WO2021141592A1

WO2021141592A1 - Three-dimensional printing with ph indicator compounds

Info

Publication number: WO2021141592A1
Application number: PCT/US2020/013013
Authority: WO
Inventors: Emre Hiro DISCEKICI; Graciela Emma NEGRI JIMENEZ; Shannon Reuben Woodruff
Original assignee: Hewlett-Packard Development Company, L.P.
Priority date: 2020-01-10
Filing date: 2020-01-10
Publication date: 2021-07-15

Abstract

The present disclosure describes multi-fluid kits for three-dimensional printing, three-dimensional printing kits, and methods of sensing pH. In one example, a multi-fluid kit for three-dimensional printing can include a fusing agent and a pH sensing agent. The fusing agent can include water and an electromagnetic radiation absorber. The electromagnetic radiation absorber can absorb radiation energy and convert the radiation energy to heat. The pH sensing agent can include water and a pH indicator compound. The pH indicator compound can be responsive to a change in pH with a visible color change.

Description

THREE-DIMENSIONAL PRINTING WITH PH INDICATOR COMPOUNDS

BACKGROUND

[0001] Methods of three-dimensional (three-dimensional) digital printing, a type of additive manufacturing, have continued to be developed over the last few decades. However, systems for three-dimensional printing have historically been very expensive, though those expenses have been coming down to more affordable levels recently. Three-dimensional printing technology can shorten the product development cycle by allowing rapid creation of prototype models for reviewing and testing. Unfortunately, the concept has been somewhat limited with respect to commercial production capabilities because the range of materials used in three-dimensional printing is likewise limited. Accordingly, it can be difficult to three-dimensional print functional parts with desired properties such as mechanical strength, visual appearance, and so on. Nevertheless, several commercial sectors such as aviation and the medical industry have benefitted from the ability to rapidly prototype and customize parts for customers.

BRIEF DESCRIPTION OF THE DRAWINGS [0002] FIG. 1 is a schematic view of an example multi-fluid kit for three- dimensional printing in accordance with examples of the present disclosure.

[0003] FIG. 2 is a schematic view of another example multi-fluid kit for three-dimensional printing in accordance with examples of the present disclosure.

[0004] FIG. 3 is a schematic view of an example three-dimensional printing kit in accordance with examples of the present disclosure.

[0005] FIG. 4 is a schematic view of another example three-dimensional printing kit in accordance with examples of the present disclosure. [0006] FIGs. 5A-5C show a schematic view of an example three- dimensional printing process using an example three-dimensional printing kit in accordance with examples of the present disclosure.

[0007] FIG. 6 is a flowchart illustrating an example method of sensing pH in accordance with examples of the present disclosure.

DETAILED DESCRIPTION

[0008] The present disclosure describes multi-fluid kits for three- dimensional printing, three-dimensional printing kits, and methods of sensing pH using three-dimensional printed pH sensors. In one example, a multi-fluid kit for three-dimensional printing includes a fusing agent and a pH sensing agent. The fusing agent includes water and an electromagnetic radiation absorber. The electromagnetic radiation absorber absorbs radiation energy and converts the radiation energy to heat. The pH sensing agent includes water and a pH indicator compound. The pH indicator compound is responsive to a change in pH with a visible color change. In some examples, the pH indicator compound can be water-soluble. In further examples, the pH indicator compound can be chemically stable at a temperature from about 70 °C to about 350 °C. In certain examples, the pH indicator compound can include an anthocyanin. In further examples, the pH indicator compound can be present in an amount from about 0.01 wt% to about 15 wt% with respect to the total weight of the pH sensing agent. In other examples, the multi-fluid kit can also include a post-processing agent having a pH that is different from the pH of the pH sensing agent and sufficient to induce a color change of the pH indicator compound. In certain examples, the fusing agent can be a colorless fusing agent or a low tint fusing agent.

[0009] The present disclosure also describes three-dimensional printing kits. In one example, a three-dimensional printing kit includes a powder bed material including polymer particles, a fusing agent to selectively apply to the powder bed material, and a pH sensing agent to selectively apply to the powder bed material. The fusing agent includes water and an electromagnetic radiation absorber. The electromagnetic radiation absorber absorbs radiation energy and converts the radiation energy to heat. The pH sensing agent includes water and a pH indicator compound. The pH indicator compound is responsive to a change in pH with a visible color change. In some examples, the polymer particles can include polyamide 6, polyamide 9, polyamide 11 , polyamide 12, polyamide 66, polyamide 612, thermoplastic polyamide, polyamide copolymer, polyethylene, thermoplastic polyurethane, polypropylene, polyester, polycarbonate, polyether ketone, polyacrylate, polystyrene, polyvinylidene fluoride, polyvinylidene fluoride copolymer, poly(vinylidene fluoride-trifluoroethylene), poly(vinylidene fluoride- trifluoroethylene-chlorotrifluoroethylene), wax, or a combination thereof. In further examples, the pH indicator compound can be chemically stable at a melting point temperature of the polymer particles. In other examples, the pH indicator compound can include an anthocyanin and can be present in an amount from about 0.01 wt% to about 15 wt% with respect to the total weight of the pH sensing agent.

[0010] The present disclosure also describes methods of sensing pH. In one example, a method of sensing pH includes exposing a three-dimensional printed pH sensor to a solution having a pH, wherein the three-dimensional printed pH sensor includes multiple fused layers of polymer particles, wherein a portion of a surface of the three-dimensional printed pH sensor includes a pH indicator compound immobilized in the fused polymer, wherein the pH indicator compound is responsive to a change in pH with a visible color change. In some examples, the method can also include making the three-dimensional printed pH sensor by iteratively applying individual build material layers of the polymer particles to a powder bed. A fusing agent can be selectively jetted, based on a three-dimensional object model, onto the individual build material layers. The fusing agent can include water and an electromagnetic radiation absorber. A pH sensing agent can be selectively jetted, based on the three-dimensional object model, onto the individual build material layers. The pH sensing agent can include water and the pH indicator compound. The powder bed can be exposed to energy to selectively fuse the polymer particles in contact with the electromagnetic radiation absorber to form a fused polymer matrix at individual build material layers. In certain examples, the fusing agent can be a colorless fusing agent or a low tint fusing agent. In other examples, the pH indicator compound can include an anthocyanin. [0011] It is noted that when discussing the multi-fluid kits, three- dimensional printing kits, and methods herein, these discussions can be considered applicable to one another whether or not they are explicitly discussed in the context of that example. Thus, for example, when discussing a fusing agent related to a three-dimensional printing kit, such disclosure is also relevant to and directly supported in the context of multi-fluid kits and methods, vice versa , etc.

[0012] It is also understood that terms used herein will take on their ordinary meaning in the relevant technical field unless specified otherwise. In some instances, there are terms defined more specifically throughout the specification or included at the end of the present specification, and thus, these terms have a meaning as described herein.

Multi-fluid Kits for Three-dimensional Printing

[0013] The multi-fluid kits, three-dimensional printing kits, and methods described herein can be used to make three-dimensional printed objects that are capable of sensing pH. In particular, the three-dimensional printed objects can have a pH indicator compound incorporated into the fused polymer particles making up the three-dimensional printed objects. This pH indicator compound can be capable of changing color when exposed to different levels of pH. Some examples of pH indicator compounds include anthocyanins, which can be derived from a variety of natural sources such as fruits, vegetables, and flowers. In one particular example, anthocyanins derived from red cabbage can change colors to indicate acidic or basic pH conditions. These anthocyanins can be purple under neutral conditions where the pH is about 7. Under acidic conditions, the anthocyanin molecule can transform to a cationic form that has a red or pink color. When exposed to basic conditions, the molecule can be deprotonated to form a quinone functionality that has a yellow color.

