WO2005102669A1 - Tenterframe stretching process and apparatus for same - Google Patents

Abstract

A method and apparatus for transverse stretching and orienting of an orientable film or polymer films wherein the apparatus comprises a tenterframe, in which both edges of the film are held by continuously advancing lateral expansion means (251) and the film is heated such that the width of the film is gradually expanded as it is advanced through the apparatus by expansion means. The orientation process at each position of the passage is essentially limited to one or two narrow neck-zones each controlled by narrow heating means (252) which, seen in the transverse dimension of the apparatus, each act over a narrow space and over which the film is passed in frictionless or low friction engagement. The longitudinal direction is angularly disposed to the film travel such that in each film cross­section the narrow neck-zone or zones gradually proceeds over the width of the film until essentially the entire width to be oriented has been passed by such zone or zones.

Description

TENTERFRAME STRETCHING PROCESS AND APPARATUS FOR SAME The invention concerns a new tenterframe stretching process for films of thermoplastic material improved mainly for the purpose of enabling a 5 wider range of physical stretching conditions and thereby use of the tenterframe process to produce product not currently available. Furthermore the improved process is energy saving when adequately carried out. In a continuously working tenterframe, there is a pair of continuously moving chains or similar each supplied with closely spaced holding devices0 which clamp the edges of the orientable film of polymer material as it enters the tentering device and open to release the film when the tentering has been completed. One edge is clamped to one chain, the other edge to the other chain. The two chains are driven and supported by suitable means, and the distance between the two chains gradually expands in the direction5 of the advance, such that the film clamped to the many holding devices gradually becomes transversely stretched, provided it is sufficiently stretchable at the temperature applied. To make the film sufficiently stretchable it is, as the tenterframe process presently always is practised, kept at a very elevated temperature during the entire tenterframe set-up, or 0 at least the film portions between the two chains with holding devices, is encased in an oven. For the purpose of stabilising the oriented film, the arrangement may at the end comprise a zone where the chains move without further expanding the film, or even allow the film some lateral contraction.5 Presently the transverse tenterframe stretching process is used entirely in the manufacture of biaxially oriented film, such as biaxially oriented PP, HDPE, nylon or PET film or coextruded film based on one of thesfe' polymers. In the so-called sequential biaxial stretching procedure, the .film js ongitudinally stretched in a roller process prior or subsequent to the o tenterframe process, and the latter is carried out in the simple way described above:,. In the so called simultaneous biaxial stretching procedure the distance between the individual holding means on each of the "chains" is gradually increased at the same time as the "chains" perform the lateral expansion, and thereby longitudinal and transverse stretching take place simultaneously. This simultaneous procedure requires particular high stretching temperatures. Most of the film compositions which today are processed in tenterframes, can be uni-axially orientated by stretching at room temperature or even lower temperatures, no matter whether the orientation takes place parallel with or transverse to the direction of the extrusion by which the film was made, provided the film to be stretched has been cut to narrow strips, e.g. strips of about 1-3 cm width. During such laboratory stretching the film will contract transversely to the longitudinal direction of the strip at the same time as its thickness becomes reduced. The amount of transverse contraction depends to some extent on the melt-orientation in the film, but normally this dependence is insignificant, and the ratio by which the width becomes reduced will approximately equal the ratio by which the thickness is reduced. The volume of the stretched material remains unchanged as any change in density is insignificant and therefore the ratio by which both thickness and width become reduced approximately equals the square root of the stretch ratio. If e.g. the stretch ratio is 4:1 , both width and thickness will become reduced to about half of the values they had before the stretching. In extreme cases in which the melt-orientation is very high, and the strip has been cut out parallel to the direction of melt-orientation, the elongation during the stretching may be evenly distributed over the entire length which is under tension, but the normal situation is that the material "necks down" in a relatively narrow "neck-zone" (i.e. the reduction of thickness and width is limited to this zone) which then gradually advances over the length of the strip until the full length under tension has become oriented. By further stretching, the strip elongates evenly over this length and eventually breaks. Normally the neck-zone will extend over between about 1 mm to 1-2 cm. When a film is stretched in a tenter-frame at about room temperature, it will similarly start to stretch in a neck-zone, or several neck-zones, but there is no possibility for this zone or these zones to develop evenly, and the tendency in the film material to contract in its length during elongation of its width cannot be satisfied, and therefore the