In the previous article, we introduced inorganic colorants. This article will introduce organic colorants.

For aesthetic reasons, in order to obtain colorful rubber products, organic colorants with bright colors that inorganic colorants cannot provide are 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 color bleed resistance like inorganic colorants, they can indeed produce a very attractive appearance.

The unit price of organic colorants is much higher than that of inorganic colorants. However, their use effect is also much better, but in general, the cost of using organic colorants is still higher than that of inorganic colorants.

1. Organic red colorant

Red may be the most popular color, but due to the chemical properties of organic red colorants, it may also be the most annoying color. There are eight major categories of organic red colorants used for colored rubber products (see Table 1), namely: Permanent Red 2B; Pyrazolone Red; Sulfur Red; Quinacridone Red; Naphthol Red; Lithol Red; Lake Red C; Lithol Red.

The most widely used Permanent Red 2B includes barium red salt and calcium red salt. Barium salt provides orange-tinted red, and calcium salt provides slightly bluish red. Mixing the two can produce a variety of shades. These colors are strong and bright and relatively easy to bleed. They have suitable heat and light stability, but they will begin to fade once the vulcanization temperature exceeds 175°C.

The hue provided by pyrazolone red is between the two types of Permanent Red 2B, barium salt and calcium red salt. The hue of pyrazolone red varies depending on the specific type, but the difference between them is not particularly large. The color of pyrazolone red is bright, bright, and transparent. It is a pure organic colorant, not a salt. Pyrazolone red has excellent resistance to soap bleed, and its heat and light stability are roughly equivalent to that of Permanent Red 2B. Pyrazolone red is an organic colorant approved for use in food contact rubber products. It is more expensive than Permanent Red 2B, but its special color tone, good bleed resistance and medical applications make it indispensable.

Sulfo red is a technically impeccable but expensive colorant. It is only used in unusual shades and is often used in blends.

Quinacridone red has excellent technical properties but is relatively expensive. This colorant is approved for use in food contact rubber products.

Naphthol red, in different salts, can provide a variety of shades from orange-red to blue-red. Compared with Permanent Red 2B, it is more expensive, so it is only used when there is an outstanding requirement for soap bleed resistance.

Lithol Ruby, Lake Red C and Lithol Red are lower-priced colorants that have been used in large quantities in the past. However, their technical performance is relatively poor, and with the increasing attention paid to the quality and service life of rubber products, this type of colorant is used less and less.

Rubber masterbatch is usually the first choice for manufacturers of extruded, molded and calendered colored rubber products. This masterbatch is generally prepared in the form of a high molecular weight binder with a colorant content of about 50%. Its dosage is generally 1% to 3% of the total rubber compound. Pyrazolone red and permanent red 2B are more difficult to disperse, so rubber masterbatch is strongly recommended for these two colorants.

2. Organic orange colorant

Commonly used organic orange colorants are divided into two major categories, namely o-dianisidine orange and diaryl orange. o-dianisidine orange provides a pure and slightly reddish orange tone, while diaryl orange has a more yellow tone. Both colorants have good non-fading properties, while diaryl orange has better light resistance. Both products are easier to disperse than the organic red colorants mentioned above, but rubber masterbatch is still recommended. The concentration and usage of the masterbatch are the same as those of the red rubber masterbatch.

3. Diaryl Yellow

Diaryl yellow colorants provide a bright, pure yellow hue. There are four major categories of such colorants, which differ in the degree of substitution on the molecule. These categories are named AAA, AAOT, AAMX and AAOA (as shown in Table 1), and they are listed in order from the worst to the best colorfastness. These colorants are considered to be the most easily dispersed colorants. However, the most widely used are rubber masterbatches, which have the same characteristics as the above colors.

4. Phthalocyanine Blue

This is the main colorant for blue rubber products. It comes in a variety of shades, and the red ones are the most widely used in rubber. This type of color has the worst thermal stability, so the vulcanization temperature must be considered when using phthalocyanine blue. Non-crystalline phthalocyanine blue has less red hues, but is much more thermally stable. Phthalocyanine blue with a green hue is much weaker than other types, but has much better thermal stability and is more expensive. Phthalocyanine blue is difficult to disperse and should be used in the form of rubber masterbatch or color paste. The properties of this rubber masterbatch are similar to those of other colors.

5. Phthalocyanine green

This colorant also has several shades, from slightly yellowish green to slightly bluish green, and is the most commonly used colorant for green rubber products. Many green rubber products are produced by mixing yellow, blue and this green. All shades of phthalocyanine green have excellent technical properties and are all difficult to disperse. For this colorant, it is recommended to use a rubber masterbatch again, which has similar properties to other colors of rubber masterbatch.