AU720595B2 - Process for longitudinal stretching in the production of oriented polypropylene films - Google Patents

Description

Description 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 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 polyprQpylene films (BOPP films). In the stenter process, the BOPP film is produced 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 Q 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 i 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- 0 set. The film may subsequently be surface-treated by flame or corona. The film is wound up and finished to give the customer-ready cut roll.

Biaxially oriented polypropylene films are distinguished by a very good property profile. Their characteristic properties are high mechanical strength, good heatsealability and a bright appearance. Owing to this good property profile and 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, 2 lamination and as electrical insulation in capacitor foils. Electrical foils generally have a thickness of less than 10 pm. The essential prerequisite for flaw-free processing of these very thin foils is a very good thickness profile.

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 m/min. 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 at which the film is oriented.

An increase in the stretching rate means greater mechanical and thermal stresses on 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 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. The different speeds of the heated and chill rolls produces longitudinal stretching of the S"film.

"o5 DE-B 1 221 786 describes an apparatus for stretching a thermoplastic film web in the 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 OT 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 3 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 arranged in such a way that they keep the tensile force away from the part of the material web lying on the heating roll. In an expedient embodiment of the invention, 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 the optimum length of the stretching zone can be set precisely depending on the type and 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 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 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 25°C, depending on the film material, is produced 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 force is in each case applied to only one film surface, albeit in an alternating o 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 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 cf 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 of the film surfaces 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 no longer giving high performance if the production speeds are increased. At increased production speeds, these processes give films having an uneven thickness profile, impaired heat-sealing properties and a worse film appearance.

Accordingly, in one aspect of the present invention there is provided 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.

In the novel process (the reference numbers below refer to Fig. 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 in the process, a) the slow-running part of the stretching machine contains the driven roll and the fast-running part of the stretching machine contains the driven roll pair the roll pair being arranged in such a way that a roll nip is formed, 'o 25 and b) the film is passed into the roll nip in such a way that it is gripped simultaneously or virtually simultaneously by the roll pair at the contact points and the film is stretched between the roll and the roll pair In accordance with the invention, the roll pair is arranged in such a way that the film is gripped virtually simultaneously at the contact points 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, 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 are driven and their novel arrangement, i.e. application of the stretching 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 film.

It has been found that, in the longitudinal stretching processes of the prior art (cf.

Figure the entire stretching force is taken up by only one film surface (Figure 2, film surface i.e. the force is applied to one side, which can result in a critical stress value in/on the film being reached or exceeded inthe 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 v 2 /vl) means that a critical stress value initially 20iO occurs in the fast-running part of the longitudinal stretching machine. The stresses o 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 therefore sufficient to improve the appearance, thickness profile and heat-sealing S 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 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 6 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 devices according 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.

The longitudinal 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 v 1 v 2 and v 3 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 v 2 and v 1 of 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 8.0. When the stretching zone 10 is reached, the film 9 has achieved a temperature Ts at which it can be stretched by 7 the particular stretching ratio. The length of the stretching zone is generally from 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 160°C. The stretching force F s is 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 v 2 and v 3 of the roll pair 2/3 are generally the same, but can a:;o differ slightly from one another by a maximum of The rolls 2 and 3 can be driven separately, although joint driving of the rolls 2 and 3 is also possible. Figure 4 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 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 o.:o 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 .O.l 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 o25 stretching machine since the stresses acting on the film are greater 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.

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 8 arrangement shown in Fig. 1 (film roll loop angle 50 to 1800) 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 onesided 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.

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 by the film 9 increases approximately proportionally with thz film web speed.

°e 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 roll surface escapes suddenly after a critical pressure has built up in the air cushion.

g 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 to improve the frictional contact in the slow-running part of the stretching machine. The nip roll can be driven or undriven. For the outlined reasons, the nip roll is expediently 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 9 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 the raw material, and the force application and the associated stresses on the two film surfaces can be divided equally.

The lornitudinal stretching in accordance with the invention is particularly advantageously used in the production of films by the stenter process, which is 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 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 heatset and, if desired, flame- or corona-treated.

