Patent Application
20080156427
This disclosure relates generally to processes for bonding together substrates using materials having an improved microwave absorbing composition, and more particularly to a process for bonding substrates to form laminated structures in which microwave energy is used to facilitate the bonding process
People rely on disposable absorbent articles to make their lives easier. Disposable absorbent articles, such as adult incontinence articles and diapers, are generally manufactured by combining several textile components.
Frequently one or more of the textile components of a disposable absorbent article are adhesively bonded together. For example, adhesives have been used to bond individual layers of the absorbent article, such as the topsheet (also known as, for example, the body-side liner) and backsheet (also known as, for example, the outer cover), together. Adhesive has also been used to bond discrete pieces, such as fasteners and leg elastics, to the article. In many cases, the bonding together of these textile components forms a laminated structure in which adhesive is sandwiched between textile substrates (such as layers of polymer film and/or layers of woven or nonwoven fabrics), thereby bonding the substrates together.
The bonding of textile substrates has conventionally been accomplished through the use of ultrasonic bonding. Ultrasonic bonding is a conventional bonding technique wherein polymeric materials are exposed to a high frequency vibration which results in a heating, melting, and flowing of the polymeric materials into each other to form a mechanical and/or chemical bond. Although commonly utilized in the production of laminated absorbent articles, ultrasonic bonding can become problematic in the presence of conventional hot melt adhesive compositions. For example, during ultrasonic bonding the adhesive composition can result in bleedthrough of the adhesive through one or both of the polymeric materials. This bleedthrough can result in at least three significant problems. First, such bleedthrough can result in a discolored end product. Such discoloration, although typically not affecting product performance, is not desirable for consumers who prefer white, uncolored, clean-looking products. Second, the bleedthrough on the end product can result in a tacky product which sticks to skin upon use, which is not desirable for consumers. Third, the bleedthrough can result in an adhesive residue build-up on the ultrasonic bonding equipment and other equipment used in the manufacturing process. Such an adhesive build-up can result in the need for frequent cleaning of the machinery, which increases costs, as numerous contaminants can adhere to, and build up on, the adhesive. Additionally, the adhesive build-up on the machinery can result in the adhesive composition being deposited on absorbent products in unintended areas.
Additionally, conventional hot melt adhesive compositions exhibit viscous flow behavior with much lower softening points. These characteristics may result in the creation of a heat sink during ultrasonic bonding. When a heat sink is created, a high percentage of the ultrasonic energy of the system is used for re-melting the adhesive in the bonded area, which may lead to bleedthrough under the combination of pressure and heat. Additionally, less ultrasonic energy remains in the system to melt the thermoplastic materials and perform the ultrasonic bond between the materials. The re-melting of the adhesive is not an optimal use of ultrasonic energy as an adhesively bonded joint is typically not as strong as an ultrasonically bonded joint as the bond strength is limited to the cohesive strength of the adhesive. Also, cohesive strength may vary significantly with temperature and, in the case of absorbent care products such as diapers and incontinence devices, body heat may be sufficient to weaken the strength of the adhesive bond to the point of failure.
Based on the foregoing, there is a need for a bonding process that does not require the use of ultrasonic energy and equipment and facilitates improved adhesion of substrates to form laminated structures.
Generally, the present disclosure provides for methods of using adhesive compositions having improved microwave absorbing properties to bond substrates forming laminated structures. Specifically, the adhesive compositions utilized in the methods of the present disclosure absorb the microwave energy, thereby heating and melting into the substrate materials and bonding the substrates together, providing for an improved laminated structure; that is, a laminated structure having a stronger adhesive bond.
As such, the present disclosure is directed to a process for bonding substrates to form a laminated structure. The process comprises: applying an adhesive composition having a dielectric loss factor at 915 MHz and 25 degrees Celsius of at least about 10 to at least a first face of a first substrate; contacting the first substrate with a second substrate to form the laminated structure; moving the laminated structure through a microwave application chamber of a microwave system; and operating the microwave system to impart microwave energy to the laminated structure in the microwave application chamber to facilitate bonding of the laminated structure.
