Pigment Selection for Plastics
How to select the right pigment for plastic applications: organic vs inorganic, heat stability, lightfastness, and regulatory compliance.
The Foundation of Plastic Coloring
Pigment selection is the most critical decision in masterbatch formulation. The pigment determines not only the color but also the heat stability, light fastness, migration resistance, regulatory compliance, and cost of the final product. A wrong pigment choice can lead to color shift during processing, fading in use, regulatory non-compliance, or excessive cost. This guide provides a comprehensive framework for selecting pigments for plastic applications.
Organic vs Inorganic Pigments
| Property | Inorganic Pigments | Organic Pigments |
|---|---|---|
| Chemical nature | Metal oxides, sulfides, complexes | Carbon-based molecules |
| Particle size | 0.2–2.0 μm | 0.01–0.5 μm |
| Heat stability | 300–1000°C | 150–350°C |
| Light fastness | Grade 7–8 | Grade 3–8 (varies) |
| Tinting strength | Medium | High |
| Opacity | High (opaque) | Low (transparent to translucent) |
| Migration resistance | Excellent | Good–Fair (depends on grade) |
| Cost | $1–15/kg | $5–60/kg |
| Color range | Limited (earth tones, white, black) | Wide (bright, saturated colors) |
Key Pigment Selection Criteria
1. Heat Stability
Pigments must survive the processing temperature of the base polymer without decomposition, color shift, or gas evolution:
| Polymer | Processing Temp | Required Pigment Heat Stability | Suitable Pigments |
|---|---|---|---|
| PE (LDPE/HDPE) | 160–220°C | 240°C+ | All inorganic; most organic |
| PP | 180–240°C | 260°C+ | All inorganic; high-stability organic |
| PS | 180–240°C | 260°C+ | All inorganic; high-stability organic |
| ABS | 200–260°C | 280°C+ | All inorganic; selected organic |
| PA (Nylon) | 240–300°C | 320°C+ | Most inorganic; few organic |
| PET | 260–290°C | 310°C+ | Selected inorganic only |
| PC | 280–330°C | 350°C+ | Specialty inorganic only |
2. Light Fastness
Light fastness is measured on a 1–8 Blue Wool Scale (8 = best):
| Grade | Performance | Application |
|---|---|---|
| 1–2 | Poor (fades in weeks) | Not recommended for any product |
| 3–4 | Fair (fades in months) | Short-life indoor products |
| 5–6 | Good (1–2 years) | Indoor consumer goods |
| 7–8 | Excellent (5+ years) | Outdoor, automotive, long-life |
3. Migration Resistance
Migration occurs when pigment molecules move within the polymer matrix or transfer to contacting surfaces. This is especially important for:
- Food contact packaging (pigment must not migrate into food)
- Stacking or nesting (pigment must not transfer between parts)
- Toys and children's products (EN 71-3 migration limits)
- Medical devices (USP Class VI biocompatibility)
Recommended Pigments by Color
White
- Titanium dioxide (rutile): The only choice. Refractive index 2.73, excellent opacity and brightness. Available in coated (alumina-silica, zirconia) and uncoated grades.
Black
- Carbon black (N330): Standard plastic coloring grade, particle size 28–36 nm, excellent UV resistance.
- Iron oxide black (Fe3O4): NIR-detectable alternative for recycling applications.
Blue
- Phthalocyanine blue: High tinting strength, excellent light fastness (Grade 7–8), heat stable to 350°C. Most common blue pigment for plastics.
- Ultramarine blue: Inorganic, excellent light fastness but lower tinting strength and limited acid resistance.
- Cobalt blue (CoAl2O4): Inorganic, excellent heat stability (500°C+), used for high-temperature polymers.
Green
- Phthalocyanine green: High tinting strength, excellent light fastness, heat stable to 350°C. The standard green for plastics.
- Chromium oxide green (Cr2O3): Inorganic, excellent heat stability (500°C+), lower tinting strength but superior weather resistance.
Red
- Quinacridone red: High-performance organic, excellent light fastness (Grade 7–8), heat stable to 300°C. The premium red for plastics.
- Iron oxide red (Fe2O3): Inorganic, excellent stability, lower tinting strength, economical. Good for earth-tone and rust colors.
- Ceramic red (complex inorganic): Heat stable to 400°C+, excellent weather resistance for engineering plastics.
Yellow
- Nickel titanate yellow: Inorganic, excellent heat stability (500°C+), good for high-temperature polymers. Limited tinting strength.
- Bismuth vanadate: Bright, high-tinting, heat stable to 300°C. Excellent for vivid yellow.
- Irgazin yellow (DPP): High-performance organic, excellent light fastness, heat stable to 280°C.
Regulatory Compliance Check
| Regulation | Scope | Key Pigments Approved |
|---|---|---|
| FDA 21 CFR 178.3297 | US food contact | TiO2, carbon black, iron oxides, ultramarine, phthalocyanine (selected) |
| EU 10/2011 | EU food contact | TiO2, carbon black, iron oxides (positive list) |
| EN 71-3 | Toys (migration) | Limited metal content (Cr, Pb, Cd, Hg, As) |
| RoHS | Electronic products | No lead, cadmium, mercury, hexavalent chromium |
| REACH SVHC | EU chemicals | Screening for substances of very high concern |
Conclusion
Selecting the right pigment for plastic coloring requires balancing color target, heat stability, light fastness, migration resistance, regulatory compliance, and cost. Inorganic pigments offer superior stability and economy for standard applications, while organic pigments provide brighter, more saturated colors for premium products. Always verify pigment compatibility with your processing temperature, end-use requirements, and applicable regulations before final selection.
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