Method to process narrow molecular weight distribution polyolefins

This invention relates to a method to process narrow molecular weight distribution polyolefins, particularly thermoplastics, polyethylenes, polypropylenes, and polyacrylates, whether they are homopolymers or copolymers. The process may be used in any extruder and is typically implemented along the extruder screw and/or barrel by heating the first zone the screw/barrel to place the polyolefin into a molten, flowable state. Thereafter, the polymer is cooled in the second heating zone to a point 50.degree. C. above or 10.degree. C. below the crystalline melting temperature of the polymer and thereafter is reheated in the final heating zone to a molten, flowable state and finally extruding it within 5 seconds or less through the die.

FIELD OF THE INVENTION 
This invention relates to the processing of polyolefin polymers and 
improvement of their processing capabilities by varying temperature 
profile along the barrel and die exit of an extruder. 
BACKGROUND OF THE INVENTION 
In the recent past there has been a new class of catalyst disclosed that 
produces new olefinic polymers and copolymers having many new and unique 
properties. In particular, these polymers offer strength properties based 
on their narrow molecular weight distribution. These polymers, however, 
are more difficult to process than their predecessors and thus methods of 
improving their processability are being sought. This invention relates to 
an improvement in the processing abilities of these and other narrow 
molecular weight distribution polymers. 
Semi-crystalline polyolefins, such as polyethylene and polypropylene, are 
processed well above their melting temperatures. In the case of 
polyethylene, the processing temperature is usually in the range of 
190.degree. to 260.degree. C. for extrusion and blow molding operations. 
For polypropylene in most of the cases, this temperature range is also 
suitable, although in case of specialty polypropylene it could be higher. 
The processing temperature and other parameters are obviously functions of 
the polymer grade and operation being sought. However, one thing is 
clearly emerging due to rapid advances in the processing and converting 
technology. It is desired that such materials be processed at the highest 
throughput rates possible with minimum energy consumption, that is, with 
the least possible head pressures during processing. 
In order to accomplish this in the industry today, processing operations 
and screw designs are altered routinely for processing both conventional 
and specialty polyethylenes. The successful processing of non-conventional 
polymers such as narrow molecular weight distribution polyethylenes 
remains a challenge to the industry. In fact, even conventional so called 
linear polyethylenes (LLDPE) having a weight average molecular weight in 
the range of 35,000 to 200,000 and a molecular weight distribution of Mw 
over Mn less than equal to 5 cannot be processed at very high speeds using 
conventional polyethylene extruders. The latter is often desired and 
demanded due to obvious economic reasons. At high speeds in polyethylenes, 
as well as other conventional polymers such as polypropylene and 
polystyrene, flow instabilities occur during all sorts of processing 
operations including fiber spinning, extrusion, coating, film blowing and 
molding. Above a critical speed or, in other words, above critical stress 
and strain rates, the melt flow instabilities yield the extruded product 
which is highly distorted. In the extreme case, it can be chaotic. At 
times, even at relatively low speeds, the distortions of the extrudate of 
some grades of polyethylene can be bad enough to be readily detectable 
with the naked eye. Distortions, if limited to the surface, are called 
sharkskin. As the name implies, sharkskin consists of regular grooves and 
cracks perpendicular to the flow direction of the extrudate. Increase in 
the speed results in the increase of the severity of distortion, first 
appearing as a more wavy fracture then grossly helical distortions 
followed by at extremely high speeds, gross melt fracture and destruction 
of the extrudate. Such behavior obviously limits the throughput rates of 
polymers. This subject has been under study for nearly 30 years and 
continues to be investigated by industry and academia. The subject, for 
example, has been discussed in various polymer rheology books and chapters 
authored or edited by Eirich, Walters, Han, Keller (independent authors) 
and chapters in Encyclopedia of Polymer Science and Technology. 
