The invention relates to a multi-layered biaxially-oriented film, mainly made of a crystallizeable thermoplastic polyester and economically produceable with a high surface quality, i.e. without pinning bubbles and without electric arcing and film breaks, at production speeds of 340 m/min or higher. According to the invention the multi-layered polyester film has an average specific electrical melt resistance over its total thickness which lies within a range from 1.5xc3x97107 to 30xc3x97107 xcexa9xc3x97cm. The invention also relates to a method of production of the film.

1. Field of the Invention
The invention relates to a multi-layered biaxially-oriented film, mainly made of a crystallizeable thermoplastic polyester. The film can be economically produced with a high surface quality and at speeds of more than 340 m/min. The invention also relates to a process for the production of this film.
During the production of multi-layered films with thicknesses lying within the range from 0.5 to 350 xcexcm plastic granulates are initially melted in extruders, the extruded melts are then led through a multi-layer slot die. The prefilm emerging from the slot die is then placed on a take-offxe2x80x94and quenching roll and, after leaving the take-off roll, longitudinally and transversely stretched in relation to the machine direction.
For the benefit of a stabilized placement of the prefilm on the take-off roll the state-of-the-art technology requires the application of additional forces to the prefilm in the area between the point at which the film exits the flat-film die and the point at which it is placed on the take-off roll. These additionally applied forces can be generated by blowing a stream of air out of an air knife at the prefilm, by spraying water into the nip of the prefilm prior to the point of application on the take-off roll, by producing a water film on the take-off roll, or by generating an electrical field by means of an electrode under high tension, with the field lines of the electrical field being directed towards the surface of the take-off roll. The additionally applied forces serve the purpose of removing the air which is enclosed between the surface of the take-off roll and the bottom side of the prefilm, i.e. that side of the prefilm facing the take-off roll, and which is underneath the nip of the prefilm, prior to placing the prefilm on the take-off roll, in order to obtain a stabilized placing of the prefilm on the surface of the roll by means of a given wrap angle. If this cannot be acomplished, enclosures of air of a more or less voluminous nature, so-called pinning bubbles, may result, which can be found between the bottom side of the prefilm and the surface of the take-off roll, adversely affecting the smoothness of the surface and, thus, the quality of the prefilm previously cooled down on the take-off roll, and subsequently the quality of the biaxially-stretched film manufactured from it.
2. Description of the Related Art
It is known that as an electrode a high-grade steel blade can be parallelly arranged to the surface of the take-off roll. For this purpose a 10 mm-wide portion of the steel blade is usually punched out or cut out from a metal blade which has a thickness of about 20 xcexcm.
The german utility model G 9402027 describes a device for the production of films, especially thin films or thinnest films by means of a rotating roll, with which a plastically formable film, previously fed into a slot die, is transported and stretched. Between the roll and a steel band parallely arranged to the roll at a short distance, electrical tension is applied by which the film is placed on the roll. The steel band in this known device has a sharp longitudinal edge which is arranged right next to the roll and pointed directly to it. This sharp edge is, similar to a cutting edge, arranged at the tapered area of the steel band, which is hereby arranged right next to the roll at a distance of less than 6 mm. The center line of this tapered area is directly pointed towards the center point of the roll. The use of such a steel band electrode helps to improve the quality of the film.
As the final speed during the film production is increased to a value of more than 340 m/min, an increased amount of pinning bubbles can be noticed. Increasing the electrical tension can counteract this development to a certain degree, but if the tensions applied are too high, electric arcing between the electrode and the quenching roll is the result. This leads to a severe defect within the film in the area affected by these arcings, namely an extremely thin area extending over the width of the film. As a result the film breaks within the machine, which is extremely unfavourable from an economical point of view. The procedural window for the production safety in the relation between pinning bubbles and electric arcing with film breaks gets smaller and smaller as the production speed is increased.
It was the object of this invention to produce a multi-layered, biaxially-oriented polyester film with a thickness lying within the range from 1 to 20 xcexcm at speeds of 340 m/min or more assuring a procedural window of a sufficient size for the production stability during the manufacturing process.
This object is achieved by the using a kind of film as previously mentioned, wherein the average specific electrical melt resistance over the total thickness of the multi-layered polyester film lies within the range from 1.5xc3x97107 to 30xc3x97107 xcexa9xc3x97cm, preferably from 2xc3x97107 to 10xc3x97107 xcexa9xc3x97cm, especially preferred from 3xc3x97107 to 6xc3x97107 xcexa9xc3x97cm.
The average specific melt resistance over the total thickness [xcfx81] (rho) is the arythmetical average of the specific electrical resistances of the individual layers based on the thicknesses of the individual layers.
