HK1260519B - Multi-ply fibrous product comprising a laminating adhesive with a dermatologically acceptable acid - Google Patents

Description

Multi-layered fiber product comprising a laminating adhesive having a dermatologically acceptable acid

Technical Field

The present invention relates to multilayer fibrous products, in particular nonwoven products, tissue products or mixtures thereof, capable of contributing to a healthy skin pH, and in particular to multilayer fibrous products in which the individual layers are bonded to one another by an adhesive comprising a dermatologically acceptable acid and optionally a salt of said acid.

Background Art

The material based on nonwoven and tissue paper is widely used in modern society. Toilet paper, paper towel such as paper towel or household (kitchen) towel, facial tissue, napkin and handkerchief are main commodities.These products contain cellulose fiber (papermaking pulp fiber) usually, and described cellulose fiber is selected from hardwood type, softwood type and non-wood type, for example bamboo or awn (Miscanthus).These products can also contain the mixture of cellulose fiber and non-cellulose fiber.

The most important physical properties of tissue paper products include their strength, softness, absorbency, mainly towards aqueous systems, and their resistance to linting and dusting. These physical properties are usually adjusted to meet common consumer demands.

Tissue products usually come into contact with human skin during use. For this reason, the art has considered treating certain tissue products, such as toilet paper or handkerchiefs, with lotions containing various types of skin care ingredients and often also pH-adjusting substances. However, facial tissue, napkins or household (kitchen) towels usually do not contain lotions, and for toilet paper or handkerchiefs, there is currently a greater demand for lotion-free grades.

For products without lotion, the inventors of the present invention have noticed a difference between the pH of conventional tissue products and the pH of healthy skin. Depending on the origin of the papermaking fibers, the pH of a raw tissue web that has not been further treated is very close to neutral (pH of about 6.5) if primarily virgin papermaking fibers are used; and slightly alkaline (pH of about 8.5) if a larger proportion of recycled fibers is included. Thus, tissue products currently have a pH of 6.5-8.5, while the pH of healthy skin is 4.5-6.0. This slightly acidic pH is caused by the acidic mantle of human skin. The acidic mantle of human skin is a very fine, slightly acidic membrane on the surface of the skin that acts as a barrier to bacteria, viruses and other potential pollutants that could penetrate the skin. Therefore, it is important to minimize the influence of external factors on the pH of the skin.

The inventors of the present invention have observed that it cannot be excluded that, in particular during prolonged or repeated contact with human skin, conventional tissue products without lotion have a negative impact on the pH of healthy skin. The same applies to nonwoven products containing cellulose fibers.

It is therefore an object of the present invention to provide a multilayer fibrous product, particularly a nonwoven product, a tissue product or a mixture thereof, which reduces the potential of conventional multilayer fibrous products to alter the pH of healthy skin.

Another object of the present invention is to provide a multilayer fibrous product, in particular a nonwoven product, a tissue product or a mixture thereof, having particularly effective means for preventing the negative influence of the pH of conventional multilayer fibrous products on the pH of the skin.

Furthermore, according to another object of the present invention, it is preferred to provide such a multi-layered fiber product without adversely affecting other properties associated with its use, such as layer strength and/or softness and/or absorbency and/or other properties known in the art.

Other objects and purposes of the present invention can be understood from the following description.

Summary of the Invention

The present invention relates to:

1. A multilayer fibrous product comprising at least two fibrous layers, preferably a multilayer nonwoven product, a tissue product or a mixture thereof, said multilayer fibrous product comprising at least two tissue layers and/or nonwoven layers comprising cellulose fibers, wherein at least two fibrous layers are bonded to each other by an aqueous adhesive composition comprising an adhesive component and a dermatologically acceptable acid and optionally a salt of said acid; and

A method for preparing a multi-layer fibrous product, preferably a tissue paper product, comprising the steps of:

a) providing at least two fibrous webs of tissue paper and/or nonwoven,

b) optionally embossing at least one of said webs so as to provide at least one raised pattern,

c) applying an aqueous adhesive composition to at least one side of at least one of said fibrous webs, said aqueous adhesive composition comprising an adhesive component and a preferably water-soluble dermatologically acceptable acid and optionally the corresponding salt of said acid, wherein, if at least one of said webs is embossed, said aqueous adhesive composition is preferably applied to the distal ends of at least a portion of said protrusions,

d) superimposing the at least two webs such that the aqueous adhesive composition is located between the at least two superimposed webs,

e) bonding the at least two webs together, and

f) optional further steps of converting the bonded web into the final multi-layer fiber product,

The characteristics of the multi-layered fiber product and/or the aqueous adhesive composition are preferably as defined in the following description and claims.

The present invention comprises the following embodiments ("items"):

2. The multi-layer fiber product as described above, wherein the acid and optionally the salt of the acid are provided in an amount such that the pH of the multi-layer fiber product measured according to ISO 6588-1:2012 (E) is 3.5 to 7, preferably 4 to 6.9, more preferably 4.3 to 6.4, even more preferably 4.5 to 6.2, for example 4.8 to 6.0. (It should be noted that, hereinafter, the description of broader and narrower ranges also discloses embodiments in which the lower and upper limits are combined in different ways. Thus, item 2 also discloses, for example, the following pH ranges: 3.5 to 6.9, 3.5 to 6.4, 3.5 to 6.2, 3.5 to 6.0, 4.0 to 7.0, 4.3 to 7.0, 4.5 to 7.0, 4.8 to 7.0, 4.0 to 6.4, etc.)

3. The multi-layer fiber product according to item 1 or 2, wherein the aqueous adhesive composition contains the acid and optionally the salt of the acid in a molar amount of 0.1 mol/l to 10 mol/l, preferably 0.5 mol/l to 5 mol/l, for example 1 mol/l to 3 mol/l.

4. A multi-layered fiber product as described in item 1, 2 or 3, wherein the amount of the aqueous adhesive composition present is 0.01 wt% to 2 wt%, preferably 0.1 wt% to 1.3 wt%, relative to the weight of the multi-layered fiber product, and the amount of the aqueous adhesive composition present is based on the total weight of all ingredients of the adhesive composition excluding water.

5. A multi-layered fibrous product as described in any of items 1, 2, 3 or 4, wherein the binder component is selected from: (a) natural polymers such as proteins and protein-based compounds, gums and gum-like materials, polysaccharide-based materials, glues derived from animal products, and (b) synthetic polymers such as PEI (polyethyleneimine), PAE (polyamide polyamine-epichlorohydrin) and PVAm (polyvinylamine), polyvinyl alcohol (PVA), polyvinyl acetate (PVAC), vinyl acetate-ethylene copolymer (VAE), polyvinylpyrrolidone (PVP), polyurethane and styrene-butadiene copolymer, and water-soluble or water-dispersible cellulose-based compounds such as carboxymethyl cellulose, sodium carboxymethyl cellulose, methyl cellulose and ethyl cellulose; or any combination thereof.

6. The multi-layered fibrous product of any one of items 1, 2, 3, 4 or 5, wherein the dermatologically acceptable acid is preferably water-soluble and is selected from:

(i) inorganic acids, such as boric acid,

(ii) an optionally hydroxylated organic acid having 2 to 24 carbon atoms, preferably an optionally hydroxylated organic acid having 3 to 6 carbon atoms, more preferably citric acid, lactic acid, isoascorbic acid and combinations thereof, and

(iii) polymeric organic acids, such as polyacrylic acid, and combinations thereof;

And the acid salt optionally present therein is a salt corresponding to the selected acid.

7. The multi-layered fibrous product of any one of items 1, 2, 3, 4, 5, or 6, wherein the aqueous adhesive composition comprises a dermatologically acceptable acid and a salt thereof, and the acid and the salt thereof are present in amounts that collectively provide pH buffering capacity.

