Flammable and inflammable are two words that are exactly the same, but one can easily conclude that they are opposite to each other. Both inflammable and flammable are used to describe things that are capable of burning, things that are combustible, especially liquid substances. Invariably, inflammable is more or less like a synonym for flammable. Although both words might be referring to the same thing, you will still notice little difference(s) between them.
The major difference between the two words is that flammable is mostly used when describing substances that can easily be ignited, or substances that attract fire very easily and faster. That's why you will see trucks carrying substances like petrol, ethanol having labels like 'highly flammable.' I think the reason why people don't use ''highly inflammable'' is the possible interpretation people might give to it; many may think since the word is starting with the prefix 'in,' then it should be regarded as the opposite of flammable.
Inflammable and flammable are terms which mean the same thing to describe objects or items that can be burned or ignited. Flammable is the preferred term for a material that catches fire readily. Inflammable means the same thing that a material can burn easily. If a material doesn't burn easily, you could say it is not flammable or non-flammable. A few examples of flammable materials include wood, kerosene, and alcohol. Some examples of non-flammable materials include helium, glass, and steel.
The source or the word Inflammable would be the word inflame, and it precedes the invention of the word flammable. Both mean capable of burning. Flammable materials have at least one component that can be oxidized and that oxidation produces enough heat that it can create radicals (which are necessary for combustion). The older and more proper term is inflammable.