Sheet materials and articles comprising TiO2 and laser marking

Abstract

A sheet of material that is marked with a pulsed laser, the sheet of material having an outer edge and an inner edge separated by a core. The distance from the outer edge to the inner edge is the sheet thickness. The sheet of material has a total concentration of TiO2 of less than about 1.00%. There is an exterior layer beginning at the outer edge and extending into the core, the thickness of the exterior layer is from about 10.00% to about 40.00% of the sheet thickness, and the exterior layer has an average concentration of from about 2.50% to about 10.00% TiO2.

Description

FIELD OF THE INVENTION

The present invention relates to sheet materials laser marked articles, and articles comprising such sheet materials and laser marking.

BACKGROUND OF THE INVENTION

Short-pulse laser decoration utilizes energy from nano, pico and femto short pulse lasers across a variety of wavelengths and energies to mark decorative patterns onto articles such as products and/or packages. Any and all other decoration techniques that may apply to the product and/or package (i.e. labels, screen print, digital print, etc.) can be used together with laser marking to achieve various decorative and functional effects. The laser technique used in short pulse laser marking is, importantly, a high through-put technique which uses a stationary laser source from which the laser beam is directed by means of electronically/mechanically controlled mirrors (i.e. “galvo” sets) and lenses (i.e. F-theta and similar lenses) to the product or package being marked. These mirrors and lenses steer the laser beam across the surface of the article (this steering is also called “scanning”) so that the laser can impart an image, such as a digital image (for example from a computer file such as a PDF file) to the surface of the package or product. This approach has further advantages over other decoration techniques in that the use of a digital image allows for customization and personalization of the decoration.

There is a great deal of interest in the possibilities presented by laser-marking articles such as by means of short-pulse laser marking. For example, replacing adhesive labels on polymer containers is not only economically beneficial, but ecologically beneficial as well. Eliminating adhesive labels on polymer containers, for example, decreases the total weight of the packaging material which reduces the amount of petroleum-derived material per package and reduces the weight of the packaging thereby requiring less fuel for shipping. Further, the absence of an adhesive label enables the polymeric container to be more easily recycled since adhesive labels often need to be removed prior to recycling due to the potential impurities which may be introduced to the recycle stream.

Recent recycling regulations for plastics restrict the amount of colorants and other chemicals that can be added to plastics being recycled. Unfortunately, many of the chemical additives aide laser marking to insure sharp, precise marks on the plastic. Removing the chemical additives or restricting their amount may be detrimental to the laser marking process. Ironically, the removal of certain chemical additives to make recycling easier for plastics, may restrict laser marking, which itself is an environmentally friendly process.

Laser marking of small articles (i.e. golf balls, etc.) and/or small regions on articles (i.e. date codes on finished packages, address labels) is known. While lasers are improving, and newer lasers have a variety of energies and wavelengths, these marking processes can still be slow and expensive. Further, they do not have the ability to mark small characters that require high-precision such as small-font text (i.e. usage instructions, ingredient listing) comprised of alphanumeric characters. For example, date codes are marked onto packages by relatively quick lasers, but they employ single lines of large, imprecisely, or unequally spaced spots (in the range of 250 μm to greater than 800 μm in diameter) and relatively large font characters. This is equivalent to printing stick figures, which are adequate for some purposes but difficult for consumers to read and almost impossible for machines to read. More specifically, single lines of large, imprecisely, or unequally spaced spots cannot currently be used to mark high-precision small font text or machine-readable graphics such as UPC or QR codes on articles.

The current state of the art for laser marking apparatuses and processes generally includes a laser which generates a laser beam and a scanner that directs the beam to the article surface to be marked. Scanners may use a set of mirrors that are directed by galvo-sets to the article surface, or may use a polygon scanner. Apparatuses that utilize galvo sets include “raster” marking processes and “vector” marking processes. These are either fast but with poor precision and resolution, or slow but with higher precision and resolution. The combination of high speed and high precision does not exist in the prior art. This problem is particularly notable when marking large areas on articles, such as when using laser-marking as a full replacement for other decoration techniques, where all the text and/or graphics provided on at least one face of the article (much of which is required for regulatory purposes) is provided via laser-marking. Large area marking can be facilitated by polygon scanners, but these lack flexibility in terms of changing images.

A raster laser marking process lays down individual laser marks in a grid, and the image is marked by the laser row by row, point by point. Each of the pulses is “gated” such that pulses are only fired for a dark pixel of the image and no pulse is fired for the light pixel of the image (or vice versa). Each of the pulses is individually gated and the pulse energy of each pulse can be varied to produce grayscale. State-of-the-art raster marking processes are effectively limited to lasers with a ˜100 kHz repetition rate given the practical limit of a ˜10 us update rate in signaling the laser's on/off function (i.e. “gating”) and can only be made faster by increasing the pulse-spacing, which can sacrifice fine detail, such as required to mark small-font text and graphics.

Vector marking processes can be run above 100 kHz as the pulses are typically gated open while the laser beam is “steered” (by mirrors) in the shape of the vector-lines being marked. Vector-marked articles comprising text can often be recognized as the marked lines are typically one-pulse wide (unless in-filled) and the pulses become closer together near the corners, where the surface velocity of the laser beam was slowed as it turned the corner. However, it has been found that the accuracy of the placement of the marks with vector-marking suffers at very high surface velocities of the laser beam.

High-speed laser-marking can be achieved by polygon scanners (e.g. High Throughput Raster Processing Polygon scanner systems from Next Scan Technology, Evergem, Belgium), which can be optimized for high speed and accuracy. The polygon scanner systems employ a rotating polygon mirror for row scanning. These scanners are typically used for full-surface processing of a regular pattern. Specifically, the field of view is typically a square, which is relatively large by printing standards, and a repeated pattern is marked in its entirety over and over again on subsequent articles. The square field of view configuration of these scanners may not lend them to accurate marking of things like small characters, alphanumeric characters, logos, pictures and the like.

While high speed is important for high throughput, high precision is important for legibility of the laser marked pattern, which is important when marking text (i.e. for human legibility) and when marking machine-readable codes such as bar codes, UPC codes, QR codes and the like (i.e. for machine legibility). The quality of the laser marks and the precision of their locations on the article are both important to legibility of the laser marked pattern.

Thus, there remains the need for materials such as sheet materials, and articles comprising such materials, that include low levels (e.g. <1%) of TiO2 that can be laser-marked with high-speed laser marking processes.

SUMMARY OF THE INVENTION

The present invention provides a solution for one or more of the deficiencies of the prior art as well as other benefits. The specification, claims and drawings describe various features and embodiments of the invention, including a sheet of material, which may be polymeric, that is marked with a pulsed laser, the sheet of material having an outer edge and an inner edge separated by a core. The distance from the outer edge to the inner edge is the sheet thickness. The sheet of material has an average concentration of TiO₂ of less than about 1.00%, preferably less than about 0.95%, even more preferably less than about 0.90%, and even more preferably less than about 0.80%. Further, there is an exterior layer beginning at the outer edge and extending into the core, the thickness of the exterior layer is from about 10.00% to about 40.00%, preferably from about 9.00% to about 35.00%, even more preferably from about 8.00% to about 32.00%, and even more preferably from about 7.00% to about 30.00%, of the sheet thickness, and the exterior layer has an average concentration of TiO₂ of from about 2.50% to about 10.00%, preferably from about 2.75% to about 9.50%, even more preferably from about 3.00% to about 9.25%, and even more preferably from about 3.25% to about 9.00%. The laser marking on the sheet of material may include a UPC, QR, Data matrix or other machine-readable symbol or code, and the machine-readable symbol or code has an overall symbol grade of 1.5 or greater on the ISO/IEC15416-1 (for verification of linear Bar Codes) or an overall symbol grade of 1 or greater on ISO/IEC15415 (for verification of two-dimensional bar codes).

In another embodiment of the present invention, the sheet of material forms an article, which can be a garbage bag, a bottle, a sachet, a tube, a film, a laminate, a bag, a wrap, a drum, a jar, a cup, or a cap.

In an embodiment, the laser marks comprise a predetermined pattern which is a line or curve and in which the spacing between adjacent laser marks varies within the line or curve.

In another embodiment, the laser marks comprise a predetermined pattern which is a grid of locations each comprising a mark or a void in a grid pattern. The grid pattern has a plurality of locations positioned in two or more rows, wherein the two or more rows are substantially parallel, each adjacent pair of locations of the plurality of locations along any of the two or more rows is separated by an X-distance and each adjacent pair of the two or more rows is separated by a Y-distance; and the X-distance and the Y-distance are the same.

