Basic Knowledge of Epoxy Resins and Epoxy Adhesives

　　(1) The Concept of Epoxy Resin:
　　Epoxy resin refers to a general term for polymeric compounds that contain two or more epoxy groups in their molecular chain structure; it is a type of thermosetting resin, with bisphenol A epoxy resin being a representative example.
　　(2) Characteristics of Epoxy Resins (typically referring to Bisphenol A epoxy resins)
　　1. High Bonding Strength: Among synthetic adhesives, epoxy resin adhesives rank among the highest in terms of bonding strength.
　　2. It exhibits low shrinkage upon curing; among adhesives, epoxy resin adhesives have the lowest shrinkage rate, which is one of the reasons why epoxy resin adhesives achieve high bonding strength after curing. For example:
　　Phenolic resin adhesive: 8–10%; organic silicone resin adhesive: 6–8%.
　　Polyester resin adhesive: 4–8%; Epoxy resin adhesive: 1–3%
　　After modification, the shrinkage rate of epoxy resin adhesive can be reduced to 0.1–0.3%, with a coefficient of thermal expansion of 6.0 × 10⁻⁵/°C.
　　3. Excellent Chemical Resistance: The ether groups, benzene rings, and aliphatic hydroxyl groups in the curing system are not easily eroded by acids or bases. It can be used for two years in seawater, petroleum, kerosene, 10% H₂SO₄, 10% HCl, 10% HAc, 10% NH₃, 10% H₃PO₄, and 30% Na₂CO₃; it can be immersed at room temperature for six months in 50% H₂SO₄ and 10% HNO₃; and after being immersed for one month in 10% NaOH (100°C), its performance remains unchanged.
　　4. Excellent electrical insulation: The dielectric strength of epoxy resin can exceed 35 kV/mm.
　　5. It exhibits excellent processing performance, stable product dimensions, good durability, and low water absorption.
　　While bisphenol A epoxy resins certainly have many advantages, they also come with certain drawbacks:
　　①. The operating viscosity is high, which can be somewhat inconvenient during construction.
　　②. The cured material is brittle and has low elongation.
　　③. Low peel strength.
　　④. Poor resistance to mechanical and thermal shock.
　　(3) Applications and Development of Epoxy Resins
　　1. The Development History of Epoxy Resins:
　　Epoxy resin was patented in Switzerland in 1938 by P. Castam, and the earliest epoxy adhesives were developed by Ciba in 1946. In 1949, S.O. Creentee in the United States developed epoxy coatings, while China began industrial production of epoxy resin in 1958.
　　2. Applications of Epoxy Resin:
　　① Coatings Industry: Epoxy resin is the largest‑volume application in the coatings industry, with waterborne coatings, powder coatings, and high-solids coatings being among the most widely used today. It can be extensively applied in industries such as pipelines and containers, automobiles, ships, aerospace, electronics, toys, and handicrafts.
　　②Electronics and Electrical Appliances Industry: Epoxy resin adhesives can be used for electrical insulation materials, such as sealing and potting of rectifiers and transformers; sealing and protection of electronic components; insulation and bonding of electromechanical products; sealing and bonding of batteries; and surface coating of capacitors, resistors, and inductors.
　　③ Hardware accessories, handicrafts, and sporting goods industries: Can be used on products such as signage, accessories, trademarks, hardware, rackets, fishing gear, sports equipment, and handicrafts.
　　④ Optoelectronics Industry: Can be used for the encapsulation, potting, and bonding of products such as light-emitting diodes (LEDs), digital tubes, pixel tubes, electronic display screens, and LED lighting fixtures.
　　⑤ Construction Industry: It is also widely used in fields such as roads, bridges, flooring, steel structures, building construction, wall coatings, embankments, engineering projects, and cultural relic restoration.
　　⑥ Adhesives, sealants, and composite materials: including bonding between various materials such as wind turbine blades, handicrafts, ceramics, and glass; the lamination of carbon fiber sheets; and the sealing of microelectronic materials, among others.
　　(4) Properties of Epoxy Resin Adhesive
　　1. Epoxy resin adhesives are produced by further processing or modifying the properties of epoxy resins to meet specific performance requirements. Typically, epoxy resin adhesives must be used in conjunction with a curing agent, and they can only fully cure after being thoroughly mixed. Generally, epoxy resin adhesives are referred to as “A adhesive” or the “main agent,” while the curing agent is called “B adhesive” or the “curing agent” (hardener).
　　2. The main characteristics of epoxy resin adhesive before curing include: color, viscosity, specific gravity, mixing ratio, gel time, working time, curing time, thixotropy (yield stress), hardness, and surface tension.
　　Viscosity: Refers to the internal frictional resistance generated within a colloid during flow, with its value determined by factors such as the type of substance, temperature, and concentration.
　　Gel Time: The curing of adhesive is the process of transitioning from a liquid to a solid state. The gel time is the duration from the onset of the adhesive’s reaction until the point at which the adhesive mass begins to approach a solid state; it is determined by factors such as the mixing ratio of epoxy resin adhesive and the temperature.
　　Thixotropy: This property refers to the phenomenon whereby, when a colloid is subjected to external forces (such as shaking, stirring, vibration, or ultrasonic treatment), it becomes thinner in response to the applied force; once the external factors cease, the colloid returns to its original viscosity.
