FI115650B - Procedure for calendering paper or cardboard - Google Patents

Description

115650

Method for calendering paper or paperboard

The present invention relates to a method for sharpening fish or paperboard.

Calendering is a process in which a fibrous web is usually pressed between two or more rolls, whereby, due to mechanical work and heat, the paper deforms both in the plane direction and in the thickness direction. There are also various belt calender solutions as disclosed, e.g., in FI application 20020159, which has a length adjustable calendering area. The length of the calendering area in the belt calender may be e.g. in the range 10 mm to 15000 mm.

15 In current calendering processes, calendering parameters include: - roll surface temperature - paper temperature - roll diameter run speed <* 20 - line loading on rolls - roll coating hardness - paper moisture (- distribution) ·, - roll surface topography - nip length 25 -

Some of the above parameters are interconnected. In the present calendering methods, different parameters are individually adjusted to obtain the desired end product. Some parameters are fixed due to the process-30 and machine layout.

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It is an object of the present invention to provide a calendering method which achieves an effective calendering result by relatively simple adjustment procedures.

To achieve this object, the process of the invention is characterized in that the process comprises subjecting the web in a calendering nip or region to a temperature above the glass transition temperature of the polymers contained in the web under such high pressure that the liquid in the web is not vaporized; the calendering effect is achieved, after which the pressure acting on the web is controlled lowered below the vapor pressure corresponding to that temperature, whereby the liquid in the web is evaporated and the web dries in a controlled manner, whereby the polymers contained in the web Said liquid in the form of said 15 may be water or an aqueous mixture.

The required pressure for each temperature can be read with sufficient accuracy from the water phase diagram or the vapor pressure table, for example jul. on Kari I. Keskinen, Tables and Drawings in Chemical Engineering, p.

20 24-35. These values may be varied depending on the composition of the aqueous mixture in the fibrous web, for example on the basis of test runs.

In the process of the invention, the web is compressed in a plastic state - i.e. to achieve the desired calendering result, after which the pressure acting on the web can be lowered in a controlled manner below the vapor pressure corresponding to that temperature. the deformation remains permanent.

: 30 The following describes briefly the polymers contained in the fibrous web and their glass transition temperature based on the publication: Mikko Tani, M.Sc.

The fibrous web typically contains three different types of natural polymers: cellulose-5, amorphous hemicellulose and amorphous lignin, the proportion of which varies depending on the type of pulp and the species of wood used to make the pulp. Coated species also include synthetic amorphous polymers or natural polymers in the coating paste, or both. The deformation of the polymers contained in the paper is 10 time dependent and partially irreversible (viscoelastic). The viscoelastic behavior of the polymers depends on the shear rate, molecular weight, structure of the polymers, amount of additives and temperature. Below a certain temperature characteristic of each polymer, the amorphous phase is in the glass space.

Thereby, the amorphous polymers and the amorphous portions of the partially crystalline polymers are solidified into hard and brittle glassy substances. In addition to the reversible deformation, a permanent deformation can occur under the influence of external force in the glass space. Permanent deformation in the glass space due to external force is called plastic deformation. The physical and mechanical properties of the amorphous phase undergo a significant change in the glass transition region, whose center is defined as the glass transition temperature Tg. The polymers contained in the paper are in the glass space at room temperature. The glass transition temperatures of cellulose under absolutely dry conditions vary from 200 ° C to 250 ° C depending on the degree of crystallinity. Corresponding values for hemicellulose are 150-220 ° C and for lignin 130-205 ° C. Humidity 25 influences these temperatures. The glass transition temperature is dependent on the plasticizing effect of water, whereby the increase in water content decreases the glass transition temperature. As the moisture content of the fiber web rises above 5%, two different glass transition temperatures have been observed with the mechanical pulp, the lower one being the glass transition temperature of the cellulose fraction and the higher one the lignin fraction. Lignin is able to absorb only a limited amount of moisture, so its glass transition temperature remains constant at a moisture content above 2.5%. As a result, heat is required for the plasticization of lignin 4 115650, whereas cellulose is plasticized with water and heat.

In the method according to the invention, the calendering of the web is achieved so that each layer of the web is at each point of time an optimum temperature-humidity-pressure operating point for the plasticity of the web. In the method of the invention, one or more of the parameters may be fixed and the remainder adjustable or, in an ideal process, all independent parameters may be freely adjustable. In an ideal calendering process, independent parameters can be considered as: - the temperature of the surfaces in contact with the web - the paper inlet temperature - the moisture (distribution) of the paper - the pressure pulse (duration of the distribution) over the web.

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Other parameters can be considered as case-specific constants.

The method according to the invention is suitable for use in both conventional calenders in which the fibrous web is pressed between two or more rolls, and in belt calendars, in particular in a metal belt calender such as the metal belt calender of application FI20020159.

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Claims (2)

5 115650 Claim