selfseal

Selfseal, also written self-seal or self-sealing, is a term used to describe a range of closures and sealants that form a seal automatically in response to a triggering condition. The triggering condition varies by application, including moisture, pressure, or contact with a particular substance.

In packaging and mail, self-sealing closures are most commonly seen on envelopes and pouches. These products carry an adhesive area that bonds when pressed, providing a quick, convenient seal without the need for additional liquids or heat. Some self-sealing packaging relies on a liner that is peeled away to expose the adhesive, while others use a pressure-activated adhesive that does not require moisture.

In defense and aerospace contexts, self-sealing designs typically refer to structures intended to reduce leakage after damage. The best-known example is self-sealing fuel tanks, which employ layers of sealant materials and resilient barriers that swell or deform when in contact with leaking fuel to seal punctures. This concept enhances survivability by limiting fuel loss and fire risk after ballistic impact.

Other uses of self-seal technology include tamper-evident security labels and certain types of protective seals on equipment and containers. Limitations include that self-sealing systems may degrade over time due to environmental exposure, temperature, and age; they can be damaged by rough handling; and they are not universally applicable to all materials or contents.

Historically, self-sealing envelopes appeared in the early 20th century, with advances in sealing technology driving broader adoption in mailrooms, packaging, and military engineering.