Patent Application
20130029138
This invention relates to grafted polymer coatings and particularly to a polymer coating deposited using a “grafting from” procedure from a plasma polymer.
International patent application WO 2006/097719 A1 discloses a polymer coating deposited using a “grafting from” procedure from a plasma polymer deposited on a substrate. In one embodiment the plasma polymer is chosen and deposited in such a way that it possesses functional groups which act as sites for a “grafting from” procedure. For example, a deposited plasma polymer may possess transferable halogen moieties which directly initiate Atom Transfer Radical Polymerisation (ATRP). In such embodiments, a surface initiated polymerisation procedure (“grafting from”) is undertaken directly after the plasma polymer deposition upon exposure of the plasma polymer to a suitable monomer(s) and suitable catalytic or mediating compound(s). In an alternative embodiment, the plasma polymer layer requires further derivatisation before it can initiate polymer growth. An example of this latter embodiment involves exposing a plasma polymer deposited from 4-vinylbenzyl chloride (4-VBC) to sodium diethyldithiocarbamate in ethanol to produce a dithiocarbamate functionalised 4-VBC surface having suitable initiator functionality to initiate photochemical Iniferter polymerisation when subsequently exposed to a methanolic solution of styrene monomer.
Whilst WO 2006/097719 A1 identifies a number of advantages and applications for the “grafting from” technique and the benefit of using a plasma polymer base layer from which the polymer coating is “grafted from” (thus avoiding some of the constraints associated with deposition of a polymer coating directly upon a surface of a substrate), its proposed coatings and manufacturing methods nevertheless suffer from a number of limitations.
One aim of the present invention is to overcome some of the limitations associated with known grafted polymer coatings and techniques.
In accordance with one of its aspects, the present invention provides a method of coating a substrate as defined in claim 1. According to this aspect of the invention, at least some of the active radicals are maintained “in an active state” that is to say in a state in which they are capable of initiating radical polymerisation of the conventional polymer without additional transformation or liberation. This is fundamentally different from the mechanism proposed in WO 2006/097719 according to which it is essential to provide particular functional groups in the plasma polymer which initially stabilise or trap the radicals in the functional group prior to a subsequently step in which the stabilised or trapped radicals are liberated so as to be able to initiate radical polymerisation.
Aspects of the invention relating to articles comprising polymer coatings are defined in other independent claims. The dependent claims define preferred or alternative embodiments.
In accordance with one aspect of the present invention, a plasma polymer serves as the base layer for the initiation of the deposition of a polymer layer for a “grafting from” or radical polymerisation process. The plasma polymer layer may be deposited on a wide variety of substrates including metal substrates (including steel and aluminium substrates) and non-metal substrates (including glass, silicon and polymer substrates). The substrate may be a sheet, a film, a surface or fibres. The plasma polymer layer may be firmly secured to the substrate, notably by cross-linking of the plasma polymer during its deposition.
The term plasma polymer is intended to denote an irregular three dimensional network of highly cross-linked molecular segments that may be formed, for example, by plasma enhanced chemical vapour deposition of an organic precursor, the high degree of cross-linking preferably contributing to properties including high mechanical resistance, thermal stability and high adherence to metal, glass and polymer substrates. The plasma polymer is preferably produced by exposing its precursor molecules to a plasma containing electrons whose energy is greater than the energy necessary to fragment the functional groups of the precursor molecule (for example anhydride functional groups).
The presence of active free radicals is induced in the plasma polymer, preferably during deposition of the plasma polymer. Such active free radicals are maintained in an active state so that they can initiate radical polymerisation when the plasma polymer is exposed to pre-cursors (monomers) of a conventional polymer. Thus, in this aspect of the invention, it is not necessary to specifically select the plasma polymer, its precursors and the deposition conditions such that the plasma polymer possesses functional groups which can subsequently act as sites for a “grafting from” procedure as radicals maintained in an active state within the plasma polymer (but not necessarily present as stable functional groups) initiate radical polymerisation of the desired polymer. For example, the plasma polymer precursors need not comprise specific functional groups, or if they do, such functional groups need not be maintained during formation of the plasma polymer to provide “grafting from” sites. Consequently, a significant simplification is provided in the nature and deposition of the plasma polymer, a wider choice of plasma polymers is made possible and the necessity of catalysing or derivising the plasma polymer to render it capable of initiating radical polymerisation is removed.
