FIELD OF THE INVENTION
This invention is in the field of fluoropolymers having
functional side groups, and is concerned with articles selected from wire insulation,
cable jacket, hose, tubing and convoluted tubing made from such fluoropolymers.
BACKGROUND OF THE INVENTION
Thermoplastic fluoropolymers are well known for outstanding
combinations of properties including chemical resistance, unique surface characteristics,
high service temperatures, and good dielectric characteristics. As a result, fluoropolymer
resins are used in a wide variety of applications including wire insulation, cable
jacket, hose, tubing, film, linings for chemical process equipment, articles for
fluid handling in laboratory and manufacturing situations, and the like. The service
temperature in some of these applications can be high. As is common for thermoplastics,
some properties of fluoropolymers change as temperature increases. Modulus and tensile
strength, for example, typically decrease with increasing temperature, and fabricated
articles such as tubing and wire insulation typically have lower cut-through resistance
at high temperature.
Fluoropolymer articles having improved properties, particularly
at high temperature, are desired.
The use of functional groups in fluoropolymers has been
aimed at modifying the surface characteristics of fluoropolymers. For example, Kerbow
in U.S. Patent 5,576,106 discloses a grafted fluoropolymer powder that is effective
in such uses as an adhesive to join dissimilar materials, Iura et al. in
European Patent Application Publication EP 0 761 757 disclose a fluorine-containing
polymer alloy of a grafted fluorine-containing polymer and a polymer containing
no fluorine, and Shimizu et al. in European Patent Application Publication
0 626 424 disclose a blend of a thermoplastic resin and a fluoropolymer having a
functional group that improves interfacial affinity with the thermoplastic resin.
SUMMARY OF THE INVENTION
This invention provides articles selected from wire insulation,
cable jacket, hose, tubing and convoluted tubing made from melt-fabricable fluoropolymer
resin and having improved mechanical properties. Preferred such articles include
tubing and wire insulation, which exhibit improved cut-through resistance.
The fluoropolymer resin from which such articles are made
has pendant functional groups. Such groups can be produced by grafting a polar-functional
compound to the fluoropolymer.
It has been discovered that addition of functional groups
to fluoropolymers can result in surprising improvement of bulk properties. Improved
properties are evident at room temperature, but are accentuated at elevated temperatures.
The articles of this invention are fabricated by conventional
melt processing techniques. For example, articles can be formed by melt extrusion,
using such techniques as pressure extrusion or extrusion accompanied by melt draw,
or by injection molding. The processes of fabricating the articles of the present
invention are new in that the melt-fabricable fluoropolymer resin used has pendant
Generally, when articles of this invention are fabricated
by melt extrusion, a cross-section of such articles normal to the extrusion direction
is characterized by a perimeter that is a closed figure. Such figures include, for
example, regular geometric shapes such as circles, ellipses, rectangles and the
like, and also irregular shapes including shapes that are either minor or major
departures from regular shapes. Preferably, extruded articles of the invention have
general axial symmetry though not necessarily exact axial symmetry. For example,
convoluted tubing having helical convolutions does not have exact axial symmetry,
but is considered for purposes of the present invention to have general axial symmetry
(or near axial symmetry). Extruded articles are wire insulation, cable jacket, tubing
including convoluted tubing, and hose.
Articles of the present invention exhibit improved mechanical
properties. Such improvements include increased stiffness and increased toughness,
the latter indicated, e.g., by increased force required to cut through a thin section.
Such improvement can be evident at room temperature, but may be more pronounced
at elevated temperature. For wire insulation of the present invention, substantial
increases in cut-through resistance have been observed at 150°C. See Example
1, which discloses an increase of about 100%. Such large increases are not required
for articles of the invention. Cut-through resistance of wire insulation or tubing
at 150°C, for example, is generally at least 25% greater than for such articles
made from comparable fluoropolymer resin but not having pendant functional groups,
and is preferably at least 50% greater. Stiffness (flex modulus) is also increased.
For example, flex modulus at room temperature is generally increased by at least
10% relative to that of the same article made from similar fluoropolymer lacking
pendant functional groups.
The fluoropolymer having pendant functional groups exhibits
a reduction in melt flow rate (increase in melt viscosity) after being exposed to
elevated temperature such as may be encountered during melt fabrication of the article.
