DK144002B - PROCEDURE FOR PREPARING A HEAT-CREATABLE FOLI LAMINATE CONTAINING AN ORIENTED FILM OF LOW OXYGEN PERMEABILITY VINYLIDEN CHLORIDE POLYMES - Google Patents

Description

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The present invention relates to a process for producing a heat shrinkable foil laminate having an oxygen permeability of less than 70 cm x 25 micrometers / m x 24 hours in air at 1 atm. and 23 ° C, consisting of an oxygen barrier layer in the form of an oriented film (A) of a vinylidene chloride polymer laminated on each side to a layer (respectively and B2) of another polymer, wherein method a film of a plasticized vinylidene chloride polymer containing 7-12% by weight of a plasticizer is extruded, oriented by stretch and laminated to the other polymer layers before or after stretching.

Homopolymers of vinylidene chloride cannot be converted to foil by conventional extrusion technique because they decompose or decompose very rapidly at the extrusion temperature. However, by copolymerizing vinylidene chloride with smaller amounts of one or more other monomers, it is possible to prepare copolymers which, when mixed with appropriate plasticizers (and possibly smaller amounts, usually less than 15% by weight, of other polymers) can extruded into film which can be oriented by stretching to obtain shrinkable sheets. The term "vinylidene chloride polymer" is used herein to denote such copolymers (including terpolymers) and mixtures thereof with other polymers well known in the art. Vinyl chloride is the most common comonomer, and other suitable comonomers include acrylonitrile and the alkyl acrylates and methacrylates, especially alkyl acrylates and methacrylates whose alkyl groups contain 1-8 carbon atoms.

Heat shrinkable vinylidene chloride polymer foil has been widely used for packaging purposes, especially for food packaging. The films are particularly useful for vacuum packaging techniques, in which the film is shrunk over the material to be wrapped. The foil is usually in the form of a bag produced by closing or sealing a tubular oriented foil and is obtained by well known extrusion technique and orientation by the bubble method. The bag is clipped or heat sealed after the material to be wrapped is placed in the bag. Copolymers containing 90 to 65% by weight of vinylidene chloride and 10 2 100002

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to 35% by weight vinyl chloride has proved particularly suitable for this purpose. (The percentages given throughout this specification are by weight).

As stated above, it is necessary to soften 5 vinylidene chloride polymers so that they can be extruded satisfactorily and stretched to oriented films, and the greater the proportion of plasticizer, the easier the polymer to extrude and orient, and the better the resistance of the final product to violence. and wear. On the other hand, the oxygen permeability of the final product is also increased with increasing plasticizer content, and for many purposes, especially in food packaging, it is vital that oxygen permeability is low. In recent years, requirements in the packaging industry have become stricter and stricter, and in current commercial practice, a permeability of less than 100 is essential and a permeability of less than 70 is highly desirable. The 2 oxygen permeabilities stated in the present specification are expressed in cm x 25 micrometers / m x 24 hours in air at 1 atmospheric pressure and a temperature of 23 ° C. For example, they can is measured by 0. User cell whose operation is explained 20 in Examples 1-4 below. In addition, the use of large proportions of the plasticizer may cause loss of clarity in storage due to increasing incompatibility between the polymer and the plasticizer.

Another serious disadvantage of increasing plasticizer content is that the foils become progressively more difficult to seal.

Another important factor is that the resistance to violence and abrasion in foil of vinylidene chloride polymers varies widely with the particular compound used as plasticizer. The softeners used in practice are, of course, those which provide good resistance to violence and wear, and it is unfortunate that these 30 softeners are also precisely those which make it difficult to combine good oxygen barrier properties with good extrusion and orientation, so that the amount of plasticizer used to obtain a permeability of less than 100 must be limited to a level which makes it difficult to control the extrusion and especially the orientation of the film, especially when the films are stretched by the conventional method, bubble orientation. This has resulted in significantly increased expenses, both in regulating the production of the oriented foil and in wastage due to interruption in production. For example, when using dibutyl sebacate, which is by far the most widely used plasticizer for this purpose, requires a copolymer of vinylidene chloride of 15-35 wt% vinyl chloride 7 to 10 wt% dibutyl sebacate to provide satisfactory extrusion and behavior by orientation; but the films made from a polymer so softened have undesirably high oxygen permeability and undesirably low resistance to wear and tear, especially at low temperatures. In addition, very careful temperature control is required to seal such foils and obtain the necessary sealing strength. Another plasticizer that behaves in much the same way as DBS (dibutyl sebacate) is di-n-hexyl azelate (DNHZ). At the other end of scale 15, acetyl tri-n-butyl citrate (ATBC) and 2-ethylhexyldiphenylphosphate (DPPh) provide a reasonably good combination of low oxygen permeability and satisfactory extrusion and orientation, but have not been used in practice. because foil softened thus has very poor resistance to violence and wear.

