Patent Application
20120043041
The present invention is related to continuous papermaking machines. More particularly, the present invention relates to papermaking belts suitable for making paper products.
Disposable products such as facial tissue, sanitary tissue, paper towels, and the like are typically made from one or more webs of paper. If the products are to perform their intended tasks, the paper webs from which they are formed must exhibit certain physical characteristics. Among the more important of these characteristics are strength, softness, and absorbency. Strength is the ability of a paper web to retain its physical integrity during use. Softness is the pleasing tactile sensation the user perceives as the user crumples the paper in his or her hand and contacts various portions of his or her anatomy with the paper web. Softness generally increases as the paper web stiffness decreases. Absorbency is the characteristic of the paper web which allows it to take up and retain fluids. Typically, the softness and/or absorbency of a paper web is increased at the expense of the strength of the paper web. Accordingly, papermaking methods have been developed in an attempt to provide soft and absorbent paper webs having desirable strength characteristics.
Processes for the manufacture of paper products generally involve the preparation of aqueous slurry of cellulosic fibers and subsequent removal of water from the slurry while contemporaneously rearranging the fibers to form an embryonic web. Various types of machinery can be employed to assist in the dewatering process. A typical manufacturing process employs the aforementioned Fourdrinier wire papermaking machine where a paper slurry is fed onto a surface of a traveling endless wire where the initial dewatering occurs. In a conventional wet press process, the fibers are transferred directly to a capillary de-watering belt where additional de-watering occurs. In a structured web process, the fibrous web is subsequently transferred to a papermaking belt where rearrangement of the fibers is carried out.
A preferred papennaking belt in a structured process has a foraminous woven member surrounded by a hardened photosensitive resin framework. The resin framework can be provided with a plurality of discrete, isolated channels known as deflection conduits. Such a papermaking belt can be termed a deflection member because the papermaking fibers deflected into the conduits become rearranged upon the application of a differential fluid pressure. The utilization of the belt in the papermaking process provides the possibility of creating paper having certain desired characteristics of strength, absorption, and softness. An exemplary papermaking belt is disclosed in U.S. Pat. No. 4,529,480.
Deflection conduits can provide a means for producing a Z-direction fiber orientation by enabling the fibers to deflect along the periphery of the deflection conduits as water is removed from the aqueous slurry of cellulosic fibers. The total fiber deflection is dependent on the size and shape of the deflection conduits relative to the fiber length. Large conduits allow smaller fibers to accumulate in the bottom of the conduit which in turn limits the deflection of subsequent fibers depositing therein. Conversely, small conduits allow large fibers to bridge across the conduit opening with minimal fiber deflection. Deflection conduits defined by a periphery forming sharp corners or small radii increase the potential for fiber bridging which minimizes fiber deflection. Exemplary conduit shapes and their effect on fiber bridging is described in U.S. Pat. No. 5,679,222.
As the cellulosic fibrous web is formed, the fibers are predominantly oriented in the X-Y plane of the web thereby providing negligible Z-direction structural rigidity. In a wet press process, as the fibers oriented in the X-Y plane are compacted by mechanical pressure, the fibers are pressed together increasing the density of the paper web while decreasing the thickness. In contrast, in a structured process, the orientation of fibers in the Z-direction of the web enhances the web's Z-direction structural rigidity and its corresponding resistance to mechanical pressure. Accordingly, maximizing fiber orientation in the Z-direction maximizes caliper.
A paper produced according to a structured web process can be characterized by having two physically distinct regions distributed across its surfaces. One region is a continuous network region which has a relatively high density and high intrinsic strength. The other region is one which is comprised of a plurality of domes which are completely encircled by the network region. The domes in the latter region have relatively low densities and relatively low intrinsic strength compared to the network region.
The domes are produced as fibers fill the deflection conduits of the papermaking belt during the papennaking process. The deflection conduits prevent the fibers deposited therein from being compacted as the paper web is compressed during a drying process. As a result, the domes are thicker having a lower density and intrinsic strength compared to the compacted regions of the web. Consequently, the caliper of the paper web is limited by the intrinsic strength of the domes. An exemplary formed paper is described in U.S. Pat. No. 4,637,859.
After the initial formation of the web, which later becomes the cellulosic fibrous structure, the papermaking machine transports the web to the dry end of the machine. In the dry end of a conventional machine, a press felt compacts the web into a single region of cellulosic fibrous structure having uniform density and basis weight prior to final drying. The final drying can be accomplished by a heated drum, such as a Yankee drying drum, or by a conventional de-watering press. Through air drying can yield significant improvements in consumer products. In a through-air-drying process, the formed web is transferred to an air pervious through-air-drying belt. This “wet transfer” typically occurs at a pick-up shoe, at which point the web may be first molded to the topography of the through air drying belt. In other words, during the drying process, the embryonic web takes on a specific pattern or shape caused by the arrangement and deflection of cellulosic fibers. A through air drying process can yield a structured paper having regions of different densities. This type of paper has been used in commercially successful products, such as Bounty® paper towels and Charmin® bath tissue. Traditional conventional felt drying does not produce a structured paper having these advantages. However, it would be desirable to produce a structured paper using conventional drying at speeds equivalent to, or greater than, a through air dried process.
