EP0397740A1 - A method and an apparatus for producing a thinned plastics film and a reel of such a film - Google Patents

Abstract

A plastic resin film (eg low density polyethylene or linear low density polyethylene) in a dimensionally stable state is stretched by a network of stretch rollers (18, 20, 22), for example 300% in the direction of the machine. Such stretched film has a strong tendency to shrink ("to regain its shape"), which means that it is only suitable for immediate use to package sturdy items, and that it is not suitable for winding . It is then granted a pause time under conditions (X; Y) such that the tendency to return to form is reduced to a maximum of 5% withdrawal. The film obtained is suitable for winding and is used in many applications. During the pause time (Y), a degree of shrinkage can be allowed, for example from 300% to 255% of stretching. The tension is carefully maintained at a low value by means of a dancer roller assembly (36) and a reaction adjustment essentially as a function of the speed of the film at the end of the pause time, and possibly also as a function of the position and speed of movement of the dancer roller.

Description

A method and an apparatus for producing a thinned plastics film and a reel of such a film

The present invention relates to thin plastic film, uses thereof, and to a method of preparing such film.

Plastics packaging films are well known. A film of, for example, low density polyethylene ("LDPE") (often blended with linear low density polyethylene, "LLDPE") is made by extrusion. Thinner films can be made by adjusting the extrusion process, e.g. by reducing the distance between the die lips, but there is a limit to this. In practical terms, it is difficult to extrude a film of much under 15μm in thickness. Furthermore it is operationally undesirable to vary the output of an extruder. The plant will function much more smoothly if the extruder is run under constant conditions, near its optimum. This would typically involve extruding film whose (constant) thickness is in the range 20-35μm.

Some materials are extruded and then thinned. The thinning may be carried out before they have set after extrusion (e.g. the "blowing" of high density polyethylene extruded as a tube), or with substantial heating of a cast film (e.g. the stretching and orientation of polypropylene). However, thinning to produce a re- reelable film has not been applied to the cheaper materials low density polyethylene ("LDPE") and linear low density polyethylene ("LLDPE"). Such materials have been stretched only immediately prior to use for stretch wrapping. In such a context, the aim is not to produce a stable film, but to produce a film with a substantial tendency to decrease in length ("elastic memory"), so that it will grip a wrapped article. If such a stretched film were wound onto a reel, the force on the reel core would be crushing. It is thus desirable to provide a thinning process applicable to such materials.

We have now found that thin films (even including films of thickness under lOμm, even 5μm) which may be suitable for packaging can be produced by extruding a plastics film and then thinning it under controlled conditions. The thinning is generally effected uniaxially, suitably by stretching in the machine direction. The film to be thinned may be cast or blown film which has not been further processed to orient it. Desirably, the film is stretched without substantial necking (i.e. reduction of the width of the film). We have found that the problem of elastic memory can be ameliorated or even solved by allowing a controlled relaxation prior to use or reeling. In one aspect the invention provides a method of producing a film, which comprises extruding a plastics resin film, allowing the film to • cool sufficiently to become dimensionally stable; stretching the stable film so as to reduce its thickness and, prior to use or reeling, allowing a dwell time under conditions such that elastic memory effects are substantially reduced so that the film is capable of being reeled up for subsequent use. Preferably the thinning is effected by passing the film through an assembly comprising tension rolls, then around a control dance roll, before the film is wound for storage or use. The dance roll may be automatically displaceable to regulate the speed of the thinning assembly to control the tension applied to the film. This can be particularly important when the film is very thin (e.g. under lOμ ) and/or in circumstances where the demand for the film tends to fluctuate rapidly. The technique may be applicable to thinning quite thick films down to a moderate thickness, and moderate films down to low thickness, e.g. under lOμm. Of course, a film can be given several successive thinning treatments to achieve a desired thinning. The step of allowing a dwell time to reduce memory effects may involve permitting some shrinkage of the film and/or holding it under controlled low tension for a relaxation period. As an example of controlled shrinkage, film may be stretched by a high speed roller with a first linear speed S, and passed (preferably via a tension control unit, e.g. a dance roll) to a wind up roller with a lower linear speed, e.g. 85% of S. During the controlled shrinkage the film may be held under the lowest tension compatible with maintaining stable control of the travelling web. As an example of holding under controlled tension, use may be made of a festoon arrangement, suitably between a dance roll and a winding station. Thus the film is suitably maintained under control before the winding step, to provide the dwell time.

