INFRARED ENERGY REFLECTING COMPOSITION AND METHOD OF MANUFACTURE

Abstract: An infrared energy reflecting enamel composition comprises a ground coat (2, 1002) of enamel on a substrate (1, 1001) and embedded TiO2 particles (3, 1003) in the ground coat of enamel. The infrared reflecting enamel composition has an infrared energy reflectivity value of at least 80 %. A method for manufacturing the composition is also disclosed for embedding the TiO2 in the enamel. The enamel may also comprise metal oxide particles that enhance the infrared energy reflectivity of the enamel. Further, the metal oxide particles in the enamel can be formed in several ways so as to enhance the reflectivity.

Description: INFRARED ENERGY REFLECTING COMPOSITION AND METHOD OF MANUFACTURE BACKGROUND OF THE INVENTION Field of the Invention This invention relates to enhanced reflectance infrared energy reflecting compositions for cooking apparatus. DESCRIPTION OF THE RELATED ART Ovens for cooking food have been known and used for thousands of years. One of the oldest cooking of food resulted when food products were left next to a fire, perhaps on a hot rock, and cooked essentially by a heat transfer method of conduction. With refinement, an enclosure surrounding the heating element entrapped the heated air, giving rise to cooking by convective heat transfer. This process was the prototype for the modern gas or electric oven. In the past century, radiant energy from energy radiation sources has been used to heat and directly cook foodstuffs. Within the past few decades, microwave ovens have become common, in which microwave radiation cooks the food. This has proved useful in allowing very short cooking times for many types of food. Ovens using infrared energy sources, for example such as quartz halogen lamps, are used for quick heating of food. These quartz halogen lamp ovens can also be used for cooking, and are common in restaurants. In these ovens, most of the heat is infrared energy. This infrared energy is reflected and a majority of the infrared energy is lost into the walls of the oven. The walls of these ovens do not reflect a sufficient amount of infrared energy cooking energy onto the food to be cooked. Attempts have been made to line the inside of the quartz halogen lamp ovens with metallized coatings, which are often highly polished coatings. However, the highly polished surfaces cannot withstand the scrubbing and cleaning processes and materials to which ovens are subjected. The cleaning leads to a degradation of the metallized coating, and a subsequent reduction in the reflective efficiency of the oven. Further, the metal reflective surfaces provide only a specular reflectance, and do not efficiently disperse and direct the energy to the food to be heated. The specular reflectance by metallic surfaces provides a direct, "angle in equals angle out" type of reflectance. Thus, the specular reflectance merely reflects around the oven, without a substantial portion of the energy impinging on the food in the oven. It was generally believed that radiation with wavelengths shorter than 1 micron is not useful in cooking or baking processes, partly because of the weaker interaction of the shorter wavelengths with the foodstuff molecules in terms of general heat transfer, and partly due to the inferior penetrating properties of such radiation. In particular, it was believed that visible light, i.e., radiation with a wavelength in a range between about 0.4 to about 0.7 micron, was not very useful in the cooking process. However, as taught by Westerberg in U.S. Patent No. 5,571 ,005, an oven is provided with a sufficiently intense source of visible and infrared radiation. Cooking time can be reduced relative to conventional cooking ovens. The combination of the deeply penetrating reflected infrared radiation and the intense visible radiation establishes a temperature gradient within the interior of the foodstuff that ensures that the surface temperature of the foodstuff is hotter than the interior, and the products of the cooking, i.e., the water vapor and gases like CO2, are quickly driven to the surface and out of the foodstuff. This process results in a very rapid and efficient cooking of the food. Using combinations of visible and infrared radiation to cook food has significant advantages. The cooking process is very fast. Bakery products, for example, can be baked 5 to 10 times faster than conventional convection ovens and conduction cooking processes. The quality of the cooking process is enhanced for many foodstuffs. Vegetables are cooked so fast that they