A sheet such as a geomembrane consisting essentially of an extruded in situ blend of two copolymers of ethylene and one or more alpha-olefins having 3 to 8 carbon atoms, said blend having a flow index in the range of about 3 to about 100 grams per 10 minutes; a melt flow ratio in the range of about 50 to about 200; a density in the range of 0.905 to 0.943 gram per cubic centimeter; an Mw/Mn ratio in the range of about 10 to about 50; and a weight average molecular weight in the range of about 180,000 to about 465,000.

TECHNICAL FIELD 
This invention relates to geomembranes and other forms of sheet extruded 
from a blend of ethylene copolymers prepared in a series of polymerization 
reactors. 
BACKGROUND INFORMATION 
There has been a rapid growth in the market for linear low density 
polyethylene (LLDPE), particularly resin made under mild operating 
conditions; typically at pressures of 100 to 300 psi and reaction 
temperatures of less than 100.degree. C. This low pressure process 
provides a broad range of LLDPE products for blown and cast film, 
injection molding, rotational molding, blow molding, pipe, tubing, and 
wire and cable applications. LLDPE has essentially a linear backbone with 
only short chain branches, about 2 to 6 carbon atoms in length. In LLDPE, 
the length and frequency of branching, and, consequently, the density, is 
controlled by the type and amount of comonomer used in the polymerization. 
Although the majority of the LLDPE resins on the market today have a 
narrow molecular weight distribution, LLDPE resins with a broad molecular 
weight distribution are available for a number of non-film applications. 
LLDPE resins designed for commodity type applications typically incorporate 
1-butene as the comonomer. The use of a higher molecular weight 
alpha-olefin comonomer produces resins with significant strength 
advantages relative to those of ethylene/1-butene copolymers. The 
predominant higher alpha-olefin comonomers in commercial use are 1-hexene, 
4-methyl-1-pentene, and 1-octene. The bulk of the LLDPE is used in film 
products where the excellent physical properties and drawdown 
characteristics of LLDPE film makes this film well suited for a broad 
spectrum of applications. Fabrication of LLDPE film is generally effected 
by the blown film and slot casting processes. The resulting film is 
characterized by excellent tensile strength, high ultimate elongation, 
good impact strength, and excellent puncture resistance. 
These properties together with toughness are enhanced when the polyethylene 
is of high molecular weight. However, as the molecular weight of the 
polyethylene increases, the processability of the resin usually decreases. 
By providing a blend of polymers, the properties characteristic of high 
molecular weight resins can be retained and processability, particularly 
the extrudability (from the lower molecular weight component) can be 
improved. 
The blending of these polymers is successfully achieved in a staged reactor 
process such as those described in U.S. Pat. Nos. 5,047,468 and 5,126,398. 
Briefly, the process is one for the in situ blending of polymers wherein a 
higher density ethylene copolymer is prepared in a high melt index reactor 
and a lower density ethylene copolymer is prepared in a low melt index 
reactor. The process typically comprises continuously contacting, under 
polymerization conditions, a mixture of ethylene and one or more 
alpha-olefins with a catalyst system in two reactors connected in series, 
said catalyst system comprising: (i) a supported titanium based catalyst 
precursor; (ii) an aluminum containing activator compound; and (iii) a 
hydrocarbyl aluminum cocatalyst, the polymerization conditions being such 
that an ethylene copolymer having a melt index in the range of about 0.1 
to about 1000 grams per 10 minutes is formed in the high melt index 
reactor and an ethylene copolymer having a melt index in the range of 
about 0.001 to about 1 gram per 10 minutes is formed in the low melt index 
reactor, each copolymer having a density of about 0.860 to about 0.965 
gram per cubic centimeter and a melt flow ratio in the range of about 22 
to about 70, with the proviso that the mixture of ethylene copolymer 
matrix and active catalyst formed in the first reactor in the series is 
transferred to the second reactor in the series. 
While the in situ blends prepared as above and the films produced therefrom 
are found to have the advantageous characteristics heretofore mentioned, 
industry continues to seek films of various thicknesses with 
characteristics tailored to particular applications. One such application 
is that of sheet of which an important example is the geomembrane. Sheet 
is characterized in this specification as having a thickness greater than 
10 mils as opposed to film, which is characterized by a thickness of 10 
mils or less. Many of the characteristics of sheet are similar to film, 
but its applications fall into those areas where film would be too fragile 
such as geomembranes, truck bed liners, radiator shrouds, wheel well 
liners, and sandbox liners. Sheet can be produced in various forms 
including those having smooth, embossed, or textured surfaces. It is also 
used in multiple layers when the nature of the application demands an even 
heavier product. 
Geomembranes, which is the application of most interest here, are made from 
continuous polymeric sheets that are impermeable and quite flexible. Their 
primary function is as a barrier to liquids and gases. 
Various operations during installation of a geomembrane can result in 
scratching or scoring, altering the geomembrane's ability to stretch or 
conform. Further, in applications where subsidence is probable, such as 
landfill caps or liners, the geomembrane's ability to multiaxially 
elongate and relieve stress is important. Conformability to surfaces can 
add to slip resistance, even with textured geomembranes, improving soil 
retention, maximum slope angles, and pullout resistance. Resistance to 
puncture by substrates can be enhanced when conformance leads to 
relaxation of the geomembrane and stress removal. To be commercially 
acceptable, then, geomembranes should have a high level of three 
dimensional extensibility; be easy to process; have relatively high 
ultraviolet (UV) resistance; and have good mechanical properties and 
chemical resistance. To meet these prerequisites, the selection of the 
particular polyethylene resin becomes paramount. 
