GB2031924A - Electrically conductive polyester coating compositions - Google Patents

Abstract

A sprayable gel coat for fibre reinforced plastic tank structures comprises a polyester resin including 40% graphite and preferably styrene. The composition provides good conductivity to electrostatic charges with good strength properties.

Description

SPECIFICATION A conductive plastics coating material and a tank structure incorporating same This invention relates to a conductive plastics coating material and to a liquid containing tank structure incorporating same. The coating would generally be used as a gel coat for a fibreglass reinforced plastics tank structure and serves the purpose of dissipating static electrical charges.

A known drawback of fibreglass reinforced tank structures is the difficulty in dissipating electrostatic charges from the surface thereof due to the low conductivity of the fibreglass material. Attempts have been made to use mesh or netting materials which are conductive by incorporating these into the structure during the manufacture. Such methods have not proved fully satisfactory in dissipating electrostatic charges and furthermore render the moulding process more complex which, to a large extent, defeats one object of fibreglass tank structures which is their ease and simplicity of construction.

It is an object of this invention to provide a coating material which has a conductivity sufficient to dissipate any electrostatic charges and to provide a tank structure of fibreglass or similar resin impregnated plastics material which incorporates the coating.

According to this invention it has been found that incorporation of graphite into a polyester resin in a proportion by weight of about forty per cent graphite relative to the weight of the resin produces a plastics material having sufficient conductivity to dissipate electrostatic charges.

The proportion referred to works out in practice to be about twenty six per cent by dry volume of graphite to resin. A particular usable composition comprises polyester resin with forty per cent by weight air spun graphite which is a powder material to which can be added per 500 gramme of resin 110 grammes of styrene which enables the material to be sprayed onto the surface of a mould using technique normal to the application of the gel coat.

Such a material has been shown to have a surface resistivity of 15K ohms per square meter using a 500 volt megger. It is to be expected that graphite of differing forms and from differing sources may require a different percentage, but this can be quite easily determined in practice.

It has been found surprisingly that as the percentage of graphite is increased in relation to the basic polyester resin the conductivity increases very little until a certain point is reached whereupon conductivity rises at a very rapid rate. The graph of Figure 1 illustrates the sort of change obtained between conductivity and percentage of graphite.

It has also been found surprisingly that the proportion of graphite cannot be increased too far because the structural strength of the coating falls rapidly beyond a certain percentage of incorporated graphite. The graph of Figure 2 illustrates the structural strength as a function of percentage of graphite.

From these two graphs it can be appreciated that there is a narrow band of useful percentages outside which either the conductivity falls to an unacceptably low level, or alternatively the structural strength of the coating is insufficient.

The results are suprising and it could not be expected that simple incorporation of a graphite material in this way would produce a coating having sufficient conductance to be used usefully for dissipating electrostatic charges.

A further aspect of the invention provides for the conductive coating according to the invention to be applied between the glassfibre reinforced plastics material and the normal cosmetic gel coat, and in this case it has been found that electrostatic charges occurring on the surface of the gel coat are dissipated by the conductive coating, notwithstanding the fact that there is no conductive path existing between the surface of the gel coat and the conductive layer. This result is also surprising and in practice it means that it is not necessary to have the conductive coating on the outermost surface of a glass reinforced plastics tank structure.

A tank structure may be manufactured according to another aspect of this invention by spray application of a normal cosmetic gel coat to the surface of a mould after application of a suitable release agent, followed by spray application of the conductive polyester resin material and this is then followed by the normal layup of the glass reinforced fibre forming the main structure. Atank made in this way has been found to dissipate all electrostatic charges occurring on the surface and no means hitherto has been able to achieve this result.

1. An electrically conductive synthetic plastics material, primarily a material for coating a fibre reinforced material, comprising a polyester resin into which has been incorporated about 40% by weight graphite.

2. A material in accordance with Claim 1, wherein the proportion of graphite lies between 25 and 55% by weight.

3. A material in accordance with Claims 1 or 2, wherein the material comprises 26% dry volume of graphite to resin.

4. A material in accordance with Claim 1 or 2, wherein the polyester resin includes 40% by weight air spun graphite and about 22% styrene to yield a sprayable composition.

5. An article of fibre reinforced plastics material produced by lay-up in a mould to the surface of which has been applied a gel coat comprising the material of any preceding claim.

6. An article of fibre reinforced plastics material with a gel coat wherein between the gel coat and article there has been provided the material according to any preceding Claim 1 to 4.