film breaks. Increase of the stretching temperature gradually reduces the problems, but as mentioned above an even stretching requires temperatures rather close to the melting range of the film. However, the inventor has found that the morphology of an oriented film very much depends on the stretching temperature, and that stretching at low temperatures promotes the kind of strength properties which is required for a high energy absorption. This is e.g. normally required in cross- laminates used e.g. for industrial packaging, cover sheets, tarpaulins and pond liners. In such products there is a need for high tear propagation resistance and high puncture resistance, which depend on energy absorption. In the most widely used manufacturing process for cross-laminates the starting material is tubular film. This is first longitudinally stretched near room temperature (which is much simpler than transverse stretching near room temperature) and then cut on bias to produce a web in which the direction of orientation forms an angle of e.g. about 45 degrees to the machine direction. Two such webs are then continuously laminated with their orientations crossing each other. Stabilization by heat can be carried out at any stage after the orientation at low temperature and does not essentially affect the energy absorbing properties. However, this and other manufacturing processes for cross-laminates, which all use spiral cutting, cannot be carried out with the different steps following one another in-line, and for that purpose it would be greatly advantageous to transversely stretch one film at a relatively low temperature in a tenter-frame, and laminate it to a longitudinally stretched film, likewise stretched at a relatively low temperature. In this way all process steps from extrusion to lamination can be carried out in-line. The manufacture of cross-laminates is an important example of an application where there is a need for a tenter-frame process which can be carried out at relatively low temperatures, but there are other applications of the present invention, mainly for manufacture of bi-axially oriented film with improved energy absorbing properties. The improved tenter-frame process according to the present invention is based on the principle that the transverse stretching is started as one or two narrow neck-zones controlled by local heating, and while the film is advanced under transverse tentering, this or these neck-zones are "scanned" over the width of the film, controlled by localized heating. This heating is produced by one or two heating means ("the orientation controlling means") which are angularly disposed to the direction in which the film is moved, and over which the film moves in frictionless or low- frictional engagement. The process is more precisely defined in claim 1. In order to localize the necking-zone or -zones as precisely as possible, the heating is preferably immediately followed by cooling using cooling means which likewise act over a narrow space, are passed by the film in frictionless or low-frictional manner and which extend longitudinally in close proximity to the orientation controlling means. If the present invention is used for biaxial stretching, it will be a sequential biaxial stretching process, and the film will be oriented in the machine direction prior to or in succession to the transverse expansion. In order to optimize stretching temperatures and/or make transverse orientation most uni-axial, a preferred embodiment of the invention is characterized in that prior to the engagement of the film edges with the lateral expansion means (the chains) the film is brought in a pleated state with the pleats laterally extending. This can e.g. be achieved by feeding the film in between rubber belts in a roller arrangement where the belts are kept highly tentered at the inlet and thereafter relaxed. In order to maintain the film most regularly pleated until it meets the orientation controlling heating means there is preferably used a multitude of pairs of narrow belts, each pair adapted to carry the pleated film close to the heating means before it releases the film (see Fig. 2). The transverse pleating gives the film a possibility to contract in the direction perpendicular to the direction of stretching, and as it will appear from the introduction this will highly facilitate the stretching. The pleating "anticipates" the contraction. If the pleating is sufficiently deep, the film can freely contract like a narrow strip, which is stretched in its longitudinal direction. If the film is longitudinally oriented by stretching before or in succession to the transverse stretching, it is preferably supplied with longitudinally extending pleats prior to this longitudinal stretching in order to enable a lower stretching temperature. Prior to the transverse pleating the film may be supplied with closely spaced transverse narrow linear zones thinner than the average by incremental longitudinal stretching e.g. between gear rollers, preferably while keeping the main body of the film at a temperature about or lower than 50°C. This helps to make the pleat fine and regular. The distance between such lines may e.g. be about 0.5-3 mm. Even a small degree of incremental longitudinal stretching will help to produce an even pleating, and furthermore it has been found that the longitudinal stretching zones can act as initiators for the transverse stretching. Without such transverse thinner lines the width of the pleats will normally be irregular and will strongly depend on the thickness and stiffness of the film. Widths beyond about 20-30 mm should normally be avoided. Simultaneously with the incremental stretching these transverse lines may be heated