The invention also relates to a calender for implementing the method.

The function of calendering is to increase the smoothness and gloss of paper and paperboard webs as well as to improve other qualities of a printing surface. In the calendering of paper and paperboard, the web is subjected to deformation in a nip formed by two opposed rolls. The rolls may have a hard-or soft-covered surface.

The surface material of soft-covered rolls is paper or other fibrous material or a polymer coating. Hard-surfaced thermorolls are generally made from cast iron permitting heating or cooling of the rolls with oil, steam or other methods such a induction heating. One of the nip rolls can be replaced by an alternative member such as a press shoe or belt.

The deformability of the web being calendered can be improved by elevating the web temperature and/or moisture content. The web is heated by a heatable thermoroll.

Additionally, the web may be wetted and heated with steam before the web enters the nip. Slippage of the web on the roll surfaces may also cause deformation of fiber thus improving the smoothing effect.

The deformability of the web being calendered is improved substantially when the polymeric components of the web reach the glass transition temperature. In paper and paperboard webs, the glass transition temperature of their polymeric components <BR> <BR> varies from room temperature (e. g. , the coating mix polymers) to about 250 °C (for polymeric compounds of wood fiber). The glass transition temperature of polymers

drops with a higher moisture content of the web.

In calendering, the temperature of the outer layers of the web is desirably elevated above the glass transition temperature of their polymeric compounds. Simultaneous- ly, the web is worked in a nip formed by calender rolls, whereby the hot superficial layers of the web are deformed more than the middle portion of the web. When the web exits the nip, its temperature falls gradually approaching the ambient tempera- ture. Herein, a recovery phenomenon known as re-swelling occurs resulting in coarser roughness of the web. Re-swelling is caused by dimensional recovery taking place in different magnitude at different points of the web. Re-swelling is particularly vigorous when the temperatures incurred in calendering remain lower than the glass transition temperature of the polymeric components of the web. Web roughness also increases when the web is calendered at a temperature above the glass transition temperature of the polymeric components and is cooled after the nip to a temperature below the glass transition temperature.

Due to the re-swelling occurring in the web, the desired smoothness of the web can be attained only by carrying out calendering in multiple successive nips, running the calendering equipment at a slower speed or using a higher lineal nip pressure.

Patent publication US 5,245, 920 describes a calendering method wherein the object is to reduce the re-swelling of the web after its leaves the press nip. The web is heated in the nip to a temperature above the glass transition temperature. When the web leaves the nip, it is rapidly cooled to a temperature below the glass transition temperature of the polymeric compounds of the web. Cooling is carried out with the help of a cooling device located immediately downstream from the nip. Such a rapid cooling reduces the re-swelling of the web. A disadvantage of this embodiment is that the web is cooled after its departure from the nip, whereby no pressing force capable of preventing re-swelling is any more imposed on the web. Moreover, the cooling device increases the footprint need and cost of the calender.

It is an object of the present invention to provide a novel type of calendering method

and a calender capable of reducing the re-swelling of the web after leaving the calender nip.

The goal of the invention is attained by way of heating a substantial portion the poly- meric compounds of at least one surface of the web being calendered at the ingoing side of the calender nip into a plastic state, i. e. , to a temperature not lower than the glass transition temperature of the polymeric compounds in the web surface layer.

Subsequently, a substantial portion of the polymeric compounds heated to their glass transition temperature is cooled at the outgoing side of the nip still under the pressure of the nip to a temperature lower than the glass transition temperature.

Respectively, the calender according to the invention comprises heating means for heating at least one surface of the web at the ingoing side of the calender nip to a temperature not lower than the glass transition temperature of the polymeric com- pounds of the heated web surface layer. The heating means may comprise, e. g. , a heated belt. The calender also comprises cooling means for cooling the web surface heated to the glass transition temperature at the outgoing side of the nip to a tempera- ture lower than the glass transition temperature of the polymeric compounds of the heated web surface layer. The cooling means may comprise, e. g. , a cooled roll.

More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1.

Furthermore, the calender according to the invention is characterized by what is stated in the characterizing part of claim 8.

The invention offers significant benefits.

A major portion of the polymeric compounds of the web that are heated to the glass transition temperature is cooled down to a temperature below the glass transition temperature simultaneously as the web is being worked in the calender nip. This arrangement reduces the substantial increase of web roughness as the web leaves the

nip thus allowing the desired smoothness of the web surface to be attained using a lesser number of successive calendering nips. Hence, savings are attained in both equipment investment and energy consumption costs.

In the following, the invention will be examined in greater detail with the help of exemplifying embodiments and making reference to the appended drawing showing schematically a side elevation view of a belt calender.

The belt calender shown in the diagram comprises a nip 6 formed by a first member 1 and a second member 4 running in a nip contact during the operation of the calender. The first member is a rotating roll 1 and the second member is an endless- loop belt 4 passing over a backing roll 2 and about guide rolls 3 adapted on both sides and below the backing roll 2. The belt 4 is pressed against the roll 1 by a backing roll 2. The roll 1 is advantageously a hard-surfaced thermoroll adapted heatable or chillable by means of a liquid circulated in the interior of the roll 1. The web 5 being calendered travels via a nip 6 formed between the roll 1 and the belt 4.

