DE3521331A1 - Process for producing a high-strength polymer film and device for carrying out the process - Google Patents

Abstract

The invention relates to a process for producing a high-strength polymer film. To this end it is proposed to draw off a polymer material continuously as a liquid film from a rotating first roller by means of a cooling roller arranged at a very close distance and having an elevated velocity and to molecularly orientate said polymer material in the thermoelastic temperature range. The polymer morphology thus forming, of needle crystals embedded in amorphous intermediate regions, is frozen-in on the cooling roller by cooling the polymer film. The time required for this must, however, be shorter than the relaxation time of the molecular chains back to the more thermodynamically stable state in which the crystals are present as folded crystals. The needle crystals result in an intrinsic reinforcement, combined with a hitherto unobtainable increase in strength, of the polymer film. In order to reduce the layer thickness of the pre-laid melt film, it is especially advantageous to feed a multi-layer film or to apply the latter to the melt roll from which the carrier layer is taken off separately and wound. The polymer material can in an advantageous manner also be applied as a polymer solution in low concentration, as a result of which, after homogeneous evaporation of the solvent, extremely thin film layers can be pre-laid. The device for carrying out the process is also described.

Description

Process for making a high strength polymer film

and apparatus for carrying out the method. The invention relates to refers to a process for making a high strength polymer film or ribbon, in particular made of polyethylene or polymer blends and on a device for Execution of the procedure.

Polymer films or foils are predominantly produced using the melt extrusion process, in particular produced by the blown film or flat film process. To increase Their strength (modulus of elasticity and tensile stress at break) are determined in a known manner Monoaxially or biaxially stretched warm and rearranged the molecular chains.

In addition, processes are also known in which the polymer in one Solvent is dissolved and through a pouring nozzle on an endless steel belt or a casting roll is deposited. The one formed after the solvent has evaporated The film is peeled off the base, dried and wound up. Such foils can also be stretched monoaxially or biaxially to increase their strength will.

In the known processes for the production of polymer films or -films are achieved strengths that are significantly lower than the theoretical possible of the special polymer.

From previous publications on high modulus fibers or films it can be seen that the theoretical strength of a partially crystalline polymer material is achieved when the slender crystals, in particular Needle crystals, the polymer are brought into a parallel position and thereby self-reinforcement of the fiber or film is generated.

In order to bring about such a morphology, it has to be thermodynamically Desired equilibrium when partially crystallizing high polymers cool from the melt, which consists of folded crystals and amorphous intermediate layers, be avoided. The formation of needle crystals can be in solutions or melts of a polymer take place under an extremely high longitudinal flow gradient, which must be greater than the reciprocal of the relaxation time of the polymer chains from stretched to the coiled state of the molecules.

It has already been proposed to produce high strength polymer films, by applying a thin layer of a solution of a polymer to a base, the solvent then evaporated and a polymer film thin layer thickness at speeds of a few m / min is removed from the base.

It is considered disadvantageous that this method of the so-called Gel stretching has so far only been carried out on a laboratory scale and, in particular, only discontinuously is executable.

The object of the invention is therefore to provide a method as described in the preamble of claim 1 specified type for the production of a high-strength polymer film, in particular of a self-reinforced polymer film to be carried out continuously can and leads to the desired products in an economically justifiable manner, in order to make these available in the quantities required by the market.

The object is achieved according to the invention with the features in Characteristic part of claim 1.

Thereafter, the polymer mass is continuously from as a liquid film a rotating first roller, which is partially wrapped around by a parallel one in addition, they are arranged with a small roller gap, in opposite directions and with a higher surface speed rotating cooling roller withdrawn, the temperature of which is below the crystallite melting temperature of the polymer mass. The ratio resulting from the speed difference of the Surface speeds of the first roller and the cooling roller on the one hand and the Thickness of the undrawn polymer film on the first roll, on the other hand, is formed is, according to the invention, is greater than the reciprocal of the relaxation time of the stretched to the coiled molecule of the polymer and is in particular greater than 103 s-1.

