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Multistep chemistry in thin films; the challenges of blocked isocyanates
Progress in Organic Coatings 43 (2001) 131–140
Multistep chemistry in thin films; the challenges of blocked isocyanates Douglas A. Wicks a,∗ , Zeno W. Wicks Jr. b a
Bayer Corporation, 100 Bayer Road, Pittsburgh, PA 15205, USA b 190 Spring View Court, Louisville, KY 40243, USA
Received 11 September 2000; received in revised form 18 April 2001; accepted 2 May 2001
Abstract The paper provides an overview of the history and progress of blocked isocyanates in coatings. Typically blocked isocyanate systems are used to obtain the performance of two-component polyurethanes in a one-component thermally cured system. Two-component polyurethanes have an established position as high performance coatings, but may be inappropriate for a given application because of equipment costs or the need to be mixed just prior to use with a limited potlife. Blocked isocyanate systems overcome these difficulties by chemically masking the isocyanate, allowing it to be mixed with coreactants in a one-package delivery form. Formation of the polyurethane coating is then accomplished by thermally decomposing the isocyanate-blocking agent bond. There are many factors that affect this reaction: • Structures of the isocyanate and blocking agent. • Thermal stability of the isocyanate-blocking agent bond. • Polarity of the reaction media (i.e. coreactant). • The nucleophylicity of the blocking agent vs. the coreactant. • Ability of the blocking agent to diffuse out of the film. • Catalysis. However, higher temperatures are required for curing, in some cases, toxic hazard of the escaping blocking agent must be considered, and in some cases the blocking agent can cause discoloration. Major efforts in the last few years have been centered on reducing these difficulties. The complexity of the deblocking reaction requires a full understanding of these factors and how they interact. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Blocked polyisocyanates; Coatings; Urethanes; Catalysis; Overview
1. Introduction We have recently completed an exhaustive review with over 1700 references of the literature and patents on blocked isocyanates [1,2]. One of the most striking things that became evident in this study was the complexity involved in the chemistry and use of blocked isocyanates. The literature abounds in unwarranted generalizations based on studies of a limited number of materials in a limited number of ways.
In this paper, we hope to make people aware of the complexities and to define the variables involved and, to the extent possible, discuss the basic mechanisms of the reactions. Commonly, one thinks of a blocked isocyanate as one in which the isocyanate group is reacted with an active hydrogen compound. When heated the blocked isocyanate decomposes to free the isocyanate that then reacts with a coreactant to form a urethane or a urea, respectively.
∗ Corresponding author. Tel.: +1-412-777-7851; fax: +1-412-777-2940. E-mail address: [email protected] (D.A. Wicks).
There are several examples of products that react like blocked isocyanates but are not called blocked isocyanates.
0300-9440/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 0 - 9 4 4 0 ( 0 1 ) 0 0 1 8 8 - 6
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Fig. 1. Preparation of TACT via alkyl carbonate.
Most of these are called carbamates. Carbamate is a synonym for urethane and carbamates could also be called alcohol blocked isocyanates. The tris(alkoxycarbonylamino)triazine (TACT) is a mixed methyl butyl carbamate derivative of melamine, that can be viewed as an alcohol blocked isocyanate. TACT is prepared by reacting melamine with dimethyl carbonate and butyl alcohol (Fig. 1) [3]. Uretdiones (isocyanate dimers) are frequently called “self-blocked isocyanates”. Uretdiones are of particular interest because they can react with a nucleophile without releasing any blocking agent; this lack of a volatile byproduct is of particular interest in powder coatings. Uretdiones can dissociate and react with alcohols to form urethanes or alcohols can react directly with the dimer to yield allophanates (Fig. 2). Formation of allophanates is favored by dibutyltin dilaurate (DBTL). With primary amines, the direct reaction with the dimer is favored and biurets are obtained [4]. The decomposition temperature depends on the parent isocyanates. The uretdiones of TDI, IPDI, and HDI decompose without catalyst at 150, 160, and 200◦ C, respectively. The reaction products of an isocyanate with an active methylene compound is called a “blocked isocyanate” but do not yield urethanes or ureas when reacted with hydroxy-functional or amine-functional coreactants. The most widely used example of this type product is made by reacting an isocyanate with diethyl malonate (DEM). When a DEM blocked isocyanate reacts with an alcohol the products are ester-amides.
Scheme 1.
2. Mechanism of reactions of blocked isocyanates There are two urethane forming reaction mechanisms by which most blocked isocyanates can react with a nucleophile (NuH). In the elimination–addition reaction (Scheme 1A), the blocked isocyanate decomposes to the free isocyanate and the blocking group (BH). The isocyanate then reacts with a nucleophile to form a final product. In the addition–elimination reaction (Scheme 1B), the nucleophile reacts directly with the blocked isocyanate to yield a tetrahedral intermediate followed by elimination of the blocking agent. In general, cross-linking is more rapid in the presence of a nucleophile that can react rapidly with the isocyanate; for example, amines react much more rapidly than alcohols. The differences in reactivity depend on the structures of
Fig. 2. Reactions of uretdione to form allophanate, urethane, or biuret groups.
D.A. Wicks, Z.W. Wicks Jr. / Progress in Organic Coatings 43 (2001) 131–140
the amines, alcohols, and blocking agent. Primary amines react more rapidly than secondary amines. Thus, blends of oxime blocked isocyanates and secondary amine-functional polymers increase in viscosity more slowly than those with primary amine-functional polymers [5]. During the curing process, low molecular weight blocking agents can evaporate from the film. Those that can diffuse more rapidly through the cross-linking film and volatilize more rapidly from the surface give more rapid (or lower temperature) cure than other blocking agents of the same class that diffuse and evaporate more slowly. For example, methyl, ethyl, and isopropyl alcohol blocked TMXDI and an acrylic polyol with a tin catalyst gave cross-linked films in increasing times in the order listed [6]. The temperature dependence of reactions is modeled by the Arrhenius equation, r = ln A −
Ea RT
Relative rates of reaction change with temperature depending on differences in the preexponential A term. For example, the measured A values for caprolactam blocked isocyanates are relatively low compared with the A values for diisopropyl and diisobutyl ketone oxime blocked isocyanates [7]. As temperature increases, the advantage in curing rate of these oxime blocked isocyanates over caprolactam increases. The pathway of the reaction has not been extensively studied. Generally, it has been assumed, and in a few cases it has been proven, that the reaction proceeds by the elimination–addition pathway. However, in other cases, addition–elimination reaction has been proposed. Caution is urged in generalizing from the results of a single combination. It is possible that a blocked isocyanate with one coreactant might react by addition–elimination and with another coreactant might react by elimination–combination. Similarly, with a constant isocyanate and coreactant but different blocking groups, the mechanisms might differ. In at
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least some cases, the reaction pathway may change with temperature.
3. Kinetics of reactions Kinetic studies are complicated by the fact that all, or almost all, the reactions are reversible and several side reactions are generally possible. Even the simplest case of heating a blocked isocyanate alone is complicated by possible side reactions of the isocyanate. At high temperatures, there are dimerization or trimerization reactions of the isocyanate or reaction with the original blocked isocyanate to form, for example, an allophanate or biuret. These in turn can also thermally decompose through different kinetic pathways. When used with a polynucleophile, the liberated isocyanate can in addition react with the urethanes or ureas produced. In some cases, the deblocking reaction may be catalyzed by another molecule of blocked isocyanate or by the blocking agent (Fig. 3). The curing schedule required for cross-linking of blocked polyisocyanates with polynucleophiles is dependent on many variables including: • structures of the isocyanate, blocking agent, and nucleophile, • relative rate of reaction of the nucleophile with the isocyanate compared to the reverse reaction rate of the isocyanate with the blocking agent, • the rates of diffusion and evaporation of the blocking agent, • polarity and hydrogen-bonding potential of the reaction medium (solvents or coreactant), • concentrations of reactive groups, • type and concentration of catalysts, • extent of side reactions and whether they lead to cross-linking or termination, and
Fig. 3. Side reactions of isocyanates after deblocking.
