GB2152961A - Method of creping a paper web - Google Patents

Abstract

A sort but relatively strong paper web is produced by first creping a web from a Yankee dryer 1 at a dryness of about 95%, applying the web by means of adhesive to a creping drum 2, and creping the web from the creping drum. The web is treated between the first and second creping steps to reduce its stretch, thereby enabling a better response to the second creping step and resulting in bulk and softness gains. The treatment may be effected by running the Yankee drier and creping drum at different speeds. The web may be passed between calendering rollers between the first and second creping steps. <IMAGE>

Description

SPECIFICATION Method of making a paper web This invention relates to a method of making a soft, light-weight paper web suitable for making sanitary paper products and having a basis weight of between 8 and 20 libs.12880 ft2. Examples of sanitary paper products which can be made from such a web are bathroom and facial tissue, paper towels, and napkins.

It is well known that the principal strength of a paper web formed from an aqueous slurry of cellulosic fibers results from the interfibre bonds which are formed by the hydrogen bonding process associated with paper making. Paper which has a substantial concentration of these bonds is characterised by both strength and stiffness.

Softness is a most desirable characteristic of a sanitary paper product. In order to achieve the required degree of softness, paper webs have been softened by reducing their strength either mechanically or chemically to disrupt and reduce the interfibre bonds in the paper web which are formed during drying.

Various proposals have been made for achieving this, such as creping the web from a drying surface with a creping blade, adding chemical debonders to the fibrous material forming the web, and creping the web a second time. All of these methods result in strength levels which are relatively low for a given degree of improved softness.

Attempts have been made to improve the strength of the sheet after creping. These have included preferential creping using notched creping blades, or discontinuous creping surfaces. In this method, the resulting sheet was shown to have low tensile areas. In another proposal, interfibre bonds formed during drying of the web were supplemented with chemical bonds before creping. While this resulted in a stronger sheet having the desired softness, the material was very expensive to manufacture. In another proposal, the strength reduction was attenuated by pressing the sheet onto a drying cylinder with a fabric to provide differential adhesion on the creping surface; this results in a differential crepe in which only parts of the sheet are subjected to the action of the creping blade.Here, the differential adhesion inevitably results in lower overall adhesion of the sheet to the creping surface, giving a lower overall creping response and smaller softness gains.

Compression of the web when it is wet encourages the formation of interfibre bonds, and a successful approach to the production of soft paper has been to prevent the formation of these bonds during manufacture of the web by avoiding mechanical compaction of the wet web at any stage. Drying of the web is achieved using through-dryers which direct air through the web, and it has been found that the internal structure of such never-pressed webs has an inherent bulk and softness which gains the maximum benefit from a subsequent creping step. Consumer demands for still softer sanitary paper products have meant it has become increasingly difficult to produce paper of the required softness from conventional soft paper-making equipment of the kind in which the web is pressed to remove water and then creped from the surface of a Yankee dryer.Although better results have been achieved using through-dryer technology, such equipment is very expensive both to install and to operate. There has therefore been a substantial demand for methods of manufacturing soft paper which are capable of utilising conventional Yankee paper machines but which result in a softer product than has hitherto been possible to achieve.

In an attempt to reach these desired quality levels, proposals have been made for adding a second creping stage to a conventional single-crepe Yankee paper machine, but generally these have not been successful in producing a iight-weight sanitary tissue having a basis weight in the range of 8 to 20 Ibs./ream with the required degree of softness. Rather, the paper of this weight produced on such machines has not generally exhibited the balance between softness and strength which is required for sanitary paper products.

The present invention provides a method of making a creped paper web which is comparable in softness and strength to never-pressed webs but which can be produced using conventional equipment incorporating mechanical de-watering and a Yankee dryer on which the web is dried and then creped. These improved results are achieved by using a dual creping process in which the first creping step is controlled so as to condition the internal and surface structure of the web in such a manner that the creping response is maximized at the second creping stage. Such an arrangement is capable of producing soft paper at lower cost than apparatus using through-dryers to promote drying of the paper.

