Application technology and process control of pigment in production

Application technology and process control of pigment in production

Yao Shengzhou Yu Jia (Wuhan Yincai Technology Co., LTD.)

Abstract: This paper outlines key performance indicators (KPIs) and classification criteria for pigments, including hiding ability, weather resistance, and dispersion property. Comparative tables illustrate the application characteristics of different pigment types. The text provides detailed explanations of the three production methods—dry mixing, bonding process, and melt extrusion—along with their applicable scenarios and critical process control points. Furthermore, it addresses common issues such as color mottling, uneven metallic effects, and poor pigment dispersion, detailing their causes and effective solutions.

I. Introduction

In coatings, pigments not only provide color and decorative properties but also directly influence multiple performance characteristics such as film opacity, weather resistance, and corrosion resistance. The selection, dispersion, and processing techniques of pigments constitute the core of coating production, which directly impacts the quality stability and application effectiveness of final products. This article systematically introduces principles for pigment classification and selection in manufacturing processes, application methods and techniques for different pigments (including dry mixing, bonding processes, and melt extrusion), along with key process control points and solutions to common challenges encountered during production. The aim is to provide practical technical expertise that offers actionable guidance for coating production.

II. Basic properties and selection of pigments

In the production process, the selection of pigment directly determines the appearance, performance and application scope of the final product. Therefore, understanding the key performance indicators of pigment and selecting it scientifically according to specific requirements is the basis to ensure the quality of coating.

2.1 Key performance indicators

a. covering ability 

 Refering to a pigment's ability to conceal the base color of a substrate. Titanium dioxide (rutile type) is the preferred white pigment for providing excellent coverage due to its high refractive index. Black pigments like carbon black achieve opacity through intense Matting. Coverage testing typically involves observing a pigment's performance on black-white checkered grids under specific concentrations and film thicknesses. The coverage curve can be clearly observed on the backside of sprayed samples (as shown in the figure).

b. weather resistance 

This refers to the pigment's ability to resist environmental factors such as sunlight (especially ultraviolet rays), high temperatures, and humidity, preventing issues like powdering or fading. For outdoor applications like architectural aluminum profiles and automotive components, it is essential to select weather-resistant inorganic pigments or specially treated organic pigments (the most commonly used today being resin-coated materials).

c. heat resistance 

Paints require curing at high temperatures (typically 180-200℃), with some applications enduring prolonged exposure to elevated heat. Therefore, pigments must maintain stability under both curing and application temperatures without decomposition or discoloration. Many organic pigments demonstrate poor heat resistance, necessitating careful selection during formulation. For outdoor coatings, choosing the right pigment is crucial for achieving optimal performance.

d. dispersity 

The dispersion efficiency and stability of pigments in resin matrices are critical factors. Poorly dispersed pigments are prone to flocculation, which affects coating gloss and color uniformity while potentially causing issues like floating color and mottling. The dispersion process comprises three key stages: wetting, depolymerization, and stabilization. During production, pigment properties vary with different stirring durations. Therefore, the formulation containing titanium dioxide combined with 2-3 types of pigments demonstrates optimal stability throughout manufacturing processes.

e. resistance to solvent 

Refers to the pigment's ability to resist dissolution by solvents or chemicals in a resin system. Pigsments with poor solvent resistance may exhibit "color seepage" -where pigments migrate into adjacent areas and contaminate other colors. (Generally, alcohol should be repeatedly wiped on the surface N times to observe the results.)

f. oil absorption 

The amount of oil absorbed per 100 grams of pigment to achieve a specific consistency (usually linseed oil). Oil absorption amount indirectly reflects the specific surface area and particle size distribution of the pigment. Pigments with high oil absorption usually require more resin to moisten, which can affect formulation costs and paint leveling.

g. weather resistance 

The weather resistance of pigment is directly related to the service life of coating in outdoor environment. The pigment with poor weather resistance is easy to fade, powder or lose luster, affecting the decorative effect and protective performance.

Finally, cost-effectiveness is also a factor that cannot be ignored on the premise of meeting all performance requirements. Through formula optimization, cost can be controlled while ensuring quality. According to my years of field production experience, rational use of paint can not only save cost, but also reflect the value of technicians.