8. A multi-layered fibrous product as described in any of items 1, 2, 3, 4, 5, 6 or 7, wherein the layer comprises as papermaking fibers: (i) only non-regenerated cellulose fibers, (ii) only regenerated cellulose fibers, or (iii) a mixture of non-regenerated cellulose fibers and regenerated cellulose fibers in a weight ratio of 99/1 to 1/99, such as 90/10 to 10/90 or 80/20 to 20/80, wherein in any of (i), (ii) or (iii), the cellulose fibers can be selected from wood fibers, such as softwood fibers or hardwood fibers, and non-wood fibers, such as fibers from annual plants, and combinations thereof.

9. The multilayer fibrous product of any one of items 1, 2, 3, 4, 5, 6, 7 or 8, wherein the number of layers, in particular the number of tissue layers, is 2 to 8, in particular 2 to 6, such as 2 to 5.

10. A multi-layer fibrous product as described in any one of items 1, 2, 3, 4, 5, 6, 7, 8 or 9, wherein the adhesive composition is applied to at least a portion of the surface of the layer, the fibrous layer, in particular a nonwoven layer or a tissue layer, in at least one regular or irregular pattern, wherein the regular or irregular pattern preferably covers at least the central area of the fibrous layer, in particular a nonwoven layer or a tissue layer.

11. A multi-layer fibrous product as described in any one of items 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10, which is preferably a tissue product, wherein the multi-layer fibrous product, preferably a tissue product, has two outer layers and optionally at least one additional layer located between the two outer layers, wherein at least one of the outer layers is embossed to form protrusions, wherein the aqueous adhesive composition is applied to the distal end of at least a portion of the protrusions, and the protrusions are arranged to bond the two outer layers and optionally at least one additional layer located between the two outer layers together.

12. The multi-layer fibrous product of any one of items 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or 11, which is preferably a tissue product, comprising at least three layers: an embossed upper outer layer and an embossed lower outer layer, and a central layer, each outer layer comprising a relief pattern, the relief pattern comprising at least a portion of discrete protrusions, at least a portion of the protrusions of each outer layer having distal regions facing the central layer, and at least one of the upper outer layer and the lower outer layer having a pattern density of at least 10 protrusions/ ^cm2 , preferably at least 20 protrusions/ ^cm2 , the lower outer layer comprises a first pattern and a second pattern, the height of the first pattern being smaller than the height of the second pattern, wherein the central layer and the upper outer layer are connected in a nested manner and are connected to the lower outer layer (1) by an applied adhesive composition (12), wherein the central layer (2) and the upper outer layer (3) are connected to the lower outer layer (1) to the extent of being connected to at least a portion of the tip of the second pattern of the lower outer layer, wherein the height of the protrusions of the second pattern is preferably 50% to 200% greater than the height of the protrusions of the first pattern.

13. A multi-layer fibrous product as described in any of items 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12, wherein the percentage of the total surface of the multi-layer fibrous product bonded by the adhesive composition is 1% to 20%, preferably 2% to 10%, more preferably 4% to 8%.

14. The multi-layer fibrous product of any one of items 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 or 13, wherein the multi-layer fibrous product is selected from wipes, hygiene products such as toilet paper, handkerchiefs, household towels, towels, facial tissue, serviettes/napkins, bed linens or clothing, wherein the multi-layer fibrous product is preferably a toilet paper product having 2 to 8 layers, preferably 2 to 6 layers.

Insofar as the present description mentions “preferred” embodiments/features, combinations of these “preferred” embodiments/features shall also be considered disclosed, insofar as such combinations of “preferred” embodiments/features make technical sense.

In the following, the use of the term "comprising" is to be understood as disclosing also the term "consisting of" as a more restricted embodiment, insofar as this makes technical sense.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic diagram showing an embossing/gluing station that can be used to prepare a three-ply tissue product of the present invention. In Figure 1, the following reference numerals represent:

(1) Layer A (first outer layer),

(2) Layer B (middle layer),

(3) Layer C (second outer layer),

(4) Adhesive unit,

(5) Gluing room,

(6) Anilox roll (engraving),

(7) Coating roller (flat rubber roller),

(8a), (8b) rubber roller,

(9a), (9b) embossing roller,

(10) Marrying roll,

(11) Three-layer tissue paper products

As in all the figures, these reference numerals apply to all the figures.The directions of rotation of the different rollers and the direction of advance of the layers are indicated by arrows.

Figure 2 is a schematic diagram showing the structure of a preferred embodiment of a three-layer tissue paper product. In Figure 2, the following reference numerals represent:

(12) Adhesives (adhesive compositions),

(13') first-level engraving (small embossing),

(14) Secondary engraving (large embossing, decorative patterns).

Figures 3a and 3b are schematic diagrams of different embossing patterns of tissue paper products. In Figure 3, the following reference numerals represent:

(15a, b) small embossed pattern,

(16) Large embossed pattern.

Figure 4 is a schematic diagram of a portion of the surface of the embossing roller (9a) shown in Figure 1. Two different height levels of engraved protrusions result in corresponding different embossing levels of the tissue paper product. In Figure 4, the following reference numerals are indicated:

(13) First-level carving,

(14) Second level carving.

Figure 5 is a schematic diagram of a preferred example of a small embossed pattern for a tissue paper product, with a dot count of 20 per ^cm2 .

DETAILED DESCRIPTION

The present invention relates to a multi-layer fiber product comprising at least two fiber layers. Preferably, the fiber layers are selected from:

(a) a multi-layer nonwoven product comprising at least two nonwoven layers, said nonwoven layers preferably comprising cellulosic fibers,

(b) a tissue product comprising at least two tissue layers, and

(c) A mixture of a nonwoven product and a tissue product comprising at least one tissue layer and at least one nonwoven layer, said nonwoven layer preferably comprising cellulosic fibres.

In the multi-layered fibrous product, at least two of the above-mentioned fibrous layers are bonded to each other by an aqueous adhesive composition comprising an adhesive component and a dermatologically acceptable acid, and optionally a salt of the acid.

The multi-ply fibrous product of the present invention is preferably a tissue paper product.

Tissue products, nonwoven products or mixtures thereof are made of two or more layers bonded to each other by a specific adhesive composition.

As used herein, the term "ply" refers to a distinguishable and generally separable cut-to-size tissue web obtained after processing ("converting") one or more base (rough) tissue webs. Each individual ply may comprise one or more plies, for example one, two, three, or four plies. In the case of nonwovens, the term "ply" as used herein refers to a distinguishable and generally separable cut-to-size nonwoven web.

With respect to tissue paper webs, the term "layer" refers to a stratum within the web having a defined fiber composition. One or more plies are formed by depositing one or more slurry feed streams onto a wire using a pressurized single-layer or multi-layer headbox. This technique is well known to those skilled in the art. It enables the use of different fiber types in each ply of the web.

As "tissue web" is understood a single-ply base tissue obtained from a tissue machine. Tissue webs are produced by a process comprising the steps of forming an aqueous suspension of pulp fibers, the so-called "furnish", depositing the aqueous suspension onto a wire to form a wet web, dewatering, drying and creping the web.

The tissue web and the resulting layer preferably have a basis weight of 8 to 50 g/m ² , in particular 10 to 30 g/m ² , especially 12 to 25 g/ ^{m 2 .} The same applies to the nonwoven web and ^the resulting ^layer .

Due to the inherent compatibility of the production process (wet-laid), "tissue" production is considered to be included in papermaking technology. Tissue production differs from paper production by its significantly lower basis weight and significantly higher tensile energy absorption index. Paper and tissue also generally differ in their modulus of elasticity, a material parameter that characterizes the stress-strain behavior of these flat products.

