Epoxy Resins and Epoxy Adhesives

Epoxy Resins and Epoxy Adhesives

 

1 Concept of epoxy resin

• Epoxy resin refers to the general term for polymer compounds containing two or more epoxy groups in the polymer chain structure. It is a thermosetting resin, and the representative resin is bisphenol A epoxy resin.

 

2 Characteristics of epoxy resin (usually bisphenol A epoxy resin)

• The application value of epoxy resin alone is very low, and it needs to be used with a curing agent to have practical value.
• High bonding strength: Among synthetic adhesives, epoxy resin glue has the highest bonding strength.
• The curing shrinkage is small. Among adhesives, epoxy resin glue has the smallest shrinkage, which is one of the reasons why epoxy resin glue has high curing bonding.

For example: phenolic resin glue: 8-10%; silicone resin glue: 6-8%

Polyester resin glue: 4-8%; epoxy resin glue: 1-3%

If the epoxy resin glue after modification can reduce the shrinkage rate to 0.1-0.3%, the thermal expansion coefficient is 6.0×10-5/℃

• 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 for two years in seawater, petroleum, kerosene, 10% H2SO4, 10% HCl, 10% HAc, 10% NH3, 10% H3PO4 and 30% Na2CO3; and soaked in 50% H2SO4 and 10% HNO3 at room temperature for half a year; soaked in 10% NaOH (100℃) for one month, the performance remains unchanged.

 

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. Although the advantages of bisphenol A epoxy resin are good, it also has its disadvantages:

①. The operating viscosity is high, which is somewhat inconvenient in construction

②. The cured product is brittle and has a low elongation.

③. The peel strength is low.

④. The resistance to mechanical shock and thermal shock is poor.

 

3 Application and development of epoxy resin

• 1. The development history of epoxy resin: Epoxy resin was applied for a Swiss patent by P.Castam in 1938, and the earliest epoxy adhesive was developed by Ciba in 1946. In 1949, S.O.Creentee in the United States developed epoxy coatings. my country began 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. Currently, water-based 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.

②Electronic and electrical industry: Epoxy resin glue can be used for electrical insulation materials, such as sealing and pouring of rectifiers and transformers; sealing and protection of electronic components; insulation treatment and bonding of electromechanical products; sealing and bonding of batteries; surface coating of capacitors, resistors, and inductors.

③Hardware accessories, handicrafts, and sporting goods industries: can be used for signs, accessories, trademarks, hardware, rackets, fishing gear, sporting goods, handicrafts and other products.

④Optoelectronics industry: can be used for packaging, pouring and bonding of products such as light-emitting diodes (LEDs), digital tubes, pixel tubes, electronic display screens, and LED lighting.

⑤Construction industry: it is also widely used in industries such as roads, bridges, floors, steel structures, buildings, wall coatings, dams, engineering construction, and cultural relics repair.

⑥Adhesives, sealants and composite materials: such as bonding between various materials such as wind turbine blades, handicrafts, ceramics, and glass, composite carbon fiber sheets, and sealing of microelectronic materials, etc.

 

4 Characteristics of epoxy resin glue

• Epoxy resin glue is a reprocessing or modification of the characteristics of epoxy resin to make its performance parameters meet specific requirements. Usually, epoxy resin glue also needs to be matched with a curing agent before use, and needs to be mixed evenly before it can be completely cured. Generally, epoxy resin glue is called A glue or main agent, and the curing agent is called B glue or curing agent (hardener).

 

• The main characteristics of epoxy resin glue before curing are: color, viscosity, specific gravity, ratio, gel time, usable time, curing time, thixotropy (stop flow), hardness, surface tension, etc. Viscosity: refers to the internal friction resistance generated by the colloid during flow, and its value is determined by factors such as material type, temperature, and concentration.

 

1. Gel time: The curing of glue is the process of transformation from liquid to solid. The time from the beginning of the reaction of the glue to the critical state when the colloid tends to solid is the gel time, which is determined by factors such as the mixing amount and temperature of the epoxy resin glue.
2. Thixotropy: This property refers to the phenomenon that when a colloid is touched by an external force (shaking, stirring, vibration, ultrasound, etc.), it changes from thick to thin with the action of the external force, and when the external factors stop acting, the colloid returns to its original consistency.
3. Hardness: It refers to the material's resistance to external forces such as imprinting and scratching. 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 a 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.
4. Surface tension: The attraction of the molecules inside the liquid makes the molecules on the surface act inward. This force makes the liquid minimize its surface area and form a force parallel to the surface, which is called surface tension. Or it is the mutual traction force 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. The magnitude of surface tension is related to the properties, purity and temperature of the liquid.