The relaxation time is the time that the crystals need in order to return to the thermodynamic position from the orderly position that arises in the flow state to reach the predetermined position in which the crystals are formed as folded crystals are. The relaxation time is less than 10 3s and is in particular 10-4 up to 10-5 s.

In other words, the invention is between the first heated roll (work roll) and the cooling roll (= take-off or take-off roll) high distortion due to the different surface speeds in the shortest possible time Way (according to the film thickness) and in direct connection to the Delays the freezing of the resulting structure below the crystallite melting point before folding crystals can form.

According to an advantageous development of the method, the first The roller is heated to the crystallite melting temperature of the polymer mass. The chill roll can be brought to a temperature below the crystallite melting temperature. the However, the temperature is preferably above the glass transition temperature of the polymer mass.

This has the advantage that during the removal of the polymer film needle crystals present in a parallel position due to the cooling roller until they are reached the freezing temperature can grow.

For the reason mentioned, the cooling roller can be controlled by a temperature control medium are flowed through.

The first heated roller can be controlled by known, controllable heating devices, such as direct and / or indirect heating devices, to the crystallite melting temperature of the polymer are heated. Resistance heaters, for example, can be used as heating devices. Induction heating, liquid or steam heating can be used. Also can Heating gas fan or radiant heater, if necessary, also open from the outside act on the surface of the first roller.

The application of the film to the first roller can be performed according to the invention several possibilities can be done, either using the homogenized polymer applied directly to the first roller from an extrusion die of an extruder, wherein as an extrusion tool in particular a flat nozzle tool with adjustable Gap width is used. Another option is to have one in one prior, separate operation produced polymer film from a supply spool is withdrawn, via a preheating section formed from a set of preheating rollers to feed and to the first roller (= melting roller) to create where it is melted at the crystallite melting temperature Tm of the polymer. the The thickness of a polymer film applied to the heated roller is preferable about 5 to 20 cm.

It is also possible to use a multilayer film instead of a monofilm to be applied via the preheating section to the heated first roller or with two Melting devices and a wide injection molding tool with a laminar adapter or the like to be applied to the first roller. The multilayer film consists of a carrier film, which does not soften at the melting temperature of the polyethylene or at the first Roller sticks, for example made of polyamide. The thickness of the coextruded polyethylene film can be set to about 5 to 10 # m.

When the polyethylene film is drawn off by the cooling roller, the polyethylene layer becomes because of the low affinity of the polymers released from the multilayer film and the Carrier film fed to a separate winder.

According to the procedure in which with a coextruded multilayer film particularly thin film layers can be applied.

It has proven to be advantageous for the implementation of the procedure, when the speed difference between the surface speed of the first Roller and the cooling roller is greater than 30 m / min, i.e. the feed speed of the polymer film on the first roller about 5 to 10 m / min and the take-off speed through the chill roll is about 40 m / min.

The process according to the invention can thus be used to produce high-strength polymer films or -bandchen in such a way that the longitudinal orientation of the polymer film or ribbon on a very short distance, i.e. as short as the thickness of the Melt film takes place, the freezing of the flow structure of the polymer, which this receives on the take-off roller, immediately after leaving the first heated one Roll is effected, especially when the cooling roll has a small diameter and the residence time on the chill roll is very short. This can be used in the end product produce a polymer morphology that is different from the usual, made of folded crystals and amorphous intermediate layers or single crystal wafers stacked one behind the other (hard elastic fibers) existing morphologies have a much higher breaking stress and has a much greater modulus of elasticity.

Another embodiment of the invention results from the identifier of claim 10. Thereafter, the polymer mass is applied as a solution to the surface of the first roller applied or sprayed and the solvent from this polymer solution evaporated homogeneously. The remaining polymer film is then peeled off the chill roll, those above the freezing temperature of the polymer, but below the crystallite melting temperature is heated and has a peripheral speed that is a multiple of the peripheral speed the first reel is.

