Bio-Based Impregnated Resin Preparation for Aldehyde-Free Decorative Paper Production

Abstract

With the growing concern for environmental protection and personal health, utilizing bio-based impregnated resin has become a sustainable approach for producing aldehyde-free decorative paper and in-house decorations. Our current work focuses on the preparation of an aldehyde-free resin (AFR) by formulating quaternized cellulose nanofibrils (QCNFs, Ave. width 10 ± 3 nm, Ave. length of and >500 nm) with aqueous acrylate emulsion. We analyzed the synthesized QCNFs, acrylate emulsion, and AFR by using various methods, including FTIR, XPS, XRD, TGA/DTG, and rheometer, to evaluate their applicability for impregnated paper processing. At a low coating weight of 4.0 g/m², a 30.8% increase and 4.9-times increase in tensile strength and contact angle were detected, respectively. Meanwhile, the free aldehyde emission from the AFR-coated paper was found to be 0.1 mg/L even at a high coating weight of 18.8 g/m², which is far below the E0 level requirement in the JAS 234:2003 criteria. Therefore, the surface coating of the decorative base paper was found to be competitive in covering the porous structure of the paper, reinforcing its mechanical strength, and providing high water resistance. Moreover, the lower free aldehyde emission from the AFR-coated paper ensures its safety and potential application in house decoration products.

1. Introduction

Exploring innovative strategies for enhancing the resistance to environmental corroding of decor paper while also lowering the unfavorable adverse effects on the environment continues to be a global challenge in the decoration industry. Impregnated paper is a type of environmentally friendly decorative paper that is produced by base paper impregnation with appropriate resins and additives, followed with/without printing processing and then surface coating [1]. The impregnated paper offers competitive resistance to environmental corrosion and ultimate customization solutions, while avoiding the plundering of precious woods [2]. Therefore, impregnated paper production received increasing attention in the decoration industry worldwide, as evidenced by the increased market demand and academic publications.

The physical and chemical properties of the impregnated resin play key roles in determining the performance of decorative paper, which has undergone development from the first generation of urea formaldehyde (UF) resin to the second generation of melamine formaldehyde (MF) resin to the latest bio-based resin [3]. Although the UF and MF resin can provide the impregnated paper with desirable adhesive strength and corrosion resistance, the high risk of the released free formaldehyde during the resin synthesis, base paper impregnation, and the end usage in house ornamentation threaten the health of the worker and user, leading to the increasing demand of environmentally friendly substitutes [4]. Therefore, the increasing awareness of human health and stricter regulations in terms of formaldehyde emission push the manufacturers to develop and replace the high formaldehyde-releasing resins with bio-based adhesives.

Bio-based adhesives and impregnated resin are adhesives that are made from renewable biological materials, such as plant-based materials or biomass. These adhesives are considered more sustainable and environmentally friendly compared to traditional adhesives that are made from petroleum-based materials. Bio-based adhesives and impregnated resin-containing biopolymers, such as starch, cellulose, lignin, proteins, tannin, and their derivatives, have been developed to replace traditional UF and MF resins [3,5,6,7,8]. For instance, a modified soy protein adhesive containing pectin and carboxylate cellulose nanofiber was fabricated via the incorporation of ultrasonicated biopolymers into the soy protein [9]. The bio-based adhesive was found to improve the wet shear strength of the plywood from 0.63 MPa to 1.15 MPa and water resistance. The covalent cross-linking between the bio-based adhesive and the plywood contributed to the mechanical reinforcement and water resistance. Similarly, Podlena et al. prepared another hybrid adhesive containing soy protein and coadjutant polymers, such as polyethylene oxide, hydroxypropyl methylcellulose, cellulose nanofibrils, polyvinyl alcohol, and lignin, and it showed competitive adhesive strength to the UF adhesive. An adhesive consisting of soy protein isolate, cellulose nanofibrils, and lignin with a weight ratio of 1:7:2 and a solid content of 9% was found to achieve the highest strength-reinforcement performance [10]. However, these hybrid adhesives still suffer from the drawbacks of decent adhesive strength but low surface strength in terms of surface corrosion resistance, thus limiting their application in interior decoration materials and furniture. To enhance the surface-bound strength and abrasion resistance, Zhu et al. designed a resin containing waterborne polyurethane and acrylic acid, as well as cellulose nanocrystal, for potential application in impregnated paper and plywood or other furniture [8]. The crystal effect of cellulose nanocrystals in the waterborne polyurethane and acrylic acid was found to improve the abrasion resistance of the impregnated paper by 41.3% and 33.3%, respectively. Unfortunately, although the author claimed that a formaldehyde-free resin was prepared, solid evidence of the low or free formaldehyde emission was missing. In addition, the increasing demand for multifunctionalities in the modern decoration industry, such as chemical resistance, dry and moist heat resistance, and antibacterial property, still need to be solved using sustainable and environmentally friendly strategies. Therefore, the current work aimed to design and fabricate a bio-based impregnated resin with desirable environmental corrosion resistance for decorative paper production towards sustainable development of the decoration industry.

