Dimensional Stability and Water Absorption of Spruce Wood ( Picea Abies Karst . ) Impregnated and Coated with Polyurethane and Acrylic Coatings

Impregnating materials may affect the stabilization of wood dimensions in two ways: blocking of free water movement paths and chemical connection with hydroxyl groups of wood. The degree of modifi cation depends on the applied material, its penetration and fi xation to the wood. The paper investigates the impact of impregnation and coating with polyurethane and acrylic coatings of spruce wood (Picea abies Karst.) on absorption of water and dimensional stability of wood (anti-shrinking effi ciency) after being immersed in water for a specifi c time. Impregnated spruce wood absorbs two times less water than untreated wood and it is dimensionally more stable than coated spruce wood. The water absorption of coated spruce wood is low, but it is not dimensionally stable.

Understanding the complexity of this problem, the investigation should contribute to improving the dimensional stability of the wood used for making interior and exterior wood products.

Preparation of test pieces 2.1. Izrada ispitnih uzoraka
Test pieces were made of spruce wood (Picea abies Karst.) in dimensions 20x20x30 mm.The selected wood was without visible fl aws, average growth in radial direction to eight annual rings of 10 mm.Test pieces were conditioned to equilibrium moisture content of the wood in accordance with ISO 554.
The moisture content and wood density were determined according to ISO 3130 and ISO 3131 standards.The average value of moisture content was W = 12 %, the average value of density in absolutely dry condition was ρ o = 0.45 g/cm 3 .Two impregnated groups and two coated groups with 32 test pieces were treated for each type of coating, as well as a control group of untreated pieces.

Coating properties 2.2. Svojstva premaza
The coatings were based on polyurethane and acrylic resins, standard products from the range of a reputable manufacturer (ICA LP152P and ICA LAC367).The preparation of the coatings was carried out according to the instructions given by the manufacturer; they were not additionally diluted.The viscosity was determined according to ISO 2431 standard, and the dry residue percentage according to ISO 1515 standard.The polyurethane coating viscosity was v pu = 15 "F4/20 °C and the acrylic coating viscosity was v ak =27 " F4/ 20 °C.During the process, the viscosity was not changed.The dry residue of polyurethane coating was N.V. pu = 49.3 %, and the one of acrylic coating was N.V. ak = 54 %.Although the amount of the dry residue of the acrylic coating was slightly higher, the difference of 4.7 % was statistically negligible.

Impregnation procedure 2.3. Postupak impregnacije
Wood impregnation was performed according to "Double Vacuum Process" procedure (Videlov, 1980;Richardson, 2003).This treatment is applied to impermeable wood species, where the impregnation pressure has a minor impact on the penetration and retention of wood materials, while the increased time of action has a more signifi cant impact (Richardson, 2003).The wood test pieces were completely immersed in the coating solution.The fi nal operation of the vacuum procedure (Figure 1 point 6 and 7) was to remove the impregnating solution from the autoclave, in order to prevent the formation of the fi lm on the wood surface.The ratio of pressure and time of the operation is shown in Figure 1.

