Presses – With additional treatment of material – Adding materials
Reexamination Certificate
1997-07-10
2001-07-17
Gerrity, Stephen F. (Department: 3405)
Presses
With additional treatment of material
Adding materials
C034S629000, C034S653000, C100S099000, C100S161000, C162S206000
Reexamination Certificate
active
06260481
ABSTRACT:
FIELD OF THE INVENTION
This invention concerns a process for increasing the gloss and/or smoothness of a web of material, especially a web of paper, whereby a web of material that is moistened with the help of steam is guided through a roller gap where it is exposed to a pressure, and this invention also concerns a device for increasing the gloss and/or smoothness of a web of material with a pair of rollers that form a roller gap and a steam dispensing device arranged upstream from the roller gap in the direction of travel of the web.
BACKGROUND OF THE INVENTION
Gloss and smoothness are characteristics of a web of material that affect not only the appearance of the material but also its processability. For certain applications, high gloss and/or smoothness values are desired but they should also be as uniformly reproducible as possible.
At first the pressure in the roller gap was increased in order to increase the gloss and/or smoothness. However, this has the negative effect that the web of material is greatly compressed on the whole and thus suffers a loss of volume. In addition the web of material may also lose stability in this way. To prevent this disadvantage from becoming too serious, there was subsequently a trend toward increasing the temperature of the rollers that form the roller gap. This made it possible to further increase the gloss and smoothness but such a procedure is very energy intensive. In order to achieve roller temperatures of 200° C., substantial quantities of energy must be supplied continuously because the rollers are constantly being cooled by the web of material passing by. In addition there have also been attempts to influence the gloss and smoothness through the moisture content of the web of material, but this has the disadvantage that the moisture supplied to the web must be removed again at least in part after the treatment, which entails additional process steps that increase the amount of time and equipment required for treatment of the web of material. Steam blasting tubes arranged between two roller gaps downstream from the deflection in a web of paper are known for influencing the moisture content when processing paper on supercalenders (U.S. Pat. No. 5,122,232). The steam emitted from the steam blasting tubes condenses in the ambient air and is deposited as a mist—for example, in the form of extremely fine water droplets—on the web of material. Another disadvantage of this procedure is that the entire vicinity around the web of material is then exposed to a very humid atmosphere that leads to corrosion of metal parts in the smoothing device.
This invention is therefore based on the problem of simplifying the method of improving the gloss and/or smoothness.
SUMMARY OF THE INVENTION
This problem is solved in a process of the type defined initially by the fact that the steam condenses on the web of material which is guided through the roller gap before the increase in moisture content of the surface due to the steam treatment has dropped below a predetermined level.
In this way it is possible to achieve not only a moistening of the web of material but at the same time an increase in temperature is achieved. The heat contained in the steam is transferred to the web of material in condensation, so this measure results in a web of material having the required temperature at the surface and also having the required moisture content. When this web of material is guided through the roller gap, the roller gap then affects essentially only the surface of the web of material without causing changes deep in the web of material, i.e., in the direction of the thickness, to any mentionable extent. Therefore, the volume of the web of material is largely maintained although the surface quality is definitely improved. The rollers need not be heated nearly as much. Furthermore, the pressure in the roller gap can be kept lower than in the past. This saves a considerable amount of energy. It is possible to determine empirically or by calculations how long it takes for the moisture to penetrate into the interior of the web. However, before this condition occurs, the web—or more precisely, its surface—has already been treated in the roller gap. Thus the steam treatment takes place immediately before the web of material enters the roller gap. This yields two advantages. First, the surface of the web is still at a relatively high temperature and has a relatively high moisture content, so the increase in gloss and/or smoothness is accomplished even at a low pressure and low temperature in the roller gap. Secondly, the web does not absorb any mentionable amount of moisture, so no expensive aftertreatments are necessary. The energy needed to treat the surface is kept in the same range where the treatment takes place, i.e., where the smoothing is to occur. The rest of the web is not affected at all or is impaired only to a slight extent.
Preferably the predetermined value for the moisture content is in the range of 12% to 25%, especially in the range of 16% to 25%. Thus a relatively large amount of moisture is supplied to the surface. However, since this supply of moisture is limited to the surface and a thin layer beneath the surface, this treatment does not result in any mentionable loss of volume nor any great increase in the moisture content of the web in general.
It is especially preferable here that the web of material is guided into the roller gap before the temperature in the middle third of the thickness of the web has reached 1/e times the surface temperature. (Note that “e” is the base of natural logarithms and is approximately equal to 2.7182818285). This permits an adequate distance between the steam treatment of the web and the roller gap which cannot be reduced to zero for structural reasons. On the other hand, the difference between the middle third of the web and the outer third of the web with regard to the temperature is so great that the smoothing treatment is limited to the outer third if the temperature has an influence there. The influence of the moisture content is limited to even thinner areas of the surface because the temperature penetrates faster than the moisture.
It is advantageous for the steam to be kept free of water droplets until it strikes the web of material. Thus, on the one hand, means are taken to assure that the steam does not contain any water droplets, but on the other hand, water droplets are not allowed to form in the steam. This can be accomplished, for example, by heating the steam until just before it strikes the web of material. In this way it is possible to assure that all the heat contained in the steam can be transferred to the surface of the web of material in condensation in order to bring about the desired increase in temperature which is associated with an increase in the moisture content at the surface. Before the steam strikes the web, no cloud of mist is formed, so there is not such a great increase in the atmospheric humidity in the vicinity of the machinery.
It is advantageous for the steam to first be distributed in a distribution chamber over an outlet area having a predetermined expansion in the direction of travel of the web of material and then for the steam to move at a high velocity within a predetermined range in the direction of the web of material. The amount of steam that condenses on the web of material can be controlled on the basis of this velocity. This velocity also depends on the velocity of the web of material and is generally 15 m/s or more. Due to the fact that the steam is first distributed in a distribution chamber, this yields the advantage that the treatment of the web of material is relatively uniform over the width of the web. The flow rate of the steam can be controlled on the basis of the pressure in the distribution chamber.
It is also advantageous to determine the gloss and/or smoothness and/or a corresponding parameter of the web of material downstream from the roller gap and to adjust the amount of steam dispensed as a function of the difference between the actual value thus
Gerrity Stephen F.
Kueffner Friedrich
V.I.B. Apparatebau GmbH
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