IMPROVED METHOD OF SOFTENING PULP AND PRODUCTS PRODUCED BY SAME

Abstract: Wood pulp sheets treated with triacetin and other compounds, or solutions or emulsions of same, having increased softness while maintaining absorbency, and methods for producing same. More particularly, the invention relates to the treatment of wood pulp useful for making a fluff pulp using a softening agent including alkyl ethers, aryl ethers and formic, ethanoic and propanoic esters of low molecular weight glycols, such as triacetin, propylene glycol diacetate and 2-phenoxyethanol.

Description: TITLE OF THE INVENTION IMPROVED METHOD OF SOFTENING PULP AND PULP PRODUCTS PRODUCED BY SAME This application is a continuation-in-part of co-pending United States Patent Application Ser. No. 08/731,142; filed October 10, 1996. BACKGROUND OF THE INVENTION Field of the Invention This invention relates to softened wood pulps with good absorption properties, and a process for making such pulps. Description of the Related Art It is desirable for many industrial applications to produce a cellulosic wood pulp which maximizes both softness and absorbency. The softness of a pulp product is greatly influenced by the degree to which the constituent wood pulp is debonded, i.e., the extent to which hydrogen bonds within the wood pulp are broken; softer pulps and pulp products typically having decreased hydrogen bonding. Wood pulp softness can be expressed in terms of properties such as Mullen strength (the strength of pulp or a pulp product, measured in kilopascals (kPa) , defined in greater detail below) , and Kamas energy (the energy required to convert a given amount of pulp or pulp product to a fluff material, measured in watt hours per kilogram (Wh/kg) , defined in greater detail below) . Lower values of Mullen strength and Ka as energy correlate to softer, increasingly debonded, pulp. Many industrial pulp applications involve the conversion of pulp to fluff pulp by mechanical means. Fluff pulp has the inherent characteristics of bulk, softness, high absorbency, and resiliency. Resiliency often depends on the length, diameter, and stiffness of the pulp fibers. Long, stiff fibers will provide greater bulk and resiliency than short, flexible fibers due to their relatively larger interfiber distances to compaction. The inter-fiber voids formed in fluff or debonded pulp determine to a large extent the absorbency of the pulp. Large void areas lead to higher absorbency since it is these void areas that hold moisture. The efficient mechanical fluffing of wood pulp requires a pulp that will debond to a desirable degree with minimum power input and little mechanical fiber damage. Such pulp must have the proper bulk and degree of inter-fiber bonding. A hard pulp sheet will increase the power needed to create fluff pulp and will therefore lead to increased fiber damage. An unduly soft pulp sheet will lead to pull-out of large pieces of pulp, causing poor fluffing. It is known that the use of cationic surfactants in the manufacture of wood pulp products, for instance sanitary papers, yields a product which has a soft hand feel. This is accomplished through the lubricating nature of the substantive softening molecule; less extensive inter-fiber bonding leading to greater bulk and the plasticizing effect of these additives. Cationic surface-active agents are used in the manufacture of fluffed debonded pulp to increase bulk, reduce Mullen strength (and hence, reduce inter-fiber bonding) , and impart softness to fibers. The lubricating effect of these agents prevents extensive inter-fiber bonding and increases the bulk of a machine formed pulp sheet. In fluffing, such agents improve the debonding characteristics of a pulp sheet. This results in lower power requirements and less fiber damage. Reduced fiber damage produces a fluffed pulp with better bulk and resiliency. In the manufacture of fluff or debonded cellulosic pulp, cationic surfactants are used primarily to reduce the inter-fiber bonding of pulp sheets. Reduced inter-fiber bonding is normally associated with a significant reduction in Mullen strength. Since a significant amount of energy is required to convert pulp to final fluff product, the use of debonded pulp reduces overall energy costs of conversion. It has long been accepted in the paper making industry that pulp softening and debonding cannot be accomplished utilizing cationic surfactants (or even nonionic debonders which enjoy limited use) without sacrificing absorbency properties of wood pulp. It is generally believed that debonding pulp with hydrophobic materials, such as cationic surfactants, results in the reduction of absorbent properties. Reductions in absorbent properties using standard cationic quaternary ammonium compounds for debonding can be quite substantial (e.g., 18% reductions for a partially debonded southern bleached kraft pulp and about 27% reductions for a fully debonded southern bleached kraft pulp) . There are four cationic chemical materials used to soften pulp to produce a fluff or debonded pulp. All of these materials are quaternary ammonium compounds typified by a nitrogen ion attached by covalent bonds to four organic groups. An anion, usually a halide (e.g., a chloride) or sulfate group, is associated with the positive ion of the quaternary nitrogen. Examples of such quaternary ammonium compounds include the following generic structures: Dialky l Dimethyl Quarternary Ammoni um Compound DIamldo Quarternary Ammonium Compound Dial yl Quarternary Ammonium Compound Imidazoline Quarternary Ammonium Compound Dialkyl Pheno l Alkoxy latθd