Microwave-induced localized heating of CNT filled polymer composites for enhanced inter-bead diffusive bonding of fused filament fabricated part

Abstract

A microwave-induced heating of CNT filled (or coated) polymer composites for enhancing inter-bead diffusive bonding of fused filament fabricated parts. The technique incorporates microwave absorbing nanomaterials (carbon nanotubes (CNTs)) onto the surface or throughout the volume of 3D printer polymer filament to increase the inter-bead bond strength following a post microwave irradiation treatment and/or in-situ focused microwave beam during printing. The overall strength of the final 3D printed part will be dramatically increased and the isotropic mechanical properties of fused filament part will approach or exceed conventionally manufactured counterparts.

Claims

1. A polymer composite comprising polymer filaments each having an outer layer in which the polymer filaments are arranged such that there are filament-filament interfaces between the outer layers of the polymer filaments, wherein (a) the outer layer of each of the polymer filaments comprises an electrically conductive percolated network of an electrically conductive nanomaterial, (b) the electrically conductive nanomaterial comprises carbon nanotubes that are mixed with polylactic acid (PLA) in the outer layer, (c) the outer layer of each of the polymer filaments has a thickness between 0.005% and 30% of a diameter of the polymer filament, and (d) the electrically conductive nanomaterial is heat-able by an electromagnetic field to generate a temperature profile in the polymer filaments that forms a weld at each of the filament-filament interfaces between the outer layers of the polymer filaments.

2. The polymer composite of claim 1, wherein the polymer filament comprises at least one polymer selected from a group consisting of styrene (ABS), polycarbonate (PC), polylactic acid (PLA), polyethylene (PE), polycarbonate(PC)/styrene(ABS), polyphenylsulfone (PPSU), polyamide (nylon), polystyrene (PS), polyetherimide, polyether ether ketone (PEEK), polytetrafluoroethylene (PTFE), and combinations thereof.

3. The polymer composite of claim 1, wherein the carbon nanotubes are multi-walled carbon nanotubes.

4. The polymer composite of claim 1, wherein the outer layer comprises between 1 wt % and 50 wt % of carbon nanotubes.

5. The polymer composite of claim 1, wherein the outer layer has a wt % of carbon nanotubes sufficient to form the electrically conductive percolated network of the electrically conductive material.

6. The polymer composite of claim 1, wherein the polymer filament has a volume and the electrically conductive nanomaterial is present throughout the volume.

7. A polymer composite comprising polymer filaments each having an outer layer in which the polymer filaments are arranged such that there are filament-filament interfaces between the outer layers of the polymer filaments, wherein (a) the outer layer of each of the polymer filaments comprises an electrically conductive nanomaterial, (b) the electrically conductive nanomaterial comprises carbon nanotubes that are mixed with polylactic acid (PLA) in the outer layer, (c) the outer layer of each of the polymer filaments has a thickness between 0.005% and 30% of a diameter of the polymer filament, and (d) the electrically conductive nanomaterial is heat-able by an electromagnetic wave, the electrically conductive nanomaterial provided at a concentration sufficient to provide a threshold dielectric loss tangent of the polymer filaments that provides fusing at each of the filament-filament interfaces between the outer layers of the polymer filaments upon the electrically conductive nanomaterial absorbing an electromagnetic wave selected to produce a predetermined temperature profile in the outer layer.

8. The polymer composite of claim 7, wherein the threshold dielectric loss tangent maximizes an electric current induced in the electrically conductive nanomaterial by the electromagnetic wave.

9. The polymer composite of claim 7, wherein the concentration of the electrically conductive nanomaterial is adjusted such that the outer layer exhibits a desired bond strength at the interface.

10. The polymer composite of claim 7, wherein the outer layer comprises polymer chains that inter-diffuse across the interface upon fusing.

11. The polymer composite of claim 7, wherein the polymer filament comprises at least one polymer selected from a group consisting of styrene (ABS), polycarbonate (PC), polylactic acid (PLA), polyethylene (PE), polycarbonate(PC)/styrene(ABS), polyphenylsulfone (PPSU), polyamide (nylon), polystyrene (PS), polyetherimide, polyether ether ketone (PEEK), polytetrafluoroethylene (PTFE), and combinations thereof.

12. The polymer composite of claim 7, wherein the outer layer has a wt % of carbon nanotubes sufficient to form an electrically conductive percolated network of the electrically conductive material.

13. The polymer composite of claim 7, wherein the polymer filament has a volume and the electrically conductive nanomaterial is present throughout the volume.

14. A three-dimensional object comprising a plurality of polymer filaments arranged in successive layers, wherein (a) the polymer filaments each comprise a coating comprising an electrically conductive percolated network of an electrically conductive nanomaterial, (b) the electrically conductive nanomaterial comprises carbon nanotubes that are mixed with polylactic acid (PLA) in the coating, (c) the coating of each of the polymer filaments is an outer layer which has a thickness between 0.005% and 30% of a diameter of the polymer filament, and (d) the electrically conductive material is heat-able by an electromagnetic wave to generate a temperature profile in the polymer filament that fuses the coating of the polymer filaments together at a filament-filament interface between the outer layers of the polymer filaments in each of the successive layers.

15. The three-dimensional object of claim 14, wherein the polymer filaments comprise at least one polymer selected from a group consisting of styrene (ABS), polycarbonate (PC), polylactic acid (PLA), polyethylene (PE), polycarbonate(PC)/styrene(ABS), polyphenylsulfone (PPSU), polyamide (nylon), polystyrene (PS), polyetherimide, polyether ether ketone (PEEK), polytetrafluoroethylene (PTFE), and combinations thereof.

16. The three-dimensional object of claim 14, wherein the outer layer has a wt % of carbon nanotubes sufficient to form the electrically conductive percolated network of the electrically conductive material.

