Добавил:
ivanov666
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Plastics technology. Часть 2. Учебное пособие.pdf
251
Figure 54 - Schematic representation of preferential sorption
capillary flow mechanism (a). Critical pore diameter for maximum
separation and permeability (b)
From an industrial standpoint, the two basic parameters
characterizing reverse osmosis systems for a given separation are:
(1) rejection factor, which involves the choice of the appropriate
chemical nature of the film surface;
(2) water flux, which depends on methods for preparing films
containing the la rg e st num bers of pores of the required size.
This approach is the basis of the successful development of porous
cellulose acetate membranes for desalination and other applications.
To achieve high flux in reverse osmosis, the membrane should
have a high surface area and be very thin (0.02-1.0 mm), but to withstand
the high applied pressure it also needs to be very strong. To meet these
requirements, which are in conflict, the semipermeable membrane is used
as a skin mounted on a support, which is another membrane that is very
porous, like a filter paper, and that is nonselective, and very thick (>100
μm). In some such bilayers each layer is of the same polymer and the
membrane is then called integral or asymmetric, but in others the layers are
of different polymers and the membrane is called a composite. Composite
membranes have the advantage of each layer being individually optimized
– the skin for its permeability and the support for its strength – but they are
harder to make them integral membranes.
Polyelectrolytes, except those which are covalently cross-linked,
are soluble in water, and their applications are based on this property.
Basically, these applications depend on the polyelectrolyte altering the fluid
properties of an aqueous medium, or modifying the behavior of particles in
aqueous slurries or colloidal suspensions.
252
Polyelectrolytes raise the viscosity of aqueous solutions thus acting
as thickeners, and the magnitude of the effect increases with the polymer’s
molecular weight. Naturally occurring gums and acidic polysaccharides
have been traditionally used as thickening agents in food stuffs and
pharmaceutical products, but, more recently, synthetic polyelectrolytes
have been used in these roles. Much use is also made of polyelectrolytes to
modify the characteristics of latex paints and similar proprietary fluids.
Polyelectrolytes can also stabilize particles in aqueous suspension
and so act as dispersants. In this reaction, the hydrophobic backbones of
polyelectrolytes are absorbed onto the surface of the particles by van der
Waals attraction, while their ions form a hydrophilic surface and interact
with water. In suspension or bead polymerization, for example, a
hydrophobic vinyl monomer is dispersed in water by agitation to form
droplets that are stabilized by a polyelectrolyte. Each stabilized droplet of
the monomer acts as a bulk polymerization system and on polymerization
gets the shape of a bead.
Polyelectrolytes, depending on their ionic charge, can interact with
colloidal particles and neutralize the stabilizing hydrophilic charges, thus
acting as flocculating agents. They have been used in this way to coagulate
slurries and industrial wastes.
There are other applications of polyelectrolytes that depend on their
behavior in water in various ways. They are thus used as sizes in the textile
industry and in paper manufacturing, and as additives to drilling muds and
to soil for conditioning purposes.
A very different kind of application of polyelectrolytes is their use
as dental cements. Here a divalent cation is added to an aqueous solution of
a polyanion to form a highly cross-link ed precip itate o f great strength.
2.5.1 Ionomers
Polyethylene ionomers
Ionomers based on polyethylene are mainly copolymers with
pendant carboxylate groups in which the polyethylene backbone is their
major component (>90%). Ethylene is directly copolymerized with
methacrylic acid in a continuous process developed from the high-pressure
method used to make low-density polyethylene by free-radical initiation.
The comonomers are mixed in appropriate proportions, allowing for the
greater reactivity of the methacrylic acid, and introduced with a peroxide
initiator to a reactor at a pressure of about 207 MPa and a temperature of
253
250-280°C. Because methacrylic acid reacts disproportionately rapidly and
has to be replenished frequently, the conversion to polymer is kept low per
pass, and at 15-20% conversion the residual monomer is removed, then
recycled with fresh monomer. This ensures that the methacrylic acid units,
and hence the ionic cross-l inks formed later, are randomly distributed and
uniform in composition.
The distinctive properties of the ionomers become manifest only
when the carboxylic acid groups are neutralized. The neutralization is a
kind of post-treatment that is performed by melting the polyacid in a mill at
150°C and adding a base or basic salt in powder form or in solution. In
these neutralizations, the melt starts as a soft, fluid, opaque mass of
polyacid and then becomes stiff, rubbery, and transparent as the ionomer is
formed.
