Selection/use of colorants in dispersions.

The production of a colored product, whether rubber, plastic, paper, textile, etc., is generally accomplished by the use of either dyes or pigments. Since the use of dyes in rubber products is highly specialized and infrequent, this article will concentrate on discussing the use of pigment colorants and the rubber compounds in which they are to be used.

Dyes are colorants soluble in at least one of the common solvents, such as water, alcohol or oil. Pigments do not have any significant solvency in such media so they are described as insoluble.

Rubber products colored black, gray or white are in the majority. These are achromatic colors. While the development of such colors requires careful laboratory work, the raw materials to use are relatively simple to choose. Specifically, carbon black is the primary colorant for black rubber products. Titanium dioxide is the primary whitening agent to produce white products. Zinc oxide, barytes and black iron oxide also find some specialized uses. Blends of these materials with other colors, as described later, may be used to produce particular shades.

The balance of this presentation will cover chromatic colors, such as reds, blues, yellows, greens, browns, pinks, etc. The red, yellow and blue colors are the primary colors. While all other colors can be produced by blends of these colors, rubber products are colored not only by their blends, but also by the use of specific colorants that are not primary colors, such as orange, green and pink pigments.

The compound

With the above as a general introduction to the subject, let us first consider the actual rubber compound that is to be used to produce a colored rubber product. First, the compound must be developed to provide particular physical characteristics regarding flexibility, hardness, strength, satisfactory service life, etc. However, if it is to be colored, all of the component parts of the compound must be selected to provide not only the appropriate physical properties, but also to produce a compound that can be properly and economically colored. This would seem to be obvious, but apparently it isn't, as many compounds have had to be changed drastically in order to provide a good colored product, as opposed to the same product colored, for example, black.

While it is an oversimplification, consider that there are two types of colored rubber products, those with light or tint tones, and those of dark or lull shades. In order to simplify the following instructions, note that where darker colors are called for, some darker colored raw materials can be used that would not be acceptable for a very bright, light color.

The following are the specific recommendations regarding the raw materials to use. These are important, not only because their consideration assists in making a good colored rubber product, but also because the color pigment itself is generally the most expensive ingredient in the compound. Therefore, it is desirable to keep the use of the colorant to a minimum and to choose the other ingredients in the rubber compound so that this can be accomplished.

The following are guidelines for the development of a good base compound for the production of a colored rubber product. Use clean, light colored rubber. The particular type of rubber polymer will be dictated by the end product use and the cost of available polymers.

Dark colored fillers and softeners should be avoided, especially where bright, light colors are desired. Calcium carbonate and silicate pigments are the preferred compounding ingredients, although kaolin clays are often used. Light colored plasticizers and resins should be selected, preferably of the non-reactive type. These should also be non-discoloring and non-staining. In addition, excessive use of fatty acids may cause mold staining, color streaking and blooming of the color. Hydrocarbon resins can be used to improve physical properties and processing. Again, they should be light in color, non-staining and have low reactivity. Antioxidants should be of the non-staining and non-discoloring type. Phenolic antioxidants are recommended.

The choice of accelerators is critical. Thiazole and thiuram types are recommended. Some sulfenamide and dithiocarbamate types can also be used. No copper-bearing materials can be used, nor should products that provide an alkaline reaction, such as ureas, aldehydeamines and diphenyl guanidine, because they may discolor the compound or consume the color. Peroxides attack some color pigments and not others. Great care must be taken where peroxide cures are the choice. Do not use any lead products, such as red lead or litharge.

Mixing and processing

With reference to the mixing and processing of the colored rubber compound, there are some precautions here as well. Both processing and curing temperatures can be critical to colored compounds that use organic color pigments. Temperatures above 325[degrees]F are not recommended, although higher temperatures are used with careful selection of the color pigment and plant testing. Cured products, especially mats or other flat products, must be cooled sufficiently before stacking to avoid heat degradation in the center of the stack that may result in color change.

The processing and mixing procedures are also very important. They may differ, depending on whether the colorant is being introduced as a powder, or as a pre-dispersed colorant, such as a rubber color masterbatch or a plasticizer paste. Where dry pigment powder is being added to the mixer, it is desirable to charge the color as one of the first ingredients to add to the rubber in the mixing cycle. Time and good sheer are required to develop full and uniform color characteristics in the mix. Where production demands may not permit this, the color can be added with other ingredients at a later time in the cycle, but with a possible sacrifice in color value and uniformity. Rubber color masterbatches or pastes can be used where later addition of the colorant is required. Since the color in these forms is pre-dispersed, such later addition does not normally cause a problem.

Some colored rubber products are colored on a second pass. In these cases, it is essential that the pre-dispersed form be used and, also, that even this procedure provide sufficient mixing time and shear to develop a uniformly colored stock.

