1 Introduction

Heat treatment of wood is an attractive alternative to improve the decay resistance and dimensional stability of low natural durability wood species (Militz 2002; Vernois 2001; Patzelt et al. 2002; Esteves and Pereira 2009). However, durability of heat-treated wood is generally not sufficient for certain end-uses, where wood is subjected to insect attack, especially in termite-infested areas (Mburu et al. 2007; Surini et al. 2012) or in ground contact in the presence of soft rot (Raberg et al. 2012). Moreover, the degradation of wood components and particularly of hemicelluloses, strongly linked with cellulose as well as lignin in the cell wall, weakens the wood structure (Yildiz et al. 2006; Korkut et al. 2008; Mburu et al. 2008; Candelier et al. 2013). Future progress of thermo-modified materials will depend on developments that permit their application in use Classes 3 and 4 (EN 335-2 2007) where the occurrence of wood-boring insects, including termites is possible. One way to reach these objectives is to combine boron impregnation and thermo-modification. Indeed, boron compounds, such as boric acid, disodium tetraborate decahydrate (borax) or disodium octaborate tetrahydrate (DOT), are considered as acceptable biocides, which have proven efficacy for non-ground contact applications. These compounds have been shown to be highly effective against insects like termites and decay fungi. Borates are non-volatile, colourless, odourless, non-corrosive, non-flammable and inexpensive. However, one restriction of their use for timber preservation is their natural solubility in water leading to their rapid depletion from treated wood under outdoor conditions. Several alternatives have been described to limit boron depletion (Obanda et al. 2008) involving the use of water repellents or polymerizable monomers (Baysal et al. 2004; Kartal et al. 2004; Mourant et al. 2009; Temiz et al. 2008), the formation of insoluble complexes (Kartal and Green 2003; Kartal and Imamura 2004), the formation of protein borates (Thévenon et al. 1997, 1998; Thévenon and Pizzi 2003), the formation of organic borates with different polyols like glycol, glycerol, mono glycerides or polyvinyl alcohol (Gezer et al. 1999; Toussaint-Dauvergne et al. 2000; Mohareb et al. 2010), or gel formation (Mohareb et al. 2011; Obounou Akong et al. 2013). Among these alternatives, several approaches involve impregnation of boron with different additives intended to assure its fixation after heating. In this context, it has recently been demonstrated by the authors that thermo-modification of wood samples impregnated with boron in the presence of different water soluble vinylic monomers allowed improvement of boron resistance to leaching (Salman et al. 2014). The aim of this paper is to report results concerning the effects of such treatments on improved durability of treated wood against different brown rot fungi and termites.

2 Experimental

2.1 Materials

Mini-blocks (15 by 5 mm2 in cross section by 20 mm along the grain) of Scots pine sapwood (Pinus sylvestris L.) were used throughout this study. Sixty replicates were used for each treatment solution. Heat treatment was performed at 220 °C and half of these samples were subjected to leaching to determinate boron fixation.

2.2 Block impregnation

Different impregnation solutions were prepared with or without polyglycerolmethacrylate (PGMA) and different boron concentrations. PGMA was synthesised according to Soulounganga et al. (2003, 2004) and used at 10 % concentration (w/w). Boron was tested at two Boric Acid Equivalent (BAE) concentrations (2 and 4 %) using sodium tetraborate decahydrate (borax). Mini-blocks (60 replicates) were oven dried at 103 °C for 48 h and weighed (m0). Wood samples were vacuum treated at 5 mbar for 15 min, impregnated with the treatment solutions and kept immersed for 30 min. at atmospheric pressure. Wood samples were removed from the solution and kept for 16 h at ambient temperature, dried at 103 °C for 48 h and weighed (m1). Weight percent gain (WPG) was calculated as follows:

$$ {\text{WPG }}\left(\% \right) = 100 \times \left( {{\text{m}}_{1} - {\text{m}}_{0} } \right)/{\text{m}}_{0} $$
(1)

where m0 is the initial dry mass of wood samples without treatment and m1 is the dry mass of treated wood samples.

