Note: Descriptions are shown in the official language in which they were submitted.
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POLYOXYMETHYLENE RESIN COMPOSITIONS HAVING
IMPROVED MOLDING CHARACTERISTICS
BACKSROUND OF THE INVENTION
The present invention relates to polyoxymethylene (also referred to herein as
polyacetal) resin compositions having improved moldability and improved
physical
properties of the molded article.
Polyacetal resin is manufactured by polymerizing a mostly formaldehyde
monomer or a formaldehyde trimer (trioxane). Acetal homopolymer is a
homopolymer of formaldehyde (for example, Delrin~ acetal resin, manufactured
by
E.I. du Pont de Nemours and Company). Acetal copolymer is obtained, for
example, by copolymerizing alkylene oxide with for example, trioxane.
Polyoxymethylene resin, because of its high mechanical strength, excellent
abrasion resistance, fatigue resistance, moldability, and the like, is
extensively used,
for example, in electrical and electronic applications, automotive
applications, and
precision machine applications.
Polyoxyrnethylene resins are the most crystalline of the engineering
polymers and as a consequence, freeze quickly in a mold. However, these resins
also have a high shrinkage. Recently, nucleated of polyoxymethylene resins
have
been introduced to improve set-up time and reduce shrinkage. But, further
improvements are desirable, especially if toughness can be improved.
Additionally,
it is desirable to eliminate or substantially eliminate voids in molded
articles.
SUMMARY OF THE INVENTION
25 The present invention comprises a blend of a polyoxymethylene resin with a
combination of a polyalkylene/ unsaturated carboxylic acid lower alkyl ester
nucleating material, a waxy de-nucleating material and a nucleant. Such
compositions have improved set-up time and reduced shrinkage and improved
elongation. Warpage is also reduced in asymmetrical parts. Voids are
eliminated or
3o substantially avoided in molded articles using the present combination.
Statistically
designed experiments have demonstrated a previously unrecognized synergy
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between the additives, i.e., an alkylene (meth) acrylate polymeric nucleatirig
material such as polyethylene methacrylate (EMA), a waxy de-nucleating
material
such as polyethylene wax and a nucleant such as talc. '
s BRIEF DESCRIPTION OF THE DRAWINGS
Figs. 1 a and 1 b show crystallization l l2 time plots for various
polyoxymethylene resin compositions.
Figs. 2a, 2b and 2c show photocopy reproductions of cut sections of bars
molded from polyoxymethylene resins containing various additives.
~o Figs. 3a and 3b show photocopy reproductions of cut sections of bars
molded from polyoxymethylene resins containing various additives.
Figs. 4a and 4b show photocopy reproductions of cut sections of bars
molded from polyoxymethylene resins containing various additives.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a composition containing a
polyoxymethylene resin and a combination comprising:
a) a polyalkylene/unsaturated carboxylic acid lower alkyl ester
nucleating material;
b) a waxy denucleating material; and
c) a nucleant.
More particularly, the instant composition contains a polyoxymethylene
resin and a combination comprising:
a) 0.5-3% by wt. of a polyalkylene/unsaturated carboxylic acid lower
z s alkyl ester nucleating material;
b) .1-1% by wt. of a waxy denucleating material; and
c) .OI-3% by wt. of a nucleant.
A preferred composition contains the polyoxymethylene resin and a
combination comprising:
3o a) 1-3% by wt. of the ester nucleating material;
b) 0.1-1 % by wt. of the waxy denucleating material; and
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c) 0.01-3% by wt. of the nucleant.
It should be understood that the percents by weight are based on the entire
composition. The remainder of the composition is comprised of the
polyoxymethylene resin and other additives.
The polyoxymethylene resins which can be used in the instant invention
include a wide variety of homopolymers and copolymers which are known in the
art. These polymers are generally polymers of formaldehyde in which the
polymer
chain, exclusive of the terminal portions of the chain, is a series of
methylene to
oxygen linkages. The polymer chain can also include moieties of the general
io formula:
R~- ~C)m- Rz
wherein m is an integer of 1 to S and R, and RZ are inert substituents which
will not cause undesirable reactions in the polymer. Such additional
components of
m the polymer chain are present as a minor proportion of the repeating units.
