Multi-scale characterization of bamboo bonding interfaces with phenol-formaldehyde resin of different molecular weight to study the bonding mechanism

Abstract

Understanding the bonding mechanism of the interfacial region between bamboo and adhesives is essential for accelerating the development of improved adhesives for advanced bamboo-based materials. In this study, Br-labelled phenol-formaldehyde (PF) resins with four different molecular weights (MWs) were used to make bamboo–adhesive interfaces for tracing the adhesives in bamboo. Ultra-depth-of-field microscopy and scanning electron microscopy in conjunction with energy dispersion spectrometry were used to access the distribution and penetration of resin in the bamboo polymer. Fourier transform infrared images and solid-state cross-polarization/magic angle spinning nuclear magnetic resonance spectra were used to access the molecular-scale interactions between PF resins and bamboo cell walls. The results showed that the PF resins with high MW infiltrated into the lumina of damaged bamboo cells near the bondline to form glue nails, while those with low MW penetrated into the bamboo cell wall to form nanomechanical interlocking. Chemical bonds and secondary forces such as polar forces and hydrogen bonds were generated between bamboo and PF resin. Finally, the twice-adhesive dispensing method combining low-MW resins with high-MW resins was used to improve the bonding strength of the interface.

1. Introduction

Currently, it is imperative to develop novel structural timber products for building materials with increasing resource demands in today's modern industrialized world [1]. Bamboo, as a highly renewable, fast-growing, sustainable, energy-efficient biomass resource [2], has the highest specific strength compared to timber, concrete and structural steel. It is used to produce engineered bamboo composites such as bamboo scrimber, laminated bamboo lumber and bamboo plywood to meet the demand for sustainable building products [3].

As an integral component of most engineered bamboo products, adhesive and the bamboo–adhesive bondline play an important role in composite material applications. Phenol formaldehyde (PF) resin, as one of the most moisture-durable wood adhesives available [4], is also commonly used in the engineered bamboo industry. The wood/PF resin bonding interface has been characterized by many researchers in various ways such as nuclear magnetic resonance (NMR) [5], infrared nanospectroscopy [6], fluorescence microscopy with nanoindentation [7], etc. Due to the porous structure of wood, PF resins penetrate through the lumens and interconnecting pits of the wood cells to form the main bonding interface while also infiltrating into cell walls to form a nanomechanical interlocking [8]. However, the structure, property and chemical composition of bamboo are quite different from those of wood. On the one hand, the wax in bamboo outer skin and the solid cells in bamboo inner skin are important factors affecting the penetration of adhesives. On the other hand, bamboo is innately lacking in transverse structure like wood rays, which is not conductive to the flow of adhesives in bamboo. Guan et al. characterized the penetration depth of PF resins and poly(vinyl alcohol) modified PF resins in bamboo by using fluorescence microscopy [9]. However, this observation remained at the cellular level. Whether the PF resins can infiltrate into the polymer structure of bamboo cell walls is not clear. Furthermore, although wood/PF resin bonding interface has been studied in various ways for decades [10,11], no consensus has been reached on whether and to what extent covalent bonds could form between PF resins and wood polymers. Huang et al. provided evidence that no new chemical shift was added to ¹³C NMR spectra of wood cured by low-molecular weight (MW) PF resins, indicating that there are no chemical reactions between wood polymer and low-MW PF resins [12]. However, Laborie et al. found that low-MW PF resins can be miscible with wood matrix at the molecular scale [5]. A recent study by Wang et al. showed that PF resins could not only infiltrate into the cell walls but also react with cell-wall polymer [6]. Therefore, it is of importance to understand whether PF resin could interact with bamboo polymer to form chemical bonds.

The permeation of PF resins in wood/bamboo is closely related to their MW [13,14]. Low-MW PF resins will over-penetrate and lead to an undesirable starved bondline while high-MW PF resins only remain in the bondline and it is difficult to form mechanical interlocking. Therefore, a good bonding interface requires some low-MW PF resins to infiltrate into cells to form glue nails and some high-MW PF to remain in the bondline to ensure its thickness and strength. In this study, in order to determine the distribution of resin, bromine-labelled PF (BrPF) resins with four different MWs were used to make bamboo/adhesive interfaces. An ultra-depth-of-field microscope was implemented to observe the permeation of PF resin in bamboo cells. A scanning electron microscope in conjunction with energy dispersion spectrometry (SEM-EDS) was used to determine the amount of Br, and therefore BrPF, that penetrated into bamboo cell walls. Fourier transform infrared (FTIR) images and solid-state cross-polarization/magic angle spinning (CP/MAS) ¹³C NMR spectra were used to access the molecular-scale interactions between PF resins and bamboo cell wall. Finally, twice-adhesive dispensing method, that is, the first gluing with low-MW PF resins and the second gluing with high-MW PF resins, was used to see whether the bondline was strengthened.

