FI121275B - Method of calendering a fiber web - Google Patents

Description

A method for calendering a fibrous web

The invention relates to a method for calendering a fibrous web in a calender having a roller having at least two rolls with a roll nip between them and at least one roller having a polymer-5 coating.

There are several factors that influence the gloss of paper. There are several options available to ensure that the paper has a high gloss level; the amount of coating in the paper and the dry solids and binder content of the coating paste can be adjusted. The amount of polishing pigment contained in the coating paste can be changed. The fish-nip nip-10 load and the temperature of the thermal rolls can be changed. Further, the roughness of the calendering base paper and the moisture of the paper before calendering can be adjusted. Specifically, in a multi-roll calender, the development of paper gloss correlates with the number of roll nipples, the speed of the paper at the calender and roll nipples, and the temperature of the heated thermal rollers in the roll. In one of the most commonly used types of multi-roll calenders, 15 alternate heatable thermal rollers and polymer-coated rolls. Typically, there are 6 to 16 such rolls and polymer-coated rolls in a multi-roll calender; e.g., the so-called Optiload and Twinline calendars used by the applicant typically have 5 to 12 rolls, of which 2-5 are thermal rolls and 4-7 are polymer coated rolls.

Often, the gloss of a paper type that develops during calendering does not meet the quality requirements set for it, even if the paper composition and calendering conditions are kept constant. Because so many different factors affect the gloss of paper, these causes of quality degradation have been difficult to find and control. The object of the invention was to find a process variable available in connection with multi-roll paper bleaching which would ensure the uniform gloss of the paper that develops during calendering.

Prior to commissioning, the coating of polymer-coated rollers is sanded to a specific surface cavity; Height, typically 0.4 µm to 0.6 µm. The surface roughness here refers to the mean surface deflection Ra defined in, for example, Aimo Pere: Machine Drawing 1 & 2 (ISBN 952-97096-7-3). Over time, the surface quality of the polymer coated roll decreases; its roughness increases with the abrasive action of the coating on the fibrous web, and in addition, imprints are formed on the surface of the fibrous web, causing undesirable vibration of the web. The time of roll replacement is generally not specified but is replaced at intervals deemed appropriate.

: 'i 2

However, the experience of replacing polymer-coated rolls has been found to be inadequate in some calendering conditions, since the quality of the paper to be calendered may well diminish well before the actual replacement date.

The invention is based on the surprising finding that in a multi-roll calender where one or both sides of a paper web are to be polished to a particular gloss, the surface roughness of the polymer coated rolls must be kept below a certain limit specified for paper type and roll nip. Previously, little attention has been paid to this issue and its relationship to paper gloss has not been understood due to the variety of parameters affecting gloss formation. In determining the paper-type surface roughness limit for each polymer-coated web, priority is given to the gloss of the fibrous web entering the multi-roll calender and to the desired improvement in the degree of polishing of the paper roll at a particular roll nip. Also, the surface roughness of the backing roll (paddle roll) of the polymer coated roll i may be taken into account when determining the maximum roughness limit of the polymer coated roll. Further, the limit value determination! Other factors that affect the gloss of the paper web mentioned above, which include both the gloss factors in the paper composition and the gloss factors in the calendering process can also be considered. >

More particularly, the invention relates to a method for calendering a fibrous web in a calender having a roller having at least two rolls between the roll nip and at least one roller. 20 of the rolls are polymer coated rolls. The maximum allowable average surface roughness of the polymer coated rolls of the roll depends on the desired gloss of the fibrous web at each roll coated polymer roll and the gloss of the fibrous web (W; W1) entering the calender. The average roughness of the surface of each polymer-coated roll is less than the maximum allowable average surface roughness of the surface determined for each polymer-coated roll.

The invention is most preferably used for coated pulp and wood-based glossy paper grades with a Hunter gloss of more than 20%, preferably 50%.

The invention is primarily directed to controlling the surface roughness of polymer-coated rolls in the manufacture of paper and board grades polished in a multi-roll calender, but the invention can also be used on uncoated papers, although their surface allowances may be relatively high due to their relatively high surface area.

By multi roll roll calender is meant a calender for paper or cardboard calendering, with a roll set comprising a plurality of parallel rolls overlapping.

3

Each time a roll nip is formed between two overlapping rolls, the surface of the fiber web passing through the calender is shaped by the nip pressure in the roll nip. In a multi-roll calender, at least the top and / or bottom rolls can be raised and lowered to open or close the roll nipples, but often also the top and bottom rollers: the so-called intermediate roll casing can be expanded / displaced to influence the nip pressure between two pairs of rolls. In a multi-roll calender, all roll nipples may have the same or different nip pressure. In addition, the multi-roll calender may include a number of conventional devices pertaining to the calendering process and nip pressures, such as: means for adjusting the moisture content of the fiber web; instruments which have been extensively described in the literature.

