WO2023075782A1

WO2023075782A1 - Binding agents with humectants for three-dimensional printers

Info

Publication number: WO2023075782A1
Application number: PCT/US2021/057208
Authority: WO
Inventors: Elizabeth GALATI; Michelle M. NIU; Thomas Craig ANTHONY; Kristopher J. ERICKSON
Original assignee: Hewlett-Packard Development Company, L.P.
Priority date: 2021-10-29
Filing date: 2021-10-29
Publication date: 2023-05-04

Abstract

In example implementations, a three-dimensional printing kit for printing a three-dimensional object in a three-dimensional printing system is provided. The three-dimensional printing kit includes a build material and a binding agent. The build material includes particles of copper or a copper alloy. The binding agent includes water, copper (II) nitrate or a hydrate thereof, and a humectant.

Description

BINDING AGENTS WITH HUMECTANTS FOR THREE-DIMENSIONAL PRINTERS

BACKGROUND

[0001] Three-dimensional (3D) printing may be an additive printing process used to make three-dimensional solid parts from a digital model. 3D printing can be often used in rapid product prototyping, mold generation, mold master generation, and short run manufacturing. Some 3D printing techniques are considered additive processes because they involve the application of successive layers of material. This is unlike customary machining processes, which often rely upon the removal of material to create the final part. 3D printing can often use curing or fusing of the build material, which for some materials may be accomplished using heat-assisted extrusion, melting, or sintering.

BRIEF DESCRIPTION OF THE DRAWINGS

[0002] FIG. 1 is a block diagram of an example three-dimensional printing kit in accordance with the present disclosure;

[0003] FIG. 2 is a block diagram of another example three-dimensional printing kit in accordance with the present disclosure;

[0004] FIG. 3 is a schematic illustration of an example three-dimensional printing system that uses the three-dimensional printing kit of the present disclosure;

[0005] FIG. 4 is a schematic illustration of another example three- dimensional printing system that uses the three-dimensional printing kit of the present disclosure; and

[0006] FIG. 5 is a flowchart illustrating an example method of selectively applying the three-dimensional printing kit of the present disclosure on a build material to print a three-dimensional object.

DETAILED DESCRIPTION

[0007] Examples described herein provide binding agents with humectants for three-dimensional (3D) printing. As discussed above, 3D printing may be an additive printing process that is used to make 3D solid parts from a digital model. 3D printing includes adding layers of build material. Layers of each object are “printed” in the build material with a binder. The binder helps to fuse the build material together when heated during a sintering or fusing process. [0008] One type of build material that can be used includes particles of a metal, such as copper. Copper or copper alloys in powder form can be used in 3D printing processes to build an object. Examples of copper alloys include copper-zinc (brass), copper-tin (bronze), and copper-nickel (cupronickel, Monel).

[0009] An example binding agent that can be used with copper is copper (II) nitrate (Cu(NOs)2) or hydrates thereof. However, mixing the copper nitrate binding agent with copper powder can lead to a chemical reaction that forms CuOx on the powder surface and can liberate gaseous by-products. Elevating the temperature of the powder and binding agent mixture to cure the binding agent may accelerate the undesirable reaction that forms CuO_x.

[0010] In addition, gas evolution beneath the part surface during curing of the binding agent may displace powder particles and create bubbles or voids within the object being printed. Voids that breach the surface may clearly be visible to the naked eye. Moreover, the porosity caused by the voids may reduce the overall strength and density of the finished object and inhibit sintering to full density.

[0011] Some solutions to address these drawbacks have included adding phosphorous to the copper nitrate binding agent. However, phosphorous has some limitations as well. For example, there is a limit to how much phosphorous can be added to maintain an acceptable thermal conductivity as well as print performance. In addition, the phosphorous-containing compounds can cause deceleration behavior in the printhead, which may reduce the printing speed and overall throughput of the 3D printing system.