In yet another embodiment, the laser marking pulsed laser comprising a predetermined pattern of locations each comprising a mark or a void in a grid pattern that forms alphanumeric characters in the form of text having a font size within the range of 6 pt to 10 pt. The grid pattern has a plurality of locations positioned in two or more rows, wherein the two or more rows are substantially parallel, each adjacent pair of locations of the plurality of locations along any of the two or more rows is separated by an X-distance and each adjacent pair of the two or more rows is separated by a Y-distance; and, the Y-distance is at least 1.2, preferably 1.5, more preferably 1.7, and even more preferably 2 times the X-distance. Alternatively, the text can have a font size within the range of 11 pt to 16 pt and the Y-distance is at least 2, preferably 2.5, more preferably 3, and even more preferably 4 times the X-distance.

In another embodiment of the present invention, in addition to the exterior layer there are two or more interior layers within the core with each interior layer having a different concentration of TiO₂.

The present invention provides many benefits over the prior art. Because the laser marking can be, for example, consumer readable alphanumeric characters, sentences, paragraphs, and other methods of visual communication which can be marked on an article without the need of traditional labels. Specifically, the sheet materials and articles of this invention can be marked with ingredient listings, use instructions, UPC codes, and the like, in a fast, cost-effective manner without labels and adhesives. This provides cost savings, is environmentally friendly (fewer wasteful stickers on a package) and allows for instantaneous change in the message communicated to the consumer. For example, if an ingredient is changed in a formula, new ingredient labels can be marked on the article as soon as the change can be made in the computer instructions to the laser apparatus. No new labels are required. Moreover, the present invention provides methods and processes to laser mark polymeric materials containing low quantities of the laser marking additive TiO₂.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an article in accordance with the present invention marked with an alphanumeric character in a grid pattern.

FIG. 2 is a schematic view of a lasing apparatus according to the present disclosure.

FIG. 3 is a grid according to the present disclosure wherein the locations in adjacent parallel rows are stacked.

FIG. 4 is a grid according to the present disclosure wherein the locations in adjacent parallel rows are offset.

FIG. 5 is an alphanumeric character marked in a grid pattern in accordance with the present invention.

FIG. 6A is an alphanumeric character in a grid pattern marked in accordance with the present invention.

FIG. 6B is an alphanumeric character in a grid pattern marked in accordance with a prior art process.

FIG. 7 is a schematic representation of a two-layer sheet of material according to the present invention.

FIG. 8 is a schematic representation of a multilayer sheet of material according to the present invention.

FIG. 9 is a graphical representation of the concentration of TiO₂ within a sheet of material of the present invention.

FIG. 10 is a modified version of FIG. 5 to illustrate the % Mismarked calculation.

DETAILED DESCRIPTION OF THE INVENTION

Article

“Article”, as used herein refers to an individual object such as an object for consumer usage, such as a container suitable for containing materials or compositions. The article may be a container, non-limiting examples of which include bottles, tubes, sachets, films, laminates, bags, wraps, drums, jars, cups, caps, and the like. The compositions contained in such containers may be any of a variety of compositions including, but not limited to detergents (e.g., laundry detergent, fabric softener, dish care, skin and hair care), beverages, powders, paper (e.g., tissues, wipes), diapers, beauty care compositions (e.g., cosmetics, lotions), medicinal, oral care (e.g., toothpaste, mouth wash), and the like. Containers may be used to store, transport, and/or dispense the materials and/or compositions contained therein. The article can be made of any a variety of common materials including PET, PETG, HDPE, PP, PVOH, LDPE, LLDPE, steel, glass, aluminum, cellulose, pulp, paper, etc.

“Sheet material” as used herein refers to any high aspect-ratio structure wherein the thickness is substantially less than the length and width. Sheet materials include flexible sheet materials such as films and laminates as well as rigid materials such as bottle walls which may be formed by blow-molding preforms or parisons. Films and laminates may be rolled up to form tubes or other containers.

“Layer”, as used herein refers to striated regions within the sheet material that includes at least portions that are roughly parallel to the external surface(s) of the sheet material. Exemplary embodiments of sheet materials include co-extruded material such as for forming films, and layered materials for bottle making such as co-blown materials such as parisons used on forming bottle, co-injected materials used in forming bottle preforms, and over-molded preforms. “Layer”, as used herein, does not include painted or coated-on materials or in-mold labels.

FIG. 1 shows an article 10 having a predetermined feature 17 laser marked as a grid 16. The predetermined feature 17 can be consumer readable, machine readable or both. Predetermined feature 17 can be, for example, an alphanumeric character, a company logo, a drawing, artwork, UPC or QR codes, and the like. In this instance, the marked locations 12 make up an alphanumeric character 14, which in this case is the number two, “2”. The unmarked locations 11 in grid 16 are shown for illustration purposes only and do not appear on the final marked article 10. Article 10 is shown as a container and has an opening 11 and a neck 13 that provides access to the interior space 15.

An article according to the present invention may be formed of a single thermoplastic material or resin or from two or more materials that are different from each other in one or more aspects. The two or more materials may comprise layers within the article. The materials making up each of the layers can be the same or different from any other layer. For example, in addition to the outer layer comprising TiO₂, the article may comprise one or more layers of a thermoplastic resin, selected from the group consisting of polyethylene terephthalate (PET), polyethylene terephthalate glycol (PETG), polystyrene (PS), polycarbonate (PC), polyvinylchloride (PVC), polyethylene naphthalate (PEN), polycyclohexylenedimethylene terephthalate (PCT), glycol-modified PCT copolymer (PCTG), copolyester of cyclohexanedimethanol and terephthalic acid (PCTA), polybutylene terephthalate (PBCT), acrylonitrile styrene (AS), styrene butadiene copolymer (SBC), or a polyolefin, for example one of low-density polyethylene (LDPE), linear low-density polyethylene (LLPDE), high-density polyethylene (HDPE), propylene (PP) and a combination thereof.

Recycled thermoplastic and/or cellulosic materials may also be used, e.g., post-consumer recycled (“PCR”) materials, post-industrial recycled (“PIR”) materials and regrind materials, such as, for example polyethylene terephthalate (PCRPET), high density polyethylene (PCRHDPE), low density polyethylene (PCRLDPE), polyethylene terephthalate (PIRPET) high density polyethylene (PIRHDPE), low density polyethylene (PIRLDPE) and others.

The thermoplastic materials may include monomers derived from renewable resources and/or monomers derived from non-renewable (e.g., petroleum) resources or a combination thereof. For example, the thermoplastic resin may comprise polymers made from bio-derived monomers in whole, or comprise polymers partly made from bio-derived monomers and partly made from petroleum-derived monomers.

Additional pigments, colorants, and laser absorption additives may be added to the material of the sheet materials and articles of the present invention. Suitable choice of the laser wavelength in combination with pigments/colorants/additives may provide for suitable marking of the article. In cases where the contrast or speed of the marking is not sufficient, these pigments/colorants/additives may facilitate absorption of the laser energy, thereby serving as laser absorption additives.

In embodiments of the present invention, the additive that facilitates the laser marking is titanium oxide (TiO2), and the amount of TiO2 incorporated into the plastic is 1% or less. Unfortunately, at this low level, the TiO2 is not easily marked, particularly in high-speed marking processes. Without being bound by theory, it is believed that low-speed laser marking processes, such as processes using continuous lasers which can be gated open for extended periods of time, or processes using a pulsed laser where the pulsed laser is allowed to impose multiple pulses over a single area of the material being marked (i.e. overlapping pulses), can successfully mark materials incorporating 1% TiO2 or less. These processes, however, are slow and require long cycle times to laser mark an article comprising the material.

Many laser absorption additives, including TiO2, may have a deleterious effect on recycling of plastics and other polymeric materials. Thus, it may be beneficial to reduce the concentration of laser absorption additives in the material being laser marked. One such solution is to reduce the amount of laser absorption additive in the overall material, while retaining enough additive on the surface to get a good clear and concise marking on the material. FIG. 7 shows a two-layer sheet material 126 with outer edge 122 and inner edge 123 defining core 124. The distance from the outer edge 122 to the inner edge 123 is the sheet thickness. The sheet thickness can be from about 10.0 microns to about 2.00 mm thick, preferably from about 20.0 microns to about 1.50 mm thick and even more preferably from about 50 microns to about 0750 mm thick. Core 124 has an exterior layer 120 and an inner layer 121. The exterior layer is at least about 10.00% of the sheet thickness and less than about 40% of the sheet thickness. Exterior layer 120 has a concentration of TiO₂ of from about 2.50% to about 10.00%, while the entire sheet has a TiO₂ concentration of less than 1%. Those skilled in the art will understand the simple mathematical relationship between the concentration of TiO₂ in the exterior and interior layers. For example, if the exterior layer has about 5.00% TiO₂ and is 10.00% of the sheet thickness, and if the interior layer has 0.0% TiO₂, then the total concentration of the TiO₂ in the sheet material is 0.50%, well below 1.00%.

