Selection and use of colorants

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Preface: Dyes are pigments that are soluble in at least one common solvent, such as water, ethanol, and oils. Colorants have no significant solubility in such media, so they are described as insoluble. The majority of rubber products are made in black, gray, or white, all of which are achromatic colors. The selection of raw materials for such products is relatively simple; specifically, carbon black is the main coloring agent for black rubber products, while titanium dioxide serves as the primary whitening agent for white products. Zinc oxide, barite, and black iron oxide can be used to mix with other colors to create special shades. The following will provide a systematic introduction to colored pigments, such as red, blue, yellow, green, brown, pink, etc. Red, yellow, and blue are the most basic colors, known as the “primary colors”. Although all other colors can be created by mixing these three colors, coloring rubber products relies not only on the mixture of these primary colors but also on specialized colorants that are not primary colors, such as those for orange, green, and pink. 1　Rubber compound: Let’s first discuss the rubber compounds used in the production of colored rubber products. Firstly, such rubber compounds must possess specific physical properties related to flexibility, hardness, strength, and service life. If it is to be made into a colored product, when designing the formula, it is also necessary to consider from technical and economic perspectives whether coloring can be carried out properly. Colorful rubber products can be simply divided into two categories: those with light shades or pale colors, and those with dark shades or saturated colors. To simplify the explanations below, especially in cases where darker colors are required, certain raw materials with darker colors that cannot accommodate bright, light shades can be used. Colorants are often the most expensive ingredients in rubber compounds; therefore, their usage should be kept as low as possible. The same principle should apply when selecting other ingredients for the rubber compound. The classification and selection of colorants will be discussed in detail below. The following are the guidelines for preparing high-quality base rubber compounds used in the production of colored rubber products. First of all, pure, light-colored rubber should be selected; the specific type of rubber polymer is determined by the intended use of the final product and the price of the available polymers. In applications using light-colored, bright rubber compounds, it is advisable to avoid using dark-colored fillers and softeners; although kaolin can also be used, calcium carbonate and silicates are the preferred additives. In addition, light-colored plasticizers and resins should be selected, with non-reactive types being preferable; such additives are less likely to fade or get contaminated. Furthermore, excessive use of fatty acids may cause contamination, colored streaks, and color flaking. Hydrocarbon resins can be used to improve physical and processing properties; they should also be light in color, pollution-free, and have low reactivity. Antioxidants should be non-polluting and non-fading; phenolic antioxidants are recommended. The selection of the accelerator is crucial. Thiazole and thiram types are recommended; certain sulfenamide and dithiocarbamate types can also be used. Copper-containing substances cannot be used, nor should products that cause an alkaline reaction be employed (such as urea, aldehyde amines, and diphenylguanidine), as they may cause the rubber compound to fade or lose its color. Peroxides can damage certain colors; extra care must be taken when choosing peroxide vulcanizers, and no lead-containing substances such as lead tetroxide (lead white) or lead monoxide (lead yellow) should be used. 2 Mixing and Processing Some suggestions regarding the mixing and processing of colored rubber compounds are also presented in the text. For colored compounds that use organic colorants, the processing temperature and vulcanization temperature are crucial; it is recommended that the temperature not exceed 160°C. Sulfidized products—especially gaskets or other flat products—must be fully cooled before stacking, to prevent thermal degradation in their center that could lead to discoloration. Processing conditions and mixing procedures are also very important; whether they are chosen correctly depends on whether the colorant is added in powder form or as a pre-dispersed colorant (such as rubber color masterbatches and plasticizer pastes). When adding dry pigment powder to the mixer, it is ideal to incorporate the pigment as the first additive during the mixing process. To fully and evenly utilize the properties of the colorant during mixing, sufficient mixing time and good shear are required. When actual production conditions do not permit this, the colorant can be added together with other additives at the later stage of the mixing process; however, this may compromise the color value and uniformity. In this case, rubber masterbatches or pastes can be used. Since these forms of colorants are pre-dispersed, adding them later generally poses no problem. In some colored rubber products, the rubber compound is colored during the second pass through the rollers. In this case, it is necessary to use a pre-dispersed form of pigment, as this provides sufficient mixing time and shear to produce a uniform colored compound. Certain colorants, especially inorganic colorants and some organic colorants, can be vulcanized at high temperatures (such as 175°C). In certain situations, it is more desirable to use a coloring agent with a higher concentration, but this will increase the cost of the rubber compound. 