Polyurethanes.pdf

POLYURETHANES

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POLYURETHANES

The polymers known as polyurethanes include materials that incorporate

the carbamate group, NHCOO , as well as other functional groups, such as

ester, ether, amide, and urea. The name polyurethane is derived from ethyl carba-

mate, known as urethane. Polyurethanes are usually produced by the reaction of a

polyfunctional isocyanate with a macroglycol, a so-called polyol, or other reactants

containing two or more groups reactive with isocyanates. Often a combination of a

macroglycol and a short-chain glycol extender is used to produce segmented block

copolymers. The macroglycols are based on polyethers, polyesters, or a combina-

tion of both. In recent years diamines have also been used as comonomers in order

to achieve higher reaction rates in molding and spray applications. In addition to

the linear thermoplastic polyurethanes, obtained from difunctional monomers,

branched or cross-linked thermoset polymers are made with higher functional

monomers. Linear polymers have good impact strength, good physical properties,

and excellent processibility, but limited thermal stability (owing to their thermo-

plasticity). Thermoset polymers, on the other hand, have higher thermal stability

but sometimes lower impact strength (rigid foams). The higher functionality is ob-

tained with higher functional isocyanates (polymeric isocyanates), or with higher

functional polyols. Cross-linking is also achieved by secondary reactions. For ex-

ample, urea groups are generated in the formation of water-blown ﬂexible foams.

An isocyanato group reacts with water to form a carbamic acid, which dissociates

into an amine and carbon dioxide, with the latter acting as a blowing agent. The

amine reacts with another isocyanate to form a urea linkage. Further reaction of

the urea group with the isocyanate leads to cross-linking via a biuret group. Water-

blown ﬂexible foams contain urethane, urea, and some biuret groups in their

network structure. Urea-modiﬁed segmented polyurethanes are manufactured

from diisocyanates, macroglycols, and diamine extenders. Polyurethane network

polymers are also formed by trimerization of part of the isocyanate groups. This

approach is used in the formation of rigid polyurethane-modiﬁed isocyanurate

(PUIR) foams.

The addition polymerization of diisocyanates with macroglycols to produce

urethane polymers was pioneered in 1937 by O. Bayer (1). The rapid formation

of high molecular weight urethane polymers from liquid monomers, which occurs

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even at ambient temperature, is a unique feature of the polyaddition process, yield-

ing products that range fromcross-linked networks to linear ﬁbers and elastomers.

The enormous versatility of the polyaddition process allowed the manufacture of

a myriad of products for a wide variety of applications.

The early German polyurethane products were based on tolyene diisocyanate

(TDI) and polyester polyols. In addition, a linear ﬁber, PerlonU, was produced from

the aliphatic 1,6-hexamethylene diisocyanate (HDI) and 1,4-butanediol. Com-

mercial production of ﬂexible polyurethane foam in the United States began in

1953. In Germany a toluene diisocyanate consisting of an isomeric mixture of

65% 2,4-isomer and 35% 2,6-isomer was used in the manufacture of ﬂexible foam,

whereas in the United States the less expensive 80:20 isomer mixture was used. In

1956, DuPont introduced poly(tetramethylene glycol) (PTMG), the ﬁrst commer-

cial polyether polyol; the less expensive polyalkylene glycols appeared by 1957.

The availability of the lower cost polyether polyols based on both ethylene and

propylene oxides provided the foam manufacturers with a broad choice of suit-

able raw materials, which in turn afforded ﬂexible foams with a wide range of

physical properties. Polyether polyols provide foams with better hydrolytic sta-

bility whereas polyester polyols give superior tensile and tear strength. The de-

velopment of new and superior catalysts, such as Dabco (triethylenediamine) and

organotin compounds, has led to the so-called one-shot process in 1958, which

eliminated the need for an intermediate prepolymer step. Prior to this develop-

ment, part of the polyol was treated with excess isocyanate to give an isocyanate-

terminated prepolymer. Further reaction with water produced a ﬂexible

foam.

The late 1950s saw the emergence of cast elastomers, which led to the devel-

opment of reaction injection molding (RIM) at Bayer AG in Leverkusen, Germany,

in 1964. Also, thermoplastic polyurethane (TPU) elastomers and Spandex ﬁbers

were introduced during this time. In addition, urethane-based synthetic leather

was introduced by DuPont under the trade name Corfam in 1963.

The late 1950s also witnessed the emergence of a new polymeric isocyanate

(PMDI) based on the condensation of aniline with formaldehyde. This product

was introduced by the Carwin Co. (later Upjohn and Dow) in 1960 under the

trade name PAPI. Similar products were introduced by Bayer and ICI in Europe

in the early 1960s. The superior heat resistance of rigid foams derived from PMDI

prompted its exclusive use in rigid polyurethane foams. The large-scale produc-

tion of PMDI made the coproduct 4,4 ,-methylenebis(phenyl isocyanate) (MDI)

readily available, which has since been used almost exclusively in polyurethane

elastomer applications. Liquid derivatives of MDI are used in RIM applications,

and work has been done since the 1990s to reinforce polyurethane elastomers with

glass, graphite, boron, and aramid ﬁbers, or mica ﬂakes, to increase stiffness and

reduce thermal expansion. The higher modulus thermoset elastomers produced

by reinforced reaction injection molding (RRIM) are also used in the automotive

industry. In 1969 Bayer pioneered an all-plastic car having RIM-molded bumpers

and fascia; in 1983 the ﬁrst plastic-body commercial automobile (Pontiac Fiero)

was produced in the United States.

