Polyurethane Polyurethane is a polymer composed of a chain
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,
poly(tetramethylene ether) glycols, which are made by polymerizing tetrahydrofuran,
are used in high performance coating, wetting and elastomer applications.
Conventional polyester polyols are based on virgin raw materials and are manufactured
by the direct polyesterification of high-purity diacids and glycols, such as adipic acid and
1,4-butanediol. Polyester polyols are usually more expensive and more viscous than polyether
polyols, but they make polyurethanes with better solvent, abrasion, and cut resistance.
Other polyester polyols are based on reclaimed raw materials. They are manufactured by transesterification
(glycolysis) of recycled poly(ethyleneterephthalate) (PET) or dimethylterephthalate (DMT) distillation
bottoms with glycols such as diethylene glycol. These low molecular weight, aromatic polyester
polyols are used in rigid foam, and bring low cost and excellent flammability characteristics
to polyisocyanurate (PIR) boardstock and polyurethane spray foam insulation.
Specialty polyols include polycarbonate polyols, polycaprolactone polyols, polybutadiene polyols,
and polysulfide polyols. The materials are used in elastomer, sealant, and adhesive applications
that require superior weatherability, and resistance to chemical and environmental attack.
Natural oil polyols derived from castor oil and other vegetable oils are used to make
elastomers, flexible bunstock, and flexible molded foam. Copolymerizing chlorotrifluoroethylene
or tetrafluoroethylene with vinyl ethers containing hydroxyalkyl vinyl ether produces fluorinated
(FEVE) polyols. Two component fluorinated polyurethane prepared by reacting FEVE fluorinated
polyols with polyisocyanate have been applied for make ambient cure paint/coating. Since
fluorinated polyurethanes contain high percentage of fluorine-carbon bond which is the strongest
bond among all chemical bonds. Fluorinated polyurethanes have excellent resistance to
UV, acids, alkali, salts, chemicals, solvents, weathering, corrosion, fungi and microbial
attack. These have become the first choice for high performance coating/paints.
Chain extenders and cross linkers Chain extenders (f=2) and cross linkers (f=3
or greater) are low molecular weight hydroxyl and amine terminated compounds that play an
important role in the polymer morphology of polyurethane fibers, elastomers, adhesives,
and certain integral skin and microcellular foams. The elastomeric properties of these
materials are derived from the phase separation of the hard and soft copolymer segments of
the polymer, such that the urethane hard segment domains serve as cross-links between the amorphous
polyether (or polyester) soft segment domains. This phase separation occurs because the mainly
non-polar, low melting soft segments are incompatible with the polar, high melting hard segments.
The soft segments, which are formed from high molecular weight polyols, are mobile and are
normally present in coiled formation, while the hard segments, which are formed from the
isocyanate and chain extenders, are stiff and immobile. Because the hard segments are
covalently coupled to the soft segments, they inhibit plastic flow of the polymer chains,
thus creating elastomeric resiliency. Upon mechanical deformation, a portion of the soft
segments are stressed by uncoiling, and the hard segments become aligned in the stress
direction. This reorientation of the hard segments and consequent powerful hydrogen
bonding contributes to high tensile strength, elongation, and tear resistance values. The
choice of chain extender also determines flexural, heat, and chemical resistance properties.
The most important chain extenders are ethylene glycol, 1,4-butanediol (1,4-BDO or BDO), 1,6-hexanediol,
cyclohexane dimethanol and hydroquinone bis(2-hydroxyethyl) ether (HQEE). All of these glycols form polyurethanes
that phase separate well and form well defined hard segment domains, and are melt processable.
They are all suitable for thermoplastic polyurethanes with the exception of ethylene glycol, since
its derived bis-phenyl urethane undergoes unfavorable degradation at high hard segment
levels. Diethanolamine and triethanolamine are used in flex molded foams to build firmness
and add catalytic activity. Diethyltoluenediamine is used extensively in RIM, and in polyurethane
and polyurea elastomer formulations. Catalysts
Polyurethane catalysts can be classified into two broad categories, amine compounds and
metal complexes. Traditional amine catalysts have been tertiary amines such as triethylenediamine
(TEDA, 1,4-diazabicyclooctane or DABCO), dimethylcyclohexylamine (DMCHA), and dimethylethanolamine (DMEA).
