Stabilized carboxyl-containing ethylene interpolymers

Abstract

The oxidative, process, and/or color stability of carboxyl-containing ethylene interpolymers is improved by incorporating therein at least one aromatic bis(organophosphorus) compound, with or without a hindered phenol antioxidant and/or a dicarboxylic acid.

Description

BACKGROUND OF THE INVENTION

Interpolymers of ethylene containing pendent carboxyl groups are inherently highly susceptible to thermal oxidation. This inherent instability translates into significantly reduced production prime rates due to oxidative gels (crosslinked polymer), polymer discoloration, and degradation by-products which can corrode process equipment. This instability also seriously limits the useful life of the polymer's end-products, e.g., films, coatings, adhesives, and the like.

U.S. Pat. No. 4,254,014 discloses the preparation of bis(cyclic phosphite) compounds and their use as thermal stabilizers for polycarbonates. The stabilizers are also identified as aromatic bis(dioxaphosphorinane) compounds represented by the formula: ##STR1## wherein Ar is a divalent aromatic radical having each of its valences (valence bonds) on an aromatic ring; each R is individually a divalent aliphatic radical and each R' is individually hydrogen or a monovalent organic radical, provided that the aromatic bis(dioxaphosphorinane) is inert to the polycarbonate under conditions of fabrication and use.

The above, relatively new, compounds are among the aromatic bis(organophosphorus) compounds which are of interest in the present invention.

There exists a need for improving the thermal stability of interpolymers of ethylene containing pendent carboxyl groups. It is an object of this invention to meet that need.

SUMMARY OF THE INVENTION

The thermal stability of carboxyl-containing ethylene interpolymers is improved by incorporating therein minor amounts of an aromatic bis(organophosphorus) compound, alone or in combination with a dicarboxylic acid and/or with a hindered phenolic antioxidant.

DETAILED DESCRIPTIONS

Ethylene interpolymers containing carboxyl groups are well known, especially those comprising interpolymers containing ethylene and olefinically unsaturated carboxylic acids, e.g., acrylic acid or methacrylic acid, such as shown in U.S. Pat. Nos. 3,239,370 and 3,520,861. The ethylene/acid interpolymers often containing relatively minor amounts of other polymerizable olefins, such as propylene, butene, hexene, octene and the like.

Other ethylene polymers containing carboxyl groups are known, such as those prepared by grafting unsaturated carboxylic acids onto ethylene polymer chains.

Suitable monomers for use in incorporating carboxyl groups into ethylene polymers are, e.g., acrylic acid, methacrylic acid, crotonic acid, 3-methyl 2-butenoic acid, alkyl acrylates, vinyl acetate, and the like. The carboxylic groups may be provided by monomers conforming substantially to those represented by the empirical formula ##STR2## where R and R' are, independently, hydrogen or methyl groups and R" is hydrogen or alkyl groups of 1-10 carbon atoms. Other carboxylates are provided by monomers conforming substantially to the empirical vinyl alkylate formula ##STR3## where R is preferably an alkyl group of 1-4 carbon atoms. One or more of such carboxyl containing monomers may be used to provide about 0.01 to about 60% by weight of the ethylene polymer. The carboxyl-containing polymers may exhibit melt flows of from 0.01 gm/10 min. as measured by ASTM-D-1238, Condition E, to about 500 gm/10 min. as measured by ASTM-D-1238, condition B; i.e., they range from very high mol. wt to very low mol. wt.

The dicarboxylic acid additives of the present invention conform generally to the empirical formula: ##STR4## where R represents an aliphatic (C.sub.0 to C.sub.24) or aromatic radical; R' and R" each independently, represent amino, mercapto, phosphato, carboxyl, or hydroxyl groups or other nucleophilic groups or hydrogen groups. Where R is C.sub.0, then the compound is HOOC-COOH and there are no R' and R" groups. Where R is from C.sub.1 to C.sub.24 there may be only one R' and/or only one R" or there may be a plurality of R' and/or R" groups other than hydrogen. Where R is C.sub.2 to C.sub.24 there may be one or more double-bonds in the carbon chain. The following list of dicarboxylic acids is representative, but not exhaustive of the compounds contemplated within the purview of this invention:

