Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE l:~ENTIO~
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Field of the Invention
This invention relates to polypropylene polymer
pellets having improved processing characteristics
for spinning, extruding and the like, as well as
methods for obtaining them, A good processing
polypropylene polymer for fiber or film form~tion
desirably has the following attributes:
1) the ability to be attenuated. when molten
without breaking - this allows high through-
put production o fine filaments and thin
films which have high s~rength relative
to unattenuated products; and
2) the ability to be pumped throu~h pipin~
and capillaries and/or be attenuated as
a fiber or film requiri~g a minimum of
energy - this implies lower ~hear and
extensional viscosities ~or ~he polymer
melt
It has been demonstrated that the ~irst attribute
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(high attenuabili-ty) can be attained with a propylene
polymer having a narrow molecular weight
distribution (defined as the polymer weight average
mo]ecular weight divided by the polymer number
average molecular weight). The second attribute
(low shear and extensional viscosities3 is attained
with a lower weight average molecular weight polymer.
Zeigler-Natta catalysts presently used in the
commerical production of polypropylene produce
polymers having too broad a molecular weight dis-
tribution coming out o~ the polymerization reactor
for production of fine fibers or thin films. Thus,
low weight average molecular weight polymer out
of such a reactor would have the desired low vis-
cosity for processing, but would not be attenuable
to the desired extent. Polypropylene suppliers
have, therefore, found it necessary to make a very
high weight average molecular weight polymer followed
by a random molecular scission step (thermal or
chemical degradation) which inherently narrows the
molecular weight distribution while at the same
time reduces the weight average molecular weight
to the d~sired level.
Polypropylene is degraded chemically by addition
of compounds that decompose forming free radicals.
Chemical stabilizers added to polypropylene to
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enhance end-use stability may interfere with free
radical generators.
However, it has been found that some free radical
generator types of chemicals, such as the specific
types of organic peroxides described in British
Patent 1,442,681 for example, are minimally a~rected
by commonly used stabilizers and are, thus, pre~erred
prodegradants.
The degree to which the polymer can be degraded
is limited, however, by the inability o the pol~mer
producer to form pellets from very low viscosity
polymers. Therefore, the poLypropylene processor
manufacturing films a~d ~ibers faces the problem
of having to use a polypropylene not optimally
suited for th~se applications. Thus, a need has
been demonstrated for a polymer having high viscosity
properties fo~ pelletizing purposes and low viscosity
properties for end use processing purposes.
Descri~tion of the Prior ~rt
Low viscosity polypropylene polymer~ desirable
to processors cannot presently be pelletized commer-
cially by polymer producers without producing an
excess of "stringers" (pellets with long tails)
that tend to plug producers' and processors' equip-
ment.
It has been suggested that the polymer viscosity
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could be increased for pelletizing by running the
pelletizer at a temperature just above the poly-
propylene polymer melting point to improve pellet
cut. This can be done only at a low throughput
to reduce the heat generated by shear forces in the
pelletizing equipmen~ and/or by cooling the molten
polymer - both adding considerably to the process
cost and complexity.
It has also been suggested that the end use
processor add additional chemical prodegradant to
the polypropylene pellets to reduce the polymer
viscosity to the desired level prior to fiber or
film formation. Elowever, there are several dis-
advantages to this approach;
1) the peroxide prodegradants are fire/
explosion hazards and require special
handling procedures and equipment;
2) to be most effective, the peroxide must
be uniformly dispersed within the polymer
before it decomposes and reacts - other-
wise a polymer with variable viscosity
may result with an even broader molecular
weight distribution than the original
polymer. The polymer producer, having
access to specialized equipment and the
fine reactor flakes rather than pellets;
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is in a much better position to achieve
this uniform distribution;
3) the processor's equipment may be damaged
by a variable viscosity polymer;
4) the peroxide is more efficient as a pro-
degradant if well dispersed before reacting;
and
5) the peroxide added to or on the pellets
rather then within them acts as a lubri-
cant in extruder ~eed sections reducing
throughput for a given rpm.
The processor may also reduce the molecular
weight by using very high temperatures to thermally
degrade the polypropylene. However, these very high
temperatures lead to:
1) reduced equipmen~ lie
2) throughput limitations because of quenching
restraints
3) excessive energy consumption
4) hazardous operating environments; and
5) additive probLems~
The additive problems include:
1) excessive additive degradation, necessit~ting
that more additive be added to the polymer
than is required in the final product;
2) limited range of u~eable additives, requiring
that more expensive or non-optimum
additives be used; and
3) polymer piping, capillaries, dies, and
the like plugging from the degradation
products.