The high tensile energy absorption index of tissue paper is caused by external or internal creping. The former is produced by the action of a creping blade pressing the paper web adhered to the drying drum, or, in the latter case, by the speed difference between two wires ("fabrics"). This causes the still moist, plastically deformed paper web to break up internally due to pressing and shearing, making it more stretchable under load than uncreped paper. A high tensile energy absorption index can also be achieved by giving the tissue paper a 3D structure through the wires themselves. Most of the typical functional properties of tissue paper and tissue paper products derive from the high tensile energy absorption index (see DIN EN 12625-4 and DIN EN 12625-5).

Tissue can be produced from papermaking fibers according to a "conventional process," such as in the preparation of "dry-creped tissue" or "wet-creped tissue," or in a "structured tissue process," such as the through-air-dried (TAD) process, the preparation of uncreped through-air-dried (UCTAD) tissue, or alternative preparation methods, such as Voith's Advanced Tissue Molding System (ATMOS), Georgia Pacific's Energy Efficient Technologically Advanced Drying eTAD, or Metso Paper's Structured Tissue Technology SST. Hybrid processes, such as NTT (New Textured Tissue), which is a variation of the conventional process, can also be used.

The traditional dry crepe preparation method includes the following steps:

- pressing and drying the wet paper fibers into a sheet on large diameter heated drums (also known as Yankee dryers); and

- The dried paper fiber sheet is then separated and creped with the aid of metal blades applied opposite to the roller and crosswise to its direction of rotation.

The creping operation creates undulations in the sheet in a direction crosswise to the direction of travel of the sheet. The creping operation increases the thickness of the sheet and imparts elasticity and tactile (soft touch) properties to the sheet.

The TAD preparation method comprises the following steps:

- molding the wet paper fiber sheet onto the fabric; and

- The sheet is then dried at least partially by a stream of hot air passing through the sheet.

The dried sheet can then be creped.

In addition, in the preparation of the tissue paper web to be used, a process as described in PCT/EP2015/059326 (filing date: April 29, 2015; title: Tissue paper comprising pulp fibers originating from Miscanthus and method for manufacturing the same), which is incorporated herein by reference, can be used. Specific reference is made to the description on pages 22 to 27, item 3 of the application and the details of the TAD process disclosed therein (e.g., 3D-shaped fabrics, permeable drying drums, etc.). The parameters described in this paragraph are also valid for using ATMOS technology.

Once the tissue paper has been produced, specialized manufacturing operations known as converting operations are typically employed to form tissue products (i.e., paper towels, toilet paper rolls, bath tissue, wipes, kitchen paper rolls, handkerchiefs, etc.).

The term "nonwoven" is very common in the art and can also be further defined for the purpose of the present invention in the manner described in ISO 9092: 2011. Typical nonwoven production technologies include air-laid technology, spunlaid technology, dry-laid technology and wet-laid long fiber technology.

According to a preferred embodiment, each nonwoven layer in the multilayer fibrous product (i.e., multilayer nonwoven product or nonwoven/tissue mixture) used in the present invention comprises cellulosic fibers. In this case, the content of cellulosic fibers is at least 20% by weight, more preferably at least 50% by weight, for example at least 80% by weight, based on the total weight of all fibers present in each layer. The remaining fibers in these cases are non-cellulosic fibers, for example synthetic fibers.

According to a preferred embodiment, the multi-layer fibrous product of the present invention comprises at least one, preferably at least two, textured nonwoven layers. The preparation of textured nonwoven webs that can be used as one or more nonwoven layers in the present invention has been described, for example, in WO2014/009784 (incorporated herein by reference). Referring to Figure 12 of this international application, a preferred embodiment of a textured nonwoven is produced as follows:

A roll of fluff pulp 30 is used as raw material. It is fed into a crusher 31. The fluff pulp is defibrated to form fluff or free papermaking fibers 32. Other raw materials can be used, such as a mixture of fluff pulp and synthetic fibers, man-made fibers or other natural fibers (such as cellulose fibers) or thermal bonding fibers. The papermaking fibers 32 are fed into a given number (for example, two) of forming chambers 33. In the forming chambers 33, the papermaking fibers 32 are transported by an air flow. The papermaking fibers 32 are deposited onto a forming fabric 34, which travels in an annular manner below the forming chambers 33. Almost before leaving the forming fabric 34, the formed web is compacted by a compactor 35. The compacted web obtained by this step is then transferred from the forming fabric 34 to a transfer fabric 36. An adhesive is sprayed onto one side of the compacted web by a first adhesive sprayer 37. The adhesive is a latex composition, for example, an ethylene and vinyl acetate copolymer composition. The compacted web is then dried in a first drying unit 38 (e.g., at a temperature of approximately 110°C to 120°C). The compacted web is then calendered in a calendering section 39. The calendering section 39 comprises a calendering roll 40 and an opposing mating roller 41, which rotate in opposite directions. The calendering roll 40 is a calendering roll having a positive pattern according to the present invention. The mating roller 41 can be a smooth-surfaced roller (e.g., made of steel or rubber). The calendering roll can be heated. This step produces a calendered web 42. An adhesive (e.g., latex) is sprayed onto the other side of the calendered web 42 by a second adhesive sprayer 43. As an alternative to using adhesive sprayed by the first and second adhesive sprayers 37, 43, thermal bonding fibers (e.g., thermal bonding fibers or thermal bonding fibers and latex) can be used and mixed with the papermaking fibers in the forming chamber 33. The calendered web 42 is then dried and cured in a second drying unit 44 (e.g., at a temperature of approximately 200°C). The resulting web 42 is further solidified and cooked by a third drying unit 45 (e.g., at a temperature of about 200° C.). The calendered web resulting from the above steps forms a textured nonwoven fabric 46 comprising papermaking fibers. It can be wound onto a reel 50 as a roll of textured nonwoven fabric comprising papermaking fibers. The reel 50 of textured nonwoven fabric comprising papermaking fibers can then be fed into a converting unit 60 to produce napkins, paper towels, toilet paper rolls, facial tissue rolls, wipes, kitchen towel rolls, skin care or cleaning wipes, handkerchiefs, and the like.

Papermaking fibers (hereinafter also referred to as "cellulose fibers") can be produced from virgin and/or recycled pulp raw materials. The cellulose fibers used in the present invention generally contain long-chain cellulose fractions as the main structural building blocks, which are present in naturally occurring cellulose-containing cells, particularly in the cells of lignified plants. Preferably, the fibers are separated from lignified plants by a digestion step to remove or reduce the content of lignin and other extractables and an optional bleaching step. The cellulose fibers can also be derived from non-wood sources, such as annual plants.

The cellulose fibers that can be used can be regenerated types (e.g. Lyocell), but other types of pulp are preferably used. The pulp used can be a primary fiber material ("virgin fiber") or a secondary fiber material (regenerated pulp). The pulp can be derived from a lignin-free or low-lignin source, such as cotton linter, esparto grass (alfa grass), bagasse (e.g. cereal straw, rice straw, bamboo or hemp), kemp fiber, miscanthus fiber or flax (also referred to as "non-wood fiber" in the specification and claims). Preferably, the pulp is made of lignocellulosic material, such as softwood (usually derived from coniferous trees) or hardwood (usually derived from deciduous trees).

Either "chemical pulp" or "mechanical pulp" can be used, with chemical pulp being preferred.

