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Файл:Plastics technology. Часть 1. Учебное пособие.pdf
111
been used for shiplap wall cladding, fencing and bench-type seating,
although in many of these cases UPVC materials have not proved
sufficiently robust.
Substantial quantities of UPVC are also used for blow moulded
containers for such diverse materials as consumable liquids such as fruit
squashes, liquids for household use such as detergents and disinfectants,
cosmetics and toiletries, and pharmaceuticals.
Plasticized PVC, referred to below as PPVC, is used in a wide
variety of applications. Originally a substitute for natural rubber when the
latter material became difficult to obtain during World War II, it is
frequently the first material to consider where a flexible, even moderately
rubbery, material is desired. This arises from the low cost of the
compounds, their extreme processing versatility, their toughness and t heir
durability.
The long-established use in wire and cable insulation remains the
largest single application area.
The market for PPVC film and sheet is only slightly less than for
wire and cable insulation. Uses are as diverse as seepage barriers, factory
doors, inflatables, baby pants, car trim, covering materials for book
bindings and document cases and shower curtains. UPVC film has also
been widely used for packaging of food, particularly for shrink-wrap
purposes, but this market has been subject to some substitution by
polypropylene.
PPVC has also retained a substantial market as a leathercloth,
where it is more durable than earlier cellulose-based products.
Tubes and profiles remain a small but important market for PPVC,
with garden hose being one well-known end-use. Other examples are trim
and edging strips for furniture.
UPVC manufactured by paste processes is used not only for
leathercloth but for sports equipment and playballs, sheathings and
protective covers. PVC i s widely used for the manufacture of “vinyl-coated
wallpaper”. The use of PVC for foam has decreased in recent years
although the material continues to find some use in rigid cellular
applications.
For many years vinyl chloride-vinyl acetate copolymers had two
main uses, in flooring compositions and for long playing gramophone
records. Whereas the former application remains strong, the use in
gramophone records has dropped sharply, particularly since the widespread
acceptance of polycarbonate-based compact discs.
112
2.3.2 Crystalline PVC
The development of stereoregular polyolefins and polydienes as the
result of the discovery of Ziegler-Natta-type catalysts not surprisingly
stimulated attempts to produce vinyl chloride polymers of increased stereo
regularity. Whilst the Ziegler-Natta catalysts cannot be used because they
react with both monomer and polymer, low-temperature free-radical
polymerization using γ-radiation and active substances such as
alkylboranes has been found to be feasible. Whereas conventional PV C is
about 55% syndiotactic and about 5% crystalline, materials polymerized at
-50ºC are about 65% syndiotactic and 20% crystalline. The Tg also
increases steadily with decrease in polymerization temperature, with a
value of about 105°C for PVC polymerized at -50°C. Melting occurs over a
range and it has been estimated than the Tm for 100% syndiotactic material
would be about 273°C.
It is interesting to note that these crystalline materials do not
dissolve in tetrahydrofuran or cyclohexanone at room temperature,
indicating that PVC is too weak a proton donor to overcome extensive
crystallization. Crystalline PVC has a greater tensile strength and creep
resistance than conventional polymer. It is, however, brittle, and whilst
most conventional impact modifiers appear ineffective, EVA polymer s are
said to be quite useful. Plasticized compounds may also be prepared
although mixing temperatures of up to 190°C are necessary.
2.3.3 Graft Polymers Based on PVC
Graft polymerization techniques have been lately employed in an
attempt to obtain improvements in toughness and thermal stability of PVC.
Commercial quantities of vinyl chloride grafted on to ethylene-vinyl acetate
copolymer and vinyl chloride grafted on to a butadiene-acrylonitrile
copolymer have become available as impact modifiers, the latter mat erial
being notable in providing tough, transparent blends with only a low
tendency to stress-whitening. Vinyl chloride-EVA graft copolymers are
used in blends with PVC homopolymers in unplasticized PVC compounds
for outdoor service, such as window frames, where good weatherability and
impact resistance at low temperature are important. In addition to b lendin g
materials, a number of ready-to-use high-impact PVC materials based on
graft polymers have become available recently. Grafts of vinyl chloride
with ethylene-propylene rubbers have also given polymers of high impact
strength. Finally one would mention a recent claim that grafting with cis-
1,4-polybutadiene sub-statially improved the thermal stability of PVC.
