DE2361368A1 - Flat thermoplastic film with uniform thickness prodn - by stepwise cooling of molten film, stretching film transversely and solidifying - Google Patents

Abstract

Flat thermoplastic film extruded from wide slot dies is obtd. with a more uniform profile contg. fewer thicker parts by (a) effecting stepwise cooling of the molten film to a temp. above the solidification temp., (b) effecting stretching of the film across the direction of extrusion, and (c) cooling the molten film to a solid film. Method enables thin films, e.g. with a final thickness of 2-50 mu to be produced with a more uniform thickness.

Description

K 2278. FP-Dr.Kn-df December 7, 1973

- Description to register the

KALLE AKTIENGESELLSCHAFT Wiesbaden-Biebrich:

for a patent

Process for reducing thick spots in the production of flat foils from thermoplastics / ■ ■ '

The present invention relates to a method in which the changes in thickness that inevitably occur during the production of the foils are reduced by four and thus improved thermoplastic foils are produced.

Melters see in the production of FoTi en from thermopTasti thick and thin spots occur due to their expansion a number of errors and undesirable accompanying

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evoke stalls. These areas are different Thicknesses run in strips in the longitudinal direction of the Foil web and keep their position over a long period of time. The strips can be different Have causes, such as minor changes in the Surface roughness and mechanical injuries the extrusion nozzle or wetting differences in The melt emerges from the extrusion nozzle.

The foils are made from the thermoplastic material then stretched longitudinally and / or transversely or vice versa, the above-described defects in the film web, still further strips are added, their The cause is uneven stretching.

Since these strips of different thickness their position in of the film web for several 10,000 m- maintain, add up the deviations when rolling up the film. This results in ring-shaped bulges on the roller of thick spots and ring-shaped grooves due to thinnest spots.

Both the beads and the grooves lead to very detrimental effects permanent embossing and distortion of the film Furthermore, they prove to be very disadvantageous when further processing (e.g. metallizing and coating) the Foil web and must therefore be prevented. 509824/0780

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In addition to the problems that arise from further processing of the foils that are defective in this respect, there is the problem of detection for quality control and classification of this flange! "-or error.

Both the processing of defective foils and the Detection of these thickness-dependent defects cause Costs that are partly avoidable.

The inventive method reduces defective Deviations from the mean film thickness already on Melt film regardless of its formation. It causes a more uniform film thickness over the entire Web width and thus serves to improve quality.

Figure 1 shows the recording of a thickness measurement of a stretched film web. You can see fluctuations in thickness, their size on the unstretched prefilm can be up to several percent of the average thickness, and the transverse extent of several centimeters range up to a few micrometers. The recorded deviations are usually time-, constant and are sometimes referred to as jet strips

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One method used to date to reduce the streaks is to deform the continuous nozzle lips with the aim of adjusting the ex.trusi ons late. Adjusting screws are used for this purpose, which tighten the nozzle lips. The chosen construction However, it proves not to be sufficiently effective in the middle area of the slot nozzle in particular, because both a reaction-free adjustment and a discrete gap width change in the area of an adjusting screw not possible. In contrast, the division of the nozzle lip into segments (Fig. 2) enables a Reaction-free setting without the previous great effort, even in the middle area of the slot nozzle. According to this construction, the deformation of the nozzle lip is also particularly suitable with regard to the subsequent explained procedures for the rough division of the Thickness profile of the melt film.

Concentrate efforts on quality improvement in this application refer to the stripes that are due its small width from the adjusting screws can be influenced directly. These thick and thin areas, like the wider ones, already appear in the melt film on. They will be extended in the following insurance even more so, because the temperature of the thick areas of the foil after the first cooling of the melt film

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-in. in practice is always less than or equal to that of the thinner areas. In the event of stretching, this always leads to greater elongation of the thinner sections, that is, the original thin areas of the prefilm become even thinner relative to the thickness of the film, and the original thick areas are relatively even thicker. So the mistakes are amplified.

A reduction or leveling of the thick places requires ■ greater elongation of the same. This is only possible if the temperature of a thicker part is always higher than that of a thinner one, since the ability to flow increases with the temperature. Efforts to achieve this state have been known for a long time, e.g. . . ".". from the US-PS 2 851 733. In the previously beschri ttenen "- Because, however, had the desired temperature condition in particular for reasons of today's popular hoh, s operational speeds not -safe verwirk ^cozy:.

The task thus arose of a "relatively technical simple process to develop the thick parts in FoI ienbah.nen closed; reduce and at the same time a .Ver - "-" - uniformity to achieve the slide.

