Experimental Investigation of Mechanical and Physical Properties on Epoxy Resin with Wood Timber for Furniture Application

ABSTRACT

Composites are attracting the attention of researchers nowadays because of their strength-to-stiffness ratio. The development of wood particle composites reinforced for practical purposes has been successfully attempted. This paper focuses on the application of an epoxy/hardener ratio with wood particle-reinforced polymers to develop high-strength wood composites. The percentage of wood particles in the composition varied as follows: 5, 15, and 25 wt%, with epoxy/hardener ratios of 5:0.3, 10:1, and 5:0.8. Appropriate surface modifications were made by different treatments with 0.6, 0.8, and 1 g of NaOH in 10 liters of distilled water and 0.2% KMnO4 for better adhesion of particles and epoxy. Flexural strength, impact strength, water absorption, and SEM were investigated. Nine experiments were investigated based on the Taguchi orthogonal array blend design. The composite was successfully fabricated using the conventional hand lay-up technique. The highest mean S/N ratio considered was found for wood content composition (15%), epoxy resin to hardener ratio (10%), and NaOH (1%), i.e. the main factors affecting the mean flexural and impact strength were the wood particle contents in the matrix. Water absorption was investigated. Water absorption was found to increase with wood particle content.

摘要

复合材料由于其强度与刚度的比值而受到研究人员的关注. 开发用于实际目的的增强木颗粒复合材料已经成功地进行了尝试. 本文重点研究了环氧树脂/固化剂与木颗粒增强聚合物的比例在开发高强度木材复合材料中的应用. 木材颗粒在组合物中的百分比变化如下: 5、15和25 wt%，环氧树脂/硬化剂的比例为5:0.3、10:1和5:0.8. 通过用0.6、0.8和1 g NaOH在10升蒸馏水中和0.2%KMnO4中的不同处理进行适当的表面改性，以更好地粘附颗粒和环氧树脂. 研究了弯曲强度、冲击强度、吸水率和扫描电镜. 基于田口正交阵列共混设计，对9个实验进行了研究. 采用传统的手工叠层技术成功地制备了复合材料. 木材成分（15%）、环氧树脂与硬化剂的比例（10%）和氢氧化钠（1%）的平均S/N比最高，即影响平均弯曲强度和冲击强度的主要因素是基质中的木颗粒含量. 研究了吸水性. 吸水率随木材颗粒含量的增加而增加.

KEYWORDS:

• ANOVA
• mechanical strength
• chemical treatment
• water absorption

关键词:

• ANOVA，机械强度，化学处理，吸水性

Introduction

Wooden furniture is commonly seen in homes, workplaces, and government buildings, including hospitals, hotels, and educational institutions. They enhance the attractiveness and beauty of the space. Furniture includes items such as doors, tables, chairs, decorations, cabinets and shelves, wardrobes, mattresses, and more. According to Ngui et al. (2011), furniture has the highest value-added component among the major wood-based items. Wood dust particles are one of the most important by-products generated during wood processing (Chaudemanche et al. 2018; Lopez et al. 2020). This byproduct, if stored under uncontrolled conditions, can be a significant cause of environmental pollution and human respiratory system harm from NO₂ (nitrogen dioxide) emissions. Cedrela Odorata from the Amazon rainforest is one of the most important woods in terms of commercial value for wood manufacturing. It generates dusty particles in various furniture industries and companies. Due to the hydrophilic nature of the wood, conventional wood items are not suitable for outdoor use. To study the engineering properties of wood particle polymer composites, the hydroxyl group must be coated or removed. Composites are made from two or more individual combinations of materials. The aim of this study was to produce a wood particle composite from a biodegradable and renewable raw material as a reinforcement with an epoxy resin matrix for outdoor, indoor, and decorative furniture products.

