WO2008002980A2

WO2008002980A2 - Laser engraveable flexographic printing article

Info

Publication number: WO2008002980A2
Application number: PCT/US2007/072246
Authority: WO
Inventors: Rustom S. Kanga
Original assignee: Stork Prints America, Inc.
Priority date: 2006-06-27
Filing date: 2007-06-27
Publication date: 2008-01-03
Also published as: WO2008002980A3

Abstract

The present invention provides a method of cross-linking a thermoplastic polyurethane (TPU) for making a directly laser engravable flexographic printing article. The printing article can be a flat printing plate or a continuous in-the-round printing sleeve. TPUs can be compounded in an extruder using various cross-linking and laser sensitive additives. The compounded TPU is extruded on a flat carrier or a round sleeve, cross-linked during or after extrusion using thermal energy or UV radiation. The extruded and cross-linked TPU is ground or machined to the dimension required for the printing process and is ready for laser engraving. The printing element is laser engraved using infrared lasers using computer generated graphics. After engraving a simple wipe-down is all that is required before the plate or sleeve is used on a flexographic printing press. Other binders such as thermoplastic polyester elastomer and thermoplastic polyamide elastomer can also be used.

Description

LASER ENGRAVABLE FLEXOGRAPHIC PRINTING ARTICLE

Technical Field of the Invention

[0001] This application claims priority to U.S. Provisional Application

No. 60/816,786, filed June 27, 2006. The invention relates to an article for use in flexographic printing, such as a plate or sleeve, and a method for laser engraving the printing article to form a relief such that the article can be used in flexographic printing. In one embodiment of the invention, the article does not require further processing, and as such can be used in a "direct-to-plate" laser engraving system.

Background of the Invention

[0002] Printing plates are well known for use in ffexographic printing, particularly on surfaces which are corrugated or smooth, such as packaging materials like cardboard, plastic films, etc. Typically, flexographic printing plates are manufactured using photopolymers which are exposed through a negative, processed using a solvent to remove the non-cross-linked areas to create a relief, which is post-cured and detackified. This is typically a very lengthy and involved process. Recently, flexographic plates have been manufactured using digital imaging of an in situ mask layer which obviates the need for a negative or a photomask to make the plate, and which has other performance benefits as well.

[0003] Recently, it has been possible to laser engrave a rubber element directly to provide the desired relief surface necessary for flexographic printing. Laser engraving has provided a wide variety of opportunities for rubber printing plates. Highly concentrated and controllable energy lasers can engrave very fine details in rubber. The relief of the printing plate can be varied in many ways. Very steep as well as gently decreasing relief slopes can be engraved so as to influence the dot gain of such plates. Ethylene propylene diene monomer (EPDM) rubber can be laser engraved to form flexographic printing plates.

[0004] The directly engraved type of flexographic printing plate is made from vulcanized rubber. Commercial rubbers can be natural or synthetic, such as EPDM elastomers. Lasers can develop sufficient power densities to ablate certain materials. Lasers such as high-power carbon dioxide (CO₂) lasers can ablate many materials such as wood, plastic and rubber and even metals and ceramics. Once the output from a laser is focused at a particular point on a substrate with a suitable power density, it is possible to remove material in depth to create a relief. Areas not struck by the laser beam are not removed. Thus, the use of the laser offers the potential of producing very intricate engravings in the proper material.

[0005] U.S. Pat. No. 3,459,733 to Caddell describes a method for producing polymer printing plates. The printing plate is made by exposing a layer of the polymeric material to a controlled laser beam of sufficient intensity to ablate the polymer and form depressions on the surface.

[0006] U.S. Pat. Nos. 5,798,202 and 5,804,353 to Cushner et al. disclose processes for making a flexographic printing plate by laser engraving a reinforced elastomeric layer on a flexible support. The process disclosed in U.S. Pat. No. 5,798,202 involves reinforcing and laser engraving a single- layer flexographic printing element comprised of a reinforced elastomeric layer on a flexible support. The elastomeric layer is reinforced mechanically, thermochemically, photochemically or combinations thereof. Mechanical reinforcement is provided by incorporating reinforcing agents, such as finely divided particulate material, into the elastomeric layer. Photochemical reinforcement is accomplished by incorporating photohardenable materials into the elastomeric layer and exposing the layer to actinic radiation. Photohardenable materials include photo-cross-linkable and photo- polymerizable systems having a photo-initiator or photo-initiator system.

[0007] The process disclosed in U.S. Pat. No. 5,804,353 is similar to

U.S. Pat. No. 5,798,202, except that the process involves reinforcing and laser engraving a multilayer flexographic printing element comprised of a reinforced elastomeric top layer, and an intermediate elastomeric layer on a flexible support. The elastomeric layer is reinforced mechanically, thermochemically, photochemically or combinations thereof. Mechanical and photochemical reinforcement is accomplished in the same manner as described by U.S. Pat. No. 5,798,202. The intermediate elastomeric layer may be reinforced as well.

