Synthesis and characterization of tailored polyurethane coatings
Aqueous polyurethane (PU) dispersions were synthesized for use in paper coatings. These PUs contained a polyester polyol soft segment (content of between 65 to 75%) and a urethane hard segment (content of between 30 to 35%). Triethylamine (TEA) was used as the neutralizing agent. The polyester polyol segment consisted of neopentyl glycol (NPG), adipic acid, 1,4-cyclohexane dicarboxylic acid (1,4-CHDCA) and 2-phosphonobutane- 1,2,4-tricarboxylic acid (PBTCA), while the urethane hard segment consisted of toluene diisocyanate (TDI), dimethylolproponic acid (DMPA) and ethylene glycol (EG) as a chain extender for increasing the hard segment content. Waxes and fillers were incorporated into the PU coating mixtures to investigate their effect on the barrier properties of the PU. Two types of fillers were used: nano-fillers and micro-fillers. The nano-fillers used included the Cloisite nano-clays NC15A, NC93A and NC30B, and the micro-fillers used included talc, kaolin clay and barium sulfate. Two different polyester polyols were synthesized: one containing a phosphate and the other containing no phosphate. The polyols were characterized in terms of their acid value, hydroxyl value and molecular mass. The PUs synthesized from the polyol containing no phosphate showed unfavourable barrier properties compared to results achieved with the phosphate-containing PU. The PU dispersions were applied to paperboard, and then dried at a maximum temperature of 130oC for 15 to 60 seconds, depending on the coating volume. The PU-coated paperboard was characterized primarily by determining the moisture vapour transmission rate (MVTR), and by scanning electron microscopy (SEM). PU films (stand alone, not supported by paper) were prepared by heating the concurrent PU dispersion in Teflon holders over three different temperature stages (60, 90 and 120oC) for about 2 days. The dried films were then characterized by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopy. The PU coatings showed self-assembly properties, which were affected primarily by the ionic content (comprising of DMPA, PBTCA and excess TEA) and emulsion viscosity. These self-assembly properties were analyzed by static contact angle (SCA) and MVTR measurements. It was found that the final coating properties were affected by the self-assembly mechanism of the PU. Generally, the phosphated PU coatings had lower MVTR values than the non-phosphated PU coatings. SEM analysis showed that the phosphated PU coatings had no pinholes, while the non-phosphated PU coatings had pinholes. DMA analysis showed that the phosphated PUs had higher Tg values than the non-phosphated PUs. Further, the inclusion of the phosphate monomer increased the emulsion stability and the compatibility between the hard and soft segments of the PU. Also, the exfoliated PU nanocomposites at 1% filler loading gave much better MVTR results compared to the PU microcomposites. It also rendered the coating to be non-blocking, with minimal change in MVTR.