The present disclosure relates to the use of specific disulfo-type fluorescent whitening agents for whitening paper or board.
It is well known that the whiteness of paper and board can be improved by the addition of fluorescent whitening agents (FWAs). The most important FWAs used in the paper and board industry are anilino-substituted bistriazinyl derivatives of 4,4′-diaminostilbene-2,2′-disulfonic acid (flavonic acid). From these FWAs disulfo-, tetrasulfo- and hexasulfo-types are known. The disulfo-type FWAs with no sulfonic acid groups at the aniline rings have a low solubility in water and a high affinity for cellulose fibres. They are especially suitable for use at the wet-end of paper making process. The hexasulfo-type FWAs with two sulfonic acid groups at each aniline ring have a high solubility in water and a low affinity for cellulose fibres. They are more specialty products when very high whiteness is desired. The tetrasulfo-type FWAs with one sulfonic acid group at each aniline ring exhibit a behaviour between the disulfo- and hexasulfo-type FWAs and are most commonly used for whitening paper or board.
For ease of handling and metering, the paper and board industry demands FWAs to be supplied in a liquid form, preferably as a concentrated aqueous solution, which should be stable to prolonged storage over a wide temperature range. Due to the low solubility of disulfo-type FWAs in water, currently solubilising auxiliaries such as urea, triethanolamine or diethylene glycol are added in amounts of up to 30% to provide storage stability for concentrated aqueous solutions of disulfo-type FWAs. These solubilising agents have no affinity to cellulose and contaminate the effluent from the paper mill, thus being undesired. EP-A-1 752 453 teaches storage stable solutions of disulfo-type FWAs which contain specific counter-ions for the sulfonic acid groups, which counter-ions are derived from specific aminoalkanols. WO 02/055646 A1 discloses concentrated aqueous solutions containing a mixture of two specific disulfo-type FWAs.
Alternatively, slurries or dispersions of disulfo-type FWAs in water are known, e.g. from EP 0 884 312 B1. However, in order to enable the metering of homogenous preparations into the papermaking process, usually stirring is required.