Compliant yet excellent

Transcription

1 Compliant yet excellent NMP- and critical-additive-free polyurethane dispersions for high performance parquet flooring applications. Waterborne polyurethane dispersions (PUDs) can produce coatings with excellent mechanical properties, ideally suited to high performance flooring applications. Environmental pressures to remove some undesirable 'traditional' additives have led to the development of NMP-free and low VOC PUDs which are able to retain excellent performance. Guru Satguru*, Peter Jansse, Marc Roelands. * Corresponding author. Contact: Dr. Guru Satguru, DSM Neoresins, Sluisweg 12, 5145 PE Walwijk, The Netherlands, Tel , Fax , guru.satgurunathan@dsm.com Environmental and legislative pressures have driven the surface coating industry to focus increasingly on waterborne polymer colloids. As a result, the design and development of waterborne polymers has been greatly advanced, resulting in coating properties that often match and sometimes exceed those provided by solventborne systems. Two major classes of waterborne polymers are in use: - Aqueous dispersions prepared by emulsion polymerisation, for example acrylic and styrene-acrylic polymers; - Aqueous dispersions of preformed polymers which are subsequently dispersed in water, for example waterborne polyurethanes. Each type has its own advantages and disadvantages in terms of achieving useful surface coating properties. For instance, acrylic emulsion polymers can readily be copolymerised, resulting in economical systems with reasonable film properties. Polyurethanes, on the other hand, create quite unique film properties especially in terms of mechanical properties, which are ideally suited for high performance surface coatings. Technical advances in both these areas have been very well documented in both academic and industrial research environments [1-4]. Making polyurethane dispersions Several routes [5, 6] are available for the synthesis of polyurethane dispersions. They include the acetone process, the prepolymer mixing process, the melt dispersion process and the ketamine process. All these have as a common first step a conventional polyurethane synthesis in which diols or polyols are reacted with diisocyanates, followed by dispersion in water. A schematic representation of the synthetic steps of the prepolymer mixing process used to produce an anionic polyurethane dispersion employed for this study is shown in Figure 1. In this process the stoichiometry (isocyanate:hydroxyl ratio) of the reaction is controlled to ensure that the isocyanate terminated prepolymer has a workable viscosity/molecular weight, in order to facilitate the aqueous dispersion step. This essential step of regulating the prepolymer viscosity is generally achieved by mixing the prepolymer with a diluent which ultimately remains in the final dispersion. N-methyl pyrrolidone (NMP) is commonly used for this purpose. The prepolymer is then neutralised and transferred to water, where spontaneous particle formation occurs. Chain extension is carried out at this stage, resulting in a high molecular weight polyurethane dispersion. Polydisperse, swollen particles enable good film formation The mode of synthesis of polyurethane dispersions typically results in particles which are polydisperse in terms of size, and swollen in terms of their internal particle morphology [7]. In fact, the particles behave as a collection of large macromolecular chains held together by intermolecular interactions. A schematic representation of the morphology of a polyurethane dispersion is shown in Figure 2 and a transmission electron micrograph of a typical dispersion is presented in Figure 3. From both figures it is evident that the particle size is small and the distribution is broad, i.e. polydisperse. This dispersion morphology clearly results from the synthesis, which is based on step growth addition polymerisation followed by the emulsification process. Also shown in Figure 4 is a schematic representation of the internal morphology of an anionic polyurethane particle. Evidence from fluorescence and small angle neutron scattering measurements suggests that the particle morphology is swollen via plasticisation by water molecules [7]. These advantageous features allow polyurethane particles to coalesce effectively to achieve excellent film formation [8]. Flexibility and bonding are both controllable The polymer chain makeup of typical polyurethanes commonly contains soft and hard segmentation. Soft segments mainly are derived from high molecular weight polyol components, while the hard segment contribution is derived from diisocyanates, low molecular weight diols and the urethane and urea bond sequences. The hard domains provide both physical interaction sites via hydrogen bonding and filler-like reinforcement to the soft segment matrix. Depending on the phase mixing via hydrogen bonding or de-mixing of the soft and hard segments, polyurethanes can yield a range of specific film properties which are uniquely suited for surface coatings [9]. In addition to the hydrogen bridge network, covalently crosslinkable functionalities can be built into the polymer chains in order to further enhance mechanical and chemical resistance properties [10, 11]. These features are ideally suited to parquet coatings where mechanical properties such as superior toughness, elongation, mar and scratch resistance properties are essential requirements. Key requirements of parquet coatings The main requirements for parquet flooring are perfect appearance combined with excellent wear and tear properties. These properties are dependent both on the type and nature of the wood substrate and the surface coating applied to it. Appropriate design of the polymer coating in relation to the substrate is therefore of paramount importance. Often three major considerations are incorporated into the design of the coating: - Appearance/subjective perception of the coating - Ease of formulation and application - Excellent mechanical and chemical resistance properties. The appearance presented by the coating should correspond to perceived end consumer needs. Here, optical appearance such as gloss or matt finish and haptic characteristics which give the right feel to the coating are often required [12]. In terms of application, factors such as wetting, lapping, recoatability and drying times are important. After application, it is essential that the coating be able to withstand damage over the period of usage. Here,