The loss in fluidity, or gain in melt strength, is a significant factor
determining the usefulness of ionomers. That the effect is due to the
formation of strong ionic cross-links is clearly shown by a comparison of
the melt viscosity of an ionomer with its acid precursor (Figure 55). The
melt viscosity of a cop olymer of eth ylene a nd metha cryli c acid co ntain ing,
for example, 2 mole per cent of acid is increased by only about 50%
compared to an otherwise equivalent polyethylene homopolymer, but when
neutralized, and therefore ionized, the melt viscosity increa ses tw enty-fold.
(The slight increase with the acid is attributed to the weak hydrogenbonded cross-links.) The melt-strength of an ionomer is such that its molten
film can be drawn over the sharp edges of a nail without it being punctured
or torn.
A striking property of the ethylene ionomers is that they are
transparent, unlike their acid precursors or polyethylene itself, which are
not. The haze in polyethylene is due to the fact that the polymer is partially
crystalline, and minute crystallites within it agglomerate into spherulites,
which are of a size to scatter light. Ionomers also contain crystallites, and
the crystallite formation is, in fact, helped by the ions, the domains of
which serve to nucleate them; but the crystallites are unable to agglomerate
because of the high viscosity of their surroundings. Thus, microcrystallinity
is enhanced by the ions, but macro-crystallinity, which causes haze, is
inhibited. The transparency of ionomers is useful in packaging applications.
254
Figure 55 - Melt viscosity of poly(ethylene-co-methacrylic acid)
Copolymer*
Ionomer
Na+ Zn
2+
Appearance
Tensile strength, MPa
Hazy
12.4
Hazy
23.4
Transparent
35.9
Transparent
29.6
and its sodium salt relative to polyethylene.
Some physical properties of polyethylene ionomers are compared
with those of polyethylene and the acid copolymer, poly(ethylene-comethacrylic acid) in Table 21. Ionomer is generally tougher and, as shown
in Table 21, relative to the acid copolymer, its tensile strength is increased
by 27-53% and its stiffness is nearly tripled.
Table 21 – Comparative physical properties of polyethylene ionomers and
their acid precursor
Property PE
Melt index, g/10 min
Elongation, %
Polyethylene ionomers are described as flexible and tough with
good impact toughness at low temperature, and with good resistance to
grease and solvents, to stress-cracking, and to abrasion. They are blowmolded into films, sheets, bottles, and blister packs, and injection-molded
into various objects. Their resistance to grease and solvents has made them
useful in meat packaging. They are very effective as external coatings on
glass bottles to contain break ag es.
-
600
(–CO2H)
5.8
553
0.03
330
0.09
313
255
Elastomeric ionomers
A number of ionic polymers exist that have a recognized elastomer
as the covalent backbone and have a small ionic content, so they may be
called elastomeric ionomers. The ions provide at least a part of the crosslinks in these polymers. Those elastic ionomers that are cross-linked
exclusively by their ions have, however, the useful feature of being
thermoplastic.
Carboxylated polybutadiene ionomers, which are close relatives of
the polyethylene ionomers described above, have an essentially
polybutadiene backbone that contains some acrylonitrile and styrene to
adjust its flexibility and toughness, and, in addition, up to 6% by weight of
acrylic or methacrylic acid. Like the polyethylene ionomers, they are
usually made by direct copolymerization with the carboxylic acid monomer
using, however, emulsion methods. Typically the monomers are slurried in
water with sodium dodecylbenzene sulfonate as the emulsifier and
potassium persulfate as the free-radical initiator. The tendency of the
carboxylic acid monomer to dissolve in the aqueous phase instead of
remaining in the butadiene-rich phase is suppressed by making the aqueous
phase acidic so that the monomer remains in the nonionized form.
The carboxylated polybutadienes, when neutralized, undergo ionic
cross-linking, producing the effect of vulcanization. The neutralization can
be done by treating with aqueous sodium hydroxide then heating, or by
heating directly with zinc oxide. The ionic cross-link formed with the
sodium ion of moderate strength at room temperature and dissociates at
100°C. With zinc, the ionic cross-link formed is much stronger, although
the polymer is capable of substantial flow at higher temperatures.
