U.S. patent application number 09/970282 was filed with the patent office on 2002-06-20 for propylene polymer foams.
Invention is credited to Hughes, Kevin R., Sammler, Robert L., Suh, Kyung W., Thoen, Johan A., Tusim, Martin H., Zhao, Jin.
Application Number | 20020077379 09/970282 |
Document ID | / |
Family ID | 22915402 |
Filed Date | 2002-06-20 |
United States Patent
Application |
20020077379 |
Kind Code |
A1 |
Hughes, Kevin R. ; et
al. |
June 20, 2002 |
Propylene polymer foams
Abstract
The present invention relates to a foam, a process for preparing
the foam and an article containing the foam. The foam contains a
coupled propylene polymer and has a density in of from 9.6 to 801
kg/m.sup.3 and has either a foamability factor of more than 1.8 to
less than 2.8 and an open cell content less than 20 percent, or a
foamability factor of at least 2.8 and less than 15 and an open
cell content of less than 50 percent. The process includes heating
a coupled propylene polymer having a melt flow rate from 0.2 to 20
g/10 min and a melt strength of at least 39 cN to a molten state to
produce a molten polymer material and mixing said molten polymer
material with a blowing agent under conditions to produce a foamed
material having a density in the range of from 9.6 to 801
kg/m.sup.3.
Inventors: |
Hughes, Kevin R.; (Hemlock,
MI) ; Sammler, Robert L.; (Midland, MI) ; Suh,
Kyung W.; (Midland, MI) ; Zhao, Jin; (Midland,
MI) ; Thoen, Johan A.; (Terneuzen, NL) ;
Tusim, Martin H.; (Midland, MI) |
Correspondence
Address: |
THE DOW CHEMICAL COMPANY
INTELLECTUAL PROPERTY SECTION
P. O. BOX 1967
MIDLAND
MI
48641-1967
US
|
Family ID: |
22915402 |
Appl. No.: |
09/970282 |
Filed: |
October 2, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60242590 |
Oct 23, 2000 |
|
|
|
Current U.S.
Class: |
521/142 |
Current CPC
Class: |
C08J 2323/12 20130101;
C08L 23/10 20130101; C08L 2203/14 20130101; C08J 2205/052 20130101;
C08J 9/141 20130101; C08J 9/106 20130101; C08J 9/14 20130101 |
Class at
Publication: |
521/142 |
International
Class: |
C08J 009/00 |
Claims
What is claimed is:
1. A foam comprising a coupled propylene polymer and having a
density in the range of from 9.6 to 801 kilograms per cubic meter
and having either a foamability factor of more than 1.8 to less
than 2.8 and an open cell content less than 20 percent, or a
foamability factor of equal to or more than 2.8 and less than 15
and an open cell content of less than 50 percent.
2. The foam of claim 1, wherein the foam has either a foamability
factor of more than 1.8 to less than 2.8 and an open cell content
less than 20 percent, or a foamability factor of equal to or more
than 2.8 and less than 6 and an open cell content of less than 50
percent.
3. The foam according to claim 1, wherein the coupled propylene
polymer has a melt strength of>39 centiNewtons, a drawability
from 15 to 60 millimeters per second, and a melt flow rate from 0.2
to 20 grams per 10 minutes.
4. The foam according to claim 3, wherein the propylene polymer is
isotactic.
5. A process for preparing a foam comprising heating a coupled
propylene polymer having a melt flow rate from 0.2 to 20 grams per
10 minutes and a melt strength of at least 39 centiNewtons,
optionally mixed with a nucleating agent, to a molten state to
produce a molten polymer material, and mixing said molten polymer
material with a blowing agent under conditions to produce a foamed
material having a density in the range of from 9.6 to 801 kilograms
per cubic meter.
6. The process according to claim 5, wherein the propylene polymer
has a melt strength of at least 39 centiNewtons and a drawability
from 15 to 60 millimeters per second.
7. An article comprising a foam according to claim 1.
Description
CROSS REFERENCE STATEMENT
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/242,590, filed Oct. 23, 2000.
FIELD OF THE INVENTION
[0002] This invention relates to propylene polymer foams, methods
for preparing the same, expandable compositions, and foamed
articles.
BACKGROUND
[0003] Polypropylene can offer better impact properties than
polystyrene, because polypropylene is a semi-crystalline polymer
which has a glass transition temperature substantially below room
temperature. In addition, polypropylene can offer good temperature
stability and high chemical resistance. However, until now, the
production of polypropylene foams has been limited because of its
low melt strength and melt elasticity and this makes it difficult
to be foamed in comparison to the other plastics. If the melt
strength and the melt elasticity are too weak, as in the case of
polypropylene, the cell walls separating the bubbles will be too
weak to bear the extensional force that is generated during the
foaming process and the bubbles will rupture very easily. As a
result, foamed polypropylene products are generally characterized
by high open cell content, which makes them unsatisfactory in many
applications.
[0004] Extruded closed cell foams of polypropylene having a
foamability factor of less than about 1.8 are disclosed in U.S.
Pat. No. 5,527,573. Branched or lightly crosslinked polymers, such
as may be obtained by chemical or irradiation branching or lightly
crosslinking, including high melt strength polypropylene are used
therein for making foams. In co-pending U.S. patent application
Ser. No. 09/133,576 filed Aug. 13, 1998 and WO-99/10424 in-situ
rheology modification of polyolefins is disclosed as applied to,
among others, polypropylenes resulting in polypropylenes having
advantageous melt processing properties. Foams are mentioned among
the possible end uses or applications of such rheology-modified
propylene polymers.
[0005] There is a continuous demand for propylene polymer foams of
a relatively high closed cell content, which do not suffer from the
bubble stability problem generally experienced with polypropylene
foams. The foams desirably are capable of being manufactured at
high foam production rates on conventional foaming equipment while
using a relatively low amount of blowing agent.
SUMMARY OF THE INVENTION
[0006] The present invention is directed in one aspect to a foam
comprising a coupled propylene polymer and having a density in the
range of from 9.6 to 801 kilograms per cubic meter (kg/m.sup.3)
(0.6 to 50 pounds per cubic foot (lbs/ft.sup.3)) and having either
a foamability factor of more than 1.8 to less than 2.8 and an open
cell content less than 20 percent (%), or a foamability factor of
equal to or more than 2.8 and less than 15 and an open cell content
of less than 50%.