The phenomena of flow instability of high polymer melts at high shear rates 
is due to their inherent viscoelastic nature. The viscoelasticity of 
particular polymers is dictated by its molecular architecture and is the 
controlling factor in processing. Generally it is observed that polymers 
having broader molecular weight distributions, i.e., an Mw over Mn of 
greater than five can be processed at relatively high shear rates as 
compared to those having narrow molecular weight distributions. At any 
given shear rate and temperature, especially those employed in commercial 
processing operations, for high molecular weight polymers, the broad 
molecular weight distribution polymers have lower viscosities than 
corresponding narrow molecular weight distribution polymers. 
Further, it is also an experimental fact that the drop to lower viscosity 
from the initial equilibrium viscosity, the so-called zero shear 
viscosity, .eta..sub.0, occurs at lower shear rates for broad molecular 
weight distribution than at narrow molecular weight distribution. Due to 
these two factors, at a given processing temperature and pressure, broad 
molecular weight polymers can be processed at corresponding higher rates 
than the narrow molecular weight counterparts. Conventional wisdom 
suggests that the lower molecular weight chains and broad molecular weight 
distribution polymers help in reducing not only the number of 
entanglements per unit volume, but also lower their rate of formation as 
well. Intuitively, thus, longer chains will have a higher probability of 
entangling than compositions with shorter chains. It is conjuncture that 
for this reason narrow high molecular distributed polymers have problems 
in processing at conventional and higher processing speeds. 
SUMMARY OF THE INVENTION 
This invention relates to a process to improve the processability of olefin 
polymer comprising cooling and heating selected section of the extruder 
barrel just prior to the final extrusion of the polymer. This process also 
relates to polymers having increased melt strength and final strength 
properties.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
In a preferred embodiment this invention relates to methods for processing 
narrow molecular weight distribution polymers, preferably having a 
molecular weight distribution ("Mw/Mn") less than or equal to five, 
preferably less than or equal to four, even more preferably less than or 
equal to three. In particularly preferred embodiments, the Mw/Mn is 
between 1.5 and 5, preferably between 2 and 4, even more preferably 
between 2.5 and 3.5. It has been observed that the more narrow the 
molecular weight distribution the more effective this method is on 
processability. This method may be applied to essentially any polyolefin; 
however, homopolymers and copolymers of ethylene and/or propylene and 
acrylates are preferred polymers. Indeed, any thermoplastic polyolefin or 
thermoplastic elastomers can be modified by this method. 
In a preferred embodiment, this invention relates to a method for 
processing narrow molecular weight distribution polymer at high speeds 
comprising heating the polymer to a flowable, molten state, typically 
about 50.degree. to 130.degree. C. above the crystalline melting point as 
determined by DSC, first peak, then quickly reducing the temperature of 
the molten polymer to the temperature that is within 50.degree. C. above 
and 10.degree. C. below the crystalline melting point of the subject 
polymer, preferably within 30.degree. C. above to 10.degree. C. below, 
even more preferably within 10.degree. above or below the crystalline 
melting point and thereafter heating the polymer back up to a flowable, 
molten state and extruding the polymer through a die. The polymer is 
preferably extruded quickly after being cooled. Typically the polymer is 
extruded within 5 seconds of entering the cooling zone, even more 
preferably within 3 seconds of entering the cooling zone, even more 
preferably within 2 seconds of entering the cooling zone, even more 
preferably within 1 second of entering the cooling zone. (The cooling zone 
is that section of the extrusion zone, typically along the extruder 
barrel, where the polymer temperature is reduced. In a preferred 
embodiment, the heating and cooling processes may take place along the 
various screw extrusion zones or barrels. The cooling of the barrels is 
typically accomplished by wrapping cooling coils along the barrel. This 
method is effective in any commercial extruder and may be practiced on any 
variety of polyolefins, particularly, thermoplastic polyolefins, even more 
particularly, ethylene homopolymer and copolymers, propylene homopolymer 
and copolymers, and acrylate homopolymers and copolymers. By molten, 
flowable state it is meant a state where the viscosity of the polymer is 
reduced such that the polymer will move through the processing equipment 
at useful speeds. The useful speeds in a extruder generally correspond to 
shear rates in the range of 10 to 10.sup.4 sec.sup.-1. 
