DIAPHRAGM FOR AN ELECTRO-ACOUSTIC TRANSDUCER

Abstract: A diaphragm for an electro-acoustic transducer compris­ es a sheet obtained by the melt processing of a mixture of a polymer and mica comprising (a) 30 to 95% by weight of a polymer whose major component is at least one olefin, polyester or polyamide or a mixture of such polymers, and (b) 5 to 70% by weight of mica having a weight-average flake diameter of 500 microns at maximum and a weight-average aspect ratio of at least 10. Addition of the specified mica improves the dynamic modulus of the sheet without sub­ stantially changing its loss tangent, so that the sheet provides an excellent diaphragm.

Description: This invention relates to diaphragms for electro-acoustic transducers and more particularly, though not exclusively, to diaphragms for use in moving-coil loudspeakers. Diaphragms for electro-acoustic transducers are fundamentally required to have a high dynamic modulus, a moderate loss tangent and a moderate density. They were previously made mainly of paper, but recently, thermoplastic film, e.g. of polyolefin, polyester or polyamide, has often been used, since it provides excellent acoustic properties, has a high degree of mouldability, and lends itself to mass production at a low cost. For example, United Kingdom Patent No. GB-A-1 563 511 discloses an electroacoustic transducer diaphragm made of polyolefin film. It has however been desired to develop a diaphragm having a higher dynamic modulus in order to provide improved acoustic properties. It is known that a reinforcing filler can be mixed into a polymeric material in order to improve its dynamic modulus. However, if a fibrous reinforcing filler, such as glass or carbon fibres, is used, an anisotropic diaphragm is formed due to the orientation of fibres that takes place during the formation of the diaphragm by extrusion. If a flaky reinforcer, such as graphite or seashell powder, is used, it is difficult to obtain a diaphragm having a satisfactorily improved dynamic modulus. Laid-Open Japanese Patent Specification No. 162695/1980 discloses a diaphragm for an electroacoustic transducer formed from a thermoplastic resin and flaky graphite. It is also known that mica can be used for making a diaphragm for an electro-acoustic transducer. Laid-Open Japanese Patent Specification No. 47816/1978 discloses a diaphragm formed by a paper-making machine from a mixture of cellulose fibres and mica dispersed in water, and Laid-Open Japanese Patent Specification No. 75316/1977 discloses a diaphragm formed by a papermaking machine from a mixture of carbon fibres and mica. These diaphragms have, however, not met any success in practice, since it is difficult to handle mica, which inherently has no entangling property, by a papermaking machine. Other methods involving the use of papermaking machines are disclosed in Japanese Specification JP-A-5 520 029, in which such a machine is used to form a mica sheet from mica and an emulsion or aqueous solution of a resin, and in German Specification DE-A-2 554 158, in which a diaphragm for an electro-acoustic transducer is produced by forming a mica sheet on a papermaking machine, impregnating the sheet with a resin and coagulating the resin. Again, the handling of the mica by the papermaking machine presents difficulties. It is also known that a diaphragm can be formed from a sheet made of a mixture of polyvinyl chloride and mica as disclosed in Laid-Open Japanese Patent Specification No. 136796/1980, but no diaphragm having satisfactory acoustical properties has been obtained by this proposal. Document GB-A-717 148 discloses a diaphragm for an electro-acoustic transducer formed from a sheet obtained by the melt processing of a mixture of a polymer and mica. The term "polymer" includes homopolymers and copolymers. The present invention relates to a diaphragm for an electro-acoustic transducer as set out in claim 1. When proceeding according to this invention, it is possible to use a polyolefin, i.e., a polymer of an aliphatic C 2 ₋₆ olefin, such as polyethylene (particularly high-density polyethylene), polypropylene (particularly isotactic polypropylene), polybutene, poly(3-methylbutene-1), and poly(4-methylpentene-1), or a crystallizable copolymer containing at least 50 mol% of the aliphatic C₂₋₆ olefin as the main component. By crystallizable copolymer is meant a copolymer having a crystallinity of at least 25%. Examples of the copolymerizable monomers other olefin monomers, vinyl acetate, maleic anhydride, methyl acrylate or methacrylate, and acrylic or methacrylic acid. These copolymerizable monomers are used in a quantity usually not exceeding 20 mol% and that does not adversely affect the crystallinity of the polymer. It is possible to use a random, block or graft copolymer. It is preferable to use isotactic polypropylene, which can easily be formed into a heat-resistant diaphragm at a low cost, or an isotactic propylene copolymer having an ethylene