CA2045136C

CA2045136C - Resealable packaging material

Info

Publication number: CA2045136C
Application number: CA002045136A
Authority: CA
Inventors: Steven J. Straus; Edward J. Zuscik; William Ben Bower; Apostol Yanidis
Original assignee: Printpack Illinois Inc
Current assignee: Printpack Illinois Inc
Priority date: 1989-01-09
Filing date: 1990-01-05
Publication date: 2002-03-19
Anticipated expiration: 2010-01-05
Also published as: BR9006997A; AU5041390A; JP2938186B2; EP0453509A4; KR910700144A; AU630838B2; CA2045136A1; JPH04502588A; EP0453509A1; WO1990007427A1

Abstract

A machinable flexible packaging material capable of forming a seal that is reclosable at ambient temperature is made by applying a tacky adhesive and a thin skin layer onto a substrate, such as polyester film. Being thin, the skin layer fractures when the seal is opened, thereby allowing the adhesive to form a resealable interface at the seal area. However, since the adhesive is encapsulated in the material initially, the material may be used like other packaging materials in form, fill and seal machines.

Description

2? PCT/US90/00101 2 ~

Resealable Packaq~inq 7~aterial Flexible packaging materials comprising layers of thermoplastic polymers are used to package a variety of products, particu~arly food. For example, cereals, potato chips and other snack foods are packaged in such materials.
The packages are formed in packaging machines in which the material is sealed to itself by heated sealing jaws.
However, when a package is opened by pulling apart one of the heat seals, nolmally at the top of the package, ~he heat seal is not reasealable without the application of heat as well as pressure. This invention pro~ides a flexible packaging material that is resealable by the appli~ation of pressure alone at the a~ea of a ~eal formed by a packaging machine.
The material of this invention comprise~ a substrate laye~, a layer of tacky adhesive adjacent the substrate layer, and a skin layer covering the layer of tacky adhesive. When a ~eal formed by sealing the packaging mate~ial tc itself i5 pulled apart, the tacky adhesive is exposed at the surface of the seal area, thereby permitting the material to be re~ealed to itself at the area of the seal by the application of manual pres~ure alone ~o reclose the package.
The substrate layer may comprise any material ~hat is conYentionally used fo~ flexible packaging aeplications, such as pla~tic film, me~al foil, paper or a combination thereof.
Plastic film substrate~ may be ~ingle layer or multilayer and may be oriented or unoriented. The film can be polyamide, polye~ter, or olefin polymer or copolymer. Suitable substrates include poly~ethylene terephthalate~ film, paper, oriented polypropylene film, and nonwoven ma~erials.
As used herein, the term "tacky adhe~ive~ ~efers eo an adhesive that, in the practice of this invention, forms a re~ealable bond by the application of manual pressure alone at room temperature. The re~eal strength is preferably between about 0.5 and 4 pounds per inch (0.1 and 0.7 kg/cm), morç preferably between about one and two pounds per inch (0.18 and 0.36 kg/cm). Such adhe~ives may include cold seals W0~0/07427 PCT/US90/00101 ~ ~ L~ ~ 6 --2--and tacky lacquers. Cold seals are formed from aqueous latex emulsions of an elastomer. The tacky adhesive is preferably a pressure sensitive adhesive, which is a blend o~ an elastomer and a tackifier. The pressure sensitive adhesive preferably comprises ~rom about 40 to 80 percent of a thermoplastic elastome~, 20 to 60 percent of a tackifier, and up to about 15 percent of other components, ~uch as a fragrance and conventional additives, such as an antioxidant and an oil.
~ hen the adhesive layer in the packaging material contains a fragrance, the fragrance is not appreciably noticeable until the material is formed into a package and the package is opened because the fragrance i~ e~sentially encapsulated in the packaging material. When the package is opened, the fragrance is liberated, thereby enhancing the appeal of the product in the package. Best re~ults are obtained u~ing an oil-based fragrance that is absorbed by the elastomer component of the adhesive.
Particularly suitable elastomers include styrene copolymers, such as styrene-butadiene copolymer~ sold under tha trademark Stereon by Fire~tone and styrene-isoprene copolymer~ sold under the trademark Kraton by Shell Chemical Company. Other suitable ela~tomers include polyurethane and ethylene-vinyl acetate copolymers having a vinyl acetate content of about 18 percent or higher.
The tackifier may be any tackifier conventionally u~ed with ela6tomers to for~ pres~ure ~en~itiYe adhesives.
Suitable tacki~ier~ include terpene resins, such as re~ins sold under the trademark Zonatac by Arizona Chemical Company, and petroleum hydrocarbon re~ing, ~uch as re~ins sold under the trademark Es~orez by Exxon Chemical Company.
The pres~ure ~en~itive adhesive is pre~erably ~oextrudable. The coextrudabili~y of the pre~sure sensitive adhe~ive may be enhanced by utilizing a thermoplastic elastomer having a low melt index and by increa~ing the proportion of the elasto~er in the adhesive. The melt index of the adhesive iB preferably le~s than 30, more preferably lé~s than 10, for coextrucion wi~h conventional polyolefins.

