The present invention relates to a method of manufacturing
heat sensitive recording material and to the heat sensitive material. In particular
the inventions is applicable in manufacturing a heat sensitive material so that
the material in question has a better bar code readability than before. Further,
the invention relates to a heat sensitive recording material having an improved
bar code readability.
Heat sensitive material is typically paper or corresponding
material composed of several layers and used typically in sheet or roll form. The
main layers are base paper or corresponding material, and coating. Further, at least
a color forming agent, a developing agent and a sensitizing agent are situated in
the coating. When heated to a suitable temperature the coating melts, in some cases
softens or sublimates, thus allowing reactions of other components of the coating,
whereby, as a consequence of the chemical reaction, a colored trace is produced
in the recording material.
In the following, a few patent documents disclosing the
use of Bisphenol S, a nowadays-popular color former, has been discussed in more
EP-A-0968837 discusses a thermally sensitive recording
medium, which comprises; on a substrate; a thermally sensitive color developing
layer comprising a colorless or a pale colored dye precursor and a color developer.
The document concentrates on developing a black color with a maximum absorption
wavelength of 420-480nm and 550-640nm. The used color former is Bisphenol S.
EP-A-0546836 discusses a thermally sensitive recording
material with improved thermal response for a given total coat weight of thermally
sensitive composition or image-forming components. The thermally sensitive recording
material has the thermally sensitive composition applied separately in two or more
adjacent layers, each of which contains color developing co reactant material, rather
than in the conventional single layer. The adjacent thermally sensitive layers are
formulated such that together they generate a substantially single color final print
in response to heat energy input. The color developing co reactant materials in
the layers may be the same or different and are typically bisphenols, like for instance
US-A-5084593 discloses a recording material using Bisphenol
S, among other color developers, as the color developer compound. The document is
mainly concerned on discussing various leuco dye alternatives.
US-A-4916111 discloses a thermo sensitive recording material
comprising a support, an intermediate layer including a foamed portion with minute
voids on the support, and a thermo sensitive coloring layer formed on the intermediate
layer is disclosed, which is particularly improved in such a manner that the voidage
of the upper portion of the intermediate layer close to the thermo sensitive coloring
layer is smaller than the voidage of the lower portion of the intermediate layer
close to the support, or the voidage of the intermediate layer is increased from
the upper portion thereof towards the lower portion thereof. Among dozens of color
developers, Bisphenol S is mentioned.
US-A-5250493 discloses a thermo sensitive recording material
having a support and a thermo sensitive coloring layer formed on the support, which
thermo sensitive coloring layer includes at least a first thermo sensitive coloring
layer containing at least one leuco dye, and a color developer capable of inducing
color formation in the leuco dye under application of heat thereto, and a second
thermo sensitive coloring layer containing at least one leuco dye, and a color developer
capable of inducing color formation in the leuco dye under application of heat thereto.
The leuco dye for use in the first thermo sensitive coloring layer has the absorption
intensity in the near infrared region, and the leuco dye for use in the second thermo
sensitive coloring layer has the absorption intensity in the visible spectrum. As
one of the color developers Bisphenol S is mentioned.
Further, patent documents EP-A-0 968 837, US-A-5,256,621
and US-A-6,093,678, among others, may be mentioned as examples of patent literature
discussing heat sensitive recording material in general.
Heat sensitive recording material, i.e. so-called thermal
paper, is used for example in various stickers, tags and labels; based on the bar
code printed on them the products or the properties, for example the price, of the
products thus marked can be recognized automatically. A separate bar code reader
is used for the recognition of the bar code. A characteristic feature in the operation
of a bar code reader is that light is directed from the reader to the bar code and
it reflects back from the white spaces between the bars of the bar code but does
not reflect, or at least reflects substantially weaker, from the bar code itself.