For a three-layered ABC-film the following formula applies   ρ  =                    ρ        A            ·                        d          A                          d                      g                                      xe2x80x83                                    e                                      xe2x80x83                                    s                                +                  ρ        B            ·                        d          B                          d                      g                                      xe2x80x83                                    e                                      xe2x80x83                                    s                                +                  ρ        C            ·                        d          C                          d                      g                                      xe2x80x83                                    e                                      xe2x80x83                                    s
dA, dB, dC: thickness of the individual layers
dges=dA+dB+dC: total thickness
xcfx81A, xcfx81B, xcfx81C: specific electrical resistance of the individual layers
During the production of films with thicknesses of 1 to 20 xcexcm at final speeds of 340 m/min or more under electrical tension, a dramatically high amount of pinning bubbles is the result every time polyester raw material with an average specific electrical melt resistance over its thickness of more than 30xc3x97107 xcexa9xc3x97cm is used. Increasing the electrical tension slightly reduces, but does not eliminate the forming of pinning bubbles, since this in turn leads to an increased amount of electric arcing.
If, however, the average specific electrical melt resistance over the thickness is below 1.5xc3x97107 xcexa9xc3x97cm, the previously described problems do reoccur whenever production speeds are high.
It was completely surprising that biaxially-oriented polyester films produced with a thickness of 1 to 20 xcexcm at final speeds of 340 m/min or more could be manufactured without pinning bubbles and free of electric arcing if its average specific electrical melt resistance [xcfx81] over the total thickness of the film is within the range from 1.5xc3x97107 to 30xc3x97107 xcexa9xc3x97cm according to the invention.
The manufactured films are multi-layered. They can have a symmetrical or an unsymmetrical structure, wherein differently composed polyesters, respectively composed or non-composed polyesters, or polyesters of the same chemical compound but with a different molecular weight and a different viscosity are combined by way of coextrusion.
The film according to the invention mainly consists of crystallizeable polyethylene terephthalates, of crystallizeable polyethylene naphthalates (PEN), or mixtures thereof.
Crystallizeable polyethylene terephthalate or crystallizeable polyethylene naphthalate means
crystallizeable homopolymers
crystallizeable copolymers
crystallizeable compounds
crystallizeable recycled material
other variations of crystallizeable polyester.
Polyesters can either be produced according to the ester interchange process, e.g. catalyzed by ester interchange catalysts such as Znxe2x80x94, Mgxe2x80x94, Caxe2x80x94, Mnxe2x80x94, Lixe2x80x94, or Gexe2x80x94salts, or according to the direct ester process (PTA method), where antimone compounds are used as polycondensation catalysts and phosphorus compounds as stabilizers. The IV-value (intrinsic viskosity) of the polyesters preferably lies within the range from 0.5 to 1.0 dl/g.
Examples of polyesters are polycondensates made of terephthalic acid, isophthalic acid or 2,6-naphthalene dicarboxylic acid containing glycols with 2 to 10 carbon atoms such as polyethylene terephthalate, polybutylene terephthalate, poly-1,4-cyclohexylene-dimethyleneterephthalate, polyethylene-2,6-naphthalene dicarboxylate, polyethylene naphthalate/bibenzoate or polyethylene-p-hydroxy-benzoate.
The polyesters can be made of comonomer units of up to 50 mol %, especially of up to 30 mol %, whereby a variation of the glycol- and/or acid component is possible. Among others the copolyesters can contain as acid components 4,4xe2x80x2-bibenzoic acid, adipic acid, glutaric acid, succinic acid, sebacic acid, phthalic acid, isophthalic acid, 5-Na-sulfoisophthalicacid orpolyfunctional acids such as trimelitic acid.
The polyester films can be produced, according to known methods, of a polyester raw material, optionally containing additional raw materials and/or further additives customarily used for making carrier films for magnetic recording media at usual quantities of 0.1 to a maximum of 20% by weight, as multi-layered, optionally coextruded films with either equally or differently structured surfaces, wherein, for example, one surface can be pigmented, and the other surface contains less pigments or no pigments at all. In that manner one or both film surfaces can be provided with a customary functional coating in accordance with known methods.
The preferred extrusion method for the production of the polyester film comprises coextruding the melted polyester layers through a slot die and chilling it as a mainly amorphous prefilm on a quenching roll with a polished surface. This film is reheated thereafter and stretched in the machine direction (=MD) and in the transverse direction (=TD), respectively in TD and in MD, respectively in MD, in TD and again in MD and/or in TD. The stretching temperatures are generally within the range from Tg+10 K to Tg+60 K (Tg=glass transition temperature). The length stretch ratio is usually within the range from 2 to 6, especially from 3 to 4.5, the transverse stretch ratio is within the range from 2 to 5, especially from 3 to 4.5, and the ratio of the optionally performed second length stretching is within the range from 1.1 to 3. As an option the first length stretching can be simultaneously performed with the transverse stretching (simultaneous stretching). This is followed by the thermofixing of the film at oven temperatures lying within the range from 150 to 250xc2x0 C, especially from 170 to 240xc2x0 C.
