DESC:Field of Invention

The present disclosure relates to a heating element for heating liquids in general, and water in particular. Specifically, the present disclosure relates to a heating element comprising a coated thermally conductive substrate having improved thermal conductivity and hydrophobicity.

Background

Scale deposition on heating elements is a common issue, especially in environments with hard water. Scale, primarily composed of calcium carbonate and other minerals, accumulates on the heating element and acts as an insulating layer and impedes heat transfer to the water, thereby reducing heat transfer efficiency. This leads to higher energy consumption. Scale buildup can also cause overheating and damage to the heating element due to metallurgical failure, leading to more frequent replacements.

One of the known ways of removing the scaling is to regularly clean the heating elements. Periodic descaling using chemical agents can help remove existing deposits and prevent further buildup. However, it is not feasible to carry out regular cleaning and maintenance of heating elements used in water heaters, specifically household water heaters.

Coating and plating on heating elements are also done to protect the heating element from the scale deposition. Coatings comprising glass, polymer and ceramic are used to create a smooth, non-stick surface that prevents minerals from adhering to the heating element. However, the coating on the heating element creates insulation layers and barriers for heat transfer. It has been observed that protective coating may reduce heat transfer by 11% compared to an uncoated heating element, and this will increase in the long run as scale deposition starts on the heating element. The coating also results in inappropriate or uneven heat transfer to water. Similarly, the plating process is not environmentally friendly and not a sustainable solution.

Accordingly, there exists a need for a heating element that exhibits improved thermal conductivity and scale deposition protection without compromising the corrosion resistance and hydrophobicity thereof.

Brief description of drawings

FIG. 1A shows a metal substrate of a heating element (HE) without any coating thereon.

FIG. 1B depicts the acid treated- metal substrate of the heating element.

FIG. 1C depicts the blasted metal substrate of the heating element.

FIG. 1D depicts a heating element prepared in accordance with an embodiment of the present disclosure.

FIG. 2 depicts the thermal conductivity, and the contact angle of a conventional copper substrate (without any coating), a PTFE coated copper substrate, and a heating element prepared in accordance with an embodiment of the present disclosure.

FIG. 3 depicts the temperature increase rate against the number of cycles performed in (i) a regular water heater with conventional copper substrate as heating element, (ii) a water heater with the PTFE coated copper substrate and (iii) water heater with the heating element prepared in accordance with the present disclosure.

FIG. 4A depicts the metallurgical condition of the conventional copper substrate (without any coating) after usage.
FIG. 4B depicts the metallurgical condition of the heating element prepared in accordance with an embodiment of the present disclosure.

Summary

A heating element comprising a thermally conductive substrate having a coating composition coated thereon is disclosed. The coating composition comprises a fluoropolymer and a nanofiller dispersed in the fluoropolymer in an amount of 0.1 to 2 weight percent based on the weight of the fluoropolymer.

Detailed Description

Reference will now be made in detail to embodiments of the present disclosure. The terminology used in the description presented herein is not intended to be interpreted in any limited or restrictive manner, simply because it is being utilized in conjunction with a detailed description of certain specific embodiments of the invention. Furthermore, embodiments of the invention may include several features, no single one of which is solely responsible for its desirable attributes, or which is essential to practicing the inventions herein described.

It will be understood by those skilled in the art that the foregoing general description and the following detailed description are explanatory of the invention and are not intended to be restrictive thereof.

The terms “a,” “an,”, and “the” are used to refer to “one or more” (i.e., to at least one) of the grammatical object of the article.

Reference throughout this specification to “an aspect”, “another aspect” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention.
The terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion and are not intended to be construed as “consists of only”, such that a process or method that comprises a list of steps does not include only those steps but may include other steps not expressly listed or inherent to such process or method.

Likewise, the terms “having” and “including”, and their grammatical variants are intended to be non-limiting, such that recitations of said items in a list are not to the exclusion of other items that can be substituted or added to the listed items.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the disclosure, the preferred methods, and materials are now described. All publications mentioned herein are incorporated herein by reference.

In an aspect, a heating element comprising a thermally conductive substrate having a coating composition coated thereon is disclosed. The coating composition comprises a fluoropolymer and a nanofiller dispersed in the fluoropolymer in an amount of 0.1 to 2 weight percent based on the weight of the fluoropolymer.

