Progress in Organic Coatings 44 (2002) 161–183 Two package waterborne urethane systems Zeno W. Wicks, Jr. a , Douglas A. Wicks b,∗ , James W. Rosthauser b b a 190 Springview Court, Louisville, KY 40243, USA Bayer Corporation Coatings Research, 100 Bayer Road, Pittsburgh, PA 15205, USA Received 3 October 2001; received in revised form 4 October 2001; accepted 21 December 2001 Abstract This article is an extensive review of the literature on two component urethane systems that use water as a carrier. It generally covers the period from 1985 through 2000 with special emphasis on patent references since 1993. It includes both ambient temperature cured and baked systems containing unmodified and modified isocyanate building blocks. The main criterion for inclusion into this paper was that the finished urethane group containing polymer reaches its ultimate molecular weight after it is applied to a substrate, so that traditional latex-type materials are not emphasized. The paper describes the raw materials and chemistry used in this field and presents some of the factors that must be considered to make this unique chemistry industrially feasible. The major concern is formulating, mixing, and applying the systems in a manner that limits the water reacting with the highly active isocyanates to avoid defects from generated carbon dioxide. The review also discusses some of the end use areas where two component urethanes have been commercially applied. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Two component waterborne urethanes; Polyurethane dispersions; Water-reducible acrylic resins; Polyisocyanates; Polyamines; Polycarbodiimides; Chemistry; Cross-linking; Applications 1. Introduction The increasing need to reduce volatile organic compounds (VOCs) and hazardous air pollutants (HAPs) emissions has led to increased efforts to formulate waterborne systems for use as coatings, adhesives and related end uses. Urethane systems have followed this trend. Several early reviews of work on aqueous urethane systems have been published; they primarily discuss one package systems and are now primarily of historical interest [1–3]. Two later reviews are available that are primarily of interest as reviews of one package systems [4,5]. Another review covers the whole field of waterborne polyurethanes with references into 1985 [6,7]. The main thrust for research in the field since that time has been to improve two common deficiencies of the one-component, mainly thermoplastic urethane systems: residual hydrophilicity leading to insufficient hydrolytic stability or water resistance and a low degree of crosslinking needed for high hardness and good solvent resistance. In recent work, thermosetting polyurethanes have become increasingly important, particularly two-component (2K) systems. Work on 2K systems began to expand rapidly after 1993. This review includes cross-linking of ∗ Corresponding author. Tel.: +1-412-777-7851; fax: +1-412-777-2940. E-mail address: [email protected] (D.A. Wicks). aqueous urethanes with a variety of cross-linkers and use of polyisocyanates to cross-link a variety of waterborne coreactants. The majority of the work has been done using polyisocyanates as the cross-linkers. Since isocyanates react relatively readily with water, it was assumed for many years that they could not be used directly in waterborne systems. The earliest work starting in the 1970s was using isocyanates as cross-linkers in waterborne adhesives and textile finishes. Starting in the early 1980s, they have been increasingly used in 2K waterborne coatings. In the last few years, 2K waterborne systems have been adopted commercially on a large scale [8–10]. Bayer Corporation received a Presidential Green Award in 2000 for their work on these systems. Sections 2–4 provide general background applicable to any of the end uses. Section 2 discusses the various classes of resins that are cross-linked in these 2K systems. Section 3 discusses the various types of cross-linkers used. Section 4 deals with the mixing and application considerations. Section 5 discusses the various end uses. In some cases, a patent will claim that the products can be used in more than one of the end uses but the examples will all be for one particular end use category. We have only referenced such patents in the end use category covered by the examples. 0300-9440/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 0 - 9 4 4 0 ( 0 2 ) 0 0 0 0 2 - 4 162 Z.W. Wicks Jr. et al. / Progress in Organic Coatings 44 (2002) 161–183 2. Types of coreactants used Terminology in the field is confusing. We are using the following terminology: • “Water-soluble” will be used only for coreactants that are truly soluble in water such as some polyether polyols. • “Latex” is used to mean polymer particles dispersed in water. Some authors refer to latexes as emulsions or dispersions. It is common practice to call dispersions of polyurethanes in water, polyurethane dispersions (PUDs). Dispersions of high molecular weight polyurethanes could properly be called latexes but this is seldom done in the literature. • “Water-reducible” is used to mean resins that when dispersed in water form aggregates of resin in water that are swollen with water and sometimes with solvent and water. The dispersions typically have a small average particle size in the range 10–200 nm and usually exhibit a Tyndall effect. Some authors call resins that we are designating as water-reducible: water-soluble, emulsions, dispersions or ionomers. Most water-reducible resins are acrylic, polyester or polyurethane resins. In the case of urethanes, we have elected to call them PUDs because this terminology is almost universally used in the literature. There are two broad categories of water-reducible resins: those in which the aqueous dispersion is stabilized by anionic groups on the resin from the neutralization of an acid group on the resin with a low molecular weight amine and those which are stabilized with cationic groups from neutralization of an amine group on the resin with a low molecular weight carboxylic acid. The resins are all so-called thermosetting resins, i.e. they have functional groups, hydroxyl, amine and/or carboxylic acid groups, which can react with a cross-linker. 2.1. Water-reducible acrylic and polyester resins Water-reducible acrylic resins are prepared from a combination of acrylic and methacrylic esters, sometimes styrene, a hydroxy-functional (meth)acrylate and (meth)acrylic acid. The non-functional monomers are selected to control the Tg of the resin and coating. A typical resin includes sufficient acid monomer and so the resin has an acid number of 35–60. The level of hydroxy-functional monomer controls the cross-link density of the coating. Molecular weights are commonly of the order of M̄w = 35 000 and M̄n = 15 000. Polymerization is carried out in a water-miscible solvent such as a glycol monoether. The resin is neutralized with an amine and diluted with water. The resin is not soluble in water and on dilution, a dispersion is obtained which is stabilized by the salt groups oriented on the surface. The morphology of water-reducible acrylic resin dispersions has been most widely studied. See Ref. [11] and the references cited therein for further discussion of the composition and morphology of such water dispersions and factors affecting the morphology. The behavior of PUDs and other water-reducible resin systems are similar. Flickinger et al. [12] discusses the rheological properties of thermoplastic PUDs; we have not found a corresponding rheological study of the thermosetting PUDs used in 2K coatings. The structure and amount of amine used in neutralizing anionic water-dispersible resins has an important effect on many properties of the systems. The effects of amine have been more extensively studied with water-reducible acrylic resins cross-linked with melamine-formaldehyde (MF) resins than with other binders. While some of the effects are specific to MF cross-linked systems, many are true with any thermosetting aqueous dispersion system. The following discussion summarizes the effects (see Ref. [11] and the references cited therein for further discussion). Mechanical stability of the aqueous dispersion can be affected by the amine used and the percent of the theoretically required amine to neutralize all the carboxylic acid groups (extent of neutralization, EN). While there may be some effect of base strength of the amine, the larger factor appears to be water solubility of the amine. Lower ENs are required for aminoalcohols, such as dimethylaminoethanol (DMAE), and for morpholine derivatives than for tertiary-alkylamines. The longer the alkyl groups in a tertiary-amine, the higher the EN required; for example, more tripropylamine is required than triethylamine. With any given amine/resin combination, there will be some minimum EN required to prevent macrophase separation. The viscosity of the diluted system is very dependent on the EN, lowest viscosities are obtained when EN is only slightly above that required for stability. As further amine is added viscosity increases since the aggregate particles become more swollen with water, increasing in the internal phase concentration of the dispersion which in turn increases the viscosity. Rate of amine loss in turn is affected by volatility of the amine and its rate of diffusion out of the film. As volatiles vaporize, evaporation becomes controlled by rate of diffusion through a film rather than vapor pressure, i.e. amine loss is controlled by the rate of diffusion to the surface of the film. The rate of diffusion of amine through a film is dependent on the base strength of the amine. As an amine molecule diffuses through the film, it can form a salt with a COOH group, which is in equilibrium with the free amine and acid. With a lower base strength amine equilibrium is shifted more to the free amine and acid, and therefore will diffuse more rapidly through the film to the surface where it can evaporate and is lost from the film more rapidly. N-Ethylmorpholine has a much lower pKa (7.4) than TEA (10.9) or NH4 OH (9.40) hence is lost more quickly from a film, despite its lower vapor pressure. We have not found a systematic study of the effects of amine structure and concentration on 2K PUDs. TEA seems to be the most widely used; ammonia, DMAE and N-methylmorpholine have been mentioned occasionally. In Z.W. Wicks Jr. et al. / Progress in Organic Coatings 44 (2002) 161–183 most cases, EN is not mentioned; presumably equivalent amounts of amine have been used. Solvent-free coatings have been prepared using waterreducible acrylic resins from which the solvent, butyl acetate, has been vacuum distilled off an aqueous dispersion of a salt of the resin; a low viscosity mixture of HDI uretdione and isocyanurate was used as the cross-linking agent [13]. Similarly acetone has been used as the solvent in the polymerization and removed from the aqueous dispersion by distillation [14]. Graft copolymer dispersions and semi-block copolymer dispersions prepared by catalytic chain transfer polymerization have the advantage of narrower molecular weight distributions as compared with water-reducible acrylic resins made by free radical polymerization [15]. The semi-block dispersions permit use of unmodified polyisocyanates such as IPDI isocyanurate. A water-dispersible acrylic resin made from propoxylated allyl alcohol, acrylic esters, methacrylic acid and styrene, neutralized with TEA, is cross-linked with a blend of HDI uretdione and IPDI isocyanurate [16]. A water-dispersible acrylic resin made using a polyethylene glycol methacrylate and hydroxyethyl acrylate as comonomers, with another hydroxy-functional acrylic resin and a polyisocyanate are used in formulating a three-component water-thinned coating [17]. Graft copolymers of non-oxidizing alkyd resins with water-reducible acrylic resins have been used in coatings with a mixture of HDI uretdione and isocyanurate [18]. Graft copolymers of alkoxylated polyols, such as propoxylated glycerol and a water-reducible acrylic, resin are used in coatings with an aqueous dispersion of HDI isocyanurate [19]. Inclusion of 10% styrene and 4-hydroxybutyl acrylate as comonomers in preparing water-reducible acrylic resins for use with TMXDI/trimethylolpropane (TMP) prepolymer gives faster drying and better hardness than similar acrylic resins without the styrene and using HEA as the hydroxy monomer [20]. To compensate for the rigidity of the TMXDI for ambient temperature cure systems, the Tg of the acrylic resin should be >0 ◦ C [21]. The hydroxyl content, acid number and EN by amine affect the particle size distribution of aqueous dispersions. Best performance was achieved with a mean particle size of 0.13–0.15 ␮m. Cationic water-reducible acrylic resins with secondary amine groups made using t-butylaminoethyl methacrylate as a comonomer, neutralized with acetic acid and dispersed in water have been patented for use with an emulsion of a solution of HDI biuret as a cross-linker in 2K coatings [22]. A hydrolytically stable polyester prepared from 2-butyl-2ethyl-1,3-propanediol, TMP, and 1,4-cyclohexanedicarboxylic acid further reacted with 1,4-cyclohexanedimethanol and trimellitic anhydride neutralized with DMAE is used with an isocyanate cross-linker in a waterborne coating [23]. Water-reducible polyester resins are made including as one-component in the esterification 5-lithiumsulfoisophthalic acid, the aqueous dispersion of the resin is used in 2K 163 coatings with a mixture of HDI uretdione and isocyanurate as cross-linking agent [24]. Solvent-free water-reducible polyester dispersions have been prepared by dissolving the polyester in acetone, neutralizing with DMAE, dispersing in water and then distilling off the acetone [14]. Waterreducible polyesters with excess amine, such as 2-methyl1,5-diaminopentane, are used with HDI isocyanurate as a cross-linker [25]. Aqueous dispersions of polyesters with surfactants have been used with hydrophilically modified polyisocyanates to formulate coatings [26]. 