As an elastomer, rubber is characterized by its ability to be temporarily deformed, often under a great degree of tensile stress, and return to its original state with little or no permanent degradation. Non-vulcanized rubbers have molecular chains that are not cross-bonded, lending them a high amount of plasticity and the capacity for forming through heat treatments. On the other hand, vulcanized rubbers remain in their fully elastic state, precluding thermal forming methods, but with greater weathering, oxidation, and wear resistance, as well as tensile strength.
Since both rubber and vulcanized rubber are greatly affected by conditions such as temperature range, presence of corrosive elements, and material stability, numerous considerations must be taken into account when coloring these materials. The presence of a toxic or destabilizing substance within a pigment or the improper application of coloring agents can severely damage a rubber product run, reducing cost-efficiency and hurting a manufacturer’s bottom-line.
Impurities
The pigments used for coloring rubber should not contain any substances that might degrade the material characteristics or inhibit the vulcanization of the rubber. Certain metals, such as manganese or copper, if introduced in even trace amounts, can greatly destabilize a rubber compound by causing accelerated aging. Rubber coloring pigments, therefore, are restricted to a 0.01 percent combined content of those two metals. However, some pigments, such as copper phthalocyanine, can have slightly higher amounts of incompatible metals, but usually no more than 0.015 percent of the total composition. Manufacturers frequently test pigments for the presence of these antagonistic substances.
Color Consistency
Aesthetic qualities, such as the uniformity and fastness of the alteration, are important considerations when coloring any material. In the case of rubber, drift resistance is one of the central criteria in choosing an appropriate pigment. Many manufacturers use a migration test to determine if a given pigment will cause colors to run, fade, or bleed into other surfaces. This test employs a range of different pigment concentrations, each of which is applied to a standard white rubber sheet. The pigmented sheet is then quickly vulcanized under a hot steam exposure process, usually for no longer than half an hour. Engineers place cotton fabric against the colored rubber to ascertain if color has bled into the fabric or into the rubber.
Heat Resistance
Since rubber is influenced by temperature changes, the pigmentation of a rubber product must be able to withstand certain heat ranges or react to thermal treatments in a particular way. Like migration tests, heat resistance tests involve multiple pigment concentrations being tested simultaneously. These concentrations usually contain between 0.01 and 1 percent color composition and a 10-to-1 ratio of chalk. After the pigments are applied, the newly colored rubber sheets undergo a hot vulcanization process that can reach nearly 300 degrees Fahrenheit. The results are then compared to an uncolored white rubber sheet that goes through an identical heating procedure. If the pigmentation cracks, fades, bleeds, or in any way degrades the material quality of the rubber base, that concentration is deemed unsuitable for coloring purposes.
Types of Coloring Agents
Industrial coloring methods are categorized by the International Organization of Standardization, which provides standards for manufacturing most kinds of pigments and dyes. This organization also delineates the chemical and physical properties of various pigments, as well as the techniques for testing coloring materials. Many coloring manufacturers also employ the Color Index International as the standard authority on naming specific colors and identifying their characteristics on the color spectrum.
Rubber coloring pigments are typically applied in powdered or granulated form. Some examples of the most commonly used types of coloring agents in rubber fabrication include:
- Pyrazolone Orange/Yellow: This pigment is useful in a wide variety of rubber products due to its capacity for efficient vulcanization and low rate of bleeding in natural rubber. Its high level of water resistance also makes it applicable for sponges, sealers, and many underwater rubber apparatuses.
- Disazopyrazolone Red: When used in rubber materials, disazopyrazolone red displays excellent lightfastness, meaning it has a low rate of fading from exposure to light. In addition, it offers reduced potential for color drift or bleeding, and high water and solvent resistance.
- Phthalocyanine Blue: Although produced in limited volumes, this additive yields a high tolerance for heating treatments, and does not bleed into rubber or fabric. It is also resistant to both hot and cold water, soaps, certain acids, and some corrosive solutions.
These coloring agents are a small portion of the wide array of pigments and dyes available in rubber production. The Color Index International lists nearly 30,000 different coloring products, including many used by rubber manufacturers. But despite the extensive variety of rubber coloring agents, nearly all of them are judged on their material stability, resistances, potential for color migration, and reactions to different temperature ranges.