2.2 Classification and selection of pigments

Paint pigments are mainly divided into inorganic pigments and organic pigments, in addition to metal pigments and pearlescent pigments and other special effect pigments. The following table summarizes the main types and characteristics:

Table: Common pigment types and their application characteristics (reference)

2.3 Principles of pigment selection

When selecting pigments, a comprehensive performance evaluation and rigorous testing and verification are required:

 Color and opacity: must meet the design requirements of the final product.

 Weather resistance, heat resistance, light resistance and chemical resistance: it should match the application scenario of the coating.

 For example: outdoor vehicle parts should be selected with excellent weather resistance high performance inorganic pigments or special organic pigments;

 Home furnishings can be more cost and color intensity.

In addition, the compatibility between pigment and resin system is also very important. Poor compatibility will lead to coating defects such as floating color, flowering and fibrillation.

2.4 Method of using pigment

In powder coating production, the method of introducing pigments into the system and achieving uniform dispersion directly determines the final coating's color, appearance, and performance. Depending on the pigment type (particularly those containing shear-sensitive effects) and desired coating characteristics, three primary methods are employed: dry mixing, bonding process, and melt extrusion. I will not elaborate on the specific technical details of these processes here.

2.5 Key control points of production process

Powder coating production is a complex process, to obtain uniform color, stable performance products,

The state of the pigment must be carefully controlled at every stage. From pre-mixing to final packaging, each step has its key process control points.

2.5.1 Pre-mixing: The starting point for homogeneity

Pre-mixing is the first step to ensure uniform distribution of all formula components, especially trace pigments and additives. If the pre-mixing is uneven, subsequent extrusion and grinding processes cannot compensate for it, ultimately resulting in color deviations, uneven performance, or even surface defects in the product.

l Equipment selection: high speed mixer is usually used.

l key parameter ：

 Mixing time: too short time will not mix evenly, too long time may cause excessive temperature rise of material or lead to different components with different densities are separated. The best time should be determined by experiment.

 Material feeding order: generally put resin, filler and other large materials first, then put pigment, additives and other small materials, which is helpful to improve the mixing efficiency and quality.

2.5.2 Melt extrusion: a dispersed core

Melt extrusion is the most critical step in pigment dispersion. In this process, pigment aggregates and agglomerates are dispersed by mechanical shear force, and are moistened and coated by molten resin to form a stable dispersion.

l equipment selection 

Double screw extruders have become the mainstream choice because of their excellent mixing and self-cleaning capabilities.

l key parameter 

 Temperature control: The temperature of each heating zone must be controlled accurately. If the temperature is too low, the resin will not melt completely and the pigment can not be dispersed effectively; if the temperature is too high, the resin pre-crosslinking or the pigment degradation may occur, affecting the flow and color.

Screw speed: The speed directly affects the shear force and material residence time. High speed is conducive to dispersion, but may cause the material residence time to be too short; low speed may not be enough for dispersion. The best balance should be found.

Feeding speed: it should match the screw speed to maintain a stable material filling rate and ensure the stability of extrusion and uniform dispersion effect.

2.5.3 Crushing and classification: control of particle size distribution

After extrusion and tablet pressing, the powder should be crushed to a suitable particle size. The particle size and its distribution directly affect the storage stability of the powder, construction performance (such as electrostatic powder rate, fluidization effect) and coating appearance (such as texture and gloss).

 Crushing equipment: commonly used air classification mill (ACM mill).

 key parameter ：

 Crusher speed: affects the particle size of powder. The higher the speed, the finer the powder is usually.

 Classifier rotation speed: precise control of the final particle size distribution (PSD) of powder. The higher the classifier rotations, the more fine powder is separated, the finer the particle size of the product.

Target particle size: The particle size of powder coating is usually controlled within the range of D50 30-40 μm. Ultrafine powder (<10 μm) content is too high, the powder is easy to agglomerate, poor fluidization, easy to fly ash when spraying; too much coarse particles will affect the smoothness of the coating film, resulting in severe orange peel.

2.5.4 Post-mixing and bonding: key processing of effect pigments

For effect pigment, the subsequent bonding process is the key to the success of the effect.

Bonding temperature: This is the most core parameter. The temperature must be precisely controlled in a narrow window slightly higher than the Tg of the base powder resin (e.g., 50-60℃). If the temperature is too low, the bonding will not be firm; if the temperature is too high, the material will be lumpy and scrapped.

Mixed time: work in conjunction with temperature to ensure that the effect pigment is fully and firmly bonded to the surface of the base powder particles.

Inert gas protection: For oxidizable metal pigments (such as aluminum powder), the bonding process is usually carried out under nitrogen protection.

