coating materials POLYURETHANE COATINGS FOR METAL AND PLASTIC SUBSTRATES BY AUGUSTIN T. CHEN AND RONALD T. WOJCIK OLIN CORP., CHESIRE, CONN. Polyurethane coatings have excellent gloss, hardness, flexibility, abrasion resis-tance, chemical resistance, and ultraviolet (UV) durability. In addition, higher solids polyurethane coatings meet current regulations for volatile organic compound (VOC) emissions without compromising a coating's performance. Because polyurethane coatings offer so many advantages, they are the market leader for coating various substrates in the following end-use markets: automotive refin-ish; aircraft; and chemical agent resistant coatings (CARC). Moreover, polyurethane coatings have especially rapid growth in automotive original equipment manufacturing (OEM); industrial maintenance (such as bridges and bulk storage tanks); and appliances. This article describes polyurethane chemistry, applications, and perfor-mance requirements for coating metallic and plastic substrates. The authors will limit discussion ro two-component solventborne, one-component solvent borne, and powder coatings based on polyurethanes as applied to the automotive refinish, automotive OEM, aircraft, and industrial maintenance end-use mar-kets for metal substrates. Due to the strong growth of plastic components for automotive OEM applications, polyurethane coatings for automotive OEM plastic components are also discussed. POLYURETHANE COATINGS FORMULATION The performance of any coatings can be divided into five elements: processing applications, coating appearance, mechanical performance, heat and UV resistance, and chemical resistance. Figure 1 outlines the elements, including test methods, used to measure per-formance. Coating scientists use Fig. 1 as a laboratory blueprint defining the important steps in the formulation process. A formulator will modify this blue-print, as required, to meet specific performance objectives. For example, in au-tomotive refinish, formulators focus first on optimizing the application and appearance, before maximizing the mechanical performance and resistance test-ing (UV and chemical). In industrial maintenance, formulators are most concerned with optimizing application and chemical resistance, whereas mechanical per-a performance property fails the formance and appearance are less important. If the chemical reactants specification, the formulator adjusts the concentrations of and restarts the design process. SOLVENTBORNE COATINGS Coating scientists begin the formulation process by selecting chemical reactants that form a good film. They then blend, adjust, and readjust concentrations fol-lowing the formulation outline, until the finished product has met all per-formance requirements. Polyurethane solventborne coatings contain some or all of the following chemical reactants: aliphatic polyisocyanates, polyols (hy-droxylated acrylic or polyester), solvents, catalyst, pigments, and additives (e.g., 108 PROCESSING VOLATD..E ORGANIC CONTENT (VOC) SPRAY VISCOSITY DRYTIME CURE PROFILE SHELF Sf ABILITY ponlFE APPEARANCE GLOSS DLSTINCTNESS OF IMAGE (DOl) YELLOWING INDEX MECHANICAl, PERFORMANCE PENCIL or PENDULUM HARDNESS ADHESION TABOR ABRASION IMPACf MANDREL BEND MAR RFSISTANCE 1'\" HEAT & CHEMICAl RESISTANCE QUV WEA TIlERlNG nORmA WEATHERING SOL VENT RFSIST ANCE ACID I BASE RESISTANCE YEl.LOWING INDEX Fig. 1. Schematic of coating formulation process. UV stabilizers, reactive diluents, rheology control). The first polyisocyanate commercialized for a solventborne coating was the re-action product of toluene diisocyanate (TOI) and trimethanol propane (TMP). Commercialized in 1955, this product is still used for metallic industrial fin-ishes as a cobinder for magnetic media (audio/video tapes) and for wood coat-ings; however, the TOI/TMP polyisocyanate yellows severely when exposed to di-rect sunlight, as do all aromatic polyisocyanates. This disadvantage led to the de-velopment oflight-stable aliphatic polyisocyanates made from hexamethylene di-ducts (Fig 2). isocyanate (HOI), isophorone diisocyanate (IPOI), and their ad Three commercial HOI-based polyisocyanates are used in polyurethane coat-ings. Finished coatings based on HOI trimer (polyisocyanurate) chemistry have significant advantages, including low viscosity (reduced VOC), excellent color, and low free monomer content. Two commercial polyisocyanates based on biuret chemistry are also available for making polyurethane coatings. Both HOI bi-urets are higher in color and higher in free monomer content than the HDI trimer. The low-viscosity biuret is higher in isocyanate content and lower in equivalent weight than HOI trimer. The high-viscosity HOI biuret is higher in vis-cosity and similar in isocyanate content and equivalent weight to the HOI trimer. 