Ó 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Polypropylene T. GEOFFREY HEGGS, current address: The Mill, Great Ayrton, TS9 6PX, United Kingdom 1. Historical Survey .................... 381 2. Polymer Structure ................... 382 2.1. Molecular and Chain Structure ......... 382 2.2. Crystallization and Morphology ........ 384 3. Raw Materials ...................... 385 3.1. Propene ........................... 385 3.2. Polymerization Diluents ............... 385 3.3. Catalyst Preparation ................. 386 3.3.1. TiCl 3 -Based Catalysts ................. 386 3.3.2. Supported Catalysts ................... 387 3.3.3. Homogeneous Catalysts ............... 388 3.3.4. Aluminum Alkyl Cocatalysts ............ 388 3.4. Hydrogen .......................... 389 4. Polymerization Mechanism ............ 389 5. Industrial Processes .................. 390 5.1. Suspension Homopolymerization (Early Diluent Processes) ................... 390 5.2. Bulk Polymerization in Liquid Propene. . . 392 5.3. Solution Polymerization ............... 392 5.4. Spheripol Process ................... 392 5.5. Hypol Process ...................... 393 5.6. Gas-Phase Processes ................. 394 5.7. Copolymerization.................... 396 5.7.1. Random Copolymerization .............. 396 5.7.2. Impact (Block) Copolymerization ........ 397 5.8. Product Finishing ................... 397 5.9. Additives .......................... 398 6. Compounding ...................... 399 7. Properties ......................... 400 7.1. Homopolymer ...................... 400 7.2. Copolymers ........................ 401 7.3. Elastomer Blends with Polypropylene .... 404 8. Uses .............................. 404 8.1. Injection Molding ................... 404 8.2. Blow Molding ...................... 406 8.3. Fibers and Flat Yarns ................ 406 8.4. Film .............................. 409 8.5. Foil and Sheet ...................... 410 8.6. Extruded Pipe ...................... 410 9. Environmental Aspects ............... 411 9.1. Recycling .......................... 411 9.2. Pyrolysis .......................... 412 9.3. Incineration ........................ 413 9.4. Environmental Interactions ............ 413 References ......................... 414 1. Historical Survey In the 1950s KARL ZIEGLER discovered new cat- alysts which were to revolutionize the plastics industry. Before then, propene could not be polymerized to high molecular mass products with the catalysts then available, namely, free- radical, anionic and cationic systems. Even the most favorable of these yielded only liquids consisting of many isomers and greaselike ma- terials unsuitable for making hard plastic products. In 1953 KARL ZIEGLER, Professor at the Max- Planck-Institut f€ ur Kohlenforschung, demon- strated that ethylene could be polymerized to a high molecular mass crystalline polymer at mod- erate pressure and temperature. His group had for some years been using aluminum alkyls to con- vert ethylene into a range of oligomers having an even number of carbon atoms. Unexpectedly, almost total conversion to butenes occurred in one experiment. The cause was eventually traced to nickel-ion contamination in the auto- clave. Tests with many other metallic com- pounds revealed that a combination of zirconi- um ion with aluminum alkyl converted ethylene to high molecular mass polymer. Further searching revealed that titanium ion was even more active in producing polyethylene (Novem- ber 1953). Such combinations of transition met- al compounds with an aluminum alkyl later became known as Ziegler – Natta Catalysts, thereby acknowledging the immense contribu- tion of GIULIO NATTA to discovering and char- acterizing polypropylene (PP) and other a-ole- fin polymers in 1954. DOI: 10.1002/14356007.o21_o04
Transcript
� 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Article No : o21_o04
Polypropylene
T. GEOFFREY HEGGS, current address: The Mill, Great Ayrton, TS9 6PX,
In the 1950s KARL ZIEGLER discovered new cat-alysts which were to revolutionize the plasticsindustry. Before then, propene could not bepolymerized to high molecular mass productswith the catalysts then available, namely, free-radical, anionic and cationic systems. Even themost favorable of these yielded only liquidsconsisting of many isomers and greaselike ma-terials unsuitable for making hard plasticproducts.
In 1953 KARL ZIEGLER, Professor at the Max-Planck-Institut f€ur Kohlenforschung, demon-strated that ethylene could be polymerized to ahigh molecular mass crystalline polymer at mod-erate pressure and temperature. His group had forsome years been using aluminum alkyls to con-
vert ethylene into a range of oligomers having aneven number of carbon atoms. Unexpectedly,almost total conversion to butenes occurred inone experiment. The cause was eventuallytraced to nickel-ion contamination in the auto-clave. Tests with many other metallic com-pounds revealed that a combination of zirconi-um ion with aluminum alkyl converted ethyleneto high molecular mass polymer. Furthersearching revealed that titanium ion was evenmore active in producing polyethylene (Novem-ber 1953). Such combinations of transition met-al compounds with an aluminum alkyl laterbecame known as Ziegler – Natta Catalysts,thereby acknowledging the immense contribu-tion of GIULIO NATTA to discovering and char-acterizing polypropylene (PP) and other a-ole-fin polymers in 1954.
DOI: 10.1002/14356007.o21_o04
NATTA, Professor and Director of the MilanInstitute of Industrial Chemistry, was a consul-tant to Montecatini, to whomNATTA assigned hispatents. The close collaboration betweenZIEGLER
and NATTA was acknowledged in 1963 by thejoint award of a Nobel prize in Chemistry fortheir outstanding contributions to polymer sci-ence. A fascinating account of ZIEGLER andNATTA’s personal approach to these discoveriesis provided by MCMILLAN [1].
In the the United States, patents on crystallinepolypropylenewere awarded originally toNATTA
et al. (on behalf of Montecatini), but were over-turned later in favor of Phillips Petroleum Com-pany. The immense legal procedings to establishinventor rights to crystalline polypropylene in theUnited States are outlined by HOGAN and BANKS
[2], with more recent comments by PINO andMORETTI on this costly litigation [3]. Phillip’spolymerization catalyst, consisting of chromi-um-ion promoted silica – alumina, has not beenused in any commercial plant to manufacturecrystalline polypropylene.
Notwithstanding the complexities and hazard-ous nature of these new coordinated catalysts,industrialists and academe alike were stimulatedby ZIEGLER and NATTA’s prompt and prolificdisclosures concerning this new, high-meltingpolymer. Industrial-scale production of PPstarted at Ferrara, Italy, in 1957 with a 6�103
t/a plant. By the end of 1994 global capacity hadrisen to 20.5�103 t/a (Table 1), spread over 35countries [4]; today, new plants are expected tobe capable of producing 200�103 t/a to achievereasonable economies of scale.
There are several reasons for this phenomenalgrowth, which exceeds that of other bulk plastics[5] (Table 2). At first propene was readily avail-able, almost at byproduct prices, from petro-chemical cracker plants making ethylene. Thepolymer itself was suitable for a wide range ofexisting and new applications such as films,
various fibers, large and small moldings fromboat hulls to instrument parts. In addition, newmanufacturing technologies based on MgCl2supported catalyst systems yielded both costsavings and improved products by eliminatingatactic PP removal steps and then the deashingstage. Some 40 years on there are expectationsthat the new metallocene catalyts will extendthese uses, even to the extent of challengingproducts such as polyamide, ABS, and flexiblePVC for part of their market [6].
2. Polymer Structure
2.1. Molecular and Chain Structure
Polypropylene is a synthetic, high molecularmass linear addition polymer of propene. In1954/5, NATTA et al. prepared sufficiently largeamounts of PP to isolate and characterize variousstereoisomers. This led to a new descriptivenomenclature still used today. Commercial in-terest lies primarily in highly crystalline PP,together with its further modifications throughcopolymerization.
In perfectly isotactic PP, each monomer unitin the chain is arranged in a regular head-to-tailassembly without any branching or 1,3 additions
Table 1. Global PP capacity (103 t/a) and its distribution
Country/region End 1990 End 1994 Growth* 1990 –
1994, %
Western Europe 4530 5705 26
North America 4400 5440 24
Japan 1870 2656 42
Other Far East/
Australia
2734 4090 50
Middle East/Africa 280 655 34
South America 450 1035 30
Eastern Europe 790 930 18
Total/average 15 054 20 511 36
*7% per annum growth for four years is 31% total growth.
Table 2. Major polymer usage in Western Europe
Polymer Consumption, 103 t/a Growth rate, %
1974 1984 1994 ’74/’84 ’84/’94
LLDPE þLDPE 3080 4089 6015 33 47
HDPE 1035 1704 3570 65 110
PS 1118 1290 1908 15 48
PVC 3360 3825 5480 14 43
PP 680 1895 5178 179 173
382 Polypropylene Vol. 29
(Fig. 1 A). This is the result of template-typeconstraints by the heterogeneous stereospecificcatalyst. In practice such perfection is hard toachieve. An occasional error (Fig. 1 B), averag-ing about 0.3 – 1.5 per hundred chain links,occurs in some, but not equally in all chains.The multisite nature of solid catalysts is thoughtto be responsible for this behavior.
Solvay and MgCl2 supported systems reducethe extent of these errors, and with them theamount of noncrystalline (atactic) polymer. Stillat the development stage, metallocene-basedsingle-site catalysts (SSCs) also produce stereo-regularity errors, but these seem to be distributedmore uniformly along the chains, regardless oftheir length (Fig. 1 E). This contrasts with mul-tisite catalysts (MSCs), for which misplacementsvary in frequency and in number. The tendencyhere is for the high molecular mass fraction tocontain fewer faults, while the short chains sufferfrom excessive dispruptions, which lead to re-duced crystallinity.
Until recently, syndiotactic PP (Fig. 1 C) wasnot commercially important because of unac-ceptable polymerization difficulties at low tem-peratures. Some metallocene catalysts overcomethis problem to produce a regular sequence ofracemic (r) propene placements yielding a crys-talline polymer melting at 130 �C.Mitsui Toatsuand Fina have entered into a joint venture toassess costs and customer reaction to syndiotac-tic PP.
Complete loss of steric regulation generatesatactic polymer chains (Fig. 1 D), incapable ofcrystallizing. Hitherto this low molecular massbyproduct had only a few commercial outlets foradhesives. The position could change with theemergence of high molecular mass versions de-liberately made with a metallocene-based cata-lyst, [(Me2SiFlu2)ZrCl2] (Flu ¼ fluorenyl)[7].
All Ziegler – Natta commercial polymershave broad molecular mass distributions withinthe range Mw/Mn ¼ 5 – 10 for ex-reactor mate-rial. Any reduction is achieved by peroxide-
Figure 1. PP chain structures
Vol. 29 Polypropylene 383
induced chain clipping in an extruder. Metallo-cene polymers have narrower distributions (Mw/Mn¼ 2 – 4) as a consequence of their single-sitemechanism. This avoids the need for peroxideadditives, which produce objectionable odor andvolatile byproducts.
2.2. Crystallization and Morphology
Polypropylene’s strength and versatility stemfrom a matrix of interlocking crystallites thatallow formation of rigid and tough articles.Highly isotactic polymer, with its regular struc-ture, forms a helical coil having three monomerunits per turn. These coils stack together into thinlamellar crystallites which force the chains tofold several times as they emerge and reenterlamellae. Polyethylene behaves similarly (see! Polyethylene, Figure 2)
While the equilibrium (i.e., thermodynamic)melting point is 188 �C, at normal analysis heat-ing rates the final melting point is ca. 160 –170 �C. Three crystalline forms are known (Ta-ble 3), of which the a-form is the most stable.Rapid quenching yields the b-form with a lowerdensity andmelting point of 150 �C. Polymers oflower stereoregularity and random copolymersusually contain g- crystallites in addition to thea-form.
Crystallization is complicated further by as-sembly of these lamellar crystals into largerstructures, called spherulites. These have radialsymmetry and can adopt five different forms(Table 4). While the size of the crystallites is inthe region of 10 – 50 nm, too small to scattervisible light (400 – 600 nm), spherulites aremuch larger (103 – 105 nm). The latter are re-sponsible for the translucency of slowly cooledPP moldings. Rapid crystallization from the melt
by fast cooling, adding nucleating agents, andreducing spherulite sizes by stretching a partiallysolidified melt all improve transparency.
Crystallization from a viscous melt is hin-dered by chain entanglements and by the needfor helices to fold as they close-pack into lamel-lae. This, and the formation of nuclei mightcontribute to the low degree of crystallizationeven in highly stereoregular polymers. Typical-ly, the onset of crystallization occurs at ca. 110 –120 �C in DSC instruments. This point coincideswith the cooling peak for polyethylene, whosepresence would be masked in blends and inethylene copolymers. Using the melting curveovercomes this analytical difficulty. Anotheraspect of tardy crystallization is the low level ofcrystallinity in fabricated items, even when high-ly stereoregular polymers are used. Commercialarticles vary from 30% crystallinity in rapidlyquenched films to 50 – 60% in moldings. Evenpurified and annealed samples of highly stereo-regular PP rarely achieve more than 70% crys-tallinity. Polypropylene is properly regarded as asemicrystalline polymer for these reasons.
