N-ALKYL METHACRYLATE MODIFIED POLYPROPYLENE

Abstract: Disclosed are graft copolymers of polypropylene contain­ ing 3 to 100% by weight based on the weight of the polypropyl­ ene backbone, preferably 3 to 30%, of an alkyl methacrylate moiety. These are useful as adhesives for bonding polypropyl ene to chlorinate-hydrocarbon polymers.

Description: This invention relates to a modified propylene polymer that is useful as an adhesive layer in polypropylene laminates, particularly in such laminates that include a chlorinated polymer gas-barrier layer. The polyolefins and particularly polypropylene (which may include crystalline copolymers of propylene with up to about 15% of ethylene or a l-olefin of 4 to 6 carbon atoms, as well as propylene homopolymer) have a number of properties that make them highly desirable as packaging materials. For example, they are inexpensive and tough, have good surface properties, are readily drawn and shaped, are inert to most chemical materials and can be used in food packaging applications. However, polypropylene is highly permeable to oxygen and other gases that can be harmful to food products. To overcome this objection for many applications, the polypropylene is laminated with another polymer that has lower gas permeability. Chlorinated hydrocarbon polymers such as polyvinyl chloride and poly(vinylidene chloride) and their copolymers are excellent gas-barrier materials. It has been proposed to employ a layer of one of these materials as an interlayer between layers of the polypropylene to enhance the gas-barrier properties of the composite structure. A layer of a film-forming saponified ethylene-vinyl acetate copolymer has also been found to be useful for this purpose. Significant differences in polarity between polypropylene and such barrier films prevent polypropylene from adhering well to the barrier film, so it is necessary to install an adhesive layer between the polypropylene and the barrier layer in order to create a laminar structure. For instance, U.S. Patent 4,182,457 discloses an adhesive resin layer between an orienting resin layer (which may be polypropylene) and an oxygen-barrier layer (which may be an ethylene-vinyl alcohol copolymer). The most popular material suggested for use as an adhesive layer appears to be an ethylene-vinyl acetate copolymer. A wide variety of thermoplastic polymers having carbonyl groups are suggested by the above U.S. Patent, including polypropylene modified with methyl methacrylate for use with oxygen-barrier layers consisting of ethylene-vinyl alcohol copolymers or nylon resins. FR-A-1 548 820 issued in 1968 discloses a packaging film comprising two surface layers of polyolefin, for example polypropylene, with a composite gas barrier layer located between the surface layers and comprising several barrier layers, these barrier layers being rendered adherent between themselves by means of a synthetic resin, the composite barrier layer itself adhering to the two surface layers. It is stated that the barrier layers may be combinations of polyvinylidene chlorides, polyvinyl chlorides, polyvinylidene fluorides and mixtures thereof. the polyolefins may have a content of copolymerizable monomers. EP-A-0 001 313 published in 1979 discloses a process for preparing a graft copolymer by reacting a polymer of an alkene-1 in an aqueous suspension with a polar monomer in the presence of a certain specified radical initiator system. The graft copolymerisation of a crystalline ethylene-propylene copolymer and butyl methocrylate is disclosed. This document also discusses blending the modified polypropylene with unmodified polypropylene. FR-A-1 420 772 issued in 1965 and EP-A-0 041 214 published in 1981 disclose a process for grafting a large number of free-radical-polymerizable monomers, which include butyl methacrylate, onto an olefin polymer backbone which includes polypropylene. It is desirable to provide alternative laminar structures and in particular adhesive layers that function better than those presently known and adhere well to chlorinated hydrocarbon polymers as well as polypropylene. According to the invention, a polymer blend comprises polypropylene and up to 35% by weight, based on the total weight of the blend of a rubbery ethylene-propylene copolymer (that is, an ethylene-propylene copolymer containing 20 to 80% by weight, based on the weight of this copolymer, of ethylene), characterized in that the blend also contains 10 to 30% by weight, based on the total weight of the blend of a graft copolymer as hereinafter defined. The said graft copolymer is defined as a graft copolymer in which 3 to 30% by weight, based on the weight of the polypropylene backbone, of an alkyl methacrylate moiety is grafted onto a