Mouldable material

DESCRIPTION

The present invention refers to a mouldable material. In particular a mouldable material having good conductive properties.

More and more electrical functions tend to be integrated in plastic products. In such so-called "Moulded Interconnect Devices" (MID) conducting structures may be provided on plastic substrates by means of different techniques, such as two-shot moulding [2C-MID], laser techniques [e.g. LDS and MIPTEC], hot embossing [HE-MID], insert moulding [IS-MID]. In those techniques patterning and metallization are subsequent processing steps: in 2C-MID [SKW-process, PCK-process, Bayer process] patterning of the structures results from injection moulding, whereas electro-plating yields the metal structures; in LDS [LPKF process] and MIPTEC [Panasonic process] a laser performs the patterning step followed by electro-plating of the conducting structures; - in HE-MID a flexible foil with a metal structure is pressed on a plastic substrate; in IS-MID a conductive metal stamped-bended sheet is used as insert in the injection moulded process. Combinations of the mentioned processes are also known.

Mouldable synthetic materials are commercially available having specific resistances of about 10 ^~ Ohm.cm. Those materials are well applicable for shielding and in antennas etc. However, for conducting substantial electrical currents a specific resistance of about 1 x 10 ^"4 Ohm.cm or less, in particular of about 10 ^"5 Ohm.cm or less will be required.

WO2005057590 discloses several metal/plastic hybrid materials comprising additives in plastic and realizing specific resistances of less than 10 ^"2 Ohm.cm. The metal/plastic mixtures

comprise a thermoplastic such as polyamide 6, a metal alloy having a rather low melting temperature, i.e. between 100 ⁰ C and 400 ⁰ C, and an electrically conductive and/or metallic filler material, consisting of fibres and/or particles of e.g. copper, steel or carbon, having e.g. a length of about 2 mm and a thickness in the range of 10-80 μm. As a preferred metal alloy WO2005057590 proposes a commercially available alloy "MCP 200" manufactured by MCP HEK GmbH in Lϋbeck, having a melting point of 200 ⁰ C.

The materials known from WO2005057590 may have rather good injection moulding capabilities for rather "bulky" parts. A severe disadvantage, however, is that, due to the dimensions of the conductive particles, in practice, the rheological behaviour does not meet the requirements for the manufacture of small conductive MID parts (including thin conductive channels) e.g. in an "In Mould Assembly" (DVIA) production process, in which those product parts (including said conductive channels) are moulded and assembled within the same mould. Increasing the amount of metal filler to decrease the resistance of materials even more makes that disadvantage more pronounced, limiting the application of such materials.

Additionally, in Michaeli and Pfefferkorn, "Material and processing behaviour of thermoplastics filled with low melting metals", Plastics and Rubber Processing and Applications, 2006 Vol. 35, 9 pp 380-386 (http://cat.inist.fr/?aModele=afficheN&cpsidt=l 8275270), it is stated that polymer blends filled with solid fillers such as metal fibres or carbon black can no longer fulfil the increasing demands on electrical and thermal conductivity. High levels of hard fillers increase the melt viscosity unacceptably, causing a minimised process window, high mechanical stresses on the polymer as well as an increased abrasion on both mould and machinery. The high electrical conductivity required for connectors, moulded interconnect devices (MID) or sensors is said to be achieved by a material developed by IKV, Aachen, and Siemens AG, Erlangen/Germany, combining fibre filled polymer blends with low melting metal alloys. These alloys have the advantage of being already molten during processing and of not solidifying before the cooling phase.

Aim of the present invention thus is to provide an improved material which is well fit for moulding well conductive parts having one or two dimensions (width/height) of less than 1 mm and, preferably, less than 100 micrometer.

Another aim is to provide a mouldable plastic mixture which meets the requirements for "In Mould Assembly".

Yet another aim is to provide a conductive plastic having good mechanical properties, like tensile strength, flexural strength etc.

According to the invention it is preferred to provide a mouldable material having good conductive properties, comprising:

- a synthetic material,

- nanotubes, - and a metal alloy having a melting temperature within or below the range of the processing temperature of the synthetic material.

Said metal alloy may hereinafter be referred to as 'low melting metal alloy'.

As used herein, the range of processing temperature of the synthetic material means the temperature range over which the synthetic material can be formed into the envisaged product shape. The lower limit of this range is in general set by the melting temperature of the synthetic material of the melting temperature range in case the synthetic material is a composition of components having different melting temperatures. The melting temperature or temperature range of the synthetic material can be determined using differential scanning calorimetry (DSC), as described in ISO 11357-1 , -2, -3 for thermoplastic polymeric materials. The upper limit of the processing temperature range is in general set by the boiling temperature or decomposition temperature of the synthetic material (whichever of these two temperatures is lower). The boiling temperature or the decomposition temperature of the synthetic material can be determined using DSC, differential thermal analysis (DTA) and/or thermal gravimetric analysis (TGA) conducted in an inert, e.g. nitrogen, atmosphere, at 1 bar.

The melting temperature or temperature range of the metal alloy can be determined according to the guidelines recommended in the NIST Special Publication 960-15, "DTA and Heat-flux DSC Measurements of Alloy Melting and Freezing", W. J. Boettinger et al, November 2006. A mouldable material according to the invention may in particular be advantageous for preparing an electrically-conductive product (which may be a part of a larger arrangement that also comprises one or more electrically-insulative parts), more in particular an electrically-conductive product with one or more conductive parts of which one or two dimensions (width/height) is less than 1 mm and, preferably, less than 100 micrometer.

