FIELD OF THE INVENTIONThis invention relates to a bulky flexible paper and a method for producing such paper.[0001]
BACKGROUND OF THE INVENTIONComic books and paperbacks have been flourishing at a rapid pace in recent years, reflecting the decreasing number of book readers-a trend that is forcing the industry to print more visually appealing, inexpensive books. Lighter, more flexible papers are therefore required for use in the production of comic books/paperbacks. Here, “lighter” means reduced weight at a fixed thickness, i.e., higher bulk (lower density). Given the public's increasing awareness of environmental problems, the production of lighter papers is essential, as it facilitates the effective use of paper pulps produced from wood resources. “Flexible” refers to the flexural rigidity of paper at a fixed thickness, i.e. the pliability of paper that makes it easy to keep the book open instead of allowing the pages to close naturally.[0002]
The methods used to increase paper bulk include those that employ cross-linked pulp (Japanese Patent Application Laid-open No. 4-185792, etc.), those that add synthetic fiber to the paper mixture (Japanese Patent Application Laid-open No. 3-269199, etc.), those that fill the space between pulp fibers with inorganic material or other filler (Japanese Patent Application Laid-open No. 3-124895, etc.), and those that use foaming granules (Japanese Patent Application Laid-open Nos. 5-230798, 11-200282, etc.). However, the use of cross25 linked pulp and synthetic fiber makes it impossible to recycle the paper, while applying a filler between the pulp fibers or using foaming granules reduces the paper's strength significantly. Methods that use general surfactants (publication of WO patent application No. 98/03730; Japanese Patent Application Laid-open Nos. 11-200283, 11-200284, 11-200285, 11-269799, 11-350380, etc.) have also been reported.[0003]
Contrastingly, the flexibility of paper is believed to manifest in accordance with the fiber type, paper density, moisture content of paper and other factors that interact with paper fibers. Paper products used in the home, such as toilet paper and facial tissue, require a certain level of flexibility to suit their purposes. Numerous softening agents have been developed to date, and glycerin, polyethylene glycol, urea, emulsified paraffin, quaternary ammonium salt and other substances are known to add flexibility to paper. Additionally, a softening agent containing di-longchain alkyl-based quaternary ammonium salt (Japanese Patent Application Laid-open Nos. 63-165597, 8-296197, etc.), softening agent containing di-longchain alkyl-based quaternary ammonium salt, glycerin and water or fatty alcohol of carbon number 4 or below (Japanese Patent Application Laid-open No. 4-100995), softening agent containing lanolin and lanolin derivative (Japanese Patent Application Laid-open No. 53-147803), softening agent containing urethane alcohol or its quaternary compound (Japanese Patent Application Laid-open No. 60-139897), softening agent containing cationic oligomer (Japanese Patent Application Laid-open No. 63-251049), softening agent containing polyamide derivative (Japanese Patent Application Laid-open No. 51-38600), and pyrolidone carboxylic acid or its salt (Japanese Patent Application Laid-open No. 7-189170) are also reported. However, these bulk-increasing agents or softening agents have often failed to exhibit sufficient bulk-increasing effect or softening effect or have only exhibited low repeatability of the bulk-increasing/softening effect in the paper-making process.[0004]
SUMMARY OF THE INVENTIONOne purpose of the present invention is to provide a method for the production of a printing paper that offers sufficient bulk and flexibility by optimizing the effect of the bulk-increasing/softening agent.[0005]
Another purpose of the present invention is to provide a printing paper that offers sufficient bulk and flexibility, even when the temperature of the paper surface has been low in the drying process during paper-making.[0006]
The inventors have carried out extensive studies in order to identify ways of achieving the above purposes, and have found that the relationship between the melting point of the bulk-increasing/softening agent and the temperature of the paper surface during the preheating drying period (a period during the drying process before the temperature of the wet paper becomes constant) is important. Moreover, the inventors have found that raising the temperature of the paper surface to the melting point of the bulk-increasing/softening agent or above would produce a sufficient bulk-increasing/softening effect. In other words, drying the wet paper under conditions in which the temperature of the paper surface remains at or above the melting point of the bulk-increasing/softening agent results in a paper offering sufficient bulk and excellent flexibility. The bulk5 increasing/softening agent to be used should desirably be an ester compound of polyhydric alcohol and higher fatty acid or long-chain alkyl ammonium salt. Additionally, the bulk-increasing/softening agent should preferably be added to between 0.1 weight-% and 5.0 weight-%, inclusive, of the material pulp. To add bulk and flexibility without changing the properties of the paper, it is even better to limit the amount of bulk-increasing/softening agent to between 0.2 weight-% and 2.0 weight-%, inclusive.[0007]
