HK1238225B - A tearable core, a roll comprinsing such a core and a tearable core manufacturing method - Google Patents

Description

Tearable core, roll comprising such a core and method for producing a tearable core

Technical Field

One aspect of the present invention relates to a tearable core, i.e., a cylindrical tube, more precisely, a cardboard cylindrical tube. Such cores find particular, though not exclusive, application in the tissue paper industry, where webs of absorbent material, such as tissue paper webs or nonwoven webs, are wound around such cores. Another aspect of the present invention relates to a roll comprising a sheet product wound around a core according to the present invention. Yet another aspect of the present invention relates to a method for manufacturing the core.

Background Art

Rolls of sheet-like products wound on a core are known in the art. Such rolls are used as paper towels, wet wipes or paper rolls for household and professional applications, for example. According to a first type of use, the sheet-like product can be distributed tangentially from the outside of the roll toward the core of the roll, for example, by rotating the roll. According to a second type of use, the sheet-like product can be distributed radially/centrally from the inside of the roll toward the outside. This is so-called center-feed distribution. In this second situation, the core must be removed from the roll before distribution begins.

Document JPH1129263 describes an inner release paper tube. This inner release paper tube facilitates disassembly and removal of the paper tube by simply pulling the inner circumferential paper tape, as well as smooth release of the wound material from the inside. The inner release paper tube consists of a spiral-shaped paper tube, wherein an outer circumferential paper tape is formed from the outer circumference of the paper tube, and an inner circumferential paper tape is formed from the inner circumference of the paper tube. These tapes overlap laterally and are spirally wound so that the seams of the tapes do not overlap. The outer and inner circumferential paper tapes are intermittently joined along the seams of the outer circumferential tapes.

Document WO 2011/092590 describes a core having an axially tearable cylindrical wall. The core consists of two superimposed strips: an outer strip that comes into contact with the product, and an inner strip. These strips are wound around one another and joined together by a zone of attachment, such as by gluing. At least one longitudinal edge of the inner strip is unattached or weakly attached over a defined width, forming a free and accessible flap facing the outer strip over at least a portion of its spiral length.

The two tearable cores described above require precise and detailed positioning of the bond between the inner and outer strips. To achieve this, the core manufacturing process must be carefully adjusted and controlled during manufacturing. This is particularly true when the manufacturing cadence changes. Therefore, there is a need to simplify the manufacturing process of tearable cores, particularly by providing a tearable core that is easy to manufacture regardless of the changing manufacturing cadence.

Summary of the Invention

The object of the present invention is to propose a tearable core which overcomes the above-mentioned drawbacks and which facilitates changes in the manufacturing rhythm in particular during the manufacturing process.

According to one aspect, a tearable core suitable for winding a sheet product is provided, comprising a cylindrical body wall defining a longitudinal hole with a longitudinal axis XX', the cylindrical body wall being tearable from an edge, the cylindrical body wall comprising an outer strip and an inner strip, the outer strip being a portion of the tearable cylindrical body wall adapted to come into contact with the wound sheet product, the inner strip being another portion of the tearable cylindrical body wall facing the longitudinal hole, the outer strip and the inner strip being spirally wound around each other and overlapping with a lateral displacement parallel to the longitudinal axis XX', the outer strip and the inner strip being fastened together by a first attachment zone via their mutually facing surfaces, wherein:

- the strips are composed of a material having properties such that the inner strip is made of a paperboard having a greater basis weight than the outer strip and a water drop absorption time less than the water drop absorption time of the outer strip, or the outer strip is made of a paperboard having a greater basis weight than the inner strip and a water drop absorption time less than the water drop absorption time of the inner strip; and

- building the cylindrical body wall into a core tube by fastening together external strips, the first end of a first turn of external strips and the second end of an adjacent turn of external strips being superposed and fastened by their mutually facing surfaces by means of a second attachment zone.

The first and second attachment zones are positioned so that the two opposing surfaces of the outer and inner strips are not attached together over a defined width at an overlapping helical region where two adjacent turns overlap, thereby forming a free and internally accessible tab at the edge of the tearable core.

The outer and inner strips and the ends of the outer strips may be fastened together by adhesive bonding of their mutually facing surfaces, the first and second attachment zones being made of an adhesive film, which may be continuous or discontinuous.