______________________________________Name Formula______________________________________oxalic HOOCCOOHmalonic HOOCCH.sub.2COOHsuccinic HOOCCH.sub.2CH.sub.2COOHtartaric HOOCCH(OH)CH(OH)COOHglutaric HOOC(CH.sub.2).sub.3COOHadipic HOOC(CH.sub.2).sub.4COOHpinelic HOOC(CH.sub.2).sub.5COOHsuberic HOOC(CH.sub.2).sub.6COOHazelaic HOOC(CH.sub.2).sub.7COOHsebacic HOOC(CH.sub.2).sub.8COOHmaleic HOOCCHCHCOOHacetone dicarboxylic HOOCCH.sub.2COCH.sub.2COOHmalic HOOCCHOHCH.sub.2COOHphthalic ##STR5##______________________________________

Other substituted or unsubstituted bis-dicarboxylic polymethylenes will become apparent to practitioners of the relevant arts.

The aromatic bis(organophosphorus) compounds for use in the present invention conform, generally, to at least one of the following empirical structures, ##STR6## where, X is O--R'--O or R';

R' is an alkylene or alkylidene of C.sub.1 -C.sub.8, such as methylene, ethylene, propylene, propylidene and the like; R is an alkylene or alkylidene of C.sub.1 -C.sub.8, such as R' or may be an ether or thioether or polyether such as alkyleneoxyalkylene, alkylenethioalkylene, poly(alkyleneoxy)alkylene, poly(alkylenethio)alkylene, and the like; Ar represents biphenylene or phenylene whether substituted or not and where any such substituent groups may be halo, amino, or alkyl of C.sub.1 -C.sub.8 such as methyl, ethyl, propyl, butyl, t-butyl, hexyl, and the like; Ar' may be, independently of each other, any of the Ar groups described above, R" represents alkyl groups of C.sub.1 -C.sub.8, such as methyl butyl, hexyl, and the like including substituted alkyls such as haloalkyl or amino alkyl; M represents P, P'O, or P.dbd.S.

For the sake of conciseness, the above compounds are referred to this disclosure as "aromatic bis(organophosphorus) compounds".

Some representative compounds are:

(a) 2,2'-[(1-methyl ethylidene)bis(4,1-phenylene oxy-2,1-ethane diyloxy)]bis 5,5-dimethyl-1,3,2-dioxaphosphorinane (b) 2,2'-[(1-methyl ethylidene)bis(4,1-phenylene-oxy-2,1-ethane diyloxy)]bis 5,5'-dimethyl-1,3,2-oxo phosphate (c) 2,2'-[(1-methyl ethylidene)bis(4,1-phenylene-oxy-2,1-ethane diyloxy)]bis 5,5'-dimethyl-1,3,2,-thio phosphate (d) 2,2'-[(1-methyl ethylidene)bis(4,1-phenylene-oxy-1-methyl-2,1-ethane diyloxy)]bis 5,5'-dimethyl-1,3,2-dioxaphosphorinane (e) tetrakis(2,4-di-tert-butyl phenyl)4,4'-biphenylene diphosphonite and the like.

Further illustrations of the above empirical structures are:

1. 2,2'-[(1-methyl ethylidene)bis(4,1-phenylene)]bis(5,5-dimethyl-2,2'-disulfide-1,3,2-dioxaphosphorinane), which is a cyclic thiophosphonate of the formula ##STR7##

2. 2,2'-[(1-methyl ethylidene)bis(4,1-phenylene)]bis(5,5-dimethyl-2,2'-dioxide-1,3,2-dioxaphosphorinane), which is a cyclic phosphonate of the formula ##STR8##

3. tetrakis(2,4-di-tert-butyl phenyl)4,4'-biphenylene diphosphonite, which is a noncyclic phosphonite of the formula ##STR9##

4. tetrakis(2,4-di-tert-butyl phenyl)4,4'-biphenylene dithiophosphonate, which is a noncyclic thiophosphonate of the formula ##STR10##

5. tetrakis(2,4-di-tert-butyl phenyl)4,4'-biphenylene diphosphonate, which is a noncyclic phosphonate of the formula ##STR11##

6. 2,2'-[(1-methyl ethylidene)bis(4,1-phenylene)]bis(5,5-dimethyl-1,3,2-dioxaphosphorinane), which is a cyclic phosphonite of the formula ##STR12##

