HK1150571B - Method and apparatus for longitudinal orientation of thermoplastic film material - Google Patents

Description

Method and device for longitudinal orientation of thermoplastic film material

Technical Field

The present invention relates to a method and apparatus for longitudinal orientation of thermoplastic film materials, and in particular to the manufacture of cross-plied films (crosslaminates) of films that have been uniaxially stretched at relatively low temperatures.

Background

It is known to obtain the best omni-directional strength properties in interleaved stacks by the following orientation steps: first, a strong, nearly uniaxial melt orientation is performed during extraction from the extrusion die, or better, a nearly uniaxial orientation is performed when the polymer material is semi-molten, and then further orientation is performed at a relatively low temperature. "all-round strength properties" as used herein refers to a combination of tensile strength, yield point, tear propagation resistance and puncture resistance. It has not been possible to give satisfactory indications why this combination of orientation steps is preferred, but it can be simply stated that when oriented in these steps, the molecular chains will exhibit a wide range of different degrees of orientation, and that those relatively low orientations will help the film reorient rather than split when it is subjected to tearing or puncturing forces.

However, stretching at low temperatures causes significant problems in films made of, for example, High Density Polyethylene (HDPE) or isotactic or syndiotactic polypropylene (PP). On the one hand, the problem is that when the film is stretched in the machine direction, it will have a high tendency to shrink in the transverse direction, while its thickness will decrease. This trend is highest in terms of the properties obtained when the temperature is low (e.g. between 10-40 ℃, which is the most desirable stretching temperature range for HDPE and PP). On the other hand, the problem is that at these low temperatures, the material tends to "neck-in" over a reasonably long area, rather than gradually forming an orientation. This means that the stretching must be performed between closely spaced stretching rollers or rods unless special precautions are taken that will prevent the film from undergoing the necessary shrinkage in the transverse direction.

In the inventor's patent US 3233029, which was published about 40 years ago, a solution to this problem has been proposed, namely that it "reserves" a substantial part of the film's tendency to shrink in the transverse direction by longitudinally pleating in one or more short stretching zones before stretching, as will be more precisely stated in the preamble of claim 1.

In the above-mentioned patent, the pleating mechanism described consists of two sets of disks spaced apart mounted on a shaft, one set above and the other below the film to be pleated, so that the disks in one set engage between the disks of the other set. Thus, the film is forced to fold or wrinkle. The patent further discloses that the film preferably passes over a crown roller that causes the stress on the edges of the film to be equal to the stress in the middle of the film. Crowning means that the roll has a maximum diameter in the middle, which gradually decreases towards its ends. Finally, the patent discloses cooling the film in the stretching zone, preferably by covering the stretching rod with a felt and keeping this felt wet. The water also helps to shrink the film in the cross direction by its lubricating action to eliminate wrinkles. No folds remain in the final product.

The inventors succeeded in applying this old invention to elastic HDPE and PP, but only over a rather narrow width and not enough for industrial manufacture of industrial bags such as ortholaminates or protective layers for ortholaminates. When attempting to apply the present invention to stiffer films, such as films made from plain HDPE or PP, or when attempting to apply to films of greater width (e.g., 1 meter width), the transverse force exerted by the film always causes the film to stretch transversely in the form of thin longitudinally extending lines. The principle of applying longitudinal pleating, thus allowing the film to shrink in the transverse direction during longitudinal stretching, has hitherto only been carried out in industry under conditions that also produce transverse stretching and weakening along narrow longitudinal lines.

Disclosure of Invention

The described problem is overcome by an improvement from the characterizing part of claim 1. By the mentioned convergent delivery of the film, the transverse forces exerted by the pleating means are reduced and can be almost completely eliminated by an optimized adjustment, so as to avoid the formation of transversely stretched, weakened longitudinal lines. The preferred degree of pleating, i.e. the ratio between the width of the film before and after pleating, measured along a line from edge to edge, will be discussed in the detailed description.

The invention is of particular importance for the longitudinal stretching of films consisting mainly of HDPE, PP or mixtures of these polymers, because of the relatively low price, stiffness and strength properties available for such films, which make them most suitable for use in the cross-lamination of industrial products such as industrial bags, protective layers, tarpaulins, reinforced linoleum, pond liners, greenhouse films and "house wrap films". However, the invention is also applicable to all other films of thermoplastic polymer material, as long as the film in the form of long narrow strips can be oriented at or near normal room temperature. By way of example, the invention can be applied to films based on polyamides and polyesters, such as polyethylene glycol terephthalate, polyvinylidene chloride and crystalline copolymers of vinyl chloride and vinylidene chloride. It is also expected to be useful for films based on biodegradable polymeric materials that are cold-stretchable.

Most suitably, the reduced area should be no longer than 3 times, preferably no more than twice, more preferably no more than the width of the original film.

