Water and water-vapour permeability of preservative treated woodcoating

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1 Water and water-vapour permeability of preservative treated woodcoating systems Matjaž Pavlič, Marko Petrič Biotechnical Faculty, Department of Wood Science and Technology, University of Ljubljana, Rožna dolina, Cesta VIII/34, SI-1000 Ljubljana, Slovenia ABSTRACT Wood products used outdoors are frequently finished with protective and decorative stains. However, low-build and high-build stains do not provide sufficient protection against biodeterioration of wood. Fungal decay risk is related to the moisture content in wood, being influenced by permeability of a coating. In order to minimize the decay risk, biocides may be applied before finishing or added into a coating formulation. However, pretreatment of wood with wood preservatives may have an effect on some important properties of a coating, such as adhesion or permeability. The aim of our research was to establish a possible influence of the new, boron based wood preservative on water and water-vapour permeability of some acrylic and alkyd stains on the preservative treated wood. We stated that the boron based preservative in wood significantly changed permeability of the tested wood-coating systems. Key words: water vapour permeability, boron based wood preservative, stain INTRODUCTION Under changes of climate conditions and weathering wood degrades very quickly. For protection of wood products used outdoors mostly surface finishing is being used which has both protective and decorative roles. One of the most important tasks of an exterior coating is to protect wood against excessive moisture which leads to dimensional changes of a woodcoating composite and subsequently to stresses in the wood substrate, at the wood-coating interface and in the coating film itself (Arnold, 1999). As a result, coating can crack and/or flake and loses its protective function (Arnold, 1999; Graystone, 1998). Therefore, water and water-vapour permeability is one of the most important properties affecting coating performance (Feist et.al., 1985; Arnold, 1999). Moisture permeability has been a subject of investigation for many years and it has been reviewed by Sell (1975) and Graystone (1998). Despite of this past work the question of the optimum moisture permeability of a coating still remains unanswered. Low permeability coating might provide good protection against external moisture but at the same time there is a risk of fast moisture entry through a crack, open end grain or open joint (de Meijer, 1999; Janotta, 1974; Derbyshire, 1999). This can establish favourable conditions for decay. On the other hand, a high permeability coating can cause rapid moisture changes and consequently, dimensional movements. Therefore, the optimum moisture permeability is largely dependent on the type of wood and the structure coating is applied on, specific end-use and climatic zone (Graystone, 1998; de Meijer, 1999). Permeability of a coating depends on several factors like its build, degree and type of pigmentation and additives, and the type of binder (de Meijer, 1999; van der Wel and Adan, 1999). Within the same category of coating, waterborne coatings usually show a higher

