FI63885C - FOERFARANDE FOER FRAMSTAELLNING AV EN BIAXIELLT STRAECKT FOLIEMED GROV YTA FRAON POLYPROPYLEN ELLER EN SAMPOLYMER AV PR OPLEN OCH EN ANNAN ALFA-OLEFIN - Google Patents

Description

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Japan-Japan (JP) 51-111795 (71) Toray Industries, Inc., 2, Nihonbashi-Muromachi 2-chome, Chuo-ku,

Tokyo, Japan-Japan (JP) (72) Koji Iwasaki, Otsu, Shiga, Norishige Okada, Otsu, Shiga, Masashi Matsubayashi, Osu, Shiga, Katsumi Bokuda, Sonoyama, Shiga, Japan-Japan (JP) (7 ^) Oy Kolster Ab (5I +) Method for making a biaxially stretched rough-surfaced film from polypropylene or a copolymer of propylene and another β-olefin

The invention relates to a method for producing a rough-surfaced plastic film having very good electrical properties and improved slipperiness compared to prior art films.

Several methods are known for making rough-surfaced films. These include, for example, a method of making films in which large amounts of inorganic additives are added, a method in which a rough surface is obtained by rubbing with roughening particles or pressing, and a method in which the surface of a film is coated with roughening agents.

However, films made by these known methods are unsuitable for electrical insulation purposes because they contain excessive amounts of additives, or because it has proven difficult to produce a uniform, rough film by rubbing or squeezing a cooled and hardened film surface because the film surface is easily distorted and

2 63885

In particular, the former method, i.e. roughening by the addition of inorganic substances, renders the film unusable due to the deterioration of the electrical properties.

The invention seeks not only to overcome the above-mentioned shortcomings in electrical properties, but also to produce a rough-surfaced polypropylene film which, in addition, has excellent non-stick properties, i.e. good slip properties.

as a solution, a method has been developed for producing a biaxially stretched rough-surfaced film from polypropylene or a copolymer of propylene and another <* olefin, in which a melt of film material is extruded from a wide nozzle and allowed to solidify in a coil The method is characterized in that in the production of the pre-film, a cooling rate of 8080 ° C / s, wherein the time interval during which the melt surface temperature is between the crystallization temperature (s) and the temperature (tc to 20 ° C) of the film material, is adjusted to at least 1/3 second.

When a copolymer is used, it contains more than 50% propylene, the remainder being c (-olefin, especially ethylene, butene, etc.) Propylene may also contain small amounts of additives, e.g. antioxidants, pigments, plasticizers and organic glidants, etc.

In the process according to the invention, the crystallization of the melt surface is promoted by reducing the cooling rate and maintaining the temperature in the crystallization range when the polypropylene melt is cast on the cooling roll.

Crystallization is further promoted by using a relatively high temperature in the first stretch between the melting point (t) and (t -50 ° C) of the polymer crystals.

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After the second stretching, which is performed perpendicularly to the first, a biaxially stretched rough-surfaced film having good non-stick properties is obtained.

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The crystallization temperature (tc) of the polymer is determined here with a Differential Scanning Calorimeter (D.S.C.). The crystallization temperature of polypropylene is usually 110-120 ° C, but may change easily due to some additions, e.g. addition of other polymer or copolymerization.

The cooling rate is calculated from the following equation: dt U. Δ t de c .ol P 5 Here we mean: _ d0 = cooling rate (° C / 1Ό t = melt temperature {° Q,) Θ = time ζ hy U = total heat conductivity ^ kcal / n3 »h · ° C) t = melt temperature minus cooling temperature {° cj c = specific heat of the melt, (kcal / kg · ° C ^

= specific gravity of the melt ^ Tcg / m y 1 = thickness of the melt (jvJ

The cooling rate of polypropylene can be calculated as follows: Cooling rate _ 235 x melt temperature - cooling temperature (° C) (° C / s) melt thickness (^ μm)

Namely, the invention must satisfy both conditions that the cooling rate of the melt at the crystallization temperature (t) of the polymer is maintained at _-80 ° C / s and that the time interval during which the melt surface temperature is in the range of tc and (tc-20 ° C ) is at least 1/3 second. Without these conditions, there is a risk that the melt will stick to the cooling roll, or crystallization will not proceed as necessary to achieve the object of the invention.

