Structural adhesives are used in high strength, permanent bonding applications. They are capable of bearing considerable loads. In many cases, a structural adhesive that has good adhesion to a substrate will not fail when the bonded joint is stressed to its yield point. They often provide the primary means of attachment in structural applications. They are generally formulated from thermosetting resins that require chemical crosslinking either with the addition of a curing agent (hardener), a catalyst and/or by heat. Some high strength thermosetting elastomers, such as polyurethanes, can also be classified as structural adhesives. Common polymeric resin families that are used to formulate structural adhesives are described below.
Epoxies are a very versatile family of structural adhesives. They bond well to many substrates and can be easily modified to achieve a wide range of properties. They provide high shear strength on a wide variety of plastics, metals and glass. When fully cured, these thermosetting adhesives offer high thermal and chemical resistance, as well as high cohesive strength and minimal shrinkage. Toughened epoxy adhesives are often an excellent choice for very damanding applications, and find extensive use in the automotive, industrial and aerospace industry.
Modified (meth)acrylic adhesives are one of the most popular structural adhesives. They are often called reactive acrylics to separate them from other acrylic resins that are normally used in pressure sensitive applications. They are thin liquids that polymerize to form tough bonds. Acrylic structural adhesives contain similar monomers as the original anaerobic adhesives. However, they have been carefully formulated to cure even in the presence of air. Cure can occur in minutes or in hours at room temperature, depending on the type of acrylic resin used. Acrylics often bond through thin layers of oil due to their great solvent power. However, most acrylics have a stronger (sometimes pungent) odor than most other types of structural adhesives.
Anaerobic acrylic adhesives are based on monomer systems that only cure when air is excluded from the resin. They are thin liquids consisting mainly of (meth)acrylic monomers that polymerize to form tough
plastic bonds when confined between closely fitting (metal) joints.
Anaerobic adhesives will bond many common substrates including
many common metals, glass, ceramics, and many thermoset plastics.
However, many anerobic adhesives and sealants will cure only when exposed to metal ions, typically coming from a metal surface.
They are mainly used to augment the seal of threads or joints and/or to provide a holding force for mechanically joined assemblies.
Cyanoacrylate adhesives, also known as ‘‘superglues", are single component liquids that have fast cure, and excellent shear strength. Although very similar in curing characteristics to the anaerobic adhesives, cyanoacrylates are much more rigid and less resistant to moisture. This class of adhesives is available only as low-viscose liquids that cure in seconds at room temperature and bond well to many substrates. Like many one-part urethanes, they require moisture to initiate polymerization and cure only in thin bondlines. Cyanoacrylates normally have poor heat and moisture resistance and have relative low peel and impact strengths, also newer developemnts resulted in adhesives that exibit some toughness. Cyanoacrylates are usually used where there is minimal environmental stress and when fast setting times are required.
Reactive urethane adhesives are available as 100% solids or as solvent based systems and as one or two part adhesives. Unlike most epoxies, polyurethanes can be formulated to be extremely flexible. They form tough bonds with high peel strength and medium to high shear strength. Since they are very flexible, they are often an excellent choice to bond films, foils, and elastomers. They also have good adhesion properties to sheet molding composites (SMC), and therefore, are used extensively in the automotive and marine industry. Unlike many other structural adhesives, urethane adhesives have excellent low temperature properties. However, urethanes do not have high temperature resistance and often do not bond well to metals unless a primer is applied to the substrate prior bonding.
Silicone adhesives are available as either solvent solutions for
pressure sensitive adhesives (PSA's) or as one or two part
semi-structural adhesives. These types of adhesives have
typically (much) lower shear strength than the other types of
structural adhesives but excellent
peel and impact strength and outstanding heat resistance.
Solvent based silicone adhesives cure via condensation or
via free-radical polymerization. These adhesives are very
tacky and exhibit only low to moderate peel strength but have excellent low and high temperature performance
even after extended exposure to elevated temperature and adhere to a large variety of substrates. Because silicones have low surface energy, they
even bond to low surface energy plastics such as polyethylene
and fluorocarbons.