CH640796A5 - HEAT SHRINKABLE, FLEXIBLE MULTILAYER FILM FOR PACKAGING PURPOSES. - Google Patents

Description

The invention provides a multilayer flexible, heat-shrinkable thermoplastic packaging film with a shrinkage tension of 7 to 28 kg / cm 2, based on a temperature range of 93 to 149 ° C, and with at least three layers, namely at least one inner layer, which in the is essentially not oriented and consists of a homopolymer or copolymer of ethylene, and at least one which is oriented, the thickness of the inner layer or the total thickness of the inner layers making up 10 to 90% of the total thickness of the multilayer film, and the films being a contains oriented layer of a polyester or copolyester.

The multilayer film according to the invention therefore comprises at least three layers, the inner layer or at least one of the inner layers, if the film has more than three layers, consisting of a homopolymer or copolymer, which may be a terpolymer of ethylene, and at least one the other layers are made of a polyester or copolyester, which for the sake of simplicity is referred to as the “polyester layer”. The «third layer» according to the definition above can consist of a polyester layer, i.e. an oriented layer of a polyester or copolyester.

The inner layer does not have to consist exclusively of an ethylene polymer. It may contain other compatible ingredients. A preferred material for the inner layer is a blend, the major component of which is an ethylene polymer and the minor component of which is a homopolymer or copolymer of butylene. The multilayer film is oriented overall, i.e. it has the properties of an oriented film and is heat shrinkable in at least one direction. Orientation is given to the polyester layer, while the inner layer of ethylene polymer is essentially unoriented. The orientation is preferably biaxial.

The invention encompasses multilayer films as indicated above with three or more layers. A preferred embodiment consists of three layers with a substantially non-oriented central inner layer, as stated above, the thickness of which makes up 10 to 90%, preferably 25 to 90% of the total thickness of the film and which is flanked by two outer layers. At least one of the two outer layers is oriented and consists of a poly

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ester or copolyester, the thickness of the two outer layers together accounting for 90 to 10%, preferably 75 to 10% of the total thickness of the film.

Another preferred film according to the invention consists of five layers, namely:

(a) an oriented central layer made of a polyester or copolyester or a homopolymer or copolymer of propylene.

(b) two substantially non-oriented inner layers on both sides of the central layer with a total thickness of 10 to 90% of the total thickness of the multilayer film, each inner layer being constructed as indicated above, and

(c) two outer layers, at least one of which consists of a polyester or copolyester,

wherein in (c) at least one of said outer layers is an oriented layer of a polyester or copolyester if said inner layer comprises a homopolymer or a copolymer of propylene.

The or each polyester outer layer is preferably oriented.

Another embodiment comprises five layers, namely

(a) a central layer made of a polyester or copolyester

(b) two inner layers on each side of the central layer, each inner layer consisting of an ethylene copolymer, preferably an "ethylene-vinyl acetate copolymer, and

(c) two outer layers, one or both of the outer layers being composed of a polyester, copolyester, polyethylene, cross-linked polyethylene or cross-linked ethylene-vinyl acetate copolymers or homopolymers or copolymers of propylene.

In their heat-shrinkable form, the central layer and optionally one or both of the outer layers are normally oriented, while the inner layers (b) are normally essentially unoriented.

One method of making the multilayer film is e.g. by pulling out all of the polymer layers required for the film and stretching the multilayer film formed in order to orient it. For this purpose the film is normally brought to a temperature of e.g. Heated 80 to 200 ° C and then stretched hot. It is not desirable to orient the polymer molecules of each layer. In the preferred method, the or each inner layer is “hot blown”, as a result of which the film is stretched at a temperature which is above the orientation temperature of the polymeric material of this layer, so that it is not oriented to a substantial degree. In other words, the orientation of the multilayer film is predominantly effected in the other layer (s).

Unless otherwise stated in the context, the term "polymer" used here means homopolymers and copolymers, which can also be terpolymers. They can be present as graft polymers, furthermore as polymers with monomer units arranged in any order or alternately. In the copolymers, the monomer listed first in the non-abbreviated name thereof is usually present in the largest proportions.

The term "melt flow" used here is otherwise known as the "melt index". It indicates the amount of thermoplastic resin that passes through a given opening under a given pressure, namely a load of 2160 g and a temperature of 190 ° C

Can be pressed for 10 minutes as described in ASTM Test D 1238, Condition E.

