Modelling the surface free energy parameters of polyurethane coats—part 2. Waterborne coats obtained from cationomer polyurethanes

Piotr Krl Boena Krl Jaromir B. Lechowicz Polyurethane cationomer coats were synthesised on the basis of typical diisocyanates, properly selected polyether polyols, HO-tertiary amines and HCOOH as quaternisation reagents. The values of their surface free energy (SFE) parameters were obtained by the van Oss-Good method, with the use of the contact angle values which had been found by the goniometric method. Based on the obtained findings, empirical models were developed which made it possible to anticipate the effects of the raw material types on the SFE values of the produced coats. The possibility was noted to adjust the SFE values within 25-50 mJ/m2 by selecting carefully suitable parent substances. The principal consequences for the formation of improved hydrophobicity coats, applicable inter alia specialised protective coatings, were found to come not only from diisocyanate and polyol types but also from the alkylammonium cation structure which results from the use of different tertiary amines. The fundamental SFE lowering effect was noted when tertiary amines with 0-15 % of the 2,2,3,3-tetrafluoro-1,4-butanediol as a fluorinated chain extender was incorporated into polymer chains. - The coatings obtained from cationomer polyurethanes (more frequently termed cationic polyurethanes in reference papers) were found applicable as waterborne polyurethanes in the past years for the production of environmentally friendly lacquers, adhesives and protective coatings, coating base paper, ink jet printing base paper, etc. [13]. The definite majority of the cationic polyurethanes referred herein which is specific is obtained by reacting typical diisocyanates, polyols and N-methyldiethanoloamine (N-MDA); they are then subjected to further quaternisation in an aqueous medium with the use of an organic acid in most cases [16]. The number of ionic groups present within a polymer is critical when one wants to achieve appropriate mechanical properties, surface properties, thermal stability or biodegradability of the coats obtained from polyurethane cationomers. That number is decisive for the ionic strength of the aqueous dispersion of an ionomer which contains bulky alkylammonium cations in its molecules [13, 7]. Because of that, polyurethane cationomers may also be used as components in stable aqueous dispersions for free radical polymerisation of styrene and acrylic monomers which yields poly(urethane-styrene-acrylic) copolymers with core-shell structures. They turned out applicable as water-dilutable emulsion paints and protective coatings [8, 9]. Research was also conducted on the use of PU cationomers in the synthesis of polymeric nanocomposites incorporating intercalated or exfoliated layer silicate clays and composites which involve graphenes [10, 11]. When formulating hydrophobic coats, it is advantageous to incorporate fluorine atoms into polyurethane chains, including polyurethane ionomers that can be done in a few ways. Fluorine may be added with the isocyanate reactant, which is relatively difficult. Also, it may be added as a fluorinated derivative of polyol or as a chain extender in the form of generally available 2,2,3,3-tetrafluoro-1,4butanediol (TFBD) [5, 1215]. Not only N-MDA is used for the manufacture of alkylammonium cations but also other more water-repellent tertiary amines or amino acids like, for example, lysine may also be used [16, 17]. Cationic biodegradable multiblock poly(-caprolactone urethane)s have been synthesised which contain gemini quaternary ammonium side groups on the hard segments. To obtain these polyurethanes, a new L-lysinederivatised diamine containing gemini quaternary ammonium side groups was first synthesised [17]. Radicals derived, e.g. from alkyl haloids, which are added when the final aqueous dispersion is produced, may additionally reduce polarity of the coats. New counterions are obtained in that way which are more active in many cases than anions derived from organic acids or from frequently employed hydrochloric acid [18]. Having in mind structural variety of cationomers and their growing importance in the production of environmentally friendly polyurethane dispersions, we started the research programme which was intended to develop empirical models to describe the structural effects of cationomers on surface free energy (SFE) values of polyurethane coats formed from those cationomers. SFE makes a measure which represents well the chemical nature of each polymer coats or film from the viewpoint of its water-repellent performance. Earlier, we conducted similar studies for the coats which had been obtained from polyurethane elastomers applied as solutions in organic solvents [19]. The problem turned out much more difficult in the present case because of higher structural complexity of cationomer chains. Moreover, Table 1 Chemical compositions of synthesised polyurethane cationomers Type of diisocy-anate Type of polyol the models had to consider not only polyurethane structural fragments which were derived from diisocyanates, polyols and chain extenders but also those derived from tertiary amines and counterions as they made considerable contributions to the structures of alkylammonium cations. Similar to the approach presented in [19], sets of raw materials which were principally decisive for the chemical structures of polyurethane cationomer chains were assumed as independent variables for a given category. Since the reaction system was more complex, we unfortunately could not develop linear models, as was the case for polyurethanes. Structural parameters exp were independent variables in the previous models; those parameters could be found with the use of the 1H NMR spectral analysis [19]. All the same, we verified chemical structures of all polyurethane cationomers covered by the present study with the use of spectral methods (FT IR, 1H NMR and 13C NMR) and 19F NMR spectroscopy if needed [15, 20]. This study also covered a number of cationomer coats which had been synthesised earlier and for which chemical structures and SFE parameters had been determined precisely. The references to those publications were given in Table 1. Some cationomers, however, had to be synthesised additionally for the needs of this study, and a few syntheses were conducted once more and a few SFE measurements were taken once more to verify our earlier findings. Fluorine content, wt % a Syntheses of the sample nos. 1, 2, 6, 8 and 15 are repeated and new samples were marked with a letter a Type of tertiary amine with wt % of TFBD Mcalc The following reagents used for the synthesis of polyurethane cationomers were characterised in part I of our work [19]: 4,4-Methylenebis(phenyl isocyanate) (M= 250.25) (MDI) I s o p h o r o n e d i i s o c y a n a t e , [ 5 - i s o c y a n a t o 1-(isocyanatomethyl)-1,3,3-trimethylcyclohexane] (M= 222.28) (IPDI) Polyoxyethylene glycols, M=600 2,2,3,3-Tetrafluoro-1,4-butanediol (M=162.08) (TFBD) 1,6-Hexamethyle (...truncated)