PUE Polyurethane Color Paste: Pigment Dispersion in Polyether Polyol, TPU and Green Foaming Applications

Keywords: PUE polyurethane color paste · polyether polyol carrier · NCO/OH cure · TPU elastomer · flexible & rigid foam · green blowing · REACH/RoHS/CBAM

1. Introduction

Polyurethane (PU) is one of the most versatile polymer chemistries in modern manufacturing: flexible foam for furniture and bedding, rigid foam for insulation, thermoplastic polyurethane (TPU) for films and cables, cast polyurethane (CPU) for rollers and wheels, and RIM parts for automotive interiors. Each process demands a colorant that disperses cleanly in the polyol stream, does not disturb NCO/OH stoichiometry, survives the foaming or casting exotherm, and carries the same compliance burden as the finished part when it ships to Europe, North America or Japan.

DENSON's PUE polyurethane color paste is engineered specifically for this chemistry. Unlike solvent-borne pastes (FLTY series), water-borne pastes (GT series), or UV-curable pastes (FLUV series), PUE uses a polyether polyol itself as the carrier. The pigment is pre-dispersed into a polyol that is chemically compatible with the customer's own polyol blend, so adding PUE is essentially dosing a little extra polyol with colour in it — no foreign resin, no solvent, no water, no photoinitiator. This article covers the carrier chemistry, dispersion mechanism, three application windows (foam, TPU, CPU/emerging markets), selection parameters, and the global compliance package (REACH, RoHS, EU 2004/42/EC, GHS/CLP, CBAM, low-halogen IEC 61249, phthalate-free, heavy-metal-free) that DENSON supplies with every PUE drum.

Figure 1. Continuous slabstock flexible foam line. PUE color paste is injected into the polyol blend head alongside the isocyanate and cures in situ via the NCO/OH reaction.

2. PUE Product Features and the Polyether Polyol Carrier

2.1 Why polyether polyol is the correct carrier

A polyurethane system is fundamentally a two-component reaction: an isocyanate (MDI, TDI, pMDI, or aliphatic HDI) reacts with a long-chain polyol to build the soft segment, and with a short-chain chain-extender (diol or diamine) to build the hard segment. If you add a pigment carrier that is anything other than a polyol, you introduce a foreign molecule into the crosslinking reaction. A solvent-borne paste adds VOCs and can plasticise the foam; a water-borne paste adds unwanted water that reacts with isocyanate to release extra CO₂ and ruins the calculated index; an epoxy-based paste (SEP series) simply does not cure in a PU matrix at all.

PUE avoids this by dispersing pigment directly into a polyether polyol that is chemically compatible with the customer's own polyol blend. The hydroxyl groups on the carrier polyol take part in the NCO/OH reaction just like any other polyol molecule, becoming a real part of the polyurethane network rather than an inert contaminant. This is why PUE is described as a polyol-borne, NCO-reactive colorant — it is neither solvent-free in the UV sense nor water-borne; it is a reactive concentrate.

2.2 Two carrier grades, two particle-size targets

DENSON formulates PUE in two carrier grades matched to shade and dispersion difficulty:

The black grade uses a high-OH difunctional polyether because carbon black is the hardest pigment to disperse: its primary aggregates (10–100 nm) cluster into agglomerates that require high shear and strong wetting. The high-OH carrier provides both viscosity for bead-milling and enough reactive sites to be incorporated into the soft segment of flexible foam and TPU. The color grade uses a trifunctional polyether at OH value 35 — the industry standard for flexible slabstock and moulded foam — because organic red/yellow/blue pigments disperse more readily and the triol contributes to crosslink density in rigid and semi-flexible systems.

2.3 High pigment loading, heat resistance, dispersibility

PUE pastes are supplied at high pigment loading (typical carbon black 25–35% in the black grade; organic pigments 20–30% in the color grades) so that typical addition rates run at 0.5–3.0 pph on polyol — a small metered shot that does not perturb the NCO/OH index by more than ±2%. The pigments selected are heat-stabilised grades designed to survive the exotherm of free-rise slabstock foam (core temperatures up to 150–170 °C), the demoulding bake of moulded foam (180–220 °C oven post-cure), and TPU extrusion at 180–220 °C barrel temperatures without bleeding, crocking, or shade shift.

Figure 2. Stainless-steel polyol reactors. PUE pigment is pre-dispersed into polyether polyol in these vessels, then bead-milled to the target particle size before being drummed.

