Wettability and color change of thermally treated 2 poplar OSB after accelerated weathering test 3

Oriented strand board (OSB) panels are widely marketed for several applications, from 13 building to packaging. The manufacturing of poplar OSB started recently in Northern Italy 14 representing a relevant innovation in the sector since this product is usually made of coniferous 15 wood. The thermal treatment is widely used for reducing the hygroscopicity of wood-based 16 products, nonetheless it influences the mechanical behavior and degrades the main components of 17 wood, which can affect their finishing. Therefore, it is important to know the properties of the 18 treated surfaces. To this purpose, in this study a lot of OSB panels, made of poplar wood, 15 mm 19 thick and with a density of 590 kg/m3, were thermally treated under vacuum conditions at 190 °C 20 for 2 h and then subjected to accelerated weathering. The changes in color and in wettability due to 21 treatment and accelerated weathering were studied. The thermal treatment determined a 22 significant darkening of the color; the accelerated weathering darkened the untreated surfaces and, 23 on the opposite, lightened the thermo-treated surfaces. The wettability decreased after thermal 24 treatment and increased after weathering, more evidently in treated panels. Overall, this study 25 improves the knowledge about the behavior of the surface of thermo-treated poplar OSB, which is 26 relevant for the industrial coating of this product. 27


Introduction
Poplar (Populus spp.) is a fast-growing tree with low economic costs in terms of logging and production.The manufacturing of poplar Oriented Strand Board (OSB) started in 2012 in Northern Italy, representing a relevant novelty for the national and European wood sector: OSB, in fact, is typically made entirely or largely of coniferous wood [1].Production of Poplar OSB has steadily increased from 2014, reaching about the 2% of the overall OSB volume manufactured in Europe [2].
In perspective this product it is also expected to contribute in sustaining the poplar cultivation sector in Italy, which in the past decades suffered a sensible reduction in plantations coverage [3,4].
The manufacturing process of poplar and coniferous OSB is similar, the main phases being wood stranding, glue spreading and panels pressing.Paraffin is commonly used to detach the panels from the plates of the continuous press adopted for manufacturing and to limit the swelling of the finished panels.Anyway, differences exist in the physical and mechanical characteristics depending on the wood used.Specifically, the density of poplar OSB varies from 500 kg/m 3 to 620 kg/m 3 , depending on its thickness; this is about 15% lower than the density of coniferous OSB, which typically ranges from 600 to 680 kg/m 3 [5].Further, poplar wood also determines a lighter and whitish color of the panel compared to that of coniferous OSB, which generally presents a yellow tone.According to product standard EN 300 [6], poplar OSB is currently available in OSB-3 type that is for load-bearing use in humid conditions.In general, OSB is intended for final applications in building and packaging, but some end-uses in furnishing are also relevant [7].Poplar OSB presents several interesting features making it suitable for various uses in the above sectors, nonetheless the low natural durability of poplar wood (Class 'DC 5 -not durable', according to EN 350 [8]) is a limiting factor in case of exposition in conditions that determine the risk of fungal decay.

Preprints
Several methods for improving the natural durability of wood exists [9].Among these, thermal treatment consists in exposing wood to high temperatures, generally from 160 °C to 230 °C, in a controlled environment.This causes a chemical degradation by heat transfer that modifies several properties of wood.In particular, natural durability and dimensional stability increase, whereas density and mechanical properties decrease; further, the color shifts towards darker tones [10].Over the past years several processes, mainly differing in inert atmosphere and curing conditions (gases, oil, steam etc.) have been developed; in this context, the thermo-vacuum treatment is performed in vacuum-conditions and is nowadays well-established on the market [11]; recently its potential for extracting chemicals from poplar wood has also been studied [12].
Overall, modified wood is adequate for non-structural uses in humid or exterior environments, for instance in furnishing, flooring and facades [13].Thermal treatment is therefore often proposed as a means for valorizing wood of species with low natural durability or for finding new applications to timber and related materials having few market outlets [14,15].
Wetting properties are frequently investigated, usually measuring the contact angle (CA), for assessing several aspects regarding processing and performance of wood and derived products [16].
Being wood a porous and hygroscopic material, in fact, its interactions with liquids play a key role in many final applications.These include adhesion, coating, waterproofing, surface modifications, weathering, degradation and liquid absorption [17,18].For instance, the contact angle is generally considered a fundamental parameter determining penetration of adhesives inside wood [19]; spreading and adhesion of coatings on wood strongly depend on CA [20]; changes in the wetting properties have been taken as a major indicator for wood weathering tests [21].In this context, exposing wood to ultraviolet (UV) light is an effective pretreatment for activation that enables to increase its wettability and it is therefore widely used in the wood coating sector [22,23].
Some relatively recent research has been performed on the thermal treatment of OSB panels.
Mendes et al. [24] showed that the physical properties of OSB made with Pinus taeda wood increased after thermal treatment, while the mechanical properties decreased.Anyway, to this day no detailed studies have been reported about the surface degradation of poplar OSB considering the combined effect of thermal treatment and accelerated weathering test (AWT).
To this purpose, in this study the contact angle was recorded on untreated and thermo-treated poplar OSB, exposed and non-exposed to accelerated weathering test.Color change was also determined as an indicator of the intensity of the performed process.

