GB2341135A

GB2341135A - Polymeric films for packaging

Info

Publication number: GB2341135A
Application number: GB9919987A
Authority: GB
Inventors: Roy Christopherson; Steven George Friedrich
Original assignee: Rexam Medical Packaging Ltd
Current assignee: Amcor Flexibles Winterbourne Ltd
Priority date: 1998-08-29
Filing date: 1999-08-25
Publication date: 2000-03-08
Also published as: GB9919987D0; GB9818869D0

Abstract

Packages (10), especially for medical apparatus (25), include a polymeric films having:- <SL> <LI>(a) a core layer comprising at least one homogeneously branched substantially linear ethylene/ a -olefin interpolymer having </SL> <SL> <LI>(i) a melt flow ratio, I<SB>10</SB>/I<SB>2</SB> / 5.63;and <LI>(ii) a molecular weight distribution, M<SB>m</SB>/M<SB>n</SB> & (I<SB>10</SB>/I<SB>2</SB>)-4.63; </SL> <SL> <LI>(b) a first outer layer on the core layer comprising a linear polyethylene or a linear polyethylene/polypropylene blend; and <LI>(c) optionally a polymeric heat seal layer on the opposite side of the core layer from said first outer layer; </SL> the said outer layer forming an outer surface of the packages, for examle including a closure web (20) heat sealed thereto. They have show good puncture and abrasion resistance the film itself having good formability.

Description

Packages Produced from Polymeric Films This invention concerns packages

produced from polymeric films, for example for items of medical equipment.

Packages consisting of a polymeric film heat sealed to a closure web are extensively used for packaging a wide range of products, and many types of film and closure webs have been proposed hitherto for the purpose. Furthermore, although essentially flat films and closure webs can be used to package many products, it is desirable in some instances to have either or both shaped to receive the product. Typically such shaping is effected thermally.

It is also highly desirable, and in some cases essential, that the packages once formed remain intact during shipping and more particularly during handling prior to use of the packaged product. In particular, it is important that the integrity of packages containing sterilized medical equipment is retained until the packages are opened for use of the equipment. Such packages should, therefore resist puncturing, tearing and 2 abrasion not only by forces applied to the exterior of the packages but also by forces applied by the product in the packages to the interior of the packaging materials, for example as a result of movement of the product within the packages.

According to the present invention there is provided a package comprising a polymeric film, the film comprising (a) a core layer comprising at least one homogeneously branched substantially linear ethylene /(x-ole fin interpolymer having (i) a melt flow ratio, 110/12 25.63; and (ii) a molecular weight distribution, Mw/Mn'(Il0/I2)-4.63; (b) a first outer layer on the core layer comprising a linear polyethylene or a linear polyethylene/polypropylene blend; and (c) optionally a polymeric heat seal layer on the opposite side of the core layer from said first outer layer; the said first outer layer forming an outer surface of the package.

Packages in accordance with the present invention have shown good puncture and abrasion resistance combined with good formability for the film itself.

The core layer of films used in the present invention is formed from at least one homogeneously branched substantially linear ethylene/cc-olefin interpolymer.

As used herein, the term "'interpolymer" includes copolymer and terpolymer. These ethylene /(x-ole fin interpolymers preferably have a density, as measured by ASTM D-792, from 0.85g/cm3 to 0.94g/cm3 and a melt index (12), as measured according to ASTM 3 - D-1238 (condition 190/2.16) from 0.01g/10 min. to lOg/10 min., and a melt flow ratio (IlO/I2) from 5.6 to 30.

In is measured in accordance with ASTM D-1238 (condition 190/10). The ethylene interpolymers, depending on their density, can also have a critical shear stress at the onset of gross melt fracture of greater than about 4 x 106 dynes/cm2. However, all of these ethylene interpolymers preferably have a critical shear rate, at onset of surface melt fracture, at least 50 percent greater than the critical shear rate at the onset of surface melt fracture of a linear ethylene polymer having about the same 12, Mw/Mn, and density. These ethylene interpolymers preferably have a molecular weight distribution (Mw/Mn) of less than about 3.5, especially from 1.5 to 2.5.