Table 3. Crystal habits of PP [8–10]
Crystal
form System
Crystal density
(20 �C), g/cm3
Chains per
unit cell mp, �C*
Iso a monoclinic 0.932 – 0.943 4 171
Iso b pseudo
hexagonal
0.922 9 150
Iso g triclinic 0.939 1 131
Smectic 0.916
Amorphous 0.85
Syndio orthorhombic 0.93 2 138
(4/1 helix)
*The enthalpy of melting for 100% crystalline isotactic PP is
variously reported, due to different techniques. WUNDERLICH [8]
the ICI DSC standard is 185 J/g, and the Shell DSC standard is
188emsp14;J/g [11, p. 590].
Table 4. Spherulitic structures
Spherulite type Crystal type Features
I a positive birefringence, lost at 160 – 170�C; epitaxial branching;always associated with type II
II a negative birefringence, lost at 160 – 170 �C;fibrillar crystals radiating from nucleus; most stable of all spherulites
III b strongly negative birefringence; associated with b-crystalsand high shear in cooling; melts and reverts to type II at 145 –150 �C
IV b special case of Type III, characterized by banded concentric rings
V g rare variant formed under high pressure with g-crystals
384 Polypropylene Vol. 29
3. Raw Materials
3.1. Propene
Sensitive Ziegler – Natta catalysts require purepropene to ensure high polymerization rates andgood yields of isotactic polymer, without waste-ful byproducts. Harmful impurities can begrouped into two main categories which havedifferent effects on the process. The most de-structive contaminants are polar and highly un-saturated compounds which react directly withthe catalyst systems, retarding polymerizationand impairing stereospecificity. Examples in-clude acetylenes, dienes, carbon monoxide anddioxide, carbonyl sulfide, water, alcohols, andammonia. The second group contains materialswhich adversely influence the process, but not thechemistry, by raising the pressure during reactiondue to an accumulation of inert substances. Thesefurther complicate the monomer recovery sec-tion and recycle streams. This group containsmaterials such as methane, ethane, propane, bu-tane, and nitrogen.
Most manufacturers use polymer-grade pro-pene as feedstock, which can be further refinedby passing it through local guard columns con-taining alkali, molecular sieve, alumina, sup-ported copper, etc. to guarantee consistently highpurity [12, 13]. Typically, monomer conformingto the following specification, is adequate formost plants, but certain sensitive supported cat-alysts require the lower levels listed for carbonylsulfide, carbon monoxide, and oxygen (this qual-ity is said to be suitable formetallocene catalysts)[14]:
Propene 99.5 wt% min.
Propadiene 5 vol ppm max.
Propyne 5 vol ppm max.
Butadienes 10 vol ppm max.
Oxygen 5 – 2 vol ppm max.
Carbon monoxide 3 – 0.3 vol ppm max.
Carbon dioxide 5 vol ppm max.
Carbonyl sulfide 0.5 – 0.03 vol ppm max.
Total sulfur 1 wt ppm max.
Water 5 wt ppm max.
Methanol 5 vol ppm max.
Ammonia 1 vol ppm max.
Ethane 500 vol ppm
Propane 0.5 wt%
Butane 500 vol ppm
N2 þ CH4 300 vol ppm
Isobutene 50 vol ppm (estimated)
Hydrogen 10 vol ppm max.
Ethylene 50 vol ppm max.
1-Butene 50 vol ppm max.
Ethylene and 1-butene copolymerize withpropene to marginally reduce the polymer soft-ening point. Hydrogen is a powerful chain-trans-fer agentwhose concentrationmust be accuratelycontrolled in the reactor.
Propene is normally manufactured outside thePP plant. An important source of both ethyleneand propene, in the approximate weight ratio of2: 1, is from steam cracking naphtha or gas oil at700 – 950 �C, followed by fractionation. Refin-ery catalytic cracking of petroleum productsyields 2 – 5% of propene, which is isolated asa chemical grade containing 90 – 95% of themonomer with much propane. This can be puri-fied further to polymer-grade specification, butsome PP plants which are able to handle the extrapropane may bypass further refining or add asimplified hydrogenation unit to reduce dienes[15]. These possibilities arise at the milder end ofthe cracking range of 400 – 500 �C, which thengenerates lower amounts of harmful highly un-saturated products. Currently there is some inter-est in building propane dehydrogenation plantsas a third source of pure propene [15, 16].
Some properties of propene relevant to poly-propylene production are as follows:
Heat of polymerization 2514 kJ/kg
bp � 47.7 �CCritical temperature 92 �CCritical pressure 4.6 MPa
Vapor pressure at 20 �C 0.98 MPa
3.2. Polymerization Diluents
Most early processes and some current plants usean inert hydrocarbon diluent in the reactor to helptransfer propene to the solid catalyst and toconvey reaction heat to the water- cooled jacket.Typically there is twice as much diluent in thereactor as polymer; thus, the purity demands onthe diluent are as important as for the monomer.Refined petroleum fractions and synthetic hydro-carbons in the C6 – C8 range performwell, whilesolvents ranging from butane to dodecane arealso used commercially. Polar impurities, such as
Vol. 29 Polypropylene 385
alcohols, carbonyl compounds, water, and sul-fur- containing compounds must be kept below1 – 5 ppm. The content of aromatics should bebelow 0.1 – 0.5 vol%, depending on the catalystemployed.HereUV spectroscopy, supplementedwith IR spectroscopy and refractive index mea-surement, is used to monitor quality. Regularchecks must be made on recycled-diluent qualityto prevent oxidized species and catalyst frag-ments accumulating unnoticed.
Polypropylene articles, such as films and con-tainers, are widely used for food packaging andhandling. In these cases the diluents and otheradditives must conform with the appropriatehealth regulations.
Since polymerizations carried out in the gasphase or in liquid propene only use smallamounts of diluent as catalyst carriers, theirpurity requirements are less stringent.
3.3. Catalyst Preparation [17]
All current commercial manufacturing processesfor highly isotactic PP use heterogeneous Zieg-ler – Natta catalyst systems. These two-compo-nent initiators consist of a solid transition-metalhalide, usually TiCl3, and an organoaluminumalkylating agent such as diethylaluminum chlo-ride (DEAC). This system was the basis of PPplants until about 1980 when supported catalystswere commercialized. An overview of the fourdevelopment phases is given in Table 5.
3.3.1. TiCl3-Based Catalysts
Of the four known crystalline forms of TiCl3,three are purple (a, g , d) while the fourth is brown(b). The latter is not used in PP manufacturebecause of its poor stereospecificity. All thepurple forms have a layer lattice consisting ofclosely packed chlorine atoms (Fig. 2) eitherhexagonal (a), cubic (g), or intermediatebetween the two (d). Preparative methods are
Table 5. Catalyst development
System Catalyst performance Plant process
Activity, kg PP/g catalyst I.I.*, wt%
1st Generation
1957 – 1970 0.8 – 1.2 88 – 93 deash and remove
TiCl3 � AlEt2Cl atactic
2nd Generation
1970 – 1980 3 – 5 92 – 97 deash/deactivate,
TiCl3 � AlEt2Cl þ atactic usually remains
Lewis base (Solvay)
3rd Generation
1980 – 1990 5 – 20 � 98 no deash and
MgCl2-supported TiCl4 � AlEt3 no atactic removal
4th Generation
1995 ca. 20 ? no deash and
Metallocene/MAO systems no atactic removal
MMD ca. 2 – 4
* Isotactic index (% insoluble in boiling heptane).
Figure 2. Layer lattice of a-TiCl3 [19] (printed with permis-sion of Harwood Academic Publishers)
386 Polypropylene Vol. 29
critical to achieve good stereoregulation and highactivity with these catalyst components.
Early work by the NATTA school used pureTiCl3 made by hydrogen reduction of TiCl4,followed by ball milling to give a high surfacearea.
TiCl4þ1=2 H2!TiCl3þHCl
Amore active form is made by reduction withaluminum powder, followed by ball milling.
3 TiCl4þAl!3 TiCl3�AlCl3
Much of the aluminum chloride is present as atrue solid solution in crystalline d-TiCl3. Thiscatalyst has a larger surface area of 10 – 40 m2/gand is commercially available from several com-panies. Nowadays, a Lewis base, such as an esteror ketone, may be introduced at the milling stageto increase stereospecificity.
Another important route dispenses with ballmilling by reducing a hydrocarbon solution ofTiCl4with an organoaluminum compound belowroom temperature. Di- and trialkylaluminumcompounds are suitable. In this process the tita-nium trichloride precipitates in the brown, bform. Heating to 80 – 120 �C completes thereduction and transforms this solid to the re-quired purple variety. Several hydrocarbonwashes remove some of the aluminum com-pounds to leave an active, stereospecific catalyst.A useful aspect of this technique is that condi-tions can be selected to precipitate sphericalcatalyst particles. Subsequent propene polymer-ization then yields spherical polymer particlesthat exhibit good flow characteristics. This abili-ty to replicate the shape of catalyst particles isquite general in Ziegler – Natta polymerization.Polymer forms throughout the agglomerate ofsmall catalyst clusters, expanding the particleuniformly in all dimensions as polymerizationcontinues.
Major refinements in alkyl-reduced TiCl3 sys-temsweremade by Solvay [18–20] in their three-stage process, which gives a 4 – 5-fold increasein activity:
1. Titanium tetrachloride solution is reducedwith diethylaluminum chloride at 0 �CTiCl4þAlEt2Cl! TiCl3 � xAlCl3 � zAlEtCl2where x ¼ 0.15, z ¼ 0.20
2. The brown b-TiCl3 solid is treated with dii-soamyl ether to dissolve out most of thealuminum compounds
3. The extracted solid is reacted with neat tita-nium tetrachloride at 65 �C to transform the bcrystallites to the violet d-form
The novelty lies in removing much of thealuminum residues at the b stage, without col-lapsing the particle, and then transforming to thestereoregulating d-form while retaining a spon-gelike structure with a surface area of 150 m2/g[18]. Analogous commercial products are avail-able in Japan and the United States.
3.3.2. Supported Catalysts
The third-generation, supported titanium cata-lysts, are based on research by Montedison (nowMontell) in Italy and Mitsui Petrochemical In-dustries (now Mitsui Sekka) in Japan. Collabo-ration and cross-licensing between these twocompanies avoided incipient interference claims.Starting around 1968 with finely milled magne-sium chloride, which has a similar layer structureto violet TiCl3, ways were discovered of deposit-ing stereoregulating titanium compounds on thesurface.
A particle-form supported catalyst may beprepared as follows [21]:
1. A spheroidal support is prepared by making afine dispersion of molten (125 �C) magne-sium chloride ethanolate complex in hot ker-osene containing sorbitan distearate as surfac-tant. The dispersion is quenched by pumpinginto kerosene at � 15 �C to give particleswith a diameter of 5 – 30 mm.
2. A slurry of the particles is added to neat TiCl4at 20 �C.
3. Diisobutyl phthalate (17% v/w) is added andthe mixture heated to 120 �C.
4. Decant, and heat with further TiCl4 to 130�C.
Isolate by hot filtration and washing. Theproduct contains 2.3 wt% Ti, 63 wt% Cl,20 wt% Mg, and 9.9 wt% diisobutylphthalate.
For the polymerization of propene, triethyla-luminum is used as a cocatalyst, and phenyl-triethoxysilane as an additional external Lewis
Vol. 29 Polypropylene 387
base stereoregulator. Potential corrosion pro-blems have been substantially reduced by usingthe chlorine-free triethylaluminum.
By ca.1980, commercial plants were achiev-ing 20 – 30 kg polymer per gram catalyst, andthe isotactic index progressively improved from88 to 99%.
These catalysts form the basis of advancedtechnology plants in which deashing (removal ofcatalyst residues) and atactic polymer separationcan be eliminated entirely [22]. It is estimatedthat by 1998 70% of global plants will beoperated with such ‘‘high mileage’’ technology.Coupled with this is the possibility of exploitingthe good handling characteristics of the Spher-ipol coarse particle-form powders, thus dispens-ing with the need for expensive pelletizing plant.The segregation of added powdered stabilizersduring storage and the problems of handling suchmixtures are addressed, to some degree, byMon-tell’s Valtec products, in which the additives aretreated to provide a hard coating on the polymer.Some customers still prefer to handle traditionalgranules from an extruder.
3.3.3. Homogeneous Catalysts [17],[23,24],
Metallocenes have been used for 30 years asmodel compounds for Ziegler – Natta reactions.They were quite unsuitable for commercial re-actors because of their extremely low activity andpoor stereocontrol. In the early 1980s, SINN andKAMINSKY serendipitously discovered that repla-cing triethylaluminum with methyaluminoxane(MAO) enormously increased the polymeriza-tion rate of ethylene when combined with asubstituted zirconocene dichloride. There wereseveral other major hurdles to overcome before asuitable candidate was available for use in PPplant. Low stereospecificity succumbed to amore stereorigid metallocene, and the poor reac-tivity and polymerization temperature restric-tions were solved by further molecular tailoring.Trials were conducted in 1996 on commercial-scale streams.