polypropylene backbone, the alkyl portion of the alkyl methacrylate moiety having the general formula - CH₂-R in which R is a 1- to 5- carbon alkyl residue. Preferably the alkyl methacrylate moiety constitutes 10 to 20% by weight, based on the weight of polypropylene backbone. Also according to the invention, a process for making a graft copolymer comprises reacting polypropylene with an alkyl methacrylate monomer in the presence of a free-radical initiator, the mole ratio of alkyl methacrylate to the initiator being within the range of 10/1 to 30/1 during the reaction, characterized in that a peroxide initiator is added to the polypropylene at the reaction temperature immediately before adding the alkyl methacrylate monomer and the monomer is then added gradually over a time period corresponding to 1 to 2 half lives of the peroxide initiator. By half-life is meant the time period during which the peroxide composition will decrease by 50% due to decomposition. The half-life of decomposition depends on the stability of the peroxide and the reaction temperature. Thus, the rate of initiator decomposition can be varied by either the choice of peroxide or the reaction temperature. Also according to the invention, as already indicated above, a polymer blend comprising polypropylene and up to 35% by weight based on the total weight of the blend, of a rubbery ethylene-propylene copolymer containing 20 to 80% by weight, based on the weight of the copolymer, of ethylene, is characterized in that it also contains 10-30% by weight, based on total weight of the blend, of a graft copolymer of polypropylene and an alkyl methacrylate in which the methacrylate moiety is present in the amount of 3 to 30% by weight, based on the weight of the polypropylene backbone, and its alkyl radical has the general formula -CH₂-R in which R is a 1 to 5 carbon alkyl residue. Also according to the invention, a laminar structure comprising a polypropylene layer, an adhesive layer and a barrier layer is characterized in that the adhesive layer is the polymer blend according to the invention. Preferably in the laminar structure according to the invention, the barrier layer is a film-forming chlorinated hydrocarbon polymer. This can be either a homopolymer of a chlorinated hydrocarbon, e.g. poly-(vinylidene) chloride or poly(vinyl chloride) or a copolymer of various other vinyl monomers with vinylidene chloride in appropriate mole ratios. Preferred are the vinyl chloride-vinylidene chloride copolymers. The chlorinated hydrocarbon polymers impart to the laminates of the invention a very high resistance to permeation by gases, including oxygen, water vapor and carbon dioxide. These polymers also undergo molecular orientation on drawing within the same approximate temperature range as does polypropylene, thus they do not detract from the favorable physical properties; i.e., tensile strength, stiffness and the like, exhibited by polypropylene. The laminar structure according to the invention may be used as a film or sheet, or it can be converted, by thermo-forming for instance, into a shaped article such as a container having the same barrier abilities. The laminated structures of this invention, which as indicated may have a minimum of three layers, preferably have at least five layers, including a second adhesive layer and a second polypropylene layer. In some cases additional layers may be desirable. As the polypropylene layer it is preferred to use a homopolymer of propylene having a molecular weight within the film forming range; i.e., having an intrinsic viscosity of about 2 to 5. However, the term "polypropylene" is used as indicated above in a broad sense to include crystalline copolymers containing copolymers of propylene with up to about 15% of ethylene or a l-olefin having 4 to 6 carbon atoms. High or low density polyethylene can be employed as a substitute for polypropylene, but these are generally not favored for use in molded containers as the polyethylenes do not have the favored surface gloss and toughness of polypropylene. To prepare the graft copolymers used in the invention, a relatively standard grafting technique is employed. The graft copolymers are prepared at a temperature below the softening point of polypropylene via a solvent-free, vapor phase reaction in the presence of a free-radical-forming catalyst. Reaction conditions appear to be important for getting optimum adhesive qualities in the resultant copolymer. Preferably the reaction conditions are maintained within a closely defined narrow range that results in the appropriate graft add-on level and most effective distribution of the polar functionality. In addition to achieving the optimal amount of graft attached to the polymer chain of about 10 to 