Keep that noise down!

Tom Epple
Senior R&D Associate
Avery Dennison
Painesville, Ohio

Edited by Victoria Reitz

Appliances, such as refrigerators and computers use pressure-sensitive adhesives to control noise.

Foam sound absorbers disrupt sound waves and absorb vibrating air molecules.

Sound barriers block sound waves from transmitting.

Free-layer damping adheres a semirigid material to a vibrating source.

In constrainedlayer damping, a viscoelastic material is sandwiched between a rigid material and a panel.

Squeaks, rattles, and hums are not music to an appliance designer's ears. These nuances, specifically those caused by vibration or the friction between two dissimilar materials, can be controlled with the use of pressure-sensitive adhesives (PSA). In dry or solvent-free form, these tapes are permanently tacky at room temperature. They can be used from simply bonding plastic foam to metal panels all the way to specialized vibration-damping applications that rely on the viscoelastic properties of certain adhesives.

Any fastener selected for sound abatement must be done with the surroundings in mind. The fasteners must work with the appliance environment, which can range from hot, dirty motors to machine panels and cannot add sound. Mechanical fasteners can be used, but must be engineered into the design. They are not easily changed. Glues often require special application equipment as well as time to dry. PSA's on the other hand, adhere to surfaces with finger or hand pressure and don't require activation by water, solvent, or heat.

Absorbers and barriers
Proven devices for sound and vibration control include sound absorbers, sound barriers, antifriction films, and vibration dampers. PSA's play an active role in each. Sound absorbers trap the sound after it's created. They are usually polyester and polyether urethane foams with varying thickness and density and are primarily used for low-frequency applications, 1,000 Hz or less. Sound absorbers allow sound waves into the material then disrupt, dissipate, and absorb the energy. This prevents sound from rebounding or reverberating onto panels or surfaces.

Foam sound absorbers are a common, low-cost approach to reducing noise and can be added after completing the design. Sound absorbers are generally located in cabinets, for instance, the metal shells of washing machines.

Sound barriers function the same as absorbers but prevent sound in a different way. They block sound waves from transmitting but let sound rebound or echo back to the source. Sound barriers are effective over a wide range of frequencies, but work especially well on frequencies greater than 1,000 Hz.

Sound-barrier materials are usually rubber or rubber-filler blends. They are much denser and usually much thinner than foam absorbers and are generally located around the noise source, such as a motor or engine. High-density barriers absorb sound energy. Such barriers are usually more expensive than absorbers, and are often used where heat and environ-mental exposures require more durable materials.

In addition to blocking sounds, barriers can also act as vibration dampers. Because barriers are typically thin, dense materials, when applied to a thin metal panel it will stiffen the metal and reduce its vibrations.

Antifriction materials
Parts rubbing together are another source of noise in an appliance. There are many metal-to-metal contact points in an appliance, and when they rub, they make noise. Equally important and often overlooked, is that contact points also propagate vibrations throughout the entire appliance, which magnifies sounds.

It should be no surprise that applying antifriction devices at points around the appliance where parts contact and rub reduces squeaks and rattles. Molded plastic buttons are the most common antifriction device. They typically snap and attach mechanically to key locations within the appliance. The molded buttons are generally made from a durable plastic. While they do reduce metal-to-metal contact, they must be engineered into the design, which limits their use. Harder plastics still transmit some noise.

One recent noise-abatement idea uses ultrahigh-molecular-weight polyethylene (UHMWPE) film, a tough and durable plastic, attached with PSA's. UHMWPE often comes die cut to an application's specific shape and can be applied almost anywhere in the appliance where materials rub together.

Vibration damping
Damping, which stops noise at the source is another way to control sound. Vibrationdamping materials are viscoelastic polymers that absorb energy and dissipate the vibration. These treatments provide a permanent solution and better noise reduction than absorbers or barriers. They come in different grades or densities to reduce vibrations over a wide range of frequencies and temperatures. Vibration dampers reduce noise from internal components associated with appliance operation and also reduce the noise associated with the sides of an appliance. A damped appliance will sound less "tinny" and more solid to a consumer rap-ping the side.

Two primary vibration-damping methods are called free layer and constrained layer. Free-layer damping adheres a semirigid material to a vibrating source, such as the outer shell of an appliance. The damping materials are typically heavy and thick. The damping material stiffens the shell, and as it flexes the damper stretches and dissipates the vibration energy. This significantly reduces the vibration and hence the associated noise.

