Press roll for paper making machines

A press roll for a paper machine suitable for use as a substitute for a granite roll includes an inner roll body having a metallic cylindrical exterior to which a coating is applied formed either only of a metallic component or of a mixture of metallic and ceramic components. Where the coating material includes only a metallic component, such a metallic component is chosen so that a separate ceramic component precipitates during the alloying of the material. The coating material is formulated to obtain a surface energy and polarity component thereof which provide the roll surface with suitable web release properties for particular pulp qualities.

BACKGROUND OF THE INVENTION 
The present invention relates generally to press rolls for use in paper 
making machines and, more particularly, to press rolls useful as 
substitutes for granite rolls in paper making machines. 
Stone rolls, often used in the center roll position of paper machine press 
sections, are usually formed of granite since a granite surface has 
generally acceptable web release properties and, additionally, is 
relatively resistant to the abrasive effects of a doctor. 
However, granite press rolls have drawbacks resulting from other properties 
of granite. Granite is a non-homogeneous, brittle material having internal 
flaws inherent in its structure. Further, the properties of granite vary 
with heating and cooling cycles. This, in conjunction with the anisotropic 
properties of granite, can cause a number of problems which must be taken 
into account both in the manufacture and use of the rolls. For example, 
special grinding techniques at elevated temperatures have been used to 
improve the quality of granite rolls. 
The fact that internal flaws exist in granite has limited the linear loads 
to which granite rolls can be subjected thereby limiting the extent to 
which a web can be dewatered in the paper machine press section. Although 
sophisticated designs, such as the use of tensioning flanges attached at 
the ends of a steel shaft passing through the thick cylindrical granite 
mantle for eliminating the possibility of tensile loads acting on the 
mantle under any loading conditions, have reduced the susceptibility of 
granite rolls to catastrophic failure, occasional cracking has occurred 
with consequent expensive down time required for repairs or replacement of 
the roll. Another disadvantage of granite rolls is their great weight 
which necessitates extensive frame constructions and which increase the 
tendency for the paper machine to vibrate during operation. 
Attempts have been made for these reasons to provide synthetic substitutes 
for granite rolls. For example, rolls have been provided with coverings 
formed of a polymer, such as hard rubber or polyurethane, to which a rock 
powder, such as quartz sand, has been added. However, such synthetic rolls 
have the drawback that the paper web has a tendency to adhere excessively 
to the roll surface. Moreover, such rolls have poor strength properties 
and cannot be used at elevated temperatures, such as above 80.degree. C. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide new and improved press 
rolls for paper machines. 
Another object of the present invention is to provide new and improved 
press rolls for paper machines which can be used in place of granite rolls 
but which are not subject to the disadvantages of granite rolls described 
above. 
Still another object of the present invention is to provide new and 
improved press rolls of the type described above which can be constructed 
from conventionally manufactured roll bodies which provide mechanical 
strength for the roll. 
A further object of the present invention is to provide new and improved 
press rolls for paper machines having web release surface properties 
suited for particular pulp qualities and which, additionally, are highly 
resistant to strains caused by temperature variations. 
The invention is based on the recognition that the release characteristics 
of a press roll depend upon the surface energy of the press roll which in 
turn is a factor of both the material of which the surface is formed as 
well as the relative proportions of hydrophilic and hydrophobic components 
in the surface material. The surface energy of the roll must be within 
certain limits in order to control the detachment or release of the web 
from the surface of the press roll. For example, granite will provide 
generally acceptable web detachment or release characteristics when its 
surface energy, which varies as a function of the roughness of the granite 
surface, is in the range of between about 41 to 50 mJ/m.sup.2. 
Additionally, the hydrophilic/hydrophobic characteristics or behavior of 
the roll surface also affects the web release characteristics of a press 
roll. Since hydrophilic substances repel hydrophobic substances and vice 
versa, and since a paper web contains hydrophilic and hydrophobic groups, 
the release charactertics of a roll surface material will also depend on 
the ratio of hydrophilic to hydrophobic components contained therein. 
