Extrusion process for preparing orientated polyolefins

An orientated polyolefin composition comprising an olefin (co)polymer having a weight average molecular weight (Mw) from 30,000 to 1,000,000 and a high molecular weight tail extending to molelcular weights above 1,000,000, and has a microstructure which is substantially parallel chain extended cores of the component polymer(s) and associated lamellar overgrowths extending in planes normal to the cores in which the lamellar overgrowths on adjacent cores intermesh and are capable of sustaining a haul off tension of at least 10 MPa without breaking during its extrusion, is produced by extruding an olefin copolymer through a die under a pressure of about 5-10 MPa, at a temperature above the self-blocking temperature thereof and by hauling off the extrudate at a tension of at least 10 MPa without breaking. The extrudates so produced have a high modulus.

The present invention relates to novel orientated polyolefins and to a 
continuous extrusion process for the production thereof. 
It is known from Odell, J.A., et al in Polymer, Vol 19, p 617, (1978) that 
orientated polyethylene plugs can be produced by extruding polyethylene 
through a capillary die under conditions of pressure and temperature such 
that fine "fibrils" are produced within the melt in the capillary, and 
then blocking the die exit so that the pressure within the capillary rises 
and causes the melt to rapidly solidify therein. The die is then cooled 
and the polyethylene plug is removed from the capillary. The orientated 
plugs so produced have a high modulus ranging from 10-100 GPa and are less 
prone to fibrillation and thermal shrinkage normally associated with other 
forms of orientated polyethylene produced, for example, by cold drawing. 
Electron microscopy of the plugs revealed an almost entirely lamellar 
microstructure with the lamellae extending in planes normal to a system of 
substantially parallel chain extended cores, the cores being orientated in 
a direction substantially parallel to the longitudinal axes of the plugs. 
Each chain extended core and its associated lamellae which are believed to 
be chain-folded overgrowths of the core had an overall shape not unlike a 
microscopic "shish-kebab". It was further observed that the lamellae 
associated with adjacent cores intermeshed with one another and this was 
considered to be a primary reason for the observed high modulus of the 
plugs. This die-blocking technique is only applicable to the discontinuous 
production of short plugs. 
It is an object of the present invention to provide novel orientated 
polyolefins having "shish-kebab" orientated microstructure and to provide 
a continuous process for the production thereof. 
Accordingly, the present invention is an orientated polyolefin composition 
comprising an olefin (co)polymer having a weight average molecular weight 
(Mw) above 30,000 and not greater than 1,000,000 and a high molecular 
weight tail extending to molecular weights greater than 1,000,000, the 
composition having a microstructure which comprises substantially parallel 
chain extended cores of the component polymer(s) and associated lamellar 
overgrowths extending in planes normal to the cores and in which the 
lamellar overgrowths on adjacent cores intermesh and being capable of 
sustaining a haul off tension of at least 10 MPa at a temperature up to 
5.degree. C. above the self blocking temperature of the olefin (co)polymer 
without breaking during extrusion thereof. 
By "olefin (co)polymer" is meant throughout this specification one or more 
olefin homo-polymers, copolymers or melt blends of two or more 
(co)polymers which either inherently or as a consequence of melt blending 
has the defined haul off tension, molecular weight and molecular weight 
distribution characteristics. References herein to `melt blends` exclude 
solution blending because such solution blends need large amounts of 
solvents which create considerable handling, recovery and polymer 
contamination problems. 
The molecular weight distribution of the olefin (co)polymers referred to 
herein are determined by gel permeation chromatography (GPC) under the 
following conditions: 
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Temperature 
140.degree. C. 
Mobile Phase 
Trichlorobenzene (TCB) 
Flow Rate 0.5 ml/min 
Injection Volume 
500 microliters 
Concentration 
Adjusted so that in the intrinsic 
C (% wt) viscosity (deciliter/g) as 
determined in trichlorobenzene at 
140.degree. C. is 0.15. 
Detector Differential Refractive Index 
Pressure 9 bar 
Columns Shodex A806/S 
A80M/S 
A804/S 
Calibration 
NBS SRM 1475 
Narrow MWD polyethylenes (PE) 
Linear hydrocarbons 
Narrow MWD polystyrene (PS) + universal 
calibration 
(K (PS) = 1.21 .times. 10.sup.-4, alpha (PS) = 0.707 
K (PE) = 4.48 .times. 10.sup.-4, alpha (PE) = 0.718 
______________________________________ 
The defined microstructure of the composition of the present invention is 
hereafter termed as `shish-kebab` morphology. 
