Polyols as solvents A new route to NMP-free high-performing PUR-dispersions for various applications. Waterborne polyurethane dispersions (PUDs) combine excellent performance with low solvent content and can offer a wide range of properties. Until recently, almost all PUDs contained N-methyl pyrrolidone (NMP) which will in future be subject to labelling requirements. A new route to NMP-free PUDs uses polyols which themselves have solvent properties. These resins are shown to have excellent properties. Dirk Mestach. Waterborne polyurethanes are one of the first choices of binder for high quality and demanding applications. They can be formulated into environmentally acceptable coatings containing low amounts of volatile organic components (VOC). The nature of the polymeric backbone makes them both hard and flexible, and so ideal for use in scratch resistant, hard-wearing coatings. These properties are the result of inter- and intra-chain interactions between polyurethane chains, attributable to hydrogen bonding between the urethane and urea linkages in the polymer backbone. Waterborne polyurethanes can be used in both one- and two- component and radiation curing coatings and may be either thermoplastic or (self-) crosslinking. Production processes for polyurethane dispersions Regardless of the application, the general synthesis route for a polyurethane dispersion is that a polymeric diol, for example a polyester, polyether or polycarbonate diol, is reacted with a hydrophilising diol and a diisocyanate. Depending on the ratio between the diols and the diisocyanate, the polyurethane obtained can be either isocyanate (NCO) or hydroxy functional. Where the polyurethane is isocyanate functional, it is often referred to as a polyurethane prepolymer. The polyurethane is then dispersed into water either directly or by means of the phase-inversion emulsification process. If the polyurethane is NCO-functional it can be chain extended after dispersion using a diamine. Optionally, a chain stopper can be used to react with the NCO functionality. This chain stopper can be used to introduce functional groups, for example for crosslinking of the polyurethane after drying of the coating. Solvent choice for PUD manufacture is very limited In most waterborne polyurethanes used in the coatings industry, at least part of the hydrophilising diol is dimethylol propionic acid (DMPAc). This has a hindered carboxylic acid group which is less reactive than most acid groups, and it therefore reacts as a diol. The free acid group can be neutralised with a base to make the polyurethane water dispersible. Until recently, waterborne polyurethanes generally contained a certain amount of volatile organic solvents, which are used in the synthesis to reduce the (pre-)polymer viscosity and also to aid in dissolving DMPAc in the polyols prior to the addition of the diisocyanate. Not all solvents can be used for this as they have to be aprotic (non-reactive with isocyanates) and hydrolytically stable. This limits the choice to ketones, (cyclic) ethers and some amines and amides. The solubility of DMPAc in these solvents varies greatly, and until recently N-methyl-2-pyrrolidone (NMP) was the preferred solvent. The trouble with NMP A problem associated with NMP is the fact that it cannot be removed by distillation after dispersing the polyurethane in water. It therefore remains in the dispersion and functions as coalescing aid. The classification of NMP as "toxicologically questionable" is currently being discussed by the European Union. It is proposed that products containing more than 5% NMP will have to be labelled as being irritant and toxic (Xi: R36/37/38, T: R 61). In the United States, California's Proposition 65 also requires special labelling of products containing NMP while other states and countries may follow. There is therefore a worldwide need to replace or eliminate NMP from polyurethane dispersions. Alternative production routes - advantages and drawbacks Several approaches have been used to replace NMP in the manufacture of polyurethane dispersions. A straightforward replacement by N-ethyl-pyrrolidone (NEP) is suggested by some companies [1]. Even though it is claimed that NEP is a useful alternative and that its toxicological profile is favourable compared to that of NMP, this does not appear to be a sustainable option. Other manufacturers have modified the well-known acetone process [2]. An important problem associated with the acetone process is that the solubility of DMPAc in acetone is virtually zero. Neutralising the carboxylic acid group of DMPAc with triethylamine, however, raises its solubility considerably [3]. A drawback of this process is that large amounts of acetone have to be stripped from the polyurethane after it has been dispersed. In order to obtain an economically feasible process, the acetone has to be recycled and used again. As acetone has a very low flashpoint, some producers prefer to work with methyl ethyl ketone (MEK). Another way to increase the solubility of DMPAc in the reaction mixture is to cap the hydroxyl groups with ε -caprolactone. This modification converts the crystalline material into a soft waxy material