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Файл:Plastics technology. Часть 2. Учебное пособие.pdf
191
heating circuits. Because of its toughness combined with its heat and
CCl
3
CCl
3
HF
CCl
2
CCl
2
F
Z
n
CF
3
CFCl
chemical resistance it also finds use for lining pumps and valves and other
equipment for the chemical industry and for laboratory ware.
Whereas Tefzel is said to be an internally stablized copolymer of
TFE and ethylene, other copolymers that are compounds of similar
copolymers with stabilizers of antioxidants are now also available
(Hostaflon ET by Hoechst and Aflon by Asahi Glass Co.). Glass-fibrefilled grades are also available.
1.7.7 Polychlorotrifluoroethylene Polymers (PCTFE) and Copolymers with Ethylene (ECTFE)
Polychlorotrifluoroethylene was the first fluorinated polymer to be
produced on an experimental scale and polymers were used in Germany
and in the United States early in World War II. PCTFE was used, in
particular, in connection with the atomic bomb project in the handling of
corrosive materials such as uranium hexafluoride.
The monomer may conveniently be produced from
hexachloroethane via trichlorotrifluoroethane
The major differences in properties between PTFE and PCTFE can
be related to chemical structure. The introduction of a chlorine atom, which
is larger than the fluorine atom, breaks up the very neat symmetry which is
shown by PTFE and thus reduces the close chain packing. It is still,
however, possible for the molecules to crystallize, albeit to a lower extent
than PTFE. The introduction of the chlorine atom in breaking up the
molecular symmetry appears to increase the chain flexibility and this leads
to a lower softening point. On the other hand the higher interchain
attraction results in a harder polymer with a higher tensile strength. The
unbalanced electrical structure adversely affects the electrical insulation
properties of the material and limits its use in high-frequency applications.
Because of the lower tendency to crystallization it is possible to
produce thin transparent films.
The chemical resistance of PCTFE is good but not as good as that
of PTFE. Under certain circumstances substances such as chlorosulphonic
acid, molten caustic alkalis and molten alkali metal will adversely affect
the material. Alcohols, acids, phenols and aliphatic hydrocarbons have
little effect but certain aromatic hydrocarbons, esters, halogenated
hydrocarbons and ethers may cause swelling at elevated temperatures.
192
The polymer melts at 216°C and above this temperature shows
CH
CH
+ HF
HgCl
3
on Charcoal
CH
2
CHF
better cohesion of the melt than PTFE. It may be processed by
conventional thermoplastics processing methods at temperatures in the
range 230-290°C. Because of the high melt viscosity high injection
moulding pressures are required.
PCTFE is more expensive than PTFE and its use is comparatively
limited. With the advent of FEP copolymers, TFE-ethylene copolymers and
the perfluoroalkoxy polymers the advantage of melt processability is no
longer, alone, a sufficient justification for its use. The particular advantages
of the material are its transparency in thin films and its greater hardness
and tensile strength as compared to PTFE and FEP copolymers. Examples
of its use include gas-tight packaging film for medical and military
applications (the main use), transparent windows for chemical and other
apparatus where glass or other materials cannot be used, seals, gaskets and
O-rings and some electrical applications such as hook-up wire and terminal
insulators. Consumption, estimated at 350-400 tonnes per annum, is only
about 1% that of PTFE.
PCTFE is marketed by Hoechst as Hostaflon C2 and in the United
States by Minnesota Mining and Manufacturing (Kel-F) and Allied
Chemical (Halon).
Copolymers of chlorotrifluoroethylene and ethylene were
introduced by Allied Chemicals under the trade name Halar in the early
1970s. This is essentially a 1:1 alternating copolymer compounded with
stabilizing additives. The polymer has mechanical properties more like
those of nylon than of typical fluoroplastic, with low creep and very good
impact strength. Furthermore the polymers have very good chemical
resista nce and electrical insulation prop erties and are resistant to burning.
They may be injection moulded or formed into fibres.
