Electrodeposition coating method using alternating current

A method for producing a coated article which comprises filling an electrodeposition coating vessel with an aqueous solution or dispersion of an electrodeposition coating composition containing as the paint binder the below-mentioned salt of a purified resin, immersing in said vessel an article to be coated (A) having an electrode area of S.sub.1 and an opposite electrode (B) having an electrode area of S.sub.2 in such proportions that the electrode area ratio S.sub.1 /S.sub.2 becomes 1.5 to 300, connecting the article to be coated (A) and the opposite electrode (B) to an alternating current electric source, and impressing an alternating current to the electrodes to deposit the aforesaid electrodeposition coating composition onto the article to be coated (A), wherein the salt of purified resin is:

• i. a salt of a polycarboxylic acid resin having an acid value of 5 to 50 which has a pK.sub.a (r) value, as defined by the formula (I) shown below, of 8.0 or more, an n value of 0.5 to 1.5 and a degree of neutralization of 0.3 to 1.5, and which contains 5 wt% or less of low molecular weight acidic impurities having a pK.sub.a (e) value, as defined by the formula (II) shown below, of at least 0.5 smaller than the pK.sub.a (r) value of the main component of the resin, ##EQU5## wherein pH is the pH value of an aqueous solution of the salt of the polycarboxylic acid resin;
• pK.sub.a (r) is the constant -log K.sub.a (r);
• K.sub.a (r) is the dissociation constant in water of the polycarboxylic acid resin;
• n is a parameter showing the degree of spreading of the polycarboxylic acid resin when the resin is dissolved or dispersed in an aqueous medium;
• and .alpha. is the degree of neutralization of the polycarboxylic acid resin with a base, ##EQU6## wherein pH is the pH value of an aqueous solution of the acidic impurities;
• pK.sub.a (e) is the constant -log K.sub.a (e);
• K.sub.a (e) is the dissociation constant in water of said impurities;
• and .alpha. is the degree of neutralization of said impurities with a base, or
• ii. a salt of a nitrogen-containing basic resin high in hydrophobic property which has a pK.sub.b (r) value, as defined by the formula (III) shown below, of 6.0 or more, an n' value of 0.5 to 1.5, and a degree of neutralization .alpha.' of 0.3 to 1.5, and which contains 5 wt% or less of low molecular weight basic impurities having a pK.sub.b (e) value, as defined by the formula (IV) shown below, of at least 0.5 smaller than the pK.sub.b (r) value of the main component of the nitrogen-containing basic resin, ##EQU7## wherein pOH is the pOH value of an aqueous solution of the salt of the nitrogen-containing basic resin;
• pK.sub.b (r) is the constant -log K.sub.b (r);
• K.sub.b (r) is the dissociation constant in water of the nitrogen-containing basic resin;
• n' is a parameter showing the degree of initial spreading of the nitrogen-containing basic resin when the resin is dissolved or dispersed in an aqueous medium;
• and .alpha.' is the degree of neutralization of the nitrogen-containing basic resin with an acid, ##EQU8## wherein pOH is the pOH value of an aqueous solution of the basic impurities;
• pK.sub.b (e) is the constant -log K.sub.b (e);
• K.sub.b (e) is the dissociation constant in an aqueous medium of said impurities;
• and .alpha.' is the degree of neutralization of said impurities with an acid.