According to the invention, there is provided a method of making a soft paper web having a basis weight of from substantially 8 to 20 lbs./2880 ft2 comprising forming a web from a slurry, applying the web to the surface of a Yankee dryer, creping the web from the surface of the Yankee dryer at a dryness of substantially between 93 and 97% by means of a first creping doctor, drawing the web from the creping doctor and treating it to reduce the elasticity of the web introduced by the said creping step such that the mean square stretch of the web, as hereinafter defined, is not more than 9%; applying the web by means of an overall adhesive to a creping drum, and creping the web from the creping drum by means of a second creping doctor at a dryness of at least 93%.

Thus, prior to the second creping step, process modifications are made to the conventional paper machine to create a web having a structural lending itself to gains greater than those possible without the modifications. In the prior art, the web is formed and dried on the conventional paper machine then creped with a high level of stretch and crepe created in the sheet for maximum bulk and flexibility. The invention calls for a web, after Yankee creping, having very low residual stretch, therefore having a low capacity to absorb energy elastically at the second creping stage, and hence less able to resist permanent deformation of its structure.

The mean square stetch of the sheet is defined as follows:

Where MDS is the percentage stretch in the machine direction, CDS is the stretch in cross-machine direction, and MSS is the mean square stretch thus defined.

Sheets produced using conventional papermaking techniques have a mean square stretch (MSS) of about 11% while the web in the invention prior to the second creping step has a MSS of less than 9% and preferably less than 6%. The lower MSS value illustrates the relative difference in energy that can be absorbed in the sheet before disrupting the interfibre bonds. Aweb having a MSS within this range can elastically absorb a lower level of potential energy from the creping blade, thus greater interfibre bond disruption occurs resulting in bulk and softness gains.

The reduction in mean square stretch of the web is conveniently achieved by drawing the web from the first creping doctor under tension, and a suitable choice of a minimum speed differential between the Yankee dryer and the creping drum can provide the necessary tension without causing rupture of the web.

An example would be to take the web from the Yankee running at 2810 fpm and adhere itto the second creping cylinder that is running at 2650 fpm. The consequent smoothing out of the web between the Yankee and the second creping step yields a structure that is quantified by a machine direction stretch MDS of 8% and a cross direction stretch CDS of 4.6% resulting in an MSS of 6.1%.

Other means for achieving tensioning of the web are possible. For example, an additional tensioning roller or roller set may be interposed between the Yankee and the creping drum.

In order to optimise the performance of the second creping stage, i.e. creping of the web from the creping drum, it is important that the web is strongly adhered to the creping drum over substantially its whole surface area, such that very high creping forces can be employed. Under these conditions maximum disruption of the interfibre bonding will be achieved with the conditioned web. To enable the maximum possible surface area of the web to be applied to the creping drum, the web is drawn from the Yankee dryer and treated to smooth out the crinkles in the web surface. This is suitably achieved by the same step of drawing the web from the first creping doctor under tension which produces the low residual stretch.As the web is pulled from the Yankee with minimum speed differential, its surface smoothness is enhanced by minimizing the surface effect of the creping action created at the Yankee blade. When a web has less surface crepe, it presents a smoother surface to the coating on the second creping cylinder, allowing for greater area of physical contact at the fibre coating interface thus creating more adhesion and increasing the total force transmitted to the sheet at the second creping step. Alternatively or in addition, the web may be passed through calendering rolls to give it even greater surface smoothness, allowing more intimate contact and higher overall adhesion.

This combination of the web's low energy absorbing capacity and smooth surface as it enters the second creping stage results in an improved response from the second crepe, giving bulk and softness gains in the finished web.

An important parameter in the invention is the web dryness prior to the second creping cylinder. In the invention, when the sheet moisture is reduced below 7%, two benefits are gained. Firstly, the ability of the sheet to adhere to the creping cylinder is much improved at high levels of dryness, and the web is thus at optimum moisture content and uniformity entering the second creping stage. Secondly, at this dryness little or no compaction occurs at the presser roll nip which would change the web structure and subtract from the overall result of the invention.