The term “oriented” used in the present context refers to the orientation of the molecules of a polymer in predominantly one direction which is sufficient to impart heat-shrinkability to the layer or film.

The “inner layer”, which is referred to as the “core layer” in the priority-based US application, means a layer in a multilayer film which is flanked on both sides by adjacent layers. The inner layer is essentially not oriented, e.g. blown hot. It does not necessarily represent a central layer of the film. For example, in the five-layer film indicated above, inner layers are on either side of a central layer.

The term "hot blown" as used herein means that the specified material has been stretched at or above its orientation temperature range and therefore generally above its melting temperature, so that the stretching process has brought about minimal stress and molecular orientation. Such a material is not considered to be heat shrinkable because it has a very low shrink tension, usually below about 7 kg / cm 2 (100 p.s.i.).

"Outer layers", which can also be referred to as "main layers", are the layers on the surface of the multilayer film, i.e. the top and bottom layers of a flat film and the radial innermost and radial outermost layers of a tubular film or sack etc.

The term "polyester" as used herein means a thermoplastic, film-forming, especially saturated polyester, e.g. Polyethylene terephthalate or poly-buylene terephthalate. The term “polyester” in particular also includes the film-forming polycarbonates.

The term "copolyester" means thermoplastic film-forming copolyesters, e.g. Ethylene terephthalate glycol, terephthalic acid / isophthalic acid cyclohexanedimethanol and butylene terephthalate tetramethylene ether terephthalate.

The term “cross-linked” means that bonds have been formed between the molecules of the individual polymers. Crosslinking of some polymers can be initiated by exposing them to ionizing radiation, such as gamma or X-rays, or electrons or β-particles. For crosslinkable polymers such as polyethylene or ethylene-vinyl acetate copolymers, the radiation dose can be related to the degree of crosslinking by using the insoluble gel, i.e. the proportion of the polymer which does not dissolve in a solvent, such as boiling toluene, is regarded as the crosslinked proportion of the irradiated polymer. Usually there is no measurable gel at a radiation dose of less than 0.5 megarads. With the above polymers, a slight increase in crosslinking is observed at more than 12 megarads. Partial crosslinking at intermediate dosage levels can have beneficial effects, e.g. an improvement in the toughness and strength of the polymer without reducing the flowability to an undesirable degree.

The term "butylene polymer" refers to high molecular weight isotactic polymers synthesized from butene-1, which is normally referred to as butylene. They are flexible crystalline thermoplastic polyolefins with a density of about 0.91 g / cm3. They differ from polymers from isobutene, which is normally referred to as butene, which are widely used as oil additives, and from amorphous atactic poly (l-butene) ) polymers, which are viscous oils to rubbery polymers.

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In the following, the "ethylene-vinyl acetate copolymer" is abbreviated as "EVA" and the "propylene-ethylene copolymer" as "EP".

The essentially non-oriented inner layer obviously dampens or moderates the relatively high shrinkage stress of the polyester layer and, if present, of the oriented EP layer. Without this moderate effect, the shrink tension of the film would be large enough to deform the packaged product in many packaging applications. It enables desired shrinkage stresses, which roughly correspond to those of PVC films, to be achieved for these applications.

The preferred ethylene polymer for the inner layer (s) is EVA. The melt flow of the copolymer is considered more important than its vinyl acetate content. It is usually 0.1 to 6.0 g / 10 min. The melt flow is believed to be a significant factor in the handling and machinability of the final multilayer film, i.e. their ability to be used in automatic packaging devices. The most useful usually has a vinyl acetate content of 4 to 18% by weight and a melt flow index of 0.1 to 6.0 g / 10 min. proven.

The EVA in Example 1, the preferred embodiment, contains 12% by weight vinyl acetate and has a melt index of 0.3 g / 10 min. For a 3-layer structure, it is a preferred EVA.

The polymeric material of an inner layer of the film according to the invention can consist of a mixture of ethylene polymer and butylene polymer, although this is less advantageous than EVA alone. A larger proportion of the mixture preferably consists of the ethylene polymer and a smaller proportion of the butylene polymer.

Preferred polymers for the larger component are low density polyethylene and EVA in proportions of 70 to 90% by weight of the total mixture, preferred polymers for the smaller component polybutylene and butylene-ethylene copolymers in quantities of 30 to 10% by weight of the total mixture.

