As suggested by the name, coloring agents are the material that can provide people the color they need. Color agents have their own specific colors, which includes a variety of colors such as white, black, as well as gray.
Colorants are grouped into two categories: dyes and pigments. The main differences between dyes and pigments are:
Solubility: In the using process, dyes can be dissolved in the application(some dyes such as disperse dyes are partially dissolved), but pigments are not soluble or its solubility is very low. Solubility is a very important item that determines the performance of pigments such as migration and color penetrating. Sometimes, to improve the application properties of certain pigments, the solubility of pigments should be improved in the application.
In the using process, pigments have particle form and crystal shape. These play important roles in the color and application properties of organic pigments. Dyes dont have requirements on this aspect.
The color of pigment almost represents its using color in the substrate. With the dilute proportion, only the shade of pigments changes, but the basic color is not changing. This is to say that the color of the colored substrate is the color of pigment itself. While for dyes, the tone of the color will be changed when there is chemical reaction. They dyeing color is different with the color of dye.
Different Properties of Pigments
Color shades of Organic Pigments
For a long time, visual judgement is the basic color evaluation way, which relies on the well-trained and professional color judge personnel. But it is difficult to judge whose eyes can be the standard judge. Along with the development of color evaluation judgement, a more scientific evaluation system is established. The use of color tester can make the color description qualitatively and quantitatively.
The definition of color cannot be easily described orally. For example, what color can be called red? No matter how many modifiers are added, it is still not so accurate. Such as magenta, brick red, poldo red. But if we can make a color sample or a color card, the color is defined more obviously.
In the actual application, people like to choose a standard pigment as per their demand. And then they will test the new sample against their standard sample in certain testing system. A sample color will be made from standard sample and new sample and the shade difference will be compared. The common color system used is CIE*LAB system, and the XYZ three-stimulus value system that can be interconverted.
The concept of color strength has absolute color strength and relative color strength. Absolute color strength is based on pigment absorbance of lights, which is the maximum absorption wavelength or the overall absorption coefficient of the whole visible spectrum (the latter can be depended on the components of the different spectra).
Relative coloring strength is the relative value obtained from comparing the light absorption coefficient between the sample and the standard pigment. The ratio of the number of sample pigments matched to the number of standard pigments when reaching the same color depth.Accurate matches are not always found, because there are inherent differences in color light between the two, which cannot be eliminated by quantity alone, and whose color differences can be formulated through the CIE*LAB system.
The color force of the pigment will have different results according to its application conditions, that is, different color exhibition methods, as well as different measurement methods, and evaluation methods.
Opaque or transparency
It refers to the ability of pigment substrate to cover the color of the target layer. This is to define the covered area by paint colored with pigments, or it can refer to the the minimum thickness of the layer needed to cover the substrate.
To effectively cover the substrate, the coating must scatter. The amount of scattering is related with the coating thickness, the light absorption inside the coating, as well as the color difference of the substrate
Solvent resistance and various fastness
According to the definition of pigment,it should not be soluble in the application. but the absolute insolubility is not possible.There is certain solubility as per different application or different processing conditions. So if the solvent in in the carrier reach certain amount, recrystallization, color penetrating, or overflow will happen. It is very important to prevent the dissolution of pigment.The factors that can control the solubility of pigments include solvent, the particle size, the chemical structure of the pigment, as well as the processing temperature.
This is a testing method to judge pigment resistance to some solvent: put the pigment powder into filter paper, put it into a tube with solvent. The weight of the solvent and pigment should be determined. After a set at room temperature for the hours, the solvent resistance of pigment can be judged from the stained situation of solvent. This method may not fully represent the actual application, but the result is similar. If the pigment is vert dissolved , it could be problem for organic pigments to use in the application containing this kind of solvent. Problems such as color penetrating, fastness or flow will occur.
Water resistance, soap washing resistance, alkali resistance, acid resistance:
For the above resistance, there are two aspects, one is about pigment’s chemical stability in these solvents and the other is the solubility in these solvents.
This resistance is mainly about pigments’s resistance situation in the processing and application stage. For example,in the water transfer process of pigment filter cake, it needs pigments to be good resistant to water. In the application of printing ink, it requires pigments to be resistant to water or alkali water. In the application of exterior wall coating, it requires pigments to be resistant to the alkali in cement or lime resistant. During car washing, it requires organic pigments in automotive paint to be resistant to alkali water or detergent, etc.
Some pigments or lake pigments contain free alkaline or acid groups, will form salt in alkali or acid and this will cause color change, etc.
Threre are standard testing methods to test this properties. And the evaluation is according to level 1-5,grade 5 is best. A suitable testing method should be selected to test pigments. The test results are not same when pigment is directly exposed to acid and alkali or when pigments are made into coating and then exposed to acid or alkali.
Printing of package
Due to different packaging materials and the items packaged, there are different fastness requirements, such as oil resistance, wax resistance, cheese resistance, drug resistance, disinfectant resistance, steaming resistance, thermal sealing resistance (including the color change of coating and physical damage to the film, adhesion to the instrument) etc.
Textile fastness refers to the colour maintaining ability of dyed filaments or dyed textile during processing or application.Such as water resistance during washing, peroxidation resistance, bleaching resistance of chlorine acid, friction resistance, sweat resistance (including enzyme), ironing resistance, acid and alkali resistance, solvent resistance, dry washing resistance, and sodium hydrosulfite resistance, etc.
Migration resistance efers to the occurrence of bleeding and frosting phenomenon.
Frosting means that the dissolved pigment will migrate to the surface again after it is erased. Bleeding means that the color will migrate to the similar material when get in touch.
The degree of migration is related to the particle distribution, chemical structure and content of the pigment.Migration is also related to all components of the material being colored, such as polymers, plasticizer, and stabilizers.