Metallocences suitable for isotactic PPmanufacture generally seem to be based onzirconocenes supported on inert solids to pre-serve particle size and shape. This makes themmore compatible with the advanced process
technologies of the major operators who referto them as ‘‘drop-in catalysts’’. Sophisticatedchemistry (Figure 3) is needed to prepare thebase metallocenes, many of which have beenmade only in laboratory-scale equipment. Anadditional constraint is the heavy patenting,already amounting to some 900 applicationssince 1984. Hoechst, Exxon, Fina, Mitsui, andBASF together hold half of these.
The great versatility of single-site metallo-cenes has been demonstrated in trials that yieldedimproved and novel polymers. It remains to beseen howmany of these desirable features can beincorporated in a single catalyst and how easilyappropriate changes can be introduced at plantlevel. Some of these polymer properties arementioned in Section 7.1 [23].
3.3.4. Aluminum Alkyl Cocatalysts
Nowadays, aluminum alkyls are purchased fromoutside manufacturers. In the case of DEACsystems it is necessary to tailor the chlorinecontent to the type of TiCl3 used. As the variousalkyls interact rapidly, it is simple to adjust thecomposition by adding AlEt3 or AlEtCl2:
2 AlEt2Cl�AlEtCl2þAlEt3
Triethylaluminum is made commerciallyfrom aluminum, ethylene, and hydrogen. Careshould be taken to avoid the high levels of thehydride that are sometimes present. Trimethyla-luminum is available from outside maufacturers,as is methyl aluminoxane (MAO). The latter ismade by the controlled addition of water to thetrialkyl with various techniques to moderate thepotentially violent reaction. One method uses a
Figure 3. Hoechst (left) and BASF metallocenes
388 Polypropylene Vol. 29
salt hydrate, such as such as hydrated potassiumaluminum sulfate (potassium alum). Variousgrades of MAO are available, possibly due tothe different manufacturing methods and thepolymeric nature of the product, which containslinear and cyclic short chains.
3.4. Hydrogen
Pure hydrogen is used at concentrations of0.05 – 1 vol% based on propene to control mo-lecular mass. Polar and highly unsaturated con-taminants should be excluded, as usual.
4. Polymerization Mechanism
Many kinetic and theoretical models have beenapplied to Ziegler catalysts. Only a very smallproportion (< 1%) of the titanium atoms inviolet TiCl3 are active for stereospecific poly-merization. COSSEE proposed that an active site isgenerated at those surface titanium atoms whichhave one chloride vacancy and one exposed andweakly bound chloride anion, the remaining fourchlorine atoms being firmly held in the lattice.These sites are located on the lateral faces ofviolet TiCl3 crystals where the protruding chlo-rine is alkylated by aluminum alkyl (Fig. 4 A).An incoming propene molecule approaches thisactive site (Fig. 4. B), whose immediate crystalgeometry controls both the initial coordinationand the configuration as themonomer inserts intothe titanium – alkyl s-bond (Fig. 4 C). Accord-ingly, this is a template type [25] polymerizationcontrolled by the surface shape, and not by thepreviously inserted monomer unit. Atactic poly-mer is formed at more open surface sites havingtwo vacancies or two weakly bonded chlorineatoms. While there are a number of alternativeproposals [17, 26, 27] the essentials of the Cos-see – Arlman model are widely accepted.
Since each active site on the crystal is unlikelyto be unique, a distribution of imperfections is tobe expected. Sometimes Ziegler – Natta cata-lysts are referred to now as multisite catalysts
to emphasize this wide distribution of polymerchain lengths and stereoregularities.
In MgCl2 supported titanium chloride sys-tems, 10 – 20% of the titanium is thought to bein the formof active sites, substantiallymore thanin Solvay’s improved catalyst, which contains ca.0.8 – 2.7% of the titanium in active form. Sur-face geometry still controls stereoregularity, butLewis bases are much involved in this regulation[28].
The new (1996) metallocene single-site cat-alysts have unique structures which secure pre-cise control and definition of the active site.Consequently, a narrow molecular mass distri-bution and better control of chain irregularities,now including new types of chain defects,may beobtained with selected metallocenes/aluminox-anes. These catalysts can be used in solution, butmanufacturers usually prefer them supported tosecure better control of particle size and shape,together with freedom from reactor build-up[24]. Apparently certain supports can be usedwithout spoiling catalyst or plant performance.Hence the name drop-in catalysts to emphasizethe ease with which they can be introduced intoexisting plant. Their considerable potential andperformance are being tested at the pilot-plantstage [29].
Figure 4. Cossee – Arlman model for polymerization siteA) Alkylation of lateral surface chlorine; B) Propene com-plex; C) Insertion into Ti – C2H5 s-bond
Vol. 29 Polypropylene 389
5. Industrial Processes
In the laboratory, it is a fairly simple matter tomake a sample of PP from a given catalyst. Allthat is necessary is to suspend the solid transitionmetal compound in dry, inert-gas-blanketed,pure heptane containing aluminum alkyl cocata-lyst and then bubble in propene gas at about60 �C and atmospheric pressure. Polymer formsas a white solid permeating the catalyst particleswhich give an overall red-pink hue (with TiCl3).Elevated pressures increase reaction rates ac-cording to a first order law.
Industrial manufacturing plants must incor-porate many other stages to secure salableproduct: some of these complications are elim-inated in modern high-technology plants byusing advanced catalysts. Here the residualcatalyst and atactic levels can be sufficientlylow not to discolor polymer nor to generateodor and smoking. Although older plants stillusing the traditional process make high qualityproducts, their higher cost base means thatthey are becoming increasingly uncompetitive,except in some highly specialized and low-tonnage applications. Some 70% of global PP
plants have installed new ’’High Mileage’’processes.
The following sections describe one olderslurry type of plant and four state-of-the-artadvanced processes. From these it will be possi-ble to visualize a number of hybrid alternatives tosuit particular catalyst characteristics [32]. Copo-lymerization is dealt with separately for eachsystem in Section 5.7, though for conveniencethe process flow sheets illustrating homopoly-merization (Figs. 6–7 and 9) include the bolt-oncopolymer plant.
Details of the individual steps shown in Figure 5vary between manufacturers as regards processconditions and type of equipment.
Polymerization. For continuous systems,which have largely superseded batch polymeri-zation, the mean residence time of catalyst in thereactor(s) is usually 1.3 – 3 h. Individual auto-claves started out as 10 – 30 m3 vessels, but as
Figure 5. Schematic of early PP manufacturing process
390 Polypropylene Vol. 29
stream capacities increased sizes up to 100 m3
have been installed. Another common practiceinvolves linking reactors together in series or inparallel configurations. The maximum outputrate of a reactor is determined by the heat remov-al system, which no longer relies simply onwater- cooled polymerizer jackets. Internal andexternal cooling circuits provide the necessaryextra cooling at these high rates (> 0.3 t h�1
m�3).It is only necessary to meter catalyst into the
first reactor of a cascade system, both because theactivity decay is fairly small, and because it is notdesirable on quality grounds to generate a secondfamily of particles downstream. Propylene andhydrogen feeds to each reactor maintain therequired pressure and gas compositions to securethe correct polymerization rate and molecularmass. Most of the reaction diluent is metered intothe first vessel in the train, but some can be addedfurther downstream to minimize fouling as thepolymer concentration increases. Slurry viscosi-ty increases rapidly during the latter stages ofpolymerization as the swollen polymer particlesapproach close packing. This point is stronglydependent on the size and shape of the catalystparticles. Seldomwill the slurry concentration beallowed to exceed 42 wt% unless a very lowdensity diluent is used.
As a rule, polymerization temperatures liebetween 50 and 75 �C; the desirability of usinga higher temperature being countered by de-creased catalyst stereospecificity causing in-creased solubles and poorer properties. Whilereactor pressures of 0.5 – 1.0 MPa typified ear-lier plants, intensified processes expand this to2 MPa.
Propene Flashing and Recovery. Twoways of removing and recovering unreacted pro-pene, stages b and k in Figure 5, are practicedcommercially. Most commonly, slurry from thelast reactor discharges into a heated vessel atlower pressure to flash off propene vapor andother volatiles. After cooling and condensation,fractional distillation recovers pure propene as anoverhead stream with a bottom discharge ofpropane and some diluent. In certain cases it ismore economical to return the flashed propene toan adjacent plant for use there. The alternativescheme simply allows polymerization to contin-ue without feeding any fresh propene to the latter
part of the reaction train. Propene in solution isgradually converted into polymer as the poly-merization rate falls exponentionally in the first-order reaction. This simple scheme entirely elim-inates the recovery stage, but it does call foradditional reactors working at low rates.
Catalyst Removal (Deashing). Catalyst re-moval, steps c and d in Figure 5, involves theextraction of highly polar particulate TiCl3 froma hydrophobic polymer particle. Procedures dif-fer appreciably, but the first step always convertstitanium and aluminum residues into complexesor alkoxides that are soluble in the diluent,usually by heating to 60 �C with 2 – 20% of analcohol, though acetylacetone has also been used.The choice of ethanol, propanol, or butanol isinfluenced by the diluent boiling point and sub-sequent recovery schemes.
Step d, Figure 5, shows a widely practisedarrangement whereby water washing is used totransfer all these catalyst complexes into anaqueous phase, leaving behind a purified polymerslurry.
It is vital to work with either a strongly acidicor strongly alkaline complexing medium to re-press hydrolysis and polycondensation, whichimpair the extraction efficiency. This techniquenormally yields polymer containing 10 –30 ppm of Ti, 10 – 40 ppm of Al, and 20 –40 ppm of Cl residues. Very pure polymer canbe made in an alternative form of step d. Here,the polymer is isolated by filtration, and is re-peatedly washed by reslurrying in diluent/alco-hol mixtures to remove all solubilized catalystwithout introducing water. This process adds tothe load on diluent and alcohol recovery, but PPquality improves.
Centrifuging. Centrifuging the deashedslurry (step e, Figure 5) removes most of thefree diluent, but the wet cake can still hold up to50 wt% (dry basis) of diluent containing dis-solved atactic polymer. Depending on the gradeof polymer being produced, it is sometimesbeneficial to introduce a washing stage withdiluent while centrifuging to reduce the amount
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of soluble polymer in the final product. Centri-fuges can be of the filtering (basket) or decant-ing type according to the powder characteri-stics.
Drying. Drying the wet cake sometimesrequires more than one stage, particularly inprocesses using high-boiling solvents. Steamdistillation removes most of the diluent to leavean aqueous slurry from which the polymer isreadily isolated by filtration. A conventionaldrier removes the small amount of water stilladhering to the hydrophobic particles. Moreelaborate driers, operating under a nitrogen at-mosphere, remove diluent directly in flash dry-ing, fluidized beds, or hot surface contact typesystems. Before the advent of high-technologysystems (Section 5.4), some manufacturers hadalready improved catalyst stereospecifity suchthat the low level of atactic polymer in the diluentcould be left in the product. In this case, the totalslurry can be dried without previous centrifuga-tion. This avoids all atactic polymer handlingproblems. All these drying stages must use nitro-gen blankets throughout to minimize hazardswith flammable hydrocarbons, as well as toprotect the polymer.
Extrusion and Pelletizing. Powder leavingthe driers at about 100 – 120 �C can be feddirectly to a pelletizing extruder, in which it isstabilized and converted into dense granules. Asintermediate powder storage is often necessary,common practice introduces a cooling stage to70 – 80 �C to prevent adventitious oxidationwhen antioxidants are absent.
Diluent and Alcohol Recovery. An essen-tial part of the schematic plant shown in Figure 5is solvent and alcohol recovery, and atactic poly-mer separation (stages h – j). Diluent from thecentrifuge contains soluble polymer and traces ofalcohol. This mixture, together with liquid fromthe driers, is separated by multistage distillation.Pure diluent is then recycled to the polymerizers,while viscous atactic polymer separates out as aheavy end.Wiped film evaporators can be used tostrip the last traces of diluent from highly viscoussolutions.
Most of the alcohol is recovered from the firstfew water washes by fractional distillation (steph) followed by azeotrope splitting.
5.2. Bulk Polymerization in LiquidPropene
A special case of the slurry is to use liquidpropene, with its poorer solvent power, insteadof an inert diluent. Polymerization rates increaseconsiderably with pressures around 3 – 4 MPa.Stirred autoclaves incorporating evaporativecooling, and loop reactors provide good heat-transfer rates. The chemistry parallels that de-scribed in Section 5.1, but the engineering de-mands are greater [32]. After polymerization, thecatalyst is solubilized with polar complexingagents to permit extraction from the polymer bycountercurrent washing with liquid propene. Hy-brid plants are known which use liquid propenefor polymerization, followed by flashing andresuspension in heptane for polymer deashing.
5.3. Solution Polymerization
In contradistinction to polyethylene manufac-ture, solution polymerization at high temperatureis rarely practiced for isotactic PP. Special cat-alysts are necessary to minimize the customaryreduction in stereospecificity above 100 �C, butthe activity and stereoregularity still remains toolow to dispense with catalyst extraction andremoval of atactic PP. The original Eastman –Kodak process makes grades for some specialmarkets, but there are no plans for further newplants along these lines. While polymerization atca. 150 – 200 �C should permit some usefulrecovery of polymerization heat, overall costsare high for isotactic PP. It is reported [33] thatthe solution process is being used to make atacticPP, for which it seems more suited. Commercialplants dedicated to make only atactic PP havebeen announced by Himont (Canada), El Paso(Texas), and Huls (Germany).
