Pigment Migration in Plastics & Coatings
Causes, testing methods, and solutions for blooming, bleeding, and plate-out — with migration-resistant pigment selection
What Is Pigment Migration?
Pigment migration is the unwanted movement of pigment molecules or particles within a substrate or from one material to another in contact. Unlike dye migration (which involves molecular dissolution), pigment migration occurs when pigment molecules have partial solubility in the binder, plasticizer, or processing environment. At elevated temperatures during processing or in-service use, soluble pigment fractions dissolve and diffuse through the polymer matrix. Upon cooling or reaching the surface, these dissolved molecules recrystallize, creating visible surface deposits (blooming) or color transfer to adjacent materials (bleeding). Migration is fundamentally a solubility-driven phenomenon — the more soluble a pigment is in a given medium, the greater its migration risk.
Three Forms of Migration
- Blooming — A white or colored powdery deposit forms on the surface of the colored article. This occurs when pigment molecules dissolved in the polymer at processing temperature recrystallize on the surface during cooling. The surface deposit can be wiped off. Blooming is most common in flexible PVC, polyolefins, and rubber compounds containing soluble organic pigments or pigment lakes.
- Bleeding — Color transfers from the colored substrate into an adjacent material in contact (over-coating, adhesive layer, packaging film, or another plastic part). Bleeding requires direct contact and is driven by the pigment’s solubility gradient between the two materials. Bleeding is the most damaging form of migration in multi-layer packaging, laminated products, and coated textiles.
- Plate-out — Pigment deposits accumulate on mold surfaces, extruder dies, or processing equipment during manufacturing. Over repeated cycles, the deposit builds up and can transfer to subsequent parts. Plate-out is driven by pigment volatility/sublimation at processing temperature combined with condensation on cooler tooling surfaces. It differs from blooming in that it occurs on processing equipment, not the final part surface.
⚡ Key Distinction: Migration vs. Rub-Off vs. Crocking
Migration is a chemical diffusion process — pigment molecules dissolve and move through the medium. Rub-off and crocking are mechanical — poorly bonded pigment particles on the surface are physically transferred by friction. Rub-off can be fixed by improving surface adhesion or applying a clear coat. Migration requires changing the pigment chemistry or reducing solubility.
Symptoms of Migration
WHITE POWDER ON SURFACE (BLOOMING)
A white, pale, or slightly colored powdery deposit appears on the surface of the finished article over time (hours to weeks after production). The powder can be wiped off with a cloth. Under microscope, the deposit consists of fine needle-like or dendritic crystals. Most common in flexible PVC with PR57:1 or PR48:2 lake pigments.
COLOR TRANSFER TO ADJACENT MATERIAL (BLEEDING)
When the colored article is pressed against a white or light-colored material (PVC film, paper, another plastic part) under light pressure and/or mild heat, visible color transfers to the light material. This is the classic “sandwich test” failure and is the most commercially damaging form of migration — food packaging, toys, and medical devices cannot tolerate any bleeding.
COLOR BUILD-UP ON PROCESSING EQUIPMENT (PLATE-OUT)
A colored deposit gradually accumulates on injection mold surfaces, extruder die lips, or calender rolls. The deposit is often tacky or powder-like. Once built up, it can cause surface defects on subsequent parts and requires production stoppage for cleaning. Plate-out is typically driven by pigments with low thermal stability and measurable vapor pressure at processing temperature.
GHOSTING / HALO AROUND PRINTED GRAPHICS
In printed or decorated plastic articles, a faint colored “halo” or ghost image appears around the printed area, extending 1–5 mm beyond the intended graphic boundary. This occurs when pigment from the ink or decoration migrates laterally through the substrate due to plasticizer or residual solvent mobility.
