Development of Innovative, Environmentally Friendly and High-Performance Hybrid Biocomposite Materials

1. Introduction and General Information

As global forest resources continue to diminish and environmental awareness increases, efficient utilization of existing forest resources is imperative given the rising demand for wood and wood-based materials. Consequently, the search for new products that can utilize waste from wood-processing industries has intensified, and wood-based composite sheet products capable of replacing solid wood are being developed. Thus, efforts to create economical, higher-strength and lightweight materials have been intensified. Composite materials, formed by various combinations of components with different properties, have therefore gained considerable importance. Materials created by combining two or more materials from the same or different groups at the macro level, either to bring together their best properties or to produce new characteristics, are called "Composite Materials." Alternatively, they can be defined as materials composed of different types of materials or phases combined to obtain superior properties by compensating for each other's weaknesses (Candan, 2014; Rosato, 1997). Figure 1 shows the reinforcement and matrix structures that constitute composite materials.

Figure 1. Reinforcement and matrix structures constituting composite materials (Aran, 1990)

Table 1 lists the types of materials used in composite structures and the forms of composite structures.

Table 1. Matrix, reinforcement element and composite structure types (Aran, 1990)

Of these two material groups, the reinforcement material determines the strength and load-bearing properties of the composite material, while the matrix material plays a preventative role in crack propagation that may occur during plastic deformation and delays the failure of the composite material (Aran, 1990). The low specific weight of composite materials provides significant advantages in lightweight structures. Additionally, the corrosion resistance, thermal, sound and electrical insulation properties of fiber-reinforced composite materials offer an important advantage for their respective applications (Onat, 2015). The aim of this study is to produce environmentally friendly, sustainable, recycled and recyclable, very low-cost, high-performance composite material.

2. Historical Development of Composite Materials

Although the production of composite materials, which have widespread application in all fields, appears to be limited to the last few hundred years, their earliest examples date back to ancient times. Their consideration as an engineering subject only began in the early 1940s. The first examples of multi-component materials date to the stage when humans began to utilize naturally occurring materials through modification. Since ancient times, people have attempted to address brittleness by placing plant or animal fibers within fragile materials. One of the best examples in this regard is adobe. In adobe production, straw and fibers such as vine branches added to clay mud increase the material's strength both during production and use. On the other hand, evidence suggests that the application of fibers commonly used today in reinforcing composite materials is not new. For example, the production of glass fibers dates back to ancient Egypt. As early as 1600 BCE, the production of fine glass fibers in Egypt was known, as evidenced by the existence of amphorae from the XVIII Dynasty decorated with glass fibers in various dark and light colors. The first industrial record of glass fiber use dates to 1877. Patents for the production of artificial stone plates using hydraulic binders and fibrous material were filed in the early part of this century. Two of the fiber-reinforced composite materials that have found the most widespread use in everyday applications are asbestos-reinforced composites and glass-fiber-reinforced polyester composites. Cement and asbestos composites, first used in thin-sheet production, have maintained their importance over the years and remain in use today. Synthetic resins reinforced with fibers began to be used in industry from the mid-1950s onward. The most well-known group of this material is "glass-fiber-reinforced polyester resin composite." Known in Turkey as "fiberglass," this material has been used since the early 1960s in applications such as liquid storage tanks, roof panels, and small-scale marine vessel construction. The carport of "Anadol," Turkey's first domestically produced serial automobile, was manufactured from this material. In Turkish, the term "Glass Reinforced Plastic (GRP)" has become established for synthetic resin matrix materials reinforced with glass fibers. In the production of glass-reinforced plastics, in addition to polyester, the most commonly used material, thermoset and thermoplastic resins are employed today.

3. Structure of Composite Materials

Composite materials are manufactured to comprise at least one base material and at least one reinforcement phase. The reinforcement and base material constituting the composite display physical cohesion depending on the production method employed during composite manufacture and the designed form. The base material and reinforcement element, brought together through various methods, form an interface region between them, which acts as a bonding area, allowing them to maintain their designed forms while exhibiting their properties substantially without complete loss.

Figure 2. Schematic representation of the components of composite material (Kaya, 1995)

Composite materials have three main elements. These are: 1. Matrix, 2. Reinforcement, 3. Additives.

3.1. Matrix

The matrix forms the continuous phase as a thermoplastic or thermoset polymer material. Since particleboard is a wood composite material, the matrix of this composite material is the adhesive used. In this study, the chips were reinforced with thermoset polyester resin. Thermoset polymers undergo continuous hardening when heated and can never be reheated and hardened again. During heating, covalent cross-linking occurs. This type of bonding prevents bending and rotational movements. Their structures are rigid and brittle (Ay, 2008). There are two types of polyester resin used in composite materials. These are: orthophthalic, which is more economical, and isophthalic polyester, which has better properties such as water resistance. There are polyester production companies in Turkey for general use applications. Advantages of polyester resins are ease of use and very low cost (USD 0.5–1/kg) (Arıcasoy, 2006).

