Plastic articles with compatibilized barrier layers

Abstract: Polyolefin - acrylic graft copolymers capable of compatibilising melt processible polyolefin and gas barrier resins are used therewith as adhesives in multilayer composites or as a third component therewith in monolithic articles so as to produce packaging materials and articles, such as films and bottles for use especially in food and drinks packaging.

Description: This invention is concerned with improved extruded and molded articles, such as films and bottles, with improved impact strength and reduced oxygen permeability and to methods of preparing such articles. Polyolefins combine advantageous properties both technically and commercially. Polypropylene, for example, exhibits greater hardness and stiffness than polyethylene, lower brittleness than standard polystyrene, outstanding resistance to hot water and to chemicals, and very good electrical properties. Thus, olefin homopolymers and copolymers have a wide range of applications in the form of containers, moldings, profiles, tubes, films, fibrillated filaments and textile fibers. Formed polyolefin articles may be prepared by injection or blow-molding. In particular, stretch blow-molding is a mass production process for plastic bottles replacing the glass variety. These are used for oil, wine, spirits, milk, still and CO₂-containing mineral water, soft drinks and beer. Although polyolefins possess many desirable properties, as enumerated above, in the particular applications of containers and packaging films, it is desirable to reduce the gas permeability below that normally exhibited by polyolefin films and bottles. Typically, an extruded film or blown bottle will be comprised of several layers of different polymers, arranged to exploit desirable properties and compensate for detrimental properties. For example, polypropylene offers structural integrity, but is permeable to many gases. A layer of a gas barrier resin, such as a nitrile barrier resin, will reduce the permeability to acceptable levels, even though the barrier resin itself may have undesirable properties, such as sensitivity to liquids. The barrier resin may be sandwiched between layers of the structural polyolefin to isolate the resin, while obtaining the advantages offered by the barrier. In particular, copolymers of ethylene with vinyl alcohol (EVOH) are used in the co-extrusion of packaging materials on account of the outstanding barrier properties against oxygen, nitrogen, carbon dioxide and fragrances imparted by the EVOH. Other polymers, such as poly(vinylidene chloride) (PVDC) may be used as the barrier resin. The selection of barrier resin is well within the purview of the skilled artisan and is not limited to EVOH and PVDC. One means for the preparation of such desirable packaging materials, such as films and bottles, is the co-extrusion of an outer layer of polyolefin, a layer of barrier resin, and an inner layer of polyolefin. However, the adhesion between the polyolefin and barrier layers is generally considered to be inadequate for most uses. Therefore, it is necessary to use an adhesive or tie layer between the polyolefin and the barrier layers in order to ensure sufficient mechanical strength to prevent delamination. Typical adhesive materials are copolymers of ethylene, such as sold under the trademark Plexar by Quantum Chemical, U.S.I. division. A representative five-layer bottle may then have an outer polypropylene layer, an adhesive layer, an EVOH barrier layer, a second adhesive layer, and an internal polypropylene layer. Typically, the percentage of adhesive will be about 10% by thickness. A representative construction is approximately 35.6x10⁻⁵m(14 mil) polypropylene, 21x10⁻⁵m(1.5 mil) adhesive, 56x10⁻⁵m (4 mil) EVOH, 1.5 mil adhesive, and 14 mil polypropylene. In extrusion blow-molding of bottles of this type, excess material must usually be removed from the finished product. This "scrap" will contain polypropylene, EVOH, and adhesive. For practical purposes, it is desirable to co-extrude this scrap layer with virgin polypropylene, adhesive and barrier resin forming a six-layer container in which the scrap layer is sandwiched between the outer polypropylene layer and first tie layer. In this way, the losses attributable to scrap are significantly reduced. This co-extrusion, however, often leads to significant problems with delamination and product quality. Since it is a critical element of these constructions to have the barrier layer, it is also important that the barrier layer be one which is state-of-the-art in terms of its properties. Although EVOH is a widely used and highly