Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PAPERMAKING BELT WITH
IMPROVED ELONGATION RESIN
FIELD OF THE INVENTION
This invention relates to a papermaking belt comprised of a resinous polymer
which exhibits improved properties.
BACKGROUND OF THE INVENTION
Papermaking belts, well known in the art, are utilized for producing patterned
paper. The paper made by utilizing a papermaking belt of the type disclosed in
this
invention is described in commonly assigned U.S. Patent Nos. 4,528,239 issued
to
Trokhan on July 9, 1985; 5,514,523 issued to Trokhan et al. on May 7, 1996;
5,503,715 issued to Trokhan et al. on April 2, 1996; 5,334,289 issued to
Trokhan et al.
on August 2, 1994; 5,554,467 issued to Trokhan et al. on September 10, 1996;
4,514,345 issued to Johnson et al. on April 30, 1985; 5,534,326 issued to
Trokhan et
al, on July 9, 1996; 5,556,509 issued to Trokhan et al. on September 17, 1996;
and
5,628,876 issued to Ayers et al. on May 13, 1997.
Papermaking belts are typically composed of two key components: a
reinforcing element; and a resinous polymer as taught by Trokhan '239 and
Johnson
et aI. '345. The resins utilized to make the papermaking belts of these
teachings suffer
from a common drawback wherein as the resins age during papennaking,
embrittlement, cracking and resin loss occur resulting in limited belt life.
It is believed
that resin elongation is the key property lost as aging occurs.
The object of an aspect of this invention is to provide a papermaking belt
comprised of a cured resinous polymer exhibiting improved ultimate elongation
defined as the elongation at the breaking point. Another object of an aspect
of this
invention is to improve papermaking belt life by providing a papermaking belt
with
improved resin elongation at elevated temperatures without an undue loss of
creep
resistance, tensile strength and/or hardness at elevated temperature relative
to the prior
art.
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SUMMARY OF THE INVENTION
This invention comprises a papermaking belt wherein the belt is comprised of
a resinous polymer. The resinous polymer is disposed in a framework. After
curing,
the polymer has an elongation at 22°C of at least about 100% and a
tensile strength at
room temperature of at least about 2600 psi.
After curing, this same polymer has an elongation of at least about 45% and a
tensile strength of at least about 700 psi wherein both the elongation and
tensile
strength of the polymer are measured at a temperature of 90°C.
The cured resinous polymer after being aged for twenty-four hours at an air
temperature of 140°C in a convection oven has an elongation measured at
22°C of at
least about 70% and tensile strength measured at 22°C of at least about
2000 psi.
According to an aspect of the present invention, there is provided a
papermaking belt wherein the belt is comprised of a resinous polymer, the
resinous
polymer being disposed in a patterned framework and wherein the resinous
polymer
after curing has an elongation of at least about 100% and a tensile strength
of at least
about 2600 pounds per square inch whereby the elongation and tensile strength
are
measured at a temperature of 22 degrees Celsius.
According to another aspect of the present invention, there is provided a
papermaking belt wherein the belt is comprised of a resinous polymer, the
resinous
polymer being disposed in a patterned framework and wherein the resinous
polymer
after curing has an elongation of at least about 45%, and a tensile strength
of at least
about 700 pounds per square inch, whereby the elongation and tensile strength
are
measured at a temperature of 90 degrees Celsius.
According to a further aspect of the present invention, there is provided a
papermaking belt wherein the belt is comprised of a resinous polymer, the
resinous
polymer being disposed in a patterned framework and wherein the resinous
polymer
after curing is aged for 24 hours at a temperature of about 140 degrees
Celsius, has an
elongation of at least about 70%, and a tensile strength of at least about
2000 pounds
per square inch, whereby the elongation and tensile strength are measured at a
temperature of 22 degrees Celsius.
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BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 Plan view of one completely assembled embodiment of a papermaking belt
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1, the present invention relates to a papermaking belt 10
comprising a resinous polymer 20 disposed within a framework. The resinous
polymer 20 after curing exhibits improved elongation without sacrificing
hardness or
creep resistance. Most preferably the resinous polymer 20 of this invention is
completely cured. A resinous polymer 20 is considered completely cured at the
point
where no additional heat from polymerization is evolved upon continuing
irradiation
of the sample. As would be well-known to one skilled in the art, a calorimeter
can be
used to make this measurement. It should be noted that even at complete cure
as
described above, polymerizable groups may be trapped within the polymeric
network
and hence inaccessible to further polymerization.
