Polyurethane (PU Foam & Elastomer)

Polyurethane is coloured through the polyol side, so the paste has to be fully compatible with polyether polyol and must not disturb the balance of catalysts, blowing agents and surfactants. Colour has to survive a fast, exothermic reaction, and the finished foam or elastomer must not release plasticiser or phthalate into the end product. Because foaming is a reactive process rather than a simple drying process, any impurity in the paste is carried into the cell structure and cannot be corrected afterwards.

Polyurethane foam blocks and moulded elastomer parts in a production area

Key Requirements in This Application

Phthalate-free carrier

Consumer articles such as shoe soles, synthetic leather and furniture foam are increasingly restricted on phthalates. A carrier based on polyether polyol rather than a phthalate plasticiser avoids adding a restricted substance to the formulation.

Compatibility with the polyol component

The paste is metered into the polyol and may be stored there before foaming. It must not separate, sediment or cream, and it must remain pumpable at the metering temperature used on the line.

No interference with the reaction

Free acids, amines or water in the paste can change cream time, rise time and tack-free time. The paste must be neutral and low in water so that the existing catalyst package stays valid.

Colour fastness in foam and elastomer

Flexible foam has a very high surface area, and elastomers are often exposed to light and abrasion. Pigments must be chosen so that shade holds through the exotherm and does not fade or migrate in service.

Migration and staining

Colour must not transfer to adjacent materials, and must not stain moulds or tooling. Migration is particularly visible on light-coloured synthetic leather and on white shoe soles.

Cell structure in flexible foam

In flexible foam the pigment particles sit in the struts and windows of the cell structure. Poorly dispersed or coarse pigment can nucleate bubbles unevenly, rupture cells or create dense spots, which shows as a mottled surface and changes the feel of the foam even when the colour reads correctly.

Uniformity in thick mouldings

In a thick moulding the reaction front travels through the part, and pigment that is not fully wetted can be pushed ahead of the front and concentrate in the last region to fill. The paste must disperse evenly enough that colour is uniform through the section rather than streaked or darker at the injection point.

Integral-skin and self-skinning foam

In integral-skin foam the outer surface forms a dense skin while the core stays cellular, and the two regions develop colour differently. The paste must be fine and well wetted so that skin and core read as the same shade, and it must not concentrate at the mould surface, where a darker skin would show as a halo around edges and radii. This is most visible on light and mid-tone colours used for steering wheels, armrests and footwear components.

Recommended DENSON Series

PUE Series

Polyether polyol colour pastes formulated for PU systems and free of phthalate plasticisers, so they meter directly into the polyol component without adding a restricted substance. Because the carrier is itself a polyether polyol, it stays in the reaction system instead of behaving as a foreign additive.

View series

Technical Considerations

Add the paste to the polyol and pre-mix before the catalyst and blowing agent, so the pigment is wetted before the reaction starts.
Check water content: added water consumes isocyanate and shifts foam density and hardness even when the colour is correct.
Confirm storage stability in the polyol blend, because sedimentation in a day tank shows up as a shade drift across a shift.
For moulded parts, verify that the paste does not stain the mould or build up release residue on the tooling surface.
Re-check cream and rise times after changing paste grade, since carrier polarity affects surfactant performance.
Confirm metering accuracy at the lowest addition level used in production, because a small pump error becomes a visible colour error in light shades.

Testing and Compliance

Reaction profile

Cream time, rise time and tack-free time are measured on the production formulation with and without the paste, to confirm that the catalyst balance is unchanged.

Colour and migration

Colour is checked on cured foam or elastomer, and migration is assessed by contact transfer onto white substrate under heat and humidity, which exposes plasticiser or pigment bleed.

Paste stability

Viscosity by GB/T 9751, fineness by GB/T 6753.1 and accelerated storage are used to confirm that the paste stays uniform and pumpable in the polyol blend.

Foam physical properties

Density, compression set and cell structure are compared with and without the paste, because a colourant that changes cell opening will change the feel and durability of the foam even if the shade is correct.

Discuss Your Application

Tell us the resin system, process conditions and performance target, and we will confirm the suitable series and starting dosage.

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