Kazuki Saitoa,
Yasushi Hirabayashib and
Shinya Yamanaka*a
aDivision of Applied Sciences, Muroran Institute of Technology, Mizumoto-cho 27-1, Muroran 050-8585, Japan. E-mail: syama@mmm.muroran-it.ac.jp
bForest Products Research Institute, Hokkaido Research Organization, Nishikagura 1-10, Asahikawa 071-0198, Japan
First published on 5th October 2021
Graphene oxide (GO) has theoretically been identified as a candidate for adsorbing formaldehyde molecules. However, whether GO can actually serve as a scavenger for formaldehyde resin adhesives must be experimentally verified due to the complex interaction between GO and formaldehyde molecules in the presence of resin, the competition between the formaldehyde emission rate and its adsorption rate on the scavenger, and other complications. From the results from this study we experimentally demonstrate that GO synthesised by the improved Hummers' method is a powerful scavenger for a urea–formaldehyde (UF) resin. We investigate the effect of the added amount of GO on the formaldehyde emission from UF resin. The emission from the UF/GO composite resin is 0.22 ± 0.03 mg L−1, which is an 81.5% reduction compared to that of the control UF resin when adding 0.20 wt% GO into the UF resin. However, adding higher amounts of GO (more than 0.20 wt%) increases the formaldehyde emission and the emission approaches that of pure UF resin (1.19 ± 0.36 mg L−1). This is likely due to the more acidic pH of the composite, which may lead to a faster curing reaction of the UF resin and acceleration of the emission.
Urea–formaldehyde (UF) resin is a practical adhesive used to manufacture wood-based panels such as particleboard, fibreboard, and plywood.9 Its advantages include economical viability, fast reaction time in a hot press, water solubility, low curing temperature, resistance to microorganisms and to abrasion, and its colourless, especially cured resins.10 Although UF resins are extensively applied as bonding agents in diverse applications, reduction of formaldehyde emission from wood-based panels in the environment remains a critical challenge in the industry.11
One promising technique to reduce the formaldehyde emission from wood-based panels is to add scavengers (catchers) such as natural compounds or amine compounds.12–14 Among these scavengers, urea is highly reactive and rapidly forms a strong bond. Adsorption is another effective method to inhibit formaldehyde emission. The addition of urea, other compounds like ammonium salts15 and inorganic nanoparticles16,17 can also be used. P. H. G. de Cademartori et al. investigated the addition of alumina nanoparticles into UF resin.16 They concluded that the nanoparticles reduced formaldehyde emission during UF curing and at environmental temperatures. The effect of TiO2 nanoparticles loading on formaldehyde emission were investigated by Y. Liu and X. Zhu.17 The use of natural, bio-based scavengers such as tannins,18,19 hydrolysis lignin,20 ammonium lignosulfonate,21 and cellulose22 has been studied to not only reduce formaldehyde emission but also improve the adhesion properties. These scavengers were summarized by review of the literatures.10,23
Most studies in the literature focus on the adsorption of formaldehyde on activated carbons24–29 and other carbon-based nanomaterials.30–32 Especially carbon-based adsorbents modified by various groups have been widely studied, and currently appear to be the most effective and practical way to remove formaldehyde.33
The physical and mechanical characteristics of wood-based panels reinforced with the addition of inorganic nanoparticles34,35 and carbon-based materials.30–32 A. Kumar et al. reported the effects of activated charcoal30 and multi-walled carbon nanotubes31,32 on the physical and mechanical properties of a medium density fiberboard. These carbon-based materials had an accelerating effect on the curing of the UF resin.
Graphene oxide (GO) is usually obtained through the oxidation of graphite by the Hummers' method.36,37 Compared with other carbon-based materials, GO has a high specific surface area and a folded structure.38,39 Thus, it can provide a huge capacity for absorbing pollutants. Density functional theory (DFT) studies are performed to understand the adsorption property of the pollutant molecules on different materials at the electronic, atomic, and molecular levels. In the past decade, many theoretical studies have addressed formaldehyde on carbon-based materials,40–46 including the interaction of formaldehyde with GO.41 Although these DFT studies have noted that GO has an excellent adsorption capacity for formaldehyde, GO adsorption on formaldehyde-resin has yet to be experimentally investigated. Very recently, W. Gul and H. Alrobei reported the physical and mechanical properties of medium density fiberboard enhanced with graphene oxide.47 However, they did not measure the formaldehyde emission.
