Adhesives for LEDs and Lighting
LEDs (Light Emitting Diodes) are semiconductor light sources originally developed with two “legs” (a cathode and an anode). When voltage is applied light is emitted – “electroluminescence”. The colour is determined by the energy band gap of the semiconductor (although often it can be changed depending on the colour lens it is encapsulated in). Originally LED use was primarily for low intensity infra-red (such as for TV remotes) but nowadays LEDs can be all colours of the visible light spectrum. An exciting development for us adhesive manufacturers has been the introduction of UV-emitting LEDs which are ideal for curing UV adhesives. Nowadays more “legs” have appeared on complex LEDs. These offer more colour output from the one LED, in fact RGB LEDs allow the generation of most colours.
The LED device market is fast moving and innovative, with advancements in energy efficiency and improving lighting output from the smallest of sources happening all the time. With this comes a strong demand for technically innovative adhesive materials for bonding, potting, and sealing lamp parts and LEDs.
LEDs are now cropping up all over the place, replacing less energy efficient incandescent lighting. Most electrical devices have LEDs built in to indicate if the device is switched on or on standby. Desk lamps, room lighting, commercial lighting, outdoor lighting, vehicles, aerospace, and the list goes on… Cities and townships are getting savvy and are now opting for LED streetlamps, cutting energy bills and reducing maintenance costs – LEDs last a gazillion times longer than the old type of bulb (well, to be a bit more exact, they are about seven times more energy efficient and last at least 5 times longer than a high pressure sodium bulb).
LEDs are also versatile; you can arrange rafts of them in different shapes to get some great lighting effects. How good are those Audi vehicle lamp clusters now with the LEDs around the edges? Take a look at this year’s Eurovision stage lighting – jaw dropping effects achieved with LED lighting.
Applications for adhesives for LEDs and Lighting
LED lights are often potted into strips which can be inserted into flooring joints both indoors and outdoors. These are very attractive and are appearing in more and more public places – they are no longer limited to chic bars or high-end homes with architectural wow factor, nowadays LED strips are even appearing in car parks and parking lots! LED potting normally requires clear adhesive with as close to 100% light transmittance as possible with excellent optical properties as well as good resistance to wear and tear, weathering, water resistance, and sunlight aging. The other factor that needs to be taken into consideration is that sometimes lighting strips can be quite long. If dissimilar materials are bonded there could be some differential thermal expansion and contraction – in these situations a slightly more flexible adhesive can be most useful.
Adhesives can be used throughout the lighting industry. Electronic PCBs and lamp holders need bonding to hold them in place. Light reflectors require bonding – sometimes high temperature adhesives are required to cope with temperature extremes for this type of application, particularly for industrial lighting and stage and theatre lighting where high powered bulbs can radiate a lot of heat. Other general applications include wire tacking, bonding heat sinks to help dissipate heat (so thermally conductive adhesives are needed here), and attaching components.
Adhesive are a great way to join parts without having to use bulky, unsightly fasteners. Given that LEDs are small, lightweight, and look good, why would you use metal clips and fasteners when you can use a clear, invisible adhesive to hold things in place? Another benefit is that adhesives provide a 100% seal against moisture and don’t rattle (think about LEDs and lamps in automotive applications that could loosen due to vibration).
Below: LEDs for interactive stage flooring held in special jig. UV cure adhesive is used to hold and protect them.
Even if applications require threaded bolts to hold lamp components together, it is a good idea to use an adhesive threadlocker to prevent vibration loosening of the lamp components. It can also help prevent corrosion, making future maintenance a lot easier. In the case of underwater pool, yacht or marina lighting, adhesive threadlockers are essential to make sure salt water, sand etc. don’t get into the threads and cause the bolt to seize – otherwise you wouldn’t be able to replace a blown bulb. These underwater lights can be quite expensive (but I guess if you own a yacht or a swimming pool, the cost of a new lamp is a drop in the ocean…quite literally).
An obvious application is sealing between a lamp lens and the external housing. Normally a clear, colourless, non-yellowing, sealant is preferred but sometimes this is not a worry if this area of the bond is not on show. Lighting for public areas and street lighting requires sealants to be resistant to rain, hail, and sunshine, and also be resistant to vandal attack and theft. Sometimes the highest strength adhesive is chosen over one that is more aesthetically pleasing in order to survive vandals and thieving scoundrels.
Types of adhesive for LED and lighting applications
Clear potting can be done with epoxy glue – a two-part epoxy resin and hardener which cures at ambient temperatures when mixed in the correct proportions. Choosing a low viscosity, slow setting adhesive can help minimise the risk of air bubble entrapment. There are other ways air bubbles can be eliminated including automatic mixing under vacuum, drawing a vacuum on the potted parts whilst the adhesive is still liquid, or using vibration tables to drive air bubbles to the surface where they can be quickly popped by passing a heat gun over the surface.
Flexible clear MS-polymers provide a fantastic seal for lengthy sections where flexibility is required. Another benefit is this type of adhesive is single part and doesn’t require mixing. Also, it doesn’t yellow in sunlight and it actually loves rain and hail as this strengthens the adhesive. Flexible clear MS-polymers adhesives provide a great water-impervious seal between lamp parts and can be used as a gasket-type seal.
High temperature cyanoacrylates
These instant adhesives are popular for reflector-type applications for lighting where there can be a lot of heat (not so vital with LEDs as these run a lot cooler, 80-90% of LED output is light rather than heat, making them much more efficient). Cyanoacrylates are great on high speed production lines as they cure in seconds and require no mixing, eliminating the need to clamp parts for hours or use dispensing guns, mixing nozzles, weighing scales, etc.
Choose a threadlocker which suits the diameter and coarseness of the thread you wish to lock. Generally for lighting applications, bolts are very small with very fine thread – so a low viscosity anaerobic threadlocker is best suited to the application, if it needs to be undone again for future maintenance then go for a medium strength product. Anaerobic threadlockers are single part and only cure on metallic surfaces when in a confined joint (excess material remains liquid and uncured).
Thermally conductive adhesives for heat dissipation / thermal management
Depending on the substrates and the preferred application process, there are thermally conductive adhesives designed to bond heat sinks and other devices where good heat transfer is required. Single part heat cure epoxy, two-part epoxy, and two-part structural acrylics with a high thermal conductivity are available.
For further help and advice, product recommendations, and information about Permabond’s adhesives for LED & lighting applications, please feel free to contact Permabond and we will arrange for our chemists to assist you further.