Bar code readers have been developed over the years in order to achieve a better,
faster, more accurate and more faultless recognition of the code. For example, the
wavelength of the light used in new bar code readers has been changed. The wavelength
used earlier in bar code readers has been shorter, approx. 600 - 630 nm, than in
the present light sources. Today, LED- or laser-type bar code readers are used which
provide a higher reading speed and also a more accurate code recognition not to
mention the lower production costs of the devices. However, LED- and laser-type
bar code readers use light with a wavelength of 660 -690 nm.
When new bar code readers have been introduced, the use
of prior art thermal paper has caused problems in the readability of the bar code.
Customers have complained about disturbances occurring with certain readers using
a LED or a laser light source. Our tests, in which perhaps the most commonly used
color forming agent, 3-(N-ethyl-N-isopentylamino)-6-methyl-7-anilinofluoran, commercial
name S205, and a new, very popular 3-(diethylamino)-6-methyl-7-(3-methylphenylamino)fluoran,
which is known under the commercial name ODB-7, were used as the color forming agent,
and the most common 4,4'-isopropylidenediphenol, commercial name BPA, and 4-hydroxy-4'-isopropoxydiphenylsulfon,
commercial name NY-DS, as the developer, have shown that the introduction of another
light wavelength has resulted in that the paper properties, more precisely the light
absorption or reflectance properties, are no longer suitable for the new type of
light and the recognition of the bar code has deteriorated.
When this was studied it was found out that the reflecting
properties of a bar code changed dramatically when moving from the prior art light
wavelength (about 630 nm) upwards. This is illustrated in Figures 1, 2 and 3. It
was found out, among other things, that with conventional bar code papers and the
light wavelength (approx. 630 nm or less) employed by prior art bar code readers,
less than 10 % of the light used was reflected from the bar code bars whereas with
the equipment of new technology, even more than 50 % of the light was reflected.
At the same time, it was found out that with the wavelength of 630 nm, there were
no significant differences between different color formers and developers in the
luminous reflectance whereas with longer wavelengths different color former- developer
compounds gave very different reflections.
At first this was thought to be at least partly due to
the density of the bar code but, to a substantial extent, that is, however, not
the reason. An increase in the density reduced the total reflectance by a few percents
as a whole, only. In any case, different color former-developer compounds still
gave very different and very high reflections.
As the problem was proved to be the reflection of the bar
of the bar code and particularly the change in it when the wavelength increased,
the study was continued by finding out a limit value curve, which is the minimum
requirement the customers have given for the bar code products. Also, color former-developer
compounds were looked for with which the results would clearly be under the limit
value curve whereby the absorption of light is so strong the reflectance of light
does not cause problems in the recognition of the bar code.
Further, a so-called target curve was determined which
is located about 7 % below the limit value curve. The values of the curve were determined
by performing a large number of tests and setting as a target an error margin of
one percent in the recognition of the bar code.
Thus, one problem, which the present invention solves,
is the readability of the bar code when using LED or laser readers. The invention
is based on the idea of using a color former and a color former- developer compound
with which the absorption of light is as strong as possible with the desired light
wavelength, i.e. 660 - 690 nm.
The characteristic features of the present invention are
disclosed more specifically in the appended patent claims.
The method of the invention is described more in detail
below with reference to the accompanying drawing figures of which
- Figs. 1 and 2 illustrate reflectance values of bar code bars having two different
degrees of optic densities obtained with prior art color formers and developers;
- Fig. 3 illustrates an initial curve of reflectance determined by using the prior
art products mentioned above and other known bar code products, with a curve for
the minimum requirements set by customers;
- Fig. 4 illustrates the sensibility of different color former-developer compounds;
- Fig. 5 illustrates a comparison of different color former-developer compounds
when they are subjected to different test stresses;
- Fig. 6 illustrates reflectance curves obtained with color former-developer compounds
according to a preferred embodiment of the invention, with both curves obtained
with prior art compounds, and the limit value curve with a certain optic density
- Fig. 7 illustrates reflectance curves obtained with color former-developer compounds
according to a preferred embodiment of the invention, with both curves obtained
with prior art compounds, the initial, the limit value and the target curve with
a certain optic density (1.30);
- Fig. 8 illustrates reflectance curves obtained with a certain wavelength with
color former - developer compounds according to a preferred embodiment of the invention,
with both curves obtained with prior art compounds, and the limit value and the
target curve with a certain optic density (1.15);
- Fig. 9 illustrates reflectance curves obtained with a certain wavelength with
color former-developer compounds according to a preferred embodiment of the invention,
with both curves obtained with prior art compounds, the initial, the limit value
and the target curve with a certain optic density (1.30).