Tertiary amine catalysts are selected based on whether they drive the urethane (polyol+isocyanate,
or gel) reaction, the urea (water+isocyanate, or blow) reaction, or the isocyanate trimerization
reaction (e.g., using potassium acetate, to form isocyanurate ring structure). Catalysts
that contain a hydroxyl group or secondary amine, which react into the polymer matrix,
can replace traditional catalysts thereby reducing the amount of amine that can come
out of the polymer. Metallic compounds based on mercury, lead,
tin, bismuth, and zinc are used as polyurethane catalysts. Mercury carboxylates, are particularly
effective catalysts for polyurethane elastomer, coating and sealant applications, since they
are very highly selective towards the polyol+isocyanate reaction, but they are toxic. Bismuth and
zinc carboxylates have been used as alternatives. Alkyl tin carboxylates, oxides and mercaptides
oxides are used in all types of polyurethane applications. Tin mercaptides are used in
formulations that contain water, as tin carboxylates are susceptible to hydrolysis.
Surfactants Surfactants are used to modify the characteristics
of both foam and non-foam polyurethane polymers. They take the form of polydimethylsiloxane-polyoxyalkylene
block copolymers, silicone oils, nonylphenol ethoxylates, and other organic compounds.
In foams, they are used to emulsify the liquid components, regulate cell size, and stabilize
the cell structure to prevent collapse and sub-surface voids. In non-foam applications
they are used as air release and anti-foaming agents, as wetting agents, and are used to
eliminate surface defects such as pin holes, orange peel, and sink marks.
Production Polyurethanes are produced by mixing two or
more liquid streams. The polyol stream contains catalysts, surfactants, blowing agents and
so on. The two components are referred to as a polyurethane system, or simply a system.
The isocyanate is commonly referred to in North America as the 'A-side' or just the
'iso'. The blend of polyols and other additives is commonly referred to as the 'B-side' or
as the 'poly'. This mixture might also be called a 'resin' or 'resin blend'. In Europe
the meanings for 'A-side' and 'B-side' are reversed. Resin blend additives may include
chain extenders, cross linkers, surfactants, flame retardants, blowing agents, pigments,
and fillers. Polyurethane can be made in a variety of densities and hardnesses by varying
the isocyanate, polyol or additives. Health and safety
Fully reacted polyurethane polymer is chemically inert. No exposure limits have been established
in the U.S. by OSHA (Occupational Safety and Health Administration) or ACGIH (American
Conference of Governmental Industrial Hygienists). It is not regulated by OSHA for carcinogenicity.
Polyurethane polymer is a combustible solid and can be ignited if exposed to an open flame.
Decomposition from fire can produce mainly carbon monoxide, and trace nitrogen oxides
and hydrogen cyanide. Liquid resin blends and isocyanates may contain
hazardous or regulated components. Isocyanates are known skin and respiratory sensitizers.
Additionally, amines, glycols, and phosphate present in spray polyurethane foams present
risks. In the United States, additional health and
safety information can be found through organizations such as the Polyurethane Manufacturers Association
(PMA) and the Center for the Polyurethanes Industry (CPI), as well as from polyurethane
system and raw material manufacturers. Regulatory information can be found in the Code of Federal
Regulations Title 21 (Food and Drugs) and Title 40 (Protection of the Environment).
In Europe, health and safety information is available from ISOPA, the European Diisocyanate
and Polyol Producers Association. Manufacturing
The methods of manufacturing polyurethane finished goods range from small, hand pour
piece-part operations to large, high-volume bunstock and boardstock production lines.
Regardless of the end-product, the manufacturing principle is the same: to meter the liquid
isocyanate and resin blend at a specified stoichiometric ratio, mix them together until
a homogeneous blend is obtained, dispense the reacting liquid into a mold or on to a
surface, wait until it cures, then demold the finished part.
Dispensing equipment Although the capital outlay can be high, it
is desirable to use a meter-mix or dispense unit for even low-volume production operations
that require a steady output of finished parts. Dispense equipment consists of material holding
(day) tanks, metering pumps, a mix head, and a control unit. Often, a conditioning or heater-chiller
unit is added to control material temperature in order to improve mix efficiency, cure rate,
and to reduce process variability. Choice of dispense equipment components depends on
shot size, throughput, material characteristics such as viscosity and filler content, and
process control. Material day tanks may be single to hundreds of gallons in size, and
may be supplied directly from drums, IBCs (intermediate bulk containers, such as totes),
or bulk storage tanks. They may incorporate level sensors, conditioning jackets, and mixers.