are virtually steamed in their own water vapor, leaving them hot, but with very little loss of any of their nutritive values. The reflectance efficiency of a material composition is dependent on several factors. These factors include the particle size of the reflecting particles and the volume fraction or coverage over the surface of the material composition. An optimum particle size and volume fraction will optimize the reflectance in the desired wavelength. The reflectance efficiency of a material for a certain energy wavelength is dependent on three primary factors. These factors are: (1) a difference in the refractive index of the high index scattering particles and the low index surrounding medium; i.e., the higher the difference between the scattering particles and the medium the better; (2) an optimum particle size (typically about 1/3 to 1/2 the subject wavelength (λ)); (3) and a volume fraction of the scattering particles to provide a required number of scatterers optimally spaced within the surrounding medium. Enamels typically contain oxide particles, for example, white enamels Q0808A, XT1056-4, T1363 and XT 1032 of the Ferro Coφoration all contain oxides. These enamels comprise a white enamel and further comprise at least one of recrystallized Anatase Tiθ2 and mill added Anatase Tiθ2- These enamels, however, are not acceptable infrared reflectors because the size and amount (volume fraction) of Anatase Tiθ2 particles do not provide a sufficient degree of reflectance. Anatase Tiθ2 is normally precipitated out of the enamel at a firing condition, for example at about 800°C to about 830°C for 3-10 minutes. Anatase T.O2 has a size equal to or less than about 0.2 μm, with a reflectivity (defined as a percentage of light reflected over light incident) of about less than 70%. This reflectivity is not sufficient for efficient heating by infrared energy, since the reflectivity is low and the food will not heat quickly, thus resulting in a waste of energy. The Anatase Tiθ2, however, provides for the desirable white color in enamels and results in a hardened enamel that is easily cleaned. Accordingly, even though enamels contain Anatase Tiθ2, they are not suitable for infrared heating and result in a significant energy loss. SUMMARY OF THE INVENTION An infrared and visible energy highly efficient reflecting enamel composition comprises a binder and high reflecting non- absorbing metal oxide particles, such as Rutile Tiθ 2 - The reflectivity of the enamel composition is at least about 80% in an infrared and visible energy range between about O.δμm to about 5.0μm. The enamel composition also diffusely reflects infrared advisible energies. A method for manufacturing a high reflective infrared and visible energy reflecting enamel composition is provided in another aspect of the invention. The method comprises forming a ground coat of enamel on a substrate; providing a layer of titania powder containing Tiθ2 particles on the ground coat; and heating the ground coat and the substrate to soften the ground coat, to form a dense layer on the substrate, and to embed Tiθ 2 particles in the ground coat. According to another aspect of the present invention, an oven comprises at least one infrared energy source; at least one internal surface; and a highly reflective infrared and visible energy reflecting enamel composition. The enamel composition comprises a ground coat of enamel provided on the at least one internal surface with Rutile T.O2 particles in the enamel composition. The Rutile T.O 2 particles are embedded in the enamel composition by heating the ground coat and substrate. The enamel composition has an infrared and visible energy reflectivity of at least about 80%. BRIEF DESCRIPTION OF THE DRAWINGS While the novel features of this invention are set forth in the following description, the invention will now be described from the following detailed description of the invention taken in conjunction with the drawings, in which: FIG. 1 is an illustration of specular versus diffuse reflectance; FIG. 2 is a flow chart illustrating formation of conventional enamels; FIG. 3 is a cross-sectional view of a fired enamel on a substrate; FIG. 4 is a cross-sectional view of an energy reflecting enamel composition, in accordance with the invention; and FIG. 5 is a side cross sectional illustration of an oven provided with reflective enamel composition, in accordance with the invention; Fig. 6 is side cross-sectional view of a substrate covered with a ground coat enamel composition; FIG. 7 is a side cross-sectional view of Fig. 6 with a layer of