Introduction

UF resins are considered as one of the major adhesives for the production of wood-based composite panels, such as plywood, particle boards, and medium-density fiberboards, because of their clear color, fast curing, and low cost (Dunky 1998; Park et al. 2006; Yang et al. 2010). The performance of an adhesive in wood-based products depends on three major factors: adhesive properties (e.g., viscosity, molecular weight, and reactivity), wood parameters (e.g., wood species, surface roughness, extractives, and morphology), and processing factors (time and temperature for cold or hot pressing, etc.). In wood adhesion, the wetting, flow, penetration, and curing of adhesives at wood surface could experience some complicated physical and chemical processes. Substantial literature has studied the mechanical, physical, and chemical properties of resin adhesives for wood adhesion (Myers 1984; Dunky 1998; Park et al. 2006; Que et al. 2007; Gao et al. 2008; Alam et al. 2012; Jeong & Park 2019b). However, wood is a heterogeneous material composed of cellulose, hemicellulose, lignin, and extractives with different chemical properties, and anisotropic material with different physical properties depending on the directions (longitudinal, tangential, and radial). These parameters make wood adhesion complex processes (Merela and Čufar 2013; Pang and Herritsch 2005; Lloyd 1978; Belt et al. 2021). When the resins adhere to wood, various initial phenomena involve the wetting, spreading, and attaching of resins to the wood surface (Collett 1972; Berg 1993). In particular, “wetting” on wood surfaces is one of the signs of resin bonding. An equilibrium contact angle is obtained on the surface when the adhesion of adhesive is balanced with the solid surface by surface tension (Whitesides et al. 1991). The wetting process includes: (1) an interfacial adhesion at the surface, (2) adhesive spreading that occurs when a liquid flows over a solid surface, and (3) penetration of a liquid deep into a porous solid (Patton, 1970). Therefore, wettability is inextricably linked to the adhesion between the resin and wood (Matuana et al. 1998). Thus, it is apparent that good wetting is an important prerequisite process for establishing proper interfacial bonding and strong adhesion (Baier et al. 1968; Shi and Gardner 2001; Shupe et al. 2001; Xu et al. 2012; Petrič and Oven 2015). After wetting, actions of resins on the wood surface such as spreading, penetration, and adhesion follow.

In most studies, it is very common to evaluate the wettability using a static contact angle which is formed at an equilibrium on the solid–liquid interface because of its accessibility and simplicity (Hse 1972b; Sun et al. 2017; Wibowo and Park 2021b). In general, a decrease in contact angle (\(\theta\)), which is a sign of the affinity for a solid in interfacial adhesion, leads to improved wettability (Freeman 1959; Collett 1972; Kamke and Lee 2007). Moreover, it is possible to measure not only the contact angle but also other variables to evaluate the wettability such as surface tension, surface energy, surface adhesion, and work of adhesion. For example, it is demonstrated that the contact angle of polymeric MDI resins is lower than that of MUF resins because of distinct polar components, regardless of wood species (Rathke and Sinn 2013).

In general, the contact angle, a measure of wettability, is established by the surface tension induced by the molecules attraction between two different materials and expressed by Young’s equation (Eq. 1) (Patton 1970). The contact angle is formed at an equilibrium of the surface energy (\(\gamma_{L}\)) of the liquid, the interfacial energy (\(\gamma_{SL} )\) between the solid and the liquid, and the surface energy (\(\gamma_{S}\)) of the solid substrate (Wang et al. 2015; Shin et al. 2018).

$$\gamma_{S} - \gamma_{SL} = \gamma_{L} \cos \theta$$
(1)

The contact angle of a droplet depends on the surface properties such as roughness, cavities, permeability, or chemical properties. If the surface has a good wetting, the spreading is improved and leads to enhanced interfacial adhesion, which reflects adhesion and adhesive penetration into wood tissues. When the adhesive properly spreads, the physically rough and porous structures of heterogeneous wood surface enhance the interfacial contact area with the resin, resulting in an improved entanglement which actually facilitates mechanical bonding (Amorim et al. 2013; Rezaee Niaraki and Krause 2020; Alade et al. 2022). Of course, the contact angle between wood and liquid depends on the characteristics of a surface such as roughness and porosity. A study demonstrated that rougher earlywood surfaces of spruce and pine wood had lower apparent contact angle (Scheikl and Dunky 1998). Similarly, an increase in the surface roughness by sanding resulted in higher surface energies up to a maximum, followed by a decrease (Sinn et al. 2004). Therefore, the physico-chemical characteristics of the resin interact with the properties of the wood substrate in relation to the bonding process (Hse 1972a).

In addition, numerous studies have shown that hydrophobic characteristics and content of extractives had a detrimental impact on the wettability of resins to wood (Chen 1970; Hse and Kuo 1988; Nussbaum and Sterley 2002; Roffael 2016). As a result of the accumulation of extractives, heartwood has a substantially higher content of extractives and increases its density greater than sapwood (Wilkes 1984; Singleton et al. 2003; Cherelli et al. 2018). The extractives influence the physico-chemical properties of the wood substrate and the wettability of heartwood (Bergström et al. 1999; Zhang et al. 2004; Matsushita et al. 2012; Metsä-Kortelainen and Viitanen 2012; Belt et al. 2017; Celedon and Bohlmann 2018).

