JPH0745230B2 - Polypropylene embossed film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to an oil resistance suitable for an insulating material for electric articles such as capacitors, cables, transformers, etc., which are used after being impregnated with oil,
The present invention relates to a polypropylene embossed film having excellent electrical and mechanical properties.

[Prior Art] The oriented polypropylene film has a dielectric loss (tan δ),
It has excellent dielectric breakdown voltage and has been used as an oil-immersed insulating material. However, since swelling during oil immersion is large,
In particular, when used for power cables and oil-filled transformers, it was necessary to reduce the swelling rate. For this reason, there is a method in which polypropylene is uniaxially highly oriented and highly crystallized and the film is embossed (JP-A-56-109406 and JP-A-58).
−102404). This is because polypropylene is highly uniaxially oriented to lower the swelling, and uniaxially oriented polypropylene is mechanically fragile, so that the elongation which is insufficient due to embossing is to be compensated.

[Problems to be Solved by the Invention] However, the above-mentioned technique has various problems as follows.

For example, in the case of a highly uniaxially oriented film, the elongation is extremely small in the orientation direction and the film is easy to tear.
Fine cracks are generated during embossing, which easily causes insulation defects. Also, the direction orthogonal to the orientation direction (usually the width direction)
However, because of its low mechanical strength, it was disadvantageous when subjected to large mechanical deformation.

The present invention intends to provide a polypropylene embossed film for oil-immersed insulation, which has excellent mechanical properties in balance and excellent dielectric breakdown properties while taking advantage of such a low swelling property of the uniaxially oriented film.

[Means for Solving Problems] The present invention is characterized by having the following configuration in order to solve the above problems.

That is, a uniaxially oriented polypropylene film (A) having a birefringence of 0.025 to 0.033 and a biaxially oriented polypropylene film (B) having a birefringence of 0.010 to 0.020 are bonded so that the principal axes of their respective refractive indexes are orthogonal. A laminated body integrated by laminating a layer (C), wherein the adhesive layer (C) is a layer having a thickness of 0.5 to 10 μm and made of a propylene copolymer having an intrinsic viscosity of 1.0 to 1.9 dl / g, and A polypropylene embossed film having a thickness ratio of (A) and (B) of 1: 1 to 7: 1 and an embossing ratio of the surface (B) side of the laminate of 10 to 50%. About.

In the present invention, polypropylene (hereinafter abbreviated as PP) that constitutes a uniaxially or biaxially oriented polypropylene film is a polymer mainly composed of propylene, and other monomers are copolymerized or other resins are used. Although they may be blended, in order to keep the swelling rate low, the copolymerization rate is preferably 5 mol% or less, and the blending rate is preferably 5 wt% or less.

Further, when the isotacticity of the PP is 93% or more, preferably 96% or more, more preferably 98% or more, not only the swelling upon oil immersion becomes small but also the dielectric breakdown voltage becomes high, which is preferable. Intrinsic viscosity [η] is preferably 1.2 to 2.8 dl / g, and more preferably 1.4 to 2.2 dl / g for good film-forming properties, good swelling rate, and good mechanical properties. preferable.

The birefringence (hereinafter abbreviated as Δn) of uniaxially oriented polypropylene (hereinafter abbreviated as PP-UO) in the present invention is 0.025.
~ 0.033, preferably 0.027 to 0.
It is 031. When Δn is smaller than the above range, the swelling ratio is large and the product cannot be used. On the other hand, when it is larger than the above range, the film becomes brittle and easily cracks in the orientation direction, and the dielectric breakdown voltage decreases.

In addition, biaxially oriented polypropylene (hereinafter referred to as PP
Δn of (abbreviated as —BO) is required to be 0.010 to 0.020, and preferably 0.012 to 0.018. When Δn is larger than the above range, not only the mechanical properties when bonded to PP-UO are not sufficiently improved, but also cleavage is likely to occur and the dielectric breakdown voltage is lowered. Further, the plane orientation of the PP-BO is preferably 0.010 to 0.024, and more preferably 0.012 to 0.020, since the swelling rate is small and the mechanical properties are good.