3. Technical Mechanism: Wetting, Dispersion and NCO/OH Cure

3.1 How hydroxyl value, functionality and molecular weight control wetting

Pigment dispersion is a three-step process: wetting of the pigment surface by the carrier, mechanical breaking of agglomerates under high shear (bead mill or high-speed disperser), and stabilisation of the newly exposed surface so particles do not re-flocculate. In a polyether polyol carrier, wetting is governed by three molecular parameters:

• Hydroxyl value (mg KOH/g). Higher OH value means shorter polyol chains and more OH end-groups per gram. This raises polarity and improves wetting of polar pigment surfaces (carbon black, treated TiO₂, organic reds), but it also raises the cured crosslink density. For carbon black, the high-OH difunctional carrier gives the best wetting and the tightest particle-size distribution below 800 nm.

• Functionality (f). A difunctional (f = 2) polyol builds linear soft segments — ideal for flexible foam and TPU where elasticity matters. A trifunctional (f = 3) polyol introduces branching and crosslink points — required in rigid insulation foam and integral-skin moulded parts where dimensional stability matters. PUE color grades are therefore triol-based so they drop into rigid and semi-flexible formulations without changing the crosslink profile.

• Molecular weight (Mn). A lower-Mn polyether (e.g. 1000 Da) has lower viscosity and easier milling, but contributes to hard segments; a higher-Mn polyether (3000–5000 Da) gives softer, more elastic cured parts but higher paste viscosity that can limit pigment loading. DENSON balances Mn against pigment loading so that the PUE paste pumps through standard gear pumps at 20–40 °C without heating.

3.2 Particle size: why 800 nm black and 1 μm color

Carbon black is a light-absorbing pigment: its tinting strength scales strongly with the degree of dispersion, because light only interacts with particles below roughly one wavelength of visible light (~400–700 nm). Milling the black grade below 800 nm ensures a deep, neutral jet rather than a brownish or washed-out grey. Organic color pigments are opacifying and light-scattering rather than absorbing; at ~1 μm they give vibrant, consistent hue without the extra milling energy that would over-shear the polyether carrier and shorten batch life.

3.3 NCO/OH curing compatibility

At the PU machine head, PUE is dosed into the polyol side (the "B-side") alongside the base polyol, catalyst (amine, tin), silicone surfactant, blowing agent and chain extender. When the mixed stream hits the isocyanate ("A-side", MDI/TDI), every hydroxyl group — whether on the base polyol, the chain extender, or the PUE carrier — reacts with an NCO group to form a urethane bond. The pigment particles are simply embedded in the rising crosslinked network.

Because the PUE carrier is itself a polyol, formulators do not need to correct the NCO/OH index for an "inert" contaminant. DENSON supplies each PUE batch with a measured hydroxyl value and moisture content; the customer's process engineer enters these into the index calculator so the stoichiometry stays at the design value (typically index 1.02–1.10 for flexible foam, 1.05–1.15 for rigid foam, 1.03–1.06 for TPU and CPU). Moisture in the paste is held below 0.1% so that no uncontrolled CO₂ is generated by the NCO + H₂O side reaction.

4. Application Case 1 — PU Flexible and Rigid Foam: Green Blowing Transition

The largest outlet for PUE color paste is slabstock and moulded flexible foam (furniture, bedding, automotive seats) and rigid insulation foam (appliance panels, construction sandwich panels, spray foam). Historically, continuous slabstock used CFC-11 and then HCFC-141b as the physical blowing agent; both are now phased down under the Montreal Protocol and its Kigali Amendment. Modern lines run on water-blown CO₂ systems, cyclopentane, HFOs (e.g. Solstice LBA) or methyl formate. PUE pastes are formulated to be compatible with all of these blowing packages.

In a water-blown flexible formulation, water (typically 2–5 pph on polyol) reacts with isocyanate to generate CO₂ in situ. The PUE paste's own moisture must therefore stay tight — any extra water in the colour shift will overblow the bun, raise porosity, and destabilise the rise. DENSON ships PUE in sealed drums with a <0.1% moisture spec and a CoA; customers on cyclopentane lines also receive a static-discharge declaration because cyclopentane is highly flammable (LEL 1.1–8.7% v/v).

For rigid appliance foam (refrigerator insulation), the trend is cyclopentane or HFO blowing with pMDI. PUE color grades here are usually black (hidden cabinet fill) or neutral grey; the triol carrier at OHV 35 matches the 450–5000 Da sucrose/glycerol-initiated rigid polyols used in the industry, so cure profile and demould time are unaffected. The 1 μm particle size prevents nozzle wear in the high-pressure mixing head.