The radicals may be provided at the surface of the plasma polymer and/or in its bulk. Radicals provided below the surface of the plasma polymer may be particularly suited to initiating radical polymerisation of a precursor in a way which provides a high level of adhesion between the plasma polymer and a grafted conventional polymer (which, in this case, may be attached within the volume of the plasma polymer rather than just at the surface of the plasma polymer).
The excitation source used to induce the presence of active radicals within the plasma polymer may be a plasma generator, for example a radio frequency coil. The plasma generator may be used in capacitive (rather than inductive) mode as under at least some conditions this may favourite generation of a large number of radicals. Possible alternative or additional excitation sources include electromagnetic wave generators, a source of gamma radiation and a source of electron radiation.
Preferably, the plasma polymer is deposited on a substrate in a controlled atmosphere, for example a reduced pressure atmosphere substantially free of oxygen and/or nitrogen and/or other reactive species which would tend to react with and/or deactivate free radicals induced in the plasma polymer. The substrate may be maintained in a controlled atmosphere until exposed to the polymer precursors (monomers) of the conventional polymer to be deposited. This may be achieved, for example, by introducing the conventional polymer precursors (monomers) in to the enclosure in which the plasma polymer is deposited (for example at the end of a plasma polymer deposition phase) or moving the substrate to a different enclosure at which the conventional polymer precursors (monomers) are deposited through a controlled atmosphere, for example in a multi chamber on-line coater.
Preferably, the plasma polymer is a highly reticulated plasma polymer. The term “highly reticulated plasma polymer” as used herein means a plasma polymer in respect of which:
This may be determined by ToF-SIMS analysis and suitable data processing.
With respect of the highly reticulated plasma polymer:
Preferably, the plasma polymer is devoid or substantially devoid of functional groups which could potentially be activated to act as “grafting from” sites.
The polymer coating, notably the conventional polymer, preferably provides a functional coating adapted to the substrate and/or its use. For example:
With respect to the use of the plasma polymer precursors comprising one or more precursors selected from the group consisting of allylamine, acrylate, butyl acrylate, methyl acrylate, ethyl acrylate, 2-ethyl hexyl acrylate, glycidyl methacrylate, aromatic and aliphatic acrylate derivatives, aromatic and aliphatic methacrylate derivatives, CH4/N2, silane derivatives (eg. Hexamethylenedisiloxane), fluorine derivatives (eg. Difluoroethylene, tetrafluoroethylene), aliphatic and aromatic organic derivatives, aliphatic or aromatic alcohols, saturated or unsaturated alcohols, aliphatic or aromatic amines, saturated or unsaturated amines, ketones, acids, aldehydes, esters, anhydrides, these may provide one or more of the following advantages:
The conventional polymer layer may be provided by one or more monomers deposited, for example selected from the following acrylate monomers:
Acrylate monomers may be used to provide a “scratch resistant” and/or “self healing” functional coating due to the thermo-mechanical properties of a polymer layer obtained by radical polymerisation of these acrylates. Where the glass transition temperature Tg of such a layer is lower than ambient temperature, the mobility of the polymer chains at ambient temperature permits them to go back to their initial configuration in the case of a scratch (ie to have a “self healing” function).
An unsaturated monomer able to polymerise via free-radical polymerisation reaction may be used as a precursor for the conventional polymer layer. The precursor(s) for the conventional polymer layer may be selected from:
The conventional polymer may comprise a copolymer derived from two or more of such precursors.
The conventional polymer is preferably secured to the plasma polymer by covalent bonding.
The term conventional polymer is intended to indicate a polymer which is not a plasma polymer, and comprising repeating structural units connected by covalent chemical bonds.
In one embodiment, the precursors of the conventional polymer are the same as the precursors of the plasma polymer. This not only simplifies the manufacturing process but, when desired, is one way of producing a graded plasma/conventional polymer. Such a graded structure may have a highly cross-linked plasma polymer deposited on the substrate with the structure of the polymer changing progressively to a conventional polymer as the distance from the substrate surface increases. This may provide a gradient transition between the plasma polymer and the conventional polymer, ie a gradual transition rather than a single step transition between the plasma polymer and the conventional polymer, particularly where the conventional polymer is arranged as a layer over the plasma polymer. Such a graded structure may provide particularly good securing of the conventional polymer to the plasma polymer. A process in which the same precursors are used for the plasma polymer and the conventional polymer and in which radical polymerisation occurs simultaneously with plasma polymerisation may produce such a coating, for example by gradually reducing the power applied during plasma polymerisation and preferably allowing for radical polymerisation to continue after the plasma has been discontinued.
