The present invention relates to a process for producing a high strength
porous film of polypropylene. More particularly, the present invention relates to
a process for producing a porous film of polypropylene excellent in moisture permeability
and tensile strength which has been obtained by preparing a non-porous film of polypropylene
without substantially adding a plasticizer or a solvent to the starting polypropylene
and subjecting the film to stretching and, optionally, to a thermal treatment for
enhancing crystallinity as well as a process for producing same.
Mentioned as a means for making a film porous is a method wherein
a biaxially stretched film or an inflation film of polypropylene is uniaxially or
biaxially stretched for making it porous, a method wherein the above uniaxially
stretching is carried out in the presence of a first liquid substantially incapable
of dissolving a polypropylene film, or a method wherein the above thermal treatment
or stretching treatment of the polypropylene film is carried out in the first liquid
substantially incapable of dissolving polypropylene and thereafter the film is dipped
into a second liquid compatible with the first liquid and lower in boiling point
than the first liquid but inferior in affinity to polypropylene and is then dried
up.
The polypropylene film produced with the process of the present invention
is characterized by possessing the following physical properties:
- (1) a tensile strength of at least 35 MPa in machine direction (MD) and transverse
direction (TD);
- (2) a porosity of not more than 80%; and
- (3) a moisture permeability of at least 20 g/m2 day and is obtainable
by being made porous by stretching and, optionally, thermally treating an air-impermeable
and biaxially stretched polypropylene film which has been obtained without using
a plasticizer or a solvent.
BACKGROUND OF THE INVENTION
From the past, a number of the methods for shaping porous polypropylene
have been proposed.
A general method for making a film involves an inflation film shaping
method and a T-die sheet shaping method. in case a polyolefin is to be made porous
according to such a method, a method wherein the polyolefin is impregnated with
an incompatible organic or inorganic matter and then the surface between the polyolefin
and the incompatible matter is destroyed by external force such as stretching or
a method wherein a component incompatible with polyolefin is treated with an organic
solvent, an acid or an alkali capable of dissolving the component is said to be
general. Thus, it is rather small that a single component polyolefin sheet obtained
from a film-shaping apparatus is directly employed as an original film for preparing
a porous film.
As an example of a porous film prepared from a single component film,
there is mentioned a film prepared from an extrusion cold stretching method as disclosed
in the Official gazette of Japanese Laid-open Patent Appln. No. Sho. 62-121737.
The cold stretching in this case is in principle uniaxial and therefore
a balance in tensile strength of the film is extremely poor so that the tensile
strength of the film in the direction perpendicular to the machine direction is
as small as 20 MPa or less, thus making its industrial use limitative.
With a view to improving the strength in the direction perpendicular
to the machine direction in this invention, Japanese Laid-open Patent Appln. No.
Sho. 55-161830 discloses a solvent-stretching method for a microporous film.
This method enables stretching the film in transverse direction up
to about 300% after stretching it in the machine direction so that a balance of
the tensile strengths seems to be improved. However, tensile strengths themselves
are extremely poor so that the strength of the stretched film is almost equal to
unstretched film or is lower than the latter and at most as high as 30 MPa.
A high molecular weight polyolefin possesses excellent properties
such as impact-resisting property, anti-abrasive property, chemicals-resisting properties
and the like, as compared with a widely used polyolefin, and is now being studied
for shaping it into films or sheets
As the high molecular weight polyolefin is extremely higher in melt
viscosity than the widely used polyolefin, however, the former cannot be shaped
into films or sheets by the aid of an ordinary extruder unlike the widely used polyolefin.
In case a film or sheet excellent in tensile strength is to be obtained
from the high molecular weight polyolefin, it is ordinary that the high molecular
weight polyolefin is impregnated with a solvent or a plasticizer compatible therewith
to decrease its melt viscosity before being shaped into a film or sheet by the aid
of a conventional extruder and is then stretched or further treated for eliminating
the solvent or plasticizer.
These solvent method and plasticizer method employ a large amount
of a solvent or plasticizer compatible with the high molecular weight polyolefin
and a much larger amount of another solvent compatible with the above solvent or
plasticizer for removing the first solvent or plasticizer to make the resultant
film porous.
