QUESTION: We have been using epoxy adhesives for many years in our assembly operations for metals and plastics. However, one of our suppliers is trying to convince us to try his polyurethanes. How do these adhesives compare to other engineering adhesives?
ANSWER: Polyurethanes do not have the extreme high performance of highly crosslinked epoxies or anaerobics, or the instant curing of the cyanoacrylates. However, they are versatile and can be formulated from hard or flexible systems to form bonds with high peel strengths and toughness. In many large-area bonding situations, they compete with reactive acrylics. They have also replaced polychloroprene contact cements in many applications.
Polyurethanes for assembly operations are usually one-component, moisture-cured systems or two-component mixable systems. The most common applications for polyurethane adhesives include the footwear industry, construction, woodworking, transportation, packaging and assembly operations.
Polyurethane chemistry is also used in reactive hot-melt urethanes (RHMUs), which are two-stage thermoset adhesives that are applied as molten liquids and subsequently crosslink after application through reaction with atmospheric moisture. Major applications for RHMUs include the assembly of large panels for recreation vehicles and campers, and the bonding of steel panels with an expanded polystyrene core for the production of insulated garage doors. 
It is necessary to look at all aspects of your bonding operation before considering a change of adhesive. Important considerations include:
  • Required rate of cure
  • Gap filling capability
  • Bonding oily or dirty surfaces
  • Tensile shear strength
  • Peel strength
  • Impact resistance
  • Load-bearing capability
  • Heat and cold resistance
  • Fluid resistance
  • Weatherability
  • Health and safety issues
  • Cost effectiveness