Method for coating a metal cookware

A method for coating a metal cookware includes the steps of (A) cleaning the metal cookware; (B) forming a scratch-resistant oxide film on the cooking surface by anodic oxidation; (C) roughening the scratch-resistant oxide film by blasting to form a roughened surface thereon, the roughened surface having valleys extending into the cooking surface; and (D) applying an anti-stick film of fluorocarbon resin on the roughened surface.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method for coating a metal cookware, more 
particularly to a method for coating a metal cookware having 
scratch-resistant and anti-stick characteristics and a relatively long 
service life. 
2. Description of Related Arts 
The common methods used for surface treatment of metal cookware are 
described as follows: 
1. The cooking surface of the metal cookware is roughened by blasting 
abrasive particles, such as aluminum oxide (Al.sub.2 O.sub.3) or steel 
grains, against the cooking surface. A hard oxide film is then formed on 
the roughened cooking surface by an anodic oxidation. During the anodic 
oxidation, an electrolytic solution containing sulfuric acid or oxalic 
acid is employed. Since the hard oxide film is provided on the cooking 
surface of the metal cookware, the cooking surface has a scratch-resistant 
characteristic. However, a main drawback of the metal cookware is that the 
cooking surface does not have an anti-stick characteristic, thereby 
facilitating the accumulation of greasy dirt thereon. 
2. The cooking surface of the metal cookware is roughened by blasting 
abrasive particles, such as aluminum oxide (Al.sub.2 O.sub.3) or steel 
grains, against the cooking surface. An anti-stick film is then formed by 
spraying suspended fluorocarbon resin upon the roughened cooking surface. 
Since the fluorocarbon resin film is provided on the cooking surface of 
the metal cookware, the cooking surface has an anti-stick characteristic. 
However, the hardness of the cooking surface of the metal cookware is 
relatively poor such that the cooking surface is susceptible to 
scratching. 
3. After the cooking surface is roughened, a hard oxide film is applied on 
the roughened cooking surface, and an anti-stick film is then applied upon 
the hard oxide film. Referring to FIG. 1, abrasive particles, such as 
Al.sub.2 O.sub.3 or steel grains, are blasted against the cooking surface 
of the metal cookware 6 to form a roughened surface 61. Referring to FIG. 
2, a porous hard oxide film 7 is coated on the roughened surface 61 by an 
anodic oxidation with the use of the electrolytic solution containing 
sulfuric acid or oxalic acid. Referring to FIG. 3, the hard oxide film 7 
of the metal cookware 6 is further coated with an anti-stick film 8. Since 
the metal cookware 6 is provided with both the hard oxide film 7 and the 
anti-stick film 8, the cooking surface of the metal cookware 6 has the 
scratch-resistant and anti-stick characteristics. Referring to FIGS. 2 and 
3, the formation of the roughened surface 61 can facilitate the attachment 
and bonding of the hard oxide film 7 to the metal cookware 6. However, 
since the roughness of the hard oxide film 7 is relatively low, the 
attachment and bonding between the hard oxide film 7 and the anti-stick 
film 8 is not strong. Therefore, the anti-stick film 8 easily peels off 
after a period of use. 
SUMMARY OF THE INVENTION 
Therefore, the objective of this invention is to provide a method for 
coating a metal cookware having scratch-resistant and anti-stick 
characteristics and a relatively long service life. 
According to this invention, the method for coating a metal cookware 
comprises the steps of (A) cleaning the metal cookware; (B) forming a 
scratch-resistant oxide film on the cooking surface by anodic oxidation; 
(C) roughening the scratch-resistant oxide film by blasting to form a 
roughened surface thereon, the roughened surface having valleys extending 
into the cooking surface; and (D) applying an anti-stick film of 
fluorocarbon resin on the roughened surface. 
The metal cookware may be made of a material selected from the group 
consisting of aluminum and aluminum alloy. An electrolytic solution used 
in the step (B) may be a mixture of sulfuric acid, magnesium sulfate and 
ferrous sulfate. The scratch-resistant oxide film may contain aluminum 
oxide, magnesium oxide and iron oxide. 
The step (A) may comprise immersing the metal cookware in a base solution 
and then in water in order to remove grease and other contaminants. The 
base solution is a sodium hydroxide solution in which a ratio of sodium 
hydroxide to water is about 3 to 1. 
The oxide film in the step (B) has a thickness of 40-80 .mu.m. The 
roughened surface after the step (C) has a roughness (Ra) of 6-8 .mu.m, 
and the depth of valleys is about 25-60 .mu.m.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 4, a metal cookware, which is a product of aluminum or 
aluminum alloy formed by punching, is subjected to the coating method of 
this invention, the latter comprising the steps of: (a) cleaning the metal 
cookware by immersing the metal cookware in a base solution and then in 
water in order to remove grease and other contaminants; (b) drying the 
cleaned metal cookware; (c) forming a scratch-resistant oxide film on a 
cooking surface of the metal cookware by anodic oxidation; (d) roughening 
the scratch-resistant oxide film by blasting to form a roughened surface 
thereon, the roughened surface having valleys extending into the cooking 
surface; (e) cleaning the roughened surface by blowing pressurized air 
thereagainst; and (f) applying an anti-stick film of fluorocarbon resin on 
the roughened surface. The step (f) can comprise the steps of (fl) 
applying a primer coating of fluorocarbon resin and heating the primer 
coating, (f2) applying a top coating of fluorocarbon resin and heating the 
top coating, and (f3) cooling forcibly the top coating after the step 
(f2). 