As already mentioned above for the advantage of stretching at lower temperatures, the stretching according to the invention should generally be carried out at a temperature not higher than 60 ℃, preferably not higher than 50 ℃, more preferably not higher than 40 ℃. The film to be stretched may be in the form of a flattened tube. This is particularly relevant for the manufacture of cross-laminates from uniaxially oriented films, since the usual manufacturing method of such cross-laminates comprises the step of helically cutting a longitudinally oriented tube.

In a preferred embodiment of the invention, the downstream roller or roller assembly comprises at least one banana roller, the convex side of which is directed towards the upstream roller assembly. "banana-type rollers" are the name commonly used for rollers having a curved axis, usually formed as a circular arc. Banana rollers are commonly used to remove wrinkles or folds, but are used here for the opposite purpose. In its simplest form, the banana-type roller consists of a slightly curved shaft that is placed into a rubber tube, on which the rubber tube can turn. The tube is usually lubricated with, for example, talc. In a more industrial design there is an array of ball or roller bearings closely side by side between the bending shaft and the rubber tube. The rubber tube may for example be replaced by an array of rings, each fitted to a bearing.

The degree of curvature of the banana roller is adjustable. The adjustment of conventional banana rollers is known and can be performed by adjusting the angular position of the shaft ends. To allow for variable bending, the shaft is made of a composite (e.g., fiberglass or carbon fibers embedded in a polymer material).

Whether using a banana roller with adjustable bow or a banana roller with fixed bow, the radius of the bend is determined by the length of the shrink zone and the selected degree of pleating. This will be explained in the detailed description.

As a technical equivalent to using at least one banana roller as the downstream roller or as part of a downstream roller assembly, a number of stub rollers can be provided, mounted independently so as to together form part of a polygon approximating a circular arc.

When the film leaves the last roll or roll assembly of the downstream portion of the reduction zone and proceeds toward the stretching roll or rod, the film is preferably directed substantially perpendicular to its movement within the reduction zone, and preferably no more than 10 ° from this perpendicular. As further explained in the description with reference to the figures, this precaution is provided in order to equalize the longitudinal tension of the film over its width.

Similar to the structure of the downstream roller or roller assembly, the upstream roller or roller assembly may suitably consist of a banana roller or a plurality of parallel banana rollers, wherein the concavity of the roller is directed towards the downstream roller or roller assembly. The banana roller preferably forms an arc with a tangent at any position of the arc perpendicular to the film tension created by the downstream roller or roller assembly. If the upstream and downstream rollers are both banana-type rollers, this means that the rollers form substantially concentric arcs. This is further elucidated in the detailed description of the preferred embodiments.

The upstream banana roller or the last of the upstream banana rollers may advantageously have an array of protruding, circular segment portions in order to start the pleating. Also like the structure downstream of the reduction zone, the curvature of each banana roller may be made adjustable.

As already mentioned, the film preferably exits from the downstream portion of the reduced area substantially perpendicular to its movement within the reduced area. It is also preferred and for similar reasons that the film is delivered towards the first upstream roll in a direction substantially perpendicular to its movement within the reduced area (and preferably up to 10 ° away from this direction).

As an alternative to using one or more banana rollers at the input to the reduced area, the upstream roller or roller assemblies may be crown rollers or roller assemblies that together form crown-shaped stub rollers on a straight shaft, said stub rollers being connected via bearings to a common shaft so as to be rotatable independently of each other.

Preferably, the tapering of the width in the reduction zone is assisted by a grooved banana roller mounted between the upstream and downstream rollers or roller assemblies, or otherwise by an array or intermeshing disks. The grooved banana-type roller used for this purpose can consist of disks of different outer diameters in alternating succession or of grooved short roller segments, so that the disks or the short roller segments are mounted on a curved shaft via bearings or the disks or the short roller segments themselves can act as bearings.

Preferably, the nip between two such grooved banana rollers is not set in a fixed manner, but is made variable by an adjustable force trying to push the two grooved banana rollers together. This adjustable force may be generated by a spring, pneumatic means or by gravity. When pleating is irregular (as is common at the beginning), the forces that attempt to increase bite act most strongly on the least pleated film. If the means of pushing the two grooved banana rollers together have been adjusted appropriately by experiment, the progressive pleating will become uniform over the entire width of the film.

If the bite between the grooved banana rollers is set in a fixed manner without special precautions being taken, or if the force trying to push the rollers together adjustably is set too high, the result may be that the grooved banana rollers will stretch in the transverse direction at the least pleated film, thus creating longitudinally extending thin lines, rather than gradually making the pleating uniform.

