GB2175246A - Process and apparatus for transversally stretching a film - Google Patents

Abstract

A process for transversally stretching and heat-setting a film using an apparatus comprising a transversal stretching zone, wherein the film is transversally stretched by being held at its lateral peripheries as it is moved forward and a heat-setting zone wherein the transversally stretched film is heat-set, said transversally stretching zone and said heat-setting zone being optionally divided into two or more sub-zones, wherein in at least one separate zone, heated air, which contains oligomers in a concentration lower than that of sublimate oligomers or substantially no oligomers, is blown onto the film surface widthwise across the film at the downstream side and/or the upstream side relative to movement of the film.

Description

SPECIFICATION Process and apparatus for transversally stretching a film This invention relates to an apparatus and a process for transversally stretching thermoplastic polymerfilms, especially polyester films. The term "transversal stretching" in this specification is used as meaning a step of widthwise stretching of a film inclusive of heat-setting thatfollows.

In the accompanying drawings, Figure 1 is a schematic plan viewofthe housing of a priorarttentering apparatus. Figures 2 and Figure 3 are respectively a schematic transversal cross-sectional view and a schematic longitudinal cross-sectional view of one zone of a prior arttentering apparatus. Figured is a schematic longitudinal cross-sectional view of a preferred embodiment of this invention. FigureS is a drawing showing the configuration of an air supply nozzle.

Polymer films, especially polyesterfilms, have excellent properties and are widely used as materials for magnetictapes and electric appliances. Ofthe materialsfor magnetictapes,for use as video tape, requirements with respect to the properties thereof have become stricter and stricter in recent years. One of the problems is deposition ofoligomers on the surface ofthe polymerfilm. Deposition ofoligomers on the surface ofthefilm is undesirable since it is a cause ofdropouts.

Various methods are proposed for the prevention of contamination with oligomers. Those methods are roughly divided into two catagories. In one, polyester materials which contain low amounts of oligomers are used. The other involves preventing deposition of oligomers, which have been sublimed from the polymer, on the film surface in the film-making stage, especially inthestep oftransversal stretching.

The transversal stretching of a polyesterfilm is commonly called "tentering" and carried out bythe method as illustrated in Figure 1. In Figure 1 a polyester film 1 is transversally stretched and heat-set in a housing 2. The housing is divided intofourzonesAto D, each zone forming an oven in which hotairis circulated. Zone A isa pre-heating zone, zone B is a transversal stretching zone, zone C is a heat-setting zone and zone D is a cooling zone. In each zone, thetemperature is regulated. As seen in Figure 1, zone B isfurtherdivided intotwo sub-zones.Zone C may be divided into a plurality of sub-zones, and the same applies with respectto zoneA and zone D. Thefourzones may be composed of a single housing or they may be conveniently separated into two or more housings. In the invention of the present invention, the apparatus comprising the four zones Ato D is called a transversal stretching apparatus. The film 1 is gripped by clips (not shown) at the lateral peripheries and is transversally or widthwise stretched. Such tentering method and apparatus are typically disclosed in U. S. Patent No. 2,823,421.

In such a tentering method, oligomers, that is, low molecular substances contained in the polymer, are sublimated in some thermoplastic polymers. The air containing a high concentration of such oligomers is entrained downstream along the line of movement ofthefilm, and the oligomers are deposited onthefllm at the cooling zone that follows the heat-setting zone, and the winding step outside ofthe tentering apparatus.

The deposits contaminate the film as streaks or powder, and adversely effect the commercial value ofthe product.

In orderto prevent this phenomenon,the following measures have been taken.

(1 ) To reduce the sublimated substance content in the entrained air by providing an air supply and exhaust device atthe upper part of the heat-setting zone; (2) To reduce the sublimated substance content in the heat-setting zone by providing a catalytic burning device in the heat-setting zone, through which the circulating air is passed; (3)To effect ventilation ofthe entrained air and exhaustion of contaminated air by introducing cooling air from outside into a cooling zone at the downstream partofthe heat-setting zone and exhausting all the introduced air; (4) To exhaust the entrained airfrom the system by providing a zone called "neutral zone" free from an air supply device between the cooling zone and the heat-setting zone, whereby exhaustion is carried out;; (5)To reduce the sublimated substance content in the entrained air by providing another zone after the heat-setting zone, wherein air supply and exhaustion is effected; or (6) To reduce the amountofthe entrained air by providing suction nozzles in a plurality of places inthe vicinityofthefilm.

However,these methods have defects as follows: (1) The amount of sublimated substances is reduced by the airsupplyand exhaust device in the heat-setting zone, but satisfactory effect cannot be expected. If a sufficient effect is intended, tremendous heat loss is caused.

(2) The catalytic burning device is effective in reducing the amount of sublimated substances. However, periodical renewal and reactivation of the catalyst is required. Also the effect is not marked at low temperatures.