As a result, a process can also be carried out continuously based on the same principle perform, in which the polymer is present dissolved in a solvent. One can thereby use the advantage that by using low concentrations of the polymer mass extremely thin layers of the solvent-free film in the applied solution of about 1 ß .mu.m and below can be generated, which is presented to and from the chill roll subtracted from.

To achieve homogeneous evaporation of the solvent is the first roller heated to a temperature that is above Room temperature and is below the boiling point of the solvent. Another heating of the first roller then takes place specifically from the outside, in the area between the application of the polymer solution and the lifting of the solvent-free film and with temperatures increasing in the direction of rotation of the roller. This is where the temperature is of the film when it is withdrawn from the first roller is well above the glass transition temperature and especially in the vicinity of the # crystallite melting point of the polymer, so that the processes involved in rearranging the crystal morphology during withdrawal and quenching of the polymer film correspond to the processes already described.

An apparatus for practicing the method is in the claims 15 to 20 indicated and described in more detail with reference to the drawing. She stands out characterized in that a rotatable, steplessly drivable and above ambient temperature heated first roller with a large roller diameter and very smooth working surface a delivery device at a small radial distance and one after the other in the circumferential direction for applying the polymer mass, a plurality of heating devices and a cooling roller are assigned, wherein the cooling roller can be driven at a higher peripheral speed is as the heated first roller. This first roller, which is in the further description also referred to as a work roll, are according to the process to be carried out still further auxiliary equipment assigned, for example a cleaning roller, a cooling device for local cooling of part of the circumference of the roll and suction devices for the solvent vapors and, if necessary, facilities for tempering and post-stretching of the film between drafting systems.

The invention is explained in more detail below with reference to the drawing.

1 shows a roller arrangement with a preheating section, first Roller and cooling roller in a schematic representation; 2 shows an enlarged section the nip between the first roll and the chill roll; Fig. 3 is a schematic Arrangement with multilayer film master; Fig. 4 shows an apparatus for continuous Processing of polymer solutions.

An apparatus for carrying out the method of manufacture of a high-strength polymer film according to that shown schematically in FIG Arrangement of a pair of pinch rollers 1 for delivering a film strip approximately 5 to 20 thick 2 from a rotatably mounted supply reel, not shown. The pair of pinch rollers 1 is unheated. From here, the film strip 2 runs into a preheating section 3, which is formed from one or more sets of wrapping rollers 4, all with are driven essentially constant, adjustable peripheral speed. The rollers 4 can be with the same or preferably with two different, be heated in the direction of the film strip 2 increasing temperatures, the but are chosen so that an adhesion or gluing of the film strip 2 on a the roll shells is avoided.

The film strip 2 is taken from the last roller of the preheating section 3 passed to the melting roller 5 (= first roller), which runs at the same peripheral speed v1 how the wrapping rollers 4 of the preheating section 3 is driven. Your temperature is, however, controllable in such a way that the respectively fed film strip 2 except for the Crystalline melting temperature Tm of the polymer at the transition from the thermoelastic to the thermoplastic state is heated. This state is intended to be achieved at a circumferential area of the melting roller 5 between 30 and 90 degrees in front of the withdrawal point 6 of the melted Polymer film 7 through the cooling roller 8 lies.

It should be noted that the heating means of the rollers 4, 5 for simplicity sake have been omitted and that all heating devices are applicable, the Heat up the rollers sufficiently quickly and precisely to the selected temperature and then on Maintain the set temperature level.

In addition to the melting roller, the cooling roller 8 is at a very close distance s arranged parallel axis. The distance can be adjusted very precisely. It should be mentioned that for this purpose the melting roller 5 and the cooling roller 8 with high precision in terms of Concentricity and surface quality machined and in accordance with the high requirements are exactly stored. The cooling roller 8 can be temperature controlled and in axially parallel channels a temperature control medium flows through it, as it is for godets in textile technology and heating / or cooling rollers are known in the plastics processing industry. The cooling roller is driven with adjustable peripheral speed v2, which is so it is chosen that the difference in surface velocities = = V2 - v1 divided through the thickness s of the polymer film on the fuser roll that specified in claim 1 Relationship fulfilled.

