Let's begin by defining “antagonist.” In literature, an antagonist is someone who gets in the way of the main character or main characters, someone who stands as an opposite to them. An antagonist is often, but not always, a villain of sorts who plots against people or puts up road blocks to keep them from meeting their goals.
With this definition in mind, let's think about Lynn Nottage's play Sweat. There are several important characters in this play but none of them really fits the definition of antagonist. Even though at times they are hostile towards each other, they are mostly on the same side. The antagonist in this case, then, is not a person but a company. The employer of most of the characters is called Olstead's, and it is constantly getting in the way of the characters in one fashion or another. Stan was hurt on the job years ago as were other people the characters know. Cynthia gets a promotion over Tracey in a way Tracey thinks is unfair. Then the company lists the names of its first round of layoffs. The US plant is too big, and it can find cheaper labor in Mexico, so it is moving much of its operation there. It also begins hiring Mexican Americans in its US plant to cut down on labor costs. The company also cuts pay and benefits for the US workers it plans to keep, and when the employees protest, it locks them out.
Indeed, Olstead's stands in the way of the success of the play's other characters, and it seems to plot against them, putting up road blacks to keep them from meeting their goals. It creates conflict, even violence, among them as they struggle to deal with the loss of their jobs and the loss of their dreams.