materials | SILICONE
Moldable optical silicones enable
LED lamps and luminaire designs
Silicone materials can withstand high heat and help deliver higher lumen density in SSL
product designs while simplifying the manufacturing process and enabling more complex
architectures, explains HUGO DA SILVA.
M
oldable silicones are emerging
as a viable option in LED-based
product designs for use as secondary optics, light pipes, light guides, and
other optical components. Indeed new formulations designed specifically for solidstate lighting offer the ability to withstand
high temperatures associated with the LED
semiconductor junction with no optical degradation. The material can also be molded
into complex shapes offering great flexibility to the product developer.
The global lighting market is on the verge
of a transformation as LEDs increasingly
replace conventional light sources. According to analysts at research firm McKinsey &
Company, the market for LED lighting will
explode at 30% per annum to exceed $81 billion by 2020, when it will represent close to
60% of the overall lighting market.
Such projections are supported by the
accelerating adoption of LED technology
in more and more general lighting applications, ranging from low-power, low-lumen
fixtures like downlight replacements –
where LEDs are increasingly replacing low
power compact fluorescent lamps and halogens – to more challenging applications like
street lights, industrial lighting, office lighting, high power halogens or illumination of
sport venues. As LED-based light sources
further penetrate applications that demand
higher lumen density and power, physics
will demand they operate at higher temperatures – even as lamp and luminaire designers seek to reduce the number of LEDs, or
pack the LEDs closer in order to develop
sources that are comparable or smaller in
HUGO DA SILVA (hugo.silva@dowcorning.
com) is Global Market Manager for Lighting
at Dow Corning, Lighting Solutions. Based in
Belgium, da Silva leads the optical, thermal
and protection business for solid state lighting
devices such as LED, OLED and innovations
related to lamps, modules and luminaires
applications.
FIG. 1. Moldable silicones enabled a design by LEDiL, a leading optics supplier,
to incorporate secondary optics that integrate dual functions – the optics shape
the light and seal the electronics against water ingress.
size than previous generation devices.
At the same time, LED designers are innovating modules, lamps and luminaires that
integrate multiple functions into fewer
parts (Fig. 1), or that incorporate smaller
or more complicated features. LED manufacturers are seeking new materials that
accelerate productivity, deliver higher yields
– particularly for larger parts – or reduce
waste.
New materials
All of these challenges effectively represent growing pains for an emerging lighting segment that is quickly evolving past
its early-generation designs and materials.
In response, the industry is exploring new
materials, such as silicones that, while less
familiar in LED applications, bring a rich
history of proven performance in dozens of
other industries, including advanced electronics, automotive and communications.
Silicones address several of the challenges
posed by next-generation LED designs,
including the ability to withstand higher
temperatures, support higher lumen density, enhance manufacturability and enable
more complex designs. Like LEDs, silicones
are evolving too. Recently, several leading
optical and LED manufacturers have been
putting a new class of optical-quality moldable silicones to the test in their designs,
and seeing positive results.
While some grades of silicone are transparent, moldable silicones such as those
recently introduced by Dow Corning represent a more advanced material engineered
expressly for LED applications, which is
to say they compare well in performance
against today’s best-in-class optical materials. Plus, like conventional silicone materials, moldable silicones exhibit low viscosity before cure, enabling them to be molded
more easily into complex shapes than either
organic polymers or glass, offering new
design options for secondary lenses, light
pipes, light guides and other optical components. This quality can also help reduce
Reprinted with revisions to format, from the October, 2012 edition of LEDs MAGAZINE
Copyright 2012 by PennWell Corporation
materials | SILICONE
manufacturing costs and cycle times in
injection molding and other processes, and
potentially reduce system costs for LEDilluminated lamps and luminaires. Lastly,
compared to many organic materials, the
chemical backbone of silicones makes
them particularly well suited to manage the
increasingly high temperatures of today’s
and tomorrow’s LED lighting systems.