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Fig. 4. Use of dynamic TGA to track curing of 1K urethane film [8].
• the extent of cross-linking required to achieve film properties. Typically, the variables have to be evaluated for each application. A combination that leads to satisfactory cure for wire enamel cured at 265◦ C may not be appropriate for an automotive clear coat at 140◦ C. In view of the many variables, comparisons of “deblocking temperatures” for different blocked isocyanates must be done with care. One should be sure to compare data obtained by the same method. Furthermore, there is not a threshold temperature; rather there is a determination of the temperature at which the analytical method used detects some extent of the deblocking or cross-linking reaction. 3.1. Determination of deblocking temperatures Many analytical techniques have been applied to study the reactions of blocked isocyanates. One needs to remember that reported deblocking temperatures frequently depend on the method of analysis, heating rate, and other variables. Different analytical techniques can give different deblocking temperatures for the same sample. • The most common methods for determining “deblocking temperature” follow some change in physical properties. For example, gel time or development of solvent resistance. • The IR spectra of isocyanates show a characteristic strong absorption band near 2250 cm−1 ; deblocking temperatures have been reported as the temperature at which this absorption is first detected. • Use of FTIR in combination with dynamic mechanical analysis (DMA). • Isothermal thermogravimetric analysis (TGA) (Fig. 4). • The changes in heat flow associated with deblocking as measured by differential scanning calorimetry (DSC). • The chemical species can be tracked directly using solid state NMR.
• The reaction of isocyanates with water leads to formation of CO2 ; blocked isocyanates are heated in the presence of molecular sieves saturated with moisture and the lowest temperature at which evolved CO2 can be detected is reported as the deblocking temperature [9]. It must be remembered that further cross-linking after reaching 200 MEK rubs has little effect on MEK rub resistance but cross-linking continues leading to further changes in the mechanical properties of films. Such further cross-linking has to be followed by film properties, changes in DMA results, and/or resistance to swelling by solvents. This is particularly critical in ambient temperature curing since the glass transition temperature increases with cross-linking and if the Tg of the film approaches the curing temperature, the rate of further cross-linking reactions becomes mobility controlled and as Tg approaches 50◦ C above cure temperature, reaction virtually ceases even though there are unreacted functional groups. Reaction rates and extent of reaction can be very dependent on volatilization of the blocking agent. Fig. 5(a) shows a time resolved series of IR spectra taken of a neat sample of a methyl ethyl ketoxime (MEKO) blocked HDI polyisocyanurate being heated at 140◦ C. The NCO peak at 2250 cm−1 can be clearly seen growing in the spectra. Fig. 5(b) shows the spectra when the blocked isocyanate was also heated to 140◦ C in a thin film but in a covered ATR cell then the cover slide was removed. Even with the test being run at 140◦ C, there was only insignificant generation of isocyanate before removal of the cover slide. The effect of removal of the cover slide can be clearly seen on spectrum (b) where it is marked with an arrow. This inhibition of reaction has clear implications for studying blocked urethane systems for use in coatings. A related effect should be expected from increased film thickness. In this case, the slow diffusion of the blocking group through the reactive medium could result in slower cure. Thus analytical techniques used for tracking the deblocking
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Fig. 5. ATR spectra of deblocking of MEKO blocked HDI isocyanurate as a function of time at 140◦ C (a) without a cover slide and (b) with a cover slide that was removed as shown by the arrow.
kinetics in thin films should: (a) allow for evaporation of the blocking group and (b) be of a film thickness comparable to that used in the final application. Not all systems that use blocked isocyanates require that the blocking group leave as it does in films. Studies related to shelf stability, adhesives, and thick section plastics all need to have retention of the blocking group to predict field use performance. Looking at the various techniques, it is possible to estimate whether or not there is loss of the deblocking group during the measurement. Some of the methods intrinsically allow for release because they are run in the open. Examples are: • TGA, • DMA. Others intrinsically retain or retard release of the blocking group because they are run in closed fashion. Examples are: • • • •
cone and plate rheology measurements, amine titration methods on solutions, NMR, CO2 evolution.
Still others can lead to different results depending on how they are run: • film property measures, • IR, FTIR and ATR. 3.2. Effect of isocyanate structure In general, blocked aromatic isocyanates deblock at lower temperatures than blocked aliphatic isocyanates. This results from the greater electron-withdrawing potential of an aromatic ring as compared to an aliphatic group. Substitution of the aromatic ring with electron-withdrawing groups, such as Cl, NO2 , and COOR, increase deblocking rates while electron donor groups, such as alkyl groups, decrease deblocking rates. There are also steric effects on deblocking rates. In one study, dimethyl ketoxime blocked TDI deblocked at 120◦ C, blocked MDI at 130◦ C, and blocked H12 MDI at 136◦ C [10]. However, the IPDI adduct deblocked at 143◦ C (Fig. 6). Blocked aliphatic isocyanates with the isocyanato group on a tertiary carbon (for example, MEKO
Fig. 6. Five isocyanates with different steric and electronic environments.
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Table 1 Deblocking temperatures of MEKO blocked monomeric isocyanates Blocked isocyanate
Deblocking T (◦ C)
HDI IPDI XDI H6 XDI TMXDI
132 121 140 147 100
blocked TMI) deblock at significantly lower temperatures [11]. A study of deblocking of a series of MEKO blocked isocyanate monomers sheds light on the combined effects of sterics and electronics [12]. The MEKO blocked versions of the isocyanate monomers listed in Table 1 were evaluated for deblocking temperature using TGA. The reported deblocking temperature is that at the onset of weight loss. In this series, IPDI was found to deblock at a significantly lower temperature than HDI. This is a result of the steric influence of the cyclohexyl ring. Of special interest within this series is the comparison of XDI, H6 XDI and TMXDI. With H6 XDI deblocking at a much higher temperature than IPDI, it is apparent that the presence of a -cyclohexyl does not lead to lower deblocking temperature. The determination of deblocking temperature was the onset of weight loss, so the low result for IPDI probably results from the secondary isocyanate position. The higher reactivity could result from being in a “neopentyl” position with a quaternary carbon  to the NCO. Moving to XDI, the effect of having the isocyanate in a benzylic position can be seen. Though it shows a lower deblocking temperature than the -cyclohexyl isocyanate, it is not as low as the pure aliphatic diisocyanates. Again, the steric crowding at benzylic position in TMXDI leads to significantly lower deblocking temperature. The contribution is clear when comparing TMXDI to XDI.