Other features of the present disclosure will be in part apparent and in part pointed out hereinafter.
Corresponding reference characters indicate corresponding parts throughout the drawings.
With reference now to the drawings and in particular to
The term “spunbond” refers to small diameter fibers which are formed by extruding molten thermoplastic material as filaments from a plurality of fine, usually circular capillaries of a spinneret with the diameter of the extruded filaments then being rapidly reduced as by, for example, in U.S. Pat. No. 4,340,563 to Appel et al., and U.S. Pat. No. 3,692,618 to Dorschner et al., U.S. Pat. No. 3,802,817 to Matsuki et al., U.S. Pat. Nos. 3,338,992 and 3,341,394 to Kinney, U.S. Pat. No. 3,502,763 to Hartman, and U.S. Pat. No. 3,542,615 to Dobo et al. Spunbond fibers are generally not tacky when they are deposited onto a collecting surface. Spunbond fibers are generally continuous and have average diameters (from a sample of at least 10) larger than 7 microns, more particularly, between about 10 and 20 microns.
The term “meltblown” refers to fibers formed by extruding a molten thermoplastic material through a plurality of fine, usually circular, die capillaries as molten threads or filaments into converging high velocity, usually hot, gas (e.g. air) streams which attenuate the filaments of molten thermoplastic material to reduce their diameter, which may be to microfiber diameter. Thereafter, the meltblown fibers are carried by the high velocity gas stream and are deposited on a collecting surface to form a web of randomly dispersed meltblown fibers. Such a process is disclosed, for example, in U.S. Pat. No. 3,849,241 to Butin et al. Meltblown fibers are microfibers which may be continuous or discontinuous, are generally smaller than 10 microns in average diameter, and are generally tacky when deposited onto a collecting surface.
Laminates of spunbond and meltblown fibers may be made, for example, by sequentially depositing onto a moving forming belt first a spunbond substrate, then a meltblown substrate and last another spunbond substrate and then bonding the layers together such as by using the methods described herein. Alternatively, the substrates may be made individually, collected in rolls, and combined in a separate bonding step using the methods described herein. Such laminates usually have a basis weight of from about 0.1 to 12 osy (6 to 400 gsm), or more particularly from about 0.75 to about 3 osy.
The bonding apparatus 21 suitably comprises an adhesive applicating device, schematically and generally indicated at 25, operable to apply the adhesive composition to at least one face 24a, 24b of a substrate 23. For example, in the embodiment illustrated in
The term “adhesive composition” as used herein refers to a substance that bonds two faces of one or more substrates together. The term “bond” refers to the joining, adhering, connecting, attaching, or the like, of two elements. Two elements will be considered to be bonded together when they are bonded directly to one another or indirectly to one another, such as when each is directly bonded to intermediate elements.
In one particularly suitable embodiment, the adhesive composition is a dye. The term “dye” as used herein refers to a substance that imparts more or less permanent color to other materials, such as to the substrate 23. Suitable dyes include, without limitation, inks, lakes (also often referred to as color lakes), pigments and other colorants. In one embodiment, the dye has a viscosity in the range of about 2 centipoises (cPs) to about 100 cPs, more suitably in the range of about 2 cPs to about 20 cPs, and even more suitably in the range of about 2 cPs to about 10 cPs.
Furthermore, the adhesive composition is of a composition that provides an enhanced absorption of microwave energy, such as by having a relatively high dielectric loss factor. For example, the adhesive composition may suitably have a dielectric loss factor a dielectric loss factor at 915 MHz and 25 degrees Celsius of at least about 10, more suitably at least about 50, and even more suitably at least about 100. For comparison purposes, the dielectric loss factor of water under the same conditions is about 1.2. In another suitable embodiment, the adhesive composition has a dielectric loss factor at 2,450 MHz and 25 degrees Celsius of at least about 25, more suitably at least about 50, and even more suitably at least about 100. Water has a dielectric loss factor of about 12 under these same conditions.
