Bike helmets are made of hard foam, which has shock-absorbing properties and serves as a "crumple zone" for your head.
Microshell helmets have a thin plastic cover and are the most common. These models are made of thin polycarbonate, which is primarily found in lightweight sports models.
The hardshell helmets have a hard plastic cover around the foam (polystyrene foam) and are mainly used for downhill riding due to their high weight. The "hardshell" material is made of ABS (acrylonitrile butadiene styrene) and is somewhat stronger and heavier, which is why it is also used in motorcycle helmets.
The straps of the helmets are made of polyester and the buckles are made of solid polyamide or POM (polyoxymethylene).
The materials used in the manufacture of the helmets can be found online in the article information or in the product instructions.