CA1314164C - Roll for use in fabrication of coated paper - Google Patents

Abstract

Abstract of the Disclosure A roll for use in fabrication of coated paper as a gloss calender roll and a machine calender roll is disclosed. The roll is provided onto its surface with a sprayed coating made of a compound of tungsten carbide and cobalt or nickel-chromium alloy, in such a way that a porosity of the sprayed coating of cermet is selected less than 1.2 % and surface roughness Rmax of the sprayed coating is selected less than 1.8 µm.

Description

1314~64 ROLL FOR USE IN FABRICATION OF COATED PAPER
Brief DescriPtion of the Drawinas Fig. 1 is a diagram showing a coating line of white paperboard;
Fig. 2 is a graph showing a relation between a pressure of fuel gas for spraying heat source supplied to a spraying gun and porosity of the coating formed by the fuel gas; and Eig. 3 is a graph showing a relation between a porosity of WC cermet sprayed coating and roughness of surface finishing of the coating.
Backaround of the Invention The present invention relates to a roll for use in fabrication of coated paper, particularly, to surface treating rolls, such as a gloss calender roll and a machine calender roll for use in a coating line of white paperboards, varlous kinds of rolls for use in the equipments of other portions in the coating line, rolls for use in a manufacturing line of various coating papers other than the white paperboards, and rolls for use in a manufacturing line of non-coated printable papers and non-coated wrapping papers.
As a characteristic of carton board for use in fabrication of boxboards and dressing cases, it is preferred to have a hard quality of paper and a large stiffness. The carton board having such a characteristic is referred to as a paperboard.

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131~164 One of these paperboards used for a high grade paperboard carton, such as boxes for cosmetics and medical supplies is referred to as a white paperboard to which a color printing is applied for use.
Most important quality characteristic of such a white paperboard is a printability of its surface. In order to obtain such a printability, the white paperboard ~ust be subjected to a coating process by coating on the surface a paint coating of kaolin (clay:

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main component Al2Si20s(OH)~) and calcium carbonate as a main com~onent, thereby improving smootnness, gloss contrast and printability of the surface of the white paperboard This coatinq step is also referred to as a coating part and positioned at downstream of a papermaking line in the coating process.
Fig. 1 shows an outline of the coating line which is explained hereinafter in accordance with the drawing. At first, a paperboard or fiberboard 1 which is made of and drled, is entered from a dryer part A
into a calender coater part a in which the paperboard 1 is subjected to a smooth coatingprocess of pulp fibers through machine calender rolls 2, a coating pan or tank 3 having coating material 4 therein, dryer rolls and machine calender rolls 6 and then made the thickness of the paperboard uniform and added gloss onto the surface of the paperboard at a gloss calender roll 7. For such a uniformization and gloss addition, hitherto, the surface of the gloss calender roll is subjected to a hard chromium plating (hereinafter, referred to as chromium plating).
The gloss calender roll having such a chromium plated coating, however, cannot add gloss to a base paper for coated paper, sufficiently, so that the paperboard must be added a super calender roll process as an after-step of the process. The roll for use in fabrication of coated paper having the chromium plated 131~64 coating, also, is susceptible of wear due to hard microparticles (for example, clay consisting of main component Al2Si20s(0H),) ln the coating material applied to the surface of the paperboard, so that the gloss of the roll diappears within a short duration and thus t.he mirror surface holding function required for gloss calender roll becomes poor.
In the coating step, moreover, the surface cleaning is performed by using a hard metal doctor blade, such as a cutting steel in order to eliminate excess coating material adhered onto the surface of the roll during operation thereof. When the chromium plated surface is subjected to cleaning process due to the doctor blade system, the chromium plated surface is susceptible of injury and its function disappears within a short duration.
From the above reason, the prior roll for use in fabrication of coated paper with chromium plated coating must be exchanged every 2 ~ 6 months or the chilled casting roll must be exchanged every 2 weeks -one month, so that maintenance cost becomes increased and productivity becomes decreased due to stopping of coating line.
~ n the while, in order to improve productivityand quality due to simplification of coating step, the feature that the surface temperature of gloss roll is incrèased from present 150 C to about 300 C have been X

