Note: Descriptions are shown in the official language in which they were submitted.
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COMPOSITE DOCTOR BLADES
Background of the Invention
The present invention relates to composite doctor blades. More particularly,
the present
invention relates to composite doctor blades for use in papermaking, for
example in calenders during
the manufacture of printing paper. The term "calender" and variations thereof,
as used herein, is
intended to refer to an apparatus used to calender paper, including stand-
alone calendering units such
as supercalenders and calendering units within a papermachine such as machine
calenders, gloss
calenders and soft nip calenders. The present invention further relates to a
method of using doctor
blades in calenders.
Doctor blades are widely used to remove various materials from the surface of
papermachine rolls. By its very nature, the process of removal of contaminants
from the roll surface
may result in significant wear to the roll surface, the doctor blade itself or
both. The components of
paper, particularly coating components, tend to be abrasive and tend to cause
wear in the surface of
the papermachine roll. Conventional doctor blades may be constructed from
metal, e.g., steel,
stainless steel, nickel or bronze, metal coated with alloy or ceramic
material, plastic, or "composite"
materials, i.e., fiber-reinforced polymeric materials. Fig. 1 shows a typical
papermachine
configuration wherein a doctor blade 2 is positioned against a surface 16 of a
papermachine roll 12,
for example a calender roll. Doctor blades typically have a 45° beveled
edge 14, as shown in Fig. 1.
Metal blades generally exhibit high stiffness in the machine direction, i.e.,
the direction
perpendicular to the rotational axis of the papermachine roll, and good wear
characteristics. The
machine direction of the papermaking process is generally known in the art as
the direction of the
paper web as it passes through the papermachine and is indicated by arrow 18
in Fig. 1. Such blades
also tend to be susceptible to corrosion and to cause excessive roll wear.
Plastic blades tend to be used in papermachine locations unsuitable for metal
blades. Plastic
blades, however, generally have significant drawbacks because they tend to
have low stiffness and
tend to degrade at the temperatures typically used in the papermaking process.
Composite blades are typically formed from a plurality of fibrous layers
impregnated with
resin, each fibrous layer generally having a woven structure such that a
certain proportion of the
fibers lay in the machine direction, while the remaining fibers lay in the
cross-machine direction, i.e.,
the direction parallel to the rotational axis of the papermachine roll. The
cross-machine direction is
generally known is the art as the direction perpendicular to the path of the
paper web and is indicated
by arrow 20 in Fig. 1. Although composite blades tend to wear more quickly
than metal blades, they
also tend to cause less wear on the roll surface. Reduced blade life is
typically viewed as a drawback
and improved wear resistance of the blade is seen as desirable for many
doctoring applications. The
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wear characteristics of composite doctor blades are generally considered
acceptable in many
conventional calendering applications because excessive roll wear may
deleteriously affect the final
properties of the paper.
Composite doctor blades are often used with on-line calenders, which are
typically run at
relatively high nip pressures and high roll surface temperatures. These
operating conditions tend to
increase the amount of coating particles and contaminants on the calender roll
surface. If the
calendering rolls are not doctored on an almost continuous basis, buildup of
coating particles and
contaminants reach unacceptable levels, directly affecting the final product
properties of the paper,
such as paper gloss and paper smoothness. Moreover, the abrasiveness of the
particles and
contaminants tend to degrade the surface of the calender roll, causing a
permanent degradation of the
roll surface. Degradation in the roll surface tends to cause a deterioration
ofthe roll profile, i.e., the
roll surface is uneven which tends to cause inconsistent calendering across
the width of the paper
web. Thus, the demand for consistent paper quality at a high production rate
and with greater
e~ciency has typically resulted in almost continuous doctoring of the
calendering rolls during
operation to remove contaminants. As a result, there have been significant
efforts to increase the
wear resistance and, consequently, the blade life of composite doctor blades.
The operating conditions for on-line calendering have also driven efforts to
increase the wear
resistance of the calendering rolls. It is becoming more common for such on-
line calendering rolls to
be coated with a thin layer of thermal spray coating, which typically exhibits
resistance to roll
surface degradation and, consequently, deterioration of the roll profile. The
term "thermal spray" and
variations thereof, as used herein, is intended to refer to one of three
standard processes: plasma,
high velocity oxygen-fuel (HVOF), and detonation gun, whereby a material,
typically in powder
form, is heated and deposited on a surface. The thermal spray coating tends to
be a ceramic or metal
matrix coating. The surface of a thermal spray coated roll may also be ground
to a very low
roughness, a highly desirable property for calendering rolls used in the
manufacture of coated
printing papers.