More specifically, the "polyoxymethylene resin" component as used herein
includes homopolymers of formaldehyde or a cyclic oligomer of formaldehyde,
the
terminal groups of which are end capped by esterification or etherification,
and
copolymers of formaldehyde or of a cyclic oligomer of formaldehyde and other
zo monomers that yield oxyalkylene groups with at least two adjacent carbon
atoms in
the main chain, the terminal groups of which copolymers can be hydroxyl
terminated or can be end capped by esterification or etherification.
The polyoxymethylene resin used in this invention can be linear or
substantially linear with only minor amounts of branching and will generally
have a
is weight average molecular weight in the range of 40,000 to 175,000,
preferably
50,000 to 150,000. It is preferred that the polyoxymethylene resin does not
contain
a measurable amount of branching. The molecular weight can conveniently be
measured by gel permeation chromatography in m-cresol at 160°C or
alternatively,
hexafluoroisopropanol at room temperature. Although polyoxymethylene resins of
3o higher or lower weight average molecular weights can be used, depending on
the
physical and processing properties desired, the polyoxymethyiene resins with
the
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above mentioned weight average molecular weight are preferred to provide
optimum balance of good mixing of various ingredients to be melt blended into
the
composition with the desired combination of physical properties in the
components
made from such compositions.
As indicated above, the polyoxymethylene resin can be either homopolymer
with different weight average molecular weights, copolymers of different
weight
average molecular weights or mixtures thereof. Copolymers can contain one or
more comonomers, such as those generally used in preparing polyoxymethylene
compositions. Comonomers more commonly used include alkylene oxides of 2-12
carbon atoms and their cyclic addition products with formaldehyde. The
quantity of
comonomer will not be more than 20 weight percent, preferably not more than 15
percent, and most preferably about 2 weight percent. The commonly used
comonomers include ethylene oxide, dioxalane, and butylene oxide. Generally,
polyoxymethylene homopolymer is preferred over copolymer because of its
greater
is tensile strength and stiffness. Preferred polyoxymethylene homopolymers
include
those whose terminal hydroxyl groups have been end capped by a chemical
reaction
to form ester or ether groups, preferably acetate or methoxy groups,
respectively.
The polyaIkylene/unsaturated carboxylic acid Iower alkyl ester polymeric
nucleating materials in general are copolymers or terpolymers of a lower
alkene (Cz-
2o C,) and a lower alkyl ester of an unsaturated acid. An example of polymeric
nucleating materials useful in this invention are ethylene-based polymers of
the
formula E/X/Y, preferably E/X. In the formula E/X, X is ethylene and Y is an
unsaturated carboxylic acid ester.
The unsaturated carboxylic acid esters include alkyl (C, to Cg, pref. C, to
C,)
is esters of unsaturated carboxylic acids having 3 to 8 carbon atoms.
Illustrative
unsaturated acids include acrylic and methacrylic acids. Particular examples
of
esters are methyl acrylate, ethyl acrylate, propyl acrylate, isopropyl
acrylate, n-butyl
acrylate, isobutyi acrylate, methyl methacrylate, ethyl methacrylate, propyl
methacrylate, isopropyl methacrylate, n-butyl methacrylate and isobutyl
so methacrylate, among which ethyl acrylate and methyl methacrylate are
preferred.
The preferred ethylene methyl acrylate copolymer ("EMA") component is, in
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general, a commercially available material and can be prepared by known means.
The amount of methyl acrylate in the EMA is generally 3-40 weight percent,
preferably 15-25 weight percent, of the EMA.
The polyalklyene/unsaturated carboxylic lower alkyl ester nucleating
additive can also be an ethylene-based random polymer of the formula E/X/Y
wherein E is ethylene, X is selected from methylmethacrylate, ethyl acrylate,
and
butyl acrylate, and Y is selected from glycidyl methacrylate and glycidyl
acrylate,
and glycidyl methacrylate is preferred for Y. E/XIY consists essentially of 5-
99%
E, 0-35% X, and 0.5-10% Y.
io An example of the ethylene-based random polymer consists essentially of
90%-99% by weight ethylene and 1%-10% by weight glycidylmethacrylate.
