Understand the basic knowledge of epoxy resin and epoxy resin adhesive

Epoxy resin refers to the general name of polymer compounds containing two or more epoxy groups in the polymer chain structure. It belongs to thermosetting resin, and the representative resin is bisphenol A epoxy resin.

1. The application value of epoxy resin alone is very low. It needs to be used in combination with curing agent to have practical value.

2. High bonding strength: the bonding strength of epoxy resin is in the forefront among synthetic adhesives.

3. The curing shrinkage rate is small, and the shrinkage rate of epoxy resin adhesive is the smallest among adhesives, which is also one of the reasons for the high curing bonding of epoxy resin adhesive.
For example: phenolic resin glue: 8-10%; Silicone resin adhesive: 6-8%
Polyester resin glue: 4-8%; Epoxy resin adhesive: 1-3%
If the epoxy resin adhesive is modified, the shrinkage can be reduced to 0.1-0.3% and the thermal expansion coefficient is 6.0 × 10-5/℃

4. Good chemical resistance: the ether group, benzene ring and fatty hydroxyl group in the curing system are not easy to be eroded by acid and alkali. It can be used for two years in seawater, petroleum, kerosene, 10% H2SO4, 10% HCl, 10% HAC, 10% NH3, 10% H3PO4 and 30% Na2CO3; While soaking in 50% H2SO4 and 10% HNO3 for half a year; After soaking in 10% NaOH (100 ℃) for one month, the performance remains unchanged.

5. Excellent electrical insulation: the breakdown voltage of epoxy resin can be greater than 35kV / MM6. Good process performance, stable product size, good resistance and low water absorption. The advantages of bisphenol A epoxy resin are good, but it also has its disadvantages: ①. The operation viscosity is large, which is inconvenient in construction; ②. The cured material is brittle and the elongation is small. ③ . low peel strength. ④ . poor resistance to mechanical shock and thermal shock.

（三）Application and development of epoxy resin

1. Development history of epoxy resin: epoxy resin was applied for Swiss patent by P. castam in 1938, and Ciba developed the earliest epoxy adhesive in 1946. In 1949, S.O. creentee of the United States developed epoxy coating, and China began the industrial production of epoxy resin in 1958.
2. Application of epoxy resin: ① coating industry: epoxy resin has the largest demand in the coating industry. At present, water-based coatings, powder coatings and high solid content coatings are widely used. It can be widely used in pipeline container, automobile, ship, aerospace, electronics, toys, handicrafts and other industries. ② Electronic and electrical industry: epoxy resin glue can be used for sealing and pouring of electrical insulating materials, such as rectifier and transformer; Sealing protection of electronic components; Insulation treatment and bonding of mechanical and electrical products; Sealing and bonding of storage battery; The surface of capacitors, resistors and inductors is covered. ③ Hardware accessories, handicrafts, sporting goods industry: it can be used for signs, accessories, trademarks, hardware, rackets, fishing tackle, sporting goods, handicrafts and other products. ④ Photoelectric industry: it can be used for packaging, pouring and bonding of light emitting diodes (LED), digital tubes, pixel tubes, electronic display screens, LED lighting and other products. ⑤ Construction industry: it will also be widely used in roads, bridges, floors, steel structures, buildings, wall coatings, dams, engineering construction, cultural relics repair and other industries. ⑥ Adhesives, sealants and composite materials: such as the bonding between wind turbine blades, handicrafts, ceramics, glass and other substances, the composite of carbon fiber plates, the sealing of Microelectronic Materials and so on.

（四）Characteristics of epoxy resin adhesive

1. Epoxy resin adhesive reprocesses or modifies its characteristics on the basis of epoxy resin to make its performance parameters meet specific requirements. Generally, epoxy resin adhesive also needs to be matched with curing agent, and can be fully cured only after mixing evenly. Generally, epoxy resin adhesive is called a adhesive or main agent, and curing agent is called B adhesive or curing agent (hardener).
2, the main characteristics of epoxy resin before curing are: color, viscosity, specific gravity, ratio, gel time, time to use, curing time, thixotropy (flow arrest), hardness, surface tension and so on. Viscosity: refers to the internal friction resistance generated by colloid in flow. Its value is determined by material type, temperature, concentration and other factors.
Gelation time: the curing of glue is the process of conversion from liquid to curing. The time from the beginning of glue to the critical state of colloid toward solid is gel time, which is determined by the mixing quantity and temperature of epoxy resin.
Thixotropy: this characteristic refers to the phenomenon that when the colloid is touched by external force (shaking, stirring, vibration, ultrasonic, etc.), it changes from thick to thin with the action of external force, and when the action of external factors stops, the colloid returns to the original consistency.
Hardness: refers to the resistance of materials to external forces such as embossing and scratches. According to different test methods, there are shore hardness, Brinell hardness, Rockwell hardness, Mohs hardness, Barcol hardness, vichers hardness, etc. The value of hardness is related to the type of hardness tester. Among the commonly used hardness testers, the shore hardness tester has simple structure and is suitable for production inspection. The shore hardness tester can be divided into type A, type C and type D. type A is used to measure soft colloids, and types C and D are used to measure semi-hard and hard colloids.
Surface tension: the attraction of molecules inside the liquid makes the molecules on the surface under an inward force, which makes the liquid minimize its surface area and form a force parallel to the surface, which is called surface tension. In other words, it is the mutual traction force per unit length between two adjacent parts of the liquid surface, which is a manifestation of molecular force. The unit of surface tension is n / m. The surface tension is related to the nature, purity and temperature of the liquid.

