GB2040804A

GB2040804A - Improved heat-sealing irradiated film

Info

Publication number: GB2040804A
Application number: GB7944135A
Authority: GB
Original assignee: WR Grace and Co
Current assignee: WR Grace and Co
Priority date: 1978-12-28
Filing date: 1979-12-21
Publication date: 1980-09-03
Also published as: FR2445212A1; ZA795410B; CA1125229A; AR219850A1; JPS5595568A; AU5268679A; NZ192133A; BR7908527A; DE2951138A1

Abstract

The invention relates to multi- layer polymeric structure, especially film, which can be produced by irradiation crosslinking of the polymer of a first layer without damage to the heat- sealability of a heat-sealable second layer. By including in the first layer a chemical component which when blended with a polymer in the melt form produces a material which crosslinks to a greater extent under irradiation than does the same polymer alone, it is possible to reduce the dosage of irradiation required for satisfactory cross-linking and thereby reduce or prevent damage to heat-sealing characteristics of the second layer. The laminate is useful for orientation to produce heat-shrink packaging material.

Description

SPECIFICATION Improved heat-sealing irradiated film The present invention relates to a co-extruded multi-layer polymeric structure having a layer cross-linked by irradiation.

The use of heat-shrinkable thermoplastic film in many packaging applications is well known. Usually a product is enclosed within a film, sealed and heated to shrink the film snugly about the product. One of the most useful and satisfactory thermoplastic, materials for packaging film is polyethylene, which has highly desirable heat-shrink properties.

It is generally known in the art that films etc, of polymers and copolymers of olefins, particularly polyethylene, become thermoset if a sufficient radiation dosage is applied to the olefin polymer. For this reason it has been customary to form articles of these polymers before irradiating them, and thereby bringing about the transition from a thermoplastic to a thermosetting material which makes forming virtually impossible. Another effect brought about by the transition from thermoplastic to thermosetting properties upon irradiation is a lessening of the ability of an olefin polymer article to heat-seal to itself or to some other object, as the radiation dosage is increased. For this reason it has generally been recognized in the film art that highly irradiated polyethylene films do not possess the ability to form reliable high strength heat-seals. The prior art has thus had to either rely upon a low radiation dosage in order to preserve heat-sealing properties or to apply a second layer or lamination which possesses desirable heat-sealing properties, to the irradiated layer after the irradiation operation. This application of a second layer at this stage in the process is not always convenient.

It is thus an object of this invention to provide a polymeric structure which possesses the desirable characteristics associated with crosslinking brought about by irradiation, while at the same time possessing heat-sealing characteristics similar to those of non-irradiated articles, yet which can be produced without the necessity of having to apply the second layer after irradiation.

The basis of the present invention is as follows: One starts with a multi-layer film, one layer of which is to be highly crosslinked and another layer of which is to serve as a heat-sealable layer. These two layers can be produced unitarily by co-extrusion. The multi-layer film is then irradiated. By providing within the crosslinkable layer an irradiation crosslinking enhancer (defined hereinafter), the crosslinking efficiency of the irradiation operation is so greatly increased that the dosage of irradiation can be reduced. Thereby the heat-sealable layer, which contains no irradiation enhancer, is only minimally crosslinked i.e. to an extent not sufficient to destroy the heat-sealing characteristics of the layer. The same principles can be applied to any layered structure, whether strictly a film or not.

In accordance with this invention there is provided a process for producing a multi-layer irradiated structure, comprising provided a multi-layer structure having a crosslinkable layer including an irradiation crosslinking enhancer and a heat-sealable layer; and irradiating said multi-layer structure at a radiation dosage sufficient to crosslink said crosslinkable layer but insufficient to destroy the heat-sealability of said heat-sealable layer.

In its broadest aspects the laminate of this invention comprises a core layer and a heat-sealable layer which have both been through an irradiation step to crosslink the core layer to a high degree due to the presence therein of an irradiation crosslinking enhancer. The invention provides particularly a co-extruded multi-layer polymeric structure having a crosslinked layer including an irradiation crosslinking enhancer or residue thereof from the cross-linking and a heat-sealable layer.

