These gusset envelopes are made of highly durable tyvek fibre from dupont an exceptionally strong material thats difficult to tear and is also impermeable to water keeping contents dry and safe even in extreme conditions because its lightweight you can save on postage compared to other packaging materials these envelopes feature an expanding gusset for thick piles of documents extra strong envelopes with expanding gusset and peel amp seal flap highly durable tyvek construction tear