Matar Sami, Hatch Lewis F. Chemistry of petrochemical processes

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–95°C ) using AlCl

cocatalyzed with a small amount of water. The

cocatalyst furnishes the protons needed for the cationic polymerization:

AlCl

+ H

(AlCl

OH)

–

The product is a linear random copolymer that can be cured to a ther-

mosetting polymer. This is made possible through the presence of some

unsaturation from isoprene.

Butyl rubber vulcanizates have tensile strengths up to 2,000 psi, and

are characterized by low permeability to air and a high resistance to

many chemicals and to oxidation. These properties make it a suitable

rubber for the production of tire inner tubes and inner liners of tubeless

tires. The major use of butyl rubber is for inner tubes. Other uses include

wire and cable insulation, steam hoses, mechanical goods, and adhesives.

Chlorinated butyl is a low molecular weight polymer used as an adhesive

and a sealant.

ETHYLENE-PROPYLENE RUBBER

Ethylene-propylene rubber (EPR) is a stereoregular copolymer of eth-

ylene and propylene. Elastomers of this type do not possess the double

bonds necessary for crosslinking. A third monomer, usually a monocon-

jugated diene, is used to provide the residual double bonds needed for

cross linking. 1,4-Hexadiene and ethylidene norbornene are examples of

these dienes. The main polymer chain is completely saturated while the

unsaturated part is pending from the main chain. The product elastomer,

termed ethylene-propylene terepolymer (EPT), can be crosslinked using

sulfur. Crosslinking EPR is also possible without using a third compo-

nent (a diene). This can be done with peroxides.

Important properties of vulcanized EPR and EPT include resistance

to abrasion, oxidation, and heat and ozone; but they are susceptible

to hydrocarbons.

The main use of ethylene-propylene rubber is to produce automotive

parts such as gaskets, mechanical goods, wire, and cable coating. It may

also be used to produce tires.

TRANSPOLYPENTAMER

Transpolypentamer (TPR) is produced by the ring cleavage of

cyclopentene.

33,44

Cyclopentene is obtained from cracked naphtha or gas

oil, which contain small amounts of cyclopentene, cyclopentadiene, and

Synthetic Petroleum-Based Polymers 357

Chapter 12 1/22/01 11:11 AM Page 357

dicyclopentadiene. Polymerization using organometallic catalysts pro-

duce a stereoregular product (trans 1,5-polypentamer):

358 Chemistry of Petrochemical Processes

Due to the presence of residual double bonds, the polymer could be

crosslinked with regular agents. TPR is a linear polymer with a high trans

configuration. It is highly amorphous at normal temperatures and has a

of about 90°C and a density of 0.85.

THERMOPLASTIC ELASTOMERS

Thermoplastic elastomers (TPES), as the name indicates, are plastic

polymers with the physical properties of rubbers. They are soft, flexible,

and possess the resilience needed of rubbers. However, they are

processed like thermoplastics by extrusion and injection molding.

TPE’s are more economical to produce than traditional thermoset

materials because fewer steps are required to manufacture them than to

manufacture and vulcanize thermoset rubber. An important property of

these polymers is that they are recyclable.

Thermoplastic elastomers are multiphase composites, in which the

phases are intimately depressed. In many cases, the phases are chemi-

cally bonded by block or graft copolymerization. At least one of the

phases consists of a material that is hard at room temperature.

Currently, important TPE’s include blends of semicrystalline thermo-

plastic polyolefins such as propylene copolymers, with ethylene-propy-

lene terepolymer elastomer. Block copolymers of styrene with other

monomers such as butadiene, isoprene, and ethylene or ethylene/propy-

lene are the most widely used TPE’s. Styrene-butadiene-styrene (SBS)

accounted for 70% of global styrene block copolymers (SBC). Currently,

global capacity of SBC is approximately 1.1 million tons. Polyurethane

thermoplastic elastomers are relatively more expensive then other TPE’s.

However, they are noted for their flexibility, strength, toughness, and

abrasion and chemical resistance.

Blends of polyvinyl chloride with

elastomers such as butyl are widely used in Japan.

