Outdoor weathering performance parameters of exterior wood coating systems on tropical hardwood substrates

Abstract

Wood coating research almost exclusively focuses on softwood as substrate despite the fact that coatings applied on tropical hardwoods show a different weathering behaviour. The objective of this study was to quantify the impact of wood substrate and coating type on the weathering behaviour of finished wooden joinery in outdoor exposure using a set of statistical methods. Therefore, a broad range of white opaque paints and mid-oak semitransparent stains were used. The coatings were industrially applied on window frames made of frequently used commercial tropical hardwood species. The general appearance of the wood/coating systems was assessed visually using a rating scale that takes into account the anatomical characteristics of hardwood. A minimum required exposure time for evaluation of 2.5 years outdoor weathering for joinery in vertical position and facing south-west was determined. At 30 months best performances were recorded for waterborne alkyd primers with an acrylic topcoat when dry coating film was sufficiently thick. The dry layer thickness also affected the impact of the hiding power on the performance of the wood/coating system. Within hardwood species the degradation rate of coatings on species with a fine texture was the lowest. The significant influence of the substrate's texture on the weathering behaviour of coated hardwood stressed the need to evaluate the wood/coating system in total.

Efficacy of Hydrophobic Coatings in Protecting Oak Wood Surfaces during Accelerated Weathering

“…Adversely, a significant change in the wood colour can be expected, as the hydrophobic coatings do not prevent the leaching of extractives and degraded lignin of the underlying wood over the long-term. The experimental works [23,26,36,78] also mentioned the important aspect of the influence of individual underlying wood species on the service life of surface treatment on its exterior. Since the degradation of the protective layer of hydrophobic coatings (A) and (B) occurred due to weathering, their significantly longer service life on an exterior with rain effects (Class 3 by EN 335 [1]) cannot be assumed-not even in the case of the application of other types of underlying wood.…”

“…When exterior oil-based coating (C) is applied, a longer service life can be expected in the case of softwoods [79] that have a morphological structure that is more simple than hardwoods with wide open vessels [57]. However, each underlying wood can affect the service life of the coating individually through the specific content of the extractives [27], and it is therefore necessary to confirm these assumptions by testing and comparing the evaluated characteristics [20,23,25].…”

“…The impact of their application on native wood [16][17][18], thermally modified or chemically treated wood [19], or application as a topcoat in coating systems [20][21][22] was investigated. Multilayer coating systems are currently the most used variants of wood coatings against weathering, and they are the focus of attention of many scientific studies [23,24]. Based on the results of experimental studies, the type of coating system influences the overall service life of coatings [25] and the underlying wood [23,26].…”

“…Multilayer coating systems are currently the most used variants of wood coatings against weathering, and they are the focus of attention of many scientific studies [23,24]. Based on the results of experimental studies, the type of coating system influences the overall service life of coatings [25] and the underlying wood [23,26]. Particularly, wood species with high extractives content are reported as problematic in the field of coating application [27].…”

Natural weathering performance of three fast-growing Eucalypt woods

“…Scientific studies have been conducted regarding the performance of native and exotic woods exposed to artificial weathering (Baysal et al 2014, Valverde and Moya 2010, Baar and Gryc 2012, Grüll et al 2014) and natural weathering (Rosu et al 2010, Mattos et al 2013, De Windt et al 2014, Akhtari and Nicholas 2014. Thus, the investigation of woods from fast-growing species with high benefit in the market is important to improve their performance.…”