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Description  |
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This invention relates to a process for the recording of information in the
form of lines, marks, alphanumerical signs or images by the ink jet
process and in particular to novel writing liquids (inks) which are
particularly suitable for this purpose.
Numerous processes for the contact-free recording of information on almost
any surfaces have become known in recent years. Particularly interesting
among these are the various forms of the ink jet process (see, for
example, R. Meyer, K. Hoffmann; 4. Intern. Kongress Reprogaphie and
Information, Plenarvortrage pages 184 to 199, 1975). In some of the
various forms of the ink jet process, a jet of writing liquid is ejected
from a fine nozzle under the action of a high hydrostatic pressure, and
the jet is modulated according to the information to be recorded and is
directed to a writing surface which generally has a relative component of
movement transverse to the direction of the jet. Modulation of the jet in
accordance with the information can be achieved, for example, by means of
a control electrode which electrically charges the jet from the nozzle
according to the information to be recorded so that the ink jet is fanned
out to varying degrees. This results in a variation in the proportion of
ink which is not deflected and therefore passes through a narrow aperture
to reach the writing surface, as described in German Pat. No. 1,271,754.
In another process, a uniform electric charge is imparted to the jet of
liquid which has broken up into droplets after its emission from the
nozzle, and the charged droplets are then deflected from their original
line of flight by deflecting electrodes according to the information to be
recorded. The part of the original jet which has not been deflected
reaches the writing surface, where it registers the information in the
form of lines, marks, alphanumerical signs or images, as described in U.S.
Pat. No. 3,596,275.
In the variations of the ink jet process mentioned here the ink must be
suitable for sufficiently rapid removal of the electric countercharge of
the electric charges imparted to the droplets into which the ink jet has
broken up. It has been found that this requires an electrical conductivity
of at least 10.sup.-4 Ohm.sup.-1 cm.sup.-1. Highly insulating writing
liquids such as the solutions of dyes in oils or petroleum hydrocarbons
described in Japanese Patent Application No. 49-50935, for example, have a
much lower electric conductivity and are therefore unsuitable for the
variations of the ink jet process described here. Also the inks described
in German Offenlegungsschrift No. 2,516,268 having a liquid medium
consisting essentially of a mixture of formamide and an aliphatic
monohydric alcohol have not the high electric conductivity of at least
10.sup.-4 Ohm.sup.-1 Cm.sup.-1 required for the ink jet process described
here. The fine nozzles used for producing the jet generally have a
diameter of less than 60 .mu.m. The ink should therefore have a very low
viscosity so as to ensure a sufficiently high rate of flow through the
fine nozzles at the given writing speeds and writing temperature, which is
generally room temperature, thereby ensuring a sufficiently high colour
density of the recording. The viscosity of the writing liquid should
suitably be below 5 cP at 20.degree. C.
If the writing surfaces are made of water absorbent materials such as
paper, absorbent textile fabrics, leather or foils coated with a
hydrophilic substance, the inks used for these variations of the ink jet
process may be aqueous solutions of dyes as described for example in
German Offenlegungsschrift Nos. 2,258,804; 2,258,836, and 2,258,837. These
aqueous inks easily fulfil the above mentioned requirements of low
viscosity and sufficiently high electric conductivity. It is well known
that if the nozzles are kept out of operation for prolonged periods during
which they are not kept supplied with fresh ink, aqueous inks tend to dry
up inside the nozzle or in the porous absorbent material forming part of
the writing head or on the control electrodes, thereby considerably
impairing the operational reliability of the apparatus. This problem has
been attributed to the relative high volatility of the solvent (water)
used in aqueous inks. It has therefore been proposed to treat aqueous inks
with certain additives designed to prevent unwanted drying up of the ink
as described in German Offenlegungsschrift Nos. 2,258,835 and 2,260,464.
This measure succeeded in further improving the operational reliability of
the process, and ink treated with such additives still dries sufficiently
rapidly and without smudging on the absorbent writing surfaces mentioned
above.
It has been found, however, that these aqueous inks are not suitable when
attempts are made to use the ink jet process for writing on
water-repellent surfaces or surfaces which absorb water only slowly.
Surfaces of this kind are found, for example, on materials such as
hydrophobic polymers metals, glass or porcelain. Aqueous writing liquids
are particularly unsatisfactory if the marks made on the writing surface
are required to dry within a very short time, for example 1 to 2 seconds.
Such rapid drying is often necessary for example for writing on rapidly
moving bands which are passed over guide rollers or rolled up at a short
distance from the writing apparatus so that the written side of the bands
is brought into contact with other surfaces, or if articles on which the
ink marks have been made (individual sheets, packets, envelopes) are
required to be stacked shortly after the writing operation without any of
the written information being indesiredly printed on the surface with
which it comes into contact.
It has been found that for these purposes, i.e. when using the ink jet
process for recording on hydrophobic surfaces or surfaces which are
difficult to wet, it is particularly suitable to use inks consisting of
solutions of organic dyes in a medium of predominantly readily volatile
solvents.
