CA1100730A

CA1100730A - Biaxially stretched five-layer film and method for manufacture thereof

Info

Publication number: CA1100730A
Application number: CA294,120A
Authority: CA
Inventors: Shinsuke Yoshikawa; Nobuyuki Hisazumi; Masataka Yamamoto
Original assignee: Kureha Corp
Current assignee: Kureha Corp
Priority date: 1976-12-29
Filing date: 1977-12-29
Publication date: 1981-05-12
Also published as: JPS552192B2; AU3196877A; NL7714477A; FR2375984A1; DE2760181C2; IT1090387B; DE2758320A1; US4226822A; DE2758320C2; BE862475A; GB1600250A; NL171682C; US4161562A; FR2375984B1; JPS5382888A; NL171682B; AU504294B2

Abstract

Abstract of the Disclosure A biaxially stretched five-layer film suitable for packaging fatty foodstuffs of irregular shape is disclosed. The film has a first (outermost) layer made of an ionomer, an olefin homopolymer, an olefin copolymer or a mixture of the olefin homopolymer with the olefin copolymer; a second layer of an ethylene copolymer; a third layer of a vinylidene copolymer; a fourth layer of an ethylene copolymer; and a fifth (innermost) layer of an ionomer. This film is obtained by preparing a laminate consisting of the layers of respective resins mentioned above by means of a specific die designed for multi-layer tubular molding and biaxially stretching the laminate under specific conditions. A film which has excellent properties for the packaging of fatty foodstuffs can thus be produced.

Description

llL)U730 This invention relates to a biaxially stretched five-layer film suitable for packaging fatty foodstuffs of irregular shape and to a method for the manufacture thereof.
Generally, fatty foodstuffs such as raw meat, pro-cessed meat and cheese, are irregular in shape and more often than not contain bones or other hard solid articles (e.g.
primary packaging materials such as, for example, plates and nets) which make packaging difficult and often result in the rupture of packaging films.
Shrink packaging methods have been used with consider-able success in the packaging of such fatty foodstuffs of irregular shape. Such methods involve wrapping the foodstuffs with shrinkable films, and then shrinking the films. These methods are very convenient, and vinylidene chloride copolymer resins (hereinafter referred to as "VDC resins" for short) have been most popular as the shrinkable films because of their suit-able gas-barrier property, oil resisting property and clipping property as well as their shrinkability.
The VDC resins which are in use today contain from 6 to 10~ by weight of additives, such as plasticizers and stabil-izers, which are incorporated therein for the purpose of imparting the necessary flexibility for enhancing the cold resisting property and the clipping property and further for the purpose of conferring an advantageous film-forming property.
However, in many circumstances, the use of VDC resins is not desirable from the point of view of hygiene because the additives present in the resins migrate into some, if not all, foodstuffs which are exposed to direct contact with the resin films. It may also happen that the resins, if used on heavy foodstuffs, fail to provide sufficient physical strength, and cold resisting properties.

- 2 11~)(~730 Various attempts have heretofore been made to relieve the VDC resins of such disadvantages. For example~ a three-layer laminated film has been proposed which is produced by interposing a layer of a VDC resin between a pair of outer layers each made of an ethylene~vinyl acetate copolymer resin (herein-after referred to as "Et-VA resin" for short). Et-VA resin has excellent cold resisting properties and adhesiveness with the VDC resins. The three layers are simultaneously co-extruded as a single construction ~as disclosed in Canadian Patent No. 982,923, for example).
A three-layer laminate film has also been suggested having a similar construction, except that one of the outer Et-VA
resin layers is cross-linked (i.e. one of the two outer layers of the laminated film) by exposure to radiation (as disclosed in Japanese Patent Laid-Open To Publication No. 34565/1972, for example). These conventional multi-layer laminated films in-variably have at least one of the outer layers made of an Et-VA
resin which is notably deficient in oil resisting properties (namely, the outer layer of Et-VA resin in a state not cross-linked by exposure to radiation). In the packaging of fattyfoodstuffs with these films, if the non-crosslinked outer layers are located on the outermost side, then such outer layers are inevitably susceptible to adhesion of fats or oils in the course of the packaging process. The innermost layers of these laminated films which are exposed to direct contact with foodstuffs, there-fore, are required to possess an oil resisting property where the foodstuffs are of a fatty nature. This means that in those multi-layer laminated films which are used for packaging fatty food-stuffs, the outermost layers and innermost layers are both required to possess an oil resisting property. The fatty food-stuffs thus wrapped in such multi-layer laminate films are then )730 dipped in a hot water bath kept about 95C so as to shrink the laminated film During this shrinking process~ the oute.rmost layers of the laminated film require a high oil resisting property~ The outermost layers are, accordingly, required to have sufficient resistance to both heat and oil at such elevated terr.peratures. In this respect, the multi-layer laminated films produced by the conventional techniques described above are not preferable in practical use.
Prior to the shrinking process, the fatty foodstuffs of irregular shape are wrapped in the multi-layer laminated films, evacuated and clipped at the opposite open ends with wire clips.
As a result, a vacuum is formed and remains between the film and the foodstuffs where the packaged article contains hollows.
During the shrinking process, the portions of the films stretched over the hollows are exposed to the stress of thermal shrinkage and to the external pressure due to the presence of the vacuum and, furthermore, they are softened up by the actions of heat and oil. Such affected portions of the films are inevitably stretched out to an excessively small thickness, with the result that the barrier property and strength are reduced, and the film may possibly even rupture. What is more, the multi-layer laminated films produced by the conventional techniques invariably have their seams heat sealed and, therefore, are not completely free from the problem that their sealed layers, upon exposure to an intense heat of 95C and in the presence of an oil, are swelled with the oil and are consequently degraded in strength occasionally to the extent of sustaining rupture along the seams.
It is, therefore, an object of the present invention to provide a multi-layer laminate film which is free from the disadvantageous oil resisting property and heat resistance suffered by the conventional multi-layer films and which has good )730 aas-barrier property, cold resisting property, clipping property, heat-sealing property and thermal shrinkability.
According to one aspect of the invention there is provided a biaxially stretched five-layer laminate film comprising: a first layer of an ionomer, an olefin homopolymer, an olefin copolymer or mixture of an olefin homopolymer with an olefin copolymer and having a thick-ness in the range of from 10 to 25~; a fifth layer of an ionomer and having a thickness in the range of from 20 to 50~; a second layer of an ethylene copolymer with a crystal-melting temperature 5 to 30C. lower than that of the ionomer of said first or the ionomer of the fifth layer and having a thickness in the range of from 0.2 to

