Introduction: The application of plastic products is quite extensive. Plastic products on the market are of various colors and colorful, which are liked by the public, and the plastic coloring can improve the appearance quality and internal performance of plastic products. So how are injection molded products processed for coloring? What factors can cause plastic discoloration? This article shares the relevant content of the plastic coloring method, the content is for the reference of friends:


Plastic coloring

Plastic coloring refers to the process of uniformly mixing and dispersing a certain amount of color material with natural plastic to make it evenly dispersed in the plastic, so that the entire plastic gets a uniform color. According to the use requirements of plastic products and the characteristics of plastics, choosing appropriate coloring methods and colorants can significantly improve the weather resistance, corrosion resistance, electrical properties, optical properties and mechanical properties of plastic products.

Coloring type
There are four types of plastic colorants according to their physical form

①Powdered colorant-toner
②Solid colorant-masterbatch
③ Paste colorant-color paste
④Liquid colorant—color oil

In the coloring of the plastics industry, toners and color masterbatches are relatively widely used. Toner coloring is a method of directly using toner (pigment or dye) to add appropriate powdery additives to color plastic particles, also known as dry coloring.

The role of coloring
① It can increase the variety of plastics, prosper the plastic commodity market, and improve the commodity value and appearance quality of plastic products.

②Improve and enhance the performance of the product: some colorants, especially carbon black, have the functions of anti-ultraviolet radiation, outdoor aging, and sun-shading.

If the surface is colored by painting, dyeing, electroplating, etc., it will form a protective layer on the surface of the plastic product, which can significantly improve the environmental and weather resistance of the product. Especially after coating with those high-performance paints, the environmental adaptability of exposed plastic products is improved by 5-10 times.

③ Endow products with certain functional characteristics, expand the application range of plastic products, and increase the added value of products. This is mainly achieved by painting, electroplating and other methods, so that the products have antistatic, conductive, anti-magnetic and optical properties. In addition, if carbon black, metal powder, etc. are used as colorants, the products will also have certain antistatic properties or conductivity.

④ It has the function of concealing and protecting the contents, which is embodied in the application of plastics in the packaging industry.

Coloring method
① Toner coloring
Toners are roughly divided into three categories: inorganic toners, organic toners, dyes

Inorganic toner: good temperature and weather resistance, but the disadvantages are poor dyeing power and poor dispersion. Mainly for mineral extraction, temperature resistance can reach over 300 degrees, not suitable for transparent and bright colors;

Organic toner: poor temperature resistance, good dispersion, can be used for soft glue. Petrochemical refining;

Dyes: Good temperature resistance, best dispersibility, and high brightness. The disadvantage is that it has high mobility and is not suitable for soft rubber.

Advantages: good dispersibility, suitable for small batch plastic processing, low cost, and saves the manpower and material resources in the processing of color masterbatches and color pastes.

Disadvantages: Blown film and spinning products are not practical. Toner is used for coloring. Since the pigment powder is evenly distributed in the resin to be colored, its dispersibility is better.

②Color masterbatch
The color masterbatch adopts a certain process and corresponding equipment. Under the action of additives, the pigment (or dye) is mixed into the carrier, and through heating, plasticizing, stirring, and shearing, the molecules of the pigment powder and the molecules of the carrier resin are finally made They are fully combined and then made into particles of similar size to the resin particles.

Advantages: improved environmental pollution caused by flying toner, easy color change during use, no special cleaning of the extruder hopper, strong pertinence, simple color matching, and easy quantification. Compared with the batch resin dry dyeing and granulating, the use of color masterbatch can reduce the resin performance aging caused by the secondary processing of plastic products, which is beneficial to the improvement of the service life of plastic products. In the process of processing, the color masterbatch is fully mixed with the carrier resin under the action of additives. When in use, it is placed in the resin to be processed in a certain proportion, and the compatibility is obviously better than the coloring of the toner.

Disadvantages: poor dispersion. Due to the small amount of addition, the processing time of plastic products is short. Due to the limitation of the extruder screw length to diameter ratio, the dispersion of color masterbatch is often not as good as that of toner. High cost, one more manufacturing process, the dyeing cost is higher than toner coloring; the performance requirements of the color masterbatch are too high, when the performance of the carrier in the color masterbatch is different from that of the resin to be colored, the surface of the plastic product often appears undispersed Dots, stains, patterns, so the use of color masterbatch is limited due to its compatibility and dispersion.

