Migration Resistance in Pigments
Understanding blooming, bleeding, and plate-out — how to prevent pigment migration, meet regulatory limits, and ensure safety in food contact, toys, and consumer products.
What Is Migration Resistance?
Migration resistance is the ability of a pigment to resist movement of its molecules from within a pigmented substrate to the surface or into adjacent materials. Unlike dyes, which are designed to be soluble and mobile, pigments are expected to remain locked within the matrix. When migration occurs, it manifests as visible defects, contamination, and — critically — potential regulatory non-compliance. Why this property matters for your application: a migrating pigment in food packaging can contaminate the food product; in a child’s toy, it can transfer to skin or saliva; in a multi-layer laminate, it can ruin the appearance of adjacent layers. Migration failure is not just a cosmetic problem — it is a safety and liability issue.
The Three Types of Migration
Migration is not a single phenomenon. Understanding which type you are dealing with is essential for diagnosis and remediation:
| Type | Definition | Visual Evidence | Root Cause |
|---|---|---|---|
| Blooming | Pigment molecules migrate to the surface of the pigmented article, forming a colored powdery or greasy film that can be wiped off. | Surface discoloration on the same article; rubs off onto a white cloth (rub test positive). | Pigment has limited solubility in the polymer; as the polymer cools and crystallizes, pigment is “squeezed out” to the surface. Exacerbated by plasticizers and residual solvents. |
| Bleeding | Pigment molecules diffuse from the pigmented substrate into an adjacent, unpigmented material — a second polymer layer, a solvent, an adhesive, or even skin. | Color transfer to white PVC, PE film, or paper in contact with the test specimen under heat and pressure. | Pigment solubility in the receiving medium. Even pigments with zero blooming may bleed if the adjacent material is a good solvent for the pigment molecule. |
| Plate-Out | Pigment deposits onto metal surfaces of processing equipment — extruder screws, die lips, calender rolls — during manufacturing. | Colored deposits on equipment that transfer back onto subsequent production runs, causing cross-contamination. | Pigment-polymer incompatibility; pigment particles are not adequately wetted or stabilized and are abraded from the melt onto metal surfaces by shear forces. |
Test Methods & Standards
Migration resistance is evaluated through a battery of tests — no single method covers all migration types or regulatory requirements.
| Standard | Scope | Method |
|---|---|---|
| ISO 105-Z06 | Textiles — assessment of bleeding | Pigmented fabric is sandwiched between two undyed fabrics, wetted, and pressed at 37°C for 4 hours. Color transfer to the white adjacent fabric is rated on the Grey Scale (1–5). |
| EN 71-3 | Safety of toys — migration of certain elements | Toy material is extracted with 0.07M HCl at 37°C for 2 hours to simulate gastric fluid. Migration limits are set for heavy metals (Sb, As, Ba, Cd, Cr, Pb, Hg, Se) in mg/kg. Critical for pigments containing heavy metals. |
| DIN 53775 | Testing of colorants in plastics — bleeding in PVC | Pigmented rigid or plasticized PVC sheet is pressed against white PVC at 80°C and 0.1 MPa for 24 hours. Bleeding is rated visually 1–5. This is the industry-standard bleeding test for PVC applications. |
| EN 71-7 | Safety of toys — finger paints | Similar to EN 71-3 but specific to finger paints. Includes limits for primary aromatic amines (PAAs) derived from azo pigment breakdown. |
| EU 10/2011 | Food contact plastics — overall and specific migration | Plastic article is exposed to food simulants (3% acetic acid, 10% ethanol, 20% ethanol, 50% ethanol, vegetable oil, or Tenax) under time/temperature conditions mimicking intended use. Specific migration limits (SML) are set per substance in mg/kg food simulant. Pigment must be listed in the positive list. |
| FDA 21 CFR | US food contact — indirect additives | Similar principle to EU 10/2011 with different simulants, time/temperature conditions, and positive lists. Key sections: 178.3297 (colorants for polymers). |
Understanding the Rating Scale
Migration resistance is typically rated on a 1–5 scale, where:
- Grade 1 — Very poor: severe migration; visible color transfer to adjacent material within hours at room temperature
- Grade 2 — Poor: moderate migration; visible transfer at elevated temperature (50–80°C)
- Grade 3 — Moderate: slight migration; faint color transfer noticeable under pressure at elevated temperature
- Grade 4 — Good: trace migration; barely perceptible color transfer only under extreme conditions (80°C + pressure for 24 h)
- Grade 5 — Excellent: no detectable migration under any test condition
For food contact, toys, and medical applications, Grade 5 is typically required regardless of cost considerations — there is no acceptable “slight bleeding” when public health is at stake.
