|Title of Invention||
PROCESS FOR PREPARING GEO-MEMBRANE APPLICATIONS
|Abstract||The invention discloses a process for preparing geo-membrane applications by flat sheet extrusion or by blown sheet extrusion of a bimodal resin consisting of medium density polyethylene (MDPE) having a density of from 0.925 to 0.945 g/cm3 and wherein the MDPE resin is prepared by chemical blending with a single metallocene catalyst system in a double loop reactor wherein the loops are operated under comonomer/hydrogen split configuration wherein a high molecular weight and high comonomer concentration is produced in one reactor and a low molecular weight, low comonomer concentratrion is produced in the second reactor.|
|Full Text||GEO-MEMBRANE APPLICATIONS.
The present invention relates to the field of geo-membranes prepared with bimodal
medium density polyethylene resins.
Polyethylene resins prepared with Ziegler-Natta (ZN) catalyst systems are generally
used for preparing geo-membrane applications. These resins such as for example
Dowlex® 2342M or Stamylan® LL0132H200 have a reasonably high stress crack
resistance, tensile properties, impact toughness and good processability, but they
leave room for improvement.
Metallocene-prepared resins having a narrow monomodal polydispersity index have
also been tested in that field but they have proven to lack the required balance of
environmental stress crack and processing properties. In the field of geo-membrane
applications stress crack resistance of over 400 h is required simultaneously with
good processabiiity. In this description, the polydispersity index D is defined as the
ratio Mw/Mn of the weight average molecular weight Mw over the number average
molecular weight Mn.
There is thus a need to prepare resins that can be used to produce geo-membrane
It is an aim of the present invention to geo-membrane appiicatons that have high
environmental stress crack resistance.
It is also an aim of the present invention to produce geo-membrane appiicatons with
a resin that is easy to process by flat sheet extrusion or by blown sheet extrusion.
It is another aim of the present invention to provide geo-membrane appiicatons
having good tensile properties.
It is a further aim of the present invention to provide geo-membrane applicatons
having high impact toughness.
It is yet a further aim of the present invention to provide geo-membranes applications
that have good resistance to tear and puncture.
Accordingly, the present invention provides geo-membrane applications produced by
flat sheet extrusion or by blown sheet extrusion with a bimodal resin consisting of
medium density polyethylene (MDPE).
In this invention, flat sheet extrusion is preferred.
The bimodal HDPE resin can be prepared from a physical blend or from a chemical
blend. Preferably, it is prepared from a chemical blend. The chemical blend can
result for example from a single catalyst system used in a double loop reactor
wherein the loops are operated under different polymerisation conditions or from two
or more catalyst systems used in a single or in a double loop reactor.
When a double loop reactor is used, it can be operated under three modes:
- hydrogen split wherein different concentrations of hydrogen are used in the
different reactors in order to produce a low molecular weight fraction in a
reactor and wherein the polydispersity is broadened in the other reactor;
- comonomer split wherein different comonomer concentrations are used in the
different reactors in order to produce a low comonomer concentration in a
reactor and a high comonomer concentration in the other reactor;
- comonomer/hydrogen split wherein a high molecular weight and high
comonomer concentration is produced in one reactor and a low molecular
weight, low comonomer concentratrion is produced in the second reactor. In
the direct configuration, the high comonomer concentration is produced in the
first reactor and vice versa, in the inverse configuration, the low comonomer
concentration is produced in the first reactor.
The last mode in direct or inverse configuration is preferred in the present invention.
Preferably, the bimodal HDPE resin is prepared with a catalyst system based on a
bridged bisindenyl metallocene catalyst component. The catalyst component is of
general formula I
wherein (Ind) is an indenyl or an hydrogenated indenyl, substituted or unsubstituted,
R" is a structural bridge between the two indenyls to impart stereorigidity that
comprises a C1-C4 alkylene radical, a dialkyl germanium or silicon or siloxane, or a
alkyl phosphine or amine radical, which bridge is substituted or unsubstituted; Q is a
hydrocarbyl radical having from 1 to 20 carbon atoms or a halogen, and M is a
transition metal Group 4 of the Periodic Table or vanadium.
