Patent Grant
8178025
The present invention relates generally to the manufacture of absorbent creped paper products including both cube embossing and substantially cross-machine direction perforate embossing. In one embodiment, the products are made from furnish incorporating at least about 15% bleached chemithermomechanical pulp (BCTMP).
Embossing is the act of mechanically working a substrate, such as a web or a cellulosic web, to cause the substrate to conform under pressure to the depths and contours of a patterned embossing roll. Generally the web is passed between a pair of embossing rolls that, under pressure, form contours within the surface of the web. During an embossing process, the roll pattern is imparted onto the web at a certain pressure and/or penetration. In perforate embossing the embossing elements are configured such that at least a portion of the web located between the embossing elements is perforated. As used herein, “perforated” refers to the existence of at least one of (1) a macro-scale through aperture in the web, (2) when a macro-scale through aperture does not exist, at least incipient tearing such as would increase the transmittivity of light through a small region of the web, or (3) a decrease the machine direction strength of a web by at least 15% for a given range of embossing depths.
Embossing is commonly used to modify the properties of a web to make a final product produced from that web more appealing to the consumer. For example, embossing a web can improve the softness, absorbency, and bulk of a final product. Embossing can also be used to impart an appealing pattern to a final product.
Embossing is carried out by passing a web between two or more embossing rolls, at least one of which carries the desired emboss pattern. Known embossing configurations include rigid-to-resilient embossing and rigid-to-rigid embossing.
In a rigid-to-resilient embossing system, a single or multi-ply substrate is passed through a nip formed between a first roll, whose substantially rigid surface contains the embossing pattern as a multiplicity of protuberances and/or depressions arranged in an aesthetically-pleasing manner, and a second roll, whose substantially resilient surface can be either smooth or also contain a multiplicity of protuberances and/or depressions that may cooperate with the rigid surfaced patterned roll. Commonly, rigid rolls are formed with a steel body which is either directly engraved upon or which can contain a hard rubber cover or other suitable rigid surface (directly coated or sleeved) upon which the embossing pattern is formed by any convenient method such as, for example, laser engraving. The resilient roll may consist of a steel core provided with a resilient surface, such as being directly covered or sleeved with a resilient material such as rubber or other suitable polymer. The resilient surface may be either smooth or engraved with a pattern. The pattern on the resilient roll may be either a mated or a non-mated pattern with respect to the pattern carried on the rigid roll.
In a rigid-to-rigid embossing process, a single-ply or multi-ply substrate is passed through a nip formed between two substantially rigid rolls. The surfaces of both rolls contain the pattern to be embossed as a multiplicity of protuberances and/or depressions arranged into an aesthetically-pleasing manner where the protuberances and/or depressions in the second roll may cooperate with those patterned in the first rigid roll. The first rigid roll may be formed, for example, with a steel body which is either directly engraved upon or which can contain a hard rubber cover or other suitable rigid surface (directly coated or sleeved) upon which the embossing pattern is engraved by any conventional method, such as laser engraving. The second rigid roll can be formed with a steel body or can contain a hard rubber cover or other suitable rigid surface (directly coated or sleeved) upon which any convenient pattern, such as a matching or mated pattern, is conventionally engraved or laser-engraved. In perforate embossing, a rigid-to-rigid embossing system is typically used; however, a rigid-resilient configuration may also be used for perforate embossing.
When substantially rectangular embossing elements have been employed in perforate embossing, the embossing elements on the embossing rolls have generally been oriented so that the long direction axis, i.e., the major axis, of the elements extend only in the machine direction. That is, the major axis of the elements is oriented to correspond to the direction of the running web being embossed. These elements are referred to as machine direction elements. As a result, the elements produce perforations which extend primarily in the machine direction and undesirably decrease the strength of the web in the cross-machine direction. This orientation improves absorbency and softness but can degrade, i.e., reduce the strength of, the web primarily in the cross-machine direction while less significantly degrading the strength of the web in the machine direction. As a result, the tensile strength of the web in the cross-machine direction is reduced relatively more, on a percentage basis, than that of the machine direction. In addition, the cross-machine direction strength of the base sheet is typically less than that of the machine direction strength. As a result, by embossing with machine direction elements only, the cross-machine direction strength is even further weakened and, accordingly, because the finished product will fail in the weakest direction, the product will be more likely to fail when stressed in the cross-machine direction.
Cross-machine direction tensile strength can be associated with consumer preference for paper toweling. In particular, consumers prefer a strong towel, of which cross-machine direction and machine direction strength are two components. Because an un-embossed base sheet is typically much stronger in the machine direction than the cross-machine direction, a process is desired which results in improved softness without sustaining excessive losses in cross-machine direction tensile strength.
The present invention addresses at least the above described problem by providing at least one embossing pattern, wherein at least a portion of the elements are oriented to provide perforating nips which are substantially in the cross-machine direction and are configured to perforate emboss (perf-emboss) the web, thereby preserving more of the cross-machine direction strength. In addition, the present invention may also provide at least two embossing rolls, where the embossing elements on at least one embossing roll are configured to impart an embossing pattern on the web, and where the embossing pattern includes elongated embosses in one or both of the machine direction and the cross-machine direction.
Additionally, in view of the rising costs of virgin fibers, the use of recycled cellulosic furnish to make towel and tissue products is often desirable, especially for facilities that produce large volumes of absorbent products. Products made from recycle furnish, however, tend to be relatively stiff, having relatively high tensile strengths and relatively low bulk leading to poor absorbency and softness properties. Moreover, these products tend to have relatively low wet/dry strength ratios. Various methods have been employed to increase the bulk and softness of products made from recycle furnish, including the use of softeners, debonders, and the like, the use of anfractuous fibers, and/or the use of new processing techniques. Many of these methods require significant capital investment and cannot be readily adapted to existing production capacity, such as conventional wet-press (CWP) paper machines with Yankee dryers.
There is disclosed in U.S. Pat. No. 5,607,551, which is incorporated herein by reference in its entirety, through-air-dried (TAD) tissues made without the use of a Yankee dryer. The typical Yankee functions of building machine direction and cross-machine direction stretch are replaced by a wet end rush transfer and the through-air-drying fabric design, respectively. According to the '551 patent, it is particularly advantageous to form the tissue with chemi-mechanically treated fibers in at least one layer. Resulting tissues are reported to have high bulk and low stiffness. Furnishes enumerated in connection with the '551 patent process include virgin softwood and hardwood as well as secondary or recycle fibers (see col. 4, lines 28-31). In the '551 patent it is further taught to incorporate high-lignin content fibers such as groundwood, thermomechanical pulp, chemimechanical pulp, and bleached chemithermomechanical pulp. Generally these pulps have lignin contents of about 15 percent or greater, whereas chemical pulps (Kraft and sulfite) are low yield pulps having a lignin content of about 5 percent or less. The high-lignin fibers are subjected to a dispersing treatment in a disperser in order to introduce curl into the fibers. The temperature of the fiber suspension during dispersion may be about 140° F. or greater. In one embodiment, the temperature may be about 150° F. or greater and, in yet another embodiment, the temperature may be about 210° F. or greater. The upper limit on the temperature may be dictated by whether or not the apparatus is pressurized, since the aqueous fiber suspensions within an apparatus operating at atmospheric pressure should not be heated above the boiling point of water.
It is believed that the degree of permanency of the curl is greatly impacted by the amount of lignin in the fibers being subjected to the dispersing process, with greater effects being attainable for fibers having higher lignin content (see col. 5, lines 43 and following). Lignin-rich, high coarseness, generally tubular fibers are further described in U.S. Pat. Nos. 6,254,725, 6,074,527, 6,287,422, 6,162,961, 5,932,068, 5,772,845, and 5,656,132, each of which is incorporated herein by reference in its entirety. The so-called uncreped, through-air-dried process of the '551 patent requires a relatively high capital investment and is expensive to operate inasmuch as thermal dewatering of the web is energy intensive and is sensitive to fiber composition.
Commercial success has also been achieved in connection with U.S. Pat. No. 5,690,788, which is incorporated herein by reference in its entirety. In accordance with the '788 patent, there is provided biaxially undulatory single ply and multiply tissues, single ply and multiply towels, single ply and multiply napkins, and other personal care and cleaning products, as well as creping blades and processes for the manufacture for such paper products. Generally speaking, there is provided in accordance with the '788 patent a creping blade provided with an undulatory rake surface having trough-shaped serrulations in the rake surface of the blade. The undulatory creping blade has a multiplicity of alternating serrulated sections of either uniform depth or a multiplicity of arrays of serrulations having non-uniform depth. The blade is operative to impart a biaxially undulatory structure to the creped web such that the product exhibits increased absorbency and softness with a variety of furnishes. Specifically disclosed are conventional furnishes such as softwood, hardwood, recycle, mechanical pulps (including thermo-mechanical and chemithermomechanical pulp), anfractuous fibers, and combinations of these (see col. 20, line 41 and following). Example 20 of the '788 patent notes the properties obtained when using the undulatory blade in the manufacture of towels including up to 30 percent anfractuous fiber high bulk additive (HBA). HBA is a commercially available softwood Kraft pulp sold by Weyerhauser Corporation that has been rendered anfractuous by physically and chemically treating the pulp such that the fibers have permanent kinks and curls imparted to them. Inclusion of the HBA fibers into the base sheet will serve to improve the sheet's bulk and absorbency.
