Method for the production of steel sheet coated with a fluororesin film

A method of producing a steel sheet coated with a fluororesin film by heat-fusing the fluororesin film onto a surface of the steel sheet, comprising heating the surface of the steel sheet to a temperature higher by 30-120 degrees C. than a melting point of the fluororesin of the fluororesin film, and contact bonding the fluororesin film onto the steel sheet surface between a pair of rolls, while a surface of a roll facing the fluororesin film, in the pair of rolls, is heated to a temperature lower by 100-210 degrees C. than the melting point of the fluororesin of the fluororesin film.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method for the production of a steel sheet 
coated with a fluororesin film, and more particularly proposes a technique 
for strongly applying a fluoro- resin film containing fluorine atom 
therein onto a surface of a steel sheet. 
2. Background and Related Art 
In general, the fluororesin is excellent in the properties such as 
corrosion resistance, stain resistance, adhesion resistance, heat 
resistance, incombustibility and the like. Therefore, if a stainless sheet 
or the like can firmly be covered with a film of this fluororesin, the 
resulting coated steel sheet possesses both the above properties inherent 
to the film and the properties inherent to the stainless steel (strength 
and workability). 
The steel sheets coated with the fluororesin film having the above 
properties can widely be applied to a toasting mold for baking breads, 
cakes, confectioneries and the like, food cooking tools and food working 
tools, heating cook tools such as an inner plate of a microwave oven, an 
inner pot of a rice cooker, a top plate of a gas table and the like and 
further kitchen tool materials such as material for oven hood and so on. 
Therefore, it is desired to establish a technique for industrially 
producing the above steel sheets. 
In this connection, materials obtained by coating stainless steel sheet or 
plated steel sheet with a fluororesin have hitherto been disclosed in 
JP-A-61-137534, JP-A-61-138567 and JP-B-63-11147. In JP-B-59-16836 and 
JP-B-59-16837 are disclosed materials obtained by coating Al-plated steel 
sheet with a fluororesin. In JP-B-54-24434, JP-B-59-10304 and 
JP-A-63-126728 are disclosed materials obtained by coating Al or Al alloy 
body with a fluororesin film. In JP-A-63-168333 is disclosed a method of 
coating a steel sheet with a fluororesin film through a thermosetting 
adhesive. 
According to these conventional techniques, however, the coating by means 
of the adhesive is difficult to achieve and also the strong covering is 
difficult because the fluororesin film has low surface tension and hence a 
strong non-adhesion property. Particularly, there has not yet been 
developed an effective method of strongly applying onto the surface of the 
steel sheet with respect to fluororesin films containing two or more 
fluorine atoms per one molecule unit such as ethylene 
tetrafluoride-perfluoroalkylvinyl ether copolymer resin (PFA), ethylene 
tetrafluoride-propylene hexafluoride copolymer resin (FEP) and ethylene 
tetrafluoride-ethylene copolymer resin (ETFE) up to the present, so that 
the practical use of these films is actually delayed. 
In this point, the inventors have already proposed in JP-A-5-162243 a 
method wherein a mixture of fluororesin and heat-resistant resin is 
previously applied onto a surface of aluminum plated steel sheet, zinc 
alloying plated steel sheet or stainless steel sheet and a resin film of 
the same kind is applied thereon and pressed through rolls, during which 
the interface between the film and the steel sheet is heated to a 
temperature just above a melting point of the film to weld to the 
undercoat resin layer. 
However, this technique adopts a simple method of conducting heat pressing 
only by single heating, so that there are problems that the adhesion force 
between the film and the steel sheet becomes insufficient and that 
melt-breaking of the film and poor adhesion are caused because only the 
surface temperature of the steel sheet is controlled. 
It is a main object of the invention to adhere a fluororesin film 
containing two or more fluorine atoms per one molecule unit and having a 
poor adhesion property onto a surface of a stainless steel sheets or the 
like at a high adhesion force. 
