I.Definition of plastic color matching:
Definition:Color matching refers to selecting three basic hues – red, yellow and blue (the primary colors), that produce pleasing results: they meet requirements set out on color cards for differences of shade between hues; match customer desired hues as closely as possible while still remaining economically feasible and unlikely to change during processing or use.
Plastic coloring provides plastics with various functions, including improving light resistance and weather resistance; providing antistatic properties; imparting special functions; protecting certain special materials like conductivity or antistatic properties; or simply serving agricultural film functions of weeding out insects for seedling cultivation purposes – to name but a few examples of their many applications! Thus color matching and coloring help achieve specific requirements as part of an application solution.
II. Colorant:
pigments and dyes:
Pigments: Pigments are insoluble colorants that cannot be dispersed using conventional solvents, making their distribution throughout a plastic structure all the more crucial for optimal coloring properties. In order to get maximum effects from pigment use mechanical heating and mixing processes as this allows pigments’ color distribution throughout its structure evenly.
Classification of pigments: Pigments can be divided into organic and inorganic categories depending upon their structures.
Inorganic pigments offer numerous advantages: good thermal stability, outstanding weather resistance, long term light stability at an economical price point and superior disperability (for instance: titanium dioxide or carbon black).
Titanium dioxide series: There are three primary forms of titanium dioxide: titanium dioxide, zinc oxide and lithopone. Titanium dioxide comes in both rutile and anatase structures for maximum hide power, stability and weather resistance. If left undispersed prior to leaving its factory of origin it would produce numerous black spots during color matching processes unless processed further with dispersant. Certain manufacturers’ grades enter the market processed so these materials may be directly used; typically used to cover transmittance resin transmission and increase whiteness.
Carbon black: Carbon black pigments are popular black hues due to its inexpensive price point and strong antioxidant and electrical conductivity properties on plastic materials. Different production processes produce various varieties of carbon black with different particle sizes and properties; their uses also vary widely; it can be divided into pigmented carbon black or rubber reinforcing carbon black depending on how it will be put to use. Pigmented carbon black can be classified into high, medium and low pigmentation levels based on its coloring ability. Due to carbon black particles’ tendency for aggregation during processing, adding dispersant is recommended in order to enhance coloring power of carbon black and increase coloring power before final processing begins.
Disadvantages:
Relatively poor color strength, high relative density, large amount of addition, and not bright colors;
Mainly used for: increasing color density (generally all kinds of resins can be used according to the needs of the color), especially in engineering plastics, such as PA, PC, PBT, POM, PPO, PPS and other materials as well as non-transparent colors, gray and other resins.
Organic Pigments:
Advantages: high coloring power, bright color, complete chromatogram, low relative density, and small addition amount.
Disadvantages: Heat resistance, weather resistance, and hiding power are not as good as inorganic pigments, dispersion effect is poor, transparency is relatively poor compared to dyes, and the price is higher.
Mainly used in: POM, PE, PP, TPU, TPE and other crystalline materials and some bright (dye brightness cannot achieve bright color products) products.
Dyes:
Advantages: Dyes are organic compounds that can be used in most solvents and dyed plastics, with advantages of low density, high color strength, and good transparency.
Disadvantages: Organic pigments tend to have small molecular structures and therefore migrate when coloring (crystalline materials). Their price tends to be more costly than inorganic ones and some prices even approach those of organic ones.
Pearl luster pigment, more commonly called mica titanium pearl luster pigment, is composed of titanium dioxide coated mica crystal. Based on hue, it can be divided into three different categories: silver-white pearlescent pigment, rainbow pearlescent pigment and color pearlescent pigment.
Attention points:
When purchasing pigments, it is important to understand the C.I. index of pigments. The C.I. is an international compilation of dye and pigment varieties co-published by the British Dyers and Colourists Association and the American Association of Textile Chemists and Colourists. Each pigment is assigned two numbers based on application and chemical structure categories to avoid misunderstandings when purchasing pigments with the same molecular structure but different names. It is also beneficial for color management during use and for using the correct pigments to facilitate troubleshooting in case of problems.
III. Color matching and coloring process:
Color matching can be achieved by directly adding toner to resin, extruding toner mixed with resin, and using color masterbatch methods.
Toner directly added to resin method:
After the toner is directly mixed with plastic resin, it is sent to the next product molding process, which has short process and low cost, but poor working environment, poor coloring power, and poor coloring uniformity and quality stability.
Toner mixing resin extrusion method:
This method is the most effective for the uniformity of resins and colors, allowing the pigments to be fully dispersed in the resin, resulting in accurate colors, clean quality, and easy processing.