This reduction in melt flow indicates that cross-linking occurs, even in the absence
of cross-linking promoter or catalytic agent.
Articles of the present invention are fabricated essentially
from fluoropolymer resin having pendant functional groups. The articles consist
essentially of such resin in the sense that other substance or material which may
react with said pendant functional groups is not present. Thus, for example, no
cross-linking promoter or co-agent or such material is present. Likewise, no fluorine-free
polymer is present as a component of a blend or alloy, or as a reinforcing component.
However, articles of the invention can contain inert additives present in minor
amount, such as pigments, or present merely as filler, such as carbon. Additives
such as thermal or oxidative stabilizers that are normally present in fluoropolymer
resins can be present in usual amounts in articles of the invention.
As used herein, "functionalized fluoropolymer" means fluoropolymer
having functional side groups or functional groups attached to side groups, i.e.,
pendant functional groups. Usually, but not necessarily, such functional units are
at the ends of the pendant side groups. Fluoropolymer that does not have such pendant
functional groups is sometimes described herein as "non-functional fluoropolymer".
Thus, non-functional fluoropolymer and functionalized fluoropolymer differ at least
by the presence in the latter of pendant functional groups. Non-functional fluoropolymer
can be a precursor to functionalized fluoropolymer, in which instance the process
of functionalizing involves addition of functional groups to the non-functional
polymer. However, "functionalizing" is also used in a broader sense herein to include
preparation of functionalized fluoropolymer which would be non-functional if pendant
functional groups were not present, even though non-functional fluoropolymer may
not be the precursor.
Functional groups, in the context of the present invention,
are groups capable of improving the mechanical properties of fluoropolymer articles
when such functional groups are present in a fluoropolymer composition. Improvement
of mechanical properties is indicated, for example, by an increase in cut-through
resistance (e.g., as measured for wire insulation or for tubing) or by an increase
in stiffness (flex modulus), as discussed above. Functional groups that improve
the properties of fluoropolymers when present are acid (including carbon-, sulfur-,
and phosphorus-based acid) and salt and halide thereof, anhydride and epoxide. Preferred
functional groups include anhydride, especially maleic anhydride. As one skilled
in the art will recognize, more than one type of functional group can be present.
Normally, however, a single type of functional group is used.
Such functional groups can be introduced, for example,
by incorporating into the fluoropolymer, during polymerization, monomer units having
such functional groups, i.e., functional monomers, or by having an ethylenically
unsaturated compound grafted thereto which imparts polar functionality to the fluoropolymer,
the polar functionality being present as part of the ethylenically unsaturated compound.
Such grafted fluoropolymer includes the grafted fluoropolymer powder described in
U.S. Patent 5,576,106 and the grafted fluoropolymer described in EP 0 650 987. Other
known methods of grafting can be used. Preferred polar-grafted fluoropolymers include
the surface-grafted powder of the '106 patent. Examples of polar functionality provided
by grafting include acids, including carboxylic, sulfonic and phosphonic acids and
salts thereof, and epoxides. Glycidyl methacrylate is an example of a grafting compound
that provides epoxide functionality. Among compounds for grafting onto and thereby
becoming part of the polar-grafted fluoropolymer, maleic acid and maleic anhydride
are preferred. Maleic anhydride can be halogen-substituted, e.g., dichloromaleic
anhydride and difluoromaleic anhydride.
The concentration of functional groups in the fluoropolymer
resin component, i.e., in the functionalized fluoropolymer or in blends of functionalized
fluoropolymer plus non-functional fluoropolymer, if non-functional fluoropolymer
is present, of the melt-fabricable fluoropolymer composition of this invention is
effective to improve mechanical properties of the article fabricated by melt extrusion
or injection molding. For example, the cut-through resistance at 150°C of wire
insulation, tubing, or any other article like in claim 1 that can be subject to
the cut-through test is improved by at least 25% relative to the same article made
from comparable fluoropolymer but having no pendant functional groups, and is preferably
improved by at least 50%. Alternatively, the flex modulus at room temperature is
increased by at least 10% over the same article made from similar fluoropolymer
having no pendant functional groups. As will be recognized by one skilled in the
art, the concentration of functional groups that is effective to achieve improvement
may vary at least with the type of functional group. The concentration of functional
groups present can be expressed relative to the number of main chain carbon atoms
in the fluoropolymer resin. Generally, the concentration of functional groups present
is at least about 25/106 main chain C atoms, based on total fluoropolymer
in the composition. The concentration of functional groups is usually in the range
of 25-2500 per 106 main chain C atoms, preferably in the range of 50-2000
per 106 main chain C atoms, based on total fluoropolymer present.
