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Файл:Plastics technology. Часть 1. Учебное пособие.pdf
131
igure 18 – Emulsion polymerization plant: A - emulsion feed
F
tank; B - polymerization reactor; C - drumming tank; F - filter; M - meter;
P - pressure gauge; and T - temperature indication .
A typical process for the preparation of a 50% methyl methacrylate,
49% butyl acrylate, and 1% methacrylic acid terpolymer as an
approximately 45% dispersion in water begins with the preparation of the
monomer emulsion charge.
The listed ingredients are added in given order while maintaining
good agitation. The reactor charge (deionized water, 30.90 parts; sodium
lauryl sulfate, 0.11 parts) is heated with good agitation under a nitrogen
atmosphere to 85°C, then the initiator charge (ammonium persulfate, 0.23
parts) is added to the reactor and the monomer emulsion feed is begun. The
monomer emulsion is fed uniformly over 2.5 h while maintaining 85°C.
After the addition is complete, the temperature is raised to 95°C to
complete the conversion of mono mer. The product is then cooled to room
temperature, filtered, and packaged.
132
deionized water
13.65
sodium lauryl sulfate
0.11
methyl methacrylate
22.50
butyl acrylate
22.05
methacrylic acid
0.45
Monomer emulsion charge Part
thacrylic dispersion polymers are shipped in bulk or in drums.
Me
Tank trucks and tank cars used for bulk shipment are constructed of
stainless or resin-coated steel and are insulated to prevent freezing.
Filament-wound glass fiber-reinforced polyester tanks are recommended
for storage because of their relatively low cost, ease of installation, and
chemical resistance. Usually storage tanks are located in an enclosed and
heated environment to prevent freezing during cold weather. Dispersion
polymers are subject to the various instability problems common to all
colloidal systems, such as sedimentation, skinning (surface film), gritting
(solid with the dispersion), gumming (deposits on walls), and sponging
(formation of an aerogel). Undesirable changes may be caused by time,
drift in pH, evaporation, high or low temperature, shear and turbulence, and
foaming. Oxidative degradation is not usually encountered with
methacrylic dispersion polymers, but bacterial attack is common and is
avoided by pH adjustment, addition of bactericidal agents, and careful
housekeeping.
Polymerization in bulk
Bulk polymerization is extensively used in the manufacture of the
sheet and to a lesser extent rod and tube. In order to produce a marketable
material it is important to take the following factors into account:
(1) The exotherm developed during cure.
(2) The acceleration in conversion rate due to increasing viscosity.
(3) The effect of oxygen.
(4) The extensive shrinkage in conversion from monomer to
polymer (~20%).
(5) The need to produce sheet of even thickness.
(6) The need to produce sheet of constant quality.
(7) The need to produce sheet free from impurities and
imperfections.
In order to reduce the shrinkage in the casting cell, and also to
reduce problems of leakage from the cell, it is nor mal practice to prepare a
“prepolymer”. In a typical process monomer freed from inhibitor is heated
133
with agitation for about 8 minutes at 90°C with 0.5% benzoyl peroxide and
then cooled to room temperature. Plasticizer, colouring agents and
ultraviolet light absorbers may be incorporated at this stage if required. The
resulting syrup, consisting of a solution of polymer in monomer, is then
filtered and stored in a refrigerator if it is not required for immediate use.
The heating involved in making the prepolymer may also be of assistance
in removing oxygen dissolved in the monomer.
The preparation of a prepolymer requires careful control and can be
somewhat difficult in large-scale operations. An alternative approach is to
prepare a syrup by dissolving some polymer in the monomer and adding
some peroxide to the mixture. As in the case of a prepolymer syrup, such a
syrup will cause less shrinkage on polymerization and fewer leakage
problems.
Acrylic sheet is prepared by pouring the syrup into a casting cell.
