Brook,
N. J.
Metal-Catalyzed Oxidation of Polyethylene A process to use off-grade polyethylene as a raw material for chemical manufacture is suggested by a study of the effect of metal salts and oxygen on polyethylene C E R T A I N pigments, usually particular iron, chromium, manganese, and cobalt compounds, may cause serious deterioration to the point where polyethylene is converted to a buttery or oily wax (2, 7 7). Infrared spectra of the products of such pigment-catalyzed reactions show an unreasonably large percentage of oxygencontaining molecules. I n the work reported here, the catalyzed low temperature oxidation (below 100" C.) of polyethylene is explored to understand the degradation reactions better and as a possible process for using polyethylene as a source of chemicals. The idea of catalyzed oxidation of paraffins is not new. The oxidation of paraffin wax has been reported (6) and during World War I1 the Germans catalytically oxidized petroleum waxes to make fatty acids and esters (72, 75).
1 Present address, Harris Research Laboratories, Washington, D. C.
The results reported for the metal-catalyzed oxidation of polyethylene are consistent with the mechanisms previously reported for model compounds; no single mechanism i s proposed. The mechanisms described and experimental results indicate that highly branched and unsaturated polyethylene will b e most susceptible to metal-catalyzed oxidation. It may be possible to predict products by starting with a polyethylene of preselected structure and employ metalcatalyzed ozone oxidation of off-grade polyethylene in chemical manufacturing. Mechanistic descriptions of the reaction indicate that conventional antioxidants may protect branched polyethylenes from pigment-catalyzed degradation.
Petroleum waxes can be oxidized with metallic or metal compound catalysts to produce a carnaubalike wax (70). Oxidation of peat with air and an alkaline permanganate catalyst at 150° C. converts up to 22y0 of the carbon to water-soluble polycarboxylic acids ( 73). The chemistry of these processes is logically extended to polyethylene.
Experimental The polyethylene used was the conventional, high pressure, branched type of 2.84 melt index, ground to 40 to 60 mesh. Observations were made on samples hot milled with metal compounds and stored at 2 5 or 60 C. Reaction rates were increased by operating at high oxygen pressures. Semiquantitative examinations of the reacted samples were made with a Model 21 Perkin-Elmer infrared spectrophotometer. The reactions were always carVOL. 52, NO. 2
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121
ried out below 80' C., because much above this temperature crosslinking and different oxidation reactions seem to predominate. High pressure oxidations were conducted in a Parr 2-liter series 4500 stainless steel stirred autoclave fitted with heater and cooling coils. Constant temperature (65' to 70" C.) was maintained by pumping water at 70' C. through the coils and compensating for heat losses by operating the heater at reduced voltage. Maximum operating pressure was 1000 p s i . A steady source of ozone a t an unknown concentration was provided by a home-made laboratory device. The extent of oxidation of the polyethylene after reaction was judged from infrared spectra. Isolated products were similarly characterized. Experimental results are reported and compared by tabulating the relative absorption of infrared energy a t wave length A, through a parameter Dx,defined as:
D X = ipog~o;] t
X 108
Dh is directly proportional to the weight per cent of the group showing absorbance a t X microns. Here, t is the thickness of the sample (usually 0.016 inch) and Z and ZOare the Beer's law definitions of the absorbances. Band assignments are (7) : Band Assignments for D k Values
Wave Length, Microns
Functional Grouping Hydroperoxide (-OOR) 2.8 Carbonyl ( = b o ) 5 . ?-5 .a tram-Internal unsaturation (t.i.u.) 10.35 Terminal unsaturation (t.u.) 11.0 Pendant methylene (=CHz) 11.25
Rough comparisons are made using
D X for convenience rather than reproducing dozens of spectra. I t is difficult to quantitate chain scission, which should be judged from melting point and molecular weight changes. For example, all samples oxidized in the presence of potassium permanganate melted more sharply and at a lower temperature than the starting material. Experimental Results
T h e infrared spectrum of polyethylene heated in contact with bright metallic copper, or copper, chromium, manganese, or lead compounds, shows a marked increase in hydroxyl, carbonyl, and ether group content of the sample and a reduction in pendant methylene content. The infrared spectra exhibit absorbance attributable to cyclohexyl or similar structures and the formation of a metallic salt. Compounds analogous
1 22
to the metallic naphthenate driers might be the oxygen carrier. The naphthenate driers of cobalt, lead, and manganese (in order of decreasing activity) are catalysts for the oxidation and chain scission of polyethylene. T h e reaction is sensitive to exposed surface and leads to embrittlement and the subsequent appearance of a soft material accompanied by the detection of infrared absorption bands characteristic of estertype waxes. The reaction products from polyethylene milled with manganese dioxide and stored for some months at 60" C. were fractionally distilled under vacuum. Three fractions of indefinite boiling points were collected. The first two fractions were oils and the predominant fraction was a highly crystalline wax having a sharp melting point about 100' C. and a low viscosity when molten. The
Table 1.
