Functionalization of adhesives and application requirements analysis II

2 Spiral Locking - Universality of Adhesives

The screw lock was initially mechanically applied and later developed into a rubber-bonded form, which is a simpler form of bonding in terms of joint form and force. Spiral locks also contain adhesives that cushion, seal, etc. on a certain strength basis. With the expansion of the application of this method, adhesives with locking ability are also changing, and some people divide this change into four generations [5]. It is still not clear whether each generation is divided by time or by technology level. However, it can be seen that performance requirements such as de-VOC, high-efficiency, and general-purpose performance are continuously improving, and the most common features are universality.

(1) Low/No VOC

Volatile organic solvents (VOCs) exist in adhesives and pose a risk of fire when used. They also have negative impacts on the environment and human health. Therefore, there is an urgent need to remove them from the adhesive and achieve low/no VOC. Accordingly, water-based adhesives, hot-melt adhesives, and 100% reactive adhesives that already have this function have been applied and promoted in other aspects; and screw-locking, the use of adhesives has therefore evolved from the first generation to the second generation.

The first generation of locking adhesives are solvent-based adhesives. The dissolving resin is dissolved in a solvent, the prepared glue solution is applied on the spiral, and the lock is used to volatilize the solvent to form a locking force. This kind of glue cures slowly and has low strength. It can only be coated evenly under low viscosity, so it will take longer to produce strength. Therefore, it is limited in application and can only be used for small screws on weak components and only Minimal locking force Universal screw locking.

The second-generation locking adhesives are anaerobic adhesives. Due to the above-mentioned problems with solvent-based locking adhesives, it came into being. Anaerobic locking adhesives consist of acrylic monomers, initiators, accelerators, and other additives. Rely on the weakening of the inhibitory effect under oxygen barrier and the solidification of the catalytic free radical polymerization of the locked metal surface. Advantages of this adhesive are: single-component, solvent-free, low viscosity, easy to operate; without light and heat, it can be cured at room temperature by air; high strength, fast curing, and both can be selected according to requirements; chemical resistance performance So good. The disadvantage is that its strength is related to the material to be adhered. Due to the difference in the catalytic action of different metals, this glue can only have good effects on active surfaces such as copper and iron, and it needs to be coated with a surface treatment agent on inactive or inhibitory surfaces. To complete the lock. Inconvenient operation and low efficiency. It is mainly used for the sealing of screws and fitting surfaces and screw locking which requires strength.

(2) High efficiency

Both of the above-mentioned two generations of locking adhesives are inconvenient to operate and have low efficiency, so the adhesive will be developed to the next generation.

The third generation of locking adhesives are microcapsule adhesives, acrylic and epoxy types. Microcapsules are made by placing one of the two components in the microcapsules and mixing them into the other. This glue is pre-coated on the spiral before use, so it is also called a pre-coated locking glue. After the screw is twisted, the microcapsules rupture, and the main agent contacts with the curing agent to react and complete the locking. The advantage of this adhesive is that it can be pre-coated, which greatly improves the assembly efficiency, and is suitable for production line operations; it can be used with a wide range of applications, and both size screws can be used; there is no limitation on the adhered material. The disadvantage is that this type of glue also requires a certain amount of solvent.

(3) Versatility

Since the third generation of lock adhesive development, it has improved the efficiency for the automotive and appliance industries that use a large number of screw assemblies to meet the operational needs of the production line, but screw fastening is not limited to this. For example, bridges, towers, trains, and large buildings sometimes cause accidents due to bolt loosening and falling off; secondly, outdoor or rainy construction and after-sales service maintenance will not operate on the production line, which raises generality issues.