The thermally conductive substrate includes any substrate capable of transferring thermal energy. In an embodiment, the thermally conductive substrate is made of metal or ceramic. In an embodiment, the thermally conductive substrate is a metal substrate selected from the group consisting of copper, nickel, and alloys thereof. In some embodiments, the metal substrate is copper.

The nanofiller is dispersed in the fluoropolymer in the amount of 0.1 to 2 weight percent based on the weight of the fluoropolymer. In an embodiment, the nanofiller is dispersed in the fluoropolymer in the amount of 1 to 2 weight percent based on the weight of the fluoropolymer. In some embodiments, the nanofiller is dispersed in the fluoropolymer in the amount of 2 weight percent based on the weight of the fluoropolymer.

In an embodiment, the nanofiller is an inorganic filler selected from the group consisting of oxides, carbides, borides or nitrides of aluminum, boron or zinc, and combinations thereof. In an embodiment, the nanofiller is a carbon-based nanomaterial selected from the group consisting of carbon nanotubes and graphene. In some embodiments, the inorganic filler is selected from the group consisting of aluminum oxide (Al2O3) and aluminum nitride (AlN), boron nitride (BN), and combinations thereof. In some embodiments, the nanofiller is one of aluminum oxide, aluminum nitride and boron nitride. In some embodiments, the nanofiller is a hybrid filler comprising of aluminium nitride and boron nitride in a w/w ratio ranging between 1:10 and 10:1.

In an embodiment, the nanofiller has an average particle size (D50) in the range of 0-100 nm. In some embodiments, the nanofiller has D50 of 75-100 nm.

The fluoropolymer includes any known fluoropolymer, now known or developed in the future, that exhibits non-adhesion and low friction properties as well as heat and chemical resistance properties. In an embodiment, the fluoropolymer is selected from the group consisting of polytetrafluoroethylene (PTFE, or Teflon®), fluorinated ethylene propylene (FEP), ethylene tetrafluoroethylene copolymer (ETFE), polyvinylfluroide (PVF), polyvinylidene fluoride (PVDF), and fluoroethylene vinyl ether (FEVE). In some embodiments, the fluoropolymer is PTFE.

In an embodiment, the fluoropolymer is in the form of micropowder having an average particle size of about 0.01 to 100 µm.

In an embodiment, the coating composition further includes one or more additives generally known to be useful for the preparation, storage, application, and curing of coating compositions. In an embodiment, said one or more additives include, but are not limited to, anticorrosive pigments, inorganic fillers, organic pigments, inorganic pigments, rheology modifiers, dispersing agents, and metallic driers.

A method for preparing the above coating composition is also disclosed. In an embodiment, the coating composition is prepared by dispersing the nanofiller in the fluoropolymer. The dispersion is carried out using a technique well known in the art. In an embodiment, the dispersion is carried out using stirring, heating, magnetic agitation, ultrasonication or a combination thereof. In an embodiment, the dispersion is carried out using continuous stirring at 350-400 rpm. In an embodiment, the dispersion of nanofiller and the fluoropolymer is carried out at a temperature in the range of 30-40ºC for a time period in the range of 15 minutes to 50 minutes. In some embodiments, the dispersion is carried out at 30-35ºC for 25 minutes.

In an embodiment, the dispersion of the nanofiller in the fluoropolymer is carried out in the presence of a solvent and a coupling agent. The solvent includes any known aliphatic or aromatic solvents. Examples of the suitable solvent include but are not limited to ethyl acetate, xylene, toluene, and other volatile organic compounds. In some embodiments, the solvent is ethyl acetate. In an embodiment, the solvent is added in an amount such that the solvent and fluoropolymer are in v/w ratio ranging between 1.5:1 to 2.5:1. In some embodiments, the v/w ratio of the solvent and the fluoropolymer is 2:1.

The coupling agent includes any suitable silane coupling agent. In an embodiment, the coupling agent is selected from the group consisting of ?-aminopropyltriethoxysilane, and methyltrimethoxysilane.

In an embodiment, the coating composition has a viscosity in the range of 20-35 Pa·s. In some embodiments, the coating composition has the viscosity of 24 Pa·s.