17. The three-dimensional object of claim 14, wherein the polymer filament has a volume and the electrically conductive nanomaterial is present throughout the volume.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For better understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings.

(2) FIG. 1A illustrates a process for coating a polymer filament exterior with carbon nanotubes by spray deposition.

(3) FIG. 1B is an illustration showing the spray deposition process illustrated in FIG. 1A.

(4) FIG. 2 is an illustration showing an alternative process for coating a polymer filament exterior with carbon nanotubes by dip coating.

(5) FIG. 3 is an illustration of a carbon nanotube/PLA-coated filament.

(6) FIG. 4 illustrates a process in which the coated polymer filaments (after the spray deposition shown in FIG. 1) are exposed to microwave heating.

(7) FIG. 5 is an illustration of polymer filaments partially coated by spray deposition (as shown in FIG. 1) and then exposed to microwave heating (as shown in FIG. 4).

(8) FIG. 6 illustrates polymer filaments (after the spray deposition shown in FIG. 1) arranged in a crosshatch stack arrangement being exposed to microwaves.

(9) FIG. 7 is a photograph of a microwave-fused crosshatch stack made by coating polymer filaments by spray deposition (as shown in FIG. 1) and arranging and then exposing the crosshatch stack of coated polymer filaments to microwaves (as shown in FIG. 6). The magnified portion in this FIG. 7 is an optical microscope image of the microwave-fused crosshatch stack 701.

(10) FIG. 8 is two photographs of similarly made 3D printed objects (object 801 and object 1102). Object 801 was made by an additive manufacturing process with pure ABS printer filament. Object 1102 was made by the same additive manufacturing process with carbon nanotube coated ABS printer filament, which was treated post-printing with microwave irradiation.

(11) FIG. 9 is a photograph of tensile dogbone samples with (902) and without (901) CNTs incorporated into the printing process.

(12) FIG. 10 is a photograph of compression testing samples with (1002) and without (1001) CNTs incorporated into the printing process. The image shows the samples after compression testing and the difference in compressive strength with the CNTs and microwave treatment applied to the sample.

(13) FIG. 11 is an SEM of a cross-sectional view of a polymer filament coated with a dispersion of carbon nanotubes.

(14) FIG. 12 is an SEM of the coated polymer filament of FIG. 11 (further magnified) showing the randomly oriented multi-walled carbon nanotubes.

(15) FIG. 13 is an SEM of a 3D printed object (viewed from the top surface) made by an embodiment of the present invention (after being irradiated with microwaves).

(16) FIG. 14 is an SEM of the 3D printed object of FIG. 13 (viewed from the top surface and further magnified) showing the randomly oriented multi-walled carbon nanotubes.

(17) FIG. 15 is an SEM of a cross-section of the 3D printed object of FIG. 7.

(18) FIG. 16A is a photograph of a bundle of multi-wall carbon nanotube coated PLA filaments. FIG. 16B is an image from a forward-looking infrared (FLIR camera) of the temperature profile of the bundle of FIG. 16A upon microwave exposure.

(19) FIG. 17 is a graph reflecting the DC conductivity (1701) and AC conductivity (1702) versus the multi-walled loading in PLA films.

(20) FIG. 18 is an image from a forward-looking infrared (FLIR camera) of a PLA/multi-walled carbon nanotube samples with a loading of 0.1 wt %.

(21) FIG. 19 is an image from a FLIR camera of a PLA/multi-walled carbon nanotube samples with a loading of 0.5 wt %.

(22) FIG. 20A is an image from a FLIR camera of a PLA/multi-walled carbon nanotube samples with a loading of 1.0 wt %. FIG. 20B is enlarged view of the image of FIG. 20A.

(23) FIG. 20C is an ANSYS simulation of volume loss density (roughly proportional to power dissipated as heat) as a function of CNT loading of 1.0 wt % in homogeneous polymer nanocomposites.

(24) FIG. 21A is an image from a FLIR camera of a PLA/multi-walled carbon nanotube samples with a loading of 2.5 wt %. FIG. 21B is enlarged view of the image of FIG. 21A.

(25) FIG. 21C is an ANSYS simulation of volume loss density as a function of CNT loading of 2.5 wt % in homogeneous polymer nanocomposites.

(26) FIG. 22A is an image from a FLIR camera of a PLA/multi-walled carbon nanotube samples with a loading of 5 wt %. FIG. 22B is enlarged view of the image of FIG. 22A.

(27) FIG. 22C is an ANSYS simulation of volume loss density as a function of CNT loading of 5 wt % in homogeneous polymer nanocomposites.

(28) FIG. 23A is an image from a FLIR camera of a PLA/multi-walled carbon nanotube samples with a loading of 10 wt %. FIG. 23B is enlarged view of the image of FIG. 23A.

(29) FIG. 23C is an ANSYS simulation of volume loss density as a function of CNT loading of 10 wt % in homogeneous polymer nanocomposites.

(30) FIG. 24 is a graph of the maximum temperature versus time of PLA/multi-walled carbon nanotube samples (with loadings of 0.1 wt %, 0.5 wt %, 1 wt %, 2.5 wt %, 5 wt %, and 10 wt %).

(31) FIG. 25A is a graph of the DSC curves for neat PLA and PLA/multi-walled carbon nanotube (10 wt %).

(32) FIG. 25B is a graph of the DSC thermogram for neat PLA and PLA/multi-walled carbon nanotube (10 wt %).

(33) FIG. 25C is a graph showing the heat capacity at constant pressure versus temperature for neat PLA and PLA/multi-walled carbon nanotube (10 wt %).

(34) FIG. 26 is a graph showing simulated results of the normalized microwave absorbed power versus the loss tangent for fixed dielectric constant of a simulated lossy dielectric in a rectangular waveguide with a constant dielectric constant.

(35) FIG. 27 shows the electric field magnitude in a bundle of PLA filaments coated with a thin conductive shell representing CNTs.