This consists of two plates of heat-resistant polished glass provided with a
separating gasket round the edges. The gasket commonly consists of a
hollow flexible tube made from a rubber, or from plasticized poly(vinyl
alcohol). The cell is filled by opening up the gasket at a corner or edge and
metering in the syrup, care being taken to completely fill the cell before
closing up the gasket. The cell is held together by spring-loaded clamps or
spring clips so the plates will come closer toget her as the reacting mixture
shrinks during polymerization. This technique will enable the sheet to be
free of sink marks and voids.
It is important to use rigid glass sheet and to apply pressure to the
plates in such a manner that they do not bow out as this would lead to sheet
of uneven thickness.
The filled cells are then led through a heating tunnel. In a typical
system the time to pass through t he tunnel is about 16 hours. For the first
14 hours the cell passes through heating zones at about 40°C. Under these
conditions polymerization occurs slowly. Any acceleration of the rate due
to either the rise in temperature through the exothermic reaction or due to
the viscosity-chain termination effect will be small. It is particularly
important that the temperature of any part of the syrup is not more than
100°C since this would cause the monomer to boil. By the end of this
period the bulk of the monomer has reacted and the cell passes through the
hotter zones. After 15 hours (total time) the cell is at about 97°C, at which
temperature it is held for a further half-hour. The sheet is then cooled and
removed from the cell. In order to reduce any internal stresses the sheet
may be annealed by heating to about 140°C and, before being dispatched to
134
the customer, the sheet is masked with some protective paper using gelatine
or, preferably, with a pressure-sensitive adhesive.
When casting large blocks, the exotherm problem is more severe
and it may be necessary to polymerize inside a pressure vessel and thus
raise the boiling point of the monomer.
In order to compensate for shrinkage, special techniques are
required in the manufacture of rod. In one process, vertical aluminium
tubes are filled with syrup and slowly lowered into a water bath at 40°C. As
the lowest level of syrup polymerizes, it contracts and the higher levels of
syrup thus sink down the tube, often under pressure from a reservoir of
syrup feeding into the tubes.
Acrylic tubes may be prepared by adding a calculated amount of
syrup to an aluminium tube, sealing both ends, purging the air with
nitrogen and then rotating horizontally at a constant rate. The whole
assembly is heated and the syrup polymerizes on the wall of the rotating
tube. The natural shrinkage of the material enables the casting to be
removed quite easily.
An interesting modification of the sheet casting process is the band
polymerization process due to Swedlow. In this process a
monomer/polymer syrup is polymerized between steel bands which pass
through heating zones and which are spaced according to the sheet
thickness required. Whilst there may be some economic attraction of the
process in some countries with high labour costs the quality of the product
is generally inferior to that of cell-cast sheet. Furthermore, where lower
optical qualities are tolerable extruded sheet is generally cheaper to
produce. The process, as with the cast cell process, does however allow for
the possibility of cross-linked polymer sheet that cannot easily be produced
by extrusion processes.
Suspension polymerization
The average molecular weight of most bulk polymerized
poly(methyl methacry-lates) is too high to give a material which has
adequate flow properties for injection moulding and extrusion.
By rolling on a two-roll mill the molecular weight of the polymer
can be greatly reduced by me chanical scission, analogous to that involved
in the mastication of natural rubber, and so mouldable materials may be
obtained. However, bulk polymerization is expensive and the additional
milling and grinding processes necessary make this process uneconomic in
addition to increasing the risk of contamination.
135
As a result the suspension polymerization of methyl methacrylate
was developed to produce commercial material such as Diakon made by
ICI. Such a polymerization can be carried out rapidly, usually in less than
an hour, because there is no serious exotherm problem.
There is, however, a problem in controlling the particle size of the
beads formed and further in preventing their agglomeration, problems
common to all suspension-type polymerizations. The particle size of the
beads is determined by the shape and size of the reactor, the type and rate
of agitation and also the nature of suspending agents and protective colloids
present. Suspending agents used include talc, magnesium carbonate and
aluminium oxide whilst poly(vinyl alcohol) and sodium polymethacrylate
are among materials used as protective colloids.
