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Файл:Plastics technology. Часть 1. Учебное пособие.pdf
121
effect on mechanical properties. Even at 20°C unfilled PTFE has a
measurable creep with compression loads as low as 2.1 MPa.
The coefficient of friction is unusually low and stated to be lower
than that of any other solid. A number of different values have been quoted
in the literature but are usually in the range 0,02-0,10 for polymer to
polymer.
The chemical resistance of PTFE is exceptional. There are no
solvents and it is attacked at room temperature only by molten alkali metals
and in some cases by fluorine. Treatment with a solution of sodium metal
in liquid ammonia will sufficiently alter the surface of a PTFE sample to
enable it to be cemented to other materials using epoxide resin adhesives.
Although it has good weathering resistance, PTFE is degraded by
high-energy radiation. Exposure to a dosage of 70 Mrad will halve the
tensile strength of a given sample. The polymer is not wetted by water and
does not measurably absorb it. The permeability to gases is low, the water
vapour transmission rate being approximately half that of low-density
polyethylene and poly(ethylene terephthalate).
Processing
PTFE is normally available in three forms:
(1) Granular polymers with median particle size of 300 and 600µm.
(2) Dispersion polymer obtained by coagulation of a dispersion. It
consists of agglomerates with an average diameter of 450 µm made up of
primary particles 0.1 µm in diameter.
(3) Dispersions (latices) containing about 60% polymer in particles
with an average diameter of about 0.16 µm.
The exceptionally high melt viscosity above the melting point
(about 1010-1011 poises at 350°C) prevents the use of the usual techniques
for processing thermoplastics. In the case of granular polymers, methods
allied to those used with cerami cs and in powder metallurgy are employed
instead. In principle this involves preforming the powder, usually at room
temperature, sintering at a temperature above the mel ting point, typically at
about 370°C, and then cooling.
Granular polymer may also be extruded, albeit at very low rates
(2.5-16 cm/min), by means of both screw and ram extruders.
PTFE mouldings and extrudates may be machined without
difficulty. Film may be obtained by peeling from a pressure sintered ring
and this may be welded to similar film by heat sealing under light pressure
at about 350°C.
122
Dispersion polymer, which leads to products with improved tensile
strength and flex life, is not easily fabricated by the above techniques. It
has, however, been found possible to produce preforms by mixing with 1525% of a lubricant, extruding and then removing the lubricant and
sintering. Because of the need to remove the lubricant it is possible to
produce only thin-section extrudates by this method.
Additives
Because of the high processing temperatures there are few
pigments suitable for use with PTFE. A number of inorganic pigments,
particularly the cadmium compounds, iron oxides and ultramarines, may,
however, be used.
The resistance of PTFE to creep can be improved by blending in up
to 25% of glass or asbestos fibre using PTFE dispersions. By the same
technique alumina, silica and lithia may be incorporated to give compounds
of improved dimensional stability coupled with good electrical insulation
properties. Molybdenum disulphide and graphite improve dimensional
stability without losing the low coefficient of friction whilst the use of
barium ferrite will produce a material that can be magnetised. The
incorporation of titanium dioxide serves to increase the dielectric constant
whilst certain compounds of boron increase the resistance to neutron
bombardment.
Applications
The use of PTFE in a great diversity of applications may be
ascribed to the following properties:
(1) Chemical inertness.
(2) Exceptional weathering resistance. Samples exposed in Florida
for 10 years showed little change in physical properties.
(3) The excellent electrica l insulation characteristics.
(4) The excellent heat resistance.
(5) The non-adhesive properties.
(6) The very low coefficient of friction.
However, world production is only about 55000 tonnes per annum
and this is a reflection of the high volume cost, the rather specialized
techniques involving lengthy processing times and to a small er extent the
high creep rate under load.
Because of its chemical inertness over a wide temperature range it
is used in a variety of seals, gaskets, packings, valve and pump parts and in
laboratory equipment.
123
Its excellent electrical insulation properties lead to its use in wire
insulation, in valve holders, in insulated transformers, in hermetic seals for
condensers, in laminates for printed ciruitry and for many other
miscellaneous electrical applications.
PTFE is used for lining chutes and coating other metal objects
where low coefficients of friction or non-adhesive characteristics are
required. Because of its excellent flexing resistance, inner linings made
from dispersion polymer are used in flexible steam hose. A variety of
mouldings are used in aircraft and missiles and also in other applications
where use at elevated temperatures is required.
