Chapter 12
Imine—Isocyanate Chemistry: New Technology for Environmentally Friendly, High-Solids Coatings Downloaded by STANFORD UNIV GREEN LIBR on May 12, 2013 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0640.ch012
Douglas A. Wicks and Philip E. Yeske Industrial Chemicals Division, Coatings Research, Bayer Corporation, 100 Bayer Road, Pittsburgh, PA 15205-9741
The chemical industry faces a major challenge in the design and manufacture of new environmentally-friendly chemicals. Within the coatings industry, these new chemicals must play a role in pollution prevention as well. The need for development of new raw materials which reduce solvent demand and yet maintain coating performance is of primary importance. The inherent high reactivity and toxicity of primary amines has limited their applicability in low volatile organic compound content (low VOC), high solids coatings. This paper details the design and use of ketimines and aldimines as blocked primary amines in such formulations. Characterized by low toxicity, viscosity and reactivity relative to the parent amine, imines are shown to be excellent reactive resins for high solids coatings. Aspects of imineisocyanate chemistry presented include the direct reaction of imines with polyisocyanates as well as the relative hydrolytic stability of aldimines versus ketimines. The impact of each of these aspects upon pollution prevention is also discussed. This paper demonstrates how safer chemicals can be designed without affecting efficacy.
As the 1990's unveil a new era of increasingly stringent environmental and safety regulations in conjunction with increased consumer demands for ecologically friendly materials, many traditional coatings systems are being eliminated as surviving systems undergo radical changes. These changes are needed to overcome traditional coatings technologies' dependence on high solvent content, and thus high volatile organic compound (VOC) content, to facilitate and control the coatings application. The technology changes now taking place in industrial markets such as automotive refinish, maintenance, architectural and OEM applications, are being primarily driven by regulatory pressures but are also coupled with increased customer demands for performance and value. VOCs undergo photochemical decomposition in the 0097-6156/96/0640-O234$15.00/0 © 1996 American Chemical Society
In Designing Safer Chemicals; DeVito, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
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environment to a variety of products that lead to the formation of smog, and contribute significantly to overall air pollution. The push to reduce emissions of VOCs has shifted resin development towards a variety of approaches as shown in Table I.
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Table I. Approaches to Low VOC Coatings Approach 1. UV cure 2. Powder 3. Waterbome 4. High Solids
Primary Characteristic using a polymerizable monomer as the solvent using heat to induce flow and crosslinking using water as solvent low viscosity reactive resins
Approaches to Low VOC Coatings Currently, the uses of UV or Powder coatings are limited as both require specialized application and cure equipment. They do, however, find use in factory situations where the applications are specifically controlled. UV cure is not applicable to pigmented coatings or for irregular shaped substrates, since in both these cases the UV radiation cannot penetrate to give an evenly cured film. Powder coatings have a major drawback in their requirement for high bake temperatures (>150 °C). These high temperatures limit the number of substrates which can be coated and have the additional disadvantage of increased energy demand and the environmental problems associated with increased fossil fuel combustion to meet the demand. Waterbome coatings have found wide acceptance as low VOC systems. In these products water acts as the carrier solvent, and film formation is facilitated by addition of coalescing solvents or through the use of low glass transition temperature resins. Consumer architectural latex paints are one successful example of how a waterbome coating can effectively displace organic solvent borne systems without a decrease in product performance or ease of application. Nevertheless, water still carries with it several drawbacks which limit its use as a universal solution to VOC reduction. First, it is