CA1126640A - Vacuum packaging method - Google Patents
Vacuum packaging methodInfo
- Publication number
- CA1126640A CA1126640A CA310,483A CA310483A CA1126640A CA 1126640 A CA1126640 A CA 1126640A CA 310483 A CA310483 A CA 310483A CA 1126640 A CA1126640 A CA 1126640A
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- Canada
- Prior art keywords
- film
- layer
- laminated film
- temperature
- elastic
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B65—CONVEYING; PACKING; STORING; HANDLING THIN OR FILAMENTARY MATERIAL
- B65B—MACHINES, APPARATUS OR DEVICES FOR, OR METHODS OF, PACKAGING ARTICLES OR MATERIALS; UNPACKING
- B65B31/00—Packaging articles or materials under special atmospheric or gaseous conditions; Adding propellants to aerosol containers
Abstract
VACUUM PACKAGING METHOD
Abstract of the Disclosure The invention relates to an improved vacuum packaging method which makes use of a novel laminated film as the packaging film. The laminated film is composed of a plas-tically deformable layer of a synthetic resin having a yield stress of more than 50 kg/cm2 at a temperature of 23°C. and an elongation of less than 30% at the elastic limit at a shaping temperature of 50 to 180°C, and an elastic layer of a synthetic resin having an elongation of more than 40% at the elastic limit at the shaping temperature. The film enables a tightly fitting package to be formed without any wrinkling or rupture, and without requiring a complicated procedure, equipment and apparatus.
Abstract of the Disclosure The invention relates to an improved vacuum packaging method which makes use of a novel laminated film as the packaging film. The laminated film is composed of a plas-tically deformable layer of a synthetic resin having a yield stress of more than 50 kg/cm2 at a temperature of 23°C. and an elongation of less than 30% at the elastic limit at a shaping temperature of 50 to 180°C, and an elastic layer of a synthetic resin having an elongation of more than 40% at the elastic limit at the shaping temperature. The film enables a tightly fitting package to be formed without any wrinkling or rupture, and without requiring a complicated procedure, equipment and apparatus.
Description
11'~1~64(~
The present invention relates to an improvement in vacuum packaging methods.
In recent years, the blister-packaging technique using metal molds has been widely used. As is well known, a blister-package can be formed by a method in which a sheet or film is at first shaped into a desired form of container by means of a metal mold and, after charging a material or an article to be packaged into the depressions of the shaped sheet or film and combining the shaped sheet with a flat base film or sheet, the assembly is tightly sealed after replacement of the air in the depressions of the shaped sheet or film with a gas, or after evacuation of the depressions, if necessary. When, blister-packaging is applied to perishables such as meat, the space which inevitably forms between the ma-terial or the article and the film promotes juice separation, thus spoiling the appearance of the product and promoting putrefaction of the pa~ ,ed material.
In order to overcome the above-mentioned disadvan-tages, several methods have been proposed, including a method in which the size of metal mold is closely coin-cided with the outer dimensions of the the material or article to be packaged, and a method in which the degree of vacuum in the shaping of the sheet is made so high that the packaging film is forced to contact the material or the article tightly along its external form. However, the former method is not practical since it is necessary to have a rigorous control of the size of materials or articles to be packed and the insertion of such material or article into the thus formed container is not easy. On the other hand, the latter method is disadvantageous in
The present invention relates to an improvement in vacuum packaging methods.
In recent years, the blister-packaging technique using metal molds has been widely used. As is well known, a blister-package can be formed by a method in which a sheet or film is at first shaped into a desired form of container by means of a metal mold and, after charging a material or an article to be packaged into the depressions of the shaped sheet or film and combining the shaped sheet with a flat base film or sheet, the assembly is tightly sealed after replacement of the air in the depressions of the shaped sheet or film with a gas, or after evacuation of the depressions, if necessary. When, blister-packaging is applied to perishables such as meat, the space which inevitably forms between the ma-terial or the article and the film promotes juice separation, thus spoiling the appearance of the product and promoting putrefaction of the pa~ ,ed material.
In order to overcome the above-mentioned disadvan-tages, several methods have been proposed, including a method in which the size of metal mold is closely coin-cided with the outer dimensions of the the material or article to be packaged, and a method in which the degree of vacuum in the shaping of the sheet is made so high that the packaging film is forced to contact the material or the article tightly along its external form. However, the former method is not practical since it is necessary to have a rigorous control of the size of materials or articles to be packed and the insertion of such material or article into the thus formed container is not easy. On the other hand, the latter method is disadvantageous in
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.
~2f~;40 that the excess portions of the filM are undesirahly wrinkled on evacuation and the packaged material or article becomes irregular in shape, thus giving a poor external appearance. In addition, the wrapping film is apt to be broken in the wrinkled portions thereof. ~oth methods are thus unfavorable from a practical point of view. There is accordingly a strong demand for improve-ments in packaging films and methods of this type.
In order to achieve a tight package while avoiding wrinkles in the surplus portions of the film, the use of heat shrinkable or elastic films as the packaging film have been proposed. In this connection, however, conventional stretched films or elastic films are unsatisfactory because of their large residual stress or strain. For example, the stretched film or the elastic film often warps the article or material packaged there-with, or squeezes a soft material such as ham, sausage or the like, or causes separation at the sealed portion.
As an attempt to produce vacuum-tight packages with reduced residual stress, Canadian ~atent No. 962,577 discloses a packaging method utîlizing the specific thermal property of a film of vinylidene chlor-ide copolymer. In this method, a vinylidene chloride copolymer is extruded in the form of a film and is immediately rapidly quenched to keep the film in an amorphous state, and this film is used for packaging. The use of such a film is advantageous in that the Eilm in the amorphous state can be prevented from beina wrinkled upon packaging due to its inherent elasticity and that, after ~0 packaging, the ~rapped film can be fixed in shape by cry-stallization thereof, which reduces the residual strain.
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In this method, the adverse effects due to residual stress or strain as experienced in ordinary shrinkable or elastic films can be avoided.
In this method, however, it is necessary that the film of vinylidene chloride copolymer formed by extrusion be immediately, rapidly quenched so that it can be used in the amorphous state. Moreover, the film-forming step and the packaging step must be carried out at the same time so that the packaging procedure can be completed within a very short time before crystallization of the rapidly cooled copolymer is induced, and the film is then cry-stallized in a subsequent step, such as by heat treatment.
Accordingly, this method has a vital disadvantage that if the speeds in related steps, particularly the film-forming speed and the packaging speed, are not synchronized pro-perly, packaging will become impossible, entailing a loss of materials. Further, owing to the inherent character-istics of the copolymer of vinylidene chloride, the packaging procedure should be carried out at sufficiently ~u low temperatures since crystallization of the film is greatly accelerated under high temperature conditions, resulting in a loss of elasticity. This poses a serious problem if bacteria remain on a film used to wrap raw ham, perishables and the like foods, for which sterilization at high temperature must be avoided after being packed.
It is therefore a main object of the present invention to provide a laminated film which can overcome the disad-vantages of the prior art and which is free from residual stress or strain and is suitable for tightly packaging materials along their profile, and also to provide an improved thermal and, vacuum packaging method using such 112~640 a laminated film.
Accor~ing to one aspect of the invention there is provided a laminated film sJitable for packaging articles or materials by vacuum forming, said film comprising: 3 plastically de~ormable la~er of synthetic resin having a yield stress of more than 50 kg/cm at a temperature o 23C and an elongation of less than 30% at the elastic limit at a temperature of 50 to 180C, and an elastic layer of a synthetic resin having an elongation of more than 40~ at the elastic limit at a temperat~re of 50 to According to another aspect of the invention there is provided a vacuum packaging method comprising the steps of: (a) shape-forming a laminated film in a metal mold at a temperature of 50 - 180C under a vacuum to form cup-shaped depressions in said film while providiny flat edge portions, said laminated film being composed of a plastical]y deformable layer of synthetic resin having a yield stress more than S0 kg/cm at a temperature of 23C and an elongation of less than 30~ at the elastic limit at the shaping temperature of 50 - 130C, and an elastic layer of a synthetic resin having an elongation of more than 40% at the elastic limit at said shaping temperature; (b) providing a base film adjacent to the metal mold; (c) placing articles or materials to be pack-aged at positions on said base film corresponding to the cup-shaped hollows in said laminate film; (d) combin;ns the flat edse portion of said shape-formed laminate film and the base film; (e) evacuating the interior of the spaces formed by the cup-shaped depressions of said lam-inate film and said base film, and (f) releasing the vacuum which had been esta~lished in step (a) between said metal ld and said cup-shaped formed Laminate film thereby producing a tight package of said articles or rnaterial.s losely ~di~eled lamina~ n.
rhc re~ whi_il dre utili~e.3 as th(~ plasticllly ;ieformable layel- in the laminated film and which are hardly cleformed plastically at a temeratu~e of 23C.
but dre plastically deformable at the shaping tempera-ture should preferably have a yield stress of more than 50 kg~cm2 and iess than 1000 k~/cm2 at a temperature of 23C., more preferably less than 500 kg/cm at the shaping temperature and an elongation of less than 30%, preferably less than 25% at the elastic limit i.e., the resin 10ws or is permanently deformed under a stress deformation of more than 30 %. In other words, the resin is of the type which is easily deformed plastically by ;~elting at the shaping temperature or under a stress less than 1/3, preferably less than 1/5, of the yield stress at a temperature of 23C and most preferably under a stress below 10 kg~cm2 at the shaping temperature even though not melted at the shaping temperature.
