This week, the ratings of four 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. The stock also earns F’s in Portfolio Grader’s specific subcategories of Earnings Momentum, Earnings Revisions, Equity and Margin Growth. To get an in-depth look at TROX, get Portfolio Grader’s complete analysis of TROX stock.

Calgon Carbon Corporation (CCC) is having a tough week. The company’s rating falls from a C to a D. Calgon Carbon is engaged in products and services for purifying water and air. For more information, get Portfolio Grader’s complete analysis of CCC stock.

Marrone Bio Innovations, Inc. (MBII) ratings are on the decline this week as the company earns an F (“strong sell”). Last week, it received a D (“sell”). The stock gets F’s in Earnings Momentum, Equity, Cash Flow and Sales Growth. To get an in-depth look at MBII, get Portfolio Grader’s complete analysis of MBII stock.

Slipping from a C to a D rating, Rockwood Holdings, Inc. (ROC) takes a hit this week. 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 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.