15 Common Issues and Solutions in Polyurethane Soft Foam Production

pu foam

During actual polyurethane foam production, various incidents and problems can arise. Each production issue is typically caused by multiple factors. In complex incident analyses, it is often difficult to list all the influencing factors or pinpoint the main contributors. Below are 15 commonly encountered problems and their causes for reference.

1. High Closed-Cell Ratio

   – Polyether Polyols: High ethylene oxide content, high reactivity, often occurring when switching between polyether polyols of varying reactivity.

   – Process Formulation: Excessive tin octoate, high reactivity of isocyanates, high crosslinking density, rapid crosslinking speed, excessive amines and physical blowing agents causing low internal pressure in the foam. When foam elasticity is too high, it fails to open the cells. A high TDI index can also lead to a high closed-cell ratio.

2. Shrinkage (Gelation Rate Faster Than Foaming Rate)

   – High Closed-Cell Ratio: Shrinkage occurs during cooling.

   – Process Conditions: Low ambient temperature, low material temperature.

   – Formulation: Excessive silicone oil, excessive physical blowing agents, and a low TDI index.

3. Internal Cracking

   – Process Conditions: Low ambient temperature, high reaction center temperature.

   – Formulation: Low TDI index, excessive tin, high early foaming strength.

   – Silicone Oil: High reactivity and low dosage.

4. Top Cracking (Imbalanced Gas Release and Gelation Speed

   – Process Conditions: Low ambient temperature, low material temperature.

   – Formulation: Insufficient catalyst dosage, low amine content, poor-quality silicone oil.

5. Bottom Corner Cracking (Excessive Amine Usage, Rapid Foaming Speed)

   – Surface Large Pores: Excessive physical blowing agents, poor-quality silicone oil and catalysts.

6. Poor Low-Temperature Performance of Foam

   – Poor inherent quality of polyether polyols, low hydroxyl value, low functionality, high unsaturation, and low TDI index with the same tin usage.

7. Poor Ventilation

   – Climate Conditions: Low ambient temperature.

   – Raw Materials: High polyether polyols, high silicone oil reactivity.

   – Formulation: Excessive tin or inadequate tin usage, low water and amine content, high TDI index.

8. Poor Resilience

   – Raw Materials: High reactivity and low molecular weight polyether polyols, high silicone oil reactivity.

   – Formulation: Excessive silicone oil and tin, high water content, and high TDI index when tin usage is the same.

9. Poor Tensile Strength

   – Raw Materials: Excessive low molecular weight polyether polyols, low hydroxyl value, and low functionality.

   – Formulation: Insufficient tin leads to poor gelation, high TDI index and low water content resulting in low crosslinking density.

10. Smoking During Foaming

    – Excessive amine promotes the reaction between water and TDI, generating significant heat, causing the evaporation of low boiling point substances that produce smoke.

    – If it’s not scorch marks, smoke typically comes from TDI, low boiling point substances, and monomeric cycloalkanes in the polyether polyols.

11. Foam with White Striations

    – Rapid foaming and gelation lead to slow transfer speeds during continuous foaming. Localized compression results in a dense layer, causing white striation. Increase transfer speed, lower material temperature, or reduce catalyst dosage.

12. Brittle Foam

    – Excessive water content in the formulation leads to a high amount of dihydropyrimidine. Poorly dissolved silicone oil, poor-quality tin catalyst, insufficient crosslinking, high content of low molecular weight polyether polyols, excessively high reaction temperature, and ether bond breakdown reduce foam strength.

13. Foam Density Below Set Value

    – Excessive foaming index due to inaccurate measurement, high ambient temperature, and low air pressure.

14. Foam with Skin at the Bottom, Sides, and Cavities

    – Excessive tin and insufficient amines slow the foaming speed, and rapid gelation leads to low temperature during continuous foaming.

15. High Elongation Rate

    – Raw Materials: High reactivity polyether polyols with low functionality.

    – Formulation: Insufficient crosslinking with a low TDI index, excessive tin and silicone oil.

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