CA1069447A

CA1069447A - Laminate film for use in vacuum packaging

Info

Publication number: CA1069447A
Application number: CA229,675A
Authority: CA
Inventors: Gian L. Argela; Riccardo Austoni
Original assignee: WR Grace and Co
Current assignee: WR Grace and Co Conn
Priority date: 1974-06-20
Filing date: 1975-06-19
Publication date: 1980-01-08
Also published as: GB1510115A; NL7507252A; CH605271A5; DE2527166A1; FR2275377B1; BE830377A; SE7507044L; AU8222675A; IT1039102B; DK278075A; FR2275377A1; JPS5113876A

Abstract

ABSTRACT OF THE DISCLOSURE
This invention relates to vacuum packaging. A backing-board laminate having in a substrate layer of irradiated polyethylene, oriented polypropylene or oriented polyamide, an oxygen-barrier layer of hydrolysed ethylene-vinyl acetate copolymer and an uppermost layer of a blend of an ethylene-vinyl acetate copolymer with a wax is heat-sealed vacuum-tightly to a thermoformed pouch film laminate the lowermost layer of which is of polyethylene or an ethylene-vinyl acetate copolymer. An ink-spot of e.g. nitrocellulose resin provided between the backing board and pouch films enables the consumer to start to pull open seal. Once the pull is started, the seal opens easily. The easy-open seal, excellent barrier properties and high mechanical strength of the package make it commercially attractive.

Description

~!D69~7 ThiS Inven~ion relate~ to packaging~ in particular to the packaging of articles, especially foodstuffs, by sealing around the article two plastics films.
In many packaging processes it is the practice to seal an article ~etween two plastics films so securely that it is very difficult to open the package ~y pulling the two plastics films apart at the seal.
This is a particular difficulty in vacuum packaging since a particularly strong seal is formed. The problem of opening the package is especially acute in one par~icular kind of vacuum packaging, namely vacuum skin packaging in which the article is placed on one barrier film termed the "backing board"J i.e. a film of low oxygen-permeability and another barrier film, termed the "pouch film", is placed above the article, ready to be wrapped around its top and sides. Vacuum is applied to remove air from around-the article and the films are heated so that the films are drawn tightly aro~md the article to be packaged and sealed to each other. The article is often packaged so tightly and sealed so securely that the consumer cannot open the seal by hand and has to resort to opening the package with a knife or other cutting instrument.
The films used in vacuum packaging are required to have a large number of different prope.rties, especially where the article packaged is a foodstuff. The film must act as a barrier to the entry of oxygen from the atmosphere to the packaged article, ~ave chemical resistance to attack ~y acids and alkalis, a high degree of transparency so t~at ~t is attractive to~the consumer ~ ~ ~ 2 _ .

b~ printable, have adequate mechanical strength to withstand the packaging operation itself and the subse~uent handling and transportation~ and be heat- :
sealable yet ea~y to open by the consumer, Because of 5 all t~ese requir~ments it ha~ been a practics to employ laminates of films hav.ing different properties. Eowever, it is di~ficult to find a laminate which considercd overall meets the r~quirements to a commercially valuable extent. .
Variou~ proposals which the present applicants do 1~ not regard as completely satisfactory ~or use as a hacking .
board ~ilm in ~acuum packaging are described in U.S.
Patent Specifications Nos~ 3,540,962 (~nzawa et al,), 395~0,325 (Sogi et al.) and 39595,740 (Gerow) and U.K. . .
Patent Specification No. l,2961013. Many of the prior ~ :.
prop~als are not oompletely acceptable because the laminate :.
lack~ sufficient barrier propertieæ andjor is dif~icult ~:
~or the con~umer to open and/or lacks overal7 mechanical ~trength.
. .
In our U.K, Patent Specification No 3 1~ 296,013 we-have described a va~uum skin pac~aging process in which an "easy-to-open" seal is ~ormed between the.two plastic~ films, Each of the plastics films is a laminate ... .... . . .

.. . . . . . . . . . .

