Pigments for Plastics - Plastics Pigments Uses in 2024

1. Organic pigmentsfor plastics

Organic pigments are made from organic compounds. They have good transparency and color stability. For plastic products, they are for dyeing and color enhancement, such as transparent plastic cups and household appliance housings. Common organic pigments include: azo pigments, phthalocyanine pigments, anthraquinone pigments, carbon black, etc.

2. Inorganic pigmentsfor plastics

Inorganic pigments are made from inorganic compounds. They have high covering power and light resistance. They are usually brighter than organic pigments, but relatively transparent. For plastic products, they are for dyeing and light resistance enhancement, such as flower pots and bicycle helmets. Common inorganic pigments include iron oxide pigments, acid and alkaline pigments, titanium dioxide, etc.

3. Metallic pigments for plastics

Metallic pigments are usually made from metal sheets or foils. They have a distinctive soft luster and strong metallic feel. For plastic products, they are for a metallic feel, such as car bodies and mobile phone shells. Common metallic pigments include aluminum powder, bronze powder, copper powder, etc.

4.Pearlescent pigmentsfor plastics

Pearlescent pigments are made of mica, silica and the like. They have the unique pearlescent effect and good transparency. For plastic products, they are for a pearlescent sense, such as colored hard plastics and key chains. Common pearlescent pigments include aluminum silver pearlescent pigments, golden red pearlescent pigments, etc.

In short, appropriate pigments for plastic products should be selected by the variety, performance, application, color and other factors. Only some pigments are introduced here for plastic products. Other pigments are also optional.

I. Dry coloring

Raw plastic materials are colored by directly mixing with plastic pigment powder (pigment or dye) and an appropriate amount of additive powder. This is called dry coloring.

Dry coloring has the advantages of good dispersion and low cost. The amount can be determined as needed to facilitate preparation. Dry coloring involves no labor or materials in the use of colorant, color paste and others, so it features low cost and no volume restriction to the purchaser and seller. Its shortcomings include dust contamination in transportation, storage, weighing and mixing, affecting the working environment and operators’ health.

II. Coloring with paste colorant (color paste)
In the paste coloring method, the colorant and liquid additive (plasticizer or resin) are usually mixed and ground into paste. Then the paste is mixed evenly with plastic, such as vinyl paste and paint.
Advantages of coloring with paste colorant (color paste) is good dispersion, without dust contamination. Its shortcomings include difficulties in colorant consumption and high cost.

III. Coloring with color masterbatch
For the color masterbatch, the pigment of corresponding color is first prepared, and then mixed with the masterbatch carrier by proportion in the formula. Then, heating, plasticizing, stirring and shearing are carried out through the granulator. Finally, pigment particles are fully combined with carrier resin particles, to form the particles equivalent to resin particles in size. Then, the particles are used to manufacture plastic products. Only a small amount of color masterbatch (1% to 4%) is required for resin coloring.

Compared with dry coloring, masterbatch coloring has the following apparent advantages: reduction of e environmental pollution caused by flying color powder; each change in color, without special cleaning of the extruder hopper; and stable formula stability, ensuring that two successive lots of color masterbatches of the same grade are relatively stable.

Shortcomings of masterbatch coloring include high costs and inflexible quantity adjustment. If color masterbatch made from pearlescent powder, phosphor, luminous powder and other color powder is used for plastic coloring, the coloring effect will be about 10% lower than that of direct use of plastic colorant. In addition, stream-like stripes and joints are likely to occur on injection molding products.

Plastic color pigment is rarely used in plastic products. Although plastic pigments are used less than fire retardants, they have some impact on the properties of products. Some colorants may adversely affect fire retardants. The impact of plastic pigments on material properties mainly lies in the following six aspects.

1. Impact on electrical properties

Inorganic pigments usually have poor electrical properties. If they are used as colorants for PVC and PE cables, their electrical properties should be considered. Particularly, PVC cables have poor electrical insulation, so pigments have greater impact on them, and colorants with better electrical properties should be used.

2. Impact of metal ions in pigments on thermal oxidizing decomposition of resin

Colorants containing copper, iron, and other metals will greatly facilitate thermal oxidizing of plastics. For examples, PP molecules contain a lot of tertiary carbon atoms, so they are very sensitive to copper ions. They will be quickly decomposed in the presence of copper ions in pigments.

3. Impact on crystallization properties

If plastic pigments are added into plastic products, especially organic pigments, the formation of polymer will be affected in the production process, such as the quantity and size. Pigments have no impact on mechanical properties. But they will increase the shrinkage rate, especially in large containers.

4. Light shielding impact of some plastic pigments

Some plastic pigments can greatly improve the light stability and weather resistance of plastic products. For example, carbon black is not only a main black pigment, but also a light stabilizer. It has good shielding impact on UV rays.

5. Impact on mechanical properties

If plastic pigment particles are large, uneven dispersion will reduce the impact strength. The proportion of plastic pigment particles should be less than 1%. In addition, fine particles should be evenly distributed in products, to reduce the impact on mechanical properties of products.

I. Chemical treatment

Chemical processing methods for pigments include: solvent treatment, water-oil phase inversion, water-gas phase inversion, and inorganic acid treatment.

II. Solvent treatment

This mainly applies to azo pigments. Coarse pigments (powder or paste) are mixed with appropriate organic solvents at a certain temperature for some time, to improve the heat resistance, light resistance, solvent resistance and covering power. The use of solvent depends on the chemical structure of pigment. For example, the molecules of azo pigments contain benzimidone. Crude pigment particles are hard, with low coloring power. If alkaline solvents such as DMF (dimethylformamide) are used, the properties of pigments will be significantly improved.

III. Water-oil phase inversion

Generally, pigments are prepared by drying and grinding of the water-pigment system. The lipophilicity and hydrophobicity of organic pigments are used in the manufacturing process. The pigment particles dispersed in water are stirred at a high speed, and then mixed with the water-insoluble organic polymer (oil phase). Pigment particles are gradually converted from water into oil. Then a small amount of moisture in oil is evaporated to get the oily paste. Color particles are prepared by high-speed stirring and shearing. Through extrusion and phase inversion, pigments have high dispersibility, brightness and coloring power.

IV. Water-gas phase inversion

An inert gas is blown into the pigment dispersed in water. It will be absorbed by the pigment. Or, the pigment will be absorbed onto the surfaces of small bubbles. Then foam will be floating on the liquid surface. Coarse particles will sink to the bottom. Soft pigments can be obtained by separating the floating foam and drying the particles. Through gas inversion with gas, pigments will have higher dispersibility.

V. Inorganic acid treatment

Sulfuric acid is often used in inorganic acid treatment. Inorganic acid treatment is divided into acid dissolution, acid pulping and acid grinding. It is mainly applied to prepare copper phthalocyanine pigments.

VI. Physical treatment

The main physical treatment method is mechanical grinding and shearing.

The plastic pigment powder with too small particles is treated with solvents to further enhance crystallization. The pigments with too large particles should be crushed to reduce the condensation and increase the dispersion.

Depending on the dispersion properties of pigments, it is necessary to sift the powder. Pigments are mechanically screened (80-400 meshes) after grinding or shearing to ensure the consistency of the particle size of color powder, greatly increase the dispersion of plastic pigment powder, and reduce color spots, traces and stripes on plastic products. Broken yarns are reduced and even avoided during the coloring of non-woven fabrics.