In addition to the LLDPE mentioned above, two other polyethylenes are of 
interest here. They are VLDPE (very low density polyethylene having a 
density of less than 0.915 gram per cubic centimeter) and MDPE (medium 
density polyethylene having a density in the range of 0.926 to 0.940 gram 
per cubic centimeter). LLDPE has a density in the range of 0.915 to 0.925 
gram per cubic centimeter. Note that VLDPE and MDPE are also linear. It is 
found that unimodal VLDPEs made with a vanadium based catalyst, when 
converted to geomembranes, meet the above prerequisites except that they 
exhibit low UV resistance; unimodal VLDPEs made with titanium based 
catalysts, when converted, are substantially higher in UV resistance than 
the vanadium based VLDPEs, but are difficult to process; and unimodal 
LLDPEs and MDPEs, when converted, have relatively poor three dimensional 
extensibility. Thus, it would be desirable to provide geomembranes based 
on polyethylenes, which, after conversion, handily meet all of the above 
prerequisites. It is apparent that these properties would also be 
advantageous in sheet destined for other applications. 
DISCLOSURE OF THE INVENTION 
An object of this invention, therefore, is to provide sheet, particularly 
geomembranes, which exhibits the aforementioned exemplary qualities. Other 
objects and advantages will become apparent hereinafter. 
According to the present invention such sheet has been discovered, the 
sheet being extruded from an in situ blend produced by a variation of the 
process outlined above. The sheet has a gauge of greater than about 10 
mils and consists essentially of an extruded in situ blend of two 
copolymers of ethylene and one or more alpha-olefins having 3 to 8 carbon 
atoms, said blend having a flow index in the range of about 3 to about 100 
grams per 10 minutes; a melt flow ratio in the range of about 50 to about 
200; a density in the range of 0.905 to 0.943 gram per cubic centimeter; 
an Mw/Mn ratio in the range of about 10 to about 50; and a weight average 
molecular weight in the range of about 180,000 to about 465,000. 
DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
The gauge or thickness of the sheet is greater than about 10 mils. The 
geomembrane, however, is preferably in the range of about 20 to about 200 
mils, and is more preferably in the range of about 40 to about 100 mils. 
The optimum gauge for geomembranes is about 60 mils. The sheet can be 
formed by extrusion or two or more sheets can be formed by co-extrusion or 
laminated together by other means. 
The extruder is a conventional one using a die, which will provide the 
desired gauge. Examples of various extruders, which can be used in forming 
the sheet are the single screw type modified with a round die or a flat 
die with continuous take off equipment. A typical single screw type 
extruder can be described as one having a hopper at its upstream end and a 
die at its downstream end. The hopper feeds into a barrel, which contains 
a screw. At the downstream end, between the end of the screw and the die, 
is a screen pack and a breaker plate. The screw portion of the extruder is 
considered to be divided up into three sections, the feed section, the 
compression section, and the metering section, and multiple heating zones 
from the rear heating zone to the front heating zone, the multiple 
sections and zones running from upstream to downstream. If it has more 
than one barrel, the barrels are connected in series. The length to 
diameter ratio of each barrel is in the range of about 16:1 to about 30:1. 
The extrusion can take place at temperatures in the range of about 175 to 
about 280 degrees C., and is preferably carried out at temperatures in the 
range of about 190 to about 250 degrees C. 
The blend, which is used in the extruder, is produced in two staged 
reactors connected in series wherein a mixture of resin and catalyst 
precursor is transferred from the first reactor to the second reactor in 
which another copolymer is prepared and blends in situ with the copolymer 
from the first reactor. 
The copolymers produced in each of the reactors are copolymers of ethylene 
and at least one alpha-olefin comonomer. The relatively high molecular 
weight copolymer is produced in what is referred to as the high molecular 
weight reactor, and the relatively low molecular weight copolymer is 
produced in what is referred to as the low molecular weight reactor. The 
alpha-olefin comonomer(s), which can be present in the high molecular 
weight reactor, can have 3 to 8 carbon atoms. The alpha-olefin 
comonomer(s), which can be present in the low molecular weight reactor, 
can also have 3 to 8 carbon atoms. The alpha-olefins are exemplified by 
propylene, 1-butene, 1-hexene, 4-methyl-1-pentene, or 1-octene. Any of the 
aforementioned comonomers can be used in either reactor. 
Preferred comonomer combinations are as follows: 
______________________________________ 
high mol wt reactor 
low mol wt reactor 
______________________________________ 
1-hexene 1-butene 
1-butene 1-butene 
1-butene 1-hexene 
1-hexene 1-hexene 
______________________________________ 
It will be understood that generally the in situ blend can be characterized 
as a bimodal resin. In some cases, however, the two components making up 
the blend are sufficiently close in average molecular weight that there is 
no discernible discontinuity in the molecular weight curve. The properties 
of bimodal resins are strongly dependent on the proportion of the high 
molecular weight component, i.e., the low melt index component. For a 
staged reactor system, the proportion of the high molecular weight 
component is controlled via the relative production rate in each reactor. 
The relative production rate in each reactor can, in turn, be controlled 
by a computer application program, which monitors the production rate in 
the reactors (measured by heat balance) and then manipulates the ethylene 
partial pressure in each reactor and catalyst feed rate in order to meet 
the production rate, the production rate split, and catalyst productivity 
requirements. 
The in situ blending can be achieved by the processes described in U.S. 
Pat. Nos. 5,047,468 and 5,126,398. A typical catalyst system used in in 
situ blending is a magnesium/titanium based catalyst system, which can be 
exemplified by the catalyst system described in U.S. Pat. No. 4,302,565 
although the precursor is preferably unsupported. Another preferred 
catalyst system is one where the precursor is formed by spray drying and 
used in slurry form. Such a catalyst precursor, for example, contains 
titanium, magnesium, and aluminum halides, and an electron donor, and is 
attached to the surface of silica. The precursor is then introduced into a 
hydrocarbon medium such as mineral oil to provide the slurry form. See 
U.S. Pat. No. 5,290,745. 
The electron donor, if used in the catalyst precursor, is an organic Lewis 
base, liquid at temperatures in the range of about 0.degree. C. to about 
200.degree. C., in which the magnesium and titanium compounds are soluble. 
