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Answers on adhesives

Answers on adhesives
Concluding our series on adhesives, we take a look at some of the typical more general questions posed by engineers. And with the help of Henkel we provide the answers.

Can substrates with different thermal expansion coefficients be successfully bonded? The answer is "yes" - and, indeed, this is one of the benefits of adhesives compared to other joining methods. By selecting the right kind of adhesive technology, it is possible to bond materials such as ceramics and plastics or glass and aluminium with no problem. A call to a technical advisor at an adhesives manufacturer will help make the right selection. And while we're discussing benefits, it is always worth remembering that using adhesives as a joining method means the weight of assemblies are generally lighter and in many instances costs are reduced when compared to more traditional fastening methods.   

How is the correct amount of adhesive determined for a joint? It is a mistake to think that applying large amounts of adhesive will give better strength. That is seldom the case and, as a general rule, only enough adhesive to fill the bondline is required. In fact, the use of excess material is not only wasteful and can lead to migration, but also can cause clean up problems. For applications that require high production throughput with extreme accuracy of dispensing, automatic application equipment will allow just the right amount of product to be applied. 

On a more general point, how can adhesive be prevented from running out of the joint? In general terms, the answer to this question revolves around employing an adhesive that has the proper viscosity for the job in hand - and then utilising the most appropriate application technique. In addition, using minimal amounts in the proper place will reduce or eliminate migration.  

In the case of adhesives applied in an assembly line situation, how long is the wait until bonded parts can be put into service? In many circumstances, components are ready for service when they reach the end of the assembly line. Instant adhesives are not called by that name without good cause - and will often be placed in service within seconds. Threadlocking products show evidence of cure within 15-20 minutes and can resist vibration within an hour.  

What advantages do Epoxies provide? Epoxy resins can offer several distinct benefits over other adhesive technologies - including excellent fatigue and thermal shock resistance, the capability of joining dissimilar materials, resistance to chemicals and corrosion. As always it is important to use the correct adhesive technology for the application. 

Staying with epoxies - how crucial is the mix/ratio accuracy? The mix ratio of two-part epoxy adhesives is perhaps the most crucial processing parameter in order to achieve the best results. Any deviation from the manufacturer's specified proportions will result in a less than perfect bond. However, this need for accuracy need not be a deterrent from using this adhesive technology. In many cases, two-part epoxies are available in pre-measured twin cartridge packaging. This configuration helps eradicate the possibility of the 'incorrect mix' - which is the foremost cause of epoxy adhesive failure.

Will adding more hardener speed up the reaction?  In some instances adding more hardener will result in the speeding up the bonding process - but this is not recommended. Adding some heat to the joint - not changing the mix ratio - is a better method of speeding up the reaction.

What is the difference between Polyurethane and Silicone adhesives? Polyurethanes provide exceptional toughness, flexibility and high elongation, excellent gap filling and high chemical resistance. Significantly, they allow the joint to be painted when the adhesive has cured. Polyurethane adhesives are employed to bond a wide range of materials, including glass, wood, concrete, metal and plastic. Unlike some other adhesive technologies, polyurethanes are suitable for materials that are both porous and non-porous. These products are specified by designers in the manufacture of items such as automotive windscreens, shower screens and sceptic tanks where the bond forms a structural part of the assembly.

Both polyurethanes and silicones vulcanise from the outside to the inside of the bondline. They both provide flexible bonding, but silicones are particularly recommended where elevated temperature (more than 230°C) applications are involved. What's more, silicone products provide effective sealing for a variety of fluids with high performance versions suitable for contact with engine and transmission oils. Curing time increases with the thickness of the sealant section, but the addition of moisture in the form of humidity will speed the cure rate. Conversely, the application of heat alone may drive off dampness and prolong the cure time.  Silicones are available for a wide variety of uses including glass to glass bonding, building functions, and boiler and automotive engine/ transmission sealing applications.
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