The features described above achieve a controlled tensile strength breakdown at the second crepe which provides the desired gains in softness and bulk. Preferably, this controlled mean tensile strength reduction in the web ranges from 20-50%, but is advantageously about 35%.

In the prior art where a second creping step is used, differential adhesion is used to limit the tensile reduction. However, as adhesion is limited to limit tensile reduction, a satisfactory creping response can only be achieved with webs having lower breaking lengths. The present invention uses a modified web structure and very high creping forces in the second crepe to produce webs having improved softness on both sides of the sheet. These high creping forces result from correspondingly high adhesion levels between the sheet and the creping drum which are generated by the high surface smoothness of the sheet, the low moisture content of the web, and the use of an adhesive which is applied overall to the creping drum. All of these factors result in adhesion levels that are more than three times higher than the adhesion levels employed in conventional Yankee creping. Examples of adhesives which are capable of achieving the required adhesion levels are: water soluble resins such as carboxymethyl cellulose, polyvinyl alcohol, and polyacrylamide; and emulsions of resins such as acrylates, vinyl acetates, vinyl chlorides, and methacrylates.

It is well known in the art that as creping adhesion increases in any drum-to-blade creping operation, the resulting creped web will exhibit increased after crepe web tension. This phenomenon is disclosed at Column 3 lines 32-37 of U.S. Patent 4,302,282 issued to Terrill A. Young. It is therefore possible to use after crepe web tension as a method to monitor the amount of adhesion between a web and a creping drum.

There are several methods to actually measure after crepe web tension and Young describes one such method at Column 10, lines 23-34.

Young shows that patterned web adhesion is a very low 1.0 g;cm of web width and that he was able to increase adhesion to 2.7 g/cm of web width.

In normal, conventional Yankee dry creping, the after crepe sheet tension ranges from values of 1.6 g/cm of web width to as high as 3.5 or 4 g7cm at very high adhesion levels. The adhesion level in process according to the invention is preferably at least 10 gicm of web width and in one example is in excess of 11.4 g;cm of web width which is some 3 to 7 times higher than adhesion levels employed in conventional yankee dry creping.

Conventional Yankee creping operations typically produce webs having 12-27 crepe bars per cm. In the invention, the finished web would preferably have at least 35 crepe bars per cm. at a nominal MD stretch of 18% for optimum surface smoothness. While the softness of both sides of the sheet is improved, the side that was adhered to the dryer has surface smoothness that has been measured to be 30 to 40% higher than the other side. Therefore, the final product softness of a light weight two-ply tissue, where the smooth sides can be turned out, can be significantly enhanced by this invention.

In one embodiment of the invention, the sheet is creped a third time using the same creping step described above with the side that was not against the dryer in the second step pressed into the coating for creping.

The resulting sheet then has further enhanced bulk and enhanced surface smoothness on both sides of the sheet.

The invention, therefore, can be summarised as a series of process modifications to a conventional Yankee paper machine followed by a creping step that creates a web having higher quality than previously achieved in the prior art at the same basis weight and strength. The higher quality sheet takes the form of a soft fibrous sheet material having a basis weight of about 8 to 20 Ibs. 2880 ft.2 with a softness measured by crush/cal.2 of 1.0 to 1.5 and density of 0.120 to 0.160 gm;cc.

Preferred embodiments of the invention will now be described with reference to the accompanying drawings in which; Figure 1 compares the performance of a method according to the invention with a conventional paper making process.

Figure 2 shows apparatus for carrying out the method according to the invention.

Figure 3 shows a modification of the apparatus shown in Figure 2.

Figures 4a-4c show apparatus for measuring the flexibiiity-softness of a web.

Table 1 below shows the benefits that can be gained from adding the current invention to conventional Yankee paper machines. These benefits are expressed in terms of bulk per unit of basis weight, indicating thickness normalized for fibre weight, and crepe bars per cm where the more crepe bars per cm present in a sheet, the smoother its surface feels to the touch.