As used herein, the “dielectric loss factor” is a measure of the receptivity of a material to high-frequency energy. The measure value of ∈′ is most often referred to as the dielectric constant, while the measured value of ∈″ is denoted as the dielectric loss factor. These values can be measured directly using a Network Analyzer with a low power, external electric field (i.e., 0 dBm to +5 dBm) typically over a frequency range of 300 KHz to 3 GHz, although Network Analyzers to 20 GHz are readily available. Most commonly, dielectric loss factor is measured at a frequency of either 915 MHz or 2,450 MHz (and at room temperature, such as about 25 degrees Celsius). For example, a suitable measuring system can include an HP8720D Dielectric Probe, and a model HP8714C Network Analyzer, both available from Agilent Technologies of Brookfield, Wis., U.S.A. Substantially equivalent devices may also be employed. By definition ∈″ is always positive, and a value of less than zero is occasionally observed when ∈″ is near zero due to the measurement error of the analyzer.
As such, the adhesive composition may include additives or other materials to enhance the affinity of the adhesive composition to microwave energy. Examples of such additives and materials include, without limitation, various mixed valent oxides, such as magnetite, nickel oxide and the like; carbon, carbon black and graphite; sulfide semiconductors, such as FeS2 and CuFeS2; silicon carbide; various metal powders such as powders of aluminum, iron and the like; various hydrated salts and other salts, such as calcium chloride dihydrate; diatomaceous earth; aliphatic polyesters (e.g., polybutylene succinate and poly(butylene succinate-co-adipate), polymers and copolymers of polylactic acid and polyethylene glycol polymers; various hygroscopic or water absorbing materials or more generally polymers or copolymers with many sites of —OH groups.
Examples of other suitable inorganic microwave absorbers include, without limitation, aluminum hydroxide, zinc oxide, barium titanate. Examples of other suitable organic microwave absorbers include, without limitation, polymers containing ester, aldehyde ketone, isocyanate, phenol, nitrile, carboxyl, vinylidene chloride, ethylene oxide, methylene oxide, opoxy, amine groups, polypyrroles, polyanilines, polyalkylthiophenes. Mixtures of the above are also suitable for use in the adhesive composition to be applied to the substrate. The selective additive or material may be ionic or dipolar, such that the applied energy field can activate the molecule. Non-limiting examples of suitable adhesive compositions that have the desired dielectric loss factor are available from Yuhan-Kimberly, South Korea under the designations: NanoColorant Cyan 220 ml (67581-11005579); NanoColorant Magenta 220 ml (67582-11005580); NanoColorant Yellow 220 ml (67583-11005581); NanoColorant Black 220 ml (67584-11005582); NanoColorant Red 220 ml (67587-11005585); NanoColorant Orange 220 ml (67588-11005586); NanoColorant Gray 220 ml (67591-11005589); and NanoColorant Violet 220 ml (67626-1006045).
The adhesive applicating device 25 according to one embodiment may comprise any suitable device used for applying adhesive composition to a substrate 23 for use in a laminated structure other than by saturating the entire substrate (e.g., by immersing the substrate in a bath of adhesive solution to saturate the substrate), whether the adhesive composition is pre-metered (e.g., in which little or no excess adhesive composition is applied to the substrate upon initial application of the adhesive composition) or post-metered (i.e., an excess amount of adhesive composition is applied to the substrate and subsequently removed). It is understood that the adhesive composition itself may be applied to the substrate 23 or the adhesive composition may be used in an adhesive solution that is applied to the substrate.
Examples of suitable pre-metered adhesive applicating devices 25 include, without limitation, devices for carrying out the following known applicating techniques:
Slot die: The adhesive composition is metered through a slot in a printing head directly onto the substrate 23.
Direct gravure: The adhesive composition is in small cells in a gravure roll. The substrate 23 comes into direct contact with the gravure roll and the adhesive composition in the cells is transferred onto the substrate.
