2 research outputs found
Insight Into Solventless Production of Hydrophobically Modified Ethoxylated Urethanes (HEURs): The Role of Moisture Concentration, Reaction Temperature, and Mixing Efficiency
In this work, we report for the first time on the influence
of
the quality of reactants and reaction conditions on the production
of hydrophobically modified ethoxylated urethanes (HEURs) and selected
prepolymers without the use of solvents. We show that the polyol water
concentration is detrimental to the progress of the main urethane
forming reaction, confirming the necessity of carefully drying the
reactants below 1000 ppm to suppress the consumption of diisocyanate
toward urea during HEUR synthesis. Increasing the mixing speed (≈30
to 750 rpm), reaction temperature (80–110 °C), and catalyst
concentration (0.035–2.1 wt % bismuth carboxylate) can significantly
increase the rate of molecular weight buildup, but their effect decreases
with time as the bulk viscosity increases and mixing limitations eventually
take over, leading to the Weissenberg effect and chain growth termination.
Consequently, for the selected formulation, the maximum product molecular
weight attained lies in the range of ≈20 000–22 000
g/mol, irrespective of the specific process conditions applied
One-Step versus Two-Step Synthesis of Hydrophobically Modified Ethoxylated Urethanes: Benefits and Limitations
Associative thickeners, such as hydrophobically
modified
ethoxylated
urethanes (HEURs), are an important class of rheological modifiers
allowing precise control and optimization of the rheology of waterborne
coatings. In this work, we present a novel, comprehensive investigation
of one-step and two-step HEUR synthesis processes, highlighting their
impact on the final HEUR properties. In the conventional one-step
process (current industrial practice), there are inherent limitations
in producing high molecular weight polymers due to the complex competition
between end-capping and polymerization. We show that the two-step
method allows for much higher molecular weight polymers than the one-step
method while using less amounts of toxic diisocyanates. Additionally,
using the two-step method, the polymerization can be simply and efficiently
controlled by the addition timepoint of the end-capping agent, which
can be tailored to provide HEURs with a wide range of molecular weight
and polydispersity index. However, the efficient end-capping of high
molecular weight polymers remains a challenge when using conventional
mixing equipment in batch reactors due to mass transfer and mixing
limitations associated with the significant increase in the bulk viscosity
of the reaction mixture. To overcome these limitations, alternative
and more efficient mixing technologies, such as reactive extruders,
should be considered for the efficient end-capping of high molecular
weight polymers