Root Causes
| Cause Category | Mechanism | High-Risk Pigments |
|---|---|---|
| Pigment Solubility in Binder/Plasticizer | All pigments have a finite solubility in organic media, however small. Pigments with low molecular weight (<500 g/mol), azo chromophores, or ionic lake structures dissolve more readily in plasticizers (DOP, DINP, DOA) and polymer matrices. The dissolved fraction is mobile and can diffuse to the surface or into adjacent materials. | PR57:1 (Lake Red C, MW ~460), PR48:2 (Lake Red 2B), PY12 (diarylide, MW ~630), PR3 (toluidine red, MW ~308), PR53:1 (Lake Red C) |
| Processing Temperature Exceeding Migration Threshold | Every pigment has a temperature above which solubility increases exponentially, dramatically increasing migration risk. This threshold is pigment- and medium-specific. Process temperatures exceeding 220°C are particularly problematic for many monoazo and diarylide pigments. | PY12 (limit ~180°C in polyolefins), PR3 (limit ~160°C), PR57:1 (limit ~200°C). Processing at 240°C+ in PP or PA will cause severe migration. |
| Low Molecular Weight | Pigment migration tendency correlates inversely with molecular weight. Small molecules (<500 g/mol) diffuse more rapidly through polymer free volume. High-molecular-weight pigments (>800 g/mol) such as phthalocyanines, DPPs, and quinacridones have negligible solubility and diffusion rates. | PR3 (MW 308) — extreme risk. PR57:1 (MW ~460) — high risk. PB15:3 (MW 576) — very low risk. PR254 (MW ~357) — low risk despite moderate MW due to strong intermolecular H-bonding and high crystallinity. |
| Plasticizer Content and Type | Plasticized PVC is the highest-risk substrate for migration. The plasticizer (typically 30–50 phr DOP/DINP) acts as a solvent for soluble pigment fractions and provides a continuous diffusion pathway. Ester-type plasticizers are particularly aggressive solvents for azo and lake pigments. Phthalate-free plasticizers (DINCH, TOTM, ESBO) may have different solubility profiles. | Flexible PVC with >30 phr DOP + PR57:1 is a classic migration failure combination. Switching to polymeric plasticizers (adipate polyesters) reduces migration but increases cost. |
| Pigment Impurities / Residual Synthesis Byproducts | Commercial pigments may contain 1–5% of unreacted intermediates, side-reaction products, or low-molecular-weight fractions from incomplete synthesis. These impurities are often more soluble than the target pigment and can migrate even when the pigment itself is nominally migration-resistant. High-purity grades are essential for demanding applications. | Residual aromatic amines in azo pigments (from incomplete coupling), residual phthalonitrile in phthalocyanines, residual metal salts in lake pigments. |
Diagnostic Tests
1. Contact Bleed Test (PVC Standard: EN 71 / ASTM F963 for Toys)
Press the colored test specimen against a white PVC sheet containing 30–40 phr DOP plasticizer. Apply a pressure of 5 kPa (0.5 N/cm²) at 80°C for 24 hours (accelerated conditions), or at 50°C for 72 hours (standard). After exposure, separate the sheets and visually inspect the white PVC for any color transfer. Use a spectrophotometer to measure ΔE of the white sheet — any ΔE > 1.5 indicates migration. For toy and food-contact applications, zero visible transfer is required.
2. Sandwich Migration Test (PE Film)
Sandwich the colored specimen between two layers of white LDPE film (100 µm thickness). Place between glass plates under a 500 g weight. Heat in an oven at 60°C for 72 hours, or at 80°C for 24 hours (accelerated). Inspect both PE film layers for color transfer. This test simulates migration in polyolefin-based packaging and multi-layer films.
3. Solvent Extraction Test
Immerse a known weight of colored specimen in a suitable solvent (ethanol, 3% acetic acid, or olive oil — depending on the intended end-use and regulatory requirements) at 40°C for 24 hours. Filter the extract and measure the absorbance at the pigment’s λmax using UV-Vis spectrophotometry. Compare to a calibration curve of known pigment concentration. The result is expressed as mg pigment extracted per dm² of surface area. Regulatory limits vary: food contact typically requires <0.01 mg/dm².
