Matar Sami, Hatch Lewis F. Chemistry of petrochemical processes

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acid with ethylene glycol or 1,4-butanediol. These materials are used to

produce film for magnetic tapes due to their abrasion and chemical resis-

tance, low water absorption, and low gas permeability. Polyethylene

terephthalate (PET) is also used to make plastic bottles (approximately

25% of plastic bottles are made from PET). Similar to nylons, the most

important use of PET is for producing synthetic fibers (discussed later).

Polybutylene terephthalate (PBT) is another thermoplastic polyester pro-

duced by the condensation reaction of terephthalic acid and 1,4-butanediol:

Synthetic Petroleum-Based Polymers 337

The polymer is either produced in a bulk or a solution process. It is

among the fastest growing engineering thermoplastics, and leads the

market of reinforced plastics with an annual growth rate of 7.3%.

The 1997 U.S. production of thermoplastic polyesters was approxi-

mately 4.3 billion pounds.

POLYCARBONATES

Polycarbonates (PC) are another group of condensation thermoplastics

used mainly for special engineering purposes. These polymers are con-

sidered polyesters of carbonic acid. They are produced by the condensa-

tion of the sodium salt of bisphenol A with phosgene in the presence of

an organic solvent. Sodium chloride is precipitated, and the solvent is

removed by distillation:

Chapter 12 1/22/01 11:11 AM Page 337

Another method for producing polycarbonates is by an exchange reaction

between bisphenol A or a similar bisphenol with diphenyl carbonate:

338 Chemistry of Petrochemical Processes

Diphenol carbonate is produced by the reaction of phosgene and phe-

nol. A new approach to diphenol carbonate and non-phosgene route is by

the reaction of CO and methyl nitrite using Pd/alumina. Dimethyl car-

bonate is formed which is further reacted with phenol in presence of

tetraphenox titanium catalyst. Decarbonylation in the liquid phase yields

diphenyl carbonate.

However, the reaction is equilibrium constained and requires a compli-

cated processing scheme.

Properties and Uses of Polycarbonates

Polycarbonates, known for their toughness in molded parts, typify the

class of polymers known as engineering thermoplastics. These materials,

designed to replace metals and glass in applications demanding strength

and temperature resistance, offer advantages of light weight, low cost,

and ease of fabrication.

Materials made from polycarbonates are transparent, strong, and heat-

and break-resistant. However, these materials are subject to stress crack-

|| ||

CO + 2 CH

ONO r CH

O—C—COCH

+ 2NO

OO O

|| || ||

—O—C—C—O— —O—C—O— + CO

Decarbon.

OO OO

|| || || ||

O—C—C—OCH

+ 2

r —O—C—C—O— + 2 CH

Chapter 12 1/22/01 11:11 AM Page 338

ing and can be attacked by weak alkalies and acids. Table 12-4 compares

the properties of polycarbonates with other thermoplastic resins.

Polycarbonates are used in a variety of articles such as laboratory

safety shields, street light globes, and safety helmets. The maximum

usage temperature for polycarbonate objects is 125°C.

POLYETHER SULFONES

Polyether sulfones (PES) are another class of engineering thermoplas-

tics generally used for objects that require continuous use of tempera-

tures around 200°C. They can also be used at low temperatures with no

change in their physical properties.

Synthetic Petroleum-Based Polymers 339

Table 12-4

Properties of polycarbonates compared

with some thermoplastics

Melting or

glass Izod

transition tensile compressive flexural impact,

temperature strength strength strength 1/8 in.

Resin (°C) (MPa) (MPa) (MPa) (J/m)

PPO, impact 100–110 117–127 124–162 179–200 43–53

modified

PC 149 65 86 93 850

PC, 30% glass 149 131 124 158 106

PC-ABS 149 48–50 76 89–94 560

Nylon 6/6, impact 240–260 48–55 160–210

modified

Nylon 6/6, 33% 265 151–193 202 282 117–138

glass

PBT 232–267 56 59–100 82–115 43–53

PBT, 30% glass 232–267 117–131 124–162 179–200 69–85

Acetal, 181 124 96 69–122

homopolymer

ABS, impact 100–110 33–43 31–55 55–76 347–400

modified

PPO, impact 135 48–55 69 56–76 320–370

modified

PPO, 30% 100–110 117–127 123 138–158 90–112

reinforced

Chapter 12 1/22/01 11:11 AM Page 339

Polyether sulfones can be prepared by the reaction of the sodium or

potassium salt of bisphenol A and 4,4

-dichlorodiphenyl sulfone.

