CA2192917A1 - Process for longitudinal stretching in the production of oriented polypropylene films - Google Patents
Process for longitudinal stretching in the production of oriented polypropylene filmsInfo
- Publication number
- CA2192917A1 CA2192917A1 CA 2192917 CA2192917A CA2192917A1 CA 2192917 A1 CA2192917 A1 CA 2192917A1 CA 2192917 CA2192917 CA 2192917 CA 2192917 A CA2192917 A CA 2192917A CA 2192917 A1 CA2192917 A1 CA 2192917A1
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- CA
- Canada
- Prior art keywords
- film
- stretching
- roll
- rolls
- driven
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
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Classifications
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
- B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
- B29C55/00—Shaping by stretching, e.g. drawing through a die; Apparatus therefor
- B29C55/02—Shaping by stretching, e.g. drawing through a die; Apparatus therefor of plates or sheets
- B29C55/04—Shaping by stretching, e.g. drawing through a die; Apparatus therefor of plates or sheets uniaxial, e.g. oblique
- B29C55/06—Shaping by stretching, e.g. drawing through a die; Apparatus therefor of plates or sheets uniaxial, e.g. oblique parallel with the direction of feed
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
- B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
- B29C55/00—Shaping by stretching, e.g. drawing through a die; Apparatus therefor
- B29C55/02—Shaping by stretching, e.g. drawing through a die; Apparatus therefor of plates or sheets
- B29C55/18—Shaping by stretching, e.g. drawing through a die; Apparatus therefor of plates or sheets by squeezing between surfaces, e.g. rollers
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- Engineering & Computer Science (AREA)
- Mechanical Engineering (AREA)
- Shaping By String And By Release Of Stress In Plastics And The Like (AREA)
- Laminated Bodies (AREA)
Abstract
Process for longitudinal strentching in the production of oriented polypropylene films The invention relates to a process for the longitudinal stretching of an at least single-layer thermoplastic film.
The slow-running part of the stretching machine contains at least one driven roll and the fast-running part of the stretching machine contains at least two driven rolls, the roll pair being arranged in such a way that a roll nip is formed, into which the film is fed in such a way that it is gripped simultaneously by the roll pair and stretched.
The slow-running part of the stretching machine contains at least one driven roll and the fast-running part of the stretching machine contains at least two driven rolls, the roll pair being arranged in such a way that a roll nip is formed, into which the film is fed in such a way that it is gripped simultaneously by the roll pair and stretched.
Description
2~92~17 -HOECHST TRESPAPHAN GMBH HOE 95/P 002 DCh.VK/As Description 5 Process for longitudinal stretching in the production of oriented polypropylene films The invention relates to a process for longitudinal stretching of biaxially oriented films made from thermoplastic polymer, in particular polypropylene. The films 10 produced by the process are distinguished by good heat-sealability, good appearance and a good thickness profile.
The prior art discloses various processes for the production of biaxially oriented polypropylene films (BOPP films). In the stenter process, the BOPP film is produced 15 by extruding the melt through a flat-film die followed by stretching in the longitudinal and transverse direction. The film generally has a multilayer structure.
In detail, the process involves first compressing, warming and melting the polymers in an extruder. The melts corresponding to the individual layers of the film are jointly 20 filtered and forced simultaneously through a flat-film die, giving a melt film as extrudate. The melt film is cast onto a chill roll, where it solidifies to give an unoriented film. The film is subsequently oriented in the longitudinal direction via rolls and in the transverse direction in a stretching oven and is subsequently heat-set. The film may subsequently be surface-treated by flame or corona. The film is 25 wound up and finished to give the customer-ready cut roll.
Biaxially oriented polypropylene films are distinguished by a very good propertyprofile. Their characteristic properties are high mechanical strength, good heat-sealability and a bright appearance. Owing to this good property profile and 30 excellent processing properties - characterized by low slip, high rigidity and good thickness profile - BOPP films have been used in a wide variety of applications. The most important market segment is packaging, which accounts for about 80% of the films produced. In addition, BOPP films are employed in significant amounts in technical applications, for example in metallization and transfer metallization, ~19~917 -lamination and as electrical insulation in capacitor foils. Electrical foils generally have a thickness of less than 10 ,um. The essential prerequisite for flaw-free processing of these very thin foils is a very good thickness profile.
5 The trend in the production of BOPP films is toward higher production speeds and wider production widths. In 1980, the machine speed at which BOPP films were produced was 100-150 m/min, while today it is 300-400 mtmin. The machine width in 1980 was about 5 m, while today it extends up to 8 m. An increase in the production speed and machine width also means an increase in the stretching rate, i.e. the rate 10 at which the film is oriented.
An increase in the stretching rate means greater mechanical and thermal stresseson the thermoplastic during orientation. In particular during longitudinal stretching, where the stretching of the film is carried out in a very short distance between two 15 rolls rotating at different speeds, the increase in the stretching rate can result in an impairment in the film quality.
During longitudinal stretching, the film web is first passed over a plurality of heated rolls which bring the material to the temperature necessary for stretching. These 20 rolls are driven at a low peripheral speed. The film then reaches one or more chill rolls, which are driven at a higher peripheral speed than the heating rolls. Thedifferent speeds of the heated and chill rolls produces longitudinal stretching of the film.
DE-B 1 221 786 describes an apparatus for stretching a thermoplastic film web inthe longitudinal direction. In this apparatus, two rolls which press the edges of the film web against a support are provided after, regarded in the stretching direction, the region warmed by the heating device. These undriven rolls have the task of countering any reduction in the film web width during the longitudinal stretching. The stretching force in this process is applied by the rolls to one side of the film.
DE-B 1 212 290 describes an apparatus for longitudinal stretching of a material web in which the material web is heated to the stretching temperature by heating rolls in - 2i92917 the longitudinal direction and subjected in the stretching region to a tensile force acting in the longitudinal direction. The transport devices for the material web are ar!anged in such a way that they keep the tensile force away from the part of the material web Iying on the heating roll. In an expedient embodiment of the invention, 5 the transport devices are formed by a transport roll driven independently of the heating roll and by an undriven counterpressure roll. The process described has the disadvantage that the tensile force is again only applied to one side of the film.
A particular embodiment of the device of DE-B 1 212 290 is described in German Patent 1 504 058. The device of DE-B 1 212 290 is refined in such a way that theoptimum length of the stretching zone can be set precisely depending on the typeand thickness of the material web, the operating speed and the desired stretching ratio. This is achieved in accordance with the invention in that, in order to change the length of the stretching zone, the pair of transport and/or the pair of tension rolls 15 is/are pivotable about the axis of the heating roll or of the chill roll. This process again has the disadvantage of application of force on one side and an unfavorable arrangement of the chill rolls 8 and 9.
German Patent 1 919 299 claims a process for the longitudinal stretching of a 20 plastic film in which longitudinal tension is kept away from the final heating roll in a particularly simple manner. The object is achieved in accordance with the invention in that a temperature drop of from 2 to 25C, depending on the film material, isproduced between the temperature of the final heating roll and the temperature of the first stretching roll. This process again has the disadvantage that the stretching 25 force is in each case applied to only one film surface, albeit in an alternating manner. In addition, the unfavorable arrangement of the rolls 7 and 8 means that the risk of air bubbles between the film and the rolls 7 and 8 is particularly great.
DE-B 2 833 189 describes a process for the longitudinal stretching of an at least 30 two-layer plastic film, in which the layer of higher melting point is oriented and the layer of lower melting point is heat-sealable. Before reaching the stretching rolls, the film is first warmed to a temperature which is both above the stretching temperature of the higher-melting layer and below the adhesive temperature of the lower-melting layer. The lower-melting layer is then shock-cooled within a short time interval to a temperature significantly below the adhesive temperature, while the higher-melting layer remains at the stretching temperature. This procedure prevents adhesion ofthe film sur~aces to the stretching rolls. This process again has the disadvantage that the force for longitudinal stretching of the film is only applied to one side of the films.
The known BOPP production processes have the disadvantage of r.a Ionger giving high performance if the production speeds are increased. At increased productionspeeds, these processes give films having an uneven thickness profile, impaired heat-sealing properties and a worse film appearance.
The object of the present invention was therefore to provide a process for the longitudinal stretching of a thermoplastic film by means of which a film having good heat-sealability, good appearance and a good thickness profile is provided at the production speeds customary today.
The object is achieved in accordance with the invention by a process for the longitudinal stretching of an at least single-layer thermoplastic film. In the novel process (the reference numbers below refer to Fig. 1), before the longitudinal stretching, the film is warmed in the slow-running part of the stretching machine to a temperature suitable for stretching and fed to a stretching zone (10). In the process, a) the slow-running part of the stretching machine contains the driven roll (1), and the fast-running part of the stretching machine contains the driven roll pair (2)/(3), the roll pair (2)/(3) being arranged in such a way that a roll nip (4) is formed, and b) the film (9) is passed into the roll nip (4) in such a way that it is grippedsimultaneously or virtually simultaneously by the roll pair (2)/(3) at the contact points (5)/(6), and the film (9) is stretched between the roll (1 ) and the rollpair (2)/(3).