Each indenyl or hydrogenated indenyl compound may be substituted in the same
way or differently from one another at one or more positions in the cyclopentadienyl
ring or in the cyclohexenyl ring and the bridge.
Each substituent on the indenyl may be independently chosen from those of formula
XRV in which X is chosen from Group 14 of the Periodic Table, oxygen and nitrogen
and each R is the same or different and chosen from hydrogen or hydrocarbyl of
from 1 to 20 carbon atoms and v+1 is the valence of X. X is preferably C. If the
cyclopentadienyl ring is substituted, its substituent groups must not be so bulky as to
affect coordination of the olefin monomer to the metal M. Substituents on the
cyclopentadienyl ring preferably have R as hydrogen or CH3. More preferably, at
least one and most preferably both cyclopentadienyl rings are unsubstituted.
In a particularly preferred embodiment, both indenyls are unsubstituted, and most
preferably they are unsubstituted hydrogenated indenyls.
The most preferred metallocene catalyst component is isopropylidene-bis-
tetrahydroindenyl zirconium dichloride.
The active catalyst system used for polymerising ethylene comprises the above-
described catalyst component and a suitable activating agent having an ionising
Suitable activating agents are well known in the art.: they include aluminium alkyls
aluminoxane or boron-based compounds.
Optionally, the catalyst component can be supported on a support.
This metallocene catalyst system is preferably used in a liquid full double loop
reactor wherein the loops are operated under different conditions in order to produce
a bimodal resin. The double loop reactor can be operated either in direct
configuration wherein the copolymer is prepared in the first reactor or in inverse
configuration wherein the homopolymer is prepared in the first reactor.
The bimodal resins of the present invention have a polydispersity index that is
preferably larger than 3, more preferably from 3.1 to 3.5. The molecular weights are
determined by GPC-DRI. In solution, long-branched polymers assume a more
compact configuration than linear chains and their molecular weight can thus be
slightly underestimated. The density is preferably of from 0.925 to 0.945 g/cm3,
preferably of from 0.934 to 0.938 g/cm3 and the melt flow indices MI2 and HLMI are
respectively ranging from 0.1 to 2 dg/min, preferably of from 0.5 to 1 dg/min and of
from 5 to 30 dg/min.
The density is measured following the method of standard test ASTM 1505 at a
temperature of 23 °C. The melt flow indices MI2 and HLMI are measured following
the method of standard test ASTM D 1238 at a temperature of 190 °C and
respectively under a load of 2.16 kg and 21.6 kg.
The methods used to prepare geo-membranes are either flat sheet extrusion or
blown sheet extrusion. In both methods, the heart of the process is the extruder.
Pellets are fed into the extruder typically by a screw system, they are then heated,
placed under pressure and formed into a hot plastic mass before reaching the die.
Once the components are in the hot plastic state, they can be formed either into a
flat sheet by a dove tail die or into a cylindrical sheet that is subsequently cut and
folded out into a flat sheet.
In the flat sheet extrusion process, the hot plastic mass is fed into a dove tail die and
exits through a horizontal straight slit. Depending upon the width of the die, one or
more extruders may be needed to feed the hot plastic mass into the die. High quality
metal oilers placed in front of the slit are used to control the thickness and surface
quality of the sheets. These rollers must be able to sustain pressure and
temperature variations without deformation and they are connected to cooling
liquids. The rollers are designed in order to control the sheet thickness to less than
3% variation over the whole width. A third roller may be used to further cool the
sheet and to improve its surface finish. The surface finish of the sheet is directly
proportional to the quality of the rollers' surface. The evenly cooled finished material
is then fed over support rollers to be wrapped onto a core pipe and rolled up.
In the blown extrusion process the hot plastic mass is fed into a slowly rotating spiral
die to produce a cylindrical sheet. Cooled air is blown into the centre of the cylinder
creating a pressure sufficient to prevent its collapsing. The cylinder of sheeting is fed
up vertically: it is then closed by being flattened over a series of rollers. After the
cylinder is folded together, the sheet is cut and opened up to form a flat surface and
then rolled up. The annular slit through which the cylinder sheet is formed is
adjusted to control the sheet's thickness. Automatic thickness control is available in
modem plants. Cooling is performed by the cool air blown into the centre of the
cylinder and then during the rolling up process.