Resin furniture is furniture made of certain plastics that can be molded into certain shapes. They weatherproof outdoor furniture that is traditionally made of wood. They have become a popular alternative to hardwood outdoor furniture for a number of reasons, including their appearance, ease of maintenance and cleaning, durability, and the fact that they do not rot in rain and snow. These factors have made resin furniture a popular option for outdoor furniture users (Ülker 2016). Epoxy resin is widely used for laminations, adhesives, coatings, and advanced composite surfaces due to its excellent mechanical, chemical, and corrosion resistance, as well as excellent thermal and dimensional stability (Ren et al. 2008). Due to its excellent stiffness, dimensional stability, and chemical resistance properties, it is a natural choice for advanced composites (Liu et al. 2009; Ren et al. 2008). Researchers are working in this field to develop composites with special properties, economic efficiency, and environmental friendliness. In composites, it serves as a matrix. Compared to other adhesive systems, epoxy adhesives have better water resistance. However, moisture resistance is highly dependent on the type of epoxy curing agent and hardener used (Broughton 2012). Automotive and aerospace applications, optical and medical applications, industrial tools, electrical systems, electronic assemblies, consumer applications, marine applications, paints and coatings, and many other applications are some of the epoxy applications (Epoxy 2023). Industrialized countries are reducing the use of natural resources to maintain the natural balance by absorbing emissions of harmful gases as natural resources are among the most important environmental protection measures (Bansard and Schroder 2021). As for thermosetting resins, the main source of industrial resins used in the production of fiberboard, particleboard composites, unsaturated polyesters, vinyl esters, etc. still comes from petroleum (Masuelli 2013). Ethiopia is currently paying close attention to environmental concerns.

Hariharan et al. (2023) investigated SiC nanofiller particles in biopolymer composites of areca fruit peel and tamarind fruit hybrid fibers, varying the proportion from 1 to 4 wt% in 1 wt% increments. The results suggest that a hybrid composite panel with a filler content of 3 wt% has the best overall properties. Therefore, the properties of filler materials are significantly influenced by the appropriate use of micro- and nano-sized filler particles and the optimization of the filler content. Siraj et al. (2022) studied the impact of silica on high-density polyethylene composite sheets. However, the toughness and elastic modulus of the material decreased with decreasing particle size. Elleithy et al. (2010) studied the morphological properties of microcalcium carbonate fillers in high-density polyethylene composites and found that CaCO3 exhibited some agglomeration. The addition of CaCo3 microparticles did not affect the shear sensitivity of the composite but increased its viscosity compared to the pure resin. Alghamdi (2022) presented fly ash filled with high-density polyethylene, which was characterized by its morphological and tensile properties. The result was that the elastic modulus of the composite increased with fly ash concentration, while the tensile strength did not increase linearly.

Szabelski et al. (2022) presented a study on the strength of adhesive joints considering the inaccuracy of hardener dosage in the context of evaluating the degradation of compounds used either at room temperature or at elevated temperature. The results show that the variation of the hardener has a significant effect on strength depending on the parameters studied. The lower the excess of hardener, the stiffer the adhesive. However, since the ambient temperature is not high, it cannot be assumed that heat curing generally improves the strength of the bond in all applications. Pereira and d’Almeida (2016) studied the effect of the ratio of curing agent to epoxy resin on water absorption properties using different diffusion models. They found that the weight increases due to water absorption increased with increasing hardener concentration, although the diffusivity showed the opposite effect. The larger portion showed a higher crosslink density, while the smaller portion showed a lower number of crosslinks.

Surface-treated polymer composites outperform untreated composites in terms of mechanical properties. The study was supported by surface modification techniques that significantly improve the overall mechanical properties of the composites while reducing their hydrophilicity (Liu et al. 2009; Maleque, Belal, and Saupam 2007). Some studies found that after NaOH treatment of raw fibers, the load-bearing capacity of the composites increased compared to the composites made from untreated fibers (Nam et al. 2011). The surfaces of these fibers were also modified by silane treatment, and their morphological properties were evaluated (Oliveira et al. 2008). This particular chemical treatment made the natural fibers or particles rougher and the bonding of the fibers or particles much easier. After alkali treatment, the particles are often treated with a potassium permanganate (KMnO₄) solution to further increase the tensile strength of the composites. KMnO₄ contains permanganate ions (MnO₄^2-), which lead to the formation of cellulose radicals. In addition, the highly reactive Mn³⁺ ions lead to copolymerization (Annie et al. 2008).

In this work, the mechanical and physical properties of a resin containing wood particles for the production of wood for furniture applications were experimentally investigated. Bending strength, impact strength, and water absorption tests were performed, and the proper composition of the resin and hardener ratio was determined. Imported water-resistant furniture (salon tables, outdoor tables, kitchen cabinets, etc.) is expensive in Ethiopia. Promoting innovative ideas, and adapting products are undeniable ways to increase domestic production. Domestic production and use should be encouraged. In this type of approach, corporations and businesses can contribute by saving foreign exchange and promoting professional development for a larger number of workers. This research addressed the main problem of the growth of local furniture products and affordability in society.

