Identification and quantification of off-odorants in acrylic paint and characterisation of acrylic esters and their analogue ketones based on their chemical structure

Since the discovery of acrylic esters in 1934 and the development of the first waterborne acrylic paints in the 1940s, acrylates have been an integral part of our economy. Due to their modifiable properties, acrylates can be used in different applications such as adhesives, sealants, varnishes, paints for indoor or outdoor application, artists' paints, road markings, coatings, textiles and various plastics. Although acrylate based products (e.g. acrylic paints or glues) can elicit an unpleasant odour, current research has not focused on the causative substances. Therefore, the present thesis aims at such odour-elucidation of acrylates and products containing acrylates.

In a first step, white and black acrylic paints were investigated with regard to their odorant composition using human-sensory methods and odour extract dilution analysis in conjunction with gas chromatography-olfactometry and mass spectrometry. A total of 26 substances contributing to the odour of acrylic paints were identified in the samples studied. The substances identified were mainly attributable to acrylic monomers (butyl acrylate, 2-ethylhexyl acrylate), solvent residues (cumene, butanol, propyl benzene) and naphthalene derivatives.

In a second step, sixteen of the identified substances were quantified in a stable isotope dilution analysis using two-dimensional gas chromatography-mass spectrometry. The analyses showed that butanol was present at the highest concentrations in each paint. Furthermore, it was found that every sample contained at least one acrylic monomer, which contributed to its overall odour. Particular attention was paid to the determination of naphthalene derivatives, since naphthalene is known to pose health risks (class 2 carcinogen). Analyses showed that eight out of ten samples contained naphthalene. Although an acute danger is not to be expected, direct contact with a class 2 carcinogen should nevertheless be avoided.

Since it was shown that acrylic monomers can be responsible for off-odours in acrylic paints, the odour variety of different acrylates was investigated in the third study. Twenty acrylates were investigated with regard to their odour qualities and odour thresholds. The synthesis of nine of the investigated substances was described for the first time in literature. The majority of the investigated substances showed an odour threshold between 0.73 and 20 ng/L air. The substances sec-butyl acrylate and 2methoxyphenyl acrylate were found to be particularly odour-active, whereas acrylates with free hydroxyl groups showed significantly higher odour thresholds, thus being less odour-active.

The investigation of the odour attributes further revealed a mushroom-like and metallic odour for butyl acrylate similar to 1-octen-3-one. Since the substances differ structurally only in one atom, the final study addressed the question of whether all acrylates, namely n-alkyl acrylates and acrylates with unsaturated side chains, and their structurally related ketones have similar odour properties. Generally, both analogous series showed a dependence of the odour on the chain length of the substance. Although the majority of the investigated substances smelled mushroomlike, geranium-like or fruity, a direct correspondence of the odour qualities could only be detected between two analogous pairs, namely butyl acrylate and 1-octen-3-one as well as hexyl acrylate and 1-decen-3-one. Regarding their odour intensity, it was found that ketones showed a stronger dependence on the chain length of the substances. In conclusion, the odour of acrylates is just as diverse as the structural diversity of the substance class itself. The investigations carried out as part of this thesis should help to optimize acrylic products, particularly with regard to their odour properties, and thus ensure a more pleasant and safer environment for consumers