CA2117574A1

CA2117574A1 - Biaxially and monoaxially oriented polypropylene cold seal release film

Info

Publication number: CA2117574A1
Application number: CA002117574A
Authority: CA
Inventors: Andrew F. Wilkie
Original assignee: Borden Inc
Current assignee: Applied Extrusion Technologies Inc
Priority date: 1994-04-07
Filing date: 1994-08-24
Publication date: 1995-10-08
Also published as: US5482780A; US5489473A

Abstract

The invention presents a new use for modified polypropylene films, wherein the films are cold sealable for heat sensitive packaging operations.

Description

C~2i 1~7~
Title: "BTA~rTATTY AND MON~ rTATTY ~k POLYPROPYLENE COLD SEAL- RELEASE FII,M"

Technical Field The invention relates to a polypropylene laminate or composite film for packaging, said laminate film having i ~ d cold seal release properties. The film comprises a core layer and a cold seal release skin layer . The laminated composite f ilm thus produced can be wound into a roll and when cold-sealed will exhibit excellent release properties.

Ba~ L ~ u-,d Art Cold seal cohesives are generally natural or synthetic rubber latex c ~ which when applied to a f lexible packaging substrate allow the package to be cohesively sealed by pressure and adequate dwell time at ambient temperatures. By "cold seal" herein is meant the sealing or bonding of two film layers to each other at a temperature of less than about 150F. Cold seal latexes are particularly useful in wrapping products which are heat sensitive, such as confectioneries. Cold seal cohesives are generally applied to the inside (cold seal receptive surface) of a film lamination.
They may also be applied to a single web film. In each case, the outer film surface (facing away from the cold seal) must repel adhesion to the cold seal on the inner web in order that the roll stock may be unwound when it is used to wrap the product. In the c~e o~ th~ slng~e web ~il~, th~ ;gr~ c~ col~

. ` CA2 1 1 7574 typically coated with an overlacquer (polyamide type) in order to provide sufficient release from the cold seal, i.e., prevent roll hloclr;n~J. In the case of lamination, the outer web film is generally modified with migratory additives which when sufficiently bloomed to the surface promote low co-efficient of friction (C.O.F. ) for r--h;n~hil ;ty and cold seal release (C.S.R. 1 . It is known from the art that certain migratory additives, particularly amides, coming in intimate contact with the cold seal surface in sufficient quantities, will cause the cold seal cohesive strength to become signif icantly weakened . This rh~n ~ is known in the trade as cold seal deadening.
Biaxially oriented polypropylene (B.O.P.P. ) film by itself does not give adequate cold seal release (C.S.R.) or coefficient of friction (C.O.F. ) and requires the addition of additives to accompligh these objectives. These additives, which are pL~ ;n:~ntly migratory, have two main problems: (1) they must bloom to the film's surface and remain there in order to be consistently effective; and (2) they have a tendency to retard the cold seal cohesive strength. The current industry standard film for this application is a mono-layer B.O.P.P. (homopolymer amide modified) film from Hercules, Inc. designated B523 or B522. In the case of B523, the slip modified film is corona discharge treated to an equal level on both surfaces and, as such, can be used on either side. In the case of B522, the film is corona discharge treated on only one side. The untreated surface is the preferred surface for cold seal release. These films, however, need to be aged for a .,, . . . ., . , .. , . ~ .... , ~ . . . . . ..

period of time after production at a certain t ~ ~lLuLe 80 that the cold seal release and C.O.F. additives sufficiently bloom to the surface for the film to be functional.
The main requirements for a - --Y;~11y oriented or a B.O.P.P.
cold seal release film include: (a) good cold seal release; (b) good anti-blocking to reverse printed inks; (c) good lamination bonds to inks and adhesives on the non-release surface; (d) good and stable optics (i.e., haze/gloss); (e) good and stable C.O.F.;
and (f) minimal/no effect on cold seal cohesiveness in the finished structure .
U.S. Patent No. 4,230,767 issued October 28, 1980 to Isaka, et al. teaches a multi-layer propylene polymer f ilm for sealing packages. However, the Isaka, et al. laminated film is a heat sealable material and Isaka does not teach cold seal release properties .
It is therefore desirable to have a B.O.P.P. film for packaging purposes which is useable in cold sealable applications and which exhibits the requirements described above.