Materials and methods

Materials

The most important and crucial element in the design process to ensure product quality is material selection. Epoxy and methyl ethyl ketone peroxide were purchased from World Glass Fiber PLC, Ethiopia, and have the chemical structure and formula shown in Figure 1. Wood particles were collected from a local furniture manufacturing company. A particle size of 0.15 mm is used.

Figure 1. Molecular structure of Epoxy and Hardener.

Manufacturing process

The sample was successfully prepared by an open-hand molding system (Figure 2). NaOH and KMnO4 were used for wood particles chemically treated with distilled water and 0.6, 0.8, and 1 g (by weight) NaOH solutions in 10 liters of distilled water for 2 hours, which cured the broken surface of the particles and improved the surface structure of the wood particles. The particles are washed with acetic acid to remove the excess base until the pH reaches 7. After alkali treatment, the particles are treated with potassium permanganate (KMnO₄). After acid treatment, the particles are washed with distilled water at least three times. Then, the particles are dried at room temperature.

Figure 2. Manufacturing of composite plate by hand lay-up method a) wood particles b) composite plate in the mold.

The combination of variables is determined based on three factors and three levels (Table 1), which are measured exactly by parts by weight. The resin is poured directly onto the base plate of the barrier container; it must cover the entire surface to the required thickness. The composites are made at room temperature; the set-up time is 5 to 9 minutes, and 24 hours are required after curing.

Table 1. Input factors and their levels.

Factors	Symbols	Level
		1	2	3
Wood particle content (Wt.%)	A	5	15	25
Epoxy/hardener ratio	B	16.7	10	6.25
NaOH concentration in distilled water for treatment of wood particles (gram)	C	0.6	0.8	1

The influence of the hardener epoxy ratio on the mechanical properties is described by various researchers (Abdel-Raheem, Halim, and Al-Khoribi 2018) due to its crosslinking ability. On this basis, three ratios were selected among six ratios. The epoxy/hardener ratios are listed in Table 2.

Table 2. Mixed designation of epoxy/hardener ratio.

	Weight personage ratio
Description	Sample S1	Sample S2	Sample S3	Sample S4	Sample S5	Sample S6
Epoxy	10	10	10	10	10	10
Hardener	0.6	0.8	1	1.2	1.4	1.6
Ratio	16.67	12.5	10	8.33	7.14	6.25

For flexural properties, water absorption, and impact strength, three tests are performed for each characteristic. In accordance with ASTM D790, specimens are prepared using a three-point setup (Figure 3) and in accordance with ASTM 570 for water absorption tests in tap water. The Izod impact test was performed using a 70 × 15 mm2 dimension cut with a circular saw. Nine tests were performed based on the Taguchi Orthogonal Array Mixture Design (L₉ (3^3) (Table 3).

Figure 3. Mechanical properties of a) flexural strength b) impact strength c).

Table 3. Taguchi orthogonal array design (L₉ (3^3)).

No. of conducted experiments	Wood particle content (Wt.%)	Epoxy/hardener ratio	NaOH concentration in distilled (g)
S1	5	16.70	0.6
S2	5	1.00	0.8
S3	5	6.25	1.0
S4	15	16.70	0.8
S5	15	1.00	1.0
S6	15	6.25	0.6
S7	25	16.70	1.0
S8	25	1.00	0.6
S9	25	6.25	0.8

The morphology of the investigated composites was studied using an SEM (Quanta FEG 250) coated with a thin layer of gold (<12 µm). The cryogenically fractured cross-sections of the samples were examined, and the images were taken at a magnification of 500 × and a voltage of 15 kV.

Results and discussion

Hardener-to-epoxy ratio analysis

Figure 4a illustrates the flexural strength of the material prepared in the epoxy/hardener ratio. The flexural strength of the composites improves from 0.6 wt% to 1.2 wt% hardener, but decreases with increasing concentration from 1.2% to 1.6%, while the elongation decreases with increasing concentration, as clearly shown in Figure 4b. The selection points for the change in epoxy/hardener ratio are 16.67, 10, and 6.25, which are used for the development of these composites.

Figure 4. Epoxy/Hardener ratio of a) flexural strength and b) elongation properties.

