DESCRIPTION

The invention relates to a method for position-selective carbonization of a substrate. The invention also relates to a device for position-selective carbonization of a substrate.

Inkless printing devices rely on the thermal process of selective carbonization to print or mark on substrates comprising cellulose such as paper and cardboard without the need of ink. This selective carbonization, i.e. the inkless printing, can be applied to regular substrates omitting the need of special coatings, special heat sensitive paper or special wavelength-sensitive paper. Another benefit is that there is no need for the use of consumables such as toners which is beneficial from environmentally point of view. This also applies to the omission of ink.

For the quality of the print it is important that the contrast between the print and the substrate is sufficient. An adequate contrast is in particular relevant when printing text, numbers and/or barcodes. Laser-based inkless devices can already achieve a desired, and therefore optimal, contrast, however, therefore the printing needs to be performed at relatively low scan speeds. Additionally, it is recommended that the print is made on a white background such as white paper or white cardboards, since further types of non white substrates, such as brown cardboards, may result in a decreased contrast between the substrate and the print, resulting in a lower quality of the print. Applying a relatively low scan speed, compared to conventional printers, is also required for optimising the blackness of the print. When operating at low scan speed the print can achieve the lowest lightness value, which corresponds to an optimum blackness. However, the low scan speed results in a relatively long printing time. A further drawback of the state of the art is that the print depth of the print with respect to the substrate for a fixed power density of the laser increases with time. Hence, when operating at low scan speed the print will be deeper into the material, and by defect decreasing the strength of the substrate material and increasing the probability of burning through the material leading till hole formation.

It is therefore desired to provide an improved method and/or device for inkless printing wherein an optimized contrast between the print and the substrate can be obtained. It is also a desired to provide an improved method and/or device for inkless printing wherein the scan speed during printing can be increased. It is a goal of the invention to provide at least one solution for abovementioned aims.

The invention provides thereto a method for position-selective carbonization of a substrate, comprising the steps of:

a) providing at least one carbonizable substrate, in particular a cellulose substrate,

b) position-selectively carbonizing said at least one part of the substrate by position-selectively irradiating of said at least one part of the substrate, by using at least one primary irradiation source to form at least one printed marking and

c) post-irradiation of at least one printed marking generated during step b) by using at least one secondary irradiation source, in particular a laser, such that the marking is darkened. The method according to the invention focusses on post-irradiation of one or more position-selectively irradiated parts of a carbonizable substrate. The position-selectively irradiation of the substrate causes the substrate temperature to rise and to exceed the (minimum) carbonization temperature of the substrate, as a result of which the irradiated part(s) is/are carbonized. In this manner one or more markings can be printed. This carbonization process is also referred to as inkless printing. However, a limitation of the current inkless printing techniques is that the carbonization step causes a physical limitation to the overall process. As described above, a factor significantly limiting the process is the printing speed which can be achieved by using the primary irradiation source. Use of multiple laser beams is possible, but this still takes a considerable amount of time in order to achieve a desired blackness level of the marking(s), and often too much time from an economic and commercial point of view. It has, however, surprisingly been found that by firstly generate a carbonized marking, which may have a (light) brown colour, at high speed, and by subsequently post-irradiating this initial marking, the blackness level (darkness level) can be increased significantly to a sufficient blackness level within a short period of time. By applying step c) each irradiated (carbonized) substrate part is irradiated at least twice to obtain one or more markings having sufficient blackness, and without requiring a significant amount of time, as a result of which the printing speed of the printing process as such can be increased significantly. This allows a reduced contact time between the beam emitted by the irradiation source(s), which results in a limited carbonization depth. A limited carbonization depth leading to superficial marking(s) is typically advantageous, since serious damage of the substrate can be prevented this way. The invention enables that only a slight modification, in particular a slight optical observable modification, of the substrate is required during step b) since completion of the full thermal process of carbonization can be effectuated during step c). Step b) thereby at least initiates the carbonization, while during step c) the carbonization is continued resulting in a relatively dark (black) print. Step c) is generally not a time- limiting factor for the printing since irradiation of step c) as a high position-selectivity as used during the inkless printing of step b) is not required. Additionally, it is possible that the area to be post-irradiated covers a larger area than merely the position-selectively irradiated parts. It is even conceivable that substantially the entire substrate is exposed to post-irradiation under step c). By applying the post irradiation of step c) a significant increase of the blackness of the irradiated part (i.e. the printed marking) can be obtained to a satisfying blackness level in a limited amount of time. Hence, by applying step c) the printing speed of step b) can be significantly increased which is favourable from an economic and commercial point of view. A satisfying blackness level is often defined by the lightness level L ^* as defined in a CIELAB colour space, which is, in this particular context, preferably equal to or below 30. Due to step c) providing for sufficient blackness of the desired print it possible that the diameter of at least one beam of the primary irradiation source, in particular a laser, is proportional to the resolution of the desired print. This will result in at least one marking on/in substrate which preferably substantially equals the beam used for the irradiation.

