Direct filling ink capable of being used for a ballpoint pen containing an aqueous ink composition

Abstract: There is provided an ink capable of being used in a ballpoint pen, the ink comprising: 2 to 10 % by weight of a dye, 20 to 30 % by weight of a wetting agent, 5 to 10% by weight of urea, 0.2 to 0.4 % by weight of cross-linked acrylic acid polymer, 0.05 to 0.2 % by weight of xanthan gum, 1.0 to 5.0 % by weight of sorbitol, and the balance, water.

Description: The present invention relates to a direct filling ink capable of being used in a ballpoint pen. A ballpoint pen where ink is filled directly by an ink tube or a coloring agent is generally known. Typically, inks used in these types of pens have a shear-thinning property, in other words, they have a high viscosity when not being used, and a low viscosity in use due to rolling (shearing force) of the ballpoint pen. Such ballpoint pens tend to have many problems. For example, one problem is that when the water content of the ink has evaporated, the water soluble (dispersible) resin in the ink becomes thickened and/or forms a layer on the pen point due to its very high molecular weight, thereby adversely affecting the quality of writing with the pen. The use of a cross-linked acrylic acid polymer ink having a relatively low thickening ratio and forming no layer when the water content is evaporated solves this problem. However, such cross-linked acrylic acid polymer inks have low thixotropy; thus viscosity is often not reduced to a desired level by shearing force during the writing operation. This causes bleeding and blurs in the writing. If a pigment is used as a coloring agent, light resistance and water resistance in the written ink tend to be excellent. However, when the water content of the ink is evaporated, the pigment cannot be dispersed, thereby clogging the ink on the pen point. Thus, it is often difficult to get the pen to write at the onset of each use, or the pen may be incapable of writing at all. What is worse, in the case of a pigment ink, once its dispersion is lost or the pigment has hardened, the pigment generally is difficult to redisperse and even if new ink is added, as such, its use typically is difficult to restore. Thus, as of yet, there is no ink suitable for use in a ballpoint pen containing an aqueous ink composition that has sufficient writing ability and resistance to drying. Due to such problems, ballpoint pens having aqueous ink compositions, must be equipped with a cap to prevent the evaporation of water from the ink. However, a capless type of ballpoint pen capable of commencing a writing operation at a single touch is urgently sought. An object of the present invention, therefore, is to provide an aqueous ink composition having excellent dry-up resistance and excellent writing ability which is capable of being used in a ballpoint pen without any bleeding or blur. In accordance with these and other objectives, there is provided an ink capable of being used in a ballpoint pen, the ink comprising: 2 to 10 % by weight of a dye, 20 to 30 % by weight of a wetting agent, 5 to 10 % by weight of urea, 0.2 to 0.4 % by weight of cross-linked acrylic acid polymer, 0.05 to 0.2 % by weight of xanthan gum, 1.0 to 5.0 % by weight of sorbitol, optionally further ingredients and the balance, water. Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims. A suitable ink in accordance with the present invention is a direct filling ink capable of being used for a ballpoint pen and has excellent dry-up resistance and can be used in writing without any significant bleeding and/or blur. A useful direct filling ink for a ballpoint pen containing an aqueous ink composition according to the present invention, essentially consists of 2 to 10 % by weight of a dye, 20 to 30 % by weight of a wetting agent for maintaining wetness, 5 to 10 % by weight of urea, 0.2 to 0.4 % by weight of cross-linked acrylic acid polymer, 0.05 to 0.2 % by weight of xanthan gum, 1.0 to 5.0 % by weight of sorbitol, and balance being water. Unless otherwise stated, all percentages expressed herein are weight percentages based on the total weight of the ink composition. One aspect of the present invention is to employ a dye as a coloring agent. If a pigment is used, ink stability may be lost by the evaporation of water. Once the ink has hardened, the pigment is generally difficult to restore because the ink is usually not redispersed even though new ink has been added, due to the presence of the pigment. However, if a dye is used as a coloring agent and the ink hardens, the dye can generally be restored to its former state when new ink