Microwave systems are widely used for the vulcanisation or "curing" of rubber products, especially for the continuous vulcanisation of rubber profiles and the pre-heating of large rubber products like bridge bearings etc. Before we explain the advantages arising from the use of microwave systems, we take a short look at the vulcanisation process and how microwave energy can aid it.
Vulcanisation is the process of converting rubber and similar elastic polymers into durable products by cross-linking the polymer molecules, i.e. building additional bridges between them. While there are also types of cold vulcanisation, microwave systems are always used for hot vulcanisation, i.e. the rubber is first mixed with vulcanisation agents, accelerators and other chemicals like fillers etc, then formed into the final shape and finally heated (170 - 220 ℃) for a certain period of time, until the chemical reaction is finished and the part obtains is durable properties. While certain polar rubber types readily absorb microwave energy, e.g. Neoprene (CR) and Nitrile Rubber (NBR), other types like Natural Rubber (NR), Ethylene Propylene Diene Monomer (EPDM) and Styrene Butadiene Rubber (SBR) do not absorb microwave energy and therefore require certain fillers to make them suitable for microwave vulcanisation. The most commonly used filler is carbon black, because it not only absorbs the microwave energy, but also increases the tensile strength and abrasion resistance of the final product. Where carbon black cannot be used, certain vulcanisation agents, accelerators or other fillers serve as absorbers for the microwave energy. Microwave systems are mainly used for sulphur based vulcanisation and cannot be used for some other types of vulcanisation, e.g. peroxide based.
The continuous vulcanisation of rubber profiles is the main application of microwave vulcanisation systems. Before the introduction of microwave systems rubber profiles were usually heated by infrared radiation or hot air / liquids. However, rubber is a poor conductor for thermal energy, and especially in the case of profiles with a larger cross-section this did not only lead to unacceptable long vulcanisation times, but also to other problems like "over-curing". Over-curing, also know as de-vulcanisation, is the degrading of the material properties due to excessive exposure to heat. In order to completely vulcanise profiles with a large cross-section they had to be heated for an extended time, which caused the surface of the profiles to start degrading. Modern multi-mode microwave vulcanisation systems, on the other side, work via volumetric heating, which, depending on the type of rubber processed, has distinctive advantages:
Because the microwave energy is capable of penetrating the rubber profile, energy transfer does not take place via the surface of the profile, but via volumetric heating. This means that the rubber profile is heated not only much more rapidly, but also much more evenly, especially in case of profiles with a large cross-section, eliminating problems like over-curing. Another advantage is that the profile leaves the microwave at vulcanisation temperature, the infrared and hot air tunnels formerly used to heat up the profile are now only needed to maintain the vulcanisation temperature for a certain time, making it possible to run them at much lower temperature, thus making the process more energy efficient. The fast heating process inside the microwave also permits to shorten infrared and hot-air tunnels, saving valuable work floor space and saving additional energy. Because of the fast and even heat-up of the profile, the final product is more evenly cured and has superior properties compared to traditionally vulcanised profiles.
Sponge or "foam" rubber is a special type of rubber which contains a considerable amount of small cells or pores filled with gas, giving it superior thermal insulation properties. The final profiles are used for the thermal insulation of hot water pipes, in refrigeration plants, to protect people from contact with hot / cold surfaces etc.
For the production of sponge rubber a blowing or foaming agent is mixed into the rubber compound. During the initial heat-up, this blowing agent releases a certain amount of gas, which forms fine gas cells in the rubber profile and causes it to swell. With traditional vulcanisation systems, where heat energy was transferred through the surface of the profile, the outer part of the profile started to "blow" first. However, because sponge rubber is an excellent thermal insulator, the blowing of the outer layer insulated the inner part of the profile, not only stopping the blowing agent from releasing gas, but also causing the profile to be only partially vulcanised. Because the outer part already reached vulcanisation temperature and hardened, even extended heating times could not lead to an acceptable cell formation.
In contrast, when heated in a microwave vulcanisation system the rubber profile is heated up evenly over the cross-section, allowing the blowing agent to release gas almost simultaneously over the whole cross-section, resulting in gas pores of even size and distribution. Because the microwave energy is not absorbed by the newly formed gas pores, it continuous to penetrate the profile and heats it up to vulcanisation temperature, resulting in an evenly blown and cured product with excellent mechanical and thermal properties.
Due to the reasons mentioned above, microwave vulcanisation systems are also the optimum solution for the production of sponge rubber sheets. However, due to the extremely large cross-section of these sheets even multi-mode microwave cavities might lead to an uneven heat-up and the problems related with it. Therefore we offer special plants for the production of these sheets, which are equipped with a tailor-made microwave energy distribution system. This distribution system not only ensures perfectly even heat-up even of thick and wide sheets, but also offers an extremely high efficiency and easy scalability, thus adapting perfectly to your production requirements.
Curing of large, solid rubber parts like bridge bearings, engine mounts etc usually takes place in heated moulds mounted into hydraulic presses. As the heat transfer only takes place through the surface of the product, the heating process is time consuming and can lead to negative effects like over-curing etc. In order to shorten down the vulcanisation time and to avoid possible problems, these parts can be pre-heated using microwave ovens. Because of the large dimensions of the parts, pre-heating usually takes place in batch-style ovens with an operating frequency of 915 MHz, not only because of the better penetration depth at this frequency, but also because the magnetrons are available with an output power ranging between 15 - 100 kW, ensuring that the parts are pre-heated within a reasonable time. Pre-heating via microwave systems leads to shorter production times, less energy consumption, and a more even vulcanisation of the final product, improving the mechanical properties like compression set etc.
Should you require any further information regarding the above applications please contact us.