DE2100330C3 - Process for the production of biaxially oriented films from polyester - Google Patents

Description

The invention relates to a method of manufacture biaxially oriented films made of polyester, in which an oriented film increases at, depending on from the orientation temperature selected temperature by applying a heated Embossing surface is embossed under pressure.

It is already known, the sliding properties of one Improving the film by treating it with heated embossing rollers at an elevated temperature (French Patent 12 83 953).

It is also known to provide embossed surfaces to achieve opaque-free embossed thermoplastic films use, which have 3,000 to 200,000 hemispherical elevations (German patent specification 12 34 005).

It is also known to roughen film surfaces by treating the film with make a brush (British patent 10 06 116).

Another known method of providing a film surface with a roughening is to to add fillers to the polymer mixture, creating roughening on the surfaces of the finished film obtained; Furthermore, foils provided with roughening were passed through by extrusion molding of the foil made a funnel nozzle with an irregular lip.

The invention is based on the object of a method for producing a biaxially stretched one To create film from polyester, which for special applications, especially in the manufacture of Magnetic tape as a recording medium high winding speeds in the manufacture of more stable

ίο Rolls of tape are permitted for this task the known method for improving the sliding properties of a film as unsatisfactory or Proven economically unacceptable.

The above-mentioned object is achieved according to the invention in that

a) the film to a temperature range R, which is between the transition temperature of the second order of the film material and about 40 ° C above

ao is the transition temperature of the second order,

is heated,

b) the application of the foil mi. * the embossing surface with a surface roughening of about 0.76 to 12.7 microns averaged with a force per cm of at least about

9.0 kp takes place, the temperature of the embossing surface being adjusted so that a film surface temperature is reached which is at least in the temperature range R , and
c) by stretching the film in the transverse direction, the embossings are converted into transverse grooves which have a depth of at least 0.25 to approximately 2.03 microns.
With the method according to the invention, without the use of fillers in the film, not only random roughening of the film surface is obtained, but mechanically stable transverse grooves which allow air to escape quickly during the winding process. Trapped air has been a major cause of uneven winding in known films and has hitherto required relatively low film winding speeds, which limited production capacity. The embossing surface has essentially only a line contact with the foils, which is why the effective force is given as force per cm.

In a further advantageous embodiment of the method according to the invention, the film is direct cooled below the transition temperature of the second order after the embossing.

The cooling is expediently carried out by placing the film over a temperature of about 15 ° C held cooling roller is passed.

The inventive method is suitable for polyester films, for example those made of polyethylene-2,6-naphthalate, Polyethylene l, 5-naphthalate, polyethylene terephthalate and their copolymers, the contain up to about 5 mol percent of other ester-forming building blocks, such as diethylene glycol, other polymethylene glycols with 1 to 10 carbon atoms, hexahydrop-xyleneglycol, other aromatic dicarboxylic acids such as isophthalic acid, diphenic acid and p-terphenyl-4,4'-dicarboxylic acid, cycloaliphatic acids such as hexahydroterephthalic acid or small amounts of aliphatic Acids such as adipic acid or a hydroxy acid such as hydroxyacetic acid.

It has been found that temperatures which are more than 40 ° C above the transition temperature of the second order

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tion, often make the film so soft that it is on the surface when embossing rollers are used of the nip or sticks to the pneumatic rollers and that higher temperatures also lead to a noticeable crystallinity in the film, which the subsequent Leg stretching in the transverse direction; outlaws.

Temperatures below the transition temperature of the second order do not allow permanent deformation the film stretched in one direction.

It is preferred that the temperature of the film mass at the center of the embossing gap is greater than about 20 ⁰ C above the transition temperature of the second order, rur most Polyäthylenterephthalaifolien is a film temperature in the middle of the embossing gap of about 100 ° C besondcs suitable.

The film can be heated by any device suitable for the particular stretching device used suitable. It has been found that radiant heaters or infrared heaters in front of the holding rollers lying on both sides of the foil are effective means of bringing the foil to the desired embossing temperature range bring to.