2. Materials and Methods

2.1. Materials

N-(2-3-epoxypropyl) trimethylammonium chloride (EPTMAC, >90%) was purchased from Sigma-Aldrich. Eucalyptus bleached kraft pulp and base paper were received from Kingdecor (Zhejiang, China) Co., Ltd. (Zhejiang, China). All other chemicals in the purity of A.R. were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

2.2. Preparation of Quaternized Cellulose Nanofibrils (Q-CNFs)

The Q-CNFs were prepared following Olszewska’s report, with modification [11]. In brief, the eucalyptus bleached kraft pulp with a hemicellulose content of 12.5 wt.% was dispersed in deionized water at 3% and stirring for 4 h, followed by filtration. The fiber filtration cake was redispersed in 5% NaOH isopropanol/water solution at room temperature for 30 min at a concentration of 5 wt.% The EPTMAC was added to the pulp slurry in a mole ratio of 1:4–1:2 to the pulp weight (oven-dried, o.d.) to start the reaction at 65 °C for 8 h, under mechanical stirring at 500 rpm. At the end of the reaction, the pulp was thoroughly washed with deionized water by centrifugation at 5000 rpm until no variation of the supernatant’s pH and conductivity was observed. The pulp slurry was diluted to 2 wt.% and then homogenized by passing through a high-pressure homogenizer (ATS AH-PILOT, ATS, Suzhou, China) at 200 bar twice and 500 bar three times to prepare the Q-CNFs hydrogel.

2.3. Synthesis of Aqueous Acrylate Emulsion

The aqueous acrylate emulsion was synthesized in two steps, with modifications, from Xu’s report [12], i.e., the pre-emulsion preparation and the aqueous acrylate emulsion synthesis. The pre-emulsion preparation was prepared by mechanically stirring (150–200 rpm) the mixture containing 3.5 mL butyl acrylate, 3.5 mL methacrylic acid, 200 mg ammonium persulfate, 2 mL nonyl phenol (NP-10), and 2 mL polyethylene glycol 400 mono oleate (PEG 400mo) at room temperature until a light blue pre-emulsion mixture was obtained. The pre-emulsion mixture in a round flask was immersed in a water bath at 80 °C, followed by dropwise adding the mixture containing 6.5 mL butyl acrylate, 6.5 mL methacrylic acid, and 400 mg ammonium persulfate in 30 min. The reaction mixture was stirred at 100–150 rpm to yield the aqueous acrylate emulsion.

2.4. Formulation of Bio-Based Impregnated Resin

The aldehyde-free impregnated resin was prepared by adding 60 mL aqueous acrylate emulsion to a round flask, which was then immersed in a water bath at 55 °C. The Q-CNFs (1.5 g, o.d.), triethylamine (5 mL), ethylenediamine (5 mL), and polyvinyl alcohol (2.5 mL) were sequentially added into the aqueous acrylate emulsion, and we kept stirring at 200–300 rpm for 30 min. The polyethylene glycol (2.5 mL) was subsequently added into the reaction mixture for a further reaction for another 30 min at 55 °C. The TiO₂ (10 wt.%, 5 mL) and Al₂O₃ (10 wt.%, 5 mL) aqueous suspension and supplementary water were added into the reaction mixture for stirring for 10 min at 55 °C to yield the bio-based impregnated resin.