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investigated. When the chromium plated coating is heated to 300 C , it is found that hardness becomes decreased and abrasion resistance becomes largely deteriorated, so that this method does not provide effective resolution.
Summary of the Invention - The present invention has for its object to provide a roll for use in fabrication of coated paper having high-functional coating which eliminate the above described defects.
According to the present invention, there is provided a roll for use in fa~rication of coarted paper having its surface with sprayed coating of cermet, characterized by comprising the sprayed coating of cermet having porosity of less than 1.2 % and having a sprayed coating surface roughness Rmax of less than 1.8 ~m.
The present invention provide a roll having good mirror surface by forming sprayed coating of cermet onto the surface of gloss calender rolls and machine calender rolls from the materials consisting of mixed compound of tungsten carbide (hereinafter referred to as "WC") having high hardness and high denseness and Co or an alloy of Ni-Cr, and then by abrasive finishing the surface of the sprayed coating as a smooth surface.
According to the present invention, sprayed coating of cermet of WC has a hardness, an order of Hv:

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1100 ~ 1300 (hardness of chromium plated layer Hv: 800 ~
900), so that the surface of the sprayed coating can not be injured even though wear effect caused by coating materials and the use of hard doctor blade. The hardness of sprayed coating according to present invention can not be decreased as in the chromium plated coating even though heated to 300 C, so that the hard doctor blade can be used over a long period of time and thus the mirror surface holding function of the roll as well as gloss addition to the paper may be held at any time, thereby obtaining the coated paper with high quality and low cost.
The roll for use in fabrication of coated paper according to the present invention such as gloss calender roll and machine calender roll is provided over its surface with a sprayed coating of carbide cermet consisting of compound of tungsten carbide as a component and cobalt or nickel-chromium alloy, in such manner that porosity of the sprayed coating of cermet is less than 1.2 % by spraying and surface roughness Rmax is less than 1.8 ~m by polishing, thereby obtaining gloss calender rolls and machine calender rolls.
The carbide cermet used in the present invention is a mixture of metal carbide such as tungsten carbide and cobalt or nickel - chromium alloy. For example, as such a mixture, following composition is used.

131~164 WC (X) - Co (Y) WC(X) - Ni(Y) - Cr(Z) wherein X = 95 wt % ~ 68 wt %
Y = 5 wt % ~ 32 wt %
Z = 5 wt % ~ 25 wt %
According to the present invention, sprayed coating of cermet of tungsten cabide (hereinafter, referred to as "WC") sprayed onto a roll surface, has a hardness larger than that of chromium plated layer (Hv800 ~ 930), and Vickers hardness being an order of Hv1100 ~ 1300, and is excellent in abrasion resistance.
The surface of the sprayed coating of cermet exhibits the condition that in view of macro-observation, WC fine particles each having an acute angle are stood together in large numbers. Therefore, the sprayed coating of cermet effects to cause scratches and dents onto the coated paper as it is, so that the coating is susceptible of adhesion of stickies and tackies.
The sprayed coating of cermet, also is a mixture of WC particles of high hardness and metals, so that coating made by plasma spraying the mixture has a metal structure that metal particles are displaced around WC particles. The WC particles, however, are hard and do not perform a plastic deformation so that WC
particles themseves are destroyed by the collision at spraying to fall a part thereof and the collision energy 13141~4 makes cracks onto the preformed coating so that voids tend to generate in the sprayed coating of cermet. If such a sprayed coating having voids therein is polished, these voids are exposed thereby forming pits onto the mirror finished surface, so that such a mirror surface becomes inappropriate as a gloss calender roll or other rolls.
Considering from such a view point, the sprayed coating for use in gloss calender roll according to the present invention, must be mirror-finished finally, so that even if the sprayed coating has a high density (porosity is low) and a large number of voids therein, as a characteristic, an indispensable condition is that the voids are fine or microscopic to an extent that does not affect the mirror finishing. If the surface of the sprayed coating rolls is too mirror~
finished, its frictional coefficient required as roll becomes decreased because of very hard WC particles, so that mirror-finish requires a great deal of labor and too much expenses.
There are importances that as a gloss calender roll, the sprayed coating is formed by taking these problems into account and a proper surface finishing is selected.