Tabulation
of
Various compounds were employed as oxidation catalysts. They were compared by suspending 25 grams of powdered polyethylene in a solution of catalyst in a liquid not susceptible to rapid oxidation-e.g., water-and subjecting the system to 1000 p.s.i. of oxygen for 7 to 8 hours at 65' to 70' C. The catalysts were compared at the same concentration of 0.13 mmole of metal per gram of polyethylene, or about 2% by weight, based on the resin. The relative effects of different catalyzing compounds may be judged from Table I ; water was used as the suspending liquid unless otherwise noted. The catalyst is necessary to cause oxidation and chain scission because D X values for polyethylene subjected to 1000 p.s.i. of oxygen for 7 to 8 hours at 65" to 70' C . with no catalyst d o not differ from those listed for unreacted polyethylene.
Shows the Effect of Various Compounds as Oxidation Catalysts
D k Values
(7-8 hours at 65-70' C., 1000 p.s.i. 02) Dl1.1sr
(=CHI)
Starting material, no catalyst Manganese stearate" Manganese naphthenate in CCh Cobalt naphthenate in CClr Cobalt naphthenateO K~Cr207 FeC12 RiClt MnOl MnOz, no suspending liquid MnOn in CCL AK-33-X* in CClr KMnO4 in CClr KMnO4 (I
20 ... ... ...49
e..
e . .
21 3 25 14 2 4 3
14 20 6
4 5 8
... 7
e..
24
1 2 0 0 1 1 2
... 1
2 ... 1
13 ... ?
2.84
0 ... 1
...09
>200
1 0
11 12 1 9 10 11
0 1
1 2
... 4
... 4
2.16
16.5
No spectra obtained when product was not wet by water. Methylcyclopentadienylmanganese tricarbonyl (Ethyl Corp.).
wax appeared to be a derivative of an unbranched aliphatic hydrocarbon, at least partially unsaturated, about 20 to 40 carbon atoms long. The infrared spectrum of the wax indicated more than one type of carbonyl (including ester), as well as ether or peroxide groups and possibly hydroperoxide or hydroxyl groups. The extent of oxidation and degradation is a function of oxygen pressure. The rate appears to be dependent on the formation of an organometallic compound or salt that acts as the catalyst. The catalytic compound is not formed in an anaerobic atmosphere up to 150" it is preferably formed in the presence of the less than 1% of unidentified methanol-soluble material on the surface of the polyethylene. The oxidation rate of methanol-washed polyethylene is less than the oxidation rate of the unwashed material.
INDUSTRIAL AND ENGINEERING CHEMISTRY
23
2 ... ... 0
...32
Melt Index
c.;
The catalytic activity of cobalt naphthenate is consistent with what was found on long-term aging of polyethylene with Nuodex driers. The ineffectiveness of cobalt naphthenate when the polyethylene was suspended in water is probably due to the poor catalyst-resin contact because of catalyst insolubility. The same holds true for manganese stearate and potassium permanganate in carbon tetrachloride. Generally, the inorganic catalysts were effective. Ferrous chloride produced a bright orange product, probably red iron oxide, which does not catalyze the oxidation. Nickel(I1) chloride catalysis yielded a product showing strong carbonyl and hydroperoxide infrared absorbance but left the pendant methylene groups intact. Potassium dichromate did not produce as much carbonyl or peroxide but attacked the pendant methylene groups. The most effective
POLYETHYLENE O X I D A T I O N rate had little effect-small pH changes from neutral may have hindered the reaction, despite the lowering of the surface tension of the suspending medium by the stearates. A surface-active catalyst would be ideal, especially if it did not coat the resin with something as insoluble and tenacious as manganese dioxide. The results concerning pH are inconclusive. When potassium permanganate is used as a catalyst, an infrared absorption band is observed between 6.25 and 6.50 microns, possibly indicative of salt formation. From 5- to 8-hour reaction a t 70" C. and 1000 p.s.i. of oxygen is required before this band shows; the melted portion from the column oxidation showed the same band. Inasmuch as the rate of oxidation appears to in-
catalysts listed in Table I are potassium permanganate and cobalt naphthenate. The potassium dichromate and nickel chloride were not only effective but did not discolor the polyethylene. Potassium permanganate leaves the polyethylene brown, probably from the formation of manganese dioxide and/or manganese(II1) compounds. Powdered polyethylene was oxidized under a variety of conditions with potassium permanganate. Oxidation was conducted in a jacketed glass column thermoregulated with benzene vapor (80" C.) by supporting the powdered material on a porous glass disk and passing filtered, compressed air through the polymer. The polyethylene was first wet with a concentrated solution of potassium permanganate and sodium
~~
Table It.
