Properties of Plywood Made of Thermally Treated Veneers Bonded with Castor Oil-Based Polyurethane Adhesive

Abstract

Wood industries use thermal and thermomechanical treatments as ecological approaches to increase the durability of wood products, avoiding the need for chemical additives. In this regard, the aim of this study was to compare the physical and mechanical properties of plywood made from veneers treated at different temperatures using thermal and thermomechanical processes, with untreated panels serving as a control. The treatment process involved Pinus taeda veneers submitted to treatment in a hot press at 1.0 MPa in a laboratory oven at temperatures of 160 °C, 180 °C, and 200 °C for 30 min. For bonding the veneers, a vegetable-based polyurethane resin derived from castor oil with a grammage of 395 g/m² was used, applying pressing conditions at 90 °C, 0.6 MPa, and 10 min. Our results indicate that temperature significantly influences plywood properties, playing a key role in the choice of equipment for the treatment process. Regardless of the method employed, the treatment resulted in an improvement in the hydrophobicity of the veneers due to the decrease in hemicellulose content. Notably, the reduction in strength and stiffness caused by the loss of cell wall polymers was not statistically significant. The treatment was successful in softening the wood material, reducing roughness, and increasing wettability. Despite a minimum of 20% reduction in glue line tension, the samples still surpassed the 1 MPa mark, showing satisfactory results. This demonstrates the feasibility of adjusting treatment variables to ensure the proper use of this adhesive.

1. Introduction

In recent decades, the construction industry has increasingly embraced engineered wood products known for their reduced environmental impact and lower energy consumption compared to conventional building materials [1]. However, these wood products often exhibit limitations, such as inadequate moisture resistance, resulting in dimensional changes and susceptibility to fungal and pest attacks [2].

To tackle these challenges, wood modification methods like heat treatment (HT) and thermomechanical treatment (TM) have gained worldwide popularity as environmentally friendly alternatives, eliminating the need for potentially toxic chemicals [3,4]. The application of heat or a combination of heat and pressure reduces the material’s water affinity, resulting in enhanced moisture resistance and improved durability [5,6].

In the context of plywood, treatment procedures may involve direct application to the finished panel or pre-treatment of veneers prior to gluing [7]. While the addition of heat-treated veneers can lead to a reduction in shear strength at the glue line and variations in panel stiffness and strength [6,8,9], some studies suggest that plywood composed of densified veneers or a combination of treated and untreated veneers does not significantly differ from untreated plywood in terms of mechanical properties [7,10,11].

The physical and chemical properties of veneers heavily rely on the degradation and densification relationship during the process, which is influenced by pressure, temperature, and treatment duration [12]. Remarkably, densified veneers subjected to TM can yield positive outcomes with shorter treatment times, even as little as a few minutes [3,13,14]. Nonetheless, it is crucial to avoid excessive densification, as it can result in adverse effects such as increased weight, greater thickness loss, or diminished bonding performance [10,15].

Thermal modification at temperatures above 150 °C induces irreversible changes in the wood’s chemical structure, affecting key components such as hemicelluloses, cellulose, lignin, and extractives [4,16]. This process has a significant impact on surface properties, including roughness and wettability, which, in turn, influence the adhesive capabilities of wood-based panels [17,18,19].

In studies conducted by Ferreira et al. [5,15], plywood panels treated at varying temperatures (160 °C, 180 °C, or 200 °C) for 60 min exhibited smoother surfaces and reduced wettability as the severity of thermal treatment increased compared to the untreated ones. Interestingly, despite these surface alterations, the mechanical properties of the plywood remained unchanged, indicating that the heat treatment of the veneers did not compromise the final product’s mechanical integrity.

Although previous research has primarily focused on formaldehyde-based adhesives, such as urea-formaldehyde, phenol-formaldehyde, and melamine-formaldehyde, studies on bonding heat-treated veneers with castor oil-based polyurethane resins have been relatively limited, despite the growing interest in this subject [20,21,22]. The selection of castor oil-based polyurethane (PU) resin for this study is based on its non-aggressive nature to humans and the environment, as well as its excellent resistance to water and ultraviolet radiation. Derived from Ricinus communis, one of the most important oilseeds in tropical regions, this adhesive cures at room temperature and can be accelerated by applying temperatures ranging from 60 °C to 90 °C. This temperature range is remarkably lower than that of formaldehyde-based adhesives (around 140 °C) [23], offering energy-saving advantages for industries.

