What are the different curing methods for different adhesives?

What are the different curing methods of different adhesives: curing of hot melt adhesives, curing of solution-type adhesives, curing of emulsion-type adhesives, curing of plastisol-type adhesives, curing of reactive adhesives, etc. Details of different curing methods of different adhesives Introductory instructions.

What are the different curing methods of different adhesives?

In order to facilitate the infiltration of the adhesive on the surface of the adherend, the adhesive must be made into a liquid state or turned into a liquid state before bonding. After bonding, it has strength only when it becomes a solid-state. The process of changing the adhesive layer from liquid to solid by appropriate methods is called adhesive curing. Different adhesives often use different curing methods:

Curing of hot melt adhesive

The thermoplastic polymer material obtains fluidity after being heated and melted, and many polymer melts can be used as adhesives. The polymer melt can be solidified by cooling after infiltrating the surface to adhere. This type of adhesive is called hot melt adhesive.

The curing of hot melt adhesive is a simple heat transfer process, that is, heating to melt the glue, and cooling to solidify. The curing process is greatly affected by the ambient temperature. The ambient temperature is low and the curing is fast. In order to allow the hot-melt adhesive to wet the adherend, the melting temperature and drying time must be strictly controlled during use. Special attention should be paid to hot-melt adhesives with crystalline adhesives, otherwise, the adhesive will not crystallize due to excessive cooling. Completely reduce the bond strength.

Curing of solution adhesives

The thermoplastic polymer substance can be dissolved in a suitable solvent to become a polymer solution to obtain fluidity. After the polymer solution infiltrates the surface of the adherend, the solvent is volatilized to generate a certain adhesion. Many polymer solutions can be used as adhesives. The commonly encountered solution adhesives are rubber solutions for repairing bicycle inner tubes. Many adhesives are solution-based.

The essence of the curing process of the solution adhesive is that the concentration of the volatile solution of the solvent increases continuously and reaches a certain strength. The curing speed of the solution glue is determined by the volatilization speed of the solvent, and is also affected by factors such as ambient temperature, humidity, the compactness and water content of the adherend, and the size of the contact surface. When preparing a solution glue, a specific solvent should be selected and changed into a mixed solvent to adjust the curing speed. Choosing a solvent that is easy to hold can easily affect the crystallization speed and degree of the crystalline material, and even cause the adhesive layer to crust and reduce the bonding strength. In addition, the rapid volatilization of cooling water at the bonding area is also detrimental to the bonding strength. The solvent selected is too slow to volatilize, the curing time is long, the efficiency is low, and it may also cause solvent retention in the adhesive layer, which is unfavorable for bonding. Strict attention should also be paid to fire and poisoning when using solution glue.

Curing of Emulsion Adhesives

Aqueous emulsion adhesive is a dispersion of polymer colloid in water, which is a relatively stable system. When the water in the emulsion gradually penetrates into the adherend and volatilizes, its concentration will gradually increase, so that the colloidal particles will coagulate and solidify due to the action of surface tension. The ambient temperature has a great influence on the coagulation of the emulsion. When the temperature is high enough, the emulsion can agglomerate into a continuous film. When the temperature is too low or below the low film-forming temperature (which is usually slightly lower than the glass transition temperature), a continuous film cannot be formed. , at this time, the film is white, and the strength is poor. The low film-forming temperature of different polymer emulsions is different, so when using this type of adhesive, the ambient temperature must be higher than its low film-forming temperature, otherwise, the bonding effect will not be good.

Curing of plastisol adhesives

The plastic paste is an unstable dispersion system of polymer compounds in plasticizers, and its curing is basically the process of polymer compounds dissolved in plasticizers. This paste has certain stability at room temperature. When heated (usually at 150-209°C), the plasticizers of polymer compounds can quickly dissolve into each other and gel completely. Increasing the temperature is conducive to the movement of polymer chains and the formation of a uniform and dense adhesive layer, but the temperature is too high. Can cause polymer decomposition.

Curing of reactive adhesives

Reactive adhesives all have active groups, and under the action of assimilation agents, initiators, and other physical conditions, the adhesives undergo chemical reactions such as polymerization and cross-linking to cure. According to the curing mode, reactive adhesives can be divided into curing agent curing type, catalyst curing type, and initiator curing type. As for the curing mechanism of photosensitive curing, radiation assimilation, etc., it generally belongs to the above types.

Epoxy resin and polyurethane adhesives are mostly cured with stoichiometric curing agents; common initiators such as second-generation acrylate structural adhesives and unsaturated polyester adhesives initiate curing; some phenolic and urea-formaldehyde resin adhesives can be catalyzed and cured by acid catalysts. Some reactive adhesives will automatically accelerate when they are assimilated. Special attention should be paid when designing formulas or using adhesives because the rapid exothermic heat during gelation will cause defects in the adhesive layer, destroy the material to have adhered, and cause the bonding to fail.

After the initial curing of the glue, the adhesive layer can generally obtain a certain bonding strength, and the bonding strength will continue to increase in a long time after the initial curing. Since the molecular movement becomes difficult after the initial curing, it is extremely beneficial for the adhesive strength to properly extend the curing time or properly increase the curing temperature after the initial curing to promote the smooth progress of the post-curing.

For a specific type of rubber, the set curing temperature cannot be lowered. The result of the temperature reduction is that the curing cannot be completed, resulting in a decrease in the bonding strength. This deterioration is difficult to compensate by extending the curing time. For the glue set to be cured at a higher temperature, it is better to use temperature-programmed curing, which can avoid the overflow of the glue, the separation of insoluble components, and reduce the internal stress of the glue layer. For the rubber seeds generated by volatile low molecular weight substances during the curing process, a certain pressure is often applied during curing. If no small molecular substances are generated during the curing process, the only contact pressure is applied to prevent the dislocation of the bonding surface.

For adhesives cured with a curing agent, the amount of curing agent is generally stoichiometric. When the amount of curing agent is insufficient, it is difficult to cure completely, and the curing agent is generally slightly excessive. When using a curing agent with a larger molecular weight, its dosage range can be slightly larger. For example, when curing epoxy resin with 650 polyamides, the dosage is 30-1100 parts by mass. For adhesives cured with initiators, increasing the number of initiators within a certain range can increase the curing speed without affecting the performance of the adhesive. Insufficient dosage will cause premature termination of the reaction and complete curing; too large a dosage will reduce the degree of polymerization and reduce the bonding strength. In order to avoid the adverse effect of gelation on the adhesion, a composite initiator can be used: that is, the initiator with low activity and high activity are used together. After adding the initiator, properly adding some special reducing substances (called accelerators) can greatly reduce the chemical energy of the reaction and increase the reaction speed. It can even be made into a fast-solidifying fat seed at room temperature, which is a redox initiation system. Since the reducing agent promotes the decomposition of the initiator and reduces the initiation efficiency, the initiator dose should be increased in the redox initiation system. The catalyst only changes the reaction speed. The catalyst-cured adhesive reacts very slowly (referring to normal temperature) when no catalyst is added and can be stored for a long time. Excessive use of the catalyst will deteriorate the performance of the adhesive layer. It is also feasible to appropriately increase the curing temperature when the amount of catalyst is small.

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