Different paper grades contain different respective proportions of 
hydrophilic and hydrophobic substances characterized by the particular 
pulp quality. For example, release problems have been found to exist in 
the case of a granite press roll and unbleached kraft, such as sack paper, 
indicating that a roll material is needed that behaves more 
hydrophilically than granite. 
Accordingly, it is believed that various paper grades should possess 
preferred matching roll materials with particular hydrophilic and 
hydrophobic compositions and surface energies that yield advantageous 
sheet release properties for that particular paper grade. 
Briefly, a press roll is provided including a roll body or mantle having a 
metallic exterior and manufactured by conventional techniques, and a 
coating applied to the exterior of the roll body to form the outer surface 
of the press roll. According to the invention, the coating is formed of a 
material comprising either a metallic component only or a mixture of a 
metallic component and a non-metallic, ceramic component. The compositions 
of the metallic and ceramic, if any, components are formulated to provide 
the coating with a surface energy and hydrophilic/hydrophobic 
characteristics (measured by the polarity component of the surface energy) 
which will yield advantageous sheet release properties for a particular 
paper grade. The surface energy is generally within the range of between 
about 35 to 50 mJ/m.sup.2. A material having a polarity component lower 
than 7.1 mJ/m.sup.2. A material having a polarity component lower than 7.1 
mJ/m.sup.2 behaves more hydrophobically than granite. More hydrophilic 
coating materials provide advantageous release properites for unbleached 
pulps whereas coating materials which behave hydrophobically produce 
advantageous release properties for newsprint pulps. 
In the case where the coating material comprises only a metallic component, 
it is formulated so that a separate ceramic component precipitates in the 
coating process during which the material is alloyed to provide corrosion 
and wear resistance. For example, the roll body may be coated by applying 
a mixture of materials in the form of a band over the roll body exterior 
and using an arc welding technique which alloys the material with a 
separate ceramic component precipitating during the welding process. Where 
the ceramic component is not provided as a result of the alloying of the 
metallic component, the coating can be provided, for example, by preparing 
an admixture band from a powder comprising a mixture of metallic and 
ceramic materials by rolling and sintering or the desired ceramic 
component can be provided in a shield powder used to alloy the welded 
surface material while in a molten state. Suitable ceramic materials 
include, for example, Al.sub.2 O.sub.3, SiO.sub.2, TiO.sub.2, ZrO.sub.2, 
SiC, TiC, Cr.sub.3 C.sub.2, WC, NbC, VC, and Cr.sub.7 C.sub.3. Thus, the 
coating applied to the roll body in any event includes a separate 
component of ceramic material so that not only is the desired surface 
energy and hydrophilic/hydrophobic behavior achieved to provide a 
controlled release of the web from the roll, but, additionally, desirable 
corrosion resistance and wear resistance properties are obtained. 
The metallic component generally has a composition comprising a base metal 
and a relatively large proportion, e.g., between about 9 to 35 percent, of 
chromium. The use of chromium is advantageous in that chromium is a 
hydrophilic material and the addition of chromium increases the surface 
energy of the metal as well as its polarity component.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In accordance with the invention, a coating of a material having the 
desired properties is applied to a roll body or mantle having a metallic 
exterior. The roll body or mantle is manufactured in conventional manner 
such, for example, as by casting, or alternatively, by forming the same 
from a metal sheet or sheets formed in a cylindrical shape with abutting 
end edges welded to each other. The roll body or mantle, including the 
ends and axle journals, provide the roll with necessary mechanical 
strength. Generally, the coating is formed from a material comprising 
either only a metallic component, in which case a separate ceramic 
component precipitates during the application of the material onto the 
roll body, or a mixture of a metallic component and a ceramic component. 
The metallic component is formulated to have a surface energy and a 
polarity component thereof which yield the desired sheet release 
properties while the ceramic component provides the roll surface with good 
resistance to wear and corrosion. 
