2.2. Methods

The graphene-doped UATR/aramid nanocomposites were prepared at room temperature, following a similar procedure to that described in previous works [3,13,14]. Graphenic nanomaterials were added to the liquid UATR matrix (2% w/w) and mechanically stirred at 800 rpm for 2 h in a screw-top flask. Graphene-doped UATR (UATR-G) was then sonicated in an ultrasonic bath (50–60 Hz) for 2 h, with the cap opened every 30 min to allow degassing, in order to prevent the formation of air bubbles in the final laminate. Once the sonification process was completed, the UATR-G dispersion was left overnight for colloidal stabilization and protected from sunlight, to prevent degradation of the polymer matrix caused by undesired photochemical reactions. No coupling or dispersing agent was used in the preparation of the matrices so as not to alter the chemistry of the UATR polymer or the possible interactions of the graphenic nanomaterials among themselves (self-aggregation) or with the rest of the components of the final nanocomposite. To ensure the homogeneity of the laminates, the matrix used for the undoped laminate was subjected to the same procedure as the UATR-Gs but without the addition of any doping agent. Figure 2 provides a schematic representation of the preparation process for the UATR-G matrices.

Schematic representation of the preparation procedure for graphene-doped UATR (UATR-G) matrices.

The catalyst (Trigonox 239, Cumyl hydroperoxide) was added to the stabilized UATR-G colloidal dispersion (2% w/w) with mechanical stirring at 250 rpm for 5 min. Laminates, reinforced with Twaron CT709 para-aramid fibers, were made by means of a combined technique of hand lay-up and vacuum-assisted molding under compression at room temperature. Once the lamination process was completed, the laminates were cured for 24 h at room temperature, followed by two post-curing steps at 80 °C (5 h) and 120 °C (3 h), according to the manufacturer’s technical specifications. After curing, the corresponding specimens were machined from the laminated plates by high-pressure water jet cutting, for the characterization of their mechanical properties. Figure 3 shows a schematic representation of the manufacturing process.

Schematic representation of the preparation procedure for UATR-G/aramid nanocomposites. RT = Room Temperature.

Tensile modulus (E_t, GPa) and tensile strength (σ_t, MPa) were characterized in accordance with the ISO 527-4: 1997 standard by using Microtest EM2/300/FR apparatus. Flexural strength (σ_f, MPa) and flexural modulus (E_f, GPa) were characterized in accordance with the ISO 14125: 1998 standard by using MicrotestEM2/300/FR apparatus. Charpy impact strength (a_cU, kJ·m⁻²) was characterized according to the ISO 179-1:2010 standard by using a Zwick I 5113.100 Charpy pendulum for composite materials. Table 3 shows the dimensions of the specimens, and the number of specimens tested.

Specimen dimensions and number of specimens tested.