EP0993964A2

EP0993964A2 - Coatings for marking by laser

Info

Publication number: EP0993964A2
Application number: EP99120010A
Authority: EP
Inventors: Frank A. Meneghini; Michael P. Secord; Richard C. Woudenberg
Original assignee: Markem Corp
Current assignee: Markem Imaje Corp
Priority date: 1998-10-16
Filing date: 1999-10-15
Publication date: 2000-04-19
Also published as: EP0993964A3

Abstract

A radiation sensitive marking coating, preferably a clear, colourless coating, has a resin, and an activatable dye system which may be activated by radiation, for marking a substrate such as a beverage can.

Description

This invention relates to marking substrates.
One technique for marking substrates is known as laser marking. In laser marking, radiation is directed onto a substrate to modify the substrate, or a coating on the substrate, in a way that induces a colour change. The radiation can be directed, or addressed, in a pattern over the substrate such that a desired image is rendered.
In a first aspect, the invention features a marking coating which has a resin and an activatable dye system.
Embodiments may include one or more of the following features. The coating is substantially colourless prior to exposure to radiation at a wavelength and intensity sufficient to induce a select thermal variation that activates the dye system. The dye system is activated at a temperature of about 80-200°C. The coating is absorbing at a non-visible wavelength. The coating is absorbing in the infrared. The resin is absorbing in the infrared. The resin is a solvent cast resin. The resin is a polyketone resin. The resin is a UV curable resin. The dye system is a thermally-activated dye system. The dye system includes a thermally sensitive acid-base variation component, and a dye which changes colour in response to an acid-base variation. The acid-base variation component is a blocked acid. The dye includes a lactone. The coating includes a base additive. The base additive is an amine.
The dye system is a thermochromic dye which undergoes structural rearrangement. The thermochromic dye is a sulphonamide.
In another aspect, the invention features a method of marking a substrate.
The method may include applying to the substrate a coating as described herein and irradiating the substrate in a desired pattern with selected radiation effective to induce a colour change. The coating may applied to a metal substrate such as a beverage can. The coating may be irradiated in a pattern indicating a date.
Embodiments of the invention may have one or more of the following advantages.
A preferred coating includes a resin which absorbs in the infrared and a dye system that includes a heat sensitive acid-base variation component, such as a blocked acid, and an acid-base sensitive dye, such as a leuco dye. The resin absorbs laser radiation, converting it to heat, which in turn induces an acid-base variation, such as unblocking the blocked acid, which in turn activates the dye.
Another preferred coating utilises a dye that undergoes an internal rearrangement when heated. The temperature required to induce the colour change is above the temperature normally encountered by the end product during use but is typically still rather mild. For example, temperature changes in the range of 100-150° C are typically sufficient. As a result, the colour change can be achieved rapidly with radiation energy levels that can be produced cheaply and safely. The coating maintains its integrity without substantial delimitation or thermal degradation.
The coatings may be colourless and clear prior to irradiation. As a result, the underlying substrate, and any markings directly on the substrate, will be visible in areas not exposed to radiation. The coatings may also be strongly adherent to a wide variety of substrate materials. For example, the coating may be strongly adherent to a metal, such as aluminium, tin, or stainless steel, as well as glass paper, and packaging film. The coating may be either cast from a solvent, followed by evaporation of the solvent, or cured, such as by heat or radiation. The radiation curing may use radiation at a wavelength different from the wavelength used to induce the colour change. For example, curing may be achieved by ultraviolet radiation, while marking is carried out with infrared radiation.
All percentages given herein are by weight unless otherwise indicated or apparent.
