CA1340148C - Method of reducing shrinkage during drying of a paper web - Google Patents
Method of reducing shrinkage during drying of a paper webInfo
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- CA1340148C CA1340148C CA 540140 CA540140A CA1340148C CA 1340148 C CA1340148 C CA 1340148C CA 540140 CA540140 CA 540140 CA 540140 A CA540140 A CA 540140A CA 1340148 C CA1340148 C CA 1340148C
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- board
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Abstract
A method is disclosed for reducing shrinkage during drying of a paper web formed from paper stock. The method includes the steps of adding a surfactant to the paper stock prior to forming the stock into the paper web. The surfactant is dispersed throughout the stock such that the surfactant is intimately mixed with the stock. The surfactant mixed with the stock is supplied to a headbox of a paper forming section and the mixture of stock and surfactant is ejected from the headbox onto a forming wire of the forming section. A first portion of water is drained from the stock such that the paper web is formed on the forming wire. The paper web is led from the forming section to an extended nip press where the web is passed through the extended nip press such that a second portion of water is removed from the web. The pressed web is conducted through a drying section for removing a third portion of water from the formed, pressed web so that the surfactant inhibits bonding between adjacent fibers within the pressed web so that shrinkage of the pressed web during drying is reduced and a dried web of the required caliper is obtained.
Description
A METHOD OF REDUCING SHRINKAGE DURING
DRYING OF A PAPER ~B 1340148 BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
This invention relates to a method of reducing shrinkage in the Z direction or thickness during drying of a paper web formed from paper stock. More particularly, this invention relates to a method of reducing shrinkage during drying of a paper web by use of a surfactant added to the paper stock prior to drying of the pressed web.
INFORMATION DISCLOSURE STATEMENT
In the production of various grades of paper and board, it is necessary that such grades meet a specified thickness, or caliper, standard. Furthermore, such grades must meet certain density standards. By way of example, fine papers may be controlled within a narrow range of caliper so that the product will run well in machines designed for specific paper thickness such as a copying machine or converting machinery for making envelopes or the like.
Various boxboards and carton stocks are made to a bulk, or density, speciflcation to maintain a required stiffness of the product. For example, the sidewalls of a milk carton must have sufficient stiffness to hold the contents within the car-ton without excessive bulging of the carton. Control of the caliper and density of paper and board grades is of great importance in the manufacture of paper products because stiff-ness can vary up to the cube of the thickness.
During recent years the extended nip press has been used commercially in the production of board for removing more water in the press section than was possible with the more conventional press arrangements. The extended nip press, by removing a larger portlon of water from the formed web, not only results in savings of energy, particularly in the drying section, but has also resulted in increased productivity.
-1- ~
Z 1~401~8 -The additional water removal achieved at the extended nip press section results from greater compaction of the web during passage through the nips of the extended nip press.
This, in turn, produces a paper or board with a lower caliper and a higher density. Such higher density is accompanied by greater fiber-to-fiber bonding and a stronger sheet.
By way of example, if a caliper, or thickness of the web, of .035 inches ( 089 centimeters) is required, it has been found that after convent10nal pressing the wet thlckness of the board may be .055 inches (.14 centimeters~. However, during drying, the board shrinks significantly in the thick-ness direction, or Z direction, to the target of .035 lnches (.089 centimeters). Such shrinking occurs as a consequence of Campbell forces. (Campbell forces are forces between fibers due to the surface tension of water.) As the water evaporates, the surface tension effects pull fibers closer together and hydrogen bonding occurs. The forces become stronger as the water present decreases. Thus, it is seen that with the conventional method of pressing a formed web, it is not uncommon for the board, or paper web, to shrink in thickness, or caliper, by as much as 40% during the evaporative drying process. Additionally, such shrinkage in the Z direction occurs due to fiber collapse.
However, with the use of the extended nip pressing tech-nique, the dryness of the resultant paper or board is improved by 7 to 14 percentage points of dryness after the press section.
This represents reduction in the water to be evaporated of between 25 and 50 percent. Although such reduction in water to be evaporated during the drying section is very desirable from the thermal energy saving viewpoint, a problem exists in that the web emerging from the extended nip press is compacted from .OSS inches (.14 centimeters) down to .04 inches ~.10 centimetersl after pressing. From these figures, it evident -- 13401~8 that the web emerging from an exte~nded nip press has a caliper, or thickness,that ls reduced during pressing from between 15 to 30 percent. At least a portion of this reduced thickness may be carried through the drying process and it is not un-common to achieve a further 40 percent reductlon ln the web thickness as the web passes through the drylng section. For example, with a web thickness of .04 inches (.lO centimeters) enterlng the drylng sect10n, a drled web may result havlng a thickness of .025 lnches ~.06 centimeters). Thus, the board produced at the desired hlgher dryness after pressing has a callper that falls below the target or deslred callper. Al-though, as a result of the hlgher density, the web has ln-creased strength propertles such as tenslle strength, such increased strength ls not always needed or, in fact, deslred.