FIG. 8 is a multilayer sheet material 136 with outer edge 137 and inner edge 138 defining core 135. Sheet material 136 is shown with an optional protective layer 134, which can be, for example a varnish. Protective layer 134 can be added to the sheet material 136 either before or after laser marking of sheet material 136. Sheet material 136 comprises exterior layer 130 and two inner layers 131 and 132, which can have the same or different concentrations of TiO₂. It is understood that there may be three or more inner layers. The two or more inner layers may have identical concentrations of TiO₂ but be different materials, or the multiple inner layers could be of the same material with different concentrations of TiO₂, or combinations of these.

FIG. 9 is a graphical representation of potential concentration profiles across a sheet material. Dashed line 145 is an approximate separation between exterior layer 140 and interior layer 141. The X-axis represents the thickness of the layers, in percent of the total thickness. The exterior layer depicted in FIG. 9 is approximately 14.00% of the total sheet thickness. The Y-axis represents the concentration of TiO₂ in layers 140 and 141. Line 142 represents a scenario where the concentration of TiO₂ in exterior layer 140 is constant at approximately 4.00%, and the concentration of TiO₂ in the inner layer 141 is 0.00%. This results in a total concentration in the sheet material of about 0.56%. In contrast, line 143 depicts a varied concentration of TiO₂ across the layers where the concentration of TiO₂ starts at about 6.00% and drops to about 4.00% in exterior layer 140, which is again, about 14.00% of the total sheet thickness. Within inner layer 141 the TiO₂ concentration starts at 4.00% and drops to 0.00% at about 30.00% of the total sheet thickness. This results in an overall concentration of TiO₂ within the sheet material of about 0.90%. Those skilled in the art will understand that there are an infinite number of ways to get to a total concentration of less that about 1.00% TiO₂ in the sheet material while having from about 2.50% to about 10.00% concentration of TiO₂ in the exterior layer that is at least about 10.00% and less than about 40.00% of the total sheet thickness.

Laser and Lasing Apparatus

A pulse laser such as a short pulse laser may be used to mark the articles according to the present invention. Lasers for use in the present invention are commercially available and include nano, pico, and femto second lasers. These short pulse lasers can emit pulses applied at high energy-densities and high repetition rates, the high energies and high repetition rates are important to allow laser-marking the article at high speed. The laser marks themselves include marks made by oxidation, reduction, ablation, etching, foaming, and carbonization to the article such as a product or package.

Any suitable laser can be used to mark the article 10. FIG. 2 shows one example of a lasing apparatus 200 comprising a laser 20 useful for marking an article in accordance with the present invention. The lasing apparatus 200 includes a laser 20 which may be any laser capable of generating sufficient energy to mark the articles, such as a UV laser, having power in the range of 1 W to 60 W, and a laser wavelength of 355 nanometers or an IR marking laser, having a power in the range of 1 W to 300 W, or even 500 W, and a laser wavelength of 1064 nanometers. Such lasers are available from various suppliers, including an IPG ULPN-355-10-1-3-M marker or YLPN-1-1×350-50-3M MOPA module, available from IPG Photonics of Oxford, MA, United States. Other makes and types of lasers are also possible and different power ranges and settings may be used. The lasing apparatus can include optics that can be used to direct the laser beam, and/or to modify the laser beam such as by changing the energy density and/or spot size of the laser beam 28, as desired.

Frequency or Repetition Rate, measured in Hz, is the number of laser pulses a single laser can deliver in a second. For instance, a 1 MHz laser delivers 1,000,000 pulses per second where a 100 kHz repetition rate laser delivers 100,000 pulses per second. Repetition rate can be important for processing a particular lasing job in a short amount of time (i.e. high-speed laser marking). The more pulses per unit time available correlates (inversely) to the time required to mark a given row for a particular job almost linearly.

Pulse Energy is the amount of energy a single laser pulse contains and is typically measured in μJ or mJ. Typically, pulse energy is in the range of 5 uj to 2000 uJ (2 mJ), preferably in the range of 7 μJ-1000 μJ, and more preferably 10 uJ-300 μJ. The average power of the laser, then, is given as the pulse energy times the repetition rate.Average power=pulse energy(J)*rep rate(Hz or 1/sec).

Peak power is equal to pulse energy divided by pulse duration, and pulse duration can be less than 100 nanoseconds, less than 50 nanoseconds, less than 20 nanoseconds, less than 10 nanoseconds, or less than 1 nanosecond. Therefore, pulse energy and pulse duration are linearly related to peak power. Shorter pulse durations achievable with nanosecond, picosecond and femtosecond lasers allow for very high peak power which aids in the ability to mark articles.

In the lasing apparatus 200 depicted in FIG. 2, the laser 20 projects laser beam 28 onto X-mirror 22 which is rotated by X-galvo 21. X-mirror 22 and X-galvo 21 collectively form an X-galvo set. Laser beam 28 is then projected onto Y-mirror 24 which is rotated by Y-galvo 23. Y-mirror 24 and Y-galvo 23 collectively form a Y-galvo set. The X and Y mirrors 22 and 24 respectively, work together to direct laser beam 28 to the location where the desired mark 29 is to be marked on article 27. Before laser beam 28 reaches article 27, it will typically go through a lens 26. The distance from lens 26 to article 27 is the focal length 25.

The combined optics of the lasing apparatus may function so as to sweep the laser beam across the surface of the article in successive passes. The laser beam may sweep across the article along a first row in the grid in an X-direction, being directed by the X-mirror, while emitting (or omitting) pulses. The combination of the sweep-speed of the laser beam across the surface of the article, also called the surface velocity of the laser beam, and the repetition rate of the laser pulses, then, determines the spacing of marks along the X-direction.X-spacing*Repetition Rate=Surface Velocity

The laser may emit a pulse or pulses while sweeping across the article at a given location thereby resulting in a marked location (or locations), or the laser may omit pulse(s) while sweeping across the article at a given location thereby resulting in unmarked location(s) (i.e. void(s)). The laser beam may be swept across the article at a constant surface velocity while emitting and/or omitting pulses. The surface velocity or sweep-speed is defined above. The laser beam may subsequently sweep across the article along a second row of the grid (such as a row adjacent to the first row) while emitting (or omitting) pulses. The laser beam may sweep across the first and second rows in the same direction or in opposite directions. For example, the laser beam may sweep across the first row from “left-to-right” and across the subsequent/adjacent row from “right-to-left”.

Those skilled in the art will appreciate that the laser energy must be absorbed by the article's material in order for the article to be marked. For example, pulse lasers utilizing 355 nm (UV) may be absorbed by TiO2 added to the article. Other pairings of laser wavelengths with base materials or laser absorption additives for the article exist and are contemplated herein.

Laser Marking

The articles of the present invention are typically marked by the process of reduction or oxidation. The term “translucent” as used herein means the material, layer, article, or portion of the article being measured has total luminous transmittance of greater than 0% and less than or equal to 90%. The term “opaque” as used herein means the material, layer, article, or portion of the article being measured has total luminous transmittance of about 0%. The total luminous transmittance is measured in accordance with ASTM D1003.

Reduction and oxidation involve the laser energy changing the oxidation state of at least one of the article's components such as a laser absorption additive or opacifying pigment, resulting in a discoloration or color change that is viewed as a mark. For instance, without being bound by theory, the energy imparted from a UV laser can promote the reduction of TiO₂ to form a titanium sub-oxide where the oxidation state of titanium has been reduced to less than +4 and whereby this reduction results in a color change from white/colorless to blue, dark blue to black.

