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Aliphatic Polyolefins other than Polyethylene, and Diene Rubbers

Aliphatic Polyolefins other than Polyethylene, and Diene Rubbers

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248 Aliphatic Polyolefins other than Polyethylene, and Diene Rubbers

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:

(i)

(ii)

(iii)

(iv)

(v)



homopolymers produced by Ziegler-Natta catalysis;

block copolymers produced by Ziegler-Natta catalysis;

random copolymers produced by Ziegler-Natta catalysis;

rubber-modified blends of the above;

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 Monte11 as Hivalloy.

Mention should be made of the nomenclature for the polymer. Industrially the

materially is invariably known in the English-speaking world as polypropylene.

However, the IUPAC name for the monomer is propene and until 1975 the

recommended IUPAC name was polypropene, a term very rarely used. The latest

IUPAC rules base the name of a polymer on the constitutional repeating unit,

which in this case is a propylene unit (c.f. a methylene unit for polyethylene) and

this leads to the name poly(propy1ene) (i.e. with brackets). In this volume the

more common, unbracketed but still unambiguous name will be used.



11.1.1 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

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 polymerisation vessel under pressure

whilst the catalyst solution and the reaction diluent (usually naphtha) are metered

in separately. In batch processes reaction is carried out at temperatures of about

60°C for approximately 1-4 hours. In a typical process an SO-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

polymerisation. The polymer molecular weight can be controlled in a variety of



Polypropylene



249



ways, for example by the use of hydrogen as a chain transfer agent or by

variations in the molar ratio of catalyst components, the polymerisation

temperature, the monomer pressure or the catalyst concentration.

At this stage of the process the following materials are present in the

polymerisation vessel :



(1)

(2)

(3)

(4)

(5)



Isotactic polymer.

Atactic polymer.

Solvent.

Monomer.

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 I 1 .I.

There have also been a number of quite substantial changes in the method of

polymerisation 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 this enables the washing stage to be dispensed

with. In particular, both liquid (bulk process) and gas-phase processes have been

developed, including methods which avoid the need for separate stages for the

removal of catalyst residues and/or atactic material. Thermal and chemical aftertreatments 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 polymerisation 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 polymers by use of metallocenes (see

below). There is current interest in the production of polymers with a bimodal

MWD (for explanations see the Appendix to Chapter 4).

Another technical development is that of high impact isotactic polypropylene

in which rubber droplets are produced in situ during the polymerisation stage.

After propylene homopolymerisation ethylene is added to the reacting mass in a



L



L.



P



E

00



G



Polypropylene



25 1



second reactor and finely dispersed ethylene-propylene rubber droplets are

formed by polymerisation 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(2-methyl-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

propylenes with monomers such as long-chain olefins, cyclic olefins and styrene

which is not possible with more conventional Ziegler-Natta catalysts.



11.1.2 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 properties 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 polypropylenes the net effect is a

melting point some 50°C higher than that of the most regular polyethylenes. 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 high-energy treatment leads to chain scission rather than cross-linking.

The most significant influence of the methyl group is that it can lead to

products of different tacticity, ranging from completely isotactic and syndiotactic

structures to atactic molecules (see Chapter 4). The isotactic form is the most

regular since the methyl groups are all disposed on one side of the molecule.

Such molecules cannot crystallise in a planar zigzag form as do those of

polyethylene because of the steric hindrance of the methyl groups but crystallise

in a helix, with three molecules being required for one turn of the helix. Both

right-hand and left-hand 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 polymers may also be formed in which a block of

monomer residues with a right-handed helix is succeeded by a block with a lefthanded helix. The frequency with which such changes in the helix direction occur

can have an important influence on the crystallisation 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



252



Aliphatic Polyolefins other than Polyethylene, and Diene Rubbers

5.000y



.





n



Is



.C 4.000*z

1 n



-



Y



I



w

*



3.000i



ISOTACTIC INDEX



r-



Figure 11.2. Effect of isotacticity on tensile properties. (Reproduced by permission of IC1 Plastics

Division)



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 polymer which is

insoluble in n-heptane and by the presence of block 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 greater the softening point, stiffness, tensile strength, modulus and

hardness, all other structural features being equal (Figure 11.2).

The influence of molecular weight on the bulk properties of polypropylene is

often opposite to that experienced with most other well-known 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

crystallise so easily as lower molecular weight material and it is the differences

in the degree of crystallisation which affect the bulk properties. It may also be

mentioned that an increase in molecular weight leads to a reduction in brittle

point (see Table 11.1).

Published data on com-mercial polypropylenes indicate that their molecular

weights arcinthe range M , = 38 000-60000 and M , = 220000-700000, with

values of M,/M, from about 5.6 to 11.9. These averages are somewhat higher

than those encountered normally with polyethylene and may help to explain the

difference in molecular weight dependence. It is in fact the case that the very high

molecular weight polyethylenes also have some difficulty in crystallising and

they too have lower tensile strength and stiffness than more conventional

polymers of lower molecular weight.

