WO2010068150A1

WO2010068150A1 - Method and apparatus for bonding

Info

Publication number: WO2010068150A1
Application number: PCT/SE2008/000704
Authority: WO
Inventors: Marcus Lehto; Robert Perneborn
Original assignee: SCA Hygiene Products AB
Current assignee: Essity Hygiene and Health AB
Priority date: 2008-12-12
Filing date: 2008-12-12
Publication date: 2010-06-17
Anticipated expiration: 2011-06-12

Abstract

The invention relates to a bonding method, comprising delivering a web comprising selected materials to be bonded in a predetermined pattern through a nip region between a pattern roller and an anvil roller to form a bonded web. The method involves using an anvil roller having a first diameter (DA) and a pattern roller having a second diameter (DP), which pattern roller is provided with raised surface elements arranged in a predetermined pattern. The anvil and pattern rollers have an equivalent radius (Re) of less than or equal to 40 mm and causes a compressive mechanical deformation in the nip between the anvil and pattern rollers by means of said raised surface elements to induce an internal heating of the web materials. The compression of the web materials are performed at a deformation rate sufficient to cause a local temperature increase within the deformed materials of the web material between the raised surface elements and the anvil roller, causing bonding between the web materials.

Description

METHOD AND APPARATUS FOR BONDING

TECHNICAL FIELD

The present invention relates to a bonding method and an apparatus for forming a bonded web. More particularly, the present invention pertains to a technique for forming a bonded web by inducing a high-rate of shear deformation.

BACKGROUND ART

Polymeric films and fabrics may be been bonded by employing conventional techniques such as adhesive bonding, thermal bonding, and ultrasonic bonding. The bonding techniques often involve passing webs of materials through the nip formed between a pair of counter-rotating bonding rollers, which roller may include bonding rollers with smooth surfaces, bonding rollers with surfaces that include distributed raised patterns of bonding elements, and combinations of rollers with smooth and patterned surfaces. The bonding techniques may involve combinations of heat and pressure to effect the desired bonding. In particular arrangements, the bonding rollers may be of a different construction or be rotated at different speeds to produce a line speed differential between the bonding rollers. Typically, it has been recognized that the bonding techniques as indicated above produce better bonding when conducted at lower bonding speeds.

At conventional previously employed, low bonding speeds, a large amount of heat can transfer away from the deformed material into the cooler parts of the web material and into the bonding rollers during the deformation period of the bonding process. US 2006/266473 discloses a method that involves increasing the bonding speed to overcome the problem of heat loss. However, for some bonding processes the line speed is limited by preceding or subsequent process steps. Also, an increased bonding speed may cause the raised patterns on a bonding roller to melt through the web material when certain material combinations are used. In such cases it may not be practically possible to increase the bonding speed.

Conventional bonding techniques, such as those described above, have not provided a desired combination of bonding speed and bond strength between the material webs. Additionally, the non-adhesive bonding techniques have had difficulty forming adequate bonds between materials that have a large difference in their melting-point temperatures. As a result, there has been a continued need for improved bonding techniques that can provide desired bond strengths and be adapted for forming a bonded web from webs of different materials and thicknesses. Further, there is a continued need for improved bonding techniques that can be adapted for use at any desired bonding speed.

DISCLOSURE OF INVENTION

The above problems have been solved by a bonding method and apparatus according to the appended claims.

The present invention relates to a bonding method and an apparatus for performing said method. The bonding method involves delivering a web comprising selected materials to be bonded in a predetermined pattern through a nip region between a pattern roller and an anvil roller to form a bonded web. The web materials to be bonded include a first web of a selected material and at least a second web of a selected material. The bonding method involves

- using an anvil roller having a first diameter D_A and being rotated at a first predetermined speed;

- using a pattern roller having a second diameter Dp and being rotated at a second predetermined speed, which pattern roller is provided with raised surface elements arranged in a predetermined pattern;

- using anvil and pattern rollers having an equivalent radius R_e of less than or equal to 40 mm; - causing a compressive mechanical deformation in the nip between the anvil and pattern rollers by means of said raised surface elements to induce an internal heating of the web materials, and

- compressing the web materials at a deformation rate sufficient to cause a local temperature increase within the deformed material of the web materials between the raised surface elements and the anvil roller, causing bonding between the web materials.

When the web materials pass through the nip between the anvil and pattern rollers the portions of the web materials passing between local areas comprising raised surface elements on the pattern roller and the anvil roller are mechanically deformed by compression in a relatively short time period. During this temporary deformation of the web materials the temperature in the local areas between the raised surface elements and the anvil roller will increase. The local increase in temperature caused by the high rate of deformation will cause bonding of the web materials by localized melting or softening of at least one the web materials in the said areas between the raised surface elements and the anvil roller.

As stated above, he relationship between the anvil roller and the pattern roller may be selected so that the equivalent radius R_e is less than or equal to 40 mm.

When selecting a suitable diameter for the pattern and anvil rollers, at least one of the first diameter and the second diameter may be selected to be equal to or less than 160 mm. Depending on process parameters, the smaller of the first diameter and the second diameter may be selected to be equal to or less than 80 mm. Simultaneously, at least one of the first diameter and the second diameter is selected equal to or greater than 20 mm.