1.7.8 Poly(vinyl fluoride) (PVF)
Poly(vinyl fluoride) was first introduced in the early 1960s, in film
form, by Du Pont under the trade name Tedlar. Details of the commercial
method of preparing the monomer have not been disclosed but it may be
prepared by addition of hydrogen fluoride to acetylene at about 40°C.
It may also be prepared by pyrolysis of 1,1-difluoroethane at
725°C over a chromium fluoride catalyst in a platinum tube or by the
action of zinc dust on bromodifluoroethane at 50°C.
193
The polymers were first described by Newkirk. Polymerization
may be brought about by subjecting acetylene-free vinyl fluoride to
pressures to up to 1000 atm at 80°C in the presence of water and a trace of
benzoyl peroxide.
Although poly(vinyl fluoride) resembles PVC in its low water
absorption, resistance to hydrolysis, insolubility in common solvents at
room temperature and a tendency to split off hydrogen halides at elevated
temperatures, it has a much greater tendency to crystallize. This is because
the fluorine atom (c.f. the chlorine atom) is sufficiently small to allow
molecules to pack in the same way as polyethylene.
PVF has better heat resistance than PVC and exceptionally good
weather resistance. It will burn slowly. Instability at processing
temperatures makes handling difficult but this problem has been
sufficiently overcome for Du Pont to be able to market their Tedlar film.
PVF film is now being used in the manufacture of weatherresisting laminates, for agricul tura l glazing and in electrical applications.
1.7.9 Poly(vinylidene fluoride)
This melt-processable homopolymer was first introduced in 1961
as Kynar by the Pennsalt Chemical Corporation (the company name being
subsequently changed to Pennwalt). Other companies now manufacturing
similar polymers are Dynamit Nobel (Dyflor), Kureha (KF), Solvay (Solef)
and Atochem (Foraflon).
The monomer is a gas boiling at -84°C which may be made by
dehydrochlorination of 1-chloro-1,1-difluoroethane:
CF2ClCH3 → CF2=CH2
or by dechlorination of 1,2-dichloro-1,1-difluoroethane:
CF2ClCH2Cl → CF2=CH2
Poly(vinylidene fluoride) is a crystalline polymer melting at
171°C. Amongst the melt-processable fluoroplastics the polymer is of
interest because of its good mechanical properties and relatively low price.
Tensile and impact strengths are good and the material is flexible in thin
sections. Although it has generally good chemical resistance, strongly polar
solvents such as dimethylacetamide tend to dissolve the polymer whilst
some strongly basic primary amines such as n-butylamine tend to cause
embrittlement and discolouration. The polymer is also attacked by some
concentrated acids. A further disadvantage of the material is that its
dielectric properties are frequency dependent and this limits its use as an
electrical insulator. The high dielectric constant is a particular feature.
194
Of greater interest in recent years have been the peculiar
piezolectric properties of poly(vinylidene fluoride). In 1969 it was
observed that stretched film of the polymer heated to 90°C and
subsequently cooled to room temperature in a direct current electric field
was 3-5 times more piezoelectric than crystalline quartz. It was observed
that the piezolectric strain coefficients were higher in the drawn film and in
the normal directions than in the direction transverse to the film drawing.
The piezoelectric phenomena have been used to generate ultrasonic
waves up to microwave frequencies using thin poly(vinylidene fluoride)
transducers. In the audio range a new type of loudspeaker has been
introduced using the transverse piezolectric effect on a mechanically biased
membrane. This development has been of considerable interest to
telephone engineers and scientists.
Poly(vinylidene fluoride) also has interesting pyroelectric
properties showing a stable and reversible polarization which persists after
several heating cycles. In consequence the film is used in pyroelectric
detectors. PVDF has a wide processing window in that there is a big
difference between the melt temperature and the decomposition
temperature. Thermal stability may, however, be drastically affected by
contaminants, and scrupulous cleanliness is important when processing.
The generation of HF should decomposition occur during processing is an
obvious hazard. Typical melt temperatures are in the range 240-260°C,
with mould temperatures being anything from 30 to 120°C.