③ Color paste
Paste is a semi-finished product made of pigments and fillers dispersed in paint. According to the purpose of classification, it can be divided into paint color paste, tape color paste, paper color paste, textile color paste, mechanical color paste, toy color paste, latex product color paste, etc.

Advantages: easy to use, can be directly used in the processing process for forming and sheet coloring; under normal circumstances, good high temperature resistance, convenient storage, no precipitation; can reduce the dust pollution problem in the use of toner, and can also solve the color masterbatch Insufficient compatibility and dispersibility, the color paste is easy to control the amount of input in use, reduce the color difference, improve the coloring power and color brightness; the environmental protection type meets the requirements of EU REACH and other regulations

Disadvantages: At present, the biggest restriction for the development of color pastes is the production process of domestic color pastes. High-quality color pastes have higher requirements on the process and the quality of raw materials, and their cost will be higher than that of color masterbatches and toners; Sometimes the screw may slip; when stored for a long time, precipitation will occur. It is better to stir before use.

④Color oil
Color oil is a liquid pigment concentrate without resin carrier. At present, color oil is mainly used in preforms in my country.

Advantages: low addition ratio, usually 0.1~0.5% for transparent series, 0.3~3.0% for pearlescent/solid color series; excellent dispersibility, more uniform and consistent colors, more brilliant colors, higher gloss of products; stable colors, reducing coloration Defects: The material only goes through a high temperature process to keep the physical properties of the raw resin unaffected; the best inventory management requires only a small space to complete the storage of color oil, which saves three-quarters of the storage space relative to color masterbatches. Reduce the pressure of overall inventory and working capital; simple color oil adding equipment, low cost, simple and fast color change process, improve the freedom of production scheduling, save personnel time, machinery and energy loss; suitable for all polyolefins (such as PP/ PE), polystyrene resin (PS), PET resin and other engineering resins.

Disadvantages: Antistatic agents cannot be added to the color oil, which causes the product to be dust-proof in the shelf or storage; the shelf life of the color oil products is shorter than that of the color masterbatch products; the color oil requires special color oil addition equipment, namely, color oil metering pump Only then can the color oil be supplied to the injection molding machine to produce the desired color product.

Fading factors
Plastic colored products will fade due to many factors. The fading of plastic colored products is related to the light resistance, oxygen resistance, heat resistance, acid and alkali resistance of the toner, and the characteristics of the resin used. The following is a detailed analysis of the fading factors of plastic coloring:

Light fastness
The light fastness of the colorant directly affects the fading of the product. For outdoor products exposed to strong light, the light fastness (light fastness) level requirement of the colorant used is an important indicator. The light fastness level is poor, and the product will quickly fade during use. The light resistance grade selected for weather-resistant products should not be lower than six grades, preferably seven or eight grades, and indoor products can choose four or five grades.

The light resistance of the carrier resin also has a great influence on the color change, and the molecular structure of the resin changes and fades after being irradiated by ultraviolet rays. Adding light stabilizers such as ultraviolet absorbers to the masterbatch can improve the light resistance of colorants and colored plastic products.

Heat resistance
The thermal stability of a pigment refers to the degree of thermal weight loss, discoloration, and fading of the pigment at the processing temperature. Inorganic pigments are composed of metal oxides and salts, with good thermal stability and high heat resistance. The pigments of organic compounds will undergo molecular structure changes and a small amount of decomposition at a certain temperature. Especially for PP, PA, PET products, the processing temperature is above 280℃. When selecting colorants, one should pay attention to the heat resistance of the pigment, and the heat resistance time of the pigment should be considered on the other hand. The heat resistance time is usually 4-10min. .

Oxidation resistance
Some organic pigments undergo macromolecular degradation or other changes after oxidation and gradually fade. This process is high temperature oxidation during processing, and oxidation when encountering strong oxidants (such as chromate in chrome yellow). After the lake, azo pigment and chrome yellow are mixed, the red color will gradually fade.

Acid and alkali resistance
The fading of colored plastic products is related to the chemical resistance of the colorant (acid and alkali resistance, oxidation-reduction resistance). For example, molybdenum chrome red is resistant to dilute acid, but is sensitive to alkalis, and cadmium yellow is not acid resistant. These two pigments and phenolic resins have a strong reducing effect on certain colorants, which seriously affect the heat resistance and weather resistance of the colorants and cause fading.

For the fading of plastic colored products, it should be selected after comprehensive evaluation of the above-mentioned properties of the required pigments, dyes, surfactants, dispersants, carrier resins and anti-aging additives according to the processing conditions and use requirements of the plastic products.