The outer and inner strips and the ends of the outer strips may be fastened together by mechanical fastening via their mutually facing surfaces.

The first attachment zone may extend from the first end of the outer strip to the second end of the inner strip.The width W33 of the outer strip may be equal to or less than the width W34 of the inner strip.

The second attachment zone may extend over a width that is less than a width of the first attachment zone.

The basis weight of the inner strip may be about 50% higher than the basis weight of the outer strip, and the water droplet absorption time of the inner strip may be several minutes while the water droplet absorption time of the outer strip may be undetermined.

The outer stripe may have a basis weight that is about 50% higher than the inner stripe basis weight, and the outer stripe may have a water droplet absorption time of several minutes while the inner stripe may have a water droplet absorption time of undetermined magnitude.

The basis weight per unit area of one strip may range from 100 to 150 g/m ² and the water droplet absorption time may be about 10 minutes, and the basis weight per unit area of the other strip may range from 400 to 450 g/m ² and the water droplet absorption time may be undetermined.

According to another aspect, there is provided a roll of sheet product comprising a sheet product wound around a tearable core of the present invention.

The roll of sheet product may be a tissue paper web.

According to a further aspect, there is provided a method for manufacturing a tearable core in the form of a cylindrical body wall defining a longitudinal hole of longitudinal axis XX', the cylindrical body wall being tearable from an edge, the manufacturing method comprising the steps of:

- the outer strip and the inner strip are composed of materials having such properties that the inner strip is made of paperboard having a basis weight greater than that of the outer strip and a water droplet absorption time less than that of the outer strip, or the outer strip is made of paperboard having a basis weight greater than that of the inner strip and a water droplet absorption time less than that of the inner strip;

- assembling the outer and inner strips by overlapping them with displacement in a lateral direction parallel to the longitudinal axis XX′, the outer and inner strips being fastened together by their mutually facing surfaces by a first attachment zone;

- spirally winding the assembled outer and inner strips to construct the cylindrical body wall into a core tube by fastening the outer strips together, the outer surface of a first end of one turn of the outer strip superimposing on the outer surface of a second end of an adjacent turn of the outer strip and fastened together by their mutually facing surfaces by a second attachment strip; and

- Cutting the tearable core into defined lengths.

The method of making a tearable core may further comprise applying adhesive in a solid or dashed line on the outer surface of the first end of the outer strip of the assembled outer and inner strips.

According to the invention, it is possible to avoid or at least significantly reduce the control and adjustments required during the core manufacturing process, especially when the manufacturing rhythm changes. This can be achieved by the combination of the materials constituting the outer and inner strips and the position of the second attachment zone.

The invention further enables the core to maintain its ability to be qualitatively easily torn before installation in the dispenser, ie before dispensing begins.

Other advantages will become apparent from the following description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and not limitation in the accompanying figures, in which like references indicate similar elements:

1 schematically shows a perspective view of a roll of tissue paper when a tearable core is torn and of a roll of tissue paper in a center-feed dispenser during radial/center dispensing, in its upper and lower parts, respectively;

FIG2 is a cross-sectional view schematically and partially illustrating a roll of a tearable core and a wound absorbent product according to an embodiment of the present invention;

FIG3 is a side view of the tearable core of FIG2 showing overlapping helical regions;

FIG4 is a partial cross-sectional perspective view of the tearable core of FIG2;

FIG5 is a cross-sectional view schematically and partially illustrating the extraction sequence of the tearable core of FIG2 ;

FIG6 is a top view of a strip comprising assembled paperboard outer and inner strips used to make the tearable core of FIG2; and

FIG. 7 schematically illustrates a manufacturing machine and method for winding and cutting the tearable core of FIG. 2 .

DETAILED DESCRIPTION

FIG1 , top, is a perspective view schematically illustrating a tissue roll 1. The tissue roll 1 comprises a wound absorbent product, such as a tissue sheet 2, wound around a tearable core 3 made of cardboard material. The tissue sheet 2 may be provided with pre-cut lines (not shown) to facilitate separation of the individual tissue sheets. Before dispensing tissue sheet portions from the roll 1, the core 3 is axially torn by applying a pulling tearing force 4 along the longitudinal axis XX' of the roll. After the core has been torn and completely removed from the roll 3, the first inner winding of the tissue roll 1 can be grasped through the hole forming the inner center of the roll. The roll is then fitted into a center-feed dispenser 5.