Additional information may be obtained by
reference to prior patents. sritish Patent 1,442,681
to Chemie Linz describes a process for the
préparation of polypropylene including degradation
with peroxide prodegradants producing a narrow
molecular weight distribution polypropylene polymer.
U.S. Patent 3~887,534 to Baba et al describes the
use of aliphatic peroxides as prodegradants for
polypropylene and discusses problems related thereto
but suggests that unreacted prodegradant is to be
avoided. U.S. Patent 3,144,436 to Greene et al
describes a process for degrading steroregular
polymers including the use of free radical initiators.
In one embodiment a two-step method is described
wherein there is controlled injection of the pro-
degradant into the me]t zone of the extruder. U.S.
Patent 3,849,241 to Buntin et al and U. S. Patent
3,978,185 also to Buntin et al describe meltblowing
processes that are improved through controlled de-
gradation of the polymers. U~S. Patent 3,755,527
to Keller et al similarly describes advantages of
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polymer degradation.
SUMMAR~
The present invention encompasses, 1) a step-
wise method of degrading polypropylene, initially
producing a polymer that is readily formed into
pellets that when heated undergo fuxther degradation
producing a low viscosity polymer that can be
conveniently processed into high quality films and
fibers; and 2) prodegradant containing polypropy-
lene polymer in the form of pellets and the likeresulting from this method.
The present invention results from the dis-
covery that when certain free radical generating
chemicals that act as polypropylene prodegradants
are added to the polymer and the pelleti~ing
equipment operated in a specified manner, a portion
of the chemical ~urvive the pelletizing process~
After extrusion to form pellets the reaction is
interrupted and the remainder of the prodegradant
will then react upon re-extrusion producing a polymer
that processes well and produces ~ilms and fibers
with excellent properties. The exact remaining
percentage of prodegradant after pelletizing
by the producer will vary depending upon pelletizing
temperature, prodegradant residence time at this
temperature, and type of prodegradant, but will
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preferably be more than half and up to 90~ of that
originally added. Ideally, for pelletizing, no
degradation takes place, but, as a practical matter,
some prode~radant will initially react during pellet-
izing. The small amount reacting initially, as
low as about 10%, only minimally reduces the polymer
viscosity at the pelletizer, permitting well formed,
free flowing pellets to be made. ~fter pelletizing,
residual prodegradant in an amount of at least
0.01 percent based on the weight of polymer is
necessary for acceptable results to be obtained.
Thus, the advantages of a two-step degradation addition
method described above in connection with the prior
art are retained, but the disadvantages are substantially
eliminated.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a graph showing the relationship
between reaction time and viscosity at extrusion
temperatures for polypropylene with two prodegradant
embodiments of the invention; and
FIG. 2 is a graph of exiting polymer viscosity
versus percent prodegradant added as a liquid and
as a concentrate.
DESCRIPTION OF TIJE PRBFERRED EMBODIMENTS
While the invention will be described in
connection with preferred embo~iments, it will be
understood that it is not intended to limit the
invention to those embodiments. On the contrary,
it is intended to cover all alternatives, modifi-
g
cations, and equivalents as may be included within
the spirit and scope of the invention as defined
by the appended claims.
The invention is applicable to the production
and processing of polypropylene. It is also
applicable to the processing of waste polypropy-
lene material to permit reuse in film and Eiber
formation. 0f course, as will be apparent to one
skilled in the art, OptilllUIII operating conditions
and concentrations will vary depending upon the
properties of the po]ymer being used and the ultimate
properties desired by the processor.
As produced, polypropylene generally has a high
weight average molecular weight in the range of from
about 250,000 to 500,000 and a molecular weiqht dis-
tribution of about 10 to 15. For high speed spinning
and fiber forming, the weight average molecular
weight distribution is preferably about 2.5 to 4.5.
However, when the molecular weight is reduced below
~out 130,000 , the polypropylene resin cannot be easily
commerically processed into pellats. The low
viscosity polymer instead produces poorly formed
pellets which are difficult to transport and handle.
Therefore, manufacturers prefer that the de~radation
of polypropylene prior to delivery be limited to
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produce a molecular weight not less than about
160,000. Many prodegradants are used to achieve this
degree of degradation in pelletizing equipment, and
most of them totally react under these conditions.
Peroxide prodegradants decompose at different rates
depending on temperature and environment. The rates
of decomposition are defined in terms of half-life.