Polycarbonate
Cyclo-olefin
copolymer
Acrylic
Hotter LED designs
High-heat applications are where moldable silicones shine. As a class of high-performance materials, they easily withstand
temperatures of 150°C and higher without significant loss of optical or mechanical performance. These qualities will
become more attractive as LED sources
increasingly deliver more intense white
light from comparatively smaller package
sizes, and as customers seek smaller lamps
and luminaires with higher luminous flux,
which will also drive up temperatures at
the device level.
As lumen densities increase, the package
temperatures within today’s high-brightness LEDs are already reaching as high as
150°C. This not only poses challenges for
epoxy encapsulants conventionally used
to seal LED packages, it is also raising heat
exposure for traditional secondary optics
materials such as polycarbonate (PC) and
acrylic. In general, the optical quality of
these plastic materials declines over time at
temperatures above 125°C and 95°C, respectively. The same applies to epoxies at temperatures above 150°C.
Such high temperatures cause traditional
optical materials used in LED lighting systems to yellow with age, which diminishes
the total system light output. This can have
a profound impact on lumen maintenance
and efficiency – dropping the expected 80%
lumen output below acceptable levels earlier
than the expected 50,000 hours of an LED
light source’s useful lifetime. Further, yellowing can adversely change an LED’s color
temperature over time. Such shifts in a light
source’s color are unacceptable to lighting
designers and end-users alike.
In comparison to conventional materials, moldable silicones retain their excellent optical stability and transparency
even after prolonged exposure to temperatures upwards of 150°C, exhibiting comparatively little or no yellowing and greater reliability across the visible spectrum. Indeed,
this emerging class of silicones enables LED
optical components to maintain their lumen
output and efficiency better over the course
of an LED’s useful lifetime.
Silicone resin
Materials aged at 200°C for 24 hrs
FIG. 2. Moldable silicones performed
well in tests that compared their
thermal and optical stability against
conventional materials, such as PC,
acrylic and epoxy.
Thermal testing
Thermal aging tests performed by Dow
Corning in an air-circulating oven at 150°C
for up to 10,000 hours demonstrated that silicone’s high optical transmission remained
steady – ranging from 90-95% in the visible spectrum – under such conditions.
Moldable silicones also retained their high
performance for other optical qualities during aging under high heat, including reflection, low haze and stable refractive index.
Moldable silicones also stood up well in
related tests that compared their thermal
and optical stability against conventional
materials, such as PC, acrylic and epoxy.
Thermally aged for 200 hours at 150°C,
moldable silicones retained their high optical quality (Fig. 2). In contrast, incumbent
organic materials exposed to identical conditions began to show significant yellowing
as temperatures exceeded 125°C.
The outstanding thermal and optical stability of moldable silicones may be enough
to inspire new LED designs. These materials
can help resolve issues such as glare control
while maintaining efficiency, color temperature stability and performance over time.
But in addition to this, silicone’s low viscosity before cure further enables designers to
consider LED components with more complex shapes, thinner wall configurations,
dual functions or very fine features.
Silicone structures
Structures like undercuts, for instance,
are difficult to easily achieve with plastics
because they cannot be easily released from
the mold. Fabricating parts that adjoin thinand thick-wall sections is also more challenging with plastic, which is more brittle
and therefore more prone to cracking or
breaking. Lastly, designs that use plastic
materials typically keep them away from
the heat of the LED light source, which
precludes configurations that shift plastic
optics closer to or even touching the LED.
Like conventional silicones, optical-grade
silicones are well-suited to precision molding applications. Before cure, the viscosity
of silicones decreases as heat increases. This
allows silicone resins to be injected into a
mold at lower pressures than what is typical for other materials, while still achieving good flow and reproduction. For example, their low viscosity enables replication of
micrometer-sized features on a lens surface
that, in turn, offers advantages in enhancing, focusing or directing light output.
In short, the physical properties of moldable silicones enable new designs that would
be otherwise very challenging to achieve
with incumbent materials, allowing for the
exploration of new shapes, styles and sizes
of LED lighting, as well as new applications.