The electron donor can be an alkyl ester of an aliphatic or aromatic 
carboxylic acid, an aliphatic ketone, an aliphatic amine, an aliphatic 
alcohol, an alkyl or cycloalkyl ether, or mixtures thereof, each electron 
donor having 2 to 20 carbon atoms. Among these electron donors, the 
preferred are alkyl and cycloalkyl ethers having 2 to 20 carbon atoms; 
dialkyl, diaryl, and alkylaryl ketones having 3 to 20 carbon atoms; and 
alkyl, alkoxy, and alkylalkoxy esters of alkyl and aryl carboxylic acids 
having 2 to 20 carbon atoms. The most preferred electron donor is 
tetrahydrofuran. Other examples of suitable electron donors are methyl 
formate, ethyl acetate, butyl acetate, ethyl ether, dioxane, di-n-propyl 
ether, dibutyl ether, ethyl formate, methyl acetate, ethyl anisate, 
ethylene carbonate, tetrahydropyran, and ethyl propionate. 
While an excess of electron donor is used initially to provide the reaction 
product of titanium compound and electron donor, the reaction product 
finally contains about 1 to about 20 moles of electron donor per mole of 
titanium compound and preferably about 1 to about 10 moles of electron 
donor per mole of titanium compound. 
An activator compound, which is generally used with any of the titanium 
based catalyst precursors, can have the formula AlR.sub.a X.sub.b H.sub.c 
wherein each X is independently chlorine, bromine, iodine, or OR'; each R 
and R' is independently a saturated aliphatic hydrocarbon radical having 1 
to 14 carbon atoms; b is 0 to 1.5; c is 0 or 1; and a+b+c=3. Preferred 
activators include alkylaluminum mono- and dichlorides wherein each alkyl 
radical has 1 to 6 carbon atoms and the trialkylaluminums. A particularly 
preferred activator is a mixture of diethylaluminum chloride and 
tri-n-hexylaluminum. About 0.10 to about 10 moles, and preferably about 
0.15 to about 2.5 moles, of activator are used per mole of electron donor. 
The molar ratio of activator to titanium is in the range of about 1:1 to 
about 10:1 and is preferably in the range of about 2:1 to about 5:1. 
The hydrocarbyl aluminum cocatalyst can be represented by the formula 
R.sub.3 Al or R.sub.2 AlX wherein each R is independently alkyl, 
cycloalkyl, aryl, or hydrogen; at least one R is hydrocarbyl; and two or 
three R radicals can be joined to form a heterocyclic structure. Each R, 
which is a hydrocarbyl radical, can have 1 to 20 carbon atoms, and 
preferably has 1 to 10 carbon atoms. X is a halogen, preferably chlorine, 
bromine, or iodine. Examples of hydrocarbyl aluminum compounds are as 
follows: triisobutylaluminum, tri-n-hexylaluminum, di-isobutyl-aluminum 
hydride, dihexylaluminum hydride, di-isobutyl-hexylaluminum, isobutyl 
dihexylaluminum, trimethyl-aluminum, triethylaluminum, tripropylaluminum, 
triisopropylaluminum, tri-n-butylaluminum, trioctylaluminum, 
tridecylaluminum, tridodecylaluminum, tribenzylaluminum, 
triphenylaluminum, trinaphthylaluminum, tritolylaluminum, dibutylaluminum 
chloride, diethylaluminum chloride, and ethylaluminum sesquichloride. The 
cocatalyst compounds can also serve as activators and modifiers. 
As noted above, it is preferred not to use a support. However, in those 
cases where it is desired to support the precursor, silica is the 
preferred support. Other suitable supports are inorganic oxides such as 
aluminum phosphate, alumina, silica/alumina mixtures, silica modified with 
an organoaluminum compound such as triethylaluminum, and silica modified 
with diethyl zinc. A typical support is a solid, particulate, porous 
material essentially inert to the polymerization. It is used as a dry 
powder having an average particle size of about 10 to about 250 microns 
and preferably about 30 to about 100 microns; a surface area of at least 
200 square meters per gram and preferably at least about 250 square meters 
per gram; and a pore size of at least about 100 angstroms and preferably 
at least about 200 angstroms. Generally, the amount of support used is 
that which will provide about 0.1 to about 1.0 millimole of titanium per 
gram of support and preferably about 0.4 to about 0.9 millimole of 
titanium per gram of support. Impregnation of the above mentioned catalyst 
precursor into a silica support can be accomplished by mixing the 
precursor and silica gel in the electron donor solvent or other solvent 
followed by solvent removal under reduced pressure. When a support is not 
desired, the catalyst precursor can be used in liquid form. 
Activators can be added to the precursor either before and/or during 
polymerization. In one procedure, the precursor is fully activated before 
polymerization. In another procedure, the precursor is partially activated 
before polymerization, and activation is completed in the reactor. Where a 
modifier is used instead of an activator, the modifiers are usually 
dissolved in an organic solvent such as isopentane and, where a support is 
used, impregnated into the support following impregnation of the titanium 
compound or complex, after which the supported catalyst precursor is 
dried. Otherwise, the modifier solution is added by itself directly to the 
reactor. Modifiers are similar in chemical structure and function to the 
activators. For variations, see, for example, U.S. Pat. No. 5,106,926. The 
cocatalyst is preferably added separately neat or as a solution in an 
inert solvent, such as isopentane, to the polymerization reactor at the 
same time as the flow of ethylene is initiated. 
U.S. Pat. No. 5,106,926 provides another example of a magnesium/titanium 
based catalyst system comprising: 
(a) a catalyst precursor having the formula Mg.sub.d Ti(OR).sub.e X.sub.f 
(ED).sub.g wherein R is an aliphatic or aromatic hydrocarbon radical 
having 1 to 14 carbon atoms or COR' wherein R' is a aliphatic or aromatic 
hydrocarbon radical having 1 to 14 carbon atoms; each OR group is the same 
or different; X is independently chlorine, bromine or iodine; ED is an 
electron donor; d is 0.5 to 56; e is 0, 1, or 2; f is 2 to 116; and g is 
1.5d+2; 
(b) at least one modifier having the formula BX.sub.3 or AlR.sub.(3-e) 
X.sub.e wherein each R is alkyl or aryl and is the same or different, and 
X and e are as defined above for component (a) 
wherein components (a) and (b) are impregnated into an inorganic support; 
and 
(c) a hydrocarbyl aluminum cocatalyst. 