219~7 In accordance with the invention, the roll pair (2)/(3) is arranged in such a way that the film is gripped virtually simultaneously at the contact points (5)/(6), causing the stretching force in the fast-running part of the stretching machine to be applied to both film surfaces 7 and 8. This produces better tension distribution within the film 9, 5 in particular within the layers of the film 9 close to the surface, compared with application of the stretching force on one side. Surprisingly, the fact that the rolls (2) and (3) are driven and their novel arrangement, i.e. application of the str~Lching force to both sides, means that the properties of the film are significantly better. This is particularly true of the optical properties and the heat-sealing properties of the 1 0 film.
It has been found that, in the longitudinal stretching processes of the prior art (cf.
Figure 2), the entire stretching force is taken up by only one film surface (Figure 2, film surface 7), i.e. the force is applied to one side, which can result in a critical 15 stress value in/on the film being reached or exceeded in the layer of the film close to the surface. The stress peaks of this type can result in damage to the surface.
The fact that the film thickness in the fast-running part of the stretching machine is less by the stretching ratio f (f = v2/v1 ) means that a critical stress value initially 20 occurs in the fast-running part of the longitudinal stretching machine. The stresses within the film in the slow-running part of the stretching machine are significantly smaller owing to the greater thickness, so that the risk of surface damage to the film is significantly less there. In accordance with the invention, application of force to both sides by the rolls 2 and 3 in the fast-running part of the stretching machine is 25 therefore sufficient to improve the appearance, thickness profile and heat-sealing properties.
In addition, the simultaneous contact of the film surfaces 7 and 8 with the rolls 2 and 3 at the points 5 and 6 ensures that the air dragged along by the film web 9 is 30 squeezed out at the contact points 5 and 6. The film is in contact with the rolls 2 and 3 over its entire surface, so that optimum force application is ensured between the rolls 2 and 3 and the film surfaces 7 and 8. If, by contrast, the roll 3 is positioned as shown in Figure 3, air is dragged in between the film 9 and the roll 2, forming an air 2 ! 9291 7 cushion which reduces the contact area for force application. The film 9 is no longer flat on the roll 2, causing the partial interruption in force application between the film 9 and the roll 2. This results in film sections being accelerated to different extents in the stretching direction in this region of the roll, which causes width and thickness differences in the film 9. In production of the film by the novel process as shown in Fig. 1, the inclusion of air is prevented by the film being gripped virtually simultaneously at the contact points 5 and 6 by the rolls 2 and 3. It has been found that the thickness profile of the film is consequently significantly better than in an arrangement as shown in Figure 2 or 3.
The invention is explained in greater detail below with reference to drawings.
Figure 1 is a diagrammatic view of the embodiment according to the invention.
Figure 1 A shows a detailed view on the area I A of Figure 1.
Figures 2 and 3 show illustrative embodiments of longitudinal stretching devicesaccording to the prior art and non-inventive embodiments.
Figure 4 A and B shows an embodiment according to the process wherein roll 3 is driven with the aid of roll 2.
Figures 5 and 6 show further embodiments of the process according to the invention.
Figure 7 shows an instrument used to conduct a hot tack test.
The 'ongitudinal stretching machine shown in Figure 1 comprises, in accordance with the invention, the three driven rolls 1, 2 and 3 by means of which the film 9 is stretched by the longitudinal stretching ratio f. The speeds of the rolls are v1, v2 and V3. The longitudinal stretching ratio f is given, for frictional contact between the rolls 1, 2 and 3 and the film 9, approximately by the ratio of the speeds v2 and v1 f the rolls 2 and 1. The longitudinal stretching ratio f for PP films is usually in the range from 4 to 9, preferably in the range from 4.5 to 8Ø When the stretching zone 10 is reached, the film 9 has achieved a temperature Ts at which it can be stretched by the particular stretching ratio. The length of the stretching zone is generally from 50 to 600 mm, preferably from 50 to 500 mm, in particular from 50 to 400 mm.
Depending on the raw material, film thickness, stretching rate and stretching ratio, the temperature Ts is preferably between 80 and 160C. The stretching force Es is 5 applied to the film 9 by means of friction between the driven roll 1 and the film surface 7 and between the driven roll pair 2/3 and the film surfaces 7 and 8. The speeds v2 and V3 of the roll pair 2/3 are generally the same, but can also differ slightly from one another by a maximum of 5%. The rolls 2 and 3 can be driven separately, although joint driving of the rolls 2 and 3 is also possible. Figure 4 10 shows diagrammatically an example of a joint drive of the roll pair 2/3. To this end, the roll 3 is expediently designed in such a way that the two rolls 2 and 3 are in contact in the edge region 13. The film 9 lies flat on the two roll surfaces in the recess 14 and is transported by friction forces. If only one film thickness is produced on the machine, the roll pair 2/3 can be made of a hard material, for example steel 15 with a chrome-plated or ceramic surface. Somewhat greater flexibility with respect to the film thickness is achieved if, for example, the roll 3 is rubber-covered.
Application force is particularly good if the rubber has a Shore A hardness of between about 50 and 100. In the production of greatly different film thicknesses, the roll 3 must be replaced, which is expediently carried out when re-setting the 20 thickness. The two-sided frictional contact between the roll pairs 2 and 3 and the film surfaces 7 and 8 means that the film surface 7 is subjected to significantly lower stresses than in the case of frictional contact on one side, i.e. in arrangements with only one driven roll in the fast-running part of the stretching machine. In the invention, the two-sided frictional contact is achieved in the fast-running part of the 25 stretching machine since the stresses acting on the film are greatef there than in the slow-running part of the stretching machine owing to the lower film thickness. With regard to achieving a critical stress value, the slow-running part of the stretching machine is insignificant.
30 In spite of force application on both sides by the novel process, the stretching force is not necessarily applied equally by the rolls 2 and 3. The further looping of film 9 around roll 2 means that the distribution of the stretching force onto rolls 2 and 3 is not equal. Experiments have shown that the stretching force in the stretching arrangement shown in Fig. 1 (film - roll loop angle 50 to 180) can be divided between rolls 3 and 2 in a ratio of up to 40:60. In spite of the unequal distribution of the force application, the stress in the film is lower in the novel process than in one-sided stretching force application.
A further feature of the invention is that the roll pair 2/3 is arranged in such a way that the air dragged along by the film 9 is squeezed out at the contact points 5 and 6 of the roll pair 2/3. To this end, the film surfaces 7 and 8 are gripped simultaneously by the roll pair 2/3 at the contact points 5 and 6 and transported further by friction.
10 This ensures that the film 9 lies flat on the rolls 2 and 3 without air being dragged in and air cushions forming between the film 9 and the roll surfaces. The contact points should not be more than 50mm, preferably not more than 40 mm, in particular not more than 20mm, measured in the film web direction, apart from one another. The risk of air inclusion increases with machine speed, since the amount of air dragged 15 by the film 9 increases approximately proportionally with the film web speed.
It is observed that positioning of the rolls 2 and 3 as shown in Figure 3 causes the film 9 to run roughly as far as the contact point 5 on the roll 2. The film web width is unstable, i.e. it is subject to variations. The air included between the film 9 and the 20 roll surface escapes suddenly after a critical pressure has built up in the air cushion.
The rough, unstable film running results from local different accelerations of the film web. Thick/thin areas are initiated in the film 9, resulting in a worse thickness profile of the film 9.
25 Figure 5 shows a further expedient embodiment of the invention. The stretching machine comprises in accordance with the invention the three driven rolls 1, 2 and 3 and the novel arrangement of the roll pair 2/3. The slow-running part of the stretching machine additionally comprises a nip roll 15, which serves further toimprove the frictional contact in the slow-running part of the stretching machine. The 30 nip roll can be driven or undriven. For the outlined reasons, the nip roll isexpediently arranged at the contact point of the film 9 with the roll 1, so that, analogously to the roll pair 2 and 3, the film comes into contact simultaneously or virtually simultaneously with roll 1 and the nip roll. The looping of the film 9 around the rolls 1 and 2 and the diameter of the roll 3 are advantageously selected so that the shortest possible stretching zone 10 can be achieved (S-shaped loop).
This is important, for example, for the production of films which are sensitive to splitting or which can only be stretched at a low longitudinal stretching ratio.
Figure 6 shows a further embodiment of the invention in which longitudinal stretching is divided over a plurality of stretching zones 10 to 12. In the embodiment shown, the rolls 16 to 19 are likewise driven. By means of the selected arrangement of the rolls, the individual stretching ratios can be matched in a targeted manner to 10 the raw material, and the force application and the associated stresses on the two film surfaces can be divided equally.
The longitudinal stretching in accordance with the invention is particularly advantageously used in the production of films by the stenter process, which is 15 known per se. It is advantageously suitable both for the production of films which are only monoaxially oriented (only in the longitudinal direction), and for the production of biaxially oriented films.
In the stenter process, the melts corresponding to the individual layers of the film 20 are extruded or coextruded through a flat-film die, the resultant melt film is taken off on one or more roll(s) for solidification, the film is subsequently stretched and heat-set and, if desired, flame- or corona-treated.
If desired, the film can be biaxially oriented in the longitudinal and transverse 25 directions. The biaxial stretching (orientation) is carried out successively, it being preferred to carry out the stretching by the novel process first in the longitudinal direction of the machine and then in the transverse direction of the machine.
