One Pinus roxburghii Sarg. tree (90cm girth) was harvested from the grounds of the Forest Research Institute, Dehradun (latitude: 30°19’N and longitude: 78°04’E). The freshly felled log was converted into small clear specimens measuring 3,8cm (Width) × 3,8cm (Thickness) ×15cm (Length). The sawn specimens were randomly selected from heartwood portion of wood. The selected straight grain and defect free specimens were submersed in fresh water to maintain a green condition prior to MW treatment.

A 900W Microwave oven (model: 30SC3) was used at a frequency of 2450 MHz for the experiment. Details of treatment are as follow:

Intensity of MW treatments: T1 (95,57), T2 (111,50), T3 (127,42), T4 (143,35W/cm²) and C1: Controls.

MW treatment time: 5 Min.

Total number of specimens: 90 including controls (Group-I: 30 specimens i.e., subjected to pH studies and Group-II: 60 specimens (30 were Microwave and 30 were MW cum Preservative treated) i.e., subjected for Termite mound test).

Replicates: six in each treatments and controls.

Percent moisture reduction caused by MW treatment was calculated from the conditioned weight of the blocks before and after MW treatment (Equation 1).

(1)

Where,

1. _^W1 = Weight of specimens before MW treatment

2. _^W2 = Weight of specimens after MW treatment.

Treated and control specimens for pH evaluation were converted into chips and pH of the wood samples was determined using the hot water extraction method described by Sinthole (2005).

Microwave treated and controls (C) specimens were subjected to preservative treatment with an aqueous solution of acid copper chromate (ACC) at 4% concentration using a full cell process i.e., an initial vacuum of 75 kPa (25 in Hg) for 30 min followed by pressure 689,47 kPa (100 lbs/in²) for 1 h. A Final vacuum of 75 kPa (25 in Hg) for 15 min was applied. Controls (C1) received no preservative treatment and served as references for the termite tests.

After treatment, specimens were weighed to determine the preservative retention (IS 401:2001). The amount of preservative solution absorbed by specimen (retention value R in kgm^-3) was calculated as Equation 2

(2)

Where; G is the weight of the preservative solution absorbed by the block (W₂-W₁) in gm, C is the concentration of the preservative solution (%) and V is the volume of the test block in cm³.

The samples were cut into test specimens (100 × 25 × 6 mm). The test specimens were buried at different places inside a termite mound of Odentotermes obesus (Rambur) in the beginning of month of May. Specimens were removed from the mound in November when activity of termites almost ceases due to fall in temperature. The specimens were visually examined for termite attack and reinstalled next year in month of May so as to have exposure to termites for two successive termite seasons. Specimens were cleaned off mud and debris, and evaluated visually to ascertain damage by termites, according to the following rating scale given in Table 1 (Shukla 1977).

Table 1:
Scale used to assess degree of termites damage (Shukla 1977).

Statistical analysis was conducted using SPSS 16 version software. Retention and pH results of P. roxburghii at different MW intensities were compared using an analysis of variance (ANOVA) and Duncan’s modified significant difference test at α=0,05.

The initial moisture content (MC) of P. roxburghii wood before microwave treatment was 40,60% and decreased to 38,40 (T1) and 35,33% (T4) respectively, after microwave treatment (Figure 1).

Figure 1:
Mean Moisture losses percentage in MW treated and untreated specimens of P. roxburghii (MSE 3,22 at (p≤0,05) level).

Increased MW treatment intensity resulted significant difference (p≤0,05) in moisture reduction from the initial values. The intensity level of the MW energy had generated steam within the wood cell that resulted removal of MC from wood (Vinden et al. 2010). The removal of steam through pores could be seen visually when specimens were removed from microwave assembly.

Wood pH steadily decreased with increasing microwave intensity and differed significantly between T2 (111,50 W/cm²) and T3 (127,42 W/cm²) MW treatments (Table 2).

Table 2:
Effect of microwave treatment on pH of wood, preservative uptake and resistance to termite attack.

Similar pH reductions have also been reported in Eucalyptus tereticornis wood by Poonia and Tripathi (2017) in MW treated wood. Niemz et al. (2010) and Chen et al. (2012) also found similar results in thermally modified wood. The pH decrease may be explained by the production of organic acids during thermal treatment, mainly by the degradation of hemicelluloses, as acetyl groups split off (Nuopponen et al. 2004). Phenolic carboxylic acid and 4-O-methyl-glucuronic and galacturonic acids are also produced as the result of wood hydrolysis adding to the acidity (Windeisen et al. 2007).

The control (C) specimens had an average retention level of 10,41 kg/m³, whereas samples treated using different MW intensities exhibited almost 2 fold increase in retentions (i.e. from 17,53 to 21,61 kg/m³). Preservative retentions increased with increased MW intensity to a maximum of 21,61 kg/m³ in samples exposed to 143,35 W/cm² whereas the lowest retention (17,53 kg/m³) was found at 95,57 W/cm². Similar effects of MW-treatments were also reported by Treu et al. (2008) on Norway spruce (Picea abies), Ramezanpour et al. (2014) on fir wood and Poonia et al. (2016) on P. roxburghii wood. Thus results reveal that retention of preservative increase with increase in intensity.

The control (C1) and microwave treated specimens were heavily attacked by termites in the first season. Attack continued and the samples were destroyed after a second season indicating that MW treatment provided no protection against termites. These results are in agreement with the results reported by Poonia and Tripathi (2017): MW treatment did not protect wood from termite attack. Similar results have also been found in thermally modified wood (Metsa-Kortelainen et al. 2011 and Duarte et al. 2012). The reason behind this non resistivity against termite is may be the change in chemical constituents and wood structure during microwave treatment. Thus, MW treatment attributes no resistance to wood against termite.

The specimens subjected to ACC preservative treatment at 689,47 kPa (100 lbs/in²) for 1h were sound after two seasons of exposure. Hence, data exhibited significant higher protection when MW treatment was given with ACC at the different intensity. These results corroborate our previous findings (Poonia et al. 2017), suggesting a synergistic effect between chemical and MW treatment for the improvement of termite durability. ACC preservative is effective even when specimens had not received any MW treatment. However, microwave treated wood resulted in remarkably high retention of preservative suggesting end use of wood even in drastic condition.

^♠Corresponding author: pooniaforestry@gmail.com