Vibration damping materials and soundproofing structures using such damping materials

Abstract: A vibration damper includes two materials, each being formed of a metal sheet having a rubber- or synthetic resin-base viscoelastic polymeric layer or layers formed on one or both sides, which are arranged in opposition to each other through said viscoelastic layers and bonded together with the use of a hot-melt-adhesive synthetic resin layer having a high melting point. Alternatively, one of said materials is arranged in opposition to another metal sheet through said viscoelastic layer or layers and bonded thereto with the use of said hot-melt-adhesive synthetic resin layer. A soundproofing structure using such a vibration damer is also provided.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention is concerned with an improvement in or relating to a composite type of vibration damping material (which may hereinafter be referred to as the vibration damper) comprising a metal sheet and a viscoelastic polymeric material and a soundproofing structure using such a material. Statement of the Prior Art As the composite type of vibration damping materials comprising a metal sheet and a viscoelastic polymeric materials, there have been known one vibration damper formed of a thin steel sheet 1 and a rubber- or synthetic resin-base viscoelastic polymeric layer 2 bonded to both sides thereof, as illustrated in Figure 9, and another vibration damper formed by sandwiching and bonding a viscoelastic polymeric layer 2 between two steel sheets 1, as illustrated in Figure 10. The vibration damper of Figure 9 is called the non-constraint type and can damp vibration due to the "extensional deformation" of the viscoelastic layer in association with flexural vibration, whereas the vibration-damper of Figure 10 is referred to as the constraint type and can damp vibration due to the "shearing deformation" of the viscoelastic layer in association with flexural vibration. From a comparison of the non-constraint and constraint types of dampers, it is found that there is a considerable difference therebetween in that the constraint type is effective with a thin viscoelastic layer, but the non-constraint type needs a viscoelastic layer three to four times as thick as a steel sheet. Said composite type of vibration dampers are used in a wide range of industrial fields such as automotive engine mounts, disc brakes, oil pans, transmissions, compressors, air cleaners, brake clutches, electronic ranges, speakers and players. As mentioned above, the aforesaid composite type of vibration dampers are now increasingly used in various industrial fields due to their superior damping capacity. In actual applications, however, they are often used in severer environments where various factors such as temperature, pressure, oils, solvents and coolants are applied alone or in combination, rather than in relatively moderate environments at normal temperature, under no load and in the atmosphere. There is thus still demanded a vibration damping material that can sufficiently meet such requirements. In particular, the aforesaid composite type of vibration damper is intimately related to the thickness of the metal sheet and viscoelastic layer forming it. One problem with this damper is that the thicker the metal sheet, the higher the mechanical strength but the lower the damping capacity. Another problem is that increases in the thickness of the viscoelastic layer result in decreases in both the resistance to heat and pressure and the mechanical strength. This vibration damper should thus be designed while taking such problems into account. SUMMARY OF THE INVENTION A main object of the present invention is to improve the conventional composite type of vibration dampers and provide a vibration damping material which is less affected by the thickness of a metal sheet and a viscoelastic layer forming it, and combines improved damping capacity with improved resistance to pressure, heat, solvents, oils and the like in severe environments. Another object of the present invention is to obtain such a vibration damping material at lower costs on a mass-production scale. A further object of the present invention is to provide a soundproofing structure using such a vibration damping material as mentioned above. According to the present invention, the aforesaid objects and problems are achieved and solved by the provision of a vibration damping material comprising in combination: a first material including a first metal sheet and first and second viscoelastic layers made of rubber-or a synthetic resin-base viscoelastic material which are formed on both sides of the first metal sheet; a second material comprising a second metal sheet; and a first hot-melt adhesive synthetic resin layer having a high melting point which is disposed between the first viscoelastic layer of the first material and the second material so as to serve to bond together the first and second materials. According to the present invention, there is also provided a soundproofing structure characterized in that a noise generating portion is surrounded by a sound absorbing material, a sound insulating material and so on, and the aforesaid vibration damping material is used as an outermost cladding panel. In the vibration damping material of the aforesaid structure, the viscoelastic polymeric layer(s) preformed on the metal sheet(s) and the synthetic resin film layer used for lamination are sandwiched between the metal sheets arranged in opposition to each other. Thus, both the layers act synergistically to produce a vibration damping effect. In the aforesaid soundproofing structure, the aforesaid composite type of vibration damping material is used as the cladding panel. Thus, without using sound insulating materials forming conventioanl soundproofing structures such as lead or iron sheets, mortar and hard cement, it is possible to obtain a soundproofing effect equivalent to that achieved therewith. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be explained with reference to the accompanying drawings, which are given for the purpose of illustration alone, and in which:- Figures 1 to 5 are side views showing various embodiments of the present invention, Figure 6 is a graphical view illustrating the sealing pressure of a vibration damper, Figure 7 is a perspective view of a soundproofing structure using a vibration damper of the present invention, Figure 8 is a perspective view showing a portion of a vibration insulating mechanism, and Figures 9 and 10 are side views showing conventional vibration dampers. DETAILED EXPLANATION OF THE INVENTION Referring now to Figures 1 to 5, there are shown various embodiments of the constraint type of vibration dampers according to the present invention. A vibration damper A, shown in Figure 1, includes a material a₂, where a₂ is obtained by forming rubber- or synthetic resin-base viscoelastic polymeric layers Z on both sides of a metal sheet 1, and a₂ is located in opposition to and laminated on a metal sheet 4 with the use of a hot-melt adhesive synthetic resin film 3, having a high melting point. A vibration damper A, shown in Figure 2, includes a material a₁ where a₁ is obtained by forming a rubber- or synthetic resin-base viscoelastic polymeric layer 2 on one side of a metal sheet 1, and having its viscoelastic layer 2 arranged in opposition to and laminated on said material a₂ with the use of a hot-melt adhesive synthetic resin film 3 having a high melting point. A vibration damper A, shown in Figure 3, includes two said materials a₂ laminated together with the use of said hot-melt adhesive synthetic resin film 3. A vibration damper A, shown in Figure 4, includes metal sheets 4 arranged in opposition to and laminated on both the viscoelastic layers 2 of said material a₂ with the use of said hot-melt-adhesive synthetic resin films 3. A vibration damper A, shown in Figure 5, includes a metal sheet 4 and said materials a₂ located in opposition to and laminated on both sides of said sheet 4 with the use of said hot-melt-adhesive synthetic resin films 3. As the metal sheet 1 forming a part of each of said vibration dampers A, use may be made of an iron sheet (a cold-rolled steel sheet), an aluminium sheet, a stainless steel sheet, a brass sheet, a copper sheet, a zinc sheet, a nickel sheet and a tin sheet. Usable as said viscoelastic polymeric layer 2 are acrylonitrile-butadiene rubber (NBR), styrene-butadiene rubber (SBR), natural rubber (NR), butyl rubber (IIR), ethylene-propylene polymer (EPDM), butadiene rubber (BR), isoprene rubber (IR), chloroprene rubber (CR), ethylene propylene rubber (EPM), acrylic rubber (ACM), silicon rubber, fluorine rubber (FKM), epichlorohydrin rubber (CO, ECO), urethane rubber (U), polynorbornene rubber, ethylene acrylic rubber and other rubber. Usable as said hot-melt-adhesive synthetic resin film 3 having a high melting point are polyamides such as nylons, polyesters, polyolefins such as polypropylene and polyethylene, fluorine resins such as FET and other resins. Usable as the metal sheet 4 to be laminated on said material a₁ are preferably a metal sheet which has been surface-treated such as a galvanized iron sheet, a colored zinc sheet, a vinyl chloride-coated steel sheet, a fluorine resin-coated steel sheet, an aluminium sheet, a colored aluminium sheet, an aluminium foil, a kraft paper-laminated aluminium sheet, a stainless steel sheet, a colored stainless steel sheet, a lead sheet and the like. When vibration is applied to the aforesaid constraint type of vibration damper, the "shearing deformation" occurs in the viscoelastic polymeric layer sandwiched between the metal sheets as already mentioned. Due to the viscoelastic behavior of the polymer, however, a part of the "shearing deformation" of the viscoelastic layer is transformed into thermal energy so that the vibration of the vibration damper is absorbed. As a part of the vibrational energy of the vibration damper is absorbed by the thermal energy of the viscoelastic layer, the vibration is gradually damped down. The vibration damping capacity of this vibration damper is quantitatively expressed in terms of a loss factor = η. For vibration damping purposes, a loss factor η of at least 0.01 may be applicable depending upon conditions, although an ideal loss factor is at least 0.05. The loss factor of each of the aforesaid vibration dampers varies depending upon the viscoelasticity of the sandwiched viscoelastic polymeric layer. Since the viscoelasticity of general viscoelastic polymeric materials vary largely in a temperature range of 0 to 150°C, the loss factor of vibration dampers varies in such a temperature region, correspondingly. In the embodiments illustrated in Figures 2 and 3 the hot-melt-adhesive synthetic resin film 3 having a high melting point, which forms a part thereof, serves to laminate together the two viscoelastic polymeric layers 2 with a strong adhesive force and causes the shearing deformation effective for vibration damping to occur easily in the viscoelastic polymeric layer. Such a synthetic resin film itself has rigidity and makes a contribution to an increase in pressure resistance, since it produces an effect as if a single thin metal sheet is interlaminated in the viscoelastic layer. The viscoelastic polymeric layer forming a part of the aforesaid contraint type of vibration dampers is generally broken down into the rubber and synthetic resin systems. In view of temperature dependency, preference