III. FAQ and Solutions

In the production and application of powder coatings, due to pigment selection, dispersion process or construction conditions and other factors, you may encounter various problems. The following is a common problem analysis and solution.

3.1 Floc and blooming

3.1.1 Problem description

Uneven spots or stripes appear on the coating surface, and the color is inconsistent.

3.1.2 Causes

The density and particle size of different pigments in the formula are quite different, and they are separated due to different movement speed during the film formation process.

The pigment dispersion is unstable and flocculation occurs.

During curing, the surface tension of the paint is uneven, causing Benard cells, which carry the pigment to the surface.

3.1.3 Solutions

Optimize the formula and select pigments with similar density and particle size.

Add appropriate wetting dispersant to improve pigment dispersion stability and prevent flocculation.

Adjust leveling agent and thixotropic agent to improve the leveling of coating during melting and suppress Bernard vortex.

3.2 Uneven effect of metal pigment

3.2.1 Problem description

The metal powder coating appears color difference, stripe or "snowflake" pattern, and the metal feel is not uniform.

3.2.2 Causes

In the dry mixing process, the metal pigment is separated from the base powder.

The bonding process is not good, and the metal pigment is not firmly bonded.

Improper construction parameters (such as voltage, powder output) lead to uneven electrostatic adsorption.

Uneven film thickness or improper curing conditions affect the orientation of metal pigment.

3.2.3 Solutions

Bonding process is preferred for producing metal powder coatings.

Optimize the bonding process parameters (temperature and time) to ensure the bonding effect.

Adjust the construction parameters. It is recommended to use the corona spray gun and set a lower static voltage and powder output.

Ensure uniform film thickness and stable curing conditions.

3.3 Poor pigment dispersion

3.3.1 Problem description

The coating has low gloss, dark color, insufficient coloring and covering power, and the surface may be rough.

3.3.2 Causes

Extrusion process parameters (temperature, speed) are not proper, pigment is not fully dispersed.

The pigment itself has poor dispersion or easy flocculation.

The resin content in the formula is too low or the wettability is poor, which can not fully cover the pigment.

3.3.3 Solutions

Optimize extruder parameters to ensure sufficient shear force and appropriate temperature.

For difficult to disperse pigments, choose high efficiency dispersant or super dispersant.

Adjust the formula to ensure sufficient resin content or select a resin with better wettability.

3.4 Insufficient coverage

3.4.1 Problem description

The coating does not completely cover the color or pattern of the substrate.

3.4.2 Causes

Insufficient pigment addition.

The pigment itself has poor hiding ability (such as some organic pigments).

The particle size distribution is not proper, or the pigment flocculation occurs, resulting in a decrease in the number of effective covering particles.

3.4.3 Solutions

Appropriately increase the amount of high coverage pigment (such as titanium dioxide).

Choose a pigment variety with stronger covering power.

Improve the dispersion process to ensure that pigments are fully utilized in primary particles.

3.5 Construction problems: Blockage of gun or poor rate of powder application

3.5.1 Problem description

Powder clogs the nozzle during spraying, or powder can not be effectively adsorbed to the workpiece.

3.5.2 Causes

The particle size of the powder is too fine or the distribution is too wide, and the superfine powder is easy to accumulate.

The electric charge characteristics of metal pigment and base powder are inconsistent (easy to occur in dry mixing method).

The powder is damp or lumps.

3.5.3 Solutions

Control the particle size of powder and reduce the content of ultrafine powder (<10 μm).

Bonding process is adopted to improve the charge consistency of metal pigment.

Strengthen packaging and storage management to prevent powder from absorbing moisture.

IV. CONCLUSION

The production of powder coatings is a complex process that integrates material science, manufacturing techniques, and practical experience. The selection and application of pigments are particularly critical in determining product success. From carefully selecting suitable pigment varieties to implementing appropriate processing methods (such as melt extrusion, dry mixing, or bonding) for their incorporation into the system and achieving uniform, stable dispersion, every step requires meticulous control.

Mastering these key principles and being capable of swiftly identifying, analyzing, and resolving emerging issues forms the foundation for producing high-quality, value-added powder coatings. With the maturation and widespread adoption of advanced technologies like powder bonding, the visual appeal and application scope of these coatings have been significantly enhanced. This advancement enables them to meet environmental standards while competing with traditional liquid-based coatings in the decorative market.