It will be noted that the term "elastic limit" used herein is defined as a stress at an elastic recovery rate of 95 ~ when a specimen with an effective length of 100 mm is pulled at a tensile velocity of 500 mm/min and immed-iatel~ the stress exerted on the specimen is released.
The term "plastic deformation" is sometimes used, in a broad sense, to mean not only plastic deformation, but also deformation due to viscous flow, and is herein used to cover such a broad interpretation.
Specific examples of the resin include homopolymers of ~_-olefins e.g. ethylene, propylene and the like, copoly-mers with a major proportion of the ~-olefins and a minor ~lZ~t;40 proportion of monomers copolymerizable with such ~-olefins, ionomers of copolymers of ~-olefins and organic acids, vinylidene chloride-base resins, saponified ethylene-vinyl acetate copolymers, acrylonitrile-base resins, and the like. These resins may be used singly or in combination. The plastically deformable layer may be made of a laminate of films obtained from different resins indicated above.
Of the above-mentioned resins, the vinylidene chloride-base resins, saponified ethylene-vinyl acetate copolymers, and acrylonitrile-base resins are prefer-able, particularly when a high impermeability to gases is required, since they are all highly impermeable to gases. The vinylidene chloride-base resins are those which comprise a major proportion of vinylidene chloride and a minor proportlon of monomers copolymerizable therewith. Preferable monomers are, for example, vinyl chloride, acrylonitrile, organic acids such as acrylic acid, methacrylic acid, maleic acid, itaconic acid, etc., alkyl esters of t~ese organic acids, vinyl acetate, iso-butylene, butadiene, and the like. In general, 60 to 95 parts by weight of vinylidene chloride is copolymerized with 40 to 5 parts by weight of one or more copolymeriz-able monomers. As a matter of course, these copolymers may be mixed with less than 10 wt ~ of additives such as harmless plasticizers, stabilizers and the like.
The saponified products of ethylene-vinyl acetate copolymer are those composed of 20-50 mole ~ of ethylene and 50 - 80 mole % of vinyl acetate and saponified to an 3v extent of at least 96 ~ or more.
The acrylonitrile-base resins are those which i40 comprises a major proportion of acrylonitrile copolymerized (or graft-copolymerized) with a minor proportion of copolymerizable monomers such as, for example, butadiene, styrene, acrylic esters, methacrylic esters and the like.
These copolymers of high impermeability to gases may be mixed with a minor proportion of butadiene copolymers or acrylic esters to improve their impact strength.
~ owever, these resins with high impermeability to gases are generally in a crystalline state and rapidly increase their fluidity at a melting point at which they turn fluid. Since the yield stress of a layer of such resin at temperatures below the melting point is lowered only slightly with increase of temperature, it is preferable that this resin layer be used in combination with a layer of a resin which has a low softening point and can thus lower the yield stress of the laminate less than 5 kg/cm2 at the shaping temperature, or preferably with a layer of a resin which makes viscous flow possible at the shaping temperature in order to increase the dif-ference between the yield stresses at a temperature of 23C. and at the shaping temperature.
The layer of a resin of lower softening temperature should preferably show an adhesiveness at the shaping temperature onto at least one surface contacting a base film and can thus serve as an adhesive when intimately contacted with the base film.
On the other hand, the resins showing a rubber-elasticity which are used as the elastic layer of the laminated film according to the present invention are those which show an elongation of more than 40~ at an o elastic limit at the shaping temperature. Examples of such resin include homopolymers of butadiene or iso-prene, random or block copolymers such as of buta-diene or isoprene with styrene and acrylonitrile, etc.
ethylene-propylene copolymer, chlorinated polyethylene, plasticized polyvinyl chloride, stretched and oriented vinylidene chloride-base resins, stretched and oriented products of saponified ethylene-vinyl acetate copolymer, and the like. Preferred are plasticized polyvinyl chloride and 1,2-polybutadiene.
It will be noted that the terms "plastically deformable resin" and "elastic resin" are used to distinguish these resins from each other only at the shaping temperature for convenience sake. For instance, resins such as butadiene-styrene copolymers serving as an elastomer at a temperature of 23C. are plastically deformed at elevated temperatures if not cross-linked.
Thus, it may be possible that the laminated film is constructed, if desired, of a plastically deformable layer composed of a non-cross-linked butadiene-styrene copolymer layer and a vinylidene chloride resin layer and of a cross-linked butadiene-styrene copolymer layer as an elastic layer. In this sense, any resin other than the afore-indicated resins for the plastically deformable layer and the elastic layer which meet the afore-mentioned requirements may likewise be used.
The laminated film according to the present invention is, as described hereinbefore, constructed of two ore more resin layers including at least one plastically deform-able resin layer and at least one elastic resin layer.
These laminated films can be made by the usual methods, i40 e.g. co-extrusion, dry laminating or combinations thereof.
The vacuum packaging method of the present invention, at least in preferred forms, using the laminated film is feasible by a series of the following steps of: forming depressions in the laminated film by means of a vacuum metal mold, placing materials or articles to be packaged in the depressions of the shaped film; contacting the flat edge portions of the shaped film with the base film;
evacuating the interior of the depressions, and then returning the pressure in the spaces between the mold and the shaped film from a vacuum to a normal pressure level, thereby ensuring a tight package.
The reason why a tight package is produced is as follows. As the atmospheric pressures on the shaped film is reduced to a vacuum, the shaped film starts to shrink by itself without causing any wrinkles because of the elastic recovery force of the elastic layer until the film tightly contacts the article or material placed in the depressions. If the outermost layer directly contacting the mold has a large friction coefficient with regard to the metal mold surface, the film may be stretched at the depressions and, in an extreme case, may be broken.
Needless to say, such trouble occurs less frequently as compared to the case when no elastic layer is used. In order to overcome this disadvantage, however, it first comes to mind to change the shape of the mold. Aside from this, several methods may be used including a method in which a suitable temperature gradient is imparted to the mold upon the shape forming, a method in which a lubricant or air is introduced between the outermost layer and the mold, and a method using as the outermost layer a resin i~`
.,~
ilZf~6~0 whicll is heat resistant and small in coefficient of friction. Most preferably, the laminated film used should have an elastic recovery rate, on shape forming, of more than ~0 %.
In order that the resin layer plastically deEormed at the shaping temperature faithfully follows the deformation recovery of the elastic resin layer, it is preferable to impart adhesiveness to the plastically deformable resin layer or the elastic layer by any known method.
After the multi-laminated film has tightly wrapped around the material or article by the above-described method, the plastically deformable layer solidifies either naturally when the material is, for example, a food cooled to low temperatures, or by forcibly cooling the material not yet cooled. As a result, the plastically deformable layer becomes so hard as to withstand the residual stress of the elastic resin layer, freeing the packaged material from a large compression force. Accordingly, even if soft materials or articles such as ham, sausage, etc., are packaged, they are free from being unduly deformed or being pressed to such an extent as to allow the inside juice to be squeezed out by an excessive compressing force exceeding that required only for a tight package.
Further, even though the adhesion to the base film is in a degree of mere vacuum adhesion, there is no fear that separation may occur in the sealing portion due to the residual strain. The laminated film or packaging film after complete solidification of the plastically deform-able layer is tough at temperatures below room temperatureof 23C. and resists deformation. This is advantageous in 4~
that the packaged articles or materials are protected from damage even when a number of the packaged articles or materials are packed in, for instance, a carton for transport.
It is practically preferable to define the thicknesses of the individual layers of the laminated film within specific ranges, in order to produce the above-mentioned unique plasto-mechanical effects of the plastically deformable layer and the elastic layer under two different 1~ temperature conditions i.e., the shaping temperature and a room temperature.
The total thickness of the multi-laminated film is preferably in the range of 40 - 150 ~. A smaller thickness than 40 ~ is practically unsatisfactory in its physical strength, while, with larger thickness than 150 ~, such films undesirably require a long heating time and are hard to deform. The plastically deformable resin layer or layers preferably have a thickness, in total, of 3 - 100 ~, more preferably of 10 - 50~ . A smaller thickness than 3 ~ is unfavorable since the resistance of the layer to the residual stress of elastic layer after cooling becomes poor. A thickness exceeding 100~ is also unfavorable since such a film will impede the elastic recovering action of the elastic layer, producing wrinkles and thus giving an objectionable appearance to the pack-aged material or article. The elastic layer or layers are preferably, in total, 10 - 100~ in thickness. With a thickness smaller than 10~ , elastic recovery of the elastic layer at the shaping temperature tends to produce wrinkles on packaging. The layer with a thickness larger than 100 ~ has high residual stress after cooling, , .