The above object is achieved by _a: method for reducing slubs and improving the large professional Ts in. Manufacture of from

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Slot dies extruded thermoplastic films, which is characterized in that one of the Slot die extruded melt film above the The solidification temperature gradually cools, thereby stretching across the direction of extrusion and only then does the melt film cool down to form a solid film. The prerequisite for the stretching process is the thickness-proportional temperature state of the film, independent from the working speed or other disturbances, safely reached,

When the melt emerges from the nozzle, the temperature of the melt film is above the melting point. The cooling off runs according to an exponential function depending on the heat content of the respective film strip (Fig. 3) Because the heat content is almost proportional in relation to the film thickness, results during cooling under the same cooling conditions over the width of the cooling roll a higher temperature for a thick film strip.

The temperature profile is across the entire width of the film proportional to the respective film strip thickness. He corresponds total of the hatched field in Figure 3 between the cooling curves of the thinnest and thickest film strips.

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The temperature behavior described is th of a accompanied by a decreasing curve of the yield stress as the temperature rises (Fig. 4 and 4a) »It follows that that during the first cooling due to the temperature differences, the yield stress of a thicker film strip is lower than that one thinner each time. , ..-.-

In connection with the step-by-step cooling according to the invention With the transverse expansion, the entire foil profile is equalized without additional heating and the associated interference safely achieved.

In terms of equipment, the temperature difference can be determined during the cooling, among other things, through the use of two Realize one behind the other rolls instead of the usual one cooling roll. Between the two chill rolls According to the invention, a transverse stretching device for the melt film is attached, which is constructed so that the transverse stretching takes place with constant force and at a constant path speed and the stretching to a necessary extent adjustable maximum can be limited.

The melt film cools down very quickly after leaving the slot die. To the working temperature of the method according to the invention, however, it follows that the first cooling path to achieve

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the necessary working temperature must be kept short. Furthermore, the S-melt peel angle should be kept as small as possible. The use of a first cooling roller with a small diameter meets both requirements. The second cooling roller has a larger diameter required for the final cooling if the same cooling conditions are assumed for both rollers.

After the first cooling, the prefilm is, without using the method according to the invention, with a indefinite spectrum of thick and thin places. To the Equalizations are therefore correspondingly different To cause constrictions. ■ ·

In Figure 5, a schematic film strip from the entire web is shown as a pattern on an enlarged scale, as it runs off the first cooling roller. At the point X-, is the temperature t ^ " the temperature-dependent yield stress & , the transverse force-dependent stress *" -? , and the cross section F ₁ . The same applies to positions Xp and X ₃ . K is the transverse stretching force acting on the strip. The internal forces act at points X ^, Xp and X ₃

^K l ^{= F} l '£> 1 » ^K 2 ^{= F} 2 ' ^ Z ^{and K} 3 ^{= F} 3 '3 ^across

l ^{= F} l '£> 1 » ^K 2 ^{= F} 2 ' ^ Z ^{and K} 3 ^{= F} 3 '3 to the direction of the path. In the example shown in Figure 5, the area F ₁ (F "<F ₃ .

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The cooling of the Schmel zefi Iniesi is now in place to interrupt, at the at t, {t, <^ t ^, and thus

^{the Differen2} between

slv * ß s2 ^ & s3

_s o - ^u ' ⁿ ^ £ l> ς. 3 may T i st is large, where ty is greater ^; , \: - "

than the glass transition temperature, ie greater than the temperature of the first transition point, the transverse stretching force K is to be dimensioned so that O \ ^ i ' G Z> 6T _s2 and 6 ₃ > 6 _s3 . G _sl , 6 - and ^ J q are the yield stresses as a function of the respective temperature of a film strip. Figure 6 shows their relationship in the stress-strain-di a-. gram. (S ■, is the tension at the desired smallest prefilm thickness. At point X. so 11 so that no change in shape occurs. \

Since the FTießgeschwindgigk & it from thermoplas ti sehen'- f4aterialien with temperature and -. "- the changing Verhäl TNI s of stretching force and stretching force increases are ^:: the requirements for di ckenproport-i onälen E i nschnürung at the right time of the strain added. Ei n _r e further / Moegli chkei t, to proportion the egg nschnürung ri chti g, is, η eb en de κ D eh η ζ eitdie D eh η ge schv / i η speed to be e in rivers, as the FIi eating resistance by therniopTasti

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Materials also depends on the flow rate.

To avoid structural damage in the film, the The amount of stretching should not be too large and the method only applies to the fine structure of the thickness profile of the film can be applied. The rough structure of the thickness profile is expediently effected by a flat nozzle that can be adjusted in segments.

To summarize, let me return to the essential points of view referenced.

In the method according to the invention described, the Melt film before its first cooling to solidification temperature Treated to reduce thick spots The other known methods are used for this purpose only 'after the first cooling down.

The applied transverse stretching takes place with constant stretching force and necessary to avoid the stress peaks limited maximum elongation and not, as is usual with transverse elongation, with constant elongation.