The carbonization initiated during step b) and completed during step c) of the method according to the invention is typically based upon pyrolysis, and hence is also referred to as pyrolytic carbonization. The advantages of pyrolytic carbonization is that carbon can be produced in a relatively simple and cost- efficient manner, without needing complicated facilities. Typically, at an early stage of pyrolysis (400°C <T<600°C), cyclization and aromatization proceed in the carbonizable substrate, typically formed by an organic precursor, with the release of various organic compounds like hydrocarbons, and inorganic matters such as CO, C0 ₂ , H ₂ 0, mainly because some of the C-C bonds are weaker than C-H bonds. Over 600°C, out-gassing is typically hydrogen (H ₂ ) due to the polycondensation of aromatics. Up to 1500°C, though this temperature doesn’t have to be necessarily reached, the residues which have “suffered” from carbonization may be called carbonaceous solids though they might still contain hydrogen. Above 1500, graphitization begins so the residues contain more than 99% of C which are thus called carbon materials. The occurrence of reactions, including cyclization, aromatization, polycondensation and graphitization, depends strongly on the substrate used as well as heating conditions. Sometimes these processes overlap with each other throughout pyrolysis and therefore, the whole process from precursor to the final carbon residues is often simply called“the carbonization”. In the method according to the invention at least cyclization and aromatization take place.

It has been found that the flame retardants could facility and stabilize the pyrolysis process of the carbonizable substrate. For example, the preferred presence of dihydrogen phosphate (GDP), ammonium phosphate (DAP), and diguanidine hydrogen phosphate (DHP) in and/or on the substrate leads to an increase of 33% on carbon yield. Moreover, water-soluble organosilicon, whether alone or mixed with other ammonium additives, also helps increasing carbon yield to an important extent and improving simultaneously mechanical resistivity of carbon particles and carbon fibres. It was also found that impregnation of the substrate with a diluted sulfuric acid solution before step b) is performed, or conducting the pyrolysis process of step c) in a hydrogen chloride (HCI) atmosphere helps increase the carbon yield to 38%. Hence, it is preferred that the substrate is treated with at least one of the aforementioned additives prior to performing step b) and preferably prior to step b) and/or to subject the substrate during at least step b) in an acidic environment. Instead of applying an acidic environment during step b), it will be clear that step b) may also be applied in air (atmospheric conditions) or in an inert atmosphere.

Carbonizable substrates refer to substrates, in particular sheets or layers, which can get carbonised at elevated temperature, typically temperatures of 270 degrees Celsius and higher, more specifically temperatures of 400 degrees Celsius and higher. Examples of carbonizable substrates are cellulose based materials like paper, brown carton, wood, etcetera. Also the use of coloured substrates is possible when applying the method according to the invention. It is also conceivable that the substrate is formed by a carbonizable polymer, like polyimide or polyamide.

