EP0426548B1 - Multi-ply sheet of absorbent paper - Google Patents

Abstract

The multi-ply sheet made up of at least two sheets of absorbent paper made of cellulose wool, whose weight per unit area is between 10 and 40 g/m<2>, obtained especially by a process consisting in goffering separately by calendering each of the said sheets according to a pattern consisting of protuberances distributed according to a first pitch (P1sm, P2sm) in a first direction and a second pitch (P1st, P2st) in a second direction forming a nonzero angle with the first, and whose end away from the plane of the ungoffered sheet comprises a bonding flattening, bonding, especially by adhesive bonding, the two sheets together via the said flattenings placed in at least partial coincidence, is characterised in that the pitches (P1sm, P2sm) in the said first direction are different and are related by the relationship <IMAGE> where L1 is a predetermined value corresponding to the measurement of a segment along which it is desired to guarantee the existence of at least one point capable of forming a bond between the two sheets, whatever the relative position of the two patterns. <IMAGE>

Description

The present invention relates to the field of absorbent paper, in particular cellulose wadding, for sanitary use as toilet paper for example or else for domestic use as household paper.

The patent US-A-3,414,459 describes a sheet of absorbent paper obtained by the combination, known as tips against tips, of at least two embossed sheets of paper of cellulose wadding. The embossing pattern consists of protuberances arranged in a regular network, the density of which is between 3 and 30 protuberances per cm². The protrusions on each sheet are obtained by mechanical deformation between a metal cylinder engraved in relief according to the desired pattern and a smooth rubber cylinder. The protrusions are arranged on the same side of the plane of the sheet, have a height of between 0.2 and 1 mm, and occupy between 10 and 60% of the total surface.

In order to give it the flexibility, compressibility and softness required by the application as sanitary and domestic paper, a mechanical calendering treatment is applied to the double sheet from the connecting rolls, by which its thickness is reduced by at least 50%. This treatment aims to reduce the compressibility modulus of the double sheet, that is to say the resistance to deformation when it is subjected to a compression force exerted perpendicular to its main plane. It follows that a person handling the leaf thus treated will feel an impression of greater softness and will attribute to it a spongy character.

As reported in the aforementioned patent, the two sheets are bonded by passing them through the clamping gap formed between the two identical metallic embossing cylinders. The latter are arranged in parallel, and driven by means of belts or other equivalent members so that their rotational speeds are equal and in opposite directions. The drive members are adjusted so that the bosses coincide as exactly as possible in the tightening interval.

• Un décalage circonférentiel entre les picots disposés sur une même génératrice théorique du cylindre. Ainsi, pour un cylindre de 2,60 m de longueur on a mesuré un décalage circonférentiel de 5/10 mm entre les bossages extrêmes d'une même génératrice. Ce décalage est indépendant de la densité du motif.
• Un décalage axial le long de cette génératrice, dont, pour un certain mode d'usinage, l'amplitude varie selon une fonction périodique sinusoïdale avec une fréquence dépendant de la finesse du motif. Plus la finesse est grande plus la fréquence des oscillations le long de la génératrice est élevée. On a mesuré une amplitude maximale de 1/10 mm.

The patent implicitly admits that the metal cylinders are engraved perfectly and that there is no variation in the arrangement of the pins relative to each other. In reality, the usual engraving techniques lead to two types of offset:

• A circumferential offset between the pins arranged on the same theoretical generator of the cylinder. Thus, for a cylinder of 2.60 m in length, a circumferential offset of 5/10 mm was measured between the extreme bosses of the same generator. This offset is independent of the density of the pattern.
• An axial offset along this generator, the amplitude of which, for a certain mode of machining, varies according to a sinusoidal periodic function with a frequency depending on the fineness of the pattern. The greater the fineness, the higher the frequency of the oscillations along the generator. A maximum amplitude of 1/10 mm was measured.

These manufacturing tolerances do not affect the quality of the connection between the two sheets when the pins have a relatively large contact surface. Indeed, the probability is low to see two spikes, of the same index on the two cylinders, so offset from each other that they no longer provide a connection.

