Graphene/graphene oxide and melamine as synergistic additives for polyester nanocomposite coatings

Ensuring mechanical robustness is crucial for polyester coatings on pre-coated metal (PCM). The incorporation of nanofillers such as graphene nanoplatelets (GNP) and graphene oxide (GO) has demonstrated potential in enhancing mechanical properties of polymeric nanocomposites. However, the utilisation of these nanofillers in saturated polyester systems remains largely unexplored. Therefore, this study developed polyester-based nanocomposite coatings incorporating GNP or GO and systematically investigated the effects of the nanofiller loading and the hexa(methoxymethyl)melamine (HMMM)/polyester ratio on the properties of the coatings. Adding nanofillers led to the reduction of both glass transition temperature (Tg) and hardness (H) of the coating. Specifically, at a 1:5 HMMM/polyester ratio, Tg reduced from 29.9 °C for plain polyester (Blank) to 26.1 °C and 22.0 °C for polyesters with 5 wt.% GNP (A-5.0GNP) and GO (A-5.0GO) respectively; H values also decreased from 274 MPa for Blank polyester to 222 MPa and 143 MPa for A-5.0GNP and A-5.0GO, respectively. The hydroxyl groups on the nanofillers affected crosslinking between HMMM and polyester by reacting with HMMM, thereby reducing the crosslink density of the cured coatings. Increasing nanofiller loading led to a more significant reduction in crosslink density, resulting in decreased Tg and H values. At the same loading level, the significantly higher hydroxyl content of GO had a more pronounced effect in reducing crosslink density compared to GNP. Furthermore, an increase in HMMM compensated for the HMMM loss to GO and facilitated the polyester crosslinking reactions, leading to significant increase in both Tg and H. Specifically, for the 2 wt.% GO nanocomposites, Tg increased significantly from 24.7 °C to 32.1 °C with a rise in the HMMM/polyester ratio from 1:5 to 1:2, while the H value increased notably from 193 MPa to 452 MPa.