W090/07427 PCT/US9~/00101 2 ~

The pressure sen~itive adhesive, o~ o~her tacky adhesive, may be applied to the substrate by methods o~her than extrusion, such as adhesive lamination, cotogravure, flexographic or silk screen printing, Meyer rod, spray coating, charged particle coating, or vacuum deposition. The adhesive may be applied as 100 percent solids or by using water or a solvent as a vehicle.
Since the tacky adhesive is required only in the area where the packa~e is to be opened, the adhesive may be coated onto the substrate in register only where it is required.
The skin layer may be similarly applied in register over the adhesive alone, such as by printing the adhesive layer and the skin layer onto the substrate sequentially u~ing a central impression press. However, the ~kin layer may be applied to other surface areas as well, including the entire surface of the packaging material.
The tacky adhe~ive may also be applied to the sub~trate by tran~ferring it from a carrying medium. This technique i~
commonly u~ed to transfer a thin layer of metal, ~uch as that formed by vapor deposition, from one substate to another in the manufac~ure of metallized packaging materials. The skin layer may al~o be tran~ferred with the adhesive by this technique. The carrying medium i~ ereferably paper coated with a silicone relea~e agent. The paper substrate may be recycled and may be an endless belt in a continuous proces~.
If the paper bearing the adhesive layer is rolled up, it is preferably coated with the release agent on both side~ ~o prevent blocking.
The thicknes6 of the layer of tacky adhe~ive is preferably between about 0.1 and 2.0 mil (2.5 and 51 microns)~ more pre~erably between about 0.3 and 0.7 mil (7.6 and 18 microns).
The skin layer perform~ two critical functions. First, it ~roduces a coefficient of friction on the surface of the packaging material such that the material i8 machinable in a conventional packaging machineO Second, the s~in layer become~ broken ~o that when the ~eal i8 pulled apart, the tacky adhe~ive i8 exposed at the surface of- the ~eal. There W~90/07427 PCT/US90/00101 are two mechanisms by which this occurs. In one mechanism (First Mechanism), the skin layer is disrupted by the shearing action of the sealing jaws in the packaging machine such that the tacky adhesive becomes present at the intec~ace where the material is sealed to itself. The ~kin layer i5 preferably disrupted over ~o~t of the area of the seal. By thi~ mechanism, it is po~sible to produce the initial seal from ehe tacky adhesive alone, thereby obviating the need to apply heat to the sealing jaws. In the other mechanism (Second Mechanism), the skin layer is heat ~ealed to it~elf, preferably using flat sealing jaws, and remains substantially intact until it ruptures through to the adhesive layer when force is initially applied to pull the heat seal apart. The ~kin layer then delaminates from the sub~trate layer when additional force is applied until the end of ~he &eal i~
reached. The delamination may occur between the ~kin layer and the adhesive layer, ba~ween ehe adhesive layer and the substrate layer, or, preferably, in a plane lying in the adhesive layer so the adhe~ive is present on both surfaces of the seal area after the ~eal ha6 been pulled apart. The latter type of delamination i5 promoted by heating both fiealing jaws when the heat ~eal i~ formed.
When the Second ~echani~m i~ employed, the tacky adhesive i~ preferably an extrudable pres~ure ~ensitve adhe~ive which is coaxtruded with the skin layer, either with the ~ubstra~e layer or onto the fiubstrate layer. When the ~ub~trate layer i~ coextruded with ehe skin layer and the adhe6ive layer, either blown film coextru~ion or ca~t film ~oextru~ion may be employed. The extrudate may contain multiple layer~ of the pres~ure sensitive adhe~i~e.
The skin layer may be a layer of a thermoel2stio film-forming polymer which i~ conventionally u~ed a~ a heat ~eal layer in packaging materials. The polymer preferably has a tensile strength such ~hat at the desired thicknesc of the Rkin layer the heat 6eal i8 easy to open, preferably by a force between about 0.4 and 0.9 kg/cm. The polymer also preferably has a low elongation at ~reak, preferably less than 400 percent. The polymes is preferably a copolymer of PCT/U590/~0101 W~0/07~27 5 ~ '~ 6 s ethylene and an ethylenically unsaturated comonomer selected fro~ the group con~isting of carboxylic acids and esters, salts and anhydrides thereof. Example~ of such comonomers include acrylic acid, methacrylic acid, methyl acrylate, ethyl acrylate, methyl methacrylate, vinyl acetate, and maleic anhydride. Such copolymers are well known commercially available polymer~ commonly used to form heat ~eal layers in flexible packaging materials. Other polymers, ~uch as polyolef in8, may be blended with the copolymer in a minor amount. The copolymer is preferably an ionomer. The term ionomer refers to a ~ilm-forming copolymer of an olefin and an ethylenically un~aturated monocarboxylic acid wherein at least 10% of the carboxylic acid yroup~ are neutralized by a metal ion, preferably zinc. The olefin ha~ the formula RCH=CH2 where R is hydrogen or an alkyl group ba~ing up to 8 carbon atoms. The olefin i~ preferably ethylene. The carboxylic acid i~ preferably acrylic acid, methacrylic acid, or methylmethacrylic acid. The proportion of olefin unit~ in the ionomer i8 at lea~t 50, preferably at least 80, mole percent. Suitable ionomer~ are described in U.S. Patent No.