~.~3ti,9~

consisting of a layer of a polyvinyl chloride and a layer of "Saran"
~Registered Trade Mark in the U.K.), a vinylidene chloride/vinyl chloride copolymer. The films are arranged so ~hat ~he seal is formed be~ween ~he two "Saran" layers. An adequate but low strength seal is obtained. To open the seal a tab is provided. This may be a protruding ribbon positioned and sealed between the plastics films in the sèal area or one of the films can have notches or cut-outs along i~s edge in the seal area to provide the tab.
The above described tab, whether in the form of a ribbon or cut-out portion is a nuisance to provide during manufacture and can look rather inelegan~. We have now devised an improved provision for opening the seal.
The present invention provides a package comprising two plastics films enclosing an article and heat-sealed together, at least one of the films having a lower seal strength at its interface with the other, whereby the films may be pulled apart at the seal by hand, and at least one of the i~ films having at its interface with the other and near the margin of the sealJ
, ~ s,4~ re s,~ O ~
a ~6~4~ of a material which adheres to one of the films and not the other and which is infusible at a temperature at which the films are heat-sealable.
The present inven~ion further provides a laminate ~ilm suitable for use as a backing board ilm in Yacuum pac~aging, the laminate film comprising in order:
(a) a substrate layer of unoriented, irradiated polyethylene, oriented polypropylene or oriented polyamide, preferably 10-25 microns thick, ~b) a barrier layer carried on the substrate, especially a barrier layer of olefin (e.g~ ethylene) vinyl alcohol copolymer, and most preferably the hydrolysis product o~ an ethylene-vinyl ace~ate copolymer con~aining 20-95 mole % of vinyl acetate units, at least 85% of which units have been hydrolysed, the preferred thickness of the barrier layer being 3 30 microns, and . - .

- 2B _ ~
.'', ''~' ".

.. . . . . . ,, , . : ~ : ~ : . . , , -(c) a heat-sealable layer, constituting an upper-most layex of the laminate film, of a blend of an ~thylene-vinyl acetate copolymer with a wag, ~specially ~ - .
paraffin wax, thi~ layer having a preferred th~cXness of 3-15 microns.

According to an em~odiment of the pre~ent invention thexe is provided a vacuum package comprising two pla3tic~ films enclo~.ing an axticle and heat-~ealed together, one of the film~ being a bac~ing board laminate film of the invention and the other a thenmofonmed pouch ~ilm adherent by a vacuum-tight heat-seal to the upper-most layer of the backing boaxd film, characteri~ed in that a 9pot ig provided between the heat-~ealed films at their interface ~with each other) and near the margin .
of the seal, the spot being made of a re~inou~ material which under the heat-sealing co~dition~ fu~ible and which adheres to one of the film~ and not the other, whereby the fiLm~ may be pulled apart by beginning the pull at ~he spotO The tenm "spot" denotes any area, of w~atever size or ~hape, covering part o the heat-sealed interface occupied ~y a depo~ît of the ~aid resinous material. Thus, the spot is not necessarily circular but may be, for example, ~haped in the form of an arrow ~ -or letter(s) of the alphabet.
The above-described laminate may be u~ed as a backing board with a thenmoformable pouch film, pre erably a laminate film a~ deqcri~ed hereinafter, for vacuum packa~ing. ..
': .
': -.
,,, " '.

. .. :. . . . ., . ,. . ,~ ., , . , .:

In vacuum ~kin packaging the bo~tom film on which the article i9 placed i~ conveniently tenmed the "backing board". The upper film which fonms the pouch and ~urround~ most of the packaged product i~ tenMed the "thermoformable pouch ~ilm". The thermoformable film is preferabl~ a multi-layer laminate having khe following compo~ition~-~c .
Preferred thickne~s Unoriented polyole~in which may 25 - 250 be irradiated or un irradiated, ionomeric resin, polye~ter, pla~ticised vinyl chloride polymer, polyamide or any other thenmoformable m~terial 2nd layer~ barrier la~e~
Copolymer of an olefin, preferably 3 - 30 ethylene, with vinyl alcohol ~ ~ - .
Any heat-sealable ~hermoplastic, 5 - 75 eOg. polyethylene or an ethylene/
vinyl acetate copolymer The backing koard may be either thermoforma~le or non-ther~ofonmable. The p.referred consti~u~ion of a non-thermoformable backing ~oard i~ a~ followso : :
Preferred thickne~ :

Orien~ed polyamide~ 10 - 25 Oriented polypropylene 15 - ~5 ~;rradiated polyolefin 25 - 50 , .:, . . ... , .. .. ~ , . . .. . , - - .

