Erasable Ink Composition for Writing on Impervious Surface

This invention relates to an erasable ink composition for writing on an impervious writing surface, and more particularly, to an erasable ink composition which contains a separating agent therein to provide writing on an impervious writing surface such as a so-called white board which is readily erasable by wiping lightly with an eraser made of, for example, felt.

A variety of erasable ink composition to write on an impervious writing surface to provide writing thereon which is readily erasable by wiping with, for example, an eraser made of felt are already known, as disclosed in, for example, Japanese Patent Application Laid-open No. 63-43981, No. 62-265377 and No. 2-129278. However, these prior erasable ink compositions contain carboxylic acid esters, liquid paraffin, silicone resins or nonionic surfactants as a separating agent therein, and thus it is known that they provide writing which is insufficiently erasable depending upon the material of impervious writing surface. Namely, the prior erasable ink compositions have erasability dependendent on the material of impervious writing surface. It is also known that such an ink composition provides writing which is not readily erasable after long standing. Namely, the prior erasable ink compositions have erasability dependendent on time.

It is an object of the invention to provide an ink composition which provides writing on an impervious writing surface readily erasable irrespective of the material of the surface and after long standing.

According to the invention, there is provided a erasable ink composition for writing on an impervious writing surface comprising an organic solvent, a resin, a pigment, and a separating agent which comprises a polyoxyethylene polyoxypropylene monoalkyl ether which is liquid at normal temperatures in an amount of 5-20% by weight.

The ink composition of the invention contains an organic solvent preferably such as an alcohol or a glycol monoalkyl ether. The alcohol is preferably an aliphatic alcohol of 1-4 carbons such as methanol, ethanol, isopropyl alcohol, n-propyl alcohol or butanol. The glycol monoalkyl ether is preferably a monoalkyl ether of glycol of 2-4 carbons such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, propylene glycol monomethyl ether or propylene glycol monoethyl ether. These solvents may be used singly or as a mixture.

The ink compositions of the invention contains such an organic solvent in an amount of 40-90% by weight, preferably in an amount of 50-75% by weight. When the amount of organic solvent is too large, the resultant ink composition has no viscosity suitable for writing, and may not provide writing of a sufficient darkness. When the amount of the organic solvent is too small, the resultant ink composition has a viscosity too large to write well, and it is also bad in storage stability.

The ink composition of the invention may further contain a dialkyl ketone wherein the alkyl has 1-4 carbons such as methyl ethyl ketone or methyl isobutyl ketone, or a lower aliphatic carboxylic acid ester wherein the carboxylic acid has 2-4 carbons and the alkyl has 1-3 carbons preferably such as ethyl acetate or butyl acetate, as a dissolving aid for a resin in an organic solvent used which will be described hereinafter.

The ink composition of the invention contains a pigment as a colorant. The amount of the pigment is determined in accordance with the pigment used and the darkness required. However, when the amount is too large, the resultant ink composition may have too large a viscosity and may be poor in writability and storability as well as erasability of writing, while when the amount is too small, the resultant ink composition may provide infeaibly faint writing, and may also writing poor in erasability. Usually the ink composition of the invention contain a pigment in an amount of 1-30% by weight, preferably of 3-15% by weight.

A resin is contained in the ink composition so that the ink composition has a viscosity suitable for writing and has a suitable adhesion onto an impervious writing surface. It is necessary that the resin is soluble in the organic solvent used in the ink composition, and such a resin includes, for example, polyvinyl butyral resin, acrylic resin, styrene-acrylic copolymer resin, styrene-maleic acid copolymer resin, shellac resin and rosin-maleic acid copolymer resin.

When the resin is contained in excess, the resultant ink compositions may have too large a viscosity and may deteriorate in writability, storability and erasability of writing, whereas when the amount is too small, the resultant ink composition may not have a viscosity suitable for writing, and may also provide writing of insufficient erasability. Usually the ink compostion of the invention contains a resin in an amount of 1-20% by weight, preferably of 5-15% by weight.