When the double sheet obtained by this process is transformed into rolls of small width, 10, 20 or 30 cm for example, no unbound sheets are noticed.

We wanted to extend this association technique to sheets of cellulose wadding paper, previously embossed in a fine pattern where the contact surfaces are very small. It was then found that entire areas, forming large bands oriented in the machine direction, were not linked. After transformation into rolls, of toilet paper for example, the width of which is less than the width of these strips, products are obtained cut inside these same strips, which are composed of a winding of two unbound sheets. This results in a significant percentage of scrap.

The invention proposes to solve this problem and aims to provide a complex sheet which we control the extent of areas where the two sheets are not linked due to the misalignment of the protrusions.

L1 étant une valeur prédéterminée, correspond à la mesure d'un segment, quelconque, parallèle à la première direction, le long duquel on souhaite garantir l'existence d'au moins un point susceptible de constituer une liaison entre les deux feuilles quelle que soit la position relative des deux motifs.The invention therefore relates to a complex sheet composed of at least two sheets of absorbent paper of cellulose wadding whose grammage is between 10 and 40 g / m² and each comprising protuberances distributed in a pattern with a first step ( P1sm, P2sm) in a first direction and a second pitch (P1st, P2st) in a second direction forming a non-zero angle with the first, said two sheets being linked together by the flats of said protuberances. This complex sheet is characterized in that the steps (P₁sm, P₂sm) in said first direction are different and are linked by the relation

L1 being a predetermined value, corresponds to the measurement of any segment, parallel to the first direction, along which it is desired to guarantee the existence of at least one point capable of constituting a connection between the two sheets whatever the relative position of the two motifs.

Thus, according to the invention, before defining the pitch of each pattern according to the chosen direction, which can be, the machine direction of production - corresponding to the direction of travel of the sheets - or else the transverse direction which is perpendicular to it, we first sets the maximum extent of the pockets that are tolerated on the complex sheet in this direction. Then, this value L1 being determined, the patterns are chosen whose steps satisfy the required condition.

This condition results from the following observation: when one moves along the chosen direction, from a connection point where two protuberances overlap, one notes, in the case of an apparently identical implantation on the two sheets, that the protrusions, which face each other along this direction, gradually move away from each other until they no longer coincide, then come closer again. When the surface of contact between two protrusions is less than a certain threshold, it has no connection possible at this location. In fact the difference varies between a maximum value of half a step and a zero value.

This misalignment is due to the inevitable manufacturing tolerances. A tiny initial difference at the level of two adjacent reliefs becomes observable by the cumulative effect and the high number of reliefs. In addition, when one arrives in an area where the protuberances are not aligned, it is necessary to count a significant number of steps before finding two protuberances capable of forming a connection. This explains the presence of large unbound pockets. This phenomenon can be compared to a certain extent with the phenomena of beats observed when proceeding to the composition of two sinusoidal vibrations of the same direction and of slightly different periods.

et $\text{PL = (n + 1) P₂sm}$

, avec (P₁ > P₂).The analysis of this phenomenon shows that, still moving in the chosen direction (sm), when the misalignment between the reliefs becomes minimum again, the number of steps, separating the protrusions, since the last minimum misalignment is equal to n for the pattern (of step P₁sm) from one of the sheets and to (n + 1) for that (of step P₂sm) of the other sheet. We can then define a distance PL satisfying the two relations: $\text{PL = nP₁sm}$

and $\text{PL = (n + 1) P₂sm}$

, with (P₁> P₂).

Cette distance PL, que l'on peut désigner par "pas de liaison", correspond donc à la distance séparant deux zones de la feuille complexe susceptibles de comporter une liaison entre les deux feuilles (pour chacune de ces deux zones la surface de contact entre les deux reliefs est suffisante pour constituer une liaison).By eliminating n between them, we obtain the equivalent relation

This distance PL, which can be designated by "no connection", therefore corresponds to the distance separating two areas of the complex sheet likely to include a connection between the two sheets (for each of these two areas the contact surface between the two reliefs is sufficient to constitute a connection).