Higher temperature cures, such as 350[degrees]F, can be used with some colorants, particularly the inorganic ones and a few carefully selected organic pigments. In some cases, the addition of a higher concentration of the colorant will provide the desired results, but the costs of the rubber compound are increased by such addition.

The colors

With reference to the pigment colorant to select, the proper color pigment is determined by the processing requirements already described and, more importantly, the end use for which the product has been engineered. The following characteristics must be considered:

* Resistance to fading (caused by sunlight, heat or weathering);

* resistance to bleeding; and

* deleterious effect on aging characteristics of the base compound. (Avoid colors that contain copper or manganese). Also, since the colored rubber product must compete in the marketplace, the color cost-per-pound of cured rubber product requires careful selection of the most economical colorant to provide the required results. There are two groups of pigment colorants, inorganic pigments and organic pigments.

Inorganic pigments

Iron oxide colors, lead pigments (such as chrome yellow), ultramarine blue, chromium oxide and cadmium pigments are the ones normally used.

Iron oxides are available as reds, yellows, browns and tans. All give dull shades. The use of synthetic iron oxides is recommended. All are supplied as a fairly fine powder with high gravities. Care must be taken in the selection of iron oxides, as some grades contain impurities, such as copper, that cause degradation of the cured rubber product on aging.

Ultramarine blue pigments can provide shades from a green shade to a red shade of blue. These colorants possess excellent heat and light stability, but are not fast to acid, which limits their use in many outdoor applications and any indoor ones that might be involved with fruit juices, etc. The largest use of ultramarine blue in rubber is for tinting white rubber compounds, just as bluing is used in washing white shirts. Where ultramarine blue is used to obtain full shades, a bright color is possible.

Chromium oxide green provides a blue-green shade, and the color possesses excellent heat and light fastness. It has a high gravity.

Lead pigments include a wide range of chromate yellow pigments and molybdate orange pigments. These provide bright colors with good heat and light fastness characteristics, but they are lead bearing compounds and their dust can flash and burn. In sulfur cured compounds, any moisture may cause lead sulfide spotting. These now have been replaced by organic pigments in most cases.

Cadmium pigments are used only for specialized applications where their particular fastness characteristics are required. The pigment is usually extended onto a base to improve dispersion and handling. They are high in gravity and expensive to use. Also, cadmium is another heavy metal, and rubber compounders tend to avoid such materials.

All inorganic pigments are weak tinctorially. The normal usage level is 2 to 5% of the total weight of the compound. Inorganic pigments are normally more easily dispersed than organic pigments and are less dusty, both because their particle size is larger and their gravity is much higher. Nevertheless, all color pigments are very dusty and manufacturing precautions must be taken.

Organic pigments

Organic pigments are used because they give bright, attractive shades of color and the technical requirements of the particular rubber product can generally be satisfied by selecting the proper pigment or combination of pigments.

For aesthetic reasons, to obtain bright colored rubber products, the organic colors are used, as opposed to the dull color provided by the inorganic pigments. This has resulted in a significant use of organic pigments in rubber compounding. Most organic pigments do not provide as good properties regarding heat, light and bleed resistance, as do inorganic pigments. However, they do provide a much more attractive appearance.

Organic pigments are much more expensive per pound than inorganic pigments, but they are also much more effective per pound. Nevertheless, organic pigments normally are more costly to use.

Table 1 lists the type of organic pigments generally used in rubber compounding by chemical types and color index numbers. These ratings indicate the primary technical characteristics, including heat stability, light stability, soap bleed and open steam curability.

Organic red pigments

Red is probably the most popular color. It can also be the most troublesome because of the chemical nature of the red organic color pigments. There are eight major groups of organic red pigments used in coloring rubber products (table 1). These are: Permanent Red 2B; Pyrazolone Red; Thio Red; Quinacridone Red; Naphthol Red; Lithol Rubine; Lake Red C; and Lithol Red.

Permanent Red 2B, the most widely used type, includes the barium and calcium red salts. The barium salt provides an orange shade red, while calcium salt provides a mote blue shade red. Blends provide a wide variety of shades. These are strong, bright colors and have relatively good fastness to bleeding. They have reasonable heat and light stability, however, they will begin to fade at curing temperatures that exceed 350[degrees]F.

Pyrazolone red provides a color shade that falls between the barium and calcium red 2B shades. The pyrazotone shades do vary, by specific type, but none are significantly different from each other. Pyrazolone reds are bright, clean and transparent colors, being a pure organic pigment and not a salt. Pyrazolone reds have excellent soap bleed resistance and light and heat stability approximately equal to the red 2B colors. Pyrazolone red is an organic red pigment qualified for use in rubber products in contact with food, listed in Chapter 1, Title XXI, Section 177.2600 of the Federal Register, "Rubber articles intended for repeated use." It is more expensive to use than red 2B but is necessary where its particular color shade, better bleed resistance or medical qualification apply,

Thio red is a technically excellent and expensive pigment. It is used only for its unusual shade, often in blends.

