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PLASTICS ENGINEERING

Founding Editor

Donald E. HudginProfessor

Clemson UniversityClemson, South Carolina

1. Plastics Waste: Recovery of Economic Value, Jacob Leidner2. Polyester Molding Compounds, Robert Burns3. Carbon Black-Polymer Composites: The Physics of Electrically Conducting

Composites, edited by Enid Keil Sichel4. The Strength and Stiffness of Polymers, edited by Anagnostis E. Zachariades

and Roger S. Porter5. Selecting Thermoplastics for Engineering Applications, Charles P. Mac-

Dermott6. Engineering with Rigid PVC: Processability and Applications, edited by I. Luis

Gomez7. Computer-Aided Design of Polymers and Composites, D. H. Kaelble8. Engineering Thermoplastics: Properties and Applications, edited by James M.

Margolis9. Structural Foam: A Purchasing and Design Guide, Bruce C. Wendle

10. Plastics in Architecture: A Guide to Acrylic and Polycarbonate, RalphMontella

11. Metal-Filled Polymers: Properties and Applications, edited by Swapan K.Bhattacharya

12. Plastics Technology Handbook, Manas Chanda and Salil K. Roy13. Reaction Injection Molding Machinery and Processes, F. Melvin Sweeney14. Practical Thermoforming: Principles and Applications, John Florian15. Injection and Compression Molding Fundamentals, edited by Avraam I.

Isayev16. Polymer Mixing and Extrusion Technology, Nicholas P. Cheremisinoff17. High Modulus Polymers: Approaches to Design and Development, edited by

Anagnostis E. Zachariades and Roger S. Porter18. Corrosion-Resistant Plastic Composites in Chemical Plant Design, John H.

Mallinson19. Handbook of Elastomers: New Developments and Technology, edited by Anil

K. Bhowmick and Howard L. Stephens20. Rubber Compounding: Principles, Materials, and Techniques, Fred W.

Barlow21. Thermoplastic Polymer Additives: Theory and Practice, edited by John T.

Lutz, Jr.22. Emulsion Polymer Technology, Robert D. Athey, Jr.23. Mixing in Polymer Processing, edited by Chris Rauwendaal24. Handbook of Polymer Synthesis, Parts A and B, edited by Hans R.

Kricheldorf

25. Computational Modeling of Polymers, edited by Jozef Bicerano26. Plastics Technology Handbook: Second Edition, Revised and Expanded,

Manas Chanda and Salil K. Roy27. Prediction of Polymer Properties, Jozef Bicerano28. Ferroelectric Polymers: Chemistry, Physics, and Applications, edited by Hari

Singh Nalwa29. Degradable Polymers, Recycling, and Plastics Waste Management, edited by

Ann-Christine Albertsson and Samuel J. Huang30. Polymer Toughening, edited by Charles B. Arends31. Handbook of Applied Polymer Processing Technology, edited by Nicholas P.

Cheremisinoff and Paul N. Cheremisinoff32. Diffusion in Polymers, edited by P. Neogi33. Polymer Devolatilization, edited by Ramon J. Albalak34. Anionic Polymerization: Principles and Practical Applications, Henry L. Hsieh

and Roderic P. Quirk35. Cationic Polymerizations: Mechanisms, Synthesis, and Applications, edited

by Krzysztof Matyjaszewski36. Polyimides: Fundamentals and Applications, edited by Malay K. Ghosh and

K. L. Mittal37. Thermoplastic Melt Rheology and Processing, A. V. Shenoy and D. R. Saini38. Prediction of Polymer Properties: Second Edition, Revised and Expanded,

Jozef Bicerano39. Practical Thermoforming: Principles and Applications, Second Edition,

Revised and Expanded, John Florian40. Macromolecular Design of Polymeric Materials, edited by Koichi Hatada,

Tatsuki Kitayama, and Otto Vogl41. Handbook of Thermoplastics, edited by Olagoke Olabisi42. Selecting Thermoplastics for Engineering Applications: Second Edition,

Revised and Expanded, Charles P. MacDermott and Aroon V. Shenoy43. Metallized Plastics: Fundamentals and Applications, edited by K. L. Mittal44. Oligomer Technology and Applications, Constantin V. Uglea45. Electrical and Optical Polymer Systems: Fundamentals, Methods, and

Applications, edited by Donald L. Wise, Gary E. Wnek, Debra J. Trantolo,Thomas M. Cooper, and Joseph D. Gresser

46. Structure and Properties of Multiphase Polymeric Materials, edited by TakeoAraki, Qui Tran-Cong, and Mitsuhiro Shibayama

47. Plastics Technology Handbook: Third Edition, Revised and Expanded, ManasChanda and Salil K. Roy

48. Handbook of Radical Vinyl Polymerization, Munmaya K. Mishra and YusufYagci

49. Photonic Polymer Systems: Fundamentals, Methods, and Applications,edited by Donald L. Wise, Gary E. Wnek, Debra J. Trantolo, Thomas M.Cooper, and Joseph D. Gresser

50. Handbook of Polymer Testing: Physical Methods, edited by Roger Brown51. Handbook of Polypropylene and Polypropylene Composites, edited by Har-

utun G. Karian52. Polymer Blends and Alloys, edited by Gabriel O. Shonaike and George P.

Simon53. Star and Hyperbranched Polymers, edited by Munmaya K. Mishra and Shi-

ro Kobayashi54. Practical Extrusion Blow Molding, edited by Samuel L. Belcher

55. Polymer Viscoelasticity: Stress and Strain in Practice, Evaristo Riande,Ricardo Daz-Calleja, Margarita G. Prolongo, Rosa M. Masegosa, and Cat-alina Salom

56. Handbook of Polycarbonate Science and Technology, edited by Donald G.LeGrand and John T. Bendler

57. Handbook of Polyethylene: Structures, Properties, and Applications, AndrewJ. Peacock

58. Polymer and Composite Rheology: Second Edition, Revised and Expanded,Rakesh K. Gupta

59. Handbook of Polyolefins: Second Edition, Revised and Expanded, editedby Cornelia Vasile

60. Polymer Modification: Principles, Techniques, and Applications, edited byJohn J. Meister

61. Handbook of Elastomers: Second Edition, Revised and Expanded, editedby Anil K. Bhowmick and Howard L. Stephens

62. Polymer Modifiers and Additives, edited by John T. Lutz, Jr., and Richard F.Grossman

63. Practical Injection Molding, Bernie A. Olmsted and Martin E. Davis64. Thermosetting Polymers, Jean-Pierre Pascault, Henry Sautereau, Jacques

Verdu, and Roberto J. J. Williams65. Prediction of Polymer Properties: Third Edition, Revised and Expanded, Jozef

Bicerano66. Fundamentals of Polymer Engineering: Second Edition, Revised and

Expanded, Anil Kumar and Rakesh K. Gupta

Additional Volumes in Preparation

Handbook of Plastics Analysis, edited by Hubert Lobo and Jose Bonilla

Metallocene Catalysts in Plastics Technology, Anand Kumar Kulshreshtha

Preface

There has been a major effort, particularly in the last five years, to develop poly-propylene-based composites to replace metals and many types of engineeringthermoplastics in high-performance applications. In addition to providing reducedcost per unit volume, interphase design of polypropylene composites can be tai-lored to suit a growing number of property specifications. The interphase regionis defined as the polymeric coating surrounding any type of dispersed particulateincorporated in the polymer matrix, for example, mineral filler particles, glassfibers, or even dispersed elastomer droplets.

This handbook is a comprehensive guide to interphase design, describing keymaterial ingredients that contribute to suitable thermal and mechanical behaviordemanded by end-use requirements. I have identified interphase design as thefoundation on which additives are the individual building blocks needed to de-velop a suitable material. The sequence of formation steps begins with the manu-facture of polypropylene resins and culminates with the aufbau (building up)of the interphase design.

The molecular structure and morphology of individual polypropylene resinscan be readily modified at the reactor stage via new catalyst systems. Postblendsof available resins with various additives promote impact resistance, controlledrheology, thermal stability, and other desirable characteristics of the polymermatrix. The incorporation of chemical coupling agents and mineral-filler or glass-

iii

iv Preface

fiber reinforcement into the modified polypropylene matrix to form a chemicallycoupled composite is the major final step in interphase design.

The final combination of ingredients promotes an adhesive bond betweenthe polymer matrix and load-bearing glass-fiber reinforcement. Consequently,the physicochemical characteristics of the microstructure based on the interphasedesign determine the ultimate mechanical and other properties of filled or glass-fiber reinforced polypropylene resins.

By virtue of significant advances in chemical coupling and glass-fiber sizingtechnology, chemically coupled polypropylene composites can be manufacturedto exhibit strength and stiffness required for elevated temperature applications,for example, in hot climates or under the hood in automobiles (60150C). Atthe other end of the temperature spectrum, elastomer modification plus filler addi-tion to maintain stiffness provides the means for low-temperature impact resis-tance in subzero Arctic locations (30 to 40C).

Even at room temperature, there are many factors that can influence the ser-vice lifetime of a molded part. A material that is suitable for swimming poolpumps, for example, requires a wide spectrum of requirements: burst strength(tensile strength), recoverable strain (ductility) under pressure cycling or creep-fatigue resistance, moisturechemical (chlorine gas) resistance, and combinedweatherability under combined moisture exposureultraviolet radiation.