[0008] A problem associated with the elastomeric elements which are reinforced both mechanically and photochemically is that laser engraving does not efficiently remove the elastomeric material to provide desired relief quality, and ultimately, printing quality. It is desirable to use an additive in the elastomeric layer which is sensitive to infrared light in order to enhance the engraving efficiency of the element. Photo-chemically reinforcing the element provides the desired properties for engraving as well as in its end-use as a printing plate. However, the presence of the additive as particulate or other absorbing material tends to reduce the penetration of the ultraviolet radiation required to photo-chemicaliy reinforce the element. If the elastomeric layer is insufficiently cured during photochemical reinforcement, the laser radiation cannot effectively remove the material and poor relief quality of the engraved area results. Further, the debris resulting from laser engraving tends to be tacky and is difficult to completely remove from the element. Additionally, if the element is not sufficiently photo-chemically reinforced, the required end- use properties as a printing plate are not achieved. These problems tend to be exacerbated with increasing concentration of the additive that enhances engraving efficacy.

[0009] U.S. Pat. No. 6,627,385 teaches the use of graft copolymers for laser engraving. U.S. Pat. No. 6,511 ,784, U.S. Pat. No. 6,737,216 and U.S. Pat. No. 6,935,236 teach the use of elastomeric copolymers for laser engraving using various infrared (IR) additives.

[0010] Many patents in the field teach the use of typical styrenic thermoplastic elastomers (TPEs) that have been used for photo-curing applications. One problem associated with these non-polar TPEs is that they have limited sensitivity to laser engraving because of their hydrocarbon backbone nature. The use of polar TPEs such as thermoplastic polyurethanes (TPUs) thermoplastic polyester elastomers (TPPE) and thermoplastic polyamide elastomers (TPAE) as both laser engravable systems and as printing elements would be desirable. However, most of the above polar TPEs on the market would not be effective either as laser engravable systems, or as printing plates because they are not crosslinked. [0011] The cross-linking of the above TPEs and especially TPUs has not been done before in flexography, and thus, TPUs have not been used in flexography. However, polyurethanes for flexography have been well known, particularly for liquid photopolymers. By definition, a TPU is solid at room temperature and can be extruded and workable at higher temperatures. This characteristic is due to the presence of hard and soft segments, which form a network at room temperature, and is thus a solid. This network structure also differentiates TPUs from traditional polyurethanes in its outstanding physical attributes and thus offers an attractive system to be used in flexo applications. However, most elastomers used in Flexo need to be cross-linked to withstand the rigors of the printing process and to minimize swells in the inks used for printing. Additionally, the elastomers used in laser engraving have to be cross-linked. Traditional flexo photopolymers have unsaturation in the backbone, which allows the cross-linking with acrylate monomers and UV photo-initiators. The TPUs on the market today do not have unsaturation. Hence, the difficulty in UV curing these for flexo applications. The issue is to find a reliable way to make a plate from these.

Summary of the Invention

[0012] Therefore, an object of the present invention is to provide a method for making a laser engravable flexographic printing article.

[0013] Another object of the present invention is to provide a reliable method for making a printing plate from thermoplastic polyurethanes (TPUs). [0014] These and other objects of the present invention can be achieved in the preferred embodiments of the invention described below. One preferred embodiment of the invention includes a method for making a flexographic printing article including the steps of providing a thermoplastic polyurethane, and cross-linking the thermoplastic polyurethane whereby the article can be used in a direct laser engraving flexographic process.

[0015] According to another preferred embodiment of the invention, the cross-linked thermoplastic polyurethane can be used in the direct laser engraving flexographic process and in flexographic printing without further processing.

[0016] According to another preferred embodiment of the invention, the printing article is laser engraved by infrared laser radiation to form a relief such that the article can be used in flexographic printing.

[0017] According to another preferred embodiment of the invention, the printing article can be a plate or a sleeve.

[0018] According to another preferred embodiment of the invention, the binder is a high performance polyester-based thermoplastic polyurethane.

[0019] According to another preferred embodiment of the invention, the thermoplastic polyurethane is extruded and thermally cross-linked during extrusion.

[0020] According to another preferred embodiment of the invention, at least one cross-linking additive for inducing the cross-linking of the thermoplastic polyurethane is provided. [0021] According to another preferred embodiment of the invention, at least one laser additive comprising such as kaolin clay, mica, antimony tin oxide, or copper oxide is provided.

[0022] According to another preferred embodiment of the invention, the thermoplastic polyurethane is thermally cross-linked after extrusion.

[0023] According to another preferred embodiment of the invention, the thermoplastic polyurethane is cured for about 8 to 12 hours at about 180- 240⁰F, and the thermoplastic polyurethane is cross-linked during the curing.

[0024] According to another preferred embodiment of the invention, the thermoplastic polyurethane is cross-linked during curing with ultraviolet radiation.

[0025] According to another preferred embodiment of the invention, reactive plasticizers, photo-initiators, monomers and/or oligomers are provided.

[0026] According to another preferred embodiment of the invention, the thermoplastic polyurethane is cross-linked during curing with electron beam radiation.

[0027] According to another preferred embodiment of the invention, the ultraviolet curable polyurethane dispersion includes urethane acrylates, photo- initiators and/or laser additives.

[0028] According to another preferred embodiment of the invention, the printing article is grounded to a desired dimension.

[0029] According to another preferred embodiment of the invention, the printing article is machined to a desired dimension. [0030] According to another preferred embodiment of the invention, the binder is an unplasticized polyethylene terephthalate based thermoplastic polyurethane / rubber alloy.

