Quality Park™ Survivor™ Catalog Mailers are made with durable 14-lb DuPont® Tyvek™, which is strong enough to withstand the most grueling conditions. The material is puncture, tear and moisture resistant—virtually indestructible. Yet, it is still lightweight which can provide a savings on shipping costs. These white mailing envelopes feature an easy Flap-Stik™ closure which requires no moisture for a secure seal and a release strip for quick, easy opening. Heavyweight 18 lb Tyvek™. Seam Type: Weld; Flap Type: Square; Security Tinted: No; Opening: Open End.