Enhanced dimensional stability of straw-based biocomposites modified with UV light-cured coatings

Yuan, Y., Sun, X., Xu, D., He, J., Wang, X., Wu, D., and Li, S. (2024). “Enhanced dimensional stability of straw-based biocomposites modified with UV light-cured coatings,” BioResources 19(2), 2749-2762.

Abstract

This study demonstrated an effective method to enhance the dimensional stability of straw-based biocomposites with modified lignosulfonate as a binder. The ultraviolet (UV) light-curable nanosol was prepared by adding 3-(trimethoxysilyl)propyl methacrylate (MEMO) as sol–gel precursor into polyvinyl alcohol (PVA) solution. The MEMO/PVA coatings were generated using 2-hydroxy-2-methyl-1-phenylpropan-1-one (Darocur 1173) as radical photo-initiator and chitosan (CS) as additive, on straw-based biocomposites via UV-curing process. The effects of the crucial steps, such as the UV-curing process, hydrolysis time, Darocur 1173 dosage, and CS dosage on the dimensional stability of straw-based biocomposites, were evaluated. The optimum preparation parameters, obtained using the Box–Behnken design, were 31.9 min hydrolysis time, 4.5% Darocur 1173 dosage, and 2.7% CS dosage. Moisture resistance of minimum TS of CS-MEMO/PVA-coated straw-based biocomposites resulted in ~23.1% reduction in dimensional stability without significant decline in the mechanical properties when compared with those without UV curing. Moreover, the glossy spherical particles underwent arrangement in a fish-scale shape with scales closely linked with each other and no agglomeration occurred in CS-MEMO/PVA hybrid film. The CS promoted the cross-linking of MEMO/PVA coating on the biocomposite surface. The resulting biocomposites can be directly applied to public humid-environment applications such as bath furniture and bathroom partitions.

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Enhanced Dimensional Stability of Straw-based Biocomposites Modified with UV Light-cured Coatings

Yuan Yuan,^a,b Xiang Sun,^c Dong Xu,^a,b Jianyu He,^a,b Xuansong Wang,^a,b Donghua Wu,^a,b and Sidan Li^a,b,*

DOI: 10.15376/biores.19.2.2749-2762

Keywords: Straw-based biocomposites; Ultraviolet-curing; Chitosan; Dimensional Stability; Box–Behnken design

Contact information: a: College of Mechanical and Resource Engineering, Wuzhou University, Wuzhou 543000, China; b: Wuzhou Engineering Research Center of Resource Recycling, Wuzhou 543000, China; c: College of Resources, Environment and Materials, Guangxi University, Nanning 530000, China;

* Corresponding author: yuan__yuan2014@sina.com

INTRODUCTION

The dimensional stability of wood or non-wood composites is an important characteristic of consumer concern that demands comprehensive understanding (Okuda and Sato 2007; Yuan et al. 2014, 2019). Under normal circumstances, a waterproof barrier can be achieved by completely covering the surface of the composite board with a dense polymer layer or coating (Gao et al. 2023; Liu et al. 2023; Tao et al. 2023). The application of nanotechnology has been shown to improve the surface properties of materials, leading to their high durability. Among various methods involved in nanotechnology, the sol–gel technique can aid in customization of surface characteristics to a certain extent and combine different functions into a single material in a convenient and cost-effective manner (Gao et al. 2015; Zhang and Guo 2023). Relevant studies on sol–gel process mainly have focused on functionalization of fiber or wood surfaces to improve their characteristics and impart new properties (Nkeuwa et al. 2014a,b; Wang et al. 2020). The nano-sol prepared using tetraethyl orthosilicate or γ-aminopropyl triethoxysilane was used as the precursor and applied to the surface of the treated material through impregnation and heat treatment to obtain good contact angle, and the surface of the treated material exhibited superhydrophobic properties (Li and Li 2018).

