Polyurethane Flexible Foam Shrinkage and Collapse: Five Core Q&A and Formula Adjustment Techniques

I. What: Defining Shrinkage, Collapse, and Closed-Cell Structure

Q1: What is the fundamental difference between shrinkage and collapse?
Shrinkage occurs during the cooling or curing stage after foaming, where the volume gradually decreases. The root cause is insufficient gelling or excessive closed-cell structure.

Collapse happens during the foaming process — the foam either fails to rise or collapses after rising. The fundamental reason is missing components or severely unbalanced formulations.

Severity: Collapse is more serious (product scrap), while shrinkage is a moderate issue (dimensional change).

Q2: How to distinguish between closed-cell shrinkage and open-cell shrinkage?
You can identify the shrinkage type using the following tests and take targeted measures:

Finger press test:

Closed-cell shrinkage: Feels bouncy and resistant when pressed. Solution: Add an open-cell agent or adjust the water/MC ratio.

 Open-cell shrinkage: Feels permeable and easy to press in. Solution: Increase the gelling catalyst.

Air permeability test:

When blowing air through the foam, closed-cell foam resists airflow, while open-cell foam allows air to pass through.

II. Why: Causes of Shrinkage in Mass Production and Countermeasures

Q3: Why does shrinkage not occur in small-scale trials but appear in mass production?
Shrinkage risk increases due to three main factors:

Heat dissipation difference: Large foam blocks in mass production cool slowly, leading to higher internal temperatures and altered reaction behavior.

Equipment difference: Automatic production lines may have different mixing parameters than small-scale manual or lab setups.

Curing condition difference: Large foam blocks stacked together restrict heat dissipation.

Solutions: Conduct pilot-scale tests before full production; allow longer curing times or horizontally cut the foam blocks for better cooling.

III. How: Formula Key Adjustment (TDI Index and Water/MC Ratio)

Q4: What should the TDI (isocyanate) index be set to?
The standard range is 1.03–1.10, depending on product type, season, and specific issues:

 Regular foam: 1.05–1.08

 Severe shrinkage: Increase the index by 0.02

 Severe closed-cell issue: Decrease the index by 0.02

 Seasonal adjustment: Increase by 0.02–0.03 in winter; decrease by 0.02–0.03 in summer.

Q5: How to control shrinkage risk by adjusting the water and dichloromethane (MC) ratio?

This adjustment balances temperature and density:

Adjustment principle: More water increases reaction heat and temperature; more MC absorbs heat and stabilizes the reaction.

Adjustment method: If the temperature is too high (causing scorching or shrinkage), reduce water by 0.1–0.2 parts and increase MC by 1–2 parts.

Balance note: When adjusting water and MC, slightly fine-tune amine catalysts to maintain balance between foaming and gelling reactions.