In the situation illustrated by Fig. 1, the bar codes were
printed on prior art recording material using prior art color former-developer compounds.
Thus, the color formers were S205 and ODB-7 and the developers BPA and NY-DS. Figure
1 illustrates in percentage the amount of light reflected by the bar code, i.e.
the dark bars of it, at a lower optic density, 1.15, as a function of the light
wavelength. It can be seen that with the light wavelength (630 nm) used in prior
art equipment, less than 10 % of the light was reflected by the bar code bars, whereas
with the equipment of the modern technology more than 40 %, even more than 50 %
of the light was reflected. Further, it should be noted that with the wavelength
630 nm, there are no significant differences in the reflectance with different color
formers and developers, whereas with a longer wavelength, different color former-developer
compounds produce very different reflections. Differences with the wavelength of
690 nm are even of the order of more than ten percent, already.
At first it was thought that this was at least partly due
to the degree of density of the bar code but Fig. 2 illustrates that, in essence,
this is not the case. In the situation illustrated in Fig. 2, the optic density
had already been raised to 1.30, which has given a total reflectance decrease of
a few percents, only. Anyway, different color former-developer compounds still give
very different and very strong reflections. However, the figures illustrate that,
of the two developers used, BPA gives with both color formers a slightly weaker
reflection (which is a desired property) than NY-DS with the same color formers.
Since the problem was proved to be the reflection of the
bar code bar and in particular the change of it with the increase of the wavelength,
the study was continued by determining a limit value curve to the Figures mentioned
above based on the requirements of the customers; with the reflection values indicated
by the curve, an interpretation of the bar code is obtained, which at least in most
cases satisfies the customers. Figure 3 illustrates, with the curves of the test
samples of Fig 1, the reflectance value curve of a commercial bar code product,
which can in a way be considered as the initial point of this development work.
The figure also includes a limit value curve drafted based on the requirements of
customers which illustrates the maximum values of the reflectance set by the customers.
In order to make the interpretation of the bar code so reliable and of so high quality
that the customers need not complain about bar code reading problems, the reflections
of the bar codes have to be clearly below the above limit value curve.
When the so-called target curve was determined in the way
described above, test were started to find a chemical compound, which would give
results on the level of the curve or even below it.
The invention is thus based on the idea that different
chemical compounds of color formers, developers and sensitizers, the most significant
of which have proved to be compounds of color formers and developers, give different
light reflection responses at different light wavelengths. The object of the invention
was thus to find a chemical compound or compounds, which would produce bar codes
not reflecting light at the light wavelength 660 - 690 nm used in LED or laser-type
bar code reading devices as much as bar codes produced with prior art chemical compounds.
This would be utilized in bar code reading, which is based on the difference between
the high reflection from the background, which is mainly white, and the reflection
from the code bar, which is as low as possible, and can be interpreted as a high
contrast between the code bar and the background. The end result is that the bar
code reader detects the information from the bar code better when the bar code has
been printed on thermal paper containing the new chemical compound according to
the invention, compared with thermal paper containing prior art chemicals. Thus,
the chemical compound according the to present invention has been developed particularly
for the light wavelength used in the new type of bar code readers.
In producing a bar code and other images on thermal papers
and other heat-sensitive recording materials, developers and color formers are used
and commonly also sensitizers, which decrease the melting point of the coating material.
Various developers, color formers and sensitizers have been compiled in the following
lists. The lists include, among others chemicals, also the developers BPA and NY-DS
and color formers S205 and ODB-7 mentioned above.