3~; a third layer of a vinylidene chloride copolymer and having a thickness in the range of from S to lS~; and a fourth layer of an ethylene copolymer with a crystal-melting point ~ to 30C. lower than that of the ionomer of said first or fifth layer and having a thickness in the range of from 0.2 to 3~, the ratio of the thickness of said first layer to that of the fifth layer being in the range of from 0.4 to 0.6 and the total thickness of the film being in the range of from 35 to 90~.
According to another aspect of the invention there is provided a method for the manufacture of a biaxially stretched five-layer laminate film, which method comprises:
(a) operating a three-layer tubular molding die having three resin flow paths and provided in each of said resin flow paths with a torpedo serving to divide the flow of resin in the path, with the flow path for the outermost resin layer and that for the intermediate resin layer being each provided internally with a tubular path having `~';'~

11~)0730 the leading end not in contact with said torpedo;
(b) feeding an ionomer, an olefin homopolymer, an olefin copolymer or a mixture of said olefin homopolymer with said olefin copol~mer as the outermost layer resin, a vinylidene chloride copolymer as the intermediate layer resin and an ionomer as the innermost layer resin respect ively to said resin flow paths and, at the same time, feeding an ethylene copolymer with a crystal-melting point 5 to 30C. lower than that of said ionomer of the first 10 layer or ionomer of the fifth layer to each of said tubu-lar paths, thereby producing a laminate; (c) rapidly cooling the resultant laminate thereby causing the vinyli-dene chloride copo]ymer present in the laminate to assume an amorphous state; and (d) biaxially stretching the thus quenched laminate at a temperature which is lower than the crystal-melting point of the outermost layer resin and that of the innermost layer resin and at least 4C. higher than the crystal-melting point of said ethylene copolymer.
In the accompanying drawings:
Fig. 1 is an explanatory diagram illustrating a die for molding a laminate by feeding the respective resins used as raw materials to the interior of the die in ac- ~-cordance with one example of the present invention; and Fig. 2 is an explanatory diagram illustrating the subsequent treatment of a laminate obtained as shown in Fig. 1 to convert it to a biaxially stretched five-layer laminate film in accordance with one example of the present invention.
The VDC resin used as the third layer in the biaxially stretched five-layer laminated film of the present inven-tion is preferably a copolymer consisting of 65 to 95 % by 73~

weight of vinylidene chloride and 5 to 35~ by weight of at least one unsaturated monomer which is copolymerizable with the vinylidene chloride. Examples of copolymerizable monomers include vinyl chloride, acrylonitrile, alkyl acrylic esters (with alkyl groups having from 1 to 18 carbon atoms), etc. This VDC resin may incorporate small amounts of plasticizers and stabilizers as the occasion - 6a -11()(~73V

demands. The desirability of providing such additives will be apparent to persons skilled in the art. Examples of such additives are dioctyl adipate and epoxydized soybean oil, etc.
which are representative plasticizers and sta~ilizers. In the present invention, this VDC resin layer serves to confer upon the produced film a gas-barrier property and a durability to the shrinking treatment. The layers of the film other than this VDC resin layer have melting points at approximately the temperature at which the shrinking treatment is carried out, so that their strengths are much reduced during the shrinking treat-ment and their inclination toward loss of strength becomes all the more conspicuous especially when they are plasticized such as by fats which adhere to the film when the film is used in practice. If the VDC resin layer having an oil resisting pro-perty and heat resistance is excessively thin, then it may fail to sufficiently withstand the stress of thermal shrinkage and even a slight degree of external impact and, as a result, may sustain rupture. For these reasons and particularly the latter reason, the VDC resin layer has a thickness of at least 5 ~, preferably more than 7 ~, and this thickness must be increased with the increasing weight of a given article to be packaged with the film of the invention. If the thickness exceeds lS ~, however, the occurrence of cracks due to the low-temperature brittleness of the film can no longer be prevented even by virtue of the outermost (first) resin layer and the innermost (fifth) resin layer. Thus, the thickness of the VDC resin layer is limited to the range of from 5 to 15 ~, and preferably to the narrower range of from 7 to 12 ~.
The ionomer used as the outermost (first~ resin layer or the innermost (fifth) resin layer in the present invention ~ is~an lOni~C copolymer having an ionic linkage, which is produced .: .

