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Corrosion, which is due to the chemical reactions between materials and their surrounds, has been well studied and expects to be avoided in metal applications. Coating is a common method used to cover the surface of metal products to protect them from wearing away caused by moisture, oxidation, radiation, biological deterioration and other environmental damages. The coating layer deposited on metal substrate acts as a physical barrier to isolate it from the corrosive environment, e. g. a thin layer of zinc coating is a typical inorganic coating which effectively prevents oxidation in metal substrate. Compared with inorganic coatings, organic coatings not only provide sufficient corrosion resistance, but also are able to produce glossy and colorful surface.
Coil coating is widely used for organic coatings deposited on metallic sheet with the application of ship bodies, car bodies, buildings and airplane wings [1-2]. It is a continuous and highly automated process for metal coating, within which, up to 1.1m wide coil of metal go through the coating line at a speed of 20m per minute, making both the top and bottom sides of the metallic sheet coated and unwound [3-4]. During the manufacturing process, the metal is firstly washed by brush or by low concentrated alkaline cleaner to remove contaminants, pretreated to improve the adhesion between the metal and the coating, and followed by primer coating and top coating, in which coating liquid is transferred from pan to applicator rolls, as shown in Figure 1. After both primer coating and top coating, the sheet intermediately passes through ovens respectively for coating to be cured. Finally, a multilayered structure of coatings is deposited on the metal substrate as shown in Figure 2, which protects the substrate from corrosion. For a thick coating, the applicator rolls rotate opposite to the movement of the sheet, while for a thinner coating, they turn in the same direction with the sheet [4']. Coil coating is widely used due to its cheap and easy application because coating on flat surfaces is preferred rather than on irregular shapes. Moreover, compared with traditionally painting technology, coil coating has the advantages of less environmental pollution as an environmentally friendly alternative, since during the manufacturing process, the generated volatile organic compounds can be collected and only a nominal amount of waste is generated .
Primer coating unit
Top coating unit
Figure 1 A typical coil coating process 
Figure 2 A typical multilayered structure of metallic sheet after coil coating
Usually, polyurethane, as well as polyesters, epoxies and vinyls are used for metal coatings in commercial applications. Polyurethane is selected mainly due to its versatility. Many research works have been done to polyurethane, finding that it have a unique combination of many unusual properties and can be processed by almost all known manufacturing techniques [5-8]. Therefore, in modern life, PU is applied in a very broad range of field, besides coating, including high performance adhesives, sealants, foams, gaskets, spandex fibres and thermoplastic elastomers [9-11]. The worldwide consumption of polyurethane raw materials has been more than 13 million tons in 2007 and the average annual growth rate is about 5% .
In polyurethane, chains of organic units are linked together by the urethane linkage -NH-CO-O-. Commonly, polyurethane is abbreviated as PU rather than the abbreviation PUR. However, it should not be confused with urethanes which are also known as ethyl carbamate. In fact, PU is neither produced by the polymerization of urethane monomers, nor does it contain predominantly urethane groups in its structure. Generally, PU is prepared by the condensation reaction between an isocyanate and a polyol with the presence of catalysts, even sometimes chain extenders, so the final structure and properties of PU greatly depends on the structure and functionality of the polyol and the isocyanate to be reacted.
Although PU has good abrasion resistance, tear strength, excellent shock absorption, flexibility and elasticity, it is weak at thermal stability and barrier properties. To overcome these advantages, one way is to modify the reacted polyol and isocyanate, another way which is more attractive is to incorporate inorganic fillers, especially nanofillers, such as layered silicates, nanotubes and nanofibres . Compared with pure polymer or conventional composites, the primary advantage of applying polyurethane nanocomposites is the significantly improvement in thermal, mechanical and barrier properties [17-23]. The key to obtain these significantly improvements is to disperse nanofillers well in the polymeric matrix to take advantage of their extremely high aspect ratio . In addition, the weight of the product can be greatly reduced when superior properties are achieved with small amounts of nanofillers, in the range of 3-5% by weight .