[0014] An example anthocyanin compound is shown in the cationic form below: OH

OH sugar

[0015] The same example anthocyanin in its deprotonated form is shown below:

OH

OH sugar [0016] In some cases, pH indicator compounds such as the example shown above can exhibit a range of colors that can correspond to a range of pH levels. For example, depending on the pH level a certain proportion of the pH indicator molecules can transform into a cationic or deprotonated form. The overall color of the pH indicator can be related to the relative number of molecules in the differently colored forms. Certain pH indicator compounds can show a reliable, reproducible color range depending on the pH. These can be used to estimate pH by visual inspection or by colorimetric analysis.

[0017] In some examples, the pH indicator compound can also be thermally stable at elevated temperatures used during three-dimensional printing. In many examples, the fluid agents and materials described herein can be used with certain three-dimensional printing processes that involve fusing layers of polymer powder to form solid layers of a three-dimensional printed object. In one process, a fusing agent can be applied onto a powder bed of polymer particles. The fusing agent can include an electromagnetic radiation absorber, which can be a material that absorbs radiant energy and converts the energy to heat. Radiant energy can be applied to the powder bed to heat and fuse the polymer particles on which the fusing agent was applied. Thus, the polymer particles can be heated to a temperature that is high enough to fuse the polymer particles together, which can be from 70 °C to 350 °C or higher, depending on the type of polymer being used as a build material. Therefore, the pH indicator compound can be capable of withstanding these elevated temperatures during printing while still maintaining the ability of changing color with changes in pH.

[0018] In certain examples of the three-dimensional printing processes describe herein, the fusing agent can be applied using jetting architecture such as an inkjet print head. Such a system can jet small droplets of the fusing agent at selected locations on the powder bed with a high resolution. This can allow for making high resolution, detailed three-dimensional printed objects.

[0019] A pH sensing agent that includes the pH indicator compound can also be applied to the powder bed during the three-dimensional printing process. This agent can also be jetted using an inkjet print head, in some examples. Using this process, detailed patterns of pH-sensitive areas can be formed with high resolution. For example, the pH sensing agent can be applied at or near portions of the powder bed where a surface of the three-dimensional printed object is formed. When the three-dimensional printed object is complete, the pH indicator compound from the pH sensing agent can be incorporated into the fused polymer at the surface of the object. With the pH indicator compound present in the fused polymer at the surface, the color of the surface of the three-dimensional printed object can change to indicate changes in pH. The three-dimensional printed object can then be used as a pH sensor that can be contacted with any material to measure the pH of the material.

[0020] In certain examples, the pH sensing agent can be applied to the powder bed around the edges of individual layers or slices of the three- dimensional printed object. These edges will become the surfaces of the final three-dimensional printed object. In further examples, the pH sensing agent can also be applied slightly inside the edges (i.e. , a few millimeters or less) so that the pH indicator compound can be present slightly beneath the surface of the final three-dimensional printed object. In still further examples, the pH sensing agent can be applied slightly outside the edges of the individual layers of the three- dimensional printed object to ensure that the polymer particles present at the surface of the three-dimensional printed object are coated or partially coated with a sufficient amount of the pH indicator compound.

[0021] In certain examples, a post- processing agent can be applied to the three-dimensional printed object. The post-processing agent can have a pH that is different from the pH of the pH sensing agent. When the post-processing agent is applied to the three-dimensional printed object, the different pH can cause the pH indicator compound to change color. In some examples, this can be used to color the three-dimensional printed object for aesthetic purposes or any other purpose. In other examples, the post-processing agent can be any material for which it is desired to measure the pH.

[0022] With this description in mind, FIG. 1 shows a schematic of an example multi-fluid kit for three-dimensional printing 100. The kit includes a fusing agent 110 and a pH sensing agent 120. The fusing agent can include water and an electromagnetic radiation absorber. The electromagnetic radiation absorber can absorb radiation energy and convert the radiation energy to heat. The pH sensing agent can include water and a pH indicator compound. The pH indicator compound can be responsive to a change in pH with a visible color change.

[0023] In some examples, the multi-fluid kit can also include a detailing agent. The detailing agent can include a detailing compound, which is a compound that can reduce the temperature of powder bed material onto which the detailing agent is applied. In some examples, the detailing agent can be applied around edges of the area where the fusing agent is applied. This can prevent powder bed material around the edges from caking due to heat from the area where the fusing agent was applied. The detailing agent can also be applied in the same area where fusing was applied in order to control the temperature and prevent excessively high temperatures when the powder bed material is fused.

[0024] In further examples, the multi-fluid kit can also include a post processing agent. The post-processing agent can have a pH that is different from the pH of the pH sensing agent. The pH can be sufficiently different that the post processing agent can induce a color change of the pH indicator compound when the post-processing agent is contacted with the pH indicator compound. In certain examples, the pH sensing agent can have a neutral pH and the post-processing agent can be acidic or basic. In other examples, the pH sensing agent can be acidic or basic, and the post-processing agent can have an acidic, basic, or neutral pH that is sufficiently different from the pH of the pH sensing agent to induce a color change in the pH indicator compound. In some examples, the post-processing agent can be an aqueous solution of an acid or base. Examples of acids that can be used in the post-processing agent can include acetic acid, citric acid, lactic acid, hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, boric acid, sulfonic acids, and others. Examples of bases that can be used can include sodium hydroxide, sodium carbonate, sodium bicarbonate, ammonia, and others.

[0025] FIG. 2 shows a schematic illustration of an example multi-fluid kit 200 that includes a fusing agent 110, a pH sensing agent 120, and a post processing agent 230. The post-processing agent can be a fluid having a pH that is different from the pH of the pH sensing agent and sufficient to induce a color change in the pH indicator compound. In some examples, the post-processing agent can be applied by dipping the three-dimensional printed object in the post processing agent, or by spraying the post-processing agent onto the three- dimensional printed object, or by another method of application such as brushing.

Three-dimensional Printing Kits

[0026] The present disclosure also describes three-dimensional printing kits. In some examples, the three-dimensional printing kits can include materials that can be used in the three-dimensional printing processes described herein. FIG. 3 shows a schematic illustration of one example three-dimensional printing kit 300 in accordance with examples of the present disclosure. The kit includes a powder bed material 340 including polymer particles and a fusing agent 110 to selectively apply to the powder bed material. The fusing agent can include an electromagnetic radiation absorber that can absorb radiation energy and convert the energy to heat. The kit also includes a pH sensing agent 120. The pH sensing agent can include water and a pH indicator compound. The pH indicator compound can be responsive to a change in pH with a visible color change. [0027] In further examples, a three-dimensional printing kit can include additional fluid agents, such as a detailing agent and/or a post-processing agent. FIG. 4 is a schematic illustration of one example three-dimensional printing kit 400 that includes a powder bed material 340, a fusing agent 110, a pH sensing agent 120, a post-processing agent 230, and a detailing agent 450. As mentioned above, the post-processing agent can be used after the final three-dimensional printed object has been formed. For example, the post-processing agent can be sprayed on the three-dimensional printed object or the object can be dipped in the post-processing agent, etc. The detailing agent can be a fluid agent that can be applied during the three-dimensional printing process in areas where it is desired to reduce the temperature of the powder bed.

[0028] To illustrate the use of the three-dimensional printing kits and multi fluid kits described herein, FIGs. 5A-5C illustrate one example of using a three- dimensional printing kit to form a three-dimensional printed object. In FIG. 5A, a fusing agent 110 and a pH sensing agent 120 are jetted onto a layer of powder bed material 340. The fusing agent is jetted from a fusing agent ejector 112 and the pH sensing agent is jetted from a pH sensing agent ejector 122. These fluid ejectors can move across the layer of powder bed material to selectively jet fusing agent on areas that are to be fused. The pH sensing agent can be jetted in areas where the pH indicating ability is desired. A radiation source 570 can also move across the layer of powder bed material.