Some properties of sodium and zinc vulcanizates are compared
with those of the acid precursor in Table 22. High tensile strength is a
characteristic of ionic vulcanizates. As a comparison, a standard
polybutadiene elastomer vulcanized with sulfur gives a strength of 1.9-5.8
MPa, whereas an equivalent copolymer containing 1.5 mole% methacrylic
acid, and vulcanized with magnesium oxide, gives 29.0 MPa. They also
respond differently to fillers such as carbon black – sulfur vulcanizates are
reinforced, while ionic vulcanizates are weakened.
Carboxylated polybutadienes have not been used much as
thermoplastic elastomers, principally because they have poor compression
set, high stress relaxation, and poor performance at higher temperatures.
Carboxylated polybutadienes, supplied as lattices, are used mainly in
256
dipping and coating processes, applications, which often do not involve
Property
Acid
copolymer
Sodium
vulcanizate
Zinc
vulcanizate
Tensile strength, MPa
0.7
11.7
41.4
ionized carboxylate, at least directly. The applications include adhesives,
paper coating, glove-dipping, carpet-backing, binding nonwoven fabrics,
and also shrink-proofing woolen garments where the carboxyl groups are
believed to react with pendant amino groups in the proteins of the wool. A
wide range of carboxylated lattices are available as well-developed items of
commerce; some of these are styrene-butadiene rubbers (SBRs) and others
are acrylonitrile-butadiene rubbers (NBRs).
Table 22 – Vulcanization of carboxylated polybutadienes
Elongation, %
1600
900
400
Mixed vulcanizations, such as with zinc oxide and sulfur or zinc
oxide and peroxide, are used for carboxylated NBRs in order to combine
the advantages of the ionic method with the conventional methods. The
mixed vulcanizates have high tensile strength and notable resistance to
abrasion, oil, and fuel. They are used in applications such as industrial
rollers and wheels and shoe soles. Dry NBRs are available in fewer grades
than lattices. Examples are Krynac by Doverstrand and Hycar by Goodrich.
Elastomeric ionomers have also been developed from ethylenepropylene-diene ternary copolymers known as EPDM rubbers. The diene is
commonly ethylidene norbornene. Du Pont made carboxylated ionomers by
free-radical grafting of maleic anhydride (0.5-5%) onto the die ne moi et y of
the polymer and neutralized the product with rosin salt. The ethylidene
norbornene can also be sulfonated, thus:
Ionomer properties of sulfonated EPDM are said to develop only
257
when the acids are neutralized. The best properties are given by zinc salts,
particularly when they are plasticized by zinc stearate.
Ionomers based on polytetrafluoroethylene
Polymers with a polytetrafluoroethylene (PTFE) backbone and
pendant perfluorosulfonate or perfluorocarboxylate groups have become
commercially important materials, although they are expensive. The
sulfonates were introduced as Nafion by Du Pont in the early 1970s and the
carboxylates as Flemion by Asahi Glass in 1978. They are made by freeradical copolymerization of tetrafluoroethylene and perfluorovinyl
monomers giving precursor copolymers, (1) and (2), which can be postfunctionalized by hydrolysis to generate sulfonic and carboxylic acid
groups. The perfluorovinyl monomers themselves are made from
tetrafluoroethylene by multi-step synthesis using hexafluoropropylene
oxide.
Nafion precursor Flemion precursor
(1) (2)
Although originally developed for use as a membrane in fuel cells,
Nafion has found more useful applications in various other electrolytic
separation processes. In these applications, use is made of its cationexchange properties and its ability to survive in extremely aggressive
chemical environments. An outstanding example is the use of Nafion as a
membrane in the chlor-alkali cell where it is gradually replacing the
traditional diaphragm and mercury cells. In the chlor-alkali process a cell is
partitioned by the polyanionic membrane into an anode compartment, to
which brine is added, and a cathode compartment to which water is added
(Figure 56). On application of electric potential, Na+ passes from the brine
around the anode and through the membrane to the water around the
cathode where it forms sodium hydroxide. The membrane keeping the brine
and water separate allows transfer of Na+ ions by ion-exchange and acts as
a barrier to Cl– and OH–.
