Добавил:
ivanov666
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Plastics technology. Часть 1. Учебное пособие.pdf
41
plastics production league. In 1998 it was estimated that global capacity
was about 26 .3∙106 tonnes, with the United States and Western Europe each
sharing about 25% of the market. As with other large tonnage materials, the
figures for these two major production regions are somewhat lower than for
ten years earlier due to the increasing share taken by the so-called
developing countries.
The application of metallocene catalysis to the preparation of
polypropylenes reached a commercial stage with the production by Exxon
of their Achieve range in 1996 and in 1997 by Targor, the BASF-Hoechst
joint venture with the introduction of Metocene. Such metallocene
polypropylenes are, however, only a small proportion of the total
polypropylene market, predicted at only about 3% of the total in 2005.
With a rapidly growing market many grades of polypropylene are
available but five main classes may be distinguished:
(1) homopolymers produced by Ziegler-Natta catalysis;
(2) block copolymers produced by Ziegler-Natta catalysis;
(3) random copolymers produced by Ziegler-Natta catalysis;
(4) rubber-modified blends of the above;
(5) homopolymers and copolymers produced by metallocene
catalysis.
Two interesting developments should also be noted: syndiotactic
polypropylene produced by a novel metallocene system and polypropylene
grafted with styrene and/or maleic anhydride marketed by Montell as
Hivalloy.
Preparation of polypropylene
There are many points of resemblance between the production of
polypropylene and polyethylene using Ziegler-type catalysts. In both cases
the monomers are produced by the cracking of petroleum products such as
natural gas or light oils. For the preparation of polypropylene the C3
fraction (propylene and propane) is the basic intermediate and this may be
separated from the other gases without undue difficulty by fractional
distillation. The separation of propylene from propane is rather more
difficult and involves careful attention to the design of the distillation plant.
For polymer preparation impurities such as water and methylacetylene must
be carefully removed. A typical catalyst system may be prepared by
reacting titanium trichloride with aluminium triethyl, aluminium tributyl or
aluminium diethyl monochloride in naphtha under nitrogen to form a slurry
consisting of about 10% catalyst and 90% naphtha. The properties of the
42
polymer are strongly dependent on the catalyst composition and its particle
shape and size.
In the suspension process, which was the first method to be
commercially developed, propylene is charged into the polymerization
vessel under pressure whilst the catalyst solution and the reaction diluent
(usually naphtha) are metered in separ ately. In batch processes reaction is
carried out at temperatures of about 60°C for approximately 1-4 hours. In a
typical process an 80-85% conversion to polymer is obtained. Since the
reaction is carried out well below the polymer melting point the process
involves a form of suspension rather than solution polymerization. The
polymer molecular weight can be controlled in a variety of ways, for
example by the use of hydrogen as a chain transfer agent o r b y va riatio ns in
the molar ratio of catalyst components, the polymerization temperature, the
monomer pressure or the catalyst concentra tio n.
At this stage of the process the following materials are present in
the polymerization vessel:
(1) Isotactic polymer.
(2) Atactic polymer.
(3) Solvent.
(4) Monomer.
(5) Catalyst.
The first step in separating these ingredients involves the transfer of
the reaction mixture to a flash drum to remove the unreacted monomer,
which is purified (where necessary) and recycled. The residual slurry is
centrifuged to remove the bulk of the solvent together with most of the
atactic material which is soluble in the naphtha. The remaining material is
then treated with an agent which decomposes the catalyst and dissolves the
residue. A typical agent is methanol containing a trace of hydrochloric acid.
The solution of residues in the methanol is removed by a centrifuging
operation and the polymer is washed and dried at about 80°C. At this stage
the polymer may be blended with antioxidants, extruded and cut into
pellets. There are a number of variations in this basic process, many of
which involve extra processes to reduce the atactic content of the polymer.
A typical flow sheet for the manufacture of polypropylene is given in
Figure 7.
43
Figure 7 – Typical flow sheet for polypropylene manufacture
There have also been a number of quite substantial changes in the
method of polymerization over the years. For example, newer catalyst
systems, such as those containing magnesium compounds, give an
appreciable improvement in the yield of isotactic material and t his enables
the washing stage to be dispensed with. In particular, both liquid (bulk
process) and gas-phase processes have been developed, including met hods
which avoid the need for separate stages for the removal of catalyst
residues and/or atactic material. Thermal and chemical after-treatments
have also been developed to reduce the width of the molecular mass
distribution.
One such system is that developed by Himont, which uses three
components:
(1) A titanium component supported on a magnesium halide.
(2) An organo-aluminium component.
(3) A Lewis base.
