A compound is a mixture of chemicals which are added together to combine their desired properties. Rubbers on their own have limited commercial application due to their mechanical properties. These properties can be improved through vulcanisation Curing and Crosslinks. Another way of improving the rubber properties is by adding different materials to the rubber in a compound.
Rubber compounding is a complicated science and rubber technologists need a very technical background to be able to create successful compounds. Not only must the materials chosen wisely but the ratio of the ingredients must be chosen specifically to reach the desired properties of the final product. It can take hundreds of iterations to perfect the properties of the rubber compound. Each ingredient included in the compound has a specific role and affects the final rubber product obtained. Ingredients can be added to the base rubber compounds to improve mechanical properties, improve processability or to lower the cost of the final product. The quantity of each ingredient in the compound is expressed per 100 parts.
The basic ingredients in a rubber compound are the rubber compound and the vulcanising agent. The rubber is the base of the compound and provides the main rubbery qualities of the final mixture. Usually, sulphur or peroxide is added as the vulcanising agent. Other vulcanising agents include amines, silanes, and anilines. These chemicals cause crosslinks to form in the rubber. To learn more about this process, read our article on Curing.
The strength of a rubber can be improved by adding a reinforcing filler such as Carbon Black to the compound. Carbon Black, shown in Figure 1, is a powder containing fine particles of mainly carbon, produced by incomplete combustion. Carbon Black has nanoparticles with various functions such as ultraviolet absorption and conductivity. The particle and structure size of the Carbon Black can affect its dispersibility and blackness. In general, increasing the size of the particle or the structure will decrease the blackness of the compound but will improve the dispersibility of the particles.
Most rubbers are black in colour due to the addition of Carbon Black. Other reinforcing fillers including white clays and mineral fillers and pigments can be used to create coloured compounds, but white fillers usually offer less reinforcement than Carbon Black.
Carbon Black increases the strength, abrasion resistance and weathering properties as it absorbs harmful UV radiation. However, adding Carbon Black to the compound can cause the material to become brittle. To counterbalance this, oils such as processing, or extender oils are added to the compound. These can also be referred to as plasticisers, softeners, or processing aids. The addition of the oils helps to incorporate the dry ingredients and can also make the resulting material easier to process by reducing its viscosity. Most rubber compounds contain a significant amount of these oils as well as Carbon Black to increase the strength of the material as much as possible without stiffening the material. The ratio of Carbon Black to oil must be chosen carefully depending on the intended purpose of the rubber.
Chemicals such as zinc oxide and stearic acids can be added to the compound. These chemicals are known as accelerators or activators as they can speed up the reaction of vulcanization. The traditional method of sulphur curing developed by Goodyear used to take hours but with the addition of accelerators it can now be done in minutes.
Speeding up the vulcanisation reaction from hours to a few minutes significantly decreases the processing time of the rubber material. However, these accelerators make the vulcanisation reaction so fast that some crosslinks will be formed before the rubber has been shaped into its final form and could lead to scorching of the material. To allow time for the rubber to be shaped properly before the vulcanisation occurs, “retarders” are added to the compound. These retarders delay the crosslinking reaction until the rubber mixture is at the curing temperature.
Many rubbers contain double bonds in their polymer chains which allows them to be easily vulcanised with sulphur. However, these double bonds present could react with the oxygen or ozone in the air. This can cause the rubber to perish over time. To prevent this phenomenon, “antioxidants” and “antiozonants” can be added to the rubber compounds. These chemicals help to improve the service life of the compound.
Antioxidants help protect the compound from high temperature while in use and while the compound is being mixed. These ingredients can absorb free radicals that can break the polymers bonds and reduce service life of the compound.
A rubber compound can become a very complicated and intricate mixture including rubber polymers, reinforcing fillers, accelerators, retarders, antioxidants, and antioxidants. Each of these ingredients and their ratios can influence the final material and therefore rubber compounders require a lot of skill and knowledge to create successful mixtures. The rubber compounds can be manipulated in many ways to tailor the compound for their specific job. Compounds can be made to have high oil resistance, higher abrasion resistance, higher operating temperatures or lower compression set for example.