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241
glucose units. The reaction may be indicated schematically as
RONa + CS
2
ROCSNa
S
The resultant yellow sodium cellulose xanthate is dispersed in an
aqueous caustic soda solution, where some hydrolysis occurs. This process
is referred to as “ripening” and the solution as “viscose”. When the
hydrolysis has proceeded sufficiently the solution it transferred to a hopper
from which it emerges through a small slit on to a roller immersed in a tank
of 10-15% sulphuric acid and 10-20% sodium sulphate at 35-40°C. The
viscose is coagulated and by completion of the hydrolysis the cellulose is
regenerated. The foil is subsequently washed, bleached, plasticized with
ethylene glycol or glycerol and then dried.
The product at this stage is “plain” foil and has a high moisture
vapour transmission rate. Foil which is more moisture proof may be
obtained by coating with pyroxylin (cellulose nitrate solution) containing
dibutyl phthalate as plasticizer or with vinylidene chloride – acrylonitrile
copolymers. A range of foils are available differing largely in their
moisture impermeability and in he at sea lin g charact eris tic s.
Regenerated cellulose foil has been extensively and successfully
used as a wrapping material, particularly in the food and tobacco industries.
Like other cellulose materials it is now having to face the challenge of the
completely synthetic polymers. Although the foil has been able to compete
in the past, the advent of the polypropylene film in the early 1960s
produced a serious competitor which led to a marked reduction in the use of
the cellulosic materials.
Regenerated cellulose does, however, have the advantage that it
biodegrades well aerobically in composting (rather more slowly
anaerobically).
2.4.4 Vulcanized Fibre
This material has been known for many years, being used
originally in the making of electric lamp filaments. In principle vulcanized
fibre is produced by the action of zinc chloride on absorbent paper. The
zinc chloride causes the cellulosic fibres to swell and be covered with a
gelatinous layer. Separate layers of paper may be plied together and the
zinc chloride subsequently removed to leave a regenerated cellulose
laminate.
The removal of zinc chloride involves an extremely lengthy
242
procedure. The plied sheets are passed through a series of progressively
more dilute zinc chloride solutions and finally pure water in or der t o lea ch
out the gelatinising agent. This may take several months. The sheets are
then dried and consolidated under light pressure.
The sheets may be formed to some extent by first softening in hot
water or steam and then pressing in moulds at pressures of 1.5-3.5 MPa.
Machining, using high-speed tools, may be carried out on conventional
metal-working machinery.
A number of grades have been available according to the desired
end use. The principal applications of vulcanized fibre are in electrical
insulation, luggage, protective guards and various types of materialshandling equipment. The major limitations are dimensional instability
caused by changes in humidity, lack of flexibility and the long processing
times necessary to extract the zinc chloride.
2.5 Ionic Polymers
Ionic polymers are polymers containing chemically bound ions
within their structure. These are specialist materials, some of which have
limited, though important, commercial applications. Those containing few
ions are melt-processable thermoplastics called ionomers; those containing
many ions are either water-soluble polymers called polyelectrolytes or
crosslinked polymers containing ionic groups called ion-exchange resins.
Polyelectrolytes and ion-exchange resins are, in general, intractable
materials and not processable on conventional plastics machinery.
Ionic polymers may be classified according to the type of the
bound ion, its position within the structure, and the amount of bound ions
present along a given length of polymer chain. They may be further
classified according to the nature of the counterion or the nature of the
supporting polymeric backbone.
The bound ion is usually either an anion such as sulfonate (–SO
phosphonate (–PO
+
(–NR
) or amine (–NH
3
2–
) or carboxylate (–CO
3
+
). But both types can occur together and the
3
–
) or a cation such as quaternary
2
–
),
3
polymer is then said to be ampholytic. In each case, the polyion may be
strong or weak according to the degree to which it ionizes. For example,
polysulfonates and quaternary type polyions are strong, polycarboxylates
and amine type polyions are weak, while polyphosphonates are
intermediate in nature.
