ADHESIVE BLENDS AND COMPOSITE STRUCTURES

Abstract: Adhesive blends especially suitable for adhesion to polypropylene and gas barrier materials are disclosed, as are composite structures incorporating such blends, and methods of making such composite structures. The blends are preferably elastomer free, and comprise a mixture of an ad­ hesion-promoting amount of high molecular weight low density polyethylene having a narrow molecular weight distribution, about 0.1 to 30 wt.% of a non-elastomeric copolymer of pro­ pylene and a polymerizable, ethylenically unsaturated car­ boxylic acid or derivative, and the balance a non-elastomer­ ic ethylene/propylene copolymer wherein ethylene comprises at least 5 wt.% of the copolymer.

Description: This invention relates generally to adhesive blends and composite structures and, more particularly, this invention relates to an adhesive blend especially suitable for adhering to propylene polymers and gas barrier materials, and to composite structures incorporating such materials and adhesives. Description of Related Art The use of propylene polymers together with a gas barrier material, such as an oxygen barrier material, for example, in semi-rigid structures is gaining widespread acceptance in such markets as the food packaging industry. In such applications, propylene polymers provide structural integrity, and the the barrier material reduces the rate and extent of transmission of molecules such as oxygen through the structure. However, propylene polymers will not bond to gas barrier materials without an intervening adhesive. Heretofore, adhesives designed to bond polypropylene to such barrier layers have exhibited a number of disadvantages. The use of maleic anhydride modified polypropylene in adhesive applications is well known. Blends of modified and unmodified polypropylene and their use as metal coatings are disclosed, for example, in U.S. Patent No. 3,483,276 assigned to Hercules. Adhesives which can bond polypropylene to a barrier material (such as ethylene vinyl alcohol copolymer, for example) are described in U.S. Patent No. 4,058,647 assigned to Mitsubishi Petrochemical and in U.S. Patent No. 4,198,327 assigned to Mitsui Petrochemical. The adhesives of U.S. Patent Nos. 4,058,647 and 4,198,327 are blends of polypropylene modified with maleic anhydride, a propylene polymer, and a hydrocarbon elastomer or rubber. However, the use of an elastomer in such an adhesive blend adds to the cost of the blend, and many cause material handling problems. Also, the presence of elastomers in such blends may result in undesirably high contents of extractable materials, which may render the blends unsuitable for use in food packaging applications. Schmukler et al U.S. Patent Nos. 4,472,555 and 4,409,364 disclose adhesive blends which provide adhesion to both polypropylene and polar substrates. These blends are mixtures of modified polyethylene, polyethylene and polypropylene. Liu U.S. Patent No. 4,510,286 assigned to Hercules discloses a bonding resin composition which comprises maleic anhydride modified polypropylene, a sequentially polymerized propylene/ethylene copolymer having an ethylene content of 15-30 wt.% and a propylene polymer or propylene/ethylene copolymer having an ethylene content of 0-14 wt.%. Adhesive blends made according to the Schmukler et al or Liu patents may not provide adequate adhesion to polar substrates (such as some gas barrier materials) for some applications. SUMMARY OF THE INVENTION It is an object of the invention to overcome one or more of the problems described above. According to the invention, an adhesive blend which can bond polypropylene to a gas barrier material is provided. The invention also comprehends composite structures comprising layers of polypropylene and a gas barrier material adhered together by the adhesive blend, and methods of making such composite structures. The adhesive blend, which is substantially elastomer-free, comprises a mixture of (a) a non-elastomeric copolymer of propylene and ethylene wherein ethylene comprises at least about 5 wt. % of the copolymer but insufficient in amount to render said copolymer elastomeric, (b) about 0.1 up to about 30 wt. % of the blend of a non-elastomeric copolymer of propylene and at least one comonomer comprising a polymerizable, ethylenically unsaturated carboxylic acid or acid derivative, and (c) an adhesion-promoting effective amount between about 2 and 30 wt. % of an ethylene homopolymer having a specific gravity in the range of about 0.915-0.930, inclusive, a melt index of less than about 1 dg/min as measured at 190°C, and a relatively