Coextrusion allows the combination of different materials into a single multi-layer
List of Figures.
Figure 1 represents the molecular weight distribution of the resins tested.
Figure 2 represents the complex viscosity expressed in Pas as a function of
frequency expressed in rad/s for several resins.
Several resins have been tested in the production of geo-membrane applications,
They were selected as follows.
Resins R1 and R2 are monomodal medium density polyethylene (MDPE) resins
prepared with isopropylidene-bis(tetrahydroindenyl) zirconium dichloride.
Resins R3 and R4 are bimodal MDPE resins prepared with isopropylidene-
bis(tetrahydroindenyl) zirconium dichloride (THI) in a double loop reactor in direct
configuration, i.e. wherein comonomer hexene is introduced into the first reactor and
hydrogen is introduced into the second reactor.
Resin R5 is a Ziegler-Natta HDPE sold by Dow under the name Dowlex® 2342 M.
Resin R6 is a Ziegler-Natta HDPE sold by DSM under the name Stamylan®
Their properties are summarised in Table I.
For the bimodal polyethylene resins according to the present invention, different
products were obtained in reactors Rx1 and Rx2 of the double loop system.
The curves representing the molecular weight distribution for all resins are
represented in Figure 1. As expected, the molecular weight distribution of all the
resins prepared with a Ziegler-Natta catalyst system are significantly broader than
those of all the metallocene-prepared resins. In addition, they include very long
chains that are characterised by a high molecular weight fraction above 106 daltons.
The metallocene-prepared resins, both monomodal and bimodal do not contain very
The molecular architecture of the resins has also been investigated and the amount
of short chain branching and long chain branching has been evaluated for each
The short chain branching (SCB) content was measured by NMR. The results for all
resins are displayed in Table III as well as the nature of the short branches.
The long chain branching content was determined by the long chain branching index
(LCBI). The method is described by SchrofF R.N. and Mavridis H. in Macromolecules,
32, 8454 (1999) and LCBI is given by empirical formula
wherein n0 is the zero shear viscosity expressed in Pa.s and [n] is the intrinsic
viscosity in solution expressed in g/mol. This method is more sensitive than the usual
Dow Rheological Index (DRI) or NMR methods and is independent of the
polydispersity. It was developed for substantially linear polyethylene such as typically
obtained in metallocene catalysis and it only requires the measurement of intrinsic
viscosity of a dilute polymer solution and the zero shear viscosity. It is equal to zero
for linear chains and deviates from zero when long chain branching (LCB) is present.
The intrinsic viscosity values were calculated from the Mark-Houwink relationship
that was developed for linear chains and it must be noted that this method only
applies to resins having a small content of long chain branching. The zero shear
viscosity was obtained by Carreau-Yasada fitting. The results are displayed in Table
II and they show that the resins prepared with Ziegler-Natta catalyst systems have
no long chain branching and that the bimodal metallocene-prepared resins have the
highest level of long chain branching.
It must be noted that for the bimodal resins according to the present invention that
are prepared in a double loop reactor, the fluff exiting the first reactor had a higher
content of long chain branching than the global product.
The complex viscosity curves as a function of angular frequency are presented in
Figure 2. The viscosity curves were analysed with a laboratory-scale rheometre at
the end of the extruder. The bimodal polyethylene resins of the present invention
were characterised by a higher zero-shear viscosity than the Ziegler-Natta
polyethylene (ZNPE) resins or the monomodal mPE because of the presence of
LCB. At high angular frequencies, the viscosity curves for all resins were very
similar, implying similar outputs at the end of the extruder as seen in Figure 2.
The processability of resin R3 according to the present invention was also tested in a
flat sheet extrusion process under two different processing conditions.