The precursor is prepared from a titanium compound, a magnesium compound, 
and an electron donor. Titanium compounds, which are useful in preparing 
these precursors, have the formula Ti(OR).sub.e X.sub.h wherein R, X, and 
e are as defined above for component (a); h is an integer from 1 to 4; and 
e+h is 3 or 4. Examples of titanium compounds are TiCl.sub.3, TiCl.sub.4, 
Ti(OC.sub.2 H.sub.5).sub.2 Br.sub.2, Ti(OC.sub.6 H.sub.5)Cl.sub.3, 
Ti(OCOCH.sub.3)Cl.sub.3, and Ti(OCOC.sub.6 H.sub.5)Cl.sub.3. The magnesium 
compounds include magnesium halides such as MgCl.sub.2, MgBr.sub.2, and 
MgI.sub.2. Anhydrous MgCl.sub.2 is a preferred compound. About 0.5 to 56, 
and preferably about 1 to 10, moles of the magnesium compounds are used 
per mole of titanium compounds. 
The electron donor, the support, and the cocatalyst are the same as those 
described above. As noted, the modifier can be similar in chemical 
structure to the aluminum containing activators. The modifier has the 
formula BX.sub.3 or AlR.sub.(3-e) X.sub.e wherein each R is independently 
alkyl having 1 to 14 carbon atoms; each X is independently chlorine, 
bromine, or iodine; and e is 1 or 2. One or more modifiers can be used. 
Preferred modifiers include alkylaluminum mono- and dichlorides wherein 
each alkyl radical has 1 to 6 carbon atoms; boron trichloride; and the 
trialkylaluminums. About 0.1 to about 10 moles, and preferably about 0.2 
to about 2.5 moles, of modifier can be used per mole of electron donor. 
The molar ratio of modifier to titanium can be in the range of about 1:1 
to about 10:1 and is preferably in the range of about 2:1 to about 5:1. 
The entire catalyst system, which includes the precursor or activated 
precursor and the cocatalyst, is added to the first reactor. The catalyst 
is admixed with the copolymer produced in the first reactor, and the 
mixture is transferred to the second reactor. Insofar as the catalyst is 
concerned, only cocatalyst is added to the second reactor from an outside 
source. 
The polymerization in each reactor is, preferably, conducted in the gas 
phase using a continuous fluidized process. A typical fluidized bed 
reactor is described in U.S. Pat. No. 4,482,687. 
A relatively low melt index (or high molecular weight) copolymer is 
preferably prepared in the first reactor, and the relatively high melt 
index (or low molecular weight) copolymer is prepared in the second 
reactor. This can be referred to as the forward mode. Alternatively, the 
relatively low molecular weight copolymer can be prepared in the first 
reactor and the relatively high molecular weight copolymer can be prepared 
in the second reactor. This can be referred to as the reverse mode. 
The first reactor is generally smaller in size than the second reactor 
because only a portion of the final product is made in the first reactor. 
The mixture of polymer and an active catalyst is usually transferred from 
the first reactor to the second reactor via an interconnecting device 
using nitrogen or second reactor recycle gas as a transfer medium. 
In the high molecular weight reactor: 
Because of the low values, instead of melt index, flow index is determined 
and those values are used in this specification. The flow index can be in 
the range of about 0.01 to about 40 grams per 10 minutes, and is 
preferably in the range of about 0.2 to about 1 gram per 10 minutes. The 
weight average molecular weight of this polymer is, generally, in the 
range of about 400,000 to about 500,000. The density of the copolymer is 
at least 0.860 gram per cubic centimeter, and is preferably in the range 
of 0.900 to 0.930 gram per cubic centimeter. The melt flow ratio of the 
polymer can be in the range of about 20 to about 70, and is preferably 
about 22 to about 45. 
Melt index is determined under ASTM D-1238, Condition E. It is measured at 
190.degree. C. and 2.16 kilograms and reported as grams per 10 minutes. 
Flow index is determined under ASTM D-1238, Condition F. It is measured at 
190.degree. C. and 10 times the weight used in determining the melt index, 
and reported as grams per 10 minutes. Melt flow ratio is the ratio of flow 
index to melt index. 
In the low molecular weight reactor: 
A relatively high melt index (or low molecular weight) copolymer is 
prepared in this reactor. The high melt index can be in the range of about 
50 to about 3000 grams per 10 minutes, and is preferably in the range of 
about 100 to about 1500 grams per 10 minutes. The weight average molecular 
weight of the high melt index copolymer is, generally, in the range of 
about 10,000 to about 30,000. The density of the copolymer prepared in 
this reactor can be at least 0.900 gram per cubic centimeter, and is 
preferably in the range of 0.925 to 0.950 gram per cubic centimeter. The 
melt flow ratio of this copolymer can be in the range of about 20 to about 
70, and is preferably about 20 to about 45. 
The blend or final product, as removed from the second reactor, can have a 
flow index in the range of about 3 to about 100 grams per 10 minutes, and 
preferably has a flow index in the range of about 5 to about 90 grams per 
10 minutes. The melt flow ratio can be in the range of about 50 to about 
200. The weight average molecular weight of the final product is, 
generally, in the range of about 180,000 to about 465,000. The density of 
the blend can be at least 0.905 gram per cubic centimeter, and is 
preferably in the range of 0.910 to 0.943 gram per cubic centimeter. 
As noted above, the blend has a broad molecular weight distribution which 
can be generally characterized as bimodal. The broad molecular weight 
distribution is reflected in an Mw/Mn ratio of about 10 to about 50, 
preferably about 20 to about 40. Mw is the weight average molecular 
weight; Mn is the number average molecular weight; and the Mw/Mn ratio can 
be referred to as the polydispersity index, which is a measure of the 
breadth of the molecular weight distribution. 
