Hook and loop fastener
Hook-and-loop fasteners or touch fasteners consist of two components: typically, two lineal fabric strips (or, alternatively, round "dots" or squares) which are attached (e.g., sewn, adhered, etc.) to the opposing surfaces to be fastened. The first component features tiny hooks; the second features even smaller and "hairier" loops. When the two components are pressed together, the hooks catch in the loops and the two pieces fasten or bind temporarily during the time that they are pressed together. When separated, by pulling or peeling the two surfaces apart, the strips make a distinctive "ripping" sound.
Touch fasteners are available in several strengths and constructions by several manufacturers.
Some touch fasteners are strong enough that a two inch square piece is enough to support a load of 175-pound (79 kg).  Fasteners made of Teflon loops, polyester hooks, and glass backing are used in aerospace applications, e.g. on space shuttles. The strength of the bond depends on how well the hooks are embedded in the loops, how much surface area is in contact with the hooks, and the nature of the force pulling it apart. If Velcro is used to bond two rigid surfaces, e.g. auto body panels and frame, the bond is particularly strong because any force pulling the pieces apart is spread evenly across all hooks. Also, any force pushing the pieces together is disproportionally applied to engaging more hooks and loops. Vibration can cause rigid pieces to improve their bond. Full-body Velcro suits have been made that can hold a person to a suitably covered wall.
When one or both of the pieces is flexible, e.g. a pocket flap, the pieces can be pulled apart with a peeling action that applies the force to relatively few hooks at a time. If a flexible piece is pulled in a direction parallel to the plane of the surface, then the force is spread evenly as it is with rigid pieces.
Three ways to maximize the strength of a bond between the two flexible pieces are:
- increase the area of the bond, e.g. using larger pieces
- ensure that the force is applied parallel to the plane of the fastener surface, e.g. bending around a corner or pulley.
- increase the number of hooks and loops per area unit.
Shoe closures can resist a large force with only a small amount of hook and loop fasteners. This is because the strap is wrapped through a slot, halving the force on the bond by acting as a pulley system (thus gaining a mechanical advantage), and further absorbing some of the force in friction around the tight bend. This layout also ensures that the force is parallel to the strips.
Advantages and disadvantages
Touch fasteners are easy to use, safe, and maintenance free. There is only a minimal decline in effectiveness even after many fastening and unfastenings. The tearing noise it makes can also be useful against pickpockets.
There are also some deficiencies: it tends to accumulate hair, dust, and fur in its hooks after a few months of regular use. The loops can become elongated or broken after extended use. Velcro often becomes attached to articles of clothing, especially loosely woven items like sweaters. Additionally, the clothing may be damaged when one attempts to remove the Velcro, even if they are separated slowly. The tearing noise made by unfastening hook and loop fasteners make it inappropriate for some applications. For example, a soldier hiding from the enemy would not want to alert the enemy to his position by opening a Velcro pocket. It also absorbs moisture and perspiration when worn next to the skin, which means it will smell if not washed.
Textiles can contain chemicals or compounds, e.g. dyes, that may be allergenic to sensitive populations. Some products have been tested according to the Oeko-tex certification standard which imposes limits on the chemical content of textiles to address the issue of human ecological safety.
- ASTM D5169-98(2010) Standard Test Method for Shear Strength (Dynamic Method) of Hook and Loop Touch Fasteners
- ASTM D5170-98(2010) Standard Test Method for Peel Strength (“T” Method) of Hook and Loop Touch Fasteners
- ASTM D2050-11 Standard Terminology Relating to Fasteners and Closures Used with Textiles
- Stephens, Thomas (2007-01-04). "How a Swiss invention hooked the world". swissinfo.ch. Retrieved 2008-05-09.
- McSweeney, Thomas J.; Stephanie Raha (August 1999). Better to Light One Candle: The Christophers' Three Minutes a Day: Millennial Edition. Continuum International Publishing Group. p. 55. ISBN 978-0-8264-1162-4. Retrieved 2008-05-09.
- "About us: History". Velcro.us. Retrieved 2013-11-13.
- Schwarcz, Joseph A. (October 2003). Dr. Joe & What You Didn't Know: 99 Fascinating Questions About the Chemistry of Everyday Life. Ecw Press. p. 178. ISBN 978-1-55022-577-8.
- Crespo et al, Contact Dermatitis, 2009