German specialty chemical maker Altana has signed a deal to acquire Rockwood Holdings’ rheology additives business for $635 million. Rheology additives are used to optimize the viscosity of materials in applications including coatings and personal care products. The Rockwood business has about 340 staffers and operates plants in Germany, the U.K., and the U.S. It generated sales in 2012 of $191 million. Altana plans to complete the deal by year’s end.