The Hunter gloss of a fiber web is more than 20% if the paper or board is uncoated, such as SC (supercalendered) for magazine paper, and more than 50% if the paper or board is coated. The boundary between paper and board is blurred and, for example, the heaviest grades of paper have a basis weight greater than that of the lighter grades; the papers are made in single layers and have a basis weight of 25-300 g / m2. The cartons are multilayer and have a basis weight of about 150 to 600 g / m2.

Polished paper grades are available in both mechanical pulp (wood-based) and sel-20-based coated papers. Typical types of paper with a high mechanical pulp for calendering according to the method of the invention are SC paper, about 40-60 g / m2, Hunter gloss 20 -50%, LWC paper, about 60-80 g basis weight. / m2, Hunter gloss 50 -80%, however, gloss is generally about 50 -65%, MWC (Medium Weight Coated) basis weight about 70-90 g / m2, Hunter gloss 25 65-75%, MFC- Machine Finished Coated, 50-70 g / m2 Hunter Gloss 25-70%, FCO 40-70 g / m2 Hunter Gloss 45-55% HWC 100-135 g / m2, Hunter gloss over 90%. Typical pulp-based paper grades containing less than 10% mechanical pulp, for which the method of the invention can be used for calendering, are WFC (Wood free coated) papers 30, whose basis weight varies considerably depending on the number of times they are coated with a basis weight of about 90-130. g / m2, Hunter gloss about 65-85%. The PPS 10 roughness (pm) of the paper surface obtained by the process of the invention ranges from 0.6 to 2.8 depending on the paper type.

Uncoated SC paper is typically used in print media such as magazines 35 and catalogs. ULWC, and LWC paper (very light / light coated) in magazines and various catalogs, SMC, MWC and HWC paper grades (medium to heavy / heavy coated) are mainly used as printing and writing paper, WFC paper (wood-free, coated) in advertisements and magazines, but also as glossy art printing paper. The basis weight of the glossy art printing papers ranges from 100 to 300 g / m.

In some cases, the invention may also be used to make matte-coated newsprint grades 5 having a gloss of about 10 and a roughness of 3.0 to 4.5.

In the invention, the maximum roughness of polymer coated rolls in a multi-roll calender is first determined by first determining the factors affecting the paper gloss, such as the amount and coating composition of the paper type, These conditions are then kept constant, and the paper web is run at the scheduled running speed in the calender for a period of time T. During this period, T is observed for wear on the polymer coated rolls. When the gloss development of the paper type during calendering no longer meets the requirements, the roughness of the rolls is checked, which directly provides the maximum allowable roughness of each roll of the multi-roll calender.

Depending on the type of paper 15 placed on each roll stack of the multiroll calender polymeeripinnoitetul- - le roll to obtain different average allowable maximum roughnesses Ra, since the development of the paper-siley and the gloss of the upper roll set and a bottom roll nips are often different. Thus, the replacement interval of each polymer-coated roll depends on its position in the roll.

The same average surface roughness can also be determined for all polymer coated rolls in a multi roll calender, after which the rolls should be replaced. If the rolls are to be replaced at the same time, the maximum permissible polymer-coated roll surface roughness must be selected according to the most consumable polymer-coated roll. Roll nipples at different heights of the track may include, for example: different nip pressure profile, which causes different wear on the rollers.

The invention will now be further illustrated in more detail with reference to the accompanying schematic figures 1 and 2, in which:

Figure 1 shows the development of paper gloss in a multi-lacquer calender observed in a test drive.

Figure 2 is a schematic representation of the multi-roll calender used in the experimental setup, viewed directly at the end of the calender.

In the experiment, a very high Hunter gloss (60-75%) and a gloss% improvement in calendering of about 20-30 units were sought for the paper web W.

5

Figure 2 shows a schematic diagram of a used so-called Optiload multi-roll calender with a roll set of 9 rolls, one of which was a potentiometer-coated roll and the other a thermo roll. In the track system, the thermo roll T and the polymer coated rolls P were thus alternated; PI .. P4 such that the thermal roll T in the odd roll nipples was pinched through and over the non-odd fiber web. The polymer-coated roll P, in turn, was located on the odd roll nipples N1, N3, N5 and N7 below the fibrous web passing through that nip and on the odd nipples N2, N4, N6 and N8 above the fibrous web. N1 to N8 and 16 pairs of rolls of polymer coated roll and thermo roll. The top and bottom rolls of the roll were the heat rolls in the heater and the fiber web (W; W1) of a certain gloss arrived at the top roll N; N1 such that the paper web entering the calender W; W1 underside came against top thermal roller T; Tl. The same nip pressure was applied to all roll nipples on the track. The gloss of the forthcoming paper web W1 was about 40% on the Hunter scale.

The gloss development below and above the calendered paper web was examined by roll nip at three different nip pressures. The nip pressures used were 200, 399 and 380 kN / m. Figure 1 shows in dashed lines connected to the upper side of the calender Hunter gloss finish developing paper and the solid line calender paper from the lower side Hunter gloss development of the various roll nips, and in the nip pressure.