(36) FIG. 28 illustrates a schematic of peel tests for microwave enhanced CNT/polymer film adhesion.

(37) FIG. 29 illustrates a process of an embodiment of the present invention wherein an in-situ microwave applicator applies microwave energy at the point of extrusion to control the localized heating and welding of the polymer.

DETAILED DESCRIPTION

(38) The present invention embodies a technique for incorporating microwave absorbing nanomaterials (carbon nanotubes (CNTs)) onto the surface (or throughout the volume) of 3D printer polymer filament to increase the inter-bead bond strength following a post microwave irradiation treatment and/or in-situ focused microwave beam during printing. In embodiments of the present invention, additive manufacturing techniques, in which successive layers are laid down additively into the desired shape, are utilized by 3D printers to manufacture thermoplastic parts. These 3D printers can print intricate product designs much faster than regular machining techniques; however, the strength of the final part may be compromised when compared to parts produced through conventional machining methods. A treatment process of the present invention is used to address the compromise of the strength of the produced part as compared to parts produced through conventional machining methods. The treatment process is used to eradicate this weakness of 3D-printed parts. The treatment process includes coating the part with a carbon nanotube (CNT)-coating and application of microwave irradiation (often successive applications of microwave irradiation). The microwave irradiation can occur during the additive manufacturing process and/or after completion of the additive manufacturing process.

(39) Coating Polymer Filaments

(40) Referring to FIG. 1A, FIG. 1A illustrates a process for coating a polymer filament 103 exterior with carbon nanotubes by spray deposition. In this embodiment, an airbrush 101 that has a reservoir 102 of a carbon nanotube dispersion is used to coat the exterior of polymer filament 103, such as an acrylonitrile butadiene styrene (ABS) filament. The polymer filament 103 can be made from other polymers, such as polycarbonate (PC), polylactic acid (PLA), high density polyethylene (HDPE), PC/ABS, and polyphenylsulfone (PPSU), polyamide (nylon), high impact polystyrene (HIPS), polyetherimide (ULTEM™), polyether ether ketone (PEEK), polytetrafluoroethylene (PTFE), and combinations thereof. The carbon nanotube dispersion was made by dissolving the thermoplastic in a suitable solvent (acetone for ABS) at a concentration of 10 mg/ml and adding multi-walled carbon nanotube powder (CheapTubes.com, Brattleboro, Vt.) at a concentration of 0.5 mg/ml followed by probe sonicating for 30 minutes. Other solvents that can be used are, for example, chloroform for PLA.

(41) Using the airbrush 101, a spray 104 of the carbon nanotube deposition is applied to polymer filament 103 to form a coating 105 upon the polymer filament 103. FIG. 1B is an illustration showing the spray deposition process illustrated in FIG. 1A.

(42) Alternative processes to coat the exterior of polymer filament 103 with a coating 105 include dip coating the bare filament in a bath of the CNT dispersion (such as shown in FIG. 2), simultaneous co-extrusion of a CNT polymer masterbatch sheath and pure polymer core coaxial filament, and combinations thereof. See U.S. Pat. No. 3,458,615, “Hydrodynamically Centering Sheath/Core Filament Spinnerette,” issued Jul. 29, 1969, to Bragaw et al.; U.S. Pat. No. 4,680,156, “Sheath Core Composite Extrusion And A Method Of Making It By Melt Transformation Coextrusion,” issued Jul. 14, 1987, to Collier; and U.S. Pat. No. 5,219,508, “Method Of Manufacturing Sheath Core Fiber,” issued Jun. 15, 1993, to Collier et al. The dip coating process shown in FIG. 2 is scalable for creating processing uniform filament coatings.

(43) For instance, the coating bath can be a MWCNT-OH/PLA/chloroform dispersion. In the case of dip coating a more viscous coating solution is used for controlling the final coating thickness. In one embodiment this viscosity increase is achieved by increasing the solids weight percent in the chloroform to 40 mg/ml of solvent. As the filament passes through the bath (such as shown in FIG. 2), a thin MWCNT/PLA shell is deposited onto the filament surface. The drying time is rapid enough such that the filament bulk is not damaged by the chloroform, and recovers its original mechanical properties after vacuum drying in an oven at 100° C. for 24 hours. The filament coating can be a MWCNT/PLA composite film because a MWCNT-only coating would generally not be sufficiently bonded to the surface to survive a 3-D printing process intact.

(44) FIG. 3 is an illustration of a carbon nanotube/PLA-coated filament made with the dip coating process described and depicted in FIG. 2.

(45) Carbon Nanotubes

(46) Types of carbon nanotubes used in the present invention include single-wall carbon nanotubes, multi-walled carbon nanotubes, functionalized carbon nanotubes, etc. Other forms of microwave absorbing nanomaterials can also be used, including carbon black, buckyballs, graphene, superparamagnetic or magnetic nanoparticles, metallic or semiconducting nanowires and quantum dots, and certain conductive polymers, such as polyaniline (PANI) or poly3,4-ethylenedioxythiophene polystyrenesulfonate (PEDOT:PSS).

(47) Microwave Heating

(48) FIG. 4 illustrates a process in which the coated polymer filaments 103 (after the spray deposition shown in FIG. 1) are exposed to microwaves 401. Although the pure polymer filaments 103 are relatively insensitive to the microwaves 401, the carbon nanotubes in the coating 105 display an extraordinary sensitivity to microwave exposure and these generate localized heating. This localized heating is discussed and described, for example, in Irin, F., et al., “Detection of carbon nanotubes in biological samples through microwave-induced heating,” Carbon, 2012, 50, (12), 4441-4449 and Vegesna, S., et al., “Non-destructive Technique for Broadband Characterization of Carbon Nanotubes at Microwave Frequencies,” Journal of Electromagnetic Waves and Applications, 2013, 27, (11), 1372-1381. The localized heating of the microwaved carbon nanotubes causes selective fusing at the filament-filament interface 402, which allows for polymer diffusion and formation of a continuous polymer structure that is markedly different than the native, as-printed, 3D structure. The polymer filaments 103 are able to fuse at the filament-filament interface 402 once adjacent polymer filaments 103 wet at the filament-filament interface 402 and the polymer chains are able to inter-diffuse across the interface. For example, polymer filaments 103 can wet at the filament-filament interface 402 by melting or, for polymers that do not melt (such as amorphous polymers), they can flow as non-Newtonian fluids when the polymer filament 103 is raised to a temperature significantly above its glass transition temperature.

