Plastic Pigments FAQ
Pigment selection answers for masterbatch producers and plastics compounders
Frequently Asked Questions
Which red pigment survives 300°C in polycarbonate?
PR254 (DPP, CI 56110) is the go-to red for PC processing at 300°C. Honor grades HP RED 2135 and HP RED 23216 are rated to 300°C with minimal ΔE shift (<1.0 after 10 min at 300°C). Also suitable: PR264 (DPP variant, slightly yellower), PR149 (perylene red, limited to 300°C), and PR177 (anthraquinone red, 300°C). Unsuitable: all azo pigments (decompose above 220-260°C), lake pigments (PR57:1 degrades at 180°C), and PR170 (naphthol, limited to 240°C).
How to test for pigment migration in PE film?
Standard sandwich test: press pigmented PE film between two white PE layers at 80°C under 5 kPa pressure for 24 hours (ISO 18314-4 method). After test, examine white PE for any color transfer — even faint transfer = migration risk ≥ grade 3. For food packaging, conduct additional tests: 10 days at 40°C (simulated shelf life), specific migration testing per EU 10/2011 (total migration <10 mg/dm²). Migration grade 4-5 required for food contact, grade 3-4 acceptable for general-purpose PE film.
Why do some pigments cause warpage in HDPE injection molding?
Organic pigments, especially phthalocyanines (PB15:3, PG7), can act as heterogeneous nucleating agents in semi-crystalline polymers like HDPE and PP. This alters crystallization kinetics — faster nucleation and different crystal morphology cause anisotropic shrinkage, resulting in part warpage. Solutions: use nucleating masterbatch to control and uniformize crystallization, reduce pigment loading, switch to less nucleating pigments (inorganic pigments like PR101, PY42 cause minimal nucleation), or adjust mold cooling to compensate.
What pigments are safe for food-contact polypropylene?
Pigments must be listed in positive lists (FDA 21 CFR §178.3297, EU 10/2011, or national equivalents) and meet specific migration limits (SML). Generally accepted: synthetic iron oxides (PR101, PY42, PBk11 — purity >97%), TiO2 (PW6, rutile grade), carbon black (PBk7, channel or furnace process with low PAH <0.5 ppm benzo[a]pyrene), select phthalocyanines (PB15:3, PG7 — must comply with copper migration <5 mg/kg food and polychlorinated biphenyl <25 ppm), ultramarine blue (PB29). NOT acceptable: most azo pigments (potential aromatic amine release), cadmium/lead/chromium pigments, PR57:1/PR48:2 (barium/calcium lakes). Always verify with the specific grade’s FDA letter or EU declaration of compliance.
How to prevent pigment plate-out during PVC extrusion?
Plate-out is pigment depositing on die lips, calibrators, or cooling rolls. Root cause: pigment incompatibility with PVC formulation — pigment migrates to surface during processing and deposits on equipment. Prevention: use migration-resistant pigments (grade 4-5): PR254 (DPP), PV19 (quinacridone), PB15:3 (phthalocyanine), PG7 (phthalocyanine), PY150 (benzimidazolone). Avoid migration-prone pigments (grade 1-2): PR57:1 (Lithol Rubine), PR48:2, PY12. Reduce processing temperature (PVC degrades above 200°C, releasing HCl that accelerates migration). Add process aids (acrylic copolymer at 0.5-1.5 phr) to reduce die friction.
What’s the difference between pigment masterbatch and liquid color?
Masterbatch: pigment pre-dispersed in a carrier resin (usually PE, PP, or EVA) at 20-50% loading, supplied as pellets. Advantages: lower cost per colored kg, excellent shelf life (2+ years), no liquid handling equipment, broader pigment compatibility (higher processing temperatures). Liquid color: pigment dispersed in a liquid carrier (plasticizer, surfactant, or oil) at 30-70% loading. Advantages: superior dispersion (particles already de-agglomerated), faster color change (purge in 5-10 min vs 30+ min), more accurate dosing for small shot weights, better for transparent/translucent colors. Tradeoff: liquid systems require specialized dosing pumps and have shelf life limitations (6-12 months before settling).