Historical Developments of Polyurethanes
While the chemistry and technology of polyurethanes are of relatively recent origin, the chemistry of organic isocyanates dates back almost 150 years in 1849, when Wurtz  was the first to synthesize aliphatic isocyanates and described a number of simple isocyanate reactions which are among the most important commercial reactions to this day. Subsequently, Hofmann  prepared the first aromatic isocyanate and Hentschel' method  became the predominant commercial method for the synthesis of organic isocyanates.
The pioneering work on PU polymers was done by Otto Bayer and his coworkers in 1937 in the laboratories of the German IG Farbenindustrie. To compete with nylon, the most versatile and practical polymer at that time, which was introduced by Carothers and DuPont at 1935 in Americam, O. Bayer  developed the first fiber-forming polyurethane which were produced from liquid polyesters derived from glycols or adipic acid and various aromatic diisocyanates by using the polyaddition principle. The prepolymer was then chain extended and crosslinked at elevated temperature. Bayer's work was regarded as a breakthrough in polymer chemistry. However at that time, the polymer was dismissed as impractical since the isocyanate groups react faster with urea groups rather than with urethane groups.
Initially, work focus on the production of PU fibres and flexible foams. DuPont  and ICI  first produce polyurethanes on an industrial scale in 1940s, recognizing their elastomeric properties. The used diisocyanate was 1,5-naphthalene diisocyanate and water was used as the chain extender. DuPont had been at the forefront of polyurethane technology in the 1940s, receiving patents covering the reactions of diisocyanates with glycol, diamines, polyesters and other active hydrogen-containing chemicals. Then the development was constrained by World War II, until 1954 that commercial production of flexible polyurethane foam began, based on toluene diisocyanate (TDI) and polyester polyols which were also used to produce rigid foams, gum rubber, and elastomers .
In the late 1950s, low cost polyether polyols were introduced by BASF and Dow Chemical. As the decade progressed, new-developed chlorofluoroalkane blowing agents, inexpensive polyether polyols, and methylene diphenyl diisocyanate (MDI) brought great development of polyurethane rigid foams as high performance insulation materials.
In the mid-1960s, a new manufacturing process called reaction injection mouldig (RIM) was introduced by Bayer  for polyurethanes. RIM, which is one of the most exciting developments in the history of polyurethanes and is still widely used today, is a rapid process where two or more reactive liquid components are injected by high pressure impingement into a close mold followed by demolding in a very short time (less than one minute). In the first generation of RIM, ethylene glycol or a mixture of diols and a catalyst are on one side while modified liquid MDI is on the other side. By far, there are many different forms of composite RIM systems have been developed, e. g. reinforced RIM (RRIM), in which fillers such as milled glass, mica, and processed mineral fibres are added to improve stiffness and thermal stability; structural RIM (SRIM), in which the stiffness is improved by incorporating preplaced glass mats into the RIM mold cavity; and more recently low density RIM (LD-RIM) .
Polyurethane used for coatings were among the earliest polyurethane products investigated. O. Bayer and coworkers  also investigated the polyurethane coatings at 1947, which were suitable for many substrates such as wood, metal, rubber, leather fabrics and paper. Later, Heiss  introduced one-component, moisture-cure urethane coatings, in which NCO-terminated prepolymers prepared by reaction of MDI and TDI with a variety of polyethers and then cured in air. To utilize the one-component system to extend the shelf life of urethane coatings, blocked isocyanated systems were investigated and later applied in coil coatings, powder coatings and cationic electrodeposition coatings [33,35].
In early work, non-ionic surfactants and strong shear forces are needed for producing polyether or polyester-based urethane emulsions, which often caused "water spotting" . The problem was overcome by D. Dieterich and coworkers  by reporting waterborne polyurethane coatings, which was also known as "self-emulsifying" systems since extraneous surfactants were not required. After that in the early 1970s, rapid curing methods were developed both in Germany  and in the USA  for urethane and urethane-acrylic coating systems by using UV or electron beam radiation.
