Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~071779
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- Case 8031-2
Th~s invention resides in the chemical arts. More
particularly, it relates to the chemical art having to do with
synthetic resins derived from alpha or 1-olefins. Specifically, it
relates to synthetic resins formed by the polymerization of
ethylene alone or with other olefins.
The synthetic resin formed by the polymerization of ethylene
as the sole monomer is called polyethylene. While "polyethylene"
has been used from time to time in the art to include a copolymer
of ethylene and a minor amount of another monomer, such as
butene-l, the term is not so used herein.
The polyethylene, such as low density polyethylene (LDPE) and
high density polyethylene (HDPE), and copolymers of ethylene with
a C3~0 alpha-olefin, (generally referred to as linear low density
polyethylene (LLDPE)), of commerce are normally solid, somewhat
flexible, thermoplastic polymers formed by the polymerization of
the particular monomer(s) by various methods well known in the art.
For example, such polymers can be prepared by free-radical
polymerization at high pressures, or by low pressure processes,
such as fluidized-bed, gas phase technology, with molybdenum-based
catalysts, with chromium-based catalysts, and with Ziegler-Natta
catalysts systems. The high pressure processes produce polymers
with long chain branching and the low pressure processes produce
essentially linear polymers with controlled levels of short chain
branching. In Ziegler-Natta catalyst systems, the catalyst is
formed by an inorganic compound of a metal of Groups I-III of the
Periodic Table, (for example, an aluminum alkyl), and a compound of
a transition metal of Groups IV-VIII of the Periodic Table, (for
example, a titanium halide). A typical crystallinity is about 21
to about 75 wt% by the method of Wunderlick & Guar, J. PhYs. Chem.
Ref. Data, Vol 10, No. 1 (1981). Also, the typical melt index of
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said ethylene homopolymers or copolymers is from 0.2 and 50 g/10
minutes (measured according to ASTM 1238, Condition E). Moreover,
the melting point of the crystalline phase of normally solid
polyethylene of commerce is about 135C.
Although the linear polyethylenes of commerce have many
desirable and beneficial properties, they are deficient in melt
strength. When molten, they exhibit no strain hardening (an
increase in resistance to stretching during elongation of the
molten material). Thus, linear polyethylenes have a variety of
melt processing shortcomings, including the onset of edge weave
during high speed extrusion coating of paper or other substrates,
sheet sag and local thinning in melt thermoforming, and flow
instabilities in coextrusion of laminate structures. As a result,
their use has been limited in such potential applications as, for
example, extrusion coating, blow molding, profile extrusion, and
thermoforming.
Some effort has been made in the art to overcome the melt
strength deficiency of the polyethylene of commerce.
Irradiation of polyethylene is known in the art, however, such
irradiation has been conducted primarily on articles fabricated
from polyethylene, such as films, fibers and sheets, and at high
dosage levels, i.e., greater than 2 Mrads, in order to crosslink
the polyethylene. For example, U.S. 4,668,577 discloses
crosslinking of filaments of polyethylene, and U.S. 4,705,714 and
4,891,173 disclose differentially crosslinking a sheet made from
high density polyethylene. The polyethylene crosslinked by these
methods is reported to have improved melt strength and decreased
solubility and melt flow. However, the crosslinking produced an
undesirable decrease in melt extensibility of the polyethylene,
thereby limiting the draw lengths typically required for film or
fiber applications.
Another attempt to improve the melt strength and melt
extensibility of polyethylene by exposing the linear polyethylene
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to low levels, i.e., 0.05 to 0.3 Mrads, of high energy radiation
is disclosed in United States 3,563,870.
European Patent Application 047171 discloses the
irradiation of ethylene polymers by heat aging the ethylene
polymer granules by pretreating them with an atmosphere of steam
in order to reduce the oxygen content in the granules, irradiating
the thus treated polymer at a dosage of less than 1.5 Mrads, and
then steaming the irradiated polymer.
British Patent No. 2,019,412 is directed to the
irradiation of linear low density polyethylene (LL~PE) film at
between 2 and 80 Mrads to provide increased elongation at break
values.