2.5. Preparation of the Impregnated Paper

The bio-based impregnated resin was surface coated onto the base paper (60 g/m²), using a wire bar coater (OSP-20, Ionroy, Shanghai, China), and then it was subjected to oven drying at 150 °C for 40 s. The dried papers were stored in a sealed PE bag for further analysis. A commercially available MF resin was used as the control for surface coating, following the same processes.

2.6. Surface Charge Density and Zeta-Potential Analysis

Conductometric titration of chloride ions with AgNO₃ (aq) was applied to estimate the surface positive charge density of the Q-CNF by following Hasani’s report [13]. In brief, 100 mg Q-CNF (o.d.) was diluted to 1 wt.% and stirred for 30 min before titration. Then, the Q-CNF suspension was titrated with 10 mM AgNO₃ (aq) at 0.1 mL/min, using an automatic titrator (Excellence T5, Mettler Toledo, Greifensee, Switzerland). The positive charge density, i.e., the trimethylammonium chloride content, in units of mmol/g, was calculated by multiplying the concentration of the AgNO₃ (aq) and its volume consumption.

The zeta potential of the QCNF was measured using a Zetasizer Nano (Malvern, Cambridge, UK) at a concentration of 0.1 wt.%, at room temperature.

2.7. Spectrometry Analysis (FTIR, XPS, and XRD)

The milled pulp fiber cellulose, Q-CNFs, acrylate emulsion, and the impregnated resin were freeze-dried at −50 °C, 10 mbar, for three days before the spectrometry analysis.

FTIR spectra were collected from the dried samples, using a Bruker Alpha II spectrometer (Bruker, Bremen, Germany) equipped with an ATR system in the range of 400–4000 cm⁻¹, with a 4 cm⁻¹ resolution and an accumulation of 32 scans.

The XRD pattern of the sample was recorded using Cu Ka radiation at 40 kV in the range of 5–40 degrees (Bruker D8 Advance, Bruker, Germany).

The XPS spectra of the samples were recorded using a photoelectron spectrometer equipped with a monochromatic A1 Ka X-ray source (Thermo Fisher Scientific, Waltham, MA, USA) operated at 12 kV, 6 mA. Binding energy calibration was carried out with respect to the C1s peak of the C-C bond at 284.8 eV.

2.8. Microscopy Analysis (TEM and SEM)

The morphology and size distribution of the Q-CNFs, acrylate emulsion, and the bio-based impregnated resin were characterized using the TEM (HT-7800, Hitachi, Japan) operated at an accelerating voltage of 200 kV and analyzed using the Image J software. The surface and cross-section of the impregnated paper were characterized using the SEM (FEI, Quanta FEG 250, Thermo Fisher Scientific, Waltham, MA, USA) operated at a voltage of 15 kV.

2.9. Rheology Analysis of the Resin

The rheological properties of the Q-CNFs, acrylate emulsion, and the bio-based impregnated resin were analyzed using a Discovery hybrid rheometer (HR 10, TA, New Castle, DE, USA) with a Peltier plate (Φ40 mm) at room temperature. The linear viscoelastic region (LVR) of the samples was identified from the oscillation amplitude sweep measurement from 0.1–1000 Pa at a frequency of 1 Hz. The oscillation frequency measurement of the samples was conducted at a stress of 10 Pa and a frequency of 0.001–1000 Hz. The shear viscosity of the samples was measured at a shear rate of 0.01–1000 s⁻¹.

2.10. Thermal Analysis

The thermal stability of the air-dried acrylate and bio-based impregnated resin was analyzed using the TGA under a nitrogen gas environment (15 mL/min) in a temperature range of 30–680 °C and heating rate of 20 °C/min.