5.4. Spheripol Process [34]
Figure 6 depicts a modern state-of-the-art plantbased on Himont technology [34] for makinghomopolymer and impact copolymers (Sec-tion 5.7.2) with supported catalysts. The latterare sufficiently active not to require any catalystextraction, nor removal of atactic polymer be-cause of the high stereospecificity. Homopoly-
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merization takes place at ca. 70 �C and 4 MPa inliquid propene circulating round one or moreloop reactors. A single axial flow agitator in eachloop maintains high flow rates to ensure goodheat transfer to the water-cooled jackets, whilstalso preventing any polymer particles settlingfrom the slurry [35]. Typically the PP concentra-tion is ca. 40 wt%.
Continuously metered catalyst, triethylalumi-num, and a Lewis base stereoregulator such as adialkyldimethoxysilane are fed into the reactor tomaintain polymerization and stereocontrol. Theinitial few seconds of polymerization with a newhigh-activity catalyst particle are quite critical tosecure good performance. For this reason, someprocesses have a prepolymerization stage inwhich the catalyst components react at lowertemperature and monomer concentration. Thiscan be either a batch or continuous pretreatmentwhich produces only small amounts of polymer(< 100 g/g) in the catalyst. This prepolymerizedcatalyst is then fed into the loop reactor as usual.Mean residence time in a single polymerizer is1 – 2 h. Two loop reactors can be operated inseries to narrow residence time distributions,modify the polymer, and increase output.
A continuous stream of polymer slurry dis-charges through a heated zone for the first stageof pressure let down in cyclone (b). For homo-polymers, this connects directly to the secondarycyclone (d), bypassing the copolymerization unit
(Section 5.7.2). Unreacted propene flashes offfrom the first cyclone and is condensed withcooling water and recycled to the reactor. Acompressor is required for gas from cyclone(d). Polymer powder from the cyclone is fed intovessel (e) for deactivation with small amounts ofsteam and undisclosed additives. Residual mois-ture and volatiles are removed by a hot nitrogenpurge in vessel (f) before conveying polymer tostorage silos for conventional finishing as stabi-lized powder or extruded pellets.
Provided the expensive extrusion stage can bedispensed with by selling powder, then it isclaimed that the capital cost of Himonts Spher-ipol process is only 50% of the suspensionprocess described in Section 5.1 [34].
5.5. Hypol Process
Mitsui Petrochemical, co-inventor with Himontof the supported catalyst system, has developed ananalogous cheap process using bulk polymeriza-tion with their own supported catalysts. It differsfrom the Spheripol system in that batch prepoly-merization is used with washing. Two conven-tional stirred polymerizers are used in series, withheat removal by evaporative cooling of liquidpropene in the reactors. Slurry is then dischargedinto a stirred, heatedflash vessel,where propene isrecovered frompolymer, as in theHimont system.
Figure 6. Spheripol processa) Loop reactors; b) Primary cyclone; c) Copolymer fluidized bed; d) Secondary and copolymer cyclone; e) Deactivation;f) Purging
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5.6. Gas-Phase Processes
In gas-phase processes, gaseous propene is con-tacted with solid catalyst intimately dispersed indry polymer powder. Industry uses two differentmethods of carrying out this reaction dependingon the chosen method of heat removal. TheUnion Carbide/Shell process uses an adaptationof the Unipol polyethylene fluidized-bed system.BASF and Amoco use mechanically agitated drypowder beds with evaporative cooling in verticaland horizontal autoclaves, respectively.
BASF Novolen Process [36]. Figure 7shows the BASF continuous process for makinghomopolymers, impact copolymers, and randompropylene – ethylene copolymers using high-ac-tivity, highly stereospecific catalysts. The reactorvessels, of 25, 50, or 75 m3 capacity, areequipped with proprietary helical agitators,which give excellent agitation. Homopolymer-ization needs only the primary reactor, intowhichthe catalyst components are fed. These must bevery well dispersed in the powder bed to avoidbuild-up. The reaction conditions of 70 – 80 �C
and 3 – 4 MPa ensure that the monomer phase isgaseous in the reactor. Low concentrations ofhydrogen are used to control molecular massover wide ranges. The temperature is controlledby removing gaseous propene from the reactorhead space, condensing itwith coolingwater, andthen recirculating it back into the reactor, whereits evaporation provides the required cooling, aswell as further aeration of the stirred powder bed.Each tonne of polymer made requires ca. 6 t ofliquid propene to be evaporated as coolant.
Powder and associated gas discharge contin-uously from the primary reactor dip tube directlyinto a low-pressure cyclone (g). Propene carriergas from this cyclone is recycled to the reactorafter compression, liquifaction, and, sometimes,distillation. The powder then passes to a purgevessel where a deactivator quenches all residualcatalyst activity, and nitrogen strips out traces ofpropene from the hot powder. From here powderis conveyed into silos for stabilization and extru-sion into granules. BASF also offers a post-granulation steam-stripping package to removeany oligomers and oxidized residues from thegranules for demanding applications.
BASF pioneered their gas-phase process withcommercial production in 1969. The productsmade, Novolen 1300 series, were based on highmolecular mass total work up polymers (i.e.,containing atactic PP and catalyst residues) hav-ing reduced stereoregularity. Today, such gradesstill find niche markets, although they are vul-nerable to competition from random copolymers.Production is to be phased out shortly.
BASF also uses their process with a cheapersecond-generation catalyst TiCl3/AlEt2Cl, whichthen requires an additional dry-powder dechlori-nation stage.
Amoco – Chisso Stirred-Bed Process.Collaboration between Amoco and Chisso re-sulted in joint licensing arrangements from 1985.This process uses a horizontally stirred reactor,Figure 8, instead of the vertical helical agitator ofthe BASF process. Condensed recycled mono-mer sprayed into the top of the reactor providescooling, while uncondensed monomer and hy-drogen injected into the base maintain the gascomposition. Figure 8 also includes a fluidized-bed deactivation system for use with second-generation, chlorine-rich catalysts. Amoco –Chisso claim that their reactor achieves some
degree of plug flow, roughly equivalent to that of2 – 3 fully back mixed reactors in series [37].
Unipol – Shell Fluidized-Bed Process[38]. The Unipol – Shell plants, commissionedin 1986, combine technologies from Union Car-bide and Shell. Most conspicuous in this process,Figure 9, is the tall fluidized-bed reactor with itsexpanded upper section to reduce gas velocityand powder entrainment. Continuous feeds ofcatalyst components, comonomer, if any, hydro-gen, and propene are thoroughly mixed in thedense-phase fluidized bed of powder. A largecooler in the gas recirculation loop removes allthe reaction heat from the considerable gas flow.In this system the fluidized bed is said to behaveas a fully back mixed reactor, without undueseparation of coarse particles. No mechanicalagitation is needed. Reaction conditions are re-ported as < 88 �C and < 4 MPa.
Product powder and associated gas are dis-charged from just above the distributor plate bytimed valves into a cyclone separator (e) and thendirectly to a purge vessel (g) to remove residualmonomer. Neither catalyst removal nor atacticpolymer extraction is necessary with the modernShell catalysts used in the Unipol process.
Figure 8. Amoco – Chisso gas-phase processa) Horizontal reactor; b) Fluidized-bed deactivation; c) Compressor; d) Condenser; e) Hold/separator tank
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5.7. Copolymerization
Copolymers of propene with other a-olefinsaccount for ca. 30% of all PP sales. There aretwo distinct classes of copolymer having differ-ent application areas. Random copolymers, ob-tained by copolymerizing mixtures of propeneand other a-olefins have lower melting pointsand improved clarity. Impact (block) copoly-mers, made in a two-stage polymerization pro-cess, are high-impact-strength grades that con-tain dispersed propene – ethylene elastomers.
5.7.1. Random Copolymerization
Random copolymers aremade in the sameway ashomopolymers, but a mixture of propene and thecomonomer is used in place of pure propene.Usually, polymers containing 2 – 6 wt% ofcombined ethylene cover most applications.Sometimes the resulting copolymers are referredto as ‘‘statistical’’ because heterogeneous Zieglercatalysts rarely yield truly random copolymers[11] with mixed feeds of monomers. 1-Butenecan also be used, especially as a termonomeralong with ethylene. The higher a-olefins are
more expensive and less reactive than propeneand can complicate the monomer recycle stage.However, attractive product properties are stim-ulating manufacture [39, 40].
In the case of suspension polymerization,either in hydrocarbon diluent or in bulk, signifi-cant amounts of soluble byproducts add to han-dling difficulties due to viscosity increases athigher comonomer contents. Drying tempera-tures must be reduced because of the lowersoftening point and more cohesive nature of theparticles.
Gas-phase systems, especially those havingsome degree of mechanical agitation, are lessadversely affected, though maximum tempera-tures still need careful control.
Random copolymers often find use in foodpackaging (see Section 7.2), where the polymermust not contain high proportions of extractablematerial. Clarity improves andmelting points fallas comonomer and solubles increase. A compro-mise is reached at about 3 wt% of ethylene.Changes to catalyst and polymerization techni-ques can raise this by securing a more uniformdisruption of the isotactic sequences, which inturn encourages formation of the lower meltingg- crystallites [41].
Impact copolymerization is regarded as the low-est cost process for making toughened PP. It canbe visualized as a method for producing anintimate blend of PP homopolymer with a tough-ening ethylene – propene elastomer. Approxi-mately 10 – 20 wt% of this elastomer sufficesfor most applications, though the recent avail-ability of products containing 30 – 40 wt% willexpand their use.
The minimum requirement for block copoly-merization is a two-stage process. In one stagehomopolymer is prepared by any of the methodsdescribed in Sections 5.2, 5.3, 5.4, 5.5, 5.6. Theother stage uses a mixed monomer feed of ethyl-ene and propene to produce a largely amorphous,elastomeric phase within the polymer particles.Many other more complex configurations haveclaimed to be advantageous for certain products,but two stages are generally adequate [42, 43].
Homopolymer, or a very low ethylene copol-ymer, is always made in the first reactor becausethe elastomer fragments catalyst particles indiluent systems and produces sticky products ingas-phase reactors. This first stage of reaction,using propene and hydrogen as molecular masscontroller, is indistinguishable from conventionalhomopolymerization. In batch slurry systems,propene is metered into a reactor until the amountof polymer formed is 80 – 90% of the intendedfinal make. The remaining 10 – 20% is made inthe second stage as follows.Normally the propeneconcentration is allowed to fall, by continuedpolymerization, to a predetermined value beforeintroducing ethylene, propene, and possibly hy-drogen. No further catalyst is needed for thesecond stage, and the reaction temperature mightbe lowered to reduce the amount of soluble poly-mer diffusing out of the particles. The high reac-tivity of ethylene promotes rapid copolymeriza-tion in a more exothermic reaction. Venting andpolymer isolation follow the procedure alreadydescribed in Section 5.1, but filtration and atacticpolymer separation are more difficult because ofadditional amounts of viscous soluble copolymer.
Gas-phase systems using modern catalystsavoid much of this difficulty. Figure 7 shows atypical BASF continuous process using two stir-red reactors for making impact copolymers [36].Active polymer and gas from the first reactor
discharge directly into the copolymerizer, oper-ated at 1 – 2 MPa lower pressure and lowertemperature. Normally, the propene/ethylene/hydrogen ratio is adjusted such that the rubberycopolymer made in this reactor contains 40 –60 wt% ethylene, depending on the particulargrade. This gas composition is not readily con-densed with cooling water alone, so a compres-sion stage is added to liquify some of the recycledgas returned to the reactor base. Copolymerpowder, with its associated carrier gas, dis-charges into the low-pressure cyclone separator(g), which passes this carrier gas to recompres-sion for return to the copolymerizer. Residualmonomer in the copolymer is removed in acombined purging and deactivation vessel (h)before conveying powder into silos for extrusion.
Analogous gas-phase techniques are used byother manufacturers. Amoco – Chisso (seeFig. 8) add a second horizontal stirred reactorwhen making impact copolymers, and El Pasoalso add a similar polymerizer after their bulkfirst stage. Unipol – Shell introduce a second,but smaller, fluidized bed (Fig. 9), operating atca. 1.5 MPa, for their impact copolymers, thustaking advantage of enhanced copolymerizationactivity. Sumitomo’s new process also uses twofluidized beds. Himont (Fig. 6) and Mitsui Pet-rochemical retain their normal bulk polymeriza-tion for the first, homopolymerization step, butuse gas-phase copolymerization in a fluidized bedfor producing impact copolymers. Such dry pow-der reactions overcome the problems arising fromextraction of soluble copolymer species in liquid-based processes. At the same time, copolymerdiffusion to the surface should be reduced by thelower degree of swelling in dry powder systems.
While copolymers containing 40%, or more,of elastomer can be handled satisfactorily in plantprocesses, their cohesiveness calls for more carethan with homopolymers. Himont patents sug-gest that their fluidized-bed technology recog-nizes this aspect. Overall, there is completeunanimity that a gas-phase copolymerizationstage is preferred for this class of copolymer inmodern plant.