In particular the composition may comprise carbon nanotubes, such as fullerenes having an elongate shape. Carbon nanotubes (CNTs) are allotropes of carbon having a nanostructure where the length-to-diameter ratio can exceed 1 ,000,000. Such cylindrical carbon molecules have novel properties that make them potentially useful in many applications in nanotechnology, electronics, optics and other fields of materials science. They exhibit extraordinary strength and unique electrical properties, and are efficient conductors of heat. Inorganic nanotubes (other than carbon nanotubes) have also been synthesized.

Carbon nanotubes are members of the fullerene family, which also includes buckyballs. Whereas buckyballs are spherical in shape, a nanotube is cylindrical, with at least one end typically capped with a hemisphere of the buckyball structure. Their name is derived from their size, since the diameter of a nanotube is in the order of only a few nanometres (nm), while they can be up to several millimeters in length. Nanotubes are categorized as single -walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs). Nanotubes naturally align themselves into "ropes" held together by Van der Waals forces.

Besides carbon nanotubes, other inorganic nanotubes could be used, e.g. copper or bismuth nanotubes, as presented by Dachi Yang et al, in "Electrochemical synthesis of metal and semimetal nanotube-nanowire heteroj unctions and their electronic transport properties", Chem. Commun., 2007, 1733-1735, www.rsc.org/Publishing/Journals/CC/article.asp?doi=b614147a.

Aluminum nanotubes and their manufacture are presented by Dae Dong Sung et al, "A New Fabrication Method of Aluminum Nanotube Using Anodic Porous Alumina Film as a Template", public of the Department of Chemistry, Dong -A University, Busan 604-714, Korea, http://j ournal.kcsnet.or.kr/main/j_search/j_download.htm?code=B06081 2.

Moreover, boron nanotubes, as presented by Xiaobao Yang, Yi Ding and Jun Ni in "Ab initio prediction of stable boron sheets and boron nanotubes: Structure, stability, and electronic properties" Phys. Rev. B 77, 041402(R) (2008) (4 pages) (DOI: 10.1103/PhysRevB.77.041402), may be an interesting choice as well.

As the low melting metal alloy, it is preferred to apply an alloy of which the melting point/range is lower than the temperature processing range of the relevant synthetic material, or which coincides with at least part of that range. The MCP 200 alloy (http://www.mcp- hek.de/pdf/mcp leg/deutsch.jpg) which is commercially available from MCP HEK GmbH in Lϋbeck, having a melting point of 200 ⁰ C, the use of which material as component for mouldable conductor is known as such from WO2005057590, might be such an alloy.

As the synthetic material, in principle any mouldable synthetic material may be used. Various mouldable synthetic materials are known in the art. In particular the synthetic material may be a thermoplastic polymeric material. Examples of suitable mouldable materials include polyolefms, such as polyethylene, polypropylene; polyvinylchloride, polystyrenes; acrylonitrile-butadiene- styrene, styrene-acrylonitrile, polyamides; polybutylene terephtalate; polycarbonate, polyether imides; polyethylene naphthalate, polyphenylsulphide, liquid crystals polymers; including mixtures and copolymers thereof.

The mouldable synthetic material usually forms 50 to 80 wt. % of the total mouldable material, in particular 70 to 80 wt. %.

The nanotubes usually form 5 to 25 wt. % of the total mouldable material, in particular 5 to 10 wt. %.

The low melting metal alloy usually forms 15 to 25 wt. % of the total mouldable material, in particular 15 to 20 wt. %.

The ratio nanotubes to low melting metal alloy usually is in the range of 1.67: 1 to 1 :5, in particular in the range of 1 : 1 to 1 :3.

The invention further relates to a product made from a mouldable material according to the invention, in particular a moulded product.

A product of the invention may be a stand-alone product or form a part of a larger structure. In particular the product may be a conducting structure in or on an electronic device, such as a photovoltaic (PV) panel or an array of such panels. Electrically conductive connections between individual PV cells and/or with electronics components specific for such a panel (for instance bypass diodes and DC/DC boost converters) could be made from the mouldable material, as well as electrically conductive connections between panels and/or between an array of panels and electronic components (for instance DC/ AC inverters). In a specific embodiment, the PV panel is a solar panel.

The invention further relates to a method for manufacturing a product, comprising forming the product from a mouldable material according to the invention, which forming is usually carried out at or above the melting temperature or temperature range of the synthetic material.

The forming preferable comprises a moulding step. The moulding step may be based on methodology known per se. Suitable moulding techniques include injection moulding, compression moulding, extrusion moulding, gas or water assisted injection moulding, more- component injection moulding and combinations of two or more of these techniques.

In an advantageous embodiment the product has a specific resistance of 1 x 10 ⁴ Ohm.cm or less, or even of 1 x 10 ^~5 Ohm.cm or less. The four-point resistivity measurement technique, as, for instance, described in "The Measurement, Instrumentation and Sensors Handbook", CRC Press

LLC, editor J.G. Webster, 1999, ISBN 0-8493-8347-1, could be used to measure the specific resistance of the product.

In an embodiment the product has one or two dimensions (width/height) of less than 1 mm and, preferably, less than 100 micrometer. In particular, the product's dimensions may be at least 2 micrometer in at least one dimension, in particular in at least two dimensions.

The invention may introduce a breakthrough in MID applications.