Although it is possible to use a pattern roller that is either greater than or smaller than the anvil roller, the diameter of the pattern roller may have a number of restrictions imposed on it. For instance, the minimum diameter of the pattern roller determines the circumference of the roller. If the desired pattern is a continuous or intermittent straight line, then the diameter is not a problem. However, for spaced repeated patterns it is often desired to allow the circumference to correspond to the length of one or more products in the machine direction of a product web. In cases where the circumference of the pattern roller corresponds to the length of multiple products, the diameter of the pattern roller may be 1 ,5 m or more. Because the anvil roller may have a simpler design, being substantially cylindrical, it is easier to allow this roller to be replaced when required in order to achieve desired process parameters. In such cases the first diameter is selected equal to or less than the second diameter.

The bonding method may also involve using at least one support roller for supporting at least the roller having the smaller of the first diameter and the second diameter. A pair of supporting rollers are preferably, but not necessarily, used for supporting a relatively small diameter roller in order to prevent the roller from deflecting. For example, one or more support rollers may be used for supporting a roller or rollers having a diameter equal to or less that 50 mm. A deflection can be caused by relatively thick material webs, high forces in the roller nip, or a relatively large pattern coverage fraction cp.

Examples of how to determine the pattern coverage fraction cp are given in connection with Figures 5-7. For instance, for a pattern roller creating a bonding pattern comprising multiple circular dots with a constant or varying spacing, having a pattern width Wp and a pattern length L_P, which pattern is shown in Figure 5, the pattern coverage fraction cp is defined as;

1 ^N cp(x) = —∑y,(x), 0 ≤ x ≤ L_p where,

L_p = 2ΉR_P where R_P is the radius of the pattern roller.

Figures 6 and 7 show bonding patterns comprising rectangular and square areas, respectively. The example in Figure 6 shows a schematic web moving in the direction of the arrow A. The web has a pattern width Wp of 9 mm and the function yi(x) is constant along the length of each pattern, here 3 mm. Hence, in the section X1 in Figure 7, the pattern coverage fraction cp is calculated as; cp = (3 + 3)/9 = 0,67 . Similarly, in the section X2 in Figure 6, the pattern coverage fraction cp is equal to; cp = 3/9 = 0,33 .

The example in Figure 7 shows a schematic web moving in the direction of the arrow A. The web has a pattern width Wp of 21 mm and the function yi(x) is constant along the length of each pattern, here 3 mm. Hence, in the section X1 in Figure 6, the pattern coverage fraction cp is calculated as; cp = 3/21 = 0,14 . Similarly, in the section X2 in Figure 7, the pattern coverage fraction cp is equal to; cp = (3 + 3 + 3)/21 = 0,43.

Alternatively, a weighted pattern coverage fraction w_n(x) may be calculated. With reference to Figure 8, which illustrates an image of a contact pattern illustrating these contact area calculations, a parameter x_eff is defined based on a predetermined contact length between the web materials to be bonded and the bonding apparatus, for example in the nip between an ultrasonic horn and a patterned anvil roll. This parameter x_etf is in the calculations assumed to be 4 mm, which has shown to be a standard value for commonly available bonding equipment of this kind. A weight function %(x) is introduced since not the entire area of contact is in maximum contact with the web materials, and thus the entire area of contact is found as a weight function. The choice of weight function w_n(x) is defined over a certain interval and is defined below;

As said above x_etf is for the purpose of this invention assumed to be 4 (mm). For the constants A and B the following values are used in the calculations: A=5000 and B=O, 1. Even for bonding equipments having x_Θf_f smaller or larger than 4 mm the values of x_eff, A and B may anyhow be used for the purpose of this invention. The boundary conditions of w_n(x) are set such that the compression has a maximum at the centre of the contact area and such that the compression of the outer edges of the contact area is far less.

The weight function w_n(x) is used for calculating the weighted mean value of the area of contact between the bonding apparatus (for example ultrasonic horn and pattern roll) and the web materials as seen in a transverse direction of the seam This weighted mean value is defined as the contact area according to the invention, and can for example be calculated by means of an image analysis of a bonding pattern as described below.

A scanning of the bonding pattern is made using a resolution of 1200 dpi (dots per inch). The pattern is evaluated as a normalized inverted black and white image with the pattern coloured black. Therefore a function defining the pattern image has to be introduced;

l,if the pixel in (xn , yn) is located in the pattern

[0, 1 if the pixel in (xn , yn) is not located in the pattern

The normalized and weighted contact area is now defined as;

X _ =

(multiply by 100 to get the percentage), where

∑w_n(Xi) j=n-ε

Y M-I x_/ = 77 ∑^ - and m m=0

ε is the integer number of pixels within the length of the pattern image.

The contact area of the bonding pattern should be between 10 and 30% of the width Wp of the bonding pattern at any given point along the length of the bonding pattern. In preferred embodiments the contact area is between 10 and 25% and preferably between 13 and 20% of the width Wp of the bonding pattern at any given point along the length of the bonding pattern.

The method involves feeding the web materials through the roller nip at a predetermined line speed and selecting the first and second speeds to match the said line speed. The desired bonding can be produced while operating at relatively low pressure values and at very high line speeds or at relatively high pressure values and at very low line speeds. In this context, a relatively high line speed is considered to be above 100 m/min, while a relatively low line speed is considered to be below 100 m/min. As will be explained in further detail below, the method is particularly advantageous at line speeds less than

100 m/min.

The relationship between the anvil roller and the pattern roller may be selected so that the equivalent radius R_e is selected to be from 5 mm to 40 mm. The equivalent radius R_e is defined as the equivalent roller radius for a single roller on a flat surface. This radius is determined by the function

(1 ) — 1 = —1 + —1 w uhere,

Re Ra Rp

R_a= D_a/2, that is, the radius of the anvil roller; and Rp= D_p/2, that is, the radius of the pattern roller.