Firstly, as usual in the extrusion process, the polymers or the polymer mixtures of 30 the individual layers are compressed and liquefied in extruders, it being possible for the additives already to be present in the polymers or in the polymer mixtures. The melts are then forced jointly and simultaneously through a flat-film die, and the extruded, single- or multilayer melt film is taken off on one or more take-off rolls, where it is cooled and solidified. The resultant film is then stretched longitudinally and preferably also transversely to the extrusion direction, which results in orientation of the molecule chains. The longitudinal stretching is carried out by the novel process described, and the transverse stretching is carried out with the aid of 5 an appropriate tenter frame. The longitudinal stretching ratios are in the range from 4 to 9, preferably from 4.5 to 8.0, and the transverse stretching ratios are in the range from 7 to 12, preferably from 8 to 11.
The stretching of the film is followed by heat-setting (heat treatment), during which the film is kept at a temperature of from 80 to 160C for from about 0.1 to 10 seconds. This can be followed by printing pretreatment, for example by means of a flame or electrical corona process. The treatment intensity is generally in the range from 36 to 50 mN/m, preferably from 38 to 45 mN/m. The film produced in this way is wound up in a conventional manner using a wind-up device.
It has proven particularly favorable to keep the take-off roll or rolls by means of which the extruded melt film is cooled and solidified at a temperature of from 10 to 120C, preferably from 20 to 100C, by means of a heating and cooling circuit. The temperatures at which the longitudinal and transverse stretching are carried out can 20 vary within a relatively broad range and depend on the particular composition of the individual layers and on the desired properties of the film. In general, the longitudinal stretching is preferably carried out at from 80 to 160C and the transverse stretching is preferably carried out at from 120 to 1 70C.
25 The film produced by the novel process can have a single-layer or multilayer structure. It can be non-heat-sealable or heat-sealable. In a particular embodiment, the film has outer layers on both sides of its base layer. In a further embodiment, the film has at least one interlayer, if desired on both sides of its base layer.
30 The base layer of the film generally contains at least 85% by weight, preferably from 90 to 99% by weight, in each case based on the base layer, of a thermoplastic polymer described below and, if desired, additives in effective amounts in each case.
Suitable thermoplastic polymers are polymers of olefins having 2 to 10 carbon atoms, preferably polypropylenes, polyethylenes and/or polybutylenes.
Thcrmoplastic polymers can also be polyethylene terephthalates, polybutylene terephthalates and other polyester derivatives. The novel process is preferably 5 suitable for the production of films having a polypropylene base layer. Of propylene-containing polymers, particular preference is given to propylene homopolymers.
Suitable propylene polymers have a melting point of from 120 to 1 65C, preferably from 140 to 162C, and a melt flow index (measurement in accordance with DIN 53 735 at 21.6 N and at 230C) of from 1.0 to 10 9/10 min, preferably from 1.5 to 6 9/10 min. Propylene polymers contain at least 80% by weight, preferably from 90 to 100%
by weight, in particular from 95 to 100% by weight, of propylene units. The n-heptane-soluble content of the propylene polymer is generally from 1 to 10% by weight, based on the starting polymer. Particular preference is given to propylene homopolymers whose n-heptane-insoluble content is isotactic. The chain isotacticity 15 index, determined by 13C-NMR spectroscopy, is greater than 85%, preferably greater than 90%. The molecular weight distribution of the propylene homopolymercan vary within broad limits, depending on its area of application. The ratio between the weight average molecular weight Mw and the number average molecular weight Mn is generally between 2 and 15.
If the film is opaque, the base layer additionally contains vacuole-initiating fillers, for example CaC03 or incompatible polymers and/or pigments, as described in the prior art. In addition, the base layer may additionally contain conventional additives, such as antistatics, antiblocking agents, lubricants, stabilizers, neutralizers, pigments 25 and/or nucleating agents in effective amounts in each case.
If an electroinsulation film (capacitor foil) is produced by the novel process, the raw material used must have high purity. A high-purity polypropylene of this type must have a low ash and chlorine content compared with the packaging raw material and30 must have the lowest possible content of ionogenic constituents. Electroinsulation film is therefore usually not provided with antistatics and lubricants. The chlorine content of the high-purity polypropylene is less than 50 ppm, and the residual ash content less than 70 ppm.
- 2 1 ~29 1 7 A preferred embodiment of the oriented polypropylene film produced by the novel process comprises at least one outer layer, preferably on both sides, of olefinshaving 2 to 10 carbon atoms. Interlayers of olefinic polymers may additionally be applied.
Examples of olefinic polymers of this type for the outer layer and/or interlayer are a propylene homopolymer a copolym~?r of ethylene and propylene or ethylene and 1-butylene or propylene and 1-butylene, or a terpolymer of ethylene and propylene and 1-butylene, or a mixture or blend of two or more of said homopolymers, copolymers and terpolymers, particular preference being given to propylene homopolymers or random ethylene-propylene copolymers having an ethylene content of from 1 to 10% by weight, preferably from 2.5 to 8% by weight, or random propylene-1-butylene copolymers having a butylene content of from 2 to 25% by weight, preferably from 4 to 20% by weight, in each case based on the total weight of the copolymer, or random ethylene-propylene- 1-butylene terpolymers having an ethylene content of from 1 to 10% by weight, preferably from 2 to 6% by weight, and a 1-butylene content of from 2 to 30% by weight, preferably from 4 to 20% by weight, in each case based on the total weight of the terpolymer, or a blend of an ethylene-propylene-1-butylene terpolymer and a propylene-1-butylene copolymer having an ethylene content of from 0.1 to 7% by weight and a propylene content of from 50 to 90% by weight and a 1-butylene content of from 10 to 40% by weight, in each case based on the total weight of the polymer blend.
The propylene homopolymer employed in the outer layer and/or interlayer containsfrom at least 98 to 100% by weight of propylene and has a melting point of 140C or above, preferably 150 to 170C, preference being given to isotactic homopolypropylene. The homopolymer generally has a melt flow index of from 1.5 to 20 9/10 min, preferably from 2.0 to 15 g/10 min.
2~92917 The above-described copolymers and terpolymers employed in the outer layer and/or interlayer generally have a melt flow index of from 1.5 to 30 g/10 min, preferably from 3 to 15 g/10 min. The ,nelting point is in the range from 120 to140C. The above-described blend of copolymers and terpolymers has a melt flow index of from 5 to 9 g/10 min and a melting point of from 120 to 150C. All the melt flow indices given above are measured at 230C and a force of 21.6 N (DIN 53 735).
If desired, the outer layers and/or interlayers can likewise contain antistatics, antiblocking agents, lubricants, neutralizers, stabilizers and hydrocarbon resins.
The overall thickness of the polypropylene film produced by the novel process can vary within broad limits and depends on the intended application. It is preferably from 2 to 120 ,um, in particular from 2.5 to 100 ,um, especially from 2.5 to 80 ,um, the base layer making up from about 40 to 100% of the overall film thickness.
The thickness of the outer layers is greater than 0.1 ,um and is preferably in the range from 0.2 to 5 ,um, in particular from 0.3 to 1.5 ,um, it being possible for the outer layers to have identical or different thicknesses. The thickness of any interlayer(s) present is greater than 0.1 ,um and is preferably in the range from 0.5 to 15 ,um, in particular from 0.7 to 10 ,um.
The film produced by the novel process is particularly suitable for processing on high-speed machines, for example for packaging, lamination or metallization. It has all the important properties required of oriented polypropylene films with respect to 25 their different applications. In particular, it has good heat-sealability, a good appearance anc an excellent thickness profile.
The following measurement methods were used to characterize the raw materials and the films:
Melt flow index The melt flow index was measured in accordance with DIN 53 735 at a load of 1.6 N
and at 230C.
- 2!92917 Melting point DSC measurement, maximum of the melting curve1 heating rate 20C/min.
Molecular weight determination 5 The mean molecular weight is determined by three-detector gel permeation chromatography. The substance is dissolved in an eluent, such as THF1 and applied to a separating colun ,n. The separating column has a length of 90 cm and is filled with a porous support material whose pore size is 5 ,um. Detection is by UV
absorption spectroscopy at various wavelengths and by means of the refractive 10 index and light scattering capacity of the fractions. The calibration is carried out by means of a standard compound of known molecular weight. Comparison of the UV
absorption of the standard substance with the absorption of the sample enables assignment of the molecular weights.
15 Determination of the minimum sealing temperature (MST) The minimum sealing temperature is determined by the peel method. Heat-sealed samples (seal seam 20 mm x 150 mm) are produced using a Brugger HSG/ET
sealing machine by sealing a film at various temperatures with the aid of two heated sealing jaws at a sealing pressure of 15 N/cm2 for 0.5 second. Test strips with a 20 width of 15 mm are cut out of the sealed samples. The T-seal seam strength1 i.e. the force necessary to separate the test strips1 is determined using a tensile tester at a peel rate of 200 mm/min, the seal seam plane forming a right angle with the direction of tension. The minimum sealing temperature is the temperature at which a seal seam strength of 0.5 N/15 mm is achieved.