It has been found that when the film passes through a smooth banana type roller or a smooth roller or a straight roller, the pleating system tends to be disrupted in a uniformly pleated film at zero or low tension. This can be a problem associated with the smooth rolls used in the present invention prior to permanent elongation of the film. To counteract this disturbance, a guide device acting immediately upstream of and in close proximity to the smoothing roller may be provided. These means may preferably be rails adapted to fold all pleats on the same side.

Preferably, the pleating described is performed in multiple steps in sets of arrays of intermeshing grooved banana rollers or discs, the pitch of the arrays in the sets progressing from coarser pleating to finer pleating.

Instead of using a grooved banana roller as described, the gradual reduction of the width in this reduced area can be assisted by a set of narrow conveyor belts following and guiding the film through at least a part of this area, so that the two sets of narrow conveyor belts gradually and increasingly engage each other in this area during the convergent advancement.

The invention also relates to any device suitable for carrying out the above method and emphasizes that a banana roller with grooves suitable for forming or controlling pleating is itself considered an invention.

Drawings

The invention will now be described in more detail with reference to the accompanying drawings.

Fig. 1 is a photograph of a perspective view showing the basic principle of the tendency of a film to shrink laterally during machine direction orientation by feeding the film into a stretching zone in a pleated state. The photographs show samples of flattened tubular film (consisting essentially of HDPE) before and after stretching as described in example 1. The samples were taken outside the cold drawing machine during the stop.

Fig. 2a and 2b are main schematic diagrams showing a machine direction (machine direction) stretch line, which includes a means for pleating the film prior to stretching. Fig. 2a shows the whole line, as a vertical cross-section including a cross-section along a-a in fig. 2 b. Fig. 2b is a horizontal cross-sectional view along b-b in fig. 2 a. For the sake of clarity, the distance between the different rolls (compared to the roll diameter) is roughly shown as being disproportionately shortened. The grooved surface pattern of the pleating roll is also not shown.

Figure 3 shows a diagrammatic cross-sectional view through an axis showing a segment of a pair of intermeshing grooved rollers that can form, adjust or control the pleating. In the case where the grooved roll is a banana-type roll, the figure should be understood as an expanded view.

Fig. 4a and b are geometric figures that are provided as a basis for calculating parameters of the pleating process.

Fig. 5 shows in detail the preferred technical structure of the grooved banana roller (generally as in fig. 3), but it is a more robust and durable structure.

Fig. 6 shows in detail the structure in the middle of the banana roller at the entrance or exit of the reduced width zone.

FIG. 7 shows in schematic perspective view a plurality of positions for pleating the film as the pleats are flattened against the smoothing roll. For the sake of clarity, the guide rails and the smoothing rollers are not shown.

Fig. 8 shows an alternative to the banana roller of fig. 6, i.e. the array of short smooth rollers forms a part of a polygon close to a circular arc.

Fig. 9 shows an alternative to the grooved banana roller similar to fig. 3 and 5, i.e. the short grooved roller array forms a part of a polygon close to a circular arc.

FIG. 10 shows a portion of a crown roll including a plurality of segments that are rotatable independently of one another. This structure replaces the first banana roller shown in fig. 2a and 2 b.

Detailed Description

In the photograph of fig. 1, the region indicated as 1 is the pleated HDPE film before any orientation has been performed. Zone 2 has passed the first step of stretching (i.e. in a ratio of 1.5: 1) at 15 ℃, so it has become oriented in the weft-lock direction within "stretch lines" that run on twill roads and criss-cross each other. The film is still mostly oriented without leaving its melt orientation. The orientation resulting from cold stretching can be directly observed, since stretching of films made of HDPE or PP at temperatures below about 40-50 ℃ produces closed microvoids (which act as white pigment particles). This is well known.

Region 3 has passed through the 3 rd drawing step (the last step in example (1)) and has turned white everywhere, while the skewed criss-cross drawing lines have gradually increased and developed into a structure that is uniform on a macroscopic scale. At that time, the film has shrunk in the transverse direction and the pleating has disappeared. The final draw ratio before relaxation was 3.8: 1 and after relaxation was 2.8: 1.

The described development of the orientation process (starting with skewed, interdigitated "stretch lines" which continue to develop and increase gradually together) means that the final oriented film becomes non-uniform when viewed on a microscopic scale. The film has micro-regions throughout that have zero or nearly zero orientation (out of melt orientation), and the film will have micro-regions throughout that have an orientation that makes a small angle with the orientation in the adjoining micro-regions. This type of micro-inhomogeneity will help the film reorient when subjected to transverse forces and is therefore very good for tear propagation resistance and puncture resistance in cross-laminates made from such stretched films.

As shown in example 1, the pitch of each of the last grooved rolls used for pleating was 15 mm, corresponding to the average "wave length" of the pleats. The fineness required for the pleating depends on the transverse contraction force during cold stretching and the friction between the film and the draw-resisting roller. A low transverse contraction force and/or a high friction force require particularly fine pleating.