3.2. Reinforcement

Reinforcement consists of aramid, carbon, graphite, boron, silicon carbide, alumina, glass and polyethylene materials used in short or long continuous fiber form and serving as reinforcing material at approximately 60% volume ratio of the matrix. In the study, the material to be treated with polyester resin is particleboard. Wood is a biomaterial with recyclable, renewable and biodegradable characteristics. Approximately 40%-45% of wood consists of cellulose, 10%-25% of hemicellulose, and 18%-35% of lignin. Although the chemical structure of celluloses in different natural fibers is similar, the degree of polymerization varies. The mechanical properties of a biofiber are meaningfully dependent on its degree of polymerization (Mohanty et al. 2005). Softwoods are generally preferred in composite material production. The reason for this is that fibers obtained from softwood material must have a high aspect ratio. To this can be added a regular lumen structure (Mohanty et al. 2005). In some sample groups in this study, particleboard was reinforced with jute plant fibers. Jute has good antistatic and insulation properties, low thermal conductivity and reasonable moisture absorption advantages. Its fibers are 17–20 microns in diameter and 1–4 meters in length.

Figure 3. Wood chips Figure 4. Jute plant fiber Figure 5. Glass fiber

3.3. Additives

Fillers, chemicals and other additives are added to the matrix to improve properties according to their characteristics (Arıcasoy, 2006). The common additives used in relation to polyester resin are as follows (Glass Fiber, 2014):

Inhibitors:

Additives in this group are used to slow the reactivity of polyester to prevent polymerization before the polyester resin is used. The most commonly used inhibitors are hydroquinone and tertiary butyl catechol (TBC). The inhibitor is typically used at parts per million relative to the resin and must be carefully balanced to prevent complete prevention of resin hardening. Generally, additional inhibitor should not be used without consulting the resin manufacturer.

Promoters (Accelerators):

These additives react with the catalyst to accelerate the polymerization reaction. While inhibitors give the resin a certain shelf life, promoters (accelerators) speed up hardening from the moment the catalyst is added. The balance between promoter (accelerator) and inhibitor is very delicate, and excess addition of any additive can upset this balance. The main promoters (accelerators) are cobalt naphthenate, cobalt octoate, dimethyl aniline (DMA) and diethyl aniline (DEA).

Catalysts:

The catalyst (more precisely, the initiator), although not part of the chemical reaction, provides the energy necessary for the process to begin. With the addition of catalyst, the balance between promoter and inhibitor is disrupted and control of the reaction order shifts to the catalyst. The catalyst releases oxygen (or creates free-radical molecules) and can be viewed as fuel for the polymerization process. The reaction initiated by catalyst addition is exothermic, meaning heat is released during network structure formation.

4. Materials and Production Method

The material to be reinforced with polyester resin is particleboard. The reinforcement material of the particleboard is wood chips. Polyester resin was chosen because it is a suitable polymer for improving the physical, mechanical and biological properties of particleboard. Chips obtained from various wood types were mixed and reinforced with polyester resin added via the VRTM method, and composites were produced. The "Vacuum-Assisted Resin Transfer Molding" method was selected as the production method. It is essentially identical to the "Resin Transfer Molding" method. Only in this method, the system is supported by vacuum. Previously prepared raw materials (reinforcement) are placed inside a mold and isolated from the external environment with a vacuum bag. Vacuum is used to ensure resin infusion.

Figure 6. VRTM production method (Ataş, 2014)

Current applications include truck body parts, automobile body panels, bus panels, spoilers, instrument panels, medical devices, storage tanks, vehicle seats, chemical pumps, small vessel and other marine parts, wind energy turbine blades, aircraft parts, projectile bodies, bicycle frames and doors (Glass Fiber, 2014).

5. Findings

Table 2 presents the results of experiments conducted on trial plates produced within the scope of the study to determine the effects on the physical and mechanical properties of the manufactured composites.

Table 2. Average test values of composite groups

6. Conclusions and Recommendations

This study has demonstrated that next-generation composites with very high-performance mechanical properties, improved physical characteristics, water resistance and resistance to biological factors can be produced. In this way, high-value-added products can be manufactured. Furthermore, the use of these newly produced high-performance composites in sectors such as construction and automotive, which demand high performance, can be considered. Small or medium-scale enterprises in the forest industry can produce these plates, composites and high-value-added products for various sectors on an order basis from wood waste chips and shavings using the VRTM production facility they establish. In this way, substantial profits can be achieved. Assoc. Prof. Zeki Candan Istanbul University Cerrahpaşa School of Forestry Department of Forest Industry Engineering   Mert Yıldırım Istanbul University Cerrahpaşa School of Forestry Department of Forest Industry Engineering   Eda Silan Yildiz Technical University Faculty of Chemistry and Metallurgy Department of Chemical Engineering  

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