Preferably, this ethylene/glycidyl methacrylate ("EGMA") random polymer
consists
essentially of 90%-97% by weight ethylene and 3%-10% by weight glycidyl
methacrylate ("GMA").
is Another preferred ethylene-based random polymer consists essentially of
60%-98.5% by weight ethylene, 0.5-35% by weight butyl acrylate ("BA"), and 1%-
10% by weight glycidyl methacrylate ("GMA"). Preferably, this ethylenelbutyl
acrylate/glycidyl methacrylate ("EBAGMA") random polymer consists essentially
of SS%-84% by weight ethylene, 15%-35% by weight BA, and 1%-10% by weight
zo GMA. Most preferably, this EBAGMA random polymer consists essentially of
57.5%-74% by weight ethylene, 25%-35% by weight BA, and 1%-7.5% GMA.
The ethylene-based random polymer component can be prepared by
techniques readily available to those in the art. An example of the EBAGMA
random polymer is provided in U.S. Pat. No. 4,753,980.
zs The waxy denucleating component is a material that is capable of being
dispersed in the polyacetal resin and may be a liquid at normal room
temperatures.
Alternatively, if this material is a solid at normal room temperatures, it
must become
fluidized at a temperature lower than the temperature at which the polyacetal
is
being processed or molded. Examples of useful wax denucleating components are
3o natural or synthetic waxes, for example hydrocarbon and polymeric waxes.
Hydrocarbon waxes include mineral, petroleum, paraffin or microcrystalline
waxes
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and synthetic waxes, such as, for example ethylenic polymers or chlorinated
maphthalenes. The polymeric waxes include polyethylenes, polypropylenes and
ethylene/propylene copolymers. Preferred materials include paraffin wax, and
polyethylene wax. Good blending is obtained if the components are mixed
together
in a twin-screw extruder, which is the preferred mixing device.
The nucleant useful in the present invention is any finely divided solid, such
as boron nitride, talc, silica, polyimides, branched or crosslinked acetal
copolymer
or terpolymer, a melamine-formaldehyde resin, calcium carbonate, diatomite,
dolomite, or other commonly known nucleants for polyoxymethylene. Boron
s o nitride, terpolymers or talc are preferred, with the branched terpolymer
being most
preferred. The nucleant can optionally be surface treated by standard
processes.
The branched or crosslinked poly(oxymethylenes) useful as nucleants in the
invention may be obtained.
a. by copolymerization of trioxane with at least one compound reacting
multifunctionally and being copolymerizable with trioxane and, optionally,
with at
least one compound monofunctionally reacting and copolymerizable with
trioxane,
or
b. by branching or crosslinking reactions performed subsequently with a
linear poly(oxymethylene) having lateral or chainlinked functional groups, or
Zo c. by copolymerization of trioxane with at least one compound reacting
monofunctionally and being copolymerizable with trioxane and a branched or
crosslinked polyether or by reaction of a linear poly(oxymethylene) with a
branched
or crosslinked polyether.
Small average particle size is preferred for the nucleant. The average
zs particle size of the nucleant should be less than 20 microns, preferably
less than 10
microns, and most preferably less than 5 microns.
The nucleant may be an encapsulated nucleant. The encapsulated nucleant
as used herein consists essentially of an encapsulant polymer and a nucleant.
The encapsulant polymer can be any moderate melting polymer, i.e., any
3o polymer which melts at the processing temperatures of the polyoxymethylene
resin
of the encapsulated nucleant. Illustrative encapsulant polymers include linear
low
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density polyethylene ("LLDPE"), high density polyethylene {"HDPE"), and
polypropylene, each of which have a solid density of less than or equal to 1
gram
per cubic centimeter, as measured by ASTM D 1505. Preferably, the encapsulant
polymer is either LLDPE or HDPE. The encapsulant polymer either lacks long
chain polymer branching in its molecular structure or it is predominantly
linear. The
lack of long chain branching is due to the method by which the encapsulant
polymer
is produced.