Brief Disclosure of Invention The invention is related to a new use of a laminate film, said new use being cold seal release of said f ilm. The invention is also related to a ~ 11y oriented or a B.O.P.P. film with good to excellent cold seal release properties achieved without the use of migratory additives . The cold seal release f ilm of the present invention is produced by co-extruding a relatively thin surface ,.~

CA2i 1 7~74 skin layer on a body or core layer of homopolymer polypropylene.
The skin layer comprises a low ethylene content polybutylene copolymer at 20-50 weight percent melt blended with an ethylene propylene random copolymer at 50-80 weight percent. This combination of polymers surprisingly provides significantly better cold seal release (with or without corona treatment) than the release obtained using homopolymer polypropylene or ethylene-propylene random copolymers which do not contain surface migratory additives. Oriented polypropylene films having similar composition have been known in the art, but only for use as a heat sealable surface and are only sealable when not corona treated. Corona discharge treating the surface of the present film substantially ~;min;s:hes the heat sealing temperature use range, however, the cold seal release properties remain i YCPl 1 ,~nt. By "heat seal"
herein is meant temperatures generally in excess of 150F, and more commonly in the range of 200 to 300F. For many packaging operations, such t~ al-uLes are unacceptable.
The inventive cold sealable film d~ l L~tes: (a) good to excellent cold seal release; (b) good anti-blocking characteristics; (c) excellent optics; (d) good lamination/ink bonds; and (e) does not poison or deaden the cold seal cohesive properties .
The f ilm-to-f ilm and f ilm-to-metal C . O . F . of the release surface of the materials of the present invention are moderately high and can bene~it from the addition of 0 . 01 to 1. 0 weight percent of a non-migratory inorganic slip agent, such as cross-._,....

C~2i 1 ~574 linked silicone and/or other inorganics. Such non-migratory slip agents useful herein can include talcs ~ syloids ~ glass beads ~
diat c,e~,us earth~ clay and the like. A preferred agent is Raolin clay. To enhance the present invention, relatively minor amounts of migratory slip/anti-block and anti-static agents, such as amides ~ stearates and amines can optionally be added to the skin and/or core layers to facilitate the main film functions. The preferred amounts of such agents is o to about 1 ~ o weight percent.
The use of these additives~ however~ is limited by the propensity to deaden the cold seal in the end use application.
The materials of the present invention have f ive main advantages over the prior art s.o.P~p. films for C.S.R. These include: (lj immediate and permanent cold seal release properties (i.e. ~ no film aging is required) at the time of manufacture; (2) p~ no~tly low C . O . F . due to itS non-migratory skin layer slip package; (3) the release film will not deaden the cold seal because it does not possess migratory additives in type or quantity to do so; (4) the release surface may be corona discharge treated or non-treated~ d~oron~linq on the application, without significantly effecting cold seal release; and (5) the non-treated surface wi readily accept code dating inks.
Brief Descri~tion of Fiqures Fig. 1 presents cold seal block force release values.
Fig. 2 presents cold sea block force release values.

