This week, the ratings of three chemicals stocks on Portfolio Grader are down. Each of these rates a “D” (“sell”) or “F” overall (“strong sell”).

Tronox Ltd.’s (TROX) rating falls to a D (“sell”) this week, down from C (“hold”) the week prior. Tronox produces and markets titanium dioxide. In Portfolio Grader’s specific subcategories of Earnings Momentum, Earnings Revisions, Equity and Margin Growth, TROX also gets F’s. For a full analysis of TROX stock, visit Portfolio Grader.

Calgon Carbon Corporation (CCC) experiences a ratings drop this week, going from last week’s C to a D. Calgon Carbon is engaged in products and services for purifying water and air. To get an in-depth look at CCC, get Portfolio Grader’s complete analysis of CCC stock.

Rockwood Holdings, Inc. (ROC) earns a D this week, falling from last week’s grade of C. Rockwood Holdings is a global developer, manufacturer and marketer of value-added specialty chemicals and advanced materials used for industrial and commercial purposes. The stock gets F’s in Earnings Growth, Earnings Surprise and Margin Growth. Shares of the stock have been trading at an exceptionally rapid pace, up fourfold from the week prior. For more information, get Portfolio Grader’s complete analysis of ROC stock.

Louis Navellier’s proprietary Portfolio Grader stock ranking system assesses roughly 5,000 companies every week based on a number of fundamental and quantitative measures. Stocks are given a letter grade based on their results — with A being “strong buy,” and F being “strong sell.” Explore the tool here.