In the experiment, it was found that N of the fourth roll nip; After N4, the gloss development at all applied nip pressures was unsatisfactory below the paper. For odd roll nipples, the lower roll polymer-coated roll has a surface roughness Ra of 0.8 and for even roll roll nips the upper roll polymer-coated roll has a surface roughness Ra of 0.5. Too rough polymer-roll used in the paper type to the underside of the development of gloss desired.

However, it should be noted that even if a lower Hunter gloss, e.g. 55%, were desired for paper grades, the surface roughness Ra = 0.8 of the polymer coated roll would still have been quite sufficient.

As can be further seen from the above test, at the beginning of the calender, the roll nipples N1, N2 and N3, where the gloss was to be increased by about 50-60%, had sufficient gloss development and approximately the same under and above the paper. Thus, a higher maximum surface roughness can be tolerated for the polymer coated rolls on the roll calipers N1, N2, N3 of the calender upstream. In contrast, in subsequent roll nipples N4 to N8, where the gloss was intended to be over 60% too rough, the surface of the polymer coated roll prevented the development of gloss. Typically, polymer coated rolls 6 - / are delivered with a surface roughness Ra <0.2 for rolled polymer coated rolls and ΐ

Ra <0.4 for cast polymer coated rolls.

Thus, the maximum permissible surface roughness limit for polymer coated rolls in the calendar depends on whether the roll is located at the beginning or end of the calender roll and on how high the gloss is to be raised in the calender.

Only some embodiments of the invention have been described above, and it will be apparent to one skilled in the art that the invention may be embodied in many other ways within the scope of the inventive idea set forth in the claims. Thus, when determining the maximum allowable surface roughness of a polymer-coated roll for a given roll, the effect of the surface roughness of the heated thermal roll opposite the 10 may also be taken into account; the average surface roughness Ra of a new thermal roll is typically <0.2, the surface roughness of a worn thermal roll is greater than that.

Claims (10)

A method of calendering a fiber web (W) in a calender (1), the rolling set of which comprises at least two parallel rolls with an intermediate roll nip (N) and wherein at least one of the rolls is a polymer coated roll, characterized in that - the paper web ( W; W1) arriving at the calender (1) is a wood or cellulose-based coated web which is intended to calender in said calender to a Hunter gloss over 20%, preferably to a Hunter gloss over 50%, or is The paper web to be calendered is a wood-based LWC, MFC, FCO, MWC or HWC paper web containing mechanical pulp or a coated cellulose-based WF paper web, the paper web Hunter smoothing after calendering is intended to be above 50%, preferably over 50%, 60%. 15. the highest permissible average surface roughness (Ra) of the polymer coated rolls (P) in the roll set is dependent on the desired degree of smoothing of the fiber web (W) at the respective polymer coated rolls of the roll nip (N) and of the smoothing degree of the fiber web (W; W1) as comes to the calendar (1) and by the fact that the average surface roughness (Ra) of each polymer coated roll is poured less than that of. respective polymer coated rolls defined maximum permissible average surface roughness (Ra), average deviation of the surface, which is 0.4 µm - 0.6 µm. The method of claim 1, characterized in that the calender (1) is a ··· multi-calender calender comprising at least 6 rolls on top of each other, preferably at least eight rolls. of which rolls every other are a thermal roll (T) and each other is a: * * *: polymer coated roll (P). ··· ···. Method according to claim 2, characterized in that the fiber web (W) is a paper or cardboard web. Method according to any of the preceding claims, characterized in that the ··· roll set comprises at least one polymer-coated roll, whose maximum average surface roughness (Ra) deviates from the maximum average surface roughness (Ra) of the other roll set. rolls. • · • · 5. A method according to claim 6, characterized in that the greatest permissible average surface roughness (Ra) of the roll set's respective polymer-coated roll (P) depends on where the polymer-coated roll is located in the vase nip (N) of the multiple roll calendars. 5 6. A method according to claim 7, characterized in that the maximum permissible average surface roughness (Ra) of the polymer-coated rollers (P) of the multiple roll calendars is greater in the roll nip at the starting end of the calender than in the roll nip at the end end of the calender. 7. A method according to claim 1, characterized in that all of the polyhedron calendars (P) have the same maximum permissible average surface roughness (Ra), which surface roughness (Ra) is defined for the roller nip in the final end of the calender to obtain the desired smoothing for the fiber web. Process according to any of the preceding claims, characterized in that in the definition of the largest permissible average surface roughness (Ra) of the polymer-coated roll (P), the coating properties of the fiber web are taken into account, such as the amount of coating for the respective paper type and the composition of the coating. Method according to any one of the preceding claims, characterized in that in the definition of the greatest permissible average surface roughness (Ra) of the polymer-coated roll (P). consideration is given to the surface roughness of the thermo roll (T) located opposite said polymer coated roll. • · · · · · · · · · · · · · .1 ·· Process according to any of the preceding claims, characterized in that in the definition ···,., Av of the largest permissible average surface roughness (Ra) of the polymer-coated roll (P), the calendering conditions for the type of paper concerned are taken into account, such as the temperature of the thermal roll and the roll nip. pressure, the fiber web's driving speed in the calender and the humidity of the fiber web coming into the calender. ··· · · · · · φ · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · ·