Methacrylic adhesives are thermosetting resin systems, also called reactive acrylics to distinguish them from other acrylic resins that are normally used in pressure sensitive applications. They are thin liquid adhesives that polymerize to form tough bonds. Acrylic structural adhesives are made up of similar monomers as anaerobic adhesives. However, they have been carefully formulated to cure even in the presence of air.
The majority of thermosetting acrylic adhesives are two-part systems that provide medium to high shear and peel strength. They bond well to a large variety of substrates, including many metals and plastics. They also bond through thin layers of oil due to their great solvent power. On the other hand, many acrylics have a strong and sometimes pungent odor (depending on the composition), often much stronger than most other types of structural adhesives.
Modern formulations are often based on methyl methacrylate (MMA).
Some other important methacylates used in acrylic adhesive
formulations are methacrylic acid (MAA), N-butyl-methacrylate
(n-BMA), iso-butyl methacrylate (i-BMA), cyclohexyl methacrylate
(CHMA), 2-hydroxyethyl methacrylate and 2-ethylhexyl methacrylate
(EHMA), to name only a few. These monomers are sometimes blended
with MMA to achieve the optimal mechanical and thermo-physical
properties.
Often rubber tougheners, similar to those used in
epoxy adhesives, are added to improve the toughness and flexibility
of the relative brittle methacrylate based adhesives. These
rubbers are sometimes terminated with reactive groups to allow for crosslinking
with the methacylate matrix. Chlorosulfonated polyethylene (Hypalon)
is another elastomer that finds uses as a flexibilizer and toughener
in thermosetting methacrylic adhesives. The toughened acrylics are
called second-generation acrylics or SGAs.
Often other
ingredients such as thickeners, antioxidants, pigments and fillers
are added to lower cost and to tailor the properties of the SGAs.
The methacrylate monomers are usually cured with a peroxide or hydroperoxide initiator. These systems cure relative slowly and have to be activated / accelerated by a third compound, usually a tertiary amine. Cure can occur in minutes or in hours at room temperature, depending on the type of acrylic resin used and the concentration of peroxide and accelerator.
The majority of SGAs are two-component adhesives. The first component contains the curing agent and the second component the reactive monomers, resins and the accelerator. However, the activator and adhesive can also be applied separately on the two surfaces to be bonded.
A major advantage of reactive acrylics is the well-defined induction time after contact of the adhesive and activator where no noticeable thickening or cure takes place, which is followed by a very rapid cure (snap cure). The induction time can be adjusted over a wide range, allowing the assembly of large and complex structures not uncommon in boat manufacturing and other assembley areas.
Methacrylate adhesives often suffer from air-inhibition, which can result in insufficient cure when the adhesive is exposed to air. This can lead to poor corrosion resistance. Furthermore, their heat resistance, maximal service temperature and bond performance is usually lower than those of high-performance structural epoxy adhesives.