Quarternary Ammonium Compound In the above drawings R is typically a C-12 to C-18 alkyl group or a C-9 aryl group, as appropriate. X is typically a halide or sulfate ion. Values for n range from 2-30. The above-depicted cationic surface-active softening and debonding agents can be supplied as liquids, pastes, powders, solutions in water and alcohol, or solutions in water alone. However, quaternary ammonium debonding and softening agents are generally applied as dilute emulsions of water. Addition of a highly diluted emulsion is preferred since this assures uniform distribution of the debonder. The range of surfactant treatment rates required for use as a debonding agent is usually between 3-10 pounds per ton of pulp. The range of treatment rates required for a softening agent is usually between 1-6 pounds per ton of pulp. As one skilled in the art would recognize from reviewing the above-depicted chemical structures, there are many species of quaternary ammonium materials which can be used to improve softness and debond wood pulp. There are advantages and disadvantages to each type. However, the use of any type of quaternary ammonium compound to soften or debond cellulosic wood pulp uniformly has the disadvantage of adverse effects on absorbency. Nonionic agents are also used to a limited extent to debond pulp in the paper industry (e.g. Berocell 587, Eka Nobel) but even they cause adverse affects on absorbency. It is believed that this effect is due to the presence of long hydrophobic side chains. Accordingly, it would be desirable to provide a method of treating pulp to form fluff pulp with improved bulk, softness and reduced inter-fiber bonding without sacrificing the absorbent properties of the pulp. OBJECTS OF THE INVENTION It is an object of the present invention to overcome the above-mentioned difficulties in the prior art. It is another object of the present invention to provide a method for improving the characteristics of a wood pulp without significantly decreasing the absorbency of the wood pulp and an improved wood pulp product by same. It is yet another object of the present invention to provide a method for softening wood pulp. It is a still further object of the present invention to provide a wood pulp product having improved bulk, softness and/or reduced inter-fiber bonding without decreased absorbency. The foregoing and other objects and advantages of the invention will be set forth in following description or shall be apparent from it. SUMMARY OF THE INVENTION The invention relates to the treatment of wood pulp useful for making a fluff pulp preferably for absorbency intensive applications. The invention also relates to pulp products having improved characteristics made by the inventive methods. More specifically, the invention relates to a method of treating wood pulp with a softening agent to soften the pulp without adversely affecting the absorbency of the material. Preferably, the softening agent is selected from alkyl ethers or aryl ethers (e.g., methyl ethers) and formic, ethanoic and propanoic esters of low molecular weight glycols (e.g. acetates) , for instance triacetin, propylene glycol diacetate and 2-phenoxyethanol, to result in a pulp that is notably softer than pulp not treated with such material. The wood pulp is treated by applying to the wood pulp a sufficient amount of a material comprising the softening agent. When the invention is practiced industrially, the inventive softening agents can be applied to wood pulp in aqueous solution which can be made up in a holding tank or prepared continuously with an in-line static mixer, or by spraying the inventive softening agents onto dried pulp sheets. In the manufacture of absorbent pulp sheets, these agents can be added to a fiber slurry at the machine chest, fan pump or head box. They can also be applied by spray application to a wet pulp sheet or can be applied via a "dip and nip" procedure in which an evacuated but not completely dried pulp sheet is dipped into a solution containing the agent and subsequently pressed. Additionally, it has also been found that excellent results are achieved when softening agents of the invention are sprayed, rolled or printed onto one or both sides of a pulp sheet, including a pulp sheet having 15% or less moisture on a weight basis. Pulp sheets containing 15% or less moisture are referred to herein as dry pulp sheets. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates a graphical representation of the dosage-Kamas energy relationship of the treatment process according to the present invention depicting triacetin dosages ranging from 0.00 - 2.50% applied to untreated southern pine kraft wood pulp. Fig. 2 illustrates a graphical representation of a dosage-Mullen strength relationship of the treatment process according to the present invention depicting triacetin dosages ranging from 0.00 - 2.50% applied to untreated southern pine kraft wood pulp, with the control pulp having a Kamas energy level of 72.2. Fig. 3 illustrates a graphical representation of a dosage-Mullen strength relationship of the treatment process according to the present invention depicting triacetin in dosages ranging from 0.00 - 1.20% by weight applied to untreated southern pine kraft wood pulp, with the control pulp having a Kamas energy level of 55.6. DESCRIPTION OF PREFERRED EMBODIMENTS Definitions The following definitions are provided to assist in understanding the true nature of the invention: "Wood pulp" as herein described refers to a cellulosic material obtained from wood produced according to a pulping process