Abstract
Dimethylolpropionic acid (DMPA) internal emulsifier has been added before, during and after prepolymer formation in the synthesis of waterborne poly(urethane-urea)s (PUDs) and their structure–properties relationships have been assessed. PUDs were characterized by pH, viscosity and particle size measurements, and the structure of the poly(urethane-urea) (PU) films was assessed by infra-red spectroscopy, differential scanning calorimetry, X-ray diffraction, thermal gravimetric analysis, plate–plate rheology and dynamic mechanical thermal analysis. The adhesion properties of the PUDs were measured by cross-hatch adhesion and T-peel test. The lowest pH value and the highest mean particle size were found in the PUD made by adding DMPA after prepolymer formation, all PUDs showed relatively ample mono-modal particle size distributions. The highest viscosity and noticeable shear thinning were obtained in the PUD made by adding DMPA during prepolymer formation. Depending on the stage of addition of DMPA, the length of the prepolymer varied and the PU films showed different degree of micro-phase separation. Because the shortest prepolymer was formed in the PU made with DMPA added before prepolymer, this PU film showed the lowest storage moduli and early melting indicating higher degree of micro-phase separation. The highest storage modulus, later melting, higher temperature and lower modulus at the cross between the storage and loss moduli corresponded to the PU made by adding DMPA after prepolymer formation, because the longer prepolymer produced during synthesis. The lowest thermal stability corresponded to the PU made by adding DMPA during prepolymer formation and the structures of all PU films were dominated by the soft domains, the main structural differences derived from the hard domains. Whereas DMPA-isophorone diisocyanate (IPDI) urethane and urea hard domains were created in the PU film made by adding DMPA during prepolymer formation, the other PU films showed DMPA-IPDI, polyester-IPDI and two different DMPA-IPDI-polyester hard domains. Finally, the adhesion properties of the PUDs and PU coatings were excellent and they were not influenced by the structural differences caused by adding DMPA in different stages of the synthesis.
Highlights
Environmental regulations of volatile organic compound (VOC) emissions in adhesives and coatings have been driving the fast growth of waterborne poly(urethane-urea) dispersions in the Polymers 2020, 12, 2478; doi:10.3390/polym12112478 www.mdpi.com/journal/polymersPolymers 2020, 12, 2478 last decades [1,2,3,4]
The hard domains are produced by reacting isocyanate with dimethylolpropionic acid (DMPA), polyol and low molecular weight amine chain extender, and the soft segments are the polyol chains; the ionic interactions are due to the interactions between the carboxylic groups on the poly(urethane-urea) dispersions (PUDs) particles and the quaternary ammonium cations in the water phase
The mean particle sizes of the PUD made by adding DMPA after prepolymer formation was higher
Summary
Environmental regulations of volatile organic compound (VOC) emissions in adhesives and coatings have been driving the fast growth of waterborne poly(urethane-urea) dispersions in the Polymers 2020, 12, 2478; doi:10.3390/polym12112478 www.mdpi.com/journal/polymersPolymers 2020, 12, 2478 last decades [1,2,3,4]. The stabilization of the particles is generally achieved by surface hydrophilic moieties of short internal emulsifier covalently bonded to the poly(urethane-urea) chains [5,6]. The synthesis of PUDs consists in three consecutive stages involving the formation of an isocyanate terminated urethane prepolymer, the chain extension with short amine and the dispersion of the poly(urethane-urea) in water. The structure of the PUD particles consists in soft and hard domains, and ionic interactions. The hard domains are produced by reacting isocyanate with DMPA, polyol and low molecular weight amine chain extender, and the soft segments are the polyol chains; the ionic interactions are due to the interactions between the carboxylic groups on the PUD particles and the quaternary ammonium cations in the water phase. The properties of the PUDs are related to their structure (i.e., structure-property relationship) which is determined by the raw materials, the nature and amount of internal emulsifier, the hard to soft segments ratio (NCO/OH ratio), and the polymerization conditions, among other [7,8,9,10,11,12,13,14,15,16,17]
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