GB-A-2251205 discloses a process to produce a microporous polypropylene
film without using solvents or plasticisers by biaxially stretching an uniform polypropylene
starting film obtained by slot casting.
SUMMARY OF THE INVENTION
In recent years, demand of a porous film of poiyolefin is being increased.
In the use for a filtering material, for example, the porous film is being watched
in the field of food industry in view of its acid-resisting and alkali-resisting
properties and hygiene. As a separator for secondary batteries to which public attention
is recently paid, the porous film is expected because of its chemical stability
and safety. There are, however, a number of problems for meeting these expectations
In the foregoing extrusion and cold stretching method, a porous film having satisfactory
mechanical strength cannot be obtained. In the art wherein the high molecular weight
polyolefin is used, a great cost is technically or economically needed for coping
with the environmental hygienics on production, safety and hygienic and mental load
of workers.
Accordingly, it is an object of the present invention to improve mechanical
strength of a porous polypropylene film obtained according to an extrusion and cold
stretching method by using a biaxially stretched film or an inflation film of polypropylene
within an ordinary molecular weight region as an original film for producing a porous
film and subjecting the original film to stretching and, optionally, to a thermal
treatment under a specific condition.
It is another object of the present invention to provide a process
for producing a porous film of polypropylene excellent in mechanical strength and
free in porous film functions such as pore size, air-permeability and porosity wherein
a polypropylene film of an ordinary molecular weight is used as starting material
and subjected to a thermal treatment and/or a stretching treatment under a specific
condition thereby eliminating inefficient steps encountered in an aspect of production
in a prior art using a high molecular weight polyolefin and anxious steps from the
viewpoint of environmental hygienics and safety.
BRIEF DESCRIPTION OF THE DRAWING
Fig. 1 shows a metal frame for fixing the circumference of a film
on carrying out a thermal treatment of the film in Experimental Examples 2-11, wherein
11 stands for a screw, 12 for a film before treatment and 13 for the metal frame.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
The present invention has been proposed to attain the above object
and its important character resides in the point that polypropylene having an ordinary
range of molecular weight is used as starting material and shaped into a biaxially
stretched film or inflation film without substantial addition of a plasticizer or
solvent and finally subjected to a special treatment. Noteworthy here is that no
substantial addition of a plasticizer or solvent to the starting polypropylene means
that the polypropylene does not contain any additive which may alter the physical
properties of the polypropylene. It is to be noted however that this does not mean
exclusion of a trace amount of an additive such as a stabilizer.
According to one aspect of the present invention, there is provided
a process, as disclosed in claim 1, one for producing a porous film of polypropylene
which comprises stretching a biaxially stretched film or inflation film of polypropylene
in uniaxial or biaxial direction for rendering the film porous.
According to the present invention, there is further provided a process
for producing a porous film of polypropylene which comprises subjecting a biaxially
stretched film or inflation film of polypropylene to a thermal treatment for increasing
the crystallinity thereof and thereafter stretching the film in at least one direction
for making the film porous.
According to the present invention, there is still further provided
a process for producing the porous film of polypropylene, wherein the starting polypropylene
resin has an intrinsic viscosity [η] of not more than 4.
According to the present invention, there is still further provided
a process for producing the porous film of polypropylene, wherein the thermal treatment
for increasing the crystallinity is carried out in the presence of a first liquid
substantially . incapable of dissolving the biaxially stretched film or inflation
film of polypropylene.
According to the present invention, there is still further provided
a process for producing the porous film of polypropylene, wherein the stretching
of polypropylene in uniaxial or biaxial direction is carried out in the presence
of a first liquid substantially incapable of dissolving the polypropylene film.
According to the present invention, there is still further provided
a process for producing the porous film of polypropylene, wherein the first liquid
is a hydrocarbon having a boiiing point higher than the treating temperature during
the thermal treatment or stretching for making the film porous.