Spot-size relates to the focused area where the laser beam contacts the article. “Spot size” is the diameter of a round spot. The spots are round, but it is possible to achieve elliptical spots by control of the laser beam optics relative to the article. The spot size can be modified by focusing or de-focusing the laser beam, but the “fluence” (=energy per unit area) within the spot decreases as the spot is enlarged or de-focused. Theoretically, the minimum spot-size achievable with any laser is the wavelength of the laser itself. As a practical matter, the minimum spot size achievable with pulse lasers is ˜7-20 μm. The spot sizes can be in the range of from about 10 μm to about 150 μm, preferably from about 20 μm to about 100 μm, more preferably from about 30 μm to about 80 μm, and even more preferably from about 40 μm to about 60 μm. As discussed in the Background of the Invention, the spot sizes for conventional laser-markings for date codes (for example using CO2 lasers) and the like are a minimum of 250 μm and can exceed 800 μm. Another way to think about spot size in a marking context is the size of the paintbrush an artist is using to paint. If you want very fine detail, then smaller spots sizes would be utilized. Larger areas to be covered may prefer larger spots sizes. However, laser marking mechanisms require a minimum fluence to achieve the desired mark so balancing pulse energy, pulse duration, pulse overlap, and spot size are critical.

Geometry of the mark spacing can contribute to the cycle time and fluence (or energy per unit area) provided to an article. For example, the spacing between marks may be such that the marks do not overlap at all and have 0% overlap. At 0% overlap, each individual laser pulse is responsible for the energy provided to mark the article. If the laser does not have sufficient pulse energy or peak power to achieve a desired mark (for example, when the concentration of the laser absorption additive it too low), then one can decrease the pulse spacing to the point where the spots overlap in either one or both the X and Y-directions; overlapping the spots includes providing more than one laser pulse to the area of the article in which the spots overlap which provides higher fluence or energy per unit area to that portion of the article. Overlapping pulses is, however, undesirable for high-throughput laser-marking as pulse spacing is a key lever for cycle time. If a laser has a fixed repetition rate or pulse frequency, then to achieve the lowest process-time (also called cycle-time) one would want to spread the pulses out as much as possible while still achieving the desired mark type and mark contrast. In one embodiment of the present invention, the pulses are non-overlapping.

Pulse Duration is the length of time a pulse remains continuously above half its maximum value. The shorter the pulse, the higher the peak power can be created with a common average power. This is because average power=pulse energy (J)*rep rate (Hz or 1/sec). Peak power is equal to pulse energy divided by pulse duration. Therefore, when pulse duration gets significantly smaller, the resulting peak pulse power is significantly higher. This peak power enables improved carbonization, foaming, oxidation, reduction, etc. on the targets being marked. Short pulse lasers take advantage of this phenomenon to mark articles and enable marking mechanisms typically not found in longer pulse lasers.

Grid

While articles of the present invention do not need to be marked in a grid pattern, the grid pattern can enable features that further provide for reduced cycle-time/high-throughput. As used herein a “grid” or a “bitmap grid” is taken to mean a regular periodic array of locations that may include the plurality of marks. The periodicity of the array includes periodicity in both the X and Y-directions. The plurality of marks within the grid may or may not be present at each of the locations within the grid. That is to say, a mark may be formed at a location within the grid or may be absent at the location (i.e. a void). As mentioned, the lasing apparatus sweeps the laser beam across the article while the laser pulses are either emitted from the laser or no pulse is emitted. A marked location occurs when the laser emits a pulse to a given location and an unmarked location results when the laser does not emit a pulse to a given location. The laser beam may be swept across the article at a constant surface velocity while the repetition rate of the laser is constant, so the periodicity of locations will be regular (i.e. the X-distance) in the direction in which the laser beam is swept across the article (i.e. the X-direction) even though the spacing of marked locations may not be equal, given the possibility of unmarked locations. In the event of unmarked locations, the distance between any marked locations along the same direction (i.e. in the X-direction) may be an integer (i.e. 2×, 3× or larger) of the smallest distance measured between marks in that direction.

The laser beam may be swept across the article in subsequent rows. The laser beam may be swept from left-to-right or from right-to-left and may sweep in the same direction as it is moved from row to row (e.g. like the carriage-return on a typewriter, as in a raster process) or may be swept in alternating directions as it moves from row to row. A key contributor to reducing cycle-time includes sweeping the laser beam in alternating directions as it moves from row to row. The rows may be generally parallel to one another. The distance between adjacent rows is the Y-distance. The locations in adjacent rows may lie directly above/below one another or may be offset relative to one another. It is appreciated that an offset that is equal to the X-distance results in a realignment of the locations between rows.

An alphanumeric character is a letter or a number, for example, in English the letters are A-Z including upper case and lower case and the universal numbers are 0-9 and combinations thereof. An alphanumeric character is not limited to any particular style or font. Chinese, Japanese (e.g. Kanji, Katakana), Russian, Arabic and other languages have different alphanumeric characters that can be marked.

Those skilled the art will appreciate that the size of a printed, or in this case marked, alphanumeric character is measured by its font. The smallest font generally accepted as readable by a consumer on a marked article is about 6 pt. Font size can increase to very large sizes, but when marking a face of a consumer package, for example, fonts in excess of 20 are impractical as a few characters can fill an entire package. The “faces” of a consumer package are typically the front or back of the package, which faces typically have different markings. For example, the product name and a general product description (shampoo, conditioner, soap, etc.) are typically found on the front, while the ingredient list, UPC code and directions for use are usually on the back. For cylindrical packages, an imaginary vertical plane can be drawn down the bottle with the front face being on one side of the imaginary plane, and the back face being on the opposite side of the imaginary plane.

As previously discussed, the laser marks may be non-overlapping to reduce the time required to mark a given pattern (i.e. “time-to-mark”). Time to mark can be further reduced by spacing-out the marks in either or both of the X- and/or Y-directions, however, this increased spacing can lead to poor legibility of any alphanumeric characters comprising the predetermined pattern. For example, increasing the X-distance allows for a faster surface velocity of the laser beam across the surface of the article when marking a given row (at a constant repetition-rate). Increasing the Y-distance allows for fewer turnarounds in the course of marking a given predetermined pattern.

For certain predetermined patterns, increasing the Y-distance (fewer turnarounds) may have a greater impact on reducing time to mark than increasing the X-distance (faster surface velocity). While conventional raster marking processes include equal X- and Y-spacings, the Y-distance that is greater than the X-distance The X-distance is preferably in the range of from about 0.005 mm to about 0.500 mm; more preferably from about 0.010 mm to about 0.100 mm; and even more preferably from about 0.040 to about 0.075 mm. The Y-distance is preferably in the range of from about 0.010 mm to about 2.0 mm; more preferably from about 0.050 mm to about 0.150 mm; and even more preferably from about 0.060 mm to about 0.075 mm.

When a predetermined alphanumeric character has a font size within the range of 6 pt to 10 pt, the Y-distance may be at least 1.2, preferably 1.5, more preferably 1.7, and even more preferably 2 times the X-distance. When the predetermined alphanumeric feature has a font size within the range of 11 pt to 16 pt the Y-distance is at least 2, preferably 2.5, more preferably 3, and even more preferably 4 times the X-distance.

FIGS. 3, 4, and 5 all show various depictions of grids. More specifically, FIG. 3 is a grid 39 illustrating the X-direction 30, Y-direction 32, X-distance 31 and Y-distance 33. Potential marking locations 36 are depicted by the empty circles making up the grid. Further, in FIG. 3 the locations 36 among parallel rows 38 are “stacked” when the angle 35 between locations in adjacent row 34 drawn in the Y-direction between two potential marking locations and the X-direction 30 is approximately 90 degrees. In other words, if one uses vectors to connect neighboring marks from the array to form a parallelogram (i.e. unit cell), when the interior angles of the parallelogram are approximately 90 degrees, the locations are stacked. When the interior angles of the parallelograms differ from 90 degree (i.e. 120 & 60 degrees), the locations are offset. X-distance is measured from the center of one location to the center of an adjacent location in the X-direction.

Those skilled in the art will appreciate that the unit cell of a grid has four symmetrical axes horizontal, vertical, and two diagonals. The laser marking discussed herein can occur along any of those four axes. The vertical and horizontal directions shown in FIG. 5 are described for simplicity. FIG. 5 could be rotated 45 degrees and then the diagonals become vertical and horizontal. Again, the laser marking occurs across one row, then the laser moves up or down to the row above or below, and marks in the opposite direction as shown in FIG. 5.

FIG. 4 is another grid 49 showing an offset 44, having an offset distance 47 between adjacent parallel rows 48. Offset 44 is defined by angle 45 between locations 46 in one row 48 and the nearest location 46 in an adjacent row 48, wherein an offset 44 exists when angle 45 is greater than or less than 90 degrees. FIG. 4 further shows X-direction 40, X-distance 41, Y-direction 42, and Y-distance 43.

