Process for the production of polymer films partially provided with stiffened segments

Abstract: The invention relates to films of polymers of high molecular weight, which have been obtained by casting polymers dissolved or swollen in solvents and partially contain stiffened segments, which films serve either for decorative, handling or fixing purposes or allow, especially when formed on the film edges, a particularly ultra­ high stretching of the films without excessive formation of lips.

Description: The invention relates to a process for the production of films that partially contain stiffened segments by dissolving or swelling a polymer of high molecular weight in a solvent, shaping the polymer solution or the swollen polymer to give a film, cooling the resulting film to a temperature below the gelling temperature, at least partially removing the solvent and, if appropriate, prestretching in the longitudinal and/or transverse direction. Thin films can be produced from polymers of high molecular weights, in particular ultrahigh molecular weights, by the dissolution process or swelling process, in such a way that the polymers are dissolved or swollen in a solvent, and the solution or swollen material is shaped to give a film which is then cooled to a temperature below the gelling temperature of the polymer, whereupon the film is stretched simultaneously or sequentially in the longitudinal and transverse directions after at least partial removal of the solvent. Especially when ultrahigh stretching ratios, that is to say stretching ratios of more than 5 in each direction in the case of simultaneous stretching, are maintained, the problem arises that the gelled films obtained after cooling are more extensively stretched in the areas, at which they are gripped by the clamps of the stretching device, than in the areas not gripped by the clamps, which entails, with increasing stretching ratio, the formation of increasingly thinning lips which can tear when higher stretching ratios are reached. Furthermore, the film segments adjoining these lips have a greater anisotropic orientation than the segments which adjoin the lateral film segments not gripped by the clamps. Accordingly, the properties are anisotropic (unequal) at the film edges, so that the edge segments of films, especially those stretched at ultrahigh stretching ratios, must be cut off to a considerable width, which causes waste quantities of 20 to 50%. Moreover, there is often a demand for the production of stretched thin films which, for decorative or handling reasons or for fixing purposes, must have stiffened segments. It is therefore the object of the invention to provide a process for producing films from polymers of high molecular weight, by casting polymers, dissolved or swollen in solvents and yielding films which, on the one hand, can be stretched without the abovementioned disadvantages for the production of thin films. This object is achieved, according to the invention, in that the film, in the segments which are to be stiffened, is heated in the solvent-free state to a temperature which is between 10°C below the crystalline melting point and the decomposition temperature, and, if the film still contains solvent, it is heated to a temperature which is above the dissolution temperature and below the decomposition temperature. Preferably, the stiffened segments are located at the edges of the films. With this embodiment, it has been found surprisingly that, when the films are stretched to obtain thin films, lips no longer appear, or only to a slight extent, since these stiffened segments permit a more uniform distribution of the tension by the pulling clamps engaging at the edges for stretching. Therefore, when the films produced according to the invention with stiffenings at the edge segments are used for the production of thin films by stretching at especially ultrahigh stretching ratios, there is, on the one hand, no longer a risk of tearing in the areas gripped by the pulling clamps and, on the other hand, the high waste, caused by cutting-off the edge segments of the finished films which have been stretched in the known manner, is considerably reduced. A further advantage is that, when a certain stretching ratio of the clamps is maintained in the case of the polymer films according to the invention which are stiffened particularly at the edges, a higher degree of stretching of the latter as compared with non-stiffened films is possible, so that better utilization of the stretching device and hence more economical working are possible. In addition to the edge segments, the films which are to be stretched can also have stiffenings in other areas, which stiffenings serve for decorative purposes, facilitate the handling of the finished stretched thin films or serve as stiffened segments for fixing the films by screwing, for fitting eyelets and