The present invention relates to a coating composition for a precoated metal sheet.
The present invention relates to a precoated metal sheet resistant to trouble due to static electricity occurring due to rubbing of the coating when used for a household electrical appliance, building material, etc. Background Art
In the fields of building materials, household electrical appliances, sundry goods, automobiles, etc., in place of the conventional postcoat system of shaping the metal sheet, then assembling and coating it, much use is now being made of the precoat system of shaping a metal sheet coated in advance (precoated metal sheet, abbreviated as "PCM") and joining it to another shaped PCM to form the final product. Such use enables the coating process at the demand side to be eliminated and the problems of pollution and the environment caused by the waste coating etc. to be solved. Further, there are the merits that the space for the coating can be turned over for use for other applications etc. The amount of demand is consequently steadily growing. On PCM processing lines, however, the static electricity arising due to rubbing between the coating and some other material frequently causes the problems of dirt being picked up on the PCM surface or an electric shock being received when touching a charged PCM. JP 5-278170 A and Jp 5-279641 A disclose the method of adhering a protective sheet containing an antistatic agent on the surface of a PCM so as to prevent the PCM from being charged. This method, however, has the problem that trouble is taken for detaching the sheet and further that the cost of production of the PCM becomes higher. JP 9-254296 A and JP 10-16134 A disclose the method of .adding
to a PCM coating fluorine or another additive so as to lower the position of the PCM coating in the triboelectric series (to facilitate minus charging) or controlling the relative triboelectric series of the front and rear surfaces of a PCM to suppress the occurrence of static electricity. JP 2—18708 B describes"
that the resistance to dirt pickup of a coated metal sheet when irradiated with electron beam, the coated metal sheet being obtained by applying and curing, on a metal sheet, an electron beam—curing coating, is correlated with a half charge decay time obtained by subjecting the coated metal sheet to measurement by a static honest meter, and the coated metal sheet does not easily pick up dirt when the half charge decay time is shorter than 100 seconds (preferably 80 seconds).
These inventions, however, have problems. The inventions relating to the triboelectric series rely on only the relative positional relationship that the position of the coating in the triboelectric series is either high or low. They are not inventions based on quantitative thinking. With just this, it is difficult to reliably suppress the generation of static electricity. For example, even with two types of coatings with the same position in the triboelectric series, there are examples where the actual degree of dirt pickup due to static electricity will differ and it is a fact that the effects of the inventions are not very reliable. The reason is that there are factors affecting the chargeability other than the triboelectric series such as the shape, hardness, tackiness, etc. of the coating and these combine to determine the chargeability of the coating and in turn the dirt pickup. To reliably obtain a PCM with a good effect of suppression of charging, an invention has to be based on a quantitative indicator comprehensively including these factors. As_ described later in detail, although the aforesaid half charge decay time by a static honest meter would be an indicator of
chargeability in a certain aspect of a PCM, it is not definitely correlated with trouble due to static electricity, which is a problem encountered in a PCM processing line, and cannot be a suitable indicator. The present invention attempts to provide a PCM able to reliably suppress charging of a coating by static electricity without greatly raising the cost. Disclosure of the Invention
To achieve the above object, it is useful to discover a quantitative indicator comprehensively including the various factors affecting the chargeability and devise means for controlling the value. The present inventors discovered that by using as such a quantitative indicator the charged voltage of a coating when charging a PCM under suitable conditions, it is possible to distinguish the degree of trouble due to charging (dirt pickup of PCM and electric shock). In addition, they measured the charged voltage for various types of PCMs and determined the boundary values where trouble does not occur. Further, they conducted various searches for additive use resins or additives useful for lowering the charged voltage and thereby perfected the present invention. The gist of the present invention is as follows:
(1) A coating composition for a precoated metal sheet resistant to trouble due to static electricity, the coating composition being for fabricating a precoated metal sheet by coating and curing it on a metal sheet as a topcoat, said coating composition characterized in that a value of charged voltage obtained when measuring said precoated metal sheet by a "Method 1" described below is less than 0.15 kV: (Method 1)
Indoors in a standard state (temperature 23°C and relative humidity 50%), a chloroprene rubber sheet cut to a 5 x 10 cm size, having a hardness of 50, having a content of carbon black of 31 wt%, and having a thickness
of 5 mm is superposed at the center of a target coated surface of a flat smooth precoated metal sheet cut to a 7 x 15 cm size. This assembly is placed on a horizontal ceramic table with the precoated metal sheet at the bottom, a 1 kg weight was placed on the chloroprene rubber sheet for 10 seconds to press bond the rubber sheet to the precoated metal sheet, then the weight is removed, and the chloroprene rubber sheet is peeled off in the vertical direction, after which the charged voltage at the center of the coated surface of the precoated metal sheet is immediately measured by a non-contact type field meter.