~ ., o deforming the packa(3ed material or the article resulting in separation ;n t-he sealing region.
The shaping Lelllt~ature useful in the method of the present invention is in the range of 50 - 180C, preferably 60 - 160C and most preferably 70 - 150C. A
temperature lower than 50C is unfavorable since the form-stabilizing effect of the laminated film by the action of the plastically deformable resin layer becomes inadequate due to a small difference between the shaping temperature and the cooling temperature. While, with temperatures higher than 130C, the film is superheated. This is not favorable since various troubles will develop, e.g., when precision instruMents are packaged, they may be damaged, or when water-containing food are packaged, the surface temperature of the food will exceed 100C even on instan-taneous contact, thus generating steam thereby hindering the vacuum tight contact with the food.
The adoption of a shaping temperature above 60C, preferably above 70C, produces the advantage that the packaging film is naturally sterilized and is thus very suitable for sanitary purposes, so that it can be safely applied to materials, such as ham, which can not be subjected to high temperature sterilization. Though a higher film temperature gives a greater sterilizing effect, psychrophilic bacteria bearing no spores are generally killed at temperatures above 60 - 70C. Pro-pagation of spores or bacteria other than the above-mentioned psychrophiles can be prevented by cold storage.
Thus, the above-indicated shaping temperature conditions are satisfactory for the practice of the present invention.
Any film may be used as a base film in the practice of the present invention, including single-layer or multi-layer films or sheets made of known various plastics, metal foil or paper sheets, or laminates of these foil and sheets with plastic films or sheets. These materials may be cup-shaped or not.
The multi-laminated film may be adhered to the base film by providing a known adhesive layer therebetween and treating under the vacuum heating conditions. Also it is possible to make the layer selected from the group of resins which deform plastically at the temperature of shaping as the outermost layer of the multi-layered film and to make the layer which thermally adheres to the afore-mentioned outermost layer as the outermost layer of the base film and then to bring both opposing layers pressure-welded together under thermal and vacuum con-ditions or self-welded.
When the resin for the both outermost layers is chosen from homopolymers of ~-olefins, copolymers of ~-olefins with vinyl acetate, and ionomers of copolymers of ~-olefins with methacrylic acid, or when a low melting point adhesive layer is provided on at least one side of the layers, the opposing outermost layers are readily self-welding at the shaping temperature without applica-tion of any mechanical pressure thereon, resulting in a favorable combination. ~xamples of such adhesives melt-ing at low temperature include vinyl acetate polymers, ethylene-vinyl acetate copolymers, ionomers of copolymers of ~olefins and methacrylic acid, and various types of rubbers such as diene-base polymers, chlorinated poly-ethylene, etc., known low melting resin compositions suchas resin mixtures with rosin, etc. as tackifier or wax, ,~
, .
hot-melt resins, and adhesive resins.
Other advantages and features of the present invention will become apparent from the following Examples, which are described by way of illustration only.
Example 1:
A laminated film which was produced having, from the outside, first and second layers both of which were plas-tically deformable, and a third layer having a rubber-like elasticity, i.e., the first layer was made of an ethylene-vinyl acetate copolymer (having a vinyl acetate contentof 10 %), the second layer made of a vinylidene chloride-vinyl chloride copolymer prepared by mixing 2 parts by weight of epoxidized soyabean oil and 2 parts by weight of dioctyl adipate with 100 parts by weight of a vinyli-dene chloride-vinyl chloride copolymer containing 80% by weight of vinylidene chloride, and the third layer made of plasticized polyvinyl chloride containing 35 wt% of poly(butanediol adipate) of average molecular weight of 1600. Three extruders were used to make the laminated film of the above-mentioned type by extruding each of the three starting materials into a common circular die from each of three flowing paths. Then, the three-layered parison of a cylindrical form in a molten state was treated by the so-called inflation technique in which air was injected into the molten parison beneath the die thereby producing a three-layered film having a first layer 20~ in thickness, a second layer of 20~ in thick-ness, and a third layer of 40~ in thickness. Table 1 shows the yield stresses at 90C, the elongations at the elastic limit, the elastic recovery rates after 40 %
uniaxial stretching, yield stresses at a normal temperature 6~0 of 23C of the three-layered film and the individual layers.
Further, a rigid polyvinyl chloride sheet was used as a base film having a thickness of ~50~ which had been applied with a holt melt adhesive composed of 100 parts by weight of ethylene-vinyl acetate copolymer with a vinyl acetate content of 40 wt %, 25 parts by weight of hydro-genated rosinglycerine ester, and 10 parts by weight of microcrystalline wax A sausage product of a cylindrical form of 10 cm in diameter and 2 cm in height was placed on the base film.
The three layered film which had been vacuum shaped by means of a mold of 12 cm in diameter and 3 cm in depth and heated to 90C was put on the cylindrical sausage together with the mold. Then, the concave part of the shaped film was evacuated, and after that the pressure in the space between the mold and the film was restored from a vacuum to the ordinary pressure to cause the film to adhere to the sausage around its shape by the elastic recovery of the three layered film. The packaged sausage was quickly removed from the chamber and allowed to cool, giving a package with a wrinkle-free, attractive appearance.
The packaged sausages thus produced were not broken and were free from separation at the boundary of adhesion after being subjected to shocks during transport or in handling. In addition, the package showed so-called easy peelability, i.e., the shaped film and the base film were readily separable by hand uniformly along the hot melt adhesive face at a temperature in the range of 0 - 40C.
Since the vinylidene chloride copolymer was used as the first layer of the laminated film, it was very good in i6~
preservability, i.e., the oxygen permeability of the film at 30C was as small as 30 ml/m2.day~
The laminatecl film of the above-mentioned type prG-duced stable and desirable packages when shaped at mol(3 temperatures of 70 - 100~C. ~se o~ temperatures beiow 70C presen-ted the disadvantages that the first and secon~
layers showed less tendency for plastic deformation ar.d could not counter-balance the elasticity of the third layer and, after cooling, the film was not suitably fixed in its shape because of high residual stress involved. ~s a consequence, packaged ham was squeezed at the corners thereof, and separation occurred at the boundary between the three-layer film and the hot melt adhesive layer, since a separating force was exerted on the boundary due to the residual stress of the film. On the other hand, with mold temperatures above 100C, the film strength was ~-educed and, in some case, breakage of the film occurred upon shape forming in the mold.
Reference 1 The elastic third layer of Example 1 was replaced by a plastically deformable layer, i.e., the first layer w~s formed of the ethylene-vinyl acetate copolymer, the second layer for~ed of the vinylidene chloride-vinyl chloride copolymer, and the third layer formed of a zinc complex of ethylene-methacrylic acid copolymer (product of E. I.
Du Pont de Nemours & Co., Inc., availa~le under the trade mark oE ''SUr'Lyne, Grade AD 8102"~.
Then, the procedure of packaging was carried out with the thickness of the film and the other arrangements tne same as in Example 1. As a result, it was found that wrinkles were produced at any mold temperature and so good packages could not be obtained. The physical properties of the layers including the third layer are shown in Table 1. From Table 1, it is clearly seen that the elastic recovery rate of the film of the above type is as low as 63 % when uniaxially stretched by 40 ~ at 90C, and thus wrinkles were produced due to an excessive permanent strain.
Reference 2:
The plasticized polyvinyl chloride which was used as the third layer in Example 1 was used alone in Reference 2 and formed into a film of 80~ in thickness. Then, the packaging procedure was repeated as in Example 1. As a result, it was found that not only the corners of a packaged sausaye were squeezed due to an excess of elas-ticity of the film, but also the rigid polyvinyl chloride s'neet used as the base sheet was deformed.
In packaging, the film started to deform to tightly contact the sausage prior to the completion of evacuation in a space between the film and the sausage, i.e., before the difference in the degree of vacuum between the opposite sides of the film reached substantially zero. Thus, air could not be completely removed. In the case of the cylindrical package as in this Reference, the packaged sausage gave an unsightly appearance because of the air remaining in the upper portion of the package.
The film used had an elastic recovery percentage of 98 % at 90C when uniaxially stretched by 40 ~.
Example 2:
As a resin layer with rubber-elasticity a film made of a composition of 100 parts by weight of vinylidene chloride-vinyl chloride copolymer containing 8~ by weight ;40 of vinylidene chloride and of ~ parts by weight of expoxidized soybean oil as a stabilizer which had been biaxially stretched 3.5 times by a usual manner of thickness in 15~ was used. Two plastically deform-able films of 35~l in thickness of a sodium complex of ethylene-methacrylic acid copolymer (product of E.I. Du Pont de Nemours & Co., Inc., available under the Trade Mark of Surlyne #1601) were, respectively, laminated on both faces of the above-mentioned elastic film with a urethane adhesive to obtain a three-layered laminate. A
base film was made by bonding a film of acrylonitrile-styrene copolymer 10~ in thickness having an acrylo-nitrile content of 75 wt % and a polypropylene film 200 in thickness by means of a urethane adhesive. The hot melt adhesive used in Example 1 was applied onto the base film in the form of a circle concentrically with the circular sausage such that the adhesive circle was located at a distance of about 1 cm from the outer diameter of the circular sausage. Then, packaging was carried out in the same manner as in Example 1. As a result, a wrinkle-free sausage package with an attractive appearance was obtained in a mold temperature range of 145 - 155C. Lower tem-peratures than 145C were found to be disadvantageous in that the sausage was compressed and deformed or its corners were squeezed due to the insufficiency in softness of the film. Temperatures higher than 155C were also disadvantageous in that the physical strengths of the film were reduced and the film was readily broken on shaping in the mold.