A method for manufacturing the heating element is also disclosed. The method comprises subjecting the thermally conductive substrate to one or more pre-treatment steps; applying the coating composition comprising a fluoropolymer and a nanofiller dispersed in the fluoropolymer in an amount of 0.1 to 2 weight percent based on the weight of the fluoropolymer, on the thermally conductive substrate; followed by drying of the coated thermally conductive substrate.

Prior to the application of the coating composition on the thermally conductive substrate, the thermally conductive substrate is subjected to one or more pretreatment steps to modify the surface properties of the thermally conductive substrate. In an embodiment, the thermally conductive substrate is subjected to a pre-treatment step to remove the impurities on the surface of the thermally conductive substrate. In an embodiment, in the pre-treatment step, the thermally conductive substrate is treated with an acid. In some embodiments, the thermally conductive substrate is dipped in an acid for a time period in the range of 15-30 minutes, followed by rinsing with distilled water. The acid includes any acid which is known for pre-treatment of the thermally conductive substrate to remove impurities therefrom. In some embodiments, the acid is citric acid. After rinsing with distilled water, the thermally conductive substrate is subjected to a drying step. Any suitable drying method can be used for drying the rinsed thermally conductive substrate. Examples of suitable method include but are not limited to hot air oven drying or tray drying.

In an embodiment, the pre-treatment comprises a blasting treatment to roughen, grind, or polish the surface of the thermally conductive substrate. In an embodiment, the blasting treatment comprises sand blasting for a time period in the range of 5-15 minutes. Sand blasting creates appropriate surface roughness to cause better bonding of the coating composition with the thermally conductive substrate. In an embodiment, post sand blasting, the obtained thermally conductive substrate is washed with a solvent for a time period in the range of 5- 15 minutes. Examples of the solvent include but are not limited to acetone, toluene, methyl ethyl ketone (MEK), dimethylformamide (DMF), glycol ethers and xylene.

In the next step, the coating composition is applied on the pre-treated thermally conductive substrate to obtain a uniform coating on the thermally conductive substrate. The coating composition is applied on the pre-treated thermally conductive substrate using any known method of coating including but not limited to spray coating and dip coating.

In an embodiment, a single layer of the coating composition is applied on the thermally conductive substrate. In an embodiment, the thickness of the single layer of the coating composition applied on the thermally conductive substrate is in the range of 20-30 microns. In some embodiments, the thickness of the coating on the coated thermally conductive substrate is 20-25 microns.

Next, the coated thermally conductive substrate is subjected to drying, followed by baking. In an embodiment, the coated thermally conductive substrate is dried at a temperature in the range of 125-180ºC for a time period in the range of 10-30 minutes. The dried coated thermally conductive substrate is then baked at an elevated temperature. In an embodiment, baking is performed at a temperature in the range of 200- 300ºC for a time period in the range of 10- 20 minutes. In some embodiments, the coated thermally conductive substrate is baked for 230ºC for 15 minutes.
Figs. 1A-1D illustrate a copper substrate going through various stages of treatment to form the heating element in accordance with an embodiment of the present disclosure.

In an embodiment, the heating element has a thermal conductivity in the range of 300 to 420 W/mK, measured as per transient plane source (TPS) method. In some embodiments, the heating element has the thermal conductivity of 350-410 W/mK.

In an embodiment, the heating element has a coating angle of at least 90 degrees. In some embodiments, the heating element has the contact angle ranging between 90-100 degrees. The contact angle was measured using Needle method.

In an embodiment, the obtained heating element has a surface energy of 20-23 Mn/M. In some embodiments, heating element has the surface energy of 22 Mn/M. The surface energy was measured using the standard measurement methods known in the art.

The following examples illustrate certain embodiments and aspects of the present invention and are not to be construed as limiting the scope thereof. All parts and percentages are on a weight basis unless otherwise stated.

Examples

Example 1: Preparation of the coating composition in accordance with an embodiment of the present disclosure

In a four- necked reactor flask equipped with a temperature controller, and an overhead stirrer, 2 grams of aluminum oxide (nanofiller), and 100 grams of PTFE micropowder were taken and dispersed in 200 grams of ethyl acetate to form a formulation. Thereafter, 2 grams of the coupling agent (KH550) was added to the formulation, followed by stirring at 400 rpm for 45-50 minutes. The stirring was maintained at 400 rpm for 40-45minutes to obtain the coating composition.