[0031] According to another preferred embodiment of the invention, at least one heat dissipation additive such as metal based nanoparticles and/or metal oxide based nanoparticles are provided.

[0032] According to another preferred embodiment of the invention, at least one additive for dissipating heat such as silver, copper, indium-tin-oxide and/or copper oxide is provided.

[0033] According to another preferred embodiment of the invention, microspheres for decreasing the density of the thermoplastic polyurethane and increasing the rate of mass transfer during laser engraving of the article are provided.

[0034] According to another preferred embodiment of the invention, a cross-linking additive such as a hexamethylene diisocyanate prepolymer is provided.

[0035] According to another preferred embodiment of the invention, a method for laser engraving a flexographic printing article includes the steps of providing a thermoplastic polyurethane, cross-linking the thermoplastic polyurethane to form a laser engravable article, and laser engraving the article to form a relief such that the article can be used in flexographic printing.

[0036] According to another preferred embodiment of the invention, the article is engraved with an infrared radiation laser, such as a carbon dioxide laser, a yttrium aluminum garnet laser and/or a diode array laser. [0037] According to another preferred embodiment of the invention, a method for making a flexographic printing article includes the steps of providing a binder such as a thermoplastic polyester elastomer or a thermoplastic polyamide elastomer, and cross-linking the thermoplastic polyurethane such that the article can be used in a direct laser engraving flexographic process and in flexographic printing without further processing.

Detailed Description of the Preferred Embodiments and Best Mode

[0038] According to a preferred embodiment of the invention, laser engraving can provide a true "direct-to-plate" technology for flexography. It is simple to apply and practice without the need for complicated processing steps during manufacturing. There is a substantial gain in productivity from laser engraving. Also, the plates are relatively inexpensive to manufacture obviating the need for a sophisticated mask coating, as needed for digitally imaged plates. Recently, there has been a decrease in flexo reliefs with the use of thin plates (~30 mil) becoming more common. This trend is very attractive and well-suited for the laser engraving of flexo plates.

[0039] However, for laser engraving plates in the market thus far, the image fidelity is not as good as current digitally imaged (laser ablation) or even conventional flexo plates. This relegates laser engraving to a niche market. Additionally, the productivity so far has not been good. Thus, there is a market need to improve both the two main deficiencies of engraving, compared with mask ablation- image quality and plate making productivity. [0040] A laser engraving article according to a preferred embodiment of the invention comprises a flat engravable plate which is mounted on a round cylinder during the printing step, or a continuous in the round engravable sleeve. Either system comprises a carrier on which there may be one or more binder layers which are laser engravable.

[0041] The carrier for the laser engraving article depends on the end product. For the flat plates a heat stabilized polyethylene terephthalate (PET) of 5-7 mils thickness is preferred. The PET may be corona treated to improve adhesion, and may also be primer and adhesive coated.

[0042] For the sleeves the carrier may be a metal sleeve, typically nickel based or a composite sleeve. The sleeve is further primer and/or adhesive coated for improved adhesion. Often, the sleeve is further coated with a polyurethane foam which acts as the in situ cushion layer.

[0043] The choice of binder system for the engraving system is governed by a combination of its performance as a printing plate and sensitivity to or behavior in laser engraving. It is believed that a cross-linked thermoplastic elastomer would provide the best performance attribute both for its printing performance and as an engravable system.

[0044] A soft, high performance polyester-based thermoplastic polyurethane such as IROGRAN® A 6OE 4902DP sold by Huntsman is an excellent binder system with regard to its physical attributes as a Flexo printing plate. Other high performance polyester-based thermoplastics polyurethanes that can be used are IROGRAN® A 70 RB 1000 and IROGRAN® A 60 RE 1000 from Huntsman. The following physical properties seen in the attached spreadsheet was seen for the 4902DP (virgin polymer) compared to the green photopolymer (UV cured).

[0045] 4902DP is a preferred binder system, because it is a polyester based TPU with high chemical resistance and low hydrolytic stability, demonstrated the best solvent resistance of the 3 TPUs evaluated, and UV curing made an even better improvement in solvent resistance, does not have plasticizer, and uncured swell results indicate that it will function adequately as a printing plate.

[0046] Other resin systems that may be used advantageously in this application are the thermoplastic polyester elastomers and thermoplastic polyamide elastomers. Examples of these polymers are the soft ester type thermoplastic polyester elastomers called Keyflex from LG Chemicals and the PEBAX polyether block amides from Arkema. A preferred printing plate/sleeve has the approximate physical properties as provided in Table I below. These physical properties can serve as a guideline for a system behaving as a printing plate. Other attributes such as ink transfer are not reflected here. It is possible that systems having physical characteristics outside these parameters may also behave as a satisfactory printing plate. Many of these properties are interlinked. Thus, a high Shore A, implies a high Modulus by nature. These physical characteristics can be easily measured on an lnstron and may be a good starting point to consider when designing a laser engravable printing plate or sleeve.

[0047] Another characteristic to consider is compatibility with inks.

