Classifications

• • G—PHYSICS
  • G10—MUSICAL INSTRUMENTS; ACOUSTICS
  • G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
  • G10K11/00—Methods or devices for transmitting, conducting or directing sound in general; Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
  • G10K11/16—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
  • G10K11/162—Selection of materials
  • G10K11/168—Plural layers of different materials, e.g. sandwiches
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B32—LAYERED PRODUCTS
  • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
  • B32B5/00—Layered products characterised by the non- homogeneity or physical structure, i.e. comprising a fibrous, filamentary, particulate or foam layer; Layered products characterised by having a layer differing constitutionally or physically in different parts
  • B32B5/22—Layered products characterised by the non- homogeneity or physical structure, i.e. comprising a fibrous, filamentary, particulate or foam layer; Layered products characterised by having a layer differing constitutionally or physically in different parts characterised by the presence of two or more layers which are next to each other and are fibrous, filamentary, formed of particles or foamed
  • B32B5/24—Layered products characterised by the non- homogeneity or physical structure, i.e. comprising a fibrous, filamentary, particulate or foam layer; Layered products characterised by having a layer differing constitutionally or physically in different parts characterised by the presence of two or more layers which are next to each other and are fibrous, filamentary, formed of particles or foamed one layer being a fibrous or filamentary layer
  • B32B5/26—Layered products characterised by the non- homogeneity or physical structure, i.e. comprising a fibrous, filamentary, particulate or foam layer; Layered products characterised by having a layer differing constitutionally or physically in different parts characterised by the presence of two or more layers which are next to each other and are fibrous, filamentary, formed of particles or foamed one layer being a fibrous or filamentary layer another layer next to it also being fibrous or filamentary
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B32—LAYERED PRODUCTS
  • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
  • B32B7/00—Layered products characterised by the relation between layers; Layered products characterised by the relative orientation of features between layers, or by the relative values of a measurable parameter between layers, i.e. products comprising layers having different physical, chemical or physicochemical properties; Layered products characterised by the interconnection of layers
  • B32B7/02—Physical, chemical or physicochemical properties
  • B32B7/022—Mechanical properties
• • G—PHYSICS
  • G10—MUSICAL INSTRUMENTS; ACOUSTICS
  • G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
  • G10K11/00—Methods or devices for transmitting, conducting or directing sound in general; Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
  • G10K11/16—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
• • G—PHYSICS
  • G10—MUSICAL INSTRUMENTS; ACOUSTICS
  • G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
  • G10K2210/00—Details of active noise control [ANC] covered by G10K11/178 but not provided for in any of its subgroups
  • G10K2210/10—Applications
  • G10K2210/118—Panels, e.g. active sound-absorption panels or noise barriers
• • G—PHYSICS
  • G10—MUSICAL INSTRUMENTS; ACOUSTICS
  • G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
  • G10K2210/00—Details of active noise control [ANC] covered by G10K11/178 but not provided for in any of its subgroups
  • G10K2210/10—Applications
  • G10K2210/128—Vehicles
  • G10K2210/1282—Automobiles
• • G—PHYSICS
  • G10—MUSICAL INSTRUMENTS; ACOUSTICS
  • G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
  • G10K2210/00—Details of active noise control [ANC] covered by G10K11/178 but not provided for in any of its subgroups
  • G10K2210/30—Means
  • G10K2210/321—Physical
  • G10K2210/3223—Materials, e.g. special compositions or gases

Definitions

• the present invention relates to a sound-absorbing material that is suitable for low frequency bands.
• noise countermeasures are implemented by incorporating or attaching sound-absorbing materials to parts in the engine compartment, around the tires, and around the passenger compartment.
• the sound absorbing mechanism of sound absorbing materials is that the energy of the sound waves that enter the sound absorbing material is converted into thermal energy through collisions and friction between the sound waves and the materials that make up the sound absorbing material, thereby attenuating the energy of the sound waves. Due to this sound absorbing mechanism, it is considered desirable for the structure of the sound absorbing material to have a perforated structure that allows sound waves to pass through, and to have a large specific surface area inside the sound absorbing material so that the viscous resistance experienced by the incident sound waves is large. Materials with a porous structure are generally used as such materials. Examples include glass wool, rock wool, expanded foam, and nonwoven fabrics formed by entangling or bonding fibers.
• the material has a thickness that corresponds to the wavelength of the sound to be absorbed. Because the wavelength of low-frequency sound is long, in order for the porous sound absorbing material to efficiently absorb low-frequency sound, the material thickness must be large. However, there are limits to the material thickness depending on the application. For example, in the automotive field, in order to absorb low-frequency sound such as road noise, a material thickness that exceeds the thickness of the vehicle body is required. Furthermore, an increase in material thickness also affects the design of the vehicle. Furthermore, as the material becomes heavier as the material thickness increases, this can affect the driving performance of the vehicle, such as reducing fuel efficiency.
• Patent Document 1 proposes a thin, lightweight sound-absorbing felt that is at least partially composed of a dense layer whose breathability and basis weight are adjusted within a certain range, with the dense layer being composed of ultrafine fibers.
• Patent Document 2 proposes a sound absorbing material in which the breathable skin material and breathable base material are composed of a structure including a fabric such as a nonwoven fabric, a perforated film with breathability, or a foam sheet, the breathable skin material is laminated onto the breathable base material, and the airflow resistance per unit thickness of the breathable skin material is greater than the airflow resistance per unit thickness of the breathable base material.
• Patent Document 3 proposes a laminated sound-absorbing material that includes a porous layer made of a fiber layer or a microporous membrane with a defined mean flow pore size and basis weight, and a base layer, where the base layer is made of either a nonwoven fabric, a woven fabric, an expanded foam, or a honeycomb core, and is arranged so that the base layer is the sound incident side and the porous layer is the sound transmitting side.
• the sound-absorbing felt described in Patent Document 1 is made by entangling ultra-fine fibers with a single fiber diameter of 1.0 ⁇ m or less and a fiber length of 20 to 150 mm, and by arranging a dense layer on the surface, in which the breathability and basis weight of the felt are adjusted within a certain range, thereby increasing the specific surface area that can come into contact with sound waves, and sound absorption properties are expressed by the collision and friction of the incident sound waves with the fibers that make up the felt.
• the sound-absorbing felt described in Patent Document 1 is a porous sound-absorbing material, and in some cases the sound absorption properties are insufficient in low frequency bands below 1,000 Hz, such as road noise.
• the sound-absorbing material described in Patent Document 2 exhibits excellent sound-absorbing performance in a wide frequency range, including high frequencies of 1,000 Hz and above, by laminating a breathable skin material with an airflow resistance that is 20 times or more and less than 2,514 times the airflow resistance per unit thickness of the breathable base material onto the breathable base material.
