Introduction

Selecting the right colorant for engineering plastics is a complex process. While many organic pigments, inorganic pigments, and solvent dyes work well for general thermoplastics, high-performance materials like Nylon (Polyamide) have strict requirements.

Below, we explore the five critical performance characteristics colorants must possess for successful Nylon injection molding¹.

1. Heat Resistance and Lightfastness

The processing temperature of the resin dictates the stability requirements for the colorant. As shown in the table below, Polyamide (Nylon) requires some of the highest processing temperatures among engineering plastics².

Processing Temperatures and Colorant Requirements by Resin Type

Note : Due to the high processing temperatures of Polyamide, colorants must exhibit exceptional thermal stability. Many organic pigments can negatively affect the properties of the Nylon melt, limiting the selection to specific high-performance pigments with good lightfastness.

2. Chemical Stability

There must be no chemical reaction between the colorant and the resin, nor should the colorant promote resin decomposition.

Polyamide resins (PA) are highly active. In their molten state, they exhibit reducing properties, which can easily cause colorants to shift in hue or degrade. Therefore, the choice of colorants for Nylon is limited to those with high chemical inertness.

3. Migration Resistance

Migration refers to the movement of the colorant within the polymer or to the surface. There are three main types:

• Solvent Extraction: Leaching of color in water or organic solvents.
• Contact Migration: Staining of adjacent objects or packaging.
• Blooming (Surface Frosting): The colorant has high solubility at high heat but low solubility at room temperature, causing it to migrate to the surface as the part cools.

Inorganic vs. Organic: Generally, inorganic pigments are heterogeneously dispersed in polymers and rarely bloom. In contrast, organic pigments have varying degrees of solubility in polymers, making them more prone to migration issues.

4. Tinting Strength

Tinting strength determines the dosage of colorant required to achieve the target color.

• Particle Size: Generally, tinting strength increases as the pigment particle size decreases.
• Organic vs. Inorganic: Organic pigments³ typically have higher tinting strength than inorganic pigments.
• Mixing: Tinting strength can be significantly improved when colored pigments are used in combination with white pigments.

5. Dispersion

For optimal coloring results, the colorant must be uniformly dispersed in the polymer as microscopic particles.

Pigments often contain many aggregated particles. To achieve the desired quality, high shear force is required during the compounding process to break down large aggregates into smaller clusters. Poor dispersion leads to color streaks and weak points in the final part.