Despite many advances in the art, there is an ever present need for further improvements to products which incorporate cellulosic fiber such as recycled fiber, especially those improvements that do so on a cost-effective basis in terms of required capital and operating costs. It has also been found that there is a benefit between the use of an undulatory creping blade and the incorporation of certain high yield fibers into a web.
As embodied and broadly described herein, the invention includes an embossing system for embossing at least a portion of a web comprising a first roll and at least a second roll, the first roll and second roll defining a first nip for embossing the web, wherein at least one of the first roll and the second roll may include elongated embossing elements extending substantially in the machine direction and at least one of the first roll and the second roll may include perforate embossing elements extending substantially in the cross-machine direction, and wherein the embossing elements are capable of imparting a perforate pattern and/or a cube embossing pattern on the web. The embossing elements extending substantially in the machine direction and the perforate embossing elements extending substantially in the cross-machine direction may be provided on the same or both of the first and the second embossing rolls. In one embodiment, the web may be a cellulosic fibrous web, wherein at least about 15% by weight of the fiber, based on the weight of the cellulosic fiber in the furnish, is lignin-rich, high coarseness fiber having generally tubular fiber configuration, as well as an average fiber length of at least about 2 mm and a coarseness of at least about 20 mg/100 m. In another embodiment, the web may be creped with an undulatory creping blade. In a further embodiment, both the first and second rolls include elongated mated embossing elements extending substantially in the machine direction. In yet another embodiment, the elongated embossing elements extending substantially in the machine direction are capable of imparting a cube embossing pattern to the web, and the perforate embossing elements extending substantially in the cross-machine direction are capable of imparting a perforate pattern to the web.
Another embodiment of the invention includes a method of embossing at least a portion of a web, including providing a first roll and providing at least a second roll, the first roll and the second roll defining a first nip, providing a cellulosic fibrous web to be embossed, and passing the web between the first nip, wherein at least one of the first roll and the second roll has elongated embossing elements extending substantially in the machine direction and/or the cross-machine direction and optionally at least one of the first roll and the second roll has perforate embossing elements, that may or may not be elongated, extending substantially in the cross-machine direction, and wherein the elongated embossing elements impart a cube embossing pattern on the web. In one embodiment, both of the substantially machine direction embossing elements and the substantially cross-machine direction perforate embossing elements are on the same roll. In another embodiment, both the first and second rolls include elongated mated embossing elements substantially in the machine direction and/or the cross-machine direction. In a further embodiment, the elongated embossing elements extending substantially in the machine direction and/or the cross-machine direction are capable of imparting a cube emboss pattern to the web, and the perforate embossing elements, that are not elongated, extending substantially in the cross-machine direction are capable of imparting a perforate emboss to the web. In yet a further embodiment, at least one of the first roll and the second roll have both elongated embossing elements extending substantially in the machine direction and elongated embossing elements extending substantially in the cross-machine direction that are capable of imparting a cube emboss pattern to the web, and no perforate embossing elements extending substantially in the cross-machine direction are capable of imparting a perforate emboss to the web. In still a further embodiment, at least one of the first roll and the second roll have both elongated embossing elements extending substantially in the machine direction and elongated embossing elements extending substantially in the cross-machine direction that are capable of imparting a cube emboss pattern to the web, and perforate embossing elements extending substantially in the cross-machine direction that are capable of imparting a perforate emboss to the web.
In another embodiment of the present invention, a first roll and a second roll are provided, the first roll and the second roll defining a first nip for embossing a web, wherein at least one of the first roll or the second roll includes elongated embossing elements substantially extending in the machine direction, wherein at least one of the first roll and the second roll includes elongated embossing elements extending substantially in the cross-machine direction, and wherein at least one of the first and the second roll includes substantially cross-machine direction embossing elements. In one embodiment, the substantially cross-machine direction embossing elements are perforate embossing elements. In another embodiment, each of the elongated substantially machine direction embossing elements, the elongated substantially cross-machine direction embossing elements, and the substantially cross-machine direction elements may be on one roll. In a further embodiment, both the first roll and the second roll include elongated mated embossing elements extending substantially in the machine direction and/or the cross-machine direction. In yet another embodiment, the elongated embossing elements extending substantially in the machine direction and the elongated embossing elements extending substantially in the cross-machine direction are capable of imparting a cube emboss pattern to the web, and the perforate embossing elements, that are not elongated, extending substantially in the cross-machine direction are capable of imparting a perforate emboss to the web.
The accompanying drawings, which are incorporated herein and constitute a part of this specification, illustrate an embodiment of the invention, and, together with the description, serve to explain the principles of the invention. Further advantages of the invention will be set forth in part in the description which follows and in part will be apparent from the description or may be learned by practice of the invention. The advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Combinations and variants of the individual embodiments discussed are both intended and fully envisioned. The invention is described in detail below for purposes of description and exemplification only. Modifications within the spirit and scope of the present invention, set forth in the appended claims, will be readily apparent to those of skill in the art.
The present invention may be used with a variety of types of wet-laid cellulosic webs, including paper and the like. In addition, the present invention may be used with a variety of types of through-air-dried (TAD) cellulosic webs, including paper and the like. The webs may be continuous or of a fixed length. Moreover, the webs may be used to produce any art recognized product, including, but not limited to, absorbent paper products, for example, paper towels, napkins, facial tissue, bath tissue and the like. Moreover, the resulting product may be a single ply or a multi-ply paper product, or a laminated paper product having multiple plies.
The present invention may be used with a web made from one or more of virgin furnish, recycled furnish, and synthetic fibers. Fibers suitable for making the webs of this invention include: non-woody fibers, such as cotton fibers or cotton derivatives, abaca, kenaf, flax, esparto grass, straw, jute hemp, bagasse, milkweed floss fibers, and pineapple leaf fibers; and woody fibers, such as those obtained from deciduous and coniferous trees, including: softwood fibers, such as northern and southern softwood kraft fibers; and hardwood fibers, such as eucalyptus, maple, birch, aspen, and the like. Papermaking fibers may be liberated from their source material by any one of a number of chemical pulping processes familiar to one experienced in the art, including sulfate, sulfite, polysulfide, soda pulping, and the like. The pulp may be bleached, if desired, by chemical means including the use of chlorine, chlorine dioxide, oxygen, and the like.
In at least one embodiment, the products of the present invention comprise a blend of conventional fibers (whether derived from virgin pulp, recycle, and/or synthetic sources) and high coarseness, lignin-rich tubular fibers.
Conventional fibers for use according to the present invention are also procured by recycling of pre- and post-consumer paper products. Fiber may be obtained, for example, from: the recycling of printers' trims and cuttings, including book and clay coated paper; post consumer paper, including office paper; and curbside paper recycling, including old newspaper. The various collected paper can be recycled using any means common to the recycled paper industry. As the term is used herein, recycle or secondary fibers include those fibers and pulps which have been previously formed into a web and then re-isolated from that web matrix by some physical, chemical, and/or mechanical means. The papers may be sorted and graded prior to pulping in conventional low, mid, and high-consistency pulpers. In the pulpers the papers are mixed with water and agitated to break the fibers free from the sheet. Chemicals may be added in this process to improve the dispersion of the fibers in the slurry and to improve the reduction of contaminants that may be present. Following pulping, the slurry is usually passed through various sizes and types of screens and cleaners to remove the larger solid-contaminants while retaining the fibers. It is during this process that such waste contaminants such as paper clips and plastic residuals are removed. The pulp is then generally washed to remove smaller sized contaminants, for instance those consisting primarily of inks, dyes, fines, and ash. This process is generally referred to as deinking. Deinking can be accomplished by several different processes, including wash deinking, flotation deinking, enzymatic deinking, and the like. One example of a deinking process by which recycled fiber for use in the present invention may be obtained is called floatation deinking. In this process small air bubbles are introduced into a column of the furnish. As the bubbles rise they tend to attract small particles of dye and ash. Once upon the surface of the column of stock they are skimmed off.
In one embodiment, the conventional fibers according to the present invention may consist predominantly of secondary or recycle fibers that possess significant amounts of ash and fines. It is common in the papermaking industry for the term ash to be associated with virgin fibers. This usage is generally defined as the amount of ash that would be created if the fibers were burned. Typically no more than about 0.1% to about 0.2% ash is found in virgin fibers. Ash, as the term is used herein, includes this “ash” associated with virgin fibers as well as contaminants resulting from prior use of the fiber. Furnishes utilized in connection with the present invention may include excess amounts of ash, for example, greater than about 1% or more. Ash originates primarily when fillers or coatings are added to paper during formation of a filled or coated paper product. Ash will typically be a mixture containing titanium dioxide, kaolin clay, calcium carbonate, and/or silica. This excess ash or particulate matter is what has traditionally interfered with processes using recycle fibers, thus making the use of recycled fibers unattractive. In general, recycled paper containing high amounts of ash is priced substantially lower than recycled papers with low or insignificant ash content.