TABLE 1 Prior Art Invention Crepe bars:cm 12-27 28+ Bulk/Basis weight cog 6 6.8 Densitygoc 0.18 < 0.16 The sheet made with the invention has higher bulk and greater flexibility and softness as measured by the lower crush/cal.2 Figure 1 compares softness of a conventional dry crepe sheet with that of the invention over a range of breaking lengths. As can be seen, the invention produces a softer product over a range of strengths.

Figure 2 shows papermaking apparatus wherein a Yankee dryer is followed by a heated creping drum 2.

The web which can either be a single layered web or a multiple layered or stratified web is delivered to the Yankee dryer by a forming and partial-dewatering section 3 which is conventional and will not, therefore, be described further. The web is pressed onto the hot surface of the Yankee dryer and adheres thereto either by virtue of its water content alone or with the aid of an adhesive which is sprayed onto the surface of the Yankee dryer upstream of the point of application of the web. The web is dried on the Yankee dryer and creped therefrom by a creping doctor 4 at a dryness of 93-97 iO A further blade 5 may be provided after the creping doctor to remove excess adhesive left on the surface of the Yankee dryer.The natural foreshortening introduced by the creping action, which could be of the order of 500it if the web was untreated after being creped from the Yankee dryer, is substantially removed from the web by the web being drawn from the creping doctor 4 under tension. The web passes around a guide roller 6 to the nip between a pressing roller 7 and the heated creping drum 2. Tension in the creped web is brought about by selecting the speed differential between the Yankee dryer 1 and the creping drum 2.An example would be take the web from the Yankee running at 2810 ft.ímin. and adhere it to the creping drum running at 2650 ft.imin; such a speed differential leads to a maximum drawing-out of the creping crinkles and provides a web having a machine direction stretch of 8% and a cross direction stretch of 4.6% resulting in a mean square stretch of 6.1%.

This reduction in stretch of the web lessens the ability of the web to absorb energy during the second crepe, and the action of drawing the web from the Yankee 1 under this degree of tension also smooths out its surface structure, allowing for a greater area of physical contact and therefore high adhesion levels on the creping drum 2.

Adhesive is applied overall to the creping drum 2 either by a mixture of compounds or as one compound in a single spray, or in separate sprays, or by a printer, at a rate of 0.5 to 1.1% adhesive solids by weight of bone-dry fibre. The materials which are capable of accomplishing the adhesive function are: water soluble resins such as carboxymethyl cellulose, polyvinyl alcohol, and polyacrylamide; emulsions of resins such as acrylates, vinyl acetates, vinyl cholorides, and methacrylates. In addition to the materials listed above, other mixtures may be used containing low molecular weight starch such as dextrin. The preferred method of applying the adhesive, as shown in the Figures, is to spray the adhesive onto the surface of the drum 2 using .018 spray nozzles at pressures ranging from 100-200 psig with preferable pressure being 150 psig.

At the pressing roller 7 on creping drum 2, the low moisture content of the web, its smooth surface, and the use of an overall adhesive ensure that high levels of adhesion between the drum 2 and the web are achieved, thus enabling higher forces to be used at the second crepe, and maximising the creping response of the web. The web is creped from the drum 2 using a creping doctor 10 positioned such that maximum resistance is applied to the adhesive coating (a 20-60 pli loading) while at the same time, producing maximum bulk with optimum surface smoothness. To produce these properties, a creping angle 4-16" above the cylinder radial line at the contact point is desired. The resulting sheet is then wound at the reel with a speed differential selected to produce a finished web having a mean square stretch of 8-15%.

A further blade 10 may follow the creping doctor to remove excess adhesive from the surface of the creping drum.

Figure 3 shows apparatus similar to that shown in Figure 2 but wherein the web 11 passes through calendering rollers 11 between the Yankee dryer and the creping drum. The effect of these rollers 11 is to further smooth out the surface of the creped web so that very intimate adhesion of the web to the creping drum is achieved.

