13 Flow Mechanics of Polyurethane Foam Formation Downloaded by PENNSYLVANIA STATE UNIV on September 9, 2012 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0172.ch013
J. T. LINDT and W. KOSTRZEWSKI Department of Metallurgical and Materials Engineering, University of Pittsburgh, Pittsburgh, PA 15261
The foam architecture is resulted from a complex interaction between the physico-chemical and operating parameters characterizing a given processing situation. While considerable attention has been devoted to the chemistry and, to a somewhat lesser extent, to the morphology of polyurethanes, only a severely limited amount of information is available on flow phenomena occurring during the foam formation. As a result, rational methods for neither rheological characterization nor scale-up have been developed. Large scale experiments are often required to analyze and solve processing problems. The inability to analyze flow-related material defects is a serious drawback; shear and normal stresses associated with the foaming process can be responsible for a variety of defects such as cell structure irregularities, large voids, excessive curvature or even splitting of the foam-free surface, local density gradients and leakages from molds. Presently, an exploratory attempt is made to examine, in phenomenological terms, the flow behavior of a water-blown urethane foam prior to appreciable chemical gelation taking place. The objectives are: - to generate a quantifiable flow field associated with appreciable (macroscopic) shear stresses, -
to investigate the nature of momentum transfer involved,
- to make a quantitative assessment of the time-dependent shear stress field leading to an evaluation of the apparent viscosity as a function of time, - to examine briefly the effect of the flow field on the terminal foam morphology. 0097-6156/81/0172-0167$05.00/0 © 1981 American Chemical Society
In Urethane Chemistry and Applications; Edwards, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
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U R E T H A N E CHEMISTRY A N D
APPLICATIONS
GROWING FOAM AS A FLUID MEDIUM
Downloaded by PENNSYLVANIA STATE UNIV on September 9, 2012 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0172.ch013
From a phenomenological standpoint, g a s - l i q u i d mixtures can be d i v i d e d i n t o d i s p e r s i o n s , macro-mixed and s t r a t i f i e d systems. Fine d i s p e r s i o n s are considered pseudo-homogeneous, t h e i r flow mechanics being e s s e n t i a l l y those e s t a b l i s h e d f o r true homogeneous fluids. Fine d i s p e r s i o n s c o n t a i n f l u i d p a r t i c l e s that are s m a l l by both geometrical and dynamic c r i t e r i a . Thus, the growing foam may be regarded as a pseudo-homogeneous mixture as long as: - the bubbles are s m a l l when compared to the c h a r a c t e r i s t i c dimension of the apparatus, and the bubble Reynolds number i s v a n i s h i n g l y s m a l l (Reg
