Rheological behavior of waterborne polyurethane/starch aqueous dispersions during cure
✍ Scribed by Yixiang Wang; Ang Lue; Lina Zhang
- Publisher
- Elsevier Science
- Year
- 2009
- Tongue
- English
- Weight
- 902 KB
- Volume
- 50
- Category
- Article
- ISSN
- 0032-3861
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✦ Synopsis
Isothermal rheological behaviors of waterborne polyurethane (WPU)/starch aqueous dispersions during cure were investigated with a small-amplitude oscillatory shear flow experiment to evaluate their crosslinked structure and to predict their mechanical properties, for the first time. An abrupt increase in the elastic storage modulus (G 0 ), the viscous loss modulus (G 00 ), the complex dynamic viscosity (h * ) and the loss tangent (tan d) was observed during the curing process of the dispersions, as a result of the formation of a fractal polymer gel. The gel point (t gel ) was determined from the intersection in tan d vs curing time for different constant shear frequencies, where tan d was frequency independent and all curves crossed over, indicating the validity of the Winter-Chambon criterion for the complex system. The values of the power law exponent (n) and the gel strength (S g ) at the gel point indicated that with an increase of starch content the crosslinked WPU/starch gels underwent a transition from weak fractal to strong elastic ones. Moreover, the WPU/starch composite sheets, obtained from the aqueous dispersions with relatively high S g values, also exhibited the increased tensile strength (s b ) and Young's modulus (E). Their structure-mechanical properties relationship and the phase transitions of dispersed starch-dualphase continuity-starch matrix were revealed. This work confirmed that the rheological characters could be used to predict the mechanical properties of the WPU materials blended with natural polymer.