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Detailed Description of the Preferred Embodiment The present inventors have studied spraying conditions to obtain a fine sprayed coating and paied attention to following means, i.e., (1) softening of the sprayed particles due to increase of atmospheric temperature and (2) increase of collision energy to the surface of a body to be sprayed. However, according to the means (1), high hardness as a carbide can not be utilized since WC is changed in quality to lower carbide or intermetallic compound or an oxide due to high temperature. The present inventors, therefore, have studied the means (2) again.
In concrete, the jet speed of combustion gas is made high and the collision energy of spraying particles is made high by using oxygen-hydrogen gas or oxygen-hydrogen gas combustion flame as a heat source and by increasing pressure supplied to a spraying gun ( the more the supplying pressure of fuel gas becomes, the faster the jet speed of combustion gas becomes,) as a result of this, the reduction of the porosity of the E~ .

-- 131~164 sprayed coating is aimed at. Fig. 2 shows a relation between a pressure of oxygen-hydrogen gas supplied to a spraying gun and porosity of sprayed coating. As shown in Fig. 2, it is found that for the sprayed coating the higher the supplying pressure of fuel gas becomes, the lower the porosity becomes.
Then, by using the rolls with WC cermet sprayed coating having various porosity, the coating process of paperboard is experimentally carried out with the use of gloss calender roll and machine calender roll shown in Fig.1 and its operating mechanism is observed.
A summary of the results is shown below.
(1) The machine calender rolls 2 provided prior to coating step are generally loaded by large nip pressure (30 ~ 80 kg/~) and a little amount of water is added to the passing paper in order to smooth fiber of the paper, give directional property and adjust the thickness of paper. The rolls according to the present invention can be tolerated such a mechanical pressure and proof against corrosion due to moisture.

(2) When the mirror finished roll having very hard WC cermet sprayed coating is used, the coated paper as well as coating surface of the roll are quitely subjected to injure, so that very beautiful coated paper can be obtained.

(3) Since WC cermet sprayed coating is very hard, this coating is proof against wear due to clay 131416~
particle included in the coating material so that the sprayed coating (roll surface) is not subjected to injure.

(4) Even though hard doctor blade of high speed tool steels is used, WC cermet sprayed coating is not subjected to injure and foreign matters such as stickies and tackies can be completely eliminated, so that the roll surface can always be held at normal condition and maintenance need not over a long period of time, thereby providing costdown of the roll.

(5) The above properties are not quite chenged even though the temperature of roll surface becomes 300 C, so that the heated roll can be used, thereby improving gloss of coated surface of the paper and referring the production of high quality coated paper.

(6) However, since for WC cermet sprayed coating with high porosity, even though the coating is subjected to mirror finishing, a large number of pits appear on the coating surface due to voids, so that mirror finished surface can not be obtained. As a result this, excess coating materials and tackies are remained in recess or uneven portions, so that the gloss of coating surface becomes poor and the thickness of coating becomes uneven, thereby decreasing utilizing effect of the doctor blade and not exhibiting its function, resulting in a decrease of value as a roll.

(7) The machine calender rolls 2 with sprayed ~- 1 0 --131~64 coating of high porosity are subjected to generation of red rust due to moisture penetrated in the interior of the voids (particularly, in case of stoppinq coating line).
As described above, for gloss calender rolls according to the present invention, it is concluded that it is important to determine a compatibility of both the porosity of WC cermet sprayed coating and the surface finishing thereof.
Fig. 3 shows a relation between porosity of WC
cermet sprayed coating and surface roughness Rmax thereof. As shown in Fig. 3, it is found that the coating having low porosity can be mirror-finished, and then the coating having the porosity of less than Rmax 0.1 ~m can also be mirror-finished. However, the sprayed coating having high porosity can not be mirror-finished, since the inherent voids appear on the surface to be finished as a pit. The polishing work is fully performed by using synthetic diamond wheels and paste, and the surface roughness is measured by instrument for the measurment of surface roughness by the stylus method. The number of porosity of sprayed coating is obtained by taking a photograph in which a cross section of the sprayed coating is magnified about 500 times by a microscope, by staining the void portions, and by obtaining the stained areas with an image analyzer. The surface roughness is fully unified by maximum height - ~ -10a-;','~