Raw Materials of Polyurethanes
Isocyanates are compounds containing the functional group -N=C=O. When an isocyanate has two isocyanate groups, it is called diisocyanate. The modern polyurethane industry is based on isocyanate chemistry, thus isocyanates are one of the most heavily produced specialty organic chemicals. Commercially available organic isocyanates include aliphatic, cycloalophatic, araliphatic, aromatic and heterocyclic polyisocyanates .
Figure 4 Structures of some common diisocyanates 
There are many different kinds of diisocyanates have been investigated in preparing polyurethanes. Structures of several important diisocyanates are shown in Figure 4. TDI (2,4- and 2,6-toluene diisocyanate and mixtures of these isomers) and MDI (diphenylmethane-2.4'- or -4,4'-diisocyanate), which are aromatic isocyanates, are the most popular precursors due to their low price and the acceptable physical properties provided by their products. However, health hazard is always a concern when using aromatic isocyanates, thus MDI is more preferred to be used rather than TDI, because MDI is less volatile. PPDI (para-phenylene diisocyanate) is applied for producing thermally stable polyurethanes. HDI (1,6-hexamethylene diisocyanate) was the first aliphatic used for polyurethane synthesis, and is still used for producing adhesives and flexible surface coatings. HMDI (4,4'-ducyclohexylmethane diisocyanate) and IPDI (3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate) are both cyclo-aliphatic diisocyanates. HMDI has some transparent applications while IPDI offers excellent light and high temperature stability for polyurethane products. Other isocyanates used for polyurethane synthesis includes NDI (1,5-naphthalene diisocyanate), XDI (4,6'-xylylene diisocyanate) and TMDI (2,2,4-trimethyl-1,6-hexamethylene diisocyanate).
The polyurethane derived from these isocyanates varies a lot due to many reasons [7, 39-40]. First, the properties of the polyurethane can be affected by the shape of isocyanate molecule. A linear diisocyanate molecule, e.g. 4,4'-MDI, HDI and HMDI provide good elastomeric and thermoplastic properties by producing two-phase block copolymers and creating physical links between the polymer chains when an asymmetric molecule, 2,4'-MDI, TDI and IPDI is more likely to produce clear and softer products. Second, the functionality of the isocyanates significantly influence the cure rate of the processing and physical properties of the products, such as hardness, toughness and gloss of the products. Third, compared with the aromatic isocyanates, the aliphatic isocyanates are less reactive whereas they provide better oxidative and ultraviolet stability . Therefore, aromatic isocyanates are used in applications with little or no exposure to light when aliphatic isocyanate is value for applications which are exposed to a lot of light, such as floor, automotive coatings. Last, the different of reactivity between the functional groups of isocyanate molecules can influence the viscosity and flow characteristics of the polyurethane during processing.
3.1.2 Isocyanate Chemistry
Isocyanates are highly reactive with alcohols, carboxylic acids and amines which have been widely studied [7, 8, 42]. The high reactivity of the isocyanates is due to the low electron density of the central carbon which is indicated by the resonance structure of NCO group as shown in Figure 5. The oxygen atom carries the greatest net negative charge, and nitrogen carries an intermediate one whilst the carbon atom carries a net positive charge. Thus, it can react with both electron donor and electron acceptor functional groups. Furthermore, in terms of reaction rate and conditions, the reactions of isocyantes fall into two classes: primary reactions and secondary reactions.
Figure 5 Resonant structure of isocyanates
Primary reactions take place with a higher reaction rate and under lower temperature compared with secondary reactions. Four common primary reactions are shown in Figure 6. The reaction (1) and (2) have the greatest importance for PU technology . When isocyanate reacts with hydroxyl group such as alcohol to produce urethane linkage, the reaction is exothermic. The reaction of isocyanates with amino groups to produce urea usually is carried out at 0-25oC. Isocyanats can also react with carboxylic acid and water to form a symmetrically substituted urea and gaseous CO2 which is undesirable in synthesis of polyurethane elastomers.
Figure 6 Primary reactions
Isocyanates can react with the secondary amino group of urethanes, ureas and amide to genetate allophanates, substituted biurets, and acyl ureas respectively as shown in Figure 7. Secondary reactions result in the polymeric systems such as chain branching and crosslinking, thus, processing temperature should be controlled to avoid the secondary reactions and thereby crosslinking during polyurethane synthesis.
