The word "propellants" refers to substances that provide the necessary thrust to propel an object forward. Antonyms for propellants would be words that denote the opposite - substances that impede or prevent movement. Such antonyms could include terms like adhesive, glue, hindrance, obstruction, blockage, or restraint, which are substances or factors that slow down, block, or restrict movement. Other antonyms for propellants could include terms like braking, decelerating, or stopping, which are words that describe the opposite of accelerating or propelling an object. Overall, antonyms for propellants would refer to anything that impedes or prevents object movement instead of aiding it.