The physical properties of the three-layer film and the respective layers including the yield stress at 150C, 4() the elongation at elastic limit, the elastic recovery rate, after 40 % uniaxial stretching, the yield stress at a normal temperature (Z3C) are shown in Table 1.
Example 3:
Four extruders were used to make a laminated film including, from the outside, a first layer serving as a plastically deformable layer composed of a zinc complex of ethylene-methacrylic acid copolymer (product of E.I. nu Pont de Nemours & Co., Inc. available under the Trade Mark Surlyne ~1557), a second and a fourth layer both serving as adhesive layer and composed of styrene-isoprene block copolymer, a third layer as a plastically deformable layer composed of terpolymer of vinylidene chloride, dodecyl acrylate and vinyl chloride (having a vinylidene chloride content of 82 wt ~ and a dodecyl acrylate content of 6 wt % and containing 2 wt % of epoxidized soybean oil), and a fifth layer as an elastic layer composed of 1,2-polybutadiene. These layers were combined in a circular die and air was blown, beneath the die, into the cylindrical five-layer parison in a molten state to form a bubble.
As a result of this procedure, the respective layers had the following thicknesses: first layer/second layer/-third layer/fourth layer/fifth layer = 20~/2~/20~/2~/50~ .
Thus, a flat film with a total thickness of 94~ was obtained. The thus obtained five-layered film and the same base film as in Example 1 were used for shape forming as in Example 1 for packaging. As a result, a wrinkle-free package with an attractive appearance was obtained at a mold temperature ranging from 60 to 90C.
With temperatures lower than 60~C, the film was too hard and squeezed the ham, while at the temperatures higher ~r~
~6~0 than 90C, the tensile strength of the polybutadiene layer was suddenly reduced and its inherent elasticity was lost, tending to produce wrinkles and, in some case, causing the film to break during shape forming in the mold. The physical properties of the five-layered film and the respective layers including the yield stress at 70C, the elongation at the elastic limit, the elastic recovery rate after 40 % uniaxial stretching, and the yield stress at normal temperature (23C) are shown in Table 1.
l~Z~
Tab_e 1 \ ~ - - _ yield stress eionga elastic recovery \ (kg/cm2) leiasttct ~rate (%) \ at~ at at 40% uniaxial \ 23~C shaping shaping stretching at \ temp,* temp.* shaping tempera-\ ture*
Ex.l first plastically 60 below 2 below 10 70 layer deformaDle layer .. . ._ .
se- plastically 300 20 20-25 60 cond deforma~le layer layer ._ . _ _ third elastic _ _ over 50 over 98 layer layer ._ _ .... .
ltahyeere- _ _ 25 85 fedilm _ . . . _ . _ Ref~l third plastically 65 belo~ 2 15 70 layer deformable layer _ ltahyeere- 125 7 15-25 - 63 ed film .. _ .. . . ._ . .
~x.2 elastic _ _ over 50 over 98 layer . _. . _.................. .
plastically 70 belo~ 1 below 5 ~impossible to deformable measure) layer _ tlahryeee _ _ over 50 94 ed film __ _ ---- . .. _ .
~s~
4~
Table 1 (cond't) yi.eld stress elonga- elastic recovery \ (kg/cm2) etlaosntiact rate (~) \ at at limit(%) 40% uniaxial \ 23C shaping shaping stretching at \ temp.* temp.* shaping tempera-`~ . ture*
Ex.3 first plastically 400 15 25 80 layer layer -se- plastically 28050 20-25 60 cond deformable layer layer _ __ _ .
fifth elastic _ ~ over 50 over 98 layer layer _ . ,_ . ~ _ _ _ ._ ~ ed ~ I J ~ 18 ~ 25-35~ 90 ~haping Temperature: 90C in Example 1 and References 1 and 2 150C in Example 2, and 70C in Example 3.
- ~3 -o Example 4:
Using a three-layered film in which the first layer witll plastic deformation in Example 1 was substituted by a copolymer of ethylene and vinyl acetate (content of vinyl acetate of 20%) and the second and the third layers were in the same condition as in Example 1 and also using the same three-layered film as the base film and after arrang-ing the two films so that the opposing layers of each other film are both the ethylene-vinyl acetate layers, a sausage was packaged as in Example 1. As a result, a stable wrinkle-free package was obtained at a temperature of 85C - 100C. Though shape forming was possible even at temperatures lower than 85C but higher than 70C, the two films were readily separated due to insufficiency in adhesion when laminated under non-pressure conditions.
However, if a heat sealing device was provided just downstream of the shaping and packaging devices, a package standing practical use was obtainable. On the other hand, the mold temperature exceeding 100C was satisfactory for adhesion but, in some cases, caused the breaking of the film upon the shape forming in the mold. A proper control of the mold temperature within the above-indicated range gave so-called easy peelability or easy separability to the sealed package at lower temperature level and gave strong adhesion of the package at higher temperature level.
Example 5:
A multi-layered film was produced in which the second layer was made from a 99.5 % saponified copolymer of ethylene and vinyl acetate containing vinyl acetate moiety of ~8 mol ~ and an adhesive layer of ethylene-vinyl acetate copolymer containing vinyl acetate of 20 ~ was ;6~() inserted between the second and third layers with respective thicknesses of 50, 5, 2, and 75 microns after inflation of the first, the second, the adhesive, and the third layer with the same condition as in Example except the above-mentioned conditions. The results obtained are illustrated in Table 2.
Table 2 _ __ .
\ (kg/cm2) elongation rate (%) \ at at at shaping 40% uniaxial \ 23C shaping temp.** stretching at **
\ temD.** sha~inq tem~erature _ . .~_ = _ first ~lastically 60 below 2below 10 70 layer deformable _ layer . _ _ _ se- plastically900 230 below 10 38 cond deformable layer layer .
third elastic _ _ over 50 over 98 layer layer .. __ _ . . . . ._ laver _ _ 20 - 25 80 film . _ _ _ ** Shaping temperature: 80C
The four~layered film and the same base sheet used in Example 1 were shape formed and used for package as in Example 1. As a result, a wrinkle~free package with an attractive appearance was obtained at a mold temperature of 80 - llO~C.
,~rc~
.
~2f~;40 that the excess portions of the filM are undesirahly wrinkled on evacuation and the packaged material or article becomes irregular in shape, thus giving a poor external appearance. In addition, the wrapping film is apt to be broken in the wrinkled portions thereof. ~oth methods are thus unfavorable from a practical point of view. There is accordingly a strong demand for improve-ments in packaging films and methods of this type.
In order to achieve a tight package while avoiding wrinkles in the surplus portions of the film, the use of heat shrinkable or elastic films as the packaging film have been proposed. In this connection, however, conventional stretched films or elastic films are unsatisfactory because of their large residual stress or strain. For example, the stretched film or the elastic film often warps the article or material packaged there-with, or squeezes a soft material such as ham, sausage or the like, or causes separation at the sealed portion.
As an attempt to produce vacuum-tight packages with reduced residual stress, Canadian ~atent No. 962,577 discloses a packaging method utîlizing the specific thermal property of a film of vinylidene chlor-ide copolymer. In this method, a vinylidene chloride copolymer is extruded in the form of a film and is immediately rapidly quenched to keep the film in an amorphous state, and this film is used for packaging. The use of such a film is advantageous in that the Eilm in the amorphous state can be prevented from beina wrinkled upon packaging due to its inherent elasticity and that, after ~0 packaging, the ~rapped film can be fixed in shape by cry-stallization thereof, which reduces the residual strain.
`..-,~
In this method, the adverse effects due to residual stress or strain as experienced in ordinary shrinkable or elastic films can be avoided.
In this method, however, it is necessary that the film of vinylidene chloride copolymer formed by extrusion be immediately, rapidly quenched so that it can be used in the amorphous state. Moreover, the film-forming step and the packaging step must be carried out at the same time so that the packaging procedure can be completed within a very short time before crystallization of the rapidly cooled copolymer is induced, and the film is then cry-stallized in a subsequent step, such as by heat treatment.
Accordingly, this method has a vital disadvantage that if the speeds in related steps, particularly the film-forming speed and the packaging speed, are not synchronized pro-perly, packaging will become impossible, entailing a loss of materials. Further, owing to the inherent character-istics of the copolymer of vinylidene chloride, the packaging procedure should be carried out at sufficiently ~u low temperatures since crystallization of the film is greatly accelerated under high temperature conditions, resulting in a loss of elasticity. This poses a serious problem if bacteria remain on a film used to wrap raw ham, perishables and the like foods, for which sterilization at high temperature must be avoided after being packed.