In one process one part of methyl methacrylate was agitated with
two parts of water and 0.2% benzoyl peroxide was employed as the
catalyst. Eight to 18 g of magnesium carbonate per litre of reactants were
added, the lower amount being used for larger beads, the larger for small
beads. The reaction temperature was 80°C initially but this rose to 120°C
because of the exothermic reaction. Polymerization was complete in about
an hour. The magnesium carbonate was removed by adding sulphuric acid
to the mixture. The beads were then filtered off, carefully washed and
dried.
Other additives that may be incorporated include sodium h ydrogen
phosphates as buffering agents to stabilize that pH of the reaction medium,
lauryl mercaptan or trichlorethylene as chain transfer agents to control
molecular weight, a lubricant such as stearic acid and small amounts of an
emulsifier such as sodium lauryl sulpha te.
The dried beads may be supplied as injection moulding material
without further treatment or they may be compounded with additives and
granulated.
Structure and properties
Commercial poly(methyl methacrylate) is a transparent material,
and microscopic and X-ray analyses generally indicate that the material is
amorphous. For this reason the polymer was for many years considered to
be what is now known as atactic in structure. It is now, however, known
that the commercial material is more syndiotactic than atactic. (On one
scale of assessment it might be considered about 54% syndiotactic, 37%
atactic and 9% isotactic. Reduction in the temperature of free-radical
polymerization down to -78°C increases the amount of syndiotacticity to
136
about 78%).
Substituents on the α-carbon atom restrict chain flexibility but,
being relatively small, lead to a significantly higher Tg than with
polyethylene. Differences in the Tg's of commercial polymers (approx.
104°C), syndiotactic polymers (approx. 115°C) and anionically prepared
isotactic polymers (45°C) are generally ascribed to the differences in
intermolecular dipole forces acting through the polar groups.
In consequence of a Tg of 104°C with its amorphous nature,
commercial poly(methyl methacrylate) is thus a hard transparent plastics
material in normal conditions of use.
Because the polymer is polar it does not have electrical insulation
properties comparable with polyethylene. Since the polar groups are found
in a side chain these are not frozen in at the Tg and so the polymer has a
rather high dielectric constant and power factor at temperatures well below
the Tg. This side chain, however, appears to become relatively immobile at
about 20°C, giving a secondary transition point below which electrical
insulation properties are significantly improved. The increase in ductility
above 40°C has also been associated with this transition, often referred to
as the β-transition.
The solubility of commercial poly(methyl methacrylate) is
consistent with that expected of an amorphous thermoplastic with a
solubility parameter of about 18.8MPa
(δ=18.6), ethylene dichloride (δ=20.0), trichloroethylene (δ=19),
chloroform (δ=19) and toluene (δ=20), all in units of MPa
1/2
. Solvents include ethyl acetate
1/2
. Difficulties
may, however, occur in dissolving cast poly(methyl methacrylate) sheet
because of its high molecular weight.
Since the polymers are unbranched (apart from the methyl and
methacrylate side groups) the main difference between uncompounded
commercial grades is in the molecular weight.
Cast material is stated to have a number average molecular weight
of about 106. Whilst the Tg is about 104°C the molecular entanglements are
so extensive that the material is incapable of flow below its decomposition
temperature (approx. 170°C). There is thus a reasonably wide rubbery
range and it is in this phase that such material is normally shaped. For
injection moulding and extrusion much lower molecular weight materials
are employed. Such polymers have a reasonable melt viscosity but
marginally lower heat distortion temperatures and mechanical properties.
General properties of poly(methyl methacrylate)
Poly(methyl methacrylate) is a hard, rigid, transparent material.
Commercial grades have extremely good weathering resistance compared
137
with other thermoplastics.
