Vibration damping material

Abstract: A constrained type vibration damping material comprising a viscoelastic material layer between two constraining plates or between a constraining plate and a vibrating body in which the viscoelastic material layer is made of a bubble-containing pressure sensitive adhesive. A preferred pressure sensitive adhesive is acrylic pressure sensitive adhesive. The vibration damping material exhibits a high vibration damping property (loss factor η) in a plurality of temperature ranges.

Description: This invention relates to an improvement in a composite type vibration damping material made up of a constraining plate and a viscoelastic material layer laminated on a plate surface of the constraining plate. Heretofore, composite type vibration damping materials made by using a thin steel plate as a constraining plate and by providing a viscoelastic material layer of rubber, resin or other material on a surface of the constraining plate have been used in various industrial fields. Composite type vibration damping materials have been designed to absorb vibrations by virtue of the "dynamic viscoelastic behavior" of the viscoelastic material. From a structural viewpoint, they are roughly divided into two types: a free-layer type made by providing a viscoelastic material layer 2 on a plate surface of a vibrating body 4 as shown in Fig. 7; and a constrained type made by sandwiching a viscoelastic material layer 2 between a vibrating body 4 and a constraining plate 1 or between two constraining plates 1 as shown in Fig. 8. The former absorbs vibrations by converting vibration energy into thermal energy as a result of tension-compression deformation of the viscoelastic material layer. The latter absorbs vibrations by converting vibration energy into thermal energy mainly as a result of shear deformation. Composite type vibration damping materials, either of the free-layer type or of the constrained type, have a viscoelastic characteristic inherent to the viscoelastic material layer. Very serious drawbacks of the composite type vibration damping materials are that the vibration damping property (loss factor) has a sharp peak near the glass transitional temperature, and that the temperature range for sufficient performance of the vibration damping property is very narrow, which means its dependency on temperature is great. Heretofore, the following three approaches have been proposed for reducing the temperature dependency of composite type vibration damping materials: (a) Mixture of Filler Fine powder of mica, graphite, magnesium silicate used as a filler is mixed with a viscoelastic material to increase the surface area of a constraining surface causing the shear deformation and increase the energy converting amount, thereby changing the shape of the loss factor with temperature from a peaked shape to a divergent (broad) shape. (b) Polymer Alloy Mixtures of different kinds of polymers are phase-separated to cause respective phases to share their peculiar glass transitional points to produce a plurality of peak temperatures in the loss factor. (c) Lamination of Different Kinds of Polymers A mechanical-structural polymer composite body is made. These approaches, however, have the problems indicated below. With the approach (a), although the temperature dependency of the loss factor is usually decreased by mixing in a filler, the loss factor at the peak temperature tends to become relatively low. Moreover, the farther the temperature is from the peak temperature, the lower is the loss factor. Therefore, this approach does not achieve a high loss factor in a plurality of temperature ranges. With the approach (b), since the effect of phase separation is not always obtained from arbitrary combination of polymers, particular consideration to the mixture of polymers is required for ensuring phase separation, and a difficulty in such mixture arises. Moreover, phase separation usually invites a decrease in the cohesive force of polymers. With the approach (c), the Young's modulus of each polymer must be taken into consideration when laminating polymers. Arbitrary combination of polymers does not always provide a high loss factor in a plurality of temperature ranges desired. Moreover, consideration of the adhesiveness between polymers is also required, and the manufacturing process is divided into a plurality of steps, which causes an increase in the manufacturing cost. The present invention therefore provides a vibration damping material having a single phase of polymer which is stable and without laminating different kinds of polymers, which has peaks of the high loss factor in a plurality of temperature ranges and an excellent loss factor in a wide temperature range. In accordance with the present invention there is provided a constrained type vibration damping material in which the viscoelastic material layer for absorbing vibrations is made of bubble-containing