It is therefore a main object of the present invention to provide a laminated film which can overcome the disad-vantages of the prior art and which is free from residual stress or strain and is suitable for tightly packaging materials along their profile, and also to provide an improved thermal and, vacuum packaging method using such 112~640 a laminated film.
Accor~ing to one aspect of the invention there is provided a laminated film sJitable for packaging articles or materials by vacuum forming, said film comprising: 3 plastically de~ormable la~er of synthetic resin having a yield stress of more than 50 kg/cm at a temperature o 23C and an elongation of less than 30% at the elastic limit at a temperature of 50 to 180C, and an elastic layer of a synthetic resin having an elongation of more than 40~ at the elastic limit at a temperat~re of 50 to According to another aspect of the invention there is provided a vacuum packaging method comprising the steps of: (a) shape-forming a laminated film in a metal mold at a temperature of 50 - 180C under a vacuum to form cup-shaped depressions in said film while providiny flat edge portions, said laminated film being composed of a plastical]y deformable layer of synthetic resin having a yield stress more than S0 kg/cm at a temperature of 23C and an elongation of less than 30~ at the elastic limit at the shaping temperature of 50 - 130C, and an elastic layer of a synthetic resin having an elongation of more than 40% at the elastic limit at said shaping temperature; (b) providing a base film adjacent to the metal mold; (c) placing articles or materials to be pack-aged at positions on said base film corresponding to the cup-shaped hollows in said laminate film; (d) combin;ns the flat edse portion of said shape-formed laminate film and the base film; (e) evacuating the interior of the spaces formed by the cup-shaped depressions of said lam-inate film and said base film, and (f) releasing the vacuum which had been esta~lished in step (a) between said metal ld and said cup-shaped formed Laminate film thereby producing a tight package of said articles or rnaterial.s losely ~di~eled lamina~ n.
rhc re~ whi_il dre utili~e.3 as th(~ plasticllly ;ieformable layel- in the laminated film and which are hardly cleformed plastically at a temeratu~e of 23C.
but dre plastically deformable at the shaping tempera-ture should preferably have a yield stress of more than 50 kg~cm2 and iess than 1000 k~/cm2 at a temperature of 23C., more preferably less than 500 kg/cm at the shaping temperature and an elongation of less than 30%, preferably less than 25% at the elastic limit i.e., the resin 10ws or is permanently deformed under a stress deformation of more than 30 %. In other words, the resin is of the type which is easily deformed plastically by ;~elting at the shaping temperature or under a stress less than 1/3, preferably less than 1/5, of the yield stress at a temperature of 23C and most preferably under a stress below 10 kg~cm2 at the shaping temperature even though not melted at the shaping temperature.
It will be noted that the term "elastic limit" used herein is defined as a stress at an elastic recovery rate of 95 ~ when a specimen with an effective length of 100 mm is pulled at a tensile velocity of 500 mm/min and immed-iatel~ the stress exerted on the specimen is released.
The term "plastic deformation" is sometimes used, in a broad sense, to mean not only plastic deformation, but also deformation due to viscous flow, and is herein used to cover such a broad interpretation.
Specific examples of the resin include homopolymers of ~_-olefins e.g. ethylene, propylene and the like, copoly-mers with a major proportion of the ~-olefins and a minor ~lZ~t;40 proportion of monomers copolymerizable with such ~-olefins, ionomers of copolymers of ~-olefins and organic acids, vinylidene chloride-base resins, saponified ethylene-vinyl acetate copolymers, acrylonitrile-base resins, and the like. These resins may be used singly or in combination. The plastically deformable layer may be made of a laminate of films obtained from different resins indicated above.
Of the above-mentioned resins, the vinylidene chloride-base resins, saponified ethylene-vinyl acetate copolymers, and acrylonitrile-base resins are prefer-able, particularly when a high impermeability to gases is required, since they are all highly impermeable to gases. The vinylidene chloride-base resins are those which comprise a major proportion of vinylidene chloride and a minor proportlon of monomers copolymerizable therewith. Preferable monomers are, for example, vinyl chloride, acrylonitrile, organic acids such as acrylic acid, methacrylic acid, maleic acid, itaconic acid, etc., alkyl esters of t~ese organic acids, vinyl acetate, iso-butylene, butadiene, and the like. In general, 60 to 95 parts by weight of vinylidene chloride is copolymerized with 40 to 5 parts by weight of one or more copolymeriz-able monomers. As a matter of course, these copolymers may be mixed with less than 10 wt ~ of additives such as harmless plasticizers, stabilizers and the like.
The saponified products of ethylene-vinyl acetate copolymer are those composed of 20-50 mole ~ of ethylene and 50 - 80 mole % of vinyl acetate and saponified to an 3v extent of at least 96 ~ or more.
The acrylonitrile-base resins are those which i40 comprises a major proportion of acrylonitrile copolymerized (or graft-copolymerized) with a minor proportion of copolymerizable monomers such as, for example, butadiene, styrene, acrylic esters, methacrylic esters and the like.
These copolymers of high impermeability to gases may be mixed with a minor proportion of butadiene copolymers or acrylic esters to improve their impact strength.
~ owever, these resins with high impermeability to gases are generally in a crystalline state and rapidly increase their fluidity at a melting point at which they turn fluid. Since the yield stress of a layer of such resin at temperatures below the melting point is lowered only slightly with increase of temperature, it is preferable that this resin layer be used in combination with a layer of a resin which has a low softening point and can thus lower the yield stress of the laminate less than 5 kg/cm2 at the shaping temperature, or preferably with a layer of a resin which makes viscous flow possible at the shaping temperature in order to increase the dif-ference between the yield stresses at a temperature of 23C. and at the shaping temperature.
The layer of a resin of lower softening temperature should preferably show an adhesiveness at the shaping temperature onto at least one surface contacting a base film and can thus serve as an adhesive when intimately contacted with the base film.
On the other hand, the resins showing a rubber-elasticity which are used as the elastic layer of the laminated film according to the present invention are those which show an elongation of more than 40~ at an o elastic limit at the shaping temperature. Examples of such resin include homopolymers of butadiene or iso-prene, random or block copolymers such as of buta-diene or isoprene with styrene and acrylonitrile, etc.
ethylene-propylene copolymer, chlorinated polyethylene, plasticized polyvinyl chloride, stretched and oriented vinylidene chloride-base resins, stretched and oriented products of saponified ethylene-vinyl acetate copolymer, and the like. Preferred are plasticized polyvinyl chloride and 1,2-polybutadiene.
It will be noted that the terms "plastically deformable resin" and "elastic resin" are used to distinguish these resins from each other only at the shaping temperature for convenience sake. For instance, resins such as butadiene-styrene copolymers serving as an elastomer at a temperature of 23C. are plastically deformed at elevated temperatures if not cross-linked.
Thus, it may be possible that the laminated film is constructed, if desired, of a plastically deformable layer composed of a non-cross-linked butadiene-styrene copolymer layer and a vinylidene chloride resin layer and of a cross-linked butadiene-styrene copolymer layer as an elastic layer. In this sense, any resin other than the afore-indicated resins for the plastically deformable layer and the elastic layer which meet the afore-mentioned requirements may likewise be used.
The laminated film according to the present invention is, as described hereinbefore, constructed of two ore more resin layers including at least one plastically deform-able resin layer and at least one elastic resin layer.
These laminated films can be made by the usual methods, i40 e.g. co-extrusion, dry laminating or combinations thereof.
The vacuum packaging method of the present invention, at least in preferred forms, using the laminated film is feasible by a series of the following steps of: forming depressions in the laminated film by means of a vacuum metal mold, placing materials or articles to be packaged in the depressions of the shaped film; contacting the flat edge portions of the shaped film with the base film;
evacuating the interior of the depressions, and then returning the pressure in the spaces between the mold and the shaped film from a vacuum to a normal pressure level, thereby ensuring a tight package.
The reason why a tight package is produced is as follows. As the atmospheric pressures on the shaped film is reduced to a vacuum, the shaped film starts to shrink by itself without causing any wrinkles because of the elastic recovery force of the elastic layer until the film tightly contacts the article or material placed in the depressions. If the outermost layer directly contacting the mold has a large friction coefficient with regard to the metal mold surface, the film may be stretched at the depressions and, in an extreme case, may be broken.
Needless to say, such trouble occurs less frequently as compared to the case when no elastic layer is used. In order to overcome this disadvantage, however, it first comes to mind to change the shape of the mold. Aside from this, several methods may be used including a method in which a suitable temperature gradient is imparted to the mold upon the shape forming, a method in which a lubricant or air is introduced between the outermost layer and the mold, and a method using as the outermost layer a resin i~`
.,~
ilZf~6~0 whicll is heat resistant and small in coefficient of friction. Most preferably, the laminated film used should have an elastic recovery rate, on shape forming, of more than ~0 %.
In order that the resin layer plastically deEormed at the shaping temperature faithfully follows the deformation recovery of the elastic resin layer, it is preferable to impart adhesiveness to the plastically deformable resin layer or the elastic layer by any known method.