Consequently, the problem has been to find a method that permlts the desired increase in water removal at the press section but that does not result ln a dried web thlckness that falls below the desired, or target, value.
Although it ls true that certain hardwood flbers, or thermal-mechanical pulp furnishes (stock or stuff) recover thickness after presslng to achieve a higher degree of caliper than with stock produced from softwood, there does not appear to be any way to achleve greater water removal by pressing ln the press sectlon without also reducing the wet thlckness of the paper or board. Apparently, the lncreased dryness is a direct consequence of the lncreased compactlon.
The present inventlon seeks to overcome the aforementloned problem by not attempting to reduce the compaction that occurs during pressing, but rather by reducing the shrinkage that occurs during evaporatlve drying of the sheet.
It has been discovered that since the caliper loss that occurs during drying is greater than the loss due to lncreased ll 1340148 ~' wet pressing, an attempt to recover the thickness by acting upon the thickness of the web through the drying section would be most effective. The present invention provides means for dealing with the cause of greatest loss in thickness which is the surface tension, or Campbell forces pulling fibers sufficiently close together to permit hydrogen bonding during evaporative drying.
An article by J. A. Yan den Akker published in TAPPI, volume 35, N 1, page 13-lS (1952) reported on the effect of drying paper by sublimation where surface tension forces are minimized. The thickness or density values were not given but tensile strength was greatly reduced and opacity increased, indicating a smaller degree of hydrogen bonding.
Broughton and Wang, published in TAPPI, volume 38, N 7, page 412 - 415 (1955~ found a reduced strength of paper made and dried in low surface tension, non-polar liquids instead of water.
J. W. Swanson, published in TAPPI 1974 Engineering Confer-ence Proceedings (Seattle, WA) page 327 - 332, reported on the effects of surfactants on fibers. Anionic surfactants were found to reduce surface tension and correlated well to reduce bonding in the sheets. Cationic surfactants were found to give marked decreases in sheet properties that depend on bonding.
The present invention seeks to overcome the aforementioned inadequacies of the prior art methods of producing board and paper by providing a method of producing board or paper using an extended nip press and yet resul ting in a product having the desired caliper and density.
Another ob~ect of the present invention is the provision of a method of reducing shrinkage during drying of a paper web formed from paper stock. The method including adding a sur-factant to the paper stock.
13~0148 Another object of the present invention is the pro-vision of a method of reducing shrinkage during drying of a paper web by adding a cationic surfactant to the paper stock for reducing both surface tension and bonding between adjacent fibers within the formed, pressed web.
While the present invention is directed primarily to the reduction of shrinkage in the Z direction of a web emerging from an extended nip press, the present invention also envisages reducing such shrinkage of a web produced by a conventional high-load press.
Furthermore, although the present invention will be described hereinafter with particular reference to adding the surfactant to the stock, the present invention is not limited to such application of the surfactant. The present invent10n, as defined by the appended claims, includes adding the surfactant as by spraying or the like onto the formed web prior to drying thereof. Alternatively, the sur-factant could be introduced at the wet end or at the press section. Also, the surfactant may be introduced even at the dryer section. In the production of a multi-ply sheet, the surfactant could be introduced onto one or more of the plys only.
Other objects and advantages of the present invention will be readily apparent to those skilled in the art by a consideration of the detailed description and the appended claims.
SUMMARY OF THE INVENTION
The present invention relates to a method of reducing shrinkage during drylng of a paper web formed from paper stock. The method includes the steps of adding a surfactant to the paper stock prlor to forming the stock into the paper web. The surfactant ls dispersed throughout the stock such that the surfactant is intimately mlxed with the stock. The mixture of surfactant and stock is supplied to a headbox of - 13401~
a paper forming section and the stock and surfactant mixture is ejected from the headbox onto a forming wire of the forming section. A first portion of water is drained from the stock such that the paper web is formed on the forming wire. The formed web is led from the forming section to an extended nip press where the formed web is passed through the extended nip press such that a second portion of water is removed from the web. The pressed web is conducted from the extended nip press to a drying section for removing a third portion of water from the formed, pressed web such that the surfactant inhibits bonding between adjacent fibers within the pressed web so that shrinkage of the pressed web during drying is reduced.
In a more specific embodiment of the present invention, the surfactant is a cationic surfactant which is effectlve for reducing both surface tension and bondlng between adjacent fibers within the formed, pressed web.