In some embodiments, the following are absent from the ends of at least some of (preferably absent from the majority of and more preferably absent from substantially all of) the conventional polymer chains: chlorine, bromine, thiocarbamate groups, and nitroxy groups. Alternatively or additionally, in some embodiments the following are absent or substantially absent from the plasma polymer and/or the interface between the plasma polymer and the conventional polymer: halogens derivatives, copper derivatives, heavy metals derivatives, thiocarbamate groups, and nitroxy groups. Such materials or groups, which are essential for the coatings of conventional polymers grafted from plasma polymers of WO 2006/097719 A1 are undesirable from a perspective of cost and/or easy of handling and/or stability and/or environmental aspects and may be avoided using the present invention. Thiocarbamate groups and/or fluorine groups may also be absent in the same ways.
The plasma polymer may be in direct contact with the substrate or a coating layer may be provided between the substrate and the plasma polymer.
A step of surface preparation of the substrate to facilitate and/or enhance deposition of the plasma polymer may be provided, for example a surface cleaning and/or surface refreshing step. This may be achieved by subjecting the substrate surface to an oxygen/argon plasma.
Non-limiting examples of aspects of the invention will now be described with reference to:
a and 4b: which are representations of ToF-SIMS analysis;
The article 10 of
Radical polymerisation of the conventional monomer is initiated from radicals 14 present at and below the surface of the plasma polymer layer 12.
The article of
The following steps and conditions may be used to deposit desired coatings on a substrate arranged in deposition chamber 21:
In preparation for a cleaning step, the pressure in the deposition chamber is lowered to 10−6 Torr. Argon and oxygen are injected into the deposition chamber with the following flows: argon flow of 25 standard cm3 by minute (sccm); oxygen flow of 25 sccm. The plasma is activated when pressure is regulated to 50 mTorr. The plasma is in capacitive mode with a power of 25 W and a self-bias is measured on the substrate. The cleaning step is operated during 10 minutes.
After the cleaning step, the pressure is lowered to 10−5 Torr in order to avoid the presence of contaminant species like oxygen or water vapour.
For the deposition of the plasma film, the precursor is vaporised into the chamber with a flow rate of 2.5 sccm. The pressure is regulated to 50 mTorr.
The plasma during the deposition of the plasma polymer is in capacitive and continuous mode to raise the amount of radical in the plasma polymer. The precursor is ethyl acrylate. The power of the plasma is higher than 10 W and lower than 500 W, preferentially higher than 25 W and lower than 100 W. The deposition step lasts more than 30 seconds and less than 3 minutes, preferentially more than 1 minute and less than 2 minutes.
For the deposition step of the conventional polymer, the plasma is cut off and the vaporisation of the precursor is maintained. The pressure is raised to a value higher than 100 mTorr. The deposition step of the conventional polymer lasts more than 10 sec and less than 24 hours.
The following tests were run:
Using the equipment of
Step 1:
Cleaning and/or preparation of the surface of the substrate using the following conditions:
Pressure and gas mixture: 50 mTorr; argon flow of 25 sccm; oxygen flow of 25 sccm
Plasma coil mode and conditions: Capacitive; 25 W, estimated self-bias: −410V.
Duration: 10 minutes
Step 2:
Deposition of plasma polymer:
Pressure and gas mixture: 50 mTorr; estimated ethyl acrylate flow of 2.5 sccm
Plasma coil mode and conditions: Capacitive and continuous; 50 W, estimated self-bias of −580V.