By contrast, some parameters of UF resins such as formaldehyde/urea (F/U) molar ratio, viscosity, molecular weight (MW), and reactivity have a significant impact on the chemical and mechanical properties of UF resins (Pizzi et al. 2003; Park et al. 2006; Steinhof et al. 2014; Liang et al. 2018; Wang et al. 2018; Jeong and Park 2019a, b; Lubis and Park 2020; Wibowo et al. 2020b; Wibowo and Park 2021a). The MW, chemical structure, and morphology of low molar ratio UF resins significantly differ from those of high molar ratio UF resins (Nuryawan et al. 2017; Wibowo et al. 2020b; Wibowo and Park 2022). Using a light microscope, they reported that higher molar ratio, MW, and viscosity resulted in an increased bondline thickness and decreased resin penetration into wood (Lubis and Park 2020; Wibowo et al. 2020a). Moreover, 1.6 UF resins contain longer branched molecular chains and higher MW than those of 1.0 molar ratio UF resins (Wibowo and Park, 2021a). Because the 1.6 UF resins have a branched structure with high molecular weight, they possess greater reactivity (shorter gelation time), stronger interaction with cellulose, higher maximum surface adhesion, and work of adhesion values than those of 1.0 UF resins, indicating a relationship between the MW and wettability, or adhesion of UF resins (Wibowo and Park, 2021b).

Although wood is highly porous, resins should remain in the bondline because excessive penetration results in a starved glue line. Higher molar mass molecules of UF resins prevent a uniform distribution over a wood surface and remain on the wood surface, resulting in a delayed resin penetration (Scheikl and Dunky 1998). However, small molecules in the resin facilitate the resin penetration and cause an over-penetration, resulting in starved bondline (Gavrilovic-Grmusa et al. 2012; Jeong and Park 2019a). The viscosity also plays an important role in the bondline thickness and bond strength of plywood (Gavrilović-Grmuša et al. 2010a; b; Gavrilovic-Grmusa et al. 2012; Hong and Park 2017).

However, there is only limited literature seeking a correlation of the MW of UF resins with their wettability, surface adhesion, interphase adhesion, and adhesion strength, even though substantial research has been done on the chemistry and structure of UF resins (Pizzi et al. 2003; Park et al. 2006; Jeong and Park 2019a, b; Lubis and Park 2020; Wibowo et al. 2020b; Wibowo and Park 2021a). Therefore, the purpose of this study is to assess the influence of the MW of UF resins at different molar ratios on their wettability, surface adhesion, interphase adhesion and adhesion strength of plywood. To achieve the purpose, this study adopted a systematic approach as shown in Fig. 1. In other words, six UF resins at two molar ratios and three MW levels were synthesized and characterized for their MW, surface tension, and contact angle on the wood surface. The wood surface properties such as surface roughness and morphology before and after the removal of extractives were measured. For the first time, the maximum surface adhesion force of UF resins on earlywood and latewood either from sapwood or heartwood before and after the extraction was also determined. Moreover, their adhesion in interphase and plywood was measured to relate to their wettability and surface adhesion force.

Fig. 1
figure 1

Schematic diagram of the experimental work in this study

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Materials and methods

Materials

Pure urea (99 wt%) and formalin (37 wt%) were purchased from Daejung Chemicals and Metals, Co., Ltd., Siheung-si, South Korea. Aqueous solutions of formic acid (20 wt%), NaOH (93 wt%), and NH4Cl (99%) were purchased from Duksan Pure Chemicals Co., Ltd., Seoul, South Korea. HPLC-grade dimethylformamide (DMF, 99%) and dimethyl sulfoxide (DMSO, 99.9%) that were used as a mobile phase for gel permeation chromatography (GPC) were obtained from Fisher Scientific, Hampton, NH, USA, and Duksan Pure Chemicals Co., Ltd., Seoul, South Korea, respectively. Aqueous solutions of formic acid (20 wt%) and NaOH (20 wt%) were used to adjust the pH of UF resins during synthesis.

As shown in Fig. 2, earlywood and latewood specimens either from sapwood or heartwood from commercially available wood films (43 × 14 × 0.15 mm thick) of radiata pine (Pinus radiata D. Don) were used for the measurement of contact angle, surface roughness, maximum surface adhesion force, and interphase adhesion. Wood veneers (2 mm thick) from the same wood species were also used for plywood preparation.

Fig. 2
figure 2

Schematic diagram of a a UF resin droplet on a wood film surface for maximum surface adhesion force measurement at the receding stage with the contact angle (θ), droplet base radius (r), and principal radii (D and R) of the droplet, and b image of earlywood and latewood from sapwood or heartwood for the maximum surface adhesion force