Hence, the main objective of this study is to investigate the application of Pinus taeda veneers treated with HT and TM in plywood production, aiming to enhance dimensional stability, durability, and water repellency, while maintaining its strength and stiffness. Furthermore, we aim to assess the compatibility of the PU adhesive with the treatment process and evaluate the consistency of the results obtained.

2. Materials and Methods

A schematic diagram of the methodology employed is shown in Figure 1. In this figure, it is possible to see that the work was carried out in three stages: (1) the treatment of the veneers; (2) the investigation of the chemical and physical changes in the veneers after treatment; and (3) the physical-mechanical characterization of the plywood. The variable considered in this research were the treatments (TM and HT) and temperatures (160 °C, 180 °C, and 200 °C). Commercial PU based on castor oil was used as adhesive. All the steps involved in the development of this study are detailed below.

2.1. Materials

Five 2.3 mm thick (nominal) veneers of Pinus taeda were used to manufacture the plywood. The veneers were partitioned in the dimensions of 450 mm × 450 mm and a moisture content of 6.6% was used in the experiments.

The resin used was a commercial two-component, 100% solid (solvent-free) adhesive based on vegetable-origin polyurethane. It was obtained from the company IMPERVEG^® (Aguaí, Brazil) and sold under the name AGT 1315. The product consists of two components: component [A], the prepolymer synthesized from diphenylmethane diisocyanate (MDI) (a dark brown substance), and component [B], the polyol derived from castor oil (a yellowish substance).

The two components were carefully cold-mixed in a 1:1 (v/v) ratio for 5 min using a mixer. After mixing, its viscosity began to increase. The pot life was approximately 15 min, after which the resin started to cure, making the material highly viscous and difficult to handle. The process was accelerated with the application of heat.

2.2. Treatment Veneers

The veneers treatment was performed using two different methods: HT, using a laboratory oven without air replacement and TM, using a hot-press. The treatments followed the methodology of Ferreira et al. [24]. The hot-press was used at a pressure of 1.0 MPa. For both treatments, the treatment time adopted was 30 min for three different temperatures.

2.3. Test Applied to Veneers

In order to investigate the physicochemical alterations in the veneers, the following tests were carried out: chemical analyses, wettability and surface roughness. Each veneer was cut into four equal parts (Figure 1), with three of them being prepared for heat treatment in the laboratory oven or hot-press. The other was left out to serve as a control sample.

2.3.1. Chemical Analyses

The veneers were converted into particles processed in a Wiley mill and later classified, selecting the ones in the range of 40 to 60 mesh. The process of determining the total extracts was executed according to the norm TAPPI T-257 [25] and consisted of three stages: Soxhlet extraction with toluene/ethanol (2:1 volume), ethanol extraction at 95% and hot water extraction in a water bath. The lignin content was determined according to ASTM D1106 [26]. The holocellulose content was determined through delignification using sodium chlorite and the cellulose content was obtained using a mixture of 24% potassium hydroxide. All samples were measured in triplicate. Data were presented as dry matter percentage.

2.3.2. Surface Roughness

The surface roughness analyses were conducted using a Dektak 150 surface profilometer (Veeco Instruments Inc., Plainview, NY, USA). The samples used were of 25 mm × 25 mm. The measurement length adopted was of 10 mm and cut-off length of 2 mm, taking the necessary precautions to obtain the roughness parameters from areas free of imperfections. Measurements were taken in five sample points, perpendicular to the grain, measuring the parameter of arithmetic mean roughness (Ra).

2.3.3. Wettability

Specimens of 25 mm × 25 mm were cut from veneers and the wetting behavior was measured with a goniometer (Ramé-Hart Instrument Co., Succasunna, NJ, USA). The test was conducted at a room temperature of 20 ± 2 °C and a drop of water of approximately 30 μL on the wood surface was added using a 1 mL syringe. Three sites were performed on each specimen, and three specimens were selected randomly for each group. The total time for each measurement was 30 s, with measurements being conducted every 0.2 s, totaling 150 measurements per sample.

2.4. Plywood Preparation

The methodology adopted and the parameters used in plywood production followed previous works [5,15,20,24]. The resin was manually applied with the help of a plastic spatula, ensuring uniform distribution of the adhesive on each veneer. The adhesive grammage was 395 g/m² in a double glue line (i.e., one veneer side filled with the adhesive).