Random block copolymers of polyesters (hard segments) and amor-

phous glycol soft segments, alloys of ethylene interpolymers, and chlori-

nated polyolefins are among the evolving thermoplastic elastomers.

Chapter 12 1/22/01 11:11 AM Page 358

Important properties of TPE’s are their flexibility, softness, and

resilience. However, compared to vulcanizable rubbers, they are inferior

in resistance to deformation and solvents.

Important markets for TPE’s include shoe soles, pressure sensitive

adhesives, insulation, and recyclable bumpers.

SYNTHETIC FIBERS

Fibers are solid materials characterized by a high ratio of length to

diameter. They can be manufactured from a natural origin such as silk,

wool, and cotton, or derived from a natural fiber such as rayon. They may

also be synthesized from certain monomers by polymerization (synthetic

fibers). In general, polymers with high melting points, high crystallinity,

and moderate thermal stability and tensile strengths are suitable for

fiber production.

Man-made fibers include, in addition to synthetic fibers, those derived

from cellulose (cotton, wood) but modified by chemical treatment such

as rayon, cellophane, and cellulose acetate. These are sometimes termed

“regenerated cellulose fibers.” Rayon and cellophane have shorter chains

than the original cellulose due to degradation by alkaline treatment.

Cellulose acetates produced by reacting cellulose with acetic acid and

modified or regenerated fibers are excluded from this book because they

are derived from a plant source. Fibers produced by drawing metals or

glass (SiO

) such as glass wool are also excluded.

Major fiber-making polymers are those of polyesters, polyamides

(nylons), polyacrylics, and polyolefins. Polyesters and polyamides are

produced by step polymerization reactions, while polyacrylics and poly-

olefins are synthesized by chain-addition polymerization.

POLYESTER FIBERS

Polyesters are the most important class of synthetic fibers. In general,

polyesters are produced by an esterification reaction of a diol and a

diacid. Carothers (1930) was the first to try to synthesize a polyester fiber

by reacting an aliphatic diacid with a diol. The polymers were not suit-

able because of their low melting points. However, he was successful in

preparing the first synthetic fiber (nylon 66). In 1946, Whinfield and

Dickson prepared the first polyester polymer by using terephthalic acid

(an aromatic diacid) and ethylene glycol.

Synthetic Petroleum-Based Polymers 359

Chapter 12 1/22/01 11:11 AM Page 359

Polyesters can be produced by an esterification of a dicarboxylic acid

and a diol, a transesterification of an ester of a dicarboxylic acid and a

diol, or by the reaction between an acid dichloride and a diol.

The polymerization reaction could be generally represented by the

esterification of a dicarboxylic acid and a diol as:

360 Chemistry of Petrochemical Processes

Less important methods are the self condensation of w-hydroxy acid and

the ring opening of lactones and cyclic esters. In self condensation of w-

hydroxy acids, cyclization might compete seriously with linear polymer-

ization, especially when the hydroxyl group is in a position to give five

or six membered lactones.

Polyethylene Terephthalate Production

Polyethylene terephthalate (PET) is produced by esterifying tereph-

thalic acid (TPA) and ethylene glycol or, more commonly, by the trans-

esterification of dimethyl terephthalate and ethylene glycol. This route is

favored because the free acid is not soluble in many organic solvents. The

reaction occurs in two stages (Figure 12-9).

Methanol is released in the

first stage at approximately 200°C with the formation of bis(2-hydrox-

yethyl) terephthalate. In the second stage, polycondensation occurs, and

excess ethylene glycol is driven away at approximately 280°C and at

lower pressures (≈ 0.01 atm):

Chapter 12 1/22/01 11:11 AM Page 360

Using excess ethylene glycol is the usual practice because it drives the

equilibrium to near completion and terminates the acid end groups. This

results in a polymer with terminal -OH. When the free acid is used (ester-

ification), the reaction is self catalyzed. However, an acid catalyst is used

to compensate for the decrease in terephthalic acid as the esterification

nears completion. In addition to the catalyst and terminator, other addi-

tives are used such as color improvers and dulling agents. For example,

PET is delustred by the addition of titanium dioxide.

