Brass-plated steel cords are used in automobile tires for their reinforcement and durability. Copper in brass and sulfur in rubber react with each other and form copper sulfide, creating strong adhesion. However, when subjected to loading conditions such as heat and humidity, the composition and distribution of the copper compounds change, resulting in adhesion aging. Direct observation of the state and reaction of the copper compounds inside rubber composite is still difficult, and reaction mechanism was yet to be clarified.
A research group led by Professor Mizuki Tada of Research Center for Materials Science at Nagoya University used X-ray absorption fine structure-computed tomography to quantitatively determine the amounts of elements in a sample. The group developed a method to three-dimensionally visualize the distribution and amounts of the copper compounds in rubber-brass composite. These measurements were performed at the synchrotron radiation facility SPring-8, suppling high-brightness hard X-rays.
For each of 802 brass particles in the rubber composite, the research group visualized the distribution and amounts of three copper chemical species, monovalent and divalent copper sulfide and metallic copper. When the time of deterioration by humidity and heat (moist-heat aging) was changed, the amount and distribution of each copper component changed significantly in the rubber composite. AI machine learning analysis suggested that copper sulfidation reactions can be classified into five different patterns.
These results make it possible to view the copper reactions related to rubber-brass adhesion. The X-ray spectroimaging provided the non-destructive and practical method to visualize when and where a reaction occurs, the extent of the reaction and how it spreads within the material. It is expected to lead to the visualization of the adhesion aging mechanism inside rubber materials, which was previously unknown, and contributed to the prediction of their lifespan, and the development of materials for longer tire life and recycling.
XAFS-CT imaging of adhesive model materials