3 Selection of colorants The choice of colorants depends not only on the aforementioned processing requirements but, more importantly, on the final use of the product. When using it, the following properties must be taken into account: fade resistance (caused by sunlight, heat, or weathering), color transfer resistance, and aging characteristics that are detrimental to the base rubber compound (avoid using colorants containing copper or manganese). In addition, costs must be taken into account, and the most economical coloring agents must be carefully selected to meet the production needs of colored rubber products. There are two groups of available colorants: inorganic colorants and organic colorants. 3.1 Inorganic pigments Commonly used inorganic pigments include iron oxide, lead-based pigments (such as chrome yellow), ultramarine, chromium oxide, and cadmium-based pigments. Iron oxide can be red, yellow, brown, or tan. All of these shades are relatively dull, and they are all supplied as quite fine, high-density powder. It is recommended to use synthetic iron oxide colorants. Special care must be taken when selecting iron oxides, as some of them contain impurities such as copper, which can cause degradation of rubber products during aging. Ultramarine pigments can provide shades ranging from green tones to bluish-red hues. These types of colorants possess excellent thermal and light stability, but their acid resistance is poor, which limits their use in many outdoor applications as well as in any indoor applications where they might come into contact with acidic substances such as fruit juices. The main use of ultramarine in rubber is in white compounds, just as indigo is used when washing shirts. Using ultramarine can yield saturated, vivid colors. Chromium green oxide can provide a blue-green shade; this color boasts excellent heat and light resistance, as well as a high relative density. Lead pigments include chrome yellow pigments and molybdenum orange pigments, which have a wide range of colors. Using these colorants yields bright colors with good heat and light resistance, but they are lead-containing compounds whose dust can emit light and burn. In sulfur-vulcanized rubber compounds, any moisture can cause lead sulfide spots. Currently, these colorants have been replaced by organic colorants in most applications. Cadmium pigments are used only in special applications where color fading is not acceptable. Such colorants are usually added in their basic form to improve their dispersion and processing. It has a high relative density and is expensive. Cadmium is also a heavy metal, and care should be taken to avoid using it in rubber compounds. All inorganic colorants have weak coloring power, and their normal usage amount is 2% to 5% of the total mass of the rubber compound. Inorganic colorants are generally easier to disperse than organic colorants; they have larger particle sizes and a much higher relative density, which results in less dust generation. But this is only in comparison to organic colorants; in fact, dust generation is a serious issue with all colorants, and preventive measures must be taken during the production process. 3.2 Organic Colorants Organic colorants impart vivid and attractive colors; by selecting appropriate colorants or combining them, it is often possible to meet the technical requirements of special rubber products. For aesthetic reasons, in order to obtain rubber products with bright colors, organic colorants that provide vivid hues and are unavailable from inorganic colorants must be used. It is for this reason that organic colorants are widely used in rubber compounding processes. Although most organic colorants cannot provide good heat resistance, light resistance, and colorfastness like inorganic colorants, they can indeed produce a quite attractive appearance. Organic colorants are much more expensive per unit than inorganic colorants, yet they offer significantly better performance. Generally speaking, however, the cost of using organic colorants remains higher than that of inorganic colorants. Table 1 shows the types of organic colorants used in rubber compounding processes (classified by chemical type and chroma index) as well as their main technical properties, including thermal stability, light stability, resistance to soap staining, and direct steam vulcanizability. 3.2.1 Organic red colorants Red may be the most popular color, but due to the chemical properties of organic red colorants, it can also be the most unpleasant color. There are 8 major categories of organic red colorants used in colored rubber products (see Table 1), namely: Permanent Red 2B ； Pyrazolinone Red ； Sulfur Red ； Quinacridone Red ； Naphthol red ； Lisuoer Baohong ； Lake Red C ； Lithosol Red. Table 1 Comparison Table of Colorant Types and Names
Colorant Type | Colorant Name | Relative Density | Thermal Stability | Light Stability | Resistance to Soap Staining | Resistance to Direct Steam |
--- | --- | --- | --- | --- | --- | --- |
Orange16 | Orange16 | 1.38 | 2 | 3 | 1 | 1 |
Red38 | Pyrazolinedone Red | 1.33 | 1 | 2 | 1 | 1 |
Red88 | Thiored | 1.82 | 1 | 1 | 1 | 1 |
Orange46 | Patent Orange | 1.71 | 1 | 3 | 1 | 2 |