• the material is a structure that includes nonwoven fabric, cloth, breathable perforated film, or foam sheet, and is a porous material, the sound-absorbing properties may be insufficient in low frequencies of less than 1,000 Hz, such as road noise.
• the laminated sound-absorbing material described in Patent Document 3 has a porous layer made of a fiber layer or a microporous film laminated onto a base layer, and specifies the average sound absorption coefficient by measuring the sound absorption coefficient from frequencies between 400 Hz and 1,000 Hz.
• the embodiment uses a porous material, and the average sound absorption coefficient is only 0.40, so the sound absorption characteristics may be insufficient in low frequency bands below 1,000 Hz, such as road noise.
• the present invention aims to solve the above problems and aims to provide a laminated structure with excellent sound absorption properties in the low frequency range.
• the present invention has the following configuration.
• (1) A laminated structure in which a dense layer is laminated on a base layer, the dense layer being composed of at least two or more sheet-like materials.
• (2) The laminate structure according to (1), wherein the two or more layers of sheet-like material are the same sheet-like material.
• (3) The laminate structure according to (1) or (2), wherein the sheet-like material has a thickness of 1 mm or less.
• (4) The laminate structure according to any one of (1) to (3), wherein the sheet-like material has an areal density of 50 g/ m2 or more and 500 g/ m2 or less.
• thermoplastic fibers include ultrafine fibers having a fiber diameter of 0.1 ⁇ m or more and 5 ⁇ m or less.
• thermoplastic fibers include ultrafine fibers having a fiber diameter of 0.1 ⁇ m or more and 5 ⁇ m or less.
• a porous layer is laminated on the dense layer.
• the porous layer has a thickness of 5 mm or more and an air permeability of 1 cm 3 /cm 2 /sec to 20 cm 3 /cm 2 /sec.
• (10) A sound absorbing material obtained by molding the laminated structure according to any one of (1) to (8) above.
• (11) A sound-absorbing part for a vehicle, the part being constituted in part by the sound-absorbing material described in (10) above.
• the present invention provides a laminated structure with excellent sound absorption properties, especially in the low frequency range.
• FIG. 2 is a cross-sectional view of a composite spinneret for producing ultrafine fibers that constitute a dense layer of the present invention.
• the inventors of the present invention conducted extensive research to achieve the above object, and discovered that in order to exhibit high sound absorption properties in the low frequency band, it is important for the sound absorbing material to have a resonant frequency that matches the frequency of the sound waves incident on the sound absorbing material.
• the sound absorption mechanism is presumed to be as follows. First, sound waves incident on the laminated structure of the present invention collide with the dense layer, causing the dense layer to vibrate. At this time, the frequency at which the dense layer vibrates at its maximum is determined by the structure of the dense layer and the structure of the base layer that acts as the air layer.
• the laminated structure of the present invention absorbs sound through the above sound absorption mechanism, so it is necessary that a dense layer is laminated on the base layer.
• the dense layer of the laminated structure of the present invention must be composed of at least two or more layers of sheet-like material.
• the dense layer of the laminated structure of the present invention vibrates at a specific frequency, thereby exhibiting excellent sound absorption properties for sounds having a frequency that resonates with the vibration frequency.
• the vibration frequency of the dense layer is single, it exhibits high sound absorption properties for specific sounds that resonate with the vibration frequency, but the sound absorption properties decrease for sounds whose frequency deviates from the vibration frequency.
• the dense layer composed of two or more layers of sheet-like material
• multiple sheets can be combined to vibrate as a pseudo-single sheet-like material composite layer, and even if the thickness is thin, the peak of the vibration frequency becomes wide, and sound absorption properties can be exhibited for sounds of a wide frequency range.
• the dense layer is composed of three or more layers of sheet-like material. The more layers of sheet-like material that compose the dense layer, the more combinations that compose the pseudo-composite layer increase, and the wider the peak of the vibration frequency becomes, which is preferable.
• the upper limit of the number of layers of sheet-like material constituting the dense layer is preferably 10 layers, more preferably 8 layers or less, and even more preferably 6 layers or less. At this preferred upper limit, there is no risk of the vibrations of multiple sheet-like material layers interfering with each other and reducing the sound absorption characteristics.
• the two or more layers of sheet-like materials constituting the dense layer of the laminated structure of the present invention can be composed of sheet-like materials with different densities as necessary, but considering ease of handling and vibration as a pseudo-composite layer, it is preferable that they are the same sheet-like materials with the same mechanical properties. In this way, by making the two or more layers of sheet-like materials constituting the dense layer of the laminated structure into the same structure, the multiple sheet-like materials vibrate at the same frequency, so that no interference occurs due to the vibration of the multiple sheet-like materials when vibrating as a pseudo-composite layer, and sound absorption properties can be expressed for sounds of a wide range of frequencies.
• the sheet-like material preferably has a thickness of 1 mm or less.
• the dense layer is more likely to vibrate due to resonance with sound waves incident on the laminated structure, resulting in high sound absorption properties.
• a thickness of 0.7 mm or less is more preferable.
• a thickness of 0.4 mm or less can be cited as an even more preferable range, as it maximizes the vibration of each layer constituting the dense layer, enabling a wide range of sound absorption as a pseudo-composite layer.
• the sheet-like material preferably has an areal density of 50 g/m2 or more and 500 g/m2 or less .
• the areal density of the sheet-like material affects the vibration frequency peak of the dense layer, and the higher the areal density of the sheet-like material, the lower the vibration frequency peak becomes, and the higher the sound absorption characteristic is for sounds in the low frequency range.
• the surface density of the sheet-like material when the surface density of the sheet-like material is 50 g/m 2 or more, the sound absorption characteristics for low-frequency band sounds, particularly sounds of 1,000 Hz or less such as road noise, are fully expressed. If this idea is carried forward, the surface density is more preferably 70 g/m 2 or more, and even more preferably 100 g/m 2 or more.
• the upper limit of the surface density of the sheet-like material at which sound absorption for low-frequency band sounds is fully expressed is 500 g/m 2 , and within this range, vibration of the dense layer due to the incident sound is expressed, and the sound absorption characteristics are also guaranteed.
• a more preferable surface density is 400 g/m 2 or less, and a more preferable surface density is 300 g/m 2 or less.
• the sheet-like material preferably has an air permeability of 1 cm 3 /cm 2 /sec or more and 100 cm 3 /cm 2 /sec or less.
• the air permeability in the present invention is measured by the method described in the examples below based on the "Air permeability” A method (Fragile type method) of JIS L 1096:2010 "Fabric test method for woven and knitted fabrics”.
