Insulating material and production thereof

An insulation material made of rigid microballons within the pores of a porous polymeric base material.

FIELD OF THE INVENTION 
The present invention relates to an insulating material which has improved 
electrical properties owing to microspheres incorporated therein, and also 
to a process for producing the same. 
BACKGROUND OF THE INVENTION 
Among conventional common insulating material fluoroethylene (PTFE) resin 
and polyethylene resin which have low dielectric constants. They are often 
used in porous form for their improvement in electrical properties. 
Unfortunately, a porous PTFE resin structure cannot be produced by 
conventional physical or chemical processes using inert gas or blowing 
agents which are commonly applied to other fluoroplastics and ordinary 
thermoplastic resins, because a PTFE resin has an extremely high melt 
viscosity. This necessitates the use of special processes, such as mixing 
a PTFE resin with a substance removable by extraction or dissolution, 
forming the mixture under pressure, and removing the substance afterward, 
or such as adding a liquid lubricant to a PTFE resin powder, extruding and 
forming the mixture by rolling or other shear force, removing the liquid 
lubricant, and drawing and sintering the molded item, or such as drawing 
an unsintered molded item of PTFE resin in a liquid (such as halogenated 
hydrocarbon, petroleum hydrocarbon, alcohol, and ketone) which sets a PTFE 
resin, and sintering the molded item afterward. 
The processes mentioned above all give rise to open-cell porous PTFE 
structures, which are liable to partly change into that of closed-cell 
structures because their pores are easily collapsed by compressive force. 
This tendency is undesirable particularly in the case where the porosity 
is increased to lower the dielectric constant. If such a porous PTFE resin 
is molded into tape or sheet as an insulating material for electric wires 
and printed-circuit boards, the resulting insulating material is hard to 
handle because of its unstable electrical properties (such as dielectric 
constant). 
In order to eliminate the above-mentioned disadvantages involved in the 
prior art technologies the present inventors previously proposed a process 
for producing a porous closed-cell PTFE structure. This process consists 
of mixing a PTFE resin with glass or silica microspheres containing an 
inert gas (such as nitrogen and carbon dioxide) sealed therein, and 
forming the mixture by rolling (or any other means that exerts a shear 
force) such that the PTFE resin matrix becomes fibers which enclose the 
microspheres, permitting the gas in the microspheres to substantially 
remain as voids, as described in Japanese Patent Publication No. 
25769/19B9 or in its UK counterpart appln. no. 8B18243.1. 
This technology solved many problems associated with the porous open-cell 
PTFE structures but did not address the fact that the dielectric constant 
does not decrease in proportion to the amount of microspheres added. To 
address this problem, the present inventors carried out a series of 
researches which led to the present invention. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention provides a new insulating material and a 
process for producing the same. The insulating material of the present 
invention is free of disadvantages involved in conventional porous 
open-cell PTFE structures and maintains its stable electrical properties 
(including low dielectric constant) under external forces (such as 
compression). 
The present invention is embodied in an insulating material which comprises 
low-dielectric open-celled porous polymeric base material and a large 
number of low-dielectric hard microspheres which are held in the pores of 
said low-dielectric porous polymeric base material for a prevention 
against pore collapse. 
The present invention is also embodied in a process for producing an 
insulating material which comprises dipping a low-dielectric open-celled 
porous polymeric base material in an ultrasonically stirred liquid in 
which, are dispersed a large number of low-dielectric hard microspheres, 
thereby causing the low-dielectric hard microspheres to enter the pore of 
the low-dielectric open-celled porous polymeric base material, and 
subsequently heating the system, thereby causing the low-dielectric porous 
polymeric base material to shrink slightly so that the low-dielectric hard 
microspheres are fixed in the pores.