Dp may be determined by the shape of a desired pattern and/or the length or size of the bonded web or laminate product to be bonded. D_A may be selected depending on process parameters, in view of materials to be joined, line speed, required power to achieve joining and allowable nip force.

The power required to achieve sufficient bonding between the web materials fed between the rollers can be expressed as;

(2) PA = - where P_A is the power per unit area (W/m²);

E_a is the energy per unit area (J/m²); and t is the time used for compressing the web materials.

The value of the energy per unit area E_a can be defined as a function of the line speed, at which the web is fed between the rollers, and the equivalent radius R_e according to the following equations;

P

(3) Ea = — where

Ap

P is the total power in the nip between the rollers (W); and Ap is the pattern area speed (m²/s).

(4) Ap = Wp*cp*VL where

Wp is the pattern width (m); cp is the pattern coverage fraction (%/100);

V_L is the line speed, at which the web is fed between the rollers (m/s).

Note that the pattern coverage fraction cp may be replaced by the weight function w_n(x) as defined above.

(5) P = F *Vzm where

F is the force in the roller nip (N); and

V_zm is the vertical compression speed mean value (m/s).

Vz

(6) Vzm = — where

V₂ is the (maximum) vertical compression speed (m/s).

(7) VZ = VL * sin α where

V_L is the line speed, at which the web is fed between the rollers (m/s); and α is the contact angle during compression in the nip (radians). (8) cc = a cos — — where

Re+ h

R_e is the equivalent radius (m), as defined in equation (1), and h is the penetration depth during compression (m).

The time t for compression can be defined as a function of the line speed and the equivalent radius R_e according to the following equations;

(9) t = — where

VL

b is the contact length of a raised element in the machine direction (m), and V_L is the line speed (m/s).

(10) b = (Re+ h) * since where

R_e is the equivalent radius (m), as defined in equation (1); h is the penetration depth during compression (m); and α is the contact angle during compression in the nip (radians).

When substituting the variables in equation (2) with the definitions stated in equations (3) - (10), this results in the following equation;

From this equation it can be seen that the power PA required for adequate bonding between the web materials can be said to be dependent on the force F in the roller nip, the line speed VL at which the web is fed between the rollers, and the geometrical design of the cooperating anvil and pattern rollers. When implementing the method on a predetermined combination of web materials, the force F in the roller nip may be constrained to a maximum value by the type and/or thickness of the materials to be bonded. The force F in the roller nip may be determined by a suitable sensor provided for one or both of the anvil roller and the pattern roller. A forcing mechanism can be provided to exert a force onto one or both of the rollers, in order to operatively urge the pattern roller and the anvil roller together with a predetermined force. For example, the forcing mechanism can be located at each end of the axis supporting the pattern roller and be configured to exert a desired force in the direction of the anvil roller. Similarly, the line speed VL may be controlled by process steps occurring before or after the roller nip. Hence, the process requirements may place limitations on the line speed VL. For a predetermined bonding pattern on the pattern roller, the pattern width W_p, pattern coverage fraction cp and the penetration depth h during compression may be determined by the combination of web materials to be bonded and/or the geometrical constraints of the desired pattern. As the bonding pattern formed by the raised elements on the pattern roller is determined by the desired pattern on the finished or intermediate laminate product leaving the roller nip, it may neither be possible nor practical to make adjustments to the pattern roller. In this case, the power P_A required for adequate bonding can be controlled by a purposive selection of the equivalent radius R_e, which is determined by the diameters of the anvil roller and the pattern roller. The anvil roller as a rule is cylindrical with no raised elements, or at least with raised elements having a limited height. Because of the relatively simple design of the anvil roller, it is preferable to replace this roller in order to select the diameter of the anvil roller to achieve said required power P_A for adequate bonding. For instance, as indicated in Equation 11 , a reduction of the equivalent radius R_e will cause an increase of the power P_A.

The bonding method involves feeding the web materials through the nip at a predetermined line speed V_L and selecting the first and second rotational speeds of the anvil roller and pattern roller, respectively, to match the said line speed. If there are no limiting process requirements, the line speed may also be selected depending on the properties of the materials to be joined to achieve said required power P_A for adequate bonding. The line speed may be determined by production steps prior to or after the bonding step, or be measured by a suitable sensor provided for one or both of the anvil roller and the pattern roller. The invention further relates to a bonding apparatus comprising a pattern roller and an anvil roller arranged to feed a web comprising selected materials to be bonded in a predetermined pattern through a nip region between the pattern roller and the anvil roller to form a bonded web. The web materials to be bonded include a first web of a selected material and at least a second web of a selected material. The anvil roller has a first diameter D_A and is rotated at a first predetermined speed. The pattern roller has a second diameter D_P and is rotated at a second predetermined speed, which pattern roller is provided with raised surface elements arranged in a predetermined pattern. According to the invention, the anvil and pattern rollers have an equivalent radius R_e of less than or equal to 40 mm.

The anvil and pattern rollers are arranged to cause a compressive mechanical deformation in the nip between the anvil and pattern rollers by means of said raised surface elements, in order to induce an internal heating of the web materials. The raised surface elements on the pattern roller and the outer surface of the anvil roller are arranged to compress the web materials at a deformation rate sufficient to cause a local temperature increase within the deformed materials of the web material between the raised surface elements and the anvil roller to bond the web materials.

