Composite material is a multiphase system of solid material compounded by matrix material and reinforcing material. It gives full play to the characteristics and potential capabilities of each component material. Through the reasonable matching and synergy of each component, it presents the excellent new performance that the original single material (homogeneous material, single-phase material) does not have, so as to achieve the comprehensive requirements of certain properties of the material. The emergence of composite materials is of epoch-making significance in the history of materials development and has received great attention at home and abroad. Its rapid development is unprecedented in history, and has been widely used in various fields such as industry, agriculture, transportation, military, science and technology and people’s life. Especially in the field of aviation, aerospace and other cutting-edge technology has become an indispensable and important structural materials. No wonder some people think that the 21st century will enter the “era of composite materials”.
Thermosetting resin matrix composite material is the most studied and widely used composite material at present. It has the following characteristics: light weight, high strength, high modulus, good corrosion resistance, excellent electrical properties, a wide range of raw materials, easy processing and molding, high production efficiency, etc.. It also has material designability and some other special properties, such as vibration damping, sound dissipation, electromagnetic wave transmission, stealth, ablation resistance, etc. It has become an important material that cannot be replaced in national economy, national defense construction and science and technology development. The most used resins in thermosetting resin-based composites are still phenolic resins, unsaturated poly-tyrosine resins and epoxy resins. The performance of these three resin orders have their own characteristics: phenolic resin has higher heat resistance, good acid resistance, fast curing speed, but more brittle, high-pressure molding; unsaturated poly cheese resin has good processability and lowest price, but poor performance; epoxy resin has high bond strength and cohesive strength, excellent corrosion resistance and dielectric properties, the best overall performance, but more expensive. Therefore, in practical engineering, epoxy resin composites are mostly used in applications requiring high performance, such as structural materials, corrosion-resistant materials, electrical insulation materials and wave-transparent materials.
- Classification of epoxy resin composites
There are many varieties of epoxy resin composites (epoxy composites, also known as epoxy reinforced plastics), and their names, meanings and classification methods are not completely unified, but generally speaking, they can be classified according to the following methods.
(1) Epoxy structural composites, epoxy functional composites and epoxy functional structural composites can be classified according to their applications. Structural composites are compounded by the mechanical properties of the constituent materials, so that they can be used as structure materials under stress, and the materials can be designed and manufactured according to the stress situation, in order to achieve the best state of the material performance register ratio. Functional composite materials are compounded by other properties of the constituent materials (such as light, electricity, heat, corrosion resistance, etc.) to obtain materials with certain ideal functions. For example, epoxy resin laminate, epoxy resin electronic plastic sealant, radome, etc. It should be noted that, no matter which functional material is used, it must have the necessary mechanical properties, otherwise the good functional material is not practical. Some functional materials also have to have high strength, such as high-voltage insulator mandrel, which requires high insulation and strength, and is an insulating structural composite material.
(2) According to the molding pressure can be divided into high-pressure molding materials (molding pressure 5-30MPa), such as epoxy engineering plastics and epoxy laminated plastics; low-pressure molding materials (molding pressure <2.5MPa), such as epoxy glass fiber reinforced plastic and high-performance epoxy composite materials. FRP and high-performance composite materials are not suitable for high-pressure molding because of the large size of the parts (up to several square meters) and the shape surface is usually not flat. Otherwise, the mold cost is too high and the press tonnage is too large, thus the cost is too expensive.
- According to the characteristics of epoxy composite materials, molding methods, products and application fields, and taking care of the customary names, they can be divided into: epoxy resin engineering plastics, epoxy resin laminated plastics, epoxy resin glass fiber reinforced plastics (general-purpose epoxy resin composites) and epoxy resin structural composites.
- Composition of epoxy resin composite material
Epoxy resin composite material is composed of epoxy resin matrix, reinforcing material and interface layer of both.
(1) Epoxy resin matrix is the curing material of epoxy resin adhesive. The epoxy resin solution is composed of epoxy resin, curing agent and accelerator, modifier, diluent, coupling agent and other additives. It needs to be matched according to different use and process requirements.