United States 4,586,995 and 4,598,128 are directed to a
method for obtaining long chain "Y" branching in ethylene polymers
by heating an ethylene polymer under non-gelling, non-oxidizing
conditions to produce terminal vinyl unsaturation in an ethylene
polymer having no unsaturated end groups or to increase the
terminal vinyl unsaturation in polyethylene containing unsaturated
end groups, irradiating the heat treated ethylene polymer at a
dosage of from 0.1 to 4 Mrad and then cooling, gradually or
rapidly, the resulting irradiated polymer.
United States 4,525,257 discloses irradiation of narrow
molecular weight, linear, low density ethylene/C3 18 alpha-olefins
copolymers at a radiation dosage of between 0.05 to 2 Mrad, to
produce copolymers that are crosslinked without gelation to an
extent sufficient to provide an increase in extensional viscosity
and substantially equivalent high shear viscosity when compared
with corresponding noncrosslinked polyethylene. The irradlated
copolymers is not de-activated to reduce or eliminate residual
radical intermediates.
This invention in one aspect comprises a normally solid,
high molecular weight, gel-free, irradiated ethylene polymer
material, a branching index of which is less than 1, and that has
significant strain hardening elongational viscosity. Preferably,
the ethylene polymer material has a density of from 0.89 to 0.97
g/cc and its molecular chains have a substantial amount of free-
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end long branches.
The ethylene polymers of this invention have areduced melt index (as evidenced by I2 at 190C) which
evidences an increase in molecular weight, increase in the
melt index ratio (Ilo/I2) which indicates a broadened
molecular weight distribution, improved melt tension and
strain hardening elongational viscosity, and branching
indexes of less than 1.
The ethylene polymers of this invention are
produced by low level radiation under controlled conditions.
Another aspect of the present invention provides a
useful article composed of a substantial quantity of the
above-mentioned ethylene polymer material, such as a film.
The film may be, for example, a blown film produced by
extruding the ethylene polymer material into a tube which is
subsequently blown into a bubble.
As used herein, "ethylene polymer material" means
ethylene polymer material selected from the group consisting
of (a) homopolymers of ethylene, (b) random copolymers of
ethylene and an alpha-olefin selected from the group
onSiSting of C3_10 alPha-olefins having a maximum
polymerized alpha-olefin content of about 20 (preferably
about 16) % by weight, and (c) random terpolymers of ethylene
and two C3 10 alpha-olefins, provided that the maximum
polymerized alpha-olefin content is about 20 (preferably
about 16) % by weight. The C3 10 alpha-olefins include the
linear and branched alpha-olefins such as, for example,
propylene, l-butene, isobutylene, l-pentene, 3-methyl-1-
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pentene. The ethylene copolymer can be a HDPE or a short chain
branched L~DPE, and the ethylene homopolymer can be a HDPE or a
LDPE. Typically the LLDPE and LDPE have densities of 0.910 g/cm3
or greater to less than 0.940 g/cm3 and the HDPE have densities of
greater than 0.940 g/cm3, usually 0.950 g/cm3 or greater. In
general, ethylene polymer materials having a density from 0.89 to
0.97 g/cc are suitable for use in the practice of this invention.
Preferably the ethylene polymers are LLDPE and HDPE having a
density from 0.89 to 0.97 g/cc.
As used in this application, "high molecular weight" means a
weight average molecular weight of at least about 50,000.
The branching index quantifies the degree of long chain
branching. In preferred embodiments the branching index is
preferably less than about 0.9 and most preferably about 0.2-0.8.
It is defined by the equation:
~ IV ] l~r
g'
[IV]LU~
Mw
in which g' is the branching index, tIV]~, is the intrinsic
viscosity of the branched ethylene polymer material and [IV]L~ is
the intrinsic viscosity of the corresponding, ethylene polymer
material, namely, normally solid, ethylene polymer material of
substantially the same weight average molecular weight and, in the
case of copolymers and terpolymers, substantially the same relative
molecular proportion or proportions of monomer units.