The invention will now be described with reference to the accompanying drawings in which:-
FIGURE 1 is a block diagram schematic of a laser marking system, and
FIGURES. 2 and 2a are a more detailed schematics of laser marking systems.
Referring to Figure 1, a laser marking system 1 includes a source of radiation 2, an optical system 4, and a controller 6. The radiation source is typically a carbon dioxide laser operating in the infrared range, preferably at 10.6 microns. The optical system is arranged to direct laser radiation 8 towards a substrate 10. The substrate 10 may be on a conveyor 12 which brings substrates in sequence into an area where they can be exposed to the radiation. For example, the substrates may be aluminium beverage cans which are oriented such that the radiation can be directed on to the bottoms of the cans for marking sell-by dates. The control system 6 is typically a computer programmed to direct the laser energy in a desired pattern on to the bottom of a can (or on another area, such as the neck of the can). The control system 6 can also be adapted to monitor and/or control the advance of the conveyor 12.
Referring to Figures. 2 and 2a, two suitable laser control techniques are vector scanning and dot matrix printing. Referring particularly to Fig. 2, in vector scanning the laser 2 is turned on, and a set of two scan mirrors M1, M2 move the beam 8 over the surface from point XlY1 to point X2Y2. The beam is turned off, the mirrors are moved to the next starting position, and the process is repeated. As the laser beam 8 is moved across the surface of the substrate, it leaves a line. A complex image is made of many individual line segments, usually straight. In "vector marking" two galvo scanners are used for vector scanning. A suitable system is described in Binge et al. USSN 08/568,269, filed December 6, 1995, entitled "Scanned Marking of Work Pieces", the entire contents of which is incorporated herein by reference.
Referring to Figure 2a, in dot matrix (also called raster) printing, the laser beam 8 is moved across the entire image in an X-Y pattern, the laser 2 is turned on only in the places where a dot should appear. In dot matrix control an optical arrangement 4, e.g. an acousto-optic scanner, is used for scanning in the X direction. Y scanning is provided by motion of the substrate 10.
In each case prior to application of laser energy, the substrates are treated with a marking coating. The coating includes a resin and an activatable dye system.
The resin, which may be a natural resin, as well as other polymers, prepolymers, and monomers, is selected so that it will adhere to the desired substrate, provide a select background colour, and is robust in its ability to withstand radiation without substantially changing colour in a way that would obscure the intended marking. Preferably, the resin is colourless and remains colourless even when exposed to laser energy. The resin may be a solvent cast resin or a heat or radiation curable resin. Suitable solvent-cast resins include polyketone based resins. A preferred resin is Krumbhaar K1717 HMP (CAS number 25054-06-2, available from Lawter, Northbrook, IL). Suitable UV curable resins include acrylates, epoxies, and vinylethers. A preferred resin is ethoxylated₆trimethylol propane triacrylate (available as Sartomer-499 from Sartomer, Exton, PA). The UV curable resin may be used in combination with a photoinitiator. Suitable photoinitiators include ketone-based components. A preferred photoinitiator is diphenol (2,4,6,-trimethyl benzoyl) phosphene oxide, (available as Darocur 4265 from Ciba-Giegy, Terrytown, NY). For thermally cured resins, the cure temperature is preferably kept below the temperature at which the dye is activated. For example, resins that can be cured at around 110°C or less are typically suitable. Suitable resins also include unsaturated polyesters, and epoxies which can be applied as prepolymers and crosslinked on the substrate surface.
The resin may be capable of absorbing radiation at a select wavelength and converting the radiation to heat. Preferably, for clear coatings, the resin is essentially colourless in the visible range but absorbs at non-visible wavelengths, such as in the ultraviolet or the infrared. The coating preferably is sufficiently absorbent at a select wavelength that the coating is heated to a desired temperature which can induce the colour change. A polyketone resin absorbs strongly at 10.6 micron, a wavelength which can be produced by a carbon dioxide laser.