(2) A coating composition for a precoated metal sheet resistant to trouble due to static electricity, the composition being for fabricating a precoated metal sheet by coating and curing it on a metal sheet as a topcoat, said coating composition characterized in that a value of charged voltage obtained when measuring said precoated metal sheet by a "Method 2" described below is less than 0.25 kV:
(Method 2)
The precoated metal sheet, chloroprene rubber sheet, ceramic table, and weight described in Method 1 of (1) are heated in a 70°C oven for 10 minutes, and then taken out, after which the process described in Method 1 is all completed within 30 seconds.
(3) A coating composition for a precoated metal
sheet as set forth in (1) or ( 2 ) , characterized in that
the coating composition contains an isocyanate
derivative.
(4) A coating composition for a precoated metal
sheet as set forth in (1) or (2), characterized in that
the coating composition simultaneously contains an
isocyanate derivative and an alkoxyamine salt.
(5) A coating composition for a precoated metal
sheet as set forth in (3) or (4), characterized by
containing as the isocyanate derivative an isophorone
diisocyanate (IPDI) derivative.
(6) A coating composition for a precoated metal
sheet as set forth in (5), characterized in that the
ratio of the isophorone diisocyanate (IPDI) derivative to
the total solid content is at least 5 wt%.
(7) A coating composition for a precoated metal
sheet as set forth in any one of (4) to (6),
characterized in that the ratio of the alkoxyamine salt
to the total solid content is at least 1 wt%.
(8) A precoated metal sheet fabricated by coating
and curing a coating composition as set forth in any one
of (1) to (7) on at least one surface of the metal sheet
as a. topcoat.
Best Mode for Carrying Out the Invention The present invention will be explained in detail next. Dirt pickup was previously mentioned first as trouble due to static electricity of a PCM. Electric shock is also sometimes a problem, but this occurs when a large amount of static electricity is accumulated due to considerable friction. The problem of dirt pickup occurring due to a relatively small amount of static electricity is more serious. A case is shown where an actual problem resulted from dirt pickup. When picking up by suction arid conveying by a chloroprene suction device a PCM shaped to a housing on an assembly line of refrigerators of a household electrical appliance manufacturer, the static electricity occurring when the suction device sticks to the PCM coating and then is pulled off caused the problem of metal dust or other dirt in the line adhering to the portion of the coated surface which the suction device touched. With dirt picked up, the value of the product falls, so work to wipe it off became necessary and the cost rose greatly. To solve this problem, it is necessary to suppress the static electricity occurring when the PCM coating contacts another substance.
As indicators of the chargeability of a coating,
several physical quantities have been introduced up to now. One is the surface resistance (JIS K 6911). For a plastic, usually when the surface resistance is not more than 10 to the 12th to 13th power, it is considered that the conductivity of the material is sufficiently high and dirt will not be picked up due to the dispersion and discharge of the static electricity generated. In PCMs used for household electrical appliances, however, if the surface resistance is reduced, it is not possible to secure the corrosion resistance and other performance required, so a surface resistances of over this value have to be used. Accordingly, in household electrical appliance PCMs, it is meaningless to use the surface resistance as an indicator of the chargeability of a coating.
Second there are the initial charged voltage and the half charge decay time by a static honest meter (JIS L 1094). This technique measures the initial charged voltage when applying voltage to a coating to forcibly charge it, and the time from when the application of voltage is stopped to when the voltage falls to half of the initial charged voltage (half charge decay time). It is considered that the higher the initial charged voltage, the easier the charging and that the longer the half charge decay time, the harder the discharge. Therefore, the lower the initial charged voltage and the shorter the half charge decay time of a PCM, it is considered, the harder trouble due to static electricity like dirt pickup is likely to occur. When measuring the initial charged voltage and half charge decay time for various types of PCMs and comparing the obtained results with actual results of dirt pickup on an actual refrigerator assembly line, however, no dirt at all was picked up by PCM coatings with a relatively high initial charged voltage and long half charge decay time. That is, the theory of initial charged voltage and half charge decay time accepted until now could not explain this
actual dirt pickup phenomenon. These values therefore cannot be used as indicators for the chargeability of a coating.