It has now been found that a heat shrinkable foil laminate can be made having both good mechanical properties and low oxygen permeability of less than 70, and without the difficulties described above, namely, by laminating each side of a sheet of plasticized vinylidene chloride polymer (herein are referred to as layers A) to other polymeric layers (herein referred to as layers B. and B2) that will absorb plasticizer from the vinylidene chloride polymers. Accordingly, the process of the invention is characterized in that the extruded film of plasticized vinylidene chloride polymer is laminated on each side to a layer of such polymer capable of absorbing plasticizer from the vinylidene chloride polymer having a total thickness of at least 75% and at most 400 % of the thickness of the vinylidene chloride polymer foil in the finished laminate. The layers and B2 fulfill the two-sided function of decreasing the oxygen permeability of the layer A after the laminate is made and of protecting the layer A so that its mechanical properties are relatively unimportant. In 402 this way, the present invention makes it possible to use vinylidene chloride polymers which initially contain such amounts of plasticizer which are desirable to promote good extrusion and orientation, but have hitherto been considered impractical in the art. because of the high oxygen permeability they cause. Furthermore, the invention makes it possible to use the plasticizers which have heretofore been considered practically unusable due to the poor mechanical properties they produce.

Non-oriented, non-heat shrinkable laminates are known in which there is a central "barrier" layer at low oxygen permeability, to which are laminated layers of materials as also described by the process of the present invention. Such polymeric films are e.g. disclosed in French Patent Specification No. 1,579,772 and No. 1,580,055. However, since these patents relate to the production of non-oriented laminates, the problems sought to be remedied by the present invention do not arise. The outer layers of ethylene / vinyl acetate copolymer disclosed in these patents are present for the particular purpose of improving the adhesion or adhesion of the barrier layer to the other layers of the known prior art laminate, or according to a particular embodiment as described in French patent specification. No. 1,579,772 as an outer layer of the laminate to ensure heat sealing ability.

By laminating to polymeric layers which absorb plasticizer from the vinylidene chloride polymer, it is obtained that the oxygen permeability is reduced to below 70 by the softening of the plasticizer from layer A to layers B- and B

These film laminates are preferably made by melting the three layers through a single extrusion head with three extrusion openings or nozzles to form a tubular extrudate which is then oriented by internal gas pressure using the well known bubble technique to produce a shrinkable laminate. The oriented tubular laminate may be cut to provide flat film, or converted into prior art bags.

The layers B ^ and B £ may be of the same or different structure or composition. Preferably, at least one of the layers (the inner layer in the case of tubular laminates) is heat sealable.

Particularly good laminates are produced by the process of the invention when the layers and B2 comprise copolymers of 0 5 144002 of ethylene and vinyl acetate (herein referred to as EVA copolymers) containing up to 40%, e.g. 5-40% and preferably 10-30%, vinyl acetate. It has been found that the higher the proportion of vinyl acetate, the better the softener migration or migration.

5 On the other hand, lower vinyl acetate content provides better resistance to violence and wear. These copolymers are well associated with the vinylidene chloride polymers and therefore exhibit good adhesion to them and great versatility in regulating oxygen permeability through the plasticizer absorption mechanism and by providing good abrasion resistance.

It has also been found that the plasticizers for vinylidene chloride polymers are absorbed by chlorinated polyethylenes obtained by chlorination of low or high density polyethylene containing up to 70%, preferably 25-50%, chlorine, and are well absorbed by methacrylate-butadiene styrene polymers and mixtures of polyethylene and polyisobutylene. Still other useful polymers may be selected from one or more of the following (i) ethylene copolymers with up to 40% ethyl, propyl and / or butyl acrylates, acrylic acid, vinyl alkanoates, butylene and propylene and 20 mixtures of such copolymers with polyethylene , (ii) acrylonitrile-butadiene-styrene terpolymers, (iii) polyisobutylene, (iv) butyl rubber and (v) polyvinyl chloride.

The proportion of plasticizer blended with the vinylidene chloride polymer prior to melt extrusion is, of course, to some extent dependent on the plasticizer and the polymer, but it is preferred that the amount is at least that which gives the layer A an initial oxygen permeability of 130 to make the polymer. easily extrudable and orientable. When dibutyl sebacate is used, its amount will often be such that layer A has an initial permeability of at least 200. The polymer contains 7-12%, and especially up to 9% of the plasticizer. In particular, the use of dibutyl sebacate has achieved good results with 7-9% and especially 8% of the plasticizer. The content of plasticizer in layer A decreases as the plasticizer migrates or migrates into the layers and B2, and the migration can be promoted by increasing the temperature, but in most cases storage at room temperature for two weeks or more leads to satisfactory migration so that the content of plasticizer is reduced to a level at which oxygen permeability is below 70. When certain plasticizers are used, especially ATBC and DPPh, which do not cause as much increase in oxygen permeability as DBS, laminates can be obtained with very low permeabilities, e.g. under 45.