In figures 2a and b, the film 4 taken from the reel 5 is passed through a nip roll 6, the nip roll 6 being measured by automatic tension and a braking system (not shown) is provided to maintain the tension at the adjusted value.

The film travels from the roller pair 6 through a section (which will be discussed in connection with fig. 4 b) to the first smooth banana roller 14 which serves as an entrance to the area of reduced width. The outlet of this zone is a smooth banana roller (15) between which three pairs of intermeshing grooved banana rollers (16, 17 and 18) are mounted. The plane determined by the circular axis of the smooth banana roller 14 is substantially the same as the plane determined by the circular axis of the smooth banana roller 15, however small deviations are allowable and similarly the plane determined by the circular axis of each grooved roller in the reduced width area lies approximately in the same plane. The circular axes of all banana rollers are substantially concentric. The selection of the radii of these circular arcs is discussed in connection with fig. 4 a.

As discussed in the general description, each of the three pairs of grooved banana rollers is always pressed together under an adjusted pressure, but this is not shown in the figure.

When passing over the smooth banana roller 15, the film changes direction and leaves the roller at an angle close to perpendicular to the direction it follows as it passes through the reduced width zone. On the way to the first roll (i.e. roll 7) in the stretching section of the machine, the film passes through two pairs of intermeshing, straight, grooved rolls 19 and 20. The two pairs of grooved rollers are also always pressed together under a regulated pressure (means not shown).

Both banana rollers 14 and 15 and the all grooved rollers (curved or straight) are idle rollers.

A guide is immediately upstream of each of the smooth rollers 15 and 7 and immediately adjacent to each of the rollers 15 and 7 to prevent the smooth rollers from disturbing the uniform pleating. For the sake of clarity, these guiding means are not shown here, but reference is made to fig. 7 and the related description.

The pleating of the film is mainly caused by the concentric arrangement of the banana rollers 14 and 15 in combination with the tension in the film. However, these approaches alone will typically produce coarse and uneven pleating.

In the arrangement described, the groove separation on the roller pair 16 is greater because less force is required to form coarse, uniform pleats or corrugations, and because if fine pleats are formed at this location, the fine pleats will tend to turn into coarse pleats as one proceeds to the roller pair 17.

The pitch on roller pair 17 is adapted to double the number of pleats formed by roller pair 16 and the pitch on roller pair 18 double the number of pleats again. The smooth banana roller 15 flattens the folds or wrinkles as described in connection with fig. 7. A guide may be provided to ensure that the flattening is performed in a uniform manner. In order to avoid that the folds become thicker on the way from roll to roll, the distance from roll pair 17 to roll pair 18 and the distance from roll pair 18 to roll pair 15 are relatively short.

On the way to the intermeshing, straight, grooved rollers 19, the pleats again take an upright form. The pitch on the rollers 19 is adapted to return the number of pleats to the number formed by the pair of rollers 17. This is chosen because, despite the described guide tracks, the smooth banana roller 15 causes some confusion in the arrangement of flattened folds, and then requires a larger pitch reconstruction sequence. The number of pleats or wrinkles is doubled again by passing through the roller pair 20 and then maintained by the smooth driven roller 7 and the rubber coated nip roller 8.

The distance between the rollers 15 and 19 is longer. The reason for this is discussed in connection with fig. 4 b. The distance between the roller pair 20 and the rollers 7 and 8 is short to avoid that fine wrinkles become thicker. (the figure is not drawn to scale at these points.)

The smooth driven roll 7 and its rubber coated counter roll 8 hold the film during stretching with the help of a smooth roll 9, which is driven at substantially the same peripheral speed as the roll 7.

The smoothing rolls 10, 11 and 12 are also driven. The roll 13 is a rubber-coated nip roll. The roller 10 moves faster than the roller 9 to perform the first stretching step; roll 11 moves faster than roll 10 to perform the second stretching step; and roll 12 moves faster than roll 11 to perform the third drawing step. Each of the rollers 7 to 12 is maintained at a fixed temperature by circulating water. This temperature may be equal to, slightly below, or slightly above normal room temperature. If stretching at 30 ℃ or 40 ℃ is chosen, for example, the film must be preheated, which can be achieved most simply by maintaining the environment at this elevated temperature.

The film proceeds from the described draw line to an annealing station where the film is heated to, for example, 60-80 c and allowed to relax. This is a conventional device and is represented in the figure by block 114. It should be noted, however, that the film will become wider when allowed to relax and will therefore tend to reform into a partially pleated form unless this is avoided, for example, by using several banana-type rollers.

The tension of the film during the relaxation is set by the speed of the rollers 115 and 116, the roller 116 being paired with a rubber coated roller 117, the tension being automatically controlled by a tension measuring banana roller 118. Finally, the film is wound on a winder 119.