[0029] FIG. 5B shows the layer of powder bed material 340 after the fusing agent 110 and pH sensing agent 120 have been jetted onto an area of the layer that is to be fused. As shown in this figure, the pH sensing agent has been jetted in areas at the edges of the layer or slice of the three-dimensional printed object that is being formed from this particular layer of powder bed material. The edges of the layer will become parts of the surface of the finished three-dimensional printed object. The fusing agent has also been jetted in these areas so that these areas of the powder bed will fuse together to become part of the solid three- dimensional printed object. The radiation source 570 is shown emitting radiation 572 toward the layer of polymer particles. The fusing agent can include a radiation absorber that can absorb this radiation and convert the radiation energy to heat. [0030] FIG. 5C shows the layer of powder bed material 340 with a fused portion 342 where the fusing agent was jetted. This portion has reached a sufficient temperature to fuse the polymer particles together to form a solid polymer matrix. The pH indicator compound from the pH sensing agent is incorporated into the solid polymer matrix in the areas at the edges of the layer. Therefore, the fused portion includes a pH sensing portion 344.

[0031] In many examples, the pH sensing agent can be applied to areas of the powder bed where the fusing agent is also applied. As mentioned above, the pH sensing agent can be applied at edges of the area where the fusing agent is applied. The edges of the individual fused layers eventually become the surface of the final three-dimensional printed object. Thus, the pH sensing agent can be applied to the portions of the powder bed that eventually become the surfaces of the three-dimensional printed object. The pH sensing agent can also be applied to a certain area within the edges. For example, the agent can be applied to an area beginning at the edge of the area where the fusing agent is printed, and extending inward from the edge to a certain distance. This will result in the pH indicator compound being present in the polymer matrix from the surface of the three-dimensional printed object and down to a certain depth beneath the surface. In some examples, the pH sensing agent can be applied within the edges to a distance of from about 10 micrometers to about 1 millimeter within the edges. In further examples, this distance can be from about 20 micrometers to about 800 micrometers or from about 50 micrometers to about 500 micrometers.

[0032] Additionally, in some examples, the pH sensing agent can be applied to the powder bed outside the edges of the area where the fusing agent is applied. In certain examples, the agent can be applied in an area that extends from the edge or border of the area where the fusing agent is applied to a certain distance outside this border. The distance can be from about 10 micrometers to about 1 millimeter, or from about 20 micrometers to about 800 micrometers, or from about 50 micrometers to about 500 micrometers. The polymer particles in this area of the powder may not be fused and incorporated into the three- dimensional printed object. However, it can be difficult to form a perfect edge when fusing the polymer particles and some of the polymer particles near the edge can often become embedded in the surface of the three-dimensional printed object. Therefore, the pH sensing agent can be applied to the neighboring polymer particles to ensure that the polymer at the surface of the three- dimensional printed object has the desired amount of the pH indicator compound present.

Powder Bed Materials

[0033] In certain examples, the powder bed material can include polymer particles having a variety of shapes, such as substantially spherical particles or irregularly-shaped particles. In some examples, the polymer powder can be capable of being formed into three-dimensional printed objects with a resolution of about 20 pm to about 100 pm, about 30 pm to about 90 pm, or about 40 pm to about 80 pm. As used herein, “resolution” refers to the size of the smallest feature that can be formed on a three-dimensional printed object. The polymer powder can form layers from about 20 pm to about 100 pm thick, allowing the fused layers of the printed part to have roughly the same thickness. This can provide a resolution in the z-axis (i.e. , depth) direction of about 20 pm to about 100 pm. The polymer powder can also have a sufficiently small particle size and sufficiently regular particle shape to provide about 20 pm to about 100 pm resolution along the x-axis and y-axis (i.e., the axes parallel to the top surface of the powder bed). For example, the polymer powder can have an average particle size from about 20 pm to about 100 pm. In other examples, the average particle size can be from about 20 pm to about 50 pm. Other resolutions along these axes can be from about 30 pm to about 90 pm or from 40 pm to about 80 pm.

[0034] The polymer powder can have a melting or softening point from about 70°C to about 350°C. In further examples, the polymer can have a melting or softening point from about 150°C to about 200°C. A variety of thermoplastic polymers with melting points or softening points in these ranges can be used. For example, the polymer powder can be polyamide 6 powder, polyamide 9 powder, polyamide 11 powder, polyamide 12 powder, polyamide 6/6 powder, polyamide 6/12 powder, thermoplastic polyamide powder, polyamide copolymer powder, polyethylene powder, wax, thermoplastic polyurethane powder, acrylonitrile butadiene styrene powder, amorphous polyamide powder, polymethylmethacrylate powder, ethylene-vinyl acetate powder, polyarylate powder, silicone rubber, polypropylene powder, polyester powder, polycarbonate powder, copolymers of polycarbonate with acrylonitrile butadiene styrene, copolymers of polycarbonate with polyethylene terephthalate, polyether ketone powder, polyacrylate powder, polystyrene powder, polyvinylidene fluoride powder, polyvinylidene fluoride copolymer powder, poly(vinylidene fluoride- trifluoroethylene) powder, poly(vinylidene fluoride-trifluoroethylene- chlorotrifluoroethylene) powder, or mixtures thereof. In a specific example, the polymer powder can be polyamide 12, which can have a melting point from about 175°C to about 200°C. In another specific example, the polymer powder can be thermoplastic polyurethane.

[0035] The thermoplastic polymer particles can also in some cases be blended with a filler. The filler can include inorganic particles such as alumina, silica, fibers, carbon nanotubes, or combinations thereof. When the thermoplastic polymer particles fuse together, the filler particles can become embedded in the polymer, forming a composite material. In some examples, the filler can include a free-flow agent, anti-caking agent, or the like. Such agents can prevent packing of the powder particles, coat the powder particles and smooth edges to reduce inter-particle friction, and/or absorb moisture. In some examples, a weight ratio of thermoplastic polymer particles to filler particles can be from about 100:1 to about 1 :2 or from about 5: 1 to about 1 :1.

Fusing Agents

[0036] The multi-fluid kits and three-dimensional printing kits described herein can include a fusing agent to be applied to the polymer build material. The fusing agent can include a radiation absorber that can absorb radiant energy and convert the energy to heat. In certain examples, the fusing agent can be used with a powder bed material in a particular three-dimensional printing process. A thin layer of powder bed material can be formed, and then the fusing agent can be selectively applied to areas of the powder bed material that are desired to be consolidated to become part of the solid three-dimensional printed object. The fusing agent can be applied, for example, by printing such as with a fluid ejector or fluid jet print head. Fluid jet print heads can jet the fusing agent in a similar way to an inkjet print head jetting ink. Accordingly, the fusing agent can be applied with great precision to certain areas of the powder bed material that are desired to form a layer of the final three-dimensional printed object. After applying the fusing agent, the powder bed material can be irradiated with radiant energy. The radiation absorber from the fusing agent can absorb this energy and convert it to heat, thereby heating any polymer particles in contact with the radiation absorber. An appropriate amount of radiant energy can be applied so that the area of the powder bed material that was printed with the fusing agent heats up enough to melt the polymer particles to consolidate the particles into a solid layer, while the powder bed material that was not printed with the fusing agent remains as a loose powder with separate particles.

[0037] In some examples, the amount of radiant energy applied, the amount fusing agent applied to the powder bed, the concentration of radiation absorber in the fusing agent, and the preheating temperature of the powder bed (i.e. , the temperature of the powder bed material prior to printing the fusing agent and irradiating) can be tuned to ensure that the portions of the powder bed printed with the fusing agent will be fused to form a solid layer and the unprinted portions of the powder bed will remain a loose powder. These variables can be referred to as parts of the “print mode” of the three-dimensional printing system. The print mode can include any variables or parameters that can be controlled during three-dimensional printing to affect the outcome of the three-dimensional printing process.

