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BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
diaphragm for an electroacoustic transducer using microfibrillated cellulose.
2. Description of the Related Art The physical properties required for diaphragms for
electroacoustic transducers such as speakers and microphones (especially those used for middle
/ low frequency band and wide band) are low in density and high in elasticity (that is, light
weight and high rigidity) And have a suitable internal loss, and also have good mechanical
resistance and good weatherability. Although various polymer materials, ceramics and the like
have been proposed and used as materials satisfying the above-mentioned physical properties,
they are expensive due to the control of physical properties and problems in production. On the
other hand, at present, synthetic resin films, metal diaphragms, paper diaphragms and the like
are often used because of easy control of physical properties and manufacture.
When manufacturing a diaphragm by molding and processing a synthetic resin film, the
manufacturing method is easy, but there is a drawback that the thickness of the diaphragm
becomes uneven in molding processing.
Moreover, as a metal diaphragm, although metal plates, such as titanium and aluminum, are used
as a diaphragm as a thing with large elastic modulus, since these metal plates have small internal
loss, they are used as a diaphragm with large amplitude. The method becomes complicated.
The mechanical properties of the paper diaphragm are determined by the physical properties of
the single fiber used, the bonding strength between fibers, the bonding area, and the processing
agent such as resin processing.
Since this paper diaphragm made of plant fibers such as wood pulp is a natural fiber, the physical
properties of single fibers are limited. For example, the elastic modulus is somewhat inferior, and
it is difficult to obtain a diaphragm of a desired thickness.
By the way, it is conceivable to use a foaming agent to improve the thickness.
However, in a normal paper diaphragm, the fiber diameter of single fibers is large and the length
is long, so as shown in FIG. 3, the density is low, and the thickness of the diaphragm is only by
foaming 3 entering into the space between fibers. It does not increase, nor can it be expected to
improve the rigidity.
Also, even if more foaming agent is added to measure the thickness increase, the trapping
property of the foaming agent is poor in ordinary pulp, so the excess added is removed together
with the drainage during paper making and is sufficient The effect can not be obtained.
In addition, since the fiber-to-fiber bonding strength and the bonding area are not sufficient, a
lightweight and highly rigid diaphragm can not be obtained.
On the other hand, as shown in FIG. 2, in order to increase the fiber-to-fiber bond strength and
the bonding area, a single fiber of wood pulp is beaten with a beater, and the beaten treatment is
advanced to microfibrillated molded article 2 It is higher in density than a simple paper
diaphragm, so it is superior to a paper diaphragm in terms of elastic modulus, but it is too high in
density, and in this case the thickness under the same weight can not be obtained sufficiently,
and It is difficult to obtain sufficient rigidity and the internal loss is also reduced.
In other words, a diaphragm using microfibrillated cellulose has higher modulus of elasticity and
internal loss than simple paper diaphragms, synthetic resin diaphragms, and metal diaphragms. A
well-balanced diaphragm with excellent balance, but a diaphragm that can not have a sufficient
thickness because it only has high density, and requires a large amplitude for middle / low range
or broadband where light weight and high rigidity are required. The problem was that they were
not suitable for
The present invention has been proposed to ameliorate the above-mentioned drawbacks of a
diaphragm using microfibrillated cellulose, and the object of the present invention is to provide a
diaphragm in which the density is reduced and a desired thickness is obtained. It is to do.
SUMMARY OF THE INVENTION The present invention achieves the above object by mixing a
foaming agent with microfibrillated cellulose of which the material is wood pulp and foaming.
The microfibrillated cellulose comprises wood pulp which has been beaten to a Canadian
standard freeness of 300 ml or less, and the foaming agent comprises thermally expandable
microcapsules to achieve the above object.
Furthermore, the amount of the thermally expandable microcapsule added is about 0.1 to 50% by
weight with respect to the microfibrillated cellulose to achieve the above object.
In the present invention, a foaming agent is mixed with a microfibrillated cellulose fiber having a
high fiber-to-fiber bond strength and a large bond area, and after forming into a sheet, it is dried
and foamed, and the foamed part has a sufficient thickness. It is possible to obtain a lightweight,
high-rigidity diaphragm that it has.
In this case, although the non-microfibrillated cellulose does not obtain the desired thickness
even if the foaming agent is used as described above, the cellulose of the present invention is
microfibrillated, so that the retention of the foaming agent is good. A diaphragm of desired
thickness can be obtained. Furthermore, depending on the foaming agent, it is possible to foam
simultaneously with drying in the forming (pressing) step after sheet forming, and it becomes
possible to provide at low cost. In addition, since the thickness of the diaphragm can be easily
adjusted by the clearance of the molding (press) mold, the amount of the foaming agent, and the
like, there is also an advantage that the control of the acoustic characteristics can be easily
[Example 1] A microfibrillated material is mixed with thermally expandable microcapsules to
cellulose made of wood pulp, and this is paper-made and then heated, as shown in FIG. I got
Specifically, the diaphragm 1 was made according to the following procedure. 1. The wood
pulp is beaten to a Canadian standard freeness of 300 ml or less, that is, preferably 200 ml, and
this cellulose is a heat-expandable microcapsule, Nippon Phillite Co., Ltd., 10 wt. % Was added to
make a solution (slurry). At this time, when an appropriate amount of a retention aid (such as a
cationic fixing agent) is added, the dispersed state of the foaming agent becomes more uniform,
and the foamed state of the product is stabilized. 2. The solution (slurry) obtained in 1 above
was made into a predetermined shape (cone, flat plate, dome shape etc.) on a wire mesh etc. and
adjusted to an appropriate water content in the dewatering step (this time the water content 80
%). 3. The paper-made product obtained in the above 2 was heated and foam-formed using a
mold of appropriate clearance. The molding conditions at this time depend on the foaming agent
used, but in this example, they were set at 185 ° C. for 60 seconds. Moreover, the clearance of
the mold was 0.2 mm. Thus, in this diaphragm 1, since the foam 3 by the foaming agent is
present in the molded article 2 of cellulose, the density per unit volume is reduced as compared
with the one without the foaming agent, and the foam 3 makes the desired. The diaphragm 1 of
thickness can be obtained.
The amount of the thermally expandable microcapsule added to the microfibrillated cellulose in
the above example is preferably in the range of about 0.1 to 50% by weight, and the amount of
foaming is adjusted according to the amount of addition. it can.
In addition, a diaphragm of any shape, such as a cone shape, a flat plate, and a dome shape, can
be manufactured by this method, and can be used as another member, for example, a dust cap.
The base material may not be 100% of microfibrillated cellulose, and carbon fibers, ceramic
fibers, liquid crystalline polymer fibers, synthetic pulp (polyethylene, polypropylene, etc.),
organic substances such as mica, It may be a mixture with inorganic physical properties.
As described above, according to the present invention, since a foaming agent is mixed with the
microfibrillated cellulose made of wood pulp to foam, a diaphragm having a desired thickness
can be obtained. In addition, since microfibrillated cellulose used is wood pulp beaten to a
Canadian standard freeness of 300 ml or less, the retention of the foaming agent is good, and as
the foaming agent, handling and foaming are good. It is easy to manufacture because the heat
expandable microcapsules are used. Moreover, the addition amount of the thermally expandable
microcapsules can be approximately 0.1 to 50% by weight, and a diaphragm having a desired
thickness and characteristics can be obtained according to the addition amount.
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