Although the present invention is particularly directed to the use of a surfactant added to the paper stock, it will be apparent to those skilled in the art that other chemicals can be added to the stock for inh~biting bonding between adjacent fibers during the drying process and that the use of such chemicals does not depart from the spirit and scope of the present invention as defined by the appended claims.
Additionally, while the present invention is primarily directed to reducing shrinkage of a web produced by an extended nip press, the present invention also includes application of such surfactant to a web produced by a high-load conventional press.
Also, the present invention includes adding the surfactant to the web at either the wet end, the press section, or even the drying section and further includes adding such surfactant to one or more plys in the case of the manufacture of a multi-ply sheet.
. 13401~8 BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a comparative chart showing typical values of caliper for a board after the press section and after drying for sheets pressed with a conventional press section and pressed with an extended nip press.
Figure 2 is a graph showing the effect of several surface-active agents on sheet thickness.
Figure 3 is a comparative chart showing the resultant thickness using conventional and extended nip pressing techniques and showing the results on thickness by the application of the method of the present invention.
Although surfactants are currently used in the paper industry in various applications, applicant is unaware of any prior use of surfactants in the paper stock for overcoming the problem of reduced caliper when using an extended nip press. Applicant is aware that surfactants have been used by manufacturers of tissue to inhibit bonding in order to produce a softer feeling product. A "fluff" pulp is made using agents that interfere with bonding and the pulp is then much easier to redisperse in water or ln air for air-formed products such as disposable diapers.
However, the present 1nvention is directed to a method of controlling the final caliper of a paper or board emerging from a drying section. Laboratory experiments have shown that by the use of surfactants in the paper stock, the final caliper can be effectively controlled as shown by the results indicated in figure 2 of the drawings. These results from figure 2 indicate that the thickness of the resultant dry web can be controlled by controlling the concentration of the surfactant added to the stock or fiber water slurry.
As shown in figure 3, use of the surfactants or other debonding agents can reduce the shrinkage occurring during drying in order to meet target values. From figure 3, it is evident that by adding the surfactant to the stock prior to pressing, the thickness of the web emerging from the drying section is greater than when no surfactant is used.
From the aforementioned method, particularly when the surfactant is a cationic agent, such controlled addition of the surfactant controls the final sheet, or board, thickness to meet a target spec~fication while, at the same time, greatly increasing the amount of water that may be removed during passage through the extended nip press. Therefore the method according to the present invention, enables great savings in ~3401~8 ,_ .
the amount of steam used in the drxing section resulting in increased productivity which more than offsets the cost of adding such surfactants in low concentrations to the pulp slurry.
Referring more particularly to the drawings, figure 1 shows by way of comparison, the change in board thickness during pressing and during drying. More specifically the thickness in thousands of an inch, is indicated by the vertical axis 10. Using a conventional press section, the thickness of the resultant pressed web is indicated by 12.
Furthermore, the thickness of the web emerging from an extended nip press is indicated by 14. This comparison shows the considerable reduction in resultant thickness using the extended nip press which provides a pressed web of greatly reduced water content and increased density.
Numeral 16 indicates the target, or required thickness of the web after passing the web through the drying section. This reduction in thickness of the web after passing through the drying section from a conventional press represents approximately a 40% reduction due to shrinkage. If an extended nip press is utilized followed by conventional drying without use of the surfactant according to the present invention, the resultant dried web will have a thickness indicated by 18 which, as can be seen, is less than the required target thickness. HoweYer, when using a conventional press, the web shrinks from the thickness shown as 12 to the required thickness as indicated at 20.
Figure 2 is a graph showing the effect of using different surfactants at various concentration levels within the stock, or slurry. The vertical axis indicates the percentage increase in thickness, or caliper, of the web emerging from the drying section while the horizontal axis indicates the percentage per unit volume of stock of the surfactant added prior to the headbox. Graph 22 indicates 13401~8 ~
the relative increase in caliper.u~ing Arquad 2HT-75 while graph 24 indicates the corresponding increase, particularly with a lower concentration of surfactant using a~cErosol C-61. Both graph 22 and 24 are applicable when using liner-board pulp. Arquad is a registered trademark and Arquad 2HT-75 was supplied by Armak Corporation, a subsidiary of Armour & Company.
Similarly, graph 26 and graph 28 respectively, indicate the corresponding increases in caliper when producing a board or paper web from bleached softwood kraft (B.S.W.K) using various concentrations of surfactant.
Figure 3 is a comparison chart in which the vertical axis represents the thickness of the paper or board web after pressing and drying. 30 indicates the thickness after pressing and 32 indicates the thickness after drying, both utilizing conventional pressing and drying.
34 indicates the thickness after pressing and 36 indi-cates the thickness after drying of the web, both using an increased pressing technique obtained by utilizing an extended nip press in conjunction with a conventional drylng system.