109 0yNiO I (CH2)6NCO Catalyst OCN(CH)6 /NyN ..... (CH2)6NCO 2o HDITrimer OCN-(CH2)6-NCO HEXAMETHYLENE DllSOCYANATE HDI Biuret OCN CHJ ,. Y' o J N / -<\\:, CH3 J CHCH2 Catalyst -OCN ISOPHORONE DllSOCYANATE I ~CHJCH \Y N~O I N CH2 \"( CH2 ,,\"\" CHJ NCO HJp CHJ IPDITrimer Fig. 2. Polyisocyanate structures. Adducts made with IPDI trimer (Fig. 2) were introduced in the late 1960s. Due to a different monomer structure, these trimers are higher in viscosity, high-er in color, higher in free monomer content, and lower in crosslinkability (crosslink sites per unit molecular weight)than the HDI trimer. Table I summarizes the important physical properties of polyisocyanate adducts used for metal coat-ings. The performance of the finished coating depends on several factors: the adduct (HDI trimer or HDI biuret), polyol, solvents, and catalyst. Table II rates and ranks the coating performance (performance test methods described in Fig. 110 Table I. Polylsocyaollte Used For Metal Coatings HDI Dlmer HDI Trimer HDI Biuret (low v) HDI Blum (high v) [PDI Trimer Solids Neat NCO COlltellt. % 21-23 NCOEq. Wt. 188 Viscosity @ 2S·C. cps 100-300 Color. APHA 80 Free MODOlIlCr. % 0.5 Neat\" 21-23 190 2.000-3.000 60 0.2 Neat 22-24 182 2.700-3.700 I 200 0.7 Near 21-23 191 8.000-15.000 200 0.7 70\" 12.2 344 1.000-1.600 200 0.7 liDI, hexamothyleue dU!KJgyanato: !PD!. IsophofQJ1D d~allltC; v. vIaI:oaIty. \"ConImercIally also available III 90\" solid in butyl acetaWaromatic 100 &olvent mixture. ~CommercIally also ayallable as 75'1& solid in butyl acetatelxylone solvent mixture. 'COIIltIlUClally available as 70% soUd in butyl acetatclllllllllll!ic 100 solvent mixture. 1) of the various commercial polyisocyanates. Each polyisocyanate offers dis-tinct advantages and disadvantages depending upon the performance require-ments of the finished product. These characteristics must be considered by the formulator prior to the start of the formulation process. Coatings made with IPDI trimer are harder, have higher tensile properties, and better chemical resistance than coatings made with HDI-based polyisocyanates, as illustrated in Table II. To take advantage of both sets of performance charac-teristics, coating formulators typically blend IPDI trimers with HDI-based adducts to enhance the performance of the finished coating. HDI and IPDI Table n. Coating Performance versus Polylsocyanates HDl Dlmer Polyisocyanate SM/f Stability HDI Tl'im4r HDI Biuret (low v) HDI Biuret (high v) IPDI Trimer Good Bcst Good Good Best Meets Current VOC Regvlations for Key End·Use Markets Exceed Yes Drytime Speed Proc,~slng Yes No No Fast Long Potlife OjlCll TImc Appearance Slow Fast Slow Short Slow Short Long Long G1oas, DOl, and NOII·Yellowillg Specifications Better Better Hardness 8IId Abrasion Spec1tication Good Bctter Adhesion, Impact. and Flexibility Specification Better Good Better Good Bcst Mechanical Performance Good Good Good Better Best Poor UV Re:lsrance Field (Test Fence) Specifications (Non. Yellowing, Non.ChalIcing, 8IId Gloss Retention) Better . Blltter Oood Good Best Chemical Res/,stance Field (Test Fence) Specifications Good Better Good Good Best DOl. diatlnctnoss 01 !mail; liDI. hexamethylone dllJocyallltC; !PDI. laophorone dUlocyanate; v. vlscoaity. 111 trimer blends are used in automotive refinish and automotive OEM to speed up dry time and reduce damage to the clear coats caused by acid rain. POlYOlS Numerous acrylic and polyester polyols are available to use as coreactants with polyisocyanates. The hydroxyl (OH) functionality on the acrylic or polyester structure reacts with the isocyanate (NCO) functionality on the polyisocyanate. Polyol selection plays an important role in achieving performance goals. Lead-ing polyols for automotive refinish and OEM, aircraft, and industrial maintenance are summarized in Table III, along with their physical properties. Acrylic polyols are generally preferred in automotive and industrial maintenance applications. Urethane acrylic coatings have excellent appearance, very good UV stability, and meet the overall performance requirements for these end-use mar-kets. Polyester polyols are preferred for use on aircraft and, to a lesser extent, in in-dustrial maintenance. Urethane polyester coatings have excellent mechanical per-formance (flexibility and toughness) as compared to acrylic polyols, but are lacking in UV resistance. CATALYSTS AND SOLVENTS Coatings are cured (hardened) by the reaction of the polyisocyanate (NCO func-tionality) with the polyol (OH functionality). Coatings can cure at room tem-perature or may be accelerated by heat. The addition of catalysts will speed up the final