Patent Description:
Enamel is a substance formed by plating glass glaze to the surface of a metal plate. Common enamel is used for cooking appliances such as a microwave and an oven. Enamel is classified into acid resistant enamel that prevents oxidation, heat resistant enamel that can resist high temperature, etc., depending on the kind and the use of glaze. Further, enamel is classified into aluminum enamel, zirconium enamel, titanium enamel, soda glass enamel, etc., depending on the materials that are added to the enamel.

For the enamel, an enamel substance, that is, a glass frit is manufactured, glass powder is formed by powdering the glass frit through a dry or wet process, and then the glass powder is coated on an object to be coated, whereby an enamel layer can be formed.

The enamel layer can be formed by coating an object and then performing a sintering process at a predetermined temperature.

The material of objects on which an enamel layer is formed may be limited, depending on the required sintering temperature, and as the sintering temperature is increased, the process efficiency and the process cost are increased. <CIT>, <CIT> and <CIT> disclose glass frit compositions.

Accordingly, there is a need for a glass composition that can solve these problems and can reduce a sintering temperature.

Embodiments of the invention provide a glass frit having a low sintering temperature.

A glass frit according to the invention is according with claim <NUM>.

The glass frit according to the invention has a low sintering temperature e.g. when it is coated on a low-carbon steel sheet.

In detail, the glass frit according to the invention may be sintered at <NUM> to <NUM> e.g. after being spray-coated on a low-carbon steel sheet.

That is, the glass frit according to the invention can form a functional layer on a low-carbon steel sheet by being sintered at <NUM> to <NUM> and can have a coefficient of thermal expansion, a softening temperature, and adhesion similar to those of a functional layer that is coated in a high-temperature process.

Therefore, the glass frit according to the invention can be coated on a low-carbon steel sheet by sintering at a low temperature and the process temperature can be decreased, so the process efficiency can be improved.

Further, since the glass frit can be sintered at a low temperature, it can be coated on various materials such as aluminum and SUS, and the materials on which the glass frit can be coated may include materials on which sintering needs to be carried out at a low temperature, so it is possible to use various base materials. In accordance with a further embodiment, the invention provides a cooking appliance comprising a functional layer on a metallic base material, which functional layer is formed by sintering a glass frit in accordance with the invention.

The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description when taken in conjunction with the accompanying drawings, in which:.

The glass frit according to the invention and a cooking appliance using the glass frit are described hereafter with reference to the drawings.

<FIG> is a front view of a cooking appliance according to an embodiment.

Referring to <FIG>, a cooking appliance <NUM> includes a cavity <NUM> having a cooking chamber <NUM>, a door <NUM> selectively opening/closing the cooking chamber <NUM>, and at least one heating source providing heat for heating an object to be cooked in the cooking chamber <NUM>.

In detail, the cavity <NUM> may have a hexahedral shape with an open front. The heating source may include a convection assembly <NUM> for discharging heated air into the cavity <NUM>, a top heater <NUM> disposed at an upper portion in the cavity <NUM>, and a bottom heater <NUM> disposed at a lower portion in the cavity <NUM>. Obviously, it is not necessary for the heating source to include the convection assembly <NUM>, the top heater <NUM>, and the bottom heater <NUM>. That is, the heating source may include at least any one of the convection assembly <NUM>, the top heater <NUM>, and the bottom heater <NUM>.

The top heater <NUM> and the bottom heater <NUM> may be disposed inside or outside the cavity <NUM>.

Referring to <FIG>, functional layers may be disposed on the inner side of the cavity <NUM> and the rear side of the door <NUM>.

The functional layers are formed by sintering a glass frit to be described below. The functional layers may be coated on the inner side of the cavity <NUM> and the rear side of the door <NUM>. That is, the functional layers may be coating layers.

The functional layers improve heat resistance, chemical resistance, and contamination resistance of the inner side of the cavity <NUM> and the rear side of the door <NUM>.

Referring to <FIG> and <FIG>, a functional layer may be disposed in the cavity.

The cavity <NUM> may include a metal layer 11a and a functional layer 11b on the metal layer 11a.

The metal layer 11a may be a base material of the cavity, e.g. a base material including low carbon steel. As used herein, the term "low carbon steel" refers to steel with a carbon content of typically <NUM> wt% or less, based on the total weight of the steel.

Referring to <FIG>, the functional layer 11b may be in direct contact with the metal layer 11a.

