ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
Items fabricated from Marlex 50 ethylene polymer
ALFRED CLARK, J. P. HOGAN, R. L. BANKS,
AND
W. C. LANNING
Research Division, Phillips Pefroleurn Co., Barflesville, Okla.
HE first commercial production of polyethylene followed rapidly in the wake of the discovery in 1939 that ethylene could be polymerized to solid polymers a t extremely high pressures in the presence of molecular oxygen or peroxides ( 4 , 8). Because of the spiraling demand for conventional polyethylene, which, i t is predicted, will reach 600,000,000 pounds annually in 1959 ( B ) , many attempts have been made to produce it more economically at lower pressures. Recently, the use of aluminum alkyls for polymerization of ethylene and 1-olefins has been reported, and modifications of the catalyst system have been used to produce high polymers in a low pressure process (11-13). The use of reduced supported molybdenum and nickel catalysts in a low pressure process has also been described in the patent literature (9, 10, 14). Phosphoric acid, copper pyrophosphate, and silica-alumina have been used commercially for many years t o produce liquid 1152
polymers from propylene and butylenes. It was known prcviously that cobalt supported on charcoal ( 2 ) and nickel oxide supported on silica-alumina (1, 6 ) and other bases would polymerize ethylene t o butylenes and normally liquid polymers. Aluminum halides are used to polymerize propylene and higher olefins to high polymers. The use of supported chromium oxide to produce high polymers of ethylene and other olefins is apparently the first use of a solid “heterogeneous-type” catalyst which is commercially practical in such reactions. The discovei y , therefore, has scientific as well as commercial significance. In addition to polymerizing ethylene, catalysis produce solid t o ’ v i s c o u s liquid polymers from a variety of f e e d s
Supported chromium oxide catalysts promote the polymerization of a variety of unsaturated hydrocarbons. I n general, they
INDUSTRIAL AND ENGINEERING CHEMISTRY
voi. 48, NO. rl
LOW PRESSURE POLYETHYLENE polymerize all 1-olefins with no branching closer than the 4position to the double bond and containing eight carbon atoms or less. For example, 1-butene, 1-pentene, and 1-hexene produce branched high polymers ranging from tacky semisolids to viscous liquids. 4-Methyl-1-pentene unexpectedly gives a hard, brittle, solid polymer. When the branching is closer than the 4-position-for example, 3-methyl-1-butene-small amounts of liquid dimers and trimers are obtained but no high polymers. In ethylene particularly, average molecular weight (intrinsic viscosity) of the polymer can be varied over a wide range, from less than 10,000 up to 140,000 or higher, predominantly by reaction temperature control. Copolymers of ethylene with 1-olefins such as propylene and I-butene contain branches, the number of branches per molecule and the number of carbon atoms per branch depending on the amount and nature of the comonomer used. Ethylene homopolymers are essentially straight chain. Diolefins can also be polymerized over supported chromium oxide catalysts. For example, butadiene and isoprene give horny, brittle solids. The rule concerning branching closer than the 4-position does not apply for diolefins. Laboratory Reaction Systems. I n fixed-bed operation, catalyst is used in granular or pelleted form. The procedure is described for ethylene polymerization. With minor modifications, the same procedure may be used for other olefins. An isothermal jacketed steel reactor 2 feet in length and 1.5 inches i.d. is used. Reactor temperature is controlled by regulating the pressure of boiling liquid in the jacket. An internal thermowell extends the length of the reactor. Purified ethylene, freed of catalyst poisons such as water vapor, oxygen, and carbon monoxide by conventional means, is metered through a Taylor flow controller. Hydrocarbon solvent of suitable boiling range is pumped by means of a Hills-McCanna pump. The two streams are passed downflow, liquid-phase over the catalyst bed. Reactor pressure is controlled by a Taylor Fulscope. Solvent and polymer are collected in a 5-liter distilling flask with fractionating column. Solvent is flashed overhead into the middle section of another small column. Unreacted gases are boiled out of the second kettle, taken overhead, and metered. Solvent is recycled to the reactor. Solvent and polymer in the first kettle are cooled to room temperature to precipitate polymer and filtered. Polymer is dried in a vacuum oven. Fixed bed operation is ordinarily limited to the production of relatively low molecular weight, brittle polymer (5000 to 20,000) a t reaction temperatures of 150" to 180' C. and ethylene concentrations in solvent of 2 to 4%. At lower temperatures and higher concentrations, higher molecular weight polymer is produced, but catalyst bed plugging and spalling becomes severe. When polymer of 10,000 to 