The present invention relates to a composition for the preparation of a polysiloxane-based coating comprising carbon nanotubes.
The present invention also relates to the use of such compositions for the preparation of an anti-biofouling coating, and more particularly, the use of such coatings for paintings in a marine environment.
Biological contamination by a marine environment, which is known as biofouling, whether it is micro-organic or macro-organic, is a major problem, not only for land-based installations that use a large amount of sea water, but also for offshore installations and more generally for any object in permanent or prolonged contact with sea water, for instance boat hulls and aquaculture ropes, cages and nets. Indeed, marine organisms, such as algae, shellfish and other crustaceans, adhere to and then grow on the exposed surfaces, which impairs the correct functioning and deteriorates said installations or objects. In particular, they may, for example, block the inlets of sea water intake valves and thus reduce the water uptake capacity of land-based installations using sea water, or they may reduce the speed of ship hulls and increase their fuel consumption by adhering to said hulls.
Marine anti-biofouling and fouling release compositions are usually applied to surfaces in permanent contact with a marine environment to control or prevent the adhesion or growth of such marine organisms or alternatively to facilitate their removal. Such a composition generally contains one or more compounds that are toxic to the marine organisms that adhere to the submerged surfaces that need to be protected. The drawback of these toxic compounds is that they need to be released into the marine environment by the marine anti-biofouling coating or paint over a relatively long period in order to be sustainably effective. As a result, such a composition is always polluting, all the more so since it generally comprises compounds such as mercury, lead, tin or arsenic.
Some marine anti-biofouling coatings or paints comprise copper-based compounds, which have been known for a long time for their toxicity to phytoplankton and other marine organisms. The copper may be, for example, in the form of cuprous oxide, copper dioxide, copper thiocyanate, copper acrylate, flaked copper powder or copper hydroxide and may be released into the marine environment in the form of copper ions. Unfortunately, the drawback of this solution is that it does not last long. Specifically, once the copper content of the coating has been depleted, the coating is no longer effective. Usually, the compositions have very high doses of copper in order to give the coating a longer life. However, the use of high concentrations of copper may also lead to pollution of the marine environment.
The trend in terms of environmental regulations in the coming years will be the banning of the use of marine anti-biofouling coatings such as those mentioned previously, but also those comprising tin(IV) derivatives, such as tin oxides or tributyltin, which are all environmentally toxic and hazardous, in favour of alternative coatings that are more environment-friendly.
Document WO 2008/046166 discloses an alternative to usual anti-biofouling coatings, using carbon nanotubes dispersed in a polysiloxane matrix. In the same way, a lot of catalysts for curing polysiloxane resins are known in the prior art, but the most commonly used are those based either on organotin-based or platinum-based compounds. However, due to toxicological problems and cost considerations, the use of such catalysts should be avoided in large-scale coating applications such as their application in anti-biofouling coating of boat hull and the like.
The present invention aims to provide a composition for the preparation of an anti-bioufouling coating that does not have the drawbacks of the prior art.
More particularly, the present invention aims to provide a composition for the preparation of an anti-bioufouling coating without any metallic catalyst.
The terms “anti-biofouling composition” are used to define a precursor composition for the preparation of a coating having anti-biofouling properties, fouling release properties or a combination thereof.
The present invention is related to a composition for the preparation of an anti-biofouling coating comprising or consisting of:
According to particular preferred embodiments, the invention further discloses at least one or a suitable combination of the following features:
A second aspect of the invention is related to a method for the protection of items in a marine environment against bio-fouling comprising the step of applying on said items the composition according to the above-mentioned features and embodiments.
The present invention also discloses a method for the protection of items in a marine environment against bio-fouling comprising the step of applying on said items a coating composition comprising polysiloxane, a curing agent and trifluoroacetic acid.
The present invention discloses a composition for the preparation of an anti-biofouling coating comprising:
The first objective of the present invention is to avoid the use of heavy metal catalyst in polysiloxane coatings for environmental reasons. Surprisingly, among all organic catalysts available, it has been shown that the use of organic acid catalysts does not only solve the environmental problem of leaching metal catalyst in a marine environment, but also improves the fouling properties of MWCNTs comprising coatings, as shown in
Even better, even without the presence of carbon nanotubes, it can be seen in
Surprisingly, the influence of the catalyst concentration on the fouling properties of the obtained coating shows a non-linear behaviour with an optimum relative catalyst concentration of about 5 wt %. This can be seen on
In order to simplify the curing process, the polymer fraction of the invention essentially consists of polysiloxane, preferably polydimethysiloxane (PDMS). By essentially consisting of polysiloxane, it is meant that small amounts of copolymer may eventually be incorporated, without substantially changing the general properties of the obtained coating. Such additional polymeric modifier may for example be incorporated in order to fine-tune the viscosity of the obtained composition.
In order to optimise the viscosity of the composition to ease its application on items to be coated, the molecular weight of the polysiloxane is preferably comprised between 15000 and 25000 g/mol, more preferably about 20000 g/mol.
Polydispersity (PDI) also allows to fine-tune the rheological properties of the composition. Preferably, the polydispersity of the polysiloxane is about 1.85. PDI calculated is the weight average molecular weight divided by the number average molecular weight. It indicates the distribution of individual molecular masses in a batch of polymers.
n = 20300 g/mol; PDI =
n = 5970 g/mol (b)
(a) As determined by GPC in THF, relative to PMMA standards.