Intrinsic viscosity, also known as the limiting viscosity
number, in its most general sense is a measure of the capacity of
a polymer molecule to enhance the viscosity of a solution. This
depends on both the size and the shape of the dissolved polymer
particle. Hence, in comparing a nonlinear polymer with a linear
polymer of substantially the same weight average molecular weight,
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it is an indication of configuration of the nonlinear polymer
molecule. Indeed, the above ratio of intrinsic viscosities is a
measure of the degree of branching of the nonlinear polymer. A
method for determining intrinsic viscosity of ethylene polymer
material is described in J. App. Poly. Sci., 21, pp 3331-3343
(1977).
Weight average molecular weight can be measured by various
procedures. However, the procedure preferably used here i~ that of
laser light scattering photometry, which i8 disclosed by McConnell
in Am. Lab., May 1978, in the article entitled "Polymer Molecular
Weights and Molecular Weight Distribution by Low-Angle Laser Light
Scattering".
Elongational viscosity is the resistance of a fluid or
semifluid substance to elongation. It is a melt property of a
thermoplastic material, that can be determined by an instrument
that measures the stress and strain of a specimen in the melt state
when subjected to tensile strain at a constant rate. one such
instrument is described in, and shown in Fig. 1 of, Munstedt, J.
Rheology, 23, (4), 421-425, (1979). A commercial instrument of
similar design is the Rheometrics RER-9000 extensional rheometer.
Molten, high molecular weight, ethylene polymer material exhibits
elongational viscosity which, as it is elongated or drawn at a
constant rate from a relatively fixed point, tends to increase for
a distance dependent on the rate of elongation, and then to
decrease rapidly until it thins to nothing - so-called ductile or
necking failure. On the other hand, the molten ethylene polymer
material of this invention, that is of substantially the same
weight average molecular weight and at substantially the same test
temperature as the corresponding, molten, high molecular weight,
ethylene polymer material, exhibits elongational viscosity which,
as it is elongated or drawn from a relatively fixed point at
substantially the same rate of elongation tends to increase over a
longer distance, and it breaks or fails by fracture - so-called
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207 1 779
brittle or elastic failure. These characteristics are indicative
of strain hardening. Indeed, the more long chain branching the
~thylene polymer material of this invention has the greater the
tendency of the elongational viscosity to increase as the elongated
material approaches failure. This latter tendency is most evident
when the branching index is less than about 0.8.
Also melt tension provides an indication of the melt strength
of the material. Melt tension is determined with a Gottfert
Rheotens melt tension apparatus from Gottfert Inc. by measuring the
tension of a strand of molten ethylene polymer in centi-newtons as
follows: the polymer to be examined is extruded at 180C through
a capillary 20 mm long and 2 mm in diameter; the strand is then
subjected to stretching using a drawing system with a constant
acceleration of 0.3 cm/sec2. The tension resulting from the above
drawing is measured (in centi-newtons). The higher the melt
tension means the greater the melt strength values which, in turn,
are indicative of the particular material 1 8 strain hardening
ability.
This invention in another aspect provides a practical process
for converting normally solid, high molecular weight, ethylene
polymer material into normally solid, gel-free, ethylene polymer
material, the branching index of which is less than 1, and that has
significant strain hardening elongational viscosity.
The process comprises:
(1) irradiating said ethylene polymer material
(a) in an environment in which the active oxygen
concentration is established and maintained at less than about
15% by volume of said environment and
(b) with high energy ionizing radiation at a dose rate in
the range from about 1 to about lx104 megarads per minute for
a perlod of time sufficient to provide an exposure of up to
2.0 Mrads of radiation, but insufficient to cause gelation of
the material;
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(2) maintaining the thus irradiated material in such an
environment for a period of time sufficient for a significant
amount of long chain branches to form; and
(3) then treating the irradiated material while in such an
environment to deactivate substantially all the free radicals
present in the irradiated material.
The ethylene polymer material treated according to the process
of this invention can be any normally solid, high molecular weight
ethylene polymer material. In general, the intrinsic viscosity of
the starting, ethylene polymer material, which is indicative of its
molecular weight, should be in general about 1-25, and preferably
1-6, to result in an end product with an intrinsic viscosity of
0.8-25, and preferably 1-3. However, ethylene polymer material
with intrinsic viscosities higher and lower than these general
values are within the broader scope of this invention. Ethylene
polymer material having a melt index of about 0.01 and higher can
be used.