Effect on the pigment processing during application
Plate-out(ductility):During mixing or rolling, the pigment is deposited on the surface of the processing equipment or the system surface. Once the film is erased, it will not appear again.The reason is that the compatibility between lubricants, plasticizer, stabilizers and PVC is not good. And it is easy to migrate to the system surface, and bring out the pigment particles at the same time. The method to avoid this is to reduce the pigment content or the processing temperature or adding hardener.）
Chalking(powder ization or pigment overflow): it mainly refers to the situation that the pigment carrier cannot completely wrap the pigment, and when it exceeds the critical volume concentration, the organic pigment will tun to the surface of the paint film. The reasons for chalking can be follows: high pigment content and less resin content; problem in the processing and pigments are not wrapped; large surface area; poor light and weather resistance of resin; poor light and weather resistance of pigments.
With the long time under UV and weathering time increasing, the pigment separated from the medium and the paint surface is damaged. The poor weather resistance of pigments or carrier materials promote pulverization.
Light resistance and weather resistance
light resistance: pigment’s ability to maintain the original color after long time sun exposure. The testing is based on the pigment dispersion system, not only the pigments. Most inorganic and all organic pigments will change color after long time light exposure.
The factors sensitive to light include the physical parameters (particle distribution, crystal type), the chemical structure of the pigment, and the properties of other components in the system.The xenon lamp is the standard test instrument for light resistance test.Pigment volume concentration,paint material, substrate, layer thickness, and other additive all have an influence on the test results.
Weather resistance: Light is not the only reason for the color change of the pigment. Other factors such as atmospheric water (or even the trace),industrial emissions, gas or climate changes also cause damage to the color of pigment. The test of weather resistance requires specific environments like temperature,light intensity, humidity, oxygen content, precipitation, air as well as the latitude and longitude and the proximity with the industrial zone. All these aspects will affect on the test result. Thus, industrial standards are formed in several countries.
The test will adopt accelerated aging instrument, which has similar condition with the outdoor environment. But for pigments with similar colors and different chemical structures, the exposure to xenon lamps or sunlight will vary due to the sensitivity of the pigments to the spectrum, especially in the UV region.
The temperature resistance in the plastic industry and in the coloring of fiber raw pulp requires 260-320℃. This is a fatal natural defect for organic pigment as most organic pigments have a temperature resistance between 100 and 200℃. The thermal stability of the pigment is depended systematically. It not only depends on the chemical composition of the substrate, but also dispersion degree, processing conditions and pigment content.
In the coloring system, the reasons for thermal discoloration are: heat decomposition of pigments, chemical reaction of pigments with the substrate, the change of physical properties of the pigments, such as particle size change and the crystalline transition.
The viscosity of a system is determined by the overall component, including the actions between different components, dispersion conditions, and selection of components. The dispersion condition is most important such as wetting of the substrate, the opening of the aggregate, , dispersion degree and surface relationship. The influencing factors of pigment are as follows: concentration, particle shape, specific surface area, and surface structure.
Particle distribution and the property of substrate
Color strength: within certain range, The smaller is the pigment particles, the higher is the coloring strength of organic pigments. But after reaching the translucent point, the color strength will not further increase.(In the transparent system, there are pigments not following this rule).
Color shade: with the particle size increasing, the color will change as per some rules.The yellow color will shift to red; The orange color will shift to red shift, yellowish red color will shift to blue, bluish red will shift to yellow, brown color will shift to red, purple color color will shift to blue, blue color will shift to red.
Light resistance and weather resistance: the larger the pigment particles, the better the light weather resistance.
Dispersion: pigment particles with narrow distribution is easier to disperse than wide distribution. Pigment with large particle size is easy to disperse. But dispersion is a complex process, disperison is also related with other factors.
Gloss: It is related with the dispersion of pigments and distribution of pigments. Bigger particle size has less gloss
Solvent resistance and migration resistance: with the bigger particle size, solubility will decrease , and migration resistance will increase
Fluidity: the bigger the particle size, the better the fluidity and the less the viscosity.
Dispersion of pigments
The dispersion of pigment powder is related with many factors:
First are pigment related characteristics: chemical composition, particle distribution, crystal form, particle shape, surface structure, and the process of the pigment powder production, especially the drying and grinding process.
Second are physical and chemical properties of chemical components, which include molecular weight, polarity, viscosity and compatibility of different components.
Popular Pigment Items for different application
The manufacturer shall provide the pigment users certificates of analysis of pigments. But this is not enough for the application. End users should test organic pigments in their own paint, ink or palstic systems to confirm on if the organic pigments are suitable in their own product system.
Organic Pigments popular for paint include:Pigment Yellow 1, Pigment Yellow 3, Pigment Yellow 17, Pigment Yellow 65, Pigment Yellow 74, Pigment Yellow 83, Pigment Red 2, Pigment Red 8, Pigment Red 22, Pigment Red 112, Pigment Red 122, Pigment Red 146, Pigment Red 170, Pigment orange 13,Pigment Orange 34,Pigment Orange 64, Pigment Violet 19, Pigment Violet 23
Organic pigments popular for Ink include:Pigment yellow 1, Pigment Yellow 12, Pigment Yellow 13, Pigment Yellow 14, Pigment Yellow 83, Pigment Red 81, Pigment Red 122, Pigment Red 254, Pigment Violet 3, Pigment Violet 27
Organic Pigment popular for Plastics: Pigment yellow 83, Pigment yellow 110, Pigment Yellow 180, Pigment Yellow 191, Pigment Red 53:1, Pigment Red 48:2, Pigment Red 122, Pigment Red 170, Pigment Red 254, Pigment Orange 64, Pigment Violet 19