With the support of Mr. Russell Dekker, I have elicited contributors fromglobally recognized experts. The concept of interphase design is the central themeof this handbook. I have arranged the chapters in a stepwise manner to cover thematerial as if one were formulating individual ingredients into a polypropylenecomposite.

Chapters 1 and 2 provide information concerning the current technology ofpolypropylene resin manufacture with the desired structureproperty attributes.Chapters 3, 5, 6, 7, 11, and 12 describe modification of polypropylene resins andmicrostructure by addition of various additives and postreactor processing.

The current state of mineral-filled and glass-fiber reinforcement technologyis outlined in Chapters 8, 9, and 14 as a prelude to making composites of polypro-pylene. Chapter 8 deals with talc-filled polypropylene, while Chapter 14 providesattributes of mica reinforcement. A comprehensive treatise on glass-fiber technol-ogy is provided in Chapter 9 and the state-of-the-art reactive extrusion and com-ponding via twin screw equipment is described in Chapter 10. In Chapters 11and 12, the concept of interphase design is discussed in detail. And in Chapter12, mega-coupledtype chemical coupling is identified as the ultimate in in-terphase design in the class of engineering thermoplastics. In Chapter 13, thecharacterization of long-term creep-fatigue properties for glass-fiber-reinforcedpolypropylene composites is presented as the true gauge of mechanical responsethat represents the actual service lifetime of the molded part. The relationship

Preface v

between sizing chemistry on glass fibers, which is a key element in interphasedesign, correlates with this long-term tensile behavior.

In addition to needing an appropriate interphase design in order to achievethe desired mechanical properties, the long-term endurance of molded parts de-pends on the prudent choice of stabilizer packages (antioxidants, UV stabilizers,metal deactivators, and so forth) to withstand elevated ambient temperatures and/or exposure to the combined effects of moisture and sunlight. Chapter 4 addressesthe effects of environmental conditions as well as the need to attain effectivestabilizer packages for flame-retardant polypropylene in order to enhance weath-erability and prolonged service lifetime of a molded part.

Although the Internet has made information more readily accessible, the typi-cal tight time frame for the development of optimal interphase design requiresone to have a more practical source of basic information on a nearby bookshelf.Research scientists and technical service engineers alike will benefit from thishandbook of pertinent and up-to-date information on polypropylene-based mate-rials.

Harutun G. Karian

Contents

Preface iiiContributors ix

1 Growth of Polypropylene Usage as a Cost-EffectiveReplacement of Engineering Polymers 1Michael J. Balow

2 Polypropylene: Structure, Properties, Manufacturing Processes,and Applications 15William J. Kissel, James H. Han, and Jeffrey A. Meyer

3 Chemical Coupling Agents for Filled and Glass-ReinforcedPolypropylene Composites 39Robert C. Constable

4 Stabilization of Flame-Retarded Polypropylene 81Robert E. Lee, Donald Hallenbeck, and Jane Likens

5 Recycling of Polypropylene and Its Blends: Economic andTechnology Aspects 115Akin A. Adewole and Michael D. Wolkowicz

vii

viii Contents

6 Impact Behavior of Polypropylene and Its Blends andComposites 157Josef Jancar

7 Metallocene Plastomers as Polypropylene Impact Modifiers 201Thomas C. Yu and Donald K. Metzler

8 Talc in Polypropylene 237William P. Steen

9 Glass Fiber-Reinforced Polypropylene 263Philip F. Chu

10 Functionalization and Compounding of Polypropylene UsingTwin-Screw Extruders 335Thomas F. Bash and Harutun G. Karian

11 Engineered Interphases in Polypropylene Composites 367Josef Jancar

12 Mega-Coupled Polypropylene Composites of Glass Fibers 421Harutun G. Karian

13 Characterization of Long-Term CreepFatigue Behavior forGlass Fiber-Reinforced Polypropylene 473Les E. Campbell

14 Mica Reinforcement of Polypropylene 499Levy A. Canova

Index 549

Contributors

Akin A. Adewole Manager, Research and Development, Montell USA, Inc.,Elkton, Maryland

Michael J. Balow General Manager, Market Development and Technology,Montell-JPO Co., Ltd., Tokyo, Japan

Thomas F. Bash Process Development Manager, Ametek Westchester Plas-tics, Nesquehoning, Pennsylvania

Les E. Campbell Senior Engineer, Owens Corning Fiberglas, Anderson, SouthCarolina

Levy A. Canova Manager, Research and Development, Franklin IndustrialMinerals, Kings Mountain, North Carolina

Philip F. Chu Product Development Manager, Reinforced Thermoplastics,Vetrotex America, Wichita Falls, Texas

Robert C. Constable Uniroyal Chemical Company, Inc., Stroudsburg, Penn-sylvania

ix

x Contributors

Donald Hallenbeck Technical Manager, FRBU, Great Lakes Chemical Corpo-ration, West Lafayette, Indiana

James H. Han Senior Research Engineer, Polypropylene Business, AmocoPolymers, Inc., Alpharetta, Georgia

Josef Jancar University Professor and Director, Institute of Materials Chemis-try, School of Chemistry, Technical University Brno, Brno, Czech Republic

Harutun G. Karian Technical Product ManagerPolypropylene, Thermofil,Inc., Brighton, Michigan

William J. Kissel Associate Research Scientist, Research and Development,Amoco Polymers, Inc., Alpharetta, Georgia

Robert E. Lee Technical Manager, Polymer Stabilizers Business Unit, GreatLakes Chemical Corporation, West Lafayette, Indiana

Jane Likens Applications Research Chemist, Great Lakes Chemical Corpora-tion, West Lafayette, Indiana

Donald K. Metzler Market Development Manager, EXACT Plastomers, Ex-xon Chemical Company, Houston, Texas

Jeffrey A. Meyer Manager, Compositional and Structure Group, AnalyticalTechnology and Laboratory Services, Amoco Corporation, Naperville, Illinois

William P. Steen Technical ManagerPlastics, Luzenac America, Engle-wood, Colorado

Michael D. Wolkowicz Senior Scientist, Polymer Science and Development,Research and Development Center, Montell USA, Inc., Elkton, Maryland

Thomas C. Yu Senior Staff Engineer, Polyethylene Technology, BaytownPolymers Center, Exxon Chemical Company, Houston, Texas

1Growth of Polypropylene Usageas a Cost-Effective Replacementof Engineering Polymers

Michael J. BalowMontell-JPO Co., Ltd., Tokyo, Japan

1.1 INTRODUCTION

Throughout the history of human civilization, improvements in materials of con-struction have been sought. Starting with the building of some of humankindsearliest monuments, mud was combined with straw to improve the performanceand strength of the composite structure. Hence, the desire to use inexpensivematerials to effectively upgrade the performance of commodity materials is nota new idea. Today, with strong competition between the various materials ofconstruction, cost-effective materials have been continuously developed and ex-panded.

Polyethylene (PE) and polypropylene (PP) exemplify a class of materialscalled polyolefins. Composites based on these resins are relatively new by thestandards of those early materials of construction. Recent efforts to use thesetypes of materials combine inherent cost-effectiveness with a wide spectrum ofend-use applications as film, fiber, and moldings. The impetus for this remarkabledevelopment is attributed to the ease of polyolefin manufacture with effectiveproduction and purification of the monomers from a variety of sources, ongoingimprovements of the catalyst, and large well-controlled polymerization units.Consequently, a strong market demand for good performance and cost-effectivematerials have spurred a multibillion dollar industry.

1

2 Balow

Table 1.1 Recent Historical Consumption of Polypropylene (ktons)

Region 1989 1991 1993 1995 1997a

Europe 3,914 4,213 4,766 5,518 6,362North America 3,022 3,435 4,202 4,928 5,750Asia 3,621 4,711 5,404 7,023 8,457South America 402 516 744 876 1,031Middle East/Africa 383 425 618 813 915Total consumption 11,342 13,300 15,734 19,158 22,515

a Estimation.

1.2 POLYPROPYLENE GROWTH AND USES

Polypropylene was initially produced commercially about 45 years ago after thesuccessful development of a suitable stereo-specific catalyst, which enabled thepolymer to have the kind of structural characteristics useful for rigid items. Tables1.1 and 1.2 describe the historical and anticipated consumption of polypropylene.In Table 1.3 you can find the expected capacity growth by region for the future.The continuous growth of polypropylene is expected to continue into the nextmillenium as raw materials in an expanding number of end-use products for theautomotive and film industries.

Earlier, the performance of polypropylene was considered only intermediateto polyethylene and polystyrene (PS). But, as of late, there is significant interma-terial competititon to replace engineering polymers as materials of constructionby polypropylene base resins.

The significant growth of PP use is attributed to a combination of manyfactors besides a good balance in physical and chemical properties. Because ofappropriate melt rheology and thermal behavior, PP-based materials are widely

Table 1.2 Future Expectations of Global Consumption (ktons)

Region 1998 1999 2000 2001 AAGRa (%)

Europe 6,661 7,011 7,363 7,714 5.3North America 6,089 6,482 6,874 7,266 6.5Asia 9,745 10,654 11,562 12,471 9.6South America 1,150 1,265 1,379 1,494 10.2Middle East/Africa 969 1,043 1,116 1,189 7.6Total consumption 24,614 26,455 28,294 30,134 7.6

a Average annual growth.Source: Phillip Townsend Associates Inc.

