SUBSTANTIALLY NON-CROSSLINKED ETHYLENE-METHYLACRYLATE COPOLYMERS GRAFTED WITH A GRAFTING AGENT AND PROCESSES FOR THEIR PREPARATION

Abstract: Disclosed is a process for preparing substantially non-crosslinked ethylene methylacrylate copolymers grafted with a grafting agent such as maleic anhydride. In particular, in the process of the present invention, it has been found that the use of very small amounts, i.e., less than about 0.2 weight percent, of a free-radical initiator in combination with the grafting agent and the ethylene methylacrylate copolymer, but in the absence of solvent, results in a grafted product having little or no crosslinked component. The grafted copolymers have utility in food packaging.

Description: SUBSTANTIALLY NON-CROSSLINKED ETHYLENE-METHYLACRYLATE COPOLYMERS GRAFTED WITH A GRAFTING AGENT AND PROCESSES FOR THEIR PREPARATION BACKGROUND OF THE INVENTION 1. Field of the Invention. The present invention is directed to substantially non-crosslinked ethylene- methylaerylate copolymers grafted with a grafting agent as well as processes for their preparation. In particular, in the process of the present invention, an ethylene- methylacrylate copolymer is combined with a grafting agent and a free-radical initiator under conditions sufficient to cause decomposition of the free-radical initiator and above both the ceiling temperature of the grafting agent and the melt temperature of the copolymer. In the present invention, it has been found that if the amount of free-radical initiator is maintained at less than about 0.2 weight percent (relative to the copolymer) , then the resulting grafted product is substantially free of crosslinking. 2 . Related Art. The reaction of unsaturated components, such as maleic anhydride, with unsaturated polymers is known in the art. For example, U.S. Patent Nos. 4,388,471 and 4,450,281 disclose that maleic anhydride will react with unsaturated polymers such as polybutene thereby resulting in a suσcinic group pendant to the polybutene polymer, i.e., polybutenyl succinic anhydride. This reaction is a thermal reaction and results in the generation on non-crosslinked products, i.e., products which do not have covalent linkages between theretofore two different polymers. The grafting of certain agents onto the backbone of saturated polymers, copolymers, terpolymers, etc. is also known in the art. Such grafted copolymers are particularly useful as barrier layers and have particular utility in food packaging, etc. Moreover, because of their enhanced adhesion properties these grafted copolymers can be attached to aluminum foil which can then be used as food wraps. In the preparation of these grafted products, it is necessary to utilize a free- radical initiator in order to derivitize the backbone so as to be reactive with the grafting agent. The grafting is generally conducted by employing a free-radical initiator generally in amounts exceeding 1 percent by weight relative to the polymer. However, a problem arises when such a grafting reaction is conducted in the absence of solvent. In particular, in the absence of solvent, the reaction results in very high levels of crosslinking. Such crosslinked products are particularly troublesome because these products do not have good flow properties and are therefore not useful in extrusion applications. On the other hand, grafted products are preferably prepared in an extruder in order to achieve higher product quality as well as to minimize costs. See, for instance, "Modern Plastics", July 1985, pages 56 et seq. However, extruder reactions are by necessity done in the absence of solvent. Thus lies the heart of the problem. In order to obtain the benefits of products prepared in the extruder it is necessary to avoid the use of solvents; on the other hand, the absence of solvents results in levels of crosslinking which renders the product unacceptable for extrusion applications. In this regard, U.S. Patent No. 4,506,056 discloses that the use of small amounts of free-radical initiator, i.e., less than 0.5 weight percent, with certain polymers and copolymers (albeit not used with the ethylene- methylacrylate copolymer) results in very high levels of crosslinking. Thi^ reference also discloses that this problem in the grafting reaction can be overcome by using an additive, i.e, a donor molecule, selected from the group consisting of certain nitrogen, phosphorus or sulfur containing compounds in the grafting reaction. See also U.S. Patent No. 4,780,228 which discloses the use of a mercaptan, i.e., a sulfur containing compound, as a chain stopping agent. While the use of such donor compounds results in products with little or no crosslinking, the presence of a donor compound in the resulting grafted polymers can cause odor problems, lead to discolorment of the polymer, provide undesirable polymer degradation mechanisms, and can result in toxicity problems especially where the product polymer is used in food wrappings. On the other hand, substantially non- crosslinked grafted polymers can be prepared from either reaction in a solvent, see, for instance, Poreiko et al.. Journal of Polymer Science, A-I, 5_, 1563 (1967), or from solvent extraction of a crosslinked and non- crosslinked mixture. However, the use of solvents in process preparations necessitates that such processes not be conducted in an extruder. Moreover, the use of organic solvents in either process preparation or in extraction processes poses environmental hazards and remaining traces of the organic solvent are invariably retained in the grafted product which can leach from the product over a period of time. This latter problem is particularly troublesome where the product is in contact with food stuffs and may result in the incorporation of trace amounts of organic solvents into the foodstuff which when ingested can pose potential harmful side effects. A particularly useful copolymer that finds application in foodwraps, etc. is a copolymer of ethylene and methacrylate. When grafted with a grafting agent such as maleic anhydride, the resulting product has enhanced adhesion properties which not only allows it to be bound to aluminum but additionally allows for printing to be done on its surface. In view of the above, it is apparent that there is a need to develop a process which can