Studies on the barrier properties of exterior wood coatings

Abstract: Coatings for exterior wood have two basic functions. One isto give an aesthetically acceptable surface appearance andcolour. The other is to provide protection against wooddegradation by microbiological or physical attack. Theseprotective properties, often called the barrier properties,play an important role in the selection of proper material forsupreme durability. The assessment of these barrier propertiesis of great importance. Within the CEN Technical Committee 139,Working Group 2, Coatings for exterior wood test methods andperformance specifications have been established. Forassessment of water protection efficiency a standard testprocedure, EN 927-5, has been launched. The present work hasfocused on its applicability in assessing water protectionefficiency in relation to the degradation of coatings duringexposure.Assessments according to EN 927-5 is shown to givesignificant differences in water absorption values fordifferent types of coatings on wood. The proposed performancespecifications in ENV 927 - 2 for the water absorption valuesfor coatings to be used in different constructions seem to beset at acceptable levels. It has been shown that there is agood correlation between the level of water absorption andpractical experience of the performance of paints inScandinavia. However, it has also been shown that thecombination of a standard procedure for water absorptionmeasurement and an artificial weathering procedure gives moreinformation regarding expected durability and longtermperformance than a single measurement of water absorption onfresh, unweathered coated wood. A combination of waterabsorption measurement and artificial weathering could become auseful tool in product development as well as in benchmarking.Together with statistical tools, such as reliability-basedservice life prediction methodologies for predicting theservice life of coating systems a reduction in testing timesmay be achieved.Surface-active substances in coatings have a negative effecton the coatings ability to prevent water ingress, which mostprobably is due to the hydrophilic character of thesesubstances. The presence of these substances, which are commonin waterborne coatings, alters the moisture sorptioncharacteristics of wood. Considering that these substancesoccur in waterborne coatings, may be mobilised during and afterfilm formation and accumulate at the coating/substrateinterface, there is a great probability that these substanceschange the moisture sorption characteristics of the woodsubstrate in an unfavourable way and create unexpected dynamicmoisture conditions at the coating/wood interface.Computerized tomography has been found to be a valuable,non-destructive tool for visualising the spatial moisturedistribution of water and moisture in coated wood panels. Themethod is suitable for recurrent testing of a specimen exposedto a series of processes of wetting, drying. This thesis showsthat high moisture contents occur locally at and around cracks.In these areas there is an increased risk of internal tensionand stress resulting in crack initiation and propagation andthat high moisture contents may occur in the first fewmillimetres under waterborne coatings despite intact coatingfilms. Even with good barrier properties of the coating,moisture may accumulate by water-vapour absorption in air gapsbehind the cladding thus causing favourable conditions formicrobiological colonization.The work that has been carried out regarding assessment ofthe water protection efficiency shows promising resultsregarding the possibility to use reliability-based service lifeprediction methodology for the assessment. The aim of futurework will be to establish more reliable techniques andprotocols for assessing service life expectancy and durability,especially for waterborne coatings with special focus ontendencies to early failure and robustness of the coatingsystems.Keywords:Coatings, surfactants, water absorption value,EN 927, paint, additives, moisture dynamics, absorption,desorption, artificial weathering, artificial exposure,computerized tomography, MRI.