Capillary type ink

Ink compositions for use in capillary type writing instruments containing amide compounds of the formula: ##STR1## wherein R.sub.1 is --H, --CH.sub.3, or --NH.sub.2 when R.sub.2 is --(CH.sub.2).sub.2 OH and R.sub.3 is --H or --(CH.sub.2).sub.2 OH, and --NH.sub.2 when R.sub.2 and R.sub.3 are --H, PA1 R.sub.4 is --H, --CH.sub.3, or --NH.sub.2, and PA1 n is 0,1, or 2. These ink compositions, when used in capillary type writing instruments, allow rejuvenation of writing points which have dried out from exposure to the atmosphere.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an ink for use in capillary type writing 
instruments, and more particularly, to aqueous ink compositions promoting 
rejuvenation after the writing tip of a capillary type writing instrument 
has dried out from exposure to the atmosphere. 
2. Description of the Prior Art 
A problem frequently encountered by people who use capillary type writing 
instruments is writing point dryout. Capillary type writing instruments 
include those pens or marking devices which have fiber, felt, or porous 
plastic type writing points which deliver ink from a reservoir to the 
writing point by means of capillary action. The inks used in capillary 
type writing instruments have generally been of two types: (1) water based 
inks containing water soluble dyes, (the inks of the present invention), 
and (2) inks which contain organic solvents and dyes which are soluble in 
an organic medium. 
Capillary type writing instruments employ a variety of types of fiber, 
felt, and porous plastic writing points. Felt is a fabric of natural 
and/or synthetic fibers worked together by pressure, heat, or chemical 
action without weaving or knitting. The fibers tend to be arranged in a 
random fashion. When felt is used as a writing point, the ink is conveyed 
through the interstices between the individual fibers by capillary action. 
A second major type of writing point is made by bonding together a bundle 
of parallel strands of either natural or synthetic fibers (e.g. polyvinyl 
chloride, polyethylene, or polyamide polymers) to form capillary pathways 
between the bundles leading to the writing point. U.S. Pat. No. 3,558,392 
is an example of such a point. The fibrous point is made by setting a 
bundle of parallel strands of synthetic fibers in a desired relation by 
drawing the bundle through a heat-setting zone to establish dimensions in 
cross-section of the final object and to create longitudinally extending 
channels. The strength of the point is subsequently increased by immersion 
in a dilute resin solution followed by heating to evaporate the solvent 
and cure the resin. Depending on the denier of the fibers of the 
particular yarn which is used to form the writing point, a number of 
channels will be formed having average or mean transverse dimensions of 
from 1.times.1 mils. to 3.times.6 mils. Channels of 1.5 to 3 mils. in one 
transverse direction and between 3 and 6 mils. in another direction are 
representative of the most desirable channel sizes. Thus, the capillary 
channels are typically between 4.5 and 18 sq. mils. in cross-sectional 
area. 
An example of another type of writing point is described in U.S. Pat. No. 
3,778,496. The point described there is an elongated unitary writing 
element made by extruding and meltdrawing a polymer (e.g. acrylic, acetal, 
polyamide, or polyester polymers) and then reforming the internal portions 
of the extruded body by a compressive drawing operation to obtain channels 
of a desired size. The writing point thus formed has a longitudinally 
extending, axial, ink-conveying channel and circumaxial, spaced, 
ink-conveying channels, the axial channel having a cross-sectional area 
equivalent to a circle with a diameter between two and four mils. The 
enlarged circumaxial ink-conveying channels have an oblong cross-sectional 
shape approximately 1 to 3 mils. wide and 3 to 5 mils. in a radial 
direction. The circumaxial ink-conveying channels are connected to the 
axial channel by capillary channels having diameters of between 0.4 mils. 
and 1.0 mils. The longitudinal channels of the rods serve to deliver ink 
to the writing point by means of capillary action. 
In each of the above cases, the ink is fed from a reservoir to the writing 
surface by means of the writing point. The reservoir which feeds the 
writing point may be either a container of liquid ink or a bundle of 
fibers saturated with ink. Either will perform equally in capillary 
writing instruments since dryout is attributable to changes occuring at 
the writing tip rather than inside the pen. 
A problem with capillary type writing instruments is that when exposed to 
the atmosphere, as when left uncapped for a period of time, the water in 
the ink may evaporate so as to change the composition of the ink and make 
it more viscous. The capillaries within the tip of the writing instrument 
will then become clogged so that the written line produced becomes 
irregular. If left uncapped long enough, the composition of the ink at the 
tip may be so changed that it will not flow at all through the capillaries 
of the tip, writing becomes impossible, and the instrument must be 
discarded. 
Attempts have been made to retard the rate at which water based inks dry 
out by adding humectant materials such as ethylene glycol, propylene 
glycol, and other higher molecular weight polyhydroxy compounds such as 
those described in U.S. Pat No. 3,705,045. However, such materials tend to 
increase the viscosity of the ink to such an extent that it will not flow 
freely through the capillaries of capillary type writing instruments. 