The desired concentration of functional groups in the functionalized
fluoropolymer resin can be achieved with a single fluoropolymer having functional
groups, or a mixture of such fluoropolymers. The desired concentration of functional
groups can also be achieved by blending functionalized fluoropolymer (or mixture
of) having a higher concentration of functional groups with non-functional fluoropolymer
(or mixture of), i.e., fluoropolymer having essentially no functional groups. In
this embodiment, functionalized fluoropolymer acts as a functional group concentrate
that can be let down (diluted) with non-functional fluoropolymer. This approach
has the advantage of permitting one to achieve a variety of functional group concentrations
with a single functionalized fluoropolymer by varying the blending ratio with non-functional
fluoropolymer, and is a preferred embodiment of the invention. Preferably, in a
functionalized fluoropolymer that is a blend, the functionalized fluoropolymer component
is in minor (lesser) amount relative to non-functional fluoropolymer component.
Thus, in one embodiment of the present invention, the cross-linkable
fluoropolymer composition contains minor amounts of functionalized fluoropolymer
and a major amount of non-functional fluoropolymer. By "major amount" is meant at
least 50 wt%, preferably at least 70 wt%, of non-functional fluoropolymer based
on combined weight of non-functional fluoropolymer and functional fluoropolymer.
In this embodiment of the invention, then, the concentration of functional groups
in the functionalized fluoropolymer will be high enough so that the average concentration
of functional groups in the functional fluoropolymer plus the non-functional fluoropolymer
will be at least about 25/106 main chain C atoms, usually in the range
of 25-2500 per 106 main chain C atoms, and preferably in the range of
50-2000 per 106 main chain C atoms.
A wide variety of fluoropolymers can be used. The fluoropolymer
is made from at least one fluorine-containing monomer, but may incorporate monomer,
which contains no fluorine or other halogen. Fluorinated monomers include those
which are fluoroolefins containing 2 to 8 carbon atoms and fluorinated vinyl ether
(FVE) of the formula CY2=CYOR or CY2=CYOR'OR wherein Y is
H or F and -R- and -R'- are independently completely fluorinated or partially fluorinated
linear or branched alkyl and alkylene groups containing 1 to 8 carbon atoms. Preferred
R groups contain 1 to 4 carbon atoms and are preferably perfluorinated. Preferred
R' groups contain 2 to 4 carbon atoms and are preferably perfluorinated. Hydrocarbon
monomers that can be used include ethylene, propylene, n-butylene, and iso-butylene.
When the fluoropolymer is to be functionalized by grafting, preferably at least
one monomer contains hydrogen, and in that regard the hydrogen/fluorine atomic ratio
in the polymer is preferably at least 0.1/1. The fluoropolymer, however, preferably
contains at least 35 wt% fluorine. Fluoropolymer resins that can be used include
copolymers of TFE with one or more copolymerizable monomers chosen from perfluoroolefins
having 3-8 carbon atoms and perfluoro(alkyl vinyl ethers) (PAVE) in which the linear
or branched alkyl group contains 1-5 carbon atoms. Preferred perfluoropolymers include
copolymers of TFE with at least one of hexafluoropropylene (HFP) and PAVE. Preferred
comonomers include PAVE in which the alkyl group contains 1-3 carbon atoms, especially
2-3 carbon atoms, i.e. perfluoro(ethyl vinyl ether) (PEVE) and perfluoro(propyl
vinyl ether) (PPVE). Preferred fluoropolymers also include the copolymers of ethylene
with perhalogenated monomers such as TFE or chlorotrifluoroethylene (CTFE), such
copolymers being often referred to as ETFE and ECTFE, respectively. In the case
of ETFE, minor amounts of additional monomer are commonly used to improve properties
such as reduced high temperature brittleness. PPVE, PEVE, perfluorobutyl ethylene
(PFBE), and hexafluoroisobutylene (HFIB) are preferred additional comonomers. ECTFE
may also have additional modifying comonomer. Other fluoropolymers that can be used
include vinylidene fluoride (VF2) polymers including homopolymers and
copolymers with other perfluoroolefins, particularly HFP and optionally TFE. TFE/HFP
copolymer which contains a small amount of VF2, which copolymer is often
referred to as THV, can also be used. Examples of perfluorinated copolymers include
TFE with HFP and/or PPVE or PEVE. Representative fluoropolymers are described, for
example, in ASTM Standard Specifications D-2116, D-3159, and D-3307. Such fluoropolymers
are usually partially-crystalline as indicated by a non-zero heat of fusion associated
with a melting endotherm as measured by DSC on first melting. Alternatively or additionally,
preferred fluoropolymers are non-elastomeric, as opposed to elastomeric.
