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Why does PVC color matching products fade?

Why does PVC color matching products fade?

2021-09-07

Plastic PVC colored products will fade under the influence of many factors. The fading of plastic colored products is related to the light resistance, oxygen resistance, heat resistance, acid and alkali resistance of toner and the characteristics of resin used.

The following is a detailed analysis of the fading factors of plastic coloring by Qingdao Sainuo pe wax manufacturer:

1. Light resistance of colorant

The Lightfastness of colorants directly affects the fading of products. For outdoor products exposed to strong light, the lightfastness (lightfastness) grade of colorants used is an important index. Due to poor lightfastness grade, the products will fade quickly in use. The light resistance grade of weather resistant products shall not be lower than grade 6, preferably grade 7 and grade 8, and grade 4 and grade 5 can be selected for indoor products.

The light resistance of the carrier resin also has a great influence on the color change. The molecular structure of the resin caused by ultraviolet radiation changes and fades. Adding UV absorber and other light stabilizers to masterbatch can improve the light resistance of colorants and colored plastic products.

2. Heat resistance

The thermal stability of heat-resistant pigment refers to the degree of thermal weight loss, discoloration and fading of pigment at 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 change their molecular structure and decompose a small amount at a certain temperature. Especially for PP, PA and PET products, the processing temperature is above 280 ℃. When selecting colorants, on the one hand, we should pay attention to the heat resistance of pigments, on the other hand, we should consider the heat resistance time of pigments. Generally, the heat resistance time is required to be 4-10min. 

3. Oxidation resistance

Some organic pigments gradually fade due to macromolecular degradation or other changes after oxidation. One is high-temperature oxidation in processing, and the other is oxidation with strong oxidants (such as chromate in chrome yellow). When lakes, azo pigments and chrome yellow are mixed, the red color will gradually decrease.

4. Acid and alkaline resistance

The fading of colored plastic products is related to the chemical resistance of Colorants (acid and alkali resistance, redox resistance). For example, molybdenum chrome red is resistant to dilute acid, but sensitive to alkali, and cadmium yellow is not resistant to acid. These two pigments and phenolic resin have a strong reducing effect on some colorants, seriously affecting the heat resistance, weather resistance and fading of colorants.

For the fading of plastic colored products, the above properties of required pigments, dyes, surfactants, dispersants, carrier resins and anti-aging additives shall be comprehensively evaluated according to the processing conditions and application requirements of plastic products.

Color matching, coloring principle and technology of plastics

1、 Definition of plastic color matching:

Color matching is based on the three basic colors of red, yellow and blue, to allocate favorite colors that meet the color difference requirements of the color card, meet the requirements of customers, are economical and do not change color in processing and use. In addition, plastic coloring can also give plastics a variety of functions, such as improving the light resistance and weather resistance of plastics; Give plastics some special functions, such as conductivity, antistatic, antibacterial and other special materials; Different colored agricultural land films have the functions of weeding or avoiding insects and raising seedlings. That is, through color matching and coloring, it can also meet some application requirements.

2、 Colorant:

Pigments and dyes

Pigment characteristics: pigment is a colorant that cannot be dissolved in ordinary solvents. Therefore, in order to obtain ideal coloring performance, it is necessary to evenly disperse the pigment in plastics by mechanical heating and mixing

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.

Disadvantages: the coloring power is relatively poor, the relative density is large, the addition amount is large, and the color is not bright;

It is mainly used to increase the color concentration (generally all kinds of resins can be used according to the needs of color), especially in engineering plastics, such as PA, PC, PBT, POM, PPO, PPS and other materials, as well as non transparent color, gray and other resins.

Organic pigments:

Advantages: high coloring power, bright color, complete chromatography, low relative density and small addition.

Disadvantages: the heat resistance, weather resistance and covering power are not as good as inorganic pigments, the dispersion effect is poor, the transparency is poor relative to dyes, and the price is high.

It is mainly used in POM, PE, PP, TPU, TPE and other crystalline materials and some bright color products (bright color products whose dye brilliance cannot be reached).

Dye:

Advantages: dyes are organic compounds that can be used in most solvents and dyed plastics. They have the advantages of low density, high coloring power and good transparency.