Example 2: Preparation of the coating composition in accordance with an embodiment of the present disclosure
In a four- necked reactor flask equipped with a temperature controller, and an overhead stirrer, 2 grams of boron nitride, and 100 grams of PTFE micropowder were taken and dispersed in 200 grams of ethyl acetate. Thereafter, 2 grams of the coupling agent (KH-550) was added to the formulation, followed by stirring at 400 rpm for 45-50 minutes. The stirring was maintained at 400 rpm for 40-45 minutes to obtain the coating composition.

Example 3: Preparation of the heating element in accordance with an embodiment of the present disclosure.

Pretreatment of the copper substrate: A copper substrate was dipped in a 4% citric acid solution for 20 minutes, followed by rinsing with distilled water for 2 minutes. Thereafter, the treated copper substrate was dried in air for 10 minutes, followed by sand blasting for 10 minutes, and solvent cleaning for 5 minutes.

Application of coating composition on the copper substrate: The coating composition prepared in the example 1 was applied as a single layer on the pretreated copper substrate by spray coating, followed by drying in a hot air oven at 150ºC for 10 minutes. The dried copper substrate was subjected to baking at 230ºC in the oven for 10 minutes to obtain the heating element of the present disclosure.

Product characterization: Five samples of copper substrate- (i) uncoated, (ii) coated with only polymer-PTFE, and (iii)-(v) coated with coating composition comprising PTFE and 0.5-2 wt% of nanofiller, were tested for thermal conductivity and contact angle. Table 1 below provides the thermal conductivity properties of the five samples.

Table 1: Thermal conductivity properties
Sample No. Sample Thermal conductivity (W/mK)
1. Conventional Copper Substrate (without any coating) 397
2. Copper substrate + PTFE Coating, without any filler 300-320
3. Copper substrate + Coating composition having 0.5 wt% of filler (Al2O3) 350-360
4. Copper substrate + Coating composition having 1 wt% of filler (Al2O3) 370-380
5. Copper substrate + Coating composition having 2 wt% of filler (Al2O3) 400-410

Observation: It was observed that the heating element of the present disclosure exhibits higher thermal conductivity as compared to both the conventional copper substrate as well as PTFE coated copper substrate.

Fig. 2 depicts the thermal conductivity, and the contact angle of the conventional copper substrate, PTFE coated copper substrate and heating element prepared in accordance with the present disclosure (sample 5). As also reported in table 1 above, it was observed that the thermal conductivity of the heating element comprising the coating composition of the present disclosure is higher as compared to both the conventional copper substrate and PTFE coated copper substrate. Further, the contact angle of the heating element comprising the single layer of the coating composition is also improved which illustrates increased hydrophobicity as compared to both the conventional copper substrate and PTFE coated copper substrate. This indicates that the fluoropolymer and nanofiller act in a synergistic manner to improve the thermal conductivity and hydrophobicity of the thermally conductive substrate.

Example 4: Assessment of temperature increase or heat transfer rate

Conventional copper substrate (without any coating), a PTFE coated copper substrate and heating element prepared in accordance with an embodiment of the present disclosure (Sample 5 of Example 3) were tested in water heaters with water having TDS of 2000 and hardness of more than 800 ppm. Total 1000 cycles were completed for each water heater. The water temperature increase rate or heating rate was calculated by using the following formula:

Product characterization: Tables 2 and 3 provide water temperature increase rate observed when conventional copper substrate (without any coating), a PTFE coated copper substrate and heating element prepared in accordance with an embodiment of the present disclosure (Sample 5 of Example 3) were used in water heaters.

Table 2: Water Temperature Increase Rate/ Heating Rate
Water Temperature increase rate (ºC/Sec)
No of cycles Regular water heater Water heater with the conventional coated metal substrate Water heater with the coated copper substrate of present disclosure
100 0.2717 0.25 0.294
200 0.2727 0.257 0.294
300 0.2768 0.257 0.285
400 0.27 0.254 0.277
500 0.268 0.254 0.284
600 0.273 0.2535 0.292
700 0.269 0.2535 0.294
800 0.267 0.2571 0.283
900 0.275 0.2583 0.288
1000 0.276 0.255 0.29

Table 3: Comparison of Heating Rate
Heating element % increase/decrease in heating rate
Conventional copper substrate (without any coating) 100%
Conventional coated copper substrate with PTFE coating 93.75%
Heating element of present disclosure 105.9%

Observation: As evidenced from results in tables 2 and 3, the water heater with heating element prepared in accordance with an embodiment of the present disclosure exhibited an improved water heating rate as compared to water heaters with copper substrate and conventional coated copper substrate.