Despite the increase in costs, the use of a higher grammage in plywood bonding provides stronger and more reliable adhesion, resulting in panels with higher strength and quality. This can be crucial for applications that demand durability and performance, such as construction panels, which are the focus of this study.

The five veneers were superimposed in orthogonal layers and previously cold-pressed in a manual pneumatic press with a pressure of 0.2 MPa for 15 min. Then, they were hot-pressed in a hydraulic press with a maximum pressure of 0.6 MPa at a temperature of 90 °C for 10 min, in order to guarantee the curing of the adhesive [24]. The pressing time was divided into 3 cycles, each of those lasting 3 min and 30 s added for pressure relief between cycles, according to the methodology of Ferreira et al. [5]. After pressing, the board was placed in a controlled room climate at a temperature of 25 °C and 60% relative humidity for 72 h before being sectioned to prepare the samples.

2.5. Physical Properties of Plywood

The produced panels were physically characterized according to Brazilian norms: determination of density (ABNT NBR 9485: 2011) [27], determination of moisture content (ABNT NBR 9484: 2011) [28], determination of water absorption (ABNT NBR 9486: 2011) [29] and determination of thickness swelling (ABNT NBR 9535: 1986) [30]. For each trial, the number of samples was ten.

Thickness swelling (TS) and water absorption (WA) of the samples were determined using test pieces with dimensions of 50 mm × 50 mm. They were immersed in distilled water for 24 h. The test samples were removed from the water, weighed, and their thickness was measured. The percent change from the original thickness represents thickness swelling and the percent weight change from the original weight represents water absorption.

2.6. Mechanical Properties of Plywood

The 3-point bending strength was determined using a Universal Testing Machine using the ABNT NBR 9533: 2013 method [31]. Static bending test was carried out in parallel (PA) and perpendicular (PE) directions. A minimum of six specimens were tested for each treatment. The plywood shear strength was determined according to ABNT NBR 12466: 2012 [32].

For the static bending test, the specimens were manufactured with dimensions of 350 mm × 75 mm × 12.0 mm. The distance between the support centers was 300 mm. The test was conducted with a single loading cycle and the loading speed was set at 0.08 mm/s.

For the glue line shear test, the specimens’ dimensions were 150 mm × 25 mm × 12.0 mm. The two grooves were 25 mm apart from each other. The loading speed used was 120 N/s⁻¹, resulting in an approximate test time of 35 s for each specimen.

2.7. Data Analysis

A hypothesis test was performed using ANOVA to verify whether there were significant differences between the means. The Tukey test was used at the 5% nominal probability significance level. The variables analyzed the different temperatures and used equipment. The linear correlations were determined based on the standard Pearson’s method. All statistical tests were performed using the statistical packages Minitab v.22 (Minitab Inc., State College, PA, USA).

3. Results

3.1. Chemical Analyses of Veneers

The heat treatment altered the wood structural chemistry, as shown in

Table 1. Average values of chemical analyses as a function of the type of heat treatment.

Treatment	Temp. (°C)	Holocellulose	Cellulose	Hemicellulose	Lignin	Extractives
	Control	67.28 (0.06) a 1	43.17 (0.38) ab	24.11 (0.34) a	26.11 (0.06) a	6.58 (0.11) a
HT	160	66.23 (0.34) ab	43.37 (0.19) a	22.85 (0.49) b	26.80 (0.34) a	6.97 (0.10) b
HT	180	64.99 (0.69) b	42.88 (0.41) ab	22.11 (0.28) bc	29.51 (0.69) c	6.50 (0.05) a
HT	200	61.64 (0.48) d	41.12 (0.13) d	20.52 (0.35) d	31.80 (0.48) d	6.56 (0.10) a
TM	160	63.54 (0.18) c	42.47 (0.24) bc	21.24 (0.16) cd	27.94 (0.18) b	8.52 (0.40) d
TM	180	62.65 (0.31) cd	41.99 (0.18) c	20.33 (0.14) d	30.03 (0.27) c	7.65 (0.12) c
TM	200	57.41 (0.76) e	41.94 (0.09) c	15.14 (0.49) e	35.24 (0.45) e	7.68 (0.09) c

¹ Averages followed by the same letter were not statistically different using Tukey test at the 5% significance level of probability. Standard deviation in parentheses.