Alternatively or in addition, an absorptive component that absorbs laserlight and converts it to heat may be added to the resin. The absorption component may be at about 1 to 5% of the coating. Adhesion promotors that absorb in the laser wavelength range may also be used. Examples include methacrylate ester derivates and carboxylic acid species. A preferred material is Ebecryl 168 (available from UCV Chemicals, Smirna, GA) which absorbs at 10.6 microns.
The heat needed to activate certain dye systems may also be provided by absorption of radiation by the substrate, rather than, or in addition to, absorption by the coating. For example, certain substrates (e.g., a metal substrate) may be highly efficient in absorbing infrared radiation and in conducting heat resulting from absorption to proximate portions of the coating.
The coating may also include a pigment so that, rather than a clear and colourless coating, the coating has a desired background colour to enhance contrast of the image. The pigment is preferably an inert material. For example, for a white coating, titanium dioxide may be added. Other suitable pigments include barium sulphate, and Rhoplex beads, (e.g., Rhoplex ac 1024, available from Rhom and Haas, Philadelphia, PA).
The activatable dye system changes colour (active colour or shade) in response to absorption of radiation by the coating. The dye system may be thermally activated or photochemically activated. The dye system is selected for compatibility with the resin and the substrate. In addition, the dye system is capable of inducing a colour change as a result of exposure to radiation at a select wavelength and intensity which does not adversely affect the resin.
The dye system is sufficiently robust in that colour change does not occur when the coating is exposed to conventional use conditions of the substrate, such as exposure to ambient temperatures in the range of 0-40°C, and ambient light. In preferred systems, the dye is activated at a relatively low temperature, e.g., 80-150°C, preferably 100-140°C. Preferably, the dye system is colourless prior to exposure to radiation.
The dye system may induce a colour change by creating an acid-base variation in the coating. Systems of this type may use a moiety, such as a heat sensitive or photochemically activated moiety which changes acid-base characteristics upon exposure to heat or radiation (e.g., UV) and a dye component that changes colour in response to the change of acid-base characteristics of the moiety. These dyes are preferably present in the amount of about 1 to 15% of the coating formulation. Suitable dyes include leuco dyes which include fluorams and lactones that interrupt conjugation until treated with an acid. Preferred acid-catalysed leuco dyes include the Copikem 20 dyes (e.g., 3,3-bis (butyl-2-methyl-lH-indol-3-yl-1-[3H]-isobenzofuranone, colour magenta, available from Hilton-Davis, Cincinnati, OH). Other dyes include phenolphthalein, and other dyes that change colour when exposed to a pH change.
Suitable heat sensitive moieties that change acid-base characteristics include blocked acids, blocked amines, and chelated amines that can be photorearranged. These moieties are preferably present at about 0.5 to 5% of the coating formulation. Blocked acids include salts of weak bases and strong acids, e.g., sulfonic acid salts. A preferred blocked acid is diethylammonium trifluoromethane sulfonate (Floured FC-520, 3M, St. Paul, MN), a salt of a strong acid which dissociates when heated. Jodonium salts can also be used as thermally or optically activated acid sources. An example is diaryliodonium hexafluoroantimonate (available as SARCAT CD-1012, Satomer, Exton, PA). The heat-sensitive moiety may also become basic upon exposure to heat or radiation to activate a dye sensitive to basic conditions. Examples include blocked amines, such as, t-butyl carbonates, which becomes unblocked by removal of the t-butyl group upon exposure to heat. In this latter system, carbon dioxide gas is evolved upon heat exposure, causing a refractive index variation, which can improve contrast. This feature may be particularly beneficial for colourless coatings.
