In this article we explore what structural adhesives are, the different types that are used in industry, and the reasons why you might want to use them instead of mechanical fasteners.
A structural adhesive is a type of adhesive that is capable of bonding two or more substrates together by bearing the forces required for the process and distributing the them across the bond line. They are referred to as “load-bearing” adhesives as a result and are responsible for securely holding the materials together through vibrations, temperature deviations, chemicals or other destructive agents.
Due to the significance of their role, a bonded product’s function and utility often depends on the bond strength offered by its structural adhesive. The adhesives are also responsible for filling gaps and blocking the entrance of liquids. One test that is used to check for a structural adhesive is whether it forms a bond with a lap shear strength greater than 10 MPa.
By being able to securely hold different, or similar, substrates together, structural adhesives can effectively replace mechanical fastening methods such as welding and riveting. There is an improved stress distribution when using adhesives compared to rivets or joints; in the latter case, all the stress and shearing forces are based around the fastener, whereas with adhesives, they are distributed along the bond line, leading to a stronger assembly.
In industry, structural adhesives may be used on boats and water sport equipment in the maritime sector, wind turbine blades in the wind industry, car and bus panels in the automotive sector, and for the construction and maintenance of trains in the railway sector and aircraft in the aerospace sector. Structural adhesives can bond plastics, metals, glass, composites and ceramics.
Types of Structural Adhesives
Various types of structural adhesives exist, each being fit for a different purpose and offering unique characteristics. The main categories have been explained below:
Epoxies make up the largest segment of structural adhesives; offering excellent chemical resistance and the highest shear strength on various substrates ranging from metals and composites to glass and wood. Epoxies are great gap-filling adhesives and can also be used for encapsulation purposes. They are available as one component adhesives that are heat cured, or two component adhesives that cure at room temperature. Most epoxies are rigid and brittle when cured. They require a thoroughly cleaned substrate surface before an attempt at bonding.
Unlike epoxies, urethanes are flexible when cured and, as a result, have superb impact and vibration resistance. They have high shear and peel strength but low chemical and temperature resistance. They are also available as two-part formulations which don’t require moisture to cure, or one-part adhesives which are moisture-curing. Urethanes are great for bonding dissimilar substrates due to their flexibility and ability to form strong bonds.
Acrylics are the best choice for bonding to composites and metals with little to no surface preparation. They offer excellent shear, impact and peel strength with a high fatigue resistance. Acrylics rapidly cure at room temperature, are very flexible and usually offer a good aesthetic to the bonded product.
Methyl Methacrylates (MMAs)
Methyl methacrylates are acrylic adhesives which provide especially high strength bonds when used on plastics. MMAs are made of a resin and hardener and can bond dissimilar substrates together while retaining flexibility and bond strength. MMAs do not require heat to cure, although they are exothermic and produce heat when cured. MMAs are the perfect replacements for welds and mechanical joints in the bonding of metals.
Advantages of Using Structural Adhesives
- Less labour required to apply adhesives compared to mechanical fasteners
- Lower material costs than mechanical fasteners
- Fewer steps involved in applying adhesives compared to fasteners
- Rapid curing adhesives save time even more
- Adhesives are lighter than mechanical fasteners
- Due to mechanical fasteners focusing all the loading forces around the joint itself, often extra material must be added to balance it out across the whole product. This extra material can be saved when using adhesives as the loading forces are distributed along the bond line already.
No Negative Effects on Product Strength
- The structural integrity of the product is not affected by applying the adhesive, as opposed to applying a mechanical fastener. This leads to a higher flexibility and tensile strength of the product overall.
- Structural adhesives are able to absorb vibrations as they pass through the bond. Therefore, they can dampen sounds and make products more acoustically pleasing and less loud. This is an important factor to consider in the construction of washers, dryers and cars.
- Mechanical fasteners are prone to rust after exposure to humidity, which affects the aesthetic of the bonded product negatively. This issue does not present itself with adhesives.
- Welds often form obvious lumps or change the texture of the product under the layers of paint.
- Black adhesives, such as Araldite 2031-1, offer a slick look when used on black or dark products as they become barely noticeable.
Despite this information, it’s crucial that you consult with an expert when choosing an adhesive for a specific application.