According to the present invention, there is yet further provided
a process for producing the porous film of polypropylene, wherein the chermal treatment
or stretching treatment is carried out in the presence of the first liquid substantially
incapable of dissolving polypropylene, the polypropylene film is then dipped into
a second liquid which is compatible with the first liquid but lower in the boiling
point than the first liquid and is inferior in affinity to polypropylene and thereafter
the film is dried.
According to the present invention, there is further provided a process
for producing the porous film of polypropylene, wherein the biaxially stretched
film or inflation film of polypropylene is obtained without use of a plasticizer
or solvent.
Below are the starting material, a method for preparing the orignal
(precursor) film, a method for the thermal treatment, a method for the stretching
and the characteristics of the resultant film with respect to the process for producing
a porous film of polypropylene of the present invention.
(Starting material)
Polypropylene used in the present invention is obtained by polymerizing
propylene alone or together with one or more combination of α-olefins having
carbon atoms of 4-8, for example, in slurry polymerization by the aid of a Ziegler
catalyst or gas phase polymerization by the aid of a Metallocene catalyst.
The polypropylene is preferably a homopolymer of propylene and, in
case of polypropylene copolymer, the amount of such copolymer is 5 mol% or less.
No limitation exists in the molecular weight of the polymer unless
any trouble takes place at the time of shaping. In order to make shaping ability
compatible with high strengthening, however, the intrinsic viscosity [η] is
desirably at least 1 dl/g, preferably at least 2 dl/g but not more than 4 dl/g.
In case the intrinsic viscosity [η] is less than 1 dl/g, the operation
for rendering the film porous may become difficult.
The polypropylene used in the present invention may be incorporated
with various additives usually employed for polypropylene, such as a heat-resisting
stabilizer, a weather-resisting stabilizer, a lubricant, an anti-blocking agent,
a slipping agent, a pigment, etc., in an amount not damaging the object of the present
invention.
(Original [precursor] film)
Among polypropylene films, those having an intrinsic viscosity [η]
of at least 1 dl/g can be obtained according to an ordinary film-shaping method
such as biaxially stretching method or inflation film-shaping method.
As the stretching method, any of the usually employed methods can
be adopted, such as a sequential biaxially stretching method wherein an ordinarily
used T-die sheet is, after shaping, stretched in a machine direction by a roll or
the like and successively stretched in a transverse direction by a tenter, and a
simultaneous biaxially stretching method wherein the sheet is biaxially stretched
simultaneously by way of a tenter clip. The inflation film-shaping method can be
carried out with the aid of a conventional shaping device by properly selecting
an expansion ratio and a draft ratio (a ratio of a film take-up rate to a resin
extrusion rate in the die).
A film thus obtained is a non-porous film having preferably an intrinsic
viscosity [η] of at least 1 dl/g, more preferably at least 2 dl/g but not more
than 4 dl/g, a tensile strength of at least 50 MPa in machine direction (MD), a
tensile strength of at least 50 MPa in transverse direction (TD), and a moisture
permeability coefficient of not more than 10 g·mm/m2·day. No limitation
exists in thickness of the resultant film so far as the resultant porous film satisfies
the moisture permeability condition, but the thickness is preferably 1-100 µm, more
preferably 10-50 µm for the convenience of handling in the subsequent stretching
step.
Crystallinity of the film determined from the heat of crystal fusion
measured by a differential scanning calorimeter (DSC) is at least 40%, preferably
at least 45%. A film obtained according to the aforementioned shaping method and
having a crystallinity of not more than 40% tends to depress its porosity and is
thus not preferable when stretched to make porous
(Method for thermal treatment)
A thermal treatment of the film obtained by the aforesaid shaping
treatment can be carried out according to a treating method and conditions under
which crystallinity of the treated film may exceed 50%, for example, within a temperature
range from 100°C to the melting or dissolving temperature of polypropylene and a
period of time over 1 minute. In this case, unstretched film is preferably be restrained
or fixed at least in one direction, preferably orthogonal directions to prevent
shrinkage.
It is a matter of course that further thermal treatment for the resultant
film obtained in the above shaping treatment having a crystallinity of at least
40% with a view to enhancing crystallinity further is preferable for maintaining
a high porosity when the film is made porous.