The encapsulant polymer is selected from a group of polymers well known
in the art. The encapsuiant polymers are commercially available or,
alternatively,
io can be prepared by techniques readily available to those skilled in the
art.
Generally, the encapsulant polymers are prepared by polymerizing ethylene or
ethylene and alpha-olefin comonomers in solution phase or gas phase reactors
using
coordination catalysts, particularly Zieglar or Phillips type catalysts.
It is preferred that the LLDPE encapsulant polymer have a melt index, as
~s measured by ASTM D1238 method, condition E, in the range of 5 to 55 grams
per
min. It is preferred that the HDPE encapsulant polymer have a melt index, as
measured by ASTM D1238 method, condition E, of about 0.5-7 grams per 10 min.
Compositions containing LLDPE or HDPE having melt indices outside the range
given above may yield stock shapes with good porosity values, but can give
rise to
zo compounded resin and extruded stock shapes having other undesirable
characteristics, such as decreased stability or separation of the
polyoxymethylene
and LLDPE or HDPE (i.e., de-lamination).
Additives or ingredients that can have an adverse effect on the oxidative or
thermal stability of polyaxymethylene should be avoided.
zs The composition useful in the present invention may optionally include, in
addition to the components described above, other ingredients, modifiers, and
additives as are generally used in polyacetal compositions, including thermal
stabilizers and co-stabilizers, antioxidants, colorants (including pigments),
toughening agents(such as thermoplastic polyurethanes), reinforcing agents,
3o ultraviolet stabilizers (such as benzotriazoles or benzophenones),
including hindered
amine light stabilizers (especially those wherein the hindered nitrogen is of
tertiary
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amine functionality or wherein the hindered amine light stabilizer contains
both a
piperidine, or piperazinone ring, and a triazine ring), glass, and fillers.
Suitable
thermal stabilizers include polyamides (including a nylon terpolymer of nylon
66,
nylon 6/10, and nylon 6 and the polyamide stabilizer of U.S. Pat. No.
3,960,984);
s meltable hydroxy-containing polymers and copolymers, including ethylene
vinyl
alcohol copolymer and the stabilizers described in U.S. Pat. No. 4,814,397 and
U.S.
Pat. No. 4,766,168; non-meltable hydroxy-containing or nitrogen-containing
polymers as described in U.S. Pat. No. 5,011,890 and in particular,
polyacrylamide;
and microcrystalline cellulose; polybeta-alanine (as described in German
published
application 3715117); polyacrylamide; or stabilizers disclosed in U.S. Pat.
Nos.
4,814,397, 4,766,168, 4,640,949, and 4,098,984; and mixtures of any of the
above.
Typical antioxidants include hindered phenols such as triethyleneglycolbis{3-
(3'-
tertbutyl-4'hydroxy-5'methylphenyl)proprionate, N,N'-hexamethylenebis(3,5-di-
tert-
butyl-4-hydroxy-hydrocinnamide), and mixtures thereof as well as antioxidants,
is including those described in U.S. Pat. No. 4,972,014.
The compositions described herein may be prepared by mixing all
components with the acetal polymer at a temperature above the melting point of
the
acetal polymer by methods known in the art. It is known to use intensive
mixing
devices, such as rubber mills, internal mixers such as "Banbury" and
"Brabender"
Zo mixers, single or multiblade internal mixers with a cavity heated
externally or by
friction, "Ko-kneaders", multibarrel mixers such as "Farrel Continuous
Mixers",
injection molding machines, and extruders, both single screw and twin screw,
both
co-rotating and counter rotating, in preparing thermoplastic polyacetal
compositions.
These devices may be used alone or in combination with static mixers, mixing
25 torpedoes and/or various devices to increase internal pressure and/or the
intensity of
mixing, such as valves, gates, or screw designed for this purpose. Extruders
are
preferred, with twin screw extruders being most preferred. Of course, such
mixing
should be conducted at a temperature below which significant degradation of
the
polyacetal component will occur. Generally, polyacetal compositions are melt .
so processed at between 170°C and 290°C, preferably between
185°C and 240°C and
most preferably 195°C and 225°C.