Migration Resistance by Pigment Chemistry
| Chemistry | CI Example | Migration Grade (1–5) | Key Risk | Regulatory Note |
|---|---|---|---|---|
| Monoazo (Lake) | PR57:1 (Lithol Rubine) | 1–2 | High risk in PVC, PE, PP; severe bleeding in plasticized PVC. Soluble in common organic solvents. | Not suitable for food contact, toys, or medical. Contains Ca salt; some grades may contain residual aromatic amines. |
| Diarylide | PY12 (AAA Yellow) | 3 | Moderate risk; bleeding increases significantly in plasticized PVC and at temperatures above 60°C. Some grades decompose above 200°C, releasing 3,3′-dichlorobenzidine. | Not permitted for food contact in EU/US. Banned in many toy applications due to potential arylamide release upon thermal decomposition. |
| Naphthol AS | PR170 | 4 | Low risk in rigid PVC and polyolefins; may bleed in highly plasticized systems or strong solvents. | Generally acceptable for consumer products (non-food, non-toy). Check specific grade certification. |
| Benzimidazolone | PY150 | 5 | Effectively zero migration in all common polymers. The hydrogen-bonded crystal lattice provides exceptional immobilization. | Preferred choice for food contact and toy applications among organic yellows. |
| DPP | PR254 | 5 | Zero migration. The DPP molecular structure forms extremely strong intermolecular hydrogen bonds and π-π stacking that render the crystal lattice essentially insoluble in any polymer or solvent below 200°C. | Gold standard for food contact reds and demanding toy/compliance applications. |
| Phthalocyanine | PB15:3, PG7 | 5 | Zero migration in all polymers. Extremely large planar molecules with strong crystal lattice energy. Only risk is trace free Cu content for EN 71-3 compliance. | Widely used in food contact, toys, and medical. Ensure low free copper grades for EN 71-3. |
| Quinacridone | PV19, PR122 | 5 | Zero migration. Highly stable polycyclic structure with extensive intermolecular hydrogen bonding network. | Excellent for compliance-driven applications. Widely used in automotive interiors (low VOC, zero migration). |
| Iron Oxide | PR101, PY42 | 5 | Zero migration. Inorganic crystal lattice is thermodynamically insoluble in organic polymers. Only risk is soluble heavy metal impurities. | Naturally suitable for food contact and toys, but verify heavy metal purity (Pb, As, Cd, Hg, Cr⁶⁺) per grade specification. |
Key Insight: Temperature and Plasticizers Are Migration Accelerators
Migration is strongly temperature-dependent — a pigment rated grade 4 at 25°C may behave like a grade 2 at 80°C because molecular solubility in the polymer increases exponentially with temperature. Similarly, plasticizers (DOP/DINP in PVC, phthalate alternatives, citrates) act as internal solvents that dramatically increase pigment solubility in the matrix. This is why many pigments that perform acceptably in rigid PVC fail catastrophically in flexible PVC. Always test at your actual processing temperature and with your full formulation — including plasticizers, stabilizers, and co-additives — not just the pigment in isolation. A “Grade 5” rating on a pigment datasheet means nothing if the test was in rigid PVC and your application is a soft-touch overmold with 30% plasticizer.