Replacement of Engineering Polymers with PP 3

Table 1.3 Worldwide Polypropylene Production Capacity by Area (ktons)

Region 1998 1999 2000 2001 AAGRa (%)

Europe 8,365 9,105 9,773 9,983 4.5North America 6,775 7,683 8,553 8,739 6.5Asia 10,699 11,576 12,006 12,241 3.4South America 1,190 1,390 1,465 1,465 5.3Middle East/Africa 1,190 1,360 1,665 1,890 12.2Total consumption 28,219 31,114 33,462 34,318 5.0

a Average annual growth.Source: Phillip Townsend Associates Inc.

processable on a variety of different equipment ranging from injection moldingto some designed for use in other industries, like calendaring and air-quenchedblow film equipment. Additionally, by having the lowest density among commod-ity plastics at approximately 0.90 g/cm3, continued market penetration of PP atthe current rate of growth is almost ensured on the basis of good mechanicalproperties at reduced cost per volume. Finally, because many major companiesare designing their products, polypropylene stands out as the main product withthe widest design flexibility and simplicity of recycling. Its excellent thermalstability, low density (assisting in separating from other materials), chemical andenvironmental inertness, and even its caloric content in the case of incinerationall add to its attractiveness as the material of construction.

The global supply from several producers located throughout the world en-sures good supply at competitive prices. From the mid-1990s to the end of the20th century, significant capacity increases will occur. The supply and demandbalance works favorably to the consumers benefit. This keeps prices in checkand ensures that suitable supply will be available. Additionally, in the next 10years, significant amounts of polypropylene from recycled sources will be avail-able. The quality of this recycled material varies widely in cleanliness but isthought to be suitable for a variety of applications, including automotive. Indus-trial waste streams are used today for compounding operations, and this willcontinue. Additionally, postconsumer waste primarily from packaging is becom-ing more available but often of lower quality (e.g., sometimes limited to black-pigmented products). Data on the major end uses in each region and the expectedgrowth of the various end uses in each region are found in Tables 1.4 and 1.5.

1.3 RAW MATERIALS

Polypropylene homopolymer consists of molecular chains with repeating unitsof propylene monomer generated in the reactor. It is derived from three major

4 Balow

Tabl

e1.

4End

Use

sforPolyp

ropylen

ein

Major

Reg

ions

(ktons

)

Con

version

Asia

Cen

tral

Middle

Eas

tSou

thNorth

proce

ssPac

ic

Europ

ean

dAfrica

America

Japan

Europ

eAmerica

Fibers

2112

210

255

345

163

1371

1798

Injectionmolding

1536

186

311

316

1336

2479

2406

Film

/extrusion

coating

1521

9316

617

149

091

562

8Blow

molding

635

726

3893

146

She

et12

930

2962

236

216

191

Other

proce

sses

207

3329

7510

232

713

6To

talc

onve

rsion

5568

557

797

995

2365

5401

5305

Sour

ce:

1996

Basi

s,Ph

illip

Tow

nsen

dAs

soci

ates

Inc.

Replacement of Engineering Polymers with PP 5

Tabl

e1.

5Exp

ected

Ann

ualG

rowth

RateforPolyp

ropylen

ebyCon

versionProce

ss(%

)

Con

version

Asia

Cen

tral

Middle

Eas

tSou

thNorth

proce

ssPac

ic

Europ

ean

dAfrica

America

Japan

Europ

eAmerica

Fibers

10.1

7.5

7.5

8.6

7.7

3.7

5.6

Injectionmolding

11.4

12.5

8.0

9.8

4.2

5.0

7.3

Film

/extrusion

coating

11.8

9.3

7.3

8.3

5.8

5.9

7.1

Blow

molding

10.1

12.5

4.2

3.6

8.1

5.8

3.7

She

et14

.87.5

7.0

6.8

5.2

6.9

6.2

Other

proce

sses

13.6

9.5

6.6

5.1

5.1

2.6

4.0

Totalc

onve

rsion

11.2

9.7

7.6

8.5

5.0

4.8

6.5

Sour

ce:

1996

Basi

s,Ph

illip

Tow

nsen

dAs

soci

ates

Inc.

6 Balow

Figure 1.1 Supply of polypropylene monomer.

sources today. Globally, most propylene monomer comes from the steam crack-ing process using naphtha, a valuable fraction of crude oil. Usually, naphthacrackers target product is ethylene monomer. Propylene is a byproduct of thecracking process produced at various ratios depending on the crude oil feedstock.Many cracking processes have a propylene plant intimately connected to effec-tively use the propylene that comes from naphtha cracking. The second largestproduction of propylene comes from the gasoline refining process. Finally, andmost recently, a new process by which propane is dehydrogenated to propylenemonomer has been used to produce propylene. Despite certain economical short-comings, when propane is readily available and transportation to markets is lessfavorable, this process is now starting to be applied. Propylene purity requirementfor the production of polymers is very high. Trace impurities in the polymeriza-tion process cause poisoning of the catalyst during production. The industrialroutes to produce propylene monomer and the region capacities with expectedgrowth are outlined in Figure 1.1 and Table 1.6.

With continued developments in the polypropylene industry, a wide variety

Table 1.6 Global Propylene Production for 1996

Region Tonnage (ktons) % of Total Growth (%)

North America 12,700 30 1.6Latin America 1,700 4 8.5West Europe 11,800 28 1.6East Europe 2,400 6 9.1Asia 11,800 28 8.6Middle East/Africa 1,200 3 34.5Others 300 1 7.4Total 41,900 100 4.9

Replacement of Engineering Polymers with PP 7

of polypropylene copolymers has followed. The original polypropylene homo-polymer is characterized by high rigidity and higher heat resistance and meltingpoint (157C) but is plagued by poor impact resistance at low temperatures(0C) and relatively poor transparency. Chapters 2, 6, and 7 describe the conse-quences of modifying PP homopolymer resin to rectify low-temperature brittle-ness and thereby enhance impact resistance.

Random copolymer resins are produced by mixing the polypropylene mono-mer at the first stages of polymerization with ethylene or with another comonomersuch as butene. With the low level incorporation of comonomer, the resultingresin exhibits somewhat lower stiffness, a lower melting point, and reduced hard-ness compared with the PP homopolymer. However, it features better transpar-ency, lower blush resistance, and slightly improved impact resistance at 0C.

Heterophasic copolymer resins (so called because their morphology typicallyshows two or more phases) have lower stiffness and improved toughness at lowtemperature, down to 40C (depending on the dispersed phase type andamount). These resins often demonstrate more complex thermal behavior (e.g.,two or more melting points and reduced stiffness at elevated temperature). Youcan find examples of typical grades of polypropylene resins in Table 1.7. Chapter2 describes propylene structureproperty relationships that suit a variety of end-use applications.

1.4 COMPETITION WITH OTHER POLYMERS

Improved performance has lead to an intense competition between the commer-cial polymers today. Polymer selection for any specific design is often governedby a complex set of characteristics. The most common design parameters arecost, temperature performance, toughnessstiffness balance, chemical resistance,electrical properties, optical properties, long-term dimensional stability, or envi-ronmental resistance. However, a particular polymer can often be chosen for lessobvious issues (e.g., specific gravity, colorability, decoratability, paint or solventsusceptibility, wear resistance, recycleability, material unification, foamability,moldability, flammability, electromagnetic properties, weldability or biodegrad-ability). These choices can often be secondary to the formability considerationto make the final articles.

Competitive penetration of polypropylene into other applications has primar-ily taken place in polyethylene, polystyrene, polyvinyl chloride (PVC), thermo-plastic polyester, nylon-6 or -6/6, and sometimes directly from metals or thermo-set polymers like phenolic or reinforced reaction injection molded (RIM)urethane. The reasons for market penetration by PP replacement vary widely withan assortment of material design options: chemical resistance, heat resistance,recycleability, processability, economics, and aesthetics.

8 Balow

Tabl

e1.

7Ty

pical

Categ

oriesof

Polyp

ropylen

eRes

ins

Test

Ran

dom

Stand

ard

Sup

erim

pac

tProperty

metho

dUnits

Hom

opolym

erco

polym

erco

polym

erco

polym

er

Melto

wrate

D-123

8dg/m

in4

6.5

44

Den

sity

D-792

Bg/cm

30.90

50.90

0.90

0.9

Tens

ilestreng

th,yield

D-638

MPa

3428

2721

Tens

ileelon

gation,

yield

D-638

%6

138

8Flex

ural

mod

ulus

,se

cant

D-790

MPa

1400

920

1200

1000

IZOD

impac

t,no

tche

dD-256

AJ/m

3956

110

54

0Hea

tde

ectio

n,45

5kP

aD-648

C

9476

9081

Discretepha

ses

Microsc

opy

11

23

Ethylen

eco

nten

tIR

analys

is%

03.5

715

Ethylen

einco

rporation

Com

pos

ition

alan

alys

esNA

C2C

3C

3

C2C

3C

2

C2C

3

C3

Replacement of Engineering Polymers with PP 9

1.4.1 Conversions from PolyethyleneReplacement of PE by PP has varied considerably. One underlying cause for thisdiversity is the difference in monomer costs. Another cause for change has beenrelated to density differences between PP and PE resins.