Thus, it is entirely possible to have more than one polymer system depending on its end use. Table I: Typical Photopolymer Plate Physicals

(After Cure)

Elongation, % 200-1000

Tensile (Break), PSI 500-5000

Modulus, PSI 250-2500

Shore A⁰ 50-70

Resilience, % 45-60

Swells, Wt% {After Cure)

Water <1 n-Propyl Alcohol <1

NPA/NPAcetate <10

UV Ink <5

[0048] For the binder to be efficient in laser engraving, the main chain needs to have labile hetero bonds which have sensitivity at 10,600 NM, and convert laser photons to heat efficiently and is removed when exposed to a laser beam of adequate intensity. The layer is preferably evaporated, or thermally or oxidatively decomposed in the process without melting, so that its decomposition products are removed from the layer in the form of hot gases, vapors, fumes or small debris particles.

[0049] In general, thermoplastic elastomers (TPEs) based on Kraton polymers currently used in typical printing plates and other carbon based polymers such as polyolefins will not be efficient as engravable binders. Hydrophilic polymers mentioned before such aspolyurethanes, polyesterspolyamides, polyvinylalcohol etc should function adequately as an engravable system. In particular, thermoplastic polyurethanes (TPUs) are particularly suitable as laser engravable sytems. However, the TPUs need to be cross-linked before they can adequately function both as a printing plate and as an engravable system.

[0050] The following are several methods of cross-linking of thermoplastic polyurethanes (TPUs) and other desirable systems such as thermoplastic polyester elastomers (TPPE) and thermoplastic polyamide elastomers (TPAE) to be used in a laser engraving system for flexography.

Thermal Cross-linking of TPUs during the Extrusion Step

[0051] One preferred method of cross-linking TPUs takes advantage of

cross-linking and chain extending of the TPU during the extrusion. The TPU can be pre-blended with the chain extending and cross-linking additive before the extrusion (single screw extrusion) or blended in the twin screw extruder as a separate stream. Other additives described below may also be similarly compounded in simultaneously. For flat plates the extrusion is done directly on a PET carrier which may have an adhesive layer. For printing sleeves the extrusion is carried out directly on the sleeve, which may have a primer or which may also have an additional cushion layer. After extrusion there may be a short post-bake step necessary. The article is then ground or machined to achieve the thickness dimensions required and to also smoothen the printing surface. It is then ready for laser engraving of the computer generated graphics. After laser engraving the plate or sleeve is ready for printing after a simple wipe down.

[0052] Commercially available TPUs from companies such as

Huntsman, BASF, Bayer and Dow in the range of the physical properties mentioned in Table I can be used. The chain extending and cross-linking additives can be obtained from companies such as Johnson Polymer and Clariant.

[0053] The hydroxy and epoxy functional chain extenders react with residual hydroxy and carboxyl groups of the TPUs created during the extrusion process. TPUs show low levels of depolymerization/ degradation during extrusion to create residual isocyanate and hydroxy groups. The chain extenders react with these groups to increase the MW and in some cases when these are multifunctional, to create a 3-dimensional cross-linked network.

[0054] The thermal curing scheme includes reactively extruding and cross-linking the TPU during the extrusion step. Applicant has investigated three TPUs in this regard: Huntsman's lrogran 4902DP, Bayer's Desmopan 6065A and, Sartomer's 3027 TPU. Cross-linking is induced using 3 different cross-linking additives, ail by Johnson Polymers (now BASF). Once extrusion was started, the cross-linking was monitored by noting the torque- higher cross-linking results in a higher viscosity and corresponding increase in the torque registered by the TSE.

[0055] Once the cross-linking additives were compounded, laser additives can be added. The laser additives can be based on Kaolin Clay or Mica. The silicates are known IR absorbers and help increase the sensitivity of the TPU to the CO₂ laser at 10,600 NM. Another additive is Antimony Tin Oxide (ATO) called "Mark-It" from Engelhard. ATO increases the sensitivity of polymers in the near IR regime typical of yttrium aluminum garnet (YAG) lasers (1060 NM) and Diode Array (830 NM). Finally, a combination of the above with nano copper oxide is formulated. Copper Oxide in the nano range increases the thermal conductivity of the polymer and acts as a heat dissipating additive. All of the additives are physically mixed.

Thermal Cross-linking of TPUs after the Extrusion Step [0056] In another cross-linking method, the TPU is compounded with the additive before or during the extrusion step. Other additives described below may also be similarly compounded in simultaneously. For flat plates the extrusion is done directly on a PET carrier which can have an adhesive layer. For printing sleeves the extrusion is carried out directly on the sleeve which may have a primer or which may also have an additional cushion layer. The cross-linking is achieved after extrusion in an extended post-cure step (8- 12 hours at 180-240⁰F). The article is then ground or machined to achieve the thickness dimensions required and to also smoothen the printing surface. It is then ready for laser engraving of the computer generated graphics. After laser engraving the plate or sleeve is ready for printing after a simple wipe down.

[0057] Commercially available TPUs from companies such as

Huntsman, BASF, Bayer and Dow can be used. The cross-linking additives which create cross-linking of TPUs post-extrusion can be obtained from PriPro Polymers.