Because of its high volume cost PTFE is not generally used to
produce large objects. In many cases, however, it is possible to coat a metal
object with a layer of PTFE and hence meet the particular requirement.
2.3.9 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:
CF2Cl•CH3 → CF
═CH
2
2
or by dechlorination of 1,2-dichloro-1,1-difluoroethane:
CF2Cl•CH2Cl→ CF2═CH
2
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 dimethyl acetamide 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.
Of greater interest in recent years have been the peculiar
piezolectric properties of poly(vinylidene fluoride). In 1969 it was observed
124
that stretched film of the polymer heated to 90°C and subsequently cooled
CH
2
C
CH
3
COOCH
3
I
CH
2
CH
COOH
II
CH
2
CH
COOR
n
III
n
CH
2
COOR
n
IV
C
CH
3
CH
2
CH
CN
n
V
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 pie zoel ect ric 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.
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.
2.4 Polymers Derivatives of Acrylic and Methacrylic Acid
Poly(methyl methacrylate) is, commercially, the most important
member of a range of acrylic polymers which may be considered
structurally as derivatives of acrylic acid (II).
This family includes a range of polyacrylates (III),
polymethacrylates (IV) and the important fibre-forming polymer,
polyacrylonit ri le (V ).
125
Methyl, ethyl and allyl acrylate were first prepared in 1873 by
Caspary and Tollens, and of these materials the last was observed to
polymerize. In 1880 Kahlbaum reported the polymerization of methyl
acrylate and at approximately the same time Fittig found that methacrylic
acid and some of its derivatives readily polymerized.
In 1901 Otto Rohm reported on his studies of acrylic polymers for
his doctoral dissertation. His interest in these materials, however, did not
cease at this stage and eventually in 1927 the Rohm and Hass concern at
Darmstadt, Germany commenced limited production of poly(methyl
acrylate) under the trade names Acryloid and Plexigum. These were soft
gummy products of interest as surface coatings rather than as mouldable
plastics materials. About 1930 R.Hill in England and W.Bauer in Germany
independently prepared poly(methyl methacrylate) and found it to be a
rigid, transparent polymer, potentially useful as an aircraft glazing material.
The first methacrylic esters were prepared by dehydration of
hydroxyisobutyric esters, prohibitively expensive starting points for
commercial synthesis. In 1932 J.W.C.Crawford discovered a new route to
the monomer using cheap and readily available chemicals – acetone,
hydrocyanic acid, methanol and sulphuric acid – and it is his process which
has been used, with minor modifications, throughout the world. Sheet
poly(methyl methacrylate) became prominent during World War II for
aircraft glazing, a use predicted by Hill in his early patents, and since then
has found other applications in many fields.
Examples of commercial poly(methyl methacrylate) sheet are
Perspex (ICI), Oroglas and Plexiglas (Atoglas). Poly(methyl methacrylate)
moulding powders include Diakon (ICI), Acry-ace (Fudow Chemical Co.,
Japan), Lucite (Du Pont) and Vedril (Montecatini).
In addition to poly(methyl methacrylate) plastics and
polyacrylonitrile fibres, acrylic polymers find widespread use. First
introduced in 1946, acrylic rubbers have become established as important
special purpose rubbers with a useful combination of oil and heat
resistance. Acrylic paints have become widely accepted particularly in the
car industry whilst very interesting reactive adhesives, including the wellknown “super-glues” are also made from acrylic polymers.
During the 1970s there was considerable interest for a time in
copolymers with a high acrylonitrile content for use as barrier resins, i.e.
packaging materials with low permeability to gases. Problems associated
with free acrylonitrile have, however, led to the virtual disappearance of
these materials from the market.
126
Other developments in recent years have been the appearance of
Trivial name
IUPAC name
Poly(acrylic acid)
Poly(methyl methacrylate)
Poly-[1-(carboxy)ethylene]
Poly-[1-methoxycarbonyl)-1-methylethylene]
CCH
3
CH
3
C
CH
3
O
+ HCN
CH
3
OH
CN
tough and heat-resistant materials closely related to poly(methyl
methacrylate) and to inter esting cro ss-linked polymers. Amongst these are
the so-called hydrophilic polymers used in the making of soft contact
lenses.
Today a very wide range of acrylic materials is available with a
broad property spectrum. The word acrylic, often used as a noun as well as
an adjective in everyday use, can mean quite different things to different
people. In the plastics industry it is commonly taken to mean poly (methyl
methacrylate) plastics, but the word has different meanings, to the fibre
chemist and to those working in the paint and adhesives industries. Unless
care is taken this may be a source of some confusion.