258
Figure 56 - Chlor-alkali cell
The Nafion membranes have ionic contents of 0.66-0.91 Eq/kg. A
bilayer of these two extremes of composition is made and also a bilayer
with a perfluorocarboxylate (Flemion) polymer. The bilayered membranes
are made by melt processing the precursor copolymer, which then, in
membrane form, is hydrolyzed.
Perfluorocarboxylates, the Flemions, were introduced with the idea
that a carboxylate at a given ion content would be less hydrophilic than a
sulfonate and so would be of help in balancing the ionic content of an ionexchange membrane with its permselectivity and hydrophilicity as required.
The bilayered Nafions also resulted from similar thinking and at tempt to
combine certain advantages of the two types.
The efficiency with which power is consumed in the electrolysis
and the highest concentration of uncontaminated sodium hydroxide that can
be produced determine the membrane performance in the chlor-alkali
process. Rapid progress in membrane performance has been made since the
introduction of carboxylate membranes, leading to an efficiency level of
95% and a concentration of 33%.
Bilayer carboxylate membranes can be produced by surface
modification of Nafion-type membranes. In a process used by Asahi
Chemical the sulfonate on a Nafion-type surface is reduced to sulfinic and
sulfenic acids, then oxidized to a carboxylate layer of 2-10 mm thickness:
259
Tokoyama Soda makes Neosepta F membranes by treating a
Nafion-type precursor with an alcohol and the oxidizing the surface in air.
Nafions in their acid forms are super-acidic, i.e., stronger than
sulfuric acid and so have high catalytic power. Nafion powders in acid form
have therefore been used as catalysts in many types of reactions, such as
esterification and Friedel-Crafts reactions at low temperatures.
Ionomers based on polysulfones
Ionomers of commercial polysulfones, principally sulfonates, are
being developed as membranes, particularly for purifying water by reverse
osmosis. In this application, they are superior to conventional membrane
materials because they are resistant to oxidation by the chlorine used in
water treatment, to harsh chemical cleaning operations, to biological
fouling, and to compaction under high operating pressures. The traditional
polymer, cellulose acetate, is less strong, while Nafion, though very inert
chemically, is too costly and difficult to fabricate in a form suitable for
reverse osmosis. The sulfones, having completely amorphous backbones
and solubility, can be easily cast into membranes from solvents. The
optimum ion content is about one eq/kg, which provides a balance of
properties between a high flux of the permeant (water) and a low leakage of
the rejected species (the dissolved salts).
Commercial polysulfones such as Udel (Union Carbide) and
Victrex (ICI) are sulfonated in a post-functionalization step to make
ionomers. When dissolved in dichloroethane and treated with a complex of
sulfur trioxide and triethyl phosphate, Udel becomes monosulfonated on
the rings marked R (as the rings connected to the sulfone group are
deactivated by the sulfone group):
Udel
260
Victrex is a random copolymer of two units in which the ring
marked R of unit B becomes monosulfonated by the simple process of
dissolving it in sulfuric acid. The other aromatic rings and the whole of unit
A are inert. The extent of the reaction is predetermined by the proportion of
unit B in the copolymer. The strength of the ionic associations in sulfonated
Victrex is indicated by the increase in Tg, which occurs linearly with the
content of unit B despite the already high value of 230°C for the parent
polymer.
Victrex
2.5.2 Polyelectrolytes
Ion-exchangers
The most important class of ion-exchangers are the organic ionexchange resins. Their framework, the so-called matrix, consists of an
irregular, macromolecular, three-dimensional network of hydrocarbon
chains. The matrix carries ionic groups such as –SO
AsO
2–
in cation exchangers and –NH
3
+
, >NH
3
–
, –COO–, –PO
3
+
, >N+<, and ≡S+ in anion
2
2–
3
, and
exchangers. Ion-exchange resins thus are cross-linked polyelectrolytes. The
first completely synthetic ion-exchange resins were prepared by
B.A.Adams and E.C.Holmes in England in 1935. Today more than 100
synthetic ion-exchange resins are marketed throughout the world by various
companies.
Ion-exchange resins are supplied as insoluble, water-swellable
beads that have either a dense internal structure (gel-type) or a porous,
multichannelled one (macroporous-type or macroreticular). The gel-type
PS resin was the first to be introduced (1947) and the macroporous-type
came later (1959).
Ion-exchangers are broadly classified as cation exchangers and
anion-exchangers. Carriers of exchangeable cations are called cation