In accordance with the invention a free radical
source prodegradant having a half-life in poly-
propylene in excess of one-half minute at ~75 F.
is added to a high molecular weight polypropylene
reactor flake polymer in an amount sufficient to
produce the final polymer properties desired by the
polymer processor. If it is desired to use a pro-
degradant with a shorter half-life or allow a
greater amount of prodegradant to make it through
the pelletization step unreacted, it is also possible
to inject the prodegradant into the molten polymer
stream. As the prodegradant rnust be dispersed U~
formly to be most effective, the injection should
be followed by a mixing step. In general,
the prodegradant should not interfere with or be
interfered with by commonly used polypropylene
stabilizers and should effectively produce free
radicals that upon decomposition initiate poly-
propylene degradation. The prodegradant should have
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a short enough halE-liEe a-t the polymer processor's
re-extrusion temperatures, however, so as to be
essentially entirely reacted before exi-ting the
extruder. Preferably they have a nalf--life in -the
polypropylene of less than 9 seconds at 550~F. so
tha-t at least 99~ of the prodegradant in the pellets
reacts before 1 minute of extruder residence time at
this temperature elapses. Such prodegradants include,
by way of example and not limitation, the following:
2,5-dimethyl-2,5-bis(tert-butyl-peroxy)hexyne-3, q-
methyl-4-tert-butyl-peroxy-2-pentanone (e.g. Lupersol
130 and Lupersol 120 available from Lucidol Division,
Penwalt Corporation), 3,6,6,9,9-pentamethyl~3-(ethyl-
acetate)-1,2,4,5-tetraoxycyclononane, (USP-138 from
Witco Chemical Corporation), 2,5-dimethyl-2,5-bis(tert-
butyl-peroxy)hexane (e.g. Lupersol 101) and ~,~'-bis-
(tert-butyl-peroxy)diisopropylbenzene (Vulcup R Erom
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Hercules, Inc.). Of these Witco USP-138 and Lupersol
130 are highly preferred. Preferred concentrations
of the free radical source prodegradants are in the
range of Erom about 0.01 to 0.4 percent based on the
weight of the polymers. Preferably the pelletizer
is operated to retain at least 75~ of the added
prodegradant in the pellets. When subjectea to
extruding temperatures by the pol~-mer user, the
degradation of the polymer will
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resume and proceed to the extent desired, essentially
completely reacting in the re-extrusion processO
Generally such extruder temperatures are in the
range of from about ~60 F. to 550 F. Alternatively,
these conditions may be obtained for degradation in
the extruder die assembly.
In the following examples melt indices were
determined using melt indexer (ASTM 123~) operated
at 177 F. with a 2160 g. weight. Samples were
allowed to heat ~o equilibrium for 5 minutes prior
to testing. The melt index is equivalent to the grams
exiting at 0.0825 inch diameter capillary in a
period of 10 minutes.
Examples
Example 1
A polypropylene reactor flake was obtained that
had a melt index of less than one. 0.275 weight
percent Lupersol 130 was added to -the flake and a
homogeneous blend made. This blend was pelletized
~0 in pelletizing equipment operated at 375 F. and the
residence time was about 2 minutes. Calculations
show about 22~ of the peroxide had reacted. The
pellets were tested and found to have a melt index
of about 5S. Approximately 10% of the prodegradant
in the pellets reacted in the melt indexer so that
actual melt index may be considered to be in the
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40-45 range for the pellets. This polymer was
easily pelletized and gave polymer pellets equivalent
to normal commercial pellets.
These pellets were then re-extruded at 460 F.
with an extruder residence time of about 3 minutes.
The extrudate was then tested and found to have a
melt inde~ of about 550. To veriy that the 460
F. extrusion step did not appreciably affect the
melt index, the extrudate was re-extruded and the
melt index increased from 550 .o 580. Thus, about
95~ of the melt index increase ~as due to the pro-
dsgradant in the pellets and about 5~ due to the
action of the extruder.
Example 2
.
The same flake and equipment was used as in
Example 1 except that 0.3~ Lupersol 130 was added
to tne flake. The pellets were found to have a melt
index of 45-50. Upon re-extrusion, the extrudate
was found to have a melt index of about 660. As
in Example 1, the pellet cut was commercially
acceptable.
Example 3
Witco Chemical USP-138 was applied to the
flake at a concentration of 0.35 weight percent.
The blend was extruded at 375 F. ~or an extruder
residen~e time of about 2 minutes. The melt index
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of the extruded sample was found to be about 15.
The sample was re-extruded at 485 F. with a 3
minute residence time and the melt index was found
to be 215. The flake without peroxide added but
processed in the above manner had a melt index of
1.7.