Which white pigment is best for plastic — TiO2 or zinc sulfide?
TiO2 rutile (PW6) is the standard: refractive index 2.73 vs polymer ~1.5 = highest opacity per gram, excellent whiteness (L* >97), UV opacity. Zinc sulfide (ZnS, PW7): RI 2.37 = lower opacity but significantly softer (Mohs hardness 3 vs TiO2 6-7), making it preferred for glass-fiber reinforced plastics where TiO2 would abrade glass fibers during compounding (reducing mechanical properties). ZnS also causes less die wear. Cost: TiO2 $2-4/kg, ZnS $5-8/kg. For most plastics, rutile TiO2 at 2-5% loading is optimal. Use ZnS only when fiber abrasion or die wear is a concern.
How to avoid pigment-related die build-up in blown film?
Die build-up (die lip deposit) is caused by low-MW organic residues in the pigment (synthesis byproducts, residual solvents, dispersants) that volatilize and condense on the die. Prevention: select pigments with low volatile content (<0.5% weight loss at processing temperature by TGA), use heat-stable pigments (no thermal decomposition), ensure complete dispersion (undispersed agglomerates trap volatiles), use polymeric dispersants (higher MW, less volatile than surfactants), conduct TGA screening of all pigment batches before production.
Can I use the same pigment for PP and PA (nylon)?
Limited overlap. PP processes at 220-240°C; PA6 at 260-280°C, PA66 at 280-300°C. Pigments that work in both: PB15:3 (phthalocyanine blue, 300°C), PG7 (phthalocyanine green, 300°C), PR254 (DPP red, 300°C), PR101 (iron oxide, 500°C), PR149 (perylene red, 300°C). Unsuitable for PA: all azo pigments (degrade 220-260°C), all lake pigments (degrade <200°C). Additionally, PA’s reducing environment at processing temperature can degrade azo chromophores even below their nominal decomposition temperature. Always verify: test pigment in PA at 300°C for 5 minutes and measure ΔE < 3.0.
How does pigment particle size affect plastic fiber spinning?
Fiber spinning (monofilament, multifilament, spunbond) is the most demanding plastics application for pigments. Requirements: D100 < 1µm (no single particle above 1µm — larger particles clog spinneret holes, cause fiber breaks), D50 < 0.5µm ideally, narrow distribution (D90/D10 < 3). Organic pigments (PR254, PB15:3, PY150) can achieve this with optimized dispersion. Inorganic pigments (PR101, PY42) are typically too coarse for fine denier fibers (<5 dpf) — use only for coarse monofilament (>50 dpf). Masterbatch dilution: 3-5% masterbatch at 20-30% pigment loading in fiber-grade carrier resin. Critical test: pressure rise test (filter pressure increase after 1 hour extrusion — should be <2 bar at 30µm filter).
Quick Reference Table
| Plastic | Process Temp | Migration Risk | Recommended | Avoid |
|---|---|---|---|---|
| PE (LDPE/LLDPE/HDPE) | 180-240°C | Medium | PB15:3, PG7, PR254, PY150, PR101 | PR57:1, PY12 |
| PP | 220-240°C | Medium | PB15:3, PG7, PR254, PY150, PR101 | PR57:1, PY12 |
| PVC | 160-200°C | High (plasticizer) | PB15:3, PG7, PR254, PV19, PR101 | PR57:1, PR48:2 |
| ABS | 200-240°C | Medium | PB15:3, PG7, PR254, PR101, PY150 | PR57:1, PY12 |
| PC | 280-320°C | Low (rigid) | PR254, PR264, PB15:3, PG7, PR101 | All azo pigments |
| PA6/66 | 260-300°C | Medium | PR254, PB15:3, PG7, PR101, PV19 | All azo, lake pigments |
Pro Tip
Never assume a pigment that works in PE will work in PC. The 80-100°C processing temperature difference is the most common cause of pigment failure in plastics. Always check the pigment’s heat stability rating against your specific resin’s processing temperature, with at least a 20°C safety margin.
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