[0007] The invention includes in a further aspect a process for
preparing a foam comprising heating a propylene polymer having a
melt flow rate from 0.2 to 20 grams per 10 minutes (g/10 min) and a
melt strength of at least 39 centiNewtons (cN), optionally mixed
with a nucleating agent, to a molten state to produce a molten
polymer material, and mixing said molten polymer material with a
blowing agent under conditions to produce a foamed material having
a density in the range of from 9.6 to 801 kg/m.sup.3 (0.6 to 50
lbs/ft.sup.3), wherein the propylene polymer is obtained by a
coupling treatment carried out before, during or after admixture
with the blowing agent.
[0008] According to yet a farther aspect the invention relates to
an expandable composition comprising a coupled propylene polymer
having a melt strength of >39 cN at a drawability from 15 to 60
millimeters per second (mm/sec), and a melt flow rate from 0.2 to
20 g/l 0 min, and a blowing agent.
[0009] Finally, the present invention relates to articles
comprising the foams according to the present invention. It has
been found that excellent foams can be obtained from certain
propylene polymers which due to their melt strength and melt
drawability characteristics provide higher thermal collapse
resistance, and therefore less open cell content and lower density
than prior art polypropylenes.
DETAILED DESCRIPTION OF THE INVENTION
[0010] As used herein the "foam density" is determined by weighing
a small representative piece of foam and dividing this by the
volumetric displacement of the foam.
[0011] As used herein the terms "melt strength" and "drawability"
refer to polymer melt properties and are measured at 190.degree. C.
according to the following procedure. Measure melt strength by
using a capillary rheometer fitted with a 2.1 millimeter (mm)
diameter, 20:1 die with an entrance angle of approximately 45
degrees. After equilibrating the samples at 190.degree. C. for 10
minutes, run the piston at a speed of 25.4 millimeters per minute
(mm/minute). The standard test temperature is 190.degree. C. The
sample is drawn uniaxially to a set of accelerating nips located
100 mm below the die with an acceleration of 2.4 millimeters per
second per second (mm/sec.sup.2). The required tensile force is
recorded as a function of the take-up speed of the nip rolls. The
maximum tensile force attained during the test (at break) is
defined as the melt strength and is expressed in cN. The limiting
wheel velocity at break is the melt drawability and reported in
units of mm/s. In the case of polymer melt exhibiting draw
resonance, the tensile force and wheel velocity before the onset of
draw resonance was taken as the melt strength and drawability,
respectively.
[0012] As used herein the "average cell size" in millimeters is
determined according to American Society for Testing and Materials
(ASTM) D3576 Standard Test Method for Cell Size of Rigid Cellular
Plastics.
[0013] As used herein the term "melt flow rate" refers to the melt
flow rate of the polymer measured according to method ASTM D 1238L,
at a temperature of 230.degree. C. under a weight of 2.16 kg and is
expressed in g/10 min.
[0014] As used herein the term "isotactic" refers to a degree of
isotacticity as measured by C.sup.13 NMR of at least about 50%.
[0015] As used herein, "propylene polymer" means propylene polymer
selected from the group consisting of (a) homopolymers of
propylene, (b) random and block copolymers of propylene and an
olefin selected from the group consisting of ethylene,
C.sub.4-C.sub.101-olefins, and C.sub.4-C.sub.10dienes, provided
that, when said olefin is ethylene, the maximum polymerized
ethylene content is less than about 20% by weight, when said olefin
is a C.sub.4-C.sub.101-olefin, the maximum polymerized content
thereof is less than about 20% by weight and when said olefin is a
C.sub.4-C.sub.10diene, the maximum polymerized content thereof is
less than about 20% by weight, (c) random terpolymers of propylene
and 1-olefins selected from the group consisting of ethylene and
C.sub.4-C.sub.8 1-olefins, provided that the maximum polymerized
C.sub.4-C.sub.8 1-olefin content is less than about 20% by weight,
and when ethylene is one of said 1-olefins, the maximum polymerized
ethylene content is less than about 20% by weight, and d) impact
propylene copolymers also referred to as heterophasic propylene
copolymers where polypropylene is the continuous phase and an
elastomeric phase is uniformly dispersed therein. Advantageously,
the impact copolymers have at least about 5 weight percent,
preferably at least about 10, preferably up to about 40, more
preferably up to about 25 weight percent, and most preferably up to
about 20 weight percent ethylene. The C.sub.4-C.sub.10 1-olefins
include the linear and branched C.sub.4-C.sub.10 1-olefins such as,
for examples 1-butene, isobutylene, 1-pentene, 3-methyl- 1-
buterie, 1-hexene, 3,4-dimethyl- 1-butene, 1-heptene, 3-methyl-
1-hexene, and the like. Examples of C.sub.4-C.sub.1o dienes include
1,3-butadiene, 1,4-pentadiene, isoprene, 1,5-hexadiene,
2,3-dimethyl-1,3- hexadiene, and the like.
[0016] The term "isotactic propylene polymer" refers to a propylene
polymer having preferably no or only a very minor percentage of
comonomers polymerized in its structure and has in general an
isotacticity of at least 50% as determined by .sup.13C NMR, more
preferably an isotacticity of at least 52%, and most preferably of
at least 54%. It preferably is a propylene homopolymer.
[0017] Surprisingly it has been found that a certain novel
combinations of propylene polymer resin properties and final
propylene polymer foam product properties produce the increased
closed cell foams of the present invention.
[0018] The foamability factor is the combination of these
properties. This factor is expressed by the following equation: 1 F
= Dn * D * ( tan ) 0.75
[0019] where
[0020] F is the foamability factor
[0021] Dn is the density of the foam in lbs/ft.sup.3 or
Dn=Dn*/16.02 wherein Dn* is the density of the
[0022] foam in kg m.sup.3;
[0023] D is the average cell size diameter in millimeters;
[0024] tan is G"/G'; where G" is the loss modulus; and G' is the
storage modulus of the polymer melt using 2.5 mm thick and 25 mm
diameter specimens at 190.degree. C. (centigrade) at one Radian per
second oscillating frequency.
[0025] According to the present invention the foams have a
foamability factor of more than 1.8 to less than 2.8, preferably
from 1.9 to 2.8, more preferably from 2.0 to 2.5 and simultaneously
an open cell content of less than 20%. Alternatively, the foams
have a foamability factor of equal to or more than 2.8 and less
than 15, preferably less than 10, more preferably less than 6,
still more preferably less than 5, even more preferably less than 4
and simultaneously an open cell content of less than 50%,
preferably of less than 45%.