Offset gravure with reverse roll transfer: Similar to the direct gravure technique except the gravure roll transfers the adhesive composition to a second roll. This second roll then comes into contact with the substrate 23 to transfer adhesive composition onto the substrate.
Curtain coating: This is a coating head with multiple slots in it. Adhesive composition is metered through these slots and drops a given distance down onto the substrate 23.
Slide (Cascade) coating: A technique similar to curtain coating except the multiple layers of adhesive composition come into direct contact with the substrate 23 upon exiting the coating head. There is no open gap between the coating head and the substrate 23.
Forward and reverse roll coating (also known as transfer roll coating): This consists of a stack of rolls which transfers the adhesive composition from one roll to the next for metering purposes. The final roll comes into contact with the substrate 23. The moving direction of the substrate 23 and the rotation of the final roll determine whether the process is a forward process or a reverse process.
Extrusion coating: This technique is similar to the slot die technique except that the adhesive composition is a solid at room temperature. The adhesive composition is heated to melting temperature in the print head and metered as a liquid through the slot directly onto the substrate 23. Upon cooling, the adhesive composition becomes a solid again.
Rotary screen: The adhesive composition is pumped into a roll which has a screen surface. A blade inside the roll forces the adhesive composition out through the screen for transfer onto the substrate.
Spray nozzle application: The adhesive composition is forced through a spray nozzle directly onto the substrate 23. The desired amount (pre-metered) of adhesive composition can be applied, or the substrate 23 may be saturated by the spraying nozzle and then the excess adhesive composition can be squeezed out (post-metered) by passing the substrate through a nip roller.
Flexographic printing: The adhesive composition is transferred onto a raised patterned surface of a roll. This patterned roll then contacts the substrate 23 to transfer the adhesive composition onto the substrate.
Digital textile printing: The adhesive composition is loaded in an ink jet cartridge and jetted onto the substrate 23 as the substrate passes under the ink jet head.
Examples of suitable post-metering adhesive applicating devices for applying the adhesive composition to the substrate 23 include without limitation devices that operate according to the following known applicating techniques:
Rod coating: The adhesive composition is applied to the surface of the substrate 23 and excess adhesive composition is removed by a rod. A Mayer rod is the prevalent device for metering off the excess adhesive composition.
Air knife coating: The adhesive composition is applied to the surface of the substrate 23 and excess adhesive composition is removed by blowing it off using a stream of high pressure air.
Knife coating: The adhesive composition is applied to the surface of the substrate 23 and excess adhesive composition is removed by a head in the form of a knife.
Blade coating: The adhesive composition is applied to the surface of the substrate 23 and excess adhesive composition is removed by a head in the form of a flat blade.
Spin coating: The substrate 23 is rotated at high speed and excess adhesive composition applied to the rotating substrate spins off the surface of the substrate.
Fountain coating: The adhesive composition is applied to the substrate 23 by a flooded fountain head and excess adhesive composition is removed by a blade.
Brush application: The adhesive composition is applied to the substrate 23 by a brush and excess adhesive composition is regulated by the movement of the brush across the surface of the substrate.
As the substrate 23 passes the adhesive applicating device 25, adhesive composition is applied to the one face 24a of the substrate 23. Typically, from about 5 grams/square meter (g/m2) to about 100 g/m2 adhesive composition is applied to the substrate. More suitably, from about 10 g/m2 to about 40 g/m2 adhesive composition is applied to the substrate.
Once the adhesive composition is applied to one face of the substrate, the substrate 23 is contacted with a second substrate 108 to form a laminated structure 106. In a further embodiment, the adhesive composition is applied to one face of the first substrate and one face of the second substrate prior to contacting the first and second substrates to form the laminated structure.
Typically, the first substrate and second substrate are contacted and then pushed through a pair of rollers to apply pressure to aid in adhering the substrates together to form the laminated structure. Typically, the first substrate and second substrate are pushed through a pair of rollers which can apply from about 0.1 pounds/linear inch to about 10 pounds/linear inch of pressure to ensure sufficient adhering of the substrates.