Example 4
2% Lupersol*130 was blended with commercially
availa~le polypropylene pellets identified as Her-
cules PC-973. This blend was then extruded at
170 C. with a one minute residence time. Cal-
culations show 98~ ~f the peroxide remained un- -
reacted in the extrudate. Various percentages of
the peroxide concentrate extrudate were then blended
with polypropylene pellets. A calculated equivalent
amount of pure peroxide was added to other pellets.
The "concentrate"/polypropylene blend and liquid
peroxide/polypropylene blend was extruded through
a ~rabender extruder at 465 F. with a seven minute
residence time. The vlscosities exiting the extruder
die tip were determined and are shown in Figure 2.
They can be seen to be equivalent.
Thus, the invention includes prodegxadant
concentrates which can be added to non-prodegradant
containing pellets to gain desired results. Concen-
trations of up to 5% by weight prodegradant can be
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formed with ease, and higher concentrations are
possible.
While it is not desired to limit the invention
to any particular theory, the significance of certain
prodegradant characteri.stics may be postulated.
From half-life determinations it can be shown that
half-life reaction rate coefficients, k, approximately
follow an Arrhenius relationship to give:
ln k = -19,700 + 40.4 for Lupersol 130
T
ln k= -19,700 + 41.6 for Lupersol 101
T
Where k = half-life reaction rate coefficient in
. -1, and T = temperature, K
polypropylene, m1n.
Having determined k, the following equation
may be used to find the amount of unreacted pro-
degradant after a given time~
CA = e~kt
CAo
where CA = concentration of unreacted prodegradant;
A = initial prodegradant concentration; and
t = reaction time, rnin.For example, after one rninute at 410 F. t483 t
50% of the original Lupersol 130 would be unrea~ted
as compared to only 10% of Lupersol 101 under
the same conditions.
In addition, it can be shown that
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the polymer viscosity exiting a piece of equip-
ment can be predicted by the following equation:
1 = 1 + KCR
Where: ~ = polymer viscosity exiting the equipment
after chemical degradation;
~ = viscosity the polymer would have had
exiting without chemical degradation;
K = chemical degradation efficiency co-
efficient; and
CR = amount of prodegradant reacted upon
exiting.
Since CR - CA - CA~ combining the above equations
gives the following equation: !
1 = 1 ~ K CA (1 - e~kt)
1~ ~1
Thus, for a constant (KCA) the ultimate polymer
viscosity will be the same after a long reaction
time regardless of prodegradant used. However,
the relationship between viscosity and time will
depend upon the half-life reaction rate coefficient,
k. For example, FIG. 1 is a graph of exiting
polypropylene polymer viscosity versus time based
on KCA - 0.005 poise~l (a typical value eOg. for
Lupersol 130 or 101) and an initial polymer vis-
cosity of 10,000 poise with the pelletizing~ex-
trusion processes carried out at 395 F. It dem-
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onstrates that the "pelletizing" viscosity of the
Lupersol 130 sample is about twice that of the
Lupersol 101 sample at a normal pelletizing time
range of 1 to 3 minutes although the ultimate
viscosities ~ould be about the same.
For a prodegradant with a short half-life,
the viscosity of the polypropylene upon exiting
a pelletizer at 395 F. after 1 minute residence
time is only 67~ of that i~ Lupersol 101 was used
and only 30~ of that if Lupersol 130 was used,
Thus, the latter is preferred, although the others
may be used.
Although, in the ~ase of Lupersol 130~ about
50% o~ the prode~radant may remain after pelletizing,
because the initial addition level is quite low,
there is little or no danger in handling the polymer.
After re-extrusion there will be essentially no
prodegradant remaining since typical proces~ing
conditions are at léast 460 F. at which the Lupersol
130 half-life coefficient is over 6 min.~l. With
an equipment residence time of only 2-1/2 minutes,
for example, only 0.000017~ of the peroxide in the
pellets would remain in the extrudate. For example,
if the polypropylene pellets had 0.2~ Lupersol
130 as received from the producer, the processor's
extrusion equipment was operated at 460 F., and the
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extruder residence time was 2-1/2 minutes, the
Lupersol 130 concentration in the polymer exiting
the extruder would be less than 1 part per billion.
Thus it is apparent that there has been pro-
vided in accordance with the invention, a poly~er
composition maintaining easy pelletization for
polymer producers while significantly improving
the processor's ability to use it and a method for
manufacturi~g the material that fully satisfy the
objects, aims and advantages set forth above.
t~hile the invention has been described in connection
with specific embodiments thereo~, it is evident
that many alternatives, modifications, and variations
will be apparent to those skilled in the art in light
o~ the foregoing description. Accordingly, it is
intended to embrace all such alternatives, modifi-
cations and variations as fall wi.hin the spirit
and broad scope of the appended claims.
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