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Methods

Synthesis of UF resins

A three-step (alkaline–acid–alkaline) procedure was used to synthesize UF resins as previously reported by Wibowo et al. (2020a, b). Formaldehyde solution was added into a four-necked glass reactor, adjusted to pH 8.0 using sodium hydroxide (NaOH, 20 wt%), and then heated to 40 °C. Subsequently, urea was added into the reactor to give an initial formaldehyde/urea (F/U) molar ratio of 2.0. The blend was then heated to 90 °C, and the pH was maintained at 8.0 for 1 h to allow the addition reaction to proceed. Subsequently, the temperature and pH were adjusted to 80 °C and 4.6, respectively, to accelerate the condensation reaction. The molecular weight (MW) of UF resins with 1.0 and 1.6 F/U molar ratio (denoted as 1.0 UF resins and 1.6 UF resins, respectively) was controlled by adjusting the target viscosity of UF resins during the acid condensation reaction, using a bubble viscometer (VG 900, Gardener-Holdt, Columbia, SC, USA). Three target viscosity scales of “D–E,” “J–K,” and “S–T” were selected to obtain low MW (L-MW), medium MW (M-MW), and high MW (H-MW) level of UF resins at two molar ratios. Thereafter, the reaction was terminated at the target viscosity level, and the pH was adjusted back to alkaline (8.0–8.2). In addition, a calculated amount of the 2nd urea was added to obtain the molar ratio of 1.0 or 1.6, and the temperature was maintained at 60 °C for 20 min. Finally, the UF resins were cooled to 25 \(^\circ\)C, and the pH was adjusted to ~ 8.2. The UF resins prepared were stored at room temperature prior to analysis.

Properties of UF resins

The viscosity of the UF resins (five resins for each of the two molar ratios) was measured on a cone–plate viscometer (DV-II + , Brookfield, Middleboro, MA, USA) using spindle no. 82 (ISO 2555, 1989) at 25 °C and 60 rpm. The non-volatile resin solids content of each resin was determined by measuring the mass of resin before and after drying in an oven for 3 h at 105 °C. For the gelation time measurement, 3% NH4Cl (added as 20 wt% solution) based on the resin solids content was added to UF resins. The gelation time of UF resins prepared was measured with a gel time meter (22A Gel Time Meter, Sunshine Instruments, Vernon Hills, IL, USA) at 100 °C boiling water.

MW measurement of UF resins

A gel permeation chromatography (GPC) system (YL9100, Young Lin Instrument Co., Ltd., Hwaseong, Korea) equipped with a refractive index detector was used to measure the apparent MWs of UF resins with different molar ratios and viscosities according to the procedures reported by Jeong and Park (2019a, b) and Wibowo et al. (2020b). Prior to the measurements, 1% UF resin solution was prepared by dissolving each sample in a mixture of 10% DMSO and 90% DMF and kept in an oven at 50 °C for 3 h. Afterward, the solution was introduced into the injection loop of the GPC system after filtration through a 0.45-μm filter. The columns used were KD-801, KD-802, and KD-806 M (Shodex, Showa Denko K.K., Tokyo, Japan). Poly (ethylene glycol) with a MW between 106 and 25,200 Da (Polymer Standards Service GmbH, Mainz, Germany) was used as calibration standards. Weight-averaged molecular weight (MW), number molecular weight (Mn), and polydispersity index (PDI) were calculated using the software YL-Clarity Version 6.1.0.130 (Young Lin Instrument Co., Ltd., Hwaseong, South Korea).

Thermal curing behavior of UF resins

A differential scanning calorimeter (DSC) (DSC 25, TA Instruments, New Castle, DE, USA) was used to analyze the curing behavior of UF resins. 3% NH4Cl based on the non-volatile solid content was mixed to prepare a 5–6 mg sample of UF resins. Then, it was sealed in a high-pressure capsule pan to suppress the evaporation of water during DSC scanning. Each sample was scanned at a heating rate of 2.5, 5, 10, and 20 °C/min in the temperature range of 30 °C–200 °C under a flow of nitrogen gas at 50 mL/min. After scanning, all thermograms of the UF resins were analyzed using a commercial software (TRIOS software, v4.3.0, TA Instruments, New Castle, DE, USA) to obtain the peak temperature at the exothermic curve.

Surface tension measurement using Wilhelmy plate

The surface tension (σ, in milli-newton per square centimeters) of UF resins is also measured because the surface tension is one of the wettability factors of adhesives at the interface (Wålinder, 2000; Wålinder and Ström, 2001; Khan et al. 2004; Lee et al. 2007). According to the procedure in DIN 53914 (1997), the surface tension of UF resins at different molar ratios and MWs was measured using a Wilhelmy plate (10 × 19.9 × 9.2 mm, PT 11, Data Physics Instruments GmbH, Filderstadt, Germany) and a weighing system attached to a tensiometer (DCAT 25, Data Physics Instruments GmbH, Filderstadt, Germany; Nayar et al. 2014). A vessel was filled with UF resins up to a height of at least 4 mm, which served as the sample stage. A measuring Wilhelmy plate (8 mg) was immersed in the UF resins to a depth of 3–30 mm at a rate of 1 mm/s and wetted by the UF resins along the perimeter of the plate. The surface tension of UF resins can be calculated using the contact angle between plate and resin. As tension force (Ftens) acts tangentially to the liquid surface at the three-phase contact line, and it can be divided into two parts (Rapp 2017): the parallel force (\(F_{\parallel } )\) to the undisturbed liquid surface and the perpendicular force (\(F_{ \bot }\)) to it are expressed by Eqs. 2 and 3:

$$F_{\parallel } = F_{{{\text{tens}}}} \times \sin \theta_{C}$$
(2)
$$F_{ \bot } = F_{{{\text{tens}}}} \times \cos \theta_{{\text{C}}}$$
(3)

where sin θc and cos θc are the parallel and perpendicular contact angle to the surface. At this point, the z-direction balance only measures the perpendicular part of the tension force, which coincides with the gravitational force (FG) of the formed surface. The balance of the tensiometer measures the weight of the attached plate at the beginning of the measurement to build the Wilhelmy equation based on this theory. If the plate surface is being contacted with liquid as it moves into the liquid, the surface tension of a liquid is automatically calculated by the Wilhelmy equation as shown:

$$\sigma = \frac{{F_{{{\text{tens}}}} }}{L} = \frac{{F_{ \bot } }}{{L \cdot \cos \theta_{C} }} = \frac{{F_{G} }}{{L \cdot \cos \theta_{C} }}$$
(4)

Based on Eq. (4), the surface tension (σ) as tensile force per length of the contact line can be obtained when the Wilhelmy plate is completely wet (contact angle = 0°).