Disadvantages: however, its general molecular structure is small, and it is easy to migrate when coloring (crystalline material) is more expensive than inorganic, and some prices are close to those of organic pigments.

Pearlescent pigment: also known as mica titanium pearlescent pigment, it is a mica wafer coated with titanium dioxide. According to different hues, it can be divided into silver white pearlescent pigments, rainbow pearlescent pigments and color pearlescent pigments.

3. Color matching and coloring process:

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

Direct addition of toner to resin:

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

Toner mixed resin extrusion method:

This method has the best effect on the uniformity of resin and color, and can fully disperse the pigment in the resin, with accurate color, clean quality and easy processing.

Color Masterbatch method:

It is a granular material with a certain concentration of colorant prepared from colorant, carrier resin, dispersant and other additives. When the product is formed, a certain amount of color masterbatch is added according to the coloring requirements to make the product contain the required coloring dose and meet the coloring requirements.

Color masterbatch can be classified according to the resin to be colored, such as ABS Color Masterbatch, PC Color Masterbatch, PP Color Masterbatch, etc; It can also be classified according to the coloring resin plus process, including injection molding, film blowing and extrusion masterbatch. Because the pigment is pretreated first, the color masterbatch has high coloring power, the dosage can be reduced, the quality is stable, the transportation, storage and use are convenient, and the environmental pollution is greatly reduced.

The dispersant removes the surface air by wetting and penetrating the pigment, disperses the condensate and aggregate into fine, stable and uniform particles, and no longer agglomerates in the processing process. The common dispersant is low molecular weight polyethylene wax, and EVA wax or oxidized polyethylene wax is used for organic pigment and carbon black that are difficult to disperse, The synthetic low molecular weight polyethylene wax is very different from the low molecular weight polyethylene wax produced by polyethylene cracking. Other additives include coupling agents, antioxidants, light stabilizers, antistatic agents, fillers, etc. the addition amount depends on the requirements and varieties, which is called multi-functional masterbatch. In addition, the addition of brightener is conducive to demoulding of molded products and improving the surface brightness of products.

4. Color matching management and instruments

The hardware of color matching management includes a colorimeter and a computer for processing the measured data. Colorimeter can be divided into spectrophotometer and color difference meter, which can replace human eyes to measure color and remove the influence of human factors on the measurement results.

Spectrophotometer is used to measure the reflection coefficient of each wavelength on the fully diffuse reflection surface. It can not directly obtain the chromaticity value or color difference, but it can evaluate the chromaticity value and other values through its data processing. Spectrophotometer can be divided into two types: diffraction grating and interference filter. The advanced spectrophotometer with built-in microprocessor has the functions of 0%, 100% automatic correction and magnification increase, so as to improve the accuracy.

The color difference meter is a simple test instrument, that is, to make a filter with light splitting characteristics equal to the color sensitivity of human eyes, and use it to measure the light of the sample. The key is to design a filter with light splitting sensitivity of photoreceptor and can measure the color difference value under a certain light source. The color difference meter has small volume and simple operation, It is more suitable for batch management of the same product with small change in spectral characteristics. The color difference meter with small microcomputer is easy to correct and output multiple color difference values with standard template.

Color matching management software includes spectral inversion curve, color difference formula, conditional isochromatic representation, hiding power representation and haze representation. The spectral reflectance curve is used for the analysis when selecting colorants, but can not be used to judge the consistency of colors. Color difference value is one of the most important indexes in color management, but the color difference obtained by different color difference formulas is different, so the chromaticity system or color difference formula used must be indicated.

Qingdao Sainuo Chemical Co., Ltd. We are manufacturer of  PE wax, PP wax, OPE wax, EVA wax, PEMA, EBS, Zinc/Calcium Stearate…. Our products have passed the REACH, ROHS, PAHS, FDA testing. 

Sainuo rest assured wax, welcome your inquiry!

Website:https://www.sainuowax.com

E-mail:sales@qdsainuo.com

              sales1@qdsainuo.com

Address:Room 2702,Block B, Suning Building, Jingkou Road, Licang District, Qingdao,China


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