2 resistance towards scratch, black heel mark (BHMR), scuffing and elasticity are important. Additionally, the coating should be resistant to chemical spillages. Standard PUDs contain undesirable additives Over and above the coating requirements discussed above, the current climate of regulatory restrictions stipulates that the coating shall be environmentally friendly. Although polyurethane dispersions supplied in water to a large degree seek to satisfy this requirement, there remains a range of less desirable chemicals that are used in the synthesis. They include: - NMP as diluent for the prepolymer; - Triethylamine (TEA) as neutralising base - Tin (Sn) as catalyst for the prepolymer reaction - Nonyl phenol based surfactants in the dispersion. It is imperative that the above chemicals be removed or substituted in order to comply with regulations. These challenges and some effective solutions will be described below, with particular reference to the production of binders suitable for ambient temperature air dried coatings and meeting the needs of the parquet flooring market. Two case studies are described. An NMP, TEA and tin free aliphatic and aromatic polyurethane binder was first produced, followed by a modification with low VOC. Acrylate monomer can replace NMP Because of concerns over the presence of NMP [13], worldwide legislation now stipulates the labelling of NMP-containing products. This move has alerted polyurethane manufacturers either to reduce the levels of NMP or ideally eliminate it. For this purpose, several more acceptable alternative diluents are being actively sought. However, in general, it has been found that these alternative diluents either do not have the solvating and viscosity regulation power of NMP or they are more expensive than it. Moreover, all these diluents including NMP remain in the final dispersion and contribute to the VOC of the dispersion. An ideal way to avoid the above scenario is to employ acrylic monomer as a diluent in place of NMP during the prepolymer reaction [14]. After the dispersion and chain extension steps, an acrylic polymerisation is performed. This route not only avoids the use of NMP or any other solvent, but also does not contribute to VOC levels. By suitable design of this synthesis process, these NMP-free urethane polymers can be made with colloidal and film properties that equal or exceed those of NMP-containing dispersions. A comparison of the properties of an aliphatic and an aromatic polyurethane dispersion with and without NMP employed in parquet coatings is shown in Table 1. In both cases, the polymer backbone composition of the respective aliphatic and aromatic polymers remained the same. While NMP was replaced with acrylic monomer, TEA was substituted by an alternative but non-toxic amine and the tin catalyst was eliminated by adjusting the prepolymer reaction conditions. It should be noted that separate and dedicated formulations were employed for the NMP-containing and NMP-free polymers. It is evident from the results presented in Table 1 that the properties of the NMP-free polymers (Pu-B and PU-D) are closely comparable to those of the NMP-containing variants. This result clearly indicates that replacement of NMP does not detract from the overall flooring properties, while improving environmental friendliness. Polymer redesign reduces coalescent demand The replacement of NMP by monomer as a reactive diluent on the one hand allows a significant reduction in VOC. On the other hand however, it is often noted that the film formation of the resulting dispersion can be adversely affected. Consequently the coalescent demand for effective film formation might be increased. This is clearly due to the lack of plasticising capability in the absence of NMP. In this situation, the challenge is to design polymers which are NMP-free yet retain a low coalescent demand. Moreover, it is imperative that the resulting polymer can provide the required flooring properties as effectively as NMP-containing systems. This conflict can be overcome by suitable design of the urethane backbone characteristics. An important feature of polyurethane composition is the ability to advantageously control the micro-phase film morphology by manipulating the level of soft and hard segments in the polymer chain. For example, an increase in the soft segment content for enhanced film formation or an increase in hard segment content to increase hardness can be achieved as appropriate to the film properties required. This is generally obtained by altering the stoichiometry of the isocyanate to hydroxyl ratio. An illustration of this effect is presented in Figure 5. Here AFM images of two NMP-free polyurethane films are shown which vary in soft segment content. It is clear that on increasing the soft segment content (II), particle coalescence is significantly increased, as evidenced by the relative smoothness and coherence of the film. Consequently the coalescent demand of PU-II is significantly lower than that of PU-I. This is also reflected in the excellent mechanical and resistance properties exhibited by PU-II over PU-I. Crosslinkable PUD achieves all target properties Based on the above design principles, a polyurethane (PU-E) was specifically developed to meet the property requirements for an environmentally friendly parquet coating. In order to further enhance scuff and black heel mark resistance of PU-E, covalently crosslinkable functionality was built in to the polyurethane backbone [11]. Results of an evaluation of PU-E are presented in Table 2. It is evident that polyurethane PU-E has the highly desirable attributes of being environmentally friendly while providing all the properties required for a high performance parquet coating. It is an 'all-free' (NMP, TEA, Sn, NPEO, surfactant and plasticiser free) and low coalescent demand binder. New PUDs meet performance and environmental demands This paper has thus highlighted the advantages and the wide scope available within waterborne urethane technology for parquet coatings. It has been shown that careful synthetic and morphological considerations can lead to tailor made products that are environmentally friendly and yet provide high performance parquet coatings. ACKNOWLEDGEMENTS The authors wish to thank their colleagues Lex Donders, Sjoerd Buil, Andre Harmsen, Ed Heijnen and Miranda Vermeer Hoffman for providing experimental and application support for this paper. REFERENCES [1] J. C. Padget, J Coatings Tech, 66 (839) 89, (1994) [2] B. K. Kim, Colloid Polym Sci, 274, (1996) [3] F. Buckman, A. Overbeek, T. Nabuurs, Eur Coatings J, 6, 53, (2001) [4] M. Hirose, J. Zhou, K. Nagai, Prog Org Coatings 38,27, (2000)

