Polyurethane Encapsulants - Enhanced Protection For LEDs in Challenging Environments
Over the past 10 years the use of LEDs for home and commercial lighting has increased exponentially, with a wide range of applications and specialist features that incandescent and fluorescent lighting systems simply cannot meet. The improvements in the efficiency of the LED over the past few years has also lead to a considerable improvement in the lifespan of the component, which is now substantially greater than both the traditional incandescent and fluorescent lighting technologies. This has meant that for a number of applications, LED lighting is now regarded as very low or even maintenance free, and is widely used for road, traffic and vehicle lighting.
The potential energy savings that can be obtained from the use of LEDs has indisputably driven the global acceptance of this comparatively new technology. However, the low energy consumption is only one of advantages available from the use of LEDs. As a result of their physical characteristics – i.e. small size and relatively low weight, they have exploded the boundaries of possibility for both lighting and lighting effects. This has effectively changed our concept of the LED as a simple white indoor bulb replacement to a multi-coloured, aesthetically pleasing lighting product with options for some really challenging environments.
The increasing variety and complexity of applications is creating new challenges in terms of protective materials and users require the highest level of performance available. In short, protective materials must ensure that the LED can perform in all environments, whether this be an internal application in Europe, outdoor street lighting in India or underwater lighting in Australia. LEDs are sensitive electronic components and often require additional protection against mechanical damage, moisture and other environmental factors.
The use of an encapsulation resin to protect LEDs has become increasingly common over the past few years. The selection of the resin is dependent upon a number of different factors; the viscosity of the mixed system, useable life and gel time for the liquid properties, as well as the hardness, density, colour and operating temperature of the cured resin. When directly protecting the LED, there are a number of essential factors; key amongst these is the clarity of the material applied to ensure the maximum utilisation of light output from the LED. In addition, any potential changes that can occur over the lifetime of the LED must also be considered.
Electrolube have developed a number of polyurethane encapsulation resins that cover a range of different property requirements that have been optimised for the LED encapsulation market. Two component (2K) resin systems are designed to offer the ease of handling and flow to allow the liquid resin to flow into the housing and around the LEDs, yet will react to form a crosslinked polymer that is tough and protects against the environment.
Polyurethane resins are available in a range of different size packs, which are designed to allow both manual and machine mixing and dispensing. For low volume or prototype work, often manual application is the best option. While for series production, machine application is by far the preferred method as it allows for the production of a consistently mixed resin, which is independent of the machine operator and the unit into which the resin is being applied. Certainly, the use of a machine is the most economical method of resin application, and there are a number of manufacturers who can offer suitable equipment dependent upon the volumes to be dispensed.
There are some important considerations to note when potting LEDs. For example, it is important that the geometry of the housing and any other pieces such as lenses are considered.
Image Caption: Geometry of LED Housing
The mixed resin is designed to flow around any obstacles in its way, however, if there are undercuts or overhangs then these can potentially trap air, which can result in poor adhesion, as well as lead to bubble formation during the curing time. If a large volume of resin is to be potted into a single unit, then it would be worthwhile considering potting the desired amount in 2 or 3 charges or shots, this then allows for any resin shrinkage to be taken into account, as well as helping to minimise any trapped air. Also the staged application approach allows for the use of a second resin, which could be an opaque or coloured layer to give the desired optical effects in the unit.
The optical properties of the cured resin have been carefully assessed to ensure that the resins preserve the colour characteristics of the LED as far as possible. When the calculated colour temperature of the LED is measured, it has been found that there is a shift in temperature, which is proportional to the depth of the resin layer applied over the top of the LED. It is, however, possible to control the colour shift by careful selection of the resin type and the depth to which it is applied.
Of course, we have mainly been discussing the LED itself, which is the most visible component of the lighting unit to the customer, but there are also a number of other components that are present that would also benefit from being encapsulated in resin to extend their service life, these include transformers, sensors, capacitors and resistors. For these components, there are a wide range of encapsulation and thermal management products which are specifically designed to maximise the service life of the complete unit. For certain applications, such as emergency, tunnel and explosive atmospheres lighting, it is possible to use flame retardant resins to encapsulate the units to meet the ATEX requirements.
The range of optically clear resins developed for LED applications are all polyurethane based resins. Polyurethane resins are highly suitable resins for the protection of LEDs in a number of different environments, in addition, they can also be adapted to offer additional benefits, such as pigmented systems used for covering the PCB up to, but not over, the LED. Such resins are used for protection of the PCB, offering an aesthetically pleasing finish whilst adding to the performance of the luminaire by reflecting the light off the PCB and increasing light output. The amount of light energy that a single LED can produce is relatively low, hence the need to cluster a number of units together in order to produce the desired amount of light. There are a number of methods for obtaining the desired colour, either with white LEDs, which produce light in a broad wavelength range, or using LEDs which produce light in a more discrete wavelength band. By combining a number of different colour LEDs together, it is possible to produce a wide colour palette.
In addition to offering clear products with a high clarity, Electrolube’s semi-rigid polyurethane resins, UR5635 and UR5634, have been developed specifically for LED lighting manufacturers to provide a high level of environmental protection for their products with additional benefits. UR5635, for instance, has a hazy/cloudy light diffusing effect, potentially eliminating the need for diffuser covers and caps. Its cloudy appearance offers a warm light diffusion effect which has seen the resin used to great success as an LED resin for both decorative and protective applications. UR5634 achieves a water-white transparency for LED potting applications and is particularly resistant to UV light making it especially useful as an outdoor LED encapsulant and for applications exposed to direct sunlight.