2 permeability (de Meijer, 1999). Permeability of a coating can both decrease or increase during weathering (Derbyshire and Miller, 1996; Mihevc et.al., 1995). Some of additives are essential for coating composition; many of them contain hydrophilic parts that contribute to the moisture affinity of the coating film (van der Wel and Adan, 1999). It is well known that improper selection, use and maintenance of coatings on wood in outdoor exposure (hazard class 3) may induce long-lasting periods of high moisture content, and consequently, the risk of fungal decay may be increased (e.g.: Rapp and Peek, 1999; Despot et.al., 1999; Arnold, 1999). The increased risk of fungal decay may be diminished by pretreatment of coated wood either with water repellents (Rapp and Peek, 1999; Plackett and Cameron, 1989; Evans et.al., 1992; Cui and Archer, 1997), by prior chemical modification of wood (Plackett et.al., 1992; Militz et.al., 1997), or probably by the most common way - with biocide containing formulations (Yalinkilic et.al., 1999 and refs. therein). When one decides for pre-treatment with a wood preservative, an environment friendly biocide should be selected. Inorganic boron compounds offer the unique advantage of exhibiting broadspectrum activity against a wide variety of pest organisms, coupled with low acute mammalian toxicity and low environmental impact (Lloyd, 1998; Schoeman and Lloyd, 1999; Peylo and Willeitner, 2001). On the other hand, boron compounds do not become fixed within the wood and therefore can be leached from the wood during service (Dinwoodie, 2000). It was shown that intact surface coatings, like varnishes could diminish leaching of boron (Peylo and Willeitner, 1995; Homan and Militz, 1995; Peylo and Willeitner, 1997). This fact enables using of boron-based preservatives also in weather-exposed sites in the hazard class 3 situations. In New Zealand for instance, borate-treated wood is approved for use in exterior exposure provided it has a three-coat paint finish (Morris, 2000). A new boron based biocide, the alkanolamine-boric acid complex was synthesised and introduced by the companies Borosol, d.o.o. and Regeneracija, d.o.o., Slovenia. The complex has an outstanding solubility in water and ethanol in comparison with solubility of boric acid and various borates. Recently, it has been shown that the new complex exhibits good fungicidal and insecticidal properties as well as fire-retardant activity (Petrič et.al., 2000a) and that it does not have negative influence on adherence of some wood finishes, in some cases adhesion is even improved (Petrič et.al., 2000). However, the influence of this new complex on water and water-vapour permeability of coating system was not known and so, it was the aim of the study presented herein, to obtain data on this impact. MATERIALS AND METHODS Test systems Water and water-vapour permeability was determined for 18 different systems (Table 1). As a substrate we used non-treated and with a boron based biocide pre-treated wood samples of Norway spruce (Picea abies (L.) Karst). Two types of preservatives were used: aqueous solution of the amine-boric acid complex (16.7 m/m %, labelled as BA) and ethanolic solution of this complex (20.0 m/m %, BR). The treatment procedure (98 minutes) in an industrial vacuum impregnation plant consisted of an impregnation in a vacuumed cylinder (90 % vacuum) and of subsequent vacuum drying period. Penetration depths of the new boron preservative were determined by the standard colouring method. The conditioned substrates were coated by brushing with one of the following, commercially obtained finishes: transparent low build acrylic water borne stain (labelled as BT), transparent high build acrylic water borne stain (BD), transparent low build alkyd organic solvent borne stain (LT),

3 transparent high build alkyd organic solvent borne stain (LD) and the white pigmented alkyd opaque paint (E). Table 1: The test systems. Preservative treatment None Aqueous solution of the new boron complex Ethanolic solution of the new boron complex Applied coating None Low build acrylic stain High build acrylic stain Low build alkyd stain High build alkyd stain White pigmented opaque paint None Low build acrylic stain High build acrylic stain Low build alkyd stain High build alkyd stain White pigmented opaque paint None Low build acrylic stain High build acrylic stain Low build alkyd stain High build alkyd stain White pigmented opaque paint System label C C/BT C/BD C/LT C/LD C/E BA BA/BT BA/BD BA/LT BA/LD BA/E BR BR/BT BR/BD BR/LT BR/LD BR/E Water-vapour permeability the dish method Water-vapour permeability of test systems (Table 1) was assessed with the dish method (modified DIN and EN ISO method). After conditioning at 23 ± 2 C and 50 ± 5 % relative humidity, the wood samples of dimensions mm, with parallel growth rings to the smaller lateral side surface were coated on the upper surface. In order to compare the results with some other data, it was decided to finish the samples by applying only one layer of a coating. After two weeks of conditioning five replicates of small cylindrical samples with a diameter of 45 mm, thickness of 2 mm and with one coated side were prepared. The test dishes with 5 ml of distilled water and test sample (Figure 1) were put in chamber with 23 ± 2 C and 65 ± 5 % relative humidity for 7 days. Water-vapour pressure difference of approximate 980 Pa was established. This caused water-vapour diffusion from inner part of the dish with approximate 100 % relative humidity to the chamber with 65 % relative humidity at constant temperature of 23 C. Adsorption and desorption of a wooden substrate was simulated with a position of a coated surface.