Corn stalk is approximately 90% cheaper than all other agricultural fibers used to date for preparation of composites (Shah 2013). The stalks left in the field are burned with the release of large amounts of smoke, which is already of major concern in northern China. In addition, from the perspective of sustainable development, the use of corn stalks is more advantageous than the use of some other natural fibers, such as jute, flax, and hemp, that require large land areas to grow (Luo et al. 2016).

Biocomposites possess a range of specialized structures with different requirements on temperature and operating conditions, which depend on their applications in different areas (Mathias et al. 2015; Noor Haris et al. 2022; Yavuz 2023). Therefore, surface hydrophobic modification is usually performed by surface coating method (Huang et al. 2021). Moreover, most adhesives and dispersants used in surface modification of biocomposites are organic solvents. These can cause environmental pollution, hindering the promotion and advancement of this process. Furthermore, volatile organic compounds (VOCs) are emitted from coating formulations, and their production cost is very high.

Ultraviolet (UV)-curing technology has emerged as an efficient alternative that is being developed rapidly (Ghazali et al. 2021; Dong et al. 2022). UV-cured coatings have high scratch and chemical resistances, involve VOC and waste reductions, and they are economical (high production speed with low footprint) (William et al. 2014). Surface coatings with desired functionalities can be obtained by layering the photocurable coatings on the substrate and curing them into films under UV irradiation (Choi et al. 2021). Noteworthy, in recent years, the integration of UV-curing technology and surface hydrophobic modification has become a research hotspot (Cui et al. 2022; Huang et al. 2022). For example, Periolatto and Ferrero (2015) combined self-assembly and UV-curing technology to prepare washable cotton fibers, and the modified fabric still exhibited a certain hydrophobic effect as well as good softness and permeability.

With the development of combined coating technology, hybrid coatings can be obtained through a dual curing process, in which the sol–gel reaction of inorganic precursors is combined with the photopolymerization of reactive monomers/oligomers (Kesmez 2019). This photopolymerization or UV-curing process involves low energy consumption, rapid curing speed, non-requirement of organic solvents, and is more environmentally friendly, durable, and does not require secondary processing compared with traditional surface spraying of resin paints. Chitosan (CS) is a deacetylation product of chitin, which is the second most abundant natural polysaccharide after cellulose and is considered an eco-friendly biomaterial. Several studies have been dedicated to enhancement of the low mechanical, thermal, gas barrier, and water resistance properties of these films (Mardyukov and Studer 2013). Moreover, the product is characterized by better processing technology. At present, it is applied only for hydrophobic treatment of fabric surface (Banerjee et al. 2019; Pakdel et al. 2022). However, the surface functionalization of biocomposites, in particular, straw-based biocomposites has rarely been reported to date.

In this study, an easy and effective coating method was used to enhance the dimensional stability of straw-based biocomposites by sol–gel method combined with UV-curing technology. The effects of key UV-curable process, as well as the hydrolysis time, photoinitiator dosage, and CS dosage on the dimensional stability of straw-based biocomposites, were systematically evaluated. The optimum preparation parameters were obtained via the Box–Behnken experimental design to provide economic, environmentally friendly, and moisture-proof treatment ideas for broadening the application range of straw-based composites.

EXPERIMENTAL

Materials and Methods

Corn straw was obtained from Anda (Heilongjiang province, China), and the skins were separated using a skin separator (XZ2020, Xingtai Hengkong Jiacheng Machinery Manufacturing Development, China). The skins were reduced to particles using a flaker (FW-100 high-speed shredder, Changzhou, China). The corn stalk particles (CP) were dried to a moisture content of 5% and then filtered through 40-mesh to 60-mesh for separation. The average chemical compositions of CP were determined to be 17.7% lignin, 45.6% cellulose, 24.5% hemicelluloses, 9.3% extractives, and 2.9% ash. Lignosulfonate was obtained from Shenyang Xingzhenghe Chemical Company (Shenyang, China). All other chemicals were of analytical grade.

