Note: Descriptions are shown in the official language in which they were submitted.
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NOVEL POLYPROPYLENE COMPOSITIONS
This invention relates to polypropylene compositions
useful in the manufacture of fibers and films. In particular, the
invention relates to polypropylene compositions containing selected
additives which provide for reduced devolatilization as evidenced by a
reduction in the amount of smoke, oil and wax generated when such
compositions are spun or extruded into fibers or films.
BACKGROUND OF THE INVENTION
Because of their superior physical properties and ease of
processing, certain higher melt flow polypropylene resins have found
widespread use in the manufacture of fibers and films. Such high melt
flow polypropylene resins have been found to be particularly useful in
processes involving melt extrusion of fine fibers such as those produced
in the "spunbond" process and in the manufacture of thin films.
However, a major disadvantage in the use of these higher melt flow
polypropylene resins in processes where the extrusion temperatures
can range as high as 280°C or higher is the evolution of volatiles
which
condense leading to a build-up of oil or waxy deposits on the extrusion
equipment requiring frequent cleaning and the formation of aerosols in
the area surrounding the extrusion equipment creating an undesirable
atmospheric condition known in the art as "smoke". While this
problem has long been recognized in the art, recent developments
including greater environmental awareness and concerns for employee
safety as well as increased demands for production efficiencies has
heightened the need for an economically acceptable solution.
Considerable effort has been directed to solving the smoke
problem by reducing the amount of low molecular weight oligomers
formed in the polymerization reactor during production of the
polypropylene resins and during the cracking of such resins where
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necessary to obtain higher melt flow products. For example, USP
5,703,203 suggests that the presence of oligomers in polypropylene
polymers causes the emission of smoke during melt extrusion and
proposes a method of stripping oligomers from the resins before
extrusion. This approach has not provided a completely satisfactory
solution to the problem of reducing smoke because while the resin
being fed to the fiber or film extrusion equipment may contain smaller
amounts of oligomers, new oligomers and other lower molecular weight
volatiles are formed as a result of thermal oxidation reactions during
the high temperature extrusion process which devolatilize and
contribute to the formation of smoke and oily or waxy build-ups on the
extrusion equipment.
It has been suggested in the prior art that the presence of
antioxidants in polyethylene resins and low melt flow polypropylene
resins can reduce the emission of volatiles resulting from
thermaloxidation reactions which occur at the high temperatures
involved in industrial extrusion processes. In an article titled
"Effectiveness of Antioxidants: Suppression of Evolution of Gaseous
Degradation Products from Low-Density Polyethylene During Thermo-
Oxidation", Madsen et al, Polymer Degradation and Stability, Elsevier
Applied Science Publishers Ltd., England, 12 (1985), pgs. 131 to 140,
the authors evaluated the effect of a number of commercially available
antioxidants in suppressing the emission of volatiles produced during
thermal degradation of polyethylene. In an article titled "Thermal
Oxidation of Polypropylene in the Temperature Range of 120-280°C",
Hoff et al, Journal of Applied Polymer Science, Vol. 29, pgs. 465-480
(1984), the authors demonstrate that the presence of large amounts of
commercially available antioxidants in low melt flow polypropylene
resins can suppress the emission of certain low molecular weight
volatiles resulting from thermal degradation at high temperatures. The
authors also found that the suppression of acetone was enhanced by
combining an organic phosphite additive with the antioxidant. Neither
of these articles suggests using antioxidants or other additives to
suppress the emission of volatiles during the extrusion of high melt
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flow resins and in fact applicants have found in their work that the use
of such commercially available antioxidants alone or in combination
with organic phosphite additives in high melt flow resins does not
provide an adequate reduction in smoke when such resins are extruded
at high temperatures.
It is well known that polypropylene without the benefit of
protective additives will degrade during processing and longer term
exposure to sunlight and the atmosphere. Various classes of chemical
compounds are recognized as being useful to protect and improve the
performance of polypropylene by functioning, for example, as
antioxidants, process stabilizers, acid acceptors, lubricants, nucleating
agents and ultraviolet stabilizers. However, other than the suggestion
in the articles discussed above that certain antioxidants alone or in
combination with organic phosphites may suppress the emission of
certain volatiles during high temperature degradation of low melt flow
polypropylene, the art has not recognized the potential benefit of using
selective additives to suppress the emission of volatiles and thus reduce
the presence of smoke during melt extrusion of high melt flow
polypropylene resins.
