Matar Sami, Hatch Lewis F. Chemistry of petrochemical processes

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The first step in the base-catalyzed reaction is an attack by the phe-

noxide ion on the carbonyl carbon of formaldehyde, giving a mixture of

ortho- and para-substituted mono-methylolphenol plus di- and trisubsti-

tuted methylol phenols:

Synthetic Petroleum-Based Polymers 347

The second step is the condensation reaction between the methylolphe-

nols with the elimination of water and the formation of the polymer.

Crosslinking occurs by a reaction between the methylol groups and

results in the formation of ether bridges. It occurs also by the reaction of

the methylol groups and the aromatic ring, which forms methylene

bridges. The formed polymer is a three-dimensional network thermoset:

The acid-catalyzed reaction occurs by an electrophilic substitution

where formaldehyde is the electrophile. Condensation between the

methylol groups and the benzene rings results in the formation of meth-

ylene bridges. Usually, the ratio of formaldehyde to phenol is kept less

than unity to produce a linear fusible polymer in the first stage.

Crosslinking of the formed polymer can occur by adding more formalde-

hyde and a small amount of hexamethylene tetramine (hexamine,

Chapter 12 1/22/01 11:11 AM Page 347

(CH

)

). Hexamine decomposes in the presence of traces of moisture

to formaldehyde and ammonia. This results in crosslinking and formation

of a thermoset resin:

348 Chemistry of Petrochemical Processes

Properties and Uses of Phenolic Resins

Important properties of phenolic resins are their hardness, corrosion

resistance, rigidity, and resistance to water hydrolysis. They are also less

expensive than many other polymers.

Many additives are used with phenolic resins such as wood flour, oils,

asbestos, and fiberglass. Fiberglass piping made with phenolic resins can

operate at 150°C and pressure up to 150 psi.

Molding applications dominate the market of phenolic resins. Articles

produced by injection molding have outstanding heat resistance and

dimensional stability. Compression-molded glass-filled phenolic disk

brake pistons are replacing the steel ones in many automobiles because

of their light weight and corrosion resistance.

Phenolics are also used in a variety of other applications such as adhe-

sives, paints, laminates for building, automobile parts, and ion exchange

resins. Global production of phenol-formaldehyde resins exceeded 5 bil-

lion pounds in 1997.

AMINO RESINS (Aminoplasts)

Amino resins are condensation thermosetting polymers of formalde-

hyde with either urea or melamine. Melamine is a condensation product

of three urea molecules. It is also prepared from cyanamide at high pres-

sures and temperatures:

Chapter 12 1/22/01 11:11 AM Page 348

Urea-Formaldehyde and Urea-Melamine Resins

The nucleophilic addition reaction of urea to formaldehyde produces

mainly monomethylol urea and some dimethylol urea. When the mixture

is heated in presence of an acid, condensation occurs, and water is

released. This is accompanied by the formation of a crosslinked polymer:

Synthetic Petroleum-Based Polymers 349

A similar reaction occurs between melamine and formaldehyde and pro-

duces methylolmelamines:

A variety of methylols are possible due to the availability of six hydro-

gens in melamine. As with urea formaldehyde resins, polymerization

occurs by a condensation reaction and the release of water.

Properties and Uses of Aminoplasts

Amino resins are characterized by being more clear and harder (ten-

sile strength) than phenolics. However, their impact strength (breakabil-

ity) and heat resistance are lower. Melamine resins have better heat and

moisture resistance and better hardness than their urea analogs.

The most important use of amino resins is the production of adhesives

for particleboard and hardwood plywood.

Compression and injection molding are used with amino resins to pro-

duce articles such as radio cabinets, buttons, and cover plates. Because

melamine resins have lower water absorption and better chemical and

heat resistance than urea resins, they are used to produce dinnerware and

laminates used to cover furniture. Almost all molded objects use fillers

such as cellulose, asbestos, glass, wood flour, glass fiber and paper. The

1997 U.S. production of amino resins was 2.6 billion pounds.

Chapter 12 1/22/01 11:11 AM Page 349

Polycyanurates

A new polymer type which emerged as important materials for circuit

boards are polycyanurates. The simplest monomer is the dicyanate ester of

bisphenol A. When polymerized, it forms three-dimensional, densly cross-

linked structures through three-way cyanuric acid (2,4,6-triazinetriol):

350 Chemistry of Petrochemical Processes

The cyanurate ring is formed by the trimerization of the cyanate ester.

Other monomers, such as hexaflurobisphenol A and tetramethyl bisphe-

nol F, are also used. These polymers are characterized by high glass tran-

sition temperatures ranging between 192 to >350°C.

The largest application of polycyanurates is in circuit boards. Their

transparency to microwave and radar energy makes them useful for man-

ufacturing the housing of radar antennas of military and reconnaissance

planes. Their impact resistance makes them ideal for aircraft structures

and engine pistons.