The ink compostion of the invention contains a polyoxyethylene polyoxypropylene monoalkyl ether which is liquid at normal temperatures, and which preferably has an oxyethylene content of not more than 40% by weight, as a separating agent.

The polyoxyethylene polyoxypropylene monoalkyl ether used in the invention is commercially available, and may be exemplified by Nikkol PBC-31 or PBC-41 (Nikko Chemicals K.K., Japan), Newpol 50HB-55, 50HB-100, 50HB-260, 50HB-400, 50HB-600 or 50HB-1000 (Sanyo Kasei Kogyo K.K., Japan), Dovanox DL-0206, DM-2007 or DH-0300 (Lion K.K., Japan), Lionol L-358, L-745, L-785 or L-950 (Lion K.K., Japan).

The polyoxyethylene polyoxypropylene monoalkyl ether is contained in the ink composition in an amount of 5-20% by weight. When the amount of the polyoxyethylene polyoxypropylene monoalkyl ether is too small, the resultant ink composition may not have a sufficient erasability, whereas when the amount is too large, the resultant ink composition may have a viscosity too large for writing well, or write faintly. In particular, it is preferred that the ink composition contains the polyoxyethylene polyoxypropylene monoalkyl ether in an amount of 7-15% by weight.

The ink composition of the invention may contain a further separating agent or a second separating agent in conjunction with the above mentioned polyoxyethylene polyoxypropylene monoalkyl ether as a first separating agent.

The second separating agent is at least one organic compound selected from the group consisting of a carboxylic acid ester and a polyoxypropylene monoalkyl ether which is liquid at normal temperatures. The carboxylic acid ester may be a carboxylic acid alkyl ester, a dibasic acid diester or a mono- or polyester of a polydric alcohol.

Thus, there is provided in accordance with the invention, a further erasable ink composition for writing on an impervious writing surface comprising an organic solvent, a resin, a pigment, and a separating agent which comprises:

(a) a polyoxyethylene polyoxypropylene monoalkyl ether which is liquid at normal temperatures in an amount of 5-20% by weight; and

(b) at least one organic compound selected from the group consisting of a carboxylic acid ester and a polyoxypropylene monoalkyl ether which is liquid at normal temperatures in an amount of 2-10% by weight.

The carboxylic acid alkyl ester is preferably such a carboxylic acid alkyl ester wherein the carboxylic acid is a saturated or an unsaturated and has 8-20 carbons, and the alkyl has 2-20 carbons. Such a carboxylic acid alkyl ester may be exemplified by cetyl octanoate, isocetyl octanoate, stearyl octanoate, hexyl laurate, isostearyl laurate, butyl myristate, isopropyl palmitate, isostearyl palmitate, isopropyl isostearate, butyl isostearate, hexyl isostearate, isocetyl isostearate, ethyl oleate or decyl oleate.

The dibasic acid diester used includes an aliphatic dibasic carboxylic acid diester or an aromatic dibasic carboxylic acid diester, preferably a dialkyl ester wherein the alkyl has 2-20 carbons. Thus, the dibasic acid diester used includes, for instance, di-2-ethylhexyl succinate, diisopropyl adipate, dioctyl adipate, diisopropyl sebacate, dibutyl phthalate, dioctyl phthalate and diisodecyl phthalate.

The ester of polydric alcohol is preferably a mono-or a polyester derived from an aliphatic polyhydric alcohol of 4-8 carbons including glycols and a saturated or an unsaturated fatty acid of 6-24 carbons. Accordingly, the ester of polydric alcohol used includes, for instance, an ester of trimethylol propane such as trimethylol propane (mono, di or tri)oleate, trimethylol propane (mono, di or tri)caproate, tri-2-ethylhexyl trimethylates (such as trialkyl esters of trimethylol compound, exemplified by trimethylol propane), trimethylol propane triisopalmitate, trimethylol propane trioctanoate or trimethylol propane triisostearate; an ester of pentaerythritol such as pentaerythritol tetraoctanoate or pentaerythritol tetra-2-ethyl hexanoate; an ester of propylene glycol such as propylene glycol monocaprilate, propylene glycol dicaprilate or propylene glycol didecanoate; or a fatty acid (mono, di or tri)glyceride or a mixture of these (esters of glycerine and fatty acids) such as olive oil, safflower oil or coconut oil.