is added due to the redissolution ability of dyes. In other words, even if the water content of a dye-containing ink is evaporated so as to make the ink harden on the pen point, the ink is redissolved when new ink is added thereto, and writing can be accomplished successfully. Another aspect of the present invention is to use a cross-linked acrylic acid polymer in combination with xanthan gum to add shear-thinning property to the ink. A cross-linked acrylic acid generally does not become thickened and form a layer if the water content of the ink is evaporated. However, cross-linked acrylic acids will tend to have low thixotropy which may prevent the viscosity of the ink from being reduced to a desired level when writing and from being kept at high levels when not in use. In addition, cross-linked acrylic acids generally have insufficient shear-thinning property. On the other hand, the xanthan gum has high thixotropy and high shear-thinning property. Therefore, the viscosity of the ink may be adjusted to obtain excellent writing quality without the necessity of dropping the ink from a ballpoint pen tip when applied to a ballpoint pen. However, xanthan gum becomes considerably thickened upon evaporation of the water in the ink, thereby causing the ink to form a layer on the pen point, and rendering the pen at least to some degree incapable of writing. According to the present invention, the cross-linked acrylic acid polymer is generally used as a shear-thinning property additive agent to substantially prevent the deterioration of writing ability in the ballpoint pen due to the evaporation of water. The xanthan gum is preferably added to complement shortcomings in the cross-linked acrylic acid polymer to reduce the likelihood of dropping ink, and to maintain high viscosity in the ink when not in use. This synergism between the cross-linked acrylic acid polymer and the xanthan gum provides an ink with excellent writing capability without incurring significant bleeding or blur when writing. The combination of these two components, inter alia, also provides an ink with excellent dry-up resistance without considerable thickening and/or formation of a layer upon evaporation of the water content in the ink. Some thickening when the ink is dropped from the pen point or upon evaporation of the water may occur. A suitable level for the thickening ratio of an ink in accordance with the present invention is preferably between 400 to 1000 mPas at 2.0 sec -1 , and between 20 to 60 mPas at 400 sec -1 . The ink of the present composition preferably has a thickening rate of within about 30% in a preferred embodiment. Yet another aspect of the present invention is the addition of urea to the ink. Urea has the ability to inhibit water evaporation as well as the ability to dissolve dye, and as such, serves as a dissolving auxiliary agent to redissolve dye that has hardened upon water evaporation in the ink. Therefore, a combination of the urea and the dye as a coloring agent contributes remarkably to improving dry-up resistance and redissolution. However, if a pigment is used as the coloring agent, the urea is preferably only included in an amount that does not significantly affect the stability of the ink. In addition, since a pigment generally cannot easily be redissolved, the inclusion of urea is not critical when pigment is used. However, the inclusion of urea is especially beneficial in inks containing a dye to improve the writing. Yet another aspect of the present invention is to add sorbitol. The sorbitol is rich in hygroscopic properties and aids in retaining the water content of the ink. In addition, its relatively low thickening ratio and drying characteristics contribute largely to maintaining good performance of the ballpoint pen over a long period of time. A direct filling ink used for a ballpoint pen containing an aqueous ink composition of the present invention comprises a dye, a wetting agent, urea, cross-linked acrylic acid polymer, xanthan gum, sorbitol and water. The dye used is included in an amount from 2 to 10 % by weight based on the total weight of the ink composition. If not more than about 2 % of dye by weight of the ink is included, the writing may in some circumstances become blurred, and in cases where more than about 10 % dye by weight is included, the writing ability may begin to deteriorate. The cross-linked acrylic acid polymer used is included in an amount from 0.2 to 0.4 % by weight based on the weight of the ink composition. When including less than about 0.2 % by weight, the writing ability and the dry-up resistance are adversely affected and produce