Bisphenol A acts as a nucleophile in the presence of the deactivated aro-

matic ring of the dichlorophenylsulfone. The reaction may also be cat-

alyzed with Friedel-Crafts catalysts; the dichlorophenyl sulfone acts as

an electrophile:

340 Chemistry of Petrochemical Processes

Polyether sulfones could also be prepared using one monomer:

Properties and Uses of Aromatic Polyether Sulfones

In general, properties of polyether sulfones are similar to those of

polycarbonates, but they can be used at higher temperatures. Figure 12-6

shows the maximum use temperature for several thermoplastics.

Aromatic polyether sulfones can be extruded into thin films and foil and

injection molded into various objects that need high-temperature stability.

POLY(PHENYLENE) OXIDE

Polyphenylene oxide (PPO) is produced by the condensation of 2,6-

dimethylphenol. The reaction occurs by passing oxygen in the phenol

solution in presence of Cu

and pyridine:

Chapter 12 1/22/01 11:11 AM Page 340

PPO is an engineering thermoplastic with excellent properties. To

improve its mechanical properties and dimensional stability, PPO can be

blended with polystyrene and glass fiber. Articles made from PPO could

be used up to 330°C: it is mainly used in items that require higher tem-

peratures such as laboratory equipment, valves, and fittings.

POLYACETALS

Polyacetals are among the aliphatic polyether family and are produced

by the polymerization of formaldehyde. They are termed polyacetals to

distinguish them from polyethers produced by polymerizing ethylene

oxide, which has two -CH

- groups between the ether group. The poly-

merization reaction occurs in the presence of a Lewis acid and a small

amount of water at room temperature. It could also be catalyzed with amines:

Synthetic Petroleum-Based Polymers 341

Figure 12-6. Maximum continuous use temperature of some engineering thermo-

plastics.

Polyacetals are highly crystalline polymers. The number of repeating

units ranges from 500 to 3,000. They are characterized by high impact

resistance, strength, and a low friction coefficient.

Chapter 12 1/22/01 11:11 AM Page 341

Articles made from polyacetals vary from door handles to gears and

bushings, carburetor parts to aerosol containers. The major use of poly-

acetals is for molded grades.

THERMOSETTING PLASTICS

This group includes many plastics produced by condensation polymer-

ization. Among the important thermosets are the polyurethanes, epoxy

resins, phenolic resins, and urea and melamine formaldehyde resins.

POLYURETHANES

Polyurethanes are produced by the condensation reaction of a polyol

and a diisocyanate:

342 Chemistry of Petrochemical Processes

No by-product is formed from this reaction. Toluene diisocyanate

(Chapter 10) is a widely used monomer. Diols and triols produced from

the reaction of glycerol and ethylene oxide or propylene oxide are suit-

able for producing polyurethanes.

Polyurethane polymers are either rigid or flexible, depending on the

type of the polyol used. For example, triols derived from glycerol and

propylene oxide are used for producing block slab foams. These polyols

have moderate reactivity because the hydroxy groups are predominantly

secondary. More reactive polyols (used to produce molding polyurethane

foams) are formed by the reaction of polyglycols with ethylene oxide to

give the more reactive primary group:

Chapter 12 1/22/01 11:11 AM Page 342

Other polyhydric compounds with higher functionality than glycerol

(three-OH) are commonly used. Examples are sorbitol (six-OH) and

sucrose (eight-OH). Triethanolamine, with three OH groups, is also used.

Diisocyanates generally employed with polyols to produce polyure-

thanes are 2,4-and 2,6-toluene diisocyanates prepared from dinitro-

toluenes (Chapter 10):

Synthetic Petroleum-Based Polymers 343

A different diisocyanate used in polyurethane synthesis is methylene

diisocyanate (MDI), which is prepared from aniline and formaldehyde.

The diamine product is reacted with phosgene to get MDI.

The physical properties of polyurethanes vary with the ratio of the

polyol to the diisocyanate. For example, tensile strength can be adjusted

within a range of 1,200–600 psi; elongation, between 150–800%.

Improved polyurethane can be produced by copolymerization. Block

copolymers of polyurethanes connected with segments of isobutylenes

exhibit high-temperature properties, hydrolytic stability, and barrier char-

acteristics. The hard segments of polyurethane block polymers consist of

(–RNHCOO)–

, where R usually contains an aromatic moiety.

Properties and Uses of Polyurethanes

The major use of polyurethanes is to produce foam. The density as

well as the mechanical strength of the rigid and the flexible types varies

widely with polyol type and reaction conditions. For example,

polyurethanes could have densities ranging between 1–6 lb/ft

for the

flexible types and 1–50 lb/ft

for the rigid types. Polyurethane foams

have good abrasion resistance, low thermal conductance and good load-

bearing characteristics. However, they have moderate resistance to

organic solvents and are attacked by strong acids. Flame retardency of

polyurethanes could be improved by using special additives, spraying a

coating material such as magnesium oxychloride, or by grafting a halo-

gen phosphorous moiety to the polyol. Trichlorobutylene oxide is

Chapter 12 1/22/01 11:11 AM Page 343

sometimes copolymerized with ethylene and propylene oxides to pro-

duce the polyol.

