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.