2.1 Material and sample origin
Samples of
Brachystegia spiciformis and
Julbernadia globiflora were obtained from 10 mature undated dominant trees (five of each species) growing in humid miombo natural forests of Cheringoma District, Sofala Province, Mozambique (S 18°45′21.9″ E 034°55′27.1″). Tree species were confirmed at the Eduardo Mondlane University xylarium through vouchered reference specimens. A batch containing 121 pre-dried boards of
B. spiciformis and 64 boards of
J. globiflora was transported to Luxhammar Ltd. (Mikkeli, Finland), where the thermal modification treatments were carried out. For the experiment, pre-kiln dried sapwood boards—nominal size (600 × 50 × 25 mm; long × radial × tang) of both species (average moisture content 12%)—were exposed to three thermal treatment levels and distributed in the sub-sets as shown in Table
1. All boards were shortened to the length of 500 mm to fit a laboratory-size (0.5 m
3) thermal modification chamber.
Table 1
Number of wood samples for each tested thermal modification
Untreated | – | – | 9 | 7 |
T1 | 215 | 2 | 43 | 27 |
T2 | 230 | 2 | 40 | 29 |
T3 | 245 | 2 | 29 |
–
|
2.2 Thermal modification of wood materials
The wood samples of both species were heat-treated using the Luxhammar thermal modification process developed by Luxhammar Corporation (Luxhammar
2019). This process consists of five different stages: Initial heating (temperature raised to 100 °C), preconditioning and drying, the actual thermal modification with high temperatures up to 250 °C, conditioning (restoration of moisture) and cooling. The heat treatment processes were carried out in an airtight stainless-steel kiln chamber using three temperature range intensities, 215 °C (T1), 230 °C (T2) and 245 °C (T3), for 2 h at a saturated steam environment, one treatment at each temperature. Due to the shortage of wood material of
J. globiflora, it was not subjected to treatment T3. However, T3 was used for
B. spiciformis to enable further studies on the effects of different thermal wood modification processes on resistance to decay and termites.
2.3 Measurement of wood properties
For all subsets of specimens, the wood properties listed in Table
2 were tested before and after each thermal treatment level, using untreated samples as reference control. Statistical differences between untreated samples and each treatment level within each wood species were calculated using Tukey’s multiple range test at p < 0.005. The mechanical tests were performed using the universal material testing machine Zwick Z050 (Germany) according to specific standards listed in Table
2.
Table 2
Tested wood properties and specific standards
Equilibrium moisture content, mass loss and oven-dry density | ISO 13061-1:2014 & ISO 13061-2:2014 |
Colour/spectral reflectance | ISO/CIE 11664-6:2014 |
Bending strength: MOE & MOR* | ISO 13061-3:2014 & ISO 13061-4:2014 |
Brinell hardness | EN 1534:2010 |
Compression strength | ISO 13061-17:2017 |
Mass loss (ML) of all specimens was calculated by weighing before and after each thermal treatment level and expressed in %. Specimens from each treatment were oven-dried to absolute dry weight and compared to monitor changes in the oven-dry density. Likewise, after each treatment, the specimens were left in standard room climate (20 °C, 65% relative humidity) until equilibrium moisture content (EMC) was achieved. The reflectance spectra of each sample were measured over three 8-mm diameter regions using a Konica Minolta CM-2600d portable spectrophotometer. Spectral data between 360 and 740 nm visible wavelength range were converted to CIEL*a*b* colour coordinates using 2° standard observer and D65 light source. Lastly, the colour difference (
ΔE*ab) between the modified and unmodified specimens was calculated using the CIE76 standard (Commission International de l’Eclairage, CIE), which corresponds to the distance between two points in the three-dimensional colour coordinate system and is calculated by the following equation:
$$\Delta E{^*}_{ab} = ((\Delta L^*)^2+(\Delta a^*)^2+(\Delta b^*)^2)^{1/2}$$
(1)
where ΔL*, Δa* and Δb* reflect the changes in lightness (L*) and the chromatic parameters redness (a*) and yellowness (b*) between the measurements on the treated samples. In each specimen, colour was measured on the same marked spot after polishing to avoid reflectance of eventual moisture or resin stains.
Brinell hardness (
HB, MPa) of wood was measured from the tangential surface with a size of 25 × 50 × 50 mm and calculated according to EN 1534 (
2010) as follows:
$$HB=2F/(\pi{^*}D^*(D-(D^2-d^2)^{1/2}),$$
(2)
where
F is the nominal force (N),
D is the diameter of the steel ball (mm) and
d is the diameter of the residual indentation (mm). As a difference from EN 1534 (
2010), the estimated value for the diameter of the residual indentation (
d, mm) was calculated from the depth of the residual indentation (
h, mm) measured by the material testing machine as follows:
$$d=2\times(10 h-h^2)^{1/2}$$
(3)
The tests for modulus of elasticity, MOE (
Ew, MPa) and modulus of rupture, MOR (
σ b,W, MPa) were carried out according to standards ISO 13061-4 (
2014) and ISO 13061-3 (
2014), respectively, using 20 × 20 × 340 mm clear wood specimens as follows:
where
P is the load equal to the difference between the upper and lower limits of loading (N),
l is the span (mm),
b is the width of the test specimen (mm),
h is the height of the test specimen (mm),
f is the deflection at the upper and lower limits of loading (mm), and:
$$\sigma_{b,W}=3P_{\max}l/2bh^2,$$
(5)
where Pmax is the maximum load (N), l is the span (mm), b is the width of the test specimen (mm) and h is the height of the test specimen (mm). The specimens were prepared so that one of the faces was as parallel as possible to the direction of the growth ring. The load was applied to the radial surface at the mid-span of the specimens.
The maximum compression strength (∂
c,0,W, MPa) parallel to the grain was determined using clear wood specimens with a size of 20 × 20 × 60 mm. The values obtained were used to calculate the compressive strength using the equation below, according to ISO 13061-17 (
2017):
$$\partial_{c,0,W}=P_{\max}/ab,$$
(6)
where Pmax is the load in (N), and a and b are the cross-sectional dimensions of the specimen (mm).
The figures describing the tested mechanical properties of MOE and MOR (before and after thermal treatments) were adjusted to 12% moisture content to address moisture variation amongst the subset of specimens modified in different thermal intensities; the following equations were used, which are valid for moisture contents of 12 ± 5%:
$$E_{12}=E_{w}/(1 - \alpha \times (W - 12))$$
(7)
$$\sigma_{b,12}=\sigma_{b,W} [1+\alpha (W-12)]$$
(8)
where α is the correction factor for the moisture content, equal to 0.04, and W is the moisture content of wood.