The present invention relates to a process for the recording of information
by the ink jet process in which a jet of writing liquid having a viscosity
of less than 5 cP at room temperature and an electrical conductivity
greater than 10.sup.-4 Ohm.sup.-1 cm.sup.-1 is ejected from a nozzle
solely by means of pressure and is directed to a writing surface having a
relative component of movement transverse to the direction of jet, and in
which the jet of writing liquid breaking up into droplets is electrically
charged and is modulated according to the information to be recorded,
characterised in that the writing liquid used is a solution of a dye in a
liquid medium consisting either completely or predominantly of one or more
organic solvents having an evaporation index of less than 11.
The finding that readily volatile solvents are suitable for the ink jet
process is surprising since it has hitherto been assumed that the risk of
blockage of the fine nozzles, which generally have a diameter of less than
60 .mu.m, is all the greater the more easily the ink dries up, i.e. the
more volatile its solvent. For this reason, the water based inks
previously used have even been treated with special substances as already
mentioned above, to prevent drying up of the inks by preventing
evaporation of the water in them. It has now surprisingly been found that
the inks according to the invention, which are dissolved in highly
volatile solvents not only achieve the desired rapid drying of information
written with them on hydrophobic or relatively non-wetting surfaces but
also do not cause blockage of the nozzles during prolonged periods of
inactivity provided that care is taken to ensure that during the periods
of inactivity, when the pressure is switched off, a certain amount of ink
is always left in the nozzle chamber which is situated behind the nozzle
and bounded by the ink inlet. If the apparatus is to be left out of action
for several days or weeks, the ink feed system may be flushed with pure
solvent after completion of the final spray process in order to prevent
drying up of the ink.
Whereas only water-soluble dryes are suitable for the known water-based
inks used in the ink jet process, the use of predominantly non-aqueous
solvents according to the invention opens up the possibility of using
numerous new classes of dyes since it is now possible also to use dyes
which are insoluble or only sparingly soluble in water. The dyes may
belong to any known class of dyes provided they are soluble in the
solvents used.
Suitable dyes include, for example, anthraquinone dyes such as Alizarine
cyanine green 5 G, C.I. 62 560; Alizarine rubinol 3 G C.I. 68 205; azo
dyes such as Zapon Fast red, C.I. 12 715; ceres black BN, C.I. 26 150;
ceres yellow GRN, C.I. 21 230; Irisol fast scarlet GN, C.I. 27 291;
xanthene dyes such as Rhodamine B. C.I. 45 170; eosin, C.I. 45 380;
pyronine G. C.I. 45 005; phenacine dyes such as Safranin T, C.I. 50, 240;
Mauvein C.I. 50 245; oxazine dyes such as Nile blue, C.I. 51 180; nigrosin
dyes such as Brilliant spirit black RM; thiazine dyes such as Methylene
blue, C.I. 52 015; acridines such as Acridine orange C.I. 46 055; indigoid
dyes such as Indorubin C.I. 73, 200; thioindigo, C.I. 73 300; triphenyl
methane dyes such as Spirit blue C.I. 42 775; Patent blue C.I. 42 045;
malachite green C.I. 42 000; phthalocyanines such as Zapon Fast blue HFL
C.I. 74 350; polymethine dyes such as Astra Violet 3 R, C.I. 48 010;
Astrazon yellow 3 G. C.I. 48 055; Astraphloxine, C.I. 48 070 etc.
Special effects and colour shades can be obtained by mixing several dyes.
UV absorbents or white toners or dyes having their main absorption in the
infrared region of the spectrum may also be used for certain purposes. The
concentration of dyes in the inks used according to the invention is not
critical and depends only on the solubility and intensity of colour of the
dye used and the intended purpose of use. It is normally sufficient to use
0.5 to 12% by weight of dye based on the total weight of the ink, although
higher concentrations may be used for special purposes.
For some purposes, in particular when the ink jet process is used in the
photographic industry, for example for marking and labelling
light-sensitive materials such as webs of film or paper, it has been found
necessary to use inks containing dyes capable of absorbing light which
does not cause any blackening i.e. imagewise recording, on the
photographic materials employed. The marks made with the ink can then be
inspected with non-actinic light at any time without the photographic
material being thereby exposed. Numerous dyes have been found which are
readily soluble in the organic solvents used according to the invention
and at the same time absorb at least 30% of the incident radiation in the
near infra-red region of the spectrum (700 to 1000 nm) to give rise to a
sufficiently high colour density. Examples of such dyes include the known
basic nigrosin dyes or dyes of the polymethine series such as penta- or
heptamethine cyanines.