. As a result, the heat treatment increased the yield of extractives and lignin and reduced the amount of holocellulose. The results follow the patterns of the ones obtained by other authors [9,33]. When the wood is exposed to high temperatures, the hemicellulose content decreases significantly because they are composed of shorter molecule chains, have a more branched structure and are generally sensitive to thermal degradation [33].

Lignin, in turn, shows an increase in its content. It is generally accepted that the increase in lignin content is due to the loss of polysaccharide material occurring during heating. However, this increase may also be related to condensation reactions in lignin with some degradation product [16,18]. Cellulose is less affected by the heat treatment, as observed in the data, although its degradation occurs mainly in its amorphous regions [34].

Regarding the extractives, an increase was observed for all temperatures in TM and 160 °C in HT. This increase in extractive content was also observed by other authors when analyzing pine wood [5,16]. Although some extractives disappear or degrade during the heat treatment, mainly the more volatile ones, new compounds are formed from the degradation of hemicelluloses. In the end, extractives remain in the biomass as molecules with weak fiber bonds [35].

The increase or decrease depends on the balance between the degradation of initial extractives and the emergence of new derivatives from the degradation of structural compounds [16]. This may be one of the possible explanations for higher quantities of extractives observed at lower temperatures and lower quantities at higher temperatures. Furthermore, the extractives content can vary as a function of wood species and the utilized treatment process [19,36].

Finally, it was noted that thermal degradation occurred more effectively in TM. The heating of central areas, and, consequently, the thermal degradation, due to the better pressed veneers thermal conductivity, happened faster when compared to the HT [37].

3.2. Roughness of Veneers

The results showed that both treatments significantly affected the surface roughness values of the veneers (Figure 2). All treated veneers had an Ra lower than the non-treated ones. These data are in agreement with other results in the literature [3,13,14]. The results suggest that the TM treatment allows for obtaining a smoother surface with low roughness.

In the visual analysis, it was possible to notice that the treatment resulted in a change in the veneer surfaces. According to the literature, after the glass transition temperature (160 °C) is achieved, the lignin becomes thermoplastic, making the wood surface a more compact solid [38]. This increase in smoothness is important to reduce the losses that occur in the plane and improve the finished products quality.

However, differences in roughness may occur for treatment due to changes in surface composition [39]. During the heat treatment, high temperature can cause degradation and volatilization of the hydrophobic extractives and volatile organic compounds that are derived from the depolymerization of wood components, especially from hemicellulose. These components may have a smoothing effect on the wood surface, and their removal or deposited can result in a rougher surface [40].

Another explanation could be that the reduction in wood moisture content can cause cell and fiber shrinkage, leading to a more irregular surface. This means that certain regions of the wood may shrink more than others, resulting in a wrinkled surface [41].

3.3. Wettability of Veneers

Figure 3 shows the change in contact angle over time. The highest droplet angle occurred in the veneer treated using HT. In this treatment, the contact angles during the 30 s after start were greater than 90°, presenting the surface hydrophobic characteristics. The different wetting behaviors in the veneer is illustrated in Figure 4. It shows the significant impact of the treatment process on the surface wettability. It was possible to observe the lower spread and fluidity of the drop on the heat-treated surface. The higher contact angles of liquids on wood means lower wettability.

This phenomenon was also observed in a similar study while heat treating Pinus taeda veneers for the period of one hour [15]. The surfaces of treated wood became less hydrophilic and significantly repellent to water. The change in properties is mainly caused by the thermal degradation of hemicelluloses and amorphous cellulose that absorb less water [11]. Moreover, it also led to the plasticization of lignin and the consequent reorganization of lignocellulosic polymeric components of wood during heating [42].

The wettability of liquid on the wood surface can also be related to solid surface roughness and wood macroscopic characteristics, whose lumen volume of vessels and fibers on the wood surface would have become narrower with the treatment [43]. Note that the contact angle test was conducted with water. When it comes to the adhesive, the difficulty of spreading and penetrating the surface irregularities of the wood due to its viscosity is even greater.

Finally, despite the veneers treated with the HT process showing lower wettability, no significant impact of this difference was observed in the produced panel. This leads to the conclusion that both processes allowed for uniform heat distribution and resulted in effective treatment methods.

3.4. Physical Properties of Plywood Samples

The physical properties of plywood manufactured with treated veneers are presented in

Table 2. Physical properties of plywood produced with treated veneers.

