Matar Sami, Hatch Lewis F. Chemistry of petrochemical processes

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chloride, dissolves in an organic solvent (such as benzene or toluene),

and the second reactant, a diamine or a diacid, dissolves in water. This

technique produces polycarbonates, polyesters, and polyamides. The

reaction occurs at the interface between the two immiscible liquids, and

the polymer is continuously removed from the interface.

PHYSICAL PROPERTIES OF POLYMERS

The properties of polymers determine whether they can be used as a

plastic, a fiber, an elastomer, an adhesive, or a paint.

Important physical properties include the density, melt flow index,

crystallinity, and average molecular weight. Mechanical properties of a

polymer, such as modulus (the ratio of stress to strain), elasticity, and

breaking strength, essentially follow from the physical properties.

The following sections describe some important properties of polymers.

CRYSTALLINITY

A polymer’s tendency to have order and form crystallites derives from

the regularity of the chains, presence (or absence) and arrangement of

bulky groups, and the presence of secondary forces, such as hydrogen

bonding. For example, isotactic polystyrene with phenyl groups arranged

on one side of the polymer backbone is highly crystalline, while the atac-

tic form (with a random arrangement of phenyl groups) is highly amor-

phous. Polyamides are also highly crystalline due to strong hydrogen

bonding. High-density polyethylene exhibits no hydrogen bonding, but

its linear structure makes it highly crystalline. Low-density polyethylene,

on the other hand, has branches and a lower crystallinity. It does not pack

as easily as the high-density polymer.

The mechanical and thermal behaviors depend partly on the degree of

crystallinity. For example, highly disordered (dominantly amorphous)

polymers make good elastomeric materials, while highly crystalline

polymers, such as polyamides, have the rigidity needed for fibers.

Crystallinity of polymers correlates with their melting points.

MELTING POINT

The freezing point of a pure liquid is the temperature at which the

liquid’s molecules lose transitional freedom and the solid’s molecules

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Chapter 11 1/22/01 11:10 AM Page 317

become more ordered within a definite crystalline structure. Polymers,

however, are non-homogeneous and do not have a definite crystalliza-

tion temperature.

When a melted polymer cools, some polymer molecules line up and

form crystalline regions within the melt. The rest of the polymer remains

amorphous. The temperature at which these crystallites disappear when

the crystalline polymer is gradually heated is called the crystalline

melting temperature, T

. Further cooling of the polymer below T

changes the amorphous regions into a glass-like material. The tempera-

ture at which this change occurs is termed the glass transition tempera-

ture, T

. Elastomeric materials usually have a low T

(low crystallinity),

while highly crystalline polymers, such as polyamides, have a relatively

high T

VISCOSITY

The viscosity of a substance measures its resistance to flow. The melt

viscosity of a polymer increases as the molecular weight of the polymer

rises. Polymers with high melt viscosities require higher temperatures

for processing.

The melt flow index describes the viscosity of a solid plastic. It is the

weight in grams of a polymer extruded through a defined orifice at a

specified time. The melt viscosity and the melt flow index can measure

the extent of polymerization. A polymer with a high melt flow index has

a low melt viscosity, a lower molecular weight, and usually a lower

impact tensile strength.

MOLECULAR WEIGHT

Polymerization usually produces macromolecules with varying chain

lengths. As a result, polymers are described as polydisperse systems.

Commercial polymers have molecular weights greater than 5,000 and

contain macromolecules with variable molecular weights. The methods

for determining the average molecular weights of polymers include

measuring some colligative property, such as viscosity or sedimentation.

Different methods do not correlate well, and determining the average

molecular weight requires more than one method. Two methods normally

determine the number average and the weight average molecular weights

of the polymer.

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Number Average Molecular Weight

The number average molecular weight (M

) is related to the number

of particles present in a sample. It is calculated by dividing the sum of

the weights of all the species present (monomers, dimers, trimers, and so

on) by the number of species present:

Polymerization 319

∑

i = degree of polymerization (dimer, trimers, etc.)

