CA1303473C

CA1303473C - Thermoplastic multilayer packaging film and bags made therefrom

Info

Publication number: CA1303473C
Application number: CA 519270
Authority: CA
Inventors: Richard Stephen Williams; Cesare Quacquarella
Original assignee: WR Grace and Co Conn
Current assignee: Cryovac LLC
Priority date: 1985-09-30
Filing date: 1986-09-29
Publication date: 1992-06-16
Anticipated expiration: 2009-06-16
Also published as: DK170038B1; DE217252T1; DK463186D0; NZ217626A; JPH0773904B2; FI88375B; AU6310286A; US4801486A; JPH0639978A; EP0217252A3; AU582902B2; MX168438B; ES2003111A6; EP0217252A2; FI863919A; DK463186A; FI863919A0; EP0217252B1; DE3682473D1; FI88375C

Abstract

THERMOPLASTIC MULTI-LAYER PACKAGING FILM
AND BAGS MADE THEREFROM

ABSTRACT

A thermoplastic multi-layer packaging film is disclosed comprising an outer polymer layer, a central layer acting as oxygen barrier and an inner heat sealing layer. The heat sealing layer comprises an ethylene /alpha-olefin copolymer wherein the alpha olefin has 4 to 8 carbon atoms per molecule and its content in the copolymer ranges of from x to y% by weight wherein x = 0.5 nC + 8 and y = 1.25 nC +
15, wherein nC is the number of carbon atoms in the alpha-olefin. The copolymer of the heat sealing layer has a density of less than 920 kg/m3.

Description

~3~P~

THERMOPLASTIC MULTI-LAYER PACKAGING FILM
AND BAGS MADE THEREFROM
The invention relates to thermoplastic multi-layer packaginq films and bags or pouches made therefrom. In particular this invention relates to films and bags havin~ superior heat shrink and cold seal properties, which give good seal strength even through contamination like grease or blood, and which allow the application of higher shrink temperatures due to their improved seal strength at elevated temperatures.
Shrinkable and heat sealable thermoplastic films are being used in packaging of non-food and food products like meat, cheese, poultry and the like. Many attempts have been made to combine good shrink properties with satisfactory sealability and good seal strength both at room temperature and at higher temperatures but there is s~ill room for improvement.
A laminate film known from U.S. patent No.
3 741 25~ comprises a core layer of a vinylidene chloride copolymer between a layer of a cross-linked ethylene-vinyl acetate copolymer used as a sealant and - 20 an outer layer of ethylene-vinylacetate copolymer which is not crosslinked. The ethylene-vinylacetate copolymer has improved properties over the previously used polyethylene, especially when it has been cross-linked by irradiation.
As disclosed in U.S. patent No. 4 064 296 the core layer may also consist of a hydrolized ethylene-vinyl ~` acetate copolymer. It has similar oxygen barrier properties as vinylidene chloride copolymers and offers 'X~

~3~?3~7 the advantage that it may be irradiated without discoloration.
Certain advantages have been achieved by using an ionomeric resin as sealing layer. They result in a higher seal strength at elevated temperatures thus allowing higher shrink temperatures. However, their cold seal strength is not fully satisfactory. This latter property is important in modern automated bag packaging processes in which the seals are subjected to substantial stress when the bags are opened and loaded with the article to be packed.
More recently low density linear polyethylenes have become commercially available and have also been proposed for packaying applications. Actually the so called linear low density polyethylenes are copolymers of ethylene and varying amounts of higher ~-olefins with e.g. 6 to 10 carbon atoms per molecule (U.S.
patent No. 4 076 698) or 3 ~o 8 carbon atoms per molecule (published European patent application 20~ 120503)~ Depending on their density these materials are referred to as linear low density polyethylene (LLDPE) or very low density polyethylene (YLDPE), the separation line being at a density of about 0.910 g/cm . Some properties of VLDPE have been described in Plas,tics Technology, September 1984, page 113 and October 1984, page 13 as well as in a company brochure published in February 1984 by DSM in the Netherlands and entitled "Stamilex PE'I~ Their properties are said to be a unique oombination between those of standard r ~ 5 30 polyethylene and polyolefinic rubbers. Their ~L3~3~3 sealability and their compatibility with other polymers has been mentioned.
It is an object of the present invention to provide a multi-layer laminated packaging Film and bags made therefrom which have improved shrink characteristics over the materials used in the past.
This means that at a given temperature the shrink in percent (the mean between the shrink in the longitudinal and transversal direction) should be higher and the tolerated maximum shrink temperature should also be higher. The maximum shrink temperature is dependent on the seal strength at elevated temperatures.
It is a further object of this invention to provide a packaging material with improved cold seal strength and thereby a reduced risk of breakages when bags made of the film material are utilized in automated loading processes.
It is yet another object of the present invention ~ to provide a packaging material which gives reliable seals even when contaminated. Especially in the packaging of meat it frequently occurs that the sealing areas are c~ontaminated with grease or blood, both of which tend to impair the sealing between hither~o used sealing layers.
- Finally and most importantly it is an object of this invention to provide a material combining the above advantages, i.e. improved shrinkability, increased maximum shrink temperature, good cold seal strength and sealability through c~ntamination.