with a low melting point and an enhanced solubility [4]. A drawback of this route is that the hard diisocyanate-dimethylol propionic acid-diisocyanate segment now becomes a soft segment. Therefore the polyurethane has to be completely redesigned in order to obtain the desired coating properties. A further way to produce solvent-free polyurethane dispersions is to use an ethylenically unsaturated monomer as a temporary solvent in the polyurethane synthesis. Esters of methacrylic acid are most suitable for this. These "solvents" are emulsion polymerised after the polyurethane is dispersed into water by adding a suitable initiator such as a persulphate or a hydroperoxide in combination with a reducing agent. Optionally, additional monomers can be added at this stage. This synthesis route leads to hybrid urethane-acrylic dispersions. This approach, however, does not solve the DMPAc solubility problem completely, because methacrylic esters are relatively poor solvents for DMPAc. The new aproach: Polyols assist in dissolving DMPAc A novel production route to NMP-free polyurethane dispersions has now been developed, which uses only small quantities of auxiliary process solvents. These solvents can be almost completely removed after production of the dispersion. The key to this process is the in-house development of polyol building-blocks that assist in
dissolving the DMPAc. Using this new process, several types of polyurethane dispersions were developed: - Chain extended high molecular weight polyurethane dispersions ("Setaqua PU-1") - Low molecular weight fatty acid modified polyurethane dispersions ("Setaqua PU-2"); - Self-crosslinking urethane-acrylic dispersions ("Setaqua UA"). High molecular weight polyurethane dispersions are most frequently used in combination with acrylic dispersions, where their main purpose is to improve the mechanical properties of the acrylics. Common applications can be found in industrial wood coatings. The chain extended dispersions, however, can also be used as the main binders in both one- and two-component coatings. Applications include parquet, furniture and plastic coatings. Cross-linkers such as polyaziridines or water-dispersible polyisocyanates can be used to enhance their performance. Fatty acid modified polyurethane dispersions are a very versatile class of binders that find application in both decorative and industrial coatings. Because of their relatively low molecular weight, they offer excellent flow and levelling. In wood primers or sealers, they allow good wetting and penetration of the wood. When used for decorative applications, such as in do-it-yourself trim paints, they offer good wet-edge and open time. Because of the fatty acid modification, the hardness and chemical resistance properties build up after drying of the coating. Most of these dispersions still contain free hydroxyl groups which can be used for reaction with, for example, water-dispersible polyisocyanates. Once again this boosts the performance of the coating. Self-crosslinking urethane-acrylic dispersions offer synergistic properties compared to simple physical blends of polyurethane and acrylic dispersions. Their applications are again numerous, ranging from parquet finishes to decorative enamels. Some application examples will now be given for these novel binders. Blends with acrylics produce good parquet finishes Parquet floors are currently in fashion. In addition to factory finished systems, many parquet floors are coated after installation. The lacquers used must meet a number of criteria: they must be non-yellowing and have a high chemical resistance, as well as fair abrasion resistance. From the application point of view, they must be fast drying and have a low odour. These lacquers can be based solely on a polyurethane, but most commonly this is combined with an acrylic dispersion. Not only does this result in the improvement of a number of coating properties, but it also brings economic benefits. In the trial one-component formulations, blends of the new chain extended PUD were produced with a self-crosslinking, surfactant-free acrylic ("Setaqua XL") and a thermoplastic acrylic ("Setaqua TP") dispersion. Both acrylic dispersions have a minimum film formation temperature of about 15 C. Test lacquers were based either on pure polyurethane, a blend with 30 % (w/w) of either of the acrylics and a blend containing 70 % of the self-crosslinking acrylic. (The corresponding blend with 70 % thermoplastic acrylic was hazy and not compatible). The formulations used are given in Table 1. It should be noted that the levels of cosolvent used in the formulations was not constant, but was based on the minimum film formation temperature (MFFT) of the binder combination. Films of the varnishes were applied onto glass (dry film thickness ca. 30 µm). The films were allowed to dry at ambient temperature (23 C) and the hardness was measured after one and seven days. These results are shown in Figure 1. The coatings were also applied onto oak veneer (150 µm wet layer thickness) and the dust-dry and tack-free times were recorded. The results are shown in Figure 2. It is quite surprising to see that the addition of 30% thermoplastic