Generally stated, the bonding method includes a delivering of a web material comprising one or more selected bonding material webs through a nip region between at least one cooperating pair of bonding rollers to form a bonded web. In a particular aspect, a pattern roller can be configured with a relatively high, pattern line speed. In another aspect, the pair of bonding rollers can be configured to provide a relatively low, bonding pressure value. The method and apparatus of the invention can more efficiently produce bonds having a desired, sufficiently high strength value. The desired bonding can be produced while operating at relatively low pressure values and at very high line speeds or at relatively high pressure values and at very low line speeds. In this context, a relatively high line speed is considered to be above 100 m/min, while a relatively low line speed is considered to be below 100 m/min. The effect of invention is particularly useful at relatively low line speeds (less than about 100 m/min). At relatively low line speed the significance of the choice of R_e for achieving a desired result increases. The deformation rate of the web materials as they pass through the roller nip is proportional to the vertical compression speed V_z. At line speeds V_L less than about 100 m/min, the deformation rate can be increased substantially if the equivalent radius R_e is reduced. This effect can also be seen at line speeds over 100 m/min, although the effect of modifying the equivalent radius R_e is less obvious at such speeds.

Adaptation of, for instance, the bonding pressure value when operating at a predetermined line speed can be achieved by selecting a suitable value for the equivalent radius of the bonding rollers. A suitable value for the equivalent radius may be selected by applying the above formulae. The desired bonding can be produced while operating at ordinary room temperatures. Additionally, the method and apparatus of the invention can produce the desired bonding between work materials that are ordinarily deemed to be incompatible when employing conventional thermal or ultrasonic bonding techniques. Compared to thermal bonding, the method according to the invention does not cause burnt welds or local melting through the materials to be bonded. Compared to ultrasonic bonding, the method according to the invention is simpler, as ultrasonic horns are not required, and is better suited for relatively wide seams and materials of different thicknesses.

The process and apparatus of the invention can have a lengthwise, machine- direction which extends in the longitudinal direction of the web passed through the machine; a lateral cross- direction which extends transversely across the width of said web; and a z-direction at right angles to the x- and y-directions. In the present invention, the machine-direction is the direction along which material webs are transported length-wise along and through a nip between a pair of cooperating bonding rollers. The cross-direction lies generally parallel to the local horizontal, and is aligned perpendicular to the local machine-direction. The z- direction is aligned substantially perpendicular to both the machine- direction and the cross-direction, and extends generally along a depth-wise, or thickness dimension of the web material. The examples given in this text relate to a substantially horizontal machine direction, but the invention is not limited to these examples. Consequently, the machine direction may also be vertical, or located in any plane between horizontal and vertical.

The method and apparatus according to the invention allows for the production of bonds having a desired, sufficiently high strength value. The desired level of bonding can be produced while operating at different pressure values and a suitable predetermined line speed. The method and apparatus can produce distinctively interconnected bonds having high, attachment strength values. The desired bonding can be accomplished while operating at ordinary room temperatures. Additionally, the method and apparatus of the invention can produce adequate bonding between web materials that may normally be considered incompatible, particularly when using conventional thermal or ultrasonic bonding techniques. The method and apparatus of the invention may be employed in any suitable manufacturing system that includes bonding of selected web materials into an assembled web. The method and apparatus are, for instance, suitable for the production of reusable articles, disposable articles or disposable absorbent articles or the like. For example, the method may involve bonding at least one layer of a web material comprising a thermoplastic material and at least one further layer of a web material comprising the same or a different material. The said web material layers can be covering materials or cover layers intended for disposable absorbent articles such as diapers, incontinence pads, sanitary towels and panty liners. An absorption body or article may be enclosed between an inner cover layer, intended to face the wearer during use, and an outer cover layer, intended to face away from the wearer during use. When worn by a user, the front and rear portions and the crotch portion of the absorbent article are made in one continuous piece which thus forms the outer cover layer. This outer cover layer can consist of one or more layers of non-woven material or other textile-like materials. Alternatively, the outer cover layer can comprise entirely or partly a liquid- impermeable material, such as a liquid-impermeable plastic film, a non- woven layer which has been coated with a liquid-blocking material, or some other flexible material layer which has the capacity to resist liquid penetration. The inner cover layer can comprise any liquid-permeable material suitable for the purpose. Examples of such materials are various types of thin non-woven material, perforated plastic films, net material, liquid-permeable foamed material or the like. The outer cover layer can be shaped so that it forms the entire outer extent of the absorbent article, such as a pant diaper or the like. The inner and outer cover layers can be interconnected around the absorption body by, for example, the bonding method according to the invention. Furthermore, one or both the cover layer(s) can be connected to one or more component parts making up the absorbent article, such as a belt, a side panel, a layer of the absorption body, or to the absorption body itself, by the bonding method according to the invention.

The web material making up the assembled web can include one or more selected materials. For example, each web material may include one or more individual webs of the same or different materials. Optionally, at least one web material can comprise a laminate including a first web of a selected, first material and at least a second web of a selected, second material. The first and second materials can be the same or different. Any suitable web material may be employed. Suitable materials are those comprising a sufficient amount of thermoplastic material that can be softened or melted in the process. Examples of suitable thermoplastic polymers for use in the method according to the invention are polyethylene, polyesters, polypropylene and other polyolefin homopolymers and copolymers and blends of thermoplastic polymers. The materials may comprise thermoplastic films, nettings, nonwoven webs or laminates of two or more layers of film, netting or nonwoven. The materials may be perforated or non-perforated. The nonwoven webs and films have a high content of thermoplastic material and may contain at least 50% thermoplastic material and preferably at least 80% thermoplastic material. The nonwoven material can also be a spunbond nonwoven material, an air-thru nonwoven material, a spunlace nonwoven (hydroentangled) material, a meltblown nonwoven material, or a combination of these. The raw material can be polypropylene (PP), polyethylene (PE) polyester (PET), polyamide (PA), or a combination of these. If a combination is used, this can be a mixture of fibres from different polymers, although each fibre can also contain different polymers (for example PP/PE bi- component fibres or PP/PE copolymers). Where appropriate, the plastic film can consist of PE or PP, PET, PLA or amyl (or any other thermoplastic polymer), or a mixture or copolymers of the aforementioned polymers.