to a temperature essentially higher than that of the main body of the film by heating the gear rollers (or equivalent devices). When the stuffing process immediately succeeds this stretching, the higher temperature in the thinner lines can promote a deep and regular pleating, and due to the cooling of these lines which will take place in the stuffing device, the pleats can become stabilized, whereby the subsequent tentering process is facilitated. When the present invention is used to manufacture crosslaminates, there may advantageously be formed a rather deeply embossed pattern in the final product by means of this incremental stretching process, e.g. when the film thickness in these lines is lower than about 80% or even lower than about 50% of the thickness of the adjacent film material. Several interesting effects can hereby be achieved, especially when the division of the lines in the final film is particularly low, i.e. no more than 2 mm, or better about 1 mm or even 0.5 mm or lower, and when further the bonding is by heatsealing and established through one or more heatseal layers under a pressure which is sufficient to establish bonding on the bosses, but insufficient for bonding between the bosses. One advantage of such a crosslaminate is that it has a textile-like appearance. A second advantage concerns the tear propagation resistance of the crosslaminate. It is known that this is best when the bonding is limited to lines or spots, but such improvement has never before been achieved by means of embossment. A third advantage concerns a possibility to use the laminate for "house-wrap-film" or for different kinds of filter material. For this purpose the plies of the laminate must be perforated prior to the lamination, and in the laminate the perforations in the different plies must be mutually displaced in such a manner that air can pass from one surface of the laminate to the other surface of the laminate but instead of passing directly through the laminate has to follow one or more of the capillaries which are formed between the plies by virtue of the embossment (the sequential stretching). For all three above mentioned purposes, the pattern of embossment should preferably be as fine as practically possible, and in this connection it is an advantage that the present transverse stretching method results in a contraction which is perpendicular to the direction of the thin lines. 5 For all three above mentioned purposes it is furthermore preferable that the longitudinally oriented film, to which the transversely oriented film is laminated, also is embossed in a fine, linear pattern which is generally parallel with its direction of orientation, in this case the longitudinal direction. For this purpose the embossment can be carried out by means of0 intermeshing grooved rollers with fine circular or helical grooves. The longitudinal stretching should preferably be carried out subsequent to the embossment and in such a manner that contraction in the transverse direction is allowed, i.e. it should preferably be preceded by pleating. The described structure of a crosslaminate, in which at least one but5 preferably more of the plies is supplied with a fine pattern of lineary embossment, generally parallel with its direction of orientation, and the bonding is limited to the linear bosses and is established through one or more lamination layers, is considered an invention in itself, independent of the present transverse stretching method. However, this method is o particularly suitable in the production of the said structure. For the sake of completeness it should be mentioned that such structures can also be manufactured by first embossing a lay-flat tubular film by passing it through a pair of intermeshing grooved rollers, having a fine pattern of circular or helical grooves, then longitudinally stretching the tubular film, preferably5 while allowing a lateral contraction, subsequently helically cutting the tube in a well known manner to form a film with orientation and embossment on bias, and finally laminating two or more helically cut films of which at least one has the described embossed pattern, with their directions of orientation crossing each other. o Going back to the description of the present transverse stretching method, the orientation controlling means in a preferred embodiment of the invention is a heated air-lubricated bar, or a pair of heated air-lubricated bars between which the film is passed. This can provide for efficient heat transfer almost as had there been real contact, at the same time as the engagement can be fully frictionless. Alternatively, the orientation controlling means may comprise one or more heated bars, which by fast vibrations are brought into and out of contact with the film. However, it is also within the scope of the invention to use, as orientation controlling means, a narrow oven with heated air. Downstream of the long, narrow heating means there may be a heated oven in which the film is further transversely stretched and/or stabilized. However, heat stabilization is normally best carried out on heated rollers. In the introduction it has been mentioned that energy saving can be achieved. This is partly because of the lower stretching temperature and partly because of the smaller heating means, in both cases enabling a limitation in the loss of heat. Most of the hot air used for the air lubrication can be collected and recycled. With particular view to the manufacture of crosslaminates or cheap biaxially oriented film, an embodiment of the invention is characterised in that at least a major layer of the film consists of HDPE or isotactic or syndiotactic PP or of a blend comprising at least 40% of such polymer.

