HARRY W. HAINES, JR.
POLYPROPYLENE n producing good quality polypropylene, monomer purity is of prime importance. Catalysts are poisoned oxygen, carbon monoxide and dioxide, sulfur compounds, and water. Crystallinity and molecular weight are altered by olefins. In fact purification plants often require a capital investment of as much as 1.5 cents pa annual pound of capacity and an operational cost of0.5 to 0.6 cent per pound. The monomer purification problem is aggrqvated by the fact that crude propylene contains a high percentage of impurities. A typical crude product may contain 2 to 3% C;s, 3 to 4% Cis, and 35 to 53% propane. This mLrture must then be purified until it contains in the neighborhood of 99.5 to 99.7% propylene (77, 73). For cxample, Sun Oil makes a product which may contain impurities amounting to no more than 4 p.p.m. oxygen, 1 p.p.m. carbonyl sulfide, 2 p.p.m. total sulfur, 1 p.p.m. carbon dioxide, 50 p.p.m. ethane, and 0.3%
I by
PmP*. I n monomer puriIication, g o d separation can be
obtained with extractive distillation, but this takes the propane-rich stream overhead. Therefore, because of the additional expense of handliing the extractive agent, rxmventional two-column or vapor recompression distillation is more economical. GonventionaI distillation provides nearly 100% overall tray efficiency for separating C , to CI compounds. The optimum d u x value is about 1.2 times the ratio wrreaponding to infinitestages. Vapor recompression is required for p r a m of 100 to 200 p.s.i.a. Here,
the overhead vapor is compressed and then condensed as the heating medium for the column reboiler. A single fractionating column is used, at a relative volatility of 1.16 to 1.22, and the compression operation can be frequently integrated with a propylene refrigeration system serving other process units. Monomer Purmcdion Schemes
Propylene, if it contains sulfur compounds, must be caustic-washed to remove hydrogen sulfide and carbon dioxide (7). But caustic washing does not remove carbonyl sulfide entirely, and a second contact with monoethanolamine solution may be required for certain feedstocks. Remaining traces of hydrogen sulfide, carbonyl sulfide, and carbon dioxide may be removed by s olid chemical adsorbent treatment applied ahead of the product dryers. For both hvo-column and vapor recompression distillation (illustrated below), complete de-ethanization is desirable. Sun eai~ploys a 40-tray column for this purpose in ita original 120 million pound-per-year plant at Marcus Hook, Pa.; Sun’s new 180 million pound-payear unit is reported to have a “super de-ethanizd’ which reduces the amount of hydrocarbons lighter than propylene from 2% to kss than 150 p.p.m. Most of the dissolved water in the feed is stripped out in the deethanizer column. De-ethanizer bottoms, containing less than 50 p.p.meach of water and ethane are fed to a conventional two.
II SPENT CAlISTlf
so
INDUSTRIAL
A N D ENGINEERING CHEMISTRY
.:
Typicalpropylcm puricofion schemes. Th mort cconomicd stporationr involve convenrional lwo-column m oapor rccomfimion di~lillnlion
.
Patent confusion, overcapacig, and unclear marketing patterns make polymerimtion technology onb a part .f the problem conaonting producers d u m n distillation system or a vapor recompression disdlation system. Superfractionation, as practiced by Sun, requires a 62-tray column and a 78-tray column, for a feed containing approximately 43% propylene, 54% propane, and 3% C, hydrocarbons. At 100 to 200 p.s.i.a., vapor recompression distillation would q u i r e 80 to 100 trays. h p y l e n e overhead from either system can be dried to about 10 p.p.m. with a suitable desiccant such as calcium aluminosilicates. Sun regenerates its dryers with hot natural gas. Carbon dioxide is apparently adsorbed on the desiccant during regeneration, with subsequent desorption into the product during a short period following regeneration. Regenerations are infrequent, so that the cost of reprocessing the contaminated product is small. This problem, however, would not occur with a COrfree regeneration gas. 