The hindered phenols for use in the present invention generally conform to empirical structures illustrated by, for example, ##STR13## or by ##STR14## and the like, where X represents one or more alkyl groups, such as (a) 1,3,5-trimethyl-2,4,6-tris(3',5'-di-tert-butyl-4'-hydroxyl benzyl)benzene

(b) 2,2'-oxamidobisethyl, 3(3',5'-di-tert-butyl-4-hydroxy phenyl)propionate (c) tetrakis[methylene-3-(3',5'-di-tert-butyl-4'-hydroxyl-phenyl)propionate]methane (d) 1,1,3-tris-(2'-methyl-4'-hydroxy-5'-tert-butyl phenyl)butane (e) 2,6-di-tert-butyl-4-ethyl phenol (f) 2,2'-oxamidobis ethyl-3(3',5'-di-tert-butyl-4-hydroxyl phenyl)propionate (g) tris(4-tert-butyl-3 hydroxy-2,6-dimethyl benzyl)-2-triazine-2,4,6-(1H,3H,5H)trione (h) 2,6-di-tert-butyl-4-methyl phenol (i) octadecyl-3,4-di-tert-butyl-4-hydroxy-hydrocinnamate and the like.

Suggested operative ranges and preferred ranges (percent by wt.) for the components of the present blends or alloys are as follows:

______________________________________Component Operative Preferred______________________________________carboxyl-containing polymer 79-99.9 99.7-99.8hindered phenol 0.01-3.0 0.02-01aromatic bis(organo- 0.01-3.0 0.02-0.1phosphorus) compounddicarboxylic acid 0.01-15 0.02-0.1______________________________________

The mixing of the interpolymers with the other components of the present invention may be done by any convenient means, such as by employing a compounding extruder, Banbury mixer, intensive mixers, or multi-screw mixing extruder, or any combination thereof, preferably by masterbatch concentrates letdown to the desired additive concentrations in an extruder equipped with one or more mixing screws.

In accordance with ordinary goals sought in polymer blending, or polymer alloying, one seeks to distribute additives substantially uniformly through the base resin.

The Oxidative Induction Period is measured by an oxygen-uptake apparatus similar to that described by Grieveson in Society of Chemical Industries, Monograph No. 13, 1961, pp. 413-431, published in London, England, and the Gardner Yellowness Index is measured by a Gardner XL-23 colorimeter according to ASTM-D-1925.

The following examples are for illustration of various embodiments, but the present invention is not limited to the particular embodiments shown.

EXAMPLE 1

An ethylene/acrylic acid copolymer prepared by free-radical initiation in a stirred autoclave under high ethylene pressure and high temperature was used in the example. The copolymer contained 6.5% by weight of acrylic acid units and had a melt flow of 2 gm./10 minutes as measured by ASTM-D-1238 condition E. Small particles of the copolymer were dry-blended with 0.25% tartaric acid and then compounded at 350.degree. F. (.about.177.degree. C.) on a 21/2 inch, 20/l L/D compounding extruder equipped with a metering, compression and mixing section. The pelletized product had an oxidative induction period of >60 minutes according to oxygen-uptake analysis at .about.149.degree. C. The oxidative induction period at .about.149.degree. C. for the base resin without the addition of tartaric acid was 9.0 minutes.

EXAMPLE 2

An ethylene/acrylic acid copolymer containing 6.5% acrylic acid by weight, a melt flow of 2 gm./10 min., and a density of 0.9325 gm/cc was used as the base resin in the following evaluations:

______________________________________ Oxidative Concentration, Induction PeriodRun Stabilizer* ppm @ 177.degree. C., minutes______________________________________1 None -- 3.52 I 2500 15.03 II 2500 11.0______________________________________ *I is tartaric acid II is a 2:1 physical blend of 1,1,3tris(2methyl-4hydroxy-5tert-butyl phenyl) butane and distearylthiodipropionate (a thioester stabilizer).

The above evaluation indicated that at an equivalent concentration, tartaric acid out-performed a conventional hindered phenol-thioester composition.

EXAMPLE 3

An ethylene/acrylic acid copolymer containing 6.5% acrylic acid, having a melt flow of 5.5 gm/10 min. and a density of 0.9325 gm/cc was used in the following evaluation.

