What Is Composite? Composite Materials and Their Properties

What is Composite?

Composite as a term refers to a material composed of two or more constituent parts. Composite materials can be briefly defined as 'materials formed when two or more constituents that differ significantly at the macroscopic scale are combined across an interface.' The constituent components that make up composite materials generally retain their properties (Rosato, 1997). Throughout history, thousands of years before modern composite materials were developed, straw-reinforced mud bricks were used in building construction. Today, composite materials serve as engineered materials for applications where conventional materials are insufficient or where property enhancement is required. Modern composite manufacturing began in the 1930s in America with the discovery of glass fiber, and glass fiber-reinforced composite materials established themselves in the global market. When evaluated from a materials science perspective, composite materials can be regarded as relatively new and advanced-technology materials. The most important characteristic of composite material is its homogeneity at the micro scale.

2. Structure of Composite Materials

In general, materials are classified into three main categories: metals, ceramics, and organic materials. Each of these three material classes has certain advantages and disadvantages. Driven by technological developments, composite materials are new materials produced by macroscopically combining two or more of these materials to gather their superior properties into a single material (Figure 1) (Kaya, 2015). Composite materials are fundamentally produced to improve one or several of the following properties of conventional materials: • Strength, • Corrosion resistance, • Thermal resistance, • Electrical conductivity, • Acoustic conductivity, • Weight, • Aesthetic appearance, • Cost. As explained above, composite material consists of a matrix primary phase and reinforcing elements dispersed within it (Hahn and Tsai, 1980). Glass, ceramics, plastics, and metals are generally used as reinforcement and base materials. The matrix structure (phase) element (material) transmits and distributes an applied force to the reinforcing phase through the interfacial bond. Thus, it holds the reinforcing phase in the intended shape and prevents damage. The mechanical properties of matrix materials are of great importance for reinforcing elements in composite materials to fulfill their load-bearing functions. The reinforcing phase controls the grain size of the matrix phase during production and resists loads by sharing them. The interfacial bond, which acts as a binder between the matrix structure and the reinforcing element, generally exhibits brittle characteristics but transmits any resultant force to the reinforcing phase without dissolution or fracture. This region is the most important area affecting the material's elastic modulus. Therefore, the durability of composite material depends on the interfacial bond being in the desired condition (Qu, 1993).

3. Classification of Composite Materials

As briefly explained, composite materials consist of different geometric components (such as fibers) that provide mechanical strength and polymeric, metal, or ceramic materials that hold these components together. Composite materials, containing many materials in their structure, can be classified in many different ways. However, the most common classification method is based on the matrix and reinforcement materials present in the structure.

3.1. Classification of Composite Materials by Matrix Material

Matrix materials can consist of metal, ceramic, or polymer materials depending on the intended use and production technique of the composite material. Metal matrix composite materials are composites in which the primary materials are various metals and metal alloys. In these composites, the second phase (reinforcement) embedded in the metal-based structure can have different geometric shapes. Metal-based materials have superior properties compared to the materials they reinforce. By combining the high elastic modulus of ceramics with the plastic deformation properties of metals, wear-resistant materials with high tensile strength can be obtained. Ceramic matrix composite materials are useful because ceramics have high temperature resistance and low weight (d= 1.5–3.0 g/cm³). Ceramic-based composite materials are generally used for parts that need to operate at high temperatures. Because they are hard and brittle, they have very low ductility and toughness, and they are susceptible to thermal shock. For this reason, they are usually reinforced with fibers. In contrast, they have very high elastic modulus and very high operating temperatures. Ceramic composites show excellent resistance to high temperatures but have a rigid and brittle structure. They also exhibit excellent electrical insulation properties (Seydibeyoğlu, 2012). Polymer matrix composite materials are mostly petrochemical-based products and are the most widely used materials today. Polymeric composites are corrosion-resistant, suitable for long-term use, easy to process, shapeable, and have high load capacity per unit mass (Vasiliev and Morozov, 2001). Polymer matrix composites are divided into two categories: thermoset and thermoplastic matrix composites. A. Thermoset matrices are more commonly used in fiber-reinforced composite manufacturing and exist in liquid form. With the addition of a curing agent, they first gel and then solidify. Thermoset resins are isotropic. In fiber-reinforced composite manufacturing, they are preferred for their generally low viscosity. Polyamides are plastics that are shapeable and have the highest thermal resistance among polymers (Itoh et al., 2002). Thermoplastic matrices are generally ductile and heat-meltable and solidify upon cooling. This gives them the ability to be reshaped. Thermoplastics can have amorphous or semi-crystalline structures. In amorphous thermoplastics, molecules are arranged in a random structure. In the crystalline region, molecules are arranged in a tightly packed ordered structure (Kaya, 2015).