content not exceeding 30% by weight, and preferably in the range of 2 to 15% by weight. Furthermore, a blend polymer obtained by mixing two or more of the above-mentioned polymers, for example, by adding low-density polyethylene or ethylene-propylene copolymer to isotactic polypropylene, may be used. The mixture of a polyolefin and mica used for the diaphragm preferably has a melt index not exceeding 3.5 g/10 min., particularly not exceeding 3.0 g/10 min. and especially not exceeding 2.0 g/10 min. The melt index may be determined in accordance with the requirements of ASTM D1238, and if the polyolefin is, for example, polypropylene, it is expressed by its melt flow rate (g/10 min.) at 230°C. If the mixture has melt index exceeding 3.5 g/10 min., a sheet formed from it is likely to develop wrinkles, or other defects when a diaphragm is formed from the sheet by vacuum forming, pressing, stamping, or otherwise. A polyolefin-mica mixture having a low melt index can be obtained by using a polyolefin having a low melt index. When proceeding according to this invention, it is also possible to use a thermoplastic polyester, for example, a polymer of an alkylene glycol ester of terephthalic or isophthalic acid. Such polyesters may contain an ester formed from C₂₋₁₀ alkylene glycols, such as ethylene glycol, tetramethylene glycol, hexamethylene glycol or decamethylene glycol. it is preferable to use a polyalkylene glycol terephthalate or isophthalate made from C₂₋₄glycol(s), or a copolyester of terephthalic and isophthalic acid containing not more than 30 mol% of isophthalic acid. Specific preferred esters are polyethylene terephthalate, polybutylene terephthalate, polypropylene terephthalate, polybutylene isophthalate, and polybutylene terephthalate-isophthalate copolymer. It is common to use polyethylene terephthalate or polybutylene terephthalate, though the latter is preferred because of its higher loss tangent. The diaphragm of this invention may also be formed from a polyamide obtained by the polymerization of a lactam or aminocarboxylic acid having 6 to 12 carbon atoms, or the polycondensate of a diamine and dicarboxylic acid, or a copolymer of such monomers or a mixture of such polymers. It is usually possible to use nylon 6, nylon 6.6, nylon 6.10, nylon 6.12, nylon 11 or nylon 12, or a copolymer or mixture of them. Nylon 6 or nylon 6.6 is preferred. It is also possible to use a crystallizable polyamide obtained by the polycondensation of a diamine such as hexamethylenediamine, metaxylenediamine, paraaminocyclohexylmethane, or 1,4-bisaminomethylcyclo hexane with a dicarboxylic acid such as terephthalic, isophthalic, adipic or sebacic acid, or a copolymer of such a condensate with nylon 6 or 6.6. It is possible to use various types of mica, such as muscovite, phlogopite or fluorophlogopite, but it is necessary to choose one having a weight-average flake diameter not exceeding 500 µm, and a weight-average aspect ratio of at least 10. Mica is crushed to some degree when it is mixed in molten form with the polymer. The terms "flake diameter and aspect ratio" of mica as herein used indicate those characteristics of mica determined after it has been mixed with the polymer. The weight-average flake diameter of mica ( D ) is obtained by the following equation: D = 2 D₅₀ where D₅₀ stands for the sieve opening that passes 50% by weight of mica flakes. The value (D₅₀) is determined by plotting various sizes of sieve opening against weights of mica flakes remaining on the sieves in a Rosin-Rammlar diagram. The weight-average aspect ratio of mica (α) is obtained by the following equation: α = D/t where t stands for the weight-average thickness of mica. The value (t) is measured by a powder film method described in C. E. Capes and R. D. Coleman: Ind. Eng. Chem. Foundam. Vol. 12, No. 1 (1973) and Nishino and Arakawa: Zairyo (text:Japanese) Vol. 27, No. 298 (1978), and calculated by the following equation, based on the result of the measurement. t = 1 ρ(1-ε)S where ρ stands for the true specific gravity of mica flakes, ε stands for the void volume, and S stands for the area of a film formed on water surface by a unit weight of mica flakes. For convenience, the value (ε) may be taken as 0.1. If mica having a weight-average flake diameter exceeding 500 µm is used to form a diaphragm, mica flakes separate easily from the diaphragm surface and it is very difficult to form the diaphragm by melt processing. It is preferable to use mica having a weight-average flake diameter of 10 to 300 µm. The weight-average aspect ratio of the mica for use in this invention may usually be in the range of 10 to 1000. If mica having a weight-average aspect ratio of less than 10 is used to form a diaphragm,the diaphragm does not have a satisfactorily improved dynamic modulus, and its acoustic properties are unsatisfactory. The mixture of the polymer and mica from which the diaphragm of this invention is formed contains 30 to 95% by weight of the