prepare substantially non-crosslinked grafted copolymers, such as ethylene-methylacrylate copolymers, in the absence of solvent and without the donor molecules used in the prior art. Accordingly, it is an object of this invention to develop a process for the preparation of substantially non-crosslinked grafted ethylene- methylacrylate copolymers. It is a further object of this invention that such a process be conducted in the absence of solvent so that the process would be adaptable for use in an extruder. It is a further object of this invention that such a process not utilize a donor molecule so as to avoid potential problems that such a molecule can pose. It is still a further object of this invention to prepare a substantially non- crosslinked grafted ethylene-methylacrylate copolymer free of any traces of grafting solvent as well as substantially free of nitrogen, phosphorus and sulfur. These and other objects are achieved by the present invention as evidenced by the attached summary of the invention, detailed description of the invention, examples, and claims. SUMMARY OF THE INVENTION The present invention is directed to substantially non-crosslinked grafted ethylene-methylacrylate copolymers as well as to processes for their preparation. In particular, in its first process aspect, the present invention is directed to a process which comprises combining in the absence of a solvent an ethylene-methylacrylate copolymer, a grafting agent, and a free-radical initiator wherein the concentration of the free-radical initiator in the reaction mixture is maintained at less than about 0.2 weight percent relative to the copolymer, under conditions sufficient to cause decomposition of the free-radical initiator and above both the ceiling temperature of the grafting agent and the melt temperature of the copolymer. In another of its process aspects, the present invention is directed to a process for preparing a substantially non-crosslinked maleic anhydride grafted ethylene- methylacrylate copolymer which comprises a) combining in an extruder in the absence of solvent i) an ethylene-methylacrylate copolymer containing about 10-6^ weight percent methylacrylate and havxng a melt index of about 0.1 to about 200, ii) maleic anhydride wherein the concentration of maleic anhydride in the reaction mixture is maintained at about 2.0 weight percent or less relative to the copolymer of i) above, and iii) a free-radical initiator wherein the concentration of said free-radical initiator in the reaction medium is maintained at less than about 0.2 weight percent relative to the copolymer of i) above, under conditions sufficient to cause decomposition of the free- radical initiator and above both the ceiling- temperature of the grafting agent and the melt temperature of the polymer; and b) removing any unreacted maleic anhydride from the grafted copolymer. In one of its composition aspects, the present invention is directed to a substantially non-crosslinked grafted ethylene-methylacrylate copolymer free of grafting solvent and substantially free of nitrogen, phosphorus, and sulfur. DETAILED DESCRIPTION OF THE INVENTION The present invention relates to substantially non-crosslinked grafted ethylene- methylacrylate copolymers as well as to processes for their preparation. In particular, by employing very small amounts of a free-radical initiator, i.e., less than about 0.2 weight percent relative to the copolymer, it has been found that the grafting of a grafting agent in the absence of a solvent onto an ethylene-methylacrylate copolymer results in substantially non- crosslinked grafted products. Moreover, it has been surprisingly found that this result appears to be specific for the ethylene- methylacrylate copolymer insofar as the use of other polymers or copolymers in place of ethylene-methylacrylate copolymer in the process of this invention results in substantial amounts of crosslinking. However, prior to discussing this invention in detail, the following terms will first be defined. "Ethylene-methylacrylate copolymers" — refer to copolymers of ethylene and methylacrylate. The specific ethylene- methylacrylate copolymer employed in the process of the present invention is not critical and can include copolymers containing high weight percentages of methylacrylate or high weight percentages of ethylene. However, preferably, these copolymers contain from about 10-60 weight percent methylacrylate and from about 90-40 weight percent ethylene; and more preferably from about 15-30 weight percent methylacrylate and 85-70 weight percent ethylene. Suitable copolymers generally have a melt index of about 0.1 to about 200; preferably from about 1 to 30; and more preferably from about 2 to 10. Additionally, suitable ethylene-methylacrylate copolymers can also contain stabilizers, such as UV stabilizers, anti-oxidants, coloring agents, etc., as well as minor amounts, i.e., 5% or less, of one or more saturated or unsaturated hydrocarbon components, such as propylene, butylene, hexa-l,4-diene, etc., provided that these additional components do not alter the basic properties of the copolymer including its reaction properties set forth in the present invention. Preferably, the copolymer contains no more than adventurous amounts of such saturated or unsaturated components. Suitable copolymers are commercially available. For example, a suitable copolymer containing about 80 weight percent ethylene and 20 weight percent methylacrylate is available as PE-2207 from Chevron Chemical Company, San Francisco, Ca. The preparation of ethylene-methylacrylate copolymers is disclosed in U.S. Patent No. 3,350,372 which is incorporated herein by reference. Also included within the term "ethylene-methylacrylate copolymers" are ethylene-methylacrylate copolymers obtained by chemical modification of the base copolymer, e.g., chlorosulfonated, chlorinated, oxidized, etc., ethylene-methylacrylated copolymers. "Grafting agent" — refers to a conjugated unsaturate carboxylic acid or anhydride compound capable of undergoing addition to saturated hydrocarbon polymers, copolymers, terpolymers, etc., in the presence of a free- radical initiator. Such saturated hydrocarbons can include minor amounts of unsaturation, i.e., 5% or less, and include hydrocarbon polymers such as