3.2. Classification of Composite Materials by Reinforcement

Classification according to the reinforcement material that comprises the composite is given in Figure 2. Fiber-reinforced composites are the most common type of composite materials. In fiber-reinforced composites, glass ranks first as the reinforcement material. Plastic resins are the most commonly used type as matrix material, with polyester ranking first due to its low cost. In fiber-reinforced composite materials, the constituent materials differ at the molecular scale and can be mechanically separated from each other. The matrix (resins) can be thermoset or thermoplastic. Reinforcing fibers can take various forms such as continuous fibers, woven cloth, or chopped fibers. Each form results in different properties. The properties of a composite material depend on how the fibers extend within the composite (Vasiliev and Morozov, 2001). Particulate-reinforced composites are reinforced by particles where the dimensions of the reinforcement material determine its contribution to the composite material properties. Particles are usually effective in increasing the composite's hardness but have limited effect on strength enhancement. Although particulate fillers are commonly used to improve physical and mechanical properties, in many cases they are used solely to reduce cost. In particulate-filled materials, many different factors affect performance. Among these, particle size, size distribution, surface energy, volume fraction, homogeneous distribution, and aspect ratio influence composite properties. Layered composites represent the oldest and most widely used type of composite structure. Very high strength values are achieved through the composition of layers with different fiber orientations. They are resistant to heat and moisture. They are preferred materials because they are lightweight compared to metals while maintaining strength. Many-layered composites maintain low cost and high strength or lightness while providing wear resistance, improved appearance, and excellent thermal expansion properties. Hybrid composites may contain two or more types of reinforcing elements in the same composite structure. These types of composites are called "hybrid composites." This field is suitable for developing new types of composites. For example, kevlar is an inexpensive fiber but has low compressive strength. Graphite, on the other hand, is an expensive fiber with low toughness but good compressive strength. A hybrid composite designed using these two fibers will have better toughness than graphite composite, lower cost, and higher compressive strength than kevlar fiber composite.

4. Some Technological Properties of Composite Materials

4.1. Insulation Properties of Composite Materials In contemporary architectural understanding, insulation has become an indispensable component of the sector due to the benefits it provides to our lives, the economy, and ecological systems. Measures taken to protect buildings and their contents from the effects of noise (sound), fire, cold, and heat can be referred to as insulation. Appropriate insulation techniques protect the structure and its contents and living organisms from harmful effects while providing comfort conditions suitable for healthy use of the space.

4.1.1. Thermal insulation

The purpose of thermal insulation is to protect building components from external factors and to create health and economically optimal comfort conditions appropriate to the intended use across changing seasonal conditions, while limiting the amount of thermal energy passing through building components. Developments in construction technologies and the use of reinforced concrete as the load-bearing skeleton have led to thinner walls. Consequently, building physics issues related to heat, moisture, sound, and combustion have emerged. Heat loss through walls in buildings can amount to 40% (in multi-story buildings) or 25% (in single-story buildings). Heat always tends to move from hot to cold environments to establish equilibrium. This transfer is a heat transfer process that reduces the quality of energy in the environment. Insulation materials exhibit resistance to this heat transfer depending on their thermal conductivity coefficients and thickness. In other words, thermal insulation in the most general sense is resistance that reduces heat transfer. The materials that provide this are called "thermal insulation materials." For thermal insulation materials to meet desired performance, they must have high void ratio, low density, and low moisture content. Insulation material created with only thermal conductivity in mind is insufficient. An ideal insulation material must simultaneously fulfill the functions of sound absorption and fire retardancy. Thermal insulation materials are classified under general headings in Table 1 (Vasiliev and Morozov, 2001). An ideal insulation material should fundamentally provide energy savings. For this condition to be realized: • Low thermal conductivity coefficients, • High vapor diffusion resistance, • Non-combustible, • Provide healthy environmental conditions, • Resistant to rot and fermentation, • Must not retain moisture. However, it is very difficult for an insulation material to possess all desired conditions simultaneously. For this reason, when selecting an insulation material, the material that is appropriate and meets the maximum common requirements according to the characteristics of the environment to be insulated must be chosen (Kaya, 2015; Vasiliev and Morozov, 2001).