The straw-based biocomposites were self-made in the laboratory, following the preparation process reported in the literature (Li et al. 2023). Briefly, CP were mixed with modified lignosulfonate binder at different proportions in a SHR-10A high-speed blender (Zhangjiagang Yunfan Machinery Co., Ltd., Zhangjiagang, China). The mixed particles were then fixed into a mat of a forming box with dimension of 250 mm × 250 mm, and then hot-pressed into biocomposites. The target density of all biocomposites was determined as 0.8 ± 0.03 g/cm³ with a target thickness of 5 mm, and then the samples were hot-pressed at 160 °C under 3.0 MPa of pressure for 5 min. 3-(Trimethoxy-silyl) propyl methacrylate (MEMO), polyvinyl alcohol (PVOH), and 2-hydroxy-2-methyl-1-phenylpropan-1-one (Darocur 1173) were obtained from Aladin Reagent Co., Ltd. (Shanghai, China). The CS and sterile deionized water were obtained from Sigma-Aldrich Company (Shanghai, China).

Preparation of UV-curable sol

Preparation of UV-curable sol was inspired by the literature (Periolatto and Ferrero 2015; Li and Li 2018). The modified processes were as follows, a PVOH solution was firstly prepared by dissolving PVOH (4 g) in deionized water (100 mL) at 75 °C for 2 h under magnetic stirring. Second, using MEMO as the sol–gel precursor, a certain amount of anhydrous ethanol was added, the contents were stirred at room temperature for 10 min, and PVOH solution was added slowly. After ultrasonic hydrolysis at 40 °C for a certain time, sol was formed. A certain amount of photoinitiator was added, the contents were stirred for 24 h, and dried at 70 °C until the solid content was 30 wt%, which resulted in the formation of UV-curable sol.

Fabrication of MEMO/PVA-coated straw-based biocomposites

Next, the UV-curable sol was coated on the surface of the straw-based biocomposites, the coating amount was 0.1 g/cm², and it was cured in an intelligent UV-curing box for 15 min. The effects of hydrolysis time, Darocur 1173 dosage, and CS dosage on the dimensional stabilities of the prepared biocomposites were systematically studied.

Characterization and Experimental Design

Properties of straw-based biocomposites

According to GB/T 17657 (2022), the test samples were cut into dimensions of 200 mm × 50 mm for the modulus of rupture (MOR) and modulus of elasticity (MOE) tests, and into 50 mm × 50 mm for the internal bonding strength (IB) test. A loading speed of 5 mm/min was selected for the MOR and the MOE tests and 2 mm/min for the IB test. For the 24-h thickness swelling (TS), the samples with dimensions of 50 mm × 50 mm were obtained. The specimens were first immersed in water at 20 ± 1 °C and then their thickness and weight changes were measured after 24 h. Moisture resistance (MR) was measured by three cycles of test treatment, where each cycle included cold water soaking, freezing, and drying. After cyclic treatment, IB_MR and TS_MR were measured by rebalancing for 24 h. For each test, the biocomposites were cut into three test samples after conditioning at 20 ± 2 °C and 65 ± 5% relative humidity (RH).

The load-bearing particleboard properties of GB/T 4897 (2015) were followed as MOR ≥ 14 MPa, MOE ≥ 1900 MPa, IB ≥ 0.45 MPa, 24-h TS ≤ 22%, IB_MR ≥ 0.18 MPa, and TS_MR ≤ 20%. The IB_MR and TS_MR were used as the dependent responses to discuss the effect of key UV-curable process on the dimensional stability of the straw-based biocomposites.

ATR-FTIR spectroscopy and SEM characterization

Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy results of straw-based biocomposites before and after the application of UV-curable coating were obtained using a spectrometer (Nicolet Nexus 670, ThermoFisher Scientific, Madison, WI, USA). The spectra were recorded at wavelengths ranging from 4,000 to 500 cm⁻¹. Each sample was scanned 40 times at resolution of 4 cm⁻¹. Scanning electron microscopy (SEM) analysis was conducted to characterize the morphological changes in straw-based biocomposites before and after the application of UV-curable coating using a scanning electron microscope (Sirion 200, FEI Company, Hillsboro, OR, USA).