including but not limited to sulfite, kraft and thermomechanical pulping processes, and in which lignin and other cellulose pulp impurities may be removed in whole or in part by a process which includes but is not limited to an oxidation or other bleaching process, wherein cellulosic hydroxyl groups naturally present in the cellulosic material have not been chemically substituted or derivatized. Cellulose ether and acetate end-use derivative products are not considered wood pulp. The term "softened pulps" refers to fibrous end-use wood pulps (for example, fluff pulps) that have some chemical agent (softener) added to soften the pulp, preferably by reducing interfiber bonding (addition of the softener results in a soft pulp sheet) . The chemical agents (softeners) are commercial products added to fluff pulps during sheet forming which make the pulp sheet softer and easier to fluff or defiber. The force with which pulp fibers bond is measured indirectly by measuring Mullen strength or the force (or energy) expended to debond or fluff a given pulp sheet. "Mullen strength" refers to the hydrostatic pressure, typically measured in kilopascals, required to produce rupture of a material under certain experimental conditions. Mullen strength is determined on some of the products presented in the examples using a method based on TAPPI T807. A TMI Monitor Burst 1000 is used to measure the hydrostatic pressure required to rupture a pulp sheet. Mullen strength is recorded as kPa at rupture . "Kamas energy" is the energy required to convert a given amount of pulp or pulp product to a fluff material measured in watt hours per kilogram (Wh/kg) . A Kamas Lab hammermill Model H-01-C was used to defiberize some of the products presented in the examples. Strips of pulp sheets 5 cm wide were fed into the hammermill, using 4200 rpm motor speed, 50% feeder speed, and an 8 mm screen. The energy required to defiberize the pulp sheet is recorded, and reported as Wh/kg of fluff, the energy of defiberization. The term "absorbency" refers to the capacity of pulp to entrain and hold liquids. References herein to increased or decreased absorbency mean changes in the time required for a pulp sample to absorb liquid using the SCAN/PFI methodology described herein. The present invention relates to a method for improving the characteristics of a wood pulp without significantly decreasing the absorbency of the wood pulp by contacting the wood pulp with a softening agent. Preferably, the softening agent is selected from the group consisting of alkyl ethers, aryl ethers, formic, ethanoic and propanoic esters of low molecular weight glycols, including materials such as triacetin, propylene glycol diacetate and 2-phenoxyethanol, having low to moderate solubility in water. These softening agents, best represented by triacetin (glycerol triacetate) , have heretofore been principally used to improve the wet stiffness of cellulose acetate filter tow, acting as a "plasticizer" . (When triacetin is applied to cellulose acetate filter tow fibers, it imparts cohesive properties to the resulting filter upon compression of the fibers.) Triacetin is attractive for application to absorbent products in that it is approved for both food-grade and pharmaceutical applications. In addition to its use in stiffening acetate filter tow, triacetin is also used in the formulation of enteric coated capsules, as a perfume fixative and as a topical anti-fungal. There are no identified effects of exposure to triacetin related to skin contact or skin absorption. This makes the use of triacetin a particularly preferred embodiment of the invention. Treatment of wood pulp, for instance bleached kraft southern pine pulp, with triacetin has been found to reduce Mullen strength and Kamas energy levels by 15- 50%. Most significantly, little or no materially adverse effects on absorbency properties result from treatment of wood pulp with triacetin. This is a unique and novel finding since, as previously discussed, the materials previously used to soften and/or debond wood pulps, quaternary ammonium compounds, have negative performance effects on absorbency properties. Further, the softening effect was wholly unexpected as triacetin is used as a stiffener for cellulose acetate tow for cigarette filters. Moreover, it has been found that treatment of wood pulp with alkyl or aryl ethers and formic, ethanoic and propanoic esters of low molecular weight glycols which have low to moderate solubility in water, for instance, not greater than 50 g per 100 g solution at 25°C, is most effective in softening wood pulp, as compared to members of the same class of compounds having high solubility in water. It should be noted that application of a softening agent to wood pulp is not limited to application in solution, and can also include application in pure form, or as an emulsion, suspension or dispersion. In another aspect of this invention, a softening agent selected from the group consisting of alkyl ethers, aryl ethers, formic, ethanoic and propanoic esters of low molecular weight glycols and mixtures thereof may be added to wood pulp to soften the wood pulp without regard to the effect, if any, of the softening agent on wood pulp absorbency. In a further aspect of this invention, triacetin, propylene glycol diacetate, 2-phenoxyethanol and mixtures thereof may be added to wood pulp to soften the wood pulp without regard to the effect, if any, of the softening agent on wood pulp absorbency. There is no prior art to suggest that the above-identified family of compounds can be used to soften wood