36 shows the thickness of the web being considerably less than the target thickness indicated by the horizonta~ dashed line 38.
indicates the thickness of the web emerging from an extended nip press and 42 shown in dotted lines indicates the acquired thickness resulting from the addition of sarfactant to the stock according to the present invention.
When such a web is passed through a conventional drying system, the thickness of the web decreases during passage through the drying system to the target thickness 38 as indi-cated by the after drying thickness 44.
According to the present invention, a method is provided for reducing shrinkage during drying of a paper web formed ~ ~a~ ~a)-k 1340148~
from paper stock. The method inc,l~udes the steps of adding a surfactant, preferably a cationic surfactant to.the paper stock prior to forming the stock into a paper or board web.
Preferably, the surfactant is added to the stock in fairly small quantities as indicated in figure 2. The stock may be linerboard or bleached softwood kraft as indicated in figure 2. However, the present invention is equally appli-cable to using a surfactant added to any type of stock and the surfactant may be anionic, or in fact, any other type of surface-active agent. The surfactant is dispersed throughout the stock so the surfactant ls intimately mixed with the stock prior to feeding the stock to the headbox of a paper forming section. The surfactant and stock is supplied to the headbox where the stock is ejected onto a forming wire of the forming section. Water is drained from the forming wire, or forming roll, in order to provide a formed web of paper or board.
The web is then passed through an extended nip press which removes a second portion of water from the formed web, the first portion of water having been drained from the stock in the forming section. The second portion of water pressed from the web is greater than the amount of water removed in a conventional press section. Consequently, the thickness, or caliper, of the resultant web is less than the caliper of a web emerging from a conventional press. Furthermore, the density of the emerging web will correspondingly be greater than that resulting from a conventional press. The pressed web is then led to a conventional drying system. However, such conventional drying system will either require less drying drums, or if the same number of drying drums are utilized, less thermal energy will be required in the drying section due to the larger amount of water removed in the extended nip press. Due to the addition of the surfactant within the stock, bonding of adjacent fibers within the web is ~ 13~ol~8 inhibited because the surfactant ~ends to inhibit hydrogen bonding which, without the use of a surfactant, would result in a web caliper less than the target caliper.
Additionally, such shrinkage in the Z direction can be reduced according to the present invention by introducing the surfactant at either the wet end, the press section or even the dryer section. Furthermore, such surfactant can be added to one or more plys when a multi-ply board is being produced.
The present invention not only provides a resultant pressed, dried web having the required thickness and density but also provides a great savings in the cost of the provision of drying steam and this more than offsets the cost of adding the surfactant in low concentration to the pulp slurry.
DRYING OF A PAPER ~B 1340148 BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
This invention relates to a method of reducing shrinkage in the Z direction or thickness during drying of a paper web formed from paper stock. More particularly, this invention relates to a method of reducing shrinkage during drying of a paper web by use of a surfactant added to the paper stock prior to drying of the pressed web.
INFORMATION DISCLOSURE STATEMENT
In the production of various grades of paper and board, it is necessary that such grades meet a specified thickness, or caliper, standard. Furthermore, such grades must meet certain density standards. By way of example, fine papers may be controlled within a narrow range of caliper so that the product will run well in machines designed for specific paper thickness such as a copying machine or converting machinery for making envelopes or the like.
Various boxboards and carton stocks are made to a bulk, or density, speciflcation to maintain a required stiffness of the product. For example, the sidewalls of a milk carton must have sufficient stiffness to hold the contents within the car-ton without excessive bulging of the carton. Control of the caliper and density of paper and board grades is of great importance in the manufacture of paper products because stiff-ness can vary up to the cube of the thickness.
During recent years the extended nip press has been used commercially in the production of board for removing more water in the press section than was possible with the more conventional press arrangements. The extended nip press, by removing a larger portlon of water from the formed web, not only results in savings of energy, particularly in the drying section, but has also resulted in increased productivity.
-1- ~
Z 1~401~8 -The additional water removal achieved at the extended nip press section results from greater compaction of the web during passage through the nips of the extended nip press.
This, in turn, produces a paper or board with a lower caliper and a higher density. Such higher density is accompanied by greater fiber-to-fiber bonding and a stronger sheet.
By way of example, if a caliper, or thickness of the web, of .035 inches ( 089 centimeters) is required, it has been found that after convent10nal pressing the wet thlckness of the board may be .055 inches (.14 centimeters~. However, during drying, the board shrinks significantly in the thick-ness direction, or Z direction, to the target of .035 lnches (.089 centimeters). Such shrinking occurs as a consequence of Campbell forces. (Campbell forces are forces between fibers due to the surface tension of water.) As the water evaporates, the surface tension effects pull fibers closer together and hydrogen bonding occurs. The forces become stronger as the water present decreases. Thus, it is seen that with the conventional method of pressing a formed web, it is not uncommon for the board, or paper web, to shrink in thickness, or caliper, by as much as 40% during the evaporative drying process. Additionally, such shrinkage in the Z direction occurs due to fiber collapse.