Contact angle measurement of UF resins

The contact angles of UF resins on different types of wood surfaces (earlywood and latewood either from sapwood or heartwood) were measured using a camera system of the tensiometer according to the procedure reported by Wibowo and Park (2021a). In brief, sessile drop contact angle measurements were taken to determine the angle formed when a droplet of UF resins was deposited on the surface of different wood types. The contact angle image was taken using the camera system in 30 s after a droplet of UF resins was added on the wood surface. Afterward, the horizontal baseline on which the droplet of UF resins was being deposited was determined by drawing the droplet edges. Finally, the software automatically calculated the gradient of the tangent of the droplet edge at the point where it met the baseline and contact angle between the baseline and a gradient.

Maximum surface adhesive force measurement of UF resins on wood surface

As shown in Fig. 2, the maximum adhesion force of a droplet of UF resins with different molar ratios and MWs on different wood types was determined using the same tensiometer according to the procedure reported by Wibowo and Park (2021a). A droplet of UF resins suspended on a Du No\(\ddot{u}\)y ring (RG 2, Data Physics Instruments GmbH, Filderstadt, Germany) was advanced to the respective wood surface, and then the liquid droplet was receded by pulling off the surface. A force–distance curve was automatically recorded during this procedure. The surface of a wood sample was moved upward toward the UF resin droplet at an advancing speed of 0.5 mm/s until the wood sample surface came in contact with the surface of the droplet. After reaching the compression distance (0.1 mm), the wood film sample was receded from the droplet at the same advancing speed until the droplet was detached from the surface of the wood film sample. Preliminary investigations determined the speed based on the maximum adhesive force with the smallest standard deviation.

The interactive force between the droplet and solid surface is called the adhesion force (F). F consists of two components. The first component is the surface tension force (FS), which is the vertical component of the liquid–gas interfacial tension along the perimeter of the triple-phase contact line, and the second component for a stretched droplet is the capillary pressure force (FL), which is created by a curvature of the liquid due to pressure difference across the liquid–gas interface. These forces can be expressed by Eq. (5) (Sun et al. 2017; Wang et al. 2020; Wibowo and Park, 2021b):

$$F = F_{S} - F_{L} = 2\pi r\gamma \sin \theta - \pi r^{2} \Delta P$$
(5)

where r is the base radius of a droplet that is in contact with the substrate, \(\gamma\) is the liquid–gas interfacial tension, \(\theta\) is the contact angle, and \(\Delta P\) is the pressure difference between the interior and exterior of a droplet interface known as Laplace pressure in Eq. 6:

$$\Delta P = {\upgamma }\left( {\frac{1}{D} - \frac{1}{R}} \right)$$
(6)

where D and R are the principal radii of the droplet. This equation is based on the equilibrium and is thus only used for quasistatic conditions. Depending on whether the shape of a droplet is convex or concave, \(\Delta P\) might be positive or negative. The contact angle at the maximum adhesion force \((F_{{{\text{max}}}} )\) is the most stable contact angle (\(\theta_{{{\text{max}}}}\)). As all of the stretched droplets have a concave shape, \(\Delta P\) was calculated by Eq. (6). Detailed information related to this equation can be found in the literature (Butt et al. 2014; Chen et al. 2015, 2016). \(F_{{{\text{max}}}}\) was recorded at the \(\theta_{{{\text{max}}}}\) identified from a typical recorded image and represented by the peak of a force–distance plot. Subsequently, the \(F_{{{\text{max}}}}\) value was calculated by Eq. (5). This experiment was repeated at least three times for each sample.

Surface roughness measurement of wood films

The surface roughness of the wood film surface (earlywood and latewood in sapwood and heartwood of radiata pine) was measured using a contact stylus surface roughness tester (HUATC SRT-6210, International Standard 4287) with a 2-μm diamond stylus, employing a cutoff length of 2.5 mm, a measuring length of 2 mm, and a speed of 1 mm/s at three replications according to the standard method (ISO 4288 1997). The surface roughness parameters were evaluated as the arithmetic mean of the sum of a roughness profile value (Ra) and the root-mean-squared value (Rq) according to Eqs. (7) and (8), respectively (ISO 4288 1997).

$$R_{a} = \frac{1}{N}\mathop \sum \limits_{i = 1}^{N} \left| {Y_{i} } \right|$$
(7)
$$R_{q} = \frac{1}{N}\mathop \sum \limits_{i = 1}^{N} \left| {Y_{i} } \right|.$$
(8)

Extractive removal of wood samples

The extractives of the wood film samples were removed by ethanol–benzene extraction according to TAPPI method (T204 cm-97, 1997). A flask containing 150 mL of ethanol–benzene mixture (1:2 v/v) was coupled to a Soxhlet extraction tube containing ~ 0.04 g of wood films. The extraction was done for at least six cycles per hour. After at least 24 cycles of extraction, the resulting extractive-free samples were used for wettability test.