It is another object of the invention to adhere continuously through roll 
pressing a fluororesin film containing two or more fluorine atoms per one 
molecule unit onto a surface of a metal without causing melt breakage and 
poor adhesion. 
SUMMARY OF THE INVENTION 
As a method for achieving the object of the invention, in view of the 
problem of the conventional technique that the melt breakage or poor 
adhesion of the film is brought about as the temperature of press roll 
becomes too low or high, the invention proposes a method of producing a 
steel sheet coated with a fluororesin film by heat-fusing the fluororesin 
film onto a surface of the steel sheet, characterized in that a surface of 
the steel sheet to be adhered with the fluororesin film is previously 
adjusted by heating to a temperature higher by 30.degree.-120.degree. C. 
than the melting point of the fluororesin, while a surface of a roll 
facing the film in a pair of rolls for contact-bonding the resin film onto 
the steel sheet surface is adjusted to a temperature lower by 
100-210.degree. C. than the melting point of the fluororesin. 
In this method, it is preferable that a primer comprised of a mixed resin 
of fluororesin and thermal-resistant resin is previously applied onto the 
surface of the steel sheet at a thickness of 3-15 g/m.sup.2 as a weight 
after the drying prior to the contact-bonding of the fluororesin film to 
the steel sheet. 
Furthermore, it is preferable that at least one heating among the steel 
sheet surface heating, the heating of primer-treated steel sheet surface 
and the roll surface heating is carried out by induction heating.

DETAILED DESCRIPTION OF THE INVENTION 
The method of the invention is characterized by the regulation of both 
temperatures, the steel sheet surface temperature and the surface 
temperature of the roll for pressing, without directing attention only to 
the regulation of the former. Thus, the resin film is contact-bonded at a 
fused state only in an interface to the steel sheet, so that the 
fluororesin film can continuously and stably be adhered to the surface of 
the steel sheet without bringing about the melt breakage and poor 
adhesion. 
The method of the invention will be described in detail below. 
In the method of the invention, it is required to adjust previously the 
surface of the steel sheet (the primer-treated steel sheet in case of the 
primer treatment) to be coated with the resin film by heating to a 
temperature higher by 30.degree.-120.degree. C. than the melting point of 
the fluororesin just before the roll pressing at a state of piling the 
fluororesin film onto the surface of the steel sheet. For example, it is 
necessary to heat the steel sheet to 340.degree.-430.degree. C. in case of 
PFA resin film having a melting point of 310.degree. C. or 
300.degree.-390.degree. C. in case of FEP resin film having a melting 
point of 270.degree. C. 
The reason why the surface temperature of the steel sheet is adjusted to 
the above range is due to the fact that when the temperature of the 
surface to be coated with the film does not reach a temperature higher by 
30.degree. C. than the melting point of the fluororesin, the surface of 
the fluororesin film to be applied is not fused and hence the adhesion 
between the film and the steel sheet surface or between the film and the 
primer becomes insufficient. On the other hand, when the surface 
temperature exceeds a temperature higher by 120.degree. C. than the 
melting point of the fluororesin, the film itself is melted and broken in 
the contact-bonding, or the possibility of degrading the adhesion property 
through thermal decomposition of the primer resin in case of the primer is 
foreseen. 
When the fluororesin film is contact-bonded to the thus 
temperature-adjusted steel sheet surface through a pair of rolls, it is 
required to adjust the surface of the roll facing the film to a 
temperature lower by 100.degree.-210.degree. C. than the melting point of 
the fluororesin. 
The reason why the surface temperature of the roll facing the film in the 
contact-bonding is adjusted to the above range is due to the fact that 
when the roll surface temperature exceeds a temperature lower by 
100.degree. C. than the melting point of the fluororesin, the cooling of 
the film surface is lacking and the film is at a molten state to cause a 
risk of melt-breaking the film, while when the roll surface temperature is 
less than a temperature lower by 210.degree. C. than the melting point of 
the fluororesin, the film is rapidly cooled and the interface to the film 
is not fused to cause the poor adhesion. 