It has been noted that the particular point within the range of 10.degree. 
below the crystalline melting point to 50.degree. above the crystalline 
melting point (Tm) to which a certain polymer is taken to obtain improved 
processing properties may be dependent on the shear rate. It has also been 
noted that a more aggressive or faster shear rate enables one to reduce 
the temperature to below the crystalline melting temperature without 
causing the polymer to become substantially solid. 
In a preferred embodiment, the cooled polymer is reheated to a molten 
state, preferably to a point 50.degree. C.-80.degree. C. above the Tm. 
Higher temperatures are within the scope of this invention. In preferred 
embodiments, the polyolefin temperature is not raised more than 70.degree. 
to 80.degree. above the crystalline melting temperature before it exits 
the die. In a preferred embodiment, polyethylene is extruded along the 
length of a screw extruder at a temperature of about 200.degree. C. or 
slightly above it to about 230.degree. C. especially in the feed, first 
compression and first metering zones. At a point before the exit of the 
extrudate through the die, the polymer extrudate temperature is reduced to 
within about 50.degree. C. of the crystalline melting point and then 
reheated to a molten, flowable state, typically about 
70.degree.-80.degree. above the crystalline melting temperature just 
before it exits the die. If desired, depending upon the grade of polymer, 
the temperature of the die could be kept higher, up to about 200.degree. 
C. in order to facilitate the processing. 
While not wishing to be bound by any theory, the inventor believes that the 
reduction in temperature minimizes entanglements and in effect "stretches" 
the polymer chains and contributes to the increased processability of the 
polymer and simultaneously contributes to the increased clarity, the 
increased strength, and the increased melt properties. In a typical 
embodiment, the cooling is practically done by lowering the temperature of 
an extruder zone intermediate between the final, usually the second 
metering section, and the first melting or compression zone. 
FIG. 1 is an illustration of a typical screw extruder. "1" describes the 
typical feed zone which is typically 7D, where D is the diameter of the 
screw, "2" describes the first compression zone which is typically 3D, "3" 
is the first metering zone which is typically 7.4D; "4" is the vent zone 
which is typically 1D, "5" is the second compression zone which is 
typically 3D, and "6" is the second metering zone which is 4.1D usually. 
"7" is zone one which has the first temperature (T1), "8" is the second 
zone which contains the second temperature (T2), "9" is the third zone 
which comprises the third temperature (T3) and "10" is the fourth zone 
which comprises the fourth temperature (T4), "11" is the die exit, and 
"12" is the die pressure. In these zones, temperature is typically 
controlled via appropriate electric heaters placed along the barrel of the 
extruder. For sake of clarity, the diagram shows four temperature zones 
only, although more or less heaters, heating elements or cooling coils 
could be installed and employed along the length of the screw in order to 
properly heat and control the temperature of an extruder and the 
extrudate. Likewise, cooling elements such as water jackets may be 
installed along the barrel to provide the cooling to a point within 
50.degree. of the Tm. In a preferred embodiment, the polymers are extruded 
within less than one second after the cooling procedure, even more 
preferably 0.75 seconds, even more preferably less than 0.5 seconds, even 
more preferably within 0.25 seconds after the cooling procedure. In 
conventional polyethylene processes, the various temperature zones 
identified as "7", "8", "9" and "10" in FIG. 1 are kept at identical 
temperatures typically at about 210.degree..+-.30.degree. C. In the 
instant invention, however, zone three which is "9" in FIG. 1 is the zone 
at which the temperature of the polymer composition is reduced to 
50.degree. C. of the crystalline melting temperature. 
Simultaneously, in order to read pressure generated along various zones, 
pressure transducers are installed. In the present case the head pressure 
was measured at position 12. 
In a preferred embodiment, this invention relates to a method to process 
narrow molecular weight distribution polyolefins, particularly 
thermoplastics, polyethylenes, polypropylenes, and polyacrylates, whether 
they are homopolymers or copolymers. The process may be used in any 
extruder and is typically implemented along the extruder screw barrel by 
heating the first zones of a barrel to place the polyolefin into a molten, 
flowable state. Thereafter, the polymer is cooled to a point within 
50.degree. C. above or 10.degree. C. below the crystalline melting 
temperature of the polymer and thereafter is reheated to a molten, 
flowable state and extruded within one second or less after being cooled. 