If desired, the film can be biaxially oriented in the longitudinal and transverse directions. The biaxial stretching (orientation) is carried out successively, it being Spreferred 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.

I5i6 Firstly, as usual in the extrusion process, the polymers or the polymer mixtures of 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 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 160 0 C for from about 0.1 to 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 120*C, preferably from 20 to 100*C, by means of a heating and cooling circuit. The temperatures at which the longitudinal and transverse stretching are carried out can 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 160*C and the transverse stretching is preferably carried out at from 120 to 170*C.

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.

11 Suitable thermoplastic polymers are polymers of olefins having 2 to 10 carbon atoms, preferably polypropylenes, polyethylenes and/or polybutylenes.

Thermoplastic polymers can also be polyethylene terephthalates, polybutylene terephthalates and other polyester derivatives. The novel process is preferably suitable for the production of films having a polypropylene base layer. Of propylenecontaining polymers, particular preference is given to propylene homopolymers.

Suitable propylene polymers have a melting point of from 120 to 165C, preferably from 140 to 162°C, and a melt flow index (measurement in accordance with DIN 53 735 at 21.6 N and at 230°C) of from 1.0 to 10 g/10 min, preferably from 1.5 to 6 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 nheptane-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 index, determined by 13 C-NMR spectroscopy, is greater than 85%, preferably greater than 90%. The molecular weight distribution of the propylene homopolymer can vary within broad limits, depending on its area of application. The ratio between the weight average molecular weight M w and the number average molecular weight Mn is generally between 2 and If the film is opaque, the base layer additionally contains vacuole-initiating fillers, for example CaCO 3 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 :.:025 and/or nucleating agents in effective amounts in each case.

9* If an electroinsulation film (capacitor foil) is produced by the novel process, the raw S 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 and 80 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.

12 A preferred embodiment of the oriented polypropylene film produced by the novel process comprises at least one outer layer, preferably on both sides, of olefins having 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 copolymer 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 by weight, preferably from 2.5 to 8% by weight, or random propylene-1butylene 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 contains from at least 98 to 100% by weight of propylene and has a melting point of 140 0 C or 30 above, preferably 150 to 170*C, preference being given to isotactic homopolypropylene. The homopolymer generally has a melt flow index of from to 20 g/10 min, preferably from 2.0 to 15 g/10 min.

13 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 melting point is in the range from 120 to 140C. 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 150 0 C. All the melt flow indices given above are measured at 230 0 C 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 pm, in particular from 2.5 to 100 pm, especially from 2.5 to 80 pm, 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 pm and is preferably in the range from 0.2 to 5 pm, in particular from 0.3 to 1.5 pm, 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 pm and is preferably in the range from 0.5 to 15 pm, in particular from 0.7 to 10 pm.

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 their different applications. In particular, it has good heat-sealability, a good S appearance and 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 230°C.

Melting point DSC measurement, maximum of the melting curve, heating rate 2 0 °C/min.

Molecular weight determination The mean molecular weight is determined by three-detector gel permeation chromatography. The substance is dissolved in an eluent, such as THF, and applied to a separating column. The separating column has a length of 90 cm and is filled with a porous support material whose pore size is 5 pm. Detection is by UV absorption spectroscopy at various wavelengths and by means of the refractive 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.

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/cm 2 for 0.5 second. Test strips with a S',O width of 15 mm are cut out of the sealed samples. The T-seal seam strength, i.e. the force necessary to separate the test strips, 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.

In addition, the tear strength is measured at a sealing temperature of 130 0 C S.likewise by the peel method.

Hot tack Hot tack denotes the strength of a still-hot seal seam immediately after opening of Sthe sealing jaws. In the test used, the separation in mm experienced by the heatseal seam at a load of 1 N (seal seam width 30 mm) is measured. The test is carried out at a sealing temperature of 1500C. The measurement is a Hoechst internal standard. There are no corresponding DIN and ASTM standards. The test instrument used is the heat contact sealing unit with grooved sealing jaws (20, 21) and deflection rolls (R 1

R

2

R

3 for a separation angle of 1800 (cf. Figure 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 .Itrips are fed around the deflection rolls (R 1

R

2

R

3 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 where the seal resulting from the sealing jaws (21) is separated at a separation angle of 1800. 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 200 or 600. A light beam hits the planar test surface at the set e° 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 angle of 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 over the 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 16 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 the film.