The polyoxypropylene monoalkyl ether is also commercially available, and may be exemplified by Newpol LB-65, LB-285, LB-385, LB-625, LB-1145, LB-1715 or LB-3000 (Sanyo Kasei Kogyo K.K., Japan).

The second separating agent is contained in the ink composition in an amount of 2-10% by weight, preferably of 3-8% by weight, based on the ink composition. When the amount of the second separating agent is too small, the resultant ink composition may not have a sufficient erasability, whereas when the amount is too large, the resultant ink composition may have a viscosity too large for writing well, or write faintly.

According to the invention, it is in particular preferred that the first and second separating agents are contained in conjunction in an amount of 10-23% by weight.

The ink composition of the invention may be produced by a conventional method, and is not limited to any specific method. By way of example, when a normal pigment is used as a colorant, the pigment is mixed with a resin in an organic solvent, the resultant mixture is milled in a beads mill to disperse the pigment finery in the solvent to form a mill base, there are added a separating agent and other components if necessary to the mill base, and then the resultant mixture is stirred. When a resin-coated pigment is used as a colorant, all the components may be mixed together at the same time, and the pigment may be dispersed in the solvent used with a paint shaker. In either methods, the resultant ink composition may be filtered or centrifuged to remove therefrom coarse materials.

Examples of the ink composition of the invention will now be described, however, the invention is not limited thereto. In the examples, parts are parts by weight.

The above fluorescent orange base has the composition below:

Such a pigment base as above may be usually prepared by adding a pigment to a solution of a resin such as polyvinyl butyral resin in an organic solvent, and mixing the resultant mixture in a bead mill.

The above white base has the composition below:

The above blue base has the composition below:

Writing was formed on various impervious writing surfaces as shown in Table 1, and its initial erasability and standing erasability were examined. The initial erasability is defined as the erasability of writing when being wiped with an eraser made of felt immediately after the writing was formed, and the standing erasability is defined as the erasability of writing when being wiped with an eraser made of felt after standing at a temperature of 40.degree. C. and a relative humidity of 40% for seven days. Among the writing boards, there were used two melamine resin boards.

The erasability was evaluated as follows. When the writing was erased by wiping five times with a sheet of felt (5 cm.times.5 cm) under a load of 200 g or 400 g, the erasability was marked "A"; when the writing was erased by wiping eight times under a load of 800 g, the erasability was marked "B"; and when the writing was not erased by wiping ten times under a load of 800 g, the erasability was marked "C". The mark "-" indicates that evaluation is not possible since the color of the writing board was the same as that of the writing formed thereon.

As indicated in Table 1, the ink composition of the invention provides writing on an impervious surface which is readily erasable after long standing. The ink composition has also erasability substantially independent on the material of impervious surface, and provide erasable writing on any impervious surface.

More specifically, the ink composition of the invention contains the polyoxyethylene polyoxypropylene monoalkyl ether as the first separting agent which improves, in particular, erasability of writing after long standing. The use of the second separting agent in conjunction with the first separating agent further improves the erasability of writing after long standing. The carboxylic acid ester is especially useful in this connection. In turn, the polyoxypropylene monoalkyl ether is less effective in initial erasability of writing than the carboxylic acid ester, however, it is more effective to improve erasability of writing after long standing. Moreover, the use of the first and second separating agent in conjunction improves coloration of ink composition on a dark color writing board such as of dark green.