Tomography gets to grip with tyres

Car tyres are mostly made from rubber, but they also contain filler materials such as carbon black and silica, to improve wear, strength, and other performance characteristics. Now there may be a better way to check that filler materials are well distributed, thanks to
testing at the ESRF by the University of Applied Sciences in Darmstadt, in cooperation with the German tyre manufacturer Continental.

The science of how filler dispersion affects rubber quality has been around for nearly 70 years. For much of that time, tests were performed on the tyre materials themselves, with filler dispersion inferred from their mechanical characteristics. More recently, researchers have turned to optical methods, in particular dark-field microscopy, to assess filler dispersion more directly. In darkfield microscopy, the central source of illumination is eclipsed, so that only rays diffracted by a sample of freshly cut rubber are observed; the bumps, or “nodges”, of filler particles on the surface of the rubber appear as white spots in the dark-field images. Estimating the size distribution of filler particles from these spots is still not wholly direct, however, because the size of the spots depends on imaging parameters, as well as where the spots’ fuzzy outlines are taken to be.

Stereo vision

Joachim Ohser and Dascha Dobrovolskij of the University of Applied Sciences in Darmstadt, Germany, together with Jorge Lacayo-Pineda of Continental, and Matthew Putman of the imaging company Nanotronics in New York, US, wanted to determine whether a new imaging platform developed by Nanotronics, nSPEC 3D, could offer an alternative, foolproof method of estimating the size distribution. Based on radiometric stereo microscopy, the method involves illuminating a sample with six LEDs around the microscope’s objective, pointing inwards at 45°. The six resultant 2D images are combined by a Nanotronics algorithm to produce a 3D, stereoscopic image of the sample’s surface.

At the ESRF beamline ID19, Ohser could compare the size distribution of filler particles in rubber samples given by nSPEC 3D with that given by the “gold standard” of 3D tomography, synchrotron microtomography. “The application of coherent illumination combined with phase-retrieval techniques [at ID19] allowed 3D image acquisition of rubber samples with high contrast between the black-carbon filler particles and the rubber matrix, the segmentation of the filler particles, and finally the accurate estimation of the filler size-distribution,” says Ohser. Working with Alexander Rack of ID19, Ohser found that the new nSPEC 3D method delivered almost exactly the same estimation of size distribution as that given by synchrotron microtomography, as well as by computer simulations (J. Microsc. 274 32). Continental’s Lacayo-Pineda believes this bodes well for the new method’s use in industry. “Radiometric stereoscopy is an efficient way of obtaining 3D information of the macrodispersion of fillers in rubber, which is indicative of both mixing efficiency and tyre performance,” he says.