cure. The catalysts of choice among formulators are diburyltin dilaurate (DBTDL), tertiary amines, alkali metal octoates, acetyl acetonates, titanates, and blends of these catalysts. Newer catalysts, such as dimethyl tin dilaurate and dibutyltin diacetate, are gaining popularity. In addition, formulators blend dif-catalysts to decrease the overall time required to complete ferent combinations of the curing process. Most common organic solvent are suitable (esters, ketones, aromatic hydrocarbons, and so on) as thinners for polyurethanes, provided the solvents do not contain groups that react with isocyanates or contain impurities (alcohols, thiols, amines, carboxylic acids, or water). TWO-COMPONENT AND ONE-COMPONENT COATINGS Two-component polyurethane coatings are formulated from the list of chemi-Table Polyols Used in Automotive Refinish and Original Equipment Manufacturing, industrial Malntenl\\Dce, and Aircraft End·use Market Polyol % Solid Vi$coslly @ (cps) 25'C m. Equivalent Welsht Color APHA T, ('C) Jnduslrlal maintenance Heavy duly Medium duty Light duty Medium duty Automotin ~iDish and original equipment manufacturing Acrylic Acrylic Acrylic Acrylic Polyester Polyester 70· 70-12,000 8,500 5,100 10,000 2,600 815 655 30 80 30 61 18 12 Aircraft 80' 75\" 80· 77\" 5.000 650 1,000 519 500 100 300 200 50 \"Propylene alyool methyl eliler acetate. h.·Butyl acetace. c.·Mcthyiamyl ketone. 112 cal coreactants outlined earlier under \"Solventborne Coatings.\" As the name im-plies, two-component coating are supplied to the end user in two separate con-tainers, and are mixed prior to application. One container (component A) holds the polyol, catalyst, solvent, and additives (rheology control agents, etc.). The second container (component B) holds the polyisocyanate, solvent, and addi-tives (UV stabilizers). Each container is individually shelf stable. The isocyanate/hydroxyl (NCO/OH) reaction begins as soon as the contain-ers are combined. The mixing ratio of the components can be based either on vol-ume or weight and is precisely calculated to achieve stoichiometry of the core-actants (NCO/OH = 1). This ratio gives a finished coating optimum mechanical performance and optimum chemical resistance. If necessary, the NCO/OH ratio may be varied to change coating properties. If the NCO/OH ratio is less than one, some OH functionality is unreacted and the coating has increased flexibility, better adhesion to substrates, and reduced solvent and chemical resistance. If the NCO/OH ratio is greater than one, some NCO is unreacted and the coating needs a longer time to dry and surface harden. The final product is harder, sol-vent and chemical resistance is increased, flexibility is decreased, and adhesion to the substrate is reduced. A rule of thumb in coatings formulation is to main-tain the NCO/OH ratio between 1.03 and 1.05. In the formulation process, dry time and pot life are determined by the amount of catalyst in component A. The amount of solvent used in the formulation de-pends upon the applications requirements, as well as on VOC regulations for the finished coating. Coating formulators will blend, adjust, and readjust the the coating reactants until the finished product has met all the concentrations of required performance goals. One-component solventborne polyurethane coatings utilize the same chem-ical reactants used in two-component coatings and are formulated by the same process except for three modifications: the polyisocyanate adduct is blocked; all chemical coreactants are blended into one container; and the coating must be heat cured. Blocked polyisocyanates utilize a chemical moiety to protect the isocyanate func-tionality from reaction during shelf storage at room temperature. The chemical moiety blocks, or caps, the isocyanate by reacting with the NCO functional groups. With the addition of heat, the blocked isocyanate breaks apart, regenerating the isocyanate functional groups (NCO) at the higher temperature (see Fig. 3). After the the NCO NCO functional group is regenerated, the coating cures by the reaction of group with the OH group (acrylic or polyester) on the polyol. One-component coat-ings are used in end use markets (such as automotive OEM) that require no mixing or metering prior to applications and that have available curing ovens. Most blocked polyisocyanates require high (>300°F) unblocking tempera-tures. This disadvantage has slowed the growth rate of one-component polyurethane coatings. Material suppliers are working on novel unblocking chemistry to lower the required temperatures. POWDER COATINGS Urethane powder coating for metallic substrates is a rapidly growing coating technology that offers significant advantages over solventborne