polyethylene, polypropylene, polybutylene, polymethylacrylate, copolymers of ethylene and propylene, terpolymers of ethylene, propylene and hexa-l,4-diene, and the like. Suitable grafting agents include maleic anhydride. acrylic acid, methacrylic acid and the like. Other suitable grafting agents are well known in the art. It is also contemplated that the grafting agent can be mixed with other components, for example, styrene. Preferred grafting agents are maleic anhydride and acrylic acid; with maleic anhydride being particularly preferred. Maleic acid may be used in lieu of maleic anhydride in reactions carried out at about 140 β C or higher since the acid is converted to the anhydride under these conditions. As a result of the addition of the grafting agent, the resultant polymer has carboxyl functionality (as either the free acid groups or as the anhydride in the case of maleic anhydride) which imparts improved adhesion properties to the polymer. "Grafted ethylene-methylacrylate copolymer" — refers to the product obtained by the addition of a grafting agent to the ethylene- methylacrylate copolymer in the presence of a free-radical initiator. In general, the addition of the qrafting agent to the copolymer will result in a product having a grafting agent content of from about 0.05 to about 5 weight percent, preferably from about 0.1 to about 1 weight percent, and more preferably from about 0.3 to about 0.6 weight percent of the grafting agent relative to the copolymer. The percent by weight of grafting agent on the polymer is determined by base titration as described in Chemical Reactions on Polymers, ACS Symposium Series, 364. 438 (1988) which is incorporated herein by reference. "Substantially non-crosslinked grafted ethylene-methylacrylate copolymers" — means that the product obtained from the free- radical addition of a grafting agent to the backbone of this copolymer by the process of this invention does not have substantial amounts of cross-linking. In this regard, most prior art free-radical grafting reactions not only result in the addition of grafting agents onto the backbone of the polymer but also result in the formation of a covalent bond between two heretofore unconnected polymers. See, for instance, Gaylord et al.. Journal of Polymer Science, 2 _, 1189-1198 (1988) . The resulting formation of a covalent bond between the two heretofore unconnected polymers is referred to in the art as "crosslinking". In general, substantially non-crosslinked grafted copolymers prepared by the process of this invention will contain less than about 5% crosslinked copolymers, preferably less than about 1% crosslinked copolymers, and even more preferably, less than about 0.5% crosslinked copolymer. Products containing 5% or less crosslinking retain good flow characteristics and are amenable to use in an extruder. The percent of crosslinked copolymer content in the resulting product is readily determined by a xylene extraction procedure described by Gaylord et al.. Journal of Polymer Science, 2 . 1 1189-1198 (1988) which is incorporated herein by reference. As previously mentioned, cross-linked copolymers possess poor flow properties which are contra-indicative for extruder preparation. "Free-Radical Initiator" — refers to a compound which is a free-radical precursor and is generally stable at ambient conditions but under reaction conditions will decompose to form a free-radical which initiates the grafting reaction. In general, the free- radical is generated from the free-radical initiator by exposing the free-radical initiator to heat at or above its decomposition temperature. Each free-radical initiator will have its own decomposition temperature and the initiator so selected for a particular reaction will of course depend on the reaction temperature to be used. In general, the free-radical initiators, which are useful in the practice of this invention have half-lives of less than about 30 minutes and preferably less than about 3 minutes at the reaction temperature and include acyl peroxides such as benzoyl peroxide, dialkyl or aralkyl peroxides such as di-t-butyl peroxide, dicumyl peroxide, cumyl butyl peroxide, 1,1- di- -butyl peroxy-3,5,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di-t-butylperoxyhexane, and bis(alpha-t-butyl peroxyisopropylbenzene) , peroxyesters such as t-butyl peroxypivalate, t-butyl peroctoate, t-butyl perbenzoate, 2,5- dimethylhexyl-2,5-di(perbenzoate) , t-butyl di(perphthalate) , dialkyl peroxymonocarbonates and peroxydicarbonates, hydroperoxides such as t-butyl hydroperoxide, p-menthane hydroperoxide, pinane hydroperoxide, and cumene hydroperoxide and ketone peroxides such as cyclohexanone peroxide and methylethylketone peroxide as well as azobisisobutyronitrile. A particularly preferred free-radical initiator is 2,5- dimethyl-2,5-di-t-butylperoxyhexane which is commercially available under the trade name Lucidol 101 (Pennwalt, Buffalo, New York) . On the other hand, any free-radical initiator having the desired half-life at the reaction temperature may be used. More detail compilations of free-radical initiators which may be used in the process of this invention are set forth at pages II-3 to 11-51 of "Polymer Handbook", Interscience Publishers (1966) and pages 696-700 of Volume 58, "Modern Plastic Encyclopedia" (1981-1982) the disclosures of which are incorporated herein by reference. While the particular free-radical initiator employed in the process of the invention is not critical, it is critical to maintain the concentration of the free-radical initiator at less than about 0.2 weight percent relative to the copolymer. While prior art processes tend to use high levels of free-radical initiator in the grafting reaction, I have found that if the concentration of the free-radical initiator is maintained at a low level, i.e., less than about 0.2 weight percent, relative to the ethylene-methylacrylate copolymer, then a substantially non-crosslinked product will be obtained. What is particularly surprising is that this result appears to be specific for this copolymer insofar as other polymers such as polyethylene, ethylene-butylacrylate copolymer, etc. , result in substantial amounts of crosslinking even when the free-radical initiator is employed at a concentration of 0.2 weight percent. In a preferred embodiment, the