______________________________________ Additive A* Additive B** Oxidative InductionRun ppm ppm Period @ 177.degree. C., mins.______________________________________1 None None 7.02 1000 None 21.03 None 1000 39.04 500 500 48.0______________________________________ *A is 1,1,3tris-(2methyl-4-hydroxyl-5tert-butyl phenyl) butane **B is tartaric acid

The above evaluation indicated that a hindered phenol-dicarboxylic acid composition is highly synergistic.

EXAMPLE 4

Using the same copolymer as in Example 3, the following evaluation was performed:

______________________________________ Additive A* Additive B** Oxidative InductionRun ppm ppm Period @ 177.degree. C., mins.______________________________________1 None None 7.02 1000 None 21.03 None 1000 35.04 500 500 78.0______________________________________ *Additive A is 1,1,3tris-(2methyl-4-hydroxyl-5tert-butyl phenyl) butane. **Additive B is 2,2[1methyl ethylidene) bis (4,1phenylene oxy2,1-ethane diyloxy)] bis 5,5dimethyl-1,3,2-dioxaphosphorinane.

This evaluation showed that a hindered phenol/aromatic bis(organophosphorus) composition is also highly synergistic.

EXAMPLE 5

Using the same copolymer as in Example 3 the following evaluation was conducted.

______________________________________ Concentration Oxidative InductionRun Stabilizer* ppm Period @ 177.degree. C., mins.______________________________________1 A 1000 23.02 B 1000 39.03 C 1000 21.04 D 1000 35.05 E 1000 >143.06 -- -- 7.0______________________________________ *A is tetrakis [methylene 3(3' ,5di-tert-butyl-4hydroxyl phenyl) propionate] methane B is tartaric acid C is 1,1,3tris-(2methyl-4-hydroxyl-5tert-butylphenyl) butane D is 2,2[(1methyl ethylidene) bis (4,1phenylene oxy2,1-ethane diyloxy)] bis 5,5dimethyl-1,3,2-dioxaphosphorinane E is tetrakis (2,4di-tert butyl phenyl)4,4biphenylene diphosphonite.

The above evaluation showed that at equivalent concentrations the dicarboxylic acid and the aromatic bis(organophosphorus) compound were more effective thermal stabilizers than either of the conventional hindered phenol stabilizers.

EXAMPLE 6

Using the same copolymer as in Example 3 the following multiple-extrusion evaluation was performed.

______________________________________Stabilizer** Other StabilizerRun A, ppm Identity ppm .sigma..sub.mi * .SIGMA..sub.mi *______________________________________1 None None None 0.147 --2 500 B 500 0.087 41.063 500 C 500 0.097 34.334 500 D 500 0.137 7.085 500 E 500 0.176 -19.67______________________________________ *.sigma..sub.mi is the standard deviation in melt indices of each sample extruded five times at 204.degree. C. on the aforementioned extruder. .SIGMA..sub.mi is the percent melt stability relative to the base resin i this evaluation and was calculated from: ##STR15## Where .sigma..sub.o was the .sigma..sub.mi of the unstabilized base resin and .sigma..sub.i was the .sigma..sub.mi of each stabilized sample independently. **A is tetrakis [methylene 3(3',5'-di-.sub.----tert-butyl-4'-hydroxylphenyl)propionate]methane B is 2,2'-[(1methyl ethylidene)bis(4,1phenylene oxy2,1-ethane diyloxy)] bis 5,5'-dimethyl1,3,2-dioxaphosphorinane C is tartaric acid D is tris(mixed mono and dinonyl phenyl)phosphite E is thiobis(6.sub.----tert-butyl-3-methyl phenol).

The above multiple-extrusion evaluation indicated that phenolic/aromatic bis(organophosphorus) compound and phenolic/dicarboxylic acid compositions offer exceptional process stability in contrast to conventional phosphite and thio-bisphenol compositions which offer little or no stability, or may even adversely affect polymer stability.

EXAMPLE 7

In this evaluation an ethylene/acrylic acid copolymer containing 6.5% by wt. acrylic acid, having a melt flow of 9.0 gm/10 min. and a density of 0.9325 gm/cc was used.

