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Файл:Plastics technology. Часть 1. Учебное пособие.pdf
141
Applications
The major uses of poly(methyl methacrylate) arise from its high
light transmission and good outdoor weathering properties. It is also a
useful moulding material for applications where good appearance,
reasonable toughness and rigidity are requirements which are considered to
justify the extra cost of the polymer as compared with the large tonnage
plastics.
For many years the market growth for poly(methyl methacrylate)
was much lower than for other major thermoplastics. For example, UK
production in 1950 was about the same as that for polystyrene, in 1965 it
was about 40% and by the end of the 1970s it was down to about 10%.
There was, however, an upsurge in the late 1980s and early 1990s and
world production capacity was estimated at 1.7·106 t.p.a. in 1996. This is
about 17% of the capacity for polystyrene. During the late 1990s there was
a considerable capacity build-up in Asia and already by 1996 this area
claimed about 38% of global capacity followed by America with 34% and
Europe 28%. While the overall market is roughly divided between
mouldings and sheet products extruded sheet is making inroads into the
cast sheet market and i n 1997 in the USA it was estimated that less than
25% of PMMA products were produced from cast (mainly sheet) materials.
In Western Europe the market has been assessed at auto applications 30%,
illumination engineering 20-25%, building industry 15%, optical industry
10-15%, household goods 8-10%, and other 15%.
The material is eminently suitable for display signs, illuminated
and non-illuminated, and for both internal and external use. The properties
of importance here are weatherability, the variety of techniques possible
which enable a wide range of signs to be produced and, in some cases,
transparency.
In lighting fittings poly(methyl methacrylate) finds an important
outlet. Street lamp housings originally shaped from sheet are now injection
moulded. Ceiling lighting for railway stations, school rooms, factories and
offices frequently incorporate poly(methyl methacrylate) housings. In man y
of these applications opalescent material is used which is effective in
diffusing the light source. Poly(methyl methacrylate) is the standard
material for automobile rear lamp housings.
The methacrylic polymer remains a useful glazing material. In
aircraft applications it is used extensively on aircraft and for m the fami lar
“bubble” body of many helicopters. On land, acrylic sheet is useful for
142
coach roof lights, motor cycle windscreens and in do-it yourself “cabins”
for tractors and earth-moving equipment. Injection mouldings are
frequently used for plaques on the centre of steering wheels and on some
fascia panelling.
Transparent guards for foodstuffs, machines and even baby
incubators may be fabricated simply from acrylic sheet. It should be
pointed out that due to rather rapid surface deterioration and the lack of
“sparkle” the material is not ideally suited as a cover for displayed goods.
Acrylic sheet is also employed for many other diverse applications,
including baths and wash-basins, which have considerable design
versatility, are available in a wide range of colours, and are cheaper and
much lighter than similar products from other materials.
Extruded sheet is cheaper than cast sheet but because there is some
residual molecular orientation, is somewhat less satisfactory optically and
more difficult to machine. On the other hand, no doubt a function of its
lower molecular weight, it may be thermoformed more easily.
Because of its excellent weathering properties, transparency and
light weight compared with glass the material is being used for the domeshaped covers of solar collectors. In this application it is important to use a
heat-resistant film between the acrylic dome and the absorbing material,
both to reduce heat loss and to protect the acrylic material if there is an
accumulation of heat due to failure of the liquid circulation in the absorber.
Decorative plaques are produced by injection moulding
poly(methyl methacrylate) and then coating the back of the transparent
moulding with a thin coat of metal by the vacuum deposition technique or
with a paint by spraying.
If the surface of an acrylic sheet, rod or tube is roughened or
carved, less light is internally reflected and the material is often rather
brighter at these non-polished surfaces. The use of this effect enables
highly attractive carvings to be produced. Similarly, lettering cut into sheet,
particularly fluorescent sheet, becomes “lit-up” and this effect is useful in
display signs.
2.4.2 Methyl Methacrylate Polymers
with Enhanced Impact Resistance and Softening Point
As with other rigid amorphous thermoplastic polymers such as
PVC and polystyrene poly(methyl methacrylate) is somewhat brittle and, as
with PVC and polystrene, efforts have been made to improve the toughness
by molecular modification. Two main approaches have been used, both of
143
which have achieved a measure of success. They are copolymerization of
methyl methacrylate with a second monomer and the blending of
poly(methyl methacrylate) with a rubber. The latter approach may also
involve some graft copolymerization.