3,355,319 and are available commercially under the trademark Surlyn.
The skin layer i8 preferably relatively thin, preferably ~hinner than the heat seal layer of conventional packaging material~ 80 the layer -fractures when the sealing jaw~ are applied or when the seal i~ pulled apart. The thicknes~ of the skin layer i8 preferably between about 0.05 and 0.5 mil (1.3 and 13 micron~), more preferably between about 0.075 and 0.2 mil (1.9 and 5 micron~).
Nhen the First Mechanism i~ employed, the ~kin layer may be any substance that produce~ the desired coefficient of ~riction and that is sufficiently disrupted by the sealing ~aw~ that the adhesive becomes present at the ~eal interface. The substance may even be finely divided solid particulate matter (dust), such as talc or starch particles.
The particles may be applied to the ad~e~ive layer in accordance with ~onventional ~ethods, such as by using Oxydry (trademar~) apparatus.

WO90/07~27 PCT~S90/00101 2 ~L.I~lt~ ~ -6-The ~kin lay~r and the layer o~ pressure sen~itive adhe~ive may be coextruded together generally in accordance with conventional coextrusion coating methods to form the packaging maeerial of the invention. The components of the ~ressure sensitive adhesive are preferably maintained under an inert atmosphere, such as nitrogen, to minimize oxidation at the extru~ion temperature, which produces degradation and odor.
The packaging material may be sealed in accordance wi~h methods typically used for sealing flexible packaging ma~erials in packaging machines. When the First Mechani~m is employed, the packaging material is preferably sealed by means of sealing jaws that impart shear to the skin layer, ~uch a~ desc~ibed in U.S. Patent 4,5B~,555. The shear promotes fracture of the skin layer and exposure of the tacky adhesive when the ~eal i~ opened. When the Fir~t Mechanism is employed, the temperature of the sealing jaws is preferably less than the melting point of the skin layer. In some embodiment~, the jaws may be at ambient temperature so the material may effectively be ~cold sealed" rather than heat sealed. When the Second Mechanism i~ employed, the skin layer is heat sealed to itself at a conventional temperature of about 230F (110C). In either mechani~m, the seal has a bond strength that is sufficient to en~ure the package does not open during normal handling. This bond ~trength is preferably at least aboue two pounds per inch (0.4 kg~cm).
However, the seal i8 alBo preferably easy to open by being pulled apart manually, so the bond ~trength is preferably les6 than about five pounds per inch (0.9 kg/cm).
Un~il the skin layer i~ fractured, the coefficient of friction of the ~kin layer is determined by the subs~ance forming the ~kin layer. Accordingly, the material of thi6 invention may be u~ed in conventional packaging machine~, such as form, fill and seal machines, in the same manner as conventional flexible packaging ma~erials. A material having a tacky adhe~ive- on its surface could not be used in such machines, of course, becau~e of the high coefficient of friction of the adhe6ive. The ability of a packaging WO90/07.J27 PCT/U~90/00101 2 ~

