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Advances in light curing and light curing adhesives

As adhesive manufacturers, we rely on our team of experienced chemists to continue to formulate state of the art products to satisfy our customers’ appetite for innovative materials which reduce manufacturing costs and increase product quality.  The Permabond motto is “Our Science … Your Success”.  We formulate – our customers succeed.

When it comes to UV light curable adhesives our chemists continue to push the technology to its limits.  Following are a few of our recent successes.

Optical Clarity PERMABOND® UV6160 is a single part, fast setting adhesive which cures rapidly on exposure to UV light.  UV6160 has been specifically formulated to provide excellent optical clarity and enhanced transparency –even when used in thick layers –  in compression or in a heavily loaded joint.

Water Resistant Adhesive PERMABOND® UV6231 is a single part, fast curing, UV curable adhesive. Its excellent optical clarity and resistance to yellowing make it ideal for bonding glass and crystal for a high quality finish. It has excellent resistance to moisture and humidity making it ideal for bathroom -type applications (such as bathroom scales, shower doors etc.) and also outdoor applications where good resistance to weathering is essential.

Tack Free Coating PERMABOND® UV681 is ideal for coating electronics (such as conformal coating) to protect against environmental and vibrational damage. Amongst various other applications, the optically clear / tack free formulation also makes PERMABOND® UV681 particularly suitable for coating smart card microchips. Its low viscosity ensures an even, bubble-free coating and high temperature resistance allows it to resist wave-soldering.

Tack Free PERMABOND® UV683 is ideal for coating and doming applications. The optically clear / tack free formulation also makes this product particularly suitable for coating smartcard microchips, as well as various other applications. Its viscosity makes PERMABOND® UV683 suitable for encapsulation of electronic components and its high temperature resistance allows it to resist wave soldering.

Although I don’t expect we’ll change our motto from “Our Science … Your Success” to “Their Science … Your Success”, our customers have obtained some recent successes with a little help from our friends Lamp Manufacturers!  The advances lamp manufacturers have made in UV curing equipment, and especially UV-LED curing technology, definitely deserves some mention.  There are a variety of options for creating UV light including LED or Arc and Microwave. LEDs give off UV light through electroluminescense when voltage is applied to each LED chip.  Many LEDs can be assembled into various configurations to produce the size and shape of the light.  Conventional UV light is created by mercury with energy (either with high voltage electric current or microwave) to excite them – as they return to their ground state they give off UV light.

LED lamps

UV light produced with LED lamps gives very consistent output over a long bulb life.  The spectral output is more narrow than light produced with mercury vapor but is in a range that is safer to use (in fact much of it is long enough wavelength to be considered visible light).  This narrow wavelength is safer but ensure the UV adhesive or ink formulation will cure appropriately in the defined wavelength – adhesives designed to cure with mercury vapor lamps may require higher intensity or longer cure times.

Mercury vapor – Mercury vapor can produce UV light over a broad spectrum, including shorter wavelengths which can be dangerous if you aren’t properly protected.

Arc & Microwave

Arc lamps – Arc lamps are popular in part because their simplicity allows for a variety of types, sizes, and styles, from small focused lamps to very long lamps used for curing UV inks on packaging and UV coatings on wood.  They are generally mounted in an enclosure with a blower to provide cooling, and a reflector.  Reflectors can be made of various materials but polished aluminum is often used.

Although they are highly versatile, the bulb life of arc lamps should be considered.  For every hour the bulb is lit the intensity of light emitted reduces.  That doesn’t mean you should shut the lamp down in between each use, as each time the lamp is turned on the intensity reduces (generally equivalent of one hour of run time) and the lamp must be cooled prior to turning it back on – the amount of time depends on the configuration but it can be anywhere from a few minutes to a half hour.

Microwave Lamps – Microwave lamps are not as versatile in size as Arc lamps, but lamps can be positioned in a line to achieve the length or width required.  They don’t degrade as quickly as arc lamps but do produce heat.

D type bulb 350 – 400 nm common for curing UV adhesives

H type bulb = 220 – 320 nm with a spike at 365 nm.  Ideal for achieving tack free, dry coatings

V type bulb = 400 – 450 nm Ideal for visible light cure adhesives

Sunshine

A good sunny day will provide about 3mW/cm2 which is quite low intensity and will require longer cure times.  However it is free and providing a rainy day won’t interrupt your manufacturing schedule greatly, it can be used.  Qualifying the cure time based on the weather is not appropriate for all applications but many artisans are using this natural UV curing technique.

Source Advantages Disadvantages
Sunshine It’s free Variable low intensity

Broad Spectrum

LED UV light Long bulb life

Instant on / off

No hazardous mercury disposal

No need to vent ozone gas

Higher initial cost

Narrow spectrum (+/- 10nm)

Mercury Vapor – Arc Broad spectrum

Versatile

 

Intensity decreases with time Bulb life of 1000 to 2000 hours

Disposal of mercury

Mercury Vapor – Microwave Consistent intensity Limit to 10” without additional units

 

Cost

UV curing systems can range from a few dollars to many thousands of dollars.  If you are implementing a high speed production line, there are a several respectable equipment companies to work with.  Contact Permabond for a referral.  If you are looking for a low cost solution, some options are listed here.

Getting the most out of which ever lamp you choose

UV light drops off inversely with the square of the distance (really fast)!  Some light sources have the highest intensity closes to the bulb itself, however others have a focused beam.  For example the highest intensity may be at 2.1” from the bulb, in which case moving the bond area closer to the lamp than 2.1” would cause the same quick decline in intensity.

For most applications it is recommended that a jig or fixture is used to maintain consistent distance between the lamp and the bond area.

When using a lamp that loses intensity over time it is important to monitor the intensity with a radiometer to ensure the bulb is still adequate to use for the application.  It isn’t advisable to simply monitor the number of hours on the bulb, as that doesn’t include intensity loss experienced from firing the bulb.  To maximize the life of the bulb, cure schedules can include an extended cure time at lower intensity.

The basic rule of thumb is “the higher the intensity the faster the cure”.  For applications in which there is a very large depth of cure needed, lower intensity for a longer time may be best.  For coating applications where the UV adhesive or coating is not contained between two surfaces, higher intensity may be desired to create a tack free surface.

Technical data sheets provide some information on the adhesive’s general energy requirements.  Unless using a dual cure system, cure of UV adhesives only occurs while it is exposed to the light.  It will not continue to cure once the light is removed.  Full cure is when the adhesive has transformed from a liquid to a solid and when further UV light exposure does not improve the adhesives properties.

To test for the lowest intensity allowable to achieve full cure, increase the distance between the bond area and the lamp to reduce the intensity.  Keep the time consistent and find the point at which the bond strength decreases.  It is perfectly acceptable to add a safety margin.

Contact Permabond technical support for information related to your application.

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