Why Use a Structural Adhesive?
Structural adhesives are chosen for a multitude of assembly operations. Unlike mechanical fastening methods, they don’t damage substrates (no need to drill holes and no heat distortion as when welding metal); can join dissimilar materials without galvanic corrosion; are amenable to a number of different geometries; don’t concentrate stress at a few localized spots (thus increasing fatigue resistance); and don’t require refinishing steps or leave protrusions (which is aesthetically more pleasing).
Compared to other types of adhesives, structural adhesives have the highest load-bearing capability; excellent environmental and chemical resistance; are generally formulated to be 100% solids (so there are no solvent emissions to deal with); and come in a range of cure times and properties. Structural adhesives cure in an irreversible process, which helps provide excellent temperature and solvent resistance. They do not need access to air to dry; nor moisture (like one-part silicone and polyurethane sealants); and thus have unlimited depth of cure.
In fact, there are so many characteristics and applications for structural adhesives that an engineer may have difficulty selecting which structural adhesive to use. Compared to other adhesives, however, structural adhesives are less intuitive to use and their performance can be widely affected by processing decisions.
Selecting a Structural Adhesive
In choosing a structural adhesive, consultation with an expert (such as a technical engineer at a supplier or an outside consultant) is invaluable. But, in some cases, preliminary decisions may be made prior to more specific discussions, or perhaps the applications are too sensitive to discuss with a range of outside experts. In that case, general principles for choosing a structural adhesive can be observed by the engineer. Regardless of the route chosen to select structural adhesives to test, the key is testing — no final decision should be made without specific validation testing. However, key principles can be used to select a set of adhesives to test.
Structural adhesives should be chosen with the end use requirements firmly in mind. Once these are known, the proper adhesive can be selected by matching the requirements to the different processing and performance characteristics of different structural adhesives.
In particular, end use conditions to consider include:
Expected conditions during end use. How hot or cold will the temperature be? Is there humidity? Will the material be exposed to rain or salt water? Will the joint be exposed to the sun and can the UV rays penetrate the substrates to reach the adhesives?
Chemical resistance required. Will the joint be in contact with fluids, such as motor oil, gasoline, diesel fluid, or jet fuel? Will it be cleaned frequently with weak acid or base cleaning solutions? Are there any specialized chemicals, which may contact the bonded part?
Cleanliness. Will it be subjected to outgassing, ionics, corrosion? Is toxicity/disposal a concern (such as in food packaging or medical devices)?
Mechanical challenges. Will the part be subjected to high impact or vibrational forces while in use? How high are the stresses on the bondline?
The general answers to the above questions will help determine which type of structural adhesive should be considered, while the specific answers to the questions (such as how many degrees of temperature does the adhesive experience in end use or how many pounds of weight must the joint support?) will help determine which specific products should be chosen based on the manufacturer’s data sheets and application test results.
Types of Structural Adhesives and Their Performance Criteria
Structural adhesives can be generally categorized by chemistry. Here, we’ll define “structural” adhesives as those routinely capable of overlap shear strengths in excess of 1,000 psi when bonding metal and testing at room temperature. Although hybrid products can be formed, in general the categories of structural adhesives are:
- Epoxies (one- and two-part formulations)
- Acrylics (two-part and two-step formulations)
- Urethanes (two-part formulations)
- Cyanoacrylates (“instant adhesives”)
Certainly each type of chemistry can be tailored to some extent, but it is also possible to roughly compare the categories based on the general properties inherent in the chemistry. The chart below shows a rough correlation across the chemistries. Properties can be varied with the addition of numerous additives, such as thickeners (to increase viscosity or stiffness), dilutents (to decrease viscosity), plasticizers, etc.; and specific curatives and accelerators can be used to vary cure times. Engineers are thus cautioned to review the specific properties for adhesives of interest.
In general, certain trends hold true: Acrylics overall provide the highest bonding strength on plastics, and may also provide good bonds to metals, including many oily metals. However, they tend to have lower vibration/impact resistance than epoxies (thus, lower fatigue resistance) and lower performance at temperature extremes. Cyanoacrylates tend to provide good shear strength on many plastics and rubbers (although primers may be required), but are rigid and show low peel and impact resistance. Urethanes tend to be quite flexible, but have lower strength in general. They can be relatively good plastic and rubber bonders and generally are lower-priced than other categories of structural adhesives.
Epoxies come in the widest range of properties and can have the best overall properties on metals. Standard five-minute rigid epoxies that are commonly available in hardware stores tend to be brittle and are best suited to applications where relatively low stress and no impact are expected. Flexible epoxies, such as 3M Scotch-Weld Epoxy Adhesive 2216, have higher peel strengths and hence better impact performance; they are also good choices for parts that may require some flex in end use. Toughened epoxies, such as 3M Scotch-Weld Epoxy Adhesive DP420 and DP460, actually incorporate elastomeric regions, which absorb impact, and thus provide the highest shear, peel, impact, vibration, and fatigue resistance; hence, they are chosen for the most demanding end-use applications. In general, however, epoxies require rigorous cleaning of oils from metal joint surfaces (exceptions include most one-part heat cure epoxies and 3M Scotch-Weld Epoxy Adhesive DP920, which can bond to most oily metals).
Based on these generalities, if an engineer is looking to bond ABS to stainless steel, for a part that will see moderate environmental stress (e.g. -20°F to 150°F, for example) but little vibration or impact, the choices would likely include epoxies and acrylics. Add vibration or impact to the criteria, and the choices would tip towards epoxies or specifically toughened acrylics. An engineer seeking to bond wood to plastic for outdoor use, where significant movement of the substrate may occur due to changes in the weather, would consider urethanes for their flexibility; if higher strength is needed, a flexible epoxy could be chosen.
Bonding a low surface energy plastic (such as HDPE) to itself or to another plastic would require a specialty acrylic, such as 3M Scotch-Weld Structural Plastic Adhesive DP8010. In some cases, of course, ultimate strength or performance in a property category may not be required. In this case, the engineer may be able to trade off performance for handling and processing capabilities.
Excerpted from a white paper by 3M
To read this white paper in its entirety, visit 3M online.