In the above study process, the inventors have also found a way to produce a paper that offers sufficient bulk and excellent flexibility, even when the temperature of the paper surface drops for some reason in the drying process during paper-making. Specifically, the inventors found a way to produce a bulky flexible paper containing a bulk15 increasing/softening agent, which has a HLB of 6 or below and is either a liquid or a solid with a melting point of 40° C. or below, wherein the desired choice of said bulk-increasing/softening agent is an ester compound of polyhydric alcohol and higher unsaturated fatty acid and/or higher branched-chain fatty acid, with a surfactant added in an amount not greater than 10.0 weight-% of the bulk-increasing/softening agent. Additionally, the bulk-increasing/softening agent should preferably be added to between 0.1 weight-% and 5.0 weight-%, inclusive, of the material pulp. Better yet, the amount of bulk-increasing/softening agent should be limited to between 0.2 weight-% and 2.0 weight-%, inclusive, of the material pulp, so as to add bulk and flexibility without a significant change in the properties of the paper.[0008]
BEST MODE FOR CARRYYING OUT THE INVENTIONThe bulky flexible paper provided through the use of the present invention, as well as its production method, are detailed below.[0009]
Generally, the process of drying in a paper-making machine involves the use of a cylinder dryer or hot-air dryer. To meet the demand for a faster paper-making process and/or coating process, in recent years the IR dryer, flotation dryer and other types of dryers have been used to increase the drying capability. All dryers use heated air as a drying heat source. Heated air is blown onto the paper sheet to raise the surface temperature and thus efficiently vaporize the moisture within the paper.[0010]
The heated air used for drying is subsequently discharged from the dryer, together with the steam from the paper sheet. Normally, heated air has a relatively small thermal capacity because there is no latent heat of condensation. Therefore, it takes some time before the paper reaches a constant rate of drying (condition in which the evaporation rate of steam is constant). On the other hand, the heated air present following the drying process contains generated steam and therefore stores a lot of thermal energy. However, this energy is often not recovered/reused but is instead discarded directly. Nevertheless, to shorten the time needed for the paper to achieve a constant rate of drying, it is effective to increase the thermal energy used in the drying process. To increase the thermal energy beyond the level provided by the heated air present following the drying process, it is important to increase the partial pressure of steam or raise the wet-bulb temperature.[0011]
The specific methods for this include adjusting the exhaust volume from the dryer in order to increase the partial pressure of steam within the dryer, or using superheated steam or a gas that contains a lot of superheated steam as a heat source. When superheated steam contacts the paper sheet within the dryer, a portion of it is condensed and becomes liquid. At this time, the latent heat of condensation causes the surface temperature of the paper sheet to rise rapidly, thereby allowing the paper to achieve a constant rate of drying within a shorter period of time.[0012]
Given the general drying process in a paper-making machine, the surface temperature of the paper is said to be between 40° C. and 55° C. until the temperature of the wet paper becomes constant (preheating drying period), after which the temperature rises to between 55° C. and 80° C. However, the mechanism by which the paper increases in bulk and becomes flexible is not clear. Although the currently available bulk-increasing/softening agents do not have the same chemical composition, it is thought that bulk and flexibility are created when the bulk-increasing/softening agent is fixed onto the pulp fibers and increases the bonding distance between fibers. Specifically, the bulk-increasing/softening agent dissolves when it receives heat during the preheating drying period, whereupon it coats the fiber surface. This has a hydrophobic effect on the fibers themselves and inhibits the hydrogen bonding between fibers, which usually occurs when the paper is dried, thus enhancing the paper's bulk and flexibility. However, if a bulk-increasing/softening agent with a melting point of 55° C. or higher is used, the agent does not dissolve sufficiently and its coatability drops. This reduces the hydrophobic effect on the fibers and the fibers undergo hydrogen bonding, which in turn reduces the paper's bulk and flexibility.[0013]