The major performance advantages that PP-based materials have over PE isthe temperature resistance and density of these materials. Although density israrely itself a deciding factor, this fact can occasionally tip the balance towardPP on a unit cost basis. From a thermal point of view, various types of PP havesome advantages. Homopolymer PP, although having greater temperature resis-tance, actually has much lower toughness properties. On the other hand, randomcopolymers and heterophasic copolymers most closely match the properties ofPE from a toughness point and still maintain higher heat resistance. For extrusioncoating or in retort packaging applications, this difference can be very significant.Even though the chemical resistance properties of the two materials are generallyquite similar in many respects, there are some subtle differences. For example,PE and PP resins have different resistance against gasoline and detergents andalso exhibit different permeability to water and oxygen.

In extruded pipe for natural gas, PE pipe has outstanding properties. How-ever, in applications like steel pipe coating where the temperature of the oil mayreach 100C, PP has a strong advantage. In applications such a water-proofingsheet where PE is used as a geomembrane, the overall weldability, dead foldrecovery, puncture resistance, and stress cracking resistance of PP-based materi-als tend to be higher in performance. Without down gauging the sheet thickness,the economic trade-off may not be enough.

In foam applications, such as those used in cushioning applications, PE canbe produced in a wide variety of densities. Without cross-linking, these PE-basedmaterials do not have suitable resistance to heat for applications like automotiveinterior cushioning foam. In these cases, occasionally blends of PE are used toimprove the overall temperature resistance. PE reacts quite differently to irradita-tion as well. Cross-linking PE with an electron beam is common practice in someapplications like wire and cable. PP cannot be easily cross-linked without incor-poration of other monomers.

1.4.2 Conversions from Polyvinyl ChloridePolyvinyl chloride has often come under environmental scrutiny. The PVC mar-ket is roughly divided into rigid and flexible materials. Rigid PVC is difficult toextrude due to its inherently high melt viscosity and tendency to degrade via shearextrusion. However, by incorporating plasticizers (commonly dioctyl phthalate orDOP), the flexible form can be fabricated more easily.

Rigid PVC applications enter into many aspects of everyday use, such as

10 Balow

flooring, textiles, pipe and conduit, siding, and rigid packaging such as bottles.In flexible applications, popular uses are in wire and cables, flexible packaging,and plastisols used for grips. Polypropylene compositions have provided a reason-able alternative for PVC materials. For the rigid applications, such as pipe andsiding, PP-based materials ranging from unfilled resins to filled, reinforced com-posites have been successfully applied. Current applications include CaCO3-filledPP copolymers for siding applications and flame-retardant PP for use in stadiumseats and building pipes. In the softer and flexible area, highly flexible PP in asingle layer or as a multiple layer structure can be applied. Applications of thistype are found in supermarket meat wrap, where the stretch and cling characteris-tics of PVC can be matched. Likewise, automotive applications with interior trimskin consisting of PP-based materials tend to perform better than plasticized PVC,especially in the hotter and sunnier environments at equatorial latitudes.

Another driving force for these replacements has been related to disposalmethods at the end of useful service lifetime of a molded part. In areas whereincineration has been the primary method of disposal, high levels of dioxin havebeen found. The transformation away from PVC has also received a lot of publicattention. In Europe, especially where the Green Peace movement has beenstrong, public outcry for replacement has been very noteworthy. Because evensmall amounts of PVC in the recycling stream can cause problems with othermaterials, monomaterial solutions have often been applied. There are examplesof this in the packaging and automotive areas. Bottle labels have been changedto polyolefins. Even automotive wire and cables are under examination, typicallyto combine with PP waste streams. Even if residual PVC remains in the wastestream, it does not preclude the use of mixed waste, whether it be for reuse orincineration.

One of the great difficulties in replacement for PVC has been it outstandingweather resistance, especially in bright colors and other applications such asroofing or siding where the flame-spread properties are important. Consequently,attempts to replace PVC by PP in these areas have experienced moderate success.

1.4.3 Conversions from Styrenic-Based MaterialsThe replacement of styrenic material has been substantial and covers a wide spec-trum of applications ranging from automotive to appliance and packaging. Thedriving forces for this change include cost reduction, performance improvement,unification to monomaterial, weatherability, and chemical resistance. In the auto-motive area, the major driving forces have been unification of materials, noisereduction, thermal stability, and weatherability. The use of acrylonitrile butadienestyrene (ABS) resin in the interior of the vehicle, particularly in the automotivedoor panels and trim, package trays, and center consoles, has primarily been

Replacement of Engineering Polymers with PP 11

converted to PP-based materials in the past 10 years. The existing trend is ex-pected to continue in the area of automotive applications.

Styrenics are amorphous in nature. Consequently, styrenic-based materialshave relatively lower shrinkage and warpage tendency than PP or PP-based com-pounds. However, with appropriate design and occasionally by the use of filledPP, these shortcomings in dimensional stability can be overcome. By incorpora-tion of resins having lower density and an appropriate choice of high fluidity orhigh melt flow rate, PP-based materials can be molded into larger internal partswith fewer gates.

High crystalline PP with inherent scratch resistance has been used to replacesome styrenics products. Some applications, like instrument panel substrates typi-cally made from styrenics copolymers or terpolymers, have frequently been usedas the substrate material for soft IP applications (foam in place polyurethanecushion foam and PVC skin covering). In these designs, the brittleness of theseproducts is used as an energy-absorbing technique for the energy managementin stryrenic materials. New head impact laws for safety of the interior of automo-biles also tend to favor the use of PP materials; however, new energy-absorbingdesigns are necessary to pass these regulations.

For small appliance applications, such as coffee pots, steam irons, and canopeners, filled PP has become the material of choice for applications requiringbetter heat resistance and thinner wall than those generally possible with ABS.Typically, these appliances have high gloss requirements normally typically at-tained by using materials based on ABS resins. Comparatively, PP-based com-pounds can come close to matching these requirements. In addition, the latteroffer better chemical resistance and heat performance at low loads.

In the packaging area where disposal and clean incineration are a factor, PP-based material has frequently been used to replace styrenic-based materials. Largevolume small containers, such as butter and yogurt cups, have been successfullyreplaced by thin-wall injection-molded PP materials. In these applications, thecritical point is the design of the tooling in which molten PP, with very highfluidity, can fill more than 24 cavities and may be injected in one shot. Thisprocess then can be very competitive to rapid in-line thermoforming of PS con-tainers. Correct design of the container is a critical consideration in this type ofapplication.

New technological developments have led to the manufacture of PP resinswith high melt strength. With an increase in melt tension, these resins can beused to improve the sag resistance characteristics in end-use applications likereheat thermoforming. Additionally, these materials can be foamed with physicalblowing agents like isobutane or isopentane to make highly expanded products(50) for cushioning foam sheet. For applications like flotation devices (lifevests, dock cushions), PP has lower inherent specific gravity than styrenics and

12 Balow

better toughness, and even the chemical resistance around boats can be importantbecause oil and fuel spills can occur.

1.4.4 Conversions from Engineering ResinsTraditionally, the primary driving force to change to PP-based materials for appli-cations using engineering resins has been cost. Applications include fan bladesin automotive and pump housing for chemical and swimming pool applications.In automotive instrument panels, polyacetal is replaced by PP. Headlamp supportframes consisting of Rynite have been replaced by glass fiber-reinforced PP. Inthese types of applications, often new tooling is used to improve the strength inthe weaker sections of the part. In a sense, existing tool design and part geometrymay not be directly applied to parts converted to PP. Increased wall thicknessand consideration for the creep characteristic differences may need to be studied;however, PP can be applied. In most cases, the cost per unit volume frequentlyweighs in favor of PP-based materials.

1.4.5 Conversions from Thermoset ResinsAlthough thermoset resins typically have quite a different profile from thermo-plastics, some of the largest growth in PP has actually come at the direct expenseof thermoset polyurethane. Automotive exterior applications such a bumpers,cladding, grills, and spoilers were originally made from steel but then moved toRIM polyurethane. This trend has been dominant during the 1980s and 1990s inthe automotive field. The major driving forces of this change have been the ex-pense of the materials and high specific gravity. Additionally, like all thermosetmaterials, if poor parts are obtained, there is little chance to salvage the material.Finally, the automotive industry as a whole has focused on PP-based materialsprimarily for their cost performance and recycling benefits.

Although paintable grades of PP resins were initially a requirement for ther-moset replacement, suitable primers and paintable products have emerged as apractical solution so that this impediment is no longer an issue. Another criterionfor material selection was the need for large runner systems. However, this matterhas been significantly reduced in recent years with the inception of hot runnertooling and sequenced gating techniques to prevent overpacking in these largeparts.

To a lesser extent, the replacement of thermosets by PP-based compoundshas also occurred in the area of thermoset polyester for large automotive partssuch as grill opening panels; however, the trend has not been so dominant asin the case of RIM. Grill opening panels, once made with thermoset polyestercompounds, have switched to filled or glass fiber-reinforced PP.

Electrical timer blocks that originally used phenolic resins have switched to

Replacement of Engineering Polymers with PP 13

other engineering resins, such as acetal or reinforced styrene acrylonitrile (SAN).Recently, there have been efforts to use PP-based compounds, but these applica-tions have not always been successful. One primary difficulty in the transitionfrom thermoset is the hardness differences between PP and most thermoset resins.RIM polyurethane is one exception of a relatively low hardness thermoset resin.From a strength point of view, with the advent of long glass injection or transfermolded techniques, PP-based materials may be able to penetrate additional ther-moset applications.

