The ratings of three chemicals stocks are down this week, according to the Portfolio Grader database. Each of these rates a “D” (“sell”) or “F” overall (“strong sell”).

This week, Tronox Ltd. (TROX) falls to a D (“sell”), worse than last week’s grade of C (“hold”). Tronox produces and markets titanium dioxide. The stock also earns F’s in Portfolio Grader’s specific subcategories of Earnings Momentum, Earnings Revisions, Equity and Margin Growth. Shares of the stock have been exchanging at an usually rapid pace, twice the rate of the week prior. To get an in-depth look at TROX, get Portfolio Grader’s complete analysis of TROX stock.

This week, Calgon Carbon Corporation’s (CCC) rating worsens to a D from the company’s C rating a week ago. Calgon Carbon is engaged in products and services for purifying water and air. Shares of the stock have been changing hands at an unusually rapid pace, three times the rate of the week prior. The trailing PE Ratio for the stock is 26.20. For more information, get Portfolio Grader’s complete analysis of CCC stock.

The rating of Rockwood Holdings, Inc. (ROC) slips from a C to a D. 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. For a full analysis of ROC stock, visit Portfolio Grader.

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.