In desired arrangements, at least one of the first and second webs can be a fabric web. Accordingly, the invention can be configured to bond a nonwoven fabric to a similar or substantially identical nonwoven fabric; to bond a nonwoven fabric to a different nonwoven fabric; to bond a nonwoven fabric to a film material; or to bond a film material to a film material.

The method and apparatus of the invention can include at least one cooperating pair of counter-rotatable or counter-rotating bonding rollers. The bonding rollers can include at least one rotatable pattern roller and at least one rotatable anvil roller. The pattern roller has an axis of rotation and can be provided with multiple pattern bonding elements, which elements may be arranged in any suitable operative distribution. Such distributions of bonding elements are conventional and well known. The individual pattern bonding elements can, for example, be pin elements, and the pin elements can have any suitable size, shape and/or cross- section.

The method and apparatus of the invention may cause a high deformation rate (length per length, per unit time), also referred to as deformation strain rate, during the relatively high-speed compression of local areas of the web materials as they pass through the nip between the anvil roller and the raised surfaces of the pattern roller. If the deformation rate is sufficiently high, the compressive mechanical deformation may induce an internal increase in temperature in the web material, and cause at least one of the web materials to soften or melt. Simultaneously, thermal conduction causes heat to be transferred away from the local areas of deformation in the deformed web material materials. As a result, the net temperature rise within the deformed web material will be determined by the difference between the internal heating due to the web deformation, and the heat loss due to thermal conduction.

The method and apparatus of the invention provides a rate of mechanical deformation or mechanical strain, which achieves a rate of internal heating that is significantly higher than the rate of heat loss due to thermal conduction. At high deformation rates induced internal heating is so rapid that only a small amount of heat is lost due to thermal conduction. Under ideal conditions, the internal heating process may be nearly adiabatic. By providing a combination of sufficiently high nip force and sufficiently large local deformation rate within the areas to be bonded, any auxiliary, pre-heating of the web material or bonding rollers may be dispensed with. Additionally, the deformation rate can be seen to increase non-linearly (cf. Equations 7 and 8) with respect to a decrease of the equivalent radius R_e. In a particular feature of the invention, the deformation rate (length per length, per unit time) during the bonding compression of the web material materials can, for example, be less than 1^*10³ sec^"1.

Another factor that may be taken into consideration is the thermal properties of the materials used in the bonding rollers. The choice of materials in the rollers can affect the rate of temperature increase produced in web materials being deformed, due to thermal conduction into the bonding rollers. A relatively large thermal conductivity of the bonding rollers may cause a reduction of the temperature increase within the deformed materials of the web material. Hence it is desirable to use materials with a relatively small thermal conductivity, in order to achieve satisfactory bonding without requiring auxiliary heating of the bonding rollers. For example, by using bonding rollers having peripheral bonding surfaces made of steel instead of bonding rollers having peripheral surfaces made of copper the thermal conductivity is reduced and the performance of the method and apparatus of the invention is improved. By using materials with lower thermal conductivity the need for auxiliary heating can be reduced or eliminated, and the bonding rollers assist in increasing the temperature of the deformed the web materials towards their softening and/or melting points in order to achieve a desired bonding strength.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be described in detail with reference to the attached figures. It is to be understood that the drawings are designed solely for the purpose of illustration and are not intended as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to schematically illustrate the structures and procedures described herein.

Figure 1 shows a schematic illustration of an arrangement for carrying out the method according to the invention;

Figure 2 shows a schematic illustration of an apparatus for carrying out the method according to the invention;

Figure 3 shows a schematic cross-section of a pattern roller.

Figure 4 shows a diagram where the contact time t has been plotted over line speed V_L.

Figures 5-8 show examples of patterns that may be imparted to an assembled web by a pattern roller;

EMBODIMENTS OF THE INVENTION

Figure 1 shows a schematic illustration of an arrangement for carrying out the bonding method according to the invention. The arrangement and the bonding method has a lengthwise, machine-direction X which extends longitudinally in the direction of a bonded web, comprising at least two web materials, a lateral cross-direction Y which extends at right angles to the machine direction, transversely across the longitudinal direction of the bonded web, and a Z- direction at right angles to the X- and Y-directions. The machine-direction X is the direction along which a particular component or material is transported lengthwise along and through a working zone of the apparatus. The cross- direction Y lies generally parallel to the local horizontal, and is aligned perpendicular to the local machine-direction X. The Z-direction is aligned substantially perpendicular to both the machine-direction X and the cross- direction Y, and extends generally along a depth-wise, thickness dimension of the appointed material targeted for work.

The bonding method and apparatus includes a delivering of a bonded web W₁- having one or more selected bonding web materials W₁, W₂ through a nip region 11 between at least one cooperating pair of rotatable bonding rollers RA, Rp, thereby forming and producing a bonded web W_B. A pattern roller R_P is provided for imparting a sufficiently high-rate of shear deformation to the bonded web W_τ in order to achieve local melting and subsequent bonding of the individual web materials W₁, W₂ as they pass the nip between the two rollers. An anvil roller R_A is provided to cooperate with the pattern roller R_P to form the said nip. The pattern roller R_P is provided with a pattern surface speed and the anvil roller RA is provided with an anvil surface speed. The anvil surface speed is preferably, but not necessarily, configured to be substantially equal to the pattern surface speed. This common surface speed will be referred to as the line speed V_L.