The terms "homogeneous branching distribution" and "homogeneously branched" refer to interpolymers and are defined herein to mean that (1) the (x-olefin monomer is randomly distributed within a given molecule, (2) substantially all of the interpolymer molecules have the same ethylene-to-a-olefin monomer ratio, (3) the interpolymer has a narrow short chain branching distribution wherein the composition distribution range index is greater than about 30 percent, preferably greater than about 50 percent, more preferably greater than about 80 percent, most preferably greater than about 90 percent, (4) the interpolymer essentially lacks a measurable high density (crystalline) polymer fraction as measured by known fractionation techniques such as, for example, a method that involves polymer fractional elutions as a function of temperature, and (5) the interpolymer is characterized as having substantially reduced levels of n-hexane extractables or substantial amorphism as determined by the FDA test method published as CFR 177. 1520(c). "Substantial amorphism" means that greater than 75 percent of the whole interpolymer is soluble under prescribed test conditions.

The term "homogeneously branched linear ethylene polymer" does not refer to high pressure branched polyethylene which is known to those skilled in the art to have numerous long chain branches. Typically, the homogeneously branched linear ethylene polymer is an ethylene /(x-ole fin interpolymer, wherein the a-olefin is at least one C3-C20 (x-olefin (for example, 1-propylene, 1-butene, 1-pentene, 4-methyl- l-pentene, 1-hexene, 1-octene), preferably wherein at least one of the (x- olefins is 1-octene. Most preferably, the ethylene/a-olefin interpolymer is a copolymer of ethylene and a C3-C20 (x-olefin, especially an ethylene/C4-C8 (x-olefin copolymer.

The term "narrow short chain distribution" as applied herein refers to interpolymers and pertains to the distribution of a-olefin monomer branches of the interpolymer as characterized by its SCBDI (Short Chain Branch Distribution Index) or CDBL (Composition Distribution Branch Index). The term is defined herein as greater than about 30 weight percent of the interpolymer molecules have an cc-olefin monomer content within 50 percent of the median total molar a-olefin monomer content. CDBI of an interpolymer can be readily calculated from data obtained from techniques known in the art, such as for example, temperature rising elution fractionation (abbreviated herein as "TREF") as described, for example, by Wild, et al., Journal of Polymer Science, Poly, Phys Ed., Vol. 20, p.441 (1982), or in US4798081. However, the preferred TREF technique does not include purge quantities in CDBI calculations. More preferably, the monomer distribution of the interpolymer and CDBI are determined using 13CNMR analysis in accordance with techniques described in USS292845 and by J.C. Randall in Rev. Macromol. Chem. Phys., C29, pp 201- 317.

Heterogeneously branched VLDPE and LLDPE are well known among practitioners of the linear polyethylene art. They are prepared using Ziegler-Natta solution, slurry or gas phase polymerization processes and coordination metal catalysts as described, for example, by Anderson et al. in US4076698. These Ziegler-type linear polyethylenes are not homogeneously branched and they do not have any long-chain branching. Also, these polymers do not show any substantial amorphism at lower densities since they inherently possess a substantial high density (crystalline) polymer fraction. At a density less than 0.9 g/cm3, these materials are very difficult to prepare using conventional Ziegler-Natta catalysis and are also very difficult to pelletize. The pellets are tacky and tend to clump together.

Commercial examples of heterogeneously branched linear interpolymers suitable for use in the present invention include ATTANE LLDPE polymers supplied by The Dow Chemical Company.

Although the core layers of films used in the present invention can consist substantially exclusively of one or more homogeneously branched substantially linear ethylene /(X-olef in interpolymer, it is generally preferred to include a minor proportion, i.e. less than 30 percent by weight, and preferably less than 20 percent by weight, of the core layer of a polyethylene which acts as a processing aid in the production and forming of films in accordance with the present invention. A preferred processing aid is low density polyethylene or ethylene/vinyl acetate.

The first outer layer of films used in the present invention are formed from at least one linear polyethylene which can be a homogeneously branched substantially linear ethylene /(x-ole fin interpolymer, for example as is used for the core layer, a linear low density polyethylene, or a linear medium density polyethylene.

