1. Introduction
The technology for plywood production has remained essentially unchanged for over a century, but ongoing research aims to enhance its quality. The primary advancements in improving plywood involve adopting modern equipment and novel adhesives. On a daily basis, the furniture business utilizes adhesives with various fillers to produce plywood, which improves its physical-mechanical, technological, and operational features. Research is focused on generating adhesives with fewer emissions and using renewable resources, signaling a move toward non-formaldehyde or low-emission formaldehyde-based resin for sustainable and equivalent adhesive solutions [
1,
2]. Fillers are characterized according to numerous parameters such as size, role, dimensions, source, and morphology [
3]. Fillers are divided into organic and inorganic [
4]. The first category includes components including wood flour, starch, wheat flour, soya flour, and lignin. Inorganic fillers include nanoparticles of various chemicals such as titanium dioxide and zinc [
4] or kaolin [
5]. Inorganic fillers, such as precipitated calcium carbonate (PCC), provide improved mechanical characteristics and greater replacement ratios, contributing to increased mechanical strength in polymer composites [
6]. Organic fillers swell in water, actively absorb moisture, enhance adhesive viscosity, and minimize veneer glue dripping [
7]. Biomaterials using natural fillers, such as bagasse, eggshell, and lemon leaves, have demonstrated superior mechanical properties, including strength under tension, resistance to indentation, and mass per unit volume, making them appropriate for various uses [
8,
9]. Inorganic-organic hybrids are frequently employed as fillers for polymer composites, having better physicochemical, thermal, or mechanical qualities [
10]. Beech bark and corn cob powder were investigated as eco-friendly fillers for UF adhesives in plywood manufacture, demonstrating good benefits on mechanical characteristics and formaldehyde emissions [
11,
12]. The application of modified corn cob powder as a filler for UF resin in plywood manufacturing successfully stopped the UF adhesive from curing prematurely and greatly lowered its viscosity, resulting in increased bonding strength [
13]. Oak and birch bark were also investigated as suitable fillers, demonstrating lower formaldehyde emissions and enhanced bonding quality in plywood manufacture [
14]. Wood flour as a filler in plywood composites showed a drop in tensile strength and impact strength with an increase in filler concentration but a beneficial effect on the modulus of elasticity at lower binder content [
15]. The investigation of using various bark species as fillers for UF resin in three-layer plywood manufacturing revealed decreased tensile strength and varied formaldehyde emissions. The results showed that the type of bark filler employed had an influence on the strength metrics and formaldehyde emissions of the resulting plywood [
16,
17], supporting the practical growth of a circular economy and assisting in environmental pollution control [
18]. Life cycle assessment (LCA) studies have demonstrated that the use of organic fillers in the plastics industry can assist in minimizing emissions to the environment, highlighting the environmental benefits of employing bio-based fillers [
19,
20]. This research implies possible economic benefits in terms of increased adhesive qualities and lower emissions. On the other hand, inorganic fillers, specifically when used in large amounts, may have a greater environmental impact, especially when considering end-of-life treatment and recycling procedures [
21].
The manufacture of nuts, which are widely available in stores, generates shells, which are typically discarded, but to avoid wasting the potential of such raw materials, for example, cashew nut shells (CNS) are converted into solid briquettes [
22], cashew nut shells can also be used to generate activated charcoal [
23]. Nutshells such as pistachio shells [
24], palm nut shells [
25], and Brazil nut shells [
26] were used to produce activated carbon. As evident, scientists typically utilize products that are readily available in their specific regions. Activated carbon, generated from diverse waste materials, provides environmental benefits such as reduced material waste and greater exploitation of natural resources [
27]. Activated carbon is a carbon-based material that has been processed to be highly porous, offering a large surface area and diverse chemical functionalities [
28]. It is produced by subjecting waste biomass or fossil resources like coal to high-temperature and chemical activation [
29,
30]. Activated carbon production involves several methods, including pyrolysis activation, physical activation, chemical activation, and steam pyrolysis [
31]. Activated carbon features a rigid carbon matrix with a high surface area and diverse functional groups, enabling it to attract and bind various molecules in both gas-phase and liquid-phase applications [
28,
29]. It is employed in a wide range of applications, such as purifying drinking water, air, and gas and medical treatments for poisoning and overdoses. Additionally, it is used in industrial processes for the removal of contaminants, in air filters for environmental control, and in the food and beverage industry for decolorization, purification purposes [
32], treatment of wastewater and leachate [
31], soil detoxification from pesticide residue [
33]. Materials made from silicone-modified activated carbon demonstrated resistance to combustion [
34]. Activated carbon aerogels created from carboxymethyl cellulose can be effectively used in energy storage applications [
35].