Box–Behnken experimental design (BBD)

The BBD was applied using Design-Expert 8.0.6 software (Stat-Ease Inc., Minneapolis, MN, USA). The three major parameters including the hydrolysis time (X_A), Darocur 1173 dosage (X_B), and CS dosage (X_C) were independent variables, and the response variables were IB_MR and TS_MR. A total of 17 experiments were carried out in Table 1. Data were expressed from three replicates along with their coefficient of standard deviation (SD).

Table 1. Experimental Design of Coded Factors and Results of Box–Behnken Design for Dimensional Stability of Straw-Based Biocomposites

RESULTS AND DISCUSSION

Data Analysis and Regression Models

The ANOVA p-values are presented using Design-Expert 8.0.6 software in Table 2. All p-values below 0.05 revealed significant model terms, while values above 0.05 indicated insignificant model terms (Alslaibi et al. 2013). Moreover, p-values below 0.0001 indicated that all models of mechanical properties were significant and there was only a 0.01% chance that such values could occur due to noise.

Table 2. ANOVA for p-Values of Parameters and their Interactions

^ns Not significant

The regression equation for IB was as follows:

IB = 0.35 + 0.016X_A + 0.031X_B + 0.085X_C + 0.012X_AX_B − 0.055X_AX_C − 0.20X_BX_C − 0.11X_A² − 0.12X_B² − 0.031X_C²

Table 2 shows that only X_AX_B was not significant. The F-value of 112.62 indicates that the model was significant. A p-value < 0.05 is indicative of the significance of the model terms. The high value of R² (0.9931) indicates a high level of model fitting. All predicted R² values (0.9974) agreed well with the adjusted R² values (0.9843). The lack of fit F-value of 0.025 indicates that the lack of Fit is not significant relative to the pure error. The low coefficient variance (CV% = 6.01) confirmed the accuracy and reliability of the experimental values in the regression model of IB_MR.

Simultaneously, the regression equation of TS is as follows:

TS = 18.31 − 1.55X_A − 1.79X_B − 3.94X_C + 0.67X_AX_B+ 2.43X_AX_C+ 0.45 X_BX_C + 5.14X_A² + 5.67X_B² − 0.081X_C².

Only X_C²was not significant in Table 2. The F-value of 502.49 presents that the model is significant. A p-value < 0.05 is indicative of the significance of the model terms. A high level of model fitting was indicated by the high value of R² (0.9985). All predicted R² values (0.9802) agreed well with the adjusted R² values (0.9965). Values of adequate precision greater than four are appropriate. The lack of fit F-value of 4.65 indicates that the lack of fit is not significant relative to the pure error. The low CV% of 1.37 confirmed the accuracy and reliability of the experimental values in the regression model of TS_MR.

Response Surface Interaction Analysis

The three-dimensional (3D) response surface and contour plots for IB_MR are shown in Fig. 1. The 3D response surface plot and the 2D response contour plot provide a visual interpretation of the interaction between two independent variables. The 3D response surface diagram shows the interaction between independent variables and response variables. The 2D response contour map can not only explain the interaction between independent variables, but also reflect the importance of the interaction between variables. That is, the circular response contour plot shows that the interaction between the corresponding variables is negligible, whereas the elliptic response contour plot shows that significant interaction occurs between the corresponding variables (Guo et al. 2010; Zhang et al. 2022).

Figure 1 demonstrates that the interaction effect between the hydrolysis time (X_A) and CS dosage (X_C) was greater than that between different Darocur 1173 dosage (X_B) and CS dosage (X_C). In turn, the interaction effect between different Darocur 1173 dosages (X_B) and CS dosages (X_C) were greater than that between different hydrolysis time (X_A) and Darocur 1173 dosage (X_B), i.e., X_AX_C > X_BX_C > X_AX_B.

