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  • 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 insseldorf, 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 usedcan 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/offdispense/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 waterHCl 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.

Polyurethane Polyurethane is a polymer composed of a chain

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