The Benefits of Using a Structural Adhesive

Based on an original 3M publication.

Structural adhesives have excellent benefits when used in particular assembly operations needing tough, durable and permanent results instead of traditional mechanical fastening methods or other types of adhesives. They include epoxies (one and two part formulations); acrylics (two-part and two-step formulations); urethanes (two-part formulations); and cyanoacrylates (instant adhesives) with overlap shear strengths in excess of 1000 psi when bonding metal and testing at room temperature.

Structural adhesives:
  • Cause no substrate damage (e.g., drill holes, heat distortion)
  • Join materials without galvanic corrosion
  • Suited for a number of different geometries
  • Reduce localized stress points — increased fatigue resistance
  • Eliminate refinishing steps; leave no protrusions (better aesthetics)

What makes them different?

With the many characteristics and applications available to you, what determines your decision? Because structural adhesives remain less predictable, processing decisions can affect overall structural adhesive performance.

Structural adhesives, when compared to other adhesives:
  • highest load-bearing capability
  • environment and chemical resistance
  • generally, formulate to 100% solids (produce no solvent emissions)
  • offer different cure times and properties, temperature and solvent resistant properties (irreversible cure process)
  • unlimited depth of cure unlike one-part silicone and polyurethance sealants.

How to choose a structural adhesive

Depending on your situation, you may want to tap a consultant or subject matter expert, however, no matter what--the structural adhesive performance should be validated by testing. When making your selections for testing consider:

End use conditions
  • Temperature — hot or cold environment
  • Humidity — rain, salt water exposure
  • Ultra violet light — exposure to sunlight with possible UV penetration through substrates to adhesive
Chemical resistance needed
  • Joint exposure to fluids (motor oil, gasoline, diesel fluid, jet fuel)
  • Joint cleaning frequency (weak acids and bases)
  • Bond exposure to specialized chemicals
  • Continual or only occasional exposure, e.g. in a filtering assembly
Cleaniness and environmental considerations during production and end use:
  • Sensitivity of bonded part to outgassing, ionics, corrosion (e.g., electronics, optics applications)
  • Adhesive usage, i.e., toxicity levels, such as regulations pertinent to food packaging or medical devices
Mechanical challenges
  • Impact and vibration exposure — will the bonded part be subject to high impact or vibrational forces in use?
  • Stress type and magnitude — height and types of stresses on bondline

Know your adhesives

Acrylics - provide the highest bonding strength on plastics and may provide good bonds to metals and many oily metals. Commonly have lower vibration/impact resistance than epoxies, therefore lower fatigue resistance and performance at high temperatures.

Cyanoacrylates - provide good shear strength on many plastics and rubbers (primers may be required), rigid, show low peel and impact resistance.

Urethanes - very flexible, however lower strength in general. Good as plastic and rubber bonders and often lower priced than other categories of structural adhesives.

Epoxies - available in the widest range of properties and possess the best properties for use on metals. Includes standard 5-minute rigid epoxies commonly found in hardware stores and can be brittle. Use with applications with moderately low stress and where little impact can be expected.

Flexible epoxies, such as 3M™ Scotch-Weld™ Epoxy Adhesive 2216 have higher peel strengths resulting in better impact performance; a good choice for parts requiring some flex in end use.

Toughened epoxies, such as 3M™ Scotch-Weld™ Epoxy Adhesive DP420 and DP460 incorporate elastomeric regions - absorb impact and provide the highest shear, peel, impact, vibration and fatigue resistance. Good choices for very demanding end-use applications. Generally, epoxies require rigorous cleaning of oils from metal joint surfaces. Exceptions include most one-part heat cure epoxies and formulas like 3M™ Scotch-Weld™ Epoxy Adhesive DP920, which bond to most oily metals.

 

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