Cross-linking of TPUs by UV Radiation Cure Mechanisms [0058] Unlike thermal cross-linking mentioned above, UV or EB curing of TPUs may have significant advantages. It is more stable during the extrusion step and obviates the need for prolonged post-cure steps. The Ultraviolet (UV) curing can be achieved during the extrusion using a traversing high intensity UV bulb. UV curing on sleeves can also be achieved post extrusion in typical clam-shell UV curing units, which are known in the industry. UV curing on plates is advantageously achieved right after extrusion using a series of bulbs. Electron beam (EB) curing of these TPUs is also possible.

[0059] Two different mechanisms can be used for UV cross-linking of

TPUs. The first types are TPUs having polybutadiene soft segments available from Sartomer. These are further compounded with reactive plasticizers, photo-initiators, and if needed monomers and oligomers also available from Sartomer. Other additives such as laser sensitive systems can also be compounded in at this point.

[0060] The second types of UV cross-linkable TPU were made by incorporating the cross-linking moiety in the main polymer chain during the extrusion step. Here, a dual functional monomer is added during extrusion. The additive gets incorporated in the TPU by reaction of the hydroxy or other functional groups with the isocyanate group created during the thermal depoiymerization of the TPU that usually happens during extrusion. This modified TPU is also simultaneously compounded with other additives such as photo-initiators, plasticizers and laser sensitive additives. After the extrusion step the article is cross-linked using UV cross-linking, and ground or machined to the dimensions. It is then ready for laser engraving of the computer generated graphics. After laser engraving the plate or sleeve is ready for printing after a simple wipe down. [0061] Commercially available TPUs from companies such as Huntsman, BASF, Bayer and Dow can be used. The cross-linking additives can be obtained from Sartomer.

[0062] In another embodiment of the invention, the liquids in UV1

(Sartomer's 3027 TPLJ) and UV2 (Huntsman TPU) formulations are blended together with the solid additive. The TPU is then added to this slurry and further mixed by hand to get a semi-uniform blend. Bayer's Desmopan TPU can be used to get the process conditions, such as temperature, etc., for a smooth extrudate. The output is approximately 10 Ibs/hr. These two extrudate films are wrapped between wax paper, and cured in the A&V unit for 15 minutes total (7.5 minutes on each side). The films physically appear to be cross-linked (tougher and lower elongation).

[0063] As such, the blending of monomers, photo-initiators, plasticizers and other additives may be possible to further cross-link and modify the TPU to be used in laser engraving. The cross-linking may be possible during the extrusion itself using a traversing UV cure unit immediately after extrusion. To ensure complete cure multiple passes of a thinner extrudate or multiple passes of the traversing UV unit may be required.

UV Curable Polvurethane Dispersions

[0064] Another method of TPU cross-linking utilizes UV curable polyurethane dispersions. UV curable polyurethane dispersions (UV PUDs) are well known in the wood industry. The UV PUDs are further formulated with various urethane acrylates to enhance the physical properties. Additives for cross-linking, such as photo-initiators and laser additives, can also be further formulated in before coating.

[0065] For flat plates, the polyurethane dispersions (PUDs) are cast on a carrier belt, dried in ovens and cured from the back using banks of UV lights. For sleeves, the PUDs are coated using various coating methods known in the industry for endless coating of sleeves such as roller coating, ring coating or spray coating. The PUD is coated directly on a bare sleeve or may be coated on a sleeve backed with a foamed cushion. The sleeve can be dried using a hot air-flow or in ovens. Several cycles of coating may be applied to build the relief.

[0066] After the required dimension is achieved, UV curing on sleeves can be achieved post extrusion in typical clam-shell UV curing units known in the industry. The articles are then ready for laser engraving of the computer generated graphics. After laser engraving the plate or sleeve is ready for printing after a simple wipe down.

2-Part Cross-linked Polvurethanes

[0067] 2-Part (2K) cross-linked polyurethanes can also be used as the laser engraving system for this application. Here the polyol and isocyanate components are mixed right before application. There are various methods of application such as spray coating, roller coating, and castable elastomers. Since this is a surface critical printing application the spray systems may have entrained air which will come out as voids after grinding/machining to achieve the final thickness and to smooth the print surface. Thus, roller coating of the admixed 2K system may be preferable. The physical properties of the final PU can be adjusted by suitable adjustment of the choice of the polyol and the polyol to isocyanate ratio. Other additives to enhance physical properties and to increase sensitivity in the laser can be added to the polyol component. [0068] A number of these 2K PU systems are available from companies such as Bayer and Dow. The polyol is initially compounded with the additives. For spray coating, the 2 separate components are loaded in a spray gun, admixed before spraying and sprayed directly onto the carrier of choice. For roller coating the components are mixed right before coating. The cross-linking of polyols with isocyanate, chain extension and polyurethane formation occurs in times varying from seconds to minutes. There may be a bake step required to push the reaction to completion. After polymer formation, the article can be ground or machined to the dimension required. It is then ready for laser engraving of the computer generated graphics. After laser engraving the plate or sleeve is ready for printing after a simple wipe down.