2. Experimental

2.1. Preparation of Br-labelled phenol formaldehyde adhesives

Because lignin and PF resins have similar chemical structures, BrPF resins were synthesized to better distinguish them and trace the impregnation and distribution of the resins by SEM-EDS. Four BrPF resins were synthesized by mixing phenol, tribromophenol, formaldehyde and sodium hydroxide in molar ratio 1:0.07:2.40:0.21. Phenol, tribromophenol and formaldehyde were mixed at once and sodium hydroxide was charged gradually according to the exotherm. Polymerization proceeded at 75°C for 2 h, 5 h, 7.5 h and 9 h, respectively, to get the four PF resins with increasing MWs. The four resins were denoted as BrPF1, BrPF2, BrPF3 and BrPF4, respectively, and table 1 lists their specific properties.

Table 1.

Specific parameters of four bromine-labelled PF resins.

	samples
properties	BrPF1	BrPF2	BrPF3	BrPF4
viscosity (mPa s/25°C)	30	67.5	307.5	630
pH	9.33	9.61	9.94	9.52
specific gravity	1.189	1.188	1.196	1.184
water solubility multiple	infinite	14.15	12.67	19.08
free formaldehyde	0.45	0.28	0.19	0.19
free phenol	2.24	1.02	0.64	0.44
solid content (%)	48.73	49.98	51.01	50.88
molecular weight	541	945	1504	1964

2.2. Sample preparation

Four-year-old moso bamboo (Phyllostachys edulis) was harvested from the forest in Anji, Zhejiang, China. The bamboo culms were harvested immediately after the rainy season and their diameters ranged from 7 to 10 cm. Within a week after harvesting, all the bamboo culms were taken to Chinese Academy of Forestry for drying. The bamboo culm was sawn into small bamboo slivers with dimensions of 50 mm (length) × 20 mm (width) × 5 mm (thickness) after removing the inner and outer bamboo skin. The bamboo sliver was dried at 85°C for 24 h to an approximate moisture content of 10% in an oven. Then, the samples were kept in a conditioned room at 20°C and 50% relative humidity for two weeks.

The four BrPFs were used to glue two-ply bamboo panels. The specimens were press-cured at 150°C for 25 min under 0.6 MPa of pressure and the glue spread was 100 g m⁻². For twice-adhesive dispensing, another two pieces of bamboo were firstly coated with BrPF1 and then BrPF4, and the glue spread was 50 g m⁻². All the specimens were conditioned at an ambient temperature of 20 ± 2°C and RH of 65 ± 2% for one week to obtain equilibration moisture content.

2.3. Shear strength of adhesive layer

Convex-shaped specimens with section sizes of 30 × 20 mm² were tested according to GB/T 17657-2013 [15]. The shear strength tests were carried out on a universal testing machine with the load applied at a speed of 2.5 mm min⁻¹ and six replicates were tested.

2.4. Ultra-depth-of-field microscope observation

A cross section in the area of interest on each specimen was created with a Leica RM2245 slicer at room temperature. Then, the section was dyed with sarranine to better distinguish bamboo matrix and resins. Ultra-depth-of-field microscopic observations were made and the images were recorded digitally. The penetration depth of resins was measured by ImageJ software and the colour phase and saturation of the images were modified by Adobe Photoshop CC 2017 software.

2.5. Scanning electron microscopy with energy dispersion spectrometry

SEM-EDS (Hitachi, SU8010, Japan) was applied to observe the diffusion of BrPF resins in cell walls with an accelerating voltage of 15 kV.

2.6. Nanoindentation tests

Nanoindentation tests were performed using a Troboscope nanomechanical testing system (Hysitron TI950, USA) at room temperature of 20°C and relative humidity of 65%. The loading speed was 50 µN s⁻¹ and the peak load was 250 µN. The holding time at peak load was 5 s and the unloading time was 3 s. Hardness and elastic modulus were calculated from the load–displacement data obtained by nanoindentation based on the Oliver–Pharr method.

2.7. Fourier transform infrared images

The FTIR images were recorded in transmission mode on a Spectrum Spotlight 400 FTIR microscope connected to a Spectrum FTIR Frontier spectrometer (Perkin Elmer Inc., USA). Transverse sections with a thickness of 10 µm were cut with a sliding microtome without any pre-treatment. Measurements provided a pixel resolution of 6.25 × 6.25 µm². The FTIR spectra were recorded with a 4 cm⁻¹ spectral resolution between 4000 cm⁻¹ and 720 cm⁻¹. Sixteen scans per pixel were averaged to increase the signal to noise ratio. The FTIR absorbance image at a size of 100 × 100 µm² was calculated from the average absorbance of the whole IR range after atmospheric compensation. Images on interphase region between bamboo and resins were recorded.