Detailed modifications in the polymerization procedure have led to
continuing developments in the materials available. For example in the
1990s greater understanding of the crystalline nature of isotactic polymers
gave rise to developments of enhanced flexural modulus (up to 2300 MPa).
Greater control of molecular weight distribution has led to broad MWD
polymers produced by use of twin-reactors, and very narrow MWD
44
polymers by use of metallocenes. There is current interest in the production
of polymers with a bimodal MWD.
Another technical development is that of high impact isotactic
polypropylene in which rubber droplets are produced in situ during the
polymerization stage. After propylene homopolymerization ethylene is
added to the reacting mass in a second reactor and finely dispersed
ethylene-propylene rubber droplets are formed by polymerization in the
porous homopolymer polypropylene pellets.
Polypropylenes produced by metallocene catalysis became
available in the late 1990s. One such process adopts a standard gas phase
process using a metallocene catalyst such as rac.-dimethylsilylenebis(2methyl-1-benz(e)indenyl)zirconium dichloride in conjunction with
methylaluminoxane (MAO) as cocatalyst. The exact choice of catalyst
determines the direction by which the monomer approaches and attaches
itself to the growing chain. Thus whereas the isotactic material is normally
preferred, it is also possible to select catalysts which yield syndiotactic
material. Yet another form is the so-called hemi-isotactic polypropylene in
which an isotactic unit alternates with a random configuration.
Metallocene catalysis can also make possible the production of
copolymers of propylene with monomers such as long-chain olefins, cyclic
olefins and styrene which is not possible with more conventional ZieglerNatta catalysts.
Propylene is readily polymerized in bulk; that is, in the liquid
monomer itself. Arco, El Paso, Phillips, and Shell are practitioners of bulk
processing in stirred or loop reactor systems. In either case, liquid
propylene (and ethylene, if random copolymer is desired) is continuously
metered to the polymerization reactor along with a high-activity/highstereospecificity catalyst system. Polymerization temperatures are normally
in the range of 45-80°C with pressures sufficient to maintain propylene in
the liquid phase (1.7–3.5 MPa). Hydrogen is used for molecular weight
control. The polymer slurry (approximately 30-50% solids in liquid
propylene) is continuously discharged from loop reactors through a series
of sequence valves into a zone maintained essentially at atmospheric
pressure an containing terminating agents. Technologies also exist for
production of propylene/ethylene block copolymers via bulk
polymerization employing stirred or loop reactors.
Modern vapor-phase polymerization is represented in one form by
the stirred gas-phase process originally developed by BASF and licensed
by Norchem in the United States. BASF process reactors contain a spiral or
45
double-helical agitator to stir the polymer bed. Cooling of the bed is
maintained by continuous injection of fresh, high-purity propylene in a
liquid or partly liquefied state into the reaction zone. The unreacted
propylene is removed from the top of the reactor during polymerization,
condensed, and reinjected with fresh propylene. Evaporation of the
unreacted propylene absorbs the heat of polymerization and also brings
about intense mixing of the solid polymer particles with the gas phase.
Energy costs of the process are economically attractive. The diluent-free
BASF process provides sufficiently high yield of polymer per unit of
catalyst so that deashing is not required. Although products made in this
way contain relatively high levels of titanium and aluminum residues, a
unique finishing step during extrusion palletizing reduces active chlorides
to an innocuous level.
With dramatic improvements in Ziegler–Natta catalyst technology,
the Spheripol process, first developed by Montedison and Mitsui
Petrochemical with simplified bulk (liquid propylene) process technology
operating with loop reactors, is capable of directly producing a relatively
large round bead with suitable density to eliminate the need for pelletizing
for many applications. Subsequently, Montedison and Hercules, Inc., which
assumed responsibility for all polypropylene operations and technology of
the parent companies.
The Himont spheripol loop reactor process is initiated by injecting
specially prepared supported catalyst and cocatalyst into liquid propylene
circulated in a relatively simply high L/D ratio loop reactor, followed by
monomer removal (Figure 8). The homopolymers so produced can be
circulated through ethylene and ethylene/propylene gas phase reactors for
insertion of copolymer fractions before final monomer stripping.
The Unipol low-pressure gas-phase fluidized-bed process, which
was introduced by Union Carbide in 1977 for LLDPE, has also been
adapted to the production of PP homopolymers and block copolymers using
Shell Chemicals high-activity (Ziegler-type) catalyst technology. The
Spheripol and Unipol processes are capable of producing polyme r in c rumb
bead or granular forms, with the potential for direct marketing without any
pelletizing finishing operation.