Higher Vicat Systems

[0069] Increasing the Vicat softening point for the TPU is expected to resolve some of the melting effects seen in typical laser engraving systems. One way to improve on the Vicat softening point is to cross-link the polymer. Another way would be to use a polymer with higher molecular weight having different and better thermal properties, such as lrogran A 70 RB1000 sold by Huntsman, lrogran A 70 RB1000 is an un-plasticized PET based TPU/Rubber alloy and has a Shore A 70-74. Other higher softening systems are also available from BASF and Bayer.

Additives

[0070] Most of the TPUs need to be further modified or compounded to be functional as a laser engraving system. The choice of additives will be dependent on the proposed effect. Additives can be classified under the following categories.

Cross-linking Additives

[0071] A number of additives are well known. Based on whether cross- linking will be thermal or photochemical, suitable cross-linkers or photoinitiators can be chosen. Likewise, choice of monomer or oligomer to create Interpenetrating Networks (IPNs) is well known and choice of these will depend on the final physical property desired. Other additives in the cross- linking package include plasticizers, antioxidants, processing aids, and surface energy modifiers.

[0072] Thermal cross-linking systems used during the extrusion step are available from Johnson Polymer, ReactAmine and Clariant Corporation. Systems for post-extrusion thermal cross-linking are available from PriPro Polymer Inc. UV cross-linkable additives are well known in the art and available from Sartomer, Ciba, and Lamberti. Cross-linking additives made from hexamethylene diisocyanate (HDI) prepolymers, such as is available from ReactAmine, have free NCO groups which tend to cross-link with the residual OH and other labile groups during extrusion and after.

Additives to Increase Laser Sensitivity

[0073] Additives to increase laser sensitivity increase the absorbttvity of the polymer at the lasing wavelength (10,600 NM). There are two areas that can be used as a resource for laser additives: Laser Marking and Solar Absorbing Glass used in automotive and greenhouse applications. Both of these use a strong IR absorber additive which acts to convert IR photons to heat. Since many of these additives are nanomaterials, uniformly and molecularly dispersing these in the binder of choice presents a challenge. Laser masterbatches are available for ease of incorporation in the binder system. There are additives that are selective for both CO₂ (10,600 NM) and some for Nd-YAG (1060 NM). The following graph illustrates this concept. These additives are available from a number of sources depending on the IR regime, such as Engelhard, Sumitomo Metal Mining and Clariant amongst others.

Wavelength/nm

[0074] The mica additives are well known but may need further optimization. Other conductive additives that can be used are carbon black or graphite although they may create other aesthetic problems. Thermal dissipative additives may also be used. All of these additives can be explored using a batch high shear polymer mixer.

Additives for Heat Dissipation

[0075] The incorporation of certain additives for charge dissipation in coating systems, films or composites can reduce the buildup of static charge. Typically these are used in the electronics industry to avoid destructive discharges that can harm electronic components or, in hazardous operations, where it may act as an ignition source. In addition, these conductive additives have also been used in films used to produce conductive display screens, such as for interactive touch screens, eliminating the need to use expensive sputtering technologies. Some of these additives can also be incorporated in our engraving polymer systems to dissipate the heat buildup in these TPUs which are known poor conductors of heat. This may help reduce the problem of polymer melting and increase the efficacy of laser engraving.

[0076] The most promising additives for heat dissipation during engraving are available from companies such as Nanophase Technologies. Nanoparticles based on metals, such as silver and copper, can be used as heat dissipators. Nanoparticles based on metal oxides such as Indium-Tin- Oxide and copper oxide, have shown high propensity of heat dissipation when used in small amounts. Nano copper oxide is the most cost effective in this application.

Microspheres

[0077] Since laser engraving is a mass transfer phenomenon, it is believed that if the bulk density of the polymer is reduced without affecting the integrity of other physical attributes, it would go a long way towards increasing the productivity in laser engraving of the printing plate which is a current shortcoming in laser engraving systems. The extreme case is of course the difference in engraving sensitivity and power requirements of steel versus a rubber, all else being constant. [0078] Microspheres decrease the density of the polymer and increase the rate of mass transfer during laser engraving. There are various microspheres, but the most promising are the unexpanded microspheres and cross-linked nanospheres. The former has liquid hydrocarbon encapsulated in a thermoplastic polymer shell, which expands during the extrusion process causing a drop in bulk density from ~1.0 to -0.2. The following graph indicates theoretically the concept of balancing the physical properties and laser sensitivity (productivity) which run counter to each other:

Density ^•

Lasina Wavelength

[0079] Much of the efforts in the industry have been focused on CO₂ lasers. CO₂ lasers typically have a spot size of around 40 μm. Thus, it is difficult to achieve image fidelity higher than 100-125 LPI. The advantage is that the lasing wavelength (10,600 NM) allows a wide use of elastomers due to their absorbtivity. YAG lasers (1060 NM) have a significantly lower spot size (<20 μm) allowing 125-175 LPI. The problem is that the lasing wavelength (1060 NM) makes choice of a binder difficult since not all binders absorb at that wavelength. Additionally, YAG lasers do not have adequate

power for engraving so productivity is not good. This shortcoming can be overcome by a judicious choice of binder and additive. Diode array lasers in

the near IR (830 NM) are also available and increasing in power capacity.