INTRODUCTION 1. UVOD
Wood products in outdoor use are exposed to both short-term and long-term water damage.The dimension stability of wood depends on its properties, processing technology, surface treatment and used materials.In a humid environment, the wood absorbs water until the moisture equilibrium is reached.At the same time, wood dimensions are changed, which has a negative impact on wood products.Therefore, there is a need for surface treatments that would protect the products and keep their dimensional stability.
Coating is a traditional method of surface treatment of wood and it is applied to provide its dimensional stabilization (Rowell, 1981).It is known that the wood coating fi lm is relatively porous, thereby absorbs water, causing resizing of the wood.This method is not suffi ciently effective and transparent coatings are not recommended as a long-term protection of wood especially wood used in the exterior, where the coatings should be renewed periodically (TRADA, 1984).A possible solution to this problem is the wood modifi cation method (Jirouš-Rajković et al., 2007), which changes the natural properties of wood with the aim of improving its properties and extending the duration of its use.A type of wood modifi cation is the impregnation with resins (Hill, 2006).
Impregnation is a capillary saturation of wood cavities with implanted viscous materials aimed at reducing its porosity.There are different types of wood modifi cation: fi lling the lumens -lumen modifi cation, penetrating in the cell wall -cell modifi cation, and chemical reaction between the implanting material and the cellulose hydroxyl groups -chemical modifi cation (Homan, 2004).Also the implanting materials can cause dimensional stabilization in two ways: by blocking the water paths in the wood and disabling the cellulose hydroxyl groups to connect with the water.The degree of modifi cation depends on the applied material penetration and fi xation in the wood (Archer and Lebow, 2006).
A possible solution that may address the problem of wood dimensional instability is the impregnation with coatings as a wood surface treatment.Using the vacuum method to implant these materials in the wood may result in the obstruction of the water movement paths or in a chemical modifi cation of the cellulose material, which will increase the wood dimensional stability.Most commonly used wood coatings are those based on polyurethane and acryl resins (Bulian and Graystone, 2009).Both resins have the ability to react with the wood, forming covalent bonds with the cellulose molecule (Rowell and Banks, 1985), which results in a modifi cation of wood surfaces.
The purpose of this investigation is to determine the infl uence of wood impregnation with polyurethane and acrylic coatings on water absorption and dimensional stability, after several days of immersion in water.It also aims to compare these properties between the untreated wood surfaces, impregnated wood sur-The dimensional stability is expressed as antishrinking effi ciency -ASE is defi ned by the formula (Stamm, 1964): (3) β vu -volume swelling of control test piece / volumno bubrenje kontrolnoga ispitnog uzorka (%) β vm -volume swelling of treated test piece / volumno bubrenje tretiranoga ispitnog uzorka (%) Statistical data processing -checking the differences between two related values; signifi cance for statistical data sets with over 30 measurements is determined by: The level of the test signifi cance α = 0.01, or the degree of reliability of 99 %, whereby the critical values are outside of the threshold of ± 2.

Postotno povećanje mase drva
The average value of the weight percentage gain of wood pieces impregnated with polyurethane coating is 20.3 % higher than the average value of wood pieces impregnated with acrylic coating, but according to the factor of signifi cance (T = 1.57), the weight increase is not signifi cant.The average WPG values of the samples are shown in Figure 2. Also, the average value of the weight percentage gain of wood coated with acrylic is 12.5 % higher than the weight percentage gain of wood coated with a polyurethane coating, but this difference is not statistically signifi cant (T = 1.5).
After the treatment, the poly-addition process was carried out until reaching the constant weight, under conditions of ISO 554 standard.
Following the impregnation and surface coating, the test pieces were immersed in water for a period of seven days.The dimensional stability was investigated throughout the mass increase as weight percentage gain, water absorption and anti-shrinking effi ciency.
The weight percentage gain -WPG is defi ned by the formula (Hill, 2006): (1) m m -mass of treated sample / masa tretiranog uzorka (g) m u -mass of sample before treatment / masa uzorka prije tretmana (g) Water absorption -W a is expressed by the percentage of humidity of the sample before and after immersion, according to the formula: (2) m 1 -mass of sample before immersion / masa uzorka prije uranjanja u vodu (g) m 2 -mass of sample with absorbed water / masa uzorka s apsorbiranom vodom (g) and of wood coated with acrylic by 12.5 %.This is about two times less than water absorption of impregnated wood and about three times less than the untreated wood.