Even when the melting point of the bulk-increasing/softening agent is between 55° C. and 60° C., if the crystallization property of the agent is high and the agent does not receive thermal energy sufficient for it to dissolve, then hydrogen bonding will occur between the fibers before the bulk-increasing/softening agent dissolves. When this happens, the paper does not have sufficient bulk or flexibility. Therefore, the paper should preferably be dried under conditions in which the paper's surface temperature is at least 10° C. higher than the melting point of the bulk-increasing/softening agent. In other words, the surface temperature of the paper should preferably be sufficiently higher than the melting point of the bulk-increasing/softening agent during the preheating drying period. For example, if the melting point of the bulk-increasing/softening agent is 40° C., the surface temperature of the paper during the preheating drying period should preferably be 40° C. or above. However, to ensure the bulk and flexibility of the paper, the drying should take place with the surface temperature of the paper being at least 10° C. higher than the melting point of the agent.[0014]
After studying the relationship between the surface temperature of the paper, especially during the preheating drying period, on one hand and the bulk and flexibility of paper on the other, it was found that by keeping the paper's surface temperature at or above the melting point of the bulk-increasing/softening agent while the paper is being dried, the fiber surface could be efficiently covered by a hydrophobic compound, thus enabling the production of a low-density paper having improved flexibility. Consequently, this finding led to the present invention. It was also revealed that if, due to a drop in drying efficiency for some reason, the paper's surface temperature cannot be raised to a sufficient level in the drying process during paper-making, the bulk-increasing/softening agent will not dissolve and the coatability will drop. This produces a reduction in the hydrophobic effect on the fibers and the fibers undergo hydrogen bonding, which in turn reduces the paper's bulk and flexibility. Even if the bulk-increasing/softening agent does dissolve, hydrogen bonding between fibers could still occur prior to dissolution if the paper does not receive sufficient heat. This also reduces the paper's bulk and flexibility. Given a general drying process in a paper-making machine, the surface temperature of the paper is between 40° C. and 55° C. until the temperature of the wet paper becomes constant (preheating drying period), after which the wet paper temperature rises to between 55° C. and 80° C. Generally, bulk and flexibility are created during the preheating drying period before bonding between the fibers takes place.[0015]
Based on these findings, the inventors examined the relationship between the surface temperature of the paper, especially during the preheating drying period, on one hand and the bulk and flexibility of paper on the other, and have found that a sufficient hydrophobic property could be added to the fiber surface and a low-density paper offering improved flexibility could be produced through the use of a bulk-increasing/softening agent that is liquid at room temperature or a solid with a melting point of 40° C. or below that easily dissolves under a small amount of heat. The inventors also found that such a bulk-increasing/softening agent should desirably have a HLB of 6 or below.[0016]
In other words, the inventors have found that a printing paper offering sufficient bulk and excellent flexibility could be obtained using a bulk-increasing/softening agent, even when the surface temperature of the paper drops for some reason in the drying process during paper-making. In this case, a bulky flexible paper containing a bulk-increasing/softening agent with a HLB of 6 or below, wherein said agent is either a liquid or a solid with a melting point of 40° C. or below, can be obtained.[0017]
The bulk-increasing/softening agent used in the present invention has both a hydrophobic group and a hydrophilic group in the molecule, which is a well-known property of surfactants. Examples of suitable surfactants include higher alcohol and higher fatty acid containing alkylene oxide, but the bulk-increasing/softening agent should not be limited to surfactant as long as the chemical is able to add bulk and flexibility to the paper. It is also known that certain aliphatic polyamide amine compounds produce a bulk-increasing/softening effect. The bulk-increasing/softening agents currently available on the market include Sursol VL by BASF, Bayvolum P Liquid by Bayer, Reactopaque (opacity-improving agent) by Sansho, and KB-08T and KB-115 by Kao.[0018]
Generally, long-chain alkyl ammonium salt and ester compound of a certain polyhydric alcohol and higher fatty acid are solid in most cases, and are also prone to the effects of the paper's surface temperature in the initial stage of the drying process due to a wide range of melting points.[0019]