As with other major plastics materials, there is at present little use
of the IUPAC systemat ic nomenclature, which is based on the nature of the
repeating unit rather than the monomer used. The following names may,
however, be noted:
Poly(acrylonitrile)
Poly(methyl acrylate)
Poly-[1-(cyano)ethylene]
Poly-[1-methoxycarbonyl)ethylene]
2.4.1 Poly(methyl methacrylate)
Preparation of monomer
This successful commercial utilization of poly(methyl
methacrylate) is due in no small measure of the process of producing the
monomer from acetone developed by Crawford of ICI which enabled the
polymer to be produced at a competitive price. Some details of the process
as operated by the Rohm and Hass Company of Philadelphia have been
disclosed.
Acetone is first reacted with hydrogen cyanide to give acetone
cyanohydrin:
The cyanohydrin is then treated with 98% sulphuric acid in a
cooled hydrolysis kettle to yield methacrylamide sulphate:
127
CCH
3
CH
3
CH
2
CH
CH
3
OH
CN
+ H
2SO4
CONH
2
H2SO
4
The sulphate is not isolated from the reaction mixture, which
CH
3
CH
2
CH
CONH
2
H2SO
4
CH3OH
CH
CH
2
CH
3
COOCH
3
+ NH
4
HSO
CH
CCH
3
CH
3
C
CH
3
CH
3
CH
2
CH
CH
2
CH
3
COOH
CH
3
Nitrogen
Oxides
COOH
-H
2
O
CH
3
OH
CH
C
CH
3
COOCH
3
b
passes into an esterification kettle and reacts continously with methanol:
The esterified stream, which may contain inhibitors to prevent
premature polymerization, is then passed to a stripping column which
separates the methyl methacrylate, methanol and some water from the
residue made up of sulphuric acid, ammonium bisulphate and the remainder
of the water. The methyl methacrylate is subsequently separated and
purified by further distillation.
Because of limitations on the ready availability of HCN,
particularly in Japan, processes involving the oxidation of C4 intermediates
have been developed and are now replacing the older route developed by
Crawford. One important process is based on the two-stage oxidation of
isobutylene or t-butyl alcohol to methacrylic acid, which is then separated
and esterified (a).
CH
CH
CH
CH
3
+
CH
CH
3
CH
3
CH
CHO
CH
CH
COOH
3
+
O
2
1/2O
+ ROH
2
2
2
CH
CH
2
a
2
CH
3
CH
+ H2O
CHO
CH
3
CH
2
COOH
CH
3
CH
2
H2O+
COOR
This process appears to be very similar to the process developed by
128
the Escambia Chemical Company which has been known for over 30 years
(b).
The monomer is a mobile liquid with a characteristic sweet odour
and with the following properties:
Boiling point (760 mmHg) 100.5°C
Density D
Refractive index n
20
4
20
D
0.936-0.940 g/cm3
1.413-1.416
Heat of polymerization 48.5 kJ/mole
Polymerization
Methyl methacrylate will polymerize readily and the effect may be
observed with non-inhibited samples of monomers during storage. In
commercial practice the monomer is supplied with up to 0.10% of an
inhibitor such as hydroquinone, which is removed before polymerization,
either by distillation under reduced pressure or, in some cases, by washing
with an alkaline solution.
Free-radical polymerization techniques involving peroxides or
azodi-isobutyronitrile at temperatures up to about 100°C are employed
commercially. The presence of oxygen in the system will affect the rate of
reaction and the nature of the products, owing to the formation of
methacrylate peroxides in a side reaction. It is therefore common practice
to polymerize in the absence of oxygen, either by bulk polymerization in a
full cell or chamber or by blanketing the monomer with an inert gas.
It has been observed that in the polymerizaton of methyl
methacrylate there is an acceleration in the rate of conversion after about
20% of the monomer has been converted. The average molecular weight of
the polymer also increases during polymerization. It has been shown that
these results are obtained even under conditions where the re is a n egli gible
rise in the temperature (<1°C) of the reaction mixture.
The explanation for this effect (known variously as the gel effect,
Tromsdorff effect or auto-acceleration effect) is that the chain termination
reaction slows down during conversion and a decrease in the termination
rate constant leads to an increase in both overall rate and molecular weight.