Fig. 3 depicts that the heating element of the present disclosure showed increased water temperature increase rate or heating rate as compared to both conventional copper substrate (without any coating), and PTFE coated copper substrate.

Example 5: Assessment of metallurgical condition of the heating element of the present disclosure

Conventional copper substrate (without any coating), and the coated copper substrate prepared in accordance with an embodiment of the present disclosure (Sample 5 of Example 3) were tested for scaling after 25000 cycles in the water heater.

FIG. 4A depicts the metallurgical condition of the conventional copper substrate when used in the water heater. Cracks were observed in the heating element that confirm metal failure.
FIG. 4B depicts the metallurgical condition of the heating element of the present disclosure. It was observed that even after 25000 cycles, heating element did not suffer a failure, and the coating was still protecting the metal surface. Further, there were no sticking or very less scaling was observed. It was also observed that the scales do not deposit on the heating element and can be easily removed therefrom. Thus, increasing the life cycle of the heating element.

Industrial application

The disclosed heating element finds application in heaters for heating of liquids, such as water. While the heating element has been described with respect to application in a water heater, it is apparent to a person skilled in the art that the heating element can be used for heating other liquids as well.

The disclosed heating element exhibits improved thermal conductivity as well as hydrophobicity, as compared to known heating elements. This heating element offers anti-scaling properties and anti-corrosive properties without compromising the heat transfer rate compared to conventional heating elements comprising uncoated thermally conductive substrates. The disclosed heating element ensures an improved and uniform heat transfer rate across the heating element, thereby reducing overheating in critical areas and minimizing the risk of heating element failure. The disclosed heating element has a longer lifespan in the water heater even during long usages in hard water.

The disclosed heating element has a high contact angle (> 90 degrees), indicating improved hydrophobicity. It also has very low surface energy (20-23 mN/m), which suggests weak attraction to external molecules, making it difficult for scale material to deposit on the thermally conductive substrate.

The disclosed coating composition exhibits improved water heating rate (increased by about 15%) compared to conventional polymeric coatings. The disclosed coating composition exhibits excellent flexibility, thermal stability, and scratch and abrasion resistance. An additional advantage of the disclosed coating composition is that it does not change the curing schedule of the polymer or the chemical structure and properties thereof.
,CLAIMS:1. A heating element comprising:
a thermally conductive substrate having a coating composition coated thereon, the coating composition comprising a fluoropolymer and a nanofiller dispersed in the fluoropolymer in an amount of 0.1 to 2 weight percent based on the weight of the fluoropolymer.

2. The heating element as claimed in claim 1, wherein the coating composition comprises the nanofiller dispersed in the fluoropolymer in the amount of 1 to 2 weight percent based on the weight of the fluoropolymer.

3. The heating element as claimed in claim 1, wherein the nanofiller is an inorganic filler selected from the group consisting of oxides, carbides, borides or nitrides of aluminium or boron, and combinations thereof.

4. The heating element as claimed in claim 3, wherein the inorganic filler is selected from the group consisting of aluminium oxide, aluminium nitride, boron nitride, and combinations thereof.

5. The heating element as claimed in claim 1, wherein the fluoropolymer is selected from the group consisting of polytetrafluoroethylene (PTFE, or Teflon®), fluorinated ethylene propylene (FEP), ethylene tetrafluoroethylene copolymer (ETFE), polyvinylfluroide (PVF), polyvinylidene fluoride (PVDF), and fluoroethylene vinyl ether (FEVE).

6. The heating element as claimed in claim 5, wherein the fluoropolymer is PTFE.

7. The heating element as claimed in claim 1, wherein the nanofiller has an average particle size (D50) in the range of 0-100 nm.

8. The heating element as claimed in claim 1, wherein the thermally conductive substrate is a metal substrate selected from the group consisting of copper, nickel, and alloys thereof.

9. The heating element as claimed in claim 1, comprising a single layer of the coating composition on the thermally conductive substrate, the single layer having a thickness of 20-25 microns.

10. The heating element as claimed in any preceding claim, having a thermal conductivity ranging between 300- 420 W/mK.