WO2015059190A1

WO2015059190A1 - Electrically conductive polyamide substrate

Info

Publication number: WO2015059190A1
Application number: PCT/EP2014/072649
Authority: WO
Inventors: Ren BAO; Michelle Elizabeth Seitz
Original assignee: Dsm Ip Assets B.V.
Priority date: 2013-10-25
Filing date: 2014-10-22
Publication date: 2015-04-30

Abstract

The invention relates to an electrically conductive system comprising a substrate and at least one conductive track adhered onto the substrate, wherein the substrate is composed of at least polyamide-66 and the conductive track is made out of an electrically conductive material and wherein the conductive track is adhered to the substrate by an jet printing technique followed by sintering. The invention further relates to a process for the production of an electrically conductive system and to its uses.

Description

ELECTRICALLY CONDUCTIVE POLYAMIDE SUBSTRATE

The invention relates to an electrically conductive system comprising a substrate and an electrically conductive track adhered to the substrate.

Conductive tracks are generally applied to a substrate by a process referred to as Laser Direct Structuring (LDS). Such a process is described for example in EP1274288. During this process, non-conductive heavy metal complexes are incorporated into the substrate material, after which the substrate is moulded.

Thereafter the substrate is irradiated with a laser beam in the pattern of the conductive tracks to be construed. As a result of the laser irradiation the surface is activated and metal seeds are generated and exposed on the surface of the substrate. The exposed metal seeds are metalized by chemical reduction in the following steps. Generally the tracks are built up in electroless plating baths with copper to form layers of 2,5 to 15 μηη thick, electroless nickel (1 -2,5 μηη), followed by plating with silver and/or gold (0,05-0,2 μηη). After each plating bath, several rinsing cycles are needed before the following plating step can be started. This process leads to the formation of metalized, conductive tracks along the pattern that the laser beam has followed.

A disadvantage of such an LDS-process is that the plating step is very critical and requires a lot of attention and control of the concentration and quality of the individual components, including sodium hydroxide, formaldehyde, chelate, and copper along with several reaction stabilizers. Clearly this process step requires a lot of knowledge and continuous attention. Additionally, the chemicals used in these baths and the waste streams generated, make the process from an environmentally point of view very unattractive and are therefore under debate. The electrically conductive systems produced with this process are therefore not very environmentally friendly. Further the apparatus needed in the LDS-process is expensive. Another disadvantage of the LDS-systems is that the product contains nickel to which many people develop or already show allergic reactions.

Therefore a need exists for electrically conductive systems whose production is less environmentally unfriendly. A process to produce those conductive systems was developed and is described for example by Kamyshny et al in "The Open Applied Physics Journal", 201 1 , 4, 19-36. The process comprises the printing of the electrical circuitry with metal-based inks in an ink-jet printing process. During this process droplets of ink are jetted from a small orifice in a printhead, directly to a specified position on a substrate. In contrast to normal home/office-use, the inks used in the printing of electronic circuitry are metal-based, more specifically the inks contain metal nanoparticles, complexes or metallo-organic compounds. The ink-jet printing step is generally followed by a sintering process to obtain conductivity in the tracks. Although the electrically conductive systems that are produced via such an ink-jet printing-based technique are more environmentally friendly, they suffer from the disadvantage that they do not provide with all kinds of substrates, electrically conductive systems with the required level of adhesion between the substrate and the conductive track.

Currently most solutions to overcome this disadvantage consist of providing new types of inks, but these solutions can be quite complex. Very little effort has been directed towards providing alternative substrates that could increase adhesion between substrate and the electrically conductive track.

Therefore a need exists for electrically conductive systems whose production is less environmentally unfriendly while these systems still meet the requirements, especially with respect to the adhesion between the substrate and the conductive track. It is the object of the present invention to provide these systems and overcome, or at least reduce, the disadvantages of the prior art.

This object has surprisingly been reached by an electrically conductive system comprising a substrate and an electrically conductive track adhered to the substrate, wherein the substrate comprises polyamide 66 (PA-66) and the electrically conductive track is obtained by jet printing.

It has been found that the electrically conductive system according to the invention comprising a substrate comprising PA-66, i.e. wherein the substrate is made of a composition comprising polyamide-66, results in a good level of adhesion between the substrate and the conductive track as determined by the so-called "cross hatching test", according to ASTM D3359-08 D, test method B, both under dry and wet conditions.