4~7 Preferred thickness 2nd layer (barrier layer) in microns .
Copolymer of an olefin, preferably 3 - 30 ethylene) with vinyl alcohol ~ a Mixture of (i) a copolymer of ethylene with vinyl acetate and (ii~ a wax, especially a paraffin wax. Layer applied from an 3 - 9 aqueous dispersion e.g. from -"Elvax D-1142"*(this resin is described in a technical bulletin dated 1~71, issued by E.I~ Du Pont de Nemours & Co. (Inc.), Electrochemical~ Dept., Wilmington~
Delaware 198989 U.S.A.).
Layer applied by melt-extrusion lO ~ 20 coating9 e.g. from "XBR"*resin (this resin is described in a technical bulletin No~ A-76939 dated August 1971 also issued by E.I. Du Pont de Nemours & Co.
(Inc.~, Electrochemicals Dept.) A ther~oformable backing board can be made by employing unoriented irradiated polye~hylene (suitable thickness, for example, 10 - 100 microns and preferred thickness 25 - 100 microns) for the substrate layer. This material will of course become oriented by thermoforming, a~d therefore will be orien~ed in the completed package.
The barrier layer o the thermoformable pouch film and/
. .

or backing board preferably has a ~ery low permeability * Trademark '. ~ :' ' 9~7 to oxygen. It is most desirably of an olefin/~inyl alcohol copolymer, obeained by hydrolysis of a copolymer containing 2-60 mole percent of units derived from one or more a -olefins, preferably from ethylene, and 20 to 95 mole percent of units derived from one or more vinyl esters of carbo~ylic acids, preferably from vinyl acetate, the degree of hydrolysis being such that at least 85 percent of the Yinyl ester uni~s are hydrolysed. A bydrolysed ethylene/vinyl acetate copolymer is preferred and is conveniently known as "HEVA"* for short~
The barrier layer of olefin/vinyl alcohol copolymer can be made very thin especially if it ~s co-extruded wlth the other layers. A preferred range of thickness is 5-25 microns.
As an altsrnative to the olefin/vinyl alcohol copolymer the barrier layer could be composed of a vinylidene chloride polymer, especially one containing 65-95% by weight of vinylidene ehloride~derived units and 5-35% by ~eight of ~inyl chloride-derived-'units.
Ni~rile barrier polymers may also be employed, especially "BAREX"*(Standard Oil)~ "PAN"*(Solvay) or 'ILPT-JLRlOO'' *
(I.C.I.).
The thenmoformable pouch f11m most preferably has a substrate of unoriented~ irradiated polyethylene, This material enables the laminar fil~ to undergo deep drawing with little or no wrinkling, i.e. elas~ic recovery is low. A polyvinyl chloridP substrate is much less suitable because the strengkh of the 12minar film is ~-* Trademark ' ' -' ~9~4~

reduced on deep drawing and it does not con~onm well to the shape of the packaged product at the corner~ thereof.
Unorientsd polyamide and polye~ter sub~trates tend to cause more wrinkling in the la~inar film and laminates having a polypropylene ~ub~trate are di~ficult to the~mofonm.
~ particularly preferred embodiment of the thermvfo~mable pouch ~i~m com~rise~o (i) a ~ub~rate layer of unoriented polyethylene which has been cro~s-linked ~y irradiatlon, preferably 25-250 microns thick, (ii) a barrier layer carried on the substrate, especially a ~arrier lay~r of ethylene/vinyl alcohol copolymer, the preferred thickns~ of the barrier layer being 3-30 micronst and (iii~ a heat-~ealable layer comprising polyethylene e.g. of density 0.91 to 0.93 g~/c.c. or an ethylene~ -vinyl acetate copolymer hav~ng a vinyl a~etate content of 0.5-35% preferably 5-1~%1 by weight.
~he thenmofo~mable pouch film could be a ~ingle ~ilm. In general, however, laminate films are preferred because o~ theîr increased toughne~ and greater ~mpenmeability to oxygen.
For the backing ~oard laminate irradiated unoriented polyethylene i3 al~o a preferred material :
for the ~ub~trate layer. The polyethylene may be irradiated with any high energy radiation which cause~
cro~-linking~ e.g~ a~ dèscribed in our United States Paten~ Sp~c~ at~n3,022,543 ~Baird et al). Generally ~ ~6~4~

