3.1. Effect of Substrate Surface Treatment Method on Bonding Strength
The birch substrate was treated with three kinds of surface treatment methods, namely, sandpaper grinding, chemical oxidation and silane coupling agent treatment. The main agent and the crosslinking agent were prepared by adjusting the adhesive of 100:18 quality and pressing the birch wood. (50 °C, RH98%) for seven cycles; each cycle was 24 h, the shear strength of the glued wood was measured for each cycle, and the change of the shear strength of the bonded joint is shown in
Figure 1. Error bars represent the standard deviation of the mean of at least three different experiments.
As can be seen from
Figure 1, under different surface treatment methods, the shear strength variation trend of the adhesive joint is different. The surface of the substrate after treatment and the glued wood shear strength reduction rate is relatively slow. The tensile strength of the adhesive joint of the sandpaper has a linear trend in the initial period. Although the sandpaper grinding treatment improves the contact area, the adsorption energy and desorption energy of the moisture between the adhesive and the binder are not changed. Due to the weak adhesion of the adhesive and wood, moisture is easily diffused from the adhesive interface into the interior of the adhesive, so the shear strength decreases rapidly. When the hygrothermal aging time reaches 144 h, the shear strength is mainly the mechanical bond between the adhesive and the wood, so the shear strength decreases slowly. The trend of compressive shear strength of the sandpaper grinding treatment is similar to that of the untreated sample. The trend of chemical oxidation and sandpaper grinding treatment is basically the same, but the degree has slowed. This is due to a small number of polar groups formed on the surface of the adhesive materials treated with chemical oxidation and formed microcracks, which improved the adhesion of the adhesive and the substrate. In contrast, the shear stress intensity curve of the silane coupling agent treatment is different from that of the former two. The compressive shear strength is kept constant when the wet compressive aging is carried out for 24 h. During this period, due to the presence of the coupling agent, the diffusion of moisture in the bonding interface is suppressed, and these small amounts of water penetrate into the adhesive, and only the plasticizing effect is made to keep the compressive shear strength constant. When the hygrothermal aging time is extended to 48 h, the water gradually spread to the inside of the adhesive, and the shear strength of the bonding joints began to decline. After 144 h of hot and humid aging treatment, the bonding strength of the bonded joint becomes very low, and the bonding strength is mainly the mechanical lock structure between the adhesive and the wood, so the shear strength decreases slowly. It has been shown that the surface treatment has a great influence on the change of the shear strength of glued wood under the condition of hot and humid aging, and the silane coupling agent treatment can effectively reduce the decrease in the compressive shear strength of the API adhesive.
3.2. XPS Analysis of Substrate
The composition of the wood is mainly C, H, and O. In this experiment, the birch substrate was subjected to different surface treatments. Because the depth of treatment was very shallow, the wide scan of the XPS can mark the inner electron bonding energy of all the elements in the wood (except H and He), so that the specific binding energy of each element can be used to identify the wood surface element composition and relative content. The XPS wide scan pattern of the surface of the different surface treatment methods is shown in
Figure 2. The composition of the wood surface elements is shown in
Table 1.
From
Table 1, the content of C element in the wood after the surface treatment was decreased, the O element content was increased, and the ratio of the atomic concentration of O/C was increased. In addition, the sandpaper treated birch substrate only slightly increased the bonding area, and the element content saw a slight change. The oxygen content of the substrate surface treated by chemical oxidation increased from 22.03% to 23.35%, indicating the surface oxidation reaction of the wood. Meanwhile, the oxygen content of the birch substrate treated by the silane coupling agent was increased from 22.03% to 25.51%, which indicated that the silane coupling agent and the polar groups on the wood surface underwent an oxidation reaction followed by the introduction of oxygen-containing groups.
The change of chemical structure in the surface treatment can be analyzed by using the change of carbon atom binding energy and intensity to determine the existence and relative content of carbon atoms in the wood. In order to characterize the change of the main groups on the surface of the birch substrate, the distribution of the carbon content in different chemical environments was analyzed. The C1s on the birch substrate were subjected to peak treatment by spectral curve fitting. The spectra of the C1s peak of the surface of birch treated with different treatment methods are shown in
Figure 3, and the position and peak area data of the carbon atoms are shown in
Table 2. Due to the influence of the electronic effect, the measured binding energy is lower than the standard value, and the calibration is basically within the standard value range.
From the experimental results, the birch substrate contains four different carbon atoms, namely C1, C2, C3 and C4, although primarily C1 and C2. The combination of carbon has undergone significant changes after the treatment. The content of C1 and C4 in the surface of the substrate is decreased, and the content of C2 and C3 is increased, which indicates that the structure of carbon-containing functional groups on the wood surface changed. It is possible that after the surface treatment, the carbon atoms contained in the wood surface are connected to the polar hydroxyl or ester groups, where the high oxidation morphology increased the electron binding energy.
The oxygen atoms and carbon atoms in the wood cellulose are mainly single bonds, which are divided into O1 and have higher binding energy. However, the binding energy of oxygen in the form of double bonds with carbon is very low, and it is classified as O2. After the chemical oxidation and silane coupling agent treatment, the content of carbon and oxygen double bonds on the wood surface increased, so we studied the change of the O1s peak in XPS spectra.
Figure 4 shows the position and peak area of the oxygen atoms in the O1s of the birch wood surface treated with different surface treatment methods, and the XPS data is shown in
Table 3.