regarded material, it suffers from the deficiency of being susceptible to water. The presence of the polypropylene protects it from water. However, in the construction of these bottles and films, it is still necessary to use the adhesive. The adhesive material adds increased cost and raises significant questions of processibility. It, therefore, is desirable to identify additional adhesive materials which will provide a better balance of properties than that currently available at improved processing efficiencies. Our copending EP-A-0335649 discloses certain graft copolymers having a polyolefin backbone with methyl methacrylate units grafted thereto which are useful, inter alia, as compatibilisers for polyolefins with other polymers. We have now found that polymers such as these, which are capable of compatibilising polyolefins with gas barrier resins are useful as an adhesive layer for multi-layer films and bottles. We have further found that the graft copolymer sufficiently improves the compatibility between barrier resins and polyolefins to permit the production of monolithic or single layer bottles comprised of a blend of those three components. The resultant products have improved physical properties while retaining acceptable gas permeability characteristics. Accordingly, the invention provides a molded or extruded plastics article with low permeability to gases, comprising: (a) a melt-processible polyolefin layer; (b) a gas barrier resin layer; and (c) polyolefin-acrylic graft copolymer which is capable of compatibilizing the barrier resin and polyolefin and which comprises (i) a non-polar polyolefin trunk having a weight average molecular weight of 50,000 to 1,000,000 and comprising units of one or more of ethylene, propylene, butylene and 4-methylpentene optionally also containing a minor amount of 1-alkene (other than those just specified), vinyl ester, vinyl chloride, methacrylic ester, and/or methacrylic acid units; and (ii) at least one poly(methacrylate) chain grafted with a covalent bond to said trunk, said chain being present in a weight ratio to said trunk of from 1:9 to 4:1, the units of said chain having a weight average molecular weight of 20,000 to 200,000 and comprising at least 80% by weight of units of methacrylic ester of the formula CH₂=C(CH₃)COOR, where R is alkyl, aryl-substituted alkyl, substituted aryl, or substituted alkaryl, and optionally up to 20% by weight of units of acrylic and/or styrenic monomer copolymerizable with the methacrylic ester; component (c) being present either in a blend with the gas barrier resin, in an amount sufficient to enhance the capability of the gas barrier resin layer to adhere to a polyolefin layer with which it may be coextruded, or as or in a tie layer between adjacent layers of polyolefin and gas barrier resin. Reference should be made to copending EP-A-0335649 for other features and characteristics of the graft copolymers, for preferred copolymers and for methods of making the copolymers. This application teaches the preparation of a variety of graft copolymers of polyolefins, in particular polypropylene, with methacrylate side chains. The use of this copolymer in a variety of resin systems has been seen to provide vastly improved compatibility, as evidenced by reduced domain sizes in polymeric blends. In a preferred multi-layer extruded film the gas barrier resin, in film form, is sandwiched between layers of polyolefin film to which it is bonded by layers of the graft copolymer. A preferred multi-layer bottle, similarly has an outer polyolefin layer, a first graft copolymer layer, a gas barrier resin layer, a second graft copolymer layer and an inner polyolefin layer and may further contain a layer of scrap polymer between the outer polyolefin layer and the first graft copolymer layer. This layer of scrap polymer preferably comprises a blend of polyolefin, graft copolymer and barrier resin. The graft copolymeric layers bond adjacent layers together. Polypropylene, polypropylene/methacrylate graft copolymer and/or ethylene/vinyl alcohol copolymer are preferred components for the multi-layer articles above described. A particularly preferred amount of graft copolymer in the extruded or molded shaped articles of the invention is about 5 percent by weight, the remainder being polymeric materials capable of forming bottles, films and other molded or extruded articles. The invention also extends to articles wherein the barrier layer has, as one component thereof, a graftcopolymer as herein described, this being effective to enhance the capability of adhesion between the barrier layer and a polyolefin layer with which it may be