Figure 3. TPU sheet extrusion line. PUE color paste is tumble-blended with TPU pellets before extrusion, or metered directly into the feed throat as a polyol-borne concentrate.

5. Application Case 2 — TPU Elastomer: Films, Cables and Footwear

Thermoplastic polyurethane (TPU) is a linear, segmented polyurethane polymer processed like a thermoplastic — extruded into film, blown into cable jackets, injection-moulded into footwear soles, or calendered into sheets. Unlike reactive foam or CPU casting, TPU is melted and shaped rather than chemically cured on the machine. PUE enters TPU production as a polyol-borne color concentrate pre-mixed into the TPU's soft-segment polyol before polymerisation, or as a pellet let-down where the paste is absorbed into TPU pellets and tumble-blended before extrusion.

Films for protective apparel, inflatables and medical tubing are typically polyester- or polyether-based TPU extruded at 180–210 °C. PUE black at <800 nm gives a clean, deep film without black specks; color grades at 1 μm give uniform hue across 50–200 μm gauge films. The high-OH difunctional carrier in the black grade matches the polyether diol (PTMEG or PPG, Mn 1000–2000) used in hydrolysis-resistant TPU grades, so the paste does not bloom or exude on storage.

Cable jacketing is the fastest-growing TPU outlet. EV high-voltage charging cables (AC Type 2 and DC CCS) use polyether-TPU jackets for hydrolysis resistance, low-temperature flexibility and abrasion resistance. PUE black is the workhorse; red and orange grades are used for colour-coded inner conductors and pilot contacts. The paste must survive the 190–210 °C extrusion barrel and any radiation or moisture cross-linking step without discolouring.

Figure 4. TPU-jacketed EV charging cable. PUE black and red grades provide the colour for outer jackets and inner conductor insulation in high-voltage DC charging leads.

Footwear (phylon, dense-sole and injection-moulded TPU) uses PUE color grades for a broad palette of fashionable shades. Because TPU shoe soles must pass flexing tests (>100,000 Bata-belt cycles without cracking), the matched-Mn difunctional carrier must not embrittle the soft segment — another reason DENSON uses a polyether-matched carrier rather than a generic solvent-borne pigment.

6. Application Case 3 — CPU Cast Elastomers, New Energy and Robotics

6.1 CPU cast elastomers

Cast polyurethane (CPU) is produced by pouring a reactive mix of prepolymer (TDI or MDI terminated) and curative (MOCA, 1,4-butanediol, or diamine) into a mould, where it cures and then post-cures in an oven at 100–120 °C for 8–24 h. CPU rollers, wheels, screen-printing squeegees, and mining screen panels are typical parts. PUE is dosed into the curative side (which is usually a polyol or chain-extender) at 0.5–2 pph. The 1 μm particle size is fine enough not to abrade the precision-machined mould surfaces, and the heat-resistant pigments survive the long post-cure without shade shift.

Figure 5. Three-component CPU casting machine. PUE color paste is metered into the curative/prepolymer stream at the mixhead and cures inside the mould via NCO/OH reaction.

6.2 New energy: battery components and charging infrastructure

Inside EV battery packs, TPU and cast PU are used for cell-gap potting, busbar covers, module compression pads, and high-voltage connector seals. These parts must meet UL 94 flammability, low smoke, and low-halogen expectations. DENSON PUE halogen-free grades use phosphorus-based flame-retardant synergists and halogen-free pigments (no chlorinated blue/green phthalocyanines in the halogen-sensitive variants) to meet IEC 61249-2-21 (Cl < 900 ppm, Br < 900 ppm, total halogens < 1500 ppm). Charging-cable TPU jackets (Figure 4) are already discussed above; PUE also colours the PU overmoulds on DC charging connectors.

6.3 Robotics: PU skin and flexible joints

Collaborative robots (cobots), prosthetic limbs and humanoid robots use soft PU elastomer "skin" over rigid internal frames to absorb impact and improve grip. These skins are often cast or dip-moulded in grey, black, or skin-tone colours and must pass skin-sensitiser and biocompatibility screening for service-contact applications. PUE supplies neutral grey and matte black shades for these parts, formulated without phthalate plasticisers and without heavy-metal pigments (lead chromates, cadmium pigments, hexavalent chromium) that would fail RoHS and REACH SVHC screening.

