The Japanese chemical makers Nippon Shokubai and Sanyo Chemical Industries say they are considering merging into a single firm that would have about $4.7 billion in annual sales and be the 11th largest Japanese chemical company. Roughly two-thirds of the sales would come from Shokubai and one-third from Sanyo. The companies say the business environment for Japan’s chemical industry “has become increasingly severe” in recent years because of a declining population and the rise of competitors elsewhere in the world with greater economies of scale. The merger would create a larger, more integrated company better able to compete, they say. Shokubai is one of the world’s largest producers of superabsorbent polymers, used in disposable diapers. Sanyo also makes superabsorbent polymers, as well as more than 3,000 performance chemicals, some of which depend on raw materials from Shokubai.