The ethylene polymer material treated according to the process
of this invention under the broadest concepts of the process can be
in any physical form, for example, finely divided particles,
granules, pellets, film, sheet, and the like. However, in
preferred embodiments of the process of this invention, the
ethylene polymer material is in pellet or spherical particle form
with satisfactory results being obtained. Spherical particulate
forms with weight average diameter of greater than 0.4 mm are
preferred.
The active oxygen content of the environment in which the
three process steps are carried out is a critical factor. The
expression "active oxygen" herein means oxygen in a form that will
react with the irradiated material and more particularly the free
radicals in the material. It includes molecular oxygen (which is
the form of oxygen normally found in air). The active oxygen
content requirement of the process of this invention can be
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achieved by use of vacuum or by replacing part or all of air in the
environment by an inert gas such as, for example, nitrogen.
The ethylene polymer material immediately after it is made is
normally substantially free of active oxygen. Therefore, it is
within the concepts of this invention to follow the polymerization
and polymer work-up steps (when the ethylene polymer material is
not exposed to air) with the process of this invention. However,
in most situations the ethylene polymer material will have an
active oxygen content because of having been stored in air, or for
some other reason. Consequently, in the preferred practice of the
process of this invention the finely divided ethylene polymer
material is first treated to reduce its active oxygen content. A
preferred way of doing this is to introduce the material into a bed
of the same blown with nitrogen, the active oxygen content of which
is equal to or less than about 0.004% by volume. ~he residence
time of the material in the bed generally should be at least about
minutes for effective removal of active oxygen from the
interstices of the particles of the material, and preferably long
enough for the material to be in equilibrium with the environment.
Between this preparation step and the irradiation step, the
prepared, ethylene polymer material should be maintained in an
environment in which the active oxygen concentration is less than
about 15%, preferably less than 5% in a gas conveyance system, and
more preferably 0.004%, by volume of the environment. In addition,
temperature of the ethylene polymer material should be kept above
the glass transition temperature of the amorphous fraction of the
material up to, but not higher than, about 70C, preferably about
60C.
In the irradiation step the active oxygen concentration of the
environment preferably is less than about 5% by volume, and more
preferably less than about 1% by volume. ~he most preferred
concentration of active oxygen is less than 0.004% by volume.
In the irradiation step, the ionizing radiation should have
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sufficient energy to penetrate to the extent desired the mass of
ethylene polymer material being radiated. The energy must be
sufficient to ionize the molecular structure and to excite atomic
structure, but not sufficient to affect atomic nuclei. The
ionizing radiation can be of any kind, but the most practical kinds
comprise electrons and gamma rays. Preferred are electrons beamed
from an electron generator having an accelerating potential of
500-4,000 kilovolts. In the case of ethylene polymer material
satisfactory results are obtained at a low level dose of ionizing
radiation of about 0.2 to 2.0 megarads, preferably 0.3 to less than
2.0 megarads and most preferably 0.5 to 1.5 megarads, delivered
generally at a dose rate of about l-lo,000 megarads per minute, and
preferably about 18-2,000 megarads per minute.
The term "rad" is usually defined as that quantity of ionizing
radiation that results in the absorption of 100 ergs of energy per
gram of irradiated material, regardless of the source of radiation.
As far as the instant invention is concerned, the amount of energy
absorbed by the ethylene polymer material when it is irradiated
usually is not determined. However, in the usual practice of the
process energy absorption from ionizing radiation is measured by
the well known conventional dosimeter, a measuring device in which
a strip of fabric containing a radiation sensitive dye is the
energy absorption sensing means. Hence, as used in this
specification the term "rad" means that quantity of ionizing
radiation resulting in the absorption of the equivalent of 100 ergs
of energy per gram of the fabric of a dosimeter placed at the
surface of the ethylene polymer material being irradiated, whether
in the form of a bed or layer of particles, or a film, or a sheet.
The second step of the process of this invention should be
performed in a period of time generally in the range from about one
minute to about one hour, and preferably about 2-30 minutes. A
minimum time is needed for sufficient migration of ethylene polymer
chain fragments to free radical sites and for combination thereat
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to reform complete chains, or to form long branches on chains. A
radical migration time less than one minute, for example, about a
half minute, is within the broader concepts of this invention, but
is not preferred because the amount of resulting free-end long
chain branching is quite low.