R value= maximum thickness minimum thickness mean thickness 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 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 A transparent, heat-sealable, three-layer film with a symmetrical structure and an S overall thickness of 40 pm was produced by coextrusion followed by stepwise orientation in the longitudinal and transverse directions. The outer layers each had a thickness of 0.6 pm.

cc r r Base layer A: 99.70% by wt.

0.150% by wt.

S 0.150% by wt.

Outer layers B: 99.67% by wt.

of isotactic polypropylene from Solvay with the tradename Eltex PHP 405 of N,N-bisethoxyalkylamine of erucamide of random ethylene-propylene copolymer having a C 2 content of 4.5% by weight of SiO 2 as antiblocking agent having a mean particle size of 4 pm.

0.33% by wt.

The production conditions in the individual process steps were as follows: Extrusion: Longidutinal stretching: Temperatures: Layer A Layers B Temperature of the take-off roll Temperature Longitudinal stretching ratio Temperature Transverse stretching ratio Temperature Convergence 280°C 280°C 130°C 160°C 140 0

C

Transverse stretching: Heat-setting: Film web speed 230 m/min '20 r 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 below.

Example 2 A film was produced by the novel process analogously to Example 1.

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.

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 for this film was a high-purity isotactic propylene homopolymer from 30 Borealis with the tradename Borealis VB 2142 E, Melt flow index 2.2 g/ 10 min.

The thickness of the film was 3.5 pm. The process conditions during production of the film which had changed compared with Example 1 were as follows: Extrusion: Temperature: 270 0

C

Temperature of the take-off roll 90 0

C

Longitudinal stretching: Temperature: 150 0

C

Longitudinal stretching ratio: The high take-off roll temperature of 90 0 C 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.

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 the thickness profile of the film has become worse.

doo *0 Comparative Example 2 S 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 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 ~as shown in Figure 1, but without driving of the roll 3, i.e. only roll 2 was driven. The Soptical properties of the film are significantly worse, and the thickness profile has become worse.

-19- 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 significantly worse.

Comparative Example Compared with Example 3, the longitudinal stretching of the film was now carried out io as shown in Figure 1, but without driving of roll 3, i.e. only roll 2 was driven. The 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 thickness profile of the film has become significantly worse.

0o 9 Throughout the specification and the claims which follow, unless the context requires 0 otherwise, the word "comprise", and variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

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Claims (20)

1. A process for the longitudinal stretching of an at least single-layer polypropylene film comprising the steps: a) providing a film having two opposed surfaces; b) providing a stretching machine having a slow-running part, a fast-running part and a stretching zone there between, the slow-running part of the machine comprising at least one driven roll and the fast-running part of the machine comprising at least one pair of driven rolls, the pair of driven rolls being arranged in such a way that a roll nip is formed such that each roll of the pair of driven rolls has a contact point which contacts a surface of the film in use; c) warming the film in the slow-running part of the machine to a temperature suitable for stretching; d) passing the film from the slow-running part of the machine into the roll nip of S the pair of driven rolls of the fast-running part of the machine such that the film is gripped substantially simultaneously on each surface thereof by the pair of driven rolls, at the contact points on each roll, thereby applying a substantially equal stretching force to each surface of the film such that the film is stretched, within the stretching zone, between the at least one roll of the slow-running part of the machine and the pair of driven rolls of the fast-running part of the machine.

2. The process as claimed in claim 1, wherein, in the fast-running part of the :000o: stretching machine, the film of drive rolls is looped further around one of either of the rolls of the pair of driven rolls. -21-

3. The process as claimed in claim 1 or 2, wherein, in the fast-running part of the stretching machine, at least 10%, preferably about 20%, in particular about 30%, of the stretching force is applied to one of the film surfaces by the roll of the pair of driven rolls, around which the film is not looped further.