The one in the thermoelastic temperature range of the polymer in the take-off direction of the cooling roller 8 molecularly oriented film strips frozen in their morphology 2 is on the cooling roller 8, for example, to room temperature cooled down and from a delivery mechanism 9, which operates at a constant speed like the cooling roller 8 is driven, withdrawn and fed to a winding, not shown.

In Fig. 2, the nip between the melting roller 5 and the Cooling roller 8 shown enlarged. It is made clear that the melted Polymer film 7 is not drawn off tangentially from the melting roller, but on this adheres and is warped to a fraction of its initial thickness s in the nip the crystallization of the polymer in a longitudinal flow gradient takes place and a self-reinforcement of the polymer by growth and freezing stretched Causes molecular chains.

It should be noted that the function of the polymer shown in FIG on the fusing roller 5 in the form of a film strip 2 can also be solved thereby may that that plasticized in a melting device such as an extruder or the like or melted polymer by means of an extrusion tool, such as a die head or pouring nozzle to which the melting roller can be applied.

In one embodiment, a 10 jwm thick HDPE film was over a pair of clamping rollers 1 and the preheating section 3 of the melting roller 5 are supplied. These had a peripheral speed of v1 = 10 m min 1 and was at one temperature heated by 1300C. The cooling roller 8 was at a peripheral speed of v2 = 40 m min 1 driven and was heated so that on the surface of the Chill roll set a temperature of about 800C. Both reels had one another a narrowest nip in the order of magnitude of the thickness of the fed polymer film. The polymer film removed from the cooling roller 8 had a thickness of about 2.5 µm.

From the values given, the ratio of the speed difference between the rollers and the stretching path can be determined: Since this value is higher than the reciprocal of the relaxation time, the specified setting satisfied the theoretical requirement for the polymer morphology to be set.

In Fig. 3 a further embodiment is shown schematically, in which the polymer film applied to the fuser roll 5 is a coextruded multilayer film 10 is.

This consists of a carrier film 11, for example polyamide, and is combined with a very thin film without the use of an adhesion promoter 12 made of HDPE from a wide injection molding nozzle 13. The wide spray nozzle 13 is in a manner known per se from two screw extruders 14 and 15, their melt lines are connected to a common laminar adapter 16, with the provided Polymers fed. The extrusion of the multilayer film 10 takes place in such a way that the film side is made of the polymer material with the higher crystallite melting point, which is used as a carrier material, facing the circumferential surface of the melting roller 5 is and is cooled and solidified on this. The pressed with the carrier layer 11 HDPE film is not completely solidified here. This is supported by the radiant heater arranged at a distance on the circumference of the melting roller 5 17, the heating power of which is controlled in such a way that the HDPE film layer on the take-off or separation point 6 of the film layers 11, 12 in the manner described above their crystallite melting temperature is maintained and by the temperature-controlled cooling roller 8 is withdrawn at increased speed v2. The two separate film strips will added by separate delivery works 18 and 19 and suitable winding devices 20 or 21 supplied.

Finally, a cleaning roller is designated by 22, which is on the circumference the fuser roll 5 between the take-off point 6 of the two monofilms and the order of the multilayer film is arranged. Here the surface of the melting roller 5, which preferably consists of a ceramic material or is plasma-coated is cleaned of any polymer residues.

Fig. 4 shows a schematic representation of a device for implementation of the method according to the invention, in which the polymer mass is in the form of a polymer solution is applied to the work roll 25 and peeled off as a solvent-free film. The work roll 25 has in comparison to the delivery or application roll 26 for the Polymer solution or compared to the cooling or lifting roller 28 a large diameter. It corresponds to the melting roller of the versions described above and was introduced at the beginning also known as the first roller. The work roll 25 is rotatably mounted and with continuously adjustable speed in the direction of the rotary arrow 31 can be driven.

It consists of a basic roller body and an outer layer a material with poor thermal conductivity, for example ceramic or an equivalent plasma coating. By heating devices not shown it can be heated to a temperature To which is below the boiling point of the solvent lies.

