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

1. Introduction and General Information

Due to decreasing forest presence worldwide and increasing environmental awareness, as well as the demand for wood and wood-based materials, existing forest resources must be used efficiently. Therefore, research is being conducted into new products utilizing wood raw materials and processing industry waste, and wood-based composite panel products that can be used as alternatives to solid wood are being developed. Studies to create both economical and high-strength, very lightweight materials have intensified. In this way, composite materials, produced by combinations of material components, have gained considerable importance. Materials formed by combining different materials at the macro level are termed "Composite Materials" to bring together the best properties of two or more similar or different material groups or to create new properties. In other words, they can be defined as materials consisting of different kinds of materials or phases combined to obtain superior properties by compensating for each other's weaknesses (Candan, 2014; Rosato, 1997). Figure 1 shows the reinforcing and matrix structures that make up composite materials.

Figure 1. Reinforcing and matrix structures forming composite materials (Aran, 1990)

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

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

From these two material groups, the reinforcing material determines the strength and load-bearing properties of the composite material, while the matrix material prevents crack propagation during the transition to plastic deformation and delays the failure of the composite material (Aran, 1990). The low specific gravity of composite materials provides significant advantages in lightweight structures. Additionally, corrosion resistance and heat, sound and electrical insulation properties of fiber-reinforced composite materials offer considerable benefits for their respective applications (Onat, 2015). The aim of this study is to produce environmentally friendly, sustainable, recyclable, low-cost, and high-performance biocomposite materials.

2. Historical Development of Composite Materials

Although composite materials, which are now widely used in every field, have been produced for several hundred years, the earliest examples date back much further. The subject began to be addressed as an engineering discipline in the early 1940s. The first examples of multi-component materials represent the stage of interventions in material properties. From the earliest times, people have attempted to overcome material brittleness by adding plant or animal fibers to fragile materials. Adobe material is one of the best examples of this approach. In adobe production, stalks and fibers such as ivy branches and straw incorporated into clay mud increase the material's resistance during both production and use. However, evidence shows that the use of fibers in composite materials is not a new application. For example, glass fiber production dates to ancient Egypt. The production of fine glass fibers in Egypt around 1600 BC is documented from the XVIII Dynasty period. Amphoras adorned with various dark and colored glass fibers from the dynasty period confirm this knowledge. The first recorded industrial use of glass fibers dates to 1877. Patents for artificial stone plate production methods using hydraulic binders and fiber materials were issued at the beginning of the twentieth century. Two of the most widely used composite materials in everyday applications are asbestos-fiber-reinforced composites and glass-fiber-reinforced polyester composites. Cement-asbestos composites, first used in thin plate construction, remain in use today. Conversely, composites with synthetic fibers have been employed in industry since the mid-1950s. The most well-known group of these materials is "glass fiber reinforced polyester resin composite". In Turkey, this material is called "fiberglass". Since the beginning of the 1960s, fiberglass has been used in applications such as small boat construction, liquid tanks, and roof plates. "Anadol", the first domestically produced automobile in Turkey, was manufactured from this material. Synthetic resin matrix materials reinforced with glass fibers are termed "Glass Reinforced Plastic (GRP)". Today, glass reinforced plastics are produced not only with polyester, the most widely used material, but also with other thermoset and thermoplastic resins.

3. Structure of Composite Materials

Composite materials are manufactured to comprise at least one primary material and at least one reinforcing phase. The reinforcement and primary material that make up the composite show a physical association depending on the production method and the design specifications during composite production. The primary material and reinforcing element, combined through various methods, maintain their designed forms by creating a connection region called the interface between them, enabling them to fully exhibit their properties without loss.

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

Composite materials have three main components: 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 is the adhesive used. In this study, wood chips are reinforced with thermoset polyester resin. When thermoset polymers are heated, continuous solidification occurs; they cannot be reheated and hardened again. During heating, covalent crosslinking occurs. This type of bonding prevents bending and rotating movements. Their structures are hard and brittle (Ay, 2008). Two types of polyester resin are used in composite materials: isophthalic polyester, which has superior properties such as water resistance, and orthophthalic polyester. Polyester production companies for general use exist in Turkey. Advantages of polyester resins include ease of use and very low cost (USD 0.5 - 1/kg) (Arıcasoy, 2006).

