Thermally Stabilized Carboxyl-Containing Ethylene Copolymers and Preparation Thereof

This invention relates to thermally stabilized carboxyl-containing ethylene copolymers and a method of preparing the same. More particularly, this invention relates to the stabilization of carboxyl-containing ethylene copolymers with hydrated compounds.

Ethylene copolymers containing carboxyl groups, such as those described in U.S. Pat. No. 3,520,861, are well known. However, ethylene copolymers containing pendant carboxyl groups are inherently susceptible to thermal instability. In the production of such ethylene copolymers, this inherent instability poses a significant problem because of crosslinking of the polymer, variation in melt flow values, and the formation of crosslinking by-products which are corrosive to process equipment. In addition, the instability seriously limits the useful life of products made from these ethylene copolymers, including films, coatings, adhesives, and the like.

Thermal stabilizing additives heretofore employed in conjunction with carboxyl-containing ethylene copolymers have not provided satisfactory results. Likewise, the injection of steam into molten ethylene copolymers containing carboxyl groups has not provided satisfactory results and, in addition, has the further disadvantages of adversely affecting the quality of products made from the copolymers and corroding processing equipment.

The invention is a carboxyl-containing ethylene copolymer composition exhibiting improved thermal stability and a method of preparing the composition. Briefly, the composition is a carboxyl-functional ethylene copolymer and a hydrated compound in intimate admixture therewith. The copolymer comprises ethylene and from about 0.01 to about 50 percent by weight of the copolymer of a carboxyl-functional comonomer polymerized therewith. The hydrate dehydrates at a temperature between about 100.degree. C. and about 300.degree. C., preferably about 160.degree.-275.degree. C. The hydrate is present in the composition in at least an amount effective to impart thermal stability thereto up to about 5 weight percent of the composition, preferably 0.01-0.45 weight percent, most preferably 0.01-0.15 weight percent.

The method of the invention includes adding a hydrated compound to a carboxyl-functional ethylene copolymer and intimately mixing the copolymer and the hydrate under conditions at which the copolymer is in a molten state. The method is suitable with copolymers comprising ethylene and from about 0.01 to about 50 percent by weight of the copolymer of a carboxyl-functional comonomer polymerized therewith. The hydrate used in the method dehydrates at a temperature between about 100.degree. C. and about 300.degree. C., preferably between about 160.degree. C. and about 275.degree. C., and is added in at least an amount sufficient to impart thermal stability upon intimate mixing of the copolymer and the hydrate, up to a rate of about 5 weight percent of the mixture, preferably 0.01-0.45 weight percent, most preferably 0.01-0.15 weight percent.

Ethylene copolymers suitable for use in the invention include copolymers of ethylene and a carboxyl-containing comonomer. By the terms "carboxyl-containing" or "carboxyl-functional" is meant any molecular species containing the moiety: ##STR1## wherein R is hydrogen or metallic cation. The comonomer may compose from about 0.01 to about 50% by weight of the copolymer. The ethylene copolymer may be random, block or graft. Suitable comonomers include olefinically unsaturated carboxylic acids. Exemplary of these are acrylic acid, methacrylic acid, methyl methacrylic acid, ethacrylic acid, crotonic acid, isocrotonic acid, tiglic acid, angelic acid, senecioic acid, and the like. Also suitable are the metal salts of such polymers, or ionomers. The polymers useful in the invention have a melt flow value of from about 0.01 to about 5000. (As used herein, melt flow value has units dg/min and is determined according to ASTM D-1238 condition E.) In addition, the copolymers may contain relatively minor amounts of other polymerizable vinyl monomers such as propylene, butene, hexene, octene, alkyl acrylates, methyl methacrylate, vinyl acetate or the like. Such polymers and the methods for making them, such as described in U.S. Pat. Nos. 3,239,370 and 3,520,861, are well known in the art.

It is essential that the composition contain a hydrate, in intimate admixture with the copolymer, present in at least an amount effective to impart thermal stability thereto up to about 5 weight percent of the composition, preferably 0.01-0.45 weight percent, most preferably 0.01-0.15 weight percent. By the term "thermal stability" is meant that the inherent tendency of the copolymer to crosslink, which is believed to occur primarily through the formation of anhydrides by the carboxyl groups, is inhibited.

Hydrates contemplated for use in the invention include hydrated metallic salts of organic acids, inorganic hydrates, bimetallic hydrates and combinations of any of such hydrates. As used herein, hydrates are defined as the products of the reaction of water with a compound, typically a salt, in which the H-OH bond is not split. It is essential that the hydrates used dehydrate above about 100.degree. C., preferably above 160.degree. C. If the hydrate dehydrates only below this temperature, dehydration of the hydrate may occur before the hydrate is in intimate admixture with the polymer and stabilization will not be complete. It is also essential that the hydrate dehydrate below about 300.degree. C., preferably below about 275.degree. C. If the hydrate does not dehydrate below this temperature, stabilization may not occur. Preferably, the hydrate will dehydrate slightly below or at approximately the temperature at which the copolymer containing the hydrate is to be processed. Typically such processing temperatures of the copolymer vary from about 175.degree. C. for film forming operations to about 230.degree.-260.degree. C. during manufacturing processing of the copolymer and extrusion coating operations.

