Introduction to Industrial Polyethylene

Properties, Catalysts, and Processes

Dennis B. Malpass

• Publisher: Wiley-Scrivener
• Year: 2010
• ISBN:
  9781118031797
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Industrial & Technical Chemistry

Demystifies the largest volume manmade synthetic polymer by distillingthe fundamentals of what polyethylene is, how it's made and processed, and what happens to it after its useful life is over.

Endorsement for Introduction to Industrial Polyethylene

"I found this to be a straightforward, easy-to-read, and useful introductory text on polyethylene, which will be helpful for chemists, engineers, and students who need to learn more about this complex topic. The author is a senior polyethylene specialist and I believe we can all benefit from his distillation of knowledge and insight to quickly grasp the key learnings."
— R.E. King III; Ciba Corporation (part of the BASF group)

Jargon used in industrial polyethylene technology can often be bewildering to newcomers. Introduction to Industrial Polyethylene educates readers on terminology commonly used in the industry and demystifies the chemistry of catalysts and cocatalysts employed in the manufacture of polyethylene.

This concise primer reviews the history of polyethylene and introduces basic features and nomenclatures for this versatile polymer. Catalysts and cocatalysts crucial to the production of polyethylene are discussed in the first few chapters. Latter chapters provide an introduction to the processes used to manufacture polyethylene and discuss matters related to downstream applications of polyethylene such as rheology, additives, environmental issues, etc.

Providing industrial chemists and engineers a valuable reference tool that covers fundamental features of polyethylene technology, Introduction to Industrial Polyethylene:

• Identifies the fundamental types of polyethylene and how they differ.
  
  

• Lists markets, key fabrication methods, and the major producers of polyethylene.
  
  

• Provides biodegradable alternatives to polyethylene.
  
  

• Describes the processes used in the manufacture of polyethylene.
  
  

• Includes a thorough glossary, providing definitions of acronyms and abbreviations and also defines terms commonly used in discussions of production and properties of polyethylene.
  
  

• Concludes with the future of industrial polyethylene.
  
  

1

Introduction to Polymers of Ethylene

1.1 Genesis of Polyethylene

Modern polyethylene has its origins in work by chemists at Imperial Chemical Industries beginning in 1933 (1). Eric Fawcett and Reginald Gibson were trying to condense ethylene with benzaldehyde at very high pressure and temperature (142 MPa^* and 170 °C). They obtained a small amount of a residue that they concluded was polyethylene, but attempts to repeat the experiment minus benzaldehyde resulted in explosions. In late 1935, ICI chemist Michael Perrin succeeded in preparing a larger amount of polyethylene. Serendipitously, Perrin used ethylene containing traces of oxygen. Either oxygen itself or peroxides formed in situ initiated free radical polymerization of ethylene. In 1939, ICI began commercial production of high pressure polyethylene (“HPPE”) now known as low density polyethylene (LDPE), and the product was used to insulate radar cable during World War II.

Work of other researchers portended the discovery of polyethylene. For example, in 1898, Hans von Pechman produced a composition he called “polymethylene” by decomposition of diazomethane. “Polymethylene” was also produced by other 2hemistries, including the Fischer-Tropsch reaction. Most of these polymers had low molecular weights. In 1930, Marvel and Friedrich produced a low molecular weight polyethylene using lithium alkyls, but did not follow-up on this finding. Descriptions of early work on polyethylene have been provided by McMillan (1), Kiefer (2) and Seymour (3, 4).

Other noteworthy milestones in the evolution of industrial polyethylene include the following:

• In the early 1950s, transition metal catalysts that produce linear polyethylene were independently discovered by Hogan and Banks in the US and Ziegler in Germany.
• Gas phase processes, LLDPE and supported catalysts emerged in the late 1960s and 1970s.
• Kaminsky, Sinn and coworkers discovered in the late 1970s that an enormous increase in activity with metallocene single-site catalysts is realized when methylaluminoxane (discussed in Chapter 6) is used as cocatalyst.
• In the 1990s, polyethylenes produced with metallocene single site catalysts were commercialized and non-metallocene single-site catalysts were discovered by Brookhart and coworkers.

A timeline of notable 20^th century polyethylene developments is provided in Figure 1.1.

This chapter introduces basic features of polyethylene, a product that touches everyday life in countless ways. However, polyethylene is not monolithic. The various types, their nomenclatures, and how they differ will be discussed. Key characteristics and classification methods will be briefly surveyed. An overview of transition metal catalysts has been included in this introductory chapter (see section 1.5) because these are the most important types of catalysts currently used in the manufacture of polyethylene. Additional details on transition metal catalysts will be addressed in subsequent chapters.

This chapter may be skipped by readers having an understanding of fundamental properties and nomenclatures of industrial polyethylene and a basic understanding of catalysts.

1.2 Basic Description of Polyethylene

Ethylene (CH₂=CH₂), the simplest olefin, may be polymerized (eq 1.1) through the action of initiators and catalysts. Initiators are most commonly organic peroxides and are effective because they generate free radicals which polymerize ethylene via a chain reaction. Transition metal catalysts (primarily Ziegler-Natta and Phillips) are also widely employed in industry but produce polyethylene with different properties and by different mechanisms. Single-site catalysts also involve transition metal catalysts, but the quantity of polyethylene produced with single site catalysts at this writing is small (<4%). Initiators, transition metal catalysts and cocatalysts are discussed in Chapters 2–6.

Figure 1.1 20^th century milestones in polyethylene.