(3) Providing a cooling zone atthe downstream partofthe heat-setting zone and introducing cooling airand exhausting it causes quenching of the film, which deteriorates the flatness of the film. Also, deposition ofthe sublimated substances occurs atthe beginning part of the cooling zone where the air is quenched, and therefore contamination ofthe film is unavoidable.

(4) Providing a neutral zone will reduce contamination in the cooling zone, but not to a satisfactory extent.

(5) Providing another zone after the heat-setting zone will bring about a higher degree of removal of sublimated substances than the neutral zone. However, as the cost of the apparatus is increased and the heat loss is also increased it is not a practical solution.

(6) Suction nozzles are effective for drawing out air from the heat-setting zone. However, the effect of reducing the entrained air is not remarkable. If the nozzles are placed close to the film in orderto enhance the suction effect, there is a dangerthatthe nozzles will contact and damage thefilm.

Therefore, none of these methods, is commercially satisfactory.

We conducted an intensive study searching for an effective method to prevent the deposition of sublimated substances which occurs in the heat-setting zone in film treatment. We found that contamination of films with oligomers can be efficiently prevented by blowing heated fresh air onto the film in one location in at least one zone so that an air curtain is formed near a partition in the housing, thus attaining this invention.

According to the present invention there is provided a process fortransversally stretching a film and heat-setting said film using an apparatus comprising a transversal stretching zone, wherein the film is transversally stretched by being held at its lateral peripheries as the film moves forward, and a heat-setting zone, wherein thetransversally stretched film is heat-set, said transversally stretching zone and said heat-setting zone being optionally divided into two or more sub-zones, wherein in at least one separate zone, heated air which contains oligomers in a concentration lowerthan that of sublimated oligomers or contains substantially no oligomers, is blown onto the film surface widthwise across the film at the downstream side and/orthe upstream side relative to the movementofthefilm.

The invention also provides an apparatus for use in the above process which apparatus comprises a transversal stretching zone, a heat-setting zone and a cooling zone, said zones being optionally divided into thetwo or more sub-zones,wherein means are provided in one of said zones or in atleastonesub-zonefor blowing heated air containing oligomers in a concentration lower than that ofsublimated oligomers originating from thefilm, or containing substantially no oligomers, onto the film surface widthwise across the film atthe downstream side and/orthe upstream side of said zone orsub-zone, relative to the movement ofthe film.

The invention will now be described in detail.

The polymericfilms to which the present invention can be applied are not specifically restricted, but polyesterfilms, especially polyethylene terephthalate film or polyethylene naphthalate film, are suitable.

The transversal stretching apparatus, which is a factor ofthe present invention, is known perse, and is composed of a pre-heating zone, a stretching zone, a heat-setting zone and a cooling zone as mentioned above, each zone being partitioned from the next exceptfora slit-like aperture through which the film passes.

In each zone, air heated to a temperature suitable forthatzone is circulated.

Figure 2 and Figure 3 are respectively a transversal cross-sectional view and a longitudinal cross-sectional view of a zone. Atransversal stretching apparatus is composed of a plurality of such units connected together longitudinally.

Afilm 1 moves through a housing 2. The film is held by clips 3. The air in the zone is heated by a heater4and circulated in the directions shown by arrows by means of a fan 6 rotated by a motor 5. The hot air is blown onto the film 1 through hot air nozzles 7. An airsupplyand exhaust duct 9 is connected to the zone. This duct is dispensable. The zone is separated from the neighboring zones by partitions 10.

In the transversal stretching apparatus, a neutral zone in which no hot air circulated, can be interposed between the adjacent zones.

The temperatures ofthe zones will vary according to the stretching conditions, but usuallythetemperature of the stretching zone is 80 C - 1 50'C and that ofthe heat-setting zone is 1 500C ~ 250 C. If each zone is subdivided, usuallythe temperature is set higher in each later step except the cooling zone. The temperature of the pre-heating zone is of course set lower than that of the stretching zone. The pre-heating zone is preferably separated into a plurality of sub-zones so thatthetemperatures in each sub-zones is set higherthan the preceding one.The cooling zone is provided to avoid sudden exposure ofthe heat-set film to the atmosphere, and this zone is also preferably separated into a plurality of sub-zones thetemperature in each sub-zone being lowerthan in the preceding sub-zone.

In the process ofthis invention, there is provided a means by which heated air is blown onto thefilm crosswise so that an air curtain is formed on the downstream side and/or upstream side of a divided zone. The zone may be the heat-setting zone orthe cooling zone. When the zone is sub-divided, said means need be provided only in one sub-zone. The higherthe temperature at which the film is heated, the larger the amounts of oligomers that are produced, and therefore the above-described means is most preferably provided in the heat-setting zone. Furtherit is preferred thatthe means be provided in a zone atwhich thetemperature is higherthan in the next zone. Th is will prevent deposition of oligomers in the nextzone.

