Adhesion of coating films on laser engraved wood surface

Abstract

The adhesion of two coating systems – hard wax oil (oil-based) and PAM lak (water-based) – were evaluated on laser-engraved wood surfaces of Norway spruce (Picea abies (L.) Karst.), European beech (Fagus sylvatica L.), and pedunculate oak (Quercus robur L.). Laser engraving was performed at two laser powers (8% = 11 W; 16% = 22 W) and three raster densities (10, 20, and 30 lines·mm⁻¹) for each power level. Adhesion was assessed using the pull-off test. The oil-based coating generally showed lower adhesion to the wood surface compared to the water-based coating. In contrast, several combinations of engraving parameters on spruce (8 × 20) and oak wood (8 × 10, 16 × 10) increased oil-based adhesion but tended to reduce water-based adhesion. On the other hand, the adhesion of the water-based coating was significantly reduced on beech wood (16 × 30) and oak wood (8 × 30, 16 × 20 and 16 × 30). In some cases, adhesion of the water-based coating exceeded the cohesive strength of the modified wood surface layers, leading to cohesive failure within the wood.

Full Article

Adhesion of Coating Films on Laser Engraved Wood Surface

Gabriela Slabejová  , ^a,* Ivan Kubovský  ,^b Jarmila Schmidtová  ,^c and

Zuzana Vidholdová  ^d

The adhesion of two coating systems – hard wax oil (oil-based) and PAM lak (water-based) – were evaluated on laser-engraved wood surfaces of Norway spruce (Picea abies (L.) Karst.), European beech (Fagus sylvatica L.), and pedunculate oak (Quercus robur L.). Laser engraving was performed at two laser powers (8% = 11 W; 16% = 22 W) and three raster densities (10, 20, and 30 lines·mm⁻¹) for each power level. Adhesion was assessed using the pull-off test. The oil-based coating generally showed lower adhesion to the wood surface compared to the water-based coating. In contrast, several combinations of engraving parameters on spruce (8 × 20) and oak wood (8 × 10, 16 × 10) increased oil-based adhesion but tended to reduce water-based adhesion. On the other hand, the adhesion of the water-based coating was significantly reduced on beech wood (16 × 30) and oak wood (8 × 30, 16 × 20 and 16 × 30). In some cases, adhesion of the water-based coating exceeded the cohesive strength of the modified wood surface layers, leading to cohesive failure within the wood.

DOI: 10.15376/biores.21.2.3300-3320

Keywords: Adhesion; Laser; Beech wood; Oak wood; Spruce wood; Oil-based; Water-based

Contact information: a: Department of Furniture and Wood Products, Faculty of Wood Sciences and Technology, Technical University in Zvolen, Masaryka 24, 96001 Zvolen, Slovakia; b: Department of Physics, Electrical Engineering and Applied Mechanics, Faculty of Wood Sciences and Technology, Technical University in Zvolen, 96001 Zvolen, Slovakia; c: Department of Mathematics and Descriptive Geometry, Faculty of Wood Sciences and Technology, Technical University in Zvolen, T.G. Masaryka 24, 96001 Zvolen, Slovakia; d: Department of Wood Technology, Faculty of Wood Sciences and Technology, Technical University in Zvolen, Masaryka 24, 96001 Zvolen, Slovakia;

