What Are the Raw Materials of Flexible Polyurethane Foam?-Part 3

Foaming Agents

PU flexible foams mainly use water as the primary blowing agent, supplemented by physical blowing agents. In block foam production, when producing low-density foam, the water dosage often exceeds 4.5 parts per 100 parts, leading to internal foam temperatures rising above 170–180°C, potentially causing spontaneous combustion. In such cases, low-boiling-point hydrocarbon foaming agents must be used to help reduce density and remove heat. When manufacturing block foam with a density above 21 kg/m³, only water (a chemical blowing agent) is typically used. For low-density or ultra-soft formulas, physical blowing agents such as dichloromethane (MC) are added.

Auxiliary blowing agents reduce foam density and hardness. Their vaporization absorbs heat, slowing curing and requiring more catalyst. This heat absorption prevents foam scorching.

Water: Equivalent Value 9

(1) Using water alone can reduce foam density but results in a stiffer feel.

(2) Excessive water leads to intense exothermic reactions, causing foam core discoloration, scorching, or combustion.

(3) Water acts as a chain terminator and reacts with TDI to generate CO₂ but does not significantly increase polymer chain length.

MC (Methylene Chloride)

Boiling point: 39.8°C; Molecular weight: 84.9.

It is easy to synthesize, low-cost, non-flammable in air, with a spontaneous ignition temperature of 605°C, and has low ozone depletion potential and short atmospheric lifespan. However, it is toxic and potentially carcinogenic; avoid frequent contact and seek medical help if ingested.

It is mainly used to regulate cell size and softness, even in high-density foams.

Drawbacks include poor system stability due to slow volatilization and long residence time, leading to foam cracking and rough cell structure. To improve results, use Dabco CS90 (strong blowing and gelling catalyst) and silicone surfactant Dabco DC5230. High purity and stability are necessary to prevent acid formation and catalyst deactivation, which can yellow the foam.

MC Foaming System Formulations

Relationship Between Water and Liquid CO₂ Usage for Different Foam Densities

Explanation:

Foaming capacity can be represented by the Foaming Index (water or equivalent per 100 parts polyether):

IF = m(water) + m(F-11)/10 + m(MC)/9 (per 100 parts polyether)

Water reacts with isocyanates to form urea linkages, releasing CO₂ and heat.

High water usage lowers density and increases hardness but weakens foam cell structure, reduces load-bearing, and may cause collapse or cracking. More TDI is needed, increasing heat and risk of scorching. If water exceeds 5.0 parts, physical agents must be added to absorb excess heat. Less water reduces catalyst need but increases density.

Foam Stabilizers (Silicone Oil)

Foam stabilizers are surfactants that disperse polyurea evenly in the foam system, acting as physical cross-linking points, improving early-stage viscosity, and preventing foam collapse.

They emulsify components, reduce surface tension, ease nucleation, and control pore size and uniformity, improving foam stability and elasticity, and preventing shrinkage or breakage.

Higher blowing agent and POP levels require more stabilizer (1–2%). When water exceeds 3.5 parts, stabilizer use increases accordingly (40% of water content). For every 5 parts of blowing agent added, increase stabilizer by 0.2–0.5 parts.

Excess stabilizer: More elastic foam walls, fine cells, but risk of closed cells.

Insufficient stabilizer: Foam rupture, collapse, and coarse, merged cells.

Typical Products:

L580: Hydrolysis-resistant, Compton USA

B4900: For medium density with excellent air permeability, Evonik Degussa (China)

Explanation:

Purpose: Adjust cell uniformity.

Models: Dabco DC5986 (high-activity fire retardant), 5950 (low-activity).

Low-activity polyethers (e.g., propylene oxide types) need high-activity stabilizers; copolymers with ethylene oxide require medium-activity stabilizers.

In special high-density foams (40–50 kg/m³), low-activity stabilizers are preferred.

Fillers

Light calcium carbonate, silica-lime: Fine, spindle-like particles, inert, dispersible in the polymer network.

They reduce chain mobility, increasing compression load. But due to lack of chemical bonding, they weaken mechanical properties and cause cracking, sinking, or collapse during foaming. Add 10ml stabilizer per 1kg of filler to mitigate.

Typical usage: 20–30% of PPG.

Pros: Increase compressive strength and density.

Cons: Reduce tensile and tear strength, shorten service life.

Flame Retardants

Phosphorus compounds promote char formation to block oxidation. Halogen compounds suppress combustion by diluting flammable gases.

The first part is about TDI, please click the following link to read: https://www.sabtechmachine.com/what-are-the-raw-materials-of-flexible-polyurethane-foam-part-1.html

The second part is about Catalysts, please click the following link to read: https://www.sabtechmachine.com/what-are-the-raw-materials-of-flexible-polyurethane-foam-part-2.html