It is preferred that air be blown onto the film by nozzles distributed widthwise across the film. The nozzles may be a long widthwise slit or a linear series of orifices. One air curtain efficiently formed widthwise in a divided zone may be sufficient. The nozzle or nozzles may be provided atthe upstream side or atthe downstream side of a zone, or at both sides, but it is more efficient to provide the nozzles atthe downstream side.

It is preferable to provide an outletthrough which the air blown from the nozzles is drawn out, since if heated air is only blown in, the airflow will be distributed in the zone. However, such an outlet is not essential. For instance, as shown in Figures 2 and 3, an air supply and exhaust duct 9 is provided,this can be used as exhaust duct. In a preferred embodiment, when air supply nozzles are provided on the downstream side of a zone, a special exhaust duct is provided immediately upstream of the supply nozzles as shown in Figure 4and explained later. When an air supply duct is provided upstream of zone, the special exhaustductcan be provided atthe immediately downstream side thereof.

A preferred embodiment ofthe invention is illustrated in Figure 4. The reference numerals 1 to 10 denote the same components and elements as in Figures 1,2 and 3.

Heated air is blown in through an air supply duct 17 by means of a fan 18 driven bya motor 19, the airis heated by a heater 16 and is blown ontothefilm through an air supply nozzle 15, andthusforms an aircurtain in the vicinity ofthe partition. The airthus blown in is drawn outthrough a duct 14 by means of a fan 1 2 driven by a motor 13.

The tip (opening) ofthe air supply nozzle is preferably inclined toward the upstream side ofthefilm movement. Figure 5 is a drawing which shows the shape of the nozzle. The tip (opening) ofthe nozzle is preferably inclined toward the upstream side when the nozzle is provided on the downstream side and toward the downstream side when the nozzle is provided on the upstream side. The inclination angle is between 50 and50'.

In the thus composed transversal stretching apparatus, the temperature ofthe air being blown from the nozzle should be the same as the temperature setforthatzone. Airsubstantiallyfreefrom oligomers orairthe oligomercontentofwhich is lowerthan that of the atmosphere in the zone is blown in, and thus replaces the air containing a high concentration of oligomers. The aircurtain formed nearthe partition prevents the flow of oligomers into the downstream side zone orthe upstream side zone.

Preferably fresh air substantially free from oligomers brought in from the outside of the stretching apparatus is used. However, the air in a zone where the temperature is set lower or the airwhich has been subjected to a treatmenttoburntheoligomerstherein may be used.

The rate ofairflowing from the nozzles depends on the distance between the tip ofthe nozzle andthefilm (preferably between Sand 20 cm) and the inclination angle ofthe nozzle end. However, the flow must not beso high that it causes the film to bend. Specifically, it should be in the range of 3 to 20 m3/min per meter of width. It is preferred that the air blow speed is higherthan the speed of hotair being blown outfromthe hotairnozzles (7 in Figure 2) which circulate hot air in the zone.

The invention will now be explained by way of working examples, which are notto be taken as limiting the invention.

Poly(ethylene terephthalate) was extruded into the form of a film, cooled and solidified. The film was longitudinally stretched at 1 000C by a factor of 3.7, and then was transversally stretched by the above-mentioned tentering process by a factor of 3.7 and heat-set. Thus a film 2000 mm in width and 6 1# Fin thickness was obtained.

In the tentering apparatus, the temperature of the pre-heating zone was 80 C, that of the transversal stretching zone was 1 000C, that of the heat-setting zone was 2300C, and that ofthe cooling zone was 600C. The speed ofthe film movement was 100 mlmin and the length of heat-setting was 3 seconds.

Aslit-like air-supply nozzle was provided over the width of the film at a position 30 cm from the downstream side partition of the heat-setting zone. The arrangement ofthe nozzle and the air supply flow rate were as indicated in Table 1. The distance between the end ofthe nozzle andthefilm was 120 mm. When an exhaust area is provided, an exhaust duct as shown in Figure 4 is provided on the upstream side of the air supply nozzle close thereto. The exhaust was regulated so that the rate of leakage ofairthrough the aperture ofthe downstream side partition was the same as in the case of the conventional stretching. The measurement ofthe sublimated substance concentration was carried out as follows.A suction part was provided at a position 500 mm downstream from the air supply nozzle and spaced from the film surface by 10 mm. Airwas drawn in therethrough and trapped with methanol. The concentration ofthe solution was measured by UV spectrometry. The air speed was measured by hot wire anemometer.

The results of the measurement are shown in Table 1.

