How much is “enough” when it comes to adhesive?
More adhesive isn’t necessarily better when you want an effective bond or seal. Covering the bond surface is the main requirement. The type of adhesive and the bonding environment have a lot to do with how much is “enough”.
These are some common issues when using too much adhesive. The probable causes of these issues and suggested solutions are good starting points for troubleshooting.
- Always contact the manufacturer for additional assistance
- When using adhesives always check the safety precautions
Cyanoacrylate (Instant) Adhesives
Problem – Slow cure time.
Cause – Low moisture to adhesive ratio. Cyanoacrylates bond when trace amounts of moisture deactivate the adhesive’s built in stabilizer. Differing amounts of trace moisture are present in the surfaces being bonded, as well as in the air. If there is too much adhesive in proportion to the amount of moisture, cure speed will be reduced.
Solution - Tests should be made to determine the smallest amount of adhesive to achieve the desired bond. Production consistency can be improved with regulated temperature and humidity controls. (Testing is also a more economic and safer process when making critical bonds.)
Problem – white haze or crust on the finished piece.
Cause - Cyanoacrylates are designed to bond two mating surfaces. The amount of adhesive required is directly related to the size of the surfaces. Excess adhesive squeezed out of the bond area cures very slowly allowing enough time for the cyanoacrylate to volatilize, cure in the air and fall back to the surface as a white residue.
Solution - Reduce the amount of adhesive used to eliminate squeeze out or use a surface activator / accelerator to cure the squeeze out.
The manufacturer’s technical support staff can help with this issue.
Problem – Slow or incomplete cure.
- large gap between mating surfaces;
- exposure to oxygen. Anaerobic adhesives cure when metal ions on the substrates surface activate the cure – but only in the absence of oxygen.
- Check the manufacturer’s recommendations for maximum gap fill. Select the grade appropriate to the application.
- Reduce the amount of adhesive used.
- Eliminate excessive squeeze out from the adhesive tube or container. Use a clean dry cloth to wipe away any excess.
When troubleshooting a bonding problem, the first step is to determine if the problem is cohesive or adhesive in nature. Simply put, you want to figure out if the problem is related to
- how the adhesive sticks to the substrates (component parts) or
- to the state of the adhesive in the bond area.
Adhesive Failure – The adhesive bond to one of the surfaces fails. This is also referred to as Interfacial Failure
Some causes of Adhesive Failure – and suggestions:
Dirt or other contaminants on substrates – clean thoroughly before applying adhesive.
Smooth surface – roughen substrate with suitable method (abrasives, chemical etching, for ex).
Hard to Bond plastics – use manufacturers recommended primer.
Joint design/bond area – modify the joint design or increase the bond area
Cohesive Failure – The tearing apart of an adhesive or sealant as the joint is stressed. The adhesive remains bonded to both surfaces but the adhesive itself cracks or tears down the middle. This occurs if the adhesive (bond) capabilities exceed its cohesive capabilities.
Likely cause of Cohesive Failure – and suggestion:
Adhesive unsuited to application – contact manufacturer for recommendation
Substrate Failure – The material being bonded fails before the adhesive.
In all cases of substrate failure, it is important to first identify the source of the stress that caused the failure. Effective adhesive choices must be suitable for the stresses on each joint as well as the substrate(s).
Examples of Substrate failure, causes – and suggestions:
Stress cracks at or near the bond area may be the result of solvent exposure. Select solvent free adhesives; wipe away excess adhesive before cure.
Delaminated or broken substrates often indicate a need for stronger substrates. In some cases, redesigning the joint may do the trick.
Surface Activated Structural Acrylic Adhesives
Problem – bond strength lower than expected.
Cause – too much initiator used. Because initiators are very thin and water like substances, they are nearly invisible once applied to some surfaces. Overuse of an initiator can reduce the strength of the cured bond.
Solution – use according to directions on the technical data sheet.