Water absorption anti-shrink efficiency and decay resistance of treated wood by silica based solutio

2.1. MATERIALS

Wood specimens from sapwood of Scots pine (Pinus sylvestris L.) with dimensions of 15 mm (radial) x 5 mm (tangential) x 30 mm (longitudinal) were prepared for the study. Specimens without any visible defects such as cracks, strain and knots were selected for the experiments, and then oven-dried prior the treatments. Tetraethoxysilane (TEOS) was sourced from Sigma Aldrich (St. Louis, MO, USA). Two SiO2 based solutions were prepared. One of the SiO2 based solutions (Sol-gel 1) was prepared by using TEOS and de-ionized water (TEOS:H2O=1:1/2). The same precursors was used for preparing the other solution (Sol-gel 2) but with a different molar ratio of TEOS:H2O=1:4.

2.2. METHOD

The specimens were first vacuum impregnated with solutions at 700 mmHg for 45 min, and were then immersed in the solutions for 60 min at atmospheric pressure. The specimens were cured at 100°C for 24h. Weight percent gain of the specimens (WPG, %) was calculated on the basis of the oven dry weight of specimens before and after impregnation. Then, specimens were conditioned at 20°C and 65% RH for 2 weeks. 244

Six replicates of treated and untreated wood specimens were placed into beakers filled with deionised water. After defined times (24h, 48h, 72h, 96h, 120h, 144h, 168h and 336h), the specimens were removed from the water, dabbed of with tissue and weighed. The dimensions of the specimens were measured. This procedure continued for a total of 14 days. Relative water uptake (WA, %) (Eq. 1) and anti-shrink efficiency (ASE, %) (Eq. 2) were determined. WA = [(W2- W1) / W1] × 100

Where, W2 = wet weight of wood specimens after wetting with water W1 = initial dry weight Su = volumetric swelling of untreated wood S = volumetric swelling of treated wood Decay test was performed according to principles of EN 113 [14] with some modifications both for leached (L) and unleached (UL) specimens with 6 replicates for each group. Water immersed specimens were used as leached specimens. Brown rot fungus, Coniophora puteana (Schumach.) P. Karst. (Mad-515) was used in the test. Malt extract agar of 4.8% concentration and specimens were sterilized in an autoclave at pressure of about 0.1 MPa at 120°C for 25 minutes. Fungi cultures were inoculated to sterile malt extract agar medium in the petri dishes. After incubation period of inoculated petri dishes, one impregnated and one control specimens were placed on the growing mycelium in each petri dish. The petri dishes were then incubated at 20ºC and 70% RH for 8 weeks. After the test, all wood specimens were removed from the petri dishes and cleaned from the surface mycelium. Then, they were dried at a temperature of 103±2°C, weighed, and the weight loss (WL, %) was calculated on the basis of oven dry weight before the test. Decay resistance of impregnated specimens defined as a percentage change was calculated based on the weight loss of control specimens.

3.1. WEIGHT GAIN (WPG), WATER ABSORPTION RATE (WA) AND ANTI-SHRINK

EFFICACY (ASE) The weight percentage gain (calculation based on the weight difference between untreated and treated wood specimens) is a good indicator of an efficient impregnation (i.e., chemical bonding or interaction between the introduced chemical and wood cell wall polymers) [15]. Weight percent gain of specimens was found to be 25.02 (±1.99) and 26.29 (±2.53)% for sol-gel 1 and sol-gel 2, respectively. Both solutions had similar weight percent gains in wood. The WPGs were consistent with the previous studies by Donath et al. [7, 16]. Donath et al. [16] found WPG value of approximately 22.3% after impregnation with three monomeric silanes in pine sapwood, and Donath et al. [7] found WPG value between 25-30% after impregnation with alkoxysilanes in beech. The average values of water absorption and anti-shrink efficiency of specimens are shown in Figs. 1 and 2. Water absorption of the control specimens increased from 69.48% to 155.04% during immersion in water. All treated specimens had lower water absorption than that of the controls after 14 days. Impregnated specimens had water absorption rate from 93.11 to 124.65%. At the end of the water immersion test, treated specimens decreased water absorption of wood as 20% in comparison with controls. In the beginning periods of the test, up to 120h, treated specimens absorbed more water than controls. A continuous increase was observed on absorbed water by controls while a slight increase on WA values was observed with treated specimens along with water immersion periods. A remarkable difference on WA was not observed between two solutions. Donath et al. [16] found that water repellence of specimens treated with three types 245

E. Dizman Tomak, E.D. Şam Parmak, Ş.S. Arpacı / Sigma J Eng & Nat Sci 11 (2), 243-248, 2020

of silanes strongly diminished after a longer submersion time (24 h). It was reported that the reduction in hydrophobicity after longer submersion time was not caused by removal of silanes during submersion. This was explained by continued condensation of unreacted silanol groups in the aqueous functional silanes during the wetting-drying cycles. Anti-shrink efficiency of Sol-gel 1 was found to be 32.12% for 24h and 25.90% for 336h. In the case of Sol-gel 2, it was found as 40.56 and 35.04% for 24h and 336h, respectively. The ASE values decreased during 72h but then they tend to increase slightly until the end of the test. The reduction of ASE from beginning to end of the test was 19% for Sol-gel 1 and 14% for Sol-gel 2. This could be due to Si–O–C-bonds are susceptible to hydrolysis [17]. Sol-gel 2 exhibited better dimensional stability than Sol-gel 1. Sol-gel 2 might penetrate and polymerize in the wood better than Sol-gel 1 since it was prepared with less molar ratio of TEOS to H 2O (1:4). High rate of unreacted silanol groups might presence on wood impregnated with Sol- gel 1.

Figure 1. Water absorption of specimens during 14 days of water immersion

Water immersion time (h) Figure 2. Anti-shrink efficiency of specimens during 14 days of water immersion 246

3.2. DECAY RESISTANCE OF SPECIMENS

Fig. 3 summarizes the average values and standard deviations of the weight loss caused by C. puteana attack. The control specimens were severely degraded by the fungus. The weight loss of the control specimens was found to be greater than 25%. All treated specimens exhibited better decay resistance than controls. Weight loss of specimens treated with Sol-gel 1 was higher than required a maximum weight loss (3%) for a candidate wood preservative according to EN 113 [14]. Anti-fungal effect could not be proven by Sol-gel 1. Only the slightly improved decay resistance was probably achieved by water repellency. Sol-gel 2 was found efficacious in suppressing C. puteana attacks when no leaching prior the decay test was used, however, it seemed to be ineffective after leaching. Goethals and Stevens [18] reported that poor activity was observed with a silylation reaction for pine and beech against C. versicolor and C. puteana attacks. The poor decay resistance of the sol-gels may also probably caused by the fact that not all of the silanol groups present react with the hydroxyl groups of wood [18]. Hydrolysis and condensation reaction of silanes was also reported [16, 17]. This finding is also compatible with ASE results. 35