These Survivor® envelopes expand to provide lightweight, inexpensive shipping for bulky items. Expandable envelopes are made of puncture, tear and moisture-resistant DuPont™ Tyvek® material. Envelopes made with Tyvek® are half the weight of heavy duty paper envelopes, so you save on postage with each mailing. Our tough expandable mailers resist overstuffing, bad weather and rough handling to deliver your contents intact. With easy to use Quality Park® Redi-Strip® envelope closures, just peel and press for an instant seal. Heavyweight 18 lb Tyvek®. Seam Type: Center; Flap Type: Square; Trade Size: #13 1/2; Security Tinted: No.