Patented release strip makes these open-end catalog envelopes easy to open. Convenient Flap-Stik closure does not require moisture to seal. Envelopes are made of DuPont Tyvek won t rip tear or puncture. They are much stronger and more secure than traditional paper envelopes. Water-resistant and moisture-resistant envelopes are lightweight to save on postage costs. Envelopes are easy to use. Simply peel and seal.