DE102013009833A1 - Silver wires in UV-curing systems. - Google Patents

Abstract

The present invention provides formulations for electrically conductive, photosensitive monomer solutions that are used in 3D printing processes. Silver nanowires are used to form a transparent, three-dimensional, percolating network.

Description

field of use

The invention relates to a formulation according to the preamble of claim 1.

Background Art - Background

Rapid prototyping is undergoing tremendous change. The modern 3D printing technology ends an aging principle. Whether toolmakers, sculptors or corporate designers: they have always cut, cut or punched workpiece blanks. So at the end - in addition to a heap of waste - the desired product was left over. Subtractive production is the name of this inefficient process.

In 3D printing, the opposite happens: plastics, glass, ceramics, steel or precious metals are glued together, melted or baked into a workpiece. The printers use only as much material as is actually needed for the product. Waste is virtually eliminated. The main processes that have been developed in recent years - and new ones are constantly added - are briefly described below.

FDM Fused Deposition Modeling: Extrusion of thermoplastic materials. The resulting surfaces, however, are of low quality. Overhangs are only possible with supporting materials (water-soluble). The method also allows experiments with foods, such as chocolate.

SLS Selective Laser Sintering: Powdered plastics, metals and ceramics are applied with a squeegee and sintered with a laser. Overhangs are easily possible due to the powder bed.

Slective Laser Melting (SLM)

Slective Laser Melting (SLM) is one of the generative manufacturing processes. In contrast to the SLA, no solution of liquid monomers is used here, much more a thin layer of a material in powder form is applied to a support and, as in stereolithography, fused layer by layer by a laser (sintering). Various metals such as steel, stainless steel or even titanium, ceramics and plastics can be used.

At the beginning of the 3D printing process, Stereolithography (SLA) was used to layer 3D objects and today is one of the best and most mature processes. The object is formed in a container which is filled with liquid base monomers of a photosensitive plastic. Photosensitive plastics, such as epoxy resins, are plastics that change their properties under the influence of a light source. This is used in stereolithography and hardens the liquid monomers of the plastic with a laser layer by layer. For this, the platform with the model always moves a few millimeters (it is also possible today a few microns) further down, so that again a thin layer of the laser can be cured.

LOM Laminated Object Manufacturing: Paper or plastic films (including aluminum) are applied. Thin layers, paper or foils are laminated on top of each other and the contours are cut with a cutting tool or a laser.

3DP 3-Dimensional Printing: As with the SLS, the initial state of the building material is also powdery in three-dimensional printing (3DP). Unlike the SLS, however, the particles are not sintered with a laser, but are connected to each other with a printhead and a liquid binder. The methods are very similar, but the use of a printhead is less expensive. Dyed binders make it possible to color the component during the construction process, thus avoiding reworking. Further advantages are the good component accuracy and the high construction speed.

LCM Lithography-based Ceramic Manufacturing: The technology is based on the selective photocuring of a photosensitive plastic mixture in which ceramic particles are homogeneously dispersed. The heart of the machine is a specially developed projection system. This ensures the exposure of the layer information with the latest LED technology. Through the use of innovative projection technology and special projection optics, it is possible to precisely reproduce even very small and fine structures. After the manufacturing process, the component is present as a green product. This must then be debindered and sintered in further work steps - just as in powder injection molding. The binder is thermally decomposed and the ceramic particles are densely sintered. This results in a 100% ceramic component that has the same material properties as conventionally manufactured components.

Multi Jet Modeling (MJM) or 3D printing is a process in which the 3D model is created by a kind of print head, similar to a commercial inkjet printer. Various materials are used, such as thermoplastics or hard waxes, which are melted and then "printed" can. Or one uses similar as with the SLM powder, which are connected however with a binder. Very fine structures are possible with 3D printing.

The printers derived from the described methods each process specific substrates adapted to the printing process, which can be solid or liquid.

The processing of substrates into a single object with different mechanical, physical or chemical functions is a task that is made possible with the printing methods described in different quality.

The company Evonik Industries AG has in the patent publication ( WO 2012/152510 A1 ) discloses a method by which the surface of a 3-dimensional object fabricated by the FDM method can be physically modified. In this case, a device is attached to the print head, which makes it possible to colorize the polymer just processed superficially and to obtain colorful objects as a result.

The company Future Carbon GmbH has in the patent DE 10 2010 013 210 A1 discloses a material which results in the addition of carbon an electrically conductive, thermoplastically processable substrate. However, the conductivity does not match the conductivity of metals.

A similar substrate is from the company Evonik Industries AG in the patent DE 10 2010 043 470 A1 described.

For example, Objet GmbH has developed a technology called PolyJet. PolyJet is a stereo lithography (SLA) process that uses two different monomer solutions to realize all mechanical material properties between hard and soft in a single component.

For SLA processes, there are currently no physically or chemically functionalized substrates in order to be able to realize electrically conductive objects.

As already described, SLA processes use organic monomers as photosensitive polymers, which can be applied very precisely by different printing technologies layer by layer, and cured in a further process step with a UV light source. For the electrically conductive functionalization of these monomer solutions, electrically conductive fillers such as carbon or metals come into question in the prior art. When using electrically conductive fillers, reaching the percolation limit, that is to say the degree of filling at which all the filler particles touch, is of decisive importance for the formation of an electrical conductivity. Fillers of spherical morphology (spherical) require a known very high degree of filling, which is usually greater than 60 percent by volume.

