The explosion happened Saturday at a factory in the coastal city of Himeji.
The factory is operated by one of the world's largest producers of acrylic acid, a primary ingredient used in disposable diapers.
Acrylic acid is a key component of superabsorbent polymers or SAP, which absorb large amounts of liquid.
The plant had been ramping up production to meet increasing global demand, especially from China, according to Japanese media reports.
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