of organic units joined by carbamate links. While most polyurethanes are thermosetting

polymers that do not melt when heated, thermoplastic polyurethanes are also available.

Polyurethane polymers are traditionally and most commonly formed by reacting a di- or

polyisocyanate with a polyol. Both the isocyanates and polyols used to make polyurethanes contain

on average two or more functional groups per molecule.

Some noteworthy recent efforts have been dedicated to minimizing the use of isocyanates to synthesize

polyurethanes, because the isocyanates raise severe toxicity issues. Non-isocyanate based

polyurethanes (NIPUs) have recently been developed as a new class of polyurethane polymers to

mitigate health and environmental concerns. Polyurethane products often are simply called

“urethanes”, but should not be confused with ethyl carbamate, which is also called

urethane. Polyurethanes neither contain nor are produced from ethyl carbamate.

Polyurethanes are used in the manufacture of flexible, high-resilience foam seating;

rigid foam insulation panels; microcellular foam seals and gaskets; durable elastomeric

wheels and tires (such as roller coaster and escalator wheels); automotive suspension bushings;

electrical potting compounds; high performance adhesives; surface coatings and surface sealants;

synthetic fibers (e.g., Spandex); carpet underlay; hard-plastic parts (e.g., for electronic instruments);

hoses and skateboard wheels. History

Otto Bayer and his coworkers at I.G. Farben in Leverkusen, Germany, first made polyurethanes

in 1937. The new polymers had some advantages over existing plastics that were made by polymerizing

olefins, or by polycondensation, and were not covered by patents obtained by Wallace

Carothers on polyesters. Early work focused on the production of fibres and flexible foams

and PUs were applied on a limited scale as aircraft coating during World War II. Polyisocyanates

became commercially available in 1952 and production of flexible polyurethane foam began

in 1954 using toluene diisocyanate (TDI) and polyester polyols. These materials were also

used to produce rigid foams, gum rubber, and elastomers. Linear fibers were produced from

hexamethylene diisocyanate (HDI) and 1,4-butanediol (BDO).

In 1956 DuPont introduced polyether polyols, specifically poly(tetramethylene ether) glycol

and BASF and Dow Chemical started selling polyalkylene glycols in 1957. Polyether polyols

were cheaper, easier to handle and more water resistant than polyester polyols, and became

more popular. Union Carbide and Mobay, a U.S. Monsanto/Bayer joint venture, also began making

polyurethane chemicals. In 1960 more than 45,000 metric tons of flexible polyurethane

foams were produced. The availability of chlorofluoroalkane blowing agents, inexpensive polyether polyols,

and methylene diphenyl diisocyanate (MDI) allowed polyurethane rigid foams to be used

as high performance insulation materials. In 1967, urethane modified polyisocyanurate

rigid foams were introduced, offering even better thermal stability and flammability

resistance. During the 1960s, automotive interior safety components such as instrument and door

panels were produced by back-filling thermoplastic skins with semi-rigid foam.

In 1969, Bayer exhibited an all plastic car in Düsseldorf, Germany. Parts of this car,

such as the fascia and body panels were manufactured using a new process called RIM, Reaction Injection

Molding in which the reactants were mixed then injected into a mold. The addition of

fillers, such as milled glass, mica, and processed mineral fibres gave rise to reinforced RIM

(RRIM), which provided improvements in flexural modulus (stiffness), reduction in coefficient

of thermal expansion and thermal stability. This technology was used to make the first

plastic-body automobile in the United States, the Pontiac Fiero, in 1983. Further increases

in stiffness were obtained by incorporating pre-placed glass mats into the RIM mold cavity,

also known broadly as resin injection molding or structural RIM.

Starting in the early 1980s, water-blown microcellular flexible foams were used to mold gaskets for

automotive panels and air filter seals, replacing PVC plastisol from automotive applications

have greatly increased market share. Polyurethane foams are now used in high temperature oil

filter applications. Polyurethane foam (including foam rubber)

is sometimes made using small amounts of blowing agents to give less dense foam, better cushioning/energy

absorption or thermal insulation. In the early 1990s, because of their impact on ozone depletion,

the Montreal Protocol restricted the use of many chlorine-containing blowing agents, such

as trichlorofluoromethane (CFC-11). By the late 1990s, the use of blowing agents such

as carbon dioxide, pentane, 1,1,1,2-tetrafluoroethane (HFC-134a) and 1,1,1,3,3-pentafluoropropane

(HFC-245fa) were widely used in North America and the EU, although chlorinated blowing agents

remained in use in many developing countries. In the 1990s new two-component polyurethane

and hybrid polyurethane-polyurea elastomers were used for spray-in-place load bed liners

and military marine applications for the U.S. Navy. A one-part polyurethane is specified

as high durability deck coatings under MIL-PRF-32171 for the US Navy. This technique for coating

creates a durable, abrasion resistant composite with the metal substrate, and eliminates corrosion

and brittleness associated with drop-in thermoplastic bed liners.