15,000 molecular weight is being produced, reaction cycles of 50 hours or longer can be obtained. The catalyst can be regenerated with air in situ and used for many cycles. Slurry-type operation is carried out in small batch reactors of '750- to 1500-cc. capacity, equipped with mechanical stirrers and electrically heated jackets. Solvent and a small amount of fine catalyst (0.2 to 0.6 weight % of solvent) are charged into the reactor before the start of the run. The reactor is closed and ethylene feed started. Ethylene pressure is permitted to build up to 400 to 500 pounds/square inch gage in less than an hour. At the end of the reaction period, polymer is removed, dissolved in additional solvent, and filtered to remove catalyst,. Polymer is recovered from the filtrate by cooling to precipitate, filtering, and drying. High molecular weight polymer (40,000 and higher) can be produced in this manner a t temperatures below 150" C. and at high ethylene concentrations. MoIecular weights as used here are based on solution viscosity (3). Catalyst Composition. Chromium concentration on silicaalumina bases has been investigated over a wide range, but no outstanding effects on polymer properties were observed. No July 1956
increase in activity is observed above 2 to 3 weight yo chromium. Silica-alumina ratio in the base has not been found to be critical over a wide range, but good results are obtained with commercial cracking catalyst bases of approximately 90: 10 ratio. Other bases for chromium oxide such as silica, alumina, zirconia, and thoria have been used successfully but show no advantages over silica-alumina. Hydrogen fluoride-treated alumina bases for chromium oxide have been investigated to a limited extent. HF-treated alumina bases produce higher molecular weight ethylene polymers than silica-alumina bases under similar fixed-bed reaction conditions. For example a t a reaction temperature of 132" C., 1.5 weight yo of ethylene in hydrocarbon solvent, 6 liquid hourly space velocity of solvent, and 450 pounds/square inch gage liquid phase pressure, molecular weight of recovered polymer from chromium oxide-silica alumina and chromium oxide-HF-trcated alumina were 14,000and 45,000, respectively. In ethylene production, activation temperature and reaction temperature and pressure control molecular weight of polymer
For the production of high molecular weight, flexible polyethylene, slurry-type operation is preferable as mentioned abovc. Three major factors control the molecular weight of the polymer: (1) temperature of catalyst activation, (2) polymerization reaction temperature, and (3) polymerization reaction pressure. I n order to investigate the effects of these factors on molecular weight, standard runs were made in slurry-type batch reactors for 4 hours duration a t 0.6 weight yoconcentration of fine catalyst dispersed in hydrocarbon solvent. Figure 1 shows the effect of temperature of activation of catalyst on molecular weight of polymer. Activation of catalyst was carried out with air a t a space rate of 100 volumes/volume/ hour. The polymerization test was carried out a t 132" C. and 450 pounds/square inch gage. Molecular weight decreases with increasing activation temperature, starting a t 46,000 a t 482" C. and going down to about 30,000 a t 760" C. Figure 2 shows the effect of polymerization rcaction temperature on molecular weight of polymer. Catalyst activation temperature was held constant at 510" C. and maximum reaction pressure was constant a t 450 pounds/square inch. Molecular weight decreases with increasing reaction temperature. From
INDUSTRIAL AND ENGINEERING CHEMISTRY
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ENGINEERING. DESIGN. AND PROCESS DEVELOPMENT
0
A 4
Po
100
IB
110
130
140
a0
1W
REACTION TEYPERATURE.'C
Figure 2.
Reaction temperature versus molecular weight of polyethylene
REACTION
Figure 3.
70
Catalyst: 2.5 wt. chromium as oxide on silica-alumina Polymerization: 4-hour tests at 4 5 0 Ib./sq. inch g a g e with slurry-type batch reactor
110' to 170" C. molecular weight drops from 100,000 to approximately 25,000. The polymers range from flexible and tough at the high molecular weights to brittle a t the lower molecular weights. Figure 3 shows the effect of polymerization reaction pressure on molecular weight of polymer. With increasing reaction pressure. molecular weight of the polymer increases. No by-products are formed in the polymerization; all the reacting ethylene is converted t o high polymer which contains less than 1.5% of polymer low enough in molecular weight to be soluble in ordinary hydrocarbon solvents a t room temperature. Polymerization of Other Olefins. Other olefins such as propylene] 1-butene] 1-pentene, 4-methyl-I-pentene, and 1-hexene have been investigated chiefly in fixed-bed operation. Average molecular weights of polymers produced from these olefins are less than those of ethylene. Table I gives typical conversions and a qualitative description of the polymers.