Interphase adhesion of UF resins/wood film composites

As shown in Fig. 3, two thin wood films (radiata pine) were bonded with UF resins at different molar ratios and MWs to prepare UF resin/wood film composites at a glue spread of 125 g/m2, which was an optimum level determined in a previous work (Jeong and Park 2019a). The prepared composite sample (100 mm × 8 mm × 0.15 mm) was used to measure maximum storage modulus (E′max) as an interphase adhesion using dynamic mechanical analysis (DMA, Q800, TA Instruments, New Castle, DE, USA) in dual cantilever mode. DMA measures modulus and damping as a function of temperature and frequency. All samples were pre-cured in an oven at 50 °C for 20 min prior to DMA analysis. The storage modulus \((E^{\prime})\) of each sample was determined at a frequency of 1 Hz, strain level of 0.005%, and heating rate of 5 °C \(/{\text{min}}\) in the scanning range of 30 – 300 °C (Jeong and Park 2019b).

Fig. 3
figure 3

Schematic diagram of UF resin/wood film composite for interphase adhesion measurement

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Measurement of tensile shear strength of plywood

Three-ply plywood was prepared using wood veneers from radiata pine (Pinus radiata D. Don). UF resins mixed with 3% NH4Cl on the basis of non-volatile resin solid content, and approximately 10% wheat flour based on the total mass of the UF resins were applied to the veneer surface at a glue spreading rate of 170 g/m2 using a rubber roller. After the pre-assembled plywood was cold-pressed under a pressure of 1.4 MPa for 20 min, it was hot pressed at 120 °C for 4 min at the same pressure. Tensile shear strength (TSS) of a plywood was determined according to a standard process (KS F 3101 2020). As shown in Fig. 4, nine specimens (25 × 80 × 6 mm) were used to determine TSS at a crosshead speed of 2 mm/min using the peak load of a universal testing machine (H50KS, Hounsfield, Redhill, England). Statistical analysis was conducted using two-way ANOVA and Duncan’s multiple range test.

Fig. 4
figure 4

Schematic diagram and dimensions of TSS specimen of plywood (unit: mm)

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Results and discussion

Table 1 shows the properties of 1.0 and 1.6 UF resins synthesized to have different MW levels. The MW levels of 1.0 and 1.6 UF resins were controlled by adjusting a target viscosity during the resin synthesis and expressed as low MW (L-MW), medium-MW (M-MW), and high-MW (H-MW) levels. MWs of 1.6 UF resins are greater than those of 1.0 UF resins even though the same target viscosity is used. As expected, the viscosity and MW of 1.6 UF resins are greater than those of 1.0 UF resins. This is due to the higher reactivity of 1.6 UF resins than 1.0 UF resins. The non-volatile solid contents of 1.0 UF resins are higher than that of 1.6 UF resins because of a higher amount of the second urea than that of 1.0 UF resins (Pizzi 1983; Park 2007; Wang et al. 2018). Moreover, the most noticeable difference is that the gelation time of 1.6 UF resins is much shorter than those of 1.0 UF resins as reported in previous studies (Park et al. 2006; Hong and Park 2017). This is probably because 1.6 UF resins are highly branched structures with many methylene ether linkages. Thus, 1.6 UF resins became a gel in a much shorter time by forming a network structure than 1.0 UF resins that contained de-branched hydroxymethyl groups and linear structures (Wibowo et al. 2020b; Wibowo and Park 2021b).

Table 1 Properties of UF resins synthesized at two F/U molar ratios and three MW levels
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Figure 5 shows the number average molecular weight (\(M_{{\text{n}}}\)), weight average molecular weight (\(M_{{\text{w}}}\)), and polydispersity index (PDI) of UF resins at different MW levels and molar ratios. Both 1.0 and 1.6 UF resins exhibited a similar trend in their viscosity and MW. As the viscosity increases, the MW also increases. All \(M_{{\text{n}}}\), \(M_{{\text{w}}}\), and PDI values of 1.6 UF resins in all viscosity scales are significantly larger than those of the 1.0 UF resins due to the branched polymer in the condensation reaction as reported in previous studies (Wibowo et al. 2020b; Wibowo and Park 2021b). 1.0 UF resins resulted in lower MW than those of 1.6 resins because the amount of the second urea decreased the degree of branching of the resins (Wang et al. 2018; Lubis and Park 2020). In addition, urea could be activated by hydroxyl ions that can react with positively charged carbons of the oxymethylene and methylene ether linkages in branched polymers so that linear polymers with a lower MW could take up a large portion of the UF resins (Wibowo and Park 2021b). The Mn and PDI of 1.0 UF resins showed a linear relationship, but this was not the case for 1.0 UF resins. An increase in the PDI values indicates that the resins have more branched molecules (Lubis and Park 2021). However, the rapidly increased PDI values and slightly decreased Mn of 1.6 UF resins with the highest molecular weight suggested that 1.6 UF resins had heterogeneous structures and highly branched molecules (Jiang et al. 2021).