(49) The microwaves can be applied during or after the 3D printing process. In some embodiments, the microwaves 401 are applied during post-fabrication of the object once the additive manufacture process is complete. In other embodiments, the microwaves are applied as the successive layers are being laid down (i.e., an “in-situ” process). Such an in-situ process can be employed by a focused microwave beam to heat the filament-filament interface during the printing process itself, which integrates the microwave exposure system into the 3D printer hardware (with appropriate electromagnetic shielding). “Welding” these filament-filament interfaces during manufacture strengthens the structure. An additional benefit is that this process enables certain shapes previously unattainable by conventional extrusion deposition processes to be manufactured. For instance, an extrusion deposition process typically cannot produce overhanging structures or large gaps, since they would be unsupported during the build. Although large unsupported bridging has been demonstrated *(http://www.thingiverse.com/thing:12925, http://youtu.be/_b6hFFcLh_Q and http://youtu.be/wK2APNwEoSk)*, printing additional layers over the bridge generally results in sagging due to the latent heat of the subsequent polymer bead and the added weight. By increasing the strength of the welds during the 3D printing process with the invention described herein, such bridge-like structures may be obtained with better dimensional tolerances. This is important for complex parts, where significant amounts of break-away or dissolvable support material are typically utilized.

(50) Polymer Filament Bundle

(51) FIG. 5 is an illustration of a polymer filament bundle 501 of polymer filaments 103 partially coated (with coating 105) by spray deposition (as shown in FIG. 1) and then exposed to microwave heating (as shown in FIG. 4). The polymer filaments 103 were made of ABS and were exposed with microwaves 401 (at around 2.45 GHz for around 20 seconds in a conventional microwave oven) to form a fused structure with minimal air gaps between the filaments (in the coated section of the polymer filaments 103). In contrast, the uncoated portion of the polymer filaments 103 (i.e., the microwave transparent ABS filament portions) were unaffected by the microwave treatment process 401 and did not fuse together.

(52) Crosshatch Stack

(53) FIG. 6 illustrates polymer filaments 103 (after the spray deposition shown in FIG. 1 to form coating 105 on the exterior) arranged in a crosshatch stack arrangement being exposed to microwaves 401. FIG. 7 is a photograph of a microwave-fused crosshatch stack 701 made by coating polymer filaments by spray deposition (as shown in FIG. 1) and arranging and then exposing the crosshatch stack of coated polymer filaments to microwaves (as shown in FIG. 6). Magnified portion 702 is an optical microscope image of the microwave-fused crosshatch stack 701. The polymer filaments 103 were ABS. The crosshatch stack of coated polymer filaments 103 were exposed with microwaves 401 (at around 2.45 GHz for around 20 seconds in a conventional microwave oven) to form a fused structure with minimal air gaps between the filaments. This filament fusion that occurs during the microwave exposure eliminates voids and improves load transfer.

(54) Variables in this process include irradiation time, power, frequency, thickness of the CNT polymer sheath, CNT weight or volume fraction in the coating, and the coating process itself. In some embodiments, the irradiation time is between about 1 second and about 5 minutes, the irradiation power is between about 1 W and about 1 kW, and the CNT polymer sheath thickness is between about 100 nm and about 0.5 mm. Frequency of the microwaves can be controlled by the type of microwave source utilized.

(55) Printed Object

(56) FIG. 8 is two photographs of similarly made 3D printed objects (object 801 and object 802). Object 801 was made by an additive manufacturing process with pure ABS printer filament. Object 802 was made by the same additive manufacturing process with carbon nanotube coated ABS printer filament, which was treated post-printing with microwave irradiation.

(57) With regard to object 802, the carbon nanotube dispersion was sprayed onto the pure ABS printer filament using an airbrush and then exposing the carbon nanotube coated printed part to microwaves at around 2.45 GHz for around 15 seconds in a conventional microwave oven. As with the other objects formed using embodiments of the present invention, this process formed a fused structure (object 1102) with minimal air gaps between the filaments.

(58) FIGS. 9-10 are photographs of other 3D printed objects made from coated filaments. Objects 901 and 1001 are 3D printed objects made with only pure uncoated PLA printer filament. These were printed using an UP! Mini 3D Desktop Printer. Object 902 and 1002 are these 3D printed objects with the CNT coating applied and after exposure to microwaves.

(59) Morphology

(60) FIG. 11 is an SEM of a cross-sectional view of a polymer filament 1101 coated with a dispersion of carbon nanotubes. The polymer filament 1101 is pure ABS. The coated polymer filament has not been irradiated with microwaves. The carbon nanotube coating 1102 (the coaxial CNT sheet) has a thickness 1103 of around 15 μm. FIG. 12 is an SEM of coated polymer filament 1101 (further magnified) showing the randomly oriented multi-walled carbon nanotubes 1201 in carbon nanotube coating 1102.