Since during step b) at least part of the position-selectively irradiated part of the substrate should be at least partially carbonized, the temperature during this irradiation should exceed at least 270 degrees Celsius, preferably at least 400 degrees Celsius. Therefore, preferably is during step b) the temperature of the at least one irradiated part of substrate brought to at least 270 degrees Celsius, preferably at least 400 degrees Celsius.

In a preferred embodiment is the wavelength of at least one beam of the secondary irradiation source, in particular a laser, smaller than the wavelength of at least one of beam of the primary irradiation source, in particular a laser. The secondary irradiation source, in particular a laser, is for example an illumination source, emitting visible light. When the term post-irradiation is used, this term is interchangeable for the term post-illumination. Typically, the secondary irradiation source, in particular a laser, emits at least one beam having a wavelength in the range of 380 to 750 nanometre, in particular 420 to 590 nanometre, more particular 450 to 570 nanometre. The secondary irradiation source, in particular a laser, is therefore configured to generate electromagnetic radiation with a wavelength within said ranges. It is beneficial if said secondary irradiation source is configured to emit at least one beam having a wavelength in the range of 450 to 490 nanometres. This embodiment will emit blue light. It is also beneficial if said secondary illumination source is configured to emit at least one beam having a wavelength in the range of 490 to 560 nanometres. This embodiment will emit green light. Since the absorbance level of the position-selective irradiated parts is higher than the absorbance level of the parts of the substrate which were not irradiated this absorbance difference can be used for selecting a secondary irradiation source for completing the carbonization. Electromagnetic radiation having a wavelength in the range of 380 to 750 nanometre, in particular 420 to 590 nanometre, more particular 450 to 570 nanometre is found out to be usable for this selective absorption. When the secondary irradiation source, in particular a laser, emits at least one beam having a wavelength in the range of 380 to 750 nanometre, in particular 420 to 590 nanometre, more particular 450 to 570 nanometre substrate reflects the irradiation and the irradiation is absorbed by the during step b) obtained marking such that the carbonization will be completed. A benefit thereof is that the substrate can be substantially entirely irradiated during step c) without negatively influencing the quality of the print and/or substrate. The secondary irradiation source is for example a laser selected from the group consisting of: a blue laser, a green laser, a blue-green laser. Typically, the secondary irradiation source, in particular a laser, comprises at least one stationary beam, sweep beam and/or a collinear beam. The desired type of beam(s) to be used is merely dependent on the area which needs to be exposed to post-irradiation.

It is possible that at least one beam of the secondary irradiation source, in particular a laser, at least partially overlaps with at least one beam of the primary irradiation source, in particular a laser. This will result in a further decrease in the required total printing time.

In a preferred embodiment of the method according to the invention are steps b) and c) successive steps. It is however also possible that step b) and step c) are substantially simultaneously performed. It is however also possible that step b) and step c) partially overlap in time. Furthermore, it is imaginable that step b) and step c) partially overlap in time. The secondary irradiation source may even form integral part of and/or may be formed by the primary irradiation source, further enabling this embodiment. The time interval between step b) and step c) is preferably chosen such that at least an optical modification of the substrate has occurred. The time interval between step b) and step c) is preferably at most 2 seconds, preferably at most 1 second.

In a preferred embodiment of the method according to the invention, during step b) a part of the substrate is position-selectively irradiated for a period of time situated in between (and including) 0 and 2 seconds, preferably between (and including) 0 and 1 seconds, more preferably between (and including) 0 and 0.5 seconds. Typically, this time interval will be sufficient to convert at least part of the substrate position- selectively irradiated into char (carbon particles/fibres) or at least initiated an activation of the carbonization. As outlined before, only a slight modification, in particular a slight optical observable modification, of the substrate is required during step b) since completion of the full thermal process of carbonization can be effectuated during step c). Possibly, during step c) a part of the substrate is position-selectively irradiated for a period of time situated in between 0 and 5 seconds, preferably between 0 and 2.5 seconds, more preferably between 0 and 1 seconds.

