Basic knowledge of color matching and process of plastic bottles

Color matching is to match the three basic colors of red, yellow and blue (three primary colors) to produce a color that is pleasing, meets the color difference requirements of the color card, meets the color that customers want, is economical and does not change color during processing and use.

01 Colorant

Pigments and dyes
Pigment characteristics: Pigments are colorants that cannot be dissolved in ordinary solvents. Therefore, to obtain ideal coloring properties, it is necessary to use mechanical heating and mixing methods to evenly disperse the pigments in the plastic.
Pigment classification: According to the structure, pigments can be divided into organic pigments and inorganic pigments.

Inorganic pigments:
Advantages: good thermal stability, very good weather resistance, excellent light stability, low price, and excellent dispersion performance. For example: titanium dioxide, carbon black, etc.

Titanium dioxide series: There are mainly three types of titanium dioxide, zinc oxide, and zinc barium white. Titanium dioxide has two structures: rutile type and anatase type. Rutile titanium dioxide has a high refractive index, high hiding power, stability, and good weather resistance. If titanium dioxide is not dispersed when it leaves the factory, a large number of black spots will be generated during the color matching process. It is in a state of dispersion that is not open, so it needs to be dispersed with a dispersant before it can be used. Some manufacturers’ brands enter the market after processing, and such materials can be used directly. It is mainly used to cover the transmittance of resin and increase the whiteness.

Carbon black: It is a commonly used black pigment with a low price. It also has ultraviolet protection (anti-aging) and conductive effects on plastics. Different production processes can produce a variety of different carbon blacks with a wide range of particle sizes, and the properties are also very different. Carbon black is divided into pigment carbon black and rubber reinforcement carbon black according to its use. Pigment carbon black is 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 aggregation. To improve the coloring power of carbon black and solve the dispersibility of carbon black, it is necessary to add a dispersant for processing before it can be used.

Organic pigments:
Advantages: high coloring power, bright color, complete color spectrum, low relative density, and small addition amount.
Disadvantages: Not as good as inorganic pigments in terms of heat resistance, weather resistance and hiding power, poor dispersion effect, poor transparency compared to dyes, and high price.

Dyes:
Advantages: Dyes are organic compounds that can be used in most solvents and dyed plastics, with advantages of low density, high tinting power and good transparency.
Disadvantages: However, its general molecular structure is small, and it is easy to migrate during coloring (crystalline material). The price is more expensive than inorganic pigments, and some prices are close to those of organic pigments.

Pearlescent pigments
Also called mica titanium pearlescent pigments, are mica wafers coated with titanium dioxide. According to different hues, they can be divided into three categories: silver-white pearlescent pigments, rainbow pearlescent pigments, and color pearlescent pigments.

Notes
When purchasing pigments, you must understand the pigment’s dye index (C.I). C.I is an international compilation of dyes and pigments compiled and published by the British Dyers’ Association and the American Textile Chemists and Dyers’ Association. Each pigment has two numbers according to its application and chemical structure. This avoids misunderstandings when purchasing pigments with the same molecular structure but different names. It is also beneficial for color management during use and using the correct pigment to find the cause when problems occur.

02 Color matching and coloring process

Color matching and coloring can be done by directly adding color powder to resin, mixing color powder with resin and extruding, and masterbatch method.

Direct adding color powder to resin method:
After the color powder is directly mixed with the plastic resin, it is sent to the next step of product molding process. The process is short and the cost is low, but the working environment is poor, the coloring power is poor, and the coloring uniformity and quality stability are poor.

Color powder mixed with resin extrusion method:
This method has the best effect on the uniformity of resin and color, and can make the pigment fully dispersed in the resin, with accurate color, clean quality and easy processing.

Masterbatch method:
It is a granular material with a certain concentration of colorant prepared by colorant and carrier resin, dispersant and other additives. When the product is molded, a certain amount of masterbatch is added according to the coloring requirements, so that the product contains the required amount of colorant to meet the coloring requirements.

Masterbatch can be classified according to the resin to be colored, such as ABS masterbatch, PC masterbatch, PP masterbatch, etc.; it can also be classified according to the coloring resin plus process, and there are three major types of masterbatch: injection molding, blown film, and extrusion grade. Masterbatch has higher tinting power due to the pretreatment of pigments. The dosage can be reduced and the quality is stable. It is easy to transport, store and use, and the environmental pollution is greatly reduced.

Dispersants remove surface air by wetting and penetrating the pigments, dispersing the agglomerates and aggregates into fine, stable and uniform particles, and no longer agglomerate during the processing. Commonly used dispersants are 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 additives include coupling agents, antioxidants, light stabilizers, antistatic agents, fillers, etc. The amount added depends on the requirements and varieties. It is called multifunctional masterbatch. For example, adding brighteners is conducive to the demolding of molded products and improving the surface brightness of products.

The performance indicators of masterbatch include color difference, whiteness, yellowness, yellowing degree, thermal stability, oxygen index, melt flow rate, etc. Of course, the fineness, migration, chemical resistance and toxicity of pigments are also related to the performance of masterbatch. Some indicators are very important in special applications, such as the filter press value (DF value) fineness of fiber-grade masterbatch.

03 Color matching management

The hardware of color matching management includes colorimeter and computer for processing measured data. Colorimeter can be divided into spectrophotometer and colorimeter, which replaces human eyes to measure color and eliminates the influence of human factors on the measurement results.
Spectrophotometer is used to measure the reflection coefficient of each wavelength on the completely diffuse reflection surface. It cannot directly obtain the color value or color difference, but the color value and other various values ​​can be evaluated through its data processing. Spectrophotometer can be divided into two types: diffraction grating spectroscopy and interference filter spectroscopy. Advanced spectrophotometer with built-in microprocessor has functions such as 0%, 100% automatic correction and magnification increase, thereby improving accuracy.

A colorimeter is a simple test instrument, that is, a filter with spectral characteristics equal to the color sensitivity of the human eye is made, and it is used to measure the light of the sample. The key is to design a filter with the spectral sensitivity characteristics of the photoreceptor and can measure the color difference value under a certain light source. The colorimeter is small in size and easy to operate. It is more suitable for batch management of the same product with small spectral characteristics. A colorimeter with a small microcomputer can easily be calibrated and output multiple color difference values ​​with a standard sample.

Color matching management software includes spectral reflection 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 the consistency of color. Color difference value is one of the most important indicators in color management, but the color difference obtained by different color difference formulas is different, so the color system or color difference formula used must be indicated.

04 Computer color matching

Color matching instrument
The use of computers for color matching formulas and their management has been successfully used in plastic color matching.
The computer color matching instrument has the following functions:
1) Color matching
Establish a database of commonly used color (dye) materials according to requirements (prepare basic color plates and input). Then input the incoming material color plates into the computer under the software menu, click on several candidate pigments on the keyboard, and immediately calculate a series of formulas, and sort them by color difference and price for color matching selection;
2) Formula correction
Correct the formula listed by the computer and other source formulas. When the color difference is unqualified, use the inconsistent reflection curve displayed on the monitor to directly increase or decrease the amount of pigment through the keyboard until the two curves basically overlap, and obtain the corrected formula;
3) Color measurement and color difference control
Measure the color strength of the colorant, the whiteness of the product, the color fastness of the product, and the color difference. Since the computer can quantitatively express the performance indicators of color, it is conducive to information communication and transmission between the two parties;
4) Color management
The color samples, formulas, process conditions, production dates, users and other information in daily work can be stored in the computer for easy retrieval, search and reference when modifying. It is convenient and fast, improves work efficiency, and is easy to keep confidential.