Furthermore, the selective evaporation of the more volatile constituents 
of the ink may cause the inks to be thickened at the exposed tip of the 
instrument by the less volatile polyhydroxy compounds which remain. If 
additional polyhydroxy compound is used in the ink formulation, the gains 
from increased humectancy are lost to the increasing viscosity. 
In an attempt to solve the point dryout problem, U.S. Pat. No. 3,519,443, 
describes inks formulated which formamide as a solvent to reduce the rate 
of point dryout. Formamide is somewhat toxic. This constitutes a 
disadvantage since capillary type writing instruments containing 
water-based inks may be used by children. In addition, formamide is 
suggested for use at concentrations from 10% to 100% of the total solvent 
content of the ink, levels at which a disagreeable odor may be produced. 
In spite of the above attempts to reduce point dryout, problems still 
exist. Therefore, a need exists for ink compositions which not only reduce 
the rate of dryout, but allow rejuvenation even after dryout has occurred. 
SUMMARY OF THE INVENTION 
The present invention comprises an ink composition adapted for use in a 
capillary type writing instrument comprising an aqueous solution 
containing: 
(a) from about 0.5% to about 15% by weight of a water soluble dye; and 
(b) from about 1% to about 25% of an amide compound of the formula: 
##STR2## 
wherein R.sub.1 is --H, --CH.sub.3, or --NH.sub.2 when R.sub.2 is 
--(CH.sub.2).sub.2 OH and R.sub.3 is --H or --(CH.sub.2).sub.2 OH, and 
--NH.sub.2 and R.sub.2 and R.sub.3 are --H, 
R.sub.4 is --H, --CH.sub.3, or --NH.sub.2, and 
n is 0, 1, or 2. 
DETAILED DESCRIPTION OF THE INVENTION 
Satisfactory operation of capillary type writing instruments is dependent 
upon a steady capillary flow of ink to the point which engages the paper 
or other writing surface. Thus, the ink used must be sufficiently fluid to 
assure free and continuous flow through the capillary passages from the 
reservoir to the writing tip. Conversely, problems can occur if the ink is 
too thin. Feathering (i.e. laterial wicking of the ink into the paper from 
the written line), and strike through (i.e. penetration of the ink to the 
back of the paper), are common problems when ink viscosity is too low. To 
make sure that the writing point is supplied a suitable amount of ink, the 
inks should be formulated to have a viscosity below 50 centipoises at 
20.degree. C. and, more preferably, a viscosity in the range between 2 and 
5 centipoises. 
In its simplest form, the present invention comprises an aqueous solution 
of a water soluble dye and a compound of the formula: 
##STR3## 
wherein R.sub.1 is --H, --CH.sub.3, or --NH.sub.2 when R.sub.2 is 
--(CH.sub.2).sub.2 OH and R.sub.3 is --H or --(CH.sub.2).sub.2 OH, and 
--NH.sub.2 when R.sub.2 and R.sub.3 are --H, 
R.sub.4 is --H, --CH.sub.3, or --NH.sub.2, and 
n is 0, 1, or 2. 
Preferred amide compounds according to the invention are: 
##STR4## 
The amide compound may be present in the composition from 1% to 25% by 
weight, preferably from 5% to 15%. 
While an aqueous solution of a water soluble ink and an amide produce an 
acceptable ink, it is preferred to include at least one polyhydric alcohol 
in the ink formulation. 
Polyhydric alcohols are used in the present invention to increase the 
viscosity of the ink thereby improving the characteristics of the line 
produced by the writing instrument and reducing strike through and 
feathering. 
Polyhydric alcohols useful in the invention include alkylene glycols (e.g., 
ethylene glycol, propylene glycol, and butylene glycol), polyalkylene 
glycols (e.g., diethylene glycol and tripropylene glycol), mono- and 
di-alkyl ethers of polyhydric alcohols (e.g., ethylene glycol methyl 
ether, ethylene glycol ethyl ether, and ethylene glycol butyl ether) and 
trihydric alcohols (e.g., glycerin). The preferred alcohols are glycerin 
and ethylene glycol, and mixtures thereof. 
The total amount of polyhydric alcohol employed in the compositions of the 
invention may be varied within wide limits as dictated by the viscosity 
considerations of the composition as discussed above. As will be 
appreciated by those skilled in the art, the amount of polyhydric alcohol 
employed depends in part on the molecular weight of the particular alcohol 
employed, since viscosity is generally directly proportional to molecular 
weight. In general, the polyhydric alcohol component of the invention 
constitutes from 1% to 50% by weight of the composition. 
While any of the water soluble dyes typically used in ink formulations may 
be employed in producing inks in accordance with the present invention, 
acid dyes are preferred due to their high tinctorial strength. Such acid 
dyes possess one or more sulfonate groups attached to an aromatic nucleus, 
and can be classified according to their chemical structure into ten 
different categories: nitro, nitroso, monoazo, monoazo (metalized), 
disazo, triphenylmethane, xanthene, anthraquinone, azine, and quionoline. 