The lower portion of FIG1 is a perspective view schematically illustrating a tissue roll 1 (shown in dashed lines) within a center-feed dispenser 5 (shown in dotted lines) during radial/central dispensing. In this exemplary embodiment, the center-feed dispenser 5 comprises a body 6, for example, of substantially cylindrical shape, for accommodating the tissue roll 1. The cylindrical body 6 comprises a dispensing opening 7. An unrolled portion of a tissue sheet 8 is dispensed from the roll 1 through the dispensing opening 7 when a user applies a dispensing force 9 substantially along the longitudinal axis XX' of the roll. The center-feed dispenser 5 may further comprise additional elements, not shown for clarity, such as a closing mechanism, a nozzle through which the sheet is dispensed, or a cutting mechanism to facilitate separation of portions of the tissue sheet 8, etc. Although the figures show the longitudinal axis XX' as a horizontal axis, other orientations are also suitable, for example, the center-feed dispenser 5 may be positioned substantially vertically.

2 to 4 schematically illustrate a tearable core 3 of the invention according to a preferred embodiment of the invention.The tearable core 3 comprises a tearable cylindrical body wall 31 defining a longitudinal hole 35 of longitudinal axis XX'.

FIG2 is a schematic and partial cross-sectional view of a tissue roll 1 comprising a tearable core 3 and a wrapped absorbent product 2. The tearable cylindrical body wall 31 of the tearable core 3 comprises an outer strip 33 and an inner strip 34. The outer strip 33 is the portion of the tearable cylindrical body wall 31 that contacts the wrapped absorbent product 2. The inner strip 34 is the portion of the tearable cylindrical body wall 31 that passes through the tissue roll 1 and faces the longitudinal aperture 35. The outer strip 33 and the inner strip 34 overlap in a lateral displacement parallel to the longitudinal axis XX'. The outer strip 33 and the inner strip 34 are helically wound around one another at a suitable winding angle φ and pitch Δ. The outer strip 33 and the inner strip 34 are fastened together via their facing surfaces by specific attachment zones 36, 37, thereby forming a tube of overlapping strips. In the present embodiment of the present invention, the outer strip 33 and the inner strip 34 are fastened together via adhesive bonding via their facing surfaces. Adhesive bonding is accomplished by applying an adhesive film to the mutually facing surfaces that need to be bonded together. The adhesive film is applied in a continuous manner. However, as an alternative, it can be applied in a discontinuous or intermittent manner.

According to the first and third embodiments of the present invention, the properties of the materials comprising the strips are such that the inner strip 34 is made of paperboard having a greater basis weight than the outer strip 33 and a shorter water drop absorption time than the outer strip 33. For example, the outer strip 33 is made of paperboard with a low basis weight and a high water drop absorption time, while the inner strip 34 is made of paperboard with a medium basis weight and a low water drop absorption time. According to the first embodiment, the basis weight of the inner strip 34 is approximately 50% greater than that of the outer strip 33, and the water drop absorption time of the inner strip 34 is several minutes, while the water drop absorption time of the outer strip 33 is an unspecified time. The water drop absorption time evaluates the water absorption rate of the paperboard material. It is measured by placing a drop of water on the paperboard surface and measuring the time in seconds/minutes for the drop to be completely absorbed. An unspecified water drop absorption time means that the water drop remains unabsorbed by the paperboard material after a specific period of time.

More precisely, the method for measuring water drop absorption time uses a free-standing measuring tube and a timer. Before starting the test, ensure that the drop height is 7 cm, plus or minus 3 mm. Prepare a cardboard sample. Place three water drops 3 cm apart on one side of the cardboard sample, and start a timer as soon as the drops are placed. Record the time when each drop is completely absorbed by the cardboard sample. Repeat the same test procedure for the other side of the sample. Calculate the average time in minutes for each side.

In the table below an example is given of a paperboard material manufactured and commercialized by the company Sonoco.