5.8. Product Finishing
Considerable capital is involved in the finishingareas of all plants after the dry powder stage. The
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great majority of this powder is extruded with avariety of additives, pelletized, tested, and thenstored in various packages for sale. Direct sale ofsuitably stabilized reactor powder circumventssome, but not all, of these costs.
Extrusion and Pelletization. Proprietaryequipment from specialist manufacturers willalways be used for high-throughput plants oper-ating at 5 – 20 t/h and 250 �C. In selecting ex-truders the following factors are relevant (inaddition to costs): powder morphology, productMFI range, melt filtration needs, devolataliza-tion, turn down ability (i.e., efficient operation atlow rates), temperature control, mixing needs,and additive feeds. The types of extruder avail-able include long, single-screw machines andtwo-stage systems in which powder is melted ina continuous high-speed mixer which dischargesthe melt into a short, pressure-generating extrud-er. Twin-screw machines with corotating andsegmented screws are increasingly used for ad-ditional flexibility. Versatility and output canalso be increased by installing a gear pumpbetween the die plate and the discharge end ofthe screw in any of these systems.
Pelletization takes two forms. Extruded lacesfrom the die plate are quenched in water to giverigid strands. High-speed cutters then chop thecontinuous strands into 2 – 5 mm lengths toproduce so-called lace-cut pellets. In the othersystem, high-speed knives rotate against theextruder die plate to cut off short lengths of themolten extrudate. Either the insulated die plate isimmersed in water, or the molten pellets areinstantly solidified by being flung into a waterring quench zone to prevent cohesion. These dieface cut granules are increasingly common inlarge plants.
Salable Powder. Densification is avoidedentirely for those outlets willing to use suitablystabilized powder. Spherical particles, up to ca.0.3 mm diameter, have been available for manyyears, but in most applications pellets are pre-ferred because of their better housekeeping (spil-lages, dust, etc.), easier handling in standardsystems, and absence of concern about possibledust explosions due to powder attrition. Emer-gence of coarse Spheripol powders fromHimont,1 – 5 mm diameter, should dispel some of theseconcerns provided adequate and nonseparating
stabilization systems are established with me-chanically robust coarse particles. This will en-tail some additional manufacturing costs, andperhapsmore product constraints, but this expen-diture should be less than that for capital andenergy intensive extrusion and pelletization op-erations. Himont is constructing a PP plant inBrazil, due for completion in 1991, which isclaimed to have no extrusion facilities. However,these have subsequently been installed.
Additive Incorporation. Extrusion pro-vides an efficient and convenient way of incor-porating antioxidants and other modifiers intopolymers. Metered amounts are mixed with themain powder feed to the extruder and homoge-nized in the molten polymer. Good mixing isessential to prevent extrusion of unstabilizedproduct at high temperature. Nitrogen blanketingis common practice. Highly specialized and low-tonnage formulations are either made on a sepa-rate plant having appropriately sized equipment,or the customer himself adds concentrates in theform of stabilizer master batches.
Liquid additives, such as some antistats, canbe metered directly into the melt, given goodsubsequent homogenization in the extruder.
5.9. Additives
Stabilizers [44]. Like most hydrocarbons,PP must be protected against oxidation, particu-larly above 100 �C. The dominant reaction ischain scission by free-radical attack at the tertiarycarbon atoms of the backbone. This generateshydroxyl and carbonyl groups, accompanied byyellow discoloration and brittleness. Incorporat-ing radical scavengers considerably delays theonset of embrittlement and increase of the MFI.Antioxidants are always incorporated into PPbefore or during extrusion at manufacturingplants. Usually a peroxide decomposer, such asa phosphite, is introduced to improve melt sta-bility, together with hindered phenols to conferlong-term aging resistance. Typically, theamount of each of these additives is 0.05 –0.2 wt%, but in more aggressive environmentsthe levels might rise to 0.5%, with thiodipropio-nate esters as peroxide decomposers. The choiceof stabilizer is influenced by the application,for example, interaction with spin finishes
398 Polypropylene Vol. 29
[45], resistance to extraction by hot water, andregulatory constraints.
Traces of copper or cobalt powerfully accel-erate thermal oxidation, especially at sustainedtemperatures above 60 �C. In these cases a che-lating agent such as a bishydrazone is added [46].
Continuous outdoor exposure also requiresprotection from the damaging effects of UVradiation. The severity of embrittlement in-creases at high temperature and with rainfall insome polypropylenes. Atmospheric pollutioncan deposit protective layers on the surface.Ultraviolet-absorbing compounds, such as al-koxybenzophenones and hydroxybenzotriazolesfor thick sections, and hindered amine lightstabilizers (HALS) for films, provide protectionat contents of 0.1 – 1.0 wt% [47]. Screeningwith some fine pigments or carbon black iseffective, but restrictive on appearance.
Gamma radiation, used for packaged food andsterilization of medical equipment, degrades PP,causing embrittlement. The effects can be miti-gated by the careful choice of stabilizer.
Antistats. The familiar static charges andresulting dust deposits on many plastics can beovercome in PP by incorporating 0.2 – 1.0 wt%of polar additives. Suitable materials, such aspolyether fatty amide and fatty amine conden-sates and glyceryl monostearate reduce the sur-face resistivity from > 1013 to ca. 107 W. Themechanism involves slow diffusion of the addi-tive to the surface where it picks up atmosphericmoisture. Hence, time and humidity are impor-tant. At 50% RH most of the recommendedadditives are satisfactory, but at 30% RH thefatty amine derivatives are advantageous. A fewdays storage at room temperature is sufficient todevelop the conducting surface layer.
Compositions containing moderate amountsof conductive carbon black are also effective, asare small amounts of polyacetylenes. The latter,and the incorporation of small amounts of finemetallic wire, are too expensive for PPapplications.
Antacids. Most of the earlier PP formula-tions contained 0.05 – 0.2 wt% of calcium stea-rate to prevent traces of acidic catalyst residuesfrom corroding customers’ equipment. It be-haves as a mild slip agent and suppresses attackofHCl on certain antioxidants. Zinc,magnesium,
and sodium stearates have also been used, andmore recently synthetic hydrotalcite (a hydratedmagnesium aluminum hydroxycarbonate). Low-er levels of such additives are still widely used.
Slip and Antiblock Agents are mainly usedin film production where high-speed handlingrequires a controlled reduction in frictionalforces. 0.1 – 0.5 wt% of oleamide, erucamide,glyceryl monostearate, etc., usually suffices. Fi-ne, spherical particles of silica incorporated at thepelletization stage prevent layers of film adheringto each other during storage.
Crystallization Nucleants. Although purePP has a final melting point of 176 �C, the meltsupercools by 50 – 60 �C before crystallizationsets in at ca. 115 �C. Small amounts of certainadditives [48] nucleate the melt, raising thecrystallization temperature by 10 – 15 �C.Smaller spherulites result, sometimes havingdifferent structures. Advantages of nucleatedgrades include shorter injection molding cycles,higher moduli, greater transparency, and higherheat distortion temperatures. Suitable additivesinclude dibenzylidene sorbitol and substitutedvariants, di-tert-butylbenzoate salts, sodiumben-zoate, finely divided talc, sodium diaryl phos-phates, and some pthalocyanine pigments. Thesematerials are somewhat selective in their effects,i.e., some are particularly good for transparencyimprovement, but less effective for modulusenhancement. Careful balancing of the overalladditives recipe is also essential because bothadverse and beneficial interactions occur be-tween some of the components [49].
6. Compounding
Compounding includes incorporating mineralfillers, glass fibers, elastomers, flame retardants,pigments, and carbon black. It is a specializedoperation dealing with relatively short runlengths in specified compounding extruders ormixers. For this reason, and for concern aboutcross contaminating unmodified PP grades, com-pounding plants usually operate separately frommain PP production plants.
Substantial property enhancements enablepolypropylene-based products to compete inquite demanding areas where unmodified PP
Vol. 29 Polypropylene 399
would be inadequate (see Table 6). Examplesinclude tough front and rear bumpers for cars,rigid coupled glass compositions for washingmachine tubs, dense grades for outdoor gardenfurniture, and temperature-resistant composi-tions for car under hood applications. Long-fiber-reinforced PP secures further increases intoughness and stiffness over short-glass-fibercompounds [50]. Fibers with a minimum lengthof between 0.8 and 3 mm, depending on thecoupling between matrix and fiber, are likely tobe satisfactory. These form an interlocking skel-etal structure which dissipates the impact energyover a large area of the molding.
The combination of lower production rates,specialized equipment, and dense additivesmeanthat PP compounds are always more expensivethan natural grades.
7. Properties
7.1. Homopolymer
Basic mechanical properties are largely influ-enced by molecular mass and molecular massdistribution (MMD), chain stereoregularity, andprocessing conditions, which introduce orienta-tion, strain, etc. Various additive packages mustbe tailored to the application to achieve good anddurable performance.
Molecular Mass. At low shear rates meltviscosities are proportional to the 3.4 power ofthe weight-average molecular mass. At the high-er shear rates found with most processing equip-ment, the dependence is still greater than a powerof 2 because much chain entanglement still ex-ists. As a result of this sensitivity, and the directrelevance to processing, the melt flow index(MFI) is always used to describe the viscositycharacteristics of each polymer grade. The de-pendence of melt viscosity on shear rate is alsorelated toMMDas a result of the strong influence
of very long chains. Hence MFI is often mea-sured at two different shear rates to give someidea of MMD. Increasing importance is beingattached to a more comprehensive rheologicalcharacterization [51, 52].
A typical, medium-flow polymer has an MFI(230 �C/2.16 kg) value of 3 dg/min, correspond-ing to a Mw of 460 000 and Mn of 54 000. Thepractical grade range of 0.3 – 50 MFI coversapplications from sheet and pipe extrusion tohigh-speed fiber spinning.
Heterogeneous Ziegler – Natta catalystsyield broad MMD polymers having Mw/Mn of5 – 10, the lower values being associated withthe more recent Solvay and supported catalysts.Very narrow MMDs are still made by preferen-tially cleaving long chains in an extruder, usuallywith the assistance of organic peroxides. Suchgrades, commonly referred to as controlled rhe-ology (CR) polymers, reduce extrusion andmolding orientation caused by high molecularmass species with their long relaxation times[53]. Note that reworked polymer is likely tohave a higher MFI and a correspondingly nar-rower MMD than the starting material.
Stereoregularity. Very high stereoregular-ity is now achievablewithmodern catalysts, evenwithout removing any soluble byproduct. Insome applications processing is helped by a smalllowering (detuning) of the isotactic index. This isreadily achieved by minor changes to the poly-merization recipe, and by introducing smallamounts of copolymerized ethylene.
Metallocene-Based PP. An expandedrange of products is iminent following the en-couraging metallocene developments. Furtherpilot trials and customer assessment of proces-sing, product, and economics is needed beforeprecise definition is possible. The advantagesalready seen include higher extrusion rates forfilms and fibers, enhanced clarity, absence ofoligomers, superior property balances, and the
ability to control further melting point reductionsby copolymerization without significantly in-creasing the low level of xylene-solublematerial.
Other Properties. Table 7 lists the proper-ties of several polymers measured by standardprocedures, with some copolymer data includedfor comparison. Caremust be taken in translatingthese exact values to fabricated products becauseproperties are dependent on built in stress andorientation. Increasing the molecular mass, i.e.,lowering MFI, raises toughness somewhat, butthe modulus and hardness decrease with thelower crystallinity.
Some general and electrical properties ofpolypropylene are as follows:
The final melting point of commercial PP liesin the range 160 – 170 �C, with purified poly-mer reaching 176 �C. However the safe upperworking temperature limit should be 100 –120 �C, depending on the stress, with shortexcursions up to 140 �C being permissible. Thiscomfortably extends the upper operating rangeavailable with high-density polyethylene by ca.20 �C, which has a similar room temperaturemodulus.
Favorable long-term creep characteristics un-der load permit useful increases in the stackingheight of PP crates or other pelletized containers.The repose angle of PP moldings on wood orcanvas, unpredictably 30% higher than for HDpolyethylene, is allegedly a useful stabilizingfeature in mechanical conveyors.
The crystalline and nonpolar nature of PPconfers good resistance to a wide range of aque-ous and polar media, including emulsifier solu-tions with their strong stress cracking abilities.Powerful oxidizing agents and highly aromatic orchlorinated solvents are too aggressive for safestorage in PP containers.
The ease with which homopolymers can bedrawn into high strength structures has beenheavily exploited in the fiber and film industries(see Section 8.3).