113
2.3.4 Vinyl Chloride-Propylene Copolymers
CH
2
CH
2
Cl
Cl
Cl
n
C
C
Cl
n
Copolymers of vinyl chloride with 2-10% propylene became
available in the USA in 1973 (Sta-Flow by Air Products and Chemicals
Inc.). Compared with the vinyl chloride homopolymer these copolymers
have a reduced tendency to dehydrochlorination and thus improved heat
stability. This is of importance both in application and during processing,
and one particular claim made for these products is their ease of moulding.
Typical products have a specific gravity of 1.3-1.4, a tensile
strength of 50 MPa and a flexural modulus of 2500 MPa.
Uses reported include motor housings for portable fans, food
blender bases, tape storage housings, tough, clear toys, injection blow
moulded products and thermoformed packages.
2.3.5 Vinyl Chloride-N-cyclohexylmaleimide Copolymers
Attempts to improve the heat deformation resistance of PVC by
chlorination or by manufacture of more crystalline grades results in a need
for much higher processing temperatures and thus reduced processing
stability. There may also be a loss in clarity. By copolymerizing vinyl
chloride with a small amount of N-cyclohexylmaleimide the softening
point can be raised without serious detriment to these properties. The
improvement is small, 5% of the comonomer raising the Vicat softening
point from 80 to 87°C. This is, however, sufficient for the material to be of
interest in applications involving short-term exposure to elevated
temperature, e.g. filling hot foodstuffs into containers. Such a copolymer
has been introduced by Hoechst (Hostalit LP HT 5060).
2.3.6 Vinylidene Chloride Polymers and Copolymers
Vinylidene chloride polymerizes spontaneously into
poly(vinylidene chloride), a polymer sufficiently thermally unstable to be
unable to withstand melt processing:
By copolymerizing the vinylidene chloride with about 10-15% of
114
vinyl chloride, processable polymer s may be obtained which are used in the
CH
2
C
Cl
C
H
H
H
Cl
Cl
CCl
2
manufacture of filaments and films. These copolymers have been marketed
by the Dow Company since 1940 under the trade name Saran. Vinylidene
chloride-acrylonitrile copolymers for use as coatings of low moisture
permeability are also marketed (Saran, Viclan).
The monomer is produced from trichloroethane by
dehydrochlorination. This may be effected by pyrolysis at 400°C, by
heating with lime or treatment with caustic soda. The trichlorethane itself
may be obtained from ethylene, vinyl chloride or acetylene.
Vinylidene chloride is a clear mobile liquid which is highly
inflammable and with the following physical properties:
Boiling point 31.9°C at 760 mmHg
Specific gravity 1.233 at 15.5°C
Refractive index 1.4246 at 20°C
Heat of polymerization 60.6 kJ/mole
Although miscible with many organic solvents it has a very low
solubility in water (0.04%).
The handling of the monomer presents a number of problems. The
monomer will polymerize on storage even under an inert gas. Polymer
deposition may be observed after standing for less than a day. Exposure to
air, to water or to light will accelerate polymerization. A number of
phenolic materials are effective inhibitors, a typical example being 0.02%
p-methoxyphenol. Exposure to light, air and water must, however, still be
avoided. The monomer has an anaesthetic action and chronic toxic
properties and care must therefore be taken in its handling.
The polymer may be prepared readily in bulk, emulsion and
suspension, the latter technique apparently being preferred on an industrial
scale. The monomer must be free from oxygen and metallic impurities.
Peroxide such as benzoyl peroxide is used in suspension polymerizations
which may be carried out at room temperature or at slightly elevated
temperatures. Persulphate initiators and the conventional emulsifying soaps
may be used in emulsion polymerization. The polymerization rate for
vinylidene chloride-vinyl chloride copolymers is markedly less than for
either monomer polymerized alone.
Conside rat ion of t he st ruct ure of pol y (vi nyl id ene c hlor ide ) ena ble s
115
certain predictions to be made about its properties.
