Optimizing Cure Process for Special Silicone-based Pressure Sensitive Adhesive

A standard method for reducing / damping vehicle brake noises involves use of what is generally referred to as a shim or insulator. In its simplest form, a shim may consist of metal substrate with a pressure sensitive adhesive (PSA) applied on one side. The PSA has two roles to play :

1. Bonds the metal skin to the brake backing plate
2. Provides noise reduction due to its viscous and elastic properties.

For applications requiring increased heat resistance, silicone-based PSA are often selected. Although application of heat is not necessary for bonding pressure sensitive adhesives, some manufacturers of brake products do when attaching shims to the backing plate. Heating during the bonding process lowers the viscosity of the PSA and causes it to flow more readily and achieve good contact. This case study describes how the cure process for one such material was optimized to achieve a partially cured product. In the bonding process, the partially cured adhesive would flow more readily than a fully cured one. During the bonding process, full cure is achieved.

In mass production, shims are stamped from coils of product. In the manufacture of finished products, the adhesives are generally solvent-borne mixtures formulated for the particular processing techniques being used. Converting a solvent-borne coating to a finished coating involves sequential process steps. The first step involves removal of solvents by controlled evaporation at a suitable low temperature and process speed. This is followed by a period of time where the temperature of the substrate and coating is increasing, and the solvent-free coating flows to yield a thin, uniform layer. Finally, the coating reaches the optimum temperature for completing the curing reaction.

When processed correctly, the material will exhibit characteristic mechanical and noise damping properties. If not done correctly, mechanical properties such as cohesive strength of the adhesive may be affected. If the cohesive strength becomes too low, the adhesive may tend to flow and ooze (leak) at the edges of the material causing a sticky mess during stamping of the shims and during bonding at the customer.

The chart in figure 1 illustrates some properties of an adhesive at three states of cure. It is very important that the required time/temperature profile be determined as part of scaling up for production, and it should be regularly verified as part of an on-going quality program.

In the process optimization study at hand, six distinct tasks led to successful production of commercial material involving a partially cured adhesive that did not exhibit oozing during stamping into shims, storage prior to use by the customer, or during bonding operations at the customer. The tasks :

• Laboratory-scale testing to establish time/temperature profiles to yield the non-oozing adhesive.
• Establish concentration of un-reacted cure agent using Differential Scanning Calorimetry (DSC).
• Production-scale trials to validate the time/temperature profile for the non-oozing adhesive.
• Establish concentration of un-reacted cure agent in production material using DSC.
• Experimentally record the time/temperature profile through the entire sequence of solvent removal, increasing temperature, and cure reaction.
• Complete documentation of processing parameters and control methods.

To see the resume of the expert associated with this case study, see the link below.