ll~)V730 by completely or ~artially neutralizing a copolyme~ of an ~-olefin, such as ethylene, with an unsaturated or~anic acid, such as acrylic acid or methacrylic acid, to form a salt witll the cation of an alkali metal, zinc or the like. In the film of this invention, this ionomer is used for the purpose of stabilizinG; the stretching operation thereby giving rise to the required shrinkability and conferring upon the produced film a heat-sealing property, cold resisting property~ oil resisting property and! particularly, seal strength in the presence of an oil. l~e innermost layer serves as the surface of the film for direct contact with foodstuffs to be packaged with the film.
~hen the thickness of the innermost layer is less than 25 ~, par-ticularly 20 ~, the film is deficient in seal strength. When the thickness `exceeds 45 ~, particularly 50 ~, the film suffers from an undesired rise of rigidity, loss of flexibility and de-graded clipping property. Because of such undesirable phenomena, the thickness of this layer is limited to the range of from 20 to 50 ~, and preferably to the narrower range of from 25 to 45 ~.
The ionor,~er, olefin homopolymer, olefin copolymer or mixture of the olefin homopolymer with the olefin copolymer, which is used as the first (outermost) resin layer, is intended to protect the third layer of the VDC resin. In addition, it serves to impart to the produced film a CO1G resisting property, sufficient heat resistance at 95C and an oil resisting property in the presence of fats and, yet, avoids obstructing the film's stretchability by inflation. For this reason, the first resin layer preferably has a crystal-melting point which is at least 4C higher than -the crystal-melting point of the second and fourth resin layers and at most 15C, preferably at most 10C, higher than that of the fifth (innermost~ resin layer. The term "crystal-melting point" as used in the present invention 110~730 refers to the temperature at ~lhich the curve of the cr~stal-meltin~ temperature measured orl an 8-mg sample at the temperature increasing rate of gC/min. b~ use of a differential scanning calorimeter ~Model lB, made by Perkin-Elmer Corp.) reaches the pea~. Examples of homopolymers or copolymers usable for the first layer resin include ethylene homopolymers, ionomer, ethylene-vinyl acetate copolymers having ethylene contents of not less than 96%
by weight, ethylene-propylene copolymershaving ethylene contents of not less 96% by weigh-t, and ethylene-acrylic ester copolymers having ethylene contents of not less than 96% by weight. Of these resins, particularly preferable are ionomer and an ethylene homo-polymer or copolymer which has a density of not more than 0.9 25 g/cm3 and a melt index of from 0.3 to 1Ø The mixture of an ole-fin hompolymer with an olefin copolymer, which is usable as the first layer resin, is particularly preferred, for the purpose of further enhancing the ethylene homopolymer's stretchability by inflation and yet avoiding possible degradation of the oil resist-ing property, to be a mixture obtained by blending an ethylene homopolymer having a density of not more than 0. 925 g/cm3 with an 20 ethylene copolymer such as, for example, ethylene-vinyl acetate copolymer, ethylene-acrylic ester copolymer or ethylene-propylene copolymer. This mixture is used advantageously when its ethylene content is not less than 92~o by weight, preferably more than 95%
by weight, and more preferably more than 97O by weight. For the purpose of enabling the produced film to acquire an improved clipping property in the use of a wire clip and an enchanced handling property, the resin used as the first layer resin may incorporate a small amount of lubricant. I~ith a view to the cold resisting property of the film, the first layer is required 11~)0730 l.l to have a thickness of not less than 7 ~, preferably 10 ~.
To avoid obstructing the stretchability of the film, however, this thickness is limited to a maximum of 25 ~. To ensure good balance among the various properties involved, the ratio of the thickness of the -Eirst layer to that of the fifth layer is limited to the range of from 0.4 to 0.6.
A film consisting of only the three layers described above exhibits lower interlayer adhesive strength after the stretching treatment than before the treatment. In the case O " of a thermally shrinkable stretched film which consists of these three layers, therefore, the selection of an adhesive agent l used for fast interlayer adhesion and the selection of a method for the interlayer adhesion itself prove quite significant.
¦! A possible reason is that the stretching operation performed on the film results in a decrease in the area of adhesion in , the interfaces- and that the orientation of molecules due to the stretching also goes to degrade the adhesive strength retained in the interfaces of the layers. Where the adhesive strength existing in the interfaces of the layers is deficient, .) ` the excellent synergistic effect of the oil resisting property and heat resistance offered by the ionomer layer and the VDC

layer does not manifest itself to advantage and the durability acquired fails to withstand various stresses caused such as by thermal shrinkage, possibly with the result that the film sus-tains rupture along the sealed portion or in the hollows. After .

J~V(~730 "

. . , an elaborate investigation in search of a method for ¦
overcoming this grave trouble, we have found that the ~; adhesive strength is particularly required in the interface between the ionomer resin layer and the adhesive layer and then in the interface between the VDC resin layer and the I :
adhesive layer. We have also ascertained that, as an ideal , adhesive agent to be used for enhancing the adhesive strength j in these interfaces, an ethylene copolymer is suitable, which ~ is of the type having a crystal-melting point which is lower 0 , than that of the ionomer in use by a difference exceeding at ¦ least 5C, preferably 7C and more preferably 11C but not exceeding 30C, preferably 20C. We have also learnt that the temperature at which the film is stretched must be lower than both the crystal-melting point of the outermost (first) resin -I layer and that of the innermost (fifth) resin layer and, yet, at least 4C higher than the crystal-melting point of the ; ethylene copolymer. The crystal-meltiny point of the ethylene copolymer used as the adhesive agent must be limited to the aforementioned range, because the adhesive strength is seriously degraded after the molecular orientation by the stretching operation when the temperature exceeds the upper limit and the adhesive strength in the interface between the ionomer layer and the adhesive layer of the ethylene copolymer is . notably degraded when the temperature falls below the lower limit.