Probability plots of flexural and impact at a 95% CI

Figure 5 shows the plots made for each response variable of flexural and notched impact strength with a confidence interval of 95%. The scatter of the experimental data for each response variable (Table 4) was determined using the probability plots. The result data points for all measured values that fall near the fitted line indicate that the distribution of results is in the middle of the range. Nevertheless, a statistical test known as Anderson-Darling (AD) was used to confirm the assumption of normal distribution of the data. The p-value of the AD test is greater than 0.05, indicating normally distributed input data. For the results, the null hypothesis states that they follow the distribution of the data. Since the p-values of AD-test for the bending and impact results are 0.634 and 0.506, respectively, which are above the significance level of 0.05, the null hypothesis is not rejected, which explains that the collected data follow the normal distribution and are suitable for optimization and further analysis.

Figure 5. Probability plots for response.

Table 4. Mechanical properties of flexural and impact strength results.

No. of conducted experiments	Flexural Strength (MPa)	Impact Strength (J/m^2)
S1	45.46	8.03
S2	62.78	1.37
S3	57.67	9.57
S4	54.58	15.27
S5	82.36	18.43
S6	64.15	13.27
S7	36.54	6.43
S8	44.83	7.50
S9	4.46	5.50

ANOVA was used to evaluate the effects of the manufacturing process parameters on the responses for wood particle composites at a 95% CI level. From (Table 5). it can be seen that the ANOVA shows that the wood particles and the ratio of epoxy resin and hardener exert a significant influence on all the mechanical properties considered. There is no significant difference between the p-values based on probability and those based on NaOH concentration in distilled water solution, as their p-values are greater than 0.05.

Table 5. ANOVA analysis factors with considered parameters.

Source	F-Value	P-Value
Flexural result
Wood particle (Wt.%) Epoxy/Hardener ratio NaOH Concentration in distilled water	49.96 22.58 4.50	0.020 0.042 0.182
Impact result
Wood particle (Wt.%) Epoxy/Hardener ratio NaOH Concentration in distilled water	134.19 12.22 5.41	0.007 0.076 0.156

Flexural strength analysis

The average S/N ratios for all nine experimental measurements are shown in Figure 6. The flexural strength is affected by the treatment and the hardener/resin ratio. The highest flexural strength was observed at S5. The ratio of NaOH (1%) to wood content (15%) and an increase in the epoxy/hardener ratio from a high level (16.7%) to a low level (6.25%) lead to an increase in flexural strength. The optimum combination of parameters and their level in this test is A₂B₂C₃ (15% wood, 10% epoxy, hardener ratio, and 1% NaOH), i.e. 82.36 MPa. The crosslinking between the epoxy resin chain and the wood particle filler decreased, which increased the stiffness (Chowdhury et al. 2018). The optimal combination of process parameters for the average flexural strength is the percentage of the wood particle type at an intermediate level (15%), the percentage weight ratio of epoxy resin/hardener at a high level (16.7), and the concentration of NaOH at an intermediate level (0.8), i.e., A₂B₃C₂ (Figure 6a).

Figure 6. Main plot of flexural strength for (a) flexural strength means and (b) S/N ratios.

Impact analysis

Composition A2B2C3 exhibits the highest impact strength (mean wood particle content, i.e., 15%; mean epoxy/hardener weight ratio, i.e., 10, and high NaOH concentration, i.e., 1.0) as shown in Figure 7b. In Figure 7, the percentage of wood particles appears to have a greater effect on the notched impact strength than the epoxy/hardener ratio and the surface treatment.

Figure 7. Main plot of impact strength for (a) impact means strength and (b) S/N ratio.

The main effect diagram for the mean value of impact strength is shown in Figure 7a. This shows that the highest notched impact strength is observed at low (5%) to medium levels (15%) and increases with increasing NaOH concentration in the distilled water. In addition, the flexural strength increases with the increase of the epoxy/hardener ratio from a high (16.7%) to a low value (6.25%). This can be attributed to the molecular flexibility of the polymers, which plays a key role in determining the relative toughness and brittleness of the composites (Sanusi, Oyinlola, and Aindapo 2013). Other possible reasons for the mechanical properties considered include the formation of microspaces at the fiber-matrix interface that promote crack propagation; the decrease in wettability due to increased particle reinforcements leads to brittle fracture in composites with lower strength (Das et al. 2018; Islam and Islam 2015). The increase in wood particle content decreased the flexural and impact strength, as shown in Figures 6a and 7a, respectively.