free-radical initiator is employed at a concentration of 0.1 weight percent or less relative to the ethylene- methylacrylate copolymer. "Ceiling temperature of the grafting agent" — refers to the temperature at or above which the grafting agent will not homopolymerize. In general, if the reaction is conducted at a temperature below the ceiling temperature of the grafting agent, then the grafting agent can homopolymerize, i.e., the grafting agent polymerizes onto itself in addition to adding to the polymer. The ceiling temperature is a function of the grafting agent concentration and can be readily determined by the skilled artisan for a particular grafting agent at a particular concentration. See, for instance, Russel. Journal of Polymer Science, Part A: Polymer Chemistry, 2J5, 2273-2280 (1980) for a discussion of the ceiling temperature of maleic anhydride. "Grafting solvent" — refers to either the solvent employed to prepare a substantially non-crosslinked grafted polymer, if the grafting process is conducted in a solvent, or to the solvent employed in the isolation of the substantially non-crosslinked grafted polymer, if this product is separated by solvent extraction from the cross-linked product. Suitable grafting solvents are known in the art and include xylene, chlorobenzene and the like. "Melt index" — is a measure of the molecular weight of a polymer and is used instead of molecular weight because of its greater practical significance to extruder processes. The higher the melt index, the lower the molecular weight and reciprocally, the lower the melt index of a polymer, the higher its molecular weight. The melt index is calculated in accordance with ASTM test D1238-86 using the following conditions - 190 β C and a weight of 2.16 kg on the piston of the plastometer. The melt index is reported in gram/10 minutes. "Melt temperature" — refers to the temperature at which the particular ethylene- methylacrylate copolymer melts. Since polyethylene melts at about 137°C, the melt temperature of the copolymer is by necessity less than 137 β C. "Psia" — refers to pounds per square inch atmospheric. In conducting the process of this invention, a mixture of free-radical initiator and grafting agent are mixed with the ethylene- methylacrylate copolymer under reaction conditions. Suitable reaction conditions include the reaction temperature which is selected so as to be above the ceiling temperature of the grafting agent so as to prevent the grafting agent from homopolymerizing. The ceiling temperature of the grafting agent will vary with the grafting agent as well as with its concentration, i.e., as the concentration of the grafting agent lowers, its ceiling temperature also lowers. Reaction temperatures of about 180 β C or more generally are above the ceiling temperatures of any concentration of grafting agent used in this invention. On the other hand, by the use of an appropriate dilute concentration of grafting agent, reaction temperatures of about 120*C are sufficient to be at or above the ceiling temperature. In general, the reaction is generally conducted at a temperature of from about 140 β C to about 250"C and preferably at about 180 β C. Also, as noted above, the reaction temperature should also be above the melt temperature of the particular ethylene- methylacrylate copolymer used as well as at or above the decomposition temperature of the particular free-radical initiator used in the reaction. Because the melt temperatures of such copolymers are generally less than the ceiling temperature of the grafting agent, the melt temperature is generally of secondary concern. However, care should be taken to ensure that in each instance the reaction is conducted at or above the melt temperature of the particular ethylene-methylacrylate copolymer employed in the reaction. On the other hand, with regard to the free- radical initiator, it is well within the skilled artisan's ability to select an initiator having a decomposition temperature appropriate for the reaction temperature selected. Such a free-radical initiator should have a half-life of less than about 30 minutes at the reaction temperature selected, i.e., one-half of the free-radical initiator will decompose at the reaction temperature in less than 30 minutes, preferably less than 3 minutes, and more preferably, less than 1 minute. The amount of free-radical initiator employed in the present process must be maintained at less than about 0.2 weight percent relative to the ethylene- methylacrylate copolymer in order to prevent substantial crosslinking of the copolymer from occurring. Preferably, the free-radical initiator is maintained at a concentration of about 0.1 weight percent or less relative to the copolymer. Because the free-radical initiator is decomposing at the reaction temperature, it is possible and indeed preferable during the reaction to add additional amounts of free-radical initiator to the reaction mixture provided that the addition does not result in a total concentration of free-radical initiator of about 0.2 weight percent or greater relative to the ethylene-methylacrylate copolymer. Such addition provides one means of maintaining the concentration of the free- radical initiator at less than about 0.2 weight percent during the reaction. The additional amounts of free-radical initiator can be added batchwise at selected intervals designed so as not to result in too large a concentration of free-radical initiator in the reaction mixture, i.e., periodically the concentration of the free-radical initiator in the reaction mixture can be determined by actual analysis or estimated by its decomposition rate and then a suitable amount of free-radical initiator added. Alternatively, the additional amounts of free- radical initiator can be added continuously by matching the addition rate to the decomposition rate of the free-radical initiator. In either procedure, the use of such small amounts of free-radical initiator results in a grafted product having substantially no crosslinking. This result is particularly surprising because the use of such small amounts of free-radical initiator with polymers other than ethylene- methylacrylate copolymers result in substantial amounts of crosslinking. See, for instance. Examples 9-14, 19-20, 23, 25 and 28- 31 herein as