[0038] The process of forming a single layer by applying fusing agent and irradiating the powder bed can be repeated with additional layers of fresh powder bed material to form additional layers of the three-dimensional printed object, thereby building up the final object one layer at a time. In this process, the powder bed material surrounding the three-dimensional printed object can act as a support material for the object. When the three-dimensional printing is complete, the object can be removed from the powder bed and any loose powder on the object can be removed.

[0039] Accordingly, in some examples, the fusing agent can include a radiation absorber that is capable of absorbing electromagnetic radiation to produce heat. The radiation absorber can be colored or colorless. In various examples, the radiation absorber can be a pigment such as carbon black pigment, glass fiber, titanium dioxide, clay, mica, talc, barium sulfate, calcium carbonate, a near-infrared absorbing dye, a near-infrared absorbing pigment, a conjugated polymer, a dispersant, or combinations thereof. Examples of near- infrared absorbing dyes include aminium dyes, tetraaryldiamine dyes, cyanine dyes, pthalocyanine dyes, dithiolene dyes, and others. In further examples, radiation absorber can be a near-infrared absorbing conjugated polymer such as poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS), a polythiophene, poly(p-phenylene sulfide), a polyaniline, a poly(pyrrole), a poly(acetylene), poly(p-phenylene vinylene), polyparaphenylene, or combinations thereof. As used herein, “conjugated” refers to alternating double and single bonds between atoms in a molecule. Thus, “conjugated polymer” refers to a polymer that has a backbone with alternating double and single bonds. In many cases, the radiation absorber can have a peak absorption wavelength in the range of about 800 nm to about 1400 nm.

[0040] A variety of near-infrared pigments can also be used. Non-limiting examples can include phosphates having a variety of counterions such as copper, zinc, iron, magnesium, calcium, strontium, the like, and combinations thereof. Non-limiting specific examples of phosphates can include M₂P₂O₇, M4P2O9, M5P2O10, M3(PC>4)2, M(PC>3)2, M2P4O12, and combinations thereof, where M represents a counterion having an oxidation state of +2, such as those listed above or a combination thereof. For example, M₂P₂C>₇ can include compounds such as CU2P2O7, Cu/MgP2C>7, Cu/ZnP2C>7, or any other suitable combination of counterions. It is noted that the phosphates described herein are not limited to counterions having a +2 oxidation state. Other phosphate counterions can also be used to prepare other suitable near-infrared pigments.

[0041] Additional near-infrared pigments can include silicates. Silicates can have the same or similar counterions as phosphates. One non-limiting example can include M₂S1O₄, M₂S1₂O₆, and other silicates where M is a counterion having an oxidation state of +2. For example, the silicate M₂S1₂O₆ can include Mg₂Si₂0₆, Mg/CaSi206, MgCuShOe, CU2S12O6, Cu/ZnShOe, or other suitable combination of counterions. It is noted that the silicates described herein are not limited to counterions having a +2 oxidation state. Other silicate counterions can also be used to prepare other suitable near-infrared pigments. [0042] In further examples, the radiation absorber can include a metal dithiolene complex. Transition metal dithiolene complexes can exhibit a strong absorption band in the 600 nm to 1600 nm region of the electromagnetic spectrum. In some examples, the central metal atom can be any metal that can form square planer complexes. Non-limiting specific examples include complexes based on nickel, palladium, and platinum.

[0043] In further examples, the radiation absorber can include a tungsten bronze or a molybdenum bronze. In certain examples, tungsten bronzes can include compounds having the formula M_xWCh, where M is a metal other than tungsten and x is equal to or less than 1. Similarly, in some examples, molybdenum bronzes can include compounds having the formula M_cMoOb, where M is a metal other than molybdenum and x is equal to or less than 1.

[0044] In still other examples, the radiation absorber can be selected to provide that the fusing agent is a “low tint fusing agent” that may be transparent, pale in color, or white. For example, the electromagnetic radiation absorber may be transparent or white at wavelengths ranging from about 400 nm to about 780 nm. In some examples, the term “transparent” as used herein, indicates that about 20% or less of the radiation having wavelengths from about 400 nm to about 780 nm is absorbed. Thus, in examples herein, the low tint fusing agent can be white, colorless, or pale in coloration so that coloring agent can be effective in coloring the polymeric powder bed material without much, if any, interference in coloration from the radiation absorber. At the same time, the low tint fusing agent can generate heat when exposed to electromagnetic energy wavelengths from 800 nm to 4,000 nm sufficient to partially or fully melt or coalesce the polymeric powder bed material that is in contact with the low tint fusing agent.

[0045] In alternative examples, the radiation absorber can preferentially absorb ultraviolet radiation. In some examples, the radiation absorber can absorb radiation in wavelength range from about 300 nm to about 400 nm. In certain examples, the amount of electromagnetic energy absorbed by the fusing agent can be quantified as follows: a layer of the fusing agent having a thickness of 0.5 pm after liquid components have been removed can absorb from 90% to 100% of radiant electromagnetic energy having a wavelength within a wavelength range from about 300 nm to about 400 nm. The radiation absorber may also absorb little or no visible light, thus making the radiation absorber transparent to visible light. In certain examples, the 0.5 pm layer of the fusing agent can absorb from 0% to 20% of radiant electromagnetic energy in a wavelength range from above about 400 nm to about 700 nm. Non-limiting examples of ultraviolet absorbing radiation absorbers can include nanoparticles of titanium dioxide, zinc oxide, cerium oxide, indium tin oxide, or a combination thereof. In some examples, the nanoparticles can have an average particle size from about 2 nm to about 300 nm, from about 10 nm to about 100 nm, or from about 10 nm to about 60 nm.

[0046] A dispersant can be included in the fusing agent in some examples. Dispersants can help disperse the radiation absorbing pigments described above. In some examples, the dispersant itself can also absorb radiation. Non-limiting examples of dispersants that can be included as a radiation absorber, either alone or together with a pigment, can include polyoxyethylene glycol octylphenol ethers, ethoxylated aliphatic alcohols, carboxylic esters, polyethylene glycol ester, anhydrosorbitol ester, carboxylic amide, polyoxyethylene fatty acid amide, poly (ethylene glycol) p-isooctyl-phenyl ether, sodium polyacrylate, and combinations thereof.

[0047] The amount of radiation absorber in the fusing agent can vary depending on the type of radiation absorber. In some examples, the concentration of radiation absorber in the fusing agent can be from about 0.1 wt% to about 20 wt%. In one example, the concentration of radiation absorber in the fusing agent can be from about 0.1 wt% to about 15 wt%. In another example, the concentration can be from about 0.1 wt% to about 8 wt%. In yet another example, the concentration can be from about 0.5 wt% to about 2 wt%. In a particular example, the concentration can be from about 0.5 wt% to about 1.2 wt%. In one example, the radiation absorber can have a concentration in the fusing agent such that after the fusing agent is jetted onto the polymer powder, the amount of radiation absorber in the polymer powder can be from about 0.0003 wt% to about 10 wt%, or from about 0.005 wt% to about 5 wt%, with respect to the weight of the polymer powder.

[0048] In some examples, the fusing agent can be jetted onto the polymer powder build material using a fluid jetting device, such as inkjet printing architecture. Accordingly, in some examples, the fusing agent can be formulated to give the fusing agent good jetting performance. Ingredients that can be included in the fusing agent to provide good jetting performance can include a liquid vehicle. Thermal jetting can function by heating the fusing agent to form a vapor bubble that displaces fluid around the bubble, and thereby forces a droplet of fluid out of a jet nozzle. Thus, in some examples the liquid vehicle can include a sufficient amount of an evaporating liquid that can form vapor bubbles when heated. The evaporating liquid can be a solvent such as water, an alcohol, an ether, or a combination thereof.