Functionalized fluoropolymers include fluoropolymers such
as those described in the foregoing paragraph and additionally containing copolymerized
units derived from functional monomers. If the concentration of functional monomer
is high enough in a TFE copolymer, however, no other comonomer may be needed. Usually,
but not necessarily, the functional groups introduced by such monomers are at the
ends of pendant side groups.
Functional monomers that introduce pendant side groups
having desired functionality have the general formula CY2=CY-Z wherein
Y is F and Z is -Rf-X, wherein Rf is a fluorinated diradical
and X is a functional group that may contain CH2 groups. The functional
groups are acid (including carbon- sulfur-, and phosphorus-based acid) and salt
and halide thereof, anhydride and epoxide. Preferred functional groups include anhydride,
especially maleic anhydride. As one skilled in the art will recognize, more than
one type of functional group can be present. Normally, however, a single type of
functional group is used. Preferably, Rf-X is linear or branched perfluoroalkoxy
having 2-20 carbon atoms, so that the functional comonomer is a fluorinated vinyl
ether. Examples of such fluorovinylethers include
In this formula, m = 0.3 and n = 1.4
Preferred comonomers that introduce pendant functional
groups include maleic anhydride, dichloromaleic anhydride, difluoromaleic anhydride,
and maleic acid.
When functionalized fluoropolymer is achieved by copolymerization,
the amount of functional monomer in the functionalized fluoropolymer of this invention
is small to achieve the desired concentration of functional groups, even when functionalized
fluoropolymer is a blend comprising non-functional fluoropolymer. Generally, the
amount of functional monomer is no more than 10 wt%, preferably no more than 5 wt%,
based on total weight of functionalized fluoropolymer, i.e., the fluoropolymer component
containing the functional monomer. In certain instances, higher concentrations of
functional monomer exceeding 10 wt% may be desired, for example, when it is not
desired to use a non-functional monomer in the functionalized melt-fabricable fluoropolymer.
While the functionalized fluoropolymer can be uniform, it is not necessary to have
a uniform concentration of functional monomer throughout the functionalized fluoropolymer.
When pendant functional groups are introduced into the
melt-fabricable fluoropolymer by, e.g., maleic anhydride or maleic acid, including
the halogen-substituted counterparts, either by grafting or by copolymerizing, the
amount of grafting compound grafted to the fluoropolymer or the amount of functional
comonomer incorporated into the fluoropolymer will generally be in the range of
0.01-1.0 wt%, preferably 0.02-0.5 wt%, based on total fluoropolymer present in the
composition. If the composition contains both non-functional fluoropolymer and functionalized
fluoropolymer, the functionalized fluoropolymer will have larger amounts of grafted
compound or copolymerized comonomer units depending on the proportion of functionalized
fluoropolymer in the composition. Generally, the amount of maleic anhydride/acid
is in the range of 0.05 wt% to 5 wt% based on the weight of functionalized fluoropolymer
in such a fluoropolymer blend. Preferably, the amount of maleic anhydride/acid in
the functionalized fluoropolymer component of a blend is 0.1-3 wt%, more preferably
The following fluoropolymers are used to fabricate articles
as described in the examples below.
The fluoropolymer having polar functional groups used in
examples below is a copolymer (ETFE) of ethylene (E), tetrafluoroethylene (TFE),
and perfluorobutyl ethylene (PFBE) having E/TFE molar ratio of about 0.9, about
1 mol% of PFBE, and melt flow rate (MFR) of 7.4 g/10 min. The polymer is prepared
generally by the method of U.S. Patent 3,624,250 as a finely-divided fluff or powder.