is given to the rubber system. That is, a synthetic resin undergoing large thermal changes (and having a low melting point) is so narrow in the secondary transition region (a transition of a viscoelastic substance from a glass region to a rubber region) that it passes sharply into the rubber region, resulting in a sharp vibration damping curve defining a narrow temperature region. Since rubber undergoes no fusion at elevated temperatures and is unlikely to change under the action of heat, on the other hand, such a transition region as mentioned above is so enlarged that a certain or higher vibration damping capacity is obtained in a wider temperature region. Thus, rubber is so less in temperature dependency than synthetic resins that it can be used in a wider temperature region. In the aforesaid constraint type of vibration dampers, said viscoelastic polymeric layer in the form of a single layer rather than a laminated layer is more advantageous in terms of pressure resistance. For instance, when a single layer of rubber is vertically compressed, the rubber swells out laterally in curved forms. However, laminated rubber is very limited in the amount of swelling-out, since its lateral movement is contrained by an adhesive force between it and a metal sheet bonded to it. With the laminated rubber, it is therefore possible to obtain large rigidity in the vertical direction. On the other hand, since adhesion presents no constraint condition in the shearing deformation occurring in the horizontal direction, the same rigidity is obtained with both the single- and laminated-layers of rubber. In other words, the laminated rubber is characterized in that it shows increased rigidity in the vertical direction but softness typical of rubber in the horizontal direction, and is restored to its orginal state due to its own restoring force. In the case of the aforesaid constraint type of vibration dampers, it is found in view of the foregoing considerations that to increase the thickness of rubber without lamination results in increased damping capacity but decreased pressure resistance. Advantageous for improved vibration damping capacity is therefore to increase the thickness of rubber by lamination, thereby maintaining the pressure resistance thereof. Referring then to the metal sheet(s) forming a part of the aforesaid constraint type of vibration dampers, it is to be noted for the determination of its thickness that an increses in its thickness leads to increased strength but decreased vibration damping capacity. Further, since how easily the vibration dampers vibrate depends upon the total weight of the metal sheets even when they are different in thickness, the vibration damping capacity is not affected by which of the metal sheets is located in opposition to a vibrator. Although no particular limitation is imposed upon the thickness of said metal sheet 1, viscoelastic polymeric layer 2, hot-melt-adhesive resin film 3 and metal sheet 4, it is general that said metal sheets 1 and 4 are in a range of 0.2 to 1.0 mm, said viscoelastic polymeric layer 2 in a range of 0.1 to 0.5 mm and said resin film 3 in a range of 0.03 to 0.1 mm. The vibration dampers of the structures shown in Figures 2, 3, 1 and 5 are best-suited for use with various equipment for which damping and sealing effects are needed simultaneously, since they are provided on one or both surfaces with the viscoelastic polymeric layer or layers 2. In other words, if a damper is used separately from a sealant in applications where the durability of equipment and the prevention of noise, resonance, etc. are required together with sealing, not only is the number of the parts involved increased but also their installation is timeconsuming and troublesome, thus leading to a rise in the cost. Although conventional joint sheets, etc. are found to have some vibration damping effect, yet they do not stand up to use because of a sharp drop of their effect at high temperatures and for insufficieny of their strength relative to enhanced performance of equipment. Figure 6 is a graphical view illustrating a comparison of the vibration damper A of the structure shown in Figure 3 with the aforesaid joint sheet J in terms of the P₁ to P₂ relation. As mentioned above, the vibration damper of the aforesaid structure can sufficiently meet resistance inevitable to various environments. The vibration dampers of the aforesaid structures may effectively be used as the noise-preventing materials for disc brakes. Heretofore, there has been known a system wherein a rubber-coated steel sheet is attached to a back side of a disc pad to reduce a high-frequency (4 to 8 KHz) noise generated during braking. However, said rubber-coated steel sheet is found to lose its effect at a relatively early stage due to rubber deteriorations occuring at high temperatures (reaching instantaneously as high as 200°C). With the vibration dampers of the present invention, however, it is possible to prevent the reduction of thier vibration damping effect by reason that thermally deteriorative rubber (or resin) portions are constrained inside. Since the present dampers are higher than the conventional ones in the damping capacity level, some redution of damping capacity, if any, poses no problem. Reference will now be made to the soundproofing structures using the vibration damming materials of the present invention. Conventional soundproofing structures known for a noise generating source such as a duct include a combination of a sound absorbing material such as rock wool or glass wool with a sound insulating material such as an iron or lead sheet, mortar or hard cement provided around a noise generating portion and an outermost protecting cladding panel such as a colored galvanized iron