Table 2 Effect of solvent on dissociation of p-nitrophenol phenyl urethane at 80◦ C
Solvent
Solvent polarity
Dissociation rate constant (×105 s−1 )
Acetone Methyl ethyl ketone Cyclohexanone Nitrobenzene Chlorobenzene Dioxane o-Xylene
20.70 18.51 18.30 34.82 5.62 2.21 2.57
95.80 15.30 3.05 3.64 1.24 1.15 0.21
be expected to be more rapid in hydrogen bond accepting solvents than in non-polar solvents. The limited data available in the literature fits with this hypothesis [13–15]. The differences can be large, for example, the deblocking temperature for a pentachlorophenol blocked IEM copolymer in benzene is reported to be 144◦ C, and in tetrahydrofuran 65◦ C [15]. For p-nitrophenol phenyl urethane it was shown (see Table 2) that the combination of solvent polarity and how protophilic the solvent is, control the homolytic dissociation [14]. The major exception to this is when nitrobenzene is used as a solvent. Its lower dissociation rate was attributed to poor hydrogen-bonding potential. Acetone, with the strongest measured hydrogen bond between the solvent carbonyl and the NH of the urethane, shows the fastest dissociation. There is need for further study of the media effect since proper solvent choice could affect package stability of 1K blocked isocyanate coatings and the hydrogen-bonding potential of coreactants could affect cure rates. 3.4. Catalysis Catalysts are usually included in blocked isocyanate formulations but commonly without consideration of what reaction or reactions they are involved in. Typically the same ones used in unblocked 2K polyurethanes are used with the blocked systems, though at higher levels. As indicated earlier, there are many reactions taking place and the catalyst can be involved in any one or more of these:
3.3. Effect of reaction medium In at least some cases, hydrogen-bonding potential of the reaction medium can have an effect on reaction rates. It is well known that reaction rates of isocyanates with alcohols and phenols are up to two orders of magnitude more rapid in weak hydrogen bond-acceptor solvents than strong hydrogen bond-acceptor solvents. Since strong hydrogen bond accepting solvents reduce k−1 , deblocking reactions would
• the deblocking reaction, • the reaction of the free isocyanate with the other nucleophile, • an addition–elimination reaction • side reactions. Engbert et al. [16] looked at the curing of an IPDI polyisocyanurate/triol system used in conjunction with a number of different blocking agents. DMA was used to track the minimum temperatures at which cross-linking starts between a trifunctional polyether with the blocked IPDI polyisocyanurate resins.
D.A. Wicks, Z.W. Wicks Jr. / Progress in Organic Coatings 43 (2001) 131–140
The study also included each system with and without catalyst. As seen in Fig. 7, there were significant differences in the systems after catalyst addition. The inclusion of 1% DBTL by weight, moved the cross-linking temperature of the dimethyl pyrazole blocked product from a not very exciting 158◦ C to the lowest of the deblocking isocyanates at 112◦ C. There is some overlap between this study and the previous one with both MEKO and caprolactam giving similar results in both cases. Such an effect was shown by Carlson et al. who studied the effect of DBTL on the deblocking of IPDI based cross-linkers without a coreactant. In this case the DBTL showed no catalytic effect on the actual deblocking reaction but did, over a longer time lead to a decrease in isocyanate concentration as a result of side reactions [17]. DBTL can catalyze the reaction of the freed isocyanate and a hydroxyl group and can be a mild allophanate catalyst. Many catalysts have been suggested for accelerating the curing rate of blocked isocyanate systems. Table 3 gives the results of screening a series of catalysts for the reaction of MEKO blocked HDI isocyanurate and a hydroxy-functional acrylic resin [18]. It should be noted that the films were intentionally undercured (20 min at 130◦ C) to emphasize the differences in catalytic activity. Several other blocked isocyanates were also tested, and the authors concluded that of the catalysts tested, bismuth tris(2-ethyl hexanoate) gave the most consistent catalyst response.
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Diazabicyclo[2,2,2]octane (DABCO) is commonly used as a catalyst; it is a better catalyst for oxime blocked isocyanates than triethylamine but quinuclidine is still better. Still stronger catalysis is obtained with the bicyclic amidine, 3,3,6,9,9-pentamethyl-2,10-diazabicyclo[4,4,0]dec-1-ene; it was found to be more effective than quinuclidine with oxime blocked aliphatic isocyanates but less effective than DABCO with aromatic isocyanates. It was also shown that the catalysts had the same relative effect on accelerating the cure rate of the deblocked analogs. It was suggested that concerted bifunctional catalysis might be the reason for the observed effectiveness.
A study of catalysis of benzyl alcohol blocked 4-isocyanatomethyl-1,8-diisocyanatooctane (TTI) has shown that, at least in some cases, there is an optimum tin concentration. Higher concentrations led to less catalytic effect as determined by number of MEK double rubs at temperatures lower than required to achieve 200 double rub resistance with films of a hydroxy-functional acrylic resin with benzyl
Fig. 7. Deblocking temperatures from [16].
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Table 3 Catalyst screening with MEKO blocked HDI isocyanurate [18] Catalyst
Metal % on solids
Film appearance
MEK 2X rubs
No catalyst Dibutyltin dilaurate Dibutyltin dilaurate Dibutyltin diacetate Bismuth tris(2-ethylhexanoate) Aluminum dionate complex Cobalt bis(2-ethylhexanonate) Zr bis(2-ethylhexanoate) Zn bis(2-ethylhexanoate) Ti tetra(ethyl acetoacetate) Calcium bis(2-ethylhexanoate) Chromium tris(2-ethylhexanoate)
0 0.05 0.09 0.18 0.065 0.04 0.05 0.18 0.048 0.50 0.05 0.255
Clear Clear Clear Clear Clear Clear Hazy Clear Clear Yellow Clear Hazy
0 43 46 38 51 19 51 12 35 52 24 43
alcohol blocked TTI baked for 30 min at 140◦ C (Table 4) [19]. In waterborne systems such as cationic E-coat, stability of the catalyst towards hydrolysis is particularly important. The hydrolytic stability of DBTDL is inadequate. Dibutyltin oxide exhibits good stability and has been commonly used, but it is a pigment and must be dispersed with the other pigments and is not a highly effective catalyst. Many other catalysts have been patented for use in E-coat. For example, bis(tributyltin)oxide is reported to be a more efficient catalyst and more easily incorporated than dibutytin oxide [20]. 3.5. Effects of functionality During the curing of blocked isocyanate coatings the deblocking and subsequent cross-linking reactions rarely go to 100% completion. Typically, bake schedules are prescribed that given a certain level of reaction that yields optimum properties. One cannot a priori predict what level of reaction is required to get the optimum performance. There are no mathematical models applicable to systems in general that can be used to estimate the effect of functionality on the attaining of properties. However, one can look at the gel point of the systems being studied. Usually the gel point is reached before the optimum properties are. Using well-established calculations one can get an idea of the degree of effect that the changes within the system may have. The degree of functional group
conversion (Pc ) required to get to the gel point can be estimated through use of Eq. (5). Where the average functionality is favg = ni fi / ni . Pc =
2 favg
(5)
All other aspects remaining equal, it is possible to decrease the amount of time required for cure by increasing the functionality of the cross-linker or polyol. For example, Fig. 8 shows the change in calculated Pc in curing a polyester of average functionality four with increasing functionality of the cross-linker. Thus, if one were to change from a difunctional blocked isocyanate to one that is tetrafunctional; the extent of reaction required to reach the gel point would reduce from more than 70% for the difunctional to approximately 50% for the tetrafunctional. 3.6. Effect of blocking groups After reading through thousands of abstracts, patents and publications, it would be satisfying to say that we could draw up a conclusive ranking of the blocking agents in terms of cure temperature. However, this is not the case, many other variables cloud the assignment of a definitive deblocking temperature. Some general guidance may be drawn from the published comparisons available.
Table 4 Effect of catalyst concentration on MEK resistance of films of hydroxy-functional acrylic resin with benzyl alcohol blocked TTI and tetra-n-butyl-1,3-diacetoxydistannoxane (Sn = 39.5%) baked at 140◦ C for 30 min Level of catalyst (Sn content, phr)
MEK rubs
0.10 0.20 0.40 1.20
30 130 105 55
Fig. 8. Critical conversion (Pc ) to gel point as a function of changing cross-linker functionality, assuming a constant average coreactant functionality of four.
