Technical Field

The present invention relates to providing liquid-applied, electrically-conductive coating compositions which are useful for the creation of electric underfloor or in wall heating systems with high energy efficiency.

Background of the Invention

Underfloor or in-wall heating is an increasingly popular technology for use in heating domestic and commercial properties. The technology is hidden, and is therefore more aesthetically pleasing than conventional radiators or free-standing heaters. Underfloor or in-wall heating distributes heat evenly around each corner of a room, is believed to be generally more efficient than traditional heating systems, and is considered to be a desirable addition to a home.

Underfloor and in-wall heating systems essentially fall into one of two categories - “dry” systems, which utilize electricity to heat the floor/wall; or“wet” systems, which utilize piped hot water to heat the floor/wall. Wet systems are more easily installed in cases where it is possible to take up an existing floor, or when new floors are being laid. Dry systems are more suitable for installation in existing rooms, as the systems are of a flatter profile and generally do not need floor heights to be adjusted.

Wet systems are reliant on hot water being pumped through plastic pipes which are installed in the sub-floor or wall. This can be disruptive and costly to install, often necessitating the raising of floor levels. However, such systems are acknowledged to be more efficient than conventional radiators by virtue of their reduced input energy requirement - typically water temperatures of 40 to 65 degrees Celsius being required to provide ambient room temperature of 23 to 32 degrees Celsius.

Dry systems essentially comprise a network of electrical heating wires or cables beneath the floor, which can be in the form of loose-lay cables, pre-formed adhesive mats or carbon films. For walls, mats are generally utilized to line the wall. The conventional cable or mat systems provide radiant heat, whereas carbon film systems provide infrared heating. Dry systems are generally much simpler to install than wet systems, and have lower associated installation costs. However, the running costs of dry systems are significantly higher than their wet counterparts. Thus, there exists a need for a dry system which still provides ease and simplicity of installation whilst affording enhanced energy efficiency and reduced running costs.

Summary of the Invention

In accordance with a first aspect of the present invention, there is provided an electrically-conductive coating composition for use in providing an electrically- resistive heating element, the composition comprising:

an electrically-conductive carbon allotrope;

a thermally-conductive powder; and

a water-borne polymeric binder.

The composition therefore preferably forms an aqueous mixture. The term“water borne” will be understood in the context of the present invention by the skilled addressee as meaning“conveyed by water”. The term“water-borne polymeric binder” may therefore refer to a polymeric binder in admixture with water, or a polymeric binder in solution.

The present invention is directed toward providing liquid-applied, electrically- conductive coating compositions which are useful for creating electric underfloor heating systems or electric in-wall heating systems with high energy efficiency. The composition also preferably provides for a moisture-breathable solution. When the liquid conductive coating dries, a porous structure is preferably formed. Preferably the porous structure permits diffusion of vapour, which may include water vapour. The composition therefore also preferably provides for a moisture- breathable solution, creating comfort in a heated room.

Preferably the electrically-conductive carbon allotrope is graphene. Preferably the graphene consists of one or more selected from: graphene powder; graphene platelets. Preferably the graphene is non-oxidised. Used in heated coatings graphene is preferably non-oxidized and comprises no defects, and it is thought that this feature aids in providing for the high electrical conductivity of graphene. It is thought that the presence of chemical compounds, oxides or defects on graphene plates leads to a sharp increase in resistance to electric current at the contact points of graphene plates. For example, the volume conductivity of non- oxidized graphene used in a heating coating such as that proposed is 50,000 S/m. The volume conductivity of oxidized graphene is thought to be around 5,000 - 6,000 S/m.

