German specialty chemicals group Altana said on Sunday it had agreed to buy the global rheology business of U.S. chemicals maker Rockwood Holdings for $635 million to expand its additives portfolio.

With the acquisition - expected to close in the fourth quarter of 2013 - Altana's sales are expected to reach about €1.9 billion from more than €1.7 billion in 2012, Altana said.

Rheology additives are used in coatings, construction materials and personal care products.

The business is part of Rockwood's additive division, which it had bundled with its titanium dioxide business.

In March it had put the bundled unit on the block, only to dissolve the combination later.

This month Rockwood has revived its efforts to hive off its titanium dioxide unit and is in talks with prospective buyer Huntsman Corp., two people familiar with the matter told Reuters on July 2.