Major markets for polyurethanes are furniture, transportation, and

building and construction. Other uses include carpet underlay, textural

laminates and coatings, footwear, packaging, toys, and fibers.

The largest use for rigid polyurethane is in construction and industrial

insulation due to its high insulating property. Figure 12-7 compares the

degree of insulation of some insulating materials.

Molded urethanes are used in items such as bumpers, steering wheels,

instrument panels, and body panels. Elastomers from polyurethanes are

characterized by toughness and resistance to oils, oxidation, and abra-

sion. They are produced using short-chain polyols such as polytetram-

ethylene glycol from 1,4-butanediol. Polyurethanes are also used to

produce fibers. Spandex (trade name) is a copolymer of polyurethane

(85%) and polyesters.

Polyurethane networks based on triisocyante and diisocyanate connected

by segments consisting of polyisobutylene are rubbery and exhibit high

temperature properties, hydrolyic stability, and barrier characteristics.

EPOXY RESINS

Epoxy resins are produced by reacting epichlorohydrin and a diphe-

nol. Bisphenol A is the diphenol generally used. The reaction, a ring

344 Chemistry of Petrochemical Processes

Figure 12-7. The comparative thickness for the same degree of insulation (dry

conditions).

Chapter 12 1/22/01 11:11 AM Page 344

opening polymerization of the epoxide ring, is catalyzed with strong

bases such as sodium hydroxide. A nucleophilic attack of the phenoxy

ion displaces a chloride ion and opens the ring:

Synthetic Petroleum-Based Polymers 345

The linear polymer formed is cured by cross-linking either with an

acid anhydride, which reacts with the -OH groups, or by an amine, which

opens the terminal epoxide rings. Cresols and other bisphenols are also

used for producing epoxy resins.

Properties and Uses of Epoxy Resins

Epoxy resins have a wide range of molecular weights (≈1,000–10,000).

Those with higher molecular weights, termed phenoxy, are hydrolyzed

to transparent resins that do not have the epoxide groups. These could

be used in molding purposes, or crosslinked by diisocyanates or by

cyclic anhydrides.

Important properties of epoxy resins include their ability to adhere

strongly to metal surfaces, their resistance to chemicals, and their high

dimensional stability. They can also withstand temperatures up to 500°C.

Epoxy resins with improved stress cracking properties can be made by

using toughening agents, such as carboxyl-terminated butadiene-acry-

lonitrile liquid polymers. The carboxyl group reacts with the terminal

epoxy ring to form an ester. The ester, with its pendant hydroxyl groups,

reacts with the remaining epoxide rings, then more crosslinking occurs

by forming ether linkages. This material is tougher than epoxy resins and

suitable for encapsulating electrical units.

Chapter 12 1/22/01 11:11 AM Page 345

Major uses of epoxy resins are coatings for appliance finishes, auto

primers, adhesive, and in coatings for cans and drums. Interior coatings of

drums used for chemicals and solvents manifests its chemical resistance.

In 1997, approximately 681 million pounds of unmodified epoxy

resins were produced in the U.S.

UNSATURATED POLYESTERS

Unsaturated polyesters are a group of polymers and resins used in

coatings or for castings with styrene. These polymers normally have

maleic anhydride moiety or an unsaturated fatty acid to impart the

required unsaturation. A typical example is the reaction between maleic

anhydride and ethylene glycol:

346 Chemistry of Petrochemical Processes

Phthalic anhydride, a polyol, and an unsaturated fatty acid are usually

copolymerized to unsaturated polyesters for coating purposes. Many

other combinations in variable ratios are possible for preparing these

resins. The 1998 U.S. production of polyesters was approximately 1.7

billion pounds.

PHENOL-FORMALDEHYDE RESINS

Phenol-formaldehyde resins are the oldest thermosetting polymers.

They are produced by a condensation reaction between phenol and

formaldehyde. Although many attempts were made to use the product and

control the conditions for the acid-catalyzed reaction described by Bayer

in 1872, there was no commercial production of the resin until the exhaus-

tive work by Baekeland was published in 1909. In this paper, he deseribes

the product as far superior to amber for pipe stem and similar articles, less

flexible but more durable than celluloid, odorless, and fire-resistant.

The reaction between phenol and formaldehyde is either base or acid

catalyzed, and the polymers are termed resols (for the base catalyzed)

and novalacs (for the acid catalyzed).

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