Only a limited amount of information is available concerning the effects of

molecular weight distribution. There is, however, evidence that the narrower the

distribution the more Newtonian are the melt flow properties. It has been noted



Polypropylene



253



Table 11.1 Some mechanical and thermal properties of commercial polypropylenes

Test method



Property



Melt flow index

Tensile strength

(lbf/in2)

(MW

Elongation at break (%)

Flexural modulus

(Ibf/in2)

(MPa)

Brittleness temperature

("C)

Vicat softening point

("C)

Rockwell hardness

(R-scale)

Impact strength (ft lbf)

(J)



Copolymers



Homopolymers



3.0



0.7



0.2



3.0



0.2



5000

34

350



4400

30

115



4200

29

175



4200

29

40



3700

25

240



190 000

1310



170000

1170



160000

1100



187000

1290



150 000

1030



+15



0



0



-15



-20



145-1 50



148



148



148



147



95

10

34



90

25

46



90

34

46



9s

34

57.5



88.5

42.5



2.16kg at 230°C.

(b) Straining rate 18 mdmin.

(c) Falling weight test on 14 in diameter moulded bowls at 20°C

(a) Standard polyethylene grader: load



that with polymers of molecular we&hts suitable for moulding and extrusion,

polymers of wide distribution (e.g. M J M , about 6 ) are stiffer and more brittle

than those with a M J M , ratio of about 2.

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.

One unfortunate characteristic property of polypropylene is the dominating

transition point which occurs at about 0°C with the result that the polymer

becomes brittle as this temperature is approached. Even at room temperature the

impact strength of some grades leaves something to be desired. Products of

improved strength and lower brittle points may be obtained by block

copolymerisation of propylene with small amounts (4-1 5%) of ethylene. Such

materials are widely used (known variously as polyallomers or just as propylene

copolymers) and are often preferred to the homopolymer in injection moulding

and bottle blowing applications.

Further variations in the properties of polyethylenes may be achieved by

incorporating additives. These include rubber, antioxidants and glass fibres and

their effects will be discussed further in Section 11.1.4.



11.1.3 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:



254 Aliphatic Polyolejins other than Polyethylene, and Diene Rubbers

(1) It has a lower density (0.90g/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 sterilising operations. For example mouldings have been sterilised in

hospitals for over 1000 hours at 135OC 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.



As shown in the previous section the mechanical and thermal properties of

polypropylene are dependent on the isotacticity, the molecular weight and on other

structure features. The properties of five commercial materials (all made by the

same manufacturer and subjected to the same test methods) which are of

approximately the same isotactic content but which differ in molecular weight and

in being either homopolymers or block copolymers are compared in Table I 1 .I.

The figures in Table I1 .I show quite clearly how an increase in molecular

weight (decrease in melt flow index) causes a reduction in tensile strength,

stiffness, hardness and brittle point but an increase in impact strength. The

general effects of isotactic index and melt flow index on some mechanical and

thermal properties are also shown graphically in Figures 11.3-11.6.' Both

random and block copolymers are now available and these show interesting

differences as indicated in Table 11.2.

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

plasticising capacity of an injection moulding machine using polypropylene is

lower than when polystyrene is used but generally higher than with a highdensity polyethylene.



LOG (MELT INDLX)



Figure 11.3. Variation of tensile yield stress with melt flow index (10kg load at 1 9 0 T ) and isotactic

index. (After Crespi and Ranalli')



255

100-



I



I



5



%



E



80

a-



-zI 6 0



\



M J I. =I



c

0

w

x



e 40-



In



u

t-



2 20



E



Figure 11.4.



3 and isotactic index.



Figure 11.5.



and melt flow index.



Figure 11.6.



isotactic index (After



256 Aliphatic Polyolefins other than Polyethylene, and Diene Rubbers

Table 11.2 Comparison of random and block copolymers (data based on three grades of Novolen

Targor Gmbh)



-



Property



Units



Test method



Novolen grade



Melt Flow Ratio

230°C/2. 16 kg

Tensile Yield

Stress

Tensile Modulus

Charpy Impact

23°C

0°C

-20°C

Haze



Random

copolymer



Block

copolymer



Homopolymer



3348SC



2600TC



1148TC



g/10 min



IS01133



30



48



48



MPa

MPa



IS0527

IS0527

1S0179/leU



23

820



19

950



1550



No break

190



103



so



No break

170

150



7



95



60



kJ/m2



%



ASTM

D1003



3s



This data indicates that the random copolymer has greater transparency but inferior low temperature impact strength



Studies of melt flow properties of polypropylene indicate that it is more nonNewtonian than polyethylene in that the apparent viscosity declines more rapidly

with increase in shear rate. The melt viscosity is also more sensitive to

temperature. Van der Wegt2 has shown that if the log (apparent viscosity) is

plotted against log (shear stress) for a number of polypropylene grades differing

in molecular weight, molecular weight distribution and measured at different

temperatures the curves obtained have practically the same shape and differ only

in position.