The melting points of PP-UO and PP-BO are preferably 164 ° C. or higher, and more preferably 166 ° C., because they have excellent dissolution resistance and swelling resistance during oil immersion. The upper limit of the melting point is not particularly limited, but in the experiments conducted by the present inventors, 178 ° C was the upper limit.

In the present invention, the propylene copolymer (adhesive layer C) for adhering PP-UO and PP-BO is a random or block copolymer of propylene and α-olefin. Among them, ethylene propylene random copolymer (rEP
C), ethylene propylene block copolymer (bEP
C), propylene butene copolymer (BPC), ethylene propylene butene ternary copolymer (EPBC) are preferred.

[Η] of the copolymer needs to be 1.0 to 1.9 dl / g, preferably 1.2 to 1.7 dl / g. When [η] is less than the above range, the amount of dissolution at the time of oil immersion increases, the viscosity of the insulating oil increases, and the insulating layer becomes brittle, so that cracks occur in the adhesive layer and dielectric breakdown easily occurs. If [η] exceeds the above range, the fluidity is poor, and sudden deformation cannot be absorbed at the time of embossing the adhesive.
A local force is applied to the UO layer, cracks easily occur in the PP-UO layer, which causes dielectric breakdown. Further, the isotacticity of the copolymer is preferably 70% or more, more preferably 80% or more, because the oil resistance is further excellent and the swelling rate is small.

Further, polar monomers such as maleic anhydride and acrylic acid may be grafted to these copolymers within a range that does not impair tan δ in practical use. This is preferable because the adhesive strength is further improved. Further, the melting point of the propylene copolymer is preferably 110 to 150 ° C., more preferably 120 to 145.
℃. In such a range, the oil resistance is excellent and the adhesive strength is good.

The thickness of the C layer needs to be 0.5 to 10 μm, and preferably 1 to 5 μm. When the thickness of the C layer is smaller than the above range, the adhesive force is insufficient and the laminated film is peeled off in oil, resulting in an insulation defect. When the thickness of the C layer is thicker than the above range, the swelling rate of the C layer is larger than that of the A and B layers, so that not only the total swelling rate increases but also the electrical characteristics, particularly tan δ, deteriorate.

PP-UO (A) and PP-BO (B) need to be bonded and integrated so that the principal axes of their respective refractive indices are orthogonal to each other. If the main axes are bonded so that they are parallel to each other, almost no improvement in mechanical properties is observed, and they are the same as those without lamination. A / C / B is basically used as the stacking method, but if B / C / A / C / B is used, curling does not occur and dimensional stability in oil is good. Therefore, it is preferable.

Further, the thickness ratio (A: B) of the A layer and the B layer needs to be 1: 1 to 7: 1, and preferably 2: 1 to 5: 1. A
When the thickness ratio of the layer is smaller than the above range, not only the swelling rate increases but also the deformation in oil becomes large and the layer cannot be used (for example, embossed sag). Further, when the thickness of the A layer is thicker than the above range, not only the mechanical properties are inferior,
During embossing, the layer A is likely to crack and the dielectric breakdown voltage is reduced.

The embossing of the film of the present invention is performed on the surface of the laminate on the B side, and the embossing rate needs to be 10 to 50%. As an embossing method, a metal roll having an uneven pattern is used.
Make contact with the side. When the embossing roll comes in contact with the A side, it is rapidly deformed, and since the breaking elongation is small, minute cracks are generated in the film, and the dielectric breakdown voltage is remarkably lowered. Conversely, from the B side, the breaking elongation is large,
No microcracks occur and the breakdown voltage does not decrease. When embossing is performed in this way, the embossing pattern on the A side is usually smoother or slightly embossed as compared with the B side. B / C /
When the layer B is on both sides of the laminate as in the A / C / B configuration, it is possible to emboss from both sides with embossing rolls to create a uniform embossed pattern, but only on one side. It may be embossed. In this way, as a result of embossing the film of the present invention, the embossing rate is 10 to 50%.
It is necessary to be, and preferably 10 to 30%. When the embossing ratio is smaller than the above range, not only the dimensional change in oil is large and the oil flowability is poor, but also the handleability is poor because the film elongation is small. Further, when the embossing ratio is larger than the above range, the dielectric breakdown voltage is remarkably lowered, and it cannot be used.

