EP1711331A1

EP1711331A1 - Method and device for longitudinal drawing of a film web

Info

Publication number: EP1711331A1
Application number: EP05706904A
Authority: EP
Inventors: Detlef Busch; Albrecht Freynhofer; Gerhard Koch; Michael Thull
Original assignee: Treofan Germany GmbH and Co KG
Current assignee: Treofan Germany GmbH and Co KG
Priority date: 2004-01-27
Filing date: 2005-01-17
Publication date: 2006-10-18
Also published as: AU2005205892A1; DE102004004084A1; JP4840817B2; CN1914025A; RU2352462C2; CA2553227C; AU2005205892B2; JP2007523767A; HK1104014A1; CN100515732C; US7596839B2; CA2553227A1; RU2006130782A; US20070157436A1; KR101227122B1; ZA200607079B; KR20060126746A; WO2005070653A1

Abstract

The invention relates to a device for drawing a film web, made from a thermoplastic plastic, comprising at least one driven roller (2), driven at a speed V1 and at least one second driven roller (3), driven at a speed V2, where V1<V2. The rollers (2, 3) are serially arranged such that a drawing gap (4) is embodied between the two rollers (2, 3). A width-maintaining device is arranged in the drawing gap (4), which mechanically grips both edges of the film web such that the width of the film web is essentially maintained during the longitudinal drawing in the drawing gap (4). The invention further relates to a method for the longitudinal drawing of films which are drawn by means of said device.

Description

Method and device for longitudinal stretching of a film web

The present invention relates to a method and a device for stretching a film web in the longitudinal direction. The process can be used for stretching biaxially oriented film, but also for stretching cast film or pre-film.

Biaxially oriented films are known in the prior art and are used in many different areas of application. In particular, biaxially oriented polypropylene films have been developed in recent years, which shrink more or less strongly in one direction or the other at elevated temperatures. The shrink properties depend on the composition of the individual layers and on the conditions during the production of the film. In particular, the temperatures during stretching, the stretching factors and the subsequent fixing are decisive. By varying these conditions, the shrink properties of a biaxially oriented film can be varied within a wide range.

For some applications it is particularly desirable that the films have high shrinkage in only one direction, while shrinkage in the other directions should be as low as possible at the same time. Applications are, for example, shrink films for all-round labels. For all-round labeling, for example, a flat film is first formed into a tube or a tube section, which is put over the container to be labeled. This tube is then shrunk at an elevated temperature. According to an alternative process, the film is processed directly from the roll. The film is first wrapped around the container or the molded body and glued. In the subsequent shrinking process, the film is fixed, the so-called roll-on shrink-on process ("ROSO"). A film with high shrinkage in the transverse direction is naturally required for the first process, and a high longitudinal shrinkage is required according to the ROSO process. So that the writing and decoration of these all-round labels are not distorted, the shrinkage in the opposite direction must be as zero as possible.

The production of such films from polypropylene presents considerable difficulties in practice. Following the usual manufacturing processes (flat film processes), the films are first extruded, cooled and then stretched biaxially. The longitudinal stretching by means of rolls running at different speeds is generally carried out first. The orientation in the transverse direction then takes place in a frame. This biaxial stretching ensures important usage properties such as mechanical strength, rigidity, transparency, a uniform thickness profile, etc. It has been shown that it is fundamentally possible to produce a film with high longitudinal shrinkage and low transverse shrinkage using this process, but extreme stretching and fixing conditions must be observed which are not readily compatible with the usual structural conditions, in particular the frame geometries , can be realized. As a rule, modifications to the systems are necessary. As a result, production changes are very time-consuming and ultimately make the product uneconomical or the quality is poor, in particular the longitudinal shrinkage is too low and the dimensional stability in the transverse direction is poor.

It is known in the prior art that the longitudinal shrinkage of a biaxially oriented film can be increased by carrying out a further stretching in the longitudinal direction after the biaxial stretching (longitudinal stretching). In the course of the studies on the present invention, however, it was found that this measure cannot simultaneously ensure that the transverse shrinkage of the film remains zero. It was found that the properties of the film in Transverse direction are significantly affected. At first it was found that the width of the film is considerably reduced in the longitudinal direction during the second stretching and a negative transverse shrinkage is produced, ie the laterally stretched film expands at elevated temperature. The reduction in width due to the longitudinal stretching is known and is also referred to as a protrusion in film technology. The subsequent subsequent stretching in the transverse direction, however, compensates for the disadvantageous effects of this entry. However, the "negative" cross-shrinkage (cross-stretch), which is caused by the longitudinal stretching, is just as unacceptable as a too high cross-shrinkage, since this ultimately results in similar distortions of the printed image when the all-round label is shrunk on.