3 [5] D. Dietrich, Prog Org Coatings 9, 281 (1981) [6] D. Lorenz, K. H. Reinmöller, K. H. Grafenschafer, Coll Polymer Sci, 259, 367 (1981) [7] R. Satguru, J. McMahon, J. C. Padget, R. G. Coogan, J Coatings Tech, 66, (830) 47, (1994) [8] I. Soutar, L. Swanson, T. Annable, J. C. Padget, R. Satguru, Langmuir, 22, 5904 (2006). [9] D. J. Hourston, G. Williams, R. Satguru, J. C. Padget, J Appl Polym Sci, 67, 1437 (1998) [10] A.Harmsen, P. L. Jansse, M. Vermeer, E. Hoogen, Eur Coatings J, 5, 14, (2003) [11] R. Tennebrooek, J. Geurts, A. Overbeek, A. Harmsen, Eur Coatings J, 11, (1998), [12] M. Schoondermark, R. Satguru, P. L. Jansse, Proceedings of Fatipec Congress 2006 [13] J. Bechera, A. Ma, A. Puelo, Paint and Coatings Industry, 20 (5), 60, (2004) [14] R. Satguru, M. Roelands, P. L. Jansse, Eur Coatings J, 12, 32, (2005) Results at a glance - The inherent polymer chain, particle and film morphology of waterborne polyurethane dispersions (PUDs) allow coatings to be developed with excellent mechanical properties, which are ideally suited to flooring applications. - Strategies that allow PUDs to be formulated without the use of 'traditional' environmentally undesirable additives are discussed. (These are NMP as a prepolymer diluent, triethylamine (TEA) as neutralising base, Tin (Sn) as a reaction catalyst and nonyl phenol based surfactants). - In particular, acrylate monomer can be used to replace NMP in the first stage of synthesis, then polymerised after the dispersion has been formed. - Further reformulation has led to a PUD which is free of all the above undesirable materials yet retains a low coalescent demand and excellent performance. The authors: > Dr. Guru Satguru obtained his PhD in polymer chemistry at the University of Lancaster and undertook postdoctoral research at the University of Bristol, UK. Since 1986 he has worked in the area of waterborne polymer dispersions. He holds the position of Business Research Associate at DSM NeoResins. > Drs. Peter Jansse holds a degree in Chemical Technology from the HTS in the Hague, and a university degree in Organic Chemistry from the University of Leiden, the Netherlands. He joined DSM NeoResins in 1985 and has mainly worked on waterborne urethane systems. Since July 2006, he has headed the research department. > Drs. Marc Roelands received a degree in Chemical Technology from the HTS in Breda and a university degree in Physical Organic Chemistry from the University of Utrecht, the Netherlands. He joined DSM NeoResins in 1997, where he has worked on the development of polymer dispersions. In November 2006 he was appointed Industry Manager for Flooring & Construction applications. This paper was presented at the European Coatings Conference "Parquet Coatings IV" Berlin, 9/10 November 2006

4 Figure 1: Waterborne polyurethane synthesis via the prepolymer mixing route. Figure 2: Schematic representation of the polyurethane dispersion morphology.

5 Figure 3: Transmission electron micrograph of a typical polyurethane dispersion.

6 Figure 4: Schematic model of the polyurethane internal particle morphology.

7 Figure 5(a): Atomic force microscopy image of the surface morphology, measured in height mode, of NMP-free urethane films with a low (PU-I) soft segment content.

8 Figure 5(b): Atomic force microscopy image of the surface morphology, measured in height mode, of NMP-free urethane films with a high (PU-II) soft segment content.

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