Table 1 (first embodiment):

External strip 33 Internal strip 34 150-200 400-450 Water drop absorption time (min) Undetermined size 10

Alternatively, according to the second and fourth embodiments of the present invention, the above characteristics of the materials constituting the strips may be reversed, meaning that the outer strip 33 is made of paperboard having a basis weight greater than that of the inner strip 34 and a water drop absorption time less than that of the inner strip 34.

Table 2 (Second embodiment):

External strip 33 Internal strip 34 400-450 150-200 Water drop absorption time (min) 10 Undetermined size

Table 3 (third embodiment):

External strip 33 Internal strip 34 180 230 Water drop absorption time (min) Undetermined size 10

Table 4 (fourth embodiment):

External strip 33 Internal strip 34 230 180 Water drop absorption time (min) 10 Undetermined size

The tearable cores according to the first and third embodiments are easier to extract than the tearable core according to the second embodiment.

According to a fifth embodiment of the present invention, the properties of the materials comprising the strips are such that inner strip 34 is made of paperboard having a basis weight substantially similar to that of outer strip 33 and a water droplet absorption time less than that of outer strip 33. For example, outer strip 33 is made of paperboard having a medium basis weight and a high water droplet absorption time, while inner strip 34 is made of paperboard having a medium basis weight and a low water droplet absorption time. According to this particular embodiment, the water droplet absorption time of inner strip 34 is several minutes, while the water droplet absorption time of outer strip 33 is unspecified.

Table 5 (fifth embodiment):

External strip 33 Internal strip 34 180-230 180-230 Water drop absorption time (min) Undetermined size 10

Furthermore, according to the present invention, the locations of the specific attachment zones 36 , 37 are selected to form a strong core 3 for supporting the wrapped absorbent product 2 while providing free and internally accessible flaps 50 to facilitate axial tearing of the tearable core 3 .

FIG6 is a top view of a strip 45 comprising assembled outer and inner strips 33, 34 for making the tearable core of FIG2 . Both the outer and inner strips are flat. The outer strip 33 is displaceably overlapped and bonded to the inner strip 34 via a first attachment zone 36 extending from a first end 38 of the outer strip 33 to a second end 39 of the inner strip 34, across their facing surfaces. Consequently, a longitudinal surface 40 at one side of the outer strip 33 and a longitudinal surface 41 at another side of the inner strip 34 are not attached together. At this stage, these longitudinal surfaces 40, 41 are surfaces to which adhesive is not applied. The width _W33 of the outer strip 33 can be the same as the width _W34 of the inner strip 34. Alternatively, the width _W33 of the outer strip 33 can be smaller than the width W34 of the inner strip ₃₄ .

FIG2 shows a cross-section of three consecutive turns A (partially shown), B, and C (partially shown) obtained by spirally winding the strip 45 of FIG6 . FIG4 is a perspective view of a partial cross-section showing the same portion. On one side, the outer strip 33 of one turn (e.g., turn B) is superimposed and connected to the outer strip of the adjacent turn (e.g., turn A) by a second attachment zone 37 between the first end 38 of the outer strip 33 of turn B and the second end 42A of the outer strip 33 of the adjacent turn (e.g., turn A). On the other side, the outer strip 33 of turn B is superimposed and connected to the outer strip of the adjacent turn (e.g., turn C) by another second attachment zone 37A between the first end 38A of the outer strip 33 of turn C and the second end 42 of the outer strip 33 of turn B. The second attachment zones 37, 37A extend over a width that is less than the width of the first attachment zone 36 that joins the outer and inner strips 33, 34. Thus, the two opposing surfaces 43 of the outer and inner strips 33, 34 are not attached together over a defined width at the overlapping helical region 44 where two adjacent turns overlap. FIG3 is a side view of the tearable core of FIG2 showing the overlapping helical region 44. Furthermore, it should be noted that the tearable cylindrical body wall 31 is held as a continuous tube only by the attachment zone 37 of the outer strip 33. According to the aforementioned embodiment (combined with the materials used for the strips and the location of the attachment zones), the union formed between the outer and inner strips 33 (first attachment zone 36) is stronger than the union between adjacent turns of the outer strip 33 (second attachment zone 37). Thus, a frangible zone is formed at the second attachment zone 37. The inner strip 34 serves as a means for applying the tearing force that separates the second attachment zone 37. A free and internally accessible tab 50 is formed at the edge (selvedge) of the core that has been cut substantially perpendicularly to the longitudinal axis XX'. This tab 50 can be easily grasped to apply the tearing force 4 shown in FIG1.