N,N-dialkylhydroxylamines are recognized in the art as
being useful process stabilizers for polyolefins including polypropylene.
See for example USPs 4,590,231, 4,876,300 and WO 94/24344. USP
4,876,300 suggests that there are advantages in the use of long chain
N,N-dialkylhydroxylamines as process stabilizers for polyolefins and
teaches the use of such hydroxylamines in combination with other
additives including organic phosphites in stabilizing polyolefins
including low melt flow polypropylene. WO 94/24344 teaches a three
component system for stabilizing polypropylene which includes a long
chain N,N-dialkylhydroxylamine, an organic phosphite and a hindered
amine. This publication suggests that the hindered amine can replace
conventionally used phenolic compounds to provide long term heat
aging stability and that the three component system provides excellent
resistance to gas fading. None of these patents suggest using the
hydroxylamines alone or in combination with other additives such as
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organic phoshites to reduce the formation of smoke during melt
extrusion of high melt flow polypropylene resins.
SUMMARY OF THE INVENTION
The present invention relates to high melt flow
polypropylene compositions which provide for reduced smoke formation
when extruded into fibers or films. The inventive composition
comprises a high melt flow polypropylene resin, a stabilizing amount of
an N,N-dialkylhydroxylamine and a stabilizing amount of an organic
phosphite or phosphonite and, optionally, other conventional additives
such as antioxidants, acid acceptors, light stabilizers, ultraviolet light
absorbers and the like.
DETAILED DISCLOSURE
The novel polypropylene compositions of the present
invention comprise a high melt flow polypropylene resin, a stabilizing
amount of an N,N-dialkylhydroxylamine and a stabilizing amount of
an organic phosphite or phosphonite and, optionally, other conventional
additives.
High melt flow polypropylene resins useful in the
composition of the present invention can be homopolymers of
propylene, random copolymers of propylene with ethylene or higher
alpha-olefins such as butene containing at least 70% by weight (w)
preferably at least 80% w propylene, and polypropylene impact
copolymers. The homopolymer phase of such impact copolymers is
preferable a polypropylene homopolymer but may contain up to 5% w of
a comonomer such as ethylene or a higher alpha-olefin. The rubber
phase of the impact copolymer is a copolymer of ethylene and propylene
with an ethylene content of between 30% w and 90% w, preferable 45%
w to 75% w. The amount by weight of rubber phase present in the
impact copolymer ranges between 5% w to 50% w and preferably from
about 10% w and 35% w.
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The high melt flow polypropylene resins useful in the
present invention have a melt flow of at least 20 dg/min (as determined
by ASTM D-1238, Cond. L), and preferably in the range of 30 to 3000
dg/min and a narrow molecular weight distribution Q in the range of
about 2 to about 6 where 4.~ is defined as the gel permeation
chromatography (GPC) parameter which is equivalent to MW/Mn as will
be understood by those skilled in the art: Such high melt flow resins
can be obtained directly in the polymerization reactor using certain
"metallocene" catalysts such as those described, for example, in
"Metallocene Catalyzed Polymers", edited by G. M. Benedikt and B. L.
Goodall, Society of Plastics Engineers, Plastics Design Library series,
1998 or by "cracking" or "vis-breaking" a low melt flow reactor product
which is normally required for polypropylene resins produced using
Zeigler-Natta catalysts. The cracking or vis-breaking of polymexs is
well known and involves thermally and/or chemically degrading the
polymers to obtain a lower molecular weight product. Representative
processes for cracking polyolefin resins, including polypropylene, are
described in US Pat. Nos. 3,144,436; 3,887,534; 4,535,125 and
5,587,434.
High melt flow polypropylene resins useful in the
compositions of the present invention are advantageously prepared by
contacting a low melt flow propylene polymer with an effective amount
of an organic peroxide at elevated temperatures in an extruder. A
peroxide particularly useful in the cracking of polypropylene resins is
2,5-dimethyl-2,5 bis(t-butylperoxy)hexane. Other peroxides known in
the art could also be used. The amount of peroxide used and the
cracking temperature will depend upon the melt flow of the starting
polymer and the desired melt flow of the final product. Typically, the
amount of peroxide used will range between 25 ppm and 5000 ppm.