As used within this disclosure the term "polymer" is used with its normal meaning to include homopolymers, bipolymers, terpolymers, other copolymers etc. The term "crosslinking" is utilized to mean the union of polymer molecules by a mechanism involving primary chemical bonds to have the effect of binding a polymer into a single network so that functionally severai molcules become bound together into a single molecule. As applied to polymers of mono-olefins, particularly polymers and coplymers of elthylene, crosslinking is detectable by an increase in the remaining residue when the sample is contacted with toluene under reflux condition for about two days. This is generally referred to as the gel content. Thus, crosslinking is detected by an increase in the gel content of the crosslinked sample as compared to a non or lesser crosslinked sample.

As used within this specification, the term "irradiation crosslinking enhancer" is a chemical component which when blended with a polymer in the melt form produces a material which crosslinks to a greater extent from a given radiation dosage than does the same polymer without the irradiation crosslinking enhancer additive. Thus an additive which when blended with a polymer to produce an irradiated crosslinked product having a higher gel content then the same polymer at the same radiation dosage level without the additive is an irradiation crosslinking enhancer.

The term "core" as used herein with reference to the core layer of the laminate of this invention is used to identify as an essential layer the layer which is crosslinkable, rather than to limit the core layer to a central or innermost layer. The laminate may comprise, however, additional layers such as additional heat-sealing layers, or a layer to inhibit the passage of gaseous or liquid substances such as oxygen or moisture. The core layer of the laminate of this invention is made of a polymer which is crosslinkable by irradiation and which can be enhanced in its crosslinking ability when blended with an irradiation crosslinking enhancer in accordance with this invention. Crosslinking using irradiation can be accomplished by various methods.

Thus there can be used electrons, x-rays, and radiation of actinic origin such as ultraviolet light having a wave length above about 2,000 angstoms and below about 2,700 angstroms. Preferably, however, electrons of at least 105 electron volts energy are applied. The irradiation source can be a Van der Graaff type electron accelerator having an operational voltage of 2 megavolts at a power output of about 5 to 10 kilowatts.

Preferably, however, the source of electrons is an electron accelerator powered by an insulated core transformer having an accelerating voltage from about 300 to 3,000 kilovolts.

The absorbed radiation within the core layer is stated by the term "RAD". The RAD is defined as an energy dosage level of 100 ergs per gram imparted by the ionizing radiation to the irradiated material at the point of interest. The core material utilized in this invention, i.e. the polymer incorporating the irradiation crosslinking enhancer, is of course a material which will when blended with an enhancer undergo sufficient crosslinkage to increase the gel content thereof at a useful irradiation dosage level, usually within the range of about 0.1 to about 5 megarads (MR). (As explained hereinafter, there are various methods of incorporating the irradiation crosslinking enhancer in the core layer, but for the purposes of the gel content test, blending is the method used).

The polymeric material of the core layer of the laminate of this invention may be selected from a broad class of materials, i.e. those materials which crosslink when subjected to irradiation. While polymers of ethylene including copolymers thereof constitute the preferred material utilized within this invention, the invention may generally be carried out with polymers of other mono-olefins particularly the l-alkenes and copolymersthereof. Particularly the invention may be carried out with polymers of propylene, butylene, pentene and hexene. The invention may additionally be carried out on vinyl chloride polymers and copolymers. The invention is particularly applicable to copolymers of ethylene and vinyl esters, e.g. vinyl acetate. The invention is applicable to any of the polymers of ethylene such as high and low density ethylene polymers produced by both high pressure and low pressure processes as well as mixtures and blends thereof.