25 In addition1 the tear strength is measured at a sealing temperature of 130C -likewise by the peel method.
Hot tack Hot tack denotes the strength of a still-hot seal seam immediately after opening of 30 the sealing jaws. In the test used1 the separation in mm experienced by the heat-seal seam at a load of 1 N (seal seam width 30 mm) is measured. The test is carried out at a sealing temperature of 1 50C. The measurement is a Hoechst internal standard. There are no corresponding DIN and ASTM standards. The test 21 92~17 instrument used is the heat contact sealing unit with grooved sealing jaws (20, 21) and deflection rolls (R1, R2, R3) for a separation angle of 180 (cf. Figure 7). The sealing time is 0.5 second and the sealing pressure is 30 N/cm 2 In the measurement process, the measurement strips (22) having a width of 30 mm are laid one on top of the other and fixed at the ends with a weight of 1 N. A flat spatula (23) is inserted between the film layers, and the measurement strips are fed around the deflection rolls (R1, R2, R3) over the sealing jaws and clamped such that the flat spatula will be positioned between the grooved sealing jaws. After completion of the sealing, the jaws open automatically. The sealed measurement strip is jerked forward to the deflection roll by the weight of 1 N (G), where the seal resulting from the sealing jaws (21) is separated at a separation angle of 180. The depth of the lamination of the seal seam in mm is assessed. The greater the numerical value, the worse the hot tack. The sealing of sealing jaws (20) remains unaffected during determination of the hot tack.
Gloss The gloss was determined in accordance with DIN 67 530. The reflectometer value was measured as a characteristic optical parameter for the surface of the film. In accordance with the ASTM-D 523-78 and ISO 2813 standards, the angle of incidence was set at 20 or 60. A light beam hits the planar test surface at the set angle of incidence and is reflected or scattered thereby. The light beams hitting the photoelectronic receiver are indicated as proportional electrical quantities. The measurement value is dimensionless and must be indicated together with the angleof incidence.
Hase The haze of the film was measured in accordance with ASTM-D 1003-52.
Assessment of the thickness profile The thickness profile was measured on-line by traversing a measurement head overthe film web width (F+H or FAG measuring instrument). The measurement head contains a beta-emitter which measures the absorption in the film and converts the value into a corresponding thickness value. The thickness values are measured over the film width and plotted as a graph. The thickness profile is assessed using the so-called R value. This is calculated from the ratio between the difference of maximum thickness value and minimum thickness value and the mean thickness of 5 the film.
maximum thickness - minimum thickness R value =
mean thickness The thickness profile is better the lower the R value.
Although the present invention has been described with preferred embodiments it is to be understood that modification may be resorted without departing from the spirit 15 and scope of this invention as those skilled in the art would readily understood.
The invention is described in greater detail now with reference to examples.
Example 1 20 A transparent, heat-sealable, three-layer film with a symmetrical structure and an overall thickness of 40 ,um was produced by coextrusion followed by stepwise orientation in the longitudinal and transverse directions. The outer layers each had a thickness of 0.6 ,um.
25 Base layer A:
99.70% by wt. of isotactic polypropylene from Solvay with the tradename Eltex 0.150% bywt. of N,N-bisethoxyalkylamine 0.150% by wt. of erucamide Outer layers B:
99.67% by wt. of random ethylene-propylene copolymer having a C2 content of 4.5% by weight 0.33% by wt. of SiO2 as antiblocking agent having a mean particle size of 4 ,um.
The production conditions in the individual process steps were as follows:
Extrusion: Temperatures: Layer A 280C
Layers B 280C
Temperature of the take-off roll 30C
Longidutinal stretching: Temperature 130C
Longitudinal stretching ratio 5 Transverse stretching: Temperature 160C
Transverse stretching ratio 10 Heat-setting: Temperature 1 40C
Convergence 15 %
Film web speed 230 m/min The film was produced by the novel process as shown in Figure 1. The properties of the films from the examples and comparative examples are shown in the table 20 below.
Example 2 A film was produced by the novel process analogously to Example 1.
25 Compared with Example 1, the random copolymer in the outer layers was replaced by a propylene homopolymer (Eltex PHP 405). The film is not heat-sealable.
The process parameters were the same. The film is distinguished by very good optical properties and a very good thickness profile.
30 Example 3 A film is produced by the novel process analogously to Example 1.
Compared with Example 1, a single-layer film was produced for electrical insulation.
The polymer ,'or this film was a high-purity isotactic propylene homopolymer from Borealis with the tradename Borealis VB 2142 E, Melt flow index = 2.2 9/ 10 min.The thickness of the film was 3.5 ~m. The process conditions during production of the film which had changed compared with Example 1 were as follows:
Extrusion: Temperature: 270C
Temperature of the take-off roil 90C
Long;tudinal stretching: Temperature: 150C
Longitudinal stretching ratio: 5.5 The high take-off roll temperature of 90C and the special longitudinal stretching temperature gave a film having a rough surface, as described, for example, in EP-A
0 497 160. The film is distinguished by an excellent thickness profile.
15 Comparative Example 1 Compared with Example 1, the longitudinal stretching of the film was now carried out as shown in Figure 1, but without driving of the roll 3, i.e. only roll 2 was driven. The hot tack and the optical properties of the film are significantly worse, and thethickness profile of the film has become worse.
Comparative Example 2 Compared with Example 1, the longitudinal stretching of the film was now carried out as shown in Figure 3, where the contact points are more than 100 mm apart and the film is no longer gripped simultaneously by the driven rolls 2 and 3. The hot tack and 25 optical properties of the film are worse, and the thickness profile of the film has become significantly worse.
Comparative Example 3 Compared with Example 2, the longitudinal stretching of the film was now carried out 30 as shown in Figure 1, but without driving of the roll 3, i.e. only roll 2 was driven. The optical properties of the film are significantly worse, and the thickness profile has become worse.
Comparative Example 4 Compared with Example 2, the longitudinal stretching of the film was now carried out as shown in Figure 3, where the contact points are more than 100 mm apart. The optical properties of the film are worse, and the thickness profile has become 5 significantly worse.
Comparative Example 5 Compared with Example 3, the longitudinal stretching of the film was now carried out as shown in Figure 1, but without driving of roll 3, i.e. only roll 2 was driven. The 10 thickness profile of the film has become worse.
Comparative Example 6 Compared with Example 3, the longitudinal stretching of the film was now carried out as shown in Figure 3, where the contact points are more than 100 mm apart. The 15 thickness profile of the film has become significantly worse.
The prior art discloses various processes for the production of biaxially oriented polypropylene films (BOPP films). In the stenter process, the BOPP film is produced 15 by extruding the melt through a flat-film die followed by stretching in the longitudinal and transverse direction. The film generally has a multilayer structure.
In detail, the process involves first compressing, warming and melting the polymers in an extruder. The melts corresponding to the individual layers of the film are jointly 20 filtered and forced simultaneously through a flat-film die, giving a melt film as extrudate. The melt film is cast onto a chill roll, where it solidifies to give an unoriented film. The film is subsequently oriented in the longitudinal direction via rolls and in the transverse direction in a stretching oven and is subsequently heat-set. The film may subsequently be surface-treated by flame or corona. The film is 25 wound up and finished to give the customer-ready cut roll.
Biaxially oriented polypropylene films are distinguished by a very good propertyprofile. Their characteristic properties are high mechanical strength, good heat-sealability and a bright appearance. Owing to this good property profile and 30 excellent processing properties - characterized by low slip, high rigidity and good thickness profile - BOPP films have been used in a wide variety of applications. The most important market segment is packaging, which accounts for about 80% of the films produced. In addition, BOPP films are employed in significant amounts in technical applications, for example in metallization and transfer metallization, ~19~917 -lamination and as electrical insulation in capacitor foils. Electrical foils generally have a thickness of less than 10 ,um. The essential prerequisite for flaw-free processing of these very thin foils is a very good thickness profile.
5 The trend in the production of BOPP films is toward higher production speeds and wider production widths. In 1980, the machine speed at which BOPP films were produced was 100-150 m/min, while today it is 300-400 mtmin. The machine width in 1980 was about 5 m, while today it extends up to 8 m. An increase in the production speed and machine width also means an increase in the stretching rate, i.e. the rate 10 at which the film is oriented.
An increase in the stretching rate means greater mechanical and thermal stresseson the thermoplastic during orientation. In particular during longitudinal stretching, where the stretching of the film is carried out in a very short distance between two 15 rolls rotating at different speeds, the increase in the stretching rate can result in an impairment in the film quality.
During longitudinal stretching, the film web is first passed over a plurality of heated rolls which bring the material to the temperature necessary for stretching. These 20 rolls are driven at a low peripheral speed. The film then reaches one or more chill rolls, which are driven at a higher peripheral speed than the heating rolls. Thedifferent speeds of the heated and chill rolls produces longitudinal stretching of the film.
DE-B 1 221 786 describes an apparatus for stretching a thermoplastic film web inthe longitudinal direction. In this apparatus, two rolls which press the edges of the film web against a support are provided after, regarded in the stretching direction, the region warmed by the heating device. These undriven rolls have the task of countering any reduction in the film web width during the longitudinal stretching. The stretching force in this process is applied by the rolls to one side of the film.
DE-B 1 212 290 describes an apparatus for longitudinal stretching of a material web in which the material web is heated to the stretching temperature by heating rolls in - 2i92917 the longitudinal direction and subjected in the stretching region to a tensile force acting in the longitudinal direction. The transport devices for the material web are ar!anged in such a way that they keep the tensile force away from the part of the material web Iying on the heating roll. In an expedient embodiment of the invention, 5 the transport devices are formed by a transport roll driven independently of the heating roll and by an undriven counterpressure roll. The process described has the disadvantage that the tensile force is again only applied to one side of the film.