• Air permeability A method (Fragile type method) of JIS L 1096:2010 "Fabric test method for woven and knitted fabrics”.
• the dense layer vibrates due to the incident sound waves, and sound absorption characteristics are exhibited due to resonance with the vibration frequency of the dense layer.
• the air permeability is 50 cm 3 /cm 2 /sec or less, and even more preferably 10 cm 3 /cm 2 /sec or less.
• the fiber assemblies referred to here include nonwoven fabrics, and fabrics such as woven fabrics and knitted fabrics. In particular, nonwoven fabrics are preferred because of the ease of adjusting the thickness, surface density, and air permeability.
• nonwoven fabrics include spunbond, meltblown, wet papermaking, needle punched felt, electrospun sheets, etc., and combinations of these are also acceptable.
• nonwoven fabric sheets obtained by wet papermaking methods are more preferred as they satisfy the above-mentioned characteristics of the sheet-like material.
• the above-mentioned sheet-like object is not limited in type or material as long as the effects of the present invention can be obtained, but it is preferable that the sheet-like object is composed of thermoplastic fibers from the viewpoint of the manufacturing and waste disposal of the sheet-like object.
• the raw material of the thermoplastic fiber may be any material used for general synthetic fibers, and examples thereof include aromatic polyesters such as polyethylene terephthalate, polytrimethylene terephthalate, and polybutylene terephthalate, aliphatic polyesters such as polylactic acid, aliphatic polyamides such as polycapramide and polyhexamethylene adipamide, semi-aromatic polyamides such as polyhexamethylene terephthalamide, thermoplastic elastomers such as thermoplastic polyurethane, and polyolefins such as polypropylene.
• aromatic polyesters or aliphatic polyamides from the viewpoints of mechanical properties, heat resistance, and ease of handling during manufacturing.
• the aromatic polyester is a high molecular weight substance having repeating units linked to the main chain via ester bonds. Combinations that form ester bonds include aromatic dicarboxylic acids and aliphatic diols, aliphatic dicarboxylic acids and aromatic diols, and aromatic dicarboxylic acids and aromatic diols.
• the aromatic polyester is preferably made of an aromatic dicarboxylic acid and an aliphatic diol. Examples of such aromatic dicarboxylic acids include, but are not limited to, terephthalic acid, isophthalic acid, 5-sodium sulfoisophthalic acid, 4,4'-diphenyldicarboxylic acid, and 2,6-naphthalenedicarboxylic acid.
• aliphatic diols examples include, but are not limited to, ethylene glycol, 1,3-propanediol, 1,4-butanediol, cyclohexanediol, diethylene glycol, and neopentyl glycol.
• the above-mentioned aliphatic polyamides are high molecular weight substances having repeating units in which so-called hydrocarbon groups are linked to the main chain via amide bonds. In general, they are synthesized by polycondensation reactions using aminocarboxylic acids and cyclic amides as raw materials, or by polycondensation reactions using dicarboxylic acids and diamines as raw materials.
• Examples of such polyamides include, but are not limited to, polycapramide, polyundecanolactam, polylauryllactam, polyhexamethylene adipamide, polyhexamethylene sebacamide, and polyhexamethylene dodecanediamide.
• the method for producing the polymer in the present invention is not limited, and the polymer may be synthesized by a general polycondensation reaction, addition polymerization reaction, etc.
• the raw materials used in production are collectively referred to as monomers, they include, but are not limited to, petroleum-derived monomers, biomass-derived monomers, recycled monomers obtained by reusing polymers as raw materials using chemical recycling methods, and mixtures of multiple types of monomers mentioned above.
• polymers may be produced from waste materials such as waste plastics using material recycling methods.
• the polymer in the present invention may be copolymerized or mixed with a second and third component in addition to the main component, as long as it does not deviate from the purpose of the present invention.
• the thermoplastic fibers that preferably constitute the sheet-like material preferably include ultrafine fibers with a fiber diameter of 0.1 ⁇ m or more and 5 ⁇ m or less.
• the fiber diameter refers to the value obtained for the fibers that constitute the sheet-like material by the method described in the Examples described later. If the fiber diameter of the ultrafine fibers is 0.1 ⁇ m or more, it is possible to suppress the reflection of sound waves incident on the laminated structure on the surface of the dense layer.
• the fiber diameter of the ultrafine fibers is more preferably 0.2 ⁇ m or more. Furthermore, if the fiber diameter of the ultrafine fibers is 5 ⁇ m or less, the fiber packing density of the sheet-like material that constitutes the dense layer is increased, the dense layer is more likely to vibrate due to sound waves incident on the laminated structure, and sound absorption characteristics due to resonance with the vibration frequency of the dense layer are manifested.
• the fiber diameter of the ultrafine fibers is more preferably 4 ⁇ m or less, and even more preferably 3 ⁇ m or less.
• the fiber length of the ultrafine fibers is preferably 0.1 mm or more and 10 mm or less.
• the fiber length of the ultrafine fibers is 0.1 mm or more, the ultrafine fibers constituting the sheet-like material are sufficiently entangled with each other, and the falling off of the ultrafine fibers can be suppressed.
• the shape of the sheet-like material is stable, the structure of the sheet-like material is less likely to change over time, and the sound absorption characteristics can be stably expressed.
• the fiber length of the ultrafine fibers is more preferably 0.5 mm or more.
• the fiber length of the ultrafine fibers is 10 mm or less, it is possible to prevent the ultrafine fibers from entangling alone to form fiber clumps, which would disrupt the structure of the sheet-like material. Therefore, the vibration frequency of the sheet-like material is stable, and the sound absorption characteristics due to resonance are stably expressed.
• the fiber length of the ultrafine fibers is more preferably 5 mm or less.
• the sheet-like material may contain, in addition to the ultrafine fibers made of the thermoplastic fibers described above, fibers of normal fineness having a fiber diameter larger than that of the ultrafine fibers.
• the normal fineness fibers are responsible for the mechanical properties as the skeleton of the sheet-like material, and play a role in ensuring the handling and moldability of the sheet-like material.
• the normal fineness fibers serve as a scaffold for the ultrafine fibers, so that the ultrafine fibers are arranged in a bridge-like manner between the normal fineness fibers inside the sheet-like material, allowing the ultrafine fibers with extremely low rigidity to exist stably inside the sheet-like material.
• the sheet-like material may contain one type of normal fineness fiber, or may contain two or more types of fibers with different fiber diameters.
• the fiber diameter is preferably 3.0 ⁇ m or more and 50 ⁇ m or less.
• the fiber diameter is more preferably 5.0 ⁇ m or more and 30 ⁇ m or less from the viewpoint of the handling and molding processability of the sheet-like product.