In order to minimise the potential of yellowing due to UV exposure, the polyurethane resins all use an aliphatic isocyanate which has been carefully selected to offer optimum long-term stability, as well as having one of the lowest hazard classifications. Since the resins are polyurethane based, the maximum service temperature is 130°C, for higher temperatures we have an optically clear silicone resin that can withstand temperatures up to 200°C.
UV stability is of key importance when considering polyurethane protection systems for LEDs and in order to verify this, Electrolube have completed weathering resistance tests on a number of encapsulation resins. Tests were conducted in accordance with ISO 4892 part 3 cycle 1 ‘Plastics Methods of Exposure to Laboratory Light Sources’ and carried out in a QUV SE Accelerated Weathering Tester. After 1000 hours’ exposure, the results indicated that the optically clear polyurethane and silicone resins, UR5634 and SC3001, have superior resistance to UV light, maintaining their clarity throughout the exposure testing. Exposure intensities will vary depending on geographical locations and therefore it is important to establish the correct accelerated exposure time for your region. As an example, this test is roughly equivalent to 4 years weathering resistance in a typical Northern European climate.
In order to determine the protection that our encapsulation resins afford, Electrolube carry out harsh water immersion tests. Under this testing regime, polyurethane resins have demonstrated excellent resistance to immersion in water, and in particular, salt water. The high resistance levels allow for a wide variety of underwater applications including underwater cable jointing, protection of underwater LED lighting in both fresh and salt water swimming pools, lighting on ship decks and protection of various sensor devices. In such cases, clear or coloured resins may be needed and must offer high adhesion and low dielectric constant throughout the lifetime of the device operating in harsh conditions. The following case study clearly demonstrates the effectiveness of polyurethane encapsulation resins for challenging and even harsh environments.
CASE STUDY – Underwater Lighting Application for Australia
An Australian customer approached Electrolube with a number of issues to address with the resin encapsulation of an LED lighting unit they had designed for swimming pool illumination. The lighting unit had to be sealed against freshwater and saltwater. Salt resistance was therefore imperative when selecting an appropriate resin for this application. The resin solution also needed to tolerate a working temperature range of 5 to 40°C, as well as being flame retardant with a specific resin colour; a light blue shade was preferred for aesthetic reasons. The customer had previously used an epoxy resin for this application from a different supplier, but due to the highly exothermic reaction associated with epoxies, some slight deformation of the base unit was occurring as a result of the high temperatures generated during resin cure, which was deemed totally unacceptable. Ultimately, the choice was narrowed down to polyurethane – but which product was going to work best?
Electrolube evaluated numerous water-resistant polyurethane options and decided to offer the customer the UR5097 encapsulation and potting compound. The cured polyurethane has high thermal conductivity – an important property as far as LED lighting units are concerned - and also has a wide temperature range. Importantly, it is flame retardant to UL94, which was another key requirement. As was the case with all the other polyurethane resins considered, the extremely low water absorption rate of polyurethane was considered the most critical property for this application.
UR5097 met all the requirements of this project with the exception of the colour, so with some re-formulation to do in order to meet the customer's requirement for a light blue, the colour was duly altered to the desired shade. The polymer used in UR5097 is also highly resistant to the transmission of water even at various pressure differences experienced due to the depth of the water. The resin’s thixotropic nature was also mildly modified in the lab to avoid any bleed of resin through the gap between the resin and LED unit and to also slow down the rate of sedimentation to an acceptable level, as well as still being easy to mix and pour into the unit. The polyurethane solution was met with full customer approval and assurance that the chosen resin will continue to protect their lighting products for years to come.
Polyurethane resins are typically elastomeric or rubbery in their cured state, which is particularly useful if the circuit to be potted contains delicate components. They also provide chemical, dust and moisture resistance, as well as excellent electrical insulation and good adhesion to most substrates, both metal and plastic. And unlike their epoxy counterparts, polyurethanes have a lower exotherm during cure, even for fast cure systems. However, cured polyurethane resins should not be allowed to rise above 130°C continuous service temperature. Polyurethane resins come in white, black, blue, clear straw, hazy/cloudy and optically clear formulations and offer a host of properties to meet the needs of high-temperature environments and those exposed to chemical contamination, mechanical stress or shock and moisture ingress.
Indeed, LEDs are the driving force today for general lighting applications. Thanks to improved luminous efficacy and brightness, along with efficiency and long life, they have revolutionised the market. They meet the need for energy-efficient lighting solutions unlike any other light source and can be found in all areas of life. Whether indoors or outdoors, for decorative or functional use, the LED offers fascinating possibilities for both now and in future. Applications include architectural, signage and street lighting, retail, leisure and office lighting, domestic and industrial lighting, medical, aerospace, automotive and marine lighting applications. The list is endless and just as the LED market is continuously growing and expanding into new areas and applications, the same is true for encapsulation resins.To overcome the practical and technically challenging problems that are frequently encountered, LED manufacturers are increasingly turning to companies such as Electrolube for help and guidance on which potting and encapsulation resin or thermal management product is right for their project. Whether the conditions provide a humidity, chemical, salt mist or corrosive gas challenge, Electrolube can offer conformal coatings and silicone, acrylic and polyurethane encapsulation resins to enhance performance and ensure reliability throughout the lifetime of the device. The growth of the LED lighting market is expected to grow into a $70 billion industry by 2020; a growth from 18% market share to 70% market share in just over 5 years (Forbes). With LED systems being increasingly used in a much broader range of applications, Electrolube are continually providing support for this ever-evolving industry, with continuous product development that recognises the importance of flame retardant optically clear resins specifically for the LED market.