matsrial to be used in packaqing ma~hine~ is referred to in the art as machinability. ~he material of this in~ention has good machinability. In pre~erred embodiments, each surface of ~he mate~ial has a coe~ficient of ~iction against ~tainless steel which is within the preferred range for machinability, which i6 from about 0.1 to 0.5.

Example 1 A laminate was formed by coextruding a skin layer and a pressure sensitive adhesive onto a substrate. The sub6trate was a polytethylene terephthalate) film haYing a thickne~s of 12.2 microns (Dupont Mylar). The skin layer, which was extruded at 218C, consisted of polyEtyrene (Dow Styron 625). The pressure senEitive adhe~ive, which was extruded at about 163C, consisted of 68 percent ~tyrene-isoprene-~tyrene block copolymer (Shell Kraton D-1107~, 30 percent aliphatic pe~soleum hydrocarbon re~in (Exxon Escorez 1310), and 2 percent phenolic antioxidant (Ciba-Geigy Irganox 1010). The hydrocarbon re6in was injected into the ba~rel of the extruder. The thick~esR of the polyztyrene laye~ wa~ about 0.1 mil (2.5 microns) and the thickne~s of the layer of pressure ~encitive adhe~ive wa~ about 0.5 mil (12.7 microns). The laminate was heat sealed to itself u~i~g the heat seal jaws described in U.S. Patent 4,582,555 by applying a pressure of BO p8i ( 5.6 kg/cm) for two ~econd~ at a temperature of 110F. A for~e of 3.3 pounds per in~h width (0.59 kg/cm) wa~ required ~o open the heat ~eal. The opened heat seal wa6 then ~esealed, opened and resealed again twenty time6 by applying a pre~sure of 14 p~i (0.99 kg/sq.em) at room temperature for les~ than 0.1 secoAd. The a~erage regeal stre~gth was 1.03 pound~ per inch width ~184 gram~cm).
~ his example illustrate~ thae the paekaging material of this in~ention is e~peGially suitable for packaging cereal~
and snack foods in packages that may be easily resealed after each intermittent use.

.

PCT~US90/0010l 2 ~ 6 8-Example 2 A lamina~e was formed by applying a coa~ing of molten pressure sensitive adhesive onto a substrate, cooling the adhe6ive, and then applying a coating of polyvinylidene chloride (PVDC) emulsion on top of the adhesive. The coatings were applied using a Meyer rod. The ~ub~trate was poly(ethylene terephthalate) film ha~ing a thickness of one mil (25 microns). The pres~ure sensitive adhesive wa~ heated to about 150C and applied at a coating weight of about 14 pounds per ream (23 grams per square meter). The pres~ure sensitive adhe~ive, which is commercially available from Findley Adhesives, has a vi~co~ity at 150C of about 42,000 centipoise. The PVDC was applied at a coating weight of about 4 pounds per ream t7 gram~ per square me~er) and dried at about 93C. The lamina~e was t~en sealed to itself using sealing jaws that impart high shear. The jaw~ were heated to 71C and applied at a pres6ure of 80 p~i (5.6 kg/sq.~m). A
force of ~.2 pounds per ineh (0.57 kg/cm) was required to open the seal. The opened 6eal was then re~ealed, opened and re~ealed 20 time~ by applying a pressure of 14 psi tO.99 kg/6q . cm) at room temperature for le~ than one second . The average re~eal strength wa~ 0.53 pound~ pPr inch (95 grams per cm).
The Example illu~rate~ the preparation of packaging material of thi~ invention employing the First Mechani~m.
The material may be u6ed to package cereals and snack foods in packages that may be easily re~ealed after each intermittent use.