20 wt % based on the weight of polypropylene, it appears to be desirable, for optimum adhesion, to ensure that the mole ratio of alkyl methacrylate to graft initiator is, as already indicated, within the range of about 10/1 to 30/1 during the reaction. It is theorized that maintenance of the relationships among the parameters that control graft chain initiation, growth and termination has a beneficial effect on adhesion by virtue of its effect on the chain length of the grafted alkyl methacrylate moieties. Thus, a low ratio of alkyl methacrylate compound to catalyst leads to formation of a relatively large number of poly(alkyl methacrylate) chains of relatively short length. On the other end of the scale, a high ratio leads to formation of relatively long poly(alkyl methacrylate) chains, but relatively few of them. For the best adhesion, there appears to be a minimum chain length whereby the grafted chains are long enough to become entangled with the chlorinated hydrocarbon polymer, as well as a minimum number of such chains to have sufficient adhering contact points between the polymer layers in the film or sheet. Thus, outside of the specified range of alkyl methacrylate-to-initiator ratios where one of these conditions is not satisfied, adhesion will result but is frequently less than optimum. The best distribution of the alkyl methacrylate grafts on the polypropylene backbone and the highest degree of uniformity of the graft chain lengths are effected if the formation of active grafting sites on the polypropylene backbone is coordinated, as already described, with the introduction of the alkyl methacrylate monomer into the reaction environment, by adding the alkyl methacrylate monomer over a time period corresponding to about 1 to 2 half-lives of a peroxide initiator. The half-life of decomposition depends on the stability of the peroxide and the reaction temperature. Thus, the rate of initiator decomposition can be varied by either the choice of peroxide or the reaction temperature. For example, it is reported that the half-life of t-butyl perbenzoate at 135°C is 15 minutes. Thus, using this initiator, the monomer should be uniformly added over a 15-30 minute charge period. A monomer addition period of 15-60 minutes is considered optimal and the peroxide half-life preferably should be compatible with the choice of the addition time within that range. The reaction can be carried out with any of the known free radical initiators as a catalyst. These include, by way of example t-butyl perbenzoate, t-butyl peroxytoluate, OO-t-butyl O-isopropyl monoperoxy carbonate, OO-t-butyl O-2-ethyl-hexyl monoperoxy carbonate, or 2,4-dimethyl[2,4-di-t-butyl-peroxy] hexane; t-butyl perbenzoate is a preferred initiator. The following examples demonstrate the preparation of the polypropylene, alkyl methacrylate graft copolymer. Example 1 To a 2-liter heated reactor equipped with a high speed blade agitator is charged 100 parts by weight of propylene homopolymer (Pro-Fax 6501 from HIMONT Incorporated). The reactor is purged and blanketed with nitrogen and the temperature within the reactor is raised to about 135°C. When the temperature is stabilized, and with the agitator running, one (1) part by weight of t-butyl perbenzoate is sprayed in. After several minutes agitation time to allow complete dispersal of the perbenzoate, 20 parts of n-butyl methacrylate is sprayed in over about a fifteen-minute-time period. After an additional 60 minutes, the reactor is evacuated at 75 mm Hg for about 50 minutes to remove any residual n-butyl methacrylate, returned to atomspheric pressure and allowed to cool to room temperature. The grafted product is removed from the reactor in the original physical form of the polypropylene. Approximately 115 parts of product are removed from the reactor following the evacuation and cooling. Extraction of a sample of this material with toluene and acetone recovers only trace amounts of extractables indicating that the n-butyl methacrylate is chemically bonded to the polypropylene backbone. The gain in weight of product over starting polypropylene indicates that about 15% n-butyl methacrylate has added onto the polymer chain. Examples 2 and 3 The procedure of Example 1 is repeated using 10 parts (Example 2) and 17 parts (Example 3) of n-butyl methacrylate. Otherwise the procedure is similar. Extraction of the product indicates that the graft efficiency was high in both cases. Examples 4-8 Using the techniques of Example 1, a series of graft copolymers are prepared as listed in the following table. Example 9 Fifty (50) pounds (22.7 kg) of homopolypropylene (Pro-Fax 6501 from Himont Incorporated) is charged to a 130 liter reactor equipped with plow blade agitators. The reactor was