= number of each polymeric species

= molecular weight of each polymer species

W = total weight of all polymer species.

depends not on the molecular sizes of the particles but on the number

of particles. Measuring colligative properties such as boiling point ele-

vation, freezing point depression, and vapor pressure lowering can deter-

mine the number of particles in a sample.

Weight Average Molecular Weight

The weight average molecular weight (M

) is the sum of the products

of the weight of each species present and its molecular weight, divided

by the sum of all the species’ weights:

ii ii

∑∑

∑

= weight of each polymeric species

= molecular weight of each polymeric species

Substituting N

= W

, the weight average molecular weight can be

defined as

∑

Larger, heavier molecules contribute more to M

than to M

. Light scat-

tering techniques and ultracentrifugation can determine M

The following simple example illustrates the difference between M

and M

: Suppose a sample has six macro-molecules. Three of them have

Chapter 11 1/22/01 11:10 AM Page 319

a molecular weight = 1.0 × 10

, two have a molecular weight = 2.0 × 10

and one has a molecular weight = 3.0 × 10

320 Chemistry of Petrochemical Processes

=×

×+×+×

×+ ×+ ×

=×

(. . .)

(. ) (. ) (. )

30 40 3010

17 10

31010 22010 13010

20 10

42 42 42

444

In monodispersed systems M

= M

The difference in the value between M

and M

. indicates the poly-

dispersity of the polymer system. The closer M

is to M

, the narrower

the molecular weight spread. Molecular weight distribution curves for

polydispersed systems can be obtained by plotting the degree of poly-

merization i versus either the number fraction, N

, or the weight frac-

tion, W

CLASSIFICATION OF POLYMERS

Synthetic polymers may be classified into addition or condensation

polymers according to the type of reaction. A second classification

depends on the monomer type and the linkage present in the repeating

unit into polyolefins, polyesters, polyamides, etc. Other classifications

depend on the polymerization technique (bulk, emulsion, suspension,

etc.) or on the polymer’s end use. The latter classifies polymers into three

broad categories: plastics, elastomers, and synthetic fibers.

Plastics

Plastics are relatively tough substances with high molecular weight

that can be molded with (or without) the application of heat. In general,

plastics are subclassified into thermoplastics, polymers that can be

resoftened by heat, and thermosets, which cannot be resoftened by heat.

Thermoplastics have moderate crystallinity. They can undergo large

elongation, but this elongation is not as reversible as it is for elastomers.

Examples of thermoplastics are polyethylene and polypropylene.

Thermosetting plastics are usually rigid due to high crosslink-

ing between the polymer chains. Examples of this type are phenol-

fomaldehyde and polyurethanes. Crosslinking may also be promoted by

using chemical agents such as sulfur or by heat treatment or irradiation

with gamma rays, ultraviolet light, or energetic electrons. Recently, high

Chapter 11 1/22/01 11:10 AM Page 320

energy ion beams were found to increase the hardness of the treated poly-

mer drastically.

Synthetic Rubber

Synthetic rubber (elastomers) are high molecular weight polymers

with long flexible chains and weak intermolecular forces. They have low

crystallinity (highly amorphous) in the unstressed state, segmental

mobility, and high reversible elasticity. Elastomers are usually cross-

linked to impart strength.

Synthetic Fibers

Synthetic fibers are long-chain polymers characterized by highly crys-

talline regions resulting mainly from secondary forces (e.g., hydrogen

bonding). They have a much lower elasticity than plastics and elas-

tomers. They also have high tensile strength, a light weight, and low

moisture absorption.

REFERENCES

1. Fessenden, R., and Fessenden, J., Organic Chemistry, 4th Ed.,

Brooks/Cole Publishing Co., 1991, p. 926.

2. Hoffman, A. S. and Bacskai, R., Chapter 6 in Copolymerization, G. E.

Ham, (ed.), Wiley-Interscience, New York, 1964.

3. Rodriguez, F., Principles of Polymer Systems, 3rd Ed., Hemisphere

Publishing Corp., New York, 1989, p. 108.

4. Wiseman, P., Petrochemicals, Ellis Horwood Ltd., England, 1986,

p. 45.

5. Seymor, R. and Corraher C. E., Jr., Polymer Chemistry, 2nd Ed.,

Dekker, New York, 1988, p. 284.