The final step of the process, the free radical deactivation
or quenching step, can be performed by the application of heat,
generally from at least 60C to about 280C, or by the addition of
an additive that functions as a free radical trap, such as, for
example, methyl mercaptan.
In one embodiment of the process the application of heat
comprises melt extruding the irradiated ethylene polymer material.
As a result, quenching of the free radicals is substantially
complete. In this embodiment, prior to the extrusion or melt
compounding, the irradiated ethylene polymer material can be
blended with other polymers, if desired, and additives æuch as, for
example, stabilizers, pigments, fillers, and the like.
Alternatively, such additives can be incorporated as a side stream
addition to the extruder.
In another embodiment of the inventive process the application
of heat is achieved by introducing the irradiated ethylene polymer
material into a fluidized bed or a staged fluid bed system in which
the fluidizing medium is, for example, nitrogen or other inert gas.
The bed or beds is or are established and maintained in a
temperature range of at least about 60C up to a temperature which
does not exceed the melting point of the polymer, with the average
residence time of the irradiated ethylene polymer material in the
fluid bed or beds being from about 5 minutes to about 120 minutes,
with about 20-30 minutes being optimum.
The product thus obtained is a normally solid, high molecular
weight, gel-free, ethylene polymer material characterized by strain
hardening. The material is also characterized by a melt index
ratio greater than 10.
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Although the process of the invention can be carried out on a
batch basis, preferably it is performed on a continuous basis. In
one continuous embodiment of the process the finely divided,
ethylene polymer material either with or without the preparation
step, dependinq on the active oxygen content of the material, is
layered on a traveling belt in the required environment. The
thickness of the layer depends on the desired extent of penetration
of the ionizing radiation into the layer and the proportion of
irradiated ethylene polymer material desired in the final end
product. The speed of travel of the traveling belt is selected so
that the layer of finely divided, ethylene polymer material passes
through the beam or beams of ionizing radiation at a rate to
receive the desired dose of ionizing radiation. After having
received the desired dose of ionizing radiation, the irradiated
layer can be left on the traveling belt in said environment for the
period of time for free-radical migration and combination to occur,
and then removed from the belt, and introduced into an extruder
operated at a melt temperature of the irradiated material, or, in
another specific embodiment introduced into a heated bed, or a
staged system of heated beds, of particles of irradiated material
fluidized with nitrogen or other inert gas. In either embodiment,
the irradiated material after at least substantially all of the
free radicals therein are deactivated is discharged into the
atmosphere and quickly cooled to room temperature. In another
embodiment, the irradiated, ethylene polymer material is discharged
from the belt and conveyed in the required environment to a holding
vessel, the interior of which has the required environment, and
held in the vessel to complete the requisite free radical migration
time. The irradiated material then is introduced into an extruder
operated at a melt temperature of the irradiated material or is
introduced into a heated, inert gas fluidized bed, or a staged
system of fluidized beds, of irradiated particles of ethylene
polymer material and, after quenching of the free radicals, the
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irradiated polyethylene is discharged into the atmosphere.
This invention in still another aspect comprises the
extensional flow use of the strain hardening, ethylene polymer
material of this invention. Extensional flow occurs when the
ethylene polymer material in the molten condition is pulled in one
or more directions at a rate faster than it would normally flow in
those directions. It happens in extrusion coating operations in
which a melted coating material is extruded on to a substrate, such
as a moving web of paper or metal sheet, and the extruder or
substrate is moving at a higher rate than the extrusion rate. It
takes place in film production when the molten film is extruded and
then stretched to the desired thinness. It is present in
thermoforming operations in which a molten sheet is clamped over a
plug mold, vacuum is applied and the sheet is pushed into the mold.
It occurs in the manufacture of foamed articles in which molten
ethylene polymer material is expanded with a foaming agent. The
strain hardening ethylene polymer material of this invention is
particularly useful as part of (for example from as little as 0.5%
by weight to as much as 95% or more by weight) or, particularly as
substantially all of the molten plastic material used in these and
other melt processing methods (for example, profile extrusion, as
in the melt spinning of fibers) for making useful articles.
