Characteristics and Application Mechanism of Epoxy Adhesives

The use of adhesives by humans has been known for thousands of years. However, epoxy adhesives (epoxy adhesives or epoxy adhesives) have only been around for more than 50 years since around 1950. However, with the advancement of various adhesive theory in the middle of the 20th century, as well as the in-depth progress of basic research such as adhesive interface chemistry, adhesive rheology, and adhesive destruction mechanism, the performance, variety, and application of adhesives have progressed rapidly. development of. Epoxy resins and their curing systems are also characterized by their unique and outstanding properties (see chapters 2 and 3) and the emergence of new types of epoxy resins, new curing agents and additives, resulting in excellent performance, variety, and adaptability. A wide range of important adhesives. Due to its high bonding strength and versatility, epoxy adhesives have been known as “all-purpose adhesives” and “strongly adhesive”. Has been widely used in aviation, aerospace, automotive, machinery, construction, chemicals, light industry, electronics, electrical appliances and daily life.

Epoxy adhesives are liquid or solid adhesives composed of epoxy resins, curing agents, accelerators, modifiers, thinners, fillers, and the like. The adhesive process of epoxy adhesive is a complex physical and chemical process, including steps such as infiltration, adhesion, and curing. Finally, a cured product of a three-dimensional cross-linked structure is formed, and the adherend is integrated into a whole. Bonding properties (strength, heat resistance, corrosion resistance, impermeability, etc.) depend not only on the structure and properties of the adhesive, but also on the structure and adhesive properties of the adherend surface, as well as the design of the joint, the preparation process of the adhesive and storage. Adhesive bonding processes are closely related, and they are also restricted by the surrounding environment (stress, temperature, humidity, medium, etc.). Therefore, the application of epoxy adhesive is a systematic project. The properties of the epoxy adhesive must be compatible with the above factors affecting the bonding performance in order to obtain the best results. The use of the same formulation of epoxy adhesive to connect different objects, or using different bonding conditions, or in different use environments, its performance will be very different. Apply sufficient attention. Due to space limitations, this chapter mainly introduces the formula design, bonding process conditions, performance and application examples of the main types of epoxy adhesives.

1, epoxy resin adhesive type, characteristics, mechanism

There are many varieties of epoxy adhesives, and their classification methods and classification criteria have not been unified. Usually classified as follows.

According to the morphology of the adhesive - such as solvent-free adhesives, (organic) solvent-based adhesives, water-based adhesives (can be divided into two types of water emulsion and water-soluble), paste adhesives, film adhesives (epoxy adhesive film), etc. .

(1) Classified by curing conditions

1) Cold curing glue (without heat curing glue). Divided into:
Low-temperature curing adhesive, curing temperature <15 °C;
Room temperature curing adhesive, curing temperature 15-40 °C.
2) Heat curing adhesive. Can be divided into:
Medium temperature curing glue, curing temperature about 80-120 °C;
High temperature curing adhesive, curing temperature >150°C.

3) Other methods of curing glues, such as light-curing glues, wet-surface and water-curing glues, latent curing glues, and the like.

(2) Classification by Adhesive Strength

1) structural adhesive, shear and tensile strength, but also should have high non-uniform tear strength, so that glued joints can withstand vibration, fatigue and impact in a long time, such as loading and unloading. At the same time, it should have high heat resistance and weather resistance. Usually steel-steel shear strength at room temperature> 25MPa, tensile strength ≥ 33MPa. Uneven tear strength >40kN/m.

2) The secondary structural adhesive can withstand medium loads. The shear strength is usually 17-25MPa and the tear strength is 20-50kN/m.

3) Non-structural adhesives, ie universal adhesives. The room temperature strength is still relatively high, but as the temperature increases, the bonding strength decreases rapidly. Can only be used for areas that are not stressed.

(3) Classification by purpose

1) Universal adhesive.

2) Special adhesives. Such as high-temperature adhesive (use temperature ≥150°C), low-temperature adhesive (resistant to -50°C or lower), strain adhesive (for strain gauges), conductive adhesive (volume resistivity 10-3～10-4Ω ·cm), sealant (vacuum seal, mechanical seal), optical glue (colorless and transparent, photo-aging resistance, refractive index matching with optical parts), anti-corrosion rubber, structural adhesive and so on.

It can also be classified according to the type of curing agent, such as amine curing epoxy glue, acid anhydride curing glue and the like. Can also be divided into two-component adhesives and one-component adhesives, pure epoxy adhesives and modified epoxy adhesives (such as epoxy - nylon adhesive, epoxy - polysulfide rubber, epoxy - nitrile rubber, epoxy - Polyurethane adhesive, epoxy-phenolic adhesive, silicone epoxy adhesive, acrylic epoxy adhesive, etc.).

Epoxy adhesive features -

Epoxy adhesives have the following advantages over other types of adhesives:

(1) Epoxy resins contain a variety of polar groups and highly reactive epoxy groups, and therefore are highly reactive with many polar materials such as metals, glass, cement, wood, and plastics, especially materials with high surface activity. The adhesive strength, together with the cohesive strength of the cured epoxy, is very high, so its bonding strength is high.
(2) When the epoxy resin is cured, substantially no low molecular volatiles are generated. The volume shrinkage of the adhesive layer is small, about 1%-2%, and it is one of the lowest curing shrinkage rates among thermosetting resins. After adding the filler can be reduced to 0.2% or less. The linear expansion coefficient of the epoxy cured product is also small. Therefore, the internal stress is small, and the impact on the bonding strength is small. In addition, the creep of the epoxy cured product is small, so the dimensional stability of the adhesive layer is good.
(3) Epoxy resins, curing agents and modifiers are available in many varieties and can be formulated with reasonable and ingenious formulas to provide the necessary technical properties (eg, rapid cure, room temperature cure, low temperature cure, water cure, low viscosity , high viscosity, etc.), and have the required performance (such as high temperature, low temperature, high strength, high flexibility, anti-aging, conductive, magnetic, thermal conductivity, etc.).
(4) It has good compatibility and reactivity with various organics (monomer, resin, rubber) and inorganic substances (such as fillers, etc.), and is easy to modify by copolymerization, cross-linking, blending, filling, etc. Improve the performance of the adhesive layer.
(5) Good corrosion resistance and dielectric properties. Can resist acid, alkali, salt, solvent and other media corrosion. The volume resistivity is 1013-1016Ω·cm and the dielectric strength is 16-35kV/mm.
(6) General-purpose epoxy resins, curing agents, and additives are produced in large quantities, yields are large, and they are easy to formulate, can be contact molded, and can be applied on a large scale.

The main drawback of epoxy adhesives -

(1) When it is not toughened, the cured product is generally brittle and has poor peeling resistance, crack resistance, and impact resistance.
(2) Low adhesion to small polar materials (eg polyethylene, polypropylene, fluoroplastics, etc.). Surface activation must first be performed.

(3) Some raw materials such as reactive diluents, curing agents, etc. have varying degrees of toxicity and irritation. Designing formulas should be avoided when possible, and ventilation and protection should be strengthened during construction operations.

Adhesive destruction mechanism -

In order to design the best formula of epoxy adhesive and achieve the required bonding performance, it is necessary to understand the adhesion mechanism of epoxy adhesive and the failure mechanism of bonding. In this regard, a lot of research has been done and many theories have been put forward. Although there are still some deficiencies, many practical problems have been elucidated and solved, which has greatly promoted the development and application of epoxy adhesives.