Pigments for Plastics - Plastics Pigments Uses in 2024

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What are plastic pigments?

Plastic pigments are industrial products. They are used for coloring plastics, to produce plastic products with specific colors. Plastic pigment should feature good color properties, heat resistance and easy dispersion. In order to increase the value of plastic products, higher requirements are put forward from the beauty only to high stability, performance and safety of colored products. Therefore, plastic colorants should have good properties for plastic products, such as weather resistance, migration resistance, no toxicity, chemical resistance, etc.

Color Pigment for Plastic

Mica Powder

Thermochromic Pigment

Photochromic Pigments

Pearl Powder

Glow in the dark powder

Chameleon Powder

Reflective Pigment

Titanium dioxide Pigment

Colour Pigments for Plastics FAQs

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.

1. Light resistance of plastic pigments

The light resistance of pigments directly affects the fading of plastics. Therefore, the light resistance (fastness) of pigments is an important indicator. In case of poor light resistance, plastics will fade quickly in use. For outdoor plastic pigments, the light resistance should be Level 6 or above and preferably Level 7 or 8. For indoor plastic pigments, the light resistance should be Level 4 or 5.

2. Heat resistance of plastic pigments

The thermal stability of pigments refers to the degree of thermal weight loss, discoloration and fading of pigments at the processing temperature. Inorganic pigments composed of metal oxide and salt have high thermal stability and heat resistance. Organic pigments are changed in molecular structure and decomposed little at a certain temperature. Especially for PP, PA and PET products processed above 280℃, importance should be attached to the heat resistance of pigments. In addition, the heat tolerance time of pigments should be considered, usually 4-10min.

3. Oxidation resistance of plastic pigments

Some organic pigments have macromolecular degradation or other changes and gradually fade after oxidation. This involves oxidation at high processing temperature and oxidation in the presence of strong oxidants (such as chromate in chromium yellow). If color lakes and azo pigments are mixed with chromium yellow, red will gradually fade.

4. Acid and alkali resistance of plastic pigments

Fading of colored plastic products is related to the chemical resistance (acid and alkali resistance and oxidation-reduction resistance) of colorants. For example, molybdenum chromium red is resistant to dilute acid, but sensitive to alkali. Cadmium yellow is not acid-resistant. The two pigments and phenolic resin have strong reduction effects on some colorants. They can greatly affect the heat resistance and weather resistance of colorants, resulting in fading.

1. Dry coloring: Plastic pigment powder is directly mixed with plastics. The pigment is evenly dispersed in plastics by melting. This low-cost method is applicable to melt extrusion, pressure molding and blow molding.

2. Wet coloring: the pigment is processed into powder and mixed with the solvent. Then the colorant is evenly sprayed or coated on plastic surfaces. This method has finer coloring effects and applies to injection molding, film blow molding, etc.

3. Dyeing: The dissolved colorant is mixed with plastics. Then the pigment is infiltrated into plastics by soaking or absorption. This method applies to the processing of amorphous plastics or fibers.

4. Pre-dyeing: The colorant is added into plastic particles before the manufacturing of plastic products. After granulation, heating and fusion molding, evenly colored plastic particles can be produced. They can be directly used in the processing of plastic products. This method applies to large-scale production. It can improve the production efficiency and product quality stability.

In addition to the above common plastic coloring methods, there are some special coloring methods, such as electric pyrolysis coloring and electron beam irradiation coloring. These methods are often adopted under special equipment and process conditions. They are applicable to plastic products with special requirements.

I. Trial mixing

  1. Select the appropriate color powder and formula according to the previous analysis.
  2. Select the soft or rigid plastic and the color powder resistant to medium or high temperature, based on the type of raw materials.
  3. If pigments are selected based on the transparency, determine the content of titanium dioxide and the degree of black and grey. Finally, select the color powder based on the hue, darkness and brightness.

II. Color matching

  1.  Determination of the proportion of titanium dioxide
  2. Determination of the proportion of black powder
  3. Determination of the proportion of main color powder
  4. Determination of the proportion of auxiliary color powder (proportion of fluorescent powder or brightener)

III. Injection molding/fine tuning

Generally, 600g resin is used. According to the proportion in the formula, color powder and resin are evenly mixed. Then, samples are made with the injection molding machine. After the samples are fully cooled down, their colors are compared and analyzed. Depending on the hue, darkness and brightness of sample colors, the amount of color powder is adjusted and repeatedly tested to meet the color requirements (if the color is tested via a computer, data analysis can be performed based on deviations of L, a and b).

A variety of pigments can be used in polypropylene, including organic pigments, inorganic pigments, pigment powders, etc. Appropriate pigments should be selected as needed for color stability and durability.

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