WO1996016787A1

WO1996016787A1 - A metallizable composite film

Info

Publication number: WO1996016787A1
Application number: PCT/US1995/013066
Authority: WO
Inventors: Riccardo Balloni; Mingliang Lawrence Tsai
Original assignee: Mobil Oil Corporation
Priority date: 1994-12-02
Filing date: 1995-10-03
Publication date: 1996-06-06
Also published as: AU3860295A

Abstract

A metallizable composite film having a polyolefin core layer and a metallizable layer on one surface of the core. The metallizable layer is an amorphous propylene homopolymer which has a high degree of atacticity. The metallizable layer of this composite film is also excellently receptive to the adherence of ink thereon without the requirement of a primer coating. A conventional heat sealable layer can be provided on the other side of the core layer. The composite film of this invention is preferably oriented, most preferably biaxially oriented. Without the metal or the ink disposed on the surface thereof, the composite film of this invention has excellent optical quality.

Description

A METALLIZABLE COMPOSITE FILM

This invention is directed to composite films comprising polyolefins. It more particularly refers to such films which have the ability to excellently adhere metals, particularly metal films, to at least one surface thereof.

Composite films of various plastics are well known. In fact there are almost an infinite variety of combinations of layers of plastic materials which can be formed into composite films. Such composite films can be made to possess the desired properties on each exposed side thereof as well as in a core layer. However, it is difficult, if not impossible, to accurately predict what the interaction of the different layers will do to the overall properties of the film before such films are actually made and tested.

Metallizable and metallized plastic films are readily available commercial materials. The problem with these films is in the adherence of the metal to the plastic substrate. The same thing is true of ink receptive films. Unmodified plastics seldom have good ink or metal adhesion. However, there are many commercially available means for modifying the surface of a plastic film to increase its ink receptivity and metal adherence. These include the well known techniques of corona discharge and flame treatment.

In the past, various polyolefin homopolymers, including some polypropylene homopolymers, have been used to enhance metallization performance of propylene polymer based films. These combinations have resulted in composite films with improved barrier properties, metal adhesion properties, and lamination properties.

United States Patent 3,674,536 has disclosed the use of coumerone-indene resin incorporated into a crystalline polyolefin film onto which the metal will be adhered by vacuum metallizing techniques. The incorporation of the coumerone-indene resin is said to improve the adhesion of the metal to the polymer surface. In United States Patents 4,345,005 and 4,604,322 there are disclosures of using an ethylene-propylene copolymer as the surface layer of a film onto which the metal will be deposited. United States Patent 4,357,383 describes a multilayer composite film suitable for metallizing in which the core layer of the film is a polyolefin and the metallizable layer is a random ethylene polymer. United States Patent 4,508,786 shows using a blend of a copolymer of crystalline polypropylene and an alpha olefin, and high density polyethylene as the metallizable skin layer.

United States Patent 4,888,237 discloses a composite film in which the metallizable skin layer comprises a completely isotactic propylene polymer which has a degree of atacticity of not more than 6%. This means that the polypropylene of the patent has an isotacticity of at least 94%. In addition, the metallizable skin layer of the patent contains up to 34% of an ethylene-propylene copolymer adhesion promoting resin, and up to 50% of a second olefin polymer having a degree of isotacticity of 6% to 15%.

United States Patent 5,049,436 discloses a multilayer film which has good heat sealability in that one surface of a highly crystalline, isotactic polypropylene core layer has coated thereon a heat sealable surface layer comprising an ethylene-propylene copolymer or an ethylene-propylene- butene terpolymer. The heat sealable surface layer preferably contains a particulate inorganic anti-blocking agent and a fatty acid amide. The disclosed composite multilayer film also has, disposed on the other surface of the polypropylene core layer, a metal receiving layer. The metal receiving layer, referred to as the ^■■A" layer in the patent, is said to be isotactic polypropylene, an ethylene- propylene copolymer, or blends of isotactic polypropylene, ethylene-propylene copolymer, and a minor amount of atactic polypropylene.

The above ^/436 patent is silent on the degree of isotacticity or the degree of atacticity which is exhibited by the particular polymers so characterized therein. However, the '436 patent does refer (in column 3, line 14) to United States Patent 4,888,237 which is, in turn, referred to hereinabove. It is stated in the ^,436 patent that a good film for use as the "A" layer of the disclosed composite film has been treated as set forth in the ^,237 patent. It is to be noted that in the '237 patent the relevant film layer is characterized as a substantially isotactic homopolymer having a degree of isotacticity of at least 94%.