* Corresponding author: slabejova@tuzvo.sk

Graphical Abstract

INTRODUCTION

Laser-treated wood has become increasingly recognized as an accessible and versatile design material. Lasers are a promising technology in wood science for architectural essence and furniture making to toys and fine crafts (Islam et al. 2023). Numerous studies have demonstrated that various technological parameters of laser treatment, as well as the operating conditions applied during processing, result in different surface qualities and visual outcomes depending on the intended aesthetic effect (Chernykh et al. 2022; Kúdela et al. 2023; Nguyen-Thi-Ngoc and Dang 2023; Jurek et al. 2025). Laser modification of wood surfaces can lead to chemical transformations within the surface layer (Dolan et al. 2015; Kúdela et al. 2020, 2023, 2024; Li et al. 2022a,b), distinctive surface characteristics, unconventional coloration (Açık 2023), and tactile surface textures (Gurău et al. 2017; Gurău and Petru 2018; Gurău et al. 2021; Kúdela et al. 2022). This technique is also gaining ground in the context of wooden buildings and architectural applications (Li et al. 2014). Furthermore, research has investigated how pre-treatment of furniture parts by laser affects laser processing performance during computer numerical controlled (CNC) machining (Açık 2024).

In practice, laser-textured wood surfaces are most frequently utilized in furniture production and interior design elements (Lungu et al. 2022). These surfaces, like untreated wood, usually require protective finishing to preserve their appearance and prevent damage from liquids and mechanical wear. Recently, protection against UV radiation has also been frequently used to maintain the color stability of the treated wood surface (Kúdela and Kubovský 2016). Traditional finishing methods involve the application of coatings that form a hardened film to protect the substrate. However, to ensure proper performance of such coatings, it is crucial to understand the adhesion behaviour between the coating film and laser-treated surfaces (Li et al. 2022a). Parameters such as surface roughness and wettability – both significantly affected by laser processing – play a vital role in coating adhesion (Li et al. 2021a). Moreover, the mechanical properties of the coatings themselves are important contributors to adhesive performance (Pavlič et al. 2021). While surface characteristics influence wettability to a certain extent, it is often the intrinsic properties of the coating that determine the final interaction (Žigon et al. 2022).

Laser-textured wood surfaces can also be applied in exterior furniture design, where additional protection is necessary due to exposure to weathering factors. Traditional wood coatings are increasingly restricted due to environmental concerns, prompting a search for eco-friendly alternatives (Varganici et al. 2021). Among these, coatings based on natural or synthetic oils and waxes – sometimes combined with water-based dispersions – belong to the class of “green” or ecological coatings. Oils penetrate the wood and enhance its natural grain and appearance. However, because oils fill the lumens and voids without forming chemical bonds with the wood cell walls, they often exhibit certain performance limitations, such as low hardness, poor resistance to detergents and chemicals, and limited photostability (Bulian and Graystone 2009; Vidholdova et al. 2021).

Recent advancements have focused on modifying wax-oil coatings to improve their performance. For example, promising results have been achieved with nanocomposite wax-oil coatings incorporating stearoyl chloride-grafted cellulose nanocrystals (SCNCs) (Wang et al. 2024). Similarly, hybrid formulations have been prepared using nanoparticles, embedded in linseed oil nanoemulsions (Bansal et al. 2022). Likewise, Kabasakal et al. (2023) reported promising results from bio-based epoxide-amine nanocoatings.

Another class of environmentally friendly finishes includes water-based coatings. Extensive research has been devoted to improving their performance, particularly by incorporating nanoparticles (Li et al. 2021b; Wang et al. 2023; Zou et al. 2023, 2024) or other functional additives (Henn et al. 2021; Calovi and Rossi 2023; Zou et al. 2025). Among the key quality parameters for such coatings is their adhesion to the wood surface – a property that has been extensively studied across various formulations and application scenarios (Vidholdova et al. 2017; Slabejová et al. 2019; Vidholdova et al. 2021; Liu and Xu 2022; Karaman et al. 2023; Angelski and Atanasova 2024; Hubbe and Laleicke 2025).

The present study investigates the influence of raster density at two power levels (8% and 16% of Pₘₐₓ = 137.5 W) during engraving on the adhesion of two different types environmentally friendly of coatings (oil-based and water-based) applied to surfaces of three wood species (spruce (Picea abies (L.) Karst.), beech (Fagus sylvatica L.), and oak (Quercus robur L.)), which are often used to make furniture. The findings are expected to contribute to optimizing the use of environmentally friendly oil- and water-based coatings on laser-engraved surfaces of different wood species.