• the fiber with the smaller fiber diameter is 1.0 ⁇ m or more and 20 ⁇ m or less.
• the composition ratio of each fiber is not particularly limited, but from the viewpoint of ensuring the strength of the sheet-like material and forming stable microscopic spaces, it is preferable that the ultrafine fibers are 2.5% by mass or more and 70% by mass or less, and the normal fineness fibers are 30% by mass or more and 97.5% by mass or less.
• the ultrafine fibers are preferably oriented at an angle of 0 degrees or more and 45 degrees or less with respect to the horizontal direction of the dense layer.
• the horizontal direction of the dense layer here refers to a direction perpendicular to the thickness direction of the dense layer. Oriented at 0 degrees with respect to the horizontal direction of the dense layer means that the fiber axis direction of the ultrafine fibers coincides with the horizontal direction of the dense layer. Also, when the ultrafine fibers are oriented at 90 degrees, it means that the fiber axis direction of the ultrafine fibers coincides with the thickness direction of the dense layer.
• the ultrafine fibers are oriented at 45 degrees or less with respect to the horizontal direction of the dense layer, sound waves entering the laminated structure efficiently collide with the ultrafine fibers, causing the dense layer to vibrate. It is more preferable that the ultrafine fibers are oriented at 30 degrees or less. Taking this idea further, the closer the orientation angle of the ultrafine fibers is to 0 degrees relative to the horizontal direction of the dense layer, the more efficiently sound waves that enter the laminated structure will collide with the ultrafine fibers, making this preferable, and the practical lower limit of the orientation angle of the ultrafine fibers is 0 degrees relative to the horizontal direction of the dense layer.
• binder fibers may be mixed as fibers of normal fineness for the purpose of improving the sheet strength and preventing the constituent fibers from falling off.
• a binder fiber mixing rate of 5% by mass or more is preferable because it makes it possible to physically bond the fibers constituting the sheet to each other, increases the rigidity of the sheet, and makes it easier for the dense layer to vibrate, thereby exhibiting high sound absorption properties. Furthermore, since the sheet strength is improved, a laminated structure with excellent moldability and handleability can be obtained.
• the upper limit of the binder fiber mixing rate is 75% by mass or less.
• the binder fiber in the present invention is not particularly limited, but is preferably a thermally adhesive binder fiber.
• An example of a thermally adhesive binder fiber is a core-sheath composite fiber in which a polymer with a melting point of 150°C or less is arranged in the sheath. After mixing the binder fibers to form a sheet-like material, the sheath component on the surface of the binder fiber is fused and bonded to other fibers by a drying process such as a Yankee dryer or an air-through dryer, or a heat treatment process such as a calendar. This thermal bonding can increase the rigidity of the sheet-like material.
• the core component of the binder fiber since the core component of the binder fiber remains in a fibrous form, it becomes a fiber with the maximum fiber diameter or a fiber with an intermediate fiber diameter depending on its fiber diameter, and can ensure the strength of the sheet-like material and play a role as a scaffold.
• the melting point of the core component of the binder fiber is 20°C or more higher than the melting point of the sheath component, the sheath component on the surface of the binder fiber is sufficiently melted during thermal bonding, and the decrease in the orientation of the core component is suppressed, so that sufficient adhesion and high rigidity can be obtained.
• the base layer of the laminated structure of the present invention has the function of maintaining the shape of the entire laminated structure, and also acts as an air layer for the dense layer, determining the vibration frequency of the dense layer that exhibits the resonance type sound absorption characteristics.
• Materials constituting such a base layer include glass wool, rock wool, foam, and fiber aggregates formed by entangling or bonding fibers, but are not limited to these. Fiber aggregates are preferred in terms of moldability, shape stability during use, and environmental impact upon disposal. Examples of fiber aggregates here include nonwoven fabrics, and fabrics such as woven fabrics and knitted fabrics, with nonwoven fabrics being preferred in terms of ease of adjusting thickness, surface density, and air permeability. Examples of nonwoven fabrics include spunbond, meltblown, air-through, spunlace, thermal bond, and needle punched felts, and combinations of these may also be used.
• the fibers constituting the base layer of the laminated structure of the present invention are not particularly limited as long as the effects of the present invention can be obtained, but thermoplastic fibers are preferred from the viewpoints of the manufacture and disposal of the base layer.
• the raw materials for the thermoplastic fibers may be any that are commonly used for synthetic fibers, and examples of such raw materials include aromatic polyesters such as polyethylene terephthalate, polytrimethylene terephthalate, and polybutylene terephthalate, aliphatic polyesters such as polylactic acid, aliphatic polyamides such as polycapramide and polyhexamethylene adipamide, semi-aromatic polyamides such as polyhexamethylene terephthalamide, thermoplastic elastomers such as thermoplastic polyurethane, and polyolefins such as polypropylene.
• the substrate layer of the laminated structure of the present invention preferably has a thickness of 5 mm or more.
• the vibration frequency of the dense layer is determined by the thickness of the substrate layer. If the thickness is 5 mm or more, the vibration frequency of the dense layer will be low, and sound absorption characteristics will be exhibited for sounds in the low frequency band of 1000 Hz or less.
• There is no upper limit to the thickness of the porous layer as long as the effects of the present invention can be obtained, but from the viewpoints of the mass of the laminated structure and ease of handling during use, it is about 40 mm.
• the surface density of the base layer of the laminated structure of the present invention is preferably 100 g/m 2 or more and 700 g/m 2 or less.
• the surface density 100 g/m 2 or more By making the surface density 100 g/m 2 or more, the function of maintaining the shape of the entire laminated structure can be obtained.
• sound absorption characteristics due to collision and friction between the incident sound wave and the material constituting the base layer can be expressed. It is preferably 150 g/m 2 or more.
• the surface density 700 g/m 2 or less the flexibility of the base layer can be improved, and it can play the role of an air layer when the sound absorption characteristics are expressed by the vibration of the dense layer.
• it since it has excellent three-dimensional tracking properties, it can maintain the shape when molding the laminated structure of the present invention. It is preferably 500 g/m 2 or less.
• the laminated structure of the present invention exhibits sound absorption properties due to the resonance between the vibration frequency of the dense layer and the incident sound waves.
• the laminated structure of the present invention can selectively absorb sounds of any frequency by adjusting the structure of the dense layer, and is particularly excellent in sound absorption properties in the low frequency band.
• sounds other than those near the vibration frequency of the dense layer for example, sounds in the high frequency band exceeding 1,000 Hz, the sound absorption properties are difficult to exhibit due to the transmission or reflection of the incident sound waves. Therefore, in order to absorb the transmitted or reflected sounds, it is preferable that the laminated structure of the present invention has a porous layer laminated on the dense layer.