CH
2
CH
2
Cl
Cl
CH
2
C
Cl
C
Cl
C
Cl
Cl
It will be seen that the molecule has an extremely regular structure
and that questions of tacticity do not arise. The polymer is thus capable of
crystallization. The resultant close packing and the heavy chlorine atom
result in the polymer having a high specific gravity (1.875) and a low
permeability to vapours and gases.
The solubility parameter is calculated at 20 MPa
1/2
and therefore
the polymer is swollen by liquids of similar cohesive forces. Since
crystallization is thermodynamically favoured even in the presence of
liquids of similar solubility parameter and since there is little scope of
specific interaction between polymer and liquid there are no effective
solvents at room temperature for the homopolymer.
The chlorine present results in a self-extinguishing polymer. It also
leads to a polymer which has a high rate of decomposition at the
temperatures required for processing.
Copolymerization, with for example vinyl chloride will reduce the
regularity and increase the molecular flexibility. The copolymers may thus
be processed at temperatures where the decomposition rates are less
catastrophic.
Vinylidene chloride-vinyl chloride polymers are also selfextinguishing and possess very good resistance to a wide range of
chemicals, including acids and alkalis. They are dissolved by some cyclic
ethers and ketones.
Because of the extensive crystallization, even in the copolymers,
high strengths are achieved even though the molecular weights are quite
low (~20000-50000). A typical 85:15 copolymer plasticized with diphenyl
ethyl ether has a melting point of about 170°C, a glass temperature of about
-17°C and a maximum rate of crystallization at approximately 90°C.
Properties and applications of vinylidene chloride-vinyl
chloride copolymers
Since some properties of the vinylidene chloride-vinyl chloride
copolymers are greatly dependent on crystallization and orientation it is
convenient to consider the applications of these copolymers and then to
discuss the properties of the products.
116
The copolymers have been used in the manufacture of extruded
pipe, moulded fittings and for other items of chemical plant. They are,
however, rarely used in Europe for this purpose because of cost and the low
maximum service temperature. Processing conditions are adjusted to give a
high amount of crystallinity, for example by the use of moulds at about
90°C. Heated parts of injection cylinders and extruder barrels which come
into contact with the molten polymer should be made of special materials
which do not cause decomposition of the polymer. Iron, steel and copper
must be avoided. The danger of thermal decomposition may be reduced by
streamlining the interior of the cylinder or barrel to avoid dead-spots and by
careful temperature control. Steam heating is frequently employed.
Additives used include plasticizers such as diphenyl diethyl ether,
ultraviolet light absorbers such as 5-chloro-2-hydroxybenzophenone (1-2%
on the polymer) and stabilizers such as phenoxy propylene oxide.
The copolymers are used in the manufacture of filaments. The
filaments are used for deck chair fabrics, car upholstery, decor ati ve radio
grilles, dolls' hair, filter presses and for sundry other applications where
their toughness, flexibility, durability and chemical resistance are of
importance.
Biaxially stretched copolymer film is a useful though expensive
packaging material (Saran Wrap-Dow) possessing exceptional clarity,
brilliance, toughness and water and gas impermeability. A number of
grades are available differing in transparency, surface composition and
shrinkage characteristics.
2.3.7 Vinylidene Chloride-Acrylonitrile Copolymers
Copolymers of vinylidene chloride with 5-50% acrylonitrile were
investigated by IG Farben during World War II and found to be promising
for cast films. Early patents by ICI and Dow indicated that the copolymers
were rigid, transparent and with a high impact strength.
The principal commercial outlet for these copolymers (Saran,
Viclan) has, however, been as coatings for cellophane, polyethylene, paper
and other materials and as barrier layers in multi-layer extruded films. Such
coatings are of value because of their high moisture and gas
impermeability, chemi cal resistance, clarity, toughness and heat sealability.