Developers are for example:
- 4,4'-isopropylidenediphenol (also know under commercial name bisphenol A
- 4,4'-sulfonyldiphenol (Bisphenol S)
- 4-hydroxy-4'-isopropoxydiphenylsulfone (NY-DS)
- 4-hydroxybenzoyloxy &agr;-naphthyl benzoate
- 4-hydroxybenzoyloxy &bgr;-naphthyl benzoate
- 4-hydroxybenzoyloxy &bgr;-phenethyl benzoate
- 4-hydroxybenzoyloxybenzyl benzoate
- 4-hydroxybenzoyloxybutyl benzoate
- 4-hydroxybenzoyloxycyclohexyl benzoate
- 4-hydroxybenzoyloxyethyl benzoate
- 4-hydroxybenzoyloxyhexyl benzoate
- 4-hydroxybenzoyloxyisopropyl benzoate
- 4-hydroxybenzoyloxymethyl benzoate
- 4-hydroxybenzoyloxynonyl benzoate
- 4-hydroxybenzoyloxyoctyl benzoate
- 4-hydroxybenzoyloxypropyl benzoate
- 4-hydroxybenzoyloxysec-butyl benzoate
- 4-hydroxybenzoyloxytert-butyl benzoate
- 4-hydroxybenzyl benzoate
- 4-hydroxybutyl benzoate
- 4-hydroxydibenzyl phthalate
- 4-hydroxydihexyl phthalate
- 4-hydroxydiisopropyl phtalate
- 4-hydroxydimethyl phthalate
- 4-hydroxyethyl benzoate
- 4-hydroxyisobutyl benzoate
- 4-hydroxyisopropyl benzoate
- 4-hydroxymethylbenzyl benzoate
- 4-hydroxypropyl benzoate
- bis-(4-hydroxy-2,3- dimethylphenyl)sulfide
- bis-(4-hydroxy-2,5- diisopropylphenyl)sulfide
- monobenzyl phthalate
- monocyclohexyl phthalate
- monoethoxybenzyl phthalate
- monoethylphenyl phthalate
- monohalogenbenzyl phthalate
- monomethylphenyl phthalate
- monophenyl phthalate
- monopropylbenzyl phthalate
- novolac type phenolic resin
As (black) color formers are used for example:
- 3 (diethylamino)-6-methyl-7-(2,4-dimethylphenylamino) fluoran
- 3-(diethylamino)-6-methyl-7-(3-methylphenylamino)fluoran (ODB-7)
- 3-(N-ethyl-N-isopentylamino)-6-methyl-7-anilinofluoran (S 205)
- 3-[N-ethyl N-(3-ethoxypropyl)amino]-6-methyl-7-anilinofluoran
Sensitizers are for example:
- 4-(4-tolyloxy)biphenyl (JKY-120)
- behenic acid amidemethylene-bis-stearic acis amide
- dibenzyl terephthalate
- dimethyl terephthalate
- dioctyl terepthalate
- ethylene-bis-stearic acid amide
- methylene-bis-stearic acid amide
- N-benzoylstearic acid amide
- N-eicosenoic acid amide
- N-methylostearic acid amide
- palmitic acid amide
- stearic acid amide
The color former, the developer and the sensitizer are
dispersed into liquid, usually water, together with a suitable binder.