~
Oxidation Was Most Effective at 1000 P.S.I. of Oxygen (2% KMnOd catalysis, 65-70' C.)
Starting material Column oxidized, melted portion Column oxidized, not melted 1 S / 2 hr., 100 p.8.i. 0 2 plus 1% benzoyl peroxide 8 hr. 100 p.s.i. 0 2 8 hr. 100 p.s.i. 0 2 , 900 p.s.i. COZ 7 hr. 1000 p.s.i. 0 2 after above 8 hr. 1000 p.8.i. 02,2 meq. stearic acid, 5 meq. RaOH 8 hr. 1000 p.8.i. 02,2 meq. stearic acid 8 hr. 1000 p.s.i. 0 2 , 2 meq. stearic acid, 20% KMnOd 8 hr. 1000 p s i . 02,methanol-washed resin 24 hr. 1000 p.s.i., 0 2 methanol-washed resin
Dii.olr
Dll.2511
(t.u.)
(=CHz)
20 6 6
3 6 7
2 2 1
2 2
13 13
1
7
0
12 24 13 16
1 1 1
1 4 1 1
0 4 10 4
6 24
1 1
1 1
7
12 9
0 2
1 1
8 12
31
0
1
11
7 8
4
...6 ...9
carbonate, so as to add 0.1 5 gram of each per gram of resin. The passage of air was continued for 52 hours with no condensable material in the exit gases. After approximately 48 hours the polyethylene in the bottom half of the tube turned deep brown and "melted" from a powder to a continuous mass. Table I1 summarizes the Dx values for the column-oxidized polyethylene for both the melted material and the portion that remained powdery and shows Dx values for polyethylene oxidized under various conditions with 270 potassium permanganate in water. The data indicate that the molten portion from the column oxidation probably underwent chain scission, for it was not otherwise much affected. The most effective potassium permanganate-catalyzed oxidation was carried out with 2% catalyst solution at 1000 p.s.i. of oxygen. The addition of stearic acid or sodium stea-
4
1
8
crease after the formation of the salt, the salt may be a potent catalyst. Seriously degraded samples, such as the one reacted 24 hours a t 70" C., show infrared bands at 8.8 and 9.3 microns that are believed due to phthalic or cyclohexyl type structures. I t may be possible to wash the salt out of the polyethylene with methylcyclohexane. The relative susceptibility to oxidation of a number of different types of polyethylenes, under the influence of manganese dioxide catalysis, was judged by infrared examination of the material after 2 months' storage at 60" C. A highly branched polyethylene of high pendant methylene content was most susceptible. Ziegler and Phillips type polyethylenes were unaffected. The products from the polyethylene oxidation have not been completely isolated nor identified. Some products, undoubtedly complex mixtures, have
been characterized as to their functional groups by means of infrared spectrum. A frequently used method of separation involved filtration of the reacted polymer, followed by evaporation of the catalyst solution. A poly01 or mixture of polyols has been separated and characterized several times this way. Permanganate catalysis yields a watersoluble, sirupy polyol or poly01 mixture of approximately six to eight carbon atoms. When benzoyl peroxide was included in the system, a polyol, or poly01 mixture, of between 6 and 12 carbon atoms was characterized. This particular product was devoid of methylene groups, much like a sugar molecule. Reaction for 8 hours at 1000 p.s.i. of oxygen gives approximately a 570 yield of polyol. Acid, ester, and alcohol functions have been noted in the infrared spectra from volatile organic products obtained by ether extraction of the catalyst solution from a number of oxidations. An interesting wax was obtained in about a 2 to 3% yield from a sample of degraded polyethylene. Material oxidized at 1000 p.s.i. of oxygen for 8 hours with 5% potassium permanganate and 0.570 stearic acid was extracted with methanol and the methanol evaporated. The wax left had a slight odor, was difficultly soluble in cold methanol, and had a softening point of about 90" C. The infrared spectrum of the wax is similar to the spectrum of carnauba wax, but the new wax does not show ester infrared absorbance at 8.2 microns. The wax has been obtained a number of times after similar reaction conditions. No quantitative data are available from the metal-catalyzed ozone oxidation of polyethylene. However, 