The weight ratio of copolymer prepared in the high molecular weight reactor 
to copolymer prepared in the low molecular weight reactor can be in the 
range of about 0.5:1 to about 2:1. The weight ratio is preferably in the 
range of about 1:1 to about 1.6:1, and the optimum weight ratio is about 
1.4:1. 
The magnesium/titanium based catalyst system, ethylene, alpha-olefin(s), 
and hydrogen are continuously fed into the first reactor; the 
polymer/catalyst mixture is continuously transferred from the first 
reactor to the second reactor; ethylene, alpha-olefin(s), and hydrogen, as 
well as cocatalyst are continuously fed to the second reactor. The final 
product is continuously removed from the second reactor. 
In the high molecular weight reactor: 
The mole ratio of alpha-olefin to ethylene can be in the range of about 
0.05:1 to about 0.4:1, and is preferably in the range of about 0.1:1 to 
about 0.25:1. The mole ratio of hydrogen (if used) to ethylene can be in 
the range of about 0.0001:1 to about 0.3:1, and is preferably in the range 
of about 0.0005:1 to about 0.15:1. The operating temperature is generally 
in the range of about 60.degree. C. to about 100.degree. C. Preferred 
operating temperatures vary depending on the density desired, i.e., lower 
temperatures for lower densities and higher temperatures for higher 
densities. 
In the low molecular weight reactor: 
The mole ratio of alpha-olefin to ethylene can be in the range of about 
0.05:1 to about 0.6:1, and is preferably in the range of about 0.2:1 to 
about 0.5:1. The mole ratio of hydrogen to ethylene can be in the range of 
about 1:1 to about 2.5:1, and is preferably in the range of about 1.2:1 to 
about 2.2:1. The operating temperature is generally in the range of about 
70.degree. C. to about 100.degree. C. As mentioned above, the temperature 
is preferably varied with the desired density. 
The pressure is generally the same in both the first and second reactors. 
The pressure can be in the range of about 200 to about 450 psi and is 
preferably in the range of about 280 to about 350 psig. 
A typical fluidized bed reactor can be described as follows: 
The bed is usually made up of the same granular resin that is to be 
produced in the reactor. Thus, during the course of the polymerization, 
the bed comprises formed polymer particles, growing polymer particles, and 
catalyst particles fluidized by polymerization and modifying gaseous 
components introduced at a flow rate or velocity sufficient to cause the 
particles to separate and act as a fluid. The fluidizing gas is made up of 
the initial feed, make-up feed, and cycle (recycle) gas, i.e., comonomers 
and, if desired, modifiers and/or an inert carrier gas. 
The essential parts of the reactor system are the vessel, the bed, the gas 
distribution plate, inlet and outlet piping, a compressor, cycle gas 
cooler, and a product discharge system. In the vessel, above the bed, 
there is a velocity reduction zone, and, in the bed, a reaction zone. Both 
are above the gas distribution plate. 
Conventional additives, which can be introduced into the blend, are 
exemplified by antioxidants, ultraviolet absorbers, antistatic agents, 
pigments, dyes, nucleating agents, fillers, slip agents, fire retardants, 
plasticizers, processing aids, lubricants, stabilizers, smoke inhibitors, 
viscosity control agents, and crosslinking agents, catalysts, and 
boosters, tackifiers, and anti-blocking agents. Except for carbon black 
discussed below, the additives are generally present in the blend in 
amounts of about 0.01 to about 10 parts by weight of additive for each 100 
parts by weight of polymer blend. 
Advantages of the invention insofar as it applies to geomembranes lie in a 
geomembrane, which will respond to subsidence without breaking or being 
punctured, and will conform to the surface to which it is applied. This is 
a result of its three dimensional extensibility. The total level of three 
dimensional extensibility of polyethylenes is a strong function of 
density. Thus, VLDPEs exhibit high levels of three dimensional 
extensibility while MDPEs and LLDPEs normally exhibit much lower levels of 
three dimensional extensibility. Subject invention, on the other hand, has 
all of the advantages of an LLDPE with the additional advantage of 
exhibiting a high level of three dimensional extensibility similar to 
VLDPE. The geomembrane of the invention further demonstrates relatively 
high UV resistance, mechanical properties, and chemical resistance, and 
can be processed with facility. Other forms of sheet have similar 
advantages. 
It should be noted that The Society of the Plastics Industry differentiates 
sheet from film using 12 mils as the boundary. As noted above, in this 
specification, the borderline is 10 mils. 
Almost all geomembrane sheets are black, and contain 1 to 45 percent by 
weight carbon black. About 2 to about 3 percent by weight carbon black is 
typically needed to protect the sheet against degradation by ultraviolet 
light, and about 5 to about 45 percent by weight is needed to make the 
sheet electrically conductive. Typical carbon blacks used for UV 
protection are N-550 and N-110 carbon blacks. N-110 is finer than N-550 
and, therefore, is more efficient in this regard. Carbon blacks are also 
used to make the sheet conductive. Electrically conductive geomembranes 
allow one to detect pin holes in the sheet. Some geomembranes, especially 
those having multiple layers, are provided with one white surface through 
the addition of titanium dioxide. The purpose of this addition is to 
prevent the geomembranes from getting too hot from solar radiation during 
installation 
Patents mentioned in this specification are incorporated by reference 
herein.

The invention is illustrated by the following examples. 
EXAMPLES 1 TO 15 
The in situ blends, which are extruded into the geomembranes of the 
invention are prepared as follows: 
The preferred catalyst system is one where the precursor is formed by spray 
drying and is used in slurry form. Such a catalyst precursor, for example, 
contains titanium, magnesium, and aluminum halides, and an electron donor, 
and is attached to the surface of silica. The precursor is then introduced 
into a hydrocarbon medium such as mineral oil to provide the slurry form. 
