Ways to form a special bond

Special fasteners are often required to meet special situations. But, as Colin Chapman of Henkel - manufacturer of Loctite products - explains, there are also 'special' adhesives available for use in exceptional situations

Surfaces to be bonded should be clean, but it is not always possible to achieve the level of cleanliness required. In some instances bonding will be needed on 'as received' parts whose surfaces may be contaminated with engine- or corrosionprevention oils, cutting fluids or other 'dirty' materials.

Oil tolerant adhesives - in the form of threadlockers and retainers - have been around for some time, but some recent developments have introduced significant improvements in the performance of these products and enabled them to be used in 'special' circumstances. In fact, today's oil tolerant threadlockers can effectively reduce the need for special mechanical fasteners such as spring washers, castle nuts and plastic inserts. And the benefits of these new threadlockers do not end there.

Coping with chrome Recent industrial health and safety regulations have prevented the inclusion of hexavalent chromates in many coatings. As a result, manufacturers are now switching to Trivalent chrome. Existing anaerobic adhesives are not compatible with the new coating and naturally this has led to new developments.

As well as curing on active substrates such as iron, copper, brass and steel, the latest generation threadlockers are also effective on passive materials including stainless steel, zinc plated surfaces, inorganic and organic coatings. This has given them far greater application scope than before.

Special silicones There have also been important developments in the area of silicones. Traditionally, standard single part silicones are slow to cure, have limited cure through volume and their optimum adhesion can be substrate dependant. In most cases, a primer is needed.

To overcome these limitations, a new range of two part products has been developed that addresses all these issues. Like all silicone products, they are extremely flexible and can be used for bonding glass, metals, ceramics and most plastics. In many cases, there is no need for surface preparation.

The main benefit however, is the speed of cure. These new two-part products allow handling and subsequent use in very short times; in some cases, handling strength is reached within minutes.

Two part form Cyanoacrylate (CA) is another adhesive technology that has been recently developed in two part form. These adhesives, previously available only as single component products, have always provided an 'instant' answer to many bonding situations. Their versatility and the capability to automate dispensing operations have made them a popular choice in a diversity of industries.

All those benefits - including an initial bond within seconds - are still available with the new two-component CA, but with the added advantage that any exposed adhesive remaining outside of the joint will harden within 2 to 4min, removing the need for a post-assembly activator.

Speedy epoxy For those special applications (especially in the area of maintenance) that demand an epoxy to be used - but where a fast handling strength is required, a new product is available for general purpose repairs or the speedy assembly of small components. It will bond to a wide variety of materials, including wood, metal, plastic and ceramic, and cures rapidly to give handling strength in approximately 5min.

Adhesive evolution The original Loctite invention, developed in 1953, solved a problem that had been faced by engineers for many years - how to stop a screw vibrating loose. It became a 'special' product for threadlocking all manner of fasteners. But developments continued apace and the anaerobic principle soon embraced other forms of bonding, such as retaining, sealing and gasketing. With these introductions, the need for many 'special' mechanical fasteners disappeared.

Since then, other adhesive technologies have been introduced that have replaced or augmented more traditional mechanical fixing methods. Today, engineers turn to adhesives for cost efficiency, automated production, lightweight components, ease of assembly and joint reliability.

Just as 'special' mechanical fasteners may be called upon in specific circumstances, so adhesives have been developed to meet equally extraordinary circumstances. Rather than being an afterthought, adhesives are now first choice for many engineers