The powerful impact of protecting future LED lighting applications, including improved growth of animals, plants and algae
Jade Bridges describes some interesting applications of LEDs and notes how modern protective compounds and thermally managed materials are improving the performance and extending the life of LED lighting units exposed to harsh indoor and outdoor environments, with extremely positive results.
It is hard to ignore the phenomenal growth of the LED market, fostered by new applications, energy-saving advertisements, and corporate policies to switch to more efficient lighting systems. Combined with the design freedom and application possibilities now offered by the technology, LEDs are expected to gain a share of nearly 70 percent of the lighting market and be the most commercially practical lighting technology by 2020(1).
The outdoor lighting market is a typical area where LEDs have opened up new structural design opportunities in a range of applications, including floor lighting in high-traffic areas. This is a particularly intensive environment for LED lighting, but its popularity is on the rise due to the long life and compact size of LED arrays, making it the best option for hard-to-reach spaces.
Protection of lighting installed under a sidewalk involves the use of encapsulation resins that completely enclose the LED array. The resin must be resistant to abrasion from pedestrian footsteps, be UV stable, and not affect the color temperature of the light emitted. When these robust resins are needed, epoxy-based systems are often considered, but standard epoxy resins are not UV stable and will yellow over time. In addition, tests have shown the vulnerability of epoxy resins to abrasion; due to their hardness, they tend to scratch and, in cases of transparent resins, leave white marks on the surface.
Clear polyurethane systems, such as Electrolube’s UR5634, offer good UV stability and a considerable degree of flexibility, which means that when the surface is scratched or worn, no marks are left as is the case with epoxy, and therefore preserves and prolongs the aesthetic appearance of a lighting unit to which it is applied. Regarding the change in color temperature, our tests here at Electrolube determined that this is directly related to the amount of material applied and the potential physical stresses (temperature, shrinkage) exerted on the LED during the curing profile.
The graph in Figure 1 illustrates how the encapsulation depth of UR5634 affects the change in color temperature. Since the change occurs only as a consequence of interaction with the LED, once the first LED is encapsulated, it can be re-encapsulated with more resin to ensure adequate protection without further affecting the color temperature of the emitted light.
Figure 1. Comparison of brightness and CCT at different encapsulation depths.
The trade-off between encapsulation depth (and, therefore, the degree of protection achieved) and color temperature change is of paramount importance. In the first case, the level of protection can be controlled by the type of material or chemical used: a standard protective coating, for example, will provide a basic level of protection from moisture, salt spray, etc. Later, a thicker two-part coating can be applied to provide greater protection from condensation and (if present) corrosive gases. Finally, an encapsulation resin can be applied and its thickness adjusted to achieve the desired level of protection. Figure 2 illustrates an example of the protection provided by these three layers of materials in a corrosive gas environment. We now consider other new and exciting applications for LED lighting and the protection challenges designers face.
Figure 2. % reduction in luminous flux after exposure toH2S
Algal cultivation
Algae produce organic food molecules, using only carbon dioxide and water, through the process of photosynthesis. An important by-product is oxygen, and these are in fact responsible for producing about 30-50% of the global net supply available for respiration. Along with food molecule formation and oxygen production, algae are also being cultivated for use in food processing, with applications ranging from reducing ice crystal formation in ice cream to clarifying beer and wine(2).
Figure 3. Close-up view of the growth of some typical algae in a photobioreactor
Environmental conditions for algae growth can be monitored and controlled by varying the wavelength of light used, simulating conditions specific to particular strains. Traditionally, high-pressure sodium and metal halide/fluorescent lighting has been used for this purpose, but the toxic materials contained within these units present a risk of contamination if they are damaged during collection. In addition, the wavelength of light emitted is not optimized for photosynthesis, which makes the process inefficient. An alternative approach is to use LEDs, which are safer for use in a food production environment. Also, very importantly, it is possible to control the wavelength of light emitted by LEDs, leading to higher growth rates and reductions in process costs.
As with outdoor architectural lighting applications, LEDs used in algal cultivation must be protected. Algae are normally grown in photobioreactors to avoid contamination or growth of unwanted species, and for LED lighting to work in these containers, it must be protected, mainly from permanent contact with water. Although it is a general practice to seal units with optically transparent encapsulation resins, a compromise must be made in terms of the depth of encapsulation, which will affect both the level of protection achieved and the desired wavelength of light emitted.