The speed of the film upon arrival at the winding machine is 340 m/min or more.
The fact that the film can be produced at high speeds, without electric arcing and free of pinning bubbles makes the production very economical. Due to its high quality the film is suitable for highly sophisticated applications, for example as carrier film for magnetic recording media, high-quality wrapping films, capacitor films, films for applications in the metallizing field, to name just a few.
The measurement of the individual properties is performed in accordance with the following standards, respectively methods.
Specific Electrical Melt Resistance
The specific electrical melt resistance is determined according to the method by Brit. J. Appl. Phys. Volume 17, pages 1149-1154 (1966). The temperature of the melts (measuring temperature) is 285xc2x0 C. and the resistance is observed immediately after applying a tension of 100 V.
Average Thickness
The average thickness dF of a film is determined by its weight based on a given length, width and density. To be measured is the weight of a film strip prepared on the cutting table, taken from the middle of a sample which extends across the entire width of the roll. The value dF is then determined by the following formula:             d      F                  (              μ                          xe2x80x83                        m            )        =                    m                          [          g          ]                                      l                                [            mm            ]                          ·                  b                                [            mm            ]                          ·                  d                                [                          g                                            /                                                          cm                3                                      ]                                ·          10      6
with the following applying:
m=mass of the piece of film
I=length of the sample
b=width of the sample
d=density of the examined material
d=1.395 g/cm3 for polyester
After the individual sample strips have been cut, their weight is determined by an analytical scale, type Mettler PM 200 (maximum weight 200 g). A computer type HP Vectra ES/12 connected to the scale determins the average thickness, based on all the necessary parameters.
IV-value (DCE)
The standard viscosity SV (DCE) is measured, according to DIN 53726, in dichloro acetic acid.
The intrinsic viscosity (IV) is determined, based on the standard viscosity, as follows:
IV(DCE)=6.67xc3x9710xe2x88x924 SV(DCE)+0.118.
Pinning Bubbles
In order to examine the film for pinning bubbles a 10 cm-wide film strip is analyzed while being led over the entire width of the original roll past the monochromatic light of a sodium arc lamp (wave length 590 nm) in front of a black background. The pinning bubbles at the surface of the film are then transmitted with a 120-fold magnification to a screen via a video camera and assessed.
A film free of pinning bubbles has a homogenous unstructured surface. A film with pinning bubbles looks slightly dull and has a surface covered with an innumerable number of the smallest of bubbles (spots). The number of bubbles, respectively spots, isxe2x89xa75000 per 100 cm2 of film.

Multi-layered, biaxially-oriented polyester film with an optimized electrical conductivity over its total thickness


The object, in a device and a method for separating a shaped substrate from a stamping tool, is to make demolding more functionally reliable and to substantially prevent damage to expensive components. A closeable chamber, the mutually moveable chamber parts of which are the support for the stamping tool and for the shapeable substrate, contains a substrate holder which, when the chamber is closed, fixes. the substrate to its support outside the stamping area, with the result that the substrate is detached from the stamping tool when the chamber is opened. The device and method can be used for the production of microengineered components.

The invention relates to the separation of a shaped substrate from a stamping tool having a pair of chamber parts, which can move with respect to one another, of a closeable chamber, of which a first chamber part serves as a support for the stamping tool and a second chamber part serves as a support for the shapeable substrate.
A method for producing microengineered components is known under the name of LIGA process (LIGA is a German acronym for Lithography with Synchrotron Radiation, Electroplating, Molding using plastics). By pressing a molding tool into a moldable material, such as for example a layer of thermoplastic, three-dimensional patterns with heights in the range from a few nanometers to a few hundred micrometers are produced, preferably under vacuum and at a temperature which lies above the softening temperature of the moldable material.
A device in accordance with DE 196 48 844 C1 which is suitable for this purpose contains a chamber with one chamber part fixed to the frame and one moveable chamber part. Setting the pressure and temperature conditions inside the chamber is linked to predetermined values of a force acting on the fixed chamber part.
After the hot-molding process has finished, the moldable material which has been applied to a substrate support on the fixed chamber part adheres to the patterns of the molding tool due to large aspect ratios, shrinkage processes and surface roughness of the molding tool caused by the process, and has to be separated in a suitable manner.
