FI109370B - Setting amount of applied web treatment agent at leveling rod unit involves utilizing computation formula provided for pre-estimating the amount of applied web treatment agent under varying process conditions - Google Patents

Abstract

An amount of applied web treatment agent at a leveling rod unit is controlled by utilizing a computation formula provided for pre-estimating the amount of applied web treatment agent under varying process conditions. Controlling an amount of applied web treatment agent at a leveling rod unit, involves applying a furnish of a web treatment agent to a moving surface; and a metering the amount of coating furnish applied to the surface (4) by way of pressing the leveling rod (1) of a leveling rod unit against the surface. The method further includes controlling the loading of the leveling rod at least partially based the loading value obtained by the equation (I), where Cw is the amount of applied treatment agent per unit area; mu is the web treatment agent viscosity; V is the speed of the moving surface; L is the lineal loading of the leveling rod; D is the specific size of furnish (3) particles; rho is the furnish density; and a, b, c and d are constants.

Description

109370

The present invention relates to a method for controlling the amount of coating or other treating agent in the manufacture of paper and board and to predicting the thickness of the applied coating.

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Paper or paperboard can be treated in many ways to improve or modify their properties to suit different uses. In most cases, the treatment agent contains solids such as calcium carbonate and kaolin to improve printing properties. The treatment agent can be applied directly to the moving web or through a special transfer surface. Usually a rotating roller is used as the transfer surface, but instead of the roller, for example, a belt or a band can be used. Usually a scraper blade or * · scraper stick is used to smooth the surface to be applied. The problem with adjusting the amount of treating agent is that • t «* · · '· * * the fact that many variables affect the final amount of' * · [treating agent, so that each piece of equipment has

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prepare a table between the amount of coating material and the scraper pressure »* • · 25 to make an initial selection of • ·. Scraper pressure on the scraper bar or blade. Final processing. the amount of material is then adjusted by measuring the actual, ··· amount of treating agent and adjusting the scraper pressure with feedback control until the process settles to the desired • »2 109370 state, ie the final amount of treating agent to be obtained corresponds to the specified treatment amount of product. Such an adjustment procedure slows down the start-up of the equipment and its response to changing conditions. On the other hand, as the properties of the treating agent 5 change, the agent reacts differently to the scraper pressure, so pre-tabulated scraper pressure values are required for a large number of different treating agent formulations, or as the recipe changes, the equipment ramp is even slower. The properties of the treating agent, such as the solids content, may change as the treating agent is recycled in the machine cycle, which may affect the amount of treating agent or layer thickness. If the properties of the treating agent are not measured, the change will only be detected by the device measuring the amount of treatment agent. As such, it is easy to measure the various properties of treating agents, but little equipment has been developed for continuous measurement on a factory scale.

It is an object of the present invention to provide a process by which the amount of treating agent remaining on a web, film transfer roller or other moving surface can be calculated to adjust the amount of treating agent to a desired value as process conditions change.

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The invention is based on a formula that can ': predict the amount of coating applied to process conditions | changes.

More particularly, the method I i 3 109370 of the invention is characterized by what is stated in the characterizing part of claim 1.

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The invention provides considerable advantages.

5 As the amount of curing agent, curing agent or web to be treated changes, a change should be anticipated to quickly adjust the curing agent to the desired level. With the current high web speeds 10 used for a long time, finding the right adjustment causes a considerable amount of waste, even in a short period of time, so that control based solely on measuring the applied application rate and measuring based adjustment is too slow. With the present invention, it is possible to calculate the change in coating quantity caused by a change in process conditions and, for example, to adjust the pressure of the scraper bar against the application surface so that the applied amount of treatment agent is set immediately after the change. The invention is capable of responding to most changes affecting the amount of * * ··· · 20 treating agents, such as changes in the amount of dry solids, changes in viscosity and, of course, the speed of the surface to be treated. The main field of application of the invention is coating, ···, with various coating pigment blends, but it can, of course, be applied to surface sizing or other treatment of paper or board.

* · I * · »:, ·. The invention will now be explained in more detail with reference to the accompanying drawing 1 * 1 *.