FIG. 5 shows an alphanumeric character 52 marked on grid 50. The alphanumeric character is the number “2” and is marked by laser marked locations 54, which are in contrast to unmarked locations 56. When multiple alphanumeric characters are printed, for example, a word, a sentence or a paragraph, the characters sharing the same line of text will also share the same horizontal rows 53 of laser markings. That is, the laser apparatus will go across one row, marking locations as needed for an individual alphanumeric character, then leave the necessary number of unmarked locations between characters, to form a row with markings associated with a plurality of characters in that row. By this method words, sentences, and paragraphs can be marked and be clearly legible to a consumer or machine.

Horizontal rows 53 and vertical columns 55 define grid 50. The distance between marked or unmarked locations (54 or 56) in horizontal row 53 defines X-distance 51. Moreover, the distance between marked or unmarked locations (54 or 56) in vertical column 55 define Y-distance 57. It is important to note that X-distance 51 and Y-distance 57 are defined with respect to the horizontal (53) and vertical (55) orientations, respectively, of the marked indicia, in this case the number “2”. But the marked locations 54 can be marked in the horizontal direction 59 or the vertical direction 58. More specifically, when marking in the horizontal direction 59, the laser moves across a horizontal row 53 either marking or leaving unmarked each location (54 and 56, respectively). Then the laser moves down or up one Y-distance and begins traveling across another row above or below the row previously marked. Likewise, when marking in the vertical direction 58 the laser moves up or down a vertical column 55 either marking or leaving unmarked each location (54 and 56, respectively). Then the laser moves across one X-distance and begins traveling up or down a vertical column adjacent the vertical column previously marked.

The aspect ratio of a number or letter is the ratio of its height to its width. The aspect ratio of the number “2” shown in FIG. 5 is greater than 1 because its height is greater than its width. One can easily see that marking the number “2” in the vertical direction 58 requires fewer turn arounds than marking in the horizontal direction 59. Thus, marking these indicia may be faster when sweeping the laser-beam while marking in the vertical direction 58. Many considerations will be taken into effect when deciding to mark in the horizontal or the vertical directions. But the X-distance 51 and the Y-distance 57 when considering the relative spacings and the font size of the alphanumeric characters will always be defined with respect to the horizontal and vertical directions, respectively, with respect to the indica being marked.

The periodicity of the locations comprising the grid includes periodicity in the X-direction and periodicity in the Y-direction. The X-direction and Y-direction may be generally orthogonal to one another. As depicted in FIGS. 3 and 4, the grids 39 and 49, respectively, may take the form of equally spaced locations along successive parallel rows. The direction parallel to the successive parallel rows is taken as the X-direction (30, 40) and the direction generally perpendicular to the X-direction is taken as the Y-direction (32, 42). The spacing between adjacent locations along any of the parallel rows (e.g. in the X-direction) is taken as the X-distance (31, 41), and the distance between adjacent parallel rows is taken as the Y-distance (33, 43).

The grid 39 may be a stacked grid as depicted in FIG. 3. In a stacked grid, the locations where the marks may be applied along a first row are directly above the locations along a second row immediately below the first row. Said another way, the angle 35 formed between the row-segment connecting a first location along the first row with an adjacent location along the first row and the row-segment connecting the first location with its nearest location along the second row is 90°. In a stacked grid, the spacing between adjacent locations along the X-axis is equal to the X-distance 31 and the shortest spacing between adjacent locations between adjacent X-axes is the Y-distance 33.

The grid may be an offset grid as depicted in FIG. 4. In an offset grid 49, the locations where the marks may be applied along a first X-axis are not directly above the locations along a second X-axis immediately below the first X-axis. Said another way, the angle 45 formed between the row-segment connecting a first location along the first X-axis with an adjacent location along the first X-axis and the row-segment connecting the first location with its nearest location along the second X-axis is greater than or less than 90°.

Those skilled in the art will appreciate that the X-direction and Y-direction are somewhat arbitrarily chosen relative to the predetermined pattern. For example, FIG. 6 B depicts an example of a “2” made by laser marking wherein the X-direction is vertical with respect to the marked “2” 61. Those skilled in the art will appreciate that the X-direction could just as easily been horizontal with respect to the marked “2” 61.

Those skilled in the art will appreciate that the grid (e.g. 39 and 49) and the regular spacing between adjacent locations assumes a planar surface of the article. Where the article surface is curved, the spacings may vary with the curvature of the surface.

The X-distance may be consistent among all the parallel rows comprising the grid. That is to say that the X-distance does not change along the X-direction of a given row, nor does it change among the rows of the grid comprising the predetermined pattern. Alternately, the predetermined pattern may include multiple regions where the X-spacing within each region is consistent but different between the regions. For example, one X-distance may be used consistently when marking alphanumeric characters and a different X-distance may be used when marking machine-readable codes such as UPC codes. Similarly, the Y-distance may not change within the pre-determined pattern or may vary between regions within the predetermined pattern. The surface velocity of the laser beam and/or the marking direction (i.e. up/down or side-to-side) may also be different between the regions. For example, it is notable that laser-marked articles using a vector marking process generally exhibit variable-spacing of locations/marks along any of their marked directions as the laser speeds-up (causing marks to be spaced further apart) and/or slows-down (causing marks to be spaced closer together) along the course of marking the article. The laser marking may be done at constant speed when the laser is marking. The movement of the laser beam stops after the end of one row along the X-direction, moves up or down to the next row in the Y-direction, and then begins marking the new row also at a constant speed. This speed may also be consistent throughout the marking of the predetermined pattern. Articles marked with the CV-bitmap grid marking process can be distinguished from articles marked with a vector marking process by the regular periodicity of marks and often by the absence of outlines or “borders” that define the marked area, see for example FIG. 6B border 63.

FIGS. 6A and 6B illustrate the difference between laser marking via bitmap grid marking with the inventive CV-bitmap process 6A, and the prior vector marking process 6B. in both cases an alphanumeric character 60 and 61 (i.e the number “2”) is marked with a laser. The alphanumeric character 60 is substantially better defined with clean, crisp edges, and very few stray markings. FIG. 6B stands in stark contrast, with largely undefined edges and a substantial number of stray markings 62 outside the border of the alphanumeric character 61. Both characters 60 and 61 were marked in approximately the same amount of time.

High Speed Laser-Marking of Human/Machine-Readable Text, Symbols and Codes

As discussed, the present invention allows for laser marking of human and machine legible text, symbols, and codes. Further, the invention enables such laser-marking such text, symbols, and codes even when laser-marking at high speed. Existing raster processes are slow, but accurate. Vector laser marking processes are faster and accurate at low speeds but can be imprecise at high speeds resulting in unclear markings that are hard to read by consumers or machines. Higher speeds with good precision relative to other marking processes may be provided by such laser marking methods as polygon scanners and CV-Bitmap scanning. As mentioned previously, the polygon scanner may not be preferred for reasons of flexibility.

FIG. 6B depicts a potential effect of running a vector-type process at high speed when marking text involving alphanumeric characters and the misplacement of marks within a row. The figure shows many rows displaced from one another where the marking either initiated too early or too late, so that the outline of the alphanumeric character is jagged and the overall appearance is blurred and potentially illegible (e.g. one cannot distinguish an “8” from an “0”). In contrast, the process and resulting patterns created by a constant surface velocity (CV) bitmap path is shown in FIG. 6A.

The surface velocity of the laser beam across the surface of the article in a CV-bitmap can be faster than those achievable with the other laser marking processes. Raster and vector processes typically exemplify surface velocity on the order of 1-2 m/s or less. CV-bitmap processes can provide for surface velocities above 8 m/s, and further surface velocities equal to or greater than 10 m/s, 15 m/s, 18 m/s, 22.5 m/s, 32.5 m/s, 45 m/s, 60 m/s and even as high as 90 m/s or higher.

The sweep path of the laser beam across the surface of the article can also contribute to reduced cycle time. Conventional raster laser marking processes sweep the laser beam across the rows in either the right-to-left or left-to-right directions, also known as unidirectional, and “jumps” the laser beam back after marking each row to start the subsequent row (like a carriage return on a typewriter). In this way, subsequent rows can be easily registered (i.e. stacked) and grid-locations can be lined-up based on this consistent starting point. To eliminate the jump distance and reduce the time between each marked row, a CV-bitmap process may use a “bi-directional” process in which marking may be done in alternating fashion in both directions (i.e. marking occurs left-to-right in a first row and right-to-left in a subsequent row).

The turnaround profile of the laser beam after completing a row can also contribute to reduced cycle time. The turnaround profile may be symmetric or asymmetric. Given the high speeds at which the laser beam sweeps across the surface of the article, an asymmetric turnaround profile may be preferred.