Specific representative examples of suitable hydrated metallic salts of organic acids include:

Sodium acetate trihydrate;

Sodium boroformate;

Trisodium citrate dihydrate;

Sodium phenolsulfate;

Sodium propionate;

Sodium tartrate dihydrate;

Potassium citrate;

Potassium glutamate;

Potassium tartrate;

Potassium sodium tartrate;

Calcium acrylate hydrate;

Calcium ascorbate;

Calcium D-saccharate;

Calcium tartrate;

Magnesium salicylate;

Magnesium citrate, dibasic; and

Magnesium acetate hydrate.

Of these, sodium tartrate and potassium citrate are preferred, with potassium sodium tartrate being especially preferred.

Specific representative examples of suitable inorganic hydrates include:

Sodium ammonium phosphate tetrahydrate;

Sodium pentaborate;

Sodium phosphite;

Aluminum chloride hydrate;

Aluminum fluoride hydrate;

Aluminum sulfate hydrate;

Zinc sulfate;

Calcium chloride hydrate;

Calcium phosphate, dibasic hydrate;

Calcium phosphate, monobasic;

Lithium tetraborate;

Magnesium phosphate, dibasic dihydrate;

Magnesium phosphate, dibasic trihydrate;

Magnesium sulfate heptahydrate;

Magnesium sulfite;

Magnesium ammonium phosphate;

Magnesium metaborate;

Potassium pyroantimonate; and

Sodium phosphate, tribasic.

Suitable hydrates also include bimetallic hydrates of the formula

wherein A and B are metallic cations with respective, independent valences a and b of from about 0 to about 10, preferably from about 1 to about 5, most preferably from about 1 to about 3, and having a sum of at least one; C is an anion with a valence c of from about 0 to about 10, preferably from about 1 to about 5, most preferably from about 1 to about 3; w and x independently have a value of from about 1 to about 8; y has a value of from about 0 to about (aw+bx-1)/a; z has a value of (aw+bx-y)/c; and m has a value of from about 0.1 to about 24, preferably from about 5 to about 15. It is understood that these valences and molecular proportions are averaged quantities, and that such quantities may vary among the individual ions and/or molecules. The following formulae are representative specific examples of such bimetallic hydrates:

Such hydrated metallic organic acid salts, inorganic hydrates and bimetallic hydrates and the methods for making them are well known.

It has also been discovered that the thermal stability of the composition is further enhanced when the composition also contains from about 0.01 to about 5, preferably from about 0.01 to about 0.15, percent by weight of the composition of conventional polymer stabilizing additives. Suitable additives include hindered phenolic antioxidants, organic phosphorus compounds, dicarboxylic acids, amine stabilizers, and combinations of any of such stabilizing additives. Representative specific examples of suitable hindered phenolic antioxidants include:

Tetrakis (methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl) propionate) methane;

2,6-Di-t-butyl-p-cresol;

2-t-Butyl-4-hydroxyanisole;

3-t-Butyl-4-hydroxyanisole; and

2,6-Di-t-butyl-4-sec-butylphenol.

Of these, tetrakis (methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl) propionate) methane is especially preferred.

Representative specific examples of suitable organic phosphorus compounds include:

Tris(nonylphenyl) phosphite;

Bis(2,4-di-t-butyl)pentaerythritol diphosphite;

Distearyl pentaerythritol diphosphite; and

Tetrakis(2,4-di-t-butyl phenyl) 4,4'-biphenylylene diphosphonite.

Representative specific examples of suitable dicarboxylic acids include: maleic acid, succinic acid, tartaric acid, adipic acid and anhydrides thereof.

Representative specific examples of suitable amine stabilizers include:

N,N'-Diphenyl-p-phenylenediamine;

N,N'-Di-beta-naphthyl-p-phenylenediamine; and

Polymerized 2,2,4-trimethyl-1,2-dihydroquinoline.

In addition, the composition may contain additional additives such as colorants, pigments, or fillers such as silica, calcium carbonate, metal oxides, talc, clay, sawdust, rice hulls, wood, flour, starch, ground bark and the like. Such inert additives exert no adverse effect upon the capacity of the hydrate to thermally stabilize the composition. When such additional additives are present, they are to be excluded from determinations of the weight percentages of carboxyl-functional copolymer, hydrate and polymer stabilizing additives employed in the composition and method of the invention.

To assure complete thermal stability of the composition, the hydrate and any other polymer stabilizing additives should be in intimate admixture with the copolymer such that the hydrate and polymer stabilizing additives are distributed substantially uniformly throughout the composition. Thus, the hydrate and any polymer stabilizing additive added to the copolymer should be in a readily dispersable form, such as, for example, in a finely divided state or dissolved in a solvent.