! l Secondly, it is imperative that the thickness of the VDC resin layer should be not less than 5 1l as already pointed out.
Thirdly, it is important that the laminate should be subjected to stretching with the VDC resin layer retained in its amorphous state. The adhesion in the interface between j the VDC resin layer and the adhesive layer can be fortified only when this requirement is satisfied.
Fourthly, it is necessary that the thickness of the adhesive layer should fall in the range of from 0.2 to 3 ~, ' preferably in the range of from 0.5 to 1~. In order for the ,I thermal shrinkage of the film to be advantageously obtained in the presence of heat and oil, the thickness of the adhesive layer must be decreased to the greatest possible extent that does not entail any degradation of the adhesive strength because the adhesive agent's lack of shrinkability is not preferable for the overall shrinking property of the multi-layer laminate film and because the adhesive agent is deficient in oil resisting property.
.l When an Et-VA resin is used as the adhesive agent, it is preferred to have an ethylene content (as one of its component unit) in the range of from 70 to 92~ by weight, ; preferably in the range of from 75 to 87~ by weight and a melt index in the range of from 2 to 10 in order to fulfil all the requirements described above. Other examples of adhesive !
.~ I

., I

., I

ll~)U730 , I

agents usable for this invention include a partially saponified I -Et-VA resin; products of grafting of Et-VA resin and/or this par-' tially saponified Et-VA resin with various polar monomers such as acrylic acid, methacrylic acid, maleic acid, maleic anhydride and vinyl chloride; two-component copolymers of ethylene with acrylic esters, methacrylic esters, etc.; and three-component ¦
'I copolymers of e-thylene with one of the esters and vinyl acetate. Although the preferable range for the ethylene content as one component unit of the adhesive agent varies O 'lli with the degree of saponification, the rate of grafting and other similar factors, it may readily be determined by any person of ordinary skill in the art on the basis of the aforementioned descriptions on the Et-VA resin. As is evident from the description given above, in the biaxially stretched I five-layer laminate film of the present invention, the second ~ -and fourth layers serving as adhesive layers are made of an ethylene copolymer which has a crystal-melting point 5 to 30C
lower than that of the ionomer in use.
The total thickness of this biaxially stretched five-laye~
laminate film is limited to the range of from 35 to 90 ~, ¦
' preferably in the range of from 40 to 80 ~, with due considera- ¦
tion to the strengths, handling property and economy of the film. Any deviation from this range may possibly bring about detrimental effects. When the thickness is less than the lower limit of 35 ~, the film suffers from insufficient strength and . , I
' 73~ 1 , 1 frequently sustains rupture durinq i-ts handling. Thus, the lower limit of the ranye may have to be elevated depending ,i '! on the weight of articles being packaged with the film in ~~~
question. When the thickness exceeds the upper limit 90 ~, the rigidity of the film may possibl~- ~ncrease excessively even to the extent of defying all efforts made to fold the film in , the clipping machine or the elasticity of the film may increase so much as to surpass the pressure with which the wire clip is tightened, with the result that formed packages permit no 'O , tight sealing.
In the manufacture of biaxially stretched multi-layer laminated films, no technique for extruding a laminate of five layers all at once has been reduced to practical use because of the difficulty involved in the adjustment of inter-layer thickness distribution. In a stretched film not so much as in an unstretched film, the adjustment of this interlayer thickness distribution must be controlled with high accuracy.
This control is so difficult particularly in the case of a tubular film that those skilled in the art have resigned O their hope of realizing this control as an impossibility.
However, we have succeeded above all in making it pos- , sible to limit the thickness of the adhesive layer ¦ -within the range of from 0.2 to 3 ~, an indispensable ,~ requirement for the thermal shrinkaye of the film to be successfully carried ut in the preserce cr oil. Now, the ,1 .

11()(;730 method of the present invention for the manufacture of the ¦
biaxially stretched five-layer laminate film will be described in full detail with reference to the attached drawings.
Fig. 1 is an explanatory diagram illustrating a typical case in which a laminate is molded by feeding the respective resins used as the raw materials into the interior of a die in accordance with the present invention. In the diagram, 1 denotes a die to which the resins are fed in accordance with this invention. The die 1 is provided with resin flow paths 2, 3 and 4 and these resin flow paths are provided respectively ' I
with torpedoes 5, 6 and 7 each adapted to divide the flow of resin in the path. In this die 1, the flow path 2 for delivery of the outerr.lost resin layer and the flow path 3 for delivery of the intermediate resin layer are respectively provided internally with tubular paths 8 and 9 whose leading ends are not in direct contact with the torpedoes 5 and 6. (It should be noted that no conventional die is provided with .
these tubular paths.) In the present invention, an ionomer, an olefin homopolymer, an olefin copolymer or a mixture of the olefin homopolymer with the olefin copolymer is fed through the flow path 2, a vinylidene chloride copolymer is fed through the flow path 3~ an ionomer is fed through the flow path 4 and, at the same time, an ethylene copolymer having a crystal-melting point 5 to 30C lower than that of the ionomer is fed through the tubular paths 8 and 9. Consequently, a five-layer laminate .

I

lll~U~30 i5 produced and extruded through the lower portion 10 of the die 1. ~rom the laminate thus eY~truded, a biaxially stretched five-la~er laminate film is manufactured as illustrated in Fig. 2.
In Fig. 2, 11 denotes an e~truder and 12 a die ~corresponding to the die 1 of Fig. 1). The resins extruded via the extruder 11 are sent thxough the die 12 and consequently '¦ extruded downwardly in the form of a five-layer tube 13.
This tubular laminate 13 is cooled in a cooling tank 16 disposed ¦
0 l directly below the die 12 and maintained at a temperature below 40C. sy means of a pair of nip rolls 15 disposed in the cooling tank 16, the liquid agent 14 placed inside the tubular laminate 13 for the purpose of preventing unwanted adhesion between the opposed portions of the inner surface of the laminate is continuously squeezed out. In the continuous tubular laminate 17 now in a folded form, the quenching produced ¦
in the cooling tank 16 enables the adhesive layer and the VDC
resin layer as well to be retained in their respectively amorphous form. The tubular laminate 17 in this state is then 0 ~' passed around guide rollers 24 disposed in the lower portion of the hot water tank 18 kept at controlled temperatures of 85 to 95C and through a pair of nip rollers 19 disposed just below the surface of the hot water tank 18. Thereafter, while the tubular laminate is travelling between the nip rollers 19 and ano-ther pair of nip rollers 22 being rotated at a speed . ~ .