In the present invention, the embossing process may be performed after the layers A, C and B are laminated and integrated, but it is preferable to perform the embossing process while the layers are integrated and integrated because the cost can be reduced.

Next, an example of the method for producing the polypropylene embossed film of the present invention will be described.

First, polypropylene as the raw material for the A layer and ethylene propylene copolymer as the raw material for the C layer are supplied to different extruders, respectively, and the molten polymer is laminated in a die to obtain 40 to 90
It is cooled and solidified on a casting drum kept at ℃. The cast sheet was preheated in a hot air oven kept at 120 to 145 ° C. and then stretched 7 to 9 times in the longitudinal direction,
Heat it while relaxing about 3 to 7% with a roll kept at 130 to 140 ° C. Then, after cutting off the end portion, a film having a thickness of 10 to 200 μm is wound on a roll to obtain a composite film.

In addition, polypropylene as a raw material for layer B and ethylene-propylene copolymer as a raw material for layer C are separately supplied to the extruder, and the molten polymer is laminated in a die,
Cool and solidify on a casting drum kept at 95 ° C. The cast sheet was preheated in a hot air oven maintained at 120 to 145 ° C., then stretched 4 to 7 times in the longitudinal direction, and then 14
Stretching 7-12 times in the width direction in a tenter kept at 0-158 ° C, heat-treating at 150-165 ° C while relaxing for a few%,
Then, after cutting off the end portion, a film having a thickness of 10 to 100 μm is obtained.

Of course, various additives such as an antioxidant, an inorganic lubricant and an antiblocking agent may be added to the film, if necessary.

In addition, it is preferable that the surface of the C layer is subjected to corona discharge treatment or plasma treatment in an atmosphere of air, nitrogen, carbon dioxide gas or the like, because it will bond more firmly.

As a heat bonding method, the layers C are laminated so that they face each other, and an embossing roll (silk 40 mesh × 100 μm) and a paper roll kept at 100 to 140 ° C. are pressed from the layer B side for embossing. Bond at the same time.

The polypropylene embossed film of the present invention can be obtained as described above.

[Characteristic Measuring Method and Effect Evaluation Method] Next, the measuring methods used in the present invention will be summarized below.

(1) Isotacticity The sample film was cut into a size of about 1 cm square, placed in a Soxhlet extractor, and boiled with methyl alcohol to 6
Extract time. The extracted sample is vacuum dried at 60 ° C. for 6 hours. A sample weighing W (mg) is taken from this, which is again placed in the Soxhlet extractor and extracted with boiling n-heptane for 6 hours. Next, this sample is taken out, thoroughly washed with acetone, vacuum dried at 60 ° C. for 6 hours, and then weighed.

If the weight is W '(mg), the isotacticity is calculated by the following equation.

Isotacticity (%) = 100 x W '/ W (2) Birefringence index The refractive index (Nx) in the longitudinal direction and the refractive index (Ny) in the width direction of the film were measured using an Abbe refractometer. , Nx is the insulation value of the difference between Ny and Ny is the birefringence of the film. In addition, sodium D line is used as a light source at the time of measurement, and methyl salicylate is used as a mount solution.

(3) Plane orientation The refractive index (Nx) in the longitudinal direction of the film, the refractive index (Ny) in the width direction, and the refractive index (Nz) in the thickness direction of the film are measured using an Abbe refractometer, and calculated by the following formula.

Plane orientation = (Nx + Ny) / 2−Nz Incidentally, a sodium D line is used as a light source at the time of measurement, and methyl salicylate is used as a mount solution.

(4) Intrinsic viscosity [η] is measured according to ASTM D1601, and is measured in 135 ° C tetralin. Unit is d
l / g.

(5) Melting temperature (Tm) Scanning calorimeter DSC-II type (manufactured by Perkin Elmer) with 5 mg of sample
Set the temperature to 20 ° C./min under a nitrogen stream at room temperature, and measure from room temperature to the melting endothermic peak temperature Tm (° C.). At this time, when a plurality of melting peaks are observed, the average value of those peak temperatures is Tm.

