Abstract:
A thermoformable acoustic sheet material that is lightweight and provides excellent sound attenuation over a wide range of frequencies includes a combination of sound attenuating materials, including an acoustic barrier having a relatively high density, a thermoformable sound absorbing layer having a relatively low density, a substantially air-impermeable layer between the barrier and the sound absorbing layer, and an acoustic performance enhancing scrim or perforated thermoplastic film on the opposite side of the sound absorbing layer. The combination is useful for preparing contoured acoustic treatments for reducing sound transmission from the engine compartment of a vehicle to the passenger compartment of the vehicle.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to high performance lightweight acoustic attenuation materials, and more particularly to multilayered acoustic attenuation materials that are thermoformable.  
       BACKGROUND OF THE INVENTION  
       [0002]     It is generally desirable to reduce the level of noise within a vehicle passenger compartment. External noises, such as road noise, engine noise, vibrations, etc., as well as noises emanating from within the passenger compartment, may be attenuated through the use of various acoustical materials. The attenuation of external noise is conventionally referred to as sound transmission loss (STL). The attenuation of internal noise is conventionally referred to as sound absorption. The acoustic impedance of a material is defined as material density times acoustic velocity, and is expressed in units of Rayls (Newton-seconds/meter 3 ). Acoustic impedance defines how easy it is for air to move through a material. Thus, for fibrous materials, acoustic impedance depends upon the density of the fibrous material and fiber diameter. Generally, the heavier the blanket and the finer the fibers, the higher the acoustic impedance. Thicker layers typically have more acoustic impedance than thin layers.  
         [0003]     Conventional acoustic treatments for reducing sound transmission at a vehicle firewall which separates the passenger compartment from an engine compartment consist of an acoustic absorber such as an open-cell polyurethane foam or a resinated fiber pad which faces the firewall, and a barrier sheet such as a heavily filled thermoplastic material.  
         [0004]     Thermoformable acoustic insulating and/or sound absorbing sheet materials are employed in substantially all mass produced motorized vehicles having a weather-tight passenger compartment. Thermoformability refers to the ability of the sheet material to be shaped in a molding tool under application of heat and, optionally, pressure, and subsequently retain the molded shape. It is highly desirable that the thermoformable acoustic sheet material used for molding sound insulating and/or sound absorbing panels for motorized vehicle applications has properties that impart resilience and flexibility to the finished panels. This combination of thermoformability, flexibility and resilience or shape-retention facilitates economical installation of the acoustic panel into the vehicle by allowing the panel to be bent during installation, such as to fit the panel into an obstructed space, without damaging or permanently deforming the shape of the panel, and by ensuring that the panel will conform as precisely as needed to the contours of a vehicle component without extensive laborious manipulation of the panel.  
         [0005]     In addition to thermoformability, flexibility and resilience, all of which are important for achieving economical manufacturing and/or installation of the acoustic panel, there is a need for progressively thinner acoustic panels in order to maximize space availability for other vehicle components, passengers and cargo. Further, there is also a progressive need for lighter weight acoustic panels in order to minimize fuel consumption.  
         [0006]     While known thermoformable acoustic sheet materials have performed adequately, there nevertheless is a desire to develop improved thermoformable, acoustic attenuation sheet materials that achieve a better combination of low cost, lightweight, reduced thickness, and outstanding acoustic attenuation properties.  
       SUMMARY OF THE INVENTION  
       [0007]     The invention is directed to lightweight, thermoformable, multilayered sheet materials that incorporate a combination of sound attenuating materials that perform various sound attenuating functions and provide excellent sound attenuation over a wide range of frequencies. The multilayered sheet materials can also be easily shaped into desired contours by application of heat, and optionally pressure, in a contoured tool cavity to provide acoustic panels that conform to various applications.  
         [0008]     The acoustic multilayered sheet materials of this invention include an acoustic barrier layer having a relatively high density, a thermoformable sound absorbing layer having a relatively low density, an airflow control layer that is essentially air impermeable, and an acoustic performance enhancement layer. The thermoformable sound absorbing layer is disposed between the airflow control layer and the acoustic performance enhancement layer, and the airflow control layer is disposed between the sound absorbing layer and the acoustic barrier layer. The sheet materials of this invention may include additional layers on either side of and/or between any of the disclosed barrier, sound absorbing, airflow control, and/or acoustic performance enhancement layers.  