Thermal spray coatings tend to resist scratching from doctoring activities
when such
doctoring activities are performed on an intermittent basis, such as the
removal of paper wrapped
around a roll after a break in the paper web. A thermal spray coated roll
will, however, generally
exhibit roll degradation when subjected to almost continuous doctoring. Over
time, thermal spray
coated rolls tend to exhibit deterioration in the roll profile and surface
finish caused by the action of
the doctor blade and the contaminants. When the roll profile and surface
finish have degraded to an
extent such that the quality of the paper is adversely affected, the roll must
be removed and
reground. Removal for grinding can result in a significant loss to production
and increased costs. In
addition, the grinding process itself removes a valuable layer of thermal
spray coating from the roll.
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Because the thermal spray coating layer of the roll tends to represent a
significant portion of the cost
of the roll and a significant monetary investment, minimizing the loss of
thermal spray coating is
highly desirable.
Efforts to increase the wear resistance of composite doctor blades may result
in more rapid
deterioration of the surface of the roll. On the other hand, an adequate level
of wear resistance is
required to minimize disruptions to production caused by the need to change
doctor blades. There
remains a need for a doctor blade that may be used almost continuously against
the surface of a
thermal spray coated roll to adequately remove surface contaminants, while
exhibiting sufficient
wear resistance to be practical in the production setting. There also remains
a need for a doctor blade
that may be used to maintain a low surface roughness of the roll with minimal
deterioration of the
thermal spray coating.
Summary of the Invention
The inventor has discovered that a composite doctor blade that includes a
plurality of
unidirectional fibers, i.e., abrasive fibers aligned in a direction parallel
to the long axis of the doctor
blade, may be used to remove surface contaminants from the surface of a roll
with minimal
deterioration of the roll surface. The inventor has found that such a doctor
blade may remain in
substantially continuous contact with the surface of a roll during operation
without significant
damage to the surface of the roll.
The composite doctor blade of the invention is suitable for use in the
manufacture of paper,
particularly for use in calenders. The composite doctor blade of the invention
provides the
abrasiveness required in paper manufacturing to adequately clean roll surfaces
without unacceptable
deterioration of the roll surfaces. The doctor blades of the invention exhibit
the structural properties
required for effectual doctoring, such as stiffness in both axes of the doctor
blade. The doctor blade
of the invention also tends to wear slowly and uniformly. Embodiments of the
doctor blade of the
invention may also be used to reduce and maintain a desired level of surface
roughness of the roll.
In one aspect, the invention provides a doctor blade including composite
material that
includes a plurality of unidirectional fibers, impregnated with a resin.
Preferred embodiments may include one or more of the following features. The
doctor blade
has a laminate structure including multiple layers of composite material. The
unidirectional fibers
are selected from the group consisting of fiberglass, ceramic, and mixtures
thereof. Preferably the
fibers are provided as long continuous filaments or multifilament strands.
Preferably the fibers are
fiberglass. The unidirectional fibers are provided in a unidirectional fabric.
At least 60% by weight
of the fibers in the unidirectional fabric are unidirectional fibers,
preferably 75% by weight, more
preferably 90% by weight. The remaining fibers, referred to herein as the
secondary fibers, are
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oriented in a direction other than parallel to the long axis of the doctor
blade. The unidirectional
fibers have diameters equal to or greater than the diameters of the secondary
fibers. Preferably the
unidirectional fibers have diameters of about 450 to 1500 pm and the secondary
fibers have
diameters of about 400 to 700 pm. The unidirectional fabric further includes
nonabrasive fibers
selected from the group consisting of carbon, i.e., graphite, rayon, aramid,
polyester and mixtures
thereof. Preferably one or more of the layers of composite material includes
carbon fibers aligned in
a direction perpendicular to the long axis of the doctor blade. The
unidirectional fabric has a weight
per unit area of about 230 to 610 g/m2. The impregnating resin is a
thermoplastic resin or an epoxy
resin, i.e., a resin containing an epoxide, oxirane or ethoxylene group. The
resin has a glass
transition temperature, Tg, of about 65 to 315°C, preferably 85 to
315°C. The resin fi~rther includes
an abrasive additive selected from the group consisting of glass microspheres,
glass fibers, crushed
glass, synthetic or industrial diamond particles, silica particles, silicon
carbide particles, boron
particles, zirconium particles, aluminum oxide particles and mixtures thereof.
In another aspect, the invention provides a method of cleaning a roll surface
including:
a) positioning a doctor blade having a long axis near the roll surface such
that the long axis of the
doctor blade is substantially parallel with the rotational axis of the roll,
the doctor blade including a
plurality of unidirectional fibers, impregnated with resin; and
b) pressing a beveled edge of the doctor blade against the surface of the
roll.
In another aspect, the invention features using the above described method, to
decrease the
roughness of a roll surface.
