The present invention concerns a copper-containing thermoplastic polyurethane (TPU) and an additive manufacturing process employing a copper-containing thermoplastic polymer, especially a TPU, as a build material.

The labeling of livestock with thermoplastic polyurethane ear marks or tags is widely practiced. Identifying information can be inscribed into the TPU by charring with a laser. For example, WO 2011/083100 A1 discloses a transparent polyurethane which can be written on by means of high-energy radiation and which contains a bismuth oxide as a contrast medium. The bismuth oxide has an average grain size of <1.5 pm. Preferably this polyurethane is a TPU.

WO 2018/150033 A1 relates to a laser-markable plastic comprising a thermoplastic polymer, bismuth oxide and a co-absorbing additive selected from the group consisting of platelet shaped silicates and inorganic copper-, cobalt-, aluminum or iron-containing pigments, wherein the amount of the co-absorbing additive relative to the bismuth oxide is from 2 to 80 wt.-%. The experimental section discloses the use of a copper hydroxy phosphate with the trademark name Fabulase® 361 as a co-absorbing additive.

US 2008/004363 A1 discloses laser-weldable polymers which consist of a laser-transparent part and a laser-absorbent part and can be welded to one another by means of laser light and are distinguished by the fact that the laser-absorbent part comprises, as absorber, copper hydroxide phosphate and/or copper phosphate. According to a dependent claim the polymer can be a thermoplastic polyurethane. This published patent application states that the absorber is preferably added to the laser-transparent polymer part in amounts of 0.001-2% by weight, in particular 0.01-1% by weight and very particularly preferably 0.05-0.5% by weight, based on the polymer part.

US 5,630,979 is directed towards a process for the inscription of moldings based on thermoplastic polyurethane elastomers or mixtures of thermoplastic polyurethane elastomers with up to 45% by weight based on the total weight of polymers, of further thermoplastics, by means of high-energy radiation, wherein the additives employed to improve the inscribability include a copper phosphate and an inorganic phyllosilicate. The amount of copper phosphate is stated to not be subject to any particular limitation but to be generally in the range from 0.02 to 5% by weight, preferably from 0.05 to 0.5% by weight, in particular from 0.05 to 0.18% by weight, based on the total weight of polymer.

Selective laser sintering (SLS) and fused deposition modeling (FDM, also FFF) processes are established additive manufacturing (“3D printing”) methods. Thermoplastic polyurethanes have also been reported as build materials, for example in US 2017/129177 A1 (use of thermoplastic polyurethane powders in powder-based additive manufacturing methods for the production of elastic articles). WO 2015/109141 A1 relates to systems and methods for solid freeform fabrication, especially fused deposition modeling, as well as various articles made using the same, where the systems and methods utilize certain thermoplastic polyurethanes which are said to be particularly suited for such processing.

In additive manufacturing a transition to lasers of the Nd:YAG type which provide laser light at a nominal wavelength of 1064 nm is observed. Then different IR absorbers than customary borides, carbon black and the like are needed in the build material. The present invention has the object of providing a TPU build material suitable for this laser type and an additive manufacturing method employing such a TPU.

This object is achieved by a TPU according to claim 1 and a method according to claim 6. The use of a thermoplastic polymer comprising copper, especially a TPU according to claim 1, as a build material is the subject of claim 14. Advantageous embodiments are the subject of the dependent claims. They may be combined freely unless the context clearly indicates otherwise.

In a thermoplastic polyurethane comprising a copper compound, copper is present in the polyurethane in an amount of > 10 ppm to < 10000 ppm, preferably > 10 ppm to < 1500, more preferably > 40 ppm to < 500 ppm based on the total weight of the polyurethane, the polyurethane is free from antimony and the polyurethane is free from bismuth, with the exception of bismuth originating from bismuth carboxylates, wherein the term “free from" is defined as disclosed below and the stated amounts of metals relate to metal atoms or ions.

In an additive manufacturing process comprising fusing layers of a thermoplastic polymer build material by irradiating the polymer with a laser according to pre-determined cross- sections of an article, the laser emits a light with a wavelength of > 1060 nm to < 1070 nm and copper is present in the polymer in an amount of > 10 ppm to < 10000 ppm, preferably > 10 ppm to < 5000 ppm, more preferably > 10 ppm to < 1500 ppm, even more preferably > 40 ppm to < 500 ppm, based on the total weight of the polymer. The pre-determined cross- section is based on the information from the digital geometry (e.g. an STL-file), according to which the component is built layer by layer.