2.3 Heat treatment

Heat treatment was performed under nitrogen in a 500 mL reactor on the different wood samples previously treated with boron with or without additives at 220 °C for 20 h in a Carlo Erba GC oven. The oven temperature was increased by 20 °C min−1 from ambient to final temperature. Temperature accuracy was estimated to ±1 % of the set temperature. Samples were weighed after treatment and mass change after impregnation and thermo-modification (ΔM) calculated according to the formula:

$$ \Delta {\text{M }}\left( \% \right)\;= \;{100 } \times \, \left( {{\text{m}}_{ 0}\;-\;{\text{m}}_{ 2} } \right) / {\text{m}}_{ 0} $$
(2)

where m0 is the initial oven dried mass of the sample before impregnation and heat treatment and m2 the oven dried mass of the same sample after heat treatment.

2.4 Boron leaching procedure

Leaching was performed according to a procedure adapted from the former European standard ENV 1250-2 (European Committee for Standardization 1994). Half of the samples (thirty replicates) were immersed in 300 mL distilled water and subjected to six leaching periods of increasing duration under continuous shaking at 20 °C. Water was replaced for each leaching period after 1, 2 and 4 h. Samples were then removed and kept air drying for 16 h. Other leaching periods were conducted for 8, 16 and 48 h with water change between each. After leaching, the blocks were dried at 103 °C for 48 h and weighed (m3). Mass change after impregnation, thermo-modification and leaching (ΔM) was calculated as previously described using the following formula:

$$ \Delta {\text{M }}\left( \% \right){ = 100 } \times {\text{ (m}}_{ 0}\;-\;{\text{m}}_{ 3} ) / {\text{m}}_{ 0} $$
(3)

where m0 is the initial dry mass of wood samples without treatment and m3 is the dry mass of the impregnated heat treated wood samples after leaching.

2.5 Decay tests

The treated specimens were tested to evaluate their resistance against fungal attack according to a procedure modified from EN 113 (1997). The Scots pine samples were exposed to three brown rot fungi: Coniophora puteana ((Schumacher ex Fries) Karsten, strain BAM Ebw. 15), Poria placenta ((Fries) Cooke sensu J. Eriksson, strain FPRL 280), and Gloeophyllum trabeum ((Persoon ex Fries) Murrill, strain BAM Ebw. 109) (9 replicates for each fungus). Sterile culture medium prepared from malt (40 g) and agar (20 g) in distilled water (1 L), was placed in a culture flask inoculated with a small piece of mycelium of a freshly grown pure culture and incubated for 2 weeks at 22 °C and 70 % relative humidity (RH) to allow full colonization of the medium by the mycelium. All wood samples were sterilized by autoclave at 121 °C for 20 min. In each culture flask, four specimens (three treated and one control) were inserted. Each experiment was replicated three times. Virulence controls were also performed on nine specimens of Scots pine. Incubation was carried out for 16 weeks at 22 °C under 70 % RH in a climatic chamber. Once the fungal exposure was completed, the mycelium was removed and the specimens were weighed in order to evaluate their moisture content at the end of the fungal exposure. The specimens were then dried at 103 °C, and their final weight was recorded. The moisture content at the end of the test (data not shown) as well as the weight loss were determined. Weight loss (WL) was expressed as a percentage of the initial oven-dry weight of the wood sample according to the formula:

$$ {\text{WL }}\left( \% \right){ = 100 } \times \, \left( {{\text{m}}_{{ 0\,{\text{ or\,2 \,or\, 3}}}} -{\text{ m}}_{ 4} } \right)\!/{\text{m}}_{{ 0\,{\text{ or\,1\,or\,2\,or\,3}}}} $$
(4)

where m4 is the final dry mass of wood samples after fungal exposure, m0 is the initial dry mass of wood samples without treatment corresponding to the controls, m2 is the dry mass of the PGMA impregnated or not wood samples cured at 220 °C before leaching and m3 is the dry mass of the PGMA impregnated or not wood samples cured at 220 °C after leaching.

2.6 Termite resistance tests

Termite resistance was evaluated using Reticulitermes flavipes (ex. santonensis) termites using a non-choice test based on the guidelines of the European standard EN 117 (2005). Prior to the test, each sample was dried at 103 °C in order to obtain its anhydrous initial weight (m0, or m1 or m2). For each set of treatments and controls, three replicates were tested for their resistance towards termites. Each sample was placed in a 9-cm diameter Petri dish containing 40 g of Fontainebleau sand (4 vol sand/1 vol deionized water). The samples were placed on a plastic mesh in order to avoid waterlogging. A total of 100 termite workers, two nymphs and two soldiers were then introduced onto the sand. Pine sapwood controls were tested in the same manner. The Petri dishes were placed in a dark climatic chamber at 27 °C with relative humidity >75 %. After 4 weeks, the samples were removed, cleaned of sand and the survival rate of the termites was calculated. The samples were dried at 103 °C and their weight loss (as a percentage of initial weight, m0, or m1, or m2) was calculated as previously described.

