Alternative formulations to potentially replace the use of hexahydrophthalic anhydride in coil coatings

A study was performed to investigate degradation of clear coats exposed to aggressive environments via accelerated and natural weathering and periodically analysed for changes in gloss and colour alongside InfraRed analysis, performed with the drive to potentially replace HHPA (Hexahydrophthalic Anhydride) in future coating systems. It was found that HHPA containing systems provided superior weathering resistance compared to other coating systems, with CHDA (Cyclohexanedicaboxylic acid)-CHDM (Cyclohexanedimethanol) coatings being the second best performing and CHDA the worst. It can be concluded that it is more likely that a co-monomer-based system would have the greater potential to produce comparable weathering resistance to HHPA than a single monomer-based coating. HHPA and CHDA containing pigmented systems were developed for weathering analysis, with the investigation showing that pigment addition did not influence coating performance, with CHDA still the poorer performing with regards to weathering. Free-standing films were developed to investigate UV absorbance of HHPA and CHDA coatings, showing that the absorbance profiles within the UV region are very similar, with the crosslinker appearing to exert a greater influence over CHDA coatings. An investigation was performed into the ability of HHPA and CHDA coatings to resist degradation in dark hydrolysis through immersion in water baths with IR, colour, and gloss analyses after 1, 3, and 7 days. It was found that, in general, the CHDA containing coatings demonstrated worse performance than the HHPA variants, indicating a greater susceptibility to moisture related degradation. Computational Chemical Modelling was performed to gain a greater degree of insight to the reasons behind the differences in weathering resistance offered by the different coating systems. Modelling aimed to calculate the reactivity of different structures within the coating systems when combined with HHPA or CHDA, in addition to identifying areas within the structures that were susceptible to electrophilic or nucleophilic attack. It was found that the CHDA containing models exhibited significantly more sites vulnerable to attack, particularly nucleophilic attack which suggests a greater weakness to moisture and moisture enhanced photodegradation. From the research carried out, it has been shown that HHPA offers superior weathering resistance in coating systems compared to other similar cycloaliphatic monomers. This means that, at present, it has not been possible to identify a suitable replacement for HHPA within coatings and as such it would be unreasonable for HHPA to be classed as a Substance of Very High Concern (SVHC) by REACH, as it would result in instances of coatings with lower work life expectancy.