[0049] In some examples, the liquid vehicle formulation can include a co solvent or co-solvents present in total at from about 1 wt% to about 50 wt%, depending on the jetting architecture. Further, a non-ionic, cationic, and/or anionic surfactant can be present, ranging from about 0.01 wt% to about 5 wt%.

In one example, the surfactant can be present in an amount from about 1 wt% to about 5 wt%. The liquid vehicle can include dispersants in an amount from about 0.5 wt% to about 3 wt%. The balance of the formulation can be purified water, and/or other vehicle components such as biocides, viscosity modifiers, materials for pH adjustment, sequestering agents, preservatives, and the like. In one example, the liquid vehicle can be predominantly water.

[0050] In some examples, a water-dispersible or water-soluble radiation absorber can be used with an aqueous vehicle. Because the radiation absorber is dispersible or soluble in water, an organic co-solvent may not be present, as it may not be included to solubilize the radiation absorber. Therefore, in some examples the fluids can be substantially free of organic solvent, e.g., predominantly water. However, in other examples a co-solvent can be used to help disperse other dyes or pigments, or enhance the jetting properties of the respective fluids. In still further examples, a non-aqueous vehicle can be used with an organic-soluble or organic-dispersible fusing agent.

[0051] Classes of co-solvents that can be used can include organic co solvents including aliphatic alcohols, aromatic alcohols, diols, glycol ethers, polyglycol ethers, caprolactams, formamides, acetamides, and long chain alcohols. Examples of such compounds include 1 -aliphatic alcohols, secondary aliphatic alcohols, 1 ,2-alcohols, 1 ,3-alcohols, 1 ,5-alcohols, ethylene glycol alkyl ethers, propylene glycol alkyl ethers, higher homologs (C6-C12) of polyethylene glycol alkyl ethers, N-alkyl caprolactams, unsubstituted caprolactams, both substituted and unsubstituted formamides, both substituted and unsubstituted acetamides, and the like. Specific examples of solvents that can be used include, but are not limited to, 2-pyrrolidinone, N-methylpyrrolidone, 2-hydroxyethyl-2- pyrrolidone, 2-methyl-1 , 3-propanediol, tetraethylene glycol, 1 ,6-hexanediol, 1 ,5- hexanediol and 1 ,5-pentanediol.

[0052] Regarding the surfactant that may be present, a surfactant or surfactants can be used, such as alkyl polyethylene oxides, alkyl phenyl polyethylene oxides, polyethylene oxide block copolymers, acetylenic polyethylene oxides, polyethylene oxide (di)esters, polyethylene oxide amines, protonated polyethylene oxide amines, protonated polyethylene oxide amides, dimethicone copolyols, substituted amine oxides, and the like. The amount of surfactant added to the fusing agent may range from about 0.01 wt% to about 20 wt%. Suitable surfactants can include, but are not limited to, liponic esters such as TERGITOL™ 15-S-12, TERGITOL™ 15-S-7 available from Dow Chemical Company (Michigan), LEG-1 and LEG-7; TRITON™ X-100; TRITON™ X-405 available from Dow Chemical Company (Michigan); and sodium dodecylsulfate.

[0053] Various other additives can be employed to enhance certain properties of the fusing agent for specific applications. Examples of these additives are those added to inhibit the growth of harmful microorganisms. These additives may be biocides, fungicides, and other microbial agents, which can be used in various formulations. Examples of suitable microbial agents include, but are not limited to, NUOSEPT® (Nudex, Inc., New Jersey), UCARCIDE™ (Union carbide Corp., Texas), VANCIDE® (R.T. Vanderbilt Co., Connecticut), PROXEL® (ICI Americas, New Jersey), and combinations thereof.

[0054] Sequestering agents, such as EDTA (ethylene diamine tetra acetic acid), may be included to eliminate the deleterious effects of heavy metal impurities, and buffer solutions may be used to control the pH of the fluid. From about 0.01 wt% to about 2 wt%, for example, can be used. Viscosity modifiers and buffers may also be present, as well as other additives to modify properties of the fluid as desired. Such additives can be present at from about 0.01 wt% to about 20 wt%. [0055] In certain further examples, the fusing agent can include from about 5 wt% to about 40 wt% organic co-solvent, from about 0 wt% to about 20 wt% high boiling point solvent, from about 0.1 wt% to about 1 wt% surfactant, from about 0.1 wt% to about 1 wt% anti-kogation agent, from about 0.01 wt% to about 1 wt% chelating agent, from about 0.01 wt% to about 1 wt% biocide, and from about 1 wt% to about 10 wt% carbon black pigment. The balance can be deionized water. pH Sensing Agents

[0056] A pH sensing agent can be included in the multi-fluid kits, three- dimensional printing kits, and methods described herein. This agent can be a fluid agent that is selectively applied to the powder bed, similar to the fusing agent described above. The pH sensing agent can include water and a pH indicator compound. The pH indicator compound can be responsive to changes in pH with a visible color change. By applying this agent to the powder bed during three-dimensional printing, the pH indicator compound can be incorporated into the fused polymer matrix making up the three-dimensional printed object. In certain examples, the pH sensing agent can be applied to areas of the powder bed that will become part of the surface of the three-dimensional printed object. Thus, the pH indicator compound can be present at certain areas of the surface. The areas of the surface can change color when exposed to different pH levels.

[0057] In various examples the pH indicator compound can be any compound that exhibits a color change when exposed to a different pH. In certain examples, the pH indicator compound can be an anthocyanin. Anthocyanins are a family of chemical compounds, many of which can be derived from natural sources such as leaves, flowers, fruits, and vegetables. In particular examples, pH indicator anthocyanin compounds can be derived from red cabbage, geranium, poppy, rose, hydrangea, blueberry, blackcurrant, rhubarb, lichen, and other natural sources. A variety of other pH indicator compounds can also be used, whether synthetic or derived from natural sources. Further examples of pH indicator compounds can include indigo carmine, alizarine yellow R, thymolphthalein, phenolphthalein, cresol red, naphtholphthalein, neutral red, phenol red, bromothymol blue, azolitmin, methyl purple, bromocresol green, methyl orange, congo red, bromophenol blue, methyl yellow, thymol blue, malachite green, gentian violet, and others. In certain examples, a universal pH indicator can be used. The universal indicator can include a mixture of several different pH indicator compounds that are sensitive to pH changes in different ranges of the pH scale. The combination of the pH indicator compounds can exhibit smooth color changes over a wide range of the pH scale.

[0058] In further examples, the pH indicator compound can be water- soluble. As used herein, “water-soluble” means that the material can be dissolved in water in an amount of about 5 wt% or more with respect to the total weight of the solution. In other examples, the pH indicator compound can be slightly water- soluble, meaning that the pH indicator compound can be dissolved in water in an amount from about 0.1 wt% to less than about 5 wt%. In certain examples, the pH indicator compound can be dissolved in the pH sensing agent. In such examples, the pH indicator compound will not negatively affect the jettability of the pH sensing agent.

[0059] The concentration of the pH indicator compound in the pH sensing agent can vary depending on the desired amount of pH indicator compound to be delivered to the build material during three-dimensional printing, and the amount of the pH sensing agent that is to be applied to the build material. In some examples, the pH indicator compound can be present in an amount from about 0.01 wt% to about 15 wt% with respect to the total weight of the pH sensing agent. In other examples, the pH indicator compound can be present in an amount from about 0.01 wt% to about 10 wt% or from about 0.05 wt% to about 6 wt%.