The ester compound of polyhydric alcohol and higher fatty acid, which is used as a bulk-increasing/softening agent in the present invention, should preferably be an ester compound of polyhydric alcohol and higher unsaturated fatty acid or ester compound of polyhydric alcohol and higher branched-chain fatty acid. Specific examples of these compounds are listed as follows:[0020]
Higher unsaturated fatty-acid esters include ethylene glycol monooleate, ethylene glycol dioleate, ethylene glycol monolinolate, ethylene glycol dilinolate, glycerin monooleate, glycerin dioleate, glycerin trioleate, glycerin monolinolate, glycerin dilinolate, glycerin trilinolate, propylene glycol monooleate, propylene glycol dioleate, propylene glycol monolinolate, propylene glycol dilinolate, pentaerythritol monooleate, pentaerythritol dioleate, pentaerythritol trioleate, pentaerythritol tetraoleate, pentaerythritol monolinolate, pentaerythritol dilinolate, pentaerythritol trilinolate, pentaerythritol tetralinolate, sorbitan dioleate, sorbitan trioleate, sorbitan tetraoleate, sorbitan dilinolate, sorbitan trilinolate and sorbitan tetralinolate, but are not limited to these compounds.[0021]
Higher branched-chain fatty-acid esters include ethylene glycol monoisostearate, ethylene glycol diisostearate, glycerin monoisostearate, glycerin diisostearate, glycerin triisostearate, propylene glycol monoisostearate, propylene glycol diisostearate, pentaerythritol monoisostearate, pentaerythritol diisostearate, pentaeriythritol triisostearate, pentaerythritol tetraisostearate, sorbitan diisostearate, sorbitan triisostearate and sorbitan tetraisostearate, but are not limited to these compounds.[0022]
Ester compounds of polyhydric alcohol and higher fatty acid include ethylene glycol monolaurate, ethylene glycol dilaurate, ethylene glycol monopalmitate, ethylene glycol dipalmitate, glycerin monostearate, glycerin distearate, glycerin tristearate, glycerin monolaurate, glycerin dilaurate, glycerin trilaurate, propylene glycol monostearate, propylene glycol distearate, propylene glycol monomyristate, propylene glycol dimyristate, pentaerythritol monolaurate, pentaerythritol dilaurate, pentaerythritol trilaurate, pentaerythritol tetraraurate, pentaerythritol monostearate, pentaerythritol distearate, pentaerythritol tristearate, pentaerythritol tetrastearate, sorbitan monopalmitate, sorbitan dipalmitate, sorbitan tripalmitate, sorbitan tetrapalmitate, sorbitan monostearate, sorbitan distearate, sorbitan tristearate and sorbitan tetrastearate, but are not limited to these compounds.[0023]
Specific examples of long-chain alkyl ammonium salt include the following compounds: lauryl trimethyl ammonium chloride, cetyl trimethyl ammonium bromide, stearyl trimethyl ammonium chloride, dimethyl distearyl ammonium chloride, dioleoyl dimethyl ammonium chloride, tridodecyl methyl ammonium chloride, trioctyl methyl ammonium chloride and trioctyl methyl ammonium bromide, but are not limited to these compounds.[0024]
Generally, higher fatty-acid esters are mostly solid at normal temperature. On the other hand, a majority of higher unsaturated fatty-acid esters and higher branched-chain fatty-acid esters are liquid at normal temperature. Higher unsaturated fatty-acid esters and higher branched-chain fatty-acid esters that are solid at normal temperature have, in most cases, a melting point of 40° C. or below.[0025]
In the present invention, a bulk-increasing/softening agent with a high HLB and thus insufficient hydrophobic property may not produce the desired bulk-increasing/softening effect because it cannot provide sufficient hydrophobicity to the fibers. If the HLB is low, specifically 6 or below in regard to the present invention, the fibers can be provided with sufficient hydrophobicity.[0026]
However, excessive hydrophobicity inhibits the efficient dispersion of water, which in turn prevents the uniform attachment of bulk-increasing/softening agent to the fibers. As a result, a sufficient bulk-increasing/softening effect cannot be achieved. Therefore, when using a bulk-increasing/softening agent with high hydrophobicity, it is important to produce a uniform emulsion by adding a surfactant offering excellent dispersibility. Some bulk-increasing/softening agents have a self-emulsification property. However, when a bulk-increasing/softening agent with high hydrophobicity is used, it is preferable that a surfactant be added to 10.0 weight-% or less of the bulk-increasing/softening agent.[0027]
Specific examples of surfactants that can be used in conjunction with the bulk-increasing/softening agent in the present invention include the following compounds: anionic surfactants such as Pelex OT-P (Kao), Demol EP (Kao), Disper TL (Meisei Chemical Works), Gohseran L-3266 (Nippon Synthetic Chemical Industry) and Aron T-40 (Toa Gosei Kagaku); nonionic surfactants such as Emulgen A-60 (Kao) and Emulmin 50 (Sanyo Chemical); and cationic surfactants such as Cation DS (Sanyo Chemical), Quartamin 86W (Kao) and Quartamin D86P (Kao), but are not limited to these products.[0028]
