How much do you know about color matching and coloring of plastic?

Time:2021-11-26 11:45:41 / Popularity: / Source:

1. Definition of plastic color matching:

Definition: Color matching is based on three basic colors of red, yellow, and blue (three primary colors), to match color that is pleasing, meets color difference requirements of color card, and meets color desired by customer, is economical, does not change color during processing and use .
In addition, plastic coloring can also give plastics many functions, such as improving light resistance and weather resistance of plastics; giving plastics some special functions, such as electrical conductivity, antistatic properties, antibacterial properties and other special materials; different colored agricultural mulches have functions of weeding or preventing insects and raising seedlings. That is, through color matching and coloring, certain application requirements can be met.
plastic color matching 

2. Coloring agent:

Pigments and dyes

Pigment characteristics: Pigments are colorants that cannot be dissolved in common solvents. Therefore, to obtain ideal coloring properties, it is necessary to uniformly disperse pigments in plastic by methods such as mechanical heating and kneading.
Pigment classification: According to structure of pigments, it can be divided into organic pigments and inorganic pigments.

Inorganic pigments:

Advantages: good thermal stability, very good weather resistance, excellent light stability, low price, superior dispersion performance. For example: titanium dioxide, carbon black, etc.
Titanium dioxide series: mainly include titanium dioxide, zinc oxide, and lithopone. Titanium dioxide has two structures: rutile and anatase. Rutile titanium dioxide has high refractive index, high hiding power, stability and good weather resistance. If titanium dioxide is not dispersed when it leaves factory, a large number of black spots will be produced during color matching process. It is in the state of dispersing that is not open, so it can be used after dispersing processing with a dispersant. Some manufacturers’ brands are processed after processing. When entering market, such materials can be used directly. It is mainly used to cover transmittance of resin and increase whiteness.
Carbon black: It is a commonly used black pigment with low price. In addition, it has UV protection (anti-aging) effect and conductive effect on plastics. Different production processes can obtain a variety of different carbon blacks with a wide range of particle sizes, and their properties vary greatly. Carbon black is classified into pigment carbon black and rubber reinforcement carbon black according to its purpose. Pigment carbon black is further divided into high pigment carbon black, medium pigment carbon black and low pigment carbon black according to its coloring ability. Carbon black particles are prone to agglomeration. To improve coloring power of carbon black and solve dispersibility of carbon black, it is necessary to add a dispersant for processing before it can be used.
Disadvantages: relatively poor coloring power, high relative density, large amount of addition, and color is not bright;
Mainly used: used to increase color density (generally all kinds of resins are available according to color needs), especially for engineering plastics with good superior performance, such as: PA, PC, PBT, POM, PPO, PPS and other materials and non-transparent colors, gray and other resins.

Organic pigments:

Advantages: high tinting strength, bright color, complete chromatogram, low relative density, small addition amount.
Disadvantages: Inferior to inorganic pigments in terms of heat resistance, weather resistance and hiding power, poor dispersion effect, poor transparency relative to dyes, and higher prices.
Mainly used in: POM, PE, PP, TPU, TPE and other crystalline materials and some bright products (bright color products that cannot be achieved by dyes).
Advantages: Dyes are organic compounds that can be used in most solvents and dyed plastics. Advantages are low density, high coloring power, and good transparency.
Disadvantages: But its general molecular structure is small, and it is easy to migrate during coloring (crystalline material). Price is more expensive than inorganic, and price of some is close to that of organic pigments.

Pearlescent Pigment:

Also called Mica Titanium Pearlescent Pigment, it is a kind of mica wafer coated with titanium dioxide. According to different hue, it can be divided into three types: silver-white pearlescent pigments, rainbow pearlescent pigments, and colored pearlescent pigments.

Points to note:

When purchasing pigments, you must understand dye index (CI) of pigment. CI is an international collection of dyes and pigments compiled and published by British Dyeers Association, American Textile Chemists and Dyeers Association. Each type of pigment is based on application and chemistry. Structure category has two numbers to avoid misunderstanding of same molecular structure and different names when purchasing, it is also conducive to color management and use of correct pigments to find cause when problems occur.

3. Color matching and coloring process:

Color matching and coloring can adopt method of adding toner directly to resin, extrusion method of toner mixed resin and masterbatch method.

Toner is directly added to resin method:

After toner is directly mixed with plastic resin, it is sent to next step of product forming process, which has a short process and low cost, but working environment is poor, coloring power is poor, color uniformity and quality stability are poor.