Therefore, the present inventors made use of the charged voltage when charging a coating by bringing it into contact with and peeling it off from a suitable object (hereinafter called the "charged voltage after charging by peeling"). The point of difference of the charged voltage after charging by peeling from the initial charged voltage by a static honest meter is that the initial charged voltage is charged by non-contact application of a constant voltage and therefore becomes a voltage based only on the physical properties inherent to the coating, while the charged voltage after charging by peeling is charged by contact with and peeling from an object, so becomes a voltage encompassing not only the physical properties inherent to the coating, but also factors relating to affinity with the object such as the shape or tackiness of the coating. As a result, the charged voltage after charging by peeling is superior as an indicator for the total evaluation of the chargeability of a coating. The charged voltage can be simply measured by a commercially available non—contact type field meter.
Further, the charged voltage corresponds substantially completely to the degree of the dirt pickup. This was confirmed by the present inventors by the following method. Various types of PCMs, such as PCMs found in practice to be susceptible to dirt pickup, PCMs resistant to pickup, PCMs with thick coatings, PCMs with thin coatings, PCMs obtained using solvent—based coatings, PCMs obtained using powder coatings, PCMs having coatings on their reverse, and PCMs not having coatings on their reverse, were cut into A4 sizes, placed perpendicularly on a nonconductive table, and rubbed in that state with choroprene rubber to forcibly charge the coatings to charged voltages of 0.1, 0.2, 0.3, 0.4, 0.5,
and 0.6 kV. To match the charged voltages to the predetermined voltages, the method was adopted of charging the coatings by friction until slightly higher voltages, and bringing a conductive bar into light contact with the PCMs to discharge them while viewing a field meter until the predetermined charged voltages. After reaching the predetermined charged voltages, 1 cm square cut pieces of OHP film were placed on the charged PCM coatings as substitutes for dirt and it was confirmed if they fell off by their own weight.
As a result of this test, it was found that completely regardless of the type of PCM, the OHP film fell off at a charged voltage of less than 0.4 kV and did not fall off and did adhere to the PCM at one of 0.5 kV or more. Of course, the way the charged voltage rose when rubbing with the chloroprene rubber differed depending on the type of PCM, but if forcibly charging to 0.5 kV even PCMs difficult to raise in charged voltage, the OHP film adhered thereto. From this fact, it can be said that the degree of dirt pickup is substantially completely determined regardless of the type of PCM once the charged voltage of the coating is determined. In other words, PCM coatings resistant to dirt pickup are low in charged voltage after charging by peeling (under the same conditions) and definitely will not be free from dirt pickup even if the charged voltage is high.
Further, once adhered OHP film will remain adhered even if the coating is susequently discharged and the charged voltage becomes 0. This is because the portion where the OHP film contacts the surface of the coating is already neutralized in charge and becomes 0, and even if discharged from this state, the charge balance at the adhered part does not change. Therefore, the charged voltage of the coating at the instant of dirt pickup determines the degree of dirt pickup. Even if using an easily dischargeable material where the charge becomes zero quickly after dirt pickup, this has no effect on the
reduction of dirt pickup. From the above discussion, it is possible to treat a low charged voltage after charging by peeling and a small dirt pickup due to static electricity as being equal.
Method 1 and Method 2 in the present invention both present conditions for measurement of the charged voltage after charging by peeling. Method 1 calls for superposing a chloroprene rubber sheet cut to a 5 x 10 cm size, having a hardness of 50, having a content of carbon black of 31 wt%, and having a thickness of 5 mm at the center of a target coated surface of a flat smooth precoated metal sheet cut to a 7 x 15 cm size indoors at a temperature of 23°C and relative humidity of 50%, placing this assembly on a horizontal ceramic table with the precoated metal sheet at the bottom, placing a 1 kg weight on top for 10 seconds to press bond the rubber sheet to the precoated metal sheet, gently removing the weight, then peeling off the neoprene rubber sheet in the vertical direction, and subsequently measuring immediately the charged voltage at the center of the coated surface of the precoated metal sheet by a non-contact type field meter. A PCM having a charged voltage as obtained by Method 1 of less than 0.15 kv will not suffer from dirt pickup due to static electricity at room temperature.