The term linear low density polyethylene (LLDPE) is used herein to refer to linear copolymers of ethylene and minor amounts of an (x-olefin having from 3 to 18 carbon atoms, preferably from 4 to 10 carbon atoms and most preferably 8 carbon atoms. The LLDPE for the polymeric used in the present invention will in general have a density of greater than 0.900 g/cm3' preferably from 0.900 to 0.940 g/cm3. In general their melt index will be less than 10g/10 min, preferably from 0.1 to l0g/10min and more preferably from 0.5 to 5g/lOmin.

When a homogeneously branched substantially linear ethylene/a-olefin interpolymer or a linear low density polyethylene is used, it is generally preferred to use it in admixture with polypropylene, such blends preferably containing from 5 to 40 percent by weight of polypropylene with the balance being substantially composed of the polyethylene. The use of polypropylene in such blends has been found to provide films in accordance with the present invention with good abrasion resistance. The polypropylene when present can be a homopolymer or one of more copolymers of propylene and up to 20 mole percent of ethylene or another a-olefin having up to 12 carbon atoms. If a copolymer is used, it can be random, block or graft. The polypropylene component of this will typically have a melt flow rate (ASTM D-1238, Condition 230/2. 16 (formerly Condition L)) of from 0.1 and 30, and preferably from 0.8 and 30.

The optional polymeric heat seal layer of films used in the present invention can be formed from a wide variety of olefin polymers known in the art as heat seal materials. Typically such materials will be copolymers of ethylene containing units derived from at least one of propylene, butene-1 and higher a-olefins, and optionally including units derived from olefinically unsaturated esters and unsaturated aliphatic carboxylic acids and their anhydrides. Preferred polymeric heat seal layers are formed from blends of LLDPE and ethylene/vinyl acetate copolymers (EVA), such blends preferably containing from 0 to 25 percent by weight of the ethylene/vinyl acetate copolymer with the balance being formed substantially by the LLDPE.

The ethylene/vinyl acetate copolymer useful for polymer compositions and blends of this invention has a weight ratio of ethylene to vinyl acetate from 2.2:1 to 24:1 and melt index of from 0.2g to lOg/lOmin. A further description of EVA is found in MQdern Plastics Encyclopaedia, Mid-October 1992 Issue, Volume 68, Number 11, page 66.

7 Films used in accordance with the present invention can include one or more intermediate layers between the core layer and the first outer layer and/or between the optional heat seal layer and the core layer. Such layers are preferably formed from at least one polyolefin which can be a homopolymer and/or a copolymer containing units derived from one or more of ethylene and at least one aliphatic (x-olefin.

Films used in accordance with the present invention can be produced by known methods. For example, they can be produced by so-called casting in which melts of polymers forming the respective layers of the films are coextruded through a slot die, following which the coextrudate is cooled and the resulting film is wound up. Alternatively, it is possible in some instances to produce films in accordance with the so-called "bubble process" in which the respective melts are coextruded through an annular die to form a polymeric tube which is inflated or blown, following which the tube is collapsed and the resulting film is wound up.

In general, casting can be used to produce all films used in accordance with the present invention whereas the "bubble" process has been found to be particularly suitable for films where the first outer layer contains polypropylene blended with a linear low density polyethylene or a homogeneously branched substantially linear ethylene/a-olefin interpolymer.

Films used in accordance with the present invention in general show good formability using thermoforming techniques used in the packaging art. In particular, the use of a homogeneously branched substantially linear ethylene/a-olefin interpolymer as the core layer has resulted in formed packages having good puncture resistance which is believed to result from a relatively low degree of thinning of the core layer where high degrees of stretching of the film takes place during the forming process.

Films used in accordance with the present invention -can be made to a wide variety of thicknesses, for example depending on the particular packaging application in which the films are to be used. For example they can be from 50 pm to 400pm thick, with thicker or thinner films being preferred for certain end uses.

The present invention further provides packages which consist of the polymeric film heat sealed by its heat seal layer to a closure web when a heat seal layer is present.

