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Special purpose bonding

Special purpose bonding Rather than being an afterthought, adhesives are now first choice among numerous engineers when it comes to that unusual application.

Let's face it, for most applications a standard adhesive product will often do the trick and create an effective bond. But for 'special needs' applications, specific performance characteristics are required. 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 - that of taking a unique liquid resin that hardened in absence of air and in presence of metal to produce a secure joint - 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, even in 'special needs' applications. 

Just as 'special' mechanical fasteners may be called upon in specific circumstances, so adhesives have been developed to meet equally extraordinary circumstances. A good example is 'dirty bonding'. Engineers know that surfaces to be bonded should be as clean as possible.  In fact, adhesives manufacturers will often provide cleaners for the very purpose of ensuring substrates are free from contamination. But, as engineers will also be aware, it is not always possible to achieve that level of cleanliness. In some instances bonding will be required on 'as received' parts whose surfaces may be contaminated with engine oils, corrosion prevention 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 enable 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. Such threadlockers are available in medium and high strength grades, and alongside their oil tolerant qualities they can be employed on high temperature applications, in some cases up to 180°C.

Coping with chrome
But the benefits of these new threadlockers do not end there. Recent industrial Health and Safety regulations mean that Hexavalent chromates can no longer be used for many coatings and, as a result, manufacturers are now switching to Trivalent chrome. This move, however, presented a challenge for adhesives as the existing anaerobic grades do not adhere very well to the new coating.

So, does that mean an end of bonding where such coatings are involved? Not at all. The formulation that was needed to ensure that adhesives cure not only on active substrates, such as iron, copper, brass and steel, but also on passive materials including stainless steel, zinc plated surfaces, inorganic and organic coatings has been incorporated into the new generation of threadlockers - making them more versatile than previously possible.

All of this means that for 'special' situations where inactive coatings and 'as received' parts are involved, there are adhesives that can provide the effective solution.

Moving away from anaerobic adhesives, there have been some 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 range of new two part products have been developed that provide unlimited cure through volume, and give good adhesion to a wide range of substrates without the need for a primer. And like all silicone products, they are extremely flexible and can be utilised for bonding glass, metals, ceramics and most plastics. In many cases, there is no need for surface preparation. The latest addition to this specialist range is a high temperature version that can withstand continuous temperatures of 220°C - and peaks up to 300°C.

However, the main benefit is the speed of cure. These new two part products allow handling and subsequent use in considerable shorter times than have previously been possible. In some cases, handling strength is reached within minutes. Typical uses for the new product include the bonding and sealing of glass and ceramic stove tops to appliance frames, securing brackets to back of stove tops, bonding glass to microwave oven doors and the sealing of washing machine drums.

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 two to four minutes, removing the need for a post-assembly activator.

It will bond together close fitting parts in 20 seconds, and even where the gap is 3mm, bonding is achieved in less than 90 seconds. Indeed, the product can be used where a more significant gap - up to 5mm - is involved.  This adhesive, which can be used on substrates such as plastic, rubber, wood, stone or metal, is capable of withstanding a force of some 20N/mm2. Application is very easy, with the adhesive available in a bi-pack syringe. The temperature range of this new two part CA is also versatile: it can be used between -20°C and +80°C.

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 now available for general purpose repairs or the speedy assembly of small components. Called Loctite Double Bubble it will bond to a wide variety of materials, and cures rapidly to give handling strength in approximately five minutes. This epoxy comes packed in a handy sachet, and is supplied complete with a mixing stick and pallet.
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