The method describing in relation to the Figures is particularly applicable to making a soft paper web which has very good tactile softness and smoothness on one side. Such a web is suitable for use in a 2-ply product wherein the softer side of each ply faces outwardly. A single ply, heavier basis web, can also be made with good tactile softness on both sides by the addition of a second creping drum so that the web may be creped on both sides.

The furnish used in making webs by the process described would produce a strong but insufficiently soft web if used in conventional prior art process. Such a furnish can be obtained either by chemical means or by refining as is well known in the art.

Definition of terms Caliper: Caliper is measured using a Federal Bulk Tester manufactured by Federal Products Corporation, Providence, Rhode Island, United States of America, which measures the thickness (expressed in .001 inch) of a 24-ply sample under a 1.0 square inch circular pad having a 235 gram load. The measured thickness is then divided by 24 to obtain the average thickness. or caliper, of a single sheet of tissue paper.

Crush!Calipel2 The crush/caliper2 is a combination of two measured paper properties which together state a relative measure of a tissue sample's stiffness or flexibility. The crush text is a method of measuring the flexibility or stiffness of a fibrous web. The test involves the measurement of the force required to collapse an annulus pre-formed in the web. The annulus is formed by the following procedure, which is illustrated in Figures 4a-4c: 1. The centre of a 4.5 in x 4.5 in. tissue sample (S) is clamped between an upper circular plate (11), 1,090 in.

diameter, and a lower circular plate (12)1.435 in. diameter.

2. As shown in Figure 4a, the clamped sample (S) is located on the axis of a circular opening 1.815 inches in diameter in an adjustable table (12) with the plane of the clamped portion of the sample even with the lower surface of a reference ring (14) 0.125 inches thick and having an inner diameter of 2.125 inches. The edges of the sample are placed within a 0.065 inch gap formed by the lower surface of the reference ring (14) and the upper surface of the adjustable table (13).

3. A fold ring (15) having a 1.502 in. inside diameter and a 1.750 in. outside diameter with the top surface having a 0.625 in. radius is located beneath the clamped sample. As shown in Figure 4b, the fold ring (15) applies an upward force against the tissue sample (S) over a distance of 0.250 inches pushing the sample (S) up through the circular slot formed by the upper circular plate (11) and the reference ring (14) while drawing the sample through the 0.065 inch gap formed by the adjustable table (13) and reference ring (14) thereby forming an annulus in the tissue sample. Ail surfaces contacting the sample have a 3 micro inch finish with all ring edges having a 0.0625 inch radius. The force required to draw the sample through the 0.065 inch gap is called the folding force orfold measured in grams.

4. To measure the crush resistance, or crush, of the sample, the 0.065 inch gap between the adjustable table (13) and the reference ring (14) is closed to clamp the edge of the sample (10) and the fold ring (15) is retracted. A platen (16) having a 2.062 in. diameter then applies a downward force on the annulus over a distance of 0.200 inches. (See Figure 4c). The force required to compress the annulus is then called the crush and measured as grams.

The crush/caliper2 is the crush measured in grams divided by the square of the caliper measure in inches.

The resulting relation is expressed as grams1 square inch.

Density Density is a measure of the total volume of space occupied by a given unit of web weight. A tissue sample's density is measured by the following procedure: 1) Slice 8 cross-sections from a single layer of the tissue sample.

2) Position the cross sections under a microscope (80x) with a camera fitted on the eyepiece and take 8 photographs of the sample cross-sections.

3) Measure the total area of the photomicrographs using a planimeter and calulate the true thickness.

True Thickness (cm) = Measured Area (cm2) Traced Length (cm) Magnification True Density (gmicc) = Basis Weight (g:m2) True Thickness - 10,000 (cm) BulklBasis Weight The ratio of Bulk, Basis Weight expresses the thickness of a sample in terms normalized by the sample's weight. Bulk is measured using the Federal Bulk Tester (which measures the thickness in 0.001 inch units of 24 sample sheets under a one square inch circular pad having a 235 gram load). Basis weight is the weight per unit area of the sample expressed in lib.'2880 ft2. The resulting ratio does not attempt to combine the units of the individual tests. The measure is sometimes used as a rough approximation of the relative density of two samples.