According to DIN 6730, "chemical pulp" is a fibrous material obtained from plant raw materials in which most of the non-cellulose components have been removed by chemical pulping without substantial mechanical aftertreatment. "Mechanical pulp" is a general term for fibrous materials produced entirely or almost entirely from wood by mechanical means, optionally at elevated temperatures. Mechanical pulp can be subdivided into purely mechanical pulps (groundwood pulp and refined mechanical pulp) and mechanical pulps that have undergone chemical pretreatment, such as chemi-mechanical pulp (CMP) or chemi-thermomechanical pulp (CTMP).

Typical properties of tissue papers include the ready ability to absorb tensile stress energy, their drapability, good fabric-like flexibility, a property often referred to as bulk softness, high surface softness, a high specific volume with a perceptible thickness, a high possible liquid absorbency and, depending on the application, suitable wet and dry strength, as well as an interesting visual appearance of the outer surface of the product. These properties allow tissue papers to be used, for example, as cleaning cloths (e.g., household towels), hygiene products (e.g., toilet paper, paper towels), handkerchiefs, cosmetic wipes (facial tissues), or napkins/serviettes.

An aqueous adhesive composition for bonding at least two fibrous layers (e.g., tissue layers and/or nonwoven layers) together comprises (i) an adhesive component and (ii) a dermatologically acceptable acid, and optionally, a corresponding salt of the acid. In one embodiment, the remainder is water. An "aqueous adhesive composition" may also be referred to and described as a "water-based adhesive" comprising a dermatologically acceptable acid and optionally, a corresponding salt of the acid.

The term "adhesive component" is understood to mean one or more chemical substances corresponding to the non-aqueous component of a water-based adhesive suitable for bonding/laminating fibrous webs of tissue and/or nonwovens. The adhesive component preferably comprises an adhesive polymer as the main component (greater than 50% by weight, e.g., greater than 80% by weight, based on the total weight of the adhesive component) or as the sole component. In addition to the adhesive polymer, the adhesive component may also contain conventional auxiliary substances used in water-based adhesives, such as tackifiers, viscosity-regulating substances, or dispersing aids.

The aqueous adhesive composition may also contain a colorant that can be used to make the pattern of application of the adhesive composition visible.

Preferably, the solids content of the adhesive component in the aqueous adhesive composition is from 2% to 50% by weight, preferably from 2.5% to 20% by weight, for example from 3% to 10% by weight.

The adhesive polymer can be a natural polymer or a synthetic polymer, preferably a synthetic polymer. It is soluble in or dispersible in the final aqueous adhesive composition. Natural polymers can be obtained from sources such as: (i) proteins and protein-based compounds, such as casein, soy protein, zein and gelatin, (ii) gums and gummy materials, such as gum arabic, tragacanth, gum ghatti, ghatti gum, mucus, etc., (iii) materials based on polysaccharides, such as starch and processed starch, dextrin, agar, pectin, etc., and (iv) glues derived from animal products (such as hides, bones and fish viscera). The synthetic polymer can be selected from PEI (polyethyleneimine), PAE (polyamide polyamine-epichlorohydrin) and PVAm (polyvinylamine), polyvinyl alcohol (PVA), polyvinyl acetate (PVAC), vinyl acetate-ethylene copolymer (VAE), polyvinyl pyrrolidone (PVP), sodium polyacrylate, polyethyl acrylate, polymethacrylic acid, polyurethane and styrene-butadiene copolymer, and water-soluble or water-dispersible cellulose-based compounds, such as carboxymethyl cellulose, sodium carboxymethyl cellulose, methyl cellulose and ethyl cellulose, or any combination thereof. Preferred polyvinyl alcohol polymers or water-soluble or water-dispersible cellulose-based compounds. Natural or synthetic polymers can have any water-soluble or water-dispersible molecular weight that is enough to form adhesives. Typically, the adhesive polymer, i.e., natural or synthetic polymers, preferably has a weight average molecular weight (measured by GPC) of about 40,000 to about 120,000, more preferably 70,000 to 90,000.

Dermatologically acceptable acids (also referred to as "skin-compatible acids") are preferably water-soluble. Preferably, the solubility in water (at 25° C.) is at least 1 g/L, particularly and increasingly preferably at least 10 g/L, at least 50 g/L, at least 100 g/L.

The acid is preferably selected from: (i) inorganic acids, such as boric acid, (ii) organic acids having 2 to 24 carbon atoms, which are optionally hydroxylated, and (iii) polymeric organic acids, such as polyacrylic acid. Optionally, each of these acids is combined with its salt, in particular its metal salt, to form a buffer.

The salt-forming metal is preferably selected from alkali metals and alkaline earth metals, including but not limited to sodium, potassium, calcium, etc. Therefore, in one embodiment, when the selected acid is lactic acid, suitable metal salts thereof include sodium lactate and potassium lactate.

The optionally hydroxylated organic acid according to item (ii) preferably has 2 to 18, more preferably 3 to 10, and in particular 3 to 6 carbon atoms. It can be saturated or unsaturated, preferably the former. It can be a monocarboxylic acid or a polycarboxylic acid (e.g. a dicarboxylic acid), preferably the former. It is preferably selected from citric acid, lactic acid, isoascorbic acid, glycolic acid, malic acid, tartaric acid, glycolide (a cyclic dimer of glycolic acid which hydrolyzes to form two glycolic acid molecules), acetic acid, dehydroacetic acid, oleic acid, palmitic acid, stearic acid, behenic acid, palm kernel oil acid, tallow acid, salicylic acid, ascorbic acid, sorbic acid, benzoic acid, succinic acid, or any combination thereof.

Preferred acids are saturated and hydroxylated, including citric acid, lactic acid, isoascorbic acid, or any combination thereof. Of these, lactic acid and its corresponding metal salt (e.g., sodium lactate or potassium lactate) are most preferably used together.

As previously mentioned, acids and corresponding acid salts can be added together (e.g., lactic acid and sodium lactate) to provide a "buffering" effect, which helps maintain the desired pH stability. Therefore, it is preferred to use an aqueous adhesive composition that contains both a dermatologically acceptable acid and a salt thereof, and the dermatologically acceptable acid and a salt thereof are present in an amount that provides pH buffering capacity.

In a preferred embodiment of the multi-layer fibrous product, the acid and optionally the salt of the acid are provided in such an amount that the pH of the multi-layer fibrous product, such as a tissue product, a nonwoven product or a mixed product, measured according to ISO 6588-1:2012(E), is from 4.3 to 6.4, preferably from 4.5 to 6.2, for example from 4.8 to 6.

In a preferred embodiment, the aqueous adhesive composition contains the acid and optionally the acid salt in a molar amount of 0.1 mol/l to 10 mol/l, preferably 0.5 mol/l to 5 mol/l, for example 1 mol/l to 3 mol/l.

The pH of the aqueous adhesive composition (measured at 25° C.) is preferably from 1 to 6, more preferably from 1.5 to 4, especially from 2 to 3.

In a further preferred embodiment, the aqueous adhesive composition has been applied to a multi-ply fibrous product, in particular a nonwoven product or a tissue product or a mixture thereof, preferably a tissue product, and the aqueous adhesive composition is present in an amount (based on the total weight of all ingredients of the adhesive composition except water) of 0.01% to 2% by weight, preferably 0.1% to 1.3% by weight, relative to the weight of the multi-ply fibrous product. These values can be calculated by dividing the amount of aqueous adhesive composition applied to the tissue product (the total weight of all ingredients of the adhesive composition except water, in g/m ² ) by the basis weight of the dry tissue product (also in g/m ² ). "Dry" tissue means that it has been conditioned as detailed in the experimental part below.