The butylene copolymer component is preferably a butylene-ethylene copolymer in which the proportion of ethylene units is advantageously up to 5% by weight. The most suitable was a polymer with an ethylene unit content of about 2% by weight. The melt flow of the butylene-ethylene copolymer or of polybutylene is preferably 1.0 to 3.0 g / 10 min.

The most advantageous mixture for the inner layer consists of EVA and a butylene-ethylene copolymer with a preferred ratio of 80:20% by weight.

An inner layer composed of a mixture of approximately 80% by weight EVA with approximately 12% by weight vinyl acetate and a melt flow of approximately 0.25 g / 10 min is particularly advantageous. and about 20% by weight of a butylene-ethylene copolymer with up to 5% by weight of ethylene and a melt flow of about 2.0 g / 10 min.

Instead of the preferred EVA, a mixture of ethylene-propylene copolymer or EP with polybutylene or a copolymer of butylene can be used, the EP component making up the larger proportion and the butylene polymer making up the smaller proportion of the mixture. Instead of the EVA dipolymer or for mixing with it, it is also possible to use EVA terpolymer, which is sold under the trade name “Plexar 3” by Chemplex Co., or the commercially available terpolymer of ethylene with 28% by weight vinyl acetate and 4 to 8% by weight methacrylic acid.

The multilayer film can include a layer of a homopolymer or copolymer of propylene.

sen, preferably from a propylene-ethylene copolymer (EP). Homopolymers of propylene can also be used. Mixtures of isotactic and atactic polypropylene can be used to achieve a desired atactic / isotactic ratio. In the most preferred embodiment according to Example 1, the EP contains approximately 3.0% by weight of ethylene units in any arrangement, but EP with 2 to 4% by weight of ethylene units and a melting point of 0.5 to 6.0 also proved to be very high satisfactory.

The total thickness of the multilayer film is usually about 2.5 to 50 [i (about 0.1 to 2.0 mils), preferably about 10 to 38 (j, (about 0.4 to 1.5 mils).

The thickness of the inner layer (s) of ethylene polymer is preferably 25 to 90% and in particular 47 to 90% of the thickness of the entire film. In a preferred embodiment of the 3-layer film, the inner layer consists of an ethylene-vinyl acetate copolymer with 4 to 18% by weight of vinyl acetate and a melt flow of 0.1 to 6.0 g / 10 min., A surface layer of a homopolymer or Copolymers of propylene, the thickness of this layer being 5 to 50% of the thickness of the entire film and the other surface layer of a polyester or copolyester, the thickness of which is 5 to 25% of the thickness of the entire film. In another preferred 3-layer film, in which the inner layer also consists of EVA, this inner layer has a thickness of 50 to 90% of the thickness of the entire film and each of the two surface layers consists of a polyester or copolyester, the thickness of the the two surface layers together is approximately 50 to 10% of the thickness of the entire film. In this preferred 3-layer film, both surface layers are preferably oriented.

A preferred 5-layer film according to the invention has an oriented central layer made of polyester or propylene polymer and oriented or non-oriented outer layers, at least one of which consists of a polyester. Both surface layers are preferably oriented and both consist of polyester. For films of this type, three sets of preferred thickness ratios are as follows:

(1) (2) (3)

a) central layer: 5-25% 5-25% 5-80%

b) two inner layers together: 15-85% 10-65% 10-85%

c) two surface layers together: 10-75% 10-65% 5-25%

An alternative 5-layer film according to the invention has the following layers:

(a) an oriented central layer made of a polyester or copolyester,

(b) two substantially unoriented layers on either side of the central layer with a total thickness of 10 to 90% of the thickness of the multilayer film, the inner layers being constructed as indicated above, and

(c) two outer layers, each consisting of a homopolymer or copolymer of propylene.

The outer layers are usually oriented. In this film, the inner layers are preferably made of EVA and the outer layers are preferably made of EP. The thickness of the polyester layer here is preferably 5 to 25% of the total film. The EP layers preferably make up 10 to 50% of the total film thickness and the EVA layers 25 to 85% of the total film thickness.

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All thickness ratios of the layers were determined on the non-oriented film.

A valuable advantage is that, according to the invention, only relatively thin layers of polyester are required, which are more expensive than olefin polymers in order to achieve shrinkage tension and / or heat-welding strength.

The polymeric material of at least one layer of all of the films according to the invention is preferably crosslinked by radiation. Irradiation improves strength and toughness. The radiation doses in the range of 1.0 to 4.0 megarads usually result in the most advantageous combination of improved strength without a significant reduction in fluidity that would inhibit welding.