It is important to note that LEDs generate heat: any air gap in the resin near the surface of the LED array will result in the development of hot zones, increasing the ambient temperature near the LED and, consequently, reducing its operating life. Here the choice of an appropriate resin is important, since certain properties, such as viscosity, will affect void formation.
At this point, we have addressed the need for protection of LED lighting arrays against potentially harmful environments, but what about secondary equipment, such as power supplies and controllers? Control units can be placed on site or installed remotely, and they too most likely require protection from the environment. These units can also be enabled for wireless connection, in which case the dielectric constant or relative permittivity of the resin must be taken into account. Normally, relative permittivity values of about 3-4 are ideal, but since applications and signal frequencies vary, it is important to test the protective compound in locoperto ensure that it meets specifications.
Control units are often equipped with internal temperature sensors, such as PTC (positive temperature coefficient) thermistors, which may require the use of thermally conductive compounds to ensure that all air gaps are minimized in order to improve heat transfer to the sensor surfaces. Thermal management pastes, room temperature vulcanization (RTV) compounds and void-filling products are often used for such applications, and the final choice depends on the structure of the sensor and the corresponding size of any air voids that may be present. The selected thermal management material must also remain stable at the application thickness in order to achieve the best thermal transfer to the sensor.
Products such as the recently introduced Electrolube HTSX can provide low thermal resistance and excellent stability over a wide operating temperature range but, as a thermal interface material, it is designed to be applied in thin layers. If the air gap present is greater than 200 microns, results can be improved if a void filling or RTV curing material, such as Electrolube HTCPX or TCOR, is used.
For power supplies, an encapsulation resin can serve two purposes: protection from the environment and heat dissipation. For example, Electrolube UR5633 provides very high levels of thermal conductivity in combination with excellent water resistance and low temperature tolerance. However, the viscosity of this two-part polyurethane resin is very high and, therefore, may not be ideal for applications in confined spaces where air voids are likely to form. Keep in mind that air gaps in an encapsulated unit can create hot spots, leading to reduced performance and durability. Low-viscosity resins, such as Electrolube’s two-part epoxy ER2221, may be more suitable in these cases, while high-performance resins, such as Electrolube’s ER2225, offer useful additional properties, including chemical resistance and high temperature tolerance.
Lighting the way to better animal welfare
LED-based lighting technology now also helps improve the health, welfare and yield of livestock on farms. Many studies have shown that improving the quality of lighting in animal pens, stables, and barns reduces stress, while using LEDs of a specific wavelength can address other issues, such as the attraction of flies to the light source. In addition, light color temperature has been shown to have an impact on the growth and positive behavior of chickens. It is not surprising, therefore, that the poultry industry is gearing up to retrofit as many as 600,000 barns with LED lighting in the coming years(3).
Lighting in these environments must meet IP66 rating for dust tightness and resistance to penetration from high-pressure water jets. In addition, it must be resistant to corrosion (especially corrosive gases that may be present in poultry barns), high humidity, and potential impact damage. Protective compounds, such as protective coatings and encapsulation resins, provide the long-term protection required to meet the rigor of these demanding environments.
The applications reviewed in this article highlight the importance of electrochemicals and their ability to protect LEDs and associated products from adverse environmental conditions while ensuring efficient heat dissipation from these components. As is the case with all electronic applications, the proper selection and implementation of protective coatings, encapsulation resins, and thermally managed materials is critical to the performance and expected lifetime of an LED product, and these issues must be considered early in the design process.
Electrolube has become a reputable global supplier to this rapidly evolving market, providing technical support and guidance on material choices for designers and manufacturers of LED lighting fixtures in a wide range of applications and environments that includes everything from weather-resistant street lighting to sleek, modern interior lighting designs.
References:
(1) McKinsey & Company: Lighting the way: Perspectives on the global lighting market, second edition, 2012
(2) https://www.britannica.com/science/algae/Ecological-and-commercial-importance [Ultimo accesso luglio 2018]
(3) https://greengage.global/nfu-poultry-article-on-greengages-led-lighting-for-livestock-farming/ [Ultimo accesso luglio 2018]