Red53:1 | Lake Red C | 1.48 | 1 | 3 | 2 | 2 |
Green36 | Phthalo Green | 2.20 | 1 | 1 | 1 | 1 |
Green7 | Phthalo Green | 2.05–2.12 | 1 | 1 | 1 | 1 |
Red49:2 | Resorcin Red | 1.65 | 3 | 3 | 3 | 3 |
Red49:1 | Resorcin Red | 1.74 | 3 | 3 | 3 | 3 |
Violet23 | Carbazole Violet | 1.46 | 1 | 1 | 1 | 1 |
Red48:2 | Permanent Red 2B | 1.70–1.76 | 1 | 2 | 2 | 2 |
Red48:1 | Permanent Red 2B | 1.98 | 2 | 2 | 2 | 2 |
Blue15:1 | Phthalo Blue (NC) | 1.62 | 1 | 1 | 1 | 1 |
Blue15 | Phthalo Blue (RS) | 1.58–1.61 | 1 | 1 | 1 | 1 |
Blue15:3 | Phthalo Blue (CS) | 1.64 | 1 | 1 | 1 | 1 |
Red23 | Naphthol Red | 1.45 | 2 | 2 | 1 | 1 |
Green8 | Colorant Green B | 1.39 | 3 | 2 | 1 | 1 |
Yellow13 | Diaryl Yellow (AAMX) | 1.35 | 1 | 1 | 1 | 1 |
Yellow17 | Diaryl Yellow (AAOA) | 1.39 | 1 | 1 | 1 | 1 |
Orange13 | Diaryl Yellow | 1.35 | 2 | 2 | 1 | 1 |
Yellow12 | Diaryl Yellow (AAA) | 1.22–1.41 | 2 | 2 | 1 | 1 |
Yellow1 | Sunfast Yellow | 1.45 | 3 | 2 | 3 | 3 |
Yellow14 | Diaryl Yellow (AAOT) | 1.40 | 2 | 2 | 1 | 1 |
Red57:1 | Resorcin Brilliant Red | 1.55 | 2 | 2 | 2 | 2 |
Blue29 | Dark Blue | 2.35 | 1 | 1 | 1 | 1 |
Green17 | Chromium Oxide | 5.10 | 1 | 1 | 1 | 1 |

Thermal Stability, Light Stability, Resistance to Soap Staining, Resistance to Direct Steam:
1 – 175°C, 1 – 200 hours, 1 – No staining, 1 – Excellent;
2 – 165°C, 2 – 100 hours, 2 – Slight staining, 2 – Good;
3 – 150°C, 3 – 50 hours, 3 – Staining, 3 – Not applicable.
Note: Thermal stability refers to the value under normal pressure; higher temperatures are generally allowed for continuous vulcanization or very short vulcanization cycles ； Light stability is tested using a fading tester; in typical rubber applications, colorants with a grade of 1–2 are generally satisfactory, while colorants with grade 3 are chosen only for certain special applications. The most widely used Permanent Red 2B includes barium red salts and calcium red salts. Barium salts impart an orange tint to red, while calcium salts give it a slight blue tint. Mixing the two can produce a variety of shades. These colors are intense and vivid, and tend to bleed more easily. They possess appropriate thermal and light stability; however, they begin to fade once the vulcanization temperature exceeds 175°C. The shade provided by pyrazoline red lies between that of barium salt and calcium red salt, both of which are forms of Permanent Red 2B. The red tones of pyrazolines vary depending on the specific type, but the differences between them are not particularly large. Pyrazoline red has a bright, vivid, and transparent color; it is a pure organic dye, not a salt. Pyrazoline Red exhibits excellent resistance to soap staining, and its thermal and light stability are roughly equivalent to Permanent Red 2B. Pyrazoline red is an organic dye approved for use in rubber products in contact with food. Its cost of use is higher than that of Permanent Red 2B, but its unique color tone, good resistance to color migration, and applications in the medical field make it indispensable. Thiorhodamine is a technically flawless but expensive dye. It is used only for uncommon shades, and is often mixed together. Quinacridone red possesses excellent technical properties, but its price is relatively high. This coloring agent is approved for use in rubber products in contact with food. Different salts of naphthol red can provide a variety of hues ranging from orange-red to blue-red. It is more expensive than Permanent Red 2B, so it is used only when there are high requirements for resistance to soap staining. Lithosol Red, Lake Red C, and Lithosol Red are coloring agents with lower prices that have been used in large quantities in the past. However, their technical performance is relatively poor, and as there is an increasing emphasis on the quality and service life of rubber products, these types of colorants are being used less and less. Rubber masterbatches are usually the preferred choice for manufacturers of extruded, molded, and calendered colored rubber products. Such color masterbatches are generally prepared in the form of a high-molecular-weight binder with a coloring agent content of about 50%. Its usage is generally 1% to 3% of the total amount of the rubber compound. Pyrazoline Red and Permanent Red 2B are difficult to disperse; therefore, rubber color masterbatches are highly recommended for these two dyes. 3.2.2 Organic Orange Colorants Common organic orange colorants are divided into two main categories, namely o-phenylenediamine orange and diaryl orange. O-phenylenediamine orange provides a pure orange shade with a slight red tint, whereas diphenyl orange has a more yellowish tone. The lightfastness of both of these colorants is quite good, whereas diaryl orange has better light resistance (see Table 1). Both of these products are easier to disperse than the organic red colorant mentioned earlier, but rubber color masterbatches are still recommended. The concentration and usage amount of its masterbatch are the same as those of the red rubber masterbatch. 3.2.3 Diphenyl yellow – Diphenyl yellow pigments can provide a bright, pure yellow shade. There are 4 main categories of such colorants, which differ in the degree of substitution of their molecules. These types are named AAA, AAOT, AAMX, and AAOA respectively (as shown in Table 1), and they are arranged in order from worst to best in terms of lightfastness. These colorants are considered to be the easiest to disperse. However, rubber color masterbatches are still widely used, and these masterbatches have the same properties as the colors mentioned earlier. 