Coloring method of rubber products
There are two main coloring methods for rubber products: surface coloring and mixed coloring
1. Surface coloring
The coloring agent is sprayed onto the surface of rubber products to color them.
The method has certain effect on static products, but for dynamic products, the coloring agent is easy to peel off and lose color.
2. Mixing coloring
Is currently the main method of rubber coloring, divided into two kinds of solution method and mixing method.
Among them, the mixing method and dry powder coloring, color paste coloring, granule coloring and master refining rubber coloring and other methods.
Solution method coloring
The method is to dissolve the rubber with its good solvent into a certain concentration of solution, and then add the coloring agent and the rubber compound except sulfur yellow into the solution and mix well, dry at a certain temperature to remove the solvent, and finally add sulfur yellow in the rubber refining machine. This coloring method is complicated, colorant and compound are not uniformly dispersed, there are color differences, solvent is difficult to recover, and pollute the environment, so it is less used at present.
Mixing method coloring
This method is the most commonly used method for coloring rubber products, which is to add the coloring agent directly or mix it with the carrier first and then add it into the rubber material, and mix it evenly through the rubber refining machine to color the rubber. Specific methods are as follows:
(1) Dry powder coloring
This method is to directly add the powder coloring agent into the rubber together with the small material, and then mix it on the open refining machine. The advantage of the method is simple operation, low cost, but the mixing of dust, pollution, and not easy to disperse evenly, there are color differences, if the particles are too hard and too coarse, but also cause color spots, stripes or chromatographic pollution and other quality problems, has been less used.
In order to facilitate the use of powder colorants and stearic acid, sulfate and other pre-dispersion of colorants, the market has been sold.
(2) color paste coloring
The method is to mix the coloring agent with the liquid compound (such as plasticizer), grind it into a paste or slurry with a three-roller machine, and then add it to the rubber in a certain proportion and mix it.
This method can avoid dust flying, and is also conducive to colorant dispersion in the rubber and uniform color. However, the content of coloring agent in color paste is low, coloring is not high, transportation is large, loss is large, and it is not convenient for users to use.
(3) Granule coloring
There are two main methods of preparing colorant granules.
The first method is the same as other powdered matching agent granulation method, first the powdered colorant with surfactant infiltration, and then extruded by wax melt or melt with resin after granulation;
The second method is to moisten the colorant with surfactant and then make the colorant particles refined by mechanical force to make a certain concentration of dispersion, and then mixed with latex co-sink, after drying and rolled tablets granulation. The surfactants are mostly anionic and nonionic, such as fatty acid salts, sulfonates, etc., and the latex usually uses natural latex.
This granular colorant is easy to use, good dispersion, no dust flying, no pollution of the environment, bright color, uniform color, no color difference, is a very promising coloring method. But the granular colorant preparation process is complex, high cost, limiting its wide application.
(4) Master refining rubber coloring
This method is the first colorant, part of the plasticizer and other rubber compounding agent and raw rubber by the mixing machine to make the concentration of about 50% of the colorant master refining rubber, and then proportionally mixed into the rubber coloring.
This method is more effective for the coloring of fine particles and difficult to disperse colorants, with uniform coloring and no color difference. Pay attention to the concentration of colorant in the masterbatch and the amount of other compounding agents in the masterbatch when matching.
Regarding the choice of pigment, the choice of the appropriate color pigment depends on the previously described processing requirements and the end use of the product, the latter being more important. The following factors must be considered:
Resistance to fading, caused by factors such as sunlight, exposure to heat or natural ageing;
Resistance to bleeding and harmful effects on the aging properties of the basic rubber, avoiding colors containing copper or manganese.
In addition, since colored rubber products must compete in the marketplace, in order to reduce the cost per pound of color for vulcanized rubber products, the most economical pigment needs to be carefully selected to achieve the desired results.
There are two types of pigment colors to choose from – inorganic and organic pigments. Today we’ll start with inorganic pigments
Inorganic pigments
Iron oxide, lead pigments (e.g. chrome yellow), ultramarine blue, chromium oxide and cadmium pigments are the commonly used types.
The iron oxide group includes red, yellow, brown and tan, all of which produce a dark light. Synthetic iron oxides are recommended. They are all available as fairly good powders with high specific gravity.
