Epoxy resins and plastics

(1) Introduction to epoxy resin
Epoxy resin refers to a class of polymers containing ether groups on the macromolecular chain and epoxy groups at both ends, which is referred to as EP in English. There are many types of EP, which can be divided into bisphenol A type, bisphenol F type, bisphenol S type, alicyclic group, aliphatic group and phenolic epoxy type according to their composition. However, the most common industrial application is bisphenol A epoxy resin, the output can account for more than 90%.
Uncured EP is linear in structure and thermoplastic. However, the strength of medium and low molecular weight EP is low, and it cannot be directly used as plastic products, but can only be used as adhesives and coatings. EP used for adhesives is known as "all-purpose glue" because of its good adhesion performance. Only high molecular weight EP has the appropriate strength and can be directly processed into plastic products, but the amount used is very small. If medium and low molecular weight EP is to be used in plastic products, it must be cross-linked and cured by adding a curing agent at a certain temperature to form a body-shaped network structure, so as to have excellent performance.
The cured EP has good mechanical properties, chemical stability, electrical insulation, dimensional stability and heat resistance, etc., and is widely used in casting, pressing, and reinforcing FRP and foam products.
(2) Epoxy plastic properties
The properties of epoxy plastics depend on factors such as the type of resin, the degree of cross-linking, the type of curing agent, and the properties of the filler.
1. Mechanical properties
The mechanical properties of FRP products made of EP are very good, which is better than that of general engineering plastics. However, the strength and modulus of EP decrease with increasing temperature. Taking E-42 epoxy FRP as an example, the bending strength is 383MPa at 23°C and only 25.3MPa at 100°C.
2. Thermal performance
EP has excellent heat resistance, depending on the type and amount of resin and curing agent. For example, the heat deflection temperature of mild cross-linked EP is only 60°C, while that of highly cross-linked EP is as high as 250°C, and the heat deflection temperature of EP cured with low molecular weight PA is 90°C, while that of EP cured with acid anhydride is 200°C.
In addition, EP has a small coefficient of linear expansion and shrinkage, resulting in good dimensional stability.
3. Electrical performance
The electrical properties of EP are excellent, the volume resistivity of EP is 1014~15Ω•cm, the dielectric loss tangent is (2.58~3)×10-2, and the dielectric constant is 4.03. However, the electrical properties of EP vary depending on the type of additives and the ambient temperature, and the electrical properties of anhydride-cured EP are greatly affected by humidity.
4. Environmental performance
EP contains benzene rings and ether bonds, which has good chemical stability and can resist general acids and alkalis. Chemical resistance is dependent on the type of hardener. Amine-cured EP has poor acid resistance, while anhydride-based curing agents have good acid resistance and poor water resistance.
(3) Processing and application of epoxy plastics
There are many molding methods for epoxy plastics, mainly including pressing, injection molding, lamination and casting.
Pressing molding often adds reinforcing materials such as glass fiber, etc., and the relative density of the product is 1.8~2.
Injection molding requires raw materials to be stored for a long time with stable stability, good fluidity and long-term plasticization, and short curing time at high temperatures.
Lamination EP laminated FRP is the largest use of EP in plastics, the substrate is mainly glass cloth, asbestos, mica and paper are also used.
Casting molding is often used for molding electronic components and molding of various parts.
Epoxy plastics are mainly used in the following applications:
1. Epoxy FRP products
Large shells, such as pleasure boats, car bodies, seats, fast dining tables, engine covers, instrument panels, chemical anti-corrosion pipes, tanks, tanks, aircraft elevators, etc.
2. Injection molding and pressed products
Automobile engine parts, headlamp reflectors, brake parts, switch housings, home appliance bases, etc.
3. Casting products
Molding of various electronic and electrical components, fixing of metal parts.
4. Styrofoam products
It is not as widely used as PF and AF foam products, and is mainly used for medium and low temperature insulation materials, lightweight and high-strength sandwich materials, shockproof packaging materials, floating materials and sound-absorbing materials on aircraft.