The substantially grooved idler roll shown in fig. 3 may be straight or curved as mentioned. Thus, for example, the two axes 20 may be understood as extending from a plane perpendicular to the plane of the paper. The pleated shape is adjusted and made uniform by the ring 22, which idles on the fixed shaft 23. The rings 24 hold the rings 22 precisely spaced apart from one another. The rings 22 and 24 are made of a self-lubricating material, such as Teflon (Teflon).

At the beginning of the pleating/stretching machine line, each pair of grooved rolls should be disengaged from each other. As the film travels along the line, a bite is gradually established (e.g., by pneumatic means), as explained in the general description. A more stable structure of the grooved banana roller is shown in fig. 5, of course, the straight grooved roller for pleating can be made to rotate in one piece with bearings at the ends.

In the following calculations relating to fig. 4a, the following approximation is made: the axes of the two smooth banana rollers (AB and CD in this figure, 14 and 15 in figure 2a, respectively) are equal to the radii of their convex and concave shapes. This approximation is permissible because their cross-section is typically less than 3 centimeters in radius.

A. E, G, I and C represent one edge of the film at different steps of the process, B, F, H, J and D represent the other edge. The distance from a to B (measured along the arc) is the width of the film as it enters the "reduced width region"; and the distance from C to D (also measured along the arc) is the width of the pleated film as it leaves the region; p is the center of the concentric axes of the 5 arcs.

The degree of pleating is the ratio between the width of the un-pleated, un-stretched film and the pleated film as it enters the roller 7 (see figure 2 a). Here, the width of the pleated membrane is measured straight from edge to edge. This ratio is substantially equal to the ratio between the arc length AB divided by the arc length CD, which in turn is substantially equal to the radius PA divided by the radius PC.

When a strip of film several centimeters wide is stretched longitudinally at a lower temperature in the ratio n: 1, it will generally tend to reduce its width and thickness almost equally, i.e. both at a ratio of about √ n: 1, but more or less depending on its melt orientation. Thus, for example, at a stretch ratio of 4: 1, both the width and thickness typically decrease at a ratio of about 2: 1. For stretching a wide film in a ratio of 4: 1 (about the best ratio has been found for HDPE or PP films that can be stretched without risk of rupture), when the stretching temperature is about 20 ℃, therefore, the degree of pleating should theoretically be about 2: 1. The stretch ratio of 4: 1 refers to the state where no relaxation occurs and the film is still under the highest tension that occurs during stretching. However, it is in fact very difficult to form perfectly uniform pleating, and in order to ensure that no pleating marks remain after stretching, it has been found that the degree of pleating is most appropriate between 1.5: 1 and 1.6: 1 at the mentioned stretching ratio of 4: 1 and at a temperature of about 20 ℃.

In the figure, the radius PA is 1.5 times the radius PC, which corresponds to a pleating degree of 1.5: 1. It has further been shown that the length of the "reduced area" is equal to the width (arc CD) of the fully pleated film, which has been found to be very suitable. Therefore, the angle between the two film edges AC and BD is 0.5 radian equal to 28.6 °.

In the following, it has further been specified that the length of the arc CD and the reduced area is 1.00 m, and that the length of the bow AB is thus 1.5 m. Then, the radius PA would be 3.00 meters and the radius PC 2.00 meters. Arc EF may be suitably located midway between arc AB and arch CD, and arc GH may be suitably located midway between EF and CD. This means that arc EF has a radius of 2.50 meters and arc GH has a radius of 2.25 meters. As mentioned in connection with fig. 2a, the arc IJ should be very close to the arc CD. It has been specified that the radius should be 2.08 meters.

The distance between the middle of the arc AB and the middle of the chord AB is 3 m x (1-cos0.25) ═ 9.4 cm. As already mentioned in the description of fig. 1, the degree of pleating required depends on the transverse contraction force and the friction between the film and the hold-up stretching roller. It has been found that a pitch of 15 mm on the roll 7 is generally suitable for HDPE or PP based tubular films if its gauge does not substantially exceed 0.10 mm. The number of folds through the smooth banana roller 15 corresponds to this pitch, as explained with reference to fig. 2 a. Thus, considering the different radii, the pitch on the roller pair 18 is 15 × 2.08 ÷ 2.00 ÷ 15.6 millimeters. The roller pair 17 is configured to produce half the number of pleats at a pitch of 30 x 2.25 ÷ 2.00 ═ 33.75 millimeters. Finally, the roller pair 16 is configured to produce this half pleat count, which will have a pitch of 60 × 2.5 ÷ 2 ═ 75 millimeters. If the pleats are prepared by projecting somewhat smoothly circular segment portions on grooved roll 14, the pitch of these projections will be 60 x 3/2-90 mm.

