This week, the ratings of three chemicals stocks on Portfolio Grader are down. 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. The stock price has dropped 11.9% over the past month, worse than the 1.7% decrease the S&P 500 has seen over the same period of time. Trade volume is up 748.1% from the previous week. To get an in-depth look at TROX, get Portfolio Grader’s complete analysis of TROX 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. Share prices fell 5% over the past month. For more information, get Portfolio Grader’s complete analysis of CCC stock.

Rockwood Holdings, Inc. (ROC) earns a D this week, falling from last week’s grade of 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. 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.