The object of the present invention was therefore to provide a method according to which an oriented polypropylene film can be produced which has a high longitudinal shrinkage at elevated temperature and at the same time does not change its dimensions in the transverse direction when this temperature acts. The process should be simple, economically efficient and can be used for various film materials, in particular for biaxially stretched polypropylene films. Furthermore, the process should also be very flexibly suitable for other starting materials. It was therefore also a task to specify a method for the longitudinal stretching of cast film or pre-film.

This object is achieved by a method for the longitudinal stretching of an at least single-layer film (1) made of thermoplastic material, which is warmed to a temperature suitable for stretching in the slow-running part of the drafting system and fed to a drafting zone, the slow-running part of the drafting system contains the driven roller (2) and the fast-running part of the drafting system contains the driven roller (3) and wherein the pair of rollers (2) / (3) is arranged such that a Drawing gap (4) is formed between these two rollers (2) / (3) and the film (1) is guided into the drawing gap (4), characterized in that the film (1) during the drawing in the area of the drawing gap (4 ) between the rollers (2) / (3) in the two edge areas (10) is mechanically gripped and fixed by a fixing device in such a way that the width of the film which it has when it enters the stretching gap (4) is at Stretching not changed significantly.

Figure 1 shows the device for holding out in a schematic cross-sectional view. The film (1) is stretched between a slow-running roller (2) and a high-speed roller (3) (stretching rollers). The area between the run-up point (17 ') of the film on the roll (2) and the run-off point (17 ") from the roll (3) forms the draw gap (4). In the draw gap (4) are the two carriages (5a) and (5b) are arranged above and below the film (1). The carriages (5) each comprise a stamp or pressure cylinder (7a) and (7b), as well as the fixing rollers (6a) and (6b). 7) the slides (5) are pressed in the direction of the film surface, in this way the film (1) is clamped or fixed between the fixing rollers (6a) and (6b). The rollers (6) touch the film surface depending on the design in one or more contact points (8).

Figure 2 shows a schematic plan view of the film web (1) with the fixing devices in both edge regions (10) of the film (1). (9) is the running direction of the film web (1). The rolls (6a) of the slides (5a) lying above the film (1) are shown. It is shown that the rollers (6a) are arranged in the edge region (10) of the film (1). Of course, this also applies to the rollers (6b), which are not shown because they are underneath.

FIG. 3 is a schematic top view of a film web (1) with a double-row arrangement of the fixing rollers. As can be seen, the rollers are arranged in two rows (19a) and (19b) lying next to one another, the position NEN of the fixing rollers (6) of each row are offset from one another.

Figure 4 shows schematically how the distance (11) between the contact points (8) shortens when the diameter of the fixing rollers is reduced. The fixing rollers (6) are in the contact points (8); (8 ¹ ); (8 ") in contact with the film (1). Due to the diameter of the rollers (6) there is a spatial distance (11) between the contact points (8). Starting from a given diameter d of the rollers (6) (see Fig. 4a) this distance decreases from (11a) to (11b) (see Fig. 4b) when the diameter d of the rollers (6) is reduced.

Figure 5 shows the consequences of tolerances in the diameter of the fixing rollers (6). A schematic illustration shows how, when the rollers (6) are rigidly fixed to the slide (5), not all rollers (6) are in contact with the film (1) if the rollers (6) are not all exactly the same Diameter.

Figure 6 / 6a shows a preferred embodiment with non-rigid, i.e. flexibly mounted rollers (6), whereby the problem according to FIG. 5 is solved. It is shown how rollers (6) with an uneven diameter are attached to the carriage (5) in a resilient or sliding manner. Rollers with a larger diameter press in the spring (13). In this embodiment, all the rollers (6) are in contact with the film (1), so that there are one or more contact points (8) for each fixing roller (6).

Figure 7 shows an arrangement with slide (5) and flexibly mounted rollers (6) and stretching rollers (2) and (3). It is shown that between the contact points (8 ') of the first fixing rollers (6'a) and (6'b) and the run-up points 17' there is a free area (14 ') in which the film (1) is not fixed , The same applies between the last fixing rollers (6 "a) and (6" b) and the drain point (17 "). Figure 7a shows an arrangement similar to Figure 7, wherein in the free area (14 '); 7, an additional slide rail (15) is arranged. This slide rail (15) is shaped in such a way that the film (1) runs in the area (14) over this slide rail (15) Guide rail (15) tapering towards the stretching roller (2) / (3) and towards the first or last fixing roller The areas (14) which remain free are significantly shorter compared to Figure (7).

Figure 8 shows an arrangement similar to Figure 7a. Here, above the slide rail (15), additional fixing rollers (18) are arranged on the slide (5a) above the film web (1), which press the film (1) against the slide rail (15) and thus immediately after leaving the opening or Also fix drain point (17). The slide rails are an optional feature and can also be omitted depending on the nature of the film.

9a and 9b show fixing rollers with a profiled surface in plan view and in cross section. In the illustrated embodiment, the fixing rollers are each covered with 4 O-rings (19), through which a profile with uniform elevations (20) and depressions (21) is created. The figures show how the profiled rolls are positioned above and below the film web so that the elevations and depressions interlock (22). Fig. 10 shows how the profiling in conjunction with the offset positioning of the rollers surrounds the profiling in a wavy manner.