D.A. Wicks, Z.W. Wicks Jr. / Progress in Organic Coatings 43 (2001) 131–140 Table 5 Rate constants for deblocking of ketoxime blocked isocyanates (k values were given in the increased rate of deblocking from top to bottom) Oxime of
k (×10−5 s−1 )
Methyl n-amyl ketone Methyl isoamyl ketone Methyl 3-ethylheptyl ketone Methyl 2,4-dimethylpentyl ketone Methyl ethyl ketone Cyclohexanone Methyl isopropyl ketone Methyl isobutyl ketone Diisobutyl ketone Methyl t-butyl ketone Diisopropyl ketone 2,2,6,6-Tetramethylcyclohexanone
4.7 5.8 8.3 9.4 10.0 10.3 11.8 12.0 15.2 19.9 21.0 300.0
The structure of blocking groups has a major effect on deblocking temperatures and cure rates of coatings. Here, we review blocking groups from the point of view of understanding as well as possible with the information available, the relationships between blocking group structures and reactivity of blocked isocyanates. There are other important aspects in addition to reactivity involved in the choice of blocking groups, these generally are related to a particular application. Deblocking temperatures are dependent on steric effects. Table 5 gives the first-order rate constants for deblocking for a series of oximes (with caprolactam for comparison purposes) when solutions of blocked cyclohexane isocyanates in toluene were heated to 107◦ C [7] and the isocyanate was trapped with dibutyl amine. With the exception of tetramethylcyclobutanedione monooxime, rate constants increase as steric bulk increases and were found to be much greater than the deblocking rate of caprolactam (for caprolactam k = 0.92 × 10−5 s−1 ). Regulski and Thomas [21] published a broad comparison of blocking groups being used on IEM copolymers. Their study covered the cross-linking of a copolymer of methyl methacrylate, ethyl acrylate and IEM (Fig. 9) with a hydroxy-functional coreactant. The polymer was prepared from the blocked monomer. The results of their findings are shown in Table 6. The systems were catalyzed with 0.5 wt.% dibutyltin diacetate, and the reported temperatures are when the system
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Table 6 Crosslinking temperatures of specific IEM derivatives Group
Specific derivative
Temperature
Phenol
Methyl salicylate Methyl p-hydroxybenzoate
110–120 130
Imidazole
Imidazole
110–130
Oxime
MEKO Acetone oxime
140–150 130–140
N-hydroxyimides
N-hydroxyphthalimide N-hydroxysuccinimide
170–175 160
Alcohols
Methoxypropanol Ethylhexanol Pentol Ethyl lactate
175–200 >200 200 200
Lactams
Caprolactam Pyrrolidinone
175–200 250
Ethyl acetoacetate
Ethyl acetoacetate (never really cured)
>175
developed MEK double rub resistance after 30 min in an air flow oven. Using monomeric blocked TMI, Lucas and Wu [11] measured the appearance of the NCO band at 2260 cm−1 as a function of temperature, without a coreactant present. They made two sets of observations for each system, the first being the initial appearance of the isocyanate band and the second being the temperature at which rapid generation of NCO is noted (Table 7). The most significant observation is the low onset temperatures that they found for the oxime blocked monomer. Their value for the MEKO adduct is 50◦ C lower than was reported for TMXDI being studied by TGA. The discrepancy may arise from the low volatility of the MEKO, in the TGA measurement, it must volatilize to be seen. This may not happen at an appreciable rate until 100◦ C. The authors reported problems polymerizing the blocked monomers in the presence of hydroxy-functional monomers due to premature cross-linking. This is traceable to the deblocking of the isocyanate at the polymerization temperatures of 60–100◦ C. They also checked the “cross-linking” temperature by looking at copolymers containing both the blocked TMI moiety and hydroxyethyl acrylate (Fig. 10) in the presence of 0.5% dimethyltin dilaurate. The reported “minimum cure temperature” is that which gave MEK double rub resistance after 30 min of cure. The self-cross-linking results do not match either of the observations for NCO appearance, but they do follow the trend with oximes being the lowest and the alcohols being the highest in terms of deblocking temperatures. 4. Outlook
Fig. 9. Blocked IEM copolymer used by Regulski and Thomas.
As discussed in this paper and dealt with more fully in Ref. [1], substantial progress has been made in understanding
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Table 7 Deblocking temperatures of various blocked TMI samples Group
Specific derivative
Temperature (◦ C) NCO onset
Rapid NCO
Self-cross-linking
Oxime
MEKO Acetone oxime Cyclohexanone oxime
50 50 65
65 80 100
<120
Lactams
Caprolactam
90
125
130
N-hydroxyimide
N-hydroxysuccinimide
115
140
130
Alcohols
Methanol n-Butanol n-Pentanol n-Hexanol Propylene glycol
155 165 165 155 >180
190 190 190 175 –
>145 >145
–
Fig. 10. Self-cross-linking blocked TMI copolymer studied by Lucas and Wu.
the complexities of the reactions of blocked isocyanates. As we cover in Ref. [2], there has been a marked increase in the use of blocked isocyanates. Not only are they major cross-linkers in coatings such as cationic electrodeposition primers and powder coatings among others, they are also used or are being investigated as cross-linkers in a wide array of other end uses. The need now is for a better understanding of the effect of various combinations of variables on the cure rates. There is need for better understanding of interactions when more than one variable is changed. For example, the effect of simultaneously changing isocyanate, blocking group, catalyst and catalyst concentration, temperature, coreactant structure, and medium effects of both solvents during storage and coreactant backbones during curing needs further study. It is to be expected that new experimental techniques will be required to fully understand the complex interactions. References [1] D.A. Wicks, Z.W. Wicks Jr., Prog. Org. Coat. 36 (1999) 148. [2] D.A. Wicks, Z.W. Wicks Jr., Prog. Org. Coat. 41 (2001) 1. [3] L.A. Flood, R.B. Gupta, R. Iyengar, D.A. Ley, V.K. Pai, US Patent 5 792 866 (1998). [4] A. Chen, L. Katz, R. Wojcik, J. O’Connor, in: Proceedings of the Waterborne, High-solids, Powder Coating Symposium, New Orleans, LA, 1996, p. 103.
[5] J.-J. Lin, G.P. Speranza, M. Cuscurida, Ind. Eng. Chem. Res. 36 (1997) 4231. [6] Y. Huang, G. Chu, M. Nieh, F.N. Jones, J. Coat. Technol. 67 (842) (1995) 33. [7] J.S. Witzeman, Prog. Org. Coat. 27 (1996) 269. [8] Th. Engbert, E. Konig, E. Jurgens, Farbe Lack 102 (7) (1996) 51. [9] S. Mohanty, N. Krishnamurti, Eur. Polym. J. 34 (1998) 77. [10] P.L. Jansse, J. Oil Colour Chem. Assoc. 89 (1989) 478. [11] H.R. Lucas, K.-J. Wu, J. Coat. Technol. 65 (820) (1993) 59. [12] I. Muramatsu, Y. Tanimoto, M. Kase, N. Okoshi, Prog. Org. Coat. 22 (1993) 279. [13] C.B. Reilly, N. Orchin, Prepr. ACS Div. Paint Plast. Printing Inks, Atlantic City, NJ, 1956. [14] M.S. Fedoseev, G.N. Marchenko, G.A. Sal’nikova, in: A.M. Schiller (Ed.), Soviet Urethane Technology, Vol. I, Technomic, Westport, CT, 1973, p. 237. [15] P.C. Boyer, Ph.D. Dissertation, Univ. So. Miss., 1986, p. 74. [16] Th. Engbert, E. Jurgens, E. Konig, FATIPEC Congr., 23rd (Vol. B), 1996, pp. B115–B143. [17] G.M. Carlson, C.M. Neag, C. Kuo, T. Provder, Polym. Sci. Technol. 36 (Fourier Transform Infrared Charact. Polym.) (1987) 197. [18] W.J. Blank, Z.A. He, M.E. Picci, Polym. Mater. Sci. Eng. 79 (1998) 399. [19] H.P. Higginbottom, G.R. Bowers, L.W. Hill, J.F. Courtier, Prog. Org. Coat. 34 (1998) 27. [20] V. Eswarakrishnan, R.R. Zwack, E.R. Coleridge, US Patent 5 356 529 (1993). [21] T. Regulski, M.R. Thomas, Org. Coat. Appl. Polym. Sci. Proc. 48 (1983) 1003.