2.7 Statistical analysis

A statistical analysis of data, based on the single-step multiple comparison Tukey–Kramer’s method was performed using the JMP program (SAS Institute Inc., Cary, NC, USA). For each degradation agent, sample resistance was studied versus applied treatment among groups containing 3–9 samples. Statistical analysis allowed classifying results in categories from A to D (Table 2). Systems not connected by the same letter are significantly different, at the 5 % level.

3 Results and discussion

Characteristics of the different samples used for biological test are presented in Table 1.

Table 1 Characteristic of treated pine sapwood samples used for durability tests
Full size table

Without any treatment, mass variation corresponded to the classical mass loss generally observed during thermal treatment due to degradation of wood cell wall polymers. In the presence of 10 % polyglycerol methacrylate, two opposite effects were observed: impregnation with PGMA led to an increase in sample mass, while thermal treatment resulted in a mass loss. Cumulative effects of PGMA impregnation followed by curing at 220 °C for 20 h resulted in a slightly lower mass loss than that recorded for thermomodification alone suggesting that mass loss was due to the degradation or transformation of impregnated polymer. Impregnation of borax before thermomodification tended to slightly reduce the effect of thermodegradation reactions and consequently the mass loss caused by thermal modification. These results confirmed those already described in the literature (Kartal 2006; Kartal et al. 2008; Kikuchi and Maeda 2007; Tomak et al. 2011). Similar effects were observed for treatments involving association of PGMA and borax, for which mass change after impregnation and thermal modification remained positive. Addition of PGMA was also associated with reduced boron depletion after leaching leading to retentions of 10–20 % of the initially impregnated boron quantity. The effect of PGMA addition on boron retention was also evaluated on leached and unleached samples exposed to different durability tests against termites and decay fungi (Table 2).

Table 2 Fungi and termite resistance of pine wood samples subjected to the different treatments
Full size table

Untreated pine samples were heavily degraded by the three brown rot fungi used as well as by termites. After thermal treatment, all the samples were, as expected, more resistant to fungi, while there was no durabulity improvment against termites. Addition of boron improved wood durability against termites even after thermal treatment. However, boron fixation was not sufficient to achieve efficient protection after leaching. Indeed, samples impregnated with 2 % BAE and subjected to thermal treatment were heavily degraded by termites after leaching. Boron levels after leaching were less than 2 kg/m3 (BAE), far below those required to protect wood. Quantities of boron retained in wood after impregnation with 4 % BAE solution after leaching partially reduced termite attack, but were insufficient to completely protect the wood, especially under more severe leaching conditions. Impregnation of boron with PGMA resulted in efficient fixation of boron allowing wood protection even after leaching at either boron concentration utilized. Mass loss of pine samples exposed to termites were low and practically all termites died at the end of test, while samples treated with boron without addition of PGMA experienced greater mass losses and higher survival rates after leaching. These results are in good agreement with the quantities of boron remaining in wood (Salman et al. 2014) illustrating the positive effect of PGMA addition on the fixation and retention of boron after leaching. More surprisingly, and contrary to heat-treated samples, samples heat-treated in the presence of 10 % PGMA without addition of boron also exhibited improved resistance to termites. This last result was of particular interest because it could permit the development of a completely non “biocidal treatment” and merits further investigations. Statistical analysis indicated in all cases a significant difference between untreated samples and treated samples subjected to fungal attack. Similarly, significant differences were also detected between all treated samples and a group of samples constituted of control and 220 °C heat-treated samples indicating efficiency of the treatments.

4 Conclusion

Boron fixation was improved by combined treatments using a first impregnation of borax (2 or 4 % BAE) with 10 % PGMA in water, followed by a thermal treatment under nitrogen at 220 °C. The treatment protected wood from both termite attack and decay. This combination would be useful in termite infested areas. In a more interesting way, wood samples treated with 10 % PGMA aqueous solution and subjected to thermal treatment at 220 °C were also totally resistant to fungi and termites suggesting that boron was not necessary for termite protection.