Temperatures in the extruder may range between 180° C and
320°C.
The selected N,N-dialkylhydroxylamines which are part of
the compositions of the present invention are known in the art as
secondary antioxidant additives useful for stabilizing polypropylene
during melt processing. Many specific examples of N,N-
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dialkylhydroxylamines useful in the present invention and methods of
preparation are specifically disclosed in U.S. Pat. Nos. 4,590,231;
4,782,105; 4,876,300 and 5,013,510, the disclosures of which are
incorporated herein by reference. Particularly useful in the
compositions of this invention are the long chain N,N-
dialkylhydroxylamines disclosed in U.S. Pat. No. 4,876,300 and having
the formula
R1R2NOH
wherein Rl and R2 are independently alkyl groups having 12 to 18
carbon atoms. The most preferred alkyl groups for R1 and R2 are the
alkyl mixture found in hydrogenated tallow amine. The amount of N,N-
dialkylhydroxylamine incorporated into the compositions of the present
invention to achieve the desired results will be in the range of 50 ppm
to 5000 ppm based on the total composition and preferably in the range
of 200 ppm to 1000 ppm.
The selected organic phosphates and phosphonites which
are part of the compositions of the present invention are known in the
art as being useful as secondary antioxidants and stabilizers for
polypropylene resins and are selected from the group consisting of:
Tris(2,4-di-tert-butylphenyl) phosphate (IRGAFOS 168);
2,4,6 tri-t-butylphenyl 2 butyl 2 ethyl 1,3 propanediol phosphate
ULTRANOX 641);
2,2',2"-nitrilo triethyl-tris(3,3',5,5'-tetra-tent-butyl-1,1'-biphenyl-
2,2'-diyl] phosphate (IRGAFOS 12);
Bis (2,4-dicumylphenyl) pentaerythritol diphosphite
(DOVERPHOS S-9228); and
Tetrakis (2,4-di-tert-butylphenyl) - 4,4' - biphenylene diphosphite
(SANDOSTAB P-EPQ).
Tris(2,4-di-tert-butylphenyl) phosphate is particularly useful and
preferred as the organic phosphate which is part of the compositions of
the present invention. The amount of organic phosphate or phosphonite
incorporated into the compositions of the present invention should be in
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the range of 100 ppm to 5000 ppm, preferably in the range of 500 ppm
to 2000 ppm .
The compositions of the invention also may contain
additives which may be generally termed stabilizers, antioxidants,
lubricants, acid acceptors, anti-static agents, nucleating additives and
additives which stabilize against radiation, such as ultraviolet (LTA
stabilizers and those that provide resistance to gamma irradiation.
Antioxidants which may be most useful in the
compositions of the present invention include primary antioxidants of
the phenolic-type. Their main function is to provide long-term thermal
stability which is usually required in fabricated articles such as fibers
and films. Preferred phenolic primary antioxidants include 1,3,5-tris-
(4-tent-butyl-3-hydroxy-2,6-dimethylbenzyl) 1,3,5-triazine-2,4,6-
(1H,3H,5H)-trione and tetrakis[methylene (3,5-di-tert-butyl-4-
hydroxyhydrocinnamate)] methane. Other useful antioxidants include
1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxy-benzyl) benzene;
octadecyl 3-(3',5'-di-tert-butyl-4'-hydroxyphenyl) propionate ; tris[3,5-di-
t-butyl-4-hydroxybenzyl) isocyanurate; 3,5-di-tert-butyl-4-
hydroxyhydrocinnamic acid triester with 1,3,5-tris(2-hydroxyethyl)-s-
triazine-2,4,6(1H,3H,5H)-trione; bis-[3,3-bis(4'hydroxy-3'tert-butyl-
phenyl)-butanoic acid]-glycolester; 2,2'-methylene-bis-(4-methyl-6-
tertiary-butylphenol)-terephthalate; 2,2 bis[4-(2-(3,5-di-tert-butyl-4-
hydroxyhydrocinnamoyloxy)) ethoxy-phenyl]propane ; calcium
bis[monoethyl(3,5-di-tert-butyl-4-hydroxybenzyl)phosphonate]; 1,2-
bis(3,5-di-tert-butyl-4-hydroxyhydrocinnamoyl)hydrazine; and 2,2-
oxamido bis[ethyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate].
