FI57550B - FOERFARANDE FOER FRAMSTAELLNING AV VAERMEKRYMPBART FOERPACKNINGSLAMINAT - Google Patents

Description

nr - ^ - Γ Λ. . .... NOTICE cnccr \ ^ i11) UTLÄGGNI NGSSKRI FT 5 7 5 5 0 C (4¾ PatentU sold 10 C9 1980 (51) Kv.ikiirrt.ci.3 B 29 D 9/00, B 32 B 27/28 FINLAND —FINLAND (21) PtMnttlhtkumu * - PttwitamBknlni 1801/72 (22) Hakemiipllvl —Anaekniniadii 22.06.72 * * (23) Alkupitvt — GiM | h «ttdaf 22.06.72 (41) Tullut lulkhukal - Bltvit offentllf 2U.

Patent Office · · ........ - ... ,. , _. ^ ',. (44) Date of Nihttvikflpuon and other public footwear—

Patent- och ragittarstyralaan Amekan utlmgd och uti.tkrift * n pubiicitrad 30.03.80 (32) (33) (31) Requested atuolkaut - Begird prlorltet 2 3.06.71 1¾.01.72 United Kingdom-Storbritannien (GB) 29532/71, 1986 / 72 (71) WR Grace & Co., 3 Hanover Square, New York, New York IOOOU, USA (72) Märio Gillio-Tos, Rho, Paolo Vietto, Rho, Italian-Italian (lT) (7 * 0 Berggren 0y Ab (5 ^) Method for producing a heat-shrinkable packaging laminate -Förfarande för framställning av värmekrympbart förpackningslaminat

The present invention relates to a process for producing a new heat-shrinkable packaging laminate.

Homopolymers of vinylidene chloride cannot be converted to films by conventional extrusion methods because they 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 suitable plasticizers (and possibly smaller amounts, usually less than 15% by weight of other polymers), can be extruded into films which can be stretched to obtain shrinkable films. The term "vinylidene chloride polymer" is used herein to mean 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 alkyl acrylates, methacrylates, especially alkyl acrylates and methacrylates having 1 to 8 carbon atoms in the alkyl groups.

Heat-shrinkable films of vinylidene chloride polymers have been widely used for packaging purposes, especially in food packaging. These films are very useful in 57550 vacuum packaging methods where the film shrinks around the material to be packaged. The film is usually in the form of a package made by sealing a tubular oriented film obtained by a well-known extrusion process and bubble orientation. The package is sealed or heat-sealed after the material to be packaged is fitted to the package. Copolymers containing 90-65% by weight of vinylidene chloride and 10-35% by weight of vinyl chloride have been found to be very suitable for this purpose. (Percentages are calculated by weight in this specification).

In the manufacture of a heat-shrinkable film, the film must be strongly stretched. In the case of a vinylidene chloride polymer film, it has been found that in order to obtain the best results in terms of ease of performance of the stretching process, it is preferable to use much more plasticizer than is commonly used in the manufacture of extruded, non-stretched (i.e. non-stretched) film. It allows, for example, the vinylidene chloride polymer to be stretched at a relatively low temperature and gives it a high shrinkage energy. Unfortunately, however, the high plasticizer content lowers the oxygen barrier properties of the vinylidene chloride polymer film. In other words, its resistance to oxygen permeation decreases. This is apparently serious when the film is to be used for packaging oxygen-sensitive materials such as food. The problem has thus been to produce a film which is both easily oriented and has the desired good oxygen barrier properties. For currently used products, a permeability of less than 100 is essential and less than 70 is highly desirable. The oxygen permeabilities shown in this specification are expressed in cnr of oxygen per 25 μm of film surface m, in 24 hours, at atmospheric pressure, at a temperature of 23 ° C. These values can be measured e.g. 0. Using a Brugger cell.

In addition, the use of larger amounts of plasticizer can cause loss of brightness during storage due to increased polymer-plasticizer incompatibility. Another significant disadvantage of the increasing plasticizer content is that it is difficult to seal the film.