The aqueous adhesive composition preferably has a viscosity of 1 to 500 mPa.s, more preferably 1 to 200 mPa.s, as measured at 25° C. using a Brookfield LV-DV-I viscometer from Brookfield Engineering Laboratories, Inc., using the spindle and speed given in the experimental section. Even more preferably, the viscosity is adjusted to 25 to 150 mPa.s, in particular 70 to 125 mPa.s.

Because the addition of a dermatologically acceptable acid (eg, lactic acid) to the adhesive composition affects its viscosity, and thus the handling and application properties of the adhesive composition, it is preferred to use an amount of acid that results in a viscosity within these ranges.

According to other embodiments of the present invention, the layers of the claimed multilayer fibrous product comprise as papermaking fibers: (i) only non-regenerated cellulose fibers, (ii) only regenerated cellulose fibers, or (iii) a mixture of non-regenerated cellulose fibers and regenerated cellulose fibers in a weight ratio of 99/1 to 1/99, such as 90/10 to 10/90 or 80/20 to 20/80, wherein in any of (i), (ii) or (iii), the cellulose fibers may be selected from wood fibers, such as softwood fibers or hardwood fibers; and non-wood fibers, such as fibers from annual plants, and combinations thereof. Since fibrous products made from regenerated cellulose fibers tend to have a higher pH value, a larger amount of acid may be required to prevent a negative impact on the natural skin pH.

In one embodiment, the present invention relates to a multilayer fiber product. The fiber layer can be 2 to 8 layers, particularly 2 to 6 layers, or 2 to 5 layers of nonwoven fabric. In the very thick product with a higher number of plies, adhesive composition is not added to an outer layer to bond all layers together usually, but adds to the second or third layer (if 2 or 3 inner layers are arranged between two outer layers). Then, adhesive composition can migrate to other layers from the layer having been applied with adhesive composition, automatically migrates if lamination is combined and contacts each other, or migrates at last if pressure is applied (usually in the roll gap (rollnip)) to those zones that carry adhesive composition. In this way, all layers can be bonded together.

The aqueous adhesive composition can be applied to whole surface or its part, i.e. so-called " application zone ".In these application zones, adhesive composition can be used as continuous coating or apply with the form of one or more regular or irregular patterns, preferably uses one or two regular patterns.Regular pattern can for example produce by the adhesive/machinery (for example by embossing) layer bonding technology of the combination explained hereinafter.Preferably, the summation of application zone or all application zones constitutes at least 50% of the surface area of multilayer fiber product, preferably at least 80%.More preferably, application zone comprises the center area of the multilayer fiber product that most likely contacts with the skin of user.

The one or more patterns preferably include discrete and/or partially overlapping elements, such as dots, circles, lines, stripes, curves, polygons, etc., which can form design elements such as flowers, leaves, etc.

Preferably, the adhesive composition is applied in a pattern, preferably in a pattern that partially or completely coincides with the pattern of protrusions produced by embossing at least one layer. In one embodiment of a multi-layer fibrous product (e.g., toilet paper), at least one outer layer preferably comprises the embossed pattern over substantially at least 50%, preferably at least 80%, of its surface (preferably including the central area of the multi-layer fibrous product).

In a preferred embodiment, the multi-layer fibrous product exhibits substantially uniform coverage of the aqueous binder composition over its surface. "Coverage" herein refers to the distribution of the aqueous binder composition between at least two layers of the multi-layer product in at least one regular or irregular pattern, preferably one or two regular patterns. This "coverage" preferably includes a central area of the multi-layer fibrous product.

The adhesive composition can be applied by techniques commonly used in the art, such as spraying, roller coating, printing, using a contactless coating system such as developed by WEKO (Weitmann & Konrad GmbH & Co. KG), or slot die coating.

After applying the aqueous binder composition, no specific drying step is required. Typically, the fibrous layers, such as tissue layers and/or nonwoven layers, preferably tissue layers, will absorb the free water content of the aqueous binder composition. Additionally, some water may evaporate when the multi-layer fibrous product is exposed to air.

In the present invention, by using the aqueous adhesive composition that comprises acid or buffer system, or by adhesive layer bonding and mechanical layer bonding (for example, by knurling or embossing) combination, by at least two fibrous layers, preferably all fibrous layers, for example tissue layer and/or nonwoven layer, preferably tissue layer is combined together.As explained in further detail below, during adhesive bonding, adhesive film is deposited on the whole surface area of at least one deck or on a part thereof, then places the surface through adhesive treatment, makes it contact with the surface of at least one other layer.By adhesive composition is applied to the inboard of one deck in preferably two layers, these two layers are bonded together.When desire will be bonded together more than two layers, only adhesive composition is applied to one deck in these layers, for example, be applied to the inboard of one outer layer in two outer layers, also be enough usually, because adhesive composition also can migrate to other layer and all layers are bonded together, particularly if adhesive layer bonding and mechanical layer bonding and those zones that carry adhesive composition apply pressure and combine.During mechanical bonding, can make layer combination by knurling or by pressing or by embossing. Embossing is the process of creating raised or depressed designs, typically through pressure between multiple engraved rollers or plates, or between an engraved roller or plate and a resilient or deformable support surface. This can result, for example, in dry-creped tissue layers having a specific relief or impression. After embossing, the thickness of the dry-creped tissue layer or layers increases compared to their initial thickness.

In a further preferred embodiment, a multilayer fiber product, such as a tissue product, a nonwoven product or a mixture thereof, has two outer layers and optionally a further layer located between the two outer layers, wherein at least one outer layer is embossed to form protrusions, and wherein the aqueous adhesive composition is applied to at least a portion of the distal ends of the protrusions, the protrusions being arranged to bond the two outer layers and optionally the further layer located between the two outer layers together. In this embodiment, the lower outer layer can also be "flat", i.e. without protrusions, such as those produced by embossing, and the aqueous adhesive composition is applied only to the distal ends of the protrusions of the upper outer layer in order to connect these outer layers (and, for example, the optional central layer) together.

Preferred techniques for embossing a fibrous layer, in particular a nonwoven layer or tissue layer comprising cellulosic fibers, and for applying the aqueous adhesive composition are explained below.

For the production of multi-layer fiber products, in particular tissue products, in particular bath tissue and household tissue, known methods for producing embossed and adhesively bonded layers can be used, such as Goffra Incolla/point embossing, DESL (double embossed single lamination) and Pin-to-Pin/Foot-to-Foot. Before the layers are bonded, the layers are usually embossed in the nip of an embossing roll and an anvil roll.

In the first-mentioned production method, Goffra Incolla, a first web is passed through the nip between an embossing roller and an anvil roller. In this nip, the web is embossed with an embossing pattern. An applicator roller then applies adhesive to those portions of the first web where the protruding embossing elements within the embossing roller are present. Adhesive is transferred from an adhesive tank to the applicator roller via an adhesive transfer roller. A second web is then fed to the first web and adhesively bonded to it in the nip between a so-called bonding roller and the embossing roller. Adhesive bonding occurs in those portions where the adhesive was applied.

The second production method (DESL) is very similar to the Goffra Incolla method described above. It comprises an additional pair of rollers, comprising a second embossing roller and a second anvil roller. The additional pair of rollers is used to emboss the second web before adhesively bonding the second web to the first web using a bonding roller. Typically, the additional pair of rollers is placed close to the first pair of rollers and the bonding roller. In a special case of the general DESL production method, the embossing elements of the first embossing roller and the embossing elements of the second embossing roller are arranged so that the embossing elements of the first embossed layer and the embossing elements of the second embossed layer cooperate with each other, similar to a gear system. This serves to achieve mutual stabilization of the two layers. However, for the DESL production method, such a correlation between the embossing elements of the first, i.e., upper, layer and the embossing elements of the second, i.e., lower, layer is not required.