(61) FIG. 13 is an SEM of a 3D printed object (viewed from the top surface) made by an embodiment of the present invention (after being irradiated with microwaves). FIG. 13 shows the interface 1302 between two extruded polymer beads 1301. FIG. 14 is an SEM of the 3D printed object of FIG. 13 (viewed from the top surface and further magnified) showing the randomly oriented multi-walled carbon nanotubes 1401 in the carbon nanotube coating. The multi-walled carbon nanotubes have diameters between around 50 nm and around 80 nm. FIG. 15 is an SEM of a cross-section of the 3D printed object of FIG. 7. This cross-section shows the polymer filament 1501 (pure ABS), carbon nanotube coating 1502, and interface 1503 between the extruded polymer beads.

(62) A comparison between the multi-walled carbon nanotubes 1201 (before irradiation with microwaves) in FIG. 12 and the multi-walled carbon nanotubes 1401 (after irradiation with microwaves) in FIG. 14 shows that the microwave irradiation process does not significantly damage or destroy the multi-walled carbon nanotubes.

(63) Additionally, the post treatment images of FIGS. 13-15 shows the carbon nanotubes and polymer are in good contact and there is no visible bubbling or damage (i.e. burning) of the polymer. This revealed that the carbon nanotubes likely transferred (via conduction) their intense heat profile to the polymer and dissipated this heat to fuse the polymer interface without causing significant degradation.

(64) Heat Response to Microwave Radiation

(65) Microwave-induced CNT heating opens up the possibility of tunable polymer mobility and adhesion in polymer nanocomposite interfaces. This phenomenon could also be used to improve these polymer welds in 3-D printed parts. By connecting dielectric properties with in situ microwave thermometry, the present invention can control how the microwave response affects polymer film and filament adhesion in an additive manufacturing context.

(66) It has been discovered that there is a non-linear, non-monotonic microwave absorbance response in the filaments as a function of carbon nanotube loading. Above a threshold carbon nanotube loading, the heating response increased sharply, indicating that the carbon nanotubes' microwave response is affected by carbon nanotube network connectivity (percolation). This was unexpected and not previously reported.

(67) It is believed that the heating response of CNT-loaded polymer nanocomposites is non-linear and non-monotonic because of the dielectric properties of electrically percolating CNT networks (i.e., due to a connection between microwave heating adsorption and the dielectric properties of the CNT/polymer nanocomposites).

(68) Polyactic acid (PLA), which is commonly utilized in additive manufacturing, was mixed with commercially available multi-walled carbon nanotubes at various weight percentages. For example, —OH functionalized multi-walled carbon nanotubes were dispersed in a PLA/chloroform solution and these dispersions were used to solution cast and hot press MWCNT/PLA films at varying levels of MWCNT loading between 0.00 and 10.0 weight percent. After casting these films, characterization of these included DC and AC electrical property testing, infrared thermometry (effectively using imaging to carry out in situ microwave calorimetry), and Differential Scanning calorimetry (DSC).

(69) These measurements indicated a dramatic increase in microwave absorbance at MWCNT loadings above the electrical percolation threshold. It is believe that this increase in heating response is caused by a transition from a disconnected network of the conducting carbon nanotubes to a connected network. The induced electric current magnitudes on the carbon nanotubes due to microwave radiation are significantly increased above this threshold, resulting in high power dissipation through heating. It is further believed that below this threshold, the gaps between the carbon nanotubes in the disconnected network inhibit current flow due to the high impedances in the matrix.

(70) FIG. 16A is a photograph of a bundle of multi-wall carbon nanotube coated PLA filaments. FIG. 16B the FLIR for microwave exposure of a bundle of the MWCNT-coated PLA filaments (198 W, 20 seconds of exposure), the selected video frame shows the heating profile after 0.1 seconds of exposure and the depicted 17° C. rise in temperature (from 18° C. to 35° C.) illustrates the rapidness of the heating process. The data show localized heating effects, confirming that the heating is restricted to the MWCNT-enriched regions, promoting inter-filament welds, similar to simulation results in FIG. 27 (discussed below). Over the course of the analysis, the spatially averaged sample temperature rose by 58.5° C. This data in FIG. 16B indicate that the prospect of coating the filament with microwave-responsive CNTs is effective.

(71) Electrical Properties

(72) The heating can be tailored based upon the electrical properties of the resulting film. Such electrical properties include conductivity, loss tangent, and dielectric properties.

(73) The DC and AC conductivities of the PLA films loaded with various multi-walled carbon nanotube loadings were measured. DC measurements were conducted using a four-point-probe. These AC measurements were performed with a low-power microwave network analyzer using coaxial lines and rectangular waveguides on the PLA/multi-walled carbon nanotube samples (rectangular PLA/multi-walled carbon nanotube samples, 0.5 mm thick, inserted in a rectangular waveguide). The scattering parameters (transmission and reflection coefficients) were measured using an Agilent microwave network analyzer and were used to extract the dielectric properties of the samples after proper calibration. Since a waveguide was used for the measurements, a thru-reflect-line (TRL calibration) was performed to correct for the coax to waveguide adapters and input and output waveguide sections.

(74) FIG. 17 is a graph reflecting the DC conductivity (1701) and AC conductivity (1702) versus the multi-walled nanotube weight percent loading in PLA films. As shown in FIG. 17, the AC (at 2.45 GHz) and DC conductivity values are the same for percolated, resistive carbon nanotube networks at high carbon nanotube loading (greater than 5 wt %), but AC conductivity is higher at low carbon nanotube loading (less than 3 wt %).

(75) Other microwave material characterization methods (such as using coaxial lines that operate over a very broad frequency range—from DC to microwaves) can be used to yield dielectric constant and conductivity data. Such data can be used to determine optimal carbon nanotube concentrations needed, and may reveal frequency dependent features (resonances) which can be exploited for tailoring the heating response.