Conditions for polymerization vary widely and polyethylene compositions, as noted above, also differ substantially in structure and properties. In eq 1.1, subscript n is termed the degree of polymerization (DP) and is greater than 1000 for most of the commercially available grades of polyethylene.

The polymer produced in eq 1.1 is known as polyethylene and, less commonly, as polymethylene, polyethene or polythene. (In the late 1960s, “polythene” became part of popular culture when the Beatles released “Polythene Pam.”) Polyethylene is the IUPAC recommended name for homopolymer. As we shall see, however, many important ethylene-containing polymers are copolymers. Nomenclatures for various types of polyethylene are addressed in section 1.3. Though some have suggested that its name implies the presence of unsaturated carbon atoms, there are in fact few C=C bonds in polyethylene, usually less than 2 per thousand carbon atoms and these occur primarily as vinyl or vinylidene end groups.

Polyethylene is the least costly of the major synthetic polymers. It has excellent chemical resistance and can be processed in a variety of ways (blown film, pipe extrusion, blow molding, injection molding, etc.) into myriad shapes and devices. Fabrication methods will be briefly discussed in Chapter 8.

As removed from industrial-scale reactors under ambient conditions, polyethylene is typically a white powdery or granular solid. In most cases, the raw polymer is then melted and selected additives are introduced. (Additives are essential to improve stability and enhance properties of polyethylene. See Chapter 8.) The product is shaped into translucent pellets and supplied in this form to processors. Pelletization increases resin bulk density resulting in more efficient packing and lower shipping costs. It also lowers the possibility of dust explosions while handling.

Raw polyethylene resin is melted and shaped into pellets. This increases bulk density, improves handling characteristics and reduces shipping costs. Pellet size is typically −3 mm (or ~ 0.1 in).

Polyethylene is a thermoplastic material. That is, it can be melted and shaped into a form which can then be subsequently remelted and shaped (recycled) into other forms. Polyethylene does not typically have a sharp melting point (T_m), but rather a melting range owing to differences in molecular weight, crystallinity (or amorphous content), chain branching, etc. Nevertheless, “melting points” between about 120 and 140 °C are cited in the literature. Because polyethylene is usually processed above 190 °C, where it is completely amorphous, melting ranges are less important than flow characteristics of the molten polymer. Molten polyethylene is a viscous fluid and is an example of what are termed “non-Newtonian fluids,” that is, flow is not directly proportional to pressure applied (see section 8.3 of Chapter 8).

Polymerization of ethylene illustrated in eq 1.1 may be terminated by several pathways leading to different end groups. The type of end group depends upon several factors, such as polymerization conditions, catalyst and chain transfer agents used. Since end groups are primarily simple alkyl groups, polyethylene may be regarded as a mixture of high molecular weight alkanes.

Chain branching in low density versions of polyethylene is common. Extent and length of branching stem primarily from the mechanism of polymerization and incorporation of comonomers. Branching is classified as long chain branching (LCB) or short chain branching (SCB). By convention, SCB implies branches of 6 or fewer carbon atoms. LDPE contains extensive LCB and branches can contain hundreds of carbon atoms. Branches on branches are also common in LDPE.

This increases amorphous content and contributes to LDPE attributes, such as film clarity and ease of processing. As branching increases, density decreases. In LLDPE, incorporation of relatively large quantities of alpha olefin comonomers results in abundant SCB and lowering of density.

Ethylene may be copolymerized with a range of other vinylic compounds, such as 1-butene, 1-octene and vinyl acetate (VA). These are termed comonomers and are incorporated into the growing polymer. Comonomers that contain oxygenated groupings such as vinyl acetate are often referred to as “polar comonomers.” Comonomer contents range from 0 to ~1 wt% for HDPE up to ~40 wt% for some grades of ethylene-vinyl acetate copolymer.

The range of suitable comonomers depends upon the nature of the catalyst or initiator. For example, Ziegler-Natta catalysts are poisoned by polar comonomers. Hence, commercial copolymers of ethylene and vinyl acetate are currently produced only with free radical initiators. However, some single site catalysts are tolerant of polar comonomers (see section 6.2.1).

When ethylene is copolymerized with substantial amounts (>25%) of propylene an elastomeric copolymer is produced, commonly known as ethylene-propylene rubber (EPR) or ethylene-propylene monomer (EPM) rubber. When a diene, such as dicyclopentadiene, is also included, a terpolymer known as ethylene-propylene-diene monomer (EPDM) rubber is obtained. EPR and EPDM are produced with single site and Ziegler-Natta catalysts and are important in the automotive and construction industries. However, EPR and EPDM are produced in much smaller quantities relative to polyethylene. Elastomers display vastly different properties than other versions of industrial polyethylene and are considered outside the purview of this text. EPR and EPDM will not be discussed further.

In copolymerizations of ethylene and α-olefins us...

Table of contents

• Cover
• Title Page
• Copyright
• Contents
• Preface
• List of Tables
• List of Figures
• Chapter 1 - Introduction to Polymers of Ethylene
• Chapter 2 - Free Radical Polymerization of Ethylene
• Chapter 3 - Ziegler-Natta Catalysts
• Chapter 4 - Metal Alkyls in Polyethylene Catalyst Systems
• Chapter 5 - Chromium Catalysts
• Chapter 6 - Single Site Catalysts
• Chapter 7 - An Overview of Industrial Polyethylene Processes
• Chapter 8 - Downstream Aspects of Polyethylene
• Glossary
• Trade Name Index
• Index
• Also of Interest