narrow molecular weight distribution as measured by the ratio of weight average molecular weight (M w ) to number average molecular weight (M n ) of about 3 to 5. Other objects and advantages of the invention will be apparent to those skilled in the art from the following detailed description taken in conjunction with the appended claims. The adhesive blend of the invention provides excellent adhesion between a polypropylene substrate and a polar or other gas barrier material even without the use of elastomeric materials in the blend. Elimination of the elastomer is advantageous both in economic and material handling considerations. Additionally, the elimination of elastomers is especially advantageous in food packaging materials, since elimination of elastomers results in the elimination or substantial reduction in the amount of extractable materials in the adhesive. This is highly desirable in materials which must be approved by government agencies for use in food packaging. The adhesive blend of the invention provides adhesion which is at least as good as elastomeric material-containing blends, and which may be superior thereto under some conditions. It is to be understood, however, that small amounts of an elastomeric material may be added to the blend if desired without departing from the spirit of the invention. In some cases, addition of small amounts of an elastomer may enhance adhesion, and improve impact strength and/or flexibility. The adhesion to polar and other gas barrier materials and to polypropylene is also superior to those exhibited by prior adhesive blends. The presence of the ethylene homopolymer, which is referred to in the art as "high molecular weight low density polyethylene" or "HMW LDPE" significantly improves the adhesion of the inventive blend to polar or other gas barrier substrates while eliminating the need for the presence of elastomers. The propylene/ethylene copolymer, which may comprise the bulk of the inventive blend, comprises at least about 5 wt. % ethylene. If desired, the polymer may be a terpolymer of propylene, ethylene and a third 1-olefin. The copolymer may be a random, block or impact copolymer, as desired. It is well recognized by the art that high ethylene contents (e.g. 35-40 wt. %) in propylene/ethylene copolymers may render the copolymer elastomeric. As used herein, the term "substantially non-elastomeric copolymer of propylene and ethylene" is understood to encompass those impact copolymers which may have somewhat rubbery microphases wherein the ethylene content of the copolymer is higher than the ethylene content of the copolymer as a whole, as long as the copolymer as a whole is not elastomeric. The modified polypropylene component of the blend, which generally comprises between about 0.1 and 30 wt. % of the blend, preferably 5 to 25 wt. % and most preferably 10 to 15 wt. %, may be any one of those which are well known in the art, and may be either a graft copolymer having a polypropylene backbone or a copolymer wherein the comonomer is incorporated into the polymeric chain of the copolymer. The comonomer is generally described as at least one polymerizable, ethylenically unsaturated carboxylic acid or acid derivative, such as an acid anhydride, an acid ester, an acid slat, amide or imide. Such comonomers include, but are not necessarily limited to the following: acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid, mesaconic acid, maleic anhydride, 4-methyl cyclohex-4-ene-1,2-dicarboxylic acid anhydride, bicyclo(2.2.2.)oct-5-ene-2,3-dicarboxylic acid anhydride, 1,2,3,4,5,8,9,10-octahydronaphthalene-2,3-dicarboxylic acid anhydride, 2-oxa-1,3-diketospiro(4.4)non-7-ene, bicyclo(2.2.1)hept-5-ene-2,3-dicarboxylic acid anhydride, maleopimaric acid, tetrahydrophthalic anhydride, norborn-5-ene-2,3-dicarboxylic acid anhydride, Nadic anhydride, methyl Nadic anhydride, Himic anhydride, methyl Himic anhydride, and x-methylbicyclo(2.2.1)hept-5-ene-2,3-dicarboxylic acid anhydride. If the copolymer is a graft copolymer, the polypropylene backbone may be a homopolymer or propylene/ethylene copolymer, but non-elastomeric, crystalline propylene homopolymers are preferred. The grafted polypropylene may be prepared in solution, in a fluidized bed reactor, or by melt grafting, as desired. A particularly preferred grafting monomer is maleic anhydride, and a maleic anhydride/crystalline polypropylene backbone graft copolymer having a maleic anhydride content of about 0.1-3 wt. % is very suitable for use in the invention. Those skilled in the art will recognize that relatively higher proportions of the modified polypropylene