pulp. Further, the discovery that triacetin can accomplish softening (i.e., reductions in Mullen strength) without negatively affecting absorbency properties contradicts conventional industry practice and knowledge. As the materials that work best in this invention are those that have limited water solubility, it is a preferred embodiment of the present invention to treat wood pulp with materials having solubility in water at 25 °C of no more than 50 g per 100 g solution, more preferably no greater than 15 g per 100 g solution, and even more preferably no greater than 9 g per 100 g solution. Various types of wood pulp (in sheet form) were tested to measure the effect that the softening agents of the present invention had on properties including Mullen strength, Kamas energy, absorption time and fluid retention. It was found that each of triacetin, propylene glycol diacetate and 2-phenoxyethanol reduce Kamas energy, reduce Mullen strength, and have a negligible effect on both absorption time and fluid retention properties. In light of the examples discussed below, and the data contained therein, the present invention provides a softened pulp wherein the Kamas energy of the pulp preferably reduced by 5 Wh/kg, more preferably is reduced by 15 Wh/kg and most preferably was reduced by 25 Wh/kg or more. Further, the present invention provides for a softened pulp wherein the Kamas energy of the pulp preferably is reduced by 5%, more preferably by 10%, even more preferably by 20%, even more preferably still by 30%, and most preferably by 40%. The present invention likewise provides a softened pulp wherein the Mullen strength is reduced by 100 kPa, more preferably reduced by 200 kPa, and most preferably reduced by 400 kPa or more. Further, the present invention provides for a softened pulp wherein the Mullen strength preferably is reduced by 5%, more preferably by 10%, even more preferably by 20%, more preferably still by 30%, even more preferably still by 40%, and most preferably by 50%. Further, the absorption time of the softened pulps of the present invention preferably increases by no more than 0.50 seconds, more preferably by no more than 0.25 seconds, and most preferably, not at all. Moreover, the retention of the softened pulps of the present invention, measured in grams retained per gram of pulp (g/g) , preferably decreases by no more than 0.50 g/g, more preferably by no more than 0.25 g/g and most preferably, not at all. Without being limited to a specific theory of operability, it is believed that hydrophobicity, an inherent quality which causes the materials of the present invention to possess only low to moderate solubility, is also responsible for the ability of these materials to interfere with intrafiber and interfiber hydrogen bonding. It is further believed that the increased interference with hydrogen bonding of low to moderate solubility is effected as follows: Because, for practical reasons, wood pulp is never fully (100%) dried, high solubility materials will remain to a large extent in solution even at substantial pulp dryness. Low to moderate solubility materials will not remain in solution, and will thus be able to more directly affect hydrogen bonding. According to one preferred embodiment, the softening agent comprises triacetin. Triacetin appears to be the most effective in softening and debonding wood pulp, as shown by the data set forth in the examples below. Triacetin has a solubility of 7 g - 7.8 g per 100 g solution (at 2 - 75 °C) . Other effective materials include propylene glycol diacetate, which has a room temperature solubility of approximately 8g per lOOg solution, and 2-phenoxyethanol, which has a room temperature solubility of approximately 3g per lOOg solution. On the other hand, triethylene glycol diacetate (TEGDA) , which (like triacetin) is a stiffener for cellulose acetate filter tow, but (unlike triacetin and the other softening agents disclosed herein) is completely water soluble, is not effective in softening or debonding pulp (see Example 3, below). Triacetin does not have long hydrophobic side chains such as those exhibited by the conventional cationic softener as well as nonionic softeners presently used in the industry. Without being limited to a theory of operability, it is believed that this is a factor that mitigates negative absorption affects. According to another embodiment, the softening agents comprise alkyl or aryl ethers (e.g., methyl ethers) and formic, ethanoic and propanoic esters of low molecular weight glycols (e.g., acetates) of low to moderate water solubility, such as propylene glycol diacetate and 2-phenoxyethanol, which are also effective in softening kraft wood pulps. The softening agents discussed above can be applied to wood pulp in a number of ways. One embodiment of the present invention relates to a method for softening wood pulp comprising the steps of applying the softening agent by dipping the wood pulp into a solution containing the softening material, pressing the wood pulp, and drying the wood pulp. Another embodiment relates to a method for softening wood pulp comprising the steps of adding the softening agent to a wood pulp slurry. Other embodiments of the present invention comprise applying the softening agent to wet or dry wood pulp by the spraying, rolling or printing it onto wood pulp sheets. Gravure type printing is a preferred method of applying the softening agent to pulp sheets. When the softening agent of the invention is applied to pulp sheets by spraying, rolling or printing, it has been found that significant reductions in the Kamas