Ask the Engineer

Sticky Questions on Adhesives

How heat-resistant are light-curing adhesives, and can these adhesives be over-cured and break down? This week’s Ask the Engineer answers these and more.

Ask The Engineer: Sticky Questions on Adhesives


QHow heat resistant are light-curable adhesives?


AThe heat resistance of light-curable adhesives varies by product, but most will be in the 200-400°F temperature range.


QCan a light-curable adhesive be over cured and can it breakdown?


AAcrylic-based light-cure adhesives are very forgiving, so it is more difficult to over cure them. One case of extreme over-curing occurred when an engineer put an adhesive under a static light source and went to lunch. Upon return, the adhesive had too much UV light at high intensity with too much resultant heat, and started to breakdown.


QCan the level of cure affect an adhesive’s durometer?


AIf you plot durometer versus cure time with a constant intensity, you will actually see the durometer level off and plateau. Over-exposing the adhesive to 2x, 3x, 4x cure times do not usually have a negative effect.


QWill light-curable adhesives continue to cure after removing them from the light source?


ASome types of hybrid systems with moisture-cure technology may continue to cure after removing the light. Some UV light-curable epoxies continue to harden slightly if they have achieved 90 percent of full cure, so take extra care when working with these materials.


QAre there recommend adhesive fillers that will not hinder the curing process?


AThere are a number of fillers available on the market that will not hinder the cure process. Selecting fillers that yield a clear or translucent adhesive is a trick that the chemists use as they formulate.


QWhat, if any, surface preparation is required to optimize the effectiveness of light-curable adhesives?


AOn some difficult to bond to substrates like PE and PP, surface preparation can improve the adhesion. Chemical primers, corona treatment, or plasma treatment (preferably with oxygen gas) are very commonly explored, as well as UV pre-treatment of the surface.


QCan pre-exposing a substrate to UV light have a similar effect to surface treatment?


APre-exposing a low-surface-energy plastic or substrate with high energy, short wavelength UV light is one effective way to increase adhesion, similar to other surface treatment options.


QTo what depth will a light-curable adhesive cure?


ATypically, applications with a thickness of <0.25 inches are common. Best bond line thicknesses are usually 0.002-0.006 inches (0.05-0.15 mm). Conformal coatings are typically 0.001-0.003 inches and some special deep-section potting can be over 0.25 inches.


QIs there any concern, other than cosmetic appearance, when yellowing of the cured adhesive occurs?


AUsually yellowing is a cosmetic phenomenon, with little impact on physical properties. Some optical applications do have a problem with yellowing as it changes the optical transmission of light through the adhesive. In that case, an optically clear adhesive designed to be non-yellowing may be a good choice.


QDoes the fluorescence of the adhesive change from pre-cure to post-cure? The heat resistance of light-curable adhesives varies by product, but most will be in the 200-400°F temperature range.


AThe fluorescence of the adhesive does not change from pre-cure to post-cure in most cases.


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