Adhesives

UV adhesives are a single component product which require no mixing or measuring. The environmental friendly adhesives offer cure-on-demand properties, eliminating extra curing times and process steps for an increase in production. With a wide selection of adhesives, they are easily incorporated into assembly process.

UV adhesives are ideal for bonding dissimilar materials and small parts. The UV adhesives have very strong bonding qualities over mechanical fasteners as stress is evenly distributed, as well as filling gaps for a neat appearance. The UV adhesives exhibit excellent impact and environmental resistance.

The photochemistry involved in UV curable materials is highly complicated and far beyond the scope of UVABC's.com. The chemistry of the UV curable material is tailored to the specific process with the method of application, UV source, and desired properties of the cured material all factored in.

UV curable materials consist of a minimum of three elements in two different mechanisms:

Elements;

Photoinitiator, oligomers, and monomers are the basic elements that make up the UV curable adhesive. The photoinitiator's role is to absorb the UV photon and transfer this energy to the curing process. Oligomers act as the backbone of the cured material to help give it the desired physical properties and the monomers create the cross linking action which cures the material. In addition to these elements, pigments may be added to achieve desired color or opacity and other additives may be used such as to control viscosity or tack .

Mechanisms;

1. Cationic
   Epoxy adhesives are typically used as a catalytic cure mechanism along with silicone compounds and some urethane materials. The catalyst is a by-product from the reaction of the photoinitiator element to UV light. By definition a catalyst is something that promotes a chemical reaction but is not consumed by the reaction. Because the catalyst is not consumed once the reaction is started it will eventually go through to completion. This capability is known as shadow curing where portions of the material that were not directly exposed to the UV light will eventually cure.
2. Free radical
   Acrylic adhesives are from an entirely different chemistry and different photoinitiator. The curing of acrylic adhesives is a result of free radical mechanism. The free radicals are produced by the photoinitiator when exposed to UV light. Because the radicals are consumed in the reaction, the cure will only happen where UV light is directly delivered and will only happen as long as light is being delivered.

(Not to scale. Representation Only.)

Filter Range (mm)	Typical Applications and Adhesives	Lamp* maximum irradiance	LED* maximum irradiance
320-500	General Purpose - suitable for most UV and visible light curing Epoxy and Acrylic adhesives on a broad variety of substrates	30,000	N/A
400-500	Visible Light Output - used with visible curing Acrylic adhesives, particularly when UV sensitive substrates are involved	19,500	N/A
320-390	Narrow band filter - some Epoxy adhesives have superior response to this filter. May also be used when unwanted substrate heating results from visible light irradiation.	12,100	N/A
365	Peak filter - May be used when unwanted substrate heating results from UV and/or visible irradiation.	10,500	8,000
250-450	Provides some UVC input that can be helpful for surface curing of Acrylic adhesives - Must be used in conjunction with an Extended Range liquid light guide, or a quartz fiber light guide.	30,000	N/A

*OmniCure® S2000 (Lamp) and OmniCure® LX300 (LED) used for comparison.

	Cationic Adhesives	Free Radical Adhesives
Cure Speed	Moderate	Fast
Strengths	High	Moderate
Resistance	High	Moderate
Surface Finish	Hard	Tacky
Shrinkage	Low	Moderate
Cure Depth	High	Moderate
Ease of Use	Moderate	Excellent
Plastics OK?	A Few	Almost All

Type	Service Temp. (°c)	Advantages	Disadvantages
Epoxies (Cationic)	-40 - 150	high bond strength good chemical and temp. resistance versatile wide variety of formulations good gap filling	short pot life requires special dispensing low peel strength high exothermic values slow fixturing toxicity
Acrylics (free radicals)	-40 - 100	versatile adhesives, fast curing good chemical and temp. resistance good impact, peel, and shear	primer required flammable toxicity
Polyurethanes	-50 - 120	good resistance to fatigue versatile good peel strength good toughness	moisture sensitive poor resistance to temperature short pot life toxicity
Cyanoacrylate	-30 - 100	fast bonding cap to plastic & rubber high tensile strength easy dispensing good adhesion excellent pot life	poor resistance to moisture and temp. poor gap filling cured adhesive is brittle poor peel strength
Anaerobics	-55 - 150	suitable for bonding coaxial joints versatile easy dispensing high strength excellent pot life good solvent resistance	typically brittle usually requires primers
Silicones	-70 - 260	flexible good for bonding glass to other substrates good moisture and temp. resistance variety of viscosities	moderate relatively low strength short shelf life corrosive slow curing limited solvent resistance
Polyimides	-45 - 300	high temp applications	demanding process during assembly