Analysis of functionalization and application requ

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Analysis of the functionalization and application requirements of adhesives II

2 screw locking - versatility of adhesives

screw locking initially used mechanical methods, and later developed to use glue locking. From the joint form and stress situation, this is a relatively simple kind of bonding. Screw locking also includes the functions of cushioning and sealing on the basis of a certain strength of the adhesive. With the expansion of the application of this method, the adhesives with locking ability are also changing, and some people divide this change into four generations [5]. Are the generations divided by time, or by technical level? It is not known yet, but it can be seen from it that the performance requirements of VOC removal, high efficiency and versatility are constantly improving, among which the versatility is the most prominent

(1) low/no VOC

volatile organic solvents (VOC) exist in adhesives, which have the risk of fire when used, and have adverse effects on the environment and human health. Therefore, it is urgent to remove them from adhesives to achieve low/no VOC. Accordingly, water-based adhesives, hot melt adhesives and 100% reactive adhesives with this function have been applied and promoted in other aspects; The glue used for screw locking has therefore developed from the first generation to the second generation

the first generation of locking adhesive is solvent based adhesive. It is to dissolve the soluble resin in the solvent, apply the prepared glue on the screw, and form the locking force by volatilizing the solvent after locking. This kind of adhesive has slow curing and low strength. It can only be evenly coated under the condition of low viscosity, so the time to produce strength will be longer. Therefore, its application is limited, and it can only be used for the locking of small screws on weak current components and general screws requiring only minimal locking force

the second generation of locking adhesive is anaerobic adhesive. Due to the above problems of solvent based locking adhesive, it came into being because of its convenient experimental operation. Anaerobic locking adhesive is composed of acrylic monomers, initiators, accelerators and other additives. Curing depends on the weakening of polymerization inhibition under oxygen isolation and the catalytic free radical polymerization on the locked metal surface. The advantages of this adhesive are: single component, solvent-free, low viscosity, easy operation; Without light and heat, it can be cured at room temperature by isolating air; High strength, fast curing, and both can be selected according to requirements; Good chemical resistance, etc. The disadvantage is that its strength is related to the bonded material. Due to the difference of catalytic action of different metals, this adhesive can only have a good effect on active surfaces such as copper and iron, and it needs to be coated with surface treatment agent to complete the locking on inactive or inhibitory surfaces. And the operation is inconvenient and inefficient. It is mainly used for sealing screws and mating surfaces and screw locking requiring strength

(2) high efficiency

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

the third generation of locking adhesive is microencapsulated adhesive, which has two types: acrylic acid and epoxy. Microencapsulated locking adhesive is to place one of the two components in the microencapsulation, and then mix it with the other component. This glue is pre coated on the screw before use, so it is also called pre coated locking glue. After the screw is tightened, the microcapsule breaks, and the main agent reacts with the curing agent to complete the locking. The advantages of this adhesive are: it can be precoated, which greatly improves the assembly efficiency and is suitable for production line operation; Wide range of applications, large and small screws can be used; There is no limit to the bonded material. The disadvantage is that certain solvents are also required for this kind of glue coating

(3) versatility

after the development of the third generation, the locking adhesive has improved the efficiency and met the operation needs of the production line for the automotive and household appliance industries that use a lot of screws, but screw fastening is not limited to this. For example, bridges, iron towers, trains and large buildings sometimes cause accidents due to bolts loosening and falling off; Secondly, construction outdoors or in the rain and after-sales service maintenance will not be operated on the production line, which raises the problem of universality