The polymerization step can be conducted in a mold, in an extruder (TPU

production), or continuously on a conveyor (block foam production). Also, spraying

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of the monomers onto the surface of a substrate produces polyurethane coatings.

The resulting polymers can be thermoplastic, which allows reprocessing by in-

jection molding, extrusion, blow molding, and other remelting processes, or they

are thermoset polymers as used in the RIM process in the molding of automotive

bumpers, or in the manufacture of cellular polyurethanes.

Polyurethanes are a primary component of the global polymer market. They

amount to about 6% of the total world plastic use. The world consumption of

polyurethanes in 2000 was about 8 million tons, with a global growth averaging

around 3–4% a year. The Western Hemisphere uses about 3 million metric tons

per annum, Western Europe approximately 2.6 million metric tons per annum,

the remainder being used in Asia and Africa.

Today’s global polyurethane industry has been reshaped by several merg-

ers of the 1980s and 1990s. Some of the familiar players, such as ICI, Upjohn,

Olin, Rhone Poulenc, Union Carbide, and Arco, sold their polyurethane busi-

nesses; Bayer, the principal global isocyanate producer, strengthened its position

in polyether polyols by acquiring the Arco polyol business in 1999. Also Dow, the

other leading producer of polyether polyols, acquiredUnionCarbide in 1999, which

further strengthened its position in polyols. The primary polyurethane players of

the newmillennium are Bayer, BASF, Dow, and Huntsman, the latter through the

purchase of the global ICI business. Lyondell, which acquired the TDI businesses

from Olin and Rhone Poulenc, sold the Arco polyol business to Bayer in 1999,

thereby indicating their intent to eventually exit polyurethanes. Over the years

the primary polyurethane chemical producers underwent forward integration by

buying primary polyurethane system houses, ie their principal customers. Re-

cent examples include the acquisition of Essex, a leading producer of automotive

windshield adhesives and sealants, and of Flexible Products and General Latex,

which are polyurethane foam system houses, by Dow; and BASF acquired IPI

International, a producer of insulation foam systems.

In Asia and South America, the primary global chemical producers formed

joint ventures with primary local companies, some of which established small

volume manufacturing sites. In contrast, Dow/Mitsubishi built an isocyanate dis-

tillation plant in Yokaichi, Japan, to separate PMDI/MDI feedstock. Dow has an-

other distillation plant in Delfzjiel, Holland, which has been increased by 60%

in 2000. In this plant feedstock from Dows Estarreja, Portugal, plant is sepa-

rated into PMDI and MDI. Although distillation plants are less costly, the other

primary producers seem to be involved in building global-size facilities in Asia.

For example, BASF plans to build a new 140-kt/a TDI plant in Yosu, South

Korea by 2003. A present MDI plant at this site will be simultaneously expanded

to 160 kt/a. Also, several major facilities are planned for mainland China. A re-

cent project by Bayer, the building of a major TDI plant in Taiwan, was can-

celled because of local opposition to the plant. Enichem in Italy, which acquired

its isocyanate technology from ICI, is a regional producer of isocyanates and

polyols.

The major producers of polyurethane chemicals also manufacture TPU elas-

tomers. DuPont was also at one time involved in polyurethanes, but it sold its TDI

technology to Dow and excited the synthetic leather business. However, DuPont is

still the principal force in the production of polyurethane ﬁbers (Lycra). Through

the acquisition of Uniroyal and Witco, the Crompton & Knowles Corp. became a

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principal force in polyurethane elastomers, which are now sold under the trade

name CKWitco. Manufacturing andmarketing arrangements include a rigid foam

system marketing deal between Huntsman and Shell, and a manufacturing joint

venture of BASF and Shell. The latter is named Basell CV, which opened a new

styrene monomer/propylene oxide plant at Moerdijk in the Netherlands with a

capacity of 250 kt/a of propylene oxide. Another plant in Singapore is scheduled to

open in 2002. Some of the new polyols are used to supply Huntsman, which is the

only primary polyurethane company without a polyol manufacturing capability.

One of the current trends in polyurethanes is the gradual replacement

of TDI by the less volatile PMDI or MDI in many applications. The produc-

tion of PMDI/MDI is a coproduct process, which is economically viable because

the market requires amounts of both isocyanates in the amounts presently pro-

duced. All primary producers remove some of the higher priced MDI (up to 50%)

by vacuum distillation. A process for the manufacture of only MDI does not

exist.

