![\"Air](\"https:\/\/permabond.com\/wp-content\/uploads\/2022\/11\/Air-in-joint-1.png\")
<\/p>\nWith the exception of Great Aunt Tilly, we don\u2019t exist in a complete vacuum.\u00a0 Air is everywhere and often finds its way into an adhesive bond joint.\u00a0 It goes without saying that adhesive bonds better than air \u2013 so minimizing or eliminating air bubbles is essential to achieving the strongest bond.\u00a0 It is also important to eliminate air if you desire a full seal \u2013 dispensing one air bubble the size of the bead of adhesive will create a leak path.\u00a0 In many applications the most important reason to eliminate air is to provide a clean aesthetically pleasing finish.<\/p>\n
Air finds itself into adhesive in many ways. It may be bounced around in a shipping truck, tossed into a tool chest, or transferred to a dispensing unit. Often simply allowing the adhesive to settle will bring air to the surface. In other cases, if the viscosity of the adhesive is too high to release air independently. You can use vacuum or centrifuge to remove it. Use care not to over-vacuum adhesive with volatile components, as it may affect the adhesive properties.\u00a0 Also, use care not to remove dissolved air from anaerobic adhesives as it is required to keep the material from curing.<\/p>\n
An adhesive bond joint design, which provides an air escape path, is ideal.\u00a0 Tongue and groove joints with equal square edges can trap air.\u00a0 A slant on one side of the tongue can allow air to escape.<\/p>\n
Manufacturing design is another avenue to vent the air.\u00a0 For example, flat on flat will trap air, but simply angling the top portion as if you were applying a coverslip to a microscope slide will allow the air to get out of the way.<\/p>\n
When dispensing two component adhesives dispense via static mix tips to avoid air entrapment.\u00a0 If mixing two component epoxy by hand, it is important to stir \u2013 not fold \u2013 to minimize the air incorporated.\u00a0 Use wooden tongue depressors \u2013 not spoons \u2013 to mix.\u00a0 Stir thoroughly, and be sure to scrape the sides and bottom of the container.\u00a0 When potting with two component epoxy adhesives, air bubbles may rise to the surface and pop.\u00a0 If they don\u2019t pop, a heat gun or hair dryer set on high heat will usually encourage them to open.<\/p>\n
Automatic dispensing equipment is a fantastic way to minimize air.\u00a0 With the right viscosity and rheology, dispensing valves that use suck back to stop the dispense can be accurately dialed in. Other valve systems are available that do not use back pressure.<\/p>\n
Where you dispense the adhesive can be just as important as how you dispense adhesive.\u00a0 Consider the blind hole effect.\u00a0 When using a threadlocker in a blind hole be sure to dispense the adhesive onto the threads within the hole.\u00a0 If the threadlocker is dispensed onto the threads of the fastener instead, the threadlocker will be pushed up the fastener and out of the joint as the air in the hole is displaced by the fastener.\u00a0 Although it is common in threadlocking, this phenomenon can happen with any adhesive and any joint configuration that will cause air to be displaced by the adherend.<\/p>\n
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Above, the threadlocker was dispensed onto the bolt.\u00a0 Note the pool of sealant pushed up and out of the threads by the trapped air.<\/p>\n
The dispense pattern is also important. Dispensing a bead of adhesive which will, upon assembly, compress to fill the bond area is ideal.\u00a0 However, when one bead won\u2019t fill the area, consider dispensing in a serpentine pattern.<\/p>\n
![\"Minimizing](\"https:\/\/permabond.com\/wp-content\/uploads\/2022\/11\/Air-in-joint-2-e1478567856216.jpg\")
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Avoid two beads which will trap air between them, or any closed shape.<\/p>\n
![\"air-in-joint-3\"](\"https:\/\/permabond.com\/wp-content\/uploads\/2022\/11\/Air-in-joint-3.png\")
![\"air-in-joint-4\"](\"https:\/\/permabond.com\/wp-content\/uploads\/2022\/11\/Air-in-joint-4.png\")
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When bonding clear substrates it is easy to see the joint, but when bonding metals or other materials you can\u2019t see through it is difficult to tell.\u00a0 If a joint fails and the adhesive appears crepey it is likely that the joint separated before the adhesive was cured.\u00a0 What\u2019s crepey?\u00a0 – well crepey isn\u2019t a technical term for adhesive.\u00a0 It is a popular word in anti-aging skin care.\u00a0 Crepey skin is creepy \u2013 it\u2019s that old skin that looks like crepe paper.\u00a0 If a bond fails and you see striations that look like crepe paper \u2013 one of two things happened.<\/p>\n
![\"crepey](\"https:\/\/permabond.com\/wp-content\/uploads\/2022\/11\/Air-in-joint-5.jpg\")
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Either the joint separated before curing or the adhesive shrank.\u00a0 Shrinkage generally isn\u2019t an issue with engineering adhesives \u2013 however, solvent or water based adhesives can have very high shrink rate.<\/p>\n