1.5 MODIFICATIONS OF POLYPROPYLENE TO MEET THEDESIGN NEEDS

In the early 1960s, an industry started to develop around the modification ofpolymers. This industry originally started as a way to simply modify polymerfor improvements of impact resistance, color, or thermal stability. It initially usedmany tools of the rubber industry to modify polymers for specific end-use appli-cations. Polypropylene with its useful balance of properties has found interestingutility in the automotive industry. This market penetration of PP was primarilydue to its good thermal resistance for under-the-hood applications, its ease ofcolorability for applications on the interior of the car, and its low raw materialcost. Today, the modification of PP is a large industry, often associated with thePP production or in some cases completely independent of the polymerizationsteps itself.

Polypropylene has been modified with any number of enhancing agents rang-ing from glass fibers and asbestos for strength improvements to conductive addi-tives like carbon black, stainless steel, and carbon fibers, with calcium carbonate,talc, and mica for stiffness improvement and a whole range of other additivesfor special effects in the compound. The affect on the overall properties of thematerial depends greatly on the type of filler, especially its shape, the chemicalmakeup of the additive, the level of dosing, and the temperature range of thefinal application. In Chapters 8, 9, and 14, PP composites consisting of talc filler,fiberglass, and mica reinforcement are discussed in detail.

1.6 CONCLUSIONS

Polypropylene has had a remarkable growth over the past 45 years. The reasonsfor this growth have been the versatility of the polymer; ability of the polymer tobe modified and tailored for specific applications; its overall balance of physical,mechanical, electrical, chemical, and thermal properties; and a competitive price.Additionally, PP-based material has very good stability over a wide temperature

14 Balow

range and has outstanding retention of mechanical properties after recycling (seeChap. 5) and also in many cases incineration ability. The competitive price comesfrom wide availability of the basic monomer and the simplicity of the polymeriza-tion systems available today. The polymer can be processed on a wide varietyof equipment, thus allowing it to be transformed easily, simply, and safely intoa wide variety of usable articles. For these reasons, polypropylene has a brightfuture as one of humankinds building materials for the next century.

2Polypropylene: Structure, Properties,Manufacturing Processes,and Applications

William J. Kissel and James H. HanAmoco Polymers, Inc., Alpharetta, Georgia

Jeffrey A. MeyerAmoco Corporation, Naperville, Illinois

2.1 TYPES OF POLYPROPYLENE

Polypropylene (PP) is a thermoplastic material that is produced by polymerizingpropylene molecules, which are the monomer units, into very long polymer mole-cules or chains. There are a number of different ways to link the monomerstogether, but PP as a commercially used material in its most widely used formis made with catalysts that produce crystallizable polymer chains. These give riseto a product that is a semicrystalline solid with good physical, mechanical, andthermal properties. Another form of PP, produced in much lower volumes as abyproduct of semicrystalline PP production and having very poor mechanicaland thermal properties, is a soft, tacky material used in adhesives, sealants, andcaulk products. The above two products are often referred to as isotactic (crys-tallizable) PP (i-PP) and atactic (noncrystallizable) PP (a-PP), respectively.

As is typical with most thermoplastic materials, the main properties of PPin the melt state are derived from the average length of the polymer chains andthe breadth of the distribution of the polymer chain lengths in a given product.In the solid state, the main properties of the PP material reflect the type andamount of crystalline and amorphous regions formed from the polymer chains.

Semicrystalline PP is a thermoplastic material containing both crystallineand amorphous phases. The relative amount of each phase depends on structuraland stereochemical characteristics of the polymer chains and the conditions under

15

16 Kissel et al.

which the resin is converted into final products such as fibers, films, and vari-ous other geometric shapes during fabrication by extrusion, thermoforming, ormolding.

Polypropylene has excellent and desirable physical, mechanical, and thermalproperties when it is used in room-temperature applications. It is relatively stiffand has a high melting point, low density, and relatively good resistance to im-pact. These properties can be varied in a relatively simple manner by alteringthe chain regularity (tacticity) content and distribution, the average chain lengths,the incorporation of a comonomer such as ethylene into the polymer chains,and the incorporation of an impact modifier into the resin formulation.

The following notation is used in this chapter. Polypropylene containing onlypropylene monomer in the semicrystalline solid form is referred to as homopoly-mer PP (HPP), and we use this to mean the i-PP form. Polypropylene containingethylene as a comonomer in the PP chains at levels in about the 18% range isreferred to as random copolymer (RCP). HPP containing a comixed RCP phasethat has an ethylene content of 4565% is referred to as an impact copolymer(ICP). Each of these product types is described below in more detail.

2.1.1 HomopolymerHomopolymer PP is the most widely used polypropylene material in the HPP,RCP, and ICP family of products. It is made in several different reactor designsusing catalysts that link the monomers together in a stereospecific manner, re-sulting in polymer chains that are crystallizable. Whether they crystallize and towhat extent depends on the conditions under which the entangled mass of poly-mer chains transitions from the melt to the solid state or how a heat-softened solidPP material is strained during a further fabrication procedure like fiber drawing.

Homopolymer PP is a two-phase system because it contains both crystallineand noncrystalline regions. The noncrystalline, or amorphous, regions are com-prised of both isotactic PP and atactic PP. The isotactic PP in the amorphousregions is crystallizable, and it will crystallize slowly over time up to the limitthat entanglement will allow. The extent of crystallization after the initial fabrica-tion step of converting PP pellets or powder into a molded article will slowlyincrease over time, as will the stiffness. A widely accepted model of HPP mor-phology likens the solid structure to a system consisting of pieces of stiff card-board linked together by strands of softer material. In the areas represented byflat pieces of cardboard, PP polymer chains weave up and down into close-packedarrays called crystallites (little crystals), which are called lamella by morphol-ogists. The soft strands linking the pieces of stiff cardboard are polymer chainsthat exit one crystallite, enter another, and then begin weaving up and down inanother crystallite. The crystallizability of the chains is one factor that determineshow thick the crystallites will be and the thickness of the crystallites determines

Characteristics of Polypropylene 17

how much heat energy is required to melt them (the melting temperature). Atypical HPP has an array of crystallites from thick ones to very thin ones, andthese manifest themselves as an array of melting points.

Homopolymer PP is marketed mainly by melt flow rate (MFR) and additiveformulation into fiber, film, sheet, and injection molding applications. Melt flowrate is an indicator of the weight-average molecular weight as measured by theASTM or ISO MFR test method.

2.1.2 Random CopolymerRandom copolymers are ethylene/propylene copolymers that are made in a singlereactor by copolymerizing propylene and small amounts of ethylene (usually 7%and lower). The copolymerized ethylene changes the properties of the polymerchains significantly and results in thermoplastic products that are sold into mar-kets in which slightly better impact properties, improved clarity, decreased haze,decreased melting point, or enhanced flexibility are required. The ethylene mono-mer in the PP chain manifests itself as a defect in the chain regularity, thus inhib-iting the chains crystallizability. As the ethylene content increases, the crystallitethickness gradually decreases, and this manifests itself in a lower melting point.The amount of ethylene incorporated into the chain is usually dictated by a bal-ance between the thermal, optical, or mechanical properties.

2.1.3 Impact CopolymersImpact copolymers are physical mixtures of HPP and RCP, with the overall mix-ture having ethylene contents on the order of 615% wt%. These are sold intomarkets where enhanced impact resistance is needed at low temperatures, espe-cially freezer temperature and below.

The RCP part of the mixture is designed to have ethylene contents on theorder of 4065% ethylene and is termed the rubber phase. The rubber phase canbe mechanically blended into the ICP by mixing rubber and HPP in an extruderor it can be polymerized in situ in a two-reactor system. The HPP is made in thefirst reactor and the HPP with active catalyst still in it is conveyed into a secondreactor where a mixture of ethylene and propylene monomer is polymerized inthe voids and interstices of the HPP polymer powder particle. The amount ofrubber phase that is blended into the HPP by mechanical or reactor methods isdetermined by the level of impact resistance needed. The impact resistance ofthe ICP product is determined not only by its rubber content but also by the size,shape, and distribution of the rubber particles throughout the ICP product. Reac-tor products usually give better impact resistance at a given rubber level for thisreason.

As the rubber content of the ICP product is increased, so is the impact resis-

18 Kissel et al.

tance, but this is at the expense of the stiffness (flexural modulus) of the product.Consequently, polymer scientists often describe a product as having a certainimpactstiffness balance. The stiffness of the ICP product is dictated by the stiff-ness of the HPP phase and the volume of rubber at a given rubber size distributionin the product. The impact resistance is dictated by the amount and distributionof the rubber phase in the ICP product.

2.2 TACTICITY

The solid-state characteristics of PP occur because the propylene monomer isasymmetrical in shape. It differs from the ethylene monomer in that it has amethyl group attached to one of the olefinic carbons. This asymmetric nature ofthe propylene monomer thus creates several possibilities for linking them togetherinto polymer chains that are not possible with the symmetrical ethylene monomer,and gives rise to what are known as structural isomers and stereochemical isomersin the polypropylene chain.

In structural isomerism, polymer scientists refer to the olefinic carbon withthe methyl group on it as the head (h) and the other olefinic carbon as thetail (t) of the monomer. The most common method of polymerization usescatalysts that link the monomers together in the head-to-tail fashion, althoughoccasionally there is a mistake made and the monomers form a head-to-head or a tail-to-tail linkage, but these tend to be rare.