Preferred methods may include one or more of the following features. The
beveled edge of
the doctor blade remains in substantially continuous contact with the roll
surface during operation.
The positioning step includes the formation of an angle of about 25 to
30° between the beveled edge
of the doctor blade and the roll surface, as measured from a tangent to the
roll where the beveled
edge touches the roll surface. The pressing step is performed at a pressure of
about 85 to 700 N/m,
preferably about 175 to 440 N/m. The surface roughness of the roll is reduced
to about 0.025 to
0.20 ~m Ra, preferably about 0.050 to 0.13 pm Ra . The surface roughness of
the roll is maintained
during the effective life of a blade at a level of about 0.025 to 0.20 pm Ra,
preferably about 0.050 to
0.13 pm Ra.
In another aspect, the invention provides a method of making a composite
doctor blade
including the step of impregnating a composite material comprising
unidirectional fibers.
Preferred methods may include one or more of the following features. The
method includes a
layering step wherein multiple layers of composite material are superimposed
on top of one another
to form a laminate structure. The method includes a curing step wherein the
resin is subjected to an
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elevated temperature and pressure. The method includes a cutting step wherein
the cured laminate
structure is cut into two or more doctor blades, each blade having a long
axis.
Other features and advantages of the invention will be apparent from the
following detailed
description, the drawings, and the claims.
Brief Description of the Drawings
Fig. 1 is a schematic perspective view showing a doctor blade in contact with
a papermaking
roll.
Fig. 2 is an exploded perspective view of a doctor blade according to one
embodiment of the
invention.
Fig. 3 is an exploded perspective view of a doctor blade according to an
alternate
embodiment of the invention.
Fig. 4A and 4B are highly enlarged schematic cross-sectional views of a
papermachine roll,
showing the surface before and after the use of an embodiment of the
invention.
Fig. 5 is a schematic side view of a calendering unit showing a method of
using a doctor
blade embodying the invention.
Detailed Description of Preferred Embodiments
Referring to Fig. 1, a composite doctor blade 2 is held against a papermaking
roll 12, which
is rotating about its axis in the direction denoted by arrow 22, such that a
leading beveled edge 14 of
the doctor blade 2 may remove contaminants from the surface 16 of the roll. In
Fig. 1 the machine
direction is denoted by arrow 18 and the cross machine direction is denoted by
arrow 20. The doctor
blades discussed below would be used in the environment and in the manner
depicted in Fig. 1.
Referring to Figs. 2 and 3, doctor blade 10 of the invention includes a
laminate structure
formed from multiple layers of composite material 32, each layer including a
plurality of
unidirectional fibers 31, and a plurality of reinforcement fibrous layers 30.
The composite material
layers 32 are arranged within the laminate structure such that the
unidirectional fibers 31 are aligned
in a direction substantially parallel to the long axis of the doctor blade 10.
Reinforcement layers 30
differ from composite material layers 32 in that the majority of the fibers in
the reinforcement layers
are not oriented parallel to the long axis of the doctor blade 10.
Reinforcement layers 30 may be
included in the laminate structure to provide reinforcement, e.g., stiffness
or strength, or to increase
the thiclrness of the doctor blade. Reinforcement layers 30 are shown
schematically, without
indicating the direction of the fibers, in Figs. 2 and 3. Reinforcement layers
30 can have a woven or
5
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nonwoven structure and the fibers may be aligned substantially in the machine
direction or in two or
more directions.
Fig. 2 and Fig. 3 illustrate embodiments of the doctor blade of the invention
that include nine
layers. Typically composite doctor blades will include five to twenty layers
but may include more
layers depending on the thickness desired for the doctor blade 10. As will be
understood by
practitioners skilled in the art, the appropriate number of layers for a
composite doctor blade is
determined by the operating requirements of the particular doctoring
application. Each composite
material layer 32 and reinforcement layer 30 is impregnated with an epoxy or
thermoplastic resin
such that the layers may be laminated, i.e., bonded under pressure and
temperature, together to form
a single laminate structure.
As shown in Fig. 2, the laminate structure of one embodiment of the doctor
blade 10 may be
formed from alternating reinforcement layers 30 and composite layers 32.
Preferably the
reinforcement layers 30 include fiberglass fibers aligned in two or more
directions in a woven
structure. The embodiment of the doctor blade 10 shown in Fig. 2 would be
suitable for doctoring
applications requiring a relatively high level of abrasiveness, such as the
calendering of a coated
paper web having a relatively high moisture content, e.g., about 4 to 10%,
which tends to cause
increased build-up of coating particles on the roll surface.
Fig. 3 shows an alternate embodiment of the doctor blade 10 in which composite
material
layers 32 are core layers and the reinforcement layers 30 are outer layers.