4. The process as claimed in any one of claims 1 to 3, wherein, in the fast-running part of the stretching machine, air dragged along by the film is squeezed out at the contact points of the pair of driven rolls. The process as claimed in any one of claims 1 to 4, wherein the stretching force is applied to the film by means of friction by the roll of the slow-running part of the machine and the pair of driven rolls of the fast-running part of the machine.

6. The process as claimed in any one of claims 1 to 5, wherein the speeds of each roll of the pair of driven rolls do not differ from one another by more than

7. The process as claimed in any one of claims 1 to 6, wherein the contact points of the pair of driven rolls are not more than 50 mm, preferably not more than 40 mm, in S particular not more than 20 mm, measured in the film web direction, apart from one another. 9

8. The process as claimed in any one of claims 1 to 7, wherein the length of the stretching zone is from 50 to 600 mm, preferably from 50 to 500 mm, in particular from 50 to 400 mm. oo..

9. The process as claimed in any one of claims 1 to 8, wherein the film temperature T i Ts in the stretching zone is from 80 to 160°C. O04JO a *a O0 -22- The process as claimed in any one of claims 1 to 9, wherein one of the rolls of the pair of driven rolls has a rubber covering.

11. The process as claimed in any one of claims 1 to 10, wherein the slow-running part of the stretching machine contains a non-driven or driven roll.

12. The process as claimed in any one of claims 1 to 11, wherein one of the rolls of the pair of driven rolls is driven by the second roll of the pair.

13. The process as claimed in any one of claims 1 to 12, wherein the film is looped in an S-shape around the roll of the slow-running part of the machine and a roll of the pair of driven rolls of the fast-running part of the machine.

14. The process as claimed in any one of claims 1 to 13, wherein the stretching is divided into a plurality of individual stretching operations. The process as claimed in any one of claims 1 to 14, wherein a biaxially oriented polypropylene (BOPP) film is produced.

16. The process as claimed in any one 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 one of claims 1 to 15, wherein the BOPP film •l produced by the process has a multilayer structure.

18. The process as claimed in any one of claims 1 to 15, wherein the BOPP film produced by the process has a multilayer structure and is not heat-sealable. A:" 9 S* i P 4-, -23-

19. The process as claimed in any one of claims 1 to 15, wherein the BOPP film produced by the process has a multilayer structure and is heat-sealable. The process as claimed in any one of claims 1 to 15, wherein the BOPP film produced by the process has a thickness of from 2 to 120 ,um, preferably from 2.5 to 120 ,um, in particular from 3 to 120 aum.

21. An apparatus for the longitudinal stretching of thermoplastic films, comprising a) at least one driven roll which is driven at speed V 1 and b) at least on pair of driven rolls one roll of the pair driven at speed V 2 and the other roll of the pair driven at speed V 3 where the speeds V 2 and V 3 differ from one another by a maximum of (based on the faster-running roll) and speed V, is lower than the speeds V 2 and V 3 and the at least one pair of driven rolls are arranged after the at least one driven roll in such a way that, during longitudinal stretching of the film by means of the apparatus, the film first comes into contact with the at least one driven roll and then into contact with the at least one pair of driven rolls, and the rolls of the at least one pair of driven rolls 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 substantially simultaneously with each roll of the pair such that a substantially equal force is applied to both surfaces of the film. ::'oo o

22. A process for the longitudinal stretching of polypropylene films, wherein part of the stretching force necessary for orientation of the film is applied via both film surfaces of the film.

23. A process according to Claim 1, substantially as herein before described with reference to the examples and drawings.

24. An apparatus according to claim 21, substantially as herein before described with reference to the examples and drawings. DATED this 20th day of March 2000 HOECHST KTIENGESELLSCHAFT By its Patent Attorneys DAVIES COLLISON CAVE 9 a 9* 9, a a.. a a *9 9. a a a a 9aa a a a a 9 a a. a. a. S 9 a a a. a a a a a aa a aa 9