SYNTHETIC RUBBER

Synthetic rubbers (elastomers) are long-chain polymers with special

chemical and physical as well as mechanical properties. These materials

have chemical stability, high abrasion resistance, strength, and good

dimensional stability. Many of these properties are imparted to the

Chapter 12 1/22/01 11:11 AM Page 350

original polymer through crosslinking agents and additives. Selected

properties of some elastomers are shown in Table 12-5.

An important

property of elastomeric materials is their ability to be stretched at least

twice their original length and to return back to nearly their original

length when released.

Natural rubber is an elastomer constituted of isoprene units. These

units are linked in a cis-1,4-configuration that gives natural rubber the

outstanding properties of high resilience and strength. Natural rubber

occurs as a latex (water emulsion) and is obtained from Hevea brasilien-

sis, a tree that grows in Malaysia, Indonesia, and Brazil. Charles

Goodyear (1839) was the first to discover that the latex could be vulcan-

ized (crosslinked) by heating with sulfur or other agents. Vulcanization

of rubber is a chemical reaction by which elastomer chains are linked

together. The long chain molecules impart elasticity, and the crosslinks

give load supporting strength.

Vulcanization of rubber has been reviewed

by Hertz, Jr.

Synthetic rubbers include elastomers that could be crosslinked

such as polybutadiene, polyisoprene, and ethylene-propylene-diene tere-

polymer. It also includes thermoplastic elastomers that are not crosslinked

and are adapted for special purposes such as automobile bumpers and wire

and cable coatings. These materials could be scraped and reused.

However, they cannot replace all traditional rubber since they do not have

the wide temperature performance range of thermoset rubber.

The major use of rubber is for tire production. Non-tire consumption

includes hoses, footwear, molded and extruded materials, and plasticizers.

Synthetic Petroleum-Based Polymers 351

Table 12-5

Selected properties of some elastomers

Tensile Temp.

Durometer strength Elongation range of Weather

hardness at room at room service resis-

range temp, psi temp, % °C tance

Natural rubber 20–100 1,000–4,000 100–700 –55–800 Fair

Styrene-butadiene

rubber (SBR) 40–100 1,000–3,500 100–700 –55–110 Fair

Polybutadiene 30–100 1,000–3,000 100–700 –60–100 Fair

Polyisoprene 20–100 1,000–4,000 100–750 –55–800 Fair

Polychloroprene 20–90 1,000–4,000 100–700 –55–100 Very good

Polyurethane 62–95 A 1,000–8,000 100–700 –70–120 Excellent

40–80 D

Polyisobutylene 30–100 1,000–3,000 100–700 –55–100 Very good

Chapter 12 1/22/01 11:11 AM Page 351

Worldwide use of synthetic rubber (not including thermoplastic elas-

tomers) in 1997 was approximately 10.5 million metric tons.

Natural rubber use is currently about 6 million tons/year and is

expected to grow at annual rate of 3.3%.

Thermoplastic elastomer consumption, approximately 0.8 million

tons, is forecasted to reach over one million tons by the year 2000.

BUTADIENE POLYMERS AND COPOLYMERS

Butadiene could be polymerized using free radical initiators or ionic

or coordination catalysts. When butadiene is polymerized in emulsion

using a free radical initiator such as cumene hydroperoxide, a random

polymer is obtained with three isomeric configurations, the 1,4-addition

configuration dominating:

352 Chemistry of Petrochemical Processes

Polymerization of butadiene using anionic initiators (alkyllithium) in a

nonpolar solvent produces a polymer with a high cis configuration.

high cis-polybutadiene is also obtained when coordination catalysts

are used.

38,39

Properties and Uses of Polybutadiene

cis-1,4-Polybutadiene is characterized by high elasticity, low heat

buildup, high abrasion resistance, and resistance to oxidation. However,

it has a relatively low mechanical strength. This is improved by incorpo-

rating a cis, trans block copolymer or 1,2-(vinyl) block copolymer in the

Chapter 12 1/22/01 11:11 AM Page 352

polybutadiene matrix.

Also, a small amount of natural rubber may be

mixed with polybutadiene to improve its properties. trans 1,4-Polybuta-

diene is characterized by a higher glass transition temperature (T

–14°C ) than the cis form (T

= –108°C ). The polymer has the toughness,

resilience, and abrasion resistance of natural rubber (T

= –14°C ).

Styrene-Butadiene Rubber (SBR)

Styrene-butadiene rubber (SBR) is the most widely used synthetic rub-

ber. It can be produced by the copolymerization of butadiene (≈ 75%)

and styrene (≈ 25%) using free radical initiators. A random copolymer is

obtained. The micro structure of the polymer is 60–68% trans, 14–19%

cis, and 17–21% 1,2–. Wet methods are normally used to characterize

polybutadiene polymers and copolymers. Solid state NMR provides a

more convenient way to determine the polymer micro structure.