CA21 1 75~4 Detailed DescriPtion The preferred B.O.P.P. cold seal packaging films of the present invention are laminated f ilms comprising at least one cold seal release layer and a cold seal receptive layer. The cold seal release skin layer or layers can have a gauge of 2-10 and comprise 50-80 weight percent of an ethylene-propylene random copolymer (2-6% ethylene). The r~ ;n~n~ 50-20 weight percent of the C.S.R.
skin layer is a polybutylene-ethylene copolymer, wherein the ethylene content is pref erably about O . 5 to 2 . 5 weight percent .
The C.S.R. skin layer also contains a cross-linked silicone at a level of from about 1,000 parts per million up to about 10,000 parts per million. The C.S.R. skin layer also optionally contains kaolin clay at levels of from O up to about 10, 000 parts per million .
The core layer of the f ilms of the present invention can comprise 40 to 200 gauge homopolymer isotactic polypropylene, ethylene-propylene random copolymer, and/or ethylene-propylene block copolymer. In addition, the core layer can optionally contain O to about 5 weight percent of high density polyethylene and various additives, such as amides, including erucamide, b~h~n~mi-l~ or glycerol monostearate, or amines. Other inorganics, calcium stearate, or silicone oil can also be added to the core ,.. . .

C~2i i 7~74 layer, but these are often more useful when used within the skin layer. The core layer, if desired, may also be corona discharge treated, or f lame treated .
Thus, the present invention is directed to a packaging laminate film having Py~pllpnt cold seal release packaging properties, which comprises a core layer selected from the group consisting of homopolymer isotactic polypropylene, ethylene-propylene random copolymer, and ethylene-propylene block copolymer, and a cold seal release skin layer comprising 50 to 80 weight percent of ethylene-propylene random copolymer, wherein the ethylene content of the random copolymer is 2 to 6 weight percent, and wherein the skin layer further comprises 20 to 50 weight percent of a polybutylene-ethylene copolymer, wherein the ethylene content is about 0 . 5 to 2 . 5 wei~ht percent.
The invention is further directed to a method of using a laminate film wherein said film comprises a core layer selected from the group consisting of homopolymer isotactic polypropylene, ethylene-propylene random copolymer, and ethylene-propylene block copolymer, and a cold seal release skin layer comprising 50 to 80 weight percent of ethylene-propylene random copolymer, wherein the ethylene content of the random copolymer is 2 to 6 weight percent, and wherein the skin layer further comprises 20 to 50 weight percent of a polybutylene-ethylene copolymer, wherein the ethylene content is about 0 . 5 to 2 . 5 weight percent.
It is surprising and llnPYrected that co-extruded B.O.P.P.
films previously used exclusively for heat sealing packaging applications can, by the present invention, be utilized in cold seal applications bearing desirable release properties. The co-extruded polypropylene films of the present invention, when utilized in cold seal applications, exhibit excellent cold seal release to commercially applied cold seals. Release is immediate from the date of manufacture, without aging, and remains permanent and consistent through aging. Unlike prior art films, no additive migration is necessary to achieve desired release. In fact, migratory release additives are not required in the present cold seal invention, but are preferred. Additionally, good anti-block to reverse printing inks and good ink adhesions and lamination bond strengths on print or laminating surfaces are obtained by the present invention. In addition, according to the present invention, minimal to zero cold seal deA~ning in the final film is observed .
The cold sealing process of the present invention is preferably achieved at a temperature of less than 150F, and a more preferred temperature range is from 65F to 85F. In the cold sealing process of the present invention, the laminate films described herein are subjected to sufficient ~Les-uL~ and for sufficient time to achieve the desired cold seal. The time needed for ~r~s,uL~ application to achieve cold sealing can be from 0.1 to 1 second, but these times are not limitations of the present invention. In one ~mho~ , the cold seal ~les:.u~e can be from about 20 to 100 psi, and more preferably is from 70 to 90 psi.

~ . .

The resulting cold sealed films of the present invention preferably exhibit a release force of from about zero to about 75 grams/inch, but 5 to 50 grams/inch is preferred.
~AsLE 1 Block Force 1 BOPP Film Type (Cold Seal Release) Comments qms / in . width Hercules 75g B523 25 Competitive fully bloomed/
aged state of the art mono-layer film, corona treated surf ace, at least 2 years old .
70g Proponite CSX 15 Inventive film (no migra-tory additives) non-corona treated surf ace .
75g Proponite AP-1 60 Coextruded, corona treated ethylene propylene copolymer surf ace, not fully bloomed.
75g Proponite AP-1 5 Same as above except fully bloomed, additive package aged 4 months Q 4 0 C .
75g Proponite A so Coextruded slip modified, corona treated surf ace, homopolymer polypropylene CommerclAl ~rhree Hunketeern cold ne~l lamination Block condition~: 2 nec. moisture/amblent temp./100 p~l. load 2 hr~ O-o, ~ lb. Io~
.

