A number of ingredients are added to both natural and synthetic rubber in order to obtain certain desirable properties. By convention, mix formulations begin with the amount of the designated elastomer—for instance, natural rubber (NR), butadiene rubber (BR), or styrene-butadiene rubber (SBR)—given as 100 parts by weight. The amount of each other ingredient is then expressed in parts by weight added per 100 parts by weight of the elastomer. If two or more elastomers are used, then they are shown in the recipe as fractions of 100 parts—for example, “NR, 60 parts; BR, 40 parts.” When the elastomer contains oil already added by the producer, allowance is made for this dilution in the recipe. For example, if SBR 1702 is used, the mix formulation may begin “SBR 1702, 137.5 parts by weight,” because that amount of SBR 1702 contains 37.5 parts by weight of oil and 100 parts by weight of SBR elastomer.
The cure package
The most important ingredients are those, known as the cure package, that cause interlinking reactions to take place when the mix is “cured.” In order to minimize the risk of premature cure, they are usually added at the end of mixing. The cure package usually consists of sulfur and one or more “accelerators” (e.g., sulfenamides, thiurams, or thiazoles), which make the sulfur interlinking reaction occur faster and more efficiently. When the ratio of sulfur to accelerator is less than one, the recipe is known as an “efficient vulcanization” (EV) system and gives products with sulfur interlinks of shorter length. EV products have improved resilience but lower strength.
Two other ingredients that play an important role in vulcanization chemistry are known as “activators,” commonly zinc oxide and stearic acid. These compounds react together and with accelerators to form a zinc sulfurating compound, which in turn is the key intermediary in adding sulfur to a diene elastomer and creating sulfur interlinks.
Other less widely used interlinking reagents are sulfur compounds known as sulfur donors—e.g., tetramethylthiuram disulfide—which introduce monosulfide interlinks between polymer molecules, and peroxides, notably dicumyl peroxide. Peroxides decompose on heating to form radicals, which abstract hydrogen from groups on the polymer molecules. Carbon radicals formed in this way on different molecules then combine to create carbon-carbon interlinks. Although products with C―C interlinks are more resistant to heat and oxidative attack, their strength is lower than products with sulfur interlinks. In addition, monosulfide links give weaker products than polysulfide links. This paradoxical result—that inherently strong C―C interlinks give the weakest products, whereas inherently weak polysulfide links give the strongest products—is attributed to the fact that weak interlinks will break under stress before the main chain does, so failure of the elastomer molecule itself is delayed.
Fillers
Almost every conceivable material has been added to rubber in attempts to cheapen and stiffen it. Two particulate fillers are outstanding because they also strengthen elastomers to a remarkable degree. The most important, used almost universally, is finely divided carbon black, prepared by incomplete combustion of oil or gas. Carbon black consists of small spherical particles having diameters of only 10–100 nanometres (10–100 billionths of a metre) and made up of concentric graphitic layers of carbon. The surface of the particles also contains some oxygen and hydrogen. During manufacture, chains of particles become fused together to create extended open “structures,” still very small in size.
Another reinforcing filler with particles of similar shape and size is finely divided silica (silicon dioxide, SiO2), prepared either by burning silicon tetrachloride or by acid precipitation from a sodium silicate solution.
Both carbon black and silica, when added to a mix compound at a concentration of about 30 percent by volume, raise the elastic modulus of the rubber by a factor of two to three. They also confer remarkable toughness, especially resistance to abrasion, on otherwise weak materials such as SBR. If greater amounts are added, the modulus will be increased still further, but the strength will then begin to fall. Disadvantages of reinforcement with carbon black or silica are lower springiness (resilience) and a decrease in the initial high stiffness after flexing.
For a filler to be reinforcing, it appears that the fundamental particles must be small—for instance, 10–50 nanometres in diameter—and that the elastomer must adhere well to them. If either of these conditions is absent, the reinforcing power will be lessened. Indeed, the smaller the particle size (and hence the greater the surface area), the greater the observed reinforcing effect. It is still not understood how fine particles are able to confer high strength and toughness on elastomer compounds. Strengthening and toughening are possibly associated with debonding of highly stressed elastomer molecules from the filler particles, reducing the stress on the polymer chains and delaying catastrophic fracture.
Protective chemicals
Certain additives confer resistance to heat, sunlight, oxygen, and ozone. Amines, particularly paraphenylene diamines, are powerful retarders of oxidation, or antioxidants. Added to rubber compounds in small amounts (1–2 percent), they appear to disrupt the free-radical oxidation reactions that lead either to molecular rupture and softening or to increased interlinking and hardening as rubber ages. Hindered phenols, another antioxidant class, are less powerful than amines but have less tendency to stain light-coloured rubber compounds. Small amounts of certain metals, notably copper, manganese, and iron, act as powerful catalysts of oxidation; sequestering agents are therefore used to block the action of these elements if their presence is unavoidable.
Atmospheric ozone reacts readily with elastomers containing C=C double bonds, leading to breakage of molecules lying in the surface. As a result, small, deep fissures, termed ozone cracks, are formed if the rubber is stretched slightly (by more than about 10 percent). Cracks one millimetre long appear in unprotected rubber after only a few weeks of exposure to a typical outdoor concentration of ozone, about 5 parts per 100 million. However, certain diamines (e.g., alkyl-aryl paraphenylene diamines) prevent cracking, probably by competing with the C=C bonds in rubber for reaction with ozone. These antiozonants “bloom” to the surface and react there, protecting the rubber. A few percent of an antiozonant is therefore commonly included in the mix formulation of rubber compounds based on unsaturated elastomers. An alternative method of protection, often employed simultaneously, is to include a few percent of a microcrystalline paraffin wax in the mix formulation. Because it is incompatible with the elastomer, the wax blooms to the surface and forms a protective skin.
Plasticizers and processing aids
Liquids are added to elastomer mixes in order to soften and plasticize the compound, either in processing or later in use. For example, elastomers with high glass transition temperatures (and correspondingly slow molecular motions) can be improved by adding low-temperature plasticizers—i.e., compatible liquids that act as internal lubricants. Plasticizers must have low vapour pressure and a high boiling point in order to be retained within the compound over long periods of service. Examples are aliphatic esters and phthalates. Phosphate plasticizers also confer a measure of flame resistance. Other liquids are added to rubber compounds as processing aids in order to make mixing and extrusion easier. Typically, 5 percent of a petroleum oil is used.