a reactive species which causes hydrolysis of resins and interferes with many common crosslinking reactions. Second, water has a fixed evaporation rate so there is a significant dependence of drying speed and the final quality of coatings applied on curing temperature and relative humidity. High solids reactive coatings bring relief from many of the difficulties mentioned above. They are the preferred method for producing high quality, weatherable, chemically resistant coatings. In many industries where high performance in terms of durability, solvent/chemical resistance or weatherability is required, high solids polyurethanes are becoming the coatings of choice. Though high solids, 2-component polyurethanes represent a robust, viable route for significantly reducing emissions while still yielding high performance coatings, formulators are beginning to reach the limit of effectiveness with currently available technology. Further solvent reductions must now be realized by reducing
In Designing Safer Chemicals; DeVito, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
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the molecular weight of the paint components, which concurrently reduces their viscosity and in turn their demand for organic solvents. One key effect of this molecular weight reduction is that the resins employed are no longer solid products at 100% concentration so the coating no longer lacquer dries but must dry through chemical reactions. Based on the limitations described above, the ideal chemistry for a high-solids crosslinking coating would be one in which the: • rate of reaction is suppressed before application (extended pot-life), • the rate of reaction is greatly accelerated after application (fast drytimes)and • the method of suppression/acceleration does not generate VOCs. We have found that high solids coatings that utilize imine-isocyanate based systems over amine-isocyanate/hydroxyl-isocyanate systems meet the regulatory demands of developing less hazardous substances, and the customer demands for product performance. Imine-isocyanate systems significantly reduce the VOCs in a coating when compared with current systems and they do not utilize primary amines, which are generally ecotoxic. The systems developed by us are less ecotoxic than amine-isocyanate based systems because the imines are stable to water hydrolysis and do not generate primary amines when combined with water. This paper will describe our approach to developing imine-isocyanate based systems as environmentally friendly, equally efficacious alternatives to existing coatings systems. Background Polyurethane Coatings. Within the coatings industry, the term "urethane" has evolved to describe a broad and often intermixed family of chemistries derived from reactions of the isocyanate group (R-N=C=0). Still, the fundamental chemical reaction taking place in urethane coatings is that of an isocyanate with an alcohol. By employing polyfunctional alcohols (polyols) and polyfunctional isocyanates (polyisocyanates) one obtains crosslinked polyurethanes (Figure 1). This reaction proceeds at a reasonable, well defined rate and can be further controlled by catalysis with tertiary amines, various organo-tin compounds and metal salts (1).
O R-NCO+ R -OH f
II
R - N C - O R ' (Urethane) H
Polyurethane Figure 1. Urethane Formation From an Alcohol and an Isocyanate. Polyisocyanates. Analysis of the solvent requirements of the individual components in a urethane coating makes apparent that the polyisocyanate portion is
In Designing Safer Chemicals; DeVito, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
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the high solids component of the coating (2). Figure 2 shows the viscosity of a typical polyisocyanate solution as a function of percent solids. The commercial polyisocyanate shown here has a neat viscosity of 3,500 mPa»s, which when reduced to a typical application viscosity of 100 mPa»s gives a solution of 84% non-volatile content, (i.e., 16% solvent by weight). This value may be compared to a 50-60% organic solvent demand for typical polyisocyanate coreactants (this will be described in more detail in the next section).
10000 HDI Isocyanurate Polyisocyanate
1000 1 T| mPa«s 100 -
i
10 40
i
60 80 Percent Solids
100
Figure 2. Viscosity of an HDI Isocyanurate Polyisocyanate as a Function of Percent Solids in a 2/1/1 Blend of MEK/MBK/Exxate 600.