Referring to <FIG>, the functional layer 11b may be in indirect contact with the metal layer 11a. In detail, a buffer layer 11c may be disposed between the metal layer 11a and the functional layer 11b. The buffer layer 11c may include a bonding layer. That is, the bonding force between the metal layer 11a and the functional layer 11b can be improved by the buffer layer 11c.

Referring to <FIG>, a functional layer may be disposed on the rear side of the door <NUM>. In detail, a functional layer may be disposed on the rear side, which faces the cooking chamber <NUM>, of the door <NUM> when the cooking chamber <NUM> is closed. The functional layer can improve heat resistance, chemical resistance, and contamination resistance of the rear side of the door <NUM>.

The door <NUM> may include a metal layer 14a and a functional layer 14b on the metal layer 14a.

The metal layer 14a may be a base material of the door, e.g. a base material including low carbon steel.

Referring to <FIG>, the functional layer 14b may be in direct contact with the metal layer 14a.

Alternatively, referring to <FIG>, the functional layer 14b may be in indirect contact with the metal layer 14a. In detail, a buffer layer 14c may be disposed between the metal layer 14a and the functional layer 14b. The buffer layer 14c may include a bonding layer. That is, the bonding force between the metal layer 14a and the functional layer 14b can be improved by the buffer layer 14c.

The functional layers may be formed by coating the glass frit on the inner side of the cavity <NUM> or the rear side of the door <NUM>, and sintering the glass frit. Preferably, the functional layers are coated on the inner side of the cavity <NUM> and the rear side of the door <NUM>, whereby they can improve heat resistance, chemical resistance, and contamination resistance of the inner side of the cavity <NUM> and the rear side of the door <NUM>.

The glass frit in accordance with the invention that may be coated on at least one of the cavity and the door of the cooking appliance is described hereafter.

SiO<NUM> is included by <NUM> wt% to <NUM> wt% of the entire glass frit.

SiO<NUM> is included in the glass frit, thereby being able to form a glass structure of the glass frit, improve the framework of the glass structure, and improve acid resistance of the glass frit.

When SiO<NUM> is included in an amount of less than <NUM> wt% of the entire glass frit, the glass structure of the glass frit may be deteriorated, so durability of a functional layer may be reduced. When SiO<NUM> is included in an amount of more than <NUM> wt% of the entire glass frit, the sintering temperature of the glass frit may be increased.

B<NUM>O<NUM> is included by <NUM> wt% to <NUM> wt% of the entire glass frit.

B<NUM>O<NUM> can increase a vitrification area of the glass frit and can appropriately adjust the coefficient of thermal expansion of the glass frit. Further, B<NUM>O<NUM> can provide sufficient fusion flow in a sintering process of the glass frit by reducing the viscosity of the glass frit.

That is, B<NUM>O<NUM> can reduce a contact angle at a high temperature and can improve propagation and fluidity in a sintering process of the glass frit, so high adhesion can be maintained even at a low temperature.

When B<NUM>O<NUM> is included in an amount of less than <NUM> wt% of the entire glass frit, the vitrification area may be reduced and the glass structure is deteriorated, so durability of a functional layer may be reduced. When B<NUM>O<NUM> is included in an amount of more than <NUM> wt% of the entire glass frit, the sintering temperature of the glass frit may be increased.

The I-group oxide is an oxide of an element of the first group of the periodic table, i.e. an oxide of an alkali metal. In detail, the glass frit includes all of Li<NUM>O, Na<NUM>O, and K<NUM>O.

The I-group oxide is included <NUM> wt% to <NUM> wt% of the entire glass frit.

The I-group oxide can reduce the sintering temperature of the glass frit by being included in the glass composition. In detail, the I-group oxide can sufficiently reduce thermal properties of the glass composition, so the glass composition can be sintered at a low temperature.

When the I-group oxide is included in an amount of less than <NUM> wt% of the entire glass frit, the sintering temperature of the glass frit may increase. When the I-group oxide is included in an amount of more than <NUM> wt% of the entire glass frit, the coefficient of thermal expansion of the glass frit may be increased, so a bonding force may be reduced due to the difference from the coefficient of thermal expansion of the base material.

ZnO is included by <NUM> wt% to <NUM> wt% of the entire glass frit.

ZnO can reinforce the glass structure of the glass frit by being included in the glass frit.