N
I
1
I
I
m
m
w
P R e U U I E . PSlG
Reaction pressure versus molecular weight of polyethylene
Catalyst: 2.5 wt. % chromium as oxide on silica-alumina Polymerization: 4-hour tests a t 132' C. with slurry-type batch reactor
The optimum temperature for polymerization of the higher olefins in fixed-bed operation is lower than that for ethylene. Ethylene polymerization is run in fixed-bed operation at 150" to 180' C., and bed plugging and catalyst spalling occur a t temperatures much below 150" C. The polymers of higher 1-olefins, being of lower molecular weight and more soluble, can be produced a t lower temperatures without mechanical difficulties. Also maximum conversion for propylene (Table 11) is obtained a t about 108" C. Conversions of other 1-olefins reach a maximum a t slightly lom-er temperatures. Practically complete conversion of ethylene is obtained a t teniperatures up to about 180" C.. after which conversion declines. The reason for the decline in activity above a certain temperature is not well understood, but there may be a very slow interaction between chromium trioxide and the olefin m-hich gradually destroys active sites. At higher temperatures the interaction is accelerated. Ethylene activity persists a t higher temperature because it polymerizes the most vigorously of all the olefins and, therefore, requires the femvest active catalyst sites. As shown in Table I, activity tends t o decrease with increasing molecular -might of the normal 1-olefin monomer. 4-Methyl-1-pentene is more reactive than the corresponding normal I-olefin
Table I.
Polymerization of Various 1 -Olefins
(Fixed bed 88" C. 500 lb./sq. inch gage 2 1.h.s.v 20 mole Yo olefin in hydrocarbo'n solveni, 6-hr. runs: catalyst, 3'wt. % C r .& oxide on RiOr.41208) Monomer A v . Conversion, % T y p e PolytneI Ethylene 100 Solid 91 T a c h seiiiisolida Propylene 77 Tack;,: elastic, seiriisolid I-Butene 82 Tackiqr than polypropylene, I-Pentene r. piniaolid
I-IIexene 4-hletbyl-1-pentene
XI 100
1xI
110
I3C
I
141
REACTION TeYPERATURE. 'C
Figure 4.
Reaction temperature versus molecular weight of ethylene propylene copolymer
70
Feed: 90 wt. ethylene, 10 wt. ?& propylene Catalyst: 2.5 wt. chromium as oxide on silica-olumina Polymerization: 4-hour tests at 450 Ib./sq. inch g a g e with slurry-type butch reactor
70
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Table II.
40-56 80
Veri- tacky transparent eel Solid a n d l(quid polymers
Effect of Temperature on Propylene Conversion
(Fixed bed, 500 lb./sq. inch gage. 2 I.h.s.v., 1 4 mole 5% propylene in hydrocarbon nolrent, ?-hr. runs; catalyst, 8 wt. 7" C r a 8 oxidc on SiOa-.41aOs) Temp., ' C. Conversion, 5% 65 89
--
uu
41
105
9;i 83 23
120 1511
INDUSTRIAL AND ENGINEERING CHEMISTRY
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LOW PRESSURE POLYETHYLENE Introduction of small amounts of higher oleflns into ethylene feed produces more flexible polymers
By incorporating minor amounts of olefins such as propylene and 1-butene into ethylene feed, copolymers can be produced. Feeds containing up to 15% of higher 1-olefins are of most interest for producing flexible, solid polymers. In general, these copolymers are more flexible than homopolymers of ethylene. Most of the copolymerization investigations were carried out in slurry-type operation. Molecular weights for ethylenepropylene copolymer (90210 weight %) are lower for the same reaction temperatures than those of ethylene homopolymers. These copolymerization runs were carried out for 4 hours a t a maximum pressure of 450 pounds/square inch gage with 0.6 weight yo of chromium oxide-silica-alumina catalyst in hydrocarbon solvent. The molecular weights of the copolymers vary from about 65,000 a t 110’ C. to 40,000 a t 132’ C. Flexibility of copolymers, as indicated by elopgation of tensile test specimens, increases with decreasing temperature of polymerization reaction.