Fabrication techniques
Silicone-based components can be fabricated
using a variety of techniques, including injection molding, casting/cavity molding and
others. While naturally very flexible, their
hardness may be tuned to either absorb
vibration or deliver excellent impact resistance. With their low moisture uptake and
ability to withstand harsh environmental
effects, conventional silicones are already
frequently used by the electronics industry to protect fragile components against
damage. Moldable optical silicones deliver
many of the same advantages.
Shrinkage is another familiar challenge
for plastics that is not as much of a problem
for moldable silicones, which need not be
cooled in the mold as long as plastic in order
to prevent warping. This helps reduce cycle
time – particularly for large parts – which is
important since the length of time that a part
must remain in the machine can represent an
important portion of its total cost, depending
on the mold, optical part design and process
factors mentioned above. In addition, the
comparatively low shrinkage of moldable silicones helps minimize or prevent warping in
components that integrate straight sections,
such as the back of semispherical optics.
The design and manufacturing advantages of moldable silicones cannot be overstated because they allow issues to be solved
materials | SILICONE
early in the LED design chain. A finished LED lamp might incorporate
over ten different silicone-based components, including adhesives, pottants, secondary optics and encapsulants (Fig. 3). Silicones are well known
for addressing challenges at the package level, and as LEDs penetrate into
general lighting those challenges will
become more common.
FIG. 3. A finished LED lamp might
Secondary optics
(diffuser)
Optics
(remote phosphor)
LED chip
encapsulants
incorporate over ten different
silicone-based components,
including adhesives, pottants,
secondary optics and encapsulants.
ships to develop innovative new applications for moldable silicones.
Conformal coating/
In terms of performance, moldwhite reflection
able optical silicones combine and
Thermal
Optics example
often exceed the best qualities of both
interface materials
Referring back to Fig. 1, LEDiL, a leadorganic polymers and glass. As demand
ing optics supplier to the world’s lightfor LED lighting accelerates over the
ing manufacturers, recently demonnext decade, moldable optical silicone
Adhesives for
strated this with the development of
materials will play a major role in the
environmental
seals
its innovative Strada-FT-TPHS lens
development of new high-performing
Conformal coating
module. The product of a collaboration
LED light sources, and help expand
between LEDiL, Dow Corning and other
design and processing options for LED
suppliers, the module features secondlamps and luminaires. Their good therary optics comprising an asymmetmal stability, moldability and mechanThermal pottants
Thermally conductive
ric lens fashioned from Dow Corning’s
ical properties offer benefits at virtufor heat control
encapsulants for
moldable silicones.
ally every stage of the LED value chain
power components
Notably, LEDiL’s application of mold– solving challenges to sealing, proable silicone technology enabled the
tecting, adhering, and shaping light.
secondary optics to perform dual funcWith the addition of their attractive
tions. In addition to creating an asymoptical qualities, moldable silicones
metric forward-throw light distribution to 150⁰C to prevent yellowing over the course can address design issues such as diffusion
pattern, the secondary optics also provide of the module’s lifetime. Neither these features and glare control, color temperature variaingress protection (IP) for the LED package. nor the secondary optic’s dual functionality tion and performance over time. Moldable
By using the lens material as a seal against would have been easy or even possible using silicones offer the potential to advance the
outside dust and moisture, LEDiL was able plastics, underscoring the versatile design adoption of LED lighting, drive down cost
to reduce the overall number of parts and possibilities that moldable silicones offer.
and help expand the technology into new
address a challenge that would normally
The performance demands may vary markets, such as general and accent lighthave fallen to its luminaire customers fur- wildly between one LED design and ing for home, office and retail spaces, trafther down the design chain. Namely, its cus- another in today’s evolving lighting indus- fic lights and other outdoor lighting, mobile
tomers would have had to seal the entire try. But moldable silicones are finding devices and automotive interior lighting.
luminaire.
application at every level of the LED design Silicone-based LED lighting could especially
Moldable silicones enabled further design chain at other top optical suppliers and benefit applications that require a cool
features for the module, such as an undercut, lighting designers. Further, Dow Corning touch and environmental toughness.
and thermal and optical stability from -45⁰ C is continuing to seek new industry partnerReflector materials