(6) A sample having a swelling degree of 100 mm square with an electric insulating oil is cut out, dried in a thermostatic layer at 120 ° C. for 20 hours, and immediately measured for its thickness to be D (μm).

This sample is crushed in dodecylbenzene oil at 100 ° C., left for 24 hours, taken out, and immediately thereafter, the thickness of the sample is measured, and this is designated as D ′ (μm). The degree of swelling is calculated by the following formula.

Swelling rate (%) = 100 x (D'-D) / D (7) Dimensional change rate A 100 mm x 100 mm sample was cut from the film and immersed in DDB oil at a temperature of 100 ° C for 24 hours, then immediately in the longitudinal direction. (MD) dimension LMD (mm), width direction (TD) dimension LTD (m
m) and measure the dimensional change rate using the following formula.

Dimensional change rate (%) = [(L-100) / 100] x 100 where L is LMD or LTD.

(8) Breakdown voltage (BDV) was measured according to ASTM D149 (20 ° C).

(9) The strength and elongation were measured according to JIS C-2330.

(10) Embossing Ratio The thickness D ₁ of the laminated film before embossing and the thickness D ₂ after embossing were measured and calculated using the following formula.

Embossing rate (%) = [(D ₂ −D ₁ ) / D ₁ ] × 100 [Example] The present invention will be described based on examples.

Example 1 As polypropylene (hereinafter abbreviated as PP), intrinsic viscosity [η] = 2.0 (dl / g), isotacticity II = 98
(%) Pellets, on the other hand, as a propylene copolymer, ethylene propylene block copolymer obtained by copolymerizing 20 mol% of ethylene ([η] = 1.6, melting peak temperature by DSC = 138 ° C, hereinafter abbreviated as bEPC. And)) were extruded from different extruders, laminated and integrated in a die, extruded in a sheet form, and cooled and solidified on a cooling drum at 45 ° C. by air pressure.

Next, after preheating the sheet in a hot air oven at 145 ° C, it is stretched 5 times in the longitudinal direction (hereinafter abbreviated as MD), and subsequently fed into a stenter type tenter, and at 160 ° C in the width direction (hereinafter abbreviated as TD). 10 times, and then heat treated for 2 seconds while allowing TD to relax 7% to obtain a biaxially oriented polypropylene film (hereinafter abbreviated as PP-BO) having a thickness of 20 μm.

The PP-BO thus obtained has a biaxially oriented PP layer (B).
/ b EPC layer (C), each thickness is 19μm, 1μm
And the birefringence of the B layer was 0.012. In addition, this film showed excellent friction coefficient of 0.6 because it used a specific bEPC, even though it did not contain a slip agent and an anti-blocking agent.

Next, the above PP and bEPC were extruded from different extruders and integrated in a die to obtain a three-layer laminated sheet consisting of bEPC / PP / bEPC. Subsequently, the sheet was preheated in a hot air oven at 145 ° C, stretched to MD 8.5 times, and then heat-treated on a Teflon coat roll at 140 ° C for 5 seconds while allowing MD to relax 5%. PP-UO
Abbreviated).

The thickness of the film thus obtained is 80 μm, and the constitution is a bEPC layer (C) / a uniaxially oriented PP layer (A) / bEPC layer (C). The respective thicknesses of the A layer are 78 μm and the C layer is 1μ each
It was m. The birefringence of the layer A was 0.030.

Next, the PP-BO was laminated on both sides of the PP-UO so that the C layers face each other, and a pyramid-shaped concavo-convex pattern with a length of 0.66 mm, a width of 0.66 mm, and a depth (height) of 0.1 mm was formed on a 1 cm square. The embossing was performed while laminating between a pair of embossing rolls consisting of 225 metal rolls and paper rolls at 130 ° C and a linear pressure of 30 kg / cm.

At this time, the main axes of the refractive indexes of PP-UO and PP-BO were laminated so as to be orthogonal to each other.