~3~34 4 ~

It has been found that the above-mentioned objects are achieved by the present invention which is directed to a thermoplastic multi-layer packaging film comprising at least an outer polymer layer, an inner heat sealing layer and optionally an oxygen barrier layer between said inner and said outer layer, and which is characterized in that the heat sealing layer comprises a copolymer of ethylene and x to yg by weight, based on the copolymer, of an CC -olefin with 4 to 8 carbon atoms, whereby x = 0.5nC + 8 and y = 1.25 nC + 15 with nC being the number of - carbon atoms in the oL-olefin , said copolymer having a density of less than 920 kg/m3.
Preferably the heat sealing layer comprises a ` 15 copolymer of ethylene and 12 to 20X by weight, based on the copolymer, of an o~-olefin with 4 to 8 carbon atoms.
It has surprisingly been found that both the heat shrink properties and the cold seal strength of the laminated packaging film depend on ~he comonomer content of the ethylene copolymer used as the sealing layer. In order to achieve optimal results the - comonomer content must increase when going from the C4 - o~-olefin (butene(1)) to the C8- C~-olefin ~5 (octene(1)). It is particularly surprising that the films of the present invention exhiblt improved shrlnk properties bo~h in irradiated (cross-linked) and non-irradia~ed condition and ~hat these properties as well as the seal strength are a function of the comonomer rr.. ~ 0 content of the linear low and very low density "polyethylene".

~ ~V3~3 When -using a copolymer of ethylene and butene (1) the comonomer content should be between 10 and 20% by weiqht, based on the copolymer. Such copolymers have a density of less than 915 kg/m3. When employing a 5copolymer of ethylene and uctene (1) the comonomer content should preferably be increased to between 12 and 25g by weight, based on the copolymer. 5uch copolymers have a density of less th~n 920 kg/m3.
According to the invention it is also possible to 10blend the ethylene/ ~-olefin copolymer with up to 50 by weight, based on the sealing layer composition, of a polymer which is compatible with said ethylene / o~ -olefin copolymer. Such further polymer may preferably be selected from the qroup comprisin~ linear Iow 15density polyethylene (LLDPE) with a density above 920 kg/m , linear high density polyethylene (LHDPE), low density polyethylene (LDPE), ethylene viny] acetate (EVA), acid modified E~A, polypropylene, ethylene/propylene copolymers, ionomeric polymers and 20ethylene/alkylacrylate copolymers wherein the alkyl moiety has 1 to 8 C atoms, in particular ethylene-methylacryla~e (EMA~, ethylene-ethylacrylate (EEA~ and ethylene-butylacrylate (EBA).
The ethylene/alkylacrylate copolymer which can be 25blended with the ethylene / o~-olefin in the heat - sealing layer can comprise 3 to 30% by weight of alkylacrylate.
Acccrding to one embodiment of the invention the ~ ethylene copolymer forming the sealing layer may be ", ~ 30cross-linked by irradiation in order to improve its ~3~?3~3 shrink characteristics and mechanical properties.
However, such cross-linking is not necessary since a very satisfactory packaging film is also obtained without irradiation. If polyvinylidene chloride is used as core layer providing an oxygen barrier the irradiation should take place prior to the lamination of the core layer to the sealing layer since PVDC may suffer when exposed to high energy irradiation. The entire multi-layer film may be irradiated if the PVDC
oore layer is substituted by a core layer of partly hydrolyzed ethylene vinyl-acetate copolymer (EVOH).
PVDC and EVOH are the preferred core layers since they show outstanding oxygen barrier properties.
The films are normally orier,ted in longitudinal and transversal direction to obtain the desired shrink properties. If a tubular film is produced by extrusion the orientation may be e~fected using the known air bubble method and further stretching the film by pulling. Al~erna~ively, orientation may be achieved by deep drawing of a planar film.
The outer layer of the multi-layer packagin~ film of the invention may be of the same ethylene copolymer as the sealing layer, i.e. an ethylene/ 0~ -olefin copolymer comprising 10 to 25X by weight, based on the copolymer, of an o~-olefin with 4 to 8 carbon atoms, said copolymer having a density of less than 920 kg/m3 ethylene vinyl acetate and polyamides, and ionomeric s resins. It may frequently be of advantage if an adhesive layer and/or an adhesion promoting layer is ~nterposed between said other layers.