Property
Acrylic sheet
Moulding
composition
Copoly
mer
n
M
Volume resistivity (20 ºC)
~106
~60000
-
The properties of three types of poly(methyl methacrylate) (sheet
based on high molecular weight polymer, lower molecular weight i njec tio n
moulding material and a one-time commercial copolymer) are given in
Table 6.
Table 6 – Some properties of methyl methacrylate polymers
Molecular weight (
Specific gravity
Tensile strength, MPa
Tensile modulus, MPa
Flexural strength, MPa
Flexural modulus, MPa
Water a bsorption [% i n 24
h (20ºC)]
Heat deflection temperature
(1.82 MPa), ºC
Refractive index n
)
1.19
-
3000
140
2750
0.2
100
20
D
1.49
>10
16
1.18
72.5
2400
-
2750
0.3
85-95
1.49
17
>10
As might be expected of a somewhat polar thermoplastics material,
mechanical, electrical and other properties are strongly dependent on
temperature, testing “rate” and humidity. Detailed data on the influence of
these variables have been made available by at least one manufacturer and
the following remarks are intended only as an illustration of the effects
rather than as an attempt at providing complete data.
Poly(methyl methacrylate) is recognized to be somewhat tougher
than polystyrene (after consideration of both laboratory tests and common
experience) but is less t ough than cellulose acetate or the ABS polymers. It
is superior to untreated glass in terms of impact resistance and although it
cracks, any fragments formed are less sharp and jagged than those of glass
and, normally consequently less harmful. However, oriented acrylic sheet
such as may result from double curvature shaping shatters with a
conchoidal fracture and fragments and broken edges can be quite sharp.
Although it is harder than most other thermoplastics the scratch resistance
does leave something to be desired. Shallow scratches may, however, be
1.17
-
2750
130
-
0.25
80
1.49
-
138
removed by polishing.
The optical properties of poly(met hyl methacrylate) are particularly
important. Poly(methyl methacrylate) absorbs very little light but there is
about 4% reflection at each polymer – air interface for normal incident
light. Thus the light transmission of normal incident light through a parallel
sheet of acrylic material free from blemishes is about 92%. The optical
properties of poly(methyl methacrylate) have been exploited in the
development of optical fibres.
Poly(methyl methacrylate) is a good electrical insulator for lowfrequency work, but is inferior to such polymers as polyethylene and
polystyrene, particularly at high frequencies.
The apparent volume resistivity is dependent on the polarization
time. The initial polarization current is effective for some t ime and if o nl y a
short time is allowed before taking measurements low values for volume
resistivity will be obtained.
As may be expected of an amorphous polymer in the middl e range
of the solubility parameter table, poly(methyl methacrylate) is soluble in a
number of solvents with similar solubility parameters. The polymer is
attacked by mineral acids but is resistant to alkalis, water and most aqueous
inorganic salt solutions. A number of organic materials although not
solvents may cause crazing and cracking, e.g. aliphatic alcohols.
Additives
Poly(methyl methacrylate) may be blended with a number of
additives. Of these the most important are dyes and pigments and these
should be stable to both processing and service conditions. Two particular
requirements are, firstly, that when used in castings they should not affect
the polymerization reaction and, secondly, that they should have good
weathering resistance.
Plasticizers are someti mes added t o the polymer, dibutyl phthalate
being commonly employed in quantities of the order of 5%. Use in
moulding powders will enhance the melt flow but somewhat reduce the
mechanical properties of the finished product.
Further improvement in light stability may be achieved by addition
of small quantities of ultraviolet absorbers. Typical examples include
phenyl salicylate, 2,4-dihydroxybenzophenone, resorcinol monobenzoate,
methyl salicylate and stilbene.
Processing
In commercial practice three lines of approach are employed in
139
order to produce articles from poly(methyl methacrylate). They are:
(1) Processing in the melt state such as by injection moulding and
extrusion.
(2) Manipulation of sheet, rod and tube.
(3) The use of monomer-polymer doughs.
