Metal bonding epoxy adhesives are some of the strongest glue for metal

We are often asked “What is the strongest glue for metal to metal?”  For most applications the strongest metal glue is one part metal bonding epoxy also known as heat activated epoxy (heat curing epoxy).  Typical bond strengths of three adhesive types are compared below.

Strongest metal glue?

• Adhesive Type                                    Typical Strength on Steel
• One part epoxy                                   6000 psi (41 N/mm²)
• Two part epoxy                                   3000 psi (20 N/mm²)
• Structural acrylic                                 3600 psi (25 N/mm²)

For the highest bond strength, one part epoxies are the best metal bonding epoxy.

With the onset of all of the new plastics and composites, metals continue to be desirable in applications that require a heat resistance.  High temperature epoxy adhesives are usually one part epoxy.  Specialty two part epoxy adhesives that have a secondary heat curing phase achieve higher temperature resistance than conventional two part epoxy.

One part epoxy adhesives are not only the strongest glue for metal to metal, they also provide the highest heat resistance.  Standard and high heat resistant adhesive types are listed below.

• Adhesive Type                         Standard Heat Resistant Adhesive     High Heat Resistant Adhesive
• One part epoxy                       350°F (180°C)                                      570°F (300°C)
• Two part epoxy                       175°F (80°C)                                        570°F (300°C)
• Structural acrylic                     300°F (150°C)                                      390°F (200°C)

Heat activated epoxy

In addition to strength and heat resistance, when selecting a one part  metal bonding epoxy adhesive, it is important to determine if and how much you’d like the adhesive to flow prior to curing.  As the temperature in the heat curing oven raises, some epoxies can flow or sag.

Three primary categories of one part epoxy adhesives are:

1. No Sag – the adhesive will not slump, sag, or flow as the temperature is raised.
2. Flowing – the epoxy will level out, spread slightly
3. Free Flowing – the epoxy will flow, if it isn’t contained physically, it may run out of the bond area.

Chemical Resistance

So far we’ve determined that one part epoxy is the strongest glue for metal to metal and it is a high heat resistant adhesive, the third factor in selecting metal glue is chemical resistance, and once again, one part epoxy lead the pack!

Below is a comparison of the same adhesive types for non-polar solvents.  For example, non-polar solvents include: motor oil, toluene, gasoline.

• Adhesive Type            Resistance to Non-Polar Solvents
• One part epoxy           Excellent
• Two part epoxy           Very Good
• Structural acrylic        Very Good

Examples of polar solvents include: water, ethylene glycol, isopropyl alcohol, and acetone.  All three types of adhesives listed above have very good resistance to polar solvents.

Limitations

In addition to strength, chemical resistance, and temperature resistance, one part epoxy adhesives also provide the following benefits.

• Rubber toughened for high-peel strength and impact resistance
• No mixing required
• Excellent environmental durability
• Rapid cure increases production rates
• Solvent free improves workplace safety
• Cost savings over brazing and welding
• Low odor improves workplace environment

So if one part epoxy adhesives are all that, why choose anything else?

1. Adhesion – although one part epoxy adhesive provide excellent bond strength on metal, other types of adhesives outperform them on plastics and other substrates. MMAs, UV cure adhesives, instant adhesives (CA glue), all provide more adhesion on plastics.

2. Cure process – one part epoxy heat cure schedules vary by product but can be too high for many plastics – an example follows.

• 266°F (130°C) 75 mins
• 300°F (150°C) 60 mins
• 338°F (170°C) 25 mins

For many applications this process is ideal, for other applications it can be cumbersome.

Although heat cure is a fairly quick and easy process, dispensing two component epoxy adhesives via static mix tips and just allowing them to cure at room temperature is preferable by many.  In other applications the instant cure of cyanoacrylates or cure on demand of UV curable make processing more efficient.  For many metal to metal or metal to magnet applications no mix structural acrylics in which an activator is applied to one surface and resin to the other – are ideal as the handling time is less than 5 minutes.

3. Appearance – one part metal bonding epoxy adhesives are generally opaque, clear products are available but can yellow slightly which can be apparent if used in a thick layer.  UV cure adhesives are ideal for transparent, optically clear, non-yellowing bond lines.

4. Viscosity – one part epoxy adhesives have relatively high viscosity. Water thin wicking grades are not available.  However, when heated, viscosity can reduce in free flowing grades  but they will not become water thin like anaerobic adhesives and sealants or cyanoacrylates.

5. Shore Hardness – one part epoxy adhesives are hard, typical shore hardness is D80 to D85. Consider modified two component epoxy or MS polymers for applications that require flexibility.

For further help, please contact Permabond.