The films according to the invention have "massive" shrinkage tension of about 7-28 kg / cm2 (about 100 to 400 p.s.i.) in contrast to "high" shrinkage tension of about 28 to 700 kg / cm2 (about 400 to 1000 p.s.i.). These shrinkage stresses refer to a temperature range of 93 to 149 ° C (200 to 300 ° F).

The following examples illustrate the invention. The layers containing EVA are essentially non-oriented ("hot-blown") inner layers. All other layers are oriented.

example 1

A preferred multilayer film according to the invention has the following layer arrangement:

EP / EVA / Copolyester / EVA / EP.

This 5-layer structure is produced using three extruders, which feed an annular injection mold to be pulled out together, so that each layer is pulled out in a ring. An extruder feeds a melt of EP with about 3% by weight ethylene into the injection mold to form the inner and outer annular layers. A second extruder supplies molten EVA with a vinyl acetate content of about 12% by weight and a melt index of about 0.3 to the extruder to form annular EVA layers adjacent to the inner EP layer and the outer EP layer. The central layer between the two EVA layers is supplied by a third extruder which feeds a molten copolyester, which in this embodiment consists of ethylene terephthalate glycol. This five-layer tube, as it leaves the extruder, is often referred to as a "strip". In this embodiment, the total thickness of the tubular tape was 0.4 mm (16 mils), of which the copolyester was 20%, the EVA layers were 47% and the EP layers were 33%.

After exiting the extruder, the tape is rapidly cooled to room temperature and below, preferably to about 10 ° C, passed through execution rollers in a water bath, inflated and heated to a temperature in the range of 110 to 135 ° C which is above the melting temperature of the EVA and is in the orientation temperature range of the copolyester and the EP. In general, the orientation temperatures for multilayer films according to the invention are in the range from 80 to 200 ° C. When the inflated tube is heated to this temperature range, it expands into a bubble under the influence of internal air pressure, causing the tube to biaxially stretch. In this embodiment, the tube expands until a wall thickness of 19 [A (0.75 mil) is reached. The bubble is then cooled by blowing frozen air on its surface to quench the polymeric materials and to freeze the molecular structure of these materials in their oriented state. Then you let the bubble collapse and slit the expanded tube into packaging films.

The multilayer film can be stretched to the desired thin wall, which depends on the specific packaging purpose for which the film is to be used. The practical lower limit to which films according to the invention can usually be stretched is about 2.5 microns (0.1 mils), the upper usable limit is about 50 microns (2.0 mils).

io The multilayer film produced according to the above method was used to wrap 20 X 25 X 2.5 cm (8 "X 10" X 1 ") gift boxes with a conventional" L "sealing device, in which a sheet of film was placed over the box folded and welded on the remaining three sides 15. With this type of sealing device, the decisive closure of a package is the expiring closure of the package immediately before the next package. The package is moved further in that in the area of the still hot seal against 20 The film is pushed, so the heat seal strength is important to ensure the integrity of the package and the packaging speed.

After the gift boxes were wrapped in the film, they were led through a "shrink tunnel". In this tunnel, they were heated to approximately the same temperature at which the film was oriented. After leaving the tunnel, the packages are “ready”. All packaging with the film according to this embodiment had an excellent appearance, did not exert excessive shrinking force on the thin-walled gift cans to deform them and showed good heat-sealing strength as well as good weldability and cutability. In all cases, the films of this embodiment proved to be as good as commercially available 35 PVC films under the same conditions.

Example 2

A second multilayer film was produced as in Example 1, with the difference that the middle layer made of 40 copolyesters made up 18%, the inner layers made of EVA together 52% and the outer layers made of EP 30% of the total thickness of the film. The film was also stretched to a thickness of about 10 microns (0.75 mil). The tensile strength and the tensile strength as well as the maximum shrinkage tension were lower than in the film according to Example 1. However, the overall properties were very satisfactory and compared with those of PVC.

Example 3

50 A 5-layer film was produced as in Example 1 with the difference that the central copolyester layer was 9.5%, the EVA layers together 57%, one of the EP layers 19% and the other 14.5% of the total thickness of 25 microns (1.0 mil) of the multilayer film. In 55 packaging tests similar to those of Example 1, the film showed lower but satisfactory heat seal strength. Their shrinkage tension was also lower.