On the other hand, Method 2 calls for heating the precoated metal sheet, chloroprene rubber sheet, ceramic table, and weight in advance in a 70°C oven for 10 minutes, taking them out, then completing the process described in Method 1 within 30 seconds. A PCM having a charged voltage as obtained by Method 2 of less than 0.25 kV will not suffer from dirt pickup due to static electricity even at a high temperature of 50 to 100°C. Resistance to dirt pickup at a high temperature is required for example in the process of fabrication of refrigerators. After shaping of a PCM into a
urethane foam is injected as an insulating
material. Due to the heat of reaction of the urethane, the temperature rises to about 80°C. It is required that no dirt pickup due to static electricity occur even in this state. If the temperature of the coating is high, generally the static electricity generated by contact and peeling becomes greater. It is believed that this is because at the time of a high temperature, the coating softens and sticks when contacting another object and therefore the effective contact area rises.
Inclusion of an isocyanate derivative in the PCM coating is extremely effective for suppressing the charged voltage after charging by peeling. The reason is not clear, but it can be understood that by including an isocyanate derivative, the position of the coating in the triboelectric series becomes lower and the relative position with respect to the chloroprene rubber becomes closer, so the amount of static electricity generated is suppressed.
The isocyanate derivatives include blocked monomers, dimers and trimers of various isocyanates and blocked prepolymers having such an isocyanate in their skeleton. Representatives of the isocyanates are IPDI (isophorone diisocyanate), TDI (tolylene diisocynate), MDI (4,4'-diphenylmethane diisocyanate), HMDI (hexamethylene diisocyanate), hydrogenated MDI, hydrogenated XDI (xylylene diisocyanate), hydrogenated TDI and the like. Blocking agents are represented by methanol, ethanol, butanols, propanols, phenol, cresols, chlorophenols, nitrophenols, hydrophenols, acetylacetone, ethyl acetoacetate, ethyl malonate, caprolactam, phosgene, 1-chloro-2-propanol, MEK oximes and the others. Types of isocyanates and types of blocking agent are not limited to the above.
Using an isophorone diisocyanate (IPDI) derivative as the isocyanate derivative is preferable since yellowing at the time of baking the PCM coating and yellowing along with the long elapse of time are
suppressed. Further, if the ratio of the IPDI derivative to the total solid content is at least 5 wt%, the effect of suppression of static electricity becomes more remarkable. With respect to the effect of suppression of static electricity, there is no upper limit to the amount of IPDI derivative added. However, it is preferable that the derivative is added at one's discretion in an amount as required taking into account that the use of too large an amount may not only lead to a saturation of the effect and be uneconomical, but also reduce a performance such as processability.
Inclusion of an alkoxyamine salt into the PCM coating along with the isocyanate derivative is effective for synergistically suppressing the charged voltage after charging by peeling. The reason is not clear, but it may be possible that the inclusion of the alkoxyamine salt causes the dielectric constant of the coating to rise and the capacitance (effect of charge accumulation) of the coating to rise, so the potential of the coating surface layer falls and the charged voltage falls. When the ratio of the alkoxyamine salt to the total solid content is at least 1 wt%, the effect of suppression of static electricity becomes particularly remarkable. The alkoxyamine salts include, but are not limited to, Flowlen AE —2 manufactured by Kyoeisha Chemical Co., Ltd., SN Stat 824 manufactured by San Nopco, Ltd., and Disparlon 1121 manufactured by Kusumoto Chemicals, Ltd., for example.
With respect to the effect of suppression of static electricity, there is no upper limit to the amount of alkoxyamine salt added. However, it is preferable that the alkoxyamine salt is added at one's discretion in an amount as required taking into account that the use of too large an amount may not only lead to a saturation of the effect and be uneconomical, but also reduce a performance such as processability.