In a second aspect the invention provides a thinning assembly comprising an array of tension rolls whereof a downstream roll has drive means; and a dance roll arranged downstream of said powered roll such that a variable film path may be provided, extending through the thinning assembly and then looping around the dance roll; and wherein the position of the dance roll is variable in response to variations in film tension to vary the size of the loop. Preferably a control means senses the position of the dance roll and/or its rate of displacement and acts to control the rate at which film is fed through the unit. To enable the dance roll to respond rapidly, it should be of low inertia and displaceable with low friction. Thus it may be tensioned via an air cylinder whose piston is able to float relative to the piston rod in a radial direction. The piston is desirably supported on an air cushion or air bearing. Preferably the thinning assembly includes a wind up roller for receiving film from the last thinning roll (via the dance roll if present), the wind up roller being arranged to operate at a lower linear film speed than the last thinning roll. Film produced by the method or apparatus of the invention preferably shows a recovery of not more than 5%. That is, if a length of film is released from tension, its shrinkage is not more than 5%. For example, LDPE film of thickness 35μm was stretched by 300% (i.e. to four times its original length) and re-reeled at a lower speed such that the stretching was reduced to 255%. When released from tension, the film shrank by only about 2% of its length. Thus overall the process converted each lm of 35μm film to about 3.5m of film of about lOμm thickness, the film being quite dimensionally stable. The film could be safely re-reeled, and used for a variety of purposes, including wrapping.

In another aspect the invention provides film

(particularly film comprising LDPE or LLDPE) which has been thinned as indicated above, and has a recovery of not more than 5%. Other films may be usable, e.g. polypropylene and coextruded films.

In another aspect the invention provides a method of wrapping an object in film, wherein the film is produced as described above and wrapped around an object, with or without an intermediate reeling step. Film embodying the invention, which has been thinned and allowed to relax to reduce memory effects, can be used to wrap even quite delicate objects. The film can consist essentially of the usual plastics materials which are used for wrapping films (such as LDPE and LLDPE), so that it can have the usual desirable properties. Thus it can be heat-sealable, and can have good slip or cling properties and satisfactory water vapour transmission rates. In a further aspect, the invention provides an object wrapped in such a film.

Embodiments of the invention will now be described in greater detail with reference to the accompanying drawings in which:

Fig. 1 is a schematic view of apparatus for producing and using film embodying the invention;

Fig. 2 is a schematic view of the dance roll system; and Fig. 3 shows an air cylinder in more detail.

Fig. 1 shows apparatus in which relatively thick film (e.g. 35 or 15μm) 10 is converted to thin film (e.g. of thickness 10 or 5μm respectively) 10', which is fed to a packaging machine or onto a reel or to a subsequent process. The thick film 10 may come direct from an extruder 12, or may first have been wound onto a reel 14. (The extruder is shown only schematically at 12. In practice it would include means for cooling the extruded film, e.g. a chill roll, so that film would be stable before being processed. ) The film 10 passes first through a thinning unit 16. In this example, this employs three tension rolls 18,20,22. The downstream one of these (22) is driven by a driye motor 24. The others (18,20) may be driven from it via a gear train (shown schematically by the broken line 26) or held in fixed speed ratios by electronic controls, such that the rolls are driven at successively higher speeds in the direction of film travel. Alternatively, the middle roll 20 may be an idler. The upstream roll 18 may be driven by a separate motor 24A. The rolls 18,20,22 have high friction surfaces, so that they can apply a consistent level of traction to the film. This together with their narrow spacing means that neck down of the film is minimised. A preferred surface for the rolls 18,20,22 is provided by rubber with a multiplicity of small protuberances. Analogous "pimply" metal surfaces can also give good traction, but with a greater risk of tearing the film. They may be satisfactory for relatively thick films or low speeds. There is desirably also a pressure roller 23 pressing the film against the final roller 22 to minimise film slip. If the film comes immediately from a reel 14, this is conveniently mounted on a pivot arm 30 such that a reel bar can be pivoted away from the thinning unit for a reel to be mounted, whereafter it can be pivoted back and urged against the upstream roll 18, suitably by a pneumatic cylinder assembly 32. The film reel 14 is then rotated by contact with the roll 18. In cases where it becomes necessary for the film to be in greater peripheral contact with the rolls 18,20,22 then shown in Fig. 1. A nip roll may be interposed between reel 14 and roll 18 to provide an extended film path around the rolls.