ZnO can function as an intermediate for keeping balance between SiO<NUM> and the I-group oxide. In detail, ZnO can function as an intermediate for keeping balance between SiO<NUM> and B<NUM>O<NUM> that function as a network former and the I-group oxide that functions as a network modifier.

When ZnO is included in an amount of less than <NUM> wt% of the entire glass frit, the glass structure of the glass composition may be deteriorated, so durability of a functional layer may be reduced. When ZnO is included in an amount of more than <NUM> wt% of the entire glass frit, the crystallization of glass may occur.

The glass frit further includes all of Al<NUM>O<NUM>, ZrO<NUM>, and TiO<NUM>.

Any one of Al<NUM>O<NUM>, ZrO<NUM>, and TiO<NUM> is included by <NUM> wt% to <NUM> wt% of the entire glass frit.

Al<NUM>O<NUM> and ZrO<NUM> can improve chemical resistance and durability of the glass frit by being included in the glass frit. In particular, Al<NUM>O<NUM> and ZrO<NUM> can supplement low chemical durability of an alkali phosphated glass structure formed by P<NUM>O<NUM>, Na<NUM>O, and K<NUM>O, through structural stabilization.

TiO<NUM> can improve hiding power of the glass composition according to an embodiment. That is, the hiding power of the coating layer of a glass composition on the function layers can be improved by TiO<NUM>.

When Al<NUM>O<NUM>, ZrO<NUM>, and TiO<NUM> are included in an amount of less than <NUM> wt% of the entire glass frit, chemical resistance and durability of the glass composition may be reduced and the hiding power of the glass composition may be deteriorated, so when the glass composition is coated on a buffer layer, the color of the buffer layer may be shown to the outside. When Al<NUM>O<NUM>, ZrO<NUM> and TiO<NUM> are included in an amount of more than <NUM> wt% of the entire glass frit, the sintering temperature of the glass frit may be increased and accordingly the process efficiency may be decreased.

The glass frit further includes a fluorine compound. The fluorine compound includes NaF. In detail, the fluorine compound may include NaF and AlF<NUM>. That is, the glass frit may include all of NaF and AlF<NUM>.

The fluorine compound can appropriately adjust the surface tension of a coating film formed by a glass composition. Further, the vitrification area of the glass frit can be increased by the fluorine compound.

NaF is included by about <NUM> wt% to <NUM> wt% of the entire glass frit.

When the fluorine compound is included in an amount of less than <NUM> wt% of the entire glass frit, the vitrification area is reduced and the glass structure may be deteriorated, so durability of a functional layer may be reduced. When the fluorine compound is included in an amount of more than about <NUM> wt% of the entire glass frit, the sintering temperature of the glass frit may be increased.

The glass frit may further include at least one of Co<NUM>O<NUM>, NiO, Fe<NUM>O<NUM>, and MnO<NUM>. The glass frit may further include all of Co<NUM>O<NUM>, NiO, Fe<NUM>O<NUM>, and MnO<NUM>.

Co<NUM>O<NUM>, NiO, Fe<NUM>O<NUM>, and MnO<NUM> can increase adhesion of the glass composition that is coated on a base material. That is, Co<NUM>O<NUM>, NiO, Fe<NUM>O<NUM>, and MnO<NUM> may be adhesion-reinforcing elements that improve adhesion when a glass composition is coated on a base material.

By Co<NUM>O<NUM>, NiO, Fe<NUM>O<NUM>, and MnO<NUM>, adhesion can be improved even if the glass composition is directly coated on a base material without a specific buffer layer when the glass composition is disposed on the base material.

Accordingly, the entire thicknesses of the cavity and/or the door on which the glass composition is coated can be reduced, so the process efficiency can be improved.

At least one of Co<NUM>O<NUM>, NiO, Fe<NUM>O<NUM>, and MnO<NUM> may be included by <NUM> wt% or less of the entire glass frit. Preferably, at least one of Co<NUM>O<NUM>, NiO, Fe<NUM>O<NUM>, and MnO<NUM> may be included by <NUM> wt% to <NUM> wt% of the entire glass frit.

The present invention is described in more detail with respect to a method of producing glass frits according to examples not representing embodiments of the invention.

As shown in the following Table <NUM>, a glass frit material was provided.

Na<NUM>CO<NUM>, K<NUM>CO<NUM>, Li<NUM>CO<NUM> were used as the raw materials of Na<NUM>O, K<NUM>O, and Li<NUM>O, CaCO<NUM> was used as the raw material of CaO, and the other elements were the same as those shown in Table <NUM>.