Once the drying phase of the papermaking process is finished, the arrangement and deflection of fibers is complete. However, depending on the type of the finished product, paper may go through additional processes such as calendering, softener application, and converting. These processes tend to compact the dome regions of the paper and reduce the overall thickness. Thus, producing high caliper finished paper products having two physically distinct regions requires forming cellulosic fibrous structures in the domes having a resistance to mechanical pressure.
It would be advantageous to provide a wet pressed paper web having increased strength and wicking ability for a given level of sheet flexibility. It would be also be advantageous to provide a non-embossed patterned paper web having a relatively high density continuous network, a plurality of relatively low density domes dispersed throughout the continuous network, and a reduced thickness transition region at least partially encircling each of the low density domes.
A first embodiment of the present disclosure provides for a papermaking belt having an embryonic-web-contacting surface for carrying an embryonic web of paper fibers and a non-embryonic-web-contacting surface opposite the embryonic-web-contacting surface. The papermaking belt comprises a reinforcing structure having a patterned framework disposed thereon. The patterned framework has a continuous network region and a plurality of discrete deflection conduits. The deflection conduits are isolated from one another by the continuous network region. The continuous network region also comprises a pattern formed therein, the pattern having a plurality of tessellating unit cells. Each cell of the plurality of unit cells comprises a center, at least two continuous land areas extending in at least two directions from the center where each deflection conduit is surrounded by a portion of at least one of the continuous land areas. At least one of the continuous land areas at least bifurcates to form a continuous land area portion having a first width before the bifurcation and at least two continuous land area portions having a second width after the bifurcation. Each of the at least two continuous land area portions has a second width in continuous communication with the continuous land area portion having the first width. Each of the at least two continuous land area portions are disposed at an angle (θ) relative to each other ranging from about 1 degree to about 180 degrees.
Another embodiment of the present disclosure provides for a papermaking belt having an embryonic-web-contacting surface for carrying an embryonic web of paper fibers and a non-embryonic-web-contacting surface opposite the embryonic-web-contacting surface. The papermaking belt has a reinforcing structure having a patterned framework disposed thereon. The patterned framework has a continuous network region and a plurality of discrete deflection conduits. The deflection conduits are isolated from one another by the continuous network region. The continuous network region has a pattern formed therein, the pattern having a plurality of tessellating unit cells. Each cell of the plurality of unit cells comprises a center and at least two continuous land areas extending in at least two directions from the center. Each deflection conduit is surrounded by a portion of at least one of the continuous land areas. At least one of the continuous land areas at least bifurcates to form a continuous land area portion having a first width before the bifurcation and at least two continuous land area portions. A first of the at least two continuous land area portions has a second width and a second of the at least two continuous land area portions has a third width after the bifurcation. Each of the at least two continuous land area portions are in continuous communication with the continuous land area portion having the first width. Each of the at least two continuous land area portions are disposed at an angle (θ) relative to each other ranging from about 1 degree to about 180 degrees.
Still another embodiment of the present disclosure provides for a papermaking belt having an embryonic-web-contacting surface for carrying an embryonic web of paper fibers and a non-embryonic-web-contacting surface opposite the embryonic-web-contacting surface. The papermaking belt comprises a reinforcing structure having a patterned framework disposed thereon. The patterned framework has a continuous deflection conduit region and a plurality of discrete land areas. The discrete land areas are isolated from one another by the continuous deflection conduit region. The continuous deflection conduit region comprises a pattern formed therein. The pattern comprises a plurality of tessellating unit cells. Each cell of the plurality of tessellating unit cells comprises a center and at least two continuous pillow areas extending in at least two directions from the center. Each discrete land area is surrounded by a portion of at least one of the continuous deflection conduit region. At least one of the continuous deflection conduit region at least bifurcates to form a continuous deflection conduit portion having a first width before the bifurcation and at least two continuous deflection conduit portions having a second width after the bifurcation. Each of the at least two continuous deflection conduit portions having the second width are in continuous communication with the continuous deflection conduit portion having the first width. Each of the at least two continuous land area portions are disposed at an angle (θ) relative to each other ranging from about 1 degree to about 180 degrees.
According to one embodiment of the present invention, an embryonic web 120 of papermaking fibers is formed from an aqueous dispersion of papermaking fibers on a foraminous forming member 11. The embryonic web 120 is then transferred to a foraminous imprinting member 219 having a first web contacting face 220 comprising a web imprinting surface and a deflection conduit portion. A portion of the papermaking fibers in the embryonic web 120 are deflected into deflection conduit portion of the foraminous imprinting member 219 without densifying the web, thereby forming an intermediate web 120A.
