What is Vulcanisation?
Vulcanisation is the process that gives rubber its elastic properties. Prior to this process rubber is has more of a gum-like consistency. It's a lot more malleable and doesn't return to shape. Vulcanisation brought about a change in public opinion with regards to rubber. The capability of rubber going back to its original shape enabled this material to be used for many applications.
What is Vulcanisation?
Vulcanisation is the irreversible process of curing elastomers. Originally, vulcanisation referred exclusively to the treatment of natural rubber with sulphur, but this has grown to include the treatment of synthetic rubbers with other curatives (such as peroxide and metal oxides). During vulcanisation, cross-links are formed between the polymer chains within the rubber, resulting in improved elastic properties over a greater temperature range. For more information on how cross-links are formed and what they do, check out our section on Curing and Crosslinks.
What are the 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. 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.
How can you vulcanise rubber with 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. The various methods of rubber moulding create the perfect environments for the rubber to be vulcanised.
The second use for hot air ovens 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.
How do autoclaves vulcanise rubber?
Autoclaves is an open vulcanisation method that is used for industrial purposes. They're cylinder-shaped pressure vessels that seal and process rubber parts. Once inside, the rubber is subjected to heat and pressure that enables crosslinks to form on the rubber. These crosslinks are what give the rubber its elasticity. The autoclave's vulcanisation process improves the rubber parts in every way with increased tensile strength, hardness and improving its longevity and durability in the harshest conditions.
Inside the autoclave saturated steam under pressure acts as an inert gas, enabling greater heat transfer, thus higher temperatures can be used, and shorter cure times are possible making this process more desirable than the air oven.
Autoclaves can also be used in the medical industry as a means to sterilise materials effectively removing all germs and pathogens.
Two benefits of autoclaves are:
- Most rubber compounds are designed for steam autoclaves
- The pressure of the steam stops bubbles trapped in the rubber compound from expanding and bursting.
- Great for producing long extrusions like fenders and bumpers.
Steam tunnels, like hot air tunnels, are a continuous version of autoclaves - passing the product through a series of steam tunnels to achieve vulcanisation.
What is water curing?
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.
What is the Liquid Curing Method (LCM)?
The continuous vulcanisation 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 commonly used with the LCM. Salt is excellent for curing due to its relatively short length curing units and good heat exchange properties. This means salt 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.
What is rotocure?
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 enabling long lengths of belting to be made. This system however is not completely continuous.
How can you vulcanise rubber with infrared tunnels?
A continuous vulcanisation method. Infrared tunnels are very similar to hot air tunnels, except that the heat is supplied by infrared bulbs.
How can you vulcanise rubber with fluidised beds?
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.
How does cold vulcanisation work?
Thin items may be vulcanized at room temperature by treatment with sulphur monochloride S2Cl2 then dipping in a solution or exposure to its vapours. The process has been essentially replaced by ultra-accelerators which also kick-start the vulcanisation process at room temperature.
How do you vulcanise rubber with 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.
How do you vulcanise rubber with microwave systems?
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 items up to 200°C within 30 seconds. This method can vulcanise rubber compounds that are electrically conductive.
What is post-curing?
Post-curing is a process that may be used for some elastomer types to improve one or more properties of the vulcanisate. 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.