The device according to the invention consists in principle of two parts which are arranged in both edge regions (10) opposite the film (1), ie above and below the film (1), also referred to below as a carriage (5). Each carriage (5a) and (5b) has a plurality of rollers (6a) and (6b) which are arranged in succession in a row (FIG. 1) or in a double row (FIG. 3). The length of the row corresponds approximately to the length of the stretching gap (4). The distance between the rollers (6) within the row or within the double row should be as small as possible. The diameter of the rollers (6) is chosen in relation to the length of the stretching gap (4) so that a plurality of rollers (6) can be arranged in the stretching gap (4), for example at least 3, preferably 5 to 20, rollers (6) per side and slides arranged, the number of rollers in double-row versions doubled accordingly. The rollers (6) are arranged evenly over the entire length of the stretching gap (4). The embodiment with a double-row arrangement of the rollers (6) (FIG. 3) is preferred. As a result of this arrangement, the distances between the contact points (8) are halved compared to a single-row arrangement, and the contact area, ie the number of contact points (8) overall, is increased or almost doubled. The two rows should be positioned as closely as possible to one another so that the width of the edge (10) remains as narrow as possible.

The two carriages (5a) and (5b) are positioned in the stretching gap (4) above and below the film (1) so that the rows of rollers (6) are aligned parallel to the running direction (9) of the film and in contact with the edge area (10) of the film, so that the rollers (6) rotate in the direction of travel (9) of the film as it passes through the film (1). Deviations of up to + 10 ° from the parallel alignment of the rolls are possible without the effect of the fixation being adversely affected, ie the rolls can also be somewhat inclined to the film web. The contact pressure of the two carriages (5a) and (5b) can be regulated by means of appropriate devices, such as a pressure stamp (7), on the two carriages (5a) and (5b), so that the pairs of rollers consist of rollers arranged above and below ( 6a) and (6b), as it were, clamp the film (1), ie fix it in such a way that no or only a small increase in width is possible when stretching in the longitudinal direction. In general, the film width in the longitudinal stretching according to the invention is reduced by a maximum of 20% compared to the original width before the longitudinal stretching, preferably 0-15%, in particular 2-12%. The large number of pairs of rollers (6a) and (6b), in particular in the case of double rows, means that a corresponding number of contact points (8) or fixing points in which the film (1) is held between the upper and lower rollers (6a) and (6b). The smaller the rollers (6), the more contact points (8) ensure the most uniform possible fixation over the length of the stretching gap (4) during the stretching. It goes without saying that there are limits here for constructional reasons of reducing the size of the rollers (6).

The film web (1) is fixed over the length of the stretching gap (4) in its edge region (10) by the roller pairs (6a) and (6b) of the upper and lower carriages (5a) and (5b). The rollers (6) of the roller pairs (6a) and (6b) are dimensioned such that their diameter is generally smaller than their width b _g , so that the term "roller" characterizes the element more accurately than the term "roller". However, it is also possible that appropriately dimensioned rollers can serve an equivalent purpose. Above all, such "rollers" are used to stretch wider film webs in order to achieve better lateral fixation. Depending on the design and width of the fixing rolls, the contact between the roll and the film edge results in a contact area across the width of the film roll of the invention, the “contact points”, which are mentioned several times in this description, also include approximately linear contact areas which arise over the width of the roll, or.

The person skilled in the art will select the size of the element, ie the roll diameter and the roll width, depending on the thickness and width of the film web, the stretching speed and stretching factors and the width of the edge area. The diameter of the rollers (6) depends in particular on the dimensions of the drafting system, in particular on the size of the drafting gap (4). In relation to the length of the stretching gap (4), the person skilled in the art will select the diameters as small as possible in order to maximize the number of contact points (8). The width of the roll depends on the thickness and the width of the film web (1) and the stretching force to be applied for longitudinal stretching. The higher the stretching force, the larger the contact area should be to ensure a good fixation. Of course, the roll width will also depend on whether you are realizing single-row or double-row designs. Double-row designs naturally have narrower rollers. The width of the film itself depends on the type of material and the specified machine dimensions and can therefore vary within a wide range. For example, the film web (1) will not be wider than 5 m, preferably 0.2 to 3 m. The extension gap (4) should be as short as possible.

So that a uniform detection of the film edge (10) over the entire length of the stretching gap (4) is ensured, it is necessary that the rollers (6) from the beginning to the end of the stretching gap (4) are arranged and that all rollers (6) are the same Have diameter. Limits are of course imposed by the usual manufacturing tolerances. In general, the diameter of the rollers within a carriage (5a) and (5b) should not differ from one another by more than 10%. This applies both to the roll itself and to the overall roll diameter, including any applied coverings.