Furnishes containing excessive ash also typically contain significant amounts of fines. Fines constitute material within the furnish that will pass through a 100 mesh screen. Ash content may be determined using TAPPI Standard Method T211 OM93. Ash and fines are most often associated with secondary, recycled fibers, post-consumer paper, and converting broke from printing plants and the like. Secondary, recycled fibers with excessive amounts of ash and significant fines are available on the market and are inexpensive because it is generally accepted that only very thin, rough, economy towel and tissue products can be made from these fibers unless the furnish is processed to remove the ash and fines. The present invention makes it possible to achieve a paper product with high void volume and good softness and/or absorbency properties from secondary fibers having significant amounts of ash and fines without any need to preprocess the fiber to remove fines and ash. While the present invention contemplates the use of fiber mixtures, including the use of virgin fibers, fiber in the products according to the present invention may have, in some embodiments, greater than about 0.75% ash, and in additional embodiments more than about 1% ash.
Lignin-rich cellulosic pulps or fibers having high coarseness and generally tubular structure used in the products and processes of the present invention are typically those known in the industry as “high-yield” pulps due to their high yield based on the cellulosic feed to the respective pulping and/or treatment processes. Thermomechanical pulp (TMP) and chemithermomechanical pulp (CTMP), as well as bleached chemithermomechanical pulp (BCTMP) and alkaline peroxide mechanical pulp (APMP), are suitable. Such pulps may have a lignin content of at least about 5% and sometimes more than about 10%. In some embodiments, the pulp has a lignin content of more than about 15% up to about 30% or more. In some embodiments the pulps are at least one of TMP, CTMP, BCTMP, and APMP having lignin contents of from about 15% to about 25%.
TMP is a mechanical pulp produced from wood chips where the wood particles are softened by preheating, before a pressurized primary refining stage, in a pressurized vessel at temperatures not exceeding the glass transition temperature of lignin. CTMP is produced from chemically impregnated wood chips by means of pressurized refining at high consistencies. APMP is produced by way of a chemimechanical pulping process, where the chemical impregnation of the wood chips is carried out by alkaline peroxide prior to refining at atmospheric conditions.
BCTMP is CTMP bleached to a higher brightness, typically about 80 GE or higher. GE brightness, as used herein, measures the amount of light reflected from the surface of a pulp and is highly dependant not only on the type of pulp but also on the degree to which it is bleached. It is measured by comparing the amount of essentially parallel light beams reflected by a pulp surface when illuminated at an angle of 45°, to the amount of same light reflected by the surface of magnesium oxide, which is the standard of 100%. The specific process for measuring GE brightness is disclosed in TAPPI T-452 “Brightness of Pulp, Paper, and Paperboard (Directional Reflectance at 457 nm).” Differences between BTCMP and recycle fiber can be appreciated by reference to Table 1 below.
It will also be appreciated from
The various high-lignin pulps employed in connection with the present invention may be prepared by any suitable method. For example, mechanical pulp may be bleached as described in U.S. Pat. No. 6,136,041 entitled “Method for Bleaching Lignocellulosic Fibers,” which is incorporated herein by reference in its entirety. Suitable bleached pulps may include BCTMP with about a 21% lignin content bleached with hydrogen peroxide, sulfite, and caustic.
Suitable lignin-rich, high coarseness, and generally tubular cellulosic fibers include fibers selected at least one of APMP, TMP, CTMP, and BCTMP, as defined herein. In one embodiment, these fibers may be present in an amount of from about 20 to about 40 percent by weight. BCTMP is a particularly suitable fiber for many products and may have a lignin content in various embodiments of at least about 15%, at least about 20%, or at least about 25% by weight. BTCMP with a lignin content of about 25% to about 35% may also be employed.
The high coarseness and generally tubular lignin-rich fiber may be derived from softwood in many embodiments and may be at least one of APMP, TMP, CTMP, and BCTMP. Moreover, these high coarseness and generally tubular lignin-rich fibers may be used in combination with virgin pulp and/or recycled fiber.
Lignin content is measured by way of TAPPI method T222-98 (acid insoluble lignin). In this method, the carbohydrates in wood and pulp are hydrolyzed and solubilized by sulfuric acid. The acid-insoluble lignin is filtered off, dried, and then weighed.
Fiber length and coarseness can be measured using a fiber-measuring instrument such as the Kajaani FS-200 analyzer available from Valmet Automation of Norcross, Ga., or an OPTEST FQA. For fiber length measurements, a dilute suspension of the fibers (about 0.5 to 0.6 percent), whose length is to be measured, may be prepared in a sample beaker and the instrument operated according to the procedures recommended by the manufacturer. The reported range for fiber lengths is set at an instrument's minimum value of, for example, 0.07 mm and a maximum value of, for example, 7.2 mm. Fibers having lengths outside of the selected range are excluded. Three calculated average fiber lengths may be reported. The arithmetic average length is the sum of the product of the number of fibers measured and the length of the fiber divided by the sum of the number of fibers measured. The length-weighted average fiber length is defined as the sum of the product of the number of fibers measured and the length of each fiber squared divided by the sum of the product of the number of fibers measured and the length the fiber. The weight-weighted average fiber length is defined as the sum of the product of the number of fibers measured and the length of the fiber cubed divided by the sum of the product of the number of fibers and the length of the fiber squared. As used herein throughout this specification and claims, unless indicated otherwise, the weight-weighted average fiber length is referred to by the terminology “average fiber length,” “fiber length,” and the like.
Fiber coarseness is the weight of fibers in a sample per a given length and is usually reported as mg/100 meters. The fiber coarseness of a sample is measured from a pulp or paper sample that has been dried and then conditioned at, for example, 72° F. and 50% relative humidity for at least four hours. The fibers used in the coarseness measurement are removed from the sample using tweezers to avoid contamination. The weight of fiber that is chosen for the coarseness determination depends on the estimated fraction of hardwood and softwood in the sample, and range from about 3 mg for an all-hardwood sample to about 14 mg for a sample composed entirely of softwood. The portion of the sample to be used in the coarseness measurement is weighed to the nearest 0.00001 gram and is then slurried in water. To insure that a uniform fiber suspension is obtained and that all fiber clumps are dispersed, an instrument such as the Soniprep 150, available from Sanyo Gallenkamp of Uxbridge, Middlesex, UK, may be used to disperse the fiber. After dispersion, the fiber sample is transferred to a sample cup, taking care to insure that the entire sample is transferred. The cup is then placed in the fiber analyzer as noted above. The dry weight of pulp used in the measurement, which is calculated by multiplying the weight obtained above by 0.93 to compensate for the moisture in the fiber, is entered into the analyzer and the coarseness is determined using the procedure recommended by the manufacturer.
In one embodiment of the present invention, predominantly recycled fiber (i.e., more than about 50% by weight based on the weight of cellulosic fiber in the sheet) with at least about 15% by weight high yield, lignin-rich cellulosic fiber is used. In various embodiments, at least about 60%, at least about 75%, or at least about 80% recycle fiber may be incorporated into the sheet if so desired. Specific features and embodiments of the invention are further described below.
The suspension of fibers or furnish may contain chemical additives to alter the physical properties of the paper produced. These chemistries are well understood by the skilled artisan and may be used in any known combination. Such additives may include surface modifiers, softeners, debonders, strength aids, latexes, opacifiers, optical brighteners, dyes, pigments, sizing agents, barrier chemicals, retention aids, insolubilizers, organic or inorganic crosslinkers, or combinations thereof; the chemicals optionally comprising polyols, starches, PPG esters, PEG esters, phospholipids, surfactants, polyamines, and the like. In addition, such additives may include any known or later developed chemistries that may be readily apparent to the skilled artisan.
The sheet may be prepared by a wet-crepe process for making absorbent sheet comprising: (a) preparing an aqueous fibrous cellulosic furnish comprising high coarseness, generally tubular and possibly lignin-rich cellulosic fiber; (b) depositing the aqueous fibrous furnish on a foraminous support; (c) dewatering the furnish to form a web; (d) applying the dewatered web to a heated rotating cylinder and drying the web to a consistency of greater than about 30% and less than about 90%; (e) creping the web from the heated cylinder at the consistency of greater than about 30% and less than about 90% with a creping blade provided with a creping surface adapted to contact the cylinder; and (f) drying the web subsequent to creping the web from the heated cylinder to form the absorbent sheet. In one embodiment, the web may be dried to a consistency of from about 40% to about 80% prior to creping the web from the heated rotating cylinder. In another embodiment, the web may be dried to a consistency of from about 50% to about 75% prior to creping from the heated rotating cylinder. In yet another embodiment, an undulatory creping blade may be used.
Another process which may be employed is a dry-crepe process that may or may not use an after-crepe dryer. A dry-crepe process for making absorbent sheet of the invention includes: (a) preparing an aqueous cellulosic fibrous furnish wherein at least about 15% by weight of the fiber based on the weight of cellulosic fiber in the ash is lignin-rich coarse fiber having a generally tubular fiber configuration as well as an average fiber length of at least about 2 mm and a coarseness of at least about 20 mg/100 m; (b) depositing the aqueous fibrous furnish on a foraminous support; (c) dewatering the furnish to form a web; (d) applying the dewatered web to a heated rotating cylinder and drying the web to a consistency of about 90% or greater; (e) creping the web from the heated cylinder at the consistency of about 90% or more with a creping blade provided with an undulatory creping surface adapted to contact the cylinder; and optionally (f) drying the web subsequent to creping the web from the heated cylinder to form the absorbent sheet. In one embodiment, the web is dried to a consistency of greater than about 95%.