may be required as the level of comonomer therein decreases in order to maintain a desired effective level of comonomer in the blend. The ethylene homopolymer is known in the art as "high molecular weight low density polyethylene" or "HMW LDPE" and is made by a high pressure process. This material has a specific gravity in the range of about 0.915 to 0.930, and a melt index of less than 1 dg/min, and preferably within the range of 0.2-0.6, as measured at 190°C according to ASTM Method D-1238. This material is also characterized as having a relatively narrow molecular weight distribution as measured by the ratio of weight average molecular weight (M w ) to number average molecular weight (M n ) of about 3 to 5, inclusive. Conventional low density polyethylene (LDPE) typically has a broader molecular weight distribution, as measured by a ratio of about 10 to 15. Two suitable high molecular weight low density polyethylene materials are sold by Enron Chemical Company of Rolling Meadows, Illinois under the trade designation NPE 3503 and NPE 3509, respectively. Each material has a melt index of 0.3 (as measured at 190°C), a M n of about 30,000, and a ratio M w /M n of about 3.1. The specific gravity of both materials is 0.925. Other useful HMW LDPE materials are Enron Chemical Company Product NPE 3505 and those sold by Neste Oy of Finland. In contrast, one conventional low density polyethylene (Enron Chemical Company NPE 940) has an M n value of about 12,000, with a ratio of M w /M n of about 13, and a melt index of 0.25. The adhesive blend contains an adhesion-promoting effective amount of the ethylene homopolymer, generally comprising at least a bout 2 wt. % and no more than about 30 wt. % of the blend. In some applications, the ethylene homopolymer should comprise at least 5 or 10 wt. % of the blend in order to provide the desired level of adhesion. The minimum effective proportion of ethylene homopolymer will be readily ascertainable by those skilled in the art. The adhesive blend will generally comprise between about 5 and 25 wt. % of the modified polypropylene and between about 10 and 30 wt. % of the ethylene homopolymer, with the balance comprising the propylene/ethylene copolymer and any desired additives, such as stabilizers. Concentration ranges of about 5 to 15 wt. % of the modified polypropylene and about 20 to 25 wt. % of the ethylene homopolymer, with the balance propylene/ethylene copolymer, are highly preferred. The adhesive resin is preferably prepared by melt compounding the three components together with the usual additives and pelletizing the blended material. The blended material is useful in bonding propylene polymers to gas barrier materials such as metals or other polar substrates such as nylons and ethylene vinyl alcohol copolymers. As used herein with respect to blend components and composite structure substrates, the terms "polypropylene" and "propylene polymer" may be used interchangeably, and denote propylene homopolymers and copolymers, whether random, block or impact copolymers, unless otherwise indicated. The invention also comprehends a method of making composite structures wherein the inventive adhesive blend is adhered to one or more solid substrates selected from propylene polymers and gas barrier materials, such as those identified herein. Suitable methods include, but are not limited to, coextrusion (e.g. blown film coextrusion), extrusion lamination, and coextrusion blow molding. The latter is a preferred method of forming bottles, for example. The following non-limiting examples illustrate the practice of the invention. In each example, adhesives were prepared by blending as described above. After blending, the adhesive was tested by forming a two layer cast film using a 1,905 cm (3/4 inch) Wayne extruder for the adhesive and a 2,54 cm (1 inch) Killion extruder for gas barrier material and polypropylene layers. The adhesion between the layers was measured using a Chatillon tensile tester by pulling apart the layers of a 2,54 cm (1 inch) wide strip of film. The film thickness in all the examples is 0,01778 to 0,02032 cm (7 to 8 mils) total thickness with a 50:50 layer distribution. Example 1. The results of Experiments 1 to 5 show the effect of the HMW LDPE homopolymer on adhesion. The compositions of these blends and the adhesions to ethylene vinyl alcohol (EVOH) copolymer are listed in Table 1, below. Experiment 1 is a comparative experiment which shows that a blend of propylene copolymer, modified polypropylene, and 40% ethylene/60% propylene copolymer (EPM rubber) gives good adhesion to the EVOH. Replacement of the ethylene