undesired ink viscosity, and if more than about 0.4 % cross-linked acrylic acid polymer by weight is used, the writing ability of the ink may be affected due to the fact that the viscosity may not be reduced sufficiently by the shearing force of the ball rolling during the writing operation. The xanthan gum is used in an amount from 0.05 to 0.2 % by weight based on the total weight of the ink. If less than about 0.05 % by weight is included, the performance of the ink may be lowered due to undesired viscosity, and if more than about 0.2 % by weight is included, a layer may be formed on the pen point which may result in deterioration of the writing. Moreover, the use of more than about 0.2% xanthan gum may produce a deposit inside the pen point when the ink dries, thus making the ink unstable. The wetting agent preferably comprises any compound which functions to keep the ink composition wet. The wetting agent is preferably at least one compound selected from a dihydric alcohol, a trihydric alcohol and a derivative of an alcohol such as ethylene glycol, propylene glycol, glycerin, diethylene glycol, triethylene glycol, dipropylene glycol, polyglycerin, thiodiglycol, poyethylene glycol, polypropylene glycol, methyl cellosolve, butyl cellosolve, methylcarbitol or ethylcarbitol. Most preferably the wetting agent is at least one compound selected from ethylene glycol, polyethylene glycol and glycerin. The wetting agent is included in an amount from 20-30% by weight based on the weight of the ink. If less than about 20 % by weight of the wetting agent is included, the dry-up resistance of the ink may be reduced and the ink may become hardened on the pen point. If more than about 30 % by weight is included, the drying resistance of the ink will still be excellent; however the writing may include blurs and/or bleeding. The urea is used in an amount from 5 to 10 % by weight based on the total weight of the ink composition. If less than about 5 % by weight is included, the hygroscopic property and dye dissolving ability are lowered, possibly causing poor dry-up resistance, and if more than 10 % by weight is included, the writing ability of the ink may be adversely affected due to the small change in viscosity when shearing force is applied. In addition, the use of excessive amounts of urea may affect the viscosity when shearing force is not applied due to the possible precipitation of urea crystals after evaporation of some of the water content of the ink. If urea crystals form, they may inhibit ink flow. The sorbitol is used in an amount from 1.0 to 5.0 % by weight based on the weight of the ink. If less than about 1.0 % by weight of sorbitol is included, the hygroscopic property and the dry-up resistance may not be as marked as required, and if more than about 5.0% by weight of sorbitol is included, the thickening ratio in the ink may become increased upon drying, thereby reducing the dry-up resistance of the ink. The present ink composition can be prepared using any suitable method known to those skilled in the art. For example, the ink may be prepared by mixing ion exchange water and 20-30 wt.% of a wetting agent (moisture maintaining agent). Then 5-10 % by weight of urea, 1.0-5.0 % by weight of sorbitol, 2-10 % by weight of dye are added and mixed for one hour at 60°C. A polymerizable acrylic acid polymer or a cross-linked acrylic acid polymer and xanthan gum are added, and the mixture is stirred with a homogenizing stirrer to form a uniform ink. Other optional ingredients which may be added to the present composition include, 0.2 to 2.0 % by weight of rustproof lubricant, 0.5 to 2.0 % by weight of a pH regulating agent and 1.0 to 2.0 % by weight of a fungusproof agent. Each of these weights are based on the total weight of the ink composition. The rustproof agent, if employed, may comprise one or more compounds selected from a phosphoric acid ester surfactants, phosphoric acid ester type surfactants, benzotriazoles and fatty acid esters. The pH regulating agent, if employed, preferably comprises triethanolamine. The fungusproof agent may comprise any compound known to function in preventing or reducing fungus as known to those skilled in the art, e.g. such as 1,2-benzoisothiazoline-3-one (PROXEL XL-2, Zeneca, Ltd.). The present invention will be described by reference to the following examples. Example 1 Direct black 154 7.5 % by weight ethylene glycol 20.0 % by weight urea 5.0 % by weight sorbitol 1.0 % by weight rustproof lubricant 0.5 % by weight pH regulating agent 0.5 % by weight fungusproof agent 1.0 % by weight cross-linked acrylic acid polymer 0.25 % by weight xanthan gum 0. 