Sealants have grown in sophistication in recent years as the markets have grown more demanding. The emergence of new materials has spawned a range of sealant types as varied as their applications. The overall market for sealants has grown dramatically as they become available to a broader spectrum of consumers, from Do-It-Yourself (DIY), to factory-made housing, to automobile manufacturing.
Elastomeric sealants can perform a variety of functions other than merely bond and seal. One of these additional functions is the damping of noise and vibration. The reduction of noise and vibration is an important and valuable consideration in many products, including automobiles, lawn mowers, industrial machinery, hard disc drives, and residential construction. The two industries that have traditionally had the most need for damping treatments are automotive and aerospace, where considerations for passenger comfort and component reliability are equally consuming.
This article will review noise-damping sealants that have created a niche market in the above applications. In the context of this article, noise and vibration will be treated much the same, for they originate from the same sources. In any situation where noise is generated, all vibration sources must be considered. Along with this, the conductive paths that transmit noise must also be addressed.
Noise – A Common Environmental Pollutant
The unwanted effects of noise can include: damage to hearing, headaches, induced stress and inefficiency in workers, as well as general displeasure of neighbors and pets. However, the control of damaging vibration is an essential factor in mechanical systems design. Uncontrolled resonant vibrations could result in excess noise, geometric distortions, structural fatigue, and possible component failure. In the auto industry, excessive noise has both comfort and safety implications, particularly for the driver of a vehicle. Effects range from simple interference with speech to the more serious onset of fatigue and loss of concentration.
The problems of noise or vibration are a common occurrence, and they can often be handled by simply increasing the mass of the offending product. The difficulty is that there is always a tradeoff that needs to be considered. Such tradeoffs generally are with regard to weight, space, cost, and aesthetics. However, usually a more efficient solution is the application of a noise or vibration damping treatment to the product.
One of the most interesting properties of viscoelastic adhesives and sealants concerns the damping behavior of these products. If the bonding is adequate, a part of the kinetic vibration energy is converted to heat and dissipated from the system through thermal conduction. Purely elastic materials store energy during loading and then fully return when the load is removed. However, viscoelastic materials have a high damping coefficient that returns only a portion of the stored energy and dissipates the rest as heat after the load is removed.
The hysteresis (energy loss) reduces the amount of “bounce back” caused by unwanted vibration and, thereby, provides effective damping. Additionally, the fluid-like viscous response of these polymers lets them deform uniformly under load. They transmit applied force in all directions and distribute a small amount of pressure over a large area.
An example of this mechanism is evident in the use of a polyurethane adhesive in bonding a steel frame to the floor of an automobile or bus. The transmission of the engine and wheel noise through the body is considerably damped by the properties of the polyurethane. The result is a noticeable reduction in the noise level in the passenger compartment.
Solutions to the Noise Problem
There are two main types of acoustical damping configurations: extensional and constrained. Extensional damping is the easiest to employ, and it is commonly used in the automotive industry. The main areas where damping materials are used in automobiles includes floor, dash panel, roof, doors, and brakes.
An extensional damping treatment is a single-layer coating in which energy dissipation occurs primarily from the flexural and extension motions of the damping layer. This is simply illustrated by a relatively thick coating of viscoelastic material on a substrate, which is prone to vibrate (Figure 1a).
A constrained layer damping system (Figure 1b) consists of a three-layer configuration. The viscoelastic material is sandwiched between the substrate that is vibrating and a relatively stiff constraining layer. The purpose of the constraining layer is to produce a shearing action with the viscoelastic layer as the composite sandwich vibrates. Constrained layer damping is used more to reduce vibration than to attenuate noise.
The extensional damping coating and the viscoelastic layer of the constrained system are based on polymeric materials, which are selected because of their relatively high mechanical loss factors under the required conditions. The effectiveness of the constraining layer depends directly on its modulus and on the relative thickness of the viscoelastic layer and the constraining layer. These parameters are optimized based on damping characteristics, cost, added weight, durability of treatment in the expected service environment, and aesthetics.