* * • * · 30 i i »» | t *! 109370! 4! j

Figure 1 is a diagrammatic representation of the operation of a rod scraper.

A stick scraper is used to limit the amount of coating layer to be applied to a moving paper or board surface or film transfer roller and to smooth the surface. In addition, it can be used in conjunction with transfer belts and other similar moving surfaces.

The rod scraper consists of a rod body 2 and a smooth or grooved rod 1 rotatably fitted thereto.

The rod 1 is rotated within the rod cage 2 and the shape of the rod surface and the direction and speed of rotation can be influenced on the manufactured surface. The rod scraper may be a separate scraper or it may be, for example, a metering member delimiting the chamber of the short-stay applicator.

However, dispensing to the surface 4 under the rod always occurs in the same way. The treatment agent 3 previously applied to the surface 4 or contained in the applicator chamber is pressed by the surface into the nip between the surface 4 and the rod 1, which limits the amount of treatment agent leaving the nip 20, leaving only a desired layer 5 on the surface.

The thickness of the treatment medium passing between the doctor blade 1 and the surface 4 is mainly controlled by changing the pressure applied to the surface of the doctor blade 1, i.e. the pressure of the doctor blade 25. Since the thickness and properties of the hydrodynamic film formed between the moving surface and the rotating scraper rod are very dependent on many variables, the film thickness is very difficult to determine numerically. This invention is based on the fact that the scraping process can be described by a limited number of correctly selected variables.

> «* 4 109370 i I 5 i ([The most important variables affecting the scratching process are! I thickness h of the scraped layer 5, solids: specific size D, viscosity μ of the detergent mixture, velocity V of the moving surface 4 and maximum pressure at nip P. and variables can be represented by a non-dimensional formula:, xh (μνΛ (1) H ™} 10 As shown in the formula, it includes an unknown function g that has to be found.

An unknown function can be determined experimentally. In a film transfer coating test on a single coating composition 15, the film transfer roll speed was varied between 800 and 2000 m / min and the scraper bar line pressure varied between 0.5 and 3 kN / m. The specific size of the solids was assumed to be constant (1 µm) because in all experiments: the same type of pigment was used. Coating • I! The viscosity of 1 1 • '·' 20 was measured with a capillary viscometer at a shear rate of 105s_1. The maximum pressure measured as a function of the measured film thickness is shown in Table 1.

SO. - - ---- 45- 40- "m +„ 9 | 3S- i. · 25 f 30-. V · m1 + f «g ·.» I, ·. ·, I »« fs.;. ·· > '- I, s -. 1 £ .0- i- .....

... s-. · O J-1 1 1 --- 1 '400 eno {00 «000 1200

Maximum pressure P fkP1l. ··· ', 30 Table 1.

6 109370

As shown in the table, the same maximum pressure is obtained; very different coating thicknesses, so that the coating thickness remaining on the moving surface 5 cannot be directly deduced from the scraper pressure. In order to determine mathematically the thickness of the scraped layer, the aforementioned unknown function must be determined. When the ratio h / D is plotted against the dimensionless factor μν / PD of function g, Table 2 shows that the relationship follows the change in dimensionless factor 10 linearly from the test results, whereby function g can be assumed to be linear and determined by linear approximation.

H -1.1.00, 'Li — riipii ·! · TmC »CO,« S ».w» * / 1B' YA * B * X ^ 15, 'Ληηββ VihR finer ·· —-—---- «« /. ^ A 0.0044 0.03322 _ _ m * 1.4 "B 0.041ft 0JJ0IQ2 R tttllft; Vr |. - .- J *> ΙΛ- tVH *

M-IK

0Λ- *** · 0.2- / * * · 1 I „i - j— 20 * 15 20 25 30 35 40 45

B / D

* · * '··· * Table 2

Now, from formula 1, the layer thickness for which. 25 gives equation 2: »| (2, * - [^ - »11:::" a * * * ·

As can be seen from Equation 2, the values 30 experimentally determined remain the constants a and b. When the constants are determined, | 109370 i i i gives the thickness of the coating layer. The constants are device specific and the values obtained in one test run I are used below; a = 0.0416 and b = 0.0484.