~3~3~3 If bags are made from the film material the film will normally be folded so that the fold forms the bottom of the bag whereafter both sides are sealed so that the side seals close both sides of the bag.
Alternatively a bag may be formed from a tubular film in that the bottom of the bag is formed by a transverse seal across the flattened tube and the mouth is formed by severing ~he tube at a pre-selected distance from said transverse seal. Pouches can be obtained by sealing two film sheets at three edges.
The invention will be further i-llustrated by reference to the following examples without being limited thereto.
The following test methods were used to investigate the properties of the packaging films and to compare them with other materials.

A. VARIABLE PRESSURE HOT BURST TEST (VPHB) This is a test to determine the seal quality of shrinkable materials at different temperatures. The approach with this test is to immerse total seal areas into hot water and after a predetermined dwell time, the pressure inside the bag is increased at a constant rate of approx. 25,4 mm of water per second until the seal fails.
The mm of water pressure, at the level at which the seal fails, are recorded, and there are minimum specifications expressed in mm for each bag width. The highest is the number, the better is the seal quality ~: at that temperature.

- ~ 3 B. PERCENT SHRINK
The percent shrink is measured immersing for a couple of econds in hot water a specimen and measuring the percent of retraction, both in the longitudinal and transversal direction. The highest is the percent of shrink, ~he better is the performance of the material in terms of packag~ presentation.
In practice, ~he maximum achievable percent of ; shrink is that measured at the temperature at which the material meets the minimum VPHB requirements expressed - 10 in mm of water pressure (see point A~. To predict the shrink behaviour it is therefore useful to plot in the same diagram the % of shrink and the VPHB as a function of the temperature.
C. COLD SEAL STRENGTH
The cold seal~strength represents the seal quality at room temperature and predicts the seal performance when the ba~ is loaded with a pro~uct, either manually or by means o~ automatic loaders, which operate with a pusher, which pushes the product into the bag. The cold seal strength is measured with a modified pusher of a commercial loader, equipped with a system to record the ; pressure necessary to open ~he bottom seal. The head of the~ pusher operates on a 6 cm length of seal and the cold seal strength ls expressed in kg/6 cm. The maximum pressure recordable with this systems is 40 kgt6 cm.
; " .
r The reported results are the averaqe of 10 ~3q~
_ g _ measuremen ts .
The following raw materials were used in the examples:

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A A q = ~ 1 A A N ~ ~ ~ N o~

' A 1 = A O ~ 0 ~V~ ~ A ~ 6 3c ;, ~; a ~ o u , a V~ ~ ~ ~ n a ~ I ~ z a I ~ ~ z ~ ~