The amount of dye used will, of course, vary with the nature and color of 
the ink to be produced and may constitute from about 0.5% to 15% by weight 
of the total ink composition. 
If desired, it is also possible to add other optional ingredients typically 
used in water based inks such as corrosion inhibitors (e.g., 
benzotriazole) and preservatives (e.g., methyl parasept). 
The inks can be produced by the simple operation of mixing and dissolving 
the ingredients in any suitable way, but it is preferred that the ink 
components (with the exception of an amide of the present invention) be 
held at 55.degree. C. with constant stirring for two hours after their 
initial mixing. When the ink cools to room temperature, an amide of the 
present invention is introduced with stirring and the ink formulation is 
then filtered. 
The filtered ink is incorporated in any of the conventional capillary 
writing instrument types described above, by well known procedures. For 
example, the ink may be injected into the end of the reservoir opposite 
the writing point in a previously assembled porous point pen by means of a 
syringe. An alternate procedure, described in U.S. Pat. No. 3,581,378, is 
to first saturate a fibrous reservoir with ink, followed by insertion into 
the barrel of the porous point pen. 
The inks of the present invention are designed for use in capillary type 
writing instruments where the writing point may be accidently allowed to 
dry out (e.g., by leaving the writing point uncapped for a period of 
time). Should that occur, the instrument may be rejuvenated, rather than 
being discarded as would be the case with pens containing prior art inks. 
Rejuvenation is accomplished by recapping the writing point and allowing 
the instrument to equilibrate for a few hours, after which the writing 
point will have regained its ability to produce a written line. 
In order to illustrate typical and preferred ink compositions embodying the 
present invention, the following examples are included:

EXAMPLE I 
Six percent by weight of a dye known as Color Index Acid Green 16 (C.I. 
#44025), 2% by weight of a dye known as Color Index Acid Yellow 11 (C.I. 
#18820), 1% by weight benzotriazole, 0.2% methyl parasept, 5% glycerin, 
and 12.5% ethylene glycol were mixed and dissolved in 73.3% by weight of 
water. The solution was held at 55.degree. C., with constant stirring for 
two hours. After the ink was cooled, 10% by weight of 
.alpha.-aminopropionic amide was dissolved in the cooled solution. The ink 
formulation was then filtered and charged into capillary type writing 
instruments having writing tips formed of closely packed polyester fiber 
strands. Each pen was tested to make sure it would produce a written line, 
after which the uncapped pens were placed in a dessicator containing a 
saturated solution of MgCl.sub.2.6H.sub.2 O to maintain a R.H. of 
approximately 33%. 
Pens removed from the dessicator four, seven, and fourteen days later would 
not produce a written line since their writing points had dried out. 
However, when the pens were recapped and allowed to equilibrate for a few 
hours, the ability to write had been restored in all cases. 
EXAMPLE II 
______________________________________ 
Ingredient % by Weight 
______________________________________ 
Color Index Acid Blue 9 
(C.I. #42090) 7.0 
benzotriazole 1.0 
methyl parasept 0.2 
glycerin 5.0 
ethylene glycol 12.5 
water q.s. to 100 
______________________________________ 
Procedure: The above ingredients are combined and heated to 55.degree. C. 
for two hours with constant stirring. The solution is allowed to cool to 
room temperature, at which time 10% by weight of 
N-(2-hydroxyethyl)acetamide is added with stirring. The ink formulation is 
then filtered and charged into capillary type writing instruments having 
writing tips formed by closely packed polyester fiber strands. 
EXAMPLE II 
______________________________________ 
Ingredient % by Weight 
______________________________________ 
Color Index Acid Red 73 
(C.I. #27290) 8.0 
benzotriazole 1.0 
methyl parasept 0.2 
glycerin 5.0 
ethylene glycol 12.5 
water q.s. to 100 
______________________________________ 
Procedure: The above ingredients are combined and heated to 55.degree. C. 
for two hours with constant stirring. The solution is allowed to cool to 
room temperature, at which time 10% by weight of 
N,N-bis(2-hydroxyethyl)acetamide is added with stirring. After filtering, 
the ink formulation is charged into capillary type writing instruments 
having writing tips formed of closely packed polyester fiber strands. 
EXAMPLE IV 
______________________________________ 
Ingredient % by Weight 
______________________________________ 
Color Index Acid Blue 9 
(C.I. #42090) 7.0 
benzotriazole 1.0 
methyl parasept 0.2 
water q.s. to 100 
______________________________________ 
Procedure: The above ingredients are combined and heated to 55.degree. C. 
for two hours with constant stirring. The solution is allowed to cool to 
room temperature, at which time 15% by weight of .alpha.-aminopropionic 
amide is added with stirring. The ink formulation is then filtered and 
charged into capillary type writing instruments having writing tips formed 
by extruding, melt-drawing, and compressively drawing polyamide. 
The pens containing the ink compositions set forth in Examples II, III, and 
IV can be rejuvenated after their writing points have dried out, by 
capping for a few hours, At that time, their ability to produce a written 
line will have been restored.