The application roll is initially located on the circumference of the work roll 26 for the polymer solution, which is in a container filled with the polymer solution 29 30 revolves and this through circumferential contact as a thin liquid film on the work roll applies. Then follow at a close radial distance in the direction of rotation 31 of the work roll 25 has several heating devices 27 to make the solvent homogeneous to evaporate until a solvent-free film on the outer surface of the work roll 25 remains. The heating devices 27 are regulated in their heating power, and in such a way that the temperatures T1 to T5 in the direction of rotation of the work roll 25 increase, the solvent and the solvent must be completely evaporated free film with a thickness of a few in up to the crystallite melting point of the Polymer is heated.

The withdrawal by the temperature-controlled cooling roller 28, the temperature of which is preferably lies between the crystallite melting point and the glass transition temperature of the polymer according to the invention in such a way that the above-described morphology is established. Further cooling to room temperature takes place due to the very thin film thickness on the way to the winding station 35 in the ambient air.

The cooling roll 28 is followed in the direction of rotation 31 by the work roll 25 another cleaning roller 22, for example in a container with solvent runs around and removes polymer residues still adhering to the roller surface. By a Cooling device 32, which is arranged in the circumferential direction in front of the polymer feed is, the surface of the work roll is finally cooled down so far that the starting temperature To is reached again, which is below the boiling point of the Solvent lies.

It should also be mentioned that the last described with reference to FIG Device is connected to suction devices, not shown, by which the evaporated solvent is sucked off to it by condensation, absorption or the like to be recovered and fed back into the solvent cycle. Such devices are known, so a detailed description is not necessary appears necessary.

Finally, it should be mentioned that the device according to FIG A particular advantage for the achievable film strength is modified in that in connection with the take-off roller 28 and in front of the winder 35, a running path is provided is, the length of which is determined for example by a cooled pair of pinch rollers will. The pair of rollers is driven at an adjustable speed. Between the lifting roller 28 and the pair of pinch rollers are arranged several heat radiators 27, so that the film according to the described arrangement at a defined temperature and can be annealed during the dwell time in this running route and Let internal tensions be reduced. Another, faster driven pair of pinch rollers, that follows the first and is also arranged in front of the winding 35, can A post-stretching of the film can also be carried out, which improves its final strength leads.

REFERENCE SIGNS 1 pair of pinch rollers 2 film strips 3 preheating section 4 wrap roller 5 melt roller (= first roller) 6 take-off point 7 melted Polymer film 8 Cooling roller 9 Delivery mechanism 10 Multi-layer film 11 Carrier film 12 HDPE film layer 13 Broad injection nozzle 14 Screw extruder 15 Screw extruder 16 Laminar adapter 17 Radiant heater 18 Delivery unit 19 Delivery unit 20 Winder 21 Winder 22 cleaning roller 25 work roller (= first roller) 26 delivery roller, application roller 27 Heating device 28 Lifting roller, cooling roller 29 Polymer solution 30 Container 31 Direction of rotation the work roll 32 cooling device 35 winding - Blank page -

Claims (22)