Quinacridone reds have outstanding technical characteristics. However, their pricing is generally above $20 per pound. This pigment is qualified for use in rubber products in contact with food, listed in Chapter 1, Title XXI, Section 177.2600, of the Federal Register, "Rubber articles intended for repeated use."

Different salts of napthol red provide shades from the orange red to the blue red. It is an expensive pigment, relative to the permanent red 2B, so it is only used where its outstanding soap bleed resistance dictates its use.

Lithol Rubine, Lake Red C and Lithol Red are lower cost pigments and, at times in the past, there have been substantial amounts used. However, the technical characteristics are relatively poor and, with the emphasis on quality and longer term service of rubber products, these types are being used less and less.

Rubber color masterbatches are often the preferred form used by producers of extruded, molded and calendered colored rubber products. These masterbatches are generally produced at approximately a 50% color pigment level in a polymeric binder. Their usage is generally 1% to 3% on the total weight of the compound. The pyrazolone red and permanent red 2B pigments tend to be hard to disperse, so the use of rubber color masterbatches for these two pigment types is highly recommended.

Organic orange pigments

There are two major types of organic orange pigments used, dianisidine and diarylide oranges. The dianisidine provides a clean and somewhat red shade, while the diarylide shade is more yellow. Both have reasonably good fastness properties, with the diarylide having the better light-fastness characteristics. Both are indicated in table 1. Both products are easier to disperse than the reds discussed earlier, but the use of rubber color masterbatches is common and recommended. The concentrations and uses are the same as for the red masterbatches.

Diarylide yellows

These provide bright, clean shades of yellow. There are four major classes for rubber use that differ in the degree of substitution on the molecule. These types are designated as AAA, AAOT, AAMX and AAOA. This listing is in the order of the least fastness characteristics to the best, as is indicated in table 1. These pigments are considered to be the most easily dispersed of the organic color pigments. However, masterbatches are widely used, and the characteristics of these masterbatches are the same as for the previous colors.

Phthalocyanine blues

This is the primary colorant for blue rubber products. There are many shades, with the red shades (RS) being the most widely used in robber. It is also the least heat stable, so curing temperatures must be considered when using this type of phthalocyanine blue. The non-crystallizing grade is less red in shade, but much more heat stable. The green shade phtalocyanine blue is considerably weaker in strength, but significantly more heat stable and more expensive than the other grades. Phthalocyanine blue pigments are very hard to disperse and should be used in rubber color masterbatch form or color paste form. Characteristics of the rubber color masterbatch are similar to those of the other colors.

Phthalocyanine green

This pigment comes in several shades, varying from a somewhat yellowish green to a somewhat bluish green. It is the most widely used single color pigment for green rubber products, although many green rubber products are produced from blends of yellow, blue or this green. All shades of phthalocyanine green provide excellent technical characteristics and all are difficult to disperse. Again, the use of a rubber color masterbatch is recommended. The characteristics of the masterbatch are similar to those of the other colors.

Color matching

The above provides the basic information regarding the rubber compound to be developed and the colors to be used in it. However, very few rubber products are produced from one color pigment, inorganic or organic. The final color of the rubber item is normally developed by matching some particular color that was deemed to be desirable by the purchaser or marketer of the rubber product being produced by the plant. Color matching, therefore, is a major requirement for the successful production of colored rubber parts. Also, since colored products of any type are related to fashions, new colors are constantly being required.

It is difficult enough to make a rubber product to match another rubber product. However, very often the rubber producer is required to match the color of an entirely different type product, such as lacquered metal chip, a piece of dyed fabric, colored paper, or a colored plastic or painted part. Such samples are usually small and almost always have a different surface texture than the rubber product will have. Thus, the difference in the degree of light reflectance makes the matching job more difficult. Furthermore, since many of these other products can and do use chemically different colorants, and since such products often give different color appearances under different light sources, it can be difficult, at best, and impossible, at worst, to provide a fully satisfactory match in the rubber product.

Blends of colors are most always required, and the blends may well involve both inorganic and organic colors. Matches must be evaluated under both artificial light and daylight sources. Newer techniques even provide several different types of artificial light. This type of matching is done by a skilled laboratory technician, using his eyes and knowledge to develop the best match.

Color computers have now been developed to the point where, in a specific rubber compound, the acceptability of a match can be determined graphically. In fact, with proper programming, a color computer can even assist the colorist in choosing the appropriate pigments and amounts to use. However, at least in the experience of the writers, the human eye still ends up making the final determination.