It is to be noted that while polymers are generally characterized as isotactic or atactic or syndiotactic, they are not 100% of any of these configurations. A characterization of ^■•isotactic" for example means that the polymer has more isotactic chains than atactic chains. Therefore, it is appropriate to look at a polymer's quantitative percentage of isotacticity or atacticity rather than at its qualitative characterization. Currently, in the plastics industry, commercial, film grade polypropylene is mainly propylene homopolymer with an isotacticity of at least 93%, a weight average molecular weight of 300,000 to 400,000, and a heptane solubility of 3 to 7%. That is, such polymers are highly isotactic with a large majority of their polymer chains in an isotactic configuration.

Applying this definition to the materials disclosed in the '436 patent, it would appear that the relevant metallizable layer is disclosed to have been made up of isotactic propylene polymer, that is, if it is commercial film grade isotactic polyporopylene, it has an isotacticity of at least 93%, that is an atacticity of not more than 7%. Considering the disclosure of the '436 patent in concert with the disclosure of the '237 patent, it is possible that the isotacticity of the propylene polymer material being used in the '436 patent was as low as 80 to 93%, and therefore it might have had an atacticity as high as 7 to 20%.

United States Patent 5,106,917 discloses a laminated film comprising a backing member and a heat sealing member. In this structure, the heat sealing member is a blend of polybutene, and a styrene-olefin block copolymer, which may also contain polypropylene. The patent notes that this composite film may have aluminum coated on the heat sealable member.

One aspect of this invention comprises a composite, self supporting film comprising a polyolefin core layer and a metallizable layer on one surface of the core. The metallizable layer is an amorphous propylene homopolymer which has a high degree of atacticity. The metallizable layer of this composite film is also excellently receptive to the adherence of ink thereon without the requirement of a primer coating. A conventional heat sealable layer can be provided on the other side of the core layer. The composite film of this invention is preferably oriented, most preferably biaxially oriented. Without the metal or the ink disposed on the surface thereof, the composite film of this invention has excellent optical quality.

The core layer of the composite film of this invention is a polyolefin. Preferably, it is an olefin homopolymer such as polyethylene or polypropylene. The thickness of this core layer is not a critical parameter. However, it is intended that the composite film of this invention will be suitable for use in packaging. Therefore, the core layer should not be so thick that it adversely affects the flexibility of the composite film. Thicknesses of from 0.30 to 3.00 mil, preferably from 0.60 to 1.00 mil, have been found to be suitable.

The metallizable layer of the composite film of this invention is an essential feature. It is characterized as being made up of amorphous, atactic polypropylene. This layer has an atacticity of at least 60% and an isotacticity of less than 40%. It is highly soluble in heptane, having a heptane solubility of at least 60%. The metallizable layer has a heat of fusion of not more than 30 joules per gram, e.g., from 15 to 30 joules per gram (compared to the heat of fusion of 100% crystalline polypropylene of 165 joules per gram) , and a relatively low density, when measured at 25*C according to ASTM procedure No. D 792, of up to 0.88 gm/cc, e.g., from 0.86 to 0.87 gm/cc.

The metallizable layer suitably comprises an amorphous propylene homopolymer. It is considered to be within the scope of this invention for the metallizable layer to contain a small proportion of other polymeric materials admixed therewith, preferably not more than 10% of other polymers. The usual additives may be incorporated with the amorphous, atactic propylene homopolymer metallizable layer in the usual proportions for the usual purposes. The amorphous polypropylene to be used as the metallizable layer is commercially available in large lumps and it is desirable to reduce the size of these lumps before compounding. This is suitably accomplished by cooling these lumps to a temperature below its glass transition temperature, e.g., -20*C, and then subjecting the cooled lumps to conventional size reduction at such low temperature.

The practice of this invention calls for the metallizable layer to be atactic amorphous polypropylene. It is preferred in the practice of this invention to use this polymer alone as the metallizable layer. However, it is considered to be within the scope of this invention to use blends of such amorphous, atactic homopolymers with other materials which improve its processability. Atactic, amorphous polypropylene homopolymer for use herein has a very low melt viscosity of less than 15,000 centipoise, e.g., from 8,000 to 10,000 centipoise, when measured at 190"C which makes it difficult to process onto a suitable core except by coextrusion. Therefore blending this essential material with a small amount of conventional polypropylene having a melt flow rate of from 1 to 10 grams/minute serves to improve its processability. It is considered to be within the scope of this invention to employ up to 30 wt.% of this conventional polypropylene, but not so much as to substantially reduce the adhesion of later deposited metal to this surface. Therefore, the metallizable layer will comprise at least 70 wt.% of atactic amorphous polypropylene.

