Ingredients in rubber compounds are a large group of components, including curing agents, accelerators and vulcanization activators, antioxidants, plasticizers, fillers and special additives (blowing agents, dyes, etc.). The list of ingredients in phr (parts per hundred rubber) called rubber compound recipe. Most of the rubber products can also contain reinforcing materials – fibers, filaments, wires, metal.
The most common curing agent is natural premium powdered sulfur in the form of yellow crystals of the orthorhombic form obtained during the processing of sulfur ores by grinding and sifting.
Vulcanization accelerators are adding to the compound to accelerate the cross-linking of sulfur and improve the physical-mechanical properties of the rubber. Some accelerators are curing agents at the same time – sulfur donors.
Accelerator activators increase activity – oxides, and hydroxides of zinc, lead, magnesium, calcium, cadmium, bismuth or combinations thereof, especially in the presence of stearic, palmitic, oleic acids and their zinc salts. Depending on the temperature of critical effect and cure rate they are divided into ultra accelerators, medium, and high activity. The most common classification of accelerators is by chemical composition.
Antioxidants in the action mechanism are divided into preventive and chain terminators. Preventive antioxidants include mercaptans, sulfides, dithiocarbamates, dithiophosphates, esters of phosphorous acid. They inhibit initiation by decomposing peroxides, absorption of light radiation or deactivation (passivation) of metals. Antioxidants that are breaking chains by binding peroxide radicals include amines and phenols. At the same time amines are dyeing rubber compounds and therefore they are recommended for use only in black rubbers.
By combining antioxidants with the same mechanism of action is usually observed additivity of their action. A synergistic effect when a combination of two antioxidants is more effective than any of them in the same concentration as observed in different mechanisms of action. There is a synergistic effect of substituted phenols and amines with phosphorous acid esters, amines with sulfides. The inhibitory effect is enhanced by the fact that one inhibitor inhibits the formation of peroxides and thus protects the second inhibitor of rapid consumption.
Plasticizers – a large group of low-molecular substances that modify the properties of the elastomer, increasing the flexibility of the macromolecules and mobility of supramolecular structures, but at the same time reduce their strength and hardness. Plasticizers are well combined with the rubber and improving the low-temperature resistance of rubber compounds. Processing aids partially soluble in rubbers, which facilitates the processing of rubber compounds and regulation of their tackiness, improves the dispersion of fillers. Plasticizers can be products of oil or coal refining (fuel oil, bitumen, oils, resins, waxes, and ceresins) herbal products (pine resin, rosin, vegetable oil, fatty acids) and synthetic compounds (polyesters, oligoester acrylates or polydienes).
Fillers are divided into active (enhancers) that enhance the strength, abrasion resistance and tensile strength of rubber compound, and inert (diluents). The main reinforcing filler is carbon black – black fine powder, a product of thermal decomposition of gaseous or liquid hydrocarbons. The inorganic fillers include natural minerals (chalk, kaolin, bentonite, shungite, talc) and synthetic reinforcing (colloidal silicic acid, magnesium oxide and zinc, calcium silicate and aluminum fluoride, calcium fluoride) allow to obtain high strength, colored, heat-resistant, non-flammable and corrosive resistant rubber compounds from non-crystallizing rubbers.
Dyes for colored rubbers are divided into inorganic and organic. The color intensity increases with decreasing particle size and their ability to be dispersed in the rubber. The color of rubber affects on its resistance to aging under the influence of light. White, yellow and green colors of rubber are protective, and blue color accelerates the aging process, and therefore rarely used. This is due to the rubber’s absorption or reflection of a certain spectrum. Titanium dioxide ( a mixture of TiO2 c BaSO4), lithopone (a mixture of ZnS with BaSO4), zinc oxide, zinc sulfide and titanium oxide are using as white pigment. The intensity of the white color increases with increasing rubber differences in refractive index between the rubber and pigment, and also depends on the size and shape of the particles and the other ingredients of the rubber compound. Coloring rubber in other colors is carried by the white tone. Iron oxide Fe2O3, often in conjunction with lithopone and sulfurous antimony, selenium, cadmium compounds, cinnabar mercury and sulfur are using as red pigments. Chromium oxide gives blue-green color, and blue – ultramarine.