Any of a variety of closure webs can be used to form such packages. For example, they can be polymeric films, which typically will form hermetically sealed packages when sealed to the films used in accordance with the present invention, or microporous webs which enable air to enter the packages. Microporous webs are, however, preferably such that they prevent the ingress of micro-organisms, this being necessary when sterilized medical equipment is to be packaged. Examples of microporous webs which can be used to form packages in accordance with the present invention include papers, papers impregnated with polymers, and non-woven and spunbonded polyolefin fabrics, for example the spunbonded polyethylene fabric as sold under the trade mark Tyvek.

Reference will now be made to the accompanying drawings which illustrate an embodiment of package in accordance with the invention:- Figure 1 is a perspective view of the embodiment of a heat sealed package in accordance with the invention; and Figure 2 is a perspective view of the embodiment of Figure 1 after it has been partially peeled open.

The peel open package 10 shown in Figures 1 and 2 consists of a shaped polymeric film or sheet 15 with a lid 20 adhered thereto along a continuous seal line 30. An article 25, a syringe being 9 illustrated as an example, is encapsulated between the film 15 and the lid 20.

As will be appreciated, the film 15 can be formed into any desired shape, for example to fit the general shape of the article 25. The film 15 can, for example, be shaped as a blister, a tray, a bag or a pouch.

The term "blister" is used herein to refer to a container formed from a flexible film which itself has been formed to have a recessed central portion and an adjacent substantially flat peripheral portion which continuously surrounds the central portion. The heat seal 30 is then formed between the film 15 and the lid 20 along a continuous line on the peripheral portion surrounding the central portion.

As will be appreciated, packages in accordance with the present invention having porous closure webs, and more particularly, their contents, can be sterilized by exposure to a sterilant gas, for example ethylene oxide. Packaged articles can be removed from the sealed bags or pouches by removing the lid 20 from the lower sheet 15.

The term "tray" is used herein to refer to containers having a generally similar shape to that of blisters but being formed from a generally stiffer film than is used to form blisters.

Blisters and trays are preferably formed by thermoforming flat sheets of film, for example using known methods.

The term "bag" is used herein to refer to containers formed from essentially flat films which have been folded at least once. In general, all but one of the non-folded edges of such bags will be sealed before an article is placed into the bag after which the bag can be heat sealed to form a continuous seal.

The term "pouch" is used herein to refer to packages produced from substantially flat films which have not been folded or formed but have been sealed to a lidding material along a continuous seal line.

In all cases, the packages are preferably sealed by applying heat and pressure to the area which is to be sealed.

Peel-open packages in accordance with the present invention can be produced by known methods, for example using known packaging machinery. For example, films in accordance with the present invention can be used with horizontal form-fill-seal machines which first produce a three dimensional series of containers from a reel of the film using heat and pressure or a vacuum to form a series of pockets in the film. Articles to be packaged are then placed in the pockets in the film and the machine is then used to heat seal a web of a lidding material under pressure over the pockets, for example using a heated platen, so that continuous seals are produced between the film and the lidding material around each of the pockets. Individual packages can then be cut from the resulting assembly. These machines can be used to form packages which use blisters or trays to contain the articles.

Hitherto proposed bag or pouch making machines can be used to form essentially two dimensional packages in accordance with the present invention. Using these machines, the first stage will usually be to unwind a sheet of film in accordance with the present invention from a first reel so that its heat seal layer is facing upwards.

The following examples are given by way of illustration only. All percentages are by weight unless stated otherwise.

In the Examples packs made from various films were subjected to a shipping test. The test was conducted as follows:- Each of the various films to be tested was converted into a series of strips with thermoformed recesses for four 10cm3 syringes. A syringe was then placed into each of the recesses of each strip, and the recesses were then closed by heat sealing a paper closure web over them.

Sixteen of such strips were then inserted into a cardboard box in which they fit tightly, and twelve of such boxes were then placed into larger cardboard container in which the smaller boxes fit tightly.

The containers were then dropped vertically on to the floor from a height of 38.1cm on to one of their smallest faces using a Gaynes drop tester. The containers were then rotated a quarter turn and dropped from the same height on to one of their next to smallest faces. Thereafter they were rotated in the same sense by two further quarter turns, the containers being dropped from the same height on to a face after each of these quarter turns. The containers were subsequently dropped in turn from the same height on to the two remaining faces.