Referring to FIGS. 5 to 7, a metal cookware 9, which is a product of 
aluminum or aluminum alloy formed by punching, is immersed in a base bath 
at a temperature of about 50.degree.-70.degree. C. for about 3-5 minutes. 
The base bath contains a sodium hydroxide solution in which a ratio of 
sodium hydroxide to water is about 3 to 1. The metal cookware 9 is then 
immersed in a water bath at a temperature of about 50.degree.-70.degree. 
C. for about 3-5 minutes so as to remove grease and other contaminants. 
Afterwards, the cleaned metal cookware 9 is dried in a drying oven at a 
temperature of about 200.degree. C. for about 3-5 minutes. 
The metal cookware 9 undergoes an anodic oxidation process in order to form 
a hard oxide film on a cooking surface thereof. During the anodic 
oxidation process, an electrolytic solution containing sulfuric acid, 
magnesium sulfate and ferrous sulfate and having a concentration of 
100-350 g/L is provided. The temperature is controlled at 
-5.degree.-20.degree. C. The current is 2 A/dm.sup.2 and can be a direct 
current or an alternating current. In addition, the electrolytic voltage 
is 20-40 V. Referring to FIG. 5, an oxide film 10 is formed on the cooking 
surface of the metal cookware 9. The oxide film 10 contains aluminum 
oxide, magnesium oxide and iron oxide, and has a thickness of about 40-80 
.mu.m. 
Referring to FIG. 6, the oxide film 10 is roughened by a blast of air that 
blows abrasive particles, such as aluminum oxide or steel grains, against 
the oxide film 10 in order to form a roughened surface 11 which has 
valleys extending into the cooking surface of the metal cookware 9. The 
roughened surface 11 has an average roughness (Ra) of 6-8 .mu.m, and the 
distance between peaks and valleys is about 25-60 .mu.m. Such a roughened 
surface 11 can facilitate the attachment and bonding of a succeeding 
coating thereon. 
The roughened surface 11 is cleaned with pressurized air which is blown 
thereagainst and is further coated with an anti-stick film of fluorocarbon 
resin. Referring to FIG. 7, suspended fluorocarbon resin is sprayed onto 
the roughened surface 11 by 0.5-3.5 kg/cm.sup.2 of pressurized air to form 
a primer coating 12. Fluorocarbon resin of the primer coating 12 embeds 
into the valleys and bonds tightly with the oxide film 10 and the cooking 
surface of the metal cookware 9. Afterwards, the metal cookware 9 is dried 
in a drying oven at about 50.degree.-120.degree. C. for about 5 minutes 
such that the primer coating 12 has a thickness of about 6-15 .mu.m and a 
high hardness. The suspended fluorocarbon resin is further sprayed onto 
the primer coating 12 to form a top coating 13. The metal cookware 9 is 
dried in a drying oven at a temperature of about 150.degree. C. for about 
10 minutes and is further heated at a temperature of about 
350.degree.-420.degree. C. for about 5 minutes such that the sintering 
process for the fluorocarbon resin is carried out. The top coating 13 has 
a thickness of about 15-25 .mu.m. The hot metal cookware 9 is then removed 
from the drying oven and is cooled forcibly with a strong fan in 
atmospheric conditions. 
The advantages of the present invention are described as follows: 
1. Since the roughness surface 11 formed on the scratch-resistant oxide 
film 10 has valleys extending into the cooking surface of the metal 
cookware 9, the fluorocarbon resin of the anti-stick film sprayed on the 
oxide film 10 embeds into the cooking surface. The fluorocarbon resin 
bonds both the oxide film 10 and the cooking surface such that the 
anti-stick film can attach tightly to the metal cookware. 
2. Owing to the sintering process for the fluorocarbon resin and owing to 
the provision of the oxide film, the resultant anti-stick film of 
fluorocarbon resin can have a good anti-stick characteristic and high 
hardness. 
3. Aside from the major component of fluorocarbon resin, the anti-stick 
film can further contain metal powders, such as titanium powder, or 
non-metallic powders, such as ceramic powder, which can combine with 
fluorocarbon resin by the use of a solvent and a surfactant. In this way, 
the resultant anti-stick film can have a hardness of 4-8H that is larger 
than a hardness of 2H of a conventional metal cookware. 
4. A cooking surface of a metal cookware which is treated by the method of 
this invention can have high hardness and a good anti-stick characteristic 
and can also have a relatively long service life when compared with a 
conventional metal cookware since the anti-stick film can attach firmly to 
the metal cookware. In using the metal cookware treated by the method of 
this invention, a metal frying utensil can be used and the cleaning of the 
metal cookware can be carried out completely and conveniently. 
While the present invention has been described in connection with what is 
considered the most practical and preferred embodiment, it is understood 
that this invention is not limited to the disclosed embodiment but is 
intended to cover various arrangements included within the spirit and 
scope of the broadest interpretations and equivalent arrangements.