This week, 4 Chemicals stocks are improving their overall ratings on Portfolio Grader. Each of these stocks is rated an “A” (“strong buy”) or “B” overall (“buy”).

NewMarket Corporation (NEU) gets a higher grade this week, advancing from a C last week to a B. NewMarket Corporation develops, manufactures, and blends fuel and lubricant additives marketed worldwide to refiners and others who sell petroleum products. The company also gets A’s in return on equity. For more information, get Portfolio Grader’s complete analysis of NEU stock.

Codexis, Inc.’s (CDXS) ratings are looking better this week, moving up to a B from last week’s C. Codexis, Inc. is a developer of proprietary biocatalysts, which are enzymes or microbes that initiate or accelerate chemical reactions. For more information, get Portfolio Grader’s complete analysis of CDXS stock.

The rating of TerraVia Holdings, Inc. (TVIA) moves up this week, rising from a C to a B. The company also gets A’s in earnings revisions. For more information, get Portfolio Grader’s complete analysis of TVIA stock.

Delta Technology Holdings Ltd (DELT) boosts its rating from a C to a B this week. The company also gets A’s in return on equity and free cash flow. For more information, get Portfolio Grader’s complete analysis of DELT 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.