Although the bonded web W_τ in Figure 1 is shown as comprising two bonding web materials W₁, W₂ the method can be applied to bonded webs having multiple web materials. The number of web materials to be joined is selected depending on the product to be manufactured and the type of materials to be included in the bonded web.

The power P_A, as defined below, required to achieve sufficient bonding between the web materials fed through the nip between the rollers is determined by the melting temperature of each individual web material, the total thickness of the bonded web, the pattern of the pattern roller, the force F in the roller nip, the line speed VL and the equivalent roller radius R_e. In some cases, the required power PA or the process in which the apparatus is used may require a relatively high line speed V_L. At the same time, the materials in the bonded web may require a relatively low bonding pressure value, thus limiting the force F. With a predetermined pattern on the pattern roller, only the equivalent roller radius R_e may be varied to achieve the required power P_A. Alternatively, the choice of web materials in the bonded web may put limitations on the maximum possible values for line speed V_L and force F ih the roller nip. As in the above case, a predetermined pattern on the pattern roller, only the equivalent roller radius R_e may be varied to achieve the required power P_A.

From the above it is obvious that the power PA can advantageously be adjusted by providing a pattern roller Rp and an anvil roller R_A providing a roller combination with a suitable equivalent roller radius R_e. In particular, the power P_A is adjusted by selecting a diameter D_A for the anvil roller RA, wherein the diameter is selected so that the equivalent roller radius R_e is equal to or less than 40 mm (see Equation 1 above). It is of course also possible to select a suitable diameter D_P for the pattern roller Rp but due to the often complex surface geometry provided on the pattern roller Rp this may not always be a practical solution. According to the invention the diameter D_P of the pattern roller Rp may be up to 160 mm if the diameter D_A of the anvil roller R_A is selected equal to the diameter D_P. If the diameter of, for instance, the anvil roller RA is decreased, then the diameter of the pattern roller Rp may be increased to any desired diameter, while maintaining the relationship of the equivalent roller radius R_e being equal to or less than 40 mm, as required by the invention. The relative diameters of the pattern roller R_P and the anvil roller R_A is selected depending on the required process parameters as outlined below.

For the purpose of providing an example, it has been assumed that the anvil roller R_A used in the bonding method has a first diameter D_A that is equal to or less than the respective second diameter D_P of the pattern roller Rp. However, the invention is not limited to this example.

The bonding method involves delivering the bonded web WT comprising selected web materials W₁, W₂ to be bonded in a predetermined pattern through the nip region 11 between the pattern roller R_P and the anvil roller R_A to form a bonded web W_B. The web materials to be bonded include a first web Wi of a selected material and at least a second web W₂ of a selected material. According to the invention, the diameter D_A of the anvil roller R_A may be selected to be equal to or less that the diameter D_P of the pattern roller R_P. Figure 1 shows an arrangement where the diameter D_A is less than the diameter D_P. In cases where diameter D_A of the anvil roller R_A is less than 50 mm it may require support from at least one support roller in order to be able to withstand the sometimes relatively large forces F in the roller nip 11. Figure 2 schematically indicates an arrangement 10 according to the invention, where a bonded web W_τ having one or more selected bonding web materials is passed through a nip region 11 between where the anvil roller R_A and the pattern roller R_P at a predetermined line speed V_L. In this example, the anvil roller R_A is supported by two support rollers 12, 13.

Figure 3 shows a schematic cross-section of a pattern roller R_P which has a diameter D_P. The pattern roller has a radius r_P , or D_P/2, and is provided with a pattern schematically illustrated by means of a number of single raised patterns

31 , 32 with a common height h. The pattern roller is arranged to cooperate with an anvil roller (not shown) in order to bond the bonded web WT comprising two bonding web materials W₁, W₂ passed between the rollers at a predetermined line speed V_L. As the single pattern 31 contacts the target web W_τ it is moving at a peripheral speed V(_rP+h) and will compress the bonded web W_τ over a distance defined as a nip width b as the single pattern 31 is rotated through an angle α. The deformation rate as the bonded web is compressed between the pattern on the pattern roller and the anvil roller is proportional to the contact time t, that is the time taken for the pattern to move across the nip width. The bonding method according to the invention involves

- using an anvil roller R_A having a first diameter D_A and being rotated at a first predetermined speed;

- using a pattern roller Rp having a second diameter D_P and being rotated at a second predetermined speed, which pattern roller is provided with raised surface elements arranged in a predetermined pattern;

- using anvil and pattern rollers having an equivalent radius R_e of less than or equal to 40 mm;

- causing a compressive mechanical deformation in the nip between the anvil and pattern rollers by means of said raised surface elements to induce an internal heating of the web materials, and

- compressing the web materials at a deformation rate sufficient to cause a local temperature increase within the deformed materials of the web material between the raised surface elements and the anvil roller, causing bonding between the web materials.

According to a preferred embodiment, the anvil roller R_A used in the bonding method has first diameter D_A that is equal to or less than the respective second diameter D_P of the pattern roller R_P. The relationship between the anvil roller and the pattern roller may be selected so that the equivalent radius R_e is selected to be from 5 mm to 40 mm. The equivalent radius R_e is defined as the equivalent roller radius for a single roller on a flat surface. This radius is determined by the function shown in Equation 1 above.

