This week, the overall grades of four 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, Taminco Corp.’s (TAM) rating worsens to a D from the company’s C rating a week ago. TAM is a holding company that provides domestic and international scheduled air travel through the subsidiaries TAM Linhas Aereas and TAM Mercosur. The stock also gets an F in Earnings Surprise. For more information, get Portfolio Grader’s complete analysis of TAM stock.

This week, Calgon Carbon Corporation (CCC) drops from a C to a D rating. Calgon Carbon is engaged in products and services for purifying water and air. The stock has a trailing PE Ratio of 26.10. For a full analysis of CCC stock, visit Portfolio Grader.

This is a rough week for Rockwood Holdings, Inc. (ROC). The company’s rating falls to D from the previous week’s 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 changing hands at an unusually rapid pace, twice the rate of the week prior. 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.