the like. The films produced according to the invention can be in a form prestretched in the longitudinal and/or transverse direction, before they are provided with the stiffened segments. Accordingly, the films can, after cooling to a temperature below the gelling temperature, be prestretched to a certain extent in any desired manner in the longitudinal and/or transverse direction, since the problem of lip formation at the edges does not arise in this case. These prestretched films can then be provided with stiffened segments at the edges and/or in other areas, depending on the intended use. The process according to the invention is characterized in that the film, in the segments which are to be stiffened, is heated in the solvent-free state to a temperature which is between 10°C below the crystalline melting point and the decomposition temperature, and, if the film still contains solvent, it is heated to a temperature which is above the dissolution temperature and below the decomposition temperature. Preferably, the heating is carried out for a period which is sufficient for re-entangling of the polymer molecules, in accordance with the equation: Another variant for making the films according to the invention comprises producing the stiffened segments by lamination with a polymer film on one side or on both sides, especially with a film of a polyolefin of low density, such as low-density polyethylene, such as LDPE and LLDPE. The films produced according to the invention are used especially for the production of thin films by uniaxial and/or biaxial stretching, in particular ultrahigh stretching. The polymers of high molecular weight, used for producing the films according to the invention, consists, for example, of polyolefins such as polyethylene and polypropylene, polysulfones, polyimides, polycarbonates and halogen-containing polymers such as polytetrafluoroethylene, polyvinylidene fluoride, polyvinylalcohols, polyacrylonitrils, and the like. All polymers can be used which can be dissolved or swollen by appropriate solvents and can be shaped in the form of solutions or swollen polymers to give a film, for example by means of a shaping die. The polymers are dissolved in an expedient manner in an extruder, especially in a twin-screw extruder, at an elevated temperature, it being possible for static mixers to be provided downstream of the extruders in order to accelerate the dissolution under the action of shear. Especially, polyolefins of high molecular weights, that is to say molecular weights of more than 1 x 10⁴ g/mol, in particular of ultrahigh molecular weights from 4 x 10⁵ to 6 x 10⁶ g/mol and higher (weight average molecular weight), can be used according to the invention. Especially, linear polypropylenes and in a very particularly preferred manner linear polyethylenes are used here. The use of copolymers is also possible, and, for example, linear polyethylenes can be used which contain minor quantities, preferably at most 5 mol%, of one or more other alkenes copolymerizable therewith, such as propylene, butylene, pentene, hexene, 4-methylpentene, octene and the like. Preferably, the polyethylene contains 1 to 10, especially 2 to 6, methyl or ethyl groups per 1000 carbon atoms. The polyolefins can also contain minor quantities or one or more other polymers, especially alkene-1 polymers. The polymers used can contain known additives such as fillers, stabilizers against degradation by ultraviolet radiation or ozone, flame-proofing agents, crystallizing agents such as calcium stearate, and the like, or all these additives can be added to the solution or swollen material. The invention is explained in more detail below with reference to polyethylenes of ultrahigh molecular weight as particularly preferred polymers for the production of the films according to the invention. The solvents used for these polyethylenes and also for other polyolefins of ultrahigh molecular weight and copolymers thereof are the halogenated and non-halogenated hydrocarbons known for this purpose, in particular aliphatic, cycloaliphatic and aromatic hydrocarbons having boiling points of at least 100¤C, such as paraffins, toluene, xylenes, tetralin, decalin, C₉-C₁₂-alkanes or petroleum fractions. It is also possible to use solvents which are solid at room temperature but are liquid at the dissolution temperature, such as paraffin waxes. The polyethylene is advantageously dissolved in these hydrocarbons in quantities of between 1 and 80% by weight, especially 10 and 50% by weight. Preferably, the dissolution takes place at a temperature between 100 and 220¤C, decalin being used as a particularly preferred solvent. The dissolution temperature depends of course on the particular polyolefin used, on the solvent used, on the quantity to be dissolved and on the equipment used for dissolution. In an advantageous manner, the dissolution takes