• a specific example is a structure in which a dense layer is laminated on a base layer, and a porous layer is further laminated on the dense layer.
• the porous layer here is a layer made of a material having a porous structure as described later. In the porous layer, collisions and friction occur between the incident sound waves and the material constituting the porous layer, and the energy of the incident sound waves is converted into thermal energy, and the energy is attenuated to absorb the sound.
• the porous layer must have a thickness corresponding to the wavelength of the sound to be absorbed, and the shorter the wavelength of the sound, the better the sound absorption characteristics are for sounds in the high frequency band.
• the laminated structure of the present invention by laminating a porous layer having the above-mentioned sound absorption mechanism on a dense layer, sound waves incident on the laminated structure are converted into thermal energy in the porous layer and absorbed, and low frequency band sounds that are difficult to absorb in the porous layer are absorbed by the vibration of the dense layer. Furthermore, because the sound reflected by the dense layer is absorbed by the porous layer, sound absorption characteristics can be expressed for sounds in a wide band.
• the porous layer which is preferably laminated on the dense layer in the laminated structure of the present invention, preferably has a thickness of 5 mm or more.
• the porous layer needs to have a thickness corresponding to the wavelength of the sound to be absorbed, and if the thickness of the porous layer is within the above preferred range, it can adequately absorb sounds in the high frequency band, which have short wavelengths.
• the thickness of the porous layer There is no upper limit to the thickness of the porous layer as long as the effects of the present invention can be obtained, but from the viewpoints of the mass of the laminated structure and ease of handling during use, it is preferable that the thickness be around 40 mm.
• Porous layers used in the laminated structure of the present invention include glass wool, rock wool, expanded foam, and fiber assemblies formed by entangling or bonding fibers, but are not limited to these. Fiber assemblies are preferred in terms of moldability, dimensional stability during use, and environmental impact during disposal.
• the fiber assemblies referred to here include nonwoven fabrics, and fabrics such as woven fabrics and knitted fabrics, with nonwoven fabrics being preferred in terms of ease of adjusting thickness, surface density, and air permeability.
• Nonwoven fabrics include spunbond, meltblown, air-through, spunlace, thermal bond, and needle punched felts, and combinations of these are also acceptable.
• the porous layer that is preferably laminated on the dense layer in the laminated structure of the present invention is not particularly limited as long as the effects of the present invention can be obtained, but it is preferable that it is composed of thermoplastic fibers from the viewpoint of the production and waste disposal of the porous layer.
• the raw material of the thermoplastic fiber may be any material that is commonly used for synthetic fibers, and examples of such raw materials include aromatic polyesters such as polyethylene terephthalate, polytrimethylene terephthalate, and polybutylene terephthalate, aliphatic polyesters such as polylactic acid, aliphatic polyamides such as polycapramide and polyhexamethylene adipamide, semi-aromatic polyamides such as polyhexamethylene terephthalamide, thermoplastic elastomers such as thermoplastic polyurethane, and polyolefins such as polypropylene.
• aromatic polyesters such as polyethylene terephthalate, polytrimethylene terephthalate, and polybutylene terephthalate
• aliphatic polyesters such as polylactic acid
• aliphatic polyamides such as polycapramide and polyhexamethylene adipamide
• semi-aromatic polyamides such as polyhexamethylene terephthalamide
• thermoplastic elastomers such
• the porous layer preferably laminated on the dense layer has an air permeability of 1 cm 3 /cm 2 /sec or more and 20 cm 3 /cm 2 /sec or less. If the air permeability of the porous layer is 1 cm 3 /cm 2 /sec or more, the reflection of the sound waves on the surface of the porous layer can be suppressed, so that the sound waves are incident on the laminated structure and the sound absorption characteristics are expressed. More preferably, it is 5 cm 3 /cm 2 /sec or more.
• the air permeability of the porous layer 20 cm 3 /cm 2 /sec or less friction in the porous layer due to the incident sound waves is induced, and the energy of the sound waves is efficiently converted into heat energy, and sufficient sound absorption characteristics are expressed. If this idea is pushed forward, the air permeability of 10 cm 3 /cm 2 /sec or less can be cited as a more preferable range.
• the sound absorption coefficient for a sound of 1,000 Hz is 60% or more.
• the sound absorption coefficient here is measured in accordance with JIS A 1405 (2007) "Measurement of sound absorption coefficient and impedance using an acoustic tube.” If the sound absorption coefficient for a sound of 1,000 Hz is 60% or more, sufficient sound absorption characteristics against noise such as road noise are exhibited even when used as part of a sound absorbing part for a vehicle. It is more preferable that the sound absorption coefficient is 70% or more, and the practical upper limit is 100%.
• the method for producing the dense layer of the laminated structure of the present invention is not particularly limited, but as an example, a method for producing a sheet-like material by wet papermaking is shown.
• short fibers that make up the sheet-like material such as ultrafine fibers, other fibers with a larger fiber diameter than the ultrafine fibers, and thermally adhesive binder fibers, are put into water and stirred with a disintegrator to prepare a fiber dispersion that is uniformly dispersed.
• this feeding process it is possible to adjust the dispersibility by adjusting the amount of fiber fed, the amount of aqueous medium, the stirring time, etc., and it is preferable for the short fibers to be as uniformly dispersed as possible in the aqueous medium.
• a post-addition method may be used in which an ultrafine fiber dispersion that is uniformly dispersed in the aqueous medium is prepared separately and then mixed with the above-mentioned fiber dispersion.
• One example of a method for producing ultrafine fibers used in the laminated structure of the present invention is to use sea-island composite fibers made of two or more types of polymers that have different dissolution rates in a solvent.
• the sea-island composite fiber referred to here has a structure in which island components made of poorly soluble polymers are scattered in a sea component made of a readily soluble polymer.
• the readily soluble polymer of the sea component is dissolved, leaving behind the poorly soluble polymer of the island component, which becomes the ultrafine fiber.
• the easily soluble polymer of the sea-island composite fiber in the present invention is selected from melt-moldable polymers and copolymers thereof, such as aromatic polyesters typified by polyethylene terephthalate, aliphatic polyesters typified by polylactic acid, aliphatic polyamides typified by polycapramide and polyhexamethylene adipamide, thermoplastic elastomers typified by thermoplastic polyurethane, and polyolefins typified by polypropylene.
• melt-moldable polymers and copolymers thereof such as aromatic polyesters typified by polyethylene terephthalate, aliphatic polyesters typified by polylactic acid, aliphatic polyamides typified by polycapramide and polyhexamethylene adipamide, thermoplastic elastomers typified by thermoplastic polyurethane, and polyolefins typified by polypropylene.