It is particularly convenient (a) as the vinylidene chloride polymer to use a compatible blend of (i) a copolymer of 90-65% vinylidene chloride and 10-35% vinyl chloride and (ii) up to 10% by weight of the polymer used to provide layers B 2 and (e.g., an EVA copolymer in an amount up to about 8%) or (b) as the vinylidene chloride polymer to use a copolymer of 90-65% vinylidene chloride and 10 -35% vinyl chloride and, as the polymer providing layers B 2 and B2, use a compatible mixture of a polymer which extracts plasticizer from the plasticized vinylidene chloride polymer layer (e.g., an EVA copolymer) and a small proportion of vinylidene chloride. In this way good adhesion is obtained between the layers, and in addition there is the important advantage that it is possible to recycle the waste laminate material for use in the manufacture of layer A or one or both layers B1 and B2.

A preferred vinylidene chloride polymer is a copolymer of vinylidene chloride and vinyl chloride containing 10 to 35 weight percent of vinyl chloride and a minor proportion, e.g.

0.5-15 and usually 3-15% of an EVA copolymer containing 5-40%, e.g. 20-40% and preferably 10-30%, vinyl acetate. This polymer blend is particularly useful when layers B 2 and B2 are composed of the same EVA copolymer. Another preferred combination 30 is a layer A containing as the only film-forming polymeric component a vinylidene chloride and vinyl chloride copolymer having a vinyl chloride content of 10-35% by weight, and layers B 2 and B2 composed of a compatible mixture of an EVA copolymer as just defined and a minor amount of the same vinylidene chloride copolymer. It will be seen that these combinations make it possible to recycle waste laminate in the manner described above.

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Surprisingly, it is not only possible, but particularly advantageous, to use a mixture containing an EVA copolymer to provide barrier layer A, since EVA copolymers have high oxygen permeability and their addition to copolymers 5 of vinylidene chloride causes an increase of its permeability.

The greater the thickness of the layers and B2, the more plasticizer they will absorb from the layer A of vinylidene chloride polymer and the better the resistance of the laminate to violence and abrasion. Therefore, it is important to ensure that the amount of plasticizer and the relative thickness of the layers are adjusted to one another so that the oxygen permeability is reduced to the desired level. The combined thickness of the layers B1l and B2 is at least 75% and especially at least 100, but not more than 15 400% of the thickness of layer A. Under this condition, the thickness of the vinylidene chloride polymer film may vary within wide limits and may e.g. may be from 2.5 to 63.5, especially from 7.5 to 63.5 micrometers, while each of the layers B 1 and b 2 may be from 5.0 to 75 micrometers and preferably 5 to 50 micrometers thick. . Appropriate total thicknesses for the laminates can go up to 220 and generally up to 127, but preferably up to 98 microns. It has been found that when the vinylidene chloride polymer is a copolymer of 90-65% vinylidene chloride and 10-35% vinyl chloride or a mixture of such copolymer with up to 15% 25 of an EVA copolymer containing 10-30 vinyl acetate and the layers Bi and B2 are composed of an EVA copolymer containing 10-30% vinyl acetate or a mixture of such a small amount of EVA copolymer, e.g. up to 8%, of the vinylidene chloride copolymer are particularly useful laminates having a layer A of 20-30 microns, each of which is at least 12 and preferably 15-30 microns thick.

Although it is preferred to use dibutyl sebacate as the plasticizer, other plasticizers may be used. It has been found that diisobutyl adipate and (2-ethylhexyldiphenyl) phosphate migrate better than dibutyl sebacate, whereas acetyl tri-n-butyl citrate migrates poorer than dibutyl sebacate.

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The layers of the laminate may, of course, contain additives or additives of the kind conventionally used for the polymers such as stabilizers, lubricants and abrasives. As a general rule, these additives should be such that the laminate becomes transparent.