(CH ) -NCO I 2
6
Y** Y OCN-(CH ) ' 2
6
N
Monoisocyanurate ofHDI X
(CH ) -NCO 2
6
Hexamethylene diisocyanate (HDI)-based polyisocyanurates and biurets are being commercially pushed to lower viscosities, with the practical limitations of these products being 1000 and 2000 mPa»s, respectively. Still newer types of polyisocyanate resins are pushing the viscosity envelope below 500 mPa s (3,4). These materials yield spray able solution viscosities (100 mPa»s) with non-volatile contents approaching 100%. For hygiene and practical purposes, it is important to remember that all of the commercial and developmental polyisocyanate adducts used in high solids coatings are #
In Designing Safer Chemicals; DeVito, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
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free of monomelic diisocyanate and that the products derived from the oligomerization of HDI are at least difunctional. Hydroxy Functional Coreactants. Acrylic polyols have been favored historically for most applications because of their superior weathering, low cost and low isocyanate demand. These products have traditionally been of relatively high molecular weight (10-20,000 g/mol) with hydroxy 1 equivalent weights of 1,000 or more. With the large number of hydroxyls present on the polymer, only a moderate degree of reaction is required to get complete network formation. Because these products are solids (without crosslinking), drying of the product was not a major concern. The major drawback of high molecular weight acrylics is their high solvent demand, yielding coatings with only 40-50% solids at application viscosities. The first step taken toward high solids OH-functional coreactants was the development of lower molecular weight acrylics and polyesters with reduced functionality. These products, with molecular weights typically below 2,500 g/mol, have been successful in providing high quality coatings with solids of 55-65% at application viscosity (for example, see Figure 3). 10000
1000 #
T| mPa s 100
io -f 40
1
1
60 80 Percent Solids
f 100
Figure 3. Viscosity of Hydroxy Functional Coreactants as a Function of Percent Solids in a Blend of Methylethyl Ketone/Methyl Isobutyl Ketone/Exxate 600. Great strides have been made in reducing solvent demand by lowering the molecular weight and changing the molecular architecture of acrylic and polyester polyols, but additional gains will be difficult without sacrificing application properties. For example, further reduction of molecular weight below current levels yields, in the case of acrylics, an unacceptable fraction of monofunctional or nonfunctional polymer chains (5) and in the case of polyesters exceedingly long dry times and/or soft coatings. Clearly new chemistries for and approaches to low viscosity coreactants for polyisocyanates are needed.
In Designing Safer Chemicals; DeVito, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
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Polyurea Coatings. The reaction of isocyanates with amines or polyamines occurs to form ureas and polyureas. Typical primary and secondary aliphatic amines and primary aromatic amines react very rapidly with isocyanates at ambient temperatures (Figure 4). In most cases this reaction is too fast to be useful in coatings applications unless very specialized application equipment and techniques are used. For example, one commercial polyurea system is formed by the reaction of amino terminated poly ethers with aromatic polyisocyanates (6). In this system, the reactants are mixed under high pressure at the tip of the spray gun and cure within seconds of application. This limits their widespread use in that die equipment is very expensive and the rapid cure precludes the flow and leveling required to make a high quality finish.
R-NCO+R'-NHs
•
o II R—NC—NR (Urea) f
H R-^NCO) + R'-^NH^
H
• Polyurea
Figure 4. Urea Formation From an Amine and an Isocyanate. Aspartic Acid Esters. Recently there have been advances in hindered amine chemistry which allow amines to be used in high solids coatings without the use of specialized equipment (7-11). These hindered amines are obtained by the Michael Addition of primary amines to dialkyl maleates. The resulting secondary amine, an aspartic acid ester, has reduced reactivity due to steric effects and hydrogen bonding. Like the parent amines, aspartic acid esters react with isocyanates to form ureas (Figure 5).
^C0 Et
^C0 Et
2
+ R R '' --NN' R-NCO + ]n T H--0
2
> R-NC-N H
COjEt
R'
Figure 5. Reaction of an Aspartic Acid Ester with an Isocyanate. The reactivity of these amines towards polyisocyanates was found to be dependent on the structure of the parent amine. As shown below in Table II, cycloaliphatic polyaspartic acid esters yield gel times of several hours, with an additional ring methyl group extending the gel time to greater than a day due to steric hindrance. Acyclic structures, such as hexamethylene diamine, are more reactive than their cyclic analogs, but still much slower than typical primary and secondary amines.
In Designing Safer Chemicals; DeVito, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
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Table II. Reactivity of Aspartic Acid Esters Towards Polyisocyanates Et02C Et02C"^,_ H
C0 Et 2
^^COgEt H
R
R Viscosity * CH3 ,CH3 1500 -t>CH