sheet, a stainless steel sheet or an aluminium sheet, all being provided by a soundproof lagging method. In order to increase the amount of noise to be reduced, such soundproofing structures as mentioned above generally make use of means for increasing the weight or number of layers of the sound absorbing and insulating materials. However, the aforesaid lagging method is disadvantageous in that since the soundproofing materials are wound directly around the noise source, vibration is propagated from the soundproofing materials to the outermost cladding panel through support fittings, from where it is re-radiated as a sound. However, if the vibration damping material of the present invention is used for the outermost cladding panel of the aforesaid soundproofing structures, it is then possible to obtain a soundproofing effect better than that achieved with the conventional soundproofing structures, even when the sound insulating material such as a lead sheet, an iron sheet, mortar or hard cement is removed. One embodiment of the soundproofing structure for a duct is illustrated in Figure 7, wherein reference numeral 5 stands for a duct casing, 6 a sound absorbing material, 7 a sound insulating material, 8 a support stay, 9 a support angle, 10 a stud bolt, 11 a clamper, and A a vibration damper of the present invention in the corrugated form. In view of design, the sound absorbing and insulating materials forming the soundproofing structure of Fig. 7 may be used in various combinations. The following effects are obtainable with the vibration damping materials of the present invention: (1) Since there is obtained a laminate of the viscoelastic layer preformed on the metal sheet with the synthetic resin film used to laminate the materials together, a vibration damping capacity more improved than that of conventional vibrations dampers is achievable. (2) An improved vibration damping capacity is achievable especially in environments where various factors such as heat, pressure, oils and solvents are applied alone or in combination. (3) Vibration dampers fit for the intended purposes can easily be obtained depending upon the combinations of said materials. For instance, with two viscoelastic layers it is possible to obtain an additional increase in the vibration damping capacity. With the two viscoelastic layers isolated from each other by a metal sheet, an improved resistance to pressure and heat is obtained. Where a thickness has to be added to the vibration damper, it is possible to improve the vibration damping capacity without incurring large changes in the flexural rigidity, thereby achieving reductions in the weight and price. If said materials are combined with a sheet of a different metal providing an intermediate layer, it is possible to maintain the mechanical strength of the whole vibration damper and reduce the weight thereof at the same time. (4) If a viscoelastic layer is formed on the surface of the vibration damper comprising the combination of said materials, it is then possible to obtain vibration damping and sealing effects at the same time. (5) If use is made of materials having a viscoelastic polymeric layers formed on both sides of a metal sheet, it is then possible to make use of conventional steps for producing vibration dampers, thereby producing the dampers at low costs on an mass-producible scale. (6) When comparing with the soundproofing structures using conventional metal cladding panels, the soundproofing structures using the vibration dampers of the structures according to the present invention can achieve not only an increased attenuation of vibration but also have a remarkable effect upon the reduction in the number of layers and weight of the absorbing and insulating materials.

Claims: A vibration damping material comprising in combination: a first material including a first metal sheet and first and second viscoelastic layers made of rubber- or a synthetic resin-base viscoelastic material which are formed on both sides of the first metal sheet; a second material comprising a second metal sheet; and a first hot-melt adhesive synthetic resin layer having a high melting point which is disposed between the first viscoelastic layer of the first material and the second material so as to serve to bond together the first and second materials. A vibration damping material as claimed in claim 1, wherein the second material consists only of a metal sheet. A vibration damping material as claimed in claim 1, which further comprises a third viscoelastic layer made of rubber- or a synthetic resin-base viscoelastic material which is formed on one side of the second metal sheet and is located adjacent to the first hot-melt adhesive synthetic resin layer. A vibration damping material as claimed in claim 3, which further comprises a fourth viscoelastic layer made of rubber- or a synthetic resin-base viscoelastic material which is formed on the other side of the second metal sheet. A vibration damping material as claimed in claim 1, which further comprises a third metal sheet and a second hot-melt adhesive synthetic resin layer having a high melting point which is disposed between the third metal sheet and the second viscoelastic layer. A vibration damping material as claimed in claim 4, which further comprises a third metal sheet located adjacent to the first hot-melt adhesive synthetic resin layer and a second hot-melt adhesive synthetic resin layer having a high melting point which is disposed between the third viscoelastic layer and the third metal sheet.

Inventors: NIWA TAKAHIRO, SHIMIZU YASUO, NIWA, TAKAHIRO, SHIMIZU, YASUO
Applicants: NICHIAS CORP, NICHIAS CORPORATION
Application year: 1989
Main classification: B32B15
IPC codes: E04B1, G10K11, F16F9, B32B15, F16F15