Poboljšanje stabilnosti dimenzija drva
The values of anti-shrinking effi ciency of impregnated and coated spruce wood are shown in Figure 4.
The anti-shrinking effi ciency of the impregnated wood and coated wood was improved, but during the seven-day immersion it decreased.
The improvement of ASE of wood impregnated with polyurethane and wood impregnated with an acrylic resin is almost equal (T = 0.67).On the fi rst day of immersion, ASE improved by 35.7 %, and on the second day it decreased to 24.5 %.Until the seventh day of immersion, there was no change.On the seventh day of immersion, the ASE of the impregnated wood was 24.2 %.The correlation coeffi cients of the impregnated wood shows (R = 0.26 and R = 0.34) that there was a weak correlative dependence between the percentage of improved ASE and the duration of immersion.The duration of treatment has a negligible impact on ASE, actually a negligible impact on the stability of wood dimensions.
The improvement of ASE of wood coated with acrylic was higher than that of wood coated with polyurethane coatings (T = 3.89).However, in a period of six-day immersion, the ASE was equaled.On the fi rst day of immersion, ASE of the coated wood was improved by 74.1 %.Then the stability of the coated wood was constantly decreasing and on the fi fth day of immersion it was equal with the dimensional stability of the impregnated wood.
In the coated wood, ASE decreased.On the seventh day, it was 17 % and was less than ASE of the The differences in WPG of the impregnated wood are followed by high coeffi cients of variation.This is confi rmed by the fact that spruce wood is not permeable and it is processed with diffi culties due to impregnation (Militz, 1993;Mamonova, 2000).It is the result of closed pore structure of spruce wood, created during the wood drying process.It is also the result of low permeability of the cell lumens (Gindl et al., 2006), and this is the reason for low penetration of the impregnating materials in the wood.

Upijanje vode
Water absorption was gradually increased in all tested groups during a seven-day immersion.By the fourth day it was intense, and with low dynamics it increased until the last day.Water absorption is shown in Figure 3.
Untreated wood absorbed water during all seven days and it increased by 41.9 %.
After a seven-day treatment, water absorption also increased in wood impregnated with polyurethane coating by 27.4 %, and in wood impregnated with acrylic by 21.6 %.Related to absorption of untreated wood, it was almost twice lower.The water absorption of wood impregnated with polyurethane coating was signifi cantly higher than the absorption of wood impregnated with acrylic coating.The difference was 5.8 %, and it started to be signifi cant on the fourth day, continuing to be signifi cant until the seventh day.The increase of water absorption in wood treated with polyurethane coating can be explained by the presence of polyester component in the coating, which makes it inconsistent with hydrolysis (Daniliuc et al., 2012).
Until the seventh day, water absorption of wood coated with polyurethane coating increased by 11.9 % As well known, when the water leaks through the coated fi lm, the wood starts swelling, causing cracks in the fi lm and weakening the fi lm adhesion.After a few such cycles, parts of the fi lm are removed from the wood surface and the water freely diffuses in, causing unhindered dimensional changes.So, it can be said that the coating is not a proper procedure to stabilize the wood exposed to conditions of increased humidity and action of water.
According to the results obtained from the two applied wood surface treatments, the best wood stability would be certainly achieved by a combination of both treatments.The surface treatment with impregnating modifi cation and fi lm coating on the wood can achieve connection of the surface fi lm with the inner part beneath the surface, where the cellular and intercellular cavities of the wood are fi lled with the same fi lm material and are able to make a chemical connection with wood.

CONCLUSIONS ZAKLJUČCI
Based on the obtained results of water absorption and dimensional stability of spruce wood impregnated and coated with polyurethane and acrylic coatings, the following can be concluded: -The water absorption of wood impregnated with polyurethane coating was 27.4 %, with acrylic coating 21.6 % and related to untreated wood, it was almost twice lower.-The water absorption of wood coated with polyurethane coating was 11.9 % and with acrylic coating 12.5 %; it was about twice less than that of impregnated wood and about three times less than that of untreated wood.-The impregnation with polyurethane and acrylic coatings improved the dimensional stability of spruce wood -anti-shrinking effi ciency of 24.2 %. -On the fi rst day of immersion, anti-shrinking efficiency of impregnated spruce wood decreased to 32.2 %, about 1.5 times less, and till the seventh day it remained unchanged.-Anti-shrinking effi ciency of spruce wood coated with polyurethane and acrylic coatings, constantly decreased during the seven-day period until reaching 77 % or 4.3 times.-Improvement of dimensional stability -anti-shrinking effi ciency of the impregnated spruce wood compared to the coated wood is 29.7 % higher.-The impregnated spruce wood absorbed water two times less the untreated.It was dimensionally stable.By absorbing water, the coated spruce wood was permanently altering the dimensions.-This research contributes to the development of a wood treatment method that will improve permeability of surfaces and dimensional stability of the wood immersed or exposed to water.