3.2.4 Phthalo Blue: This is the main coloring agent used in blue rubber products. It comes in a variety of shades, with the red variants being the most widely used in rubber. Colors of this type have the poorest thermal stability; therefore, the vulcanization temperature must be taken into consideration when using phthalo blue. Amorphous phthalo blue has a less intense red hue, but its thermal stability is much better. Compared to other types, phthalo blue in green tones has a relatively low intensity, but it possesses much better thermal stability and is also more expensive. Phthalo blue pigments are difficult to disperse and should be used in the form of rubber masterbatches or color pastes. The properties of this rubber masterbatch are similar to those of rubber masterbatches in other colors. 3.2.5 Phthalo Green – This coloring agent is also available in several shades, ranging from slightly yellowish-green to slightly bluish-green; it is the most commonly used coloring agent for green rubber products. Many green rubber products are made by mixing yellow, blue, and this green color together. All shades of phthalo green possess excellent technical properties, and all of them are difficult to disperse. For this type of colorant, rubber color masterbatches are once again recommended; the properties of these masterbatches are similar to those of rubber color masterbatches in other colors. 4 Color combinations The above is the basic information regarding rubber compounds and the colorants used in them. However, few rubber products are produced using a single coloring agent (inorganic or organic). Under normal circumstances, the final color of rubber products is determined by the combination of certain specific colors; therefore, color matching is a fundamental requirement for successfully producing colored rubber products. Furthermore, since any colored product is related to current fashion, there is an increasing demand for novel colors. Making the color of one rubber product match that of another is indeed a difficult task, not to mention the fact that it is often required for the rubber product to match the color of materials that are completely different in nature, such as metal shavings coated with polish, colored fabrics, colored paper, colored plastics, or painted items. They are usually small and almost always have surface textures that differ from those of other rubber products. As a result, the differences in light reflectivity make color matching more difficult. Moreover, since such different types of products typically use different chemical colorants, they often exhibit different colors under various light sources, which further increases the difficulty of matching colors in rubber products. As for the mixing of pigments, the color combinations must be evaluated under artificial light as well as under sunlight. This combination is determined by laboratory technicians with extensive knowledge of color pairing and skilled in their craft, through visual inspection. Currently, color-matching computers have been developed for the field of specialty rubber compounds, allowing one to see on screen whether the color combination is satisfactory. If properly programmed, a color-matching computer can even assist colorists in selecting the appropriate pigments and their quantities, but the final decision still has to be made by a human using their eyes. In some cases, it is required that the color of rubber products closely match the color of materials colored with completely different types of colorants, which necessitates the use of conditional color matching methods. It is a pairing method that displays different colors under various light sources. The evaluation of this combination effect will depend on the light source used, especially on the specific testing methods employed by the customer. When it is technically impossible to achieve an exact color match, in order to obtain a color as close as possible under various testing conditions, the colorist must adjust his working procedures and materials. This often requires the use of 3 or more pigments, along with careful adjustment of other additives related to them. The type of titanium dioxide and the adjustment of its dosage are likely to be the most critical factors. 5 Conclusion Generally speaking, inorganic colorants have lower costs and can provide rubber process engineers with good heat resistance, light resistance, and colorfastness. These colorants are usually in powder form, with an addition amount of 2% to 5% of the total rubber compound mass. Its drawback is that the colors are usually not very vivid or pure. To achieve brighter and purer colors, rubber engineers must use organic colorants. These colorants are expensive, have poor thermal stability, even worse light resistance, and inferior colorfastness compared to inorganic colorants. Generally, it is much more difficult to disperse organic colorants in rubber compounds; when used in powder form, the problem of dust generation becomes even more pronounced. In actual production, they are usually used in the form of rubber color masterbatches. This pre-dispersed form completely eliminates the dust problem; in terms of color, it **improves the uniformity of the color in the final product, and it also makes operations such as weighing easier. Colorants used in rubber products, especially those supplied in the form of special masterbatches, are also widely used in cross-linked polyethylene.