Photochemically activated moieties, which vary acid-base characteristics in response to absorption at select wavelengths, by, e.g., photo rearrangement, are described in Kutal U.S. 5,691,113, U.S. 5,652,280, and Palmer et al. Macromolecules, Vol. 28, No. 4, 1995, (P.1326), the entire contents of all of which are incorporated herein by reference.
The dye system may also include an acid or base additive to neutralise components within the coating which might induce a partial colour change prior to application of laser energy. For example, a coating with a blocked acid may also be provided with a base, such as an amine, to neutralise any unwanted residual acidity in the coating. The base may be added in sufficient amount to produce a clear, colourless mixture prior to coating or curing. A preferred amine is n-methyldiethanolamine.
Other suitable dye systems include acidic or basic components that are physically separated from the dye in the resin matrix. When the coating is heated the acid-base component flows, permitting interaction that leads to activation of the dye. An example is a system including bisphenol A and a leuco dye which can be incorporated in the resin in particulate form. Formulas for the particulate systems, available from, Ciba-Giegy, are provided in Appendix A.
Dyes which change colour without an acid-base variation can also be used. These species typically undergo internal molecular rearrangement upon exposure to heat or photochemical reaction. These dyes are preferably present at about 2 to 8% of the coating formulation. Examples include thermochromic dyes including a sulphonamide groups that interrupt conjugation until heated. Preferred dyes are described in U.S. 5,451,478, (e.g. col.22), by Polaroid, the entire contents of which is incorporated herein by reference.
Suitable dye systems are also discussed in the following: U.S. Patent Nos. 5,539,446, 5,451,478, 5,424,475, 5,422,230, 5,350,870, 5,342,816, 5,236,884, 5,210,064, 5,206,208, 5,192,645, 4,960,901, 4,839,335, 4,826,976, 4,745,046, 4,720,450, 4,745,046, 4,720,450, 4,720,449, 4,663,518, 4,602,263, 5,795,981, 5,656,750, 5,627,014, 5,492,795, 5,405,9736, 5,354,873, 5,262,549, 5,231,190, 5,227,499 and 5,227,498, the entire contents of all of which are incorporated herein by reference.
The coating may also include multiple dye species or systems of different colours that can be activated to provide a desired blended colour Alternatively, dye systems that are activated under different conditions, e.g., heat level or wavelengths, can be used to provide coatings that can be marked in a variety of different colours. For example, a coating may incorporate a first system that uses a dye of one colour that is activated by an acid condition using a blocked acid which unblocks at a first temperature and a second dye system of a different colour that is activated at a second, higher temperature. Marking in the first colour can be achieved by heating to the first temperature but below the second temperature. Marking in a colour that is the combination of the first and second colours can be achieved by heating to the second temperature. In addition, different portions of an image can be marked in different colours by heating the different portions to either the first temperature or above the second temperature.
The coating may be applied as a solvent cast coating or heat or radiation cured coating. The coating is typically 4-13 microns thick and can be applied by air brush and/or drawn by a doctoring arrangement. Preferred solvents are acetone, butylacetate, cyclohexane, water, and aqueous solvents. The dye may be dissolved first in a solvent, e.g., cyclohexane, with the other components added subsequently. The substrate may be cleaned prior to application of the coating by, for example, a solvent treatment or hydrogen flame treatment. For heat or UV curable coatings, the coating is exposed to an oven or UV station. The rate of cure and intensity of UV radiation may be varied by the use of a varying amounts of a photoinitiator.
Laser irradiation may be carried out using 100 watt carbon dioxide laser at 10.6 microns, which provides a power level at the film of 2 to 50 watts with a 64 to 128 microsecond pulse width. A preferred operating condition is 25 watts of 175 microsecond pulse width, one pulse per pixel and pixel diameter of 0.010 inch. Other suitable lasers include neodymium-YAG at a wavelength of 1.064 microns.
The following examples are illustrative.