Color masterbatch method:
Masterbatch is a granular material comprised of colorant, carrier resin and dispersant in various concentrations; during molding processes it may also contain dispersants for increased dispersion of colorants and additives. A certain quantity of masterbatch will then be added according to color requirements so that finished products contain sufficient amounts of pigments that meet these specifications.
Color masterbatch can be divided into different categories depending on the resin being colored, such as ABS masterbatch, PC masterbatch and PP masterbatch. Furthermore, injection molding, blown film and extrusion grade masterbatches may all use color masterbatch in some capacity for coloring purposes. Due to pretreatment with pigments, colour masterbatch boasts high coloring power at lower dosage with stable quality that offers ease of transportation storage use that further decreases environmental pollution.
Dispersant can effectively eliminate surface air by wetting and penetrating pigments, dispersing aggregates into fine, uniform particles without creating surface air pockets, as well as not aggregate during processing. Common dispersants include low molecular weight polyethylene waxes. Organic pigments and carbon black that are difficult to disperse require EVA waxes or oxidized polyethylene waxes as dispersant agents, although there can be significant variance between synthetic low molecular weight polyethylene waxes and those made by polyethylene pyrolysis compared with each other. Other additives used as fillers or additives include coupling agents, antioxidants, light stabilizers, antistatic agents and fillers. Their quantity depends on grade requirements or requirements and is known as multifunctional masterbatch. Brighteners may help enhance mold release as well as surface brightness on finished products.
Color masterbatches feature various performance indicators as performance measures; such as color difference, whiteness, yellowness and degree of yellowing as indicators; thermal stability index index melt flow rate rate are among them; fineness migration chemical resistance toxicity pigment fineness migration chemical resistance chemical resistance are related. Furthermore, special indicators like pressure filtration value or fineness (DF value) of fiber masterbatch fineness is particularly vital.
IV.Color matching management and instruments
The hardware for color matching management includes colorimeters and computers that process the measured data. Colorimeters can be divided into spectrophotometers and colorimeters, which replace the human eye in determining color and remove the influence of human factors on the measurement results.
Spectrophotometer is used to determine the reflection coefficient of each wavelength on the completely diffuse reflecting surface, and cannot directly obtain the chromaticity value or color difference. However, by processing the data, it can evaluate the chromaticity value and other various values. Spectrophotometer can be divided into two types: diffraction grating and interference filter. The advanced spectrophotometer with built-in microprocessor has the functions of automatic correction at 0% and 100% and magnification increase, which improves the accuracy.
The colorimeter is a simple testing instrument that makes a filter with spectral characteristics equivalent to the human eye’s color sensitivity, and uses it to measure the light of the sample. The key is to design a filter with spectral sensitivity characteristics of the photoreceptor and capable of measuring color difference values under certain light sources. The colorimeter is small in size, easy to operate, and suitable for batch management of the same product with small changes in spectral characteristics. The colorimeter with a small microcomputer is easy to calibrate using standard samples and output multiple color difference values.
Color matching management software includes spectral reflectance curve, color difference formula, conditional isochromatic representation, opacity representation, and haze representation. The spectral reflectance curve is used for analysis when selecting colorants and cannot be used to judge color consistency. Color difference is one of the most important indicators in color management, but different color difference formulas produce different color differences, so it is necessary to specify the chromaticity system or color difference formula used.
V. Computer color matching:
The use of computers for color mixing formulas and management has been successfully applied to plastic color matching.
The computer color matching instrument has the following functions:
(1) Color matching: Create a database of commonly used pigments (dyes) according to requirements (prepare a basic color palette and enter it). Then, under the software menu, enter the incoming color palette into the computer, click on several candidate pigments on the keyboard, and instantly calculate a series of formulas, which are listed according to color difference and price for color matching options;
(2) Formula correction: The computer lists the formula and other sources of formula. When the color difference is not acceptable, the inconsistent reflection curve displayed on the monitor is used to directly increase or decrease the amount of pigment through the keyboard until the two curves are basically coincident, resulting in a corrected formula;
(3) Color measurement and color difference control measure the coloring strength of colorants, whiteness of products, color fastness of products, and color difference. Since computers can quantitatively express the performance indicators of color, it is beneficial for information communication and transfer between both parties;
(4) Information such as color swatches, formulas, process conditions, production dates, and users in daily color management work can be stored in a computer for easy retrieval, search, and reference during modifications, which is convenient, fast, and efficient, and also facilitates confidentiality;
