CA1068431A

CA1068431A - Self-sealing films

Info

Publication number: CA1068431A
Application number: CA210,392A
Authority: CA
Inventors: Mario Borsatti; William J. Busby; Frederico Argurio
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1973-10-01
Filing date: 1974-09-30
Publication date: 1979-12-18
Also published as: BE820606A; NO142222C; SE7411142L; NO743519L; NL7412710A; US3932563A; DK517374A; SE399902B; JPS5077447A; NO142222B; FI274574A; DE2445608A1; GB1472059A; LU71023A1; IT1022423B; FR2246591B1; FR2246591A1

Abstract

ABSTRACT

Compositions suitable for fabricating into a self-sealing film comprise (i) 90-99.95 wt.% of a copolymer of ethylene and a vinyl (or hydrocarbyl substituted vinyl) ester of a C1-C30 monocarboxylic acid, (ii) either 0.05 to 10 wt.% of a hydrocarbon resin, or 0.05 to 5 wt.%
of polybutylene or polyisobutylene, or a combination of 0.05 to 10 wt.%
of the hydrocarbon resin and 0.05 to 5 wt.% of polybutylene or polyisobutylene, provided the combined weight of the hydrocarbon resin and polybutylene or polyisobutylene is not more than 10 wt.% and (iii) 0.01 to 3.0% by weight based on the combined weight of (i) and (ii) of a partial carboxylic acid ester of a polyol, e.g. sorbitan monolaurate.

Description

Thi8 invention relates to self-sealing packaging films.
Self-sealing packaging films are known ant are used in shQps for example, for wrapping and display of food in refrigerated display unito.
~ ariou~ materials havo beèn used for such films, the most common being PVC and polyolefin. However these films suffer from various dioadvantage~
and have not been completely satisfactory in use.
W8 have now discovered a compositon which when made into film has good antifog, roll wind-off and plate out properties. According to this invention a composition suitable for fabricating into a self-sealing film comprises (i) 90-99.95 wto% of a copolymer of ethylene and a vinyl (or hydrocarbyl substituted vinyl) Qster of Cl-C30 monocarboxylic acid, (ii) either 0.05 to 10 wt.% of a hydrocarbon resin, or 0.05 to 5 wt.% of polybutylene or polyi90-butylene, or a combination of 0.05 to 10 wt.% of the hydrocarbon resin and 0.05 to 5 wt.X of poIybutylene or polyisobutylene, provided the combined weight of hydrocarbon resin and polybutylene or polyisobutylene is not more than 10 wto%, and (iii~ 0.01 to 300% by weight based on the combined weight of (i) and (ii) of a partial carboxylic acid Qster of a polyol.
Concerning the ethylene copolymer the carboxylic acid is preferably aliphatic, and saturated and preferably monocarboxylic. Thus one may use vinyl propionate, vinyl hexoate, vinyl octaoate, vinyl dodecanoate, vinyl behenate, or isopropenyl acetateO The particularly preferred ester is vinyl acetate.
The rasulting copolymer should preferably contain from 85-97 wto%, eOgO 91 to 95 wt.% of ethylene.

~06843~
One method of prepsring the copolymers involves feeding the monomers into a tubular reactor which has been previously purged with nitrogenO A
small amount of oxygen, usually 00005 to 0005 wt~% based on the weight of ethylene i8 also introduced into the reactor~ Alternatively a peroxide initiator, eOgO di-t-butyl peroxide, or a mixture of peroxide initiator and oxygen may be introduced înto the reactor in place of oxygen alone. A
solvant teOg. benz~ne, water, sa~urated hydrocarbons, methanol) may also bo employed in the reactionO The prossure is maintained between 60 and 2700 stmospheres (900 and 40,000 pOs.iOgO), proferably between 135 and 2000 atmospheres t2,000 and 30,000 p~s~i.gO). The temperature should be maintained between 40C and 300C, preferably between tOC and 250Co Another method of preparing the copolymers i8 via a batch proceosO Such a process requires a solvQnt for the reactants, the solvent being for examplè
toluene or hexane~ The preferred solvent however i8 benzeneO Tho reaction initiator may be any pero~y compound, preferably di-t-butyl peroxideO The temperature of the pQlymeri~stion reaction is dependent upon the particular peroxide initiator employed and should be high enough for suff;cient decompo-sition of the initiator to occurO This temperature will usually be between 40C and 300Co For the preferred initiator, iOe, di-tsrt-butyl peroxide, the most suitable temperature is between 13QC and 160Co The pressure should be between 60 and 1000 atm~spheres (900 and 15000 pOsOiOgO)~ and preferably being between 75 and 470 atmospheres (1100 and 7000 pOsOi~gO)O The autoclave or ~imilar equipment containing the solvent, initiator and vinyl or hydrocarbyl substituted vinyl ester is purged with nitrogen and then with ethylene before charging with a sufficient amount of ethylene to yield the desired pressure when heated to the reaction temperatureO During the polymerization additional ethylene is added to maintain the pressure at the desired levelO Further amounts of initiator and/or solvent, and/or vinyl and hydrocarbyl substituted vinyl ester may also be adted during the reactionO On completion of the reaction free solvont and unreacted monomers are removed by stripp;ng or some other suitable process yielding the desired polymer~

The ethylene copolymer should have a molecular weight correspond-ing to a melt index (as defined in European specification ISOR292 which is analogous to ASTM1238) of 0.2 to 20, preferably 1.5-3Ø
The hydrocarbon resin may be a polyterpene resin or a resin 5 obtained by the polymerization of a cracked petroleum distillate fraction. The polymer is preerably hydrogenated to yield a very light coloured product.
Although various petroleum distillate fractions may be used the preferred feed-stocks are those having a relatively wide 10 boiling range, e.g. 20C to 280C, Before polymerization takes place the cracked petroleum distillate fraction is preferably heat-soaked, e.g. at a temp-erature above 100C e.g. 130C to 140C, for a period of less than one hour so as to dimerize the cyclic dienes. The heat-15 soaked feed is thereafter distilled and the desired fraction e.g. one boiling between 80C and 180C, vacuum distilled to obtain the desired cut to be polymerized.
The preferred fractions are those which comprise compounds having one or more unsaturated rings in the molecule, such as 20 cyclodienes, cycloalkenes, indenes etc,, in particular the fractions boiling above 100C.
Polymerization preferably takes place without a catalyst, i.e. thermal polymerization at a preferred temperature of 240C
to 320C, e.g. about 250C. The thermal polymerization is 25 preferably carried out in an inert atmosphere, e.g. N2 at a pressure of 10-12 atmospheres for 1 to 9 hours. If desired polymerization can take place using a Friedel-Crafts catalyst, such as AlC13, BF3, SnC14, TiC14 at a temperature of -100 to +95C, e.g. 0C to 55C.
When the polymer is subjected to hydxogenation, the pol~mer is~first preferably dissolved in a saturated hydrocarbon sol-vent such as heptane, in a proportion of, for example, 20 to 60~ by weight in relation to the polymer solution.
Suitable hydrogenation catalysts include nickel, reduced nickel, nickel-tungsten sulphide, molydenum sulphide, and a preferred catalyst is a pre-activated catalyst on a Kieselguhr support containing about 58% by weight of nickel, the specific area being about 140 m2/g and the weight ratio: reduced nickel total nickel being about 0.66. Suitable proportions of catalyst are from 5 to 20% by weight, e.g. 7 to 13% by weight, basea on weight of resin.
Hydrogenation preferably takes place at 215 to 270C e.g.
about 215C at a pressure of 40 to 60 atmospheres, e.g. 45 atmospheres, for a period up to 3 hours, e.g. about 2 hours.

After cooling the solution of polymer should be filtered to eliminate traces of catalyst and obtain a clear solution free from all impurities.
The solution is then distilled under nitrogen and there-after steam distilled at a temperature not exceeding 260C.

The resin thus obtained usually has a Gardner colour of 1 to 3, a bromine number of 1 to 20 and a ball-ring softening point of about 100C.

-~ _ 5 _ A prefexred resin derived from a petroleum distillate fraction is a hydrogenated polycyclopentadiene resin. Suit-able hydrocarbon resins also include hydrogenated coumarone-indene resins.
Suitable polyisobutylenes (e.g. Vistanex*) are highly paraffinic hydrocarbon polymeric compositions with long straight molecules having a terminal unsaturation only. They are relatively low molecular weight, very viscous, clear tacky and semi-solids.
The polyisobutylene which may be used is preferably of relatively low molecular weight, e.g. of Staudinger viscosity avera~e molecular weight from 8,700 to 10,000 and a viscosity average MW (Flory) of 30,000 to 50,000 e.g. about 35,000. The polyisobutylene may be a homopolymer or may contain small amounts of other copolymerised monomers such as styrene.
The preferred proportions of ethylene copolymer and hydro-carbon resin or polybutylene or polyisobutylene are 96 to 99 wt.% of ethylene copolymer and 1 to 4 wt.% of hydrocarbon resin, polybutylene or polyisobutylene and 92 to 96 wt.% of ethylene copolymer and 3 to 6 wt.~ of hydrocarbon resin plus 1 to 2 wt.%
of polybutylene or polyisobutylene.
The partial carboxylic acid ester of a polyol (by which we mean that at least one hydroxyl group derived from the polyol is unesterified) is preferably derived from a (monobasic) fatty acid and preferably from a C10-C24 carboxylic acid e.g. a C10-C20 fatty acid. The carboxylic acid can be saturated or unsaturated.
Examples of suitable acids are lauric, myristic, palmitic, stearic, * Trade Mark ~ - 5A -behenic, oleic or erucic. Dicarboxylic acids such as adipic or pimelic acids could also be used.