EXPERIMENTAL

Materials and Sample Preparation

Test samples were prepared from spruce wood (Picea abies (L.) Karst.), beech wood (Fagus sylvatica L.), and oak wood (Quercus robur L.) logs, cut approximately 1.3 m above the ground. From each log, a single plank with final dimensions 1000 × 70 × 15 mm³ was selected. These planks were then used to prepare eighty-four test specimens from each wood species with either radial even radial-tangential orientation and dimensions of 115 × 70 × 15 mm³. Six specimens were randomly selected for each combination (Wood × Laser Power × Raster Density) from each plank samples without macroscopic wood defects, and with all samples were and subsequently conditioned to 6% moisture content before sanding the surfaces with coarse and finally fine P180-grit abrasive paper.

Both laser-treated and untreated boards from all three wood species were. The test specimens were divided into two groups:

1. Untreated wood – natural (non-engraved) reference samples.

2. Laser-modified wood – samples treated by surface engraving.

Laser Engraving

Laser engraving was performed according to the methodology described in (Kúdela et al. 2022). Each sample was positioned under the lens of a CO₂ laser system (CM-1309, Shenzhen Reliable Laser Tech, Shenzhen, China), with a fixed distance of 17 mm between the lens and the wood surface used (Fig. 1a, b). The laser head moved along the wood grain direction at a constant speed of 350 mm·s⁻¹. The maximum output power of the laser system was Pₘₐₓ = 137.5 W, which was measured at the resonator output as described in Kúdela et al. (2024).

Fig. 1. The laser engraving: a) The sample positioned under the lens of a CO₂ laser system; and b) the laser-treated three wood species (spruce, beech and oak wood)

Laser treatment was conducted at two power levels: 8% = 11 W and 16% = 22 W of Pₘₐₓ. For each wood species, samples were engraved using three raster densities: 10, 20, and 30 lines·mm⁻¹, resulting in a total of seven treatment combinations per species (six laser treatments plus one reference (untreated control)). The engraving was executed parallel to the grain.

Surface Finishing

Two types of coating materials (Table 1) were used for surface finishing:

1. Oil-based finish – Hard wax oil Osmo Original 3032, Osmo Holz und Color GmbH & Co. KG, Warendorf, Germany), a high-solids coating based on natural oils and waxes (Osmoshop 2025).

2. Water-based finish – PAM lak (PAM-lak, Čierna Voda – Triblavina, Slovakia). It was composed of aqueous acrylic dispersions, coalescing agents, and special additives (PAM 2020).

Table 1. Characteristics of Coating Materials and their Applications

All coating applications were carried out in accordance with the manufacturers’ technical data sheets. The finishes were applied to all test specimens after laser treatment and prior to adhesion testing. After application of the coating materials, the surface treatment cured under the same conditions (at 23˚C, RVV 55%) for 21 days.

Thickness of the Coating Film

To document the visual changes of the coated wood surfaces after laser engraving, high-resolution images were acquired using a Color Laser Jet Pro MFP M477fdw (Fig. 2a).

The non-destructive method was chosen to measure the film thickness (µm) using the ultrasonic instrument PosiTector 200 (DeFelsko Corporation, Ogdensburg, NY, USA). Thickness measurements were conducted with surface of 100 × 50 mm² at 12 points per specimen across the top surface (Fig. 2b).

Fig. 2. The instruments: a) multifunction printer Color Laser Jet Pro M477fdw; and b) the PosiTector 200 non-destructively measures the film thickness

Adhesion

The adhesion of the coating films on both laser-modified and unmodified wood surfaces was evaluated using the pull-off test in accordance with the STN EN ISO 4624 (2024) standard (Slovak Technical Standard adopted as European Standard, which adopted the International ISO Standard). For each wood species and coating material, seven combinations of laser treatment (including untreated) were tested. In each combination, six replicates were tested, as specified by the STN EN ISO 4624 (2024) standard.