It is advantageous if at least one primary irradiation source is a laser, and preferably a diode laser and/or C0 laser. Carbon dioxide lasers are the highest-power continuous wave lasers that are currently available. And they are also quite efficient: the ratio of output power to pump power can be as large as 20%. The C0 laser typically produces a beam of infrared light with the principal wavelength bands centering on 9.4 and 10.6 micrometres (pm). Lasers typically operate relatively fast and, moreover, are flexible, as a result of which lasers are ideally suitable to create different track, pads, or electronic circuits, or parts thereof, within a short time frame. Instead of using a laser, it is also imaginable that the substrate is irradiated position-selectively in another manner, for example by using a heated stamp to physically burn, position-selectively, the substrate. Alternatively, a mask may be applied onto the substrate after which the uncovered parts of the substrate are heated, for example by means of a heated air flow, to temperature above the carbonization temperature. Stamps and masks are typically useful in case a standard track layout and/or pad layout would be desired.

It is commonly advantageous in case, during step b), the substrate and the at least one irradiation source, preferably a laser, and more preferably a diode laser and/or C0 laser, are mutually displaced by using a speed which is at least 100 mm/s, preferably at least 1000 mm/s, more preferably 2500 mm/s, even more preferably at least 5000 mm/s, in particular preferably at least 6000 mm/s. This speed is also called the printing speed, the marking speed, or the carbonization speed. Figure 2 shows that it is experimentally found that these printing speeds are feasible when applying the method according to the invention without encountering limitations in the darkness and/or contrast of the print.

It is possible that the substrate is substantially entirely irradiated by at least one secondary irradiation source during step c). In a preferred embodiment is the total area of a substrate exposed to post irradiation during step c) at least equals the total area of said substrate which is position-selectively carbonized during step b). It is also conceivable that the total area of a substrate post-irradiation during step c) is larger the total area of said substrate which is position-selectively carbonized during step b). It is also possible that the total area of a substrate post-irradiation during step c) is equal to or smaller than the total area of said substrate which is position-selectively carbonized during step b).

It could be advantageous in case the method comprises step d), comprising of preheating the substrate, preferably to a temperature situated in between 200 and 250 degrees Celsius, prior to performing step b). Experiments have shown that preheating the substrate prior to executing step b) could improve the char yield, and hence the conductivity. This preheating could be realized, for example, by means of an oven, an infrared heating source, and/or by the same irradiation source as used during step b). In this latter embodiment, the to be preheated part of the substrate will typically be exposed to a reduced power density to prevent premature carbonization of the substrate. This may, for example, by achieved by so- called beam-shaping, wherein the irradiating beam of the irradiation source is broadened to reduce the power density of said beam.

An embodiment of the method according to the invention is possible, wherein at least part of the substrate is subjected to at least one photochemical bleaching step, preferably prior to step b). By applying such chemical bleaching step at least part of the substrate will be whitened. With the term bleached or whitened it is meant that the lightness value (L ^* ), as defined in the CIELAB colour space, is increased. An increased lightness value of a substrate corresponds to a lighter colour of the substrate. This advantageous since the contrast between the print and the substrate is substantially dependent on the difference is lightness value of the substrate and the lightness value of the print. A larger difference between these values consequently corresponds to a better contrast. The photochemical bleaching step is preferably applied before selective carbonization of the substrate. Applying at least one photochemical bleaching step is in particular useful when using an non-white substrate, such as but not limited to, brown cardboard. This embodiment is for example in particular beneficial when printing a (matrix) bar code or QR code, since these codes require a sufficient contrast between the print and the substrate.