Monomer solutions which are UV-curing or light-curing can no longer be processed with such a high degree of filling of fillers, on the one hand because the stability of the chemically polymerized layer is too low, and on the other optical transparency for complete curing of the exposed monomer layer is needed ,

It is therefore an object of the present invention to describe a substrate which is used in stereo lithography as a rapid prototyping method for producing 3-dimensional objects and has electrically conductive function.

A further object is to introduce any conductive structures into an object which can subsequently be contacted and used as electrical conductors.

Another object is the optical transparency of the substrates according to the invention.

Other tasks not explicitly mentioned emerge from the overall context of the following description, claims and examples.

Description of the invention

This is where the present invention begins.

The invention provides materials for overcoming the aforementioned disadvantages of the prior art. In particular, it is the object of the invention to describe advantageous materials, production process for such materials, treatment and purification processes for the materials, intermediates produced from the materials such. As masterbatches, coating solutions or inks, processing methods, coated with the materials according to the invention or equipped objects as well as products or applications made therefrom.

The inventors have surprisingly discovered that the use of small amounts of finest metallic wires in a 3D photosensitive printing substrate does not hinder the necessary curing of the monomers by light, but confers an electrically conductive function on the polymer.

The finest metallic wires here are to be understood as nanowires, which are to be defined briefly below:
In this document, the term "nanowire" refers to all structures which have similar extensions in the range from 1 nm to 1000 nm in at least two spatial directions and have an extension of at least 5 times the other two dimensions in the third spatial direction.

In general, all metals in the form of nanowires which are known to have a good electrical conductivity are suitable for overcoming the prior art. The electrical conductivity is given in Siemens per meter (SI units: S / m). The conductivity of the metals or metallic compounds according to the invention is above 20 × E6 S / m, preferably above 40 × E6 S / m, more preferably above 60 × E6 S7m. The metals suitable for achieving the above electrical conductivity are silver, copper, gold, aluminum and their alloys, which may also contain other metallic constituents.

In comparison, the electrical conductivity of carbon is 3 × E6 S / m. Thus, the carbon modifications (carbon nanotubes, CNTs) also available as nanowires are not suitable for meeting the requirements of the present invention.

Surprisingly, it has been found that formulations with metallic nanowires to exceed the percolation limit require a few percent by mass of nanowires so that the formulation as a whole becomes electrically conductive. The proportion of silver nanowires is in the range of 1-100 weight percent, preferably in the range of 1-25 weight percent, more preferably in the range of 1-10 weight percent.

One consequence of the low admixtures of conductive components, such as metallic nanowires, is a concomitant optical transparency in the entire wavelength range. The entire wavelength range here consists of the shorter wavy, ultraviolet range (UV, wavelength less than 350 nm), the visible range with a wavelength of 350-700 nm and the infrared range with wavelengths greater than 700 nm.

Another condition for the quality of transparency is the morphology of the metallic nanowires used. It has been found that the aspect ratio, as a quotient of length to thickness, decisively influences the transparency of formulations produced therefrom. A suitable morphology is achieved when the nanowires have a thickness of 20-200 nm, more preferably 20-120 nm, and most preferably 20-80 nm.

The length of the metallic nanowires is in the range of 1-80 microns, preferably in the range of 1-50 microns and most preferably in the range of 1-35 microns.

To illustrate the invention is based on the 1 and 2 directed.

1 shows a micrograph of a percolating network of silver nanowires, which forms an electrically conductive layer with a transparency of more than 80%.

2 shows a recording of a scanning electron microscope with the silver nanowires according to the invention, which can be used to formulate a transparent, electrically conductive monomer solution.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 2012/152510 A1 [0014]
• DE 102010013210 A1 [0015]
• DE 102010043470 A1 [0016]

Claims (15)

Formulation of photosensitive monomer substrates for the production of 3D printed objects, characterized in that the formulation contains metallic nanowires. Formulation according to claim 1, characterized in that the metallic nanowires have an electrical conductivity in the range greater than 20 × E6 S / m. Formulation according to one of claims 1-2, characterized in that the metallic nanowires have an electrical conductivity in the range greater than 40 × E6 S / m. Formulation according to one of claims 1-3, characterized in that the metallic nanowires have an electrical conductivity in the range greater than 60 × E6 S / m. A formulation according to any one of claims 1-4, characterized in that the metallic nanowires are contained in a proportion of 1 to 100 weight percent. A formulation according to any one of claims 1-5, characterized in that the metallic nanowires are contained in a proportion of 1 to 25 weight percent. Formulation according to one of claims 1-6, characterized in that the metallic nanowires are contained in a proportion of 1 to 10 weight percent. Formulation according to one of claims 1-7, characterized in that the metallic nanowires have a thickness or a diameter of 20 to 200 nm. Formulation according to one of claims 1-8, characterized in that the metallic nanowires have a thickness or a diameter of 20 to 120 nm. A formulation according to any one of claims 1-9, characterized in that the metallic nanowires have a thickness or a diameter of 20 to 80 nm. A formulation according to any one of claims 1-10, characterized in that the metallic nanowires have a length of 1 to 80 microns. Formulation according to one of claims 1-11, characterized in that the metallic nanowires have a length of 1 to 50 microns. A formulation according to any one of claims 1-12, characterized in that the metallic nanowires have a length of 1 to 35 microns. Formulation according to one of claims 1-13, characterized in that the metallic nanowires consist of the elements silver, copper, gold and aluminum. Formulation according to one of claims 1-14, characterized in that the metallic nanowires consist of silver.

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