(IV) Characteristics of Epoxy Resin Adhesive
Epoxy resin adhesive is obtained through further processing or modification of epoxy resin. It usually needs to be used in combination with a curing agent and must be mixed evenly to fully cure. The epoxy resin adhesive is commonly referred to as Adhesive A or the main agent, and the curing agent is called Adhesive B or the hardener.
Characteristics before Curing
The main characteristics before curing include color, viscosity, specific gravity, mixing ratio, gel time, pot life, curing time, thixotropy, hardness, and surface tension, etc. Among them:

• Viscosity reflects the internal frictional resistance when the colloid flows.
• Gel time is the critical time from the start of the glue's reaction until the colloid tends to solidify.
• Thixotropy refers to the phenomenon where the colloid changes from thick to thin under the action of external force and returns to its original consistency when the external force stops.
• Hardness is the material's resistance to external forces. Hardness is the ability of a material to resist external forces such as indentation and scratching. According to different test methods, there are Shore hardness, Brinell hardness, Rockwell hardness, Mohs hardness, Barcol hardness, Vickers hardness, etc. The hardness value is related to the type of hardness tester. Among 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 colloid surface tensions. Surface tension: The attractive force of the molecules inside the liquid causes the molecules on the surface to be under an inward - acting force. 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 per unit length between two adjacent parts of the liquid surface, which is a manifestation of molecular forces. The unit of surface tension is N/m. The magnitude of surface tension is related to the nature, purity, and temperature of the liquid.

Characteristics after Curing
The main characteristics after curing include resistance, breakdown voltage, water absorption rate, compressive strength, tensile strength, shear strength, peel strength, impact strength, heat distortion temperature, glass transition temperature, internal stress, chemical resistance, elongation, shrinkage coefficient, thermal conductivity, dielectric constant, weather resistance, and aging resistance, etc. Among them:

• Resistance describes the electrical resistance characteristics of the material.
• Breakdown voltage is the minimum voltage that can break down an insulator.
• Water absorption rate is a measure of the degree of water absorption of a substance.
• Tensile strength is the maximum tensile stress when the colloid is stretched until it breaks.
• Shear strength is the maximum load that can be borne per unit bonding area.
• Peel strength is the maximum failure load that can be borne per unit width.
• Elongation is the increase in the length of the colloid under the action of tensile force.
• Heat distortion temperature and glass transition temperature are indicators of heat resistance.
• Shrinkage rate is the percentage of the shrinkage amount to the size before shrinkage.
• Internal stress is the stress generated inside the colloid due to defects, temperature changes, etc.
• Chemical resistance is the ability to resist corrosion.
• Flame resistance is the ability to resist combustion.
• Weather resistance is the tolerance when exposed to climatic conditions.
• Aging is the phenomenon of the gradual deterioration of material properties.
• Dielectric constant is an indicator to measure the charge - storage capacity of a material.

Tensile strength: Tensile strength is the maximum tensile stress when the colloid is stretched until it breaks. Also known as tear - off force, tear - off strength, tensile force, and tensile strength. The unit is MPa.
Shear strength: Also known as shear resistance strength, it refers to the maximum load parallel to the bonding area that can be borne per unit bonding area. The commonly used unit is MPa.
Peel strength: Also known as peel resistance strength, it refers to the maximum failure load that can be borne per unit width and is used to measure the linear force - bearing capacity. The unit is kN/m.
Elongation: It refers to the increase in the length of the colloid under the action of tensile force, expressed as a percentage of the original length.
Chemical resistance: It refers to the ability to resist acids, alkalis, salts, solvents, and other chemical substances.
Flame resistance: It refers to the ability of a material to resist combustion when in contact with a flame or to prevent continued combustion when away from the flame.
Weatherability: It refers to the tolerance of a material when exposed to climatic conditions such as sunlight, heat and cold, wind, and rain.
Aging: After curing, during the processing, storage, and use of the colloid, due to the action of external factors (such as heat, light, oxygen, water, radiation, mechanical force, and chemical media), a series of physical or chemical changes occur. These changes cause the polymer material to cross - link and become brittle, crack and become sticky, change color, crack, become rough and blister, powder on the surface, delaminate and peel off, and its performance gradually deteriorates until it loses its mechanical properties and cannot be used. This change phenomenon is called aging.
Most epoxy resin adhesives are thermosetting adhesives, characterized by faster curing at higher temperatures, faster curing with a larger one - time mixing amount, and heat release during the curing process.