Figure 7 Secondary reactions
Other basic reactions are that isocyanates can react with themselves to form dimers, trimmers and other polymeric structures, e. g. dimerization and trimerization of isocyanates result in uretediones and isocyanurates respectively with in presence of catalysts. Isocyanurates have a ring structure and is thermal stable.
Polyols are another important reactive component to produce polyurethanes, which are usually liquid, containing at least two isocyanate-reacting groups (reactive hydrogen atoms). Generally, the average molecular weight of polyols ranges from 200 to 10000, depending on their applications. The structure, as well as the chain length of polyols, has a profound effect on the processing and properties of produced polyurethanes. In fact, the majority of the linkages found in polyurethanes are derived from the linkages found in the polyol. Therefore, when polyester or polyether polyols are used for the polyurethane manufacturing, the polyester-based or polyether-based polyurethanes are obtained respectively.
The polyols used fall into four major categories :
Polyether polyols. These materials produce very high quality polyurethane elastomers and foams. The commercially available polyether polyols includes polyBD, polypropylene oxide glycol (PPO), polybutylene oxide glycol (PBO), and polytetramethylene ether glycol (PTMEG) has a special application for the production of hydrolysis-resistant polyurethane elastomers, the structures of which are shown in Figure 4. It can be found that these glycols are modified by ethylene oxide, which aims at improving their reactivity with electrophiles such as isocyanates.
Figure 4 The structures of important polyether polyols 
Amine-terminated polyethers. These are based on polyether polyols with the terminal hydroxyl groups replaced by primary or secondary amino groups.
Polyester polyols. They are polyalkylene glycol esters such as polybutylene adipate or caprolactone polyesters shown in Figure 5. Polyalkylene glycol adipates are derived from the condensation polymerization of the alkylene glycol and the corresponding diester or diacid whilst caprolactone polyesters are prepared by ring open polymerization of caprolactone monomer with a glycol, e. g. ethylene glycol. It is found caprolactone polyesters provide good hydrolytic stability for the polyurethane products.
Polycarbonate, such as polyhexamethylene carbonate.
Figure 5 The structures of important polyester polyols 
Polyether and polyesters polyols are both commonly used for the production of polyurethane coatings, to meet different market requirement. Compared with polyester polyols, the polyethers have lower cost and provide better hydrolysis resistance for the polyurethane products. However, polyester-based polyurethanes exhibit better mechanical properties [42-44].
Chain extenders are low-molecular-weight reactants which are applied in the polyurethane process to produce elastomeric properties. They can be either hydroxyl-terminated or amine terminated and its molecular weight ranges from 40 to 300. They also should be difunctional to link isocyanate groups, otherwise when compounds are with functionality three, they are regarded as cross-linkers. The chain extenders improve hard-segment content in the polyurethane backbone, thus control the mechanical properties such as modulus and ultimate strength and influence the thermal and hydrolytic stability of the finished product.
Three popular chain extenders are shown in Figure 6. Chain extenders containing amine groups generally react fast but sometimes too fast and are possible to impart an odor into the finishing product. Hydroxyl-containing chain extenders are usually slow to react with isocyanates and require the addition of catalyst, such as organometallic catalysts. Another problem of applying hydroxyl-containing chain extenders is the limited amounts used in the polyurethane formulation which is due to their poor solubility in polyols.
Figure 6 Three popular chain extenders
Catalysts are used to catalyse the reaction of isocyanates with polyols in polyurethane synthesis. The most commonly used catalysts are tertiary amines like diaminobicyclooctane (DABCO), dibutyltin dilaurate (DBTDL), organometallics (primarily tin compounds) like stannous octoate, and carboxylic acid salts.
Tertiary amines, which are one of the nucleophilic catalysts, are the most commonly used flexible polyurethane foam catalysts. They can catalyse both isocyanate-polyol and isocyanate-water reactions. Organometallic compounds, which are one of the electrophilic catalysts, also represent good catalytic effect in urethane reactions. The catalysis mechanism of tertiary amines and organometallic compounds are shown in Figure 7.
