Speeding up cure. Seite/Page: 164

Transcription

1 Speeding up cure New hydrophilic isocyanate gives fast drying in waterborne two-pack PUR coatings Philippe Olier, Mathias Dubecq, Philippe Barbeau, Eugénie Charrière Two-pack (2K) waterborne polyurethanes are versatile high-performance coatings, but with some limitations in terms of reconciling fast drying speed with adequate pot life A newly developed aliphatic polyisocyanate - a hybrid of the HDI and IPDI structures - is shown to provide fast development of hardness in a wide range of formulations, while maintaining an extended pot life without loss of gloss or properties in the final coating Since they were first introduced to the market in the 1960s, the performance and quality of aliphatic polyisocyanates has been constantly improved Starting with biurets, manufacturers then developed trimers of hexamethylene diisocyanate (HDI), which offered improved stability and viscosity All these products found place of choice in many coating applications Later on, environmental constraints led to the development of alternatives to solvent-borne coating technologies Thus, water-borne coatings, high solids or UV coatings were developed to satisfy environmental regulations Throughout these developments, the same questions always arose: how would it be possible to obtain products which would be easy to use, with low viscosity and fast drying as well as being environmentally friendly and of course at reasonable cost? Today, 2-component waterborne polyurethanes are among the fastest-developing coatings technologies [1] This paper reports on a recently developed hydrophilic grade of polyisocyanate derivatives with a hybrid structure based both on HDI and IPDI (isophorone diisocyanate), which should provide the optimum combination of good reactivity and fast drying behaviour A comparison with other representative hydrophilic hardeners is also made The main characteristics of the polyisocyanate hardeners and resins used in this study are described in tables 1 and 2, respectively The hardeners were formulated with both waterborne acrylic polyols and polyurethane dispersions (PUD), and, as table 2 shows, representatives of resins for different coatings applications were used An NCO/OH ratio of 16/1 was chosen for the acrylic polyol formulations, taking into account side reactions with water All coatings were dried under controlled conditions (23 C and 50% relative humidity) Typical formulations for general industrial and wood coatings are given in table 3 Very often, the formulation of aqueous coatings requires the presence of organic cosolvents in order to decrease the viscosity of the hardener [2] If the cosolvent is not properly selected, it will disturb the hydrophobic/hydrophilic balance and large diameter particles can be produced, leading to poor optical properties such as haze or low gloss This can occur in some physical blends, where hydrophobic IPDI derivatives are added to improve the drying behaviour, or when polyisocyanates with different hydrophilic components are mixed Emulsions with narrow particle size distribution Laser diffraction granulometry was used to investigate the emulsification properties of Hardener A upon dilution with various co-solvents in the presence of water The technique is based on the diffraction of laser light, carried out after dilution in water For particle sizes larger than the wave length, the diffraction and diffraction angle of the laser light can be correlated to the particle size As the viscosity of the polyisocyanate is quite low (1200 mpas), the dilution was simply done by hand mixing Figure 1 shows the particle size of the emulsions made with Hardener A (average diameters obtained for 50 vol% and 90 vol% of the particles were plotted) The emulsification leads to sub-micron particles even when the polyisocyanate is not further diluted with any co-solvent, indicating a good emulsification of the hardener in aqueous media, which will lead to high gloss and optimum film formation The addition of co-solvent does not affect the particle size distribution of the emulsion, with the exception of "Shellsol A", which is a very apolar solvent (not recommended for aqueous formulations) In addition, the diameters representative for 50 vol% and 90 vol% of the particles are quite similar, which is indicative of a narrow particle size distribution This will provide a good stability of the emulsion and an improved pot life, as well as good optical properties of the film, which is particularly important when hydrophilic or hydrophobic IPDI derivatives are used to reduce drying times It may be difficult to find the right hydrophilic/hydrophobic balance, when different kinds of isocyanates and/or emulsifying technologies are used Improving appearance and pot life The improved emulsifying properties of the new isocyanate should provide additional benefits such as high gloss or improved pot life Thus, Hardener A and B were compared in a general industrial clearcoat formula (Table 3) In 2K-solvent-borne PUR formulations the pot-life can be clearly related to an increase in viscosity, but this parameter is not reliable for waterborne formulations because crosslinking occurs inside the droplets without changing the viscosity significantly In this case, the pot-life of the system can be determined by monitoring a decline in the properties of the coating (such as a loss of gloss or increase in haze) The most noticeable aspect is the influence of the hardener on the gloss and haze development (shown in Figure 2) Whereas Hardener A provides a high initial gloss and low haze, coatings made with the formulation based on Hardener B suffer from poor appearance For the latter, the pot life can be estimated as between 2 and 3 hours while the properties obtained with the formulation based on the new isocyanate remain relatively stable over time, reflecting its hybrid nature Faster hardness development The drive towards higher productivity in coatings markets requires the development of new systems with improved drying properties There are many ways to characterise drying phenomena, with measurements of chemical resistance, hardness or drying times being among the simplest ones In this work, the drying properties were evaluated by monitoring the drying times of the coatings according to NF EN ISO 1517 and the Persoz Hardness according to NF EN ISO 1522, which also provides a good indication of the curing kinetics of the system After the curing stage, the coating has not developed its final properties yet, but needs to be resistant enough for further handling To test the performance of the formulations described in Table 3 in this respect, the dust free time was measured, as well as the Persoz hardness 24 hours after the coating application (Figures 3 top and bottom), because they are fairly representative of this period of time The benefit of Hardener A can be seen very clearly from