Adhesion was measured using a PosiTest® AT-A Pull-Off Adhesion Tester (DeFelsko Corporation, Ogdensburg, NY, USA). Circular metal dollies (20 mm in diameter) were bonded to the coated surfaces using a two-component epoxy adhesive (Pattex® Repair Epoxy, Henkel AG & Co. KGaA, Düsseldorf, Germany). The adhesive was allowed to cure for 72 h at 20 °C and 60% relative humidity. Prior to testing, the edges of the bonded dollies were carefully incised with a blade to localize the failure within the test area and avoid propagation outside the bonded zone (Fig. 3a).

The pull-off force was applied at a rate of 1 mm·min⁻¹ until detachment occurred. Each failure was then visually assessed using both a table magnifier and a VHX-7000 digital microscope (Keyence Corporation, Osaka, Japan) (Fig. 3b).

Fig. 3. The adhesion of the coating films was evaluated using the pull-off test: a) the PosiTest AT-A Pull-off Adhesion Tester measures adhesion of coatings; and b) the digital microscope Keyence VHX-7000

The failure modes were classified based on their location: within the wood substrate, the coating film, the adhesive joint, or at the interface with the metal dolly. Microscope inspection of the fracture surfaces was carried out to determine the nature of the failure as follows:

A – Cohesive failure within the substrate (wood).

A/B – Adhesive failure between the substrate and the first coating layer.

Y/Z – Adhesive failure between the adhesive and the dolly.

The area of each type of fracture was calculated according to the content of the circular sector (Fig. 4a) and segment (Fig. 4b):

A1 = πr2. α / 360° (1)

A2 = r2/2 . (arc α – sin α) (2)

In these equations, A1 and A2 represent the content (m²), C represents the centre of the circle, π represents pi (Archimedes’ constant), r represents the circle radius (m), and α represents the central angle (°). The area of each type of fracture was a percentage of the total fracture area, rounded to the nearest 10%.

Fig. 4. The area of type of fracture was calculated according to the content of: a) the circular sector and b) the circular segment

RESULTS AND DISCUSSION

Statistical Analysis

In the statistical processing of data, a three-way Analysis of Variance was applied to examine the effects of three independent factors – wood, number of lines and laser power on the response variable – adhesion. This approach made it possible to assess both main effects and interaction effects among the factors. Duncan’s post-hoc test was applied as part of the pairwise simultaneous comparison. To complement the ANOVA results, confidence intervals were constructed for the mean values of adhesion for each group, providing an 95% estimate of the population mean values. All analyses were performed under the assumption of normality and homogeneity of variances. As a decision rule in hypothesis testing, the commonly accepted 5% significance level was applied. All calculations were performed using the statistical software STATISTICA 14.

Dry Film Thickness and Surface Wood after Laser Engraving

Table 2 presents the visual documentation (scans) of the engraved surfaces of spruce, beech, and oak wood with coating films.

Table 2. The Surface Change after Laser Engraving with Surface Finishes (Oil-based and Water-based) on Spruce, Beech, and Oak Wood

Engraving altered the colour of the wood surface (Kúdela et al. 2022, 2023, 2024) and transformed the originally flat surfaces into wavy ones. The undulation was caused by the uneven loss of material during engraving. These same samples were analysed in detail in previous studies by Kúdela et al. (2020, 2021), where roughness parameters both parallel and perpendicular to the grain were quantified. The present article therefore builds directly on this published research by the project team, highlighting the most relevant characteristics to explain adhesion behaviour. The results of Kúdela et al. (2020, 2021) showed that increases in roughness and waviness were significant already at 8% laser power. Across the entire raster density range, increasing raster density produced higher roughness parameters both parallel and perpendicular to the grain, with significantly greater variance recorded perpendicular to the grain. The most pronounced changes were observed in spruce, whereas beech exhibited the lowest variance.