the dosage o radiation given will be within the range 7 to 75 megareps~ preerably 8-20 m~garep~. GenerallyJ
the film will be obtained by blowing a tube by the "bubble" process and slitting the tube into f i~mO The blowing is insufficient to cauRe orientation of the polyethylene. ~rradiation 3hould be ef~ected after blowingO
Th~ heat-~ealable layer of low ~eal ~trength o~ the non-thermoformable backing board i~ o- a mixture of ethylene/vinyl acetate copolymer with wax. The wax c~ntent of the resin will not u~ually exceed 70% by wei~ht and generally at least 5% will be re~uired. The preferred range of wax content i~ 10-60% by weight. The ~pecific preferred materials "Elvax D-1142" and t'XBR
re~in" confonm to the above definition. In pri~ciple the heat-sealable layer o~ the thenmoformable pouch film and/or a thenmofonmable backing board could be of the ~ame ow seal ~trength matexial, although it is 30m2tImes ~en~itive to the heat used in the thermoforming. For this xea~on it may be de~irable to u~e for the heat~
~aalable layer in the thermoformable pouch film polyethylene or an ethylene/vinyl acetate copolymer having a fairly low vinyl acetate content~ usually 0.5 - 15~ e.g. 12% vinyl acetate unit~ by weight, whereas the ethylene/vinyl acetate copolymer employed in ~he backing board film may have a vinyl acetate content of 20-30% by weight~
~he low sea~ strength heat-~ealable layer o~ :
the ba~king board film need not be thick to provide the - 8 - .

~6~47 ea~y-to-open seal. It is particularly advantageou~ to apply a very thin coating e.g. not more than about 10 microns, for maximum economy of materials. A preferred thic~ne~s i~ 3-9 microns. It i~ indeed unnece~ary to apply a thicker coati~g~ ~ince the coating serves no other purpose than to provide an ea~y~to-open seal. on tha other hand, it i~ not easy to extrude coating5 le~
than 10 microns thick and w~ere the coating is to be fonmed by ~xtrusion ~or conv~ni~n~e in the preparative procedure, a greater thicknes~, e.g. 10-15 micron~ may be required ~or practi~al re~o~.
~he backin~ board ~i}m o~ the invention conveniently consist~ only of the thre~ ~pecified layers . -.:
(~lus the ink spot w~ere desired) but other compatible ..
layers could be included~ Thus, ~or example, the heat- ~ .
~ealable layer of low seal ~trength may const~tute the fourth layer o~ a four-layer laminate backing board~
It will be under~tood that the invention include~ al~o a method of vacuum pac~a~ing in which air is removed ~rom around an article, e~pecially a ~ood~tu~f, to be packaged, a thermoformable pouch film is fonmed to the required shape and the backing boaxd film and the thermo~ormed pou~h film are heat-sealed around the :.
article, characteri~ed in that th~ backing board film ~5 i~ a laminate film according to the inve~tion~
The invention of cQurse fur~her includeq ~he vacuum pa~kages compri~ing the two film~ heat ~ealed ~`
~acuum-tigh~ly around an ar~icle. me initially ~nori~ed material$ will nonmally become oriented in the _ g _ .