The unit 16 works essentially under conditions of constant speed. Since the output film 10' may be very thin, large variations in tension cannot be tolerated. Therefore, there is a dance roll assembly 34 immediately downstream of the unit 16. This uses a displaceable dance roll 36 and a stationary roll 38, such that a variable film loop 40 is defined. The dance roll 36 is displaced by small tension changes, as will be described later.

The film 10 will typically have been extended to about three times its original length (and, therefore, reduced to about one third of its original thickness) by its passage through the stretching unit 16. A film, particularly of an LLDPE/LDPE blend, which has been extended like this may show considerable elastic memory. That is, it has a strong tendency to return at least some way towards its original configuration. It is desired to make it lose a substantial part of this tendency. This is achieved by permitting controlled relaxation. Thus, as shown in path X, the apparatus shown in Fig. 1 may incorporate a festoon arrangement designed to interpose a suitable delay between the thinning of the film and its eventual wrapping onto or about an object or reel. The festoon arrangement 42 may comprise a multiplicity of rollers 44 defining a serpentine path for the film. The rollers 44 must have a very low frictional drag, or substantial tension will be applied to the film. There may be a further dance roll assembly downstream of the rollers 44 for stabilising the tension. The festoon may be such as to delay the film 10' by several seconds. preferably at least 3 and suitably 7 or more.

Fig. 1 shows the film 10' being rewound onto a core to form a reel 45. The usual variations in extruder behaviour will lead to random fluctuation in the input to the thinning unit. (This is true even if the unit 16 is fed from a reel 14, since the film on this reel will have thickness variations as a result of the behaviour of the extruder when it was produced. ) If it were not for the dance roll assembly 34, there would be severe variations in the tension of the film downstream of the stretching unit 16. Since the film 10' is there likely to be thin and travelling at high speed, such variations carry a serious risk that the film will break, leading to a shutdown of the apparatus. The dance roll assembly and the associated control apparatus are shown in more detail in Fig. 2. The dance roll 36 is mounted on a distal end of a pivot arm 50. This has a pivot 52 at its other end, and an intermediate pivotal connection 54 to the piston rod 56 of an air piston/cylinder assembly 58. The pivot arm 50 is also coupled at an intermediate region to a position transducer P (which provides an output signal indicative of the angular position of the arm 50, and hence of the position of the dance roll 36 and the size of the loop 40); and also to a rate transducer R, which provides an output signal indicative of the rate at which the arm 50 moves. The signals from the transducers P,R are fed to a differential amplifier A, which provides an output signal which controls the stretch drive motor M (24). The speed of this motor is determined by a tachometer T, which provides a further input to the differential amplifier A. Thus the control circuitry is under feedback loop control to provide operational stability.

If the apparatus is run in line with an extruder, the thinning unit 16 has to follow the extruder output speed (e.g. using electronic speed reference). The dance roll assembly and control apparatus may then act to control the rewind unit, and hence the rewind tension.