(b) As claimed by the supplier.
All listed materials are used as received.
For the preparation of all examples (see the following Tables) two parts (Part A and Part B) were formulated according to the commercially available two-part PDMS-based resins. In a typical experiment, part A consists of silanol-terminated PDMS (Polysiloxane 1), hexamethyldisilazane-treated silicone dioxide (Si) and a curing agent (VTiPOS). According to the supplier's recommendations, the polysiloxane/VTiPOS ratio is fixed to 95/5 (wt %/wt %). The amount of Si is fixed to 2 wt % in the final formulations in order to regulate the viscosity.
In the case where Polysiloxane 2 was used (0.5 wt % MWCNTs) for the binary coating preparation, the MWCNTs contribution to the final mixture was adjusted to 0.1 wt % by diluting said MWCNTs with Polysiloxane 1.
For all compositions, all components were mechanically mixed at 1200 rpm for 30 min.
In all experiments, part B consists of PDMS-CH3 added with the requested amount of catalyst in order to reach the final composition as marked in Tables 2 to 4.
For all sample preparations, parts A and B are mixed together in a weight ratio of 10:1 in order to obtain a cross-linked PDMS via a condensation reaction at room temperature (after a 48 h reaction time, which is a predefined time selected to compare the studied formulations).
The samples were characterized in terms of viscosity (η, cP) of the binary compositions before cross-linking, tensile modulus (E, MPa) and equilibrium degree of swelling (Gsw) and the extractable value (Esw) of the cross-linked samples (see the following Tables).
The viscosity of the compositions was determined with a Brookfield DV-II viscometer at 25±0.1° C. and at a rate of 1 rpm.
The tensile modulus of the samples was analysed at room temperature with a dynamic mechanical analyser DMA (TA Instrument 2980) operating in tensile strain-sweep mode. A frequency of 1 Hz, a preload of 0.1 N and an amplitude from 0.5 to 27 mm were used. The results are the average of at least three measurements.
For the swelling tests, pre-weighed dry cylindrical samples of the cross-linked formulations were immersed in 100 ml of heptane at room temperature during 48 h. To determine the equilibrium degree of swelling (Gsw %), samples were taken out at regular time intervals, the heptane remaining in excess at the sample surface was gently removed with filter paper and the swollen samples were weighed. After reaching the equilibrium, the samples were dried during 16 h at 50° C. under vacuum. The Esw. the Gsw % and the Esw were calculated as follows:
where w0 is the weight of the sample after curing, before immersion in heptane and drying under vacuum;
where w1 is the weight of the sample after immersion in heptane during 48 h;
where w2 is the weight of the sample after drying during 16 h at 50° C. under vacuum.
The values of Gsw and Esw are the average of at least three samples. It can be seen in table 3 that all samples catalysed with an organic acid in the presence of carbon nanotubes were almost fully cross-linked, as they were not dissolved by hexane, and the level of extractible matter was limited to low values. Regarding inorganic acids, only samples catalysed with sulphuric acid were not fully dissolved by the hexane, but the level of extractible (up to about 56%) showed insufficient levels of cross-linking.
In the case of samples containing no carbon nanotubes (table 2), the situation is more complex, as TCA does not fully cross-link the polysiloxane at low levels of catalyst.
The biofouling behavior of such coatings has been checked in an assay described below.
The assay procedure regarding the studies of cellular colonization is in accordance with section 4.2 of the Biological Workshop Manual (BWM, AMBIO Biological evaluation workshop, University of Birmingham UK 21-22 Apr. 2005; Ulva Sporeling Growth). Spores are released from plants collected from the seashore. The concentration of spores is adjusted to a standard concentration, for example 1×106 spores/ml. Each coating sample (table 4) is immersed in 30 liters of distilled water for one week, and then in artificial sea water for 1 hour in darkness, in the presence of the colonizing cells (spores of the marine alga Ulva) before the growth medium is added. The samples are then incubated in an illuminated incubator for 6 days, the medium being refreshed every 2 days. The biomass on each slide is quantified by measuring the amount of chlorophyll present. This is quantified directly through in situ fluorescence using for example a plate reader.
After 6 days of growth, the amount of cells (or biomass) that adhere to the surfaces is evaluated by in situ fluorescence determination (section 4.2.1 of the Biological Workshop Manual, AMBIO Biological evaluation workshop, University of Birmingham, UK, 21-22 Apr. 2005) by virtue of the auto-fluorescence of the photosynthetic pigment chlorophyll by means of a fluorescent reader which emits light of a wavelength of 430 nm, exciting the chlorophyll contained within the chloroplasts of the algal cells growing on the sample surface, and then measures the 630 nm light, which is emitted as the pigment returns to ‘resting state’. This method of biomass quantification has the advantage of being relatively quick and non-destructive.
This procedure has been applied to coatings of the present invention. Results are summarized in
The samples are then washed out and the removal rate is measured. The removal rate obtained is represented in
Number | Date | Country | Kind |
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09015835.3 | Dec 2009 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2010/069345 | 12/10/2010 | WO | 00 | 9/10/2012 |