Benefits of Engineered Pressure-Sensitive Adhesives for Vibration Damping

Flexcon has been designing and manufacturing pressure-sensitive adhesive (PSA) containing composites for nearly 70 years. These products range from material used for bumper stickers to the highly engineered PSAs used for sound and vibration damping or for bonding to low surface energy materials such as silicone rubber. 

Our engineered PSAs cover a wide variety of application requirements. They include adhesives that:

• meet aerospace, medical, and security requirements
• are designed to form a bond in sub-zero cold, or to maintain adhesion at temperatures in excess of 500ºF (260ºC)
• bond to low surface energy substrates
• resist “out gassing”
• respond to low frequency AC
• greatly resist dielectric breakdown
• act as a thermal insulator, or facilitate heat transfer
• are designed to lift a fingerprint, or can leave a fingerprint on the substrate if removed
• can be cleanly (no residue) removed
• can survive the strict requirements for LOCA (loss of coolant accident)
• can bond sound damping materials to the inner skin of aircraft or can be the noise reducing component itself.

Vibration Damping Adhesives

Flexcon supplies engineered adhesives for use as the viscoelastic layer in constrained layer damping.

Viscoelastic Material

Materials can change their state or phase with changes in temperature. Water, for example, at moderate temperatures is a “liquid.” When the temperature cools to 32ºF (0ºC), water freezes into ice. When you increase the temperature to 212ºF (100ºC), water boils into a “gas” (vapor).

Polymeric materials also undergo significant changes in their state with changes in temperature. While measurable, the changes in state may not be as clearly obvious as melting ice or boiling water.

People often use the term “melting point” or “melt flow” for some polymers. However, it may not accurately describe the polymer's property. This is especially true when pressure affects the polymer or when the polymer undergoes cross-linking (thermoset).

In general, polymers have a more complicated relationship to temperature. For example, unlike water, they are also sensitive to the rate of temperature change.

Given these facts, just what do polymers do? What are their changes in state with respect to temperature changes? And how does all this relate to “vibration damping”?

Polymer transition states:

Glassy Phase - The Glassy Phase is roughly equivalent to the solid phase in water. The polymer is at its hardest in this phase. It does not work well for sound damping in the Glassy Phase. The polymer tends to let noise pass through the vibrating structure.

Elastic Phase - When you heat a polymer, it transitions from its Glassy Phase to its Elastic Phase. We call the temperature at which this occurs “the glass transition temperature,” designated Tg. This phase marks the beginning of vibration damping.

Viscous Phase - This phase is still good for damping. However, as the temperature rises at a constant frequency, the ability to recover starts to decrease.

Polymers are more complex than simple materials like water. They behave differently when the temperature changes. Polymer response to changes in temperature can vary by the rate of change in temperature.

Simply stated, the faster the rate of temperature change, the higher the Tg.

If the change to a polymer is mechanical, like a vibration, the same rules apply. The faster the change, or the higher the frequency, the higher the apparent Tg. This phenomenon is familiar to everyone who has ever played with Silly Putty. 

Roll the Silly Putty into a cigar shape, then hold it by the ends, and slowly draw it out. It seems as though we could stretch it forever or at least until our arms are at full extension.

However, when we pull the cigar quickly, in effect creating a “vibration,” the Silly Putty snaps like a piece of chalk. The higher the vibrational frequency, the more “solid” the polymer is at any given temperature. In other words, to keep the same viscosity of a polymer as vibrational frequency is increased, the temperature must be increased.

To measure vibration damping, we measure the Tg dynamically. This means we measure it at different frequencies and temperatures.

Loss Factor and Storage Modulus

The most useful metrics in evaluating a polymer’s damping potential are its loss factor and storage modulus.

When a ball drops from a height, the ball deforms, then bounces back easily. The bounce-back (recovery) is a measure of how much of the impact energy the ball could store and release to bounce back. This is the “storage modulus.”

The difference between the starting height and the recovery height is the “loss factor” or a measure of how much of the energy was lost to the surrounding environment as heat. The ratio of these two quantities is a useful measure of damping ability. We define the tan d (tan delta) as the ratio of the loss factor to the storage modulus.

One way to show how temperature and frequency interact is to run Tg tests at a set temperature. This process occurs over a range of vibrations. We then go to another temperature region and again evaluate the Tg as a function of changing frequency. 

Please note that, from the polymer’s point of view, it would make no difference if the test were conducted at a constant frequency, then a varied temperature, then the cycle was repeated again at another constant frequency, varying the temperature again, and so forth. Thus, we see the limits on the use of viscoelastic liquid for vibration damping at a given frequency.

If the temperature of the polymer is too low, it may be in its Glassy state. If the temperature of the polymer is too high, it may be in the very fluid part of the Viscous state and have too low a storage modulus to recover.

Reduced Frequency Nomograms

Polymer chemists have long used the concept of the “master curve” to graphically represent complex relationships in a concise, useful fashion. The master curve of the relationship between temperature and frequency as they relate to Tg changes and vibration damping is called the “Reduced Frequency Nomogram."

Here we note that a reference point was chosen to begin construction of the nomogram. While it is possible to start the nomogram at any point, it is most useful to select a point around a key element, such as the “loss factor” approaching a value of 1 (101).

Reading the nomogram, the Flexcon SA6000 series adhesive seems to maintain good damping properties over a temperature range of -180°F to -280°F. Furthermore, the frequency range is indicated as roughly 500-15000 cycles/sec (Hz). This turns out to be in a good range for certain disc brakes on cars. Indeed, manufacturers have used Flexcon silicone adhesives in this application for a number of years.

Flexcon’s tacky silicone adhesives in the SA6000 Series offer the same pressure-sensitive features as traditional PS adhesives. However, they also work well in very low or high temperatures where acrylic adhesives fail.

Using Viscoelastic Materials into a Damping Treatment

Measuring a polymer's properties is important. Creating its reduced frequency nomogram is also key.

These steps help define a damping treatment for noise and vibration issues. But we need one more step -- we must construct the actual device. By placing a viscoelastic layer between two more rigid layers, the composite forms a constrained layer damping system. 

The constrained layer damping system uses the displacement/recovery of the viscoelastic material. The vibrational input is transmitted through the rigid constraining layer to the viscoelastic layer.

The change in this viscoelastic layer helps turn vibrational energy into thermal energy. This process is known as the loss factor. As the final stage in the cycle, the viscoelastic layer recovers, and is thus ready for the next vibration (storage modulus).

Flexcon has adhesives that can work with many different vibration and temperature combinations. There may not be one viscoelastic material for all frequency and temperature needs.

Flexcon can provide these adhesives in different formats. They can be in transfer tape, as part of a single-coated product, or in a double-coated design. The double-coated option uses an extra adhesive to bond two surfaces that do not stick well together.

Conclusion

Flexcon offers adhesive solutions and technical expertise that benefit many industries, from aerospace to automotive to appliance. By working closely with OEM Design Engineers, we can use Flexcon’s special PSA skills to solve tough problems in noise and vibration control. Contact us for technical information on Flexcon’s vibration damping adhesives and materials.