Sodium hypochlorite–based binderless technology for solid wood adhesion: application of a commercial bleaching agent for adhesion and observation of surface characteristics

Abstract

In this study, several methods for a novel binderless technology were explored, the effects of treatment conditions on the adhesive performance were examined and the surface was characterized during adhesion. The method entailed a three-step process for bonding solid wood, which involved immersion in sodium hypochlorite–containing treatment solutions, surface scratching, and drying with pressing. Various treatment conditions were investigated, including solution types, immersion durations, scratch treatment materials, and pressing methods. The resulting shear strength was 9.7 MPa, which is well above the Japanese Agricultural Standards value for laminated wood (6 MPa). In this method, the specimens were immersed in a bleach solution containing a surfactant for 16 h, followed by pressurization at 1 MPa for 24 h at 60 ℃ using a pressure-controlled press. Around the bonding area in the specimens, the cells in the immersed area were significantly compressed, and the aldehyde groups of the coniferyl aldehyde in the lignin were denatured or lost. Based on these findings, a new binderless technology was developed for solid wood without a high-temperature, high-pressure pressing step.

Introduction

Adhesives commonly used in wood products are typically derived from petrochemical sources. However, as societies have strived for sustainability in recent years, efforts to reduce the use of fossil resources have intensified. One approach to achieve this goal is binderless adhesion. Self-adhesion techniques have been investigated primarily for molding small elements, including wood particles and fibers. In recent years, these techniques have been extended to larger elements, such as veneers and solid wood.

For plywood, researchers have reported a method that involves wetting the surface of Japanese cedar veneer with water, followed by scratching with a brass brush and adding heat and pressure at 220 ℃ with a maximum of 5 MPa for 20 min [1]. However, the results fell slightly below the Type II requirement of the Japanese Agricultural Standard (JAS) [2]. Furthermore, parallel laminated veneers subjected to heat and pressure at 240–300 ℃ under 4.0–5.5 MPa for 80–360 s exhibited shear strengths of 2.00–5.85 MPa [3]. Notably, these methods require high temperatures (≥ 220 ℃) and pressures (≥ 4–5 MPa) during the pressing process.

Regarding solid wood, a patent [4] has been obtained for a process involving immersion in a 10–50% sodium hydroxide solution for 15–120 min, followed by immersion in allyl bromide at 70 ℃ for 30–180 min, and finally immersion in a water and methanol (1:1 ratio) solution. This process results in wood surface alkylation, providing thermoplasticity. After the treated surfaces are combined, heat and pressure are applied at 120–180 ℃ for 1–30 min, followed by 5 min of cooling. The shear strength of woods bonded without adhesives is equivalent to that of the material. However, this method is complex due to the treatment process, the requirement for alkylation reaction equipment, and the challenges in obtaining and recovering allyl bromide. Similarly, another self-adhesion technique [5, 6] has been reported that involves rapidly rubbing wood surfaces together to melt the surfaces in close contact. This friction welding method has a short processing time (40 s) and an adhesive layer shear strength three times that of the material. Nevertheless, thermoplastic resin friction welding equipment is needed to generate frictional motion, reaching temperatures as high as 400 ℃ during processing.

In recent years, a technique for strengthening wood through delignification and compression was reported [7]. In the published paper and related patents [8], it was shown that different wood pieces can be integrated after they are boiled with NaOH and Na₂SO₃ for 7 h and compressed at approximately 5 MPa and 100 ℃ for one day. To apply this principle to solid wood adhesion, researchers conducted experiments involving sandwiching surface-polished capacitor paper or filter paper between wood pieces. After immersion in a sodium hydroxide solution and surface rubbing with a melamine sponge, adhesion occurred in both cases [9]. In contrast to previous approaches, this method does not use high thermal energy for bonding.

In the present study, a novel binderless bonding technique was explored that uses an immersion solution primarily composed of sodium hypochlorite, a common bleaching agent.

The objective of this research was to develop several methods for applying this new binderless bonding technology and assess the effects of various surface characteristics on the binderless bonding performance.

Materials and methods

Bonding methods

The bonding technique included three processes: (1) chemical treatment involving immersion in the solutions; (2) physical treatment to scratch the surface; and (3) pressing and drying. For each treatment condition (n = 2), two-layer specimens were prepared.

The processing conditions are detailed in Table 1. The treatment solution type, immersion duration, physical treatment material used for surface scratching, and pressing conditions were investigated.

Table 1 Adhesion methods for the preparation of binderless specimens

For comparison, specimens were bonded using water-based polymer isocyanate adhesives (PI-bond™, OSHIKA CORPORATION). The main component was 5340S, and the crosslinking agent was H-50. The amount used was 250 g/m². The condition is G in Table 1.