Graphene is obtained by a low-temperature method from natural graphite, without the use of any chemical reagents. It is thought that graphene obtained by this method has a high electrical conductivity due to the absence of defects and deformations of graphene platelets. Without wishing to be bound by theory, graphene oxide (GO) has extremely low electrical conductivity, which is why it is generally called as electrical insulator. However, GO is oxidized from graphene which has a very high electrical conductivity (also called original or pristine graphene). GO conducts very low amounts of electrical current depending upon the level of its oxidation. A highly oxidized GO is an electrical insulator. To improve electrical conductivity, GO is generally reduced using several methods. Following reduction, the reduced GO cannot generally conduct electricity to the level of pristine graphene, due to remaining defects in the reduced GO after reduction. During reduction, oxygen-containing groups are reduced and the carbon-containing groups are increased. Among other factors, this particular feature preferably promotes electrical conduction (Hongtao Liu et al. , Reduction of graphene oxide to highly conductive graphene by Lawesson's reagent and its electrical applications., J. Mater. Chem. C, 2013, 1 , 3104-3109).

In preferred embodiments of the invention, the graphene powder comprises graphene nanoplatelets each having a nanoplatelet thickness less than about 5 nm; and a nanoplatelet size less than about 50 microns. In particularly preferred embodiments, the graphene nanoplatelets preferably have a nanoplatelet thickness less than 2 nm; and a nanoplatelet size in the range: 5 microns to 20 microns. The term“nanoplatelet size” in the context of the present invention will be understood by the skilled addressee to be the distance along the longest dimension of the nanoplatelet.

Graphene powders and nanoplatelets have a tendency to clump together. A water borne polymeric binder is preferably advantageous in providing for easier dispersion of graphene throughout the composition and therefore provide a more homogeneous mixture. It is thought that the homogeneous mixture provides better electrical and heat dispersion.

The graphene platelets preferably have an electrical conductivity greater than about 10 ³ Siemens/metre (S/m). Preferably the graphene platelets preferably have an electrical conductivity in the range: 10 ³ S/m to 10 ⁶ S/m. An electrical conductivity selected from the range: 10 ⁴ S/m to 10 ⁵ S/m is particularly preferred.

Suitable graphene powders preferably include, for example, Graphene Nanoplatelets GS-030P and GS-030W, commercially available from Graphene Star Ltd.

Preferably, the thermally-conductive powder is selected from the group consisting of: ferrosilicon, titanium dioxide, carbon microspheres and glass microspheres. In some embodiments, the composition comprises more than one of said thermally- conductive powders in combination.

In preferred embodiments of the invention, the thermally-conductive powder comprises atomised ferrosilicon. The term“atomised” will be understood by the skilled addressee to mean“converted to fine particles”, the relative size of said particles being inferable by the skilled addressee. Preferably the ferrosilicon comprises a silicon content selected from the range: 10 % wt to 20 % wt; with a silicon content selected from between 14 % wt and 16 % wt being particularly preferred. The atomised ferrosilicon preferably comprises ferrosilicon particles each having a particle size, the particle size being less than about 300 microns; with a particle size selected between 50 microns and 200 microns being particularly preferred. The term “particle size” in the context of the present invention will be understood by the skilled addressee to refer to the average distance along the longest dimension of a particle.

Suitable ferrosilicon powders preferably include, for example, Atomised Ferrosilicon 15%, commercially available from M & M Alloys Ltd.

The thermally-conductive powder (TCP) is preferably combined with the electrically-conductive carbon allotrope (ECC) in a weight ratio of TCP : ECC. In an example embodiment the weight ratio is preferably selected from between 5:1 and 15: 1 ; with a weight ratio selected from between 8: 1 and 12: 1 being particularly preferred. In preferable such examples, the TCP is ferrosilicon powder, and the ECC is graphene. In preferred embodiments of the invention, the thermally- conductive powder and electrically-conductive carbon together constitute 50 to 90 weight percent of the aqueous mixture. In particularly preferred embodiments, the thermally-conductive powder and electrically-conductive carbon together constitute 65 to 75 weight percent of the aqueous mixture.