Fig. 5
figure 5

Mn, Mw, and PDI of 1.0 and 1.6 UF resins at different molar ratios and MW levels; a 1.0 UF resins and b 1.6 UF resins

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Figure 6 shows changes of the peak temperature (Tp) at which the highest conversion rate occurs during the curing reaction to compare the curing behavior of UF resins with different molar ratios and MWs. All DSC thermograms of UF resins are presented in Fig. S2 in Supplementary Material. The Tp of UF resins corresponds to a maximum polycondensation reaction of the polymeric moieties between the primary amino groups and hydroxymethyl groups of the resin (Siimer et al. 2003). As expected, the Tp also increased as the heating rate increased from 2.5 °C/min to 20 °C/min, regardless of the MW level and molar ratio of UF resins. In addition, Fig. 6 shows that Tp of 1.0 UF resins is higher than that of the 1.6 UF resins, indicating that more heat is released from the maximum polycondensation reaction of the primary amino groups of unreacted urea with the hydroxymethyl groups (mono-,di-, or tri-hydroxymethyl) in 1.0 UF resins. Therefore, the resin reactivity of resin curing increased as the F/U molar ratio increased. In addition, the Tp tends to decrease as the MW increases, which is consistent with other results (Siimer et al. 2003; Park et al. 2006; Lubis and Park 2018).

Fig. 6
figure 6

Change of Tp of UF resins with a 1.0 and b 1.6 F/U molar ratios at different MW levels and heating rates

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Figure 7 shows the surface tension obtained from the Wilhelmy plate, depending on the molar ratio and MWs. The surface tension of 1.0 UF resins is greater than those of 1.6 UF resins even though the MWs of 1.0 UF resins are smaller than those of 1.6 UF resins. The surface tension of 1.0 UF resins is statistically different at the MW levels, while it is not the case for 1.6 UF resins (see Table S1 in Supplementary Material). This could be due to a difference in the resin solids content. In other words, a higher resin solids content of 1.0 UF resins has greater surface tension than 1.6 UF resins that have a lower resin solids content. These results show that the resin solids content of UF resins clearly influences their surface tension. The surface tension values obtained in this study are comparable to the published value of 63.0 dyn/cm for pure UF resins (Lee et al. 2007).

Fig. 7
figure 7

Surface tension of 1.0 and 1.6 UF resins with different MW levels using Wilhelmy plate method. *The statistical data related to this figure are presented in the supplementary information (Table S1)

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Figure 8 shows the surface roughness (Ra and Rq) of the earlywood and latewood either from sapwood or heartwood of the wood film. Rq values are calculated as the square of the amplitude and are more sensitive to the peaks and valleys of a surface. Thus, it always maintains greater values than Ra. Earlywood has higher Ra and Rq values than latewood, regardless of sapwood or heartwood, which is comparable with reported results, regardless of surface treatment (Follrich et al. 2010). However, there is no difference in the surface roughness between sapwood and heartwood.

Fig. 8
figure 8

Ra and Rq values of different wood types: a earlywood and latewood from sapwood, b earlywood and latewood from heartwood. The symbol (***) shows that they are significantly different at a p-value of 0.01 by T test

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Earlywood either from sapwood or heartwood shows greater surface roughness than latewood as the earlywood from sapwood or heartwood has larger lumens and a greater number of inter-wall cracks than latewood (see Figs. S3 and S4 in Supplementary Material). In addition, the surface morphology between sapwood and heartwood is not much different because there is no significant difference in the microstructures between sapwood and heartwood. The difference in the mesopore structures of the earlywood and latewood from sapwood could be related to structural features of the cell wall. The density of latewood is 2.3 times higher than that of earlywood from sapwood because of differences in the cell wall thickness. However, the surface roughness of heartwood may not be affected because a similar amount of micropores and mesopores exists in earlywood and latewood, which are highly affected by extractives (Plomion et al. 2001; Dehon et al. 2002; Yin et al. 2015).

The contact angle of 1.0 and 1.6 UF resins with L-, M-, and H-MW levels on the surface of earlywood and latewood either from sapwood or heartwood before the extractive removal is presented in Fig. 9. As expected, the contact angles of 1.0 and 1.6 UF resins on the earlywood from sapwood are relatively lower than those of UF resins on latewood. This could be due to the large lumens exposed in earlywood and an increase in the surface roughness, indicating that wood type affects their wettability (Hse 1972b; Shupe et al. 1998). In addition, the contact angle on heartwood is smaller than those on sapwood because heartwood in dark color has a lower permeability and significantly higher extractive than sapwood. 1.0 UF resins with the highest MW have the largest contact angle on earlywood. However, 1.0 UF resins with M-MW have the largest contact angles on latewood (both from sapwood and heartwood), indicating that UF resins with M-MW have greater wettability on latewood in contrast to the earlywood because the surface roughness of earlywood produces topographic heterogeneity on wood surfaces. In addition, the contact angle could be related to the resin penetration into wood because a rougher surface with porous structures enhances capillary forces allowing for small adhesive molecules to penetrate into the pores in a greater depth (Cheng and Sun 2006).

Fig. 9
figure 9

Contact angles of 1.0 and 1.6 UF resins with different MW levels on different wood types; a and b 1.0 UF resins on the earlywood and latewood from sapwood and heartwood, respectively; c and d 1.6 UF resins on the earlywood and latewood from sapwood and heartwood, respectively

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The maximum surface adhesion force is the adhesion force between a resin droplet and wood surface at the peak of the force–distance curves in four steps (Sun et al. 2017; Wibowo and Park 2021b). A typical force-distance curve of UF resins on the wood surface is presented in Fig. S1 in Supplementary Material. Figure 10 shows the maximum surface adhesion force of 1.0 and 1.6 UF resins with L−, M−, and H-MW on the surface of different wood types. Regardless of wood types, 1.0 UF resins showed greater maximum surface adhesion force than 1.6 UF resins. This could be partially responsible for the high resin solids content of 1.0 UF resins. In other words, a greater polymer fraction of 1.0 UF resin per unit volume could provide more adhesion to the wood surface than 1.6 UF resins did because 1.6 UF resins had lower resin solids content. Moreover, the polymeric species of high MW could have resulted in strong cohesion as the polymer chain becomes longer due to an increase in molecular entanglements (Myers 1999; Wibowo and Park 2021a).