The slot nozzle is used to adjust the die with a coarse structure adjustable in individual segments over the entire width. However, it only enables

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Influence of constant thickness fluctuations, whereas -., the method according to the invention also occurs irregularly Corrected thickness fluctuations.

The film produced according to the described process as a pre-film and in stretched form has significantly fewer sources of defects in terms of thickness, profile in number and size, which have a negative effect on the final film and would interfere with further processing ^:

In terms of apparatus, the method by means of conventional to- "operating clips or endless twin belts or chains can with Anpreßrädern which grasp the melt film by passing the other can-to the transverse elongation of _the:. Melt film also divergently extending bevel gears werden.- taken Preferably, in this case the Web guiding elements (clips, tapes, discs, chains) to be designed in such a way that the required constancy of the force is ensured during stretching These apparatuses are state of the art and do not limit the method according to the invention.

The method according to the invention is basically applicable to all Flat films made of thermoplastics can be used, but is preferred for polyesters, polyolefins, such as.

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Polypropylene and polyamides, used in particular for those films whose final thickness is 2 to 50, u should be, since precisely these thin films are particularly subject to damage from thick places.

The following examples are intended to illustrate the invention again explain without, however, being limited thereto should exist.

example 1

Polyethylene terephthalate was 30 m / min mm in a width of 450 extruded from a T-die at 29o C ^0. Below the nozzle a, cooled to 2O ⁰ C roll was located at a distance of 50 cm, on which the melt was solidified for the film. On the route between the nozzle and the roller, the melt was continuously cooled from 29o C to 24O ⁰ C ^0. The solidified film was then stretched in the longitudinal direction at 90 ^° C. in the ratio 1: 3.5 and then in the transverse direction at HO ^° C. in the transverse direction in the ratio 1: 3.5. The stretched 12 u thick film was heat set at 22O ⁰ C. The thickness diameter of the film produced according to the prior art is shown in FIG.

Example 2

Example 1 was repeated with the difference that the melt was used between the nozzle and the cooling roller

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divergent driven pulleys by 10%
was stretched. The thickness deviations from the standard value ■; /.-: (Figure 1) were reduced by 30%. ■

Example 3

Example 2 was repeated with the difference that.,

an elongation of 20% was made. /.".-""-- "

The thickness deviations from the sol! Value (Figure 1) was

reduced by 40%.

Example 4 ■: ■ '■''■ ^: - ■: ■' ■.

Example 2 was repeated with the difference that ^:

an elongation of 30% was made.

The thickness deviation from the target value (Figure 1) was around

50% reduced. _;

Example 5 ,

Polypropylene was produced at 10 m / min in a width of

250 mm at 25O ⁰ G from a slurry extruded. ..

Below "the" Düs.e-v / a-r - at a distance of 50 cm one

at 2O ⁰ C cooled roll angeordnetj on which the melt was solidified for the film. On the route between the nozzle and the roller, the melt was continuously cooled by 25Q ⁰ C to 215 ⁰ C abge. The solidified film was then applied lengthways at 140 ° C. in a ratio of 1: 6 and thereafter

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stretched in the transverse direction at 150 ^° C. in a ratio of 1: 7. The stretched 12.u strong film was heat-set at 16O ⁰ C.

The thickness diameter of the prior art The film produced corresponded in the order of magnitude to the illustration in FIG. 1.

Example 6

Example 5 was repeated with the difference that the melt between the nozzle and the cooling roller is by means of divergent driven pulleys was stretched by 10%. The thickness deviation from the target value compared to the prior art was reduced by 25%.

Example 7

Example 6 was repeated with the difference that an elongation of $ 20 was made.

The thickness deviation from the nominal value was reduced by 37%

Example 8

Example 6 was repeated with the difference that an elongation of 30% was carried out.

The thickness deviation from the nominal value was reduced by 45%

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Claims (6)

PA TE N T A NSPR O G H 'E -1.} Procedure for the reduction of thickest parts and for Improving the thickness profile in the production of thermoplastic extruded from slot dies, see foils, characterized in that the Melt film pressed out of the slot die above the solidification temperature in stages cools and thereby carries out an elongation transversely to the extrusion direction and only then cools the melt film to form the solid film. - - · - ■ 2. The method according to claim 1, characterized in that that you then the solidified film simultaneously or gradually stretches. 3. The method according to any one of claims 1 or 2, characterized characterized in that one has a longitudinal, transverse stretch undertakes. ".--. 4. The method according to any one of claims 1 to 3, characterized characterized in that polyester is used. 5. The method according to any one of claims 1 to 4, characterized characterized in that a transverse stretching force (K) 5 0 9 8 2 4/0 7 8 0 is expended in which the yield stresses 6 ₁ << o _sl »^ 2> 6 _s2 and ... <£ ₃ > G _s3 be. 6. The method according to any one of claims 1 to 5, characterized characterized in that films with a thickness of 2 to 50 µ are produced. 50982 47 0780