The third production method (pin-to-pin/foot-to-foot) is similar to the DESL method. The upper and lower layers are embossed separately by means of two pairs of rollers. The adhesive is applied to the embossed protrusions ("protuberances") of the first layer. However, the layer bonding is not achieved by means of bonding rollers as in the DESL method, but directly by means of the protruding embossing elements of the second embossing roller. To this end, the gap width between the first and second embossing rollers needs to be precisely adjusted, which is primarily defined by the respective thicknesses of the two webs (upper and lower layer). Furthermore, the embossing rollers must be designed so that at least some of the protruding embossing elements of the two rollers face each other. This is why the terms pin-to-pin or foot-to-foot embossing are used.

Further knurling may be used.

A suitable lamination technique for the present invention is described in WO 2011/035803 which produces a three-layer fiber product comprising a first layer, an intermediate layer whose projections nest into the pillow-like cells of the first layer, and a bottom layer which preferably has no projections.

A preferred lamination technique of the present invention is illustrated in the accompanying drawings and explained in the examples and is related to the teachings of US 2003/0129363 assigned to Georgia Pacific France. This embodiment, also illustrated in Figures 1 to 4, relates to a multi-layer fibrous product, such as a tissue product, a nonwoven product or a mixture thereof, preferably a tissue product, comprising three layers, an embossed upper outer layer (3) and an embossed lower outer layer (1), and a central layer (2), each outer layer comprising a relief pattern, the relief pattern comprising at least a portion of discrete protrusions, the distal region of at least a portion of the protrusions of each outer layer (1, 3) facing the central layer, and at least one of the upper outer layer (3) and the lower outer layer (1) having a pattern density of at least 10 protrusions/ ^cm2 , preferably at least 20 protrusions/ ^cm2. , the lower outer layer (1) comprises a first pattern (15a, 15b) and a second pattern (16), the height (13) of the first pattern being less than the height (14) of the second pattern, wherein the central layer and the upper outer layer are connected in a nested manner and are connected to the lower outer layer (1) by an applied adhesive composition (12), wherein the central layer (2) and the upper outer layer (3) are connected to the lower outer layer (1) to the extent of being connected to at least a portion of the tip of the second pattern of the lower outer layer, wherein the height of the protrusions of the second pattern is preferably 50% to 200% greater than the height of the protrusions of the first pattern.

In the present invention, the aqueous adhesive composition is preferably applied in such a way that the "adhesive surface" (or "bonding surface") is from 1% to 20%, preferably from 2% to 10%, more preferably from 4% to 8%. As "adhesive surface" is understood the proportion of the total surface of the outer layers of a multilayer fibrous product (preferably a multilayer nonwoven product, a tissue product or a mixture thereof) comprising at least two fibrous layers which are bonded to each other by the aqueous adhesive composition or, if applicable, the proportion of the total surface of the outer layers of said multilayer fibrous product which are bonded to one or more inner layers by the aqueous adhesive composition. If there are two or more adhesive units applying the mixed acid/adhesive in different non-overlapping patterns to one or more fibrous layers, e.g. tissue layers and/or nonwoven layers, the corresponding adhesive surfaces are added together. With reference to e.g. FIG. 2 , the "adhesive surface" can be determined (e.g. visually or with the aid of a microscope) by adding the individual adhesive bonding areas (12) corresponding to the large embossed areas (16) shown in FIG. 3 a and 3 b and dividing this sum by the total surface area of the tissue product (plan view). With reference to the designs shown in e.g. figures 3a and 3b, the "adhesive surface" (in %) can also be calculated by dividing the total area of the macroembossments (16) as a flat two-dimensional view by the total surface area of the fibrous product.

Various techniques exist in the art for adjusting the amount of aqueous adhesive composition applied to a multi-layer fiber product, and these techniques can also be used in the present invention. If, as described above, the aqueous adhesive composition is applied to the distal ends of at least a portion of the protrusions (generated by embossing), the aqueous adhesive composition is preferably supplied to the embossing unit by a roller arrangement as shown in Figure (1). This arrangement comprises a roller (6) having an engraved surface, such as an anilox roller, and a coating roller (7), such as a rubber roller having a flat surface. In this case, the amount of aqueous adhesive composition can be adjusted by: (i) changing the surface volume of the engraved unit of the roller (6) to a value of 9 ^cm3 / ^m2 to 30 ^cm3 / ^m2 (preferably a volume of 18 ^cm3 / ^m2 to 21 ^cm3 / ^m2 ); and/or (ii) adjusting the speed of the engraved roller (6) to a value of 3% to 40% of the line speed (preferably a speed of 7% to 15%, for example 10%).

The multi-layer fiber product, such as a tissue product, a nonwoven product or a mixture thereof, preferably a tissue product, is preferably selected from wipes, hygiene products such as toilet paper, handkerchiefs, household towels, towels, facial tissue, napkins/serviettes, bedding or clothing. More preferably, the multi-layer fiber product is a toilet paper having 2 to 8 layers, preferably 2 to 6 layers.

The present invention also relates to a method for producing a multi-layer fiber product, preferably a tissue product, a nonwoven product or a mixture thereof, comprising the steps of:

a) providing at least two fiber webs, such as tissue paper webs and/or nonwoven webs, preferably at least two tissue paper webs

b) optionally embossing at least one of said webs so as to provide at least one raised pattern,

c) applying an aqueous adhesive composition to at least one side of at least one of said fibrous webs, said aqueous adhesive composition comprising an adhesive component and a preferably water-soluble dermatologically acceptable acid and optionally the corresponding salt of said acid, wherein, if at least one of said webs is embossed, said aqueous adhesive composition is preferably applied to the distal ends of at least a portion of said protrusions,

d) superimposing the at least two webs such that the aqueous adhesive composition is located between the at least two superimposed webs,

e) bonding the at least two webs together, and

f) Optional further steps of converting the bonded web into the final multi-layer fiber product.

With regard to steps a) to e), reference is made to the above description of the multilayer fiber product, preferably a tissue product, a nonwoven product or a mixture thereof, and to its preparation.

In the present invention, a combination of multiple layers (at least two layers) can be used to impart specific properties, such as thickness, bulkiness, softness, and strength to the final multi-layer fiber product.

When processing the bonded fiber web, for example a tissue web and/or a nonwoven web comprising cellulosic fibers, into a final product (optional conversion step f), the following steps can be used alone or in combination: cutting to size (machine direction and/or cross direction), folding, embossing, perforating, applying lotion, smoothing, stacking, rolling and packaging.

Example

The following test method is used to evaluate the produced tissue paper. Prior to testing, the test sample is conditioned at 50% relative humidity and 23°C for at least 12 hours. When the description or claims of the present invention refer to the weight of the raw tissue paper or tissue paper product, the weight is preferably determined after conditioning in the manner described above.

4.1. Basis weight

Basis weight is determined according to EN ISO 12625-6:2005, Tissue paper and tissue products, Part 6: Determination of grammage.

4.2. Paper thickness (Caliper)

The measurement is performed using a precision micrometer (0.001 mm accuracy) according to a modified method based on EN ISO 12625-3:2014, Part 3. To do this, the distance between the sample and a fixed reference plate and a parallel pressure foot is measured. The diameter of the pressure foot is 35.7 ± 0.1 mm (nominal area 10.0 cm ² ). The applied pressure is 2.0 kPa ± 0.1 kPa. The pressure foot moves at a rate of 2.0 ± 0.2 mm/s.

A useful instrument is a L&W Model SE050 Thickness Gauge (available from Lorentzen & Wettre, Europe).

