Chemical Resistance
Acid, alkali, and solvent resistance determine whether a pigment survives its chemical environment — from concrete alkalinity to aggressive solvent-based coating systems.
What Is Chemical Resistance?
Chemical resistance is the collective term for a pigment’s ability to withstand exposure to acids, alkalis, organic solvents, and reactive chemicals without color change, dissolution, or chemical degradation. It encompasses acid resistance (resistance to acidic environments), alkali resistance (resistance to alkaline/basic environments), and solvent resistance (resistance to bleeding/extraction in organic solvents).
Chemical resistance is rated on a 1-5 scale (1 = poor/severe discoloration, 5 = excellent/no change), tested according to ISO 2812 (determination of resistance to liquids) and ASTM D1308 (chemical spot test). Solvent bleeding is separately tested via contact bleed test — placing the pigmented system in contact with a white substrate saturated with solvent at 50°C for 24 hours and measuring color transfer.
Test Methods
- Acid/Alkali Resistance (ISO 2812): Immerse pigmented coating in 5% H₂SO₄ (acid) or 5% NaOH (alkali) for 24 hours. Measure ΔE and visual change.
- Solvent Resistance (Spot Test, ASTM D1308): Apply solvent (xylene, MEK, ethyl acetate, ethanol, mineral spirits) to coated surface for 1 hour covered with watch glass. Measure ΔE, blistering, softening.
- Solvent Bleeding (Contact Test): Press pigmented film against white PVC containing 30% DOP plasticizer at 80°C/24h. Measure color transfer to PVC.
- Chemical Spot Test (DIN 53775): Apply 10% HCl to pigmented PVC sheet, observe color change over 24h.
Chemical Resistance by Pigment Chemistry
| Pigment (CI) | Chemistry | Acid (1-5) | Alkali (1-5) | Solvent (1-5) | Key Vulnerability |
|---|---|---|---|---|---|
| PR57:1 | Monoazo Lake (Ca) | 2 | 2 | 2 | Sensitive to all three — lake structure breaks down; metal ions exchange in acid; azo bond cleaves in alkali |
| PR48:2 | Monoazo Lake (Ca) | 2 | 2 | 2 | Similar to PR57:1 — calcium salt dissolves in acidic conditions |
| PY12 | Diarylide Yellow | 2 | 3 | 3 | Acid-sensitive — azo group protonates and cleaves; moderate alkali/solvent |
| PY13 | Diarylide Yellow | 2 | 3 | 3 | Same as PY12 — acid sensitivity limits use in acid-cure systems |
| PY83 | Disazo Condensation | 3 | 3 | 3 | Better than PY12/13 but still moderate — avoid strong acids |
| PR170 | Naphthol AS | 4 | 4 | 4 | Good all-around — limited solvent bleeding in very aggressive solvents (MEK, DMF) |
| PR254 | DPP (Diketopyrrolopyrrole) | 5 | 5 | 5 | Excellent all-around — DPP ring is chemically inert; no ionizable groups |
| PB15:3 | Cu-Phthalocyanine | 5 | 5 | 5 | Excellent — phthalocyanine macrocycle is one of the most chemically stable organic structures known |
| PG7 | Cu-Polychloro Phthalocyanine | 5 | 5 | 5 | Excellent — additional chlorine atoms increase chemical inertness further |
| PV19 (Quinacridone) | Linear Trans-Quinacridone | 4 | 5 | 5 | Excellent alkali and solvent resistance; slight acid sensitivity in concentrated H₂SO₄ |
| PY150 | Benzimidazolone | 5 | 5 | 4 | Good — moderate solvent bleeding in aromatic solvents (xylene, toluene) |
| PR101 | Iron Oxide (α-Fe₂O₃) | 5 | 5 | 5 | Excellent — chemically inert oxide, survives pH 1-14 |
Real-World Chemical Exposure Scenarios
| Scenario | Chemical Challenge | Required Resistance | Recommended Pigments | Pigments to Avoid |
|---|---|---|---|---|
| PVC Processing | HCl evolution at 180-200°C from PVC dehydrochlorination | Acid resistance 4-5 | PR254, PB15:3, PG7, PV19, PY150, PR101 | PR57:1, PR48:2, PY12, PY13 |