[0026] The parameter tan may be determined by using a mechanical
spectrometer, such as model RMS-800, available from Rheometrics,
Inc. in Piscataway, New Jersey, USA. In the evaluation of
rheological characteristics, such as tan , G' and G", of
viscoelastic materials, such as polymer melts, a disk-like
specimen, measuring 2.5 mm in thickness and 25 mm in diameter is
placed between opposed, axially spaced apart, radially-extending
surfaces and is coupled to each surface, filling the axial spacing
between the surfaces. One of the surfaces then is rotated about the
axial direction relative to the other to place the test specimen in
shear and the torque resulting from the shear is measured. The
shear may be steady shear, in which case the measured torque is
constant, or the shear may be dynamic shear, in which case the
measured torque changes continuously with time. The measured torque
is proportional to the viscous, or loss component of the modulus
(G") of the material. For the purpose of this invention the shear
is steady shear, meaning the measured torque, and thus G", is
constant at the given temperature. As a result of the nature of the
forces applied to the test specimen in this procedure, the test
specimen has a tendency to expand axially, thereby placing axially
directed forces upon the relatively rotating surfaces to which the
specimen is coupled. This axial force exerted upon the surfaces by
the test specimen under shear conditions is proportional to the
elastic, or storage component of the modulus (G') of the material.
The parameter tan for the present invention is then calculated as
G" divided by G' at the stated temperature and oscillating
frequency.
[0027] In addition to meeting the foamability factor requirement,
the mixture of the blowing agent and propylene polymer material
(foaming gel) is cooled down to the optimum foaming temperature,
which is easily determined experimentally.
[0028] The melt flow rate of the propylene polymer used for
preparing the foams according to the present invention is generally
from 0.2 to 20 g/10 min, preferably from 0.3 to 10 g/10 min and
most preferably from 0.4 to 5 g/10 min as measured by ASTM D1238L
at 230.degree. C./2.16 kilograms (kg). At melt flow rates below
these ranges the polymer will be more difficult to extrude, while
above these ranges it will be difficult to make foam having large
cross-sections.
[0029] The propylene polymer in general has a melt strength of at
least about 39 cN, preferably at least about 40 cN, most preferably
at least about 50 cN, and in some instances at least about 60 cN.
The propylene polymer exhibits these melt strength values
preferably at a melt drawability of from 15 to 60 mm/sec. The
drawability property allows the drawing of the cells to small
diameters, whereas the melt strength provides sufficient strength
to form closed cells. Propylene polymers having such suitable melt
strength and melt drawability properties allow the production of
foams of a broad density range and of good mechanical properties
with the possibility of controlling cell size and open cell
content.
[0030] The propylene polymer is obtained from conventional
propylene polymers that have been treated to provide the desirable
properties melt properties by chemical treatment with coupling or
branching agents. Preferred coupling treatments are disclosed in
U.S. patent application Ser. No. 09/133,576 filed Aug. 13, 1998 and
WO 99/10424 published Mar. 4, 1999 (which are both incorporated by
reference herein in their entirety).
[0031] The propylene polymers used in the current invention are
thermoplastic polymers and not crosslinked polymers. Crosslinking
(otherwise known as "vulcanization") results in a thermoset polymer
characterized by high gel levels. Crosslinking is typically
evidenced by gel formation which is measured in the case of
polypropylene by xylene insolubility, or in the case of films by
optically evident gels in a film, for instance as analyzed by a
laser gel counter commercially available from Optical Control
System, Inc. under the trade designation FS-3.
[0032] The polymer materials that are subjected to such a treatment
are suitably of any molecular weight distribution (MWD). MWD is
calculated as the ratio M.sub.w/M.sub.n9, where M.sub.w, is the
weight average molecular weight and M.sub.n is the number average
molecular weight. Those skilled in the art are aware that polymers
having a MWD less than about 3 are conveniently made using a
metallocene or constrained geometry catalyst or using electron
donor compounds with Ziegler Natta catalysts. In the practice of
the invention, the MWD is preferably at least from about 3.5 to
about 15 and more preferably from about 6 to about 9.
[0033] The most preferred way of preparing the propylene polymers
used for making the foams of the current invention is by a coupling
treatment such as disclosed in U.S. patent application Ser. No.
09/133,576 filed Aug. 13, 1998 and WO 99/10424 published Mar. 4,
1999. As used herein, "coupling" refers to modifying the rheology
of a polymer by reacting the polymer with a suitable coupling
agent. A "coupled polymer" is a rheology modified polymer resulting
from a coupling reaction. Coupled polymers are also referred to
herein as "modified polymers" or "rheology modified polymers." A
coupled polymer differs from a crosslinked polymer in that the
coupled polymer is thermoplastic and has a low gel level.
[0034] According to this preferred treatment, the propylene polymer
resin is reacted with a polyfunctional compound capable of
insertion reactions into C-H bonds. Such polyfunctional compounds
have at least two, preferably 2, functional groups capable of
forming reactive groups that are capable of C-H insertion
reactions. Those skilled in the art are familiar with C-H insertion
reactions and reactive groups capable of such reactions. For
instance, carbenes as generated from diazo compounds, as cited in
Mathur, N.C.; Snow, M.S.; Young, K.M., and Pincock, J.A.;
Tetrahedron, (1985), 41(8), pages 1509-1516, and nitrenes as
generated from azides, as cited in Abramovitch, R.A.; Chellathurai,
T.; Holcomb, W.D; McMaster, I.T.; and Vanderpool, D.P.; J. Org.
Chem., (1977), 42(17), 2920-6, and Abramovitch, R.A., Knaus, G.N.,
J. Org. Chem., (1975), 40(7), 883-9.
[0035] Compounds having at least two functional groups capable of
forming reactive groups that are capable of C-H insertion under
reaction conditions are referred to herein as coupling agents. Such
coupling agents include alkyl and aryl azides (R-N.sub.3), acyl
azides (R-C(O)N.sub.3), azidoformates (R-O- C(O)-N.sub.3),
phosphoryl azides ((RO).sub.2-(PO)-N.sub.3), phosphinic azides
(R.sub.2-P(O)-N.sub.3)and silyl azides (R.sub.3- Si-N.sub.3).
Preferably, the coupling agent is a poly(sulfonyl azide). U.S.
patent application Ser. No. 09/133,576 filed Aug. 13, 1998 and WO
99/10424 published Mar. 4, 1999 contain additional teaching
regarding azides and their use for modifying polymers.