place in an extruder, as described above, the residence time in the extruder being advantageously between 1 and 25 minutes, especially between 3 and 15 minutes. The polyolefin goes into solution, assisted by the kneading action in the extruder and in the static mixers which may be provided downstream. The solution is then extruded in an advantageous manner through a shaping die which preferably has a width between 60 and 700 mm and a height between 100 µm and 10 mm, a tape being formed which is rapidly cooled to a temperature below the dissolution temperature of the polyolefin, a gel being formed. The cooling can be carried out by introducing the film into a cooling bath, by blowing a cooling gas onto the film or by using cooling rolls. If a cooling bath is used, the coolant used is preferably water, but the solvent which has been employed for the dissolution or swelling of the polyethylene can also be used as the coolant. It is also possible to use an agent which extracts the solvent. Preferably, the temperature of the cooling bath is 15 to 20¤C. The solvent is at least partially removed, for example by evaporation, by squeezing or by extraction. The gelled film obtained can then be prestretched to a certain slight extent, in the longitudinal and/or transverse direction as appropriate, before the stiffening treatment according to the invention is carried out. For example, the degree of prestretching in one or both directions can be between 1.1 and 3. According to a particularly preferred embodiment, the stiffening treatment is carried out in such a way that the film obtained by dissolution in a solvent or by swelling in a solvent is heated, after the cooling described and at least partial removal of the solvent, in the segments to be stiffened to a temperature which, in the case of solvent-free films is between 10¤C below the crystalline melting point and the decomposition temperature and, if the film still contains solvent, is at a temperature above the dissolution temperature and below the decomposition temperature. Preferably, the stiffening is carried out in the edge parts of the film where the latter, for the subsequent stretching for the production of thin films, is gripped by clamps of a stretching device. The width of the stiffening segment along the film edges is here preferably 0.5 to 15 mm. For decorative purposes, the stiffenings can be carried out in any desired design. Heating can be carried out in such a way that, in a continuous process, the film is passed through one or more gaps formed by pairs of heated rolls, if appropriate under pressure, and the film can also be run in contact with heated metal plates for the stiffening treatment. Contactless heating by infrared irradiation is also possible. In terms of time, temperature, process and equipment, the heating must be carried out in such a way that re-entangling of the polymer molecules occurs, in accordance with the equation given above. In this way, mechanical stiffening takes place in the heat-treated areas of the film, which can also still contain solvent. As a result of this stiffening, it is possible to stretch the film simultaneously or sequentially at ultra-high stretching ratios with the formation of thin films, without lips being formed thereby to an excessive extent, which lips can tear off at high stretching ratios, disturb the isotropy of the films and cause large quantities of waste. The stiffened segments on the film can be formed in any desired configurations. In order to avoid the problems in stretching already mentioned, stiffened edge strips are generally formed on the film edges gripped by the clamps of the stretching device. The stiffenings can also be produced on the film edges in a controlled manner only in those areas at which the clamps of the stretching device engage. This can be effected, for example, in such a way that the edges of the film are brought into contact with rolls which, around their periphery in certain spacings which correspond to the engagement spacings of the clamps of the stretching device, have heatable protrusions which stiffen the film pointwise on contacting. The film can also be provided with stiffened segments in any other desired areas by the heat treatment described, for example in areas where a stretched sheet produced from these films is to be provided with eyelets or is to be fixed by screwing down and the like. Stiffened segments can also be produced in any desired arrangement for decorative purposes. According to another embodiment of the invention, the stiffened segments in any desired configurations can also be produced in such a way that the films are provided on one side or both sides with a reinforcing layer, in particular of a polymer film, polyethylenes of low density (LLDPE) being particularly suitable reinforcing polymers. These films can usually be laminated on or fused on, if