The rollers (6) can in principle be made of any material or composite material that meets the requirements. The surface should be designed such that a positive connection between the film web (1) and the roller surfaces is promoted. A relative movement (slip) between the material web and the roller surfaces should be avoided. The surface of the rollers (6) is preferably provided with a rubber coating (or rubber rings see FIGS. 9 and 10), which ensures the necessary static friction to avoid slippage. For example, the surface can also be covered with a special metal, such as copper. The combination of different types of rolls (metal on rubber) is also conceivable, depending on the film web. In a further embodiment, the surface or the roll shape is designed in such a way that a frictional connection between roll-film-roll is further improved. Rollers with a profiled surface have proven successful for this, ie each roll surface has depressions and elevations which are evenly distributed over the width of the roll. At the same time, these profiled rolls are positioned above and below the film web in such a way that the elevation of the rolls below is opposite the recesses of the rolls above (or vice versa) and interlock when the rolls are pressed on. As a result, the edge of the film wraps around this profile in a wave-like manner and a larger contact area is generated with increased contact pressure. This creates a groove-like pattern in the edge area of the film. Since this border area is hemmed later, this "deformation" is not disadvantageous.

Such embodiments are, for example, fixing rollers which are covered with a number of O-rings. A similar effect can also be achieved by engraving thick roller coverings made of metal or rubber. Alternatively, knob-like coverings can be applied to the roll surface. Rubber or plastic surfaces are also preferred for such profiled roller surfaces.

The rollers (6) generally have no independent drive. They are driven via the running film web (1) and then have a peripheral speed v _R , which is established by the web speed of the film web (1) in connection with the stretching.

It was found in the course of the investigations of the present invention that a uniform gripping of the film edges (10) by the roller pairs (6a) and (6b), optionally profiled and offset, is particularly advantageous over the entire length of the stretching gap (4) , Ideally, both film edges would have to be continuous over the length of the stretching gap. tes (4) are held with constant force to prevent the unwanted entry and its disruptive effects. The continuous detection as possible is taken into account by a large number of very small-sized rollers (6), as a result of which many contact points (8) which are close to one another are generated (FIG. 3). The smaller the rollers (6), the shorter the "free" areas (11) between the individual contact points (8) (Fig. 4). However, it has been shown that many very small roles (6) create another problem. The film (1) may not always be fixed with the same force in all contact points or contact areas (8), ie a uniform pressure of all rollers (6) is not always given. Basically, the entire carriage (5a) / (5b) with all the rollers (6a) and (6b) is pressed downwards or upwards towards the film (1) via a pressure cylinder (7), so that in principle all of the rollers (6) are included the same force on the foils or pressed against each other. However, it was found that due to manufacturing tolerances, the roller diameter and fluctuations in the thickness of the roller coverings applied, for example the rubber coating, not all the pressure rollers press against one another with the same force. The consequence of this is that the film (1) is not positively grasped in all areas or the contact pressure in the individual contact point (8) is not the same for each pair of rollers (6a) and (6b) (Fig. 5). This disadvantage could be eliminated in a preferred embodiment according to FIG. 6. In this embodiment (FIG. 6), the rollers (6) are not fixed rigidly, but flexibly, for example via a cylindrical sliding bolt (12), on the slide (5) and preferably at the same time mounted against a resilient pressure piece. In the simplest case, this resilient pressure piece can be realized by means of grub screws with an underlying spring. The flexible or resilient mounting (13) of the rollers (6) enables the entire slide (5) to be pressed onto the edge of the film to such an extent that all the rollers (6) are in non-positive contact with the edge of the film (10). The excessively large diameter of individual rollers (6) can now be compensated for by resiliently pressing the rollers in the direction of the carriage (5), so that as a result all rollers (6) apply the same force to the fo Line edge (10) are pressed (Fig.6a).

In a further particularly advantageous embodiment according to FIGS. 7a and 8, the design of the slide (5) is modified in such a way that the uniform, continuous detection of the film (1) over the entire length of the stretching gap (4) is ensured even better. As already explained above, many small rollers (6) over the length of the stretching gap (4) ensure that the areas (11) between the contact points (8) remain as small as possible. It has been shown that, despite this measure, comparatively large "free" areas (14) cannot be avoided when entering and leaving the film (1) into or out of the stretching gap (4). This problem arises from the different diameters of the fixing roller pairs (6a) and (6b) and the stretching rollers (2) / (3). The film (1) is guided over a slow-running roller (2) when it runs into the drawing gap (4) and only hits the first fixing pair of rollers (6'a) and (6'b) of the two after a distance (14 ') Carriage (5a) and (5b). This distance cannot be avoided from a construction point of view, since even when the first pair of rollers (6'a) and (6'b) approaches the stretching roller (2) at maximum, due to the different diameters of the fixing rollers (6'a) and (6'b) and stretching roller (2) remains a gap (14 '). (Fig. 7) This applies analogously when it runs out of the drawing gap (4). The film (1) leaves the last pair of fixing rollers (6 "a) and (6" b) and remains free until it hits (17 ") the fast-rotating stretching roller (3). These gaps (14) cause that in the and a reduction in the width of the run-out area can only be insufficiently prevented This disadvantage can be eliminated by a further preferred embodiment of the slides (5a) and (5b).