4. Blooming Test (Temperature Cycle)
Subject the colored specimen to 5 temperature cycles: 4 hours at 80°C (above Tg to promote diffusion), then 4 hours at 4°C (to promote recrystallization). After cycling, wipe the surface with a clean white cloth soaked in ethanol. Any color on the cloth indicates blooming.
5. Plate-Out Simulation
Process the colored compound through a two-roll mill at the intended processing temperature for 30 minutes. Inspect the mill rolls for color deposit. Alternatively, run 20–30 injection molding cycles and inspect the mold surface for deposit buildup. Quantitative: weigh a clean aluminum foil placed in the mold before and after processing to measure deposit mass.
Corrective Actions & Solutions
Switch to Migration-Resistant Pigments
The most reliable solution is pigment replacement. Replace high-risk pigments with chemically robust, high-molecular-weight alternatives that have negligible solubility in typical polymer matrices.
| Migrating Pigment (Replace) | CI | Migration-Resistant Alternative | CI | Shade Compatibility |
|---|---|---|---|---|
| Lake Red C | PR57:1 | DPP Red (Diketopyrrolopyrrole) | PR254 | Similar bluish-red; PR254 is slightly more opaque |
| Toluidine Red | PR3 | Naphthol Red AS / Azo Condensation Red | PR170 / PR144 | PR170 is yellower; blend with PR122 for shade match |
| Diarylide Yellow AAA | PY12 | Benzimidazolone Yellow H4G / Isoindolinone Yellow | PY151 / PY110 | PY151 is greener; PY110 is redder — blend to match |
| Diarylide Yellow AAMX | PY13 | Benzimidazolone Yellow H3G / Isoindoline Yellow | PY154 / PY139 | PY154 is very close match, slightly greener |
| BON Arylamide Red (Lithol Rubine) | PR57:1 | Quinacridone Magenta / DPP Rubine | PR122 / PR264 | PR122 is excellent bluish-red match for PR57:1 |
Reduce Plasticizer Content or Switch Type
- Reduce plasticizer loading: Decreasing plasticizer from 50 phr to 35 phr in flexible PVC reduces the solvent volume available for pigment dissolution. However, this also stiffens the final product — verify mechanical property requirements.
- Switch to polymeric plasticizers: Replace monomeric plasticizers (DOP, DINP) with polymeric types (adipate polyesters, MW 2,000–8,000). Polymeric plasticizers are much poorer solvents for pigment molecules and have lower diffusion rates themselves, reducing the transport medium for dissolved pigment. Typical trade-off: 20–40% higher cost, slightly reduced low-temperature flexibility.
- Use non-migrating plasticizers: Permanently plasticized compounds (PVC-PA graft copolymers, reactive plasticizers, or plasticizer-free flexible PVC using high-MW internal plasticization) eliminate the plasticizer migration pathway entirely.
Lower Processing Temperature
Reduce processing temperature by 10–20°C where possible, especially in extrusion and injection molding. Every 10°C reduction roughly halves the diffusion rate (Arrhenius relationship). Add processing aids (acrylic process aids in PVC, waxes in polyolefins) to maintain flow at lower temperatures. In PVC, reducing processing temperature from 190°C to 170°C can dramatically reduce migration of PR57:1.
Use a Barrier Layer
In multi-layer applications (packaging films, laminated products, coated textiles), insert a barrier layer between the colored layer and the surface. EVOH (ethylene vinyl alcohol), PVDC, or aluminum foil layers are effective barriers against migrating pigment molecules. In PVC flooring, a clear PVC wear layer containing only inorganic pigments (no organics) can act as a barrier between the printed design layer and the surface.