appropriate with the use of adhesion promoters. The invention is illustrated in more detail by the attached drawings in which, in plan view: Figure 1 shows the appearance of an edge part of a film which has been obtained by stretching, using a film of the generic type, claimed according to the invention, without stiffened segments on the edges, and Figure 2 shows the appearance of the edge part of the same film which has been obtained by stretching a film with stiffened segments on the edge parts gripped by the clamps of the stretching device. As shown in Figure 1, lips 2 are formed on the film, which has not been provided with stiffened segments on its stretching edges, as a result of stretching by the clamps 1, which lips become increasingly thinner in the tension area with increasing stretching ratio and can finally tear off. These lips form to a greater extent, the greater the distance L between two clamps 1 engaging on the film 3. α is the angle between the tangent, placed through the upper right corner of the pulling clamps at the inner flank of the lips, and a line parallel to the original film edge at the distance of the travel covered by the pulling clamps, whereas R is the radius of curvature of the lips produced by the stretching, assuming a semicircular form of the resulting lips. The examples which follow explain the invention. Example 1 (comparative example) A polyethylene having a weight-average molecular weight of 1.5 x 10⁶ g/mol (Hostalen GUR 412®) is dissolved in decalin in a concentration of 8% by weight, with the addition of 1% by weight of an antioxidant, relative to the polyethylene, in a twin-screw extruder (Berstorff ZE 40 x 33 D) of 25 mm internal diameter at a temperature of 185¤C during a residence time of 7 minutes. The solution obtained is extruded through a shaping die to give a tape which is cooled by introducing it into water of a temperature of 15¤C. A piece of 200 x 200 mm dimensions is cut out of this tape. This piece is clamped into an elastic frame and dried, a shrinkage of 15% in the machine direction and 31% in the transverse direction being allowed. Simultaneous biaxial stretching is then carried out in a biaxially operating stretching device in an oven at a temperature of 131¤C at a rate of 33%/second (= 40 mm/second) until a stretching ratio of 8 x 8 is reached. Before stretching, the distance of the mutually opposite clamps is 70 mm. The table which follows summarizes the results of measurements of 4 lips produced by the stretching, L being the distance between the centre of adjacent pulling clamps in each case, α being the angle between the tangent, placed through the upper right corner of the pulling clamps at the inner flank of the lips, and a line parallel to the original film edge at the distance of the travel covered by the pulling clamps, and R being the radius of curvature of the lips produced by the stretching. The smaller the angle α and the larger the radius of curvature R (in mm), the smaller are the lips formed (compare Figure 1). Even though the film has been biaxially stretched at a stretching ratio of the clamps 8 x 8, it shows in its middle part only a stretching ratio of about 6 x 6, because of the very pronounced lips formed. On further stretching, these lips tear in the vicinity of the clamps. Example 2 The procedure described in Example 1 is repeated, the piece of film being heat-treated, after drying, at its 4 side edges for a length of 140 mm and a width of 4 mm during a period of 8 seconds by means of a hot metal piece under a load of 100 N at 150¤C. Simultaneous biaxial stretching at a rate of 33%/second (= 40 mm/second) up to a stretching ratio of the clamps of 8 x 8 is carried out in an oven at a temperature of 131¤C. During stretching, the film thickness decreases from originally 150 µm to 2.2 µm. In contrast to comparative Example 1, the stretching ratio of the film in its middle part is 7.5 x 7.5. The results are summarized in the table which follows. 1 2 3 4 Clamp distance L (mm) 80 78 79 79 α 20¤ 24¤ 23¤ 18¤ R (mm) 70 65 61 73 On further stretching, the film breaks. The film described above can, without stiffened edges, be stretched up to a stretching ratio of 62 in one direction before it tears, whereas the uniaxial stretching ratio, after the stiffened edges have been produced, decreases to 12 (before tearing), which demonstrates the reinforcing effect of the stiffening treatment. Example 3 The procedure described in Example 2 is repeated, the film being stretched first in the machine direction and then in the transverse direction. During stretching, the film thickness decreases from 150 µm to 2.3 µm. The results are shown in the table which follows: 1 2 3 4 Clamp distance L (mm) 81 79 79 80 α 21¤ 24¤ 19¤ 20¤ R (mm) 71 67 69 76 Example 4 (comparative example) A polyethylene of a weight-average molecular weight of about 1.5 million g/mol (Hostalen GUR 412®) is dissolved in decalin, together with 1% by weight of an antioxidant, relative to the polyethylene, in a quantity of 20% by weight in a manner described in Example 1 and, from the solution, a film is produced likewise in the manner described in Example 1, which film is allowed to shrink during drying by 8% in the machine direction and by 24% in the transverse direction. In an oven at a temperature of 131¤C, stretching is carried out according to Example 1 at a stretching rate of 33%/second (= 40 mm/second) up to a stretching ratio of 8 x 8. The results are shown in the table which follows. 