Activated carbon is also used in wood composite technology. It has been found that the addition of activated carbon to particleboards improves their mechanical properties, indicating better bonding and increased strength. This study investigated the effect of incorporating different proportions of activated carbon (0%, 1.5%, 4.5%, 7.5%) on the properties of particleboard. The results demonstrate that density increased with higher activated carbon content while moisture content decreased, suggesting better dimensional stability and water resistance. Mechanical properties, such as internal bond strength, modulus of rupture, and modulus of elasticity, significantly improved with the addition of activated carbon, indicating enhanced bonding and increased strength. Additionally, thermal conductivity decreased as the activated carbon content increased, leading to improved insulation performance. In summary, incorporating activated carbon at a ratio of 4.5% in particleboard significantly enhances its physical, mechanical, and thermal properties [
36]. Activated carbon was utilized as a filler in epoxy resin, resulting in smoother surfaces and enhanced characteristics [
37]. The impact of activated charcoal on the curing kinetics and crosslink density of UF resin was studied using differential scanning calorimetry. The results revealed that activated charcoal accelerates the curing process of UF resin, increases the crosslink density, and reduces the activation energy. Notably, in medium-density fiberboard, the addition of activated charcoal enhanced the modulus of rupture and internal bond strength, indicating improved performance with even a small amount of activated charcoal. Additional benefits include: the activation energy of UF resin decreases with increasing concentration of activated carbon, suggesting lower curing temperatures for the resin and the crosslink density of UF resin improves significantly with activated carbon addition. Additionally, formaldehyde emissions significantly decreased by incorporating activated carbon [
38,
39].
Based on the literature review conducted, it can be stated that the conversion of hazelnut shells into activated carbon is not a widely adopted solution. However, experiments have used hazelnut shells and ground them into powder form in research on wood materials. Walnut shells and dregs were utilized as fillers in plywood, with the dregs demonstrating better characteristics and fewer formaldehyde emissions than the shell filler [
40]. Palm Kernel Meal and Palm Shell have been studied as fillers for wood adhesives. Their addition positively affected the shear strength in plywood [
41]. Hemp flour and rye flour were both investigated as fillers for urea-formaldehyde resin. Hemp flour showed promise in reducing formaldehyde emissions without compromising the mechanical properties of plywood [
42].
Global hazelnut yield was 1.0 tons per hectare in 2021, marking a 1.2% decline compared to the previous year. Despite irregular yield fluctuations, the growth trend persisted during 2016-2021, with an average annual increase of 7.0%. The growth in the hazelnut industry is driven by increasing market demand. The hazelnut market is projected to grow from USD 474.21 million in 2023 to USD 700 million by 2028, with a compound annual growth rate (CAGR) of 8.1%. Demand for hazelnuts is rising due to consumer preferences for their beneficial health properties [
43]. The unused biomass after the shelling process, known as hazelnut shells (HS), accounts for approximately 50-55% of the product weight in shells and is currently predominantly used as boiler fuel [
44].
To summarize the literature review prepared, hazelnut cultivation worldwide is increasing, leading to a higher volume of waste in the form of shells. To prevent an increase in carbon dioxide emissions into the atmosphere through the incineration of this waste and simultaneously recognize the potential of activated carbon, the aim of this study was to apply the hazelnut shells in various stages (native, chemically treated/delignified, activated carbon from hazelnut shells) as a filler in three-layer plywood technology.
4. Conclusions
This study confirmed the effective use of hazelnut shells as fillers in three-ply ply-wood production. The introduction of 10 pbw hazelnut shell flour significantly enhanced the modulus of rupture and modulus of elasticity, indicating increased strength and flexibility of the material.
Chemically modified hazelnut shell flour showed optimal results at a 5 pbw concentration, demonstrating a beneficial impact on the mechanical properties of plywood. However, higher concentrations of chemically modified flour resulted in a deterioration of strength parameters.
Carbonized flour derived from hazelnut shells, even at a 1 pbw concentration, significantly improved plywood's bending strength and thermal properties. Carbon particles effectively reduced formaldehyde emissions and could improve water resistance, indicating better dimensional stability and a reduced environmental impact.
The studies confirmed that excessive filler content negatively impacts the strength parameters of plywood. Optimizing the filler concentration is crucial for achieving the material's best mechanical and operational properties.
The use of hazelnut shells as filler promotes sustainability and waste reduction. This solution supports a circular economy, reducing carbon emissions and encouraging the use of renewable resources.
The research indicates the practical feasibility of using hazelnut shell flour in plywood production. This approach could revolutionize the wood materials industry by providing a greener and more efficient solution.
The research demonstrates that hazelnut shells can be a valuable, eco-friendly additive in plywood production. Their use improves plywood's mechanical and operational properties while reducing its environmental impact, supporting sustainability, and promoting the use of renewable resources.