How to Identify the Function of an Infinitive in a Sentence

As you know, an infinitive functions as an adjective, adverb or noun in a sentence. However, some infinitive seemingly can function as an adjective as well when it functions as an adverb. Please look at the following sentence.

I need a sunglass to protect my eyes.

Here, the above sentence is generally understood that "I need a sunglass so that I can protect my eyes. (Adverbial uses of infinitives)" By the way, it can also be read that "I need a sunglass which can protect my eyes. (Adjectival uses of infinitives)" So, the infinitive, "to protect my eyes," can be interpreted as adjectival or adverbial uses of infinitives at the same time. Could you confirm whether this observation and conclusion are right? If I misunderstand something and an infinitive cannot be used as an adjective and an adverb at a time in a sentence, please explain to me about it.
Thank you.