7.2. Copolymers
Random Copolymers. Incorporating 2 –7 wt% of ethylene into the main chain lowers
Vol. 29 Polypropylene 401
Table
7.Mechanical
andthermal
properties
ofPPOrigin
‘‘Propathene’’(w
ithpermissionofICIChem
icals&
Polymers)
Property
Method
Tem
perature
Homopolymer
Impactcopolymer
Random
copolymer
GSE16
GWM
22
LYM42
LZM
55/52
GWM
101
GWM
213
LXF301
Meltflow
index
(230
� C/2.16kg),
g/10min
(dg/m
in)
ISO
1133
1.00
4.00
12.00
33.00
6.00
2.00
7.50
Tensile
yield
stress,MPa
ISO
527
33.50
34.50
34.50
34.00
27.00
23.50
25.00
Flex.modulus,GPa
ISO
178(10mm/m
in)
1.45
1.50
1.50
1.55
1.15
1.00
0.85
Izodim
pactstrength,kJ/m
2ISO
180(0.25mm
notchradius)
23
� C4.50
4.50
4.00
3.00
9.50
nobreak
5.00
0� C
3.00
3.00
2.50
2.00
5.50
10.00
2.50
�20
� C2.00
2.00
2.00
3.50
7.50
�40
� C5.00
Instrumenteddropweight
impactstrength,J
ICImethod
23
� C7.00
10.00
10.00
0� C
6.00
11.00
2.00
�20
� C6.00
11.00
�40
� C9.50
Embrittlementtemperature,
� CICImethod
>23
>23
>23
>23
�20.00
�45.00
Rockwellhardness
ISO
2039/2,ASTM
D785R
scale
93.00
95.00
95.00
95.00
90.00
75.00
Vicat
softeningtemperature
(10N
force),
� CISO
306A,BS2782:120A
154.00
154.00
154.00
154.00
147.00
143.00
135.00
Heatdistortiontemperature,� C
ISO
75/A
&/B
A–1.8
MPa
65.00
65.00
65.00
68.00
60.00
50.00
46.00
B–0.45MPa
100.00
100.00
100.00
102.00
95.00
90.00
71.00
402 Polypropylene Vol. 29
the overall crystallinity, gives a broader soft-ening range with reduced melting points(Figs. 10 A, 10 B), increases the fraction ofsoluble polymer, and improves transparencyand surface gloss. These changes are dependenton the amount of comonomer and its distribu-tion along the chain. Table 7 shows how themodulus decreases for a random copolymercontaining ca. 3 wt% of combined ethylene.
Several applications take advantage of thelower melting point to make thermoweldablefilm. Ideally, it would be useful to have a lowmelting range without undue increases in theproportion of xylene-soluble residues. This isclaimed to be possible with butene copolymers,perhaps because the solubles are produced atsemi-exposed catalyst sites more easily accesibleto ethylene [40]. Some advantages are alsoclaimed for propene – ethylene – butene terpo-lymers [34, 39]. Butene helps the development oflow-melting g-triclinic crystallites. Random co-polymers are somewhat tougher than homopo-lymers, and do not exhibit the familiar stress-whitening behavior of the tougher copolymerimpact grades.
Impact Copolymers. Section 5.7.2 de-scribes block copolymerization systems for mak-ing high impact strength grades of PP. These are
often referred to as block copolymers, toughenedpolypropylene, or more pedantically heteropha-sic copolymers [34].
Essentially, these products are best viewedas an intimate dispersion of elastomer in a PPmatrix. The particulate rubber phase confersgood toughness, while the homopolymer matrixis responsible for retaining good high-tempera-ture performance and adequate stiffness. In thisrespect they are far superior to random copo-lymers, and become the first choice when im-pact strength and rigidity are called for. Inextruded pellets or molded articles the rubberycopolymer phase is well distributed in the formof droplets 1 – 5 mm in diameter. Such a struc-ture (Fig. 11) is produced during melting andrecrystallization. Powder direct from the reac-tor has elastomer permeating throughout theparticle in a more continuous form. Many ofthe complex interactions which control thestability of this phase structure and the resultingtoughness, await further study, but currentviews are very well discussed by VAN DER VEN
[43].Toughness, stiffness, and softening points are
given in Table 7 for two impact copolymerscontaining ca. 10 and 20% of elastomer. Gas-phase polymerization systems are able toincrease the rubber content to 40%, but the
Figure 10. A) DSC heating endotherm; B) DSC coolingexothermCourtesy of the ‘‘Propathene’’ Business, ICI Chemicals &Polymers
Figure 11. Electron micrograph of high impact copolymermolding after hexane surface wash
Vol. 29 Polypropylene 403
products are not available in large commercialquantities at present. These impact copolymersnaturally contain more soluble polymer thanhomopolymer, and this second phase reduces thetranslucency. Severe deformation causes somewhitening or blushing [54] in the stressed re-gions, particularly in very high impact grades. Inother respects the rules relating processing andproperties to MFI, nucleation, controlled rheolo-gy, stabilization, etc., are the same as for homo-polymer.
7.3. Elastomer Blends withPolypropylene
Originally, PP was toughened by blending withca. 10 wt% of polyisobutylene (PIB) elastomer.In some situations the same technique is usedtoday, with ethylene – propene rubber (EPR) orethylene – propene – diene rubber (EPDM) re-placing PIB.
As rubbers are incompatible with PP, consid-erablemechanicalwork is necessary to secure thevital fine dispersion of elastomer. Good blendingalso requires a reasonable viscosity match be-tween the two components. Common practice isto use a two-stage process in which the elastomeris first compounded with PP to give a 50 –70 wt% masterbatch of the rubber as free flow-ing granules. Such masterbatches can be pur-chased at extra cost. Good mixing devices areessential for the final compounding step with PP.Twin-screw extruders or high-speed mixing de-vices perform satisfactorily.
Well-prepared blends have physical proper-ty combinations only just short of those ofdirectly polymerized impact copolymers. How-ever, the latter are generally preferred on ac-count of consistency and grade versatility. Elas-tomer blending is also part of the broad spec-trum of compounds referred to in Section 14.Here they are used to uprate the toughness ofhomopolymer and impact copolymers in thesecomplex formulations. Some manufacturers al-so introduce HD polyethylene, in addition torubber, to secure further toughness improve-ments. In these cases, electron microscopy re-veals that in well-dispersed systems the poly-ethylene is always located inside and towardsthe center of these rubber droplets. This encap-
sulation of PE makes it behave more like rubbertowards the matrix, thereby increasing tough-ness at the expense of modulus in such ternaryblends.
8. Uses
Polypropylene is readily processed in conven-tional equipment used for other thermoplastics.Injection molding, commonly using screw-typereciprocating machines, accounts for 40 – 50%of all applications. Extrusion processes accountfor the remainder with domination by fiber andfilm.
A more detailed picture is obtained by ana-lysing the North American 1996 sales data (in103 t/a) listed in the following [56] :
Blow molding
Containers 74
Others 5
Total blow molding 79
Extrusion
Coatings 5
Fibers and filaments 1463
Film (up to 0.25 mm)
Oriented 503
Unoriented 111
Pipe and conduit
Sheet (over 0.25 mm) 110
Wire and cable 5
Others 43
Total extrusion 2240
Injection molding
Appliances 139
Consumer products 565
Rigid packaging 523
Transportation 186
Others 131
Total injection molding 1544
Export 503
All others 1177
Total 5543
8.1. Injection Molding
An exceptionallywide range of injection-moldedproducts stems from the rigidity, toughness, andchemical and temperature resistance of PP. Ex-amples include automotive trim and ventilationcomponents, bottle crates, industrial containers,washing machine tops and tubs, kitchenware,tool handles, domestic waste systems, and small
404 Polypropylene Vol. 29
boat hulls. Such a variety of articles requirescareful grade selection according to the requiredimpact strength of the product and the melt flowconstraints of the processing equipment. AnMFI (230 �C/2.16 kg) range of 2 – 20 dg/minmeets most needs. Increasing the MFI not onlyassists mold filling and reduces cycle times, butit helps manufacture of complex moldings hav-ing high flow ratio geometries (flow ratio is theratio of longest path to the section thickness).The inevitable fall in impact strength at highMFI can be recouped, in some cases, by movingfrom homopolymer to impact copolymergrades.
While molding conditions vary with the sizeand complexity of the article, the followingmachine settings will cover most products [53]:
Mold temperature: 40 – 80 �C, lower still for thin walled items
The low moisture absorption of PP largely dis-penses with any need to dry granular feedstock.Neither do gaseous or carbonaceous decomposi-tion products form in extruder dead spots. In-stead, polymer remaining there becomes pro-gressively more fluid through chain scission.Machine shot weight capacities normally referto performance with polystyrene, i.e., the poly-styrene yardstick figure. The lower melt densityof PP (0.75 g/cm3) lowers the machine’s weightcapacity by ca. 25% when molding PP. Plasti-cizing capacity is slightly greater than for poly-ethylene, but is lower than for amorphous poly-styrene. Machine outputs with PP benefit fromthe ability to eject moldings at higher tempera-tures than with their lower softeningcounterparts.
Linear post-molding shrinkage of PP is 1 –2%, of which 85% occurs within the first 24 h.Distortion in molded products reflects internalstress caused by nonuniform cooling and poly-mer orientation. The latter is especially pro-nounced in surface skins where the viscous meltstretches as it is pulled along by the advancingmelt front adjacent to cool walls. Controlledrheology (CR) polymer, containing fewer of theorientation-prone long chains, reduces this typeof warping [53].
An important practical aspect of PP concernsits ability to form strong integral hinges, such as ina lidded box, where the lid is permanently at-tached to the base by a thin web of polymer alongthe whole of one edge (Fig. 12). This web, about0.25 – 0.6 mm thick, can be produced directly inthe molding process or by post forming opera-tions. An initial flexing of the hingewhile slightlywarm induces the correctmolecular orientation topermit repeated opening, even at subzero tem-peratures. In the laboratory, such hinges havewithstood 23�106 flexes without failure [57].
Various types of structural foamed moldingscan be produced having a cellular core sand-wiched between solid integral skins giving anoverall density in the range 600 – 800 kg/m3
[58]. These lightweight, but rigid parts can varyin size from a paint brush handle to a sack palletor car body panel. Foaming results from using atumble-blended feedstock of granules and achemical blowing agent. Alternatively, high-pressure nitrogen can be injected into the melt.Molding techniques are altered to achieve ex-tremely rapid polymer injection rates into themold. This prevents premature foaming beforethe solid skin forms. Foaming in the core arisesby allowing the melt pressure to fall rather thanmaintain the customary pressure hold to keep themold full.
Four alternative foam processes exist:
1. In the ICI sandwich molding process, differ-ent polymers are used for the skin and core by
Figure 12. Integral hingeCourtesy of the ‘‘Propathene’’ Business, ICI Chemicals &Polymers
Vol. 29 Polypropylene 405
using two separate injection barrels. As theskin is unfoamed, high-quality surfaces result.
2. In the Varitherm Process (BASF, KraussMaffei), improved surface finish is achievedby using a hot mold (100 – 120 �C) followedby rapid cooling after a short shot of the melt(i.e., an injection of polymer insufficient to fillthe mold).
3. In the counterpressure method the mold ispressurized prior to filling to prevent degas-sing at the melt front.
4. In hollow section molding (e.g., CINPRES)nitrogen is injected in a continuous streaminto the mold at the same time as the polymermelt. Under appropriate conditions, hollow orribbed moldings are produced when gas flowspreferentially down thick, hot sections of themolding. This technique is of increasinginterest.
8.2. Blow Molding
The extrusion blow molding process for PPcopies that already well established for low- andhigh-density polyethylene.However, applicationto PP has been slow while suitably viscous andtough polymers were developed at attractiveprices. At present, the important areas are smallerpackaging containers which take advantage ofgood transparency, good form stability up to140 �C for hot filling or sterilization, and greaterfreedom fromenvironmental stress crackingwithaggressive products. Brake fluid reservoirs ex-emplify this. Very large containers are dominat-ed by high-density polyethylene.
Extrusion blow molding employs two linkedprocesses [59] in which an extruded moltenparison is transferred to a mold for inflation withlow-pressure air at 0.4 – 1.0 MPa. High molec-ular mass polymer, MFI(230 �C/2.16 kg) 0.4 –2.0 dg/min, combined with low melt tempera-tures at the die of 200 – 210 �C, provide a suit-ably stiff melt. If the melt stress in a heavy PPparison exceeds ca. 20 kPa, then the resultingtension thinning will be troublesome as regardsparison stability and uniform wall thickness.
Injection blow molding replaces the extrudedparisonwith an injection-molded preform.Blow-ing takes place in a secondmold whichmaintainsthe high-definition neck of the preform. Thisprocess eliminates scrap and makes it easier to
producewidemouth containers having improveddimensional accuracy and appearance.
Injection stretch blow molding is a furtherrefinement which introduces a plunger stretchingstage to elongate a conditioned parison or pre-form just before the final blow (see ! Plastics,Processing, 1. Processing of Thermoplastics).Better mechanical properties and enhancedtransparency stem from partial orientation. Thistechnique is sometimes referred to as the meltphase process.
In stretch blow molding, the extruded ormolded preform is carefully conditioned to justbelow the polymer melting point before stretch-ing. Axial stretching of the preform with simul-taneous or almost immediate blowing in themoldproduces a very high quality container. Correctlycarried out, this process secures increased rigidi-ty and toughness, high transparancies, and re-duced permeabilities as a direct result of signifi-cant amounts of orientation. The more complextechnique increases capital and operating costs,but it is of growing interest [60] and is alreadybeing applied extensively to biaxially orientedPETP bottles.