TAsLE 2 Block Force (gms/inch width) Commercial Kraft 16 slice American Singles Cold Seal Lamination Film TvPe Aqinq Condition of Film 1 Day 7 Days 1 Day 5 Days Ambient Ambient 40C 40C
Hercules 75g B523 2+ years old) 50 50 60 40 Proponite 75g AP-3 coextruded, corona treated, 120 120 45 15 slip modif ied commercial BOPP
Proponite 75gl inventive film APC-93-lA
non-treated surface 35 25 15 15 treated surface 25 35 25 10 Proponite 75g2 inventive film, homopolymer polypropylene 160 150 140 45 surf ace, treated 5kln LaYer: 60~ 5~ c EP random copolymer core LaYer: 99.19, isotatic homopolymer s . 5 gauge 40~ po~--butylene 69 . s gauge polypropylene 3,000 ~-)m high density polypropylene l,ooo ppQ high density 5, 000 ~ n calcium stearatO polyethylene _, 500 ~ Q U~e~L A~ 3, 500 ppm ULC . L
:,500 ~m kaolin clay 3,500 ppm hoL- ~lo '',000 ~ ~D 31~ cross linked silicone l,ooo ppm erucamide 2skln LaYer: 97~ homopolymer polypropylene core LaYer: 99.2~ i80tatic homopolymer 4 gauge 3~ high den~ity polyethylene 71 gauge polypropylene 2,000 ppm 211 cros8 linked silicone l,ooO pp~D erucamide 3, 000 ppm erucamide 7, 000 ppm 7, 000 ppm ~

~ .

C~21 1 7574 Cold Seal Block Force (~ms/inch width) Kraft 16 slice M~M/
Film Type Aged Am. Singles Mars Twix Comments Condition Cold Seal Cold Seal 100g APC-93-lA Ambient 25 25 Inventive film 2 days surf ace corona treated 100g APC-93-lA Ambient 15 15 Inventive f ilm 2 days surf ace untreated Hercules B523 Ambient 55 20 Competitive 2+ years state of the art, fully aged f ilm corona treated Hercules B523 Ambient 140 70 Competitive (4/91) [alcohol 2+ years state of the wiped lOx to , art f ilm with remove amides ] ~ surf ace amides removed Mobil LCM-W Ambient 150 80 Competitive 1+ year slip f ilm corona treated Mobil LCM-W Ambient 120 75 Competitive [alcohol wiped 1+ year slip film lOx to remove untreated amide ] sur f ace Commercial Ambient 140 - Commerciall front of Kraft <4 mos. acceptable lamination lamination ( Borden 7 5g AP--1) Commercial Ambient - 70 Commercially front of 3 mos. acceptable M&M/Mars lamination Twix Lamina-tion (Borden lOOg AP-1) 11 C~21 1 7574 Examl~le 1: Example 1 is illustrated in Table 1 which shows the excellent cold seal release properties of the f ilms of the preser~t invention. The f ilm, a 70 gauge Borden Proponite f ilm product, contains essentially no migratory additives associated with cold seal release. This is cold seal laminated to a commercial M~M/Mars Three Musketeers film, which, as such, is the prime example of the skin layer composition of ethylene propylene copolymer and ethylene polybutylene copolymer showing its inventive C.S.R. property. This laminated composite film is a B.O.P.P.
product with the surface being non-treated and heat sealable. The state of the art film, a conventional 75 gauge Hercules B523, also shows excellent C.S.R in Table 1. However, the Hercules film represents the optimum performance for a film containing a fully matured migratory additive package. The Hercules f ilm showed a block force reading of cold seal release of 25 grams per inch width, while a standard 70 gauge Borden Proponite C.S.X. film surface, non-corona treated, and comprising no migratory additives, exhibited a more desirable block force cold seal release measurement of only 15 grams per inch width. Further, a 75 gauge Borden Proponite AP-l film with a fully bloomed additive package showed excellent C.S.R. value of 5 grams per inch width, a value clearly superior to the value exhibited by the Hercules B523 film.
Also included in Table 1 is a standard slip-modified, corona treated homopolymer 75 gauge Borden Proponite A film. This film demonstrated the deficiency of a homopolymer surface with an insufficient migratory additive package for C.S.R. yielding an .. . . . . .

