The preparation of epoxy resin/H.sub.3 PO.sub.4 reaction products which--when amine-salified--have utility as water-borne film-forming materials is disclosed in U.S. patent application Ser. No. 19,958, filed Mar. 12, 1979 in the name of Patrick H. Martin as inventor, and now issued as U.S. Pat. No. 4,289,812. (The disclosure of said patent is incorporated herein, by reference, for all purposes which thereby may be legally served.)
In the preferred version of the disclosed process, about 1 phr of H.sub.3 PO.sub.4, is reacted, in the form of the 85% acid (.about.0.2 phr H.sub.2 O) for about 4 hours with a nominally difunctional epoxy resin which is a copolymer of bisphenol A with its diglycidyl ether and has an EEW (epoxide equivalent weight) of about 1500-2000. The reaction is carried out in a relatively hydrophobic solvent for the resin, such as an acetone/CH.sub.2 Cl.sub.2 mixture, which does not strongly solvate the acid or water and results in the formation of phosphate ester groups comprising from one to three epoxide residues each. This is accompanied by hydrolysis of higher ester to monoester groups and concurrent formation of terminal glycol groups on the cleaved-off resin molecules. Acid catalyzed, direct hydrolysis of oxirane groups (to glycol groups) also occurs to a limited extent.
The reaction mixture is mixed with water, triethylamine added in excess and the solvent and unsalified amine stripped off to yield an aqueous dispersion of the amine-salified product resin having a solids content of about 50 wt. %.
For efficient operation of the foregoing process on a plant scale, it must be carried out in a continuous manner and the recovered solvents and amine separated and recycled. This necessarily requires a relatively complicated plant installation incorporating separate vessel, pump, line and control instrument assemblies for each of several stages, i.e., reaction, water and amine addition, stripping and separation stages. It is thus evident that a simpler process, requiring a less complicated and costly plant installation, would be highly desirable. Also, a reduction in the reaction time would be desirable.
For the presently most important contemplated application of the coating resins made in the foregoing manner--i.e., as sprayed-on, beverage can interior coatings--the aqueous resin dispersion is formulated with certain "coupling" solvents, such as n-butanol, glycol monoethers, etc., which are high boiling but readily volatilized at the temperatures required (with curing agents such as melamine resins, for example) to cure the coatings. Examples 11, 18 and 20in the above-referred-to application disclose such formulations comprising glycol monoethers and said application also teaches that glycol ethers are suitable media for the epoxide/phosphoric acid reaction ("adduction"). Also, U.S. Pat. No. 4,059,550 (Shimp) discloses the use of the monobutyl ethers of ethylene and diethylene glycols as media for such adductions and the use of the resulting solutions (including an organic base) as catalysts in multi-component resin systems.
Thus, the possibility of eliminating the need for solvent removal from the reaction mixture formed in the above-described process, by utilizing a formulation solvent as the reaction medium, was considered--an implicit assumption being that the amount of solvent required would not be such as to result in an unacceptably high solvent-to-water or solvent-to-solids ratio for the contemplated application. That is, because the solvent would not be removed, a high enough solids content must be attainable in the reaction mixture so that the solids level after dilution with water and any other solvents in the final formulation will still be adequate for the requisite mode of application and the essential film-forming properties of the coating formulation. Also, the content ot volatilizeable organic components (VOC) in the final formulation would have to be low enough not to result in an unacceptable level of emissions in the curing step. The matter of reaction mixture viscosity at adequately high solids levels is also of concern (with regard to stirring power requirements).
The monobutyl ether of ethylene glycol (DOWANOL-EB; or simply "EB" hereinafter) was tried as the medium for reactions of 1 phr of H.sub.3 PO.sub.4 (as 85% aq. H.sub.3 PO.sub.4) with an epoxy resin (D.E.R.-667) represented by the following ideal formula, n having a value therein of from about 12 to 14, R.sub.1 being H and Q being the nucleus of bisphenol A: ##STR1##
The reactions were carried out under the conditions known to be suitable when acetone/CH.sub.2 Cl.sub.2 is employed as the reaction medium. The free H.sub.3 PO.sub.4 content of the reaction product was found to be low enough for applications not posing stringent water-resistance requirements but undesirably high for can coatings to be subjected to conditions commonly employed for pasturization. Correspondingly lower phosphomonoester contents were also experienced. These results were believed due to greater retardation of the adduction reaction than of the hydrolysis reactions, as a consequence of differences in (acid and water) solvation effects by the hydrophilic reaction medium employed.
This was despite the fact that the solvent only constituted about 40 wt. % of the initial reaction mixture (60% "solids" or non-volatiles).
At this point, the use of an anhydrous form (100% or higher) of phosphoric acid, which is taught as an operable option in the above-referred-to application, was considered, as this would permit the adduction (phosphorylation) to be carried out in the absence of water. However, the only example (#10) in said application of using such an acid source material (pyrophosphoric acid) discloses that gelling of the reaction mass occurred, requiring excessive dilution with more of the solvent (dioxane) in order to have a stirrable mixture, which is essential to usefully rapid, subsequent hydrolysis of phosphopolyesters (and any residual P--O--P groups).
The rapid development of a very high viscosity observed in the absence of concurrent hydrolysis is believed attributable primarily to formation of esters by adduction of two or three oxiranes per P.dbd.O moiety, which occurs even if 100% H.sub.3 PO.sub.4 (essentially no P--O--P groups) is used as the acid.
U.S. Pat. No. 2,541,027 (Bradley) discloses monoalkylorthophosphates as an alternative to the use of H.sub.3 PO.sub.4 in preparing adducts with epoxy resins. This was considered as a way of attaining some reduction in the number of long-chain branches per phosphopolyester group, thereby reducing the viscosity of the reaction mixture. However, this would also result in introduction of not necessarily desirable or readily hydrolyzed alkyl groups in the product resin. Furthermore, it was not apparent that excessive amounts of solvent would not still be required.
Consideration was also given to the use of dialkyl orthophosphates as the acid, but this would result in triesters containing two alkyl groups not necessarily desirable or more readily hydrolyzed than the &gt;PO--OC-- group formed by P--OH/oxirane adduction--thus posing the problem of whether the PO--OH required for salification could be formed without cleavage of the latter PO--OC group. A substantial reduction in the adduction rate would also be anticipated, at a given P/oxirane reactant ratio.
The prior art itself does not contemplate a "one-pot" process or address itself to the problems inherent in attempting to use a formulation solvent as the reaction medium for preparing H.sub.3 PO.sub.4 /epoxide adducts of high phosphomonoester contents and low free acid contents. Neither does it make obvious a solution to those problems. However, the process of the present invention, as defined in the following summary, does provide such a solution.