8-hour oxidation at 70" C. of polyethylene in an oxygen stream containing a small amount of ozone, a t I-atm. total pressure, is ineffective without a metal compound catalyst. Polyethylene was reacted under identical conditions in a solution of carbon tetrachloride containing 0.13 mmole of cobalt as cobalt naphthenate per gram of resin. Within 8 hours the polymer was completely degraded and unrecognizable. It is difficult to estimate the relative rates of oxygen and ozone reaction. However, 8-hour catalytic oxidation with an ozone-enriched oxygen stream at 1-atm. total pressure was more effective over-all than 24-hour catalytic oxidation of polyethylene at 7-atm. pressure. The product of the ozone reaction was the familiar soft, buttery, and oily wax. The oxidation proceeds faster in the presence of water. Preferably, the suspending medium-e.g., carbon tetrachloride-should swell the resin so as to drive the catalyst (and the oxygen if it is soluble in the liquid phase) into the inVOL. 52, NO. 2
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123
terior of the resin particle. The oxidation is accompanied by stripping of the pendant methylenes and chain scission. The terminal carbon on the cleaved chain may react with oxygen to form oxygenated products or may not, accounting for the few cases that show infrared evidence for increases in terminal unsaturation.
RH + n M n + radical + R R + 0, Roz RH +R (3) RO, + RO2 products +
ROOH
-
M(n+1)+
+
+ ROOH
+
M(n+l) RO+OH+
+R02
(I)
+ H + + Mn+ (2)
Equation 1 is probably rate-determining (76, 78), after which M(“+’)+ will be rapidly reduced to Mn+ via 2. According to Woodward and Mesrobian (78), the scheme proceeds as follows:
1 24
0
+
2
OH
The catalytic oxidation and conversion of polyethylene to low molecular weight compounds containing functional groups may be the antithesis of the FisherTropsch synthesis, representing a new source of organic chemicals. Like the Fisher-Tropsch reaction, and many related reactions, the reaction reported here is catalyzed by compounds of copper, chromium manganese, iron, cobalt, nickel, lead, and possibly others. Any mechanistic description of this reaction has to account for certain salient observations; the reaction proceeds in the solid state, in carbon tetrachloride solution and in aqueous slurry-either homogeneous or heterogeneous media. Readily ionizable compounds, such as potassium permanganate and dichromate, are as effective as cobalt naphthenate or manganese dioxide. There appears to be a rate-determining step involving oxygen associated with the detection of infrared absorption bands attributable to metal compounds and ring formation. The pendant methylene and olefin group concentrations of the polymer are decreased. The experimental evidence presented is insufficient to make possible conclusions regarding the mechanism of the oxidation. However, the mechanisms proposed for metal-catalyzed oxidations of lower molecular weight compounds may apply to the various structural units along the polyethylene chain. The role of the metal atom in the oxidation scheme has been given by Walling (76) and in greater detail by Woodward and Mesrobian (78). The major path by which transition metal salts act as catalysts is by reaction with the intermediate peroxides to produce radicals. The radicals are formed by some redox scheme and they in turn initiate additional autoxidation chains. Autoxidation rates are accelerated and reaction becomes autocatalytic. A complete scheme for participation of a metal ion would be:
+ ROOH
+ Ro2
This scheme can account for observed autocatalysis, because the hydroperoxide formed in 3 will subsequently react with metal ion and continue the chain process.
Discussion
Mn+
postulates a cyclic intermediate of permanganate with the olefin leading to glycols and chain scission. Equation 4 can fit into an autocatalytic scheme. Equation 5 is not autocatalytic by itself unless the manganese dioxide formed catalyzes the continued oxidation of the polyethylene by a mechanism different from Equation 5.