The bound ion can be pendant to the polymer’s covalent backbone,
as in a polysulfonate (1) or it can be integral or enchained, as in an ionene
243
(2)
(1) (2)
Almost any conventional polymer can be modified to form an ionic
polymer of the pendant type. However, ionic polymers of the integral type,
like the ionenes, are specialized structures whose backbones can exist only
in the ionic form.
Ionic polymers contain counterions that neutralize the charges on
the bound ions. The counterions may be grouped into three types:
(1) univalent;
(2) di- or trivalent;
(3) polymeric. Polymers with polymeric counterions are often
called polysalts, polyelectrolyte complexes, polyion complexes, simplexes,
or coacervates.
The methods used for synthesis of ionic polymers can be divided
mainly into three types:
(1) direct synthesis;
(2) post-functionalization of a standard preformed polymer;
(3) post-functionalization of a special preformed polymer.
Direct synthesis is used for polyelectrolytes such as carboxy
polymers, e.g., poly(acrylic acid) and poly(methacrylic acid), which are
probably the most common ionic polymers of all. Post-functionalization of
pre-formed polymers is a frequently used synthetic method, provided that
the polymer is sufficiently reactive. Common examples are the sulfonation
of polystyrene and the grafting of thioglycollic acid on to polybutadiene
using free radicals.
Special post-functionalizable copolymers have also been used to
derive acid ionomers by hydrolysis, thus avoiding the difficulties of
copolymerizing ionic and nonionic monomers. To this end there are many
examples where carboxylic acid polymers are formed by hydrolyzing
copolymers containing acrylate esters, acrylonitrile, or maleic anhydride. A
sulfonic acid ionomer, Nafion, is formed by hydrolysis of
tetrafluoroethylene copolymerized with a sulfonyl fluoride.
All of the aforementioned ionomers are anionic polymers. Cationic
polymers are less common, though equally important. Pendant cations are
usually of the quaternary ammonium type and made by a multi-step post-
244
functionalization in which a precursor containing labile chloride is reacted
Ion
Designation
Dominant
Application
Example
>0.5
Ionomer
Ionic
Thermoplastic
Ethylene-acrylic
1-1.5
Ionomer
Ion-
Membranes
PTFE copolymer
1-1.5
Ionomer
Hydrophi-
Membranes
Sulfonated
>4
Polyelectro
Water
Thickeners,
Polyacrylic acid
>4
Polyelectro
Ion-
Ion-exchange
Sulfonated
polystyrene
14
Polyelectro
Ionic
linking
Dental cements
Polyacrylic acid
with a tertiary amine:
Sometimes, the inverse of this is also done when a pre-formed
polyamine is quaternized by reacting with an alkyl halide:
Physical properties and applications
Synthetic ionic polymers have three distinctive properties that
dictate their usage: (a) ionic cross-linking, (b) ion-exchange capability, and
(c) hydrophilicity. Major applications for organic ionic polymers in relation
to their ion content and dominant property are summarized in Table 20.
Table 20 – Applications of organic ionic polymers in relation to the amount
of ion present
content,
equiv/kg
property
crosslinking
exchange
(electrodialysis, etc.)
acid copolymer
(Nafion,
Flemion)
lyte
lyte (crosslinked)
lyte
licity
solubility
exchange
cross-
(reverse osmosis)
dispersants, flocculants and sizes
resins
polyethersulfones
or copolymer
polystyrene,
aminated
245
Ionic cross-linking
Since the ions in ionic polymers are held by chemical bonds within
a low dielectric medium consisting of a covalent polymer backbone
material with which they are incompatible, the polymer backbone is forced
into conformations that allow the ions to associate with each other. Because
these ionic associations involve ions from different chains they behave as
crosslinks, but because they are thermally labile they reversibly break down
on heating. Ionomers therefore behave as cross-linked, yet meltprocessable, thermoplastic materials, or if the backbone is elastomeric, as
thermoplastic rubbers. It should be noted that it is with the slightly ionic
polymers, the ionomers, where the effect of ion aggregation is exploited to
produce meltprocessable, specialist thermoplastic materials. With highly
ionic polymers, the polyelec-trolytes, the ionic cross-linking is so extreme
that the polymers decompose on melting or are too viscous for use as
thermoplastics.