(76) Temperature Profile

(77) Temperature profiles of the PLA/multi-walled carbon nanotube samples set forth above were also examined to show their thermal response to high power (10-100 W) microwave radiation at 2.45 GHz in a controlled environment. The in-situ microwave calorimetry setup used a forward-looking infrared (FLIR) camera to image the temperature increase at the center of the homogeneous films placed inside a rectangular waveguide, which in turn was connected to a microwave generator. (FLIR® Systems, Inc.). The FLIR looks at the sample through a metal mesh window covering the open end of the waveguide. An electromagnetic RF meter is used to ensure safety during microwave radiation.

(78) PLA/multi-walled carbon nanotube samples (with loadings of 0.1 wt %, 0.5 wt %, 1 wt %, 2.5 wt %, 5 wt %, and 10 wt % were tested. The FLIR camera showed thermal images of PLA films (inside the waveguide) at varying MWCNT loadings after 30 seconds of 20 W microwave exposure at 2.45 GHz in a custom waveguide. FIGS. 18, 19, 20A, 21A, 22A, and 23A. As shown in these Figures, the heating response dramatically increases as the loading is increased to 2.5 wt % carbon nanotubes. It is believed that this was related to the onset of a resistive (rather than capacitive) percolating network.

(79) FIGS. 22A and 23A also show that the heating responses decreases (and changed in spatial distribution) as the loading changes from 5 wt % to 10 wt %. It is further believed that this is related to the transition from power absorbance to microwave power reflectance associated with high conductivity networks. The unusual, non-monotonic heating progression indicates an overall picture of microwave power transmission (in sub-percolation samples), microwave power absorbance (samples with nanofiller loadings just above percolation), and microwave power reflectance in highly conductive samples. This further indicates a beneficial range of CNT content for microwave heating.

(80) FIG. 24 shows the maximum temperature versus time of the PLA/multi-walled carbon nanotube samples (with loadings of 0.1 wt %, 0.5 wt %, 1 wt %, 2.5 wt %, 5 wt %, and 10 wt %) (2401-2406, respectively).

(81) Differential Scanning Calorimetry

(82) Differential Scanning calorimetry (DSC) was used to measure the melting point, crystallization temperature and glass transition temperatures of the neat PLA and PLA/multi-walled carbon nanotube samples set forth above. FIG. 25A shows the DSC curves for neat PLA (2501) and PLA/multi-walled carbon nanotube (10 wt %) (2502). FIG. 25A shows the DSC thermogram for neat PLA (2503) and PLA/multi-walled carbon nanotube (10 wt %) (2504). These measurements indicate the precise temperatures at which the polymer is likely to form strong welded interfaces. Most thermoplastic polymers must be heated beyond their glass transition temperature in order to form a bond or weld. The DSC also yielded estimates of the differentiable heat capacity as a function of temperature that correlated with the results of the temperature profiles obtained through FLIR imaging. FIG. 25C shows the heat capacity at constant pressure versus temperature for neat PLA (2505) and PLA/multi-walled carbon nanotube (10 wt %) (2506). The DSC measurements indicated that the T.sub.g of the PLA is correlated with a plateau in the dynamic temperature versus time response in the FLIR-imaged samples (˜60° C.).

(83) Microwave Interaction with Carbon Nanotube-Loaded Polymers

(84) The heating behavior of the microwaves is based on the sample geometry, the applied electromagnetic field and the materials' dielectric properties. Simulation and modeling was utilized to investigate the underlying science of microwave response of CNT-loaded polymers. Simulation and modeling aids in the investigation of the effect of various parameters (CNT loading, microwave power, sample thickness) on the heating response.

(85) Modeling microwave heating involves electromagnetic modeling as well as thermal/heat transfer modeling. Commercial software from ANSYS (High Frequency Structure Simulator, HFSS) for the electromagnetic simulation coupled with ANSYS Mechanical for the thermal simulation is utilized. COMSOL Multiphysics is also utilized for comparison with ANSYS results. Modeling CNT-loaded polymers exposed to microwaves is a challenging problem due to the drastic difference between the dimensions of CNTs (nanometers) and the bulk sample (centimeters). Direct simulation is not possible due to the vast computational resources needed.

(86) Simulation results using ANSYS HFSS are shown in FIGS. 20C, 21C, 22C, and 23C (alongside experimental data for comparison in FIGS. 20B, 21B, 22B, and 23B, respectively) for simulations of slabs of a lossy dielectric in a rectangular waveguide. The loadings of the samples in the simulations of FIGS. 20C, 21C, 22C, and 23C (and experiments of FIGS. 20B, 21B, 22B, and 23B) were 1 wt %, 2.5 wt %, 5 wt %, and 10 wt %, respectively. The simulations required an input value of loss tangent, which was experimentally measured for samples of varying CNT loading. The loss tangent and dielectric constant for these samples were taken from waveguide measurements on the nanocomposite films with varying CNT loading. These measured loss tangent and dielectric constant values were used for the simulations, and the resulting spatial distributions again show the microwave transmission, absorbance, and reflectance.

(87) The similarity between the two datasets (of (a) FIGS. 20C, 21C, 22C, and 23C and (b) FIGS. 20B, 21B, 22B, and 23B) was remarkable. As CNT loading increased, the samples' microwave response underwent two transitions: from power transmission to absorbance to reflectance. This remarkable agreement between simulation and experiment for CNT/microwave interactions is completely new and not previously reported.

(88) The relationship between carbon nanotube concentration and total absorbed power was computed for tuning the microwave absorption in the carbon nanotube layer. ANSYS HFSS was used to simulate a slab of a lossy dielectric placed in a rectangular waveguide with a constant dielectric constant. FIG. 26 shows the total normalized microwave power absorbed by the slab versus the material's loss tangent tan δ (the loss tangent is related the material's electrical conductivity tan δ=σ/(2πfϵ), where σ is the conductivity, ϵ is the dielectric constant, and f is the frequency). As shown in FIG. 24, there is an optimum value for the loss tangent (controlled by the CNT concentration amount) that maximizes the power dissipated in the material (converted to heat). This can be used to identify the parameters for achieving the desired bonding strength in the manufactured components using the CNT/PLA substrates after microwave exposure. Such parameters include CNT concentration, microwave power, frequency, and exposure time.