The present invention can be used in a variety of different processes, including conventional wet press processes and through-air-drying processes. In addition, to increase the smoothness of the resulting product, the web may be calendared. Moreover, to increase the bulkiness of the product, an undulatory creping blade may be used, such as described in U.S. Pat. No. 5,690,788, which is herein incorporated by reference in its entirety. Those of ordinary skill in the art will understand the variety of processes in which the above-described invention can be employed.
A forming fabric is supported on rolls 18 and 19, which are positioned relative to the breast roll 15 for pressing the press wire 12 to converge on the foraminous support member 11. The foraminous support member 11 and the wire 12 move in the same speed and at the same direction, which is in the direction of rotation of the breast roll 15. The pressing wire 12 and the foraminous support member 11 converge at an upper surface of the forming roll 15 to form a wedge-shaped space or nip into which one or more jets of water or foamed liquid fiber dispersion (furnish) provided by single or multiple headboxes 20, 20′ is pressed between the pressing wire 12 and the foraminous support member 11 to force fluid through the wire 12 and into a saveall 22 where it is collected to reuse in the process.
According to the embodiment in
A pit 44 is provided for collecting water squeezed from the furnish by the press roll 16 and a Uhle box 29. The water collected in the pit 44 may be collected into a flow line 45 for separate processing to remove surfactant and/or fibers from the water and to permit recycling of the water back to the papermaking machine 10.
According to the present invention, an absorbent paper web may be made by dispersing fibers into an aqueous slurry and depositing the aqueous slurry onto the forming wire of a papermaking machine. Any suitable forming scheme might be used. For example, an extensive but non-exhaustive list includes a crescent former, a C-wrap twin wire former, an S-wrap twin wire former, a suction breast roll former, a Fourdrinier former, or any art-recognized forming configuration. The forming fabric can be any suitable foraminous member, including single layer fabrics, double layer fabrics, triple layer fabrics, photopolymer fabrics, and the like. A non-exhaustive list of background art in the forming fabric area includes U.S. Pat. Nos. 4,157,276; 4,605,585; 4,161,195; 3,545,705; 3,549,742; 3,858,623; 4,041,989; 4,071,050; 4,112,982; 4,149,571; 4,182,381; 4,184,519; 4,314,589; 4,359,069; 4,376,455; 4,379,735; 4,453,573; 4,564,052; 4,592,395; 4,611,639; 4,640,741; 4,709,732; 4,759,391; 4,759,976; 4,942,077; 4,967,085; 4,998,568; 5,016,678; 5,054,525; 5,066,532; 5,098,519; 5,103,874; 5,114,777; 5,167,261; 5,199,261; 5,199,467; 5,211,815; 5,219,004; 5,245,025; 5,277,761; 5,328,565; and 5,379,808, all of which are incorporated herein by reference in their entireties. One forming fabric particularly useful with the present invention is Voith Fabrics Forming Fabric 2164 made by Voith Fabrics Corporation, Shreveport, La.
Foam-forming of the aqueous furnish on a forming wire or fabric may be employed as a means for controlling the permeability or void volume of the sheet upon wet-creping. Suitable foam-forming techniques are disclosed in U.S. Pat. No. 4,543,156 and Canadian Patent No. 2,053,505, the disclosures of which are incorporated herein by reference in their entireties.
In accordance with the present invention, creping of the paper from a Yankee dryer may be carried out using an undulatory creping blade, such as that disclosed in U.S. Pat. No. 5,690,788, the disclosure of which is incorporated herein by reference in its entirety. Use of the undulatory crepe blade has been shown to impart several qualities when used in production of tissue products. In general, tissue products creped using an undulatory blade tend to at least have higher caliper (thickness), increased CD stretch, and/or a higher void volume than do comparable tissue products produced using conventional crepe blades. All of these changes effected by use of the undulatory blade tend to correlate with improved softness perception of the tissue products.
The undulatory creping blade, as shown as blade 70 in
As illustrated in
The number of teeth per inch may be taken as the number of elongate regions 82 per inch and the tooth depth may be taken as the height, H, of the groove indicated at 81 adjacent surface 88.
Several angles are used in order to describe the geometry of the cutting edge of the undulatory blade. To that end, the following terms are used:
Creping angle“α”—the angle between the line of contact of a rake surface 78 of the blade 70 and the plane 52 tangent to the Yankee at the point of intersection between the undulatory cutting edge 73 and the Yankee.
Axial rake angle “β”—the angle between the axis of the Yankee and the undulatory cutting edge 73 which is the curve defined by the intersection of the surface of the Yankee with indented rake surface of the blade 70.
Relief angle “γ”—the angle between the relief surface 72 of the blade 70 and the plane 52 tangent to the Yankee at the intersection between the Yankee and the undulatory cutting edge 73, the relief angle measured along the flat portions of the present blade is equal to what is commonly called “blade angle” or “holder angle”, that is, “γ” in
Blade bevel angle—the angle the rake surface 78 defines with a perpendicular 54 to the blade body.
Based on the above terms, and referring to
α=90+blade bevel angle−γ.
While the creping angle for a conventional blade will be constant over the entire creping surface, these parameters vary over the creping surface of an undulatory blade.
The value of each of these angles may vary depending upon the precise location along the cutting edge at which it is to be determined. The remarkable results achieved with the described undulatory blades in the manufacture of the absorbent paper products are due to those variations in these angles along the cutting edge. Accordingly, in many cases it will be convenient to denote the location at which each of these angles is determined by a subscript attached to the basic symbol for that angle. As noted in the '788 patent, the subscripts “f,” “c,” and “m” refer to angles measured at the rectilinear elongate regions, at the crescent shaped regions, and the minima of the cutting edge, respectively. Accordingly, “γf”, the relief angle measured along the flat portions of the present blade, is equal to what is commonly called “blade angle” or “holder angle.” In general, it will be appreciated that the pocket angle αf at the rectilinear elongate regions is typically higher than the pocket angle αc at the crescent shaped regions.
While the products of the invention may be made by way of a dry-crepe process, they may also be made by way of a wet-crepe process, and in one embodiment with respect to a single ply towel. When a wet-crepe process is employed, the after-drying section, for example that of after-drying section 30 in
When an impingement air after dryer is used, in one embodiment the after drying section 30 of
There is shown in
After wet shaping, the web W may be transferred over the vacuum roll 110 impingement air-dry system as shown. The apparatus of
Yet another after-drying section is disclosed in U.S. Pat. No. 5,851,353, which is incorporated by reference herein in its entirety and which may likewise be employed in a wet-creped process using the apparatus of
Still yet another after-drying section 30 is illustrated schematically in
A second felt 132 may likewise form an endless loop about a plurality of after-dryer drums and rollers as shown. The various drums may be arranged in two rows as shown and the web may be dried as it travels over the drums of both rows and between rows as shown in the diagram. The second felt 132 carries the web W from drum 134 to drum 136, from which the web W may be further processed or wound up on a take-up reel 138.
In another embodiment of the present invention, the web may be a creped or recreped web as depicted in
The crepe frequency count for a creped base sheet or product may be measured with the aid of a microscope. For Example, the Leica Stereozoom® 4 microscope has been found to be suitable for this procedure. The sheet sample is placed on the microscope stage with its Yankee side up and the cross direction of the sheet vertical in the field of view. Placing the sample over a black background improves the crepe definition. During the procurement and mounting of the sample, care should be taken that the sample is not stretched. Using a total magnification of 18-20, the microscope is then focused on the sheet. An illumination source is placed on either the right or left side of the microscope stage, with the position of the source being adjusted so that the light from it strikes the sample at an angle of approximately 45 degrees. It has been found that Leica or Nicholas Illuminators are suitable light sources. After the sample has been mounted and illuminated, the crepe bars are counted by placing a scale horizontally in the field of view and counting the crepe bars that touch the scale over a one-half centimeter distance. This procedure is repeated at least two times using different areas of the sample. The values obtained in the counts are then averaged and multiplied by the appropriate conversion factor to obtain the crepe frequency in the desired unit length.
It should be noted that the thickness of the portion of the web 150 between the longitudinally extending crests 158 and the furrows 156 may, on average, typically be about 5% greater than the thickness of portions of the web 150 between the ridges 152 and the sulcations 160. Suitably, the portions of the web 150 adjacent the longitudinally extending ridges 152 (on the air side) are in the range of from about 1% to about 7% thinner than the thickness of the portion of the web 150 adjacent to the furrows 156 as defined on the air side of the web 150.