Primary antioxidants such as those specified above may
advantageously be combined with secondary antioxidants such as
organic phosphites or phosphonites to provide the desired long term
thermal stability for the compositions of this invention. To achieve the
best results, it is desirable to keep the total amount of primary and
secondary antioxidants present under about 1.5% by weight of the
composition.
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Lubricants or mold release agents are typified by fatty
acid amides, examples of which include ethylene bis stearamide,
oleamide and erucamide.
Acid acceptors may be categorized as salts of fatty acid,
lactic acid salts and related derivatives, hydrotalcite-like compounds,
and certain metal oxides. Examples of each type in order include
calcium stearate, calcium lactate, DHT-4A, and zinc or magnesium
oxide.
Anti-static agents enhance static decay on molded parts.
Key examples include glyceryl monostearate and glyceryl distearate, as
well as mixtures thereof.
Nucleating additives are typified by benzoic acid salts
such as sodium, lithium or aluminum benzoate, minerals such as talc,
and organophosphorous salts such as NA-11 and MARK 2180.
Ultraviolet stabilization is provided by light absorbers
such as TINUVIN 327 or by hindered amine types such as CYASORB
3346, TINUVIN 622, TINUVIN 770 DF and CHIMASSORB 944.
Resistance against gamma irradiation is provided by
combinations of additives such as phosphorous containing secondary
antioxidants or the lactone type (e.g. HP-136), and hindered amines.
Additionally, Milliken's RS 200 additive is of benefit, as are mobilizing
additives such as mineral oil (cited in U. S. Patents Nos. 4,110,185 and
4,274,932). The latter is used in combination with a non-phenolic
secondary antioxidant and a hindered amine.
The various types of additives discussed above may be
used separately or blended with the primary antioxidants. This applies
to all the above additive types and further includes fillers like barium
sulfate, clays, calcium carbonate, silicates, pigments, such as titanium
dioxide, zinc oxide, lead chromate, cadmium sulfides, cadmium
selenide, zinc sulfide, basic carbonate of white lead; stabilizers such as
tribasic lead sulfate, basic lead chlorosilicate, dibutyl tin oxide and
other salts of lead, zinc, cadmium, tin, and the like; flame retardants
such as antimony oxide; ultra-violet stabilizers, slip agents, anti-block
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agents, and other solid additives which enhance the properties and
processability of the polymer to which they are added.
While the above listing seeks to provide key examples of
the different additive types, it is not to be viewed as limited by the
examples in scope. It is also recognized that certain of the above
additives are multi-functional, e.g., an acid acceptor such as calcium
stearate may also provide mold release performance, as may also be the
case with glyceryl monostearate. Further, combinations of any or all
types of additives given, or of additives within a given class, are
considered to be within the scope of the present invention.
Any additives, including the N,N-dialkylhydroxylamine
and organic phosphite or phosphonite compounds which are part of the
compositions of the present invention may be incorporated into the
high melt flow polypropylene resin by conventional techniques at any
convenient stage prior to or during the extrusion of the compositions
into shaped articles such as fibers or filins. For example, the additives
may be mixed with the polypropylene resin in dry powder form or as a
solution or suspension in an extruder prior to or during the process of
cracking the resin or the process of pelletizing the resin.
The effectiveness of the N,N-dialkylhydroxylamine and
organic phosphite or phosphonite additive system in reducing smoke
may be enhanced by employing the technique of "inerting" in the
equipment used to extrude the novel polypropylene compositions of this
invention and/or in the area surrounding such equipment. This
technique involves maintaining an inert gas atmosphere such as
nitrogen or carbon dioxide in the extrusion equipment including the
extruder and die and/or in an enclosed space surrounding such
equipment.
The following examples are intended to illustrate the
present invention but should not be construed as in any way as
limiting the scope of the invention. The amount of smoke generated in
the process of extruding several polypropylene compositions was
measured and compared. The polypropylene resin component was the
same in each composition tested and is identified as a polypropylene
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homopolymer having a melt flow (MF) of 38 dg/min which was obtained
by peroxide cracking a nominal 3 MF polypropylene homopolymer
produced in a gas phase reactor using a Zeigler-Natta catalyst. The
amount of various additives present in each composition shown in
Table I below is given in parts per million (ppm) by weight.