2.8. CP/MAS ¹³C NMR

Solid-state CP/MAS ¹³C NMR spectra were obtained on a Bruker AVIII 400 MHz spectrometer with a 4 mm CP/MAS probe at 25° after 30 000 scans.

3. Results and discussion

3.1. Mechanical interlocking

Figure 1 shows BrPF bondline with different MWs in bamboo bonding interface. The left column of figure 1 is the original image obtained by ultra-depth-of-field microscope. To better observe the penetration of PF resins in bamboo, the colour phase and the saturation of the bitmap were modified, where the colour of bamboo matrix was changed to neutral grey while that of resins to warm colour. It was difficult for BrPF to penetrate into the cavity of bamboo thin-wall cells except into the cell lumen of damaged cells near the bondline. Most BrPF resins remained in the bondline, with only a few penetrating into nearby cell corners. This phenomenon was totally different from that of wood. BrPF could fill the wood cell lumina near the bondline through pits and ray cells. As seen from figure 1, except for BrPF1, the penetration depth of BrPF resins in bamboo is negatively correlated with their MWs. BrPF1 rarely permeated into the interior of bamboo (figure 1e), and its maximum penetration depth was about one cell length (68 µm). In addition, it was obvious that the BrPF1/bamboo bondline was seriously damaged and divided into two parts. This was because BrPF1 had the lowest MW and the best fluidity. During hot-pressing, BrPF1 resins were extruded and preferentially flowed out from both sides rather than into the interior of bamboo, leading to an undesirable starved bondline. In the process of slicing, the bondline could be easily cut apart. BrPF2 penetrated the deepest in the bamboo interior and can permeate into the cell corners four cell lengths away from the bondline (227 µm) (figure 1f). Meanwhile, the bondline was relatively strong without separation after slicing. With further increasing the MWs of BrPF resins, the penetration depth of BrPF resins in bamboo gradually decreased owing to poor fluidity. BrPF4 can only reach cell corners (figure 1h), which were one cell length away from the bondline (74 µm). Compared with the other three kinds of BrPF, its bondline was fuller and thicker.

Figure 1.

Ultra-depth-of-field microscope images of BrPF bondline with different MWs in bamboo bonding interface (a, BrPF1; b, BrPF2; c, BrPF3; d, BrPF4) and modified images by changing the colour phase and saturation (e, BrPF1; f, BrPF2; g, BrPF3; h, BrPF4). (Online version in colour.)

Figure 2 shows the SEM-EDS line scans of the cell wall of bamboo thin-wall cell near the bondline. The presence of Br element was detected only in thin-wall cells near BrPF1 and BrPF2 bondline. This indicated that BrPF can infiltrate into bamboo cell walls when its MW was lower than 1000. As to why the content of Br in the cell wall near BrPF2 bondline was higher than that near BrPF1, it has been explained previously that most of the resin flowed out from both sides during hot-pressing due to its excellent fluidity rather than infiltrating into bamboo interior. The average elastic modulus (E_r) and hardness (H) of the cell wall of bamboo thin-wall cells near BrPF2 bondline were further evaluated by nanoindentation (figure 3). The E_r and H values of the cell wall increased by 9.7% and 17.9%, respectively, after the infiltration of BrPF resins. This indicated that the low-MW resins infiltrated into the cell wall to form an interpenetrating polymer network, which had a positive effect on the mechanical properties of bamboo cell wall.

Figure 2.

SEM-EDS line scans of the cell wall of bamboo thin-wall cell near the bondline (a, BrPF1; b, BrPF2; c, BrPF3; d, BrPF4). (Online version in colour.)

Figure 3.

Scanning probe microscopy image (a), curve of load versus displacement (b) and nanoindentation mechanics (c) of cell wall of bamboo thin-wall cell near BrPF2 bondline. (Online version in colour.)

3.2. Intermolecular interactions

In composite materials, the interfacial bonding mechanisms mainly include mechanical interlocking and chemical reactions. It has been proved above that PF resins could infiltrate into the damaged cells, cell walls (only for low-MW PF) and cell corners, thus forming mechanical interlocking with bamboo. It is important to make clear as to how bamboo cell-wall polymers interact with resins by obtaining chemical information about the interphase region. Figure 4 shows the IR image and a series of IR spectra collected at seven points along a line from resin to bamboo cell wall. Figure 4b presents the position of the seven points in the cross section of the interface region and figure 4c gives the corresponding spectra collected at these points. The green, red and violet spectra represent the bamboo cell wall region, resin region and interphase, respectively. Following recognized peak identifications of the IR spectra of polymer of bamboo cell wall and PF resin were identified and listed in table 2.