acrylic does not affect the drying times, even though the level of cosolvent in the blend is higher. Blends with the self-crosslinking acrylic have longer drying times, although still short enough for this application. The chemical resistance properties of the dried varnishes were determined (on oak veneer, two coats after drying for seven days at ambient temperature). These results are shown in Table 2. The blend with the thermoplastic acrylic does not offer much advantage with respect to chemical properties, and some resistance properties even deteriorate. Blends with the self-crosslinking acrylic, on the other hand, offer interesting improvements in properties. The blend containing the higher level of acrylic performs particularly well. NMP-free metal primers Air-drying, fatty acid modified polyurethane dispersions are very suitable as industrially applied metal coatings. So far, however, these types of coatings have often contained NMP. A low VOC NMP-free metal primer formulation was developed and its main properties were studied. Good adhesion is crucial for a primer, and the tests showed that adhesion was excellent on virtually all metal substrates. Primers based on this resin, designated "Setaqua PU-2" can easily be overcoated with both one- and two- component water-borne and solvent-borne topcoats. Even the primer alone shows very good salt spray resistance (Table 3). Urethane-acrylic hybrids Urethane-acrylic dispersions offer advantages over simple blends of a polyurethane dispersion and an acrylic dispersion. Because the acrylic part is polymerised in the presence of the dispersed polyurethane, grafting reactions occur, resulting in the formation of true hybrid particles where the polyurethane and acrylic polymer chains are present in one particle. This is clearly shown in Figure 3, where atomic force microscopy picture of a film cast from such a hybrid is shown. The acrylic-urethane hybrid polymer is modified with carbonyl groups in order to cross-link via the reaction with a polyhydrazide component. As described in numerous previous papers, it takes about one week at ambient temperature for such a system to reach full conversion. Dynamical Mechanical Thermal Analysis (DMTA) was used to study the mechanical properties after crosslinking (see Figure 4). Even though only one type of particle can be seen using AFM, the DMTA plot shows different transitions, suggesting a core-shell like morphology. A clear furniture lacquer can be formulated using the formulation shown in Table 4. The coating, applied at a wet layer thickness of 150 µm, showed a Persoz hardness of 150 s after only one day of drying at ambient temperature. After one week, hardness had increased to 200 s, indicating that full crosslinking had taken place. Chemical resistance properties were tested after 2 and 7 days of drying. The results are given in Table 5. As the table shows, resistance properties are quite satisfactory after short drying times and excellent after full cure has been obtained. REFERENCES [1] K. Ott et al, Patent Application WO2005/090447 A2 to BASF AG, 2005.
[2] The Bayer Scientific Magazine, issue 17, p 92, 2005 (Internet: http://www.research.bayer.com/medien/pages/4003/wood_c oating.pdf) [3] H. Schurmann, J. Bung, H. VanAlsten, US Patent 4,096,127 to Akzona Inc., 1978. [4] R. L. Scriven, W. Chang, US Patents 4,066,591 and 4,147,679 to PPG Industries Inc. 1979. Results at a glance - Waterborne polyurethanes (PUDs) combine excellent performance with low solvent content and can be produced with a wide range of properties. - Until recently, almost all PUDs contained N-methyl pyrrolidone (NMP) which will in future be subject to labelling requirements. - A new route to the production of NMP-free PUDs has been developed, by using novel polyols which themselves act as solvents. - Test results on these resins and their blends with acrylic dispersions show that a good range of properties can be obtained in wood finishes and metal primers. - Hybrid systems, in which acrylates are polymerised in the presence of the PUD, can provide even better performance than blends of PUD and acrylic resins. The author: -> Dr. Dirk Mestach obtained his doctorate in polymer chemistry at the University of Gent (Belgium). In 1989 he joined Akzo Nobel, first in Belgium in the coatings division and later on with Akzo Nobel Resins, today Nuplex Resins, in the Netherlands. He has been active in the development of waterborne binders for the coatings and printing inks industry. At present he is R&D manager at Nuplex Resins. This paper was presented at the European Coatings Conference "Polyurethanes for High Performance Coatings IV", Berlin, 23/24 March 2006
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Figure 2: Dust-dry and tack-free times as well as co-solvent amounts of parquet varnishes.
Figure 3: Atomic force microscopy views of a waterborne urethane acrylic hybrid resin ("Setaqua UA"); left: topographic, right: tapping mode.
Figure 4: Dynamic mechanical thermal analysis on the waterborne urethane acrylic hybrid resin "Setaqua UA" (frequency 11 Hz) after drying at ambient temperature for 7 days.
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