See U.S. Pat. No. 5,290,745 ('745). The catalyst composition and method of 
preparing same used in these examples is of the same composition and 
preparation method as example 1 of '745 except that diethylaluminum 
chloride and tri-n-hexylaluminum are not used. 
Polyethylene is produced using the following standard procedure: Ethylene 
is copolymerized with 1-hexene (first reactor) and 1-butene (second 
reactor). Triethylaluminum (TEAL) cocatalyst is added to each reactor 
during polymerization as an 11.2 weight percent solution in isopentane. 
The pressure in each reactor is 300 psia. Each polymerization is 
continuously conducted after equilibrium is reached. 
Polymerization is initiated in the first reactor by continuously feeding 
the above catalyst precursor and cocatalyst triethylaluminum (TEAL), and 
into a fluidized bed of polyethylene granules together with ethylene, 
1-hexene, and hydrogen. The TEAL is first dissolved in isopentane (5 
percent by weight TEAL). The resulting copolymer mixed with active 
catalyst is withdrawn from the first reactor and transferred to the second 
reactor using nitrogen as a transfer medium. The second reactor also 
contains a fluidized bed of polyethylene granules. Ethylene, 1-butene, and 
hydrogen are introduced into the second reactor where they come into 
contact with the copolymer and catalyst from the first reactor. Additional 
cocatalyst is also introduced. The product blend, which is referred to as 
Resin A, is continuously removed. Resin A is the resin used to prepare the 
geomembrane embodiment of the invention. The variables are set forth in 
Table I. The properties of Resin A are shown as the second reactor resin 
properties in Table I. 
TABLE I 
______________________________________ 
reaction conditions 
first reactor 
second reactor 
______________________________________ 
temperature (.degree.C) 
86.9 83.0 
pressure (psia) 300 300 
C2 partial 21.4 70.7 
pressure(psia) 
H2/C2 molar ratio 
0.002 1.70 
C6/C2 molar ratio 
0.148 0.0023 
C4/C2 molar ratio 
-- 0.349 
catalyst precursor 
13 -- 
feed rate(lbs/hr) 
TEAL feed rate 84 48.5 
(lbs/hr) 
production rate 20,100 13,200 
(lbs/hr) 
bed weight(lbs) 56,700 70,300 
bed level (feet) 
38.2 40.3 
residence time 2.82 2.11 
(hrs) 
space/time/yield 
5.04 3.30 
(lbs/hr/cu ft) 
recycle hexane 2 -- 
(mol %) 
weight percent 8.4 -- 
condensing 
resin properties 
flow index 0.42 8.3 
(g/10 min) 
melt index (I.sub.5) 
-- 0.28 
(g/10 min) 
density(g/cc) 0.9142 0.9207 
residual 4.8 3 
titanium(ppm) 
bulk density 22.3 27 
(lbs/cu ft) 
average particle 
0.032 0.029 
size(inch) 
split (% by wt) 60 40 
______________________________________ 
Notes to Tables: 
1. Values for the second reactor resin properties are for the final product 
blend, i.e., Resin A. 
2. Density is measured by producing a plaque in accordance with ASTM 
D-1928, procedure C, and then testing as is via ASTM D-1505. 
3. Melt index is determined under ASTM D-1238, Condition E. It is measured 
at 190.degree. C. and 2.16 kilograms and reported as grams per 10 minutes. 
At 2.16 kilograms, the melt index can also be referred to a I.sub.2. At 5 
kilograms, the melt index is referred to as I.sub.5. 
4. Flow index is determined under ASTM D-1238, Condition F. It is measured 
at 190.degree. C. and 10 times the weight used in determining the melt 
index, and reported as grams per 10 minutes. The flow index can also be 
referred to as I.sub.21. 
5. Melt flow ratio is the ratio of flow index to melt index. 
6. The molecular weight distribution is determined via Size Exclusion 
Chromatography using a Waters.TM. 150C with trichlorobenzene as solvent at 
140 degrees C. with a broad molecular weight distribution standard and 
broad molecular weight distribution calibration method. 
7. Py=air pressure at yield. 
8. Pb=air pressure at burst. 
9. Vb=air volume at Pb. 
10. PbVb=Work, which is the total amount of energy needed to burst a 
geomembrane specimen via the test method defined in paragraph 11. 
11. Three dimensional extensional behavior is determined by a burst test. 
The burst test involves clamping a 7 inch (17.87 centimeters) diameter 
sample of a geomembrane, which is 20 mil (0.5 millimeter) thick, over a 
plate equipped with a computer controlled air inlet valve and sensors that 
monitor air flow and pressure. Air is introduced in increments of 1 psi 
(0.0069 MPa), and each increment is held for 15 seconds, until the sample 
yields, enlarges, and bursts. During each increment of pressure, makeup 
air is introduced as necessary to maintain the pressure. The real time 
changes of air volume and pressure are recorded. The shape and also the 
size of an enlarged sample, especially before burst, is normally related 
to its density, i.e., a half ellipsoid surface for typical MDPEs; a 
truncated sphere for typical VLDPEs; and a hybrid of an ellipsoid and 
sphere for typical LLDPEs. 
A typical sample yields at Py, enlarges substantially thereafter, and 
bursts when air volume reaches Vb at an air pressure Pb. The ultimate 
volume of air that a geomembrane can withstand, Vb, is important where its 
three dimensional extensibility is necessary for the particular 
application. Nevertheless, this property alone, without specifying Pb, is 
not sufficient to comprehensively describe the three dimensional 
extensional behavior of the geomembrane. In this invention, ultimate work, 
i.e., PbVb, of geomembranes is used primarily to differentiate the three 
dimensional extensional behavior of different polyethylenes while Vb can 
also be used, if desired. 
12. The tests in Table III are carried out on plaques under ASTM D 638. It 
should be noted that the uniaxial performance tested here is not a 
predictor of multiaxial capability. 
