Polyurethane to Non-Polyurethane Bond Failure

In many applications a polyurethane is bonded to a steel or aluminum insert, hub or core. This can be difficult to accomplish, as many variables come into play to ensure a strong bond. Bonding is usually accomplished by the use of an adhesive applied to the insert prior to the casting of the polyurethane system.

Bond failure caused by release agent contamination.

Potential Causes

Poor Initial Bond
In parts which require bonding, precautions must be taken during the application of the mold release. Contamination of the bonding surface with mold release can weaken the bond line.

Too Much Stress at the Bond Line
Bond strength is dependent on many factors. At times, due to the design of the part, stresses are localized at the bond line, causing the bond to break.

Overheating
The adhesives generally used in these applications have an upper use temperature limit in the range of 200°C to 300°C. If the system temperatures exceed these levels, the bond strength may be compromised.

Water/Solvent Permeation
In some roller applications where the part is immersed in water or another solvent, the water or solvent may eventually migrate through the polyurethane and collect at the bond interface. This may cause degradation of the adhesive, or it may cause enough stress at the bond line to rupture the bond.

Solutions

Choose the Correct Primer or Adhesive
There are many different primers and adhesives that can be used for typical polyurethane applications. Choosing the correct adhesive to withstand the operating environment is critical. Consult your adhesives supplier for the proper recommendations.

Prepare the Surface Properly
Be sure the surface of the substrate is properly roughened to help improve the mechanical bonding of the system. Also ensure that the substrate has been properly cleaned after blasting and that all grease and oils have been removed. In addition, make sure that the mold release does not come into contact with the properly prepared substrate surface. Contact your adhesives supplier for additional recommendations.

Design the Part to Minimize Stress
When designing parts, there are many techniques that can be used to minimize the stress at the bond interface. We recommend the following: avoid sharp corners; design-in fillets and overhangs; and equalize stress profiles. See Elastomer Design for further details.

Provide Cooling
In cases where overheating of the bond line may be an issue, providing cooling to either the substrate or the polyurethane would be advantageous.

Choose the Correct System to Minimize Permeation
The correct choice of polyurethane system to minimize heat buildup is essential. See Blowout for further information on how to minimize heat buildup.