titania powder thereon; FIG. 8 is a side cross-sectional view of Fig. 8 after heat treating; and Fig. 9 is side cross-sectional view of Fig. 8 with a layer of enamel after heat treating. DETAILED DESCRIPTION OF THE EMBODIMENTS The invention is directed to an enamel composition with reflecting particles possessing a particle size and volume fraction to efficiently reflect energy, for example visible and infrared energies. A material's reflectance, such as enamel, is dependent on several factors, including: a size of reflecting particles, for example oxide particles; a volume fraction of the reflecting particles over the surface of the material; and a difference in the refractive index of the particles and the surrounding medium. The larger the size and volume fraction of oxide particles in the material, the greater the reflectance for the material. Conventional enamel compositions normally have a white coloration and comprise titania, T.O2, that in part provides an enamel with its white color and appearance. Tiθ2 in conventional enamel compositions is normally Anatase Tiθ2 that is precipitated out at a firing condition, by conventional firing for example in a range between about 800°C to about 830°C for a time period of about 3 to about 10 minutes. Anatase Tiθ2 possess a particle size less than or equal to about 0.2μm, and with reflectance values for conventional enamel compositions are about 70%. It has been determined that a diffuse reflectance value of at least about 80% is desirable for applications in cooking ovens relying on intense visible and infrared radiation for reduced cooking time. In accordance with the invention, it is desirable to increase the particle size and the volume fraction of reflecting oxide particles in the material to increase the reflectance of the material. An size and volume fraction of the reflecting particles in the material will result in an acceptable reflectance of energy in a desired wavelength (λ). For example, an optimum size for reflecting particles is in a range between about 1/3 wavelength (λ) to about 1/2(λ) of a desired radiation, and an optimum volume fraction is in a range between about 15% to about 40% of the enamel. Reflecting oxide particles are added to an enamel composition to enhance the reflectivity. These particles comprise metal oxide particles, such as Na2θ, K2O, S_2θ, Tiθ2, ZnO, Z1O2, Sb2θ3 and compounds thereof, all of which are effective energy reflectors. The above metal oxides and their compounds (metal oxide) are especially effective reflectors for energy with a wavelength range between about 0.04μm to about 5.0μm. These wavelengths are emitted by visible and infrared energy sources commonly used in rapid cooking ovens (including, but are not limited to, lamps, such as quartz halogen lamps). These metal oxides diffusely reflect and disperse visible and infrared energy, so as to efficiently and effectively heat food in an oven with a reflectivity above at least about an 80% reflectance value. In addition to simply reflecting the energy, reflecting oxide particles also diffuse and disperse the energy, when compared to the direct "angle in equals angle out" specular reflectance of known materials. Diffused energy reflectance causes more energy to impinge on the food, compared to a specular reflection. The difference in specular versus diffused reflection is illustrated in Fig. 1. Rutile Tiθ2 particles provide effective reflecting particles within the scope of the invention. Rutile Ti0 possesses a high reflective index and a large particle size, greater than about 0.2 μm. Rutile Tiθ2 has an index of refraction in a range between about 2.65 to about 2.75. Accordingly, Rutile Tiθ2 will exhibit a higher reflectance of infrared energy and will act as a suitable reflective particle for enamel compositions, when compared to conventional enamels containing Anatase Tiθ2. Enamel compositions, according to the invention, comprise at least a frit (glass) that includes a binder and oxides; mill additives, such as clay; and other materials for other properties, such as thickness, color, and appearance. Oxides in the frit may include Na2θ, K2O, S.2O, T.O2, ZnO, Zrθ2, Sb2θ3, oxides and compounds thereof that occur naturally in the frit. Altematively, non-naturally occurring oxides can be added to the frit during preparation. The metal oxide particles in an enamel, at least in part, provide reflectance of visible and infrared energies. An enamel is often placed on a substrate for use. Fig. 2 is a flow chart illustrating the formation of an enamel-coated substrate. At step S1 , the individual