After the multi-laminated film has tightly wrapped around the material or article by the above-described method, the plastically deformable layer solidifies either naturally when the material is, for example, a food cooled to low temperatures, or by forcibly cooling the material not yet cooled. As a result, the plastically deformable layer becomes so hard as to withstand the residual stress of the elastic resin layer, freeing the packaged material from a large compression force. Accordingly, even if soft materials or articles such as ham, sausage, etc., are packaged, they are free from being unduly deformed or being pressed to such an extent as to allow the inside juice to be squeezed out by an excessive compressing force exceeding that required only for a tight package.
Further, even though the adhesion to the base film is in a degree of mere vacuum adhesion, there is no fear that separation may occur in the sealing portion due to the residual strain. The laminated film or packaging film after complete solidification of the plastically deform-able layer is tough at temperatures below room temperatureof 23C. and resists deformation. This is advantageous in 4~
that the packaged articles or materials are protected from damage even when a number of the packaged articles or materials are packed in, for instance, a carton for transport.
It is practically preferable to define the thicknesses of the individual layers of the laminated film within specific ranges, in order to produce the above-mentioned unique plasto-mechanical effects of the plastically deformable layer and the elastic layer under two different 1~ temperature conditions i.e., the shaping temperature and a room temperature.
The total thickness of the multi-laminated film is preferably in the range of 40 - 150 ~. A smaller thickness than 40 ~ is practically unsatisfactory in its physical strength, while, with larger thickness than 150 ~, such films undesirably require a long heating time and are hard to deform. The plastically deformable resin layer or layers preferably have a thickness, in total, of 3 - 100 ~, more preferably of 10 - 50~ . A smaller thickness than 3 ~ is unfavorable since the resistance of the layer to the residual stress of elastic layer after cooling becomes poor. A thickness exceeding 100~ is also unfavorable since such a film will impede the elastic recovering action of the elastic layer, producing wrinkles and thus giving an objectionable appearance to the pack-aged material or article. The elastic layer or layers are preferably, in total, 10 - 100~ in thickness. With a thickness smaller than 10~ , elastic recovery of the elastic layer at the shaping temperature tends to produce wrinkles on packaging. The layer with a thickness larger than 100 ~ has high residual stress after cooling, , .
~ ., o deforming the packa(3ed material or the article resulting in separation ;n t-he sealing region.
The shaping Lelllt~ature useful in the method of the present invention is in the range of 50 - 180C, preferably 60 - 160C and most preferably 70 - 150C. A
temperature lower than 50C is unfavorable since the form-stabilizing effect of the laminated film by the action of the plastically deformable resin layer becomes inadequate due to a small difference between the shaping temperature and the cooling temperature. While, with temperatures higher than 130C, the film is superheated. This is not favorable since various troubles will develop, e.g., when precision instruMents are packaged, they may be damaged, or when water-containing food are packaged, the surface temperature of the food will exceed 100C even on instan-taneous contact, thus generating steam thereby hindering the vacuum tight contact with the food.
The adoption of a shaping temperature above 60C, preferably above 70C, produces the advantage that the packaging film is naturally sterilized and is thus very suitable for sanitary purposes, so that it can be safely applied to materials, such as ham, which can not be subjected to high temperature sterilization. Though a higher film temperature gives a greater sterilizing effect, psychrophilic bacteria bearing no spores are generally killed at temperatures above 60 - 70C. Pro-pagation of spores or bacteria other than the above-mentioned psychrophiles can be prevented by cold storage.
Thus, the above-indicated shaping temperature conditions are satisfactory for the practice of the present invention.
Any film may be used as a base film in the practice of the present invention, including single-layer or multi-layer films or sheets made of known various plastics, metal foil or paper sheets, or laminates of these foil and sheets with plastic films or sheets. These materials may be cup-shaped or not.
The multi-laminated film may be adhered to the base film by providing a known adhesive layer therebetween and treating under the vacuum heating conditions. Also it is possible to make the layer selected from the group of resins which deform plastically at the temperature of shaping as the outermost layer of the multi-layered film and to make the layer which thermally adheres to the afore-mentioned outermost layer as the outermost layer of the base film and then to bring both opposing layers pressure-welded together under thermal and vacuum con-ditions or self-welded.
When the resin for the both outermost layers is chosen from homopolymers of ~-olefins, copolymers of ~-olefins with vinyl acetate, and ionomers of copolymers of ~-olefins with methacrylic acid, or when a low melting point adhesive layer is provided on at least one side of the layers, the opposing outermost layers are readily self-welding at the shaping temperature without applica-tion of any mechanical pressure thereon, resulting in a favorable combination. ~xamples of such adhesives melt-ing at low temperature include vinyl acetate polymers, ethylene-vinyl acetate copolymers, ionomers of copolymers of ~olefins and methacrylic acid, and various types of rubbers such as diene-base polymers, chlorinated poly-ethylene, etc., known low melting resin compositions suchas resin mixtures with rosin, etc. as tackifier or wax, ,~
, .
hot-melt resins, and adhesive resins.
Other advantages and features of the present invention will become apparent from the following Examples, which are described by way of illustration only.
Example 1:
A laminated film which was produced having, from the outside, first and second layers both of which were plas-tically deformable, and a third layer having a rubber-like elasticity, i.e., the first layer was made of an ethylene-vinyl acetate copolymer (having a vinyl acetate contentof 10 %), the second layer made of a vinylidene chloride-vinyl chloride copolymer prepared by mixing 2 parts by weight of epoxidized soyabean oil and 2 parts by weight of dioctyl adipate with 100 parts by weight of a vinyli-dene chloride-vinyl chloride copolymer containing 80% by weight of vinylidene chloride, and the third layer made of plasticized polyvinyl chloride containing 35 wt% of poly(butanediol adipate) of average molecular weight of 1600. Three extruders were used to make the laminated film of the above-mentioned type by extruding each of the three starting materials into a common circular die from each of three flowing paths. Then, the three-layered parison of a cylindrical form in a molten state was treated by the so-called inflation technique in which air was injected into the molten parison beneath the die thereby producing a three-layered film having a first layer 20~ in thickness, a second layer of 20~ in thick-ness, and a third layer of 40~ in thickness. Table 1 shows the yield stresses at 90C, the elongations at the elastic limit, the elastic recovery rates after 40 %
uniaxial stretching, yield stresses at a normal temperature 6~0 of 23C of the three-layered film and the individual layers.
Further, a rigid polyvinyl chloride sheet was used as a base film having a thickness of ~50~ which had been applied with a holt melt adhesive composed of 100 parts by weight of ethylene-vinyl acetate copolymer with a vinyl acetate content of 40 wt %, 25 parts by weight of hydro-genated rosinglycerine ester, and 10 parts by weight of microcrystalline wax A sausage product of a cylindrical form of 10 cm in diameter and 2 cm in height was placed on the base film.
The three layered film which had been vacuum shaped by means of a mold of 12 cm in diameter and 3 cm in depth and heated to 90C was put on the cylindrical sausage together with the mold. Then, the concave part of the shaped film was evacuated, and after that the pressure in the space between the mold and the film was restored from a vacuum to the ordinary pressure to cause the film to adhere to the sausage around its shape by the elastic recovery of the three layered film. The packaged sausage was quickly removed from the chamber and allowed to cool, giving a package with a wrinkle-free, attractive appearance.
The packaged sausages thus produced were not broken and were free from separation at the boundary of adhesion after being subjected to shocks during transport or in handling. In addition, the package showed so-called easy peelability, i.e., the shaped film and the base film were readily separable by hand uniformly along the hot melt adhesive face at a temperature in the range of 0 - 40C.
Since the vinylidene chloride copolymer was used as the first layer of the laminated film, it was very good in i6~
preservability, i.e., the oxygen permeability of the film at 30C was as small as 30 ml/m2.day~
The laminatecl film of the above-mentioned type prG-duced stable and desirable packages when shaped at mol(3 temperatures of 70 - 100~C. ~se o~ temperatures beiow 70C presen-ted the disadvantages that the first and secon~
layers showed less tendency for plastic deformation ar.d could not counter-balance the elasticity of the third layer and, after cooling, the film was not suitably fixed in its shape because of high residual stress involved. ~s a consequence, packaged ham was squeezed at the corners thereof, and separation occurred at the boundary between the three-layer film and the hot melt adhesive layer, since a separating force was exerted on the boundary due to the residual stress of the film. On the other hand, with mold temperatures above 100C, the film strength was ~-educed and, in some case, breakage of the film occurred upon shape forming in the mold.
Reference 1 The elastic third layer of Example 1 was replaced by a plastically deformable layer, i.e., the first layer w~s formed of the ethylene-vinyl acetate copolymer, the second layer for~ed of the vinylidene chloride-vinyl chloride copolymer, and the third layer formed of a zinc complex of ethylene-methacrylic acid copolymer (product of E. I.
Du Pont de Nemours & Co., Inc., availa~le under the trade mark oE ''SUr'Lyne, Grade AD 8102"~.