Rising costs of petrochemical feedstocks and an enhanced public desire for environmentally

friendly green products raised interest in polyols derived from vegetable oils. One of

the most vocal supporters of these polyurethanes made using natural oil polyols is the Ford

Motor Company. Chemistry

Polyurethanes are in the class of compounds called reaction polymers, which include epoxies,

unsaturated polyesters, and phenolics. Polyurethanes are produced by reacting an isocyanate containing

two or more isocyanate groups per molecule (R-(N=C=O)n ≥ 2) with a polyol containing

on average two or more hydroxy groups per molecule (R'-(OH)n ≥ 2), in the presence

of a catalyst. The properties of a polyurethane are greatly

influenced by the types of isocyanates and polyols used to make it. Long, flexible segments,

contributed by the polyol, give soft, elastic polymer. High amounts of crosslinking give

tough or rigid polymers. Long chains and low crosslinking give a polymer that is very stretchy,

short chains with lots of crosslinks produce a hard polymer while long chains and intermediate

crosslinking give a polymer useful for making foam. The crosslinking present in polyurethanes

means that the polymer consists of a three-dimensional network and molecular weight is very high.

In some respects a piece of polyurethane can be regarded as one giant molecule. One consequence

of this is that typical polyurethanes do not soften or melt when they are heated...they

are thermosetting polymers. The choices available for the isocyanates and polyols, in addition

to other additives and processing conditions allow polyurethanes to have the very wide

range of properties that make them such widely used polymers.

Isocyanates are very reactive materials. This makes them useful in making polymers but also

requires special care in handling and use. The aromatic isocyanates, diphenylmethane

diisocyanate (MDI) or toluene diisocyanate (TDI) are more reactive than aliphatic isocyanates,

such as hexamethylene diisocyanate (HDI) or isophorone diisocyanate (IPDI). Most of the

isocyanates are difunctional, that is they have exactly two isocyanate groups per molecule.

An important exception to this is polymeric diphenylmethane diisocyanate, which is a mixture

of molecules with two-, three-, and four- or more isocyanate groups. In cases like this

the material has an average functionality greater than two, commonly 2.7.

Polyols are polymers in their own right and have on average two or more hydroxyl groups

per molecule. Polyether polyols are mostly made by co-polymerizing ethylene oxide and

propylene oxide with a suitable polyol precursor. Polyester polyols are made similarly to polyester

polymers. The polyols used to make polyurethanes are not "pure" compounds since they are often

mixtures of similar molecules with different molecular weights and mixtures of molecules

that contain different numbers of hydroxyl groups, which is why the "average functionality"

is often mentioned. Despite them being complex mixtures, industrial grade polyols have their

composition sufficiently well controlled to produce polyurethanes having consistent properties.

As mentioned earlier, it is the length of the polyol chain and the functionality that

contribute much to the properties of the final polymer. Polyols used to make rigid polyurethanes

have molecular weights in the hundreds, while those used to make flexible polyurethanes

have molecular weights up to ten thousand or more.

The polymerization reaction makes a polymer containing the urethane linkage, -RNHCOOR'-

and is catalyzed by tertiary amines, such as 1,4-diazabicyclooctane (also called DABCO

or TEDA), and metallic compounds, such as dibutyltin dilaurate or bismuth octanoate.

This is often referred to as the gellation reaction or simply gelling.

If water is present in the reaction mixture (it is often added intentionally to make foams),

the isocyanate reacts with water to form a urea linkage and carbon dioxide gas and the

resulting polymer contains both urethane and urea linkages. This reaction is referred to

as the blowing reaction and is catalyzed by tertiary amines like bis-(2-dimethylaminoethyl)ether.

A third reaction, particularly important in making insulating rigid foams is the isocyanate

trimerization reaction, which is catalyzed by potassium octoate, for example.

One of the most desirable attributes of polyurethanes is their ability to be turned into foam. Making

a foam requires the formation of a gas at the same time as the urethane polymerization

(gellation) is occurring. The gas can be carbon dioxide, either generated by reacting isocyanate

with water. or added as a gas or produced by boiling volatile liquids. In the latter

case heat generated by the polymerization causes the liquids to vaporize. The liquids

can be HFC-245fa (1,1,1,3,3-pentafluoropropane) and HFC-134a (1,1,1,2-tetrafluoroethane),

and hydrocarbons such as n-pentane. The balance between gellation and blowing

is sensitive to operating parameters including the concentrations of water and catalyst.