Thus, a particularly preferred laminate comprises identical inner and outer layers, each approx. 18 microns thick and composed of an ethylene / vinyl acetate copolymer (EVA) containing 18% by weight vinyl acetate together with a small amount of a commercially available release agent. These layers initially do not contain any plasticizer, but plasticizer is migrated into them from the central layer. This central layer is 20-30 microns and preferably approx. 25 microns thick and is composed of a mixture of (i) approx. 96% by weight of a copolymer of vinylidene chloride (78% by weight) and vinyl chloride (22% by weight) prepared by emulsion polymerization and (ii) approx. 4% by weight of an EVA copolymer containing approx. 18% by weight of vinyl acetate, together with a plasticizer (eg, dihexylazelate or dibutyl sebacate), a heat stabilizer (preferably an organic material such as "Epikote" ® 1001) and optionally a wax as a lubricant.

This preferred laminate is prepared as follows. The three layers are extruded through a single extrusion head to give a tubular laminate which is quenched in a bath at 5 ° C and then stretched by the bubble method after reheating by passing through another bath at 40-45 ° C. As a result of this manufacturing method, the layers are oriented.

The following examples serve to elucidate the method of the invention.

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Examples 1-4

Foil laminates are prepared by coextruding in tubular form followed by stretch orientation by the bubble method. The vinylidene polymer used is a copolymer comprising 75% vinylidene chloride and 25% vinyl chloride, and the oxygen permeabilities are measured with Bruger's cell two weeks after extrusion. The user's cell is a 1U002 9

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small chamber which is divided by a sample of the foil whose permeability is to be measured. The foil sample is extended across the opening of one chamber half, and the other chamber half is closed tightly against the foil so that the foil sample divides the chamber into two halves.

5 Then both chamber halves are evacuated to remove any impurities in the air. Next, at atmospheric pressure, air is allowed to fill one half of the cell, and a highly sensitive pressure gauge such as a mercury capillary tube is used to measure the air pressure change. On the other side of the film sample, the remaining half of the cell is filled with pure oxygen gas, and a constant pressure difference is maintained between the oxygen side and the air side. Since the volume of the cell is known and the area of the film sample is also known, the pressure increase on the air side after 24 hours can be directly related to the volume of gas passed through the film during this period. It will be appreciated that the volume of gas passing through the foil sample depends on the area of the sample, its thickness, the period, the pressure difference and the temperature. Thus, the oxygen permeability is determined by the volume (in cm) of oxygen which 2 has passed a standard area of the foil sample (im) with a standard thickness of the foil (25 microns) over a period of time (which in this sample is set at 24 hours), over a pressure difference (set at 1 atmosphere at the sample) and at a certain temperature (set at 23 ° C at the sample). This is the basis for the maximum oxygen permeability stated in the requirement of 3 2 25 70 cm x 25 microns / m x 24 hours in air at 1 atmosphere and 23 ° C.

The following table lists data for the different layers of the foil laminate and the results obtained with the foil laminate. Thus, the initial composition of each layer is listed, using the following abbreviations, namely VDC for vinylidene chloride polymer, EVA (28) for ethylene / vinyl acetate copolymer containing 28% vinyl acetate, EVA (18) for ethylene / vinyl acetate copolymer containing 18% vinyl acetate, ATBC for acetyl-tri-n-butyl citrate and DBS for dibutyl sebacate. Furthermore, Table 35 shows the thickness of each layer (measured by orientation) and the amount of plasticizer determined by standard analysis technique, which is finely divided into each layer in freshly extruded and stretched condition and after storage at room temperature for two weeks after orientation; from this, the migration rate is calculated. Furthermore, the table gives the measured oxygen permeability of layer A as a single layer in fresh stretched state, ie. before migration of the plasticizer (0x "T),

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and the corresponding oxygen permeability found after two weeks, i. after migration (0xFD).

Ex. Initial layers Thickness f Softening Migra- 0xSL 0xFD

ir. session composition see μ% tion% 1 Βχ EVA (28) 17.5 1.15 14.4

A. VDC 8% DBS

8% EVA (28) 32.5 6.40 - 235 6o E »EVA (28) 5.1 0.35 4.4 15 ^ 2 Βχ EVA (l8) 15.2 0.9 11.2

A. VDC

8% DBS 27.9 6.2 - 130 68 B2 EVA (1.8) 14.0 0.9 11.2 20 ________ - 3 a eva (25) 15.0 1.05 13.1

A. VDC

8f DBS 23.0 5.47 - 130 48

Bg EVA (1.8) 23.0 1.48 18.5 25-4 B1 EVA (1.8) 45.0 0.42 4.7

A. VDC 9% ATBC

4f EVA (18) 85.0 8.0 - 85 42 B2 EVA (18) 45.0 0.53 5.9 30 _—---- u --- M--

The foil laminates prepared by the process of the invention preferably have a shrinkage of 15-60%, as determined by immersion in water at 96-98 ° C for 3 seconds.