The premix produced according to the invention is then stretched in a known temperature range, i. by a first stretch of the polymer film, preferably performed between the melting temperature (t) and (tm-35 ° C) of the polymer crystals, the polymer film is caused to crystallize and cloud, (t) is similarly measured with a differential thermal analyzer (DSC), and polypropylene it is usually 150-170 ° C. The first stretch is performed at a stretch ratio of 3.5-6.

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The second stretch (perpendicular to the first) is preferably performed in the temperature range between (t -25 ° C) and (t + 15 ° C) m m with a stretch ratio of 6.0-12.0. If necessary, a surface treatment, e.g. corona treatment, is then performed.

Of the stretched films prepared under the described conditions, a thin film, e.g. a film having a thickness of less than 2 μm (preferably less than 15 μm). The surface roughness of the film is then 0.3-2.0 μm.

The production of the film according to the invention is described in more detail by means of a drawing:

The figure illustrates a practical example based on the film manufacturing method according to the invention.

It can be seen from the figure how the melt 2 is extruded through the die 1, cooled by a cooling roller into a pre-film for subsequent stretching. The pre-film 4 is stretched longitudinally by means of rollers 5-7 rotating at a lower speed and rollers 8-9 rotating at a higher speed. A bilaterally stretched film is obtained when the film is stretched in a direction perpendicular to the first direction by a cross-stretching frame not shown in the figure. Reference numeral 10 denotes a longitudinal stretch heating device. The film is removed pituusvenytyslaittees-direction of the arrow and then passed poikkivenytykseen.

The following example illustrates the invention:

Example

Isotactic polypropylene (intrinsic viscosity in tetralin at 135 ° C = 2.20, t = 116 ° C, t = 160 ° C) is extruded as a melt at 280 ° C from a T-die. The polymer melt is cooled by a cooling roll using different surface temperatures and different residence times, and then stretched at different temperatures between 12 5 ° C and 13 5 ° C individually at a constant stretching ratio of 4.5.

A bidirectionally stretched film having a final thickness of 6 is then prepared by stretching at a stretch ratio of 8.5 and 150 ° C and thermosetting at 165 ° C.

The following table shows the results comparing the surface roughness, turbidity, non-stick properties, appearance, film productivity of the films produced, in which the latter evaluates the uniformity of the thickness and non-tearability of the films produced.

Table

Comparative films according to the invention _, _____ rn_ (2) _m_ (2) _O) Cooling rate + 85 85 80 40 40 (° C / s)

Dwell time ++ (s) 0.25 0.25 0.35 0.80 0.80 125 135 125 125 133 ° C (° C)

Turbidity (%) 0.20 0.50 1.00 2.10 3.50

Clamping force +++ (blocking 2500 700 500 300 250 shear force) (kg / 12 cm2)

Surface roughness. (/ and) 0.1 0.5 0.8 1.0 1.2

Appearance U NU U U U

Film quality

(film produc- SB G G G G

tivity) 6 63885

In the table mean: U = homogeneous NU = not homogeneous SB = less satisfactory G = good

Notes: +: Melting surface cooling rate at crystallization temperature ++: Time interval between film surface temperature between tc and (t -20 ° C), c +++: value measured after corona treatment,

The measurements were performed as follows:

Turbidity: ASTM (D 1003-52, D 1003-61)

Blocking shear force: The measurement was performed by compression. 2 by applying 30 x 40 mm pieces of film with a compressive force of 480 g / 12 cm at 40 ° C and 85% relative humidity for 24 hours and measuring with a traction device (Chopper).

Surface roughness: JIS B 0601

The results obtained show that the surface roughness of the film must be 0.3-2.0 μm, the turbidity more than 1.0% and the cooling conditions within the above-mentioned limits in order to obtain a surface with a uniform appearance.

The films made according to the invention are suitable for electrical insulation purposes, in particular for the manufacture of capacitors, because their surface is roughened without inorganic additives.