polymer, and 5 to 70% by weight of mica. If the mixture contains less than 5% by weight of mica, the diaphragm does not have a satisfactorily improved dynamic modulus. If the mixture contains more than 70% by weight of mica, it is difficult to mould a sheet from which the diaphragm is formed. It is particularly advisable to use 10 to 60% by weight of mica and 40 to 90% by weight of polymer. In order to increase the dynamic modulus of the diaphragm and prevent separation of mica flakes from the diaphragm surface, thereby improving the interfacial bonding strength between the polymer and the mica, it is advisable to use mica having a surface treated with a surface-treating agent such as a silane coupling agent. Examples of applicable silane coupling agents include γ-aminopropyltrimethoxysilane, N-(β-aminoethyl-γ-aminopropyltrimethoxysilane, γ-mercaptopropyltriethoxysilane, and γ-glycidoxypropyltrimethoxysilane. In order to apply the surface-treating agent to mica, it is possible to immerse mica powder in a solution of the agent in water or an organic solvent, and dry it. Alternatively, the agent can be incorporated directly into a mixture of the polymer and mica when the mixture is prepared. Although there is no particular limitation on the quantity of the surface-treating agent to be used, it is usually satisfactory to use 0.1 to 3% by weight of the agent based on the weight of the mica. When the diaphragm of this invention is manufactured, it is possible to use in addition to mica an auxiliary filler, such as talc, calcium carbonate, wollastonite, glass beads, magnesium hydroxide, silica, graphite, glass flakes, barium sulphate, alumina, or fibres of potassium titanate, processed mineral, glass, carbon or aramide, usually in a quantity not exceeding 40% by weight of the polymer and mica, and not exceeding 95% (preferably 50%) by weight of mica. it is also possible to add such material as a pigment, a plasticizer, a stabilizer and/or a lubricant if required. The diaphragm of this invention is manufactured of a sheet formed from the polymer and mica, preferably from a molten mixture of the polymer and mica by extrusion in a customary manner, as this method facilitates sheet forming. Furthermore, the sheet is moulded into a desired shape, e.g. by vacuum forming, pressing or stamping according to need. Although there is no particular limitation on the thickness of the diaphragm according to this invention, it is suitable for it to have a thickness of 0.1 to 0.9 mm, and particularly 0.2 to 0.7 mm. A diaphragm having a thickness less than 0.1 mm is low in strength while a diaphragm having a thickness greater than 0.9 mm is too heavy, and requires a strong and expensive magnet. The diaphragm thus obtained is incorporated into a loudspeaker of any type known in the art. United Kingdom Patent No. GB-A-1,563,511 discloses the construction of a typical moving-coil loudspeaker in which the diaphragm is used in the form of a hyperbolic cone or tweeter dome. The diaphragm of this invention has a drastically higher dynamic modulus than that of any conventional diaphragm formed solely from a polymer and a substantially unchanged loss tangent. Furthermore, it is easy to manufacture and therefore provides an excellent loudspeaker diaphragm. The diaphragm of this invention can maintain its high dynamic modulus even at a high temperature, and it is, therefore, fully capable of withstanding any elevation in the ambient temperature that will occur to an acoustic apparatus in which the diaphragm is used, or any temperature elevation that will occur when any such acoustic apparatus is assembled, for example, when the diaphragm is bonded to a base. The invention will now be described in further detail with reference to the examples, which are illustrative. As will appear, the diaphragms in accordance with the present invention prepared in the Examples have no anisotropy, are easy to mould, have excellent acoustic properties and retain the properties, particularly the loss tangent, of the polymer. Comparative Examples, which are not in accordance with the invention, are also presented. Example 1 Phlogopite having a weight-average fiake diameter of 21 µm and having a surface treated with 0.5% by weight, based on the mica, of γ-aminopropyltriethoxysilane, and crystalline polypropylene having a melt index of 1 g/10 min. were mixed in molten form by a single screw extruder at 230°C to form pellets. The pellets were extruded at 240°C into a polypropylene-mica sheet containing 60% by weight of phlogopite and having a thickness of 300 µm. The mica in the sheet had a weight-average flake diameter of 18 microns and an aspect ratio of 12. The dynamic modulus E′ and loss tangent tan δ of the sheet thus obtained were measured at a frequency of 110 Hz and a temperature of 20°C by using a Toyo Baldwin Vibron DDV-2. Its density ρ was measured by using ethanol in accordance with the method specified by Japanese Industrial Standard JIS K7112A. The