[0012] The present disclosure provides a binding agent mixed with a humectant. A humectant may be defined as a compound that can inhibit evaporation of water from the binding agent. The humectant may have a relatively high boiling point, thereby preventing clogging of a print nozzle. [0013] The mixture of the binding agent and the humectant may eliminate or significantly reduce the chemical reactions that form unwanted CuO_x and the gas evolution beneath the surface of the object being printed. In addition, the mixture of the binding agent and the humectant avoids the deceleration behavior associated with phosphorous added to the binding agent.

[0014] FIG. 1 illustrates a schematic illustration of an example three- dimensional printing kit 100 of the present disclosure. Three-dimensional printing kits can be used to make copper or copper alloy three-dimensional printed objects. A certain three-dimensional printing, or additive manufacturing, process can be performed using the materials described herein. In an example, a binding agent can be applied to layers of metal particles that are made of copper or a copper alloy. Successive layers of the metal particles can be added, and a binding agent can be applied on the layers to bind the particles together to form layers of a three-dimensional printed green body. The green body can later be fused, such as by sintering, to form a metal object.

[0015] The binding agents used in the three-dimensional printing process can include an aqueous solution of copper (II) nitrate trihydrate. The binding agent can be applied to certain areas of the layers of metal particles. The metal particles and the applied binding agent can then be heated to an elevated temperature at which the copper (II) nitrate trihydrate can decompose (or partially decompose) to form copper hydroxynitrate (Cu2(OH)3NOs). The copper hydroxynitrate can bind the metal particles together in the green body. When the green body is subsequently fused at a high temperature, the hydrogen, oxygen, and nitrogen in the copper hydroxy nitrate can be driven off as gases, and the copper can remain as a part of the fused metal object.

[0016] In some cases, chemical reactions can occur between the metal particles and the binding agent when the binding agent is applied to the metal particles. For example, some binding agent formulations that include water and copper (II) nitrate trihyrate can cause a reaction with copper or copper alloy particles. As noted above, the reaction can produce gases such as nitric oxide (NO) and nitrogen dioxide (NO2). The reaction may also oxidize the copper or copper alloy, forming copper (I) oxide (CU2O) and/or copper (II) oxide (CuO). If enough gas is released by this reaction, the gas can reduce the density of the three-dimensional printed green body. For example, the gas can become trapped and form bubbles between particles in the green body. The gas can also cause dimensional instability, such as bulging in the surface of the green body. These defects can persist through the sintering process. Thus, the gas released by the reaction can affect the appearance and density of the final sintered metal object. Furthermore, voids created by the gas can negatively affect properties of the final sintered metal object, such as thermal conductivity, electrical conductivity, strength, and others.

[0017] The binding agents described herein can include an additive that inhibits the reaction described above. Binding agents that include the reaction inhibition additive can reduce gas evolution when applied to the metal particles, compared to binding agents that do not include the reaction inhibition additive. Thus, three-dimensional printed green bodies made using the binding agents described herein can have higher density compared to three-dimensional green bodies made using other binding agents. The sintered metal objects that are made by sintering the green bodies can also have better properties compared to sintered metal objects made using other binding agents. In some cases, the reaction inhibition additive can also remove oxidation that is already present on the metal particles. This can also help the metal particles sinter together with a higher density and increased properties such as thermal conductivity and electrical conductivity.

[0018] In an example, the three-dimensional printing kit 100 of the present disclosure may include a build material 110 and a binding agent 120 that includes an inhibition additive, as described above. In an example, the inhibition additive may be a humectant. The humectant may be a compound that can be added to the binding agent 120 to increase a boiling point of the binding agent 120. The humectant may inhibit evaporation of water in the binding agent to prevent solids from forming in the binding agent 120 (where such solids could clog a print nozzle that ejects the binding agent 120 onto the build material 100). Examples of the humectant and formulations of the humectant in the binding agent 120 are discussed in further detail below with respect to FIG. 2.

