High resilience polyurethane foam is a high-quality cushioning material with excellent rebound properties and low cost. However, during the actual production process, various defects such as foam shrinkage, internal hollowing, residual odors, surface imperfections, and poor resistance to humid heat aging are often encountered. In recent years, we have explored solutions to these issues.
1. Foam Shrinkage
Foam shrinkage is a common and difficult problem in actual production, mainly caused by tooling, molds, and raw materials, which are interrelated.
1.1 Tooling and Mold Factors
When the mold sealing is inadequate, material leakage can occur, preventing the foam from reaching the desired density and leading to shrinkage. Additionally, foam products near the mold seams may develop hard edges. This issue can be resolved by improving the mold sealing or increasing the mold clamping force.
1.2 Raw Material Factors
If the bubble membrane is highly elastic during foaming and the bubbles expand without rupturing, most of the bubbles will be closed cells, leading to high closed-cell rates. When the foam cools, the pressure inside the bubbles decreases, causing foam shrinkage and deformation. Solutions to this issue include:
Adjusting the catalyst dosage to control foam cell size and open-cell ratio. Typically, amine catalysts promote the reaction between isocyanates and water (foaming), while tertiary amine or organotin catalysts promote the reaction between isocyanates and polyols (gelation). Excessive gelation catalysts can cause premature gelation, leading to robust bubble membranes and closed cells. Reducing the gelation catalyst dosage can lower the closed-cell ratio.
Reducing the average functionality of polyether polyols, as higher functionality leads to faster reticulation and more elastic bubble membranes, increasing the closed-cell rate.
Using an appropriate amount of foam stabilizer, as excessive amounts can lead to overly stable bubbles that do not open, causing shrinkage.
Controlling the isocyanate index, as a high index can exacerbate closed-cell formation and shrinkage.
2. Internal Foam Hollows and Foam Collapse
Internal hollows and foam collapse during production are mainly caused by two factors:
2.1 Imbalance in Gelation and Foaming Reaction Rates
In the final stages of foaming, the bubble membrane’s viscosity is high but elasticity is low. If gas generation is extensive and the bubble membrane cannot withstand the stretching, it will rupture, leading to cell opening. If the bubble membrane ruptures extensively and the foam lacks sufficient strength, it may collapse. Increasing the gelation catalyst or reducing the foaming catalyst dosage can improve the balance between gelation and foaming reactions, enhancing bubble membrane strength and reducing hollows or collapse.
2.2 Inadequate Foam Stabilizer Dosage
Organosilicone foam stabilizers are crucial in polyurethane foam production as they reduce surface tension and stabilize the foaming process, resulting in fine and uniform cell structures. If the stabilizer dosage is too low, foam pores may open prematurely, causing collapse or hollows. Appropriate foam stabilizer dosage can coordinate cell opening, which should occur when the foaming and gelation reactions are complete and in equilibrium, typically when the foam has reached its peak height and strength.
3. Residual Odors in Foam
Residual odors can stem from:
Excessive isocyanate, leading to residual toluene diisocyanate (TDI) with an irritating odor.
High volatile compound content in the chosen polyether, resulting in a “polyether smell.”
Residual amine catalysts causing strong amine odors. Solutions include high-temperature storage to volatilize residual catalysts or using alternative amine catalysts that participate in chemical reactions, though these may increase production costs.
4. Surface Pores on Foam Products
Surface pores or internal voids can be caused by:
Insufficient mold surface smoothness, affecting material flow and resulting in rough surfaces and pores. Improving mold surface smoothness and using high-quality mold release agents can resolve this.
High viscosity and poor flowability of the material system, leading to air bubble retention on the foam surface. Lowering the viscosity of the combined polyether can address this issue.