Materials Get Involved
There are many examples of extensional damping layers. Most of the commercial products have centered on the use of vinyl, polyurethane, and butyl materials. These materials can be applied as coatings or sealants; however, they are also manufactured into products that take the form of sheets, foam, and tiles. In all of the products it is necessary that the damping material be well bonded to the substrate for maximum vibration attenuation. Vehicle acoustic systems reduce interior noise by isolating and absorbing noise vibrations generated by the engine, suspension and tires—the principal sources of in-car noise.
Sealants used in the automotive industry must have excellent adhesion to many substrates over a wide range of test temperatures. These sealants must also have excellent corrosion resistance and other properties that are associated with the automotive environment. Industry-accelerated testing environments are very demanding, requiring resistance to hot, humid atmospheres as well as salt spray exposure.
Vinyl damping sheets are made in a wide range of thicknesses and service temperature ranges. These can be fabricated to fit particular locations and needs. Many products of this type come with pressure-sensitive adhesive for easy installation. Vinyl damping sheets typically are mass loaded and have an applied weight of one pound per square foot of applied area. It is possible with good mass-loaded vinyl to achieve stand-alone noise isolation reduction of 27 dBA, resulting in 5-8 dBA of loss in a wall system.
Butyl elastomer-based materials can be used as mass-loaded extensional damping in similar fashion. Blended polyisobutylene and halogenated butyl are two polymers that can be used alone, as a filled composite, or blended with other elastomers. Typical applications include mats, non-cured tapes, and sealants.
Whereas vinyl and butyl products operate principally by mass-loading, other polymers function mainly by viscoelastic energy absorption. One of the best materials in this respect is polyurethane because its glass transition characteristics can be “engineered” for specific applications. They can be formulated not only to provide a vibration damping, but also retain their flexibility and mechanical properties over a wide range of temperatures and environments. Table 1 provides properties of polyurethane and butyl sealants relative to other sealants that are commonly used in the auto industry.
The creation of interfaces within the extensional damping layer is a technique that has also found significant application for noise reduction. Open cell foams and fiberglass reinforced damping materials, for example, are excellent at removing sound energy from an air space. The energy waves that impinge on this type of material are absorbed at the many interfaces before it can pass through the material.
Acoustical foams manufactured out of polyurethane and other polymers achieve optimum sound absorption by controlling the air permeability of the foam cell structure. The air flow resistance in the foam often indicates effectiveness of a given product. Design flexibility is often achieved with a variety of facing materials. These add durability, resistance to contamination and acoustical performance. The addition of a high modulus facing can result in a constrained layer type of damping configuration as shown in Figure 1b.
Foams and reinforced mats generally minimize noise by reducing the reflection of sounds from walls and ceilings. They do not provide as much extensional viscoelastic damping as solid materials. As a result, foams and mats make excellent sound absorbers within a space, but they do not make a very good sound barrier.
Acoustical properties of barriers are best at middle and high frequencies Existing sound absorptive materials generally serve as good barriers for acoustical noise above 500 HZ, but they perform poorly at lower frequencies. By using barriers in conjunction with acoustical foams, performance can be altered to achieve specific reductions across a discrete frequency range.
Recently, researchers developed a class of material that is particularly effective at blocking low frequency acoustical energy. The patented process adapts to a variety of materials, including silicones, rubber, polyurethanes, and epoxy resins. Potential applications range from hearing protection to automotive, building, electronic packaging, and sporting goods.
Table 1. Comparative Properties of Different Sealants
|Noise and vibration damping||2||4||2||2||4|
|1 = No rating 2 = Poor 3 = Good 4 = Excellent|
Edward M. Petrie is the sole proprietor of EMP Solutions, a Cary, N.C.–based consulting firm focused on solving problems in the adhesives and sealants industry. He also works as a technical expert for SpecialChem. For more details, please visit www.specialchem4adhesives.com.