With reference now back to
The microwave system 101, with reference to
In a particular embodiment, illustrated in
The application chamber 107 in one particularly suitable embodiment is a tuned chamber within which the microwave energy can produce an operative standing wave. For example, the application chamber 107 may be configured to be a resonant chamber. Examples of suitable arrangements for a resonant application chamber 107 are described in U.S. Pat. No. 5,536,921 entitled SYSTEM FOR APPLYING MICROWAVE ENERGY IN SHEET-LIKE MATERIAL by Hedrick et al., issued Jul. 16, 1996; and in U.S. Pat. No. 5,916,203 entitled COMPOSITE MATERIAL WITH ELASTICIZED PORTIONS AND A METHOD OF MAKING THE SAME by Brandon et al, issued Jun. 29, 1999. The entire disclosures of these documents are incorporated herein by reference in a manner that is consistent herewith.
In another embodiment, the effectiveness of the application chamber 107 can be determined by measuring the power that is reflected back from the impedance load provided by the combination of the application chamber 107 and the target material (e.g. the laminated structure 106) in the application chamber. In a particular aspect, the application chamber 107 may be configured to provide a reflected power which is not more than a maximum of about 50% of the power that is delivered to the impedance load. The reflected power can alternatively be not more than about 20% of the delivered power, and can optionally be not more than about 10% of the delivered power. In other embodiments, however, the reflected power may be substantially zero. Alternatively, the reflected power may be about 1%, or less, of the delivered power, and can optionally be about 5%, or less, of the delivered power. If the reflected power is too high, inadequate levels of energy are being absorbed by the laminated structure 106 and the power being directed into the laminated structure is being inefficiently utilized.
The application chamber 107 may also be configured to provide a Q-factor of at least a minimum of about 200. The Q-factor can alternatively be at least about 5,000, and can optionally be at least about 10,000. In other embodiments, the Q-factor can be up to about 20,000, or more. If the Q-factor is too low, inadequate electrical field strengths are provided to the laminated structure. The Q-factor can be determined by the following formula (which may be found in the book entitled Industrial Microwave Heating by R. C. Metaxas and R. J. Meredith, published by Peter Peregrinus, Limited, located in London, England, copyright 1983, reprinted 1993):
Q-factor=fo/Δf
where: fo=intended resonant frequency (typically the frequency produced by the high-frequency generator), and
Δf=frequency separation between the half-power points.
In determining the Q-factor, the power absorbed by the laminated structure 106 is deemed to be the power delivered into the application chamber 107 to the laminated structure, minus the reflected power returned from the application chamber. The peak-power is the power absorbed by the laminated structure 106 when the power is provided at the intended resonant frequency, fo. The half-power points are the frequencies at which the power absorbed by the laminated structure 106 falls to one-half of the peak-power.
For example, a suitable measuring system can include an HP8720D Dielectric Probe, and a model HP8714C Network Analyzer, both available from Agilent Technologies, a business having offices located at Brookfield, Wis., U.S.A. A suitable procedure for determining the Q-factor is described in the User's Manual dated 1998, part number 08712-90056. Substantially equivalent devices and procedures may also be employed.
In another aspect, the application chamber 107 may be configured for selective tuning to operatively “match” the load impedance produced by the presence of the target material (e.g. the laminated structure 106) in the application chamber. The tuning of the application chamber 107 can, for example, be provided by any of the techniques that are useful for “tuning” microwave devices. Such techniques can include configuring the application chamber 107 to have a selectively variable geometry, changing the size and/or shape of a wave-guide aperture, employing adjustable impedance components (e.g. stub tuners), employing a split-shell movement of the application chamber, employing a variable frequency energy source that can be adjusted to change the frequency of the energy delivered to the application chamber, or employing like techniques, as well as employing combinations thereof. The variable geometry of the application chamber 107 can, for example, be provided by a selected moving of either or both of the end walls 128 to adjust the distance therebetween.
