1) Waterjet-milled ground tire rubber from the tread of truck tires was chosen for the research (provided by Aquajet Ltd.). The reason is that the water milling provides larger specific surface compared to other milling methods. This means more efficient irradiation: more functional groups per mass of material is expected. 2) Irradiation was carried out at room temperature and in an air atmosphere. The dose rate was 2 kGy/h, and the room's air was changed six times an hour to always have enough oxygen to create functional groups on the surface. The examined doses based on literature data were: 20, 40, 60, 80, 100, 250, 500, 750, and 1000 kGy. 3) The surface modification effect of this irradiation was determined indirectly. The irradiated grind was mixed into fresh rubber and then vulcanized. The surface properties were deduced from the mechanical properties developed, of which the increase in tear strength is the best indicator of the interfacial bonding. Based on measurements, the optimal irradiation dose is around 40-60 kGy. 4) In order to gain a deeper understanding of the changes that took place in the ground tire rubber due to radiation, we performed a Horikx analysis using the results of Soxhlet extraction and swelling tests. This analysis can be used to determine the ratio of chain-scission and the selective crosslink break in a degraded cross-linked polymer. Based on the results it can be concluded, that up to a dose of 100 kGy, the the selective scission of cross-links was dominant, which means that the sulphur cross-links were selectively broken. The cross-link density decreases as the absorbed dose increases while the soluble content remains unchanged. Above 100 kGy, the calculated values suggests that the cross-links breaks were less dominant, but chain-scission become to the fore.

Tensile strength of the ground tire rubber containing rubber mixtures as a function of gamma radiation dose.

1) For air irradiation of ground tire rubber, our aim was to activate the surface, which was effective up to a dose of 100 kGy. Above this, the degradation of the ground tire rubber limited its potential applications. On this basis, irradiation experiments in aqueous media were also performed up to 100 kGy. For the treatment to be effective and successful, continuous air bubbling was added to the aqueous medium and a small amount of surfactant was used to achieve a stable suspension. 2) The advantage of aqueous irradiation is that it can be incorporated into the production process after a water jet grinding step, but after this the grind must be dried for further processing. After the treatment, water-based suspensions were filtered through 615 MN pleated filter paper, then air-dried at room temperature until constant weight. Subsequent tests confirmed that this gentle drying method preserved the active groups on the surface. 3) Structural changes in the irradiated samples after drying were revealed by Horikx analysis. It was shown that, as in air irradiation, the cross-link density decreases with increasing dose, while the soluble solids content does not deviate significantly from the reference. This confirms the characteristic of selective cross-linking breakdown, which is highly favourable for the recycling. 4) The treated ground tire rubber was mixed with fresh rubber and tested for mechanical and morphological properties. The results were similar to those obtained with air irradiation, with the major difference that the favourable mechanical properties were already observed at low doses (20-40 kGy). This demonstrated that surface activation in aqueous media is also effective. Furthermore, results similar to those obtained with air irradiation can be obtained at lower doses, i.e. this method is also more economical.

Tensile strength of the vulcanizates containing ground tire rubber.