The base tissue paper (web) to be measured is cut into 20 x 25 cm pieces and conditioned in an atmosphere of 23°C and 50% RH (relative humidity) for at least 12 hours. For measurement, a stack of 10 base tissue papers is prepared and placed under the press plate, which is then lowered. The thickness of the stack is then read 5 seconds after the pressure stabilizes. The thickness measurement is then repeated nine times, with additional samples processed and prepared in the same manner.

The average of the 10 values was taken as the thickness of the 10 sheets of the measured base thin paper (hereinafter referred to as "10-ply paper thickness").

The finished product to be measured (ie, a single-ply or multi-ply tissue product) is cut into pieces of 20 x 25 cm and conditioned in an atmosphere of 23°C, 50% RH for at least 12 hours.

To measure, place a sheet under the platen and lower it. After the pressure stabilizes, read the sheet's thickness 5 seconds later. Repeat this measurement nine times, with additional samples processed in the same manner.

The average of the 10 values obtained was taken as the thickness of one sheet of the measured finished product (eg, two-ply paper towel) ("thickness of one sheet").

Viscosity

The viscosity of the compositions was measured at 25° C. using a Brookfield LV-DV-I viscometer from Brookfield Engineering Laboratories, Inc. Depending on the viscosity range expected, the following spindles (sp) and speeds (v) were used:

- For x < 0.1 Pa s, sp 1/v 100 rpm

-For 0.1Pa s＜x＜1Pa s, sp 2/v 100rpm

-For 1Pa s＜x＜6Pa s, sp 3/v 100rpm

- For x > 6 Pa s, sp 4/v 100 rpm.

4.4 pH measurement (rough tissue/thin paper products)

As described in the appended claims, the following test method is preferably used to measure the pH of a tissue product.

ISO 6588-1:2012(E)-Cold extraction method

This method is based on extracting a representative sample of the corresponding tissue product (2 g) with high-purity water (100 mL) at a temperature of 20°C to 25°C for 1 hour. The extract is prepared, filtered, and processed according to this international standard. The pH of the extract is measured at a temperature of 20°C to 25°C. This method yields the average pH of the entire tissue product.

4.5 pH measurement on the skin

Flat electrode methodology

A skin surface pH study was conducted to investigate the effect of the low pH tissue product of the present invention on skin pH. Measurements were taken on the volar forearm before and after application of the test product. Skin surface pH was measured using a flat-end electrode, calibrated in standard buffer solutions of pH 4.0 and pH 7.0. Before each measurement, the electrode was immersed in 0.9 wt% NaCl in deionized water. 200 μL of 0.9 wt% NaCl was added to the skin and wiped with the tissue of the present invention. A five-person panel was used to test the effect of the tissue product of the present invention on forearm skin surface pH.

Example 1

Preparation of aqueous adhesive compositions

A potassium lactate buffer solution of approximately pH 2 was prepared by dissolving 85% lactic acid (264 g, 15% water) and KOH (5 g) in H ₂ O (736 g). The molar content of lactic acid calculated from these values ("acid molarity") was 2.366 mol/l. The acid molarity refers to the total amount of acid added, regardless of whether a portion of it is present as lactate due to the addition of KOH.

The buffer solution was then mixed with 429 g of a polyvinyl alcohol-based aqueous adhesive (Tak 1004, approximately 18.0% solids; available from H.B. Fuller, Europe, pH approximately 4.5) to give a ratio of 70% potassium lactate buffer solution to 30% Tak 1004 by weight. After mixing, the solids content of the adhesive component was approximately 5.4% by weight based on the total aqueous adhesive composition. The pH of the resulting aqueous adhesive composition was approximately 2.3.

Preparation of tissue paper products

A non-embossed rough tissue paper web having a thickness of 0.37 mm/sheet and a basis weight of 56.8 g/m ² produced by a conventional dry creping process was prepared from a stock mixture comprising the following components.

The roll consists of three layers, using only non-recycled (virgin) papermaking fibers (100% virgin fibers). The composition of each layer is as follows:

The two outer layers are made of 9% softwood kraft fiber, 88% eucalyptus kraft fiber and 3% other hardwood kraft fiber;

One inner layer is made of 73% softwood kraft fiber, 20% hardwood kraft fiber, and 7% CTMP.

The pH of the rough tissue paper web measured according to ISO 6588-1:2012(E) was 7.7.

Layer bonding steps

Three prepared tissue paper webs were provided to the embossing and gluing station shown in FIG. 1 .

The first web (1) and the third web (3) are each advanced to rubber rollers (8a and 8b) and embossed by engraved rollers (9a and 9b). The second web (2), which will later form the middle layer of the three-layer product, is first advanced to a gluing unit (4), where the aforementioned aqueous adhesive composition is applied to one side of the web by means of (secondary) protrusions (14) of the engraved roller (9a), which press the web against the adhesive application roller (7). The adhesive (aqueous adhesive composition) is applied only to the secondary engravings (14), which are approximately twice as high as the primary engravings (13).

The gluing unit (4) comprises an adhesive chamber (5) containing an aqueous adhesive composition, an engraved anilox roller (6) that transfers the aqueous adhesive composition (at room temperature) from the adhesive chamber to an applicator roller (7), and a rubber roller with a flat surface that transfers the adhesive composition to the second web (2). A doctor blade (not shown in FIG1 ) meters the amount of adhesive on the anilox roller, which runs at different speeds, to reduce the amount of adhesive transferred to the applicator roller. The applicator roller runs at a linear speed. The surface of the anilox roller has small holes that can deliver a specific volume of adhesive per square meter.

According to a preferred embodiment, the volume of the anilox roller is adjusted to 18 to 21 ^{cm 3} /m ² , the speed of the anilox roller is adjusted to 10% of the line speed, and the adhesive bonding area occupies 4 to 6% of the total surface of the tissue product, the adhesive bonding area corresponding to the total area of all ^bonding areas (12) as shown in FIG2 .

The three tissue paper webs are superimposed in the nip between rollers (9a) and (9b), first webs (1) and (2) and then web (3), however no pressure is applied in the nip, and the webs are further advanced to the nip between the engraved roller (9a) and the bonding roller (10), in which all three webs are bonded together by pressing the adhesive covered areas (12) together.

The bonded web is then cut to size, perforated, and further converted into toilet paper rolls.

As can be seen from Figures 2 and 3, the resulting three-ply toilet paper shows three embossing patterns: a small embossing pattern (15a, 15b) (see primary engraving, 13) and a large embossing pattern (16) (see secondary engraving, 14) which is also used as a decorative pattern.

The tissue paper product is produced such that all embossed patterns cover the entire surface of the tissue paper product.

The adhesive composition was applied in an amount of 0.39% by weight based on the basis weight of the tissue product, and the adhesive composition was applied in an amount based on all ingredients of the adhesive composition except water.

The pH value of the tissue paper product was 5.6 as measured by ISO 6588-1:2012(E) method.

Comparative Example 1 and Examples 2 to 12

Twelve different three-ply tissue products (bathroom tissues) were produced in the same manner as described in Example 1, except for the differences shown in Table 1 and explained below.

In Comparative Example 1 and Examples 2 to 5 and Examples 8 to 12, the same papermaking fibers and layer compositions as in Example 1 were used.

In Examples 6 and 7, the wrapping paper consists of three layers, containing 26% recycled fiber and 74% virgin fiber. The composition of each layer is as follows:

The two outer layers are made of 60% bleached eucalyptus kraft pulp and 40% bleached softwood kraft pulp

An inner layer made of 20% bleached eucalyptus kraft pulp and 80% deinked pulp fibers

In Comparative Example 1 (CEx 1), no acid was added to the aqueous adhesive composition.

In Examples 6 and 7, coarse tissue webs were prepared from a pulp mixture containing 26% recycled fibers and 74% virgin fibers according to a conventional dry crepe process.