Figures 5A, 5B, and 5C are schematic and partial cross-sectional views illustrating the extraction sequence of the tearable core of Figure 2. The user grasps the free and internally accessible flap 50 and applies a tearing force 4 in a direction substantially parallel to the longitudinal axis XX'. The tearing force 4 has the effect of breaking the joint zone 52A between the overlapping ends 38, 42 of the outer strip 33, near the edge 51 of the tearable core 3, at the second attachment zone 37. Upon further application of the tearing force 4, the breaking effect continues to propagate spirally and successively breaks the second attachment zone 37 (different broken joint zones 52B, 52C, and 52D are shown in steps in the extraction sequence) until the other edge of the tearable core 3 is reached. At the same time, the diameter of the torn core decreases and the cylindrical body wall 31 is no longer in contact with the sheet product 2. As a result, the tearable core 3 is axially torn and the user is able to access the first inner sheet product 2 from the interior 35 of the roll 1.

FIG7 schematically illustrates the manufacture of a core 3 in the form of a cylindrical tube. The spirally wound tube core 3 is formed by spirally winding a strip 45 comprising preassembled outer and inner strips 33, 34 of cardboard on a cylindrical mandrel at a given spiral winding angle. The strip 45 is wound so that the outer surface of the first end 38 of one turn of the outer strip 33 overlaps the outer surface of the second end 42 of the adjacent turn of the outer strip and is joined together in the overlapping spiral region 44 to form a cylindrical tube.

For example, the core has a diameter of approximately 80 mm, a width W33 of approximately 120 mm, a width W34 of approximately 140 mm, an opposing portion 43 of approximately 10 mm width, and an overlapping helical region 44 of approximately 40 mm width.

More precisely, Figure 7 schematically shows a machine for winding and cutting a tearable core tube 3. The tearable core 3 is spirally wound in a winding unit 60 and subsequently cut in a cutting unit 64 to the desired length.

The winding unit 60 includes a glue deposition nozzle 61 , a mandrel 62 , and a winding belt 63 .

A glue deposition nozzle 61 is positioned upstream of the mandrel 62 and the wrapping tape 63. The glue deposition nozzle 61 applies adhesive (glue) in a solid line (as shown) or a dashed line (not shown) on the outer surface (longitudinal surface 41) of the first end 38 of the outer strip 33 of the preassembled outer and inner strips 33, 34. The line may have a width of several millimeters.

The tearable core tube is formed by wrapping the strip 45 around a mandrel 62. The mandrel 62 may be an elongated metal cylinder. The wrapping angle φ may be 45°, but any other angle is also acceptable.

The wrapping tape 63 joins the strips 45 and forms the tearable core tube 3. Due to this joining, the core tube is advanced along the mandrel in the manner of a screw. The outer surface of the first end 38 of the outer strip 33 is applied so as to adhere to the outer surface (longitudinal surface 40) of the second end 42 of the adjacent outer strip (see the details shown in FIG2). This forms a well-defined tearable core tube 3 downstream of the wrapping tape 63.

The cutting unit 64 is positioned downstream of the winding unit 60. The cutting unit 64 cuts the tearable core tube 3 of the desired length L. The cutting unit 64 may comprise a circular saw, a blade, a crushing device, etc. FIG3 schematically and partially shows the tearable core tube 3 obtained by the machine shown in FIG7.

The overall process may be continuous or discontinuous, meaning that the strips 45 may be individually manufactured (pre-assembled) and then temporarily stored as rolls (not shown) before being wound into cores in the machine described above.

According to the invention, manufacturing is simplified, resulting in the possibility of increasing the production rhythm from 30-35 m/min to 80 m/min.

The drawings and their above description illustrate rather than limit the invention.