The polymer, like many fluorine-containing polymers has very
good weathering resistance and may also be used continuously up to
150°C. Outside of the electrical field it finds use in fluid handling, in hot
water piping systems, in packaging and in chemical plant. A widely used
specific application for PVDF is in ultra-pure water systems for the
semiconductor industry.
2 PLASTICS BASED ON CHEMICALLY MODIFIED POLYMERS
2.1 General Patterns of Polymer Chemical Modification
Polymer modification is a directed change in the properties of
polymers in order to give them new valuable technical features. During
chemical modification, the original polymer is subjected to physical or
chemical effects. As the result, it turns i nto a new polymer of a different
chemical structure. Chemical modification of polymers is accomplished by
195
substitution of either hydrogen atoms or atoms of other elements, or various
groups of the polymer chain by other atoms and groups. Due to these
transformations, the resulting polymer acquires new properties. Thus, the
polymer can acquire the ability to dissolve in organic solvents and to soften
when heated, therefore, the polymer can be processed into various products
(films, fibers, cast and extruded products, paints, and etc.). Chemical
modification of polymers gives them acidic or basic properties due to the
introduction of the relevant groups. The substitution of hydrogen atoms in
polyethylene, poly(vinyl chloride) and ot her vinyl polymer s by the atoms
of chlorine is widely used to produce chlorinated polyethylene, chlorinated
polyvinyl chloride, and etc. The process of chlorination is often combined
with the simultaneous influence of sulfur dioxide, resulting in the formation
of a chlorosulfonated polymer. The reaction of hydrogen atoms substitution
in the benzene ring of polystyrene and its homologues in the synthesis of
ion-exchange polymers is widely spread. For this purpose polystyrene is
treated with nitric acid and then the resulting nitro group is reduced to the
primary amino group, which is then subjected to alkylation, and thus turns
into the secondary or tertiary amine.
Sulfonation of polystyrene results in ion-exchange resins
containing sulfonic acid benzene ring.
An important reaction widely used for modification of hydroxyl
polymers is the replacement of hydrogen atoms in the hydroxyl group by
the alkyl or acyl groups. In case of proper processing of cellulose,
poly(vinyl alcohol) and other alcohols, polymer ethers and esters of
cellulose, as well as acetals o f poly (vinyl alcohol) are obtained.
Features of polymer chemical modification
Chemical modification of polymers is one of the most important
ways to obtain polymers with a given set of properties. It is based on the
substitution reaction of the hydrogen atom near the carbon, oxygen or
nitrogen atoms by other atoms or groups. Although these reactions are
common organic synthesis transformations, and proceed in the same
conditions as in the case of low molecular weight compounds, the final
product depends on the polymer nature of the initial substance.
A characteristic feature of the processes of polymer modification is
the fact that all reactions of this type tend not to complete; in addition, they
are accompanied by side effects. This is due to the influence of several
physical and chemical factors, such as the nature of reagents and catalysts,
the structure of polymer chains, and etc. Theref ore, as the result of these
196
reactions, a mixture of different macromolecules is formed, each of them
C
6
H
7
O
2
(OH)
3
m
C
6
H7O2(OH)
2
(OR)
n
C
6
H
7
O
2
(OH)(OR)
2
p
C
6
H
7
O
2
(OR)
3
q
CH
2
CH
NO
2
m
CH2CH
NO
2
n
contains various chain links in a variety of combinations. The resulting
polymer is chemically inhomogeneous since its macromolecule consists of
various links. Thus, due to substitution of hydroxyl groups in cellulose, the
obtained polymer contains glucose units, in which three, two, or one of the
hydroxyl groups can be substituted; at the same time, a number of units can
not be substituted.