Thus the present invention provides an electrically conductive system as described above, wherein the adhesion between the substrate and electrically conductive track has classification 4B or 5B according to ASTM D3359-08 D, test method B.

As will be shown later, it is surprising that the substrate comprising

PA-66 provides such good adhesion since other high performance polymers, such as liquid crystal polymer (LCP) and polyethylene terephthalate (PET), as well as polyacarbonate and aliphatic polyamides like PA 46 and PA 6 fail in the adhesion test. Moreover, PA-66 used in the invention is able to withstand the relatively high sintering temperatures of above 200 °C.

Also, even after exposure to high humidity and high temperature conditions for a prolonged period of time, the adhesion remains high. This makes the electrically conductive system particularly suitable for certain applications in electronic equipment, such as antennae for mobile devices.

A further advantage of the electrically conductive system that is prepared with a jet printing process according to the invention is that the height of the conductive track can be lower than with an electrically conductive system that is applied via an LDS-process. Generally a height of approximately a few μηη is sufficient when silver is used as the material for the conductive track in an ink-jet based process, while the track height obtained in the LDS-process is generally at least 15-20 μηη. A lower height of the tracks means that less precious metal or metal alloy is needed in the preparation, therefore the costs of the total electrically conductive system is reduced compared to prior art systems.

According to the invention the substrate comprises PA-66. Thus the substrate may also comprise other components or polymers. However, it is preferred that the substrate comprises at least 30 wt.% of PA-66, based on the total weight of the substrate, preferably at least 40 wt.%.

In particular the invention provides an electrically conductive system, wherein the substrate, respectively the composition the substrate is made of, consists of

(A) 30 - 100 wt.% of PA-66,

(B) 0 - 50 wt.% of at least one other polymer, wherein the at least one other polymer may comprise another polyamide in an amount of less than 30 wt.%;

(C) 0 - 60 wt.% of reinforcing agents; and

(D) 0 - 15 wt.% of at least one additive.

Herein the weight percentages (wt.%) are relative to the total weight of the substrate, respectively relative to the total weight of the composition.

Beside PA-66 the substrate may also comprise other polymers. A preferred further polymer is a polyamide other than PA-66. The other polyamide can be an aliphatic polyamide, or a semi-aromatic polyamide, or a combination thereof.

However, the amount of other polyamide has to be less than 30 wt.% based on the total weight of the substrate. The other polyamide is preferably compatible with PA-66.

Examples of suitable aliphatic polyamides are homopolyamides such as PA-6, PA-1 1 or PA-12, PA-46, PA-69, PA-610, PA-612 and PA-1212, and copolyamides thereof.

Examples of suitable semi-aromatic polyamides include

homopolyamides like PA-6T, PA-7T, PA-9T, PA-10T and PA-12T having a melting temperature in the range of 270-350 °C, and copolyamides of PA-4T, PA-5T, PA-6T and /or PA-8T , with for example PA-7T, PA-9T, PA-10T, PA-1 1 T, PA-12T, PA-6, PA- 66, and/or PA-MXD6. Suitable copolyamides include PA-6/6T, PA-6I/6T, PA-106/10T, PA-66/6T, PA-46/4T, PA-10T/6T, PA-9T/M8T, PA-6T/5T, PA-6T/M5T and PA-6T/10T. The polyamides may comprise other repeat units of further diamines and diacids, next to those mentioned in the copolyamides hereabove, thus forming more complex copolyamides. For further examples of suitable semi-aromatic copolyamides see Kunststoff Handbuch, (Carl Hanser Verlag 1998) Band 3/4 Polyamide chapter 6.

The substrate can also contain one or more reinforcing agents, preferably fibrous reinforcing agents. Examples of fibrous reinforcing agents are graphite fiber, carbon fiber, glass fiber, silica fiber, aluminum silicate fiber, processed mineral fiber, phosphate fiber, calcium sulfate fiber or potassium titanate fiber.

Preferably, the reinforcing agents are selected from graphite fiber, carbon fiber, glass fiber and combinations thereof. The amount of reinforcing agent that is present in the substrate can be chosen between wide ranges. The amount is generally determined based on the mechanical properties, such as for example the stiffness that an envisaged application requires; the man skilled in the art knows what range of stiffness applies for which application. For example for an application in stiffeners and enclosures a range of 0-60 wt%, preferably 20-50 wt%, can be added. For an application in add-on and screw-on components a generally applicable range is 0- 25wt%. The mentioned weight percentages for the amount of reinforcing agent are relative to the total composition of the subtrate.