coatings. The core-actants used in polyurethane powder coatings are: a solid blocked polyisocyanate (blocked with -caprolactam), a solid polyester resin, catalyst (dibutyltin dilau-113 o R·N·C·BL H I \" R·NCO + BL·H BL = BLOCKING COMPOUND t o R·NCO + R'·OH POLYOL Fig. 3. Unblocking/curing reaction of blocked polyisocyanates. ---~ R·N·C-OR' I II H rate), and pigment and flow aids. Isophorone diisocyanate is reacted with trimethylolpropane (IPDI/TMP)and then blocked with -caprolactam (Fig. 4).This polyisocyanate, which is the lead-ing choice for powder coatings,is coreacted with polyesters. The crosslinker must be a solid with aT of 55 ° C. The polyester must also be a solid with aT between g g 50 and 60°C. The coreactants are melt blended,extruded, cooled, and pulver-ized to produce the final powder coating. Powder coatings can be applied by fluidized bed or electrostatic spray. A coated metal substrate requires heat to melt and flow the powder and to unblock the isocyanate to complete the cure. Standard curing conditions are 10 minutes at 360°F for catalyzed coatings and 30 minutes at 360°F for uncatalyzed coatings. Table IV lists the physical prop-the isophorone diisocyanate blocked polyisocyanate and polyester resin erties of used in powder coatings. Urethane powder coatings for metals are a rapidly growing segment of the coat-ings industry. Finished coatings have good chemical resistance, corrosion resis-tance, and mechanical performance. In addition, they have very good exterior col-or and gloss retention. Because finished products are available in all colors, clear coats and textured coatings, they are in demand for appliance, automotive wheel covers and trim, playground equipment, and garden tractors. POLYURETHANE COATINGS FOR PLASTIC SUBSTRATES Since the invention of phenolic resin 80 years ago, plastics have come a long way, replacing traditional materials such as metal, wood, and glass in many appli-cations. In 1992, more than 60 billion pounds of plastics were used in the United States. Compared with traditional materials, plastics are cost effective, inherently corrosion resistant, and can be easily processed into complicated shapes. With these advantages, plastics are used today in almost all major in-dustries, ranging from construction to health care. Unlike metal or wood, the colored surface finishes of plastic products for most applications usually are achieved not by painting of the products. In-stead, they are usually accomplished by making the products from colored resins that are manufactured by compounding or mixing solid pigment or liq-uid colorant into the resins during the resin manufacturing stage. One of the 114 CHzOH CH)CHZCCHzOH CHzOH TRIMETHYLOLPROPANE I I + CHOCN~ J CHJ + QH 0 e-CAPROLACl'AM CHzNCO ISOPHORONE DIISOCYANATE (1) (3) (3) .. Fig. 4. Powder coating blocked polyisocyanate. IPDI I TMP BLOCKED POLYISOCYANATE CROSSLINKER few major exceptions are plastic products used in transportation applications, such as automotive exterior or interior parts. In such applications, where a high degree of surface aesthetic quality such as gloss, distinctness of image (DOl), and color matching/harmony are required, this can be achieved only by painted finishes. In some aspects, coatings formulated for plastic substrates may require different considerations compared with the coatings designed for metal substrates. For ex-ample, plastics have lower surface tension, which makes the wetting and leveling of coatings on plastic substrates more difficult than on metal or wood sub-strates. Unlike metals, plastics will deform or melt at high temperature; therefore, the curing of painted plastic parts is carried our either at room temperature or at lower temperatures than metal parts. The typical industrial practice is 180°F for low-bake plastics, and 250 ° F for high-bake plastics. Some plastic products, such as those made from the SMC (sheet molding compounds) and glass-or fiber-reinforced composites, may need special preparation to obtain good surface evenness before painting. Furthermore, some plastics may be sensitive to cer-tain types of coating solvents. For these reasons, coating formulators have to be aware of these differences when designing coatings for plastics. Today, the designers in the transportation industry not only need to select appropriate plastics to produce components, they also need to select appropriate coatings to paint these plastic components. In addition to obtaining high gloss and DOl, other factors such as weatherability, acid-rain etching resis-tance, scratch/mar-resistance,and chemical resistance are also important in determining the appropriate coatings. Polyurethane coatings, with a desirable Table IV. Powder Coatings (200'C) (poise) ICI V~coslty Gardner Viscosity Equivalent Weight 1.020-1.250 Combining Weighl 240 Blocked polyisocylUlate Polyester MAK. Methyl amyl ketone. 