Thirteen other resins are prepared, extruded into 20 mil thick 
geomembranes, and compared with the 20 mil thick geomembrane made from 
Resin A. The extrusions are effected on a 21/2 inch single screw 
Prodex.TM. extrusion line (60 mil die gap) under the conditions described 
below. It is found that the processability of Resin A is very good for 20 
mil geomembranes and good for 40 mil geomembranes. 
The following additives are added to each resin prior to or during 
extrusion in percent by weight based on the weight of the resin: 
______________________________________ 
additive percent by weight 
______________________________________ 
octadecyl 3,5-di-tert-butyl-4- 
0.045 
hydroxyhydrocinnamate 
zinc stearate 0.075 
silica gel 0.005 
bis(2-hydroxyethyl)stearylamine 
0.075 
tris(nonylphenyl)phosphite 
0.15 
vinylidene fluoride- 
0.08 
hexafluoropropene polymer 
______________________________________ 
The extrusion is conducted on a single screw 21/2 inch Prodex.TM. flat die 
extrusion line having a 60 mil die gap under the following operating 
conditions: die width is 2 feet; lip setting is 60 mils; chill roll 
opening is 40 mils; screen pack in mesh size is 20/60/20; barrel 
temperature in degrees F. for the rear zone is 342, the center zone 378, 
and the front zone 380; adapter temperature in degrees F. is 400; melt 
temperature in degrees F. is 461; die temperature in degrees F. for the 
left side is 406, the gate 351, the center 402, and the right side 402; 
the amp meter reading is 25; the extruder screw rpm is 200; the head 
pressure is 3570 to 3580 psi; the roll temperature in degrees F. for the 
top is 218, for the center 210, and for the bottom 250; the sheet linear 
speed is 72.25 inches per minute; the rate is 120 pounds per hour; the 
gauge variability is 38 to 41; the sheet width is 21.1 inches; and the 
bottom roll is open. 
There are five types of resins. Type I is a bimodal LLDPE. The Type I resin 
used in example 1 is Resin A. Type II is a bimodal VLDPE and is made by 
essentially the same procedure as Resin A. Type III is a unimodal LLDPE 
prepared with a titanium based catalyst according to the preferred process 
described in U.S. Pat. No. 4,302,565. Type IV resins are unimodal VLDPEs 
prepared with a vanadium or a titanium based catalyst according to the 
preferred process described in U.S. Pat. Nos. 4,508,842 and 4,302,565, 
respectively. Type V resins are unimodal MDPEs prepared with a 
conventional chromium based catalyst. The material properties of the 
resins are set forth in Table IIA; the three dimensional extensibility 
performance is set forth in Table IIB; and various test results are set 
forth in Table III. 
TABLE IIA 
______________________________________ 
MI FI Melt 
Resin Density (g/10 
(g/10 
Flow Molecular 
Example 
Type (g/cc) min) min) Ratio 
Mw/Mn Weight 
______________________________________ 
1 I 0.9229 0.07 9 129 28.4 330,000 
2 II 0.9140 0.966 
81 84 -- -- 
3 III 0.917 1 25 25 -- -- 
4 III 0.916 0.5 13 26 -- -- 
5 III 0.921 0.8 -- -- -- -- 
6 III 0.917 2.5 -- -- -- -- 
7 III 0.923 0.3 -- -- -- -- 
8 III 0.920 0.95 24 25 -- -- 
9 IV 0.905 0.5 23 46 -- -- 
10 IV 0.900 0.4 -- -- -- -- 
11 IV 0.908 0.2 17 85 12.7 291,000 
12 IV 0.910 0.5 -- -- -- -- 
13 V 0.939 0.17 18 106 21.2 242,000 
14 V 0.939 0.2 24 120 22.3 234,000 
______________________________________ 
Table IIB 
______________________________________ 
Resin Py Pb Vb PbVb 
Example Type (psig) (psig) (liter) 
(ft lb) 
______________________________________ 
1 I 20 9 4.5 542.4 
2 II 15 6 6.0 631.6 
3 III 18 12 0.45 
61.1 
4 III 17 11 0.7 91.5 
5 III 17 10 1.0 125.6 
6 III 18 12 0.7 95.0 
7 III 22 19 0.36 
61.7 
8 III 18 14 0.4 58.4 
9 IV 13 7 6.0 662.1 
10 IV 11 5 5.5 551.0 
11 IV 14 6 6.0 631.6 
12 IV 14 7 5.5 606.9 
13 V 31 30 0.3 68.2 
14 V 31 30 0.3 68.2 
______________________________________ 
TABLE III 
______________________________________ 
Two Percent 
Yield Ultimate Ultimate 
Secant 
Stress Stress Elongation 
Modulus 
Example (psi) (psi) (percent) 
(psi) 
______________________________________ 
1 1879 4315 794 38,374 
3 1782 4432 740 38,358 
4 1768 4517 688 36,391 
______________________________________ 
The resin of example 1 has a yield strain of 15 percent; an ultimate strain 
of 794 percent; a melting point of 125.66 degrees C.; and a heat of fusion 
of 120.1 Joules per gram. The yield stress, ultimate stress, yield strain, 
and ultimate strain are determined under ASTM D 638 using an average of 5 
measurements. The melting points and heat of fusion are determined by 
differential scanning calorimeter (DSC) using an average of 3 
measurements. 
EXAMPLES 15 TO 18 
Four geomembrane sheets are prepared from Resin A plus additives as in 
example 1 except that the thicknesses are in the neighborhood of 40 mils. 
The geomembranes are then tested for various mechanical properties. The 
results are set forth in Table IV. The NSF-54 (NSF International Standard 
54 as revised in 1993) Test Method is used. NSF stands for National 
Sanitation Foundation. NSF provides a set of standards for geomembranes 
(or flexible membrane liners) made from various polymers in NSF-54. This 
standard includes, among other things, the minimum requirements for 
various geomembranes and the test methods for measuring geomembrane 
properties. 