For the purposes of the present invention, a variety of water-borne polymeric binders are preferably suitable, with those based on polyurethane, acrylic, styrene- acrylic or styrene-butadiene polymers and co-polymers being preferred. Suitable commercially available materials preferably include, for example, the following:

Cempolay Universal Primer, available from Bostik Ltd;

- Acrylic Primer Undercoat, available from PPG Architectural Coatings UK Limited; and

503 SBR Bond, available from Everbuild Building Products Ltd (Sika Company).

In accordance with a second aspect of the present invention, there is provided an electrically-conductive coating for use in an underfloor or in-wall heating system, the coating comprising a first electrically-conductive layer comprising an electrically-conductive carbon allotrope, a thermally-conductive powder, and a polymeric binder; and

a second dielectric layer adjacent the first electrically-conductive layer, the dielectric layer comprising a thermally-cured coating.

Preferably the first electrically-conductive layer is formed from an electrically- conductive composition according to the first aspect of the present invention described herein.

Preferably the thermally cured coating is a two-part coating which may comprise one or more resins selected from the group: epoxy; polyurethane; poly-urea.

A variety of commercially available, 2-part, thermosetting coatings are preferably suitable for use as the dielectric layer of the second aspect of the present invention. Examples of suitable coatings include those based on epoxy, polyurethane or poly urea resins. Preferably the coatings contain minimal volatile constituents, with 100% solids (solvent free) materials being particularly preferred. For the purposes of the invention, poly-urea compositions comprising at least one aliphatic poly isocyanate component and at least one poly-aspartic acid ester component are preferred on account of their rapid curing characteristics.

In accordance with a third aspect of the present invention, there is provided an electrically-conductive heating system comprising:

an electrically-conductive coating according to the second aspect of the present invention; and

a plurality of electrical contacts in electrical communication with the first electrically-conductive layer.

Preferably the system further comprises a power supply in communication with the plurality of electrical contacts, the power supply being arranged to provide an electrical current to the contacts. Preferably the electrical contacts are electrically-conducting strips. Preferably the conductive strips comprise copper. Preferably the conductive strips each comprise a conductive strip width selected from between 10 mm and 20 mm, and is most preferably 12.5 mm. The conductive strips are preferably a maximum of about 1 m wide. Preferably the conductive strip width of the plurality of conductive strips is the same. Embodiments will be appreciated wherein the conductive strip width of the plurality of conductive strips may be varied. In alternative embodiments, flat conductive wires may be employed. Preferably the flat conductive wires comprise copper. In preferable embodiments, the plurality of electrically-conductive contacts are at least partially embedded within the first electrically-conductive layer.

In accordance with a fourth aspect of the present invention, there is provided a method of forming an electrically-resistive heating element, the method comprising the steps of:

a) providing a pre-prepared surface by applying a network of electrically- conductive strips or wires to a surface to be treated;

b) applying an aqueous mixture of an electrically-conductive coating composition in accordance with the first aspect of the present invention over the pre-prepared surface, said aqueous mixture being substantially uniformly applied over the prepared surface; and

c) allowing the aqueous mixture to dry, forming a dried electrically- conductive layer.

In some embodiments, the method may further comprise a step of connecting said network to a power supply. Preferably in step a) the network of electrically- conductive strips or wires is adhered to the surface to be treated. Preferably the network of electrically-conductive strips or wires are self-adhesive. Preferably the network of electrically-conductive strips or wires comprise copper. Embodiments will be appreciated wherein the electrically-conductive strips or wires are pure copper. The electrically-conductive strips or wires in preferable embodiments may be superconductive. In step b) the aqueous mixture is preferably applied in liquid form. In example embodiments wherein the electrically-conductive carbon allotrope is graphene, the water-borne binder aids in dispersing graphene platelets which may otherwise have a tendency to clump together with poor dispersion in less-aqueous mixtures.

Preferably the aqueous mixture is applied at an application rate selected from between 0.5 kg and 1.5 kg per square metre of pre-prepared surface.

Preferably the method further comprising the steps of:

d) applying a thermally curable dielectric coating to the dried electrically- conductive layer; and

e) thermally curing the thermally curable dielectric coating, so as to seal the surface of the electrically-conductive layer, and form an electrically- conductive coating with an electrically-insulative surface.