As binders are used for example:
- amide-modified polyvinylalcohol
- carpoxy-modified polyvinyl alcohol
- petroleum resins
- polyacrylic acid
- polyacrylic acid esters
- polyamide resins
- polyvinyl acetate
- polyvinyl alcohol
- silicone-modified polyvinyl alcohol
- styrene-butadiene copolymer
- styrene-maleic acid copolymer
- sulfonic acid-modified polyvinyl alcohol
- terpene resins
Further, fillers are used in paper production in manners
known per se; filler are for example:
- aluminium hydroxide
- calcined caolin
- calcium carbonate
- diatomaceous earth
- nylon powder
- styrene microballs
- titanium oxide
- ureaformaline resin
Further, "lubricants" are used in paper production, which
are for example:
- polyethylene wax
- stearic acid ester wax
- zinc stearate
Test we have performed have proved that it is possible
to use as a color former in the thermal paper according to our invention a color
former, which is known per se under the commercial name ODB-7; the light absorption
with this chemical at a wavelength of over 630 nm is higher (and correspondingly
reflectance is weaker) than with for example the coler former S205 which is the
most popular today (cf. for example Figs. 6 and 7). As a developer can be used for
example 4,4'-isopropylidenediphenol, commercial name Bisphenol A or a shorter
name BPA, which is commonly used. However, according to our studies, another
possible developer is surprisingly 4,4'-sulfonyldiphenol, commercial name
Bisphenol S or BPS. BPS is the cheapest of the chemicals used and
also the least sensitive, and because of this unsensitive property it has not been
extensively used in the production of thermal paper. Fig. 4 illustrates a comparison
between the sensitivity of BPS and that of other known developers. Sensitivity here
means the amount of energy required for producing an image; thus, the smaller the
energy required is the better is the sensitivity. The figure illustrates that the
use of BPS calls for open-mindedness as it's behavior deviates so much from that
of the other developers. However, it has been surprisingly found out that the use
of the compound ODB-7 - BPS, and in particular the use of BPS as the developer,
produces a substantially better result in the bar code readability than the use
of other possible chemical compounds (cf. Figs. 6 - 9). It has also been found out
that the preservability of the bar code has improved significantly.
The test we have performed are described below with reference
to the accompanying tables. At first the test apparatus is described.
The bar code reader was a PSC Quick check®
PC600 PC bar code reading system, which is commonly used and produced by PCS
Inc. The thermal printers were devices of two different manufacturers. One was thermal
printer Model 200 by Atlantek, where the print head is KYOCERA KST-216-8MPD1
with R=639 &OHgr;. The other printer was thermal printer Model MP104 by
Markpoint, having a print head Mitsu EP104-8X, 800 ohm, produced by Mitsubishi.
The optic density was determined with a Macbeth meter RD-918 without a filter.
The reflectance values were measured with an L&W Elrepho spectrophotometer
by Lorenzen & Wettre.
The light used in the tests was C/2, which corresponds
to a normal office lighting. The filter was an UV filter run by light C.
In the first test, the results of which are presented in
Table 1, the degree of density of a bar code was measured at different heat energies
projected on a square millimeter of a thermal paper and by using different chemical
compounds in the heat sensitive material In the test, samples of thermal papers
were used which had been produced by using BPA, BPS or NY-DS as the developer and
either ODB-7 or S205 as the color former. The amount of energy directed to the thermal
paper was changed and the degree of density of the printed color was measured.
3-1/NY-DS + S205
3-3/NY DS+ +ODB-7
* Not within the
scope of the invention.
The results in Table 1 indicate that BPS as a developer
is very unsensitive which in practice means that in order to achieve the same degree
of density BPS requires about twice the amount of heat energy compared with other
developers. Not until at very high degrees of density, more than 1.25, the heat
energies required by other developers reach BPS. in practice, Fig. 4 shows to a
person of ordinary skill in the art that BPS is not worth using as a developer as
it imposes greater demands on the printer and increases the energy consumption of
For Table 2, the thermal paper samples described in Example
1 were tested to find out the preservability of the color. The samples, on which
the bar code had been printed with black color, were treated as follows:
- Test 1: soaking in water for two hours,
- Test 2: sample covered with oil, measuring after one hour of treatment
- Test 3: sample covered with oil, measuring after 24 hours of treatment
- Test 4: sample wrapped in a PVC plastic foil, temperature 23°C, measuring
after two hours;
- Test 5: sample wrapped in a PVC plastic foil, temperature 23°C, measuring
after 24 hours; and
- Test 6: sample wrapped in a PVC plastic foil, temperature 40°C, measuring
after 24 hours.
Both oil and the softeners in the PVC tend to restore the
reversible color reaction to its initial state.
Table 2 shows in percentage the share of the print image
density remaining in the paper at the measuring moment.
*Not within the
scope of the invention
When comparing different developers with each other it
can be seen that BPS is clearly different from BPA throughout the whole test range.