[0019] In certain examples, the binding agent 120 described herein can be particularly useful with metal powder build materials that do not already include reaction inhibition additives. For example, some copper powders are available that include a small phosphorus content (such as less than 1 wt%). The phosphorus content in such copper powder can have a similar effect of inhibiting the reaction with the binding agent 120. However, different copper powder formulations may be made up of pure copper or copper alloys without any phosphorus or other reaction inhibition additives. The binding agent 120 described herein can be used to form three-dimensional printed green body objects from such copper powders without the negative effects of gas evolution from the reactions described above.

[0020] In an example, the build material 110 may comprise copper or a copper alloy, as described above. In addition, the three-dimensional printing kit 100 may include additional components not shown. For example, the three- dimensional printing kit 100 may also include surfactants. For example, the surfactants may be added to be approximately 0.01 weight percent (wt%) to 1 .0 wt% of the binding agent 120. Examples of surfactants may include alkyldiphenyloxide disulfonate (also known by the tradename Dowfax 2A1), nonionic fluorosurfactant (also known by the tradename Capstone FS-35), ethoxylated alcohol (also known by the tradenames Tergitol TMN-10 or Tergitol TMN-6), and the like.

[0021] FIG. 2 illustrates a schematic illustration of another example three- dimensional printing kit 200 of the present disclosure. In an example, three- dimensional printing kit 200 may include a build material 210 and a binding agent 220. The build material 210 may comprise particles of copper or a copper alloy.

[0022] In an example, the binding agent 220 may include compounds that can inhibit gas evolution when the binding agent 220 is applied to metal particles or to the build material 210. The binding agent 220 may include water 222, copper (II) nitrate 224 or a hydrate thereof, and a humectant 226.

[0023] In an example, the humectant 226 may be a glycol or glycerol based compound. In other words, the humectant 226 may be a compound that contains two hydroxyl (-OH) groups that can inhibit the evaporation of water from the print ink. Example glycols and glycerols that may be used as the humectant 226 may include ethylene glycol, propylene glycol, glycerol, glycerol ethoxylate, 2-Ethyl-2-(hydroxymethyl)-1 ,3-propanediol (EHPD), and 1 ,2 butanediol. In an example, the humectant 226 may be propylene glycol.

[0024] In an example, the humectant 226 may be added to approximately between 2 wt% to 14 wt% of the total weight of the binding agent 220. In an example, the humectant 226 may be added to approximately 5 wt% to 9 wt% of the total weight of the binding agent 220. In an example, the humectant 226 may be added to approximately 7.5 wt% of the total weight of the binding agent 220.

[0025] In an example, the binding agent 220 may be prepared by mixing a solution of the copper (II) nitrate trihydrate 224, water, and the humectant 226. For example, a 60 wt% of the copper (II) nitrate trihydrate 224 solution may be prepared in advance. The appropriate amount of the humectant 226 and water 222 may then be added to the solution of copper (II) nitrate trihydrate 224 and mixed to form the binding agent 220 of the present disclosure. In an example, the final composition of the binding agent 220 may comprise approximately 40 wt% of the copper (II) nitrate trihydrate 224, 7.5 wt% of the humectant, and the remaining wt% of water and/or surfactants.

[0026] The binding agent 220 of the present disclosure was shown to reduce the reaction between the copper (II) nitrate trihydrate 224 and the build material 210. As a result, the reduction in reaction led to an increase in green density and an increase in green part break strength. For example, parts printed with the binding agent 220 were shown to have a break strength that was, on average, two or more times greater than parts printed with a binding agent that did not include the humectant 226. In addition, parts printed with the binding agent 220 were shown to have a green density that was 3% to 10% higher than parts printed with a binding agent that did not include the humectant 226.

[0027] Moreover, the binding agent 220 of the present disclosure was shown to facilitate formation of closed porosity during sinter, increase final part density after hot isostatic pressing (HIP) treatment, and provide an increase in thermal and electrical conductivity.