Immersion in treatment solutions

Todomatsu (Abies sachalinensis) specimens (thickness 13 × width 100 × length 60 mm) were used, and the following two bleaches were used for immersion: Hospital Haiter™ (treatment solution I), which contained sodium hypochlorite (6%) and sodium hydroxide (Kao Corporation, Japan), and Kitchen Haiter™ (treatment solution II), which contained sodium hypochlorite, surfactant, and sodium hydroxide (Kao Corporation). The effective chlorine concentrations of these solutions were measured via iodometric titration [10]. The sodium hypochlorite concentrations were calculated to be 4.0% and 4.9% for solutions I and II, respectively. The pH of these solutions was determined using a handheld pH meter (HORIBA Ltd. Japan, LAQUAtwin-pH-33B). The pH values were 13.2 and 13.1 for solutions I and II, respectively.

Each specimen was immersed in solution I or II for 16 or 24 h. The laminae were allowed to float while immersed on one side in the treatment solutions.

Surface scratching

Surface scratching was performed to loosen and fibrillate the wood fibers. The aim was to achieve 3D entanglement and bonded fibers between the treated surfaces. After immersion, the surfaces of the samples were rinsed with tap water. The surface was manually scratched with a nylon hook fastener or a coconut palm fiber scrubbing brush for 30 s each while water was poured over the wood surface. Conditions were also examined without these treatments, in which the surface was simply flushed with tap water. Laminae in Conditions A and B were scratched using an electrically powered nylon hook fastener. An electric sander equipped with a nylon fastener was vibrated and placed on the treated laminae surface for 30 s.

Pressing method

After the scratch treatments, the following pressing methods were tested: screw clamping or automatic pressure control using a hydraulic press. In the manual screw clamping method, the treated surfaces faced each other, and the two treated laminae were sandwiched between stainless steel plates. Bolts were threaded through the threaded holes in the four corners and tightened to 1 MPa pressure with a torque wrench. The specimens were then placed in an air dryer at 60 ℃ for 24 h without retightening. For automatic pressure control, the treated surfaces were faced with each other and pressed together for 24 h at 1 MPa pressure with a hot plate temperature of 60 ℃ using an automatic pressure-controlled press.

Characterization of bonded wood

Block shear test

Block shear specimens (n = 6) were obtained for each condition (Fig. 1). The samples were then subjected to block shear tests [11]. The density was calculated by the weight and dimension of the block shear test samples. The moisture contents of the samples were calculated by the weight of the bonded sample after the block shear test and 24 h of oven drying at 105 ℃.

Fig. 1

Diagram of specimen preparation. L: longitudinal direction; T: tangential direction; R: radial direction

Observation of the treated surface texture

To assess the scratching effect, three-dimensional images of the surfaces after treatment were taken using a digital microscope (High-Speed Microscope VW-9000; KEYENCE Corp.). The length between the minimum and maximum values in the depth direction of the fibers where the focus can be set is shown in Fig. 2. Earlywood and latewood cells were observed in the cross-sections of the samples that were air-dried after immersion. Cross-sections of the glued sections of the bonded specimens were observed.

Fig. 2

Three-dimensional magnified view of wetted surfaces after scratching

Color reaction with phloroglucinol hydrochloride

To evaluate lignin denaturation or loss due to treatment, laminae before or after bonding were subjected to a color test with phloroglucinol hydrochloride. The test solution included 1 g of phloroglucinol, 50 ml of ethanol, and 25 ml of concentrated hydrochloric acid. This solution was uniformly spread onto the surface of the specimens, and observations were made under a microscope. The thickness of the nondyed area was measured using PC software (PC software ver. 1.5 for the VW series, copyright ©2010, KEYENCE Corp.).

Results and discussion

Effects of different solutions on the shear strength

Table 2 shows the density, moisture content of the block shear test specimens, shear strength, and number of specimens. The moisture content of the binderless specimens was 6.70–8.66%, the densities ranged from 384–440, and the sample size was 6. Although each shear strength was different, adhesion was successful under all conditions.

Table 2 Density, moisture content, shear strength and number of block shear test specimens

For Conditions A and B, the average shear strength of solution I was 2.08 MPa; in comparison, the average shear strength of solution II was more than twice as high (5.02 MPa) (Conditions A and B in Fig. 3) Surfactants were included in solution II but not in solution I. Sodium hypochlorite solution exhibited poor wettability, and it was difficult for the solution to penetrate into wood [12]. The surfactant likely enhanced the ability of the treatment solution to penetrate the wood.

Fig. 3

Shear strength test results. Legend: ×: mean; 〇: plots; ―: median

Surface scratching and observations of treated surface texture

The average shear strengths for Conditions D, E, and F were 8.69, 9.69, and 9.22 MPa, respectively (Fig. 3), and they all exhibited strong bonding. The F condition indicates no surface scratching. However, the values were equivalent to those obtained with surface treatment and surpassed the standard value for Todomatsu specified by JAS for glued laminated timber (6.0 MPa) [11].