When compared with NY-DS, there is no difference in the water soaking, but in the
oil tests and in particular in the plastic foil tests the difference was very clear.
The result of test 4 with a compound of BPS and ODB-7 indicates that the print image
has even become denser than the original print. The test results have been illustrated
also in Fig. 5, which indicates even more clearly the superiority of BPS in all
tests compared with other developers.
In examples 3 and 4 the reflectance of light from bar codes
was studied with the thermal paper samples described in example 1. In example 3,
the thermal energy used in the printing of the bar code was the average whereas
in example 4, a higher thermal energy was used to produce the image. The bar code
reflections were determined in both examples by using wavelengths 630 - 690 nm with
intervals of 10 nm. The light and the filter used in all the test were the same.
In the tests of this example, samples of essentially the
same bar code density, i.e. 1.13-1.16, were used.
The test results indicate that with all the wavelengths,
all the color formers and all the developers, the BPS developer gave as a rule the
The average reflection with BPS was about 10-20 % lower
than with other developers.
The results of the test are presented in Fig. 6, where
additionally both a limit value curve representing the minimum requirements of the
customers and a curve representing the target value determined by ourselves are
included. The reflectance values obtained with the target curve result according
to our tests in the bar code reading in an error margin of less than one percent,
only, which is clearly smaller than the error margin accepted by the customers.
We have determined a mathematical function the descriptor of which sets with a reasonable
accuracy on the limit value curve of the customers. The function in question is
of the form
where Hp is the reflection percent (%) and Ap the light wavelength (nm) employed
by the bar code reader.
From this, we have come to the target curve by moving the
curve 7 % down which is performed by multiplying the values of the limit value curve
by 0.93. Then the target curve is of the form
Table 3 (herein * means outside the scope of the invention)
1-3/BPA + ODB-7
2-1/BPS + S205*
2-3/BPS + ODB-7
3-1/NY- DS + S205*
3-3/NY-DS + ODB-7
Wave length /nm
In this test, higher thermal energy was used to form the
image whereby the degree of density of the samples varied between 1.30 -1.33.
Table 4 (herein * means outside the scope of the invention)
1-3/BPA + ODB-7
2-1/BPS + S205*
3-1/NY- DS + S205*
Wave length/ nm
Test results indicate that also with a higher degree of
density, BPS still gives a substantially lower reflectance than other developers.
In practice, the difference is of the same order as above, i.e. 10 - 20 %.
Figures 8 and 9 illustrate the test results of the above
examples 4 and 5 in a little broader scale by showing on the co-ordinates only the
light wavelength range 660 - 690 nm the more modern bar code readers use. The figures
indicate that by using BPS as the developer, the reflectance at the wavelength 690
nm is at its best less than 35 % while the color former-developer compounds used
earlier have hardly reached a reflectance of below 40 %.
In the situation of Fig. 8, where the optic density was
the average, i.e. about 1.15, only two of the tested color former-developer compounds
reached below the target curve. The color former in both of these was ODB-7 and
in one compound the developer was BPA and in the other one BPS.
In the situation of Fig. 9, where the optic density was
about 1.31, four color former-developer compounds reached below the target curve.
In three of the cases the color former was ODB-7 and in one it was S205. In two
of the cases the developer was BPS and once both BPA and NY-DS.
Thus it may be concluded that a recommendable color former
would be ODB-7 or any color former with corresponding properties of the ones described
on pages 10 and 11. Correspondingly, a recommendable developer is BPS or any developer
with corresponding properties of the ones listed on pages 7 - 10.
The solution according to the invention allows using conventional
prior art sensitizers, which have been listed above on pages 11 and 12 as far as
it does not prevent the desired effect of the invention.
In addition to the improved readability of the bar code,
the solution according to the invention provides also for an improved preservability
of the bar code. Further, if BPS is used as the developer, the coating is very inexpensive.
The invention may be employed in all applications of thermal
paper where bar codes and LED or laser readers are used, such as for example price
tags, entrance tickets and in weighing fruit.