Modification of Shellac with Clove (Eugenia caryophyllata) and Thyme (Satureja hortensis) Essential Oils: Compatibility Issues and Effect on the UV Light Resistance of Wood Coated Surfaces

Abstract

Shellac (SL) is a natural resin employed for wood finishing, while clove (Eugenia caryophyllata) and thyme (Satureja hortensis) essential oils (C-EO, T-EO) are organic natural products of vegetal origin with antifungal, antimicrobial and antioxidant properties. The present paper aims at exploring the potential of modifying alcoholic shellac solutions with essential oils (C-EO, T-EO), focussing on the recurrent effects of this modification on the traditional wood finishing technique, the colour and the UV light resistance of the wood coated surfaces. The compatibility of C-EO and T-EO with ethyl alcohol and the alcoholic reference SL solution was evaluated, and modified SL solutions with a content of 5% and 10% C-EO and T-EO were prepared. Wood samples of European maple (Acer pseudoplatanus) and European walnut (Juglans regia) were finished with reference and modified SL solutions. An accelerated UV ageing test was run, and the samples were evaluated after 24, 48 and 72 h of exposure. Colour measurements in the CIELab system and FTIR–ATR investigation were employed for monitoring colour and surface chemistry changes. Coating with SL resulted in visible colour changes for both wood species (ΔE values of 31.06 for maple and 13.00 for walnut) and increased UV resistance, reducing colour changes after 72 h UV exposure (by 83% for maple and by 59% for walnut) as compared to the uncoated controls. Modification of SL solutions with C-EO and T-EO only slightly influenced the colour of finished surfaces (ΔE = 1.88–5.41 for maple, ΔE = 1.36–3.41 for walnut) and their UV resistance. The colour changes (ΔE) of coated surfaces after 72 h exposure varied in the range 1.63–4.53 for maple and 2.39–3.58 for walnut, being generally slightly higher (by 1.7–2.9 units for maple and 0–1 unit for walnut) for the modified SL solutions. FTIR investigation highlighted only minor chemical changes of the shellac coating films after 72 h of UV exposure. A photo-induced oxidative process of eugenol seemed possible in the case of SL modified with C-EO.

11th Hardwood Conference Proceedings

“…Each cycle of 24 h UV exposure consisted in four steps of 6 h UV irradiation at 40°C alternated with dark periods of 0.5 h. This procedure was repeated four times, resulting 24, 48, 72 and 96 hours of exposure. More details related to procedure were previously published by the authors (Timar and Beldean 2022). Two replicates from each treating and finishing variant, alongside corresponding two uncoated controls, were tested simultaneously.…”

“…Coating with shellac hinders this particularity due to the fact that actually only the surface coating layer is analysed by the FTIR-ATR technique. More details on the spectra of C-EO, T-EO and shellac were previously reported (Timar and Beldean 2022).…”

“…A good compatibility with alcoholic shellac solutions was found, alongside a slight influence of modification on the colour of finished maple surfaces and their colour changes by UV-VIS light simulating indoor conditions. (Timar and Beldean 2022). The present research aimed at studying the effect of pre-treatments of maple (Acer pseudoplatanus) wood surfaces with alcoholic solutions (10%) of clove and thyme essential oils on their colour, compatibility with shellac and beeswax in traditional finishing technologies and the colour stability of the coated surfaces when exposed to light in indoor conditions.…”

Light-Induced Colour Changes in Wood Surfaces in Indoor Conditions Determined by an Artificial Accelerated Test: Influence of Wood Species and Coating Materials

“…A Feutron 400 FKS environmental climatic chamber (Feutron Klima Simulation-GmbH-Greiz, Germany), equipped with a UVA Spot 400 T lamp (Hoenle UV Technology-Gilching, Germany), fitted with a glass H2 filter, was employed to expose the wood samples to UV-VIS light in the range of 295 to 600 nm, simulating natural light filtered by window glass, similarly to the previously reported research [23,53]. Considering the maximum emission of the lamp in the UV range (mainly UV-A, as indicated also by its name), this type of radiation with higher energy was considered to be the main ageing factor causing colour and chemical changes in our experiments.…”

“…The samples were placed vertically on a rack at a distance of 60 cm from the UVA light source. The actual irradiation procedure, similar to the previously reported research [53], included an initial conditioning of 0.5 h (20 • C, 55% RH, no light), followed by 3 cycles of 24 h UV exposure separated by conditioning phases of 24 h (20 • C, 55% RH, no light), inserted to allow the removal of samples for measurements. Each cycle of 24 h UV exposure consisted of four steps of 6 h UV irradiation at 40 • C, alternated with dark periods of 0.5 h. Cycles of 24 h UV exposure are in accordance with a standardised procedure of exposure to daylight behind window glass (method B) [54].…”