This fluff is refined by passing it through a comminuting machine (Fitzmill®,
Fitzpatrick Co.) using a screen with 0.04-inch (1.0-mm) openings. Average particle
size is about 100-120 µm as determined by U.S. Standard screen analysis. Unless
otherwise specified, the powder exiting the Fitzmill® (ETFE powder)
is used without fractionation, i.e. without classification. Maleic anhydride is
grafted to the ETFE powder surface by the irradiation process disclosed by Kerbow
in U.S. Patent 5,576,106 to obtain a grafted maleic anhydride concentration of about
0.30 wt%. The grafted ETFE (g-ETFE) powder is formed into granules by compacting
with a roll compacter and then granulating with a hammer mill (FitzMill®).
The g-ETFE granules prepared as outlined above are blended
in nominally 20/80 and 50/50 ratios by weight with cubes of an ETFE resin having
E/TFE molar ratio of about 0.9, about 1 mol% of PFBE, and MFR of about 6.0 g/10
min, and having no pendant functional groups (ETFE-3, following paragraph). The
granules and cubes are dry-blended, and the resultant mixtures are melt-blended
using a standard single-screw extruder with a metering screw having a mixing section,
and the extrudate is strand-cut into cubes. Measured concentrations of grafted maleic
anhydride in the blends are 0.034 wt% for the blend (g-blend-20) containing 20 wt%
of g-ETFE and 0.12 wt% for the blend (g-blend-50) containing 50 wt% of the g-ETFE.
The resultant blends have MFR of 6.4 and 4.0 g/10 min, respectively.
Three ETFE resins in cube form and having no pendant functional
group are also used. ETFE-1 has E/TFE molar ratio of about 0.9, about 1 mol% of
PFBE, and MFR of 7.4 g/10 min, and thus is similar to the base resin used for g-ETFE
powder. ETFE-2 has E/TFE molar ratio of about 0.9, about 1 mol% of PFBE, and MFR
of about 11 g/10 min. ETFE-3 has E/TFE molar ratio of about 0.9, about 1 mol% of
PFBE, and MFR of about 6.0 g/10 min.
The concentration of grafted maleic anhydride in ETFE compositions
used is determined by the method of Kerbow in U.S. Patent 5,576,106 using the infrared
absorption peak at about 1795 cm-1 and a multiplicative factor of 3.8
to convert absorbance/mil of sample thickness (0.97 to convert absorbance/mm) to
concentration in wt%.
The concentration of copolymerized maleic anhydride (MAn)
in the TFE/PEVE copolymer exemplified below is estimated by Fourier transform infrared
(FTIR) spectroscopy. A solution of 0.1 g of succinic anhydride in 10 mL of ethanol
in a 0.102-mm CaF2 cell gives absorptivities of 1765 cm2/g
at 1867 cm-1 and 10,894 cm2/g at 1790 cm-1. When
cold pressed as thin films, the TFE/PEVE/MAn terpolymers showed peaks at about 1897
cm-1 and about 1820 cm-1. The latter is used to estimate copolymerized
MAn concentration, assuming that copolymerized maleic anhydride has the same absorptivity
as ethanolic succinic anhydride. An absorption band at about 2365 cm-1
that has been widely used as an internal thickness band is used to measure film
thickness. The spectrum of a commercial TFE/PPVE copolymer (Teflon®
PFA fluoropolymer resin grade 340) control sample is subtracted prior to calculation.
Melt flow rate (MFR) of ETFE resins is measured at 297°C
according to ASTM D-3159.
Dynamic cut through measurements on insulated wire samples
are made by the method of ASTM D-3032. The same method is used for tubing samples
by inserting a mandrel into the tubing.
Flex modulus is measured according to ASTM D-790.
Insulation 0.006 inch (0.15 mm) thick is extruded onto
AWG 22 conductor from g-blend-20 and ETFE-2, and onto AWG-20 conductor from ETFE-1,
using a 1.25-inch (32-mm) Entwistle wire extrusion line equipped with a standard
metering screw having a mixing torpedo (U.S. Patent 3,006,029) to provide a uniform
melt, and a melt draw technique. Dynamic cut through force is measured at room temperature
and at two elevated temperatures. Table 1 shows improved cut-through resistance
for the insulation of fluoropolymer having pendant functional groups, with notably
better retention of cut-through at the high temperatures.