Lee and colleague have suggested that surface functional groups, including oxygen atoms, of activated carbon fibres decrease the adsorbed amount of formaldehyde in humid conditions due to their affinity to water.48 Thus, an intermediary resin and a formaldehyde emission kinetics during the hydrolysis reaction of resin make it difficult to reproduce the DFT predictions because DFT studies focus on an ideal system (i.e., the interaction between the formaldehyde molecule and functional groups on the GO surface).
Herein we demonstrate that GO is an excellent scavenger and propose a new composite UF-based adhesive. The proposed UF/GO resin exhibits a low formaldehyde emission.
We not only investigate the effect of GO addition on the formaldehyde emission from UF resins, but also show that the pH of UF/GO resins plays a crucial role in extracting the GO ability.
Fig. 1 (a) FT-IR spectrum, (b) XRD pattern, and (c) UV-vis spectrum of GO prepared by the improved Hummers' method. The insert figure in (b) is XRD pattern of raw graphite. |
Graphite as the raw material had a sharp diffraction peak at 2θ = 26.5°. This peak corresponded to the (002) plane of hexagonal graphite structure. XRD analysis confirmed the crystalline nature and phase purity of the synthesised GO (Fig. 1(b)). The relatively wide diffraction peak at 10.7° corresponded to GO,49,50 revealing an expansion of the interlayer spacing from 0.34 nm (graphite) to ∼0.8 nm. The peak at 26.5° disappeared, confirming that almost all the raw graphite was converted to GO.
The UV-vis spectrum had a strong absorption peak at 232 nm (Fig. 1(c)). This peak was attributed to the π–π* transition of the C–C conjugated aromatic domains and weak absorption with a shoulder at 305 nm due to n–π* transition of CO bond. The UV-vis spectrum with GO peaks at 232 nm underwent a colour change from black to brown.50
Fig. 2 depicts a typical TEM image of GO. Submicron to several microns of a few layer sheets were observed. The FT-IR, XRD, UV-vis, and TEM observations all provided evidence that the prepared sample contained a few layers of GO.
Scavengers | Added amount [wt%] | Formaldehyde emission [mg L−1] | Rate of decrease [%] | Ref. | |
---|---|---|---|---|---|
With scavengersd | Without scavenger | ||||
a Based on the total weight of the solid resin, the scavenger, and the curing agent.b Unknown whether based on (a) or not.c Volume percent.d It is noted that the curing reaction was carried out using an oven in this study. According to our previous study,58 this simple evaluation method for measuring a formaldehyde emission from UF resin was in good agreement with the emission from plywood.e The unit is mg/100 g. | |||||
Urea modified scallop shell | 83.8a | 3.9 | 11.4 | 65.8 | 51 |
Propylamine | 0.7a | 0.32 | 0.7 | 54.3 | 53 |
Chitosan nanoparticles | 1a | 0.22 | 0.54 | 59.3 | 54 |
Alumina nanoparticles | 2a | 3.7 ppm | 4.3 ppm | 14.0 | 16 |
Copolymer | 7.5b | 1.20 | 2.00 | 40.0 | 55 |
Pozzolan | 10b | 5.3 | 9.9 | 46.5 | 56 |
Ethyl cellulose microcapsules | 68.1b | 0.49 | 1.37 | 64.2 | 57 |
Multiwalled carbon nanotubes | 0.52c | 7.7e | 12.3e | 37.4 | 30 |
GO | 0.20a | 0.22 | 1.19 | 81.5 | This study |
Fig. 3 shows the formaldehyde emission from UF/GO. Formaldehyde emissions of 1.12 ± 0.51, 0.67 ± 0.09, and 0.22 ± 0.03 mg L−1 were achieved upon adding 0.10, 0.15, and 0.20 wt% of GO, respectively. The emission clearly decreased compared with the UF and UF/graphite resin. In particular, 0.20 wt% GO addition gave the lowest formaldehyde emission of 0.22 ± 0.03 mg L−1.