The height of the ridges 152 correlates with the tooth depth H formed in the undulatory creping blade 70. At a tooth depth of about 0.010 inches, the ridge height is usually from about 0.0007 to about 0.003 inches for sheets having a basis weight of about 14 to about 19 pounds per ream. At double the depth, the ridge height increases to from about 0.005 to about 0.008 inches. At tooth depths of about 0.030 inches, the ridge height is from about 0.010 to about 0.013 inches. At higher undulatory depths, the height of the ridges 152 may not increase and may decrease. The height of the ridges 152 also depends on the basis weight of the sheet and strength of the sheet.
The average thickness of the portion of the web 150 adjoining the crests 158 may be significantly greater than the thickness of the portions of the web 150 adjoining the sulcations 160. Thus, the density of the portion of the web 150 adjacent the crests 158 can be less than the density of the portion of the web 150 adjacent the sulcations 160. The process of the present invention may produce a web having a specific caliper of from about 2 to about 8 mils per 8 sheets per pound of basis weight. The usual basis weight of the web 150 is from about 7 to about 35 lbs/3000 sq. ft. ream.
Suitably, when the web 150 is calendared, the specific caliper of the web 150 may be from about 2.0 to about 6.0 mils, per 8 sheets per pound of basis weight, and the basis weight of the web may be from about 7 to about 35 lbs/3000 sq. ft. ream. In one embodiment, the caliper of the sheet of the invention may be at least about 7.5% greater than that of a like or equivalent sheet prepared without the use of an undulatory creping blade or at least about 5% more than that of a sheet made without high coarseness tubular fibers creped with an equivalent undulatory creping blade. Calipers reported herein are 8 sheet calipers unless otherwise indicated. Thus, eight sheets are stacked and the caliper measurement taken about the central portion of the stack. Preferably, the test samples are conditioned in an atmosphere of 23°±1.0° C. (73.4°±1.8° F.) at 50% relative humidity for at least about 2 hours and then measured with a Thwing-Albert Model 89-II-JR or Progage Electronic Thickness Tester with 2-in (50.8-mm) diameter anvils, 539±10 grams dead weight load, and 0.231 in/sec descent rate. For finished product testing, each sheet of product to be tested must have the same number of plies as the product to be sold. For napkin testing, the napkins are completely unfolded prior to stacking. For base sheet testing off of winders, each sheet to be tested must have the same number of plies as produced off the winder. For base sheet testing off of the paper machine reel, single plies are used.
In one embodiment, the invention is directed to a creped absorbent cellulosic sheet incorporating from about 15% to about 40% by weight of high coarseness, generally tubular and lignin-rich cellulosic fiber based on the weight of cellulosic fiber in the sheet prepared by way of a process comprising applying a dewatered web to a heated rotating cylinder and creping the web from the heated rotating cylinder with an undulatory creping blade. When a lignin-rich, high coarseness and generally tubular cellulosic fiber is used, it may comprise at least about 10% by weight lignin based on the weight of the lignin-rich cellulosic fiber. In one embodiment, the lignin-rich, high coarseness and generally tubular cellulosic fiber may comprise at least about 15% by weight lignin based on the weight of the lignin-rich cellulosic fiber. In another embodiment, the lignin-rich, high coarseness and generally tubular cellulosic fiber may comprise at least about 25% by weight lignin based on the weight of the lignin-rich cellulosic fiber. In a further embodiment, the lignin-rich, high coarseness generally tubular fiber comprises from about 25% to about 35% by weight lignin based on the weight of the lignin-rich, high coarseness and generally tubular cellulosic fiber in the sheet. The lignin-rich, high coarseness and generally tubular fiber may have an average fiber length of at least about 2.25 mm and the fiber length may be from about 2.25 to about 2.75 mm. According to one embodiment, the coarseness can be from about 20 to about 30 mg/100 m.
The water absorbent capacity (WAC) of the sheet of the present invention may be at least about 5% greater than that of a like or equivalent sheet prepared without the use of an undulatory creping blade or at least 5% more than that of a sheet made without high coarseness tubular fibers creped with an equivalent undulatory blade. WAC is defined as the point where the weight versus time graph has a “zero” slope, i.e., the sample has stopped absorbing. In one embodiment, the WAC of the product may be greater than about 170 g/m2.
The WAC of the products of the present invention may be measured with a simple absorbency tester. The simple absorbency tester may also be a useful apparatus for measuring the hydrophilicity and absorbency properties of a sample of tissue, napkins, or towel. In this test a sample of tissue, napkins, or towel 2.0 inches in diameter is mounted between a top flat plastic cover and a bottom grooved sample plate. The tissue, napkins, or towel sample disc is held in place by a ⅛ inch wide circumference flange area. The sample is not compressed by the holder. De-ionized water at 73° F. is introduced to the sample at the center of the bottom sample plate through a 1 mm diameter conduit. This water is at a hydrostatic head of minus 5 mm. Flow is initiated by a pulse introduced at the start of the measurement by the instrument mechanism. Water is thus imbibed by the tissue, napkin, or towel sample from this central entrance point radially outward by capillary action. When the rate of water imbibation decreases below 0.005 gm water per 5 seconds, the test is terminated. The amount of water removed from the reservoir and absorbed by the sample is weighed and reported as grams of water per square meter of sample.
A Gravimetric Absorbency Testing System may be used to determine WAC, which is obtainable from M/K Systems Inc., Danvers, Mass. WAC is actually determined by the instrument itself. The termination criteria for a test are expressed in maximum change in water weight absorbed over a fixed time period. This is basically an estimate of zero slope on the weight versus time graph. The program uses a change of 0.005 g over a 5 second time interval as termination criteria.
A series of one-ply wet-creped towels were prepared as indicated in Table 2 below.
As will be appreciated from Table 2, the use of BCTMP together with an undulatory creping blade of 12 tpi, 30 mil tooth depth exhibited synergy. Data for the towels also appears plotted on
The synergies are calculated based on Examples A and B, as well as measurements based on a sheet made from the same composition in terms of fiber and the same approximate basis weight. In the first step in calculating the percent synergy, the expected creping blade delta is calculated as the difference between examples A and B. For example, a 142−137 or 5 g/m2 increase in WAC is expected based on the use of an undulatory blade. Next, the synergy is calculated as the difference between the observed value and the expected value divided by the expected delta times 100%. For WAC in Example 1, this calculates as: (162−(152+5))/5×100% or 100% greater than the expected increase based on additive effects. As can be seen from Table 2, large absorbency synergies as well as significant caliper increases may be achieved in accordance with the invention. Likewise, products made with BCTMP and an undulatory creping blade exhibit remarkable increases in water absorbency rates (WAR). The differences seen in Table 2 and
In another embodiment of the present invention, the sheet may be embossed with a plurality of embossing patterns having their major axes generally along the cross-machine direction of the sheet. Embossed products may include perforate embossed products with a transluminance ratio (hereinafter defined) of at least about 1.005. The embossed products may have a dry MD/CD tensile ratio of less than about 2. In one embodiment, the dry MD/CD tensile ratio may be less than about 1.5. Cross-machine direction perforate embossing systems are described in U.S. Pat. No. 6,733,626 and U.S. patent application Ser. No. 10/236,993, each of which is incorporated herein by reference in its entirety.
In one embodiment, the converting process may include an embossing system of at least two embossing rolls, the embossing rolls defining at least one nip through which a web to be embossed is passed. The embossing elements may be patterned to create perforations in the web as it is passed through the nip.
Generally, for purposes of this invention, perforations are created when the strength of the web is locally degraded between two bypassing embossing elements resulting in either (1) a macro scale through-aperture, (2) in those cases where a macro scale through-aperture is not present, at least incipient tearing, where such tearing would increase the transmittivity of light through a small region of the web, or (3) a decrease the machine direction strength of a web by at least 15% for a given range of embossing depths.
As shown in
Not being bound by theory, it is believed that the superior strength reduction results achieved using the present invention are due to the location of the local degradation of the web when perforate embossing as compared to when non-perforate embossing. When a web is embossed, either by perforate or non-perforate methods, the portion of the web subject to the perforate or non-perforate nip is degraded. In particular, as a web passes through a non-perforate nip for embossing, the web is stressed between the two embossing surfaces such that the fiber bonds are stretched and sometimes, when the web is overembossed, which is not desired when non-perforate embossing a web, the bonds are torn or broken. When a web is passed through a perforate nip, the web fiber bonds are at least incipiently torn by the stresses caused by the two bypassing perforate elements. As stated above, however, one difference between the two methods appears to be in the location of the at least incipient tearing.
When a web is over-embossed in a rubber to steel configuration, the male steel embossing elements apply pressure to the web and the rubber roll, causing the rubber to deflect away from the pressure, while the rubber also pushes back. As the male embossing elements roll across the rubber roll during the embossing process, the male elements press the web into the rubber roll which causes tension in the web at the area of the web located at the top edges of the deflected rubber roll, i.e., at the areas at the base of the male embossing elements. When the web is over-embossed, tearing can occur at these high-tension areas. More particularly,
When a web is perforate embossed, on the other hand, the areas of degradation 242, as shown in
In one embodiment according to the present invention, the embossing rolls capable of imparting a cross-machine direction embossing pattern have substantially identical embossing element patterns, with at least a portion of the embossing elements configured such that they are capable of producing perforating nips which are capable of perforating the web. As the web is passed through the nip, an embossing pattern is imparted on the web. In one embodiment, the embossing rolls may be either steel, hard rubber, or other suitable polymer. In another embodiment, the embossing elements are mated. The direction of the web as it passes through the nip is referred to as the machine direction. The transverse direction of the web that spans the emboss roll is referred to as the cross-machine direction. In one embodiment, a predominant number, i.e., at least about 50% or more, of the perforations are configured to be oriented such that the major axis of the perforation is substantially oriented in the cross-machine direction. As used herein, an embossing element is substantially oriented in the cross-machine direction when the long axis of the perforation nip formed by the embossing element is at an angle of from about 60° to about 120° from the machine direction of the web. As used herein, an embossing element is substantially oriented in the machine direction when the long axis of the perforation nip formed by the embossing element is at angle outside of from about 60° to about 120° from the machine direction of the web.