2.2. Polyurethane dispersions PUDs are prepared with a water-dispersing–stabilizing group built into the polymer. While many PUDs are thermoplastic film formers, virtually all of those used in 2K coatings have reactive functional groups, most commonly hydroxyl groups. Three broad classes of stabilizing groups are anionic groups such as carboxylic acid or sulfonate salts, cationic groups such as amine or ammonium salts, and nonionic stabilizing groups, such a long chain polyethers. For anionic PUDs, several monomers with carboxylate or sulfonate groups have been investigated. Those most commonly used are 2,2-dimethylolpropionic acid (2,2-DMPA) neutralized with a tertiary-amine before dilution with water- or sulfonate-substituted polyamines such as H2 NCH2 CH2 NHCH2 CH2 SO3 − Na+ . The hindered carboxylic acid group on 2,2-DMPA is almost completely unreactive with isocyanates at the temperatures used in preparing PUDs so that the reaction is limited to the hydroxyl groups. Some work has been reported with cationic groups such as amine salts. The majority of PUDs have been made with aliphatic isocyanates. Not only are color retention and exterior durability superior to aromatic derivatives, but also aromatic isocyanates are generally more reactive with water. HDI and IPDI have been most widely used. HDI tends to give relatively low viscosity PUDs that are more easily dispersed in water, and give higher gloss, more flexible films with good scratch resistance. IPDI generally provides fast drying and harder coatings [8]. In some cases, combinations of HDI and IPDI have been used. Due to its low reactivity with water, H12 MDI is also extensively used; it provides properties intermediate between HDI and IPDI. For nonionic stabilization, diols or polyisocyanates with side chains from polyalkylene glycol monoethers are used. The ionic stabilizers function primarily by charge repulsion whereas the nonionic stabilizers function primarily by entropic repulsion. The ionic type are less stable to addition of electrolytes, especially multivalent electrolytes than the nonionic type. They also generally show lower freeze-thaw stability, and have lower mechanical stability. On the other hand, they are more stable at temperatures above 70 ◦ C. The most common way to prepare thermosetting PUDs is by what is called the prepolymer mixing process (making a prepolymer and then adding water) or inverse process 164 Z.W. Wicks Jr. et al. / Progress in Organic Coatings 44 (2002) 161–183 (adding prepolymer to water). Most PUDs used for 2K coatings are hydroxy-functional made by adding an aminoalcohol to terminate the formation of the polyurethane. For example, a prepolymer prepared by reacting H12 MDI with 2,2-DMPA and a polyester diol in N-methylpyrrolidone (NMP) neutralized with TEA and dispersed in water then terminated with ethanolamine, diethanolamine, trimethanolaminomethane, or aminomethylethanolamine were cross-linked with a hydrophilically modified HDI isocyanurate [27]. Use of a hydroxy-functional PUD made with a polyester, H12 MDI, and 2,2-DMPA, neutralized with TEA, and terminated with diethanolamine with a nonionic hydrophilically modified HDI isocyanurate has been patented [28]. Another way to prepare PUDs is by the acetone process. The process has the advantage that the polymerization is completed in acetone before addition of water so that the competitive reaction with water is not a factor as in some other methods of preparation. On the other hand, processing cost is relatively high as a result of the acetone removal step with its long processing time as well as a result of foaming during the early stage of distillation. Also since the reaction is run at relatively low solids, batch size of final product is limited, requiring greater processing time per unit of solids. Hydroxy-functional PUDs can also be made by making the PUD with excess diol; higher functionality can be achieved by including a trifunctional polyol such as TMP in the formulation. Uniformity of the extent of branching and molecular weight can be difficult to control with this approach and viscosity is higher [29]. A PUD prepared with a non-oxidizing alkyd resin, 2,2-DMPA, and hydrogenated TDI (H6 TDI), neutralized with DMAE, and dispersed in water is cross-linked with a mixture of an HDI isocyanurate and a nonionic hydrophilically modified HDI isocyanurate [30]. A hydroxy-functional non-oxidizing alkyd or polyester resin is dissolved in acetone, 2,2-DMPA and IPDI are added resulting in a polyurethane which is neutralized with ammonia and diluted with water, the acetone is distilled off the resulting PUD [31]. In another case, a polyester, TDI, HDI and a sodium salt of the Michael adduct of acrylic acid and ethylenediamine are reacted in acetone, diluted with water, and the acetone distilled off to give a PUD used with a water-dispersible polyisocyanate and triethanolamine in adhesives [32]. Cationically stabilized hydroxy-functional PUDs have also been prepared by the acetone process. A polyester diol, H12 MDI, and N-methyldiethanolamine are dissolved in acetone and reacted followed by termination with diethanolamine and neutralized with lactic acid, dispersed in water and the acetone removed by distillation [33]. An anionic PUD prepared from a polyester diol, 2,2-DMPA and H12 MDI neutralized with TEA dispersed in water and terminated with diethanolamine, formulated with tripropylene glycol and a nonionic hydrophilically modified HDI isocyanurate gives higher gloss than the comparable coating without the added glycol [34]. PUDs with polyester backbones and 2,2-DMPA have been widely used. However, they are subject to hydrolysis, or saponification at basic pH, which can lead to breaking the polymer backbone and separation into two phases, especially at elevated temperatures [35]. The hydrolysis is autocatalytic since carboxylic acid of 2,2-DMPA catalyzes the hydrolysis. The stability is lowest for PUDs not chain extended; as the amount of chain extension (with ethylenediamine) is increased, stability improves. Polycarbonate diols are reported to be more resistant to hydrolysis than polyester diols [36]. A PUD prepared using a polycarbonate diol, IPDI, 2,2-DMPA, chain extended with isophoronediamine (IPDA) and terminated with ethanolamine has been used with a combination of IPDI isocyanurate and HDI isocyanurate–uretdione [37] or a HDI prepolymer with 2-n-butyl-2-ethyl-1,3-propanediol that was converted to a mixed isocyanurate–uretdione [38]. Since polycarbonate diols are more expensive than polyesters, compromise systems based on mixed diols have been evaluated. A PUD from a polyester, a polycarbonate, 2,2-DMPA, and HDI, neutralized with DMAE is cross-linked with a nonionic hydrophilically modified HDI isocyanurate [39]. A PUD prepared from a water-reducible polyester, a polycarbonate diol, and IPDI, neutralized with DMAE is cross-linked with nonionic hydrophilically modified polyisocyanates based on HDI [40]. Hybrid PUDs containing hydroxy-functional acrylicpolyurethanes have been used [41]. A PUD made by reacting a polyester polyol with H12 MDI and 2,2-DMPA, neutralizing with TEA, reducing with water, and then polymerizing a combination of (meth)acrylates and hydroxyethyl methacrylate in the dispersion and terminated with diethanolamine is used with a nonionic hydrophilically modified HDI isocyanurate [42]. Also a combination of a water-reducible acrylic resin and a PUD prepared from a saturated fatty acid alkyd, 2,2-DMPA, and IPDI, both resins were neutralized with ammonium hydroxide and dispersed in water and an HDI uretdione/isocyanurate was used as a cross-linker [43]. Another route to hydrolysis-resistant PUDs is demonstrated by reacting a BPA epoxy resin with p-aminobenzoic acid in the monomethyl ether of propylene glycol, neutralizing with TEA, diluting with water, then reacting with H12 MDI, the PUD is cross-linked with a polyisocyanate [44]. Hydroxy-functional PUDs can be made by a non-isocyanate process [45,46]. The preparation involves reacting an aliphatic polyester polyol with bis-␤-hydroxypropyl carbamate. Carboxylic acid functionality was incorporated by reaction with an anhydride, the resulting product is neutralized with a tertiary-amine and dispersed in water [47]. PUDs made by this process and a conventional PUD made by the prepolymer mixing process in NMP were evaluated using HDI isocyanurate, IPDI isocyanurate [48], and a hydrophilically modified HDI isocyanurate [46,48]. The conventional PUD with the hydrophilically modified polyisocyanate gave the hardest films, but VOC is substantially higher than the others. With the non-isocyanate PUDs, use of a blend of Z.W. Wicks Jr. et al. / Progress in Organic Coatings 44 (2002) 161–183 HDI isocyanurate and hydrophilically modified polyisocyanate gave the best overall results. Non-isocyanate-derived PUDs with high hydroxyl, carboxyl and urethane contents are reported to be excellent pigment dispersion vehicles [46]. A combination of a hydroxy- and trimethoxysilyl-functional water-reducible acrylic resin and a hydroxy- and triethoxysilyl-functional PUD are used with a nonionic hydrophilically modified polyisocyanate in formulating adhesives and coatings [49]. Amine-terminated PUDs can be made by reacting an MEKO-blocked isocyanate-terminated polyurethane with aminoethylethanolamine and dispersing in water [50]. Surprisingly, it has been found that an anionic PUD made with 2,2-DMPA but without hydroxyl groups gives cross-linked films with a hydrophilically modified HDI isocyanurate [27]. The authors propose that an interpenetrating network is responsible for the cross-linking effect, perhaps enhanced by intermolecular hydrogen bonding between urethane groups. Increased amounts of isocyanate cross-linker decreased elongation and increased modulus. 2.3. Epoxy resin-based coreactants A cationic hydroxy-functional resin is prepared by reacting a BPA epoxy resin with diethanolamine, neutralizing with formic acid and dispersing in water is used with a polyisocyanate in 2K waterborne coatings [51]. 2.4. Polyamines In most systems, aliphatic amines are too much reactive to use in even 2K coatings; however, hindered amines have been developed that can be used. A hydrophilically modified polyisocyanate has been used with a dispersion of a hindered amine synthesized from IPDA and trimethylolpropane triacrylate [52] and another made from diethyl maleate and bis(4-amino-3-methylcyclohexyl(methane)) [53] dispersed in an aqueous solution of acetic acid. Cationic water-reducible acrylic resins with secondary amine groups made using t-butylaminoethyl methacrylate as a comonomer, neutralized with acetic acid and dispersed in water have been patented for use with an emulsion of a solution of HDI biuret as a cross-linker in 2K coatings [22]. A nonionic hydrophilically modified prepolymer prepared from a mixed HDI uretdione/isocyanurate, a monomethyl ether of polyethylene glycol, 2,2-dimethyl-1,3-propanediol and 2-ethylhexyl alcohol is cross-linked with ethylene diamine [54,55]. Another approach to waterborne 2K coatings is to use amines as part of the coreactants in the coating; since amines react more rapidly with isocyanates than with either hydroxyl groups or water, to the extent that this reaction can be used water reaction should be reduced. The fraction of cross-linking by amine permissible is limited since excessive fast reaction would interfere with coalescence. A patent discloses use of a mixture of polyester 165 polyols, polyurethane polyols both with carboxylic acid groups with tertiary-amines to stabilize the water dispersion; then adding mixtures of amines of different reactivity [56]. 