Adhesion
Adhesive bonds are often categorised into two types: primary bonds and mechanical bonds. Primary bonds form when epoxy is applied to a layer of epoxy that hasn’t fully cured yet. In this case, the layers bond as if they were mixed together at the same time. Mechanical bonds, on the other hand, depend on the epoxy adhering to a substrate. To visualise this, think of the epoxy flowing into all the imperfections and sanding scratches of the substrate, locking into place. The actual forces involved are much more complex and beyond the scope of this discussion.
The key principles for achieving strong secondary bonds remain consistent, regardless of the understanding of interatomic forces. These principles include:
- Wet out the substrate: The epoxy must be able to flow and wet the surface. Surface contaminants can prevent this from happening.
- Allow the epoxy to cure in place: Clamps must remain in place, and the joint should not be stressed until the epoxy cures.
- Use a high-strength epoxy: WEST SYSTEM® Epoxy is a structural adhesive once it has cured.
Check out our useful Epoxycraft Article: A Guide To Epoxy Bonding.
How Does Tensile Adhesion Impact Adhesive Performance?
Tensile adhesion refers to the stress experienced by an adhesive when a load is applied perpendicular to its surface. This type of stress can cause adhesion failure at levels much lower than the adhesive’s actual tensile strength. It’s crucial to distinguish between tensile adhesion value and tensile strength. For instance, WEST SYSTEM Epoxy is engineered for high tensile strength, making it ideal for creating durable fibreglass laminates. It also boasts high tensile adhesion to common substrates, ensuring strong bonds. Similarly, WEST SYSTEM G/flex® is formulated to achieve superior adhesion values.
In practical applications, pure tensile adhesion loads are rare. When a tensile load is applied, it often causes the substrate to deflect, introducing peel stress that can significantly weaken the assembly. Moreover, it’s uncommon for components to be assembled in a way that the load path is purely tensile. Even slight off-axis loads can induce considerable peel stress.
To measure tensile adhesion, a PATTI device (Pneumatic Adhesive Tension Testing Instrument) is used. This device pulls bonded studs perpendicular to the surface and measures the load required to detach them. The PATTI device operates by using compressed air to inflate a bladder, lifting a metal stud glued to a substrate. A gauge then measures the tensile force needed to break the bond.
Tensile Adhesion of G/Flex Epoxies
| Material | G/flex | Surface Prep/Conditions | Tensile Adhesion (PSI)/(MPa) |
|---|---|---|---|
| G-10 high-density laminate | 650 | 80-grit sand / dry surface | 3459 / 23.85 |
| G-10 high-density laminate | 655 | 80-grit sand / wet surface | 2473 / 17.05 |
| 1018 steel | 650 | 80-grit sand / dry surface | 3562 / 24.55 |
| 1018 steel | 655 | 80-grit sand / wet surface | 1772 / 12.22 |
| Galvanised steel | 650 | 100-grit wet sand | 2562 / 17.66 |
| Galvanised steel | 655 | 100-grit wet sand | 2929 / 20.19 |
| Copper | 650 | 80-grit sand | 2334 /16.09 |
| Copper | 655 | 80-grit sand | 2685 / 18.51 |
| Bronze | 650 | 80-grit sand | 2782 / 19.18 |
| Bronze | 650 | Scotch Brite™ pad sand | 2962 / 20.42 |
| Bronze | 655 | 80-grit sand | 2936 / 20.24 |
| 316 Stainless Steel | 655 | 80-grit sand | 2522 / 17.38 |
| ABS plastic | 655 | 80-grit sand | 1535 / 10.58 |
| ABS plastic | 655 | 80-grit sand, flame treat | 1813 / 12.50 |
| ABS plastic | 655 | Alcohol wipe, flame treat | 3288 / 22.66 |
| PVC | 655 | 80-grit sand | 1780 / 12.27 |
| PVC | 655 | 80-grit sand, flame treat | 1813 / 12.50 |
| PVC | 655 | Alcohol wipe, flame treat | 2081 / 14.34 |
| Polyethylene (HDPE) | 655 | 80-grit sand | 400 / 2.75 |
| Polyethylene (HDPE) | 655 | 80-grit sand, flame treat | 1890 / 13.03 |
| Polyethylene (HDPE) | 655 | Alcohol wipe, flame treat | 2312 / 15.94 |
| Polycarbonate (Lexan®) | 655 | 80-grit sand | 1870 / 12.89 |
| Ipe | 650 | 60-grit sand | 2134 / 14.71 |
| Ipe | 655 | Plane, isopropyl alcohol wipe x 3 | 2223 / 15.32 |
| Teak, vertical grain | 650 | 80-grit sand parallel to grain | 1413 / 9.74 |
| Teak, vertical grain | 655 | 80-grit sand parallel to grain | 1381 / 9.52 |
| Teak, vertical grain | 655 | 80-grit sand, alcohol wipe x 2 | 1503 / 10.36 |
| White oak, vertical grain | 650 | 80-grit sand | 1935 / 13.34 |
| White oak, vertical grain | 655 | 80-grit sand | 1780 / 12.27 |
| White oak, vertical grain | 655 | Alcohol wipe x 2 | 2212 / 15.25 |
| Purpleheart | 650 | 60-grit sand parallel to grain | 1731 / 11.93 |
CHOOSING THE RIGHT ADHESIVE?
When selecting a WEST SYSTEM Epoxy product for bonding, consider the following factors:
- Working Time
- Assembly Time
- Time Required Until Clamps Can Be Removed
- Cure Time
Failures are rarely due to the epoxy’s strength. Issues are more likely to arise if the epoxy cures too much before assembly or isn’t allowed to cure sufficiently before applying a load. Success is achieved when customers choose a WEST SYSTEM Epoxy labelled as a structural adhesive with a cure profile that suits their process and working conditions. Additionally, best practices include proper surface preparation and achieving the correct bond line thickness.
When designing, building or repairing a structure that relies on bonded components for strength, please call our Technical Advisors at +44 (0)1794 521 111 or fill out our Contact Form. We would be happy to discuss your project with you.