The choice of the orientation of the marking direction can affect job cycle time, particularly when marking features with a high (or low) aspect ratio. The aspect ratio of the feature is generally taken as the ratio of the height to the width of the feature. Where the height and the width are nearly similar, the aspect ratio is close to 1 and the impact (to job cycle time) of choosing the marking direction relative to the dimensions of the feature may be minimal. However, for features that have a high aspect ratio (e.g. height>>width) or low aspect ratio (e.g. width>>height), job cycle time can be reduced by selecting the marking direction relative to the dimensions of the feature. For example, the marking direction may be chosen to be generally parallel to the longer dimension of the feature (i.e. the major axis) or the marking direction may be chosen to be generally parallel to the shorter dimension of the feature (i.e. the minor axis), see again FIG. 5. While there are many factors affecting job cycle time, it is believed, that corresponding the marking direction to the major axis of the feature reduces the number of turnarounds required when marking the feature, thereby reducing job cycle time.

The choice of the orientation of the marking direction can also affect job cycle time at very high surface velocity. At very high surface velocities, the turnaround time can increase to the point where it dominates the job cycle time. Selecting the marking direction to be generally parallel to the longer dimension of the feature minimizes turnarounds and can reduce job cycle time. As previously discussed, the X- and Y-distances may be different, and this difference can contribute to reduced job cycle, and any loss in clarity of the image(s) of the feature, such as alphanumeric characters, can be compensated by reducing the X-distance while increasing the Y-distance.

Precision Metrics

As discussed, the precision of the placement of the laser-marks can be a contributor to both human-legibility of a marked text or image, and machine-legibility of a marked code. A number of precision metrics can be used to characterize the precision of the placement of the laser-marks. These are elaborated, below.

Precision metrics, generally, rely on determining the dispositions of marked locations. The precision of the laser marks may be determined relative to the predetermined pattern, relative to the grid, or relative to each other. The disposition of the marks can be determined using image analysis, though one of skill in the art will appreciate that any means of determining the dispositions could be used (e.g. pencil and graph paper).

Microscopy

Images for image analysis can be generated by microscopy. A stereomicroscope such as a motorized Zeiss SteREO Discovery. V20 (Carl Zeiss Microscopy, LLC, Thornwood, NY) equipped with color camera such as the Axiocam 305 (5 megapixel CMOS, Carl Zeiss Microscopy, LLC, Thornwood, NY) is used to image characters, digits and images of interest of a sample using reflected light illumination such as achieved with a LED ring light and light source such as a Cold-light source CL 6000 LED lamp (Carl Zeiss Microscopy, LLC, Thornwood, NY). A typical light intensity of 80-100% of the maximum light intensity is used. The individual laser markings that combine to form the character, digit, or images of interest are resolved using suitable magnification using an objective lens such as an Achromat S 1.5×FWD 28 mm (Carl Zeiss Microscopy, LLC, Thornwood, NY) combined with a zoom factor such that the total magnification is between 10× and 345×. By way of example, characters, digits, or images with a font size of 10 pt, the total magnification is about 40× magnification. After the digit of interest is brought into view of the camera, the character, digit, or image is brought into focus using manual skill or, preferably using the autofocus module via the user interface platform (such as Zen V2.6 Blue Edition or higher with Zen Autofocus module, Carl Zeiss Microscopy, LLC, Thornwood, NY). Prior to collecting an image of the characters, digits, or image, the imaging settings are optimized by using an auto exposure option from the user interface platform along with the lamp intensity. Images are collected in the highest resolution format possible, such as ZVI, then exported as Tiff files having resolution of about 2464×2056 pixels. Furthermore, the marked rows of the character, digit, or artwork should be nearly parallel with the horizontal borders of the image. If required, multiple images taken at higher magnification may be accurately stitched together to encompass the full area of the character, digit or image.

Image Analysis

The images from the microscope appear gray but are captured in color. The images are converted to gray scale using an NTSC protocol. A suitable image analysis software is required to perform this and several other image processing steps. Analysis functions implemented by MATLAB available from The Mathworks, Inc., Natick, MA are referenced in this method description.

The microscopy and subsequent image analysis may be pursued over one or more predetermined patterns, a portion of a predetermined pattern, or an individual image within a predetermined pattern such as a graphic or an alphanumeric character. Where the image analysis is to be performed over a portion of a predetermined pattern, prior to the analysis, the portion (such as an individual graphic or alphanumeric character) must be separated from any surrounding images, characters or artwork. A mask may be drawn around the character or image of interest in the predetermined pattern. The mask separates the character or image from other partial characters, digits, bar codes, artwork, dirt or other imperfections that may occur in the image.

The image analysis relies first on identifying the laser marks that comprise the image. The laser marks can be identified by any reasonable means. For example, by repeatedly thresholding the grayscale image from the microscopy. The start threshold is set to capture only a few pixels that fall in some of the markings. The threshold value then progressively changes, capturing an ever-increasing area of the marks. The progressive thresholding continues from the start threshold to a stop threshold. The stop threshold may be determined automatically such as by using MATLAB's “multithresh” function (i.e. Otsu's method). Progressive thresholding can be advantageous in the analysis because the area of markings may overlap and/or merge and the background may not be perfectly uniform. The direction of the threshold progression (i,e, light-to-dark or dark-to-light) can be used to identify dark markings versus a relatively light background or light marking versus a relatively dark background. In the example presented, dark marks are identified versus a relatively light background.

Connected components may then be used to identify individual marks once the area reaches a certain size. A connected-components algorithm is executed with each new threshold to group touching pixels into blobs. When a blob reaches 50% of the area for a mark, it is identified as a mark. The center coordinates of the marks are found using a centroid method as implemented in MATLAB's “regionprops” function. The centers are subsequently used (see below) to determine spacings among adjacent marks in a row (e.g. the X-distance) and spacing between adjacent rows of marks (e.g. the Y-distance).

Determining the Observed X- & Y-Distances and Standard Deviations

A means of expressing the precision of the marked text, image or code in a grid pattern is by the observed X-distance and Y-distance of the grid, as well as their standard deviations. These parameters can be determined using the image analysis, though one of skill in the art will appreciate that any means of determining these distances and standard deviations may be used. One means of determining these values by image analysis includes the use of “Delaunay Triangles”. For the Delaunay Triangle method, center coordinates of the marks are passed to MATLAB's “Delaunay Triangulation” function which creates a triangulation based on their center points. Edges of a Delaunay triangulation never cross and the center points are connected in a nearest-neighbor manner.

The X-distance is taken as the distance between adjacent marked locations along a given row within the grid. The adjacent marked locations along a given row result in a horizontal edge within the Delaunay Triangulation data structure. These horizontal edges can be separated from other edges in the triangulation by calculating the angle of the edge. A horizontal edge in a row of the grid will be within +/−10 degrees of the horizontal edge of the image. The grid consists of a periodic spacing of locations along the rows, so the X-spacings should be relatively consistent (e.g. have a low standard deviation). In this analysis a horizontal edge with a length greater than 2 times the programmed distance can be eliminated from consideration as indicative of a non-adjacent location. The observed X-distance determined when analyzing an image such as an alphanumeric character is, then, taken as the average length of horizontal edges between adjacent marks for all marks/rows within the given image or character. The X-distances for a plurality of characters in a macroscopic image can then be averaged further to provide an average X-distance for a given marking condition and a given image or predetermined pattern.

The Y-distance can be determined as the vertical distance between adjacent rows. In the Delaunay Triangulation a horizontal edge can be part of 2 adjacent triangles. Each base edge contributes 2 vertices to each of the triangles and the third vertex is the nearest-neighbor mark in the adjacent row either above or below the base edge.

For each of the base edges, the perpendicular distance to the nearest mark above and below the base edge is determined. Only the minimum (i.e. nearest) of these two distances is recorded. Using only the minimum distance helps ensure that the row is adjacent and helps prevent double-counting of rows. The average and standard deviation of these perpendicular distances over a given image is then taken as the average Y-distance and standard deviation for the image. The topmost and bottommost rows of the character/digit are not used as the base of measured triangles as they have only one adjacent row. The Y-distances for a plurality of images in a predetermined pattern or portion thereof can then be averaged further to provide an average Y-distance and standard deviation for a given predetermined pattern or portion thereof (such as for a given alphanumeric character).