A particular embodiment of the device of DE-B 1 212 290 is described in German Patent 1 504 058. The device of DE-B 1 212 290 is refined in such a way that theoptimum length of the stretching zone can be set precisely depending on the typeand thickness of the material web, the operating speed and the desired stretching ratio. This is achieved in accordance with the invention in that, in order to change the length of the stretching zone, the pair of transport and/or the pair of tension rolls 15 is/are pivotable about the axis of the heating roll or of the chill roll. This process again has the disadvantage of application of force on one side and an unfavorable arrangement of the chill rolls 8 and 9.
German Patent 1 919 299 claims a process for the longitudinal stretching of a 20 plastic film in which longitudinal tension is kept away from the final heating roll in a particularly simple manner. The object is achieved in accordance with the invention in that a temperature drop of from 2 to 25C, depending on the film material, isproduced between the temperature of the final heating roll and the temperature of the first stretching roll. This process again has the disadvantage that the stretching 25 force is in each case applied to only one film surface, albeit in an alternating manner. In addition, the unfavorable arrangement of the rolls 7 and 8 means that the risk of air bubbles between the film and the rolls 7 and 8 is particularly great.
DE-B 2 833 189 describes a process for the longitudinal stretching of an at least 30 two-layer plastic film, in which the layer of higher melting point is oriented and the layer of lower melting point is heat-sealable. Before reaching the stretching rolls, the film is first warmed to a temperature which is both above the stretching temperature of the higher-melting layer and below the adhesive temperature of the lower-melting layer. The lower-melting layer is then shock-cooled within a short time interval to a temperature significantly below the adhesive temperature, while the higher-melting layer remains at the stretching temperature. This procedure prevents adhesion ofthe film sur~aces to the stretching rolls. This process again has the disadvantage that the force for longitudinal stretching of the film is only applied to one side of the films.
The known BOPP production processes have the disadvantage of r.a Ionger giving high performance if the production speeds are increased. At increased productionspeeds, these processes give films having an uneven thickness profile, impaired heat-sealing properties and a worse film appearance.
The object of the present invention was therefore to provide a process for the longitudinal stretching of a thermoplastic film by means of which a film having good heat-sealability, good appearance and a good thickness profile is provided at the production speeds customary today.
The object is achieved in accordance with the invention by a process for the longitudinal stretching of an at least single-layer thermoplastic film. In the novel process (the reference numbers below refer to Fig. 1), before the longitudinal stretching, the film is warmed in the slow-running part of the stretching machine to a temperature suitable for stretching and fed to a stretching zone (10). In the process, a) the slow-running part of the stretching machine contains the driven roll (1), and the fast-running part of the stretching machine contains the driven roll pair (2)/(3), the roll pair (2)/(3) being arranged in such a way that a roll nip (4) is formed, and b) the film (9) is passed into the roll nip (4) in such a way that it is grippedsimultaneously or virtually simultaneously by the roll pair (2)/(3) at the contact points (5)/(6), and the film (9) is stretched between the roll (1 ) and the rollpair (2)/(3).
219~7 In accordance with the invention, the roll pair (2)/(3) is arranged in such a way that the film is gripped virtually simultaneously at the contact points (5)/(6), causing the stretching force in the fast-running part of the stretching machine to be applied to both film surfaces 7 and 8. This produces better tension distribution within the film 9, 5 in particular within the layers of the film 9 close to the surface, compared with application of the stretching force on one side. Surprisingly, the fact that the rolls (2) and (3) are driven and their novel arrangement, i.e. application of the str~Lching force to both sides, means that the properties of the film are significantly better. This is particularly true of the optical properties and the heat-sealing properties of the 1 0 film.
It has been found that, in the longitudinal stretching processes of the prior art (cf.
Figure 2), the entire stretching force is taken up by only one film surface (Figure 2, film surface 7), i.e. the force is applied to one side, which can result in a critical 15 stress value in/on the film being reached or exceeded in the layer of the film close to the surface. The stress peaks of this type can result in damage to the surface.
The fact that the film thickness in the fast-running part of the stretching machine is less by the stretching ratio f (f = v2/v1 ) means that a critical stress value initially 20 occurs in the fast-running part of the longitudinal stretching machine. The stresses within the film in the slow-running part of the stretching machine are significantly smaller owing to the greater thickness, so that the risk of surface damage to the film is significantly less there. In accordance with the invention, application of force to both sides by the rolls 2 and 3 in the fast-running part of the stretching machine is 25 therefore sufficient to improve the appearance, thickness profile and heat-sealing properties.
In addition, the simultaneous contact of the film surfaces 7 and 8 with the rolls 2 and 3 at the points 5 and 6 ensures that the air dragged along by the film web 9 is 30 squeezed out at the contact points 5 and 6. The film is in contact with the rolls 2 and 3 over its entire surface, so that optimum force application is ensured between the rolls 2 and 3 and the film surfaces 7 and 8. If, by contrast, the roll 3 is positioned as shown in Figure 3, air is dragged in between the film 9 and the roll 2, forming an air 2 ! 9291 7 cushion which reduces the contact area for force application. The film 9 is no longer flat on the roll 2, causing the partial interruption in force application between the film 9 and the roll 2. This results in film sections being accelerated to different extents in the stretching direction in this region of the roll, which causes width and thickness differences in the film 9. In production of the film by the novel process as shown in Fig. 1, the inclusion of air is prevented by the film being gripped virtually simultaneously at the contact points 5 and 6 by the rolls 2 and 3. It has been found that the thickness profile of the film is consequently significantly better than in an arrangement as shown in Figure 2 or 3.
The invention is explained in greater detail below with reference to drawings.
Figure 1 is a diagrammatic view of the embodiment according to the invention.
Figure 1 A shows a detailed view on the area I A of Figure 1.
Figures 2 and 3 show illustrative embodiments of longitudinal stretching devicesaccording to the prior art and non-inventive embodiments.
Figure 4 A and B shows an embodiment according to the process wherein roll 3 is driven with the aid of roll 2.
Figures 5 and 6 show further embodiments of the process according to the invention.
Figure 7 shows an instrument used to conduct a hot tack test.
The 'ongitudinal stretching machine shown in Figure 1 comprises, in accordance with the invention, the three driven rolls 1, 2 and 3 by means of which the film 9 is stretched by the longitudinal stretching ratio f. The speeds of the rolls are v1, v2 and V3. The longitudinal stretching ratio f is given, for frictional contact between the rolls 1, 2 and 3 and the film 9, approximately by the ratio of the speeds v2 and v1 f the rolls 2 and 1. The longitudinal stretching ratio f for PP films is usually in the range from 4 to 9, preferably in the range from 4.5 to 8Ø When the stretching zone 10 is reached, the film 9 has achieved a temperature Ts at which it can be stretched by the particular stretching ratio. The length of the stretching zone is generally from 50 to 600 mm, preferably from 50 to 500 mm, in particular from 50 to 400 mm.
Depending on the raw material, film thickness, stretching rate and stretching ratio, the temperature Ts is preferably between 80 and 160C. The stretching force Es is 5 applied to the film 9 by means of friction between the driven roll 1 and the film surface 7 and between the driven roll pair 2/3 and the film surfaces 7 and 8. The speeds v2 and V3 of the roll pair 2/3 are generally the same, but can also differ slightly from one another by a maximum of 5%. The rolls 2 and 3 can be driven separately, although joint driving of the rolls 2 and 3 is also possible. Figure 4 10 shows diagrammatically an example of a joint drive of the roll pair 2/3. To this end, the roll 3 is expediently designed in such a way that the two rolls 2 and 3 are in contact in the edge region 13. The film 9 lies flat on the two roll surfaces in the recess 14 and is transported by friction forces. If only one film thickness is produced on the machine, the roll pair 2/3 can be made of a hard material, for example steel 15 with a chrome-plated or ceramic surface. Somewhat greater flexibility with respect to the film thickness is achieved if, for example, the roll 3 is rubber-covered.
Application force is particularly good if the rubber has a Shore A hardness of between about 50 and 100. In the production of greatly different film thicknesses, the roll 3 must be replaced, which is expediently carried out when re-setting the 20 thickness. The two-sided frictional contact between the roll pairs 2 and 3 and the film surfaces 7 and 8 means that the film surface 7 is subjected to significantly lower stresses than in the case of frictional contact on one side, i.e. in arrangements with only one driven roll in the fast-running part of the stretching machine. In the invention, the two-sided frictional contact is achieved in the fast-running part of the 25 stretching machine since the stresses acting on the film are greatef there than in the slow-running part of the stretching machine owing to the lower film thickness. With regard to achieving a critical stress value, the slow-running part of the stretching machine is insignificant.
30 In spite of force application on both sides by the novel process, the stretching force is not necessarily applied equally by the rolls 2 and 3. The further looping of film 9 around roll 2 means that the distribution of the stretching force onto rolls 2 and 3 is not equal. Experiments have shown that the stretching force in the stretching arrangement shown in Fig. 1 (film - roll loop angle 50 to 180) can be divided between rolls 3 and 2 in a ratio of up to 40:60. In spite of the unequal distribution of the force application, the stress in the film is lower in the novel process than in one-sided stretching force application.