This invention is further illustrated by the accompanying
drawings which form a material part of these disclosures, and by
the following examples.
In the drawings
Fig. 1 is a schematic flow sheet of a preferred embodiment of
a continuous process for converting, for example, normally solid,
polyethylene into a normally solid, gel-free, polyethylene with
strain hardening;
Fig. 2 are plots of elongational viscosities versus
elongational times of samples of two, free-end long chain branched
high density polyethylene obtained by the process of this invention
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and of a high density polyethylene control.
Fig. 3 are plots of elongational viscosities versus
elongational times of a sample of a free-end long chain branched
low density polyethylene obtained by the process of this invention
and a LLDPE control.
In greater detail, Fig. 1 depicts a fluid bed unit 10 of
conventional construction and operation into which finely divided,
high molecular weight, polyethylene is introduced by way of conduit
11, nitrogen gas is introduced by way of conduit 13, and from which
substantially active oxygen free, high molecular weight,
polyethylene is removed by way of a solids discharge conduit 15
which also has a solids flow rate controller 16. The solids
discharge conduit 15 leads to a conveyer belt feed hopper 20.
The conveyer belt feed hopper 20 is a capped structure of
conventional design. It is operated so that its interior contains
a nitrogen atmosphere. It has a bottom solids discharge outlet
through which the polyethylene particles move and form a layer on
the top horizontal run of an endless conveyer belt 21.
The conveyer belt 21 is generally horizontally disposed, and
continuously moves under normal operative conditions. It is
contained in radiation chamber 22. This chamber completely
encloses the conveyer belt, and is constructed and operated to
establish and maintain a nitrogen atmosphere in its interior.
In combination with the radiation chamber 22 is an electron
beam generator 25 of conventional design and operation. Under
normal operative conditions it generates a beam of high energy
electrons directed to the layer of polyethylene particles on the
conveyer belt 21. Below the discharge end of the conveyer belt is
a solids collector 28 arranged to receive the irradiated
polyethylene particles falling off the conveyer belt 21 as it turns
into its path of opposite travel. Irradiated polyethylene
particles in the solids collector 28 are removed therefrom by a
rotary valve or star wheel 29 and delivered thereby to a solids
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transfer line 30.
The transfer line 30 leads to a gas-solids separator 31. This
unit is of conventional construction and usually is a cyclone type
separator. Gas separated therein is removed as by gas discharge
conduit 33 while separated solids are discharged therefrom as by a
rotary valve or star wheel 32 into a solids discharge line 34. The
solids discharge line 34 can lead directly to an extruder hopper
3S.
The extruder hopper 35, which feeds an extruder 36, is
conventional in construction and operation. It too is an enclosed
structure adapted for establishing and maintaining a nitrogen
atmosphere in its interior. The extruder 36 is of conventional
construction, and is operated in normal fashion. The solids in the
extruder hopper 35 move therefrom into the extruder which is
operated at a rate of extrusion to result in the period of time
between irradiation of the polyethylene and its entry into the
extruder being sufficient for a significant amount of free-end long
chain branches to form. Accordingly, the volume of the extruder
hopper 35 is selected to provide, if necessary, the desired amount
of hopper storage time to meet this condition. The extruder 36 is
designed (length of extruder barrel and screw) and operated at a
melt temperature and at a pressure sufficient to maintain the free
radical containing polyethylene therein for the amount of time
needed to deactivate substantially all of the free radicals
present.
The thus treated, finely divided polyethylene is characterized
by being substantially gel-free, having a density of from 0.89 to
0.97 g/cc , and being substantially branched with free-end long
chains of ethylene units. It can be used as is, or introduced, for
example, directly into a pelletizing and cooling unit 37 and
conveyed away therefrom as by solids transport line 38 as solid
pellets which can be stored and then used, or used without storage.
Similar results are achieved when other specific embodiments
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of high molecular weight, ethylene polymer material are treated
according to the continuous process just depicted.
The following examples illustrate the high molecular weight,
polyethylene of this invention, and the foregoing preferred
embodiment of a process for making it.
The melt indexes, I2 and Ilo~ are measured according to ASTM
D-1238. The melt index ratio is determined by dividing Ilo by I2.