CA2i i75/4 undesirably high block force cold seal release value of 90 grams per inch width.

ExamPle 2: Example 2, illustrated in ~able 2, shows data on a production film specially produced for cold seal applications.
The example shows that the B . o . P . P . f ilm has immediately good C.S.R. without the need for migratory additives to be bloomed to the surface. The conventional Hercules B523 f ilm shows moderately good C.S.R. to a commercial cold seal lamination which does not improve upon heat aging the f ilm. This demonstrates that the competitive Hercules film has a fully mature additive package. The cold seal inventive film, a Borden Proponite 75 gauge APC-93-lA, is presented in Table 2 for both corona-treated and non-corona-treated samples . These f ilms also contain an additional, but not essential, migratory additive package. However, the cold seal f ilms of the present invention demonstrate ~ood to excellent C . S . R.
immediately without the need for migratory additives to be fully bloomed. There is a slight improvement in C.S.R. with heat aging.
Both treated and non-treated surfaces are shown to be effective for cold sealing in this example.

Example 3: Example 3, illustrated in Table 3, shows the results of cold seal block force testing of films of a different gauge . The f ilms of Example 3 include the inventive f ilm formulation APC-93-lA and two different commercial cold seal films.
Th~ C . ~ . R . surf ~ce ~f the inventlve f i lm i5 aorona -t~e/l ~ to a CA~
miniml~m level and shows excellent immediate C.S.R. to both cold seals, that is, a value of 25 grams per inch on the cold seal test and 25 grams per inch on the M&M/Mars Twix cold seal test. The same f ilm without corona treatment exhibited even lower cold seal block f orce values of 15 grams per inch f or both tests . The Hercules B523 film also shows good to excellent C.S.R. to both cold seals, however, after wiping the film surface with a solvent to substantially remove the bloomed surface additives, the C.S.R.
properties of the Hercules B523 film became significantly impaired.
This condition represents Hercules B523 when it is not fully aged.
It also represents a case where the surface additives are driven back into the film through processing thereby negatively affecting both C.S.R. and C.O.F. This phenomenon has been reported by converters and is considered a negative feature of the prior art film.

Examl~le 4: Example 4 , illustrated in Figure 1, compared B . o . P . P . f ilms where the concentration of the ethylene propylene random copolymer was varied from 60-90 weight percent and the ethylene polybutylene copolymer component was varied from 10-40 weight percent. These inventive films contained no additives of any kind and the film surfaces were not corona treated. Two commercial cold sealed film structures were used to measure C.S.R.
The results indicated that good C.S.R. was achieved when the level of polybutylene copolymer was greater than 10 weight percent and preferably gre~ter than Zo yeight perc~nt. Specifically, ~r a 75 - - CA ? i i 7~ :74 gau~e B.O.P.P. film of the present invention with 60 weight percent ethylene propylene random polymer and 40 weight percent polybutylene copolymer, a C.S.I~. value of 50 grams per inch cold seal block force was obtained. On a 60 gauge B.O.P.P. film of the present invention, also bearing 60 weight percent ethylene propylene random copolymer and 40 weight percent polybutylene copolymer, a cold seal block force measurement of about 30 grams per inch was obtained. These values were obtained for the Three Musketeer cold seal test. Cold seal block force values for the preceding films were each 20 grams per inch. These values compare quite favorably to, or are better than, the values obtained for Hercules B523.