+
OH
OH OH (4)
I
I
I
HC-0 \Mn/O
H&OH 1
HC-O/
HC-OH
I
-C
I
‘
\O-
I
II IH,O L-
+
-
I
H&O
Mn04-
H&O
+ MnO,
(5)
I The catalysis requires metal ions that can exist in more than one oxidation state with a suitable redox potential. This scheme is not meant to exclude the reaction of metal ion with hydrocarbons to initiate oxidation chains ( 4 , 7). The radicals resulting from the autocatalysis will attack labile groups along the polyethylene chain or these labile groups will enter directly into the above reaction scheme. Experimental evidence indicates that branch points and unsaturation are particularly labile to the metalcatalyzed oxidation, consistent with previous reports (8, 9,74, 76). The possible course of the reactions of labile groups along the chain has recently been reviewed ( 3 ) . A slightly different reaction scheme may also describe the observed catalyzed oxidation of polyethylene. Direct attack of a metal ion at a double bond will result in electron transfer with the formation of a reactive radical ion ( 5 ) . R-CH=CH2
+ M(”+’)++ RCH -
d ~+,M“+
The subsequent reactions to yield the observed products would consist of a series of steps in which the carbonium ion reacts with water to give hydroxylic compounds. These are then rapidly oxidized by the metal ion in successive stages until stable products are formed. Chalk and Smith (7) have described such a scheme in which glycol is readily oxidized by cobaltic salts and glycol fission ensues by a one-electron transfer and the formation of a free radical. The resulting products would be an aldehyde or ketone and another radical, =C-OH. Equations 4 and 5 are summaries to compare a scheme of Chalk and Smith (7) with a similar one proposed by Wiberg and Saegebarth (77) for the potassium permanganate oxidation and cleavage of olefins. Their mechanism
INDUSTRIAL AND ENGINEERING CHEMISTRY
Acknowledgment
Acknowledgment is due the late Ralph
G. Fulton. literature Cited
(1) Aggarwal, S. L., Sweeting, 0. J., Chem. Reus. 57, 665 (1957). (2) Balky, E., Gould, R., in “Polythene,” A. Renfrew and P. Morgan, eds., p. 55, Interscience, New York, 1957. (3) Bawn, C. E. H., Chemical Society, London, Spec. Pub. 9,1957. (4) Bawn, C. E. H., Pennington, A. A., Tipper, C. F. H., Discussions Faraday Soc. 10, 232 (1951). (5) Bawn, C. E. H., Sharp, J. A., J . Chem. Sac. 1957, p. 1854. f6) Brooks. B. T.. “The Chemistrv of Non-Benzoid Hydrocarbons,” Reinhold, \
,
N.e w . ..
Ynrk. - -. . . 1950. -.--I
(7) Chalk, A. J., Smith, J. F., Trans. Faraday Sac. 53,1214 (1957). ( 8 ) Farmer, E. H., Zbid., 38, 340 (1942). (9) . . Farmer, E. H., Sutton. D. A . , J . Chem. Sac. 1942; p. 139. (10) Fronmuller, G., Mirra, M. J., U. S. Patent 2,798,085 (1957). (11) Goodwin, W. J., Modern Plastics 31, No. 4, 104 (1954). (12) Keunecke, J., Ger. Patent 721,945 .( 1942). (13) Piret, E. L., Hein, R. F., Besser, E. D., White, R. G., IND.ENG. CHEM.49, 737 (1957). (14) Tobolskv. A. V.. Mesrobian, R. B., “Organic Peroxides;” Interscience, New Ynrk. 1954. (1f)-v:S.- Dept. Commerce, PB Repts. 100, 225, 1315, 1850, 2422, 4091, 4291, 23753, 42659, 49196, 85149, 526595, FIAT-905, BIOS-748, CIOS-XXXI-79. (16) Walling, C., “Free Radicals in Solution,” Chap. 9, Wiley, New York, 1957. (17) Wiberg, K . B., Saegebarth, K. A., J . Am. Chem. SOC.79, 2822 (1957). (18) Woodward. A. E., Mesrobian, R. B., ‘ Ibid., 75, 6189‘(1953j. RECEIVED for review May 19, 1959 ACCEPTED October 20, 1959 Division of Paint, Plastics, and Printing Ink Chemistry, Symposium on Stabilization of Polyolefins, 135th Meeting, ACS, Boston, Mass., April 1959.
