Selecting the best adhesive for an engineering project
17 October 2013
Engineering adhesives and compounds are playing an increasingly important role in the build quality of manufacturing equipment and machinery and their ongoing reliability. These products are used structurally at the design and manufacturing stage to achieve more static and dynamic strength, to save weight, prevent leak paths and eliminate problems associated with corrosion.
During the service life of the plant, they offer many benefits over traditional methods for applications – such as thread locking, thread sealing, gasketing and retaining – ensuring systems work optimally and for longer.
There is huge range of engineering adhesives from which to choose and selecting the right one for the job can be a little daunting. All good manufacturers will give you advice, but it still helps to have a basic grounding on the merits of the various options.
So, starting with structural bonding, here is a brief introduction to the most popular products that fall into five basic technology groups; epoxies, acrylics, polyurethanes, silicones and silane modified polymers.
A structural bond is used to join critical load-bearing parts of an assembly. Adhesives developed to meet this need require a combination of high shear, tensile and peel strength, together with maximum stress, impact and shock absorbent characteristics. What’s more they need to bond a diverse selection of materials including composite, wood, metal and glass.
Different formulations provide different characteristics and chemistries as we shall see later, but first we should consider the benefits that adhesives provide over conventional joining methods.
Reduced weight, improved fatigue resistance, uniform stress distribution and the ability to join dissimilar material are the primary advantages. And in many instances, less critical tolerances can be specified as many structural adhesives also have gap filling capability.
It is also worth bearing in mind that adhesive and other joining methods can also work together. Whilst adhesive bonding provides sufficient strength, the addition of a few discretely placed rivets provides instant alignment, jigging and clamping advantages.
In broad terms, structural adhesives fall into five groups – epoxies, acrylics, polyurethanes, modified silanes and silicones. So why the variety and where is each used?
When it comes to high strength structural bonding, epoxy resins are the acknowledged leader. Epoxy formulation comes with a huge degree of freedom and virtually any characteristic can be formulated on demand.
Structural epoxies are generally slower to cure than other technologies. It is the ideal choice for rigid bonding where the bonded parts can be considered to be structurally linked. They also have an ability to fill large gaps and provide good chemical resistance.
Epoxies will bond most materials and whilst they may not be the best choice in every instance they are a good all rounder. Therefore applications are quite varied, epoxies can be used for bonding brackets and hinges onto machinery, magnets into motors or can be used for potting terminals so seal them against moisture ingress.
If a small degree of flexibility is required for the otherwise rigid bond, acrylics are a good choice. They are suitable for bonding a wide range of materials and are regularly used for sheet metal, especially on applications where the substrates might flex.
Toughened acrylics are often used in place of spot weld or blind rivets, especially where the appearance of the finished product is important, or where there is a requirement to spread any load over a wider area and reduce stress concentrations. It should be remembered though the correct joint design is essential, something that has previously been spot welded will most likely be suitable for bonding whereas something that has previously been butt welded will not.
A key characteristic of the acrylic family is their speed of set. Handling strength is generally achieved in less than 30 minutes at room temperature. Another major benefit of using toughened acrylics on sheet metal applications is their ability to deaden sound when compared to traditionally fastened metal sections.
Acrylics are ideal for bonding reinforcing struts and top hat sections onto panel work, the adhesive leaves the exterior appearance free from defects and in many cases gives a superior joint strength. Other applications could be bonding badges and labels onto equipment removing the need to drill the parts and for mechanical fasteners.
The key advantages of high performance polyurethanes include high strength, their suitability for bonding and sealing of a wide range of substrates, excellent gap fill capability and low temperature performance.
Elastomeric versions are widely used for direct glazing applications, but the structural versions are typically specified for building construction, machine assembly and ventilation systems.
Elastomeric polyurethanes are best known for their ability to fill large gaps and for resisting flexing, these products have been used in the motor industry for around 40 years where they are typically used for bonding windscreens into vehicles. Similar applications exist in industrial plant where inspection windows are bonded to the frame.
For elastic bonding, silicone is king. Products in this class are selected for their ability to absorb and tolerate dynamic stress. They are available in one- and two-part versions and allow the bonding of a wide variety of substrates.
The key characteristics of silicones are low modulus, high elongation to failure (200-600 per cent) and are generally capable of withstanding temperatures of up to 250°C. Some grades even offer performance up to 350°C. They have a high gap filling and sealing capacity, good durability and are resistant to humidity, ozone and UV radiation.
In a maintenance environment silicone products are generally used for sealing parts and with operating temperatures in excess of 200°C they are the obvious choice when high temperatures are involved.
- Modified silanes
An ability to bond just about any substrate and compatibility with most paint systems make modified silanes another good option for flexible bonding. Mainly one-component products, they cure by reaction with moisture so no primer is needed to achieve a good bond.
Typically they are used to seal joints and seams on a broad variety of structures including air conditioning and ventilation equipment and they can be used for interior and exterior applications.
Although application areas are very similar to those for polyurethanes, modified silanes do not require a primer. They are a good general-purpose product where larges surfaces are involved.
The benefit of instant adhesives is clear. They cure very quickly when confined between surfaces and are chosen for bonding small parts to achieve extremely fast fixturing. Historically, this group of adhesives was only suitable for close-fitting surfaces, but now formulations are also available that are able to gap fill and this extends their potential in maintenance engineering considerably.
Instant adhesives come in a variety of types, optimised for specific application requirements such as the parts to be bonded, the loads to be resisted, the joint geometry and the process parameters. New high temperature instants will also withstand exposure up to 120◦C for short periods.
- Threadlocking and sealing
These applications are where adhesive technology has perhaps the greatest foothold in maintenance and repair. It is widely used but, as with adhesive bonding, there are a variety of options from which to choose the best product for the task.
The main benefits of threadlocking adhesives over traditional mechanical locking devices is that they prevent self-loosening and secure any threaded fastener against vibration and shock loads. They are easy flowing or semi solid formulations which completely fill the gaps between mating threads, eliminating fretting corrosion by creating a unitised assembly.
Thread sealants do a similar job but are primarily designed to create an instant, low pressure seal. The substrates involved are the main criterion when selecting the most appropriate thread sealant for the job.
As a rule of thumb anaerobic sealants are used on metal fittings and silicones on plastic and plastic/metal combinations. Sealing cord is the best choice for metal and plastic tapered threads and offers the benefit of post assembly adjustment.
Gasketing sealants provide an instant seal that resists high pressure when fully cured. Many factors influence adhesive choice but broadly anaerobic products are for rigid flanges where the sealing gap is zero or very small. Silicone gasketing materials are best suited for large gap applications and assemblies where flange movement occurs.
For more information or advice on selecting the best adhesive product for your application, visit www.loctite-maintenance.co.uk.http://www.engineersjournal.ie/2013/10/17/selecting-the-best-adhesive-for-an-engineering-project/http://www.engineersjournal.ie/wp-content/uploads/2013/10/Glue-1024x768.jpghttp://www.engineersjournal.ie/wp-content/uploads/2013/10/Glue-300x300.jpgChemmachinery,manufacturing