TRANSPORTATION SOLUTIONS
SILICONES: BRIGHT IDEAS FOR
ADVANCED AUTOMOTIVE LIGHTING
SYSTEMS
BY
Throughout motoring history, starting with oil lanterns on
early “horseless carriages,” safety has been the priority for
exterior lighting systems for transportation vehicles of all
types. While this remains true today, advanced automotive
lighting technologies also are adding a new dimension to
vehicle styling. Significant changes in component packaging and light sources are opening exciting opportunities for
enhanced driving safety, vehicle visibility and occupant comfort, along with such benefits as smaller packaging and lower
power consumption.
Alexandre Lembi
DOW CORNING CORPORATION
SÃO PAULO, BRAZIL
Early oil and gas lamps were replaced with incandescent
bulbs, which remained standard for decades. Now, tungstenfilament headlamps are being challenged by high-intensity
discharge (HID) and light-emitting diode (LED) technologies
as preferred light sources. In lamp packaging, glass, metals
and mechanical fasteners have given way to lighter-weight
plastics, assembly adhesives and lens sealants.
The speed of change in vehicle-lighting technologies has
accelerated, and Dow Corning innovations are helping the
industry keep pace. Silicone adhesives and sealing materials
from Dow Corning are providing more durable, lighter-weight
and cost-efficient lamp assembly solutions. And, the company’s
silicon-based science is spurring the growth and availability of
low-energy, high-brightness LED lighting innovations. Some of
the key advantages of silicones in advanced automotive
lighting systems are explained here.
OCT
01
2013
TRANSPORTATION SOLUTIONS
DYNAMIC CHANGE IN
AUTOMOTIVE LIGHTING
In the past few years, automotive exterior
lighting has taken on a new role – still
focused on vehicle safety, but also
gaining importance for brand design
style. Traditionally, low-wattage incandescent lightbulbs have been used for
most automotive signaling and marker
lamps. Tungsten-halogen lightbulbs
have been a very common light source
for headlamps, and some recent models use small halogen bulbs for signaling
and marking functions as well. Such
incandescent light sources are taking on
more stylish looks, made possible with
new materials and packaging, yet they
are no longer the only option.
Today’s automotive lighting technologies such as HID Xenon projection
lamps, adaptive front lighting systems
(AFLS) and LED lighting are driving
major changes in safety, comfort and
convenience – not to mention new
dimensions in vehicle exterior and interior
styling. Sharper, brighter and faster-acting
vehicle illumination is available, while
drivers can look into curves and better
anticipate the road ahead. Safety is being
enhanced with better road visibility for
the driver, as well as with improved
attention to vehicle visibility for pedestrians and other drivers.
Clear-look headlamps, complex-shaped
lamps and light color effect are now
common design features. And LEDs in
front lighting are gaining acceptance for
headlamps, for daytime running lights
(DRL) and as indicators. Vehicle OEMs
and lighting suppliers are challenged to
optimize packaging costs, styling flexibility and service life across a full range of
vehicle platforms and global markets.
SILICONES HELP PACKAGE
INNOVATIVE LIGHTING
With automotive designers working hard
to improve performance, reduce vehicle
weight, increase fuel efficiency and trim
emissions – without sacrificing safety or
increasing cost of ownership – polymer
adhesives and sealants have become
materials of choice for lighting component assembly. In fact, Dow Corning®
7091 Adhesive/Sealant has been in use
for more than 15 years by a number of
lighting-system manufacturers for headlamp assembly. Silicone adhesives also
are widely used for sealing foglamps.
Other advanced silicone bonding and
sealing materials are not only opening
new lighting-design opportunities;
they also are contributing to lightweighting, faster cycle times, lower
costs, reduced fogging and extended
lamp-assembly life.
High-performance silicone adhesives
and sealants from Dow Corning include
proven, effective moisture-cure RTV
(room-temperature-vulcanizing) products,
as well as neutral-cure hot-melt options.
The RTVs are available as one- and
two-component systems for automated
lens bonding and sealing applications.