Then, the procedure of packaging was carried out with the thickness of the film and the other arrangements tne same as in Example 1. As a result, it was found that wrinkles were produced at any mold temperature and so good packages could not be obtained. The physical properties of the layers including the third layer are shown in Table 1. From Table 1, it is clearly seen that the elastic recovery rate of the film of the above type is as low as 63 % when uniaxially stretched by 40 ~ at 90C, and thus wrinkles were produced due to an excessive permanent strain.
Reference 2:
The plasticized polyvinyl chloride which was used as the third layer in Example 1 was used alone in Reference 2 and formed into a film of 80~ in thickness. Then, the packaging procedure was repeated as in Example 1. As a result, it was found that not only the corners of a packaged sausaye were squeezed due to an excess of elas-ticity of the film, but also the rigid polyvinyl chloride s'neet used as the base sheet was deformed.
In packaging, the film started to deform to tightly contact the sausage prior to the completion of evacuation in a space between the film and the sausage, i.e., before the difference in the degree of vacuum between the opposite sides of the film reached substantially zero. Thus, air could not be completely removed. In the case of the cylindrical package as in this Reference, the packaged sausage gave an unsightly appearance because of the air remaining in the upper portion of the package.
The film used had an elastic recovery percentage of 98 % at 90C when uniaxially stretched by 40 ~.
Example 2:
As a resin layer with rubber-elasticity a film made of a composition of 100 parts by weight of vinylidene chloride-vinyl chloride copolymer containing 8~ by weight ;40 of vinylidene chloride and of ~ parts by weight of expoxidized soybean oil as a stabilizer which had been biaxially stretched 3.5 times by a usual manner of thickness in 15~ was used. Two plastically deform-able films of 35~l in thickness of a sodium complex of ethylene-methacrylic acid copolymer (product of E.I. Du Pont de Nemours & Co., Inc., available under the Trade Mark of Surlyne #1601) were, respectively, laminated on both faces of the above-mentioned elastic film with a urethane adhesive to obtain a three-layered laminate. A
base film was made by bonding a film of acrylonitrile-styrene copolymer 10~ in thickness having an acrylo-nitrile content of 75 wt % and a polypropylene film 200 in thickness by means of a urethane adhesive. The hot melt adhesive used in Example 1 was applied onto the base film in the form of a circle concentrically with the circular sausage such that the adhesive circle was located at a distance of about 1 cm from the outer diameter of the circular sausage. Then, packaging was carried out in the same manner as in Example 1. As a result, a wrinkle-free sausage package with an attractive appearance was obtained in a mold temperature range of 145 - 155C. Lower tem-peratures than 145C were found to be disadvantageous in that the sausage was compressed and deformed or its corners were squeezed due to the insufficiency in softness of the film. Temperatures higher than 155C were also disadvantageous in that the physical strengths of the film were reduced and the film was readily broken on shaping in the mold.
The physical properties of the three-layer film and the respective layers including the yield stress at 150C, 4() the elongation at elastic limit, the elastic recovery rate, after 40 % uniaxial stretching, the yield stress at a normal temperature (Z3C) are shown in Table 1.
Example 3:
Four extruders were used to make a laminated film including, from the outside, a first layer serving as a plastically deformable layer composed of a zinc complex of ethylene-methacrylic acid copolymer (product of E.I. nu Pont de Nemours & Co., Inc. available under the Trade Mark Surlyne ~1557), a second and a fourth layer both serving as adhesive layer and composed of styrene-isoprene block copolymer, a third layer as a plastically deformable layer composed of terpolymer of vinylidene chloride, dodecyl acrylate and vinyl chloride (having a vinylidene chloride content of 82 wt ~ and a dodecyl acrylate content of 6 wt % and containing 2 wt % of epoxidized soybean oil), and a fifth layer as an elastic layer composed of 1,2-polybutadiene. These layers were combined in a circular die and air was blown, beneath the die, into the cylindrical five-layer parison in a molten state to form a bubble.
As a result of this procedure, the respective layers had the following thicknesses: first layer/second layer/-third layer/fourth layer/fifth layer = 20~/2~/20~/2~/50~ .
Thus, a flat film with a total thickness of 94~ was obtained. The thus obtained five-layered film and the same base film as in Example 1 were used for shape forming as in Example 1 for packaging. As a result, a wrinkle-free package with an attractive appearance was obtained at a mold temperature ranging from 60 to 90C.
With temperatures lower than 60~C, the film was too hard and squeezed the ham, while at the temperatures higher ~r~
~6~0 than 90C, the tensile strength of the polybutadiene layer was suddenly reduced and its inherent elasticity was lost, tending to produce wrinkles and, in some case, causing the film to break during shape forming in the mold. The physical properties of the five-layered film and the respective layers including the yield stress at 70C, the elongation at the elastic limit, the elastic recovery rate after 40 % uniaxial stretching, and the yield stress at normal temperature (23C) are shown in Table 1.
l~Z~
Tab_e 1 \ ~ - - _ yield stress eionga elastic recovery \ (kg/cm2) leiasttct ~rate (%) \ at~ at at 40% uniaxial \ 23~C shaping shaping stretching at \ temp,* temp.* shaping tempera-\ ture*
Ex.l first plastically 60 below 2 below 10 70 layer deformaDle layer .. . ._ .
se- plastically 300 20 20-25 60 cond deforma~le layer layer ._ . _ _ third elastic _ _ over 50 over 98 layer layer ._ _ .... .
ltahyeere- _ _ 25 85 fedilm _ . . . _ . _ Ref~l third plastically 65 belo~ 2 15 70 layer deformable layer _ ltahyeere- 125 7 15-25 - 63 ed film .. _ .. . . ._ . .
~x.2 elastic _ _ over 50 over 98 layer . _. . _.................. .
plastically 70 belo~ 1 below 5 ~impossible to deformable measure) layer _ tlahryeee _ _ over 50 94 ed film __ _ ---- . .. _ .
~s~
4~
Table 1 (cond't) yi.eld stress elonga- elastic recovery \ (kg/cm2) etlaosntiact rate (~) \ at at limit(%) 40% uniaxial \ 23C shaping shaping stretching at \ temp.* temp.* shaping tempera-`~ . ture*
Ex.3 first plastically 400 15 25 80 layer layer -se- plastically 28050 20-25 60 cond deformable layer layer _ __ _ .
fifth elastic _ ~ over 50 over 98 layer layer _ . ,_ . ~ _ _ _ ._ ~ ed ~ I J ~ 18 ~ 25-35~ 90 ~haping Temperature: 90C in Example 1 and References 1 and 2 150C in Example 2, and 70C in Example 3.
- ~3 -o Example 4:
Using a three-layered film in which the first layer witll plastic deformation in Example 1 was substituted by a copolymer of ethylene and vinyl acetate (content of vinyl acetate of 20%) and the second and the third layers were in the same condition as in Example 1 and also using the same three-layered film as the base film and after arrang-ing the two films so that the opposing layers of each other film are both the ethylene-vinyl acetate layers, a sausage was packaged as in Example 1. As a result, a stable wrinkle-free package was obtained at a temperature of 85C - 100C. Though shape forming was possible even at temperatures lower than 85C but higher than 70C, the two films were readily separated due to insufficiency in adhesion when laminated under non-pressure conditions.
However, if a heat sealing device was provided just downstream of the shaping and packaging devices, a package standing practical use was obtainable. On the other hand, the mold temperature exceeding 100C was satisfactory for adhesion but, in some cases, caused the breaking of the film upon the shape forming in the mold. A proper control of the mold temperature within the above-indicated range gave so-called easy peelability or easy separability to the sealed package at lower temperature level and gave strong adhesion of the package at higher temperature level.
Example 5:
A multi-layered film was produced in which the second layer was made from a 99.5 % saponified copolymer of ethylene and vinyl acetate containing vinyl acetate moiety of ~8 mol ~ and an adhesive layer of ethylene-vinyl acetate copolymer containing vinyl acetate of 20 ~ was ;6~() inserted between the second and third layers with respective thicknesses of 50, 5, 2, and 75 microns after inflation of the first, the second, the adhesive, and the third layer with the same condition as in Example except the above-mentioned conditions. The results obtained are illustrated in Table 2.
Table 2 _ __ .
\ (kg/cm2) elongation rate (%) \ at at at shaping 40% uniaxial \ 23C shaping temp.** stretching at **
\ temD.** sha~inq tem~erature _ . .~_ = _ first ~lastically 60 below 2below 10 70 layer deformable _ layer . _ _ _ se- plastically900 230 below 10 38 cond deformable layer layer .
third elastic _ _ over 50 over 98 layer layer .. __ _ . . . . ._ laver _ _ 20 - 25 80 film . _ _ _ ** Shaping temperature: 80C
The four~layered film and the same base sheet used in Example 1 were shape formed and used for package as in Example 1. As a result, a wrinkle~free package with an attractive appearance was obtained at a mold temperature of 80 - llO~C.