Preferably the
reinforcement layers 30 include carbon fibers. The embodiment of the doctor
blade 10 of the
invention shown in Fig. 3 would be appropriate for a doctoring application
demanding a less
abrasive blade, such as an on-line calendering unit where the paper web is
relatively dry, e.g., about
1 to 4% moisture content, and tends to cause minimal contamination of the roll
surface.
The arrangement of the layers within the laminate structure of the doctor
blade 10 is
generally symmetrical around the central core layer 34, shown as a
reinforcement layer 30 in Fig. 2
and as a composite material layer 32 in Fig. 3. If the arrangement of the
layers is not symmetrical
about the central core layer 34, the doctor blade may bend or twist along its
long axis.
Suitable fibers for the composite material layers 32 include fiberglass,
ceramic fibers, and
mixtures thereof, preferably fiberglass. As used herein, the term "fiber" is
intended to encompass an
individual filament or a multifilament strand having a length greater than its
width. The composite
material layers may include relatively short fibrous segments or long
continuous fibers, i.e., fibers
that run the length of the doctor blade. Preferably, the composite material
layers include
predominantly long continuous fibers.
Suitable fibers for the composite material layers are sufficiently abrasive to
materials
typically used to form the surface of papermaking rolls, e.g., cast iron,
chilled iron, cast steel, or a
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thermal spray coating including a ceramic or metal matrix material, so that
they will clean and/or
reduce the roughness of the roll surface. Suitable fibers for the composite
material layers are
generally sufficiently rigid so as to provide strength in the longitudinal
direction to the doctor blade.
If the fibers comprising the composite material layers are not sufficiently
rigid, the flexibility of the
doctor blade itself will increase, which may result in ineffectual doctoring
of the roll surface because
the doctor blade will tend to flex when pressure is applied to clean the roll
surface.
The unidirectional fibers are generally provided in the form of a fabric.
Suitable fabrics
including unidirectional fibers are generally referred to in the art as
"unidirectional fabrics" even
though such fabrics may have woven structures such that a certain proportion
of the fibers are
aligned in a different direction. As used herein, the term "secondary fibers"
is intended to refer to the
fibers included in the unidirectional fabric but are not aligned in a
direction substantially parallel to
the long axis of the doctor blade. Secondary fibers are generally used in
unidirectional fabric to
provide a rudimentary framework for the unidirectional fibers so that the
fabric does not fall apart
during processing, e.g., during impregnation with resin and during lamination.
Such secondary fibers
tend to be abrasive to materials typically used to form the surface of
papermaking rolls. Suitable
unidirectional fabrics contain at least 60% by weight unidirectional fibers.
Preferably, at least 75%
by weight of the unidirectional fabric are unidirectional fibers, most
preferably 90% by weight.
Unidirectional fabrics preferably have a woven structure, so that the fabric
is able to retain
its shape through impregnation with resin and the manufacture of the doctor
blade. During
manufacture of the doctor blade, large sheets of unidirectional fabric, and,
if desired, reinforcement
layers, are impregnated with resin. ABer impregnation, the impregnated layers
are layered so that
multiple layers are superimposed on top of one another to form a laminate
structure. The laminate
structure is then subjected to an elevated temperature and pressure to cure
the resin and bond the
layers together. The cured laminate structure is then cut into two or more
doctor blades, each blade
having a long axis.
The unidirectional fabric may fizrther include a small proportion of
nonabrasive fibers, such
as carbon, rayon, cotton, aramid, i.e., aromatic polyamide, polyester and
mixtures thereof. Such
nonabrasive fibers may be used to provide other properties such as reduced
abrasiveness or
structural strength. The nonabrasive fibers may be aligned fixlly in the cross
machine or the machine
direction, or in more than one direction. Orientation of the nonabrasive
fibers becomes important
when they are used to provide strength to the doctor blade structure. For
instance, an embodiment of
the invention may have carbon fibers woven into the unidirectional fabric in
order to reduce the
abrasiveness of the doctor blade and to provide strength. If strength is
needed in the long axis of the
blade, the carbon fibers should extend in a direction substantially parallel
with the long axis. If
strength is needed across the width of the doctor blade, the carbon fibers
extend in a direction
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substantially perpendicular to the long axis. If the nonabrasive fibers have a
glass transition
temperature, Tg, it should be greater than the surface temperature of the roll
against which the
doctor blade is applied. If the Tg of the fibers is equal to or less than the
temperature of the roll
surface, the fibers tend to melt and contaminate the roll surface.
Practitioners skilled in the art are
aware of how to select appropriate nonabrasive fibers exhibiting the
properties desired for a
particular doctoring application such as reduced abrasiveness and/or increased
strength.