Displacement

Another means to parameterize the precision of the laser marking when a grid is used is by the percent displacement. As shown in FIG. 6B, imprecise laser-marking may result in substantial displacement or overhang of marks or voids within a row, resulting in blurred images with a jagged outline. Any of a number of means can be used to express the displacement within the rows including simple visual inspection. For example, such a means may include simply observing the character (or other element of a predetermined pattern) and assessing whether it is legible or not, given prior knowledge of the intended marked pattern (e.g. the alphanumeric character). The displacement can also be quantified. Importantly, human- and machine-readable patterns generally include a “smooth” outline (versus the jagged outline shown in FIG. 6B). Said another way, for user-readable and machine-readable patterns, the left-most and right-most marked locations within a given row (or marked portion of a row), generally are not substantially displaced (in the X-direction) from the left-most and right-most marked locations (respectively) in the adjacent rows (above and below) relative to the X-distance.

In this quantification of displacement, the start-point of each marked portion within a row (for a given character or pattern element) is taken as the left-most mark and the finish-point of each marked portion within a row taken as the right-most mark. The start-points and finish-points for each marked portion within a row is determined relative to the corresponding start-point and finish-point (respectively) of the adjacent rows above it and below it. The row under consideration is determined to be “displaced” on the left-side of the character/pattern element if the start-points of both the row above and the row below are left of the measured row's start-point, and the row under consideration is determined to be “displaced” on the right-side if the finish-points of both the row above and the row below are right of the measured row's finish-point. The horizontal distance from the start-point (and finish-point) of the row to the start-points (and finish-points, respectively) of the rows above and below are determined, and the displacement is taken as the shorter of these two distances. The left-side displacement being the displacement determined by the start-points and the right-side displacement being the displacement determined by the finish-points. A marked portion within a row may include no displacement, either left- or right-side displacement or both left- and right-side displacement. The top-most and bottom-most rows comprising the image being analyzed (i.e. an alphanumeric character) are omitted from the analysis, as they do not have two adjacent rows. One means to identify the start and finish points of each row uses the Delaunay Triangulation analysis previously discussed for determining the X- and Y-distance(s) and standard deviation(s).

The “% Displacement” for a given image such as an alphanumeric character is the sum of the displacements (each displaced mark counts as one displacement) for the rows comprising the character divided by the number of rows making up the character.% Displacement=(total displacement in the character)/(number of rows in the character)*100

The “A % D” or Average % Displacement for a predetermined pattern comprising multiple alphanumeric characters as text, is simply the sum of the % Displacement for each character in a sample set divided by “n” the number of characters in the sample.

% Mismarked

Yet another method of quantifying the precision of the laser marking when a grid is used is the percent of mismarked locations, or “% Mismarked”. Referring now to FIG. 10, which is a modified version of FIG. 5 for illustrative purposes. FIG. 5 shows the alphanumeric character “2” marked in a grid pattern. There are approximately 130 marked locations 54. In FIG. 10, there are 8 voids 101 that should be marked locations. Further, there are 4 marked locations 100 that should be voids. As should be apparent, both the voids that should be marked locations and marked locations that should be voids are mis-marked mistakes, accordingly they are added together and compared to the number of marked locations. In the example of FIG. 10 there are 12 mistakes (8+4) out of a total of 130 desired marked locations. The % Mismarked=the number of mistakes divided by the number of desired marked location times 100, ((12/130)*100)=9.23%. FIG. 6B shows an egregious mismarked alphanumeric character (“2”) wherein the % Mismarked is greater than 20%.

The “average % Mismarked” for a predetermined pattern comprising text is simply the sum of the % Mismarked for each alphanumeric character divided by the number of alphanumeric characters. To achieve the desired readability of text comprised of the alphanumeric characters the average % Mismarked of the alphanumeric characters is less than about 20%, preferably less than about 15%, more preferably less than about 10% and even more preferably less than about 5%.

Method for Total % TiO₂ in an Article

A series of thermoplastic resin chips containing a suitable grade of pigmentary TiO₂ are prepared at concentrations of 0.5, 0.75, 1, 1.5, 2, 3, 5, 7.5 and 10% by use of a single or double screw extruder. Flat parts such as plaques or chips are produced using an injection molding machine such as available from BOY Machines, Inc (Exton, PA 19341 USA). Pigmentary TiO₂ is best selected from grades which contain coatings to enable compatibility with the resin to ensure good dispersion and homogeneity across the part. Examples of pigmentary TiO₂ intended for use with polyolefins include TIOXIDE® TR48 (Venator, Wynyard Park, UK) and Ti-Pure™ R-103 (The Chemours Company FC LLC, Wilmington, Delaware USA). Since many examples of pigmentary TiO₂ have surface coatings, the percent powder added in the extrusion process could be slightly different from expected. For this reason, the % TiO₂ content of the chips are confirmed by Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) and the measured values are used for the Raman Spectroscopy method herein.

Samples from the 0.5, 0.75, 1, 1.5, 2, 3, 5, 7.5 and 10% TiO₂ standard chips are cut into smaller pieces using ceramic scissors and knife. Scissors and knife are cleaned between samplings. A blank resin sample (same resin as used for the TiO₂ standards) and TiO₂ standard samples are prepared (n=1-n=3). Approximately 0.05-0.1 g of sample is transferred and weighed into a pre-cleaned 15 mL TFM UltraWAVE tube (Milestone Srl, Bergamo, Italy). Each sample is combined with digestion agents such as nitric acid, hydrochloric acid and fluoroboric acid. Samples are then fully digested using an UltraWAVE Microwave Digestion System heating to a final temperature of 250-260° C. Samples are then transferred to 50 mL polypropylene tubes, combined with internal standard, and diluted to 50 mL with deionized water. The resulting test solutions are visually checked to confirm full digestion of the sample such that the solution yielded a transparent solution with no fine particulates present. The transparent solutions are analyzed by ICP-OES for Ti. Working standards are prepared by combining appropriately diluted reference standards (Inorganic Ventures, Christiansburg, Virginia, USA) covering the concentration range of interest (45 ppm-145 ppm Ti), together with selected internal standard (Y). The final composition of these working standards are matched to the prepared samples, in terms of acid content, for matrix-matching purposes. Results from analysis of these working standards yield the requisite data to prepare calibration curves for quantitation of titanium. The prepared standard and test samples are analyzed using an ICP-OES analyzer such as an Agilent 5110 ICP-OES (Agilent Technologies, Inc. Santa Clara, CA, USA) using multiple wavelengths to demonstrate adequate selectivity with one wavelength chosen for reporting purposes (Ti 336 nm) and with Yttrium (Y 371 nm) used as the internal standard.

Method to Determine % TiO₂ within Specific Layers

Standards of thermoplastic resin with varying amounts of TiO₂ are analyzed by Raman spectroscopy using a Raman microscope such as the Renishaw Virsa™ Raman Analyzer (Renishaw plc., Gloucestershire, United Kingdom) equipped with a continuous 785 nm laser with fiber-optic-coupled probe including motorized x/y/z stage, live video camera and halogen or LED light source for white-light sample viewing, and 20×/0.40NA LE Plan Nikon microscope objective. The surface of the sample is located by utilizing the live, white-light video image prior to data acquisition. Spectral acquisition is achieved by means of two-dimensional (2D) mapping by scanning the sample in x, y with respect to the incident laser beam, or by line scans of the standards in the x or y direction. A minimum of 300 of Raman spectra covering an area of 300 um² or greater are obtained per calibration standard. For each spectrum collected, the area of a unique TiO₂ Raman peak that does not interfere with any Raman peak from the host resin is determined, and the resulting value is divided by the area of a unique Raman peak from the host resin. For HDPE, as an example, for each spectrum collected, the % TiO₂ is determined by calculating the area of the TiO₂ Raman peak centered at around 417 cm−1 and dividing by the spectral signature area of HDPE between 1390 and 1504 cm−1. A calibration curve is generated by calculating the average value of all normalized TiO₂ and plotting the result against the % TiO₂ as determined from the ICP-OES method. This calibration equation is used to determine % TiO₂ within specific layers of an article.