         [0009]     The acoustic multilayered sheet materials of this invention are useful for preparing contoured sound attenuating panels that conform with and are attached to the metal wall separating the engine component from the passenger compartment of a typical automobile. In a particular aspect of the invention the acoustic multilayered sheet material is installed with the barrier layer directly adjacent to the metal wall.  
         [0010]     These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, the claims and the appended drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a schematic cross section of an embodiment of the thermoformable acoustic sheet material of the invention.  
         [0012]      FIG. 2  is a schematic cross section of an alternative embodiment of the invention.  
         [0013]      FIG. 3  is a schematic cross section of a motor vehicle employing the thermoformable acoustic sheet material of  FIG. 2 .  
         [0014]      FIG. 4  is a cross-sectional diagrammatic view of a vertically-lapped nonwoven fibrous mat that may be utilized in the thermoformable acoustic sheet materials of this invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     A thermoformable acoustic sheet material  10  comprises at least four layers, including an acoustic barrier or mass layer  12  or  22 , a sound absorbing layer  14  or  24 , at least one substantially impermeable layer  40 ,  45 , or  28 , and a performance enhancement layer  16  or  26 , as shown in  FIGS. 1 and 2 . Sound absorbing layer  14  or  24  is located between barrier layer  12  or  22  and acoustic performance enhancement layer  16  or  26 . Substantially, impermeable layer  40 ,  45  and/or  28  is located between sound absorbing layer  14  or  24  and barrier layer  12  or  22 .  
         [0016]     Barrier layer  12  or  22  is a relatively high density, thin, flexible layer, selected from resonated cotton shoddy, mineral filled foamed plastic, and, most desirably, a dense fibrous layer.  
         [0017]     Preferably, barrier layer  12  or  22  comprises a relatively high density layer of nonwoven fiber that has been compressed to form a sheet or layer having an area weight of from about 40 grams per square foot to about 200 grams per square foot and a thickness of from about 1 millimeter to about 15 millimeters, and more typically a thickness of from about 6 millimeters to about 8 millimeters. Barrier layer  12  or  22  may be vertically-lapped, air-laid, cross-laid, needle-punched or the like.  
         [0018]     Alternatively, barrier layer  12  or  22  may comprise a mineral filled foamed plastic material. The mineral filler typically has a specific gravity of from about 4 to about 20. Suitable mineral fillers include barium sulfate and tungsten. Suitable thermoplastic materials that may be employed for fabricating a mineral filled foamed plastic barrier layer  12  or  22  include polyolefins such as polyethylene and polypropylene, rubber modified polyolefins, ethylene-vinyl acetate copolymers, and polyvinylchloride. The mineral filler may comprise up to about 70% of the weight of barrier layer  12  or  22 , such that the resulting filled thermoplastic barrier layer  12  or  22  has a relatively high density, typically greater than 1 g/cc. Typically, a mineral filled foamed plastic barrier layer  12  or  22  has a thickness of from about 1 millimeter to about 15 millimeters, and more typically from about 6 millimeters to about 8 millimeters.  
         [0019]     As a further alternative, barrier layer  12  or  22  can be fabricated of resonated cotton shoddy. Various resins may be employed in suitable amounts, including thermoplastic resins and/or thermosetting resins, to impart a suitable density and cohesiveness to the barrier layer.  
         [0020]     Sound absorbing layer  14  or  24  is a relatively low density fibrous material that is typically lofted to achieve an area weight of from about 25 grams per square foot to about 100 grams per square foot for a thickness that is at least about 15 millimeters, more typically from about 20 to about 40 millimeters (prior to thermoforming of the multilayered composite), although greater thicknesses may be used for certain applications.  