,~rc~
Claims (14)
1. A vacuum packaging method comprising the steps of:
(a) shape-forming a laminated film in a metal mold at a temperature of 50 - 180°C under a vacuum to form cup-shaped depressions in said film while providing Flat edge portions, said laminated film being composed of a plastic-ally deformable layer of synthetic resin having a yield stress more than 50 kg/cm2 at a temperature of 23°C and an elongation of less than 30% at the elastic limit at the shaping temperature of 50 - 180°C, and an elastic layer of a synthetic resin having an elongation of more than 40% at the elastic limit at said shaping temperature;
(b) providing a base film adjacent to the metal mold;
(c) placing articles or materials to he packaged at positions on said base film corresponding to the cup-shaped hollows in said laminate film;
(d) combining the flat edge portion of said shape-formed laminate film and the base film;
(e) evacuating the interior of the spaces formed by the cup-shaped depressions of said laminate film and said base film, and (f) releasing the vacuum which had been established in step (a) between said metal mold and said cup-shaped formed laminate film thereby producing a tight package of said articles or materials having closely adhered laminate film.
(a) shape-forming a laminated film in a metal mold at a temperature of 50 - 180°C under a vacuum to form cup-shaped depressions in said film while providing Flat edge portions, said laminated film being composed of a plastic-ally deformable layer of synthetic resin having a yield stress more than 50 kg/cm2 at a temperature of 23°C and an elongation of less than 30% at the elastic limit at the shaping temperature of 50 - 180°C, and an elastic layer of a synthetic resin having an elongation of more than 40% at the elastic limit at said shaping temperature;
(b) providing a base film adjacent to the metal mold;
(c) placing articles or materials to he packaged at positions on said base film corresponding to the cup-shaped hollows in said laminate film;
(d) combining the flat edge portion of said shape-formed laminate film and the base film;
(e) evacuating the interior of the spaces formed by the cup-shaped depressions of said laminate film and said base film, and (f) releasing the vacuum which had been established in step (a) between said metal mold and said cup-shaped formed laminate film thereby producing a tight package of said articles or materials having closely adhered laminate film.
2. The method according to claim 1 in which the plastic-ally deformable layer and/or the elastic layer of said lam-inated film are composed of a plurality of resinous layers.
3. The method according to claim 1 in which the total thickness of the plastically deformable layer or said laminated film is in the range of 3 to 100µ .
4. The method according to any one of claims 1 to 3, in which the total thickness of the elastic layer of said laminated film is in the range of 10 to 100µ .
5. The method according to any one of claims 1 to 3, in which the total thickness of the laminated film is in the range of 40 to 150µ .
6. The method according to claim 1, in which the plastically deformable layer of said laminated film comprises a first resinous layer having a yield stress less than that of said first resinous layer at the shaping temperature of 50 to 180°C.
7. The method according to claim 6, in which the second resinous layer serving as a plastically deformable layer of said laminated film has a yield stress less than 5 kg/cm2 at said shaping temperature.
8. The method according to claim 1, in which the plastically deformable layer of said laminated film is formed of a resin which is readily plastically deform-able at the shaping temperature by a stress which is less than 1/3, preferably less than 1/5 of the yield stress of the film at a temperature of 23°C.
9. The method according to claim 8, in which the plas-tically deformable layer of said laminated film is formed of a resinous material which is easily deformable plas-tically by a stress below 10 kg/cm2 at said shaping temperature.
10. The method according to any one of claims 1 to 3, in which the laminated film has an elastic recovery rate of above 80 % at said shaping temperature.
11. The method according to any one of claims 1 to 3, in which the plastically deformable layer of said laminated film is composed of at least one resinous material selected from the group consisting of vinylidene chloride copolymer, saponified ethylene-vinyl acetate copolymer, and acrylonitrile resin.
12. The method according to any one of claims 1 to 3, in which the elastic layer of said laminated film is composed of at least one resinous material selected from the group consisting of plasticized polyvinyl chloride, 1,2-poly-butadiene and stretched and oriented vinylidene chloride-vinyl chloride copolymer.
13. The method according to any one of claims 1 to 3, in which said shaping temperature is in the range of 70 to 150°C.
14. The method according to any one of claims 1 to 3, in which said laminated film is shape-formed under a stress between elastic limits of the plastically deformable layer and the elastic layer.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US06/074,828 US4275544A (en) | 1977-09-05 | 1979-09-12 | Vacuum packaging method |
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP105787/1977 | 1977-09-05 | ||
| JP10578777A JPS5449288A (en) | 1977-09-05 | 1977-09-05 | Method of heating and wrapping to vacuum |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1126640A true CA1126640A (en) | 1982-06-29 |
Family
ID=14416837
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA310,483A Expired CA1126640A (en) | 1977-09-05 | 1978-09-01 | Vacuum packaging method |
Country Status (8)
| Country | Link |
|---|---|
| JP (1) | JPS5449288A (en) |
| BE (1) | BE870203A (en) |
| CA (1) | CA1126640A (en) |
| DE (1) | DE2838226C2 (en) |
| FR (1) | FR2401849A1 (en) |
| GB (1) | GB2005628B (en) |
| IT (1) | IT1098491B (en) |
| NL (1) | NL182211C (en) |
Families Citing this family (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2145363B (en) * | 1983-08-23 | 1986-09-24 | Grace W R & Co | Vacuum skin package |
| JPS60155701U (en) * | 1984-03-28 | 1985-10-17 | 三菱重工業株式会社 | Steam turbine exhaust chamber spray device |
| JPS60155702U (en) * | 1984-03-28 | 1985-10-17 | 三菱重工業株式会社 | Steam turbine exhaust chamber spray device |
| KR20060026574A (en) | 2004-09-21 | 2006-03-24 | 삼성광주전자 주식회사 | Cyclone dust collecting appartus |
Family Cites Families (13)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR1493353A (en) * | 1965-09-23 | 1967-08-25 | Grace W R & Co | Hot-formed packaging |
| DE1511714C3 (en) * | 1966-12-12 | 1978-07-13 | Oscar Mayer & Co., Inc., Chicago, Ill. (V.St.A.) | Machine for the continuous packaging of objects |
| US3663240A (en) * | 1969-03-24 | 1972-05-16 | Mayer & Co Inc O | Package and method of making same |
| BE754507A (en) * | 1969-08-08 | 1971-02-08 | Dow Chemical Co | PERFECTED ARTICLE PACKAGING PROCESS |
| CA926760A (en) * | 1970-02-09 | 1973-05-22 | W.R. Grace And Co. | Thermoplastic vacuum packaging process and apparatus |
| CA962577A (en) * | 1971-06-09 | 1975-02-11 | Robert L. Goller | Food package and method of making same |
| GB1385196A (en) * | 1971-06-23 | 1975-02-26 | Grace W R & Co | Process for the production of laminates |
| GB1431047A (en) * | 1972-05-09 | 1976-04-07 | Gist Brocades Nv | Packaging of materials |
| DE2262708C2 (en) * | 1972-12-21 | 1985-07-11 | The Dow Chemical Co., Midland, Mich. | Method of making a laminate |
| US3835618A (en) * | 1973-01-22 | 1974-09-17 | Grace W R & Co | Apparatus for producing vacuum skin packages in multiples |
| DE2364565C2 (en) * | 1973-12-24 | 1983-01-05 | Multivac Sepp Haggenmüller KG, 8941 Wolfertschwenden | Method and vacuum packaging device for producing a package |
| US3950919A (en) * | 1974-08-22 | 1976-04-20 | W. R. Grace & Co. | Apparatus and process for vacuum skin packaging |
| US3966045A (en) * | 1975-05-12 | 1976-06-29 | W. R. Grace & Co. | Skin package |
-
1977
- 1977-09-05 JP JP10578777A patent/JPS5449288A/en active Granted
-
1978
- 1978-08-30 NL NLAANVRAGE7808910,A patent/NL182211C/en not_active IP Right Cessation
- 1978-09-01 DE DE2838226A patent/DE2838226C2/en not_active Expired
- 1978-09-01 CA CA310,483A patent/CA1126640A/en not_active Expired
- 1978-09-05 BE BE190251A patent/BE870203A/en not_active IP Right Cessation
- 1978-09-05 GB GB7835664A patent/GB2005628B/en not_active Expired
- 1978-09-05 IT IT27316/78A patent/IT1098491B/en active
- 1978-09-05 FR FR7825556A patent/FR2401849A1/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| IT7827316A0 (en) | 1978-09-05 |
| DE2838226C2 (en) | 1984-11-22 |
| BE870203A (en) | 1979-01-02 |
| JPS5649206B2 (en) | 1981-11-20 |
| IT1098491B (en) | 1985-09-07 |
| NL7808910A (en) | 1979-03-07 |
| GB2005628B (en) | 1982-03-03 |
| DE2838226A1 (en) | 1979-03-15 |
| FR2401849B1 (en) | 1983-07-22 |
| FR2401849A1 (en) | 1979-03-30 |
| NL182211C (en) | 1988-02-01 |
| NL182211B (en) | 1987-09-01 |
| JPS5449288A (en) | 1979-04-18 |
| GB2005628A (en) | 1979-04-25 |
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