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Answers on adhesives

Answers on adhesives
Engineering adhesives are widely employed throughout industry - yet questions often arise about some aspects of their use. In this series of articles, Industrial Technology, with the help of Henkel, manufacturer of Loctite brand products, will provide the answers to those questions.

Surely, a glue is a glue is a glue. Yet we read about different adhesive technologies. Won't they all simply 'stick' things together? While it might be true as a very general statement that all adhesives 'stick' things together, that simply isn't good enough when it comes to engineering adhesives. Each adhesive technology has a specific purpose, and while there may be some overlap in applications, using an inappropriate adhesive can result in problems - and sometimes joint failure.  

Broadly speaking, the adhesive technologies that interest design, production and maintenance engineers include anaerobics, cyanoacrylates (instant adhesives), epoxies, polyurethanes and silicones.  Within those categories are adhesives that cure using light, heat or activator techniques. If that sounds a little daunting, do not panic. Help desks at adhesives suppliers can provide specific details of which adhesive is suitable for which type of job.  

Just how much adhesive should be used for applications involving cyanoacrylate and anaerobic products? Of course, usage will vary from application to application. However, when it comes to these particular adhesive technologies, some general guidelines are possible.  As far as cyanoacrylates are concerned, the bead length varies according to the thickness of the adhesive. Typically, a bead length between 500mm and 1500mm can be applied per one gram of adhesive. For anaerobic threadlocking or threadsealing applications, using an M3 with a 2.5mm engagement, 160 components per ml of adhesive is a reasonable guide figure. For M6 x 4mm engagement, between 30 and 50 components per ml is possible, while for an M10 x 8mm engagement the figure is between 10 and 16 components per ml. When it comes to retaining applications, around 30 parts with a 25mm diameter x 15mm engagement and a 0.05mm clearance can be treated with 1ml of adhesive. 

Anaerobics are generally supplied in 50ml or 250ml bottles and CAs in 20g or 500g bottles, so there are a high number of applications per bottle.  For example, a 50ml bottle would lock 1500 M6 fasteners.

Talking about bottle contents, why are anaerobic 50ml and 250ml bottles only half full? The volume of product in each bottle is carefully controlled during the filling procedure. Bottles are only filled halfway because air is needed in the package to prevent anaerobics from solidifying. In addition, containers also allow anaerobics to breathe by allowing air to permeate through the plastic bottles. Having said all of that, users are not being short-changed. The contents in the bottle are what are stated on the label.

How can you remove the white haze that sometimes appears around a bond line when an instant adhesive is used - and how can you prevent it from occurring again? This phenomenon is known as 'blooming' or 'frosting', and it can occur with cyanoacrylates during their cure cycle. Blooming usually occurs in the summer time when humidity is at its peak. It can also occur when a customer uses an excessive amount of adhesive which would remain liquid outside the joint for a long time allowing minute quantities of cyanoacrylate monomer to evaporate then settle on to the parts.  

Where an area already has blooming it can be removed through the use of a proprietary clean up solvent or acetone - although this is not possible when bonding plastics. The first stage to prevent blooming would be to ensure the correct amount of adhesive is applied and there is not unnecessary excess outside of the joint. 

Next would be to improve the ventilation and air movement around the compounds during the cure process. This can often be achieved by altering the way the parts are stored during the cure process. In many cases the use of 'Low Odour', 'Low Bloom' products can be used to control any difficulties. But beyond that, changing to light-curing cyanoacrylates will eliminate blooming in most cases. These one-part solvent-free adhesives cure immediately when exposed to low-intensity UV or visible light. Because they cure so quickly, blooming is not a problem. Instead, the adhesives form a dry, glasslike surface even with short-term, low-intensity light exposure.
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