WO1994026811A1

WO1994026811A1 - Producing a layer on a substrate

Info

Publication number: WO1994026811A1
Application number: PCT/GB1994/001019
Authority: WO
Inventors: Thomas Ronald Thomas
Original assignee: British Technology Group Limited (Gb)
Priority date: 1993-05-12
Filing date: 1994-05-12
Publication date: 1994-11-24
Also published as: GB9310009D0; AU6686194A

Abstract

A chemically homogeneous layer, such as hard carbon, is formed on a polymeric substrate, such as polycarbonate, by removing all the non-carbon atoms from the polycarbonate and leaving the carbon skeleton, which remains covalently bound to the rest of the polycarbonate substrate. The non-carbon atoms are removed by cold plasma treatment in an evacuated chamber using argon as a process gas.

Description

PRODUCING A LAYER ON A SUBSTRATE

This invention relates to producing a layer on a substrate.

The substrate is polymeric and the layer is in effect the polymer minus its non-skeleton atoms. The layer is therefore chemically homogeneous, optionally with a transitional underlayer between it and the substrate.

Traditionally, when it was required to apply a layer such as a hard carbon coating to a polymeric susbtrate, a plasma-decomposed hydrocarbon would be deposited on to the substrate. Although adhesion of the deposited layer to the substrate could be satisfactory, the adhesion depended on careful preparation of the substrate and on careful control of the deposition conditions.

In the electrochemical field, porous high-surface-area carbon composites are produced by admixture of carbon dispersions upon porous substrates using largely proprietary methods including compression and subsequent heat treatment. Such processes are essentially 'wet' methods and give rise to various manufacturing problems. According to the present invention, a chemically homogeneous layer (with possible transitional underlayer) is produced upon a polymeric substrate by cold plasma treatment of the substrate, such as to remove all atoms different from those forming the polymer backbone, whereby the desired chemically homogeneous layer remains. Preferably, the chemically homogeneous layer is the substantially unaltered skeleton of the polymer.

Typically, the polymer backbone is mainly or wholly carbon, the polymer being for example polycarbonate or a fluorocarbon such as polytetrafluoroethylene (PTFC). The polymeric substrate may itself be in the form of a coating or deposit on a base. Thus, for example, a thin layer of PTFC could be deposited on a base and converted into a carbon layer as set forth above. It will be appreciated that the layer produced according to the invention is covalently and integrally bound to the polymer substrate. A hard carbon infra-red-transparent scratch-resistant coating integrally bound to a polycarbonate substrate might be of interest as a windshield, especially if the carbon skeleton is rearranged into a vitreous or diamond-like structure. A carbon-like layer upon a fluorocarbon polymeric substrate (made by cold plasma defluorination) could be of interest to industries concerned with electronics, catalysts and fuel cells, where conductive or active films generated on a non-conducting or otherwise inert substrate would be of practical importance. The fluorocarbon could be porous or microporous PTFE.

It is known that treatment of fluorocarbon polymers such as

PTFE with alkali metals and complexes derived from alkali metals like Li, Na, can give rise to chemically etched surfaces. This technique has been used to enable such polymers to be more easily bonded by virtue of the polar groups created at the surface by reaction of the defluorinated surface with water, oxygen and carbon dioxide. This technique is generally regarded as a progressive defluorination, the loss of fluorine generating a reactive carbon site which can then react further. It has also been demonstrated that the use of lithium as its amalgam will electroche ically corrode PTFE ultimately yielding carbon with high surface area. The present invention, by contrast, relates to the manufacture of, for example, a composite material consisting of a carbonaceous layer attached to fluorocarbon body, especially to PTFC and particularly to porous and microporous fluoropolymers. This can be carried out in the gas phase, and the vacuum technique which can be employed, described hereafter, is cleaner, quicker and allows better control over the product parameters than the 'wet¹ methods mentioned earlier.

The invention will now be described by way of example. A sample of fluorocarbon polymer, namely microporous PTFE film made according to European Patent 247771B, is placed in an evacuated chamber. The fluorocarbon polymer is subjected to a cold plasma - so called because the process temperature at the substrate is low, about 300°K. The plasma is generated in the evacuated chamber utilising a process gas; the gas is so described, although present in only very low concentrations and although it may be any of several gases (e.g. a noble gas such as argon or neon, or dihydrogen), since it effects defluorination at the exposed surface of the polymer.

Different gases at different energy levels have been investigated to determine optimum defluorination conditions and, with the noble gas argon, the C(1S) spectrum shows the major product to be a carbon-to-carbon bond. The electrical conductivity of certain forms of elemental carbon can arise from the presence of ir bonds between the atoms resulting in delocalisation of electrons between layers of condensed aromatic rings, analogous with the material graphite.

In this way, a porous composite is created by the controlled defluorination of the microporous fluoropolymer in an evacuated chamber using a cold plasma. The carbon remaining after loss of fluorine can be utilised as an electrical conductor or a catalyst.

Claims

1. Producing a chemically homogeneous layer upon a polymeric substrate by cold plasma treatment of the substrate, such as to remove all atoms different from those forming the polymer backbone, whereby the desired chemically homogeneous layer remains.

2. Producing a layer according to Claim 1, wherein the chemically homogeneous layer is the substantially unaltered skeleton of the polymer.

3. Producing a layer according to Claim 1 or 2, wherein the polymer backbone is mainly or wholly carbon.

4. Producing a layer according to Claim 3, wherein the polymer is polycarbonate or a fluorocarbon.

5. Producing a layer according to any preceding claim, wherein the polymeric substrate is in the form of a coating or deposit on a base.