When using some conventional composite doctor blades, coating particles and
contaminants
removed from the surface of a roll tend to plug up crevices and interstices
between the fibers of the
working surface of the doctor blade, i.e., the surface of the blade that is
against the roll surface,
reducing the effectiveness of the blade. Doctor blade 10 tends to exhibit less
"plugging up" with
particles and contaminants because there are fewer crevices and interstices
available due to the cross
machine orientation of the unidirectional fibers. The composite material
layers 32 (Fig. 2 and Fig. 3)
also expose a greater surface area of fiber to the roll surface because the
unidirectional fibers are
aligned in the cross machine direction, parallel to the roll surface upon
which they are acting. During
operation, the unidirectional fibers at the working surface of the doctor
blade tend to disintegrate,
exposing adjacent unidirectional fibers. This disintegration provides a
refreshed working surface on
the beveled edge 14 (Fig. 1) of the doctor blade. The unidirectional fibers
typically disintegrate into
very small particles which are removed from the roll surface with the doctored
contaminants.
Conventional composite doctor blades often have a propensity to form scratches
on the
surface of the roll because the "ends" of the abrasive fibers are acting upon
the surface of the roll.
Because a significant proportion of the fibers in a typical doctor blade are
aligned in the machine
direction, the surface of the roll is subjected to doctoring by the "ends" of
the fibers rather than the
"sides" of the fibers. Under near continuous doctoring conditions, such action
by the fiber ends tends
to increase the roughness of the roll surface significantly, eventually
causing an unacceptable
deterioration in roll surface and final product properties.
While doctor blades of the invention may include secondary fibers or
nonabrasive fibers
aligned in the machine direction, the propensity to form scratches is
significantly reduced because
there are fewer fibers in the machine direction, and therefore, fewer fiber
"ends" acting upon the roll
surface. The composite material layers of the doctor blade of the invention
tend to reduce the
roughness of the roll surface, as shown in Fig. 4A and Fig. 4B. Fig. 4A is a
roll profile of a section
of the surface 16 of roll 12 (Fig. 1) before the use of an embodiment of
doctor blade 10, providing a
magnified view of the peaks and valleys caused by contaminants and/or the ends
of fibers aligned in
the machine direction. Fig. 4B shows the roll profile of the same section of
roll surface 16 after the
use of an embodiment of doctor blade 10. The action of doctor blade 10 on the
roll surface 16 tends
to remove the tops 40 of the peaks to a uniform level L, resulting in a
decrease in surface roughness.
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Embodiments of doctor blade 10 may reduce the surface roughness of the roll
about 0.025 to 0.20
pm Ra, preferably about 0.050 to 0.13 pm Ra. Embodiments of doctor blade 10
may also be used to
maintain the surface roughness of the roll during the effective life of a
blade at a level of about 0.025
to 0.20 pm Ra., preferably about 0.050 to 0.13 pm Ra. The level of surface
roughness achievable
through use of the doctor blade of the invention will depend on the materials
used to form the surface
of papermaking rolls and the operating conditions of the particular doctoring
application.
The greater surface area of unidirectional fiber exposed in the composite
layers also
provides a more uniform abrasive surface with which to doctor the surface of
the roll. A more
uniform abrasive surface generally results in a more uniform roll surface
profile. Because a greater
l0 area of abrasive material, i.e., the longitudinal length of the
unidirectional fibers, is exposed during
doctoring, the doctor blade also wears more slowly and uniformly.
Preferred unidirectional fabrics tend to have a relatively open structure,
woven in the plain
weave style, in which the unidirectional and secondary fibers cross
alternatively. Unidirectional
fabrics are available in weaves having different degrees of openness. The
weight per unit area of a
15 unidirectional fabric provides an indication of the openness of the weave.
The weight per unit area of
the unidirectional fabrics is preferably about 230 to 610 g/m2. A loose, low
weight weave tends to be
less abrasive and to disintegrate faster than a tighter, high weight weave
under the same operational
demands. However, higher weight unidirectional fabrics may be more suitable in
doctoring
applications requiring a more abrasive blade to prevent rapid disintegration
that would result in
20 significant wear of the blade and a shorter blade life.
A loose, low weight unidirectional fabric tends to be more suitable for
doctoring applications
requiring a less abrasive blade, such as an uncoated paper web having a low
moisture level that
creates relatively little contamination of the roll surface. A tighter, high
weight unidirectional fabric
tends to be more suitable for doctoring applications requiring high
abrasiveness and high resistance
25 to wear, such as a coated paper web having a high moisture level that
creates significant
contamination of the roll surface. In a tighter, high weight material there is
an increased proportion
of secondary fiber "ends" per unit area of material exposed to the roll
surface. A tighter, high weight
material will exhibit a high abrasiveness per unit area because kinks are
created in the unidirectional
fibers as they weave over and under the secondary fibers, exposing a greater
surface area of abrasive
30 material. Practitioners skilled in the art would understand how to
determine whether a particular
weave is loose or tight using the guidelines provided above.
