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Файл:Plastics technology. Часть 1. Учебное пособие.pdf
91
Initiator
Caproyl peroxide
Demineralised water
0.001 parts
90 parts
In addition, buffer salts such as disodium h ydrogen phosphate may
be used to prevent the pH of the aqueous phase falling during
polymerization. Small amounts of an anti-foam a gent may be employed to
reduce frothing when discharging from the vessel at the end of the
polymerization process.
The trichlorethylene is a solvent transfer agent used to control
molecular weight.
Over the past years considerable attention has been paid to the
dispersing system since this controls the porosity of the particle. This is
important both to ensure quick removal of vinyl chloride monomer after
polymerization and also to achieve easy processing and dry blendable
polymers. Amongst materials quoted as protective colloids are vinyl
acetate-maleic anhydride copolymers, fatty acid esters of glycerol, ethylene
glycol and pentaerythritol, and, more recently, mi xed cellulose ethers and
partially hydrolyzed poly(vinyl acetate). Much recent emphasis has been on
mixed systems.
There has also been a trend in recent years to the use of free-radical
initiators that decompose more rapidly than the traditional initiators such as
caproyl and lauryl peroxides. Currently used initiators include
peroxydicarbonates, t-butylperpivalate, azobis-(2,4-dimethylvaleronitrile)
and acetyl cyclohexylsulphonyl peroxide.
Because of its low water solubility (0.09% at 20°C) vinyl chloride
may be polymerized in emulsion. Using secondary alkyl sulphonates or
alkali salts of alkyl sulphates as emulsifiers rapid polymerization can occur
in oxygen-free environments. The use of “redox” initiating systems has
made possible rapid reaction at temperatures as low as 20°C whilst in
recent laboratory work sub-zero temperatures have been used. Watersoluble initiators are employed. Ammonium persulphate, potassium
persulphate and hydrogen peroxide are typical initiators whilst bisulphites
and ferrous salts are useful reducing agents. Modifiers are often employed
to control the molecular weight. Reaction times are commonly of the order
of 1-2 hours. After polymerization the particles are normally spray dried.
There will thus be residual emulsifier which will adversely affect clarity
and electrical insulation properties. Some improvement is, however,
obtained using special washing operations.
92
Vinyl chloride is occasionally copolymerized with monomers,
CH
2
CH
CH
2
CH
2
Cl
notably with vinylidene chloride, vinyl acetate and propylene. Where
vinylidene chloride is used as comonomer there is a reduction in the overall
polymerization rate. In addition, since vinylidene chlorode radicals add
preferentially to a vinylidene chloride molecule during chain growth a
heterogenous product is formed. This can be overcome by drip feeding the
vinylidene chloride through the reaction at such a rate as to give a constant
monomer composition. In the case of vinyl chloride-vinyl acetate
copolymerizations the vinyl chloride is consumed preferentially and this
will, without special steps being taken, also lead to some heterogeneity.
Structure of poly(vinyl chloride)
It is useful to compare the structures of PVC and polyethylene
since this enables predictions of the properties of the former to be made.
Both materials are linear polymers and substantially thermoplastic. The
presence of the chlorine atom causes an increase in the inter chain
attraction and hence an increase in the hardness and stiffness of the
polymer. PVC is also more polar than polyethylene because of the С–Cl
dipole. Thus PVC has a higher dielectric constant and power factor than
polyethylene, although at temperatures below the glass transition
temperature (+80°C) the power factor is still comparatively low (0.01-0.05
at 60Hz) because of the immobility of the dipole.
The solubility parameter of PVC is about 19.4 MPa
1/2
and the
polymer is thus resistant to non-polar solvents which have a lower
solubility parameter. In fact it has very limited solubility, the only solvents
that are effective being those which appear to be capable of some form of
interaction with the polymer. It is suggested that PVC is capable of acting
as a weak proton donor and thus effective solvents are weak proton
acceptors. These include cyclohexanone (δ=20.2MPa
tetrahydrofuran (δ=19.4 MPa
1/2
). There are many materials that are suitable
1/2
plasticizers for PVC. They have similar solubility parameters to PVC and
are also weak proton acceptors. These are of too high a molecular weight
and too large a molecular size to dissol ve the polymer at room temperature
but they may be incorporated by mixing at elevated temperatures to give
mixtures stable at room temperature. The presence of chlorine in large
quantities in the polymer renders it flame retarding. The presence of
) and
93
plasticizers, however, reduces the resistance to burning.