It is for example possible that the photochemical bleaching is performed by irradiation of at least part of the substrate with an irradiation source using a power density in a range of 20k W/cm2 to 140 kW/cm2 and applying a irradiation time of at most 55 microseconds. The irradiation source can be either the primary irradiation source used for the carbonization of the substrate or a further secondary irradiation source. The irradiation of at least part of the substrate with an irradiation source, preferably a laser, with a power density in the range of 20k W/cm2 to 140 kW/cm2, preferably 30k W/cm2 to 120 kW/cm2, and an irradiation of 55 microseconds or below results in a thermal shock of the substrate. The thermal shock provides the photochemical bleaching effect. As a further result of the thermal shock at least part of the water content of the treated area will be evaporated. At least part of the evaporated water may optionally be removed via an extractor. The photochemical bleaching step is generally performed as an additional step, however, it is also possible that the chemical bleaching step replaces step b) of the method according to the invention.

The method according to the invention preferably comprises step e), comprising the step of increasing the bond strength between at least one marking printed and/or to be printed during step b) and the substrate. This will lead to an improved fixation of the printed marking(s) onto the substrate. Increasing the bond strength can be realized in different manners, wherein step e) can be performed prior and/or after step b), and wherein step e) can be performed prior and/or after step c). In particular in case step e) is performed prior to step b), step e) is preferably based upon treating the substrate with a bond strength improving coating, which can, for example, by spraying, preferably by using one or more spray nozzles, onto the substrate prior to step b). This bond strength improving coating may also be applied after carbonization according to step b). The coating may be configured to react with the marking(s) to intensify the bond between the marking and at least one of the substrate and the coating. It is also imaginable that step e) comprises the step of further irradiating the at least one marking, such that the bond strength between said at least one marking and the substrate is improved (intensified). It is also imaginable that step e) comprises the step of apply mechanical pressure onto the at least one marking formed during step c), which may also lead to an increase of the bond strength of said at least one marking onto the substrate. Applying a pressure may, for example, be realized by using a roller.

The invention furthermore relates to a printing device for selective carbonization of a substrate, preferably by using a method according to the present invention, at least one primary irradiation source, in particular a laser, such as a C02 laser, being configured to position-selectively carbonize at least one part of the substrate by position-selectively irradiating of said at least one part of the substrate to form at least one printed marking and at least one secondary irradiation source, in particular a laser, being configured to post-irradiate of at least one printed marking generated by means of the primary irradiation source, such that the marking is darkened.

The benefits as described above for the method according to the present invention also apply to the corresponding device according to the invention. The device preferably comprises at least one control unit for controlling at least one primary irradiation source and/or at least one secondary irradiation source. Typically the control unit is configured to control the printing device as such. The control unit is preferably programmed to execute at least step b) and step c) of the method according to the invention.

The invention will be elucidated on the bases of non-limitative exemplary embodiments shown in the following figures, wherein:

figure 1 a shows a schematic representation of a print obtainable via selective carbonization of a substrate;

figures 1 b-1 e show examples of the parts to be exposed to post-irradiation;

figure 2 shows the effect in lightness of the inkless print as a function of the marking speed of the laser when applying the method according to the invention; and

figures 3a and 3b show possible embodiments of a printing device according to the invention.

Figure 1 a shows a schematic representation of an example of a print (1 ) obtainable via position-selective carbonization of a substrate (2) via the method according to the present invention. The figure shows the carbonized area (1 ) or print (1 ) in order to be able to indicate the area(s) of the substrate which will be exposed to post-irradiation under step c) after completion of step b) of the method according to the invention. Examples of the areas exposed to post-irradiation are illustrated in figures 1 b-1 e.

Figure 1 b shows the substrate (2) as shown in figure 1 a, wherein an area (3b) to be exposed to post irradiation is indicated via highlighting (3b). The determination of the area (3b) is based upon the surface enclosed by the print (1 ) which is to be position-selectively carbonized (printed). As can be seen in the figure, the area (3) to be post-irradiated encloses the print (1 ) entirely.

