UV Adhesives

UV-Curing Adhesives

High performance adhesives consist most often of at least two components, which have to be mixed before application. Their pot life is often less than half an hour.

Ultraviolet-curing adhesives contain additional premixed components, but no solvents. A dormant photo-initiator is the secret which allows to store them for at least six months in the dark. After application of these adhesives, UV/light will awaken the initiator, transform it into radicals which can start a rapid polymerisation reaction.

Types of UV Adhesives

There are a wide variety of UV-curing materials available, for a broad range of applications.

The first type of adhesive is an epoxy-based material.

Epoxy adhesives use a catalytic cure mechanism. The catalyst is a by-product from the reaction of the photoinitiator to UV light. By definition, a catalyst is something that promotes a chemical reaction, but is not consumed in the reaction. One consequence of this is that UV-curing epoxy adhesives can exhibit a shadow curing capability: after having been initiated by UV, they can cure between two opaque materials (metal, filled plastic).

Additional performance properties of epoxy-based adhesives that can be modified include impact resistance, shrinkage, glass transition temperature, high temperature strength, surface-specific adhesion characteristics and chemical or moisture resistance.

Acrylic adhesives result from an entirely different chemistry and a different type of photoiniator. Curing of acrylic adhesives results from a free radical mechanism. The free radicals are produced by the photoinitiator when it is exposed to ultraviolet light. However, the free radicals are consumed in the adhesive cure process, so acrylic adhesives can only cure where UV light is delivered. At least one of the components being bonded must be UV-transparent to some degree. Another consequence of this cure mechanism is that no shadow cure capability is evident.

Modification of properties in acrylic adhesives is more often conducted at the chemical level, through changes in formulation or combination with other base resins. Wide ranging properties can include impact resistance, surface insensitivity, environmental resistance and others. The emergence of urethane acrylate adhesives, as well as acrylated epoxies, begins to make simplistic adhesive classifications more challenging.

UV Cure Conditions

Curing equipment should be engineered to ensure repeatability in critical UV parameters, such as intensity and duration of exposure. The ability to adjust the spectral profile of a UV light source can also be very important, as different adhesives may respond more favorably to different spectral inputs. Similarly, when working with a variety of bonding surfaces-notably plastics - changing the UV parameters can avoid excess heating of the parts.

General remarks for users

The following text is almost a BLOG, to give hints about the selection, testing and evaluation of UV-adhesives as you seldom find or receive them from the suppliers!

Exchange of Informations for adhesive selection

Your supplier should be informed about the substrates to be combined, preferably receive samples for preliminary checking. He should suggest a product with good adhesion to the substrates, free of stress corrosion risks. He should supply clear informations about:

  the correct UV-spectrum
  the needed irradiance, dose to achieve a correct cure
  typical range of illumination times.

Data sheets often suggest to work at 365nm, 50mW/cm2 during 2 minutes; such indications for spot curing applications are useless particularly for automated assembly!

You must receive adapted instructions: UVA (320-390nm), if possible with additional visible UV (390-500nm), times from less than 1 to 5s, irradiance values in W/cm2, typically 2W/cm2 and more.

Some simple adhesive tests

Apply three small adhesive drops on your substrate or on a microscope glass. Illuminate them one after the other for 5s, with a lightguide distance of 10mm.

Do you observe white fumes? In this case the adhesives contains reminders of solvents or other very volatile substances. This should not be the case, because they may be toxic, irritating (Xi), they risk to deposit on the hot lightguide end, and reduce transmission. See also under "cleaning". If the drop becomes yellow or brown you should increase the distance of the lightguide or reduce the irradiance. (this happens with some epoxy-adhesives and may not be a problem for technical bonding).

Test adhesion: push the droplet from one side with a needle or a small screw-driver. If it does not move, the adhesion is ok, if it moves, adhesion is poor or base layer is still liquid, due to insufficient cure.

Scratch the surface: trace left gives an idea of the cure or he hardness of the adhesive. Check the surface with a magnifying glass: if surface is liquid, the adhesive shows surface inhibition by oxygen which hinders polymerisation. Ask for an adhesive which cures with UVA and/or visible UV, because most spot-curing sources do not emit (a lot) of UVB (below 320nm).

Reactivity: Apply adhesive droplet between two microscope glasses. Make scissor movements, 10mm below fixed lighguide; by an increase of the viscosity/polymerisation this rapidly becomes impossible. (after 1s= fast adhesive, <5s= ok, more than 10s adhesive is "too slow", ask for more reactive formulation! This is a simple quite spectacular demo, also to show the effect of UV light curing.

Adhesive producers and research institute make more complex and expensive testing; from them you can obtain additional information and results.

Other remarks are in preparation.