In the composition of the present invention the distance between the 
adjacent cores is suitably less than 5000 Angstroms, and is preferably 
below 2000 Angstroms and most preferably from 500-2000 Angstroms. The 
lamellae associated with adjacent cores preferably taper in thickness from 
the core outwards and intermesh thereby forming a "zip-like" structure. If 
the cores are too far apart disorientation can occur due to lamellar 
twisting which adversely affects the physical properties of the 
composition. 
The olefin (co)polymer to be extruded has a density which is at least 910 
kg/m.sup.3, suitably above 920 and preferably from 925 to 960 kg/m.sup.3 
as determined by British Standard BS 2782-620D. The density of the olefin 
(co)polymer should be such that it is able to crystallize from a melt 
thereof. 
The present invention further provides a continuous process for producing 
an extruded polyolefin composition which comprises an olefin (co)polymer 
which has a weight average molecular weight (Mw) above 30,000 and not 
greater than 1,000,000 and a high molecular weight tail extending to 
molecular weights greater than 1,000,000, said process comprising 
continuously extruding the polyolefin under pressure at a temperature 
above its self-blocking temperature through a die, cooling the extrudate 
at the die exit and continuously hauling off the extrudate at a tension of 
at least 10 MPa without breaking so as to avoid die swell. 
The olefin (co)polymer is suitably polyethylene having a weight average 
molecular weight (Mw) from 30,000 to 1,000,000, preferably not greater 
than 800,000 and a high molecular weight tail. Further the molecular 
weight distribution of the polymer should have a high molecular weight 
`tail` component but preferably should not have a corresponding `tail` 
component at the low molecular weight end of the distribution curve. The 
molecular weight of the high molecular 
weight `tail` component is preferably from 1,000,000-5,000,000 and that of 
the low molecular weight `tail` component, which should preferably be 
absent, ranges, if present from 500-10,000. An example of a suitable 
olefin polymer is an unblended polyethylene having a weight average 
molecular weight (Mw)=400,000 and a number average molecular weight (Mn) 
=73,000 of which 9% w/w has a molecular weight above 1,000,000 but which 
has no low molecular weight tail below 2000. The high molecular weight 
tail component is responsible for the formation of the core fibrils which 
are the chain extended nuclei and which promote formation of the 
shish-kebab morphology. On the other hand, presence of a low molecular 
weight tail is detrimental to such morphology because it tends to reduce 
melt strength thereby preventing the use of relatively high haul-off 
stresses during extrusion. 
The relative concentration of the high molecular weight tail component in 
the olefin (co)polymer is suitably below 20% w/w, preferably below 10% 
w/w. Within the ranges of concentration specified above it is preferred 
that the higher the average molecular weight of the polyolefin in the tail 
(i.e. above 1,000,000), the lower its concentration in the olefin 
(co)polymer. Preferably the olefin (co)polymer contains at least 0.5% w/w 
of high molecular weight tail component. 
The polyolefin composition should be capable of sustaining a haul-off 
tension of at least 10 MPa, preferably greater than 50 MPa, most 
preferably from 50-75 MPa without breaking during extrusion. 
The extruded product in its fibrous form suitably has a modulus of at least 
3 GPa, preferably greater than 5 GPa. 
The polyolefin composition may also contain small quantities of an 
antioxidant or other conventional additives, for example to prevent 
oxidation of the olefin (co)polymer during storage or use which may affect 
its molecular weight and hence the physical properties thereof. 
An olefin (co)polymer which inherently possess the desired haul off 
tension, weight average molecular weight, and the stated requirements of 
the high and low molecular weight tail components is suitably an unblended 
homo- or copolymer as obtained from a polymerization reactor. Such an 
unblended polyolefin can be prepared, for example, by a Ziegler or a 
Phillips process using appropriate catalysts and polymerization conditions 
known in the art. 
The olefin (co)polymer is preferably dried prior to extrusion. 
During the extrusion it is necessary to maintain the olefin (co)polymer in 
the die just above but not more than 7.degree. C. above its self-blocking 
temperature which is the temperature at which the melt of the olefin 
(co)polymer solidifies in the die and stops extruding. The extrusion 
temperature may for example be maintained at not more than 5.degree. C., 
for example not more than 2.degree. C., above the self-blocking 
temperature. The precise value of the self-blocking temperature will 
depend upon various extrusion conditions such as the extrusion rate, die 
geometry and the composition of the olefin (co)polymer. However, in all 
cases the self-blocking temperature is higher than the melting temperature 
of the olefin (co)polymer at normal (1 atmosphere) pressure due to the 
fact that increased pressure raises the melting temperature. The 
self-blocking temperature of a given melt can be determined by simple 
trial and error techniques known to those skilled in the art. The 
extrusion pressures used in the present invention can be relatively low 
e.g. 5-10 MPa in comparison with conventional solid state extrusion 
processes which use pressures of around 200 MPa. 
