|Construction||Mass Spring System|
No. 334, January/February 2008
The secret of polyurethane foams
How small air bubbles absorb vibrations – a contribution by Kristina Pfeil
DIEPHOLZ. The worlds of work and life are moving together closer and closer. The coexistence of industry, commerce, houses, apartments and traffic brings along a multitude of problems. Ever higher demands are being placed on sound and vibration isolation, for example. Only in this way the proximity of a manufacturing company and a residential building is possible, a hotel can be built next to the light rail. The plastics industry supplies an effective and economical material for isolating vibrations with polyurethane, PUR for short.
This material is not only resistant, durable and versatile, but also impresses with its insulating effect. A spring is built into polyurethane foams and tiny, closed air bubbles make them insensitive to short-term peak loads under static and dynamic stress. Thanks to the gas springs, the polymer structure returns almost completely to its original position even after temporary extreme stress. This distinguishes this material of non-cellular elastomers (e.g. rubber), in which the spring stiffness depends exclusively on the Shore hardness and its shape (e.g. as grooves or nubs). PUR foams are therefore also suitable for in-situ concrete construction.
Another advantage of this material: under dynamic alternating load, a part of the mechanical energy is turned into heat. This means that additional, complex damping elements can be dispensed with if the elastomer springs are suitably designed and constructed. The risk of a resonance catastrophe, which can occur in particular when steel springs are used, is significantly reduced. PUR foams can be used in a variety of ways as compression- or shear-loading springs – whether directly on the vibration exciter or on the object to be protected. Whenever vibrations and structure-borne noise need to be well insulated, they are used: in building construction and industry as well as in rail, road, tunnel and bridge construction. Ventilation systems, elevator motors, pumps, emergency power generators or combined heat and power units, but also stair flights and platforms can be sound- and vibration-insulated with the aid of polyurethane foams.
In addition to vibration damping under machine foundations, passive insulation of highly sensitive laboratory equipment or entire laboratory rooms is another field of application.
Under highly-stressed floors, for example in warehouses or factories, PUR foams are used for sound insulation as well as under the helipad on the roof of a hospital. Where all-round sound insulation is required, for example in cinemas, concert halls, discotheques or sound studios, room-in-room constructions are also possible.
Even entire buildings can be mounted on this material to protect against ground vibrations – a technology used by the planners of a residential and commercial building in Berlin-Lichterfelde on Kranoldplatz. Here not only the vibrations of the S-Bahn running about 20 meters away should be absorbed. As a precaution, the project was planned in such a way that even a possible resumption of ICE traffic on a track only five metres away would not reduce the quality of living. The project also had to meet the requirements of comply with DIN 4109 „Sound insulation in building construction“ and DIN 4150 „Vibrations in the building industry“.
„The engineers of Kötter Consulting Engineers KG said: „The insulation against external noise was easy to implement in terms of construction. For shock protection, however, a cheaper alternative to the commonly used steel springs was sought – and found in the elastomer suspension. On the usual cleanliness layer PUR sheets were laid (dipoelast material with a density of 300 kilograms per cubic meter up to 680 kilograms per cubic meter according to DIN EN ISO 845). The foundation was then concreted over. Lateral PUR panels installed wherever the building is located under the earth‘s surface finally led to complete decoupling of the building from its surroundings.
Depending on requirements, bearing is applied in point, strip or full-surface form. The foams are usually offered as flat sheets with different densities (typical bulk densities vary between 150 kilograms per cubic meter and a thousand kilograms per cubic meter) and damping properties (from about one ton per square meter to 200 tons per square meter). The selection of the different types is load-dependent. Almost all load cases and areas of application can be covered by the combination of different thicknesses or by special designs of matched density and cross-linking. The individual adaptation is simple and – after a systematic analysis of the sources of interference – takes place via the density and thickness of the material as well as via the selected contact surface. Sophisticated techniques in the use of polyurethane foams for vibration isolation were developed in rail construction. Intermediate layers, for example, which are mounted directly under the rail foot, increase the elasticity of the ballast track. This not only protects them,
but also improves driving comfort. Another possibility is the installation of PUR intermediate plates between the ribbed and concrete support plates in ballastless track systems. Vibrations due to road unevenness are thus reduced. In addition, sub-ballast mats are common. They also increase the elasticity of the track, protect the track system and reduce audible noise and vibrations. Another advantage: The panels are not only easy to lay, they can also be driven on with heavy construction equipment. Other areas of application are sleeper bearings and inlay plates for sleepers.
Wherever the residents living near a railway line are to be effectively protected from noise and vibrations, the use of mass-spring-systems has proven itself. The track can be completely decoupled from its surroundings by using PUR floor and side mats. The Belgian transport company De Lijn, for example, decided to build a 1,360 meter long, two-lane tram line in Ghent for this technology. The selection criteria were not only the desired sound and vibration insulation, but also an easy installation and longevity of the material. The Belgian company was responsible for planning and execution CDM. After a thorough examination of the requirements, polyurethane panels (dipoelast material) were laid horizontally and vertically. A low-density material was chosen which was specially produced for this construction project. The polyurethane foams are suitable for service temperatures from minus 30 degrees Celsius to plus 70 degrees Celsius. Temperature-related changes in the static and dynamic modulus of elasticity at plus 20 degrees Celsius deviating temperature must be taken into account in the design.
PUR foams are normally flammable (fire class B2 according to DIN 4102). In the event of fire, however, no corrosive flue gases are produced, but gases similar to those produced when wood or wool is burned.
Polyurethane foams are resistant to water, concrete, oils and greases as well as diluted acids and alkalis. So-called integral foams are a special type of foam: They have a closed, thick outer skin and a cellular core. Their density thus decreases from the outside to the inside. All these properties make polyurethane foams an important material for the construction industry. The author of the article, Kristina Pfeil, works as a freelance journalist.