As a result, an embossed film having an apparent thickness of 150 μm and an embossing rate of 25%, in which the C layer was partially melted and integrated, and which had a structure of B / C / A / C / b was obtained.

The characteristics of the film thus obtained were as follows.

Swelling index: 2.1% dimensional change: (MD) 0.5% (TD ) 0.5% breakdown voltage: 92kv / mm ^{Strength: (MD) 24.8kg / mm 2} (TD) 15.6kg / mm 2 elongation: (MD) 46% (TD) 60% As described above, it is an oil-impregnated insulating material that has a small swelling rate and a high dielectric breakdown voltage, as well as a small directivity in mechanical properties (strength, elongation). .

Comparative Example 1 Using the raw material of the PP layer of PP-UO of Example 1, this time, a film was formed without laminating the propylene copolymer on the surface layer, and a thickness of 120
A PP-UO having a thickness of μm and a birefringence of 0.030 was obtained. The PP-UO alone was sent to the embossing device of Example 1 to give an apparent thickness of 15
An embossed film having 0 μm and an embossing rate of 25% was obtained.

The characteristics of the film thus obtained are summarized in Table 1 in comparison with Example 1. PP-UO, which has excellent swelling properties, occupies the entire film layer, so the swelling rate is small, but PP-UO
Since there is no BO layer, the dielectric breakdown voltage is reduced by embossing, and the mechanical properties are unbalanced between MD and TD, making it unsuitable as an electrical insulating material.

Example 2 As PP, [η] = 1.9 dl / g, II = 98% pellets, as propylene copolymer, [η] = 1.5 dl / g, melting point = 142
An ethylene-propylene copolymer (rEPC) at 0 ° C was prepared, and PP-BO composed of PP / rEPC was produced according to Example 1. The P
The total thickness of P-BO was 15 μm, the rEPC thickness was 1 μm, and the birefringence of the PP layer was 0.013. On the other hand, PP-UO also has the same P
Manufactured according to Example 1 using P and rEPC, consisting of PP / rEPC, total thickness 60 μm, rEPC thickness 1 μm, birefringence of PP layer
A film having the properties of 0.031 was obtained.

These PP-BO and PP-UO were overlapped so that the rEPC surfaces faced each other, and laminating embossing was performed in the same manner as in Example 1.
The film thus obtained has an apparent thickness of 98 μm,
The embossing rate was 30%. The embossing was performed so that the embossing roll was on the PP-BO side, and the main axes of the refractive indexes of PP-BO and PP-UO were laminated so as to be orthogonal to each other.

The various properties of the embossed film thus obtained are summarized in Table 1. The film has excellent swelling properties upon oil immersion and dielectric breakdown voltage, and has a balanced mechanical property. However, when compared with Example 1, the curl of the film after oil immersion was slightly larger.

Comparative Example 2 Regarding PP-BO and PP-UO of Example 2, the thickness constitution was PP-BO.
(PP: 10 μm / rEPC: 5 μm), PP-UO (PP: 50 μm / rEPC: 10
(μm) except that the characteristics were not changed. Lamination embossing was performed using these materials in the same manner as in Example 2 to obtain a film having an apparent thickness of 98 μm and an embossing rate of 30%.

The various properties of the embossed film thus obtained are summarized in Table 1. The swelling ratio is as large as 4.5% because the crystallinity is low and the ratio of the rEPC layer having poor swelling property is high.
Further, since the dielectric breakdown voltage is inferior, it is found to be unsuitable as an oil-immersed electrical insulating material.

Comparative Example 3 An embossed film was produced in the same manner as in Example 2, except that the embossing roll was pressed from the PP-UO side in the laminating embossing step.

The various characteristics of this embossed film are summarized in Table 1. The embossed shape is large on the PP-UO side and small on the PP-BO side, so the swelling rate is slightly larger than in Comparative Example 2. . Further, it can be seen that since the embossing roll is in direct contact with the PP-UO layer, fine cracks are easily generated in the PP-UO, and the dielectric breakdown voltage is lower than that in the second embodiment.

As is clear from the comparison between Example 2 and Comparative Example 3 above, PP
By embossing PP-UO via the -BO and propylene copolymer layers, an embossed film having an excellent dielectric breakdown voltage can be obtained.