1 2 3 4 Clamp distance L (mm) 79 80 80 81 α 40¤ 43¤ 36¤ 41¤ R (mm) 42 41 38 49 On further stretching, the film breaks in two clamps, while one clamp is no longer able to hold the film and the latter slides off. Example 5 The procedure described in Example 4 is repeated, the film sample being stiffened at the edges by the procedure described in Example 2. The sample is then simultaneously biaxially stretched in an oven at a temperature of 131¤C up to a stretching ratio of 8 x 8 at 33%/second (= 40 mm/second). During stretching, the film thickness decreases from 190 µm to 2.7 µm. The results are shown in the table which follows. 1 2 3 4 Clamp distance L (mm) 78 78 79 80 α 22¤ 19¤ 21¤ 21¤ R (mm) 74 71 69 68 On further stretching, the film tears in the middle part. A comparison of the stretching ratio in one direction of the film sample, which has not been provided with stiffening edges, with a film sample having stiffening edges according to Example 2 shows a decrease of the stretching ratio from 43 to 18, which again demonstrates the reinforcing effect of the stiffening treatment according to the invention. Example 6 (comparative example) Polypropylene of a weight-average molecular weight of about 1,000,000 g/mol is dissolved in decal in a quantity of 4% by weight by the procedure described in Example 1 and is processed by the method described therein to give a film sample which, at an oven temperature of 159¤C, is simultaneously biaxially stretched up to a stretching ratio of 7 x 7 at a rate of 33%/second, the film thickness decreasing from 82 µm to 1.3 µm. In this case, the distance of the mutually opposite clamps before stretching is 90 mm. The results are shown in the table which follows. 1 2 3 4 Clamp distance L (mm) 95 94 94 95 α 67¤ 73¤ 55¤ 68¤ R (mm) 32 35 41 32 Example 7 The procedure followed is as described in Example 5, the stiffening treatment being carried out at 175¤C for a period of 6 seconds, stiffened side edges of 34 mm x 140 mm dimensions being produced under a load of 100 N. The results are shown in the table which follows. 1 2 3 4 Clamp distance L (mm) 80 79 80 81 α 35¤ 31¤ 29¤ 33¤ R (mm) 60 64 63 54 A comparison of the uniaxial stretching ratios of a film sample free of stiffening edges with a film sample provided with stiffening edges (compare Example 2) shows that the stretching ratio decreases from 59 to 44 on stretching in one direction.

Claims: Process for the production of films that partially contain stiffened segments by dissolving or swelling a polymer of high molecular weight in a solvent, shaping the polymer solution or the swollen polymer to give a film, cooling the resulting film to a temperature below the gelling temperature, at least partially removing the solvent and, if appropriate, prestretching in the longitudinal and/or transverse direction, characterised in that the film, in the segments which are to be stiffened, is heated in the solvent-free state to a temperature which is between 10°C below the crystalline melting point and the decomposition temperature, and, if the film still contains solvent, it is heated to a temperature which is above the dissolution temperature and below the decomposition temperature. Process according to Claim 1, characterised in that the stiffened segments are located at the edges of the film. Process according to Claim 1 or 2, characterised in that the polymer comprises a polyolefin having a molecular weight (weight average molecular weight) of more than 100,000 g/mol. Process according to Claim 3, characterized in that the polymer comprises polyethylene, preferably linear polyethylene, having a molecular weight (weight average molecular weight) from 400,000 to 6,000,000 g/mol. Process according to any one of Claims 1-4, characterized in that heating is carried out for a period which is sufficient for re-entangling of the polymer molecules to occur, in accordance with the equation: Process according to any one of Claims 1-5, wherein the films containing the stiffened segments are ultrahigh stretched.

Inventors: PLUYTER PIETER BOUDEWIJN, RUTTEN HENDRICUS JOHANNES JOZE, PLUYTER, PIETER BOUDEWIJN, RUTTEN, HENDRICUS JOHANNES JOZEF
Applicants: DSM NV, DSM N.V.
Application year: 1988
Main classification: C08J5
IPC codes: B29C71, C08J5, B32B3, B29C55