[0028] In addition, the binding agent 220 provides improved print performance over phosphate containing inks. For example, the binding agent 220 exhibits no deceleration behavior. For example, deceleration may occur when the print inks cannot be dispensed at the frequency used to match the desired print head speed. Print inks that include a phosphate based binding agent may have trouble being dispensed as quickly as the desired printhead speed. Thus, the printhead speed is slowed down to accommodate the limitation on drop dispensing frequency, a phenomenon referred to as deceleration. In contrast, the binding agent 220 does not exhibit the deceleration behavior and can be dispensed at frequencies used for high speed printing (e.g., print head speed greater than 15 inches per second). As a result, three-dimensional print systems are able to print at a higher firing frequency, and thus provide faster print speeds.

[0029] In some examples, the build material 210 and the binding agent 220 can be co-packaged in separate containers. Specifically, a container containing the build material 210 and a container containing the binding agent 220 can be packaged together.

Examples

[0030] The examples below provide comparative examples of the performance of different examples of the binding agent 120 or the binding agent 220 of the present disclosure compared to previously used binding agents. In an example, a small amount of powder was mixed with the ink and heated on a hotplate at 60 degrees Celsius (°C) to induce a reaction. The samples were then spread onto glass slides for observation and comparison. All inks containing Cu(NOs)2 also contained some surfactants (e.g., Dowfax 2A1 (0.5 wt%) and Capstone FS-50 (0.025 wt%). Table 1 illustrates the different combinations of inks and humectants that were used.

TABLE 1 : EXAMPLE INK AND HUMECTANT COMBINATIONS

[0031] In an example, MSV-20, NaCI (0.1 M), and water were previous formulations of inks that were used. Ink ID A, B, C, D, and E represent examples of new ink formulations with the humectants described above. Ink ID B represents the same formulation of the previously used MSV-20 except that 2- pyrollidione is replaced with propylene glycol.

[0032] Visual observations were made to classify the severity of the powder reaction to the nitrate in the binding agent. As discussed above, the copper nitrate in the binding agent may oxidize the copper powder into CuO_x. The oxidation of the copper powder into CuO_x can liberate gaseous by-products that create bubbles or voids within the object being printed with the copper powder. Reducing or suppressing the oxidation of the copper powder with the added humectant can lead to a stronger printed part. To observe the effectiveness of the humectant in the binding agent to prevent the oxidation of the copper powder in to CuO_x, three criteria were used: 1) powder color darkening, 2) rate of color change, and 3) the strength of the resulting deposit.

[0033] With regards to criteria 1), a darker color is associated with more oxidation and is less desirable. A lighter color may mean less oxidation and that the humectant is more effective in suppressing the oxidation of the copper powder. With regards to criteria 2), a faster rate of color change may mean a faster rate of oxidation or that greater oxidation of the copper powder is occurring. Faster oxidation is less desirable. With regards to criteria 3), the samples with the strongest reactivity (e.g., darker colors and faster rate of color change) yielded very loosely bound powder. In contrast, the samples with the lowest reactivity had deposits which were held together well. The worst ink ID was found to be water. The inks with added humectants, especially glycol-type humectants, had lower reactivity. Ink ID B with the propylene glycol humectant was found to have a low reactivity.

[0034] Based on the performance of Ink ID B, MSV-EG-7B was selected for comparison against the previously used formulation of MSV-20 to observe improvements in green part strength of copper molded bars. For certain powders that react with Cu(NOs)2 in the binder, it was shown that the MSV-EG- 7B binding agent could provide up to 3 times the fracture stress strength and up to double the density compared to the previously used formulation of MSV-20. [0035] In addition, parts were printed on a Dalmata system with MSV-20 and MSV-EG-7B binding agents. In an example, bars were printed using an 80 micron layer thickness and a 4-pass print mode where the ink was jetted on passes 1 and 3. Maximum print temperatures of 117 to 120 °C were used. Inks with and without the propylene glycol humectant were used to print bars at contone (ctn) levels of 250, 400, and 510 simultaneously to compare improvements in print performance. The bars underwent a one hour in-bed anneal process at 85 °C.