The travel direction of web 5 is marked with an arrow in the diagram. The length of the calender nip 6 may be adjusted by changing the position of the guide rolls 3. For instance, when both guide rolls 3 at the opposite sides of the backing roll 2 are elevated upward, the belt 4 can be guided to travel a longer distance on the periphery of the thermoroll 1, whereby the length of the nip 6 increases. In practice, the length of the calender nip 6 according to the invention can be increased up to about 95 % of the circumference of roll 1. Herein, the nip length must be understood to refer to the travel distance along which both ones of members 1,4 forming the nip 6 are in contact with the web 5 so as to press the web 5. Typically, the temperature of the web 5 entering the calender varies from about 30 °C (off-line calenders) to 100 °C (on-line calenders).

An essential portion of one side of web 5 is heated in the nip 5 to a temperature not lower than the glass transition temperature of the polymeric compounds contained in the heated surface. Herein, a substantial quantity of the polymeric compounds con- tained in the surface being heated is gradually brought to a temperature not lower

than their glass transition temperature. Normally, also the polymeric compounds con- tained in the inner portion of web 5, yet close to the surface layer being heated, are heated to their glass transition temperature. Prior to passing the web 5 out from nip 6, a substantial portion of the heated surface layer with its polymeric compounds heated in nip 6 to their glass transition temperature is cooled to a temperature below the glass transition temperature. As the cooling of the polymeric compounds below their glass transition temperature takes place in the nip 6 under a pressing force, the re- swelling of web 5 after leaving nip 6 remains minimal.

The heating and cooling of web 5 in nip 6 is implemented, e. g. , so that the web 5 pressing surface of the second member 4 is made from a material of low thermal capacity. Herein, the surface temperature of the second member 4 pressing the web 5 is kept at the ingoing end of nip 6 higher than the glass transition temperature of the polymeric compounds contained in the surface layer of web 5 facing the pressing member. Furthermore, the thermal conductivity of the surface of the second member 4 pressing web 5 must be high in order to achieve rapid transfer of the heat content of the member from its surface to the surface of web 5 thus allowing the surface layer of web 5 and the polymeric compounds thereof to reach at least their glass transition temperature in the ingoing end of nip 6. However, due to its low thermal capacity, the heat content of the surface pressing web 5 can be kept so small that the temperature of the second member 4 pressing web 5, together with the surface temperature of web 5, can fall below the glass transition temperature of the polymeric compounds contained in the surface layer of web 5 by the time the web reaches the outgoing end of nip 6.

A preferred embodiment of the invention is carried out using a belt calender such as the one shown in the diagram comprising a nip 6 formed between a belt 4 dimen- sioned to fulfill the above-described requirement and cooperating with a cooled roll 1. The outer surface of roll 1 can be cooled by way of, e. g. , circulating chilled cooling liquid in the interior of the roll perimeter. The roll 1 may also be of an uncooled type. The outer surface of belt 4 is heated with the help of a heater device 7 at a point exterior to the nip 6. The belt 4 is preferably heated as close as possible to

the point where the web 5 is taken into nip 6, whereby the outer surface of belt 4 is prevented from cooling substantially prior to its entry into nip 6. Typically, the temperature of the outer surface of belt 4 at its entry point into nip 6 is in the range of 70-400 °C. Herein, the belt entry point into nip 6 must be understood to refer to the point where both members 1,4 forming nip 6 come into contact with web 5. Heat stored in belt 4 is transferred to web 5 in the ingoing end of nip 6 wherein a substan- tial quantity of polymeric compounds contained in the surface web 5 facing belt 4 is heated so as to reach a plastic state. Simultaneously, web 5 is subjected to pressing in nip 6 formed between belt 4 and roll 1. A portion of the heat transferred from the surface of belt 4 to the surface of web 5 is conducted through web 5 to the cooled surface of roll 1. A substantial quantity of the polymeric compounds of web 5 heated into a plastic state is again cooled within the outgoing end of nip 6 to a temperature below the glass transition temperature of the polymeric compounds.

The belt 4 can be fabricated from a thermally insulating material having its outer surface, that is, the surface facing the web 5, covered with a thin layer of a material of high thermal conductivity. Such an outer layer may be of a metallic material, for instance.

The invention may also be carried out using embodiments different from those described above.

For instance, the above-discussed treatment, wherein the surface of the web 5 is heated to a temperature not lower than the glass transition temperature of the poly- meric compounds contained in the web surface layer and thereupon is cooled below the glass transition temperature in nip 6, may be carried out simultaneously on both sides of the web 5. Herein, both members 1,4 forming the nip 6 are dimensioned to meet these requirements. When necessary, the surface of the web 5 before its entry into the ingoing side of the nip 6 may be heated with the help of a preheater 8 to a temperature lower than the glass transition temperature of polymeric compounds contained in the web surface layer. The web 5 may also be wetted just prior to calendering in order to reduce the glass transition temperature of the polymeric compounds. In lieu of a belt calender, also any other calender type of sufficiently long nip may be used, such as a shoe calender, for instance.