Many irradiation crosslinking enhancers are known to the art. Any such enhancer may be utilized within the core layer of the laminate of this invention. While not meant to be limiting, the following is a general listing of some suitable classes of irradiation crosslinking enhancers; dialkyl maleates and fumarates in which the alkyl group contains 4 or more (usually 4 to 20) carbon atoms; vinyl esters of fatty acids in which the fatty acid contains 3 or more (usually 3 to 20) carbon atoms; vinyl alkyl ethers in which the alkyl group contains 18 or more (usually 18 to 30) carbon atoms; alkyl acrylates in which the alkyl group contains 1 or more (usually 1 to 20) carbon atoms and; alkyl methacrylates in which the alkyl group contains 3 or more (usually 3 to 20) carbon atoms.

Other particularly preferred irradiation crosslinking enhancers for use in this invention are allyl methacrylate, allyl acrylate and diallyl maleate. The irradiation crosslinking enhancer can be incorporated in the core layer by several conventional techniques, e.g. mixing the materials from the powder form, blending the enhancer into the polymer in the molten state or by diffusing the irradiation crosslinking enhancer in the gaseous or liquid form into the solid polymer. It is preferred, however, to mix the irradiation crosslinking enhancer in the powder form before the melt blending of the precursor of the core layer which takes place before melt extrusion. The irradiation crosslinking enhancer in the solid polymer will not cause any crosslinking until the layer has been acted upon by a source of radiation as discussed above. It is normally employed in a small amount from about 0.05 to about 10 weight percent of the overall composition of the core layer, but in any event in an amount sufficient to increase the gel content of the core layer.

The heat-sealable layer of the laminate structure of this invention can be of any material conventionally used for its heat-sealing capability. Such materials generally heat-seal to themselves at a temperature within the range of about 70 to about 1 50 C under a pressure from about 5 to about 50 Ib. per square inch (35 KPa to 350 KPa). Such materials then have a peel strength about 1 Ib. per inch of width (17.9 kg/m) as measured by American Society for Testing Materials specification D-903. Such materials preferably have a peel strength above about 10 Ibs. per inch of width (179 kg/m).

Materials which can be utilized for the heat-sealable layer of the laminate of this invention include both high and low density polyethylene, polyproplene and other polymers of 1-mono-olefins. Particularly effective heat-sealable substances include unsaturated ester polymers such as ethylene/unsaturated ester copolymers, e.g. ethylene/vinyl acetate, ethylene/vinyl propionate, ethylene/methyl methacrylate, ethylene/ ethyl methacrylate, ethylene/ethyl acrylate, ethylene/isobutyl acrylate, and the like; unsaturated carboxylic acid copolymers, e.g. ethylene/unsaturated carboxylic acid copolymers, e.g. ethylene/acrylic acid, ethylene/ methacrylic acid, ethylene/maleic acid, ethylene/fumaric acid, ethylene/itaconic acid, and the like. The carboxylic acid groups of the acid copolymers may be either partly or wholly neutralized to form what is commonly referred to as an ionomer. Particularly preferred heat-sealable materials are copolymers of ethylene and vinyl acetate containing from about 5 to about 40 weight percent of vinyl acetate. This listing, however, is not meant to be exhaustive but merely exemplary of the materials which might be utilized in a heat-sealable layer within the laminate of this invention. These materials generally tend to diminish in their heat-sealing ability upon exposure to irradiation sufficient to crosslink an otherwise non-irradiation enhanced layer. By utilizing a core layer having an irradiation enhancer, however, the heat-sealable layer is not exposed to sufficient radiation dosage to significantly deleteriously affect the heat-sealing quality or characteristics of the heat-sealable layer. Preferably the heat-sealable layer is not exposed to sufficient radiation to adversely affect its heat-sealing characteristics to any substantial extent. The characteristic of chief interest is peel strength and preferably this will be at least 17.9 kg/metre width, more preferably 170 kg/metre width, as mentioned above.

Preferably irradiation is carried out to a dosage level of about 0.1 to about 4 MR. This dosage level is normally sufficient to ensure crosslinking within the core layer without substantially adversely affecting the heat-sealing characteristics of the heat-sealable layer. This dosage level refers to the entire structure and can be measured by reference to the gel content of any of the layers (in the case of the core layer, making due allowance for the enhanced crosslinking).