However, with the use of the extended nip pressing tech-nique, the dryness of the resultant paper or board is improved by 7 to 14 percentage points of dryness after the press section.
This represents reduction in the water to be evaporated of between 25 and 50 percent. Although such reduction in water to be evaporated during the drying section is very desirable from the thermal energy saving viewpoint, a problem exists in that the web emerging from the extended nip press is compacted from .OSS inches (.14 centimeters) down to .04 inches ~.10 centimetersl after pressing. From these figures, it evident -- 13401~8 that the web emerging from an exte~nded nip press has a caliper, or thickness,that ls reduced during pressing from between 15 to 30 percent. At least a portion of this reduced thickness may be carried through the drying process and it is not un-common to achieve a further 40 percent reductlon ln the web thickness as the web passes through the drylng section. For example, with a web thickness of .04 inches (.lO centimeters) enterlng the drylng sect10n, a drled web may result havlng a thickness of .025 lnches ~.06 centimeters). Thus, the board produced at the desired hlgher dryness after pressing has a callper that falls below the target or deslred callper. Al-though, as a result of the hlgher density, the web has ln-creased strength propertles such as tenslle strength, such increased strength ls not always needed or, in fact, deslred.
Consequently, the problem has been to find a method that permlts the desired increase in water removal at the press section but that does not result ln a dried web thlckness that falls below the desired, or target, value.
Although it ls true that certain hardwood flbers, or thermal-mechanical pulp furnishes (stock or stuff) recover thickness after presslng to achieve a higher degree of caliper than with stock produced from softwood, there does not appear to be any way to achleve greater water removal by pressing ln the press sectlon without also reducing the wet thlckness of the paper or board. Apparently, the lncreased dryness is a direct consequence of the lncreased compactlon.
The present inventlon seeks to overcome the aforementloned problem by not attempting to reduce the compaction that occurs during pressing, but rather by reducing the shrinkage that occurs during evaporatlve drying of the sheet.
It has been discovered that since the caliper loss that occurs during drying is greater than the loss due to lncreased ll 1340148 ~' wet pressing, an attempt to recover the thickness by acting upon the thickness of the web through the drying section would be most effective. The present invention provides means for dealing with the cause of greatest loss in thickness which is the surface tension, or Campbell forces pulling fibers sufficiently close together to permit hydrogen bonding during evaporative drying.
An article by J. A. Yan den Akker published in TAPPI, volume 35, N 1, page 13-lS (1952) reported on the effect of drying paper by sublimation where surface tension forces are minimized. The thickness or density values were not given but tensile strength was greatly reduced and opacity increased, indicating a smaller degree of hydrogen bonding.
Broughton and Wang, published in TAPPI, volume 38, N 7, page 412 - 415 (1955~ found a reduced strength of paper made and dried in low surface tension, non-polar liquids instead of water.
J. W. Swanson, published in TAPPI 1974 Engineering Confer-ence Proceedings (Seattle, WA) page 327 - 332, reported on the effects of surfactants on fibers. Anionic surfactants were found to reduce surface tension and correlated well to reduce bonding in the sheets. Cationic surfactants were found to give marked decreases in sheet properties that depend on bonding.
The present invention seeks to overcome the aforementioned inadequacies of the prior art methods of producing board and paper by providing a method of producing board or paper using an extended nip press and yet resul ting in a product having the desired caliper and density.
Another ob~ect of the present invention is the provision of a method of reducing shrinkage during drying of a paper web formed from paper stock. The method including adding a sur-factant to the paper stock.
13~0148 Another object of the present invention is the pro-vision of a method of reducing shrinkage during drying of a paper web by adding a cationic surfactant to the paper stock for reducing both surface tension and bonding between adjacent fibers within the formed, pressed web.
While the present invention is directed primarily to the reduction of shrinkage in the Z direction of a web emerging from an extended nip press, the present invention also envisages reducing such shrinkage of a web produced by a conventional high-load press.
Furthermore, although the present invention will be described hereinafter with particular reference to adding the surfactant to the stock, the present invention is not limited to such application of the surfactant. The present invent10n, as defined by the appended claims, includes adding the surfactant as by spraying or the like onto the formed web prior to drying thereof. Alternatively, the sur-factant could be introduced at the wet end or at the press section. Also, the surfactant may be introduced even at the dryer section. In the production of a multi-ply sheet, the surfactant could be introduced onto one or more of the plys only.