2.5. Latexes Latexes are aqueous dispersions of high molecular weight polymers. Most latexes of interest in 2K systems have functional groups that can react with isocyanate groups, most commonly hydroxyl or carboxylic acid groups. In general, the latexes are film formers even without cross-linking and only a relatively low cross-link density need be reached to have good film properties. The cross-linker has to be a separate dispersion so that achieving uniform films can be difficult. Many latex systems include a coalescing solvent in the formulation to reduce the Tg of the polymer permitting film formation at lower temperatures. In conventional latex systems after film formation, the coalescing solvent evaporates increasing the Tg so that a harder film is obtained. Many of these coalescing solvents are hydroxy-substituted and can therefore react with an isocyanate. For example, it has been shown that mono-n-butyl ether of propylene glycol used as coalescing solvents react with isocyanate groups [57]. A statistical study of the effect of several variables on performance of hydroxy-functional acrylic latexes cross-linked with a nonionic hydrophilically modified polyisocyanate finishes for wood kitchen cabinets has been published [58]. It was found that high hydroxy content of the latex (hydroxyl number 52), small particle size of the latex, core-shell preparation of the latex and low Tg (obtained by increasing level of coalescing solvent) enhanced performance. An extensive study of the effect of carboxylic acid structure and level in latexes made by emulsion polymerization of methyl methacrylate, butyl acrylate, hydroxyethyl methacrylate with acrylic acid, methacrylic acid or ␤-carboxyethyl acrylate neutralized with DMAE in 2K urethane coatings [59]. It was shown that the COOH groups react with isocyanate groups. There was a dramatic difference in CO2 bubble formation between acrylic acid and ␤-carboxyethyl acrylate containing latexes with the former being much more likely to give serious bubble formation. The effect was attributed to the large amount of polyacrylic acid in the water phase of the latex due to the solubility of acrylic acid in water compared with little polyacid in the water phase with the less water-soluble ␤-carboxyethyl acrylate. Hydroxy-functional fluorinated vinyl ether latexes are cross-linked with hydrophilically modified polyisocyanates to give coatings with excellent exterior durability [60]. Vinyl acetate/butyl acrylate latex cross-linked with an emulsion of HDI isocyanurate partially reacted with isocetyl alcohol has been patented as plywood adhesives [61]. HDI isocyanurate is used with a copolymer of vinyl acetate and n-butyl maleate to formulate an adhesive for laminating wood [62]. A nonionic hydrophilically modified MDI made by partially reacting it with a monoether of poly- 166 Z.W. Wicks Jr. et al. / Progress in Organic Coatings 44 (2002) 161–183 ethylene glycol is used as a cross-linker for various latexes, such as vinyl acetate/ethylene latex, for laminating PVC plastics [63]. Poly(vinyl alcohol)-stabilized vinyl chloride/ethylene/hydroxyethyl acrylate latexes are cross-linked with a nonionic hydrophilically modified polymeric MDI for use as adhesives in laminating wood [64]. Adhesives for rubber are formulated with a chlorosulfonated polyethylene latex and a nonionic hydrophilically modified HDI isocyanurate [65]. Acetoacetoxyethyl methacrylate latex copolymers can be cross-linked with nonionic hydrophilically modified isocyanates [66]. An extensive study showed that stain and chemical resistance of coatings was superior to comparable formulations made with hydroxy-functional latexes. Acrylic latexes that are polymerized in the presence of water-reducible polyester resins and with a small amount of hexanediol diacrylate to give a low cross-link density are used with TMP and a nonionic hydrophilically modified HDI trimer to formulate coatings [67]. The water, solvent and abrasion resistance of latex-based paints are improved by emulsifying into the paint a combination of HDI isocyanurate and dimer [68]. The pot lives of the mixtures are 2 h. The latexes used are conventional acrylic and styrene/butadiene latexes without functional group substitution. No explanation of reasons for the improvements is given; presumably they result from reactions with nonionic surfactants and water-soluble polymeric thickeners such as hydroxyethylcellulose or formation of polyurea gel particles. A polyester polyol-stabilized latex of methyl methacrylate and hexanediol diacrylate in a 2K coating with a hydrophilically modified HDI polyisocyanate gives films with good properties even though there are no functional groups on the acrylic polymer [69]. 3. Types of cross-linkers used 3.1. Isocyanates 3.1.1. Unmodified isocyanates Use of many unmodified isocyanates in 2K water systems is limited because they can be difficult to mix into a system thus increasing the probability of phase separation before reaching the substrate. Most isocyanates can only be used in proportioning spray guns that permit mixing immediately before spraying because their pot lives are too short. In attempting to use unmodified polyisocyanates, it is desirable to use as low viscosity isocyanates as possible. The viscosity of HDI isocyanurate with a functionality of 3.3 is 1.7 Pa s at 28 ◦ C compared to 8.5 Pa s for HDI biuret with the same functionality making it easier to disperse [70]. The viscosity of a combination of HDI uretdione and isocyanurate has a lower viscosity than HDI isocyanurate that makes it easier to incorporate into a dispersion [62]. A mixed uretdione and isocyanurate obtained by reacting a HDI 2,2,4-trimethylpentane-1,3-diol adduct with butyl phosphine catalyst has a viscosity of 0.15 Pa s. The color of the product is reduced by treating with MEK peroxide [71]. Use of a proprietary water-reducible acrylic resin with conventional HDI-based isocyanates is reported to be satisfactory [72]. 4-Isocyanatomethyl-1,8-diisocyanatononane diisocyanate is said to stir into water and aqueous systems more easily than other polyisocyanates [73]. Monomeric IPDI has been used as the cross-linking agent for solvent-free aqueous dispersions of acrylic and polyester resins [14]. The polyisocyanate component is sometimes diluted with solvent such as butyl acetate to reduce its viscosity. It has been reported that using solvents such as cyclic carbonates or lactones give smaller particle size more uniform dispersions in water than solvents like butyl acetate [74]. Alkoxyethanol acetates are patented for use as non-environmentally damaging solvents for isocyanates in 2K coatings [75]. HDI biuret is mixed into a diglyme solution of DMAE neutralized water-reducible acrylic resin before dispersion in water, pot life is not stated [76]. TMXDI/TMP adduct can be used with 1/1 NCO/OH ratios with low Tg water-reducible acrylic resins and DBTDL catalyst and then mixed with water to give coatings that do not blister from CO2 release even in thick films cured at high humidity and with performance equal to 2K solventborne coatings [77–81]. The system has the advantage that the mixing procedure assures that the aggregates will be uniform in composition. Ambient temperature curing is satisfactory with 1:1 indexing and pot life is >7 h due to the slow reaction of the tertiary-isocyanate with water. The Tg of the acrylic resin should be 0 ◦ C or lower, primary hydroxyl substitution and an hydroxyl content of 3.0–4.5%. The low Tg is required for ambient temperature cure since the rigid aromatic rings of TMXDI lead to more rapid increase in Tg as curing progresses which can lead to mobility restricted curing with higher Tg resins. Dimethyltin dicarboxylates are more effective catalysts than DBTDL at ambient temperature and force dry temperature (60 ◦ C) [21]. TMXDI is stirred into an aqueous dispersion of a graft copolymer made from a polymer of maleated polybutadiene reacted with diethylamine and styrene grafted with a hydroxybutyl and butyl acrylates in a 2K coating with a pot life of approximately 4 h [82]. Use of semi-block copolymer dispersions with dispersions of acrylic resins prepared by catalytic chain transfer polymerization is reported to permit use of unmodified polyisocyanates such as IPDI isocyanurate, resulting in 2K top coats with improved film appearance and humidity resistance [15]. 3.1.2. Hydrophilically modified polyisocyanates Modifying isocyanates so that they are more readily dispersible in water permits easier mixing of the two packages. Most commonly nonionic modification is used but there are also several examples of ionic modifications. 3.1.2.1. Nonionic modifications. Nonionic hydrophilically modified polyisocyanates made by reacting a fraction of the –NCO groups on a polyisocyanate (such as HDI or Z.W. Wicks Jr. et al. / Progress in Organic Coatings 44 (2002) 161–183 IPDI isocyanurate) with a polyglycol monoether are more easily mixed. The modified polyisocyanate is stirred into an aqueous dispersion of a coreactant to form a heterogeneous dispersion, in which the polyisocyanate and coreactant are in separate dispersed particles [41,83]: The ease of mixing and stability of the dispersions increases as the length of the polyether modifier is increased [84]. It is reported that with HDI isocyanurate, the monomethyl ether of polyethylene glycol used must have a molecular weight greater than 120 (n = 2) and less than 1040 (n = 24) for good aqueous dispersion [85]. If a monoether of polyglycol with n higher than 10 –(CHRCH2 O)n – is used to make the nonionic hydrophilically modified isocyanate, crystallization is likely to occur. Adequate water dispersibility without crystallization can be obtained using ethers with n > 5 butn < 10. For example, 0.08 eq. of the monomethyl ether of polyethylene glycol with n = 6–8 per eq. of HDI isocyanurate gives a modified isocyanate that does not crystallize and gives films that are less water sensitive than when ethers with higher n-values are used [86]. Use of polyester/polyethers as modifiers for the isocyanate overcomes the crystallization problem and decreases the water sensitivity of final films. For example, reacting a monomethyl ether polyethylene glycol (n = 7) with ε-caprolactone gives a polyester/polyether that is used to modify HDI isocyanurate [87]. The viscosity of the dispersions in water increases as the polyether content of the nonionic hydrophilically modified isocyanate increases as shown in Fig. 1. This difference results from the increase in internal phase concentration, which increases for two reasons: the use of longer chain monoether polyglycols increases the amount of polyisocyanate derivative and more water dissolves in the isocyanate phase. The increase in internal phase volume leads to the increase in viscosity, as would be expected. 167 High molecular weight polyol substitution gives polyisocyanate dispersions, which undergo more rapid decrease in pH on standing as shown in Fig. 2. Solubility of water in the modified polyisocyanate increases with the amount of polyether present; reaction with the isocyanate with water increases generation of CO2 which decreases pH of the dispersions over time [88]. Hardness of cured films decreases as the polyether content increases due to the plasticizing effect of the polyether chains [89]. Moisture resistance of films directly on metal decreased as hydrophilic modification increased and acid resistance was poorer. A combination of TDI isocyanurate and HDI isocyanurate partially reacted with a methyl ether of polyethylene glycol [90] and a TDI prepolymer with propoxylated TDI partially reacted with a methyl ether of polyethylene glycol [91] are used as cross-linkers for water-reducible acrylic resins. Hydrophilically modified polyisocyanates are made by preparing a prepolymer of HDI and a polyether polyol with an average NCO functionality of 5.4 and partially reacting with a methyl ether of polyethylene glycol [92]. The adduct of 4-isocyanato-1,8-octane diisocyanate with the methyl ether of polyethylene glycol has also been patented [93]. A nonionic hydrophilically modified polymeric MDI is used as a cross-linker for polyether or polyester polyols [94]. Stability of the dispersion of a hydrophilically modified polyisocyanate can be maintained while decreasing the rate of reaction with water by incorporating a hydrophobic substituent on the isocyanate. Water sensitivity is decreased by using a combination of hydrophilic monoether of a polyglycol and a hydrophobic alcohol to modify a polyisocyanate [95]. A polyisocyanate is made by trimerizing an HDI 1,3-butanediol adduct, then reacting with a monomethyl ether of polyethylene glycol and a monoester of ricinoleic acid [96]. The nonionic hydrophilically modified polyisocyanates were used to cross-link anionic PUDs for coatings or adhesives. Stability of hydrophilically modified polyisocyanates in water can be increased by mixing them with an anionic emulsifying agent and a solvent. For example, an aqueous Fig. 1. Viscosity of nonionic hydrophilically modified isocyanate dispersions as a function of concentration and polyether content. 