CH
CH
2
Cl
CH
CH
2
Cl
Zn
CH
CH
2
+
ZnCl
2
CH
CH
CH
2
CH CH
2
CH
2
CH
2
CH
CH
Cl
CH
Cl
Cl
a
Cl
Cl
б
CH
CH
2
CH
CH
Cl
CH
2
CH
CH
CH
CH
Much work has been carried out in order to elucidate the molecular
structure of poly(vinyl chloride). In 1939, Marvel, Sample and Roy
dechlorinated PVC with zinc dust to give linked cyclic structures:
By noting the amount of chlorine that could be removed in this way
they were able to determine whether the polymer was formed by head-totail linkage (a) or head-head and tail-tail li nk a g e (b).
It would be expected that if linkage was by the latter mechanism
complete dechlorination would occur, the adjacent chlorine atoms being
removed together. In the case of head-tail polymerization it would be
expected that because the reaction does not occur in steps along the chain,
but at random, a number of unreacted chlorine atoms would become
isolated and thus complete dechlorination could not occur:
It was found that the amount of chlorine that could be removed (8487%) was in close agreement to that predicted by Flory on statistical
grounds for structure (a). It is of interest to note that similar statistical
calculations are of relevance in the cyclisation of natural rubber and in the
formation of the poly(vinyl acetals) and ketals from poly(vinyl alcohol).
Since the classical work of Marvel it has been shown by di verse techniques
that head-to-tail structures are almost invariably formed in addition
polymerizations.
X-ray studies indicate that the vinyl chloride polymer as normally
prepared in commercial processes is substantially amorphous although
some small amount of crystallinity (about 5% as measured by X-ray
diffraction methods) is present. It has been reported by Fuller in 1940 and
Natta and Carradini in 1956 that examination of the crystalline zones
indicates a repeat distance of 5,1 Å which is consistent with a syndiotactic
(i.e. alternating) structure. Later studies using NMR techniques indicate
that conventional PVC is about 55% syndiotactic and the rest largely atactic
in structure.
94
Over the years there has been considerable controversy over the
w
M
n
M
nw
M/M
nature and extent of branching in poly(vinyl chloride). At one time it was
believed that in normal commercial PVC there might be up to 16 long
chain branches per molecule. Recent studies, however, suggest that the
existence of more than one long branch per molecule is not very probable.
There may, however, be a few (usually <10) short branches per molecule.
It is of interest to note that free-radical polymerization at lower
temperatures, e.g. at -40°C using γ-ra diatio n or by th e use of hi ghly act ive
initiators such as the alkylboranes, increasingly favours the formation of
syndiotactic structures. These more regular polymers also show freedom
from branching. The greater regularity and absence of branching result in
crystalline polymers with decreased solubility in cyclohexanone. The
polymers also have, as would be expected, greater density and a high
softening point. Such polymer s, however, are more difficult to process and
the products generally more brittle. It has, however, been claimed that
through careful control of molecular weight and crystallinity useful
materials can be obtained.
PVC has a rather limited thermal stability. This is rather surprising
since it is known that low molecular weight materials containing similar
structures are far more stable. It would thus appear that this instability is
due to imperfections or weak points in the structure at which degradation
can commence.
Characterization of commercial polymers
As indicated in the previous section, commercial PVC polymers are
largely amorphous, slightly branched molecules with the mono mer residues
arranged in a head-to-tail sequence. Individual grades of materials do,
however, differ in average molecular weight, molecular weight distribution,
particle shape, size and size distribution, and the presence of impurities.
Some grades may also contain small quantities of comonomer residues.
The molecular weights of commercial polymers are in the range
=100000-200000,
=45000-64000, although values may be as low
as 40000 and as high as 480000 for the weight average molecular weight.
The ratio
is usually about 2 for the bulk of commercial material
although it may increase with the higher molecular weight grades.
In the early days of the commer cial development of PVC, emulsion
polymers were preferred for general purpose applications. This was
95
because these materials exist in the form of the fine primary particles of
diameter of the order of 0.1-1.0 µ m, which in the case of some commercial
grades aggregate into hollow secondary particles or cenospheres with
diameters of 30-100 µm. These emulsion polymer particles have a high
surface/volume ratio and fluxing and gelation with plasticizers is rapid. The
use of such polymer s was, however, restricted because of the presence of
large quantities of soaps and other additives necessary to emulsion
polymerization which adversely affect clarity and electrical insulation
properties.
If PVC polymer particles are mixed, at room temperature, with
plasticizers the immediate product may take one of two forms. If there is
insufficient plasticizer to fill all the gaps between the particles a “mush”
will be produced. If all the voids are filled then the particles will become
suspended in the plasticizer and a paste will be formed. In the case of
conventional granular polymer, or with emulsion polymer cenospheres, the
particles are too large to remai n in suspension and will settle out. Therefore
compounds used in “paste-processes” must use polymers with a small
particle size. On the other hand there is a lower limit to this, since small
particles will have a very high surface/volume ratio and measurable
plasticizer absorption will occur at room temperature to give a paste whose
viscosity will increase unduly with time. As a consequence paste polymers
have an average particle size of about 0.2-1.5 µm.
