Blocked polyisocyanates containing monofunctional polyhedral oligomeric silsesquioxane (POSS) as crosslinking agents for polyurethane powder coatings

https://doi.org/10.1016/j.porgcoat.2012.08.008Get rights and content

Abstract

Blocked polyisocyanate crosslinkers for powder coatings were synthesized using alicyclic diisocyanates (TMDI and IPDI), formic acid, (methylaminopropyl)hepta(isobutyl)Si8O12 (POSS), ɛ-caprolactam, dibutyltin dilaurate as well as triethylamine as catalysts. The chemical structures of these compounds were characterized by means of IR, 1H NMR and 13C NMR spectroscopy. The three-dimensional surface topography and surface chemical structure of the resulting powder coatings were investigated by using confocal microscope and ATR FT-IR. The values of surface roughness parameters were calculated. The surface topography was correlated with the chemical structure of the coatings and macroscopic surface behaviour: surface free energy, abrasion resistance, hardness, adhesion to the steel surface and impact resistance. Thermogravimetric analysis was employed to assess the hardening property of powder coatings and the thermal decomposition processes.

Highlights

► A method of the synthesis of blocked polyisocyanates containing monofunctional polyhedral oligomeric silsesquioxane (POSS) was proposed. ► Incorporation of the POSS particles to polyurethane powder coatings, resulted in better coating properties. ► An improvement in the thermal stability of POSS modified coatings was observed.

Introduction

In recent years there has been growing interest in organic–inorganic hybrid polymers. It is possible to obtain such polymers for instance by incorporating inorganic groups into the chains of organic polymers. The inorganic components applied for this purpose include polyhedral oligomeric silsesquioxanes (POSS) [1], [2], [3], [4], [5]. Polyhedral silsesquioxanes comprise a large group of compounds built of trifunctional silicate units, with the proportion of one atom of silicon to one and a half atom of oxygen ([RSiO3/2]n). In this formula “R” stands for any organic substituent or an atom of hydrogen, while “n” most frequently adopts the following values: 6, 8, 10 or 12. In order to build POSS molecules into the polymer chain, it is necessary to synthesize compounds containing R substituents ending with various functional groups, e.g. hydroxyl, epoxy, vinyl or amino. Depending on how many of the R substituents contain reactive functional groups, POSS molecules can be incorporated into the polymer chain as a node of the polymer network, as a part of the main chain, or side chains. Oligomeric silsesquioxanes have been used for modifying such organic polymers as: polypropylene [6], cellulose acetate [7], polyester/cotton blends [8], polyamide [9], and polyurethane [10], [11].

Products of this kind combine properties of organic polymers with typical qualities of inorganic compounds. In comparison with traditional polymers their most significant advantages include: reduced combustibility, thermal conductivity, heat emission during combustion and increased hydrophobicity, chemical resistance, mechanical strength, abrasion resistance, and hardness, temperature of thermal decomposition [12], [13]. Due to their unique qualities polymers of this type can be applied in various domains. They can be used for instance to obtain coating films with enhanced resistance to external factors [14], [15], [16]. Introduction of POSS particles into polyurethane coatings results in their improved performance properties, such as increased hydrophobicity, abrasion resistance and chemical resistance [17].

In this paper I would like to report a convenient way of synthesizing POSS-containing blocked polyisocyanate, which allows for an easy and accurate approach to prepare polyurethane powder coating systems. The synthesis of these polyisocyanates covered four stages: the synthesis of ureaisocyanate, biuret formation, POSS addition and the blocking reaction. Spectroscopic techniques such as Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance spectroscopy (1H NMR and 13C NMR) were used in order to characterize the chemical structure of these polyisocyanates. POSS particles containing one amine group were built into the structure of polyisocyanates to form side chains, which facilitates migration towards the air/film interface to minimize the interfacial energy.

This paper focuses on the results of the study aimed at determining the impact of POSS particles on the properties of polyurethane powder coatings. Confocal microscopy (CLSM) was employed for visualizing the surface topography of obtained coatings. In addition to the surface topography measurements, ATR FT-IR was used to reveal chemical structure at the surface. In order to examine the way the presence of POSS particles impacts the properties of polyurethane powder coatings, the author carried out the measurements of the contact angle with the liquids: diiodomethane and water, abrasion resistance, hardness, adhesion to the steel surface and impact resistance. Thermogravimetric analysis was applied to assess powder coatings in terms of their hardening property as well as their thermal decomposition process. The relationship between coating properties and POSS content was discussed.

Section snippets

Raw materials and reagents

Isophorone diisocyanate (IPDI) – Desmodur I from Bayer A.G. (Leverkusen, Germany). 2,2′,4- and 2,4,4′-trimethyl-1,6-hexamethylene diisocyanate (TMDI) – Vestanat TMDI from Evonik Degussa G.m.b.H. (Marl, Germany). (Methylaminopropyl)hepta(isobutyl)Si8O12 (POSS) from Hybrid Plastics (Hattiesburg, USA). ɛ-Caprolactam (C) from Zakłady Azotowe w Tarnowie – Mościcach S.A. (Tarnów, Poland). Benzoin from Aldrich (Buchs, Switzerland). Rucote 102 (RU) – polyester resin based on isophthalic acid and

Results and discussion

Aliphatic (TMDI) and alicyclic diisocyanates (IPDI) were employed in the syntheses because the coatings produced there from offer high resistance to yellowing. The synthesis itself consisted of four stages (Fig. 1). During the first stage ureaisocyanate was formed in the reaction of diisocyanate with formic acid. In course of this process, initially mixed anhydride of carbamic and formic acid was formed, which after heating to temperature of 75 °C emitted CO2 producing formamide. Formamide

Conclusions

The process of biurethization of IPDI and TMDI by means of formic acid, followed by an addition of POSS and ɛ-caprolactam allowed for synthesizing blocked polyisocyanates which in their structure contained POSS particles. On the basis of the synthesis reactions, it was concluded that the optimum conditions for obtaining blocked polyisocyanates containing POSS particles include a four-stage process carried out in the presence of two catalysts: dibuthyltin dilaurate and triethylamine, in a

Acknowledgements

This work was supported by the Polish Ministry of Science and Higher Education under grant no. N N507 503338.

The author would like to thank Mr Ireneusz Niemiec, from NanoFocus, Oberhausen Germany, for performing confocal micrographs, Ms Dorota Naróg, Ph.D., from the Faculty of Chemistry, Rzeszów University of Technology, for taking FT-IR spectra, as well as Bayer A.G., Evonik Degussa G.m.b.H., Worlée – Chemie G.m.b.H and Zakłady Azotowe in Tarnów – Mościce S.A. for sending free samples of raw

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