A comparison of the shrink properties of the films of Examples 1 to 3 with polyvinyl chloride (PVC), polypropylene 60 (PP) and cross-linked polyethylene (PE) is shown in the table below.

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Foil shrink tension range, kg / cm2 (p.s.i.)

from 93 to 149 ° C (200-300 ° F)

PVCa

11.2-18.3

(160-260)

example 1

20.0-27.4

(285-390)

Example 2

20.7-23.2

(295-330)

Example 3

13.4-17.6

(190-250)

PPb

30.9-49.9

(440-710)

PEC

24.6-29.5

(350-420

a “Reynolon 4155” from Reynolds Metals Corporation.

h Propylene-ethylene copolymer film, which predominantly contains propylene and has been approved by W.R. Grace & Co. is marketed as "CP-900".

u Cross-linked polyethylene, which has been Grace & Co. is sold under the trademark “D-Film”.

It is noteworthy that the shrinkage stress ranges of the films according to Examples 1 to 3 were lower than that of the simple films made of polyolefins, such as polypropylene and polyethylene, and approached those of PVC. The films of Examples 1 to 3 also had sufficient heat seal strength at high packaging speeds. The optical properties and the appearance of the packaging from the films of Examples 1 to 3 were excellent.

Example 4

In accordance with the above examples, a 5-layer film with the following layer arrangement was pulled out:

Polyester / EVA / Polyester / EVA / EP.

In this multilayer film, the polyester or copolyester layers make up 10 to 50%, the EVA layers 10 to 50% and the EP layer 5 to 25% of the total thickness of the film. The 5-layer film is desirable if an additional polyester content is required for better heat sealing and one of the surfaces of the film should have the hardness and scratch resistance provided by the polyester.

Example 5

According to the preferred embodiment, a further 5-layer film can be produced with the following layer arrangement:

Polyester / EVA / EP / EV A / polyester.

The polyester or copolyester makes up 10 to 50%, the EVA layers make up 10 to 85% and the EP layer makes up 5 to 80% of the total thickness of the film. This structure is advantageous if both surfaces of the film should have the hardness and scratch resistance of the polyester and an additional polyester content is required for better heat sealing resistance.

Example 6

A 3-layer film was pulled out in a manner similar to that in the preferred embodiment. Three extruders were required for the three materials and the ring-shaped mold for pulling out together was designed so that the EVA / layer resulted in the central or middle layer. The structure was as follows:

EP / EVA / copolyester.

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This three layer film was biaxially stretched to a thickness of 19 microns (0.75 mils) so that the two outer layers were 4 microns (0.15 mils) thick and the middle layer 11 microns (0.45 mils) thick Mol), gift cans were packed in this 3-layer film as in the preferred embodiment. However, this film was less satisfactory due to its unbalanced nature, i.e. the EP layer and the co-polyester layer have different shrink properties, so that the film curled. The crimp appeared on the inside of the copolyester side, so that the copolyester surface was on the inside of the vertebra. In some cases, such crimp may be desirable, e.g.

if sacks are made from the film and a herb on the eyelet leaf of the sack makes opening the sack easier.

Example 7

A balanced 3-layer film can be produced with the following 25 structure.

Polyester / EVA / polyester.

Instead of the polyester, a copolyester can of course also be used for this structure. The

Polyester layers make up 10 to 50% and the EVA layer makes up 50 to 90% of the total thickness of the film.

Example 8

35 According to the invention, a further 5-layer film can be produced as follows:

PE / EVA / polyester / EVA / PE.

The abbreviation «PE» denotes polyethylene, which, depending on the wishes of 40, can be of low, medium or high density. If low or medium density polyethylene is used, it may be necessary to provide additional strength by crosslinking. As indicated, the middle layer can consist of polyesters or of a Co-45 polyester.

Example 9

A 5-layer film was produced with the following preferred thickness ratio for the individual layers: 1/2/1/2/1. The outer or skin layers consisted of the propylene-ethylene copolymer "WO 7-1" from ARCO Polymers, Inc. with 3.5 to 4.0% by weight of ethylene and a melt flow of about 4.0. The central layer was made of polyethylene terephthalate. The remaining two layers 55 consisted of the preferred EVA / butylene-ethylene copolymer composition in the ratio 80% / 20%. The EVA consisted of "Alathon 3135" by du Pont with 12% by weight vinyl acetate and a melt flow of approximately 0.25 and the butylene-ethylene copolymer from "Witron-8240-2" from Witco 60 Chemical Corporation with approximately 2% by weight. -% ethylene and a melt flow of 1.0 to 2.0. This film has good shrink properties and good "heat seal resistance".