[0036] When the poly(sulfonyl azide) reacts with the propylene
polymer resin, at least two separate propylene polymer chains are
advantageously joined and the molecular weight of the polymer chain
is increased. In the preferred case when the poly(sulfonyl azide)
is a bis(sulfonyl azide) (hereinafter "BSA"), two propylene polymer
chains are advantageously joined.
[0037] The poly(sulfonyl azide) is any compound having at least two
sulfonyl azide groups (-SO.sub.2N.sub.3) reactive with the
propylene polymer. Preferably the poly(sulfonyl azide)s have a
structure X-R-X wherein each X is SO.sub.2N.sub.3 and R represents
an unsubstituted or inertly substituted hydrocarbyl, hydrocarbyl
ether or silicon-containing group, preferably having sufficient
carbon, oxygen or silicon, preferably carbon, atoms to separate the
sulfonyl azide groups sufficiently to permit a facile reaction
between the propylene polymer and the sulfonyl azide, more
preferably at least 1, more preferably at least 2, most preferably
at least 3 carbon, oxygen or silicon, preferably carbon, atoms
between functional groups. While there is no critical limit to the
length of R, each R advantageously has at least one carbon or
silicon atom between X's and preferably has less than about 50,
more preferably less than about 20, most preferably less than about
15 carbon, oxygen or silicon atoms. Silicon containing groups
include silanes and siloxanes, preferably siloxanes. The term
inertly substituted refers to substitution with atoms or groups
that do not undesirably interfere, at the coupling reaction
conditions, with the desired reaction(s) or desired properties of
the resulting coupled polymers. Such groups include fluorine,
aliphatic or aromatic ether, siloxane as well as sulfonyl azide
groups when more than two propylene polymer chains are to be
joined. R is suitably aryl, alkyl, aryl alkaryl, arylalkyl silane,
siloxane or heterocyclic, groups and other groups that are inert
and separate the sulfonyl azide groups as described. More
preferably R includes at least one aryl group between the sulfonyl
groups, most preferably at least two aryl groups (such as when R is
4,4' diphenylether or 4,4'-biphenyl). When R is one aryl group, it
is preferred that the group have more than one ring, as in the case
of naphthylene bis(sulfonyl azides). Poly(sulfonyl)azides include
such compounds as 1, 5-pentane bis(sulfonylazide), 1,8-octane
bis(sulfonyl azide), 1,10-decane bis(sulfonyl azide),
1,10-octadecane bis(sulfonyl azide), 1-octyl-2,4,6-benzene
tris(sulfonyl azide), 4,4'-diphenyl ether bis(sulfonyl azide),
1,6-bis(4'-sulfonazidophenyl)hexane, 2,7-naphthalene bis(sulfonyl
azide), and mixed sulfonyl azides of chlorinated aliphatic
hydrocarbons containing an average of from 1 to 8 chlorine atoms
and from about 2 to 5 sulfonyl azide groups per molecule, and
mixtures thereof. Preferred poly(sulfonyl azide)s include
oxy-bis(4-sulfonylazidobenzene), 2,7- naphthalene bis(sulfonyl
azido), 4,4'-bis(sulfonyl azido)biphenyl, 4,4'-diphenyl ether
bis(sulfonyl azide) and bis(4-sulfonyl azidophenyl)methane, and
mixtures thereof.
[0038] Sulfonyl azides are commercially available or are
conveniently prepared by the reaction of sodium azide with the
corresponding sulfonyl chloride, although oxidation of sulfonyl
hydazines with various reagents (nitrous acid, dinitrogen
tetroxide, nitrosonium tetrafluoroborate) has been used.
[0039] The following discussion regarding the coupling reaction
mechanism provides the inventors current theories but is not
intended to limit the scope of this invention. Sulfonyl azides
decompose in several ways, but for the practice of the invention,
the reactive species, believed to be the singlet nitrene, as
evidenced by insertion into C-H bonds is desired. Thermal
decomposition is reported to give an intermediate singlet sulfonyl
nitrene, which will react readily by insertion into carbon-hydrogen
bonds. The high temperatures necessary for efficient formation of
the sulfonyl nitrene is usually greater than about 150.degree. C.
When BSA such as, 4,4'-Oxydibenzenesulfonyl azide (DPO- BSA) is
used for the coupling agent, polymer stream temperatures of greater
than 250.degree. C. are preferably avoided while there is
significant unreacted azide in the reaction mixture.
[0040] The poly(sulfonyl azide) is preferably mixed with the
propylene polymer before the resulting mixture is heated to the
decomposition temperature of the poly(sulfonyl azide). By
decomposition temperature of the poly(sulfonyl azide) is meant that
temperature at which a substantial percentage of the azide is
converted to the sulfonyl nitrene, eliminating nitrogen and more
heat in the process. The decomposition temperature may be
determined by differential scanning calorimetry (DSC). For
instance, a differential scanning calorimeter (DSC) thermogram of
the DPO-BSA shows no change in the heat flow until a sharp
endothermic melting peak is observed at 100.degree. C. The baseline
is flat again (no heat flow) until a broad exothermic peak is
observed that begins about 150.degree. C, peaks at 185.degree. C.
(referred to herein as the peak decomposition temperature) and is
complete by 210.degree. C. The total amount of energy released due
to decomposition of the sulfonyl azide groups is about 1500
Joules/gram. Preferably, the poly(sulfonyl azide) is heated to at
least the peak decomposition temperature. The poly(sulfonyl azides)
used advantageously have a peak decomposition temperature greater
than about 150.degree. C., preferably greater than about
160.degree. C., more preferably greater than about 180.degree.
C.
[0041] The amount of poly(sulfonyl azide) is preferably at least
about 50 parts per million by weight (ppm), more preferably at
least about 75 ppm, most preferably at least about 100 ppm, and in
some instances, preferably at least about 150 ppm. In the practice
of the invention, formation of crosslinked networks to an extent
that would result in intractable propylene polymer is to be
avoided; therefore, poly(sulfonyl azide) is preferably limited to
that amount which results in chain coupled or rheology modified
(but not substantially crosslinked) propylene polymer, preferably
less than about 2000 ppm, more preferably less than about 1500 ppm,
most preferably less than about 1300 ppm poly(sulfonyl azide) based
on the total weight of propylene polymer. Substantial crosslinking
is characterized by the presence of gels of sufficient size or
weight percentage such that the processing of the film is
detrimentally affected. Such detrimental effects include increased
operating amperage, discontinuities in or undispersed materials in
the film, increased back pressure, and/or, partial die plugging due
to gels or black specs. The amount to be used depends on the melt
flow rate of the starting and targeted propylene polymers and can
be determined by the skilled person.
