Method of producing an acoustic diaphragm

Abstract: An acoustic diaphragm obtained by forming micro-fibrillated cellulose into a web by a process similar to a paper-making process is disclosed. the micro-fibrillated cellulose is the cellulose obtain by beating to a Canadian standard freeness of not more than 300 ml, or the bacterial cellulose. Since the micro-fibrillated cellulose has only poor wet strength, it is reinforced by a reinforcement element and, in this state, is formed into the web on a wire screen. The reinforcement element may be detached after forming the web, or may be left laminated with the cellulose web so that the resulting composite product is used as the acoustic diaphragm.

Description: This invention relates to a method for producing an acoustic diaphragm used for a loudspeaker or the like, in particular to an acoustic diaphragm which employs a micro-fibrillated cellulose. Previously, the paper pulp has been extensively used for an acoustic diaphragm such as the case of a loudspeaker or the like. The steps involved in the preparation of the paper for a loudspeaker are beating the pulp, dispersing and swelling the beaten pulp in water, and forming the pulp dispersed in water into a web shape by a process similar to that used in the paper making process. However, the web obtained by simply dispersing the pulp obtained from wood in water by the process similar to the paper-making process is unsuitable for use as a diaphragm because it is devoid of a crisp feel and has insufficient mechanical strength, because individual fibres making up the pulp are not affixed strongly together. The affixing force may be developed by softening and disintegrating the fibres into component fibrils (fibrillation) to increase the number of contact points between the fibres so as to increase the number of hydrogen bonds. Such mechanical fibrillation of the individual fibres is termed beating and usually performed by an apparatus known as a beater. The material of an acoustic diaphragm is required to have a relatively high longitudinal wave propagating velocity or sound propagating velocity C is required of the acoustic diaphragm, which makes it advantageous to use a material which is both light and has a large Young's modulus. The physical properties of the cone paper, such as the Young's modulus or tensile strength, are determined by the degree of beating, as mentioned above, such that, in order to produce cone paper exhibiting higher values of the Young's modulus, it is necessary to employ a cellulose exhibiting the advanced degree of beating and hence of fibrillation. It is thought that, in the paper used as the diaphragm material, the higher the degree of beating of the cellulose used for making the web, the higher the Young's modulus of the cone paper becomes. However, an excessive degree of beating of the cellulose used for making the web of the cone paper results in the strength of the cellulose in the wet state during the web-making process being drastically lowered, so that difficulties arise in respect of handling and shape retention. For example, in an attempt to transfer the formed web in the wet state to another metallic mould, the web's shape may collapse. On the other hand, the cellulose tends to intrude into the meshes of the wire screen of the web-making apparatus, so that, when an attempt is made to peel off the formed web (cone paper) from the wire screen after drying, an excess force tends to be applied momentarily to the web causing the web to be destroyed due to the fact that the wire screen is more rigid than the web. On the other hand, when a flat web is formed and moulded to a desired shape by press working with the aid of a metallic mould, excess force tends to destroy the web. Therefore, owing to production difficulties, it is thought to be difficult to make the web for an acoustic diaphragm from cellulose which has been fibrillated to a sufficiently high degree as to achieve satisfactory physical characteristics for the diaphragm produced from it. Above all, it is thought to be extremely difficult to produce the diaphragm with a reduced thickness, which is desireable from a performance viewpoint. EP-A-0,200,409 discloses a moulded material made from bacteria produced cellulose which has a high dynamic strength. Patent Abstracts of Japan Vol. 8 No. 227 (E-285) referring to JP-A-59,144299 discloses a diaphragm made from a reinforcing polymer layer and a layer formed from cellulose pulp. It is therefore a principal object of the present invention to provide an acoustic diaphragm having superior physical properties, such as Young's modulus and tensile strength. It is another object