[0060] As mentioned above, in some examples the pH indicator compound can be chemically stable at the elevated temperatures used during three- dimensional printing. As used herein, “chemically stable” can be used with reference to the pH indicator compound to describe compounds that do not chemically decompose or react in such a way that compromises the pH indicating properties when heated to the melting point temperature of the polymer powder. Or, if the pH indicator compound begins to decompose or react at the melting point temperature, the decomposition or reaction can be sufficiently slow that less than 10 wt% of the pH indicator compound decomposes or reacts while the polymer particles are being fused together. The time that the pH indicator compound is exposed to high temperatures during the 3D printing processes described herein can be sufficiently short that the pH indicator compound is not significantly degraded. In certain examples, the pH indicator compound can be chemically stable at a temperature from about 70 °C to about 350 °C. In other examples, the pH indicator compound can be chemically stable at a temperature from about 100 °C to about 250 °C or from about 120 °C to about 200 °C.

[0061] The pH sensing agent can also include ingredients to allow the pH sensing agent to be jetted by a fluid jet print head. In some examples, the agent can include jettability imparting ingredients such as those in the fusing agent described above. These ingredients can include a surfactant, dispersant, co solvent, biocides, viscosity modifiers, materials for pH adjustment, sequestering agents, preservatives, and so on. These ingredients can be included in any of the amounts described above.

[0062] In certain examples, the pH sensing agent can include water and an organic co-solvent. In some examples, the amount of organic co-solvent in the pH sensing agent can be from about 2 wt% to about 10 wt%, or from about 2 wt% to about 8 wt%, or from about 3 wt% to about 6 wt% with respect to the total weight of the pH sensing agent. In other examples, the pH sensing agent can consist of water and the pH indicating compound dissolved in the water.

Detailing Agents

[0063] In further examples, multi-fluid kits or three-dimensional printing kits can include a detailing agent. The detailing agent can include a detailing compound. The detailing compound can be capable of reducing the temperature of the powder bed material onto which the detailing agent is applied. In some examples, the detailing agent can be printed around the edges of the portion of the powder that is printed with the fusing agent. The detailing agent can increase selectivity between the fused and unfused portions of the powder bed by reducing the temperature of the powder around the edges of the portion to be fused.

[0064] In some examples, the detailing compound can be a solvent that evaporates at the temperature of the powder bed. In some cases the powder bed can be preheated to a preheat temperature within about 10 °C to about 70 °C of the fusing temperature of the polymer powder. Depending on the type of polymer powder used, the preheat temperature can be in the range of about 90 °C to about 200 °C or more. The detailing compound can be a solvent that evaporates when it comes into contact with the powder bed at the preheat temperature, thereby cooling the printed portion of the powder bed through evaporative cooling. In certain examples, the detailing agent can include water, co-solvents, or combinations thereof. Non-limiting examples of co-solvents for use in the detailing agent can include xylene, methyl isobutyl ketone, 3-methoxy-3-methyl-1- butyl acetate, ethyl acetate, butyl acetate, propylene glycol monomethyl ether, ethylene glycol mono tert-butyl ether, dipropylene glycol methyl ether, diethylene glycol butyl ether, ethylene glycol monobutyl ether, 3-Methoxy-3-Methyl-1- butanol, isobutyl alcohol, 1 ,4-butanediol, N,N-dimethyl acetamide, and combinations thereof. In some examples, the detailing agent can be mostly water. In a particular example, the detailing agent can be about 85 wt% water or more. In further examples, the detailing agent can be about 95 wt% water or more. In still further examples, the detailing agent can be substantially devoid of radiation absorbers. That is, in some examples, the detailing agent can be substantially devoid of ingredients that absorb enough radiation energy to cause the powder to fuse. In certain examples, the detailing agent can include colorants such as dyes or pigments, but in small enough amounts that the colorants do not cause the powder printed with the detailing agent to fuse when exposed to the radiation energy.

[0065] The detailing agent can also include ingredients to allow the detailing agent to be jetted by a fluid jet print head. In some examples, the detailing agent can include jettability imparting ingredients such as those in the fusing agent described above. These ingredients can include a liquid vehicle, surfactant, dispersant, co-solvent, biocides, viscosity modifiers, materials for pH adjustment, sequestering agents, preservatives, and so on. These ingredients can be included in any of the amounts described above.

[0066] In certain examples, the detailing agent can include from about 1 wt% to about 10 wt% organic co-solvent, from about 1 wt% to about 20 wt% high boiling point solvent, from about 0.1 wt% to about 2 wt% surfactant, from about 0.1 wt% to about 5 wt% anti-kogation agent, from about 0.01 wt% to about 5 wt% chelating agent, from about 0.01 wt% to about 4 wt% biocide, and the balance can be deionized water.

Methods of Sensing pH

[0067] The present disclosure also describes methods of sensing pH. FIG. 6 shows a flowchart illustrating one example method 600 of sensing pH. The method includes: exposing a three-dimensional printed pH sensor to a solution having a pH, wherein the three-dimensional printed pH sensor includes multiple fused layers of polymer particles, wherein a portion of a surface of the three- dimensional printed pH sensor includes a pH indicator compound immobilized in the fused polymer, wherein the pH indicator compound is responsive to a change in pH with a visible color change 610. As mentioned above, the area where the pH indicator compound is present can be designed and controlled on the voxel level during three-dimensional printing. Thus, any pattern, design, or image can be formed with the pH indicator compound. The area that includes the pH indicator compound can change color when the three-dimensional printed pH sensor is exposed to different pH levels.

[0068] In further examples, the methods can also include making the three-dimensional printed pH sensor. In a particular example, a method of making the three-dimensional printed pH sensor can include: iteratively applying individual build material layers of the polymer particles to a powder bed; based on a three-dimensional object model, selectively jetting a fusing agent onto the individual build material layers, wherein the fusing agent includes water and an electromagnetic radiation absorber; based on the three-dimensional object model, selectively jetting a pH sensing agent onto the individual build material layers, wherein the pH sensing agent includes water and the pH indicator compound; and exposing the powder bed to energy to selectively fuse the polymer particles in contact with the electromagnetic radiation absorber to form a fused polymer matrix at individual build material layers.

[0069] The methods of sensing pH and making the three-dimensional printed pH sensors can be performed using any of the fluid agents, powder bed materials, and other components of the multi-fluid kits and three-dimensional printing kits described above. In some examples, applying individual build material layers can be repeated multiple times to form multiple layers or slices of the three-dimensional printed object. The fusing agent and pH sensing agent can be applied to the individual layers as desired. The areas where the pH sensing agent is applied can become portions of the surface that have the pH indicator compound incorporated into the fused polymer in the finished three-dimensional printed pH sensor.

[0070] In some examples, the surface of the final three-dimensional printed pH sensor can have a pattern of pH sensing areas and areas that do not react to pH. Applying the pH sensing agent using precise methods such as fluid jetting can allow for voxel-level control over the placement of the pH sensing agent. Therefore, high resolution patterns of pH sensitive areas can be formed on the surface of the three-dimensional printed object.

[0071] In further examples, the three-dimensional printing process can be performed using a colorless or low tint fusing agent. Thus, the fusing agent itself can have a minimal effect on the color of the three-dimensional printed object. Therefore, the color change exhibited by the pH indicator compound can be easily visible when the fusing agent is low tint or colorless.

[0072] The fusing agent, pH sensing agent, and detailing agent can be jetted onto the powder bed using fluid jet print heads. The amount of the fusing agent used can be calibrated based on the concentration of radiation absorber in the fusing agent, the level of fusing desired for the polymer particles, and other factors. In some examples, the amount of fusing agent printed can be sufficient to contact the radiation absorber with the entire layer of polymer powder. For example, if individual layers of polymer powder are 100 microns thick, then the fusing agent can penetrate 100 microns into the polymer powder. Thus the fusing agent can heat the polymer powder throughout the entire layer so that the layer can coalesce and bond to the layer below. After forming a solid layer, a new layer of loose powder can be formed, either by lowering the powder bed or by raising the height of a powder roller and rolling a new layer of powder.