Stereochemical isomerism is possible in PP because propylene monomerscan link together such that the methyl groups can be situated in one spatial ar-rangement or another in the polymer. If the methyl groups are all on one sideof the chain, they are referred to as being in the isotactic arrangement, andif they are on alternate sides of the chain, they are referred to as being in thesyndiotactic arrangement. Each chain has a regular and repeating symmetricalarrangement of methyl groups that form different unit cell crystal types in thesolid state. A random arrangement of methyl groups along the chain provideslittle or no symmetry, and a polymer with this type of arrangement is known asatactic polypropylene.

When polymer scientists discuss the stereochemical features of PP, they usu-ally discuss it in terms of tacticity or percent tacticity of polypropylene,and in the marketplace the term polypropylene is generally used to refer toa material that has high tacticity, meaning high isotactic content. The high tac-ticity PP materials have desirable physical, mechanical, and thermal propertiesin the solid state. Atactic material is a soft, sticky, gummy material that is mainlyused in sealants, caulks, and other applications where its stickiness is desirable.Syndiotactic PP, not a large volume commercial material, is far less crystallinethan isotactic PP.

Characteristics of Polypropylene 19

2.3 MOLECULAR WEIGHT AND MOLECULARWEIGHT DISTRIBUTION

Unlike pure simple compounds, whose molecules are all of the same molecularweight, polymer samples consist of molecules of different molecular weights.This is a reflection of the fact that a polymer sample is a collection of moleculesof differing chain lengths. Therefore, an average molecular weight concept wasadopted for polymers. No single average, however, can completely describe apolymer sample, and a number of different averages are used. Ratios of some ofthese averages can be used to calculate a molecular weight distribution (MWD),which describes the breadth of the molecular weights represented.

Molecular weight averages of polypropylene are measured by the techniqueof gel permeation chromatography (GPC), a chromatographic technique that sortsout the polymer chains by chain length after the PP is dissolved in a solvent.When dissolved, the PP is no longer a thermoplastic but instead is a bunch oflong molecules dispersed in a solvent. From the GPC data, one can calculatethe number-average (Mn), weight-average (Mw), and z-average (Mz) molecularweights. In PP, the Mn relates to physical properties of the solid, the Mw relatesto viscosity properties of the melt, and the Mz to elastic properties of the melt.Because the GPC chromatogram contains a lot of data that is not easy to tabulateand communicate, it is convenient to use a ratio, especially Mw/Mn, because itgives a good estimate of the MWD and is a simple number to tabulate and store.It is a good estimate because the Mn is very sensitive to short chains and the Mwis very sensitive to long chains in the products.

The GPC equipment is fairly expensive and prone to failure, and the actualexperiment is slow, labor intensive, and requires dissolution of the PP at hightemperatures in solvents like xylene and trichlorobenzene. Thus, other methodsto estimate the molecular weight have been developed. The most popular one istermed the MFR test, and it gives a number that is easily correlatable to the Mwaverage. Most HPP products are sold with MFR numbers ranging from 0.2 to45, and these correspond to Mws from 1,000,000 down to 100,000. Note that themolecular weight averages are inversely proportional to MFR numbers.

2.4 MECHANICAL PROPERTIES

The mechanical properties of most interest to the PP product design engineer areits stiffness, strength, and impact resistance. Stiffness is measured as the flexuralmodulus, determined in a flexural test, and impact resistance by a number ofdifferent impact tests, with the historical favorite being the Izod impact at ambientand at subambient temperatures. These mechanical properties are mostly used topredict the properties of molded articles. Strength is usually defined by the stress

20 Kissel et al.

at the yield point rather than by the strength at break, but breaking strength isusually specified for fiber or film materials under tensile stress.

To understand the use and comparison of mechanical property data, one mustremember that mechanical properties are not measured on the resins themselvesbut instead on specimens fabricated from the resin, and it is from the physicsgoverning the fabrication and mechanical testing procedures that the mechanicalproperties are derived. Because there are so many variables that can affect me-chanical properties, consensus testing organizations like ASTM and ISO wereformed to bring some uniformity and consistency to specimen preparation andmechanical testing. Because the ASTM and ISO fabrication and testing methodsallow some freedom within their guidelines, when one is asked what the mechani-cal properties of a material are, the first answer should be to ask by what tests,what specimens, and under what conditions. The latter includes such things asthe exact specimen type, age of specimen, how the specimen was conditioned,testing speed, testing temperature, data acquisition procedure, and method of cal-culation.

Flexural modulus or stiffness typically increases as the level of crystallinityincreases in a PP product, but it also depends on the type of crystal morphology.Thus, stiffness generally decreases as the crystallizability (tacticity) decreases or,in random copolymers, as the ethylene content increases because this tends todecrease crystallizability.

2.5 RHEOLOGY

Rheology is the science that studies the deformation and flow of matter, and inPP there is interest in both viscosity and elasticity of the melt state and the solidstate. The rheological properties of PP are important because of the broad rangeof processing techniques to which PP is subjected, including fiber and film extru-sion, thermoforming, and injection molding. The viscosity of PP is of most impor-tance in the melt state because it relates to how easily a PP product can be ex-truded or injection molded. In fiber extrusion, melt elasticity is important toprocessability of a PP product because it relates to how easily a material can bedrawn into a fiber. In contrast to PP, most engineering resins are used mainly ininjection molding operations.

The viscosity of a PP product is related to its Mw, and a good estimation ofit at low shear rates can be obtained from the MFR test. This is only a singlepoint test, and more information about the viscosity at different strain rates isneeded to completely understand and characterize the processability of a product.The strain rate dependence of melt viscosity in PP is related to its molecularweight distribution, which is commonly described by the ratio of the Mw to Mn

Characteristics of Polypropylene 21

averages. As the MWD of PP gets broader, it shear thins (becomes less viscous)more than a narrower MWD PP at the same strain rate.

As indicated above, the rheological properties in the melt are related to theMWD. In PP, these are controlled mainly by the process used, although withZiegler-Natta catalysts there is a small effect due the catalyst. Typical MWDsare in the 56 range. The MWD can be made more narrow by using postreactorpolymer chain shortening. This may be accomplished by adding a peroxide inthe extrusion compounding manufacturing step, in which stabilizers and otheradditives are normally incorporated into the PP reactor product before pelletiza-tion. These controlled rheology (CR) resins have higher MFR and reduced MWDthan the unmodified reactor product. In the CR process, also known as visbreak-ing (for viscosity breaking), the longer higher-molecular-weight molecules arepreferentially (statistics) broken.

The MWD can be made broader by using a two-reactor configuration thatproduces different melt flow rates in each reactor. Recently, metallocene PP cata-lysts have shown the ability to produce PPs with very narrow molecular weightdistributions, on the order of 23. These resins have a great deal of value in fiberextrusion applications where less shear sensitivity of the viscosity is important.

2.6 MORPHOLOGY

Homopolymer PP exists as a two- and possibly a three-phase system of crystallineand amorphous phases with the amorphous phase being comprised of a crystalliz-able isotactic portion and a noncrystallizable atactic portion. The noncrystalliz-able, gummy, atactic PP phase has small amounts of a low molecular weight oilymaterial at a level of 1% and lower. The latter has been characterized in someproducts as having some structural inversions of propylene monomers and somebranches other than methyl. Typical levels of crystallinity in extruded PP pelletsare in the 6070% range. One way to describe the morphology of PP is to con-sider it an assemblage of crystallites that act as physical cross-links in an amor-phous matrix.

In the crystalline phase, the alpha or monoclinic phase is the dominant crystalform of PP with a melting point of about 160C. The beta or hexagonal phaseis less common and less stable. The latter has a melting point of about 145C.Typical levels of beta crystallites are less than 5% in injection molded parts.

2.7 THERMAL ANALYSIS

A number of techniques fall under the thermal analysis heading. For PP charac-terization, one of the most useful is differential scanning calorimetry (DSC). A

22 Kissel et al.

technique giving essentially the same information, although data are developedbased on a somewhat different principle, is differential thermal analysis (DTA).In DSC, thermal transitions are recorded as a function of temperature, which iseither increased or decreased at a defined heating or cooling rate.

Some of the useful information derived from DSC heating scans includesthe melting temperature, which is taken as the maximum of the endothermic peak,and the heat of fusion, determined by integrating the area under the endothermicpeak. The melting temperature of PP homopolymer is about 160C, whereas thatof usual PP random copolymers is about 145C. Polypropylene impact copoly-mers exhibit the same melting temperatures as homopolymers, the rubber constit-uent not affecting the melting temperature. Impact copolymers do, however, havelower heats of fusion than homopolymers because the heat of fusion is relatedto the proportion of crystalline polymer present. The rubber portion is essentiallynoncrystalline and therefore does not melt.

In the DSC cooling of PP from the melt, crystallization occurs. The minimumof the exothermic peak defines the crystallization temperature. This temperatureis an indication of how rapidly the PP crystallizes. The higher the temperature,the more rapid the crystallization. Nucleating agents added to PP increase thecrystallization rate of PP, resulting in a higher crystallization temperature. PPcrystallizes such that crystalline structures called spherulites are formed. Nucle-ation results in the formation of smaller spherulites than would otherwise havebeen formed. This, importantly, results in increased clarity and stiffness but alsoimparts some possibly undesirable features, such as warpage or brittleness.