The intermediate web 120A is carried on the foraminous imprinting member 219 from the foraminous forming member 11 to a compression nip 300 formed by opposed compression surfaces on first and second nip rolls 322 and 362. A first dewatering felt 320 is positioned adjacent the intermediate web 120A, and a second dewatering felt 360 is positioned adjacent the foraminous imprinting member 219. The intermediate web 120A and the foraminous imprinting member 219 are then pressed between the first and second dewatering felts 320 and 360 in the compression nip 300 to further deflect a portion of the papermaking fibers into the deflection conduit portion of the imprinting member 219; to densify, a portion of the intermediate web 120A associated with the web imprinting surface; and to further dewater the web by removing water from both sides of the web, thereby forming a molded web 120B which is relatively dryer than the intermediate web 120A.
The molded web 120B is carried from the compression nip 300 on the foraminous imprinting member 219. The molded web 120B can be pre-dried in a through air dryer 400 by directing heated air to pass first through the molded web, and then through the foraminous imprinting member 219, thereby further drying the molded web 120B. The web imprinting surface of the foraminous imprinting member 219 can then be impressed into the molded web 120B such as at a nip formed between a roll 209 and a dryer drum 510, thereby forming an imprinted web 120C. Impressing the web imprinting surface into the molded web can further densify the portions of the web associated with the web imprinting surface. The imprinted web 120C can then be dried on the dryer drum 510 and creped from the dryer drum by a doctor blade 524.
Examining the process steps according to the present invention in more detail, a first step in practicing the present invention is providing an aqueous dispersion of papermaking fibers derived from wood pulp to form the embryonic web 120. The papermaking fibers utilized for the present invention will normally include fibers derived from wood pulp. Other cellulosic fibrous pulp fibers, such as cotton linters, bagasse, etc., can be utilized and are intended to be within the scope of this invention. Synthetic fibers, such as rayon, polyethylene, polyester, and polypropylene fibers, may also be utilized in combination with natural cellulosic fibers. One exemplary polyethylene fiber which may be utilized is Pulpex™, available from Hercules, Inc. (Wilmington, Del.). Applicable wood pulps include chemical pulps, such as Kraft, sulfite, and sulfate pulps, as well as mechanical pulps including, for example, groundwood, thermomechanical pulp and chemically modified thermomechanical pulp. Pulps derived from both deciduous trees (hereinafter, also referred to as “hardwood”) and coniferous trees (hereinafter, also referred to as “softwood”) may be utilized. Also applicable to the present invention are fibers derived from recycled paper, which may contain any or all of the above categories as well as other non-fibrous materials such as fillers and adhesives used to facilitate the original papermaking.
In addition to papermaking fibers, the papermaking furnish used to make paper product structures may have other components or materials added thereto as may be or later become known in the art. The types of additives desirable will be dependent upon the particular end use of the paper to product sheet contemplated. For example, in products such as toilet paper, paper towels, facial tissues and other similar products, high wet strength is a desirable attribute. Thus, it is often desirable to add to the papermaking furnish chemical substances known in the art as “wet strength” resins.
A general dissertation on the types of wet strength resins utilized in the paper art can be found in TAPPI monograph series No. 29, Wet Strength in Paper and Paperboard, Technical Association of the Pulp and Paper Industry (New York, 1965). The most useful wet strength resins have generally been cationic in character. Polyamide-epichlorohydrin resins are cationic wet strength resins which have been found to be of particular utility. Suitable types of such resins are described in U.S. Pat. Nos. 3,700,623 and 3,772,076. One commercial source of useful polyamide-epichlorohydrin resins is Hercules, Inc. of Wilmington, Del., which markets such resin under the mark Kymeme™ 557H.
Polyacrylamide resins have also been found to be of utility as wet strength resins. These resins are described in U.S. Pat. Nos. 3,556,932 and 3,556,933. One commercial source of polyacrylamide resins is American Cyanamid Co. of Stanford, Conn., which markets one such resin under the mark Parez™ 631 NC.
Still other water-soluble cationic resins finding utility in this invention are urea formaldehyde and melamine formaldehyde resins. The more common functional groups of these polyfunctional resins are nitrogen containing groups such as amino groups and methylol groups attached to nitrogen. Polyethylenimine type resins may also find utility in the present invention. In addition, temporary wet strength resins such as Caldas 10 (manufactured by Japan Carlit) and CoBond 1000 (manufactured by National Starch and Chemical Company) may be used in the present invention. It is to be understood that the addition of chemical compounds such as the wet strength and temporary wet strength resins discussed above to the pulp furnish is optional and is not necessary for the practice of the present development.
The embryonic web 120 is preferably prepared from an aqueous dispersion of the papermaking fibers, though dispersions of the fibers in liquids other than water can be used. The fibers are dispersed in water to form an aqueous dispersion having a consistency of from about 0.1 to about 0.3 percent. The percent consistency of a dispersion, slurry, web, or other system is defined as 100 times the quotient obtained when the weight of dry fiber in the system under discussion is divided by the total weight of the system. Fiber weight is always expressed on the basis of bone dry fibers.
