Making solid gains Novel acrylic and polyester polyols reduce VOCs in solventborne urethanes. Paul Vandevoorde Ad van Gaans. High solids solventborne polyurethanes can offer excellent performance, but VOC levels must be reduced to 420g/l to produce 'compliant' coatings. Different types of acrylic and polyester polyol were synthesised and their effects on coating properties examined. The combination of a low molecular weight acrylic polyol and a polyester reactive diluent allows hard clearcoats to be produced with low VOC content even at low spraying viscosities. New environmental and health regulations are changing the coatings world irreversibly. In April 2004 the European Parliament approved a new Directive (2004/42/CE), prescribing the maximum allowed VOC contents in decorative and automotive refinishing paints. What are the most appropriate responses to this legislation? Low VOC solutions: a range of options Low VOC coatings are generally high solids solventborne, waterborne, radiation curing or powder coatings. The choice depends on the needs of the customer, application facilities and local circumstances. Powder coatings require high temperature stoving and thus are obviously unsuitable for vehicle refinishing. Radiation (UV) curing coatings are normally applied at 100% solids. The first UV-curable products for vehicle spot repair work have been announced recently. Under well-controlled application and drying conditions, waterborne coatings can perform at almost the same level as their solventborne counterparts. Waterborne 2K PUR is increasingly used in demanding markets including automotive OEM and refinishes [1]. For the coatings user, higher solids have the distinct advantages that the chemistry, application, cure behaviour and film performance are similar to those of older products. However, to meet legislative requirements, VOC levels for 'compliant coatings' must not exceed 420 g/l at application viscosity. The continuous demand for improved performance is equally important in coatings development. For automotive topcoats better etch and scratch resistance [2] is required as is applicability over waterborne basecoats. Many developments have also been undertaken to ensure higher productivity and profitability for the user. For 'performance' read 'polyurethane' In high-performance finishes, solventborne 2K polyurethanes are the dominant technology. Good ambient cure can be combined with attractive appearance and excellent mechanical chemical and ageing resistance. The transfer to higher solids automotive refinish coatings meeting new European legislation requires lower viscosity binder components. Design aspects and performance of new high solids polyols for 2K PUR coatings, with particular emphasis on automotive refinish clearcoats, will be explored. Viscosity reduction must focus on polyols Lower viscosity aliphatic polyisocyanate hardeners have become available. As they are the binder part with the lowest viscosity, the viscosity reduction must now come from the polyol, usually an acrylic type made by conventional free radical polymerisation and with a molecular mass Mn in the range 2500-5000 and polydispersity of around 2.5-5. With few exceptions, only acrylic polyols with Mn in the range 1500-2000 or lower have a low enough viscosity to produce compliant coatings. Low molecular mass is associated with a lower average hydroxyl functionality and a less crosslinked film with inferior properties. Increasing the fraction of hydroxy functional monomer is not a solution, because more hydrogen bonding will increase the viscosity, and demand for isocyanate will be higher giving higher formulation costs. Non-functional resin molecules act as plasticisers while mono-functional species at best will act as dangling chains attached to the network. A very high degree of conversion is needed for the new generation of low molecular polyols compared with the older polyol types having a high functionality. Using a special simulation program, the functionality distributions of acrylic polyols can be calculated. In Figure 1 the effect of molecular weight and hydroxyl content on calculated crosslink density is shown for model acrylic polyols. Pure HDI trimer is chosen as crosslinker and reaction of all NCO groups is assumed, as is the absence of side reactions other than urea formation from contact with water (Figure 1b). The significance of glass transition temperature High glass transition temperatures (Tg) promote short dustand tack-free times and early hardness, giving faster application. Higher Tg resins are always more viscous, but at low molecular weights this is less evident. The measured Tg of low molecular resins is far below the calculated value. With low molecular polyols, viscosities and VOC are lower but initial physical drying is not observed because the fluidity of the isocyanate dominates. After solvent evaporation, the coating is still a viscous liquid, although higher crosslink densities can largely compensate for this in the final stages of cure. Strategies for reducing molecular weight A number of approaches can be used to reduce the molecular weight of acrylic polyols during manufacture. Low polydispersity (expressed as Mw/Mn) is generally desirable as it means less low-functionality oligomers and a smaller high viscosity fraction. Recently, methods giving better control of radical polymerisation (including narrower molecular weight