Another important transition detected by DSC is the glass transition. Thisis the transition that amorphous (noncrystalline) materials undergo in changingfrom the liquid to rubbery state. In i-PP this is difficult to detect by DSC becausethe concentration of amorphous PP is small, but detection is easy in a-PP, theglass transition temperature being in the vicinity of 15C.

There are several other thermal analysis techniques. In thermomechanicalanalysis (TMA), mechanical changes are monitored versus temperature. Expan-sion and penetration characteristics or stressstrain behavior can be studied. Indynamic mechanical analysis (DMA), the variations with temperature of variousmoduli are determined, and this information is further used to obtain fundamentalinformation such as transition temperatures. In thermogravimetric analysis(TGA), weight changes as a function of temperature or time (at some elevatedtemperature) are followed. This information is used to assess thermal stabilityand decomposition behavior.

2.8 MANUFACTURING PROCESSES

The process technology for PP manufacture has kept pace with catalyst advancesand the development of new product applications and markets. In particular, the

Characteristics of Polypropylene 23

Figure 2.1 Early slurry process technology.

relationship between process and catalyst technology was clearly symbiotic andthat of a partnership. Advances in one technology had always exerted a strongpushpull effect on the other to improve its performance. The progress in processtechnology has resulted in process simplification, investment cost and manufac-turing cost reductions, improvement in plant constructability, operability, andbroader process capabilities to produce a wider product mix.

The simplified block diagrams in Figs. 2.12.3 serve to illustrate the ad-vances in PP process technology from a complex process in Fig. 2.1 to one thatis simpler in Fig. 2.3. The slurry process technology as illustrated in Fig. 2.1 istypical of manufacturing units built in the 1960s and 1970s. This technology wasdesigned for catalysts of the first and second generations. It required a solventsuch as butane, heptane, hexane, or even heavier isoparaffins. The solvent served

Figure 2.2 Bulk (slurry) process technology.

24 Kissel et al.

Figure 2.3 Gas-phase process technology.

as the medium for dispersion of the polymer produced in the reactors and fordissolving the high level of atactic byproducts for removal downstream. The useof a solvent also facilitated the catalyst deactivation and extraction (or deashing)step, which required contacting the reactor product with alcohol and caustic solu-tions. Plants based on this technology required a large amount of equipment, agreat deal of space, and complicated plot plans. They were high in both capitaland operating costs, labor intensive, and energy inefficient. Moreover, there wereenvironmental and safety issues associated with the handling of a large volumeof solvent and the disposal of the amorphous atactic byproducts, and a largewastewater stream containing residual catalyst components. With the advent ofthird- and fourth-generation catalysts, many of these older slurry plants stayedviable by cost reduction aided by the higher catalyst activities and lower atacticproduction. They also benefitted from plant capacity creeps and debottleneck-ing.

The slurry process technology evolved into the more advanced slurry process(Fig. 2.2) in the late 1970s to take advantage of the higher performing third-generation catalysts initially and later the even better fourth-generation catalysts.The improved slurry processes were commonly referred to as the bulk (slurry)process. One major change from the older slurry technology was the substitutionof liquid propylene in place of the solvent system. This became possible becausecatalyst de-ashing and atactic removal were no longer needed to produce accept-able PP resins. With very few exceptions, virtually all slurry plants built overthe last two decades were based on bulk process technology. Montells Spheripolprocess represents technology of this type, using pipe loop reactors operated liq-uid full, with a PP slurry in liquid propylene. Additionally, a fluidized bed reactoris used by Spheripol downstream of the bulk pipe loop reactors when impactcopolymers are in the product slate.

The emergence of gas-phase process technology for PP occurred about thesame time as the bulk processes. Gas-phase technology was revolutionary in that

Characteristics of Polypropylene 25

it completely avoided the need for a solvent or liquid medium to disperse thereactants and reactor product. This process eliminates the separation and recoveryof large quantities of solvents or liquid propylene required in slurry or bulk reac-tors. The PP products from the gas-phase reactors are essentially dry, requiringonly deactivation of the very low level of catalyst residues before the incorpora-tion of additives and pelletization. Thus, this process technology reduced themanufacturing of PP to the bare essential steps. Representatives of commercialgas-phase process technology include Amoco, Union Carbide (Unipol), andBASF (Novolen).

Amocos technology features a horizontal stirred bed reactor system thatuses mild mechanical agitation for reactor mixing and temperature control. Theheat of polymerization is removed by the use of quench cooling or evaporativecooling using a spray of liquid propylene. The Unipol process is based on a gasfluidization principle that relies on a large volume of fluidizing gas for reactormixing, polymerization heat removal, and temperature control. According totrade literature, Unipol has claimed that the gas cooling can now be supplementedby some amount of liquid evaporation in the fluidized bed, referred to as thecondensing mode cooling. The BASF gas-phase reactor is a vertical stirredbed reactor in which the polymerization heat is removed by vaporization of liquidpropylene in the bed. In the above three gas-phase processes, a second reactorof a similar design as the first reactor is added for the production of impactcopolymers. A sketch of the reactor systems associated with the four types ofcommercial PP process technology described aboveAmoco, Spheripol, BASF,Unipolis shown in Fig. 2.4. The Amoco gas-phase process technology is morecompletely depicted in Fig. 2.5.

In summary, over four decades, PP process technology has never stoppedcreating value for the resin customers through both incremental and generationalchanges. The changes came about through a partnership with advancements incatalysts to result in better manufacturing economics and simpler plants, makingthem easier to operate and at higher efficiencies. At the same time, the improvedprocess technology has also added enhancements to many product properties andexpanded the product applications.

2.8.1 World-Scale TechnologyThe PP industry is exciting and will continue to grow globally at a rate attractiveto making new investments. Obviously, it is also highly competitive, and theresin customers have high expectations. To favorably compete and to satisfy cus-tomers, PP producers must have access to world-scale technology when newinvestment is being considered. The criteria for world-scale technology are thefollowing:

26 Kissel et al.

Figure 2.4 Reactor systems in polypropylene technologies.

1. Simple and efficient process;2. Attractive economics for resin manufacture: low plant investment and

operating costs;3. Efficient and high performance with fourth-generation catalysts;4. Capability for a wide range of products, with the ability to allow easy

product transitions in manufacturing;5. Environmentally clean and safe operations;6. Capability of plant design for high single-line capacity;7. Commitment of technology provider to continuous improvements and

innovations.

To improve capital utilization and remain competitive, we believe that all newplants should have production capacity no less than 150,000 metric ton/yr. Anew trend is to build larger units with production capacity over 200,000 metricton/yr.

Characteristics of Polypropylene 27

Figure 2.5 Amoco gas-phase process technology.

2.9 POLYPROPYLENE APPLICATIONS

As is obvious from the preceding discussion in this chapter, PP should really beconsidered a group of polymers, not just a single polymer. Because the propertiesof PPs cover a substantial range, the applications of PP are quite diverse. This,of course, belies the usual classification of PP as a commodity resin. The mostimportant applications of PP are discussed in this section.

Organizing a discussion on applications is challenging because the questionarises as to whether similarity of uses or similarity of the fabricated products orsimilarity of the fabrication techniques should be used as the criterion for arrang-ing information. None of the methods is perfect. Here the material is organizedin a fashion that intertwines these, but it seems logical to the authors based onour experience.

2.9.1 Fibers and FabricsA great volume of PP finds its way into an area that may be classified as fibersand fabrics. Fibers, which broadly speaking includes slit-film or slit-tape, are

28 Kissel et al.

produced in various kinds of extrusion processes. The advantages offered by PPinclude low specific gravity, which means greater bulk per given weight, strength,chemical resistance, and stain resistance.

Slit-FilmIn slit-film production, wide web extruded film, which is oriented in the machinedirection by virtue of the take-up system, is slit into narrow tapes. These tapesare woven into fabrics for various end uses.

In general, non-CR homopolymer of about 24 g/10 min is used in thisapplication. Higher flow rate resins permit higher extrusion speeds, but lowerMFR resins result in a higher tenacity at a given draw ratio.

A major application of slit-film is in carpet backings, both the primary andsecondary types. The primary carpet backing is not the one that is seen on theback of a carpet; that is the secondary backing. The primary backing is the onethat is between the secondary backing and the face yarns and is the one to whichthe face yarns are tufted. The secondary backing protects the tufted fibers andadds substance to the carpet. Today, more carpet backing is produced from PPthan from the natural jute fibers, which at one time were dominant. Jute suffersfrom its unsteady supply situation, being affected by weather and producing-country politics. Moreover, PP is not subject to damaging moisture absorptionand mold attack in high humidity.

Slit-film also finds its way into many other applications. These include twine,woven fabrics for feed and fertilizer sacks, sand bags and bulk container bags,tarpaulins, mats, screens for erosion prevention, and geotextiles to stabilize soilbeds. Fibrillated slit-film is used as a face yarn in outdoor carpets and mats.

Continuous Filament FibersContinuous filament (CF) fibers are more conventional fibers than slit-film fibersin that each strand results from extrusion through its own die hole. Polypropylenehomopolymer is extruded through rather small holes in a die called a spinneret,each spinneret containing somewhere around 150 holes each, and spinning speedsare often high. The resulting filaments are very fine, being on the order of 5denier per filament (dpf; a denier being defined as 1 g/9000 m). Therefore, toreduce viscosity, relatively high melt flow rate PP (ca. 35) is used, and highprocess temperatures (ca. 250 C) are used. Usually, a CR resin is the choicebecause a narrow molecular weight distribution is desired.