The addition of a bulk-increasing/softening agent inhibits the bonding of pulp fibers, which generally results in lower paper strength and the increased bulk/flexibility of paper. However, the effect of adding a bulk-increasing/softening agent hits a plateau at a certain amount. In other words, it is preferable that a bulk-increasing/softening agent be added to between 0.1 weight-% and 5.0 weight-%, inclusive, of the material pulp. It is even better to limit the amount of bulk-increasing/softening agent to between 0.2 weight-% and 2.0 weight-%, inclusive, since an excessive addition of such agent may affect the general paper properties, for example by triggering a substantial change in the friction coefficient of the paper, while an insufficient amount may not fully produce the desired bulk-increasing/softening effect, depending on the type of bulk-increasing/softening agent used.[0029]
The bulky flexible paper provided through the present invention is produced from various types of pulp through the normal paper-making process. Material pulps include chemical pulp (bleached or unbleached kraft pulp from softwood, bleached or unbleached kraft pulp from hardwood, etc.), mechanical pulp (groundwood pulp, thermomechanical pulp, chemi-thermomechanical pulp, etc.), or recycled, ink-removed pulp, wherein these material pulps may be used alone or in combination at arbitrary ratios of blending. The pH level of the pulp slurry may be in the acid, neutral or alkali range. The bulky flexible paper provided through the present invention may contain filler.[0030]
There are no limitations as to which fillers must or must not be used, and common fillers can be employed. Examples include inorganic fillers such as clay, fired clay, silicious marl, talc, kaolin, fired kaolin, delaminated kaolin, heavy calcium carbonate, light calcium carbonate, magnesium carbonate, barium carbonate, titanium dioxide, zinc oxide, silicon oxide, amorphous silica, aluminum hydroxide, calcium hydroxide, magnesium hydroxide and zinc hydroxide; and organic fillers such as urea-formalin resin, polystyrene resin, phenol resin and fine hollow grains, wherein such fillers can be used alone or in any combination.[0031]
To obtain a paper containing a bulk-increasing/softening agent as defined in the present invention, add a bulk-increasing agent or its emulsion internally during the normal paper-making process (as opposed to applying the agent to the exterior of the paper after it has been made). Addition of bulk-increasing agent may take place at any step, as long as the uniform mixing of agent and pulp slurry can be achieved.[0032]
When producing a bulky flexible paper as defined in the present invention, various nonionic or cationic internal auxiliaries traditionally used in the paper-making process, such as retention-aiding agent, freeness-improving agent, paper-strength enhancer and sizing agent, may be selected and used as appropriate.[0033]
Internal auxiliaries normally used in the paper-making process include aluminum sulfate; aluminum chloride; sodium aluminate; basic aluminum compounds such as basic aluminum chloride and basic aluminum polyhydroxide; water-soluble aluminum compounds such as alumina sol that dissolve easily in water; polyvalent metals such as ferrous sulfate and ferric sulfate; and silica sol.[0034]
The sizing agent can be chosen from various types, including alkyl ketene dimer compounds, alkenyl succinate anhydride compounds, styrene-acryl compounds, higher fatty acid compounds, petroleum resin-based sizing agents, and rosin-based sizing agents.[0035]
Other auxiliaries normally used in the paper-making process include various starches, polyacrylamide, urea resin, melamine resin, epoxy resin, polyamide resin, polyamide, polyamine resin, polyamine, polyethylenimine, vegetable gum, polyvinyl alcohol, latex, polyethylene oxide and hydrophilic-crosslinked-polymer-particle dispersion, as well as their derivatives and transformed versions.[0036]
Additionally, dye, fluorescent whitener, pH regulator, defoaming agent, pitch controller, slime controller and other agents normally used in the paper-making process may be added as appropriate for the given application. The method of producing paper provided through the present invention applies to all papers made using different schemes, including papers produced in the acid range of around pH 4 to 5 or those produced in the neutral range of around pH 6 (weak acidity) to 9 (weak alkalinity), wherein the slurry usually contains calcium carbonate or other alkali filler. The paper-making machine may be a Fourdrinier paper machine, twin-wire machine, Yankee machine, or a machine of another suitable type.[0037]
The bulky flexible paper obtained in the manner explained above is ideal, for example, as a product for offset printing. It can also be used as a letterpress printing paper, electrophotographic paper, ink-jet printing paper, thermal paper, pressure-sensitive paper or other type of paper used for the recording of information. The paper obtained through the present invention is also suitable as the base paper of a coated stock.[0038]