Fig. 10
figure 10

Maximum surface adhesion forces of 1.0 and 1.6 UF resins with different MW levels on different wood types; a and b 1.0 UF resins on the earlywood and latewood from sapwood and heartwood, respectively; c and d 1.6 UF resins on the earlywood and latewood from sapwood and heartwood, respectively

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In addition, 1.0 UF resins showed the strongest adhesion force on the latewood from sapwood, while the surface adhesion force of earlywood from sapwood was smaller than those of latewood surface. This could be due to a greater surface roughness of the earlywood than that of latewood. The maximum surface adhesion force of 1.0 UF resins increased for the sapwood only as the MW level of UF resins increased (Fig. 10a). In general, high surface roughness reduces the effective contact area of the surface and maximum pull-off traction, leading to a low adhesion at the surface (Henry et al. 2012; Joe et al. 2020; Wibowo and Park 2021a). However, it is contrasting to the reported results that a greater amount of extractives comprising small, non-wettable organic molecules in the latewood of Southern pine explains the higher contact angle of the UF resins on latewood than on earlywood (Šernek et al. 2004). However, 1.6 UF resins did not show a clear trend in the surface adhesion force on earlywood and latewood from sapwood or heartwood.

The extractives in radiata pine are more abundant in heartwood than sapwood, and their quantity and composition are dependent on the percentage of inherent heartwood and the age of the wood (Hillis 1972). Moreover, resin acids made up a considerable amount of the total extractives in the heartwood of radiata pine, accounting for 70–80% of the total extractives, while sapwood resin acids made up 40% (Hillis 1962). More specifically, mature and juvenile (sapwood and heartwood) 19–28-year-old radiata pine have 0.2% and 0.6% of extractives, respectively (Hillis 1962). Latewood from heartwood contains more extractives than earlywood because of the high amount of resin acids due to concentration caused by the formation of heartwood for a long time (Lloyd, 1978). In addition, the toxic compounds of extractives increased the wood density. Thus, the fine texture of denser wood reduced wettability (Santoni and Pizzo 2011). Moreover, previous studies have shown that sapwood is more wettable than heartwood of Douglas fir and Southern pine, which both contain non-wettable extractives (Shi and Gardner 2001). Wood species with high extractive content, such as sessile oak and spruce, have higher contact angles than those with low extractive content (Timell 1986). Therefore, it is possible to compare the effect of the presence/absence of extractives on the surface properties of wood and its wettability.

To understand the effect of extractives on wettability, extractives of wood were removed by extraction with an ethanol–benzene mixture. Figure 11 shows the surface roughness of the earlywood and latewood from heartwood or sapwood before and after the extractive removal. The surface roughness of the earlywood from sapwood or heartwood increased after the extraction, whereas that of latewood decreased slightly after the extraction. As presented in Figs. S3 and S4 in Supplementary Material, the surface roughness of earlywood qualitatively shows larger cavities after the extraction, which could be responsible for increased surface roughness (Santoso et al. 2019). However, the surface roughness of heartwood is smaller than that of earlywood from sapwood or heartwood, which could be due to an improved homogeneity of the surface after the extractive removal.

Fig. 11
figure 11

Ra and Rq values of wood types before and after the removal of extractives: a and b Ra of earlywood and latewood from sapwood and heartwood, respectively; c and d Rq of earlywood and latewood from sapwood and heartwood, respectively

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In addition, Figs. S3 and S4 in Supplementary Material show SEM images of the earlywood and latewood from sapwood or heartwood before and after the extraction. Even though it is a qualitative comparison, Fig. S4 illustrates that the earlywood images from sapwood or heartwood show more cavities after the extraction than before. However, the SEM images of the earlywood and latewood from heartwood look not much different to before and after the extraction, which results in a slight change in their surface roughness. A more detailed chemical analysis will help to determine how the chemical composition of extractives affects wettability.

To understand the wettability of UF resins on the surface of different wood types after the extraction, the contact angle of UF resins was measured, and the results are presented in Fig. 12. After the extraction, the contact angles on earlywood from sapwood and heartwood decreased, while those of latewood increased for both wood types. Moreover, the contact angles of sapwood were relatively higher than those of heartwood, regardless of earlywood or latewood. This result is very consistent with the surface roughness of wood after the extraction. In other words, the contact angles of earlywood from sapwood or heartwood decreased as the extractive removal increased the surface roughness. This result corresponds to an improved wettability of other woods (Subcordate alnus and Zelkova carpinifolia) after the extractive extraction (Ghofrani et al. 2016). By contrast, the contact angles on latewood slightly increased after the extraction except for the latewood from heartwood with 1.6 UF resins. This could be due to the fact that the latewood has fewer cavities and lower surface roughness than earlywood. These results suggest that the extractive removal in the earlywood film affects the contact angle by changing the surface roughness and chemical composition.