Coextruded parisons can be used as precursersfor blowmolding when special barrier propertiesare required [61]. In particular, a central layer ofethylene vinyl alcohol copolymer (EVOH) toreduce oxygen diffusion, combined with mois-ture protective outer layers of propene – ethyl-ene random copolymer, give containers suitablefor storing oxygen-sensitive food products.
8.3. Fibers and Flat Yarns
Very large amounts of polypropylene homopo-lymers are used for fiber manufacture in Europeand the United States. These applications exploitthe wide range of physical forms, includingincreasing amounts of versatile nonwoven fab-rics [62–64]. Monoaxial orientation can be ap-plied to conventional spinneret yarns, as withpolyamides and polyesters, and to flat tapesmadefrom extruded film. These differ in the followingrespects (Table 8).
SpinneretTypeYarns. Heremelt is extrud-ed through a die plate perforated by many smallholes to generate individual thread lines. In thelong spin process [65] (see ! Fibers, 7. Poly-
406 Polypropylene Vol. 29
olefin Fibers, Section 1.2.2), which has integralspinning and finishing stages as well as out of linedrawing options, theremay be 50 – 250 holes perspinneret.An air cooling gapof 2 – 5 m is neededbetween the die plate and the wind-up roll. Linespeeds up to 1000 m/min at the spinning stage,increasing to 3000 m/min during solid drawingover hot rolls, call for complex and expensivehaul off, drawing, and wind-up sections. Productmay be packed off as continuous yarn, tow, orstaple. Suitable polymers have MFI(230 �C/2.16 kg) in the range 12 – 25 dg/min, with nar-rowermolecularmass distribution grades offeringsome advantage with such high rates of meltdraw. These are high-throughput plants, bestsuited to long runs of a single grade of product.
In a variation of this technique [66], theintegrated spunbonded process (see ! Fibers,3. General Production Technology, Chap. 6),yields a continuous bonded mat of partiallyoriented yarn as a consequence of some draw inthe venturi type haul off operating at up to5000 m/min. There is no further drawing as thefiber is collected as a mat on the take-off belt forconversion into nonwoven fabrics. These areused, for example, in geotextiles, furnishings,carpet backing, etc. The balance of drawdownrate and orientation in this process is influencedby the polymer molecular mass distribution.
Figure 13 depicts a compact shortspin pro-cess for staple production using a die containingca. 40 000 holes. Cooling air jets freeze the meltwithin about 20 mm so that Godet haul off rollscan be placed only 1 m away from the die.Orientation is achieved by the in-line stretchingof this tow in an air oven, followed by choppingin line to produce staple [61]. Broad molecularmass distribution polymers give the necessarymelt strength, while MFI(230 �C/2.16 kg) re-quirements increase from 7 to 20 dg/min as the
filaments become finer. These find use in geo-textiles, carpet face fiber, and diaper cover stockapplications. A useful feature of PP fabrics madefrom fine fibers is the ability of underwear gar-ments to reduce claminess by transferring mois-ture away from the skin to an outer absorbentlayer. Polypropylene based fabrics are also usedfor sports clothing and socks, but the lowmeltingpoint of PP calls for special attention to the heatsetting of the average domestic iron.
In the third, and most recent production pro-cess (Fig. 14), air is blown through a very fluidpolymer melt maintained at high temperature toassist chain scission in the spinning process itself[66, 67]. Polymer MFI(230 �C/2.16 kg) at thenozzle can range from 35 to 300 dg/min or high-er. The specially designed die, having a row ofnozzles along its width, sprays a stream of short,
Table 8. Polypropylene fiber processes
Process Filament count, tex* Product
Long spin 0.2 – 3.0 high-tenacity monofilament; drawing integral or separate; high output
BCF yarn 0.2 – 2.0 special case of long spin making only bulked continuous fiber
Spunbonded 0.2 – 2.0 venturi haul off; no 2nd stage draw. bonded mat output
Shortspin 0.2 – 40 compact unit; tow stretched and cut in line for staple
Melt blown 0.002 – 0.02 low orientation, very fine fiber; only bonded mat output
Fibrillated yarn 110 – 500 oriented slit film; fibrillated for baler twine, rope, etc.
Weaving tape ca. 110 nonfibrillated slit film for carpet backing, sacks, etc.
Strapping tape 500 – 1000 thick, oriented tape as a steel alternative
* tex ¼ weight in grams of 1000 m of yarn; equivalent cylindrical fiber diameter in mm ¼ 37ffiffiffiffiffiffitex
p.
Figure 13. Shortspin processa) Spinneret; b) Quenched fibers; c) Air-jet coolingCourtesy of the ‘‘Propathene’’ Business, ICI Chemicals &Polymers
Vol. 29 Polypropylene 407
low-orientation fibers onto a moving belt wherethey are bonded together and wound onto a roll.This process produces nonwoven fabrics whichare of increasing importance for industrial wipesand surgical wrap, and for a variety of filtrationapplications, such as face masks made frombonded mats of very fine fibers.
Flat Yarns [68] (see also! Fibers, 7. Poly-olefin Fibers, Chap. 2). In this process (Fig. 15),extruded film is the basis of a cheap and differenttype of product to that achievable with filament-based yarns. Melt from a conventional slot die israpidly quenched with cold water or water-cooled steel rolls to give low- crystallinity film,50 – 250 mm thick. Razor blade type knives thenslit this broad film into strips 5 – 20 mm wide,which are oriented in a hot air oven at 120 –180 �C by drawing between Godet rolls at ratiosin the range 5 : 1 to 10 : 1. Both film thicknessand width diminish in this operation by approxi-mately the square root of the draw ratio.
Fibrillated Tape (see also ! Fibers, 7.Polyolefin Fibers, Chap. 2). While flat yarns arequite strong in the direction of draw, they are veryweak in the transverse direction. With normalhomopolymer, simply twisting high-draw-ratiotape induces spontaneous fibrillation. A web of
irregular fine fibers is formed as the film splitsdown its length. The resulting product resemblesa coarse sisal twine, for which it is a goodsubstitute. Such fibrillated yarns can be tailoredto specific applications by controlling draw ra-tios, thickness, and polymer. Nowadays, pinfibrillation, in which a corotating spiked rollcontacts the film just after slitting, offers moreprecise control over this step. The products arefamiliar as baler twine, string, and some ropes.
Weaving Tapes. High strength tapes, with-out fibrillation, are made by controlling extru-sion, orientation, and polymer type. Introductionof finely divided chalk, together with some poly-ethylene, usually as the chalk masterbatch, isparticularly effective in reducing splitting. Im-pact copolymers are amore expensive, and rarelyused, alternative to homopolymer for thisapplication.
Both tapes and fibers contain residual stressfrom the extrusion and orientation processes.This results in linear shrinkages of up to 8%immediately after manufacture, but after storagefor a few days at room temperature the value fallsto about 2.5%. For those applications, such asweaving tapes for carpet backing, where zeroshrinkage is often needed, a heat setting stage isessential. Out-of-line storage of the tapes onbobbins at 130 – 135 �C for several hours oftensuffices.
Thick, 0.3 – 0.9 mm, oriented tapes are es-tablished alternatives to steel bands in manystrapping applications. They are generally madeby the water quench process operating at drawratios of 9 – 10. Thicker tapes are made fromfoil or individually extruded film tapes. Highmolecular mass, MFI(230 �C/2.16 kg) 0.4 –2.0 dg/min, homopolymers and block copoly-
Figure 14. Melt blowing processa) Extruder; b) Gear pump; c) Heated die; d) Hot air manifold;e) Collector; f) Take-off roll
Figure 15. Flat tape/yarn production unita) Wind up frame; b) 3rd Godet stand; c) Hot air annealing oven; d) 2nd Godet stand; e) Hot air orientation oven; f) 1st Godetstand; g) Simple film costing unit; h) Extruder; i) Filmdie; j)Water- cooled chromium-plated rolls orwater bath; k) Slitting unit;l) Starting drum; m) Individual precision flangeless bobbin wind upCourtesy of the ‘‘Propathene’’ Business, ICI Chemicals & Polymers
408 Polypropylene Vol. 29
mers with added chalk and polyethylene largelysuppress fibrillation. A further precaution here isto emboss the finished tape with a diamondpattern. A useful aspect of PP strapping tapes istheir higher elongation and elastic recoverywhich helps them to remain tight on packagesprone to shrink or settle.
8.4. Film
Packaging film is an important outlet for PP,accounting for about one sixth of polymer salesin Europe, and rather more in Japan (see Sec-tion 6). By far the major proportion of this isbiaxially oriented PP (BOPP), an establishedreplacement for regenerated cellulose. Althoughthe equipment needed for orientation is complexand expensive, it secures superior physicalproperties.
Good clarity in unoriented film is dependenton very rapid quenching of the melt to reducespherulite size. The chill roll extrusion process(see ! Films, Section 2.2.2) for making 30 –250 mm film extrudes a mobile, hot melt from aslot die onto water-cooled, highly polished steelrolls. Extrusion temperatures of 240 – 270 �Ccombinedwith anMFI(230 �C/2.16 kg) of about7 dg/min minimize surface irregularities in themelt. Random copolymers containing 2 –3 wt% of combined ethylene enhance tough-ness, clarity, and gloss, with some loss of modu-lus. These soft-feel transparent films are used astextile packages, bakery wraps, and documentsleaves at thicker gauges.
Tubular quenched (TQ) film made in a down-wardly extruded bubble (Fig. 16) uses someexternal water cooling which achieves reason-able clarity in film for packaging. In contrast, theless common tubular film from the air- cooledupwardly extruded bubble ismore hazy, but findsuse in the production of weaving tapes (Sec-tion 8.3). For most film applications, slip andantiblock additives are needed.
Biaxially oriented polypropylene (BOPP)film [69, 70], typically 12 – 40 mm thick, hasexcellent transparency, toughness, and stiff-ness, and is widely used in cigarette and biscuitpackages, potato chip bags, and carton over-wrap. Film modulus is three times that of theunoriented cast product. These complex filmmanufacturing plants are usually proprietary
systems based either on a bubble process, or ona flat die (stenter) process. In the bubble process(Fig. 17), thick cast tube is reheated and thenblown into a bubble with simultaneous stretch-ing in the transverse and machine directions toproduce a balanced film. The stenter process(see ! Films, Section 2.3.2) uses thick castsheet which is reheated and conditioned fordrawing. Longitudinal and lateral stretching areperformed separately and can be adjusted tosecure the required orientation balance. Thetypical area draw in these processes is 30 : 1to 50 : 1.
It is impossible to heat seal such highlyoriented material because unacceptable shrink-age then occurs. Low-melting surface coatingsovercome the problem by reducing the sealingtemperature to a level which the films can safelywithstand. Suitable materials are vinylidenechloride copolymers, polyethylene, poly(1-bu-tene), as well as butene, propene, and ethyleneco- and terpolymers. Some of these may beapplied as aqueous emulsions, but common
Figure 16. Tubular quenching processa)Melt from extruder; b) Die; c) Air ring; d) Air flow; e) Film;f) Water quench weir; g) Cooling skirt; h) Nip rolls;i) Dewatering, slitting, wind up etc
Vol. 29 Polypropylene 409
practice is to coextrude the low-melting poly-mer as an outer layer of the cast tube or sheet.After orientation, this layer can be less than1 mm in thickness. Thesemore compatible poly-olefin coextruded coatings also simplify scraprecovery and recycling without fear of the coat-ing instability inherent in halogenated poly-mers.
Polypropylene film, however made, is notreceptive to printing inks and adhesives becauseof the low polarity. Mild surface oxidation bycorona discharge treatment overcomes theseproblems.
8.5. Foil and Sheet
Slowheat transfer rates in sheet and thick foil callfor extended cooling times at the extrusion stage.Foil, up to about 0.7 mm thick, can be made bythe cast tube process (Section 8.4), but the moreversatile three roll stack system (Fig. 18) is ableto produce sheet with thicknesses of 0.3 –15 mm. Recent techniques involving additionaland larger diameter rolls extend this range to40 mm. Thicker sheets are made by compressionmolding of granules or large molded slabs, andby injection – compression molding. This canmake 2 m square slabs with thicknesses up to240 mm. High molecular mass homo- and co-polymer,MFI(230 �C/2.16 kg) ca. 0.3 – 1.0 dg/min, provide higher dimensional stability inslow- cooling, thick-section melts whose tem-perature at the die exit is ca. 210 �C. Higherflow grades with MFI(230 �C/2.16 kg) of up to5 dg/min may be used for thin foil, depending onthe available equipment.
Commercial sheets, normally up to 2 m wideprovide stock for fabricating into welded tanksfor storing corrosive liquids. They are also usedas linings for steel and concrete containers, forducting construction, and as an inert constructionstock for chemical plant. It is possible to bondwoven glass or polyester fabric to the heatedsurface for subsequent further strengtheningusing traditional glass fiber lamination techni-ques. Punched sheets can be biaxially oriented togenerate heavy duty meshlike structures for geo-textiles [71].
Thick sheet, as made, has a nonuniform struc-ture across its thickness as a result of consider-able differences in cooling rate.
