"We already own more than 15 percent and we're not done buying," White Tale investors David Millstone and David Winter told Finanz und Wirtschaft in a joint interview released on Friday.
The next disclosure threshold would be 20 percent. They had previously reported a stake of just over 15 percent.
Calling themselves "long-term oriented investors" who are "here to stay", Millstone and Winter reiterated their opposition to the planned merger that would give Clariant 52 percent of the combined entity should shareholders approve.
"We want Clariant to become a better and stronger company, and we don't see that happening with Huntsman."
The proposed deal, they said, significantly undervalued Clariant and overvalued Huntsman.
The Swiss chemical manufacturer should instead sell its plastics and coatings business, they said, and reinvest the proceeds into acquisitions within the higher-margin specialty chemicals businesses.
(Reporting by Brenna Hughes Neghaiwi; Editing by Michael Shields)