(a), the concept of epoxy resin:
Epoxy resin refers to the general name of polymer compounds with two or more epoxy groups in the polymer chain structure, which belongs to thermosetting resin, and the representative resin is bisphenol A epoxy resin.
(2) Characteristics of epoxy resin (usually bisphenol A epoxy resin)
1. The application value of epoxy resin alone is very low, and it needs to be used with curing agent to have practical value.
2. High adhesive strength: among synthetic adhesives, the adhesive strength of epoxy resin adhesive ranks in the forefront.
3. The curing shrinkage is small, and the shrinkage of epoxy resin adhesive is the smallest among adhesives, which is one of the reasons for the high curing adhesion of epoxy resin adhesive. For example:
Phenolic resin adhesive: 8-10%; Silicone glue: 6-8%
Polyester resin adhesive: 4-8%; Epoxy resin adhesive: 1-3%
The shrinkage of modified epoxy resin adhesive 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 easily corroded by acid and alkali. It can be used in seawater, petroleum, kerosene, 10%H2SO4, 10%HCl, 10%HAc, 10%NH3, 10%H3PO4 and 30%Na2CO3 for two years. At the same time, it was soaked in 50%H2SO4 and 10%HNO3 at room temperature for half a year. Soaked in 10%NaOH( 100℃) for one month, the properties remained unchanged.
5. Excellent electrical insulation: the breakdown voltage of epoxy resin can be greater than 35kV/mm..
6. Good process performance, stable product size, good tolerance and low water absorption.
The advantages of bisphenol A epoxy resin are good, but it also has its disadvantages:
① High working viscosity and inconvenient construction.
② The physical properties after curing are brittle and the elongation is small.
③ Low peel strength.
④ Poor resistance to mechanical shock and thermal shock.
(3) Application and development of epoxy resin
1. Development history of epoxy resin;
Epoxy resin is a Swiss patent applied by P.Castam in 1938. The earliest epoxy adhesive was developed by Ciba Company in 1946, and the epoxy coating was developed by S.O.Creentee in 1949. The industrial production of epoxy resin began in 1958.
2. The application of epoxy resin:
① coating industry: epoxy resin is most needed in the coating industry. At present, waterborne coatings, powder coatings and high solid coatings are widely used. It can be widely used in pipeline containers, automobiles, ships, aerospace, electronics, toys, handicrafts and other industries.
Electronic and electrical industry: epoxy resin glue can be used to seal and pour electrical insulation materials, such as rectifiers and transformers; Sealing protection of electronic components; Insulation treatment and bonding of mechanical and electrical products; Sealing and bonding of storage battery; Surface coating of capacitors, resistors and inductors.
(3) Hardware ornaments, handicrafts, sporting goods industry: it can be used for signs, ornaments, trademarks, hardware, rackets, fishing tackle, sporting goods, handicrafts and other products.
④ Optoelectronic industry: It can be used for packaging, potting and bonding of LED, digital tube, pixel tube, electronic display screen, 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 restoration and other industries.
⑥ Adhesives, sealants and composite materials: such as the bonding of wind turbine blades, handicrafts, ceramics, glass and other substances, the compounding of carbon fiber plates, and the sealing of microelectronic materials.
(4) Characteristics of epoxy resin adhesive
1, epoxy resin adhesive is based on the epoxy resin to re-process or modify its characteristics, so that its performance parameters meet specific requirements. Usually, epoxy resin adhesive also needs curing agent to be used, and it needs to be stirred evenly to be completely cured. Generally, epoxy resin glue is called glue A or main agent, and curing agent is called glue B or curing agent (curing agent).
2. The main characteristics of epoxy resin adhesive before curing are: color, viscosity, specific gravity, ratio, gel time, available time, curing time, thixotropy (stop flow), hardness, surface tension, etc.
Viscosity: refers to the internal friction resistance generated by colloid in flow, and its value is determined by factors such as material type, temperature and concentration.
Gel time: the curing of glue is a process from liquid to curing. The time from the beginning of the glue reaction to the critical state of the colloid is the gel time, which is determined by the mixing amount of epoxy resin glue, temperature and other factors
Thixotropic: This characteristic refers to the phenomenon when colloid is contacted by external force (shaking, stirring, vibration, ultrasonic wave, etc.). ), with the external force from thick to thin, when the external force stops, the colloid returns to its original consistency.
Hardness: refers to the resistance of materials to external forces such as stamping and scraping. According to different testing methods, there are Shore hardness, Brinell hardness, Rockwell hardness, Mohs hardness, Pasteur hardness, Vickers hardness and so on. The hardness value is related to the type of hardness tester. Among the commonly used hardness testers, the Shore hardness tester is simple in structure and suitable for production inspection. Shore hardness tester is divided into type A, type C and type D. Type A is used to measure soft colloid, and type C and type D are used to measure semi-hard and hard colloid.