Preferably the thermally curable dielectric coating is substantially volatile-free (100% solids) and is a 2-part thermally curable material. In step d) the thermally curable dielectric coating is preferably applied in liquid form. In preferable embodiments, following step c), the dried electrically-conductive layer remains sorbent at the time of performing step d), such that the thermally curable dielectric coating is at least partially absorbed and/or adsorbed into the dried electrically- conductive layer.

It is thought that during manufacture of an electric underfloor or in-wall heating system according to the fourth aspect of the present invention, when the electrically-conductive carbon allotrope and thermally-conductive powder are mixed, a composite with a uniform structure and high conductivity is preferably obtained. Most preferably the desired uniform structure and high conductivity is obtained when the electrically-conductive carbon allotrope comprises graphene platelets and the thermally-conductive powder comprises ferrosilicon. The resulting composite is preferably easily mixed with aqueous polymeric binders, forming a mixture suitable for application to a prepared surface. Before applying the electrically-conductive coating layer, it is often necessary to first pre-prepare the surface with the installation of a plurality of conductive strips. Preferably the conductive strips comprise copper. Preferably the conductive strips each comprise a conductive strip width selected from between 10 mm and 20 mm, and is most preferably 12.5 mm. Preferably the conductive strip width of the plurality of conductive strips is the same. Embodiments will be appreciated wherein the conductive strip width of the plurality of conductive strips may be varied. In alternative embodiments, flat conductive wires may be employed. Preferably the flat conductive wires comprise copper.

After applying a uniform layer of coating composition, and drying this layer, a conductive coating is formed. After drying of the conductive coating layer, a dielectric coating is preferably applied. This dielectric coating is preferably partially absorbed by/into and/or adsorbed onto the conductive coating, thus forming a fused coating with high dielectric properties on the surface.

Once the power supply is connected to the copper conductors and an electrical current is applied by the power supply, the coating is generating heat through electrically-resistive heating. The resultant heating is highly efficient and has a power consumption of less than about 180 Watts/square metre to heat the coating up to about 32 degrees Celsius.

Preferably the surface to be treated may comprise one or more selected from the group: a floor or wall surface; an underside of a floor covering; a removable floor underlayment. Embodiments will be appreciated wherein the surface to be treated may be any suitable surface required to radiate heat generated by electrical conduction.

Preferably the electrically-resistive heating element has an energy efficiency (Pt/P) of 0.90 to 1.00. It is thought that in today's conventional convection or oil heaters, the value of Pt/P is about 0.8-0.9. Determination of the energy efficiency of the coating is carried out by measuring the electricity consumption and measuring the heat produced in a conventional unit of time. The amount of heat produced is the sum of the amount of heat spent on heating the base layer (the electrically- conductive layer) of the coating, heating the ambient air and heating the surface of the coating. The ratio of the produced thermal power (Pt - in Watts) to the electrical power consumed for heating (P - in Watts), is an indicator of the energy efficiency (Pt/P).

Detailed Description

Specific embodiments will now be described by way of example only, and with reference to the accompanying drawings, in which:

FIG. 1 shows a sectional view of an example embodiment of an electrically- conductive coating in accordance with the second aspect of the present invention, comprising an electrically-conductive coating composition according to the first aspect of the present invention;

FIG. 2 shows a perspective view of the example embodiment of FIG. 1 ;

FIG. 3 shows a schematic view of an example underfloor heating system in accordance with the third aspect of the present invention employing a electrically- conductive coating in accordance with the second aspect of the present invention; and

FIG. 4 shows a flow diagram describing an example embodiment of a method according to the third aspect of the present invention.

Referring to FIG. 1 , a sectional view of an electrically-conductive coating 10 according to the second aspect of the present invention is shown. The coating 10 forms a planar coating on a floor 12. The floor surface 14 is pre-prepared with a network of elongate copper strips 16, over which a liquid-applied coating composition 18 according to the first aspect of the present invention is layered prior to drying. The liquid-applied composition 18 comprises nanoplatelets of an electrically-conductive carbon allotrope (in this case graphene nanoplatelets), and a thermally-conductive powder (in this case atomised ferrosilicon) suspended in a water-borne polymeric binder (in this case a polyurethane based polymeric binder). The upper surface 20 of the dried composition 18 is layered with a 2-part thermosettable dielectric layer 22 comprising a poly-urea composition having an aliphatic poly-isocyanate component and a poly-aspartic acid ester component. The thermally cured dielectric layer 22 forms an insulative surface 24 of the coating 10. A perspective view of the coating 10 is shown in FIG. 2.