168 Z.W. Wicks Jr. et al. / Progress in Organic Coatings 44 (2002) 161–183 Fig. 2. Decrease in pH as a function of time and polyether content of dispersions of hydrophilically modified polyisocyanates. dispersion of an HDI isocyanurate modified with a methyl ether of a polyether polyol, mixed with an anionic surfactant at 50% solids in a solvent such as butyl acetate retains 96% of the original NCO content after 6 h while a comparison sample without the surfactant and solvent loses all the NCO content after 2 h [97]. Films made with the solvent containing dispersion and an aqueous dispersion of an acrylic polyol are more resistant to water after curing 24 h at ambient temperature than corresponding films without the solvent. Of course, there is a significant increase in VOC. Presumably the solvent lowers the Tg of the dispersed phase permitting easier coalescence and greater mobility in the film leading to more uniform distribution of cross-linking agent and coreactant. A nonionic hydrophilically modified HDI isocyanurate is allophanized to make a water-dispersible polyisocyanate [98]. Such allophanates have two advantages over the starting isocyanurate: a lower ratio of polyether to NCO is required to achieve good water dispersibility and the product has a higher average functionality. Thus, coating films are harder and less water-sensitive. A different approach to making hydrophilically modified polyisocyanates is the use of polyvinylpyrrolidone as the hydrophilic component. For example, HDI isocyanurate is reacted with polyvinylpyrrolidone to give a polyisocyanate that is used to cross-link a water-reducible acrylic resin [99]. HDI isocyanurate, a methyl ether of polyethylene glycol, and N-(3-trimethoxysilylpropyl)aspartic acid diethyl ester are reacted to make a water-dispersible polyisocyanate [100]. Water sensitivity of films is reduced by partially reacting a hydrophilically modified polyisocyanate with an aminoalkyltrialkoxysilane. For example, HDI [101] or HDI isocyanurate [102] is reacted with a methyl ether of polyethylene glycol (n ≥ 10) and an amine-functional silane made by reacting diethyl maleate with 3-aminopropyltrimethoxysilane. Primers and top coats formulated with the trialkoxysilylated isocyanate and water-reducible acrylic resins show substantial advantages in gloss retention and reduced blistering on water immersion than corresponding films without the silane. 3.1.2.2. Ionic modifications. The N-methylmorpholine salt of an adduct of 2,2-DMPA and HDI uretdione/isocyanurate is used with water-reducible acrylic resins [103]. Use of HDI isocyanurate (with an average functionality of 3.3) reacted with 0.5 meq. of 2-hydroxyethane sulfonic acid has also been patented [104]. Combined ionic and nonionic stabilizers have also been used. Water sensitivity of films can be reduced while avoiding crystallization when only low levels of monoether of polyol are used along with 2,2-DMPA to give an modified isocyanate that gives good dispersions in tertiary-amine water solutions. For example, reaction of 1 eq. of HDI isocyanurate with 0.05 eq. of 2,2-DMPA and 5.5 meq. of the butyl ether of a polyethylene/polypropylene glycol with NMM gives a modified polyisocyanate that shows no crystallization and gives cross-linked coatings with superior properties [105]. The amount of polyglycol ether used in making a hydrophilically modified isocyanate can be reduced by partially reacting the polyisocyanate with a salt-substituted group. For example, a polyisocyanate is reacted with the TEA salt of monobutyl phosphate to make a water-dilutable isocyanate for use with water-reducible acrylic resins [106]. An anionic water-dispersible polyisocyanate made by reacting HDI isocyanurate with a mixture of mono- and diesters of phosphoric acid and a mono(nonylphenol) ether of polyethylene glycol (n = 9) is used with a water-reducible acrylic resin [107]. Cationically stabilized water-dispersible polyisocyanates have also been made. For example, IPDI trimer, ethoxylated 3-ethyl-3-hydroxymethyloxetane and N-hydroxyethylmorpholine are reacted and then alkylated with dimethyl sulfate and neutralized with lactic acid to give a cationically stabilized water-dispersible polyisocyanate [33]. When used with a cationically stabilized PUD, the pot life is over 8 h, much longer than the corresponding anionically stabilized system. Z.W. Wicks Jr. et al. / Progress in Organic Coatings 44 (2002) 161–183 3.1.3. Emulsions of polyisocyanates Emulsions of unmodified polyisocyanates have been used. For example, an emulsifier made by reacting IPDI isocyanurate with a monomethyl ether of polyethylene glycol and 1-isobutylisopentyloxypolyethyleneoxyethyl alcohol has been used to prepare emulsions of HDI biuret that are used with a cationic water-reducible acrylic resin to make 2K coatings with superior film properties [108,109]. Vinyl acetate/butyl acrylate latex can be cross-linked with an emulsion of HDI isocyanurate partially reacted with isocetyl alcohol [61]. 3.1.4. Blocked isocyanates Most blocked isocyanate systems are one package. However, there are systems, especially with amines or aromatic isocyanates, in which reactivity is too fast to permit their formulation. In such cases, 2K-blocked isocyanate systems have been used. Blocked polyisocyanates have been used in 2K systems to cross-link amine-functional coreactants. A stable PUD is made of diethyl malonate-blocked HDI biuret and sodium N-methylaminoethane sulfonic acid is cross-linked with bis(4-aminocyclohexyl)methane [110]. 2K waterborne sealants have been patented using an aqueous dispersion of a relatively high molecular weight polyether diol part of which was terminated with imidazoleblocked TDI [111]. The sealant remains spreadable for 2 h after the dispersion of the blocked isocyanate is made. Waterborne sealants with good alkali, blocking, efflorescence and water resistance for cement and slate products are formulated with a water-reducible acrylic resin and a blocked isocyanate [112]. 2K pastes of 2-methylimidazole-blocked TDI/polyether polyol adduct and a ZnO aqueous dispersion [113] or polyester fabric tapes impregnated with a blocked isocyanate polyether polyol [114] are patented for use as surgical casts and braces. Methylimidazole-blocked aromatic polyisocyanate prepolymers with a polyetherpolyol are used as gelling agents to treat wastewaters [115]. 3.1.5. Factors affecting cross-linking reactions versus reactions with water Isocyanates react with water as well as the coreactant, which are usually hydroxy-functional. While the rate constants of reaction with water are lower than with most primary alcohols, the large excess of water means that a significant amount of the isocyanate will react with water. Since the reaction with water also gives cross-links (urea instead of urethane), the problem can be minimized by use of a large excess of isocyanate; the so-called “indexing” is frequently 2:1 or even higher isocyanate:hydroxyl. However, urea linkages can lead to turbidity in films due to the relatively low compatibility and crystallinity of urea as compared to urethane groups in the polymer matrix. The cost of the isocyanate is almost always higher than that of the polyol. Also, the reaction with water leads to the evolution of CO2 , which may give foaming or blistering of applied films. Hence, in 169 most cases, it is desirable to reduce the reaction with water so that indexing need be only a little greater than 1. The required excess of water is affected by the hydroxy-functionality, f¯n , of the coreactant. For example, a series of PUDs with f¯n of 2, 4 and 6 were cured with 1:1 and 2:1 indexing of isocyanate. In each case the 2:1 indexing resulted in higher cross-link density as a result of self-condensation with water in addition to cross-linking through the hydroxyl groups. At 1:1 indexing cross-link density increased with the hydroxy functionality of the PUD. With a functionality of 6OH, the effect of higher indexing was relatively small [27]. In general, the reactivity of aromatic isocyanates with water is so high that they are difficult to use in 2K water systems. Use of a mixed HDI/TDI isocyanurate-derived hydrophilically modified isocyanate as a cross-linker gives coatings with faster drying than a similar product prepared from HDI isocyanurate and a clearer coating with better properties than one derived from TDI isocyanurate [116]. In aliphatic isocyanates NCO groups on primary carbons are more reactive with water than those on secondary carbons. 2-Methylpentane diisocyanate (2-MPDI) shows a differential reactivity since one of the NCO groups is on a secondary carbon and one on a primary carbon. When it is trimerized the NCO on the primary group reacts preferentially. No difference between the trimers of HDI and 2-MPDI with catalyst in rate of reaction with water was found in a model compound study of the reactions with n-octyl alcohol. The reaction with the HDI trimer had to be stopped after 20% conversion due to precipitation from the butyl acetate solvent whereas the 2-MPDI trimer gave no precipitation up to 80% conversion. The reaction of 2-MPDI with octyl alcohol was slower but could be compensated for by increasing catalyst concentration. 2-MPDI trimer gives faster development of hardness and higher ultimate hardness in films of waterborne acrylics than HDI trimer [117]. The reactivity of the tertiary-isocyanate groups on TMXDI with water is even slower than that of secondary groups giving it an advantage in some applications [83]. A diisocyanate in which one of the isocyanate groups is attached to a tertiary-carbon, such as 1isocyanato-1-methyl-4(3-isocyanatomethyl)cyclohexane or 1-isocyanato-1-methyl-4(4-isocyanatobut-2-yl)cyclohexane gives the advantage of differential rates of reaction [118]. Since the rate of reaction of isocyanates generally decreases in the following order: primary alcohols, secondary alcohols, 2-hydroxyethyl ether alcohols, 2-hydroxypropyl ether alcohols, the required excess of NCO over OH tends to increase in that order. Also, in particles containing both a hydroxy-functional material and a polyisocyanate, urethane-forming reactions can occur just as in solventborne 2K coatings limiting pot life for faster reacting hydroxy components. Most of the work reported has been with hydroxy-functional coreactants; however, amine-functional cross-linking has also been investigated. Primary amines are generally too reactive for practical application but useful systems have been reported using hindered secondary amines (see Section 2.4). Assuming that nonionic substitution in 170 Z.W. Wicks Jr. et al. / Progress in Organic Coatings 44 (2002) 161–183 Table 1 Dependence of relative selectivities (urethane/urea) on catalyst [121] Catalyst Relative selectivity Zr(AcAc) Mn(AcAc) None DBTDL Al(AcAc) DBTDA Zn(AcAc) Mn octanoate Co(AcAc) Ni(AcAc) Bi octanoate Zn octanoate Ti(AcAc)chelate Co octanoate 6.25 3.75 2.8 2.1 1.7 1.4 1 0.8 0.65 0.2 0.15 0.1 <0.1 <0.1 Scheme 1. Ref. [120]. hydrophilic isocyanates reverses the reactivity with alcohols and water is real, it should be included in this section too. 3.1.6. Catalyst effects Almost all 2K coatings contain a catalyst, DBTDL appears to have been the most widely used. However, DBTDL is relatively readily hydrolyzed. Since many of the coreactants are anionic resins stabilized with amine salts of carboxylic acids, it is important to recognize that catalysis by DBTDL, and similar tin compounds, is inhibited by carboxylic acid. While DBTDL favors reaction of H12 MDI with butyl alcohol as compared with water at a ratio of 2:1, dibutyltin dichloride gives approximately equal reactivity [119]. Relative reaction rates of NCO with hydroxyl groups and water depend on the catalyst used. Zr(AcAc)4 (King Industries, ZrCAT) catalyst shows better selectivity than DBTDL [120,121]. Since Zr(AcAc)4 hydrolyzes in water over time, the catalyst was blended in the isocyanate package; it was shown that the package stability of this solution was satisfactory. A waterborne coating formulated with DBTDL showed gassing after only 30 min and gelled after 1 h, while one formulated with Zr(AcAc)4 was stable over 4 h. Also when dried at 70% RH, DBTDL films lost most of their gloss whereas those with the Zr catalyst retained most of theirs; at 90% RH all films were flat [121]. Table 1 gives the relative reactivity of butyl isocyanate with 2-ethylhexyl alcohol and water (molar ratio 1.0/1.0/2.0) in a THF solution for a series of catalysts. The catalyst concentrations were adjusted so that the isocyanate would be completely reacted in 5 h. The relative rates at ambient temperature were determined by comparing the ratios of integrated FT-IR peaks for urethane and urea. Zirconium complexes with 6-methyl-2,4-heptadione and 2,2,6,6,-tetramethyl-3,5-heptadione are even more active catalysts than the acetylacetone complex [121]. A mechanism for catalysis by Zr(AcAc)4 is given in Scheme 1. No significant catalytic synergy has been observed between Zr(AcAc)4 and tertiary-amines [120,121]. Amines catalyze the reaction of isocyanates with water; the higher the pKa value, the greater is the catalytic activity [121]. DABCO is reported to be a slightly more active catalyst for reaction of TDI in diglyme with water than DBTDL [122]. This effect is more important when aromatic isocyanates are used than with aliphatic isocyanates. 