The thermal treatment of the film prior to stretching is carried out
under restraint of the film, or in other words, by fixing the film at least in one
direction, preferably orthogonal directions.
In case the inflation film is used as an original film, the pore diameter
of the resultant porous film may be adjusted by its draft ratio. If the draft ratio
is small, the pore diameter of the film tends to become larger. In contrast, if
the draft ratio becomes larger, the pore diameter will become smaller.
In case the original film prior to the thermal treatment is fixed
in two directions, selection of an optimum condition in the aforesaid treatment
leads to omit the subsequent stretching treatment and the film will be made porous
at that stage. When a specific solvent undermentioned is employed, the treated film
can be allowed to dry under the fixing condition thereby obtaining a porous film.
Atmosphere for the thermal treatment may be open air, but the thermal
treatment may be carried out in a first liquid substantially incapable of dissolving
polypropylene. By the wording "substantially incapable of dissolving" is meant that
at least a part of the polypropylene is swollen or dissolved at the treating temperature
but is kept as a whole in the form of a film. Accordingly, a solvent capable of
dissolving the whole polypropylene is excluded from the first liquid.
Namely, crystallinity of polypropylene can be increased when the prepared
polypropylene precursor film is restrained at least in one direction and dipped
in the first liquid which does not substantially dissolve polypropylene at the treatment
temperature
Illustrative of such first liquid are lower aliphatic alcohols such
as ethanol, propanol, butyl alcohol and amyl alcohol; lower aliphatic ketone such
as acetone, methyl ethyl ketone and cyclohexanone; lower aliphatic esters such as
ethyl formate and butyl acetate; halogenated hydrocarbons such as carbon tetrachloride,
trichloroethylene, perchoroethylene and chlorobenzene; hydrocarbons such as heptane,
cycohexane, octane, decane and dodecane; nitrogen-containing organic compounds such
as pyridine, formamide and dimethylformamide; and ethers such as methyl ether, ethyl
ether and dioxane. Glycols such as monoethylene glycol, diethylene glycol and triethylene
glycol as well as silicone oils generally utilizable as a warming heat medium are
also preferable first liquid
These liquids may be used as a mixture of two or more of them. Warm
or hot water containing a surfactant is also effective as the first liquid but benzene,
xylene and tetralin are not preferable for the reason that these solvents dissolve
polypropylene at the treatment temperature. The first liquids preferable for polypropylene
are n-decane and n-dodecane.
(Dipping into a low boiling point liquid and drying)
The film subjected to the thermal treatment in the first liquid is
then subjected to a drying treatment. According to the sort of liquid used for the
treatment. the liquid may be evaporated by direct drying with warm or hot blast
if the film is fixed in two directions to prevent shrinkage. In case of the first
liquid which is relatively slow in drying speed, it is preferable that the treated
film is dipped into a second liquid compatible with the first liquid and lower in
boiling point than the first liquid but inferior in affinity to polypropylene and
then dried up. On drying, the treated film is fixed preferably at least in one direction,
more preferably in two directions
Illustrative of the utilizable second liquid are low boiling point
hydrocarbons such as hexane and heptane; chlorine-substituted low boiling point
hydrocarbons such as methylene chloride; and chlorine and fluorine-substituted low
boiling point hydrocarbons such as 1,2-dichloro-2,2,2-trifluoroethane, 1,1-dichloro-1-fluoroethane,
1,3-dichloro-1,1,2,2,3-pentafluoropropane and 2,2,3,3,3-pentafluoropropanol. Concerning
the dipping temperature and the dipping time, a minimum temperature and shortest
period of time are selected so long as substitution of the liquids is completely
carried out below the thermal treatment temperature
The dried porous film thus obtained may be subjected to a heat set
treatment in order to remove wrinkles on the film, to adjust the porosity and the
thickness of the film, and to minimize the surface abrasion coefficient of the film.
As conditions for the heat set treatment, a temperature, a treating time and the
like are suitably selected in a gaseous (air) atmosphere.