The glass frit material was mixed, melted at about <NUM> for one to two hours, and then rapidly cooled by a quenching roller, thereby obtaining a glass cullet.

Organopolysiloxane in an amount of about <NUM> wt% to <NUM> wt% was put into the glass cullet, milled and pulverized by a ball mill for about <NUM> hours, and then put through a <NUM>-mesh sieve to have a particle diameter of about <NUM> or less, thereby producing a glass frit.

The glass frit was sprayed to a low-carbon steel sheet of <NUM>×<NUM> (mm) and <NUM>(mm) thickness by a corona discharge gun. The voltage of the discharge gun was controlled between <NUM> to <NUM> kV and the amount of the glass frit sprayed to the low-carbon steel sheet was <NUM>/m<NUM>.

A functional layer was formed on a side of the low-carbon steel by sintering the low-carbon steel sheet with the glass frit sprayed thereon at <NUM> to <NUM> for <NUM> to <NUM> seconds, and then adhesion of the functional layer was measured.

The adhesion test device was Dupont Impact Tester (ASTM D1794, JIS K5400), in which a sample was put on the center of a sample stage, a steel ball having a diameter of about <NUM> (<NUM> inch) was put on the center of the plane of the sample, and then a weight of <NUM> was freely dropped from a height of <NUM>, thereby examining the state of peeling.

The reference for determining adhesion levels was as follows.

Further, a pellet type specimen was sintered under the same condition as that for sintering the glass frit to measure thermal properties of glass, and both sides of the specimen was ground and then a Td (softening temperature) and a CTE (coefficient of thermal expansion) were measured at a temperature rising speed of <NUM>/min by a TMA (Thermo Mechanical Analyzer).

As shown in Table <NUM>, a functional layer was formed in the same way as Example <NUM> except that glass frit materials were provided as listed in the table. Adhesion, softening temperature, and a coefficient of thermal expansion of the functional layer were measured.

As shown in Table <NUM>, a functional layer was formed in the same way as Example <NUM> except that glass frit materials were provided. Adhesion, softening temperature, and a coefficient of thermal expansion of the functional layer were measured.

Referring to Table <NUM>, it can be seen that the functional layers formed by the glass frits according to Examples have high softening temperature and coefficient of thermal expansion. That is, it can be seen that the functional layers of Examples have a softening temperature of about <NUM> and a coefficient of thermal expansion of <NUM>(× <NUM>-<NUM>/°C) or more.

That is, it can be seen that the functional layers formed by the glass frits according to Examples have improved durability and chemical resistance.

Further, it can be seen that the functional layers formed by the glass frits according to Examples have improved adhesion.

That is, referring to Table <NUM>, it can be seen that the glass frits according to Examples have improved adhesion on a low-carbon steel even at a sintering temperature of <NUM> to <NUM>.

The glass frits according to the invention may have a low sintering temperature when they are coated on a low-carbon steel sheet.

In detail, the glass frits according to the invention may be sintered at <NUM> to <NUM> e.g. after being spray-coated on a low-carbon steel sheet.

That is, the glass frits according to the invention can form a functional layer on a low-carbon steel sheet by being sintered at <NUM> to <NUM> and can have a coefficient of thermal expansion, a softening temperature, and adhesion similar to those of a functional layer that is coated in a high-temperature process.

Claim 1:
A glass frit including SiO<NUM>, B<NUM>O<NUM>, ZnO, an I-group oxide, and further including NaF,
wherein the SiO<NUM> is included by <NUM> wt% to <NUM> wt% of the entire glass frit,
the B<NUM>O<NUM> is included by <NUM> wt% to <NUM> wt% of the entire glass frit,
the ZnO is included by <NUM> wt% to <NUM> wt% of the entire glass frit,
the I-group oxide is included by <NUM> wt% to <NUM> wt% of the entire glass frit, and includes Li<NUM>O, Na<NUM>O and K<NUM>O,
the NaF is included by <NUM> wt% to <NUM> wt% of the entire glass frit,
wherein the glass frit further includes Al<NUM>O<NUM>, ZrO<NUM>, and TiO<NUM>, and
wherein the Al<NUM>O<NUM>, ZrO<NUM>, and TiO<NUM> is included by <NUM> wt% to <NUM> wt% of the entire glass frit.