An early approach was to use butadiene as the comonomer but the
resultant copolymers have largely been used only in latex form in paper and
board finishes and are no longer believed to be important.
Copolymers of methyl methacrylate and butyl acrylate gave
polymers that were somewhat tougher and slightly softer than the
homopolymers.
Rather more recently Rohm and Haas GmbH have introduced
Plexidur plus which is a copolymer of acrylonitrile and methyl
methacrylate. It is best considered as a glazing material for use in schools,
sports halls and vehicles. The material also has good clarity, rigidity and
surface hardness.
Following the success in blending rubbery materials into
polystyrene, styrene-acrylonitrile and PVC materials to produce tough
thermoplastics the concept has been used to produce high-i mpact PMMAtype moulding compounds. These are two-phase materials in which the
glassy phase consists of poly(methyl methacrylate) and the rubbery phase
an acrylate polymer, usually poly(butyl acrylate). Commercial materials of
the type include Diakon MX (ICI), Oroglas DR (Rohm and Haas) and Plex
8535-F (Rohm GmbH).
In comparison with the styrene-based and better known ABS and
ASA materials the high-impact methacrylates have generally lower values
for mechanical properties such as tensile strength, impact strength and
modulus. However, long-term weathering tests show the marked
superiority of the methacrylates over ABS and even ASA materials to
degradation. In a typical test the impact strength of unnotched high-impact
PMMA rods was about sixfold that of both ABS and ASA materials.
Over the years many attempts have been made to produce
commercial acrylic polymers with a higher softening point than PMMA.
The usual approach was to copolymerise MMA with a second monomer
such as maleic anhydride or an N-substituted maleimide which gave
homopolymers with a higher Tg than PMMA. In this way copolymers with
Vicat softening points as high as 135°C could be obtained.
Such materials, known as poly(methyl methacrylimides) or PMMI,
are marketed by Rohm and Haas in the USA as Kamex, and there is a small
production by Rohm in Europe, where the product is marketed as Pleximid.
144
Hard-coated poly(methyl methacrylimide) sun-roofs have already been
specified for American sports cars, whilst the polymer might be expected to
make some inroads into the polycarbonate market, with one specific target
being auto headlamp diffusers.
2.4.3 Acrylic Adhesives
Methyl methacrylate has been used for many years as a reactive
adhesive for joining together poly(methyl methacrylate). To reduce curing
shrinkage it is usually thickened with its polymer although alternative
materials could be used which might be cheaper but generally cause a loss
in clarity. The bond sets by polymerization which may be brought about by
ultraviolet light or by the use of peroxides. Room temperature setting with
peroxides is achieved by the use of amines as promoters.
The alkyl 2-cyanoacrylates have become well-known adhesives,
often popularly known as super-glue.
In dry air and in the presence of polymerization inhibitors methyl
and ethyl 2-cyanoacrylates have a storage life of many months. Whilst they
may be polymerized by free-radical methods, anionic polymerization is of
greater significance. A very weak base, such as water, can bring about rapid
polymerization and in practice a trace of moisture on a substrate is enough
to allow polymerization to occur within a few seconds of closing the joint
and excluding the air. (As with many acrylic monomers air can inhibit or
severely retard polymerization).
Cyanoacrylate adhesives are particularly valuable because of their
speed of action, which allows the joining of intricate parts without the need
for complex jigs and fixtures. Within very broad limits the more monomer
that is used to make a j oint the less will be the strength. These adhesives
have in fact no gap-filling ability, nor can they be used on porous
substrates. Whilst they have good heat and solvent resistance their
weathering behaviour is limited and joints should not be in frequent contact
with water.
The reluctance of acrylic monomers to polymerize in the presence
of air has been made a virtue with the anaerobic acrylic adhesives. These
are usually dimethacrylates such as tetramethylene glycol dimethacrylate.