One of the primary reasons for carrying out a crosslinking reaction is to enhance the ability of the core layer to undergo an orientation process so as to provide a heat-shrinkable film. It is not necessary to the concept of this invention that an orientation process be carried out, however, since the irradiation crosslinking process enhances other desirable characteristics of the laminate film other than the ability to undergo orientation. It is preferred, however, that the laminate film produced by this invention be oriented e.g. by conventional techniques, so as to provide a film having heat-shrink characteristics. It is especially preferred that such heat-shrink characteristics exist at a temperature at or below the boiling point of water, i.e. 100"C. For most purposes of this invention the laminate will be biaxially oriented, desirably to a free shrinkage at 96"C of at least 10% in both the longitudinal and transverse directions.

The laminate of this invention is preferably produced by coextrustion of the layers, and desirably in a tubular form. A suitable preparation of tubular laminate is described in our U.S. Patent 3,874,967. A process of producing a laminate film of this invention is illustrated schematically in the accompanying drawing. The drawing shows two extruders, 2 and 3, supplying coextrusion die 4, for providing the various layers of the laminate. Thus extruder 2 may be supplied with the core layer polymer including the irradiation enhancer and extruder 3 may be supplied with the material for the heat-sealable layer. The extruders supply coextrustion die 4 so as to produce a tubular film 6. The die is positioned to extrude the tube downwardly into a cooling bath 8 of water or other inert liquid maintained at a temperature of between -18"C and +21"C.

The cooled and flattened tubing of tape 16 is fed through feed rolls 18 into an irradiation vault 20 which houses and encloses a source of electrons 22. Following irradiation, the tape is fed by feed rolls 20 to a hot bath 30 in racking tank 46 which contains water or other liquid inert to the polymer layers. This liquid is maintained at a temperature sufficient to heat the tape to a desirable orientation temperature for the inner core layer. This temperature will vary somewhat depending upon the composition of the core layer.

Orientation techniques are well known in the art, however, and a desirable orientation temperature is readily ascertainable for a particular polymer film material. For example, crosslinked low density polyethylene orients well at a temperature of about 88 to 1 02"C. The preferred temperature, however, is about 96"C. Air or other gasses are introduced into the heated tape to form a bubble 38 between the surface of the hot bath and the upper deflate rolls 34. The amount of inflation is preferably sufficient to provide a stretch of from about 3 to 1 to about 8 to 1 in each direction.

While the above description has primarily in mind a coextruded laminate having only the two layers, it is readily apparent that other materials may be coextruded along with the two essential layers. The most readily extrudable layer would be an additional heat-sealable layer identical in composition to the primary heat-sealable layer in order to provide a three layer laminate which sandwiches the core layer. Other materials, however, such as layers to inhibit oxygen permeation, may be included along with the core and heat-sealable layer or layers.

The following examples illustrate the invention.

Example 1 Several samples were produced having a core layer containing an irradiation crosslinking enhancer, in accordance with this invention. Structures comprising three concentric layers were coextruded from a 4 inch (10 cms) die utilizing an equal weight blend of high and low density polyethylenes within each layer. The central ("core") layer of the structures, however, contained a small percentage of diallyl maleate as an irradiation enhancer. All samples were oriented by the bubble technique at a temperature of approximately 250 F (120 C) to provide a stretch of about 5 to 1 in both the machine and transverse directions. The samples were immediately chilled after orientation. Each of the samples had a thickness of about 1 mil (25 microns) after the orientation step. The properties of each sample as well as an identification of each sample is contained in the following Table 1.

TABLE 1 Shrink tension, p.s.i.