TABLE IV 
______________________________________ 
elong- tensile 
elong- tensile 
thick- ation at 
strength 
ation at 
strength 
ness yield at yield 
break at break 
Example 
MD/TD (mil) (%) (psi) (%) (psi) 
______________________________________ 
15 MD 40.8 24.4 1623 653 4242 
TD 40.7 20.4 1617 738 4067 
16 MD 40.2 24.4 1628 639 4218 
TD 39.9 20.2 1655 730 3862 
17 MD 40.4 27.9 1611 682 4256 
TD 40.3 21.9 1642 787 4530 
18 MD 45.4 27.2 1625 673 4303 
TD 45.5 22.9 1614 878 4600 
______________________________________ 
MD= machine direction and 
TD= transverse direction. 
EXAMPLE 19 
In this example, the ultraviolet (UV) resistance of the Example 1 resin 
composition is determined by continuously weathering dog-bone specimens 
cut out from 80 mil plaques prepared as above. A QUV Weatherometer.TM. is 
used with A-340 light bulbs running on a cycle of 20 hours at 70 degrees 
C. under UV light (dry) and 4 hours at 55 degrees C. dark (wet). The 
tester is also referred to as "The Q-U-V Accelerated Weathering Tester". 
It is manufactured by the Q-Panel Company, Cleveland, Ohio. Specimens are 
taken out after every 200 to 300 hours of weathering and their tensile 
strengths and elongation values at break are measured, 50 percent loss of 
elongation at break is the criterion of failure. The values reported are 
the average values of three measurements or specimens. The composition 
contains 2.25 weight percent carbon black (N-550). 
Typical commercial black medium density polyethylene geomembrane sheets are 
produced by either adding the carbon black via black masterbatch into the 
polymer during the sheet conversion process or by directly converting 
polymer, which already contains carbon black, into sheets. In either case, 
the amount of carbon black in the sheets varies from 2 to 3 weight 
percent. The typical amount of carbon black is 2.25 to 2.5 weight percent, 
and this amount is considered sufficient to protect the sheets from 
ultraviolet degradation. 
The results are reported in Table V. As shown, the composition does not 
fail for up to about 4100 hours. The composition of example 11, however, 
fails at about 1200 hours even though it contains the same carbon black in 
the same amount. 
TABLE V 
______________________________________ 
tensile elongation 
QUV time strength at 
at break 
(hours) break (psi) 
(percent) 
______________________________________ 
0 4125 746 
(control) 
1580 4168 793 
1817 4040 790 
2072 4359 760 
2432 4183 774 
2626 3938 775 
2879 4313 807 
3120 4262 797 
3364 3936 796 
3607 3798 634 
3863 3686 737 
4096 3911 774 
______________________________________ 
EXAMPLES 20 TO 26 
The geomembrane industry specifies that a geomembrane composition should 
exhibit a minimum of 100 minutes of oxidation induction time (OIT). Resin 
A is tested with an antioxidant only or with an additive package. Resin A 
is mixed with the additives in a Brabender.TM. mixer at 180 degrees C. for 
5 minutes, and double pressed on a slow-cool press at 15 degrees C. OIT is 
measured at 200 degrees C. using a DuPont.TM. DSC. The variables and 
results are set forth in Table VI. 
TABLE VI 
______________________________________ 
OIT 
example formulation (minutes) 
______________________________________ 
20 Resin A plus 0.025 
2.2 
weight percent 
Antioxidant A 
21 Resin A plus additive 
7.2 
package A 
22 Resin A plus additive 
21.2 
package B 
23 Resin A plus additive 
23.3 
package C 
24 Resin A plus additive 
79.2 
package D 
25 Resin A plus additive 
84.8 
package E 
26 Resin A plus additive 
114.6 
package F 
______________________________________ 
Antioxidant A is octadecyl 3,5-di-tert-butyl-4-hydroxyhydrocinnamate. 
Additive package A contains 0.2 percent by weight of a mixture of 22.5 
weight percent octadecyl 3,5-di-tert-butyl-4-hydroxyhydrocinnamate, 2.5 
weight percent silica gel, 37.5 weight percent zinc stearate, and 37.5 
weight percent bis (2-hydroxyethyl)stearylamine; 0.15 percent by weight 
tris (nonylphenyl)phosphite; and 0.08 percent by weight vinylidene 
fluoride-hexafluoropropene polymer. 
Additive package B contains 0.3 percent by weight of a mixture of 50 weight 
percent tetrakis 
methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)!methane and 50 
weight percent tris(2,4-di-tert-butylphenyl) phosphite; 0.125 percent by 
weight 
poly6-(1,1,3,3-tetramethyl-butyl)amino!-s-triazine-2,4-diyl!2,2,6,6-te 
tramethyl-4-piperidyl)imino!hexamethylene2,2,6,6-tetramethyl-4-piperidyl)i 
mino!; and 0.1 percent by weight of a mixture of 25 weight percent zinc 
stearate, 25 weight percent zinc oxide, and 50 weight percent 
1,3,5-tris(4-tert-butyl-3-hydroxy-2,6 
dimethylbenzyl)-1,3,5-triazine-2,4,6-(1H, 3H, 5H)-trione. 
Additive package C contains 0.1 percent by weight 
tris(2,4-di-tert-butylphenyl) phosphite and 0.2 percent by weight 
octadecyl 3,5-di-tert-butyl-4-hydroxyhydrocinnamate. 
Additive package D contains 0.1 percent by weight 
tris(2,4-di-tert-butylphenyl) phosphite and 0.2 percent by weight 
1,3,5-tris(4-tert-butyl-3-hydroxy-2,6 
dimethylbenzyl)-1,3,5-triazine-2,4,6-(1H, 3H, 5H)-trione. 
Additive package E contains 0.1 percent by weight 
tris(2,4-di-tert-butylphenyl) phosphite and 0.2 percent by weight tetrakis 
methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)!methane. 
Additive package F contains 0.125 percent by weight 
tris(2,4-di-tert-butylphenyl) phosphite and 0.25 percent by weight 
tetrakis methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)!methane. 
All of the above percentages are based on the weight of Resin A except 
where components of a mixture are mentioned.