The standard melt flow index machine is often used for characterising the flow

properties of polypropylene and to provide a rough measure of molecular weight.

Under the conditions normally employed for polyethylene (2.16 kg load at

190OC) the flow rate is too low for accurate measurement and in practice higher

loads, e.g. 10 kg, and/or higher temperatures are used. It has been found3 that a

considerable pressure drop exists in the barrel so that the flow towards the end

of a test run is higher than at the beginning.

The moulding shrinkage of polypropylene is less than that experienced with

polyethylenes 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 melts which have not been

heated much above the crystalline melting point. Such regions of order would

provide sites for crystal nucleation and hence crystallisation would be more rapid

when cooling was carried out.

The electrical properties of polypropylene are very similar to those of highdensity polyethylenes. In particular the power factor is critically dependent on

the amount of catalyst residues in the polymer. Some typical properties are given

in Table 11.3 but it should be noted that these properties are dependent on the

antioxidant system employed as well as on the catalyst residues.



Polypropylene 257

Table 11.3 Some typical electrical properties of a high

heat stability grade of polypropylene4

Dielectric constant at 5 X lo6 Hz

Volume resistivity ( a m )

Power factor at 10’ Hz

103 HZ

104 HZ

105 HZ

lo6 Hz

5 X lo6 Hz



2.2s

21019



0.0009

0.001

0.0009

0.006

0.0004



0.0005



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 the same liquids. In both cases the

absence of any possible interaction between the crystalline polymer and the liquid

prevents solution of the polymers in any liquids at room temperature. In some

instances polypropylene is more affected than polyethylene but in other cases the

reverse is true. Similar remarks may be made concerning the permeability of the

two polymers to liquids and gases. With many permeants polypropylene shows the

lowest permeability but not, for example, with hexane. It may be mentioned in this

context that although high-density polyethylene is usually intermediate between

low-density polyethylene and polypropylene, where the permeant causes stress

cracking (as with a silicone oil), the high-density polyethylenes often have the

highest permeability. The fact that polypropylene is resistant to environmental

stress cracking has already been mentioned.

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. Some estimate of the difference between the

two polymers can be obtained from Figure 11.7, which comparess the rates of

oxygen uptake of each polymer at 93°C. Substantial improvements can be made

by the inclusion of antioxidants and such additives are used in all commercial

compounds. Whereas polyethylene cross-links on oxidation, polypropylene

degrades to form lower molecular weight products. Similar effects are noted



TIME AT 200.F



IN hours



Figure 11.7. Comparison of oxidation rates of unstabilised polyethylene and polypropylene (After

Kresse? )



258 Aliphatic Polyolefins other than Polyethylene, and Diene Rubbers

Table 11.4 Effect of percentage stretch on tensile properties of

polypropylene film'

Stretch (%)



Tensile strength

I



I



None

200

400

600

900



5 600

8 400

14000

22 400

23 800



EB (%)



I



500

250

115

40

40



39

58

97

155

165



when the polymer is exposed to high-energy radiation and when heated with

peroxides (conditions which will cross-link polyethylene).

Although a crystalline polymer, polypropylene mouldings are less opaque

when unpigmented than corresponding mouldings from high-density polyethylene. This is largely due to the fact that the differences between the amorphous

and crystal densities are less with polypropylene (0.85 and 0.94g/cm3

respectively) than with polyethylene (see Chapter 6). Clarity may also be affected

by the use of nucleating agents (see also Sections 3.3 and 11.1.4). Biaxially

stretched film has a high clarity since layering of the crystalline structures

reduces the variations in refractive index across the thickness of the film and this

in turn reduces the amount of light scattering.6

Biaxial stretching also leads to polymers of improved tensile strength. The

effect of increasing the amount of stretching on the tensile strength and breaking

elongation are given in Table 11.4.

There are other differences between cast, monoaxially oriented, and balanced

biaxially oriented film. Typical figures illustrating these effects are given in Table

11.5.

When film is produced by air-cooled tubular blowing methods cooling rates

are slower and larger degrees of crystallinity result. Hence tubular film is slightly



Table 11.5 Comparison of cast, monoaxially oriented and biaxially oriented polypropylene film'

Property



Tensile strength (Ibf/in*)

Machine direction

Transverse direction

Tensile strength (MPa)

Machine direction

Transverse direction

Elongation at break (%)

Machine direction

Transverse direction

ASTM D.523 gloss (45°C head)

Low temperature brittleness

Coefficient of friction



polymer



Monoaxially

oriented



Balanced

oriented



5700

3200



8 000

40 000



26 000

22 OOO



39

22



55

280



180

152



425

300

75-80

brittle at "C

0.4



300

40

>80

excellent

0.4-0.5



80

65

> 80

excellent

0.8



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