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The papermaking belts 10 of this invention may be made according to commonly
assigned U.S. Patent Nos. 5,334,289 issued to Trokhan et al. on August 2,
1994;
4,514,345 issued to Johnson et al. on April 30, 1985; 5,527,428 issued to
Trokhan et al.
on June 18, 1996 and 4,529,480 issued to Trokhan on July 16, 1985, which show
how to
make papermaking belts 10 for use with the present invention. In the preferred
method
for producing a papermaking belt 10, the four key materials required include:
a
reinforcing element 30 such as a woven screen; a barrier film such as a
thermoplastic
sheet; a mask comprising a framework of transparent and opaque regions wherein
the
opaque regions define a preselected pattern of gross foramina in the
framework; and a
liquid photosensitive resin which is cured during the beltmaking process in
order to form
a resinous polymer 20.
The reinforcing element 30 may be made according to commonly assigned U.S.
Patent Nos. 5,500,277, issued March 19, 1996, to Trokhan et al. or 5,496,624,
issued
March 5, 1996, to Stelljes Jr. et al. Examples of suitable reinforcing
elements 30 include
paper machine clothing such as forming fabrics, wet press felts and dryer
fabrics.
Alternatively, a Jacquard weave reinforcing element 30 may be utilized for the
papermaking belt 10 having a framework made of the resinous polymer 20
according to
the present invention.
A method of producing a papermaking belt 10 includes applying barrier film to
the working surface of the belt forming unit; juxtaposing a reinforcing
element 30 to the
barrier film so that the barrier film is interposed between the reinforcing
element 30 and
the forming unit; applying a coating of liquid photosensitive resin to the
surfaces of the
reinforcing element 30; controlling the thickness of the coating to a
preselected value;
juxtaposing in contacting relationship with the coating of liquid
photosensitive resin a
mask comprising a framework of both opaque and transparent regions; exposing
the
liquid photosensitive resin to light having an activating wavelength through
the mask
thereby inducing curing of the liquid photosensitive resin in those regions
which are in
register with the transparent regions of the mask; and removing from the
reinforcing
element 30 substantially all of the uncured liquid photosensitive resin. The
exact
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apparatus or equipment used in the practice of the present invention is
immaterial so long
as it can, in fact, be used to practice the present invention.
Properties of the resinous polymer 20 which are deemed to be important to
papermaking belt 10 life include elongation, tensile strength, hardness and
creep
resistance at both room temperatures and elevated temperatures. In order to
maximize
the life of the papermaking belt 10 it is especially desirable for the
resinous polymer 20
at elevated temperatures, including those temperatures to which the belt 10 is
exposed
during use, to exhibit elongation without unduly sacrificing creep resistance,
tensile
strength, or hardness relative to the prior art. The resinous polymer 20 of
this invention
has a room temperature elongation measured at 22°C of at least about
100%, more
preferred of about 110% and even more preferred of 125%. The resinous polymer
20 of
this invention exhibits improved ultimate elongation while resisting creep and
without
undue loss of tensile strength and hardness relative to the prior art.
The preferred liquid photosensitive resin composition of this invention is
comprised of four key classes of components: a prepolymer; monomers;
photoinitiator
and antioxidants. A preferred liquid photosensitive resin is Merigraph L-055
available
from MacDermid Imaging Technology, Inc. of Wilmington, Delaware. The
prepolymer
of this liquid photosensitive resin is made from a methacrylated or acrylated
polyurethane formed from polyethers and is substantially free of polyesters.
Preferably
the prepolymer is a polyurethane derived from the reaction of a diisocyanate
compound
with a polyol. The polyol is preferably comprised of a hydroxy terminated
polyether
compound which is substantially polyester free. A polyol containing
substantially all
polyether is preferred over polyols which contain either all polyester or a
mixture of
polyether and polyester as it is believed that ester segments increase the
possibility of
transesterfication. Transesterfication is a mechanism which can result in
molecular
weight loss of the prepolymer. The molecular weight loss of the prepolymer can
in turn
result in a loss of resinous polymer 20 elongation. Additionally, without
being bound by
theory, it is believed that polyethers offer more hydrolytic stability than
polyesters or
blends of polyesters and polyethers. The preferred polyurethane is endcapped
or
functionalized with a methacrylate or acrylate group. The polyurethane-based
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prepolymer is preferably comprised mainly of polyethers and is substantially
free of
polyesters. The polyethers are comprised preferably of ethylene oxide,
propylene oxide
and butylene oxide.