A second step in the practice of the present invention is forming the embryonic web 120 of papermaking fibers. Referring again to
The forming member 11 is supported by a breast roll 12 and plurality of return rolls, of which only two return rolls 13 and 14 are shown in
The embryonic web 120 can be formed in a continuous papermaking process, as shown in
A third step in the practice of the present invention comprises transferring the embryonic web 120 from the foraminous forming member 11 to the foraminous imprinting member 219, to position the second web face 124 on the first web contacting face 220 of the foraminous imprinting member 219. Although the preferred embodiment of the foraminous imprinting member 219 of the present invention is in the form of an endless belt, it can be incorporated into numerous other forms which include, for instance, stationary plates for use in making hand sheets or rotating drums for use with other types of continuous process. Regardless of the physical form which the foraminous imprinting member 219 takes for the execution of the claimed invention, it is generally provided with the physical characteristics detailed infra.
A fourth step in the practice of the present invention comprises deflecting a portion of the papermaking fibers in the embryonic web 120 into the deflection conduit portion 230 of web contacting face 220 of the foraminous imprinting member 219, and removing water from the embryonic web 120 through the deflection conduit portion 230 of the foraminous imprinting member 219 to form an intermediate web 120A of the papermaking fibers. The embryonic web 120 preferably has a consistency of between about 10 and about 20 percent at the point of transfer to facilitate deflection of the papermaking fibers into the deflection conduit portion 230 of the foraminous imprinting member 219.
The steps of transferring the embryonic web 120 to the imprinting member 219 and deflecting a portion of the papermaking fibers in the web 120 into the deflection conduit portion 230 of the foraminous imprinting member 219 can be provided, at least in part, by applying a differential fluid pressure to the embryonic web 120. For instance, the embryonic web 120 can be vacuum transferred from the forming member 11 to the imprinting member 219, such as by a vacuum box 126 shown in
A fifth step in the practice of the present invention comprises pressing the wet intermediate web 120A in the compression nip 300 to form the molded web 120B. Referring again to
The nip rolls 322 and 362 can have generally smooth opposed compression surfaces, or alternatively, the rolls 322 and 362 can be grooved. In an alternative embodiment (not shown) the nip rolls can comprise vacuum rolls having perforated surfaces for facilitating water removal from the intermediate web 120A. The rolls 322 and 362 can have rubber coated opposed compression surfaces, or alternatively, a rubber belt can be disposed intermediate each nip roll and its associated dewatering felt. The nip rolls 322 and 362 can comprise solid rolls having a smooth, bonehard rubber cover, or alternatively, one or both of the rolls 322 and 362 can comprise a grooved roll having a bonehard rubber cover.
The term “dewatering felt” as used herein refers to a member that is absorbent, compressible, and flexible so that it is deformable to follow the contour of the non-monoplanar intermediate web 120A on the imprinting member 219, and capable of receiving and containing water pressed from an intermediate web 120A. The dewatering felts 320 and 360 can be formed of natural materials, synthetic materials, or combinations thereof.
A preferred but non-limiting dewatering felt 320, 360 can have a thickness of between about 2 mm to about 5 mm, a basis weight of about 800 to about 2000 grams per square meter, an average density (basis weight divided by thickness) of between about 0.35 gram per cubic centimeter and about 0.45 gram per cubic centimeter, and an air permeability of between about 15 and about 110 cubic feet per minute per square foot, at a pressure differential across the dewatering felt thickness of 0.12 kPa (0.5 inch of water). The dewatering felt 320 preferably has first surface 325 having a relatively high density, relatively small pore size, and a second surface 327 having a relatively low density, relatively large pore size. Likewise, the dewatering felt 360 preferably has a first surface 365 having a relatively high density, relatively small pore size, and a second surface 367 having a relatively low density, relatively large pore size. The relatively high density and relatively small pore size of the first felt surfaces 325, 365 promote rapid acquisition of the water pressed from the web in the nip 300. The relatively low density and relatively large pore size of the second felt surfaces 327, 367 provide space within the dewatering felts for storing water pressed from the web in the nip 300. Suitable dewatering felts 320 and 360 are commercially available as SUPERFINE DURAMESH, style XY31620 from the Albany International Company of Albany, N.Y.
The intermediate web 120A and the web imprinting surface 222 are positioned intermediate the first and second felt layers 320 and 360 in the compression nip 300. The first felt layer 320 is positioned adjacent the first face 122 of the intermediate web 120A. The web imprinting surface 222 is positioned adjacent the second face 124 of the web 120A. The second felt layer 360 is positioned in the compression nip 300 such that the second felt layer 360 is in flow communication with the deflection conduit portion 230.
Referring again to
The molded web 120B is preferably pressed to have a consistency of at least about 30 percent at the exit of the compression nip 300. Pressing the intermediate web 120A as shown in
A sixth step in the practice of the present invention can comprise pre-drying the molded web 120B, such as with a through-air dryer 400 as shown in
Referring to
A seventh step in the practice of the present invention can comprise impressing the web imprinting surface 222 of the foraminous imprinting member 219 into the molded web 120B to form an imprinted web 120C. Impressing the web imprinting surface 222 into the molded web 120B serves to further densify, the relatively high density region 1083 of the molded web, thereby increasing the difference in density between the regions 1083 and 1084. Referring to
One of ordinary skill will recognize that the simultaneous imprinting, dewatering, and transfer operations may occur in embodiments other than those using dryer drum such as a Yankee drying drum. For example, two flat surfaces may be juxtaposed to form an elongate nip therebetween. Alternatively, two unheated rolls may be utilized. The rolls may be, for example, part of a calendar stack, or an operation which prints a functional additive onto the surface of the web. Functional additives may include: lotions, emollients, dimethicones, softeners, perfumes, menthols, combinations thereof, and the like.