1 110(~730 , several times -that of the nip rollers 1~ it is cooled in an atmosphere at room temperature and simultaneously inflated with the continuous introduction of air in the direction of forming a bubble, with the result that the tubular laminate is stretched by the biaxial inflation effected simultaneously in the direction of length and that of diameter.
As a result of the operation described above, there is formed a shrinkable five-layer tubular film 21 of which the Il innermost layer and the outermost layer are biaxially oriented.
¦I This tubular film 21 may be cut open to form a sheet to suit the occasion.
ll In this operation, the temperature at which the - li stretching is effected must be lower than both the crystal- ¦
melting point of the outermost layer resin and that of the i~ innermost layer resin and at least 4C higher than the crystal-meltiny point of the adhesive agent. The bubble stability is impaired when the temperature exceeds the limit, and the adhesive strength in the interface between the resin layers and the ' adhesive layer is degraded when the temperature falls below the O 1ll lower limit.
As the liquid agent 14 used for preventing the unwanted fast adhesion between the opposite portions of the inner surface of the tubular laminate, any substance may be adopted insofar as it is nonpoisonous and resistant to heat. Examples of suitable agents include propylene glycol, glycerin and .. ~

~ 110(J730 I! various veyetable oils.
By the method described above, there can easily be manu-factured a film having a coefficient of thermal shrinkage of not less than 25~ at 85C and not less than 45~ at 95C.
The five-layer stretched tubular film obtained by the present invention is shrinkable and excels in oil resisting property, cold resisting property, preservability and clipping , property and, the~efore, can be advantageously used for packaging I¦ fatty foodst~ffs, particularly those of irregular shape.
O ¦¦ Usually, the tubular film is cut to a desired length, heat sealed on one open end, packed with a given article, and ` j¦ clipped on the remaining open end by means of a wire clip.
Because of its outstanding sealing property, the remaining open end of the tubular film may otherwise be tightly closed by heat sealing. Alternatively, the tubular film may be il sealed at a freely selected position and cut at other suitable position to produce a bag having an open mouth, which may be tightly closed afterward by clipping or heat sealing. Thus, li various uses are found for the tubular film. Working ExampleS
'O of the present invention are provided herein below.
In the Examples, the polymers indicated below were used. Whenever there are mentioned parts and percents, they are eant as parts by weight and percents by weight.

., ~ - 18 -11~0730 li l (1) Olefin homopolymers and olefin copolymers . Symhol Kind of ~olymer* ~lelt index Density¦Percent C -- - ~
_ _ _ monomer point (~C) A LDPE 0.8 0.925 _ 108 . B LDPE 6.5 0.917 _ 100 I.
C Et-VA 3 0.92VA 5 97 . D Et VA 6 0.93VA10 88 . E Et-VA 3 0.93VA15 84 ) I F Et-VA 3 0.95VA25 70 G Et-EA 6 0.93EA18 89 . ~l Et-VA-A _ 0 95VA28 64 l, ,', * LDPE stands for low-density polyethylene, Et-VA-AA for , ethylene-vinyl acetate copolymer having acrylic acid grafted thereto, and Et-EA for ethylene-ethyl acrylate.

~ , ', i ~. l ~ ' , , jl I

~j (2) Ionomer resins Il I I
Melt Crystal-Symbol index Kin~ of polymer Ionization melting tdg/min.) point (~C) i i . .. . . . _ ll J 0.9 Ethylene/methacrylic Conversion 88 .j acid copolymer of meth-1~ acrylic aci to Na salt ii X ~.2 " ,. 96 L 2.8 _ _ _ 88 , (3) VDC resins i ll Symbol _. _ _ _ _ _ ., .__ . . _ __ _ ... .__ ._ I' Vinylidene chloride/vinyl chloride = 70/30 M (copolymer) 100 parts . Epoxydized soybean oil 1 part ... _ . . ................. . _ ........... _ Vinylidene chloride/vinyl chloride = 80/20 (copolymer) , N 100 parts Epoxydized soybean oil 1 part Dioctyl adipate 0.4 part !
1, ii l .
i I' .
Ii !

. . .

In the working ExampleS described herein below, the innermost resin layer which is exposed to direct contact with foodstuffs is designate~ as ~the fifth layer" and the remaining resin layers are designated as "the fourth, third, ... layers"
;~ in the order of their succession (The outermost layer, there- ~
.! I
! fore, is designated as "the first layer.").

~xample l:
., .
The film was composed of the following materials.
'il Fifth layer - Ionomer resin (K) ! 0 Fourth layer - Ethylene copolymer (E) ' Third layer - VDC resin (~5) j Second layer - Ethylene copolymer (E) i First layer - Olefin homopolymer or olefin ¦I copolymer (B) ' `
These materials were processed by the method illustrated in Fig. 1 and Fig. 2 to produce a biaxially stretched multi-layer film. In this case, the first and second lavers were I
I ~l united and the third and second layers were similarly united within the annular die 12 and the five layers were laminated ~0 , within the die and extruded all together. At the die lip, the resin temperature of the tubular laminate 13 was 170C. The tubular laminate 13 was cooled in a cooliny tank 16 kept at 15C and was consequently converted into a tubular laminate 17 having a flattened width of 130 mm and a thickness of about l 615 ~. (Inside the tubular laminate 13 which was held in.