As the resin used for the coating of the present
invention, an organic resin, such as a type of a high molecular weight polyester resin, polyester resin, epoxy resin, acrylic resin, urethane resin, fluororesin, vinyl chloride resin, olefin resin, or ketone resin, or an inorganic resin, such as a type of siloxane, boron, or borosiloxane, or organic/inorganic compound type resin having a siloxane, borosiloxane, or other inorganic skeleton introduced into the organic resin may be used. As the curing agent, a melamine resin, phenol, isocyanate, or a combined system thereof may be used. The metal sheet used as the base material of the present invention may be any of a cold rolled steel sheet, hot rolled steel sheet, various types of plated steel sheet (for example, zinc plated, zinc alloy plated, tin plated, lead plated, aluminum plated, chrome plated steel sheet etc.), stainless steel sheet, titanium sheet, aluminum sheet, etc. These may be used as they are or with usual chemical conversion treatment. Further, to improve the adhesion between the metal sheet and coating, it is possible to coat as an undercoat of the metal sheet for example nylon, polyacryl, polyethylene,
polypropylene, polyester, polyurethane, epoxy, polyamide, phenol, polyolefin, etc.
As the method of producing the PCM of the present invention, it is possible to produce it by a similar method as the usual one on a line for producing an ordinary PCM. As the method for coating the surface of the metal sheet, dipping, curtain flow, roll coat, bar coat, electrostatic coat, brushing, T-die, laminate, or any other method may be used.
As the baking method, a heating methods using hot air, ordinary temperature, near infrared rays, far infrared rays, high frequency induction heating, or a combination thereof may be mentioned. Examples
The present invention will be explained by the following examples and comparative examples .
As the original sheet of the PCM fabricated, a 0.6 mm thick hot dip galvanized steel sheet (YP: 19 kg/mm2, TS: 34 kg/mm2, EL: 45%) (hereinafter abbreviated as "GI"), a 0.6 mm thick electrolytic galvanized steel sheet (mechanical characteristics same as GI, abbreviated as "EG"), and 0.6 mm thick cold rolled steel sheet (mechanical characteristics same as GI, abbreviated as "CR") were used. As pretreatment, coating type chromate treatment and zinc phosphate treatment (bonderizing treatment) were performed on the front and reverse by the same specifications under standard conditions.
The coating on the front surface was made as a 2-coat, 2-bake coating of a type of an undercoating and a topcoating. The top coating corresponds to the topcoat. As a primer, a polyester-based coating A or epoxy—based coating B was used. This was coated by the roll coat method to a dry thickness of 5 µm, and then baked in a
hot air oven at a PMT (plate maximum temperature) of 215°C. On top of this, the various types of coatings shown in Table 1 were coated as topcoats by the roll coat method to a dry thickness of 15 µm and then baked in a hot air oven at a PMT of 230°C. The reverse surface was in each case coated with a melamine alkyd-based reverse surface coating by the roll coat method to a dry thickness of 5 µm/ then baked twice in a hot air oven at PMTs of 215°C and 230°C.
The coatings for the topcoat shown in Table 1 were all white coatings made by Nippon Paint Co., Ltd. colored by titanium pigments. The resins used were C (high molecular weight polyester/melamine curing system), D (high molecular weight polyester/phenol curing system), and E (acrylic/melamine curing system). The added isocyanate derivatives used were monomers of IPDI (isophorone diisocyanate), MDI (4,4'-diphenylmethane diisocyanate), and HMDI (hexamethylene diisocyanate) respectively blocked with caprolactam. As an alkoxyamine
salt, F (a tertiary ammonium salt: Disparlon 1121 manufactured by Kusumoto Chemicals, Ltd.) was used. Further, as other additives, G (aforesaid chioroprene rubber which was freeze crushed and classified: maximum particle diameter 200 micrometers), and H (PTFE (polytetraftuoroethylene) powder), each of which was added in an amount of 10 wt% to resin C, were prepared.
The charged voltages according to Method 1 and Method 2 were measured as follows:
In Method 1, a flat smooth PCM cut to a 7 x 15 cm size was placed on an insulating ceramic cup with its coated surface for measurement up indoors at 23°C and a relative humidity of 50% and temporarily grounded to reduce the charge of the PCM to zero. Next, a chioroprene rubber sheet (product number: Black 350, made by Katsura Roller Mfg. Co., Ltd.) cut to a 5 x 10 cm size, having a hardness of 50, having a content of carbon black of 31 wt%, and having a thickness of 5 mm was placed at the center of the PCM, and a 1 kg weight was placed on top for 10 seconds to press bond the rubber sheet to the PCM, the weight having a flat bottom face so as to equally apply a load to the entire surface of the chioroprene rubber sheet. The weight was gently removed, then the neoprene rubber sheet was peeled off within 3 seconds in the vertical direction and the charged voltage at the center of the coated surface of the precoated metal sheet was then measured within 3 seconds by a Field Meter FMX-002 made by the Simco Company, Inc. The voltage was measured five times under the same conditions and the average taken. In Method 2, five sets of the PCM, chioroprene rubber sheet, ceramic cup, and weight were prepared. These were heated in a 70°C oven for 10 minutes, then taken out one set at a time. The work for measurement of the charged voltage of Method 1 was completed within 30 seconds of taking out a set. The values thus obtained in five tests were averaged.
