Structural Bonding: Epoxy or MMA (Methyl Methacrylate)?
Recommending the Right Bonding Technology for Structural Bonding Specs
Ask yourself the following questions to find the right solution:
- One-part epoxy?
- Two-part epoxy?
- Standard MMA?
- MMA Core-Shell®(some MMA adhesives are filled with elastomer beads that have hard shells and soft centers to give them greater flexibility and unique fatigue-resistance properties).This patented technology (Core-Shell® technology) is available only in the ITW Plexus range.?
- Should a room-temperature-cure adhesive be used? Or should the parts be heated (to 150°C/302°F, for example)? One-part epoxies can be immediately ruled out if it is not possible for the user to heat the parts.
- What is the maximum temperature the bond will be exposed to?Only one-part epoxies are suitable for high temperatures (up to 180°C [356°F]. Methyl methacrylates withstand temperatures up to 120°C (248°F) while two-part epoxies withstand temperatures up to 80°C (176°F) (120°C [248°F] in the case of Permabond ET540and Permabond ET5401).
- What is the required mechanical strength? 1K (one-part) epoxies have a shear strength of up to 40 MPaversus 20-25 MPa for 2K (two-part) epoxies and standard MMAs. Note that Core-Shell® MMAs have a lower shear strength of around 12 MPa.
- Is flexibility a requirement? Although not as flexible as an adhesive sealant, Core-Shell® MMAs and some two-part epoxies are relatively flexible and withstand vibrations better than one-part epoxies and standard MMAs, ensuring better fatigue resistance.
- What is the gap to fill? Two-part epoxies do not yield good results with gaps measuring less than 1 mm.
- What is the substrate to be bonded? All the technologies are possible with standard metals. The Plexus range of special adhesives (MA-3940 and MA-3940LH) works best for composites and plastics. Note that glass is not an easy substrate for structural bonding. Some epoxies may work.
- Will the bond be in contact with chemicals? If the answer is yes, one-part epoxy is the best solution. MMAs (methyl methacrylates) are more susceptible to attack.
- Is clarity a requirement? If the answer is yes, ET500, ET515 and MA685 are the only suitable products.
Summary table
| One-part epoxy | Two-part epoxy | MMA Standard |
MMA Core Shell |
|
| RTV curing | No | Yes | Yes | Yes |
| Maximum temperature | 180°C (356°F) | 80°C (176°F) (120°C/248°F for ET540 and ET541) |
120°C (248°F) | 120°C (248°F) |
| Mechanical strength | 40 MPa | 20 MPa | 25 MPa | 15 MPa |
| Flexibility | No | Depending on grade | No | Yes |
| Fatigue resistance | – | + | + | +++ |
| Chemical resistance | Excellent | Good | Average | Average |
| Clarity | No | ET500/ET515 | MA685 | No |
| Min. gap | 0.25 mm | 1 mm | 0.5 mm | 0.5 mm |
| Max. gap | 5 mm | 5 mm | 4 mm | 10 mm |
| Metal bonding | +++ | +++ | + | +++(MA830 / MA832) |
| Glass bonding | – | – | – | – |
| Plastic bonding | – | + | ++ | +++(MA3940 / MA3940LH) |
| Composite bonding | + | + | +++ | +++ |