2 these graphs: Hardener A leads to an improvement in the hardness development and to a reduction of the dust free time Naturally, these properties also depend on the polyol since these account for a large part of the final formulation The improvement in hardness development and dust free time is particularly notable with the acrylic polyol 5, whose glass transition temperature is below 23 C For polyols with higher glass transition temperature such as acrylic polyol 1 with a Tg of 38 C, an effect can still be seen but with a lesser magnitude Blocking resistance is greatly improved The drying behaviour can be expected to affect other properties as well Thus, blocking resistance is very important when coating wood panels, because good stackability of coated wood is required to ensure high productivity Logically, fast drying should result in enhanced blocking resistance (the ability of two coated pieces not to stick to each other immediately after curing) Blocking resistance was evaluated using a formulation based on PUD 3 (Table 3), with an in-house method based on the NF EN ISO 4622 standard Again, a comparison was made between Hardener A and Hardener C Lenata gloss cards were coated at 70 µm dry film thickness and then dried at 60 C After cooling for 10 minutes, pieces were positioned face to face (black to white) with a 2 x 2 cm surface under a pressure of 1250 kg/m 2 After one hour at 60 C, the blocking resistance was evaluated according to a numbering scheme (5: very bad, the two coated faces stick together; 0: very good, the two coated faces can be separated without any damage to the films) The formulation containing Hardener C achieved a rating of 3 while Hardener A achieved a 0 rating with no visible defects at the end of the test The stackability is thus dramatically improved with Hardener A compared to Hardener C Fast drying is compatible with extended pot life Even though environmental constraints are a key point in the process of choosing a coating technology, technical performances are also accounting for a very large part of it The latest developments in 2-pack waterborne polyurethane systems offer products which are technically effective as well as environmentally friendly Drying properties in particular, which often have been considered as less favourable in comparison with solvent-borne systems, can be significantly improved using the new hybrid hydrophilic polyisocyanate, which allows the combination of fast drying properties with ease of use, without degrading the pot life or the film appearance, and is particularly suited for applications where air forced drying can not easily be used - for instance because of the size or sensitivity of the parts that have to be coated (eg in the coating of trains, general industrial coatings or wood coatings) - or also for outdoor applications HDI and IPDI - The hybrid structure provides easy dispersion in the presence of almost any cosolvent - Tests on a wide range of formulations show that drying and hardness development are exceptionally fast - Despite the fast drying properties, the pot life is very long, without loss of gloss or development of haze in the final coating The authors: -> Philippe Olier joined the Rhodia group (then Rhône-Poulenc) in 1989 In 1997 he graduated from the CNAM (Conservatoire National des Arts et Métiers) In 2003 he moved to the industrial coatings laboratory in Lyon where he is now in charge of the application section for Rhodia's aliphatic isocyanates range -> Mathias Dubecq joined PPG in 1990, and graduated from the CNAM with an engineer's diploma in 1999 He joined Rhodia in 2001 at the industrial coatings laboratory in Lyon, where he is now in charge of technical support for Rhodia's aliphatic isocyanates range in several countries -> Dr Philippe Barbeau received his engineering degree from the Ecole Supérieure de Physique et de Chimie industrielle in Paris, France, in 1994 He obtained his PhD from the Institut National des Sciences Appliquées in Lyon (France) in 1998 He then worked for Cray Valley (Total Company) where he held several positions in R&D In 2002 he joined Rhodia, working on several (poly)isocyanates development projects and is now Global Technical Manager for Rhodia's Industrial Coatings business unit -> Eugénie Charrière graduated in Macromolecular Chemistry & Paint Formulation from ITECH high school (France) in 1994 She joined Rhodia Recherches (formerly Rhône-Poulenc Industrialisation) in 1996 After two years in the Application laboratory of the Industrial Coatings R&D departement, she took over the Technical Service responsability for Rhodia's aliphatic isocyanates ranges in several European and Export countries In 2002, she moved to Rhodia PC&S as the European Marketing & Development manager of this range of products Acknowledgement The authors would like to thank Marie Laure Bonnefoy for her contribution to this work References [1] ML Roux, 4 th international Woodcoating Congress, The Hague, 2004, proceedings paper 1 [2] U Wustmann, 7 th Nürnberg Congress (2003), proceedings, paper 1 Results at a glance - A hydrophilic grade of polyisocyanate derivatives has been developed with a hybrid structure based on a combination of

3 Figure 1: Self-emulsifying properties of Hardener A in the presence of water showing the effects of different cosolvents (75 % dry content) as measured by laser diffraction granulometry Figure 2: Development of gloss and haze over time for a general industrial clear coat based on acrylic polyol 2 (formulation as given in table 3)

4 Figure 3: Comparison of dust free time and Persoz hardness development of hardeners A and C with various binders

5

6

7