Granular automatic dishwashing detergent compositions and their components, e.g. builders, alkaline salts, sodium silicate, low-foaming surfactants, chlorine bleach, etc., are well known in the art. A number of processes have been described for the production of such dishwashing detergent compositions.
Various processes can be used in manufacturing a granular automatic dishwashing detergent composition. For example, U.S. Pat. No. 4,379,069, Rapisarda et al., issued Apr. 5, 1983 describes a mechanical mixing process whereby a silicate free alkaline blend of detergent ingredients is prepared followed by mixing of solid alkali metal silicate. Another example involves agglomeration of detergent ingredients (see U.S. Pat. Nos. 4,427,417, Porasik, issued Jan. 24, 1984, and 3,888,781, Kingry et al., issued Jun. 10, 1975).
Any residue from automatic dishwashing detergents that remains on the dishware after washing can be a problem. This residue has been evaluated analytically and has been found to be predominantly silicate. Alkali metal silicate is known to form insoluble matter when exposed to less alkaline environments and/or other conditions which promote polymerization (CO.sub.2 absorption, dehydration, etc.).
It has recently been found that a significant improvement in the solubility (i.e. decreased insoluble residue) of an agglomerated automatic dishwashing detergent composition can be achieved by using a liquid binder other than alkali metal silicate solution, such as an aqueous solution of a water-soluble polymer like sodium polyacrylate (Copending U.S. patent application Ser. No. 550,420, filed Jul. 19, 1990). It is known that during drying of the wet agglomerates, the water-soluble polymer does not form insoluble residue like alkali metal silicates do. Further, granules agglomerated with a water-soluble polymer such as polyacrylate will not develop insoluble particles during storage as do base granules which are agglomerated using an aqueous solution of silicate. The alkali metal silicate can be post-added as a dry solid to the agglomerated base product to lower the amount of insoluble residue formation.
Preferably, a relatively high level of nonionic surfactant is desired in an automatic dishwashing detergent because of its cleaning function as well as a "water sheeting" effect. The latter function is important in that it allows for water to more easily drain from tableware thus leaving the tableware with a spotless appearance. However, problems arise relating to nonionic surfactant levels when a concentrated granular automatic dishwashing detergent composition is made. In order to form a concentrated automatic dishwashing detergent composition, less filler, i.e. sulfate, is used in the agglomeration or manufacturing process, and significantly more active ingredients, including liquid ingredients, must be packed into the formula. There are fewer solids onto which these higher levels of liquid ingredients can be loaded. Because of the reduced amount of filler and the higher level of liquids, the amount of nonionic surfactant that can be added in the agglomeration or manufacturing process is reduced dramatically. It was thought that adding nonionic surfactant onto solid silicate would lower the pH in localized areas of the silicate particles, resulting in polymerization of the silicate and formation of insoluble residue (see U.S. Pat. No. 4,379,069, Rapisarda, issued Apr. 5, 1983, column 6, lines 46-52).
It has now been found that incorporating heated low foaming nonionic surfactant (which is solid at room temperature) into silicate particles, before the silicate is admixed with base granules, improves the solubility of an automatic dishwashing detergent composition. It also provides a means for incorporating a sufficient amount of nonionic surfactant into a concentrated detergent composition. Without meaning to be bound by theory, it is believed that the nonionic surfactant prevents further polymerization of the silicate thereby preventing the formation of insoluble residue.