Menu

Threadlocking Adhesives

Posted on 2/6/2013 9:48:13 AM By Sandy Niks
  

Threadlocking adhesives are single component materials generally formulated using monomers in the methacrylate ester family, plus initiator, accelerator, stabilizer, modifiers, and (sometimes) surface activator. They are anaerobic adhesives, that is, they cure upon the exclusion of oxygen. Thus the need for one or more stabilizers that prevent the adhesive from polymerizing prematurely within the container or before the adherends are properly mated.

Threadlocking adhesives are used on the threads of nuts and bolts to keep the assembly from self-loosening due to vibrations the assembly is exposed to. Threadsealing adhesives are similar, but generally are more thixotropic to better fill the spaces to prevent leakage of any fluids. A better seal also reduces the opportunity for corrosion. The formulations are modified to provide specific properties, such as a needed torque. Some formulations allow for the adhesive to be preapplied to threads of the fasteners, simplifying the assembly process in the automobile manufacturing plant.

ASTM D5363, Standard Specification for Anaerobic Single-Component Adhesives (AN), was originally approved in 1993. It was written partially as a replacement for at least three US military specifications as part of a project to reduce government and military specifications and standards when an industry standard would fulfill the requirements. This specification identifies an adhesive by using coded tables of properties, first by adhesive, then Group, Class, and Grade. Suffixes may also be added for additional properties or a property that supersedes a property defined in the table. The Group is a description, such as “Threadlocking and retaining adhesives; slow curing; Newtonian flow properties” and “Threadlocking adhesives; fast curing; thixotropic flow properties; lubricating”. Class and Grade identify a combination of properties values in color, torque strength, storage stability, solvent resistance, hot strength, heat aging, cold strength, viscosity, and speed of cure. Additional requirements not part of the table properties include ultraviolet fluorescence (for non-destructively determining if the adhesive is present), flash point, toxicity (Material Safety Data Sheet submission), and wicking. Example of a suffix requirement could be packaging. Most threadlocking adhesives can be identified using this coding system.

There are several test methods that have been developed primarily, if not exclusively, to evaluate these adhesives. They are ASTM D4562, Standard Test Method for Shear Strength of Adhesives Using Pin-and-Collar Specimen; ASTM D5648, Standard Test Method for Torque-Tension Relationship of Adhesives Used on Threaded Fasteners (Lubricity); ASTM D5649, Standard Test Method for Torque Strength of Adhesives Used on Threaded Fasteners; and ASTM D5657, Standard Test Method for Fluid Tightness Ability of Adhesives Used on Threaded Fasteners.

Arguably, the most recognized producers of threadlocking adhesives are Loctite™, now a Henkel company, and Permabond™. Additional producers are ITW Devcon and Hernon Mfg., Inc. This is not an endorsement of any company. Nor, I am sure, is this a complete list of suppliers.

Companies:

ITW Devcon

Hernon Mfg., Inc.

Loctite™, Henkel

Permabond™


References:

Handbook of Adhesive Technology, edited by A. Pizzi, K. L. Mittal, Marcel Dekker, Inc., 1994, ISBN:  0-8247-8947-1, Chapter 29, “Anaerobic Adhesives”, by Richard D. Dick.

Handbook of Adhesives, Third Edition, edited by Irving Skeist, Ph.D., Van Nostrand Reinhold, 1990, ISBN:  0-442-28013-0, Chapter 26, “Anaerobic Adhesives”, by John M. Rooney and Bernard M. Malofsky.

Article in Popular Mechanics, How to Secure Bolts Using Threadlocker: Auto Clinic - Popular Mechanics