The dark colour obtained with the heat treatment gives the light-coloured wood an attractive appearance, as well as a better behaviour towards thermo-hygrometric variations in the outdoor environment. The heat treatment modifies the wood material by reducing hygroscopicity and consequently promotes greater dimensional stability; it also prolongs the service life of wooden products. The wood characteristics due to weather conditions change over time, particularly the colour, that reflects chemical changes. Natural ageing is usually a relatively slow process, therefore artificial ageing plays an important role to assess the performance by shortening the time compared to natural weathering conditions. The aim of this research is to evaluate the colour and reflectance variation of wooden surfaces due to artificial weathering obtained through a solar box chamber simulating outdoor conditions and subsequent water leaching. A Model 1500E Solar Box (Erichsen Instruments) was used to simulate the exposure to solar radiation and a horizontal shaking agitator with a glass container of distilled water to simulate water leaching.
Untreated and heat-treated samples were used. The thermally modification was conducted on planks of Ayous from Cameroon in an industrial system that used a slight initial vacuum in an autoclave (Maspell WDE Model TVS 6000) and a treatment temperature of 215 °C for three hours. After cutting, the samples were stored in the darkness. To simulate weathering, specimens were subjected to 6 cycles of combined solar box and water leaching. The weathering cycles were performed as follows: first cycle 72 h (3 days) of UV irradiation, followed by 5 h of water leaching; second cycle 168 h (7 days) of irradiation followed by another 5 h cycle of water leaching; third cycle 336 h (14 days) of UV irradiation followed by 5 h of water leaching; fourth cycle 504 h (21 days) of UV irradiation and 5 h of water leaching; fifth cycle 672 h (28 days) of UV irradiation and 5 h of water leaching; and sixth cycle 1008 h (42 days) of UV irradiation and 5 h of water leaching. The solar box chamber is equipped with a 2.5 kW xenon-arc lamp operating at 550 Wm-2, 55°C and UV filter at 280 nm. After each cycle colour was monitored using an X-Rite CA22 reflectance spectrophotometer under the following conditions, according to the CIELAB colour system: illuminant D65, standard observer 10°, geometry of measurement 45°/0°, spectral range 400-700 nm, measurement diameter 4 mm, white reference supplied with the instrument. The points of colour measurement were 30 for each specimen. Three measures were acquired for each point, so that 90 measures were obtained for each specimen. Reflectance of the surface was obtained through the use of an innovative multispectral hypercolorimetric imaging (HMI) technique developed by Profilocolore srl and for the first time applied on wood samples. HMI was performed at time 0 h, 504 h and 1008 h of irradiation and after the water leaching. This imaging technique allowed for obtained calibrated multispectral images, from 300 to 1000 nm, of the entire surface of the specimens that were subsequently processed through the software PickViewer®. The tools of PickViewer® could perform comparison of reflectances and colour coordinates, and PCA on the images, etc. The water, after each cycle of leaching, was also measured for obtaining pH and conductivity values. Moreover, Fourier transform infrared (FTIR) spectroscopy was used to characterize the compound extracted by water leaching.
To summarize the main results, the weathering cycles change significantly the colour of the specimen surface. Solar box irradiation causes darkening of the untreated samples, whereas it causes lightening of the thermally treated ones. On the other hand, the water leaching causes darkening in all cases. With the increase of weathering times, untreated specimen surfaces darken whereas treated specimens lighten so as to tend to have a similar colour or in any case to decrease the chromatic difference that was at the beginning of the weathering tests. The measured values of conductance are higher in the leaching water of untreated specimens and tend to decrease after the first cycles. The values of pH range between 4.00 and 4.52 in both cases (untreated and treated specimens). FTIR spectroscopy demonstrated that water leaching caused loss of materials from the specimens, mainly from those thermally treated. FTIR spectra exhibit signatures of polysaccharides materials as main compounds. Bands of lignin and extractives are also visible. Water leaching seems to remove degraded surface micro-particle of wood.