The embossing roller is heated to a temperature range R which is between the transition temperature of the second order of the foil material and about 40 ° C. above the transition temperature of the second order in order to avoid a temperature drop in the pre-heated foil during embossing. The embossing roller can be heated above this temperature range in order to further increase the film surface temperature by momentarily touching the embossing roller and the film when the embossing surface is applied to the film. The surface temperature of the film should not rise by more than 40 ° C above the transition temperature of the second order. The increased temperature of the embossing roller leads to the fact that the film assumes a deformation in the plastic state and that the plastic deformation is maintained, due to the heat jacketing of the surface of the embossed film in order to prevent relaxation of the embossed pattern. The respective degree of preheating and the pre-roll temperature can vary as long as the temperature of the film mass in the middle of the embossing nip is within the temperature range R and as long as the surface temperature at the embossing nip is not more than 40 ° C above the transition temperature of the second order; For polyethylene terephthalate, the surface temperature is preferably kept at 110 to 115 ° C, because higher temperatures result in noticeable crystallinity after embossing. The film surface temperature can be calculated by conventional methods, taking into account the variable parameters, such as the specific polymer, the polymer temperature and the roller temperature, the film speed and the heat transfer rate

The embossing surface can be made of any material that is strong enough to hold its shape Maintain the required embossing temperatures and pressures. In general, rollers were used found to be most suitable with metal surfaces. To the desired degree of surface roughening to achieve in a biaxially oriented film under most process conditions, the embossing surface shows vertical deviations between 3.8 and 15.2 microns. The surface of the The embossing roller can be heated by blowing sand, glass balls, aluminum oxide stones or balls are blown surface roughness by one of about 6.8 to 12 microns in arithmetic Get funds. Other methods of roughening a surface are coating, etching, Application of electrical discharge and engraving. An embossing roller with a roughened surface from about 3.8 to 6.35 microns in the arithmetic mean already gives a film with an extraordinary good take-up performance with most film thicknesses. A blown one used for the embossing roller Steel surface, tile by blowing and chrome plating with a surface roughness of about 0.76 to 12.7 microns in the arithmetic mean was provided, leads to a significant winding improvement and is particularly preferred for embossing which two relatively smooth surfaces on the finished Foil are required. In general, the surface roughening on the embossing roller is less a comparable improvement with regard to the winding properties of films of lower thickness to lead.

In the embodiments of the invention Process in which the embossing surface is a surface of a pair of holding rollers that is used for stretching the film is used in the machine direction, the pressure applied to the film is sufficient to to apply a firm clamping effect to the film to achieve the desired degree of stretching to effect.

As mentioned, a pressure of at least about 9 kp / cm is applied along the nip and preferably about 18 kgf / cm; the pressure required in each case varies with the composition the polymer, the film thickness, the film speed and the degree of stretching. The second roller .._ of the retaining roller pair generally consists of a more resilient material than the embossing roller, for example made of silicone rubber. Important properties the second of the two holding rollers is hard enough to apply the embossing pressure and to press the pattern to be embossed into the surface of the film. It is preferably a Shore durometer hardness A from 50 to 70. The second roller is generally relatively smooth around a film surface Get relatively free of imperfections and roughness, ensuring the best possible performance is guaranteed when used as a magnetic tape. According to a preferred embodiment of the invention In the process, the second back hold roller is below the embossing temperature range cooled to facilitate rapid cooling of the film surface in contact with it, and to prevent the rear roller from damaging that surface. The cooling down the rear roller can be effected by connecting the rear roller with an independent roller, which is kept at a significantly lower temperature, is kept in contact.

Immediately after passing through the embossing nip, the film is preferably under the Second order transition temperature cooled, more preferably below 70 ° C, in order to relax the embossed pattern to diminish. The film can be cooled by any coolant that the is adapted to the respective stretching and the embossing device used. To carry out the

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For example, a cooled air stream is used, which is applied to the embossed film is directed immediately outside the embossing roller gap. The embossed film is expediently cooled down, by passing over a chill roll which is kept at a temperature well below the transition temperature second order of the polymer is passed. Chill roll temperatures of about 15 ° C is particularly effective in reducing the relaxation of the embossed pattern.

The film is then stretched in the transverse direction. The extent of such stretching determines the external surface properties; for example an increase in transverse stretching from 3.4 times to 4.0 times leads to a 25 percent increase Reducing the depth of the grooves in the finished Foil.

The biaxially oriented films obtained according to the invention have a surface which is relatively smooth and a second surface with a transverse groove pattern. The transverse grooves represent an enlargement of the embossed pattern, which were obtained by the transverse orientation of the film after the embossing. Since the films are generally oriented in the transverse direction by at least the! their original dimensions are stretched, the transverse grooves have a length-to-width ratio of at least 2: 1. However, post-stretching of the biaxially oriented film sets in in a similar manner the depth of the grooves down and also the length-to-width ratio of the transverse grooves, so that the ratio is less than 2: 1 but is greater than 1: 1. The transverse grooves should have a depth of at least about 0.25 microns and up to about 2.03 microns when embossing rollers with a roughened surface of about 12.7 microns on average can be used. Preferably the depth is Grooves less than about 1.27 microns. Foils with an embossing surface with a surface roughening of 3.8 microns in the arithmetic mean show transverse grooves with a depth of 0.38 to 0.89 microns on the embossed surface.

