Working in collaboration with Isocoll Chemicals, Steinbeis develops an elastomer surface coating system based on filled butyl rubber.
There has been a sharp rise in demand for new materials with predefined electrochemical and mechanical properties, primarily because of the inability of existing materials to meet current environmental requirements. There is particularly strong interest in natural rubbers and synthetic rubber polymers, whose properties can be altered by using modified fillers and adapted production methods. Butyl rubber is made from different types of synthetic rubber materials and in many respects is superior to other kinds of artificial rubbers, as well as epoxides and silicones. Intelligent Functional Materials, Welding and Joining Techniques, Implementation, the Steinbeis Innovation Center in Dresden, joined forces with chemicals specialist Isocoll to develop a butyl rubber compound capable of meeting high electromagnetic protection requirements.
With its low shear modulus and low elastic linear deformation, butyl rubber does not count as a structural adhesive and is frequently used in automotive fields, air conditioning systems, ventilation, metal engineering, construction, the glass panel insulation industry, and solar modules – particularly for adhesive, coating, or sealing purposes. It is even used to shield elements from corrosion on bridge cables and other components found on bridges. Depending on actual needs, the properties of butyl rubber sealants can be adapted by adding fillers, and this opens the door to new applications. For example, by mixing in metallic particles, the specific electric resistance of natural rubber can be adjusted to achieve values ranging from 1012 Ωcm up to < 2 MΩ.
Taking a closer look at butyl rubber
As part of a research and development project with Isocoll, the Steinbeis Innovation Center experts at Intelligent Functional Materials, Welding and Joining Techniques, Implementation, succeeded in embedding mineral and metallic fillers in rubber to create a butyl rubber material that delivers corrosion protection, electrical conductivity, and electromagnetic shielding properties. The aim of the collaborative project was to determine new application options for the butyl rubber material by introducing fillers and adapting electromagnetic compatibility (EMC) to different application scenarios. To achieve this, the specialists introduced highly conductive and anti-corrosive fillers to the butyl rubber material, focusing mainly on carbon black, graphite, aluminum, copper, nickel, and iron. In addition to examining EMC protection, the adhesive properties of the processed butyl rubber were investigated for corrosion on the surfaces of engineered metal housings. There are a large number of areas in which such materials may be used, from surface coatings on electrical modules (where shielding properties are required), to electromagnetic housings and module shields, and even corrosion protection coatings with electrical dissipation properties between joined metal sheets.
The developed butyl rubber material was subjected to extensive testing and evaluations using special test rigs. The aim was to look at corrosion properties and electromagnetic protection. The experts working with Isocoll conducted a comprehensive battery of tests on material and technological performance. To understand EMC shielding properties, it was necessary to introduce particles to a sealing material. Each filler was chosen for its particular diamagnetic properties and low residual magnetism. In keeping with technical requirements of Isocoll, defined quantities of carbon black, graphite, copper, and aluminum were mixed into butyl rubber according to defined shapes and geometries. To do this, the team laser-welded metal sandwich structures using thermal joining processes, filling the sandwiches with different types of butyl rubber. Each sample then underwent artificial aging processes and was subjected to corrosive stress in special housing boxes.
Fillers influence electrical conductivity
Measuring and testing techniques were defined for determining the electrical conductivity and electromagnetic shielding properties as a function of sample thickness, the nature and size of embedded filler particles, and the proportion of particles compared to the butyl rubber filling medium. To assess electrical conductivity, material resistance was measured based on so-called real and imaginary impedance. Impedance is an important parameter for the characterization of electronic switches, components, and materials used to make technical parts. The technique chosen to take measurements was the RF I-V method. The measurements showed that two filler mixes achieved the lowest scores. One was a mixture consisting of butyl rubber and 5% graphite, 45% Cu CH S24 and 10% Cu CH-L7 E10. The other consisted of 1/3 of butyl rubber, carbon black, and graphite respectively with an aluminum foil. Introducing electrically conductive fillers significantly raised the electrical conductivity of the butyl rubber material. Depending on the type of filler used and which material it was combined with, compared to unfilled butyl rubber materials, electrical resistance dropped by up to 99.99%.
Boosting shielding properties
In addition to looking into electrical conductivity, the experts also examined the shielding properties of butyl rubber samples for the research project. To do this, measurements were carried out using reflection measurement methods. The material parameters were then used to calculate shielding effectiveness. To determine the required material parameters, each sample was placed in a coaxial gauging cubicle and measured using a network analyzer. These measurements produced excellent results for all examined samples. Changes in the shielding properties of the rubber resulted in a rise in the shielding effectiveness (SE) of the butyl rubber material from around 0 to 55 dB. This modification was the result of adding filler powder particles and aluminum foil to the butyl rubber filler. Introducing filler particles without the metal foil also delivered excellent shielding properties, with an SE value of 14 dB for samples consisting of butyl rubber and 45% graphite. By adding a high proportion of materials that are highly conductive to electricity, such as copper, aluminum, and graphite powder, with the addition of aluminum foil, excellent results were achieved for the composite samples. Changing powder particle sizes and varying proportions within the butyl rubber matrix resulted in further improvements to shielding properties.
Producing fillers by using material fibers that are highly conductive to electricity, such as copper and aluminum fibers, also resulted in good shielding properties. Using highly permeable filler materials makes it possible to improve electromagnetic absorption properties while maintaining reflection values. Compared to unfilled butyl rubber materials, the project team achieved a positive change in butyl rubber shielding properties of between 96 and 99.999%.
Steinbeis and Isocoll Chemicals have now achieved the goals they set for themselves and consider the project a success on all fronts. Introducing electrically conductive fillers to butyl rubber used as embedded components raised electromagnetic conductivity scores (EMC values) as hoped, and delivered corrosion protection and joining properties. At the same time, the demonstration units set up to assess results also led to some interesting observations. “By forming, welding, and soldering the materials, we gained detailed insights into the potential processing options offered by the demonstration units, and the kinds of effects we can expect when it comes to electromagnetic shielding,” concludes Steinbeis Entrepreneur and associate professor Dr.-Ing. habil. Khaled Alaluss. “Overall, the project allowed us to prove that introducing fillers to butyl rubber contributes significantly to EMC shielding effects and it’s possible to continue the processing of sandwiched structures, based on the butyl rubber that was used, with laser welding processes and laser soldering.”
PD Dr.-Ing. habil. Khaled Alaluss (author)
Steinbeis Innovation Center Intelligent Functional Materials, Welding and Joining Techniques, Implementation (Dresden)
Isocoll Chemie GmbH – Butyl Rubber Works (Nördlingen)