A constrained-layer damper is a viscoelastic material sandwiched between a rigid material and a panel. Similar to free layer dampers, the damping material stiffens the shell of the appliance and dissipates vibration energy through viscoelastic stretching, thereby reducing vibration and noise. Viscoelastic materials in constrainedlayer dampers are almost exclusively pressure-sensitive adhesives. The rigid material, or constraining layer, is typically aluminum or steel. The computer hard-disk industry extensively uses constrained-layer dampers to quell vibrations and reduce noise from hard drives.

Selecting a PSA
When using a PSA-based product for sound control, the first question to ask is what materials will be bonded. As obvious as this seems, it is crucial to understand the surfaces that contact the PSA tape. Plastics are a common substrate, but there are many different compositions with varying surface energy levels. Surface energy is a measure of how well an adhesive "wets out" or uniformly flows over the substrate surface. High-surface-energy materials allow excellent wet out and provide good adhesion. Examples include stainless steel, glass, copper, and aluminum. High-surface-energy plastics include nylon, polyester, ABS, polycarbonate, and rigid acrylics. Low surface energy materials, such as molded polypropylene, do not allow the adhesive to wet out and require more aggressive adhesives such as rubber-based and modified acrylic formulations. Some low-surface-energy materials may require special treatments to promote better adhesion, such as a corona treatment or a primer topcoat.

Surface texture also impacts the PSA bond. Textured materials do not allow 100% surface contact which results in lower adhesion. Using a heavier, softer, or more conformable adhesive construction, or heating the laminate improves a PSA's performance on a textured material.

Rigorous conditions and service environments require special considerations in selecting the right PSA. For example, acrylic adhesives are better suited for applications exposed to direct sunlight. Exposure to direct sunlight and UV rays are likely to prematurely age a rubber-based PSA and can discolor, embrittle, and weaken its bond. If temperature resistance and elevated temperature performance are needed, acrylics and UHA acrylics are ideal choices, since general rubber-based PSA's tend to become viscous

and weaken bonds at temperatures greater than 120°F. However, at low temperatures, acrylic PSA's tend to solidify. Below 40°F PSA's begin to lose their initial tack and become more difficult to apply. In such cases, rubber-based PSA's with typically lower crystalline points are recommended. Gasoline or oil applications can adversely impact and deteriorate the PSA bond. In such cases, a firm acrylic is suggested to withstand the harsh environments.

An application can have a critical or noncritical bond. A noncritical bond holds an item in place until it can be mechanically fastened. In such cases, there are many low cost rubber-based PSA's that do the job provided the surface is clean and uncontaminated. When a bond is critical and requires higher levels of adhesion, tack, or shear, more care must be taken to identify the right PSA formulation. PSA tapes are engineered for specific applications. Adding different and varying amounts of resins causes variance in adhesion, tack, and shear values, often at the expense of other values.

Pressure-sensitive adhesives are generally categorized as rubber-based, acrylic, and modified acrylic. Rubber-based adhesives are natural or synthetic rubbers blended with tackifying resins, oils, and antioxidants and typically produce the most cost effective PSA systems. These adhesive systems provide high initial stick but generally are not suitable for temperatures over 140°F or harsh environments.

Acrylic polymers and other additives are formulated together to produce adhesive systems that have better aging properties and environmental resistance. Acrylic adhesives are firmer and offer a low initial tack and adhesion. Typically they are more costly than rubber-based systems.

Modified acrylic polymers are used as the base component but include certain components found in rubber-based systems such as tackifiers. The additives improve the initial tack and adhesion levels while decreasing resistance to solvents, plasticizers, and high temperatures.

PSA performance reference guide
MATERIAL CHARACTERISTIC	RUBBER BASED	ACRYLIC	MODIFIED ACRYLIC
Tack	Med-high	Med-low	High
Temperature resistance	Low	High	Low-med
Adhesion	Med-high	Med-high	High
Cohesion	Medium	Med-high	Low
Solvent resistance	Low	High	Low-med
UV resistance	Low	High	Low
Plasticizer resistance	Low	Med-high	Low-med
Bonding to low-surface energy materials	High	Low-med	High
Bonding to high-surface energy materials	High	High	High
Cost	Lowest	Med-high	Med-high