However, the method of the invention is also suitable e.g. for stretching of film consisting of polyethylene terephthalate, polyamide or copolymers of vinylidene chloride. The invention also comprises the apparatus for carrying out the method. The invention shall now be explained in further detail with reference to the drawings. Fig. 1 a is a diagrammatic representation of a tenterframe showing one embodiment. Fig. 1 b represents the unit in Fig. 1a which performs the transverse pleating. Fig. 1c shows schematically the cross-section of the two narrow heating means ("ovens") with connected cooling means. Fig. 2 is generally similar to Fig. 1a but illustrating more efficient and complex means for transverse pleating, and further showing a stabilization at the end of the process. The purpose of the apparatus shown in Figs. 1a, b and c is manufacture of a mainly uniaxially transversely oriented film formed of thermoplastics material, e.g. for subsequent lamination with a mainly uniaxially longitudinally stretched film. As mentioned above, the usual tenterframe process is not very suitable in such connection. This is partly because it requires relatively high stretching temperatures, and partly because it is difficult to limit the amount of stretching. In order to obtain fully satisfactory energy absorbing strength properties in crosslaminates, the stretching temperature should preferably be lower, e.g. around or lower than 50°C, and the degree of orientation should normally be far from the ultimate limit e.g. between about 2:1 and 4:1. To make the transverse orientation generally uniaxial, the film is first supplied with fine pleats, which will enable its longitudinal contraction during the transverse extension. This process step is labelled "stuffing between rubber belts" - see the box in Fig. 1 a - and is shown in Fig. 1 b. The film (211 ) is fed into the nip between two rollers (229 and 230) and supported by the two endless rubber belts (231 and 232). It is delivered in pleated form from the nip between two rollers (233 and 234). The first pair of belt rollers (229 and 230) are driven at a circumferential velocity which is significantly higher than that of the second pair of belt rollers (233 and 234) e.g. about twice as high. The ratio between the two velocities is adjustable. Thus the two rubber belts become stretched out when they leave the nip between the second pair of belt rollers (233 and 234) and return to a less stretched state when they leave the nip between the first pair of belt rollers (229 and 230). Since the belts convey the film, the film is stuffed (becomes transversely pleated) between the belts (231 and 232). The degree of pleating should normally be adjusted so that it corresponds with the tendency to longitudinal contraction during the transverse stretching. The rollers (246) of a relatively small diameter serve to firmly hold the pleated film between the rubber belts such that the pleats remain fine and regular. They are idle rollers. Prior to the stuffing process, the film may pass a pair of intermeshing gear rollers, which stretches it incrementally in the longitudinal direction and hereby forms transversely extending lines which are thinner than the average thickness. As the pleated film leaves the stuffing device, its edges are gripped by holding clamps in endless tenter-chains (251 - see the schematic sketch Fig. 1 a) which can be a conventional tenterframe construction. The preferred embodiment of the novel concept of heating devices