The invention is therefore based on the consideration that the unbound zones being included within a segment of length equal to the connecting pitch of the two patterns, the extent of this undesirable zone can be reduced at will by reducing the value of the connection step.

It therefore suffices to choose the steps of each pattern so that the link step is smaller than the maximum unbound, tolerated extent L compatible with the application to which is intended for the complex sheet and the constraints linked to its transformation. It is certain that, over this distance, the connection points do not only exist at the places where the protuberances have a minimum or zero deviation. Connection points will no doubt be formed on either side of these protrusions, as long as the contact surface is sufficient. But the invention provides the guarantee of the existence of at least one link over this distance, which is sufficient to keep the two sheets integral.

The invention is of general application regardless of the pattern, but it is of certain interest when the sheets are embossed with a fine pattern, that is to say a pattern whose density is greater than 20 protuberances per cm² and less than 300. It will be noted that beyond 300 the distance separating the protuberances is too small to allow the deformation of the sheet of absorbent paper. The leaf is smooth in appearance.

According to an additional characteristic, the surface of the flats is between 5 and 30% of the surface of the sheet before embossing.

According to a particular application, the complex sheet is produced in a strip of width of 2.60 m, then cut lengthwise into strips of width of 100 mm, to be transformed into rolls of toilet paper whose rolled strip is pre-cut, at regular intervals, so as to obtain sheets of format 100 x 125 mm². The steps of the two patterns are chosen so that at least one of the connecting steps in a direction SM or ST is less than 100 mm. This guarantees that each 100 x 125 format sheet will include at least one connection point. This is of course the minimum requirement. In practice, we prefer to choose a sufficiently small connection pitch so that the sheets are linked at several points.

The steps can be chosen very different from each other. However, there then follows a difference in appearance on the faces of the complex sheet. According to a particular embodiment, an additional condition is imposed which makes it possible to avoid the sheet having a place and a backing: for the same direction, the ratio between the pitches of the patterns of the two sheets is between 1 and 2, preferably between 1 and 1.5.

In accordance with an additional characteristic of the invention, the number of points effectively linked by bonding is less than the number of points on the sheet capable of constituting a connection between the two sheets.

Indeed, in order to reduce as much as possible the rigidity imparted to the sheet by the adhesive, it is possible to ensure only a partial sizing compatible with the constraint of obtaining a binding inside surfaces of determined format.

According to a first embodiment, this partial gluing is ensured by means of an applicator cylinder, the effective application surface of which represents only a fraction of the surface of the sheet to be glued. This is achieved by etching the surface of the cylinder in an appropriate pattern.

According to a second embodiment, at least one of the sheets to be bound is embossed with protrusions of different heights, two for example, so that the connection with the other sheet is only ensured by the protrusions, the height is the highest.

• la figure 1 représente en plan un motif de protubérances gravées sur un cylindre,
• la figure 2 est une coupe selon II-II de l'arrangement de protubérances de la figure 1,
• la figure 3 est une représentation schématique, en coupe transversale, d'une feuille complexe obtenue par l'association de feuilles gaufrées avec des motifs à pas différents conformément à l'invention,
• la figure 4 est une vue en plan, à plus grande échelle, de la feuille complexe de la figure 3 sur laquelle on a visualisé les zones susceptibles de comporter des liaisons,
• la figure 5 est une représentation schématique d'une installation de gaufrage et de liage.
• la figure 6 est une représentation d'un mode de réalisation particulier des bossages sur les cylindres gaufreurs.

Other characteristics and advantages will be developed in the following description:

• FIG. 1 shows in plan a pattern of protuberances engraved on a cylinder,
• FIG. 2 is a section on II-II of the arrangement of protuberances in FIG. 1,
• FIG. 3 is a schematic representation, in cross section, of a complex sheet obtained by the association of embossed sheets with patterns of different pitch in accordance with the invention,
• FIG. 4 is a plan view, on a larger scale, of the complex sheet of FIG. 3 on which the zones liable to contain connections have been visualized,
• Figure 5 is a schematic representation of an embossing and binding installation.
• Figure 6 is a representation of a particular embodiment of the bosses on the embossing cylinders.