coextruded. The present application discloses and claims a significant advance in the film- and bottle-making art based upon the use of this novel copolymer as a compatibilizer for known resins. Other details and preferred features of the invention will now be illustrated in the following Examples in which all parts and percentages are by weight unless otherwise specified. Example 1 Preparation of acrylic/polypropylene graft copolymer compatibilizer A polypropylene-acrylic graft copolymer was made by polymerizing a 5% ethyl acrylate-95% methyl methacrylate monomer mixture in the presence of polypropylene (weight ratio of polypropylene to monomer = 0.67:1). Radicals were generated from di-t-butyl peroxide at the rate of 0.000070 moles/liter/minute (radical flux). Monomer and initiator were fed over 120 minutes and the theoretical (100% conversion) solids at the end of the reaction is 50%. A 378,5 ℓ (100-gallon) reactor equipped with a pitched blade turbine agitator was charged with 86.2 kg (190 lb.) of Isopar E (a mixed aliphatic hydrocarbon solvent) and 34.5 kg (76 lb.) of polypropylene (Himont 6523, Wilmington, Delaware). This mix was deoxygenated by applying vacuum to degas, followed by pressurizing the nitrogen to atmospheric pressure for three cycles. Finally, the mix was pressured to 102.75 x 10³ N/m² (15 psig) with nitrogen and heated to 150°C over 2 hours. A pressure of 239.75 x 10³ N/m² (35 psig) was maintained while the batch was held at 150°C for 3 hours. Two solutions were added over a 15-minute period.The first consisted of 59 g of di-t-butyl peroxide in 841 g of Isopar E. The second consisted of 0.32 kg of ethyl acrylate and 6.14 kg of methyl methacrylate. An additional 103 g of di-t-butyl peroxide and 1479 g of Isopar E were added over 105 minutes. At the same time, 2.26 kg of ethyl acrylate and 43.0 kg of methyl methacrylate were added over 150 minutes. The reaction exotherm increased the temperature to about 160°C. After the feed was complete, 5 kg (11 lb.) of Isopar E was fed into the reaction mixture. The reaction mixture was held in the reaction kettle for an additional 30 minutes. It was then transferred to a second kettle, which was also under pressure at 150°C. During the transfer, a solution of 80 g of di-t-dodecyl disulfide in 320 g of Isopar E was added to the second kettle. Also during this transfer, three 4.54 kg (10 lb.) batches of Isopar E were fed into the reaction kettle. The material in this second kettle was fed to a 2.03 cm (0.8 inch) twin screw extruder, where devolatilization occurred. During the devolization, the next batch was prepared in a reaction kettle. It was transferred to the extruder feed kettle while extrusion continued. In this way, several batches were made in a "semi-batch" manner, batch-wise in the reactor with continuous feed to the extruder. Three samples of this material isolated at different times during the extrusion were blended with Himont 6523 polypropylene in a ratio of 4:96, pressed and tested for sag as described in U.S. Serial No. 07/315,501. All three gave properties within the range of that expected for acceptable material. A second batch of graft copolymer was prepared in a similar manner, except that the radical flux was 0.000050 (42 g of di-t-butyl peroxide plus 858 g of Isopar E in first feed, 73 g of di-t-butyl peroxide and 1502 g of Isopar E in second feed). Measurements of properties on this batch were also within the expected ranges. The graft copolymer (GCP) used for the experiments described in the following examples was prepared by blending pellets from thirteen batches prepared in the first run and one batch of material prepared in the second run. Example 2 Preparation of a co-extruded five-layer bottle using the compatibilizing additive A five-layer bottle was extruded through five separate Battenfeld Fischer extruders. Each extruder is independently controlled. Two molds for continuous extrusion of the parison were present. The materials used were polypropylene (Fortilene 41x11, Sultex), a layer of The graft copolymeric compatibilizer (GCP) of Example 1, a barrier layer of EVOH (EVAL SC-F101, Eval Corporation of America), a second layer of The graft copolymeric compatibilizer (GCP) of Example 1 and a final inner polypropylene (Fortilene 41X11) layer. The polypropylene was extruded at a temperature between 190 and 200°C. The EVOH was extruded at a temperature between 205 and 210°C. An acceptable five-layer bottle was produced in these experiments. It was determined that a GCP melt temperature of only 172°C was necessary to extrude coherent bottles which would withstand delamination using this new combination.