(89) This modeling and simulation also indicated the inhomogeneous heating effects in the nanocomposites. FIG. 27 shows the electric field magnitude in a bundle of PLA filaments coated with a thin conductive shell representing CNTs (shown in FIGS. 16A-16B). The bundle was placed in a rectangular waveguide operating in TE.sub.10 mode which has a vertically polarized electric field. The amount of heating corresponded to the electric field magnitude. The electric field was stronger in the thin CNT shell region than in the PLA core, resulting in stronger heating of the shell. This indicates that the heating was confined to the shell, allowing for heating and diffusion at the interface rather than the bulk.

(90) Microwave-Induced Polymer Adhesion

(91) The mechanical effects of microwave heating on welds in CNT/polymer nanocomposites were also examined. The CNT-rich polymer coatings enhance polymer substrate adhesion upon exposure to microwave irradiation. In addition to polymer interface mechanics, such phenomena open up new avenues in additive manufacturing by allowing site-specific heating/bonding at the nanoscale.

(92) To examine this, MWCNT/PLA films were applied on two PLA substrates, the two substrates were then adhered, and then exposed the sample to a microwave field. A commercial 1250 W microwave oven for microwave exposure was utilized. Since microwave ovens do not have a uniform electromagnetic field distribution, the sample experienced inhomogeneous heating. However, a microwave exposure chamber can be utilized that provides uniform electromagnetic field distribution to better ensure that the parts of the sample are uniformly heated.

(93) As the multi-walled carbon nanotubes act as local heat sources when exposed to microwaves, polymer diffusion and increased mobility at the interface will allow for seamless weld, forming rapidly without warping the bulk structure. ASTM peel tests (conducted using a conventional tensile tester) as illustrated in FIG. 28 can provide a macroscale indication of the effect of microwave irradiation on the enhanced adhesive strength in the MWCNT/polymer layer. The coating thickness and CNT content can be varied (as well as the microwave exposure) to control the effects of the coating on polymer diffusion. In addition to the non-monotonic microwave response of the CNT film, it is believed that extremely high CNT loadings will inhibit polymer diffusion from the two bulk phases on either side of the interface. Accordingly, the coating thickness and CNT content can be controlled for better polymer diffusion. Hence the degree of chain inter-diffusion and effectiveness of the welding process is tunable and controllable.

(94) Manufacturing Process

(95) The process of coating normal PLA filaments with carbon nanotubes allows that these PLA filaments can be used in existing 3-D printers. The concept of coating filaments with carbon nanotubes for microwave-induced welding is entirely new. The data indicate such coated filaments can be used as a feedstock for 3-D printing a variety of structures. For instance, these CNT-loaded 3-D structures can be formed and then be exposed to microwave irradiation (in the range of 1 W to 1 kW) for a period of time (in the range of 1 second to 5 minutes) such that the CNT-rich polymer coatings will be heated and induce localized melting, interfacial diffusion, and increased adhesive strength. These can be controlled for advanced properties, including those reflected above. The resulting objects can have (by design) enhanced compressive and tensile strength (such as can be measured on an Instron mechanical tester).

(96) The data (reflected above) show that the CNT coating is maintained throughout the extrusion process that occurs during 3-D printing. These structures have similar surface conductivity and microwave responsiveness before and after printing if both CNT and PLA are present in the coating.

(97) FIG. 29 illustrates an in-situ microwave magnetron-waveguide-coaxial applicator system 2901 to heat the filament-filament interface at the newly printed location during the printing process itself that can be utilized in an embodiment of the present invention. In system 2901, the printing filament 2902 (such as carbon nanotube coated PLA) is extruded through the extruder nozzle 2903 of the additive manufacturing device (such as a 3D printer, i.e., like the 3D printer produced by Helix). (In FIG. 29 only the extruder nozzle 2903 and the printing filament 2902 of the additive manufacturing device is shown in detail. The remainder of the standard additive manufacturing device, such as the standard parts of a 3D printer are represented by block 2904).

(98) System 2901 also has a microwave source 2904 (such as a rectangular waveguide that has a 2.45 GHz microwave input (controllable with a frequency tuning plunger). The microwave source can be focused upon the newly printed location using a coaxial applicator tip 2905 located at that location. The application of microwaves at the coaxial locator tip provides localized heating (AT) 2906.

(99) Electromagnetic shielding (not shown) can also be incorporated into the system for various reasons, including safety and integrity concerns). This allows the microwave exposure system to be integrated into the 3-D printer hardware with appropriate electromagnetic shielding measures.

(100) The coaxial applicator tip 2905 can be attached to the printer head directly (which is where the extruder nozzle 2903 is located) in order to move with the printer head. This localized microwave heating allows for the microwave power to be applied during printing, with only the immediately-printed area being exposed rather than the entire sample.

(101) This will allow for direct modifications to the printer architecture to allow for microwave exposure immediately after filament deposition.

(102) Such a system will yield (a) a novel system and process that utilizes microwave radiation response of CNTs and CNT/polymer nanocomposites; (b) efficient modeling techniques suitable for materials with CNT dispersions; (c) processing-structure-property relations for CNT/polymer nanocomposite welds after microwave exposure; (d) development of a 3D printing apparatus incorporating optimized CNT loading and a microwave exposure process.

(103) Manufacturing Technology

(104) Embodiments of the present invention overcome the current disparity between (1) traditionally manufactured polymer parts utilizing injection molding, extrusion molding, machining and (2) parts printed with additive manufacturing techniques such as Fused Deposition Modeling (FDM™, Stratasys, Inc.) and Fused Filament Fabrication (FFF). Such processes (a) increase the overall strength of 3D printed thermoplastic parts, (b) allow for the rapid diffusive bonding of polymer chains between beads (thus increasing strength), without distorting or heating the bulk part, and (c) improve the isotropic mechanical properties of 3D printed thermoplastic parts so that they can rival (or even exceed) their manufactured counterparts.

