How to Design the Optimal Epoxy Adhesive Formula

Epoxy adhesives are the most widely used and have the largest usage volume among adhesives. To design the optimal formula for epoxy adhesives and achieve the required bonding performance, it is necessary to understand the adhesion mechanism and the failure mechanism of the bond.

Regarding the basic principles of epoxy adhesive formula design, the following three aspects should be well managed:

一、Relationship Between Adhesive Properties and Bonding Performance

The properties of the adhesive have a decisive impact on the bonding performance and are crucial for the formula design of the adhesive. The performance of the adhesive layer and interface layer in the joint mainly depends on the structure, properties, and curing process of the adhesive, as well as the surface structure and properties of the adherend.

The properties of the adhesive discussed here refer to the properties of the cured adhesive layer and interface layer. The main properties of the adhesive that affect bonding performance include:

(1) The strength and toughness of the adhesive. The former is the ability of the adhesive to resist external forces, while the latter is the ability to reduce stress concentration and resist crack propagation. Improving the strength and toughness of the adhesive is beneficial to enhancing the bonding strength of the joint.

(2) The modulus and elongation at break of the adhesive. These two affect the stress distribution of the bonded joint. An adhesive with low modulus and high elongation at break can greatly improve the bonding strength under "linear force" conditions. However, too low modulus and too high elongation at break can often reduce the cohesive strength, which may reduce the bonding strength. For these two opposing factors, only by finding their optimal values under their joint influence can the best "linear force" bonding strength be achieved.

(3) The stability and durability of adhesives. This is its ability to resist the deterioration of adhesive performance and structural damage caused by the surrounding environment (temperature, humidity, aging, medium erosion, etc.). It plays a decisive role in improving the heat resistance, moisture and heat resistance, aging resistance, corrosion resistance, and safety and reliability of joints. Shear strength (surface stress) and peel strength (line stress) are obviously two different types of properties. The former belongs to the category of stress and is the ultimate stress (failure stress) of the material; The latter is related to the deformation energy of the adhesive and belongs to the energy category, which is the fracture energy (fracture work) of the material. So some people list peel strength as a toughness parameter. Zhongwei Yizong et al. measured the relationship between adhesive layer thickness, temperature, and testing speed with peel strength, and found that these parameters can be converted. The number of peel strength peaks in the curve is related to the number of transition points of the adhesive.

The relationship between the hardness, modulus, and bonding performance of epoxy adhesives can be divided into four regions based on their hardness: non structural adhesives, flexible adhesives, general structural adhesives, and heat-resistant adhesives.

It must be pointed out that the performance of the adhesive and the bonding performance are interrelated and mutually restrictive. Only by comprehensively considering and weighing can the optimal formula for the required epoxy adhesive be designed.

二、Main Basis for Determining the Key Performance of the Required Epoxy Adhesive

(1) Select the performance of the adhesive according to the stress state and magnitude in the joint. For "planar force," it is advisable to choose an adhesive with high cohesive strength and adhesion strength, and good toughness. For "linear force," it is advisable to choose an adhesive with good toughness, lower modulus, and higher elongation at break. When subjected to fatigue or impact loads, it is advisable to choose an adhesive with good toughness.

(2) Select the adhesive according to the properties of the adherend. Rigid and brittle materials (such as glass, ceramics, cement, stone, etc.) should use an adhesive with high strength, high hardness, and modulus, and not easily deformed. Sheet metal parts and structural parts and other tough, high-strength rigid materials, due to large load and the presence of peeling stress, impact, and fatigue stress, should use high-strength, tough structural adhesives, such as epoxy-nitrile adhesives. Soft and elastic materials (plastic films, rubber, etc.) generally do not use epoxy adhesives. A flexible epoxy adhesive can also be selected. Porous materials (foam plastic, marine damage, etc.) should use a more viscous, flexible epoxy adhesive. Materials with low polarity (polyethylene, polypropylene, fluoroplastics, etc.) should be surface activated before bonding with epoxy adhesive.

(3) Select the adhesive according to the usage temperature. The glass transition temperature Tg of the adhesive should generally be higher than the maximum usage temperature. The usage temperature of general-purpose epoxy adhesives is about -40～+80℃. When the usage temperature is higher than 150℃, heat-resistant adhesives should be used. When the usage temperature is below -70℃, a tough low-temperature resistant adhesive should be used, such as epoxy-polyurethane adhesive, epoxy-nylon adhesive, etc. Alternating cold and heat have a greater destructive effect on the joint, and a tough high and low-temperature resistant adhesive should be used, such as epoxy-nylon adhesive, etc.

(4) Select the adhesive according to other performance requirements. Such as water resistance, moisture resistance, aging resistance, corrosion resistance, dielectric properties, etc.

(5) Select the adhesive according to process requirements (curing temperature, curing speed, viscosity, curing on damp surfaces or in water, etc.). The selected adhesive often cannot meet all requirements at the same time. This requires correctly judging which properties are the main properties (key properties) of the required adhesive, and which are secondary properties. And design the adhesive formula according to the principle of ensuring the main properties and taking into account other properties.

三、Steps and Methods for Epoxy Adhesive Formula Design

First, it should be determined whether it is possible to use an epoxy adhesive based on the usage performance and allowable curing process conditions, and whether there is an advantage in terms of performance-price ratio. Then, the formula design can be carried out in the following steps.

(1) Preliminary judgment of which are the main properties and which are the secondary properties required for the epoxy adhesive.

(2) Based on the principle of ensuring the main functions and taking into account other functions, determine the preliminary formula of the adhesive (the group assembly and ratio of the adhesive) according to the relationship between the structure and properties of the component materials and the properties of the adhesive. The cost and source of component materials should also be considered.

First, select the epoxy resin curing system. Calculate the theoretical amount of resin and curing agent according to the chemical equivalent. For catalysts and accelerators, refer to empirical data.

Then, select other additives. Determine the preliminary amount (ratio) according to empirical data or trial assembly. When selecting component materials, also pay attention to the mutual influence between them.

(3) Optimize the preliminary formula according to the principle of optimal main function and appropriate other functions. Such as using orthogonal regression analysis, and with the aid of computer-aided design, finally determine the best formula after comprehensive consideration.