The overall ratings of three 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. Shares of the stock have been changing hands at an unusually rapid pace, up 977.6% from the week prior. For more information, get Portfolio Grader’s complete analysis of TROX stock.

Calgon Carbon Corporation’s (CCC) rating weakens this week, dropping to a D versus last week’s C. Calgon Carbon is engaged in products and services for purifying water and air. To get an in-depth look at CCC, get Portfolio Grader’s complete analysis of CCC stock.

The rating of Rockwood Holdings, Inc. (ROC) declines this week from a C 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, four times 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.