Example 10

65 The same film as in Example 9 was produced with the difference that the central layer consisted of a polycarbonate resin. Again, good shrink properties and "heat sealing resistance" were achieved.

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Claims (4)

640796 1 or 2 or 3 15 a) Central shift: 5-25% 5-25% 5-80% b) the two inner ones Layers together: 15-85% 10-65% 10-85% 20th c) the two outer layers together: 10-75% 10-65% 5-25% 13. A film according to claim 1, characterized in that it has five layers, namely: (a) an oriented central layer made of a polyester or copolyester (b) two essentially unoriented layers on the two sides of the central layer with a total 30 thickness from 10 to 90% of the total thickness of the multilayer film, the composition of the inner layers corresponding to that specified in claim 1, 2, 3 or 4 and (c) two outer layers, each consisting of a homopolymer or copolymer of propylene, and 35 oriented or not oriented. 40 A “shrink film”, also known as “heat shrink film”, is a film that shrinks when exposed to heat and does not stretch. When the film is stretched again, shrink tension is generated in it. Ver-45 packs a product in a shrink film and heats it, e.g. by passing through a hot air tunnel or a hot water bath, the film shrinks around the product, forming a tight-fitting envelope that corresponds to the contour of the product. If a transparent shrink film is used, an aesthetically pleasing packaging is created, which protects the product from the loss of components, parts being removed, the product being damaged during handling and transport, or from dirt, bacteria or the like. 55 is contaminated. A shrink film is made by heating a film to an orientation temperature, i.e. a temperature at which the polymeric material of the film is oriented. The orientation temperature is below the melting temperature of the polymer and usually above room temperature. The film is then stretched, usually biaxially, i.e. in the longitudinal direction (LR) and in the transverse direction (QR). The stretching can take place between pairs of nip rollers or e.g. be effected by stenter. The stretched film is rapidly cooled to freeze the molecules of the polymeric material in their oriented state. When the film is reheated, the tension forces of the polymer molecules become in their oriented configuration 1. Heat-shrinkable, flexible multilayer film for packaging purposes with a shrinkage tension of 7 to 28 kg / cm 2 based on a temperature range of 83 to 149 ° C and of at least three layers of thermoplastic polymeric materials, with at least one inner layer essentially not oriented and is made of a homopolymer or copolymer of ethylene or is based on such a polymer and is oriented at least one of the other layers, characterized in that the thickness of the inner layer or the total thickness of the inner layers 10 to 90% of the total thickness of the multilayer film and that the film has an oriented layer made of a polyester or copolyester. 2. Film according to claim 1, characterized in that the or each inner layer consists of a mixture of a homopolymer or copolymer of ethylene with a homopolymer or copolymer of butylene. 3. Film according to claim 1 and 2, characterized in that the ethylene polymer of the inner layer consists of a copolymer of ethylene with vinyl acetate. 4. Film according to claim 3, characterized in that the ethylene-vinyl acetate copolymer contains 4 to 18 wt .-% vinyl acetate and a melt flow of 0.1 to 6.0 g / 10 min at a load of 2160 g and a temperature of 190 ° C according to ASTM No. 1238, condition E. 5. Film according to claim 1 to 4, characterized in that the inner layer consists of a hot blown layer. 6. Film according to claim 1 to 5, characterized in that it has at least a third layer made of a homopolymer or copolymer of propylene. 7. Film according to claim 1 to 6, characterized in that the thickness of the inner layer (s) makes up 25 to 90% of the total thickness of the multilayer film. 8. Film according to claim 7 with three layers, characterized in that one or both of the two outer layers is oriented and consists of a polyester or copolyester, the thickness of the two outer layers together accounting for 75 to 10% of the total thickness of the multilayer film. 9. Film according to claim 8, characterized in that the inner layer consists of an ethylene-vinyl acetate copolymer according to claim 4, an outer layer consists of a homopolymer or copolymer of propylene, the thickness of this layer making up 5 to 50% of the total thickness of the multilayer film and the other outer layer consists of a polyester or copolyester, the thickness of which makes up 5 to 25% of the total thickness of the multilayer film. 10. Film according to claim 8, characterized in that the inner layer of an ethylene-vinyl acetate copolymer according to claim 4 with a thickness of 50 to 90% the total thickness of the multilayer film and each of the two outer layers consists of a polyester or copolyester, the thickness of the two outer layers together accounting for 50 to 10% of the total thickness of the multilayer film. 11. Film according to claim 1 with five