Fig. 3 Formaldehyde emission from UF/GO resins measured according to the desiccator method47 for different GO contents. |
It should be noted that the –OH and –COOH groups of GO may react with free formaldehyde in the resins. According to DFT calculations, M. D. Esrafili and L. Dinparast has been pointed out the most stable adsorption configuration of formaldehyde is when it interacts with O atoms of surface via its H atom.41 Additionally, its adsorption energy is very low (−2.1 kcal mol−1) indicating the interaction of formaldehyde molecule with the GO surface is physisorption.41 On the other hand, M. Chavali et al. performed molecular dynamics simulation for formaldehyde–graphene oxide system.52 They reported that one formaldehyde molecule of adsorption heat was −76.4 kcal mol−1, which was close to the chemical adsorption.
Although an adsorption mechanism of formaldehyde on GO surface is not clear, we are not convinced that small quantity of GO could reduce more than 80% formaldehyde emission since there is physical adsorption between them only. Formaldehyde may react with the –OH and –COOH groups on the GO surface.
Previous studies have employed various materials, including natural, carbon-based, and other inorganic/organic materials, as a scavenger. Table 1 lists formaldehyde emission from the UF/scavenger resins. Here, even a small amount of added GO produced a high reduction effect. It should be noted that the reported formaldehyde emission from the control UF resin varies in the literature (without scavenger in Table 1). In this study, the resin with a 0.20 wt% GO content had an average formaldehyde emission of 0.22 mg L−1, which was an 81.5% reduction compared to that of the control UF resin. Additionally, the decrease ratio of the formaldehyde emission was quite high compared with previously reported scavengers.
DFT calculations have predicted that GO is a potential candidate for excellent formaldehyde adsorbent.41 However, this is the first experiment to demonstrate that GO effectively prevents formaldehyde emission from UF resin.
Increasing the GO content did not decrease the formaldehyde emission. The emission was 0.38 ± 0.09, 0.75 ± 0.08, and 1.12 ± 0.39 mg L−1 for a GO content of 0.40, 1.0, and 1.9 wt%, respectively. Moreover, formaldehyde was emitted at almost the same level as the UF resin (1.19 ± 0.36 mg L−1) with a GO amount of 1.9 wt%. Xing et al. have reported the effect of pH value on the UF resin gel time. They demonstrated that the gel time of the UF resin exponentially decreased with decreasing pH.59 Table 3 lists the pH values for each UF/GO liquid after the curing treatment. The pH gradually decreased as the GO addition amount increased, indicating that the UF resin before curing was more acidic. It is thought that the rate of formaldehyde emission is accelerated in acidic conditions due to the faster curing reaction.
Fig. 4 shows the relation between pH and the gel time for UF resin. When the pH was adjusted around 4.5, the gel time was 35–40 min, while the gel time was 60–65 min at pH = 6.0–6.5. The faster curing was observed under acidic conditions, indicating an acceleration of formaldehyde emission.
With 0.20, and 1.9 wt% GO content, the pH of UF/GO was regulated (see Experimental section: preparation of urea resin/GO and test for formaldehyde emission). As shown in Table 2, the formaldehyde emission dramatically increased from 0.22 ± 0.03 mg L−1 (unadjusted pH) to 0.91 ± 0.09 mg L−1 (adjusted pH) with 0.20 wt% GO content. On the other hand, the emission dramatically decreased from 1.12 ± 0.39 mg L−1 (unadjusted pH) to 0.43 ± 0.05 mg L−1 (adjusted pH) with 1.9 wt% GO content. When the pH of UF/GO was adjusted, the formaldehyde emission was improved or worsened, suggesting that the formaldehyde emission is sensitive to the pH of the UF/GO resin.
Fig. 5 illustrates the effect of GO addition on the formaldehyde emission from the UF resin. Similar to the case of adding raw graphite, formaldehyde adsorption did not proceed when the amount of GO was small due to the limited number of GO adsorption sites (Fig. 5(a)). However, in the case of adequate GO addition into the UF resin, GO could adsorb formaldehyde before it diffused (Fig. 5(b)). The pH of urea resin dropped to the acidic conditions when the amount of GO was large. It is speculated that the curing is more likely to occur and formaldehyde more predominantly diffuses into the atmosphere than is adsorbed on the GO surface (Fig. 5(c)).