'The Csfeedstock, containing less than 2% C zand C, contaminants, is readily obtained from refinery and chemical plant streams by two-step, series distillation at moderate reflux. ratios. Typical facilities would indude a 30-to 40-tray de-ethanizer and a 30-to 40-tray depropanizer. Crude propylene (40 to 60% propane) Leaves the depropanizer column as overhead and goes directly to the propylene purification unit. Alternatively, 90% propylene obtained from butane craeking can m e as feedstock, in which case the purilication system would consist of a 70-tray column operated at 300 p.s.i.a., with the same pretreatment and posttreatment facilities. However, the cost of upgrad-
ing 90% propylene may equal 60% of the cost of purifying a 50-50 mixture, and consideration must be given to the value of 90% propylene for other uses. Ziqler Catalysts
The actual mechanism of polymer formation has been debated among various researchers. However, it appears that a solid catalyst phase is required to obtain stereospecific read&,precluding the use of hydrocarbonsoluble compounds. Isotactic polypropylene is insoluble a t temperatures belowabout 212O F. (depending to some degree on the type of solvent) and precipitates during polymerization; the atactic fraction remains in solution along with the activator. Almost any element of the fourth, fifth, and sixth groups in the periodic table is a useful metal catalyst; these metal halides have been mentioned in the literature or patented. But titanium is the most effective metal, and titanium trichloride is the preferred halide. Chromium trichloride has been studied, with triethylaluminum and triisobutylaluminum dissolved in n-heptane (2). This work indicates that triethylaluminum is a better activator than triiibutylaluminum, but chromium trichloride is a less active catalyst than titanium trichloride. Titanium trichloridetriethylaluminum and titanium trichloride-triisobutylaluminum systems were recently reported (4) in studies conducted at temperatures above 212' F., using cyclohexane as the dilution solvent. Under these conditions the isotactic polymer fraction
V O L 5 5 NO. 2 FEBRUARY 1 9 6 s
31
dispolved in the dilution solvent while the metal halide catalyst remained insoluble. Triethylaluminumwas also found to be the best activator. Although this work may have little commercial significance, it tends to w&m Natta's conclusions (at lower temperatures) regarding heterogeneous reaction mechanism. Chain gmwth apparently occurs a t the surface of the metal halide; the organoaluminum compound is probably important in the active center ofchain growth. Molecular weight and isotactic content of the polymer (in the Natta catalyst systems) do not vary appreciably at polymerization temperatures between 85" and 160' F. h r e has little effect on molecular weight at propylene partial pressures above 1atm., but an increase in catalyst and activator concentration decreases molecular weight, without greatly altering regularity of the polymer. Molecular weight can also be controlled by the addition of a third component or chain terminating agent. Reaction rates are apparently a function of temperature, propylene pressure, catalyst concentration, and method of catalyst preparation. At;low propylene prcsnres, the over-all rate of polymy formation is proportional to catalyst concentration and propylene prapure, but independent of activator concentration (abwc a minimum activation level). Reaction rate, apparehtly first ords, inmaseswith temperature. Catalyst preparation is a highly developed. and complex art. Witanium trichloride, for example, w n forms (75) and can be prepared exist in four d&-t by a number of reactions. I _
.
AUTHOR Harq W. Hainq'Jr., is a consulting chemical m g k m and oumm of Hainet & Associates in Hauston, Tex. 32
INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
METHODS OF CATALYST PREPARATION TYP~
Alpha
Beto
CIysldFan
Violet
2TiCl.
Hexagonal
3TiCI4
Brown
2TiCI4
+ HI -+ 2TiCla + 2HCI d
+ Ti
+ HZ
750' F.
.'olymer
4TiClr
electric
2TiClg
Linear
,
&a&"
+ 2HCI
+ AlEta TiCls + EtzAlCl + Et. TiClr + EtrAlCl Tic14
TiClr
+ EtAlCln + Et.
4800
-
Me.