Figure 4.

Image of bamboo cells near BrPF2 bondline based on CCD camera (a), pseudo-colour full-spectra FTIR absorbance image of BPP2 bondline (b), and a series of IR spectra collected at seven points located in (b) along a line from resins to bamboo cell wall (c). (Online version in colour.)

Table 2.

Peak identifications of the FTIR spectra of bamboo and PF resin.

wavenumber (cm−1)	peak assignment
1732–1730, 1724	C=O stretch (acetyl group in xylan and formaldehyde)
1597, 1505, 1453	benzene ring stretch (lignin and PF resin)
1423	aliphatic CH2 scissor bend (cellulose)
1370	aliphatic CH bend (cellulose and hemicellulose)
1325	OH in-plane bend
1237	CO–OR stretch (acetoxyl in hemicellulose) and benzene–oxygen bond stretch (lignin)
1160	C–O–C stretch (cellulose and hemicellulose)
1108	OH association
1048	C–O stretch (cellulose and hemicellulose)

For the bamboo cell wall, the intense band at 1732 cm⁻¹ originated from the C=O stretching in acetyl groups in hemicellulose. The peak at 1507 cm⁻¹ as well as two main shoulder peaks at 1503 cm⁻¹ and 1452 cm⁻¹ was due to stretching vibration of benzene rings in lignin [16]. The prominent peaks at 1237 cm⁻¹ and 1048 cm⁻¹ were ascribed to the C–O stretching vibration in hemicellulose and cellulose. By contrast, the IR spectra of PF resins were less complex. The band at 1730 cm⁻¹ was assigned to the C=O stretching vibration in free formaldehyde in PF resin. The benzene ring in PF resin also presented three characteristic peaks at 1597 cm⁻¹, 1505 cm⁻¹ and 1453 cm⁻¹ in the IR spectra. As for the interphase region, its IR spectrum retained the characteristic peaks of bamboo cell wall and PF resin. The decrease in intensity of the C=O stretching band at 1724 cm⁻¹ could be attributed to the deacetylation caused by the cleavage of groups linked as an ester group to the hemicelluloses with an increase of the temperature during the curing of PF resin (reaction (3.1)) [17]. When the bamboo cell wall was mixed with PF resin, the main change was that a relatively broader band between 1160 cm⁻¹ and 1000 cm⁻¹ corresponding to C–O–C aliphatic ether appeared in the spectrum, which could be ascribed to the crosslinking reactions between OH groups in polymer of bamboo cell wall and PF resin (reaction (3.2)) [6].

3.1

3.2

Intermolecular interactions at the bamboo/PF resin interphase were further evaluated by CP/MAS NMR. Figure 5 shows the CP/MAS ¹³C NMR spectra for the cured PF resin, bamboo and bamboo/PF composite, and table 3 lists the resonance assignments of ¹³C NMR spectra. The methylene carbons were detected in the 30–40 ppm region in the cured PF spectrum, which consisted of two overlapping resonances. One near 33.5 ppm was corresponding to ortho–ortho methylene bridges and the other at 40 ppm was the characteristic of para–para methylene bridges [18]. The sharp resonance near 130 ppm represented aromatic carbon atoms and another centred at 151.3 ppm was the characteristic of phenolic hydroxyl carbons. As for the spectrum of bamboo, the resonance peaks at 110–60 ppm were assigned to the cellulose carbons. The resonance at 153.2 ppm was ascribed to the C3 and C5 of syringyl lignin. The peaks at 21.4 ppm and 171.1 ppm were related to acetyl methyl carbon and acetyl carbonyl carbon in hemicellulose, respectively. The NMR spectra of bamboo/PF composites retained all the resonance peaks of PF resin and most peaks of bamboo. The methylene carbon signals were better resolved in the spectrum of bamboo/PF resin, indicating a conformation modification of PF resin by bamboo [18]. The resonances at 21.4 ppm and 171.1 ppm attributed to the acetyl carbons in hemicellulose disappeared, indicating the deacetylation of hemicellulose during curing of PF resin.

Figure 5.

CP/MAS ¹³C NMR spectra for the cured PF resin, bamboo and bamboo/PF composite. (Online version in colour.)

Table 3.