[0036] It was found that break strengths (measured in Megapascals (Mpa)) of the bars printed with the propylene glycol humectant were, on average, about two or more times greater than that of the bars that used binding agents without the propylene glycol humectant. The results are shown in Table 2 below. Table 2 also shows that bars printed with the propylene glycol humectant had a higher green density than those printed with the previous formulation without the propylene glycol humectant. This suggests improved sinter density and end- use metrics such as thermal and electrical conductivity, as discussed above.

TABLE 2: SUMMARY OF GREEN PART STRENGTH AND DENSITY

[0037] Table 3 below shows the applicability of the propylene glycol containing binding agents for various copper powders (e.g., <22 urn, and GKN <25 urn) as well as other ink formulations. A variety of surfactants were used including Dowfax 2A1 , Capstone FS-50, Tergitol TMN-10, and Tergitol TMN-6. For all examples, there was shown to be an improved green strength and density, as well as final sintered density, compared to the parts printed with MSV-20.

TABLE 3: SUMMARY OF GREEN PART STRENGTHS AND DENSITY

[0038] Those entries in Table 3 with an asterisk (*) also include 0.025 wt% Capstone FS-50. Thus, the comparative examples, and the corresponding measured characteristics of the comparative examples, illustrate how binding agents with humectants can improve the overall strength and density of parts printed with metal powders.

[0039] FIG. 3 illustrates an example of a three-dimensional printing system 300 that can use the three-dimensional printing kit 100 or 200 described above. The three-dimensional printing system 300 can be used with three-dimensional printing kit 100 or 200 described herein to make three-dimensional printed objects. In some examples, a three-dimensional printing system can include a powder bed for holding layers of the build material. A binding agent applicator can be positioned to selectively apply the binding agent 120 or 220 onto the layers of build material 110 or 210. For example, the binding agent applicator can be controllable to apply the binding agent at specific x,y coordinates of the layer of build material. Additionally, the three-dimensional printing system can include a curing heater. As used herein, “curing” can refer to a process of heating the build material 110 or 210 and the binding agent 120 or 220 so that solvents in the binding agent 120 or 220 evaporate, and the copper (II) nitrate in the binding agent (e.g., the copper (II) nitrate 224 in the binding agent 220) is dehydrated or partially dehydrated. In a specific example, the binding agent can include copper (II) nitrate trihydrate, and curing can include heating the binding agent until the copper (II) nitrate trihydrate decomposes to form copper hydroxynitrate.

[0040] In an example, the three-dimensional printing system 300 may include a powder bed 310. The example illustrated in FIG. 3 uses the build material 210 and the binding agent 220. However, it should be noted that the three- dimensional printing system 300 may also use the build material 110 and the binding agent 120.

[0041] In an example, the powder bed 310 includes a layer of the build material 210. As noted above, the build material 210 includes particles of copper or a copper alloy. The printing system 300 may also include a binding agent applicator 320. The binding agent applicator 320 is fluidly coupled to a binding agent 220. The binding agent applicator 320 can be controlled to iteratively apply the binding agent 220 to layers of the build material 210. [0042] The printing system 300 may also include a curing heater 330 positioned to heat the powder bed 310 to a curing temperature. For example, the curing heater 330 may heat the powder bed 310 to temperatures up to 400 degrees Celsius (°C). In an example, the curing heater 330 may heat the powder bed 310 to a temperature between 25 °C to 250 °C. Although the curing heater 330 is illustrated as being positioned below the powder bed 310, it should be noted that the curing heater 330 may also be positioned above the powder bed 310.

[0043] It should be noted that the three-dimensional printing system 300 has been simplified for ease of explanation and can include a variety of additional components besides the components shown in FIG. 3. Examples of additional components include a build material distributor, a supply of additional build material, a fluid applicator for applying a second fluid agent, a hardware controller to send instructions to other components in the system, a non- transitory computer readable medium having stored computer executable instructions to cause the hardware controller to send instructions to other components of the system to perform a three-dimensional printing method, a sintering oven, and the like.

[0044] FIG. 4 illustrates another example three-dimensional printing system 400. The example printing system 400 illustrated in FIG. 4 uses the build material 210 and the binding agent 220. However, the printing system 400 may also use the build material 110 and the binding agent 120.

