Quick to achieve a lasting bond

Modern instant adhesives can help to make production processes faster, safer and more economical, as Colin Chapman of Henkel, manufacturer of Loctite brand products, explains

Adhesive bonding is one of the fastest growing techniques employed to join materials in modern manufacturing processes. When properly selected and applied, adhesives achieve lasting bonds between virtually all materials and in any combination.

So-called 'instant adhesives' are playing an increasingly important role in industrial manufacturing. They offer advantages over traditional material joining processes such as ultrasonic and solvent welding, or mechanical fastening methods such as clipping or bolting. Benefits include the ability to cure quickly, no discolouration or distortion of substrates, and a weight reduction of finished articles. In addition, they can be used to bond similar or dissimilar combinations of plastic, rubber, metal, textile, wood or paper materials - without damaging the materials either chemically or thermally.

Adhesives allow full-surface bonding, rather than joining only at isolated, highly stressed contact points.

Instant adhesives, also known as cyanoacrylates (CAs), are suitable for bonding small and medium-sized parts to achieve a close fit in production situations.

Parts can normally be processed without any additional surface preparation, although for best results the substrate should be clean and free of grease. Once applied, the cyanoacrylate cures within seconds by reacting with the moisture on the surfaces of the parts. In fact, sufficient strength is created to allow further processing after only 5 to 20s, making CAs suitable for fast production turnarounds.

Cyanoacrylates can be found in a diversity of applications, including bonding rubber feet to ABS housings, attaching clips and switches to mobile electronic devices, and forming flexible gaskets on steel.

Multipurpose products While multipurpose adhesives deliver good results for bonding a variety of different material combinations, specialty products have specifically engineered characteristics which extend the spectrum of possible applications. For instance, within this category are adhesives for difficult-to-bond plastics such as PP, PE, acetal, PTFE or silicones. In fact, choosing the right product is essential to get the best results.

For example, the elastomer-modified cyanoacrylates offer good impact and shock resistance and are, therefore, used to bond loudspeaker parts, the housings for rechargeable batteries in power drills, or similar applications.

Flexible products, on the other hand, can reliably withstand repeated twisting or bending of the bonded materials. As a result, they are suitable for bonding plastic sheaths on the metal temples of flexible-frame sunglasses or for attaching rubber profiles to refrigerator doors made of painted metal.

For transparent parts requiring very good optical performance and clear bondlines, light-curing cyanoacrylates are a good choice.

Gap-filling bonds Cyanoacrylates work by utilising the condensed humidity on the substrates to initiate the curing reaction. This reaction originates from the substrate surfaces and migrates towards the middle of the joint.

Since conventional cyanoacrylates cure from the surface and mostly have low viscosity, they are used only to join close-fitting parts.

However, the scope of applications in production processes has been broadened by the addition of a gap-filling cyanoacrylate.

Known as Loctite 3090, Henkel believes this product enhances the use of instant adhesives through its ability to fill gaps up to 5mm. This new adhesive can be used on metals, most plastics, rubber and ceramics.

Even porous substrates such as wood, paper and cork can be reliably bonded.

Formulated as a high-viscosity gel, this CA allows easier application to difficult vertical surfaces - without dripping. Unlike onecomponent systems, Loctite 3090 does not depend on the moisture on the surfaces for curing, but uses an additional activator. The two components are dispensed into an integrated static mixer, which ensures that curing is triggered within the defined timeframe, regardless of moisture or temperature and the component parts of the adhesive are always correctly mixed.

So, cyanoacrylates, instant adhesives - whatever you want to call them - were an instant success when they were introduced and I believe that they have now formed a 'secure bond' with both industrial users and consumers.