30-45 S·W@6S%(MAK) 115 combination of excellent aesthetic surface appearance (wet-look), acid-rain etching resistance, and weatherability, are the best choice. In recent years, the use of two-component solvent-based polyurethane coatings has grown rapid-ly in plastic exterior component applications, replacing traditional two-com-ponent acrylic/melamine coatings. In the meantime, environmental concerns and VOC regulations have provided an excellent opportunity for waterborne polyurethanes to become the dominant choice for plastic interior component applications in the years to come. The painting process of plastic products is quite similar to the process used for metal substrates. For example, today's automotive plastic component paint lines use the same sequence of primer, base or color coat, and clearcoat as is used for metal body frame; however, there are two unique plastic characteristics that require pre-treatment of plastic components. First, plastics in general have low surface tension; therefore, the adhesion of coatings to a plastic surface may be more difficult than on metal substrates. This is especially true for the olefinic plastics such as TPO or polypropylene. For these plastic substrates, standard three-stage or five-stage cleaning prior to painting operation may not be enough. In order to improve surface wetability and adhesion of coatings to these plastic substrates, additional surface pretreatment such as plasma discharge, flaming, and chemical or solvent pretreatment, may be needed. All these pretreatment methods not only remove the problematic they also increase the surface weak boundary layer from the plastic substrates, roughness of plastic substrates to facilitate the diffusion of coatings. Furthermore, a thin layer «O.4mil) of adhesion promoter is often used to improve the adhe-sion between coating film and olefinic plastic substrates. On the other hand, plastic components made from thermoplastic polyurethane (TPU) and reaction injection molded (RIM) polyurethane have good adhesion with polyurethane coatings without these pretreatments. Unlike metal, plastic is generally nonconductive. Today, most of the paint lines designed to have high transfer efficiency use an electrostatic spray gun. In order to make this process work, plastic substrates have to either be prepared with conductive precoat conductive pigment or filler, or be coated first with a layer of before they can be painted using an electrostatic spray gun. Plastic substrates are more flexible than metal substrates; therefore, coatings designed for plastic substrates also have to be flexible, with comparable elonga-tion to the plastic substrates. Otherwise, any dynamic deformation of the sub-strates, such as the impacting or denting of plastic parts, will cause premature adhesion failure of the coatings. The typical rwo-component solvent-based polyurethane coatings formulated for plastics most likely are based on flexible polyester resins. For the same reasons, the polyisocyanates most often used are the biuret or trimer adducts derived from HOI. These polyurethane coatings provide excellent flexibility and film properties, low baking temperature re-quirements, good surface gloss and 001, and excellent acid-etching resistance and weatherability; however, these coatings also have lower surface hardness and lower mar resistance, which may not withstand repeated automatic commer-cial car washes. Today, more plastics are used in exterior applications, and cus-tomers demand longer good appearance and maintenance-free products. How to improve the scratch and mar resistance of polyurethane coatings for plastic substrates while maintaining their other performance features will be the chal-lenge for coating formulators and their raw material suppliers. 116 In addition to the continuous demands in performance improvement, current and future environmental regulation and legislation continue to mandate fut-ther VOC reduction for all coatings. For solvent-based polyurethane coatings, ear-lier efforts to formulate low-VOC, high-solid coatings were mainly focused on the development of resins with lower viscosity; however, the reduction of resin vis-cosity is often achieved by the reduction of the molecular weight of resin. Un-fortunately, many of the low molecular weight resins also cause the resulting coatings to have less than satisfactory properties; therefore, to develop high-sol-id and low-VOC coatings that also have high performance is quite difficult by us-ing low-viscosity resins alone. In the past few