They are made from plastic and are widely used by Pathology labs and hence are called as Pathology envelopes. They are available in 250 and 350 micron and are branded as Natural Delux and Natural Super Delux respectively.
Also opaque variant of 350 micron called Milky Super Delux is available with 350 micron.
These envelopes are made up of LDPE and are used for mailing many types of materials. They are available with POD Jacket which enables sender to put mailing address. They are temper evident which means once sealed they cannot be opened without tearing the envelope. They are available above 60 micron.
They are also known as Damar envelopes. They are sturdier than normal envelopes as they are made by sandwitching coal tar (damar) between two paper sheets. Also fibre cloth is mounted on it which holds paper and coal tar (Damar) together and provides superior strength to envelope.
Bubble envelopes are brown colour envelopes where bubble roll is mounted on brown paper coated with HDP (high density polyethylene) by hot sealing process. It is available with seal and peel flap. It is specially used to mail delicate material.