Thus, the cellulose macromolecule contains four types of chain
links. In addition, these links are randomly distributed in the
macromolecule, this affects the polymer properties. By nitration,
sulfonation and phosphorylation of a polymer (e.g., polystyrene),
monosubstituted isomer chain links a re fo rmed:
The heterogeneity of the system is of great importance. If you carry
out similar reactions in a homogeneous solution, we can obtain a very
homogenous highly substituted polymer. However, in some cases, even
under homogeneous conditions, you can not achieve full substitution of the
reactive groups. This is due to mutual steric or electrostatic (polar)
influence of various groups in the immediate vicinity of the polymer chain
– the so-called "neighbor effect". The "neighbor effect" has a significant
influence on the chemical activity of the reactive centers and can lead either
to passivation of the neighboring groups, or to their activation. In obtaining
poly(vinyl al cohol) acetals under the influence of aldehydes on poly(vinyl
alcohol), due to the "neighbor effect", no more than 86% of hydroxyl
groups can be substituted, and the resulting polymer possesses various
chain links:
197
CH
2
CH
OH
m
CH
2
HC
O
CH
O
CH
CH
2
R
n
CH
2
CH
O
CH
O
R
p
In polyethylene chlorination, the substitution of hydrogen by
CH
2
C
CH
3
CO
O
CH2C
CH
3
CO
O
R
-RO
-
C
OC
O
CO
C
CH
2
CH
3
H3C
CH
2
H2O
CH
2
C
CH
3
CH2C
CH
3
CO
O
-
CO
O
-
HC CH
CH
2
H
2
C
O
+
H
H
O
C
CH
3
O
CH CH
CH
2
O
CH
O
CH
CH
2
H2C
O
+
H
H
CH
3
chlorine occurs until the chlorine content reaches 60%, then the reaction
slows down and stops when the chlorine content reaches 73%. The cont ent
does not reach the theoretical value of 85.5%. These results can be
explained by the fact that only the –CH2CHCl– (56.8% of chlorine) groups
are formed during the first stage, then the –СНС1СНС1– (73.2% of
chlorine) chain links appear, afterwards, the chlorination process stops due
to the induction effect of chlorine atoms in the chain.
A typical example of the reaction acceleration due to the "neighbor
effect" is the saponification of the methacrylate and methacrylic acid
copolymer:
In case of a partially saponified poly(vinyl acetate), or poly(vinyl
acetal), this interaction takes place through the formation of a cycle by
hydrogen bonding, which facilitates the saponification of acetals and
acetates by increasing their reactivity:
It should be noted that the polymer chemical modification reactions
198
occur in fairly harsh conditions under the influence of active agents and
may be accompanied by destruction of the polymer macromolecules. This
is especially important for the cellulose, which is easily hydrolyzed due to
the presence of acetal bonds. Therefore, the conditions under which the
reaction occurs should be chosen taking into account the characteristics of
the original polymer, in order to minimize the destructive processes.
In addition, various undesirable side effects, negatively affecting
the properties of the product, can occur during modification.
Methods of polymer chemical modification
Substitution reactions of individual atoms and groups during the
chemical modification of polymers can occur in a homogeneous system,
i.e., in solution, or in a heterogeneous system under the influence of low
molecular weig h t liq u id re a g e nts onto the solid polymer.
The reactions in a homogeneous system tend to be more complete
and are seldom accompanied by polymer destruction processes.
The reactions in a heterogeneous system do not proceed so fast as
in a solution, and, therefore, they require more time for completion. The
resulting product is less homogeneous, this depends on the velocity of the
reactants diffusion into the solid polymer. The substitution degree increases
in the course of the reaction.
The reactions of substitution in insoluble polymers (cellulose) or in
three-dimensional polymers (crosslinked copolymers of styrene) occur in
heterogeneous systems.
2.2 Chemically Modified Polymers of Unsaturated Hydrocarbons and their Halogen Derivatives
2.2.1 Cross-Linked Polyethylene
Cross-linking of a crystalline thermoplastic polymer has, in
general, two distinct effects. Firstly it interferes with molecular packing,
reducing the level of crystallization, and consequently the polymer has a
lower modulus, hardness and yield strength than the corresponding noncross-linked material. More importantly, because the network structure still
exists above the crystalline melting point the material retains a measure of
strength, typical of a rubber material. Polyethylene is typical in such
behaviour and because of the enhanced heat resistance (in terms of
resistance to melt flow) cross-linked or vulcanized polyethylene finds
199
application in the cable industry both as a dielectric and a sheathing
material.