[0073] In some examples, the entire powder bed can be preheated to a temperature below the melting or softening point of the polymer powder. In one example, the preheat temperature can be from about 10°C to about 30°C below the melting or softening point. In another example, the preheat temperature can be within 50°C of the melting of softening point. In a particular example, the preheat temperature can be from about 160°C to about 170°C and the polymer powder can be polyamide 12 powder. In another example, the preheat temperature can be about 90°C to about 100°C and the polymer powder can be thermoplastic polyurethane. Preheating can be accomplished with a lamp or lamps, an oven, a heated support bed, or other types of heaters. In some examples, the entire powder bed can be heated to a substantially uniform temperature.

[0074] The powder bed can be irradiated with a fusing lamp. Suitable fusing lamps for use in the methods described herein can include commercially available infrared lamps and halogen lamps. The fusing lamp can be a stationary lamp or a moving lamp. For example, the lamp can be mounted on a track to move horizontally across the powder bed. Such a fusing lamp can make multiple passes over the bed depending on the amount of exposure to coalesce printed layers. The fusing lamp can be configured to irradiate the entire powder bed with a substantially uniform amount of energy. This can selectively coalesce the printed portions with fusing agent leaving the unprinted portions of the polymer powder below the melting or softening point.

[0075] In one example, the fusing lamp can be matched with the radiation absorber in the fusing agent so that the fusing lamp emits wavelengths of light that match the peak absorption wavelengths of the radiation absorber. A radiation absorber with a narrow peak at a particular near-infrared wavelength can be used with a fusing lamp that emits a narrow range of wavelengths at approximately the peak wavelength of the radiation absorber. Similarly, a radiation absorber that absorbs a broad range of near-infrared wavelengths can be used with a fusing lamp that emits a broad range of wavelengths. Matching the radiation absorber and the fusing lamp in this way can increase the efficiency of coalescing the polymer particles with the fusing agent printed thereon, while the unprinted polymer particles do not absorb as much light and remain at a lower temperature.

[0076] Depending on the amount of radiation absorber present in the polymer powder, the absorbance of the radiation absorber, the preheat temperature, and the melting or softening point of the polymer, an appropriate amount of irradiation can be supplied from the fusing lamp. In some examples, the fusing lamp can irradiate individual layers from about 0.5 to about 10 seconds per pass.

[0077] The three-dimensional printed object can be formed by jetting a fusing agent onto layers of powder bed build material according to a three- dimensional object model. Three-dimensional object models can in some examples be created using computer aided design (CAD) software. Three- dimensional object models can be stored in any suitable file format. In some examples, a three-dimensional printed object as described herein can be based on a single three-dimensional object model. The three-dimensional object model can define the three-dimensional shape of the object. In some examples, the three-dimensional object model can also include a particular three-dimensional portion of the object that is desired to include the pH indicator compound from the pH sensing agent. Other information may also be included, such as structures to be formed of additional different materials or color data for printing the object with various colors at different locations on the object. The three-dimensional object model may also include features or materials specifically related to jetting fluids on layers of powder bed material, such as the desired amount of fluid to be applied to a given area. This information may be in the form of a droplet saturation, for example, which can instruct a three-dimensional printing system to jet a certain number of droplets of fluid into a specific area. This can allow the three-dimensional printing system to finely control radiation absorption, cooling, color saturation, concentration of the pH indicator compound, and so on. All this information can be contained in a single three-dimensional object file or a combination of multiple files. The three-dimensional printed object can be made based on the three-dimensional object model. As used herein, “based on the three-dimensional object model” can refer to printing using a single three- dimensional object model file or a combination of multiple three-dimensional object models that together define the object. In certain examples, software can be used to convert a three-dimensional object model to instructions for a three- dimensional printer to form the object by building up individual layers of build material.

[0078] In an example of the three-dimensional printing process, a thin layer of polymer powder can be spread on a bed to form a powder bed. At the beginning of the process, the powder bed can be empty because no polymer particles have been spread at that point. For the first layer, the polymer particles can be spread onto an empty build platform. The build platform can be a flat surface made of a material sufficient to withstand the heating conditions of the three-dimensional printing process, such as a metal. Thus, “applying individual build material layers of polymer particles to a powder bed” includes spreading polymer particles onto the empty build platform for the first layer. In other examples, a number of initial layers of polymer powder can be spread before the printing begins. These “blank” layers of powder bed material can in some examples number from about 10 to about 500, from about 10 to about 200, or from about 10 to about 100. In some cases, spreading multiple layers of powder before beginning the print can increase temperature uniformity of the three- dimensional printed object. A fluid jet printing head, such as an inkjet print head, can then be used to print a fusing agent including a radiation absorber over portions of the powder bed corresponding to a thin layer of the three-dimensional object to be formed. Then the bed can be exposed to electromagnetic energy, e.g., typically the entire bed. The electromagnetic energy can include light, infrared radiation, and so on. The radiation absorber can absorb more energy from the electromagnetic energy than the unprinted powder. The absorbed light energy can be converted to thermal energy, causing the printed portions of the powder to soften and fuse together into a formed layer. After the first layer is formed, a new thin layer of polymer powder can be spread over the powder bed and the process can be repeated to form additional layers until a complete three- dimensional object is printed. Thus, “applying individual build material layers of polymer particles to a powder bed” also includes spreading layers of polymer particles over the loose particles and fused layers beneath the new layer of polymer particles.

Definitions

[0079] It is noted that, as used in this specification and the appended claims, the singular forms ”a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. [0080] As used herein, “colorant” can include dyes and/or pigments.

[0081] As used herein, “dye” refers to compounds or molecules that absorb electromagnetic radiation or certain wavelengths thereof. Dyes can impart a visible color to an ink if the dyes absorb wavelengths in the visible spectrum.

[0082] As used herein, “pigment” includes pigment colorants, magnetic particles, aluminas, silicas, and/or other ceramics, organo-metallics or other opaque particles, whether or not such particulates impart color. Thus, though the present description describes the use of pigment colorants, the term “pigment” can be used to describe pigment colorants, and also other pigments such as organometallics, ferrites, ceramics, etc. In one specific aspect, however, the pigment is a pigment colorant.

[0083] As used herein, “applying” when referring to fusing agent and/or detailing agent, for example, refers to any technology that can be used to put or place the respective fluid agent on or into a layer of powder bed material for forming three-dimensional objects. For example, “applying” may refer to “jetting,” “ejecting,” “dropping,” “spraying,” or the like.

[0084] As used herein, “jetting” or “ejecting” refers to applying fluid agents or other compositions by expelling from ejection or jetting architecture, such as ink-jet architecture. Ink-jet architecture can include thermal or piezo architecture. Additionally, such architecture can be configured to print varying drop sizes such as from about 3 picoliters to less than about 10 picoliters, or to less than about 20 picoliters, or to less than about 30 picoliters, or to less than about 50 picoliters, etc.

[0085] As used herein, “average particle size” refers to a number average of the diameter of the particles for spherical particles, or a number average of the volume equivalent sphere diameter for non-spherical particles. The volume equivalent sphere diameter is the diameter of a sphere having the same volume as the particle. Average particle size can be measured using a particle analyzer such as the MASTERSIZER™ 3000 available from Malvern Panalytical (United Kingdom). The particle analyzer can measure particle size using laser diffraction. A laser beam can pass through a sample of particles and the angular variation in intensity of light scattered by the particles can be measured. Larger particles scatter light at smaller angles, while small particles scatter light at larger angles. The particle analyzer can then analyze the angular scattering data to calculate the size of the particles using the Mie theory of light scattering. The particle size can be reported as a volume equivalent sphere diameter.