Moreover, the surface temperature of the roll facing the steel sheet which 
does not directly contact with the film is not particularly restricted, 
but it is desirable that the temperature is kept in the range between the 
temperature by 100.degree. C. lower than the melting point of fluoro-resin 
film and that by 50.degree. C. higher than the melting point of the 
fluoro-resin. 
In this method, rubber rolls or the like having good wettability to the 
steel sheet surface and elasticity are favorably used as a pair of rolls 
for the contact-bonding. As a result, the rolls made from such a material 
are degraded by heat from the steel sheet, so that it is important that 
the roll itself should be avoided to use at a temperature not lower than 
300.degree. C. and is adequately cooled to maintain a level of not higher 
than 210.degree. C. 
In the method of the invention, as mentioned above, what extent the surface 
temperature of the steel sheet or the like is selected within the above 
range is controlled by the type and thickness of the film, surface 
temperature of the roll for contact-bonding and the like. For example, 
when the steel sheet or the primer-treated steel sheet is heated by 
passing through a tunnel furnace and thereafter taken out from the furnace 
and the film is pressed through the rolls, it is required to heat it to a 
higher temperature previously, anticipating a temperature drop. According 
to experience, the temperature drop is dependent upon factors such as 
sheet thickness, sheet passing rate, distance between furnace and press 
roll and the like, but is usually 10.degree.-50.degree. C., so that it is 
important to conduct the extra heating including the drop quantity. 
In the method of the invention, the heating of the steel sheet may be 
usually carried out by using a hot air type tunnel furnace, but is 
preferably conducted by using an electromagnetic induction heating 
furnace. Because, in case of the electromagnetic induction heating, only 
the interface (adhesion surface) between the steel sheet and the film may 
sufficiently be heated to improve the adhesion property of the film. 
Furthermore, when the primer is used, the primer is heated from the inside 
(steel sheet side) and the solvent residual quantity is comparatively 
small after heating. 
In the method of the invention, the fluororesin film-coated steel sheet is 
then cooled by slow cooling in air or quenching with water, and the 
influence upon the adhesion strength can be ignored irrespective of 
cooling methods. 
In the method of the invention, galvanized steel sheet, hot dip zinc-5% 
aluminum alloy plated steel sheet, hot dip galvannealed steel sheet, 
zinc-nickel alloy plated steel sheet, hot dip aluminum plated steel sheet, 
hot dip aluminum-silicon alloy plated steel sheet, 55% aluminum-zinc alloy 
plated steel sheet, stainless steel sheet or the like is preferably used 
as the steel sheet substrate. 
These steel sheets are favorable to have a thickness of about 0.2-1.6 mm, 
while the type thereof is not particularly restricted and is selected in 
accordance with use purpose. For instance, aluminum plated steel sheet and 
stainless steel sheet are suitable to the applications requiring high 
thermal resistance such as a top plate for a gas cooking stove and the 
like. 
Furthermore, in order to obtain a high film adhesion property in case of 
fluororesin film having a high melting point such as PFA, the steel sheet 
surface is heated to be a high temperature of about 400.degree. C. so that 
the galvanized steel sheet and the like are not suitable for such a 
requirement because the plated layer is molten. 
As the fluororesin film covering the above steel sheet material in the 
method of the invention, fluororesin films containing 2 or more fluorine 
atoms per 1 molecule unit and having a poor adhesion property, which 
include, for example, films of thermoplastic resins such as ethylene 
tetrafluoride-perfluoroalkylvinyl ether copolymer resin (PFA), ethylene 
tetrafluoride-propylene hexafluoride copolymer resin (FEP) and ethylene 
tetrafluoride-ethylene copolymer resin (ETFE) are effectively used. 
The thickness of the fluororesin film is not particularly restricted, but 
is preferably 10-100 .mu.m. 