3、The main characteristics reflecting the cured characteristics of epoxy resin adhesive are: resistance, voltage resistance, water absorption, compressive strength, tensile (tensile) strength, shear strength, peel strength, impact strength, thermal deformation temperature, glass transition temperature, internal stress, chemical resistance, elongation, shrinkage coefficient, thermal conductivity, electric induction rate, weather resistance, aging resistance, etc.

Resistance: surface resistance or volume resistance is usually used to describe the resistance characteristics of materials. Surface resistance is simply the resistance measured between two electrodes on the same surface, in Ω. Combining the electrode shape and resistance value, the surface resistivity per unit area can be obtained by calculation. Volume resistance, also known as volume resistivity and volume resistance coefficient, refers to the resistance value through the thickness of the material. It is an important index to characterize the electrical properties of dielectric or insulating materials. Ω is the leakage current of the dielectric, or 1cm. The greater the resistivity, the better the insulation performance.
Proof voltage: also known as withstand voltage strength (insulation strength). The higher the voltage applied at both ends of the colloid, the greater the electric field force on the charge in the material, and the more prone to ionization collision, resulting in colloid breakdown. The lowest voltage at which an insulator breaks down is called the breakdown voltage of the object. When a 1mm thick insulating material is broken down, the voltage kV to be added is called the insulation withstand voltage strength of the insulating material, which is referred to as withstand voltage for short, and the unit is kV / mm. The insulation performance of insulating materials is closely related to temperature. The higher the temperature, the worse the insulation performance of the insulating material. In order to ensure the insulation strength, each insulating material has an appropriate maximum allowable working temperature. Below this temperature, it can be used safely for a long time. If it exceeds this temperature, it will age rapidly.
Water absorption: refers to the measurement of the degree of water absorption of a substance. It refers to the mass percentage increased by soaking the substance in water for a certain time at a certain temperature.
Tensile strength: the tensile strength is the maximum tensile stress when the colloid is stretched to fracture. Also known as breaking force, breaking strength, tensile strength and tensile strength. The unit is MPa.
Shear strength: also known as shear strength, it refers to the maximum load that can bear parallel to the bonding area per unit bonding area. The commonly used unit is MPa.
Peel strength: also known as peel strength, it refers to the maximum failure load that can be borne per unit width. It is a measure of line stress capacity, and the unit is kn / m.
Elongation: refers to the increase in the length of the colloid under the action of tension, expressed as a percentage of the original length.
Heat deflection temperature under load: it refers to a measure of the heat resistance of the cured product. It is to immerse the cured product sample in a suitable heat transfer medium with constant temperature rise, and measure the temperature when the bending deformation of the sample reaches the specified value under the action of simply supported beam static bending load, which is the thermal deformation temperature, referred to as HDT.
Glass transition temperature: it refers to the approximate midpoint of the narrow temperature range of the cured product from glass form to amorphous or highly elastic state or flow state (or the opposite transformation). It is called glass transition temperature, which is usually expressed in TG and is an indicator of heat resistance.
Shrinkage ratio: defined as the percentage of the ratio between the shrinkage amount and the size before shrinkage, and the shrinkage amount is the difference between the size before and after shrinkage.
Internal stress: refers to the stress caused by defects, temperature changes, solvent action and other reasons in the colloid (material) without external force.
Chemical resistance: refers to the ability to resist acid, alkali, salt, solvent and other chemical substances.
Flame resistance: refers to the ability of a material to resist combustion when it comes into contact with a flame or to hinder continued combustion when it leaves the flame.
Weatherability: refers to the tolerance of materials exposed to sunlight, cold and hot, wind and rain and other climatic conditions.
Aging: during the processing, storage and use of solidified colloid, a series of physical or chemical changes occur due to the action of external factors (heat, light, oxygen, water, ray, mechanical force and chemical medium, etc.), resulting in crosslinking embrittlement, cracking and sticking, discoloration and cracking, rough foaming, surface pulverization, delamination and peeling, and gradual deterioration of properties of polymer materials, so that they lose their mechanical properties and cannot be used, The phenomenon of this change is called aging.
Dielectric constant: also known as permittivity and permittivity. It refers to the amount of "electrostatic energy" that can be stored by an object per "unit volume" under the "potential gradient" of each unit. The greater the "electrical permeability" of the colloid (the worse the quality), and when there is current in the two adjacent wires, the more difficult it is to achieve the effect of complete insulation, in other words, the more likely it is to produce a certain degree of leakage. Therefore, the dielectric constant of insulating materials should be as small as possible under normal circumstances. The dielectric constant of water is 70. Little water will cause significant changes.

4、Epoxy resin adhesive is mostly thermosetting adhesive, which has the following main characteristics: the higher the temperature, the faster the curing; The more the amount of one-time mixing, the faster the curing; Exothermic phenomenon during curing.