("ovens") (252) for a tenterframe is schematically shown in Fig. 1a, and its cross-section is shown in detail in Fig. 1 c. The "oven" consists of two parts (235 and 237) made from microporous metal each with support parts (239 and 240) made from compact metal. The component (235) and the part (237) are heated to an appropriate, controlled stretching temperature, e.g.

50°C, by means of electrical heating elements (241 ). Pressurised air is distributed through the channels (242) between parts (235/236) and (237) on the one hand and the support parts (239 and 240) on the other for lubrication. The component (236) which is of similar construction to the part (235) is not heated, and the air lubrication through this will provide cooling while the air lubrication through the adjacent parts (235) and (237) will provide heating. The heating component (235) is insulated from the cooling component (236) by means of a heat insulating wall (247) of compact material. The cooling component (236) is on the side of each "oven" which faces its nearest tenterchain. The spacing between components (235) and (237) is adjustable as indicated by the double arrows (243). This adjustment may be a simple setting of the distance, or may be through springs with adjustable tension. A preferable spacing between (235) and (237) is between about 0.5 and 2mm. Fig. 1c shows the film entering the "oven" in pleated form (244) at the righthand side and leaving the oven tentered and with the pleats stretched out (245) in the lefthand side. In a conventional tenterframe oven, the film is stretched simultaneously over its full width, and carrying this out evenly requires a rather high temperature, which as mentioned is generally not adequate e.g. in the manufacture of crosslaminates. With the present constructions, when the parameters are properly adjusted to each other, the tentering takes place within a very narrow zone, a necking zone, usually only a few mm wide or less, located on each of the cooling components (236) associated with the two narrow "ovens" (252) shown. Due to the air lubrications, the film moves through the "ovens" in a frictionfree manner, while the necking zone gradually develops from a position close to each of the chains with holding clamps towards the middle of the film. Thus all film between a narrow "oven" and the nearest chain (except a narrow zone near the chain) is fully transversely stretched up to the stretch ratio set, while the film between the two narrow, long ovens are not at all transversely stretched. At the downstream end of the two narrow, long ovens, all the film is generally even stretched. When the stretching conditions are properly set, the necking zone will as mentioned be located on the cooling part. This has the effect that the degree of stretching better can be limited, as is generally desired e.g. in the manufacturing of crosslaminates, for the purpose of allowing further stretching under impact actions (shock) on the final product. In Fig. 2 each of the two rubber belts (231 and 232) has been substituted by a large number of narrow pairs of belts (248 a, b, c, etc.). The roller pair (229) and (230) is shown as an interrupted line (229/230) which indicates the position of their centrelines. The roller pair (233) and (234) is substituted by a big number of roller pairs, each pair also indicated by an interrupted line (249 a, b, c, etc.). The construction also comprises support rollers like (246) in Fig. 1 b, but for the sake of clarity these are not shown. Fig. 2 also shows that there may be a conventional oven, indicated by the interrupted lines (250) which covers the entire width of the film (normally with exception of the clamped portions) during the last part of the transverse stretching. Hereby irregularities in the previous stretching can be equalized. Especially there may occur such irregularities in the middle of the film where the two "ovens" (252) meet. Fig. 2 further shows that the "chains" (251 ) may converge to some extent at the very end of the process to allow shrinkage of the film before the latter is released. As mentioned in the foregoing, longitudinal stretching by means of rollers may be carried out prior to or subsequent to the described tentering. At the very end of the process line the film may be stabilized by passage over heated rollers, followed by passage over a cooled roller. In the drawings Fig. 1a and Fig. 2 the transverse stretching begins close to each of the "chains" and gradually progresses towards the middle of the film. This is judged to be preferable, but alternatively the "ovens" may be arranged such that the transverse stretching begins in the middle of the film and gradually progresses towards each of the edges. Finally there may be only one long "oven" such that the transverse stretching begins close to one of the "chains" and gradually develops towards the other "chain".

Claims

CLAIMS 1. A method for transverse stretching and orienting of an orientable film of polymer materials in a tenterframe apparatus, in which both edges of the film are held by continuously advancing lateral expansion

5 means, (251 ), and the film is heated such that the width of the film gradually is expanded while it is advanced through the apparatus by the expansion means, characterised in that the orientation process at each position of the passage essentially is limited to one or two narrow neck-zones each controlled by long, narrow heating means (252) which, seen in the0 transverse dimension of the apparatus, each acts over a narrow space and over which the film is passed in frictionless or low-friction engagement, and has its longitudinal direction angularly disposed to the film travel in such way that in each film cross-section the narrow neck-zone or zones gradually proceeds over the width of the film until essentially the entire width to5 become oriented has been passed by such zone or zones. 2. A method according to claim 1 , characterised in that at each position of the transverse orientation process, the heating (235, 237) is immediately followed by cooling, using cooling means (236) which likewise act over a narrow space, are passed by the film in frictionless or low- 0 fhctional manner and which extend longitudinally in close proximity to the heating means. 3. A method according to claim 1 or 2, characterised in that the film is oriented in the machine direction prior to or in succession to the transverse expansion.5 4. A method according to any of the preceding claims, characterised in that prior to the engagement of the film edges with the lateral expansion means (251 ), the film is brought in a pleated state with the pleats laterally extending. 5. A method according to claim 4, characterised in that the o pleated state is achieved by feeding the film in between rubber belts (231 , 232) in a roller arrangement (229, 230, 233, 234) where the belts are kept highly tentered at the inlet and thereafter relaxed. 6. A method according to claim 5, characterised in that there is used a multitude of pairs of narrow belts (248 a, b, c ....), each pair adapted