Extrusion takes place through several spinnerets at the same time. The fila-ments are quenched with air, and the group from each spinneret is then taken upto produce yarn, the draw ratio and degree of orientation depending on the take-up equipment, on whether the process is single or two stage, and on the end use.Continuous filament fibers are not crimped or texturized as produced. Such bulk-

Characteristics of Polypropylene 29

ing can be imparted in a secondary process or the CF fiber yarns can be used asis, often in combination with other types of yarns.

Bulked Continuous FilamentBulked continuous filament (BCF) processes are similar to CF processes, but onemain difference is that in BCF a texturizer is an integral part of the process, toimpart bulk to the fibers, through crimps or kinks. Commonly, BCF fibers areof ca. 20 dpf, with yarns being in the vicinity of 2000 denier. Polypropylenehomopolymer with MFRs in the range of 1220 MFR and normal MWD is typi-cally used. BCF yarns are mainly used as carpet face yarns and in fabrics forupholstery.

CarpetsThe carpets constructed with PP face yarns are currently largely of the commer-cial type. For residential carpeting, dyability and deep pile construction havebeen desirable. Polypropylene cannot readily be dyed, and therefore PP fibersare colored via the addition of pigments during extrusion. Pigmented fibers havea somewhat different appearance compared with dyed fibers, such as those fromwool, nylon, and polyester. Polypropylene fibers have also been less resilient indeep pile than those from wool or nylon. However, with advances in technologyin PP resins, fiber, and carpet construction, PP usage in residential carpets issteadily growing.

Commercial carpets are of short pile, dense construction, and therefore resil-iency of PP fibers is not an issue. Furthermore, for the same reason that PP cannotbe dyed, it resists staining and soiling. Because muted tones are desirable forcommercial carpeting, the colors available from pigmentation are ideal. Also,pigmented fibers are more colorfast and fade less in sunlight. Another plus is thatin pigmented fibers, the color does not just reside on the surface, it is distributeduniformly throughout; therefore, fiber breakage in rough treatment does not resultin loss of color.

Staple FibersStaple fibers are short fibers, ranging from less than an inch to a little less thana foot in length, depending on the application. Staple fibers are spun fibers thatare produced in either of two somewhat different processes. In the long-spintraditional process, fiber is spun similarly to CF fibers, wound undrawn in a tow(bundle) in one step and then drawn (if desired), crimped (if desired), and cutin a second step.

The other staple fiber process is known as the short-spin or compact spinningprocess. This is a one-step process with relatively slow spinning speeds of about

30 Kissel et al.

300 ft/min, but one in which the spinnerets have an extremely large number ofholes (50,000100,000 or more). Overall, the process is more economical thanthe traditional process.

Staple fibers can be carded and drawn into spun yarns in the same way asnatural fibers, or they can be used in nonwoven fabrics. The PP resins used areusually homopolymers with MFRs from 1030 g/10 min, depending on the appli-cation.

Nonwoven FabricsNonwoven fabrics account for more PP usage than any other single fiber applica-tion. There are three types of nonwoven fabrics: thermobonded, from staple fi-bers; spunbonded; and melt-blown. The spunbonded and melt-blown processesare discussed below. The fabrics from each process differ from each other inproperties and appearance, and often combinations of two types are used together.Spunbonded fabrics are strong, whereas melt-blown fabrics are soft and havehigh bulk.

Nonwoven fabrics are used in several areas, probably the most well knownbeing for the liners in disposable diapers. Similar fabric is also used in femininehygiene products. At the other extreme, civil engineering fabric and tarpaulinsare also produced from nonwoven fabrics.

Spunbonded FabricIn this process, molten filaments are air quenched and then drawn by air at highpressure. To form a fabric, the filaments are then deposited on a moving porousbelt to which a vacuum is applied underneath. Bonding of the fibers is accom-plished by passing the fabric through heated calender rolls. The type of resinusually used is a CR homopolymer of ca. 35 g/10 min MFR.

Melt-Blown Fiber/FabricIn the melt-blown process, polymer is extruded through a special melt-blowingdie. The die feeds high-temperature air to the exiting filaments at a high velocity,and, in addition, the exiting filaments are quenched with cool air. In the process,drawn solidified filaments are formed very close to the die. Formation of thefabric is accomplished by blowing the filaments onto a moving screen. The melt-blowing process requires resins that have very high melt flow rates, on the orderof 500 g/10 min or higher. A resin of narrow molecular weight distribution isalso desirable.

The fibers formed in the melt-blown process are very fine and allow for theproduction of lightweight uniform fabrics that are soft but not strong. Fabricsfrom fine melt-blown fibers can be used in medical applications because they

Characteristics of Polypropylene 31

allow the passage of water vapor but prevent the penetration of liquid water andaqueous solutions.

MonolamentAs the name implies, single filaments are extruded; the molten filaments arecooled and solidified in a water bath and then drawn. Typically, monofilamentsare fairly large, being on the order of 250 dpf. Rope and twine are produced bytwisting bundles of monofilament together. Polypropylene rope and twine arestrong and moisture resistant, making them very useful in marine applications.

Low MFR (usually between 1 and 4 g/10 min) homopolymer is most oftenused for monofilament. This provides the high tensile strength (tenacity) requiredin this application.

2.9.2 StrappingStrapping is similar to slit-film but thicker, being on the order of 20 mils. As thename implies, strapping is used to secure large packages or boxes or to hold stackstogether. It takes the place of steel strapping, and its most important property isstrength, although the moisture resistance of PP is also an important attribute. Itis produced from either direct extrusion or from sheet that is slit. Uniaxial orienta-tion is applied by drawing. Homopolymer resins of low MFR (between 1.0 and1.5 g/10 min) are used for this application.

2.9.3 FilmBy definition, film is less than 10 mils thick. There are two broad classes of film,cast film and oriented film.

Cast FilmIn cast film processes, polypropylene is extruded through a die onto a chill rolland the resulting film is eventually taken up on winding equipment. Cast film isessentially unoriented but is still fairly clear because of the quench cooling thatoccurs. Film thickness usually ranges between 1 and 4 mils. An important featureof cast film is its softness and lack of cellophane-like crispness. Both homopoly-mers and random copolymers are used in cast film, the MFR most commonlybeing around 8 g/10 min. Random copolymers give slightly clearer, softer, andmore impact-resistant film.

Cast film is converted to products that include bags for clothing articles suchas mens shirts, pocket-pages for photographs, sheet protectors, and as the outernonporous sheet on disposable diapers. Cast film also is found in some tapes andpressure-sensitive labels.

32 Kissel et al.

Biaxially Oriented Polypropylene FilmTwo methods are widely used for producing biaxially oriented PP (BOPP) film.One is the tenter process, and the other is the tubular or bubble process. In both,homopolymer of about 3 g/10 min MFR is most widely used, although randomcopolymer is used for better heat sealability.

In the tenter process, extruded sheet is drawn sequentially, first in the ma-chine direction and then in the transverse or cross direction. Draw ratios of 4 7 to 6 10 are common. Film thickness ranges from 0.5 to 2.5 mils.

In the tubular process, a tube is extruded downward through an annular dieover a mandrel that cools the tube. The tube is taken up through a water bathfor further cooling. The flat tube is moved by rolls through an oven where it isreheated to a temperature close to the melting temperature. The tube is then in-flated while being stretched in the machine direction, imparting orientationthrough roughly sixfold biaxial stretching. Films typically range in thickness from0.25 to 2 mils.

Biaxially oriented PP film has excellent clarity and gloss and is printablethrough the use of some additional surface treatment technology. The main appli-cations for BOPP film are in flexible packaging. A major use is in snack-foodpackaging, where the BOPP film is used in one or more layers in a multilayerbag construction. The BOPP film provides resistance to moisture vapor to keepsnacks crisp and fresh tasting and provides a heat sealable layer. Biaxially ori-ented PP film by itself is used for packaging of bakery products (e.g., bags forfreshly baked bread). Many adhesive tapes are also produced from BOPP film.

A special kind of BOPP film, known as opaque film, is used for packagingproducts such as candy, chocolate bars, and soaps and for labels, such as thosenow used on most soft drink bottles. Opaque film is produced in the tenter processfrom PP to which a fine filler (e.g., calcium carbonate) or an incompatible poly-mer has been added.

2.9.4 Sheet/ThermoformingSheet is an extruded product that is greater than 10 mils in thickness (below thisthe product is identified as film), 40 mils being typical. Resin is extruded througha die and passes through a cooling roll stack and conveyed to nip rolls, afterwhich sheet is wound on rolls or cut and stacked or conveyed directly to a thermo-forming machine. Sheet width is usually between 2 and 7 ft.

Although there are other applications for sheet, the production of thermo-formed containers for rigid packaging applications predominates. However, PP,being a semicrystalline polymer, is not as ideal a material for the conventionalthermoforming process as is polystyrene, an amorphous polymer. Polystyrenehas been used with great success in the thermoforming of rigid packages for food

Characteristics of Polypropylene 33

for certain dairy and deli applications. Usually polystyrene is first extruded intosheet that is reheated as it is sent to the thermoformer. The heated sheet is forcedinto the cavities of a multicavity mold by a combination of vacuum and pushingby a plug. The cooled containers are cut