well as Gaylord, U.S. Patent No. 4,506,056, which sets forth in several of its examples that the use of small amounts of free-radical initiator, i.e., less than 0.5 weight percent relative to the polymer, in the grafting reaction with maleic anhydride and a polymer (albeit not ethylene-methylacrylate) in the absence of solvent and a donor molecule results in substantial amounts of crosslinking. The grafting agent is employed in the present process in amount sufficient so as to provide a grafted ethylene-methylacrylate copolymer having a grafting agent content of from about 0.05 to about 5 weight percent relative to the copolymer, preferably from about 0.1 to about 1 weight percent, and even more preferably, from about 0.3 to about 0.6 weight percent. Under the reaction conditions, the grafting agent will either react with the copolymer or be lost by evaporation. In either case, the concentration of reactable grafting agent diminishes over time. Accordingly, it is possible and indeed preferable during the reaction to add additional amounts of grafting agent to the reaction mixture. The additional amounts of grafting agent can be added batch wise at selected intervals in an amount which approximates reactable grafting agent depletion by evaporation and reaction. Alternatively, the additional amounts of grafting agent can be added continuously by matching the addition rate to the sum of the evaporation and reaction rates. In either case, because a portion of the grafting agent added to the reaction mixture is lost by evaporation, it is desirable to add more grafting agent into the reaction mixture than is intended to be incorporated into (grafted onto) the copolymer. In general, the concentration of grafting agent in the reaction mixture is maintained at about 8 weight percent or less, preferably at about 5 weight percent or less, and even more preferably at about 2 weight percent or less. When the concentration of the grafting agent is about 2 weight percent or more relative to the copolymer, it may be necessary to limit the reaction time to less than about 10 minutes in order to avoid undesirable crosslinking. As noted above, maintaining the appropriate concentration of the grafting agent (relative to the copolymer) may entail either the incremental or continuous addition of additional grafting agent. In a preferred embodiment, the additional amounts of grafting agent are added simultaneously with the additional amounts of free-radical initiator. After reaction completion, unreacted maleic anhydride can be removed from the grafted copolymer by evaporation, i.e., heating the product to a temperature of about 140°C or greater, or by treatment with water. The anhydride groups appended to the polymer are converted to carboxylic acid groups on hydrolysis and, if desired, may be regenerated by heating to eliminate water. Alternatively and less preferably, the grafted polymer may be freed of unreacted maleic anhydride by solution in a suitable solvent and precipitation in a non-solvent. Such a process is not preferred because it can add a solvent in trace amounts to the grafted copolymer. The process of this invention can be conducted in a suitable mixing device such as a Brabender Plasticorder, a roll mill, a single or multiple screw extruder or any other of the well-known mechanical mixing equipment normally used in the mixing, compounding, processing or fabrication of low or high molecular weight thermoplastic, elastomeric or thermosetting polymers or mixtures thereof in the absence of solvent. An extruder having one or more ports is a particularly desirable reaction vessel, although it is by no means necessary insofar as the grafting reaction can be done in the absence of solvent in other reaction vessels. The solid polymer, e.g., pellets or powder, may be premixed with the grafting agent and the free-radical initiator, and the resulting mixture added to the mixing device. Alternatively, the mixture of reactants may be added to the molten polymer. The ixture of grafting agent and free- radical initiator is prepared in the conventional manner and may be in the form of a mixture of powdered solids when both of the ingredients have melting points above room temperature, a slurry or paste when the free- radical initiator is a liquid at room temperature, or a liquid or fluid when the grafting agent is soluble in the liquid free- radical initiator. Alternatively, the powder, slurry or paste may be converted to a liquid by heating above the melting point of grafting agent and/or the free-radical initiator. The mixture is dropped continually or intermittently onto the surface of the melted polymer. When the mixture is solid, it may be added mechanically, e.g., from a hopper, or may be blown in with an inert gas. When the mixture is a paste, slurry or fluid, it may be added mechanically or may be pumped and sprayed onto the surface of the melted polymer, or a roll mill, or may be pumped and injected into one or more parts in an extruder. The numerous methods of adding solids, slurries, pastes or liquids to reaction vessels, mills and extruders are well known to those skilled in the art and may be used in the practice of this invention. The mixture is generally added continuously or in several portions over a period of time to promote homogeneous distribution of carboxyl groups throughout the mass of the polymer. The carboxylation reaction is extremely rapid and occurs to a major extent when the mixture comes in contact with the heated polymer. However, the reaction can continue when the molten polymer is conveyed away from the point of injection, particularly if the free-radical initiator's half life is at least 10 seconds at the reaction temperature. Generally, the reaction is conducted from about 10 seconds to about 20 minutes. Preferably, in order to ensure reaction completion, the reaction is conducted for a period of at least 30 seconds. Furthermore, when the concentration of the grafting agent is about 2 weight percent or more relative to the copolymer, it may be necessary to limit the reaction time to less than about 10 minutes in order to avoid undesirable crosslinking. An extruder containing an entry port for the addition of the polymer, one or more reduced pressure zones with injection orifices at points where the polymer is molten