distributions) have been developed and progress continues [1]. Atom Transfer Radical Polymerisation (ATRP) has been shown to yield acrylic polyols with significantly lower polydispersity. At similar VOC levels, ATRP polyols dried faster in 2K PUR formulas and gave better initial hardness [3]. The synthesis of low molecular acrylic polyols with better controlled functionality and the performance of these binders in low VOC topcoats has recently been presented [4]. In addition, special (meth)acrylic monomers having tertiary alkyl or bulky cycloalkyl substituents have positive effects on the hardness-viscosity balance, though in most cases the high material cost is prohibitive. Balancing the properties of acrylic polyols Automotive refinish topcoats, in particular clearcoats, have to meet severe requirements. Easy application, fast cure, early hardness and fuel resistance are needed for fast repair work. Compromises on appearance, resistance or durability are not acceptable. Two new acrylic polyols were developed
in a bid to meet the conflicting requirements for this application. Reducing Tg gives lower viscosity Viscosity reduction was first obtained by a drastic lowering of Tg, giving a hydroxyl number of 170 mg KOH/gram (5.1% OH) and a molecular weight close to that of a standard more viscous medium solids polyol (referred to as MSAC). This new product is denoted as HSAC-1. Using a low viscosity HDI trimer at an NCO/OH ratio of 1.0, clearcoats can be formulated with VOC of 420 g/l at a spraying viscosity of 20 sec. DIN 4 cup. Reduced molecular mass is combined with fast drying A second acrylic polyol (designated HSAC-2) was designed combining the lowest possible molecular mass with the highest possible Tg. The aim was to equal the favourable potlife/drying time balance and high initial hardness of currently used polyols. A higher acid number was chosen, as carboxylic groups tend to moderate the catalytic effect of the usual dibutyltin dilaurate (DBTL) catalyst. Low acid numbers, as in HSAC-1, give short potlife with DBTL, though adding a volatile monoacid such as acetic acid or a volatile complex former such as acetyl acetone can help. The hydroxyl number of HSAC-2 was lowered to 150 mg KOH/gram (4.5 % OH). Stoichiometric crosslinking then means that less of the diluting isocyanate component is used, so more solvent is needed to reach the same application viscosity. Consequently the VOC level of 435-455 g/l at low spraying viscosities of 18-20 sec. (23ºC) is higher than the 420 g/l goal. The characteristics and performance properties of both HSAC-1 and HSAC-2 together with the reference polyol MSAC and a polyester (HSPE) are listed in Table 1. A low viscosity HDI trimer was used as hardener, curing conditions were 23 C at 55% relative humidity and test results relate to approximately 40µm film thickness. In comparison with MSAC, the initial hardness of HSAC-1 is significantly lower. The harder HSAC-2 practical equals the hardness of the reference material for nearly the same cost-effective low NCO demand, but the 420 g/l VOC target is not fully achieved. Reactive diluents can further reduce VOC To bring application VOC levels down to 420 g/l, various types of reactive diluents can be considered. Most commercial diluents for 2K PUR are amines or their derivatives. They react well with isocyanates, though premature deblocking of blocked amines shortens pot life and may give poor appearance due to rapid reaction of amine groups with the isocyanate. Monomeric or oligomeric polyalcohols have a lower raw material cost and react with isocyanates in a similar way to the binder polyols, though they are not the most effective diluents because they form additional hydrogen bridges. Oligoester diluents are considered the most useful type As the NCO demand of monomeric polyalcohols is high, hydroxylated oligoesters (OES) were studied. At a constant hydroxyl number of 300 mg KOH/g the following parameters were varied: - hydroxyl functionality: 2.0 to 3.0 with calculated Mn in the range 350-550; - diol type: 1.6-hexanediol (HD), 1.2-propanediol (PD), ethanediol (ED), neopentyl glycol (NPG), 2-butyl-2-ethyl propanediol 1.3 (BEPD) and cyclohexane dimethanol (CHDM). In all formulations, a cycloaliphatic anhydride was used to provide some basic hardness. The oligoesters were cured with a standard medium viscosity HDI trimer at 23 C, 50 % relative humidity. Hardness was measured on glass at 40µm dry film thickness. VOC data relates to calculated values after thinning with butyl acetate to 30 sec. DIN 4 at 23 C. Experimental VOC values (ASTM D 2369-92 method) in general are slightly lower, indicating that the fraction of free diol (up to 10 % on mass) present in all OES behaves as solid resin. The results are shown in Figure 2. Hexanediol gives the lowest viscosity and VOC rather soft films. NPG, EG and CHDM give more or less similar properties: high hardness but also high VOC values. BEPD seems to offer the best balance of VOC and hardness. Alternatively, mixtures of soft and hard diols should give better properties than individual diols. With OH-number set at 300, all OES are characterised by an isocyanate demand twice as high as for HSAC-2. For most applications in car refinishing or high performance metal topcoats the raw material cost will be significantly higher in comparison with current medium solids coatings. Studying the effects of varying oligoester functionality