanate in the foam, creating an irritating odor.
(2) If the selected polyether in the raw material formulation has high volatility, the foamed product may have a “polyether odor.”
(3) The strong amine odor caused by residual amine catalysts in the foam. To address these odors, two approaches can be taken. First, the foam can be stored at a high temperature for a period to allow the residual catalyst to volatilize. However, this method is challenging to implement in practice. Second, adding amine catalysts that participate in chemical reactions in the foam system can reduce the amine odor caused by conventional amine catalysts, but the cost of foam production will increase accordingly.
4.Pores on the Surface of Foam Products
Surface pores or internal voids in foam products may be caused by five main factors.
(1) Insufficient mold surface smoothness affects the flowability of the material system, resulting in rough and porous foam surfaces. This can be addressed by improving mold surface smoothness, careful operation, and the use of better mold release agents.
(2) If the viscosity of the material system is too high, resulting in poor flowability, it can cause residual air bubbles on the surface of foam products. This can be addressed by lowering the viscosity of the combined polyether. The suitable viscosity in practical work is around 1500-1800 mPa·s.
(3) If the gelation rate during foaming is too fast and the time is too short, the viscosity of the material system rapidly increases, leading to poor flowability and the formation of surface pores. Gelation time is generally controlled between 55-65 seconds. However, gelation time should not be excessively long; otherwise, if mold sealing is inadequate, it can lead to material waste.
Addressing these defects and implementing the suggested solutions can contribute to improving the overall quality of high rebound polyurethane foam production.