for addition of the reactant mixture, and a reduced pressure zone for venting off any unreacted maleic anhydride or volatiles formed during the process, may be used advantageously in the practice of this invention. In this case, the extrudate may be removed as ribbon or rod and cut into pellets or as fibers or extruded or blown film. The film may be utilized as a self-supporting film or may be extrusion laminated onto a substrate such as paper, aluminum foil or an unoriented or oriented polymeric film, or may be coextruded with a thermoplastic polymer to form a laminate. When the reaction is conducted in an extruder, extruder pressures will generally be in the range of about 500 to 7000 psia. Because the process of the present invention does not employ a grafting solvent to either prepare or isolate the substantially non- crosslinked copolymer, the product of this invention is by necessity free of such grafting solvent. Likewise, because the process of the present invention does not utilize a donor molecule, which is defined by Gaylord, U.S. Patent No. 4,506,056, as sulfur, nitrogen or phosphorus containing compounds or a chain-stopping agent, which is defined by Gardiner et al, U.S. Patent No. 4,780,228, as a mercaptan, the product of this invention does not contain added amounts of sulfur, nitrogen or phosphorus. In general, as used herein the term "substantially free of nitrogen, phosphorus and sulfur" means that no exogenous nitrogen, sulfur or phosphorus has been added to the process for preparing the grafted products of this invention. Any concentration of sulfur, nitrogen and phosphorus in the products of this invention results from impurities in the reactants including the ethylene-methylacrylate copolymer and possibly from the equipment used in the process to prepare these compounds. In any event, such impurities can be minimized by selecting very clean reactants and equipment. Because no exogenous nitrogen, sulfur or phosphorus has been added during product preparation, the concentration of sulfur, nitrogen, and phosphorus in the products of this invention are generally less than about 100 parts per million (ppm) . Preferably, the products of this invention have a sulfur, nitrogen and phosphorus concentration of less than about 10 ppm; more preferably, less than about 5 ppm; and most preferably, less than about 1 ppm. The present invention will be illustrated in greater detail by the following examples, but it is to be understood that these examples are only illustrative in nature and not intended to limit the invention in any manner. As used in these examples, the following abbreviations have the following meanings: AA = acrylic acid MA = maleic anhydride E-M = ethylene-methylacrylate copolymer E-B = ethylene-butylacrylate copolymer PE = polyethylene MI = melt index EXAMPLES Example 1 The hopper of a Haake Rheomex 3/4 inch single screw extruder (L/D = 20:1) was charged with ethylene-methylacrylate copolymer pellets (PE 2207 - available from Chevron Chemical Company, San Francisco, Ca and which contains 0.05 weight percent of 2,6-di-t-butyl-4- ethylphenol, an antioxidant, and having a melt index of 6 g/10 minutes) which had been coated on the surface with 1 weight percent maleic anhydride and 0.1 weight percent Lucidol 101 [2,5-dimethyl-2,5-di(tert-butylperoxy)hexane— a liquid free-radical initiator available from Pennwalt, Buffalo, New York]. The surface coating of the pellets was carried out by tumbling the pellets with finely ground maleic anhydride and Lucidol 101 in a closed jar at room temperature for 30 minutes. The extruder barrel was heated to 180*C and a one-eighth (1/8) inch diameter continuous strand was extruded at 30 rpm screw speed. The strand was cooled to room temperature and cut up into pellets of approximately 3 mm diameter. The resulting product is maleic anhydride grafted ethylene-methylacrylate copolymer containing some unreacted maleic anhydride. A 5 g sample of the extrudate was extracted with 250 ml of boiling xylene, filtered and the filtrate precipitated into 600 ml of acetone. From the weight increase of the filter it was calculated that the extrudate contains 0.2 weight percent crosslinked insoluble polymer. Filtration and drying of the acetone precipitate yielded the non- crosslinked maleic anhydride grafted ethylene- methylacrylate copolymer having a maleic anhydride content of 0.5 weight percent as determined by base titration. Example 2 A Brabender Mixer equipped with a vacuum suction funnel located above the mixing chamber, preheated to 180°C, was charged with 40 g of ethylene-methylacrylate copolymer (PE 2207) . The copolymer was melted by rotating the mixing blades to 60 rpm for 2 minutes. A mixture of 2 g maleic anhydride and 0.2 g dicumyl peroxide was divided into 4 equal increments. Each of these increments was added at 2 minute intervals. After the last addition, the reaction mixture was mixed for an additional 2 minutes. During the reaction, some of the maleic anhydride was lost due to evaporation and most of the dicumyl peroxide was lost due to decomposition prior to addition of subsequent increments. The reaction mixture was then cooled to about 100°C and removed from the mixing chamber and cooled to room temperature. Extraction with boiling xylene (as described in Example 1) gave no insoluble crosslinked copolymer. The maleic anhydride content of the maleic anhydride grafted ethylene-methylacrylate copolymer was 1.4 weight percent. The following additional examples are offered to further illustrate the invention. Examples 3-8 below illustrate grafted copolymers of this invention which were prepared from ethylene-methylacrylate copolymer whereas Examples 9-14 illustrate grafted polymers which were prepared from polyethylene and accordingly are not examples of this invention. These examples were extruder prepared following the procedures set foτ-th in Example 1 but with the reagent amounts indicated below in Table I. Because of the extensive amount of cross-linking obtained in comparison Examples 9-14, no determination was made of the weight percent maleic anhydride in the crude and extracted products and no determination was made of the melt index. In any event, this data demonstrates that the preparation of a grafted ethylene-methylacrylate copolymer by the process of this invention does not result in substantial amounts of crosslinking whereas polyethylene does. TABLE I WT. % ADDED, WT. PERCENT WEIGHT PERCENT MA IN EXAMPLE ANTIOXIDANT 1 RPM'S' CROSSLINKING CRUDE PRODUCT EXTRACTED PRODUCT MI POLYM 0.6 0.5 1.3 E-N O.