Acknowledgment
Grateful acknowledgment is made to Phillips Petroleum Co.
for permission t o publish this work.
literature cited (1) Bailey, G. C., Reid, J. A. (to Phillips Petroleum Co.), U. S. Patents 2,581,228 (Jan. 1, 1952) and 2,606,940 (Aug.9,12, 1952). ( 2 ) Cheney, H. A., McAllister, S. H., Fountain, E. B., Anderson, J., Peterson, W. H., IND. ENG.CHEY.42, 2580-6 (1950). (3) Dienes, J., Klemm, H. F., J . Applied Phys. 17, 458 (1946). (4) Fawcett, E. W., Gibson, R. O., Perrin, M. W. (to Imperial Chemical Industries Ltd.), U. S.Patent 2,153,553 (April 1 1 , 1939). (5) Hogan, J. P., Banks, R. L., Lanning, W. C., Clark, Alfred, IND. ENG.CHEM.47, 752-7 (1955). (6) Modern Plastics 32, No. 5, p. 71 (1955). (7) Natta, G., Corradini, P., Atti acad. nazl. Lincsi, Rend., Classe sei. fiz., mat. e nat., 18, 19-27 (1955). (8) Perrin, M. W., Paton, J. G., Williams, E. G. (to Imperial Chemical Industries Ltd.), U. S. Patent 2,188,465 (Jan. 30, 1940). (9) Peters, E. F., Evering, B. L. (to Standard Oil Co. Indiana), U. S. Patent 2,692,261 (Oct. 19, 1954). (10) Roebuck, A. K., Zlete, Alex (to Standard Oil Co. Indiana), U. S.Patent 2,692,258 (Oct. 19, 1954). (11) Ziegler, K., Angew. Chem. 64, 323 (1952). (12) Ziegler, K., Belg. Patent 533,362 (Nov. 16, 1954). (13) Ziegler, K., Brennstof-Chem. 33, 193 (1952) and 35, 321 (1954). (14) Zletz, Alex (to Standard Oil Co. Indiana), U. S.Patent 2,692,257 (Oct. 19, 1954). RECEIYED for review November 22, 1955. ACCEPTEDApril 11, 1956. Division of Petroleum Chemistry, 129th Meeting, ACS, Dallas, Tex., April 1966.
Properties of Marlex 50 Ethylene Polymer R. VERNON JONES A N D P. J. BOEKE Research Division, Phillips Petroleum Co., Barflesville, Okla.
I
MPROVEMENTS in the basic nature of ethylene polymers have recently been made. These developments, based on polymerizing ethylene a t low pressure utilizing active catalyst systems (8, 18, 25), permit a host of new ethylene polymers with a wide range of physical and chemical properties. In general, physical properties bear a direct relationship to the density of ethylene polymers (84). Certain of the important physical properties which vary with the density spectrum are illustrated in Figure 1. Basic properties such as crystallinity, rigidity, softening temperature, and tensile strength increase as the density increases. Elongation and impact strength show an inverse variation with density; however, it must be considered that these properties are also importantly dependent on chain length. The Phillips process is capable of producing a series of polymers whose properties span this density spectrum. One of the most interesting polymers of this series is a high molecular weight homopolymer of ethylene which has been designated Marlex 50 ethylene polymer. It is a tough, white, opaque, rigid material having a high melting point and density. It possesses high tensile strength and low permeability t o liquids and gases. This article describes various properties of this high density ethylene polymer. Marlex 50 polyethylene is a general-purpose resin. As such it is compared here with typical general-purpose high pressure polyethylenes of the types exemplified by the products marketed under the trade names DYNH and Alathon 10 (density 0.92 and melt index 2). These products are referred to as “conventional” polyethylenes. High pressure polyethylenes of both July 1956
lower and higher melt indexes are commercially available. The former are designed to meet the need for improved stress cracking resistance in certain uses whereas the latter are designed for improved processing characteristics and surface gloss although with some sacrifice in other physical properties. Very recently several companies have announced the availability of intermediate density ethylene polymers which, by implication, may be produced in high pressure processes. Properties of polymer suggest applications i n films, bottles, insulation, industrial moldings, etc.
Physical Properties. That the basic physical properties of this unique ethylene polymer are markedly different from the properties of conventional high pressure type polyethylene is readily apparent. Table I lists a range of some of the more important properties as compared to those for a typical high pressure resin. The physical characteristics of the Marlex 50 polymer are correlatable with the structural picture (83) as presented by infrared spectroscopy, nuclear resonance, and x-ray diffraction. Density and hardness are increased over conventional high pressure polyethylene. With greater intermolecular cohesive forces tensile strength is doubled and elongation is decreased. Impact strength is decreased; yet the notched Izod test specimens still show a typical ductile surface on fracture. Apparently, the crystalline/amorphous ratio and relaxation time prevent the polymer specimen from deforming rapidly enough to absorb the kinetic energy of the pendulum without fracture. Heat dis-
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