Multistep chemistry in thin films; the challenges of blocked isocyanates Douglas A. Wicks a,∗ , Zeno W. Wicks Jr. b a
Bayer Corporation, 100 Bayer Road, Pittsburgh, PA 15205, USA b 190 Spring View Court, Louisville, KY 40243, USA
Received 11 September 2000; received in revised form 18 April 2001; accepted 2 May 2001
Abstract The paper provides an overview of the history and progress of blocked isocyanates in coatings. Typically blocked isocyanate systems are used to obtain the performance of two-component polyurethanes in a one-component thermally cured system. Two-component polyurethanes have an established position as high performance coatings, but may be inappropriate for a given application because of equipment costs or the need to be mixed just prior to use with a limited potlife. Blocked isocyanate systems overcome these difficulties by chemically masking the isocyanate, allowing it to be mixed with coreactants in a one-package delivery form. Formation of the polyurethane coating is then accomplished by thermally decomposing the isocyanate-blocking agent bond. There are many factors that affect this reaction: • Structures of the isocyanate and blocking agent. • Thermal stability of the isocyanate-blocking agent bond. • Polarity of the reaction media (i.e. coreactant). • The nucleophylicity of the blocking agent vs. the coreactant. • Ability of the blocking agent to diffuse out of the film. • Catalysis. However, higher temperatures are required for curing, in some cases, toxic hazard of the escaping blocking agent must be considered, and in some cases the blocking agent can cause discoloration. Major efforts in the last few years have been centered on reducing these difficulties. The complexity of the deblocking reaction requires a full understanding of these factors and how they interact. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Blocked polyisocyanates; Coatings; Urethanes; Catalysis; Overview
1. Introduction We have recently completed an exhaustive review with over 1700 references of the literature and patents on blocked isocyanates [1,2]. One of the most striking things that became evident in this study was the complexity involved in the chemistry and use of blocked isocyanates. The literature abounds in unwarranted generalizations based on studies of a limited number of materials in a limited number of ways.
In this paper, we hope to make people aware of the complexities and to define the variables involved and, to the extent possible, discuss the basic mechanisms of the reactions. Commonly, one thinks of a blocked isocyanate as one in which the isocyanate group is reacted with an active hydrogen compound. When heated the blocked isocyanate decomposes to free the isocyanate that then reacts with a coreactant to form a urethane or a urea, respectively.
∗ Corresponding author. Tel.: +1-412-777-7851; fax: +1-412-777-2940. E-mail address: [email protected] (D.A. Wicks).
There are several examples of products that react like blocked isocyanates but are not called blocked isocyanates.
0300-9440/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 0 - 9 4 4 0 ( 0 1 ) 0 0 1 8 8 - 6
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Fig. 1. Preparation of TACT via alkyl carbonate.
Most of these are called carbamates. Carbamate is a synonym for urethane and carbamates could also be called alcohol blocked isocyanates. The tris(alkoxycarbonylamino)triazine (TACT) is a mixed methyl butyl carbamate derivative of melamine, that can be viewed as an alcohol blocked isocyanate. TACT is prepared by reacting melamine with dimethyl carbonate and butyl alcohol (Fig. 1) [3]. Uretdiones (isocyanate dimers) are frequently called “self-blocked isocyanates”. Uretdiones are of particular interest because they can react with a nucleophile without releasing any blocking agent; this lack of a volatile byproduct is of particular interest in powder coatings. Uretdiones can dissociate and react with alcohols to form urethanes or alcohols can react directly with the dimer to yield allophanates (Fig. 2). Formation of allophanates is favored by dibutyltin dilaurate (DBTL). With primary amines, the direct reaction with the dimer is favored and biurets are obtained [4]. The decomposition temperature depends on the parent isocyanates. The uretdiones of TDI, IPDI, and HDI decompose without catalyst at 150, 160, and 200◦ C, respectively. The reaction products of an isocyanate with an active methylene compound is called a “blocked isocyanate” but do not yield urethanes or ureas when reacted with hydroxy-functional or amine-functional coreactants. The most widely used example of this type product is made by reacting an isocyanate with diethyl malonate (DEM). When a DEM blocked isocyanate reacts with an alcohol the products are ester-amides.
Scheme 1.
2. Mechanism of reactions of blocked isocyanates There are two urethane forming reaction mechanisms by which most blocked isocyanates can react with a nucleophile (NuH). In the elimination–addition reaction (Scheme 1A), the blocked isocyanate decomposes to the free isocyanate and the blocking group (BH). The isocyanate then reacts with a nucleophile to form a final product. In the addition–elimination reaction (Scheme 1B), the nucleophile reacts directly with the blocked isocyanate to yield a tetrahedral intermediate followed by elimination of the blocking agent. In general, cross-linking is more rapid in the presence of a nucleophile that can react rapidly with the isocyanate; for example, amines react much more rapidly than alcohols. The differences in reactivity depend on the structures of
Fig. 2. Reactions of uretdione to form allophanate, urethane, or biuret groups.
D.A. Wicks, Z.W. Wicks Jr. / Progress in Organic Coatings 43 (2001) 131–140
the amines, alcohols, and blocking agent. Primary amines react more rapidly than secondary amines. Thus, blends of oxime blocked isocyanates and secondary amine-functional polymers increase in viscosity more slowly than those with primary amine-functional polymers [5]. During the curing process, low molecular weight blocking agents can evaporate from the film. Those that can diffuse more rapidly through the cross-linking film and volatilize more rapidly from the surface give more rapid (or lower temperature) cure than other blocking agents of the same class that diffuse and evaporate more slowly. For example, methyl, ethyl, and isopropyl alcohol blocked TMXDI and an acrylic polyol with a tin catalyst gave cross-linked films in increasing times in the order listed [6]. The temperature dependence of reactions is modeled by the Arrhenius equation, r = ln A −
Ea RT
Relative rates of reaction change with temperature depending on differences in the preexponential A term. For example, the measured A values for caprolactam blocked isocyanates are relatively low compared with the A values for diisopropyl and diisobutyl ketone oxime blocked isocyanates [7]. As temperature increases, the advantage in curing rate of these oxime blocked isocyanates over caprolactam increases. The pathway of the reaction has not been extensively studied. Generally, it has been assumed, and in a few cases it has been proven, that the reaction proceeds by the elimination–addition pathway. However, in other cases, addition–elimination reaction has been proposed. Caution is urged in generalizing from the results of a single combination. It is possible that a blocked isocyanate with one coreactant might react by addition–elimination and with another coreactant might react by elimination–combination. Similarly, with a constant isocyanate and coreactant but different blocking groups, the mechanisms might differ. In at
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least some cases, the reaction pathway may change with temperature.
3. Kinetics of reactions Kinetic studies are complicated by the fact that all, or almost all, the reactions are reversible and several side reactions are generally possible. Even the simplest case of heating a blocked isocyanate alone is complicated by possible side reactions of the isocyanate. At high temperatures, there are dimerization or trimerization reactions of the isocyanate or reaction with the original blocked isocyanate to form, for example, an allophanate or biuret. These in turn can also thermally decompose through different kinetic pathways. When used with a polynucleophile, the liberated isocyanate can in addition react with the urethanes or ureas produced. In some cases, the deblocking reaction may be catalyzed by another molecule of blocked isocyanate or by the blocking agent (Fig. 3). The curing schedule required for cross-linking of blocked polyisocyanates with polynucleophiles is dependent on many variables including: • structures of the isocyanate, blocking agent, and nucleophile, • relative rate of reaction of the nucleophile with the isocyanate compared to the reverse reaction rate of the isocyanate with the blocking agent, • the rates of diffusion and evaporation of the blocking agent, • polarity and hydrogen-bonding potential of the reaction medium (solvents or coreactant), • concentrations of reactive groups, • type and concentration of catalysts, • extent of side reactions and whether they lead to cross-linking or termination, and
Fig. 3. Side reactions of isocyanates after deblocking.
