Whether we’re talking about Sony’s Walkman or a 3M’s Post-it Note, there are some of the landmark brand names that made it so far that their trademarks turn into common nouns. And this should be the good part of branding: a brand name on everybody’s lips.
Meanwhile there are cases that we use such a noun without even thinking that the word itself used to be a registred brand name, say escalator for example. This is the downside in terms of branding.
Well, these words are called eponyms.
An eponym is a general term used to describe from what or whom something derived its name. Therefore, a proprietary eponym could be considered a brand name (product or service mark) which has fallen into general use.
So, what leads a brand name to become eponym? Well, for one thing, other brands of similar nature must exist; but even more importantly, the original product, even if discontinued, must still function pronominally. In other words, a specific can be used to designate a class of generics with no loss in meaning. A usual result: lower case transcription of the brand name.
In this matter there is the American Proprietary Eponyms website, which have some of the most common eponyms in english language.