5 to carry the pleated film close to the heating means before it releases the film. 7. A method according to any of the claims 4-6, characterised in that prior to the pleating the film is supplied with closely spaced transverse narrow linear zones thinner than the average by incremental longitudinal0 stretching e.g. between gear rollers, preferably while keeping the main body of the film at a temperature about or lower than 50°C. 8. A method according to any preceding claim, characterised in that the heating means is a narrow oven with heated air. 9. A method according to any of the claims 1 -7, characterised in5 that the heating means is a heated airlubricated bar, or a pair of heated airlubricated bars (235, 237) between which the film is passed. 10. A method according to any of the claims 1 -7, characterised in that the heating means comprise one or more heated bars, which by fast vibrations are brought into and out of contact with the film. 0 11. A method according to any of the preceding claims, characterised in that downstream of the long, narrow heating means there is a heated oven (250) in which the film is further transversely stretched and/or stabilized. 12. A method according to any of the claims 1 to 11 , characterised 5 in that at least a major layer of the film consists of HDPE or isotactic or syndiotactic PP or of a blend comprising at least 40% of such a polymer. 13. Apparatus for transverse stretching a film of polymer material, comprising a tenterframe comprising continuous advancing lateral expansion means (251) on both lateral sides, to which both edges of the film may be o held, and means for heating the film, characterised in that the heating means comprise one or two long heaters (252), over which the film can pass in substantially frictionless or low friction engagement the or each heater being narrow in the transverse direction relative to the width of the apparatus extending generally in but at angle to the machine direction, and the said heater(s) extending across substantially the width of the stretching 5 apparatus. 14. The apparatus according to claim 13 in which the heating means comprise two long narrow heaters. 15. The apparatus according to claim 14 in which the heaters are arranged symmetrically on opposite lateral sides of the apparatus.0 16. The apparatus according to claim 15 in which the upstream ends of the heaters are positioned close to or at the lateral edges of the film position. 17. The apparatus according to claims 13 to 16 which further comprises cooling means (236) arranged adjacent to and downstream from5 the heating means. 18. The apparatus according to any of claims 13 to 17 comprising a longitudinal orientation station for the film upstream or downstream from the tenterframe. 19. The apparatus of any of claims 13 to 18 which further o comprises a pleating station immediately upstream from the lateral expansion means, for providing laterally extending pleats in the film whereby the film is supplied in pleated form as the lateral expansion is initiated. 20. The apparatus according to claim 19 in which the pleating station comprises at least one pair of continuous rubber belts between which5 the film passes and each of which is driven by a set of rollers which maintain the respective belt highly tentered at the upstream portion and more relaxed further downstream. 21. The apparatus according to claim 20 in which the pleating station comprises a multitude of such pairs (248 a, b, c) of rubber belts, o arranged side-by-side across the width of the apparatus and extending downstream to a position close to the heating means whereby the film is released by the belts before reaching the heating means. 22. The apparatus of any of claims 19 to 21 further comprising a longitudinal stretching station upstream from the pleating station at which

5 incremental stretching of the film in a longitudinal direction across the width of the film is carried out in closely spaced zones, for instance comprising gear rollers, preferably adapted to keep the main body of the film at a temperature controlled to less than 50°C. 23. The apparatus according to any of claims 13 to 22 in which the0 or each heater is an oven for heating air and directing it at the film. 24. The apparatus according to any of claims 13 to 22 in which the or each heater is a heated air-lubricated bar, or pair of such bars (235, 237) between which the film can pass. 25. The apparatus of any of claims 13 to 22 in which the or each5 heater is a vibrating heated bar, arranged for intermittent contact with the film. 26. The apparatus of any of claims 13 to 25 which further comprises a stabilisation station downstream of the said heating means in which the film can be transversely stretched or stabilised whilst being o heated. 27. The apparatus according to claim 26 in which the stabilisation station is either an oven or comprises heated rollers over which the film can be passed.