Treatment of the wood surface with a CO₂ laser caused changes in the physical, mechanical, and chemical properties of the wood. Dolan et al. (2015) reported that the transition from translucent wood to dark discolorations, concentrated at the top 25 microns of the surface, provided evidence of laser penetration and surface modification. Ablation and wood penetration are known to depend strongly on wood cellular structure. Although laser modification altered native morphology, nearly two-thirds of the surface still consisted of cellulose, which ensures mechanical performance and contributes to adhesion.

This finding indicate that laser engraving, similarly to steaming, as reported by Adamčík et al. (2022), influences the quality of semi-finished and finished wood surfaces. Heat treatments (laser, steaming, etc.) thus represent an important tool for improving adhesion of coatings.

Based on the analysis of published knowledge, it can be stated that heat treatments of wood (laser engraving, steaming, etc.) affect the quality of the wood surface. It was necessary to verify how laser engraving affects the adhesion of coating films on three different types of wood. The following articles (Dolan et al. 2015; Kúdela et al. 2022, 2023, 2024) present the results of chemical analysis of the wood surface after laser treatment.

After compositional analysis, Dolan et al. (2015) reported that the isolated solids portion of the laser modified material contained an increased amount of lignin due to laser treatment. Hemicelluloses were less thermally stable compared to other wood components and were susceptible to degradation and potentially volatilization during laser modification dependent upon the degree of degradation. Residual hemicellulose monosaccharides are known to create sticky surfaces and may provide initial tack for adhesion. Kúdela et al. 2022, 2023, 2024) present the chemical analysis of wood surfaces after laser engraving and subsequently the discolouration of surface due to the laser action on spruce, beech, and oak wood, on which adhesion was determined by using the Pull-off test.

For the untreated reference samples of all the wood species, the penetration of both coating materials (oil-based and water-based) into the wood was the lowest compared with the laser-engraved surfaces. Measurements of film thickness using the non-destructive ultrasonic method showed that the coatings on untreated samples remained mostly on the surface, forming a thicker continuous film. In contrast, the coatings on engraved surfaces formed thinner films, as the surface layers of the charred wood after engraving were more impregnated (Table 3).

Table 3. Thickness of the Coating Film of the Oil-based and Water-based Surface Finishes on Spruce, Beech, and Oak Wood After Laser Engraving

Coating Adhesion Strength on Oil-based Surface Finish

Table 4 presents the results of three-factor analysis of variance for the oil-based coating film. The factors monitored were wood (spruce, beech, oak); laser power (8% and 16% of Pₘₐₓ); and raster density (10, 20, and 30 lines·mm⁻¹). At significance level of α = 0.05, the differences in mean adhesion values were found to be significant. The interaction effect of the investigated factors on adhesion was also significant (p = 0.020).

Table 4. Results of Three-factor ANOVA – Influence of the Interaction of the Investigated Factors (Wood × Laser Power × Raster Density) on Adhesion

For the oil-based coating film, the basic descriptive statistics and 95% interval estimates for the mean adhesion values for each combinations of experimental factors are presented in Table 5.

Table 5. Descriptive Statistics and 95% Confidence Intervals of Adhesion of Oil-based Coating Film Means for Individual Combinations of Experimental Factors (Wood × Laser Power × Raster Density)

The sample means and 95% confidence intervals are illustrated as box plots in Fig. 5. For spruce, the combination 8 × 20 showed a significant increase compared to the reference sample (p = 0.017). For beech, all tested factor combinations differed significantly from the reference (p < 0.05). For oak, significant differences were observed for combinations 8 × 10 (p = 0.027) and 16 × 10 (p = 0.011).