(Stretching method)
Stretching is carried out at a temperature below the meting point
of the precursor film. The lower limit of the stretching temperature is preferably
150°C-168°C, more preferably 160°C-168°C, although it depends on the sort of polypropylene
and the melting point of the precursor film. A stretch ratio in this case is not
definite and varies according to the precursor film but a higher stretch ratio is
preferable in view of strength and permeability.
The stretching treatment may be carried out in atmosphere of air or
under contact with the first liquid as described in the foregoing thermal treatment,
which is substantially incapable of dissolving the film before stretching at a stretching
treatment temperature.
A method for stretching the film may be any one of the uniaxial stretching
wherein shrinkage in transverse direction (width loss) is minimized, the uniaxial
stretching wherein shrinkage in transverse direction is prevented by way of tenter
clips, the sequential or simultaneous biaxial stretching wherein a total tenter
clip system is used as conducted in an ordinary biaxial stretching testing apparatus,
the continuous and sequential biaxial stretching wherein a first stretching step
is conducted by way of a pair of rolls and a second stretching step is then conducted
by way of tenter clips in transverse direction, and the continuous and simultaneous
biaxial stretching wherein a continuous tenter clip system is used.
After completion of the stretching treatment or the thermal treatment
and the stretching, the film may be subjected to a thermal treatment under the condition
of fixing the film ends in air with a view to preventing shrinkage or formation
of wrinkles in the film.
(Porous polypropylene film)
The film produced according to the present invention is a porous film
wherein microfibrils have uniformly been dispersed and possesses a porosity proper
as a porous film, a moisture-permeability based thereon as well as excellent tensile
strength.
Tensile strength of the porous film of polypropylene obtained according
to the present invention is at least 35 MPa, preferably at least 50 MPa, more preferably
at least 70 MPa in machine and transverse directions, as calculated on the basis
of actual sectional area of the film.
Porosity of the film thus obtained is not more than 80%, preferably
not more than 60%, more preferably not more than 50% and especially preferably within
the range from 10% to 50% by properly selecting the thermal treating and the stretching
conditions of the starting film from the aforesaid ranges.
Moisture-permeability of the porous film of the present invention
is preferably at least 20 g/m2 day, preferably at least 40 g/m2
day. In the stretching and thermal treatment of the film according to the present
invention, any changes in the intrinsic viscosity [η] is within the range of
measurement errors and is thus small.
The aforesaid characteristics in the present invention were measured
according to the following methods:
(Thickness of film)
Thickness of the film was measured by the aid of a film-thickness
measuring apparatus named Miniax (Model DH-150) made by Tokyo Seimitsu KK.
(Porosity)
A film sample was weighed and the thickness as a dense film was determined
by calculation, deeming density of the film as 0.91 g/cc Polosity was determined,
using the following equation, in relation with the value determined by the aforesaid
film-thickness measuring apparatus:
Porosity (vol.%) = T0 - Tw / (T0) × 100
wherein T0 stands for thickness of the actual film measured by the film-thickness
measuring apparatus, and Tw for thickness of a dense film of 0% in porosity
determined by calculation from the weight.
(Tensile strength) and (Fracture point Elongation)
These factors were measured and calculated at room temperature (23°C)
by the aid of a tensile strength tester named Tensilon (Model RTM 100) made by Orientec
Sha in accordance with ASTM D882, Method A (width of test samples: 15 mm). (Moisture
Permeability) and (Moisture permeability coefficient)
Calcium chloride was placed in a moisture permeable cup and tightly
sealed with a test sample. The calcium chloride became hygroscopic through the test
sample and changed its weight. The changed weight was measured by a precision balance
and the moisture permeability was calculated according to the following equation:
Moisture permeability (g/m2·day) = P / (Q)
wherein P stands for a changed weight of the test sample per day in terms of g
and Q for a moisture permeable area of the test sample in terms of m2.
The moisture permeability coefficient was calculated according to
the following equation.
Moisture permeability coefficient (g·mm/m2·day) = X × Y (mm)
wherein X stands for the value of the moisture permeability calculated above and
Y for a thickness of the test sample.