The percentage of acrylonitrile used is normally in the range 5-15%. Higher
quantities facilitate solubility in ketone solvents whereas lower amounts,
i.e. higher vinylidene chloride contents, increase the barrier properties. The
barrier properties of these copolymers are of the same order as those of the
117
vinylidene chloride-vinyl chloride copolymers, and they are claimed in the
Cl
a
C
C
H
H
F
F
F
F
F
F
C
C
F
F
C
C
b
H
c
d
C
C
F
H
H
F
trade literature to be between 100 and 1000 times more impermeable than
low-density polyethylene in respect of CO2, nitrogen and oxygen
transmission. The development of multilayer packaging films has led to
widespread use of vinylidene chloride-based polymers as barrier layers. For
example, a multi-layer system polystyrene-vinylidene chloride polymerpolystyrene exhibits low permeability to gases, water vapours and odours
and is used for packaging dairy produce. The system polystyrenevinylidene chloride polymer-polyethylene additionally exhibits good
chemical resistance, stress cracking resistance and heat sealability (on the
polyethylene surface) and is used for dairy produce, fruit juices,
mayonnaise, coffee and pharmaceuticals.
2.3.8 Polytetrafluoroethylene
The high thermal stability of the carbon-fluorine bond has led to
considerable interest in fluorine-containing polymers as heat-resistant
plastics and rubbers. The first patents, taken out by IG Farben in 1934,
related to polychlorotri-fluoroethylene (PCTFE) (Figure 17 (a)), these
materials being subsequently manufactured in Germany and the United
States. PCTFE has been of limited application and it was the discovery of
polytetrafluoroethylene (PTFE) (Figure 17 (b)) by Plunkett in 1938 which
gave an impetus to the study of fluorine-containing polymers.
The inability to process PTFE by conventional thermoplastics
techniques has nevertheless led to an extensive search for a meltprocessable polymer but with similar chemical, electrical, non-stick and
low-friction properties. This has resulted in several useful materials being
marketed, including tetrafluoro-ethylene-hexafluoropropylene copolymer,
poly(vinylidene fluoride) (Figure 17 (d)), and, most promisingly, the
copolymer of tetrafluoroethylene and perfluoropropyl vinyl ether. Other
fluorine-containing plastics include poly(vinyl fluoride) and polymers and
copolymers based on CTFE.
Figure 17 –Polychlo ro tr if luoroethylene (PCTFE) (a);
Polytetrafluoreoethylene (PTFE) (b);
Poly(vinyl fluoride) (c); Poly(vinylidene flu or id e ) (d)
118
The fluororubbers also form an important class of speciality
elastomers and although the market is dominated by the vinylidene
fluoride-hexafluoro-propylene copolymers a wide range of materials has
been produced over the past 40 years.
World-wide capacity for fluoropolymers in the late 1990s has been
estimated at 90000 t.p.a. divided roughly equally between Western Europe,
North America and the rest of the world. This total is dominated by PTFE
although this has decreased from about 80% of the total in 1980 to about
60% in the late 1990s.
In addition to the presence of stable С–F bonds, the PTFE molecule
possesses other features which lead to materials of outstanding heat
resistance, chemical resistance and electrical insulation characteristics and
with a low coefficient of friction. It is today produced by a number of
chemical manufacturers such as Du Pont (Teflon), ICI (Fluon), Hoechst
(Hostaflon TF), Rhone-Poulenc (Soreflon), Montecatini (Algoflan), Nitto
Chemical-Japan (Tetraflon) and Daikin Kogyo-Japan (Polyflon).
Preparation of monomer
Tetrafluoroethylene was first prepared in 1933. The current
commercial syntheses are based on fluorspar, sulphuric acid and
chloroform. The reaction of fluorspar (CaF2) and sulphuric acid yields
hydrofluoric acid
CaF2 + H2SO4 → CaSO4 + 2HF
Treatment of chloroform, obtained by reacting methanol and
chlorine, with the hydrofluoric acid yields monochlorodifluoromethane,
also used as a refrigerant, which is a gas boiling at -40.8°C.
CHCl3 + 2HF → CHClF2 + 2HCl
The monochlorodifluoromethane may be converted to
tetrafluoroethylene by pyrolysis, for example by passing through a
platinum tube at 700°C.
2CHClF2 → CF
═CF
2
+2HCl
2
Other fluorine compounds are produced during pyrolysis, including
some highly toxic ring structures. Since very pure monomer is required for
polymerization, the gas is first scrubbed to remove any hydrochloric acid
and then distilled to separate other impurities. Tetrafluoroethylene has a
boiling point of -76.3°C. For safe storage under pressure the oxygen
content should be below 20 ppm. Traces of compounds which react
preferentially with oxygen such as 0.5% dipente ne , ben z a l d e h yd e or methyl
methacrylate may be added as stabilizers.