i~ l 731) i the cooling tank 16, soybean oil 14 was placed to a level substantially the same as that of the water bath.) I
Then, the tubular laminate 17 was conveyed through a hot water tank 18 controlled with hot water of 93C at a feed rate of 5 m/min. so as to be heated with the water bath for about 12 seconds, and then passed thorugh the first nip rollers 19 operated at a rotation speed of 5 m/min. The heated tubular laminate was cooled in the atmosphere of room temperature and '' passed through the second nip rollers 22 operated at a rotation 0 speed of 16.5 m/min. While it was thus cooled and passed between the two pairs of nip rollers, the tubular laminate was stretched in the longitudinal direction to 3.3 times the original size and, at the same time, inflated in the lateral direction to 3.1 times the original diameter of the tubular laminate 17 with the air continuously introduced into the tubular laminate interior. The biaxially stretched five-layer laminate film 21 thus obtained had a flattened width of 400 mm and a thickness of 60 ~. ~he thickness of the fifth layer was 35 ~ , that of the fourth layer 0.7 ~, that of the 3 ,I third layer 8 ~ , that of the second layer 0.7 ~ and that of ~ the first layer 15 ~ , respectively. Then, the tubular film 21 , .
was cut into pieces 800 mm leng-th and each tubular piece had one open end heat sealed to produce a bag. The conditions for the heat sealing were 170C of seal bar temperature and 0.8 second of sealing time.
,;

.1 ' 0l~730 i`
,, I
'I The film or bag ob-tained in Example 1 was tested for physical properties as indica-ted in Table 1. Examples 2 through 9 were carried out in the same way of the procedure of E~ample 1, except using the combinations of materials, layer thicknesses and temperatures of stretching indicated in Table 2.
For the purpose of comparison, Comparison Examples 1 through 7 were similarly carried out to produce films of descriptions as . indicated in Table 2. The films obtained in Examples 2 through ~l 9 and Comparison Examples 1 through 7 were also tested for L0 ~ physical properties similarly in Example 1. The results of the test are shown in Table 3. The VDC resin samples taken ¦
from the tubular laminate 17 and the stretched film 21 which were obtained in the process of Example 1 were measured for 1~ specific gravity at 30C. The values were both 1.63. When ; i the VDC resin sample taken from the stretched film 23 five hours after the stretching treatment was measured for specific gravity at 30C, the value was 1.69, indicating that crystal-, lization of the VDC resin layer proceeded after the stretching ;~ treatment.
The product of Example 1 had a suitable interlayer :! ` !
thickness distribution for the flattened bag width of 400 mm which is most widely used in packages used for wrapping raw beef, etc. and that of Example 2 had an interlayer thickness distribution suitable for packages used for wrapping relatively light articles, wi-th the flattened bag width generally ranging 11~(J1730 from 150 to 250 mrn. Contrary to that of Example 2, the product of Example 3 was usable as a bag of a largc width suitable for packages of heavy articles, with the flattened bag with falling in the neighborhood of 650 mm, for example. For the convenience of measurement of physical properties, the films produced ' -in these examples were invariably produced with a fixed flattened bag width of 400 mm. All these products were found to fulfil completely the objects of this invention similarly l to the product of Example 1.
,i In Example 8, the component layers of the film were identical with those of the film of Example 1, except for the first layer which was formed by blending LDPE with Et-VA resin, with the vinyl acetate content at 1.5%. The product of this example had both oil resisting property and heat resistance within tolerable ranges and could be used safely and satis-factorily even in the presence of fats in the ordinary shrinking treatment performed on fatty foodstuffs of irregular shape as primarily aimed at by the present invention.
Example 9 represents a case wherein the first layer ~0 of the laminate film was formed of the ionomer resin. In the stretching hot water tank kept at 84C, the laminate exhibited excellent stretch stability and outstanding oil resisting property. It had a transparent, glossy appearance.
Comparison Examples 1 and 2 represent cases in which the third layers had thicknesses both deviating from the fixed 1, ,, ,.

, l, l )730 i .
ranges, and Comparison Example 3 represents a case wherein , the fifth layer was given a thickness smaller than the lower ; limit of the fixed range of this invention. As shown in j Table 3, a]l the products of these examples were found to j ,-entail some problem or other.
Comparison Example 7 represents a case in which the innermost layer was formed of the Et-VA resin deficient in oil resisting property. The film was found to exhibit absolutely no resistance to the actions of heat at 95C and oil.
0 ll Examples 4, 5, 6, 7 and 8 represent cases in which the component layers of the films were identical with those of the film of Example 1, except for the second and fourth layers which were made of varying materials. The products of these examples exhibited properties comparable with those of the lf ' product of Example 1. The data on the adhesive strength obtained of the products of Comparison Examples 4, 5 and 6 are shown in Table 4 for the purpose of comparison with the data similarly obtained of the products of Examples 4, 5, 6 and 9.
The adhesive strength was invariably measured in the interface 0 ,¦ between the fifth and fourth layers. This measurement was performed in the atmosphere at 23C by means of 180C peel test (with the rate of drawing fixed at 200 mm/min.).

, .
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~)0730 Table 1 !
measurement Method of measurement _ ~
Coe~ficient of Twenty pieces o~ film cut to a fixed size of shrinkage in 10 cm x 10 cm were left to`stand in a hot ! hot water water bath at 95C for three seconds to be shrunk in their Eree state. After the standing, they were measured in longitu~inal and lateral directions. The 0 1 measurements were compared with the original ! measurements (10 cm x 10 cm), with the differences used for calculation of ratios of shrinkage. The shrinkage of a given film was indicated with the smallest and largest , values so calculated.
_ _ __ _ . ._ .. _ _ .__ Clipping By use of a clipping machine and a rnetal cliF
!; property (Type Z411) both made by Reem Corp-of the , U.S., about 300 parts of raw beef was vacuum ' packaged. The packages were subjected to 0 a shrinking treatment at 95C for three " secondsand left to stand at 0 + 2C for 14 days. After the standing, the packages were tested for loss of vacuum. The clipping property was rated by the frequency of the occurrence of such loss of vacuum.
~: . . ___ . _ _ i Cold Bags of film having a flattened bag width of resisting 400 mm and a length of 800 mm were filled property each with about 5 kg of raw beef ~round) and vacuum packaged by means of clipping. They 0 were subjected to a shrinking treatment at 95C for three seconds, cooled at 0 + 2C
and dropped repeatedly onto a rigid vinyl chloride board 20 mm in thickness from a ;; height of 1.5 m in a room kept at 0 + 2C, with the number of drops counted, until the film sustained rupture such as pinholes.
The counts taken of a total of 20 sample bags were averaged and reported as the cold resisting property.
' . . ._ _ . __ _ _ Gas-barrier The amount of oxygen which permeated through property a sample film was measured at 30C and 100%
o~ RH and was reported as the gas-barrier property.
_ ._ _ ._ i. !