The zelura adhesive used in Example 12 is a methylcellulose based adhesive available as a solid from Henkel. 33.3 parts by weight of the solid was dissolved in 966.7 parts by weight of water to produce approximately 1 liter of aqueous adhesive.

From these results it can be seen that all examples resulted in an effective change in pH. Despite the presence of a greater amount of acid/buffer, the adhesion of the prepared aqueous compositions was good.

Example 13

The effect of the tissue product of Example 7 on forearm skin surface pH was tested using the five-person panel and flat-face electrode method described above. The results are listed in Table 2 below.

Table 2

The above results show that the pH of healthy skin can be stabilized by the tissue paper of the present invention even if the rough tissue paper used therein has a relatively high pH (such as pH 9.0 in the case of Example 7).

Claims (20)

1. Multi-layer paper products containing at least two thin paper layers. At least two layers are bonded together by an aqueous adhesive composition comprising an adhesive component, a dermatologically acceptable acid, and optionally, a salt of said acid. The amount of said acid and optionally present salts of said acid provided is such that the pH of said multilayer paper product, as measured according to ISO 6588-1:2012(E), is between 4.0 and 6.4. The dermatologically acceptable acids mentioned herein are selected from isoascorbic acid, glycolipid, acetic acid, dehydroacetic acid, oleic acid, palmitic acid, stearic acid, behenic acid, palm kernel oil acid, tartrate, salicylic acid, ascorbic acid, sorbic acid, benzoic acid, succinic acid, and any combination thereof. The multi-layer paper products mentioned above are selected from wiping products, hygiene products, handkerchiefs, towels, facial tissues, napkins, bedding, or clothing. 2. The multilayer paper product according to claim 1, wherein the amount of said acid and optionally present salts of said acid provided is such that the pH of said multilayer paper product, as measured according to ISO 6588-1:2012(E), is 4.5 to 6.2. 3. The multilayer paper product according to claim 1 or 2, wherein the aqueous adhesive composition contains the acid and optionally a salt of the acid in a molar amount of 0.1 mol/L to 10 mol/L. 4. The multilayer paper product according to claim 1 or 2, wherein the amount of the aqueous adhesive composition is from 0.01% to 2% by weight relative to the weight of the multilayer paper product, and the amount of the aqueous adhesive composition is based on the total weight of all components of the adhesive composition except water. 5. The multilayer paper product according to claim 1 or 2, wherein the adhesive component is selected from: (a) natural polymers and (b) synthetic polymers; or any combination thereof. 6. The multilayer paper product according to claim 1 or 2, wherein the aqueous adhesive composition comprises a dermatologically acceptable acid and its salt, and the acid and its salt are present in an amount that collectively provides pH buffering capacity. 7. The multilayer paper product according to claim 1 or 2, wherein the layer comprises the following substances as papermaking fibers: (i) only non-regenerated cellulose fibers, (ii) only regenerated cellulose fibers, or (iii) a mixture of non-regenerated cellulose fibers and regenerated cellulose fibers in a weight ratio of 99/1 to 1/99, wherein in any one of (i), (ii) or (iii), the cellulose fibers are selected from wood fibers and non-wood fibers and combinations thereof. 8. The multilayer paper product according to claim 1 or 2, wherein the number of the thin paper layers is 2 to 8. 9. The multilayer paper product according to claim 1 or 2, wherein the adhesive composition is applied to at least a portion of the surface of the paper layer in at least one regular or irregular pattern. 10. The multilayer paper product according to claim 9, wherein the regular or irregular pattern at least covers the central region of the thin paper layer. 11. The multilayer paper product according to claim 1 or 2, wherein the multilayer paper product has two outer layers and optionally at least one additional layer located between the two outer layers, wherein at least one of the outer layers is embossed to form a protrusion, wherein the aqueous adhesive composition is applied to at least a portion of the distal end of the protrusion, the protrusion being arranged to bond the two outer layers and optionally at least one additional layer located between the two outer layers together. 12. The multilayer paper product according to claim 1 or 2, wherein the multilayer paper product comprises at least three layers: an embossed upper outer layer (3) and an embossed lower outer layer (1), and a center layer (2), each outer layer comprising an embossed pattern comprising at least a portion of discrete protrusions, the distal regions of at least a portion of the protrusions of each outer layer (1, 3) facing the center layer, and the pattern density of at least one of the upper outer layer (3) and the lower outer layer (1) being at least 10 protrusions/ ^cm² , the lower outer layer (1) comprising a first pattern (15a, 15b) and a second pattern (16), the height (13) of the first pattern being less than the height (14) of the second pattern, wherein the center layer (2) and the upper outer layer (3) are nested together and connected to the lower outer layer (1) by an applied adhesive composition (12), wherein the degree of connection between the center layer (2) and the upper outer layer (3) and the lower outer layer (1) is such that at least a portion of the tip of the second pattern is connected to the lower outer layer. 13. The multilayer paper product according to claim 1 or 2, wherein the percentage of the total surface area of the multilayer paper product bonded by the adhesive composition is 1% to 20%. 14. The multi-layer paper product according to claim 1, wherein the multi-layer paper product is a toilet paper product having 2 to 8 layers. 15. The multi-layer paper product according to claim 1, wherein the multi-layer paper product is selected from toilet paper and household towels. 16. The multilayer paper product of claim 5, wherein the natural polymer is selected from proteins and protein-based compounds, gums and gum-like materials, polysaccharide-based materials, and gums derived from animal products; the synthetic polymer is selected from polyethyleneimine (PEI), polyamide-polyamine-epimerol (PAE) and polyethyleneimine (PVAm), polyvinyl alcohol (PVA), polyvinyl acetate (PVAC), vinyl acetate-ethylene copolymer (VAE), polyvinylpyrrolidone (PVP), polyurethane and styrene-butadiene copolymer, and water-soluble or water-dispersible cellulose-based compounds; wherein the water-soluble or water-dispersible cellulose-based compounds are selected from carboxymethyl cellulose, sodium carboxymethyl cellulose, methyl cellulose, and ethyl cellulose. 17. The multilayer paper product according to claim 7, wherein the wood fiber is softwood fiber or hardwood fiber, and the non-wood fiber is fiber from an annual plant. 18. The multilayer paper product according to claim 8, wherein the number of paper layers is 2 to 6. 19. A method for preparing a multilayer paper product, the method comprising the following steps: a) Provide at least two fiber webs of tissue paper. b) Optionally, at least one of the said webs is embossed to provide at least one raised pattern. c) Applying an aqueous adhesive composition to at least one side of at least one of the fiber webs, the aqueous adhesive composition comprising an adhesive component and a dermatologically acceptable acid and, optionally, a corresponding salt of the acid. The amount of said acid and optionally present salts of said acid provided is such that the pH of said multilayer paper product, as measured according to ISO 6588-1:2012(E), is between 4.0 and 6.4. The dermatologically acceptable acids mentioned herein are selected from isoascorbic acid, glycolipid, acetic acid, dehydroacetic acid, oleic acid, palmitic acid, stearic acid, behenic acid, palm kernel oil acid, tartrate, salicylic acid, ascorbic acid, sorbic acid, benzoic acid, succinic acid, and any combination thereof. d) Overlap the at least two webs such that the aqueous adhesive composition is located between the at least two overlapped webs. e) Bonding the at least two webs together, and f) Optional steps for converting the bonded web into the final multilayer paper product. The multi-layer paper products mentioned above are selected from wiping products, hygiene products, handkerchiefs, towels, facial tissues, napkins, bedding, or clothing. 20. The method of claim 19, wherein the multi-layer paper product is selected from toilet paper and household towels.