Improve Pigment Purity
Request high-purity pigment grades from suppliers. These grades undergo additional washing, solvent extraction, or thermal treatment steps to remove soluble impurities that can migrate. High-purity grades typically carry a 10–30% price premium but are essential for food-contact, toy, and medical applications. Specify “low-soluble-matter” grades with maximum extractable content <0.1% by weight.
Pigment Migration Resistance Reference Table
| CI Number | Chemistry | Migration Grade (1–5) | Safe Processing Temp (°C) | High-Risk Substrates | Notes |
|---|---|---|---|---|---|
| PB15:3 | β-Cu-Phthalocyanine | 5 (Excellent) | ≤300 | None | Essentially insoluble in all polymers. Gold standard for migration resistance. |
| PG7 | Cu-Phthalocyanine Green (polychloro) | 5 (Excellent) | ≤300 | None | Same class as PB15:3. Exceptional chemical and migration resistance. |
| PR254 | Diketopyrrolopyrrole (DPP) | 5 (Excellent) | ≤280 | None | Excellent migration resistance despite moderate MW. Strong intermolecular H-bonding prevents dissolution. |
| PV19 | Quinacridone (γ-phase) | 5 (Excellent) | ≤280 | None | Quinacridones are highly crystalline and insoluble. Excellent all-around performance. |
| PR122 | 2,9-Dimethylquinacridone | 5 (Excellent) | ≤280 | None | Preferred PR57:1 replacement for high-performance applications. |
| PR101 | Synthetic Iron Oxide Red | 5 (Excellent) | ≤600 | None | Inorganic pigments are inherently migration-resistant. No solubility in organic media. |
| PY110 | Isoindolinone Yellow | 4–5 (Very Good) | ≤260 | Plasticized PVC above 30 phr | Excellent PY12 replacement. Slight migration at high plasticizer levels. |
| PY151 | Benzimidazolone Yellow H4G | 4–5 (Very Good) | ≤240 | Plasticized PVC above 35 phr | Greenish-yellow. Good lightfastness and migration resistance. |
| PR170 | Naphthol Red AS | 4 (Good) | ≤220 | Plasticized PVC, polyolefins above 250°C | Good cost-performance balance. Better migration resistance than lake pigments. |
| PY12 | Diarylide Yellow AAA | 2 (Poor) | ≤180 | Plasticized PVC, polyolefins, polyurethane | High migration risk. Avoid in flexible PVC, food contact, toys. Replace with PY151 or PY110. |
| PY13 | Diarylide Yellow AAMX | 2–3 (Fair) | ≤200 | Plasticized PVC, polyolefins above 220°C | Slightly better than PY12 due to higher MW. Still high risk in flexible PVC. |
| PR57:1 | Lake Red C (Ca salt) | 1–2 (Very Poor) | ≤200 | Plasticized PVC, polyolefins, polyurethane, polystyrene | Classic migration failure pigment. The calcium lake structure dissolves readily in plasticizers. Never use in flexible PVC. |
| PR48:2 | Lake Red 2B (Ca salt) | 1–2 (Very Poor) | ≤180 | Plasticized PVC, polyolefins, polyurethane | Similar to PR57:1. Lake structure with Ca counter-ion is highly soluble in ester plasticizers. |
| PR3 | Toluidine Red | 1 (Extremely Poor) | ≤160 | All substrates above 160°C | Lowest MW of common organic pigments (308 g/mol). Extreme migration risk. For room-temperature applications only. |
Migration Grade Scale: 5 = No migration in any substrate at any temperature. 4 = Negligible migration in most substrates. 3 = Some migration in demanding conditions (high plasticizer, high temperature). 2 = Significant migration in common conditions. 1 = Severe migration in nearly all conditions — avoid.
Prevention Checklist
- Pre-select pigments by migration grade for the application: Food contact = Grade 5 only. Toys (EN 71/ASTM F963) = Grade 4+ and must pass specific extraction tests. Medical devices = Grade 5. Automotive interiors = Grade 4+ at 100–120°C. General industrial = Grade 3+.
