As a type of improved acrylate-based adhesive, the core of the second-generation (reactive) acrylate adhesive is the free radical graft copolymerization technology of (meth) acrylic acid, which constitutes its unique two-component structure. This innovation is compared with the traditional first-generation acrylate adhesive (FGA), which is mainly composed of a simple mixture of acrylate monomers, catalysts and elastomers, and there is no chemical cross-linking between the monomers and elastomers during the curing process, so it has limited performance in water resistance, solvent resistance, heat resistance and impact resistance.
Reactive acrylate adhesives, also known as acrylate structural adhesives, are an outstanding achievement of DuPont in the 1970s. They were successfully launched on the market in 1975 to overcome the limitations of FGA. The origin of FGA can be traced back to 1955, when Eastman Company in the United States accidentally discovered its viscosity while studying ethylene compounds. It was originally composed of acrylic monomers, catalysts and rubber elastomers (such as acrylonitrile rubber, butadiene rubber, etc.). Its curing process is based on the polymerization of initiators, but the lack of chemical bonding between monomers and elastomers limits its overall performance.
In order to improve the shortcomings of FGA, researchers successfully developed a second-generation acrylic adhesive (SGA) by introducing rubber for modification, which significantly improved the peel strength. SGA and FGA are similar in composition, but the key difference is that SGA achieves a chemical reaction between the monomer and the elastomer during the polymerization process. This breakthrough greatly improves its performance.
As time goes by, based on the technological progress of SGA, the third generation of acrylic adhesives (TGA) came into being. The main difference between TGA and SGA lies in the curing mechanism: SGA relies on chemical cross-linking with a curing agent for curing, while TGA uses ultraviolet or electron beam irradiation to initiate free radical polymerization for rapid curing. Although there is little difference between the two in terms of physical and chemical properties, the innovation of curing methods has brought more application possibilities to TGA.
In my country, the second generation (reactive) acrylic adhesive is vividly called "room temperature fast-curing acrylic adhesive". Its 100% reactive two-component characteristics ensure efficient bonding effects. As early as the 1950s, patents for methacrylate-based adhesives have appeared in the world, but their development has been slow due to limitations in curing speed and performance. It was not until 1975 that DuPont launched the two-component primer-type Cavalon adhesive with chlorosulfonated polyethylene and (meth)acrylate monomer as the core, which promoted significant development in this field. By the 1980s, the technology had matured internationally. Not only had the scale of industrial production expanded, but it was also widely used in various fields, achieving a leap from research and development to practical use.
Reactive acrylate adhesives are mainly divided into two categories: primer-type and dual-primer-type. Primer-type adhesives are composed of two independent components: a main agent and a primer. The main agent is rich in key ingredients such as polymers (such as elastomers), acrylate monomers (oligomers), oxidants and stabilizers; while the primer mainly contains auxiliary ingredients such as accelerators (reducing agents), co-accelerators and solvents. In contrast, dual-primer-type adhesives do not require a primer. Both of their components are main agents, one of which contains an oxidant and the other contains an accelerator and a co-accelerator. In order to ensure rapid and complete curing at room temperature, the selected redox system must be highly matched and have high performance.
Examples of specific ingredients in the base are varied and include, but are not limited to:
1. Polymer: Use unvulcanized rubber as the main component, such as synthetic rubbers such as chlorosulfonated polyethylene, chloroprene rubber, nitrile rubber, acrylic rubber, and resin materials such as ABS, AMBS, MBS, and polymethyl methacrylate.
2. Acrylate monomers (oligomers): including methyl methacrylate, ethyl methacrylate, butyl methacrylate, 2-ethylhexyl methacrylate, β-hydroxyethyl methacrylate (or propyl methacrylate), glycidyl methacrylate, etc. These monomers provide excellent bonding properties for adhesives.
3. Oxidant: Use efficient oxidation systems, such as diacyl peroxides (such as BPO, LPO), hydrogen peroxides (such as isopropylbenzene hydroperoxide, tert-butyl hydroperoxide), ketone peroxides (such as methyl ethyl ketone peroxide) and peroxide esters, etc. These oxidants play a key role in the curing process.
4. Stabilizer: To ensure the stability of the adhesive during storage and use, stabilizers such as hydroquinone, hydroquinone monomethyl ether, phenothiazine, and 2,6-di-tert-butyl-p-cresol are often added.