In addition to the polyurethane-based prepolymer, the liquid photosensitive
resin
of this invention is also comprised of a mixture of monomers including mono-
functional,
difunctional and trifunctional monomers containing acrylate or methacrylate
groups. The
preferred monofunctional monomers are hydroxyalkyl acrylates or hydroxyalkyl
methacrylates.
A photoinitiator, most preferably comprised of 2,2-dimethoxy-2-
phenylacetophenone is also utilized. The photoinitiator is added to the liquid
photosensitive resin formulation in an amount of preferably about .OS%- 1.0%
of the
total weight of the resin formulation. When exposed to LJV light the
photoinitiator
generates free radicals which in turn initiates the polymerization reaction. A
suitable
photoinitiator is available from Giba Geigy Corp. of Hawthorne, New York as
Irgacure
651.
The antioxidant component of the liquid photosensitive resinous polymer may be
carried out according to commonly assigned U.S. Pat. Nos. 5,059,283 issued to
Hood et
al. on October 22, 1991 and 5,073,235 issued to Trokhan on December 17, 1991.
Antioxidants are added to the liquid photosensitive resin formulation in order
to prevent
the resinous polymer 20 from oxidizing and causing degradation of the
papennaking belt
resulting in premature belt 10 failure. Suitable chemicals which may be used
as
antioxidants include but are not limited to: high molecular weight hindered
phenols;
secondary amines, phosphates, phosphites, thioesters, sulfur-containing
compounds and
secondary sulfides. Preferred antioxidants used in the present invention
include:
IrganoxTM 1010 marketed by Ciba Geigy Corp. of Hawthorne, New York and
CyanoxTM
1790 marketed by Cytec Industries Inc. of West Paterson, New Jersey.
Antioxidants are
preferably added in a concentration of from about .001 % to S.0% by weight.
The type of papennaking belts 10 described in this invention may be used in
conjunction with a variety of different types of paper machines systems and
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configurations well known in the art including but not limited to fourdrinier
forming
sections, twin wire formers, crescent formers, through air drying systems and
conventional press sections.
Properties of the resinous polymer 20 including tensile strength, elongation,
hardness and creep resistance are measured on cured resinous polymer coupon
samples.
The resinous polymer coupons are prepared by casting a .040 inch layer of
liquid
photosensitive resin over a 1 mil thick polypropylene film and covering it
with a .004
inch thick polyester film, on a Merigraph 2228 photopolymer exposure unit
available
from MacDermid Imaging Technology of Wilmington, Delaware. The sample is first
exposed for 30 seconds to the upper lamps and then exposed for 400 seconds to
the lower
lamps. Both films are removed after curing.
For purposes of tensile testing and elongation, resinous polymer coupons are
tested according to ASTM test method D-638. Each coupon is die cut by using a
standard type IV dumbell die. The resinous polymer coupon is cut by striking
the die
with a hammer. The coupon is cut so as to have an overall length of 4.5
inches, a width
at the narrowest section of the coupon of 0.25 inches and an overall width of
0.75 inches.
A suitable die is available from Testing Machines Inc. of Amityville, New,
York.
For measuring tensile strength and elongation, a resinous polymer coupon is
inserted in a tensile tester such as an Instron tensile tester model No. 1122
made by the
Instron Corporation of Canton, Massachusetts. A cross-head separation speed of
2
inches per minute and a gauge length of 2.5 inches are selected. The sample is
loaded
into the tensile tester and tested to breakage by straining the coupon sample
until it
reaches its breaking point. The elongation at the point of breakage, defined
as the
ultimate elongation, is measured directly from the tensile tester or,
alternatively may be
measured using a chart recorder as is well known in the art.
Hardness of the resinous polymer coupons is measured according to ASTM test
method D-2240 using a Shore D durometer gauge and a leverloader stand
available from
the Shore Instrument and Manufacturing Company of Freeport, New York. Resinous
polymer coupons used for hardness testing are cut with a circular die of 1
inch in
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diameter. The circular coupons are stacked to achieve a total sample thickness
of at least
.250 inches prior to testing.