Fig. 5 Illustration of the formaldehyde emission from UF/GO resins containing (a) 0.10, 0.15 wt%, (b) 0.20 wt%, and (c) 0.40, 1.0, 1.9 wt% GO. The upper magnified image expresses an interaction of O atom of GO surface with H atom of formaldehyde based on the knowledge of ref. 41 and 52. |
Therefore, even if a large amount of GO is added, formaldehyde emission is not reduced.
This study suggests that the pH of UF/GO must be carefully regulated. Although we should investigate adhesive strength of plywood, and also formaldehyde emission from plywood using the UF/GO resin, GO will be a potential candidate for scavenger of plywood production. We believe that this study opens a new practical application of GO as an adhesive scavenger.
The obtained suspension was centrifuged at 9280 × g and washed with ion-exchanged water. This operation was repeated at least three times. After the final centrifugation, the supernatant was discarded and the wet GO sediment was processed into powder by freeze-drying.
To study the crystal phase, functional groups, transparency, and exfoliation level, the sample was characterised by XRD, FT-IR, UV-vis, and TEM, respectively.
To estimate the surface functional group on the GO powder, the FTIR spectra (FT/IR-460PlusK; JASCO, Tokyo, Japan) were acquired using a KBr pellet technique with a scan range from 400 to 4000 cm−1. The KBr pellets contained 1–2 wt% of the GO powder. X-ray diffractometer, XRD (MultiFlex; Rigaku, Tokyo, Japan) powder pattern of the GO powder was obtained with Cu Kα radiation (40 kV, 20 mA). The scanning rate was set at 5° min−1 from 5° to 50°. The GO powder was placed on a reflection-free sample holder. For the diluted GO dispersion, UV-Vis spectra measurements were performed on a UV-1800 spectrophotometer (Shimadzu, Kyoto, Japan). Transmission Electron Microscopy (TEM) was performed using a field emission transmission electron microscope (JFM-2100F; JEOL, Tokyo, Japan). Images were acquired in the TEM mode using a 200 kV acceleration voltage. Samples were prepared by placing a droplet of the diluted GO dispersion directly onto the TEM grid.
GO contentb [wt%] | GO [mg] | pHc [—] |
---|---|---|
a Added amount of liquid urea resin (including volatile content) and curing agent were 4.00 g and 0.04 g, respectively.b Weight ratio of the solid GO scavenger to the total weight of the solid content of UF resin, solid GO, and solid curing agent.c pH measurement was conducted after adding the curing agent. | ||
0 | — | 5.73 |
0.10 | 2.0 | 5.66 |
0.15 | 3.0 | 5.64 |
0.20 | 4.0 | 5.61 |
0.40 | 8.1 | 5.52 |
1.0 | 20.2 | 5.29 |
1.9 | 40.4 | 4.73 |
For comparison, as-received graphite was also used as a scavenger. The addition of graphite with 0.20–1.9 wt% to the total weight of the solid UF resin, solid graphite, and solid curing agent.
After curing the UF/scavenger for 1 h in an oven at 105 °C, the formaldehyde emission was measured by the desiccator method.60 It is noted that the curing reaction was carried out using an oven in this study. According to our previous study,58 this simple evaluation method for measuring a formaldehyde emission from UF resin was in good agreement with the emission from plywood. Specifically, the absorbance was measured at 412 nm using a UV-Vis spectrophotometer (UV-1800; Shimadzu, Japan). Emission tests were repeated six times for the control UF resin and three times for the UF/scavenger composite resins.
With 0.20, and 1.9 wt% GO content, the pH of UF/GO was regulated. Citric acid or calcium hydroxide, GO, UF resin, and a curing agent were combined for 1 min at 465 × g using a planetary centrifuge mill. After mixing, the mixture was cured for 1 h in an oven at 105 °C. Then the formaldehyde emission was evaluated as mentioned above.
This journal is © The Royal Society of Chemistry 2021 |