(105) Embodiments of the present invention can be used and incorporated into industrial and consumer 3D printing processes. Embodiments of the present invention can enhance the manufacturing of parts, such as parts pertaining to the aerospace, automotive, defense, and space industries.

(106) The system and processes of the present invention and the resulting materials produced are important for a range of engineering fields, both fundamental (such as polymer dynamics, microwave physics) and applied (nanocomposite processing). These outcomes dovetail with national manufacturing priority areas including additive manufacturing, vehicle and antenna design, radar-absorbing coatings, and electromagnetic shielding. Defense and aerospace needs in particular have substantial needs that can be met by additive manufacturing if the difficulties with weld mechanical properties are also addressed.

(107) The present invention also has significant impact on other industrial and scientific applications that utilize nanoparticles for electrical property modification. Accurate and efficient nanoparticle dispersions in a host medium to controllably utilize their electrical properties is a challenging problem and is very much needed in multitude of applications.

(108) The examples provided herein are to more fully illustrate some of the embodiments of the present invention. It should be appreciated by those of skill in the art that the techniques disclosed in the examples which follow represent techniques discovered by the Applicant to function well in the practice of the invention, and thus can be considered to constitute exemplary modes for its practice. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments that are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention.

(109) While embodiments of the invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the invention. The embodiments described and the examples provided herein are exemplary only, and are not intended to be limiting. Many variations and modifications of the invention disclosed herein are possible and are within the scope of the invention. Accordingly, other embodiments are within the scope of the following claims. The scope of protection is not limited by the description set out above.

RELATED PATENTS AND PUBLICATIONS

(110) The following patents and publications relate to the present invention: U.S. Pat. No. 7,754,054, “Microwave Induced Functionalization Of Single Wall Carbon Nanotubes And Composites Prepared Therefrom,” issued Jul. 13, 2010, to Mittra et al. U.S. Pat. No. 8,080,199, “Interaction Of Microwaves With Carbon Nanotubes To Facilitate Modification,” issued Dec. 20, 2011, to Tour et al. U.S. Patent Publication No. 2011/0223343, “Novel Nanocomposite For Sustainability of Infrastructure,” published Sep. 15, 2011, to Wang et. al. Chowdhury, S. R., et al., “Microwave-Induced Rapid Nanocomposite Synthesis Using Dispersed Single-Wall Carbon Nanotubes as the Nuclei,” Journal of Materials Science, 2009, 44, (5), 1245-1250, doi:10.1007/s10853-009-3259-4. Higginbotham, A. L., et al., “Carbon Nanotube Composite Curing through Absorption of Microwave Radiation,” Composites Science and Technology, 2008, 68, (15-16), 3087-3092, doi:10.1016/j.compscitech.2008.07.004. Irin, F., et al., “Detection of carbon nanotubes in biological samples through microwave-induced heating,” Carbon, 2012, 50, (12), 4441-4449. Nikzad, M, et al., “Thermo-Mechanical Properties of a Highly Filled Polymeric Composites for Fused Deposition Modeling,” Materials Design, 2011, 32, (6), 3448-3456, doi:10.1016/j.matdes.2011.01.056. Partain, S. C., “Fused Deposition Modeling with Localized Pre-Deposition Heating Using Forced Air,” Vasa, 2007, http://medcontent.metapress.com/index/A65RM03P4874243N.pdf; Montana State University, 2007, http://scholarworks.montana.edu/xmlui/handle/1/2016. Paton, Keith R., et al., “Efficient Microwave Energy Absorption by Carbon Nanotubes,” Carbon, 2008, 46, (14), 1935-1941, doi:10.1016/j.carbon.2008.08.001. Shim, Hyung Cheoul, et al., “Enhancement of Adhesion Between Carbon Nanotubes and Polymer Substrates Using Microwave Irradiation,” Scripta Materialia, 2009, 61, (1), 32-35, doi:10.1016/j.scriptamat.2009.02.060. Shofner, M. L., “Nanofiber-reinforced Polymers Prepared by Fused Deposition Modeling, Journal of Applied Polymer Science, 2003, 89, (11), 3081-3090 http://onlinelibrary.wiley.com/doi/10.1002/app.12496/full. Vegesna, S., et al., “Non-destructive Technique for Broadband Characterization of Carbon Nanotubes at Microwave Frequencies,” Journal of Electromagnetic Waves and Applications, 2013, 27, (11), 1372-1381. Wang, C. Y., et al., “Strong Carbon-Nanotube—Polymer Bonding by Microwave Irradiation,” Advanced Functional Materials, 2007, 17, (12), 1979-1983, doi:10.1002/adfm.200601011. Wang, C. Y., et al., “Flexible Field Emitter Made of Carbon Nanotubes Microwave Welded onto Polymer Substrates,” Applied Physics Letters, 2007, 90, (10), 103111, doi:10.1063/1.2711771. Wohlers Associates, “Wohlers Report 2012: Additive Manufacturing and 3d Printing, State of the Industry.” 2012, 1-271 (Ft. Collins, Co.) http://wohlersassociates.com/state-of-the-industry-reports.html. Ziemian, C., et al., “Anisotropic Mechanical Properties of ABS Parts Fabricated by Fused Deposition Modelling” Mechanical Engineering, 2012, Chapter 7, 159-181 (edited by Gokcek, Murat), doi: 10.5772/34233.

(111) The disclosures of all patents, patent applications, and publications cited herein are hereby incorporated herein by reference in their entirety, to the extent that they provide exemplary, procedural, or other details supplementary to those set forth herein.