energy required to convert the treated pulp to a fluff state are achieved, with little or no reduction of pulp absorbency. Excellent results are achieved when the softening agent is applied to one or both sides of pulp sheets. Significant reductions in Mullen values are also obtained when pulp sheets are sprayed, rolled or printed on one or both sides with the inventive softening agent. The present invention also relates to compositions of matter produced by application of the presently disclosed softening agents to wood pulp. One embodiment of the present invention provides a composition of matter comprising treated wood pulp, wherein the wood pulp is treated by applying to wood pulp a sufficient amount of a material selected from the group consisting of alkyl ethers, aryl ethers, formic, ethanoic and propanoic esters of low molecular weight glycols, each having solubility in water of no more than 50g per lOOg solution. Another preferred embodiment of the present invention thus provides for a composition of matter comprising treated wood pulp, wherein said wood pulp is treated by applying to wood pulp a sufficient amount of a material selected from the group consisting of triacetin, propylene glycol diacetate, and 2-phenoxyethanol. Individual features or a plurality of individual features describing the inventive products or processes can also themselves form independent solutions according to the invention and one or more of the features can also be combined in any way. EXAMPLES Each of examples 1-10 described below demonstrate the surprising and advantageous results obtained by treating pulp with the presently disclosed softening agents (Example 3 is comparative) ; that is, a dramatic decrease in Mullen strength and/or a dramatic decrease in Kamas energy without the significant decrease in absorbency times and liquid retention associated with the conventional cationic or nonionic surfactants presently used as softening agents in the pulping industry. Test Procedures & Definitions In the tests described hereinafter, industry- employed standard test procedures have been used. If any deviations from standard test procedure have been made, such deviations have been identified. For purposes of evaluating the products obtained and described by the present disclosure as well as the invention herein, several tests were used to characterize the desirable fibrous wood pulp end-use performance improvements resulting from use of the presently disclosed softening agent treatment, and to describe some of the analytical properties of the pulp products. A summary of these tests and definitions follows: "Rayfloc-J-LD" is an untreated southern pine kraft pulp sold by Rayonier Inc. for use in applications requiring high absorbency. "Rayfloc-J-LD-E" is an untreated southern pine kraft pulp sold by Rayonier Inc. for use in applications requiring high absorbency. Rayfloc-J-LD-E differs from Rayfloc-J-LD only by its being processed principally without the use of elemental chlorine. "Rayfloc-J" and "Rayfloc-J-E" are slightly different versions of "Rayfloc-J-LD" and "Rayfloc-J-LD- E". "SCAN testing" of fluff pulp properties are carried out on some of the products presented in the examples. The test uses SCAN/PFI methodology (SCAN-C 33:80) and test equipment to form a uniform fluff sample, and to measure its resiliency, fluid retention and rate of absorption. The fluff samples are conditioned for at least 2 hours under standard conditions (23 +/- 1 degree C and 50% +/- 2% relative humidity) prior to testing and are kept in the conditioning atmosphere throughout the test. Typically, a cylindrical fluff sample (3.00 +/- 0.05 g and 5 cm diameter) is prepared using special equipment. The height of the cylinder under a 260 g/1.3 kPa load is measured and reported as resiliency. The sample is placed in contact with a water bath. The time required for the water to migrate vertically up the cylinder to the top is reported as absorption time. The fluid retention or absorption capacity per gram of sample is calculated by weighing the saturated fluff sample. "Pad Integrity" tests are carried out on some of the products presented in the examples. Pad integrity is a measure of the strength of the fiber network in fluffed pulps, and indicates how well the fluff will maintain pad integrity in a dry formed absorbent product. The method is based on PFI method 1981, "Measurement of Network Strength in Dry, Fluffed pulps". During the test, a cylindrical test pad of 1.0 +/- 0.05 gram and 50 mm diameter is prepared in a pad former. The test pad is placed in a burst chamber, which is then installed in a stress-strain apparatus. A burst body is vertically forced through the test pad. The force required to rupture the fiber network in the test pad is reported as pad integrity. Several experiments were conducted to demonstrate the effects of triacetin on the aforementioned debonding and absorbency properties of wood pulp. The following examples are illustrative of some of the methods and products made from the methods falling within the scope of the present invention. They are, of course, not to be considered in any way limitative of the invention. Numerous changes and modifications can be made with respect to the invention. Example l Highly absorbent southern pine kraft pulp, in the form of Rayfloc-J-LD-E pulp sheets, was treated with various triacetin levels ranging from 0.66 - 2.54% by weight of O.D. (oven dried) pulp. These samples were prepared by immersing dry machine-made pulp sheets in an aqueous solution of triacetin and then mechanically pressing the sheets