The method provided by the present invention is particularly useful for making paper webs having a basis weight of between about 10 grams per square meter to about 65 grams per square meter. Such paper webs are suitable for use in the manufacture of single and multiple ply tissue and paper towel products.
The foraminous imprinting member 219 has a first web contacting face 220 and a second felt contacting face 240. The web contacting face 220 has a web imprinting surface (or land area) 222 and a deflection conduit portion 230, as shown in
In one embodiment the foraminous imprinting member 219 can comprise a fabric belt formed of woven filaments. The foraminous imprinting member 219 can comprise a woven fabric. As one of skill in the art will recognize, woven fabrics typically comprise warp and weft filaments where warp filaments are parallel to the machine direction and weft filament are parallel to the cross machine direction. The interwoven warp and well filaments form discontinuous knuckles where the filaments cross over one another in succession. These discontinuous knuckles provide discrete imprinted areas in the molded web 120B during the papermaking process. As used herein the term “long knuckles” is used to define discontinuous knuckles formed as the warp and weft filaments cross over two or more warp or weft filament, respectively. Suitable woven filament fabric belts for use as the foraminous imprinting member 219 are disclosed in U.S. Pat. Nos. 3,301,746; 3,905,863; 4,191,609; and 4,239,065.
The knuckle imprint area of the woven fabric may be enhanced by sanding the surface of the filaments at the warp and weft crossover points. Exemplary sanded woven fabrics are disclosed in U.S. Pat. Nos. 3,573,164 and 3,905,863.
The absolute void volume of a woven fabric can be determined by measuring caliper and weight of a sample of woven fabric of known area. The caliper can measured by placing the sample of woven fabric on a horizontal flat surface and confining it between the flat surface and a load foot having a horizontal loading surface, where the load foot loading surface has a circular surface area of about 3.14 square inches and applies a confining pressure of about 15 g/cm2 (0.21 psi) to the sample. The caliper is the resulting gap between the flat surface and the load foot loading surface. Such measurements can be obtained on a VIR Electronic Thickness Tester Model II available from Thwing-Albert, Philadelphia, Pa.
The density of the filaments can be determined while the density of the void spaces is assumed to be 0 gm/cc. For example, polyester (PET) filaments have a density of 1.38 g/cm3. The sample of known area is weighed, thereby yielding the mass of the test sample.
In another exemplary but non-limiting embodiment shown in
Suitable shapes for the openings 239 include, but are not limited to, circles, ovals, and polygons formed by the boundaries circumscribed by the portions that form the web imprinting surface 222 as exemplified in
Alternatively, as shown in
Referring again to
Examples of geometric shapes suitable for use with the present invention and forming the continuous network web imprinting surface 222 include fractals and constructals. Because they appear similar at all levels of magnification, fractals are often considered to be infinitely complex (in informal terms). Images of fractals suitable for use with the present invention and capable of providing the desired continuous network web imprinting surface 222 can be created using fractal-generating software. Images produced by such software are normally referred to as being fractals even if they do not have the above characteristics, such as when it is possible to zoom into a region of the fractal that does not exhibit any fractal properties. Also, these may include calculation or display artifacts which are not characteristics of true fractals. Exemplary, but non-limiting techniques for generating fractals are: 1. Escape-time fractals (also known as “orbits” fractals and are defined by a formula or recurrence relation at each point in a space, for example Mandelbrot set, Julia set, the Burning Ship fractal, the Nova fractal and the Lyapunov fractal), 2. Iterated function systems (have a fixed geometric replacement rule, for example Cantor set, Sierpinski carpet, Sierpinski gasket, Peano curve, Koch snowflake, Harter-Highway dragon curve, T-Square, Menger sponge), 3. Random fractals (Generated by stochastic rather than deterministic processes, for example, trajectories of the Brownian motion, Lévy flight, fractal landscapes and the Brownian tree), and 4. Strange attractors (Generated by iteration of a map or the solution of a system of initial-value differential equations that exhibit chaos).
An exemplary but non-limiting fractal, the Mandelbrot set, is based on the multiplication of the complex numbers. Start with a complex number z0. From z0 define z1=(z0)2+z0. Assuming that is known, zx+1 is defined to be (zx)2+zx. The points in the Mandelbrot set are all those points which stay relatively close to the point 0+0i (in the sense that they are always within some fixed distance of (0+0i) as we repeat this process. As it turns out, if zx is ever outside of the circle of radius 2 about the origin for some n, it won't be in the Mandelbrot set.
