The ratings of three chemicals stocks are down this week, according to the Portfolio Grader database. 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. Shares of the stock are changing hands at twice the rate they were a week ago. For more information, get Portfolio Grader’s complete analysis of TROX stock.

This is a rough week for Calgon Carbon Corporation (CCC). The company’s rating falls to D from the previous week’s C. Calgon Carbon is engaged in products and services for purifying water and air. The stock has a trailing PE Ratio of 26.60. To get an in-depth look at CCC, get Portfolio Grader’s complete analysis of CCC stock.

Rockwood Holdings, Inc. (ROC) gets weaker ratings this week as last week’s C drops 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. Shares of the stock have been changing hands at an unusually rapid pace, twice the rate of the week prior. 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.