Water absorption analysis

Moisture absorption should be considered in the design of composites because it is important to improve the stability of dimensional change (Wang and Morrell 2004). The moisture absorption rate of a composite was measured by soaking in distilled water. To evaluate the absorption rate, the composite sample was placed in a container of distilled water for 300 hours (Figure 8) until saturation occurred. The water absorption of the composites increased with immersion time as shown in Figure 9. This shows that the water uptake of the composites increased linearly as the wood particle loading of water increased, while the NaOH concentration in the distilled water increased, causing the water uptake to decrease. Water uptake is an important component in determining the durability of wood particle composites because moisture leads to cracking at the interfaces caused by swelling of the wood particles (Chaudemanche et al. 2018). Surface porosity and absorption by wood particles lead to water absorption. As reported in other work (Asyraf et al. 2022; Wan Badaruzzaman et al. 2022), excessive fillers tend to agglomerate, leading to the formation of voids, reducing mechanical strength, and allowing water absorption. There is a strong correlation between the density and void content of a composite and the rate of water absorption (Atiqah et al. 2017).

Figure 8. Immersing and weighting samples set up for moisture absorption test.

Figure 9. Water absorption test of designed samples.

Morphology analysis

Figure 10 shows SEM microscopic images of the samples. According to Hill et al. (1998), fibers are a natural polymer composed mainly of cellulose, hemicellulose, and lignin. These elements contribute to mechanical strength in different ways. As shown in Figure 10a, wetting of the wood particles by the polymer matrix can be seen, indicating agglomeration formation, which is due to a decrease in flexural and hardness strength. According to Obasi et al. (2021, 3) and Borsi et al. (2014), this is due to an uneven distribution of fibers in the matrix. The main defects observed in the morphology are the formation of voids, microholes (enclosed in ovals), and agglomeration (Figure 10d) of the particles.

Figure 10. SEM micrography of a) 25%, b) 15%, and c) 5% wood waste particles reinforced composite.

Conclusion

The mechanical properties such as flexural strength, impact strength, and water absorption were studied. The properties were evaluated by varying the content of wood particles, the ratio of epoxy resin and hardener, and the NaOH concentration by weight. The composite was successfully prepared using the conventional hand lay-up technique. The epoxy/hardener ratio was determined based on the flexural strength properties. Thus, the strength was increased from 0.6 to 1.2 wt% of hardener, while the amount of hardener was decreased from 1.2 to 1.6%. Among the epoxy/hardener ratios, three were selected, which are 16.7, 10, and 6.7. The flexural and impact strength of the composite was highest for composition A2B2C3 (15% wood/10% epoxy/hardener ratio/1% NaOH) based on the mean value of the S/N ratio. Moreover, the flexural strength increases with the increase of the epoxy/hardener ratio from a high level (16.7%) to a low level (6.25%). The water absorption of composites increased with the immersion time. This showed that the water absorption rate of the composites increased linearly as the wood particle loading of water increased, while the NaOH concentration in the distilled water increased and the water absorption rate decreased. The main defects observed in the morphology were the formation of voids, micro-holes, and agglomeration of particles.

Highlights

• A composite material from a biodegradable and renewable resource of wood waste particle as reinforcement with Epoxy resin was developed successfully by hand lay-up method at room temperature for resin furniture application.

• Investigated the effect of epoxy resin with hardener ratio on considerable mechanical strength, which are flexural and impact strength.

• Investigated the effect of NaOH concentration in solution for wood particle surface treatment and used to optimize the surface linkage by KMnO4 treatment.

• Based on the Taguchi Orthogonal Array Mixture Design, nine experiments are investigated. ANOVA analysis was used to discuss the significant effect of variables.

• The water absorption analysis was performed in distilled water until the saturation phase for 300 hours.

Authors Contribution Statement

Belay Taye Wondmagegnehu: Investigation, Conceptualization, Methodology, Software, Validation, Visualization, Analysis, Resources, Data carnation, Writing – original, draft. Addisalem Adefris Legesse: Formal, Writing – review & editing, Supervision.

Acknowledgments

Thank you to all my colleagues and friends of the manufacturing engineering team, who supported me in all possible forms. In particular, special thanks go to my spouse, Tigist Tekletsadik Gesese and mechanical and automotive engineering department staff, We are very grateful. We sincerely acknowledge the courtesy of the authorities of Dilla University’s Research and Dissemination office by encouraging them to accept and work at their center. Finally, above all, we extend our special thanks to the almighty God for everything of merit is due to his benevolence.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.