The resistance to dirt pickup and the resistance to
electric shock were investigated by bringing various PCMs into an actual refrigerator assembly line. For the resistance to dirt pickup, the degrees of dirt pickup after conveyance of the housing by a chloroprene suction device in the step before injection of urethane foam (ordinary temperature) and the step after injection of urethane foam (where surface of PCM coating rises in temperature to about 70°C) were evaluated. Examples with remarkable dirt pickup are shown by "Poor", ones with some dirt pickup observed by "Fair", and ones with no dirt pickup observed as "Good.
The resistance to electric shock was evaluated by whether an arc discharge occurred when touching the end face of a PCM by a metal rod after a process of vigorously rubbing a PCM coating and covering material of a belt conveyor. Examples where arc discharges occurred are shown by "Poor", ones where they occurred somewhat as "Fair", and ones where they did not occur as "Good".
Regarding yellowing at the time of baking, using Comparative Example 1 where no isocyanate derivative was added and yellowing was not observed as a reference, Examples exhibiting remarkable yellowing of the baked coating when seen visually are shown by "Poor", ones where some yellowing is observed as "Fair", and ones where no yellowing at all is observed in the same way as in Comparative Example 1 as "Good".
(Table Removed)
EG: electrolytic galvanized steel sheet, GI •. hot dip galvanized steel sheet, CR: cold rolled steel sheet A: polyester, B: epoxy
C: high molecular weight polyester/melamine curing system, D: high molecular weight polyester/phenol curing system, E: acryl/melamine curing system
IPDI: isophorone diisocyanate, MDI: 4,4'-diphenylmethane diisocyanate, HMD!: hexamethylene diisocyanate F: tertiary ammonium salt (Disparone 1121 made by Kusumoto Kasei} G: pulverized chloroprene rubber (maximum particle size 200 µm.) H: PTFE (polytetrafluoroe thylene) powder *1 All isocyanate derivatives used were caprolactarn-blocked monomers.
*2: The contents in the table axe the solid content of additive in wt% for the total solid content of the coating.
Looking at Examples 1 to 42 and Comparative Examples 1 to 10, it is understood that there is a high correlation between the charged voltage after charging by peeling and the resistance to dirt pickup. If the charged voltage according to Method 1 is over 0.15 kv or the charged voltage according to Method 2 is over 0.25 kV, the resistances to dirt pickup at ordinary temperature and at 70°C fall, respectively.
Examples 1 to 42 have charged voltages according to Method 1 of less than 0.15 kv, so are excellent in resistance to dirt pickup at ordinary temperature. An increase in the amount of addition of the isocyanate derivative and an increase in the amount of addition of the alkoxyamine salt result in a fall in the charged voltage after charging by peeling as a general trend. Further, when using together the isocyanate derivative and the alkoxyamine salt, in particular with the Method 2, the charged voltage after charging by peeling falls synergistically. If these effects are sufficient, the resistance to dirt pickup at 70°C becomes excellent as in Examples 9 to 12, 15 to 17, 20 to 22, 27, 32, 33, 35, 36, 38, 39, 41 and 42. In Examples 23 and 24, which correspond to Comparative Example 1 in which the crushed chloroprene rubber was added and Comparative Example 1 in which the PTFE was added, respectively, the charged voltage after charging by peeling is lowered. It is considered that the addition of the chloroprene rubber or PTFE caused the relative position in the triboelectric series of the chloroprene rubber sheet, which was an object to be measured its charged voltage, to become close to that of the coating. As in these Examples, the resistance to dirt pickup can be improved, as long as the charged voltage after charging by peeling is adapted to fall within a certain range, with the method for the adaptation being not limited to the addition of the isocyanate derivative or alkoxyamine salt. Even if changing the type of the original sheet from GI to EG or
CR (Examples 37 to 42, Comparative Examples 9 and 10), even if changing the pretreatment from chromate to zinc phosphate (Examples 34 and 36, Comparative Example 6), or even if changing the undercoat from a polyester type to an epoxy type (Examples 31 to 33, Comparative Example 7), there is no significant change in the charged voltage after charging by peeling of the PCM and there is no change in the resistance to dirt pickup. From this, it is seen that the properties of the topcoat predominately affect the charged voltage after charging by peeling. If the resin of the topcoat is changed from the high molecular weight polyester/melamine curing system to the high molecular weight polyester/phenol curing system or acryl/melamine curing system (Examples 25 to 30, Comparative Examples 5 and 6 ) , a tendency is seen for the charged voltage after charging by peeling to rise as a whole, but the correlation between the charged voltage and resistance to dirt pickup is on the same line.