A further feature of the invention is that the roll pair 2/3 is arranged in such a way that the air dragged along by the film 9 is squeezed out at the contact points 5 and 6 of the roll pair 2/3. To this end, the film surfaces 7 and 8 are gripped simultaneously by the roll pair 2/3 at the contact points 5 and 6 and transported further by friction.
10 This ensures that the film 9 lies flat on the rolls 2 and 3 without air being dragged in and air cushions forming between the film 9 and the roll surfaces. The contact points should not be more than 50mm, preferably not more than 40 mm, in particular not more than 20mm, measured in the film web direction, apart from one another. The risk of air inclusion increases with machine speed, since the amount of air dragged 15 by the film 9 increases approximately proportionally with the film web speed.
It is observed that positioning of the rolls 2 and 3 as shown in Figure 3 causes the film 9 to run roughly as far as the contact point 5 on the roll 2. The film web width is unstable, i.e. it is subject to variations. The air included between the film 9 and the 20 roll surface escapes suddenly after a critical pressure has built up in the air cushion.
The rough, unstable film running results from local different accelerations of the film web. Thick/thin areas are initiated in the film 9, resulting in a worse thickness profile of the film 9.
25 Figure 5 shows a further expedient embodiment of the invention. The stretching machine comprises in accordance with the invention the three driven rolls 1, 2 and 3 and the novel arrangement of the roll pair 2/3. The slow-running part of the stretching machine additionally comprises a nip roll 15, which serves further toimprove the frictional contact in the slow-running part of the stretching machine. The 30 nip roll can be driven or undriven. For the outlined reasons, the nip roll isexpediently arranged at the contact point of the film 9 with the roll 1, so that, analogously to the roll pair 2 and 3, the film comes into contact simultaneously or virtually simultaneously with roll 1 and the nip roll. The looping of the film 9 around the rolls 1 and 2 and the diameter of the roll 3 are advantageously selected so that the shortest possible stretching zone 10 can be achieved (S-shaped loop).
This is important, for example, for the production of films which are sensitive to splitting or which can only be stretched at a low longitudinal stretching ratio.
Figure 6 shows a further embodiment of the invention in which longitudinal stretching is divided over a plurality of stretching zones 10 to 12. In the embodiment shown, the rolls 16 to 19 are likewise driven. By means of the selected arrangement of the rolls, the individual stretching ratios can be matched in a targeted manner to 10 the raw material, and the force application and the associated stresses on the two film surfaces can be divided equally.
The longitudinal stretching in accordance with the invention is particularly advantageously used in the production of films by the stenter process, which is 15 known per se. It is advantageously suitable both for the production of films which are only monoaxially oriented (only in the longitudinal direction), and for the production of biaxially oriented films.
In the stenter process, the melts corresponding to the individual layers of the film 20 are extruded or coextruded through a flat-film die, the resultant melt film is taken off on one or more roll(s) for solidification, the film is subsequently stretched and heat-set and, if desired, flame- or corona-treated.
If desired, the film can be biaxially oriented in the longitudinal and transverse 25 directions. The biaxial stretching (orientation) is carried out successively, it being preferred to carry out the stretching by the novel process first in the longitudinal direction of the machine and then in the transverse direction of the machine.
Firstly, as usual in the extrusion process, the polymers or the polymer mixtures of 30 the individual layers are compressed and liquefied in extruders, it being possible for the additives already to be present in the polymers or in the polymer mixtures. The melts are then forced jointly and simultaneously through a flat-film die, and the extruded, single- or multilayer melt film is taken off on one or more take-off rolls, where it is cooled and solidified. The resultant film is then stretched longitudinally and preferably also transversely to the extrusion direction, which results in orientation of the molecule chains. The longitudinal stretching is carried out by the novel process described, and the transverse stretching is carried out with the aid of 5 an appropriate tenter frame. The longitudinal stretching ratios are in the range from 4 to 9, preferably from 4.5 to 8.0, and the transverse stretching ratios are in the range from 7 to 12, preferably from 8 to 11.
The stretching of the film is followed by heat-setting (heat treatment), during which the film is kept at a temperature of from 80 to 160C for from about 0.1 to 10 seconds. This can be followed by printing pretreatment, for example by means of a flame or electrical corona process. The treatment intensity is generally in the range from 36 to 50 mN/m, preferably from 38 to 45 mN/m. The film produced in this way is wound up in a conventional manner using a wind-up device.
It has proven particularly favorable to keep the take-off roll or rolls by means of which the extruded melt film is cooled and solidified at a temperature of from 10 to 120C, preferably from 20 to 100C, by means of a heating and cooling circuit. The temperatures at which the longitudinal and transverse stretching are carried out can 20 vary within a relatively broad range and depend on the particular composition of the individual layers and on the desired properties of the film. In general, the longitudinal stretching is preferably carried out at from 80 to 160C and the transverse stretching is preferably carried out at from 120 to 1 70C.
25 The film produced by the novel process can have a single-layer or multilayer structure. It can be non-heat-sealable or heat-sealable. In a particular embodiment, the film has outer layers on both sides of its base layer. In a further embodiment, the film has at least one interlayer, if desired on both sides of its base layer.
30 The base layer of the film generally contains at least 85% by weight, preferably from 90 to 99% by weight, in each case based on the base layer, of a thermoplastic polymer described below and, if desired, additives in effective amounts in each case.
Suitable thermoplastic polymers are polymers of olefins having 2 to 10 carbon atoms, preferably polypropylenes, polyethylenes and/or polybutylenes.
Thcrmoplastic polymers can also be polyethylene terephthalates, polybutylene terephthalates and other polyester derivatives. The novel process is preferably 5 suitable for the production of films having a polypropylene base layer. Of propylene-containing polymers, particular preference is given to propylene homopolymers.
Suitable propylene polymers have a melting point of from 120 to 1 65C, preferably from 140 to 162C, and a melt flow index (measurement in accordance with DIN 53 735 at 21.6 N and at 230C) of from 1.0 to 10 9/10 min, preferably from 1.5 to 6 9/10 min. Propylene polymers contain at least 80% by weight, preferably from 90 to 100%
by weight, in particular from 95 to 100% by weight, of propylene units. The n-heptane-soluble content of the propylene polymer is generally from 1 to 10% by weight, based on the starting polymer. Particular preference is given to propylene homopolymers whose n-heptane-insoluble content is isotactic. The chain isotacticity 15 index, determined by 13C-NMR spectroscopy, is greater than 85%, preferably greater than 90%. The molecular weight distribution of the propylene homopolymercan vary within broad limits, depending on its area of application. The ratio between the weight average molecular weight Mw and the number average molecular weight Mn is generally between 2 and 15.
If the film is opaque, the base layer additionally contains vacuole-initiating fillers, for example CaC03 or incompatible polymers and/or pigments, as described in the prior art. In addition, the base layer may additionally contain conventional additives, such as antistatics, antiblocking agents, lubricants, stabilizers, neutralizers, pigments 25 and/or nucleating agents in effective amounts in each case.
If an electroinsulation film (capacitor foil) is produced by the novel process, the raw material used must have high purity. A high-purity polypropylene of this type must have a low ash and chlorine content compared with the packaging raw material and30 must have the lowest possible content of ionogenic constituents. Electroinsulation film is therefore usually not provided with antistatics and lubricants. The chlorine content of the high-purity polypropylene is less than 50 ppm, and the residual ash content less than 70 ppm.
- 2 1 ~29 1 7 A preferred embodiment of the oriented polypropylene film produced by the novel process comprises at least one outer layer, preferably on both sides, of olefinshaving 2 to 10 carbon atoms. Interlayers of olefinic polymers may additionally be applied.
Examples of olefinic polymers of this type for the outer layer and/or interlayer are a propylene homopolymer a copolym~?r of ethylene and propylene or ethylene and 1-butylene or propylene and 1-butylene, or a terpolymer of ethylene and propylene and 1-butylene, or a mixture or blend of two or more of said homopolymers, copolymers and terpolymers, particular preference being given to propylene homopolymers or random ethylene-propylene copolymers having an ethylene content of from 1 to 10% by weight, preferably from 2.5 to 8% by weight, or random propylene-1-butylene copolymers having a butylene content of from 2 to 25% by weight, preferably from 4 to 20% by weight, in each case based on the total weight of the copolymer, or random ethylene-propylene- 1-butylene terpolymers having an ethylene content of from 1 to 10% by weight, preferably from 2 to 6% by weight, and a 1-butylene content of from 2 to 30% by weight, preferably from 4 to 20% by weight, in each case based on the total weight of the terpolymer, or a blend of an ethylene-propylene-1-butylene terpolymer and a propylene-1-butylene copolymer having an ethylene content of from 0.1 to 7% by weight and a propylene content of from 50 to 90% by weight and a 1-butylene content of from 10 to 40% by weight, in each case based on the total weight of the polymer blend.
The propylene homopolymer employed in the outer layer and/or interlayer containsfrom at least 98 to 100% by weight of propylene and has a melting point of 140C or above, preferably 150 to 170C, preference being given to isotactic homopolypropylene. The homopolymer generally has a melt flow index of from 1.5 to 20 9/10 min, preferably from 2.0 to 15 g/10 min.