[0045] In an example, the printing system 400 includes a powder bed 410 having a build material platform 402 and side walls 404. A build material applicator 408 is configured to deposit individual layers of the build material 210. [0046] The printing system 400 may also include a binding agent applicator 420 that is positioned above the powder bed 410. The binding agent applicator 420 may be moveable so that the binding agent applicator 420 can apply the binding agent 220 onto the layers of the build material 210.

[0047] A curing heater 430 may be positioned to heat the powder bed 410. In this example, the curing heater 430 may heat the individual layers of the build material 210 after the binding agent 220 is applied to selected areas of a layer of the build material 210 to form individual green body layers 412. The green body layers 412 may be made up of bound metal particles of the build material 210.

[0048] The printing system 400 may also include a hardware controller 440 or processor. The hardware controller 440 may communicate with the curing heater 430, the binding agent applicator 420, and the build material applicator 408 to send instructions to the curing heater 430, the binding agent applicator 420, and the build material applicator 408 to perform a three-dimensional printing method (e.g., the method 500 illustrated in FIG. 5, and described below).

[0049] In some examples, the binding agent applicator 420 can be moveable along two axes, such as an x-axis and a y-axis, to allow the binding agent 220 to be selectively applied to any desired location on the layers of build material 210. In other examples, the binding agent applicator 420 can be large enough to extend across one entire dimension of the powder bed 410, and the binding agent applicator 420 can be moveable along one axis.

[0050] For example, the binding agent applicator 420 can include a plurality of nozzles along the length of the binding agent applicator 420, and the binding agent 220 can be selectively jetted from the individual nozzles. The binding agent applicator 420 can then scan across the powder bed 410, and the binding agent 220 can be selectively jetted from the nozzles to allow the binding agent 220 to be applied to any desired location on the powder bed 410.

[0051] In other examples, the powder bed 410 itself can be moveable. For example, the powder bed 410 can be moveable and the binding agent applicator 420 can be stationary. In either example, the binding agent applicator 420 and the powder bed 410 can be configured so that binding agent 220 can be selectively applied to specific portions of the powder bed 410.

[0052] The binding agent applicator 420 can be configured to print drops of the binding agent 220 at a resolution ranging from about 300 dots per inch (DPI) to about 1200 DPI in some examples. Higher resolutions or lower resolutions can also be used. The volume of individual drops of binding agent 220 can be from about 1 Pico liters (pL) to about 400 pL in some examples. The firing frequency of nozzles of the binding agent applicator can be from about 1 kilohertz (kHz) to about 100 kHz in certain examples.

[0053] FIG. 5 illustrates a flow diagram of an example method 500 for selectively applying the three-dimensional printing kit on a build material to print a three-dimensional object of the present disclosure. In an example, the method 500 may be performed by the printing system 300 illustrated in FIG. 3 or the printing system 400 illustrated in FIG. 4 using the three-dimensional printing kit 100 illustrated in FIG. 1 or the three-dimensional printing kit 200 illustrated in FIG. 2.

[0054] At block 502, the method 500 begins. At block 504, the method 500 selectively applies a binding agent onto a build material comprising particles of copper or particles of a copper alloy, wherein the binding agent comprises water, copper (II) nitrate, and a humectant. For example, a layer of the build material may be deposited onto a powder bed. The build material may be leveled to provide a smooth even layer of the build material.

[0055] In an example, the copper (II) nitrate may also include hydrates thereof. For example, copper (II) nitrate trihydrate may be used in the binding agent.

[0056] The binding agent may be applied to desired locations on the layer of the build material. The desired locations may be based on a computer generated model of a three-dimensional object that is to be printed. The layer may represent a slice of the three-dimensional object. The desired locations may be based on the shape or design of the slice of the three-dimensional object that is to be printed for the current layer of build material.

[0057] In an example, the binding agent may include approximately between 2 wt% to 14 wt% of the humectant. In an example, the binding agent may include approximately 5 wt% to 9 wt% of the humectant. In an example, the binding agent may include approximately 7.5 wt% of the humectant.

