1. What is an initiator?
Initiator, also known as starter, is a substance that can generate free radicals (radicals) or ions and other active species, which can initiate monomer chain polymerization. Free radical initiators are easily decomposed into free radicals (i.e. primary free radicals) by heat, which can be used to initiate free radical polymerization and copolymerization of olefin and diene monomers, and can also be used for cross-linking and curing of unsaturated polyesters and cross-linking reactions of polymers.
Initiators are generally compounds with weak bonds that are easily decomposed into active species, among which covalent bonds have two forms: homolytic and heterolytic. The active centers of unsaturated monomer polymerization initiated by them include free radical type, anionic type, cationic type and coordination compound, etc. At present, the most widely used in the adhesive industry is the free radical type, which exhibits unique chemical activity. Under the action of heat or light, the covalent bond is homolytically cleaved to generate two free radicals, which can initiate polymerization reactions.
Initiators play an important role in the research and production of adhesives and sealants. Initiators must be used in the polymerization of acrylates, vinyl acetate solutions and emulsions, synthetic styrene-acrylic emulsions, ethylene-propylene emulsions, VAE emulsions, styrene-butadiene latex, chloroprene latex, etc., grafted chloroprene adhesives, SBS grafted adhesives, unsaturated polyester resin cross-linking curing, anaerobic adhesive curing, fast-curing acrylic structural adhesive curing, etc. Initiators can directly affect whether the polymerization process can proceed smoothly, the polymerization reaction rate, and the storage period of the product. Therefore, the initiator is one of the essential additives for preparing adhesive base materials, that is, synthetic adhesive resins.
2. Classification of initiators
There are many types of initiators. Free radical initiators are commonly used in adhesives, including peroxide initiators, azo initiators and redox initiators. Peroxide initiators are further divided into organic peroxide initiators and inorganic peroxide initiators.
(1) Organic peroxides
The general structural formula of organic peroxides is R-O-O-H or R-O -O-R, where R is an alkyl group, an acyl group, a carbonate group, etc.
(2) Inorganic peroxides
Inorganic peroxides are soluble in water and are mostly used in emulsion and aqueous solution polymerization reactions. They are mainly persulfates, such as potassium persulfate, sodium persulfate, and ammonium persulfate. Among them, ammonium persulfate and potassium persulfate are the most commonly used.
(3) Azo
Azo initiators include azobisisobutyronitrile and azobisisoheptanenitrile, which are low-activity initiators. The most commonly used one is azobisisobutyronitrile, which has an operating temperature range of 50-65°C, decomposes evenly, forms only one free radical, and has no other side reactions. It is relatively stable and can be safely stored in its pure state, but it also decomposes rapidly at 80-90°C. Its disadvantage is that the decomposition rate is low and the isobutyronitrile free radical formed lacks dehydrogenation ability, so it cannot be used as an initiator for graft polymerization.
(4) Redox initiation system
Peroxide initiators and azo initiators have high decomposition temperatures (50-100°C), which limits their application in low-temperature polymerization reactions. Redox initiation systems use free radicals generated by electron transfer between oxidants and reductants to initiate polymerization reactions. Therefore, compared with thermal decomposition initiators, redox initiators have the advantage of initiating polymerization reactions at lower temperatures (0-50°C), which can increase reaction rates and reduce energy consumption.
3. Selection and use of initiators
Different initiators have different effects in different use environments and conditions. Choosing a suitable initiator is conducive to the progress and control of the polymerization reaction, promotes process optimization, and improves the output and quality of the product. Different polymerization methods, different process conditions, and different product uses should select different initiators. The selection of initiators is mainly considered from the following aspects.
① For solution polymerization, organic peroxides or azo initiators with appropriate solubility should be selected; for aqueous solution or emulsion polymerization, inorganic peroxides or water-soluble redox system initiators and water-soluble azo initiators should be selected.
② Select an initiator with appropriate activation energy and half-life according to the polymerization reaction temperature
③ In order to reduce the polymerization temperature, try to use a redox initiation system.
④ The selected initiator should not have an adverse effect on the quality and performance of the polymer.
⑤ The amount of initiator should be appropriate
⑥ Storage is stable, safe and reliable.
⑦ Non-toxic,
The cationic initiators we produce have excellent performance and long storage time. Our cationic photoinitiators can be cured under different light sources.