years, polyisocyanate producers began to commercialize a new gen-eration of low-viscosity polyisocyanates such as those based on HDI dimer (ure-tidione). Without solvent, these low-viscosity polyisocyanates by themselves have vis-cosities in the range of a few hundred centipoise. Compared with the convention-al biurets or trimers that have viscosities in the range of thousands of centipoise, these low-viscosity polyisocyanates provide exciting new potential to prepare low VOC coat-ings. Unlike low viscosity resins, these new low viscosity polyisocyanates are man-ufactured by forming different isocyanate oligomers from the diisocyanates; there-fore, they can achieve significant viscosity reduction without affecting polyurethane reaction and the performance of resulting coatings. Recently, reactive diluents such as those based on oxazolidine and aldimine chemistry are also becoming available. These reactive diluents can be used to replace conventional solvents typically used to reduce the viscosity of resins. Be-cause the reactive diluents also react with the polyisocyanates when the polyurethane coatings are cured, they will not evaporate like a conventional sol-vent; therefore, by replacing some solvent in the coating system with reactive diluent, the overall VOC value of that coating is also reduced. Combining the benefits from all these developments, the new generation of polyurethane coatings formulated using an appropriate mixture of resin and reactive dilu-ent and low-viscosity polyisocyanates were developed that can achieve VOC reduction while maintaining excellent surface properties, weatherability, and flexibility, which are essential for plastic substrates. SAFE APPLICATION OF POLYURETHANE COATINGS Like any other coatings, polyurethane coatings are made from a variety of chem-icals. Due largely to misunderstanding and confusion regarding the terms \"iso-cyanates,\" \"diisocyanates,\" \"polyisocyanates,\" \"urethanes,\" and \"polyurethanes,'\" there may be some misconception on the safety of handling polyurethane coat-mgs. Polyurethane coatings, which sometimes are mistakenly called \"urethane coat-ings,\" are formed by the reaction of polyalcohol and polyisocyanate. In some cas-es, a certain amount of polyurea, from the reaction of amine terminated polymer with polyisocyanate, is also present in the polyurethane coatings. The monomer-ic urethanes are not part of the polymer backbone of polyurethane coatings. Polyurethane coatings can be formulated either as one-component or two-com-ponent coatings. The one-component polyurethane coatings can be either ful-ly reacted polyurethane in water-based suspension or emulsion, or as solvent-based systems that cure after application. The solvent-based one-component polyurethane coatings are further divided into two different categories. The moisture-cured one-component polyurethane coatings contain isocyanate ter-117 minated polymer. Once applied, the polyisocyanates react with water in the at-mosphere to form the final coatings. The one-component heat-cured polyurethane coatings coatings are formulated using blocked polyisocyanates. These kinds of do not contain free isocyanate functional groups. They will not react with water in the atmosphere, and have to be cured by heat. When heated at an appropriate temperature, the blocked isocyanates in these coatings will unblock to generate free isocyanate functional groups that react with active hydrogens to form the cured coatings. The two-component polyurethane coatings consist of a resin component that has active hydrogen, and a polyisocyanate component. These two components are packaged separately and are mixed immediately before application. Both one-component and two-component polyurethane coatings can be formulated as solvent based or waterborne. To cure polyurethane coatings, polyisocyanates need at least two isocyanate functional groups per polymer chain to react with resins that also have mul-tiactive hydrogens; therefore, monoisocyanates such as methyl isocyanate, are never used in any polyurethane coatings. Oiisocyanates that are commonly used are either aromatic diisocyanates such as MOl (diphenylmethane diiso-cyanate), TOI, or aliphatic diisocyanates such as HOI, IPOI, and H1zMOI [bis(4-isocyanatocyclohexyl) methane, which is sometime also called HMDI (hydro-genated MOl)]. Some of these diisocyanates such as HOI, IPOI, and TO! have relatively higher vapor pressure; therefore, in order to ensure the safety in coat-ing applications, these diisocyanates typically are either oligomerized into polyisocyanates, such as biuret, dimer, or trimer, or reacted into higher mole-cular weight prepolymers before they are actually used in polyurethane coat-ings. The