Graphene layers functionalized with a janus pyrrole-based compound in natural rubber nanocomposites with improved ultimate and fracture properties

The ultimate properties and resistance to fracture of nanocomposites based on poly(1,4-cis-isoprene) from Hevea Brasiliensis (natural rubber, NR) and a high surface area nanosized graphite (HSAG) were improved by using HSAG functionalized with 2-(2,5-dimethyl-1H-pyrrol-1-yl)propane-1,3-diol (serinol pyrrole) (HSAG-SP). The functionalization reaction occurred through a domino process, by simply mixing HSAG and serinol pyrrole and heating at 180 °C. The polarity of HSAG-SP allowed its dispersion in NR latex and the isolation of NR/HSAG-SP masterbatches via coagulation. Nanocomposites, based either on pristine HSAG or on HSAG-SP, were prepared through traditional melt blending and cured with a sulphur-based system. The samples containing HSAG-SP revealed ultimate dispersion of the graphitic filler with smaller aggregates and higher amounts of few layers stacks and isolated layers, as revealed by transmission electron microscopy. With HSAG-SP, better stress and elongation at break and higher fracture resistance were obtained. Indeed, in the case of HSAG-SP-based composites, fracture occurred at larger deformation and with higher values of load and, at the highest filler content (24 phr), deviation of fracture propagation was observed. These results have been obtained with a moderate functionalization of the graphene layers (about 5%) and normal lab facilities. This work reveals a simple and scalable way to prepare tougher NR-based nanocomposites and indicates that the dispersion of a graphitic material in a rubber matrix can be improved without using an extra-amount of mechanical energy, just by modifying the chemical nature of the graphitic material through a sustainable process, avoiding the traditional complex approach, which implies oxidation to graphite oxide and subsequent partial reduction

On the experimental measurement of fracture toughness in SENT rubber specimens

This work provides a direct comparison of several experimental approaches used in the literature to measure fracture toughness of rubber of rubber using single edge notched in tension (SENT) specimens, with the final aim to provide guidelines for an optimal testing procedure. Digital image correlation measurements were used to get new insights into the fracture process. SENT is experimentally advantageous because of the simple preparation from laboratory plates and the small amount of material required. The most common experimental approaches to measure fracture toughness of rubber rely on the energy release rate, measured by the tearing energy or the J-integral parameters. This work points out the importance of experimental conditions and test procedures: long specimens and short notches are preferred, identification of fracture initiation from the front view is necessary, strain energy density should not be evaluated from un-notched specimens at the critical stretch level, rather alternative strategies are shown in this work

Fracture toughness of rubber in quasi-static conditions: How to experimentally obtain results intrinsic to the material?

Fracture toughness, defined as the energy dissipated to advance a crack by a unit area, is an intrinsic material property, which should not depend on the geometry and dimensions of the specimen used to obtain it. One of the most frequently used test configurations to determine rubber fracture toughness at crack initiation in quasi-static conditions is the single edge notched in tension specimen (SENT). The aim of this work is to quantitatively define the limitations of specimen dimensions needed to obtain an intrinsic value of fracture toughness for a nitrile-butadiene rubber (NBR), and to investigate the possible effects of the test procedure on the results. These show that a minimum width of 10 mm and a minimum height/width ratio of 4.6 are necessary to obtain a fracture toughness independent of specimen dimensions

Viscoelastic behavior of glass-fiber-reinforced silicone composites exposed to cyclic loading

The aim of this work was to analyze the influence of fibers on the mechanical behavior of fiber-reinforced elastomers under cyclic loading. Thus, the focus was on the characterization of structure-property interactions, in particular the dynamic mechanical and viscoelastic behavior. Endless twill-woven glass fibers were chosen as the reinforcement, along with silicone as the matrix material. For the characterization of the flexible composites, a novel testing device was developed. Apart from the conventional dynamic mechanical analysis, in which the effect of the fiber orientation was also considered, modified step cycle tests were conducted under tensile loading. The material viscoelastic behavior was studied, evaluating both the stress relaxation response and the capability of the material to dissipate energy under straining. The effects of the displacement rate of the strain level, the amplitude of the strain applied in the loading-unloading step cycle test, and the number of the applied cycles were evaluated. The results revealed that an optimized fiber orientation leads to 30-fold enhanced stiffness, along with 10 times higher bearable stress. The findings demonstrated that tailored reinforced elastomers with endless fibers have a strong influence on the mechanical performance, affecting the structural properties significantly

Edge functionalized graphene layers for better ultimate properties of elastomer nanocomposites

This work considers elastomer nanocomposites based on natural rubber (NR) and high surface area graphite (HSAG), either pristine or functionalized with a biobased janus molecule, 2-(2,5-dimethyl-1H-pyrrol-1-yl)-1,3-propanediol (SP) also named as serinol pyrrole (HSAG-SP). Pristine HSAG nanocomposites were prepared through melt blending while the ones containing HSAG-SP were prepared through latex blending. The functionalization of HSAG with SP appears to lead to: lower viscosity, a comparable vulcanization kinetics, a lower Payne effect from shear strain sweep analyses, better tensile ultimate properties and an improved fracture behavior. All these experimental findings seem to suggest that functionalization of HSAG with SP allows a better dispersion of the filler in the rubber matrix. Such better dispersion is probably due to the synergistic effect of the functionalization of HSAG with polar groups and the preparation of the composite through latex blending. Functionalization therefore appears to be a promising tool for the reduction of the amount of filler in the composite, allowing the design of high performance, lighter and more sustainable materials for tyre applications

Sustainable materials for tyres. 1. Natural rubber compounds with better mechanical properties thanks to functionalized nanosized graphite

Natural rubber (NR), consisting in poly(1,4-cis-isoprene), is a highly stereoregular polymer. Its structure and properties actually make it the most important rubber, with a worldwide consumption of more than 12 million tons/year. The energy consumption required for the production of NR is much lower than the one required for producing a synthetic rubber: 8 vs 110 (MJ/kg) respectively. Also, the carbon footprint is lower for natural rubber: 0.4 vs 5 (kg CO2/kg). Nowadays it is clear that to have more sustainable materials for tyres, renewable sources should be preferred and therefore NR remains a material of primary importance. This work was aimed at promoting better mechanical properties for NR - based compounds by using a carbon nanofiller, a high surface area nanosized graphite (HSAG) in particular. Nanocomposites were prepared with NR and HSAG where HSAG was either pristine or functionalized with a bio based janus molecule, 2-(2,5-dimethyl-1H-pyrrol-1-yl)-1,3-propanediol (SP), commonly named serinol pyrrole (HSAG-SP) Pristine HSAG nanocomposites were prepared through melt blending while those containing HSAG-SP were prepared through latex blending. Three levels of filler, 5.2, 15 and 24 phr, were used. HSAG-SP lead to the following properties: a lower viscosity, a comparable vulcanization kinetics, a lower Payne effect according to shear strain sweep tests, better tensile ultimate properties and an improved fracture behavior. TEM analyses revealed that HSAG-SP allowed a better dispersion of the nanofiller in the rubber matrix. Such better dispersion was probably obtained thanks to the synergistic effect of the functionalization of HSAG and the preparation of the composite through latex blending. Functionalization therefore appears to be a promising tool for the reduction of the amount of filler in a rubber composite, allowing the design of lighter materials, which are required for a lower impact on the environment