Resonance assignment of ¹³C NMR spectrum of bamboo.

chemical shift (ppm)	resonance assignment
171.1	acetyl carbonyl carbon in hemicellulose
153.2	C3 and C5 carbon of syringyl lignin
151.3	phenolic hydroxyl carbons
129.8	aromatic carbon
105.3	C1 carbon in cellulose
84.0, 88.5	C4 carbon in cellulose
80.0–73.1	C2, C3 and C5 carbon in cellulose
72.1	PF hemiformal or dibenzyl ether linkages
64.7	C6 carbon in cellulose
63.0	PF hydroxymethyl carbons
57.0	methoxy group in lignin
40.0	para–para methylene bridges
33.5	ortho–ortho methylene bridges
21.4	acetyl carbons in hemicellulose

3.3. Bonding mechanism

Generally, mechanical interlocking and chemical reactions are responsible for bamboo/PF adhesion and the former seems to play a dominant role. Mechanical interlocking is closely related to the MWs of PF. High-MW PF resins infiltrate into the lumina of damaged bamboo cells near the bondline to form glue nails, while low-MW ones penetrate into the bamboo cell wall to form nanomechanical interlocking. During the curing process of PF, cellulose in bamboo can cross-link with resins. Also, the secondary forces such as polar forces and hydrogen bonds were generated between bamboo and PF resins.

3.4. Twice-adhesive dispensing

Based on the above results, twice-adhesive dispensing was used to improve the bonding properties of bamboo. The low-MW PF resins were coated for the first time, and the high-MW ones were coated for the second time. In this way, bamboo surface could be covered by two layers of PF resins. During the subsequent hot-pressing process, low-MW resins can penetrate into the interior of bamboo, even cell wall, while high-MW ones remained in the bondline, filling damaged cells to form a firm bond. As seen from the ultra-depth-of-field microscope image of the bondline by twice-adhesive dispensing (figure 6a), there was no glue deficiency like BrPF1 at the bondline. Meanwhile, the penetration depth of resin in bamboo was greater than that of BrPF4. It can be seen from the SEM-EDS line scans of the cell wall of bamboo that PF resins infiltrated into the interior of bamboo cell wall after twice-adhesive dispensing (figure 6b). Accordingly, the bonding shear strength of bamboo/resin interface with different MWs and twice-adhesive dispensing was tested. As expected, the bonding shear strength for BrPF1 was low due to the starved bondline. BrPF4 had the lowest bonding shear strength because almost all of the resin remained in the bondline without penetrating into the interior of bamboo to form a firm bond. After twice-adhesive dispensing, the bonding interface had the highest bonding shear strength (13.7 MPa), 13.2% and 42.7% higher than BrPF1 and BrPF4, respectively (figure 6c). It was proved that a good bonding interface required not only low-MW resins penetrating into the bamboo interior to form an interpenetrating polymer network, but also high-MW resins remaining in the adhesive layer to ensure the strength of the adhesive layer.

Figure 6.

Modified ultra-depth-of-field microscope image of bondline by twice-adhesive dispensing (a), SEM-EDS line scans of the cell wall of bamboo that PF resin infiltrated into the interior of bamboo cell wall after twice-adhesive dispensing (b) and the bonding shear strength of bamboo/resin interface with different MW and twice-adhesive dispensing (c). (Online version in colour.)

4. Conclusion

Ultra-depth-of-field microscopy and SEM-EDS were applied to map BrPF penetration into bamboo cells and bamboo cell walls at different scale. IR and NMR were used to reveal molecular-scale interactions between PF resin and bamboo cell wall. Four different BrPF resins with different MWs were studied. The results showed that low-MW PF resins penetrated most deeply in bamboo and they can infiltrate into the bamboo cell wall to reinforce the cell wall matrix while high-MW resins almost remained in the bondline. In addition to mechanical interlocking, it was revealed that covalent bonds and hydrogen bonds were formed by the reactions between the bamboo components and PF resins during the curing process. Based on the above results, the twice-adhesive dispensing method combining low-MW resins with high-MW resins was used to improve the bonding strength.

Data accessibility

The data used to support the findings of this study are available as electronic supplementary material.

Authors' contributions

Y.H. did the experiments and wrote the manuscript. Q.L. helped do the experiments. C.Y. and G.B. helped to synthesize the Br-labelled PF resin. Y.Z. and W.Y. designed the experiments and Y.Z. helped revise the manuscript.

Competing interests

We declare we have no competing interests.

Funding

The authors gratefully acknowledge the financial support from Forestry Science and Technology Extension Project through the project ‘Extension and demonstration of manufacturing technology for large-scale weatherproof bamboo-based reconstituted structural materials' ([2019]43).