O O ~
ao E o~ U ~ E

~3~ 3 Multi-layer packagin~ films of the composition summarized below in table I were prepared in the following manner. The first two layers A and B were coextruded through a conventional tubular extruder to form a tube. After leaving the die the substrate was cooled and flattened. It was then sent- through an irradiation vault where it was irradiated by high energy electrons to a dosage of about 4.5 MR. ~epending on the characteristics desired this dosage could vary ; 10 from 2 to 20 MR. After leaving the irradiation vault the substrate was gain inflated and sent through a first tubular extrusion coating die where it received a coating of vinylidene chloride copolymer. Thereafter the still inflated and now triple-walled film passed through a second tubular extrusion coating die where it received a layer of ethylene-vinyl acetate copolymer.
After the final coating the film was cooled, collapsed and rolled-up. This tape was subsequently unrolled, fed through a bath of hot water, held at a temperature of abo~ut 80 to 98 C, and as it left the water it was inflated and blown into thin ~ubing with a total wall ~; thickness of about 59 microns. This so-called "trapped bubble" technique is well known in the art. The fiIm was then rapidly cooled to set the orientation and then rolled up ~or further processing.
The polyvinylidene chloride and the ethylene-, v-inylacetate copolymer can alternatively be coated onto the substrate using a two-ply coextrusion die.
The ~est res~lts are summarized in the following - table II.

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3tP3 ~'73 When comparing the results it should be borne in mind that the shrink temperature should not exceed a value at which the hot-burst pressure drops below 650 mm/H2 0. For some food products the maximum shrink temperature may be more limited but a hi~her hot-burst will then give additional safety. It is clear from the data in table II that comparison 1 gives good cold seal strength but the hot seal strength and the percent shrink leave something to be desired. Further, this material results in poor seal strength through contamination like grease or blood. Comparison 2 shows improved shrink characteristics but the cold seal strength is considerably lower and this material cannot b sealed reliably through blood contamination.
- 15 Examples 1, 2 and 3 show that excellent shrink character~stics and a very good seal strength both at room temperature and at elevated temperatures are being obtained. It has further been found that the seal strength is not significantly affected by contamination like grease or blood. Comparisons 3 and 4 show that a very significant decrease both in seal strength and percent shrink occur when the comonomer content in the ethylene o~ -olefin copolymer drops below a certain level, i.e. the minimum comonomer content is an essential feature if the combination of desired properties is to be achieved.
:

COMPARATI YE EXAMPLES 5 AND 6, EXAMPLES 4 TO 1 O
.~ .
~ Multi-layer packagin~ films of the composition .. ~ 3 ~

summarized in the following table III were produced by conventional techniques, i.e. by coextruding a tubular film of layers A to F and subsequent stretching and orientation, e.g. in a blown-up bubble, so as to obtain a heat shrinkable film. No irradiation was employed in this case. The test results obtained are tabulated in the following table IV.

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" ~31~3~L~3 `- - 18 -The results are essentially similar to those ob~ained with the irradia~ed material. Only the films of the present inYention combine excellent shrink characteristics with outstanding cold and hot seal strength. In addition thereto they may be sealed - without impairing the seal strength through contamination like grease or blood which are always encountered in the packaging of meat products. Further~
the non-irradiated structures show the same effect, namely a strong dependency of the shrink and seal characteristics on the comonomer content of the ethylene/ d -o lefin copolymer.

.

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Claims

1. A thermoplastic multi-layer packaging film having superior heat shrink and cold seal properties comprising at least an outer polymer layer, an inner heat sealing layer and an oxygen barrier layer between said inner and said outer layer, characterized in that the heat sealing layer comprises a copolymer of ethylene and x to y% by weight, based on the copolymer, of an .alpha.-olefin with 4 to 8 carbon atoms per molecule, wherein x=0.5nC+8 and y=1.25nC+15 with nC being the number of carbon atoms in the .alpha.-olefin, said copolymer having a density of less than 915 kg/m3.

2. The film of claim 1, characterized in that the heat sealing layer comprises a copolymer of ethylene and 12 to 20%
by weight, based on the copolymer, of an .alpha.-olefin with 4 to 8 carbon atoms.