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Shaped articles may be made from the compositions of the present invention
using methods known in the art, including compression molding, injection
molding,
extrusion, blow molding, rotational molding, melt spinning, and thermoforming.
Injection molding is preferred.
Examples of shaped articles include sheet, profiles, rod stock, film,
filaments, fibers, strapping, tape, tubing, and pipe. Such shaped articles can
be post
treated by orientation, stretching, coating, annealing, painting, laminating,
and
plating. Such shaped articles and scrap therefrom can be ground and remolded.
It will be understood by those skilled in the art that various modifications
io and substitutions may be made to the invention as described above without
departing from the spirit and scope of the invention. Accordingly, it is
understood
that the present invention has been described by way of illustration and not
limitation.
Experiments
is The following experiment was devised using experimental design software
known as "EChip", available from ECHIP inc., Hockessin Delaware. The designed
experiment includes duplicates which allows the statistician to estimate
experimental error. Because this was a designed experiment, it is not always
possible to compare pairs of experiments with just one variable changing.
Instead,
Zo one must rely on the statistical analysis for interpretation of the
results. The samples
were prepared using acetate capped polyoxymethylene (POM) having a weight
average molecular weight of nominally ?2,000. The examples listed in the table
were produced using a Werner and Pfleiderer 40 mm twin screw extruder. Fifty-
pound samples were prepared at 200 pounds per hour. The machine was run at 225-
Zs 400 rpm with barrel temperature settings of 200-225C. Samples containing
P561
wax and no EMA were difficult to run because the wax interfered with melting
of
the POM so the higher temperatures and machine RPM were used for these. EMA
is EMAC~ SP2245 from Chevron Chemical which is a copolymer of 80% ethylene,
20% methyl acrylate with a melt index of 2g110 min. P561 is a non-polar
3o hydrocarbon wax sold by Moore and Munger. Celcon~ U-l0 is a branched acetal
terpolymer manufactured by Hoeschst-Celanese (now Ticona). Ultratalc~ 609 is a
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talc supplied by Barretts Minerals, Inc. Zemid~ 641 is 40% talc in
polyethylene
supplied by DuPont. U-Talc 609/EMA is a 40% dispersion of the Ultratalc in the
EMA. U-Talc 609/P56I is a 40% dispersion of the Ultratalc in the P561.
s ~ TABLE 1
CompositionEMA P561% PercentU-Talc DSC Crystfn Mold
Number % U- Source T1/2 Cryst Time
Talc 158C (min)(sec)
I 0 0 0 ZEMID 641 2.9 6.7
2 3 1.5 0 ZEMID 641 3.28 7.3
3 0 1.5 0.08 ZEMID 641 2.18 6.5
4 3 1.5 0 U-TALC 3.31 6.9
609
S 3 0 0.08 U-TALC 2.39 5.6
609/P561
6 0 1.5 0.08 U-TALC 2.12 6.8
6091P561
7 0 0 0.08 U-TALC 1.96 5.9
609
8 3 0 0 U-TALC 7.2 7
609IEMA
9 0 1.5 0.08 U-TALC 2.03 6.3
6091EMA
3 l.S 0 U-TALC 13.43 7.2
609/PS61
11 3 0 0.08 U-TALC 2.14 5.8
609
12 3 0 0.08 ZEMID 641 2.74 5.6
13 0 1.S 0 U-TALC 8.42 6.4
609
i4 0 0 0 U-TALC 3.26 7
609/P561
3 1.S 0.08 U-TALC 2.26 6.1
609/EMA
i6 0 0 0 U-TALC 10.47 6.8
609IEMA
17 1.5 0.75 0.04 ZEM1D 641 2.87 6.6
18 L5 0.75 0.04 U-TALC 1.75 S.S
609
19 1.5 0.75 0.04 U-TALC 6.24 S.7
609/P561
L5 0.75 0.04 U-TALC 5.74 5.8
609/EMA
21 0 0 0 ZEMID64t 3.69 6.3
22 3 1.5 0 U-TALC 3.25 6.9
609
23 3 0 0.08 U-TALC 2.43 S.S
609IP561
24 3 0 0 U-TALC 7.94 6.6
A I I I 609IEMA I I
I 1
The crystallization half times were determined using a differential scanning
calorimeter (DSC). Ten mg of resin was heated in a Perkin-Elmer DSC 7 at
SOC/min. to 210°C, held for 3 minutes, then rapidly cooled at
200°Clmin. to a
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' crystallization temperature of 158°C, and held there until
completion. The half
times are taken from the peak positions of crystallization exotherms.