TABLE 1 Examples & Structure of apparatus Incli- Air speed (m/sec) Flow rate Sublimated Comparative nation of substance Examples Air supply Evacuation angle Hot air Airsupply Air supply Concen Nozzle Area Nozzle Nozzle tration ( ) (m3/mm) (mg/m3) Ex.1 Vertical None 0 10 12 11 2.2 Ex.2 Vertical Provided 0 10 12 11 0.7 Ex.3 Vertical Provided 0 10 20 18 0.5 Ex.4 Inclined None 20 10 12 11 1.4 Ex.5 Inclined Provided 10 10 12 11 0.5 Ex.6 Inclined Provided 20 10 12 11 0.3 Ex.7 Inclined Provide 30 10 12 11 0.3 Ex.8 Inclined Provided 45 10 12 11 0.6 Ex.9 Inclined Provided 60 10 12 11 1.6 Ex.10 Inclined Provided 20 10 20 18 0.2 Comp. Ex. 1 None None - 10 - 11 22.7 In the present invention, deposition of oligomers on the film in the tentering thereof was remarkably reduced by supplying hot air on the downstream side and/or upstream side in at least one zone ofthetentering apparatus.

Claims (20)

1. A processfortransversally stretching and heat-setting a film using an apparatus comprising a transversal stretching zone, wherein thefilm is transversally stretched by being held at its lateral peripheries as it is moved forward and a heat-setting zone wherein the transversally stretched film is heat-set, and said transversally stretching zone and said heat-setting zone being optionally divided into two or more sub-zones, wherein in at least one separate zone, heated air, which contains oligomers in a concentration lower than that of sublimated oligomers or contains substantially no oligomers, is blown onto the film surface widthwise across the film atthe downstream side and/orthe upstream side relative to the movement ofthefilm.

2. A process as claimed in claim 1 wherein in a separate zone in said transversally stretching zone and/or said heat-setting zone, said heated air is blown onto the film on the downstream side and is drawn outthrough an evacuation duct provided on the upstream side ofthe heated airsupply.

3. A process as claimed in claim 1 or claim 2 wherein the film is a polyesterfilm.

4. A process as claimed in claim 3 wherein the film is a polyethylene terephthalate film or a polyethylene naphthalate film.

5. A process as claimed in any one of the preceding claims wherein said means is provided in a zone or sub-zoneatwhichthetemperature is higherthan in the next zone orsub-zone.

6. A process as claimed in any one of the preceding claims wherein the said means is provided in the heat-setting zone.

7. A process as claimed in any one of the preceding claims wherein air is blown onto the film by nozzles distributed widthwise across the film.

8. A process as claimed in claim 7 wherein each nozzle is inclined towards the upstream sideofthefilm movement when the nozzle is on the downstream side ofthe zone orsub-zone and inclined towards the downstream side when the nozzle is on the upstream side ofthe zone orsub-zone.

9. A process as claimed in claim 1 substantially as hereinbefore described with reference to and as illustrated in Figure 4 or Figure 5 of the accompanying drawings.

10. A process as claimed in claim 1 substantially as hereinbefore described with reference to any one ofthe Examples 1 to 10.

11. Transversally stretched film whenever produced by a process as claimed in any one of the preceding claims.

12. An apparatus for use in the process as claimed in claim 1 which apparatus comprises a transversal stretching zone, a heat-setting zone and a cooling zone, said zones being optionally divided into the two or more sub-zones, wherein means are provided in one ofsaidzones or in at least one sub-zoneforblowing heated air containing oligomers in a concentration lowerthan that of sublimated oligomers originating from the film, or containing substantially no oligomers, ontothefilm surfacewidthwise across the film atthe downstream side and/orthe upstream side of said zone orsub-zone, relative to the movement ofthefilm.

13. An apparatus as claimed in claim 12 wherein said means is provided in azoneorsub-zoneatwhichthe temperature is higher than in the next zone or sub-zone.

14. An apparatus as claimed in claim 12 or claim 13 wherein said means is provided in the heat-setting zone.

15. An apparatus as claimed in any one of claims 12to 14 wherein said means comprises nozzles distributed widthwise across the film.

16. An apparatus as claimed in claim 1 wherein each nozzle is inclined towards the upstream side ofthe film movement when the nozzle is on the downstream sideofthezone orsub-zone and is inclinedtowardsthe downstream side when the nozzle is on the upstream side of the zone orsub-zone.

17. An apparatus as claimed in claim 1 6 wherein the angle of inclination ofthe nozzle is between 50 and 500.

18. An apparatus as claimed in any one of claims 1 5 to 17 wherein the distance between the tip ofthe nozzle andthefilm path is between 5 and 20 cm.

19. An apparatus as claimed in any one of claims 12to 18 wherein said means also comprises an exhaust duct located close to the film path.

20. An apparatus as claimed in claim 12 substantially as hereinbefore described with reference to Figure 4 or Figure 5 of the accompanying drawings.