Other objects and advantages of the present invention will be readily apparent to those skilled in the art by a consideration of the detailed description and the appended claims.
SUMMARY OF THE INVENTION
The present invention relates to a method of reducing shrinkage during drylng of a paper web formed from paper stock. The method includes the steps of adding a surfactant to the paper stock prlor to forming the stock into the paper web. The surfactant ls dispersed throughout the stock such that the surfactant is intimately mlxed with the stock. The mixture of surfactant and stock is supplied to a headbox of - 13401~
a paper forming section and the stock and surfactant mixture is ejected from the headbox onto a forming wire of the forming section. A first portion of water is drained from the stock such that the paper web is formed on the forming wire. The formed web is led from the forming section to an extended nip press where the formed web is passed through the extended nip press such that a second portion of water is removed from the web. The pressed web is conducted from the extended nip press to a drying section for removing a third portion of water from the formed, pressed web such that the surfactant inhibits bonding between adjacent fibers within the pressed web so that shrinkage of the pressed web during drying is reduced.
In a more specific embodiment of the present invention, the surfactant is a cationic surfactant which is effectlve for reducing both surface tension and bondlng between adjacent fibers within the formed, pressed web.
Although the present invention is particularly directed to the use of a surfactant added to the paper stock, it will be apparent to those skilled in the art that other chemicals can be added to the stock for inh~biting bonding between adjacent fibers during the drying process and that the use of such chemicals does not depart from the spirit and scope of the present invention as defined by the appended claims.
Additionally, while the present invention is primarily directed to reducing shrinkage of a web produced by an extended nip press, the present invention also includes application of such surfactant to a web produced by a high-load conventional press.
Also, the present invention includes adding the surfactant to the web at either the wet end, the press section, or even the drying section and further includes adding such surfactant to one or more plys in the case of the manufacture of a multi-ply sheet.
. 13401~8 BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a comparative chart showing typical values of caliper for a board after the press section and after drying for sheets pressed with a conventional press section and pressed with an extended nip press.
Figure 2 is a graph showing the effect of several surface-active agents on sheet thickness.
Figure 3 is a comparative chart showing the resultant thickness using conventional and extended nip pressing techniques and showing the results on thickness by the application of the method of the present invention.
Although surfactants are currently used in the paper industry in various applications, applicant is unaware of any prior use of surfactants in the paper stock for overcoming the problem of reduced caliper when using an extended nip press. Applicant is aware that surfactants have been used by manufacturers of tissue to inhibit bonding in order to produce a softer feeling product. A "fluff" pulp is made using agents that interfere with bonding and the pulp is then much easier to redisperse in water or ln air for air-formed products such as disposable diapers.
However, the present 1nvention is directed to a method of controlling the final caliper of a paper or board emerging from a drying section. Laboratory experiments have shown that by the use of surfactants in the paper stock, the final caliper can be effectively controlled as shown by the results indicated in figure 2 of the drawings. These results from figure 2 indicate that the thickness of the resultant dry web can be controlled by controlling the concentration of the surfactant added to the stock or fiber water slurry.
As shown in figure 3, use of the surfactants or other debonding agents can reduce the shrinkage occurring during drying in order to meet target values. From figure 3, it is evident that by adding the surfactant to the stock prior to pressing, the thickness of the web emerging from the drying section is greater than when no surfactant is used.
From the aforementioned method, particularly when the surfactant is a cationic agent, such controlled addition of the surfactant controls the final sheet, or board, thickness to meet a target spec~fication while, at the same time, greatly increasing the amount of water that may be removed during passage through the extended nip press. Therefore the method according to the present invention, enables great savings in ~3401~8 ,_ .
the amount of steam used in the drxing section resulting in increased productivity which more than offsets the cost of adding such surfactants in low concentrations to the pulp slurry.
Referring more particularly to the drawings, figure 1 shows by way of comparison, the change in board thickness during pressing and during drying. More specifically the thickness in thousands of an inch, is indicated by the vertical axis 10. Using a conventional press section, the thickness of the resultant pressed web is indicated by 12.
Furthermore, the thickness of the web emerging from an extended nip press is indicated by 14. This comparison shows the considerable reduction in resultant thickness using the extended nip press which provides a pressed web of greatly reduced water content and increased density.
Numeral 16 indicates the target, or required thickness of the web after passing the web through the drying section. This reduction in thickness of the web after passing through the drying section from a conventional press represents approximately a 40% reduction due to shrinkage. If an extended nip press is utilized followed by conventional drying without use of the surfactant according to the present invention, the resultant dried web will have a thickness indicated by 18 which, as can be seen, is less than the required target thickness. HoweYer, when using a conventional press, the web shrinks from the thickness shown as 12 to the required thickness as indicated at 20.