[0042] The propylene polymer and coupling agent are suitably
combined in any manner which results in desired reaction thereof,
preferably by mixing the coupling agent with the polymer under
conditions which allow sufficient mixing before or during reaction
to avoid unnecessary or undesirable amounts of localized reaction.
An undesirable amount is an amount that interferes with the purpose
of the final product. In a preferred embodiment the process of the
present invention takes place in a single vessel, that is mixing of
the coupling agent and polymer takes place in the same vessel as
heating to the decomposition temperature of the coupling agent. The
vessel is most preferably a twin-screw extruder, but preferably a
single-screw extruder or advantageously a melt mixer, including a
batch mixer. The reaction vessel more preferably has at least two
zones of different temperatures into which a reaction mixture would
pass.
[0043] In the most preferred embodiment, the propylene polymer and
the coupling agent are physically mixed at a temperature that is
low enough to minimize the reaction between the coupling agent and
the polymer. Such physical mixing can occur in any equipment, such
as V-blenders, ribbon or paddle blenders, tumbling drums, or
extruders, which will mix the coupling agent and the propylene
polymer. The term extruder is used for its broadest meaning to
include such devices as a device that extrudes pellets as well as
an extruder which produces the extrudate for forming into articles,
such as a film.
[0044] Preferably, this physical mixing occurs in the early stages
of an extruder, most preferably a twin screw extruder. In
particular, this embodiment may be practiced by simultaneously
introducing the propylene polymer resin and the coupling agent into
the feed section of an extruder. The extruder is configured to have
a first section that physically mixes and conveys the coupling
agent and polymer in a manner that minimizes the reaction between
the coupling agent and the polymer. The conveying first section is
followed by at least a second section where the coupling agent and
polymer are rapidly further mixed and sufficient heat is added to
cause significant reaction between the coupling agent and
polymer.
[0045] In another embodiment, the mixing is preferably attained
with the polymer in a molten or at least partially melted state,
that is, above the softening temperature of the polymer, or in a
dissolved or finely dispersed condition rather than in a solid mass
or particulate form. Any mixing equipment is suitably used in this
embodiment, preferably equipment which provides sufficient mixing
and temperature control in the same equipment, but advantageously
practice of this embodiment takes place in such devices as an
extruder, melt mixer, pump conveyor or a polymer mixing devise such
as a Brabender melt mixer. While it is within the scope of this
embodiment that the reaction take place in a solvent or other
medium, it is preferred that the reaction be in a bulk phase to
avoid later steps for removal of the solvent or other medium.
[0046] Melt phase mixing is advantageous for forming a
substantially uniform admixture of coupling agent and polymer
before exposure to conditions in which chain coupling takes place.
Conveniently for this embodiment, the formation of a substantially
uniform admixture occurs along a temperature profile within
equipment such as an extruder. The first zone is advantageously at
a temperature at least equal to the softening temperature of the
polymer(s) and preferably less than the decomposition temperature
of the coupling agents and the second zone is at a temperature
sufficient for decomposition of the coupling agent. Especially in
the case of propylene polymers, most preferably the propylene
polymer(s) and coupling agent are exposed to a profile of melt
stream temperatures ranging from about 160.degree. C. to about
250.degree. C.
[0047] Those skilled in the art recognize that a polymer, or
mixture thereof, typically melts over a range of temperatures
rather than melting sharply at one temperature. For the practice of
this embodiment, it is sufficient that the polymer be in a
partially melted state. For convenience, the temperature of this
degree of melting can be approximated from the differential
scanning calorimeter (DSC) curve of the polymer or mixture thereof
to be treated.
[0048] Conveniently, when there is a melt extrusion step between
production of the polymer and its use, at least one step of the
process of the invention takes place in the melt extrusion step.
The heat produced during the extrusion step provides the energy
necessary to cause the reaction between the coupling agent and the
target polymer.
[0049] For all embodiments, a temperature of at least the
decomposition temperature of the coupling agent is preferably
maintained for a time sufficient to result in decomposition of at
least sufficient coupling agent to avoid later undesirable
reaction, preferably at least about 80, more preferably at least
about 90, most preferably at least about 95 weight percent of the
coupling agent is reacted. Those skilled in the art realize that
this time is dependent on whether the temperature is one at which
the coupling agent slowly decomposes or one at which it very
rapidly decomposes. Preferably, the time will be at least about 5
seconds, more preferably at least about 10 seconds to avoid
unreacted coupling agent, and subsequent undesirable reactions, or
to avoid the need for inconveniently, possible destructively high
temperatures. Conveniently, the reaction time is about 20
seconds.
[0050] Additives are optionally included in the propylene polymers.
Additives are well within the skill in the art. Such additives
include, for instance, stabilizers including free radical
inhibitors and ultraviolet wave (UV) stabilizers, neutralizers,
nucleating agents, slip agents, antiblock agents, pigments,
antistatic agents, clarifiers, waxes, resins, fillers such as
silica and carbon black and other additives within the skill in the
art used in combination or alone. Effective amounts are known in
the art and depend on parameters of the polymers in the composition
and conditions to which they are exposed.
[0051] In addition, other thermoplastic polymers may be blended
with the propylene polymers provided the desired foam properties
are achieved. Examples of these include high- and low- density
polyethylenes, ethylene-vinyl aromatic interpolymers, polybutene-1,
ethylene-vinyl acetate copolymer, ethylene-propylene rubber,
styrene-butadiene rubber, ethylene-ethyl acrylate copolymer and the
like, that may be mixed into the above-mentioned propylene polymer,
so long as the latter is the main component in the resulting
mixture and the mixture is of uniform quality.