Care must be taken when selecting iron oxides as some grades contain impurities such as copper – which can accelerate the aging of vulcanized rubber products. Ultramarine blue pigments produce a range of blues from greenish to reddish. These pigments have excellent thermal stability and lightfastness. However, they are not acid resistant, as acids can limit their use in many outdoor applications as well as any indoor applications containing fruit juices etc.
The greatest use of ultramarine blue in rubber is for coloring white rubber compounds. The use of ultramarine blue produces a fully chromatic light, which may then create a bright color.
Chromium oxide green provides a blue-cyan light, a color that has good heat resistance and light fastness. It has a high specific gravity.
Lead pigments include numerous chromate yellow pigments and molybdate orange pigments. They both provide a bright color with good heat resistance and light fastness, but they are lead-containing compounds and the dust in them can shimmer and burn.
In compounds using sulfur sulfide, any moisture can cause lead sulfide stains. They have now been replaced by organic pigments in most cases.
Cadmium pigments are only used in specialist applications where specific fastness characteristics are required. Such pigments are often used as the basis for improved dispersion and feel. However, they are large in proportion and expensive, and cadmium is also a heavy metal, which should be avoided in rubber compounds.
All inorganic pigments have a weak coloring power. They are usually used in amounts of 2 to 5 percent of the weight of the compound. Under normal circumstances, inorganic materials are more easily dispersible and less dusty than organic materials, all because of their larger particle size and greater specific gravity. Nevertheless, all color paints are dusty and care must be taken to prevent this before production.
The use of organic pigments, because of their ability to create bright and attractive color hues, can be used to meet the technical requirements of specific rubber products by selecting the right pigment or mix of pigments.
From an aesthetic point of view, the use of organic pigments produces bright colors. This reason has led to the extensive use of organic pigments in rubber compounds. However, most organic pigments do not produce the same properties as inorganic pigments in terms of heat resistance, lightfastness and color bleeding. In fact, their superiority lies in the more beautiful appearance of the resulting product.
Organic Red Pigments
Red is probably the most popular color. Because of the chemical properties of red organic pigments, it can also be the most problematic color. There are eight types of organic pigments that are commonly used when coloring rubber products. They are: Permanent Red 2B; Pyrazolone Red; Sulfur Red; Quinazolinone Red; Naphthol Red; Risolbauer Red; Precipitous Red C and Risol Big Red.
Permanent Red 2B is the most widely used one, which includes barium and calcium salts. The barium salt forms an orange red, while the calcium salt forms a particulate blue red light. The mixture of the two salts can form a variety of color light, they are very dark bright colors and have a fairly good resistance to color bleeding.
Pyrazolone red can form a color light between the 2 color lights of barium and calcium salts. Different types form different hues, but there is no significant difference between these hues. It is bright, clean, transparent, and a pure organic pigment rather than a salt. This organic pigment can be used in rubber products that have contact with food.
Sulfur Red is a superior and expensive pigment. It is mostly used in mixed pigments and is used only for the purpose of obtaining special color light.
Different salts of naphthol red can form different color lights such as orange red and blue red. Similar to permanent red 2B, it is an expensive pigment.
Risol Red, Golden Red C and Risol Big Red are all lower cost pigments. These 3 pigments were used in large quantities in the past. Their technical characteristics are relatively poor and with the increasing emphasis on quality and longevity of rubber products, these 3 pigments are being used less and less.
Organic Orange Pigments
There are 2 main types of organic orange pigments: anisidine and benzidine orange.
Benzidine orange has a bright, clean yellow color, and benzidine amine has a clean, slightly red color.
Phthalocyanine Blue
It is the pigment of choice for blue rubber products. There are many kinds of color light, among which red color light (RS) is the most used in rubber. Also, it has the worst thermal stability, so the vulcanization temperature must be considered when using this phthalocyanine blue.
The intensity of green light phthalocyanine blue is quite weak, but the thermal stability is very good and it is more expensive than all other grades. Phthalocyanine blue pigment is difficult to disperse and should be used in the form of rubber color masterbatch mixes or in the form of color pastes.
Phthalocyanine Green
This pigment can produce several color lights, with a slightly yellowish green light and a slightly blueish green light. Although many green rubber products are made in a mixture of yellow, blue or green, phthalocyanine green is the most widely used single species color pigment in green products.
All color lights of phthalocyanine green have good technical properties and are difficult to disperse. It is recommended to use the master mix method, which has properties similar to those of other colors.
In practice, few rubber products are made from one pigment, either inorganic or organic. The final color of a rubber product is determined by the buyer or seller of the product.