　　Hardness: Refers to a material’s ability to resist indentation, scratching, and other external forces. Depending on the testing method used, there are various types of hardness, including Shore hardness, Brinell hardness, Rockwell hardness, Mohs hardness, Barcol hardness, and Vickers hardness. The numerical value of hardness is related to the type of hardness tester used. Among commonly used hardness testers, the Shore hardness tester features a simple design and is well suited for production inspection. Shore hardness testers can be divided into Type A, Type C, and Type D: Type A is used to measure soft elastomers, while Types C and D are used to measure semi‑hard and hard elastomers.
　　3. The main properties that reflect the characteristics of epoxy resin adhesives after curing include: resistivity, dielectric strength, water absorption, compressive strength, tensile (pull) strength, shear strength, peel strength, impact strength, heat deflection temperature, glass transition temperature, internal stress, chemical resistance, elongation at break, coefficient of thermal expansion, thermal conductivity, dielectric constant, weather resistance, and aging resistance.
　　Resistivity: The resistance characteristics of a material are typically described using either surface resistivity or volume resistivity. Simply put, surface resistivity is the resistance measured between two electrodes on the same surface, with the unit being ohms (Ω). By combining electrode geometry with resistance values and performing calculations, one can determine the surface resistivity per unit area—expressed as the resistance offered by 1 cm² of dielectric to leakage current, with units of Ω·m or Ω·cm. The higher the resistivity, the better the insulating performance.
　　Proof Voltage (also known as Dielectric Strength): The higher the voltage applied across the ends of a dielectric material, the greater the electric force acting on the charges within the material, making it more prone to ionization collisions and ultimately leading to dielectric breakdown. The minimum voltage required to cause dielectric breakdown is referred to as the breakdown voltage of that material. When a 1-millimeter-thick insulating material is subjected to dielectric breakdown, the voltage in kilovolts required to achieve this is called the dielectric strength of the insulating material, commonly abbreviated as “proof voltage,” with the unit being kV/mm. The insulating properties of an insulating material are closely related to temperature: the higher the temperature, the poorer the insulating performance of the material. To ensure adequate insulation strength, each type of insulating material has a specific maximum allowable operating temperature—below this temperature, the material can be used safely over the long term; once this temperature is exceeded, the material will age rapidly.
　　Water absorption: refers to a measure of the degree to which a substance absorbs water. It is defined as the percentage increase in mass when a substance is immersed in water for a specified period at a given temperature.
　　Tensile Strength: Tensile strength is the maximum tensile stress a material can withstand before it fractures when subjected to tension. It is also referred to as breaking force, breaking strength, tensile resistance, or tensile strength. The unit is MPa.
　　Shear strength: Also known as shear resistance, it refers to the maximum load that a given bonded area can withstand when the load is applied parallel to the bonding surface, with MPa being the commonly used unit.
　　Peel strength: Also known as peel resistance, it refers to the maximum failure load that a unit of width can withstand. It is a measure of a wire’s load‑bearing capacity and is expressed in kN/m.
　　Elongation: Refers to the increase in length of a colloid under tensile stress, expressed as a percentage of its original length.
　　Heat Deflection Temperature under Load: This is a measure of the heat resistance of a cured material. A specimen of the cured material is immersed in a suitable heat-transfer medium that is heated at a constant rate, and under a static bending load applied in a simply supported beam configuration, the temperature at which the specimen’s deflection reaches a specified value is recorded as the heat deflection temperature, abbreviated as HDT.
　　Glass transition temperature: The approximate midpoint of a relatively narrow temperature range during which a cured material transitions from a glassy state to an amorphous, highly elastic, or fluid state (or vice versa) is referred to as the glass transition temperature, typically denoted by Tg, and serves as an indicator of heat resistance.
　　Internal stress refers to the stress that develops within a colloid (material) due to defects, temperature changes, solvent action, and other factors, even in the absence of external forces.
　　Chemical resistance: refers to the ability to withstand acids, bases, salts, solvents, and other chemical substances.
　　Flame resistance refers to a material’s ability to resist ignition when exposed to flame or to inhibit continued combustion after being removed from the flame.
　　Weatherability: Refers to a material’s ability to withstand exposure to climatic conditions such as sunlight, extreme temperatures, and wind and rain.
　　Aging: After curing, the adhesive undergoes a series of physical or chemical changes during processing, storage, and use due to the influence of external factors such as heat, light, oxygen, water, radiation, mechanical stress, and chemical media. These changes cause polymer materials to become cross‑linked and brittle, degrade into sticky fragments, discolor and crack, develop roughness and blistering, experience surface chalking, delaminate and peel off, and gradually deteriorate in performance—eventually losing their mechanical properties and becoming unusable. This phenomenon is known as aging.
　　Dielectric Constant: Also known as permittivity, it refers to the amount of electrostatic energy that a material can store per unit volume under a given unit of electric potential gradient. The higher a colloid’s permittivity (indicating poorer quality), the more difficult it becomes to achieve complete insulation when current is flowing between two closely spaced conductors—meaning that some degree of leakage is more likely to occur. Therefore, in most cases, the lower a material’s dielectric constant, the better. Water has a dielectric constant of 70, and even a small amount of moisture can cause significant changes.
　　4. Most epoxy resins are thermosetting adhesives, and they have the following key characteristics: the higher the temperature, the faster the curing; the larger the amount mixed at one time, the faster the curing; and exothermic reactions occur during the curing process.