The ratio is indicative of the molecular weight distribution and
the higher the number, the broader the molecular weight.
Example 1
Soltex T50-200 polyethylene in pellet form having a MI of 2
and density of 0.95, is introduced into the enclosed radiation
chamber 22 and purged with nitrogen until an oxygen level of 40 ppm
was achieved.
The material is distributed on the moving stainless steel
conveyer belt 21 to form a bed of polyethylene powder 1.3 cm high
and 15 cm wide. The bed is passed by the conveyer belt 21 through
an electron beam generated by a 2 MeV Van de Graff generator
operating at a 50 microamp beam current. The resulting absorbed
sùrface dosage is 0.40 Mrad. In addition, the active oxygen
content of the environment or atmosphere within the enclosed
radiation chamber 22 and in the remaining part of the system
comprising the irradiated polyethylene- transfer line 30, the
solids-gas separator 31 and the separator discharge line 34 is
established and maintained below 140 ppm.
After irradiation, the polyethylene falls off the end of the
conveyer belt 21 into the belt discharge collector 28 and through
the rotary valve 29 into the transfer line 30. After separation of
gas from the irradiated polymer, the polymer is fed through the
separator discharge line 34 into a bag purged with nitrogen.
The irradiated polyethylene is held for 30 minutes at room
temperature in the absence of oxygen. The material is introduced
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into an extruder feed hopper that was purged with nitrogen and then
extruded in a 2.5" single screw extruder at 260C.
The properties of the gel-free end product of Example l are
summarized in Table I.
ExamPles 2-4
Examples 2-4 are produced using the method and ingredients of
Example l except that the resulting absorbed dosage was 0.6, 0.9,
and l.0 Mrad, respectively. The oxygen level is maintained below
ppm in the enclosed rad~ation chamber 22, the irradiated
polyethylene transfer line 30, the solids gas separator 31 and the
separator discharge line 34. The properties of the gel-free end
products of Examples 2-4 are 81 -rized in Table I.
Examples 5-8
Examples 5-8 are produced using the method and ingredients of
Example l except that Dow 04052N polyethylene in pellet form having
a MI of 4 and density of 0.952, and the resulting absorbed dosage
was 0.5, 0.7, 0.8, and l.0 Mrad, respectively. The oxygen level is
maintained below lO0 ppm in the enclosed radiation chamber 22, the
irradiated polyethylene transfer line 30, the solids gas separator
31 and the separator discharge line 34. The properties of the gel-
free end products of Examples s-8 are summarized in Table I set
forth below.
Control Examples l and 2
Controls l and 2 are non-irradiated 6amples of Soltex T50-200
and Dow 04052N, respectively.
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TABLE I
Dosage
ExamPle (Mrad) Ilo I2 I~L~2
C-1 o 4.1 1.8 2.z7
1 0.4 lo.s -l.o 10.4
2 0.6 10.0 0.89 11.2
3 0.9 8.7 0.67 13.0
4 1.0 7.9 0.64 12.5
C-2 0 31.5 4.30 7.32
0.5 15.6 1.27 12.3
6 0.7 9.70 0.682 14.2
7 0.8 8.48 0.521 20.7
8 l.o 6.49 0.333 19.5
Example 9 and Control ExamPles 3-8
To demonstrate the criticality of the irradiation atmosphere
(IA), corresponding to the irradiation step (1) of the process, and
radical intermediate aging (RIA) in a controlled environment having
an oxygen content of less than 15~, corresponding to second step
(2) of the process, and the radical intermediate deactivation step
(RID), corresponding to the final step of the process of the
invention, Example 9 and Control Examples 4-8 are produced using
the method of Example 1 with the following exceptions:
- high density polyethylene having a MI of 6.90 in spherical
particulate form from HIMONT Italia S.r.l. was used
ln~tead of Soltex T50-200;
- the resulting absorbed dosage is 1.1 Mrad;
- the processing environment for Controls 3-8 are varied as
reported in Table II, and
- the radical deactivation is carried out on a 3/4"
Brabender Extruder at about 260C.
The results are set forth below in Table II.
*Trade-mark
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27651-29
- .
207177S
.