The diameters of the unidirectional fibers tend to be greater than the
diameters of the
secondary fibers. The diameters of the unidirectional fibers may be equal to
or greater than,
preferably greater than, the diameters of the secondary fibers. Preferably,
the diameters of the
35 unidirectional fibers are about 450 to 1500 pm, and the diameters of the
secondary fibers are about
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400 to 700 pm. If the secondary fibers have a greater diameter than the
unidirectional fibers, the
propensity to form scratches on the surface of the roll, and the width of such
scratches, is increased.
Suitable unidirectional fabrics including a plurality of unidirectional fibers
are available
from Fibre Glast Developments Corporation, Brookville, Ohio, e.g., 1093 E-
Glass Fabric, and from
Brunswick Technologies Inc., Brunswick, Maine, e.g., E-LPb 425 and E-LPb 567
0° Uni-
Directional fabrics.
Suitable impregnating resins include a thermoplastic or epoxy resin.
Preferably, an epoxy
resin system, comprising an epoxy resin and a curing agent, or hardener, is
employed to form the
laminate structure of the doctor blade of the invention. Resins used in the
doctor blade of the
invention are selected to withstand the operating temperatures used in the
particular doctoring
application. During operation, the resin used to manufacture the doctor blade
will be in contact with
the surface of the roll. The resin used should not melt and contaminate the
roll surface but should
wear away exposing the unidirectional fibers. Because the resin is not
abrasive, it is preferable that
the resin wears away faster than the unidirectional fibers.
The glass transition temperature, Tg, of the resin provides an indication of
the operating
temperatures it is designed to withstand. Resins suitable for use in the
doctor blade of the invention
have a Tg of about 55 to 315 °C, depending on the temperature of the
roll surface to be doctored.
For high temperature calendering applications, preferred resins are epoxy
resins having a Tg ranging
from about 65 to 315 °C, more preferably about 85 to 315 °C. If
the Tg of the cured resin is too low
for a particular doctoring application, the resin tends to melt and
contaminate the surface of the roll.
A resin with a high Tg would generally be an unnecessary expense for a doctor
blade used against a
roll operating at a relatively low temperature.
Suitable epoxy resin systems are commercially available from Fiber Glast
Developments
Corporation, e.g., the System 2000 Epoxy Resin used with 2020, 2060, or 2120
Epoxy Hardeners,
and from Resolution Performance Products, Houston, Texas, e.g., EPON Resin 828
used with EPI-
CURE Curing Agent 9552 or EPON Resin 862 used with EPI-CURE Curing Agent W.
Alternatively, an epoxy resin such as EPON Resin 828 or EPON Resin 826,
manufactured by
Resolution Performance Products, may be cured by other curing agents, such as
ETHACURE 100
Curative, available from Albemarle Corporation, Baton Rouge, Louisiana, or
methylene dianiline.
Practitioners skilled in the art are aware of how to select an appropriate
resin exhibiting a Tg
suitable for a particular doctoring application and for ease of use, e.g., the
time required to cure and
safety precautions.
The doctor blade of the invention includes about 50 to 75% fibrous material by
weight,
preferably about 60 to 70 %, and about 25 to 50% resin by weight, preferably
about 30 to 40%. As
the percentage of fibrous material in the doctor blade increases, the Tg of
the doctor blade tends to
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increase because the fibrous materials tend to have higher glass transition
temperatures than the
resins. The doctor blade of the invention should have a Tg of about 75 to 315
°C, depending on the
temperature of the roll surface to be doctored. For high temperature
calendering applications, the Tg
of the blade preferably is about 100 to 315 °C, more preferably about
150 to 315 °C. An increased
proportion of fibrous material also tends to increase the abrasiveness of the
doctor blade.
Typically, the thickness of each layer prior to bonding into the laminate
structure ranges
from about 0.20 to 0.50 mm for the composite material layers, and from about
0.09 to 0.50 mm for
the secondary layers. The thickness of the doctor blade 10 may range from
about 1.50 to 3.20 mm,
depending on the location of the doctor blade within the papermaking process
and the operating
conditions to which it is subjected. Thinner doctor blades tend to clean the
surface of rolls more
effectively over the life of the blade. Because the beveled edge of a thinner
blade is generally thinner
than the beveled edge of a thick blade, it provides a higher pressure per unit
area than the beveled
edge of a thick blade. Thicker doctor blades tend to have greater mechanical
strength and longer
blade life. The width of the doctor blade is also dependent on the location of
the doctor blade within
the papermaking process and the operating conditions to which it is subjected,
and may range from
about 50 to 100 mm. Practitioners skilled in the art are aware of how to
select the appropriate doctor
blade thickness and width that balances the desired life of the doctor blade
and level of contamination
of the roll surface.