[0086] As used herein, the term “substantial” or “substantially” when used in reference to a quantity or amount of a material, or a specific characteristic thereof, refers to an amount that is sufficient to provide an effect that the material or characteristic was intended to provide. The exact degree of deviation allowable may in some cases depend on the specific context. When using the term “substantial” or “substantially” in the negative, e.g., substantially devoid of a material, what is meant is from none of that material is present, or at most, trace amounts could be present at a concentration that would not impact the function or properties of the composition as a whole.

[0087] As used herein, the term “about” is used to provide flexibility to a numerical range endpoint by providing that a given value may be “a little above” or “a little below” the endpoint. The degree of flexibility of this term can be dictated by the particular variable and determined based on the associated description herein.

[0088] As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though individual members of the list are individually identified as separate and unique members. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary.

[0089] Concentrations, amounts, and other numerical data may be expressed or presented herein in a range format. It is to be understood that such a range format is used merely for convenience and brevity and thus should be interpreted flexibly to include the numerical values explicitly recited as the limits of the range, and also to include individual numerical values or sub-ranges encompassed within that range as if numerical values and sub-ranges are explicitly recited. As an illustration, a numerical range of “about 1 wt% to about 5 wt%” should be interpreted to include the explicitly recited values of about 1 wt% to about 5 wt%, and also include individual values and sub-ranges within the indicated range. Thus, included in this numerical range are individual values such as 2, 3.5, and 4 and sub-ranges such as from 1-3, from 2-4, and from 3-5, etc. This same principle applies to ranges reciting a single numerical value. Furthermore, such an interpretation should apply regardless of the breadth of the range or the characteristics being described.

EXAMPLES

[0090] The following illustrates examples of the present disclosure. However, it is to be understood that the following are merely illustrative of the application of the principles of the present disclosure. Numerous modifications and alternative devices, methods, and systems may be devised without departing from the spirit and scope of the present disclosure. The appended claims are intended to cover such modifications and arrangements.

Example 1 - pH Sensing Agent

[0091] A sample pH sensing agent was prepared with the composition shown in Table 1 . The red cabbage extract included naturally occurring anthocyanins that acted as the pH indicator compound. Table 1

Example 2 - Two-dimensional Print Test

[0092] The sample pH sensing agent from Example 1 was loaded in a two- dimensional inkjet printer to test the jettability of the agent. A test pattern was printed on white paper. The test pattern printed successfully, demonstrating that the agent had acceptable jettability. When printed on the paper, the pH sensing agent had a purple color, which indicates a pH of 6-7. To test the pH sensing capability of the agent, a drop of acetic acid solution was placed onto the printed area. The color turned pink when the pH sensing agent contacted the acetic acid solution. A drop of potassium hydroxide solution was then dropped onto another printed area to test the reaction to a basic solution. The color turned yellow when the pH sensing agent contacted the potassium hydroxide solution.

Example 3 - Three-dimensional Printed pH Sensors

[0093] A set of three sample three-dimensional printed pH sensor cubes were made using an HP MULTI-JET FUSION 3D® test printer (HP, Inc, USA). The powder bed material was polyamide 12 polymer particles. The fusing agent used for printing was a low tint fusing agent with a very light bluish gray color.

The sample pH sensing agent from Example 1 was also loaded in a fluid jet pen of the test printer.

[0094] Three cubes were printed. The pH sensing agent was applied to the powder bed in areas that became the surfaces of the cubes. When the cubes were finished printing, the cubes had a pink color. This suggests that three- dimensional printed objects made using this combination of fusing agent and polymer particles are initially somewhat acidic. After the cubes were finished, a drop of deionized water was dropped onto one of the cubes. This caused the surface of the cube to turn a purple color. This indicates that adding the deionized water change the pH at the surface from acidic to a more neutral pH. A drop of potassium hydroxide solution was dropped onto another cube. This caused the surface of the cube to turn yellow. A drop of acetic acid solution was dropped onto the third cube, but the color remained approximately the same pink color because the cubes had an acidic pH to begin with.

[0095] The color changes of the cubes indicate that the pH sensing agent retained its ability to change color in response to pH changes after the three- dimensional printing process. Thus, the pH indicator compound used in the sample pH sensing agent was sufficiently chemically stable at the melting temperature of the polyamide 12 polymer particles.

Claims

What is claimed is: 1. A multi-fluid kit for three-dimensional printing comprising: a fusing agent comprising water and an electromagnetic radiation absorber, wherein the electromagnetic radiation absorber absorbs radiation energy and converts the radiation energy to heat; and a pH sensing agent comprising water and a pH indicator compound, wherein the pH indicator compound is responsive to a change in pH with a visible color change.

2. The multi-fluid kit of claim 1 , wherein the pH indicator compound is water-soluble.

3. The multi-fluid kit of claim 1 , wherein the pH indicator compound is chemically stable at a temperature from about 70 °C to about 350 °C.

4. The multi-fluid kit of claim 1 , wherein the pH indicator compound comprises an anthocyanin.

5. The multi-fluid kit of claim 1 , wherein the pH indicator compound is present in an amount from about 0.01 wt% to about 15 wt% with respect to the total weight of the pH sensing agent.

6. The multi-fluid kit of claim 1 , further comprising a post-processing agent having a pH that is different from the pH of the pH sensing agent and sufficient to induce a color change of the pH indicator compound.

7. The multi-fluid kit of claim 1 , wherein the fusing agent is a colorless fusing agent or a low tint fusing agent.

8. A three-dimensional printing kit comprising: a powder bed material comprising polymer particles; a fusing agent to selectively apply to the powder bed material, the fusing agent comprising water and an electromagnetic radiation absorber, wherein the electromagnetic radiation absorber absorbs radiation energy and converts the radiation energy to heat; and a pH sensing agent to selectively apply to the powder bed material, the pH sensing agent comprising water and a pH indicator compound, wherein the pH indicator compound is responsive to a change in pH with a visible color change.

9. The three-dimensional printing kit of claim 8, wherein the polymer particles comprise polyamide 6, polyamide 9, polyamide 11 , polyamide 12, polyamide 66, polyamide 612, thermoplastic polyamide, polyamide copolymer, polyethylene, thermoplastic polyurethane, polypropylene, polyester, polycarbonate, polyether ketone, polyacrylate, polystyrene, polyvinylidene fluoride, polyvinylidene fluoride copolymer, poly(vinylidene fluoride- trifluoroethylene), poly(vinylidene fluoride-trifluoroethylene- chlorotrifluoroethylene), wax, or a combination thereof.

10. The three-dimensional printing kit of claim 8, wherein the pH indicator compound is chemically stable at a melting point temperature of the polymer particles.

11. The three-dimensional printing kit of claim 8, wherein the pH indicator compound comprises an anthocyanin and is present in an amount from about 0.01 wt% to about 15 wt% with respect to the total weight of the pH sensing agent.

12. A method of sensing pH comprising: exposing a three-dimensional printed pH sensor to a solution having a pH, wherein the three-dimensional printed pH sensor comprises multiple fused layers of polymer particles, wherein a portion of a surface of the three-dimensional printed pH sensor includes a pH indicator compound immobilized in the fused polymer, wherein the pH indicator compound is responsive to a change in pH with a visible color change.

13. The method of claim 12, further comprising making the three- dimensional printed pH sensor by: iteratively applying individual build material layers of the polymer particles to a powder bed; based on a three-dimensional object model, selectively jetting a fusing agent onto the individual build material layers, wherein the fusing agent comprises water and an electromagnetic radiation absorber; based on the three-dimensional object model, selectively jetting a pH sensing agent onto the individual build material layers, wherein the pH sensing agent comprises water and the pH indicator compound; and exposing the powder bed to energy to selectively fuse the polymer particles in contact with the electromagnetic radiation absorber to form a fused polymer matrix at individual build material layers.

14. The method of claim 13, wherein the fusing agent is a colorless fusing agent or a low tint fusing agent.

15. The method of claim 12, wherein the pH indicator compound comprises an anthocyanin.