Figure 2 illustrates that the interaction effect between the hydrolysis time (X_A) and CS dosage (X_C) was greater than that between different hydrolysis time (X_A) and Darocur 1173 dosage (X_B). In turn, the interaction effect between different hydrolysis time (X_A) and Darocur 1173 dosage (X_B) was greater than that between different Darocur 1173 dosage (X_B) and CS dosage (X_C), i.e., X_AX_C > X_AX_B > X_BX_C.

Fig. 1. Response surface plots and response contour plots showing the interaction effect of hydrolysis time (X_A), Darocur 1173 dosage (X_B), and CS dosage (X_C) on IB_MR

Fig. 2. Response surface plots and response contour plots showing the interaction effect of hydrolysis time (X_A), Darocur 1173 dosage (X_B), and CS dosage (X_C) on TS_MR

Optimization and Verification Experiment

The main objective was to figure out the optimal preparation conditions that could aid in providing maximum value of IB_MR and minimum value of TS_MR. The predicted experimental conditions with the highest desirability were selected for verification. Based on the optimization analysis, validation experiments were performed following the same method that was performed under the optimum conditions, i.e., hydrolysis time of 31.9 min; Darocur 1173 dosage of 4.5%; and CS dosage of 2.7%. Under these conditions, the optimum values of manufacturing factors and their responses of straw-based biocomposites before and after UV curing were obtained (Table 3).

Table 3. The Optimum Solution Suggested by Using Design Expert Software

The results show that the mechanical properties of the biocomposites after surface hydrophobic modification improved to a certain extent, with the increase in MOR, MOE, and IB by 7.3%, 7.0%, and 15.5%, respectively, and 24-h TS decreased 32.3%. Furthermore, IB_MR increased 70.0% and TS_MR decreased 23.1% after cyclic test, which fully meets the requirements of furniture particleboard under the humid state of GB/T 4897 (2015) and can be directly applied to public humid-environment applications such as bath furniture and bathroom partition.

ATR-FTIR Spectroscopy Analyses

The ATR-FTIR spectra of straw-based boards before and after UV curing are shown in Fig. 3. The strength of the –OH vibration peak at 3292 to 3337 cm⁻¹ and the C–N stretching vibration peak at 1415 cm⁻¹ increased, while that of the methene C–H vibration peak at 2890 to 2938 cm⁻¹ decreased. The absorbance band at 1640 cm⁻¹ assigned to the stretching vibration of the C–C bond approached zero, which indicates that the polymerization of the UV-cured film (Periolatto et al. 2013) had occurred. The new absorbance bands of Si–C, Si–O–Si, C–O, and C–H bonds were located at 845, 1076, 1730, and 2930 cm⁻¹, respectively. For example, the Si–O–Si absorbance peak at 1076 cm⁻¹ indicates the condensation reaction of hydrolyzed silane, which demonstrates the formation of SiO₂ in MEMO/PVA coating (Li and Li 2018).

For improving the stability of MEMO/PVA coating, the effect of additive CS on the formation of UV-cured film on the surface of straw-based biocomposite was further accessed. After the addition of CS, the C–N stretching vibration peak at 1415 cm⁻¹ enhanced, indicating that CS promoted the cross-linking of MEMO/PVA on the surface of biocomposite to form CS-MEMO/PVA (Sims et al. 2022).

Fig. 3. ATR-FTIR spectra of straw-based boards before and after UV curing

The possible formation mechanism of the CS-MEMO/PVA-modified composite is shown in Fig. 4. The strategy involves the activation of MEMO by hydrolyzing the alkoxy groups off, thereby forming the reactive silanol groups, similar to the hydrolysis process of organosilane (Li et al. 2017). Then, the reactive silanol reacts with the hydroxyl groups, which undergo condensation reaction to form macromolecular networks. The differences of surface morphologies between uncoated biocomposite and CS-MEMO/PVA-modified biocomposite indicate that the MEMO successfully created a chemical bridge between the particles at each interface.