         [0021]     Sound absorbing layer  14  or  24  and barrier layer  12  or  22  may comprise generally any combination of synthetic fibers, natural fibers and/or mineral fibers. However, in order to impart thermoformability, sound absorbing layer  14  or  24  and/or barrier layer  12  or  22  will typically include a sufficient quantity of melt-fusible thermoplastic fibers that fuse with other thermoplastic fibers or adhesively bond upon thermal activation with natural or mineral fibers to form a three-dimensional fibrous network in which intersecting fibers are bonded with one another after the thermoforming process to impart resilient, flexible shape-retention characteristics. Alternatively, mineral, natural or a combination of mineral and natural fibers may be employed without any fusible thermoplastic fibers, provided that a heat activatable thermoplastic material is coated onto the fibers, distributed through the fibrous network, or otherwise applied to the fibrous material to impart appropriate thermoformability, flexibility, shape retention and resilience. Suitable synthetic thermoplastic fibers include fibers comprised of homopolymers and copolymers of polyester, nylon, polyethylene, polypropylene and blends of fibers formed from these polymers and copolymers. Particularly suitable are composite or bicomponent fibers having a relatively low melting binder component and a relatively higher melting strength component. Bicomponent fibers of this type are advantageous since the strength component imparts to and maintains adequate strength of the fiber while the bonding characteristics are imparted by the low temperature component. A variety of bicomponent fibers of this type are commercially available from various sources. One suitable fiber for use in the present invention is a sheath-core bicomponent construction wherein the core is formed of a relatively high melting polyethylene terephthalate (PET) polymer and the sheath comprises a PET copolymer having a lower melting temperature which exhibits thermoplastic adhesive and thermoformability properties when heated to a temperature of about 110 to 185° C. Examples of suitable natural fibers include kenaf, grasses, rice hulls, bagasse, cotton, jute, hemp, flax, bamboo, sisal, abaca and wood fibers. Examples of suitable mineral fibers include glass, ceramic and metal fibers. However, in order to provide a preferred combination of lightweight, efficient sound absorption, and appropriate thermoformability properties, it is desirable to utilize at least some synthetic thermoplastic fibers.  
         [0022]     Although the thermoformable acoustic sheet material preferably includes sufficient melt-fusible or adhesive synthetic fibers to impart suitable thermoformability and shape retention properties, it is possible to achieve satisfactory thermoformability and shape retention properties for certain applications by incorporating only a very small percentage of adhesive synthetic fibers, or possibly none at all, by partially impregnating or coating the fibers with either a heat-fusible thermoplastic resin or a thermosettable resin (such as a thermosettable resin in which curing is initiated by application of heat).  
         [0023]     It has been discovered that a highly efficient absorber layer  14  or  24  that is lightweight and relatively thin can be achieved by utilizing vertically-lapped synthetic fiber, natural fiber, mineral fiber or any combination of synthetic, natural, and mineral fiber. The vertically-lapped fibrous layer has been shown to provide improved sound absorption as compared with a conventional high loft material using the same fibers and same weight and/or density. A vertically-lapped fibrous layer or batt is a nonwoven fibrous layer or batt that has been repeatedly folded back and forth onto itself (i.e., pleated) to produce a vertically folded sheet material in which the fibers are predominantly or at least preferentially oriented with the length direction of the fibers being parallel with the thickness direction of the layer or batt. Vertically-lapped nonwoven materials are also referred to as variable compression fabric. Vertically-lapped materials may be produced by utilizing standard textile fiber blending equipment (if a mixture of fibers is used) and standard textile carding equipment to form a nonwoven web. The carded nonwoven web is then fed into a vertical lap machine which folds the web back onto itself to form a vertically-lapped or pleated structure. The vertical laps are preferably thermally bonded together, such as by using a flatbed conveyor convection oven. A vertically-lapped nonwoven fibrous mat that may be employed in the thermoformable sheet materials of this invention is shown in  FIG. 4 . The illustrated vertically-lapped nonwoven fibrous mat (used for absorbing layer  14 ) comprising a carded fiber web  52  that is repeatedly folded upon itself to form a multiplicity of adjacent vertical laps or pleats  54 . This vertically-lapped structure may be utilized as the sound absorbing layer  14 ,  24  of the embodiments illustrated in  FIGS. 1 and 2 . Vertically-lapped materials may also be employed in the barrier layer  12 ,  22 .  
         [0024]     Alternatively, a suitable sound absorbing layer  14  or  24  and/or barrier layer  12  or  22  may be fabricated from fibrous materials utilizing other techniques such as cross-lapping, thermobonding, needle-punching, air-laying, etc.  
         [0025]     As another alternative, sound absorbing layer  14  may comprise a foam (e.g., foamed thermoplastic) having an area weight of about 25 to 100 grams per square foot and a thickness of about at least 15 millimeters.  