~0~47 pac~aging process and so ~ill be oriented in the f~nished pac~ages.
~ 11 the laminates referred to ~erein may be prepared by various met~ods. One met~od is to stick together pre-formed films of the individual materlals (as specified above for the various layers~ with an adhesive. Anot~er is to co-extrude two or more of the layers. Such a proceduxe has the advantage of enabling ver~ thin barrier films to be produced, e.g. a layer of HEVA only 5 microns thick.
The backing board laminate film according to the second aspec~ of ~he invention is not to be confused with the laminates of various prior patent specif~cations in which the heat-sealable layer does not include a wax as a constituent t~ereof and/or in which the substrate is of polyethylene not cross-linked b~ irradiation. Suc~
specifications are United States Patent Specif~cations 3,540,962 CAnza~a et al), 3,560,325 CSogi et al) and 3,595,740 (Cerow). Moreover, in the Gerow patent, for example, the minimum thickness for the heat-sealable layer of the laminate i.5 given as 12.5 microns C0.5 mil~, to be contrasted ~ith the preferred range of 3-9 microns in the present invention.
Any of the well known procedures for effecting the vacuum packaging may be used, e.g. those described in United States Patent Specifications 3,491~504 and 3,6~4,~91.
.
Referring to t~e aspect of the invention in ' ' ' lQ - ~ ~
' ' ' ;" ' ' which a spot is provided at the interface of the film~, the provision of the spot is an invention of general applicability to any plastics films (~ingle ~ilm~ or laminates~ which are to be heat-qealed together, although S i~ is of practical value only when the heat ~eal of the films i~ of relatively low ~treng~h, QO ~hat the film~
~ould be peeled apart b~ hand if only one were able to initiate the peeling operation.
It will be under~tood that any resinou~ material can be u~ed ~or the spot which does not melt at the heat-sealing temperaturP o~ the seal-~orming fi~ms~
~itrocellulose is a generally suitable material ~or application to most pla~tic~ films. In it9 unplastici~ed ~ :
form it doe~ not melt, Thu~, for example, when a ~pot is applied on a film made of ethylene-vlnyl acetate copolymer and a wax and thi~ film is sealed to a polyethylene film, the area of the spot has nil seal .
stren~th, thu~ enabling the customer to begin pulling open the ~eal at that point. . `~
Since most pla~tics ~ilm9 used for packaging are transpaxent, the spot pre~erably containq a dye or pigment 90 that the cu~t~mer can readily identi~y the ar a of ~ery low or nil seal ~trength at which to ~egin to pult open the Qeal. H~wever~ other means of identification, e.g4 printing on another layer, could be u~ed~
The thi~kne~3 of tha spot will depend on its composition, but 1-3 micron~ will u~ually be adequatea I~he ~pot can be applied in a variety o~ ways, e.g. by printing it at fixed intervals on a sheet of the packaging film before it is cut to size ~or packaging or by printin~ it on the film as part of the packaging opexation~ It will normally be applied to the backing board, film, rather than to the pouch film.
EXAMPLES
Several laminate~ were prepared and uBed for subse~uent packaging trials. All the laminate~ were prepared by the conventional adhe~ion lamina~ion, extrusion lamina~ion, ex~ru8~0n coating or co-extru~ion ~oating technique3, except that the "XBR" re~in wa3 applied by extru~ion coating and "Elvax D-1142" applied from ag~eou~ disper~ion.

~hree-layer laminate film~ were prepared having the structure o substrate layer~EEVA barrier layer/heat-~ealable layer. Each of the following film~ wa~ u3ed in turn a~ substrate layer of the above ~tructure Irradiated unoriented 30 to 100 polyethylene Plasticised pol~vinyl chloride 70 to ~00 Polyamide 4 ~ 75 Polye8~er 25 ~o 50 ~ -Ca~t polypropylene 50 ~o 100 High den~ity polyethylene 50 to 100 Ionomer re~in 50 to 125 Ihe "HEVA" ma~erial used was hydroly~ed ethylene- `
vinyl acetate copolymer containing about 20-25 weight ~ ~

: .

' ~
:

- . .~

~6g~4~

percent ethylene units and 75 - 80 weight percent hydrolysed vinyl acetate units, the degree of hydrolysis being 98%.
Each of the following films was used in turn as heat-sealable layer of the above structure.
Layer material Thickness o film _~microns) Low density polyethylene 25 to 50 Ethylene/vinyl acetate 25 to 50 copolymer (~inyl acetate content 12 weight percent) B. Backing Board Besides the materials listed above for the thermo-formable pouch film, the following three-layer laminate films were prepared of structure: substrate layer/HEVA barrier layer/
heat-sealable layer.
Each of the follo~ing films was used in turn as the substra~e layer of the above structure:
Layer material Thickness of film (microns) .

Rigid polyvinyl chloride 20 to 500 Regenerated cellulose 20 to 50 c"cellophane") Oriented polyamide 10 to 25 Oriented polyester 12 to 25 Oriented polypropylene 15 to 25 Unoriented irradiated polyethylene` 25 to 50 The HEVA layer was the same as or the thermoformable pouch film.

Each of the following ilms was used in turn as heat~

sealable layer of the above structure:

Layer material ickness of film Imicrons~
~ .
Polyethylene 20 to 50 -~
:,'''':' - 13 ~

. .