oligomer form of HOI and IPOI polyisocyanates or TDI prepoly-mers have very low monomer levels, so the possibility of vaporized monomer-ic diisocyanatein polyurethane coatings is greatly reduced. Polyisocyanates contain isocyanate functional groups that have high reactiv-ity toward chemicals with active hydrogens such as alcohol, amine, and water; therefore, polyisocyanate coatings should be stored under an atmosphere free of moisture. Similar to many other chemicals, isocyanates are irritants to eye, skin, and respiratory system. For some people, exposure to isocyanate could also cause skin and/or respiratory sensitization, resulting in asthmatic symptoms. As a generally practiced safety operation tule, personal protective equip-ment, such as butyl rubber gloves, goggles, coveralls, and respirators, should always be used during a painting operation involving polyurethane coatings. When safe painting procedure and waste disposal procedure are followed, and proper painting equipment used, the application of polyurethane coatings should not cause more industrial hygiene concern than other kinds of sol-vent-based coatings. Fully cured polyurethane coatings do not have any special waste management issues that are not encountered by other coating ingredients such as solvents, pigments, and others. The uncured polyurethane coatings contain isocyanate groups. They should be allowed to react with water or moisture to form inert polyurea before they ate disposed. Over the years, polyurethane coating man-ufacturers and polyisocyanate suppliers have developed an in-depth under-standing on the safe handling of isocyanates and related industrial hygiene knowledge. Many of them also have a wide range of education programs that can help coating users handle and apply polyurethane coatings safely. 118 FUTURE PERSPECTIVES In addition to the on going demand from end users for products that continue to perform better, the enactment of the Clean Air Act Amendments in 1990 and oth-er related environmental legislation highlighted the need for more environmentally friendly systems. In order to comply with these regulations, VOC reduction has be-come the most important goal for all coating manufacturers. Urethane-based coatings, due to the flexibility and versatility of urethane chemistry and the de-velopment of new generations of isocyanate adducts and derivatives, allow coating formulators greater freedom in designing new high-solid systems for two-com-ponent or one-component coatings and VOC-free waterborne or powder coat-ings. Urethane-based coatings also allow the coating formulators to tailor-make coat-ing formulations for specific applications. Today, many of these emerging ure-thane coatings are either already in use or are under development for various ap-plications. For the automotive industry, the superior weathering and acid rain etching re-sistance of polyurethane-based topcoats and clearcoats has been widely recognized; therefore, despite their premium prices, polyurethane coatings continue to pen-etrate large segments of the European automotive topcoat market. In the Unit-new cars currently use polyurethane topcoat, primarily ed States, about a quarter of for high-end models. With the changing attitudes of today' s car-buying cus-tomers, who increasingly demand better performance, such as excellent surface appearance and acid rain etching protection for their purchases, more American models will certainly use urethane topcoats in the near future. Recently, one-component heat-curable OEM coatings were developed for clear coat applications using blocked polyisocyanates. These coating formulations not only perform as well as two-component systems, but they also do not re-quire on-site mixing, have long pot life, and have the benefit of reducing the paint-line cleaning and preparation time. Furthermore, these one-component coat-curing agents containing free isocyanate groups ings also eliminate handling of in the paint lines.With these benefits, the one-component automotive OEM coatings systems based on blocked polyisocyanates are expected to grow rapid-ly at the expense of conventional acrylic melamine systems. Recently, newer blocked polyisocyanates are becoming available that not only offer lower un-blocking temperatures and faster heat cure time, but also offer additional per-formance benefits such as better yellowing resistance at overbake conditions, and no VOC increase. In addition to the emerging applications of urethane coatings for the auto-motive clear topcoats, anticorrosive electrode position primers for automobiles have relied on polyurethane systems for quite some time. With the need to reduce VOC, the development of waterborne polyurethane systems for