Plastic-coated metal plate for car body

Details Plastic-coated metal plate for car body Plastic-coated metal plate for car body

2000-04-14

2003-05-20

Thibodeau, Paul (Department: 1773)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Physical dimension specified

C428S418000, C428S457000, C428S334000, C180S089100

Reexamination Certificate

active

06565966

ABSTRACT:

The present invention relates to a plastic-coated metal plate which is used for forming a car body and has an excellent chipping resistance.
In car bodies of passenger cars and light cars, a part to which equipments such as an engine and a chassis are not installed and which is composed mainly of sheet metal is called “a shell body” and is composed usually of “a main body” composed of an underbody, a side member, a roof, a cowl, an upper back and a lower back and “an outer covered part” such as a hood, a front balance, a front fender, a cowl louver, doors and a luggage (back doors).
These main body and outer covered part have so far been formed by cutting, molding and assembling metal plates into sizes and shapes of the respective constitutional parts, immersing them in a cation electrodeposition paint bath to coat the front sides, back sides and end faces thereof to form primer coat paint films and then applying an intermediate coat paint and a top coat paint on the outside parts.
In recent years, however, step saving, energy saving and a reduction in CO
2
in a coating line are strongly desired in coating car bodies. In addition thereto, a chipping resistance and a corrosion resistance of a combined paint film are requested to be further improved. In order to elevate this chipping resistance, it is proposed to provide a barrier coat for forming a visco-elastic paint film in a layer between these paint films, but there are the defects that a coating step is added and the production cost goes up.
Intensive investigations repeated by the present inventors in order to solve the problems described above in car bodies have resulted in finding that the problems described above can be solved at a stroke by using a metal plate coated with a plastic film having specific physical property values, and thus they have come to complete the present invention.
Then, provided according to the present invention is a plastic-coated metal plate for an automobile exterior plate which is a laminate metal plate coated on both faces or one face of the metal plate with a plastic layer, wherein the plastic layer has a surface tension of at least 30 mN/m, an elongation percentage of at least 30% at a tensile rate of 20 mm/minute/20° C. and a rupture strength of at least 600 kg/cm
2
at a tensile rate of 20 mm/minute/20° C.
The plastic-coated metal plate of the present invention for an automobile exterior plate shall be explained below in detail.
The plastic-coated metal plate according to the present invention can be applied to passenger cars and light cars and can similarly be applied as well to trucks, buses, auto-bicycles, special installation automobiles and the like.
Those which have so far been used for car bodies can similarly be used as the metal plate for coating with plastics and, materials therefor include, for example, metal plates of iron, steel, stainless steel, aluminum, copper and alloys containing these metals, and metal plates obtained by plating the surfaces thereof with zinc, zinc
ickel and iron, and they can be used by working into the form of a coil or a cut plate. The metal plates have suitably a thickness falling in a range of usually 0.3 to 2.0 mm, particularly 0.5 to 1.0 mm. The surfaces of these metal plates are preferably subjected in advance to suitable polishing treatment, degreasing treatment and phosphate treatment in order to elevate an adhesive property with a plastic layer and an anticorrosive property.
Plastics used for coating the metal plates shall not specifically be restricted in terms of materials therefor, and there can be used, for example, conventionally known thermoplastic resins or thermosetting resins including polyolefin resins such as polyethylene and polypropylene, polyester resins such as polyethylene terephthalate (PET), polycarbonate resins, epoxy resins, vinyl acetate resins, vinyl chloride resins, fluorine-containing resins, polyvinyl acetal resins, polyvinyl alcohol resins, polyamide resins, polystyrene resins, acryl resins, polyurethane resins, phenol resins and polyether resins. These resins may suitably contain color pigments and extender pigments.
These plastics in a state that they are coated on the surface of the metal plate in a layer have a surface tension of at least 30 mN/m, particularly 35 to 55 mN/m and more particularly 38 to 50 mN/m, an elongation percentage of at least 30%, particularly 50 to 300% and more particularly 60 to 250% at a tensile rate of 20 mm/minute/20° C. and a rupture strength of at least 600 kg/cm
2
, particularly at least 1000 kg/cm
2
and preferably 1000 to 2500 kg/cm
2
at a tensile rate of 20 mm/minute/20° C. (“mN” described above is an abbreviation of milli-Newton). If the plastic layer has a surface tension of smaller than 30 mN/m, the adhesive property with an intermediate coat paint film and a top coat paint film which are formed on the surface thereof is reduced, and interlayer peeling is caused in a certain case. Also, if the plastic layer has an elongation percentage of smaller than 30%, the paint film does not become satisfactorily flexible. Further, if it has a rupture strength of smaller than 600 kg/cm
2
, deformation and cracking rupture of the paint film are liable to be caused by an impact force, and therefore there is the possibility that the chipping resistance is reduced.
A method for allowing the plastic layer formed on the metal plate to fall in characteristic values having the ranges described above includes, for example, methods in which 1) a composition of the plastic layer is suitably selected; 2) the plastic layer is treated with chemicals or organic solvents or coated with a primer, a coupling agent or a surfactant; 3) the plastic layer is subjected on the surface thereof to grafting or colloid treatment; and 4) the plastic layer is subjected to irradiation with UV rays, vacuum discharge treatment, flame treatment, ozone treatment, plasma contact treatment or corona discharge treatment. The physical property values of the plastic layer can readily be controlled in the ranges described above by carrying out these methods suitably in combination. Among them, the corona discharge treatment can be carried out, for example, by carrying out three pass treatment of 40 to 50 KHZ at a maximum output of 1.0 Kw by means of a high frequency power source CG-102 type (manufactured by Kasuga Co., Ltd.).
The surface tension (&ggr;S) of the plastic layer is a value obtained by dropping deionized water and paraffin on the surface of the prepared plastic layer to determine a contact angle (&thgr;) to the plastic layer and applying it to the following equation. The contact angle (&thgr;) can be measured at 20° C. by means of CONTACT ANGLE METER (trade name, manufactured by KYOWA KAIMEN KAGAKU Co., Ltd.):
surface tension of deionized water:
&ggr;LW=72.8 mN/m
&ggr;LWd=29.1 mN/m
&ggr;LWp=43.7 mN/m
&thgr;w: contact angle of deionized water
surface tension of paraffin:
&ggr;LP=24.4 mN/m
&thgr;p: contact angle of paraffin
&ggr;
Sd=
¼
·&ggr;LP
(1+cos &thgr;
p
)
2
&ggr;
Sp
=[(1+cos &thgr;
w
)×&ggr;
LW−
2
{square root over ( )}&ggr;LWd·&ggr;Sd]
2
/4×&ggr;
LWp
&ggr;
S=&ggr;Sd+&ggr;Sp
The elongation percentage and the rupture strength of the plastic layer are values obtained by forming a plastic layer on a tin plate by a suitable method, separating it by a mercury amalgam method and then measuring the plastic layer of length 20 mm and width 5 mm at a tensile rate of 20 mm/minute and a temperature of 20° C. by means of a tensile tester (TENSILON UTM-II-20, trade name, manufactured by TOYO BOLDWIN Co., Ltd.).
The metal plate can be coated with the plastic film by conventionally known methods. It can be coated, for example, by such a method that a film-shaped or sheet-shaped plastic molded by extrusion molding, injection molding, calendar molding or compression molding is adhered on a metal plate; plastic which is molten by heating is extruded into a film or sheet, and it is adhered on a metal plate by pressing; or powd

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