It is found that the viscosity of a paste made from a fixed
polymer/plasticizer ratio depends to a great extent on the particle size and
size distribution. In essence, in order to obtain a low-viscosity paste, the
less the amount of plasticizer required to fill the voids between particles the
better. Any additional plasticizer present is then available to act as a
lubricant for the particles, facilitating their general mobility in suspension.
The polymer particles shown in Figure 13 pack more closely and with less
voids than those in Figure 14 and hence give a lower viscosity polymer.
Figure 13 – PVC paste polymer particles with distribution
of size-efficient packing
96
Figure 14 – PVC paste polymer with homogeneous
particle size-less efficient p ack ing
The success of “filler” polymers used in increasing quantities in
PVC technology paste can be considered as an extension of this principle.
These filler polymers are made by suspension (granular, dispersion)
polymerization and by themselves the particles are too large to make stable
pastes. However, in the presence of paste-polymer particles they remain in
stable suspension. Being very much larger than paste-polymer particles
they take up comparatively large volumes in which no plasticizer is
required whatsoever. This is shown in Figures 15 and 16 where it is seen
that the replacement in space of a mixture of paste-polymer particles and
plasticizer by a large granular polymer particle releases plasticizer for use
as lubricant, i.e. a viscosity depressant.
Figure 15 – Paste polymer suspended in plasticizer
Figure 16 – PVC containing filler polymer.
Less plasticizer required to fill voids in unit volume
97
In addition to homopolymers of varying molecular and particle
structure, copolymers are also available commercially in which vinyl
chloride is the principal monomer. Comonomers used commercially
include vinyl acetate, vinylidene chloride, propylene, acrylonitrile, vinyl
isobutyl ether, and maleic, fumaric and acrylic esters. Of these the first
three only are of importance to the plastics industry. The main function of
introducing comonomer is to reduce the regularity of the polymer structure
and thus lower the interchain forces. The polymers may therefore be
processed at much lower temperatures and are useful in the manufacture of
gramophone records and flooring compositions.
Compounding ingredients
In the massive form poly(vinyl chloride) is a colourless rigid
material with limited heat stability and with a tendency to adhere to
metallic surfaces when heated. For these, and other, reasons it is necessary
to compound the polymer with other ingredients to make useful plastics
materials. By such means it is possible to produce a wide range of products,
including rigid piping and soft elastic cellular materials.
A PVC compound may contain the following ingredients:
(1) Polymer. (6) Fillers.
(2) Stabilizers. (7) Pigments.
(3) Plasticizers. (8) Polymeric processing aids.
(4) Extenders. (9) Impact modifiers.
(5) Lubricants.
Other miscellaneous materials also used occasionally include fire
retardants, optical bleaches and blowing agents.
Stabilizers
It is an observed fact that heating PVC at temperatures above 70°C
has a number of adverse effects on the properties of the polymer. At
processing temperatures used in practice (150-200°C) sufficient
degradation may take place during standard processing operations to render
the product useless. It has been found that incorporation of certain materials
known as stabilizers retards or moderates the degradation reaction so that
useful processed materials may be obtained.
There is a great deal of uncertainty as to the mechanism of PVC
degradation but certain facts have emerged. Firstly dehydrochlorination
occurs at an early stage in the degradation process. There is some infrared
98
evidence that as hydrogen chloride is removed polyene structures are
CH
2
CH
CH
2
CH
2
CH
CH
CH
Cl Cl
Cl
CH
CH
-HCl
formed:
The first physical manifestation of degradation, used in the widest
sense, is a change in the colour of PVC. Initially water-white, on heating it
will turn, in sequence, pale yellow, orange, brown and black. Further
degradation causes adverse changes in mechanical and electrical properties.
For most commercial purposes, the “end-point” is in fact the formation of
colour. With some applications some colour change can be acceptable; in
other cases little or no change may be tolerated. Thus in practice changes in
colour on heating provide a simple and readily obtainable criterion of
degradation.
The choice of a stabilizer thus becomes an empirical yet systematic
process. The following factors are the most important which must be
considered:
(1) The grade of polymer used.
(2) The nature of other ingredients present.
(3) T he cost of stabilizer required to give adequate stabilization for
the processing and anticipated service life of the compound.
(4) The clarity required of the compound.
(5) Toxicity.
(6) The effect on lubrication, printing, heat sealing and plate-out.
Many stabilizers are useful in improving the resistance of PV C to
weathering, particularly against degradation by ultraviolent radiation.
The most important class of stabilizers is the lead compounds
which form lead chloride on reaction with hydrogen chloride evolved
during decomposition. As a class the lead compounds give rise to products
of varying opacity, are toxic and turn black in the presence of certain
sulphur-containing compounds but are good heat stabilizers.