8.6. Extruded Pipe
Compared with polyethylene, tubing applica-tions for PP are quite small. However there areseveral areas where PP’s increased resistance to
Figure 18. Three roll stack extrusiona) Extruder; b) Slot die; c) Polishing stack; d) Nip rolls;e) Wind up
410 Polypropylene Vol. 29
aggressive media and its better performance atelevated temperatures are advantageous.
Extrusion at ca. 225 �C is straightforward; acentral torpedo is used to make the hollow pipe[72]. Homogeneous melts require an extruderhaving a length-to-diameter ratio of 25 : 1 anda compression ratio of 4 : 1 to reduce the risk oftrapping air in the extrudate. Pipe up to 300 mmoutside diameter is generally vacuum sized in awater bath: semi-molten pipe passes through afinely perforated steel tube having vacuum ap-plied to its external surface. Larger pipes aremade by pressure sizing where nitrogen or air isinjected via a small capillary into the torpedo toexpand the pipe to the sizing die form. Bothhomopolymers and impact copolymers are usedin the MFI(230 �C/2.16 kg) range 0.3 to 2.0 dg/min, increasing to 4.0 for drinking straws andballpoint pen tubing.
Copolymers, providing superior impactstrength with some flexibility, find use in domes-tic waste systems. Homopolymers have higherresistance to stress. It is essential to have a heavyduty stabilization system which will withstandextraction for applications involving aggressiveand hot environments, such as chemical plantsand effluent pipes. Manufacturers offer guidance[72] on polymer selection and pipe design con-forming to the various pressure classes inBS 4991, and to BS 52 254 and BS 5255 fordomestic waste pipes and fittings. Creep data areavailable [72] for pressure pipes predicting per-formance for up to 50 years.
Pipe jointing uses all the common techniques,with the exception of solvent welding. Ring sealswith rubber inserts allow some movement indomestic waste pipes. Also used are flangedjoints, hot plate butt welding, and socket fusionwelding, the latter being optionally supplemen-ted with hot-gas bead welding. Compressionjoints involving copper alloys should not be usedat temperatures exceeding 60 �C because ofreduced life expectancy.
9. Environmental Aspects
9.1. Recycling
The structure and properties of PP make it wellsuited to recycling operations. There is no fearabout cross-linking, nor of complications with
plasticizers or chlorine-rich species.Re-extrudingPP usually lowers itsmolecular mass and narrowsthe molecular mass distribution, particularly inaged feedstock. This can be helpful in someapplications that require enhanced flow. Adversefactors which must be assessed include some lossof strength from reprocessing/ageing, oxidationstability, discoloration, and contamination.
PP Manufacturers. Manufacturers usuallyproduce small proportions of PP which do notmeet their full quality requirements. Examplesinclude deviations in MFI/MMD, antioxidantlevels, copolymer characteristics, and gels. Pro-vided these faults arewithin predetermined limitsin the quality protocol, the material may berecycled in small proportions, sometimes withfurther corrective adjustments to the composi-tion. This operation calls for additional work, andcan reduce plant output by having to process thepolymer twice. For this reason, and in those caseswhere recycle is not permitted or the deviationsare too large, manufacturers either reclassify theproduct to a lower grade, or sell to an outsidecompounder for further reprocessing. This mini-mizes disturbances to continuous manufacturingprocesses.
A second source of much lower grade materi-al, arises when manufacturing conditions areseverely disturbed. This can be due to instrumen-tal and human error, blockages, equipment fail-ure, or extrusion disturbances. In these casesscrap can vary from substandard reactor powderto blocks of polymer from extruder dumps. Ex-ternal reprocessors convert such products intosurprisingly useful materials by blending andrecompounding. To them, the availability ofessentially pure PP is the prime requirement.
Plastics Converters. Turning PP intomold-ings, pipe, film, etc., invariably involves somescrap production. Apart from substandard arti-cles, there are sprues from moldings, edge trimfrom film and sheet, parison waste, and stampedsheet from thermoforming. These are recoveredfor recycling by the converter himself if theapplication allows, or more often by an outsidecompounder. In favorable situations involvinglong runs of a standard product, the convertersegregateswaste, grinds it, and then recycles it byblending small amounts with virgin feedstockwhen specifications permit. Occasionally there is
Vol. 29 Polypropylene 411
an undemanding application which can acceptlarge proportions of such regrind without diffi-culty. Storage and segregation problems are re-duced when the converter simply collects hisscrap together in the form of regrind for sale toa compounder.
TheBritishPlasticsFederation (BPF)estimates[73] that there are about 60 reprocessing compa-nies in the United Kingdom handling at least150 000 t/a of polymer. Forty three of these pro-cessanestimated25 000 t/aofPP[74].Onlya fewof these also process recovered scrap into moldedproducts such as horticultural, highway, safety,and street furniture, which do not make excessivedemands on polymer behavior. Included in theestimated figure are 15 000 t of recovered usedbattery cases and crates which can be convertedinto quite respectable compounds.Most reproces-sors only handle single polymers, not mixtures.Their main supply sources are plastics convertersand industrial or commercial establishments assuppliers of fairly well defined PP scrap [75].
Recycling PP After a Full Service Life.Nearly half the thermoplastics used in Europe areconverted into packaging [76]. In the UnitedKingdom 13% of this is based on PP, involvingan estimated 142 000 t of polymer in 1988.
Polymers used in United Kingdom for pack-aging in 1988 (in 103 t) broke down as follows[76]:
Low-density polyethylene (LLDPE and LDPE) 510
High-density polyethylene 255
Polypropylene 142
Polystyrene 78
Poly(vinyl chloride) 68
Poly(ethylene terepthalate) 53
Expanded Polystyrene 21
Total 1127
Inevitably, much of this has only a short lifebefore it ends up as waste in some form. Domes-tic waste contains 5 – 7 wt% (up to 20% byvolume) of such products [74], very little ofwhich is currently recovered. Public concernabout the environment naturally includes highlyvisible plastic litter, which is made unduly prom-inent by its low density and durability. This is anadditional factor accelerating legislation for re-covery of all types of packaging, including glass,metal, paper, as well as plastic [77–79].
In the case of PP, notable success rests withrecycling old plastic bottle crates and batteryboxes in Western Europe and the United States.This is mainly attributable to the ease with whichlarge amounts of a clean single polymer can becollectedwithout cross contamination fromotherplastics. Cooksons Plastics [80] and others [81,82] have expanded on lead recovery to retrievePP battery cases themselves. A mechanised sort-ing system has to separate stones, metal, wood,string, ebonite paper, and residual acid. TheCookson process involves crushing, severalstages of washing, flotation, and screening fol-lowed by drying and extrusion. The final pro-ducts include good-quality PP capable of beingused in automotive products and for certain typesof battery cases. Success here rests in qualityupgrading by adding elastomer to enhance tough-ness, stabilizers, and antacid incorporation torecoup oxidative stability, and molecular masstailoring with peroxides for specific applications.Hoechst [83] plan to collect plastic waste fromscrap cars, household appliance industries, andpresorted packaging waste. The 5000 t/a plantnear Cologne should be in operation by mid-1992. ICI studies on scrap recycling fromused PPcrates stress the need to incorporate additionalstabilizer in weathered products, and to allow forsome reduction in impact strength for seven-year-old crates.
Recovery and reuse of mixed plastics is moredifficult. The real need is to develop applicationsand fabrication processes for such comingledpolymers, which are essentially incompatibleand contaminated. Generally, they will be chan-neled into some massive products, such as posts,construction boards, and other bulky structureswhere resistance to water and decay is moreimportant than high inherent strength. Some 20different European schemes aim to make re-cycled plastic components as wood and concretereplacements for agricultural applications [77,84]. Comingled scrap can be upgraded by blend-ing with other recycled polymers to assistprocessing.
9.2. Pyrolysis
PP can be cracked to liquid hydrocarbons bythermal pyrolysis at 400 – 550 �C. In a recentprocess developed in Japan [85], zeolite catalysts
412 Polypropylene Vol. 29
enable this temperature to be reduced to 200 –420 �C by using a two-stage heating process.Other polyolefins, such as polyethylene andpolystyrene, and some chlorinated plastics canalso be pyrolized to liquid products. Process costestimates are $ 235 per ton of plastic, whichyields 0.5 m3 of gasoline, 0.5 m3 of kerosene,and diesel products. Plants are expected to bebuilt in Taiwan and Korea in 1991.
9.3. Incineration
Municipal waste is normally disposed of inlandfill sites, with incineration being the nextmost important method [86]. As suitable landfillsites become rarer and more expensive, pressureis exerted to reduce waste volume either byincineration or by scrap recovery.
Modern incineration technology, with energyrecovery in the formof electricity generation, cantake advantage of the high calorific value ofthermoplastics. Municipal waste calorific valuesaverage 10 kJ/g, compared with 30 – 35 kJ/g forcoal. At 44 kJ/g, PP and other polyolefins havethe same calorific value as fuel oil. Moreover, PPhas an exceptionally low sulfur content, makingit a very clean fuel with no apprehensions abouttoxic flue-gas emissions. These incinerationplants are costly to build. Even after allowingfor the sale of electricity, the net disposal cost inthe United States is estimated at 40 – 70 $/t ofprocessed waste [78]. Special burners have beendeveloped to handle atactic and isotactic wastePP feedstock, but more profitable outlets are nowavailable.
9.4. Environmental Interactions
Overall PP manufacture, like any other product,involves some interaction with the environment.Various essential inputs, such as raw materialsand services need to be considered along with allthe plant output streams.
Inputs. The principal input for PP is purepropene, with lesser amounts of ethylene forcopolymer grades. Energy is required to manu-facture these monomers from petroleum feed-stocks, and still more is needed for conversioninto polymer. At present, most of this power
comes from burning fossil fuel which then dis-charges carbon dioxide and water vapor to theatmosphere. Table 9 compares the overall ener-gy input needed to make various raw materials.This can be expressed either as the weight of oilto produce this energy, or in terms of joules perunit volume. Both values include energy formonomer production, but exclude the hydrocar-bon content of the monomer, which is trans-formed into polymer without combustion. On aweight basis, polyolefin energy requirements arebroadly similar to those of most alternative rawmaterials. On a more realistic volume basis, thestrength and lightness of plastics gives energysavings. Usually this is enhanced further in thefinal article where, for example, thin and strongplastic bottles show savings over their thickerglass counterparts.
Propene and ethylene monomers themselvesare nontoxic asphyxiant gases whose flammablenature calls for the usual extensive precautions.Ignition by static electricity is possiblewith high-pressure leaks and accidental discharges.
Autoxidation of hot, unstabilised PP powderin storage silos containing air is another knowncause of fire.
Effluents. Although catalyst componentsconstitute only a very small proportion of plantmaterials, both their manufacture and disposalinvolve noxious substances. Titanium tetrachlo-ride is used for all catalysts, sometimes in con-siderable excess for high-activity systems.Whereas expensive recycle and recovery stagesmay be installed as capacities increase, some
Table 9.U.K. energy requirements for the manufacture of natural and
synthetic materials [86]
Material Energy requirements
TOE*, overall
conversion
kJ/cm3
Aluminum 5.6 661
Tinplate 1.3 427
Copper billet 1.2 469
Glass bottles 0.5 46
Paper and board 1.4 50
Cellulose film 4.4 293
Low-density polyethylene 1.2 51
PP, old process, with extrusion 1.5 61
PP, new process, with extrusion 1.3 52
*TOE ¼ tonnes of oil equivalent to the energy requirement for
overall conversion.
Vol. 29 Polypropylene 413
remaining complexed residues will require dis-posal. Often, this involves hydrolysis with waterto generate corrosive HCl and titanium dioxidesuspensions. After neutralization with alkali,these slurries are either discharged as aqueouseffluent or settled out and used in landfill. Someincineration is planned. Disposal facilities alsoneed to be capable of receiving occasional faultycatalyst batches. Older plants still using catalystextraction processes simply use aqueous slurrydischarges, or dump concentrated residues fromthe base of stills.
Lower aluminum alkyls are pyrophoric, reactviolentlywithwater, and are intensely aggressivetowards exposed skin. Safe handling usually callsfor full protective clothing. Disposal can be byburning to generate alumina, or quite often, bycontrolled hydrolysis with water after deactiva-tion with alcohols. Some alkyl manufacturingplants will accept returned waste alkyls for dis-posal in their own dedicated facilities.
Atactic PP. Older plants needed to disposeup to 5% of their output as low molecular massatactic residues from solvent recovery streams.Landfill was a common outlet, with some incin-eration. Useful low- cost applications appearedin adhesives and roofing felt compositions, whichnow consume nearly all of the increasingly rareatactic PP byproduct.
Gaseous Effluent. Traditional flare stacksreceive most of the voluntary and involuntaryprocess gaseous discharges. Normally, thesecomprise small flows from distillation columnpurges which can be dealt with by screened lowlevel flares. These reduce the visual impact offlares which some people find objectionable, es-pecially at night. Some nitrogen purges contain-ing traces of hydrocarbons are discharged to theatmosphere. Scrubbing systems and carbon ad-sorption beds reduce hydrocarbon emissions, butonly a fewmanufacturers use them because of thecost.
References
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2 J. P. Hogan, R. L. Banks in R. B. Seymour, Tai Cheng
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