Fig. 4b of the geometry is drawn on a plane perpendicular to the plane of fig. 4a and passes through the points referred to as K and L in fig. 4 a. M is the point at which the film leaves the roller pair 6, see fig. 2 b. N on line ML is plotted so that MK is MN.

MK is the path of the film passing from the roller pair 6 to the banana roller 14 in the middle and ML is the path of the film passing between these rollers at the edges. Therefore, LM is the difference between the two routes, which creates a difference in tension. It has been specified that a difference of 1% is allowable, and the purpose of the following calculation is to establish a minimum length of the distance KM.

Angle LKN is a circumferential angle and thus half the angle KMN, since both are small angles, the following equation can be applied:

converting into:

KM×LN＝1/2×KL²

another equation (which represents a maximum 1% distance difference) is:

LN＝1/100×KM。

combining two equations can be obtained:

KM²＝50KL²，KM＝7.07×KL

as calculated in connection with fig. 4a, KL is 9.4 cm, and hence KM is 7.07 × 9.4 is 66 cm.

A similar calculation can be made for the difference in path length from roller 15 to roller 7.

In fig. 5, the rotatable wave-like part of the grooved banana roller consists of a number of rings 25 (the rings 25 are fixed to the circular bending shaft 23 via ball bearings 26). The washers 24 and 24a ensure proper performance of the ball ring.

The structure of the smooth banana roller (as shown in fig. 6) is similar to that shown in fig. 5, except that the ring 25 does not have an undulating shape and the arm 27 connected to the machine frame supports the curved shaft 23 at the middle of the curved shaft 23. Without this support, the tension in the film would distort the plane defined by the bending axis of the shaft. To simplify the drawing, the support arm 27 is shown parallel to this plane (i.e. the plane of the paper), but in practice it should be arranged inclined with respect to this plane in order to counteract the film tension as much as possible.

The "half ring" 28 corresponds to the support arm 27, which is an extension of this arm or is fixed to the shaft 23. The film slides over this half-ring and frictional heat is removed by pumping cooling water through ducts (not shown) in the arm.

In fig. 7, the standing pleats 101 are gradually transformed into flattened pleats 103 all on the same side. One in between position 102 is shown. The folds are thus all flattened to the same side on the smooth banana-shaped roll 15 and the smooth straight roll 7 (see fig. 2). The device doing this may be a comb array of thin metal plates that are gradually twisted. This means that at their upstream ends they are substantially perpendicular to the axis of the smoothing roll and towards their downstream ends their angle changes to be parallel to the axis. If there are no guides immediately upstream of each of the two smoothing rolls 15 and 7, these rolls will tend to disturb the pleats. This disruption can be counteracted to some extent by a simple comb or free-rotating disk array, but this simple arrangement will not flip the pleats to the same side.

In fig. 8, the grooved banana roller is replaced by a number of short, straight grooved rollers 29, each supported at its ends by ball bearings 30 and 31. Each pair of adjacent ball bearings is housed in a housing 32 which is fixed by an arm 33 to the machine frame or to the means for opening and closing the nip between the rollers.

Fig. 9 is the same as fig. 8 except (as in fig. 7) there is a non-rotatable, water-cooled "half-ring" 34 over which the film slides.

In fig. 10, 35 is a fixed shaft on which the short segment 36 is idly rotatable and connected to the shaft via a ball bearing 37. As mentioned, this may be a suitable alternative for the first banana roller 14. The advantage of creating a crown roll with many short, independently moving segments is that each segment can follow the film speed with hardly any slip on the roll surface.

Examples

A 100 micron thick tubular film was extruded from the following ingredients:

intermediate layer, 70% in total: 100% HMWHDPE;

inner surface layer, 10% in total: an LLDPE of m.f.i. ═ 1.

Outer surface layer, 20% in total: 60% metallocene PE + 40% LLDPE; m.f.i. ═ 1.

Width of the flattened film: 54 cm. The flattened film was pleated and stretched in the apparatus shown in fig. 2a and b at 15 ℃ with the following modifications:

the radius of the banana roller 15 is 1.00 meter and the radius of the banana roller 14 is 1.50 meters, still giving a degree of pleating of 1.5: 1. The stretching is performed in only two steps. The grooved roller pair 16 is omitted. Roll pair 18 has a radius of 1.06 meters and roll pair 17 has a radius of 1.15 meters. The pitch of the driven grooved roll 7 was 15 mm and the pitch of the other grooved rolls was calculated from this similarly to the calculation in connection with fig. 4 a.

The temperature of the stretching roll was maintained at 15 ℃ by circulating water. The film is processed at 70 c with low tension (controlled by roll 118 and its associated equipment) in oven 114.

The draw ratio (measured as the ratio between the speeds of the last set of draw rolls and the first set of draw rolls) was 2.8: 1, and the final draw ratio after relaxation was 2.8: 1.