The extrudate, as it emerges from the die exit, is cooled to ambient 
temperature. In some cases it may be necessary to quench the extrudate as 
it leaves the die exit so as to crystallize the sample before the oriented 
structures relax. As a qualitative guide, the extrudate retains its 
transparency when the sample is crystallized. The extrudate may be cooled 
using liquid or gaseous coolant but it is necessary to ensure that the die 
itself is not cooled. 
The extrudate, if it is in the filament or fiber form, is preferably wound 
on a mandrel which may also be cooled. The speed of winding of the 
extrudate is so controlled that the extrudate is held taut and hauled off 
so as to avoid any die swell as the extrudate emerges from the die. 
Preferably the haul-off speed is such that the extrudate has a diameter 
which is substantially the same as or less than that of the die. 
The extrudate derived from the olefin (co)polymer by the present invention 
has higher modulus compared with extrudates derived from olefin polymers 
which do not have the defined average molecular weight and molecular 
weight distribution characteristics and which are extruded at temperatures 
very much above the self blocking temperature of the polymer with little 
haul-off stresses during wind up. The relevant data in this context are 
described below with reference to the Examples and Comparative tests.

EXAMPLES 
Materials used 
(a) A conventional polyethylene, Rigidex 006-60 (Regd. Trade Mark, 
polyethylene Mw=130,000 and a number average molecular weight, Mn=19,200) 
and (b) an unblended polyethylene (Mw=400,000, and a number average 
molecular weight, Mn=73,000) with a high molecular weight tail component 
of which 9% w/w had a molecular weight greater than 1,000,000 and which 
had 0% component of molecular weight less than 2000. 
EXAMPLE AND COMATIVE TESTS 
Continuous Extrusion of an Orientated Extrudate from (a) Rigidex 006-60 and 
(b) unblended Polyethylene with a High Molecular Weight Tail 
The preparation of extrudates from both Rigidex 006-60 alone and 
polyethylene with a high molecular weight tail is described below: 
The Method of Extrusion 
The polyolefin melt was extruded through the Instron capillary rheometer 
with a die, 0.85 mm in diameter and 10 mm long as represented in FIG. 1. 
The extrudate was passed over a pulley and then wound up on a motor driven 
drum. The extrudate was quenched by cold air from a pipette directed about 
20 mm from the die exit. The extrusion conditions are shown in Table 1. 
For comparison, conventional Rigidex 006-60 which does not have the stated 
high and low molecular weight tail characteristics was extruded at just 
above the self-blocking temperature (see Table 1) at a melt speed of 
2.times.10.sup.-3 m/s. There was maximum die swell just before 
self-blocking and chain extended fibrils were present; if the die was 
blocked with a needle valve shish kebabs were retained in the plug in the 
capillary. However, when an attempt was made to pull the extrudate in 
order to spool it on the wind-up system, it would not draw or sustain a 
tension, highlighting the difficulties of extruding conventional molecular 
weight distribution polymers near the self-blocking temperature 
(138.degree. C). When the temperature was raised to 150.degree. C. (Table 
1) and the melt extruded, there was less die swell and it was possible to 
pull the melt, spool it and wind it up. If the drum take up speed was 
increased above the melt exit velocity, the extrudate drew down to a 
diameter smaller than the die diameter. High haul-off stresses were not 
obtained during wind-up of the extrudate. The extrudate was white, had 
little orientation and had low modulus (Table 2). 
In contrast to the above, there was a marked difference in the extrusion 
behavior of the unblended polyethylene which had a high molecular weight 
tail in accordance with the present invention. Once again, there was 
maximum die swell just above self-blocking temperature and due to the 
increase in viscosity, the self-blocking temperature was much higher for 
the same extrusion rate compared to Rigidex 006-60 alone. The most 
important difference was that whereas Rigidex 006-60 melt could not be 
pulled without breaking if extruded just above the self-blocking 
temperature, the unblended polyethylene which had a high molecular weight 
tail sustained a tension of 50 MPa and could be pulled and wound up. As 
the drum take up speed was increased the die swell was lost and the 
extrudate thinned down to approximately the die diameter. At this stage 
the extrudate was translucent. Applying a stream of cold air to the taut 
extrudate as it wound up, through a pipette, about 20 mm below the die 
exit, caused the extrudate to become transparent. The extrudate so 
produced had a high degree of transparency and orientation. 