3.2. Other cross-linkers for waterborne polyurethanes While the major thrust of formulating 2K waterborne urethanes is with polyisocyanates, urethane properties can also be achieved using other reactive cross-linkers with PUDs. 3.2.1. Polyaziridines TMP tris(2-aziridinylpropionate) has been used to cross-link anionic PUDs by reaction with the carboxylic acids [123]. Solvent swelling tests on cured films show that cross-linking is more complete with the aziridine than comparison tests with carbodiimide or MF resin cross-linked PUD. The reaction of carboxylic acids with aziridines is much faster than reaction with water, but the reaction rate with water is such that pot lives are 48–72 h [123]. Aziridines hydrolyze to an aminoalcohol, but it is said that there is no indication that the hydrolyzed aziridine adversely affects film properties. Additional cross-linker can be added to restore reactivity. A PUD cross-linked with a triaziridine gave better properties than obtained cross-linking with polyisocyanates, polycarbodiimides or a triepoxide [124]. In a comparison of various cross-linkers for 2,2-DMPA PUDs, tris(2-aziridinylpropionate) was reported to give the highest cross-link density [27]. TMXDI-based PUDs can be cross-linked with polyaziridines [124]. Pentaerythritol tris(2-aziridinylpropionate) is used as a cross-linker for acrylic/urethane PUDs [125,126]. TMP tris(2-aziridinylpropionate) was shown to give better properties when used as a cross-linker for PUD or water-reducible adhesives than a polycarbodiimide or an epoxy cross-linker [127]. Cross-linking of a waterborne polyurethane and a waterborne polyester with TMP tris(2-aziridinylpropionate) Z.W. Wicks Jr. et al. / Progress in Organic Coatings 44 (2002) 161–183 has been studied in comparison with a carbodiimide. Properties were better with the aziridine but the films still had insufficient Skydrol resistance [128]. A PUD extended with ethylenediamine and a novolak phenolic resin can be cross-linked with a polyaziridine [129]. Polyaziridines have been patented that are prepared from ethoxylated or propoxylated polyols instead of the polyols themselves, easier incorporation into waterborne systems and better properties for the coatings cross-linked with them are claimed [130]. For example, ethoxylated TMP is converted to the corresponding 2-aziridinylpropionate triester is used as a cross-linker for a H12 MDI-based carboxylic acid-functional PUD with a carboxylic acid-functional acrylic latex. A dual cure coating system has been reported in which a PUD with both amine and carboxylic acid functionality is first reacted with a diepoxy compound and later crosslinked with a polyaziridine [131]. The diepoxy compound reacted with the amine groups within the PUD aggregates leading to further chain extension. The aziridine compound, TMP tris(2-aziridinylpropionate) or hexanediol bis(2-aziridinylpropionate), was added just before application leading to cross-linked films with superior mechanical and thermal properties. Phosphoramide polyaziridines, prepared by partially reacting POCl3 with ethylene glycol or phenol and then reacting the residual acid chloride groups with ethylene imine or 2,2-dimethylethylene imine, have been used to cross-link carboxylic acid-functional PUDs [132]. The ethylene imine derivatives were more reactive than the dimethylethyleneimine derivatives. The cross-linked films were more fire-resistant than those prepared with conventional polyaziridines. Polyaziridines are skin irritants, and some individuals may become sensitized. Mutagenicity of polyaziridines is controversial; however, dilution by the coating vehicles reduces their possible toxic effects [123]. 3.2.2. Polycarbodiimides Carbodiimides react with carboxylic acids by two pathways to yield N-acylureas or ureas plus the anhydride of the acid, as shown in Fig. 3, depicting the reaction of acetic 171 acid with 1,3-dicyclohexylcarbodiimide. A model compound study with these two reactants of factors affecting the ratio of the two products obtained has been published, as shown in Table 2 [133]. Polycarbodiimides are synthesized by heating diisocyanates with 3-methyl-1-phenyl-2-phospholine-1-oxide catalyst. In many cases, a mixture of mono- and diisocyanate is used. The ratio of the two controls the molecular weight and gives a product with no free isocyanate groups. For example, a polycarbodiimide is made with a mixture of butyl isocyanate and IPDI [134]. Aliphatic polycarbodiimides are said to react more rapidly with COOH groups than aromatic polycarbodiimides [135]. Mixed aliphatic/aromatic isocyanates have higher reaction rates with COOH as compared with aromatic polycarbodiimides, with the rates approaching those with straight aliphatic polycarbodiimides at a lower cost [135]. Reasons for the reactivities are not discussed. While the differences could result from actual differences in reactivity, they might also result from the lower Tg of the aliphatic products or differences in solubility. A variety of approaches have been used to use polycarbodiimides in water systems. In latexes, they are just stirred into the latex with the surfactants in the latexes acting as emulsifying agents. They can be emulsified using surfactants [136]. Terminal NCO groups can be reacted with methyl ethers of polyglycols [136]. Polycarbodiimides substituted with polyglycol ether are more readily dispersed in water. In contrast to the patents cited above, where it is said that aliphatic carbodiimides react more rapidly with COOH groups than aromatic ones; it is reported that hydrophilically modified aromatic polycarbodiimides react more rapidly than those based on aliphatic or cycloaliphatic isocyanates. For example, TDI partially reacted with a monomethyl ether of polyethylene glycol is catalytically converted to a polycarbodiimide with an average functionality of 5 is used to cross-link carboxylic acid-functional PUDs [137]. When used in a coating for aluminum, the aromatic carbodiimide gave superior cross-linking as compared with an aliphatic carbodiimide and equal to that obtained with a polyaziridine cross-linker. Another approach to making hydrophilically modified carbodiimides is to convert Fig. 3. Reactions of acetic acid with 1.3-dicyclohexylcarbodiimide [133]. 172 Z.W. Wicks Jr. et al. / Progress in Organic Coatings 44 (2002) 161–183 Table 2 Effect of solvent, temperature and triethylamine concentration on reactions of acetic acid and 1,3-dicyclohexylcarbodiimide [133] Solvent Carbodiimide (mol%) Anhydride (mol%) N-acylurea (mol%) Acylurea/anhydride Solvent Cyclohexane Acetonitrile Tetrahydrofuran 59 20 3 24 34 8 17 46 89 0.7 1.4 11.1 Temperature effect (◦ C)b 0.0 30.0 48.0 36 4 11 18 8 4 46 88 85 2.6 11.1 21.3 effecta Effect of triethylamine (eq.)c 0.0 0.5 1.0 10.0 17.6 41.6 a 30 ◦ C for 45 h at 0.040 M for each reactant. 44 h in tetrahydrofuran at 0.040 M for each reactant. c 30 ◦ C for 23 h in tetrahydrofuran at 0.045 M for each reactant. b 1,3,6-tri(N-cyclohexyl-N′ -methylene thiourea)hexane to a polycarbodiimide by heating with hypochlorite solution or with bromotriphenylphosphine bromide/triethyl amine solution, then partially reacting with adducts of IPDI partially capped with MeO(CH2 CH2 O)68 H [138–141]. Another approach to increasing the ease of incorporation of carbodiimides in water systems is by partial reaction of a terminal isocyanate groups on TDI polycarbodiimide with the adduct of NaHSO3 to 2-butenediol to make an anionic-stabilized water dispersion. The product is used to cross-link a carboxylic acid-functional PUD [142]. Similarly, sodium hydroxypropanesulfonate is used with a variety of diisocyanate polycarbodiimides [136]. Cationically stabilized polycarbodiimide dispersions in water are made by reacting the polycarbodiimides of a variety of diisocyanates with 2-dimethylethanolamine and neutralizing with p-toluenesulfonic acid [136]. Polycarbodiimides have been used to cross-link carboxylic acid functional PUDs. A PUD made by reacting 2,2-DMPA with IPDI, then polymerizing a styrene/methyl methacrylate/butyl acrylate polymer in it, adding DMAE, dispersing in water and chain extending with 1,6-hexanediamine can be cross-linked with a dicarbodiimide [143]. Polycarbodiimides were effective in cross-linking a 2,2-DMPA PUD but cross-link density was lower than obtained using a polyaziridine [27,123,127]. Similar results were obtained with a PUD and a water-reducible polyester [128]. A PUD made by polymerizing a perfluoroalkyl acrylate/ hydroxyethyl methacrylate copolymer in the presence of a polyurethane prepared by reacting a polyester diol with HDI and 2,2-DMPA is blended tetramethylxylene diisocyanate carbodiimide to make a release coating [144]. cross-linkers for a 2,2-DMPA PUD in comparison with a polyaziridine [27]. The cross-link densities obtained were not as high as with the polyaziridine as determined by solvent swell results; however, resistance to a 10 min solvent spot was higher than with the polyaziridine. The author attributes the superior performance of the silanes to the spot test to enhanced adhesion to the substrate resulting from the trialkoxysilane interaction with the substrate. In cross-linking a hydroxy-functional 2,2-DMPA PUD, a hydrophilically modified polyisocyanate was the most efficient at ambient temperatures, followed by the polyaziridine, which was in turn more effective than a carbodiimide or a carbodiimide silane, least effective was aminopropyltrimethoxysilane [27]. N-(Trimethoxysilylpropyl) aspartate ethyl ester is reacted with HDI isocyanurate and a monomethyl ether of polyethylene glycol which is used as a cross-linker for a waterreducible hydroxy-functional polyester-polyurethane PUD [145]. 3.2.3. Silane derivatives Trialkoxyalkylsilanes have been investigated as crosslinkers for 2,2-DMPA PUDs. 3-Glycidyloxypropyltrimethoxysilane, a water-dispersible carbodiimide silane, and a water-dispersible epoxy silane were evaluated as 4. Mixing and application considerations 3.2.4. Polyepoxides Polyepoxides can be used as cross-linkers for carboxylic acid-functional PUDs; however, only baking systems are possible. An anionic PUD has been cross-linked with a BPA epoxy resin in an adhesive, while cross-linking did occur, properties were inferior to those obtained with TMP tris(2-aziridinylpropionate) [127]. Use of a combination of epoxy and aziridine cross-linkers was also reported. A triepoxy cross-linker has been used with a PUD [124]. Amine-terminated PUDs are cross-linked with liquid BPA resin [50]. It is relatively easy to make 2K waterborne coatings with good properties in the laboratory; however, production use is more difficult. There are several potential problems. In some Z.W. Wicks Jr. et al. / Progress in Organic Coatings 44 (2002) 161–183 173 Fig. 4. Schematic comparison of the changes of viscosity of 2K waterborne and solventborne coatings. Fig. 6. Schematic representation of the change in viscosity of the internal phase of a 2K waterborne coatings after mixing. systems, it is difficult to assure that uniform stoichiometric ratios are obtained throughout the film. If reaction occurs to a significant extent before application, coalescence will be inhibited, and therefore, film properties may be poor. As with solventborne 2K coatings, waterborne 2K coatings can be expected to have limited pot life. In solventborne coatings, pot life can be determined by monitoring viscosity increases. In many 2K waterborne coatings, viscosity does not increase as reactions between NCO and OH take place, since they change the viscosity inside the aggregate particles, but not the bulk viscosity. In fact, the generation of CO2 by the reaction of isocyanate with water decreases the pH, leading to shrinking of the aggregate particles, which can result in viscosity reduction [146]. Fig. 4 shows schematically the change in viscosity as a function of time to be expected for a 2K waterborne coating as compared to that for a 2K solventborne coating. Fig. 5 shows schematically the change of pH and urea content to be expected with a 2K waterborne coating over time. Fig. 6 shows schematically the change of viscosity over time of the internal phase to be expected with a 2K waterborne coating. The generation of CO2 leads to bubble formation in the coating, frequently called gassing. Noting the time after mixing when gassing is observed can compare the stability of coatings towards reaction with water. When a film is applied, even relatively low levels of bubble formation can significantly reduce gloss of the films. Hence pot life can be determined by following changes in gloss of films with time of storage [121]. CO2 bubble formation can lead to blistering, especially as thicker films are applied. To minimize problems of mixing two dispersions, they should start out with similar viscosities. Generally this means that it is desirable to use as low viscosity isocyanates as possible. The polyisocyanate component is sometimes diluted with solvent such as butyl acetate to reduce its viscosity. As mentioned earlier, using solvents such as cyclic carbonates or lactones give smaller particle size and more uniform dispersions in water than