FIGS. 1 and 2 show an example of an embossing pattern for the two sheets of absorbent paper made of cellulose wadding.

The protrusions or bosses (A) are distributed uniformly in two directions: a first direction Psm and a second direction Pst forming a non-zero angle with it. It can conveniently be the machine direction and the cross direction. In the embodiment shown, the pattern is staggered, but other arrangements are possible.

Machine direction, the protuberances on each of the two sheets are distributed uniformly with a first step, P₁sm and P₂sm respectively, while cross direction the steps are P₁st and P₂st. We choose P₁sm different from P₂sm and P₁st different from P₂st. The protuberances have at their top a flat (B) in a plane parallel to the plane of the sheet.

In Figure 3, there is shown, in section along the direction ST, the complex sheet obtained after association of the two sheets, tips against tips. When moving along the direction of the cut from a connection point where the bosses overlap, the successive connection points are more or less solid depending on the degree of overlap of the bosses. However, at a distance equal to the connection pitch PLst corresponding to n not P₁st and (n + 1) not P₂st, the bosses are again superimposed with a maximum contact surface. By choosing a sufficiently small PL value, the width of the zones where the bosses do not overlap is sufficient to ensure the connection.

Example

A complex sheet in accordance with the invention was produced from two sheets of creped cellulose wadding absorbent paper, of the same grammage: 15 g / m², which were embossed on cylinders whose patterns consist of protuberances ovals distributed in staggered rows at different machine direction Psm and cross direction Pst, and with a density ratio close to unity. Machine direction, the distance L1 over which we require the presence of at least one link, is 100 mm. Direction across the distance L2 is 125 mm. The density ratio between the cylinders (5) and (1) is 1.08. Cylinder 1 Cylinder 5 Pst (mm) 3.15 2.55 Psm (mm) 1.1 1.25 Pattern density (per cm²) 58 62.7 Embossed surface 7% 7% Protrusion height (mm) 0.6 0.6

The calculated machine direction link pitch pLsm is 9.16 mm, and the cross direction link direction PLst: 13.38 mm.

After association, it was found that the complex sheet obtained did not exhibit any bonding defect. The sheets of format L1 x L2 = 100 x 125 mm² which were cut at various places all consisted of two linked sheets.

There is shown in Figure 4 the result obtained on the complex sheet, showing only the protrusions capable of forming an at least partial connection. It can be seen that these connecting zones themselves form a pitch pattern PLst = 13.38 mm in one direction and PLsm = 9.16 mm in the other.

The glue bringing a certain rigidity to the sheet, one seeks to reduce as much as possible the quantity brought. Thus, the softness of the complex sheet is improved by gluing only some of the binding zones defined in the pattern of FIG. 5 for example. This objective is very simply achieved by choosing a sizing cylinder depositing the glue, on a sheet, in a pattern compatible with the binding pattern.

An installation known per se is described below, making it possible to produce the sheet of the invention.

In Figure 5, there is shown a first pair of cylinders (1, 3) having an engraved metal cylinder (1) on the surface of which are embossed in the desired pattern. The metal cylinder is rotated about a horizontal axis and is associated with a rubber cylinder (3) which is parallel to it, and with which it provides a tightening interval (1-3). While passing in this interval, a sheet of absorbent paper in cellulose wadding undergoes permanent mechanical deformations by the pressure exerted by the reliefs of the metal cylinder.

The installation comprises a second pair of embossing cylinders with a metal cylinder (5) of the same diameter and rotating in the same horizontal plane as the cylinder (1), it cooperates with a rubber cylinder (7) for embossing.

The cylinders (1) and (5) provide between them a tightening interval (1-5) and are driven at opposite, synchronous speeds of rotation, so as to roll over one another without sliding.

The installation also includes a gluing system (10) with applicator cylinder (11) (made of rubber or other equivalent material) bearing on the cylinder (1) upstream of the tightening interval (1-5). A transfer cylinder (13) transfers the adhesive from a plunger cylinder (15) to the applicator cylinder (11). The plunger cylinder (15) collects the adhesive in a container.

