Understand the composition, classification, and bonding process of adhesives
2021-07-26 3595
Adhesive bonding is a method of firmly connecting the same or different materials together through substances with adhesive properties. The substance with adhesive ability is called adhesive or adhesive, the object being bonded is called adhesive, and the component composed of adhesive and adhesive is called adhesive joint.
Its main advantages are simple operation and high productivity; Flexible, fast, and simple process; The joint is reliable, firm, and aesthetically pleasing. The product structure and processing technology are simple; Material saving, labor saving, low cost, and small deformation. Easy to implement repair and reuse technology can be effectively applied to the connection between different types of metals or non-metals.
Composition of adhesive
The adhesives currently used are all synthetic resin adhesives with multiple components, and single component adhesives can no longer meet the requirements of use. Synthetic adhesives are composed of a main agent and an auxiliary agent, which is also known as the main material, base material, or adhesive; Additives include curing agents, diluents, plasticizers, fillers, coupling agents, initiators, thickeners, anti-aging agents, polymerization inhibitors, stabilizers, complexing agents, emulsifiers, etc. Depending on the requirements and uses, they can also include components such as flame retardants, foaming agents, defoamers, colorants, and mold inhibitors.
1. Main agent
The main agent is the main component of the adhesive, which dominates the adhesive performance and is also an important indicator for distinguishing the types of adhesives. The main agent is generally composed of one or two, or even three types of high polymers, which require good adhesion and wetting properties. The commonly used adhesives include:
·Natural polymer compounds
Such as protein, skin gum, fish gum, rosin, peach gum, bone gum, etc.
2) Synthetic polymer compounds
① Thermosetting resins, such as epoxy resin, phenolic resin, polyurethane resin, urea formaldehyde resin, organic silicone resin, etc.
② Thermoplastic resins, such as polyvinyl acetate, polyvinyl alcohol and acetal resins, polystyrene, etc.
③ Elastic materials, such as nitrile rubber, chloroprene rubber, polysulfide rubber, etc.
④ Mixtures or grafts of various synthetic resins, synthetic rubbers, inlays, and copolymers.
2. Additives
In order to meet specific physical and chemical properties, various auxiliary components are added as additives. For example, in order to form a mesh or body structure of the main adhesive and increase the cohesive strength of the adhesive layer, curing agents are added (they react with the main adhesive and produce cross-linking effect); Adding curing accelerators or catalysts to accelerate curing and reduce reaction temperature; To improve the resistance to atmospheric aging, thermal aging, arc aging, ozone aging, and other properties, anti-aging agents are added; Adding fillers to endow adhesives with specific properties and reduce costs; Adding toughening agents to reduce the rigidity of the adhesive layer and increase toughness; In order to improve the process, reduce viscosity, and extend service life, diluents are added. include:
1) Curing agent
Curing agent, also known as hardener, is a component that accelerates the curing of bonding materials through chemical reactions. It is the most important ingredient in adhesives. Its function is to react directly or through a catalyst with the main polymer, and after curing, introduce the curing agent molecules into the resin, transforming the originally thermoplastic linear main polymer into a tough and hard network structure.
There are many types of curing agents, and different resins and requirements require different curing agents. The processability and performance of adhesive bonding are determined by the performance and quantity of the curing agent added.
2) Toughening agent
The active group of the toughening agent directly participates in the curing reaction of the adhesive and enters the chain structure of a large molecule formed by the curing product. After curing, adhesives without toughening agents have brittle properties, are prone to cracking, and have poor practicality. The adhesive with toughening agent has good impact strength and peel resistance. Different toughening agents can also reduce their internal stress, cure shrinkage to varying degrees, and improve low-temperature performance.
Common toughening agents include polyamide resin, synthetic rubber, acetal resin, polysulfone resin, etc.
3) Diluent
Diluents, also known as solvents, are mainly used to reduce the viscosity of adhesives, increase their wetting ability, and improve their process performance. Some can reduce the activity of the adhesive, thereby extending its service life. But adding too much will reduce the bonding strength, heat resistance, and medium resistance of the adhesive.
Commonly used diluents include various solvents that are compatible with adhesives such as acetone and paint.
4) Filler
Fillers generally do not undergo chemical reactions in adhesives. The use of fillers can improve the strength, impact toughness, wear resistance, aging resistance, hardness, maximum operating temperature, and heat resistance of bonded joints, while reducing linear expansion coefficient, curing shrinkage rate, and cost. Common fillers include copper oxide, magnesium oxide, silver powder, porcelain powder, mica powder, asbestos powder, talc powder, etc.
5) Modifying agent
A modifier is a component added to improve the performance of an adhesive in order to meet special requirements. For example, to increase bonding strength, coupling agents can be added, as well as preservatives, mold inhibitors, flame retardants, and stabilizers.
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