The unblended polyethylene (b) was extruded as follows: the extrusion was 
started at about 7-10.degree. C. above the self blocking temperature to 
avoid die blockage problems. An extrusion rate was chosen so that a smooth 
and uniform extrudate was obtained. The extrudate was wound up 
continuously. At this initial stage, the extrudate had little orientation, 
was opaque or translucent, and the haul off stress was low. The 
temperature of the extruder was then allowed to drop to the self blocking 
temperature, while extrusion and wind up was allowed to continue. As the 
critical temperature was reached, the extrudate became transparent and the 
haul off stress increased dramatically, attaining values as high as 50 MPa 
or more. If the temperature was allowed to drop any further, the extrudate 
broke (thus, the self blocking temperature may be regarded as that at 
which the highest sustainable haul off stress is obtained without the 
extrudate breaking). 
The electron micrograph of the extrudate produced from the unblended 
polyethylene which had a high molecular weight tail clearly indicated the 
presence of shish kebabs diagrammatically shown in FIG. 2. Again 
intermeshing of lamellae had occurred when the cores were sufficiently 
close and there were areas of disorientation where the cores were far 
apart. This structure is different from the morphology of "as spun" 
Rigidex 006-60 alone, where no shish-kebabs were present. 
The melting behavior of the extrudates from Rigidex 006-60 alone and the 
unblended polyethylene which had a high molecular weight tail were studied 
with a differential scanning calorimeter (DSC, IIB, Perkin and Elmer). The 
relevant traces showed that there is only one broad melting peak, similar 
to the one observed with pellets of spherulitic Rigidex 006-60. However, 
after extruding the unblended polyethylene which had a high molecular 
weight tail into an oriented extrudate, the melting behavior invariably 
showed two peaks (and in some cases three peaks) consisting of one main 
peak at ca 132.degree. C. formed from the melting of the lamellae and a 
subsidiary peak at a higher temperature (at ca 142.degree. C.) 
attributable to the melting of the cores. If the sample was cooled after 
heating to 180.degree. C. and remelted only one broad peak was obtained; 
clearly the shish-kebab morphology was largely lost during 
recrystallization after heating above 160.degree. C. 
Thus a two melting peak behavior with a separation of about 10.degree. C. 
between peaks appears to be associated with a shish-kebab morphology, as 
shown by the extrudates made continuously from the unblended polyethylene 
which had a high molecular weight tail. 
The mechanical properties of the extrudate shown in Table 2 also show that 
the unoriented Rigidex 006-60 extrudate has tensile modulus of 0.5-0.7 
GPa. The extrudates from unblended polyethylene which had a high molecular 
weight tail had moduli of up to 10 GPa. Thus, the extrudate from the 
unblended polyethylene is stiffer than an extrudate of ordinary Rigidex by 
a factor 10-20, but lower by a factor of 5 compared to the extensional 
moduli of the oriented plugs. 
TABLE 1 
__________________________________________________________________________ 
Extrusion Conditions 
Self blocking 
Extrusion 
Melt Extrusion 
Wind Up 
Polymer Temperature 
Temperature 
Speed Speed Comments 
__________________________________________________________________________ 
Rigidex* 006-60 
138.degree. C. 
139.degree. C. 
2 .times. 10.sup.-3 m/s 
-- Melt failure. 
" 138.degree. C. 
150.degree. C. 
" much Melt draws down; extrudate 
greater than 
diameter less than die 
2 .times. 10.sup.-3 m/s 
diameter. Extrudate has 
little orientation. 
Polyethylene (b) 
142.degree. C. 
147.degree. C. 
" about There is higher tension on 
with a high 2 .times. 10.sup.-3 m/s 
extrusion line. Very stable 
molecular weight extrusion possible. 
tail 
Polyethylene (b) 
142.degree. C. 
143.degree. C. 
" about There is higher tension on 
with a high 2 .times. 10.sup.-3 m/s 
extrusion line. Very stable 
molecular weight extrusion possible. 
tail 
__________________________________________________________________________ 
*Regd. Trade Mark. 
TABLE 2 
______________________________________ 
Extensional Moduli of Polyethylene Extrudates 
The sample gauge length was 100 mm and 
the cross-head speed (Instron) was 2 mm/min. 
Extensional 
Polymer modulus Comments 
______________________________________ 
Rigidex* 006-60 
0.7 GPa Extruded at 150.degree. C. 
See Table 1. 
Polyethylene (b) with 
4.5 GPa Extruded at 147.degree. C. See 
a high molecular Table 1. 
weight tail 
" 10 GPa Extruded at 143.degree. C. See 
Table 1. 
______________________________________ 
*Regd. Trade Mark.