There is quite an interesting twist in this subject to do with
registered trademarks. The law states that a
company cannot trademark a
generic term. For example a house builder cannot trademark the word 'house'. Therefore when a company's trademark becomes a
generic term the company loses its trademark and anybody is allowed to sell a product under this name.
In the early 20th Century a lot of companies did not realise this and there was a fashion to advertise their brand as though it was a generic term. For example "If you're looking for a Kodak then buy ours because we make the best Kodaks in the world" - Kodak cameras (this is not correct word for word but you get the idea).
There was a court battle once where Kodak almost lost their trademark (and so cameras would forever be called 'Kodaks') but they managed to win it. Unlike Thermos who did lose their trademark so it is now perfectly legal for anybody to sell a vacuum insulated flask and call it a thermos.