Thermal treatment
Ten poplar OSB panels of nominal thickness 15 mm were drawn from the standard manufacturing process of the producer.Panels were subjected to thermal treatment under vacuum conditions using the Termovuoto  process.Treatment temperature of 190 °C was maintained for 2 h.
At the end of the process, 30 specimens with dimension of 50x50x15 mm, equally distributed between untreated and thermally-treated panels were randomly cut for sampling and further testing.

Color measurement
Surface color of poplar OSB before and after accelerated weathering test, including the corresponding references, was determined on middle spot of each specimens; five replicas have where: ΔE* is the overall color change, ΔL*, Δa* and Δb* are the differences between the initial and the final values (between thermo-treated and non-treated panels, before and after accelerated weathering test).

Contact angle
For the analysis of the wettability, contact angle measurements of distilled water on the OSB surface were performed through the sessile drop method by Drop Shape Analyzer System, Krüss software module (Hamburg, Germany).Sessile drops of 2 μl have been deposited on each specimen for 5 repetitions.15 replicas for both treated and untreated OSB panels have been done for a total of 150 drops.The contact angle was evaluated for a total duration of 120 s for each drop, while the right and left angle of the drops on the wood surface were collected at intervals of 0.5 s.

Accelerated weathering exposure
AWT of poplar OSB surface was performed by a QUV Accelerated Weathering Tester.The aim was to reproduce the damage caused by sunlight, rain and dew.The exposure of the wood surfaces of the samples lasted for 168 hours.The cycle consisted of a condensation period followed by a sub-cycle of water spray and UV-A 340 irradiation according to EN 927-6 [27].

Statistical analysis
Effects of thermal treatment and AWT on color and contact angle (after 120 sec) of poplar OSB were investigated through two-way ANOVA using the software IBM SPSS ver.24.(ΔE* = 14.14).Similar trends have already been reported [30] and show how UV irradiation omogenizes the color of wood.In particular, the lightnening of thermo-treated wood is similar to that occurring in dark colored woods exposed to UV for long time.Overall, the color change of untreated and treated specimens can be mainly attributed to the degradation of lignin, which is extremely susceptible to UV irradiation [30,31].The thermal treatment determined a significant (p<0.01)decreasing in wettability.In fact, after 120 sec the pre-AWT CA of thermo-treated OSB (120.7°)resulted almost twice the pre-AWT CA of untreated OSB (68.3°).The lower wettability of the panels surface after thermal treatment can be mainly ascribed to the phenomena of degradation occurring during the process, which modifies some chemical features of wood.In particular, the main reason are the migration of non-polar extractives to the surface during heating [32] and the cross-linking between the lignin and polysaccharides, with a consequent reduction in free hydroxyl groups among the hemicelluloses that changes during heating and may undergo oxidation reactions [33].

3.2.Contact angle
For untreated OSB, after 120 sec the pre-AWT CA (68.3°) resulted significantly lower (p<0.01)than the post-AWT CA (87.5°) .On the contrary, for treated OSB after 120 sec the pre-AWT CA (120.7°) is significantly higher (p<0.01)than the post-AWT CA (65.1°), highlighting a loss of the effect previously gained through thermal treatment.It can also be noted that, after weathering, the CA line of untreated OSB shows a constant trend in time (Fig. 1), which could further decrease, whereas that of thermo-treated OSB becomes rather stable after about 70 sec (Fig. 2).Overall, the exposure to UV irradiation can represent a method for reactivating the surface of thermo-treated poplar OSB in view of industrial coatings and finishing processes.

Conclusions
The aim of the present work was to evaluate the wettability and the color change of poplar OSB, taking into account the effects of heating and accelerated weathering test.The results obtained give some evidence in terms of color and wettability change.In this regard, the hydrophobicity of poplar OSB increased after thermal treatment, whereas the surface degradation due to weathering test caused a decreasing slightly stronger in thermally treated panels, at least after 120 sec.The results are interesting for several industrial applications in which it is essential to take into account any change in wetting properties.In particular, the weathering enabled to increase the wettability of thermally treated OSB and this effect can be further evaluated or exploited for improving the coating of this panel and similarly treated wood materials whose use is recently recording a wide market diffusion.

Figure 1 Figure 1 .
Figure1and 2 show the average values of contact angle, determined using water as liquid test, for untreated and treated specimens, pre-and post-weathering.

Figure 2 .
Figure 2.Contact angle variation in treated OSB panels before and after weathering.

Table 1
shows the average color coordinates measured for untreated (NT) and treated (T), non subjected to wethering test (Pre AWT) and after wethering test (Post AWT) specimens groups, whilst color changes (ΔE*) are shown in Table2.Preprints (www.