DETAILED DESCRIPTION OF THE INVENTION 
The insulating material of the present invention contains a large number of 
low-dielectric hard microspheres which are held in the pores of the 
low-dielectric porous polymeric base material for prevention against pore 
collapse. These microspheres those of hard insulating material such as 
glass, particularly glass containing more than 80% silicon dioxide, and 
have a particle diameter from 0.1 to 50 um, which is selected according to 
the diameter of pores in the base material. They contain a gas such as 
nitrogen and carbon dioxide sealed therein, so that they have a low 
dielectric constant, low loss tangent, and low specific gravity. The 
amount of the microspheres in the insulating material is not specifically 
limited; but it is usually in the range of 0.1 to 20 wt%, preferably 1 to 
10 wt%. 
The low-dielectric, i.e. dielectric constant below 2, porous polymeric base 
material, which has an open-celled fine porous structure, may be produced 
from a fluorocarbon resin or polyolefin resin by any known method such as 
leaching method, emulsion method, irradiation method, sintering method, 
and stretching or expanding method. According to the stretching and 
expanding method, the drawing gives rise to fibers and nodes 
interconnected by fibrils which form fine open cells. The fibrils vary in 
diameter and length and the nodes vary in size and number depending on the 
drawing and sintering conditions. The low-dielectric open-celled porous 
polymeric base material may have a properly selected pore diameter and 
porosity. 
The insulating material of the present invention is constructed such that 
the polymeric base material holds in its pores low-dielectric rigid 
microspheres of a size smaller than the pore sizes, e.g. 20 um or less, 
which effectively prevent the pores from being collapsed by compressive 
force. Moreover, the insulating material has a low dielectric constant and 
stable electrical properties because the pores are not completely filled 
by the microspheres. 
The method of the present invention utilizes ultrasonic vibration to 
disperse the microspheres into the pores of the base material, so that the 
base material is exempt from compressive force in the manufacturing step. 
Therefore, the insulating material of the present invention has good 
electrical properties based on the original low dielectric constant of the 
base material which is enhanced by the microspheres filled therein. 
EXAMPLES 
The invention will be described in more detail with reference to the 
following examples, which are not intended to restrict the scope of the 
invention. 
A sample of the insulating material of the present invention was prepared 
in the following manner from a low-dielectric porous polymeric base 
material which is an open-celled porous PTFE resin sheet having a porosity 
of 75.4%, a thickness of 100 um, and a dielectric constant of 1.20. formed 
by the known stretching method, and low-dielectric rigid microspheres 
which are glass microspheres (made by Emerson & Cumming Co., Ltd.) having 
a dielectric constant of 1.20 and a particle diameter smaller than 15 um. 
The microspheres were dispersed in acetone held in a metal container 
placed on an ultrasonic vibrator (50 kHz frequency and 30W output). In the 
ultrasonically stirred dispersion was dipped the open-celled porous PTFE 
sheet for 5 minutes. This dipping operation was repeated with the sheet 
turned up side down. After the dipping operation, the PTFE sheet was dried 
and then heated at 200.degree.C. for 1 minute without restraint so that 
the sheet shrank slightly and fixed the microspheres in the pores. 
FIGS. 1 and 2 are electron micrographs taken respectively before and after 
the polymeric base material was treated with microspheres. It is noted 
that the microspheres are not broken in the pores of the base material but 
are fixed by the fibrillated PTFE. It is also noted that there is liberal 
space around the microspheres so that the insulating material retains a 
high porosity. The sheet had a dielectric constant as low as 1.25 owing to 
the glass microspheres filled in the pores which accounted for 10 wt%. 
This sheet was tested for compression resistance by measuring the 
dielectric constant after compression under a load of 10kg/cm.sup.2 for 30 
minutes. The results are shown in Table 1. 
In Comparative Example 1, an insulating material in sheet form was prepared 
by rolling from a PTFE resin powder and glass microspheres mixed in the 
same ratio as in Example 1 mentioned above. (This insulating material is 
that which the present inventors proposed in the above-mentioned Japanese 
Patent Publication No. 25769/1989.) It has the internal structure as shown 
in FIG. 3. In Comparative Example 2, the same test as in Example 1 was 
performed on an open-celled porous PTFE sheet which contains no glass 
microspheres but has almost the same dielectric constant as that of the 
insulating material in Example 1. The results are shown in Table 1. 
TABLE 1 
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Example 1 Comparative Example 1 
Comparative Example 2 
Before After Before After Before After 
compression 
compression 
compression 
compression 
compression 
compression 
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Dielectric constant 
1.25 1.27 1.60 1.70 1.20 1.45 
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It is noted from Table 1 that the porous insulating material of the present 
invention prevents its pores from being collapsed by compressive force and 
hence keeps its original electrical properties almost unchanged owing to 
the low-dielectric hard microspheres filled in the pores. In addition, it 
has a lower dielectric constant than those produced by the conventional 
method. Therefore, it can be used in the form of film or sheet for 
insulation of electric wires, cables, and printed-circuit boards. Its low 
dielectric constant and high stability to external force contribute to the 
improvement of their performance.