Two types of ionic aggregates are found in ionomers, called
multiplets and clusters, which coexist in equilibrium within the matrix of
covalent backbone material. The multiplets, very small and very numerous,
are associations of ion-pairs up to eight in number that are purely ionic and
are devoid of trapped segments of the covalent backbone, while the clusters
are large associations of these multiplets between which are trapped
segments of the backbone. Evidence for this theory of microphase
separation into multiplets and clusters is provided by many types of
physical measurement, the important techniques being small-angle x-ray
scattering, and infrared and Raman spectroscopy.
Ion-exchange
Ionic polymers contain two types of ions, namely bound ions,
which are part of the structure, and the counterions, which are free. In a
medium in which the ionic polymer is insoluble, the counterions are
exchanged for similar ions from the surrounding medium and an
equilibrium is established, the kinetics of the process being dependent on
factors such as the physical form of the insoluble polyion, its porosity, and
surface area. The fundamental fact in this exchange is that the counterions
have free movement into and out of the polymer, while ions of the same
type of charge as the bound ion do not. This barrier to ion movement is
known as Donnan exclusion.
The property of ion-exchange has important consequences and
three di ffe re nt typ es of a ppl i cat io n d epe nd on i t. T h ey a r e ( 1) io n-exchange
246
resins, (2) ion-exchange membranes, and (3) heterogeneous catalysis.
Ion-exchange resins are insoluble beads of an ionic polymer. Their
bestknown use is in deionization or demineralization of tap water to
distilled water. In this process, the water is contacted with two types of
resin, one a polyacid and the other a polybase, so that the net effect is to
exchange M+ and X– from the tap water for H+ and OH–, i.e., H2O, from the
polyion beads:
To be effective, ion-exchange resins should have high ionic
content. They are therefore polyelectrolytes and, since polyelectrolytes are
inherently water-soluble, they are chemically cross-linked during
manufacture to make them insoluble. When immersed in water they
undergo swelling, which facilitates rapid ion-exchange. The degree of
swelling is, however, inversely related to the density of the cross-links.
A polyion in the form of a thin membrane is used as ion-exchange
membrane in another application of the ion-exchange phenomenon. When
exposed to an electrolyte, an ion-exchange membrane will allow
counterions to pass through it, but will act as a barrier to the
complementary ion, and is therefore said to be permselective. Thus a
polyanionic membrane will allow passage of cations and a polycationic
membrane that of anions, so that under the influence of an electric current,
continuous fluxes of cations and anions, respectively, can be set up across
these membranes. This principle is exploited in electrodialysis and in chloralkali cells.
The dialyzer used for desalination of water by electrodialysis
(Figure 52) is an electric cell divided into a series of sub-cells that are
separated by alternate polyanion and polycation membranes. While the ions
move under the influence of the applied electric potential, the
complimentary permselectivities of the ion-exchange membranes insure
that salt is removed from alternate sub-cells and concentrated in the others.
Desalination dialyzers used in practice have up to 100 such sub-cells.
247
Figure 52 - Desalination of water by electrodialy sis.
Other applications of dialyzers include concentration of brine and
the desalting of cheese whey. (It is important not to confuse electrodialysis
with ordinary dialysis, which is not an ion-exchange process but is a
separation based on differences in size and hence diffusivity between large
and small molecules through a membrane, which, generally speaking, is not
ionic.)
A related application of ion-exchange membrane is in ion-selective
electrode. Thus when an electrode with an ion-exchange membrane
encasing is immersed in an ionic medium, it will develop a potential
proportional to the activity of the ion to which the membrane is
permselective.