Toner mixed resin extrusion method:

This method has the best effect on uniformity of resin and color. It can fully disperse pigment in resin, color is accurate, quality is clean, and it is easy to process.

Masterbatch method:

It is colorant, carrier resin, dispersant, and other auxiliary agents to formulate a certain concentration of colorant pellets. When product is formed, a certain amount of masterbatch is added according to coloring requirements to make product contain required amount of coloring to meet coloring requirements.
Color masterbatch can be classified according to resin to be colored, such as ABS masterbatch, PC masterbatch, PP masterbatch, etc.; it can also be classified according to coloring resin addition process. There are three types of masterbatch for injection molding, blown film, and extrusion. . Color masterbatch has high tinting strength due to pretreatment of pigment first, dosage can be reduced, quality is stable, transportation, storage, and use are convenient, environmental pollution is greatly reduced.
Dispersant removes surface air by wetting and penetrating pigment, disperses aggregates and agglomerates into fine, stable and uniform particles, and no longer aggregates during processing. Commonly used dispersant is low molecular weight polyethylene wax. For organic pigments and carbon black that are difficult to disperse, EVA wax or oxidized polyethylene wax is used. There is a big difference between synthetic low-molecular-weight polyethylene wax and low-molecular-weight polyethylene wax made by polyethylene cracking. Other auxiliaries include coupling agents, antioxidants, light stabilizers, antistatic agents, fillers, etc., depending on requirements and varieties, called multi-functional masterbatch. If brightener is added, it is beneficial to demolding molded products and improving surface brightness of products.
Performance indicators of color masterbatch include color difference, whiteness, yellowness, yellowness, thermal stability, oxygen index, melt flow rate, etc. Of course, fineness, migration, chemical resistance, and toxicity of pigment are also related to color masterbatch. Particle performance is related, and some indicators are very important in special applications, such as filter pressure value (DF value) fineness of fiber-grade masterbatch.

4. Color matching management and instruments

Hardware for color matching management includes a color meter and a computer for processing measured data. Colorimeters can be divided into two types: spectrophotometer and colorimeter, which can replace human eye to measure color and remove influence of human factors on measurement results.
Spectrophotometer is used to measure reflection coefficient of each wavelength to completely diffuse reflection surface. Chromaticity value or chromatic aberration cannot be obtained directly, but chromaticity value and various other values can be evaluated through data processing. Spectrophotometers can be divided into two types: diffraction gratings and interference filters. Advanced spectrophotometer with built-in microprocessor has functions of 0%, 100% automatic correction and magnification increase, thereby improving accuracy.
Color difference meter is a simple test instrument, that is, to make a filter with spectral characteristics equal to color sensitivity of human eye, use it to measure light of sample. The key is to design a photoreceptor with spectral sensitivity characteristics and can measure color difference value under a certain light source. Color difference meter is small in size and easy to operate. It is more suitable for batch management of same product with small changes in spectroscopic characteristics. Color difference meter with a small microcomputer is easy to calibrate and output multiple color difference values with standard models.
Color matching management software has spectroscopic inversion rate curve, color difference formula, conditional color representation, hiding power representation and haze representation. Spectral reflectance curve is used for analysis when selecting colorants, and cannot be used to judge consistency of colors. Color difference value is one of the most important indicators in color management, but color difference obtained by different color difference formulas is different, so chromaticity system or color difference formula used must be indicated.

5. Computer color matching:

Computer toning formula and its management have been successfully used in plastic color matching.
Computer color matching instrument has following functions:
(1) Color matching. According to requirements, establish a database of commonly used colors (dyeing) materials (preparation and input of basic color palettes). Then enter incoming color palette into computer under software menu, click on a few candidate pigments in keyboard, immediately calculate a series of formulas, list them in order of color difference and price for color matching;
(2) Formula correction. Computer lists formulas and formulas from other sources. When color difference is unqualified, inconsistent reflection curve displayed on display is used to directly increase or decrease amount of pigment through keyboard until two curves basically overlap, and revised formula is obtained;
(3) Color measurement and color difference control measure color strength of colorant, whiteness of product, color fastness of product, and color difference. Since computer can quantitatively express color performance index, it is conducive to communication and transmission of information between two parties;
(4) Color sample, formula, process conditions, production date and user information in daily work of color management can be stored in computer, which is convenient for retrieval, search and reference when modifying, which is convenient and fast, improves work efficiency, facilitates confidentiality.

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