Figure 7 The catalysis mechanisms of (a) amine catalysts [45, 46] and (b) organometallic catalyst, where L symbolizes a ligand substituent to the tin molecule .
Stoichiometry in Polyurethane Synthesis
When mixing the raw materials, a precise stoichiometry is necessary for further investigation related to the structure and properties of polyurethanes. The calculations needed are listed below :
Functionality. It indicates the number of groups in a molecule can be used to react with other groups. For polyols, the functionality is based on the number of active hydrogen atoms, for example, the functionality of glycol is 2.
Hydroxyl value (OHV). It is defined as the number of milligrams of potassium hydroxide necessary to neutralize the acetic acid which combines on the acetylation of one gram of sample and is usually supplied by the manufacturers for a given glycol. It is affected by acid value (AV).
Weight percentage of hydroxyl (OH%). When OHV is not supplied, OH% is used to describe polyols. The relationship between OHV and OH% is
Molecular weight (Mw) of polyols. It can be calculated below, where 56.1 indicates the molecular weight of KOH.
NCO content (NCO%). For TDI, NCO% = 422/174 = 48%.
For MDI, NCO% = 422/250 = 33.6%,
where 42 is the molecular weight of NCO.
Equivalent weight. It presents the molecular weight for each unit of functionality, As one NCO group reacts with one OH group, 1g equivalent isocyanate = 1g equivalent polyol.
Equivalent weight of isocyanate = molecular / functionality = 42100 / NCO%
Equivalent weight of polyol = 56100 / OHV
Equivalent weight of water = 18 / 2 = 9
Equivalent weight of acid = 56100 / AV
Equivalent weight of primary amine = molecular weight / 2
Equivalent weight of secondary amine = molecular weight
Isocyanate index. It equals to (actual quantity of isocyanate / calculated quantity of isocyanate)100%.
Polyurethane Coating Classification
Polyurethane coatings are classified into six types by ASTM in the ASTM D16 Standard, which is summarized in Table 1 with the characteristics of these different PU coatings [51, 52]. The ASTM D16-type V PU coating is suitable for high performance application and corrosion protection, thus applied in the coil coating for metallic sheet.
Table 1 Six ASTM polyurethane coating types 
Thermoplastic Polyurethane Coatings
Thermoplastic polyurethane is regarded as a linear structural block copolymer consists of hard segments (HS) and soft segments (SS), and its structure can be described as (SH)n. Phase separation is observed from this structure since the hard and soft segments are instinct incompatible or thermodynamic immiscible. In 1959, Schollenberger  first studied these segmented polyurethanes. Then in 1966, Cooper and Tobolsky  found that the hard segments acted as filler particles as well as crosslinker in the structure to restrict the motion of amorphous soft segments.
Therefore, many factors, e. g. the composition of soft and hard segments, lengths of soft and hard segments and the sequence of length distribution, chemical nature of the units composing the polymer, anomalous linkages, molecular weight and the morphology in the solid state, all decide the properties of thermoplastic polyurethane coatings. Hard segments are below their Tg and impart the polyurethane dimensional stability, hysteresis, high modulus and tensile strength while soft segments are above its Tg and provide elastomeric properties . Many investigations have been done to study these factors and show that structure of soft and hard segments , type and chain length of chain extenders [57, 58], chain length of soft segments , crystallizability of either segment  and thermal history of polyurethanes  influence the degree of phase segregation, phase mixing, hard segment organization, and subsequent polyurethane coating properties .
Thermoset Polyurethane Coatings
Compared with thermoplastic polyurethane coatings, thermoset ones have enhanced mechanical, barrier properties and better high temperature stability due the presence of crosslinking in the thermoset PU. To increase the crosslinking concentration, one effective way is to increase the functionalities of isoyanates and polyols . Other methods include introducing peroxides or tri-functional chain extenders . Besides promoting mechanical and barrier properties, higher crosslinking improves phase mixing and ease of processing.