3.2. Reinforcement

Reinforcement materials include aramid, carbon, graphite, boron, silicon carbide, alumina, glass and polyethylene in the form of short or continuous long fibers, constituting approximately 60% by volume of the matrix. In this study, the material to be reinforced with polyester resin is particleboard. Wood is a recyclable, renewable, and biodegradable biomaterial. Approximately 40%-45% of wood comprises cellulose, 10%-25% hemicellulose, and 18%-35% lignin. Although the chemical structure of cellulose in different natural fibers is similar, the degree of polymerization varies. The mechanical properties of a biofiber depend on its degree of polymerization (Mohanty et al. 2005). Softwoods are generally preferred in composite material production because the fibers obtained from softwood have a high aspect ratio. The regular lumen structure can be added to these advantages (Mohanty et al. 2005). In this study, some sample groups were reinforced with jute plant fibers. Jute has good antistatic and insulating properties, low thermal conductivity and moderate humidity absorption. 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 based on their characteristics (Arıcasoy, 2006). General additives used in polyester resin are as follows (Cam Elyaf, 2014):

Inhibitors:

Additives in this group slow down polyester reactivity to prevent polymerization of polyester resins before use. The most commonly used inhibitors are hydroquinone and tertiary butyl carbonate (TBC). The inhibitor is generally used in millisecond amounts relative to the resin and must be carefully balanced to avoid completely blocking resin hardening.

Promoters:

These additives react with the catalyst to accelerate the polymerization reaction. While inhibitors provide the resin with a certain shelf life, promoters accelerate hardening following catalyst addition. The balance between promoter and inhibitor is very sensitive, and excessive amounts of either additive can disrupt this balance. Major promoters include cobalt naphthenate, cobalt octoate, dimethyl aniline (DMA), and diethyl aniline (DEA).

Catalysts:

The catalyst (initiator) is not part of the chemical reaction but provides the energy necessary to initiate the process. By adding the catalyst, the balance between promoter and inhibitor is disrupted and reaction control passes to the catalyst. The catalyst releases oxygen (or forms free-radical molecules) and can be viewed as fuel for the polymerization process. The reaction initiated by catalyst addition is exothermic, meaning heat is generated as the network structure forms.

4. Material and Production Method

Particleboard panels are to be reinforced with polyester resin. The reinforcement material of the composites is wood chips. Polyester resin was selected because it is a suitable polymer for improving the physical, mechanical, and biological properties of wood chips. Wood chips obtained from various wood types were reinforced with polyester resin using VRTM and composites were produced. "Vacuum Assisted Resin Transfer Molding" was chosen as the production method. It is fundamentally similar to the "Resin Transfer Molding" method. In this method, the system is powered by vacuum. Previously prepared reinforcement materials are placed in the mold and isolated from the external environment by a vacuum bag. Pressure is provided via vacuum assistance.

Figure 6. VRTM production method (Ataş, 2014)

Current areas of application include truck body parts, automobile body panels, bus panels, spoilers, dashboards, medical devices, storage tanks, vehicle seats, chemical pumps, marine parts such as small boats, wind turbine blades, aircraft parts, armor bodies, bicycle frames and doors (Cam Elyaf, 2014).

5. Findings

The results of tests performed on experimental panels produced to determine the effect on the physical and mechanical properties of the composites are presented in Table 2.

Table 2. Average test values of composite groups

6. Conclusion and Recommendations

This study demonstrated that new-generation composites with very high-performance mechanical properties, improved physical properties, water absorption resistance, and biological factor resistance can be produced. In this way, value-added products will be created. Additionally, these new-generation composites can be used in high-performance demanding sectors such as construction and automotive. These high value-added products can be produced by small and medium-sized enterprises in the forest products industry using their process boards, composites, wood waste chips, and VRTM production facilities for different sectors. Asst. Prof. Zeki Candan Istanbul University Cerrahpaşa Forest Faculty Forest Industry Engineering Department   Mert Yıldırım Istanbul University Cerrahpaşa Forest Faculty Forest Industry Engineering Department   Eda Silan Yıldız Technical University Chemistry Metallurgy Faculty Chemical Engineering Department  

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