Table 1. Cut Through Resistance Results for Example 1
Cut Through Force
T (°C) \ Insulation
Small tubing having nominal outside diameter of 0.400 inch
(10.2 mm) and wall thickness of 0.025 inch (0.64 mm) is extruded from g-blend-20,
g-blend-50, and ETFE-3. The tubing made from the blends is stiffer than tubing made
from ETFE-3, based on hand examination. Additionally, repeated cutting of the tubing
made from g-blend-20 with tubing cutters requires more effort than for tubing made
from ETFE -3. These observations indicate that the tubing made from fluoropolymer
having pendant functional groups has improved properties. The three tubings are
cut into small pieces and subjected to melt flow measurement. MFR values in Table
2 show that cross-linking occurs for the fluoropolymers having pendant functional
groups, counteracting and in the case of g-blend-50 overcoming the molecular weight
reduction that takes place across the tubing fabrication process for fluoropolymer
having no pendant functional groups.
Table 2. MFR Values for Example 2
MFR (g/10 min)
A 1-liter vertical stirred reactor is charged with 0.5
g of maleic anhydride (MAn) and is closed. The reactor is purged with CO2
by several times charging with CO2 and venting. The reactor is heated
to 40°C, and the agitator is started at 800 rpm. The reactor is then charged
to a pressure of 1300 psig (9.1 MPa) with a TFE/CO2/ethane mixture of
185 g of TFE, 470 g of CO2 and 2.19 g of ethane, and 30 mL of PEVE are
injected. Then, 15 mL of a 0.68 wt% solution of [CF3CF2CF2OCF(CF3)COO]2
(HFPODP) initiator in CF3CF2CF2OCF(CF3)CF2OCFHCF3
are injected. When this amount of initiator solution has been injected, the rate
of addition of the same solution is reduced to 0.16 mL/min and this initiator feed
is continued to the end of the polymerization. A feed of a TFE/CO2 mixture
is also started at the rate of 116 g/hr of TFE and 77 g/hr of CO2 and
is continued for 1.5 hr. After 1.5 hr, all feeds and the agitator are stopped, the
reactor is vented and opened, and 134 g of polymer solids are recovered as a white
powder after devolatilizing for 1 hr at 100°C in a vacuum oven. The TFE copolymer
contains 3.4 wt% of PEVE and 0.08 wt% of MAn. Melt viscosity based on MFR measurement
using a 2160 g load but otherwise measured according to ASTM D-3307 at 372°C
is 2.35 x 103 Pa·s, and Tm is 306°C. MFR behavior
upon further high-temperature exposure indicates that cross-linking occurs.
A 400 mL autoclave is charged with 1 g of MAn, then chilled
to a temperature of less than -20°C, and 5 mL of 0.16-molar HFPODP in CF3CF2CF2OCFHCF3
are added. The autoclave is kept cold, sealed, and evacuated. Then, 64 g of VF2,
50 g of TFE, and 150 g of CO2 are condensed in. Cooling is removed and
the autoclave is agitated overnight at ambient temperature, with an exotherm of
polymerization carrying the reaction mixture to 45°C. The autoclave is vented
and product polymer is recovered as white chunks. After devolatilizing the polymeric
product under pump vacuum for 3-4 days, the recovered polymer weighs 90.4 g. The
TFE/VF2/MAn copolymer contains 0.3 wt% of MAn and has a melting point
of 160°C with heat of fusion of 25 J/g as determined by differential scanning
calorimetry on second heating. The polymer (1 g) dissolves in 50 mL of acetone or
tetrahydrofuran (THF) at room temperature, giving clear viscous solutions after
rolling in a bottle. Inherent viscosity in THF at 25°C is 3.537. When a 4.33
g sample of the polymer is heated for 1 hr in a vacuum oven at 200°-205°C,
the sample weight is reduced by 0.02 g (0.5% weight loss). Rolling a 0.14 g sample
of the heat-treated polymer in a bottle with 10 mL of acetone for 1 week causes
swelling but little dissolution of the polymer; the weight of the acetone-swollen
polymer is 0.11 g after recovery and drying. The reduced solubility indicates that
thermal cross-linking occurs during the heat treatment. Thus, fluoropolymer having
pendant functional groups introduced during polymerization can be used in articles
of the present invention.