Figure 4 shows microphotographs of the earlywood and latewood surfaces before pressing. Normal wood was used as control. Solution I for 24 h and Solution II for 16 h or 24 h were used. No significant cell shape changes were observed in the latewood. In the earlywood, the shape of the cells changed compared to that of the control in all conditions. The cells in the control group were square. The cells were rounded after immersion in solution II for 24 h, which may result from the loosening of intercellular bonds because the intercellular layer contains a high percentage of lignin [13]. The cells were distorted after immersion in solution II for 16 h or solution I for 24 h, possibly because the intercellular bonds did not loosen in a uniform manner. These changes occurred because the immersion time was shorter or the solution permeability was lower than that obtained after 24 h in solution II.

Fig. 4

Microphotographs of air-dried material after immersion treatment

The cross-sections of the bonded specimens are shown in Fig. 5. Cells near the boundary between bonded materials were compressed. The cells inside each material maintained their original shape (A–F). In Condition G, the cells near the boundary were not compressed despite of the same pressure. Compared to the other conditions, Condition A led to a smaller compressed area, possibly because Solution I was less permeable. In latewood exposed on the boundary line, cells on the opposing surface in contact with the boundary were greatly compressed, and early wood compressed before late wood. The density around the bonding area appeared to be increased, and the wood strength increases though compression [14]. The compressed cells near the boundary may have contributed to the increased shear strength.

Fig. 5

Micrographs of side views from the cross section near the boundary line

Figure 2 shows 3D images of the treated surfaces after scratching. The fibers on the surface of some samples were separated and randomly arranged by scratching. The number in the upper right corner of the figure is the depth of roughness. The roughness depth was coarser in B and E than in A, C, D, and F. The roughness of B was coarser than that of A because solution II might soften wood more than solution I. In Condition E, the roughness was coarse because the straight fibers of the brush could act more deeply between the fibers. In Condition F, the surface was smoother when scratching was not performed. The shear strength was equivalent between scratched and nonscratched samples. The press method contributes more to the shear strength than to the surface roughness. However, surface scratches may influence adhesive durability and water resistance, which was not examined in this study.

Pressing methods

For Conditions C and D, in which the material was pressed with screw clamps or an automatic pressure-controlled press, the average shear strength values were 5.52 and 8.69 MPa, respectively (Fig. 3). The shear strength of Condition D was greater than that of Condition C. After immersion in solution and water flow, the moisture content before pressing was approximately 50%. During pressing, the samples exhibited a drastic decrease in thickness. The screw clamps were not periodically retightened, the pressure had not maintained. A lower shear strength was applied to the specimen with screw clamps because the pressure could not accommodate the different thicknesses of the specimens. To ensure stable adhesive strength, it was necessary to maintain pressure during the pressing process.

Color reaction with phloroglucinol hydrochloric acid

Phloroglucinol hydrochloric acid reacts with the aldehyde group of coniferyl aldehyde in lignin, producing a reddish-purple color [15]. The immersed areas are uncolored, whereas the nonimmersed areas are colored (Fig. 6). This result indicates that the aldehyde groups of coniferyl aldehyde, the terminal constituent unit in lignin, were present in the nonimmersed areas. Conversely, in the immersed area, these groups were denatured or lost. Next, the thickness of the nondyed area was measured (Table 3). Conditions A and B were electrically scratched on the surface. Therefore, the thickness of the undyed area before bonding was smaller than others. The nondyed thickness of the sample after bonding was approximately half to 70 percent of the thickness of the two samples before bonding. Surface scratching and immersion affected approximately 0.1 mm and 1 mm of the sample thickness, respectively. Compared to the area affected by surface scratching, the layer softened by immersion is thicker.

Fig. 6

Micrographs of the bonded material after dying with phloroglucinol hydrochloride. Left: before bonding; right: after bonding

Table 3 Thickness of the nondyed area after treatment with phloroglucinol hydrochloride

Conclusions

In conclusion, binderless bonding was achieved under the following conditions: (1) immersion in a commercial bleaching solution containing a surfactant for 16 h; (2) in conjunction with the immersion method in (1), the shear strength of the structural adhesive could be achieved regardless of the surface scratching; and (3) the pressing method keeping up with the thickness decreasing was suitable for this method, and the pressing pressure was 1 MPa for 24 h at 60 ℃. In the nonimmersed area, the specimens exhibited no notable changes in cell shape or lignin coloration. However, near the boundary, the cells compressed to approximately one-third of their original thickness, causing the aldehyde groups within the coniferyl aldehyde of the lignin to denature or disappear. Therefore, lignin denaturation and consolidation near the adhesive layer may contribute to the development of binderless bonding.