In an embodiment according to the present invention, and as shown in
The end product characteristics of a cross-machine direction perforated embossed product can depend upon a variety of factors of the embossing elements that are imparting a pattern on the web. These factors can include one or more of the following: embossing element height, angle, shape, including sidewall angle, spacing, engagement, and alignment, as well as the physical properties of the rolls, base sheet, and other factors. Following is a discussion of a number of these factors.
An individual embossing element 234 has certain physical properties, such as height, angle, and shape, that affect the embossing pattern during an embossing process. Various of these properties are depicted in
The angle of the cross-machine direction elements 234 substantially defines the direction of the degradation of the web due to cross-machine perforate embossing. In one embodiment, when the elements 234 are oriented at an angle of about 90° from the machine direction, i.e., in the absolute cross-machine direction, the perforation of the web may be substantially in the direction of about 90° from the machine direction and, thus, the degradation of web strength is substantially in the machine direction. In another embodiment, when the elements 234 are oriented at an angle from the absolute cross-machine direction, degradation of strength in the machine direction will be less and degradation of strength in the cross-machine direction will be more as compared to a system where the elements 234 are in the absolute cross-machine direction.
The angle of the elements 234 may be selected based on the desired properties of the end product. Thus, the selected angle may be any angle that results in the desired end product. In an embodiment according to the present invention, the cross-machine direction elements 234 are oriented at an angle of at least about 60° from the machine direction of the web and less than about 120° from the machine direction of the web. In another embodiment, the cross-machine direction elements 234 are oriented at an angle from at least about 75° from the machine direction of the web and less than about 105° from the machine direction of the web. In yet another embodiment, the cross-machine direction elements 234 are oriented at an angle from at least about 80° from the machine direction of the web and less than about 100° from the machine direction of the web. In still another embodiment, the cross-machine direction elements 234 are oriented at an angle of about 85° to about 95° from the machine direction.
A variety of element shapes may be successfully used in the present invention for embossing the web in a cross-machine direction. The element shape is the “footprint” of the top surface of the element, as well as the side profile of the element. The elements 234 may have a length (in the cross-machine direction)/width (in the machine direction) (L/W) aspect ratio of at least about 1.0, however the elements 234 may have an aspect ratio of less than about 1.0. In a further embodiment, the aspect ratio may be about 2.0. One element shape that can be used in this invention is a hexagonal element, as depicted in
In one embodiment for embossing the web in the cross-machine direction, at least a portion of the elements 234 are beveled. In particular, in one embodiment the ends of a portion of the elements 234 are beveled. Oval elements with beveled edges are depicted in
The cross-machine direction sidewall of the elements 234 defines the cutting edge of the elements 234. According to one embodiment of the present invention, the cross-machine direction sidewalls of the elements 234 are angled. As such, when the cross-machine direction sidewalls are angled, the base of the element 234 has a width that is larger than that of the top of the element. In one embodiment, the cross-machine direction sidewall angle may be less than about 20°. In another embodiment, the cross-machine direction sidewall angle may be less than about 17°. In still another embodiment, the cross-machine direction sidewall angle may be less than about 14°. In still yet another embodiment, the cross-machine direction sidewall angle may be less than about 11°. In various embodiments, the cross-machine direction sidewall angle may be between about 7° and 11°.
When the opposing elements 234 of the embossing rolls are engaged with each other during an embossing process, the effect on the web may be impacted by at least one of element spacing, engagement, and alignment. When perforate embossing, the elements 234 may be spaced such that the clearance between the sidewalls of elements of a pair, i.e., one element 234 from each of the opposing embossing rolls 222, creates a nip that perforates the web as it is passed though the embossing rolls 222. If the clearance between elements 234 on opposing rolls is too great, the desired perforation of the web may not occur. On the other hand, if the clearance between elements 234 is too little, the physical properties of the finished product may be degraded excessively or the embossing elements themselves may be damaged. The required level of engagement of the embossing rolls is a function of at least one of one or more embossing pattern properties (i.e., element array, sidewall angle, and element height) and one or more base sheet properties (i.e., basis weight, caliper, strength, and stretch). The clearances between the sidewalls of the opposing elements of the element pair should be sufficient to avoid interference between the elements. In one embodiment, the minimum clearance is about a large fraction of the thickness of the base sheet. For example, if a conventional wet press (CWP) base sheet having a thickness of 4 mils is being embossed, the clearance may be at least about 2 to about 3 mils. If the base sheet is formed by a process which may result in a web with rather more bulk, such as, for example, a through air dried (TAD) method or by use of an undulatory creping blade, the clearance may desirably be relatively less. Those of ordinary skill in the art will be able to determine the desired element spacing of the present invention based on the factors discussed above using the principles and examples discussed further herein.
As noted above, in one embodiment the height of the cross-machine direction embossing elements 234 may be at least about 30 mils. In another embodiment, the height may be from about 30 to about 65 mils. Engagement, as used herein, is the overlap in the z-direction of the elements from opposing embossing rolls when they are engaged to form a perforating nip. The engagement overlap should be at least 1 mil. In one embodiment, the engagement is at least about 15 mils. In another embodiment, the engagement is at least about 35 mils. In yet another embodiment, the engagement is at least about 45 mils. In yet a further embodiment, the engagement is at least about the depth of a Taurus blade.
In one embodiment, the engagement between the cross-machine direction embossing elements is at least about 15 mils. Various engagements are depicted in
In one embodiment, where the element height is about 42.5 mils and the elements have sidewall angles of from about 7° to about 11°, the engagement range between the cross-machine direction embossing elements may be from about 16 to about 32 mils.
The element alignment also affects the degradation of the web in the machine and cross-machine directions. Element alignment refers to the alignment in the cross-machine direction within the embossing element pairs when the embossing rolls are engaged.
As noted above, the elements can be both in the machine direction and cross-machine direction.
In another embodiment, depicted in
Those of ordinary skill in the art will understand that numerous different configurations of the above described element parameters, i.e., element shape, angle, sidewall angle, spacing, height, engagement, and alignment, may be employed in the present invention. The selection of each of these parameters may depend upon the base sheet used, the desired end product, or a variety of other factors.
One factor that impacts these parameters is “picking” of the web as it is embossed. Picking is the occurrence of fiber being left on an embossing roll or rolls as the web is embossed. Fiber on the roll can diminish the runability of the process for embossing the web, thereby interfering with embossing performance. When the performance of the embossing rolls is diminished to the point that the end product is not acceptable or the rolls are being damaged, it is necessary to stop the embossing process so that the embossing rolls can be cleaned. With any embossing process, there is normally a small amount of fiber left on the roll which does not interfere with the process if the roll is inspected periodically, i.e., weekly, and cleaned, if necessary. For purposes of the invention, picking is defined as the deposition of fiber on a roll or rolls at a rate that would require shut down for cleaning more frequently than once a week.
The following examples exhibit the occurrence of picking observed in certain arrangements of cross-machine direction perforate embossed patterns. This data was generated during trials using steel embossing rolls engraved with the cross-machine direction beveled oval embossing pattern at three different sidewall angles. In particular, the embossing rolls were engraved with three separate regions on the rolls—a 7° sidewall angle, a 9° sidewall angle, and an 11° sidewall angle. Two trials were performed. In the first trial, the embossing rolls had an element height of 45 mils. The base sheet, having a thickness of 6.4 mils, was embossed at engagements of 16, 24, and 32 mils. In the second trial, the steel rolls were modified by grinding 2.5 mils off the tops of the embossing elements, thereby reducing the element height to 42.5 mils and increasing the surface area of the element tops. The base sheet having a thickness of 6.2 mils was embossed at engagements of 16, 24, 28, and 32 mils. For each trial, embossing was performed in both half step and full step alignment.
The element clearances for each of the sidewall angles of the first and second trials have been plotted against embossing engagement in
Based on the observed data, it appears that picking is a function of the element height, engagement, spacing, clearance, sidewall angle, alignment, and the particular physical properties of the base sheet, including base sheet caliper. An example of element clearance can be seen in
This may be compared to the clearances shown in
Thus, based on the collected data, picking may be controlled by varying element height, engagement, spacing, clearance, alignment, sidewall angle, roll condition, and the physical properties of the base sheet. Based upon the exemplified information, those of ordinary skill in the art will understand the effects of the various parameters and will be able to determine the various arrangements that will at least achieve a non-picking operation, i.e., the configuration required to avoid an unacceptable amount of picking based on the factors discussed above, and, hence, produce acceptable paper products with a process that does not require excessive downtime for roll cleaning.