The polymers that are produced after the instant cooling treatment have 
undergone changes which may be observed by standard DSC techniques. 
Changes in the crystalline melting temperature and the enthalpy of fusion 
are noted and, in particular, these changes lead to polymer products with 
unexpected properties. 
All references including testing procedures described above are 
incorporated by reference herein in their entirety. All molecular weights 
are weight average unless otherwise stated. All temperatures are in 
degrees Celsius unless otherwise stated. 
EXAMPLES 
Table I shows the processing conditions of a commercially-made narrow 
molecular weight distribution polyethylene produced by Exxon Chemical 
Company having about 10 weight % butene, a molecular weight distribution 
(Mw/Mn) of 2.4, a density of 0.904 g/cc, a crystalline melting point of 
103.5 (by DSC, 10.degree. C. per minute) and a melt index value of 4.4 at 
190.degree. C./2.13 kg Density is measured by ASTM D-792. Melt Index is 
measured by ASTM 1238 condition e. Example 1 is a polyethylene control 
example having a broad molecular weight distribution of greater than 2.5. 
TABLE 1 
__________________________________________________________________________ 
Screw/barrel 
Zone Temp.'s (.degree.C.) 
Die Pressure 
Surface Roughness 
Surface 
Example 
T1 T2 T3 T4 bar/MPa 
(.mu.m) Comments 
__________________________________________________________________________ 
1 (cont) 
220 
220 
212 
215 
182/18.2 
0.7 excellent 
2 (comp) 
220 
190 
173 
158 
225/22.5 
47.8 very bad 
3 (comp) 
190 
190 
186 
159 
241/24.1 
35.4 bad 
4 (comp) 
190 
170 
156 
153 
240/24.0 
34.4 poor 
5 (comp) 
220 
170 
159 
148 
240/24.0 
18.0 poor 
6 220 
160 
154 
219 
195/19.5 
4.4 very good 
7 220 
160 
152 
179 
225/22.5 
22.8 good 
8 (comp) 
220 
160 
160 
140 
240/24.0 
21.8 very good 
9 (comp) 
220 
160 
160 
145 
-- 13.0 -- 
10 (comp 
180 
220 
152 
141 
248/24.8 
14.2 good 
__________________________________________________________________________ 
(cont) = control; 
(comp) = comparison 
The polymers was passed through four zones along the extruder barrel. 
Temperatures T1 and T2 are the set temperatures. The actual temperatures 
are typically within .+-.5.degree. C. Temperatures T3 and T4 are actual 
temperature readings. The surface roughness is a measure of the average 
groove depth and was determined using a needle Profilometer following 
DIN-4768 ; and DIN-4769. The instrument used was a surface texture 
measuring instrument. It employs a stylus diamond 5 micron tip and is 
manufactured by Advanced Metrology Systems, LTD., Leicester, England. 
Lower numbers indicate smoother extrudates. The data in table 1 were 
gathered in a Brabender Plasti-corder PL-2000 with a 90.degree. round die 
L/D=15, D=1 mm. All Examples were extruded at nearly the same rate of 
throughput. The throughput range for the examples was 5-10 gm/min. 
Further, from the data appearing in Table 1, it can be readily noted that 
if the extrusion operation is carried out according to the instant 
invention, the materials that are extruded are not only of good quality 
but the processing is achieved at milder conditions. Particularly, it is 
observed that the head pressures are low. For example, the head pressure 
in Example 6 (195 bar/19.5 MPa) is similar to the head pressure in control 
Example 1 (182 bar/18.2 MPa), processed under conventional processing 
conditions. Head pressure of the extruder indicates the energy required 
for processing. Lower pressures mean lower energy consumption which 
translates to faster production and lower costs. It also enables one to 
operate the machine at faster speeds. 
As is apparent from the foregoing description, the materials prepared and 
the procedures followed related to specific embodiments of the broad 
invention. It is apparent from the foregoing general description and the 
specific embodiments that, while forms of the invention have been 
illustrated and described, various modifications can be made without 
departing from the spirit and scope of this invention. Accordingly, it is 
not intended that the invention be limited thereby.