Patently Apple:

The protective coatings may incorporate a binder material such as polymer (sometimes known as a matrix or silicone matrix) and may comprise particles embedded inside the binder. The particles could have hardness to enhance the durability of the coatings.

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This week the US Patent & Trademark Office released a patent application from Apple that relates to brand new protective coatings being inserted to Apple apparatus in the future that will prevent damage to housing structures, transparent members connected with optical component windows and displays, along with other device structures.