Small ether polycarboxylate and acetal polycarboxylate builders are reviewed with respect to their synthesis, calcium and magnesium sequestration, and biodegradation. JSD 1, 105-108 (1998).KEY WORDS: Biodegradation, builder, calcium, ether or acetal polycarboxylate, magnesium, stability or binding constants.Builders have several functions in detergents: sequestration of calcium and magnesium, peptization and suspension of soil, and inhibition of inorganic incrustation. The first is most important, but most difficult to achieve. It is most important because the other functions are accomplished if free hardness is below 10 −5 mol/L (1,2). In this hardness regime, surfactants are not precipitated, flocculation of soils by calcium is reversed, and calcium carbonate is dissolved.Effective sequestration is most difficult to achieve because of the number of criteria that must be met: a calcium binding constant greater than about 10 5 [depending on the builder-to-hardness ratio (3)], weight efficiency, bleach stability, low cost, ready biodegradability, and low toxicity for humans and aquatic life. Small-molecule polycarboxylates composed only of carbon, hydrogen, and oxygen can potentially meet all these criteria as well as avoid the toxicological and metal mobilization issues of nitrogen-containing builders, such as nitrilotriacetic acid (4). In liquid formulations, they are soluble and chemically stable, and effectively bind calcium at the lower wash pH.At present, citric acid is the only small-molecule polycarboxylate builder in commercial use, even though it does not meet the sequestration criteria and is not low-cost. Current formulations are designed around two lower-cost builders: zeolite A, in granular detergents, and fatty acid, in liquid detergents. Citric acid is well suited as a cobuilder in both cases. While ion exchange in zeolites and calcium precipitation of fatty acid can be slow, citrate can reduce free hardness levels rapidly. It also binds magnesium effectively (which zeolite A does not) and can shuttle calcium from wash solution to zeolite A (5,6). Nonetheless, the zeolite A/citric acid builder system does not achieve the low free hardness levels required for good soil suspension/peptization and prevention of incrustation, so polymeric dispersants are added. The surfactant system is also adjusted to be more calcium-tolerant.A small-molecule polycarboxylate builder has yet to be found that can replace zeolite A and fatty acid. The primary reason is high cost. Nevertheless, much has been learned about builder design and will be relayed in this review. Synthesis will be covered in depth because of its impact on cost.
SEQUESTRATION OF CALCIUM AND MAGNESIUMDesign principles can be stated as follows: at least three 5-or 6-membered chelate rings are necessary; oxygen substituents are preferred in the following order: carboxylate > ether > hydroxyl > acetal; and at least three carboxylate groups are required. Stereochemistry of the chelate rings should avoid strong steric interactions. The principles can be...