to about 47% dryness. The amount of triacetin remaining in the wet pulp sheet after pressing was readily calculated from the increased weight of the wet sheet. The wet pulp sheets were placed into a hot Emerson dryer for about 30 minutes and brought to near dryness (-95% O.D.). By way of an illustration, a sheet weighing 59.7g O.D. was placed into a 2.4% aqueous solution of triacetin. After pressing, the same sheet weighed 123 g prior to drying. This corresponds to a triacetin dose rate on an O.D. basis of 2.54%. Results of evaluations on these pulps are presented in Table I, below, and in Figures 1 and 2. Table I. Treatment of Rayfloc-J-LD-E Pulpsheets With Triacetin Over the triacetin treatment range of 0.6 - 2.5% based on dry pulp, Kamas energy and Mullen strength were reduced by about 30 to 50%, with no negative impact on absorbency. SCAN absorbency characteristics including pad integrity were equal to the control pulp. Heated aged absorption times were actually improved in the treated pulps. Clearly, triacetin has a marked ability to increase pulp softness and to ease the defibering of pulp as reflected by the reductions in Mullen strength that it imparts. When a wood pulp is relatively hard prior to treatment with the inventive compositions, this Mullen strength reduction effect is accompanied by significant Kamas energy reduction in preparation of Fluff for absorbent products. Example 2 Untreated southern pine kraft pulp, in the form of Rayfloc-J-LD sheets, was treated with triacetin in the 0.3 - 1.2% range in the same manner as Example 1. The results of fluff absorbency tests on these samples, including Kamas energy and Mullen strength values, are presented in Table II, below, and in Figure 3. Table II. Treatment of Rayfloc-J-LD Pulp Sheets with Triacetin The control pulp itself, in this case, had a lower Kamas energy level (55.6) compared to the control in Example 1 (72.2). For this reason, the Kamas energy reductions are not as notable as they were in Example 1, but they are clearly discernable at the 1.16% dosage level. However, the Mullen strength reductions observed are notable over the full range of dosing and consistent with the trends previously observed. At the 0.3% dosage level, for instance, the Mullen strength reduction was about 17%. Over the dosage range of 0.3 - 1.2%, Mullen strengths were reduced by about 17-33%. There were no adverse effects on fluff absorbency properties, such as would have been found had the same pulp been treated with cationic surfactants to equivalent softness. Example 3 Absorbent southern pine kraft pulp, in form of Rayfloc-J-LD sheets, was treated with TEGDA (triethylene glycol diacetate) , to yield pulp sheets with TEGDA dosages in the 0.6 - 2.6% range, using the method described in Example 1 above. These sheets were tested to determine their Mullen strength, Kamas energy, absorption time and retention. The results are shown at Table III. Table III. Treatment of Rayfloc-J-LD Pulp Sheets with TEGDA The above tabulated results show no effect on Mullen strength. No significant Mullen reductions occurred as a result of treatment with TEGDA. This material is less effective than triacetin. As postulated previously, this lack of effectiveness may be related to a lack of hydrophobic character of TEGDA compared to triacetin and other substances effectively used in the present invention. Example 4 Triacetin and other alkyl and aryl ethers and esters of low molecular weight glycols with low to moderate water solubility are added to southern pine kraft wood pulp slurries at the machine chest, fan pump or head box of a sheet forming machine. Pulp sheets are formed from the treated pulps by standard methods, and the physical properties of these sheets are tested by accepted industry methods, described above, for Mullen strength, Kamas energy, absorption time and retention. It is noted that Mullen strength and Kamas energy of the treated pulps are reduced for concentrations of softening agent in the range of 0.1% to 10.0% by weight, while absorption time values and retention values are maintained without a significant decrease. Example 5 Triacetin, and other alkyl and aryl ethers, and esters of low molecular weight glycols with low to moderate water solubility are sprayed onto an evacuated but not yet dried sheets of absorbent southern pine kraft wood pulp. The pulp sheets are dried after treatment with the softening agents. The physical properties of the pulp sheets are tested by accepted methods described above for Mullen strength, Kamas energy, absorption time and retention. It is noted that Mullen strength and Kamas energy are reduced for concentration of softening agents in the range of 0.1% to 10.0% by weight, while absorption time values and retention values are maintained without a significant decrease. Example 6 Evacuated but not yet dried sheets of absorbent southern pine kraft pulp are dipped into solutions of triacetin, and other alkyl and aryl ethers and esters of low molecular weight glycols with low to moderate water solubility at various concentrations and then pressed and subsequently oven dried. The physical properties of the treated sheets are tested as described above for Mullen strength, Kamas energy, absorption time, and retention. It is noted that Mullen strength and Kamas energy are reduced for concentration of softening agents in the range of 0.1% to 10.0% by weight, while absorption time values and retention values are maintained without a significant decrease. Example 7 Propylene glycol diacetate and triacetin were separately applied to southern pine kraft