As representatively shown in
With reference to
In the embodiment illustrated in
To tune the application chamber 107, the appointed tuning components are adjusted and varied in a conventional, iterative manner to maximize the power into the load (e.g. into the laminated structure), and to minimize the reflected power. Accordingly, the tuning components can be systematically varied to maximize the power into the laminated structure 106 and minimize the reflected power. For example, the reflected power can be detected with a conventional power sensor, and can be displayed on a conventional power meter. The reflected power may, for example, be detected at the location of an isolator. The isolator is a conventional, commercially available device which is employed to protect a magnetron from reflected energy. Typically, the isolator is placed between the magnetron and the wave-guide 105. Suitable power sensors and power meters are available from commercial vendors. For example, a suitable power sensor can be provided by a HP E4412 CW power sensor which is available from Agilent Technologies of Brookfield, Wis., U.S.A. A suitable power meter can be provided by a HP E4419B power meter, also available from Agilent Technologies.
In the various configurations of the application chamber 107, a properly sized aperture plate 130 and a properly sized aperture 132 can help reduce the amount of variable tuning adjustments needed to accommodate a continuous product. The variable impedance device (e.g. stub tuner 134) can also help to reduce the amount of variable tuning adjustments needed to accommodate the processing of a continuous laminated structure 106. The variable-position end walls 128 or end caps can allow for easier adjustments to accommodate a varying load. The split-housing 126a, 126b (e.g., as illustrated in
In another embodiment, illustrated in
As one example of the size of the application chamber 107, throughout the various embodiments the chamber may suitably have a machine-directional (indicated by the direction arrow in the various embodiments) length (e.g., from the entrance 102 to the exit 104, along which the web is exposed to the microwave energy in the chamber) of at least about 20 cm. In other aspects, the chamber 107 length can be up to a maximum of about 800 cm, or more. The chamber 107 length can alternatively be up to about 400 cm, and can optionally be up to about 200 cm.
Where the microwave system 101 employs two or more application chambers 107 arranged in series, the total sum of the machine-directional lengths provided by the plurality of chambers may be at least about 40 cm. In other aspects, the total of the chamber 107 lengths can be up to a maximum of about 3000 cm, or more. The total of the chamber 107 lengths can alternatively be up to about 2000 cm, and can optionally be up to about 1000 cm.
The total residence time within the application chamber 107 or chambers can provide a distinctively efficient dwell time. The term “dwell time” in reference to the microwave system 101 refers to the amount of time that a particular portion of the laminated structure 106 spends within the application chamber 107, e.g., in moving from the entrance opening 102 to the exit opening 104 of the chamber. In a particular aspect, the dwell time is suitably at least about 0.0002 sec. The dwell time can alternatively be at least about 0.005 sec, and can optionally be at least about 0.01 sec. In other embodiments the dwell time can be up to a maximum of about 3 sec, more suitably up to about 2 sec, and optionally up to about 1.5 sec. In one particularly preferred embodiment, the application chamber provides a dwell time of the laminated structure within the chamber of a range of from about 0.01 seconds to about 3 seconds.
In operation, after the laminated structure 106 is formed, the laminated structure is moved (e.g., drawn, in the illustrated embodiment) through the application chamber 107 of the microwave system 101. The microwave system 101 is operated to direct microwave energy into the application chamber 107 for melting of the adhesive composition (e.g., which in one embodiment suitably has an affinity for, or couples with, the microwave energy). The adhesive composition is thus heated rapidly, thereby substantially speeding up the rate at which at the adhesive composition melts and flows into the first and second substrates, thereby binding the first and second substrates together to form the laminated structure (e.g., as opposed to conventional heating methods such as ultrasonic bonding).
When introducing elements of the present invention or preferred embodiments thereof, the articles “a”, “an”, “the”, and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including”, and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.
As various changes could be made in the above constructions and methods without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.