2~92917 The above-described copolymers and terpolymers employed in the outer layer and/or interlayer generally have a melt flow index of from 1.5 to 30 g/10 min, preferably from 3 to 15 g/10 min. The ,nelting point is in the range from 120 to140C. The above-described blend of copolymers and terpolymers has a melt flow index of from 5 to 9 g/10 min and a melting point of from 120 to 150C. All the melt flow indices given above are measured at 230C and a force of 21.6 N (DIN 53 735).
If desired, the outer layers and/or interlayers can likewise contain antistatics, antiblocking agents, lubricants, neutralizers, stabilizers and hydrocarbon resins.
The overall thickness of the polypropylene film produced by the novel process can vary within broad limits and depends on the intended application. It is preferably from 2 to 120 ,um, in particular from 2.5 to 100 ,um, especially from 2.5 to 80 ,um, the base layer making up from about 40 to 100% of the overall film thickness.
The thickness of the outer layers is greater than 0.1 ,um and is preferably in the range from 0.2 to 5 ,um, in particular from 0.3 to 1.5 ,um, it being possible for the outer layers to have identical or different thicknesses. The thickness of any interlayer(s) present is greater than 0.1 ,um and is preferably in the range from 0.5 to 15 ,um, in particular from 0.7 to 10 ,um.
The film produced by the novel process is particularly suitable for processing on high-speed machines, for example for packaging, lamination or metallization. It has all the important properties required of oriented polypropylene films with respect to 25 their different applications. In particular, it has good heat-sealability, a good appearance anc an excellent thickness profile.
The following measurement methods were used to characterize the raw materials and the films:
Melt flow index The melt flow index was measured in accordance with DIN 53 735 at a load of 1.6 N
and at 230C.
- 2!92917 Melting point DSC measurement, maximum of the melting curve1 heating rate 20C/min.
Molecular weight determination 5 The mean molecular weight is determined by three-detector gel permeation chromatography. The substance is dissolved in an eluent, such as THF1 and applied to a separating colun ,n. The separating column has a length of 90 cm and is filled with a porous support material whose pore size is 5 ,um. Detection is by UV
absorption spectroscopy at various wavelengths and by means of the refractive 10 index and light scattering capacity of the fractions. The calibration is carried out by means of a standard compound of known molecular weight. Comparison of the UV
absorption of the standard substance with the absorption of the sample enables assignment of the molecular weights.
15 Determination of the minimum sealing temperature (MST) The minimum sealing temperature is determined by the peel method. Heat-sealed samples (seal seam 20 mm x 150 mm) are produced using a Brugger HSG/ET
sealing machine by sealing a film at various temperatures with the aid of two heated sealing jaws at a sealing pressure of 15 N/cm2 for 0.5 second. Test strips with a 20 width of 15 mm are cut out of the sealed samples. The T-seal seam strength1 i.e. the force necessary to separate the test strips1 is determined using a tensile tester at a peel rate of 200 mm/min, the seal seam plane forming a right angle with the direction of tension. The minimum sealing temperature is the temperature at which a seal seam strength of 0.5 N/15 mm is achieved.
25 In addition1 the tear strength is measured at a sealing temperature of 130C -likewise by the peel method.
Hot tack Hot tack denotes the strength of a still-hot seal seam immediately after opening of 30 the sealing jaws. In the test used1 the separation in mm experienced by the heat-seal seam at a load of 1 N (seal seam width 30 mm) is measured. The test is carried out at a sealing temperature of 1 50C. The measurement is a Hoechst internal standard. There are no corresponding DIN and ASTM standards. The test 21 92~17 instrument used is the heat contact sealing unit with grooved sealing jaws (20, 21) and deflection rolls (R1, R2, R3) for a separation angle of 180 (cf. Figure 7). The sealing time is 0.5 second and the sealing pressure is 30 N/cm 2 In the measurement process, the measurement strips (22) having a width of 30 mm are laid one on top of the other and fixed at the ends with a weight of 1 N. A flat spatula (23) is inserted between the film layers, and the measurement strips are fed around the deflection rolls (R1, R2, R3) over the sealing jaws and clamped such that the flat spatula will be positioned between the grooved sealing jaws. After completion of the sealing, the jaws open automatically. The sealed measurement strip is jerked forward to the deflection roll by the weight of 1 N (G), where the seal resulting from the sealing jaws (21) is separated at a separation angle of 180. The depth of the lamination of the seal seam in mm is assessed. The greater the numerical value, the worse the hot tack. The sealing of sealing jaws (20) remains unaffected during determination of the hot tack.
Gloss The gloss was determined in accordance with DIN 67 530. The reflectometer value was measured as a characteristic optical parameter for the surface of the film. In accordance with the ASTM-D 523-78 and ISO 2813 standards, the angle of incidence was set at 20 or 60. A light beam hits the planar test surface at the set angle of incidence and is reflected or scattered thereby. The light beams hitting the photoelectronic receiver are indicated as proportional electrical quantities. The measurement value is dimensionless and must be indicated together with the angleof incidence.
Hase The haze of the film was measured in accordance with ASTM-D 1003-52.
Assessment of the thickness profile The thickness profile was measured on-line by traversing a measurement head overthe film web width (F+H or FAG measuring instrument). The measurement head contains a beta-emitter which measures the absorption in the film and converts the value into a corresponding thickness value. The thickness values are measured over the film width and plotted as a graph. The thickness profile is assessed using the so-called R value. This is calculated from the ratio between the difference of maximum thickness value and minimum thickness value and the mean thickness of 5 the film.
maximum thickness - minimum thickness R value =
mean thickness The thickness profile is better the lower the R value.
Although the present invention has been described with preferred embodiments it is to be understood that modification may be resorted without departing from the spirit 15 and scope of this invention as those skilled in the art would readily understood.
The invention is described in greater detail now with reference to examples.
Example 1 20 A transparent, heat-sealable, three-layer film with a symmetrical structure and an overall thickness of 40 ,um was produced by coextrusion followed by stepwise orientation in the longitudinal and transverse directions. The outer layers each had a thickness of 0.6 ,um.
25 Base layer A:
99.70% by wt. of isotactic polypropylene from Solvay with the tradename Eltex 0.150% bywt. of N,N-bisethoxyalkylamine 0.150% by wt. of erucamide Outer layers B:
99.67% by wt. of random ethylene-propylene copolymer having a C2 content of 4.5% by weight 0.33% by wt. of SiO2 as antiblocking agent having a mean particle size of 4 ,um.
The production conditions in the individual process steps were as follows:
Extrusion: Temperatures: Layer A 280C
Layers B 280C
Temperature of the take-off roll 30C
Longidutinal stretching: Temperature 130C
Longitudinal stretching ratio 5 Transverse stretching: Temperature 160C
Transverse stretching ratio 10 Heat-setting: Temperature 1 40C
Convergence 15 %
Film web speed 230 m/min The film was produced by the novel process as shown in Figure 1. The properties of the films from the examples and comparative examples are shown in the table 20 below.
Example 2 A film was produced by the novel process analogously to Example 1.
25 Compared with Example 1, the random copolymer in the outer layers was replaced by a propylene homopolymer (Eltex PHP 405). The film is not heat-sealable.
The process parameters were the same. The film is distinguished by very good optical properties and a very good thickness profile.
30 Example 3 A film is produced by the novel process analogously to Example 1.
Compared with Example 1, a single-layer film was produced for electrical insulation.
The polymer ,'or this film was a high-purity isotactic propylene homopolymer from Borealis with the tradename Borealis VB 2142 E, Melt flow index = 2.2 9/ 10 min.The thickness of the film was 3.5 ~m. The process conditions during production of the film which had changed compared with Example 1 were as follows:
Extrusion: Temperature: 270C
Temperature of the take-off roil 90C
Long;tudinal stretching: Temperature: 150C
Longitudinal stretching ratio: 5.5 The high take-off roll temperature of 90C and the special longitudinal stretching temperature gave a film having a rough surface, as described, for example, in EP-A
0 497 160. The film is distinguished by an excellent thickness profile.
15 Comparative Example 1 Compared with Example 1, the longitudinal stretching of the film was now carried out as shown in Figure 1, but without driving of the roll 3, i.e. only roll 2 was driven. The hot tack and the optical properties of the film are significantly worse, and thethickness profile of the film has become worse.
Comparative Example 2 Compared with Example 1, the longitudinal stretching of the film was now carried out as shown in Figure 3, where the contact points are more than 100 mm apart and the film is no longer gripped simultaneously by the driven rolls 2 and 3. The hot tack and 25 optical properties of the film are worse, and the thickness profile of the film has become significantly worse.
Comparative Example 3 Compared with Example 2, the longitudinal stretching of the film was now carried out 30 as shown in Figure 1, but without driving of the roll 3, i.e. only roll 2 was driven. The optical properties of the film are significantly worse, and the thickness profile has become worse.
Comparative Example 4 Compared with Example 2, the longitudinal stretching of the film was now carried out as shown in Figure 3, where the contact points are more than 100 mm apart. The optical properties of the film are worse, and the thickness profile has become 5 significantly worse.
Comparative Example 5 Compared with Example 3, the longitudinal stretching of the film was now carried out as shown in Figure 1, but without driving of roll 3, i.e. only roll 2 was driven. The 10 thickness profile of the film has become worse.