The flattened tubular film is then helically cut to produce a single film in which the primary orientation direction forms an angle of 45 ° with the machine direction and is continuously laminated to a similar film between nip rolls at 70 ° so that the metallocene-containing layer acts as a lamination layer.

Claims (47)

1. An improved process for providing longitudinal orientation to a thermoplastic polymer film by stretching in the solid state, the stretching being carried out in one or more short regions between and/or on two or more stretching rollers or bars, in which process the width as measured in a straight line from edge to edge is reduced prior to the stretching, such reduction being in the form of a regular pattern of longitudinally extending pleats, wherein the reduction in width and the length of the stretching regions are adapted such that the film fully straightens the pleats by the inherent tendency of the polymer material to contract laterally when stretched longitudinally, wherein said pleats are formed between at least one pair of intermeshing grooved rollers or intermeshing disc sets, the improvement being characterised in that the reduction in width is carried out progressively in reduced regions which are not shorter than half the original film width and which regions are defined by upstream and downstream rollers or roller sets mounted in varying rotational axis directions, this direction makes a 90 ° angle with the cis-filament direction at the middle of the film and gradually changes towards the edges of the film to deliver the film in a convergent manner within this reduced area.

2. The method of claim 1, wherein the film consists essentially of HDPE, PP or a mixture of these polymers.

3. A method according to claim 1 or 2, wherein the reduced area is no longer than 3 times the original film width.

4. A method according to claim 1 or 2, wherein the reduced area is no longer than twice the original film width.

5. The method of claim 1 or 2, wherein the reduced area is not greater than the original film width.

6. The method of claim 1, wherein the film is in the form of a flattened tube.

7. The method of claim 1, wherein the stretching is performed at a temperature of not greater than 60 ℃.

8. The method of claim 1, wherein the stretching is performed at a temperature of not greater than 50 ℃.

9. The process as claimed in claim 1, wherein the stretching is carried out at a temperature not exceeding 40 ℃.

10. The method of claim 1, wherein the downstream roller or roller assembly comprises at least one banana roller and the convex side thereof is directed toward the upstream roller or roller assembly.

11. The method of claim 1, wherein the downstream roller assembly is comprised of a plurality of stub rollers mounted separately and together forming a portion of a polygonal periphery for the desired pleating pattern.

12. A method according to claim 1, characterised in that the film is guided in a direction deviating no more than 10 ° from perpendicular to its movement within the reduction zone when leaving the last roll or roll assembly of the downstream part of the reduction zone.

13. Method according to claim 1, characterized in that the upstream roller or roller assembly consists of a banana roller or a plurality of parallel banana rollers and the concave side is directed towards the downstream roller or roller assembly.

14. The method according to claim 13, wherein the banana roller or rollers form an arc and the tangent to the arc at any location is perpendicular to the film tension created by the downstream roller or roller assembly.

15. The method of claim 1, wherein the film is delivered toward the first upstream roll in a direction that deviates from perpendicular to its movement within the reduced width zone by no more than 10 °.

16. A method according to claim 1, characterised in that the upstream roll or roll assembly is a crown roll or an assembly of stub rolls forming together a crown on a straight shaft, said stub rolls being connected via bearings to a common shaft so as to be able to rotate independently of each other.

17. A method according to claim 1, wherein the tapering of the width in the reduction zone is assisted by intermeshing discs in the form of grooved banana roller pairs mounted between upstream and downstream rollers or roller assemblies.

18. The method according to claim 17, characterized in that the banana roller consists of alternating successive discs of different outer diameters or short roller segments with corrugations, and the discs or short roller segments are mounted on a curved shaft.

19. The method of claim 17 wherein some or all of the meshing discs are individually adjustable in their meshing.

20. The method of claim 17, wherein the tapering is aided by at least one pair of intermeshing grooved banana rollers, wherein the intermeshing can be varied by an adjustable force for moving the two rollers together.

21. A method according to claim 1, characterized in that the width is gradually reduced in said reduced area by means of a set of narrow conveyor belts following and guiding the film through at least a part of said reduced area, and in that the two sets of narrow conveyor belts gradually get into engagement with each other in said area during the advance of the conveying.

22. The method of claim 1 wherein pleating is performed in multiple steps with sets of grooved rollers, the pitches of the grooves in said sets being different from each other to develop from coarser pleating to finer pleating.

23. The method of claim 1, wherein the guide means acts immediately upstream of and in close proximity to the smooth banana-type roll as the film in a pleated state passes over the roll before permanent elongation to counteract disruption of pleating by said roll.