propylene copolymer with 2 melt index (MI) linear low density polyethylene (Experiment 2), a 3 HLMI (high load melt index) linear low density polyethylene (Experiment 3), or a poly(butene-1) (Experiment 4) all give poorer adhesion in the adhesive blends. However, use of a 0.3 MI low density polyethylene in Experiment 5, gives adhesion as good as found in Experiment 1 with the EPM. Comparison of Blend "Third Component" Experiment Third Component Adhesion to EVOH (lb./in.) (g/cm) #1 Vistalon 404 (Exxon) (EPM Rubber) 3.3 571,45 #2 LL1002.09 (Exxon) (LLDPE - 2 MI) 0.4 71,43 #3 K1124 (Enron Chemical) (LLDPE - 3HLMI) 1.2 214,29 #4 Shell 8340 [Poly (butene-1)] 1.0 178,58 #5 NPE3509 (HMW LDPE) 3.3 589,31 All blends contain 70 wt. % of Cosden 7350, 10 wt. % of maleic anhydride-modified polypropylene, and 20 wt. % of the "third component". The blends each contain the usual stabilizers as are well known in the art. Cosden 7350 is a random ethylene/propylene copolymer having about 6 to 8 wt.% ethylene in the copolymer chain. (Enron Chemical Co. products NPP 8602 and NPP 8752 are impact copolymers which are useful alternatives to the Cosden 7350 random copolymer). The modified polypropylene is a graft copolymer having a crystalline polypropylene homopolymer backbone, and a maleic anhydride content of about 1 to 3 wt.%. Example 2. Experiments 6 through 12 illustrate the range of base propylene polymers which can be used in the adhesive blends. In Experiment 6 a propylene homopolymer (NPP 1006) was used in the blend with resulting poor adhesion. A low ethylene content random copolymer, NPP 3303, also gave poor adhesion as shown in Experiment 7. The adhesion improved with the use of NPP 3333 (a random terpolymer) or Hercules 7623 (an impact copolymer) in Experiments 8 and 9. Cosden 7350 used in Experiment 10 results in good adhesion. The best adhesion was found when using impact copolymers (NPP 8602 and NPP 8752) as the base resin as shown in Experiments 11 and 12. The blends of Experiments 11 and 12 are also shown to be superior to that of Experiment 16 (Table 3, below) which is identical to that of Experiments 11 and 12 but for the identity of the propylene polymer. Resins for the Adhesive Matrix Experiment Matrix Resin Wt. % Adhesion to EVOH (lb./in.) (g/cm) #6 NPP 1006 (Enron) 70 0.1 17,86 #7 NPP 3303 (Enron) 70 0.2 35,72 #8 NPP 3333 (Enron) 70 1.0 178,58 #9 Hercules 7623 70 1.1 196,44 #10 Cosden 7350 70 3.2 571,45 #11 NPP 8602 (Enron) 65 CNS #12 NPP 8752 (Enron) 65 CNS CNS - could not separate All blends contain 10 wt. % modified polypropylene (as in Example 1) and the balance is NPE 3509 HMW LDPE. The blends also contain the usual stabilizers. Example 3. Experiments 13 through 16 show the concentration of the HMW LDPE necessary to promote good adhesion in the three-component blends of the invention. The experiments range from 10 wt. % in Experiment 13 to 25 wt. % in Experiment 16. The results are shown in Table 3. The adhesion of the blends increases as the concentration of the ethylene homopolymer increases. However, addition of too much HMW LDPE results in poorer physical properties for the blend. HMW LDPE Concentration in Blend Experiment Wt. % NPE 3509 (Enron) Adhesion to EVOH (lb./in.) (g/cm) #13 10 1.4 250,01 #14 15 1.9 339,30 #15 20 3.2 571,45 #16 25 3.7 660,74 These adhesive blends contain 10 wt. % of modified polypropylene (as in Example 1) and the balance is Cosden 7350 and the usual stabilizers. Example 4. The experiments in Table 4 show a portion of the concentration range of modified polypropylene which may be used in the adhesive blends of the invention. Experiments 17 through 20 show that the adhesion to EVOH increases with additional modified polypropylene to a maximum and then starts to decrease. The optimum range appears to be 10 to 15 wt. % of the modified polypropylene in the blend. Effect of Modified Polypropylene Concentration Experiment Wt. % Modified PP Adhesion to EVOH (lb./in.) (g/cm) #17 8 2.1 375,02 #18 15 3.6 642,88 #19 20 2.1 375,02 #20 25 1.3 232,15 The adhesive blend of each Experiment 17-20 contains 25 wt. % of HMW LDPE, the modified polypropylene of Example 1, and the balance Cosden 7350. The usual stabilizers are added. These examples also show a method for making a multilayer structure using the adhesive of the invention and novel multilayer structures containing the adhesive bonded to a polar substrate. The foregoing detailed description is given for clearness of understanding only and no unnecessary limitations should be understood therefrom, as modifications within the scope of the invention will be obvious to those skilled in the art.