Of these materials basic lead carbonate (white lead) has been, and
probably still is, the most important stabilizer for PVC. It may be
considered as typical of the lead compounds and has a low weight cost. It is
appreciated that weight cost is not, however, the best criterion to be
considered in assessing the economics of a stabilizer. Far more relevant is
the cost required to stabilize the material to an acceptable level for the
processing and service conditions involved. One additional disadvantage of
the lead carbonate is that it may decompose with the evolution of carbon
99
dioxide at the higher range of processing conditions, thus leading to a
porous product.
For this reason tribasic lead sulphate, a good heat stabilizer which
gives polymer compounds with better electrical insulation properties than
lead carbonate, has increased in popularity in recent years at the expense of
white lead. Its weight cost is somewhat higher than that of lead carbonate
but less than most other stabilizers. This material is used widely in rigid
compounds, in electrical insulation compounds and in general purpose
formulations.
Other lead stabilizers are of much specific applications. Dibasic
lead phosphite gives compounds of good light stability but because of its
higher cost compared with the sulphate and the carbonate its use is now
restricted. In spite of its even greater weight cost dibasic lead phthalate
finds a variety of specialized applications. Because it is an excellent heat
stabilizer it is used in heat-resistant insulation compounds (for example in
105°C wire). It is also used in high-f idelity gramophone records, in PVC
coatings for steel which contain polymerizable plasticizer s and in expanded
PVC formulations which use azodicarbonamide as a blowing agent. In this
latter application the phthalate stabilizer also acts as a “kicker” to accelerate
the decomposition of the blowing agent.
Normal and dibasic lead stearate have a stabilizing effect but their
main uses are as lubricants. Lead silicate is sometimes used in leathercloth
formulations but is today of little importance. Other lead compounds now
of negligible importance are coprecipitated lead orthosilicates and lead
salicylate.
Whilst lead compounds have been, and still are, the most important
class of stabilizer for PVC the metallic soaps or salts have steadily
increased in their importance and they are now widely used. At one time a
wide range of metal stearates, ricinoleates, palmitates and octoates were
offered as possible stabilizers and the efficiency of many of them has been
examined. Today only the compounds of cadmium, barium, calcium and
zinc are prominent as PVC stabilizers.
Another group of stabilizers are the organo-tin compounds. These
materials found early applications because of their resistance to sulphur and
because they can yield crystal-clear compounds. The older organo-tin
compounds such as dibutyltin dilaurate, however, give only limited heat
stability and problems may arise with high processing temperatures.
Dibutyltin maleate imparts somewhat greater heat resistance. The
availability of a number of sulphur-cont aining organo-tin compounds, such
100
as dibutyltin di-iso-octylthioglycollate, which impart excellent heat
resistance and clarity, has to some extent increased the scope of organo-tin
compounds. They are, however, more expensive in terms of weight cost. It
should be noted that the sulphur-containing organo-tin compounds should
not be used where lead derivatives are present in the PVC compound since
cross-staining will occur to form black lead sulphide. Such lead compounds
could be present as added stabilizer or even because the polymer on
manufacture was washed with water fed through lead pipes.
Mention has already been made of epoxide stabilizers. They are of
two classes and are rarely used alone. The first class is the epoxidized oils,
which are commonly employed in conjunction with the cadmium-barium
systems. The second class is the conventional bis-phenol A epoxide resins.
Although rarely employed alone, used in conjunction with a trace of zinc
octoate (2 parts resin, 0.1 part octoate) compounds may be produced with
very good heat stability.
A further class of stabilizers are the amines, such as diphenylurea
and 2-phenylindole. These materials are effective with certain emulsion
polymers but rather ineffective with many other polymers.
In addition to stabilisers, antioxidants and ultra-violent absorbers
may also be added to PVC compounds.
Plasticizers
The tonnage of plasticizers consumed each year exceeds the annual
tonnage consumption of most plastics materials. Only PVC, the
polyolefins, the styrene polymers, the aminoplastics and, possibly, the
phenolics are used in large quantity.
These materials are essentially non-volatile solvents for PVC.
Because of their molecular size they have a very low rate of diffusion into
PVC at room temperature but at temperatures of about 150°C molecular
mixing can occur in a short period to give products of flexibility varying
according to the type and amount of plasticizer added.
All PVC plasticizers have a solubility parameter similar to that of
PVC. It appears that differences between liquids in their plasticizing
behaviour is due to differences in the degree of interaction between
polymer and plasticizer. Thus such phosphates as tritolyl phosphate, which
have a high degree of interaction, gel rapidly with polymer, are more
difficult to extract with solvents and give compounds with the highest
brittle point. Liquids such as dioctyl adipate, with the lowest interaction
with polymer, have the converse effect whilst the phthalates, which are