In this preferred embodiment of the slide (5) according to FIG. 7a, the first and the last pairs of rollers (6'a) / (6 "a) and (6'b) / (6" b) are separated by one in the entry and exit area Added slide rail (15) over which the film (1) is guided. This slide rail (15) is on at least one, preferably on both sides or ends (16) of the respective shape of the fixing rollers and / or the stretching rollers are adjusted so that the slide rail (15) tapers towards the respective contact point (8) and / or run-up point (17) towards one or both sides. The film (1) thus leaves the roller (2) at point (17) and hits the surface of the slide rail (15) immediately after it is no longer in contact with the slow stretching roller (2). The film (1) runs over this slide rail (15) until it is gripped by the first fixing roller pair (6'a) and (6'b). This shortens the area (14 ') that was originally left free to a few millimeters (14'") according to FIG. 7a. The area (14 '") now results from the distance between the one tapered end (16) of the slide rail (15) and the outlet point (17) of the stretching roller (2). The same applies analogously to the end of the stretching zone. Here the slide rail bridges the distance between the contact point (8) of the last fixing roller pair (6 "a) / (6" b) and the run-up point (17 ") of the fast stretching roller (3).

The surface of the slide rail (15) is coated by a suitable covering, which on the one hand has sufficient adhesive force with respect to the film material, but at the same time allows the film (1) to slide over this surface. For example, polished stainless steel or plastic coverings are used. So that the fixing of the film width in the area of the slide rail (15) is particularly effective, the opposite slide can have additional fixing rollers (18) in this area, so that the film (1) between the slide rail (15) and the additional rollers (18 ) is fixed, which largely prevents a width reduction in this area (Fig. 8). In the same way as described above, these rollers (18), which press against the slide rail (15), are preferably resiliently mounted in order to build up a controlled and uniform contact pressure.

The film web (1) is by means of the device described above during the longitudinal stretching in its edge region (10) of the, optionally profiled and offset, pairs of rollers (6a) and (6b) of the upper and lower carriages (5a) and (5b ), possibly in conjunction with the slide rail (15) and the additional rollers (18) are fixed so that the width of the film (1) essentially does not change during the stretching. For this purpose, the two carriages (5a) and (5b) are pressed against the surface of the film from above and from below, the contact pressure being able to be controlled via corresponding cylinders (7). As a result, the film edge (10) between the pairs of rollers (6a) and (6b), or between the slide rail (15) and rollers (18), is clamped, so that the usual increase in width is effectively prevented. At the same time, the freely rotating rollers (6) of the two pairs of slides (5b) ensure unhindered transport of the film web (1) in the running direction (9). The fixed edge area (10) is usually narrow in relation to the total width of the web. The exact width of such an edge area (10) will depend on the type of material and the total width of the web. An edge region (10) is generally understood to mean the outer regions of the web, which together can make up up to 20% of the total width, ie each edge region (10) makes up 1 to 10% of the overall width of the film web (1). It goes without saying that each film web (1) has two edges (10) which run parallel to the running direction (9). All information about "the edge area (10)" in this description naturally also apply in the same way to the opposite edge.

In the sense of the present invention, the longitudinal direction is the direction in which the film web (1) runs; this direction is also called the machine direction. For the purposes of the present invention, the transverse direction is that direction which is at an angle of 90 °, i.e. runs perpendicular to the machine direction.

The process according to the invention is surprisingly outstandingly suitable for the longitudinal stretching of biaxially oriented films, in particular polypropylene films, which were previously produced in a separate production process, ie the process for the longitudinal stretching according to the invention is preferably operated offline. Similarly, the device device for stretching a cast film can be used after its manufacture. Cast films are essentially undrawn, ie after extrusion they are removed on a roll without additional measures being taken to stretch the film. In another possible variant, the spreading device can also be used "in-line" for longitudinal stretching. In this case, the longitudinal stretching process according to the invention is combined with the production process of the films, so that the production and the stretching according to the invention are combined without a width increase into a single continuous process in which the process according to the invention forms a process step. The implementation of the method according to the invention is explained in more detail below.

For longitudinal stretching, the film web (1) is optionally unwound and fed to the longitudinal stretching unit according to the invention with at least two driven stretching rollers (2) / (3) and fixing device. The film (1) is stretched by the longitudinal stretching ratio f between the rotating rollers (2) / (3) and at the same time fixed in width by the device described above. The speeds of the stretching rollers (2) / (3) are v1 and v2. On the one hand, these speeds determine the speed at which the film (1) runs through the process and, on the other hand, the stretching tension which is introduced in the longitudinal direction of the film web (1). The orientation of the cast film or pre-film or the additional orientation of the already biaxially oriented film (1) in the longitudinal direction (longitudinal stretching) takes place via this stretching tension.