Examples for suitable polymers in the additive manufacturing process include polyolefins such as PP, polyamides such as PA 6,6, PA 6,66, PA 12 and PA 12,12 as well as polyesters such as PBT. Preferably, according to one embodiment of the process, the polymer is a polyurethane.

It has surprisingly been found that thermoplastic polymers, especially TPUs with such low amounts of copper IR absorber can be used as build materials in CO2, Nd: YAG or ytterbium (fiber) laser-assisted additive manufacturing processes without the (intended) discoloration customary in the cow ear tag marking technology. In particular, bismuth oxides are excluded from the thermoplastic polymers, especially TPU, according to the invention.

It is understood that the stated amounts of metals relate to metal atoms or ions, i.e. without accompanying ligands or the like of metal compounds.

The absence of antimony is an added benefit from an environmental and recyclability standpoint.

In the context of the present invention the term “free from” is meant to include technically unavoidable impurities but exclude the deliberate addition of Sb and Bi compounds. For example, “free from” can be understood as less than 1 ppm Sb and less than 1 ppm Bi, based on the total weight of the thermoplastic polymer (respectively polyurethane). The Cu, Sb and Bi contents can be determined by atomic absorption spectroscopy (AAS), if so desired. In practice a TPU formulator will be able to calculate the Cu contents from the recipe for the TPU formulation. An exception is made for bismuth originating from bismuth carboxylates. Such carboxylates can be used as urethane catalysts and may be present in the TPU; they are not active as IR absorbers. The phrase “originating from bismuth carboxylates” is to be understood that only bismuth derived from bismuth carboxylates and its degradation products are still covered by the term "free from". Consequently, only bismuth carboxylates and/or their degradation products can be found and are allowed to be found in the product. The TPU or the thermoplastic polymer in the instance of the process according to the invention may also be free from inorganic phyllosilicates such as mica in order to provide a transparent or translucent 3D printed article. If so desired, the TPU or thermoplastic polymer in general may be colored by the addition of dyestuffs, for example.

In an embodiment of the polyurethane or the process the copper is present in the TPU or thermoplastic polymer, respectively, in an amount of > 40 ppm to < 500 ppm. Preferred are amounts of > 50 ppm to < 250 ppm.

In another embodiment of the polyurethane or the process the copper is present as copper(II) hydroxide phosphate. Particular preference is given to a compound having the empirical chemical formula Cu3(P04)2 · Cu(OH)2, the mineral of which is known as libethenite.

In another embodiment of the polyurethane it is in the form of a powder having an average powder size (dso) of < 200 pm or in the form of a fdament having a diameter of < 5 mm. More preferred are average powder sizes (dso) of < 150 pm and fdament diameters of < 3 mm.

In another embodiment of the polyurethane or the process the polyurethane or thermoplastic polymer, respectively, is an aromatic polyesterpolyol-polyetherpolyol polyurethane, an aliphatic polyesterpolyol-polyetherpolyol polyurethane or an aromatic polyesterpolyol polyurethane. In general, TPUs suitable for the present invention can be synthesized by reacting a) at least one organic diisocyanate with b) at least one compound having groups which are reactive toward isocyanate groups and a number average molecular weight (Mn) of from 500 g/mol to 6000 g/mol and a number average functionality of the totality of the components under b) of from 1.8 to 2.5 and with c) at least one chain extender having a number average molecular weight (M _n ) of from 60 to 450 g/mol and a number average functionality of the totality of the chain extenders under c) of from 1.8 to 2.5.

Preference is given to using hexamethylene 1,6-diisocyanate, cyclohexane 1,4-diisocyanate, isophorone diisocyanate, dicyclohexylmethane diisocyanate, diphenylmethane diisocyanate isomer mixtures having a diphenylmethane 4,4'-diisocyanate content of more than 96% by weight and in particular diphenylmethane 4,4'-diisocyanate and naphthylene 1,5- diisocyanate. As relatively long-chain isocyanate-reactive compounds under b), mention may be made by way of example of ones having an average of from at least 1.8 to 3.0 Zerewitinoff-active hydrogen atoms and a number average molecular weight of from 500 to 10 000 g/mol. These include compounds bearing not only amino groups but also thiol groups or carboxyl groups, in particular compounds having from two to three, preferably two, hydroxyl groups, especially those having number average molecular weights M _n of from 500 to 6000 g/mol, particularly preferably those having a number average molecular weight M _n of from 600 to 4000 g/mol, e.g. hydroxyl-containing polyester polyols, polyether polyols, polycarbonate polyols and polyester polyamides.