Fig. 4. Schematics showing the formation mechanism of the CS-MEMO/PVA-coated straw-based biocomposites

Surface Morphologies

The SEM images of uncoated biocomposite, MEMO/PVA-coated biocomposite, and CS-MEMO/PVA-modified biocomposite are shown in Fig. 5.

Fig. 5. SEM images of straw-based biocomposites before and after UV curing (a: untreated surface; b: MEMO/PVA-coated surface; and c: CS-MEMO/PVA-coated surface)

The figure clearly illustrates the dispersion of the SiO₂ particles in non-porous hydrophobic MEMO/PVA hybrid film (Fig. 5b). More interestingly, the glossy spherical particles were arranged in a fish-scale shape with scales closely linked with each other, and no agglomeration occurred in CS-MEMO/PVA hybrid film (Fig. 5c). Such arrangement is conducive to the formation of hydrophobic network structure between the particles at each interface (Li and Li 2018).

CONCLUSIONS

The three major parameters including the hydrolysis time, 2-hydroxy-2-methyl-1-phenylpropan-1-one (Darocur 1173) dosage, and chitosan (CS) dosage were independent variables, and the response variables were internal bonding strength (IB_MR) and thickness swelling (TS_MR). All variables and interactions between each other were significant for IB_MR and TS_MR.
The response surface methodology based on the Box–Behnken design was employed to obtain the optimal conditions, i.e., hydrolysis time of 31.9 min; Darocur 1173 dosage of 4.5%; and CS dosage of 2.7%. The optimum modulus of rupture (MOR), modulus of elasticity (MOE), IB, 24-h TS, IB_MR and, TS_MR of the biocomposites complied with the requirement of load-bearing particleboard, which were 33.5 MPa, 3542 MPa, 1.12 MPa, 13.4%, 0.34 MPa and 22.5%, respectively.
The TS_MR of C-MEMO/PVA-coated straw-based biocomposites resulted in ~23.1% reduction in dimensional stability without any significant decline in the mechanical properties when compared with those before UV curing. C-MEMO/PVA-coated straw-based biocomposites can be directly applied to public humid-environment applications such as bath furniture and bathroom partition.
The attenuated total reflection – Fourier transform infrared (ATR-FTIR) spectroscopy and scanning electron microscopy (SEM) results demonstrated that the C-MEMO/PVA coating formed a hydrophobic macromolecular network on straw-based biocomposites. Chitosan promoted the cross-linking of non-porous hydrophobic MEMO/PVA on the surface of biocomposite.

ACKNOWLEDGMENTS

The authors are grateful for the support of Special Project for Young Innovative Talents in Project of Guangxi Science and Technology Base and Special Talent (Grant Nos. Guike AD22080018 and AD22080019), Youth Science Project of Guangxi (Grant No. 2023GXNSFBA026289), Initial Fund of Talent Introduction of Wuzhou University in 2023 (Grant Nos. WZUQDJJ30119 and WZUQDJJ30122), Doctoral Foundation of Scientific Research Project of Wuzhou University (Green construction and electrochemical performance of carbon film based on Chinese medicine residue, Grant No. 2022A001), the Counterpart Aid Project for Discipline Construction from Guangxi University (Grant No. 2023W02), and Accelerated Project of Basic Ability of Scientific Research for Young and Middle-aged Teachers (Dye degradation properties of biomass magnetic aerogel, Grant No. 2022KY0678).

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Article submitted: January 24, 2024; Peer review completed: February 24, 2024; Revised version received: February 28, 2024; Accepted: March 4, 2024; Published: March 14, 2024.

DOI: 10.15376/biores.19.2.2749-2762