Regardless of the polymerization process used, the PP homo- and
co-polymer must be stabilized to some degree to prevent oxidative
degradation. The general practice is to incorporate a small quantity of
stabilizer in the polymer prior to the first exposure to elevated temperat ures
of a drying operation or long-term storage.
46
Figure 8 – A simplified flow diagram
of the Himont spherical loop reactor process
Inert-gas (nitrogen) blanketing is also used in some storage/transfer
systems. Additional stabilizers, up to 1%, are added to the polymer during
pelletizing. Most commercial PP compositions contain mixtures of
hindered phenols and hydroperoxide decomposers or various phosphates.
Structure and properties of polypropylene
Polypropylene is a linear hydrocarbon polymer containing little or
no unsaturation. It is therefore not surprising that polypropylene and
polyethylene have many similarities in their properties, particularly in their
swelling and solution behaviour and in their electrical properties. In spite of
the many similarities the presence of a methyl group attached to alternate
carbon atoms on the chain backbone can alter the proper ties of the polymer
in a number of ways. For example it can cause a slight stiffening of the
chain and it can interfere with the molecular symmetry. The first effect
leads to an increase in the crystalline melting point whereas the interference
with molecular symmetry would tend to depress it. In the case of the most
regular polypropylene the net effect is a melting point some 50°C higher
than that of the most regular polyethylene. The methyl side groups can also
influence some aspects of chemical behaviour. For example the tertiary
carbon atom provides a site for oxidation so that the polymer is less stable
than polyethylene to the influence of oxygen. In addition, thermal and highenergy treatment leads to chain scission rather than cross-linking.
The most significant influence of the methyl group is that it can
47
lead to products of different tacticity, ranging from completely isotactic and
CH
2
CH
CH
2
CH
CH
CH
2
CH
CH
2
CH
3
CH
3
CH
3
CH
3
CH
2
CH
CH
2
CH
CH
CH
2
CH
CH
2
CH
3
CH
3
CH
3
CH
3
CH
2
CH
CH
2
CH
CH
CH
2
CH
CH
2
CH
3
CH
3
CH
3
CH
3
syndiotactic structures to atactic molecules.
isotactic form
syndiotactic form
atactic form
The isotactic form is the most regular since the methyl groups are
all disposed on one side of the molecule. Such molecules cannot crystallize
in a planar zigzag form as do those of polyethylene because of the steric
hindrance of the methyl groups but crystallize in a helix, with three
molecules being required for one turn of the helix. Both right-hand and lefthand helices occur but both forms can fit into the same crystal structure.
Commercial polymers are usually about 90-95% isotactic. In these
products, atactic and syndiotactic structures may be present either as
complete molecules or as blocks of varying length in chains of otherwise
isotactic molecules. Stereo-block polymer s may als o be formed in which a
block of monomer residues with a right-handed helix is succeeded by a
block with a left-handed helix. The frequency with which such changes in
the helix direction occur can have an important influence on the
crystallization and hence the bulk properties of the polymer. In practice it is
difficult to give a full description of a specific propylene polymer although
there has been marked progress in recent years. Many manufacturers
simply state that their products are highly isotactic, others quote the
polymer crystallinity obtained after some specified annealing treatment,
whilst others quote the so-called “isotactic index”, the percentage of
polymer insoluble in n-heptane. Both of these last two properties provide
some rough measure of the isotacticity but are both subject to error. For
example the isotactic index is affected by high molecular weight atactic
48
polymer which is insoluble in n-heptane and by the presence of block
n
M
w
M
nw
M/M
copolymers of isotactic and atactic structures which may or may not
dissolve, according to the proportion of each type present.
In spite of these problems the general effects of varying the degree
of isotacticity are well known. Whereas the atactic polymer is an
amorphous somewhat rubbery material of little value, the isotactic polymer
is stiff, highly crystalline and with a high melting point. Within the range of
commercial polymers, the greater the amount of isotactic material the
greater the crystallinity and hence the gr eater the softening point, stiffness,
tensile strength, modulus and hardness, all other structural features being
equal.
The influence of molecular weight on the bulk properties of
polypropylene is often opposite to that experienced with most other wellknown polymers. Although an increase in molecular weight leads to an
increase in melt viscosity and impact strength, in accord with most other
polymers, it also leads to a lower yield strength, lower hardness, lower
stiffness and softening point. This effect is believed to be due to the fact
that high molecular weight polymer does not crystallize so easily as lower
molecular weight material and it is the differences in the degree of
crystallization which affect the bulk properties. It may also be mentioned
that an increase in molecular weight leads to a reduction in brittle point.
Published data on commercial polypropylene indicate that their
molecular weights are in the range
=38000-60000 and
=220000-
700000, with values of
from about 5.6 to 11.9.