the 4th generation locking adhesive is a solvent-free locking adhesive based on the 3rd generation microcapsule technology, aiming at versatility and long-term preservation. It is convenient to operate because its state is between liquid and solid (raw rubber paste); Because it is microencapsulated, it can be stored for 200 days at 60 ℃ and has a long storage period; Due to non anaerobic type, large-diameter spiral and wide gap are also applicable, with bonding and sealing functions; Also due to non anaerobic type, there is no limit to the material to be adhered, and plastic screws can also be locked. Generally speaking, due to the requirements of high locking force, that is, high performance, the locking adhesive has entered the second generation from the first generation; Due to the requirements of production efficiency, it develops from the second generation to the third generation; Due to the versatility requirements based on high reliability, it has developed to the fourth generation, and the transformation from solvent-based to solvent-free has been completed in the development process of the first to the fourth generation. But up to the fourth generation, the development is not over, its category is expanding, and its versatility is still in progress. Compatible with the environment and suitable for screw cycle, the effect of average grain size on small crack growth rate is not obvious; The use of long crack stage rings, known as the 5th generation of locking glue, is about to appear

3 structural bonding - balance of performance

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

(1) rapid curing at room temperature

structural bonding, especially in the bonding of large structural parts, curing at room temperature is very important. It is difficult to realize the heating condition of large parts, and more importantly, it will waste energy when heating. Secondly, some structural parts need to be bonded in batches, and the long curing time will affect the progress of the whole process. The second generation acrylic adhesive (SGA) can meet this requirement

the two-component SGA is composed of one component containing acrylic monomers, elastomers, organic peroxide initiators and stabilizers and another component containing acrylic monomers, elastomers, curing agents and other components. The free radical polymerization reaction starts when the two components contact. Even at a low temperature, it can be cured in a short time (from a few minutes to more than ten minutes) [6]. General elastic adhesives can also be cured at room temperature [7]. In terms of bonding between metal and plastic structures, a kind of rapid curing adhesive has also been developed [8]

(2) balance of performance

in the foregoing binding and packaging adhesives, it has been mentioned that the balance between water resistance and hydrolysis, brittleness and flexibility is the opposite performance, and this problem also exists in structural bonding. Because the joints of structural bonding have to bear various forces in use, which requires not only shear strength, but also peel strength; It can not only bear static load, but also eliminate the influence of irregular samples on the sensor, and bear dynamic load, that is, it requires the balance of performance. Therefore, the adhesive must have a certain elasticity, and the relative elongation should reach 25% - 50%. Elastic structural adhesives have this function mainly depends on their special structure. For example, the elastic epoxy resin adhesive composed of liquid bisphenol A epoxy resin (DGEBA) and silyl terminated polyepoxypropane (dmsi PPO) has this performance because it has a phase separation structure after curing: dmsi PPO is a continuous phase; DGEBA solidified product is a dispersed phase, which is dispersed in the continuous phase in the form of particles. The solidified product acts as an organic filler and meets the requirements of performance balance [9]. SGA also has a similar "island structure". The "sea" phase is an elastomer, and the "island" phase is a cured acrylic polymer. This structure is tough, with good shear strength, peel strength and impact strength [6]

(3) special diversification

structural bonding itself requires high performance of the adhesive. If it is used in specific parts, it will have higher requirements, which must be endowed with new functions and become a special functional structural adhesive [10]. For a certain application, there is usually more than one kind of special glue, and the diversification of special glue appears. The following takes the bonding of automobile structure as an example to illustrate this. ① The weldbonding technology of high elastic epoxy and spot welding is used to improve the rigidity of the car body without major changes to the car body structure; ② The preformed adhesive with dispersed acrylic polymer particle epoxy resin as the main body is used as curling adhesive with temporary fixing function. It has been used in car body, engine hood, trunk cover and other parts. It can be used for oil surface bonding, which is also an important function; ③ Using high attenuation epoxy adhesive to improve vibration attenuation energy and vibration characteristics; ④ Use dimethylacrylic acid anaerobic adhesive to lock the screw in the assembly of engine and gearbox, and use microencapsulated epoxy adhesive to lock the bolts that have been disassembled for many times; ⑤ When the flexible polyurethane glue outer panel is used for the rear door, it can absorb the stress caused by the control and data processing software shape with windows as the operation interface during the molding

(to be continued)

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