A. Preparation of sheets having a thickness of 1.5 mm with resins R1 to R3.
Temperature profile: 200/210/220/230/230 °C
Apparent shear rate of about 100 s-1
Rollers temperature: 60 °C
Stretching rate: 1.33
Targeted output rate: 95 kg/h
The real output rates Q are reported in Table III. It can be observed that output
target was reached for resins R1 and R3, but not for resin R2 characterised by a
higher viscosity than resins R1 and R3 as seen in Figure 2. The thickness control
and surface smoothness were excellent for resin r3 according to the present
B. Preparation of sheets having a thickness of 2.5 mm with resin R3.
Temperature profile: 230/230/230/230/230 °C
Apparent shear rate of about 100 s-1
Rollers temperature: 70 °C
Stretching rate: 1.12
Mechanical tests routinely performed in the field of geo-membrane applications were
carried out on compression moulded specimens.
Stress crack resistance was evaluated following the Single Point Notched Constant
Tensile Load (SPNCTL). The test uses a notched dumb-bell-shaped specimen to
determine the resistance of material to brittle fractures caused by long-term, low-
level tensile stress. The test following the method of standard test ASTM D 5397
requires that the specimens be placed in a surfactant solution, selected here as an
Ipegal 10 % solution, at a temperature of 50 °C, for an extended period of time, and
be subject to a tensile stress equal to 15 % of the material's yield stress. In the field
of geo-membrane applications, failure may not occur before at least 400 hours of
exposure. Results for the mean failure time are presented in Table IV.
Bimodal metallocene-prepared polyethylene resins produce a dramatic improvement
over the monomodal mPE. They are comparable to and often better than the
reference ZNPE generally used in the field. Increasing the molecular weight and the
content of small chain branching led to the observed increase in stress crack
resistance. Long molecules are more likely to have a high content of tie molecules
and a high efficiency in tie molecule entanglement and anchoring in the crystalline
lamellae, thereby increasing the stress crack resistance. Incorporation of
comonomer also contributes to increasing the content of tie molecule and tie
molecule entanglement. In addition, metallocene catalyst systems provide a very
homogeneous distribution of short chain branching on the longest molecules.
Tensile properties were evaluated following the method of standard test ASTM D
638 that gives information on the yield strength oy, the elongation at yield ϵy, the
break strength αB and the elongation at break ϵB. The results are presented in Table
V together with the Young modulus.
The tensile properties are at least maintained and improved in some cases for the
bimodal resins according to the present invnetion.
1. A process for preparing geo-membrane applications by flat sheet extrusion or by
blown sheet extrusion of a bimodal resin consisting of medium density polyethylene (MDPE)
having a density of from 0.925 to 0.945 g/cm3 and wherein the MDPE resin is prepared by
chemical blending with a single metallocene catalyst system in a double loop reactor wherein
the loops are operated under comonomer/hydrogen split configuration wherein a high
molecular weight and high comonomer concentration is produced in one reactor and a low
molecular weight, low comonomer concentration is produced in the second reactor.
2. The process as claimed in claim 1 wherein the metallocene catalyst system is based on
a bridged bis-indenyl catalyst component.
3. The process as claimed in claim 2 wherein the bridged bis-indenyl catalyst component
is a bis-tetrahydro-indenyl catalyst component.
PROCESS FOR PREPARING GEO-MEMBRANE APPLICATIONS
The invention discloses a process for preparing geo-membrane applications by flat sheet
extrusion or by blown sheet extrusion of a bimodal resin consisting of medium density
polyethylene (MDPE) having a density of from 0.925 to 0.945 g/cm3 and wherein the MDPE
resin is prepared by chemical blending with a single metallocene catalyst system in a double
loop reactor wherein the loops are operated under comonomer/hydrogen split configuration
wherein a high molecular weight and high comonomer concentration is produced in one
reactor and a low molecular weight, low comonomer concentratrion is produced in the second
|Indian Patent Application Number||2162/KOLNP/2007|
|PG Journal Number||29/2012|
|Date of Filing||13-Jun-2007|
|Name of Patentee||TOTAL PETROCHEMICALS RESEARCH FELUY|
|Applicant Address||ZONE INDUSTRIELLE C, B-7181 SENEFFE (FELUY)|
|PCT International Classification Number||C08J 5/18|
|PCT International Application Number||PCT/EP2005/057022|
|PCT International Filing date||2005-12-21|