• aromatic polyesters typified by polyethylene terephthalate
• Polyesters copolymerized with polyethylene glycol or sodium sulfoisophthalic acid, either alone or in combination, are more preferred.
• polyesters copolymerized with 3 mol% to 20 mol% of 5-sodium sulfoisophthalic acid and polyesters copolymerized with 3 mol% to 20 mol% of 5-sodium sulfoisophthalic acid and 5% to 15% by mass of polyethylene glycol having a weight average molecular weight of 500 to 3,000 are particularly preferred.
• the preferred method for producing the sea-island composite fiber in the laminated structure of the present invention is melt spinning using a composite spinneret, as this method provides excellent productivity and control over fiber diameter and cross-sectional shape.
• the spinning temperature is set to a temperature at which the polymers used, primarily those with high melting points or high viscosity, exhibit fluidity. This fluidity temperature varies depending on the molecular weight, but stable production is possible when it is set between the melting point of the polymer and melting point + 60°C.
• the easily soluble sea component polymer and the sparingly soluble island component polymer are melted separately, metered and transported by a gear pump, and a composite flow is formed by a known method so as to have a specific composite structure, which is then discharged from a spinneret.
• the yarn discharged from the spinneret is cooled to room temperature by blowing cooling air onto it using a yarn cooling device such as a chimney, and is then oiled and focused using an oiling device.
• the focused yarn is then entangled using a fluid entanglement nozzle device and passed through a take-up roller and a stretching roller. During this process, the yarn is stretched according to the ratio of the peripheral speeds of the take-up roller and the stretching roller.
• a further method is to heat set the yarn using a stretching roller and then wind it up using a winder (winding device).
• Another example is a two-step method in which the peripheral speeds of the take-up roller and the stretching roller are set to the same, and the yarn is then wound up using a winder that also operates at the same speed to produce an unstretched yarn, which is then stretched in a separate process.
• the sea-island composite fibers obtained by the above manufacturing method are bundled into a tow of several tens to several millions of fibers, and cut to the desired fiber length using a cutting machine such as a guillotine cutter, slicer, or cryostat to obtain short sea-island composite fibers.
• a cutting machine such as a guillotine cutter, slicer, or cryostat to obtain short sea-island composite fibers.
• the solvent used in this treatment is preferably an aqueous solution containing an alkali such as sodium hydroxide as a solute.
• the bath ratio of the sea-island composite fiber to the aqueous alkali solution is preferably 1/10,000 or more and 1/5 or less.
• the bath ratio in this range, it is possible to suppress entanglement of the ultrafine fibers when dissolving and removing the sea component.
• the alkali concentration of the aqueous alkali solution is 0.1% by mass or more and 5% by mass or less. By setting the alkali concentration in this range, it is possible to complete dissolution of the sea component in a short time and suppress deterioration of the island component.
• the temperature of the alkaline aqueous solution during the dissolution treatment is not particularly limited, but by setting it to 50°C or higher, the dissolution of the sea component can be accelerated.
• the cross-sectional shape of the ultrafine fibers in the laminated structure of the present invention can be a wide variety of cross-sectional shapes, including not only a round cross-section, but also flat, Y-shaped, T-shaped, hollow, rice field-shaped, and well-shaped.
• the cross-sectional shape can be adjusted when forming the composite flow in the spinneret to have a specific composite structure.
• the post-treatment liquid after dissolving the sea component from the sea-island composite fiber may be used as an ultrafine fiber dispersion.
• the post-treatment liquid may be filtered to separate the ultrafine fibers, which are then washed with water and dried to form a clay-like material, which may then be added to the fiber dispersion as a raw material.
• a dispersant may be added to improve the dispersibility of each fiber in water.
• the type of dispersant is not particularly limited, but examples of additives that suppress the aggregation of fibers include cationic compounds, anionic compounds, and nonionic compounds. In particular, it is preferable to use anionic compounds from the viewpoint of electrical repulsion in aqueous media.
• the amount of these dispersants added is preferably 0.001 to 10 equivalents relative to the mass of the ultrafine fibers in order to ensure dispersibility without impairing processability during wet papermaking.
• Equipment used for wet papermaking includes cylinder papermaking machines, fourdrinier papermaking machines, inclined short wire papermaking machines, and combinations of these.
• the fiber dispersion prepared as described above is diluted to a certain concentration, and then dehydrated on an inclined wire or cylinder to form a wet papermaking sheet.
• a three-dimensionally homogenous sheet can be produced by adjusting the papermaking speed, fiber amount, and aqueous medium amount to control the accumulation of the fibers during filtration.
• the surface density and thickness of the wet papermaking sheet can be adjusted by appropriately changing the supply amount of the fiber dispersion, which is the papermaking stock, and the papermaking speed.
• drying methods include passing hot air through the sheet (air-through) and contacting the sheet with a heated rotating roll (such as a heated calendar roll).
• the method for manufacturing the base layer of the laminated structure of the present invention is not particularly limited, but an example is the following method.
• the short fibers constituting the base layer are weighed.
• the type of short fibers may be one or more.
• a heat-adhesive binder fiber made of a core-sheath composite fiber in which the melting point of the core component is higher than the melting point of the sheath component may be included.
• the weighed short fibers are opened and mixed using air or the like, and the mixed short fibers are aligned with a clothed roller to obtain a mixed fiber web.
• the surface density and thickness of the mixed fiber web are adjusted to adjust the surface density and thickness of the base layer.
• the mixed fiber web is heat-treated at a temperature higher than the temperature at which the binder fiber softens or melts using a hot air dryer, a hot air circulation heat treatment machine, an infrared heater, a heat roll, or the like, to obtain a nonwoven fabric that will become the base layer.
• the short fibers may be entangled by a mechanical entanglement method using needles or water flow.
• the manufacturing method of the porous layer of the laminated structure of the present invention is not particularly limited, but an example of a manufacturing method by melt blowing is shown below.
• the thermoplastic resin that is the raw material of the nonwoven fabric is melted and supplied to a die, and hot air is blown onto the threads extruded from the die to obtain thin nonwoven fibers.
• Short fibers made of thermoplastic resin and binder fibers are blown onto the fiber flow of this nonwoven fiber so that they merge, and collected on a net as a mixed web.
• the thermoplastic resin that is the raw material of the nonwoven fabric and the thermoplastic resin that constitutes the short fibers and binder fibers may be the same polymer or different types of polymers.
• the mixed web is heat-treated at a temperature higher than the temperature at which the binder fibers soften or melt using a hot air dryer, a hot air circulation heat treatment machine, an infrared heater, a heat roll, or the like, to obtain a nonwoven fabric that becomes a porous layer.