______________________________________Run Stabilizer* Conc. ppm .SIGMA..sub.mi______________________________________1 None None --2 A 1000 16.23 B 1000 72.54 C 1000 43.8______________________________________ *A is tetrakis [methylene 3(3',5'-di-tert-butyl-4-hydroxylphenyl) propionate] methane B is 2,2'-[(1methyl ethylidene) bis (4,1phenylene oxy2,1-ethane diyloxy)] bis 5,5'-dimethyl1,3,2-dioxaphosphorinane C is tartaric acid

The above evaluation showed that at equivalent concentrations the dicarboxylic acid and the aromatic bis(organophosphorus) compound were significantly more effective process stabilizers than the conventional hindered phenol.

EXAMPLE 8

Following the multiple-extrusion evaluations of Example 6 above, Gardner Yellowness Index determinations of the samples were made according to ASTM-D-1925.

__________________________________________________________________________ Gardner YellownessStabilizer* Index, Pass:Run A, ppm B, ppm C, ppm D, ppm 0 1 2 3 4__________________________________________________________________________1 1000 -- -- -- -3.4 6.2 6.4 8.97 11.72 500 -- 500 -- -3.4 -2.8 -1.2 0.54 4.033 500 500 -- -- -2.06 -0.85 0.75 1.88 3.344 500 -- -- 500 -0.90 -0.25 0.50 1.56 3.07__________________________________________________________________________ *Stabilizers A, B, and C are same as in Example 6. Stabilizer D is same a E in Example 5.

This evaluation showed that the dicarboxylic acid and the aromatic bis(organophosphorus) compound compositions imparted good color stability to an ethylene/acrylic acid copolymer while a conventional hindered phenol stabilizer imparted less than acceptable color stability as denoted by the relatively high measurement, 11.7; the higher the value the more objectionable the color.

EXAMPLE 9

Using the same base resin as in Example 7 above the following evaluation was made.

______________________________________ Stabilizer*Run A, ppm B, ppm C, ppm .SIGMA.mi______________________________________1 500 500 -- 30.82 500 250 250 72.2______________________________________ *A is 1,1,3tris-(2'-methyl-4-hydroxyl-5'-tert-butyl phenyl) butane. B is tetrakis (2,4di-tert-butyl phenyl)4,4'-biphenylene diphosphonite. C is 2,2'-[(1methyl ethylidene)bis(4,1phenylene oxy2,1-ethane diyloxy)]bi 5,5dimethyl-1,3,2-dioxaphosphorinane.

This evaluation showed that the combination of two aromatic bis(organophosphorus) compounds and a hindered phenolic antioxidant was very effective.

EXAMPLE 10

Using the same base resin as in Example 7 above the following evaluation was made:

______________________________________ Stabilizer*Run A, ppm B, ppm C, ppm .SIGMA.mi______________________________________1 500 500 -- 30.82 500 250 250 58.13 -- 1000 -- 19.14 1000 -- -- 16.1______________________________________ *A is 1,1,3tris-(2'-methyl-4-hydroxyl-5'-tert-butyl phenyl) butane. B is tetrakis (2,4di-tert-butyl phenyl)4,4'-biphenylene diphosphonite. C is tartaric acid.

This evaluation showed that the combination of a dicarboxylic acid, an aromatic bis(organophosphorus) compound, and a hindered phenol antioxidant significantly improved the process stability of a carboxyl-containing ethylene interpolymer. The evaluation also illustrated another synergistic aromatic bis(organophosphorus) compound/hindered phenol composition.

Within the purview of the present invention, dicarboxylic acids and aromatic bis(organophosphorus) compounds, such as described herein, significantly improve the thermal-oxidative and color stability of carboxyl-containing ethylene interpolymers during their manufacturing processes, storage, fabrication, and end-usage. The applications of such heat stable ethylene interpolymers include such things as extrusion films (e.g., for paper, paperboard, metallic substrates, etc.), blister packages, flexible packages, adhesives (hot melts), primers, binders, laminates, adhesive composite structures, barrier films, metallized films, injection molded articles, rotomolded articles, wire and cable insulation, shielding tapes, powder molding, and the like, including those embodiments wherein the polymers contain colorants, pigments, or fillers. Fillers include such things as silica, calcium carbonate, metal oxides, talc, clay, sawdust, rice hulls, wood flour, starch, ground bark, and the like.