It has been found that, although it is common knowledge in the plastic film industry that vacuum deposited metal does not adhere well to conventional isotactic, crystalline polypropylene, when a composite of a polyolefin core layer and an amorphous, atactic propylene homopolymer metallizable layer is assembled, metal adheres better to the deposited metallizable layer than it did to prior art composite film structures which had a different core and/or metallizable layer composition and structure. In particular, the adherence of metal to amorphous, atactic polypropylene is far superior to the adherence of deposited metal to crystalline, isotactic polypropylene. Further, the metallized composite film of this invention has excellent moisture barrier properties.

While it is known that the poor adhesion of metal to crystalline, isotactic polypropylene can be improved by blending the polypropylene with other, low melting point polyolefins, such as, for example, polyethylene or polybutene, due to the lower softening points of these added polymers, a problem of metal crazing during extrusion lamination is created by their use. The specific composite film of this invention does not suffer from this disadvantage. When the metallizable layer is limited to at least 70 wt.% amorphous, atactic propylene homopolymer, metal can be extrusion laminated to the film without crazing being a problem. Amorphous, atactic polypropylene has the exact combination of properties, i.e., low crystallinity, low molecular weight, tackiness, and high softening point, that prevents the skin layer from fracturing during the lamination operation. The softening point of the amorphous polypropylene of this invention is from 140"C to 160'C, e.g., 157°C, as compared to the lower softening points of prior art polymers which have been sought to be used as a metallizable layer, e.g. , ethylene- propylene copolymer having a softening point of 135"C and ethylene-propylene-butene terpolymer having a softening point of 125*C. This higher softening point of the essential amorphous, atactic propylene homopolymer metallizable layer is extremely important in the practice of this invention.

The metals which can be deposited on the metallizable layer of the composite film of this invention include one or more of aluminum, copper, chromium, magnesium, nickel, zinc, tin, silver, gold, titanium, silicon, and bismuth. Combinations of metals can be simultaneously or sequentially applied. The metal can be applied in conventional thicknesses depending on the particular use which is to be made of the film. Suitably, the metal should be deposited in thicknesses of from 10 nm (100 Angstroms) to 80 nm (800 Angstroms) . This metallized film according to this invention provides a substantial light barrier and as such is particularly well suited to use in food wrapping application, particularly where the food is easily oxidized, or where the food is such that light catalyzes its spoilage. Gas and moisture barrier properties are substantially improved as well.

The composite film of this invention may also have a heat sealable layer on the side thereof away from the metallizable layer. The heat sealable layer is generally per se conventional. It may comprise a polyolefin sealant which is suitably an olefin homopolymer or copolymer which may have two or more monomer components. It may comprise an ionomer, such as Surlyn*. It may alternatively comprise a mixture of heat sealable polymers as may be required for any particular application.

The composite film of this invention may be produced by the method which comprises coextruding a first polymer composition corresponding to the core layer and a metallizable second polymer composition comprising at least 70 wt.% of the amorphous, atactic polypropylene homopolymer having a degree of isotacticity of less than 40%, a degree of atacticity of at least 60%, a heptane solubility of at least 60%, a heat of fusion of up to 30 joules/gram, and a solid density, measured at 25*C, of up to 0.88 gm/cc into a -8- coextrudate film, and quenching the coextrudate film at a temperature of from, for example, 20°C to 50"C. Since it is preferable that the improved film of this invention be oriented, the coextrudate film may be subjected to reheating to a temperature of from, for example, 125^βC to 145"C, stretching the reheated film in the machine direction an amount sufficient to machine direction orient said film, and then stretching said oriented film in a direction normal to said machine direction an amount sufficient to transversely orient said film.

The method for manufacture of the present improved composite film may include the step of applying the metal layer to the exposed surface of the second polymer composition comprising the amorphous, atactic polypropylene homopolymer. Of course, an ink layer may be applied as a further manufacturing step to the exposed surface of the second polymer composition.

The method for manufacture of the present improved film may also include a further step of applying a heat sealable layer on the side of the core layer opposite the side on which the second polymer is disposed. In fact, the core layer, metallizable second layer, and the heat sealable layer may be coextruded.

SPECIFIC EXAMPLES

The composite films of this invention are suitably made by a coextrusion technique. Amorphous, atactic polypropylene homopolymer (specifically Rexene RT 2180, a material having a degree of atacticity of 82 to 85%, a heptane solubility of higher than 80%, a weight average molecular weight of 60,000, a number average molecular weight of 18,000 as determined by gel permeation chromatography, and a viscosity of 8,000 cp) was compounded with conventional polypropylene homopolymer (specifically Shell PDC 1120 or Amoco 10-6400, which have a degree of atacticity of from 3% to 7% and a degree of crystallinity of from 57% to 64%) in the relative proportions set forth in the following Table. The compounded polymers were coextruded with a polypropylene homopolymer (specifically Fina 3371X having a degree of isotacticity of from 3% to 7% and a degree of crystallinity of from 57% to 64%) and with a heat sealable ethylene-propylene copolymer resin to make three- layer composite films of a 0.7 mil thickness. These films were metallized with aluminum to an extent sufficient to provide an optical density of 2.3.

