Currently, the adhesives in use are all synthetic resin adhesives composed of multiple components. Single-component adhesives can no longer meet the requirements in application. Synthetic adhesives consist of a main agent and additives. The main agent is also known as the main ingredient, base material or adhesive material; 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 applications, ingredients such as flame retardants, foaming agents, defoaming agents, coloring agents and mildew inhibitors can also be included.
 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 different types of adhesives. The main agent is generally composed of one, two or even three kinds of polymers, and is required to have good adhesion and wettability, etc.
Commonly used adhesive materials include:
1) Natural macromolecular compounds Such as proteins, hide glue, fish glue, rosin, peach gum, bone glue, etc.
2) Synthetic macromolecular compounds
① Thermosetting resins, such as epoxy resin, phenolic resin, polyurethane resin, urea-formaldehyde resin, organosilicon resin, etc.
② Thermoplastic resins, such as polyvinyl acetate, polyvinyl alcohol and acetal resins, polystyrene, etc.
③ Elastomeric materials, such as nitrile rubber, neoprene rubber, polysulfide rubber, etc.
④ Mixtures of various synthetic resins and synthetic rubbers or grafted, interpenetrating and copolymerized products, etc.
2. Additives In order to meet specific physical and chemical properties, various auxiliary components added are called additives. For example, in order to make the main adhesive material form a network or three-dimensional structure and increase the cohesive strength within the adhesive layer, curing agents are added (they react with the main adhesive material and produce a cross-linking effect); in order to accelerate curing and reduce the reaction temperature, curing accelerators or catalysts are added; in order to improve the performance against atmospheric aging, thermal aging, arc aging, ozone aging, etc., anti-aging agents are added; in order to endow the adhesive with certain specific properties and reduce costs, fillers are added; in order to reduce the rigidity of the adhesive layer and increase its toughness, toughening agents are added; in order to improve processability, reduce viscosity and extend the service life, diluents are added.
They include:
1) Curing Agents Curing agents, also known as hardeners, are components that promote the bonding substances to accelerate curing through chemical reactions. They are the most important auxiliary materials in adhesives. Their role is to react directly or through catalysts with the main polymer. After curing, the curing agent molecules are introduced into the resin, turning the originally thermoplastic linear main polymer into a tough and rigid three-dimensional network structure. There are many types of curing agents. Different resins and different requirements call for different curing agents. The processability of bonding and its performance in use are determined by the properties and quantities of the added curing agents.
2) Toughening Agents The active groups of toughening agents directly participate in the curing reaction of the adhesive and enter into the chain structure of the final large molecule formed by the cured product. After the adhesive without the addition of a toughening agent is cured, its performance is brittle, prone to cracking and has poor practicability. Adhesives with toughening agents added all have better impact strength and peel resistance. Different toughening agents can also reduce internal stress and curing shrinkage to varying degrees and improve low-temperature performance. Commonly used toughening agents include polyamide resin, synthetic rubber, acetal resin, polysulfone resin, etc.
3) Diluents Diluents, also known as solvents, mainly serve to reduce the viscosity of the adhesive, increase the infiltration ability of the adhesive and improve process performance. Some can reduce the activity of the adhesive, thus prolonging the service life. However, if the amount added is too large, it will reduce the bonding strength, heat resistance and resistance to media of the adhesive. Commonly used diluents include acetone, paint and many other solvents that are compatible with the adhesive material.
4) Fillers Fillers generally do not undergo chemical reactions in adhesives. The use of fillers can improve the strength of the bonded joint, impact toughness, wear resistance, aging resistance, hardness, maximum service temperature and heat resistance, and reduce the linear expansion coefficient, curing shrinkage rate and cost, etc. Commonly used fillers include copper oxide, magnesium oxide, silver powder, porcelain powder, mica powder, asbestos powder, talc powder, etc.
5) Modifiers Modifiers are some components added to improve a certain aspect of the performance of the adhesive to meet special requirements. For example, in order to increase the bonding strength, coupling agents can be added. Preservatives, mildew inhibitors, flame retardants and stabilizers can also be added.