Another important factor to keep in mind is that the damage resistance of vinyl chloride polymer films varies widely depending on the compound used as the plasticizer. In practice, 3,57550 naturally use those plasticizers which provide good damage resistance and unfortunately these plasticizers make it most difficult to combine low oxygen permeability with good extrudability and orientation, so that the amount of plasticizer used to limit 100 film orientation, especially when stretching films with conventional bubble orientation. This has entailed significant additional costs both in controlling the production of directional films and in terms of losses due to production interruptions. · For example, when dibutyl sebacate (DBS), the most widely used plasticizer for this purpose, is used, a vinylidene chloride polymer with 15-35 weight percent vinyl chloride. requires 7-10% by weight of dibutyl sebacate to achieve satisfactory extrusion and orientation, but films made from such a plasticized polymer are too permeable to oxygen and have detrimentally low wear resistance, especially at low temperatures. In addition, very precise temperature control is required to seal such films and to provide sufficient film strength.

Another plasticizer that behaves much the same as DBS is di-n-hexyl azelate (DNHZ). At the other end of the scale, acetyl tri-n-butyl citrate (ATBC) and 2-ethylhexyl diphenyl phosphate (DPPA) give a relatively good combination of low oxygen permeability, satisfactory extrudability and orientation, but these substances have not been used in practice because plasticized films have very poor resistance to damage.

It has been found that a heat-shrinkable laminate having excellent mechanical properties and oxygen permeability of less than 70 can be produced without the above-mentioned difficulties. According to the present invention, a three-layer heat-shrinkable packaging laminate having an oxygen permeability of less than 70 cm 2 per mm of film surface is prepared in 24 hours with a pressure gradient of 1 atm at 23 ° C, wherein the laminate has a plasticized vinylidene chloride polymer. an oxygen barrier layer laminated on both sides with layers of another polymer which absorb a plasticizer from the barrier layer, and wherein the barrier layer 11 57550 is co-extruded with another layer of other polymer and the resulting tube is biaxially stretched, and the invention is characterized in that co-extruded together and that the vinylidene chloride polymer of the barrier layer contains 7-12% by weight of plasticizer during extrusion, which concentration decreases after a relatively short time to the level at which the laminate has said oxygen permeability, and that the surface layers are substantially copolymer, e.g., an ethylene / vinyl acetate copolymer, optionally containing a small amount of a vinylidene chloride copolymer. The invention makes it possible to use vinyl chloride polymers which initially contain large amounts of the desired plasticizers to achieve good extrudability and orientation, but which have hitherto been considered impractical because of the high oxygen permeability they cause. In addition, the invention makes it possible to use plasticizers which have hitherto been considered impractical because of the poor mechanical properties they cause.

A preferred embodiment of the invention is thus a method wherein the vinylidene chloride polymer feedstock contains sufficient plasticizer to impart at least 130 initial oxygen permeability to the unstretched film and the laminate is maintained under conditions such that its oxygen permeability decreases to less than 70 due to migration of the plasticizer to vinyl

In the manufacturing process of the invention, all three layers are extruded using molten plastic through a single extrusion head with three extrusion orifices to form a tubular product which is then directed by internal gas pressure using a well known "bubble method" to produce a shrinkable laminate. The oriented tubular laminate can be cut to form a flat film or converted into bags using known techniques.

Layers B1 and B2 may be similar or different in structure. Preferably, at least one of the liquid layers (the inner layer of tubular laminates in question) is heat sealable.

Excellent laminates are obtained according to the invention when the copolymers of ethylene and vinyl acetate (hereinafter referred to as EVA copolymers) used for layers B1 and B2 always contain 40%> e.g. 5-50% y preferably 10-30%, vinyl acetate . It has been found that the higher the vinyl acetate produced, the better the plasticizer moves, and on the other side of the lower vinyl acetate provide better vaurioitumiskestävyyden. These copolymers have good compatibility with vinylidene chloride polymers and, as a result, adhere well to them, and a wide range of oxygen permeability can be obtained due to the absorption mechanism of the plasticizer and also good damage resistance.