Figure 6. Cobot end-effector with matte black PU skin. PUE black grade gives a uniform, non-blooming colour on cast PU elastomer joints.

7. Engineering Selection Parameters and Global Compliance Crosswalk

7.1 International compliance crosswalk

Every drum of DENSON PUE ships with a Certificate of Analysis (pigment loading, fineness, OH value, moisture), a SDS compliant with EC 1907/2006 Annex II as amended by (EU) 2020/878, and a declaration of conformance covering REACH SVHC, RoHS, phthalate-free, heavy-metal-free and IEC 61249 low-halogen status. For customers shipping into the EU under CBAM, DENSON provides a product-level carbon-footprint statement on request. Explore the full range on the DENSON homepage or browse the product listing.

8. FAQ

Q1. Is PUE a solvent-free or water-borne color paste?
Neither. PUE is polyether-polyol-borne: the carrier is a reactive polyol that becomes part of the polyurethane network during NCO/OH cure. It contains no solvent (unlike DENSON FLTY series) and no water (unlike GT series), and it is not UV-curable (FLUV) or epoxy-based (SEP). This is why it must not be substituted into coatings, inks or epoxy systems that do not contain isocyanate.

Q2. Why is black milled below 800 nm while color pigments only go to ~1 μm?
Carbon black is a light-absorbing pigment whose tinting strength depends on aggregate size below visible wavelengths; under 800 nm it gives a deep, neutral jet. Organic red/blue/yellow pigments are light-scattering rather than absorbing, and ~1 μm already gives full vibrancy. Milling finer would waste energy, over-shear the polyol carrier, and shorten the paste's shelf life without improving colour.

Q3. Does adding PUE change my NCO/OH index?
Slightly, but predictably. Because the carrier polyol is itself hydroxyl-functional, every batch is shipped with a measured OH value and moisture content. Enter those numbers into your index calculator; the drift is typically under ±2% at 0.5–3 pph addition. Moisture is held below 0.1% so the NCO + H₂O side reaction does not generate unexpected extra CO₂.

Q4. Can PUE run on cyclopentane or water-blown foam lines that have phased out HCFC-141b?
Yes. PUE contains no CFC/HCFC-compatible solvent and is fully compatible with water-blown, cyclopentane, HFO (Solstice LBA) and methyl-formate blowing packages. For cyclopentane lines, DENSON also supplies a static-discharge declaration because cyclopentane is highly flammable and the paste must not introduce additional static sources.

Q5. Is PUE suitable for RoHS and REACH-sensitive applications like EV charging cables and battery components?
Yes. Standard PUE already meets RoHS (2011/65/EU) and REACH SVHC (<0.1%), is phthalate-free and heavy-metal-free. For electronics and EV battery parts, specify the halogen-free grade, which meets IEC 61249-2-21 (Cl < 900 ppm, Br < 900 ppm). DENSON supplies third-party test reports (SGS/Intertek) on request for each shade.

Key Conclusions

1. Polyether polyol is the carrier — not a contaminant. PUE disperses pigment into a reactive polyol (high-OH difunctional for black, OHV-35 trifunctional for color) that cures in place with the isocyanate. There is no solvent, no water, no foreign resin, and no index surprise beyond the measured OH value printed on each CoA.

2. Particle size is tuned to pigment physics. Black is milled below 800 nm for deep, neutral jet; color pigments sit at ~1 μm for vibrant hue without over-shear. High pigment loading (25–35% black, 20–30% color) keeps addition at 0.5–3 pph and survives foam exotherm, TPU extrusion at 180–220 °C and CPU post-cure.

3. One compliance package for global PU markets. A properly specified PUE drum ships with REACH SVHC/Annex XVII declaration, RoHS 2011/65/EU compliance, EU 2004/42/EC VOC attestation (essentially zero), GHS/CLP SDS, CBAM carbon statement on request, IEC 61249 low-halogen grades, phthalate-free and heavy-metal-free documentation — covering flexible/rigid foam on green blowing, TPU films and EV cables, CPU cast parts, and robotics PU skin.

About Dongguan DENSON Functional Materials Co., Ltd. — DENSON formulates and manufactures polyol-borne (PUE), solvent-borne (FLTY), water-borne (GT), solvent-free (RFNC), UV-curable (FLUV), epoxy-based (SEP) and electron-beam-cured color pastes for polyurethane, coatings, inks and composite customers worldwide. For technical data sheets, samples and compliance declarations, visit www.densonchem.com or browse the product range.