By the way, the test was carried out under the following conditions:
temperature 40°C and humidity 90%, and a cylindrical cap of 60 mm in diameter was
used as the cap.
(Air permeability)
Air permeability of the film was measured according to ASTM D726 by
the aid of a standard Gurley Densometer (Toyo Seiki Seisakusho: Gurley Densometer
Type B) for measuring Gurley seconds.
(Crystallinity)
Crystallinity referred to in the present invention was determined
by calculation as a ratio of a heat of fusion to a theoretical heat of crystal fusion,
the heat of fusion being simultaneously measured at the time of measuring the melting
point according to ASTM D3417 with the aid of a differential scanning calorimeter
(DSC).
EXAMPLES
The present invention will now be illustrated in more derailed by
way of Examples, but it is construed that these Examples are given only for explanation
of preferable embodiments of the invention and that the invention is not limited
to to these Examples unless it oversteps the scope as defined in the claims.
Experimental Example 1
A biaxially stretched film of polypropylene [made by Tocello Brand(OP
U-O) No. 20, intrinsic viscosity 2.9 dl/g (measured according to ASTM D4020)] was
used as a precursor film.
Table 1 shows a restilt of measurement made for the physical properties
of the film used.
Exp. No.
Thickness
(µm)
Tensile strength
(MPa)
Elongation
(%)
Crystallinity
(%)
Moisture Permeability
(g/m2.day)
Moisture P. C*
(gmm/m2. day)
MD
TD
MD
TD
1
19.9
141
291
179
31
50.3
11.5
0.23
(Remarks) MD: Machine direction TD: Transverse direction *P.
C.: permeability coefficient
Experimental Examples 2-11 - (Not Examples of the invention)
(Making porous by the thermal treatment)
Using the film of Experimental Example 1, a thermal treatment was
carried out in the following manner: As is evident from Fig. 1, the precursor film
(12 ) was held between a pair of metal frame (13) made of a stainless steel and
fixed in all directions by fixing the up-and-down frames with a screw (i). The precursor
film in such state was immersed into a tank filled with a treating medium and allowed
to stand for a given period of time. The film fixed with the metal frame was dipped
as such into a vessel filled with hexane kept at room temperature (23°C) and allowed
to stand for 10 minutes to replace the treating medium therewith. The metal frame
was then taken up from the hexane and air dried at room temperature (23°C). The
film was then taken out of the metal frame and used as a test sample for measurement.
Tables 2 and 3 show the treating conditions and results thereof.
In Experimental Examples 5 and 9, the treating temperature was so
excessively high that the polypropylene was dissolved.
In Experimental Examples 10 and 11 wherein a silicone oil was used
as the treating medium, moisture permeability of the film was not satisfactory only
by immersion into the solvent.
Exp. No.
Treating medium
Treating temp. (°C)
Treating time (min.)
2
liquid paraffin *
140
1
3
liquid paraffin *
152
1
4
liquid paraffin *
154
1
5
liquid paraffin *
156
1
6
liquid paraffin
152
5
7
n-decane
120
1
8
n-decane
132
1
9
n-decane
140
1
10
silicone oil **
160
1
11
silicone oil **
168
1
(Remarks) * made by Witco: Carnation
(viscosity index: cSt/40°C = 11-14)
** made byToshiba Silicone KK, Silicone oil TSF451-200
Exp. No.
State of film
Thickness
(µm)
Porosity
(%)