The sheets (20, 30) of absorbent paper to be associated are fed from rollers (21, 31). The sheet (20) is guided around the rubber cylinder (3) and passes through the tightening interval (1-3) from which it emerges embossed, following the raised surface of the metal cylinder (1). The applicator cylinder (11) then deposits adhesive, in metered quantity, on the surface of the protuberances of the sheet, forming flats.

The second sheet (30) also undergoes a mechanical embossing treatment, by passing through the tightening interval (5-7), then is associated with the sheet (20) in the tightening interval (1-5). The resulting complex sheet is rolled pending further processing.

One way to reduce the amount of glue deposited is to glue the sheet (20) not on all the vertices of the reliefs, but only on a fraction. For this purpose, an applicator cylinder can be used, the surface of which has been etched in an appropriate pattern. The reliefs covering only a fraction of the total surface of the applicator cylinder, the adhesive carried by it is reduced by the same amount.

Another way is to engrave one of the cylinders, for example (1) so that the reliefs are not all at the same height. Only the highest reliefs receive the glue.

Such an example is shown in Figure 6 where we see the metal cylinders (1 ′) and (5 ′) with their bosses (A₁) and (A₅). The two sheets (20) and (30) formed on these bosses are glued together in the tightening interval (1′-5 ′) by the tops of the reliefs which come into contact at this level. According to the embodiment of Figure 6, the bosses of the cylinder 1 'are at two levels. Only the bosses (A₁₁), the height of which is greater by a value "d" than that of the bosses (A₁₂) come into contact with the smooth gluing cylinder (11) then in contact with the bosses of the cylinder (5 ′). It follows that the sheets (20) and (30) are only linked by these bosses. We can thus reduce at will the number of points likely to form a connection compared to those reproduced in Figure 4. These connection points will be formed at the intersections between the pattern of Figure 4 and the pattern reproduced by the bosses (A₁₁) .

Claims (9)

1. Complex sheet, composed of two or more sheets of absorbent cellucotton paper, having a substance of between 10 and 40 g/m² and each possessing protuberances distributed according to a pattern with a first pitch (P₁sm, P₂sm) in a first direction and a second pitch (P₁st, P₂st) in a second direction forming an angle with the first direction not equal to zero, the said two sheets being bonded together by the flat faces of the said protuberances, characterized in that the pitches (P₁sm, P₂sm) according to the said first direction are different and are connected by the relationship

   

   wherein L1 is a predetermined value corresponding to the measurement of a segment along which it is sought to guarantee the existence of at least one point capable of constituting a bond between the two sheets, irrespective of the relative position of the two patterns.
2. Complex sheet as in Claim 1, characterized in that the pitch intervals (P₁st, P₂st) according to the said second direction are likewise different, and selected in relation to a second predetermined value L2.
3. Complex sheet as in either of Claims 1 or 2, characterized in that the values L1 and L2 correspond to the format of elementary rectangular sheets which are to be punched in the said complex sheet.
4. Complex sheet as in either of Claims 1 or 3, characterized in that the pitches (P₁sm, P₂sm) and/or (P₁st, P₂st) are determined so that the ratio of the pitches between the pattern with the highest density and the pattern with the lowest density is between 1 and 2, and preferably between 1 and 1.5.
5. Complex sheet as in any one of the Claims 1 to 4, characterized in that the density of the patterns is between 20 and 300 protuberances per cm².
6. Complex sheet as in any one of the Claims 1 to 5, characterized in that the surface area of the flat faces constitutes between 5 % and 30 % of the surface area of the sheet before embossing.
7. Complex sheet as in any one of the Claims 1 to 6, characterized in that the geometry of the protuberances differs between the two sheets of absorbent paper.
8. Complex sheet as in any one of the Claims 1 to 7, characterized in that the number of points effectively bonded by adhesive is less than the number of points on the sheet capable of constituting a bond between the two sheets.
9. Complex sheet as in Claim 8, characterized in that at least one of the two sheets possesses protuberances (A₁₁, A₁₂) of different heights, such that bonding with the other sheet of absorbent paper is provided only by the higher protuberances (A₁₁).

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