Food Contact and Regulatory Implications
For pigments used in food contact materials, migration resistance is not just about aesthetic quality — it is a legally mandated safety requirement. The key concepts are:
Overall Migration Limit (OML)
The total mass of all non-volatile substances that can migrate from the packaging into food. EU Regulation 10/2011 sets OML at 10 mg/dm² of contact surface area (or 60 mg/kg food for infant foods). The pigment contributes to this total along with the polymer, additives, and processing aids. A poorly migrating pigment can push a formulation over the OML even if the pigment itself is toxicologically benign.
Specific Migration Limit (SML)
The maximum permitted quantity of a specific substance that may migrate into food — expressed in mg per kg of food simulant (mg/kg). For pigments, SMLs apply to the pigment molecule itself, any residual starting materials (aromatic amines, heavy metals), and any degradation products. A pigment must be on the EU positive list (Regulation 10/2011 Annex I) or the FDA positive list (21 CFR) to be legally used. Unlisted pigments may not be used regardless of their migration test results.
Frequently Asked Questions
Why do some red pigments bleed in PVC?
The classic example is PR57:1 (Lithol Rubine, also known as Permanent Red 2B or BON Arylamide Red). This pigment bleeds severely in PVC — particularly plasticized PVC — for two reasons: molecular structure and processing conditions. PR57:1 is a monoazo pigment with a relatively small, low-molecular-weight chromophore (~430 g/mol) that has measurable solubility in the amorphous regions of PVC. The calcium salt form (the “lake”) increases insolubility somewhat but does not eliminate molecular solubility. During PVC processing at 160–200°C, the amorphous fraction of the PVC matrix increases, and plasticizers (phthalates) create mobile liquid-like domains within the polymer. The pigment molecules partition into these domains and, over time, diffuse to the surface (blooming) or into adjacent materials (bleeding). The solution: for PVC applications requiring migration resistance, switch to naphthol AS pigments (PR170, grade 4), benzimidazolones (PY150, grade 5), DPP (PR254, grade 5), or phthalocyanines (grade 5) depending on your color target.
How to test for pigment migration?
A comprehensive migration test program includes multiple methods, but the most practical starting point is the contact bleed test adapted from DIN 53775: prepare two plaques of your pigmented formulation, sandwich a white (unpigmented) plaque of the same polymer between them, apply 0.1 MPa pressure in a heated press at your application’s maximum use temperature (or 80°C as a standard reference), and hold for 24 hours. Visually inspect the white plaque for any color transfer. For a more sensitive test, measure the ΔE* of the white plaque before and after contact. Any ΔE* > 1.0 indicates measurable migration. For food contact, this must be followed by formal simulant extraction testing per EU 10/2011 or FDA protocols in an accredited laboratory — home-brew tests do not satisfy regulatory requirements.
Is migration resistance the same as solvent resistance?
No, but they are closely related. Migration resistance measures the pigment’s ability to resist diffusion through a solid or semi-solid polymer matrix. Solvent resistance measures the pigment’s ability to resist dissolution (bleeding) when the pigmented article comes into direct contact with a liquid solvent — for example, a coating applied over a pigmented primer, or a cleaning solvent on a painted surface. A pigment with excellent migration resistance (no movement through solid polymer) may still bleed into an aggressive solvent like MEK or toluene because the solvent can swell the polymer matrix and directly solvate pigment molecules at the surface. Conversely, a pigment with poor migration resistance will almost certainly have poor solvent resistance, because solubility in the polymer is the fundamental driver of both phenomena. Think of them as two manifestations of the same underlying property — solubility — tested under different conditions. For most practical pigment selection, migration resistance is the more conservative and stringent test.
Need Migration-Safe Pigments for Compliance-Critical Applications?
Our technical team can help you select food-contact-compliant, toy-safe, and medical-grade pigments with full migration resistance documentation and regulatory certifications.