Elimination of chlorinated ﬂuorocarbon (CFC) blowing agents and the re-

duction of emission of volatile organic compounds (VOCs) have been ongoing. The

latter leads to a rapid increase in the use of water-based polyurethane dispersions

in coating applications. Flexible foam producers have eliminated auxiliary blow-

ing agents, and the rigid foam producers use water-blown formulations in com-

bination with hydrochloroﬂuorocarbons (HCFCs), hydroﬂuorocarbons (HFCs), or

hydrocarbons. Adhesives and sealants are reformulated from solvent-based prod-

ucts to 100% solid-and water-based systems.

Isocyanates

The synthesis, reactions, and manufacture of isocyanates were reviewed in 1997

(2), and the chemistry and technology of isocyanates is the subject of a recent

book (3).

The standard method of synthesis of isocyanates is the phosgenation of

amines or amine salts. The phosgenation of amines to isocyanates was pio-

neered by Hentschel in 1884 (4). Using this method, a solution of the diamines

in chlorobenzene is added to excess phosgene in the same solvent below 20 ◦ C.

The resultant slurry consisting of the dicarbamoyl chloride ( 1 ) and the diamine

dihydrochloride ( 2 ) is treated with excess phosgene at temperatures up to 130 ◦ C.

Upon heating above 65 ◦ C the dicarbamoyl chloride dissociates to generate diiso-

cyanate ( 3 ). The conversion of 2 is very slow, and the use of polar solvents or higher

pressures increases the rate of reaction.

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In the laboratory a slurry of the diamine salts, obtained by treating a solu-

tion of the diamines with hydrogen chloride or carbon dioxide, is treated above

100 ◦ C until a clear solution is obtained. Instead of the toxic phosgene gas, the liq-

uid trichloromethyl chloroformate (diphosgene) (5) or the solid bistrichloromethyl

carbonate (triphosgene) (6) can be used in the laboratory. The phosgene oligomers

have to be used with caution because the toxic monomer can be generated readily

and all reactions have to be performed under a fume hood.

In the continuous manufacture of diisocyanates, the by-products (hydrogen

chloride and excess phosgene) are vented and separated. The recovered phosgene

is recycled and part of the hydrogen chloride is used in the aniline/formaldehyde

condensation. The solvents used in the phosgenation of the diamines are aromatic

hydrocarbons, especially chlorobenzene and o -dichlorobenzene. Occasionally, more

polar solvents, such as ethyl acetate, dioxane, nitrobenzene, or dimethylsulfone,

are used. Excess phosgene can also be used as solvent if the reaction is conducted

under high pressure. Dimethylformamide (DMF) and phenyltetramethylguani-

dine catalyze the phosgenation reaction (7).

Aliphatic diamines are also phosgenated in a two-phase reaction using

methylene chloride and aqueous sodium hydroxide. The diamine and phosgene

are dissolved in methylene chloride and the form 2 is instantaneously neutralized

with sodium hydroxide. The generated diisocyanate remains in the solvent phase,

and excess phosgene is also neutralized with sodium hydroxide, which enhances

the safety of phosgene handling. The highly exothermic reaction requires efﬁcient

cooling. A disadvantage of this process is the use of a slight excess of phosgene,

which cannot be recovered.

Instead of phosgene and its oligomers, oligomeric t -butylcarbonates are

also used to convert diamines into diisocyanates. For example, sterically hin-

dered aromatic diamines react with di- t -butyldicarbonate in the presence of

dimethylaminopyridine in acetonitrile at room temperature to give sterically hin-

dered aromatic diisocyanates. In this manner 3,6-3 ,6 -tetramethyl MDI is ob-

tained in 93% yield (8). Also, aliphatic diamines react with di- t -butyltricarbonate

at room temperature to give a high yield of the corresponding diisocyanates

(9).

Since the early 1970s, attempts have been made by the principal global pro-

ducers of isocyanates to avoid the use of the toxic phosgene in the manufacture of

isocyanates. Attempts to produce TDI and PMDI by nonphosgene processes have

failed. However, two aliphatic diisocyanates, CHDI and TMXDI, aremanufactured

using nonphosgene processes. Huls and BASF have also announced plans to use

nonphosgene processes for the manufacture of IPDI in their new plants which are

under construction. In the new, nonphosgene chemistry, isocyanic acid, generated

by thermolysis of urea, reacts with diamines to give a bis-urea derivative. Subse-

quent reaction with diethylamine affords tri-substituted urea derivatives, which

are thermolyzed in an inert solvent in the presence of an acidic catalyst to give the

diisocyanate (10). Gaseous ammonia is the only by-product in this process. Also, re-

action of aliphatic diamines with carbon dioxide, in the presence of triethylamine,

affords biscarbamate salts, which can be dehydrated with phosphoryl chloride to

give the diisocyanate (11).

Another laboratory method of synthesis of diisocyanates is the thermolysis of

bisacylazides ( 4 ) (Curtius reaction). For example, dicarboxylic acid chlorides react

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