β 0.5 2.5 E-M 0.7 0.5 2.7 E-M 0.S1 0.44 E-M 0.70 0.45 E-M 0.74 0.44 E-M PE PE PE PE PE PE ' Antioxidant is 2,*-dl-tart-butyl-4-βthylphβnol. 1 RPM'a refer to t a acrew apaad in the axtrudar in rotationa par ainuta. Examples 15-20 below illustrate different grafted polymers. Examples 15-18 were prepared from ethylene-methylacrylate copolymer and in particular certain of these examples use reagent amounts outside the scope of this invention. On the other hand. Examples 19 and 20 were prepared from polyethylene and therefore are not examples of this invention. These examples were prepared following the procedure set forth in Example 2 above but with the reagent amounts set forth in Table II below. This data demonstrates that using reagent amounts outside the scope of this invention or using a polyethylene results in unacceptable grafted products. 5 The free-radical initiator (dicumyl peroxide) wan added in 4 equal increments at 2 minute intervals result in the total indicated. Because at lβO'C the half-life for decoapoβition of this init .or is 31 secon the concentration of initiator at any one time was no more than about 0.125 weight percent in Examples 15, 19 and 20 and no more than about 0.25 weight percent in Examples 16 and lθ. Exa ples 21-23 below illustrate different grafted polymers. Examples 21 and 22 were prepared from ethylene-methylacrylate copolymers and are examples of this invention whereas Example 23 was prepared from polyethylene and is not an example of this invention. These examples were prepared following the procedure set forth in Example 2 above but with the reagent amounts set forth in Table III below. This data demonstrates that the presence of an antioxidant in the polymer does not prevent crosslinking. TABLE III 4 The free-radical initiator (dlcuayl peroxide) waa added In 4 equal incraaenta at 2 ainuta intervale to raault in the total indicated. Becauaβ at lβO'C the half-life for decoapoaitlon of thla initiator ia 31 aaconda, the concentration of initiator at any one tlaa waa no aore than about 0.125 weight percent. 5 Antioxidant ia 2,6-di-tert-butyl-4-ethylphenol. In Exaaple 21, thla antioxidant vaa extracted froa the polyaer prior to conducting theaβ exaaplee. Examples 24-25 below illustrate different grafted polymers wherein the grafting agent is acrylic acid. Example 24 was prepared from ethylene-methylacrylate copolymer and is an example of this invention whereas Example 25 was prepared from polyethylene and is not an example of this invention. These examples were prepared following the procedure set forth in Example 2 above but using acrylic acid in lieu of maleic anhydride and with the reagent amounts set forth in Table IV below. This data demonstrates that the results of this invention are not specific to use of maleic anhydride as the grafting agent. TABLE IV * Tha free-radical initiator (dlcuayl peroxide) waa added in 4 equal incrementa at 2 minute interval* to result in the total indicated. Bacaua* at 180'C the half-llf* for decoapoaitlon of thi* initiator ia 31 aaconda, the concentration of initiator at any on* tia* waa no aora than aoout 0.125 weight percent. Examples 26-31 below illustrate different grafted polymers. In particular. Examples 26 and 27 were prepared from ethylene- methylacrylate copolymer and are examples of this invention. Examples 28 and 29 were prepared from ethylene-butylacrylate copolymer (16% butylacrylate, 0.05% 2,6-di-tert-butyl-4- ethylphenol—antioxidant and having a MI of 2 g/10 minutes) and are not examples of this invention. Examples 30 and 31 were prepared from polyethylene and are not examples of this invention. These examples were extruder prepared following the procedure set forth in Example 1 but with the reagent amounts indicated in Table V below. This data substantiates that the results of this invention (substantially non-crosslinked grafted products) are specific to ethylene- methylacrylate copolymers. TABLE V 7 Free-radical initiator is Lucidol 101 — 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane. " RPM's refer to the screw speed in the extruder in rotations per minute. Examples 32-59 below illustrate the effect of varying reaction parameters, i.e., reagent concentrations, reaction temperatures, reaction times, etc., on the resulting grafted copolymers. All of these examples employed PE-2207, ethylene-methylacrylate copolymer. These examples were extruder prepared following the procedure set forth in Example 1 above but with the reagent amounts and reaction conditions set forth in Table VI below. In all of these examples, the free- radical initiator employed was Lucidol 101, 2,5-dimethyl-2,5-di(tert-butylperoxy) hexane. TABLE VI Continued In Table VI above. Examples 34, 42 and 56 are taken from the same experiment. Likewise for Examples 35, 43 and 57; for Examples 36 and 48; and for Examples 37 and 49. These experiments are repeated and given new example numbers solely for the sake of comparison with examples either immediately above and/or below it. The above data demonstrates that the grafting reaction is preferably carried out at concentrations of maleic anhydride of less than about 2.0 weight percent although Examples 37-39 show acceptable results. The above data also demonstrates that for concentrations of the maleic anhydride of about 2.0 weight percent and greater, it may be necessary to limit the reaction duration to less than about 10 minutes, i.e., see Examples 36-37. Example 60 Examples 34-35, 40-43, and 56-57 were subjected to an aluminum adhesion test to test the adhesion characteristics of the grafted products of this invention. The grafted polymers of these examples were compression molded at 180"C/25 tons/1 minute between two pieces of aluminum foil. After cooling to room temperature (ten tons) , the films were found to adhere very strongly to the aluminum and could be separated only by dissolving the aluminum in dilute HC1. On the other hand, non-grafted ethylene-methylacrylate copolymer films, compression molded in the same manner, could be peeled off easily from the aluminum foil.

