Proof of concept investigation of alternative and less harmful boron compounds for epoxy-based hydrocarbon intumescent coatings

This work presents a proof-of-concept study of two boron compounds, tributyl borate (TBB) and 4-tert-butylphenylboronic acid (4TBPBA), for applications in epoxy-based hydrocarbon intumescent coatings. In comparison with two commonly-used and very effective boron compounds, zinc borate (ZB) and boric acid (BA), the alternatives are less harmful for applicators and the environment. Using a range of concentrations of the boron compounds, the thermal insulation performance and char properties were evaluated.

The 4TBPBA formulated well into the epoxy base and was able to provide the required temperature-triggered chemistry. Results demonstrated that 4TBPBA is a promising alternative to ZB or BA. In particular, the formulation containing 13 wt% 4TBPBA provided the same time to failure to reach 550 ˚C as the formulations containing ZB or BA with twice as high a molar concentration of boron. Furthermore, compared to the ZB- or BA-containing coatings, the ones containing 4TBPBA exhibited remarkably lower steel heating rates. Based on rheological measurements, this was attributed to an earlier initiation of reactions and expansion at around 100 ˚C. The hydroxyl groups in 4TBPBA are anticipated to play crucial roles in the overall chemistry and coating performance. Contrary to 4TBPBA, TBB-containing coatings showed an unsatisfactory performance.