Electric shock resistance is also substantially correlated with the charged voltage after charging by peeling of a PCM. The yardstick for excellence of electric shock resistance can be said to be a charged voltage according to Method 1 of less than 0.1 kV.
when using IPDI, MDI, and HMDI as the isocyanate, a uniform effect of reduction of the charged voltage after charging by peeling is observed, but IPDI is best in resistance to yellowing at the time of baking. With HMDI (Examples 18 to 22), the resistance to yellowing falls somewhat, while with MDI (Examples 13 to 17), the resistance to yellowing falls greatly. Therefore, when resistance to yellowing is required, it is preferable to use IPDI.
Comparative Examples 1 to 10 all have charged voltages after charging by peeling outside of the range of the present invention, so were poor in resistance to dirt pickup.
For information, values of half charge decay times
measured by a static honest meter and measured values of surface resistances, which have been hitherto regarded as indicators for resistance to dirt pickup, are shown in the Table. The half charge decay time values are shown only for some of the Examples and some of the Comparative Examples. For the measurement, a static honest meter S-4104 made by Shishido Electrostatic, Ltd. was used, with an applied voltage of 8 kv, and a surface resistance meter ST-3 made by the Simco Company, Inc. was used.
With respect to the half charge decay time, a tendency for the addition of IPDI to make the half charge decay time longer and for the addition of alkoxyamine salt to contrarily make the half charge decay time shorter can be seen. Although the addition of IPDI as well as the addition of alkoxyamine salt have an advantageous effect on the resistance to dirt pickup, they have opposite tendencies with respect to the half charge decay time. Comparative Examples 1 to 4, which show a poor resistance to dirt pickup, show a relatively short half charge decay time. Accordingly, the prior view that the shorter the half charge decay time, the better the resistance to dirt pickup is not true here, and no correlation is found between the half charge decay time and the resistance to dirt pickup. It can thus be understood that the half charge decay time is not an indicator for the resistance to dirt pickup. On the other hand, with respect to the surface resistance values, all PCMs have a value of 10 to the 14th power ohms or larger, and it can be seen that this is also not an indicator for the resistance to dirt pickup.
Industrial Applicability
As explained above, according to the present invention, it becomes possible to provide a PCM able to reliably suppress charging of the coating by static electricity without greatly raising the cost.

WE CLAIM:
1. A coating composition for a precoated metal sheet resistant to
trouble due to static electricity, said coating composition comprising a
combination of a base resin and a curing agent selected from (1) a
combination of a high molecular weight polyester and a melamine curing
agent, (2) a combination of a high molecular weight polyester and a
phenol curing agent, and (3) a combination of an acrylic resin and a
melamine curing agent, characterized in that it contains an isocyanate
derivative in an amount of 5-15% by mass of the total solids of the
composition.
2. A coating composition for a precoated metal sheet as claimed in
claim 1, wherein the coating composition optionally contains an
alkoxyamide salt in an amount of 1-2% by mass of the total solids of the
composition.
3. A coating composition for a precoated metal sheet as claimed in
claim 1 or 2, wherein said isocyanate derivative is an isophorone
diisocyanate (IPDI) derivative.
4. A precoated metal sheet fabricated by coating and curing a coating
composition as claimed in any one of claims 1 to 3 on at least one
surface of the metal sheet as a topcoat by a method of the kind such as
herein described and giving a charged voltage of static electricity less
than 0.25 KV or 0.15 KV measured using the methods such as herein
described.