 

• The main characteristics reflecting the properties of epoxy resin glue after curing are: resistance, withstand voltage, water absorption, compressive strength, tensile (tension) strength, shear strength, peel strength, impact strength, thermal deformation temperature, glass transition temperature, internal stress, chemical resistance, elongation, shrinkage coefficient, thermal conductivity, dielectric constant, weather resistance, aging resistance, etc.

 

1. Resistance: Surface resistance or volume resistance is usually used to describe the resistance characteristics of materials. Surface resistance is simply the resistance value measured between two electrodes on the same surface, and the unit is Ω. The surface resistivity per unit area can be obtained by combining the electrode shape and resistance value. Volume resistance, also known as volume resistivity or volume resistivity, refers to the resistance value through the thickness of a material. It is an important indicator for characterizing the electrical properties of dielectrics or insulating materials. It indicates the resistance of 1cm2 dielectric to leakage current, in units of Ω•m or Ω•cm. The greater the resistivity, the better the insulation performance.
2. Proof voltage: also known as withstand voltage (insulation strength). The higher the voltage applied to both ends of the colloid, the greater the electric field force on the charges in the material, and the easier it is for ionization collisions to occur, causing the colloid to break down. The lowest voltage that causes the insulator to break down is called the breakdown voltage of the object. The kilovolts of voltage that need to be added to break down a 1 mm thick insulating material is called the insulation withstand voltage of the insulating material, referred to as withstand voltage, in units of 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. To ensure insulation strength, each insulating material has an appropriate maximum allowable operating temperature. Below this temperature, it can be used safely for a long time, and it will age rapidly if it exceeds this temperature.
3. Water absorption: refers to the measure of the degree of water absorption of a substance. It refers to the percentage of mass increase when a substance is immersed in water for a certain period of time at a certain temperature.
4. Tensile strength: Tensile strength is the maximum tensile stress when the colloid is stretched to break. It is also called tear force, tear strength, tensile strength, and tensile strength. The unit is MPa.
5. Shear strength: also known as shear strength, refers to the maximum load that can be borne parallel to the bonding area per unit bonding area, and the commonly used unit is MPa.
6. Peel strength: also known as peel strength, refers to the maximum destructive load that can be borne per unit width, which is a measure of the linear force bearing capacity, and the unit is kN/m.
7. Elongation: refers to the increase in the length of the colloid under tension, expressed as a percentage of the original length.
8. Heat deflection temperature under load: refers to a measure of the heat resistance of a solidified material. It is the temperature at which the bending deformation of a solidified material reaches a specified value when the solidified material sample is immersed in a suitable heat transfer medium with a constant temperature rise. Under the static bending load of a simply supported beam, the temperature is measured, which is the heat deflection temperature, referred to as HDT.
9. Glass transition temperature: refers to the approximate midpoint of the narrow temperature range in which a solidified material changes from a glassy state to an amorphous or highly elastic state or a fluid state (or vice versa), called the glass transition temperature, usually expressed as Tg, and is an indicator of heat resistance.
10. Shrinkage ration: defined as the percentage of the shrinkage to the size before shrinkage, and the shrinkage is the difference between the size before and after shrinkage.
11. Internal stress: refers to the stress generated inside a colloid (material) due to defects, temperature changes, solvent effects, etc. in the absence of external forces.
12. Chemical resistance: refers to the ability to resist acids, alkalis, salts, solvents and other chemicals.
13. Flame resistance: refers to the ability of a material to resist burning when it comes into contact with a flame or to prevent continued burning when it leaves the flame.
14. Weatherability: refers to the tolerance of a material to weather conditions such as sunlight, heat, cold, wind and rain.
15. Aging: During the processing, storage and use of the cured colloid, due to the action of external factors (heat, light, oxygen, water, radiation, mechanical force and chemical media, etc.), a series of physical or chemical changes occur, causing the polymer material to become cross-linked and brittle, cracked and sticky, discolored and cracked, rough and bubbling, powdered on the surface, delaminated and peeled, and gradually deteriorated in performance, and even lose mechanical properties and cannot be used. This phenomenon of change is called aging.
16. Dielectric constant: also known as dielectric constant and permittivity. It refers to the amount of "electrostatic energy" that can be stored per "unit volume" of an object under each unit of "potential gradient". When the "permeability" of the colloid is greater (indicating that the quality is worse), and there is current working in two adjacent wires, it is more difficult to achieve a complete insulation effect, in other words, it is easier to produce a certain degree of leakage. Therefore, the dielectric constant of the insulating material should be as small as possible under normal circumstances. The dielectric constant of water is 70, and a small amount of water will cause significant changes.

 

• Most epoxy resin glues are thermosetting glues, which have the following main characteristics: the higher the temperature, the faster the curing; the more the amount mixed at one time, the faster the curing; there is heat release during the curing process, etc.