Figure 2 is a graph showing the effect of using different surfactants at various concentration levels within the stock, or slurry. The vertical axis indicates the percentage increase in thickness, or caliper, of the web emerging from the drying section while the horizontal axis indicates the percentage per unit volume of stock of the surfactant added prior to the headbox. Graph 22 indicates 13401~8 ~
the relative increase in caliper.u~ing Arquad 2HT-75 while graph 24 indicates the corresponding increase, particularly with a lower concentration of surfactant using a~cErosol C-61. Both graph 22 and 24 are applicable when using liner-board pulp. Arquad is a registered trademark and Arquad 2HT-75 was supplied by Armak Corporation, a subsidiary of Armour & Company.
Similarly, graph 26 and graph 28 respectively, indicate the corresponding increases in caliper when producing a board or paper web from bleached softwood kraft (B.S.W.K) using various concentrations of surfactant.
Figure 3 is a comparison chart in which the vertical axis represents the thickness of the paper or board web after pressing and drying. 30 indicates the thickness after pressing and 32 indicates the thickness after drying, both utilizing conventional pressing and drying.
34 indicates the thickness after pressing and 36 indi-cates the thickness after drying of the web, both using an increased pressing technique obtained by utilizing an extended nip press in conjunction with a conventional drylng system.
36 shows the thickness of the web being considerably less than the target thickness indicated by the horizonta~ dashed line 38.
indicates the thickness of the web emerging from an extended nip press and 42 shown in dotted lines indicates the acquired thickness resulting from the addition of sarfactant to the stock according to the present invention.
When such a web is passed through a conventional drying system, the thickness of the web decreases during passage through the drying system to the target thickness 38 as indi-cated by the after drying thickness 44.
According to the present invention, a method is provided for reducing shrinkage during drying of a paper web formed ~ ~a~ ~a)-k 1340148~
from paper stock. The method inc,l~udes the steps of adding a surfactant, preferably a cationic surfactant to.the paper stock prior to forming the stock into a paper or board web.
Preferably, the surfactant is added to the stock in fairly small quantities as indicated in figure 2. The stock may be linerboard or bleached softwood kraft as indicated in figure 2. However, the present invention is equally appli-cable to using a surfactant added to any type of stock and the surfactant may be anionic, or in fact, any other type of surface-active agent. The surfactant is dispersed throughout the stock so the surfactant ls intimately mixed with the stock prior to feeding the stock to the headbox of a paper forming section. The surfactant and stock is supplied to the headbox where the stock is ejected onto a forming wire of the forming section. Water is drained from the forming wire, or forming roll, in order to provide a formed web of paper or board.
The web is then passed through an extended nip press which removes a second portion of water from the formed web, the first portion of water having been drained from the stock in the forming section. The second portion of water pressed from the web is greater than the amount of water removed in a conventional press section. Consequently, the thickness, or caliper, of the resultant web is less than the caliper of a web emerging from a conventional press. Furthermore, the density of the emerging web will correspondingly be greater than that resulting from a conventional press. The pressed web is then led to a conventional drying system. However, such conventional drying system will either require less drying drums, or if the same number of drying drums are utilized, less thermal energy will be required in the drying section due to the larger amount of water removed in the extended nip press. Due to the addition of the surfactant within the stock, bonding of adjacent fibers within the web is ~ 13~ol~8 inhibited because the surfactant ~ends to inhibit hydrogen bonding which, without the use of a surfactant, would result in a web caliper less than the target caliper.
Additionally, such shrinkage in the Z direction can be reduced according to the present invention by introducing the surfactant at either the wet end, the press section or even the dryer section. Furthermore, such surfactant can be added to one or more plys when a multi-ply board is being produced.
The present invention not only provides a resultant pressed, dried web having the required thickness and density but also provides a great savings in the cost of the provision of drying steam and this more than offsets the cost of adding the surfactant in low concentration to the pulp slurry.
Claims (7)
1. A method of reducing shrinkage during drying of paper board formed from paper stock, the method comprising the steps of:
adding a cationic surfactant to the paper stock prior to forming the stock into the paper board;
dispersing the surfactant throughout the stock such that the surfactant is intimately mixed with the stock;
supplying the surfactant mixed with the stock to a headbox of a paper board forming section;
ejecting the mixture of stock and surfactant from the headbox onto a forming wire of the forming section;
draining a first portion of water from the stock such that the paper board is formed on the forming wire;
leading the formed board from the forming section to an extended nip press;
passing the formed board through the extended nip press such that a second portion of water is removed from the formed board; and conducting the pressed board having a first caliper from the extended nip press to a drying section for removing a third portion of water from the formed, pressed board such that the surfactant inhibits bonding between adjacent fibres within the pressed board during drying so that rather than the board shrinking to a second caliper, said second caliper being less than a target caliper, shrinkage of the pressed board during drying is reduced and a dried board of the target caliper is obtained while the board maintains a target moisture content and density.