Example 5: Example 5, illustrated in Figure ~, demonstrated cold seal block force test mea~uL~ Ls in grams per inch comparing Hercules B523 film to two cold seal films of the present invention, 75 gauge and 60 gauge. The tests were run on 3 l~usketeers cold seal and Kraft Light cold seal. The Hercules prior art film had a cold seal test value of 30 grams per inch, as did the inventive 60 gauge film. The Kraft Light cold seal test value for both 75 and 60 gauge inventive f ilm was signif icantly lower, exhibiting values of 20 grams per inch.

While certain preferred embodiments of the present invention have been disclosed in detail, it is to be understood that various modifications may be adapted without departing from the spirit of th~ lnve ~ion or ~oope o~ th- ~o l~inq

Claims

1. A packaging laminate film having improved cold seal release properties, which comprises a core layer selected from the group consisting of homopolymer isotactic polypropylene, ethylene-propylene random copolymer, and ethylene-propylene block copolymer, and a cold seal release skin layer comprising 50 to 80 weight percent of ethylene-propylene random copolymer, wherein the ethylene content of the random copolymer is 2 to 6 weight percent, and wherein the skin layer further comprises 20 to 50 weight percent of a polybutylene-ethylene copolymer, wherein the ethylene content is about 0.5 to 2.5 weight percent.

2. The packaging film of claim 1 which can be cold sealed at a temperature of less than about 150° F.

3. The packaging film of claim 1 wherein the film is a monoaxially oriented film.

4. The packaging film of claim 1 wherein the film is a biaxially oriented film.

5. The packaging film of claim 1 wherein at least one side of the film has been corona discharge treated.

6. The packaging film of claim 1 further comprising a non-migratory inorganic slip agent in the skin layer in an amount of 0.1 to 1.0 weight percent.

7. The packaging film of claim 1, said film being an extruded film which exhibits immediate and permanent cold seal release properties without aging of said film.

8. The packaging film of claim 1, wherein the skin layer further comprises Kaolin clay at levels up to about 10,000 parts per million.

9. A method of using a laminate film wherein said film comprises a core layer selected from the group consisting of homopolymer isotactic polypropylene, ethylene-propylene random copolymer, and ethylene-propylene block copolymer, and a cold seal release skin layer comprising 50 to 80 weight percent of ethylene-propylene random copolymer, wherein the ethylene content of the random copolymer is 2 to 6 weight percent, and wherein the skin layer further comprises 20 to 50 weight percent of a polybutylene-ethylene copolymer, wherein the ethylene content is about 0.5 weight percent, wherein said method comprises the steps:

(a) positioning the film around an article to be packaged, whereby the film at least partially overlaps itself;

(b) applying sufficient pressure to the overlapped portion of the film at a temperature below about 150° F to thereby seal said film to itself.

10. The method of claim 1 whereby the resulting film when contacted with a cold seal under pressure exhibits a release force of 5 to 50 grams per inch.

11. The method of claim 9 wherein the film is a monoaxially oriented film.

12. The method of claim 9 wherein the film is a biaxially oriented film.

13. The method of claim 9 wherein at least one side of the film has been corona discharge treated.

14. The method of claim 9 wherein the film further comprises a non-migratory inorganic slip agent in an amount of 0.1 to 1.0 weight percent.

15. The method of claim 9 wherein the film is an extruded biaxially oriented film which exhibits immediate and permanent cold seal release properties without aging of the film.

16. The method of claim 9 wherein the skin layer of the film further comprises Kaolin clay at levels up to about 10,000 parts per million.

17. The method of claim 9 wherein the pressure applied to cold seal the film is from 60 to 100 psi.

18. The method of claim 9 wherein the film does not contain migratory release additives.

19. The method of claim 9 wherein zero cold seal deadening in the final film is observed.