The dimensions, density, angles, and positions of the respective strips in the illustrated embodiments are not intended to be limiting examples. Those skilled in the art will readily recognize that these dimensions, density, angles, and positions may be varied as desired or deemed necessary to achieve the desired tearability effect or to adjust the core cross-section to other dimensions. Furthermore, the examples provided in the table are non-limiting examples based on currently commercialized paperboard materials; one can envision paperboard materials with basis weights as low as 80 g/ ^m² being possible in the future.

While the illustrated and described embodiments show outer and inner strips with the ends of the outer strips being fastened together by adhesive bonding via their mutually facing surfaces, the strips or portions of the strips may alternatively be fastened together by mechanical fastening (e.g., by knurling) via their mutually facing surfaces. In this case, the deformation of the mutually facing surfaces of the strips or strip portions resulting from the knurling process constitutes the first and second attachment zones.

The core of the present invention can be used to wrap sheet products such as "sheet product rolls." This has a wide range of implications including, by way of example, paper towel rolls, toilet paper, plastic sheets, and the like, metal sheets (e.g., aluminum), and the like, food storage bags, packages, and the like. The sheet product can be wound as a continuous, non-perforated sheet or a pre-perforated sheet. The "sheet product roll" can be used for residential or commercial applications. A dispenser assembly can dispense sheet products from the sheet product roll manually (a user can pull the sheet of material by hand) or motor-assisted (a motor can assist in the automatic dispensing of the sheet as desired). Such dispenser assemblies and structures are not germane to the present invention and will not be described in detail.

Any reference signs in the claims should not be construed as limiting the claims. The word "comprising" does not exclude the presence of other elements than those listed in the claims. The quantifier "a" or "at least one" preceding an element does not exclude the presence of a plurality of such elements.

Claims (15)