components, are provided. In step S2, the components provided in step S1 are mixed to an enamel slurry. Next, the enamel slurry is coated on an appropriate substrate in step S3. In step S4, the coated enamel substrate is fired to densify, harden, and glassify the enamel composition. An enamel composition 100 on a substrate 101 is illustrated in Fig. 3. A ground coat 102, if desired, is provided on the substrate 101. Firing of the enamel composition 100, ground coat 101 (if provided), and substrate 101 results in a smooth glassified hardened enamel surface with the enamel composition 100 softening and flowing over the substrate 101. An enamel composition that is fired (heated) for a long time and at a high temperature changes Anatase Tiθ2 in it into Rutile Tiθ2- The long heating process recrystallizes the Anatase Tiθ 2 to Rutile Tiθ2 at temperatures greater than conventional firing temperatures for periods longer than convention firing time periods. Altematively, the heating temperature can be increased higher than conventional firing temperatures, with the heating time period unchanged. In other words, with an increase of one of the heating time and temperature, the other of the heating time and temperature need not be increased. The Rutile phase of Ti0 2 has a smaller weight percentage compared to the Anatase phase. While at first glance this may appear to be contrary to Rutile Tiθ2 having a higher infrared reflectance, Rutile Tiθ2 has a larger particle size than Anatase Tiθ2. An enamel composition comprising Rutile T.O2 possess a higher reflectivity than Anatase Tiθ2 because of the respective particle sizes. Therefore, the particle size of a Rutile Ti0 2 , according to the invention, increases the visible to infrared reflectance of the enamel composition by increasing one or both of volume fraction and particle size. An enamel composition with a Tiθ2 metal oxide according to the invention comprises a layer of Rutile Ti0 2 . The reflectivity of the enamel is enhanced because of a high volume fraction of particles and by the size of the Rutile Tiθ2 particles. Separated recrystallized and precipitated Rutile Tiθ2 particles are blended into a binder system of an enamel composition, for example in its slurry form, to enhance the reflectivity of the enamel composition. The blended enamel composition with Rutile Tiθ2 is provided on an appropriate substrate, for example, by known processes, such as dry or wet processes. Altematively, the reflectance by oxide particles in an enamel composition, according to the invention, is enhanced by placing separated and recrystallized Rutile Tiθ2 oxide particles into an enamel composition 1 , as illustrated in Fig. 4. The enamel composition 1 comprises Rutile Tiθ2 particles 2 formed by recrystallizing and precipitating the particles. The particles are ground out into separate Tiθ2 particles and sorted. The sorted Rutile Tiθ2 particles 2 are placed in the enamel composition as a mill addition to a binder phase (binder solution) so the enamel is saturated with T.O2 particles. While a binder phase 3 may originally contain some T.O2 in solution, the amount of the T.O2 in solution is normally at a minimum solubility. Rutile Tiθ2 particles are added to the binder phase at a volume percent to a saturation point. The binder phase 3 comprises at least one of mixed alkali borosilicate, suitable glass compositions, including precursors of the enamel oxides, such as at least one of nitrates, hydroxides, chlorides, alkoxides, and carboxylates. The saturated enamel composition causes precipitation of Tiθ2 as Rutile Tiθ2 particles. The preparation of the enamel for optimization of Rutile Tiθ2 is also achieved by at least one of removing and lowering a phosphate composition in the enamel for example in a phosphate-stabilized (4% P2O5) enamel. Reflectance may be further enhanced by the addition of additional enamel constituents that promote the recrystallization and growth of Rutile Tiθ2. The additional enamel constituents that are added to the enamel comprise further amounts of constituents already within the enamel composition, and alternatively comprise mill additions not normally present in an enamel. In accordance with another aspect of the invention, an enamel composition with enhanced reflectance comprises a Tiθ 2 particle, either Anatase or Rutile, coated with a glassy coating. The coating comprises at least one of binder solutions, and a mixture of at least one of alkali/alkaline earth siiicate/borates, phosphates, and fluorine, and altematively comprises nucleation and growth aids. Nucleation and growth