Duration: 5 minutes
Using the equipment of
Step 1:
Cleaning and/or preparation of the surface of the substrate using the following conditions:
Pressure and gas mixture: 50 mTorr; argon flow of 25 sccm; oxygen flow of 25 sccm
Plasma coil mode and conditions: Capacitive; 25 W; self-bias −422 V
Duration: 10 minutes
Step 2:
Deposition of plasma polymer:
Pressure and gas mixture: 50 mTorr; estimated ethyl acrylate flow of 2.5 sccm
Plasma coil mode and conditions: Capacitive and continuous; 50 W; self bias −583 V
Duration: 1.5 minutes
Step 3:
Deposition of conventional polymer:
Pressure and gas mixture: 100 mTorr; continuous ethyl acrylate vaporisation after stopping the plasma
Plasma coil mode and conditions: Off
Duration: 21.5 hours
Using the equipment of
Step 1:
Cleaning and/or preparation of the surface of the substrate using the following conditions:
Pressure and gas mixture: 50 mTorr; argon flow of 25 sccm; oxygen flow of 25 sccm
Plasma coil mode and conditions: Capacitive; 25 W; Bias −417 V
Duration: 10 minutes
Step 2:
Deposition of plasma polymer:
Pressure and gas mixture: 50 mTorr; estimated ethyl acrylate, flow of 2.5 sccm
Plasma coil mode and conditions: Capacitive and continuous; 50 W; Bias −587 V
Duration: 1.5 minutes
Step 3:
Deposition of conventional polymer:
Pressure and gas mixture: 3.3 Torr; continuous ethyl acrylate vaporisation after stopping the plasma
Plasma coil mode and conditions: Off
Duration: 22 hours
In each example, each step in the process followed immediately from the previous step; after completion of the last step, the sample was removed from the deposition chamber and analysed. The ethyl acrylate vapour was supplied from a reservoir connected to the polymer entry port.
For examples 2 and 3, at the end of step 2 (deposition of plasma polymer) the plasma was stopped but the vaporisation and supply of the ethyl acrylate precursor was continued. On example 2, when step 3 of the experiment was stopped after 21.5 hours it was observed that the ethyl acrylate reservoir was empty.
An XPS spectrum of the product from example 1 showed a signal at 287.9 eV characteristic of ketone functions. Although the chemical precursor did not contain ketone functionalities, because of the fragmentation of the precursor molecules in the plasma during the deposition process ketone functions are formed and trapped in the film. Therefore, in our synthesis conditions, the signal of ketone functions at 287.9 eV is characteristic of the ethyl acrylate plasma polymer film.
An XPS spectrum of the product from example 2 revealed a ketone signal at 288.1 eV and a new signal at 289.3 eV. Following the literature, this new signal at 289.3 eV is representative of the acrylate function. The appearance of this signal demonstrates that the conventional poly(acrylate) have been grafted on the plasma polymer.
Taking into account the XPS analysis depth (˜10 nm) and the XPS spot (˜1 mm2), the occurrence of both these signals at 288.1 eV and at 289.3 eV could mean that the conventional poly(acrylate) layer is either thinner than 10 nm or inhomogeneous in thickness or both.
The spectrum from an XPS analysis of the product from example 3 is shown in
The presence of the acrylate signal and the occurrence of these signals in the same proportions clearly demonstrates the presence of conventional poly(acrylate) at the surface of the sample. The absence of the ketone signal reveals that the conventional poly(acrylate) layer is homogenous through the area analysed (˜1 mm2). Moreover, taking into account the XPS analysis depth (˜10 nm), it also demonstrates that the conventional poly(acrylate) layer thickness is higher than 10 nm.
Two isopropanol plasma polymer films were deposited to investigate differences between a plasma polymer film deposited at a power of 50 W in capacitive mode (example 4b which was found to be highly reticulated) and a plasma polymer film deposited at a lower power of 30 W in inductive mode (example 4a).
ToF-SIMS spectral analysis of these sample films are shown in
A PCA (principal component analysis) was conducted on the intensity of the peaks from the Tof-SIMS analysis using SIMCA software. Six points were recorded for each of example 4a and 4b and the PCA analysis allowed the definition of two principal components with the first principal component PC1 taking account of 91.3% of the variance. The PCA analysis is represented graphically in
A representation of the loadings from the PCA analysis is shown in
A calculation of the average hydrocarbon fragment taking account of the lowest loading limit of 0.90 gave:
This shows that the average hydrocarbon fragment of example 4b is significantly smaller than that of example 4a, furthermore, a comparison of the PCA analysis for examples 4a and 4b shows that the more representative or characteristic peaks for example 4a are oxygenated fragments characteristic of the precursor, indicating a lower level of reticulation for example 4a and a greater level of reticulation for example 4b.
It should be noted that the power delivered to the system for deposition of the plasma polymers of examples 4a and 4b was significantly greater than the average power of 0.26 W used in the examples of WO 2006/097719 A1.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1001355.5 | Jan 2010 | GB | national |
| 1001390.2 | Jan 2010 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2011/051080 | 1/26/2011 | WO | 00 | 10/12/2012 |