Three main approaches are used for cross-linking polyethylene:
(1) Radiation cross-linking;
(2) Peroxide cross-linking;
(3) Vinyl silane cross-linking.
Radiation cross-linking requires expensive equipment and
extensive protective measures. The technique is being used commercially
and is most suitable with thin sections. Equipment requirements for
peroxide curing are somewhat simpler but the method requires close
control. At elevated temperatures the peroxide molecules break up,
producing free radicals. These abstract hydrogen from the polymer chain to
produce a polymer free radical. In the case of polyethylene the most likely
reaction is that two radicals will combine and thus cross-link two chains
but other reactions may lead to chain scission. It is important that the
peroxide be sufficiently stable thermally to withstand compounding and
shaping operations without degradation in order to avoid premature crosslinking. Dicumyl peroxide is frequently used for low-density polyethylene
but more stable peroxides are necessary for higher density materials. For
cable covering, high production rates require high curing temperatures in
the absence of oxygen and this normally involves the use of high-pressure
steam in a long curing tube set into the extrusion line. Large amounts of
carbon black may be incorporated into polyethylene that is to be crosslinked. The carbon black is believed to take part in the cross-linking
process and the compounded product has superior mechanical properties in
many respects to the unfilled material. It is also to be noted that
copolymers of ethylene with small amounts of vinyl acetate are often
preferred for peroxide cross-linking.
The third process for cross-linking is the Sioplas process
developed by Dow. The first stage of this involves the grafting of an easily
hydrolysable trialkoxyvinylsilane onto the polyethylene chain, the site
activation having been achieved with the aid of a small amount of
peroxide. The compound is then extruded onto the wire, which is collected
on a drum. The drum is then exposed to hot water, or, more commonly,
low-pressure steam. The water hydrolyses the alkoxy groups, which then
condense to form a siloxane cross-link. The cross-linking stage is
facilitated by the use of a cross-linking catalyst, which is typically an
organo-tin compound. A number of variations of this process exist and in
one of these compounding, grafting and extrusion onto wire are carried out
in the same extruder.
200
OR
PE
Molecule
RO
RO
Si
RO
RO
Si
+
CH
CH Si
2
OR
OR
+ H2O
OR
Cross-linking
Catalyst
OR
Grafing
Initiator
CH2CH2Si(OR)
Si
O
Si
3
+ 2ROH
There has been interest, particularly in Japan, in the production of
cross-linked low-density polyethylene foam. Some processes, such as the
Furukawa process and the Hitachi process, use chemical cross-linking
techniques whilst others, such as the Sekisui process, involve radiation
cross-linking.
These cross-linked cellular materials have been used in the
automotive industry for carpeting, boot mats and sound deadening. They
have also found use for pipe insulation and as flotation media for oilcarrying and dredging hose.
2.2.2 Chlorinated Polyethylene
The first patent on the chlorination of polyethylene was taken out
by ICI in 1938. In the 1940s scientists of that company carried out
extensive studies on the chlorination process. The introduction of chlorine
atoms onto the polyethylene backbone reduces the ability of the polymer to
crystallize and the material becomes rubbery at a chlorine level of about
20%, providing the distribution of the chlorine is random. An increase in
the chlorine level beyond this point, and indeed from zero chlorination,
causes an increase in the Tg so that at a chlorine level of about 45% the
polymer becomes stiff at room temperature. With a further increase still,
the polymer becomes brittle.
Chlorination may be carried out with both high-density and lowdensity polyethylene. When carried out in solution the chlorination is
random but when carried out with the polymer in the form of a slurry the
chlorination is uneven and due to residual crystalline zones of
unchlorinated polyethylene the material remains a thermoplastic.