Rmax. As a sign indicating the surface roughness, generally ,there are three ways, Ra, Rz and Rmax. Even though Ra showing an arithmetical mean deviation of the roughness curve and Rz showing ten point height are small, that is, the roll surface having small mean roughness is abtained, if the s~rface having large roughness exists at one portion, this large roughness portion makes scratches and dents on the coated surface, poor gloss, uneven gloss, defect to doctor blade, the present invention attaches importance to Rmax and adopted this maximum height.
The reasons that in the gloss calender roll and machine calender roll according to the present invention, the porosity is limited to less than 1.8%, and the surface roughness Rmax of the coating is limited to less than 3.0~m are explained hereinafter.
Example 1 Fig.1 shows a coating line of white paperboard used in this example. In Fig. 1, the paperboard 1 is sent from the right side of the drawing through predetermined papermaking process such as a papermaking step and a drying step. This white paperboard 1 passes through a stagemachine calender rolls for smoothing pulp fiber, that is, the surface of the paperboard and is entered into a coating tank 3 which accommodates coating materials 4 (main component:kaoline, calcium carbonate acryl resin and wax emulsim). After passing through the - ~ -lOb-.;i 1314~64 casting materials 4, the white paperboard 1 is dried by driers 5 and passed through 4 stagemachine calender rolls 6 and thus entered into a gloss calender roll 7.
This gloss calender roll 7 which forms a pair together with a touch roll 8, acts to give an excellent gloss surface to the paperboard by strongly depressing the coating portion of the paperboard onto the roll surface of the gloss calender roll 7 to the touch roll 8 in case of passing the paperboard between contact surface of both rolls 7 and 8.
The gloss calender roll 7 is engaged to the tip of a doctor blade 9 consisting of high speed tool steels thereby eliminating stickies and tackies adhered onto the surface of gloss calender roll 7.
The white paperboard with gloss given by the above steps is wound up through a cooling roll 10 or sent to a printing step.
In this example, the roll of cast iron having a dimension of 1200mm in diameter and 2000mm in length is provided with WC cermet sprayed coatings 1 ~ 9 (88 wt % WC - 12 wt % Co, 73 wt % WC - 20 wt % Cr - 7 wt % Ni) having adjusted porosity and surface roughness shown in Table 1a.
The operating or driving temperature of the gloss calender roll 7 is 280 to 310 C, and the material of the doctor blade 9 is steel stock of JIS G 4401 carbon tool steel SK7 having hardness HRC57. In the ~10c-1314~64 compared examples (No. lO, No. 11), the chromium plated rolls (plated thickness 100 ~m, surface roughness Rmax 0.1, 0.3 ~m, hardness Hv 820 ~ 890) are used under the same condition as the above condition.
The gloss of coated surface of the white paperboard after passing through the gloss calender roll 7, stain and injury or the surface of the gloss calender roll 7, damage to the doctor blade 9 and change in hardness of the coating are observed and investigated as evaluation items.
Table 1b shows results obtained by investigating and continuously operating respective gloss calender rolls over about 3 months. As is seen from this table 1, the chromium plated rolls used in the compared example have very smooth and mirror finished surface excellent gloss is given to the white paperboard (particularly at begining of case), but following defects are also observed. That is, stains to roll surface, the adhesions of stikies and tackies to the roll surface become gradually large as operating duration proceeds, so that when these stickies and tackies are eliminated with the use of doctor blades the softened plated coating is susceptible of scratches.
When scrathes are caused onto the plated coating, and tackies are further susceptible of adhesion onto the scratche portions, so that the utilizing frequencies of the doctor blade 9 become more large and thus the plated ~-10d-,~

coatlng as well as doctor blade itself are subjected to injury thereby exchanging the plated coating and the doctor blade after operation over two months.
On the contrary, in gloss calender rolls according to the present invention adhesion of stickies and tackies thereto becomes small and the roll surface is always held under clean condition so that the damage to doctor blade itself becomes small. The hardness of WC cermet sprayed coating is not changed at all even though the coating subjected to exposure of high temperature (280 C ~310 C) during operation term, so that the WC cermet holds excellent hardness and the injury of the coating due to the doctor blade 9 can not be recognized at all. Only the gloss calender rolls with coating of examples No. 6 - 9 do not give gloss to the white paper board, sufficiently, resulting in decrease of market value as white paperboard.
As is seen from the above example the porosity and the surface roughness of WC cermet sprayed coating for gloss calender roll corresponding to the object of the present invention, are correspond to the region shown in Fig. 3 by an oblique line. That is, WC cermet sprayed coating with porosity of 1.2 wt % or less and surface roughness of Rmax 1.8 ~m or less can be applied to the roll. m e difference between kinds of spraying materials WC (88 wt %) - Co (12 wt %) and WC (73 wt ~) -Cr (20 wt %) - Ni (7 wt %) can not be recognlzed, so , -lOe-,,,~

-` 1314164 that both spraying materials exhibit excellent performance.