[0058] At block 506, the method 500 heats the build material and the binding agent that is selectively applied to bind a layer of the three-dimensional object. For example, a curing heater may heat the build material and the binding agent to form green body layers or bound metal particles of the build material.

[0059] The humectant in the binding agent can reduce the reaction between the copper (II) nitrate and the build material. As a result, the reduction in reaction between the copper (II) nitrate and the metal particles of the build material can eliminate the formation of gas bubbles that can form between particles in the green body. As noted above, the gas can cause dimensional instability, such as bulging in the surface of the green body. In addition, the gas bubbles can create voids that can negatively affect properties of the final sintered object. As a result, the reduction in reaction between the copper (II) nitrate and the metal particles of the build material can lead to an increase in green density and an increase in green part break strength.

[0060] The method 500 may repeat blocks 504 and 506 for multiple layers. Each layer may include a bound portion that forms a portion of the three- dimensional object that is to be printed. The method 500 may then sinter the layers that are bound to form a sintered three-dimensional printed object or the final form of the three-dimensional printed object. At block 508, the method 500 ends.

[0061] It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.

Claims

1 . A three-dimensional printing kit, comprising: a build material comprising particles of copper or particles of a copper alloy; and a binding agent, comprising: water; copper (II) nitrate or a hydrate of copper (II) nitrate; and a humectant.

2. The three-dimensional printing kit of claim 1 , wherein the humectant comprises a glycol.

3. The three-dimensional printing kit of claim 2, wherein the glycol comprises propylene glycol.

4. The three-dimensional printing kit of claim 1 , wherein the humectant comprises at least one of: ethylene glycol, propylene glycol, glycerol, glycerol ethoxylate, 2-Ethyl-2-(hydroxymethyl)-1 ,3-propanediol, or 1 ,2 butanediol.

5. The three-dimensional printing kit of claim 1 , wherein the humectant comprises 2 weight percent (wt %) to 14 wt% of a total weight of the binding agent.

6. The three-dimensional printing kit of claim 1 , wherein the humectant comprises 5 weight percent (wt %) to 14 wt% of a total weight of the binding agent.

7. The three-dimensional printing kit of claim 1 , wherein the humectant comprises approximately 7.5 weight percent of a total weight of the binding agent.

8. A three-dimensional printing system, comprising: a powder bed comprising a layer of build material comprising copper particles or copper alloy particles; a binding agent applicator fluidly coupled to a supply of a binding agent, wherein the binding agent applicator is to iteratively apply the binding agent to the layer of build material, wherein the binding agent comprises water, copper (II) nitrate, and a humectant; and a curing heater positioned to heat the powder bed to a curing temperature.

9. The three-dimensional printing system of claim 8, wherein the copper (II) nitrate comprises copper (II) nitrate trihydrate.

10. The three-dimensional printing system of claim 8, wherein the humectant comprises 2 weight percent (wt %) to 14 wt% of a total weight of the binding agent.

11 . The three-dimensional printing system of claim 10, wherein the humectant comprises at least one of: ethylene glycol, propylene glycol, glycerol, glycerol ethoxylate, 2-Ethyl-2-(hydroxymethyl)-1 ,3-propanediol, or 1 ,2 butanediol.

12. A method of printing a three-dimensional object, comprising: selectively applying a binding agent onto a build material comprising particles of copper or particles of a copper alloy, wherein the binding agent, comprises water, copper (II) nitrate, and a humectant; and heating the build material and the binding agent that is selectively applied to bind a layer of the three-dimensional object.

13. The method of claim 12, wherein the humectant comprises approximately 7.5 wt% of propylene glycol. 18

14. The method of claim 12, further comprising: sintering layers of the three-dimensional object that are bound to form a sintered three-dimensional printed object.

15. The method of claim 12, wherein the selectively applying the binding agent and the heating the build material and binding agent are repeated for a plurality of layers of the three-dimensional object until printing of the three- dimensional printed object is completed.