2.11. Formaldehyde Emission Determination

The emission of formaldehyde from the bio-based impregnated decorative paper was measured by the desiccator method, following the JIS A 1460:2015 standard procedures. In brief, the coated decorative papers were cut into sizes of 150 mm × 50 mm. Twelve pieces of the papers were placed in a test desiccator with a volume of 1000 mL and containing 300 mL DI water for formaldehyde absorption for 24 h at 20 °C. The dissolved formaldehyde in the DI water was determined using a spectrophotometer (SP-723, Shanghai Spectrum Instrument, Shanghai, China) at 412 nm.

2.12. Resistance Performance of the Impregnated Paper

The tensile strength of the samples was measured by following the ISO 1924-2 constant rate of elongation method (20 mm/min) [14], using a tensile tester (WZL-300, Shanghai, China). The water resistance of the papers was monitored using a dynamic contact angle tester (SDC-500, Sindin, Guangdong, China), following the T 558 om-97 method.

3. Results and Discussion

The development of sustainable impregnated decorative paper production depends on the formulation of a green resin. The incorporation of bio-based materials, such as QCNF, in the resin can contribute to the sustainability of the process. The current work aimed to fabricate a bio-based aldehyde-free impregnated resin with desirable environmental corrosion resistance for decorative paper production, aiming towards the sustainable development of the decoration industry. As shown in the schematic illustration in Figure 1, an aqueous acrylate emulsion was first synthesized and then formulated with quaternized cellulose nanofibrils (Q-CNFs) to prepare the bio-based impregnated resin. The resultant resin was characterized using FTIR and XPS spectrum analysis, rheology, and thermal-stability analysis. The resistance performance and emission of formaldehyde from the resin-coated decorative paper were evaluated and quantified by following relevant standard methods.

3.1. Chemical Characterization of the Bio-Based Impregnated Resin

Figure 2 depicts the FTIR spectra of cellulose powder and the QCNF, the butyl methacrylate, and the formulated aldehyde-free resin. Absorption bands at 3328 cm⁻¹, 2879 cm⁻¹, 1424 cm⁻¹, 1310 cm⁻¹, 1022 cm⁻¹, 899 cm⁻¹, and 666 cm⁻¹ are assigned to the OH stretching, CH stretching, CH₂ bending, OH bending, C-O stretching, CH bending, and the OH out-of-plane bending in the spectrum of cellulose, respectively [15]. Minor shifts in these bands are observed after the quaternization reaction. The peak at 1480 cm⁻¹ in the FTIR spectrum of the QCNF confirms the successful introduction of trimethylammonium chloride groups onto the cellulose fibril surface [16,17]. The bands at 1729 cm⁻¹ and 1248 cm⁻¹ can be assigned to the characteristic stretching of C=O and C-O-C of the synthesized butyl methacrylate [18,19,20]. The bands at 2956 cm⁻¹ and 1450 cm⁻¹ are attributed to CH₃ stretching band and CH₂ bending bond. The spectrum of the formulated aldehyde-free resin (AFR) combined the characteristic peaks of the QCNF and the acrylate without any further appearance of new bands, suggesting no covalent bonds formed among these components in the AFR. The XRD pattern of the QCNF shows the typical Cellulose Iβ crystalline structure with the (002), (101), and (040) crystal plane at 15.8°, 22.6°, and 34.8°, respectively [21]. The crystallinity index of the QCNF was calculated following Seagal’s method and gave a value of 54%, similar to early reports by Pei et al. [17]. The zeta potential (+35 mv) and surface charge density (0.55 mmol/g) determined by conductometric titration further quantified the introduction of the trimethylammonium chloride groups onto the cellulose fibril surface.

The XPS analysis (Figure 3) was applied to confirm the successful polymerization and synthesis of the butyl methacrylate. The broad-scan spectrum in Figure 3 shows the peaks in the regions of ~532, ~401, and ~284 eV, which can be assigned to oxygen (O1s), nitrogen (N1s), and carbon (C1s) atoms, respectively. The high-resolution deconvolution peaks of these spectra reveal the chemical state of these elements in the synthesized butyl methacrylate. The deconvolution peaks of O1s at 532.0 eV (O2) and 533.2 eV (O1) can be assigned to the C=O and C-O bonds [22], respectively. The deconvolution peaks of C1s at 284.2 eV (C1), 284.9 eV (C2), 286.1 eV (C3), and 288.7 eV (C4) can be assigned to the C-C/C-H, C-C, C-O, and the O=C-O bonds [22,23,24]. The deconvolution peaks of N1s at 400.8 eV (N1), and 401.8 eV (N2) can be assigned to C-N and the N-O bonds [22].

