Solution modification of polyolefins

Details Solution modification of polyolefins Solution modification of polyolefins

2000-03-14

2001-07-17

Sergent, Rabon (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

C525S297000, C525S263000, C525S273000, C526S351000, C526S331000

Reexamination Certificate

active

06262182

ABSTRACT:

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not Applicable
REFERENCE TO A MICROFICHE APPENDIX
Not Applicable
1. Field of the Invention
This invention relates to a process for the modification of a polyolefin with an unsaturated anhydride, unsaturated acid, or unsaturated ester in the presence of a radical initiator in a suitable organic solvent. By choosing the appropriate solvent, modification of the polyolefin can be achieved with high efficiency without the formation of undesirable impurities resulting from the competing reaction of the solvent with the unsaturated anhydride, unsaturated acid, or unsaturated ester. Polyolefins modified by this process are useful as adhesion promoters for the application of paints and other coatings to substrates derived from polyolefins, such as polypropylene, thermoplastic olefin and polyethylene.
2. Background of the Invention
The modification of polyolefins by reaction with maleic anhydride and other unsaturated anhydrides, acids, and esters is well known in the polymer art. The process typically involves the reaction of an unsaturated anhydride, unsaturated acid, or unsaturated ester with a polyolefin in the presence of a radical initiator. The radical initiator is typically an organic peroxide or azo compound. The choice of initiator used in the reaction is generally dictated by the selection of the desired processing temperature. The reaction can either be carried out in solution or in the molten state.
The choice of solvent for this reaction is somewhat limited due to solubility constraints of the polyolefin. Polyolefins derived from ethylene and propylene are most soluble in aromatic solvents and chlorinated solvents, and partially soluble in selected aliphatic solvents. Chlorinated solvents are not the solvents of choice for this reaction due to toxicity and environmental issues. Consequently, toluene and xylene are typically used as the reaction solvent. These solvents are readily available and relatively inexpensive. In addition, these solvents possess relatively low boiling points that allow for their efficient removal via distillation at reduced pressure following the reaction, should that be desired.
U.S. Pat. No. 4,299,754 discloses a process for preparing a surface treating agent which comprises dissolving a propylene-ethylene copolymer having a propylene content of 50-75 mole % and a crystallinity determined by an X-ray diffraction method of 2 to 20% in an organic solvent and graft polymerizing it with maleic acid or maleic anhydride in the presence of a radical generator to provide an organic solvent solution of the modified copolymer having a concentration of 10 to 100 kg per cubic meter of the solvent. Examples of the organic solvent which are disclosed include aromatic hydrocarbons such as benzene, toluene and xylene, aliphatic hydrocarbons such as hexane, heptane, octane, and decane and chlorinated hydrocarbons.
Japanese patent 59-8282 describes a melt polymerization method for making a modified polyolefin having improved strength and flowability. However, this patent discloses that the polyolefin must have an ethylene content of no less than 50 mole % in order to obtain a modified polyolefin having improved strength. The polyolefin is kneaded at a temperature of 160 degrees C. in the presence of an unsaturated carboxylic acid or its anhydride and an aromatic hydrocarbon having a tertiary alkyl group or a secondary alkyl group or an aromatic hydrocarbon having no less than three primary alkyl groups. The aromatic hydrocarbon is present in an amount of 0.5 to 30 wt. parts per 100 wt. parts of polyolefin.
Japanese patent 2,049,578 describes a method of preparing an adhesive composition comprising subjecting a polyolefin-based resin dissolved in an organic solvent to graft polymerization using substantially equimolar amounts of maleic anhydride or maleic acid and a styrene-based monomer in the presence of a radical generating agent to obtain a graft resin solution. Micro-particles of the graft resin are then precipitated from the solution. Among the suitable organic solvents listed are benzene, toluene, xylene and “other alkyl benzenes.” In all examples xylene is used as the solvent.
The typical aromatic solvents used to modify polyolefins by reaction with maleic anhydride are inherently reactive under the reaction conditions. Both toluene and xylene contain benzylic hydrogens that are easily abstracted by free radicals produced from the decomposition of the radical initiator used in the reaction. The resulting benzylic radical can then react with the unsaturated anhydride, unsaturated acid, or unsaturated ester to generate an undesirable impurity in the reaction. For example, one possible reaction sequence is illustrated by the reaction of xylene with maleic anhydride to produce a maleated xylene compound.
Competition between the solvent and the polyolefin with respect to the reaction with the unsaturated anhydride, unsaturated acid, or unsaturated ester results in an overall lower yield for the desired reaction and the generation of undesirable impurities that may need to be removed from the reaction mixture.
Impurities resulting from the reaction of maleic anhydride with solvent are typically removed by precipitating the modified polyolefin from solution through the addition of an anti-solvent, e.g., acetone. The impurities are soluble in the anti-solvent whereas the modified polyolefin is not. As a consequence of the purification step, additional time, waste, and process costs are generated in the process.
It has been reported that the radical grafting of malcic anhydride on low-density polyethylene is strongly dependent on the kind of solvent used. (Eur. Polym. J., Vol. 30, No. 9, pp. 1047-1050, 1994). This article discloses that if benzylic hydrogens are present in the solvent molecule, they take part in the reaction with maleic anhydride. For instance, when p-xylene was used as the solvent to graft maleic anhydride to low density polyethylene, only 10% of the maleic anhydride reacted with the polyolefin while the remainder reacted with the solvent. However, when solvents free of benzylic hydrogens, such as t-butylbenzene and o-dichlorobenzene were used, maleic anhydride conversion was low but nearly all was bound to the polymer. Running the reaction under more forcing conditions (i.e., higher concentrations of maleic anhydride and initiator, longer reaction times) did not increase the grafting efficiency but rather it resulted in intermolecular coupling (i.e., crosslinking) with the formation of a gelled polymer.
It would be highly desirable to modify a polyolefin with an unsaturated anhydride, unsaturated acid or unsaturated ester in a reaction solvent which is not environmentally toxic and does not compete with the polyolefin, while still achieving a high grafting efficiency. Fewer impurities would be generated and the purification step could possibly be avoided.
SUMMARY OF THE INVENTION
It has been discovered that polyolefins may be modified in solution with an unsaturated anhydride, unsaturated acid, or unsaturated ester, with high grafting efficiency without the formation of undesirable impurities resulting from the competing reaction of the solvent with the unsaturated anhydride, unsaturated acid, or unsaturated ester.
Accordingly, the invention provides an improved process for the solution modification of polyolefins. The process entails dissolving a polyolefin selected from the group consisting of polypropylene and a copolymer comprised of propylene and up to but not including 50 mole percent of a co-monomer, in a solvent to form a solution. An unsaturated anhydride, unsaturated acid or unsaturated ester and a free-radical initiator arc then added to the solution to form a reaction mixture. The solvent used to dissolve the polyolefin is a substituted aromatic solvent which does not contain benzylic hydrogens or a mixture of such solvents. The reaction mixture is reacted under suitable conditions to produce a modified polyolefin.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
Not Applicable
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