119
Polymerization
n
F
F
F
C
F
C
Pure uninhibited tetrafluoroethylene can polymerize with violence,
even at temperatures initially below that of room temperature. There is little
published information concerning details of commercial polymerization. In
one patent example a silver-plated reactor was quarter-filled with a solution
consisting of 0.2 parts ammonium persulphate, 1.5 parts borax and 100
parts water, and with a pH of 9.2. The reactor was closed and evacuated,
and 30 parts of monomer were let in. The reactor was agitated for one hour
at 80°C and after cooling gave an 86% yield of polymer.
PTFE is made commerci ally by two major processes, one leading
to the so called “granular” polymer and the second leading to a dispersion
of polymer of much finer particle size and lower molecular weight. One
method of producing the latter involved the use of a 0.1% aqueous
disuccinic acid peroxide solution. The reactions were carried out at
temperatures up to 90°C. It is understood that the Du Pont dispersion
polymers, at least, are produced by methods based on the patent containing
the above example.
Structure and properties
Polytetrafluoroethylene is a linear polymer free from any
significant amount of branching:
Whereas the molecule of polyethylene is in the form of planar
zigzag in the crystalline zone this is sterically impossible with that of PTFE
due to the fluorine atoms being larger than those of hydrogen. As a
consequence the molecule takes up a twisted zigzag, with the fluorine
atoms packing tightly in a spiral around the carbon-carbon skeleton. A
complete turn of the spiral will involve over 26 carbon atoms below 19°C
and 30 above it, there being a transition point involving a 1% volume
change at this temperature. The compact interlocking of the fluorine atoms
leads to a molecule of great stiffness and it is this feature which leads to the
high crystalline melting point and thermal form stability of the polymer.
The intermolecular attraction between PTFE molecules is very
small, the computed solubility parameter being 12.6 (MJ/m3)
polymer in bulk does not thus have the high rigidity and tensile strength
which is often associated with polymers with a high softening point.
1/2
. The
120
The carbon-fluorine bond is very stable. Further, where two
fluorine atoms are attached to a single carbon atom there is a reduction in
the С–F bond distance from 1.42 Å to 1.35 Å. As a result bond strengths
may be as high as 504 kJ/mole. Since the only other bond present is the
stable С–С bond, PTFE has a very high heat stability, even when heated
above its crystalline melting point of 327°C.
Because of its high crystallinity and incapability of specific
interaction, there are no solvents at room temperature. At temperatures
approaching the melting point certain fluorinated liquids such as
perfluorinated kerosenes will dissolve the polymer.
The properties of PTFE are dependent on the type of polymer and
the method of processing. The polymer may differ in particle size and/or
molecular weight. The particle size will influence ease of processing and
the quantity of voids in the finished product whilst the molecular weight
will influence crystallinity and hence many physical properties. The
processing techniques will also affect both crystallinity and void content.
The weight average molecular weights of commercial polymers
appear to be very high and are in the range 400000 to 9000000. The degree
of crystallinity of the finished product will depend on the rate of cooling
from the processing temperatures. Slow cooling will lead to high
crystallinity, with fast cooling giving the opposite effect. Low molecular
weight materials will also be more crystalline.
It is observed that the dispersion polymer, which is of finer particle
size and lower molecular weight, gives products with a vastly improved
resistance to flexing and also distinctly higher tensile strengths. These
improvements appear to arise through the formation of fibre-like structures
in the mass of polymer during processing.
There has been some recent interest in polymers containing very
small proportions (<2000 ppm) of a second comonomer. These can
interfere with crystallization and the resulting products are claimed to have
improved compression strength, electrical insulation properties, weldability
and transparency compared with the unmodified homopolymers.
General properties
PTFE is a tough, flexible, non-resilient material of moderate tensile
strength but with excellent resistance to heat, chemicals and to the passage
of an electric current. It remains ductile in compression at temperatures as
low as 4K (-269°C).
As with other plastics materials, temperature has a considerable