Table II
I2 Q I2 Q I2 Q
Ex. IA RIA RID Day 1 DaY 3 Day 13
C-3 - - - 6.90
C-4 N2 Air Air3.56 3.13 3.23
C-5 N2 ~ ~ 2.44 3.67 4.58
C-6 Air Air Air4.62 4.30 4.51
C-7 Air N2 N2 4.39 4.14 4.13
C-8 Air - - 4.56 5.90 8.40
9 N~ N2 N2 2.86 2.63 2.60
Poor stability of the melt flow-rate over 13 days was
exhibited by Control Examples 5 and 8 as compared to Example 9 of
the invention. Control Examples 4, 6 and 7 exhibited a somewhat
better stability of the melt flow rate than Controls 5 and 8, but
the melt flow rate is not as low as Example 9 of the invention.
Examples 10-15 and Control Examples 9 and 10
Examples lo to 15 are produced using the method of Example 1
except that the ethylene polymer used is fed through the separator
discharge line 34 into a nitrogen purged hopper of a 3/4"
Brabender extruder at 210C in first zone, 215C in the second
zone, 220C at the die and then pelletized in a conventional
manner. The pellets are then re-extruded with 0.1~ Irganox 1010
stabilizer in a 1-1/4" Killian single screw extruder with a
Maddoch mixer at 100 rpm at a flat profile of 420F. The hold
time between irradiation and extrusion is an average of about 15
minutes. The properties of the end products of Examples 10 to 15
are summarized in Table III, and the elongational viscosities,
measured using a Rheometrics RER 9000 extensional rheometer at
180C at strain rates between 0.05 to 2.0 sec~, of Examples 10, 12
and 14 and Control Examples 9 and 10 are illustrated in Figs. 2
and 3.
--19--
20 7 1 779
o ~ ~ o ,~",
cn ~ O o o I I I I o o C ,~
- ~, X
o C O
o o r I ~ c ~ J~ r
c~ O 1 a
c ~ 3
o U~ ~ I I I I , V ~
~ ~ ~ o
~v ~ o ~ ~ C ~ 3 ~ _ ,
o O ~ O W ~ 9 .,~
c
o O ~ 'c c ~`
. ~ C ~ ~ ~
'~ .1 ,C C
Z ~ ~ ~ o c~
al ` ,. r~ a
a . . . . . ~ :~ v ~ ~
.. I ~ r o o _~ C ,,C~ r ~ t ~ ~
. . C E ~0 ,~ ~~ X ~_ ~ e
J a~ cr~
~ jOO ,-,, O-",,
S O ~ O s :~ ,0, ~ ~
a ~
O ~ N ~ ~ ~ ~ . ~ C~ 1
-- 20 --
27651 - 29
Y~ '
.
21~71779
.
The results set forth in the above table show that the
ethylene polymers of this invention have increased melt index
ratios indicating an increase in the molecular weight
distribution, increased melt tension evidencing an increase in
melt strength and branching indexes showing the formation of long
chain branches.
The results set forth in Figures 2 and 3 show that the
ethylene polymers of this invention have extensional viscosities
indicative of the materials' strain hardening ability. The
tendency towards strain hardening elongational viscosity increases
as the branching index decreases, i.e., increases with increasing
degree of long chain branching.
The free-end long chain branched ethylene polymer material of
this invention has utility in melt processing operations to form
useful articles, for example, foamed and thermoformed articles
such as foamed and thermoformed sheet materials, extrusion coated
articles and fibers. Indeed, the strain hardening ethylene
polymer material of this invention is useful in all melt
processing operations in which a high molecular weight, ethylene
polymer material of enhanced melt strength is desired.
Other features, advantages and embodiments of the invention
disclosed herein will be readily apparent to those exercising
ordinary skill after reading the foregoing disclosures. In this
regard, while specific embodiments of the invention had been
described in considerable detail, variations and modifications of
these embodiments can be effected without departing from the
spirit and scope of the invention as described and claimed.
The expression "consisting essentially of" as used in this
specification excludes an unrecited substance at a concentration
sufficient to substantially adversely affect the essential
properties and characteristics of the composition of the matter
being defined, while permitting the presence of one or more
unrecited substances at concentrations insufficient to
2071779
.
substantially adversely affect said essential properties and
characteristics.
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