Comparative Example 4 In the laminating embossing step of Example 2, PP-BO and PP-
An embossed film was produced in exactly the same manner except that the bonding direction with UO was set so that the principal axes of the respective refractive indexes were parallel.

The various characteristics of this film are summarized in Table 1. The swelling characteristics are not different from those in Example 2, but not only the MD and TD of the strength are unbalanced, but the dielectric breakdown voltage is inferior. I understand.

Example 3 As PP, pellets having [η] = 1.9 dl / g and II = 97%, and as a propylene copolymer, [η] = 1.7 dl / g and melting point = 145 ° C. rEPC were prepared. PP-UO, P according to 1.
Formed P-BO film. The structure of PP-UO obtained in this way is rEPC.
(2 μm) / PP (76 μm) / rEPC (2 μm), and the birefringence of PP was 0.028. The PP-BO configuration is rEPC.
(1 μm) / PP (14 μm), and the birefringence of PP was 0.013.

Next, make these PP-BO / PP-UO / PP so that the rEPCs face each other.
-BO is laminated and laminated in the same manner as in Example 1 for embossing. Apparent thickness 132 μm, embossing rate 20%
Got the embossed film. The evaluation results of this film are summarized in Table 1, and it is found that the swelling degree is as small as 2.0% and the dielectric breakdown voltage and mechanical properties are well balanced, which makes it suitable as an oil-immersed insulating material.

Comparative Example 5 In the laminating embossing step of Example 3, three films were laminated using a mirror-finished metal roll instead of the embossing roll. As a result, the obtained film is a flat film with no embossing (embossing rate = 0.
%).

When the characteristics of this film were examined, the emboss rate was 0%.
Therefore, although the dielectric breakdown voltage is high, the swelling ratio is 6
%, And the elongation is as small as 10% in MD, so it was found that when winding in a roll, there was an extremely large amount of winding disorder that was thought to be due to uneven thickness, and there was a practical problem.

Comparative Example 6 In Example 3, as the propylene copolymer, [η]
The embossed film was produced in the same manner except that rEPC having a melting point of 2.3 dl / g and a melting point of 145 ° C. was used.

Various properties of the film thus obtained are summarized in Table 1, and not only the swelling rate is increased but also the dielectric breakdown voltage is decreased as compared with Example 3. This is because the rηPC used as the heat-bonding layer has a large [η], and as a result, it easily swelled with insulating oil and the fluidity of the heat-bonding layer during bonding embossing decreased, resulting in embossing. This is because the stress of time becomes stronger on the PP-UO layer.

[Effects of the Invention] The effects of the present invention are listed below.

(1) Mechanical properties are balanced in both directions (MD, TD), and it is possible to prevent cracks and wrinkles due to bending and the like, which were easy to occur with uniaxially oriented PP embossed film.

(2) Since the stress during embossing is transmitted to the biaxially oriented PP layer through the biaxially oriented PP layer and the propylene copolymer layer that undergoes appropriate plastic deformation during embossing, it has been a problem in the past when finely embossed. The dielectric breakdown voltage can be kept high without generating cracks.

The film according to the present invention has swelling characteristics at the time of oil immersion, mechanical characteristics, and characteristics having a balance with respect to dielectric breakdown voltage. Preferred as an oil-filled transformer insulation material.

Claims (1)

[Claims] 1. A uniaxially oriented polypropylene film (A) having a birefringence of 0.025 to 0.033 and a biaxially oriented polypropylene film (B) having a birefringence of 0.010 to 0.020 have their respective principal axes of refractive indexes orthogonal to each other. Thus, the adhesive layer (C) is a layered body integrated through the adhesive layer (C), and the adhesive layer (C) is a layer having a thickness of 0.5 to 10 μm and made of a propylene copolymer having an intrinsic viscosity of 1.0 to 1.9 dl / g. And the thickness ratio of (A) to (B) is 1: 1 to 7: 1, and the emboss rate of the (B) side surface of the laminate is 10 to 50%. Polypropylene embossed film.