Research The Ageing Of Exterior Coated Wood By The Help Of Measuring Permeability And ESEM

Wood remains an important material for use in many applications. As a nature and traditionalmaterial, wood keeps some disadvantages such as the loss and the erosion when the woodproducts are used in the exterior environment. Exterior wood product undergoes dimensionalchanges when exposed to moisture and it is susceptible to biological attack. The use of coatingson the wood product surface allows the decoration and the protection of the wood. The coatingcan protect the exterior wood products against photochemical degradation of the wood surface;the uptake of moisture and the related dimensional changes; and prevent the microbiologicaldegradation by blue-stain and mould fungi. The coating should be anti-aging and keep on itsprotection during aging. Researching the weathering of the exterior coated wood is to prolongthe service life of coating and extend the interval between two maintenances.The weathering process of exterior coated wood is such a complex phenomena which is quitedifficult or even impossible to be described or be concluded as a single mode. The weatheringof a coating system on wood is contingent on many factors. Each factor is a large and complexresearch scope which means each factor has many influence details aspects and all these aspectwill function the weathering process synthetically but not single and separately.Though there is no clear mode which can describe all the weathering process of the exteriorcoated wood, many definitive factors can be studied to get some conclusion about theweathering such as moisture permeability of the exterior coated wood.Wood is a hygroscopic and porous material which can lose or take again moisture according tothe relative humidity of the environment and temperature. Absorbing and de-absorbing water ofthe wood can cause the wood dimension changing, and the water in the wood will provide thesuitable environment for blue-stain and mould fungi. Coating as a polymer material withoutporous, it can prevent the water invade the wood from exterior well. But a good coating shouldnot be airtight totally because the wood is a hygroscopic and porous material. The coatingswhich are used in wood applications or porous structures should give complete blockage ofliquid water from exterior while at the same time should have ability to breathe out watervapours from interior.This paper focus on the air permeability of hardwood beech (Fagus sylvatica), researching theair permeability of untreated beech and the beech after coated by four kinds of exterior coatings. Then research the relationship between the air permeability and the durability of coated woodduring the artificial weathering process.The conclusion of this paper shows the variation of air permeability in beech is very large withmany factors. The permeability of beech wood in the radial direction is generally greater ofseveral times than in the tangential direction. In any case of the directions and the type of thecoatings, the air permeability decreased significantly after the wood samples were coated. Butthe changing of the permeability between untreated wood and coated wood varies with eachsample.During the artificial weathering process, the air permeability of coated wood grows with theageing time, but including some small fluctuations in details. The durability and service life ofcoated wood are influenced by three main factors: coating, wood and weathering condition.Different types of coatings have different failure mode and service life. The durability ofcoating on the radial plane wood is stronger than on the tangential plan wood in any case of thecoating types. And different ageing condition will affect ageing result differently. The airpermeability of exterior coated wood seems has no clear corresponding relation of theanti-ageing ability of coating, in any case we compared the durability of coatings with theaverage air permeability value (k_a), the changing of air permeability (F) and the air permeabilityuptake coefficient (k_t).