With respect to chain extenders c), preference is given to using aliphatic diols having from 2 to 14 carbon atoms, e.g. ethanediol, 1,2 -propanediol, 1,3 -propanediol, 1,4-butanediol, 2,3- butanediol, 1,5-pentanediol, 1,6-hexanediol, diethylene glycol and dipropylene glycol.

The relative amounts of isocyanate groups and groups which are reactive toward isocyanate are preferably selected so that the ratio is from 0.9:1 to 1.2:1, preferably from 0.95:1 to 1.1:1.

In embodiment of the process the process is a selective laser powder sintering process.

In another embodiment of the process the process is a fused deposition modeling process. The laser can assist in the deposition of the molten material from the print head by irradiating the surface of the polymer layer (especially TPU layer) onto which the molten material is to be deposited.

In another embodiment of the process the laser is operated at a power of < 501 W. Preferred is a power of < 250 W.

In another embodiment of the process the energy density of the laser, expressed as J/mm ³ , is

> 95% (preferably > 95% to < 400%, more preferred > 95% to < 300%, even more preferred

> 95% to < 200%, most preferred > 95% to > 105%) of the value calculated by the formula 0.606x ⁰⁷⁸⁵ with x being the copper content in the polymer, expressed as weight-percentage based on the total weight of the polymer.

In another embodiment of the process the polymer (especially the polyurethane) is free from antimony and the polymer (especially the polyurethane) is free from bismuth, with the exception of bismuth originating from bismuth carboxylates, wherein the term “free from" is defined as disclosed above and the stated amounts of metals relate to metal atoms or ions. The exception is made for bismuth originating from bismuth carboxylates as such carboxylates can be used as urethane catalysts and may be present in the TPU and they are not active as IR absorbers. The polymer, in particular if it is a TPU, may also be free from inorganic phyllosilicates such as mica in order to provide a transparent or translucent 3D printed article. If so desired, the polymer, in particular the TPU, may be colored by the addition of dyestuffs, for example.

Another aspect of the invention is the use of a thermoplastic polymer comprising a copper compound, preferably a polyurethane according to the invention, as a build material in an additive manufacturing process. Copper is present in the polymer in an amount of > 10 ppm (preferably > 40 ppm) to < 10000 ppm (preferably < 5000 ppm, more preferred < 1500 ppm, most preferred < 500), based on the total weight of the polyurethane, the polymer is free from antimony and the polymer is free from bismuth, with the exception of bismuth originating from bismuth carboxylates, wherein the term “free from” is defined as disclosed above and the stated amounts of metals relate to metal atoms or ions. Preferably the thermoplastic polymer, preferably the polyurethane, is in the form of a powder having an average powder size (dso) of < 200 pm or in the form of a filament having a diameter of < 5 mm. More preferred are average powder sizes (dso) of < 150 pm and filament diameters of < 3 mm. The present invention will be further described with reference to the following examples and figure without wishing to be limited by them.

Experiments using 50 ppm of copper

The first TPU (TPU-1) used was an aromatic extrusion- and injection molding grade polyesterpolyol-polyetherpolyol polyurethane having a Shore A hardness of ca. 77. The masterbatch was a masterbatch based on a universal polymer carrier containing 40 weight-% of copper(II) hydroxide phosphate-IR absorber. Compounded TPU-1 was obtained by compounding the TPU-1 and the masterbatch to yield a product with 50 ppm (weight/weight) of copper. No bismuth compounds were present in the TPU or the masterbatch. Powder 1-1 was produced by cryogrinding the uncompounded TPU-1 to an average particle size (dso) of 137 pm. Powder 1-2 was produced by cryogrinding the compounded TPU-1 to an average particle size (dso) of 130 pm.

To test the effect of the formulation of powder 1-2 in comparison to the pure TPU powder 1- 1 for laser sintering applications, both powders were each knife coated onto a glass plate

(layer height = 3000 pm) and applied to a laser system.

For the experiments a fiber laser device with a pulsed Nd:YAG (1064 nm, 12 W) laser source was used (manufacturer: ACI Laser GmbH, Germany; type: DPL Nexus Marker) and operated by applying a constant laser power output of 12W and other fixed properties shown in Table 1.

Table 1 : Fixed parameters of test setup

In order to evaluate the enhanced sintering properties of Powder 1-1 vs. Powder 1-2, the glass plates with the powder layers were exposed to different laser intensities, controlled by adjusting both laser pulse frequency and laser speed as shown in Table 2.

Table 2: Variable parameters of test setup, resulting laser power of pulse

The process settings were applied by shaping a 7 x 7-matrix pattern of equally sized square fields. The overall size of the matrix pattern was 30 mm x 30 mm. Each square of the matrix pattern was shaped by applying the settings described in Table 1 for each pair out of laser speed and pulse frequency, respectively.