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111~0730 , 1, . i ~! Sealing In bags of film havin~ a flattened bag width strength of 900 mm and a length of 800 mm, bacon slabs cut to a width of about 5 cm, a width of about 30 cm and a length of 50 cm were placed in such a manner that the bacon slahs are pack-ed closely to the sealed portion of the bags.
, The packages thus produced were evacuated in ;i a vacuum chamber and, at the same time, heat sealed on their open ends. (The packaging machine used was Type AG-8 made by MultiPak il of West Germany). Then, the vacuum packaged bacon slabs were immersed in a hot water bath at 95C and left to stand in the bath until the sealed portion of the bag or the sealed i end formed by the packaging machine sustained i rupture. The interval between the time the !, samples were immersed in the bath and the time they sustained the rupture was measured.
, A total of ten measured intervals were averag-jl ed and reported as the sealing strength.
Oil resisting Similarly to the measurement of sealing streng-property and th, vacuum packaged bacon slabs were used. A
! heat resis- small amount of oil was intentionally left to 11 tance float on the surface of a hot water bath kept i at -temperatures in the range of from 80C to ! 95C. The packages were immersed in the hot water bath for three seconds and then left to ll cool off. The packages were examined to rate i the oil resisting property of the outermost i layer of the film and detect any injury sus-tained on the film where bacon slabs had dents.
Then, the bacon slabs were removed from the packages and the innermost layer of film was inspected for oil resisting property.
.
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~ll 11~ 730 1, Tabl~ 2 , _ _ Combinàtion of layers (the value in Temper parr~ntheses indicatiny layer thickness) o~ ho-t Example 2 K(28) ~ ) ~(~ ~ 3.~(v~e3) wa=rr bath Example 3K(40) E(l.0) M(10) E(1.03 B(20) 93 I
Example 4 K(35) D(0.7) M(8) D(0,7) B(15) 93 ' Example 5 K(35) F(0.7) M(8) F(0.7) B(15~ 93 r) ,, Example 6 K(35) G(0.7) M(8) G(0.7) B(15) 93 ¦
,' Example 7 K(35) H(0.7) M(8) H(0.7) B(15) 93 !
Example 8 K(35) E(0.7) N(6) E(0-7) C30~(1 ) 93 i !~ Example 9 J(35) F(0.7) M(8) F~0.7) L(15) 84 , ! ! _ _ _ _ __ _ . . ___ _ . _ ..
" Comparison K(35) E(0.7) M(3) E(0.7) B(15) 93 Example 2 K(35) E(0.7) M(20) E(0.7) B(15) 93 Comparison K(15) E(0.7) M(81 E(0.7) B(15) 93 O Comparison K(35) C(0.73 M(8) C(0.7) B(15) 93 Comparison J(35) E(0.7) M(8) E(0.7) L(15) 84 Comparison J(35) D(0.7) M(8) D(0.7) L(15) 84 Examplr~ 7 D~35) E(0.7) M(8) E(0.73 J(19) 84 ,1 ~ - 28 -I

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l' ble 4 Temperature _ _ Materiai of adhesive layer of hot water _ ( f ourth layer ~ _ _ stretching Comparison ExamE~le E E__ ExampIe _ . __ _ Examp 1 e 4 4 _ 1 5 ,~ Dif f erence ., from stret-ching tempe -4 5 9 23 ! ' rature *
. ~ Degradatior 93C of adhesiv~ <1% 5-6.5 9~ 12~14 %10~12 %
s tre ngth * *
Adhe s ive strength ~ 5 3 0 ~ 4 0 7 0 ~ 8 0 5 0 ~ 6 0 il width) ll _ .. .. _ ~ _ .. _ i Compari- Compari- Example I _ son Ex- son Ex-i _ _ ample 6 ample 5 i Difference , f rom stret ching tempe- -4 0 14 ; rature *
Degradatio 84C of adhesiv _ <1. 8 % <2. 7 10%
strength * .
Adhes ive .
strength _ 7~10 10~15 50 . . __ wid th ) __ _ _ . _ _ * Stretching temperature (C) - crystal-melting point !~ of fourth layer (C) ** Adhesive strength of stretched film Adhesive strength before stre-tching .

,

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A biaxially stretched five-layer laminate film comprising:
a first layer of an ionomer, an olefin homopolymer, an olefin copolymer or mixture of an olefin homopolymer with an olefin copolymer and having a thickness in the range of from 10 to 25µ; a fifth layer of an ionomer and having a thickness in the range of from 20 to 50µ; a second layer of an ethylene copolymer with a crystal-melting tempera-ture 5° to 30°C. lower than that of the ionomer of said first or the ionomer of the fifth layer and having a thickness in the range of from 0.2 to 3µ; a third layer of a vinylidene chloride copolymer and having a thickness in the range of from 5 to 15µ; and a fourth layer of an ethylene copolymer with a crystal-melting point 5° to 30°C. lower than that of the ionomer of said first or fifth layer and having a thickness in the range of from 0.2 to 3µ, the ratio of the thickness of said first layer to that of the fifth layer being in the range of from 0.4 to 0.6 and the total thickness of the film being in the range of from 35 to 90µ.

2. The biaxially stretched five-layer laminate film according to Claim 1, wherein said olefin homopolymer is an ethylene homopolymer.

