Color in Plastics: Pigments vs Dyes, CIE Lab* & Masterbatch Technology

Color is often the first property a consumer evaluates. The Indian masterbatch industry (Plastiblends, Cabot India, Clariant Masterbatches) serves every plastics sector from food packaging to automotive. A polymer engineer must understand the physics of color, why pigments and dyes behave differently at processing temperatures, and how to use CIE Lab* color measurement to achieve color matching within specification.

• Explain the difference between pigments (insoluble, particle scattering) and dyes (soluble, molecular absorption).
• Interpret CIE Lab* coordinates and calculate ΔE color difference.
• Select pigment or dye based on heat stability, lightfastness, and migration resistance.
• Calculate masterbatch let-down ratio for production.
• Identify ASTM D2244 and ISO 11664 color measurement standards.

3.1 Pigments vs Dyes

3.2 CIE Lab* Color Space

The CIE Lab* system (ISO 11664-4) represents color as three coordinates:

• L*: Lightness (0 = black, 100 = white)
• a*: Red-Green axis (+a* = Red, −a* = Green)
• b*: Blue-Yellow axis (+b* = Yellow, −b* = Blue)

Color Difference ΔE (Delta E):

$$\Delta E^* = \sqrt{(\Delta L^*)^2 + (\Delta a^*)^2 + (\Delta b^*)^2}$$

3.3 Masterbatch Technology

A masterbatch (MB) is a concentrated mixture of pigments (15–60% by weight) dispersed in a carrier resin pellet. Let-down ratio:

$$\text{MB Addition (\%)} = \frac{\text{Target pigment level in part (\%)}}{\text{Pigment concentration in MB (\%)}} \times 100$$

Example: MB contains 40% TiO₂; target = 2% TiO₂ in final part:

$$\text{MB Addition} = \frac{2\%}{40\%} \times 100 = 5\%$$

→ 5 kg masterbatch + 95 kg natural resin per 100 kg production batch.

3.4 Pigment Performance Parameters

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Problem: PP crate measurement vs standard:

• Standard: L* = 52.4, a* = −6.8, b* = +18.3
• Sample: L* = 53.7, a* = −7.1, b* = +17.5

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Solution:

$$\Delta L^* = 53.7 - 52.4 = +1.3$$ $$\Delta a^* = -7.1 - (-6.8) = -0.3$$ $$\Delta b^* = 17.5 - 18.3 = -0.8$$ $$\Delta E^* = \sqrt{(1.3)^2 + (-0.3)^2 + (-0.8)^2} = \sqrt{1.69 + 0.09 + 0.64} = \sqrt{2.42} = \textbf{1.56}$$

Interpretation: ΔE = 1.56 — slightly visible difference side-by-side. If specification is ΔE ≤ 1.0, this batch fails. Colorist must increase yellow pigment (b* is low) and slightly reduce white (L* is high).

Plastiblends India Ltd (Mumbai/Daman) — India's largest masterbatch manufacturer — supplies 2,000+ custom colour masterbatches to packaging, agriculture, and automotive sectors.

Cabot India (Baroda, subsidiary of Cabot Corporation) specialises in carbon black masterbatches for HDPE pipe, wire, and film applications requiring UV stabilisation and electrical conductivity (ASTM D1248 requirement for PE pipe: ≥2.0% carbon black by ISO 18553 dispersion Category 1–3).

• Pigment: Insoluble colorant; dispersed as particles; good heat stability and migration resistance.
• Dye: Soluble colorant; dissolved at molecular level; transparent tints but prone to migration.
• CIE Lab*: Perceptually uniform color space; ΔE measures total color difference numerically.
• ΔE ≤ 1.0: Industry-standard acceptable color tolerance for most packaging applications.
• Let-down ratio: Dilution factor from masterbatch concentration to final part pigment level.
• Lightfastness: UV resistance (Blue Wool Scale 1–8; 8 = excellent).

1. ISO 11664-4:2019 — CIE Lab* color space
2. ASTM D2244 — Calculation of Color Tolerances and Color Differences
3. ISO 105-B02 — UV lightfastness (Blue Wool Scale)
4. ISO 18553 — Degree of pigment dispersion in polyolefins
5. DIN 53775 — Migration testing of colorants in plastics

1. A sample shows L* = 45.2, a* = +8.1, b* = −12.4. Reference: L* = 44.0, a* = +7.8, b* = −11.9. Calculate ΔE.
2. Explain why dyes are preferred over pigments for transparent tinted PC or PMMA parts.
3. A masterbatch contains 50% carbon black. Target in HDPE pipe = 2.5%. Calculate MB addition %.

Q1. CIE Lab* axis representing yellow-blue direction:

• A) L* B) a* C) b* D) ΔE

Q2. Acceptable ΔE tolerance for packaging color matching:

• C) ΔE ≤ 1.0

Q3. MB contains 40% pigment. Target = 2%. MB addition = ?

• C) 5%

Q4. Blue Wool Scale (ISO 105-B02) tests:

• B) Lightfastness (UV resistance)

Q5. India's largest masterbatch manufacturer:

• A) Plastiblends India