I. Polyurethane curing principle
Due to the presence of carbamate group (NHCOO-) in the main chain, commonly known as polyurethane adhesives, the structure contains the group -NCO, which improves the adhesion to various materials.
1、Polyisocyanate adhesives are mainly moisture-absorbing and curing.
2、Prepolymer isocyanate adhesives are divided into two categories: one-component and two-component.
One-component isocyanate and both ends of the polyester or polyether containing hydroxyl reaction, to get isocyanate (-NCO) elastomeric adhesives, and then add the appropriate amount of catalysts, fillers to produce one-component room temperature vulcanization polyurethane sealant, curing mechanism with polyisocyanate adhesives, curing moisture in the air.
Two-component adhesives consist of prepolymer containing -NCO group and polyester (or polyether) resin. The -NCO component is the hard chain segment, and the -OH component is the soft chain segment, mainly composed of polyether, polyester, cross-linking agent (such as MOCA di-o-chlorodiphenylmethane), and catalyst (such as stannous zincate). Adjustment of -NCO/-OH content can produce prepolymer with different relative molecular mass and different -NCO content. The reaction is hydroxyl aldehyde condensation, a kind of coupling reaction, mainly -NCO and -OH reacting and curing under the action of catalyst to form a good bonding joint.
Second, epoxy resin curing principle
The curing reaction of epoxy is ring-opening curing, ring-opening and polymerization under the action of initiator or catalyst. The initiator is usually a hydroxide (such as NaOH) and a wangi compound (such as CH3ONa).
Epoxy resin itself is a linear structure of thermoplastic resin, its curing reaction is realized by adding curing agent. Curing agent can make the epoxy resin molecules cross-linked to form a body structure of the macromolecule, into insoluble and non-melting curing material, so its curing agent is also known as cross-linking agent. In the selection of curing agent, should consider the product performance requirements (bonding strength and electrical properties), construction environment (temperature, humidity and working medium) and allow the curing conditions (can be heated, pressurized, the length of curing time). High-temperature curing with aromatic amines, anhydride and dicyandiamide, etc.; medium-temperature curing with imidazole; room temperature curing, bonding strength requirements are not high, with aliphatic amines and modified amine curing agent, in humid conditions, with ketimine and phenolic modified imine curing agent, etc.
Third, acrylate curing principle
1, acrylate adhesive curing reaction for the anionic polymerization of free radical polymerization, through the chain initiation, chain growth, chain transfer, chain termination and other stages.
Chain initiation and chain growth: When the β-carbon atom of the monomer molecule is attacked by nucleophilic reagents, it produces stable negative carbon ions, such as hydroxide anion and amine is a good nucleophilic reagent, which can trigger the cyanoacrylate monomer, the anion formed after initiation attacks another monomer molecule to produce a dimer, and further react with more monomers, chain growth reaction occurs to generate polymers.
Chain transfer and chain termination The anion in growth reacts with other substances (chain transfer agents) instead of with the monomer to produce an inert polymer and a new anionic chain, and if the latter has the ability to initiate further polymerization, then chain transfer occurs.
If the active growth chain encounters other substances other than the monomer during the polymerization process, such as water, alcohol, acid, etc., which can protonate the anion, the polymerization reaction is quickly prevented, causing chain termination.
2、Initiator, accelerator
Generally organic peroxide, such as benzoyl peroxide, diisopropyl benzene peroxide, isopropyl benzene hydrogen peroxide, cyclohexanone peroxide, etc.. The selection should pay attention to whether it has a high critical temperature, long half-life, good stability at room temperature and other characteristics. Promoters generally have transition metal soaps (such as cobalt naphthenate), tertiary amines (such as dimethyl phenylamine), organosulfur compounds (such as ethylene urea) and other compounds. Promoter can be used alone or several common use, when used together should be selected as the main promoter, the rest of the auxiliary promoter, the choice of initiator and promoter, in addition to consider the curing effect of initiation, should also pay strict attention to their presence on the impact of the storage stability of the glue.
