1. Indicators of epoxy groups This is the most important characteristic index of epoxy resin, which is used to indicate the content of epoxy group in the resin molecule, and there are three main ways of expression, including  epoxy value, epoxy index and epoxy equivalent. Epoxy value is defined as the amount of epoxy groups(mol) per 100g of epoxy resin，unit is mol/100g. The definition of the epoxy value is mainly for the purpose of calculating the amount of curing agent to be added to the epoxy resin for curing. The amount of curing agent is the mass of curing agent to be added per 100g of epoxy resin cured. Epoxy index is the amount of epoxy groups(mol) per 1kg of epoxy resin, the unit is mol/kg. In terms of the International System of Measurement (SI units), the epoxy index is more appropriate than the epoxy value, which is 10 times larger than the epoxy value. Epoxy equivalent is the mass (g) of an epoxy resin containing 1 mol of epoxy groups ,the unit is g/mol. The chain segments between the epoxy groups become longer and longer as the molecular weight of the epoxy resin increases, so the epoxy equivalent of epoxy resins with a high relative molecular mass is also elevated. The physical quantity of epoxy equivalent is usually used to describe the epoxy group of epoxy resin in the United States, Japan and Europe.   2. Hydroxyl content Bisphenol A-type epoxy resin molecular chain contains a large number of secondary hydroxyl structure, the larger the polymerization degree n value, its molecular weight is also larger, the higher the hydroxyl content. It can cross-link with phenolic resins, amino resins or Poly isocyanates, and it can promote the curing reaction. Therefore, when controlling the curing process of epoxy resin paint, the hydroxyl content of epoxy resin must be determined. There are two most used methods to express the hydroxyl content. The hydroxyl value F is the amount of hydroxyl contained in 100g of epoxy resin, the unit is mol/100g. And the hydroxyl equivalent H is the mass(g)of epoxy resin containing 1mol of hydroxyl, the unit is g/mol.   3. Softening point Epoxy resin is a mixture of homologous prepolymers with different degrees of polymerization and has no fixed melting point or melting process. The softening point generally refers to the temperature at which the epoxy resin turns from hard to soft and shows a certain fluidity in the process of heating. The softening point of epoxy resin can characterize the average molecular weight size and distribution of the resin, the molecular weight of the high softening point is large, and the molecular weight of the low softening point is small. Epoxy resins can be broadly classified according to the softening point into three types. Type Softening point Degree of polymerisation Low molecular weight epoxy resin <50°C <2 Medium molecular weight epoxy resin 50~95°C 2~5 High molecular weight epoxy resin >100°C >5     4. Viscosity The viscosity of epoxy resins affects the fluidity and workability of resins and coatings. Viscosity increases as the average molecular weight of the epoxy resin increases and decreases as the molecular weight distribution decreases. The viscosity of epoxy resins is extremely sensitive to temperature and decreases rapidly with increasing temperature, so it is generally expressed as the viscosity at a specific temperature.   5. Chlorine value The amount of chlorine contained in an epoxy resin (including organic chlorine and inorganic chlorine) is called the chlorine value. The chlorine in the epoxy resin is divided into organic chlorine and inorganic chlorine by the form of its existence. Organic chlorine comes from the residue of insufficient ring closure in the manufacture of the epoxy resin, which is called easily hydrolysable chlorine. Inorganic chlorine comes from the residual sodium chloride that is not washed sufficiently when manufacturing epoxy resins. Organic chlorine measures the resin reaction, and inorganic chlorine measures the level of post-production treatment processes for epoxy resins. Both are detrimental to the electrical properties of the cured substance and to corrosion resistance.    