In contrast to fractal models of phenomena, constructal law is predictive and thus can be tested experimentally. Constructal theory puts forth the idea that the generation of design (configuration, pattern, geometry) in nature is a physics phenomenon that unites all animate and inanimate systems. For example, in point-area and point-volume flows, constructal theory predicts tree architectures, such flows displaying at least two regimes: one highly resistive and a less resistive one. Constructal theory can be applied at any scale: from macroscopic to microscopic systems. The constructal way of distributing any system's imperfection is to put the more resistive regime at the smallest scale of the system. The constructal law is the principle that generates the perfect form, which is the least imperfect form possible.
In order to mathematize the constructal law new properties for a thermodynamic system were defined that distinguish the thermodynamic system from a static (equilibrium, nothing flows) system, that does not have configuration. The properties of a flow system are:
The global external and internal sizes (L, V) mean that a flow system has at least two length scales L and V1/3. These form a dimensionless ratio—the svelteness Sv—which is a new global property of the flow configuration (Lorente and Bejan, 2005).
Constructal law is the statement that summarizes the common observation that flow structures that survive are those that morph (evolve) in one direction in time: toward configurations that make it easier for currents to flow. This statement refers strictly to structural changes under finite-size constraints. If the flow structures are free to change), in time they will move at constant L and constant V in the direction of progressively smaller R. Constructal law requires:
R
2
≦R
1(constant L, V)
If freedom to morph persists, then the flow structure will continue toward smaller R values. Any such change is characterized by:
dR≦0(constant L, V)
The end of this migration is the “equilibrium flow structure”, where the geometry of the flow enjoys total freedom. Equilibrium is characterized by minimal R at constant L and V. In the vicinity of the equilibrium flow structure we have:
dR=0 and d2R>0(constant L, V)
The R(V) curve generated is the edge of the cloud of possible flow architectures with the same global size L. The curve has negative slope because of the physics of flow: the resistance decreases when the flow channels open up:
The evolution of configurations in the constant-V cut (also at constant L) represents survival through increasing performance—survival of the fittest. The idea of constructal-law is that freedom to morph is good for performance.
The same time arrow can be described alternatively with reference to the constant-R cut through three-dimensional space. Flow architectures with the same global performance (R) and global size (L) evolve toward compactness and svelteness—smaller volumes dedicated to internal ducts, i.e., larger volumes reserved for the working “tissue” (the interstices). The global external and internal sizes (L, V) mean that a flow system has scales L and V1/3. These form a dimensionless ratio (svelteness, Sv) that is a property of the flow configuration. For a system with fixed global size and global performance to persist in time (to live), it must evolve in such a way that its flow structure occupies a smaller fraction of the available space. This is survival based on the maximization of the use of the available space. Survival by increasing Sv (compactness) is equivalent to survival by increasing performance.
A third equivalent statement of the constructal law becomes evident if the constant-L design is recast in constant-V design space. The contribution of the shape and orientation of the hyper-surface of non-equilibrium flow structures provides for the slope of the curve in the bottom plane (∂R/∂L)V is positive. This is because the flow resistance increases when the distance traveled by the stream increases. The flow structures of a certain performance level (R) and internal flow volume (V) morph into new flow structures that cover progressively larger territories. Again, flow configurations evolve toward greater Sv.
The geometries of the continuous network web imprinting surface 222 shown in
In the example provided in
Since Ln, rn, Ln+1, and rn+1 are typically used to describe the relationships in naturally occurring capillary-like systems having 3-dimensions, it should be readily clear to one of skill in the art the land areas of the continuous network regions of the description herein will reference a width (W) because the structures of the instant disclosure are essentially macroscopically mono-planar in the machine and cross-machine directions. It would be understood by one of skill in the art that in such a circumstance that 2r=W. It should also be understood by one of skill in the art that in order to account for design choice (e.g., linear, tapered, curvilinear, etc.) and/or deal with the nuances of manufacturing, the width (W) shown and used for the basis of the present disclosure is preferably an average width of the region. Further it should be understood by one of skill in the art that even though the exemplary representative capillary-like systems depicted herein are shown as having linear characteristics, the capillary-like systems of the present disclosure could have any shape including curvilinear, combinations of linear and curvilinear designs, and the like.
Additionally, in the example provided in
W
1
=W
2
+W
3, where W2 and W3≠0; 1
W
1
<W
2
+W
3, where W2 and W3≠0; 2
W
1
=W
2
+W
3, where W2≠W3, and where W2, W3>0; and, 3
W
1
<W
2
+W
3, where W2≠W3, and where W2, W3>0. 4
It was found advantageous that the values of L, W, and θ be selected in order to provide the best correlation between repeating tessellating unit cells. While one of skill in the art could provide any value of L, W, and θ to suit the need, it was found that L1 (pre-bifurcation) and L2, L3 (post bifurcation) could range from between about 0.005 inches to about 0.750 inches and/or about 0.010 inches to about 0.400 inches and/or about 0.020 inches to about 0.200 inches and/or about 0.03 inches to about 0.100 inches and/or about 0.05 inches to about 0.075 inches. It was also found that W1 (pre-bifurcation) and W2, W3 (post bifurcation) could range from between about 0.005 inches to about 0.200 inches and/or about 0.010 inches to about 0.100 inches and/or about 0.015 inches to about 0.075 inches and/or about 0.020 inches to about 0.050 inches. It was also found that θ could range from about 1 degree to about 180 degrees and/or from about 30 degrees to about 140 degrees and/or from about 30 degrees to about 120 degrees and/or from about 40 degrees to about 85 degrees and/or from about 45 degrees to about 75 degrees and/or from about 50 degrees to about 70 degrees.