aids include, for example, ZnO, Ceθ2 and others, as known in the art. Fig. 5 is a front cross section of an oven provided with an enamel coating prepared as in the invention. The oven includes an outer enclosure 100 which includes an inner wall 120 coupled to the outer enclosure 100. An insulating layer 140 is formed between the outer enclosure 100 and the inner wall 120, where the insulating layer 140 may be a layer of air. The energy for cooking is supplied by a heating lamp 16 and a lamp 18. These lamps are generally quartz body, tungsten- halogen lamps such as 1.5 KW 208 V quartz-halogen lamp, and the oven includes any number of lamps. Surfaces 121 of the inner wall 120 are provided with an enamel composition 122, according to the invention, to form a reflecting surface 121 , which will disperse the infrared energy to the food to be heated. A control circuit 34, shown as a circuit block, controls the operation of lamps 16 and 18. The lamps 16 and 18 produce very high intensity radiation, such as visible and infrared radiations. The use of high intensity visible and infrared radiations provides a very rapid method of high-quality cooking and baking. The radiant energy from the lamps 16 and 18 radiates in all directions. A portion of the energy radiates directly onto the food item 32 and the remainder will reflect until it impinges onto the food item 32. Another method for forming the infrared and visible energy reflecting enamel composition, in accordance with the invention, will now be described with reference to Figs. 6-9. In Figs. 6- 9, an enamel composition is formed by placing a ground coat of enamel 1002 on a substrate 1001 , for example as an electrocoated powder coating by an electrostatic process. The substrate 1001 may be formed from any appropriate material usable as an infrared heating oven internal surface, such as, but not limited to, one of aluminum, steel, and alloys thereof. A layer of titania powder 1030 is then placed on the ground coat 1002. The substrate 1001 , ground coat 1002, and titanium powder layer 1030, are heated for a first heat cycle at a first temperature. This first heat cycle softens the ground coat 1002 and causes the ground coat 1002 flow and spread over the substrate 1001. The first heat cycle ends with cooling of the elements. The ground coat 1002, upon cooling, forms a densified, hardened layer. The first heat cycle causes a majority of the Tiθ2 particles 1003 of the titanium powder layer 1030 to sink into and embed in the ground coat 1002. The embedding occurs at a top surface, while the ground coat 1002 is soft and flows. Altematively, the first heat cycle duration can be extended to cause a majority of the Tiθ2 particles to sink into and become embedded in the ground coat 1002 below the top surface 1008 of the ground coat 1002. This process also form heat treated infrared reflecting enamel composition 1005. A suitable white enamel 1006, which comprises a glass enamel composition, is then layered on the heat treated infrared reflecting enamel composition 1005, as illustrated in Fig. 9. This enamel 1006 provides for enhanced cleaning without harm of enamel. The enamel 1006 is formed with a suitable thickness, for example in a range between about 0.00127 m to about 0.00762 m. The enamel 1006 and the heat treated infrared reflecting enamel composition 1005 forms an enamel assembly 1010. The enamel assembly 1010 is heat treated, for a second heat cycle, at a second temperature, for example, above a range between about 830°C to about 855°C. The second heat cycle occurs for an appropriate time period, for example, longer than at least 3 minutes, which softens the enamel assembly 1010. The enamel 1006 flows to provide a hard, glazed, and recrystallized surface. This second heat cycle also grows or recrystallizes Anatase Tiθ2 to Rutile Tiθ2. Thus, the high reflective infrared reflecting enamel composition, according to the invention, now comprises high reflective infrared and visible light reflecting Rutile Tiθ2- The reflectivity value of the resultant enamel assembly 1010 is preferably at least in a range between about 80% to about 90%, with Rutile Tiθ2 particles possessing a size in a range between about 0.2μm and 2.0μm, in a volume fraction up to about 15 wt%. The Rutile Tiθ2 particles scatter and reflect energy incident on the ground coat. While the embodiments described herein are preferred, it will be appreciated from the specification that various combinations of elements, variations or improvements therein may be made by those skilled in the are that are within the scope of the invention.