adding a cationic surfactant to the paper stock prior to forming the stock into the paper board;
dispersing the surfactant throughout the stock such that the surfactant is intimately mixed with the stock;
supplying the surfactant mixed with the stock to a headbox of a paper board forming section;
ejecting the mixture of stock and surfactant from the headbox onto a forming wire of the forming section;
draining a first portion of water from the stock such that the paper board is formed on the forming wire;
leading the formed board from the forming section to an extended nip press;
passing the formed board through the extended nip press such that a second portion of water is removed from the formed board; and conducting the pressed board having a first caliper from the extended nip press to a drying section for removing a third portion of water from the formed, pressed board such that the surfactant inhibits bonding between adjacent fibres within the pressed board during drying so that rather than the board shrinking to a second caliper, said second caliper being less than a target caliper, shrinkage of the pressed board during drying is reduced and a dried board of the target caliper is obtained while the board maintains a target moisture content and density.
2. A method as set forth in claim 1 wherein the surfactant is a cationic surface-active agent.
3. A method as set forth in claim 1 wherein the surfactant is an anionic surface-active agent.
4. A method as set forth in claim 2 wherein the surfactant reduces the surface tension effects during drying, thereby reducing the shrinkage of the resultant dried web.
5. A method as set forth in claim 4 wherein the surfactant attaches to fibres within the formed, pressed web such that the surfactant interferes with the hydrogen bonds between adjacent fibres.
6. A method as set forth in claim 1 wherein the surfactant is effective for reducing both surface tension and bonding between adjacent fibres within the formed, pressed web.
7. A method of reducing shrinkage during drying of a paper web formed from paper stock, the method comprising the steps of:
supplying the stock to a headbox of the paper forming section;
ejecting the stock from the headbox onto a forming wire of the forming section;
draining a first portion of water from the stock such that the paper web is formed on the forming wire;
leading the web from the forming section to a high-load press;
passing the web through the press such that a second portion of water is removed from the web;
conducting the pressed web from the press to a dryer section for removing a third portion of water from the formed, pressed web; and adding a cationic surfactant to the stock during any of the foregoing steps such that the cationic surfactant inhibits bonding between adjacent fibres within the pressed web, the surfactant being effective for reducing both surface tension and bonding between adjacent fibres within the formed, pressed web so that shrinkage of the pressed web in the Z direction during drying is reduced and a dried web of required caliper is obtained.
supplying the stock to a headbox of the paper forming section;
ejecting the stock from the headbox onto a forming wire of the forming section;
draining a first portion of water from the stock such that the paper web is formed on the forming wire;
leading the web from the forming section to a high-load press;
passing the web through the press such that a second portion of water is removed from the web;
conducting the pressed web from the press to a dryer section for removing a third portion of water from the formed, pressed web; and adding a cationic surfactant to the stock during any of the foregoing steps such that the cationic surfactant inhibits bonding between adjacent fibres within the pressed web, the surfactant being effective for reducing both surface tension and bonding between adjacent fibres within the formed, pressed web so that shrinkage of the pressed web in the Z direction during drying is reduced and a dried web of required caliper is obtained.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US89588386A | 1986-08-12 | 1986-08-12 | |
| US895,883 | 1986-08-12 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1340148C true CA1340148C (en) | 1998-12-01 |
Family
ID=25405219
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 540140 Expired - Fee Related CA1340148C (en) | 1986-08-12 | 1987-06-19 | Method of reducing shrinkage during drying of a paper web |
Country Status (2)
| Country | Link |
|---|---|
| JP (1) | JPS6350595A (en) |
| CA (1) | CA1340148C (en) |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS5928676A (en) * | 1982-08-10 | 1984-02-15 | Showa Electric Wire & Cable Co Ltd | Tester for dc dielectric strength |
| FI833133A (en) * | 1982-09-30 | 1984-03-31 | Beloit Corp | PRESS MED FOERLAENGT NYP |
| JPS59173398A (en) * | 1983-03-23 | 1984-10-01 | ディック・ハーキュレス株式会社 | Paper sizing method |
| US4735685A (en) * | 1984-09-13 | 1988-04-05 | Ciba-Geigy Corporation | Process for sizing paper or cardboard with anionic hydrophobic sizing agents and cationic retention aids |
-
1987
- 1987-06-19 CA CA 540140 patent/CA1340148C/en not_active Expired - Fee Related
- 1987-08-12 JP JP20261187A patent/JPS6350595A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| JPS6350595A (en) | 1988-03-03 |
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