         [0026]     Substantially impermeable layer  28 ,  40  and/or  45  is a relatively thin layer having a very low air-permeability. A substantially air-impermeable material, for purposes of this invention, generally has an airflow resistance of at least 4000 Rayls and preferably at least 5000 Rayls. Layer  28  can be fabricated from a sheet or film of thermoplastic material, or it can be formed as an integral surface part of barrier layer  12  and/or sound absorbing layer  14 , such as by melting fibers at the surface of a fibrous or foamed plastic layer so that the thermoplastic material at the surface flows together to form a relatively thin, substantially continuous and substantially air-impermeable layer at the surface of barrier layer  12  and/or sound absorbing layer  14 . Suitable polymer films include polyolefin films (e.g., polyethylene or polypropylene), polyethylene terephthalate films, etc. The substantially impermeable layer can be conveniently used for adhesively attaching barrier layer  22  and sound absorbing layer  24  ( FIG. 2 ) together. This may be achieved by utilizing a polymer film  28  having a pressure sensitive adhesive disposed on each of the opposite sides of the film. Alternatively, polymer film  28  may be used as a hot melt adhesive for bonding layers  22  and  24  together. The primary function of polymer film layer  28  and integrally formed layers  40  and  45  is to enhance the acoustic properties of thermoformable acoustic sheet material  10  and  20 . While there is not a precise upper or lower limit for thickness of polymer film  28 , polymer film  28  typically has a thickness of from about 0.5 to 80 mils. Similarly, when a substantially air-impermeable layer  40  and/or  45  is formed on the surface of barrier  12  or sound absorbing material  14 , layer  40  and/or layer  45  each have a thickness of from about 0.5 to about 80 mils and are substantially air-impermeable. However, it is possible to use thinner and/or thicker layers if desired. An example of a commercially available polymer film that may be employed is INTERGRAL™ 906 polyolefin multilayer adhesive film, which is an impermeable film available from the Dow Chemical Company. In this embodiment, layers  22  and  24  may each, independently comprise vertically-lapped, air-laid, cross-lapped, needle-punched, or other nonwoven fibrous arrangements. Alternatively, barrier layer  22  may comprise a mineral filled (e.g., barium sulfate filled) thermoplastic material, as described above.  
         [0027]     Acoustic performance enhancement layer  16  is comprised of a relatively thin, lightweight, sheet of material having a relatively low air-permeability. Examples of suitable sheet materials that may be employed as an acoustic performance enhancement layer include various scrims, such as those comprised of spun-bonded thermoplastic filaments (e.g., polyethylene terephthalate filament scrims), and perforated polymer films (e.g., a perforated polyolefin film). A scrim is a relatively thin and durable woven or nonwoven fabric which may be comprised of synthetic or natural fibers. Suitable scrims and perforated polymer films typically have a thickness of from about 0.01 millimeters to about 4 millimeters, more typically from about 0.04 millimeters to about 0.5 millimeters, and an airflow resistance of at least about 500 Rayls.  
         [0028]     A five-layer structure in accordance with the invention is shown in  FIG. 1 . In this embodiment, a relatively thin, substantially air-impermeable layer  40  is formed at the surface of barrier layer  12  which faces sound absorbing layer  14 , and another thin, substantially air-impermeable layer  45  is formed at the surface of sound absorbing layer  14  facing barrier layer  12 . Layers  40  and  45  are placed in abutment, and are typically bonded together with an adhesive or fused together by application of heat. Acoustic sheet material  10  is completed by adding acoustic performance enhancing layer  16  (typically a scrim or perforated polymer film) to the side of sound absorbing layer  14  opposite layer  45 . Layer  16  may be adhesively bonded or thermally fused to sound absorbing layer  14 .  
         [0029]     In an alternative embodiment, shown in  FIG. 2 , a four layer thermoformable acoustic sheet material in accordance with the invention is prepared by disposing an impermeable polymer film  28  between barrier layer  22  and sound absorbing layer  24 , with acoustic enhancement layer  26  disposed on the side of layer  24  opposite the side adjacent layer  28 .  
         [0030]     The thermoformable acoustic sheet materials of this invention exhibit an outstanding sound attenuation property when utilized as an acoustic treatment for attenuating sound transmission through a metal firewall between the engine compartment and passenger compartment of a motor vehicle. In accordance with this aspect of the invention, a thermoformable acoustic sheet material of the invention is appropriately formed or shaped to conform with the contours of a metal firewall  36  separating an engine compartment  34  from a passenger compartment  32  of a motor vehicle  30  ( FIG. 3 ). The shaped acoustic material 10/20 can be positioned adjacent firewall  36 , with either acoustic barrier side  12  or  22  facing and abutting firewall  36 , or with acoustic enhancement performance layer  16  or  26  facing and abutting firewall  36 .  
         [0031]     The above description is considered that of the preferred embodiments only. Modifications of the invention will occur to those skilled in the art and to those who make or use the invention. Therefore, it is understood that the embodiments shown in the drawings and described above are merely for illustrative purposes and not intended to limit the scope of the invention, which is defined by the following claims as interpreted according to the principles of patent law, including the doctrine of equivalents.