pulp, in form of Rayfloc-J sheets. The pulp sheets were dipped into aqueous solutions of propylene glycol diacetate or triacetin, then blotted with paper toweling to remove excess water. The sheets were then weighed and hung up in a hood to air dry. More specifically, the aforedescribed pulp sheets, supported on a flexible wire-mesh screen, were dipped into an aqueous solution containing either 1.1% by weight propylene glycol diacetate or 1.1% by weight triacetin for 45 seconds. After blotting with paper toweling, applied under and over the sheet to remove excess water, the wet sheet weight was obtained and the sample was hung up in a hood to air-dry. A control group consisted of untreated Rayfloc-J pulp sheets. The Rayfloc-J pulp sheets treated with either the propylene glycol diacetate or the triacetin were softer to the touch, as well as bulkier, than control sheets without propylene glycol diacetate or triacetin. As detailed in the below Table IV, there were no statistically significant negative effects on absorption for wood pulps treated with propylene glycol diacetate or triacetin. Table IV. Treatment of Rayfloc-J with Triacetin and Propylene Glycol Diacetate (PGDAc) Kamas energy was reduced from 49.7 Wh/kg (control) to 43.5 Wh/kg (a 12% reduction) with treatment with PGDAc. Mullen strength was likewise reduced from 1051 kPa to 991 kPa (a 6% reduction) . In all cases, these wood pulp sheets were observed to be softer and thicker, and improved absorption times actually appeared from the treatment of the pulp sheets with propylene glycol diacetate or triacetin (10.9 seconds for triacetin, all 11.0 seconds for PGDAc, as compared to 15.0 seconds for the control). Example 8 2-phenoxyethanol and triacetin were separately applied to southern pine kraft pulp, in the form of Rayfloc-J-E sheets. The pulp sheets were dipped into aqueous solutions of 2-phenoxyethanol or triacetin, then blotted with paper toweling to remove excess water. The sheets were then weighed and hung up in a hood to air dry. More specifically, the aforedescribed pulp sheets, supported on a flexible wire-mesh screen, were dipped into an aqueous solution containing either 1.1% by weight 2-phenoxy-ethanol or 1.0% by weight triacetin for 45 seconds. After blotting with paper toweling, applied under and over the sheet to remove excess water, the wet sheet weight was obtained and the sample was hung up in a hood to air-dry. A control group consisted of untreated Rayfloc-J-E pulp sheets. The Rayfloc-J-E pulp sheets treated with either the propylene glycol diacetate or the triacetin were softer to the touch, as well as bulkier, than control sheets without 2-phenoxyethanol or triacetin. As detailed in the below Table V, there were no statistically significant negative effects on absorption for wood pulps treated with 2-phenoxyethanol or triacetin. Table V. Treatment of Rayfloc-J with Triacetin and 2-Phenoxyethanol (2-PETOH) Kamas energy was reduced from 52.2 wh/kg to 41.8 wh/kg with 2-PETOH treatment (a 20% reduction). Mullen was slightly reduced from 1029 kPa to 988 kPa. In all cases, these wood pulp sheets were observed' to be softer and thicker, with no negative absorption effects from the treatment of the pulp sheets with 2- phenoxyethanol or triacetin. Example 9 Dry Rayfloc-J pulp sheets (-62. lg "as-is", or ~58.4g o.d. weight) were sprayed evenly with ~0.35g of triacetin on each side of the sheets. The Kamas Energy and Mullen values were then obtained on these triacetin treated sheets and on control Rayfloc-J sheets that were untreated. Absorption properties were then measured for the control and treated fluff pulp. The results of these measurements are shown below in Table VI . The Kamas Energy value for dry pulp sheets sprayed with triacetin on both sides was decreased by about 33% which indicates that defibrization of dry pulp treated with triacetin will be much easier. The Mullen reduction was about 23%. There was no decrease in absorption rate for treated dry pulp sheet either before or after heat aging relative to Controls - in fact absorption rate improvements were achieved relative to performance of Controls that is, absorption times were faster for treated pulp sheets. Table VI. Treatment of Dry Rayfloc-J Sheets with Triacetin Example 10 Dry Rayfloc-J pulp sheets (-62. lg "as-is", or -58.4g o.d. weight) were sprayed with ~0.35g and -0.75g of triacetin on one side only; the side sprayed was the side of the sheet that had been in contact with the Fourdrinier wire on the pulp machine. The Kamas Energy and Mullen values were then obtained on the triacetin treated sheets at the two dose levels and also for untreated control Rayfloc-J sheets. The results shown below in Table VII, indicate that for the dose level of ~0.75g triacetin corresponding to a concentration of 1.3% by weight, the Kamas Energy value was decreased by about 33%, and Mullen values were reduced by about 21%. These results are very close to those obtained when dry pulp sheets were sprayed on both sides with the same total triacetin dosage per sheet, as described in Example 9. At the dose level of 0.35g, corresponding to a triacetin concentration of 0.6%, the Kamas energy value was decreased by about 22% and the Mullen values by 16%. Table VII. One-Sided Treatment of Dry Rayfloc-J Sheets with Triacetin * * * * * The above descriptions of the inventions are intended to be illustrative and not limiting. Various changes or modifications in the embodiments described may occur to those skilled in the art. These can be made without departing from the spirit or scope of the invention.