The properties of these composite films are reported in the following Table.

Percentage of Treatment Amorphous pp in Level, Lamination Skin Resin. %"' dyne/ Metal Bond Q/in'" Metal

Example cm'¹" OTR"' WV R"" Pickoff, % practure

1 0 36 2.495 17.5 lo 40 No

(16.10) (0.113)

2 2 36 1.009 8.5 < 10 100 No (6.51) (0.055)

3 5 36 0.505 5.3 < 5 Tear No (3.26) (0.034)

4 10 36 0.329 4.0 < 5 Tear No (2.12) (0.026)

5 15 36 0.505 3.9 < 5 Tear No (3.26) (0.025)

6 25 36 0.391 3.4 < 5 Tear NO (2.52) (0.022)

^•** All films were made to a thickness of 0.70 mil. Metallizable skin thickness was 0.03 mil.

^<b) Base films were treated on metallizable skin side.

^(c) Oxygen transmission rate at 23°C (73"F), 0% RH in cc/100 cm²/day (cc/100 in/day) .

^(d> Water vapor transmission rate at 37.8°C (100*F), 90% RH in g x 10"³/100 cm²/day (g/100 in²/day) .

•^•> Tear represents polymer was torn apart before lamination bond value could be obtained. It generally means excellent bonds.

Claims

1. A composite film comprising a polyolefin core and a metallizable layer comprising polypropylene on at least one surface of said core, said metallizable layer comprising at least 70 wt.% of amorphous, atactic propylene homopolymer having a softening point of from 140*C to 160*C; a melt viscosity, measured at 190"C, of less than 15,000 cp; a degree of isotacticity of less than 40% and an atacticity of at least 60%; a heat of fusion of not more than 30 joules/gram; and a density, measured at 25"C, of up to 0.88 gm/cc.

2. The composite film of claim 1 having metal deposited on the exposed surface of said metallizable layer.

3. The composite film of claim 2 wherein said metal comprises aluminum, copper, chromium, magnesium, nickel, zinc, tin, silver, gold, titanium, silicon, or bismuth.

4. The composite film of claim 1 which is optically clear.

5. The composite film of claim 1 having ink adhered to an exposed surface of said metallizable layer.

6. The composite film of claim 1, 2, 3, 4, or 5 wherein said core layer comprises polyethylene.

7. The composite film of claim 1, 2, 3, 4, or 5 wherein said core layer comprises polypropylene.

8. The composite film of claim 1, 2, 3, 4, or 5 having a heat sealable layer on the side of said core layer opposite to the side thereof which has said metallizable layer disposed thereon.

9. The composite film of claim 8 wherein said heat sealable layer comprises a polyolefinic homopolymer.

10. The composite film of claim 8 wherein said metallizable layer consists essentially of atactic, amorphous polypropylene homopolymer.

11. The composite film of claim 8 wherein said heat sealable layer comprises a copolymer comprising an ionomer.

12. A method of forming a composite film comprising at least one core layer and at least one metallizable layer on at least one surface of said core layer, which method comprises: coextruding a first polymer composition corresponding to said core and a second polymer composition comprising at least 70 wt.% of an amorphous, atactic polypropylene homopolymer having a degree of isotacticity of less than 40%, a degree of atacticity of at least 60%, a heptane solubility of at least 60%, a heat of fusion of up to 30 joules/gram, and a solid density, measured at 25^SC₍ of up to 0.88 gm/cc into a coextrudate film; quenching said coextrudate film at a temperature of 20^βC to 50°C; reheating said quenched film to a temperature of 125°C to 145"C; stretching said reheated film in the machine direction an amount sufficient to machine direction orient said film; and then stretching said oriented film in a direction normal to said machine direction an amount sufficient to transversely orient said film.

13. The method of claim 12 including the further step of applying a metal layer to the exposed layer of said second polymer.

14. The method of claim 12 including the further step of applying an ink layer to the exposed layer of said second polymer.

15. The method of claim 12 including the further step of applying a heat sealable layer on the side of said core opposite to the side on which said metallizable layer is disposed.

16. The method of claim 12 wherein said metallizable layer consists essentially of said amorphous, atactic polypropylene homopolymer.

17. The method of claim 15 wherein said core layer, said heat sealable layer, and said metallizable layer are coextruded.