Pumps can be sized to meter in single grams per second up to hundreds of pounds per minute.
They can be rotary, gear, or piston pumps, or can be specially hardened lance pumps to
meter liquids containing highly abrasive fillers such as wollastonite, chopped or hammer milled
glass fibres. The pumps can drive low-pressure (10 to 30
bar, ~1 to 3 MPa) or high-pressure (125 to 250 bar, ~12.5 to 25.0 MPa) dispense systems.
Mix heads can be simple static mix tubes, rotary element mixers, low-pressure dynamic
mixers, or high-pressure hydraulically actuated direct impingement mixers. Control units may
have basic on/off – dispense/stop switches, and analogue pressure and temperature gauges,
or may be computer controlled with flow meters to electronically calibrate mix ratio, digital
temperature and level sensors, and a full suite of statistical process control software.
Add-ons to dispense equipment include nucleation or gas injection units, and third or fourth
stream capability for adding pigments or metering in supplemental additive packages.
Tooling Distinct from pour-in-place, bun and boardstock,
and coating applications, the production of piece parts requires tooling to contain and
form the reacting liquid. The choice of mold-making material is dependent on the expected number
of uses to end-of-life (EOL), molding pressure, flexibility, and heat transfer characteristics.
RTV silicone is used for tooling that has an EOL in the thousands of parts. It is typically
used for molding rigid foam parts, where the ability to stretch and peel the mold around
undercuts is needed. The heat transfer characteristic of RTV silicone tooling is poor. High-performance,
flexible polyurethane elastomers are also used in this way.
Epoxy, metal-filled epoxy, and metal-coated epoxy is used for tooling that has an EOL
in the tens-of-thousands of parts. It is typically used for molding flexible foam cushions and
seating, integral skin and microcellular foam padding, and shallow-draft RIM bezels and
fascia. The heat transfer characteristic of epoxy tooling is fair; the heat transfer characteristic
of metal-filled and metal-coated epoxy is good. Copper tubing can be incorporated into
the body of the tool, allowing hot water to circulate and heat the mold surface.
Aluminum is used for tooling that has an EOL in the hundreds-of-thousands of parts. It
is typically used for molding microcellular foam gasketing and cast elastomer parts, and
is milled or extruded into shape. Mirror-finish stainless steel is used for
tooling that imparts a glossy appearance to the finished part. The heat transfer characteristic
of metal tooling is excellent. Finally, molded or milled polypropylene is
used to create low-volume tooling for molded gasket applications. Instead of many expensive
metal molds, low-cost plastic tooling can be formed from a single metal master, which
also allows greater design flexibility. The heat transfer characteristic of polypropylene
tooling is poor, which must be taken into consideration during the formulation process.
Applications In 2007, the global consumption of polyurethane
raw materials was above 12 million metric tons, the average annual growth rate is about
5%. Revenues generated with PUR on the global market are expected to rise to approximately
US$80 billion by 2020. Effects of visible light
Polyurethanes, especially those made using aromatic isocyanates, contain chromophores
which interact with light. This is of particular interest in the area of polyurethane coatings,
where light stability is a critical factor and is the main reason that aliphatic isocyanates
are used in making polyurethane coatings. When PU foam, which is made using aromatic
isocyanates, is exposed to visible light it discolors, turning from off-white to yellow
to reddish brown. It has been generally accepted that apart from yellowing, visible light has
little effect on foam properties. This is especially the case if the yellowing happens
on the outer portions of a large foam, as the deterioration of properties in the outer
portion has little effect on the overall bulk properties of the foam itself.
It has been reported that exposure to visible light can affect the variability of some physical
property test results. Higher-energy UV radiation promotes chemical
reactions in foam, some of which are detrimental to the foam structure.
Degradation When chlorine gas reacts with water, HCl and
HOCl form and HCl causes the decomposition of the inner part of the polyurethane tubing
into amine salts and the polyol. Two species of the Ecuadorian fungus Pestalotiopsis
are capable of biodegrading Polyurethane in aerobic and anaerobic conditions such as found
at the bottom of landfills. Degradation of polyurethane items at museums has been reported
by many researchers. Polyester type polyurethanes (ES-PU) are more
easily biodegraded by fungus than polyether type.