The molecular weight of the polymer is a function of the extent of

polymerization and could be monitored through the melt viscosity. The

final polymer may be directly extruded or transformed to chips, which

are stored.

Batch polymerization is still used. However, most new processes use

continuous polymerization and direct spinning.

An alternative route to PET is by the direct reaction of terephthalic

acid and ethylene oxide. The product bis(2-hydroxyethyl)terephthalate

reacts in a second step with TPA to form a dimer and ethylene glycol,

which is released under reduced pressure at approximately 300°C.

Synthetic Petroleum-Based Polymers 361

Figure 12-9. The Inventa AG Process for producing polyethylene-terephthalate.

Chapter 12 1/22/01 11:11 AM Page 361

This process differs from the direct esterification and the transesterifi-

cation routes in that only ethylene glycol is released. In the former two

routes, water or methanol are coproduced and the excess glycol released.

Properties and Uses of Polyesters

As mentioned earlier, polyethylene terephthalate is an important ther-

moplastic. However, most PET is consumed in the production of fibers.

Polyester fibers contain crystalline as well as noncrystalline regions.

The degree of crystallinity and molecular orientation are important in

determining the tensile strength of the fiber (between 18–22 denier) and

its shrinkage. The degree of crystallinity and molecular orientation can

be determined by X-ray diffraction techniques.

Important properties of polyesters are the relatively high melting tem-

peratures (≈ 265°C ), high resistance to weather conditions and sunlight,

and moderate tensile strength (Table 12-6).

Melt spinning polyesters is preferred to solution spinning because of

its lower cost. Due to the hydrophobic nature of the fiber, sulfonated

terephthalic acid may be used as a comonomer to provide anionic sites

for cationic dyes. Small amounts of aliphatic diacids such as adipic acid

may also be used to increase the dyeability of the fibers by disturbing the

fiber’s crystallinity.

Polyester fibers can be blended with natural fibers such as cotton and

wool. The products have better qualities and are used for men’s and

women’s wear, pillow cases, and bedspreads. Fiberfill, made from poly-

esters, is used in mattresses, pillows, and sleeping bags. High-tenacity

polymers for tire cord reinforcement are equivalent in strength to nylon

tire cords and are superior because they do not “flat spot.” V-belts and fire

hoses made from industrial filaments are another market for polyesters.

POLYAMIDES (Nylon Fibers)

Polyamides are the second largest group of synthetic fibers after poly-

esters. However, they were the first synthetic fibers that appeared in the

market in 1940. This was the result of the work of W. H. Carothers in

USA who developed nylon 66. At about the same time nylon 6 was also

developed in Germany by I. G. Farben. Both of these nylons still domi-

nate the market for polyamides. However, due to patent restrictions and

raw materials considerations, nylon 66 is most extensively produced in

USA and nylon 6 is most extensively produced in Europe.

362 Chemistry of Petrochemical Processes

Chapter 12 1/22/01 11:11 AM Page 362

Synthetic Petroleum-Based Polymers 363

Table 12-6

Important properties of polyesters

(Mechanical properties at 21°C, 65% relative humidity, using 60% minute strain rate)

Polyethyleneterephthalate PET

Poly (cyclo-

Filament yarns Staple and tow hexanedi-

methylene

Regular High Regular High terephtha-

tensile tensile tensile tensile late) staple

Property strength strength strength strength and tow

Breaking strength 2.8–5.6 6.0–9.5 2.2–6.0 5.8–6.0 2.5–3.0

(g/denier)*

Breaking elongation 19–34 10–34 25–65 25–40 24–34

(%)

Initial modulus 75–100 115–120 25–40 45–55 24–35

(g/denier)*

Elastic recovery 88–93 90 75–85 — 85–95

(%r) (at 5% (at 5% (at 5% (at 2%

elongation) elongation) elongation) elongation)

Moisture regain 0.4 0.4 0.4 0.4 0.34

(%)**

Specific gravity 1.38 1.38 1.38 1.38 1.22

Melting temper- 265 265 265 265 290

ature (°C)