The experiment with powder 1-1 showed no visible sintering at all. Results of the experiment with powder 1-2 are depicted in FIG. 1. Results consistent with 5 the requirements for 3D printing via laser sintering were defined as uniformly sintered, transparent or translucent, not discolored material with sharp edges of the squares that the laser source had melted into the powder layer. These are indicated as underlined in Table 2.

Experiments using higher amounts of copper

The second TPU (TPU-2) used was an aliphatic calandering grade polyesterpolyol- polyetherpolyol polyurethane having a Shore A hardness of ca. 86A. The third TPU (TPU-3) used was an aromatic coating grade polyesterpolyol polyurethane having a Shore A hardness of ca. 93A. The masterbatch was a masterbatch based on a universal polymer carrier containing 40 weight-% of copper(II) hydroxide phosphate-IR absorber. Compounded TPUs were obtained by compounding the masterbatch and both the TPU-2 and TPU-3, respectively, to yield products with different amounts of copper as stated in table 3. No bismuth compounds were present in the TPU or the masterbatch. T able 3 : Investigated material variants

Powders 2-1, 2-2 and 2-3 were produced by cryogrinding the compounded TPUs 2-1, 2-2 and 2-3 to an average particle size (dso) of 80, 75 and 80 pm, respectively. Powders 3-1 and 3-2 were produced by cryogrinding the compounded TPUs 3-1 and 3-2 to an average particle size (dio) of 78 and 74 pm, respectively. To test the effect of the different Cu concentrations shown in table 3 within powders 2-x and 3-x for laser sintering applications, all powders were each knife coated onto a glass plate (layer height = 3 mm) and applied to a laser system.

For the experiments a laser device with a Nd:YAG (1064 nm, 12 W max.) laser source was used (manufacturer: ACI Laser GmbH. Germany; type: DPL Nexus Marker) and operated by applying a pulsed laser beam characterized by fixed properties shown in table 4.

Table 4: Fixed parameters of test setup

In order to evaluate the enhanced sintering properties of all previously described powders, the glass plates with the powder layers were exposed to different laser intensities, controlled by adjusting both laser power and laser scanning speed as shown in tables 5 to 9. Results consistent with the requirements for 3D printing via laser sintering were defined as uniformly sintered, transparent or translucent, not discolored material with sharp edges of the squares that the laser source had melted into the powder layer. These are indicated as underlined in the following tables 5 to 9.

Table 5: Powder 2-1 - Variable parameters of test setup, resulting average laser energy density and laser power of pulse

Table 6: Powder 2-2 - Variable parameters of test setup, resulting average laser energy density and laser power of pulse

Table 7: Powder 2-3 - Variable parameters of test setup, resulting average laser energy density and laser power of pulse

Table 8: Powder 3-1 - Variable parameters of test setup, resulting average laser energy density and laser power of pulse

Table 9: Powder 3-2 - Variable parameters of test setup, resulting average laser energy density and laser power of pulse

The process settings were applied by shaping either a 7 x 7- or a 9 x 9-matrix pattern of equally sized square fields. The overall size of the matrix patterns was 25 mm x 25 mm and 30 mm x 30 mm, respectively. Each square of the matrix pattern was shaped by applying the settings described in Table 2 for each pair out of laser speed and laser power, respectively. Results of the experiment with all evaluated powders are depicted in FIG. 2 to 6 with FIG. 2 relating to powder 2-1, FIG. 3 to powder 2-2, FIG. 4 to powder 2-3, FIG. 5 to powder 3-1 and FIG. 6 to powder 3-2.

From the experiments, the resulting sintered specimens are correlated to each laser intensity being derived from the laser power E _L , the scanning speed v _s of the laser beam, the hatch distance h _s . and the effective layer thickness d _L by applying following equation:

Effective layer thickness d _L was assumed to be 110 pm since this is a typical value for powder bed-based 3D-printig processes.

Based on the results of the previously described investigations, the relationship between the minimum required laser energy density and absorber concentration in TPUs 2-1, 2-2, 2-3, 3- 1 and 3-2 can be identified as depicted in FIG. 7. The values for 2-2 and 3-1 are virtually identical and their data points lie on top of each other; this was to be expected as the copper concentration is the same. With the double-logarithmic axes of FIG. 7 a linear relationship between the copper concentration (x-axis) in the polymers and the minimum required laser energy density (y-axis) can be seen. The function equation is y = 0.606x ° ⁷⁸⁵ .