• the fibers may be entangled by a mechanical entanglement method using needles or water flow.
• the mixed web may be configured to have another nonwoven fabric bonded to it via a thermally adhesive binder as a cover material.
• the method of laminating two or more layers of structures in the dense layer can be appropriately selected.
• Examples include a method of simply stacking multiple wet-laid paper sheets, a method of stacking multiple wet-laid paper sheets and then melting the binder fibers contained in the wet-laid paper sheets with heat to bond them, and a method of bonding multiple wet-laid paper sheets via a binder agent.
• the method of bonding multiple wet-laid paper sheets via a binder agent can also be appropriately selected.
• Examples include a method of applying a low-melting point polymer in a dot shape and thermally bonding them, a method of thermally bonding them using a spider web-like (web-like) low-melting point polymer sheet (thermal bonding sheet), and a method of bonding multiple laminated wet-laid paper sheets by impregnating them in a binder agent solution and drying them.
• the method of laminating the dense layer and the base layer, and the method of laminating the porous layer and the dense layer can be appropriately selected.
• Examples include a method of simply laminating the dense layer and the base layer, and the porous layer and the dense layer, a method of melting binder fibers contained in the porous layer and/or the dense layer and/or the base layer, and a method of bonding the dense layer and the base layer, and the porous layer and the dense layer, via a binder agent.
• the method of bonding the dense layer and the base layer via a binder agent, and the method of bonding the porous layer and the dense layer can also be appropriately selected.
• Examples include a method of applying a low-melting point polymer in a dot shape and heat bonding, a method of heat bonding using a spider web-like (web-like) low-melting point polymer sheet, and a method of bonding the laminated dense layer and base layer, and the porous layer and dense layer by impregnating them in a binder agent solution and drying them.
• the lamination of the dense layer and the base layer and the lamination of the porous layer and the dense layer may be performed in separate steps or in the same step.
• the laminated structure of the present invention may be subjected to known processes such as dyeing, brushing, water repellency, flame retardancy, and flame retardant.
• the shape of the laminated structure of the present invention can be adjusted as appropriate and is not limited, but may be, for example, a round shape, an oval shape, a square shape, a rectangular shape, or the like.
• corrugation, pleating, rolling, cutting, punching, drilling, or partial notching may be performed within the scope of the object of the present invention.
• the sound-absorbing material formed from the laminated structure of the present invention can be used as an excellent sound-absorbing material in various applications such as automobiles, electronic devices, buildings, and homes.
• the sound-absorbing material formed from the laminated structure of the present invention has excellent sound-absorbing properties, particularly in the low frequency range, and sound-absorbing parts for vehicles partially composed of the sound-absorbing material of the present invention are suitable for noise control in the low frequency range of 1,000 Hz or less, such as road noise, by using them in floor undercovers, floor carpets, dash inners, covers for air conditioner compressors, fender liners, inside door panels, trunk liners, etc.
• the method of use as a sound-absorbing material can be appropriately selected according to the application, such as pasting on a wall surface or inside equipment, or filling inside a wall, but a mode in which it comes into contact with a wall surface or equipment via a base layer or a porous layer is preferable so that the dense layer can vibrate with the incident sound waves.
• Tm Melting Point of the Polymer Using a TA instruments differential scanning calorimeter (DSC) Q2000 model, 20 mg of a polymer sample was heated from 20°C to 280°C at a heating rate of 20°C/min, held at 280°C for 5 minutes, cooled from 280°C to 20°C at a heating rate of 20°C/min, held at 20°C for 1 minute, and then heated from 20°C to 280°C at a heating rate of 20°C/min. The peak top temperature of the endothermic peak observed when this was taken as the melting point. When multiple endothermic peaks were observed, the endothermic peak top on the highest temperature side was taken as the melting point.
• DSC differential scanning calorimeter
• Fineness A fiber sample was wound 200 times on a measuring machine with a frame circumference of 1.125 m to prepare a hank, and after drying in a hot air dryer (105 ⁇ 2°C x 60 minutes), the mass of the hank was measured on a balance and multiplied by the official moisture content to calculate the fineness. The measurement was performed four times, and the average value was taken as the fineness.
• the fiber length referred to here is the length of one fiber in the longitudinal direction from a two-dimensionally taken image, measured in mm to three decimal places, and rounded off to one decimal place.
• the above operation was carried out for 10 images taken in the same manner, and the simple number average of the evaluation results of the 10 images was taken as the fiber length.
• Fiber diameter Images of the surface of the fiber sample were taken with a scanning electron microscope (SEM) at a magnification where 150 to 3,000 fibers could be observed, and the fiber diameters of 150 fibers randomly extracted from the taken images were measured. The fiber diameter was measured in ⁇ m units to the second decimal place, with the fiber width in the direction perpendicular to the fiber axis taken from the two-dimensionally taken image being taken as the fiber diameter.
• the obtained image was analyzed using computer software WinROOF manufactured by Mitani Shoji to measure its area, and the value that could be calculated by converting it into a circle having the same area was adopted. The above operation was performed for 10 images taken in the same manner, and the simple number average value of the evaluation results of the 10 images was rounded off to the second decimal place to obtain the fiber diameter.
• the dense layer was embedded in an embedding agent such as epoxy resin, frozen with a Reichert FC-4E cryosectioning system, cut with a Reichert-Nissei ultracut N (ultramicrotome) equipped with a diamond knife, and the cut surface was photographed at a magnification that allowed the cross section to be recognized with a Hitachi H-7100FA transmission electron microscope (TEM).
• the vertical direction of the obtained photograph was the vertical direction of the dense layer, and the fiber orientation angle was measured using image analysis software (WINROOF). This measurement was performed on 100 randomly extracted single fibers, and the average value obtained by rounding off the decimal point was taken as the fiber orientation angle.
• the orientation angle of the part with the longest straight line distance in the photograph was measured, and this value was taken as the orientation angle of the single fiber.
• the orientation angle was determined by taking the horizontal direction of the photograph as 0 degrees and the vertical direction as 90 degrees, and determining the acute angle between the horizontal direction of the photograph and the single fiber.
• Thickness The thickness was measured in mm using a dial thickness gauge (TECLOCK SM-114, probe shape 10 mm ⁇ , graduation 0.01 mm, measuring force 2.5 N or less). Measurements were taken at five random locations per sample, and the average was rounded off to two decimal places to determine the thickness.
