Thread-locking fluid (threadlocker) is a thin, single-component adhesive, applied to the threads of fasteners such as screws and bolts to prevent loosening, leakage, and corrosion. Typically, thread-locking fluids are methacrylate-based, and cure anaerobically. Thread-locking fluid is a thixotropic fluid: under shear stress, it exhibits a time-dependent decrease in viscosity. This allows it to flow well over time, yet still resist short-duration shearing, as in vibration or shock.
Thread-locking fluid was originally developed by American professor Vernon Krieble in 1953. His company, American Sealants, founded the Loctite brand.
Thread-locking fluid may be applied before or after assembly, depending on the type. Threadlockers are available in "permanent" and "removable" formulas, with heavy-duty permanent threadlockers rated to withstand as much as 3,000 psi (21 MPa) in shear. Some threadlockers are removable with the application of heat. Because thread locking adhesives typically rely on the electrochemical activity of a metal substrate to form a bond, substrates often require thorough cleaning, and in the case of less electrochemically active metals such as aluminum, priming.
Because electrochemical activity is one of the two triggers that cause polymerization of the threadlocker fluid, care must be taken to avoid contaminating the entire container of threadlocker with threadlocker that has had contact with metal, otherwise the material in the container may polymerize.
Thread-locking fluid is typically sold in small containers, in amounts from 5 millilitres (about one teaspoon) to 250 millilitres (8.5 US fl oz), and are often color-coded to indicate their strength. Threadlocker is also sold in sticks, and in tape form, similar to Teflon tape.
Typical properties of thread-locking fluids  Type Typical color-code Torque to break free Torque to continue turning Temperature range Low strength Purple 62 in-lb (7 N-m) 27 in-lb (3 N-m) −54 to 149 °C Medium strength Blue 115 in-lb (12 N-m) 53 in-lb (6 N-m) −54 to 149 °C Medium strength surface insensitive Blue 180 in-lb (20 N-m) 62 in-lb (7 N-m) −54 to 149 °C High strength Red 230 in-lb (25 N-m) 225 in-lb (25 N-m) −54 to 149 °C High temperature Red 180 in-lb (20 N-m) 270 in-lb (30 N-m) −54 to 232 °C Penetrating Green 90 in-lb (10 N-m) 310 in-lb (35 N-m) −54 to 149 °C
- The Loctite Story by Kenneth W. Butterworth. New York: Newcomen Society of the United States, 1988.
- "Hernon Introduces a New Surface Insensitive Thread Locking Adhesive". ThomasNet Industrial Newsroom. June 26, 2002. Retrieved 2009-07-06.
- "Permatex Medium Strength BLUE Threadlocker Gel Technical Data Sheet". Retrieved 2009-11-16.
- "Devcon Permatex Product Selector". Retrieved 2009-11-13.
- Allen, Mike (January 2007). "How to Secure Bolts Using Threadlocker: Auto Clinic". Popular Mechanics.
- Current Loctite Threadlockers