The following examples describe particular configurations of the invention, and are presented to provide a more detailed understanding of the invention. The examples are not intended to limit the scope of the present invention in any way. From a complete consideration of the entire disclosure, other arrangements within the scope of the claims will be readily apparent to one skilled in the art. The examples involve joining two material webs comprising 90 g/m² spunbond polypropylene (PP), respectively, using a rotary bonding system as described in connection with Figure 2. Support rollers were only used for the example using an anvil roller diameter D_A of 20 mm.. The bonding process was carried out at an ambient temperature of 23 ⁰C. Four roller combinations were employed using anvil and pattern roller diameters as indicated in Table 1. All examples used the same pattern layout with respect to penetration depth h, pattern width Wp and coverage of pattern cp. The variables used in Table 1 have been defined above and are indicated on a schematic cross-section of a pattern roller shown in Figure 3. The calculated equivalent radius R_e has been truncated to the nearest millimetre.

Table 1

As illustrated in Figures 1 and 2, the pressure bonding apparatus included a cooperating pair of bonding rollers provided by the representatively shown pattern roller RP and an anvil roller RA. The anvil roller was substantially rotatably fixed on a base, and the pattern roller was configured to ride on the top of the anvil. A forcing mechanism exerted a force which operatively urged the pattern roller towards the anvil roller. For example, the forcing mechanism was located at each end of the axis of the pattern roller and configured to exert a desired force in the direction shown in Figure 1. The two rollers counter- rotated at surface speeds which were substantially equal to that of the two webs of target material moving forward through the bonder. The two material webs were bonded together when they passed through the nip region of the bonding system.

As configured to produce the bonded webs of the examples, the forcing mechanism included pneumatic cylinders pressurized to the selected pressure values set forth in the present disclosure. Depending upon the employed pattern of bonding elements (e.g. bonding pins) on the pattern roller, as defined in Table 1 , the pressurized cylinders would be controlled produce a desired linear pressure, or nip force value F.

Example 1

Example 1 employed an anvil roller having a diameter D_A=20 mm, and a pattern roller having a diameter D_P=150 mm. This roller combination gives an equivalent radius R_e of approximately 9 mm.

Two material webs comprising 90 g/m² spunbond polypropylene (PP) were passed through the roller nip at five different line speeds VL and the nip force F was adjusted to achieve the required power P_A=450 W/mm² for adequate joining of the web materials.

Example 2

Example 2 employed an anvil roller having a diameter DA=76 mm, and a pattern roller having a diameter DP=76 mm. This roller combination gives an equivalent radius Re of approximately 19 mm.

Two material webs comprising 90 g/m2 spunbond polypropylene (PP) were passed through the roller nip at five different line speeds VL and the nip force F was adjusted to achieve the required power PA=450 W/mm2 for adequate joining of the web materials.

Example 3

Example 3 employed an anvil roller having a diameter DA=76 mm, and a pattern roller having a diameter DP= 150 mm. This roller combination gives an equivalent radius Re of approximately 25 mm.

Two material webs comprising 90 g/m2 spunbond polypropylene (PP) were passed through the roller nip at five different line speeds VL and the nip force F was adjusted to achieve the required power PA=450 W/mm2 for adequate joining of the web materials. Example 4

Example 4 employed an anvil roller having a diameter DA=150 mm, and a pattern roller having a diameter DP= 150 mm. This roller combination gives an equivalent radius Re of approximately 38 mm.

From the above examples it can be seen that for each roller combination, the nip force is reduced with increasing line speed. The reason for this is that the compressive deformation of the web materials occurs over a shorter time period with increasing line speed V_L, as indicated by the contact time t in the above examples. Hence the deformation rate increases and the local temperature within the deformed materials of the web material between the raised surface elements and the anvil roller increases. This requires a reduction of the nip force F in order to maintain the required power P_A at a desired level, while avoiding excessive local melting and or shear of the web materials.

When operating an apparatus as described above using the method according to the invention, the process parameters can be fixed by the materials to be bonded and/or a line speed determined by a process prior to or after the roller nip of the bonding rollers. In operation, where the line speed V_L is constant and the required power P_A for bonding is known for the material combination used, it may be necessary to adjust the nip force F to avoid excessive local melting and or shear of the web materials. According to the invention, this is done by selecting a suitable value for the equivalent radius R_e. With a given value for the pattern roller diameter D_P, the anvil roller diameter D_A can be calculated. It has been found that an anvil roller with a diameter D_A up to 160 mm is preferable for the method according to the invention, where the preferred range for the equivalent radius R_e is 5-40 mm. The anvil roller diameter D_A can be selected between 20 mm and 160 mm. Pattern roller diameters can be relatively large, as discussed above, and the desired effect is increased as the anvil roller diameter D_A is decreased. By collating the measured values for each line speed for each of the Examples 1-4 above, it can be shown that the nip force F can be controlled by selecting different roller diameters to vary the equivalent radius R_e. The measured values for each line speed are shown in Tables 2-6 below.

As indicated by the Tables 2-6, an arrangement operated using the method according to the invention, where the line speed VL and the power PA maintained constant, the nip force F can be reduced by reducing the equivalent radius Re. This can be done by reducing the diameter of both the anvil roller R_A and the pattern roller R_P. However, due to the often complex surface geometry provided on the pattern roller R_P it is more practical and less expensive to reduce the diameter of the anvil roller R_A.