The amount of plasticizer that is blended with the vinylidene chloride polymer prior to extrusion will, of course, depend to some extent on the quality of the plasticizer and polymer, but as mentioned above in a preferred embodiment of the invention, this amount should be at least such that Layer A has an initial oxygen permeability of 130. and to be directed. When dibutyl sebacate is used, this amount is often such that the initial permeability of layer A is at least 200. When most plasticizers are used, the polymer contains 7-12% y, e.g. 7-9%, of plasticizer. When using dibutyl sebacate, we have obtained good results with 7-9%> especially 8% of this plasticizer. The plasticizer concentration in layer A decreases as the plasticizer enters the layers and B £ and this transition can be promoted by raising the temperature, but in most cases storage at room temperature for 2 weeks or longer provides a sufficient transition so that the plasticizer concentration decreases to less than 70. certain plasticizers, especially ATBC and DPPh, which do not cause as high an increase in oxygen permeability as DBS, can be used to obtain laminates with very low permeabilities, e.g. less than ^ 5.

It is very practical to use (a) as a vinylidene chloride polymer a mixture of (i) a copolymer of 90-65% vinylidene chloride and 10-35% vinyl chloride, and (ii) up to 15% by weight of the mixture of the polymer used for the layers B to form ^ and B2 (e.g., EVA copolymer up to about 8%), or (b) use as the vinylidene chloride polymer a copolymer of 90-65% vinylidene chloride and 10-35% vinyl-6,57550 chloride, and use as a polymer to form layers B and B2. a compatible polymer blend that extracts the plasticizer from the plasticized vinylidene chloride polymer layer (e.g., EVA copolymer) and a small amount of vinylidene chloride dipolymer. In this way, good adhesion between the layers is achieved, and in addition, an important advantage is that it is possible to recycle the waste laminate and use it to make layer A or one of layers B1 and B2, or both.

A preferred vinylidene chloride polymer is a copolymer of vinylidene chloride and vinyl chloride containing 10-35% by weight of # vinyl chloride and mixed with a smaller amount, e.g. 0.5-15 #, usually 3-15 #, of an EVA copolymer having 5-40 #, e.g. 20-40 #, preferably 10-30 #, vinyl acetate. This polymer blend is very useful when layers B1 and B2 are formed from the same EVA copolymer. Another preferred combination is layer A, which is the only film-forming polymeric component of a copolymer of vinylidene chloride and vinyl chloride having 10 to 35% by weight of vinyl chloride, and layers B1 and B2 are formed of a compatible blend of a freshly defined EVA copolymer and less. amount of the same vinylidene chloride copolymer. It can be seen that these combinations make it possible to reuse the waste laminate as described above.

It is very surprising that it is possible to use a mixture containing EVA copolymer; to provide a barrier layer A because EVA copolymers have a high oxygen permeability and their addition to vinylidene chloride copolymers provides an increase in permeability.

The greater the thickness of layers B1 and B2, the more plasticizer they absorb from the vinylidene chloride polymer layer A and the better the damage resistance of the laminate. As a result, it is important to ensure that the amount of plasticizer and the relative thicknesses of the layers are proportional so that the oxygen permeability decreases to the desired value and the laminate has the desired damage resistance. The combined thickness of the layers B1 and B2 is preferably at least 75%, in particular at least 100%, but not more than 400% of the layer A

7 57550 of thickness. In this case, the thickness of the layer derived from the vinylidene chloride polymer can vary within wide limits, and can be e.g. 2.5-63.5, e.g. 7.5-63.5 microns, while the thickness of each layer and Bj can be 5.0-75 microns, preferably 5-50 microns. A suitable total thickness of the laminates can be up to 220, generally up to 127 and preferably up to 89 microns. We have found that when the vinylidene chloride polymer is a copolymer of 90-65% vinylidene chloride and 10-35% vinyl chloride, or a blend of such a copolymer with up to 15% EVA copolymer containing 10-30 % vinyl acetate, and layers B 1 and B 2 are formed from an EVA copolymer containing 10-30% vinyl acetate, or a mixture of such an EVA copolymer, with a small amount e.g. e.g. up to 8% vinylidene chloride / vinyl chloride copolymer, in highly useful laminates, the thickness of layer A is 20 to 30 microns and the thickness of the layers and the combined thickness is at least 30 microns, and the thickness of each of these layers is at least 12 microns, preferably 15 to 30 microns.

