A Guide to Choosing Which Structural Adhesive is Correct
A structural adhesive refers to a type of adhesive that hardens or cures into a substrate that is capable of holding two or more materials together under different levels of stress and over varying periods of time.
Common adhesives that come under the category of ‘structural adhesive’ include:
Epoxies come in both one-part and two-part varieties, both offering structural properties. One-part epoxies are rigid adhesives, offering temperature-resistance and environmental durability but require thermal curing in order to harden.
Two-part epoxies also provide temperature-resistance and environmental durability but cure by mixing; this means that there are different formulations offering different properties. For example, rigid epoxies deliver high tensile strength but are best suited for applications that require lower elongations. Flexible epoxies have low tensile strength but are able to withstand more movement and have a higher adhesion to most plastics.
Polyurethanes have the ability to bond to a range of materials such as plastics, metals and wood. This flexibility extends to high flex fatigue applications, meaning that they are suited to higher elongations. Other beneficial properties of polyurethanes include tensile, compression and tear strength, hardness, and impact and abrasion resistance.
UV Curing Adhesives
When exposed to UV light, these adhesives cure with very high strength and a seamless finish. With no need to mix or heat-cure, non-flammable and no solvents, UV curing adhesives are a simple bonding solution for a variety of materials such as: steel, aluminium, zinc, glass, acrylic, and other plastics.
Acrylics are strong with high tensile strength, yet flexible with high elongation; they are used as a low-cost alternative to epoxies and polyurethanes. They are suitable for bonding most plastics, aluminium, steel and other metal surfaces and for both static and dynamic loads.
So, why are structural adhesives becoming a more popular choice and alternative to other bonding solutions? Some benefits to using them instead of mechanical fastenings are:
- Due to an irreversible cure process and no need for air-drying, welding or drilling, they won’t damage the substrate and are aesthetically more pleasing.
- They have the ability to join dissimilar materials and several different geometries with an unlimited depth of cure.
- They do not concentrate stress at localised spots leading to increased fatigue resistance.
- They offer environmental and chemical resistance along with the highest load bearing capability.
- There is a wide range of cure times and properties to suit different applications.
With such a variety to choose from, there are equally a number of considerations you should think about when deciding which structural adhesive is correct for you. The end use requirements should always be in mind and some questions you can ask yourself include:
Weather and Temperature Conditions
- Will the temperature be high or low during end use?
- Will humidity levels be high during end use?
- Will the substrate be exposed to rain or salt water?
- Will the substrate be exposed to the sun / high UV levels?
- Will any fuel fluids come into contact with the substrate?
- Will the substrate have to be regularly cleaned?
- If specialised chemicals come into contact with the substrate, will it be constant or occasional?
- Are there certain regulations you will have to follow during production?
- Is the substrate sensitive to issues such as: outgassing, ionics or corrosion potential?
- Will the substrate be subject to high impact or vibration?
- Will the substrate be subject to thermal cycling?
- What kind of stress type and magnitude will the substrate be subject to?
Once these questions have been answered, the specifications will help narrow down which structural adhesive would be the best choice for you. If you are still unsure or would like some further guidance, an adhesive manufacturer would be able to offer knowledgeable advice to help. At BDK, we are able to provide you with bespoke adhesive solutions that are tailored to your application; contact us today to find out how we can assist you.