| Concrete / Cementitious Coatings | Ca(OH)₂, pH 12-13 (highly alkaline) | Alkali resistance 5 | PR101, PY42, PB15:3, PG7, PR254, PV19, PW6 | PR57:1, PY12, PY13, PR48:2 |
| Epoxy Coatings | Amine curing agents (basic pH ~10-11 during cure) | Alkali resistance 4-5 | PB15:3, PG7, PR254, PY150, PR101, PY42 | PR57:1, PY12, PR48:2 |
| 2K Polyurethane Coatings | Isocyanate crosslinker + solvent exposure (xylene, butyl acetate) | Solvent resistance 4-5, alkali 4-5 | PR254, PB15:3, PG7, PV19, PY150, PR101 | PR57:1, PY12, PR48:2 |
| Automotive Clearcoat / Basecoat | Solvent pop, acid rain (pH 3-4), UV + moisture cycling | Acid 4-5, solvent 5, weather 4-5 | PR254, PV19, PB15:3, PG7, PR122, PY150 | All azo and lake pigments |
| Coil Coating (250°C Cure) | Thermal + chemical stress during 60-90 sec peak metal temperature | Heat 250°C + acid/alkali 4-5 | PR254, PB15:3, PG7, PY150, PR101, PY42 | All azo pigments, PR57:1 |
| Solvent-Based Gravure Ink | Continuous exposure to ethyl acetate, ethanol, MEK, toluene | Solvent resistance 4-5 (no bleeding) | PB15:3, PG7, PR254, PR57:1 (moderate but acceptable for short-term), PY12 | Solvent dyes (designed to dissolve!) |
Frequently Asked Questions
Which pigments survive concrete alkalinity?
Concrete and cementitious materials have a saturated Ca(OH)₂ environment at pH 12-13, which is extremely aggressive to many organic pigments. Safe choices with alkali resistance grade 5: iron oxides (PR101, PY42, PBk11), phthalocyanines (PB15:3, PG7), DPP (PR254), TiO₂ (PW6), and ultramarine blue (PB29). Pigments that survive but may show some shift: PY150 (grade 5, but slight yellowing over years), PV19 (grade 5). Fail quickly: PR57:1 (grade 2 — azo bond cleaved, turns brown within weeks), PY12/13 (grade 2-3), all lake pigments. For colored concrete, iron oxides are the gold standard — 50+ year track record with zero chemical degradation in concrete.
Why do lake pigments fail acid resistance tests?
Lake pigments (PR57:1, PR48:2) are water-soluble sulfonic acid dyes precipitated as insoluble calcium or barium salts. In acidic conditions, the metal counter-ion (Ca²⁺, Ba²⁺) is displaced by H⁺, converting the insoluble lake back to the water-soluble dye form. The pigment literally dissolves. This is a fundamental chemical limitation of the lake structure — it cannot be “fixed” by formulation or additives. If acid resistance is required, replace PR57:1 with PR122 (quinacridone magenta, grade 4 acid) or PR254 (DPP, grade 5 but bluer shade).
How to select pigments for two-component polyurethane coatings?
2K PU systems have high chemical resistance demands for two reasons: the isocyanate crosslinker is sensitive to basic contaminants (amines, moisture), and the final film must withstand aggressive environments. Selection criteria: alkali resistance 4-5 (isocyanate + amine reaction compatibility), solvent resistance 5 (PU films are exposed to industrial solvents), no reactive functional groups (no -NH₂, -OH groups that consume isocyanate). Best pigments: PR254 (DPP), PB15:3, PG7, PV19, PR101, PY42. Acceptable: PY150 (check solvent bleeding in polyol component). Avoid: all lake pigments, azo pigments with free amine impurities.
Pro Tip
Chemical resistance is often the forgotten property — formulators prioritize color, cost, and lightfastness, then discover chemical failure only after field complaints. When specifying pigments for any system that contacts chemicals (and most do — even water-based architectural paints face household cleaners at pH 2-12), always request the pigment supplier’s chemical resistance data sheet. A pigment that looks perfect for your application on color grounds can fail catastrophically if it can’t handle your system’s chemical environment.
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