Definition of epoxy resin Epoxy resin refers to the molecular structure of the molecular structure contains two or more epoxy groups and in the appropriate chemical reagents under the action of the compound can form a three-dimensional mesh curing material. Epoxy resin is an important class of thermosetting resins. Epoxy resins include both epoxy oligomers and low molecular compounds containing epoxy groups. Epoxy resins are widely used in the fields of water conservancy, transport, machinery, electronics, home appliances, automotive and aerospace as the resin matrix for adhesives, coatings and composites.   Characteristics of epoxy resins and their curing compounds 1. High mechanical properties. Epoxy resin has strong cohesion, dense molecular structure, so its mechanical properties are higher than phenolic resin and unsaturated polyester and other general-purpose thermosetting resin. 2. Strong adhesion. Epoxy resin curing system contains very active epoxy group, hydroxyl group and ether bond, amine bond, ester bond and other polar groups. So epoxy cured products have excellent adhesion to polar substrates such as metal, ceramics, glass, concrete and wood. 3. Curing shrinkage is small. Generally its shrinkage is 1% to 2%. It is one of the smallest varieties of curing shrinkage in thermosetting resins (phenolic resins for 8% to 10%, unsaturated polycool resins for 4% to 6%, silicone resins for 4% to 8%.) The coefficient of linear expansion is also very small, generally 6*10-5/°C, so there is little change in volume after curing. 4. Good processability. Epoxy resin curing basically does not produce low molecular volatiles, so it can be low-pressure molding or contact pressure molding. It can cooperate with various curing agents to manufacture solvent-free, high solid, powder coatings and water-based coatings and other environmentally friendly coatings. 5. Excellent electrical insulation is excellent. Epoxy resin is one of the best varieties of thermosetting grease intermediary electrical properties.   6. Good stability and excellent resistance to chemicals. Epoxy resin without alkali, salt and other impurities is not easy to deteriorate. If it is stored properly (sealed, not moisture, not meet high temperature), its storage period can reach 1 year. If the test is qualified after the period, it can still be used. Epoxy curing material has excellent chemical stability. Its resistance to alkali acid, salt and other media corrosion performance is better than unsaturated polyester resins, phenolic resins and other thermosetting resins. Therefore, epoxy resin is used as anti-corrosion primer. Because the epoxy resin cured material is three-dimensional mesh structure, and it can resist the impregnation of oil and so on, so it is used in a large number of tanks, tankers, aircraft, the overall fuel tank lining and so on.   Disadvantages of epoxy resin Epoxy resin also has some disadvantages, such as poor weather resistance. Epoxy resin generally contains aromatic ether bond, its cured material is easy to be degraded after sunlight irradiation to break the chain, so the usual bisphenol A-type epoxy resin cured material is easy to lose luster in outdoor sunlight and gradually chalking, so it is not suitable to be used as outdoor topcoat. In addition, epoxy resin low temperature curing performance is poor, generally need to be cured at 10 ° C or more. Below 10°C, the curing is slow, which is very inconvenient for large objects such as ships, bridges, harbours, oil tanks and other cold season construction.   History of epoxy resin development Epoxy resin research began in the 1930s, in 1934 Germany I.G. Farben company's P. Schlack found that amines can react with epoxy groups to polymerise polymers to produce low shrinkage plastics, which was awarded a German patent. Later, Switzerland Gebr. de Trey Pierre Castan and the United States Devoe & Raynolds S.O. Greelee, they use bisphenol A and epichlorohydrin by polycondensation reaction to produce epoxy resin, with organic polyamines or phthalic anhydride can make the resin curing, the cured material has excellent adhesive properties. Soon, Switzerland's Ciba, the United States of Shell and Dow Chemical Company have begun the industrial production of epoxy resins and application development research. Into the 1950s, the production and application of ordinary bisphenol A epoxy resin at the same time, some new epoxy resins have come out. 1960 years ago, the emergence of thermoplastic phenolic epoxy resin, halogenated epoxy resin, polyolefin epoxy resins.  The development of epoxy resins in China started in 1956, and the first successes were obtained in Shenyang and Shanghai, and the industrial production started in 1958 in Shanghai and Wuxi. In the mid-1960s, some new alicyclic epoxy began to study, including phenolic epoxy resin, polybutadiene epoxy resin, glycidyl ester epoxy resin, glycidyl amine epoxy resin, etc. By the end of the 1970s, China has formed a complete industrial system from the monomer resins, auxiliary materials, from scientific research and production to application.