According to the invention, the marks formed on the recording surfaces
should be dry in a short time. The solvents must therefore evaporate very
rapidly. Low boiling organic solvents with an evaporation index of less
than 11 meet this requirement. For definition of evaporation index
reference may be had to Losemittel Hoechst, 5th Edition 1974, page 313;
German Standards DIN 53 170, I 1954). Mixtures of two or more miscible
solvents having each an evaporation index if less than 11 may be used
instead of pure solvents. Suitable solvents include, for example, alkanes
and cycloalkanes, preferably having 5 to 8 carbon atoms such as pentane,
hexane, cyclohexane or isooctane; lower alcohols such as methanol ethanol
or isopropanol; ethers such as diethyl ether, dioxane or tetrahydrofuran;
aromatic hydrocarbons such as benzene or toluene; halogenated hydrocarbons
such as methylene chloride, chloroform, carbon tetrachloride, ethylene
chloride, trichloroethylene or 1,1,2-trichloro-1,2,2-trifluoroethane;
ketones such as acetone, methyl ethyl ketone or methyl isobutyl ketone; or
esters such as ethyl formate, methyl acetate, propyl acetate, butyl
acetate or ethyl propionate.
The liquid medium in which the dyes are dissolved consist completely or
mainly, i.e. up to at least 85% by weight, of such organic solvents having
evaporation indices of less than 11. It may contain up to 15% by weight of
solvents having an evaporation index above 11. The proportion of such less
easily volatile solvents depends mainly on their evaporation index as well
as on their miscibility with the main solvent components which have
evaporation indices below 11. For example, if the added solvent has a very
high evaporation index, it may added only at a minor proportion, e.g. up
to 2 or 5% by weight. Thus in, the specific case of added formamide or
glycol the amount of these added solvents must not exceed 0.5% by weight
of the total liquid medium. If the solvents of the liquid medium are
miscible with water, up to 15% by weight of water may be added to increase
the electrical conductivity although the required electrical conductivity
is generally obtained even without water when the dye is dissolved in
readily volatile solvents.
Another suitable method of obtaining the necessary electrical conductivity
consists of adding conductivity salts to the dye solution. The salts used
for this purpose are preferably readily soluble and highly dissociated
salts of organic acids and bases, for example ammonium acetate.
If very low viscosity solvents such as acetone or methyl ethyl ketone are
used, it may be advantageous to add small quantities, e.g. up to 2%, of a
thickener or film forming substance such as Celite to the writing liquid
in order to improve the image formed by the ink jet on the smooth surface
of the writing material.
In view of the small diameter of the nozzles used in the ink jet process,
it is advisable to filter the inks before use to preventing blockage of
the nozzles by coarse particles of dust in the ink. The filter pores must,
of course, be smaller by an order of magnitude than the diameter of the
nozzle (e.g. filter pores of 5 .mu.m for nozzle diameter of 40 .mu.m). To
ensure prolonged trouble-free operation of the filters, it is advisable to
use only inks consisting of true chemical solutions not containing any
solid particles such as toners or pigments.
Examples of suitable inks according to the present invention are given
below.
EXAMPLE 1
2.5 g of phenol black L (Commercial product of Bayer AG Leverkusen)
100 ml of acetone and
0.8 g of Cellit (acetone soluble acetyl cellulose) are mixed at room
temperature in a vessel with stirrer. 10 ml of water are added.
The solution is first course filtered through a fluted filter (Schleicher
and Schull No. 1406 1/2) and then fine filtered through an acetone
resistant membrane filter having a pore size of 1 .mu.m (Millipore
DURALON).
EXAMPLE 2
25 g of Astra violet 3 R (C.I. 48 010),
1000 ml of acetone and
6 g of Cellit are mixed as in Example 1, and
100 ml of water are added. Filtration as in Example 1.
EXAMPLE 3
62.5 g of Brilliant spirit black RM (C.I. 50 415; (Commercial product of
Bayer AG) are mixed with 1000 ml of ethanol, heated in a water bath at
75.degree. C for 30 minutes and stirred. After cooling to room
temperature, the solution is rough filtered as in Example 1 and then fine
filtered through an alcohol resistant pore filter (RC 60, Schleicher and
Schull).
EXAMPLE 4
255 g of Brilliant spirit black RM are added to 1500 ml of methylene
chloride with stirring over a period of 15 minutes. The solution is then
stirred for a further 15 minutes, filtered through a fluted filter
(Schleicher and Schull No. 589.sup.2) and then through a pore filter with
a pore size of 1 .mu.m (RC 69 Schleicher and Schull), using a glass fibre
filter as preliminary filter (GF/B Whatman). The solution is diluted to
6000 ml with ethanol denaturated with a small amount of toluene.
EXAMPLE 5
40 g of heptamethine cyanine
##STR1##
are dissolved in 1000 ml of methanol. An ink ready for spraying is
obtained after filtration through a filter having a pore size of 1 .mu.m.
The inks described in Examples 1 to 5 are very suitable for use in the ink
jet process described in German Auslegeschrift No. 1,266,002 for applying
a line code to polyethylene coated paper which can subsequently be read
with the aid of a photoelectric reading head. The inks are dry 1 to 3
seconds after application.
The inks described in Examples 1,3,4 and 5 also produce marks on
polyethylene surfaces which can be read with the aid of an infra-red
reading head, i.e. under illumination with light from an infra-red
luminous diode (Example Litronix Lit 40).
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Description |
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