Preferred metallic components for a roll coating in accordance with the 
invention are materials alloyed with relatively large quantities of 
chromium, e.g., wherein the proportion of chromium in the metallic 
component is in the range of between about 9 to 35 percent. Chromium is a 
hydrophilic material and the addition of increasing amounts of chromium to 
the metallic component increases the surface energy and its polarity 
component. A particularly advantageous metallic component comprises 
stainless steel containing chromium in proportions ranging from about 9 
percent to about 35 percent. Stainless steel containing chromium in the 
higher end of this range is a hydrophilic substance whereas stainless 
steel containing chromium in the lower end of the range exhibits 
hydrophobic behavior. By adjusting the amount of chromium, the surface 
energy and polarity component of the coating can be controlled in 
accordance with the web release properties suitable for a particular pulp 
quality. Additionally, wear and corrosion resistant chromium carbides are 
formed during the alloying of the stainless steel metallic component. The 
formation of a separate ceramic component during alloying makes it 
unnecessary to add a separate ceramic material. 
EXAMPLE 1 
An experimental roll having a diameter of 1220 mm and a length of 1150 mm 
was provided with a stainless steel coating having the following 
composition (&): 
______________________________________ 
C Si Mn P S Cr Ni Mo 
______________________________________ 
0.19 0.55 0.17 0.035 
0.010 13.15 
0.15 0.01 
______________________________________ 
The chemical composition of the coating was selected in order to achieve, 
owing to the chromium content, adequate wear resistance and corrosion 
resistance (though the precipitation of chromuim carbide during the 
alloying) and a surface energy and polarity component particularly suited 
for newsprint pulps. Optimum web release for newsprint pulps is obtained 
by coating materials which are slightly more hydrophobic than granite. The 
surface energy of the coating of Example 1 is 39.3 mJ/m.sup.2 and its 
polarity component is 7.1 mJ/m.sup.2 which is at a lower limit as compared 
with granite, i.e., the coating is slightly more hydrophobic than granite. 
Referring to FIG. 1, the release properties for newsprint furnish of the 
experimental roll provided with the coating having the components of 
Example 1 are illustrated compared to those of a conventional granite 
roll. 
The vertical axis represents the release angle in degrees and the 
horizontal axis represents the web release speed difference in percent. 
The speed of the roll surface was 15 m/s. Other details of the procedure 
appear in FIG. 1. It is noted from FIG. 1 that the coating of Example 1 is 
less sensitive to process changes than granite as evidenced by the 
narrower scatter band. It is also noted from FIG. 1 that the slope of the 
release curve is steeper for the granite roll than for the press roll 
provided with the coating of Example 1. This indicates that any change in 
web tension will cause a larger fluctuation of the release angle in the 
case of the granite roll than in the case of the press roll of the 
invention. Consequently, sheet flutter is reduced and runnability is 
improved in the case of the roll of the invention. 
Referring to FIG. 2, the release properties for newsprint pulp of the 
coating material of Example 1 are illustrated compared to those of a 
conventional polymer-based coating. The vertical axis represents the 
release angle in degrees and the horizontal axis represents the web 
release speed difference in meters per second and in percent, the speed of 
the roll surface being 20 m/s. The speed difference is the difference 
between the speed of the web being released from the roll surface and the 
speed of the roll surface. Other details of the process appear in FIG. 2. 
After one year of experimental operation, no wear or corrosion in the 
surface of the press roll of the invention could be discerned. The roll 
exhibited excellent resistance to mechanical and thermal damage, is light 
weight and the hardness of coating is 500HV. 
The invention is of course not limited to this composition. For example 
variations in the chromium content will vary the surface energy, polarity 
component, hardenability, and carbide precipitation. 
EXAMPLE 2 
Another alloy was formulated which was more hydrophobic than the alloy of 
Example 1 so as to exhibit release characteristics suitable for newsprint 
pulps. The composition (&) of the alloy was as follows: 
______________________________________ 
Cr W C Co 
______________________________________ 
26 5 1 balance 
______________________________________ 
The surface energy of this cobalt-based alloy is 36.5 mJ/m.sup.2 and the 
polarity component is 3.5 mJ/m.sup.2. Thus, a coating formed of this alloy 
provides web release characteristics suitable for newsprint pulps. A 
separate carbide component of the coating precipitates during the alloying 
of the coating material. 