T. S.
(MPa)
Elongation
(%)
Air permeability
(sec/10 ml)
Moisture permeability
(g/m2·day)
Crystallinity
(%)
MD
TD
MD
TD
2
Slightly blue white
22.1
2.3
133
292
224
37
*
27
52.9
3
Blue white
24.6
18.5
118
241
270
43
892
108
55.9
4
White
23.1
21.5
95
152
254
26
417
367
55.6
5
Dissolved
-
-
-
-
-
-
-
-
-
6
Blue white
25.2
18.5
115
233
265
41
1472
85
55.8
7
Slightly blue white
22.8
4.8
130
296
223
40
*
42
54.7
8
Blue white
25.0
13.9
107
221
243
48
1054
61
55.8
9
Dissolved
-
-
-
-
-
-
-
-
-
10
Transparent
20.0
-
133
290
165
35
*
13
53.5
11
Transparent
20.1
-
135
285
160
32
*
15
53.7
(Remarks)
T.S.: Tensile strength, MD: Machine direction, TD: Transverse direction,
* at least 2000 sec./10 ml
Experimental Examples 12-15
(Stretching to make the film porous)
The film obtained in Experimental Example 1 was subjected to a definite
width uniaxial stretching or a sequential biaxial stretching under the conditions
as shown in Table 4 in the presence of a silicone oil (Toshiba Silicone KK, Brand:
TSF451-200) by the aid of a tenter-clip type biaxial stretcher to render the film
porous. The stretching operation was initiated one minute after the film was charged
into a stretching tank maintained at a predetermined temperature. A stretching rate
was constant whereby the initial rate was 500%/min. in distortion rate for the length
of the test sample.
Table 4 shows the stretching conditions for the film while Table 5
shows the characteristics of the stretched film.
Exp. No.
Stretching temp.
(°C)
Stretch ratio
MD
TD
12
160
1.5
1.0
13
160
2.0
1.0
14
168
1.5
1.0
15
160
1.2
1.2
(Remarks) MD: Machine direction, TD: Transverse direction
Exp. No.
State of film
Thickness
(µm)
Porosity
(%)
T. S.
(MPa)
Elongation
(%)
Air permeability
(sec/10 ml)
Moisture permeability
(g/m2·day)
Crystallinity
(%)
MD
TD
MD
TD
2
Blue white
17.8
20.8
167
218
91
36
57
2460
52.1
3
Blue white
16.5
32.2
148
144
41
30
17
6530
53.3
4
White
17.4
24.8
167
174
92
25
32
3250
54.7
5
Blue white
19.5
30.1
123
159
63
39
21
5320
52.8
Remarks) T.S.: Tensile strength, MD: Machine direction, TD: Transverse direction
Experimental Example 16
(Thermal treatment followed by stretching to make the film porous)
A thermal treatment was carried out in the same manner as described
in Experiment No. 8 in n-decane as treating medium Prior to dipping into hexane,
a fixed width uniaxial stretching treatment was carried out in N-decane maintained
at the same temperature (132°C) as the treating temperature. The stretching treatment
was carried out in the same manner as in the case of Experimental Examples 12-15
whereby the stretch rate was 2 times as much as in machine direction.
Table 6 shows the characteristics of the stretched film.
Exp. No.
State of film
Thickness
(µm)
Porosity
(%)
T. S.
(MPa)
Elongation
(%)
Air permeability
(sec/10 ml)
Moisture permeability
(g/m2·day)
Crystallinity
(%)
MD
TD
MD
TD
16
Blue white
23.5
35.0
141
137
42
31
14
7230
54.3
(Remarks)
T. S.: Tensile strength, MD: Machine direction, TD: Traverse direction
Experimental Example 17-19
Using a conventional inflation film apparatus (made by Thermoplastics,
Extruder: 30 mmϕ, L/D = 25, Take-up machine: Model 4-18), a film was prepared
under the following conditions:
Using polypropylene (made by Mitsui Petrochemical Industries, Ltd.
Brand name: B200, MFR = 0.5 g/10 min., intrinsic viscosity: 3.5 dl/g) as starting
material, an inflation film having a folded width of 200 mm was prepared by adjusting
the predetermine temperatures of the extruder, adapter and die portions to 230°C,
230°C and 230°C, respectively, a ratio of the film take-up velocity to the resin
extrusion velocity at the die portion (draft ratio) to 10, and an expansion ratio
to 2. An intrinsic viscosity [η] of the resultant film was 3.3 dl/g. Table 7
shows the characteristics of the resultant film
Exp. No.