Comparative Example 6 Compared with Example 3, the longitudinal stretching of the film was now carried out as shown in Figure 3, where the contact points are more than 100 mm apart. The 15 thickness profile of the film has become significantly worse.
Claims (23)
1. A process for the longitudinal stretching of an at least single-layer thermoplastic film, which, before stretching, is warmed in the slow-running partof the stretching machine to a temperature suitable for stretching and fed to a stretching zone (10), wherein a) the slow-running part of the stretching machine comprises the driven roll (1), and the fast-running part of the stretching device comprises the driven roll pair (2)/(3), the roll pair (2)/(3) being arranged in such a way that a roll nip (4) is formed, and b) the film (9) is passed into the roll nip (4) in such a way that it is grippedsimultaneously or about simultaneously by the roll pair (2)/(3) at the contact points (5)/(6), and the film (9) is stretched between the roll (1) and the roll pair (2)/(3).
2. The process as claimed in claim 1, wherein, in the fast-running part of the stretching machine, the film (9) is looped further around either the roll (2) orthe roll (3).
3. The process as claimed in claim 1 or 2, wherein, in the fast-running part of the stretching device, at least 10%, preferably at least 20%, in particular at least30%, of the stretching force is applied to one of the film surfaces (7) and (8) by the roll around which the film (9) is not looped further.
4. The process as claimed in any of claims 1 to 3, wherein, in the fast-running part of the stretching machine, the air dragged along by the film is squeezed out at the contact points (5) and (6).
5. The process as claimed in any of claims 1 to 4, wherein the stretching force is applied to the film (9) by means of friction by the roll (1) and the roll pair (2)/(3).
6. The process as claimed in any of claims 1 to 5, wherein the speeds of the roll pair (2)/(3) do not differ from one another by more than 5%.
7. The process as claimed in any of claims 1 to 6, wherein the contact points (5) and (6) are not more than 50 mm, preferably not more than 40 mm, in particular not more than 20 mm, measured in the film web direction, apart from one another.
8. The process as claimed in any of claims 1 to 7, wherein the length of the stretching zone (10) is from 50 to 600 mm, preferably from 50 to 500 mm, in particular from 50 to 400 mm.
9. The process as claimed in any of claims 1 to 8, wherein the film temperature T s in the stretching zone (10) is from 80 to 160°C.
10. The process as claimed in any of claims 1 to 9, wherein one of the two rolls (2) and (3) has a rubber covering.
11. The process as claimed in any of claims 1 to 10, wherein the slow-running part of the stretching machine contains a non-driven or driven roll (15).
12. The process as claimed in any of claims 1 to 11, wherein the roll (3) is driven by the roll (2).
13. The process as claimed in any of claims 1 to 12, wherein the film (9) is looped in an S-shape around the rolls (1) and (2).
14. The process as claimed in any of claims 1 to 13, wherein the stretching is divided into a plurality of individual stretching operations.
15. The process as claimed in any of claims 1 to 14, wherein a biaxially oriented polypropylene (BOPP) film is produced.
16. The process as claimed in any of claims 1 to 15, wherein the BOPP film produced by the process has a single-layer structure.
17. The process as claimed in any of claims 1 to 15, wherein the BOPP film produced by the process has a multilayer structure.
18. The process as claimed in any of claims 1 to 15, wherein the BOPP film produced by the process has a multilayer structure and is not heat-sealable.
19. The process as claimed in any of claims 1 to 15, wherein the BOPP film produced by the process has a multilayer structure and is heat-sealable.
20. The process as claimed in any of claims 1 to 15, wherein the BOPP film produced by the process has a thickness of from 2 to 120 ,?m, preferably from 2.5 to 120 ,?m, in particular from 3 to 120 ,?m.
21. The process as claimed in any of claims 1 to 4, wherein a polyethylene terephthalate or polybutylene terephthalate film is produced.
22. An apparatus for the longitudinal stretching of thermoplastic films, comprising a) at least one driven roll (1) which is driven at speed V 1, and b) at least two driven rolls (2) and (3) driven at speeds V 2 and V 3, where the speeds V 2 and V 3 differ from one another by a maximum of 5%
(based on the faster-running roll) and speed V 1 is lower than the speeds V 2 and V 3, and the rolls (2) and (3) are arranged after the roll 1 in such a way that, during longitudinal stretching of the film by means of the apparatus, the film first comes into contact with roll 1 and then into contact with rolls (2)/(3), and rolls (2) and (3) are arranged with respect to one another in such a way that, during longitudinal stretching of the film by means of the apparatus, the film comes into contact virtually simultaneously with the rolls (2) and (3).
(based on the faster-running roll) and speed V 1 is lower than the speeds V 2 and V 3, and the rolls (2) and (3) are arranged after the roll 1 in such a way that, during longitudinal stretching of the film by means of the apparatus, the film first comes into contact with roll 1 and then into contact with rolls (2)/(3), and rolls (2) and (3) are arranged with respect to one another in such a way that, during longitudinal stretching of the film by means of the apparatus, the film comes into contact virtually simultaneously with the rolls (2) and (3).
23 23. A process for the longitudinal stretching of thermoplastic films, wherein part of the stretching force necessary for orientation of the film is applied via both film surfaces of the film.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE1995146671 DE19546671A1 (en) | 1995-12-15 | 1995-12-15 | Process for longitudinal stretching in the production of oriented polypropylene films |
| DE19546671.3 | 1995-12-15 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2192917A1 true CA2192917A1 (en) | 1997-06-16 |
Family
ID=7780106
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2192917 Abandoned CA2192917A1 (en) | 1995-12-15 | 1996-12-13 | Process for longitudinal stretching in the production of oriented polypropylene films |
Country Status (5)
| Country | Link |
|---|---|
| EP (1) | EP0779144A3 (en) |
| AU (1) | AU720595B2 (en) |
| CA (1) | CA2192917A1 (en) |
| DE (1) | DE19546671A1 (en) |
| ZA (1) | ZA9610534B (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE102019124865A1 (en) * | 2019-09-16 | 2021-03-18 | Brückner Maschinenbau GmbH & Co. KG | Pressure roller arrangement, in particular for a longitudinal stretching system and associated stretching system and method for operating such a pressure roller arrangement |
| CN114536726B (en) * | 2022-02-24 | 2023-11-07 | 宁波长阳科技股份有限公司 | Film longitudinal stretching device and film longitudinal stretching method |
Family Cites Families (13)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US323029A (en) * | 1885-07-28 | William m | ||
| GB890005A (en) * | 1959-06-23 | 1962-02-21 | Du Pont | Improvements in or relating to the stretching of organic polymer film |
| DE1221786B (en) | 1961-12-07 | 1966-07-28 | Brueckner Trocknerbau Inh Gern | Device for stretching a sheet of thermoplastic material in the longitudinal direction |
| DE1212290B (en) | 1963-10-15 | 1966-03-10 | Brueckner Trocknerbau | Device for stretching a stretchable material web |
| DE1919299B2 (en) | 1969-04-16 | 1977-02-24 | Brückner-Maschinenbau Gernot Brückner, 8221 Siegsdorf | PROCESS FOR LONGITUDINAL STRETCHING A PLASTIC FILM |
| US3619460A (en) * | 1969-04-28 | 1971-11-09 | Chevron Res | Process for uniaxially orienting polypropylene films |
| US3786127A (en) * | 1969-07-23 | 1974-01-15 | Du Pont | Stretching polyethylene terephthalate film in a shortened span |
| CA939479A (en) * | 1969-07-23 | 1974-01-08 | Robert G. Peet | Method of and apparatus for stretching polymeric film |
| DE1957578A1 (en) * | 1969-11-15 | 1971-05-19 | Dornier Gmbh Lindauer | Longitudinal stretching of plastics |
| US3734994A (en) * | 1971-02-09 | 1973-05-22 | Du Pont | Two-stage uniaxial orientation of polyethylene terephthalate films |
| GB1419972A (en) * | 1972-11-23 | 1975-12-31 | Mccall J D | Method and apparatus for stretching plastic material |
| DE2833189C2 (en) | 1978-07-28 | 1984-09-20 | Brückner - Maschinenbau Gernot Brückner GmbH & Co KG, 8221 Siegsdorf | Method for longitudinally stretching an at least two-layer thermoplastic plastic film and device for carrying out the method |
| CA1171225A (en) * | 1982-02-23 | 1984-07-24 | Brian L. Hetherington | Machine direction orientation of nylon film |
-
1995
- 1995-12-15 DE DE1995146671 patent/DE19546671A1/en not_active Withdrawn
-
1996
- 1996-12-03 EP EP96119326A patent/EP0779144A3/en not_active Withdrawn
- 1996-12-10 AU AU74272/96A patent/AU720595B2/en not_active Ceased
- 1996-12-13 CA CA 2192917 patent/CA2192917A1/en not_active Abandoned
- 1996-12-13 ZA ZA9610534A patent/ZA9610534B/en unknown
Also Published As
| Publication number | Publication date |
|---|---|
| MX9606449A (en) | 1997-10-31 |
| EP0779144A3 (en) | 1998-01-07 |
| EP0779144A2 (en) | 1997-06-18 |
| AU7427296A (en) | 1997-06-26 |
| AU720595B2 (en) | 2000-06-08 |
| DE19546671A1 (en) | 1997-06-19 |
| ZA9610534B (en) | 1997-06-17 |
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