Although we prefer to use dibutyl sebacate as a plasticizer, we have also performed experiments with other plasticizers and found that diisobutyl adipate and (2-ethylhexyldiphenyl) phosphate migrate better than dibutyl sebacate, while acetyl tri-n-butyl citrate transfers better than acetyl tri-n-butyl citrate. .

The laminate layers may, of course, contain such additives commonly used in polymers, such as stabilizers, lubricants and lubricants. As a general rule, such additives should be used in such a way that the laminate remains transparent.

A highly preferred laminate made by the process of the invention comprises identical inner and outer layers, each about 18 microns thick, formed of an ethylene / vinyl acetate copolymer (EVA) having 18 weight percent. vinyl acetate along with a small amount of a commercially available lubricant. These layers do not initially contain a plasticizer, but the column plasticizer is transferred to them from the middle layer. The middle layer has a thickness of 20 to 30 microns, preferably about 25 microns, and consists of a blend of (i) about 96% by weight of a vinylidene chloride copolymer (78% by weight of vinylidene chloride and 22% by weight of vinylidene chloride) prepared from an emulsion polyol. And (ii) about 4% by weight of an EVA copolymer having about 13% by weight of 2-vinyl acetate together with a plasticizer (e.g., dihexylates in lettuce or dibutyl sebacate) as a heat stabilizer (preferably an organic substance such as " Epicote 1001 ") and possibly using wax as a lubricant.

This preferred laminate is prepared as follows. The three layers are successively extruded through a single extrusion head to give a tubular laminate which is cooled in a bath at 5 ° C and then stretched by bubbling after reheating by passing through a second bath at 40-45 ° 0. As a result of this method, oriented layers.

The following examples further illustrate the present invention. Laminates were made using simultaneous extrusion in the form of a tube followed by bubble orientation. The vinylidene polymer used was a copolymer of 75% vinylidene chloride and 25% vinyl chloride, and oxygen permeability was measured two weeks after extrusion. The table shows the initial composition of each layer (VDC is a vinylidene chloride polymer, EVA (28) is an ethylene / vinyl acetate copolymer with 28% vinyl acetate, EVA (18) is an ethylene / vinyl acetate copolymer with 18% vinyl acetate, ATB is tri-n-butyl citrate and DBS is dibutyl sebacate) and the thickness of each layer (after orientation); the amount of plasticizers detected in each layer after two weeks of storage at room temperature after orientation and the percentage of transition; the assumed oxygen permeability of layer A as a single layer, i. without plasticizer transfer (0xSL) and oxygen permeability of the laminate after two weeks (0xpD).

57550

Eg Layer Thickness% Plasticize Transition Ox ^ Ox No. Preliminary (Micronutrient% Coating) 1 Βχ EVA (28) 17.5 1.15 14.4

Λ. VDC

i% EVA (28) 52.5 6, AO - 235 60 B2 EVA (23) 5.1 0.35 V »2 E1 EVA (18) 15.2 0.9 11.2

A. VDC

8> TBS 27> 9 6> 2 '13 ° 68 B2 EVAU8) 14.0 0.9 11.2 3 Βλ EVA (25) 15.0 1.05 13.1

A. VDC

ό% DBS 23 '° 5> 47 - 130 B2 EVA (18) 23.0 1.48 18.5 4 B1 EVA (18) 45.0 0.42 4.7

A. VDC

9¾ ATBC An At ii5> H% EVA (18) 83.0 8.0 "8j 42 B2 EVA (13) 45.0 0.53 5.9 The laminates of this invention have a shrinkage of 15-60% t when immersed in 96- 98 C degrees for 3 seconds in water.