The monomers are supplied with a curing system comprising a peroxide
and an amine as part of a one-part pack. When the adhesive is placed
between mild steel surfaces air is excluded, which prevents air inhibition,
and the iron present acts as a polymerization promoter. The effectiveness as
a promoter varies from one metal to another and it may be necessary to use
145
a primer such as cobalt naphthenate. The anaerobic adhesives have been
CH
2
CH
CN
n
widely used for sealing nuts and bolts and for a variety of engineering
purposes. Small tube containers are also available for domestic use.
To overcome brittleness these materials are sometimes blended
with rubbery materials and with polyurethanes. These polymers may
contain unsaturated groups, particularly at the chain ends, so that graft
structures may be produced rather than simple mixtures.
2.4.4 Hydrophilic Polymers
The successful development of eye contact lenses led in turn to a
demand for soft contact lenses. Such a demand was eventually met by the
preparation of copolymers using a combination of an acrylic ester monomer
such as methyl methacrylate, a cross-linkable monomer such as a
dimethacrylate, and a monomer whose homopolymer is soluble or highly
swollen in water such as N-vinyl pyrrolidone. Such copolymers swell in
water (hence the term hydrophilic), the degr ee of swe lli ng bein g cont rolle d
by the specific type and amount of the monomers used. In use the lens is
swollen to equilibrium in water, a typical soft lens having a water cont ent
of about 75%.
Such lenses may be made by machining from rod. More recently
processes have been developed where the monomers are cast polymerized
in tiny plastics moulds whose cavity corresponds to the dimensions of the
lens.
2.4.5 Polyacrylonitrile
Polyacrylonitrile and closely related copolymers have found wide
use as fibers. The development of acrylic fibers started in the early 1930s in
Germany. In the United States they were first produced commercially about
1950 by Du Pont (Orlon) and Monsanto (Acrilan).
In polyacrylonitrile appreciable electrostatic forces occur between
the dipoles of adjacent nitrile groups on the same polymer molecule. This
restricts the bond rotation and leads to a stiff, rodlike structure of the
polymer chain. As a result, polyacrylonitrile has a very high crystalline
melting point (317ºC) and is soluble in only a few solvents, such as
dimethylformamide and dimethylacetamide, and in concentrated aqueous
solutions of inorganic salts, such as calcium thiocyanate, sodium
146
perchlorate, and zinc chloride. Polyacrylonitrile cannot be melt processed
CH
2
CH
n
CONH
2
because its decomposition temperature is close to the melting point. Fibers
are therefore spun from solution by either wet or dry spinning.
Fibers prepared from straight polyacrylonitrile are difficult to dye.
To improve dyeability, manufacturers invariably add to monomer feed
minor amounts of one or two comonomers, such as methyl acrylate, methyl
methacrylate, vinyl acetate, and 2-vinyl-pyridine. Small amounts of ionic
monomers (sodium styrene sulfonate) are often included for better
dyeability. Modacrylic fibers are composed of 35-85% acrylonitrile and
contain comonomers, such as vinyl chloride, to improve fire retardancy.
Acrylic fibers are more durable than cotton, and they are the best
alternative to wool for sweaters. A major portion of the acrylic fibers
produced are used in apparel (primarily hosiery). Other uses include pile
fabrics (for simulated fur), craft yarns, blankets, draperies, carpets, and
rugs.
2.4.6 Polyacrylamide
Polyacrylamide exhibits strong hydrogen bonding and water
solubility. Most of the interest in this polymer is associated with this
property. Polymerization of acrylamide monomer is usually conducted in
an aqueous solution, using free-radical initiators and transfer agents.
Copolymerization with other water-soluble monomers is also
carried out in a similar manner. Cationic polyacrylamides are obtained by
copolymerizing with ionic monomers such as dimethylaminoethyl
methacrylate, dialkyldimethylammonium chloride, and
vinylbenzyltrimethylammonium chloride. These impart a positive charge to
the molecule. Anionic character can be imparted by copolymerizing with
monomers such as acrylic acid, methacrylic acid, 2-acrylamido-2-methylpropanesulfonic acid, and sodium styrene sulfonate. Partial hydrolysis of
polyacrylamide, which converts some of the amide groups to carboxylate
ion, also results in anionic polyacrylamides.