5 For the above formula to be useful, the pressure in the nip must be represented by the force exerted on the rod. There are many different ways of loading the rod, and the force to be applied can be, for example, the pressure of the loading hose or the mechanical force that forces the rod cradle. The pressure at nip 10 is obtained, for example, by measuring, whereby the linear approximation gives the pressure of the formula: (3) P = cL + d 15 In the formula L is the bar load (bar) and constants in our example c = 407 and d = 38. Fitted with formula 2 gives: ; (4) A = ——-- M) l- | _407L + 38 and a 20 * · · * · * 'Formula 4 is obtained directly from the density of the treatment agent

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multiplied by the amount of coating applied or scraped Cw: • «(5) cw = \ - ^ -«>] £ [407L + 38 and ....: 25. · ”. Formula 5 can be used to calculate the amount of treatment agent • * ·. as a function of the load on the scraper bar. So, in generalized form, ··· 'has four constants a, b, c and must be experimentally determined. Because the properties of the treating agent, such as viscosity density and particle size, remain approximately the same unless the formulation of the agent is modified, the moving variables velocity of the moving surface and the force exerted by the rod remain variable variables. The speed of the surface is obtained directly from the control system and the properties of the coating 5 can be measured relatively easily, so that the desired amount of coating can be adjusted by means of formula 5 by varying the load on the rod. Since the formula can be used to accurately calculate the correct load on the scraper bar before the change in the coating amount, the adjustment of the coating quantity 10 is substantially faster than the adjustment method according to the coating quantity measurement and the subsequent load change. Coating quantity control and load control can further be used in conjunction with the invention, for example, in that the proactive control is computed and the final fine-tuned based on the measurement.

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It is essential for the invention that the ratio h / D is: presented in a form where the measurement results are plotted linearly, so that the solution of the unknown function g is reasonably easy. The coating amount calculated according to the invention as a function of the coating amount ... Table 3.

Iti · ♦ »1 ·» 9 109370 ί 70 -, --- 1

-Υ · A ♦ β * X

fP hfMWff VriM Enof ^ | E 60 ---- / 3¾ A 3.14869 1.24171, jpg 'a B 0.90939 0.03137 | O 50 · ft 0.94736 p «y ·" 2 -------. Ι · Λ! ISfi 5 8 »-" rtV '"I .: /.

8 v 10 - / 0-1 1 1 1 1 1. , 1 1 1 1 1 0 10 20 30 40 50 60 70

Measured coat weight [g / mj 10

Table 3 shows that there is a good correlation between measured and calculated coatings. Because the dispersion of the 15 dimensional variables shown in Table 2 is also small, only a small number of test points can be used to determine the variables in Formula 2. Of course, the number of test points must be • selected according to the desired accuracy.

f i. * f, *; 20

Claims (5)

109370 A method for setting the amount of curing agent with a rod scraper, comprising: - applying a curing agent to a moving surface, and - limiting the amount of coating applied to the surface by pressing against the scraper bar (1) surface of the rod scraper, characterized in that "cf.d ~ H ~ cL + d J ai where Cw is the amount of treating agent per unit area μ is the viscosity of the treating agent: V is the velocity of the moving surface * · * ·« '·' · 20 L rod load * D is the specific particle size p is the density ... a, b, c and d constants, »» · 2. given by the load. * *. Method according to claim 1, characterized in that the constants a and b are determined experimentally by measuring the pressure in the nipple of the scraper bar (1) and the surface (4) and plotting h / D as a function of μν / PD, where 109370! this graph can be used to determine | where h is the thickness of the scraped overcoat! μ is the viscosity of the treating agent V is the velocity of the moving surface 5 P is the pressure at the rod and surface nip D is the characteristic particle size function g represented by the constants a and b. Method according to Claim 1 or 2, characterized in that the constants c and d are determined from the pressure P of the nip and the load L of the corresponding scraper rod by plotting the pressure as a function of load, whereby the constants are obtained from P = cL + d. 15 Method according to one of the preceding claims, characterized in that the movable surface is the surface of a film transfer roll. • · · '·' · 20 A process according to any one of claims 1 to 4, characterized in that the moving surface is a paper or board web. • 1 k • · · k »« 25 * · »» »i · 109370