To determine the % TiO₂ within specific layers of the article, a suitably flat cross-section of the sample for viewing the internal structure of a potentially layered article is prepared by a method known by those skilled in the art of microscopy. Suitable methods can include using a sharp single edge razor blade and slicing parallel to the layers (cutting perpendicular to the layer thickness direction is not acceptable), cryo-fracture of the article (removal a small region, clamping by using forceps, placing in a liquid nitrogen bath, and breaking when brittle), and/or low energy broad beam cryo-Ion Milling such as by use of the Hitachi IM4000Plus lon Milling System (Hitachi High-Tech America, Inc., Dallas, Texas). The cross section of the sample is visualized in live time with a white light video camera. Two dimensional (2D) chemical mapping of the cross-section is obtained by focusing the laser beam at or below the cross-sectional surface and scanning across the thickness direction of the cross-section by collecting a data point every 5 um, preferably every 1 um, with a detector exposure time up to 10 s, and laser power at sample no greater than 20 mW, adjusted accordingly to maximize Raman signal while preventing saturation of the detector or burning of sample. In this manner, spectral data is collected across a region having a width of about ½ of the total cross-sectional thickness dimension (ie. if the thickness of a film is 200 μm, the region analyzed should be 200 μm in the thickness direction×100 μm in the orthogonal direction as viewed via microscope). The full Raman spectral data set is corrected for cosmic rays and baseline prior to image generation. Hyperspectral Raman images highlighting distribution of TiO₂ from spectral features unique to TiO₂ and host resin such as HDPE as described in above are generated using suitable software such as the Renishaw WiRE™ 5 software. The layers and layer thicknesses are determined by techniques known to one skilled in the art. The concentration of % TiO₂ within different layer(s) of the article is obtained by selecting a minimum of 300 Raman spectra from evenly spaced unique locations within a square/rectangle across each interface(s) of the cross-sectional plane containing TiO₂. For HDPE, as an example, for each spectrum collected, the % TiO₂ is determined by calculating the area of the TiO₂ peak centered at around 417 cm−1 and dividing by the spectral signature area of HDPE between 1390 and 1504 cm−1. The calibration curve is used to obtain the average % TiO₂ within the discrete layers of the article.

Method for Overall Bar Code Symbol Grade

For verification of linear bar codes, 7 verification parameters (decode, symbol contrast, minimum reflectance, minimum edge contrast, modulation, defects, and decodability) specified by ISO/IEC15416-1 are measured by use of an ISO/IEC compliant barcode verifier such as the Axicon 15500 (Axicon Auto ID Limited, Oxfordshire, UK). “Bar codes” are synonymous with the term “symbols” as defined in ISO/IEC15416-1. For each of these parameters, a grade from 0 to 4 is assigned per ISO/IEC15416-1 where 0 represents failure and 4 represents the highest quality. At least 10 scans which collect the reflectance profile across the full width of the bar code are measured using a ISO/IEC compliant barcode verifier. The scan reflectance profile grade for each scan reflectance profile shall be the lowest grade of any parameter evaluated for that profile. An overall bar code symbol grade is computed by the arithmetic mean of each of the scan reflectance profile grades. The overall bar code symbol grade is reported to the nearest 0.1. For linear barcode symbols, the measurements may be taken using any combination of ISO/IEC recommended aperture diameter, wavelength of light, and angle of incident light.

For verification of two-dimensional bar code symbols including two-dimensional multi-row bar code symbols and two-dimensional matrix symbols, grading is performed as per ISO/IEC15415 by use of an ISO/IEC compliant 2D barcode symbol verifier such as the Axicon 15500 (Axicon Auto ID Limited, Oxfordshire, UK). Each parameter receives a grade from 0 to 4 where 0 represents failure and 4 represents the highest quality. The parameter having the lowest grade becomes the overall grade for the 2D symbol. For 2-dimensional barcode symbols, the measurement may be taken using any combination of ISO/IEC recommended aperture diameter, wavelength of light, and angle of incident light.

All percentages are weight percentages based on the weight of the composition, unless otherwise specified. All ratios are weight ratios, unless specifically stated otherwise. All numeric ranges are inclusive of narrower ranges; delineated upper and lower range limits are interchangeable to create further ranges not explicitly delineated. The number of significant digits conveys neither limitation on the indicated amounts nor on the accuracy of the measurements. All measurements are understood to be made at about 25° C. and at ambient conditions, where “ambient conditions” means conditions under about one atmosphere pressure and at about 50% relative humidity.

The dimensions and values disclosed herein are not to be understood as being strictly limited to the exact numerical values recited. Instead, unless otherwise specified, each such dimension is intended to mean both the recited value and a functionally equivalent range surrounding that value. For example, a dimension disclosed as “40 mm” is intended to mean “about 40 mm.”

Every document cited herein, including any cross referenced or related patent or application and any patent application or patent to which this application claims priority or benefit thereof, is hereby incorporated herein by reference in its entirety unless expressly excluded or otherwise limited. The citation of any document is not an admission that it is prior art with respect to any invention disclosed or claimed herein or that it alone, or in any combination with any other reference or references, teaches, suggests or discloses any such invention. Further, to the extent that any meaning or definition of a term in this document conflicts with any meaning or definition of the same term in a document incorporated by reference, the meaning or definition assigned to that term in this document shall govern.

While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.

Claims

1. A sheet of material that is marked with a pulsed laser, the sheet of material comprising an outer edge and an inner edge separated by a core, the distance from the outer edge to the inner edge being the sheet thickness, wherein the sheet of material has a total concentration of TiO2 of less than about 1.00%, and there is an exterior layer beginning at the outer edge and extending into the core, the thickness of the exterior layer is from about 10.00% to about 40.00% of the sheet thickness, and wherein the exterior layer has an average concentration of from about 2.50% to about 10.00% TiO2.

2. The sheet of material according to claim 1, wherein the sheet of material is polymeric.

3. The sheet of material according to claim 1, wherein the laser marking on the sheet of material is a UPC, QR, Data matrix or other machine-readable code or symbol.

4. The sheet of material according to claim 3, wherein the machine-readable symbol is a linear Bar Code symbol and has an overall symbol grade of 1.5 or better based on verification according to the ISO/IEC15416 (2016).

5. The sheet of material according to claim 3, wherein the machine-readable code is a two dimensional bar code symbol and has a grade of greater than or equal to 1 based on verification according to ISO/IEC15415 (2011).

6. The sheet of material according to claim 1, wherein the sheet of material forms an article selected from the group consisting of a garbage bag, a bottle, a sachet, a tube, a film, a laminate, a bag, a wrap, a drum, a jar, a cup, or a cap.

7. The sheet of material according to claim 1, wherein the sheet of material is from about 10.0 microns to about 2.00 mm thick.

8. The sheet of material of claim 1, wherein the pulsed laser marking comprises a predetermined pattern of locations each comprising a mark or a void in a grid pattern: wherein the predetermined pattern comprises alphanumeric characters in the form of text having a font size within the range of 6 pt to 10 pt;wherein the grid pattern comprises a plurality of locations positioned in two or more rows, wherein the two or more rows are substantially parallel, each adjacent pair of locations of the plurality of locations along any of the two or more rows is separated by an X-distance and each adjacent pair of the two or more rows is separated by a Y-distance; and,wherein the Y-distance is at least 1.2 times the X-distance.

9. The sheet of material of claim 1, wherein the pulsed laser marking comprises a predetermined pattern of locations each comprising a mark or a void in a grid pattern: wherein the predetermined pattern comprises alphanumeric characters in the form of text having a font size within the range of 11 pt to 16 pt;wherein the grid pattern comprises a plurality of locations positioned in two or more rows, wherein the two or more rows are substantially parallel, each adjacent pair of locations of the plurality of locations along any of the two or more rows is separated by an X-distance and each adjacent pair of the two or more rows is separated by a Y-distance; and,wherein the Y-distance is at least 2 times the X-distance.

10. The sheet of material of claim 1, wherein in addition to the exterior layer there are two or more interior layers within the core with each interior layer having a different concentration of TiO2.

11. The sheet of material of claim 1, wherein the majority of the pulsed laser markings do not overlap adjacent markings.

12. The sheet of material of claim 1, wherein the laser marking pulsed laser comprises a predetermined pattern of locations each comprising a mark or a void in a grid pattern, and wherein the grid pattern comprises a plurality of locations positioned in two or more rows, wherein the two or more rows are substantially parallel, each adjacent pair of locations of the plurality of locations along any of the two or more rows is separated by an X-distance and each adjacent pair of the two or more rows is separated by a Y-distance; and, wherein the majority of the marks in the X-direction are non-overlapping.

13. The sheet of material of claim 1, wherein the laser marks comprise a predetermined pattern which is a line or curve; and wherein the spacing between adjacent laser marks varies within the line or curve.

14. The sheet of material of claim 1, wherein the laser marking pulsed laser comprises a predetermined pattern of locations each comprising a mark or a void in a grid pattern: wherein the grid pattern comprises a plurality of locations positioned in two or more rows, wherein the two or more rows are substantially parallel, each adjacent pair of locations of the plurality of locations along any of the two or more rows is separated by an X-distance and each adjacent pair of the two or more rows is separated by a Y-distance; and,wherein the Y-distance is equal to the X-distance.