4 Cover Seal Sample Distilled water Dish Diffusion area F = 12,56 cm 2 Figure 1: A schematic presentation of an assembled dish In the first 3 days a constant transmission of water-vapour was establishing. Water-vapour transmission was monitored by weighing the test dishes after 3, 4, 5, 6 and 7 days. According to Wassipaul and Janotta (1972) for each test system water-vapour transmission rate (P) was calculated: P = N d F t ( p 1 p2 ) [kg/mhpa] N.... mass of transmitted water-vapour [kg], d.... thickness of test sample-system [m], F.... diffusion area [m 2 ], t.... time [h], p 1, p 2. water-vapour pressure [Pa]. Water permeability the Rosenheim method Water permeability of the test systems (Table 1) was assessed according to the Rosenheim method (Institut für Fenstertechnik e.v., 1999). After conditioning at 23 ± 2 C and 50 ± 5 % relative humidity, wood samples of dimensions mm, with parallel growth rings to one surface and with rounded edges with a radius of 5 mm were coated on four lateral surfaces with two layers according to directions of the coating producer. End-grain surfaces were sealed with a two-component low permeability epoxy coating. For each test system, five replicate samples were prepared. Following application and drying, samples were again conditioned at 23 ± 2 C and 50 ± 5 % relative humidity for about two weeks. The water absorption-desorption test was carried out over a period of 28 days. Samples were fully immersed in water for 14 days and conditioned at 20 ± 2 C and 65 ± 5 % relative humidity for further 14 days. Absorption was monitored by weighing after 1, 2, 3, 4, 7, 8, 9, 10, 11 and 14 days, and desorption after 15, 16, 17, 18, 21, 22, 23, 24 and 28 days.

5 According to the test protocol relative moisture permeability (RMP) and dynamic moisture permeability (DMP) for each tested system were calculated: RMP = ( F B / F R ) 100 [%] DMP = ( F`B / F B ) 100 [%] F R area under the absorption curve of a control sample (water uptake), F B area under the absorption curve of a test sample (water uptake), F`B area above the desorption curve of a test sample (water loss). Control samples were represented by the test system C (Table 1): non-impregnated, noncoated samples. In addition, relative dynamic moisture permeability (RDMP) was calculated: RDMP = (DMP T / DMP C ) 100 [%] DMP C dynamic moisture permeability of a control sample, DMP T dynamic moisture permeability of a test sample. RESULTS AND DISCUSION Water-vapour permeability the dish method Pre-treated spruce wood samples were completely impregnated with the boron based preservative, small differences occurred only in retentions of the preservative, determined by weighing the specimens prior and immediately after the impregnation process (aqueous solution g/m 2, ethanolic solution g/m 2 ). Table 2 shows water-vapour transmission rates (P) of the test systems in simulated adsorption and desorption process.

6 Table 2: Average water-vapour transmission rates (P) of the test systems in simulated adsorption and desorption process, determined by the dish method. Water-vapour transmission rate (P) Test system [kg/mhpa] Adsorption Desorption C C/BT C/BD C/LT C/LD C/E BA BA/BT BA/BD BA/LT BA/LD BA/E BR BR/BT BR/BD BR/LT BR/LD BR/E From the table above it can be seen that pre-treatment of wood with the boron based preservative have an influence on water-vapour permeability of wood-coating system. Generaly, the permeability is increased. This phenomenon was observed with both ethanolic and aqueous solution of the new alkanolamine boric acid complex, and it is the most typical in the cases of the transparent high build alkyd organic solvent borne stain and of the white pigmented alkyd opaque paint (BA/LD, BR/LD vs. C/LD and BA/E, BR/E vs. C/E). The reasons for this observation may be various. Firstly, the curing process of a coating could be somehow influenced by the new alkanolamine boric acid complex in pre-treated wood. For instance curing process could not reach its final stage. Furthermore it is known that boron compounds do not become fixed within the wood (Dinwoodie, 2000). Therefore, their diffusion into a coating during the curing process may occur and consequently, the structure of a coating could be changed. Secondly, as can be seen from our results, water-vapour permeability of pre-treated uncoated samples is changed too (BA, BR vs. C), showing a considerable influence of the substrate itself, especially when the water-vapour permeability of the whole system (preservative treated wood coating system) is assessed. It is known that amines are hygroscopic and therefore, alkanolamine in the formulation can influence on the permeability of treated wood (Sonoda et.al., 1998). Thirdly, some previous investigations showed that the adhesion of alkyd coatings on preservative pre-treated wood was higher than adhesion on untreated samples (Petrič et.al., 2000). It is known that adhesion of a coating is influenced by its penetration into wood (Turkulin et.al., 2000; Bardage and Bjurman, 1998). So, it could be deduced that penetration of the tested alkyd coatings into pre-treated wood is improved, which may also be the reason for its increased water-vapour permeability. From the results it can also be seen that the water-borne coatings have higher permeability than the solvent-borne ones, which confirms data from literature (Ahola et.al., 1999; de Meijer, 1999; Ekstedt and Östberg, 2001) The lowest permeability was exhibited by the white