The air piston/cylinder assembly 58 is preferably of the type shown in Fig. 4. The air cylinder 60 contains a piston 62 that is able to float relative to the piston rod 56 in a radial direction. There is a radial clearance between the piston and the cylinder wall, such that there are no rubbing seals on the piston which is supported on an air cushion or air bearing. Air at a predetermined pressure is fed to the cylinder 60 through a large diameter air pipe 64, fed from an air reservoir 66, which is supplied via a pressure regulator 68. The dance roll and pivot arm are arranged to be of low inertia. This together with the special air piston/ cylinder assembly (with very low stiction and friction effects) means that the dance roll responds rapidly to varying conditions, while the feedback loop in the control circuitry minimises the tendency to "hunt". Even when the film is being used to feed a packaging machine "on line", the dance roll can compensate for the rapidly occurring velocity transients. (It would not be possible to achieve the same effect by adjusting the speed of the motor 24, since this could not respond quickly enough. )

Preferably the nominal output velocity of the unit 16 is regulated to maintain the dance roll position within a convenient range for its pivoting movement.

During load packaging or reeling, it is advantageous to be able to vary the tension with which the film is applied. This can be done easily by adjusting the force applied to the dance roll 36 by the air piston/cylinder assembly 58.

The control circuitry can automatically determine if a break in the film occurs (causing the dance roll to move rapidly to the extent of its possible travel in one sense), and may automatically shut down operation.

As an alternative or in addition to providing relaxation time by means of a delay device such as the festoon arrangement 42, control of relaxation may involve winding up the thinned film at a speed which is controlled to be lower than the speed at which is leaves the thinning unit 16. Thus there is an interval while the film travels to where it is wound up during which it undergoes a predetermined amount of shrinkage, constituting relaxation. During this time its tension can be carefully controlled by the dance roll assembly. Path Y in Fig. 1 represents the use of only this differential speed relaxation. The film is wound on a reel 45 at a (preferably constant) linear speed which is detected (and may be controlled) by a monitor roller 70. Preferably this is coupled to a rate transducer R' (Fig. 2) which provides a further input to the differential amplifier A. The relative weights of the different inputs can be adjusted to optimise the operating conditions. We have found that the rewinding data from the monitor roller 70 should dominate, e.g. so that there is 75% feedback control from R', and 25% from R + P. This stabilises the dance roll's position and reduces hunting in the system due to changes in film tension or speed.

With optimum conditions (e.g. feedback control as just described; centre roller 20 closely spaced from the adjacent driven rollers 18,22; pressure roll 23; stretch rollers with high traction rubber surfaces), the film can pass around the dance roll with a low (and constant) tension, so it can undergo substantial recovery before it is rewound. We find that (particularly for LDPE-based films), a film must not have more than 5% recovery remaining, if it is to be reelable.

In some trial runs, 35μm cast LLDPE film was stretched and rereeled (via path Y) under the conditions shown in Table 1. Table 1: Thinning of 35μm LLDPE Film

Speed at Stretch Stretch roller 70 % at % at

(m/min) roller 22 roller 70

The force exerted by the dance roll in its mid position was 23.5N, i.e. the tension in the film was 11.75N. • This is about half the tension conventionally used for reeling film of this width (about 50cm). Over its intended range of pivoting the dance roll force varies from 28N (at one end) to 12N (at the other end).

The recovery left in the film was found to be from 2 to 3%.

The material was found 'to be heat sealable, using a hot bar sealer or an impulse sealer.

A further range of tests employed 22μm cast LLDPE film of width 506mm, thinned to three different extents. Details are shown in Table 2.

Table 2; Thinning of 22um LLDPE Film

Mach

Sample % elongation Width (mm) Mean thickness (jam) tens sides centre stre

MN/m

Al 0 (untreated) 506 23 21 34.1 A2 200 431 12 11 55.6

A3 250 428 10 9 ^• 62.0 A4 300 425 7 7 102.

The samples for tensile testing came from the centre of the web.