The substrate in the electrically conductive system according to the invention, respectively the composition the substrate is made of, can contain only PA- 66 referred to above, however it is also possible to incorporate into the substrate, next to the polyamide and the optionally added one or more reinforcing agents further additives. One type of additives are one or more black pigments. Suitable examples of black pigments are carbon black, graphite, nigrosine or CuCr₂0₄. It is also possible to use a combination of black pigments. The amount of black pigment that is present in the substrate can be chosen between wide ranges. A suitable range for the amount of black pigment is between 0.1 -2 wt%, preferably 0.2-0.7 wt%, more preferably 0.3-0.5 wt% relative to the total composition of the substrate. The man skilled in the art of compounding is familiar with the techniques and possibilities for adding and blending several components into a polymeric base material. The mentioned black pigment can for example be added to the polyamide in the form of a masterbatch. The carrier polymer used to introduce the black pigment to the polyamide is not particularly critical. A suitable carrier polymer is for example another type of polyamide, such as for example polyamide-6 (PA-6). An especially advantageous substrate for use in the present invention contains next to the polyamide both the fibrous reinforcing agent and the black pigment.

Other examples of additives are fillers, flame retardants, sizing agents, non-electrically conducting additives and auxiliary additives. With auxiliary additives is meant those additives that are known to the person skilled in the art of making polyamide moulding compositions to be usually comprised in said polyamide composition. Auxiliary additives can for example be UV stabilizers, heat stabilizers, antioxidants, colorant processing aids and impact modifiers. The amount of these other components that may be present next to the polyamide may vary over a wide range, but is suitably is in the range of 0-10 wt% relative to the total weight of the composition. Preferably the amount of other components is 0.5-5 wt%, more preferably 0.5-3 wt%.

The composition out of which the substrate according to the invention is made, can be prepared by a process wherein PA-66, the optional reinforcing agent, the optional black pigment and the optional other component or components are melt- blended. Part of the materials may be mixed in a melt-mixer, and the rest of the materials may then be added and further melt-mixed until uniform. Melt-blending may be carried out using any appropriate method known to those skilled in the art. Suitable methods may include using a single or twin-screw extruder, blender, kneader, Banbury mixer, moulding machine, etc. Twin-screw extrusion is preferred, particularly when the process is used to prepare compositions that contain additives such as flame retardants, and reinforcing agents.

The substrate itself can be shaped by means of conventional moulding techniques, e.g. by means of melt processing, depending on the use of the final electrically conductive system. The final shape can be three dimensional, such as an enclosure for an electronic device, but it can also be two dimensional, e.g. a flat plaque or film of material.

According to one aspect of the invention, the substrate has the shape of a thin film, e.g. a film with a thickness of 0.5 to 1000 μηη, preferably 5 to 100 μηη.

The electrically conductive system comprises next to the substrate at least one conductive track adhered onto the substrate. The conductive track is formed on the substrate by a process that comprises at least the following steps:

• providing a substrate composed of at least one polyamide,

• optionally pre-treating the substrate,

• applying a conductive track precursor on the substrate by a jet printing

technique,

• sintering the conductive track precursor on the substrate at an elevated

temperature so as to obtain a conductive track on the substrate, • cooling down the substrate with the conductive track.

The material of which the conductive track is made can be rather freely chosen as long as they provide good electrical conductivity of the printed conductive track. The material for the conductive track is generally a metal or metal alloy. Examples of suitable materials for the conductive track are silver (Ag), copper (Cu), gold (Au), palladium (Pd), platinum (Pt), nickel (Ni) and aluminum (Al) and any combination of two or more of them. Preferably the material is Ag, Cu, Ni or Au or any combination of two or more of them. More preferably the material is Ag.

The ink will contain the metal or its precursor in a suitable liquid carrier. The suitable liquid carrier can for example be water or an organic solvent. The metal or its precursor will generally be available in the ink in a dispersed or dissolved state. A preferred state is the form of a nanoparticle. With nanoparticle is meant a particle with at least one of its dimensions in the nanometer range. A preferred material for use as the conductive track is nano-silver. With "nano-silver" is meant a silver particle of which at least one of its dimensions lies in the nanometer range. The man skilled in the art of jet printing inks for preparing conductive tracks knows how to prepare and handle these kinds of metals and/ or their precursors. Inks for jet printing techniques are for example described in the book "The chemistry of inkjet inks", edited by S. Magdassi, World Scientific Publishing UK, November 2008.