Claims: WHAT IS CLAIMED IS: 1. A process for preparing a substantially non-crosslinked grafted ethylene- methylacrylate copolymer which comprises combining in the absence of solvent: a) an ethylene-methylacrylate copolymer; b) a grafting agent; and c) a free-radical initiator wherein the concentration of said free-radical initiator in the reaction mixture is maintained at less than about 0.2 weight percent, based on the weight of said copolymer, under conditions sufficient to cause decomposition of said free-radical initiator and above both the ceiling temperature of said grafting agent and the melt temperature of said copolymer. 2. The process according to Claim 1 wherein the grafting agent is selected from the group consisting of maleic anhydride, acrylic acid and methacrylic acid. 3. The process according to Claim 2 wherein the grafting agent is maleic anhydride. 4. The process according to Claim 1 wherein the amount of free-radical initiator is about 0.1 weight percent or less, based on the weight of said copolymer. 5. The process according to Claim 1 wherein the concentration of said grafting agent in the reaction mixture is maintained at about 8 weight percent or less, based on the weight of the copolymer. 6. The process according to Claim 5 wherein the concentration of said grafting agent in the reaction mixture is maintained at about 2.0 weight percent or less, based on the weight of said copolymer. 7. The process according to Claim 1 wherein said ethylene methylacrylate copolymer contains 10-60 weight percent methylacrylate. 8. The process according to Claim 7 wherein said ethylene-methylacrylate copolymer contains 15-30 weight percent methylacrylate. 9. The process according to Claim 1 wherein said copolymer has a melt index of from about 0.1 to about 200. 10. The process according to Claim 9 wherein said copolymer has a melt index of from about 1 to about 30. 11. The process according to Claim 10 wherein said copolymer has a melt index of from about 2 to 10. 12. A process for preparing a substantially non-crosslinked grafted ethylene- methylacrylate copolymer as described in Claim 1 further comprising the step of removing any unreacted grafting agent from the grafted copolymer. 13. The process according to Claim 12 wherein said unreacted grafting agent is removed by evaporation. 14. The process according to Claim 1 wherein said process is conducted at a temperature of about 140" to about 250"C for a period of at least ten seconds. 15. The process according to Claim 14 wherein said process is conducted at about 180 β C. 16. The process according to Claim 1 wherein said process is conducted in an extruder. 17. A process for preparing a grafted ethylene-methylacrylate copolymer which comprises a) combining in an extruder in the absence of solvent i) an ethylene-methylacrylate copolymer containing 10-60 weight percent methylacrylate and having a melt index of from about 0.1 to about 200, ii) maleic anhydride wherein the concentration of maleic anhydride in the reaction mixture is maintained at about 2.0 weight percent or less based on the weight of the copolymer of i) above, and iii) a free-radical initiator wherein the concentration of said free-radical initiator in the reaction mixture is maintained at less than about 0.2 weight percent or less, based on the weight of the copolymer of i) above, under conditions sufficient to cause decomposition of said free-radical initiator and above both the ceiling temperature of said maleic anhydride and the melt temperature of said copolymer; and b) removing any unreacted maleic anhydride from the grafted copolymer. 18. The process according to Claim 17 wherein the amount of free-radical initiator is about 0.1 weight percent or less, based on the weight of the copolymer. 19. The process according to Claim 17 wherein said ethylene-methylacrylate contains 15-30 weight percent methylacrylate. 20. The process according to Claim 17 wherein said copolymer has a melt index of from about 1 to about 30. 21. The process according to Claim 20 wherein said copolymer has a melt index of from about 2 to about 10. 22. The process according to Claim 17 wherein said unreacted maleic anhydride is removed by evaporation. 23. The process according to Claim 17 wherein said process is conducted at a temperature of about 140° to about 250 β C for a period of at least ten seconds. 24. The process according to Claim 23 wherein said process is conducted at about 180 β C. 25. A substantially non-crosslinked grafted ethylene-methylacrylate copolymer free of grafting solvent and substantially free of nitrogen, phosphorus, and sulfur. 26. The grafted ethylene-methylacrylate copolymer as defined in Claim 25 wherein said copolymer contains 10-60 weight percent methylacrylate. 27. The grafted ethylene-methylacrylate copolymer as defined in Claim 26 wherein said copolymer contains 15-30 weight percent methylaery1ate. 28. The grafted ethylene-methylacrylate copolymer as defined in Claim 27 wherein said copolymer is grafted with a grafting agent selected from the group consisting of maleic anhydride, acrylic acid and methacrylic acid. 29. The grafted ethylene-methylacrylate copolymer as defined in Claim 28 wherein said grafting agent is maleic anhydride. 30. The grafted ethylene-methylacrylate copolymer as defined in Claim 25 wherein said grafted copolymer has a grafting agent content of from about 0.05 to about 5 weight percent based on the grafted copolymer. 31. The grafted ethylene-methylacrylate copolymer as defined in Claim 30 having a grafting agent content of from about 0.1 to about 1 weight percent. 32. The grafted ethylene-methylacrylate copolymer as defined in Claim 31 having a grafting agent content of from about 0.3 to about 0.6 weight percent.

Inventors: BACSKAI ROBERT, BACSKAI, ROBERT
Applicants: CHEVRON RES & TECH, CHEVRON RESEARCH AND TECHNOLOGY COMPANY
Application year: 1990
Main classification: C08F265
IPC codes: C08F8, C08F255, C08F265