transmission speed of sound was determined by a dynamic modular tester. The temperature at which the sheet had a dynamic modulus E′ of 10 9 dynes/cm 2 (100 MPa) was obtained in accordance with the temperature dependence of the dynamic modulus E′ to provide a standard for the evaluation on heat resistance. The specific modulus, sound velocity, loss tangent and heat resistance of the sheet determined as hereinabove described were all very satisfactory as shown in Table 1 below. Twenty loudspeaker cones were vacuum formed from the sheet at a temperature of 190°C. The sheet showed an excellent degree of vacuum formability and did not produce any defective product. Examples 2 and 3 Sheets having a thickness of 500 µm, and containing 30% by weight (Example 2) or 10% by weight (Example 3) of phlogopite were formed by using phlogopite having a weight-average flake diameter of 40 µm (Example 2) or 230 µm (Example 3). in all the other respects, the procedures of Example 1 were repeated for the manufacture and testing of the sheets. The results are shown in Table 1. The specific modulus, sound velocity, loss tangent and vacuum formability of the sheets were all quite satisfactory. Example 4 A sheet having a thickness of 200 µm was formed from a mixture of a propylene-ethylene block copolymer having a melt index of 3.5 g/10 min., and an ethylene content of 6% by weight, and phlogopite powder having a weight-average flake diameter of 90 µm, and occupying 30% by weight of the mixture. In all the other respects, the procedures of Example 1 were repeated for the manufacture and testing of the sheet. The test results are shown in Table 1. As is obvious from Table 1, the specific modulus, sound velocity, loss tangent, heat resistance and vacuum formability of the sheet were all quite satisfactory. Examples 5 and 6 Sheets having a thickness of 400 µm were formed from a mixture of polypropylene having a melt index of 5 g/10 min. (Example 5) or a propylene-ethylene block copolymer having an ethylene content of 6% by weight and a melt index of 5 g/10 min. (Example 6), and 30% by weight of phlogopite powder having a weight-average flake diameter of 40 µm. In all the other respects, the procedures of Example 1 were repeated for the manufacture and testing of the sheets. The specific modulus, sound velocity and heat resistance of the sheets were satisfactory as shown in Table 1, but the sheets sagged when they were heated for vacuum forming into loudspeaker cones. Twenty loudspeaker cones were formed from each sheet, but wrinkles were found in five cones formed from the sheet of Example 5 and four cones formed from the sheet of Example 6. The sheets of Examples 5 and 6 were both inferior in vacuum formability to those of Examples 1 to 4. Example 7 High-density polyethylene having a melt index of 2g/10min. and 50% by weight of phlogopite powder having a weight-average flake diameter of 90 µm were mixed and extrusion-moulded at 160°C to form a sheet. In all the other respects, the procedures of Example 1 were repeated for the manufacture and testing of the sheet. The test results are shown in Table 1. The specific modulus, sound velocity, loss tangent and heat resistance of the sheet were quite satisfactory. it also showed superior vacuum formability when it was vacuum-formed at 130°C into a diaphragm of cone form. Comparative Examples 1 to 3 Sheets were formed from polypropylene, and phlogopite powder having a weight-average flake diameter of 19 µm (Comparative Examples 1 and 3) or 15 µm (Comparative Example 2). In all other respects, the procedures of Example 1 were repeated for the manufacture and testing of the sheets. The composition of the sheets and the test results are shown in Table 2, from which it will be seen that the sheet of Comparative Example 1 was unsatisfactory in both specific modulus and heat resistance and that of Example 2 was unsatisfactory in specific modulus. Comparative Example 3 encountered difficulty in the extrusion-forming of the sheet and the vacuum-formation of a loudspeaker cone from the sheet. The properties of the sheets showed improvements over those of the sheets formed solely from polypropylene, but the improvements were not so distinct as those achieved in the Examples of this invention: in particular, the specific modulus and loss tangent were lower than in any of the Examples. Comparative Example 4 A sheet was formed solely from polypropylene of the type used in Example 1. The results are shown in Table 2. Its specific modulus was unsatisfactory for forming a diaphragm for an electro-acoustic transducer. Comparative Example 5 A sheet was formed solely from high density polyethylene of the type used in Example 7. The results are shown in Table 2. Its specific modulus and heat resistance were unsatisfactory for forming a diaphragm for an electro-acoustic transducer. Example 8 Polybutylene terephthalate (PBT) of intrinsic viscosity 1.0 dl/g and muscovite having a surface treated with