The reason for the drop in termination rate is that as the reaction mixture
becomes more viscous the radical ends of the polymer chain s find increa sed
difficulty in diffusing towards each other, leading to the important mutual
termination reaction. Small monomer molecules on the other hand find
little difficulty in diffusion at moderate conversion so that propagation
reactions are relatively little affected, until the material becomes semi-solid,
129
when the propagation rate constant also decreases. It is of interest to note
that the gel effect may be induced by the addition of already formed
poly(methyl methacrylate) or even another polymer such as cellulose
tripropionate because such additions increase the viscosity of the system.
The auto-acceleration effect appears most marked with polymers
that are insoluble in their monomers. In these circumstances the radical end
becomes entrapped in the polymer and termination reactions become very
difficult. It has been suggested that, in thermodynamic terms, methyl
methacrylate is a relatively poor solvent for poly(methyl methacrylate)
because it causes radicals to coil while in solution. The termination reaction
is then determined by the rate at which the radical ends come to the surface
of the coil and hence become available for mutual termination.
Emulsion polymerization
The principal markets for aqueous dispersion polymers made by
emulsion polymerization of methacrylic esters are the paint, paper, textile,
floor polish, and leather industries where they are used principally as
coatings or binders. Copolymers of methyl met hacrylate with either ethyl
acrylate or butyl acrylate are most common.
Most of the lower alkyl methacrylates readily polymerize in water
in the presence of a surfactant and a water-soluble initiator. The final
product is an opaque, gray, or milky-white disperson of high molecular
weight polymer at a concentration of 30-60 wt % in water. The particle size
of methacrylic–acrylic copolymer dispersions ranges from 0.1 to 1.0 μm.
These emulsion polymerizations are usually rapid and give high molecular
weight polymers at high concentration and low viscosity. Difficulties in
agitation, heat transfer, and material transfer, which are often encountered
in the handling of viscous polymer solutions, are gready decreased with
aqueous dispersions. In addition, the safety hazards and the expense of
flammable solvents are eliminated.
The surfactants used in the emulsion polymerization of acrylic or
methacrylic monomers are classified as anionic, cationic, or nonionic.
Anionic surfactants, such as alkyl sulfates and alkylarene sulfonates and
phosphates, or nonionic surfactants, such as alkyl or aryl polyoxyethylenes,
are most common. Mixed anionic nonionic surfactant systems are also
widely utilized.
Water-soluble peroxide salts, such as ammonium or sodium
persulfate, are the usual initiators. The initiating species is the sulfate
radical anion generated from either the thermal or redox cleavage of the
130
persulfate anion. The thermal dissociation of the persulfate anion, which is
a first-order process at constant temperature, can be gready accelerated by
the addition of certain reducing agents and small amounts of polyvalent
metal salts, or both. By using redox initiator systems, rapid polymerizations
are possible at much lower temperatures (25-60°C) than are practical with
thermally initiated systems (75-90°C).
Methacrylic emulsion polymerizations are usually conducted by
batch processes in jacketed stainless steel or glass-lined ketdes designed to
withstand an internal pressure of at least 446 kPa. Agitators are constructed
from the same materials as the reactor. Versatility in controlling agitation is
provided by use of variable speed drive. A baffle is sometimes used in the
kettle to improve mixing; however, excessive shear must be avoided to
control formation of coagulum. The temperature of the reactants is
controlled by circulating steam and cold water through the jacket. A
schematic diagram of a typical plant installation is given in Figure 18. A
feed line for emulsified monomer enters through the top of the kettle along
with feed lines for adding aqueous solutions of initiators and activators or
both. Additional equipment includes a temperature recorder, manometer,
sightglass, and emergency stack equipped with a rupture disk.
Monomer emulsions are prepared in separate stainless steel
preemulsification tanks, which are usually equipped with an agitator,
manometer level gauge, cooling coils, temperature recorder, rupture disk,
flame arrester, and various nozzles for charging the ingredients. Monomer
emulsions maybe charged in one shot to the reactor or, more commonly,
fed continuously throughout the polymerization.
A simple stainless steel drumming tank is used to receive the
polymerized emulsion and hold it until it is packaged into drums or tank
cars. This tank also is employed to adjust the solids content and pH, for the
addition of preservatives, stabilizers, and thickeners, and for blending
operations. A paddle-type low speed agitator is used for mi xing. A cooling
jacket on the drumming tanks permits the finished dispersion to be
discharged hot from the reactor, thus increasing the productivity of the
kettle. The cooled finished dispersion is passed through a coarse filter prior
to packaging.