[0052] The foams of the present invention may be prepared by a
process comprising heating a propylene polymer having a melt flow
rate from 0.2 to 20 g/10 min and a melt strength of at least 39 cN,
optionally mixed with a nucleating agent to a molten state to
produce a molten polymer material, and mixing said molten polymer
material with a blowing agent under conditions to produce a foamed
material having a density in the range of from 9.6 to 801
kg/m.sup.3 (0.6 to 50 lbs/ft.sup.3), wherein the propylene polymer
is obtained by a coupling treatment carried out before, during or
after admixture with the blowing agent. The molten polymer material
is most preferably mixed with a blowing agent under conditions that
preclude foaming to form a foamable gel, and then exposing the
foamable gel to conditions conducive to foaming to produce a foamed
material. A foam is conveniently prepared by heating a propylene
polymer having has a melt strength >39 cN at a drawability from
15 to 60 mm/sec, to form a plasticized or melt polymer material,
incorporating therein a blowing agent to form a foamable gel, and
extruding the gel through a die to form the foam product. Prior to
mixing with the blowing agent, the polymer material is heated to a
temperature at or above its glass transition temperature or melting
point. The blowing agent is optionally incorporated or mixed into
the melt polymer material by any means known in the art such as
with an extruder, mixer, blender, or the like. The blowing agent is
mixed with the melt polymer material at an elevated pressure
sufficient to prevent substantial expansion of the melt polymer
material and to generally disperse the blowing agent homogeneously
therein. Optionally, a nucleator is blended in the polymer melt or
dry blended with the polymer material prior to plasticizing or
melting. The foamable gel is typically cooled to a lower
temperature to optimize physical characteristics of the foam
structure. The gel is optionally cooled in the extruder or other
mixing device or in separate coolers, preferably to a temperature
above, more preferably at least about 5.degree. C. above, most
preferably up to about 40.degree. C. above, the softening point or
crystallization temperature of the propylene polymer. The gel is
then extruded or conveyed through a die of desired shape to a zone
of reduced or lower pressure to form the foam structure. The zone
of lower pressure is at a pressure lower than that in which the
foamable gel is maintained prior to extrusion through the die. The
lower pressure is optionally superatmospheric or subatmospheric
(evacuated or vacuum), but is preferably at an atmospheric level.
Preferably, the gel is extruded into a zone of atmospheric pressure
at a temperature that is about the melting temperature of the
polypropylene, or less than the melting temperature, but above the
crystallization temperature of the polypropylene. The melting
temperature and crystallization temperatures are determined by a
DSC test method.
[0053] Other suitable processes are a coalesced foam process as
described in U.S. Pat. No. 4,824,720 and an accumulating extrusion
process described in U.S. Pat. No. 4,323,528, both hereby
incorporated by reference.
[0054] U.S. Pat. No. 4,824,720, which describes the coalesced (or
strand) foam process. This patent describes a method for providing
a closed cell foam structure comprising a plurality of coalesced
extruded strands or profiles by extrusion foaming of a molten
thermoplastic composition utilizing a die containing a multiplicity
of orifices. The orifices are so arranged such that the contact
between adjacent streams of the molten extrudate occurs during the
foaming process and the contacting surfaces adhere to one another
with sufficient adhesion to result in a unitary structure. The
individual strands of coalesced foam should remain adhered into a
unitary structure to prevent strand delamination under stresses
encountered in preparing, shaping, and using the foam.
[0055] U.S. Pat. No. 4,323,528 describes an accumulating extrusion
process. In this accumulating extrusion process low density,
elongated cellular bodies having large lateral cross-sectional
areas are prepared by: 1) forming, under pressure, a mixture of a
thermoplastic polymer and a blowing agent, with the mixture having
a temperature at which the viscosity of the mixture is sufficient
to retain the blowing agent when the mixture is allowed to expand;
2) extruding the mixture into a holding zone maintained at a
temperature and pressure which does not allow the mixture to foam,
the holding zone having an outlet die defining an orifice opening
into a zone of lower pressure at which the mixture foams, and an
openable gate closing the die orifice; 3) periodically opening the
gate; 4) substantially concurrently applying mechanical pressure by
a movable ram on the mixture to eject the mixture from the holding
zone through the die orifice into the zone of lower pressure, at a
rate greater than that at which substantial foaming in the die
orifice occurs and less than that at which substantial
irregularities in cross-sectional area or shape occurs; and 5)
permitting the ejected mixture to expand unrestrained in at least
one dimension to produce an elongated thermoplastic cellular
body.
[0056] In the practice of the invention where a propylene polymer
is used which is obtained by reaction with a coupling agent, such
as a poly(sulfonyl azide), this coupling treatment may be carried
out before, during or after admixture with the blowing agent.
Conveniently the coupling agent is admixed with the polymer
preferably before or optionally during admixture with the blowing
agent and the admixture is heated at least to the decomposition
temperature of the poly(sulfonyl azide) for a period sufficient to
result in coupling before the foam is formed.
[0057] Blowing agents (also referred to herein as foaming agents)
useful in making the present foam include inorganic agents, organic
blowing agents and chemical blowing agents. Suitable inorganic
blowing agents include carbon dioxide, nitrogen, argon, water, air,
nitrogen, and helium. Organic blowing agents include aliphatic
hydrocarbons having 1-9 carbon atoms, aliphatic alcohols having 1-3
carbon atoms, and fully and partially halogenated aliphatic
hydrocarbons having 1-4 carbon atoms. Aliphatic hydrocarbons
include methane, ethane, propane, n-butane, isobutane, n-pentane,
isopentane, neopentane, and the like. Aliphatic alcohols include
methanol, ethanol, n-propanol, and isopropanol. Fully and partially
halogenated aliphatic hydrocarbons include fluorocarbons,
chlorocarbons, and chlorofluorocarbons. Examples of fluorocarbons
include methyl fluoride, perfluoromethane, ethyl fluoride,
1,1-difluoroethane (HFC-152a), 1,1,1-trifluoroethane (HFC- 143a),
1,1,1,2-tetrafluoroethane (HFC-134a), 1,1,2,2-tetrafluoromethane
(HFC-134), 1,1,1,3,3-pentafluorobu- tane (HFC-365mfc),
1,1,1,3,3-pentafluoropropane (HFC_245fa), pentafluoroethane,
difluoromethane, perfluoroethane, 2,2-difluoropropane,
1,1,1-trifluoropropane, perfluoropropane, dichloropropane,
difluoropropane, perfluorobutane, perfluorocyclobutane. Partially
halogenated chlorocarbons and chlorofluorocarbons for use in this
invention include methyl chloride, methylene chloride, ethyl
chloride, 1,l,1-trichloroethane, 1,1-dichloro-l-fluoroethane
(HCFC-141b), 1-chloro-1,1-difluoroethane (HCFC-142b),
chlorodifluoromethane (HCFC-22), 1,1-dichloro-2,2,2-
trifluoroethane (HCFC-123) and 1-chloro-1,2,2,2-tetrafluoroethane
(HCFC-124). Fully halogenated chlorofluorocarbons include
trichloromonofluoromethane (CFC-11), dichlorodifluoromethane
(CFC-12), trichlorotrifluoroethane (CFC-113),
1,1,1-trifluoroethane, pentafluoroethane, dichlorotetrafluoroethane
(CFC-114), chloroheptafluoropropane, and dichlorohexafluoropropane.
