Water normally contains alkali earth metal cations such as calcium, barium, magnesium, etc., and water "hardness" is directly related to the concentration of these cations in the water. Hard water reduces the effectiveness of detergents and soaps because these multivalent cations form insoluble salts with the anionic components of the detergent or soap, which precipitate from the detersive system and occlude dirt onto fabrics and form deposits ("scale") on sufaces and machine parts.
Detergent compositions generally contain one or more detergent builders capable of chelating or sequestering the multivalent cations. Detergent builders not only prevent the formation of insoluble precipitates (e.g., calcium soaps) but also improve detergency by deflocculating soil aggregates coagulated by the cations and breaking up the cation-enhanced binding of dirt to substrates.
Until the 1970's, the most widely used detergent builders were the condensed phosphates, especially sodium tripolyphosphate. However, with the discovery of the role of phosphates in the eutrophication of water systems, severe restrictions have been imposed on the use of phosphates in detergent compositions, and as a result there is a continuing need for the discovery of detergent builder compounds not based on phosphorus.
Many alternative detergent builders have been proposed, notably the alkali metal salts of nitrilotriacetic acid, e.g., trisodium nitrilotriacetate (NTA). Also, oxygen-based analogs of NTA, e.g., 1-oxacyclopropane-2,3-dicarboxylic acid and organic polymers such as polyacrylates, polymaleates, and polymethacrylates. See, e.g., U.S. Pat. Nos. 3,393,150, 3,666,664, 3,707,502, 3,839,215, and 4,067,816, all of which are incorporated herein by reference.
The aforementioned polymeric detergent builders are effective; however as a general rule their usefulness increases as their molecular weight increases and at high molecular weight, synthetic organic polymers give rise to further environmental concerns. This is because even though such polymers do not contribute any phosphorus to water systems, the organic polymers are not biodegradable and consequently remain in the environment, virtually unchanged, for an extremely long time.
A primary reason for the non-biodegradability of synthetic polymers is that since they do not occur in nature, no enzymes or microorganisms have yet evolved that can attack the synthetic polymer chains or utilize the polymers as food. Recent research indicates that the uninterrupted carbon backbone of common synthetic organic polymers is only sparingly susceptible to biological cleavage.
Recent research indicates that introduction of a hydrolyzable group such as an ester linkage into the backbone of organic addition polymers gives a copolyester which is biodegradable on a reasonable time scale but retains many of the physical properties of the original, non-biodegradable addition polymer. (See, e.g., W. J. Bailey, "The Design of New Biodegradable Polymers", 6th International Symposium on the Stabilization and Controlled Degradation of Polymers, Lucerne, Swiz. (June 6, 1984); W. J. Bailey and B. Gapud, "Synthesis of Biodegradable Polyethylene", American Chemical Society Symposium Series No. 280: Polymer Stabilization and Degradation, pp. 423-31 (1985).
It has now been discovered that copolymers can be prepared from methylene-substituted heterocyclic compounds and acrylate compounds via a ring-opening addition reaction to provide high molecular weight, carboxy-functional polymers useful, e.g., as detergent builders. The copolymers are characterized by active carboxylic acid groups and also by repeated ester linkages in the organic polymer chain. The carboxy functionality of the copolymers makes them effective metal cation chelators, and the ester units in the copolymer chain make the copolymers biodegradable.