of the present invention to provide a method for producing an acoustic diaphragm wherein the web of the cone paper may be handled even though the web has a low wet strength, and wherein the acoustic diaphragm having a high Young's modulus may be formed from the micro-fibrillated cellulose. According to the present invention, there is provided a method for producing an acoustic diaphragm comprising placing a reinforcement element on a wire screen and forming cellulose, preferably having a Canadian standard freeness of not more than 300 ml, on said reinforcement element for forming a composite web. According to the present invention, an acoustic diaphragm having superior physical properties, such as Young's modulus and tensile strength, is provided by making a web for the diaphragm from micro-fibrillated cellulose. According to the present invention, the web for the diaphragm is formed from the micro-fibrillated cellulose, by a process similar to a paper-making process. The cellulose may be reinforced by a reinforcement element placed on a wire screen of a web-forming apparatus. Thus, the web may be handled even if the web exhibits a low wet strength, so that the acoustic diaphragm having superior physical properties may be produced with high profitability. Since the cone paper of the acoustic diaphragm of the present invention is constituted by micro-fibrillated cellulose, the number of contact points between the fibres and hence the number of hydrogen bonds may be increased to improve the physical properties of the diaphragm, such as the Young's modulus or the tensile strength. Moreover, the cellulose is superimposed on and unified with the reinforcing element for further improving the mechanical strength of the cone paper. On the other hand, in accordance with the process for producing the acoustic diaphragm of the present invention, the micro-fibrillated cellulose is formed into a web on the reinforcement member placed on the web-forming wire screen. It is noted that the web formed from the micro-fibrillated cellulose, even though it is low in wet strength, is reinforced by the above mentioned reinforcing member, so that it may be handled easily even under the wet state, while the web shape may be retained. When peeling the reinforced member from the web after drying, the reinforcing member may be gradually peeled off the web because of the pliability of the reinforcing member, without application of an inadvertently large force to the web. The invention will be further described by way of non-limitative example with reference to the accompanying drawings in which:- Figure 1 is a diagrammatic cross-sectional view showing the web-forming process with the aid of a cone-shaped wire screen. Figures 2a to 2c show the process for producing a dome-shaped diaphragm by a drawing press, wherein Figure 2a is a diagrammatic cross-sectional view showing a cellulose-woven cloth composite body in the form of a flat plate; Figure 2b is a diagrammatic cross-sectional view showing a drawing press working process; and Figure 2c is a diagrammatic cross-sectional view showing the cellulose-woven cloth composite body moulded to a dome shape. The cellulose employed for web making according to the illustrated embodiment of the present invention is the highly micro-fibrillated cellulose which herein has a value of Canadian standard freeness of not more than 300 ml. The cellulose having a value of Canadian standard freeness of not more than 300 ml is to be used because, with value of Canadian standard freeness in excess of 300 ml, the produced web of the acoustic diaphragm has an insufficient Young's modulus. Cellulose having a Canadian standard freeness of not more than 300 ml, referred to hereinafter as micro-fibrillated cellulose , can be prepared as beaten pulp, that is, pulp mechanically beaten by a beater. A value of the Canadian standard freeness of not more than 300 ml may be easily achieved by suitably setting the beating conditions by the beater, such as the beating time or the intensity of the force applied during beating. Bacterial cellulose produced microbially by culturing certain types of bacteria under predetermined conditions may also be used advantageously as the micro-fibrillated cellulose. The above mentioned bacterial cellulose is constituted by a cellulose having high crystallinity and exhibits extremely high strength owing to its extremely strong superficial orientation properties. Also it is 200 to 500 angstroms thick and hence is extremely thin. Typical of the bacteria producing the bacterial cellulose is the acetic acid bacteria, examples thereof being Acetobacter aceti, Acetobacter xylinium, Acetobacter