EXAMPLE 3 
An alloy was formulated with a view toward providing a coating material 
which exhibits advantageous release behavior for unbleached paper pulp, 
i.e., which is more hydrophilic than granite. In this connection, 
unbleached paper pulp has large contact angles with granite indicative of 
the fact that such pulps require a coating that behaves more 
hydrophilically than granite. The composition (&) of the coating was as 
follows: 
______________________________________ 
Cr W C Mo Fe Ni 
______________________________________ 
17 7 0.1 17 6 Balance 
______________________________________ 
The surface energy of this nickel-based composition alloyed mainly with 
chromium and molybdenum is 45.5 mJ/m.sup.2 and the surface energy polarity 
is 13.7 mJ/m.sup.2, i.e., more hydrophilic and therefore suitable, for 
example, for unbleached paper pulp. 
EXAMPLE 4 
Coatings comprising different mixtures of the same metal and ceramic 
components were applied to the roll body by plasma spraying in order to 
vary the surface energy of the coating substantially over a desired range. 
The compositions of the metal and ceramic components are shown below along 
with the surface energy and polarity component corresponding to each of 
the four different mixtures of the metal and ceramic components: 
______________________________________ 
A B 
87% Al.sub.2 O.sub.3, 13% TiO.sub.2 
10% Al, 90% Cu 
Surface Polarity 
Particle Size 
Particle Size 
Energy Component 
5-45 .mu.m (%) 
45-106 .mu.m (%) 
m J/m.sup.2 
m J/m.sup.2 
______________________________________ 
0 100 39.1-40.7 
5.5-7.6 
20 80 39.5-41.1 
6.0-8.3 
50 50 42.6-46.6 
9.9-14.5 
80 20 47.1-47.6 
15.4-16.1 
______________________________________ 
A schematic illustration of the application of a coating in accordance with 
the invention to a roll body using a band welding technique is shown in 
FIG. 3. The roll body includes a cylindrical mantle 1 formed of metallic 
material and axle journals 2 at its ends. A welding band formed partially 
or entirely or a metallic component is supplied by a welding head 3 onto 
the surface of mantle 1 and the roll rotates in the direction of arrow 5. 
The welding band 4 is supplied at a speed that is greater than the rate at 
which the band is welded onto the surface of the mantle 1. 
The band 4 is melted in a shield powder arc, designated 6, so that a 
coating 7 is thereby applied to the surface of mantle 1. The surface of 
mantle 1 may be pre-heated by means of a blower 8 prior to coating. 
In order to increase the resistance of the coating to mechanical strains 
and to improve the resistance of the coating to corrosion, a ceramic 
component may be added to shield powder 6 whereupon the shield powder 6 
becomes alloyed with the molten material of the band 4. Alternatively, a 
ceramic material may be included in the coating by preparing the admixture 
band 4 to include both metallic as well as ceramic components. 
Referring to FIG. 4, the preparation of an admixture band 4' that contains 
both metallic and ceramic components is illustrated. A powder 9 comprising 
a mixture of the metallic and ceramic components is supplied at a suitable 
rate between two rolls 10 and 11 which compress the powder 9. For example, 
the powder 9 may comprise a mixture of chromium-containing stainless steel 
and a ceramic material such, for example, as one of Al.sub.2 O.sub.3, 
SiO.sub.2, TiO.sub.2, ZrO.sub.2, SiC, TiC, Cr.sub.3 C.sub.2, WC, NbC, VC, 
and Cr.sub.7 C.sub.3. 
The compressed powder 12 is then sintered at 13 to form the admixture band 
4' which is reeled into a roll. Alternatively, the ceramic material may be 
separated from the metallic component itself, such as where 
chromium-alloyed stainless steel is used with chromium carbide being 
produced without the need for separately adding the ceramic material. 
The band welding technique of the invention provides important advantages, 
such, for example, as a high coating speed, uniform penetration to only a 
relatively small extent, improved reproducibility and workability, and 
simple construction for the manufacturing equipment. Moreover, the method 
is suitable for coating in a workshop, is economical, provides good joint 
strength between the coating and the surface of the cylindrical mantle and 
requires relatively small investments for tooling up. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the claims appended hereto, the 
invention may be practiced otherwise than as specifically disclosed herein 
.