24. The method of claim 23, wherein said guide means is a threaded track adapted to fold all pleats onto the same side.

25. A device for the longitudinal orientation of a thermoplastic film (4), comprising, in succession in the direction of the lock of the filament:

i) a width reduction station comprising at least one pair of intermeshing pleating rollers (16) comprising intermeshing grooved rollers or intermeshing disc sets for applying regular longitudinally extending pleats across the film width; and

ii) a longitudinal stretching station for longitudinally stretching the film in the solid state, the longitudinal stretching station comprising one or more pairs of spaced stretching rollers (9, 10) or bars, the spacing between each said pair being relatively short;

characterised in that the width reduction station comprises an upstream roller (14) or roller assembly and a downstream roller (15) or roller assembly, a width reduction zone being provided between the upstream and downstream rollers or roller assemblies, wherein the respective axes of rotation of the upstream and downstream rollers or roller assemblies have a direction which varies from 90 ° to the in-line lock direction at the centre of the film and gradually varies towards the edge of the film on both sides of the centre such that the film is guided through the width reduction station with the edges of the film converging towards the centre.

26. An apparatus according to claim 25, characterized in that the distance between the upstream roll (14) or roll assembly and the downstream roll (15) or roll assembly is not longer than 3 times the original film width.

27. An apparatus according to claim 25, characterized in that the distance between the upstream roll (14) or roll assembly and the downstream roll (15) or roll assembly is not longer than 2 times the original film width.

28. Apparatus according to claim 25, wherein the distance between the upstream roller (14) or roller assembly and the downstream roller (15) or roller assembly is no greater than the original film width.

29. Device according to claim 25 or 26, characterized in that the downstream roller (15) is a banana-type roller and its convex side is directed towards the upstream roller or roller assembly.

30. Apparatus according to claim 25 or 26, wherein the downstream roller assembly consists of a plurality of stub rollers (29) mounted separately and together forming a portion of the polygonal periphery for the desired pleating pattern.

31. The apparatus according to claim 25, wherein the film is directed from said downstream roll (15) to the longitudinal stretching station in a direction not exceeding 10 ° from perpendicular to the direction of its movement through the width-reducing station.

32. Device according to claim 25, characterized in that the upstream roller (14) is a banana-type roller and its concave side is directed towards the downstream roller or roller assembly.

33. The apparatus of claim 32, wherein the banana roller forms an arc with a tangent that is perpendicular to the film tension created by the downstream roller or roller assembly.

34. The apparatus of claim 25, further comprising a film supply (6) upstream of the width reduction station, wherein the film is directed to said upstream roller (14) or roller assembly in a direction which deviates by no more than 10 ° from perpendicular to its movement within the width reduction zone.

35. Device according to claim 25, characterized in that the upstream roll (14) or roll assembly is a crown roll or an assembly of short rolls forming together a crown on a straight shaft, said short rolls being connected to a common shaft by bearings so as to be rotatable independently of each other.

36. The apparatus according to claim 25, wherein the width reducing station further comprises at least one pair of rollers comprising intermeshing discs (22) disposed between the upstream and downstream rollers or roller assemblies, and the at least one pair of rollers is in the form of a grooved banana roller pair.

37. The apparatus of claim 36, wherein said grooved banana roller is comprised of alternating successive disks of different outer diameters or grooved short roller segments mounted on a curved shaft.

38. The apparatus of claim 36 wherein some or all of the engaged discs are individually adjustable in their engagement.

39. The apparatus of claim 36, comprising at least one pair of intermeshing grooved banana rollers, wherein the intermeshing can be varied by an adjustable force moving the two rollers together.

40. Apparatus according to claim 25, wherein the width reducing station further comprises a set of narrow conveyor belts following and guiding the film through at least a portion of the region between the upstream roller (14) or roller assembly and the downstream roller (15) or roller assembly, the two sets of narrow conveyor belts progressively engaging each other in the region during the convergent advancement of the film.

41. The apparatus of claim 25, wherein the width reducing station comprises a plurality of pairs of intermeshing tucker rolls (16, 17, 18), and the pitch of the corrugations between successive pairs is reduced.

42. The apparatus according to claim 25, wherein a tucking guide is provided immediately upstream (8) of the at least one smoothing roll (7) downstream of the tucking roll and upstream of the longitudinal stretching station, the tucking guide being adapted to fold all of the tucks (101) in one direction (103).

43. The apparatus of claim 42, wherein the tucking guide comprises a threaded track.

44. An apparatus for processing films includes a grooved banana roller.

45. The apparatus of claim 44, comprising a pair of intermeshing grooved banana rollers.

46. The apparatus of claim 45, wherein the nip is variable by an adjustable force for moving the two rollers together.

47. The device of claim 44 or 45, which consists of disks of different outer diameters or short grooved roll segments in alternating succession, wherein the disks or roll segments are mounted on a curved shaft via bearings or the disks or roll segments themselves act as bearings.