In preparing the composition of the invention, the hydrate is first added to the carboxyl-functional copolymer in an amount effective to impart thermal stability thereto upon intimate mixing therewith at a concentration of up to about 5 weight percent of the composition, preferably 0.01-0.45 weight percent, most preferably 0.01-0.15 weight percent. If more than about 5 weight percent is employed, the physical properties of the copolymer may be adversely affected. The hydrate and the copolymer are then intimately mixed under conditions at which the copolymer is in a molten state. The hydrate addition and mixing are performed in the manufacture of the copolymer, or alternatively, may be performed following the manufacture of the copolymer. If desired, the addition and mixing steps may be substantially simultaneous.

When the composition is prepared in the manufacturing process of the carboxyl-functional copolymer, the hydrate may be added to the copolymer at any convenient point downstream of the copolymerization reactor vessel. Typical equipment downstream of the reactor includes separators in which the unreacted monomers and ethylene are removed from the molten polymer, primary extruders, and finishing extruders. Mixing of the hydrate and copolymer is accomplished by the shear imparted to the composition in the processing equipment, such as in lines, valves, pumps, vessels and extruders. The primary advantage of adding the hydrate to the copolymer in the manufacture thereof is that a higher quality product is obtained by an early inhibition of the tendency of the copolymer to crosslink. In addition, the corrosion to process equipment is significantly abated by substantially reducing the formation of corrosive crosslinking by-products.

When the composition is prepared following the manufacture of the carboxyl-functional copolymer, the hydrate addition and mixing may be performed in conventional polymer processing equipment, including compounding extruders, Banbury mixing extruders, intensive mixing or multi-screw extruders, roll mills or the like.

When polymer stabilizing additives are employed, they are added to the carboxyl-functional copolymer in the aforementioned proportions and intimately mixed therewith. The addition of the polymer stabilizing additives may be at the same point and in the same manner as the hydrate addition, or at any other point suitable for the hydrate addition as described above.

The invention is illustrated by way of the following examples.

A pelletized ethylene-acrylic acid copolymer prepared by free radical initiation in a stirred autoclave reactor under high pressure was used in preparing the examples. The copolymer contained 6.4 percent acrylic acid by weight as determined according to ASTM D-4094 and had a melt flow value of 7.68 as determined according to ASTM D-1238 condition E. The copolymer was dry blended with 2.0 percent by weight of a hydrate of the formula Mg.sub.4.5 Al.sub.2 (OH).sub.13 PO.sub.4 .multidot.3.5 H.sub.2 O and compounded at 177.degree. C. on a 1-inch diameter, 24/1 L/D compounding extruder equipped with metering, compression and mixing sections. During compounding, the dry blended mixture was diluted with additional copolymer to yield a substantially homogenous composition containing 0.10 percent by weight of the hydrate. The product is referred to herein as Example 1.

An additive-free composition was prepared for comparison purposes by passing the same copolymer through the extruder at duplicated conditions without adding any hydrated compound. This sample is referred to herein as Comparative 1.

Following the same procedure for preparing Example 1, a composition was prepared of the copolymer containing 0.10 percent by weight of a hindered phenolic antioxidant, tetrakis(methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl) propionate) methane. This sample is referred to herein as Comparative 2.

Again following the above procedure, a fourth composition was prepared which contained equal amounts of the hydrate used in Example 1 and the antioxidant used in Comparative 2, with a total additive concentration of 0.10 percent by weight. This sample is referred to herein as Example 2. The composition of the samples are summarized in Table I.

Specimens of Example 1 were placed in a Blue M Electric Co. Model No. E-514A-2 muffle furnace with a dry nitrogen atmosphere at a specified temperature. Separate thermocouples lodged in the specimens recorded the rising temperature thereof up to the specified temperature. At this time (t.sub.T) some of the specimens were withdrawn and evaluated for reduction in melt flow values, generally indicating the extent of cross-linking. The remaining specimens were withdrawn and tested after thermally degrading for an additional 5 minutes (t.sub.5) or 15 minutes (t.sub.15) at the specified temperature. The procedure was repeated for various temperatures with Examples 1 and 2 and Comparatives 1 and 2. The results are shown in Table II.

It is observed from the data in Table II that the hydrate demonstrated excellent thermal stabilization of the composition at the higher temperatures, but was not particularly effective in thermally stabilizing the composition at 240.degree. C. It is believed that this variation in effectiveness is because of the relatively high dehydration temperature of the hydrate, .about.260.degree. C.

Following the procedures outlined above, additional examples employing various hydrates and polymer stabilizing additives were prepared and evaluated for thermal stability. The results are presented in Tables III, IV and V.

The data presented above demonstrates the thermal stability of the composition of the invention containing 1000 ppm hydrate or 500 ppm hydrate and 500 ppm antioxidant in contrast to heretofore known compositions containing no additives or only conventional antioxidants.

Having described our invention above, many variations in the materials, ratios and method of preparation, as well as in the illustrated details of the composition and method will occur to those skilled in the art. It is intended that all such variations which fall within the scope of the appended claims be embraced thereby.