Claims: A molded or extruded plastics article with low permeability to gases, comprising: (a) a melt-processible polyolefin layer; (b) a gas barrier resin layer; and (c) polyolefin-acrylic graft copolymer which is capable of compatibilizing the barrier resin and polyolefin and which comprises (i) a non-polar polyolefin trunk having a weight average molecular weight of 50,000 to 1,000,000 and comprising units of one or more of ethylene, propylene, butylene and 4-methylpentene optionally also containing a minor amount of 1-alkene (other than those just specified), vinyl ester, vinyl chloride, methacrylic ester, and/or methacrylic acid units; and (ii) at least one poly(methacrylate) chain grafted with a covalent bond to said trunk, said chain being present in a weight ratio to said trunk of from 1:9 to 4:1, the units of said chain having a weight average molecular weight of 20,000 to 200,000 and comprising at least 80% by weight of units of methacrylic ester of the formula CH₂=C(CH₃)COOR, where R is alkyl, aryl-substituted alkyl, substituted aryl, or substituted alkaryl, and optionally up to 20% by weight of units of acrylic and/or styrenic monomer copolymerizable with the methacrylic ester; component (c) being present either in a blend with the gas barrier resin, in an amount sufficient to enhance the capability of the gas barrier resin layer to adhere to a polyolefin layer with which it may be coextruded, or as or in a tie layer between adjacent layers of polyolefin and gas barrier resin. A multi-layer article as claimed in Claim 1 in the form of (a) an extruded film in which the gas barrier resin in film form is bonded to and between layers of the polyolefin film by layers of the graft copolymer, or (b) a bottle having an outer polyolefin layer, a first graft copolymer layer, a gas barrier resin layer, a second graft copolymer layer, and an inner polyolefin layer wherein the graft copolymeric layers bond adjacent layers together, said bottle further optionally comprising a layer of scrap polymer, between the outer polyolefin layer and the first graft copolymer layer, said scrap layer comprising a blend of polyolefin, graft copolymer and barrier resin. An article as claimed in Claim 1 or 2 in which said gas barrier resin comprises copolymer of ethylene and vinyl alcohol, the polyolefin is polypropylene, and the graft copolymer is a methacrylate/polypropylene graft copolymer. An article as claimed in Claim 2 which is a film suitable for use in food packaging or a bottle suitable for carbonated beverages, wherein the polyolefin is polypropylene and the graft copolymer is polypropylene-methacrylate graft copolymer. A barrier layer containing as one component an amount of a graft copolymer as defined in Claim 1 and sufficient to enhance adhesion between said barrier layer and a co-extruded polyolefin layer. A process for preparing a multi-layered shaped article as claimed in any of Claims 1 to 4, comprising: (a) simultaneously co-extruding the layers so that each of them is in contact with an adjacent layer; and (b) allowing said extruded product to cool in the desired form.

Inventors: FRANTZ THOMAS MICHAEL, FREED WILLIAM THAYER, ILENDA CASMIR STANISLAUS, FRANTZ, THOMAS MICHAEL, FREED, WILLIAM THAYER, ILENDA, CASMIR STANISLAUS
Applicants: ROHM & HAAS, ROHM AND HAAS COMPANY
Application year: 1990
Main classification: C08L23
IPC codes: C08L51, B32B27, B29L9, C08L23, B32B7, C08L57, B29K23