Currently, more SBR is produced by copolymerizing the two

monomers with anionic or coordination catalysts. The formed copolymer

has better mechanical properties and a narrower molecular weight distri-

bution. A random copolymer with ordered sequence can also be made in

solution using butyllithium, provided that the two monomers are charged

slowly.

Block copolymers of butadiene and styrene may be produced in

solution using coordination or anionic catalysts. Butadiene polymerizes

first until it is consumed, then styrene starts to polymerize. SBR pro-

duced by coordinaton catalysts has better tensile strength than that pro-

duced by free radical initiators.

The main use of SBR is for tire production. Other uses include

footwear, coatings, carpet backing, and adhesives.

The l997 U.S. production of SBR was approximately 940 mil-

lion pounds.

NITRILE RUBBER (NBR)

Nitrile rubber is a copolymer of butadiene and acrylonitrile. It has the

special property of being resistant to hydrocarbon liquids.

The copolymerization occurs in an aqueous emulsion. When free rad-

icals are used, a random copolymer is obtained. Alternating copolymers

are produced when a Zieglar-Natta catalyst is employed. Molecular

weight can be controlled by adding modifiers and inhibitors. When the

polymerization reaches approximately 65%, the reaction mixture is vac-

uum distilled in presence of steam to recover the monomer.

Synthetic Petroleum-Based Polymers 353

Chapter 12 1/22/01 11:11 AM Page 353

The ratio of acrylonitrile/butadiene could be adjusted to obtain a poly-

mer with specific properties. Increasing the acrylonitrile ratio increases

oil resistance of the rubber, but decreases its plasticizer compatibility.

NBR is produced in different grades depending on the end use of the

polymer. Low acrylonitrile rubber is flexible at low temperatures and is

generally used in gaskets, O-rings, and adhesives. The medium type is

used in less flexible articles such as kitchen mats and shoe soles. High

acrylonitrile polymers are more rigid and highly resistant to hydrocar-

bons and oils and are used in fuel tanks and hoses, hydraulic equipment,

and gaskets. In 1997, the U.S. produced 86 million pounds of solid

nitrile rubber.

POLYISOPRENE

Natural rubber is a stereoregular polymer composed of isoprene units

attached in a cis configuration. This arrangement gives the rubber high

resilience and strength.

Isoprene can be polymerized using free radical initiators, but a random

polymer is obtained. As with butadiene, polymerization of isoprene can

produce a mixture of isomers. However, because the isoprene molecule

is asymmetrical, the addition can occur in 1,2-, 1,4- and 3,4- positions.

Six tactic forms are possible from both 1,2- and 3,4- addition and two

geometrical isomers from 1,4- addition (cis and trans):

354 Chemistry of Petrochemical Processes

Stereoregular polyisoprene is obtained when Zieglar-Natta catalysts or

anionic initiators are used. The most important coordination catalyst is α-

TiCl

cocatalyzed with aluminum alkyls. The polymerization rate and cis

Chapter 12 1/22/01 11:11 AM Page 354

Synthetic Petroleum-Based Polymers 355

Figure 12-8. A process for producing 1,4-polyisoprene (>99%) by a continuous solution polymerization.

Chapter 12 1/22/01 11:11 AM Page 355

content depends upon Al/Ti ratio, which should be greater than one.

Lower ratios predominantly produce the trans structure. Figure 12-8 shows

a process for producing cis-polyisoprene by a solution polymerization.

Properties and Uses of Polyisoprene

Polyisoprene is a synthetic polymer (elastomer) that can be vulcanized

by the addition of sulfur. cis-Polyisoprene has properties similar to that

of natural rubber. It is characterized by high tensile strength and insensi-

tivity to temperature changes, but it has low abrasion resistance. It is

attacked by oxygen and hydrocarbons.

trans-Polyisoprene is similar to Gutta percha, which is produced from

the leaves and bark of the sapotacea tree. It has different properties from

the cis form and cannot be vulcanized. Few commercial uses are based

on trans-polyisoprene.

Important uses of cis-polyisoprene include the production of tires, spe-

cialized mechanical products, conveyor belts, footwear, and insulation.

POLYCHLOROPRENE (Neoprene Rubber)

Polychloroprene is the oldest synthetic rubber. It is produced by the

polymerization of 2-chloro-1,3-butadiene in a water emulsion with

potassium sulfate as a catalyst:

356 Chemistry of Petrochemical Processes

The product is a random polymer that is vulcanized with sulfur or with

metal oxides (zinc oxide, magnesium oxide etc.). Vulcanization with sul-

fur is very slow, and an accelerator is usually required.

Neoprene vulcanizates have a high tensile strength, excellent oil

resistance (better than natural rubber), and heat resistance.

Neoprene rubber could be used for tire production, but it is expensive.

Major uses include cable coatings, mechanical goods, gaskets, conveyor

belts, and cables.

BUTYL RUBBER

Butyl rubber is a copolymer of isobutylene (97.5%) and isoprene

(2.5%). The polymerization is carried out at low temperature (below

Chapter 12 1/22/01 11:11 AM Page 356