Typical transverse grooves have widths of about 100 to 250 microns, lengths of about 250 to 1250 microns and depths of about 0.25 to 0.38 microns.

An apparatus which can be used in the method according to the invention is schematic shown in the single figure with which a film 10 is stretched in the machine direction by this is first passed through the nip formed by the rollers 11 and 12. (The reels work at a low speed and 14, which lie on both sides of the film 10. The heated film 10 is then through passed a nip formed by rollers 15 and 16. The rollers 15 and 16 rotate with it a faster speed than the roller: 11 and 12, so that the film 10 is stretched. The roller surface 16 can be obtained by blasting it as described above, to allow embossing of a surface of the preheated foil. The roller 16 is at a temperature Heated from about 95 to 140 ° C to create a permanent set in the machine direction oriented To effect foil and at the same time to heat-cure the pattern embossed in the foil surface. the Roller 15 can have a silicone rubber layer and is preferably in contact with a cooling roll 17, which is at a temperature of about 15 ° C is maintained. The cooling roll 17 keeps the roll surface at a temperature which is lower is than the temperature of the roll surface 16. Typical surface temperatures of the roll 15 are at about 95 ° C when it is in contact with the cooling roller 17. An embossing pressure of about 18 kp / cm is applied applied to the nip along the heated film 10 at the nip opening.

A chill roll 18 adjoins roller 15, cools the embossed surface of the machine direction oriented film 10 and at the same time removes the embossed film from the surface of the holding roller. the Cooling roll 18 is generally on a temperature

ao temperature of about 15 ° C kept. The slide 10 is then oriented in the transverse direction.

The method according to the invention is further illustrated by the following example:

as example

A polyethylene terephthalate film is oriented by using it between two sets of holding rollers different speeds, in a device similar to that in the Figure is shown. The film is heated to a temperature of around 100 ° C by means of an infrared heater.

The holding roller 16 is a steel roller with aluminum oxide grains, which pass through a sieve with a mesh size of 0.161 mm, has been blasted, and has a roughened surface from about 1.0 to 1.27 microns in the arithmetic mean up to a maximum of 5.7 microns. the Holding roller is at a temperature of 120 ° C held.

The machine direction stretched film, about 76 microns thick, is fed through the Nip between rollers 15 and 16 at a pressure of about 18 kgf / cm and a speed passed through at about 20.8 m / min. The second holding roller 15 is on the surface with a silicone rubber provided and is in contact with a cooling roller 17 to the holding roller a To give a surface temperature of about 90 ° C.

After passing through the embossing nip, the film is from the roller 15 through the on a Temperature of 15 ° C maintained cooling roller 18 removed. The pattern that appears on the embossed surface of the is copied from a film stretched in the machine direction, has a roughened surface after cooling of about 1.4 microns. The slide is then reheated and oriented in the transverse direction, and a biaxially oriented film is obtained with a smooth surface and an embossed surface. The embossed surface is characterized by transverse grooves 0.25 to 0.38 microns deep, about 100 microns wide, and about 100 microns long of about 400 microns.

1 sheet of drawings

Claims (3)

Patent claims: 1. Process for the production of biaxially oriented films from polyester, in which one oriented Foil at an elevated temperature, selected depending on the orientation temperature is embossed by applying a heated embossing surface under Drack, thereby characterized in that embossing the film after it has been reinforced in the machine direction is made such that a) the film is heated to a temperature range R which is between the transition temperature of the second order of the film material and about 40 ° C above the transition temperature of the 7th order, b) the application of the foil with the embossing surface with a surface roughness of about 0.76 to 12.7 microns on the arithmetic mean with a force per cm of at least about 9.0 kp, the temperature of the embossing surface being set so that a Foil surface temperature is reached, which is at least in the temperature range R , and c) the embossments are converted into transverse grooves by stretching the film in the transverse direction which have a depth of at least 0.25 up to about 2.03 microns. 2. The method according to claim 1, characterized in that immediately after the embossing the film is cooled below the transition temperature of the second order. 3. The method according to claim 2, characterized in that the cooling is carried out by passing the film over a cooling roller kept at around 15 ° C.