A device in accordance with DE 195 09 452 A1, which is intended to keep flexural stresses at a low level during demolding and to avoid geometric limitations on the molding-tool surface available is used for this demolding.
A mold insert whose microstructure is transferred to a molded part is surrounded, in its edge area, by an ejector which, in order to demold the molded part, can be moved between an at-rest position and a working position. A space which can be filled with a pressurized fluid is provided on that side of the ejector which is remote from the molded part. In the working position of the ejector, the intention is that the fluid should act beneath the edges of the molded part, build up a pressure between the mold insert and the molded part which is to be demolded and, as a result, assist with demolding. Moreover, it is intended that the ejector be moved into its working position with the aid of the pressure from the fluid.
However, in the solution disclosed a method of operation of this nature, in which the space provided for the dual function of the fluid has to be designed as a pressure chamber and, in order to displace the ejector, has to act as a pneumatic cylinder, involves considerable unreliability. This is because the mold insert and the adjoining ejector, together with the layer of the molded part lying above it, are provided as wall elements of the space. The undefined adhesion of the molded part on the ejector may lead to unsealed areas and may have a considerable adverse effect on the function of the device.
Also, the control of the demolding process is undefined, both as a result of the force control applied and the termination by means of mechanical stops. The behavior of the shaped part remains largely uncontrollable.
A further drawback is the edge-side matching of the ejector to the mold insert, since such a direct interface with an expensive component unnecessarily increases the risk of damage or of some other adverse effect on the tool.
The primary object of the invention is to make demolding more operationally reliable and to largely prevent damage to expensive components.
According to the invention, the object is achieved by means of a device for separating a shaped substrate from a stamping tool having a pair of chamber parts, which can be moved with respect to one another, of a closeable chamber, of which a first chamber part serves as a support for the stamping tool and a second chamber part serves as a support for the shapeable substrate. The chamber contains a substrate holder which, when the chamber is closed, fixes the substrate to its support outside the stamping area, with the result that the substrate is detached from the stamping tool when the chamber is opened.
Furthermore, the substrate holder can be displaced parallel to the direction of movement of the chamber parts and, together with the support for the stamping tool, encloses a first pressure chamber. Advantageously, the first pressure chamber is machined into the support for the stamping tool as an annular groove which the substrate holder delimits.
The pressure chamber contains the substrate holder as a moveable part and operates, with a high level of reliability, as a pneumatic cylinder with which the substrate holder is moved towards the substrate by increasing the volume of the chamber.
It is also advantageous if the substrate holder surrounds a tool holder on the support for the stamping tool, which tool holder engages around the peripheral face of the stamping tool. This avoids a direct interface between the expensive stamping tool and a moveable part, thus considerably reducing the risk of damage.
Furthermore, the substrate holder, where it rests on the substrate, delimits, together with the tool holder, a second pressure chamber, into which at least one duct for compressed air is guided, through the tool holder, in order to assist with detaching the substrate from the stamping tool when the chamber is opened.
The support for the stamping tool contains feed ducts for compressed air, a first of which ducts is connected to the first pressure chamber and a second of which ducts is connected to the duct in the tool holder.
Moreover, the invention also relates to a method for separating a shaped substrate from a stamping tool inside a closed chamber which contains a pair of chamber parts which can move with respect to one another, of which a first chamber part serves as a support for the stamping tool and a second chamber part serves as a support for the shapeable substrate. The substrate is fixed to its support in an edge area which lies outside a stamping area and is detached from the stamping tool by opening the chamber.
It is advantageous if an initial detachment area which is formed in the fixed edge area when the chamber is opened is increased in size by means of compressed air and further opening of the chamber until the substrate is completely separated from the stamping tool.
In contrast to the known solution in accordance with DE 195 09 452 A1, in which the molded part is detached from the mold insert in functional terms only by means of force control by way of the ejector, the detachment of the substrate from the stamping tool is divided into various functional sequences which can be controlled differently. While the substrate is held to its support in the edge area, it is possible to separate it from the stamping tool by suitably regulating the opening of the chamber. The performance of the molded part remains largely controllable. The substrate is not stripped off or ejected, but rather is initially held in place and is then pulled off.
For the functional sequences which can be carried out by means of compressed air, separate pressure chambers are provided, in which stable pressure conditions prevail at any time. Advantageously, the detachment operation can at any time be reliably controlled in such a way that the moldable material is removed evenly, without spring-back effects and without tearing off patterns from the tool. The avoidance of manual rework, together with the possibility of automating the separate process steps which can be reproduced subsequent to molding, lead to an increase in productivity.
The invention is to be explained in more detail below with reference to the diagrammatic drawings.

Device and method for separating a shaped substrate from a stamping tool