The morphological structure of polypropylene is rather complex
and at least four different types of spherulite have been observed. The
properties of the polymer will depend on the size and type of crystal
structure formed and this will in turn be dependent on the relative rates of
nucleation to crystal growth. The ratio of these two rates can be controlled
by varying the rate of cooling and by the incorporation of nucleating
agents. In general the smaller the crystal structures the greater the
transparency and flex resistance and the less the rigidity and heat
resistance.
Properties of isotactic polypropylene
Although very similar to high-density polyethylene, isotactic
polypropylene differs from the former in a number of respects of which the
following are among the most important:
49
(1) It has a lower density (0.90 g/cm3).
(2) It has a higher softening point and hence a higher maximum
service temperature. Articles can withstand boiling water and be subject to
many steam sterilizing operations. For example mouldings have been
sterilized in hospitals for over 1000 hours at 135°C in both wet and dry
conditions without severe damage.
(3) Polypropylene appears to be free from environmental stress
cracking problems. The only exception seems to be with concentrated
sulphuric and chromic acids and with aqua regia.
(4) It has a higher brittle point.
(5) It is more susceptible to oxidation.
Many features of the processing behaviour of polypropylene may
be predicted from consideration of thermal properties. The specific heat of
polypropylene is lower than that of polyethylene but higher than that of
polystyrene. Therefore the plasticizing capacity of an injection moulding
machine using polypropylene is lower than when polystyrene is used but
generally higher than with a high-density polyethylene.
Studies of melt flow properties of polypropylene indicate that it is
more non-Newtonian than polyethylene in that the apparent viscosity
declines more rapidly with increase in shear rate. The melt viscosity is also
more sensitive to temperature.
The moulding shrinkage of polypropylene is less than that
experienced with polyethylene but is dependent on such processing factors
as mould temperature, melt temperature and plunger dwell time. In general,
conditions which tend to reduce the growth of crystal structures will tend to
reduce shrinkage; for example, low mould temperatures will encourage
quenching of the melt. It is also found that low shrinkage values are
obtained with high melt temperatures. This is probably due to the fact that
high melt temperatures lead to a highly disordered melt whereas some
molecular order may be present in mel ts which have not been heated much
above the crystalline melting point. Such regions of order would provide
sites for crystal nucleation and hence crystallization would be more rapid
when cooling was carried out.
The electrical properties of polypropylene are very similar to those
of high-density polyethylene. In particular the power factor is critically
dependent on the amount of catalyst residues in the polymer.
As with electrical properties the chemical resistance of
polypropylene shows many similarities to high-density polyethylene. The
two polymers have similar solubility parameters and tend to be swollen by
50
the same liquids. In both cases the absence of any possible interaction
nw
M/M
between the crystalline polymer and the liquid prevents solution of the
polymers in any liquids at room temperature.
Polypropylene differs from polyethylene in its chemical reactivity
because of the presence of tertiary carbon atoms occurring alternately on
the chain backbone. Of particular significance is the susceptibility of the
polymer to oxidation at elevated temperatures. Whereas polyethylene crosslinks on oxidation, polypropylene degrades to form lower molecular weight
products. Similar effects are noted when the polymer is exposed to highenergy radiation and when heated with peroxides (conditions which will
cross-link polyethylene).
Properties of metallocene isotactic and syndiotactic polypropylene
Isotactic polypropylene produced by metallocene catalysis are now
being produced by a number of different manufacturers and because
different systems are used there is some variation in properties. Typically
however such materials have similar density, hardness and tensile strength
to conventional homo-polymers but differ in having
(1) a narrower molecular weight distribution
с 1.9;
(2) better optical properties such as 93% ASTM D1003
transparency (cf. 40% for a conventional homopolymer) and higher
specular gloss (77% cf. 57% by ISO 2813);
(3) lower Tm (typically about 15°C less using DSC methods);
(4) lower extrac ta bles;
(5) lower toughness.
The narrow molecular weight distribution means that the melts are
more Newtonian and therefore have a higher mel t viscosity at high shear
rates than a more pseudoplastic material of similar molecular dimensions.
In turn this may require more powerful extruders. They are also more
subject to melt irregularities such as sharkskin and melt fracture. This is
one of the factors that has led to current interest in metallocenepolymerized polypropylenes with a bimodal molecular weight distribution.
The lower melting point can be advantageous in that film sealing
temperatures may be lower thus allowing faster production rates of
packaging products.
The higher transparency coupled with the rigidity expected of a
conventional polypropylene homopolymer is of particular interest in thinwall moulding applications.