(kilopascals) Layer Radiation Wt % diallyl transverse Longitudinal thickness dosage maleate 200 F 300 F 200 F 300 F Sample No. ratio MR in core layer (120 C) (150 C) (120 C) (150 C) 1 1/2/1 2 0.3 439(3027) 300(2068) 189(1303) 249 (1717) 2 1/1/1 3 0.2 377(2599) 388(2675) 242(1686) 326(2248) 3 1/1/1 3.5 0.3 481(3316) 357(2461) 173(1193) 274 (1889) 4 1/1/1 4 0.3 332(2289) 415(2861) 324(2234) 341(2351) Free shrink % (longitudinal) (transverse) 200 F 300 F 200 F 300 F Sample No. (120 C) (150 C) (120 C) (150 C) 1 6 83 12 78 2 8 82 12 79 3 6 82 12 80 4 8 81 10 78 Example lI Two samples (1 and 2) were produced having a core layer containing an irradiation crosslinking enhancer, in accordance with this invention. The two structures were produced of the same material in substantially the same ways as the samples in Example 1. Two further samples (3 and 4) were produced from polyethylene blends similar to the blends utilized in samples 1 and 2 but did not contain an irradiation crosslinking enhancer and were produced as a single layer having a thickness comparable to that of three-layer samples 1 and 2. All samples had a total thickness of about 0.75 mils (about 19 microns). The coextruded structures had three layers of equal thickness.

The gel content of each sample were determined by refluxing samples in toluene for 21 hours in order to ensure complete solution of all soluble portions. These data are tabulated in Table 2 along with an identification of the samples, the dosage and an indication as to whether the gel content was determined from the outer heat-sealable ("skin") layer or core layer. The outer layers of the samples 5 and 6 having no irradiation crosslinking enhancer had no gel while the core layer which contained the enhancer had gel percentages higher than Sample No 7 which contained no irradiation crosslinking enhancer. Samples 5 to 8 were tested for seal strengths utilizing ASTM standard F88-68 for both lap seals and trim seals at various electric current levels within the electrical heating element. The lap seals were formed using a nickel-chromium heating ribbon covered with a sheet of polytetrafluoroethylene. The heating ribbon was operated in impulse fashion using a 2 second heating and 2 second cooling cycle. The trim seals were formed using a heating wire of the same alloy heated continuously with a six second interval between trim seals. The data are tabulated in Tables 3 and 4 along with some retabulation of identifying data from Table 2.

It is seen that the seal strength of the highly irradiated samples 7 and 8 is significantly less at comparable amperage than the lesser irradiated samples containing a core layer with an irradiation crosslinking enhancer.

TABLE 2 Sample no. Radiation % % Gel in % Gel in % Gel in dosage Diallyl total skin layer core layer MR maleate structure in core layer 5 2 0.2 3.8 0 12 6 3 0.3 5.4 0 16 7 5 0 5.4 - 5.4 8 10 0 42.6 - 42.6 TABLE 3 Dosage % Diallyl Sample MR maleate Seal strength (lap seals) 5 2 0.2 Amperage 22 Strength, lb (g) 0.42(191) 6 3 0.3 Amperage 21.9 Strength, lb (g) 0.35(159) 7 5 0 Amperage 22.5 Strength, lb (g) 0.72(327) 8 10 0 Amperage 23 Strength, lb (g) 0.74(336) Seal strength (lap seals) Amperage 22 24.2 - - 26.3 - 28.7 Strength, lb (g) 0.42(191) 0.87(395) - - 3.01(1365) - 3.22(1461) Amperage 21.9 24.4 - - 26.5 - Strength, lb (g) 0.35(159) 1.14(517) - - 3.25(1474) - Amperage 22.5 23.5 25.3 25.7 - 27.8 28 Strength, lb (g) 0.72(327) 0.58(263) 1.11(503) 2.13(966) - 2.41(1093) 3.22(1460) Amperage 23 - 25.2 - - 27.5 Strength, lb (g) 0.74(336) - 1.66(753) - - 1.99(903) - TABLE 4 Sample no. Dosage (MR) Trim seal strength 5 2 Amperage - 7.3 8.0 8.8 Strength Ib - 5.66 5.14 2.93 (kg) (2.57) (2.33) (1.33) 6 3 Amperage 6.9 7.6 8.3 Strength Ib 5.76 5.48 4.30 (kg) (2.61) (2.49) (1.95) 7 5 Amperage 6.7 7.4 8.1 Strength Ib 5.58 4.80 3.47 (kg) (2.53) (2.18) (1.57) 8 10 Amperage - - 8.3 9.1 Strength Ib - - 1.73 1.65 (kg) (0.78) (0.75) CLAIMS 1. A process for producing a multi-layer irradiated structure comprising: providing a multi-layer structure having a crosslinkable layer including an irradiation crosslinking enhancer and a heat-sealable layer; and irradiating said multi-layer structure at a radiation dosage sufficient to crosslink said crosslinkable layer but insufficient to destroy the heat-sealability of said heat-sealable layer.