Chemical blowing agents include azodicarbonamide,
azodiisobutyro-nitrile, benzenesulfonhydrazide, 4,4-oxybenzene
sulfonyl-semicarbazide, p-toluene sulfonyl semi- carbazide, barium
azodicarboxylate, N,N'-dimethyl-N,N'-dinitrosoterephthalamide, and
trihydrazino triazine.
[0058] Though the present process optionally employs any known
blowing agent, preferred blowing agents are volatile blowing agents
having a boiling point temperature range of-50.degree. C.
to+50.degree. C. and include, but are not limited to aliphatic
hydrocarbons such as n-pentane, isopentane, neo-pentane, isobutane,
n-butane, propane and the like; fluoro-chlorinated hydrocarbons
such as dichlorotetrafluoroethane, trichlorotrifluoroethane,
trichloromonofluoromethane, dichlorodifluoromethane,
dichloromonofluoromethane and the like; and so on. Among them, the
non-fully halogenated hydrocarbons are preferable in point of
environmental considerations. Particularly preferred among the
non-fully halogenated hydrocarbons are partially or fully
fluorinated hydrocarbons and non-fully halogenated
fluoro-chlorinated hydrocarbons. Examples of these include
1-chloro-1-fluoroethane and 1,1-difluoroethane. Particularly
preferred among the aliphatic hydrocarbons are isobutane and
isobutane/n-butane mixtures. Also contemplated are combinations of
these blowing agents with minor amounts of CO.sub.2, H.sub.2O,
N.sub.2 and argon in the mixtures.
[0059] In general, incorporation of a greater amount of blowing
agent results in a higher expansion ratio (the term "expansion
ratio" herein referred to means the ratio (density of
resin)/(density of expanded product)) and thus a lower foam
density. However care must be taken not to incorporate an amount of
blowing agent that causes a separation between resin and blowing
agent during the foam conversion process. When this happens,
"foaming in the die" occurs, the surface of the expanded product
becomes rough and no good expanded product is obtainable. The
amount of blowing agent incorporated into the polymer melt material
to make a foam-forming polymer gel preferably is from about 0.04 to
6.0 gram-moles per kilogram of polymer, more preferably from about
0.05 to 6.0 gram-moles per kilogram of polymer, and most preferably
from about 0.055 to 6.0 gram-moles per kilogram of polymer.
[0060] A nucleating agent is optionally added to control the size
of foam cells. Preferred nucleating agents include inorganic
substances such as calcium carbonate, talc, clay, titanium dioxide,
silica, barium stearate, calcium stearate, diatomaceous earth,
mixtures of citric acid and sodium bicarbonate, and the like. The
amount of nucleating agent employed advantageously ranges from
about 0.01 to about 5 parts by weight per hundred parts by weight
of a polymer resin. The preferred range is from 0.1 to about 3
parts by weight.
[0061] In general, an increased amount of nucleating agent gives a
smaller diameter of cell. However, if the amount exceeds 5 parts by
weight, agglomeration or insufficient dispersion of nucleating
substance occurs, so that the diameter of the cell becomes greater.
On the contrary if the amount is less than 0.01 part by weight, the
nucleating action is too feeble to decrease the diameter of the
cells.
[0062] The present foam preferably has a density of from about 9.6
to 801 kg/m.sup.3 (0.6 to 50 lbs/ft.sup.3) measured according to
ASTM D-1622-88, more preferably from about 9.6 to 721 kg/m.sup.3
(0.6 to 45 lbs/ft.sup.3), most preferably from 9.6 to about 641
kg/m.sup.3 (0.6 to about 40 lbs/ft.sup.3). Preferred low density
foams have a foam density ranging from 16 to 320 kg/m.sup.3 (1 to
20 lbs/ft.sup.3).
[0063] The foam preferably has an average cell size of about 6 mm
or less measured according to ASTM D3576-77.
[0064] The foam optionally takes any physical configuration known
in the art such as bead, sheet or plank or coalesced strands. The
foam is particularly suited to be formed by extrusion into a region
formed of air, vacuum or partially vacuumed environment, or
water.
[0065] The foam is optionally closed cell or open cell. A typical
amount of closed cells is more than 50 percent. Preferred
closed-cell foams have greater than 80 percent closed cell content
measured according to ASTM D2856-87.
[0066] Various additives are optionally incorporated in the foam
structure such as inorganic fillers, pigments, dyes, antioxidants,
acid scavengers, ultraviolet absorbers, flame retardants,
processing aids, extrusion aids, and the like.
[0067] According to a further aspect the present invention relates
to an expandable composition comprising a coupled propylene polymer
having a melt strength of>39 cN, a drawability from 15 to 60
mm/sec, and a melt flow rate from 0.2 to 20 g/10 min, and a blowing
agent. The expandable composition may comprise the blowing agent,
as well as a nucleating agent and other additives as mentioned in
this specification, in the amounts described herein.
[0068] The foam according to the present invention is particularly
suited for use in automotive interior and exterior applications for
energy absorption and comfort cushioning, such as automotive
headliners, door liners, energy absorption counter measure. It can
be used in general cushion packaging and comfort cushioning market.
It can be used as thermal insulation foams for insulating tanks,
trucks, and cars as well as walls and roofs in residential and
commercial buildings, refrigerators, etc. It can be used as sound
insulation foams in automobiles as well as in buildings.
[0069] Additionally, expanded or expandable beads can be formed
that can be subsequently molded into desired shapes for any of the
above applications, including impact energy management, cushion
packaging, bulk dunnage packaging and the like. The beads can also
be molded into panels for use in insulating applications such as
for roofing, walls, tanks, and the like.
EXAMPLES
[0070] The following examples are to illustrate this invention and
do not limit it. Ratios, parts, and percentages are by weight
unless otherwise stated. For the examples below, unless otherwise
indicated: melt flow rate (MFR) is measured in accordance with ASTM
D 1238, at a temperature of 230.degree. C. under a weight of 2.16
KG. The yield modulus as described in the examples refers to the
compressional yield modulus [MPa] defined as the stress at yield
divided by strain at yield as measured in a standard compression
test as determined using method ASTM D3575 Suffix D.