The graphene nanoplatelets in the electrically-conductive composition 18 have a thickness of less than 5 nm and a size of less than 50 microns, and the atomised ferrosilicon comprises a particle size of between 50 microns and 200 microns. The nanoplatelets and powder are distributed evenly in the polymeric binder to form a homogeneous mixture. The ferrosilicon and graphene are present in a weight ratio of 10 : 1 respectively, and together constitute 70 weight percent of the aqueous mixture. The network of copper strips have a width of 12.5 mm.

In use, a coating such as the embodiment 10 shown in FIG. 1 and FIG. 2 would form part of an electrical heating system, an example 30 of which can be seen in the schematic view provided in FIG. 3. In the example 30, the network of copper strips 32 are embedded within the electrically-conductive layer 34 formed using a composition according to the first aspect of the present invention. The strips 32 are electrically connected to one another at a first end 36. At a second end 38 the strips 32 are connected to a power supply 40 arranged to provide an electrical current to the strips 32. Upon provision of an electrical current to the strips 32, electricity is distributed about the electrically-conductive layer 34 by the graphene nanoplatelets therein. Resistive heat generated as a result is maintained and distributed by the thermally- conductive ferrosilicon powder. The dielectric layer 42 provides an electrically- insulative surface such that the coating layer is made safe and efficient. As such an electrically-conductive heating system is provided which is easily applied whilst affording enhanced energy efficiency and reduced running costs.

Referring to FIG. 4, a flow chart is shown describing an example method of manufacturing an electrically-resistive heating element 50 according to the fourth aspect of the present invention, the method comprising the steps of: a) providing a pre-prepared surface by applying a network of self-adhesive, conductive copper strips or wires to a surface to be treated 52;

b) applying an aqueous mixture of an electrically-conductive coating composition over the pre-prepared surface, said aqueous mixture being substantially uniformly applied over the prepared surface 54; and c) allowing the aqueous mixture to dry, forming a dried electrically- conductive layer 56.

In the example embodiment 50, the electrically-resistive heating element manufactured is substantially the same as that described 10 in relation to FIG. 1 and FIG. 2, and the aqueous mixture of an electrically-conductive coating composition is therefore equal to that of the composition 18 therein.

The method 50 in the example embodiment shown also comprises the steps of: d) applying a thermally curable dielectric coating to the dried electrically- conductive layer 58; and

e) thermally curing the thermally curable dielectric coating, so as to seal the surface of the electrically-conductive layer, and form an electrically- conductive coating with an electrically-insulative surface 60.

Following the example method 50 described, and in use, a power supply is connected to the conductive copper strips, and an electrical current applied by the power supply to the strips such that electrically-resistive heating is performed.

Examples

The present invention will also be understood with reference to the following examples, which are provided for illustrative purposes only and are not intended to limit the scope of the invention as set out in the appended claims.

Example 1 To obtain a conductive/heating coating according to the second aspect of the present invention, an electrically-conductive composition mixture is prepared according to the first aspect: 240 grams of aqueous graphene paste with a graphene content of 25.5%; 54.4 grams of acrylic dispersion; 54.4 grams of SBR dispersion (product discussed in glossary below); 708 grams of ferrosilicon powder. The resulting mixture is thoroughly mixed for 5 minutes. After that, the resulting mixture is applied evenly over an area of 1 square meter with copper conductors. The layer is dried for 6-8 hours.

After drying, a dielectric layer is prepared, using poly-aspartic which is applied to the resulting electrically-conductive layer at an application rate of 320 grams per square meter. The drying time of the poly-aspartic layer is 1 -1.5 hours.

The electrical resistivity of the obtained combined coating is 330-360 Ohms/square metre. The power consumption for heating the coating to 32 degrees Celsius is 130-150 Watts/square meter. The energy efficiency (Pt/P) is 0.99.