solvents like butyl acetate [74]. Results are also dependent on the process used to mix the two packages together. High, but not excessive, shear is required to obtain relatively uniform particle size with an average diameter of 150 nm [147]. The problems and techniques for emulsifying high viscosity materials like some isocyanurates is discussed as an important aspect of making low VOC cross-linking 2K coatings [148]. In mixing two dispersions, one containing isocyanate and another containing coreactant, maximum physical stability of the dispersion will generally be reached with the smallest particle size. However, smaller particle size means that high intensity agitation is required to break up the initial particles, which, in turn increases the possibility of isocyanate groups and water being brought into contact. The probability of new particles being formed that contain both reactants is also increased. The composition will be non-uniform when the coating is applied with some parts being high in isocyanate, others high in reactant and others with varying amounts of the two components. In making dispersions of hydrophilically modified polyisocyanates in dispersions of water-reducible acrylic resins it has been shown that at low shear rates, bimodal particle size distribution of small particles and larger particles, at high shear rate there was a single wider small particle size distribution, and with excess shearing the particle size increased and distribution broadened (Fig. 7) [146,149]. At the low shear rate the isocyanate is in small droplets and the acrylic in the larger particles, at higher shear rate the isocyanate and acrylic are together with the ionic groups on the acrylic resin stabilizing the dispersion. With excessive shear the temperature of the dispersion increases, particle size of the dispersion is too large for stability, CO2 foaming has occurred, and gel particles are found in a final film. However, small particle size means that there is a substantial increase in surface area again increasing the possibility of isocyanate interacting with water. One might Fig. 5. Schematic representation of the changes in urea content and pH of a 2K waterborne coating after mixing. 174 Z.W. Wicks Jr. et al. / Progress in Organic Coatings 44 (2002) 161–183 Fig. 7. Effect of shear on particle size distribution. expect that the pot life of fine particle size particles could be reduced because of increased urea formation. Homogeneous mixtures have also been obtained by putting an ultrasoundand cavitation-generating device in the components [150]. Doing the high intensity mixing inline with application can minimize the problems. Two-component mixing spray guns are reported to give dependable results [29]. Special equipment has been designed to permit inline mixing [149]. Spray equipment designed to provide inline intensive mixing of the two components has been used to apply polyisocyanate 2K water-reducible coatings [151]. In order to assure needed mixing of 2K coatings, a specially designed jet mixer is recommended [152]; equipment for jet mixing has been patented [153]. Performance can be affected by the rate of loss of water from films after application. If water is lost slowly there will be a larger extent of reaction of isocyanate with water requiring higher indexing to achieve full cross-linking of the coreactant groups. Since loss of water is affected by film thickness, thicker films tend to increase the relative reaction with water. A comparison of the lower part of films made by reacting HDI isocyanurate dispersed in a proprietary polyol showed that water loss from the bottom of thin films required 20 min as compared to 100 min for thick films [154]. For ambient cure coatings, relative humidity affects rate of water loss. Above 70% RH, gloss is reduced due to bubble formation caused by increased evolution of CO2 . FT-IR ATR analysis of films of 2K waterborne urethane coatings shows that reaction near the surface of isocyanate with water to form urea groups increases as RH increases [155]. The effect of humidity on curing can be seen by comparing changes in reactions near the surface of a film dried under 50 RH air and nitrogen atmosphere, the water vapor in the air led to more isocyanate conversion to urea than occurred in a nitrogen atmosphere [154]. Baking coatings can generally be formulated with lower indexing than ambient cure coatings because of the faster removal of water from the films. A large fraction of the water evaporates during flash off and initial baking, reducing the extent of isocyanate reaction with water, minimizing CO2 evolution and permitting lower ratios of NCO/OH. In one system, it was reported that indexing of 1.3–1.8 for ambient cure coatings while indexing of 1.1–1.3 could be used for heat cure coatings [156]. A comparison of ambient cure films with oven-baked films of the same coating showed much less urea formation in the baked films [146]. It was also found that some of the COOH was reacted with NCO to give amide bonds. While we have not found any experimental investigation, it seems reasonable to assume that water loss from films made with hydrophilically modified polyisocyanates is slower than when conventional polyisocyanates are used. Hydrolysis of the isocyanate in 2K water urethane coatings is said to be minimized by spraying using as the carrier gas for spraying 25% CO2 and 75% air [157]. As a result the films can be prepared that are free of blisters caused by the evolution of CO2 from the hydrolysis of NCO when the coating is sprayed with just air. 5. End uses for 2K waterborne systems 5.1. Coatings Refs. [158–160] provide general reviews of 2K waterborne urethane coatings. A comparison of 2K water coatings with 2K solvent coatings is discussed in Ref. [161]. 2K Z.W. Wicks Jr. et al. / Progress in Organic Coatings 44 (2002) 161–183 waterborne coatings are said to dry faster at ambient temperature than solventborne coatings. The greater reactivity is attributed to acceleration by the carboxyl groups and amines in the coating. Reaction is said to stop after 1 day instead of continuing for several days as in the solventborne coating. However, the films from the waterborne coating are softer and have a lower Tg indicating that cross-linking is not as complete. Free isocyanate groups were found to be present; the lack of complete reaction is attributed to mobility control of the reaction. Also the remaining volatile material in the films is significantly higher than with the solvent coating. On the other hand, force dry coatings (cured at 80 ◦ C for 30 min) the waterborne coatings show equivalent properties to those obtained with solventborne coatings. It is suggested that hydrophilic polyisocyanates, while easier to incorporate, may shorten pot life or increase CO2 development. 5.1.1. Automotive coatings Refs. [162,163] review 2K waterborne clear coatings for automobiles with comparisons of performance with 2K solventborne and 1K-blocked isocyanate coatings. 5.1.1.1. OEM coatings. Vehicles for use in automotive primers, primer-surfacers and top coats are formulated with PUDs made from a polyester diol, 2,2-DMPA and H12 MDI terminated with either diethanolamine or TMP, neutralized with DMAE using HDI isocyanurate as cross-linker [164]. A coating system using a cationic electrodeposition coating primer containing no cross-linking agent is applied to a car body and then dried at ambient temperature for 30 min. It is subsequently coated with a 2K waterborne coating made from a water-reducible acrylic resin with a sufficient excess of a nonionic hydrophilically modified HDI polyisocyanurate to cross-link both the primer and the top coat [165]. The system has the advantage of permitting cure at 130 ◦ C and providing excellent adhesion between the top coat and the primer. A chip-resistant primer is formulated with a styrene/butadiene/acrylic acid rubber latex, a PUD and a polyaziridine cross-linker [166]. Automotive primer-surfacers have been formulated with a cationic water-reducible acrylic resin and an emulsion of HDI biuret [108] or HDI isocyanurate [167] using a urethane-based emulsifying agent (see Section 2.1). The acrylic resin was a copolymer of 2-(t-butylaminoethyl)methacrylate, styrene and butyl acrylate neutralized with acetic acid and dispersed in water. The coating showed reduced gassing and improved resistance to wet sanding. 2K waterborne urethane primer-surfacers formulated with catalytic chain transfer prepared acrylic dispersions and semi-block copolymer dispersions are reported to have drying performance and properties equal to a solventborne primer [15]. Water-reducible acrylic resins with secondary amine groups incorporated using t-butylaminoethyl methacrylate, neutralized with acetic acid and dispersed in water have been used in formulating 2K primers [22]. An 175 emulsion of a solution of HDI biuret is used as the second package. An automotive metallic base coat is formulated with an anionic PUD and a nonionic hydrophilically modified HDI isocyanurate [168]. An automotive base coat is formulated with an anionic PUD prepared from an unsaturated dimer acid polyester, 2,2-DMPA, H12 MDI and TMP and a water-reducible acrylic resin neutralized with DMAE [169]. A waterborne base coat is formulated with a thermoplastic PUD, a hydroxy-functional acrylic resin or PUD and a polyisocyanate [170]. Automotive coatings have been formulated with a PUD prepared from a coconut oil modified polyester, trimethylhexamethylene diisocyanate, and 2,2-DMPA neutralized with TEA and a mixture of TMXDI/TMP prepolymer and H12 MDI [171]. 2K waterborne urethane coatings have been evaluated for automobile clear coats, best properties were obtained with a combination of HDI- and IPDI-derived polyisocyanates [162]. 2K automotive clear coats have been formulated with a water-reducible acrylic resin and a combination of HDI isocyanurate and uretdione [172]. Aqueous 2K urethane coatings for automotive top coats have been developed using a combination of water-reducible acrylic resins and a PUD with HDI isocyanurates and IPDI isocyanurates [162]. A clear coat for automobiles is formulated with HDI isocyanurate and a water-reducible acrylic resin grafted to a polyester [173]. The latter is made by reacting a hydroxy-functional polyester resin with the glycidyl ester of versatic acid and polymerizing the acrylic resin in this solution. It is reported that gassing is less likely to occur in this same type of coating if the HDI isocyanurate is partially reacted with dodecanol and a fraction of high flash naphtha is emulsified into the polyester-acrylate dispersion [174]. 2K automotive clear coats have been prepared using a TMXDI/TMP prepolymer with an anionic water-reducible acrylic resin and water [108]. Automotive top coats have been formulated with a water-reducible acrylic resin with a mixture of TMXDI/TMP prepolymer and H12 MDI [175]. A mixture of a nonionic hydrophilically modified TMXDI/TMP prepolymer with unmodified TMXDI/TDI prepolymer is used with water-reducible acrylic resins in automotive coatings [176]. Automotive coatings have been formulated with a water-reducible polyester neutralized with TEA and a liquid polyisocyanate [177]. Inclusion of a agent to reduce surface tension gives more reliable results in wet-on-wet coating [178]. A latex made from a carboxylic acid- and trimethoxysilyl-functional acrylic resin emulsion polymerized with methyl methacrylate and hydroxyethyl methacrylate is used with a nonionic hydrophilically modified polyisocyanate to formulate a clear coat [179]. A water-reducible acrylic resin prepared from vinyl t-decanoate, acrylic acid, butyl methacrylate, isobutyl methacrylate, t-butyl acrylate and hydroxypropyl methacrylate is used with a 90% solids solution of a HDI isocyanurate to make clear coats [180]. A combination of proprietary acrylic polyol with a hydroxy equivalent weight of 890 at 176 Z.W. Wicks Jr. et al. / Progress in Organic Coatings 44 (2002) 161–183 62% solids and a Tg of 30 ◦ C with water-dispersible HDI isocyanurate are reported to give excellent properties in clear coats even at indexing ratios of NCO/OH less than 1.25 [181,182]. Tack-free and through dry times decreased as the amount of tin carboxylate catalyst increased from 0 to 0.015% of solids. Pot life of the coatings obtained as measured by gloss were 2 h for the higher Sn content samples and >4 h for a catalyst-free sample. 