This week, the ratings of five Chemicals stocks on Portfolio Grader are down. Each of these rates a “D” (“sell”) or “F” overall (“strong sell”).

CVR Partners’ (NYSE:UAN) rating falls to a D (“sell”) this week, down from C (“hold”) the week prior. CVR Partners engages in the production, distribution, and marketing of nitrogen fertilizers in North America. For Portfolio Grader’s specific subcategory of Earnings Momentum, UAN also gets an F. For more information, get Portfolio Grader’s complete analysis of UAN stock.

Zoltek (NASDAQ:ZOLT) earns an F (“strong sell”) this week, moving down from last week’s grade of D (“sell”). Zoltek is an applied technology and advanced materials company. The stock gets F’s in Earnings Revisions and Earnings Surprise. As of Feb. 8, 2013, 17.2% of outstanding Zoltek shares were held short. To get an in-depth look at ZOLT, get Portfolio Grader’s complete analysis of ZOLT stock.

Cabot (NYSE:CBT) is having a tough week. The company’s rating falls from C to a D. Cabot is a global specialty chemicals and performance materials company. The stock also gets an F in Earnings Surprise. The stock price has dropped 12.4% over the past month, worse than the 3.3% increase the S&P 500 has seen over the same period of time. For a full analysis of CBT stock, visit Portfolio Grader.

This week, Calgon Carbon (NYSE:CCC) drops from a D to an F rating. Calgon Carbon is engaged in products and services for purifying water and air. The stock receives F’s in Earnings Growth, Earnings Momentum, and Earnings Revisions. Earnings Surprise and Margin Growth also get F’s. The stock currently has a trailing PE Ratio of 47.20. To get an in-depth look at CCC, get Portfolio Grader’s complete analysis of CCC stock.

This is a rough week for DuPont (NYSE:DD). The company’s rating falls to F from the previous week’s D. Dupont is involved with science and innovation across a range of disciplines, including agriculture and industrial biotechnology, chemistry, biology, materials science, and manufacturing. The stock gets F’s in Earnings Growth and Sales Growth. For a full analysis of DD 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.