Surface tension: the attraction of molecules inside the liquid makes the molecules on the surface suffer an inward force, which minimizes the surface area of the liquid and forms a force parallel to the surface, which is called surface tension. Or the mutual traction between two adjacent parts of the liquid surface per unit length, which is a manifestation of molecular force. The unit of surface tension is n/m. Surface tension is related to the nature, purity and temperature of liquid.
3. The main characteristics reflecting the curing performance of epoxy resin adhesive are: resistance, voltage resistance, water absorption, compressive strength, tensile (tensile) strength, shear strength, peeling strength, impact strength, thermal deformation temperature, glass transition temperature, internal stress, chemical resistance, elongation, shrinkage coefficient, thermal conductivity, electrical conductivity, weather resistance and aging resistance.
Resistance: Surface resistance or volume resistance is usually used to describe the resistance characteristics of materials. The surface resistance is simply the resistance measured between two electrodes on the same surface, and the unit is ω. Combining the electrode shape with the resistance value, the surface resistivity per unit area can be obtained by calculation. Volume resistance, also known as volume resistivity and volume resistivity coefficient, refers to the resistance through the thickness of the material and is an important index to characterize the electrical properties of dielectric or insulating materials. Represents the resistance of 1cm2 dielectric to leakage current, in ω? M or ω? Cm. The greater the resistivity, the better the insulation performance.
Withstand voltage: Also called compressive strength (insulation strength). The higher the voltage applied at both ends of the colloid, the greater the electric field force applied to the charge in the material, and the easier it is for ionization collision to occur, leading to colloid breakdown. The lowest voltage of insulator breakdown is called the breakdown voltage of the object. When the insulation material with a thickness of 1 mm is broken down, the voltage to be added is called the insulation compressive strength of the insulation material, which is referred to as voltage resistance for short, and the unit is kV/mm. The insulation performance of the insulation material is closely related to the temperature. The higher the temperature, the worse the insulation performance of insulating materials. In order to ensure the insulation strength, each insulation material has an appropriate maximum allowable working temperature, below which it can be safely used for a long time, and beyond which it will rapidly age.
Water absorption: refers to the measurement of the degree of water absorption of a substance. It refers to the mass percentage increased by soaking a substance in water for a certain time at a certain temperature.
Tensile strength: tensile strength is the maximum tensile stress when colloid is stretched to break. There are nominal breaking force, breaking strength, tensile strength and tensile strength. The unit is megapascals.
Shear strength: also known as shear strength, refers to the maximum load parallel to the bonding area that can be borne by unit bonding area, and the commonly used unit is MPa.
Peel strength: also known as peel strength, refers to the maximum failure load per unit width, and is an index to measure the stress capacity of the line, in kn/m. ..
Elongation: refers to the increase in the length of colloid under tensile force, expressed as a percentage of the original length.
Thermal deformation temperature under load: it is a measure of the heat resistance of cured products. It means that the sample of cured product is immersed in a suitable heat transfer medium, heated at a constant temperature, and the temperature when the bending deformation of the sample reaches a specified value is measured under the static bending load of simply supported beam, which is called thermal deformation temperature, or HDT for short.
Glass transition temperature: refers to the approximate midpoint of the narrow temperature range in which the cured product changes from glass shape to amorphous or highly elastic state or fluid state (or vice versa), which is called glass transition temperature, usually expressed as Tg, and is an index of heat resistance.
Shrinkage rate: defined as the percentage of shrinkage to the size before shrinkage, and shrinkage rate is the difference between the size before and after shrinkage.
Internal stress: refers to the stress of colloid (substance) due to defects, temperature change, solvent action and other reasons without external force.
Chemical resistance: refers to the ability to resist acids, alkalis, salts, solvents and other chemicals.
Flame resistance: refers to the ability of a material to resist combustion when it is in contact with a flame, or to hinder the continuous combustion when it leaves the flame.
Weather resistance: refers to the tolerance of materials exposed to sunlight, cold and heat, wind and rain and other climatic conditions.
Aging: Cured colloid is affected by external factors (heat, light, oxygen, water, radiation, mechanical force and chemical medium, etc.) during processing, storage and use. ), a series of physical or chemical changes occurred, which made the polymer materials brittle, cracked and sticky, discolored and cracked, rough and foamed, the surface was powdered, delaminated and peeled off, and the properties gradually deteriorated, so that the mechanical properties could not be lost.
Dielectric constant: also known as dielectric constant and permittivity. Refers to the amount of "electrostatic energy" that can be stored under the "potential gradient" of each "unit volume" object. When the "electro-permeability" of the colloid is greater (indicating that the quality is worse), when there is current in two close wires, it is more difficult to achieve the effect of complete insulation, in other words, it is easier to produce some degree of leakage. So in general, the dielectric constant of insulating materials should be as small as possible. The dielectric constant of water is 70, and less water will cause significant changes.
4. Epoxy resin adhesives are mostly thermosetting adhesives, which have the following characteristics: the higher the temperature, the faster the curing; The more the amount of one-time mixing, the faster the curing; There is exothermic phenomenon in the curing process.