2K waterborne urethane top coats formulated with catalytic chain transfer prepared acrylic dispersions and semi-block copolymer dispersions with unmodified IPDI isocyanurate are reported to result in better film appearance and humidity resistance than similar coatings formulated with hydrophobically modified polyisocyanates [15]. In a review of the status of 2K waterborne urethane clear coats, the advantages of using unmodified polyisocyanates and using inline mixing of the two components is emphasized [183]. Nonionic hydrophilically modified polyisocyanates that have been further reacted with amine-functional trialkoxysilanes have been used in formulating OEM automotive primer-surfacers, base coats and clear coats [101]. Gloss retention is improved and reduction of blistering when panels are stored immersed in water as compared to corresponding panels of coatings without the silane. The trialkoxysilane also acts as a cross-linker with hydroxyl groups and undergoes self-condensation in the presence of water. A clear coat with excellent environmental etch and abrasion resistance has been patented that consists of a hydroxy-functional acrylic resin, an adduct of isocyanatopropyltrimethoxysilane and cyclohexyldimethanol, with a HDI isocyanurate [184]. Trialkoxysilyl-functional water-reducible acrylic resins have been used with HDI isocyanurate [185]. Coatings for plastics are discussed in Section 5.1.4. 5.1.1.2. Automobile refinishing. 2K water polyurethanes using water-reducible acrylic resins are being studied for use in refinishing automobiles. VOC emissions are much lower than with solventborne 2K coatings and comparable properties have been obtained [8]. Waterborne 2K polyurethane base coats for automobile refinishing have been developed and commercialized [186,187]. An important requirement for base coats is good orientation of the aluminum flake pigment for metallic coatings, it is reported that polyurethane resins based on H12 MDI show better flop than polyurethanes with other diisocyanates. Another key requirement is for high pigment concentrations for use in custom matching of the many different colors required in automobile refinishing. Polyurethanes made with TMXDI had lower viscosities than other polyurethanes permitting higher levels of pigmentation. The lower viscosity apparently results from reduced intermolecular hydrogen bonding as a result of steric hindrance. The refinishing system developed uses an H12 MDI polyurethane as the vehicle for the mixing clear and for inorganic and aluminum pigment pastes and a TMXDI polyurethane for organic pigment pastes [187]. Nonionic hydrophilically modified polyisocyanates are used with acrylic resins in refinish primer-surfacers and clear coats [162]. A combination of HDI isocyanurate and an allophanate modified HDI polyisocyanate [188] or an IPDI isocyanurate with a 90% solids solution of HDI isocyanurate in butyl acetate and uretdione [189] is used as cross-linker for a water-reducible acrylic resin as a 2K clear coat for refinishing. 2K waterborne clear coats for refinish using polyester-acrylic hybrid dispersions with a isocyanate package of HDI uretdione and HDI isocyanurate have good properties and are easily mixed without need for special high shear mixing equipment [190]. A combination of HDI uretdione/isocyanurate with a carbodiimide prepared from TMXDI with the terminal isocyanate groups reacted with ethyl alcohol or a combination of polypropylene glycol and ethyl alcohol has been patented for use as a clear coat for refinishing that cures at 80 ◦ C [191]. Specially designed water-reducible acrylic resins with TMXDI/TMP prepolymer, and dimethyltin dicarboxylate catalyst are used in formulating clear coats for refinishing with dry time and early hardness equal to solventborne clear coats [20]. Waterborne camouflage coatings for military vehicles are formulated with a hydroxy-functional PUD and an HDI polyisocyanate [192]. High indexing (NCO:OH = 3.5) is used since the hybrid matrix of polyurethane/polyurea segments provides the chemical durability and flexibility required [159]. To achieve adequate resistance to chemical warfare agents, a PUD with a higher functionality than in the earlier work and with 5:1 indexing were required [193]. 5.1.2. Wood finishing Some applications, such as wood kitchen cabinets, do not require that cross-linking be complete; in such cases, lower NCO/OH ratios can be used, permitting lower cost and longer pot life; higher indexing is required for applications such as office and laboratory furniture [83,149]. Nonionic hydrophilically modified HDI isocyanurate and hydroxyand carboxylic acid-functional acrylic latexes were used. An associative thickener was used in the coatings; it was shown that the Tg of the film was increased by inclusion of the associative thickener in the film, perhaps indicating that the associative thickener is also cross-linked [149]. A statistical study of the effect of several variables on performance of coatings for wood kitchen cabinets using an acrylic latex cross-linked with a nonionic hydrophilically modified HDI isocyanurate has been published [58]. It was found that high hydroxy content of the latex (hydroxyl number 52), small particle size of the latex, core-shell preparation of the latex and low Tg (obtained by increasing level of coalescing solvent) enhanced performance. Comparisons with solventborne nitrocellulose lacquer and alkyd-urea conversion varnishes showed that the overall performance of optimized 2K waterborne urethane coatings were superior with the added advantage of substantially reduced VOC and HAP solvent emission. Another statistical analysis has been reported of the inter-relationships between performance on Z.W. Wicks Jr. et al. / Progress in Organic Coatings 44 (2002) 161–183 a variety of tests and the use of various additives to 2K acrylic latex/nonionic hydrophilically modified isocyanate wood coatings [194]. Wood coatings have been formulated with hydroxy-functional acrylic latexes and nonionic hydrophilically modified polyisocyanates [57]. Indexing of 1:1 is adequate for applications such as kitchen cabinets and home furniture but 2:1 indexing is recommended for applications where high solvent and stain resistances are required, such as office and laboratory furniture. Pot life as determined by physical properties of the applied coating is reported to be 6 h. In clear coats, it is reported that coatings can be “recharged”, i.e. when the pot life limit for good properties has been reached, further polyisocyanate can be added and coating performance is restored. Wood finishes that cure overnight at 50 ◦ C have been formulated with a core-shell acrylic latexes with acetoacetoxyethyl methacrylate as one of the shell monomers cross-linked with a nonionic hydrophilically modified HDI isocyanurate [66]. Stain and chemical resistance were superior to those obtained with a formulation using a similar hydroxy-functional latex. Wood finishes are formulated with a water-reducible acrylic resin with a small amount of propoxylated TMP cross-linked with a mixture of HDI uretdione and isocyanurate [195]. The added polyol increases gloss and abrasion resistance. In coatings for flooring, a driving force for adoption of 2K waterborne urethane systems has been the need to eliminate solvent odor, while retaining the outstanding abrasion resistance exhibited by urethane coatings. The coatings are formulated with a nonionic hydrophilically modified HDI isocyanurate with water-reducible polyesters or blends of polyesters and polyacrylates [159]. It is reported that the level of polyether content necessary for ease of mixing the two components in wood flooring coatings is lower using allophanate-modified polyisocyanates than with straight polyisocyanates [196]. 5.1.3. Maintenance coatings A review of 2K waterborne maintenance coatings has been published [72]. Use of a proprietary water-reducible acrylic resin with conventional HDI-based isocyanates is particularly recommended. Hydroxy-functional acrylic latexes with nonionic hydrophilically modified polyisocyanates have been recommended for use in maintenance coatings [57]. Pot lives of various coatings were 4–8 h; pot life decreased as the PVC of TiO2 was increased from 17 to 21%. Rate of developing film hardness and ultimate hardness obtained increased as the ratio of isocyanate to acrylic polyol was increased but recharging is not recommended when using indexing of 2:1. Coalescing solvent selection affects not only film formation, but also gloss of the coating. The mono-n-butyl ether of propylene glycol gave faster drying and slightly higher gloss than the monophenyl ether and distinctly better gloss and faster drying than NMP. The hydroxyl groups on the glycol ethers react with isocyanate groups. 177 A comparison of the effects of humidity, temperature and film thickness on the performance of 2K waterborne urethane maintenance coatings to solventborne coatings has been published [197]. At temperatures of 25 ◦ C and below, performance was similar but the waterborne system cured more rapidly, at 37 ◦ C and a relative humidity of 90% both systems cured more rapidly but the performance of the solventborne coating was more severely affected. Waterborne coatings applied at thicknesses up to 125 ␮m without gassing while solventborne coatings were limited to 50 ␮m thickness. Maintenance coatings that cure in 5 days at ambient temperatures have been formulated with the acetoacetoxyfunctional latexes with a nonionic hydrophilically modified HDI isocyanurate [66,198]. 5.1.4. Coatings for plastics A study of the effect of Tg of the urethane component in coatings for TPO plastics cross-linked with a hydrophilically modified polyisocyanate has been published [199]. Scratch resistance was higher with the higher (but still low) Tg urethane. Aqueous polyurethanes have been developed for use on a wide range of plastic substrates [200]. A coating for polycarbonate and ABS plastics has imparts a “soft feel” and improved resistance to sun tan lotion is formulated with a monobutyl ether of polyethylene glycol-modified HDI isocyanurate reacted with a hydroxy-functional polyester diol prepolymer as a cross-linker for a PUD made from polyethers, polyesters, 2,2-DMPA and HDI neutralized with DMAE and dispersed in water [201]. The coating is cured at 75 ◦ C for 30 min. A coating for plastics is formulated with a water-reducible polyester resin and HDI isocyanurate [202]. 2K waterborne primer-surfacers, base coats, clear coats and coatings for automobile plastic components have been developed [203]. A primer for application to polycarbonate plastics that cures in 30 min at 80 ◦ C is formulated with a water-dispersible HDI-based polyisocyanate and a cationic water-reducible acrylic resin prepared from dimethylaminopropyl methacrylate, hydroxypropyl methacrylate, other acrylic esters solubilized by making a salt with 2,2-DMPA [204]. A primer for polypropylene plastics has been formulated with a water-reducible acrylic resin, an emulsion of a chlorinated polyolefin, and HDI isocyanurate/biuret [205,206] or a nonionic hydrophilically modified polyisocyanate [206]. A hydroxy-functional acrylic latex is used with a nonionic hydrophilically modified HDI isocyanurate in coatings for TPO OEM automobile bumper coatings [207]. Coatings for polyphenylene oxide/polystyrene plastics have been formulated with a core-shell acrylic latex containing acetoacetoxyethyl methacrylate as one of the monomers in the shell with a nonionic hydrophilically modified HDI isocyanurate [66]. A water-dispersible acrylamide, acrylic acid, acrylic resin with a polyisocyanate cross-linker is used to coat plastic films to impart computer imprintability properties [208]. 178 Z.W. Wicks Jr. et al. / Progress in Organic Coatings 44 (2002) 161–183 A prime coat for nylon sheets that improves adhesion of glue adhesive used in laminating the sheets to cardboard is formulated with a PUD with a polyaziridine cross-linker [209]. Coatings for polycarbonate and polyphenylene oxide plastics have been formulated with a mixture of an anionic PUD and a carboxylic acid-functional acrylic resin neutralized with ammonia using TMP tris(2-aziridinylpropionate) [210]. A group of patents covers use of trifunctional aziridine cross-linkers for PUDs for use in coating plastics [211–214]. 5.1.5. Other coatings A nonionic hydrophilically modified HDI isocyanurate is used with a hydroxy-functional PUD made from HDI, a hydroxy-functional polyester, TMP, and 2,2-DMPA, neutralized with a mixture of ammonia and DMAE and diluted with water. Such a 2K coating had a pot life of 8 h, gave dust-free films in 4 h at ambient temperature and cured in 7 days [215]. A hydroxy-functional PUD prepared from a polyester diol, 2,2-butylethylpropanediol, 2,2-DMPA, H12 MDI, IPDI, neutralized with TEA, and terminated with diethanolamine is cross-linked with a polyisocyanate mixture of an allophanate-modified HDI and H12 MDI uretdione and isocyanurate [216]. A PUD made with a polyester polyol, IPDI and 2,2-DMPA is used with HDI uretdione and HDI isocyanurate [217]. A PUD prepared from a coconut oil alkyd, IPDI, and 2,2-DMPA, neutralized with DMAE and chain extended with IPDA has been used with a conventional polyisocyanate to make coatings for temperature-sensitive substrates having a pot life of 4 h [218]. A polyester PUD and a water-reducible acrylic resin neutralized with DMAE is used with a mixture of an HDI allophanate polyisocyanate and HDI isocyanurate to give a good with good flow and appearance [219]. A 2K primer is formulated with a water-reducible acrylic resin and a hydrophilically modified HDI isocyanurate [220]. Primers and fillers are formulated with an amine-terminated PUD and a waterborne epoxy resin [50]. A corrosion-resistant primer for aluminum is formulated with a PUD and a trifunctional aziridine [221]. 