7. An electrically conductive material substantially as described in the specification.

8. A tank structure including an electrically conductive coating manufactured substantially as described in the specification.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION A conductive plastics coating material and a tank structure incorporating same This invention relates to a conductive plastics coating material and to a liquid containing tank structure incorporating same. The coating would generally be used as a gel coat for a fibreglass reinforced plastics tank structure and serves the purpose of dissipating static electrical charges. A known drawback of fibreglass reinforced tank structures is the difficulty in dissipating electrostatic charges from the surface thereof due to the low conductivity of the fibreglass material. Attempts have been made to use mesh or netting materials which are conductive by incorporating these into the structure during the manufacture. Such methods have not proved fully satisfactory in dissipating electrostatic charges and furthermore render the moulding process more complex which, to a large extent, defeats one object of fibreglass tank structures which is their ease and simplicity of construction. It is an object of this invention to provide a coating material which has a conductivity sufficient to dissipate any electrostatic charges and to provide a tank structure of fibreglass or similar resin impregnated plastics material which incorporates the coating. According to this invention it has been found that incorporation of graphite into a polyester resin in a proportion by weight of about forty per cent graphite relative to the weight of the resin produces a plastics material having sufficient conductivity to dissipate electrostatic charges. The proportion referred to works out in practice to be about twenty six per cent by dry volume of graphite to resin. A particular usable composition comprises polyester resin with forty per cent by weight air spun graphite which is a powder material to which can be added per 500 gramme of resin 110 grammes of styrene which enables the material to be sprayed onto the surface of a mould using technique normal to the application of the gel coat. Such a material has been shown to have a surface resistivity of 15K ohms per square meter using a 500 volt megger. It is to be expected that graphite of differing forms and from differing sources may require a different percentage, but this can be quite easily determined in practice. It has been found surprisingly that as the percentage of graphite is increased in relation to the basic polyester resin the conductivity increases very little until a certain point is reached whereupon conductivity rises at a very rapid rate. The graph of Figure 1 illustrates the sort of change obtained between conductivity and percentage of graphite. It has also been found surprisingly that the proportion of graphite cannot be increased too far because the structural strength of the coating falls rapidly beyond a certain percentage of incorporated graphite. The graph of Figure 2 illustrates the structural strength as a function of percentage of graphite. From these two graphs it can be appreciated that there is a narrow band of useful percentages outside which either the conductivity falls to an unacceptably low level, or alternatively the structural strength of the coating is insufficient. The results are suprising and it could not be expected that simple incorporation of a graphite material in this way would produce a coating having sufficient conductance to be used usefully for dissipating electrostatic charges. A further aspect of the invention provides for the conductive coating according to the invention to be applied between the glassfibre reinforced plastics material and the normal cosmetic gel coat, and in this case it has been found that electrostatic charges occurring on the surface of the gel coat are dissipated by the conductive coating, notwithstanding the fact that there is no conductive path existing between the surface of the gel coat and the conductive layer. This result is also surprising and in practice it means that it is not necessary to have the conductive coating on the outermost surface of a glass reinforced plastics tank structure. A tank structure may be manufactured according to another aspect of this invention by spray application of a normal cosmetic gel coat to the surface of a mould after application of a suitable release agent, followed by spray application of the conductive polyester resin material and this is then followed by the normal layup of the glass reinforced fibre forming the main structure. Atank made in this way has been found to dissipate all electrostatic charges occurring on the surface and no means hitherto has been able to achieve this result. CLAIMS

1. An electrically conductive synthetic plastics material, primarily a material for coating a fibre reinforced material, comprising a polyester resin into which has been incorporated about 40% by weight graphite.

2. A material in accordance with Claim 1, wherein the proportion of graphite lies between 25 and 55% by weight.

3. A material in accordance with Claims 1 or 2, wherein the material comprises 26% dry volume of graphite to resin.

4. A material in accordance with Claim 1 or 2, wherein the polyester resin includes 40% by weight air spun graphite and about 22% styrene to yield a sprayable composition.

5. An article of fibre reinforced plastics material produced by lay-up in a mould to the surface of which has been applied a gel coat comprising the material of any preceding claim.

6. An article of fibre reinforced plastics material with a gel coat wherein between the gel coat and article there has been provided the material according to any preceding Claim 1 to 4.

7. An electrically conductive material substantially as described in the specification.

8. A tank structure including an electrically conductive coating manufactured substantially as described in the specification.