In a separate study, oligoesters were prepared from identical monomers but with varying hydroxyl functionality (F OH) of 2.0 to 3.0). Increased F OH is achieved by the use of more branching triols and is associated with a broader molecular weight distribution. Viscosity, hardness development and VOC were investigated to find the best possible balance for these key properties. All tests were carried out as for the first series of OES. The results in Figure 3 show that at fixed OH content, properties depend heavily on the hydroxyl functionality: increasing F OH increases viscosity due to higher polydispersity, increases hardness, but also increases VOC levels. Taking this data into account, a new low molecular polyester polyol with a lower hydroxyl content was developed, (identified as HSPE). Raw material and key resin parameters such as Tg, molecular mass and hydroxyl content were chosen so that adding HSPE effectively lowers the VOC but hardly affects the excellent initial film performance of HSAC-2. Table 2 shows test results for a number of polyol blends. The data relate to stoichiometric crosslinking with low viscosity HDI-trimer at NCO/OH = 1.0 (23 C, 55% R.H.). Choosing the optimum balance of properties HSPE levels in the range of 25 to 50 % showed the best performance balance concerning most properties. VOC levels of 420 g/l are possible, even at low spraying viscosities of 18-20 sec DIN 4 cup. Drying is retarded with more polyester, but the longer pot life suggests that more catalyst can be used [4]. Hardness is very high for all blends, with higher HSPE levels giving slightly harder films than SAC-2 alone. It may be that the higher OH content gives stronger cohesion through more urethane H-bonds in the network. Xylene resistance gradually weakens at high polyester levels, but at up to 50% HSPE content, meets the levels required for high-end applications. Both "UVCON" and Xenon accelerated weathering tests show better results for the blends than for the individual polyols. The polyester has a positive effect on colour retention. To improve discrimination, no UV absorber or HALS stabilisers were added to the clearcoats. However, in the presence of stabilisers, the durability of both polyols individually and the acrylic-polyester blend is excellent, as shown in Figure 4. The clearcoat composition is given in Table 3.
Further evaluation of these HSAC-2/HSPE blends in clearcoats applied over both solventborne and waterborne basecoats has shown excellent levelling, gloss, absence of haze and strike-in. In Table 4 a starting formulation for a high gloss, low VOC pigmented topcoat is presented. VOC is approximately 400g/l at 20 sec. DIN 4. Polyester/acrylic combination achieves the desired targets Low molecular weight acrylic and polyester polyols have been designed for use in VOC compliant, high performance 2K urethane coatings. Model oligoesters were used in the development of a polyester diluent resin for a high Tg acrylic polyol to minimise VOC levels. Adequate performance is retained even with relatively large amounts of the polyester. ACKNOWLEDGEMENTS We wish to thank Jack Suijkerbuijk and Jaap Geluk for their contributions. REFERENCES [1] E. Brinkman, P. van de Watering, P. Vandevoorde, 8th Nürnberg Congress, 2005, Paper XII.5 [2] F. Van Wijk, M. Bosma, H. Schellekens, C. Vijverberg, J. Schutyser, 8th Nürnberg Congress, 2005, Paper V.1 [3] D. Mestach, A. van Gaans, R. Brinkhuis, P-J. Elfrink, 7th Nürnberg Congress, 2003, Paper XI.6 [4] E. Bzowej, M. Shalati, G. Haldankar, R. Brinkhuis, P-J. Elfrink, paper presented at 82nd Annual Meeting of the FSCT, October 2004, Chicago [5] D. Mestach, A.van Gaans, P. Vandevoorde, T. Buser, Proceedings European Coating Conference?"Polyurethanes for High Performance Coatings III", Berlin 2004, p.199 Results at a glance - Solventborne 2K polyurethanes offer excellent performance, but it is difficult to combine good properties with the low VOC content now being required by environmental regulations. - Low molecular weight acrylic and polyester polyols have therefore been designed for use in VOC compliant, high performance 2K urethane coatings. - A polyester diluent was designed, which in combination with a high Tg acrylic polyol maximises VOC reduction but gives adequate performance even when using relatively large amounts of the polyester. The authors: -> Paul Vandevoorde studied chemistry at the University of Ghent, Belgium. He has been active in coating resins research for since 1971. As Product Development Manager of Nuplex Resins his current responsibility is the development of high solids and waterthinnable resins for automotive refinishes and high performance metal coatings. -> Ing. Ad van Gaans is Technical Service Manager for Vehicle Refinish and Commmercial Transport with specialism in two component high solids systems. He joined Nuplex Resins in 1984. This paper was presented at The 8th Nürnberg Congress in Nuremberg, April 2005
Figure 1: Calculated crosslink density for acrylic polyols with HDI trimer (all NCO converted) 1a (left) no.urea formation; 1b (right): 20 % of NCO groups consumed in urea formation. Figure 2: Effect of different diol types on oligoester properties.
Figure 3: Effect of hydroxyl functionality on oligoester properties. Figure 4: Weathering properties of stabilised clearcoat (Blend shown is 70/30 blend HSAC-2/ HSPE).
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