Polyacrylamides are used as primary flocculants or coagulant aids
in water clarification and mining application. They are effective for
clarification of raw river water. The capacity of water clarifiers can be
147
increased when the polymer is used as a secondary coagulant in
conjunction with lime and ferric chloride. Polyacrylami des, and especially
cationic polyacrylamides, are used for conditioning municipal and
industries sludges for dewatering by porous and empty sand beds, vacuum
filters, centrifuges, and other m echani cal devices.
Certain anionic polyacrylamides are approved by the U.S.
Environmental Protection Agency for clarification of potable water.
Polymer treatment also allows filters to operate at higher hydraulic rates.
The function of clarification is not explained by a simple mechanism. The
long-chain linear polymer apparently functions to encompass a number of
individual fine particles of the dispersed material in water, attaching itself
to the particles at various sites by chemical bonds, electrostatic attraction,
or other attractive forces. Relatively stable aggregates are thus produced,
which can be removed by filtration, settling, or other convenient means.
Polyacrylamides are useful in the paper industry as processing aids,
in compounding and formulating, and as filler-retention aids.
Polyacrylamides and copolymers of acrylamide and acrylic acid are used to
increase the dry strength of paper.
Polyacrylamides are used as flooding aids in secondary oil recovery
from the producing oil well. Water, being of low viscosity, tends to finger
ahead of the more viscous oil. However, addition of as little as 0,05%
polyacrylamide to the waterflood reduces oil bypass and give significantly
higher oil to water ratios at the producing wellhead. Greatly increased
yields of oil result from adding polymer to waterflooding.
Solutions containing polyacrylamide are very slippery and can be
used for water-based lubrication. Small amounts of polymer, when added to
an aqueous solution, can significantly reduce the friction in pipes, thereby
increasing the throughput or reducing the power consumption.
Other applications include additives in coatings and adhesives and
binders for pigments.
2.5 Polymers of Complex and Simple Vinyl Ethers
2.5.1 Poly(vinyl acetate) and its Derivatives
Because of its high cold flow, poly(vinyl acetate) is of little value
in the form of mouldings and extrusions. However, because of its good
adhesion to a number of substrates, and to some extent because of its cold
flow, a large quantity is produced for use in emulsion paints, adhesives and
various textile finishing operations. A minor proportion of the material is
148
also converted into poly(vinyl alcohol) and the poly(vinyl acetal)s which,
+
CH
CH
+ CH
3
COOH
CH
2
CH
OOC
CH
3
-118kJ/mole
CH
3
OOC
CH
CH
2
CH3COOH
CH
3
COO
CH
CH
3
CH3COO
are of some interest to the plastics industry.
The IUPAC systematic name for poly(vinyl acetate) is poly-(1acetoxyethylene) and that for poly(vinyl alcohol) is poly-(lhydroxyethylene). As with other common pol ymers the IUPAC names are
not in general use.
Preparation of the monomer
Vinyl acetate was originally prepared industrially by the reaction of
acetylene with acetic acid or by oxidation of ethylene.
The first reaction may be carried out either in the liquid or vapour
phase although the liquid phase route is now commercially obsolete. In a
typical liquid phase preparation, acetylene is passed through an agitated
solution of glacial acetic acid and acetic anhydride containing mercuric
sulphate, preferably formed in situ, in a finely divided state as catalyst.
Owing to the tendency for ethylidene diacetate to be formed at
elevated temperatures, care is taken for the rapid removal of vinyl acetate
from the reaction vessel as soon as it is formed:
In a typical system the reaction vessel is at 75-80°C and the vinyl
acetate formed is swept out into a condenser at 72-74°C by means of
circulating excess acetylene. This prevents distillation of higher boiling
components but allows the vinyl acetate and acetylene through. The former
is separated out by cooling and the acetylene recycled.
Vapour phase synthesis may be carr ied out by passing a mixture of
acetylene and acetic acid through a reaction tube at 210-215°C. Typical
catalysts for this reaction are cadmium acetate, zinc acetate and zinc
silicate. The monomer in each of the above ment ioned processes is purified
by distillation.