Moreover, these films are naturally colorless and transparent, but there 
may similarly be utilized films subjected to any treatment, e.g. film 
containing pigment powder for coloring, film containing additives for 
imparting functionality and the like. 
In the method of the invention, if necessary, an under coat treatment of a 
metal is carried out prior to the adjustment of the steel sheet surface by 
heating. The under coat treatment is desirable to be carried out by the 
application of the primer. 
In addition to the application of the primer, the under coat treatment may 
be conducted by the other pretreatment such as a general method in which 
the steel sheet is polished by shot burst or wire blushing and then 
subjected to pickling or electrolytic etching, or a roll-on type chromate 
treatment. In case of the stainless steel sheet, when the sheet is 
subjected to the roll-on type chromate treatment and further primer 
treatment instead of the mere primer treatment, the film adhesion 
strength, thermal resistance and durability are considerably improved. 
As the primer for the under coating the steel sheet matrix, use may be 
preferably made of aqueous solution or organic solution of primers each 
comprised of a mixed resin of fluororesin powder selected from the group 
consisting of polyethylene tetrafluoride (PTFE), ethylene 
tetrafluoride-perfluoroalkylvinyl ether copolymer resin (PFA), ethylene 
tetrafluoride-propylene hexafluoride copolymer resin (FEP), ethylene 
tetrafluoride-ethylene copolymer resin (ETFE), chlorotrifluoroethylene, 
polyvinylidene fluoride and the like and a heat-resistant resin selected 
from the group consisting of polyether sulfone (PES), polyphenylene 
sulfide (PPS), polyimide, polyamideimide, epoxy resin, silicone resin, 
phenolic resin and the like. 
The mixing ratio of the fluororesin powder to heat-resistant resin is 
preferably within a range of 1:99-95:5 by weight ratio. 
The thickness of the primer is favorably 3-15 g/m.sup.2 after the drying. 
When it is less than 3 g/m.sup.2, the effect on improving the adhesion 
strength as the primer is poor, while when it exceeds 15 g/m.sup.2, the 
effect is saturated and also there is a possibility of lowering the film 
adhesion property in the working or of creating blister of the primer due 
to remaining gas in the primer. 
The examples according to the invention will be described as compared with 
comparative examples. 
EXAMPLE 1 
A fluororesin film is heat-fused to each of steel sheets treated with 
primer having different resin compositions shown in Table 1 under various 
conditions to produce a steel sheet coated with the fluororesin film (see 
FIG. 1). 
The primary adhesion property of the thus fluororesin film coated steel 
sheet are evaluated to obtain results as shown in Table 2. 
TABLE 1 
__________________________________________________________________________ 
Under coating Heat-fusing 
conditions conditions (.degree.C.) 
Roll-on Heating 
Temperature 
Film 
type Resin 
temperature 
of roll 
Melting 
Material 
chromate baking 
of steel 
facing 
point of 
Kind 
Kind 
treatment 
Resin method 
sheet the film 
film 
of 
No 
*1 *2 composition 
*3 (.degree.C.) 
(.degree.C.) 
(.degree.C.) 
film 
Remarks 
__________________________________________________________________________ 
1 1 absence 
PFA:1 A 340 208 308 PFA 
Invention 
2 1 presence 
PES:2 A 380 120 270 FEP 
Example 
3 2 absence 
PPS:1 A + B 
380 180 308 PFA 
4 3 absence A 400 170 308 PFA 
5 3 presence 
FEP:1 A 390 130 308 PFA 
6 3 presence 
PES:2 A 420 108 308 PFA 
7 3 presence 
-- A 360 165 270 FEP 
8 3 presence 
-- A 360 150 308 PFA 
9 3 presence 
-- B 350 160 308 PFA 
10 
3 presence 
-- B 330 160 270 FEP 
11 
1 absence 
PFA:1 A 320 260 308 PFA 
Comparative 
12 
2 absence 
PES:2 A 390 240 308 PFA 
Example 
13 
3 presence 
PPS:1 A 290 130 270 FEP 
14 
3 presence 
-- B 450 80 308 PFA 
15 
3 presence 
-- B 380 40 308 PFA 
16 
3 presence 
-- B 410 110 265 ETFE 
__________________________________________________________________________ 
*1: 1: 55% Al--Zn plated steel sheet, thickness 0.6 mm, plated amount 
AZ150 2: hotdip Al--Si alloy plated steel, thickness 0.6 mm, plated amoun 
40 g/m.sup.2 (oneside) 3: SUS430 stainless steel sheet, thickness 0.6 mm 
*2: Rollon type chromate solution ZMDS-1300AN, made by Japan Parkerizing 
Co., Ltd. 