The longitudinal stretching ratio f results in the case of a force fit between the stretching rollers (2) / (3) and the film (1) approximately from the ratio of the speeds v2 and v1 of the rollers (3) and (2). The longitudinal stretching ratio f depends, among other things, on the starting material used. In the longitudinal stretching of biaxially oriented films, the longitudinal stretch factor is> 1 to less than 5, preferably 1, 2 to 3. In the case of cast films or Before stretching, the stretching factor varies from 2 to 7, preferably 3 to 5. Before stretching, the film is heated by means of heated rollers or by an air heater. When the stretching zone (14) is reached, the film (1) has reached an elevated temperature TS, at which it can be stretched with the respective stretching ratio. Depending on the raw material, film thickness, stretching speed and stretching ratio, the temperature TS during the stretching is 5 - 40 ° C below the melting point of the respective polymer, preferably between 80 and 160 ° C. The stretching force FS is transmitted to the film (1) by force closure (static friction) between the driven roller (2) and the film surface and the driven stretching rolls (3). The power transmission is particularly good if the fixing rollers have, for example, a rubber coating with a rubber hardness of about 50 to 100 Shore hardness A.

If necessary, the drafting system contains additional nip rollers, which serve to further improve the frictional connection between film (1) and stretching roller (2) / (3). The nip roller can be additionally arranged on the first slower and / or on the second high-speed roller and can be driven or not driven itself. The film (1) is gripped by the stretching roller (2) / (3) and the nip roller on both surfaces, so that it lies flat on the surface of the stretching roller (2) / (3) and good adhesion is ensured

The web speed v _{F of} the film web (1) in the method according to the invention is determined by the desired processing speed. Depending on the type of material, usual web speeds are between 1 to 1500 m / min, preferably 5 to 1000 m / min. For films made of thermoplastic polymers, speeds of 10 to 500 m / min are common.

After the longitudinal stretching according to the invention, the film is hemmed and wound up in the usual way. In general, the film is around the width of the respective border area. In individual cases, the longitudinal stretching can be combined with further processing steps, for example a surface treatment using corona or flame or plasma. The inventive method for longitudinal stretching is basically suitable for all foils made of plastics, in particular of thermoplastic plastics. Films made of thermoplastic materials are, for example, films made of polyester and polyolefins, such as polyethylenes, polypropylenes, cycloolefins, polycarbonate, polyamides, etc. Films of this type can be constructed in one or more layers. The method is particularly suitable for biaxially stretched film webs made of the above-mentioned materials with a thickness of 5 to 100 μm, preferably 2 to 80 μm. In particular, biaxially stretched polypropylene films with a thickness of 20 to 100 μm or undrawn polypropylene cast films with a thickness> 50 μm, in particular 80 to 200 μm, are preferred. The polypropylene films generally contain propylene homopolymers or propylene copolymers, such as propylene-ethylene copolymer or ethylene-propylene-butylene terpolymer or a mixture of these propylene polymers in the layer or layers. The proportion of comonomers in the copolymers is 1 to 10% by weight. These propylene polymers are usually used in boPP and PP cast films.

In general, biaxially stretched films have been stretched longitudinally by a factor of 3 to 7 and have been stretched crosswise by a factor of 5 to 10. In principle, both single-layer and multi-layer embodiments can be subjected to the longitudinal stretching. Biaxially stretched polypropylene films as such, processes for their production and their composition are known to the person skilled in the art. For cast films or pre-films which are stretched by the process according to the invention, the thickness is generally 20 to 500 μm, preferably 30 to 200 μm. Of course, these thickness specifications relate to the thickness of the films before they are stretched lengthwise by the process. The process according to the invention can be an integral part of the production process (in-line stretching in the longitudinal direction) or preferably in a separate working step after the production and winding up of the film

Basically, the biaxially stretched starting material can be produced according to the usual known film production processes. A flat film process with sequential longitudinal / transverse stretching is generally preferred. In principle, the film starting material can also be produced using the double-double process. Such methods are known in the prior art and are described in detail in numerous patents and the specialist literature. In the same way, processes for the production of a guest film are part of the specialist knowledge.

The invention opens up a simple way to fix the film in the longitudinal direction during stretching. This prevents a spread in width and the shrinkage properties in the transverse direction can be controlled excellently. The method according to the invention makes it possible to produce a film with very high longitudinal shrinkage and at the same time to obtain excellent dimensional stability in the transverse direction. This means that the products manufactured using the process can be used excellently as all-round labels for wrapping cylindrical vessels, molded articles or everyday objects.