The resin used to impregnate the composite fibrous layers may include abrasive
additives,
such as glass microspheres, glass fibers, crushed glass, synthetic or
industrial diamond particles,
silica particles, silicon carbide particles, boron particles, zirconium
particles, aluminum oxide
particles and mixtures thereof The impregnating resin may also include fi-
iction reducing additives,
such as carbon particles and powdered polytetrafluoroethylene. Reducing the fi-
iction between the
doctor blade and the surface of the roll tends to reduce the heat generated
during doctoring thereby
extending the life of the doctor blade. Practitioners skilled in the art are
aware of how to select
suitable additives to meet the requirements of a particular doctoring
application, e.g., to increase or
decrease abrasiveness or reduce friction, and to achieve the desired final
product attributes.
The reinforcement layers generally include carbon fibers, aramid fibers,
ceramic fibers,
fiberglass and mixtures thereof, preferably fiberglass. The reinforcement
layers may have a woven or
nonwoven structure and the fibers may be aligned substantially in the machine
direction or in two or
more directions. A woven structure tends to provide a greater level of
abrasiveness than a nonwoven
structure. The reinforcement layers may be woven in a plain, satin or twill
weave style, preferably a
plain or satin weave. Preferably, the weight per unit area of the
reinforcement layers is about 60 to
350 g/m2.
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Reinforcement layers comprising carbon fibers are typically used to reduce
friction and to
increase the strength of the doctor blade in the machine direction. Carbon
fibers are characterized by
high tensile strength and high stiffness but they are not considered abrasive.
Therefore, although the
ends of the carbon fibers may act upon the roll surface, they do not tend to
form scratches in the roll
surface. Aramid fibers may be used to provide tensile strength and abrasion
resistance to the doctor
blade. Ceramic or fiberglass reinforcement layers provide additional
abrasiveness to the doctor
blade. In view of the guidance above, practitioners skilled in the art would
understand how to select
the appropriate composition and number of the reinforcement layers within the
laminate structure to
meet the requirements of a particular doctoring application, e.g., to reduce
friction, to increase
to stiffness or to increase abrasiveness.
Suitable materials for the reinforcement layers are available from Fibre Glast
Developments
Corporation, e.g., 241 Woven Fiberglass Fabric, 530 3K Graphite Fabric, and
549 5HS Kevlar~
Fabric, and from Brunswick Technologies Inc., e.g., CBX 300 6k Carbon and ARBX
350 Aramid
fabrics.
A typical on-line calender is shown in Fig. 5, including two units 50, each
unit including two
soft rolls 52 and one metal roll 54. The soft rolls 52 are typically comprised
of a resilient or
yieldable material, such as fiber reinforced epoxy resin. Metal roll 54 may be
comprised of cast iron,
chilled iron, ductile iron, forged steel or cast steel. Metal roll 54 may be
further coated with a
thermal spray coating comprising a ceramic or metal matrix material, e.g., a
carbide containing
metal matrix material. The thickness of a thermal spray coating is typically
about 75 to 305 pm.
Generally the thermal spray coating is capable of being ground to a roughness
of less than about
0.20 pm Ra. The direction of rotation for each metal roll 54 is denoted by
arrow 22. A paper web 60
is passed through the calender units 50, with the aid of guide rolls 62.
Two doctor blades are used, a first doctor blade 56 positioned against the
metal roll 54 of
the first unit 50, and a second doctor blade 58 positioned against the metal
roll 54 of the second unit
50. The doctor blades 56 and 58 may be located anywhere on the circumference
of the metal roll 54,
provided that the beveled edge 14 (Fig. 1) operates against the rotational
direction of the metal roll,
as shown in Fig. 5. Preferably, each doctor blade is positioned after the
paper web 60 has passed
through both nips of each unit 50 formed by the soft rolls 52 and the metal
roll 54. Such a location
ensures that the doctor blade cleans the metal roll after one full pass of the
paper web. Practitioners
skilled in the art are aware of the most appropriate location for a doctor
blade taking into account
unique operational considerations, such as the process path of the paper web
and instrumentation or
other equipment in the vicinity of the roll to be doctored, safety
considerations, and maintenance
considerations.