*3: A: induction heating B: hot air heating 
TABLE 2 
______________________________________ 
Film adhesion 
No. property Remarks 
______________________________________ 
1 5 Invention 
2 5 Example 
3 5 
4 5 
5 5 
6 5 
7 5 
8 5 
9 4 
10 4 
11 2 Compar- 
12 4 * 1 ative 
13 1 Example 
14 3 * 1 
15 1 
16 2 
______________________________________ 
* 1: Melt breakage of film is created in the adhesion of film. The 
production stability is less. 
Moreover, as the heat pressing conditions of the fluororesin film in No. 
1-10 and No. 11-16 of Example 1, the temperature of the press roll facing 
the steel sheet is 150.degree.-250.degree. C. and line pressure of the 
roll is 0.3 kg/cm. And also, the roll-on type chromate treating weight is 
30 mg/m.sup.2 and the chromate coating is dried at 150.degree. C. for 30 
seconds. 
Further, the test method and evaluating method for the adhesion property in 
the above example are conducted under the following conditions. 
Test for adhesion property! 
It is carried out according to #-type Erichsen test method of JIS Z6744. 
Three test specimens used in the #-type Erichsen test method are provided 
by previously immersing two specimens in a boiling water for 1 or 2 hours 
and then drying in air for 30 minutes or without immersion in the boiling 
water. Each of #-type portion of these specimens is pushed out by 6 mm 
with Erichsen testing machine, and then the cut surface of the pushed film 
portion is floated by means of a knife and the film is forcedly peeled off 
therefrom by means of tweezers. 
The evaluation standard is shown in Table 3, in which the case of not 
floating all of three specimens is 5 evaluation point. 
TABLE 3 
______________________________________ 
Immersing time in boiling water 
No immersion 1 hour 
2 hours 
______________________________________ 
Evaluation 
5 .smallcircle. .smallcircle. 
.smallcircle. 
point 4 .smallcircle. .smallcircle. 
.DELTA. 
3 .smallcircle. .DELTA. 
x 
2 .smallcircle. x x 
1 x x x 
______________________________________ 
.smallcircle.: No float of film 
.DELTA.: Film is floated but can not be peeled off 
x: Film is floated and peeled off 
As seen from the results of Table 2, all of the fluororesin film coated 
steel sheets produced by the method of the invention are excellent in the 
film adhesion property as compared with those of the fluororesin film 
coated steel sheets according to the comparative examples. Furthermore, 
according to the method of the invention, the fluororesin film coated 
steel sheets can be produced continuously without creating the melt 
breakage of the film in the pressing of the resin film. 
INDUSTRIAL APPLICABILITY 
As mentioned above, according to the method of the invention, the 
fluororesin film containing 2 or more fluorine atoms per 1 molecule unit 
can be adhered strongly onto the surface of the steel sheet, and also the 
fluororesin film coated steel sheets having an excellent adhesion property 
can be produced continuously and economically. 
Therefore, the fluororesin film coated steel sheets according to the 
invention are applicable to not only materials for cooking tools, kitchen 
tools, an inner plate of a microwave oven and a top plate of a gas table 
but also materials for electron parts, printed circuit boards and the like 
and can be supplied as a cheap and excellent material suitable for wide 
applications.