Chemistry Reference
In-Depth Information
central region of such loops because considerations of evaporation rates, costs,
and other properties may also influence the choice of solvents.
The design of blended solvents is facilitated by use of these subparameters,
along with graphical analyses. Thus, referring again to
Fig. 5.4
, the polymer will
be insoluble in solvents A and B but a mixture of the two should be a solvent. It
has been suggested also that a plot of
δ
p
versus
δ
H
should be sufficient for most
practical purposes, since
δ
d
values do not vary greatly, at least among common
solvents.
The procedures outlined have a practical use, but it should be realized that the
subparameter models have some empirical elements. Assumptions such as the geo-
metric mean rule (
Eq. 5-6
) for estimating interaction energies between unlike mole-
cules may have some validity for dispersion forces but are almost certainly
incorrect for dipolar interactions and hydrogen bonds. Experimental uncertainties
are also involved since solubility “loops” only indicate the limits of compatibility
and always include doubtful observations. Some of the successes and limitations of
various versions of the solubility parameter model are mentioned in passing in the
following sections which deal briefly with several important polymer mixtures.
5.3
Solvents and Plasticizers
5.3.1
Solvents for Coating Resins
The most widespread use of the solubility parameter has been in the formulation
of surface coatings. Single solvents are rarely used because the requirements for
evaporation rates, safety, solvency, and so on generally mean that a solvent blend
is more effective and less costly. Further, use of nonsolvents is often effective for
cost reductions. The cheapest organic solvents are hydrocarbons, whereas most
solvents for film-forming polymers are moderately hydrogen bonded and have
δ
20 (MPa)
1/2
. The simple example that was given in con-
nection with
Fig. 5.4
illustrates how such blends can be formulated. The proce-
dure can be used to blend solvents with nonsolvents or even to make a solvent
mixture from nonsolvents. The latter procedure must be used with caution for sur-
face coatings, however, since the effective solubility parameter of the system will
drift toward those of the higher boiling components as the solvents evaporate. If
these residual liquids are nonsolvents the final coalesced polymer film may have
poor clarity and adhesion to the substrate. The slowest evaporating component of
the solvent blend should be a good solvent for the polymer in its own right, since
the last solvent to leave the film has a strong influence on the quality of the film.
values in the range 16
5.3.2
Plasticization of Polymers
A plasticizer is a material that enhances the processability or flexibility of the
polymer with which it is mixed. The plasticizer may be a liquid or solid or