layers of thermoplastic polymeric materials, characterized in that these: (a) an oriented central layer made of a polyester or copolyester or a homopolymer or copolymer of propylene (b) has two substantially non-oriented inner layers on each side of the central layer with a total thickness of 10 to 99% of the total thickness of the multilayer film, the composition of the inner layers corresponding to that specified in claim 1, 2, 3 or 4 and. (c) two outer layers, at least one of which consists of a polyester or copolyester and each outer layer can be oriented or not, in (c) at least one of said outer layers is an oriented layer of a polyester or copolyester if said inner layer comprises a homopolymer or a copolymer of propylene. xo 12. Film according to claim 11, characterized in that the thickness of the individual layers, based on the total thickness of the multilayer film, makes up the following percentages: 2nd PATENT CLAIMS 3rd 640796 tion is released and the film begins to shrink back to its original non-oriented dimension. Polyolefins and polyvinyl chlorides are the two main classes of synthetic resins from which the majority of the commercially available shrink films for packaging are made. Other resins from which shrink films can be made include ionomers, polyester polystyrenes and polyvinylidene chlorides. The polyolefin shrink films currently on the market consist of crosslinked or non-crosslinked oriented polyethylene, oriented polypropylene or oriented propylene-ethylene copolymers. The polyvinyl chloride and polyolefin shrink films provide a wide range of physical and performance properties, such as shrink force (the force a film exerts per unit area of its cross section during shrinking), the degree of free shrinkage (the reduction in the surface of the film when one is shrinks without stretching), tensile strength (the maximum force that can be applied to a unit area of the film before it tears), sealability, shrink-temperature curve (the shrink-to-temperature relationship) , the onset of cracking and toughness (the force at which a film begins to tear and continues to tear), visual appearance (gloss, ambiguity and transparency of the material) and dimensional stability (the ability of the film to maintain its original dimensions under all types of Storage conditions). The optimal type of shrink film depends, among other things. on the size, weight, shape and strength of the product to be packaged, the number of product components, other packaging materials that can be used with the film, and the type of packaging device available. Polyolefins are best suited for applications where moderate to high shrink tensions are preferred, and on new, high speed, automatic packaging devices where shrink and seal temperatures are more closely controlled. The polyolefins leave less deposits and residues, thereby extending the life of the device and reducing maintenance of the device. PVC films generally have better optical properties and lower shrinkage stresses, weld more firmly at elevated temperatures, and shrink over a much wider temperature range than the polyolefins. The polyolefins usually do not release corrosive gases when welding like the PVC films and are also cleaner in this respect than the PVC films. So far, polyolefins have not been able to compete with packaging made of PVC films if the products to be packaged require the low shrinkage stresses of the PVC films because the products for the application of polyolefins, the shrinkage stresses from to 4 times the shrink tension of PVC films are too fragile. PVC films are also the shrink film of choice for older, manually operated welding devices and semi-automatic packaging devices, in which the temperatures can be varied widely. Older, low-maintenance packaging devices of all types usually work better with PVC films than with single-layer polyolefin films, since the PVC films generally have broader shrinking and sealing temperature ranges. In addition, products with sharp or dotted dimensions often require PVC films due to the high resistance of PVC films to initial tearing compared to the resistance of the polyolefins, i.e. about 7 g are required to cause tear in PVC, while 2 to 3.5 g are sufficient for a typical single-layer polyolefin shrink film. For health reasons, however, it would be desirable to find a replacement for PVC for some or all of these uses, particularly to avoid its harmful fumes and corrosive by-products. A film is now shown that is heat shrinkable and has many of the desired properties of PVC shrink films, but avoids many of its disadvantages, especially the harmful fumes and corrosive by-products. The films according to the invention have shrinkage tensions which correspond approximately to those of the PVC films, good optical properties, a wide shrink temperature range, have good weldability, good resistance to crack formation and a heat seal strength which is greater than that of the polyolefin films which are widely used. The invention consists in the provision of multilayer films with at least three layers which differ from the known multilayer films which are not used as a replacement for PVC.