To establish the effectiveness of the various element patterns in perforating the web in the cross-machine direction, and thereby degrading machine direction strength while maintaining cross-machine direction strength, a test was developed, the transluminance test, to quantify a characteristic of perforated embossed webs that is readily observed with the human eye. A perforated embossed web that is positioned over a light source will exhibit pinpoints of light in transmission when viewed at a low angle and from certain directions. The direction from which the sample must be viewed, i.e., machine direction or cross-machine direction, in order to see the light, is dependant upon the orientation of the embossing elements. Machine direction oriented embossing elements tend to generate machine direction ruptures in the web which can be primarily seen when viewing the web in the cross-machine direction. Cross-machine direction oriented embossing elements, on the other hand, tend to generate cross-machine direction ruptures in the web which can be seen primarily when viewing the web in the machine direction.
The transluminance test apparatus, as depicted in
The test is performed by placing the sample 250 in the desired orientation on the light table 248. The detector 246 is placed on top of the sample 250 with the long axis of the tube 244 aligned with the axis of the sample 250, either the machine direction or cross-machine direction, that is being measured and the reading on a digital illuminometer 252 is recorded. The sample 250 is turned 90° and the procedure is repeated. This is done two more times until all four views, two in the machine direction and two in the cross-machine direction, are measured. In order to reduce variability, all four measurements are taken on the same area of the sample 250 and the sample 250 is always placed in the same location on the light table 248. To evaluate the transluminance ratio, the two machine direction readings are summed and divided by the sum of the two cross-machine direction readings.
To illustrate the results achieved when perforate embossing with cross-machine direction elements as compared to machine direction elements, a variety of webs tested according to the above-described transluminance test. The results of the test shown in Table 3.
A transluminance ratio of greater than 1.000 indicates that the majority of the perforations are in the cross-machine direction. For embossing rolls having cross-machine direction elements, the majority of the perforations are in the cross-machine direction. And, for the machine direction perforated webs, the majority of the perforations are in the machine direction. Thus, the transluminance ratio can provide a ready method of indicating the predominant orientation of the perforations in a web.
As noted above, perforated embossing in the cross-machine direction preserves cross-machine direction tensile strength. Thus, based on the desired end product, a web perforate embossed with a cross-machine direction pattern will exhibit one of the following when compared to the same base sheet embossed with a machine direction pattern: (a) a higher cross-machine direction tensile strength at equivalent finished product caliper, or (b) a higher caliper at equivalent finished product cross-machine direction tensile strength.
Dry tensile strengths (MD and CD) are measured with a standard Instron test device which may be configured in various ways, using 3-inch wide strips of tissue or towel, conditioned at 50% relative humidity and 23° C. (73.4° F.), with the tensile test run at a crosshead speed of 2 in/min. Tensile strengths are sometimes reported herein in breaking length (BL, km).
Following generally the procedure for dry tensile, wet tensile is measured by first drying the specimens at 100° C. or so and then applying a 1½ inch band of water across the width of the sample with a Payne Sponge Device prior to tensile measurement.
Alternatively, for testing the wet tensile strength, a Finch cup tester can be used. A Finch cup is a constant-rate-of-elongation tensile tester and is available from High-Tech Manufacturing Services, Inc., Vancouver, Wash.
Furthermore, the tensile ratio (a comparison of the machine direction tensile strength to the cross-machine direction tensile strength—MD strength/CD strength) of the cross-machine perforate embossed web typically will be at or below the tensile ratio of the base sheet, while the tensile ratio of the sheet embossed using prior art machine direction perforate embossing typically will be higher than that of the base sheet. These observations are illustrated by the following examples.
Higher cross-machine direction strength at equivalent caliper is demonstrated in Table 4. This table compares two products perforate embossed from the same base sheet—a 29 pounds per ream (lbs/R), undulatory blade-creped, conventional wet press (CWP) sheet.
As shown in Table 4, the cross-machine direction perforate embossed web has approximately the same caliper as the machine direction perforate embossed web (144 vs. 140 mils, respectively), but its cross-machine direction dry tensile strength (3039 g/3″) is considerably higher than that of the machine direction hexagonal-embossed web (1688 g/3″). In addition, compared to the tensile ratio of the base sheet (1.32), the cross-machine direction perforate embossed web has a lower ratio (1.16), while the machine direction perforate embossed web has a higher ratio (2.58). Thus the method of the present invention provides a convenient, low cost way of “squaring” the sheet—that is, bringing the tensile ratio closer to about 1.0.
Higher caliper at equivalent finished product cross-machine direction tensile strength is illustrated by three examples presented in Table 5. For each example a common base sheet (identified above each data set) was perforate embossed with a cross-machine direction and a machine direction oriented pattern (Hollow Diamond is a machine direction oriented perforate emboss).
In each case, the cross-machine direction perforate embossed product displays enhanced caliper at equivalent cross-machine direction dry tensile strength relative to its machine direction perforate embossed counterpart. Also, the cross-machine direction perforate embossed product has a lower tensile ratio, while the machine direction perforate embossed product a higher tensile ratio, when compared to the corresponding base sheet.
By employing cross-machine direction perforate embossing, the current invention further allows for a substantial reduction in base paper weight while maintaining the end product performance of a higher basis weight product. As shown below in Table 6, wherein the web is formed of recycled fibers, the lower basis weight cross-machine direction perforate embossed towels achieved similar results to machine direction perforate embossed toweling made with higher basis weights.
In Table 6, two comparisons are shown. In the first comparison, a 24.1 lbs/ream machine direction perforated web is compared with a 22.2 lbs/ream cross-machine direction perforated web. Despite the basis weight difference of 1.9 lbs/ream, most of the web characteristics of the lower basis weight web are comparable to, if not better than, those of the higher basis weight web. For example, the caliper and the bulk density of the cross-machine direction perforated web are each about 10% higher than those of the machine direction perforated web. The wet and dry tensile strengths of the webs are comparable, while the Sintech modulus of the cross-machine direction perforated web (i.e., the tensile stiffness of the web, where a lower number may be preferred) is considerably less than that of the machine direction perforated web. In the second comparison, similar results are achieved in the sense that comparable tensile ratios and physicals can be obtained with a lower basis weight web. Paradoxically, consumer data indicates that the 28#29C8 product was rated equivalent to the 30.5#HD product while the 22#30C6 product was at statistical parity with the 20204 product, but was possibly slightly less preferred than the 20204 product.
In one embodiment, a web formed of lignin-rich, high coarseness generally tubular fiber, such as BCTMP, is embossed with at least a cross-machine direction embossing pattern. A series of one-ply wet-creped towels were prepared using different creping blades and furnish compositions, including BCTMP. Specifically, the furnish composition was predominantly recycled fiber supplemented by various amounts of BCTMP as shown in Table 7. In each of the examples in Table 7 the amount of wet strength resin (in pounds/ton) was optimized and the basis weight was 28.0 lbs/ream. After the towel was manufactured, it was embossed with a cross-machine direction oval design, as indicated in
It can be seen from
The CD wet tensile strength of the product may be greater than about 500 g/3″. In one embodiment, the CD wet tensile strength may be greater than about 700 g/3″. The sheet may have a wet/dry CD tensile ratio of at least about 20%. In one embodiment, the wet/dry CD tensile ratio may be at least about 25%. In yet another embodiment, the wet/dry CD tensile ratio may be at least about 30%.
Following generally the procedures set forth above, a series of one-ply wet-creped towels were prepared and embossed as indicated in Table 8. The various properties of the towels were then measured.
The “void volume ratio,” as referred to hereafter, is determined by saturating a sheet with a non-polar liquid and measuring the amount of liquid absorbed. The volume of liquid absorbed is equivalent to the void volume within the sheet structure. The percent weight increase (PWI) is expressed as grams of liquid absorbed per gram of fiber in the sheet structure times 100, as noted hereinafter. More specifically, for each single-ply sheet sample to be tested, a 1 inch by 1 inch square (1 inch in the machine direction and 1 inch in the cross-machine direction) is cut out of each of eight selected sheets. For multi-ply product samples, each ply is measured as a separate entity. Multiple samples should be separated into individual single plies and 8 sheets from each ply position used for testing. The dry weight of each test specimen is weighed and recorded to the nearest 0.0001 gram. The specimen is placed in a dish containing POROFIL™ liquid having a specific gravity of 1.875 grams per cubic centimeter, available from Coulter Electronics Ltd., Luton, England (Part No. 9902458). After 10 seconds, the specimen is grasped at the very edge (1-2 millimeters in) of one corner with tweezers and removed from the liquid. The specimen is held with that corner uppermost and excess liquid is allowed to drip for 30 seconds. The lower corner of the specimen is then lightly dabbed (less than ½ second contact) on #4 filter paper (Whatman Lt., Maidstone, England) in order to remove any excess of the last partial drop. The specimen is immediately weighed, i.e., within 10 seconds, and the weight recorded to the nearest 0.0001 gram. The PWI for each specimen, expressed as grams of POROFIL per gram of fiber, is calculated as follows:
PWI=[(W2−W1)/W1]×100%
wherein
“W1” is the dry weight of the specimen, in grams; and
“W2” is the wet weight of the specimen, in grams.
