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Rubber compounds or raw rubber are semi-finished products consisting of elastomers, curing agents, fillers, plasticizers, stabilizers, and other ingredients that, in specific compositions and ratios, provide certain properties to the rubber products obtained through the vulcanization of the rubber mixture.
The technological process of producing rubber compounds involves mechanical, often sequential mixing of rubber and ingredients on special technological mixing equipment, such as mixing rolls. The technological lines used for mass production of rubber compounds include:
•Kneader machine for rubber mixing – for mixing rubber and primary components.
•Mixing rolls or a set of two or three rolls for refining the rubber composition into a mixture and adding the vulcanizing agent (typically sulfur, or less commonly, thiuram D) and forming the mixture into sheets (giving it a convenient stock form).
•Festoon-type cooler or a cold-water bath to reduce the temperature of the rubber compound and prevent it from burning.
Rubber compounds are supplied in the form of sheets of various sizes with a thickness ranging from 0.5 to 30 mm, suitable for processing into products through molding and vulcanization. Depending on their purpose, rubber compounds are produced in different grades.
Ingredients for Rubber Mixing
The ingredients of rubber compounds form a large group of components, including curing agents, accelerators, and activators for vulcanization, antioxidants, plasticizers, fillers, and special additives (blowing agents, dyes, etc.). The list of ingredients is given in phr (parts per hundred rubber), commonly referred to as the rubber compound recipe. Most rubber products can also contain reinforcing materials—fibers, threads, wires, and metal.
The most common curing agent is high-quality powdered sulfur, in the form of yellow orthorhombic crystals, obtained through grinding and sieving during sulfur ore processing.
Vulcanization accelerators are added to the compound to speed up sulfur cross-linking and improve the physical and mechanical properties of the rubber. Some accelerators also act as curing agents by donating sulfur.
Activator accelerators increase the activity of the process—oxides and hydroxides of zinc, lead, magnesium, calcium, cadmium, bismuth, or their combinations, especially in the presence of stearic, palmitic, or oleic acids and their zinc salts. Depending on the critical temperature and curing speed, accelerators are divided into ultra-accelerators, medium, and highly active types. The most common classification of accelerators is based on their chemical composition.
Antioxidants are divided by their mechanism of action into preventive and chain terminators. Preventive antioxidants include mercaptans, sulfides, dithiocarbamates, dithiophosphates, and phosphoric acid esters. They inhibit initiation by decomposing peroxides, absorbing light radiation, or deactivating metals. Chain-breaking antioxidants that bind peroxy radicals include amines and phenols. Amines, however, can discolor rubber compounds and are therefore recommended only for black rubbers.
When combining antioxidants with the same mechanism of action, additive effects are usually observed. A synergistic effect occurs when a combination of two antioxidants is more effective than either one in the same concentration. This synergism is observed between substituted phenols and amines with phosphoric acid esters or sulfides, which enhance the inhibition process.
Plasticizers are a large group of low-molecular-weight substances that modify the elastomer’s properties by increasing the flexibility of macromolecules and the mobility of supramolecular structures, though they also reduce their strength and hardness. Plasticizers improve the low-temperature resistance of rubber compounds and aid in processing by regulating tackiness and dispersing fillers. They can be derived from petroleum or coal (fuel oil, bitumen, oils, resins, waxes, and ceresins), plant products (pine resin, rosin, vegetable oil, fatty acids), or synthetic compounds (polyesters, oligoester acrylates, or polydienes).
Fillers are divided into active (reinforcing) agents that enhance the strength, abrasion resistance, and tensile strength of the rubber compound, and inert (diluting) fillers. The main reinforcing filler is carbon black, a fine black powder produced by thermal decomposition of gaseous or liquid hydrocarbons. Inorganic fillers include natural minerals (chalk, kaolin, bentonite, shungite, talc) and synthetic reinforcements (colloidal silicon dioxide, magnesium oxide, zinc oxide, calcium silicate, aluminum fluoride, calcium fluoride). These fillers allow for the production of strong, heat-resistant, non-flammable, corrosion-resistant rubber compounds in a variety of colors.
Pigments for coloring rubber are divided into inorganic and organic types. The intensity of color increases with the reduction in particle size and their dispersibility in the rubber. The color of the rubber also affects its resistance to aging due to light exposure. White, yellow, and green colors offer protection, while blue accelerates the aging process and is thus rarely used. The white pigment commonly used is titanium dioxide (a mix of TiO2 and BaSO4), lithopone (a mix of ZnS and BaSO4), zinc oxide, zinc sulfide, and titanium oxide. The whiteness intensity increases with the difference in refractive index between the rubber and the pigment and depends on the size, shape, and composition of the particles. The rubber is colored in other shades starting from a white base. Red pigments include iron oxide (Fe2O3), often combined with lithopone, antimony sulfides, selenium, cadmium compounds, cinnabar mercury, and sulfur. Chromium oxide gives a blue-green color, and ultramarine gives blue.
Many other ingredients can be used to impart different properties to the compound. Custom rubber mixing is a science, focusing on providing the necessary rubber characteristics by adjusting the ratio of components and compensating for their interactions. As a result, most manufacturers consider their rubber compound recipes proprietary. Improving mechanical properties while maintaining processability can require hundreds of hours of testing with various formulations.