3.2. Thermal Analysis of the Bio-Based Impregnated Resin

The thermal stability of the resin is a key parameter when applied in the impregnation and subsequent drying processes, as well as in the customer end-use. Thermogravimetric (TGA) and differential thermogravimetric (DTG) curves of the synthesized butyl methacrylate and the formulated bio-based impregnated resin (AFR) are shown in Figure 4. During the heating program, the temperature at which the mass loss reaches 5% was defined as the degradation temperature of the materials (T_5%), and at which the maximum decomposition rate observed was defined as T_dmax [25]. The T5% and T_dmax are recorded at 206 and 391 for the acrylate sample and at 131 and 397 for the AFR sample, respectively. The decreased thermal stability of the formulated AFR can be ascribed to the incorporation of the QCNF, which has a high affinity with moisture. A higher weight residual in the AFR sample over 600 is ascribed to the inorganic TiO₂ and Al₂O₃.

3.3. Rheological Analysis of the Bio-Based Impregnated Resin

The rheological properties of the resin have an important impact on its pumping and flow behavior during application. The flow behavior of the QCNF and the formulated bio-based impregnated resin was characterized by monitoring the viscosity under different shear rates. Data on the butyl methacrylate are not available because it presented typical Newtonian fluid behavior. However, after the incorporation of the QCNF, a non-Newtonian shear-thinning behavior was observed. The QCNF hydrogel (1.0 wt.%) was reported to show high zero-shear viscosity and stability due to the electrostatic repulsion of the cationic quaternary ammonium groups on the surface of the nanofibrils [26,27]. With the increase in shear rate, the viscosity of the QCNF and the formulated resin gradually decreased, allowing smooth and homogeneous coating processing (Figure 5A). The higher storage modulus against the loss modulus of the QCNF and the resin at a lower shear frequency (<10 Hz) suggest that both samples behave in an elastic state, which then turns to a fluid state at a higher shear frequency (Figure 5B).

3.4. Morphological Analysis of the Decoration Paper Coated with Resins

The ideal impregnated resin in decorative paper production should provide deep penetration and smooth surface coverage, conferring the decoration paper with an enhanced resistance performance. The TEM image in Figure 6 shows the morphology of the QCNF with an average width and length of 10 ± 3 nm and >500 nm. The surface and cross-section SEM images of the base paper show a porous structure with tangled cellulose fibers. Surface coating using the aldehyde-free resin for one round with a coating weight of 4.0 g/m² covered ca. 80% of the porous structure. After coating for a triple round with a coating weight of 18.8 g/m², the paper showed a smooth surface and condensed layer structure, providing the base paper with competitive coverage and potential enhanced resistance performance. In contrast, the surface coating of the base paper with common MF resins, even after triple rounds with a coating weight of 19.0 g/m², still could not cover all porous structures of the base paper. A further coating with MF resin may be needed to achieve the equivalent surface coverage, which will increase the process complexity and cost of production.

3.5. Mechanical Strength and Free Formaldehyde Emission Evaluation

The tensile strength of the coated paper was determined by following the standard method of ISO 1924-2 [14], and it is presented in

Table 1. Emission of formaldehyde from the impregnated decorative paper.