Example 1
[0071] Isotactic homopolypropylene pellets, commercially available
from Montell USA under the name Profax 6231 having a MFR of 20 g/10
min, a Melt Strength: 0.76 cN at 190.degree. C., a Drawability of
106.9 mm/s at 190.degree. C. is used as the base resin (also
referred to as Resin A). Resin A was tumble blended with 0.17 parts
of silicone oil Dow Corning 550 fluid at room temperature for 30
minutes followed by tumble blending with 0.1 parts
oxy-bis(4-sulfonyl azide) ("BSA") and 0.1 parts phenolic
antioxidant commercially available from Ciba Geigy under the trade
designation Irganox 1010 for an additional 30 minutes at room
temperature (parts are relative to 100 parts of base resin). The
resulting mixture was then fed into a 30 mm co-rotating,
intermeshing twin-screw extruder (200 rpm), with a temperature
profile of 150, 200, 200, 200 .degree. C. from the rear to the
front of the extruder. It had a length to diameter ratio of 30,
four heating zones, and was powered by a 11.2 kW (15 horsepower)
motor. The extruder had a die-temperature of 230-250.degree. C. to
ensure full reaction of the BSA with the propylene polymer. The die
pressure was 2.83-2.96 MPa (410-430 psi). The torque was: 27.1-28.3
NM (240-250 in.lb). Polymer was metered to the extruder (about 13.6
Kg/hr (30 lb/hr)) using a AccuRate volumetric feeder. The extrudate
from the two strand (3.0 mm dia. die holes) die was passed through
two water baths in series, and granulated using a Conair model 304
pelletizer. The resulting material is referred to herein as the
Resin B. The resulting sample is characterized by: MFR: 2.36, Melt
Strength: 69 cN at 190 .sup.2C, Drawability: 35 mmls at 190.degree.
C.
[0072] Foam was produced from Resin B, using a continuous process,
by blending Resin B in an extruder with 0.5 pph talc, 0.5 pph
calcium stearate agent and 0.1-0.3 pph antioxidant (for instance
Irganox 1010). 6 pph of iso-butane blowing agent was added to the
molten polymer at high pressure and temperature to ensure
homogeneous distribution. The melt temperature was then cooled
down, at which temperature nucleation of the bubble in the die can
occur as the pressure decreases. This temperature is referred to as
"foaming temperature". The melt was extruded through a die to room
temperature and pressure, which then results in an expanding foam
and cell stabilization.
[0073] Foam was produced on a pilot foaming line in the form of a
strand foam. The extrusion rate was constant at 18.1 kg/h (40 lb/h)
of polymer. The line had a 44.5 mm (1 3/4") single screw extruder,
a mixer, coolers and a die. The die was a standard strand-foam-die
with 1.1 mm (0.042") diameter holes and 3.7 mm (0.144")
hole-to-hole distance. Each hole has a 60.degree. included approach
angle (30.degree. from the centerline). Cooler exit temperature:
156.degree. C. Die face temperature: 160.degree. C. Die pressure:
2.94 MPa (426 psi). Extruder pressure: 6.28 MPa (911 psi). Rollers
at the die exit were used on both top and bottom of the strand
foam. Line speed: 0.071 m/s (14 fpm).
[0074] The resulting foam is characterized by: density of 45
kg/m.sup.3 (2.81 pcf) (measured 1 hr. after foaming); an average
cell size of 0.96 mm; an open cell content of 36% (Beckman number);
and a porosity of 13.1% (water absorbance).
Example 2
[0075] Analogous to the procedures in Example 1, Resins C, D, and E
are prepared by: reacting an isotactic polypropylene resin of a
melt flow rate of 20 g/10 min with 1200 ppm of BSA in a 40-mm-
diameter extruder (Resin C); reacting an isotactic polypropylene
resin (Profax 6231 available form Montell) of a melt flow rate of
20 g/10 min with 1200 ppm of BSA in a 92-mm-diameter extruder
(Resin D); and reacting an isotactic polypropylene resin of melt
flow rate 20 g/10 min (Dow Polypropylene H701-20) with 1000 ppm of
BSA in a 30-mm-diameter extruder (Resin E).
[0076] Foams are prepared from Resins C, D, and E under conditions
analogous to those in example 1, using the specific conditions
listed in Table 1. As comparative Examples, foams are prepared from
Profax PF814 and Profax 6823 (both available from Montell),
designated as resins F and G, respectively.
[0077] These examples demonstrate that according to the present
invention foams can be prepared with a lower open cell content, or
a higher closed cell content, than the comparative foams at the
same foamability factor. This is surprising because according to
U.S. Pat. No. 5,527,573 a lower foamability factor is required to
achieve a similar low open cell content: a foamability factor of
less than 1.8 for an open cell content of less than 20% or a
foamability factor between 1.8 and 2.8 for an open cell content in
the range of 20-50%.
[0078] A comparison of the foams of Resin D, Run 5 and of
Comparative Resin F, Run 2 shows that according to the present
invention while using a lower amount of blowing agent a foam of the
same foamability factor is obtained yet with remarkably decreased
content of open cells.
1TABLE 1 Density Cell Open Yield Foaming isobutane [kg/m.sup.3
Size, tan .delta., Foamability Cell, modulus, Resin Run T,
[.degree. C.] [wt. %] (pcf)] [mm] resin Factor [%] [MPa] C 1 165 6
46.1 1.03 1.033 3.0 35 -- (2.88) C 2 162 6 48.1 0.91 1.033 2.8 26
-- (3) C 3 160 6 48.9 0.9 1.033 2.8 24 -- (3.05) C 4 158 6 49.2
0.95 1.033 3.0 28 -- (3.07) C 5 158 6 53.0 0.96 1.033 3.3 25 --
(3.31) D 1 154 7.5 44.1 0.96 1.24 3.1 43 -- (2.75) D 2 156 7.5 41.5
1.24 1.24 3.8 34 -- (2.59) D 3 154 7.5 42.9 1.24 1.24 3.9 43 --
(2.68) D 4 152 7.5 47.4 0.97 1.24 3.4 32 -- (2.96) D 5 162 5.8 38.4
0.8 1.24 2.3 15 4.64 (2.4) E 1 158 6 45.0 0.96 1.18 2.9 36 4.43
(2.81) F* 1 158 6.5 42.6 0.8 1.17 2.4 22 -- (2.66) F* 2 158 6.5
40.4 0.82 1.17 2.3 37 -- (2.52) F* 3 155 6.5 38.3 0.85 1.17 2.3 26
-- (2.39) G* 1 152 16 17.6 0.3 1.24 0.4 11 -- (1.1) *Comparative
examples
* * * * *