Example 1

The following is an example of a solvent-cast colourless coating using an acid-base sensitive dye component.

Grams

Acetone 4.884

Hilton-Davis Copikem 20 0.501

Magenta

Krumbhaar K1717 HMP 4.615

Flourad FC-520 0.200
The Krumbhaar K1717 HMP resin is used as an energy gatherer due to its ability to absorb the energy at 10.6 microns. FC-520, a blocked acid catalyst, deblocks when the temperature reaches approximately 130° C, and creates the acid which turns the dye on. The resin and dye are dissolved in solvent in separate factions which are combined. The mixing may be facilitated by mild heating. The blocked acid is added to the combined solution. The dye is substantially colourless when in solution and prior to application of laser energy. The coating can be marked with a carbon dioxide laser.

Example 2

The following is an example of a colourless coating formulation using an acid-base sensitive dye system with an amine additive.

Grams

Acetone 4.884

Hilton-Davis Copikem 20 0.501

Magenta

Krumbhaar K1717 HMP 4.615

Flourad FC-520 0.200

N-Methyldiethanolamine 0.020
The amine was added to create a more colourless fluid. The FC-520 is slightly acidic and may turn the dye on slightly. The formulation was prepared similar to Example 1. The amine was added to the combined resin-dye solution. Colour change of the solution toward colourless can be observed as the amine is added. Alternatively, the blocked acid and amine can be prepared in a separate solution, which is added to the dye-resin solution. The coating can be marked with a carbon dioxide laser.

Example 3

The following is an example of a coating using a thermochromic dye.

Grams

Krumbhaar K1717 HMP 4.44

N-Butyl Acetate 5.56

Polaroid 34402 Magenta 0.30
Krumbhaar again acts as the energy gatherer and at approximately 140° C, the coating turns into a deep magenta. The resin was solvated with the solvent using a high speed disk dispenser to produce a light amber fluid. The dye is addition to the fluid forming an emulsions which can be applied to the substrate. The coating can be marked with a carbon dioxide laser.

Example 4

	Grams
SR 499	10
Ebecryl 168	0 1
Darocure 4265	0.2
Polaroid 34402 magenta	0.3
N-Methyldiethanolamine	0.02

This composition uses a curable acrylate resin. Ebecryl 168 is used as the energy gatherer, and the Darocure 4265 is a free radical photo initiator. Due to the slightly acidic nature of the 168, an amine is added to create a clearer film. The components are mixed together. The base is added last to neutralise the mixture. The coating is cured using a UV curing station. (Fusion 300, Fusion Technologies, Marblehead, MA). The system uses an H-bulb, at 300 watts/linear inch. The substrate may be passed under the beam (about 1" wide) at 35"/min. The coating can be marked with a carbon dioxide laser.
In other embodiments, heat may be provided by radiation with non-laser sources or by application of heat directly to the coating, e.g. with a resisting thermal element positioned closely adjacent to the coating.
The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

A marking coating, characterised in that the coating includes a resin and an activatable dye system.
The coating of claim 1 characterised in that said coating is substantially colourless prior to exposure to radiation at a wavelength and intensity sufficient to induce a select thermal variation that activates said dye system.
The coating of claim 2 characterised in that said dye system is activated at a temperature of about 100-150°C.
The coating of claim 2 or claim 3 characterised in that said coating is absorbing at a non-visible wavelength.
The coating of claim 4 characterised in that said coating is absorbing in the infrared.
The coating of claim 5 characterised in that said resin is absorbing in the infrared.
The coating of any one of claims 1 to 6 characterised in that the resin is a solvent cast resin.
The coating of claim 7 characterised in that the resin is a polyketone resin.
The coating of anyone of claims 1 to 3 characterised in that the resin is a UV curable resin.
The coating of claim 1 characterised in that said dye system includes a thermally sensitive acid-base variation component, and a dye which changes colour in response to an acid-base variation.
The coating of claim 10 characterised in that said acid-base variation component is a blocked acid.
The coating of claim 11 characterised in that said dye includes a lactone.
The coating of any one of claims 10 to 12 characterised in that the coating includes a base additive.
The coating of claim 13 characterised in that said base additive is an amine.
The coating of claim 1 characterised in that said dye system is a thermochromic dye which undergoes structural rearrangement.
The coating of claim 15 characterised in that said thermochromic dye is a sulphonamide.
The coating of claim 1 characterised in that said coating absorbs in the UV and said dye system is photochemically activated.
A method of marking a substrate comprising: applying to the substrate a coating as claimed in any one of the preceding claims, and irradiating the substrate in a desired pattern with selected radiation effective to induce a colour change.
The method of claim 18 characterised in that the method includes applying said coating to a metal substrate.
The method of claim 19 characterised in that the method includes applying said coating to a beverage can.
The method of any one of claims 18 to 20 including irradiating said coating in a pattern indicating a date.