Ethylene/~inyl acetate 25 to 30 copolymer (vinyl acetate content 12 weight percent) Du Pont's "XBR" resin 10 to 20 S Du Pont~ "E~vax 1142"~bout 3 to 10 micron~
~vinyl acetate content of ethylene-vinyl acetate copolymer 28% by weight, applied from aquaouq di~per~ion~
The best packaging results, mo~tly with regard to thermoformability and package appearance, out o~ the 40-S0 different combination~ teqted were obtaine~ using as thermoformable pouch film a ~aminate o~ the following filmsl in ~equence. irradiated unoriented polyethylene, 75 mi~ron~ thick~HEVA, as deqcri~ed above, 17 microns thick/low density polyethylene 40 micronq thic~ and using a~ the backing board a laminate cvnsi~ting of the -~
following ~ilm~ in sequence. bi-oriented pol~propylene, 25 micron~ thick~HEVA~ a~ deqcribed a~ove, 17 microns thick/"Elvax D-1142", as described abvve, 4 gxams/~quare metre, i.e. about 4 micron~ thick. The low density polyethylene film of the thermoformable laminate wa9 sealed to the "Elvax D-1142" layer at ~he interface . .
between ~he ~wo lamina~es. The~e are therefore the specific pr~ferred laminateq of the invention ~or use in vacuum packagingr :
Tests wexe done on la~ratory equipment made according to U.S. Patent Sp cifi~ation~ ~os. 3~491,504 - 14 - :~

.

~69~9~7 assigne~ to E.I~ ~u Pon~ de Nemour~ & Co. and 3~694,991 assigned to W~R. Grace & Co.
With regard to barrier properties and product pre~erva~i~n~ it wa~ found that vacuum ~kin pack~ fr~m these two lam~na~e~ allowed for a medi~m shelf-life bette~ than tha~ o~ traditional thenmo~ormed pack~ made from laminates of ~he polyamide nylon-6 and polyethylene~
The ~omparative oxygen-barrier propertie3 are as follow~.

O~ygen permeabilit~ in CaCc 0 o~ygen at S.T.~./m 24 hour~
atmosphere mea~ured at 23~C. and 50% relative humidity Un~tretche~ L~minate stretched 4~1 ~ during th~rmoforming ~i.e~ so that thickne~
i~ one fourth of the ~L~
Traditional lamin~te of nylon-6 ~50 microns ~0 thick/polyethylene (50 microns thick~ 15 - 20 ~0 - 60 The specific preferred Les~ than 1 Ab~u~ 1 ~hermofo~ma~e pouch laminate of irradiated uno~ien~ed polyethylene/
~EVA~.low den~ity polye~hylene as de~cribed above The 3pecific pre~erred Le~ than 1 - :`
bacXing koard laminate of bi-oriented polypropylene/HEVA/
'~lvax D-1142" ~:
Product pre3er~ation tests were per~ormed on ~ -several packaged foGd~uffs, stored a~ 8~Co and g~0 relative humidity. The following are typical ~hel~
li~e data l~ame quali~y o~ p~oduc~s a~ the end of th~ :
reported periods~: -.

"

.

s~e~ O ~'Y:I
Foodstuff ~bove-described Specific preferred nylon-~/poly- laminates of the ethylene laminate French pate ~t~rrine 20 30 campagnarde) Garli~ ~au~age (Saucisson 25 35 ail) Smo~ed bacon 45 60 With regard to the easy-opening aspect, the result~ obtai~ed were completely sati~f~ctor~ with the specific pre~err~A package accordin~ to the invention.
Its seal stren~th is 300 to 500 gram~ per inch versus min 3500 grams per inch for a seal between ~wo poly-ethylene film9 which does not permit separation o~ the ~ealed films.
The initiation of easy-opening operation, not possible on conventional pack~ u~ed in vacuum sk.in packaging, is obtained by means o~ an ink ~pot (spot .

2~ of pi~ented resin) applied over th~ "Elvax D-1142"
coating on the backin~ ~oard. Over the ink ~pot which i applied onto a ba~ ~ng board area corresponding to a package corner, the ~eal strength i~ nil and package :
oepning can be easily started~
A e~plained above, the spot can be applied either on the roll stock via roto or 1exo printin~ (a) i : -or during the packa~ing opexation with a flexographic multiprint-like system (b). Method (a) was successully tried using "white Multibon~ MB581" ink supplied by ICIFICIS CO~, Milan, Italy. ~his i~k i3 based on ni*rocellulose resin, pigments and ethyl acetate/ethanol ': '-''," .~
,'' :' ... . ,.. . .. . ,. . ~ . . . ..

