1. Introduction

Polyurethane (PU) colorant is a pigment dispersion system in which organic and inorganic pigments are wetted, de-agglomerated and stabilized in a polyether or polyester polyol carrier before being metered into two-component PU systems. It is widely used in cast elastomers, TPU processing, rollers, seals, mining screens and automotive interior parts. In industrial practice, the quality of a PU colorant is not judged by pigment loading alone, but by whether pigment particles remain below the micron threshold, stay dispersed during shelf life, and do not disrupt the urethane reaction. This article, prepared by DENSON, explains the dispersion mechanism, three stabilization points, three application cases and the key selection parameters, referencing the dispersion principles reported in ACS Applied Materials & Interfaces and RSC Advances on pigment surface treatment and colloidal stability.

For more product and literature information, please visit the literature list on the DENSON website.

Figure 1. Multi-color TPU masterbatch pellets used as PU colorant carrier reference.

2. Technical Features and Mechanism

A PU colorant is defined as a pre-dispersed pigment concentrate in a reactive polyol carrier, designed to be injected into PU formulations without changing stoichiometry. Its dispersion follows three stages: wetting, de-agglomeration and stabilization. In the wetting stage, the polyol carrier and dispersant replace air on the pigment surface; in the de-agglomeration stage, bead milling breaks aggregates below 5 µm; in the stabilization stage, adsorbed dispersant chains provide steric hindrance (and, in polar polyols, weak electrostatic repulsion) so particles cannot re-flocculate.

Because the polyol carrier bears hydroxyl groups, it participates in the subsequent isocyanate reaction, which distinguishes a PU colorant from a generic plastic masterbatch. Poor dispersion produces specks, color shift, weakening and blooming; good dispersion delivers Delta-E stability, abrasion resistance and consistent batch-to-batch color.

3. Application Case Study 1: Cast PU Rollers

A Chinese roller manufacturer switched from generic color paste to DENSON PU colorant for green conveyor rollers. The previous product showed specks after 72 h of shelf storage and a Delta-E drift of > 3.0 between batches. After switching, fineness was controlled at ≤ 5 µm, and the measured Delta-E between three consecutive batches was ≤ 1.2 (GB/T 11186.2). The rollers passed 100 000 cycles of shore A hardness check with no blooming.

Figure 2. Finished green PU-coated rollers on a wooden pallet.

4. Application Case Study 2: PU Cast Molds

For a precision PU casting job (blue elastomer parts poured into metal molds), the customer reported surface bubbles and poor wetting when using a low-quality colorant. The issue traced back to residual water and high polarity in the carrier. A DENSON PU colorant with moisture ≤ 0.1% (Karl Fischer) and controlled viscosity 2000–4000 mPa·s (GB/T 2794) eliminated the bubbles, and the cast parts showed tensile strength ≥ 25 MPa and tear strength ≥ 50 kN/m.

Figure 3. Casting blue PU liquid into a metal mold in production.

5. Application Case Study 3: TPU Granulate Coloring

A TPU extruder compounded engineering-grade TPU granules with DENSON colorant in a 1–3% loading. The granules were pelletized and then injection-molded into watch bands and phone cases. Test data showed ≥ 95% retention of base TPU tensile strength after coloring, and QUV 1000 h color change ΔE ≤ 2.0 (ISO 4892-3), meeting outdoor product requirements.

6. Key Selection Parameters and Usage Recommendations

When selecting a PU colorant, buyers should compare carrier compatibility, fineness, moisture, viscosity and heat stability rather than price alone. The table below summarizes the typical window for a cast-PU elastomer colorant.

Usage recommendations: pre-mix the colorant with the polyol side for 3–5 min before adding isocyanate; do not overheat above 60 °C; keep containers sealed to avoid moisture pick-up. For more technical data, refer to the DENSON product pages.

7. Conclusion

A high-quality PU colorant is engineered around three stabilization points: wetting, de-agglomeration and steric stabilization in a reactive polyol carrier. Buyers should specify fineness ≤ 5 µm, moisture ≤ 0.1% and verified QUV performance rather than relying on price alone; DENSON PU colorants meet these criteria for cast elastomers, TPU and roller applications.

8. FAQ

Q1: Why do PU cast parts show specks after adding colorant?
A: Specks usually come from under-milled pigments (> 10 µm) or poor dispersant anchoring. Use a colorant with fineness ≤ 5 µm per GB/T 6753.1 and check the dispersant compatibility with your polyol.

Q2: How much PU colorant is typically added?
A: For solid colors, 1–3% by weight on the polyol side is typical; pastels and deep shades may go up to 5%. Always pre-blend with polyol before adding isocyanate.

Q3: Why do bubbles appear after casting?
A: Bubbles are most often caused by moisture in the colorant reacting with NCO. Keep moisture ≤ 0.1% and store sealed; pre-dry the polyol if the shop environment is humid.

Q4: Can the same colorant be used for TPU granulation and cast PU?
A: They are related but not identical. TPU coloring needs heat stability at 180–220 °C, while cast PU needs low viscosity and low moisture. Choose the grade matched to your process.

Q5: How should color batch-to-batch variation be controlled?
A: Use a spectrophotometer under D65 light, target Delta-E ≤ 1.0 between production and master, and keep the milling and metering parameters fixed. Report measured values per GB/T 11186.2.