Purified monomer is usually inhibited before shipment by such
materials as copper resinate, diphenylamine or hydroquinone, which are
generally removed before polymerization. The monomer is a sweetsmelling liquid partially miscible with water and with the following
149
properties: boiling point at 760 mmHg, 72.5°C; specific gravity at 20°C,
CH
3
CHO
+
O
OC
CH
3
OC
CH
3
FeCl
3
CH
3
CH
OOC
CH
3
OOC
CH
3
+
CH
2
OOC
CH
2
CH
3
2CH
3
COONa
+
PbCl
2
CH3COOH
CH
2
CH
+
Pb
+ 2NaCl
+ CH
3
COOH
0.934; refractive index n
20
, 1.395; vapour pressure at 20°C, 90 mmHg.
D
In 1953 the Celanese Corporation of America introduced a route
for the production of vinyl acetate from light petroleum gases. This
involved the oxidation of butane which yields such products as acetic acid
and acetone. Two derivatives of these products are acetic anhydride and
acetaldehyde, which then react together to give ethylidene diacetate:
Exposure of the ethylidene diacetate to an aromatic sulphonic acid
in the presence of five times its weight of acetic anhydride as diluent at
136°C will yield the following mixture: 40% vinyl acetate; 28% acetic
acid; 20% acetic anhydride; 4% ethylidene diacetate; 8% acetaldehyde.
The latter four products may all be reused after separation.
In recent years vinyl acetate has been prepared in large quantities
by oxidation of ethylene. If ethylene is passed into a solution of palladium
chloride in acetic acid containing sodium acetate, then vinyl acetate,
ethylene diacetate and acetaldehyde are produced, the vinyl acetate being
obtained in good yields by the reaction shown in Figure 19.
Figure 19
The ethylene oxidation process can be carried out in either a liquid
or a vapour phase but the latter method is often preferred because it avoids
corrosion problems and the use of solvents.
A one-stage process for producing vinyl acetate directly from
ethylene has also been di sclosed. In this process ethylene is passed through
a substantially anhydrous suspension or solution of acetic acid containing
cupric chloride and copper or sodium acetate together with a palladium
catalyst to yield vinyl acetate.
Polymerization
Vinyl acetate may be easily polymerized in bulk, solution,
emulsion and suspension. At conversions above 30%, chain transfer to
polymer or monomer may occur. In the case of both polymer and monomer
transfer two mechanisms are possible, one at the tertiary carbon, the other
150
(illustrated in Figure 20) at the acetate group.
CH
2
CH
OOC
CH
3
Radical
+
CH2C
H
CH
3
OOC
Polymer
CH2CH
2
CH
3
OOC
Polymer
+
CH
2
C
H
Radical
OOC
CH
2
Figure 20
The radical formed at either the tertiary carbon atom or at the
acetate group will then initiate polymerization and form branched
structures.
Since poly(vinyl acetate) is usually used in an emulsion form, the
emulsion polymerization process is commonly used. In a typical system,
approximately equal quantities of vinyl acetate and water are stirred
together in the presence of a suitable colloid-emulsifier system, such as
poly(vinyl alcohol) and sodium lauryl sulphate, and a water-soluble
initiator such as potassium persulphate.
Polymerization takes place over a period of about 4 hours at 70°C.
The reaction is exothermic and provision must be made for cooling when
the batch size exceeds a few litres. In order to achieve better control of the
process and to obtain particles with a smaller particle size, part of the
monomer is first polymerized and the rest, with some of the initiator, is
then steadily added over a period of 3-4 hours. To minimise the hydrolysis
of vinyl acetate or possible comonomers during polymerization, it is
necessary to control the pH throughout reaction. For this purpose a buffer
such as sodium acetate is commonly employed.
Properties and uses
Poly(vinyl acetate) is too soft and shows excessive “cold flow” for
use in moulded plastics. This is no doubt associated with the fact that the
glass transition temperature of 28°C is little above the usual ambient
temperatures and in fact in many places at various times the glass
temperature may be the lower. It has a density of 1.19g/cm3 and a refractive
index of 1.47. Commercial polymers are atactic and, since they do not
crystallize, transparent (if free from emulsifier). They are successfully used
in emu lsion paints, as adhesives for textiles, paper and wood, as a sizing
material and as a “permanent starch”. A number of grades are supplied by
manufacturers which differ in molecular weight and in the nature of
comonomers (e.g. vinyl maleate) which are commonly used.
The polymers are usually supplied as emulsions which also differ