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Fig. 4. Use of dynamic TGA to track curing of 1K urethane film [8].
• the extent of cross-linking required to achieve film properties. Typically, the variables have to be evaluated for each application. A combination that leads to satisfactory cure for wire enamel cured at 265◦ C may not be appropriate for an automotive clear coat at 140◦ C. In view of the many variables, comparisons of “deblocking temperatures” for different blocked isocyanates must be done with care. One should be sure to compare data obtained by the same method. Furthermore, there is not a threshold temperature; rather there is a determination of the temperature at which the analytical method used detects some extent of the deblocking or cross-linking reaction. 3.1. Determination of deblocking temperatures Many analytical techniques have been applied to study the reactions of blocked isocyanates. One needs to remember that reported deblocking temperatures frequently depend on the method of analysis, heating rate, and other variables. Different analytical techniques can give different deblocking temperatures for the same sample. • The most common methods for determining “deblocking temperature” follow some change in physical properties. For example, gel time or development of solvent resistance. • The IR spectra of isocyanates show a characteristic strong absorption band near 2250 cm−1 ; deblocking temperatures have been reported as the temperature at which this absorption is first detected. • Use of FTIR in combination with dynamic mechanical analysis (DMA). • Isothermal thermogravimetric analysis (TGA) (Fig. 4). • The changes in heat flow associated with deblocking as measured by differential scanning calorimetry (DSC). • The chemical species can be tracked directly using solid state NMR.
• The reaction of isocyanates with water leads to formation of CO2 ; blocked isocyanates are heated in the presence of molecular sieves saturated with moisture and the lowest temperature at which evolved CO2 can be detected is reported as the deblocking temperature [9]. It must be remembered that further cross-linking after reaching 200 MEK rubs has little effect on MEK rub resistance but cross-linking continues leading to further changes in the mechanical properties of films. Such further cross-linking has to be followed by film properties, changes in DMA results, and/or resistance to swelling by solvents. This is particularly critical in ambient temperature curing since the glass transition temperature increases with cross-linking and if the Tg of the film approaches the curing temperature, the rate of further cross-linking reactions becomes mobility controlled and as Tg approaches 50◦ C above cure temperature, reaction virtually ceases even though there are unreacted functional groups. Reaction rates and extent of reaction can be very dependent on volatilization of the blocking agent. Fig. 5(a) shows a time resolved series of IR spectra taken of a neat sample of a methyl ethyl ketoxime (MEKO) blocked HDI polyisocyanurate being heated at 140◦ C. The NCO peak at 2250 cm−1 can be clearly seen growing in the spectra. Fig. 5(b) shows the spectra when the blocked isocyanate was also heated to 140◦ C in a thin film but in a covered ATR cell then the cover slide was removed. Even with the test being run at 140◦ C, there was only insignificant generation of isocyanate before removal of the cover slide. The effect of removal of the cover slide can be clearly seen on spectrum (b) where it is marked with an arrow. This inhibition of reaction has clear implications for studying blocked urethane systems for use in coatings. A related effect should be expected from increased film thickness. In this case, the slow diffusion of the blocking group through the reactive medium could result in slower cure. Thus analytical techniques used for tracking the deblocking
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135
Fig. 5. ATR spectra of deblocking of MEKO blocked HDI isocyanurate as a function of time at 140◦ C (a) without a cover slide and (b) with a cover slide that was removed as shown by the arrow.
kinetics in thin films should: (a) allow for evaporation of the blocking group and (b) be of a film thickness comparable to that used in the final application. Not all systems that use blocked isocyanates require that the blocking group leave as it does in films. Studies related to shelf stability, adhesives, and thick section plastics all need to have retention of the blocking group to predict field use performance. Looking at the various techniques, it is possible to estimate whether or not there is loss of the deblocking group during the measurement. Some of the methods intrinsically allow for release because they are run in the open. Examples are: • TGA, • DMA. Others intrinsically retain or retard release of the blocking group because they are run in closed fashion. Examples are: • • • •
cone and plate rheology measurements, amine titration methods on solutions, NMR, CO2 evolution.
Still others can lead to different results depending on how they are run: • film property measures, • IR, FTIR and ATR. 3.2. Effect of isocyanate structure In general, blocked aromatic isocyanates deblock at lower temperatures than blocked aliphatic isocyanates. This results from the greater electron-withdrawing potential of an aromatic ring as compared to an aliphatic group. Substitution of the aromatic ring with electron-withdrawing groups, such as Cl, NO2 , and COOR, increase deblocking rates while electron donor groups, such as alkyl groups, decrease deblocking rates. There are also steric effects on deblocking rates. In one study, dimethyl ketoxime blocked TDI deblocked at 120◦ C, blocked MDI at 130◦ C, and blocked H12 MDI at 136◦ C [10]. However, the IPDI adduct deblocked at 143◦ C (Fig. 6). Blocked aliphatic isocyanates with the isocyanato group on a tertiary carbon (for example, MEKO
Fig. 6. Five isocyanates with different steric and electronic environments.
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Table 1 Deblocking temperatures of MEKO blocked monomeric isocyanates Blocked isocyanate
Deblocking T (◦ C)
HDI IPDI XDI H6 XDI TMXDI
132 121 140 147 100
blocked TMI) deblock at significantly lower temperatures [11]. A study of deblocking of a series of MEKO blocked isocyanate monomers sheds light on the combined effects of sterics and electronics [12]. The MEKO blocked versions of the isocyanate monomers listed in Table 1 were evaluated for deblocking temperature using TGA. The reported deblocking temperature is that at the onset of weight loss. In this series, IPDI was found to deblock at a significantly lower temperature than HDI. This is a result of the steric influence of the cyclohexyl ring. Of special interest within this series is the comparison of XDI, H6 XDI and TMXDI. With H6 XDI deblocking at a much higher temperature than IPDI, it is apparent that the presence of a -cyclohexyl does not lead to lower deblocking temperature. The determination of deblocking temperature was the onset of weight loss, so the low result for IPDI probably results from the secondary isocyanate position. The higher reactivity could result from being in a “neopentyl” position with a quaternary carbon  to the NCO. Moving to XDI, the effect of having the isocyanate in a benzylic position can be seen. Though it shows a lower deblocking temperature than the -cyclohexyl isocyanate, it is not as low as the pure aliphatic diisocyanates. Again, the steric crowding at benzylic position in TMXDI leads to significantly lower deblocking temperature. The contribution is clear when comparing TMXDI to XDI.
Table 2 Effect of solvent on dissociation of p-nitrophenol phenyl urethane at 80◦ C
Solvent
Solvent polarity
Dissociation rate constant (×105 s−1 )
Acetone Methyl ethyl ketone Cyclohexanone Nitrobenzene Chlorobenzene Dioxane o-Xylene
20.70 18.51 18.30 34.82 5.62 2.21 2.57
95.80 15.30 3.05 3.64 1.24 1.15 0.21
be expected to be more rapid in hydrogen bond accepting solvents than in non-polar solvents. The limited data available in the literature fits with this hypothesis [13–15]. The differences can be large, for example, the deblocking temperature for a pentachlorophenol blocked IEM copolymer in benzene is reported to be 144◦ C, and in tetrahydrofuran 65◦ C [15]. For p-nitrophenol phenyl urethane it was shown (see Table 2) that the combination of solvent polarity and how protophilic the solvent is, control the homolytic dissociation [14]. The major exception to this is when nitrobenzene is used as a solvent. Its lower dissociation rate was attributed to poor hydrogen-bonding potential. Acetone, with the strongest measured hydrogen bond between the solvent carbonyl and the NH of the urethane, shows the fastest dissociation. There is need for further study of the media effect since proper solvent choice could affect package stability of 1K blocked isocyanate coatings and the hydrogen-bonding potential of coreactants could affect cure rates. 3.4. Catalysis Catalysts are usually included in blocked isocyanate formulations but commonly without consideration of what reaction or reactions they are involved in. Typically the same ones used in unblocked 2K polyurethanes are used with the blocked systems, though at higher levels. As indicated earlier, there are many reactions taking place and the catalyst can be involved in any one or more of these:
3.3. Effect of reaction medium In at least some cases, hydrogen-bonding potential of the reaction medium can have an effect on reaction rates. It is well known that reaction rates of isocyanates with alcohols and phenols are up to two orders of magnitude more rapid in weak hydrogen bond-acceptor solvents than strong hydrogen bond-acceptor solvents. Since strong hydrogen bond accepting solvents reduce k−1 , deblocking reactions would
• the deblocking reaction, • the reaction of the free isocyanate with the other nucleophile, • an addition–elimination reaction • side reactions. Engbert et al. [16] looked at the curing of an IPDI polyisocyanurate/triol system used in conjunction with a number of different blocking agents. DMA was used to track the minimum temperatures at which cross-linking starts between a trifunctional polyether with the blocked IPDI polyisocyanurate resins.