2K coatings are formulated with water-reducible acrylic resins and a low viscosity HDI uretdione/isocyanurate [222]. Coatings are formulated with a hydroxy-functional PUD and a combination of HDI uretdione and HDI isocyanurate [31]. 2K coatings are formulated with water-reducible acrylic resin and a nonionic hydrophilically modified polyisocyanate at a NCO/OH ratio of 1.25; pot life as determined by gloss of applied films is between 2 and 3 h [223,224]. 2K coatings are formulated with a water-reducible acrylic resin and an anionic PUD made from a non-oxidizing alkyd resin and IPDI using a mixture of HDI uretdione and isocyanurate as cross-linker [225]. Self-healing, scratch-resistant coatings are formulated with various combinations of acrylic, polyester and/or polyurethane resins in waterborne 2K coatings using hydrophilically modified isocyanates [226]. An abrasion-resistant traffic paint is formulated with a nonionic hydrophilically modified HDI isocyanurate, a hydroxy-functional acrylic latex and aminoethylaminopropyltrimethoxysilane [227]. Nonionic hydrophilically modified HDI isocyanurate with polyether polyols having functionalities of 3 or more has been patented for use as concrete coatings [228]. Concrete floor sealers are formulated with a combination of an HDI uretdione/isocyanurate and a TDI prepolymer with TMP and diethylene glycol as cross-linkers for a polyester triol [229]. A statistically designed experiment investigating the effect and interactions of the additions of various additives to 2K waterborne urethane coatings has been published [230]. Golf ball coatings with excellent abrasion resistance that are free from CO2 gassing and have the low viscosity necessary for good flow are formulated using a nonionic hydrophilically modified isocyanate and a hydroxy-functional water-reducible polyester [231] or a nonionic hydrophilically modified HDI isocyanurate and an anionic hydroxyfunctional PUD [232]. Primers for golf balls have been formulated with an anionic PUD prepared from TMXDI and a polyester polyol and a carboxylic acid-functional acrylic latex with TMP tris(2-aziridinylpropionate) [233,234]. Another golf ball primer is formulated with an anionic PUD and the same triaziridine [235]. Coatings for PVC tile having high conductivity are formulated with a PUD and TMP tris(2-aziridinylpropionate) [236]. Electrically conductive coatings are formulated with a trifunctional aziridine, a polyurethane rubber and a carboxyl acid-functional acrylic resin [237]. Coatings for brass clothing snaps and hardware are formulated with a PUD and TMP tris(2-aziridinylpropionate) [238]. A corrosionresistant coating for Zn/Ni-coated steel is formulated with a PUD and a polyaziridine [239]. Aziridine and carbodiimide cross-linkers cross-linked waterborne urethanes and a water-reducible polyester have been evaluated for aircraft finishes; Skydrol resistance was deficient [128]. Polycarbodiimides are used with carboxylic acid-functional latexes in formulating a variety of coatings, including roof coatings, hardboard coatings, coil coatings and coatings for plastics [134,135]. Coil coatings are formulated with various aqueous dispersions of polycarbodiimides with carboxylic-acid functional PUDs, water-reducible polyesters and acrylic resins [136]. Coatings for stainless steel have been formulated with a carboxylic acid-functional PUD, a water-reducible acrylic resin using glycidoxypropyltrimethoxysilane as a crosslinker [240]. A coating for use over retroreflective signs is formulated with an H12 MDI-based PUD and an acrylic latex cross-linked with TMP tris(2-aziridinylpropionate) [241]. 5.2. Adhesives and sealants 2K waterborne adhesives for laminating plywood were among the first uses for 2K waterborne urethane systems. A major motivating force was the need to reduce or eliminate formaldehyde emissions. The 2K waterborne adhesives give excellent resistance to heat and boiling water. Z.W. Wicks Jr. et al. / Progress in Organic Coatings 44 (2002) 161–183 Various latexes, such as butadiene/acrylonitrile latex, were cross-linked with solvent solutions of polyisocyanates such as tris(4-isocyanatophenyl)methane [242]. Vinyl acetate/butyl acrylate latex cross-linked with an emulsion of HDI isocyanurate partially reacted with isocetyl alcohol [61] and hydroxy-functional acrylic latexes cross-linked with solvent solutions of polymeric MDI [243] have been patented as plywood adhesives. HDI isocyanurate is used with a copolymer of vinyl acetate and n-butyl maleate to formulate an adhesive for laminating wood [62]. A hydroxy-functional acrylic latex stabilized with poly(vinyl alcohol) with polymeric MDI gives adhesives for wood with good green strength, an excellent adhesion and water resistance [244]. Solvent-free adhesives with a long pot life formulated with a poly(vinyl alcohol)-stabilized vinyl chloride/ethylene/hydroxyethyl acrylate latex with polymeric MDI as cross-linker are used for laminating wood [64]. An adhesive for laminating wood is formulated with poly(vinyl alcohol), an ethylene/vinyl acetate latex and polymeric MDI [245]. A combination of a urethane/acrylic PUD and a vinyl acetate/ethylene latex is used with a nonionic hydrophilically modified HDI isocyanurate to formulate an adhesive for laminating vinyl plastic to wood [246]. Waterborne adhesives formulated with a PUD prepared from HDI, a hydroxy-functional polyester, 1,4-butanediol, the sodium salt of N-(aminoethyl)aminoethane sulfonic acid and 2,2-DMPA is used with a mixture of HDI isocyanurate and uretdione [62]. A PUD, a nonionic hydrophilically modified polyisocyanate and triethanolamine are used in formulating an adhesive for laminating fiber boards [32]. A 2K waterborne adhesive is formulated with water-dispersible coreactants and a nonionic hydrophilically modified polyisocyanate [247]. An adhesive for laminating plastics is formulated with a nonionic hydrophilically modified H12 MDI and a PUD [248]. A PUD prepared by reacting a water-reducible polyester with the sodium salt of 5-sulfoisophthalic acid as the solubilizing group, 2,2-DMPA, IPDI and HDI is neutralized with NaOH and reduced with water, then chain extended with a combination of ethylene diamine and diethylene triamine is cross-linked with TMP tris(2-aziridinylpropionate) or a nonionic hydrophilically modified polyisocyanate is used as an adhesive for laminating PET film to polypropylene film [249]. An isocyanato-terminated polyurethane made with polyethylene glycol, polypropylene glycol, neopentyl glycol, TMP and TDI dissolved in propylene carbonate is used with a PUD for laminating plastic films or a plastic film to aluminum foil [250]. Performance is reported to be superior to that obtained with a nonionic hydrophilically modified HDI isocyanurate. Adhesives giving good clarity and water resistance are formulated with a nonionic hydrophilically modified HDI isocyanurate [251]. The adhesive is applied to a plastic film, dried and then the film is heat laminated to either another plastic film or aluminum foil. A PUD made with MDI and polytetrahydrofuran and a nonionic hydrophilically modified 179 HDI isocyanurate are used to formulate an adhesive with a pot life of >7 h for bonding vinyl sheets to ABS plastics while thermoforming [252,253]. A nonionic hydrophilically modified MDI made by partially reacting it with a monoether of polyethylene glycol is used as a cross-linker for various latexes, such as vinyl acetate/ethylene latex, for laminating PVC plastics [63]. Addition of a nonionic hydrophilically modified HDI isocyanurate to a PUD prepared from a polyester diol, HDI, IPDI and the sodium salt of N(2-aminoethyl)-2-aminoethanesulfonic acid, terminated with diethanolamine increases heat resistance when used as an adhesive for laminating PVC sheets [254]. An adhesive for laminating polyethylene film to a nonwoven fabric is formulated with a carboxylic acid-functional styrene/butadiene latex and a polyfunctional isocyanate [255]. A nonionic hydrophilically modified polyisocyanate and 3-(2,3-epoxypropoxy)propyltrimethoxysilane were reacted and used with a TDI-based PUD in formulating an adhesive for polypropylene films [256]. Adhesives for laminating polyester film to polyethylene film are formulated with an amine-terminated PUD and a liquid BPA epoxy resin [50]. Waterborne adhesives have been formulated using TMP tris(2-aziridinylpropionate to cross-link PUDs by reaction with the carboxylic acids [124]. They were particularly useful in laminating different films such as polyester/ polypropylene and polyester/aluminum foil for packaging that must withstand immersion in boiling water. Properties, especially the resistance to boiling water immersion of laminates were superior to those obtained using adhesives with the same PUD but cross-linking with a polyisocyanate, a polycarbodiimide or a triepoxide. Aziridine, carbodiimide and epoxy cross-linkers for a TMXDI-based anionic PUD have been used in 2K adhesives for laminating PET to polypropylene film [127]. A hydroxy-functional latex with a polyisocyanate that has been partially reacted with a hydrophobic monoalcohol, such as isocetyl alcohol, is used to formulate structural adhesives for laminating wood [257]. Adhesives for rubber have been formulated with a cationic hydrophilically modified polyisocyanate, such as HDI isocyanurate partially reacted with 2-hydroxyethylmorpholine followed by quaternization with dimethyl sulfate, and a cationic PUD such as disclosed in Refs. [70,258]. Adhesives for bonding rubber to fabrics, fiber cords and other substrates are formulated with a nonionic hydrophilically modified HDI isocyanurate, a chlorosulfonated polyethylene latex, and a vulcanizing agent such as dinitrosobenzene [65]. Adhesives for bonding SBR rubber are formulated with a nonionic hydrophilically modified IPDI isocyanurate or IMCI isocyanurate trimer with an polychloroprene latex with an increased hydroxy-functionality obtained by alkali treatment [259]. Polycarbodiimides are used with a carboxylic acid-functional latex in formulating pressure sensitive adhesives with greater peel strength [134,135]. A 2K aqueous putty 180 Z.W. Wicks Jr. et al. / Progress in Organic Coatings 44 (2002) 161–183 is formulated with a PUD and a hydrophilically modified allophanized HDI [260]. 5.3. Textiles, paper and leather Use of hydroxy-functional latexes with an emulsion of an isocyanate-terminated prepolymer were patented as crease-resistant textile finishes [261]. For example, a hydroxy-functional acrylic latex was used with an emulsion of an ethyl acetate solution of an HDI/polypropylene glycol prepolymer as the cross-linker. Shrink resistance treatments for wool are based on acrylamide copolymer acrylic latexes with an emulsion of an ethyl acetate solution of an HDI/propoxylated TMP prepolymer [262]. A nonionic hydrophilically modified HDI isocyanurate partially reacted with a monomethyl of polyethylene glycol was been used with a perfluorourethane to treat cotton/polyester fabric to make it water and oil repellent [263]. The treatment has package stability of at least 24 h and has the advantage over blocked isocyanate cross-linked treatments that are widely used, of curing in 10 min at 100 ◦ C instead of 5 min at 150 ◦ C. The same polyisocyanate was used with a perfluoroacrylic resin to give improved dirt resistance. A coating for fabric conveyor belts that gives a low coefficient of friction when wet with water increases the wear life and rate of transport of aqueous slurries such as coal slurries [264]. The coating is formulated with the reaction product of polyvinylpyrrolidone, a polyethylene glycol and octadecylisocyanate using a hydrophilically modified polyisocyanate prepared from H12 MDI and propoxylated glycerol. A nonionic hydrophilically modified isocyanate made with HDI isocyanurate and a monobutyl ether of a polyglycol diluted with ethylene carbonate is used with a PUD in formulating a pigment pad dyeing treatment for dyeing cotton cloth [74]. Crock fastness of the dyed fabric is equal to the crock fastness of a similar treatment prepared with the solvent-free isocyanate with only 70% of the isocyanate component and superior to the crock fastness of the same level of polyisocyanate that had been diluted with NMP. Improved efficiency is attributed to the finer particle size of the aqueous dispersion of the polyisocyanate. A leather top coating has been formulated with an aliphatic carbodiimide (see Section 3.2.2) and a carboxylic acid-functional PUD [137]. A leather substitute is made by laminating a plastic film to a napped nylon tricot fabric using an adhesive based on a PUD and a nonionic hydrophilically modified polyisocyanate [265]. 5.4. Other uses An acrylic/urethane PUD with pentaerythritol tris(2-aziridinylpropionate) as a cross-linker is used to coat aziridine primed PET sheets to make base sheets for abrasive coated sheets [125]. 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