There are a number of general points to be borne in mind when
processing the polymer in the molten state which may be summarized as
follows:
(1) The polymer granules tend to pick up moisture (up to 0,3%).
Although most commercial grades are supplied in the dry condition,
subsequent exposure before use to atmospheric conditions will lead to
frothy mouldings and extrudates, owing to volatilization of the water in the
heating cylinders. Particular care should be taken with reground scrap.
(2) The melt viscosities at the processing temperatures employed
are considerably higher than those of polystyrene, polyethylene and
plasticized PVC. This means that the equipment used must be robust and
capable of generating high extrusion and injection pressures. The injection
moulding of poly(methyl methacrylate) (PMMA) has been made much
easier by the widespread use of the reciprocating screw in-line injection
moulding machines. The use of a screw with a decompression zone and a
vented barrel may be useful both for injection moulding and extrusion,
since it is possible to remove unwanted moisture and even monomer which
has been produced by depolymerization of the polymer because of
overheating.
The melt viscosity is more sensitive to temperature than that of
most thermoplastics and this means that for accurate, consistent and
reproducible results, good temperature control is required on all equipment.
(3) Since the material is amorphous the moulding shrinkage is low
and normally less than 0.008 cm/cm.
A great number of poly(methyl methacrylate) products are
produced by manipulation of sheet, rod and tube. Such forms may easily be
machined using drills, circular saws and bandsaws, providing care is taken
not to overheat the polymer. It is very difficult to weld the sheet
satisfactorily but cementing techniques have been highly developed.
Acrylic parts may be joined using solvents such as chloroform or by use of
solutions of polymer in a suitable solvent. Generally, however, the best
results are obtained, particularly where there is a gap-filling requirement,
140
by use of a monomer-polymer solution. Commercial cements of this type
either contain a photocatalyst to allow hardening by ultraviolet light
polymerization or contain a promoter so that on addition of a peroxide,
polymerization of the monomer is sufficiently rapid at room temperature to
harden the cement in less than one hour.
When heated above the glass transition temperature (~100°C),
acrylic sheet from high molecular weight polymer becomes rubbery. The
rubbery range extends for 60°C. Further raising of the temperature causes
decomposition rather than melting. The reasonably wide r ub ber y r an ge, c. f .
cellulose acetate, high-impact polystyrene and polyethylene, enables the
sheet to be heated in ovens rather than having to be heated while clamped
to the shaping apparatus. Poly(mefhyl methacrylate) is not widely suitable
for normal vacuum forming operations since the modulus of the material in
the rubbery state is too great to allow shaping of fine detail simply by
atmospheric pressure. As a result a large number of techniques have been
devised using air pressure, mechanical pressure, or both in combination,
and sometimes also involving vacuum assistance.
The use of monomer-polymer doughs has been largely confined to
the production of dentures. A plaster of Paris mould is first prepared from a
supplied impression of the mouth. Polymer powder containing a suitable
polymerization initiator is then mixed with some monomer to form a
dough. A portion of the dough is then placed in the mould, which is closed,
clamped and heated in boiling water. After polymerization, which usually
takes less than half an hour, the mould is cooled and opened. This
technique could also be usefully employed for other applications where
only a few numbers-off are required but does not seem to have been
exploited.
A novel technique has been developed for the manufacture of tiles
and sanitary ware. A disper sion of a ground sand in methyl methacrylate
monomer is prepared with a solids content of about 72% by weight. The
particle size is such that the dispersion has reasonable stability but is
pourable. When required for use the dispersion is blended with a freeradical initiator, usually based on a peroxide, and fed into metal moulds
heated to about 70°C. As the monomer polymerizes there is a shrinkage of
about 11% by volume and this is compensated through a reduction in the
volume of the mould cavity, with one mould half moving towards the other
and into the other like a piston in a cylinder. The polymerized products
have a remarkably good finish, are virtually stress free and have
considerable flexibility in part desig n.