The properties of the present invention and the prior art measured at
22°C are
set forth in Table I below.
Table I
Present Present
Prior Art Invention Present Prior Art Invention
Resin Resin Prior Invention Resin Resin
Art
ultimate ultimate Resin Resin TensileHardness Hardness
Tensile
Elong. Elong. Strength Strength (Shore (Shore
% (%) (psi) (psi) D) D)
measured measured measured measured measured measured
at 22C at 22C at 22C at 22C at 22C at 22C
76.2 125 3906 3930 48 45
Resinous polymer 20 properties including tensile strength, elongation, creep
and
Shore D hardness are also measured at elevated temperatures. Tensile strength
and
elongation are measured at 90°C on an Instron Tensile Tester in which
the crosshead
grips of the Instron are enclosed in an environmental test chamber heated to
90°C ~ 1 °C.
Suitable environmental test chambers are available from Instron
Corp. of Canton, Massachusetts. The resinous polymer coupon to be tested is
also placed
in the test chamber for three minutes and then immediately tested on the
Instron.
For hardness measurements done at 90°C, the leverloader stand and
resinous
polymer coupon samples are preheated to 90°C in a forced draft
laboratory oven for 30
minutes and then tested in the oven according to the procedure described
above.
Creep resistance is measured using a Bohlin CVO Controlled Stress rheometer
manufactured by Bohlin Corporation of Cranbury, New Jersey. For creep testing
at 90°
C, the resinous polymer coupon samples are heated to 90°C for ten
minutes in the
rheometer and then tested. Creep measurements are taken at 25% strain and 100
seconds
after the initial load has been applied. The resinous polymer 20 of this
invention at 90°C
and 25% strain will exhibit a creep modulus of greater than about 2 x 107
dynes/cm2
wherein the modulus decreases less than 10% in the initial 100 seconds after
the stress
*rB
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has been applied. The properties of the resinous polymer 20 tested at
90°C according to
the present invention and the prior art are set forth in Table II below.
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Table II
Present
Prior Invention
Art
Resin Resin
Creep
Creep Modulus
Modulus (dynes/cm2)
Present Present (dynes/cm2)measured
Prior inventionPrior Invention Present measuredat: 25%
Art Art at:
Resin Resin Resin Resin Prior Invention25% strain,strain,
Art 100
UltimateUltimateTensileTensileResin Resin 100 secondsseconds
Elong. Elong. StrengthStrengthHardnessHardnessafter after
initial initial
(%) (%) (psi) (psi) (Shore (Shore load load applied
D) D) applied
measuredmeasuredmeasuredmeasuredmeasuredmeasuredat temp.at temp.
of of
at 90C at 90C at 90C at 90C at 90C at 90C 90C 90C
36 60 1161 980 29 27 2.6 x 2.7 x
10 ~ 10 i
In accordance with another important property of the present invention a
resinous
polymer coupon made according to the procedure described above is aged for
twenty-
four hours in a convection oven at a temperature of 140 ~ 2°C . The
coupon is removed
after twenty-four hours and tested as soon as reasonably practical as
described above
after allowing the coupon to cool to 22°C. This same test is repeated
on a coupon aged
for ninety-six hours. The properties of the resinous polymer 20 aged at
elevated
temperatures according to the present invention and the prior art are set
forth in Table III
below.
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Table III
Present
Hrs. SamplePrior Art Pres. Inv. Prior Art Invention
is Resin Resin Tensile Tensile
Maint. at Elong. (%) Elong. (%) Strength Strength
140C (psi) (psi)
24 62.5 89.0 2929 2600
96 44.3 80.0 1808 2100
Tables II and III show that contrary to conventional wisdom, tensile strength
is
not the determinative property for improving belt 10 life. It is to be
recognized that the
above described resin can be used for other applications as well as the
papermaking belts
described herein. While particular embodiments of the present invention have
been
illustrated and described, it would be obvious to those skilled in the art
that various other
changes and modifications can be made without departing from the scope and
spirit of
the invention. It is therefore intended to cover in the appended claims all
such changes
and modifications that are within the scope of this invention.