γ-aminopropyltriethoxysilane (0.5% by weight based on the mica) and having a weight-average flake diameter of 140 µm were mixed in a single-screw extruder at 250°C to form pellets. The pellets were extrusion-moulded at 240°C to form a polyester-mica sheet containing 40% by weight of muscovite and having a thickness of 400 µm. The mica in the sheet had a weight-average flake diameter of 90 µm and an aspect of 35. Table 3 shows the results of the tests conducted on the sheet thus obtained. The specific modulus, loss tangent and heat resistance of the sheet were all quite satisfactory. A loudspeaker cone diaphragm could easily be vacuum-formed from the sheet at 250°C. Comparative Example 6 A sheet was formed solely from polybutylene terephthalate of the type used in Example 8. The test results are shown in Table 3. Its specific modulus and heat resistance were unsatisfactory. Example 9 A sheet having a thickness of 200 µm was formed, by melt-mixing and extrusion-forming at 270°C and otherwise repeating the procedures of Example 8, from polyethylene terephthalate (PET) of intrinsic viscosity 0.75 dl/g and muscovite powder having a weight-average flake diameter of 140 µm. Its specific modulus, loss tangent and heat resistance were quite satisfactory as shown in Table 3. A loudspeaker cone could easily be formed from the sheet by vacuum-forming at 250°C. Comparative Example 7 A sheet was formed solely from polyethylene terephthalate of the type used in Example 9. The test results are shown in Table 3. Its specific modulus and heat resistance were unsatisfactory. Example 10 A sheet having a thickness of 300 µm was formed from nylon 6 having a melt index of 5 g/10 min. and muscovite powder having a weight-average flake diameter of 140 µm by melt mixing and extrusion forming at 250°C and otherwise repeating the procedures of Example 8. The specific modulus, loss tangent and heat resistance of the sheets were quite satisfactory as shown in Table 3. Loudspeaker cones could easily be formed from the sheets by vacuum forming at 230°C. Example 11 A sheet having a thickness of 300 µm was formed from nylon 6.6 having a melt index of 5 g/10 min. and muscovite powder having a weight-average flake diameter of 140 µm by melt mixing and extrusion forming at 270°C and otherwise repeating the procedures of Example 8. The specific modulus, loss tangent and heat resistance of the sheets were quite satisfactory as shown in Table 3. Loudspeaker cones could easily be formed from the sheets by vacuum forming at 230°C. Comparative Examples 8 to 11 Sheets were formed from polypropylene and a filler other than mica, such as talc or flaky graphite, and also from a resin of the type not used in this invention, mainly polyvinyl chloride, and mica. The composition of the sheets and the test results are shown in Table 4. None of the sheets thus obtained was satisfactory in performance.

Claims: A diaphragm for an electro-acoustic transducer and formed from a sheet obtained by the melt processing of a mixture of a polymer and mica, comprising (a) 30 to 95% by weight of (i) a polymer whose major component is at least one aliphatic olefin having 2 to 6 carbon atoms, or (ii) a polyester, or (iii) a polyamide which is obtained by the polymerisation of a lactam or aminocarboxylic acid having 6 to 12 carbon atoms, which is the polycondensate of a diamine and a dicarboxylic acid, or which is a copolymer of such monomers, or (iv) a mixture of such polymers, and (b) 5 to 70% by weight of mica having a weight-average flake diameter of 500 µm at maximum and a weight-average aspect ratio of at least 10, the flake diameter and aspect ratio being determined after mixing the mica with the polymer. A diaphragm as claimed in Claim 1, in which the polymer is a polyolefin. A diaphragm as claimed in Claim 2, in which the polyolefin is polypropylene, or a crystallizable copolymer containing at least 50 mol% of propylene units. A diaphragm as claimed in Claim 3, in which the mixture has a melt index of 3.5 g/10 min. at maximum. A diaphragm as claimed in Claim 1, in which the polymer is a polyester. A diaphragm as claimed in Claim 5, in which the polyester is polyethylene terephthalate or polybutylene terephthalate. A diaphragm as claimed in Claim 1, in which the polymer is a polyamide. A diaphragm as claimed in Claim 7, in which the polyamide is nylon 6 or nylon 6.6. A diaphragm as claimed in any preceding claim in which the mica is treated with a silane coupling agent. A diaphragm as claimed in any preceding claim in which the sheet comprises 40 to 90% by weight of the polymer and 10 to 60% by weight of the mica. A diaphragm as claimed in any preceding claim in which the sheet is formed by melt extrusion. A moving-coil loudspeaker including a diaphragm as claimed in any one of Claims 1 to 11.

Inventors: FUJII, KIYONOBU, IKEDA, KOTARO, OHARA, OSAMU, OKUNO, KENJI, SAITO, KOICHI
Applicants: KURARAY CO., LTD.
Application year: 1982
Main classification: H04R7
IPC codes: H04R7