D.A. Wicks, Z.W. Wicks Jr. / Progress in Organic Coatings 43 (2001) 131–140
The study also included each system with and without catalyst. As seen in Fig. 7, there were significant differences in the systems after catalyst addition. The inclusion of 1% DBTL by weight, moved the cross-linking temperature of the dimethyl pyrazole blocked product from a not very exciting 158◦ C to the lowest of the deblocking isocyanates at 112◦ C. There is some overlap between this study and the previous one with both MEKO and caprolactam giving similar results in both cases. Such an effect was shown by Carlson et al. who studied the effect of DBTL on the deblocking of IPDI based cross-linkers without a coreactant. In this case the DBTL showed no catalytic effect on the actual deblocking reaction but did, over a longer time lead to a decrease in isocyanate concentration as a result of side reactions [17]. DBTL can catalyze the reaction of the freed isocyanate and a hydroxyl group and can be a mild allophanate catalyst. Many catalysts have been suggested for accelerating the curing rate of blocked isocyanate systems. Table 3 gives the results of screening a series of catalysts for the reaction of MEKO blocked HDI isocyanurate and a hydroxy-functional acrylic resin [18]. It should be noted that the films were intentionally undercured (20 min at 130◦ C) to emphasize the differences in catalytic activity. Several other blocked isocyanates were also tested, and the authors concluded that of the catalysts tested, bismuth tris(2-ethyl hexanoate) gave the most consistent catalyst response.
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Diazabicyclo[2,2,2]octane (DABCO) is commonly used as a catalyst; it is a better catalyst for oxime blocked isocyanates than triethylamine but quinuclidine is still better. Still stronger catalysis is obtained with the bicyclic amidine, 3,3,6,9,9-pentamethyl-2,10-diazabicyclo[4,4,0]dec-1-ene; it was found to be more effective than quinuclidine with oxime blocked aliphatic isocyanates but less effective than DABCO with aromatic isocyanates. It was also shown that the catalysts had the same relative effect on accelerating the cure rate of the deblocked analogs. It was suggested that concerted bifunctional catalysis might be the reason for the observed effectiveness.
A study of catalysis of benzyl alcohol blocked 4-isocyanatomethyl-1,8-diisocyanatooctane (TTI) has shown that, at least in some cases, there is an optimum tin concentration. Higher concentrations led to less catalytic effect as determined by number of MEK double rubs at temperatures lower than required to achieve 200 double rub resistance with films of a hydroxy-functional acrylic resin with benzyl
Fig. 7. Deblocking temperatures from [16].
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Table 3 Catalyst screening with MEKO blocked HDI isocyanurate [18] Catalyst
Metal % on solids
Film appearance
MEK 2X rubs
No catalyst Dibutyltin dilaurate Dibutyltin dilaurate Dibutyltin diacetate Bismuth tris(2-ethylhexanoate) Aluminum dionate complex Cobalt bis(2-ethylhexanonate) Zr bis(2-ethylhexanoate) Zn bis(2-ethylhexanoate) Ti tetra(ethyl acetoacetate) Calcium bis(2-ethylhexanoate) Chromium tris(2-ethylhexanoate)
0 0.05 0.09 0.18 0.065 0.04 0.05 0.18 0.048 0.50 0.05 0.255
Clear Clear Clear Clear Clear Clear Hazy Clear Clear Yellow Clear Hazy
0 43 46 38 51 19 51 12 35 52 24 43
alcohol blocked TTI baked for 30 min at 140◦ C (Table 4) [19]. In waterborne systems such as cationic E-coat, stability of the catalyst towards hydrolysis is particularly important. The hydrolytic stability of DBTDL is inadequate. Dibutyltin oxide exhibits good stability and has been commonly used, but it is a pigment and must be dispersed with the other pigments and is not a highly effective catalyst. Many other catalysts have been patented for use in E-coat. For example, bis(tributyltin)oxide is reported to be a more efficient catalyst and more easily incorporated than dibutytin oxide [20]. 3.5. Effects of functionality During the curing of blocked isocyanate coatings the deblocking and subsequent cross-linking reactions rarely go to 100% completion. Typically, bake schedules are prescribed that given a certain level of reaction that yields optimum properties. One cannot a priori predict what level of reaction is required to get the optimum performance. There are no mathematical models applicable to systems in general that can be used to estimate the effect of functionality on the attaining of properties. However, one can look at the gel point of the systems being studied. Usually the gel point is reached before the optimum properties are. Using well-established calculations one can get an idea of the degree of effect that the changes within the system may have. The degree of functional group
conversion (Pc ) required to get to the gel point can be estimated through use of Eq. (5). Where the average functionality is favg = ni fi / ni . Pc =
2 favg
(5)
All other aspects remaining equal, it is possible to decrease the amount of time required for cure by increasing the functionality of the cross-linker or polyol. For example, Fig. 8 shows the change in calculated Pc in curing a polyester of average functionality four with increasing functionality of the cross-linker. Thus, if one were to change from a difunctional blocked isocyanate to one that is tetrafunctional; the extent of reaction required to reach the gel point would reduce from more than 70% for the difunctional to approximately 50% for the tetrafunctional. 3.6. Effect of blocking groups After reading through thousands of abstracts, patents and publications, it would be satisfying to say that we could draw up a conclusive ranking of the blocking agents in terms of cure temperature. However, this is not the case, many other variables cloud the assignment of a definitive deblocking temperature. Some general guidance may be drawn from the published comparisons available.
Table 4 Effect of catalyst concentration on MEK resistance of films of hydroxy-functional acrylic resin with benzyl alcohol blocked TTI and tetra-n-butyl-1,3-diacetoxydistannoxane (Sn = 39.5%) baked at 140◦ C for 30 min Level of catalyst (Sn content, phr)
MEK rubs
0.10 0.20 0.40 1.20
30 130 105 55
Fig. 8. Critical conversion (Pc ) to gel point as a function of changing cross-linker functionality, assuming a constant average coreactant functionality of four.