Table 3 - Line speed VL = 30 m/min

Equivalent radius Re mm 38 25 19

Line speed VL m/min 30 30 30 30

Line speed m/sec 0,5 0,5 0,5 0,5

Vertical speed Vz m/s 0,04 0,04 0,05 0,07

Mean vertical speed V: zm m/s 0,02 0,02 0,03 0,04

Nip force N 11280 7600 5736 2680

Linear pressure N/m 1128000 760000 573600 268000

Induced Power W 206 169 147 100

Pattern area speed An m²/s 0,005 0,005 0,005 0,005

Induced Energy J/m² 41107 33739 29309 20005

Contact time 10^"6s 91 ,3 74,9 65,1 44,4

Power W/mm² 450 450 450 450

Table 4 - Line speed VL = 60 m/min

Equivalent radius Re mm 38 25 19

Line speed VL m/min 60 60 60 60

Line speed m/sec

Vertical speed V₇ m/s 0,07 0,09 0,10 0,15

Mean vertical speed V zm m/s 0,04 0,04 0,05 0,07

Nip force N 5640 3800 2868 1340

Linear pressure N/m 564000 380000 286800 134000

Induced Power W 206 169 147 100

Pattern area speed Ap m²/s 0,01 0,01 0,01 0,01

Induced Energy J/πrf 20553 16869 14655 10002

Contact time 10^'6s 45,7 37,5 32,5 22,2

Power W/mm² 450 450 450 450

Table 5 - Line speed VL = 120 m/min

Table 6 - Line speed VL = 240 m/min

Figure 4 shows a diagram where the contact time t has been plotted over the line speed V_L. The diagram indicates that the effect of invention is particularly noticeable at relatively low line speeds of about 100 m/min. The diagram shows that choice of R_e is significant to achieve desired result at low speed. The deformation rate in the nip between the rollers is proportional to the contact time t. At line speeds V_L less than about 100 m/min, the deformation rate can be substantially doubled if the equivalent radius Re is reduced from R_e=38 to Re=8. In this example, this is achieved by maintaining the pattern roller diameter at 150 mm and reducing the anvil roller diameter from 150 mm to 20 mm.

Claims

1. A bonding method, comprising delivering a web comprising selected web materials to be bonded in a predetermined pattern through a nip region between a pattern roller and an anvil roller to form a bonded web; where the web materials to be bonded include a first web of a selected material and at least a second web of a selected material, characterized by

- using an anvil roller having a first diameter (D_A) and being rotated at a first predetermined speed;

- using a pattern roller having a second diameter (D_P) and being rotated at a second predetermined speed, which pattern roller is provided with raised surface elements arranged in a predetermined pattern;

- using anvil and pattern rollers having an equivalent radius (R_e) of less than or equal to 40 mm;

2. A bonding method according to claim 1, characterized in that the equivalent radius (R_e) is selected from 5 mm to 40 mm.

3. A bonding method according to claim 1 or2, characterized in that at least one of the first diameter (DA) and the second diameter (D_P) is equal to or less than 160 mm.

4. A bonding method according to claim 3, characterized in that at least one of the first diameter (D_A) and the second diameter (D_P) is equal to or less than 80 mm.

5. A bonding method according to claim 3 or 4, characterized in that at least one of the first diameter (DA) and the second diameter (D_P) is equal to or greater than 20 mm.

6. A bonding method according to any one of claims 1-5, characterized in that the first diameter (D_A) is equal to or less than the second diameter (D_P).

7. A bonding method according to any one of claims 1-6, characterized in using at least one support roller for supporting at least the roller having the smaller of the first diameter (D_A) and the second diameter (D_P).

8. A bonding method according to any one of claims 7, characterized in using at least one support roller for supporting a roller or rollers having a diameter equal to or less that 50 mm.

9. A bonding method according to any one of claims 1-8, characterized in feeding the web materials through the nip at a predetermined line speed and selecting the first and second speeds to match the said line speed.

10. A bonding method according to claim 9, characterized in that the predetermined line speed is less than 100 m/min.

11. A bonding method according to any one of claims 1-10, characterized i n that it involves bonding at least one layer comprising a thermoplastic material and at least one further layer, said layers being covering material intended for disposable absorbent articles such as diapers, incontinence pads, sanitary towels and panty liners.

12. A bonding apparatus comprising a pattern roller and an anvil roller arranged to feed a web comprising selected materials to be bonded in a predetermined pattern through a nip region between the pattern roller and the anvil roller to form a bonded web; where the web materials to be bonded include a first web of a selected material and at least a second web of a selected material, characterized in that the anvil roller has a first diameter (D_A) and is rotated at a first predetermined speed;

- that the pattern roller has a second diameter (Dp) and is rotated at a second predetermined speed, which pattern roller is provided with raised surface elements arranged in a predetermined pattern;

- that the anvil and pattern rollers have an equivalent radius (R_e) of less than or equal to 40 mm;

- that the anvil and pattern are arranged to cause a compressive mechanical deformation in the nip between the anvil and pattern rollers by means of said raised surface elements, in order to induce an internal heating of the web materials, and

- that the raised surface elements and the anvil roller are arranged to compress the web materials at a deformation rate sufficient to cause a local temperature increase within the deformed materials of the web material between the raised surface elements and the anvil roller to bond the web materials.

13. A bonding apparatus according to claim 12, characterized in that the equivalent radius (R_e) is selected from 5 mm to 40 mm.

14. A bonding apparatus according to claim 12 or 13, characterized in that at least one support roller is provided for supporting at least the smaller of the anvil and/or the pattern roller.

15. A bonding apparatus according to claim 14, ch a racte rized in that at least one support roller is arranged to support a roller or rollers having a diameter equal to or less that 50 mm.