Examples 1A-1J- Laser engraving of a Cross-linked TPU (during the extrusion)

[0080] Examples 1 A-1 H (see Table 2) teach the use of a cross-linked

TPU in laser engraving. The cross-linking was carried out during the extrusion step. The TPU from Huntsman or Bayer was compounded with the

additives and extruded in a TSE (or single screw) keeping manufacturer recommended extrusion temperatures. For a flat plate the extrusion was

done on a PET which was previously coated with a primer. For a sleeve, strip extrusion was carried out on the primer coated nickel and composite sleeves. The extruded sleeve was allowed to reach room temperature and then machined on a grinder to the dimensions required. The article was then laser engraved on a CO₂ laser commonly available in the market. The laser engraved article was then ready for printing.

[0081] Examples 11-1J (see Table 2) teach the use of a cross-linked

TPU in laser engraving using either a YAG (1060 NM) or a Diode Array (830 NM) laser.

Example 2- Laser engraving of a Cross-linked TPU (post- extrusion)

[0082] Example 2 teaches the use of a cross-linked TPU in laser engraving. The cross-linking is carried out after the extrusion step. The TPU from Huntsman or Bayer was compounded with the additives from PriPro Polymers and extruded in a TSE (or single screw) keeping manufacturer recommended extrusion temperatures. For a flat plate the extrusion is done on a PET which is previously coated with a primer. For a sleeve, strip extrusion is carried out on the primer coated nickel and composite sleeves. The cross-linking is achieved after extrusion in an extended post-cure step (8- 12 hours at 180-240⁰F). The article is then ground or machined on a grinder to the dimensions required. The article is then laser engraved on a CO2 laser commonly available in the market. The laser engraved article is then ready for printing. Laser additives similar to Example 1 are used for all 3 different types of lasers mentioned before.

Example 3- Laser Engraving of a UV Cross-linked TPU

[0083] Example 3A in Table 3 teaches the use of a cross-linked TPU in laser engraving. The UV cross-linking was carried out after the extrusion step. The TPU from Sartomer was compounded with additives as shown in Example 3A. The TPU from Huntsman was compounded with additives as shown in Example 3B-3C in Table 3. For a flat plate the extrusion was done on a PET which was previously coated with a primer. For a sleeve, strip extrusion was carried out on the primer coated nickel and composite sleeves. The cross-linking is achieved after extrusion by UV curing. For flat plates, a bed of UV lamps cured the extruded plate. For the sleeve, traversing sets of UV bulbs were used on the extruder to UV cure the strip as it comes out on the sleeve. Alternately, a clamshell UV unit common in the sleeve market can be employed. After curing, the article is then ground or machined on a grinder to the dimensions required. The article is then laser engraved on a CO₂ laser. The laser engraved article is then ready for printing. Examples 3D-3F in Table 3 teach use of a cross-linked TPU in laser engraving using either a YAG (1060 NM) or a Diode Array (830 NM) laser.

Examples 4- Laser engraving of a UV Cured PUD

[0084] Examples 4A-4D in Table 4 teach the use of a UV curable PUD to be used in a laser engraving system. The formulation is applied on the carriers using various methods. For flat plates, the PUDs are cast on a carrier belt, dried in ovens and cured from the back using banks of UV lights. Several passes are applied one on top of the other. For sleeves, the PUDs are coated using various coating methods known in the industry for endless coating of sleeves such as roller coating, ring coating or spray coating. Here, also several coating passes are applied to build the relief. After coating, the sleeve is subjected to a flood exposure in typical clamshell UV exposure units known in the industry. The article was then laser engraved on a CO₂ laser commonly available in the market. The laser engraved article was then ready for printing. Examples 4E-4F in Table 4 teach use of a cross-linked TPU in laser engraving using either a YAG (1060 NM) or a Diode Array (830 NM) laser.

Example 5- Laser Engraving of a 2-Part Cross-linked Polvurethane System

[0085] Example 5 teaches the use of a 2-Part (2K) cross-linked polyurethane to be used in a laser engraving application. The polyol and isocyanate components are mixed immediately before application. There are various methods of application, including spray coating and roller coating. This is a surface critical printing application. The spray systems may have entrained air which will come out as voids after grinding/machining. Thus, roller coating of the admixed 2K system may be preferable. The physical properties of the final PU can be adjusted by suitable adjustment of the choice

of the polyol and the polyol to isocyanate ratio. Other additives to enhance physical properties and to increase sensitivity in the laser can be added to the

polyol component. For this Example 5 Bayer's Baytec SPR 066A is used. After coating either on the flat PET carrier or round sleeve, the coating may be

subjected to an intermediate bake step.

[0086] Laser additives similar to Example 1 are used for all three different types of lasers mentioned above. The additives are dispersed in the

polyol component of the 2K system.

Example 6- Laser engraving of a Cross-linked TPU System modified with Microspheres

[0087] Examples 6A-B (see Table 5) teach use of a cross-linked TPU modified with microspheres in laser engraving. The cross-linking and polymer compounding were carried out during the extrusion step. The TPU from Huntsman or Bayer was compounded with the additives including a cross- linked nanospheres available from Sekisui having particle size 80-300 NM and extruded in a TSE (or single screw) keeping manufacturer recommended extrusion temperatures. For a flat plate the extrusion was done on a PET which was previously coated with a primer. For a sleeve, strip extrusion was carried out on the primer coated nickel and composite sleeves. The extruded

sleeve was allowed to reach room temperature and then machined on a grinder to the dimensions required. The article is then laser engraved on a CO₂ laser commonly available in the market. The laser engraved article is

then ready for printing. [0088] Example 6-C (see Table 5) teaches use of a cross-linked TPU in laser engraving using either a YAG (1060 NM) or a Diode Array (830 NM) laser. The laser engraving productivity and quality in above Example 6 TPUs modified with microspheres is much improved, without significant loss in physical properties of the printing plate.

