Learn Portuguese (Portugal): A seda delicada estraga-se na máquina de lavar roupa. - Delicate silk gets ruined in the washing machine.

This is the anti-causative or middle voice. The active verb estragar means to ruin or to damage something. By adding se, we remove the external agent (the person doing the washing). It does not mean the silk intentionally ruins itself, but rather that it simply undergoes the process of getting ruined as a natural change of state.

Could I use the passive voice and say é estragada instead of estraga-se?

Grammatically yes, but it changes the nuance. A seda é estragada sounds like a specific, albeit unnamed, person is actively ruining the silk. Using estraga-se emphasizes the natural tendency or vulnerability of the fabric itself, which is the most natural way to state a general rule or fact like this in Portuguese.

Does estragar-se only apply to objects getting physically damaged like this fabric?

Not at all! While here it means the silk gets ruined, estragar-se is incredibly versatile. It is very commonly used for food going bad or rotting, such as a fruta estraga-se (the fruit goes bad), or even an abstract situation, mood, or plan getting ruined.

Is it strictly necessary to include roupa at the end of máquina de lavar roupa?