The reaction to generate carbon dioxide involves water reacting with an isocyanate first forming

an unstable carbamic acid, which then decomposes into carbon dioxide and an amine. The amine

reacts with more isocyanate to give a substituted urea. Water has a very low molecular weight,

so even though the weight percent of water may be small, the molar proportion of water

may be high and considerable amounts of urea produced. The urea is not very soluble in

the reaction mixture and tends to form separate "hard segment" phases consisting mostly of

polyurea. The concentration and organization of these polyurea phases can have a significant

impact on the properties of the polyurethane foam.

High-density microcellular foams can be formed without the addition of blowing agents by

mechanically frothing or nucleating the polyol component prior to use.

Surfactants are used in polyurethane foams to emulsify the liquid components, regulate

cell size, and stabilize the cell structure to prevent collapse and surface defects. Rigid

foam surfactants are designed to produce very fine cells and a very high closed cell content.

Flexible foam surfactants are designed to stabilize the reaction mass while at the same

time maximizing open cell content to prevent the foam from shrinking.

An even more rigid foam can be made with the use of specialty trimerization catalysts which

create cyclic structures within the foam matrix, giving a harder, more thermally stable structure,

designated as polyisocyanurate foams. Such properties are desired in rigid foam products

used in the construction sector. Careful control of viscoelastic properties

— by modifying the catalysts and polyols used —can lead to memory foam, which is

much softer at skin temperature than at room temperature.

Foams can be either "closed cell", where most of the original bubbles or cells remain intact,

or "open cell", where the bubbles have broken but the edges of the bubbles are stiff enough

to retain their shape. Open cell foams feel soft and allow air to flow through so they

are comfortable when used in seat cushions or mattresses. Closed cell rigid foams are

used as thermal insulation, for example in refrigerators.

Microcellular foams are tough elastomeric materials used in coverings of car steering

wheels or shoe soles. Raw materials

The main ingredients to make a polyurethane are isocyanates and polyols. Other materials

are added to help processing the polymer or to change the properties of the polymer.

Isocyanates Isocyanates used to make polyurethane must

have two or more isocyanate groups on each molecule. The most commonly used isocyanates

are the aromatic diisocyantes, toluene diisocyanate (TDI) and methylene diphenyl diisocyanate,

MDI. TDI and MDI are generally less expensive and

more reactive than other isocyanates. Industrial grade TDI and MDI are mixtures of isomers

and MDI often contains polymeric materials. They are used to make flexible foam (for example

slabstock foam for mattresses or molded foams for car seats), rigid foam (for example insulating

foam in refrigerators) elastomers (shoe soles, for example), and so on. The isocyanates may

be modified by partially reacting them with polyols or introducing some other materials

to reduce volatility (and hence toxicity) of the isocyanates, decrease their freezing

points to make handling easier or to improve the properties of the final polymers.

Aliphatic and cycloaliphatic isocyanates are used in smaller volumes, most often in coatings

and other applications where color and transparency are important since polyurethanes made with

aromatic isocyanates tend to darken on exposure to light. The most important aliphatic and

cycloaliphatic isocyanates are 1,6-hexamethylene diisocyanate (HDI), 1-isocyanato-3-isocyanatomethyl-3,5,5-trimethyl-cyclohexane

(isophorone diisocyanate, IPDI), and 4,4'-diisocyanato dicyclohexylmethane, (H12MDI or hydrogenated

MDI). Polyols

Polyols can be polyether polyols, which are made by the reaction of epoxides with an active

hydrogen containing starter compounds, or polyester polyols, which are made by the polycondensation

of multifunctional carboxylic acids and hydroxyl compounds. They can be further classified

according to their end use. Higher molecular weight polyols (molecular weights from 2,000

to 10,000) are used to make more flexible polyurethanes while lower molecular weight

polyols make more rigid products. Polyols for flexible applications use low

functionality initiators such as dipropylene glycol (f=2), glycerine (f=3) or a sorbitol/water

solution (f=2.75). Polyols for rigid applications use high functionality initiators such as

sucrose (f=8), sorbitol (f=6), toluenediamine (f=4), and Mannich bases (f=4). Propylene

oxide and/or ethylene oxide is added to the initiators until the desired molecular weight

is achieved. The order of addition and the amounts of each oxide affect many polyol properties,

such as compatibility, water-solubility, and reactivity. Polyols made with only propylene

oxide are terminated with secondary hydroxyl groups and are less reactive than polyols

capped with ethylene oxide, which contain a higher percentage of primary hydroxyl groups.

Graft polyols (also called filled polyols or polymer polyols) contain finely dispersed

styrene-acrylonitrile, acrylonitrile, or polyurea (PHD) polymer solids chemically grafted to

a high molecular weight polyether backbone. They are used to increase the load-bearing

properties of low-density high-resiliency (HR) foam, as well as add toughness to microcellular

foams and cast elastomers. Initiators such as ethylenediamine and triethanolamine are

used to make low molecular weight rigid foam polyols that have built-in catalytic activity

due to the presence of nitrogen atoms in the backbone. A special class of polyether polyols,