What is composite material?
Composite material, as the name implies, is a type of material that emerges when multiple materials come together and transform into a different form. To explain it in more detail, it is a material type that is formed by the combination of a structural material with a matrix material, which cannot provide strength on its own but becomes functional when combined with the other.
Therefore, composite materials can exist in many different types. For example, the material formed by the combination of steel bars and concrete is the most familiar and commonly used composite material type in various aspects of our lives.
The ones we refer to as high-tech composite materials are composed of glass, basalt, aramid, or carbon fibers and polyester, vinylester, or epoxy resin. Moreover, composite materials made of thermoplastic resin are finding their place in the market with increasing momentum.
There are three important factors that affect the properties of these materials. The first is the structural material, which we call fiber. The most commonly used types are generally glass and carbon. Additionally, there are various types such as aramid, fiber produced from a volcanic rock called basalt, and many types like polyethylene. These fibers, including carbon fibers, have many different types. For instance, carbon fibers are classified into four groups: standard modulus, intermediate modulus, high modulus, and ultra-high modulus.
In terms of resins, the basic classification is thermoset and thermoplastic. The commonly used resins are thermosets. The main ones include polyester, vinylester, epoxy, polyurethane, and acrylic.
Each of the aforementioned materials has its advantages and disadvantages. For example, polyester is the most affordable in terms of price but has the lowest mechanical properties. Vinylester is the most resistant to corrosion, thus it is preferred in a corrosive environment. Epoxy, on the other hand, is expensive in terms of price and requires more labor-intensive application, but it has the highest level of mechanical and physical properties.
Regarding applications, which involve combining resin and fiber to create composite material, there are many different options. The difference between these options is related to the cost of the application and the quality of the composite product.
The most common and simple application is hand lay-up. In this application, fabrics are soaked with resin and adhere to the previous layer with their own weight. The quality of the finished product is low, but the application is simple and inexpensive. The two most important factors that determine the quality of the finished product in this case are the excess fiber ratio compared to the resin and the amount of trapped air.
If vacuum pressure is used in the application (infusion or prepreg), the resin ratio decreases, the fiber ratio increases, and the amount of trapped air also decreases. As the pressure increases, the air ratio continues to decrease and the fiber ratio continues to increase.
For example, in a good hand lay-up, the air ratio is 10%, and the fiber-resin distribution is 50/50. In infusion, the air ratio is around 3%, and the fiber-resin distribution is about 68/32. When curing is done under 3-4 bar pressure in an autoclave oven, the air ratio drops to 1% and the fiber-resin ratio becomes 70/30. Of course, as these ratios improve, the costs increase parabolically. Therefore, the determination of the material and application needs to be based on the requirements.