Claims

2. A process according to claim 1, wherein the heat-sealable layer is sealable at a temperature within the range 70 to 150"C and a pressure of 35 to 350 kilopascals and the radiation dosage is insufficient to give a seal strength of less than 17.9 Kg. per metre width, as measured by American Society for Testing Materials specification D-903.

3. A process according to claim 1 or 2, wherein said step of irradiating is carried out so as to provide a dosage level of 0.1 to 4 megarads.

4. A process according to claim 1, 2 or 3, wherein said crosslinkable layer is ofa polymer of a 1 -mono-olefin.

5. A process according to claim 1, 2, 3 or 4, wherein the irradiation crosslinking enhancer is selected from dialkyl maleates, dialkyl fumarates, vinyl esters of fatty acids, vinyl alkyl ethers, alkyl acrylates and alkyl methacrylates.

6. A process according to claim 1,2,3 or 4, wherein said irradiation crosslinking enhancer is allyl methacrylate, allyl acrylate or diallyl maleate.

7. A process according to any preceding claim which further comprises orienting said multi-layer structure.

8. A process according to any preceding claim, wherein the multi-layer structure is a film and is provided by co-extrusion of the layers.

9. A process according to claim 1 substantially as described in Examples I and II with reference to any one of Samples 1 to 6.

10. A multi-layer structure produced by a process claimed in any one of claims 1 to 9.

11. A coextruded multi-layer polymeric structure having a crosslinked layer including an irradiation crosslinking enhancer or residue thereof from the crosslinking and a heat-sealable layer.

12. A multi-layer structure according to claim 11 wherein the crosslinked layer has been crosslinked by irradiation at a dosage level of 0.1 to about 4 megarads.

13. A multi-layer structure according to claim 11 or 12, wherein said heat sealable layer has heat sealing characteristics substantially similar to a non-irradiated layer of the same material.

14. A multi-layer structure according to claim 11, 12 or 13, wherein said crosslinked layer is of a polymer of a 1-mono-olefin.

15. A multi-layer structure according to any one of claims 11 to 14, wherein the irradiation crosslinking enhancer is selected from dialkyl maleates, dialkyl fumarates, vinyl esters of fatty acids, vinyl alkyl ethers, alkyl acrylates, and alkyl methacrylates.

16. A multi-layer structure according to any one of claims 11 to 14 wherein the irradiation crosslinking enhancer is selected from allyl methacrylate, allyl acrylate and diallyl maleate.

17. A multi-layer structure according to any one of claims 11 to 16, wherein the heat sealable layer is sealable at a temperature of 70 to 150"C, at a pressure of 5 to 50 psi to produce a seal having a peel strength of greater than 17.9 Kg per metre of width.

18. A multi-layer structure according to any one of claims 11 to 17 in the biaxially oriented state.

19. A multi-layer structure according to claim 18 having a free shrinkage of at least 10% in both axial and transverse directions at 96"C.

20. A multi-layer structure according to any one of claims 11 to 19 in the form of a film.

21. A multi-layer structure according to claim 11 substantially as hereinbefore described.