9~9~7 ~:

solvents. Method (b) was successfully tried in the laboratory by simulating the myltiprint operation with the same kind of ink.
The accompanying drawing illustrates a vacuum skin package according to the invention~ It shows a schematic sectional view taken through a plane perpendicular ~o the plane of the seal of the package.
The package illustrated consists of a backin~
board film 1 and a pouch film 2 thermoformed in vacuo around a piece of cheese 3 (shown on a reduced scale~ and hea~-sealed together at interface 4. The backing board film 1 is a three-layer laminate consisting of a layer 5 of bi-oriented polypropylene 25 microns thick9 a layer 6 of HEVA, as described above, 17 microns thick and a ;~
layer 7 of "Elva~ D-1142" about 4 microns thick. The thermoformed pouch film consists of a layer 8 of low density polyethylene 40 microns thick, a layer 9 of HEVA ~ ~
as described above 17 microns thick and a layer 10 of ~ -irradiated, initially unoriented polyethylene 75 microns thick. All the layers within each laminate are gluéd together with a polyurethane adhesive except layer 7 which is coated from an aqueous dispersion of the copolymer. An ink spot or deposit 11 of the above~described "Multibond MB581" nitrocellulose ink, applied before the films we~e heat-sealed together to form a seal at ~nterface 4, is presen~ at the edge or margin 12 of the package (this margin is shown on a reduced scale relative to the thickness of the films).

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A package comprising two plastics films enclosing an article and heat-sealed together, at least one of the films having a lower seal strength at its interface with the other, whereby the films may be pulled apart at the seal by hand, and at least one of the films having at its interface with the other and near the margin of the seal, a spot of a resinous material which adheres to one of the films and not the other and which is infusible at a temperature at which the films are heat-sealable.

2. A package as claimed in claim 1, one of said films being a backing board laminate comprising in order:
(a) a substrate layer of unoriented, irradiated polyethylene, oriented polypropylene or oriented polyamide, (b) a barrier layer, (c) an uppermost, heat-sealable, layer of a blend of an ethylene-vinyl acetate copolymer with a wax.

3. A package as claimed in claim 2, said backing board laminate comprising in order:
(a) a substrate layer 10-100 microns thick of a material selected from the group consisting of unoriented, irradiated polyethylene, oriented polypropylene and oriented polyamide, (b) an oxygen-barrier layer 3-30 microns thick of the hydrolysis product of an ethylene-vinyl acetate copolymer containing 20-95 mole % of vinyl acetate units, at least 85% of which units have been hydrolysed, and (c) an uppermost, heat-sealable, layer 3-15 microns thick of a blend of 40-90% by weight of an ethylene-vinyl acetate copolymer with 60-10%
by weight of a wax, percentages being by weight of total resin solids in the layer.

4. A package as claimed in claim 2 with the exception that if said substrate layer is of irradiated polyethylene it is oriented and is a thermoformed pouch film, the uppermost layer of the backing board film being adhered to the thermoformed pouch film by a vacuum-tight heat-seal.

5. A vacuum package according to claim 4 wherein the thermoformed pouch film is a laminate film comprising (i) a substrate layer of oriented thermoformed polymeric material, (ii) a barrier layer, and (iii) an uppermost layer of polyethylene or an ethylene-vinyl acetate copolymer having up to 35% by weight of vinyl acetate units, the uppermost layers of the two laminate films being adhered together by the heat-seal.

6. A vacuum package according to claim 5 wherein the thermoformed pouch film comprises in order:
(i) a substrate layer 25-250 microns thick of oriented polyethy-lene which has been cross-linked by irradiation, (ii) an oxygen barrier layer 3-30 microns thick of the hydrolysis product of an ethylene-vinyl acetate copolymer containing 20-95 mole % of vinyl acetate units, at least 85% of which units have been hydrolysed, and (iii) a heat-sealable layer of a material selected from the group consisting of polyethylene of density 0.91 to 0.93 g./c.c. and an ethylene-vinyl acetate copolymer having a vinyl acetate content of 5-35% by weight.

7. A vacuum package according to claims 1, 2 or 3 wherein said deposit contains a visually identifiable dye or pigment.