It was surprisingly found that a web product formed by the use of a web imprinting surface 222 having a continuous network web imprinting surface 222 with a geometry exhibited by equation 2 (above) and the values of L, W, and θ described above exhibited several remarkable performance enhancements. This included a surprising increase in the observed VFS, and SST values and a surprising decrease in the observed residual water values (RW) over other commercial products tested.
Referring again to
The area of the web imprinting surface 222, as a percentage of the total area of the first web contacting surface 220, should be between about 15 percent to about 65 percent, and more preferably between about 20 percent to about 50 percent to provide a desirable ratio of the areas of the relatively high density region 1083 and the relatively low density domes 1084. The size of the openings 239 of the deflection conduits 230 in the plane of the first face 220 can be expressed in terms of effective free span. Effective free span is defined as the area of the opening 239 in the plane of the first face 220 divided by one fourth of the perimeter of the opening 239. The effective free span should be from about 0.25 to about 3.0 times the average length of the papermaking fibers used to form the embryonic web 120, and is preferably from about 0.5 to about 1.5 times the average length of the papermaking fibers. The deflection conduits 230 can have a depth which is between about 0.1 mm and about 1.0 mm.
The caliper of the woven fabric may vary, however, in order to facilitate the hydraulic connection between the molded web 120B and a dewatering felt 320, 360 the caliper of the imprinting fabric may range from about 0.011 inch (0.279 mm) to about 0.026 inch (0.660 mm).
Preferably, the continuous network web imprinting surface 222 extends outwardly (i.e., has an overburden) from the reinforcing element 243 of greater than about 0.006 inch and/or greater than about 0.010 inch and/or greater than about 0.015 inch and/or greater than about 0.020 inch and/or greater than about 0.030 inch and/or greater than about 0.050 inch. However, it may be possible to provide the continuous network web imprinting surface 222 with an overburden that is less than about 0.15 mm (0.006 inch), more preferably less than about 0.10 mm (0.004 inch) and still more preferably less than about 0.05 mm (0.002 inch), and most preferably less than about 0.1 mm (0.0004 inch). It is believed that the continuous network web imprinting surface 222 could be substantially coincident (or even coincident) with the elevation of the reinforcing element 243.
Exemplary continuous network web imprinting surfaces 222 having fractal and constructal geometries are shown in
As shown in
A paper product produced according to the apparatus and process of the present invention has at least two regions. The first region comprises an imprinted region which is imprinted against the web imprinting surface 220 of the foraminous printing member 219. The imprinted region is preferably an essentially continuous network. The relatively low density region 1084 deflected into the deflection conduit portion 230 of the imprinting member 219 provides bulk for enhancing absorbency.
It was surprisingly found that a web product formed by the use of a web imprinting surface 222 having a continuous network web imprinting surface 222 with a geometry exhibited by equation 2 (above) (and alternatively and correspondingly the web imprinting surfaces 222a of
The difference in density between the relatively high density region 1083 and the relatively low density region 1084 is provided, in part, by deflecting a portion of the embryonic web 120 into the deflection conduit portion 230 of the imprinting member 219 to provide a non-monoplanar intermediate web 120A upstream of the compression nip 300. A monoplanar web carried through the compression nip 300 would be subject to some uniform compaction, thereby increasing the minimum density in the molded web 120B. The portions of the non-monoplanar intermediate web 120A in the deflection conduit portion 230 avoid such uniform compaction, and therefore maintain a relatively low density. However, without being bound by theory, it is believed the relatively low density region 1084 and the relatively high density region 1083 may have generally equivalent basis weights. In any regard, the density of the relatively low density region 1084 and the relatively high density region 1083 can be measured according to U.S. Pat. Nos. 5,277,761 and 5,443,691.
The molded web 120B may also be foreshortened, as is known in the art. Foreshortening can be accomplished by creping the molded web 120B from a rigid surface such as a drying cylinder. A Yankee drying drum can be used for this purpose. During foreshortening, at least one foreshortening ridge can be produced in the relatively low density regions 1084 of the molded web 120B). Such at least one foreshortening ridge is spaced apart from the MD/CD plane of the molded web 120B in the Z-direction. Creping can be accomplished with a doctor blade according to U.S. Pat. No. 4,919,756. Alternatively or additionally, foreshortening may be accomplished via wet micro-contraction as taught in U.S. Pat. No. 4,440,597 and/or by fabric creping as would be known to those of skill in the art.