Developments of Polyurethane Nanocomposites
As new nanocomposite product of thermosetting resins have a potential market in aerospace, automotive and coating applications, many research works have been done on polyurethane-clay nanocomposite in the recent years. The pioneering work was done by Wang and Pinnavaia , who synthesized intercalated nanocomposites based on elastomeric polyurethanes. A modified organo-montmorillonite, a commercial methylene-diphenyl-diisocyanate prepolymer and ethylene glycol were used, forming the nanocomposites with an interlayer spacing of 5 nm. It was attractive to find that the addition of nanoclay enhanced both strength and toughness of the polyurethane.
Subsequently, Chen et al.  modified montmorillonite with 12 aminolauric acid (12COOH) and benzidine (BZD), which was then mixed with a solution of separately polyurethane, succeeded in yielding the fully exfoliated nanocomposites. The segmented structures of polyurethane were not interfered by the presence of the organoclays in these nanocomposites.
Tien and Wei  found that the hydrogen bonding in the hard segments of the synthesized organoclay/polyurethane nanocomposites decreased with the increasing amount of clay. They also found the strength and elongation at break of PU/organoclay nanocomposites increased dramatically compared with pure polyurethane.
Yao and Song  synthesized polyurethane nanocomposites with 4,4'-di-phenymethylat diisocyanate (M-MDI), Na+-montmorillonite, modified and modified polyether polyol (MPP). Besides strength and strain at break, the storage modulus below the Tg of the soft segments in the polyurethane increased by more than 350% with the addition of nanoclay.
Kim  studied the reinforcing effect and enhanced thermal and water resistance of the waterborne polyurethane/clay nanocomposites, which is derived from aqueous emulsion of polyurethane ionomers, based on poly(tetramethylene glycol) and isophorone diisocyanate. Kim and Moon  also prepare polyurethane/organoclay nanocomposites from highly crystalline poly(butylene succinate)/poly(ethylene glycol) polyols and 4,4â€²-dicyclohexylmethane diisocyanate, using 1,4-butanediol and organoclay hybrid as chain extenders.
Rehab and Salahuddin  synthesized polyurethane-organoclay nanocomposite by swelling the organoclay into different kinds of diols followed by addition of diisocyanate.
Organocaly is an organically modified layer silicate, which is derived from a natural mineral. Montmorillonite, hectorite and saponite are the most promising layered silicates to produce nanocomposites, but it is necessary to purify a natural montmorillonite before modifying it due to the existence of many other minerals. There are two reasons why these clays are selected to make nanocomposites [70, 71]: First, they are cheap since they are easily required. Second, they can be modified to be compatible with organic polymers due to their rich intercalation chemistry.
Structure of Layered Silicates
The layered silicates, for example montmorillonite, which are commonly used in nanocomposites belong to the family called 2:1 phyllosilocates, which consists of layers made up of two tetrahedrally coordinated silicon atoms fused to an edge-shared octahedral sheet of either aluminum or magnesium hydroxide as shown in Figure 10. The layers in the lattice crystal structure are 1nm thick and the lateral dimension of these layers varies from 30nm to several microns depending on different type of layered silicate provided in Table 2. Therefore, the aspect ratio of the layered silicate is extremely high, which results in high in-plane strength and stiffness. A regular gap called interlayer presents in the structure due to the stacking of the layers and the inter-stack Van de Waals force.
Figure 10 The ideal structures of montmorillonite 
CEC (mequiv/100 g)
Particle length (nm)
Table 2 Chemical structure and characteristic parameter of commonly used 2:1 phyllosilicates
Isomorphic substitution within the layers, for example, in the tetrahedral sheet, Si4+ replaced by Al3+, in the octahedral sheet, Al3+, replaced by Si4+ or Mg2+, led to negative surface charges which are balanced by alkali and alkaline cations such as Ca2+ and Na2+ which are inside the clay galleries. The number of substituted metals within the layer is characterized by a moderate surface charge called the cation exchange capacity (CEC), which is expressed in the unit of meq/100g. For example, for clays with a CEC calue of 100, there is a negative charge at each 0.7nm distance on both surfaces of the layers. The clay has a hydrophilic nature due to the hydration of these exchangeable cations and the polar nature of surface silanol group .
7.2 Organic Modification of Layered Silicate