106843~
The polyol may have two hydroxyl groups per molecule, e.g. ethylene glycol, propylene glycol; three hydroxyl groups per lecule, eOgO glycerol or trimethylol propane or four hydroxyl groups per molecule, e.gO pentaery-thritolO Further examples are sorbitol (6 hydroxyl groups), sorbitan ~corbitol anhydride - 4 hydroxyl group~) and dipentserythritol (6 hydroxyl groups)~
Since the ester must be a partisl ester at least ona of the hydroxyl groups must be un-esterified, and usually only one of the hydroxyl groups is estQrifiQd, Preferred esters are mono-esters of sorbitol or sorbitan, e.g~ sorbitan monolaurate, sorbitan monostearate or sorbitan monopalm;tateO
The preferred amount of carboxylic acid ester 0.~ to 1.0 wto%, based on the combined weights of ethylene copolymer and hydrocarbon resin and/or polybutylene or polyisobutylene.
After mixing the constituents of the composition of the invention the desired stretch and seal film can be~made either by tubular blowing or by chill roll castingO In the tubular blowing method the composition in the molten state is forced around a mandrel inside a die and extruded through the die opening as a tubeO The tube whilst still in the melt state is expanded to a hollow cylinder of desired diameter by blowing with the a;r admitted through the centre of the mandrelO Alternatively the extruded tube can be cooled by a water bath system wherein the extruded tube i8 cooled by passing it through a water bath containing flowing waterO The chill roll casting method results in a more tacky, more elastic film with better visual appearance. In the chill roll casting method the hot melt extruded through a die slot is cooled by the surface of two or more water-cooled chill rolls.
Films having thicknesses of between 10 and 100 microns are preferredO
If desiret the films can be coloured prov;ded they remain transparentO
It has been found that the presence of a partial carboxylic acid e~ter of a polyol, in particular sorbitan monolaurate and sorbitan mono-palmitate, improves the following properties of the film iO68431 1, Antifog: a normal polyolefin film covering food or a moist product will become covered with a layer of opaque-fine droplets when placet in a refrigerator normally at a temperature of 3-8 C0 This is a great disatvantage to the use of ~uch films for wrapping food as it prevents the buyer, e.g. in a supermarket, being able to see the food easilyO It is also clearly an advantage in the home if the wrapped food can be seen clearly.