Gamma
Violet
TiClalSl
TiCla
Delta
Cubic Violet; disordered
MeTiClr A TiClg Me. Prolonged grinding of the alpha and gamma forms
+
Alpha, gamma, and delta Ticla produce 80 to 90% isotactic polypropylene, whereas TiCla (beta) and TiClr produce 40 to 50% isotactic polymer (7,3). Thus some plants may use TiCl, as the raw material for catalyst preparation, and TiClr (beta) may be manufactured as a n intermediate in the production of Ticla (gamma). The active catalyst, however, is one of the three formsof Tic18 known to produce highly isotactic polymer. Hydrogen-reduced and aluminnm-reduced Ticla are commercially available as alternate source8 of catalyst. Titanium trichloride d m not seem to promote polymerization in the absence of an aluminum alkyl or aluminum alkyl halide activator. For obvious reasons
of secrecy, it is difficult to determine the exact activator and/or chain terminating agents employed by individual companies. One company now has a catalyst system claimed to produce essentially 100% isotactic fraction. Safety methods for handling titanium trichloride and organometallic compounds have been described in the literature (9, 10). Further instruction can be obtained from titanium trichloride producers (Anderson Chemical) and aluminum alkyl producers (Continental 424 sthyl Corp., and Texaa ALEyls). Polymerization Technology
Solvents are closely guarded secrets, although Csand Cr n-parailiiw appear to be in general usage. At least three machant solvent producers sell such materials: American Mineral Spirits Co., n-hexane and n-heptane; Humble Oil & Refining Go., n-heptane and Isopar E (240' to 290" F. boiling range i s o p a r a h ) ; and Phiuips Petroleum Co., n-hexane. For effective polymerization, solvents should be as free of catalyst poisons (water, carbonyls, peroxides, and nitrogen and sulfur compounds) and chain terminating agents (chlorides and olefins) as possible. After drying to about 10 p.p.m. of water, and pretreatment where required, a good solvent will contain less than 50 p.p.m. d reactive contaminants. Typical specifications indicate very low tolerance levels--less than 5 p.p.m. carbonyls, 1 p.p.m. pemxides, 1 p.p.m. nitrogen compounds, 10 p.p.m. sulfur compounds, 5 p.p.m. chlorides, and 25 p.p.m. olefins. Naphthenes apparently do not S e c t polymerizsttion adversely, but ammatics cause solvency problems when present in small quantities. Good solvents do not
contain more than 0.1 to 0.2 volume % of aromatics, whereas naphthenes may run as high as 45%. Solvents and better methods of recycle purification are being studied extensively, since 2 to 4 pounds of solvent are recycled per pound of isotactic product. Polymerization is only half the problem of making polypropylene. Catalyst does not dissolve in the dilution solvent and must be removed from the insoluble isotactic polymer leaving the reactor section. Patents disclose a number of methods but the most practical approach involves a series of steps aimed at dissolving the catalyst and washing the resin. Catalyst treatment procedures are applied after the dilution solvent, containing soluble activator and soluble atactic polymer, is removed from the resin. Alcohols, or alcohols containing hydrochloric acid, will effectively dissolve titanium trichloride and are commonly used as purification solvents. At this point the resin may still contain low-molecularweight, amorphous, high-molecular-weight amorphous, and partially crystalline polymer fractions soluble in acetone, ether, and n-heptane, respectively. Separate, or combined, organic and water washings are required to produce an isotactic fraction ready for drying. Actually, an entire series of washing solvents may be employed, including acetone, diethyl ether, n-pentane, n-hexane, n-heptane, and 2-ethylhexane which have increasing solubility for noncrystalline polymer. Final product crystallinity, therefore, can be controlled by organic washing procedures in the plant. Typical Polypropylene Processing Scheme. Most catalyst systems consist of two or three components, usually a titanium trichloride catalyst, an aluminum alkyl activator, and in some instances a chain terrninating (molecular weight control) agent. These components are dissolved and/or slumed in the dilution (or other suitable) solvents. Catalyst preparation is conducted batchwise in one or more propeller-agitated vessels. Purified propylene, catalyst slurry, and the dilution solvent are fed to the agitated polymerization vessel which operates at temperatures of 120" to 212" F. and pressures up to 5 atm. Residence times may vary from about a half to several hours. Heat of reaction is removed by cooling coils and/or cooling jadrets. Catalyst addition, to some extent, is influenced by monomer and solvent purity and may range from 0.25 to 0.50 weight % of the solvent charged. Approximately equal quantities of the activator are required. Polypropylene precipitates from solution during polymerization; m a t plants produce a reactor &ent containing 20 to 30% solids. The reactor slurry is transferred to a flash tank where unreacted monomer is removed as a vapor. Solvent vapors are condensed and recycled to the reactor, or fed to the dilution solvent cleanup system. After recompression, unreacted propylene goes to a distillation column and is recycled to the reactor. (Contintud on next page) VOL 55
NO. 2
FEBRUARY 1963
33
(Millions ofpounds p Canba"
I
Rmm
Avisun New Castle, Del.
Sun Oil
Dow Chemical
BASF
I
7957
I 7058
I 1959
I-
20
I
25
125
100
10
40
40
40"
5
5
10
20
80
110
40
M)
80
100
16.0
__--
Torrance, Calif. Firestone Tire & Rubber Oronae. Tex. ~~
Hercules Powder Lake Charles, Lo. Humble Oil & Refining Bavtown, Tex.
I I
Firestone Hercules
20 20 20
Esse
Monsunto Chemical Texas City, Tex.
Monsnnto
Novamont (Montecotinil Neal, W. Va.
Montecatini
Rexall Chemical Odesso. Tex.
I 1 I I
l
l
100 50
--__
I
10
I
I
I
I
25
25
50
I
Shell Chemical Woodbury, N. J. Texas Eastman Longview, Tex.