7 pigmented alkyd opaque paint (E). This result was expected because of the highest build and pigment concentration in this case. These results are preliminary and further research is ongoing. It would be useful to assess coating characteristics on pre-treated wood like penetration, wetting and surface tension, stackability, hardness, curing and observation of their structure. Water permeability the Rosenheim method In Table 3 water permeability coefficients of the test systems are presented. These results lead to similar conclusions as in assessing the water-vapour permeability. Pre-treatment of wood with the alkanolamine - boric acid compound increases relative moisture permeability (RMP) of the preservative treated wood-coating system (e.g. BA/LD, BR/LD vs. C/LD and BA/LT, BR/LT vs. C/LT). This increase is the most typical in the case of wood, pre-treated with ethanolic solution of the preservative. Highest relative moisture permeabilities among all tested systems were exhibited by the systems BR, BR/BT and BR/LT. The stronger influence of ethanolic treatment solutions may be related to higher uptake values and penetration depths than at aqueus solutions (Table 3). Dynamic moisture permeability (DMP) could be interpreted as the percentage of absorbed water that is released after 14-day drying period (EN 927-4, 2000). To show desorption efficacy of the test systems according to control (non-treated and non-coated system) the relative dynamic moisture permeability has been introduced. Two coating systems can have similar permeabilities but still show very different RDMP. We observed an increase of the relative dynamic moisture permeability of the same test systems as in the case of RMP. However, it is interesting that in the case of the test systems BA/E and BR/E the RDMP is significantly decreased. Table 3: Average uptake values and penetration depths of preservative, and water permeability coefficients of the test systems according to the Rosenheim method. Test system Uptake [g/m 2 ] Pen. depth [mm] RMP [%] DMP [%] RDMP [%] C C/BT C/BD C/LT C/LD C/E BA BA/BT BA/BD BA/LT BA/LD BA/E BR BR/BT BR/BD BR/LT BR/LD BR/E

8 According to the Rosenheim recommendations (Institut für Fenstertechnik e.v., 1999) for coating use in joinery, including windows and dors, the RMP value should be above 50 % and DMP below 50 %. From our results we can see that only C/LD and C/E test systems fit into these limitations. CONCLUSIONS Permeability of a wood-coating system was significantly influenced by pre-treatment of wood with a new boron based biocide, the alkanolamine boric acid complex. As determined by the dish and the Rosenheim methods, water-vapour and water permeabilities of the preservative treated wood-coating systems are increased in comparison with permeabilities of untreated wood-coating systems. Further research to explain this phenomenon is ongoing. REFERENCES AHOLA P, DERBYSHIRE H, HORA G and de MEIJER M (1999). Water protection of wooden window joinery painted with low organic solvent content paints with known composition: Holz als Roh- und Werkstoff 57: ARNOLD M (1999). Moisture content of wood painted with low VOC coatings during outdoor exposure test. In: Turkulin H (ed.) Surface properties and durability of exterior wood building components: international conference. Faculty of Forestry, Zagreb BARDAGE S L and BJURMAN J (1998). Adhesion of Waterborne Paints to Wood: Journal of Coating Technology 70 (878): CUI F and ARCHER K (1997). Treatment of lumber with preservative/water repellent emulsions - the significance of shear stability on penetration. The International Research Group on Wood Preservation. Document No.: IRG/WP DERBYSHIRE H (1999). Surface coatings: protecting wooden joinery against moisture. In: Turkulin H (ed.) Surface properties and durability of exterior wood building components: international conference. Faculty of Forestry, Zagreb DERBYSHIRE H and MILLER E R (1996). Moisture conditions in coated exterior wood. Part 1: An investigation on the moisture transmission characteristics of exterior wood coatings and the effect of weathering on coating permeability: Journal of the Institute of Wood Science 14 (1): DESPOT R, TRAJKOVIC J and SINKOVIC T (1999). The influence of type and colour of coat on the durability of exterior fir wood joinery. In: Turkulin H (ed.) Surface properties and durability of exterior wood building components: international conference. Faculty of Forestry, Zagreb de MEIJER M (1999). Interactions between wood and coatings with low organic solvent content. PhD thesis, Wageningen University, Wageningen

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