The 4th generation of locking adhesives is aimed at versatility and long-term preservation, and is based on the third generation microcapsule technology to improve the formation of solvent-free locking adhesives. Because its state is between liquid and solid (rubber paste), easy to operate; because it is a microcapsule type, can be stored at 60 °C for 200 days, long storage period; due to non-anaerobic type, large diameter spiral, The wide gap is also suitable for both adhesive and sealing functions; it is also non-anaerobic type, and the material to be adhered is not limited, and it can be locked for plastic screws. In a nutshell, due to the requirement of high locking force, ie, high performance, the locking glue enters the 2nd generation from the 1st generation; because of the requirement of production efficiency, it is developed from the 2nd to the 3rd generation; Requirements, but also to the fourth generation, in the first to fourth generation of the development process to complete the solvent-free conversion to a solvent-free type. However, as of the fourth generation, the development has not ended, its category has expanded, and the multifunctionality is still in progress. It is compatible with the environment and is suitable for screw recycling. It is called the 5th generation of locking adhesives.

3 Structural Bonding - Balance of Performance

Structural bonding is a complex form of bonding between stress conditions and joint types. And its performance requirements can not be the same as in the past, as long as a certain strength to meet the requirements will be successful. Due to different applications, new requirements have been put forward for structural bonding, such as rapid curing at room temperature, balance of performance and special diversification, etc. Here, the balance of performance is more important.

(1) rapid curing at room temperature

Bonding of structures, especially curing of large structural parts at room temperature, is very important. It is difficult to achieve the conditions for the heating of large parts, and it is more important to waste energy when heating. Followed by some structural parts glued to be carried out in batches, a long curing time will affect the progress of the entire process. The second-generation acrylic adhesive (SGA) meets this requirement.

The two-component SGA is composed of one component containing an acrylic monomer, an elastomer, an organic peroxide initiator and a stabilizer, and another component containing an acrylic monomer, an elastomer, a curing agent, and the like. The contact begins with free radical polymerization and can be cured within a short time (minutes to ten minutes) even at lower temperatures [6]. General elastic adhesives can also be cured at room temperature [7]. In the bonding of metal and plastic structures, a class of fast-curing adhesives has also been developed [8].

(2) Balance of performance

In the aforementioned glue for bookbinding packaging, the balance between water resistance and hydrolyzability, brittleness, flexibility, and the like has been discussed, and structural bonding also has this problem. Because the structurally bonded joints are subjected to various forces in use, not only the shear strength but also the peel strength are required; not only can they withstand static loads but also can withstand dynamic loads, that is, require a balance of properties. To this end, the adhesive must have a certain degree of elasticity, and the relative elongation should reach 25% to 50%. The elastic structure adhesives and so on have this function mainly determined by its special structure. For example, an elastic epoxy adhesive consisting of a liquid bisphenol A epoxy resin (DGEBA) and a terminal silylpropylene oxide (DMSi-PPO) has this property because of its phase separation structure after curing: DMSi - PPO is a continuous phase; DGEBA cured material is a dispersed phase, dispersed in the form of particles in the continuous phase, the cured material acts as an organic filler to meet the performance balance requirements [9]. The SGA also has a similar "island structure". The "sea" phase is an elastomer and the "island" phase is a cured acrylic polymer. This structure has toughness, shear strength, peel strength and impact strength are very good [6].

(3) Dedicated diversification

The structural adhesive itself has a high performance requirement for the adhesive, and if it is used for a specific part, there will be higher requirements, which must be assigned to its new function and become a special functional structural adhesive [10]. For a certain application, this special glue is generally not only a kind of diversification of special glue. The following is an example of the adhesion of automotive structures. 1 The use of high elasticity epoxy and spot welding with the use of spot welding (weldbonding) technology, in the body structure without major changes to improve the rigidity of the body; 2 will be dispersed acrylic polymer particles of epoxy resin as the main The pre-cured adhesive, used as a curling adhesive with temporary fixing function, has been used in body parts, hoods, rear lids, etc. It is also an important function for oil-bonded surface; 3 uses highly attenuating ring Oxygen adhesive to improve vibration attenuation energy, improve vibration characteristics; 4 using dual methacrylic acid anaerobic adhesive in the engine, gearbox assembly screw lock, with micro-capsule epoxy adhesive to remove the bolts several times; 5 The flexible polyurethane adhesive outer cover is used for the rear door, which absorbs the stress caused by the deformation during molding.


(to be continued)