• TECLOCK SM-114 dial thickness gauge
• Example 1 At a spinning temperature of 290°C, polyethylene terephthalate (melt viscosity 160 Pa s, melting point 254°C) was used as the island component, and polyethylene terephthalate copolymerized with 8.0 mol% of 5-sodium sulfoisophthalic acid and 10 mass% of polyethylene glycol having a number average molecular weight of 1,000 (melt viscosity 121 Pa s, melting point 234°C) was used as the sea component. After each polymer was melted separately, it was weighed so that the composite ratio of the island component and the sea component was 50:50 in mass ratio, and was introduced into a spinning pack incorporating the composite spinneret shown in Figure 1.
• the incoming polymer was discharged from the discharge hole (hole diameter 0.3 mm, number of holes 14 holes) using a sea-island composite spinneret (number of islands 2000) with round island component shapes.
• the discharged composite polymer flow was cooled and solidified by a cooling device, and a water-containing oil agent was fed by an oiling device. Thereafter, the yarn was taken up by the first roll, a take-up roller, at a peripheral speed of 1000 m/min and a temperature of 85° C.
• the yarn taken up by the take-up roller was then taken up by the second roll, a stretching roller, at a surface temperature of 130° C., to be stretched at a draw ratio of 3.40 times, which is expressed as the ratio of the peripheral speeds of the take-up roller and the stretching roller, and was simultaneously heat-treated by the stretching roller.
• the heat-treated yarn was taken up by a winder with a winding speed of 3,400 m/min, to obtain a drawn yarn of 35 dtex-14 filaments.
• the resulting drawn yarn was cut to a fiber length of 0.6 mm.
• the cut drawn yarn was then placed in an aqueous sodium hydroxide solution (concentration: 1% by mass) with a mass 100 times that of the drawn yarn, and heated at 90°C to obtain a dispersion of ultrafine fibers.
• short fibers core component fiber diameter 10 ⁇ m, fiber length 5.0 mm
• heat-fusible core-sheath composite fiber sheath component: low melting point polyethylene terephthalate (melting point 110°C), core component: polyethylene terephthalate (melting point 255°C)
• short fibers of polyethylene terephthalate fiber diameter 4 ⁇ m, fiber length 3.0 mm
• a papermaking stock solution containing 30% by mass of heat-fusible core-sheath composite fibers, 65% by mass of polyethylene terephthalate short fibers, and 5% by mass of ultrafine fibers.
• This papermaking stock solution was made into paper using a square sheet machine (250 mm square) manufactured by Kumagai Riki Kogyo Co., Ltd., and then dried and heat-treated in a rotary dryer with a roller temperature set to 110°C to obtain a wet-laid papermaking sheet with an areal density of 100 g/ m2 . As described later, this wet-laid papermaking sheet was made into a sheet-like product to form a dense layer.
• the fibers were mixed and opened using a carding machine to form a uniform web.
• the formed web was laminated to a predetermined thickness, and while pressing the web with a press roll to a thickness of 20 mm, the fibers were heat-fused in a heat treatment furnace having upper and lower net conveyors at 215°C, and the surface density was adjusted to 250 g/ m2 and the thickness to 20 mm, to obtain a nonwoven fabric.
• the laminated structure To prepare the laminated structure, three of the wet-laid paper sheets prepared above were stacked as they were to form a dense layer, and the nonwoven fabric was used as the base layer. First, one thermal adhesive sheet was placed on top of the nonwoven fabric emerging from the heating furnace, and three wet-laid paper sheets were stacked on top of that. The entire structure was pressed down with a 130°C heated roller while the thermal adhesive sheet was melted, bonding the wet-laid paper sheets and the nonwoven fabric layer, and obtaining a laminated structure. A circular sample with a diameter of 39.5 mm was cut out from the obtained laminated structure and evaluated as a sound-absorbing material. The evaluation results are shown in Table 1.
• Example 2 A laminated structure was produced in the same manner as in Example 1, except that two wetlaid paper sheets were directly stacked together to form a dense layer, and the laminated structure was evaluated as a sound absorbing material. The evaluation results are shown in Table 1.
• Example 3 A laminated structure was produced in the same manner as in Example 1, except that when producing the sea-island composite fiber in Example 1, a sea-island composite spinneret in which the island component fibers had a round shape (number of islands: 500) was used, and the laminated structure was evaluated as a sound absorbing material. The evaluation results are shown in Table 1.
• Example 4 A laminate structure was produced in the same manner as in Example 1, except that a sea-island composite spinneret (number of islands: 50) in which the shape of island components was round was used to produce a drawn yarn of 140 dtex-14 filaments when producing the sea-island composite fiber in Example 1, and the laminate structure was evaluated as a sound absorbing material. The evaluation results are shown in Table 1.
• Example 5 when preparing a wetlaid paper sheet as a sheet-like material constituting the dense layer, the polyethylene terephthalate short fibers were 60 mass% and the ultrafine fibers were 10 mass% in Example 5, and the polyethylene terephthalate short fibers were 67.5 mass% and the ultrafine fibers were 2.5 mass% in Example 6. Except for this, a laminated structure was prepared in the same manner as in Example 1, and evaluated as a sound absorbing material. The evaluation results are shown in Table 1.
• Example 7 A laminate structure was produced in the same manner as in Example 1, except that a melt-blown nonwoven fabric (manufactured by 3M, product name: TC1503) made of polypropylene resin was laminated as a porous layer on the laminate made of a wetlaid paper sheet as a sheet-like material constituting a dense layer and a nonwoven fabric as a base layer produced in Example 1, and evaluated as a sound absorbing material. The evaluation results are shown in Table 1.
• Example 8 In producing the sea-island composite fiber in Example 1, a composite ratio of island components to sea components was set to 80:20 by mass, and a drawn yarn of 56 dtex-48 filaments was obtained using a sea-island composite spinneret (where the shape of the island components was flat) in which 36 layers of island components and sea components were alternately laminated. The drawn yarn obtained was cut to a fiber length of 3.0 mm.
• a wetlay papermaking sheet was obtained in the same manner as in Example 1, except that a papermaking dope was prepared in the wetlay papermaking process so that the thermally adhesive core-sheath composite fibers were 30% by mass, the polyethylene terephthalate staple fibers were 30% by mass, and the ultrafine fibers were 40% by mass.
• a laminated structure was produced and evaluated as a sound absorbing material. The evaluation results are shown in Table 1.
• Example 1 A sound absorbing material was produced and evaluated using only the base layer made of the nonwoven fabric produced in Example 1. Since there was no dense layer, the sound absorbing properties were poor in the low frequency range of 1000 Hz or less. The evaluation results are shown in Table 1.
• Example 2 A laminated structure was produced in the same manner as in Example 1, except that the dense layer was formed from only one wetlaid paper sheet, and evaluated as a sound absorbing material. Since there was only one dense layer, the sound absorbing properties were poor in the low frequency band of 1000 Hz or less. The evaluation results are shown in Table 1.

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