A high ion content is not necessary for an ion-exchange membrane,
since its function is not governed by its exchange capacity. Though a high
ion content can be advantageous as it decreases the electrical resistance, it
can in fact be counterproductive if the membrane is so highly swollen by
water that its permselectivity is reduced. The ion content of an ionexchange
membrane is thus often intermediate between that of an ionomer and an
ion-exchange resin.
While chemical cross-linking is used for ion-exchange resins to
limit this swelling or to prevent the resin from dissolving, this method is
generally not desirable for membranes, because cross-linking interferes
with the process of membrane fabrication.
Ion-exchange dialysis (or Donnan dialysis) like ordinary dialysis is
248
a diffusion-controlled separation process, but unlike the latter it involves
ionexchange and so needs an ionic membrane (though without an applied
current). If a polyanionic membrane is used, as shown in Figure 52, the
cations diffuse each way across the membrane but the anions cannot so
that, in effect, cations are separated as they swap over, but the amount of
electrolyte on each side of the membrane remains constant. This type of
dialysis, using continuous cells, is being developed as a means of stripping
and concentrating radioactive ions from dilute solutions of radioactive
wastes.
Figure 52 - Ion-exchange dialysis.
Hydrophilicity
Ionic polymers are hydrophilic. Those of moderate ion content
swell o n contact wit h water and th ose of high i on content di ssolve, un less
they are cross-linked. With moderately ionic polymers, which swell
without dissolving, the hydrophilicity has the advantage of making the
structure more permeable to ions for ion-exchange. The hydrophilicity of
moderately ionic polymers leads to another type of membrane application,
that of reverse osmosis.
The method of reverse osmosis i nvolves application of pressure to
the surface of a saline solution, thus forcing pure water to pass from the
solution through a semipermeable membrane that does not permit passage
of ions. Since the natural osmotic pressure tends to force the water form the
region of low ionic concentration to that of the high, to achieve a flow in
249
the opposite direction,as in reverse osmosis, a pressure has to be applied
exceeding the osmotic pressure.
In the process used for desalination, a saline, brackish water or sea
water is forced at very high pressure through a hydrophilic membrane
resulting in water of drinkable quality (Figure 53). Reverse osmosis is also
utilized for recovering waste water for paper mill operations, pollution
control, industrial water treatment, chemical separations, and food
processing.
Figure 53 - Schematic representation of reverse osmosis process (a).
Tubular configuration system for wastewater treatment by reverse
osmosis (b)
The compartments indicated in Figure 53a are a schematic
representation of reverse osmosis process. In practice, the reverse osmosis
process is conducted in a tubular configuration system (Figure 53 b). Raw
waste water flows under high pressure (greater than osmotic pressure)
250
through an inner tube made of a semipermeable membrane material and
designed for high pressure operation. Purified water is removed from the
outer tube that is at atmospheric pressure and is made of ordinary material.
Although the membrane used for reverse osmosis may be ionic and
ion-exchange can occur, there is, however, no overall transmission of salt
because of Donnan exclusion (and unlike electrodialysis no electric current
is applied). Thus, while an essential requirement of a membrane for reverse
osmosis is hydrophilicity, it need not be ionic. In fact, the most successful
reverse osmosis membranes developed are made of nonionic cellulose
acetate. Certain ionic polymers, such as the sufonated polyaromatics, has
been used because of their greater chemical stability and resistance to
biological degradation.
Several mechanisms have been proposed to explain reverse
osmosis. According to the preferential sorption-capillary flow mechanism,
reverse osmosis separation is the combined result of an interfacial
phenomenon and fluid transport under pressure through capillary pores.
Figure 54a is a conceptual model of this mechanism for recovery of fresh
water from aqueous salt solutions. The surface of the membrane in contact
with the solution has a preferential sorption for water and/or preferential
repulsion for the solute, while a continuous removal of the preferentially
sorbed interfacial water, which is of a monomolecular nature, is effected by
flow under pressure through the membrane capillaries. According to this
model, the critical pore diameter for a maximum separation and
permeability is equal to twice the thickness of the preferentially sorbed
interfacial layer (Figure 54b).