Example 1 :

A biaxially stretched polypropylene film was produced by the stenter process. The film was built up in one layer from polypropylene and had a thickness of 75 μm. The film was stretched longitudinally by a factor of 5 and transversely by a factor of 9 during its manufacture. After stretching, the film was fixed and then wound up. The film produced in this way showed a shrinkage of less than 3% in at 130 ° C./5 minutes in a forced air oven both directions.

This film was stretched longitudinally according to the method of claim 1. The film width before the longitudinal stretching was approximately 490 mm. The longitudinal stretching was carried out at a temperature of approximately 100 to 120 ° C. and with a stretching factor of 2.1. The film was fixed by pressing the carriages (5a) and (5b) in both edge areas during the stretching.

In this way, a film web with an unseamed width of approximately 400 mm was obtained. The film showed shrinkage values at 130 ° C./5min of approx. 30% in the longitudinal direction and a negative transverse shrinkage (elongation) of 1.5%.

Example 2:

A biaxially stretched polypropylene film was produced by the stenter process. The film was made of three layers of polypropylene and had a thickness of 61μm. The base layer consisted essentially of propylene homopolymer. The cover layers on both sides, each about 1 μm thick, consisted essentially of propylene-ethylene copolymers with an ethylene content of about 4.5% by weight. The film was stretched longitudinally by a factor of 5 and transversely by a factor of 9 during its manufacture. After stretching, the film was fixed and then wound up. The film produced in this way showed a shrinkage of approx. 3% in both directions at 130 ° C./5 minutes in a forced air oven.

This film was stretched longitudinally according to the method of claim 1. The film width before the longitudinal stretching was approximately 580 mm. The longitudinal stretching was carried out at a temperature of approx. 90 to 110 ° C. and with a stretching factor of 1.3. The film was fixed by pressing the carriages (5a) and (5b) in both edge areas during the stretching. In this example, the two chutes had profiled fixing rollers, which were each covered with 4 O-rings (FIGS. 9a and 9b). The sledges were under and positioned above each other in such a way that the depressions of the upper carriage were opposite to the ridges of the lower carriage, so that the film had a groove-like pattern in the edge region after the longitudinal stretching.

In this way, a film web with a thickness of 48 / m and an unseamed width of approx. 575 mm was obtained. The film showed shrinkage values at 130 ° C / 5min of approx. 24.5% in the longitudinal direction and a negative transverse shrinkage (elongation) of 0.4%.

Example 3:

An undrawn three-layer polypropylene cast film was produced by extrusion from a slot die using the customary cast method. The film was three layers and had a thickness of approximately 150 μm. The cover layers were each approximately 30 μm thick and essentially consisted of a propylene-ethylene copolymer with an ethylene content of approximately 2% by weight. The base layer consisted of a mixture of 45% by weight propylene homopolymer and 55% by weight propylene copolymer of the top layer. The film was not stretched when it was manufactured. The film produced in this way showed a shrinkage of about 0.5% in both directions at 130 ° C./5 minutes in a forced air oven.

This film was stretched longitudinally according to the method of claim 1. The film width before the longitudinal stretching was approximately 580 mm. The longitudinal stretching was carried out at a temperature of approximately 118 ° C. and with a stretch factor of 3.2. The film was fixed by pressing the carriages (5a) and (5b) in both edge areas during the stretching. In this example, the two slides had profiled fixing rollers, which were each covered with 4 O-rings (FIGS. 9a and 9b). The slides were positioned to each other below and above the film web so that the depressions of the upper slide were opposite each other to the ridges of the lower slide, so that after the longitudinal stretching the film entered the edge area had a groove-like pattern.

In this way, a film web with a thickness of 47 μm and an unlined width of approximately 570 mm was obtained. The film showed shrinkage values at 130 ° C / 5min of approx. 27% in the longitudinal direction and a negative transverse shrinkage (elongation) of 0.2%.

Film Shrinkage:

The longitudinal and transverse shrinkage values relate to the respective length of the film before the shrinking process Lo and Qo. The test specimen is shrunk in a forced air oven at a temperature of 130 ° C. for 5 minutes. The remaining lengths and widths of the test specimen are then determined L1 and Q1. The determined change in length in relation to the original length of the test specimen in the longitudinal and transverse directions is given as shrinkage:

Longitudinal shrinkage: Ls = (Lo - L-ι) / Lo

Cross shrinkage: Qs = (Qo - Qι) / Qo

Claims

claims

1. A process for the longitudinal stretching of an at least single-layer film made of thermoplastic, which is warmed to a temperature suitable for stretching in the slow-running part of the drafting system and fed to a stretching zone (10), the slow-running part of the drafting system being fed with at least one driven roller (2) and the fast-running part of the drafting system contains at least one driven roller (3) and wherein the pair of rollers (2) / (3) is arranged such that a drafting gap (4) between these two rollers (2) / (3) is formed and the film (9) is guided into the drawing gap (4), characterized in that the film during the drawing in the area of the drawing gap between the rollers (2) / (3) in the two edge areas by means of a fixing device - Direction is mechanically gripped and fixed in such a way that the width of the film, which it has when it runs into the stretching gap (4), is not essential when stretching i changed

2. Device for stretching a sheet of film made of thermoplastic comprising at least one driven roller (2) which is driven at speed V1 and at least one second driven roller (3) which is driven at speed V2, where V1 <V2 and the rollers (2) / (3) are arranged one behind the other in such a way that a drawing gap (4) is formed between the two rollers (2) / (3), characterized in that between the two rollers (2) / (3 ) a spreader device is arranged which mechanically grips both edge areas of the film web such that the width of the film web essentially does not change during the longitudinal stretching in the stretching gap (4).