(2) Reinforcing materials mostly use fibers and their fabrics, as well as particulate (powder) materials. Enhancement effect generally increases with the increase in the length-to-diameter ratio of reinforcing materials. The reinforcement effect of particulate materials is low, mostly used for functional compounding, such as Si02 powder for insulation materials, colloidal graphite for plastic bearings, etc.. However, with the current rapid development and application of nanoscale materials, it is expected that the strengthening effect and functional composite effect will be substantially improved. A large number of fiber materials used is glass fiber and its fabric. Early had used cotton fabric, due to the limitations of the source, now except for special needs have been rarely applied. Can also use chemical fibers such as polyester fiber. In the epoxy engineering plastics mostly use short glass fiber, can also use asbestos fiber, cotton fiber, etc.. In the high-performance epoxy composites mainly use carbon fiber, as well as it and aramid fiber, high-strength glass fiber hybrid fiber. In addition to using E-glass fiber and D-glass fiber in the radome, quartz fiber with better dielectric properties can also be used.
- In the process of epoxy resin and reinforcing material compounding, an interface layer is formed between the epoxy resin matrix and the reinforcing material. The structure and performance of the interfacial layer are different from both the epoxy resin matrix and the reinforcing material. The high quality performance of the interfacial layer ensures the high performance of the potential ability of the matrix and fiber and the full realization of the composite effect.
- Characteristics of epoxy resin composites
(1) Small density, high specific strength and specific modulus. The specific strength of high modulus carbon fiber epoxy composite material is 5 times of steel, 4 times of aluminum alloy and 3.2 times of drill alloy. Its specific modulus is 5.5-6 times that of steel, aluminum alloy and Chin alloy. Therefore, the weight of carbon fiber epoxy composite members can be greatly reduced with the same strength and stiffness. This is incomparable to any existing metal materials in terms of energy saving and improving the performance of the components.
(2) High fatigue strength and good breakage safety characteristics. Epoxy composites are first damaged at the weakest point under static or fatigue load, such as transverse cracks, interface delamination, delamination, fiber fracture, etc.. However, the numerous fibers and interfaces will prevent or retard the crack expansion, and the matrix will quickly redistribute the load and transfer it to the last broken fiber through the interface, so that the whole member can continue to carry the load and will not fracture immediately as a whole. Crack expansion is slow during the fatigue process and does not fracture rapidly until about 90% of the fatigue life. The overall fracture is preceded by obvious foreshadowing, so the breakage safety characteristics are good. And metal materials under fatigue load is often no obvious warning of sudden damage.
(3) good vibration damping performance. The white vibration frequency of the structure is related to the shape of the structure itself, but also proportional to the square root of the specific modulus of the material. Epoxy composite material has high specific modulus, so it also has high self-vibration frequency. The high self-oscillation frequency is not easy to cause resonance at work, which can avoid the early breakage due to resonance. At the same time, the interface between the fibers and the matrix in the composite material has the ability to absorb vibration, so it has a high vibration damping. The vibration test of light metal alloy beam and carbon fiber composite beam with the same shape and size shows that the light alloy beam needs 9s to stop vibration, while the composite beam only needs 2.5s to be stationary.
(4) Good corrosion resistance, dielectric properties, electromagnetic wave permeability and overall performance, and good heat resistance.
(5) The whole member can be molded in one time, thus reducing the number of parts, fasteners and joints, improving the force state, saving raw materials and reducing the weight of the member. The tooling used is simple, the production cycle is short, and the cost can be greatly reduced.
(6) Anisotropy and designability of material properties. This is the outstanding feature of composite materials, especially high-performance composite materials. The formulation design and layup design of composite materials can be carried out according to the load distribution and use conditions of engineering structures. Reasonably and effectively play the role and potential performance of each constituent material, meet the predetermined requirements of material performance, realize the optimal design of the component, and achieve safety, reliability, economy and reasonableness.
(7) The main disadvantages of epoxy composites are: the dispersion of material properties is large, the aging resistance is poor, the humidity and heat resistance is not very high, the lateral properties and interlayer shear strength are not good enough.
- Application of epoxy resin composites
Epoxy resin engineering plastics are mainly used for engineering plastic parts that require high strength, good impact toughness, and certain heat resistance, such as rocket tail fins, armor-piercing ammunition rests, sonar fins, etc.. Also used as functional plastics, such as electronic components of the plastic sealing materials, water-lubricated plastic bearings.
Epoxy resin laminated plastics are mainly used as insulation structural parts of electric motors and appliances, of which the amount of epoxy copper laminate is great. Epoxy resin FRP is mainly used as corrosion-resistant containers, such as storage tanks, tank cars, electrolytic tanks, acid washing towers, etc., and also used as radomes.
High-performance epoxy composite materials are mainly used as structural parts of aircraft, satellites, spacecraft, solid rocket engine shells, as well as advanced sporting goods such as rackets, ball cupping, fishing rods, rowing boats, etc..
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