The PWI for all eight individual specimens is determined as described above and the average of the eight specimens is the PWI for the sample.
The void volume ratio is calculated by dividing the PWI by 1.9 (density of fluid) to express the ratio as a percentage.
The water absorbency rate (WAR) of the sheet of the present invention may be at least about 10% less than that of an alike or equivalent sheet prepared without the use of an undulatory creping blade or at least about 10% less than that of an alike or equivalent sheet made without high coarseness, tubular fibers. These differences are particularly apparent from
The towels described above and in Table 8 were submitted for consumer testing and given an overall rating. Testing was conducted by consumers who rated the products for drying hands, feel, overall appearance, thickness, strength when wet, absorbency, speed of absorbency, texture, ease of dispensing, being clothlike, softness, durability, among other factors. An overall rating was also assigned. Results for this test appear in
In
In one embodiment of the present invention, the web may be embossed with two embossing rolls, with at least one roll having both perforate embossing elements extending substantially in the cross-machine direction and elongated embossing elements extending substantially in the machine direction. For example, as shown in
The cube emboss pattern depicted in
In one embodiment, the elongated embossing elements may have a length of at least about 0.25″. In another embodiment, the elongated elements may have a length of at least about 0.50″. In one embodiment, the element engagement range with the web when cube embossing can be from about 18 mils to about 90 mils. In another embodiment, the element engagement range with the web when cube embossing can be from about 30 mils to about 80 mils. And in yet another embodiment, the element engagement range with the web when cube embossing can be from about 50 mils to about 70 mils.
As shown in the following tables, CWP paper towel products made with various combinations of cube embossing, cross-machine direction embossing, undulatory creping, and BCTMP are equivalent or superior to TAD paper towel products, regardless of whether virgin pulp or recycled fibers are used. Table 9 includes various combinations of cross-machine direction embossing, cube embossing, and undulatory creping. Table 10 adds the additional variable of a web containing lignin-rich, high coarseness, generally tubular fiber, specifically, BCTMP. In each table, the CWP paper towel products are compared to TAD paper products (samples G and H) and to a CWP product (sample F) not within the scope of the present invention.
In one embodiment of the present invention, the web may be both cube embossed and additionally embossed in substantially the cross-machine direction. Specifically, in one embodiment, a first roll and a second roll are provided, the first and second rolls defining a nip. At least one of the first and second rolls may include elongated embossing elements extending in substantially the machine direction, at least one of the first and second rolls may include elongated embossing elements extending in substantially the cross-machine direction, and at least one of the rolls may include substantially cross-machine direction embossing elements. The substantially cross-machine embossing elements may be perforate embossing elements. Those of ordinary skill in the art will readily appreciate that the various embossing elements may be provided on any of the embossing rolls in any combination.
As noted above, embossing only in the cross-machine direction reduces the machine direction tensile strength while maintaining the cross-machine direction tensile strength, as evidenced by the Dry MD/CD tensile ratios. Specifically, sample F, a CWP paper towel having no cross-machine direction embossing, has a dry MD/CD tensile ratio of approximately 2.75, while the cross-machine direction embossed samples in Tables 4 and 5 have dry MD/CD tensile ratios ranging from 1.16 to 1.88. When the paper towel is then cube embossed in the machine direction, the machine direction tensile strength is decreased less than the cross-machine direction strength. Likewise, when the paper towel is perforate embossed in the cross-machine direction, the cross-machine direction tensile strength is decreased less than the machine direction strength. Thus, the effect of combining the two emboss patterns is a machine direction to cross-machine direction tensile ratio that is comparable to that found in TAD towels. Specifically, samples B and C, above, have dry MD/CD tensile ratio of 1.53 and 1.34, respectively, while the TAD towels, samples G and H, have ratios of 1.43 and 1.94, respectively. Moreover, the effect of using the cube emboss alone is a paper towel product having dry MD/CD tensile ratios comparable to TAD towels. Specifically, samples C and D have dry MD/CD tensile ratios of 1.65 and 1.66, respectively. Not being bound by theory, it is believed this is the result of the cube emboss having a portion of its embossing elements oriented in the cross machine direction.
Because the perceived strength of a paper towel is often determined by the consumer when the towel is wet, the wet properties of a towel have an impact on the overall consumer acceptance of a product. Comparing samples A, B, and C with the TAD samples G and H, as well as with a traditional CWP towel, sample F, shows that the wet CD tensile of samples A, B, and C may approach or exceed that of the prior art TAD and CWP-paper towels. Additionally, CD wet/dry ratio is an indication of the perceived softness and strength of the towel. Specifically, the higher the CD wet/dry ratio, the greater the perceived softness and strength. As indicated above, the CD wet/dry ratio of the paper towel sample A, having machine direction and cross-machine direction embossing and being creped with an undulatory blade, is generally equal to or greater than the ratios for the TAD paper towels and the prior art CWP paper towel. Finally, the Sintech modulus of the paper towels of the present invention (i.e., the tensile stiffness of the web, which relates to softness and where a lower number may be preferred) is generally equal to or less than that of the TAD and prior art CWP towels when the web is embossed in both the machine direction and cross-machine direction.
The addition of BCTMP to the pulp does not adversely affect the results discussed above. Regarding dry MD/CD ratio, sample J in Table 10, which was cross-machine direction embossed, but not cube embossed, had a ratio of 1.07. Additionally, samples I and K in Table 10, which were both cross-machine direction and cube embossed, each had dry MD/CD ratios lower than the commercially available CWP towel. And sample K in Table 10, which was formed from recycled fibers, had a dry MD/CD ratio that was lower than the TAD products. Moreover, the paper towel products of samples I and K achieved or exceeded the CD wet/dry ratio of the commercially available CWP towel, as well as the TAD products. As noted above, CD wet/dry ratio is an indication of the perceived softness and strength of the towel. Finally, the Sintech modulus of the paper towels of the present of samples I and K is less than that of the TAD and prior art CWP towels.
Consumer testing supports the physical data set forth above. Specifically, six paper towel products were tested in a consumer setting. Each selected consumer sampled five of the six towels and was asked to evaluate the towel overall, as well as on key attributes. Additionally, observational data on the number of towels used, tabbing, and dispensing was recorded by the observer. Table 11 presents the results of the data. Samples F and G in Table 11 are current commercial products.
Based on the consumer tests, sample H in Table 11, a CWP paper towel having both cross-machine directional and cube embossing and 38% BCTMP, was comparable overall to the two current commercial products against which it was compared. Not only was the overall rating for the towel comparable, but the ratings on other characteristics, such as drying hands, appearance, hand feel, softness, and texture, were also comparable. Moreover, sample E, a TAD paper towel having both cross-machine directional and cube embossing, also compared overall to the current commercial products. As with sample H, not only was the overall rating comparable, but also the ratings of the characteristics noted above.
The combination of cube embossing and cross-machine direction embossing of a web also results in a CWP product having equivalent or superior softness as compared to a TAD product, as evidenced by an increased drape angle of the cube embossed/cross-machine direction embossed product. Drape angle, as used herein, is the angle of the non-supported portions of a web as the web rests on a rod. An exemplary drape angle measurement tester is depicted in
In the drape test, four different paper towel products were tested. Additionally, for each of the products, two different test comparisons were made. In the first test, the towels were cut such that the weights of the towels were similar. In the second test, the dimensions of the tested towels were identical. The results are shown in Tables 12 and 13, respectively.
The results of the test indicate unexpected softness in paper formed by CWP methods when the towel is embossed with cross-machine direction embossing and cube emboss. Specifically, sample B, which contained 38% BCTMP, was creped with an undulatory creping blade, and then cross-machine direction and cube embossed, had a substantially lower drape angle than the TAD product and, hence, was substantially softer than the TAD product. Moreover, the uncreped CWP towel exhibited similar draping characteristics as the TAD towel when similar sized sample portions were used.
The towels of the present invention may be folded, unfolded, or rolled. Moreover, a folded towel may be folded longitudinally, i.e., in the machine direction, or transversely, i.e., in the cross-machine direction, or folded both longitudinally and transversely. In one embodiment of the present invention, the paper towel is folded using a conventional automated folder. Suitable folders are manufactured by G. C. Bretting Manufacturing Co. and are also described in U.S. Pat. Nos. 6,547,909, 6,539,829, 6,508,153, 6,488,194, 6,431,038, 6,372,064, 6,322,315, 6,296,601, 6,254,522, 6,227,086, 6,138,543, 6,051,095, 6,000,657, 5,941,144, 5,820,064, 5,772,149, 5,755,146, 5,643,398, 5,584,443, 5,299,793, 6,226,611, 4,997,338, 4,917,665, 4,874,158, 4,778,441, 4,770,402, 4,765,604, 4,751,807, 4,475,730, 4,270,744, 4,254,947, and 3,709,077, each of which is incorporated herein by reference in its entirety.
While the invention has been described in connection with numerous examples, modifications thereto within the spirit and scope of the present invention will be readily apparent to those of skill in the art.
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