Claims 1. A method for producing a high strength polymer film or ribbon made of thermoplastic polymer materials, in particular made of polyethylene or polymer mixtures, characterized in that the polymer mass is a liquid Film continuously from a rotating first roller through one parallel to it arranged with a small nip, in opposite directions and with a higher surface speed rotating chill roll, the temperature of which is below the crystallite melting temperature is subtracted so that the ratio of the speed difference A v the surface speeds of the first and second rollers to the film thickness s des undrawn film: LS av / s is greater than the reciprocal of the relaxation time from the stretched to the coiled state of the polymer molecule and especially larger is as 1Q3 s 1. 2. The method according to claim 1, characterized in that the first The roller is heated to at least the crystallite melting temperature of the polymer mass. 3. The method according to claim 1 and 2, characterized in that the The temperature of the chill roll is adjustable and preferably between the crystallite melting point and the glass transition temperature of the polymer mass. 4. The method according to claim 1 to 3, characterized in that the Film from the tool of an extruder applied directly to the first roller will. 5. The method according to claim 1 to 3, characterized in that a Film is unwound from a supply spool, passed through a preheating section and then is fed to the heated first roller. 6. The method according to any one of the preceding claims, characterized in, that the thickness of the presented film is 5 to 20 $ m. 7. The method according to any one of the preceding claims, characterized in, that the speed difference between the surface speed of the heated first roller and the cooling roller is more than 30 m / min. 8. The method according to any one of the preceding claims, characterized in, that the film presented is a multilayer film consisting of a carrier film, preferably made of polyamide, which is coextruded with a polyethylene film, the thickness of which 5 to 10 t m. 9. The method according to claim 8, characterized in that the layers the multilayer film is separated from the heated first roller as it runs off, wherein the carrier film is independent of the polyethylene film peeled off by the chill roll is wound up. 10. The method according to claim 1, characterized in that the polymer mass with a concentration of less than 10 and in particular less than 3% by weight is dissolved in a solvent that the solution on the surface of the first roller is applied or sprayed that the solvent from the polymer solution on the circumferential surface of the first roller is evaporated homogeneously and the film formed is withdrawn from the cooling roller, the cooling roller having a peripheral speed which is a multiple, in particular more than ten times, the peripheral speed the first reel is. 11. The method according to claim 10, characterized in that the first Roller is heated to a temperature that is above ambient and below the boiling point of the solvent is at atmospheric pressure. 12. The method according to claims 10 and 11, characterized in that that the peripheral surface of the first roller from the outside between the application of the polymer solution and the removal of the solvent-free polymer film is additionally heated in such a way that the temperature rises steadily in the direction of rotation. 13. The method according to claim 12, characterized in that the solvent-free Polymer film on the first roller heated above the freezing temperature of the polymer will. 14. The method according to claims 10 to 13, characterized in that that the solvent evaporated from the polymer solution is sucked off the first roller and is recovered. 15. The method according to any one of the preceding claims 10 to 14, characterized in that the film withdrawn from the chill roll is at a temperature Annealed above the chill roll temperature and then between before winding two delivery plants is subsequently stretched. 16. Apparatus for carrying out the method, in particular according to claim 10, characterized by a rotatably mounted, continuously drivable and over Ambient temperature heated first roll (work roll 25) with large roll diameter, with a small radial distance and in the circumferential direction one after the other at least a delivery roller (26), a plurality of heating devices (27) for generating temperatures increasing in the direction of rotation of the first roller, and a cooling roller (lifting roller 28), which can be driven at a higher peripheral speed than the first roller, assigned. 17. The device according to claim 16, characterized in that the first roll (work roll 25) has a surface made of ceramic material, in particular a ceramic or plasma coating with very low surface roughness and poor Has thermal conductivity. 18. Device according to claims 16 or 17, characterized in that that the delivery roller (26) in a container filled with a polymer solution (29) (30) rotates, is arranged parallel to the first roller (25) and with its The outer surface rests on the circumference of the first roller and the polymer solution on this applies. 19. The device according to claim 16, characterized in that the Heating devices (27) distributed over about half the circumference of the first roller (25), heat emitters or hot air nozzles that can be regulated in terms of their performance are directed radially towards the first roller. 20. Device according to one of the preceding claims 16 to 19, characterized in that viewed in the direction of rotation of the first roller (25) on the Cooling roller (lifting roller 28) a cleaning roller rotating in a liquid bath (22) follows. 21. Device according to one or more of claims 16 to 20, characterized in that seen in the direction of rotation (31) of the first roller (25) the delivery roller (26) a cooling device (32) is arranged through which the Surface temperature of the first roller (25) at least around 15 to 200C, in particular can be reduced below the boiling point of the solvent for the polymer composition. 22. Device according to one or more of claims 16 to 21, characterized in that between the lifting roller (28) and the winding (35) two or more pinch roller arrangements that can be driven with adjustable speed, are arranged in particular for post-stretching the film and that over the length the stretching path distributed heating devices, in particular heat radiators provided are.