Claims: What is claimed is: 1. An energy reflecting enamel composition (1) comprising: a binder (3); and metal oxide particles (2), wherein a reflectivity of the enamel composition is at least 80% for infrared and visible energy, in an energy possessing a wavelength in a range between about 0.6μm to about 5.0μm, and the enamel composition diffusely reflects infrared and visible energies. 2. A composition according to claim 1 , wherein the metal oxide particles (2) comprise titanium oxide Tiθ2 particles, said Ti0 2 particles comprise Rutile Tiθ 2 . 3. A method for providing an energy reflecting enamel composition (1) comprising: providing a binder (3); forming metal oxide particles (2); and adding the particles to the binder to form the enamel composition, where the energy reflecting enamel composition is provided with a reflectivity of at least 80% in an energy range between about 0.6 to about 5.0μm. 4. A method according to claim 3, wherein the forming metal oxide particles (2) comprises: selecting particles from the group consisting of: titanium oxide, zinc oxide, zirconium oxide, antimony oxide, sulfides, halides and compounds thereof. 5. A method according to claim 3, wherein the forming metal oxide particles (2) further comprises: coating said titanium oxide particles with a coating; and selecting the binder composition from the group consisting of: mixed alkali silicate; mixed alkali borosiiicate; and glass compositions comprising precursors of oxides, such as nitrates, hydroxides, chlorides, alkoxides, and carboxylates. 6. An apparatus for heating food, the apparatus comprising: at least one internal surface; and an energy reflecting enamel composition (120') on the at least one internal surface of the apparatus, the energy reflecting enamel composition (120') comprising: a binder (3) ; and metal oxide particles (2), wherein reflectivity of the reflecting enamel for an energy possessing a wavelength in a range between about 0.6μm to about 5.0μm, and the infrared energy reflecting enamel composition diffusely reflects energies. 7. A method for manufacturing a high reflective infrared energy reflecting enamel composition, comprising: forming a ground coat (1002) of enamel on a substrate (1001); providing a layer of titania powder (1030) on the ground coat, where the layer of titania powder contains T.O2 particles (1003); and heating the ground coat (1002) and the substrate (1001), during a first heat cycle wherein the heating softens the ground coat (1002), forms a layer on the substrate (1001) and embeds the Ti0 2 particles (1003) in the ground coat. 8. A method according to claim 7, further comprising: providing a cover enamel (1006) on the ground coat (1002) and T.O2 particles (1003) after the heating. 9. A method according to claim 8, further comprising: heating, during a second heat cycle, the ground coat (1002), the substrate (1001) and the cover enamel (1006) for a temperature and time to soften the ground coat, provide a glazed recrystallized dense surface, and recrystallize the Tiθ2 particles. 10. A composition comprising: a substrate (1001); ground coat (1002) of enamel provided on a substrate (1001); the ground coat (1002) disposed on the substrate; and a cover enamel (1006) provided on the ground coat enamel; wherein the ground coat of enamel comprises Rutile Tiθ2 particles (1003) embedded in the ground coat of enamel by heating the ground coat, the enamel composition in an energy possessing a reflectivity of about 80% for energy having a wavelength in a range between about 0.6μm to about 5.0μm

Inventors: GEISE DOUGLAS DWIGHT, VENKATARAMANI VENKAT SUBRAMANI, GEISE, DOUGLAS, DWIGHT, VENKATARAMANI, VENKAT, SUBRAMANIAM
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1998
Main classification: C23D5
IPC codes: C03C8, C23D5, F24C15