This week, the overall grades of three chemicals stocks are lower, 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. 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.

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. The share price of $20.79 is within reach of the 52-week high of $16.21. For more information, get Portfolio Grader’s complete analysis of CCC stock.

Rockwood Holdings, Inc. (ROC) experiences a ratings drop this week, going from last week’s 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. To get an in-depth look at ROC, 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.