pressure sensitive adhesive. In this vibration damping material, a first peak is produced in the loss factor near the glass transitional point of the pressure sensitive adhesive, and a second loss factor peak is produced near the temperature range above the glass transitional point by virtue of consumption of vibration energy caused by friction at the interface between bubbles and polymer attendant to expansion and contraction of the bubbles. Referring to the drawings: Fig. 1 is a schematic structural diagram of a constrained type vibration damping material embodying the invention. Fig. 2 presents graphs each showing vibration damping properties (loss factors) of an existing vibration damping material and the vibration damping material according to the invention, in which: (a) shows changes in loss factor with temperature in the existing vibration damping material; and (b) shows changes in loss factor with temperature in the vibration damping material according to the invention. Fig. 3 presents graphs showing Young's moduli of the existing vibration damping material and the vibration damping material according to the invention, in which: (a) shows a graph showing changes in Young's modulus with temperature in the existing composite vibration damping material; and (b) shows changes in Young's modulus with temperature in the composite vibration damping material according to the invention. Fig. 4 is a side elevation of the vibration damping material embodying the invention. Fig. 5 is a graph showing temperature characteristics of vibration damping properties (loss factors) by the embodiment and a comparative example. Fig. 6 is a graph showing changes in Young's moduli of composite vibration damping materials according to the embodiment and the comparative example. Fig. 7 is a structural diagram of an existing free-layer type vibration damping material. Fig. 8 is a structural diagram of an existing constrained type vibration damping material. An embodiment of the invention will now be described in more detail by way of illustration. Fig. 1 schematically shows the structure of a constrained type vibration damping material according to the invention. In the drawing, reference numeral 1 refers to a constraining plate, 3 to a viscoelastic material layer made of bubble-containing pressure sensitive adhesive, and 4 to a vibrating body. A metal plate such as an iron plate (cold-rolled steel plate), aluminum plate, stainless steel plate, copper plate, and so forth, or a plate material from the ceramic industry such as calcium silicate plate, slate plate, plaster board, and so on, may be used as the constraining plate of the vibration damping material. It is recommended to use an acrylic pressure sensitive adhesive as the pressure sensitive adhesive which is the basic material of the bubble-containing pressure sensitive adhesive. Also usable are rubber-based, silicon-based, acetic acid vinyl-based, EVA-based, urethane, vinyl ether-based, nitrile rubber-based and neoprene-based material. In order for the pressure sensitive adhesive to contain bubbles, there are, for example, a method of agitating the pressure sensitive adhesive so that bubbles automatically enter therein, a method of adding a foaming agent to the pressure sensitive adhesive to generate thermal decompositional gas, a method of thermally expanding a microcapsule, and a method of spreading the pressure sensitive adhesive on a porous surface and applying heat to generate bubbles in the pressure sensitive adhesive on the porous surface. A pore rate in the range of 5% to 80% and a pore diameter in the range of 10 to 300µm are preferable for the bubble-containing pressure sensitive adhesive. With the vibration damping material whose viscoelastic material layer is made of a bubble-containing pressure sensitive adhesive, as shown in the graph of Fig. 2(b), the pressure sensitive adhesive layer changes from a glass status to a rubber status near the glass transitional point (Tg), where a first peak of the loss factor (η) appears. With an increase in temperature above the transitional point, the pressure sensitive adhesive is changed to a rubber status in which bubbles can readily expand or contract. As a result, with vibration applied, the bubbles are deformed to oval shapes via the pressure sensitive adhesive existing in the rubber status region, or repeat expansion and contraction while, in contrast, causing contraction and expansion of the pressure sensitive adhesive. Due to friction produced thereby at the interface of the pressure sensitive adhesive and the bubbles or between respective bubbles, the vibration is converted to heat and absorbed, which causes a second peak of the loss factor to appear. A graph of Fig. 3(b) shows changes in Young's modulus (rate of elasticity) occurring thereupon. The Young's modulus (E) is changed in two steps in response to the second peak of the loss factor. When the temperature