Claims: CLAIMS I 1. A method of softening a wood pulp useful for making fluff pulp comprising the step of applying to the pulp a softening agent selected from the group consisting of alkyl ethers, aryl ethers, and formic, ethanoic and propanoic esters of low molecular weight glycols, and mixtures thereof, having solubility in water at 25°C of less than 50 g per 100 g aqueous solution, wherein the Mullen strength of the pulp is decreased by at least 5%, the Kamas energy of the pulp is decreased by at least 5%, and the liquid absorption rate of the wood pulp is not decreased by more than 5%. 2. The method defined in claim 1, wherein said softening agent has a solubility in water at 25°C of no greater than 15 g per 100 g aqueous solution. 3. The method defined in claim 1, wherein said wood pulp is in sheet form, further comprising the steps of: dipping said wood pulp sheet into a solution containing said softening agent; pressing said wood pulp sheet; and, drying said wood pulp sheet. 4. The method defined in claim 1, wherein said wood pulp is in sheet form, further comprising the step of: applying said softening agent to said wood pulp sheet by spraying, rolling or printing. 5. The method of claim 1 wherein the step of applying softening agent to said wood pulp comprises adding said softening agent to a slurry of said wood pulp. 6. The method of claim 1, wherein said softening agent is selected from the group consisting of triacetin, propylene glycol diacetate, 2-phenoxyethanol and mixtures thereof. 7. The method of claim 4, wherein said softening agent is selected from the group consisting of triacetin, propylene glycol diacetate, 2-phenoxyethanol and mixtures thereof. 8. The method of claim 1, wherein said softening agent is triacetin. 9. The method of claim 4, wherein said softening agent is triacetin. 10. The method of claim 1, wherein the amount of said softening applied to said wood pulp is no greater than 3% by weight. 11. The method of claim 9, wherein the amount of triacetin applied to said wood pulp sheet is not less than about 0.1% by weight and not more than about 3.0% by weight. 12. A composition of matter comprising treated wood pulp useful for making fluff pulp produced by a method comprising the step of applying to wood pulp not more than 5% by weight of a softening agent selected from the group consisting of alkyl ethers, aryl ethers, and formic, ethanoic and propanoic esters of low molecular weight glycols, and mixtures thereof, said softening agent having solubility in water at 25°C of no more than 50g per lOOg aqueous solution. 13. The composition of matter as defined in claim 12 wherein the softening agent is selected from the group consisting of triacetin, propylene glycol diacetate, 2-phenoxyethanol and mixtures thereof. 14. The composition of matter as defined in claim 12 wherein the softening agent is triacetin. 15. A wood pulp sheet useful for making fluff pulp produced by a method comprising applying to a sheet of wood pulp not more than 5% by weight of a softening agent selected from the group consisting of alkyl ethers, aryl ethers, and formic, ethanoic and propanoic esters of low molecular weight glycols, and mixtures thereof, said softening agent having solubility in water at 25°C of no more than 50g per lOOg aqueous solution. 16. The wood pulp sheet of claim 15, wherein said softening agent comprises triacetin, propylene glycol diacetate, 2-phenoxyethanol and mixtures thereof. 17. The wood pulp sheet of claim 15, wherein said softening agent comprises triacetin.

Inventors: ABITZ PETER R, SEARS KARL D, ABITZ, PETER, R., SEARS, KARL, D.
Applicants: RAYONIER INC, RAYONIER INC.
Application year: 1997
Main classification: D21H17
IPC codes: D21C9, D21H17, D21H21