6. Kutz, I. and Berber, A., J. Polymer Science, Vol. 42, 1960, p. 299.

7. Stevens, M. P., Polymer Chemistry, Addison Wesley Publishing Co.,

London, 1975, p. 156.

8. Huheey, J. E., Chapter 11 in Inorganic Chemistry, 3rd Ed., Harper and

Row Publishers, Inc., New York, 1983.

9. Allison, M. and Bennet, A., “Novel, Highly Active Iron and Cobalt

Catalysts for Olefin Polymerization,” CHEMTECH, July, 1999,

pp. 24–28.

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Chapter 11 1/22/01 11:10 AM Page 321

10. Watt, G. W., Hatch, L. F., and Lagowski, J. J., Chemistry, New York,

W. W. Norton & Co., 1964, p. 449.

11. Baum, R. “Elastomeric Polypropylene Oscillating Catalyst Controls

Microstructure,” Chemical and Engineering News, Jan., 16, 1995,

pp. 6–7.

12. Arlman, E. and Cossee, P. J., Catal. Vol. 3, 1964, p. 99.

13. Wagner, P. H., “Olefin Metathesis: Applications for the Nineties,”

Chemistry and Industry, 4 May 1992, pp. 330–333.

14. Parshall, G. W. and Nugent, W. A., “Functional Chemicals via

Homogeneous Catalysis,” CHEMTECH, Vol. 18, No. 5, May 1988,

pp. 314–320.

15. Banks, R. L., in “Applied Industrial Catalysis,” Leach, B. E. (ed.)

Academic, New York, 1984, pp. 234–235.

16. Platzer, N., CHEMTECH, Vol. 9, No. 1, 1979, pp. 16–20.

17. Tsonis, C. P., “Catalyzed Polymerization of Cycloolefins,” Journal of

Applied Polymer Science, Vol. 26, 1981, pp. 3525–3536.

18. Dagani, R. “Superhard-Surfaced Polymers Made by High-Energy Ion

Irradiation,” Chemical and Engineering News, Jan. 9, 1995, pp. 24–26.

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CHAPTER TWELVE

Synthetic Petroleum-Based

Polymers

INTRODUCTION

The synthetic polymer industry represents the major end use of many

petrochemical monomers such as ethylene, styrene, and vinyl chloride.

Synthetic polymers have greatly affected our lifestyle. Many articles that

were previously made from naturally occurring materials such as wood,

cotton, wool, iron, aluminum, and glass are being replaced or partially

substituted by synthetic polymers. Clothes made from polyester, nylon,

and acrylic fibers or their blends with natural fibers currently dominate

the apparel market. Plastics are replacing many articles previously made

of iron, wood, porcelain, or paper in thousands of diversified applica-

tions. Polymerization could now be tailored to synthesize materials

stronger than steel.

For example, polyethylene fibers with a molecular

weight of one million can be treated to be 10 times stronger than steel!

However, its melting point is 148°C. A recently announced thermotropic

liquid crystal polymer based on p-hydroxybenzoic acid, terephthalic

acid, and p, pv-biphenol has a high melting point of 420°C and does not

decompose up to 560°C. This polymer has an initial stress of 3.4 × 10

kg/mm

, even after 6,000 hours of testing.

The polymer field is versatile and fast growing, and many new poly-

mers are continually being produced or improved. The basic chemistry

principles involved in polymer synthesis have not changed much since the

beginning of polymer production. Major changes in the last 70 years have

occurred in the catalyst field and in process development. These improve-

ments have a great impact on the economy. In the elastomer field, for

example, improvements influenced the automobile industry and also

related fields such as mechanical goods and wire and cable insulation.

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Chapter 12 1/22/01 11:11 AM Page 323

This chapter discusses synthetic polymers based primarily on monomers

produced from petroleum chemicals. The first section covers the synthe-

sis of thermoplastics and engineering resins. The second part reviews

thermosetting plastics and their uses. The third part discusses the chem-

istry of synthetic rubbers, including a brief review on thermoplastic elas-

tomers, which are generally not used for tire production but to make

other rubber products. The last section addresses synthetic fibers.