3. The film of claim 1, characterized in that the heat sealing layer comprises a copolymer of ethylene and 10 to 20%
by weight, based on the copolymer, of butene (1), said copolymer having a density of less than 915 kg/m3.

4. The film of claim 1, characterized in that the heat sealing layer comprises a copolymer of ethylene and 12 to 25%
by weight, based on the copolymer, of octene (1), said copolymer having a density of less than 915 kg/m3.

5. The film of claim 1, characterized in that the heat sealing layer comprises further up to 50% by weight, based on the layer composition, of a polymer which is compatible with said ethylene/.alpha.-olefin copolymer and is selected from the group comprising linear low density polyethylene (LLDPE), linear high density polyethylene (LHDPE), low density polyethylene (LDPE), ethylene/vinyl acetate (EVA), acid modified EVA, polypropylene, ethylene/propylene copolymers, ionomers and ethylene/alkylacrylate copolymers having 1 to 8 carbon atoms in the respective alkyl radical.

6. The film of claim 5, wherein said polymer compatible with said ethylene/.alpha.-olefin copolymer is an ethylene/alkylacrylate copolymer selected from ethylene-methylacrylate, ethylene-ethylacrylate and ethylene-butylacrylate.

7. The film of any one of claims 1 to 3, characterized in that the outer polymer layer is of a polymer selected from the group comprising a copolymer of ethylene and 10 to 25% by weight, based on the copolymer, of an .alpha.-olefin with 4 to 8 carbon atoms, said copolymer having a density of less than 915 kg/m3, EVA, polyamide, and ionomeric resins.

8. The film of any one of claims 1 to 3, characterized in that the oxygen barrier layer comprises a polymer selected from the group comprising polyvinylidene chloride (PVDC) and hydrolized ethylenelvinylacetate copolymers (EVOH).

9. The film of any one of claims 1 to 3, characterized in that an adhesive layer and/or an adhesion promoting layer is interposed between said outer layer and said inner layer.

10. The film of claim 1, characterized in that at least the heat sealing layer has been cross-linked by irradiation.

11. The film of claim 10, characterized in that the oxygen barrier layer comprises EVOH and that the entire multi-layer film has been irradiated.

12. A side-sealed bag formed from the film of any one of claims 1 to 3, said film being folded so that the fold forms the bottom of the bag and side-seals close both sides of the bag.

13. A bag formed from a tubular film of any one of claims 1 to 3, wherein the bottom of the bag is formed by a transverse seal across the flattened tube and the mouth is formed by severing the tube at a pre-selected distance from said transverse seal.

14. A multi-layer thermoplastic packaging film having improved heat shrink and cold seal properties, said film comprising:
(a) a first surface layer which is a heat sealing surface, said layer comprising:
(1) a copolymer of ethylene and an .alpha.-olefin having 4 to 8 carbon atoms per molecule; and, (2) said copolymer comprising 90 to 75% ethylene and 10 to 25% .alpha.-olefin having 4 to 8 carbon molecules and a density of less than 915 kg/m3;
(b) a second surface layer, said layer comprising a polymer selected from the group consisting of amide polymers, ethylene/vinylacetate copolymers, the copolymer of the first layer, and ionomers;
(c) a first core layer comprising an ethylene/vinyl-acetate copolymer;
(d) a second core layer comprising a polymer selected from the group consisting of copolymers of vinylidene chloride and hydrolyzed copolymers of ethylene/vinylacetate;
(e) said core layers being internal film layers positioned between said first and second surface layers;
(f) at least one adhesive layer to promote adhesion of the second core layer to another layer; and (g) said film comprising at least five layers.

15. The film of claim 14, wherein the first surface layer is cross-linked.

16. The film of claim 14, wherein the first surface layer comprises a blend of the copolymer defined in (a)(1) and (2) with a copolymer selected from the group consisting of copolymers of ethylene/vinylacetate and copolymers of ethylene/butylacrylate.

17. The film of claim 14, wherein the second core layer comprises a vinylidene chloride copolymer and the second surface layer comprises a polyamide.

18. A bag formed from the film of claim 14.