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Файл:Plastics technology. Часть 2. Учебное пособие.pdf
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Following exhaustion of a mixed bed and prior to regeneration, the
resins in the bed are separated by applying backwash at a flow rate
sufficiently high to fluidize the bed. By virtue of that fact that anion
exchange resins have a lower density than cation exchange resins, hydraulic
separation into two layers occurs. The regenerant caustic soda then contacts
the upper layer of anion resin and the acid regenerant flows through the
lower layer of cation resin. Then following a rinse with relatively pure
water, the resins are air-mixed prior to the next ion-exchange run.
Ion-exchange methods are established for treating various effluents
arising from the metal finishing processes such as plating and anodizing.
The use of strong-base resins for decolorizing sugar liquors is widely
practiced. The coloring bodies are organic anions that are sorbed by weakly
cross-linked strong-base gel resins.
Even wines are sometimes treated by column cation exchange.
Potassium hydrogen tartrate, which causes an unpleasant precipitate in
wines, is converted to the more soluble sodium salt by treatment with
polystyrene sulfonic acid resin in the sodium (Na+) form.
Ion-exchange resins are used for metal recovery from low-grade
ores and dilute leach liquors. One of the best examples is the recovery of
uranium.
Ion-exchange chromatography is well known for separating
mixtures of ions in solution. Possibly the best-known organic analytical
ion-exchange application is the chromatographic separation and isolation of
amino acids. Commercially, the most significant application is the recovery
of antibiotics such as streptomycin and neomycin. The fermentation broth
containing the impure antibiotic is treated with a polyacrylic weak-acid
resin on which the antibiotic is sorbed to the exclusion of other organic
impurities. The product is recovered by elution with dilute mineral acid.
Polycarboxylates
Polyacrylate-type homopolymers are polyelectrolytes and are the
most ionic of the organic polymers. They dissolve in water giving aqueous
solutions with unusual and useful physical properties. They are generally
made by free-radical polymerization in aqueous solution. Very-high-
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molecular weight (e.g., 4×106) polymers can be obtained that give very
viscous solutions. Polyacrylic (1), polymethacrylic (2), and polyitaconic (3)
acids are the three main types having theoretical capacities of 13.9, 11.6,
and 15.4 eq/kg, respectively. Aqueous solutions or dry powders of these
materials are commercially available.
(1) (2) (3)
Versicols (Allied Colloids) and Texigels (Scott Bader) are
homopolymers of acrylic or methacrylic acids or their copolymers with
acrylamide. They are used as stabilizers, and protective colloids and
thickeners for aqueous dispersions, binders, and flocculants. Carbopols
(B.F.Goodrich) are different grades of polyacrylic acid of varied molecular
weight having excellent suspending, thickening, and gel-forming
properties. Carbosets (B.F.Goodrich) are acrylic copolymers and have a
similar carboxylic content but generally can be dissolved in alkaline
solutions. They are used mainly in coating applications. In some
applications, they are covalently cross-linked with epoxides and so on, but
some applications use ionic cross-links made with zinc ions.
A notable application of polyacrylic acid is for cements in
dentistry. These are made by mixing an aqueous solution of the polymer
with zinc oxide when the zinc salt precipitates as a highly cross-linked gel
that rapidly sets to a hard mass under oral conditions. In a variation of this
reaction, the zinc oxide is replaced with a tooth-colored glass powder that
releases Al3+ and Ca2+ ions. These cements, called ASPA (aluminosilicate
polyacrylic acid) or glass ionomer, set very rapidly, bond well to tooth
enamel, and are compatible with living tissue.
Integral polyelectrolytes
Polyelectrolytes having bound ions integrated in the polymer
backbone are called ionenes. Some ionenes have been studied for their
bacteriostatic and bactericidal activity. Ionenes with segments of
polypropylene oxide in the backbone have been evaluated as thermoplastic
elastomers.
Polyethylenimine (PEI) is an integral polyelectrolyte that is
available commercially, e.g., Polymin (BASF). It is formed by the ring-
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opening polymerization of ethyleneimine (aziridine). The resulting
polyamine has about 50% of the expected secondary-amine functionality
and about 50% primary and tertiary due to branching:
PEI has typical polyelectrolyte properties; it is a highly viscous
hygroscopic liquid, completely miscible with water and lower alcohols,
insoluble in benzene, and reactive toward cellulose. PEI is mainly used as a
size, flocculating agent, or protective colloid, notably in the paper and
textile industries, because of its ability to bind to cellulosic fibers.
Membranes based on PEI were introduced for use in reverse
osmosis to desalinate water. These membranes, known as NS100 and
NS101, are made by forming a PEI skin on polysulfone support and
insolubilizing it by treatment with toluene di-isocyanate or phthaloyl
dichloride to produce a polyurea or polyamide (Figure 59).
Figure 59 - Insolubilization of polyethylenimine by treatment with toluene
diisocyanate or phthaloyl dichloride.
274
This tutorial was written using the follo wing l iterat u re:
1. J.A.Brydson. Plastics Materials. – Butterworth Heinemann. –
Oxford. – 1999. – 954 p.
2. Manas Chanda, Salil K.Roy. Plastics Technology Handbook. –
Taylor&Francis Group. – USA. – 2007. – 816 p.
3. Kroschwitz J., Howe-Grant M. Encyclopedia of Chemical
Technology (vol.1-27). – 1998. – 4-th edition.
275
УЧЕБНОЕ ИЗДАНИЕ
С.Ю. Софьина
PLASTICS TECHNOLOGY
Ответственный за выпуск С.Н. Русанова
276
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