131~164 Table 1a Classifil No. Porosity j Surface cation of cermet roughness sprayed ~Rmax.~m) __ _ __ _ _ coat ng Present 1 0.3 0.3 inven- 2 0.18 1.6 tion 3 U 7 ; 8 ~ ~'6 ~'1 Example 10 0 3 to be 11 _ 0.1 compared emarks 1) Hardness shows as micro-Vichkhers hardness and shows average value at 10 measuring points 2) 88WC-12Co : No.1, No.3, No.5, No.7, No.9 73WC-20Cr-7Ni : No.2, No.4, No.6, No.8, Table 1b Classifi No. Gloss Stain Injury Damage Chang~ Befor~
cation of to on to in use white roll coating doctor hard- after paper- surfac~ by blade ness use board doctor coat-blade ing Befor~ After use use __ --. F _ __. ~____ __ _ ~ .~_ __--= ~_=
Present1 fine Nothinc Nothing Nothing 1130 1150 inven- 2 N P /1 11 1260 1270 __ _ _ , _ _ tion 3 N N N 11 1170 1160 ___ ______ _____ .______ .
4 N N _ __ ~_ 1260 1270 5 N N __ _ __ _ 1 1 9 0 1210 6 partial L__~ __ _ 1250 1240 7 semi- N ____ N 1260 1240 8 gloss N N N1 170 1160 _ __ N N 1220 1220 Example 10 fineSlightGreat Great820 390 to be 11 N N exchange exchange 890 420 compared after after _ 2 month 1 month (Remarks) 1) Hardness shows as micro-Vichkhers hardness and shows average value at 10 measuring points 2) 88WC-12Co : No.1, No.3, No.5, No.7, No.9 73WC-20Cr-7Ni : No.2, No.4, No.6, No.8, 131~164 Example 2 In this example (experiences Nos. 12 ~ 16), experiences Nos. 1 - 5 of WC cermet sprayed coating according to the present invention are applied to machine calender rolls 2 and 6 (roll dimension: diameter 400 mm X length 2000 mm) and drying rolls 5 (roll dimensions: diamater 1000 mm X length 2000 mm) as it were. In the comparing examples (No. 17 and 18), current chromium plated rolls (the same as specific example 1) and a chilled casting abrasive roll ~No.19) are used at the same condition over about 6 months.
Tables 2a and 2b show the result of the above example. As is seen from the tables 2a and 2b, for machine calender roll and drying roll, the surface of the WC cermet sprayed coating according to the present invention is not contaminated by tackies and stickies and is not subjected to injury at stickies and is not subjected to injury at all, thereby performing very stable operation without stopping the coating line (because of unnecessary cleaning. On the contrary, the chromium plated roll in the compared example must be performed cleaning every about one month due to adhesion of tackies and stickies threreto, so that the plated surface of the chromium plated roll is forced to lean by each time stopping the coating line. When roll surface is observed after operating over 6 months, the surface of chromium plated roll is subjected to wear due to kaolin particles included in the coating material, thereby causing slight scratches, so that it is presumed durability of the roll coating is terminated within a very short term. The chilled casting abrasive roll in another compared example is forced to exchange in the term of 2 weeks to one month because of surface abrasion and scratche generation. On the contrary, in this example, there is no difference between two kinds of spraying materials so that above excellent performance can be exhibited.

Table 2a Classifi No. Porosityl Surface cation of cermet roughnes~
sprayed (Rmax.~m) coating c _ _ _ 12 0.3 c= ___ _ inven- 13 0.18 1.6 tion 14 _ 1.2 1.2 1.8 16 1.1 0.4 17 _ ~ _ 0.3 I~9 _ D ~
emarks : Cleaning of the roll according to the present invention is notn ecessary,particularly,and by using the chance in which the line is stopped to clean the chromium plated roll to be compared and abrasive roll of chilled casting, the roll is cleaned for caution's sake.