increases after the second peak appears, the inner pressure of a bubble exceeds the cohesive force of the pressure sensitive adhesive. Then the bubbles begin expansion, and friction at the interface of the pressure sensitive adhesive and the bubbles decreases. Therefore, since the pressure damping material cannot convert the vibration energy to frictional heat (energy), η decreases rapidly. Graphs of Fig. 2(a) and Fig. 3(a) show characteristics of loss factor and Young's modulus of an existing vibration damping material in comparison with Fig. 2(b) and Fig. 3(b). The invention is illustrated by the following examples. In this example, as shown in Fig. 4, a 0.6mm thick cold-rolled steel plate is used as the constraining plate 1. Between the constraining plate 1 and a vibrating plate 4 in the form of a 3.00mm thick steel plate is laminated a 0.1mm thick film-like bubble-containing pressure sensitive adhesive made by providing bubbles in an acrylic pressure sensitive adhesive. Upon preparing the lamination, after the respective elements are laminated in sequence, tension is applied to opposite ends of the respective elements so as to remove undesirable matters such as buckling or blister, and the elements are then bonded together by a hot press at the temperature of 50°C. A structure having a pressure sensitive adhesive without bubbles in lieu of the bubble-containing pressure sensitive adhesive 3 in the foregoing experimental embodiment was taken as a comparative example. Vibration damping properties were tested by a mechanical impedance method to measure the loss factor. Young's modulus obtained is Young's modulus of a composite body, which was calculated from a resonant frequency obtained upon measurement of the loss factor. It is therefore different from Young's modulus of the pressure sensitive adhesive itself. A graph of Fig. 5 shows vibration damping properties (temperature characteristics) of vibration damping materials by the foregoing experimental embodiment and the comparative example. A graph of Fig. 6 shows changes in Young's modulus (temperature characteristics) of the composite body. As shown in the graph of Fig. 5, the vibration damping property of the foregoing embodiment is such that, in addition to a peak (3°C) of the loss factor near the glass transitional point of the bubble-containing pressure sensitive adhesive, a second peak appears near 98°C, and the temperature dependency is decreased. In contrast, in the comparative example, although a peak of the loss factor appears at the glass transitional point of the pressure sensitive adhesive, the loss factor decreases as it goes to the high temperature range. That is, the temperature dependency is high. On the other hand, Young's modulus of the composite body according to the embodiment, as shown in the graph of Fig. 6, represents a two-step change (from 14 to 3.5x10¹⁰N/m²). The comparative example, however, represents a one-step change. Results of the foregoing vibration damping property tests are shown in the following table 1: first peak second peak temperature °C loss factor temperature °C loss factor embodiment 3 0.09 98 0.40 comparative example 19 0.11 - - According to the invention, while using a polymer in a single phase which is stable and without laminating different kinds of polymers, a vibration damping material having an excellent loss factor in a wide temperature range, with peaks of the loss factor in a plurality of temperatures, can be obtained. The vibration damping material according to the invention may be used for preventing, for example, a squeal noise caused by coupled oscillation of a disk braking system of a car. In such a case its effect is maximized when used as a squeal noise preventing vibration damping material (braking system) in which the temperature at the back of the brake pad ranges from -10 to 30°C upon starting and ranges from 80 to 120°C in normal use.

Claims: A constrained type vibration damping material characterised in that the viscoelastic material layer for absorbing vibrations is made of a bubble-containing pressure sensitive adhesive. A vibration damping material as claimed in claim 1 in which the pressure sensitive adhesive comprises acrylic pressure sensitive adhesive. A vibration damping material as claimed in claim 1 or claim 2 in which the bubble-containing pressure sensitive adhesive has a pore rate of 5% to 80%. A vibration damping material as claimed in any one of claims 1 to 3 in which the bubble-containing pressure sensitive adhesive has a pore diameter of 10 to 300µm. A disk braking system which includes a constrained type vibration damping material according to any one of claims 1 to 4.

Inventors: NIWA TAKAHIRO, SAKURADA SEIJI, YANO KUNIHIKO, NIWA, TAKAHIRO, SAKURADA, SEIJI, YANO, KUNIHIKO
Applicants: NICHIAS CORP, NICHIAS CORPORATION
Application year: 1992
Main classification: F16F9
IPC codes: B32B27, B32B15, B32B7, F16F9, F16D65, B32B5, F16F15