ExamPle_3 A laminate was formed by coextruding a ~kin layer and a layer of pre~sure sensitive adhegive onto a film of high density polyethylene having a thieknezs of two mils (51 microns). The skin layer, which wa~ extruded at 230C, consist2d of an ionomer available co~mercially from Dupont.
The pres~ure sensitive adhesive, which wa~ extruded at 196C, i8 a~ailab-le commercially ~rom Findley Adhesive~. The W090/0~427 PCT/US90/0010l 2 9 ~

adhesive ha~ a viscosity of over one million centipoises at 177C. The basis weight of the adhesive was S pounds per ream (8 grams per square meter). The basis weight of the skin layer was 4 pounds per ream (7 grams per ~quare meter).
The laminate was then heat sealed to itself using flat 6ealing jaw~ applied at a pressure of 40 psi (2.~ kg/cm) for one second, with one jaw being at room temperature and the other jaw at 116C. A force of 2.4 pounds per inch (0.43 kg/cm) was required to open the heat seal. The opened heat ~eal was then resealed, opened and resealed again 20 times by applying a pres~ure of 14 p8i (O. 99 kg/sq.cm) at room temperature for less than one second. The average reseal strength was 1.03 pounds per inch (184 grams per cm).
Thi Example illustrate~ the preparation of packaging material of this inven~ion employlng the Second Mechanism.
The material may be u~ed to package cereals and snack foods in packages that may be easily resealed after each intermi~ent use.

Claims

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

1. A flexible packaging material comprising a substrate layer, a layer of tacky adhesive adjacent the substrate layer, and a skin layer covering the layer of tacky adhesive, the skin layer and the layer of tacky adhesive being coextruded together either with the substrate layer or onto the substrate layer, each surface of the packaging material having a coefficient of friction such that the material is machinable in a packaging machine, the material being capable of being sealed to itself by the sealing jaws of the packaging machine to form a package having a seal which has an initial bond strength such that the package remains closed during normal handling but which can be readily pulled apart manually to open the package, the skin layer being fractured when the seal is pulled apart such that the tacky adhesive is exposed at the surface of the seal area, whereby the packaging material may be resealed to itself at the area of the seal by the application of manual pressure alone to reclose the package.

2. The packaging material of claim 1, wherein the coefficient of friction to stainless steel is between about 0.1 and 0.5.

3. The packaging material of claim 1, wherein the tacky adhesive contains a fragrance.

4. The packaging material of claim 1, wherein the tacky adhesive is a pressure sensitive adhesive.

5. The packaging material of claim 4, wherein the skin layer is fractured by rupturing through to the adhesive layer and then delaminating from the substrate layer when the seal is pulled apart, thereby exposing the pressure sensitive adhesive at the seal area.

6. The packaging material of claim 5, wherein the pressure sensitive adhesive comprises from about 40 to 80 percent of a thermoplastic elastomer and from about 20 to 60 percent of a tackifier.

7. The packaging material of claim 5, wherein the skin layer is a layer of a thermoplastic film-forming polymer capable of forming a heat seal when the jaws have a temperature of 110°C.

8. The packaging material of claim 7, whrein the skin layer comprises an ionomer.

9. The packaging material of claim 7, wherein the heat seal has an initial bond strength between about 0.4 and about 0.9 kg/cm.

10. The packaging material of claim 9, wherein the strength of the resealable bond formed by the pressure sensitive adhesive is between about 0.1 and about 0.7 kg/cm.

11. The packaging material of claim 7, wherein the thickness of the layer of pressure sensitive adhesive is between about 2.5 and about 51 microns.

12. The packaging material of claim 1, wherein the skin layer is fractured by the sealing jaws such that the jaws disrupt the skin layer such that the tacky adhesive becomes present at the interface where the material is sealed to itself.

13. The packaging material of claim 12, wherein the temperature of the sealing jaws is less than the melting point of the substance forming the skin layer.

14. The packaging material of claim 12, wherein the seal has an initial bond strength between about 0.4 and about 0.9 kg/cm.

15. The packaging material of claim 14, wherein the strength of the resealable bond formed by the tacky adhesive is between about 0.1 and about 0.7 kg/cm.

16. The packaging material of claim 12, wherein the skin layer consists of finely divided particulate matter.

17. The packaging material of claim 16, wherein the particulate matter is talc or starch particles.

18. A method of forming the packaging material of claim 12, comprising transferring the layer of adhesive to the substrate from a carrying medium.

19. The method of claim 18, wherein the skin layer is transferred to the substrate with the adhesive and the carrying medium is paper coated with a silicone release agent.