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The beveled edge 14 (Fig. 1) is typically cut at a 45° angle from the
horizontal plane formed
by the base of the doctor blade. The operating angle A, of the doctor blades
56 and 58 should
generally range between about 25 to 30°, measured from the horizontal
formed by a tangential line to
the surface of the roll where the beveled edge of the doctor blade is
positioned. The pressure of the
doctor blade against the roll is typically about 85 to 700 N/m, preferably
about 175 to 440 N/m.
The doctor blades 56 and 58 may be applied intermittently to the surface of
the metal roll 54
for specific cleaning activities. It is generally preferable that the doctor
blades are applied to the
surface of the metal roll 54 on a substantially continuous basis while the
roll is in operation. Use of
the doctor blade on a near continuous basis ensures that abrasive contaminants
are continuously
removed from the surface of the roll. Consequently, deterioration of the roll
surface is significantly
reduced. Such use on a substantially continuous basis also tends to reduce the
roughness of the roll
surface and to maintain a consistent roll profile. Reducing surface
deterioration, reducing the surface
roughness and maintaining a consistent profile tend to result in consistent
product quality at a high
production rate with greater efficiency. The ability to minimize deterioration
of the roll surface and
to maintain the desired level of roughness tends to increase the life of the
roll, or in the case of a
thermal spray coated roll, the life of the surface coating.
The first calendering unit 50 generally requires a more abrasive doctor blade
56, such as the
embodiment depicted in Fig. 2, because the paper web has just been coated with
a paper coating and
dried, and, therefore, tends to contain more moisture. High moisture levels in
the paper and paper
coating increase the likelihood of adhesion of the paper web to the metal roll
54, resulting in hazing,
i.e., a thin layer of coating particles adhering to the surface of the roll.
Hazing reduces the transfer of
heat from the surface of the roll to the paper which reduces the gloss levels
of the paper web. Hazing
also increases the roughness of the roll surface which also reduces the gloss
levels of the paper. If a
less abrasive blade is used in the first unit, pressure of the doctor blade
against the face of the roll
tends to be increased in an effort to scrape the residue off the roll surface.
Increased blade pressure
tends to decrease the life of a doctor blade.
A less abrasive doctor blade 58, such as the embodiment shown is Fig. 3, is
typically
employed for the second unit 50 because the paper web is drier and adhesion to
the metal roll 54 is
reduced. Because the roll is relatively clean of contaminants, the pressure of
a more abrasive blade
against the roll must be controlled closely to prevent excessive roll wear. If
a more abrasive doctor
blade is used against the roll, an excessive amount of the roll surface may be
removed, decreasing
the life of the roll surface. As discussed above, the abrasiveness of the
doctor blade is typically
affected by the use and composition of reinforcement layers, abrasive
additives, the ratio of resin to
fibrous material, and the openness of the weave for the composite material.
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Example
Composite doctor blades including 7 layers were manufactured using 4
reinforcement layers
and 3 composite material layers. The laminate structure was formed from
alternating reinforcement
and composite material layers, as depicted in Fig. 2 where the outer layers
and the core layer are
formed from reinforcement layers. The reinforcement layers were formed from a
fiberglass fabric,
woven in a plain weave style, supplied by Essco, Inc. The unidirectional
fabric used in composite
material layers was 1093 E-Glass Fabric, a fiberglass fabric in which 95% by
weight are
unidirectional fibers, available from Fibre Glast Developments Corporation.
The impregnating resin
was an epoxy resin, supplied by Essco, Inc., and had a Tg of about 90 to
115°C. The doctor blades
were used in the second calender unit, similar to the second unit 50 shown in
Fig. 5. The pressure of
the doctor blade against the metal calender roll, which was coated with a
thermal spray coating, was
about 350 N/m and the operating angle was about 27°. The roughness of
the surface of the roll after
doctor blades had been applied against the surface of the roll on a
substantially continuous basis for
about 21 days was about 0.05 to 0.08 pm Ra. After an additional 27 days, the
roughness of the
surface of the roll was about 0.08 to 0.13 pm Ra. Surface roughness was
measured using the
Surtronic 3+ instrument, manufactured by Taylor Hobson Inc., Rolling Meadows,
Illinois. While the
surface roughness of the roll increased slightly, the level of roughness
remained very low, indicating
minimal deterioration of the roll surface. The surface roughness obtained by
the doctor blades would
be considered advantageous for the manufacture of coated printing papers. The
doctor blades each
had a life of about 5 to 7 days, which would be considered an acceptable level
of resistance to wear
when used on a substantially continuous basis.
Other embodiments are within the claims. For instance, the doctor blades of
the invention are
suitable for use in other web manufacturing industries which employ rolls,
e.g., printing, polymer
film, flooring, and textile. Various modifications of this invention will
become apparent to those
skilled in the art without departing from the scope or spirit of this
invention.
What is claimed is:
14
