The invention relates to method of electrodeposition employing resinous reaction products containing cationic groups. More particularly, this invention relates to cationic resinous reaction products which are prepared from chain extended epoxide resins.
Cationic electrodepositable resins are known in the art. A preferred class of resins are those prepared from epoxy resins such as disclosed in U.S. Pat. No. 4,104,147 to Marchetti, Jerabek and Zwack.
This patent discloses chain extension of polyepoxides with organic polyols such as polymeric polyols. The chain extended products can then be reacted with a secondary amine and solubilized with acid to form cationic electrodepositable compositions. The resins have excellent properties such as high rupture voltage, good film forming properties and deposit as films with good flexibility.
One problem associated with polymeric polyol chain extension is competing reactions. Under chain extension reaction conditions, which are usually in the presence of an amine catalyst, epoxy-epoxy reaction and epoxy-secondary hydroxyl reactions compete with the desired polymeric hydroxyl-epoxy reaction. The competing reactions may consume too much of the epoxy functionality resulting in the presence of excess amine in the reaction product which adversely affects the dispersion properties of the resin as well as its throw power and film-forming properties. Also, these competing reactions if not controlled can present manufacturing difficulties, for example, undesirably high resin viscosities which are believed to be due to polymer branching.
An indication of this can be seen in FIG. 2 which is a plot of the reduced Gardner-Holdt viscosity (50 percent resin solids in 2-ethoxyethanol) versus time in hours of the reaction mixture which involves chain extension of a polyglycidyl ether of a polyphenol with a poly(oxytetramethylene) glycol having a molecular weight of 650. As shown in FIG. 2, the viscosity increases rapidly with time. If the viscosity is not carefully monitored, the reaction mixture could easily go to gelation. This is a possibility in a commercial production situation where the operator in charge of the reaction may not be able to monitor carefully the viscosity of the reaction with time.
It has been found that these problems can be significantly minimized by chain extending with a polymercapto compound, particularly a polymeric polymercapto compound. It is believed that under chain extension reaction conditions, the mercapto-epoxy reaction goes in relatively high yield with a minimum amount of competing side reactions. This results in a resin which has better properties and which is easier to manufacture, particularly on a commercial scale.
Reference is made to FIG. 1 which is a plot of the reduced Gardner-Holdt viscosity versus time for Example I of the present invention. Example I involves chain extension of a polyglycidyl ether of a polyphenol with a dimercapto polymer obtained from reacting poly(oxytetramethylene) glycol having a molecular weight of 650 with mercapto propionic acid in a molar ratio of 1:2. As shown in FIG. 1, the viscosity increases relatively slowly over the period of about 11/2 hours. At this point, the viscosity remains essentially constant with time. In commercial production, this can be important because if the operator in charge of the reaction is distracted and loses track of the time of the reaction, the viscosity of the reaction mixture will level out and not proceed to gelation.