Thickness
(µm)
Tensile strength
(MPa)
Elongation
(%)
Crystallinity
(%)
Moisture Permeability
(g/m2·day)
Moisture P. C.
(gmm/m2·day)
MD
TD
MD
TD
17
62.0
67
63
947
950
46.5
10
0.62
(Remarks) MD: Machine direction, TD: Transverse direction, and P. C.: permeability
coefficient
The inflation film was subjected to a thermal treatment for 3 minute
in an air oven maintained at 160°C and then subjected to a fixed width uniaxially
stretching treatment or a sequential biaxially stretching treatment in a silicone
oil (made by Toshiba Silicone KK, TSF-451-200). As the stretching treatment could
not be carried out uniformly at a higher temperature, it was carried out under a
relatively low temperature condition. The stretching treatment was initiated one
minute after the film was dipped into a stretching tank regulated at a predetermmed
temperature. A stretching rate was constant whereby the initial rate was 500%/min.
in distortion rate for the length of the test sample.
Table 8 shows the stretching conditions while Fig. 9 shows the characteristics
of the stretched film
Exp. No.
Stretching temp.
(°C)
Stretch ratio
MD
TD
18
80
2.0
1.0
13
80
2.0
2.0
(Remarks) MD: Machine direction TD: Transverse direction
Exp. No.
State of film
Thickness
(µm)
Porosity
(%)
T. S.
(MPa)
Elongation
(%)
Air permeability
(sec/10 ml)
Moisture permeability
(g/m2·day)
Crystallinity
(%)
MD
TD
MD
TD
18
Blue white
45.1
48.1
103
38
970
130
27
4510
52.4
19
Blue white
42.1
54.3
52
51
430
600
10
8540
52.7
(Remarks) MD: Machine direction TD: Transverse direction
Experimental Example 20
An inflation film was prepared in the same manner as described in
Experimental Example 17 except that only, the draft ratio was changed to 18. Table
10 shows the characteristics of the resultant film.
Exp. No.
Thickness of film
(µm)
Tensile strength
(MPa)
Elongation
(%)
Crystallinity
(%)
MD
TD
MD
TD
20
34.7
76
60
720
780
44.9
(Remarks) MD: Machine direction TD: Transverse direction
(Thermal treatment for making the film porous and Thermal treatment followed
by stretching for making the film porous)
Using the film of Exp No. 20, a thermal treatment was carried out
in the same manner as illustrated in Experiment Examples 2-11. A part of the film
subjected to the thermal treatment was further treated with a tenter-clip type biaxial
stretcher to stretch the film biaxially Tables 11 and 12 show the conditions for
the thermal treatment and the optional stretching treatment and a result of the
treatments, respectively.
Exp. No.
Treating medium
Treating temperature
(°C)
Treating time
(min.)
Stretching treatment
Stretch rate
MD
TD
21
Liquid paraffin
140
1
None
-
-
22
Liquid paraffin
154
1
Yes
2
2
(Remarks) MD: Machine direction TD: Traverse direction
Exp. No.
State of film
Thickness
(µm)
Porosity
(%)
T. S.
(MPa)
Elongation
(%)
Air permeability
(sec/10 ml)
Crystallinity
(%)
MD
TD
MD
TD
21
White
34.2
18.3
45
18
313
112
161.0
50.5
22
White
14.5
47.0
52
51
430
600
9.1
52.1
(Remarks) T.S.: Tensile strength, MD: Machine direction, TD: Transverse direction
It was found that a satisfactory strength was not achieved in Experiment
Example 21 wherein only the thermal treatment was carried out. By the way, the moisture
permeability of these film was recognized to be at least 20 g/m2·day
in view of the values of air permeability.
(Effect of the invention)
From the process of the present invention it is provided a porous
film of polypropylene excellent in tensile strength which has been obtained by subjecting
an air-impermeable polypropylene film obtained without substantial use of a plasticizer
and a solvent to stretching and/or thermal treatment. This film is preferably utilizable
for various fields of industry such as packaging materials for foods, filtering
materials for precision filtration and separators for battery, especially as a film
for printing use, a base film for adhesion, an air-permeable film and the like.