2~ ~oll wind-off: during use or further processing operations, such as slitting and re-winding of large rolls, it i8 important ~hat the film can be unwound easily from the rollO If the additive makes it more difficult to unwind the film this is a 8reat disadvantageO
Plate out: the additives normally used for antifogging should act on the surface of the film and are therefore normàlly chosen to be only partly compatible with the film substrate. There is usually partial migration of the additive to the surface of the film where they can be most effective, but if the layer which is formed is hazy, thè appearance of the film is spoilt, and in the worst examples the film is quite unsuitable.
Tack. some additives which result in improved anti-fog properties unfortunately spoil ~he tack or cling of the film, but others improve the tack.
Example A blend was prepared of:

~~~ 9602 wt.% of a copolymer of ethylene and vinyl ' ~n~tS
acetate containing 95 wt~% of ethylen~.

3 wto~ hydrogenated polycyclopentadiene resin, and 008 wto~ sorbitan monolaurate The blend was extruded in a single screw extruder at 170C forming strands which were water cooled and then cut into pellets~
The pellets were used to blow films in air using the following conditions-Extruder screw length = 24:1 length to diameter ratio Extruder screw compression e ratio 3.5:1 Extruder barrel te~perature profile = 170/200/230 C

_ 7 _ Die temperature - 220-225 C

Blow up ratio - 2/1 or more Thickness ~ 12-20 microns Other blends were prepared and extruded by the same method using 0.8 wt.X of other partial esters. The results obtained were as follows:

Additive Tack Plate Out Wind Off Anti~

Sorbitan monolaurate Excellent Good Fair Excellent Sorbitan monopalmitate Good Good Good Good Polyethylene glycerol Excellent Very Poor Very Poor Very Poor mono ricinoleate Glycerol mono oleate Excellent Fair Fair Excellent Sorbitan mono oleate Good Good Fair Good Glycerol monoricinoleate Good Good Poor Fair ~ Yi ~olYo~tvethyle~e~
Po1ye~hyl~ne monoricinoleate Fair Good Poor Very Poor Octyl Ph noxy Eth~ol Poor Poor Good Excellent Polyoxyethylene Sorbitan Very Poor Very Goot Very Good Poor ~onostearate Polyoxyethylene Sorbitan Very Poor Good Very Good Very~Poor ~te~te ~o~oste~n2t~

Polyoxyethylene Sorbitan No Tack Hazy Excellent Poor Monolaurate Incompatible As can be seen, films prepared with partial fatty acid esters of polyols give sati~factory results.
It has also been found possible to improve some of the partial fatty acid ester results by regulating the processing conditions, In fact the various additives have different molecular weights and their migration propertiss usually depend on the temperature at which the compositions are extruded when making the films,

Claims

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

1. A composition suitable for fabricating into a self-sealing film comprising (i) 90-99.95 wt.% of a copolymer of ethylene and a vinyl (or hydrocarbyl substituted vinyl) ester of a C1-C30 monocarboxylic acid, (ii) either 0.05 to 10 wt.% of a hydrocarbon resin, or 0.05 to 5 wt.% of poly-butylene or polyisobutylene, or a combination of 0.05 to 10 wt.% of the hydrocarbon resin and 0.05 to 5 wt.% of polybutylene or polyisobutylene, provided the combined weight of the hydrocarbon resin and polybutylene or poly-isobutylene is not more than 10 wt.% and (iii) 0.01 to 3.0%
by weight based on the combined weight of (i) and (ii) of a partial carboxylic acid ester of a polyol.

2. A composition according to claim 1 wherein the vinyl ester from which the copolymer is derived is vinyl acetate.

3. A composition according to either of claims 1 and 2 wherein the ethylene copolymer has a melt index (as defined in European specification ISOR292 analogous to ASTM1238) of 1.5 to 3Ø

4. A composition according to claim 1 wherein the hydrocarbon resin is a resin obtained by the polymerization of a cracked petroleum distillate fraction.

5. A composition according to claim 4 wherein the cracked petroleum distillate fraction has been hydrogenated.

6. A composition according to either of claims 4 and 5 wherein said fraction comprises compounds having one or more unsaturated rings in the molecule.

7. A composition according to either of claims 1 and 2 wherein the hydrocarbon resin has a Gardner colour of 1 to 3, and a bromine number of 1 to 20.

8. A composition according to claim 5 wherein the hydrocarbon resin is a hydrogenated polycyclopentadiene resin.

9. A composition according to either of claims 1 and 2 wherein the polyisobutylene has a viscosity average MW (Flory) of 30,000 to 50,000.

10. A composition according to claim 1 which comprises 96 to 99 wt.% of the ethylene copolymer and 1 to 4 wt.% of the hydrocarbon resin, polyisobutylene or polybutylene.

11. A composition according to claim 1 which comprises 92 to 96 wt.% of ethylene copolymer and 3 to 6 wt.% of hydrocarbon resin plus 1 to 2 wt.% of polyisobutylene or polybutylene.

12. A composition according to either of claims 10 and 11 wherein the ethylene copolymer contains from 85-97 wt.% of ethylene.

13. A composition according to either of claims 10 and 11 wherein the ethylene copolymer contains from 91 to 95 wt.% of ethylene.

14. A composition according to either of claims 10 and 11 wherein the partial carboxylic acid ester is derived from a (monobasic) fatty acid.

15, A composition according to either of claims 10 and 11 wherein only one of the hydroxyl groups of the polyol from which the partial carboxylic acid ester is derived is esterified.

16. A composition according to either of claims 10 and 11 wherein the ester is a saturated C10 to C20 fatty acid ester of sorbitol.

17. A composition according to either of claims 10 and 11 wherein the ester is sorbitan monolaurate or sorbitan monopalmitate.

18. A composition according to either of claims 10 and 11 wherein the amount of carboxylic acid ester is 0.2 to 1.0 wt.% based on the combined weights of ethylene copolymer and hydrocarbon resin and/or polybutylene or polyisobutylene.

19. A film prepared from a composition according to any one of claims 1, 10 and 11.