Controlling the quality of rubber compounds!
The compounds are analyzed during extrusion (for example during straining or shaping) in a pulse transmission process with the help of ultrasound waves. Fillers such as carbon black and silica have characteristic attenuation properties for sound wavesparticularly in the range of 1 MHz to 20 MHz, so that the filler content can be determined with a precision of up to 1 phr. Unlike with many other analytical methods that use electromagnetic waves, carbon black does not disturb this process. As a result, even black compounds can be volumetrically analyzed. Owing to the sound scattering, large inhomogeneities (> 10 µm) can also be determined.
Using a marker with a high density, e.g. zinc oxide, the distribution of Rhenogran® AP additionally can be monitored. Rhenogran® AP products are customer-specific additive packages made from crosslinking chemicals, and are polymer-bound, dust-free and easily dispersible. Rhenowave® is fast, both used alone or in combination with Rhenogran® AP. Following the exit of the compound from the extruder, an assessment of the compound quality is performed within a few seconds (inline control). Where array technology is used, up to 100 percent of the compound can be analyzed. Small areas where compound quality is insufficient are recognized immediately rather than later in the product. The method is representative. With the aid of special ultrasound transducers, transduscer distances of between 5 mm and max. 30 mm can be realized, depending on the compound and the compounding temperature T > 100 °C. This means that Rhenowave® can be used in a production scale of 1,000 kg/h. Rhenowave® is absolutely non-destructive.
On account of rising competitive pressure in the rubber processing industry, production processes are increasingly being accelerated to achieve higher productivity. “At the same time, the demands on the quality of rubber compounds also are on the rise,” says Dr. Andreas Schroeder, resposnible for the development of Rhenowave®. “To ensure low reject rates even under these conditions, the content, dispersion and distribution of the fillersand the crosslinking chemicals must be evaluated rapidly and in representative fashion,” he adds.
Today, rubber compounds usually are characterized by their Mooney viscosities, their crosslinking behavior and stress strain curves. These tests are destructive, meaning that only a very small percentage of the rubber compound < 0.1 % is tested. This kind of sampling therefore is not representative, but rather random. Areas displaying inadequate compound quality frequently are not detected. In addition, quality control is carried out offline, minutes, hours or even days after the compound is manufactured. As a result, errors in the compounding process are not identified until a late stage, let alone eliminated, so that in the worst case scenario rejects continue to be produced.