Dolan et al. (2015) reported that laser modification creates a surface phenomenon that physically and chemically alters the natural biopolymer organization of lignocelluloses materials in a way that promotes adhesion. The present results confirmed the conclusions in Dolan et al. (2015); thus, the adhesion of oil-based coating film to laser engraved wood surfaces increased. In Table 4, it is apparent that the lowest adhesion was on spruce surfaces with laser engraving 16 × 10 (1.41 MPa) and the highest on beech surfaces with laser engraving 16 × 10 (3.64 MPa).

Fig. 5. Confidence intervals (95%) for means of adhesion of oil-based coating film at three raster densities (10, 20, and 30 lines·mm⁻¹). a) reference sample; b) laser power 8%; c) laser power 16%

Coating Adhesion Strength on Water-based Surface Finish

The results of the three-factor ANOVA for the adhesion strength of the water-based coating layer are shown in Table 6. The factors monitored were wood (spruce, beech, oak); laser power (8% and 16% of Pₘₐₓ); and raster density (10, 20, and 30 lines·mm⁻¹). At α = 0.05, the interaction effect of the investigated factors on adhesion was significant (p = 0.013), indicating that the mean adhesion values differed significantly across factor combinations.

Table 6. Results of Three-factor ANOVA –Influence of the Interaction of the Investigated Factors (Wood × Laser Power × Raster Density) on Adhesion

For the water-based coating film, the descriptive statistics and 95% confidence intervals of the mean adhesion values for each combination of experimental factors are presented in Table 7.

Table 7. Descriptive Statistics and 95% Confidence Intervals of Adhesion Means of Water-based Coating Film for Individual Combinations of Experimental Factors

The sample means and 95% confidence intervals are illustrated in Fig. 6. Reference samples are shown on the left, and factor combinations on the right. For spruce, no significant differences from the reference were observed. For beech, the combination 16 (laser power) × 30 (raster density) differed significantly from the reference (p = 0.002). For oak, the combinations 8 × 30, 16 × 20, and 16 × 30 showed significant differences (p < 0.05).

The present results on the adhesion of water-based coating film to laser engraved wood surface did not confirm the conclusions in the work of Dolan et al. (2015). The laser engraved wood surface did not promote adhesion of water-based coating film on spruce wood. Adhesion on beech wood was both increased and decreased, and on oak wood it was reduced. In Table 6, it can be seen that the lowest adhesion was on spruce surfaces with laser engraving 16 × 10 (2.84 MPa) and the highest on beech surfaces with laser engraving 16 × 10 (9.60 MPa).

Fig. 6. Confidence intervals (95%) for means of adhesion of water-based coating film at three raster densities (10, 20, and 30 lines·mm⁻¹). a) reference sample; b) laser power 8%; c) laser power 16%

Fig. 7. Confidence intervals (95%) for means of adhesion of oil- and water-based coating film at two laser powers (8%; 16% of P_mₐₓ = 137.5 W)

Coating Adhesion Strength on Laser Engraving Surface Wood

The results showed (Fig. 7) that laser engraving power level 8% and 16% without influence of raster density (10, 20, or 30 lines·mm⁻¹) on spruce, beech, and oak wood did not significantly increase the adhesion of the oil-based coating film. The laser engraving power level 8% and 16% on spruce and beech wood did not significantly reduce the adhesion of the water-based coating film. However, the laser engraving power level 16% on oak significantly reduced the adhesion of the water-based coating film.

Coating Adhesion Strength: Fracture Pattern

After the Pull-off test, the dolly surface and the surface of the laser-treated and non-treated wood that were and subsequently coated with coatings were microscopically inspected (Table 8.).

Table 8. Nature of the Fracture, and the Type of Fracture for Oil-based and Water-based Surface Finish on Engraved Spruce, Beech, and Oak Wood

On the spruce wood surface without laser treatment, the type of fracture was 100% adhesive failure between the substrate and the first coating layer (Fig. 8a). The oil-based system on surfaces of the laser-treated spruce exhibited combination adhesive failure (Fig. 5a) and cohesive failure within the substrate (Fig. 8c).