Treatment	Temp. (°C)	Density (kg/m3)	EMC (%)	WA (%)	TS (%)
	Control	592 (57) a 1	7.59 (1.02) a	41.18 (9.27) a	4.90 (0.58) a
HT	160	566 (43) ab	6.95 (0.56) a	33.71 (5.09) b	4.26 (0.21) ab
HT	180	528 (42) b	6.34 (0.42) ab	34.30 (7.29) b	3.81 (0.22) ab
HT	200	567 (45) ab	6.11 (0.41) ab	29.55 (6.98) b	3.65 (0.55) ab
TM	160	563 (19) ab	6.99 (1.33) a	30.67 (5.74) b	4.68 (0.62) ab
TM	180	553 (27) ab	6.42 (1.08) ab	30.38 (5.05) b	4.67 (0.48) ab
TM	200	525 (12) ab	5.78 (0.78) b	30.94 (4.06) b	3.46 (0.38) b

¹ Averages followed by the same letter were not statistically different using Tukey test at the 5% significance level of probability. Standard deviation in parentheses.

. The ABIMCI (Brazilian Association of the Mechanically Processed Wood Industry) [44] has set specific mass values between 476 kg/m³ and 641 kg/m³ for structural panels with five pine veneers, which were met by all treatments in this study.

Although the treated panels showed lower densities, significant values were only found in the HT of veneers at 180 °C. These reductions were associated with the degradation of wood constituents. Small reductions in panel density after heat treatment are normal according to the literature. Lovric et al. [6] studied panels produced with poplar veneer treated at temperatures 190 °C, 200 °C, 210 °C and 215 °C and observed that the mass loss is more significant above 200 °C.

The heat treatment resulted in a reduction in the equilibrium moisture content (EMC), indicating improved dimensional stability. However, these values were below the minimum classification requirements [44]. The greatest reduction was observed in the TM at 200 °C, which observed the greatest degradation in the chemical analyses. It is clear that the cause of decrease in equilibrium moisture is due to the lower amount of water adsorbed by the wood as a consequence of the depolymerization of hemicelluloses into oligomeric and monomeric units, reducing the amount of hydroxyl groups [45].

The results of the physical tests lead to the conclusion that the veneer treatment under the suggested conditions is efficient, resulting in panels with similar density. However, the low hygroscopicity observed in the veneers due to the thermal treatment may negatively impact bonding since moisture is crucial for the development of adequate adhesive strength. When moisture is not adequate, the adhesive’s ability to penetrate the wood is compromised, resulting in a weak glue [46].

According to the research findings, thermal treatment of wood veneers in plywood panels results in a significant reduction in water absorption and thickness swelling, making the panels more resistant to moisture and dimensionally stable. In addition, the high adhesive weight prevents water from penetrating the panels and thus reduces TS and WA. Although the technical catalog for pine plywood panels made available by ABIMCI does not provide reference values for this property, these findings align with other studies on the subject [5,14].

3.5. Mechanical Properties of Plywood Samples

The bending strength parallel to the grain direction is summarized in Figure 5. All treated groups met the minimum required specifications (indicated by the red line on the graphs) for MOE and MOR in structural plywood panels with five plies. In the perpendicular direction, only the panels produced with veneers treated at 160 °C and 200 °C using TM (Figure 6) failed to meet the ABIMCI catalog specification [44]. However, the lower values found may be attributed to variations in veneer quality as no statistically significant differences were observed between treatments.

These results conflicted with those reported by Lovric et al. [6], which showed significant effect of thermal modification. Different conclusions are probably due to the different treatment conditions and raw materials used. Similar results of this experiment were found by Ferreira et al. [5], adopting the same species, temperature and longer exposure to heat (1 h).

A reduction in glue line tensile was observed for both treatments (Figure 7). The reduction during HT reached 41.1% and 38.3% for the TM method. It is worth noting that if a sample has two letters of the Tukey test, it means that it would be in the transition between the correlated values.

Although PU is known to provide excellent tensile shear strength in wood, heat treatment has affected its effectiveness. In other words, the heat exposure caused the chemical bonds on the veneer surface to break, resulting in a less receptive surface for adhesion. Similar results regarding the influence of heat treatment on bonding quality were also observed for other types of adhesives [9].