inerted with N₂ and the polypropylene agitated at 190 ppm. After the system was inerted, the reactor and its contents were heated to 140°C. When the polypropylene temperature reaches 135°C, 0.5 lbs. (0.227 kg) of t-butyl perbenzoate is sprayed onto the flake. Within 10 minutes, the addition of 10 lbs. (4.54 kg) of n-butyl methacrylate is begun. The monomer is sprayed on the polypropylene and peroxide mixture over a 25 minute addition period. After the monomer addition is completed, the material is reacted out for 15 additional minutes, when vacuum stripped for 1 hour. The alkyl methacrylate graft copolymers used in this invention can be employed by themselves as adhesive layers in polypropylene/chlorinated hydrocarbon polymer composites. The presence of the polypropylene backbone promotes compatibility with polypropylene and thus good adhesion to the polypropylene layer. Likewise the presence of the relatively polar carbonyl oxygen in the methacrylate moiety promotes adhesion to the chlorinated hydrocarbon polymer. However, for economic reasons it is desirable to blend the graft copolymers with polypropylene for use as the adhesive layer. It has been found that dilution of the grafts with polypropylene yields adhesive strengths that are well beyond the commercially required minimum. In general, the blends with polypropylene will contain about 10 to 30% by weight of graft copolymer. It is believed that the relative incompatibility of the polar methacrylate group with polypropylene causes the methacrylate groups to be rejected by the body of the polymer and thus concentrated at the interface between the adhesive layer and the barrier layer, where they are available to contact the chlorinated hydrocarbon layer to which it is intended to promote adhesion. It has also been noted that the reaction conditions employed in the grafting reaction sometimes lead to degradation of the polypropylene chain so that the extrusion and film forming properties of the grafted product are not as good as might be wanted in a coextrusion operation. Any potential problem on this point is avoided by use of the blend with polypropylene. Further improvement in the adhesion with the chlorinated hydrocarbon polymer layer is effected by including an elastomeric or non-crystalline ethylene-propylene copolymer in the formulation. A level up to about 35%, preferably about 30% by weight is optimum. The ethylene-propylene copolymer is one containing about 20 to 80% by weight of ethylene. It can be present as ethylene-propylene rubber (EPR), a commercially available product. It can also be present in the form of in-situ copolymer in which non-crystalline ethylene-propylene copolymer blocks are polymerized in with propylene homopolymer blocks. In this latter instance, the block copolymer can be employed to provide both the polypropylene and the ethylene-propylene copolymer. In the examples that follow, the good adhesion achievable between polypropylene and chlorinated hydrocarbon polymers by the use of this invention is shown. Adhesion is determined by means of the "T-Peel" adhesion test (ASTM D-1876) in which the bond between the adhesive layer and the adjacent polypropylene and polyvinylidene chloride layer is measured. This is accomplished by clamping the polypropylene side of the laminate in the jaw of an Instron Universal Tester, clamping the polyvinylidene chloride side of the laminate in the opposing jaw, and increasing the jaw separation distance at a rate of 10 inches/minute. The movement may be accomplished by moving one or both jaws in opposite directions. This movement causes the two sides of the laminate to "peel" from each other and the force required to accomplish this movement is recorded as the "T-peel" value. Example 10 An adhesive blend is prepared by dry-blending varying amounts of the graft copolymers of Examples 1, 2, and 3 with various levels of ethylene-propylene copolymer rubber (EPR) (Royalene 100 - low M.W. EPR from Uniroyal) and a block copolymer containing propylene homopolymer blocks and noncrystalline ethylene--propylene copolymer blocks (Pro-Fax 8501 from HIMONT Incorporated, Wilmington, Delaware). This blend is added to a compounding device at 200°C and mixed until it is completely melted and substantially uniform. After cooling, a portion of the blend is placed between teflon sheets and the teflon sheets placed between aluminum plates. The plates are preheated for two minutes at 220°C, then pressed at 10 tons (9071.8 kg) force for two minutes to prepare a six (6) mil (0.15 mm) film. The film is then removed from the press and quenched. The adhesive films are compression laminated between polypropylene films and polyvinylidene chloride films in a press at 190° to 205°C at 0.5 to 1 ton (453.9 to 907.2 kg) force for about 15 or 30 seconds, then quenched in cold water. Pertinent data concerning formulations and their adhesive properties are recorded in Table 2. Example 11 Using the method of Example 10, a series of adhesive resin blends is prepared using graft copolymers of other alkyl methacrylates prepared in Examples 4, 5, 6, 7 and 8. These are laminated to poly(vinylidene chloride) via compression lamination to demonstrate their adhesive properties. Particulars of these tests are recorded in Table 3. Examples 12-15 The graft copolymer described in Example 9 is extrusion compounded into adhesive blends useful for bonding polypropylene to poly-vinylidene chloride polymers. These blends consist of mixtures of the graft copolymer with ethylene-propylene rubber (EPR) and crystalline polypropylene copolymers containing various levels of ethylene comonomer. The composition of these blends is listed in Table 4. Examples 16 and 17 Films made from the blends identified as Examples 12 and 13 are compression laminated between polypropylene and polyvinylidene chloride (PVDC) polymer layers. Results are presented in Table 5. Examples 18-21 Adhesive films are prepared using the formulations described in Examples 12-15. These materials are compression laminated to PVDC at 1-ton (907.2 kg) pressure and 205°C. The same formulations are also coextruded into five-layer shells with two outside polypropylene, two adhesive layers, and a center PVDC layer using a 24-inch (61 cm) wide coextrusion die. The adhesion in both the coextruded sheet and the laminate sheet is measured by the T-peel test. Results are presented in Table 6. The inter layer adhesion between the polypropylene and PVDC layers achieved via coextrusion is very high, approaching the value achieved by compression lamination. Examples 22 and 23 The adhesives described in Examples 12 and 14 are provided to a major food packager for a coextrusion trial. This 35-mil (0.889mm), 5-layer composite sheet is made for subsequent thermoforming. The resultant sheets are tested for interlayer adhesion, giving the results listed in Table 7. This composite sheet is subsequently thermoformed into shallow food trays. The sheet made using these adhesives displays excellent formability and the final product demonstrates good adhesion between the PVDC and polypropylene layers.

Claims: A polymer blend comprising polypropylene (which may be a propylene homopolymer or may be a crystalline copolymer of propylene with up to 15% of ethylene or a 1-olefin of 4 to 6 carbon atoms) and up to 35% by weight, based on the total weight of the blend, of a rubbery ethylene-propylene copolymer (that is, an ethylene-propylene copolymer containing 20 to 80% by weight, based on the weight of this copolymer, of ethylene), characterized in that it also contains 10-30% by weight, based on total weight of the blend, of a graft copolymer of polypropylene and alkyl methacrylate in which the methacrylate moiety is present in the amount of 3 to 30% by weight, based on the weight of the polypropylene backbone, and its alkyl radical has the general formula -CH₂-R in which R is a 1 to 5 carbon alkyl residue. A polymer blend as claimed in claim 1, further characterized in that the alkyl methacrylate moiety constitutes 10 to 20% by weight, based on the weight of the polypropylene backbone. A polymer blend as claimed in claim 1 or 2, further characterized in that the polypropylene backbone is a polypropylene homopolymer. A polymer blend as claimed in any of claims 1-3, further characterized in that the alkyl methacrylate is n-butyl methacrylate. A polymer blend as claimed in any of claims 1-3, further characterized in that the alkyl methacrylate is n-pentyl methacrylate. A laminer structure including a polypropylene layer, an adhesive layer and a barrier layer, characterized in that the adhesive layer is a polymer blend as claimed in any of claims 1-5. A laminar structure as claimed in claim 6, further characterized in that the barrier layer is a film-forming chlorinated hydrocarbon polymer. A process for making a graft copolymer by reacting polypropylene with an alkyl methacrylate monomer in the presence of a free-radical initiator, the mole ratio of alkyl methacrylate to the initiator being within the range of 10/1 to 30/1 during the reaction, characterised in that a peroxide initiator is added to the polypropylene at the reaction temperature immediately before adding the alkyl methacrylate monomer and the monomer is then added gradually over a time period corresponding to 1 to 2 half lives of the peroxide initiator.

Inventors: KLOSIEWICZ, DANIEL WILLIAM
Applicants: HERCULES INCORPORATED
Application year: 1986
Main classification: C08F255
IPC codes: C09J151, B32B27, C08F255