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Technical Field of the Invention The present invention relates to a polypropylene resin
composition for an acoustic diaphragm represented by a speaker cone having improved acoustic
characteristics. [Technical background of the invention and its problems] It is generally known
that a polypropylene-based resin composition is used for an acoustic diaphragm. 0 For example,
JP-A-57-154994, JP-A-58-3499 are polypropylene or their coweights Mica of a specific shape is
mixed and compounded in combination, but in these compositions, the main characteristics
required for the acoustic diaphragm are: low specific gravity, high rigidity, internal loss (tan δ)
Among the large-sized ones, the effect of lowering the specific gravity of 1) is not obtained, US
Pat. No. 4,190,748 simply laminates low density polyethylene or non-productive polypropylene
onto polypropylene and its copolymer sheet, or The material is an acoustic diaphragm material,
which has insufficient rigidity. Also, the compositions used in these prior art are compositionally
different from the compositions of the present invention described below. [A) Outline of AJ As a
result of intensive studies on a polypropylene resin composition suitable for an acoustic
diaphragm, the present inventors mix a filler mainly composed of inorganic hollow microspheres
into a polypropylene resin. As a result, it has been found that the reduction in specific gravity, the
increase in rigidity, and the increase in jan δ can be measured, and the present invention has
been made. [Object of the Invention 1] The object of the present invention is to provide a novel
polypropylene resin composition for an acoustic diaphragm in which low specific gravity, high
rigidity and high tan δ are balanced. The present invention has the following main components
(1) and (2) or (3). (1) A composition for an acoustic diaphragm comprising 5 to 30% by weight of
a polypropylene resin and a hollow fine spherical inorganic filler. (2) The composition according
to item (1), wherein 1 to 25% by weight of the composition weight (excluding hollow
microspheres) is blended. (3) The composition according to the above item (1), wherein the
average particle diameter of the hollow fine spherical inorganic filler particles is 10 to 300 gm
and the true specific gravity is 0.2 to 0.8 g / crnj. The polypropylene-based resin which is the
basis of the composition of the present invention is a crystalline propylene homopolymer, and an
α-olefin such as propylene and ethylene, butene-1,4-methylpentene-1, hexene-1, octene-1, etc.
Copolymers with species or more, modified polypropylene resins obtained by graft
polymerization of unsaturated carboxylic acids such as acrylic acid and maleic anhydride, and
mixtures thereof. These polypropylene resins preferably have a high degree of crystallinity, such
as 90% by weight or more.
When the composition of the present invention is formed into a diaphragm by injection molding,
it is preferable to use a highly flowable polypropylene resin, for example, one having a melt flow
rate of more than 33/10 m1 n. In the case of forming a diaphragm by vacuum forming, pressure
forming, stamping forming and the like by using a diaphragm, a polypropylene resin having low
flowability, for example, an inorganic hollow microspheroidal filler used in the composition of the
present invention Examples thereof include glass balloons, silica balloons, alumina balloons,
aluminosilicate balloons, shirasu balloons, carbon balloons and the like. The average particle
diameter of the hollow microspheroidal filler particles is preferably about equal to or less than
the thickness of the diaphragm which is the final molded product. When the average particle size
exceeds about the thickness of the diaphragm, the surface smoothness of the diaphragm is
reduced and the appearance is deteriorated. The true specific gravity is preferably 0.2 to 0.6 g /
cm 3. If the true specific gravity becomes too small, the pressure resistance decreases and the
hollow body is broken in the process of melt-kneading, extruding or injection molding the
composition, so it is difficult to reduce the specific gravity. In addition, even if the true specific
gravity is too large, it is difficult to exert the effect of reducing the specific gravity. The
compounding amount is 5 to 30% by weight, preferably 10 to 25% by weight. If it is less than 5%
by weight, the improvement effect of low specific gravity and high rigidity is small, and if it
exceeds 30% by weight, besides the decrease of tan δ, the uniform dispersion is lowered and the
ripening property is also unfavorably lowered. As the non-hollow filler which is a component of
the composition of the present invention, talc, calcium carbonate, mica, magnesium phosphate,
aluminum hydroxide, barium sulfate, calcium sulfate, calcium silicate, titanium oxide, potassium
titanate, carbon Graphite, silicon nitride, wood powder, rice husk powder etc. may be mentioned.