D.A. Wicks, Z.W. Wicks Jr. / Progress in Organic Coatings 43 (2001) 131–140 Table 5 Rate constants for deblocking of ketoxime blocked isocyanates (k values were given in the increased rate of deblocking from top to bottom) Oxime of
k (×10−5 s−1 )
Methyl n-amyl ketone Methyl isoamyl ketone Methyl 3-ethylheptyl ketone Methyl 2,4-dimethylpentyl ketone Methyl ethyl ketone Cyclohexanone Methyl isopropyl ketone Methyl isobutyl ketone Diisobutyl ketone Methyl t-butyl ketone Diisopropyl ketone 2,2,6,6-Tetramethylcyclohexanone
4.7 5.8 8.3 9.4 10.0 10.3 11.8 12.0 15.2 19.9 21.0 300.0
The structure of blocking groups has a major effect on deblocking temperatures and cure rates of coatings. Here, we review blocking groups from the point of view of understanding as well as possible with the information available, the relationships between blocking group structures and reactivity of blocked isocyanates. There are other important aspects in addition to reactivity involved in the choice of blocking groups, these generally are related to a particular application. Deblocking temperatures are dependent on steric effects. Table 5 gives the first-order rate constants for deblocking for a series of oximes (with caprolactam for comparison purposes) when solutions of blocked cyclohexane isocyanates in toluene were heated to 107◦ C [7] and the isocyanate was trapped with dibutyl amine. With the exception of tetramethylcyclobutanedione monooxime, rate constants increase as steric bulk increases and were found to be much greater than the deblocking rate of caprolactam (for caprolactam k = 0.92 × 10−5 s−1 ). Regulski and Thomas [21] published a broad comparison of blocking groups being used on IEM copolymers. Their study covered the cross-linking of a copolymer of methyl methacrylate, ethyl acrylate and IEM (Fig. 9) with a hydroxy-functional coreactant. The polymer was prepared from the blocked monomer. The results of their findings are shown in Table 6. The systems were catalyzed with 0.5 wt.% dibutyltin diacetate, and the reported temperatures are when the system
139
Table 6 Crosslinking temperatures of specific IEM derivatives Group
Specific derivative
Temperature
Phenol
Methyl salicylate Methyl p-hydroxybenzoate
110–120 130
Imidazole
Imidazole
110–130
Oxime
MEKO Acetone oxime
140–150 130–140
N-hydroxyimides
N-hydroxyphthalimide N-hydroxysuccinimide
170–175 160
Alcohols
Methoxypropanol Ethylhexanol Pentol Ethyl lactate
175–200 >200 200 200
Lactams
Caprolactam Pyrrolidinone
175–200 250
Ethyl acetoacetate
Ethyl acetoacetate (never really cured)
>175
developed MEK double rub resistance after 30 min in an air flow oven. Using monomeric blocked TMI, Lucas and Wu [11] measured the appearance of the NCO band at 2260 cm−1 as a function of temperature, without a coreactant present. They made two sets of observations for each system, the first being the initial appearance of the isocyanate band and the second being the temperature at which rapid generation of NCO is noted (Table 7). The most significant observation is the low onset temperatures that they found for the oxime blocked monomer. Their value for the MEKO adduct is 50◦ C lower than was reported for TMXDI being studied by TGA. The discrepancy may arise from the low volatility of the MEKO, in the TGA measurement, it must volatilize to be seen. This may not happen at an appreciable rate until 100◦ C. The authors reported problems polymerizing the blocked monomers in the presence of hydroxy-functional monomers due to premature cross-linking. This is traceable to the deblocking of the isocyanate at the polymerization temperatures of 60–100◦ C. They also checked the “cross-linking” temperature by looking at copolymers containing both the blocked TMI moiety and hydroxyethyl acrylate (Fig. 10) in the presence of 0.5% dimethyltin dilaurate. The reported “minimum cure temperature” is that which gave MEK double rub resistance after 30 min of cure. The self-cross-linking results do not match either of the observations for NCO appearance, but they do follow the trend with oximes being the lowest and the alcohols being the highest in terms of deblocking temperatures. 4. Outlook
Fig. 9. Blocked IEM copolymer used by Regulski and Thomas.
As discussed in this paper and dealt with more fully in Ref. [1], substantial progress has been made in understanding
140
D.A. Wicks, Z.W. Wicks Jr. / Progress in Organic Coatings 43 (2001) 131–140
Table 7 Deblocking temperatures of various blocked TMI samples Group
Specific derivative
Temperature (◦ C) NCO onset
Rapid NCO
Self-cross-linking
Oxime
MEKO Acetone oxime Cyclohexanone oxime
50 50 65
65 80 100
<120
Lactams
Caprolactam
90
125
130
N-hydroxyimide
N-hydroxysuccinimide
115
140
130
Alcohols
Methanol n-Butanol n-Pentanol n-Hexanol Propylene glycol
155 165 165 155 >180
190 190 190 175 –
>145 >145
–
Fig. 10. Self-cross-linking blocked TMI copolymer studied by Lucas and Wu.
the complexities of the reactions of blocked isocyanates. As we cover in Ref. [2], there has been a marked increase in the use of blocked isocyanates. Not only are they major cross-linkers in coatings such as cationic electrodeposition primers and powder coatings among others, they are also used or are being investigated as cross-linkers in a wide array of other end uses. The need now is for a better understanding of the effect of various combinations of variables on the cure rates. There is need for better understanding of interactions when more than one variable is changed. For example, the effect of simultaneously changing isocyanate, blocking group, catalyst and catalyst concentration, temperature, coreactant structure, and medium effects of both solvents during storage and coreactant backbones during curing needs further study. It is to be expected that new experimental techniques will be required to fully understand the complex interactions. References [1] D.A. Wicks, Z.W. Wicks Jr., Prog. Org. Coat. 36 (1999) 148. [2] D.A. Wicks, Z.W. Wicks Jr., Prog. Org. Coat. 41 (2001) 1. [3] L.A. Flood, R.B. Gupta, R. Iyengar, D.A. Ley, V.K. Pai, US Patent 5 792 866 (1998). [4] A. Chen, L. Katz, R. Wojcik, J. O’Connor, in: Proceedings of the Waterborne, High-solids, Powder Coating Symposium, New Orleans, LA, 1996, p. 103.
[5] J.-J. Lin, G.P. Speranza, M. Cuscurida, Ind. Eng. Chem. Res. 36 (1997) 4231. [6] Y. Huang, G. Chu, M. Nieh, F.N. Jones, J. Coat. Technol. 67 (842) (1995) 33. [7] J.S. Witzeman, Prog. Org. Coat. 27 (1996) 269. [8] Th. Engbert, E. Konig, E. Jurgens, Farbe Lack 102 (7) (1996) 51. [9] S. Mohanty, N. Krishnamurti, Eur. Polym. J. 34 (1998) 77. [10] P.L. Jansse, J. Oil Colour Chem. Assoc. 89 (1989) 478. [11] H.R. Lucas, K.-J. Wu, J. Coat. Technol. 65 (820) (1993) 59. [12] I. Muramatsu, Y. Tanimoto, M. Kase, N. Okoshi, Prog. Org. Coat. 22 (1993) 279. [13] C.B. Reilly, N. Orchin, Prepr. ACS Div. Paint Plast. Printing Inks, Atlantic City, NJ, 1956. [14] M.S. Fedoseev, G.N. Marchenko, G.A. Sal’nikova, in: A.M. Schiller (Ed.), Soviet Urethane Technology, Vol. I, Technomic, Westport, CT, 1973, p. 237. [15] P.C. Boyer, Ph.D. Dissertation, Univ. So. Miss., 1986, p. 74. [16] Th. Engbert, E. Jurgens, E. Konig, FATIPEC Congr., 23rd (Vol. B), 1996, pp. B115–B143. [17] G.M. Carlson, C.M. Neag, C. Kuo, T. Provder, Polym. Sci. Technol. 36 (Fourier Transform Infrared Charact. Polym.) (1987) 197. [18] W.J. Blank, Z.A. He, M.E. Picci, Polym. Mater. Sci. Eng. 79 (1998) 399. [19] H.P. Higginbottom, G.R. Bowers, L.W. Hill, J.F. Courtier, Prog. Org. Coat. 34 (1998) 27. [20] V. Eswarakrishnan, R.R. Zwack, E.R. Coleridge, US Patent 5 356 529 (1993). [21] T. Regulski, M.R. Thomas, Org. Coat. Appl. Polym. Sci. Proc. 48 (1983) 1003.