Fig. 12
figure 12

Contact angles of 10 and 1.6 UF resins with M-MW on different wood types: a and b earlywood and latewood from sapwood and heartwood, respectively, for 1.0 UF resins; c and d earlywood and latewood from sapwood and heartwood, respectively, for 1.6 UF resin. *The averages are statistically different at a p value of 0.05

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Figure 13 also shows the maximum surface adhesion force of UF resins with M-MW level on wood surfaces before and after the extraction. The surface adhesion force of earlywood and latewood decreased after the extraction, except for latewood from sapwood and heartwood with 1.0 UF resins. These results could be due to the fact that the extraction increased the surface roughness of earlywood, leading to lower surface adhesion force.

Fig. 13
figure 13

Maximum surface adhesion force of 1.0 and 1.6 UF resins with M-MW level on different wood types before and after the extractive removal: a and b 1.0 UF resins; c and d 1.6 UF resins

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As discussed, it seems that the surface roughness of wood dominantly affects the contact angle and maximum surface adhesion force of UF resins on wood surfaces. Thus, a correlation between the contact angle and surface roughness (Ra values) is made and the result is presented in Fig. 14. As expected, the contact angle revealed a negative relationship with the surface roughness of wood. This result is similar to other results reported (Papp and Csiha 2017). However, the linear relationship between them is weak with a low R-square value (0.334).

Fig. 14
figure 14

Correlation between the contact angle of UF resins and surface roughness of the wood surface

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In order to relate the contact angle and maximum surface adhesion force to the adhesion of UF resin in the interphase, the composites prepared by bonding two wood films of sapwood or heartwood together with UF resins were tested with DMA. Figure 15 shows the maximum storage modulus (E′max), which is regarded as interphase adhesion of the UF resins/wood composites. The interphase adhesion of 1.0 UF resins roughly increased with an increase in the MW level, leading to the highest E′Max values at H-MW level for sapwood and heartwood. As expected, the E′Max of 1.6 UF resins was higher than those of 1.0 UF resins. However, the interphase adhesion of 1.6 UF resins was the highest at M-MW level, not at H-MW level. This could be due to the fact that low MW species deeply penetrated into amorphous wood polymers, whereas high MW species mainly remained in the bondline (Laborie et al. 2006; Nuryawan et al. 2014). In general, it is well known that 1.0 UF resins comprise more linear and low MW molecules than 1.6 UF resins (Wibowo and Park, 2022). However, 1.6 UF resins had higher MW species and provide strong cohesive force but poor penetration (Jeong and Park 2017). However, it seems that the effect of sapwood and heartwood on the interphase adhesion is overwhelmed by the resin properties such as MW, gelation time, and molecular structures. In addition, the interphase adhesion of 1.0 UF resins for the earlywood from sapwood or heartwood follows a similar trend to the surface maximum adhesion force of the same resins as shown in Fig. 10. Moreover, the interphase adhesion of 1.6 UF resins also had a similar trend to the maximum surface adhesion force as presented in Fig. 10.

Fig. 15
figure 15

E′Max of UF resins with L-MW, M-MW, and H-MW on different wood types: a 1.0 UF resins; b 1.6 UF resins

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Figure 16 shows tensile shear strength (TSS) results of plywood bonded with UF resins at different MW levels. According to the statistical analysis using T test, there is no significant difference for TSS values of UF resins with different MWs and F/U molar ratios (see Table S2). Moreover, the higher the MW level, the higher the tensile shear strength value, which is independent of the F/U molar ratios. UF resins with a high F/U molar ratio and a considerable quantity of formaldehyde emission (FE) have strong molecular reactivity and numerous cross-linking structures, which promote remarkable bond strength (Myers 1984; Li and Zhang 2021; Wibowo and Park 2021b). However, unlike the wettability and interface adhesion, the TSS of plywood displays a significant difference between high and low F/U molar ratio resins. This could be due to the fact that UF resins with high F/U molar ratio have higher MW than those of the UF resins with low F/U molar ratio, and they are more viscous, have a thick bondline, and penetrate poorly (Nuryawan et al. 2014). Therefore, as the UF resins with higher molar ratio are not thoroughly wetted on the surface, their contact angle is much higher than that of the UF resins with lower molar ratio.

Fig. 16
figure 16

TSS of plywood bonded with 1.0 and 1.6 UF resins with L-MW, M-MW, and H-MW. *The statistical data related to this figure are presented in the supplementary information (Table S2)

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Conclusion

This study attempted to understand the influence of MW on the wettability, surface adhesion, interphase adhesion, and adhesion strength in wood bonding using UF resins at two molar ratios, different types of wood (earlywood, latewood, sapwood, and heartwood) before and after the extractive removal. As the molar ratio increases, the viscosity and MW of UF resins increase. As the MW increases, the surface tension of 1.0 UF resins slightly increases, whereas that of 1.6 UF resins decreases even though the surface tension of the former is greater than that of the latter. Regardless of the MW of UF resins, the contact angle and maximum surface adhesion force are smaller on the earlywood surface with higher roughness values than those on the latewood surface. The contact angle shows a weak negative correlation with the surface roughness of wood, leading to a decrease in the maximum surface adhesion force. The wettability and surface adhesion force of UF resins with different MWs on wood surface did not proportionately improve their adhesion in interphase and plywood. These results suggest that the MW, wettability, and surface adhesion are indirectly related to interphase adhesion and cohesion in wood bonding, and the surface roughness is a dominant factor for the wettability and surface adhesion force.