Figure 1 c shows a further example how the area (3c) of the substrate (2) to be exposed to post irradiation can be defined. The predefined area (3c) substantially follows the contours of the final print (1 ). A benefit of this example is that a smaller area (3c) has to be exposed to post-irradiation compared to the example of figure 1 b, resulting in a reduced energy requirement for applying this step.

Figure 1 d shows a third example of defining the area (3d) of the substrate (2) which has to be exposed to post-irradiation post to the selective carbonization (1 ) according to the method according to the invention. The figures shows that multiple areas (3d) are indicated, wherein each area (3d) substantially follows the contours of the print (1 ). For this embodiment, the total area of a substrate (2) which is to be exposed to post-irradiation substantially at least equals the total area of said substrate (2) which is position- selectively carbonized (1 ). Figure 1 e shows a fourth example falling within the scope of the invention of defining the area (3e) of the substrate (2) which has to be exposed to post-irradiation after to the position-selective carbonization (1 ). The predefined area (3e) to be exposed to post-irradiation is further reduced compared to the previous examples. The figure shows that the predefined areas (3e) are substantially localized with respect to the print (1 ). For this embodiment, the total area of a substrate (2) which is to be exposed to post-irradiation substantially equals the total area of said substrate (2) which is position-selectively carbonized (1 ).

Figure 2 shows the effect in lightness of the inkless print as a function of the marking speed of the laser when applying the method according to the invention. The y-axis of the graph shows the lightness level

(L ^* ) of the print. The L ^* values are measured by using a calorimeter. A lightness level below 30 corresponds to an acceptable blackness, and therewith acceptable contrast, of the print. The x-axis shows the marking speed of the laser (in mm/s). A minimum laser marking speeds of 5000 mm/s is desired in order to be compatible in the printing market. The figure indicates a series of measurement points for printing where only step b) is applied and a series of measurement points where both step b) and step c) are applied. The effect of step c) in the lightness level, and therewith the darkness of the print, can be clearly observed in the graph. Even for relatively high printing speeds, exceeding 5000 mm/s the desired darkness can be obtained. Figures 3a and 3b show possible embodiments of a printing device (4) according to the invention. The device (4) is configured for selective carbonization of a substrate (2). The device comprises a primary irradiation source (5), in particular a laser, for at least partially irradiating the substrate (2) such that carbonization of at least part of the substrate (2) occurs and a secondary irradiation source (6), in particular a laser, for post-irradiation of said substrate (2). Where the embodiment of figure 3a makes use of separate irradiation sources for steps b) and c) according to the invention, the embodiment of figure 3b makes use of an integrated irradiation source for both the primary irradiation source (5) and the secondary irradiation source (6). The device (4) furthermore comprises a heating source (7) for at least partially heating the substrate (7) and a control unit (8) for controlling the irradiation source(s). Optionally, the device (4) comprises a colour sensor, an extractor for removing volatile compounds and/or a non- contact temperature sensor (not shown). The substrate (2) is in the shown embodiment positioned on a moving stage (9).

It will be apparent that the invention is not limited to the working examples shown and described herein, but that numerous variants are possible within the scope of the attached claims that will be obvious to a person skilled in the art.

The verb“comprise” and conjugations thereof used in this patent publication are understood to mean not only“comprise”, but are also understood to mean the phrases “contain”, “substantially consist of”, “formed by” and conjugations thereof. Where the term“print” is used a selective carbonized marking is meant. Where the term“irradiation” is used, this may be interpreted as“direct irradiation”, wherein an, optionally, shaped, irradiated beam directly (without intervention of an intermediate layer or intermediate component) hits the substrate, and may also be interpreted as “indirect irradiation”, wherein an, optionally, shaped, irradiated beam indirectly, via at least one intermediate layer or intermediate component, hits the substrate. An example of an intermediate layer could be, for example, a transparent plate and/or another substrate.