The test was continued by dropping the containers from a height of 38.1cm on to successive edges starting with the longest edge of the smallest face and thereafter the longest edges of successive faces following successive quarter turns as were carried out for the four smallest faces of the containers.

Finally, the containers were dropped from a height of 38.1cm on to four successive corners starting with one of the corners of the smallest face, then the adjacent corner along the longest side of the same face, followed by the two other corners around the edge of the same large face of the containers.

The containers were then placed on a Gaynes vibration table with the largest face defined by the corners on to which the containers had been dropped in contact with the table, and the table was shaken for 30 minutes at a speed of 266 rpm. The containers were rotated by 90' and then shaken for a further 30 minutes at the same speed.

The drop test was then repeated as previously described but from a height of 76.2cm for each of the fourteen drops.

The containers and boxes were then opened and the individual strips of packaged syringes were placed in a vacuum desicator with the paper webs face down. The pressure in the desicator was then reduced and a note was made of which packs which did not inflate. These packs were then inspected individually to ascertain whether the paper web or the thermoformed film had been perforated, the failures being expressed as the number of holes per thousand syringes, the maximum allowable number being 2 per thousand syringes (i.e. 0.2%).

Z=ap 1 e S 1 - P a ri-sj2n) Six different films each 120pm thick was produced by casting mono-webs of AFFINITY PL1880, LLDPE (density 0.915g/cm3), LLDPE (density 0.935g/cm3), the MDPE/ADFLEX blend of Example 4, HDPE (density 0.945g/cm3), and ADFLEX alone.

Attempts were then made to thermoform these various films to produce wells for 10CM3 syringes using a Multivac thermoform-fill-seal machine to produce trays as illustrated in the accompanying drawings. Those which could be thermoformed were then closed with a closure web consisting of a 60g/M2 medical grade paper which was heat sealed over the trays after a syringe had been placed in each of them. The sealed packages were then subjected to the shipping test described above. The formability was assessed by the range of reduced thicknesses of the films where thermoforming had taken place, the results obtained in these tests being shown in Table 1.

Tahl P I Example Polymer Formability (pm) Shipping Test 1 PL1880 26-50 0.26% 2 LLDPE (0.915) 18-40 0.91% 3 LLDPE (0.935) no - 4 MDPE + ADFLEX 16-35 6.4% HDPE (0.945) no - 6 ADFLEX 28-50 3.2% It was impossible to thermoform two of the films, and none of those which could be thermoformed passed the shipping test.

Examples 7 to 9 (Comparison) Three layered polymeric films were produced substantially as described in Example 1 except that different materials were used for the respective layers as shown in Table 2.

The formability of these films was then assessed. Where possible, packages were produced substantially as described in Example 10 and they were also subjected to the shipping test described above.

As can be seen from Table 1, two of the packages with good formability failed the shipping test.

Example 10

A three layered film was produced by extruding through a slot die a core layer of a blend of a homogeneously branched substantially linear ethylene/cc-olefin interpolymer (AFFINITY 1880; 90 weight percent of the blend), and low density polyethylene( 10 weight percent of the blend) as a processing aid, with a first outer layer on one side of the core layer consisting of a blend of AFFINITY 1880 (90 weight percent of the blend) and polypropylene (10 weight percent of the blend), and a second outer layer consisting of a blend of a linear low density polyethylene (density O.qlg/cm3' MFI=4.6) to form a heat seal layer.

The resulting cast film was cooled and then wound up. Its total thickness was 120mm, the core layer representing 50% of the total thickness of the film, and each outer layer representing 25%.

These packages passed shipping test described above.

Tabl e 2 Example Core First Second Forma- Shipping outer outer bility test layer layer (PM) M 7 HDPE (BOwt%) + EVA EVA small AFFINITY 1880 holes (20wt%) 8 LLDPE LMDPE LLDPE 18-50 fail (>0.2) 9 MDPE (75%) + LMDPE LLDPE 22-40 fail ADFLEX (25%) (>0.2) AFFINITY 1880 Core(90%) LLDPE 25-50 0.0 PP(10%) (ADFLEX is an ethylene/propylene block copolymer with 25 percent by weight of units derived from ethylene).

