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. China has done a lot of research in this area and put forward many theories. Although there are still some deficiencies, many practical problems have been clarified and solved, which greatly promoted the development and application of epoxy adhesives. With regard to the basic principles of epoxy resin adhesive formula design, the following three aspects should be grasped.
First, the relationship between adhesive properties and adhesive properties. The properties of the adhesive have a decisive influence on the adhesive properties and are crucial for the formulation of the adhesive. The properties of the adhesive layer and the interface layer in the joint mainly depend on the structure, properties and curing history of the adhesive, and of course, the surface structure and properties of the adherend. The properties of the adhesives discussed in this section refer to the cured adhesive layer and interfacial layer properties. The adhesive properties that affect adhesive properties include:
(1) Adhesive strength and toughness. 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 growth. Increasing the strength and toughness of the adhesive helps to improve the bonding strength of the joint.
(2) Modulus and elongation at break of the adhesive. Both affect the stress distribution of glued joints. Adhesives with low modulus and high elongation at break can greatly increase the bond strength when “line stressâ€. However, if the modulus is too low and the elongation at break is too large, the cohesive strength tends to decrease, but the bonding strength decreases. For these two factors that have the opposite effect, the best "strength" of the bond can only be obtained if they find the best value under their common influence.
(3) The stability and durability of the adhesive. This is its ability to withstand the surrounding environment (temperature, humidity, aging, media erosion, etc.) to deteriorate the adhesive properties and structural damage. It plays a decisive role in improving the heat resistance, heat and moisture resistance, aging resistance, corrosion resistance, and safety and reliability of the joints. Shear strength (face stress) and peel strength (line stress) are clearly two types of properties with different properties. The former belongs to the stress category and is the ultimate stress (damage 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 someone listed peel strength as a toughness parameter. The mid-tail was measured for the thickness of the adhesive layer, temperature, and the relationship between the test speed and the peel strength. It was 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. Epoxy adhesive hardness, modulus and adhesive properties, can be divided into four areas according to the hardness: non-structural adhesives, flexible adhesives, general structural adhesives and heat-resistant adhesives.
It must be pointed out that the properties of the adhesive and the adhesive properties are interrelated and mutually restrictive. Only comprehensive consideration and comprehensive trade-offs can be used to design the best formula for the required epoxy adhesive.
Second, determine the main basis for the key properties of the required epoxy adhesive
(1) Select the adhesive performance according to the force state and size of the adhesive layer in the joint. If the "surface force," should use cohesive strength and adhesion strength, good toughness adhesive. If the "line stress" should choose good toughness, small modulus, high elongation at break adhesive. The toughness adhesive should be used when subjected to fatigue or impact loading.
(2) Select the adhesive according to the nature of the adherend. Rigid and brittle materials (such as glass, ceramics, cement, stone, etc.) are suitable for adhesives with high strength, high hardness and modulus, and not easily deformed. For tough, high-strength rigid materials such as sheet metal parts and structural parts, due to their large load-bearing stress, impact stress, and fatigue stress, it is advisable to use structural adhesives with high strength and high toughness, such as epoxy-nitrile rubber. Soft and elastic materials (plastic film, rubber, etc.) generally do not need epoxy adhesive. Flexible epoxy glue can also be used. Porous materials (foam plastics, sea damage, etc.) should be taught with a large, flexible epoxy glue. Materials with small polarity (polyethylene, polypropylene, fluorine plastics, etc.) should be surface-activated and then bonded with epoxy adhesive.
(3) Select the adhesive according to the use temperature. The glass transition temperature Tg of the adhesive should generally be greater than the maximum use temperature. The use of general-purpose epoxy adhesive temperature is about -40 ~ +80 °C. When the use temperature is higher than 150°C, heat-resistant adhesives are preferred. The use of temperature below -70 °C should be good toughness of low temperature adhesives, such as epoxy - polyurethane glue, epoxy - nylon rubber and so on. Heat and cold changes on the joint damage is greater, it is appropriate to use a good toughness resistance to high and low temperature plastic, such as epoxy - nylon rubber and so on.
(4) Select adhesive according to other performance requirements. Such as water resistance, heat resistance, aging resistance, corrosion resistance, dielectric properties.
(5) According to the process requirements (curing temperature, curing speed, education, tide surface or water curing, etc.) select the adhesive. The selected adhesive often cannot meet all requirements at the same time. This requires a correct determination of which properties are the main properties (critical properties) of the desired adhesive and which are the minor properties. And in accordance with the principle of ensuring the main performance, taking into account the other properties of adhesive formulations.
Third, epoxy adhesive formula design steps and methods. First of all, it should be judged whether the use of epoxy adhesive is possible based on the performance and allowable curing conditions, and whether there is an advantage in the cost performance ratio. Then follow the steps outlined below for recipe design.
(1) Initially determine which major properties of epoxy adhesives are needed and what are the minor properties.
(2) Based on the principle of ensuring the main functions and taking into account other functions, the initial formulation of the adhesive (adhesive combination and ratio) is determined according to the relationship between the structure and performance of the component materials and the properties of the adhesive. The cost and source of component materials should also be considered. 1 Select epoxy curing system first. Calculate the theoretical amount of resin and curing agent by chemical equivalent. Refer to empirical data for catalyst and accelerator usage. 2 Select other additives. According to empirical data or trial method, select the initial dosage (proportion). Component materials should also pay attention to the mutual influence between component materials during the selection.
(3) Optimize the preliminary formula according to the principle of optimal main function and appropriate other functions. Such as the use of orthogonal regression analysis, etc., and the use of computer-aided design, after a comprehensive balance, finalized the best formula.
It must be pointed out that the bond strength measured by the standard method is not the strength of the actual joint. This is because the bonding strength is not only related to the properties of the adhesive and the adherend, but also affected by the form and geometry of the joint, the bonding process conditions, the ambient temperature and humidity, the loading method and the speed. Actual joints and standard specimens are not exactly the same in these respects. Therefore, the strength test of the simulated parts must be performed on the actual structural parts. If necessary, the actual adhesive parts must be directly subjected to the destructive strength test.
Source: 21st Century Fine Chemicals Network