The reduction in glue line resistance in the heat treatment is complex and can be explained by the influence of various factors: moisture content, temperature, adhesive, the raw material itself, etc. The reason for such decrease in property may be associated with the increased lignin and holocellulose degradation upon the heat treatment which decreases wood hygroscopicity. Plywood is a layered composite and the value of bonding strength is affected by both the wood and the adhesive [8]. The reduction in hygroscopic sites and veneers roughness may have prevented the adhesive from an adequate spreading and made it more difficult for the adhesive to bond with the wood [46,47].

Therefore, the exposure of veneers to a temperature above 160° is enough to cause wood degradation, as observed in the analyses carried out. It is understandable that the higher strength properties of the treated material will also affect the bond quality if the prerequisite for successful adhesion is fulfilled. Finally, it is essential to mention that the samples continued to meet the values proposed in the catalog and normative documents with the tensile values obtained greater than 1 MPa.

4. Discussion

As stated in the chemical analyses, heat treatment leads to degradation of hemicellulose, increases lignin and leads to loss of equilibrium moisture content. The equilibrium moisture content was more influenced by temperature than by equipment. Furthermore, the reduced roughness and wettability increased the values of contact angles. As a result, there is a reduction in tensile rupture in the glue line. Therefore, it was important to find out which parameters have the greatest influence.

Table 3. Pearson’s correlation coefficient between the analyzed parameters.

	Cellulose	Hemicellulose	Lignin	Extractives	Roughness	EMC	Contact Angle
Roughness	0.661 n.s. 1	0.422 n.s.	−0.576 n.s.	0.139 n.s.	x	x	x
EMC	0.601 n.s.	0.979 ***	−0.882 ***	−0.631 ***	0.368 n.s.	x	x
Contact angle	−0.652 n.s	−0.744 *	0.764 *	−0.392 n.s.	−0.651 n.s.	−0.748 *	x
Glue line	0.502 n.s	0.643 n.s.	−0.655 n.s.	−0.360 n.s.	0.740 *	0.618 n.s.	−0.891 ***

¹ n.s.—non significant; * p-value < 0.05; *** p-value < 0.01.

shows the Pearson correlations and demonstrates that the decrease in roughness and the increase in the contact angle result in a significant negative impact on the glue line shear property (Figure 8).

Regarding the wetting capacity, it was confirmed that this property is significantly influenced by the moisture content. This content is related to the reduction in hydrophilic groups in the wood, which is supported by the lower hemicellulose content found in this study. Furthermore, condensation reactions in lignin with some degradation products may be partially responsible for water repellency in the veneer, which is strongly indicated by the negative correlation.

Moreover, all the results of the parameters analyzed in this study indicate that the equipment’s contribution to the treatment was modest when compared to the influence of temperature. The application of heat on the veneers offers significant physical benefits as it results in panels that are more moisture-resistant and dimensionally stable, increasing the durability and overall quality of the final product, which are desirable characteristics especially for construction materials. On the other hand, it hinders adhesive penetration due to lower moisture content in the veneers, resulting in lower bonding strength.

For future research, it is recommended to conduct lamination tests and other chemical analyses to investigate the bonding strength between layers of wood veneers with different treatments. This study will provide a better understanding of how the thermal treatment has affected the adhesive’s capability to form a strong bond with the substrate. Further adhesive tests with lower weights and shorter pressing times are recommended to optimize cost-effectiveness.

5. Conclusions

The objective of this research was to study the effect of thermal modification process of veneers using the TM and HT processes. Based on this study, the following conclusions could be drawn:

• There were no significant differences between the veneers treated using the HT and TM processes under the conditions carried out in this experiment;
• Heating above 160 °C for 30 min is sufficient for changes in the veneer, such as to soften the wood material and reduce its roughness;
• The treatment increased the lignin content, while decreasing hemicellulose. In consequence, there was a reduction in the equilibrium moisture content and the contact angles with water, expressing the wettability, thereby producing veneers with hydrophobic characteristics;
• The reduction in strength and stiffness was not statistically significant;
• There was a reduction in tensile rupture in the glue line of at least 20% affected due to the heat treatment. This means that the glue ability of veneers can simultaneously be dependent on the combination of several parameters, including its roughness and wettability;
• This study provides a technical baseline that can help in the development of new panel protection technologies. In terms of treatment, although the results in general were not so different, the treatment in hot-press at 180 °C indicated better performance at analyzing all parameters.