Example 11 (Comparison) A three layered film similar to that of Example 4 was produced but with the outer layers both being formed from linear low density polyethylene (0. 935g/cm3, MFI=4. 6) rather than one being formed from LMDPE and the other from LLDPE. The core layer of the 120pm thick film was 70pm thick and each of the outer layers was 25pm thick.

This film showed a formability as assessed in Examples 5 to 10 of 25-54pm, but it failed the shipping test with a failure rate of 7.2%.

Example 12

A three layered polymeric film 120pm thick was produced by extruding through a slot die a core layer of AFFINITY PL1880 with an outer layer of linear low density polyethylene on each surface of the core layer, one of the linear low density polyethylene layers having a density of 0.915g/cm3 and the other a density of 0.930g/cm3. The polymers of both outer layers had melt flow index values of 4.6.

The core layer of this film was 60pm thick, each outer layer being 30pm thick.

This film was then thermoformed as described in Example 11 and then subjected to the previously described shipping test.

The formability of the film was 25 to 50pm, and the failure rate was 0.2%, thereby passing the test.

Example 13 (Comparison) A film similar to that produced in Example 12 was produced but instead of being cast through a slot die, it was formed as a bubble. The overall thickness of the film and the thicknesses of the respective layers of this film were the same as in Example 12.

This film failed the formability test.

Example 14 (Comparison) A three layered film was blown as described in Example 13, but the linear low density of the two outer layers had a density of 0.92g/cm3. The film thermoformed satisfactorily using the method described in Example 12 but it failed the shipping test.

Examp e 15 Example 14 was repeated but one of the outer layers was blended with 5. 3wt% of a propylene homopolymer.

The resulting film thermoformed satisfactorily using the method described in Example 12 and it also passed the shipping test.

Claims

1. A package comprising a polymeric film, the film comprising (a) a core layer comprising at least one homogeneously branched substantially linear ethylene /cc-o le fin interpolymer having (i) a melt flow ratio, 110/12 25.63; and (ii) a molecular weight distribution, Mw/Mn(IlO/,2)-4.63; (b) a first outer layer on the core layer comprising a linear polyethylene or a linear polyethylene/polypropylene blend; and (c) optionally a polymeric heat seal layer on the opposite side of the core layer from said first outer layer; the said first outer layer forming an outer surface of the package.

2. A package according to claim 1, wherein the substantially linear ethylene/(x-olefin interpolymer is a copolymer of ethylene and a C3-C18 (x-olefin.

3. A package according to claim 1 or claim 2, wherein the substantially linear ethylene/(x-olefin interpolymer is a copolymer of ethylene and octene-1.

4. A package according to any of the preceding claims, wherein the molecular weight distribution Mw/Mn of the interpolymer is from 1.

5 to 2. 5.

19 - 5. A package according to any of the preceding claims, wherein the melt flow ratio 110/12 of the substantially linear ethylene/(x-olefin interpolymer is at least 7.

6. A package according to any of the preceding claims, wherein the substantially linear ethylene /(x-ole fin interpolymer has a melt index 12 of from 0.01 to 10g/10 min, a density of 3 from 0.85 to 0.94g/cm, a molecular weight distribution Mw/Mn Of from 1. 5 to 2.5, and a melt flow ratio 110/12 Of at least 7.

7. A package according to any of the preceding claims, wherein the polyethylene of the first outer layer has a density of from 0.900 to 0.940g/cm3.

8. A package according to any of the preceding claims, wherein the first outer layer contains a propylene homopolymer.

9. A package according to claim 8, wherein the first outer layer contains up to 40wt% of a propylene homopolymer.

10. A package according to any of the preceding claims, wherein the heat seal layer when present has a density of from 0.905 to 0.925g/cm3.

11. A package according to any of the preceding claims, in the form of a bag or pouch.

12. A package according to any of claims 1 to 10, in the form of a blister pack or tray.

13. A package according to claim 12, wherein the film has a closure web adhered thereto.

14. A package according to claim 13, wherein the closure web comprises a polymeric film, paper, a paper impregnated with a polymer, or a non-woven or spunbonded polyolefin fabric.

15. A package according to any of the preceding claims, wherein the film has been thermoformed.

16. A package substantially as herein described with reference to the accompanying drawings.