this application has already made good inroads. In the past several years, heat-cured waterborne chip-resistant coatings have also been reported. Some of the newly developed wa-terborne polyurethane coatings are two-component polyurethanes that improve performance by allowing further crosslin king to occur during the droplet coa-lescence stage. The development of these systems in many respects is made pos-sible by the development oflow-viscosity polyisocyanates. For auromotive refinish applications, two-component polyurethane coatings have prevailed for many years as the best coatings in terms of performance and productivity. The trend toward using more plastic automotive OEM body pan-els, which are easy to coat with polyurethanes, will only further enhance the 119 domination of polyurethane-based coatings. Similar to other applications, au-tomotive refinish coating formulators also face the challenges of reducing haz-ardous emissions. To comply with increasingly stringent environmental regu-lations, automotive refinish formulators are moving toward higher-solid sys-tems. Because the surface appearance will always be the most important re-quirement for automotive refinish applications, new high-solid polyurethanes must reduce VOC without significantly increasing the viscosity of the resulting coatings. The development oflow-viscosity polyisocyanates will allow polyurethane coatings to continue to dominate the auromotive refinish market in the future. With the improvements in VOC recovery systems, the coil coating industry is able to compete with powder coatings for many metal finishing applica-tions. The benefits of precoated metal have been recognized by many large users, including manufacturers of appliances, automotive under-the-hood parts, and prefinished building panels.Currently, polyurethanes have only a mi-nor presence in these applications; however, in order for the coil coating industry to be competitive in these important applications, it must provide coatings with better flexibility in combination with better chemical and solvent resis-tance. To address these demanding challenges, formulators are developing new technologies that combine urethane with other traditional epoxy or poly-ester formulations. These emerging technologies provide coatings with desir-able performance and good performance/cost balance. Because of the high-performance requirements in durability, flexibility, and corrosion protection, aerospace and military camouflage coatings have tradi-tionally used two-component polyurethane systems. As in other coating appli-cations, the need to reduce VOC emissions is leading to new developments. In ad-dition to the one-or two-component waterborne polyurethane systems, high-sol-these id solvent -based coatings were also developed for many applications. Some of high-solid systems are also being developed as a two-component self-priming top-coat. With the ability to eliminate use of a separate primer, the one-coat polyurethane systems can further reduce their VOC levels. In architectural and industrial maintenance applications, the labor cost has increased rapidly in recent years. For maintaining outdoor metal structures that are exposed to harsh environments, the cost of the paint represents a much smaller portion of the total cost than the time-consuming, labor-intensive re-painting process. Polyurethane coatings offer the best weathering and chem-ical resistance under these environments. The ability to extend repaint cycle times gives urethane protective coatings significant competitive benefits in these applications. Furthermore, longer repainting cycles have the additional benefit of also reducing emissions to the environment. air quality regulations, today's high-solid maintenance To meet the challenge of polyurethane coatings have already achieved 2.8 to 3.5 pounds per gallon VOC levels. In the future, combining the developments in low-viscosity polyisocyanates, lower molecular weight resins, reactive diluents, and waterborne urethane coat-ings, these emissions will be reduced further without compromising the coating performance. SUMMARY Polyurethane coatings offer many advantages for metal substrate applications. Polyurethanes have excellent appearance (\"wet look\"), mechanical performance, UV resistance, and chemical resistance compared with other coatings. 120 Polyurethanes can be applied as higher solids or powder coatings without loss of performance. As a result, polyurethanes are the market leader for coating metallic and plastic substrates in numerous high-end and highly visible appli-cations today. In the furure,the performance of urethane coatings will allow them to be used in a broader spectrum of applications. 121