I
Eostman
Total
a 70 millha w
s 01 Bq Cit,, Mi&
Polypropylene slurry is pumped to a surge tank and then to a centrifuge, for removal of the dilution solvent. The dduent (a normal paraffin or isoparaffin) can be purified in a single distillation column. Two columns are preferred, so that a heads and tails cut can be removed. The tails contain atactic polymer which is soluble in the dilution solvent. Bottoms from the second deanup column pass to a desiccant or other suitable d+g system. Make-up solvent enters the system ahead of the first distillation column. After drying, the solvent stream is recycled to the reactor. Most, but not all, of the dilution solvent is removed from the isotactic fraction in the first centrifuge. This material is reslurried with a purification solvent (C, to CI aliphatic saturated alcohol) which in some plants may contain HCl for efficient catalyst destruction. Methanol and isopropanol seem to be the mmt popular alcohols. This alcohol-resin slurry goes to a centrifuge for liquidsolids separation. The alcohol stream is purified in a distillation column where catalyst residues and dilution solvent are removed as bottoms, and the overhead is returned to the purification v w d . Polymer from the second centrifuge is washed, dried, and stored. Antioxidants may be added prior to drying or prior to extrusion. The extruded resin is chopped and blended prior to packaging in bags. 31
40
k 20 m'llh p . l r at Pmiin, N. J .
I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
POLYPROPYLENE THE ECONOMIC PICTURE Two years ago, plant capacity for polypropylene was only about 100 million pounds. Now it is nearing 475 million, and headed for a 30% increase in 1963. Already, American capacity is twice that of the remaining Free World, and the end of the dizzy spiral still is not in sight (table above). Monsanto, Rexall, and Alamo Plastics should come on stream this year, and rumors persist that California Chemical may announce plans. The status of other potential producers remains uncertain-e.g., Allied, D u Pont, Spencer, W. R. Grace, and Union Carbide. However, a curious situation exists. Although new plants continue to come on stream, sales are only about 30% of capacity and prices are falling (about 65 centS per pound in 1957 to 38 to 40 cents in 1962). Even by 1965,sales may amount to only 60% of capacity. Admittedly, part of this discrepancy may result from
. . I
- CONSUMPTION
I
monofilaments have made substantial progress in rope and webbing. Now, growth of multifilaments for carpers and apparel is promihg. Also, improved dyeing techniques may broaden the market considerably, particularly for drapery fabrica and clothing. Most of the activity in fibers is centered in the U. S., Italy, and Japan
FIBER COMPANIES
IO
1957
1959
1961
1963
1965
YEAR Polypropyltlu Slrpply and h n d a.
v) LL
65 30 n
t 2 3
w IW PLANT CAPACIN, MILLIONS OF POUNDShR.
4.
, L 1
Plant imshnad
MOlMjild
MdhJld
American Manufacturing American Thermoplastics Dawborn Brothers Firestone Synthetic Fibers Industrial Plastic Products Industrial Wire & Plastics National Plastic Products* Plymouth Rope Southern Lus-Trus Vogt Manufacturing
Beaunit Mills G. F. Chemical Hercules Powder Reeves Brothers
. ,
J b I l y d HulltJe ~ Wrrl J . P.Sumu 81 a.
Until recently, unmodi6ed polypropylene ewid not accept dyes used fos cdba q-nthetic catiomic,ora2oic). Thiswas conon and wool (rrdd, piamek& ' reactive). Now, however, new dyes for unmodified as well as modified polypropylene are bemg developed by National Anilime in the U. S. and Nitto Spinning in Japan. However, polypropylene is not easy to spin, because of pseudoplastic behavior in the fluid state. Its apparent viscosity decreases rapidly with increasing velncity gradient, and increases with increasing polymer intrinsic viscosity. Polypropylene also enlarges more than most fibers as it leaves the spinning hole. Minor process disturbances may cause the fiber to break while spinning fine filaments. Unliie other fibers, polypropylene crystallizes readily during spinning and the degree of crystallinity, crystal size, and crystal orientation must be controlled to obtain the desired physical properties. Redling P m d i c r
Some companies have established themselves as resellem. AU of these companies except California inaccurate estimates, because polypropylene markets are difficult to evaluate (5, 6). At first, it was thought that 45 to 55% of the market would be for fdms and sheet, and 25% for molding, with the balance far fibers and miscellaneous. Actually, the reverse happened (see illustration, page 37). Fibn
hope that, by 1965, fiber consumption will reach 70 to 80 million pounds annually. Fibers may eventually become the largest end use, about 40 to 45% of the consumption pattern. Polypropylene fibers are firmly established in the carpeting field, and producers
