Handbook of Solvents - George Wypych - ChemTech - Ventech!

694 George Wypych
Such a blend is intended as a replacement for previous blends of polyamide and polyethylene
used in gas tanks and other containers. These older blends were exposed to a mixture of
hydrocarbons but now oxygenated solvents (e.g., methanol) have been added which cause
an unacceptable reduction of barrier properties. A replacement technology is needed. Improved
barrier properties in this design 4 comes from increased crystallinity of the material
brought about by controlling laminar thickness of polyethylene crystals and processing material
under conditions which favor the crystallization of poly(vinylidene fluoride).
Another approach involves the use of difunctional telechelic polyolefins with ester,
hydroxyl, and amine terminal groups. 5 Telechelic polyolefins can be used to make polyesters,
polyamides, and polyurethanes with low permeability to solvents and gases.
Separation processes such as ultrafiltration and microfiltration use porous membranes
which allow the passage of molecules smaller than the membrane pore size. Ultrafiltration
membranes have pore sizes from 0.001 to 0.1 μm while microfiltration membranes have
pore sizes in the range of 0.02 to 10 μm. The production of these membranes is almost exclusively
based on non-solvent inversion method which has two essential steps: the polymer
is dissolved in a solvent, cast to form a film then the film is exposed to a non-solvent. Two
factors determine the quality of the membrane: pore size and selectivity. Selectivity is determined
by how narrow the distribution of pore size is. 6,7 In order to obtain membranes with
good selectivity, one must control the non-solvent inversion process so that it inverts
slowly. If it occurs too fast, it causes the formation of pores of different sizes which will be
non-uniformly distributed. This can be prevented either by an introduction of a large number
of nuclei, which are uniformly distributed in the polymer membrane or by the use of a
solvent combination which regulates the rate of solvent replacement.
Typical solvents used in membrane production include: N-methylpyrrolidinone,
N,N-dimethylacetamide, N,N-dimethylformamide, dimethylsulfoxide, tetrahydrofuran,
dioxane, dichloromethane, methyl acetate, ethyl acetate, and chloroform. They are used
alone or in mixtures. 6 These are used most frequently as non-solvents: methanol, ethanol,
1-propanol, 2-propanol, 1-butanol, 2-butanol, and t-butanol. Polymers involved include:
polysulfone, polyethersulfone, polyamide, polyimide, polyetherimide, polyolefins,
polycarbonate, polyphenyleneoxide, poly(vinylidene fluoride), polyacrylonitrile, and cellulose
and its derivatives.
A second method of membrane preparation is based on the thermally-induced phase
separation process. 8 The goal
of this process is to produce a
membrane which has an
ultrathin separation layer (to
improve permeation flux) and
uniform pores. 9 In this process,
polymer is usually dissolved
in a mixture of solvents
which allow the mixture to be
Figure 12.1.22. PEI fiber from
NMP. [Adapted, by permission
from D Wang, K Li, W K Teo,
J. Appl. Polym. Sci., 71, No.11,
1789-96 (1999).]
Figure 12.1.23. PEI fiber from
DMF. [Adapted, by permission
from D Wang, K Li, W K Teo,
J. Appl. Polym. Sci., 71, No.11,
1789-96 (1999).]
processed either by spinning
or by coating. The desired
membrane morphology is obtained
through cooling to induce
phase separation of