Protect what's inside with these specially designed envelopes with open side closure. Superior water, chemical, moisture, burst, tear and puncture resistant protection. Ideal for mailing bulky items and materials. Excellent alternative to boxes and cartons. Offers a convenient, secure and no moisture required Grip-Seal flap. Simply peel off release strip and press down for a quick, secure seal. Opens on side. Made of heavy-duty 18 lb. Tyvek material.