Breaking Length The geometric mean tensile strength of a sample, divided by its Basis weight, is sometimes called Breaking Length Ad Hoc, BLAH.

BLAH = GMT Basis Weight where;

MDT = Machine Direction Tensile sarength expressed in oz. in.

CDT = Cross Direction Tensile strength expressed in oz. in.

Basis Weight = Weight per unit area of sample expressed in lb.?2880ft2, The Breaking Length of a sample can be expressed in meters by the following calculation: Breaking Length = BLAH x 658.37 metres.

Claims (17)

1. A method of making a soft paper web having a basis weight of from substantially 8 to 20 libs/2880 ft2, comprising the following steps, in sequence; (a) forming a web from a slurry; (b) applying the web to the surface of a Yankee dryer; (c) creping the web from the surface of the Yankee dryer at a dryness of substantially between 93 and 97% by means of a first creping doctor; (d) drawing the web from the creping doctor and treating the web to reduce the elasticity of the web introduced by said first creping step such that the mean square stretch of the web, as defined above, is not more than 9%; (e) applying the web by means of an overall adhesive to a creping drum; and (f) creping the web from the creping drum by means of a second creping doctor at a dryness of at least 93%.

2. A method as claimed in Claim 1, wherein the web is treated to reduce the elasticity of the web introduced by said first creping step by drawing the web under tension from the first creping doctor.

3. A method as claimed in Claim 1 or Claim 2, wherein the web is treated to reduce the elasticity of the web introduced by said first creping step such that the mean square stretch of the web is substantially 6% or less.

4. A method as claimed in any of Claims 1 to 3, wherein the web is treated between said first and said second creping steps to smooth out its surface structure so as to enable a maximum surface area thereof to be adhered to said creping drum.

5. A method as claimed in Claim 4, wherein the web is treated to smooth out its surface structure by passing the web between calendering rolls.

6. A method as claimed in Claim 2, wherein the web is drawn from the first creping doctor under tension by maintaining a minimum speed differential between the Yankee dryer and the creping drum.

7. A method as claimed in any one of the preceding claims, wherein the creping drum is heated and the adhesive is applied to the creping drum at a point upstream of the point at which the web is applied to the drum.

8. A method as claimed in any one of the preceding claims, wherein the web is adhered to the creping drum with an adhesion level as measured by a sheet tension of at least 10 g/cm of web width, and said second creping step reduces the tensile strength of the web by between 20 and 50%.

9. A method as claimed in any one of the preceding Claims, wherein the creping angle at the second creping doctor is between 4 and 16 above the creping drum radial line at the point of contact.

10. A method as claimed in any one of the preceding Claims, wherein the second creping doctor is applied to the creping drum with a loading of between 20 and 60 pounds per lineal inch.

11. A method as claimed in any one of the preceding Claims, wherein the web is drawn from the second creping doctor under tension so as to reduce the mean square stretch of the finished web to between 8 and 15%.

12. A method as claimed in any one of the preceding claims, wherein the finished web has a crepe of at least 28 crepe bars'cm. measured art a MD stretch of between 16 and 18%

13. A method as claimed in any one of the preceding Claims, wherein the finished web has a density of substantially between 0.12 and 0.16 g cc.

14. A method as claimed in any one of the preceding claims, wherein the finished web has a ratio of bulkXbasis weight of at least 6.8 ce gm.

15. A method as claimed in any one of the preceding Claims, wherein the finished web has a softness measured by crush cal.2 of between 1 and 1.5.

16. A method as claimed in any one of the preceding Claims, wherein the web is adhered to a second creping drum on the side of the web opposite to that which was adhered to the first creping drum, and is creped therefrom by means of a third creping doctor.

17. A method of making a soft paper web, substantially as hereinbefore described with reference to the accompanying drawings.