There are metal adhesives and plastic adhesives but what to do when you want to bond plastic to metal? There are metal adhesives that bond some plastics and plastic adhesives that bond some metals. The specific substrates, as well as the surface condition of the substrates, the types of stresses on the bond, and the environment the bond must withstand help in selecting the best adhesive.
For selection purposes, the best place to start is generally the type of plastic you need to bond. Some difficult to bond plastics will leave you with only one answer. For example, plastics such as polyethylene and polypropylene can be difficult to bond without preparation of surfaces unless you use Permabond polyolefin bonders such as TA4610.
A variety of adhesives that bond metal also bond plastics with higher surface energy. Note that mold releases and plasticizers that leach to the surface can affect the surface energy of the plastic.
Metals have unique properties in terms of creating adhesion problems. Particulates, chemicals, and oxidized layers, can affect the surface of metals. Additionally, almost invisible area scarring and pitting that may create adhesion problems on the surface. Some alloys need treatments like abrasion and cleaning for better bonding. To determine if a surface is properly prepared for bonding and to increase adhesion and durability of bonds to metals, Permabond 2K Primer is recommended.
When bonding dissimilar materials, tensile, shear, peel, and compression loads placed on the joint may not be the most significant stress on the bond. Every type of metal and plastic expands and contracts with temperature change at different rates. The size of the bond area, as well as the size of the parts, can create additional stresses.
In cases where the environmental temperature is expected to vary considerably, a flexible adhesive can absorb stresses from the different expansion and contraction rates.
There are a variety of options to bond plastic to metal. Two-component structural acrylics like TA4610 mentioned above are one option. Another is a surface activated MMA such as TA4246. The initiator is applied to one surface and the adhesive to the other. In other cases, instant adhesives such as the original, Permabond 910.