1. A tearable core (3) suitable for winding a sheet product (2), comprising a cylindrical body wall (31) defining a longitudinal hole (35) with a longitudinal axis XX', the cylindrical body wall (31) being tearable from an edge (51), the cylindrical body wall (31) comprising an outer strip (33) and an inner strip (34), the outer strip (33) being a portion of the tearable cylindrical body wall (31) adapted to contact the wound sheet product (2), the inner strip (34) being a tearable core (33)... The other portion of the cylindrical body wall (31) facing the longitudinal hole (35), the outer strip (33) and the inner strip (34) are spirally wound around each other and laterally displaced and overlapped along the longitudinal axis XX', the outer strip (33) and the inner strip (34) are fastened together by a first attachment zone (36) through their mutually facing surfaces, characterized in that: the inner strip (34) and the outer strip (33) are made of a material having the following properties: - First characteristic, such that the inner strip (34) is made of cardboard having a water droplet absorption time that is shorter than that of the outer strip (33) and a basis weight that is greater than or equal to that of the outer strip (33). - Or a second characteristic, such that the outer strip (33) is made of cardboard having a water droplet absorption time that is shorter than that of the inner strip (34) and a basis weight that is greater than that of the inner strip (34); and Its feature is that, by fastening the outer strips (33) together, the cylindrical body wall (31) is constructed as a core tube, and the first end (38) of the outer strip (33) of one ring (A) and the second end (42A) of the outer strip (33) of the adjacent ring (B) are overlapped and fastened through a second attachment zone (37) through their mutually facing surfaces. 2. The tearable core (3) according to claim 1, wherein the first and second attachment zones (36, 37) are positioned such that the two opposing surfaces (43) of the outer strip (33) and the inner strip (34) are not attached together in the overlapping spiral region (44) where two adjacent loops overlap, thereby forming free and internally accessible flaps (50) at the edge (51) of the tearable core (3). 3. The tearable core (3) according to claim 1 or 2, wherein the outer strip (33) and the inner strip (34) and the ends (38, 42A) of the outer strip (33) are fastened together by an adhesive bond through their mutually facing surfaces, and the first and second attachment zones (36, 37) are made of an adhesive film, which may be continuous or discontinuous. 4. The tearable core (3) according to claim 1 or 2, wherein the outer strip (33) and the inner strip (34) and the ends (38, 42A) of the outer strip (33) are fastened together by mechanical fastening through their mutually facing surfaces. 5. The tearable core (3) according to claim 1 or 2, wherein the first attachment zone (36) extends from a first end (38) of the outer strip (33) to a second end (39) of the inner strip (34), and wherein the width W ₃₃ of the outer strip (33) is equal to or less than the width W ₃₄ of the inner strip (34). 6. The tearable core (3) according to claim 1 or 2, wherein the second attachment zone (37) extends over a width less than that of the first attachment zone (36). 7. The tearable core (3) according to claim 1, wherein the inner strip (34) and the outer strip (33) are made of a material having a first characteristic, the basis weight of the inner strip (34) is 50% higher than that of the outer strip (33), and the water droplet absorption time of the inner strip (34) is several minutes while the water droplet absorption time of the outer strip (33) is undetermined. 8. The tearable core (3) according to claim 1, wherein the inner strip (34) and the outer strip (33) are made of a material having a second property, wherein the basis weight of the outer strip (33) is 50% higher than that of the inner strip (34), and the water droplet absorption time of the outer strip (33) is several minutes while the water droplet absorption time of the inner strip (34) is undetermined. 9. The tearable core (3) according to claim 1, wherein the inner strip (34) and the outer strip (33) are made of a material having a first characteristic, the basis weight per unit area of the outer strip (33) is in the range of 150 to 200 g/ ^m² , the basis weight per unit area of the inner strip (34) is in the range of 400 to 450 g/ ^m² , the water droplet absorption time of the outer strip (33) is undetermined, and the water droplet absorption time of the inner strip (34) is 10 min. 10. The tearable core (3) according to claim 1, wherein the inner strip (34) and the outer strip (33) are made of a material having a second property, the basis weight per unit area of the inner strip (34) is in the range of 150 to 200 g/ ^m² , the basis weight per unit area of the outer strip (33) is in the range of 400 to 450 g/ ^m² , the water droplet absorption time of the inner strip (34) is undetermined, and the water droplet absorption time of the outer strip (33) is 10 min. 11. The tearable core (3) according to claim 1 or 2, wherein the inner strip (34) and the outer strip (33) are made of a material having a first characteristic, the basis weight per unit area of the inner strip (34) is in the range of 180 to 230 g/ ^m² and the water droplet absorption time of the inner strip (34) is 10 min, and the basis weight per unit area of the outer strip (33) is in the range of 180 to 230 g/ ^m² and the water droplet absorption time of the outer strip (33) is of an undetermined magnitude. 12. A sheet product roll (1) comprising a sheet product (2) wound on a tearable core (3) according to any one of claims 1 to 11. 13. The sheet product roll according to claim 12, wherein the sheet product (2) is a cotton paper web. 14. A method of manufacturing a tearable core (3), said tearable core being in the form of a cylindrical body wall (31) defining a longitudinal hole (35) with a longitudinal axis XX', said cylindrical body wall (31) being tearable from an edge (51), said manufacturing method comprising the following steps: - The outer strip (33) and the inner strip (34) are made of materials having the following properties: The first characteristic is that the inner strip (34) is made of cardboard having a water droplet absorption time that is shorter than that of the outer strip (33) and a basis weight that is greater than or equal to that of the outer strip (33). Or a second feature, such that the outer strip (33) is made of cardboard having a water droplet absorption time that is shorter than that of the inner strip (34) and a basis weight that is greater than that of the inner strip (34); - The outer strip (33) and the inner strip (34) are assembled by overlapping the outer strip (33) and the inner strip (34) in a laterally displaced manner parallel to the longitudinal axis XX', and the outer strip (33) and the inner strip (34) are fastened together by a first attachment zone (36) through their mutually facing surfaces; - The assembled outer strip (33) and inner strip (34) are spirally wound to construct the cylindrical body wall (31) as a core tube by fastening the outer strip (33) together. The outer surface of the first end (38) of one loop of the outer strip (33) overlaps the outer surface of the second end (42A) of the adjacent loop of the outer strip (33) and is fastened together by a second attachment zone (37) through their facing surfaces; and - Cut the tearable core (3) to a defined length (L). 15. The method of manufacturing a tearable core (3) according to claim 14, further comprising applying an adhesive in solid or dashed lines to the outer surface of the first end (38) of the outer strip (33) of the assembled outer strip (33) and inner strip (34).