- Avoid lake pigments entirely (PR57:1, PR48:2, PR53:1, PR49:1) in any application involving plasticized PVC, polyolefin processing above 200°C, or any food-contact/toys/medical. The lake structure is inherently soluble.
- Match processing temperature to pigment capability: Never process diarylide yellows above 200°C, naphthol reds above 240°C, or toluidine reds above 160°C. Verify the pigment manufacturer’s recommended maximum processing temperature before production.
- Pre-test every new pigment-substrate combination with the contact bleed test at the worst-case temperature and contact time expected in service. Do not rely on supplier data sheets alone — test in your specific formulation.
- Request Certificates of Analysis for pigment purity (extractable content, heavy metals, aromatic amines) for regulated applications. High-purity grades are essential for compliance, not just performance.
- For flexible PVC, always conduct migration testing at the maximum plasticizer level to be used in production. Test at both the standard and accelerated conditions to establish a safety margin.
- Monitor plate-out during production. If colored deposit appears on tooling, reduce processing temperature by 10–20°C, switch to a higher-thermal-stability pigment, or increase mold surface temperature to reduce condensation.
Summary: Symptom → Root Cause → Diagnosis → Solution
| Symptom | Root Cause | Diagnosis Method | Solution |
|---|---|---|---|
| White/colored powder on surface (blooming) | Pigment dissolved at processing temperature recrystallizes on cooling. High pigment solubility in medium. | Temperature cycle test (80°C/4°C, 5 cycles). Wipe surface with ethanol-soaked cloth — color on cloth confirms blooming. Microscope shows crystalline surface deposit. | Switch to Grade 4–5 pigment (PR254, PV19, PR122). Reduce processing temperature. Add 0.5–1.0% polyethylene wax to create surface barrier. |
| Color transfer to adjacent white material (bleeding) | Pigment solubility + plasticizer content creating continuous diffusion pathway between materials | Contact bleed test: PVC/80°C/24h. Sandwich test: PE/60°C/72h. Measure ΔE of recipient surface. | Replace pigment with migration-resistant grade. Reduce plasticizer or switch to polymeric type. Insert EVOH/PVDC barrier layer between colored and contact surfaces. |
| Colored deposit on mold/extruder (plate-out) | Pigment volatility/sublimation at processing temperature. Condensation on cooler tooling. | Process 20–30 cycles and inspect mold. Weigh Al foil insert before/after. Temperature-programmed TGA to identify volatilization temperature. | Switch to pigment with higher thermal stability. Increase mold temperature to reduce condensation. Add mold release agent containing anti-plate-out additive. |
| Halo/ghosting around printed graphics | Lateral migration through substrate via plasticizer or residual solvent mobility | Print test pattern, age at 60°C/48h. Measure halo diameter. Compare different pigment grades in same ink system. | Use higher-MW pigment in ink. Reduce plasticizer in substrate. Apply heat-cure step to remove residual solvent before lamination. |
| Migration failure in flexible PVC with lake pigment | PR57:1 / PR48:2 dissolving in DOP/DINP plasticizer at processing temperature | Contact bleed test at 80°C/24h with standard PVC sheet. Extraction in DOP at 80°C — measure dissolved pigment by spectrophotometry. | Replace PR57:1 with PR122 (quinacridone magenta) or PR254 (DPP red). Reduce DOP from 50→35 phr if replacement not possible. Switch plasticizer to adipate polyester. |
| Intermittent migration — passes some batches, fails others | Variable pigment purity between lots. Soluble impurities (unreacted intermediates, byproducts) causing inconsistency. | Compare extraction test results across multiple batches. Request purity analysis (HPLC, residual amine content) from supplier. | Source high-purity grade with certified low extractable content (<0.1%). Implement incoming QC with contact bleed test on each lot. Use single qualified pigment source. |
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