Title 1	Base Paper	AFR-X1	AFR-X3	MF-X1	MF-X3
Tensile strength (kN/m)	2.6 ± 0.1	3.4 ± 0.2	3.2 ± 0.2	3.2 ± 0.3	5.0 ± 0.7
Coating weight (g/m2)	0	4.0 ± 1.2	18.8 ± 0.3	4.5 ± 0.4	19.0 ± 3.2
Contact angle (°, 0.1 s)	14.8	87.2 ± 2.1	78.1 ± 1.9	35.1 ± 8.0	74.6 ± 3.3
Contact angle (°, 1 s)	ND	86.0 ± 2.1	67.4 ± 5.0	25.9 ± 6.2	68.1 ± 7.1
Contact angle (°, 10 s)	ND	83.9 ± 3.3	56.0 ± 4.7	ND	57.5 ± 10.1
Formaldehyde emission (mg/L)	0.1	0.1	0.1	0.8	1.3

. The surface coatings with AFR and MF resins all enhanced the mechanical strength of the base paper, especially for the later yielded 92% increment at a high coating weight of 19.0 g/m². Self-condensation of the MF resin in the presence of the curing agent (NH₄Cl) and the formation of hydrogen bonds with cellulose fibers contributed to the enhanced tensile strength [28]. In the case of the AFR-resin-coated paper, interactions among the QCNF, butyl methacrylate, and the cellulose fibers formed a dense hydrogen network, leading to the increase of the mechanical strength [29]. However, a higher coating weight did not result in further reinforcement of the tensile strength, and this might be ascribed to the presence of inorganic TiO₂ and Al₂O₃, which tend to weaken the hydrogen networks of the cellulose fibers.

The free formaldehyde emission evaluation of the coated paper is critical for the potential end users of the impregnated paper for house decorations, e.g., application in the laminate flooring. The E0 level of the aldehyde emission from the laminate floor should be lower than 0.3 mg/L to meet the JAS 234:2003 criteria [30]. The low levels of formaldehyde emission from the AFR-coated paper ensure its safety in end-use. In contrast, an 8–13-times-higher formaldehyde emission was determined for the MF-resin-coated paper due to the presence of unreacted formaldehyde in the resin and the high temperature during the curing of the resin [31,32]. Therefore, although the MF resin shows a better reinforcement effect, the higher formaldehyde emission weakens its application potential in house decoration products that have high environmental and safety requirements.

The dynamic water contact angle evaluation provides insight into the surface or interface interaction of the water with the impregnated decorative paper. The base paper shows poor water resistance due to the hydrophilic nature of the cellulose fiber. Surface coatings with AFR or MF resins are helpful in enhancing the contact angle, especially the AFR-X1 coating, which provided a stable near-hydrophobic surface (contact angle ~90°). Increasing the coating weight did not result in the further enhancement of the contact angle, probably due to the increased amount of the QCNF, which is hydrophilic in nature and scribed to the smooth surface, as shown in Figure 6. The MF resin coating shows a limited and shorter water-resistance property when compared to the AFR resin coating; thus, a much higher coating dosage may be needed to achieve an equivalent value.

Overall, this work offers a solution for developing bio-based resin in impregnated decorative paper production. The incorporation of the QCNF in the resin resulted in an improved resistance performance and increased the biomass ratio, promoting the sustainability. However, the synthesis of the aqueous acrylate emulsion still involves some non-bio-based chemicals. Fully replacing these reagents with bio-based alternatives that are cost-effective and perform well remains a challenge for future research.

4. Conclusions

Developing and applying bio-based resin in impregnated decorative paper production is a critical goal and trend for green and sustainable indoor decoration. In the current work, the FTIR and XPS analysis confirmed the successful synthesis of the butyl methacrylate and the QCNF-incorporated bio-based aldehyde-free resin (AFR). The AFR exhibited high zero-shear viscosity and shear thinning behavior, allowing for smooth and homogeneous coating processing. The SEM imaging analysis revealed that the ARF coating can provide a competitive porous structure coverage, mechanical-strength enhancement, and potential water-resistance performance, which was confirmed by the dynamic water contact angle measurement. The surface coating of the bio-based resin at a low coating weight of 4.0 g/m² (AFR-X1) was found to show a better resistance performance when compared with the high coating of the bio-based resin (AFR-X3) or the MF resin. Overall, the AFR coating provided the decorative base paper with competitive strength, resistance performance, and low levels of aldehyde emission that meet JAS 234:2003 criteria, demonstrating promising application in impregnated decorative paper production.