3. The biaxially stretched five-layer laminate film according to Claim 1, wherein said olefin copolymer is an ethylene-propylene copolymer having an ethylene content of not less than 96% by weight, an ethylene-acrylic ester copolymer having an ethylene content of not less than 96%
by weight or an ethylene-vinyl acetate copolymer having an ethylene content of not less than 96% by weight.

4. The biaxially stretched five-layer laminate film according to Claim 1, wherein said mixture of an olefin homopolymer with an olefin copolymer is a mixture which is formed by blending an ethylene homopolymer having a density of not more than 0.925 g/cm3 with an ethylene-vinyl acetate copolymer, an ethylene-acrylic ester co-polymer or an ethylene-propylene copolymer and which has an ethylene homopolymer content of not less than 92% by weight.

5. The biaxially stretched five-layer laminate film according to Claim 1, wherein the crystal-melting point of said first layer resin except for said ionomer is at least 4°C. higher than the crystal-melting point of the second and fourth layer resin and no more than 15°C. higher than that of the fifth layer resin.

6. The biaxially stretched five-layer laminate film according to claim 1, wherein said second and fourth layers are of an ethylene-vinyl acetate copolymer having an ethylene content of from 70 to 92% by weight and a melt index in the range of from 2 to 10; a partially saponified ethylene-vinyl acetate copolymer; a product of grafting of said ethylene-vinyl acetate copolymer and/or partially saponified copolymer with a polar monomer such as acrylic acid, methacrylic acid, maleic acid, maleic anhydride or vinyl chloride; a two-component copolymer of ethylene with an acrylic ester or methacrylic ester; or a three-component copolymer of ethylene with one of said esters and vinyl acetate.

7. The biaxially stretched five-layer laminate film according to Claim 1, wherein said vinylidene chloride copolymer is a copolymer consisting of from 65 to 95% by weight of vinylidene chloride and from 5 to 35% by weight of at least one unsaturated monomer copolymerizable with said vinylidene chloride.

8. The biaxially stretched five-layer laminate film according to Claim 7, wherein said unsaturated monomer is vinyl chloride, acrylonitrile or an alkyl acrylic ester having an alkyl group of from 1 to 18 carbon atoms.

9. A method for the manufacture of a biaxially stretched five-layer laminate film, which method comprises:
(a) operating a three-layer tubular molding die having three resin flow paths and provided in each of said resin flow paths with a torpedo serving to divide the flow of resin in the path, with the flow path for the outermost resin layer and that for the intermediate resin layer being each provided internally with a tubular path having the leading end not in contact with said torpedo;
(b) feeding an ionomer, an olefin homopolymer, an olefin copolymer or a mixture of said olefin homopolymer with said olefin copolymer as the outermost layer resin, a vinylidene chloride copolymer as the intermediate layer resin and an ionomer as the innermost layer resin respect-ively to said resin flow paths and, at the same time, feeding an ethylene copolymer with a crystal-melting point 5 to 30°C. lower than that of said ionomer of the outermost layer or ionomer of the innermost layer to each of said tubular paths, thereby producing a laminate;
(c) rapidly cooling the resultant laminate thereby causing the vinylidene chloride copolymer present in the laminate to assume an amorphous state; and (d) biaxially stretching the thus quenched laminate at a temperature which is lower than the crystal-melting point of the outermost layer resin and that of the inner-most layer resin and at least 4°C higher than the crystal-melting point of said ethylene copolymer.

10. The method for the manufacture of a biaxially stretched five-layer laminate film according to Claim 9, wherein said olefin homopolymer is an ethylene homopolymer.

11. The method for the manufacture of a biaxially stretched five-layer laminate film according to Claim 9, wherein said olefin copolymer is an ethylene-propylene copolymer having an ethylene content of not less than 96% by weight, an ethylene-acrylic ester copolymer having an ethylene content of not less than 96% by weight or an ethylene-vinyl acetate copolymer having an ethylene content of not less than 96% by weight.

12. The method for the manufacture of a biaxially stretched five-layer laminate film according to Claim 9, wherein said mixture of an olefin homopolymer with an olefin copolymer is a mixture which is formed by blending an ethylene homo-polymer having a density of not more than 0.925 g/cm3 with an ethylene-vinyl acetate copolymer, an ethylene-acrylic ester copolymer or an ethylene-propylene copolymer and which has an ethylene homopolymer content of not less than 92% by weight.

13. The method for the manufacture of a biaxially stretched five-layer laminate film according to Claim 9, wherein the crystal-melting point of said outermost layer resin is at least 4°C. higher than the crystal-melting point of said ethylene copolymer and at most 15°C. higher than that of said innermost layer resin.

14. The method for the manufacture of a biaxially stretched five-layer laminate film according to Claim 9, wherein said ethylene copolymer having a crystal-melting point 5°C. to 30°C. lower than that of the ionomer of the outermost layer or ionomer of innermost layer is an ethylene-vinyl acetate copolymer having an ethylene content in the range of from 70 to 92% by weight and a melt index in the range of from 2 to 10; a partially saponified ethylene-vinyl acetate copolymer; a product of grafting of ethylene-vinyl acetate copolymer and/or said partially saponified copolymer with a polar monomer selected from acrylic acid, methacrylic acid, maleic acid, maleic anhydride and vinyl chloride; a two-component copolymer of ethylene with an acrylic ester or methacrylic ester; or a three-component copolymer of ethylene with one of said esters and vinyl acetate.

15. The method for the manufacture of a biaxially stretched five-layer laminate film according to Claim 9, wherein said vinylidene chloride copolymer is a copolymer consisting of from 65 to 95% by weight of vinylidene chloride and from 5 to 35% by weight of at least one unsaturated monomer copolymerizable with said vinylidene chloride.

16. The method for the manufacture of a biaxially stretched five-layer laminate film according to Claim 15, wherein said unsaturated monomer is vinyl chloride, acrylonitrile or an alkyl acrylic ester having an alkyl group of from 1 to 18 carbon atoms.