Table 2b Classifi- No.Tsurface Stain cleaning roll cation ¦condi- to frequen- surface tion roll cy for condi-of surface roll tion white surface after baaPed- 6 months c____ __ __ __ = _ __ __ _ _ Present 12 fine Nothing Evry fine inven- 13 N N 3 ~ ,, tion 14 __ ~ monthes _ _ 16 ~ _ ~ _ ~ __ __ _ ~ = _ 17 ___ present Evry Large incidenc~
18 _ _ _ _ N 2 weeks of fine scrath~
19 slaight- great Large surface ly poor roughening pulled surfac~
emarks : Cleaning of the roll according to the present invention is not necessary,particularly,and by using the chance in which the line is stopped to clean the chromium plated roll to be compared and abrasive roll of chilled casting, the roll is cleaned for caution's sake.

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The sprayed coating according to the present invention is applied to the machine calender rolls 2 at front stage of paper coating (in this machine calender rolls, the fiber of white paperboard which is not coated with coating materials is smoothened, made directional quality and adjusted thickness of paper) and the sprayed rolls are operated over about 10 months. As a result of this, very good surface conditions can be held over a long period of time, irrespective of circumstances in which more or less moisture applied to the passing paper, and the nip pressure and erosion due to the moisture are present during operation.
It is found from the above result that the machine calender rolls 2,6 provided with the sprayed coating according to the present invention can exhibit an excellent performance irrespective of presence of coating material onto the passing paperboard.
While the invention has been particularly shown and described herein with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various other modifications and changes may be made to the present invention from the principles of the invention as herein described without departing from the soirit and scope as encompassed in the accompanying claims.

Claims (10)

1. A roll for use in fabrication of coated paper having its surface with sprayed coating of cermet, characterized by comprising the sprayed coating of cermet having porosity of less than 1.2 % and having a sprayed coating surface roughness Rmax of less than 1.8 µm.

2. A roll for use in fabrication of coated paper as claimed in claim 1, wherein the sprayed coating of cermet is made of a sprayed carbide cermet consisting of compound of tungsten carbide and cobalt .

3. A roll for use in fabrication of coated paper as claimed in claim 2, wherein the sprayed carbide cermet is made of a compound of 95 ? 68 weight % of tungsten carbide and 5 ? 32 weight % of cobalt.

4. A roll for use in fabrication of coated paper as claimed in claim 2, wherein the sprayed carbide cermet is made of a compound of 88 weight % of tungsten carbide and 12 weight % of cobalt.

5. A roll for use in fabrication of coated paper as claimed in claim 1, wherein the sprayed coating of cermet is made of a sprayed carbide cermet consisting of compound of tungsten carbide and nickel-chromium alloy.

6. A roll for use in fabrication of coated paper as claimed in claim 5, wherein the sprayed carbide cermet is made of a compound of 95 ? 68 weight % of tungsten carbide, 5 - 25 weight % of chromium and 5 - 32 weight %
of nickel.

7. A roll for use in fabrication of coated paper as claimed in claim 5, wherein the sprayed carbide cermet is made of a compound of 73 weight % of tungsten carbide, 20 weight % of chromium and 7 weight % of nickel.

8. A gloss calender roll or a machine calender roll, each adapted for use in a coating line of white paperboard manufacturing process, the said roll comprises a sprayed cermet coating on a surface thereof, wherein the sprayed cermet coating consists essentially of (i) tungsten carbide and (ii) Co or an Ni-Cr alloy, has a Vickers hardness (Hv) of from about 1100 to about 1300 and has such a smooth surface that a porosity is less than 1.2% and a surface roughness Rmax is less than 1.8 µm.

9. A roll as claimed in claim 8, wherein the sprayed carbide cermet is made of a compound of 95 - 68 weight % of tungsten carbide and 5-32 weight % of cobalt.

10. A roll as claimed in claim 9, wherein the sprayed carbide cermet is made of a compound of 95 - 68 weight % of tungsten carbide, 5 - 25 weight % of chromium and 5 - 32 weight %
of nickel.