3. Apparatus according to claim 2, characterized in that the wide Hold device from two pairs of slides (5a) and (5b), that is, a total of four slides, with a pair of slides (5a) and (5b) positioned on each film edge and each of the four slides has several rollers (6a) and (6b) , which are arranged one after the other in a row, one slide above each and one slide opposite each other is arranged below the film web on each film edge, and the respective slides (5a) and (5b) arranged above and below a film edge are positioned relative to one another, that the rollers (6a) and (6b) are aligned in the running direction (9) of the film and the opposing pairs of rollers (6a) and (6b) grasp / touch the film web lying in between in the edge region (10).

4. Apparatus according to claim 2 or 3, characterized in that the length of the row of rollers corresponds approximately to the length of the drafting gap (4), so that the rollers (6) of the carriage (5) are arranged over the length of the drafting gap.

5. Device according to one of claims 2 to 4, characterized in that each carriage (5) has a double-row arrangement of rollers (6), the two rows of the double row being arranged offset to one another, so that the distance between the contact points (8) is halved with the film compared to an identical single-row arrangement.

6. Device according to one of claims 2 to 5, characterized in that the rollers (6) are freely rotatable and not driven.

7. Device according to one of claims 2 to 5, characterized in that the rollers (6) are provided on their surface with a rubber or metal coating.

8. Device according to one of claims 2 to 7, characterized in that the carriage (5a) and (5b) can be moved by means of a printing cylinder away from the film web or towards the film web, and that the contact pressure lying above and below the film web Roller pairs (6a) and (6b) can be regulated via the pressure cylinder.

9. Device according to one of claims 2 to 8, characterized in that the rollers (6) are movably connected to the carriage (5).

10. The device according to claim 9, characterized in that the rollers (6) via a cylindrical sliding pin (12) with the carriage (5) are connected.

11. The device according to claim 10, characterized in that the rollers (6) are mounted against a resilient pressure piece.

12. The device according to one of claims 2 to 11, characterized in that in each case one slide of each pair of slides (5a) and (5b) additionally has a slide rail (15), so that a pair of slides has a slide with slide rail (15) and a slide without slide rail (15), which are positioned opposite one another above and below the film web.

13. The apparatus according to claim 12, characterized in that the slide rail (15) is arranged in the region between the first roller in the stretching zone and the slow roller (2) and a second slide rail (15) between the last roller in the stretching zone and the high-speed roller (3) is arranged.

14. The apparatus according to claim 12 or 13, characterized in that the slide rails have a tapering end towards the rollers (6) and towards the roller (2) and / or (3).

15. The apparatus according to claim 13 or 14, characterized in that the slide without slide rail (15) have additional roller (18) which are opposite the slide rail (15).

16. The device according to one of claims 3 to 15, characterized in that the rollers (6) have a profiled surface.

17. The apparatus according to claim 16, characterized in that the profiled rollers are coated with O-rings made of metal or rubber.

18. The apparatus according to claim 16, characterized in that the surface of the rollers are profiled by engraving.

19. The apparatus according to claim 18, characterized in that the rollers have a profiled rubber covering.

20. A method for the longitudinal stretching of a film web, by means of a device according to claim 1, characterized in that the film is first passed over the slow-running roller (2), then passes through the drawing nip (4) and then guided over the high-speed roller (3) , characterized in that during the stretching in the stretching gap (4) both edges of the film are fixed between the rollers (6) of the two pairs of slides.

21. The method according to claim 20, characterized in that the film is stretched by means of a device according to one of claims 3 to 19.

22. The method according to claim 20 or 21, characterized in that a biaxially stretched film is stretched in the longitudinal direction.

23. The method according to any one of claims 20 to 22, characterized in that the biaxially stretched film in its manufacture along with a

Factor in the range of 3 to 6 and across with a factor in the range of 5 to 12 was hidden.

24. The method according to any one of claims 20 to 23, characterized in that the film is elongated by a factor of> 1 to 5.

25. The method according to any one of claims 20 to 24, characterized in that the film is a biaxially stretched polypropylene film which has a thickness of 20 to 100 microns.

26. The method according to claim 20 or 21, characterized in that a guest film is stretched in the longitudinal direction.

27. The method according to claim 20 or 21, characterized in that a pre-film is stretched in the longitudinal direction.