**Grams per denier-grams of frorce per denier. Denier is linear density, the mass for 9,000 meters of fiber.

**The amout of moisture in the fiber at 21°C, 65% relative humidity.

Chapter 12 1/22/01 11:11 AM Page 363

Numbers that follow the word “nylon” denote the number of carbons

present within a repeating unit and whether one or two monomers are

being used in polymer formation. For nylons using a single monomer

such as nylon 6 or nylon 12, the numbers 6 and 12 denote the number of

carbons in caprolactam and laurolactam, respectively. For nylons using

two monomers such as nylon 610, the first number, 6, indicates the num-

ber of carbons in the hexamethylene diamine and the other number, 10,

is for the second monomer sebacic acid.

Polyamides are produced by the reaction between a dicarboxylic acid

and a diamine (e.g., nylon 66), ring openings of a lactam, (e.g., nylon 6)

or by the polymerization of w-amino acids (e.g., nylon 11). The produc-

tion of some important nylons is discussed in the following sections.

Nylon 66 (Polyhexamethyleneadipate)

Nylon 66 is produced by the reaction of hexamethylenediamine and

adipic acid (see Chapters 9 and 10 for the production of the two

monomers). This produces hexamethylenediammonium adipate salt. The

product is a dilute salt solution concentrated to approximately 60% and

charged with acetic acid to a reactor where water is continuously

removed. The presence of a small amount of acetic acid limits the degree

of polymerization to the desired level:

364 Chemistry of Petrochemical Processes

The temperature is then increased to 270–300°C and the pressure to

approximately 16 atmospheres, which favors the formation of the poly-

mer. The pressure is finally reduced to atmospheric to permit further

water removal. After a total of three hours, nylon 66 is extruded under

nitrogen pressure.

Nylon 6 (Polycaproamide)

Nylon 6 is produced by the polymerization of caprolactam. The

monomer is first mixed with water, which opens the lactam ring and

gives w-amino acid:

Chapter 12 1/22/01 11:11 AM Page 364

Caprolactam w-Amino acid

The formed amino acid reacts with itself or with caprolactam at approx-

imately 250–280°C to form the polymer:

Synthetic Petroleum-Based Polymers 365

Temperature control is important, especially for depolymerization, which

is directly proportional to reaction temperature and water content. Figure

12-10 shows the Inventa-Fisher process.

Nylon 12 (Polylaurylamide)

Nylon 12 is produced in a similar way to nylon 6 by the ring opening

polymerization of laurolactam. The polymer has a lower water capacity

than nylon 6 due to its higher hydrophobic properties. The polymeriza-

Figure 12-10. The Inventa-Fisher process for producing nylon 6 from caprolac-

tam

: (1) Melting station, (2,3) polymerization reactors, (4) extruder, (5) interme-

diate vessel, (6) extraction column, (7,8) extraction columns, (9) cooling silo.

Chapter 12 1/22/01 11:12 AM Page 365

tion reaction is slower than for caprolactam. Higher temperatures are

used to increase the rate of the reaction:

366 Chemistry of Petrochemical Processes

The monomer (laurolactam) could be produced from 1,5,9-cyclododeca-

triene, a trimer of butadiene (Chapter 9). The trimer is epoxidized with

peracetic acid or acetaldehyde peracetate and then hydrogenated. The

saturated epoxide is rearranged to the ketone with MgI

at 100°C.

This

is then changed to the oxime and rearranged to laurolactam.

Nylon 4 (Polybutyramide)

Nylon 4 is produced by ring opening 2-pyrrolidone. Anionic polymer-

ization is used to polymerize the lactam. Cocatalysts are used to increase

the yield of the polymer. Carbon dioxide is reported to be an excellent

polymerization activator.

Nylon 4 has a higher water absorption capacity than other nylons due

to its lower hydrophobic property.

Nylon 11 (Polyundecanylamide)

Nylon 11 is produced by the condensation reaction of 11- aminounde-

canoic acid. This is an example of the self condensation of an amino acid

where only one monomer is used. The monomer is first suspended in

water, then heated to melt the monomer and to start the reaction. Water

is continuously removed to drive the equilibrium to the right. The poly-

mer is finally withdrawn for storage:

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