The in-mold crystallization times were obtained from plots of the melt
pressure curves during injection molding of the samples into 1X8X.070 inch
bars.
s The DSC and in-mold crystallization data measured for these materials was
then
statistically analyzed using the "EChip" DOE software to provide the graphical
illustration of the effect of EMA and P561 wax. Note in the presence of a talc-
based
nucleating agent, "Zemid", the wax which normally has the effect of slowing
crystallization (DSC half time increases) now has the opposite effect, it
increases
io crystallization rate (the time for half the DSC sample to crystallize at
158°C
decreases).
Note also inspection of the data in Table 2, below shows that EMA and wax
individually or in combination have little
effect on in-mold crystallization time, the combination with just 0.04% talc
nucleant
is has a greater effect than 0.08% nucleant by itself.
TABLE 2 - AVERAGE DATA
~.~
EXAMPLE EMA h P561 Ptrccnt TALC Cryst Ttmt (S)
% avg.
1 0 0 0 6.7
2 3 0 0 6.8
3 0 1.5 0 6.4
4 3 1.5 0 7.1
_...
.
0 0 0.08 5.9
6 3 0 0.08 5.8
7 0 1.5 0.08 6.8
8 1.5 0.75 0.04 5.7
9 1.5 0.75 1/.CeloonUlO 5.6
A plot of crystallization half time in a composition containing varying
amounts of EMA and P561 in a polyoxymethylene resin is shown in Fig. 1 a. Fig.
zo 1 b shows a plot of crystallization half time for a polyoxymethylene
composition
containing 0.080% nucleant (Zemid) and varying amounts of EMA and P561.
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Samples containing polyoxymethylene resins and the additives as shown in
Table 3 below were formed by injection molding to form bars on a cycle that is
too
short for an unnucleated resin to produce void-free parts. The bars were cut
lengthwise to reveal their internal structure. Photocopy reproductions of the
cut bars
s are shown in Figs. 2a, 2b and 2c. It can be seen that sample 1 (Fig. 2a), an
example
of the present invention is substantially void free as compared to the bars
shown in
Figs. 2b and 2c which do not contain the three additives used in the present
invention.
TABLE 3
nucleant/% EMA {PE Wax
1. Celcon U-10*/2% 1.5% .75%
i
2. O.I7% Zemid~ contains0 0
0.08% Talc
3. ~ 0 ( 0 ~ 0
~ o T drancned terpolymer of mw = 1 Uy,UUU
The additives shown in Table 4, below were compounded together on a
40mm W&P twin screw extruder and then molded into 1/4 inch thick tensile bars
on
a molding cycle which is too short for the unmodified control resin, #3. The
bars
were then machined to 1/8" thickness to reveal the internal porosity. Note the
is control, sample #3 has many voids while the sample 14, which is the object
of this
invention has very few. The latter is the more desirable for molding
applications.
TABLE 4
Run No. Acetal MW Additive Branched terpolymer
MW=109,000
3 85000 NONE NONE
6 85000 EMA/2,P561/.4NONE
12 85000 NONE 1
14 85000 EMA/2,P561L4 1
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Photocopy reproductions of the bars machined to show the internal porosity
are shown in Figs. 3a, 3b and Figs. 4a and 4b. Fig. 3a corresponds to sample 3
of
Table 4; Fig. 3b corresponds to sample 6 of Table 4; Fig. 4a corresponds to
sample
12 of Table 4; and Fig. 4b corresponds to sample 14 of Table 4.
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