To conduct the smoke measurement tests, the
polypropylene resin composition to be tested was fed to a 5 inch slit
film die with a 0.020 inch die gap using a Brabender 3/4 inch single
screw extruder. The die was completely surrounded by an enclosure
having an exhaust chimney at the top which was connected to an
exhaust blower. The open bottom of the enclosure through which the
polymer extrudate exits was attached to a fabric enclosure which
surrounds the extrudate for a distance of 3 feet. With this
arrangement, the atmosphere surrounding the die is continuously
withdrawn through the exhaust blower. The amount of smoke
generated in the atmosphere surrounding the die during the extrusion
of each formulation tested was measured using a model HAM-1010
particle detector manufactured by PPM, Inc. This particle detector,
which works on a light scattering principal, was connected by
sampling tubing to the exhaust chimney which is attached to the die
enclosure. A flow meter and vacuum pump were connected to the
particle detector.
Various process parameters such as melt temperature, air
flow through the particle detector, extruder throughput and air flow
past the die were held constant to the extent possible during the
testing of all of the various polypropylene compositions. In each test,
the polypropylene composition was extruded through the slit die at a
throughput of approximately 2.25 kg/hr. The extrudate melt
temperature, measured with a hand held pyrometer near the die exit,
was 250°C. The exhaust blower was set to provide an air flow through
the exhaust chimney of approximately 50 liters per minute (lpm). The
vacuum pump and flow meter were adjusted to provide a flow of
approximately 1.5 lpm through the particle detector. Sufficient time
was allowed for the system to reach steady state conditions before
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commencing the collection of data. For each test run, the output of the
particle detector was recorded for 20 minutes. The recorded data was
then analyzed to determine an "average smoke density" expressed as
mg (smoke) per cubic meter of air.
To make a fair comparison of the smoke generated during
the extrusion of the various polypropylene compositions tested, three
20 minute runs were made for each composition. The "average smoke
density" was determined for each run from the data collected by the
particle detector and then the average of the three runs was calculated
and used as the "average smoke density" for that composition.
A Control average smoke density value was calculated
and used to determine the Smoke Indicator number for the
compositions tested which appears in Table I. The Control average
smoke density value was determined by calculating the average of the
average smoke densities of the first three compositions in Table I. The
Control average smoke density value thus represents the average of the
average smoke densities for the 9 test runs involved in Examples 1-3 in
Table I. The compositions in Examples 1-3 are essentially the same and
involve the presence of a known phenolic antioxidant process stabilizer,
Irganox 3114 sold by Ciba. The Smoke Indicator number is calculated
by dividing the average smoke density of the polypropylene
composition of each Example by the Control average smoke density.
The lower the Smoke Indicator number, the more resistant the
composition is to the generation of smoke during melt extrusion.
Nine polypropylene compositions were tested by the
procedure described above and their Smoke Indicator number was
determined. The additive content and the Smoke Indicator number for
each composition are presented as Examples 1-9 in Table I below.
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~.
o, o ~ ~ m o~omocmco
0 0 0 0 0 0
'a' 0 0 0
o , , , 0 0 0 , , ,
, , , o 0 0 ' ' °o
o~ 0 0
'"' , , , , , , o , °o
0
o o o
r..,, , , , o
m o 0 0 , ,
a~
H
0
d' 0 0
c~ , , , , , ~ , ~ ,
u~ ~°r~ u°~~ , ,°~ , 0 0
°o o° o° °o o° , o 0
N ~ ~t cat c~ ~7
a~
catm rr~ cc~. oo
v~
W
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The additives listed in Table I are identified as follows:
Zn0 - Zinc oxide
Cast - Calcium stearate
P 1240 - Calcium lactate
3114 - Irganox 3114 - Ciba - 1,3,5-Tris(3,5-di-tert-butyl-4-
hydroxybenzyl)-s-triazine-
2,4,6(1H,3H,5H)trione
1010 - Irganox 1010 - Ciba - tetrakis[methylene (3,5-di-
tert-butyl-4-hydroxyhydro -
cinnamate)] methane
168 - Irgafos 168 - Ciba - tris(2,4-di-tert-butylphenyl)
phosphate
FS 042 - Ciba - bis(hydrogenated tallow alkyl )
hydroxylamine
As can be readily seen from the Smoke Indicator numbers
in Table I, Examples 4-6, which are representative of the high melt
flow polypropylene compositions of the present invention, are
considerable better than those of the other compositions tested.
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