PHR stands for parts per hundred.

Table 6: Solvent Swells for TPUs

[0089] A laser engravable flexographic printing article and methods for making and using same are described above. Various details of the invention may be changed without departing from its scope. Furthermore, the foregoing description of the preferred embodiments of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation- the invention being defined by the claims.

Claims

What is claimed is:

1. A method for making a flexographic printing article comprising the steps of:

(a) providing a thermoplastic polyurethane; and

(b) cross-linking the thermoplastic polyurethane whereby the article can be used in a direct laser engraving flexographic process.

2. A method according to claim 1, wherein the cross-linked thermoplastic

polyurethane can be used in the direct laser engraving flexographic process and in flexographic printing without further processing.

3. A method according to claim 1 , further comprising the step of laser

engraving the printing article by infrared laser radiation to form a relief whereby the article can be used in flexographic printing.

4. A method according to claim 1 , wherein the article is selected from the

group consisting of a plate and a sleeve.

5. A method according to claim 1 , wherein the step of providing a

thermoplastic polyurethane comprises providing a high performance polyester-based thermoplastic polyurethane.

6. A method according to claim 1 , further comprising the step of extruding the thermoplastic polyurethane, and further wherein the thermoplastic polyurethane is thermally cross-linked during extrusion.

7. A method according to claim 1 , further comprising the step of providing at least one cross-linking additive for inducing the cross-linking of the thermoplastic polyurethane.

8. A method according to claim 7, further comprising the step of providing at least one laser additive Comprising one or more selected from the group consisting of kaolin clay, mica, antimony tin oxide, and copper oxide.

9. A method according to claim 1 , further comprising the step of extruding the thermoplastic polyurethane, and further wherein the thermoplastic polyurethane is thermally cross-linked after extrusion.

10. A method according to claim 9, further comprising the step of curing the thermoplastic polyurethane for about 8 to 12 hours at about 180-240⁰F, wherein the thermopiastic polyurethane is cross-iinked during the curing.

11. A method according to claim 1 , wherein the step of cross-linking the thermoplastic polyurethane comprises curing the thermoplastic polyurethane with ultraviolet radiation.

12. A method according to 11 , further comprising the step of adding one or more selected from the group consisting of reactive plasticizers, photo- initiators, monomers and oligomers.

13. A method according to claim 1 , wherein the step of cross-linking the thermoplastic polyurethane comprises curing the thermoplastic polyurethane with electron beam radiation.

14. A method according to claim 1 , wherein the curable thermoplastic polyurethane is obtained with an ultraviolet curable polyurethane dispersion.

15. A method according to claim 14, wherein the ultraviolet curable polyurethane dispersion includes one or more selected from the group consisting of urethane acrylates, photo-initiators and laser additives.

16. A method according to claim 1 , further comprising the step of grinding the article to a desired dimension.

17. A method according to claim 1 , further comprising the step of machining the article to a desired dimension.

18. A method according to claim 1 , wherein the step of providing a thermoplastic polyurethane comprises providing an unplasticized polyethylene terephthalate based thermoplastic polyurethane / rubber alloy.

19. A method according to claim 1 , further comprising the step of providing at least one heat dissipation additive selected from the group consisting of metal based nanoparticles and metal oxide based nanoparticles.

20. A method according to claim 1 , further comprising the step of providing at least one additive for dissipating heat selected from the group consisting of silver, copper, indium-tin-oxide and copper oxide.

21. A method according to claim 1 , further comprising the step of providing microspheres for decreasing the density of the thermoplastic polyurethane and increasing the rate of mass transfer during laser engraving of the article.

22. A method according to claim 1 , further comprising the step of providing a cross-linking additive comprising a hexamethylene diisocyanate prepolymer.

23. A flexographic printing article made according to the method of claim 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 , 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 or 22.

24. A method for laser engraving a flexographic printing article comprising the steps of:

(a) providing a thermoplastic polyurethane;

(b) cross-linking the thermoplastic polyurethane to form a laser engravable article; and

(c) laser engraving the article to form a relief whereby the article can be used in flexographic printing.

25. A method according to claim 24, wherein the step of laser engraving the article comprises engraving the article with an infrared radiation laser.

26. A method according to claim 23, wherein the step of laser engraving the article comprises engraving the article with at least one selected from the group consisting of a carbon dioxide laser, a yttrium aluminum garnet laser and a diode array laser.

27. A method for making a flexographic printing article comprising the steps of:

(a) providing a binder selected from the group consisting of a thermoplastic polyester elastomer and a thermoplastic polyamide elastomer; and

(b) cross-linking the thermoplastic polyurethane whereby the article can be used in a direct laser engraving flexographic process and in flexographic printing without further processing.