Handbook of Solvents - George Wypych - ChemTech - Ventech!

382 Semyon Levitsky, Zinoviy Shulman
The diagram of the liquid-vapor phase equilibrium is characterized by a decrease in the derivative
dp/dT with the polymer concentration (dp/dT →0atk→0). This leads to increase
in both the nucleation energy and the detachment size of a bubble (equation [7.2.59]) and,
consequently, to reduction of the bubbles generation frequency. Note that in reality the critical
work, W cr, for a polymeric liquid may exceed the value predicted by the formula [7.2.59]
because of manifestation of the elasticity of macromolecules.
As known, 62 the heat transfer coefficient in the case of developed nucleate boiling of
low-molecular liquids is related to the heat flux, q, by the expression α =Aq n where
n≈0.6-0.7. For concentrated polymeric solutions the exponent n is close to zero. The decrease
in heat transfer is explained by the increase in the viscosity of the solution near the
heating surface, resulting from the evacuation of the solvent with vapor. Another reason for
the decrease of α in such systems is the reduction of the bubble growth rate with lowering k 0
and the impossibility to achieve large Ja numbers by rising the solution superheat.
Since in boiling of concentrated polymer solutions the α value is small, the superheat
of the wall at a fixed q increases. This can give rise to undesirable phenomena such as burning
fast to the heating surface, structure formation, and thermal decomposition. Usually, in
this case the heat transfer is intensified by mechanical agitation. Note that one of the promising
trends in this field may become the use of ultrasound, the efficiency of which should be
evaluated with account for considerable reduction in real losses at acoustically induced
flows and pulsations of bubbles in viscoelastic media. 68,69
Specific features of boiling of high-molecular solutions are important for a number of
applications. One of examples is the heat treatment of metals, where polymeric liquids find
expanding employment. The shortcomings of traditionally used quenching liquids, such as
water and oil, are well known. 70 Quenching in oil, due to its large viscosity and high boiling
temperature, does not permit to suppress the perlite transformation in steels. From the other
hand, water as a quenching medium is characterized by high cooling rate over the temperature
ranges of both perlite and beinite transformations. However, its maximum quenching
ability lies in the temperature range of martensite formation that can lead to cracking and
shape distortion of a steel article. Besides, the quenching oils, ensuring the so-called “soft”
quenching, are fire-hazardous and have ecological limitations.
The polymeric solutions in a certain range of their physical properties combine good
points of both oil and water as quenching liquids
and permit to control the cooling process
over wide ranges of the process
parameters. For the aims of heat treatment a
number of water-soluble polymers are used,
e.g. PVA, PEO, PAA, polymethacrylic acids
(PMAA, PAA) and their salts, cellulose
compounds, etc. 71-73 The optimal concentration
range is 1 to 40% depending on molecular
mass, chemical composition, etc. Typical
Figure 7.2.19. Cooling curves for a silver specimen
quenched in a polymer aqueous solution at 20 o C.
[Adapted, from S.P. Levitsky, and Z.P. Shulman, Bubbles
in polymeric liquids, Technomic Publish. Co.,
Lancaster, 1995, with permission from Technomic
Publishing Co., Inc., copyright 1995]
data 73 are presented in Figure 7.2.19, where
curves 1-5 correspond to the solution viscosity
η p = (1.25, 2.25, 3.25, 5, 11)×10 -3 Pas,
measured at 40 o C. The quenchants based on
water-soluble polymers sustain high cooling