Elastically effective devulcanized ground tyre rubber (GTR) for high recyclate content rubber blends, and their compatibility with nanofillers
This paper demonstrates that mechanical devulcanization techniques, such as solid-state shear milling (S3M), can produce a recycled rubber powder that is elastically effective when blended into a virgin rubber matrix. Furthermore, blends produced via this route match the equivalent virgin rubber modulus, despite high recyclate loadings (30/70 vNR/GTR blend). The addition of nanofillers (carbon black and graphene nanoplatelets) to these blends was evaluated to see how their impact differed from a virgin matrix and how they compared with filler reinforcement theories. Stiffness of the filled blend could be related to the aspect ratio of graphene nanoplatelets or shape factor for carbon black. Results pointed to differences between standard vulcanization of a virgin material and revulcanization of a recycled blend, highlighting scope for further development of recycled rubbers. This process may allow for incorporation of devulcanized rubber into existing engineering products, promoting a circular economy for rubber.
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Rheology was performed to determine the optimum cure time of the prepared compounds, using an Anton-Paar Physica MCR 301 rheometer (Graz, Austria) with a 25 mm parallel plate, at 1Hz and 1% strain and a temperature of 150 °C. Temperature was controlled using a P-ETD400 plate and an H-ETD400 hood. Test pieces were discs of 25.4 mm diameter and 2.5 mm thickness that were trimmed at the start of the torque-time measurement. Total crosslink density was measured via the equilibrium swelling method. Each material was swollen in toluene until equilibrium (~3 days), these samples were then lightly dried using a paper towel and weighed. Crosslink density (χc) was calculated according to the Flory-Rehner equation Tensile testing for rubber materials was performed according to ISO 37 with an extension rate of 500 mm/min using a 1kN load cell. The specimens were stamped out using a modified ISO 37-2 shape cutter, the dimensions for which are given in Figure 1. This design simply increases the head surface area whilst maintaining the gauge dimensions. It was created to help prevent tearing of the head at high extension when using pneumatic grips. Hardness testing was performed using a durometer. For ease of repetition, the durometer was attached to an Instron 5565 universal testing machine and set to depress the material at a rate of 500 mm/min to a depth of 3 mm, on rubber stacked to a thickness of >6 mm. The highest value and resting value after rapid relaxation were recorded. Scanning electron microscopy - SEM was performed using an FEI Quanta400 environmental SEM at 20 kV on samples that were sputter coated with gold following cryo-fracture in liquid nitrogen. Atomic Force Microscopy (AFM) was used to measure the lateral dimensions and thickness of the 5 and 20 μm graphene nanoplatelets (GNPs). AFM was performed on an Oxford Instruments MFP-3D. The tips used were AC160 purchased from Asylum Research. GNPs were drop-cast onto a silicon wafer according to the graphene good practice guide, using 0.1 mg/ml in a 50/50 water/IPA mix 44. Measurements were made from edge to edge of the silicon wafer using a combination of 50-90 μm images in AC mode. Particle dimensions were measured using the Igor software package developed by Oxford Instruments for use with their AFM. Initially a mask was created and then the background was flattened using either a 0th or 1st polynomial. Particle analysis was then performed on the masked particles to obtain thickness (t), length (l) and width (w) of the GNPs. To confirm the accuracy of the particle analysis results from this software, many flakes were also sectioned and manually measured from the height retrace images. Average lateral length, d, (in the reinforcing direction) of the GNPs, which were assumed to be approximately rectangular, was calculated using In all cases error bars and ± represent one standard deviation, where the sample result is averaged over a minimum of 3 specimens.
Engineering and Physical Sciences Research Council