The conductive track precursor is applied with a jet printing technique.

Examples of suitable jet printing techniques are ink jet printing and aerosol jet printing. The technique of aerosol jet printing is well-known and is for example described in the article "3D aerosol jet printing-adding electronics functionality to RP/RM" by Martin Hedges et al. presented at the DDMC 2012 Conference held on 14-15 March 2012 in Berlin.

Thus, the invention also relates to a process for the production of an electrically conductive system comprising a substrate and a conductive track adhered to the substrate, comprising the steps of:

• providing a substrate comprising PA-66,

· applying a conductive track precursor on the substrate by a jet printing

technique,

• sintering the conductive track precursor on the substrate at a temperature of at least 150 °C so as to obtain a conductive track on the substrate.

The conductive track is formed from the conductive track precursor after sintering of the applied ink so as to obtain a continuous connectivity. With sintering is here and hereinafter meant a process of welding conductive track precursor particles together at a temperature below its melting point. The ink applied by the ink- jet printing may comprise metal nano-particles as the conductive track precursor particles. Sintering can be effected by thermal sintering, photonic sintering, microwave sintering, plasma sintering, electrical sintering or sintering by chemical agents. The man skilled in the art of applying conductive tracks is familiar with these techniques and knows how to determine the best method for each case. If thermal sintering is used, preferred temperatures are at least 150°C, preferably at least 180 °C, more preferably at least 200 °C. Maximum temperatures are determined by the thermal degradation of the materials used. In general the temperature will be at most 350 °C. A suitable sintering process can take place at a temperature between 150 and 260°C for 10-30 minutes.

The electrically conductive system according to the present invention has surprisingly good adhesion between the substrate and the conductive track. The adhesion between the substrate and the conductive track can be determined by applying the method as described in the Standard Test for Measuring Adhesion by Tape Test, ASTM D3359-08 D, method B, as described below.

Figure 1 provides the Classification of the Adhesion Test results according to this method.

Depending on the application in which the electrically conductive system will be used, a suitable PA-66 composition will be determined and prepared. Part of this PA-66 composition can be other components than polyamide, for example one or more fibrous reinforcing agents, carbon black and/or other components. All of these components for the polyamide composition are described above and what is described there also applies here. The polyamide composition is made into a substrate by one or more processes well-known to the man skilled in the art of making polymeric substrates.

The substrate can be pre-treated before it is used in the process for the production of an electrically conductive system; however it is also possible to use the substrate without further pre-treatment steps. An example of a pre-treatment could be a cleaning step or a plasma-treatment.

In a next step the conductive track precursor is applied onto the substrate by an jet printing technique. The exact nature of the conductive track precursor will depend on the type and required properties of the conductive track.

The final thickness of the conductive track will vary depending on the application. In general the thickness is between 10 nm and 100 μηη, preferably between 0.5 μηη and 10 μηη. It is also possible to apply the conductive track in several consecutive layers, with a sintering and cooling step between each application of the layers. The invention also relates to the use of an electrically conductive system according to the invention. As the adhesion of the electrically conductive system on the substrate is improved compared to prior art systems new application areas become available next to the previously known applications. An example of an advantageous application is in antennas, such as for example in mobile devices, e.g. phones. It was previously impossible with the then available techniques, such as laser direct structuring, to produce an antenna on a substrate with carbon fiber in it.

Further applications include electrical circuits and connectors that can be used in a variety of objects, e.g. in solar cells, transistors, OLED's and RFID technologies. If the substrate is a film, the electrically conductive system of the invention is particularly suitable for flexible printed circuits (FPC).

The invention will be demonstrated in the following examples, without being limited to them. Examples

Methods

Adhesion

In order to determine adhesion of the printed coating on the substrate, ASTM test ASTM D3359-08 D: "Standard Test Methods for Measuring Adhesion by Tape Test" is applied. According to test method B, a lattice pattern with six cuts in each direction is made in the coating to the substrate. Pressure-sensitive tape is applied over the lattice. The tape is peeled away at an angle of 180° with constant speed and force. The adhesion is evaluated according to the Classification of Adhesion Test Results (see Figure 1 ) and Table 1 below. For further detail the ASTM standard is referred to.