Example 2

To obtain a conductive/heating coating according to the second aspect of the present invention, an electrically-conductive composition mixture is prepared according to the first aspect: 302 grams of aqueous graphene paste with a graphene content of 25.5%; 93.2 grams of acrylic dispersion; 40.0 grams of SBR (product discussed in glossary below) dispersion; 800 grams of ferrosilicon powder. The resulting mixture is thoroughly mixed for 5 minutes. After that, the resulting mixture is applied evenly over an area of 1 square meter with copper conductors. The layer is dried for 6-8 hours.

After drying, a dielectric layer is prepared, using poly-aspartic which is applied to the resulting electrically-conductive layer at an application rate of 300 grams per square meter. The drying time of the poly-aspartic layer is 1 -1.5 hours. The electrical resistivity of the obtained combined coating is 600-650 Ohms/square metre. The power consumption for heating the coating to 32 degrees Celsius is 160-180 Watts/square meter. The energy efficiency (Pt/P) is 0.98.

Example 3

To obtain a conductive/heating coating according to the second aspect of the present invention, an electrically-conductive composition mixture is prepared according to the first aspect: 225 grams of aqueous graphene paste with a graphene content of 25.5%; 102 grams of acrylic dispersion (an off-the-shelf white primer comprising titanium dioxide microspheres); 606 grams of ferrosilicon powder. The resulting mixture is thoroughly mixed for 5 minutes. After that, the resulting mixture is applied evenly over an area of 1 square meter with copper conductors. The layer is dried for 6-8 hours.

After drying, a dielectric layer is prepared, using poly-aspartic which is applied to the resulting electrically-conductive layer at an application rate of 350 grams per square meter. The drying time of the poly-aspartic layer is 1 -1.5 hours.

The electrical resistivity of the obtained combined coating is 800 Ohms/square metre. The power consumption for heating the coating to 32 degrees Celsius is 160-180 Watts/square meter. The energy efficiency (Pt/P) is 0.99.

Example 4

To obtain a conductive/heating coating according to the second aspect of the present invention, an electrically-conductive composition mixture is prepared according to the first aspect: 225 grams of aqueous graphene paste with a graphene content of 25.5%; 100 grams of SBR dispersion (product discussed in glossary below); 605 grams of ferrosilicon powder. The resulting mixture is thoroughly mixed for 5 minutes. After that, the resulting mixture is applied evenly over an area of 1 square meter with copper conductors. The layer is dried for 6-8 hours. After drying, a dielectric layer is prepared, using poly-aspartic which is applied to the resulting electrically-conductive layer at an application rate of 350 grams per square meter. The drying time of the poly-aspartic layer is 1 -1.5 hours.

The electrical resistivity of the obtained combined coating is 750-770 Ohms/square metre. The power consumption for heating the coating to 32 degrees Celsius is 150-170 Watts/square meter. The energy efficiency (Pt/P) is 0.99.

Glossary of materials used

Acrylic dispersion - Cempolay Universal Primer, Bostik Ltd.

SBR dispersion - 503 SBR Bond, Everbuild Building Products Ltd.

Aqueous graphene paste - Graphene Nanoplatelets GS-030W, Graphene Star Ltd.

Ferrosilicon powder - Atomised Ferrosilicon 15% (Cyclone 60) - M&M Alloys Ltd. Poly-aspartic coating - MS 870, lOBAC Ltd.

Example embodiments, such as those described in FIG. 1 to FIG. 4, will be appreciated having an electrically-conductive composition and/or eventual coating as described in any of Examples 1 to 4.

It will be appreciated that the above described embodiments are given by way of example only and that various modifications thereto may be made without departing from the scope of the invention as defined in the appended claims. For example, in the embodiments shown and described, copper strips form electrical contacts. Embodiments will be appreciated wherein the electrical contacts are any suitable electrically-conductive material. Embodiments may be appreciated wherein the electrically-conductive coating is all that is required to transmit electricity in an electrically-conductive heating system of the present invention.