Table 3 compares the properties of linear polyethylene of 20μm thickness produced by Teno AB, with the same material after passage through the illustrated apparatus (Fig. 1, path Y). The passage reduced its thickness to about one third of its original value. Table 3: Thinning of 20μm LDPE Film

MPa « Mega Pascal

Nm/mm - Newton Metre Per millimeter

MD - Machine direction

TD = Transverse direction

It can be seen that the strength of the material is reduced only slightly in the transverse direction, while it is greatly increased in the machine direction. The tear resistance is also only slightly lessened in the transverse direction. The film still has some extendibility in the machine direction, and can be used for wrapping. Of course, it need not be used directly, but can be reeled and stored for subsequent use. Extruded films inevitably contain occasional small holes. When film is stretched according to the present invention, any holes are enlarged and can easily be detected by known hole detection apparatus located between the thinning unit and the rewinding or utilisation station. When the input film comes from a reel 14, it is easy to stop the process when a hole is detected and, for example, cut out the holed portion of film and possibly start winding a new reel 45. Thus it is possible to produce reels of film which are hole-free.

Film produced by a method embodying the invention has many uses. For example, it can be slit into strands for use in knitting (e.g. of sacks). It can be fibrillated (generally after irradiation, usually with an electron beam) to produce fibres. These can be extremely fine, particularly if the film is very thin. Film may be used in the production of laminates, e.g. with paper or board, typically by means of a water-based vinyl acetate copolymer adhesive. The illustrated apparatus can give the following advantages:

( 1) uniform film tension;

(2 ) operation independent of end utilisation speed;

(3 ) possibility of adjustment of film delivery tension during operation;

(4) operation independent of film type or thickness ; (5) high thinning ratios can be used without the risk of film rupture;

(6) possibility of automatic detection of film break and/or holes; (7) economy: use of less than half the amount of plastics material;

(8) ability to wrap fragile or crushable loads.

Claims

CLAIMS :

1. A method of producing a film, which comprises extruding a plastics resin film (12), allowing the film to cool sufficiently to become dimensionally stable; and stretching (16) the stable film so as to reduce its thickness; characterised in that, prior to use or reeling (45) the stretched film is allowed a dwell time under conditions (X;Y) such that elastic memory effects are substantially reduced so that the film is capable of being reeled up (45) for subsequent use.

2. A method according to claim 1 wherein the dwell time is at least 3 and preferably at least 7 seconds.

3. A method according to claim 1 or 2 wherein the film is maintained (36) under low tension during the dwell time.

4. A method according to any preceding claim wherein, during the dwell time, the film is allowed to lessen its length by a controlled amount.

5. A method according to any preceding claim wherein the dwell time conditions are selected so that the resulting film has a tendency to recover in the machine direction by shrinking by not more than 5%.

6. A method according to any preceding claim wherein during the dwell time the film is passed around a dance roll (36) whose position is automatically displaceable (50-68) to stabilise film tension.

7. A method according to claim 6 wherein the film is stretched by a series of stretching rollers (18,20,22) and the speed (24) of the last of them (22) is controlled in dependence on the position of the dance roll (36) and the speed of the film. 5

8. A method according to claim 7 wherein the speed of the film at the end of the dwell time is measured (70) and used for said control.

9. A method according to any of claims 6 to 8 wherein the position of the dance roll" (36) is determined by an

10 air cylinder (60) employing a floating piston (56,62).

10. A method according to any preceding claim wherein the film is stretched by a series of stretching rollers (18,20,22) whose film contact surfaces are of textured rubber.

15 11. A method according to any preceding claim wherein the film comprises LDPE and/or LLDPE.

12. Apparatus for use in producing a thinned plastics resin film comprising in succession a thinning unit (16), means defining a film path (36,38,44,70), and a reeling or

20 utilisation station (45), characterised in that the means defining a film path are adapted for providing a dwell time under conditions such that elastic memory effects are substantially reduced so that the film is capable of being reeled up for subsequent use.

25 13. Apparatus according to claim 12 including a respective roller (22,70) at each end of said film path, and control means for causing the downstream roller (70) to run with a lower film speed than the upstream roller (22).

14. Apparatus according to claim 12 or 13 wherein the means defining a film path includes a dance roll (36) and means (50-68) for automatically displacing it to stabilise film tension.

15. A reel of extruded plastics resin film which has been stretched in the machine direction and allowed to recover so that its residual tendency to recover is not more than 5% shrinkage in the machine direction.