Table 1

Materials used

Polymers P1 Polyamide 66 obtainable under the Akulon®, from DSM Engineering Plastics. P2 Polyamide 46 obtainable under the tradename Stanyl® from DSM Engineering Plastics.

P3 Polyamide 6, obtainable under the tradename Akulon®, from DSM Engineering Plastics.

P4 Polycarbonate, obtainable under the tradename Xantar from Mitsubishi

Engineering Plastics.

P5 Liquid Crystal Polymer (LCP), obtainable under the tradename Vectra® from

Ticona.

P6 Polyethylene Terephthalate (PET), obtainable under the tradename Arnite® from DSM Engineering Plastics.

For the glass fibers, a standard grade standard spherical glass fibers were used.

Examples 1 and 2 and Comparative Examples 1 to 5

A plaque molded from the polymer composition, including flame retardant and glass fibers as listed in Table 2 below was prepared, using standard preparation techniques. Molding was performed by using a common injection molding machine with barrel heating conditions ranging from 275-345 °C and with tool temperatures varying between 50 °C and 140 °C. Drops of a silver ink composition (ink based on nanosilver particles less than 10 μηη, polyvinyl pyrolidon, glycerine, available under the trade name Cabot CSD66) were dropped on the plaque surface. The ink was spread by means of a Mayer bar to obtain a layer thickness of 5 μηη.

Thereafter the plaques were sintered in an oven at 230 °C (unless otherwise indicated in Table 2) for 15 minutes. The adhesion was tested by means of the Adhesion test described above. The results are indicated in Table 2 below as "Adhesion result dry".

A second plaque coated with silver ink was prepared as described above. After sintering, the plaque was submitted to a damp heat treatment as follows. The plaque was placed in an oven at a relative humidity of ca. 95% and submitted to the following temperature cycle: the temperature was gradually increased in 1 .5 hours to 65 °C. The plaque was maintained at 65°C for 4 hours. Then the temperature was gradually decreased to 30 °C in 1 .5 hours. The temperature was maintained at 30 °C for 1 hour. This 8 hour cycle was repeated 8 times (total 72 hours). Thereafter the oven temperature was set at +25°C and relative humidity of 50% and the plaques were maintained there for 2 hours. The adhesion of the plaques thus treated was tested by means of the adhesion test described above. The results are indicated in Table 2 below as "Adhesion result wet".

Table 2

¹ Sintering temperature 200 °C

² Sintering temperature 150 °C

The results in Table 2 show that both the initial adhesion and the adhesion after treatment in humid conditions was very good for the polymer compositions according to the inventions. For the polymers in the comparative examples, results were insufficient. The examples also show that addition of glass fibers does not alter the results observed.

Claims

Electrically conductive system comprising a substrate and an electrically conductive track adhered to the substrate wherein the substrate comprises polyamide-66 and the electrically conductive track is obtained by jet printing, wherein the substrate is made of a composition consisting of

(A) 30 - 100 wt.% of polyamide-66;

(C) 0 - 60 wt.% of reinforcing agents; and

(D) 0 - 15 wt.% of at least one additive;

wherein the weight percentages (wt.%) are relative to the total weight of the composition.

Electrically conductive system according to claim 1 , wherein the electrically conductive track comprises a metal or metal alloy.

Electrically conductive system according to claim 2, wherein the metal is silver, gold, copper or nickel or an alloy thereof.

Electrically conductive system according to any of the preceding claims, wherein the electrically conductive track has a thickness of 10 nm to 100 μηη. Electrically conductive system according to any of the preceding claims, wherein the substrate contains another polyamide in an amount of less than 30 wt.%.

Process for the production of an electrically conductive system comprising a substrate and a conductive track adhered to the substrate, comprising the steps of:

• providing a substrate made of a composition as mentioned in claim 1 ,

• applying a conductive track precursor on the substrate by a jet printing technique, and

Process according to claim 6, wherein the jet printing technique is aerosol jet printing.

Electrically conductive system obtained by the process according to claim 6 or 7.

Antenna for use in a mobile device, comprising an electrically conductive system according to any of claims 1 -5. Use of an electrical conductive system according to any of claims 1 -5 in the manufacture of electrical circuits or contacts in solar cells, transistors, Organ Light Emitting Diodes (OLED), Flexible Printed Circuits (FPC) and Radio Frequency Identification (RFID) systems.