Where rubber products must be produced to closely match materials that were colored with totally different types of colorants, one may well obtain a metameric match. This is a match that may appear perfect under one light source and quite different under another. The acceptability of the match will be determined by the light source of the application, especially, and the particular testing procedures used by the customer.

Where an exact match is not technically possible, the colorist must adjust his work and his raw materials to provide as close a match as possible under the various testing conditions. This often requires the use of three or more colors and careful adjustment of the other compounding ingredients involved. Adjustments in types and amounts of titanium dioxide can be important.

Conclusion and summary

In conclusion, and to summarize the above information in a general way, inorganic pigments offer the rubber compounder good heat, light and bleed resistance at low cost. These colorants are usually used as powder pigments at 2% to 5% on the total compound weight. The shades provided are usually less bright and clean.

To get brighter, cleaner colors, the rubber compounder must use organic pigments. These are tinctorially much stronger, and they are more expensive per pound. The organic pigments are less heat stable, less light fast and may be less bleed resistant than the inorganic pigments. They are almost always more expensive to use.

Generally, organic pigments are much more difficult to disperse in the rubber compound, are significantly more dusty if used as a powder, and are, in fact, generally used as a rubber color masterbatch. This pre-dispersed form does eliminate dust problems, as far as the color is concerned, greatly improves the uniformity of the color in the final product and provides the colorant in a much more easily weighed and handled form.

The colorants used for rubber products, particularly in special masterbatch forms, are also widely used in crosslinked polyethylene applications. New uses are constantly being developed in new polymer systems, many of which are rubber-like, even if they are not cured by peroxide or sulfur systems.

Table 1--colors

Pigment type             Pigment      Specific      Heat
                            name       gravity      stab.

Dianisidine             Orange 16          1.38         2
Pyrazolone                 Red 38          1.33         1
Thio red                   Red 88          1.82         1
Proprietary             Orange 46          1.71         1
Lake red C               Red 53:1          1.48         1
Phthalocyanine           Green 36          2.20         1
Phthalocyanine            Green 7     2.05-2.12         1
Lithol                   Red 49:2          1.65         3
Lithol                   Red 49:1          1.74         3
Carbazole               Violet 23          1.46         1
Perm. red 2B             Red 48:2     1.70-1.76         1
Perm. red 2B             Red 48:1          1.98         2
Phthalocyanine (NC)     Blue 15:1          1.62         1
Phthalocyanine (RS)       Blue 15     1.58-1.61         1
Phthalocyanine (GS)     Blue 15:3          1.64         1
Naphthol                   Red 23          1.45         2
Pigment green B           Green 8          1.39         3
Diarylide (AAMX)        Yellow 13          1.35         1
Diarylide (AAOA)        Yellow 17          1.39         1
Diarylide               Orange 13          1.35         2
Diarylide (AAA)         Yellow 12     1.22-1.41         2
Hansa                    Yellow 1          1.45         3
Diarylide (AAOT)        Yellow 14          1.40         2
Lithol rubine            Red 57:1          1.55         2
Ultramarine blue          Blue 29          2.35         1
Chromium oxide           Green 17          5.10         1

Pigment type               Light        Soap        Open
                        stab. (1)      bleed       steam
                                      resist.     resist.

Dianisidine                     3           1           1
Pyrazolone                      2           1           1
Thio red                        1           1           1
Proprietary                     3           1           2
Lake red C                      3           2           2
Phthalocyanine                  1           1           1
Phthalocyanine                  1           1           1
Lithol                          3           3           3
Lithol                          3           3           3
Carbazole                       1           1           1
Perm. red 2B                    2           2           2
Perm. red 2B                    2           2           2
Phthalocyanine (NC)             1           1           1
Phthalocyanine (RS)             1           1           1
Phthalocyanine (GS)             1           1           1
Naphthol                        2           1           1
Pigment green B                 2           1           1
Diarylide (AAMX)                1           1           1
Diarylide (AAOA)                1           1           1
Diarylide                       2           1           1
Diarylide (AAA)                 2           1           1
Hansa                           2           3           3
Diarylide (AAOT)                2           1           1
Lithol rubine                   2           2           2
Ultramarine blue                1           1        1(A)
Chromium oxide                  1           1           1

1 - masstone

Heat stability

1 - 350[degrees]F
2 - 330[degrees]F
3 - 300[degrees]F

Light stability

1 - 200 hours
2 - 100 hours
3 - 50 hours

Soap bleed

1 - no bleed
2 - slight bleed
3 - bleed

Open steam

1 - excellent
2 - good
3 - don't use

Heat stability refers to conventional press cures. CV cures or very
short cure cycles usually permit higher temperatures satisfactorily.
Light stability refers to fadeometer tests. Colors with a rating of
2 to 1 are usually satisfactory in general rubber applications.
Colors with a rating of 3 are used in selected applications.