The overall ratings of four chemicals stocks are down on Portfolio Grader this week. 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. For more information, get Portfolio Grader’s complete analysis of TROX stock.

Calgon Carbon Corporation (CCC) gets weaker ratings this week as last week’s C drops to a D. Calgon Carbon is engaged in products and services for purifying water and air. 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 CCC, 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 receives F’s in Earnings Momentum, Equity, Cash Flow and Sales Growth. For more information, get Portfolio Grader’s complete analysis of MBII stock.

Rockwood Holdings, Inc.’s (ROC) rating weakens this week, dropping to a D versus last 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. 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.