15 % by weight ion exchange water rest Ion exchange water was mixed with ethylene glycol followed by adding urea, sorbitol, dye, rustproof lubricant, a pH regulating agent and a fungusproof agent to mix by stirring, and followed by adding cross-linked acrylic acid polymer and xanthan gum to mix, to prepare ink. Examples 2 to 10 Inks were prepared as in Example 1 but the proportions of the components in each composition and dye are changed as shown in Table 1. Ink performance is also shown in Table 1. Writing performance: o ○ --- Especially excellent writing without any bleeding or blur ○ --- Available for practical use Δ --- Somewhat poor × --- Especially poor Dry-up resistance ability: Evaluated by writing a line with a ballpoint pen, which was left sideways without a cap at 50°C under 30% RH for more than 3 monts and then the ink receiving tube (inner core) thereof was at room temperature o ○ --- Capable of beginning writing within 3 cm without a cut line, blur or the like ○ --- Capable of beginning writing within 3 to 5 cm Δ --- Capable of beginning writing within 5 to 10 cm × --- Incapable of beginning writing within 10 cm Ink stability: Evaluated by status after leaving the ballpoint pen sideways without a cap at 50°C under 0% RH o ○ --- Stable for 3 months or longer (No deposit in ink not less than 3 months and the thickening rate is within 30 %) ○ --- Stable for 2 to 3 months Δ --- Stable for 1 to 2 months × --- Out of the above conditions within 1 month As is apparent from the Examples, according to the present invention, a dye is preferably used as a coloring material, a wetting agent is beneficial to include for maintaining the moisture content of the ink, urea and sorbitol are beneficially used to obtain excellent dry-up resistance and cross-linked acrylic acid polymer is beneficially used in combination with xanthan gum to obtain both excellent writing ability and ink stability when drying up. Comparative Examples 1 to 19 Ink was prepared as in Example 1 but ink compositions were as shown in Table 2. Performance is also shown in Table 2. Comparative Example 1 shows an example of an increased concentration of dye, and Comparative Example 14 shows an example of a decreased concentration of dye. The increased concentration of dye exhibits weak pseudoplastic ability, whereby desirable ink viscosity is difficult to obtain. The writing ability is adversely affected, ink stability is poor when drying up, and dry-up resistance ability is reduced. In addition, the reduced concentration of dye exhibits excellent ink stability, but has reduced writing ability at the beginning of a writing operation due to the properties of the resin when drying up. Comparative Examples 2, 3, 15 and 16 show examples of using a pigment instead of a dye as coloring material. The pigment ink is not completely restorable and has reduced ink stability when drying up. In addition, the use of pigment may have a wearing effect on the ball portion of a ballpoint pen, thereby potentially shortening the writing lifetime of the pen. Comparative Example 5 shows an example which employs an increased amount of the wetting agent, and Comparative Example 4 shows an example of a reduced amount of the wetting agent. The example employing the increased quantity thereof exhibits an excellent dry-up resistance ability, but has slightly poorer writing ability with some blurs and bleeding. In addition, the ink has reduced stability due to abnormal thickening in the ink after drying. On the other hand, the reduced amount thereof exhibits an absence of ink lubricity, causing some blurs or cuts in the writing, and also exhibits a low dry-up resistance ability due to the reduced amount of hygroscopic wetting agents. Comparative Example 8 shows an example of an increased amount of the urea, and Comparative Example 6 shows an example of a reduced amount of the urea. The increased additive amount thereof exhibits a poorer writing ability due to the small difference between the viscosity of the ink when shearing force is applied and the viscosity when shearing force is not applied. In addition, crystals may be precipitated on the pen point when drying up. On the other hand, the reduced additive amount thereof exhibits reduced dry-up resistance ability caused by the lower amount of hygroscopic properties and dye dissolving power. Comparative Example 9 shows an example of an increased additive amount of the sorbitol, and Comparative Example 7 shows an example of a reduced amount of the sorbitol. The increased amount thereof exhibits weak pseudoplastic ability, thereby writing ability is adversely affected. In