THERMOPLASTICS AND ENGINEERING RESINS

Thermoplastics are important polymeric materials that have replaced

or substituted many naturally-derived products such as paper, wood, and

steel. Plastics possess certain favorable properties such as light weight,

corrosion resistance, toughness, and ease of handling. They are also less

expensive. The major use of the plastics is in the packaging field. Many

other uses include construction, electrical and mechanical goods, and

insulation. One growing market that evolved fairly recently is engineer-

ing thermoplastics. This field includes polymers with special properties

such as high thermal stability, toughness, and chemical and weather

resistance. Nylons, polycarbonates, polyether sulfones, and polyacetals

are examples of this group.

Another important and growing market for plastics is the automotive

field. Many automobile parts are now made of plastics. Among the most

used polymers are polystyrene polymers and copolymers, polypropylene,

polycarbonates, and polyvinyl chloride. These materials reduce the cost and

the weight of the cars. As a result, gasoline consumption is also reduced.

Most big-volume thermoplastics are produced by addition polymer-

ization. Other thermoplastics are synthesized by condensation. Table 12-1

shows the major thermoplastics.

POLYETHYLENE

Polyethylene is the most extensively used thermoplastic. The ever-

increasing demand for polyethylene is partly due to the availability of the

monomer from abundant raw materials (associated gas, LPG, naphtha).

Other factors are its relatively low cost, ease of processing the polymer,

resistance to chemicals, and its flexibility. World production of all poly-

ethylene grades, approximately 100 billion pounds in 1997, is predicted

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to reach 300 billion pounds in 2015, the largest increase for linear low

density polyethylene.

High-pressure polymerization of ethylene was introduced in the 1930s.

The discovery of a new titanium catalyst by Karl Ziegler in 1953 revolu-

tionized the production of linear unbranched polyethylene at lower pres-

sures. The two most widely used grades of polyethylene are low-density

polyethylene (LDPE) and high-density polyethylene (HDPE). Currently,

Synthetic Petroleum-Based Polymers 325

Table 12-1

Major thermoplastic polymers

Chapter 12 1/22/01 11:11 AM Page 325

a new LDPE grade has been introduced. It is a linear, low-density grade

(LLDPE) produced like the high-density polymer at low pressures.

Polymerizing ethylene is a highly exothermic reaction. For each gram

of ethylene consumed, approximately 3.5 KJ (850 cal) are released:

nCH

=CH

[–CH2–CH

]–

∆H = –92KJ/mol

When ethylene is polymerized, the reactor temperature should be well

controlled to avoid the endothermic decomposition of ethylene to carbon,

methane, and hydrogen:

=CH

2C + 2H

=CH

C + CH

Low-Density Polyethylene

Low-density polyethylene (LDPE) is produced under high pressure in

the presence of a free radical initiator. As with many free radical chain

addition polymerizations, the polymer is highly branched. It has a lower

crystallinity compared to HDPE due to its lower capability of packing.

Polymerizing ethylene can occur either in a tubular or in a stirred auto-

clave reactor. In the stirred autoclave, the heat of the reaction is absorbed

by the cold ethylene feed. Stirring keeps a uniform temperature through-

out the reaction vessel and prevents agglomeration of the polymer.

In the tubular reactor, a large amount of reaction heat is removed

through the tube walls.

Reaction conditions for the free radical polymerization of ethylene are

100–200°C and 100–135 atmospheres. Ethylene conversion is kept to a

low level (10–25%) to control the heat and the viscosity. However, over-

all conversion with recycle is over 95%.

The polymerization rate can be accelerated by increasing the tempera-

ture, the initiator concentration, and the pressure. Degree of branching and

molecular weight distribution depend on temperature and pressure. A

higher density polymer with a narrower molecular weight distribution

could be obtained by increasing the pressure and lowering the temperature.

The crystallinity of the polymer could be varied to some extent by

changing the reaction conditions and by adding comonomers such as vinyl

acetate or ethyl acrylate. The copolymers have lower crystallinity but bet-

ter flexibility, and the resulting polymer has higher impact strength.

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