Among these, talc, mica, carbon graphite, and rice hulls powder are particularly preferable in that
they have a large effect of improving rigidity. These may be used alone or in combination of one
or more types. The 0 particle diameter is preferably about equal to or less than the thickness of
the diaphragm, generally 150 p. M or less is preferred. The compounding amount is up to 25%
by weight. When it exceeds 25% by weight, the decrease of tan δ is large and the specific gravity
is also large, and the characteristics as an acoustic diaphragm are largely lost. When hollow
microspheres and non-hollow fillers are used in combination, the blending amount of the nonhollow fillers is preferably equal to or less than the blending amount of the hollow microspheres
in order to obtain a low specific gravity. In the composition of the present invention, as other
additional components, stabilizers, youthful agents, lubricants, nucleating agents, antistatic
agents, UV absorbers, flame retardants, anti-copper agents, polyethylene, synthetic rubbers,
styrenic synthetic rubbers Additives or compounding agents generally used for polypropylene
resins such as rosin and petroleum resins and derivatives thereof, coupling cracks, etc. can be
appropriately used.
Although the composition of the present invention can be directly formed by tri-blending the
constituent components, it is previously kneaded by a conventional kneading method such as a
roll, Banbury mixer-1 high speed rotary mixer, extruder, etc. to obtain a composite composition.
The method used for post-forming is general. The composition of the present invention is
generally formed into a sheet by injection molding or extrusion, and the sheet is generally
formed by thermoforming such as vacuum forming into a diaphragm of a final shape. At that
time, other materials, for example, plastic films such as polyester film and polycarbonate film,
paper, non-woven fabric, metal foil and the like can be used as an end layer. Also, in order to
improve the adhesion, the surface of the molded article can be treated by a corona treatment or a
method generally known for flame treatment rings. The invention will now be described by way
of example. [Examples of the Invention] Each test of the present invention was conducted in the
following manner. Q Melt flow rate (MFR): JIS K87580 Specific gravity (density): JIS K 67580
Jang rate: ASTM 08820 Internal loss (tan δ): Preparation of vibration lead method 0 sheet:
Composite polypropylene resin set at 240 ° C Using an 85m / wφ extruder, extrusion cooling is
carried out between a metal roll with a diameter of 400m / mφ and a rubber roll with a
diameter of 15h / mφ set at 30 ° C from a T-die with a width of 800 ■. A 35 mm sheet was
made. Examples 1 to 12. Comparative Examples 1 to 5 A homopolymer having a melt flow rate of
1.0 g / 10 ml was used as a polypropylene resin. As hollow microspheres, hollow glass made by
Nippon Sheet Glass (trade name: Caroon) C-10 (true specific gravity 0.38 g / c, average particle
diameter 4 GBm), C-30 (true specific gravity 0.221 / cm ′ ′) , An average particle diameter of
40 p-m) or an aluminosilicate hollow sphere (trade name: Fillite) manufactured by Nippon Phillite
■, having a true specific gravity of 0.4 g / crn ', an average particle diameter of 401 L, 200 gm,
0.7 g / 0.7 A material having an average particle diameter of 40 JL cm '' was used. As fillers, talc
(average particle size 2.m), mica (average particle size 60%, aspect ratio 55), carbon graphite
(average particle size 4SIL ■) is used and blended at a mixing ratio of Table 1 Thereafter, the
mixture was melt-kneaded and extruded in a 40-preferred 1-layer φ extruder to form pellets,
which were then sheeted by the above method. On the other hand, as a comparative example, a
composition which did not satisfy the compositional conditions of the present invention was
tested in the same manner as in the example, and the results are shown in Table 1.
In addition, to each compounding amount, BHT as O, 15 PHR and calcium stearate as 0.20 PHR
were added as a stabilizer. As shown in Table 1, the comparative examples which do not satisfy
the compositional conditions of the composition of the present invention have defects in any of
low specific gravity, high rigidity (Young's modulus), high tan δ, and sheet surface property, It
can be seen that all the characteristic values maintain good levels.
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