rancens, Sarcina ventriculi, Bacterium xyloides, Acetobacter pasteurianus and Agrobacterium tumefaciens. Further examples of the bacteria are those belonging to the genus Pseudomonas and the genus Rhizobium. The above mentioned bacterial cellulose may be produced as a gel-like substance of a certain thickness in the interface between the culture surface and the air, or by an aeration and agitation culture. The bacterial cellulose so produced may be disagregated in water to form a web. For forming the web, high-polymer fibres such as carbon fibres, glass fibres, aramide fibres, polyolefin fibres, ultradrawn polyolefin resins or polyester resins, may be mixed as reinforcements into the micro-fibrillated cellulose . Additives for paper, such as so-called sizing agents or fillers, may also be added to the micro-fibrillated cellulose, if necessary or desired. On the other hand, a reinforcement element placed on the wire screen may be employed to make up for the wet strength of the web formed from the micro-fibrillated cellulose. For example, woven or non-woven cloths exhibiting certain pliability or flexibility may be conveniently employed as the reinforcement element. The material type or the thickness of the woven or non-woven cloths may be arbitrarily selected if the element is used simply as the reinforcement of the web. However, if the woven or non-woven cloths are directly unified with the web of the micro-fibrillated cellulose, as will be explained later, the material type or the thickness of the element may be selected as a function of the desired properties of the acoustic diaphragm. Meanwhile, if the reinforcements element is used simply as the reinforcement for the web, it is preferred that the reinforcement elements, such as the woven or non-woven cloth, can be readily peeled off from the micro-fibrillated cellulose. On the other hand, if the reinforcement element is to be directly unified with the web of the micro-fibrillated cellulose, it is preferred that the elements can be readily brought into tight contact with the micro-fibrillated cellulose, while being of a higher strength and a higher modulus of elasticity. More specifically, woven or non-woven cloths of carbon fibres, glass fibres, polyester fibres, aramide fibres or silk, may be selectively employed by taking the above requirements into account. The micro-fibrillated fibres may be formed into a web by first placing a wire screen 2 on the bottom of a paper-making machine 1, as shown in Figure 1, placing the above mentioned reinforcement element 3 on the wire screen 2, and supplying thereto a liquid suspension 4 containing the micro-fibrillated cellulose dispersed therein to produce a web 5. The web 5 thus produced is subject to a drying step for drying. At this time, the web 5 formed by micro-fibrillated cellulose may be transferred to the drying process while it is still placed on the wire screen 2. Alternatively, the web may be detached from the wire screen 2 along with the reinforcement element 3 and re-placed on another metal mould before the web is transferred to the drying process. In the latter case, since the web 5 formed by the micro-fibrillated cellulose is handled simultaneously with the reinforcement element 3, there is no risk of destruction or warping of the web 5 even though the web has inferior wet strength. After drying, the reinforcement element may be peeled off from the web of the micro-fibrillated cellulose (cone paper) so that the web formed solely by the micro-fibrillated cellulose may be used as the acoustic diaphragm. Alternatively, the reinforcement element may be unified directly to the web so that the resulting web-woven fabric or web-non-woven fabric composite body may be used as the composite acoustic diaphragm. With the above described method, the shape of the resulting cone paper is determined by the shape of the wire screen 2. However, the web of the micro-fibrillated cellulose in the form of a flat plate may have a desired shape imparted to it by drawing with the use of, for example, a metallic mould. In any of the above methods, an ordinary wire screen 2 may be employed, such as a wire mesh or a punched or perforated metallic plate. It will be seen from the above that, by using a web of the micro-fibrillated cellulose and unifying the reinforcing element to the web in a laminated fashion, there may be provided an acoustic diaphragm which has significantly improved physical properties, such as the Young's modulus or tensile strength. Also, in accordance with the presently proposed method, since the reinforcement element is placed on the wire screen and the cellulose is placed on the reinforcement element