It can be seen from the experimental results that the peak area of O1 increased and the peak area of O2 decreased, indicating that the hydroxyl groups in the cellulose and hemicellulose were oxidized after the surface treatment, and the silane coupling agent treatment causes the polar groups on the wood surface to undergo a chemical reaction, resulting in an increase in the oxygen-containing functional groups.
3.3. Birch Surface Treatment FT-IR Analysis
The change of the main groups and the improvement of the interface properties after the surface treatment of the birch substrate are of great significance to improve the aging resistance of the glued materials. The FT-IR spectra of untreated and surface-treated birch substrates are shown in
Figure 5. It can be seen from
Figure 5 that the spectra of the four samples have characteristic absorption peaks at 1047.04 cm
−1, 2910.30 cm
−1 and 3426.30 cm
−1, respectively. After chemical oxidation and coupling agent treatment, the wood surface absorption peak disappeared at 2364.51 cm
−1, while the sandpaper polished surface was enhanced here. In addition, the absorption peaks of the surface of the wood treated by the coupling agent at 1748.38 cm
−1 and 1246.65 cm
−1 also disappeared, indicating that the introduction of certain groups and the groups of the wood itself occurred after chemical oxidation and coupling agent treatment.
3.4. Effect of Surface Treatment on the Water Absorbency of Adhesive in Bonding Joint
A bonded joint was prepared using a surface treated birch substrate and the starch-based API adhesive. The adhesive joint was then treated in a hygrothermal aging environment (temperature: 50 °C, relative humidity: 98%) for a total of seven cycles. EDS was used to observe the change of oxygen distribution and content on the surface of the adhesive joint. The EDS spectrum of the bonding joint with different surface treatment methods is shown in
Figure 6, and the oxygen content is shown in
Table 4.
From the experimental results, the surface oxygen distribution and the oxygen content of the bonded joint changed after the hygrothermal aging treatment. The content of oxygen element in the API adhesive of the bonding joint was increased, which varied among different surface treatment methods. With the hygrothermal aging cycle extended, the oxygen content which is treated with the same method is also gradually increased. There is an isochronous relationship between the surface treatment method and the hygrothermal aging cycle.
Surface treatment can improve the surface bonding energy of glued products, which can be slowed by the penetration rate of water. The oxygen content of the adhesive in the adhesive was measured by EDS at 50 °C and RH98% under the condition of hygrothermal aging. The water absorption of the adhesive in the glued wood treated by different surface treatment methods was calculated according to the formula. The relationship between heat aging time is shown in
Figure 7.
As can be seen from
Figure 7, after the hygrothermal aging, the starch-based API adhesive joint water absorption in the first cycle increased very slowly. When the aging treatment reached 48 h, the adhesive joints increased rapidly owing to the water that had penetrated into the bonding joint. The water absorption of the adhesive in the bonding joint of the untreated and polished sandpaper is higher than the other two treatment methods, mainly because the lower bonding interface energy made the water permeate faster. After 120 h, the water absorption rate of the four methods slowed down, and the trends were similar. By this time, the water has been through the bonding interface into the internal of the adhesive. As the adhesive structure is the same, the water permeate velocity from the outer surface of the adhesive to the internal is the same, but from the bonding interface to penetrate in the internal is different. The overall degree of moisture spread into the interior of the adhesive is still different, and this leads to the low bonding energy of the untreated and sandpaper polished adhesive joints, showing the faster trend of the automatic fracture, combined with the higher bonding energy of the chemical oxidation and coupling agent treatment of the adhesive joints, which will delay their breaking.
It is possible to determine the damaged form of the adhesive joint prepared by the starch-based API adhesive by analyzing the content of the surface element after the failure of the bonded joint by EDS. The oxygen content of the bonded joint is shown in
Table 5 and the distribution form is shown in
Figure 8.
It can be seen that the oxygen content of the surface of the adhesive is 28.3% at 50 °C and RH 98%, which is determined by its chemical structure. When the adhesive joint was hygrothermally treated for 48 h, the oxygen content of the treated and sandpaper polished adhesive joint was 27.1% and 26.9%, respectively, which was lower than the oxygen content on the surface of the adhesive. The cohesive strength of the adhesive is still higher than the birch’s cohesive strength and interfacial bonding strength. Therefore, the damage of the bonding joint occurs inside the birch, so the oxygen content is less than that before the hot and humid aging. While the content of oxygen in the fracture of the adhesive joint treated by chemical oxidation and the coupling agent is increases, it is still lower than the oxygen content of the adhesive surface at room temperature. It has been shown that the cohesive strength of the adhesive is the same as the interfacial adhesion strength of the birch substrate with API adhesive, and the failure mode is mixed destruction. With the hygrothermal aging, the fracture oxygen content of untreated and polished sandpaper treatment of the bonding joint is increasing, and it is higher than the oxygen content of the adhesive. During this period, the cohesive strength of the adhesive is lower than that of the birch material and the interfacial bonding strength, so the failure of the bonding joint occurs inside the adhesive. Because of the water absorption of the bonding joint, the oxygen content increased, and it is higher than the surface oxygen content of the adhesive before the aging. After 72 h of the chemical oxidative treatment and 96 h of the coupling agent treatment, the oxygen content of the joint fracture was higher than that of the adhesive at room temperature, and then the failure mode of the adhesive joint was changed. There is also an isochronal relationship between the oxygen content of the fracture surface of the bonded joint and the different surface treatment methods.