Claims

1. A method for improving the thermal stability of ethylene/acrylic acid or methacrylic acid interpolymers containing pendant carboxylic acid groups, said method comprising

incorporating into said interpolymers a minor amount of at least one aromatic bis(organophosphorus) compound selected from those conforming to the formulae ##STR16## Ar represents biphenylene or phenylene whether substituted or not and where any such substituent groups may be halo, amino, or alkyl of C.sub.1 -C.sub.8

Ar' represents, independently of each other, any of the Ar groups described above,

R represents alkyl groups of C.sub.1 -C.sub.8 including substituted alkyl groups, and

M represents P, P.dbd.O, or P.dbd.S, and, in addition thereto, a minor amount of at least one hindered phenolic antioxidant and/or a minor amount of at least one dicarboxylic acid of the formula ##STR17## or

HOOC--COOH,

where R' represents an aliphatic or aromatic grouping containing C.sub.1 to C.sub.24, and where R" and R'" each, independently, represent amino, mercapto, phosphato, carboxyl, or hydroxyl group or hydrogen, said carboxyl-containing interpolymer constituting 79 to 99.9% by weight, said hindered phenol constituting 0.01 to 3.0% by weight, said aromatic bis(organophosphorus) compound constituting 0.01 to 3.0% by weight, and said dicarboxylic acid constituting 0.01 to 15% by weight, all based on the total weight of the composition.

2. The method of claim 1 wherein the amount of the aromatic bis(organophosphorus) compound is in the range of about 0.02% to about 0.1% by weight of the interpolymer.

3. The method of claim 1 wherein the aromatic bis(organophosphorus) compound is at least one of formula I.

4. The method of claim 1 wherein the aromatic bis(organophosphorus) compound is at least one of formula II.

5. The method of claim 1 wherein the interpolymer comprises ethylene/acrylic acid copolymer.

6. The method of claim 1 wherein at least one hindered phenolic antioxidant is used in addition to the bis(organophosphorous) compound.

7. The method of claim 1 wherein at least one of the dicarboxylic acids is used in addition to the bis(organophosphorous) compound.

8. The method of claim 1 wherein the hindered phenolic compound comprises about 0.02% to about 0.1% by weight of the total and is at least one compound corresponding to the formula ##STR18## where X represents one or more alkyl groups.

9. A composition of matter comprising an ethylene/acrylic acid or methacrylic acid interpolymer containing pendant carboxylic acids groups having incorporated therein a minor amount of at least one aromatic bis(organophosphorous) compound conforming to the formula ##STR19## Ar represents biphenylene or phenylene whether substituted or not and where any such substituent groups may be halo, amino, or an alkyl or C.sub.1 -C.sub.8,

Ar' represents, independently of each other, any of the Ar groups described above,

R represents alkyl groups of C.sub.1 -C.sub.8, including substituted alkyl groups, and

M represents P, P.dbd.O, or P.dbd.S,

and in addition thereto, a minor amount of at least one hindered phenolic antioxidant and/or a minor amount of at least one dicarboxylic acid of the formula ##STR20## or

HOOC--COOH,

where R' represents an aliphatic or aromatic grouping containing C.sub.1 to C.sub.24, and where R" and R'" each, independently, represent amino, mercapto, phosphato, carboxyl, or hydroxyl group or hydrogen, said carboxyl-containing interpolymer constituting 79 to 99.9% by weight, said hindered phenol constituting 0.01 to 3.0% by weight, said aromatic bis(organophosphorus) compound constituting 0.01 to 3.0% by weight, and said dicarboxylic acid constituting 0.01 to 15% by weight, all based on the total weight of the composition.

10. The composition of claim 9 wherein the amount of the aromatic bis(organophosphorous) compound is in the range of about 0.02% to about 0.1% by weight of the interpolymer.

11. The composition of claim 9 wherein the aromatic bis(organophosphorous) compound is at least one of formula I.

12. The composition of claim 9 wherein the aromatic bis(organophosphorous) compound is at least one of formula II.

13. The composition of claim 9 wherein the interpolymer comprises ethylene/acrylic acid copolymer.

14. The composition of claim 9 wherein at least one hindered phenolic antioxidant is used in addition to the bis(organophosphorous) compound.

15. The composition of claim 9 wherein at least one of the dicarboxylic acids is used in addition to the bis(organophosphorous) compound.

16. The composition of claim 9 wherein the hindered phenolic compound comprises about 0.02% to about 0.1% by weight of the total and is at least one compound corresponding to the formula ##STR21## where X represents one or more alkyl groups.