How to Produce High Load-Bearing Foam and Ultra-Soft Foam?

This article introduces high load-bearing and ultra-soft foam as hardness-modified types of general-purpose soft foam.

① Hardness

Hardness is a key physical performance index used to measure the softness or firmness of polyurethane flexible foam. Due to differences in how force is applied to the surface, hardness can be expressed in various forms, which are not mutually convertible.

Flexible polyurethane foam is an elastomeric material that deforms easily. Its hardness is commonly tested and expressed in two ways: Shore hardness and Indentation Load Deflection (ILD).

  *Shore hardness (also known as Shore A or HA): Measured with a Shore durometer. It is expressed in degrees (symbol: H or HA). Specialized “foam hardness testers” are available for this purpose.

  *ILD (Indentation Load Deflection): Measures the force (in Newtons, N) required to achieve a specific deformation under set conditions and equipment. This is also called the ILD value.

② Factors Affecting the Hard Segment

Hard Segment + Hard Segment Aggregation

Diagram: Relationship Between Foam Hardness and Water Content

Physical Blowing Agent

Diagram:Relationship Between Foam Hardness and Physical Blowing Agent Dosage

Isocyanate Index

Diagram:Relationship Between Foam Hardness and Isocyanate Index

Soft Segment

Diagram:Relationship Between Soft Segment Molecular Chain Content and Hardness

Fillers and Polymer Polyols

Diagram:Relationship Between Stone Powder and POP Dosage and Hardness

Morphology of Solid Fillers in Foam Network

Polymer Polyol Structural Diagram

I 

Catalysts like amines and tin have minimal effect. Hardening agents increase the degree of branching.

Approximate Hardness Increase from Formulation Variables

(Values recorded in Shore A hardness; calculated using golden ratio-based equations.)

*Every 0.1 increase in water dosage contributes 1 HA:

Δm(H₂O) = 0.1 → ΔH = 1 (HA)

Or

Δm(H₂O) = 1 → ΔH = 10 (HA)

*General-purpose polyether (PPG) in unit density contributes 0.618 HA:

Δd = 1 → ΔH·ω(PPG) = 0.618 (HA)

*For polymer polyol (e.g., POP):

Δm(POP) = 1 → ΔH = 0.618 (HA)

*For calcium carbonate (stone powder), assuming ratio m(PPG₁):m(CaCO₃) = 1:1:

Δm(CaCO₃) = 1 → ΔH = 0.382 (HA)

*For MC physical blowing agent:

Δm(MC) = 1 → ΔH = –1.618 (HA)

*For TDI index (baseline at 1.05):

ΔIn(TDI) = 0.01 → ΔH = 1 (HA)

③ Hardness Enhancement Methods for High Load-Bearing Foam

Ways to increase foam hardness include:

1.Increasing water dosage

2.Enhancing hard segment content and aggregation

3.Reducing MC usage

4.Adding fillers

5.Using polymer polyols

6.Raising TDI index

7.Adding hardening agents

Among these, increasing polymer polyol (POP) is highly effective and operationally convenient.

④ Ultra-Soft Foam

Ultra-soft foam is made using the opposite methods of hardening:

*Softeners (both organic and inorganic silicones): Reduce surface tension and limit hard segment aggregation.

*Plasticizers: Have a minor softening effect.

*Lowering the TDI index and increasing MC dosage can soften foam but present operational risks.

⑤ Conventional Foam

Due to the widespread use of hardening and softening methods, foam types are generally identified by density and hardness. As a result, high load-bearing and ultra-soft foams have gradually disappeared from independent classification and are now grouped with general-purpose soft foams under the term “conventional foam.”