for web making, cellulose having a lower wet strength, such as micro-fibrillated cellulose, may be handled easily, so that the acoustic diaphragm with a high Young's modulus may be produced efficiently. In addition, a diaphragm formed of a composite material formed by micro-fibrillated cellulose and various additives, if desired, may be produced easily with various desired properties according to the intended usage and applications. The present invention will be hereinafter explained with reference to several illustrative examples. Example 1 Bacterial cellulose produced by acetic acid bacteria was disagregated using a mixer, and then formed into a web on a web-forming screen 11 fitted with a woven polyester fibre cloth 12 as shown in Figure 2a. The web thus formed was formed by the cellulose 13 and the woven cloth 12. In the web-forming process, the micro-fibrillated cellulose was the desegregated bacterial cellulose produced by the acetic acid bacteria, while the woven polyester fibre cloth 12, used as the reinforcement element, was the product NO 120S with the 100 mesh size (pore diameter, 200 µm) manufactured by NBC Co. Ltd. The concentration of the web was 1g/l. The drying conditions were five minutes of drying with a mould temperature of 140°C. Then, as shown in Figure 2b, the composite body of the cellulose 13 and the woven cloth 12 was processed by drawing by means of a metal mould half 14A having a hemispherical recess and a mating metal mould half 14B having a projection in register with the recess to produce a dome-shaped composite diaphragm as shown in Figure 2c. Example 2 The web-forming and drawing process steps were carried out in the same way as in Example 1. The woven polyester fibre cloth 12 was then peeled off from the cellulose to produce a dome-shaped diaphragm formed solely by the cellulose 13. Example 3 The bleached Kraft pulp (N.B.KP) from needle-leaved trees was beaten by a Hollender type beater to the Canadian standard freeness of 300 ml and processed by web forming and drawing process steps in the same way as in Example 1 to produce a composite diaphragm formed by the cellulose and the polyester fibres. It is noted that, in the above Examples 1 and 3, a binder manufactured by Nippon Zeon Co. Ltd. under the trade name of Nipol Latex and a yield improver (wet web strength improver) manufactured by Dick Hercules Co. Ltd. under the trade name of Kaimen 557-N, were added to the liquid cellulose suspension prior to being formed into a web, in amounts of 10wt. % and 5wt. % related to the quantity of the solid cellulose, respectively, for improving adhesitivity between the cellulose and the woven polyester fibre cloth. The internal loss (tan δ), Young's modulus E and the sound velocity C were measured of the diaphragm obtained by the above technique in accordance with the vibration reed method. The results are shown in the Table below. The results obtained with a customary paper diaphragm, produced by forming the cellulose having a Canadian standard freeness of 560 ml, are also shown in the Table by way of a comparative Example. Comparison between the characteristics of the diaphragm obtained in the Examples and those of the customary paper diaphragm shows that the Young's modulus obtained in the Examples 1 to 3 is two or three times that obtained with the conventional paper diaphragm according to Comparative Example. In addition, since the diaphragms of the Examples 1 to 3 are film-shaped and free of pin holes, as distinct from a conventional paper diaphragm, the coating or impregnation of a joint-filling material which was previously indispensable in a paper diaphragm, may be dispensed with, such that it become possible to produce a thin-film diaphram with a thickness of the order of 10 µm.

Claims: A method of producing an acoustic diaphragm characterised by the steps of placing a reinforcement element on a wire screen of a web-forming machine and forming micro-fibrillated cellulose on said reinforcement element for forming a composite web. The method for producing the acoustic diaphragm according to claim 1 wherein the micro-fibrillated cellulose is obtained by beating pulp to a Canadian standard freeness of not more than 300 ml. The method for producing the acoustic diaphragm according to claim 1 or 2 wherein said micro-fibrillated cellulose is bacterial cellulose produced by bacterial culturing.

Inventors: KURIHARA NOBORU C O AJINOMOTO, URYU MASARU C O SONY CORP, KURIHARA, NOBORU, C/O AJINOMOTO CO. INC., URYU, MASARU, C/O SONY CORP.
Applicants: AJINOMOTO KK, SONY CORP, AJINOMOTO CO., INC., SONY CORPORATION
Application year: 1991
Main classification: H04R7
IPC codes: H04R31, H04R7