Claims: An adhesive blend, comprising a mixture of: (a) a substantially non-elastomeric copolymer of propylene and ethylene wherein said ethylene comprises at least about 5 wt.% of said copolymer but insufficient in amount to render said copolymer elastomeric; (b) about 0.1 to 30 wt.% of said blend of a non-elastomeric copolymer of propylene and a comonomer comprising at least one polymerizable ethylenically unsaturated carboxylic acid or acid derivative; and (c) and adhesion-promoting effective amount between about 2 and 30 wt.% of said blend of an ethylene homopolymer having a specific gravity in the range of about 0.915-0.930, inclusively, a melt index of less than about 1 dg/min as measured at 190°C, and a narrow molecular weight distribution as measured by a ratio of weight average molecular weight (M w ) to number average molecular weight (M n ) of about 3 to 5, inclusively. The blend of claim 1, wherein said blend is substantially free of elastomeric material. The blend of claim 1, comprising said copolymer of (b) in an amount of about 5 to 25 wt.% of said blend and said homopolymer of (c) in an amount of about 10 to 30 wt.% of said blend. The blend of claim 1 wherein said copolymer of (a) is selected from the group consisting of random, block and impact copolymers. The blend of claim 1 wherein said copolymer of (a) is a terpolymer of propylene, ethylene and another 1-olefin. The blend of claim 1 wherein said comonomer of (b) is selected from the group consisting of acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid, mesaconic acid, maleic anhydride, 4-methyl cyclohex-4-ene-1,2-dicarboxylic acid anhydride, bicyclo(2.2.2.)oct-5-ene-2,3-dicarboxylic acid anhydride, 1,2,3,4,5,8,9,10-octahydronaphthalene-2,3-dicarboxylic acid anhydride, 2-oxa-1,3-diketospiro(4.4)non-7-ene, bicyclo (2.2.1)hept-5-ene-2,3-dicarboxylic acid anhydride, maleopimaric acid, tetrahydrophthhalic anhydride, norborn-5-ene-2,3-dicarboxylic acid anhydride, Nadic anhydride, methyl Nadic anhydride, Himic anhydride, methyl Himic anhydride, and x-methylbicyclo(2.2.1)hept-5-ene-2,3-dicarboxylic acid anhydride. The blend of claim 1 wherein said comonomer of (b) is either incorporated into the polymeric chain of said copolymer of (b) or grafted onto a propylene polymer backbone. The blend of claim 7 wherein said grafting monomer comprises between about 0.1 and 3.0 wt. % of said copolymer of (b). The blend of claim 1 wherein said homopolymer of (c) has a specific gravity of about 0.925 and a melt index of about 0.2 to 0.6. A composite structure, comprising: (A) a first substrate comprising a layer of gas barrier material adhered to (B) a second substrate comprising a layer of a propylene polymer by (C) an adhesive blend of any one of claims 1 to 9 disposed between said first and second substrates. The composite structure of claim 10, wherein said gas barrier material is a polar material, preferably selected from the group consisting of metals, nylon and ethylene vinyl alcohol copolymers. The composite structure of claim 10, wherein said second substrate is a propylene homopolymer or comprises a random, block or impact copolymer of propylene. A method of making a composite structure comprising the step of adhering: (A) a first substrate comprising a layer of gas barrier material to (B) a second substrate comprising a layer of a propylene polymer by (C) an adhesive blend disposed between said first and second substrates, said adhesive blend comprising a mixture of any one of claims 1 to 9. The method of claim 13 wherein said adhering step is carried out by coextrusion, blown film coextrusion, extrusion lamination or coextrusion blow molding. The method of claim 13 wherein said gas barrier material is a polar material, preferably selected from the group consisting of metals, nylon and ethylene vinyl alcohol copolymers. The method of claim 13 wherein said second substrate is a propylene homopolymer or comprises a random, block or impact copolymer of propylene.

Inventors: BECKER, PETER D., JACHEC, KEVIN V.
Applicants: QUANTUM CHEMICAL CORPORATION (A VIRGINIA CORP.)
Application year: 1987
Main classification: C08L23
IPC codes: B32B27, B32B7, C09J151, C09J123, B32B15, C08J5, C09J4, C08L23, C09J133