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 & moisture resistantvirtually 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 & a release strip for quick, easy opening. Heavyweight 18 lb Tyvek. Seam Type: Weld; Flap Type: Square; Security Tinted: No; Opening: Open End.