A pilot scale Fourdrinier papermaking machine is used in the present example. A 3% by weight aqueous slurry of northern softwood kraft (NSK) pulp is made up in a conventional re-pulper and may be diluted to a ≈0.1% consistency in a stock chest. The NSK slurry is refined gently and a 2% solution of a permanent wet strength resin (i.e. Kymene 5221 marketed by Hercules incorporated of Wilmington, Del.) is added to the NSK stock pipe at a rate of 1% by weight of the dry fibers. The adsorption of Kymene 5221 to NSK is enhanced by an in-line mixer. A 1% solution of Carboxy Methyl Cellulose (CMC) (i.e. FinnFix 700 marketed by C.P. Kelco U.S. Inc. of Atlanta, Ga.) is added after the in-line mixer at a rate of 0.2% by weight of the dry fibers to enhance the dry strength of the fibrous substrate. A 3% by weight aqueous slurry of Eucalyptus fibers is made up in a conventional re-pulper. A 1% solution of defoamer (i.e. BuBreak 4330 marketed by Buckman Labs, Memphis Tenn.) is added to the Eucalyptus stock pipe at a rate of 0.25% by weight of the dry fibers and its adsorption is enhanced by an in-line mixer.
The NSK furnish and the Eucalyptus fibers are combined in the head box and deposited onto a Fourdrinier wire homogenously to form an embryonic web. The Fourdrinier wire Dewatering occurs through the Foudrinier wire and is assisted by a deflector and vacuum boxes. The Fourdrinier wire is of a 5-shed, satin weave configuration having 84 machine-direction and 76 cross-machine-direction monofilaments per inch, respectively. The embryonic wet web is transferred from the Fourdrinier wire, at a fiber consistency of about 15% to about 25% at the point of transfer, to a to photo-polymer fabric having a fractal pattern cells, about 25 percent knuckle area and 22 mils of photo-polymer depth. The speed differential between the Fourdrinier wire and the patterned transfer/imprinting fabric is about −3% to about +3%. Further de-watering is accomplished by vacuum assisted drainage until the web has a fiber consistency of about 20% to about 30%. The patterned web is pre-dried by air blow-through to a fiber consistency of about 65% by weight. The web is then adhered to the surface of a Yankee dryer with a sprayed creping adhesive comprising 0.25% aqueous solution of Polyvinyl Alcohol (PVA). The fiber consistency is increased to an estimated 96% before the dry creping the web with a doctor blade. The doctor blade has a bevel angle of about 25 degrees and is positioned with respect to the Yankee dryer to provide an impact angle of about 81 degrees; the Yankee dryer is operated at about 600 fpm (feet per minute) (about 183 meters per minute). The dry web is formed into roll at a speed of 560 fpm (171 meters per minutes).
Two plies of the web are formed into paper towel products by embossing and laminating them together using PVA adhesive. The paper towel has about 53 g/m2 basis weight and contains 65% by weight Northern Softwood Kraft and 35% by weight Eucalyptus furnish.
The NSK furnish and the Eucalyptus fibers are prepared by a method similar to that of Example 1, combined in the head box and deposited onto a Fourdrinier wire, running at a velocity V1, homogenously to form an embryonic web.
The web is then transferred to the patterned transfer/imprinting fabric in the transfer zone without precipitating substantial densification of the web. The web is then forwarded, at a second velocity, V2, on the transfer/imprinting fabric along a looped path in contacting relation with a transfer head disposed at the transfer zone, the second velocity being from about 5% to about 40% slower than the first velocity. Since the wire speed is faster than the transfer/imprinting fabric, wet shortening of the web occurs at the transfer point. Thus, the wet web foreshortening may be about 3% to about 15%. The web is then adhered to the surface of a Yankee dryer, having a third velocity (V3) by a method similar to that of Example 1. The fiber consistency is increased to an estimated 96%, and then the web is creped from the drying cylinder with a doctor blade, the doctor blade having an impact angle of from about 90 degrees to about 130 degrees. Thereafter the dried web is reeled at a fourth velocity (V4) that is faster than the third velocity (V3) of the drying cylinder.
Two plies of the web made according to Example 1 can be combined to form a multi-ply product by embossing and/or by laminating them together using a PVA adhesive. The paper towel can have about 53 g/m2 basis weight and contains 65% by weight Northern Softwood Kraft and 35% by weight Eucalyptus furnish.
Any dimension and/or value disclosed herein is not to be understood as strictly limited to the exact numerical values recited. Instead, unless otherwise specified, each dimension and/or value is intended to mean both the recited dimension and/or value and a functionally equivalent range surrounding that dimension and/or value. For example, a dimension disclosed as “40 mm” is intended to mean “about 40 mm.”
Every document cited herein, including any cross referenced or related patent or application is hereby incorporated herein by reference in its entirety unless expressly excluded or otherwise limited. The citation of any document is not an admission that it is prior art with respect to any invention disclosed or claimed herein or that it alone, or in any combination with any other reference or references, teaches, suggests or discloses any such invention. Further, to the extent that any meaning or definition of a term in this document conflicts with any meaning or definition of the same term in a document incorporated by reference, the meaning or definition assigned to that term in this document shall govern.
While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.