addition, crystals may be precipitated when water content is evaporated because of lack of solubility. On the other hand, the reduced additive amount thereof exhibits a reduction in the ink lubricity, resulting in a reduction in dry-up resistance, and also exhibits a poorer writing ability due to reduction in the viscosity of the ink. Comparative Example 10 shows an example of an increased amount of the cross-linked acrylic acid polymer. Comparative Example 11 shows an example of an increased amount of the xanthan gum. The large additive amount of the cross-linked acrylic acid polymer affects the writing ability because the viscosity is not sufficiently lowered due to shearing force of the ball rolling when writing. In addition, writing ability at the beginning of a writing operation is also lowered for the same reason. If the amount of the xanthan gum is too high, not only is a layer easily formed on the pen point, resulting in poor writing at the beginning, but also a deposit is produced inside the pen point when drying up, resulting in unstable ink. Comparative Example 12 shows an example of a reduced additive amount of the cross-linked acrylic acid polymer. Comparative Example 10 shows an example of a reduced amount of the xanthan gum. In the Comparative Example 12, even if the xanthan gum is added at a maximum amount in view of the layer forming ability of the ink, the desired ink viscosity is difficult to obtain, thus causing adverse effects on writing ability. In addition, the water retention power of the cross-linked acrylic acid polymer is reduced, resulting in a reduction in the dry-up resistance ability. The reduced amount of the xanthan gum exhibits lower performance because the desired viscosity is difficult to set. Comparative Example 18 shows an example of a reduced amount of both the cross-linked acrylic acid polymer and the xanthan gum. In this case, ink is easily dropped from the pen, high viscosity when the ink is not in use is difficult to maintain, and writing quality is adversely affected. Comparative Example 19 shows an example of adding only the cross-linked acrylic acid polymer not including the xanthan gum as a viscosity adjusting agent. Without the xanthan gum, adequate shearing decrease effect is difficult to obtain. To prevent blurs, cuts in the writing or drop of the ink, it is necessary to add additional amounts of the cross-linked acrylic acid polymer, thus resulting in deterioration of the dry-up resistance of the ink. Therefore, the use of both xanthan gum and the cross-linked acrylic acid polymer is preferential. Ink of the present invention has excellent dry-up resistance ability and writing ability without substantial bleeding and/or blur and is most suitable for use in a ballpoint pen application.

Claims: An ink capable of being used in a ballpoint pen, said ink comprising: 2 to 10 % by weight of a dye; 20 to 30 % by weight of a wetting agent other than sorbitol; 5 to 10 % by weight of urea; 0.2 to 0.4 % by weight of cross-linked acrylic acid polymer; 0.05 to 0.2 % by weight of xanthan gum; and 1.0 to 5.0 % by weight of sorbitol, optionally further ingredients and the balance water. An ink according to claim 1, comprising as further ingredient 0.2 to 2.0 % by weight of a rustproof lubricant. An ink according to claim 1 or 2, comprising as further ingredient 0.5 to 2.0 % by weight of a pH regulating agent which is not the wetting agent defined in claim 1. An ink according to any one of the preceding claims, comprising as further ingredient 1.0 to 2.0 % by weight of a fungusproof agent. An ink according to any one of the preceding claims, wherein the wetting agent comprises at least one selected from dihydric alcohol, trihydric alcohol and derivatives of dihydric alcohols and trihydric alcohols. An ink according to any one of the preceding claims, wherein the wetting agent comprises at least one selected from ethylene glycol, polyethylene glycol and glycerin. An ink according to any one of claims 2 to 6, wherein the rustproof agent comprises at least one selected from phosphoric acid ester surfactants, benzotriazoles and fatty acid esters. An ink according to any one of claims 3 to 7, wherein the pH regulating agent is triethanolamine. A ballpoint pen whenever containing an ink as claimed in any one of the preceding claims. A refill tube for a ballpoint pen whenever containing an ink as claimed in any one of claims 1 to 8.

Inventors: FUKASAWA TERUAKI, ONO TAKASHI, YOSHII KAZUHIRO, FUKASAWA, TERUAKI, ONO, TAKASHI, YOSHII, KAZUHIRO
Applicants: PILOT KK, KABUSHIKI KAISHA PILOT
Application year: 1997
Main classification: C09D11
IPC codes: C09D11