Different Methods of Vulcanising Rubber
There are many acceptable ways to cure rubber compounds. Vulcanisation is achieved by heating the compound at a fixed temperature for a fixed length of time under a fixed pressure to produce covalently bonded cross-links. So, the three vital variables are time, temperature, and pressure. Changing any one of these variables will affect how long it will take for the rubber to cure and its quality.
There are two branches of methods within vulcanisation. Open vulcanisation and continuous vulcanisation. Open vulcanisation is when a finite amount of the rubber product is produced and placed into an oven or autoclave to be vulcanised. Continuous vulcanisation is a single-line operation, where the rubber product continuously feeds straight into the vulcanising medium and is vulcanised.
Let’s have a look at the different methods.
The rubber moulding processes (compression, transfer, injection, blow or vacuum) create the perfect environments for the rubber to vulcanise. See our article on rubber moulding for a better understanding of each of the moulding processes.
Hot Air Ovens and Tunnels
An open vulcanisation method. Hot air ovens can be used to vulcanise thin products. They can also be used in the post vulcanisation stage after a product has been precured in a mould. This second use is to remove peroxide decomposition products from items cured with peroxides. Hot air ovens are not very efficient due to the poor heat transfer of the air. On top of this, longer cure times at lower temperatures are necessary to prevent the development of porosity and deformations in the unvulcanised products.
Hot air tunnels are a continuous version of hot air ovens where the product is passed through a series of heated tunnels at a rate that will ensure that vulcanization is complete by the time it reaches the end.
An open vulcanisation method. Autoclaves, also known as “curing pans” or “vulcanisers”, are pressure vessels filled with steam that are used to cure rubber products not contained in moulds (hose, extruded sections, coated cloth, and small cable batches). The process involves saturated steam under pressure, which acts as an inert gas, enabling better heat transfer, thus higher temperatures can be used, and shorter cure times are possible making this process more desirable than the air oven.
Two benefits of this method are, first, most rubber compounds are designed for steam autoclaves and second, the pressure of the steam stops bubbles trapped in the rubber compound from expanding and bursting.
Steam tunnels, like hot air tunnels, are a continuous version of autoclaves – passing the product through a series of steam tunnels to achieve vulcanisation.
An open vulcanisation method. Water cures can be used for products not affected by immersion. This method is useful for large items like rubber lined containers and is especially useful for hard rubber compositions. Due to the product having direct contact with the water, better heat transfer than the hot air or autoclave methods can be achieved. Consequently, less deformation and faster cures are obtained.
Liquid Curing Method (LCM)
The continuous vulcanization in liquid baths is called the liquid curing method. In this process, the extrudate is run through a suitably hot bath immediately after leaving the extruder. The bath temperature typically ranges between 200°C-300°C consisting of salt mixtures, potassium nitrate mixtures, polyglycols, silicone oil and metal alloys.
Salt baths are a commonly used LCM. Salt is excellent for curing due to its relatively short length curing units; good heat exchange properties, meaning it can be used at extremely high temperatures; and can vulcanise compounds that are electrically conductive. This method is particularly suited for peroxide cured compounds, which must be cured in the absence of oxygen. There is also a pressurised version of the liquid salt vulcanisation methods (PLCV).
A continuous vulcanisation method. Flat rubber goods such as conveyor belts and floor coverings are vulcanized continuously by the Rotocure method. This process involves the use of a wide steel band that presses the product against large, slowly rotating heated drums. Slow rotation of the drum allows vulcanization to occur after approximately 10 minutes of contact time. Belt curing presses are also used; however, this system is not completely continuous although it enables long lengths of belting to be made.
A continuous vulcanisation method. Infrared tunnels are very similar to hot air tunnels, except that the heat is supplied by infrared bulbs.
A continuous vulcanisation method. Fluidized beds are efficient vulcanization systems. They consist of small particles suspended in a stream of heated air. They are normally used for continuous vulcanization of extrusions. Heat transfer is approximately 50 times greater than with hot air alone.
Thin articles may be vulcanized by treatment with sulphur monochloride S2Cl2 by dipping in a solution or exposure to its vapours. The process has been essentially replaced by ultra-accelerators, which are capable of curing at room temperature.
High Energy Radiation
Systems using either gamma radiation from cobalt 60 or electron beams have been used for vulcanization. The electron beam method has been used to cure both polyethylene and silicone rubbers, generally accomplished by passing the material through a beam on a conveyor.
A continuous vulcanisation method. Ultra-high frequency fields developed by alternating electromagnetic circuits can warm up large sections from the inside out, curing the rubber quickly and evenly. The process requires polar rubber mixtures since nonpolar materials will not absorb the energy produced. It is possible to warm articles up to 200°C within 30 seconds. This method can vulcanise rubber compounds that are electrically conductive.
Post-curing is a process that may be used for some elastomer types to improve one or more properties of the vulcanizate. It requires circulation in hot-air ovens with a constant fresh air supply and can last several hours at a high temperature. Some rubber parts may then be painted to protect against ozone, oils, acids, chemicals etc.