4.1.2. Sound insulation

Sound can be defined as a stimulus to the sense of hearing that is perceived by the ear as pressure fluctuation in air, water, or a similar elastic medium. The human ear can hear sounds between 20 Hz and 20,000 Hz. For sound to be audible, its intensity must reach a certain level. If the sound wave is in an irregular spectrum, the term noise is used. Noise is defined as unpleasant, unwanted sounds. Sound absorption (absorption and attenuation) and sound insulation are two concepts often confused with each other. In sound absorption, air particles friction within the insulation material converts part of the sound into heat energy, thereby reducing the sound's energy. However, sound absorption between two rooms cannot affect the sound insulation between them. The term sound absorption refers to how much of the sound emanating from the sound source is absorbed in the room where the source is located. If sound waves encounter a barrier with different density or elasticity from the medium through which they travel, part of the energy is reflected, some is absorbed after converting to heat energy. The remaining part completes its passage. Insulation materials that form the outer envelope of buildings act as such barriers, preventing to greater or lesser degrees the passage of noise from external sources into the interior, depending on their structure and design (Kaya, 2015). The material used for sound insulation must have a fibrous structure or be open-porous. Sound striking surfaces covered with fibrous material causes air molecules between the fibers to convert part of the sound more or less to heat energy. The magnitude of the sound absorption coefficient depends on the fibers being fine and close together, as this will positively affect vibration movements. Table 2 shows the sound absorption coefficients of some materials for comparison. In dense (non-porous), single-layer, homogeneous structural elements, sound insulation is essentially proportional to weight per unit area (kg/m²). Sound reduction is generally related to density. High-density materials demonstrate better sound reduction efficiency. As the mass of structural elements increases, it becomes more difficult for them to transmit sound through vibration. Natural fibers are generally good sound absorbers. Among natural fibers, it is possible to find a material suitable for every type of sound absorption need. Many recycled materials are also used as noise control components. Many natural materials such as cellulose waste, kenaf, flax, hemp, sheep wool, bamboo, or coconut fibers can be used as sound absorbers for room acoustics and noise barriers because they exhibit good sound absorption performance (Vasiliev and Morozov, 2001). Energy intensity will decrease along the path the sound travels. In porous materials, sound absorption is related to the material's density, thickness, the air gap behind the material, and surface structure. Dry loose cellulose fibers are suitable for high sound wave absorption. They create excellent acoustic walls. They are effective in controlling noise coming from external sources.

4.1.3. Fire insulation

Fire insulation involves the fire triangle of combustible material, oxygen, and ignition temperature. Combustion can be defined as a physical and chemical reaction. The reaction with oxygen results in the emission of light and heat. Additionally, low-energy products are released (water, carbon dioxide, and carbon monoxide). The temperature increase of a material depends on its specific heat, density, thermal conductivity, latent heat, and heat of vaporization. The objective in flame retardants is to alter the chemistry of combustion. Thus, carbonization will occur without flame propagation, the decomposition of combustible gases will be prevented, and the spread of combustion heat will be blocked. The flame retardant additive content, thickness, density, and flammability of a material are factors affecting its combustion resistance. Various test methods have been developed to measure the behavior of building and insulation materials during fire. The combustion behavior of the material is measured and classified following the conducted tests. The DIN 4102 standard in Germany and BS 476 standard in Britain are used as the basis for describing these tests and classifications. The combustion and flammability properties of insulation materials are given in Table 3 (Kaya, 2015).

5. Conclusion and Recommendations

Composite materials derived from different raw materials can be successfully used in many fields on their own. However, especially with the recent fluctuations seen in energy prices and the need for more careful energy use, certain views have emerged in society. In this context, new composite materials can be developed and made available for use in the construction and building industry. In this regard, applications in developed countries can be taken as a model. For general impressions of the Composite Fair, you can visit our Youtube page. Assistant Professor Ali İhsan Kaya School of Technical Sciences / Mehmet Akif Ersoy University

References

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