POLYPROPYLENE SALES AGREEMENTS
Raralbr
Borir M u c n
Avisun
Allied Chemical Colifornia Chemical Rexall Chemical" Monsanto Chemical" Du Pont Spencer Chemical W. R. Groce & Co. Union Carbide
Dow Chemical Hercules Powder Humble Oil & Refining Novamont IMontecatiniI Shell Chemical s T k s J m d h o a tbir _n #.mu in Im3. VOL 5 5
NO. 2
FEBRUARY 1965
35
Chemical are makers of polyethylene, and therefore they can round out their line of polyolefins and at the same time test-market without major capital expenditure. But resellers have their problems too-low profit margins and difficulty in controlling quality of material allocated to them. Some have pilot plants which can supply most of the resin they need. Patents and Foreign Competition
Other than Japan, no single country overseas has more than 20% of the non-U. S. Free World capacity. At best, Italy, where polypropylene reached commercial status at an early date, cannot claim more than 10% of American potential output. American companies have capitalized on Montecatini’s failure to obtain a strong patent position here. However, Montecatini seems to have a good grip on the patent situation elsewhere-strong enough to acquire at least 10 licensees. Some Japanese companies, however, are uorking hard to break Montecatini’s grip in Japan. \Vithin a few years Japan should have about 20y0 of U. S. capacity, roughly twice that of any European nation. A bitter struggle is taking place to break through Montecatini’s patent position. The principal participants in this battle are Shin Nippon clliisso, Showa Denko, and Tokuyania Soda. The Russians appear to be actively commercializing resin and fiber production (16). Polypropylene is one area where they do not claim co-hTatta or pre-Natta discovery. Soviet scientist A . I . Dintses says he obtained a high molecular polymer of ethylene, by high pressure polymerization, at about the same time and independently of English research workers, Fawcett and Gibson.
M a k i n g polypropylene thread i n Montacatini’s plant at Terni, Italj
Coiintry Austria
1 1
FOREIGN POLYPROPYLENE PLANTS Mzilion I Comjany I Pounds I
I
Danubia-Montecatini
England
Imperial Chemical Industries Shell Chemical
France
Pechiney
Germany
C h emisc h e W e r ke
‘
H u Is
Farbwerke Hoechst Holland
10
Montecatini
JaDan
Mitsubishi Petrochemical’ Mitsui Petrochemical Shin N i p p o n Chisso’ S h o w a Denko‘ Sumitomo Chemicalh Tokuyama S o d a
20
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Ziegler Ziegler Montecatini
I
60
20 26
20 20
4 Planned.
I
20
Total
36
20
Montecatini
C
Montecatini M o n t e c a tini M o n t e c a tini
Italy
a N . V. Rotterdamse Polyolejnen Zfoolschappr]. b M I 7 Z approved and under conslniction.
I
30 15
30 lo
i
She I I-M o nt e c a tini’
Process License
325
I
Montecatini Montecatini Montecatini Avisun Eastman Montecatini Tokuyama S o d a
that they can cross-license themselves out of any future patent problems. Although American firms are confident about their position at home, they have been extmnely cautiotu toward overseas licensing. Only Avisun and Eastman have granted licenses in Japan, for example. Montecatini and Ziegler (Germany), seem to control in foreign countries. The Fulun
1965
A
New uses will continue to be developed for polypropylene, and better methods will be perfected for handling existing products. For example, biaxial orientation greatly improves the strength of polypropylene film, making it a dabable packaging material. It doubles the strength in the lengthwise direction and strengthens the film as much as eight times mosswise. Stiffness, too, is more than doubled. Oriented polypropylene film has about twice the reaiatance to moisture and gas seepage as woriented film. And reduction in impact strength at low temperatures does not occur with oriented polypropylene film. Oriented film resembles cellophane in cleamem, gloss, and ease of handling on high-speed over-wrapping equipment. Avisun, Dow, W. R. Grace, H d e s , Humble Oil, U. S. Indusrrial Chemicals, and Union Carbide now manufacture these materials. They are working hard to solve heat-sealing problems encountered during use. By 1965, consumption of oriented film could reach 20 million pounds-one fourth of polypropylene film usage. Avisun, Dow, D u Pont, Humble Oil, Novamont, Rexall, Union Carbide, and fiumerous other firms make cast film which has established itself firmly in the breadwwmh morket. Recently a more optinistie outtook hastrprojectedforthefuturegrowthofca'atfitm. . .
- ___..._
(d &.;a,
Q Wme, R. A,, C h . En& 67,7841 (May 30.1960). 9 6 9 (June 27, 1960). (9) Lema. R W.,IND. ENS. Cmu. 53, 7OA-71A @mm8a