Typically, these silicones can be used
for assembly bonding and sealing of
headlamps, foglamps, DRL and CHMSL
(center high-mount stoplight) applications, and various signaling and marker
fixtures. Some are especially formulated for sealing LEDs with excellent
high-temperature resistance and optical
transparency.
Crystal-clear Dow Corning® brand
hot-melt assembly sealants are 100
percent silicone products that require no
mixing and provide high green strength
in automated applications. As neutralcure sealants, they provide immediate,
strong-bonding adhesion and can be
tack-free in 10 to 30 minutes. They allow
immediate on-line lamp-assembly
pressure testing without interrupting
production and can help reduce labor
costs, assembly cycle times and production floor space requirements. They
can provide durable, flexible, leak-free
seals with significant manufacturing
improvements.
OCT
02
2013
In general, silicones resist harsh environments and withstand high temperatures
to form strong structural bonds to glass,
metal, plastic and difficult-to-bond
substrates. Some key advantages include:
• Silicones are resistant to thermal
shock and retain strength in extreme
heat and cold.
• Silicones weigh less than and can
outperform polyurethanes without
softening in extreme heat or cracking
in extreme cold.
• Silicones are softer and more flexible
than highly crosslinked acrylic and
epoxy formulations.
• Silicones are compatible with lens
substrates, including polycarbonates
and polypropylenes.
• Silicones resist degradation from
ultraviolet (UV) light, salt spray and
occasional contact with nonpolar
solvents like gasoline and oil.
• Silicones have good transparency,
very low moisture absorption and
excellent noise- and vibrationdamping capabilities.
Selecting the best silicone bonding and
sealing solution can depend on several
factors, including the substrates to be
joined, lamp temperatures, and production and processing requirements.
Low-VOC (volatile organic compound)
choices reduce potential for fogging. Lens
sealing grooves also can be designed
to direct volatiles outside the lamp
assembly to minimize fogging.
SILICONES: TOP POLYMERS FOR
LED LIGHTING
LED technology has changed dramatically
since its initial limited uses in automotive media and instrumentation
applications. Today, high-brightness
LEDs offer stylish good looks, low
energy use and long life for a full range
of exterior lighting possibilities, as well as
for brand differentiation involving interior
design ambience. Yet, the brighter LEDs
also generate much more heat in their
TRANSPORTATION SOLUTIONS
components, which can cause traditional
design materials like epoxy to yellow
with age, reduce light output and even
change color temperature.
As such, the thermal, optical and protective requirements for these newer
LEDs are spurring the use of new material sets, and silicones are emerging as
the material of choice. Silicones manage
heat better than epoxy, urethane, acrylic
and other materials for fabricating LEDbased lighting systems. They can
provide excellent optical stability and
transparency in LED applications with
little or no yellowing and greater reliability
across the visible light spectrum.
Silicones are one of the only material
chemistries that can survive temperatures from -40 to 260°C (-40
to 500°F). Organics are generally
unable to withstand such high heat.
Epoxy, polycarbonate and cyclo-olefin
copolymers may provide stable light
output at junction temperatures up to
120°C (248°F), but higher heat fluxes in
new LED designs are pushing junction temperatures to 180°C (356°F)
and beyond. Exposed to such high
heat flux, organics are being pushed
beyond their physical limits, causing
degradation and less transmission of
visible light. In contrast, silicones can
withstand exposure to lead-free solder
reflow temperatures of up to 260°C
(500°F) during processing and higher
junction temperatures in service – with
no significant degradation of physical
or optical properties.
Another important feature in improving
device reliability is moisture resistance.
Hydrophobic by nature, silicones do not
easily absorb water, yet their gas permeability allows any trapped moisture
vapor to escape. Silicones for LED
applications can be supplied with very
low ionic impurity levels. As such, silicones can eliminate potential for
corrosion or adhesion problems.
In addition, by closely matching the
refractive index of the silicone material to that of the substrate, these LED
designs can minimize interfacial light
losses. As a result, silicones can deliver
some of the highest optical transmission values of any polymer material.
The combination of clarity and matched
refractive index contributes to increased
light output and improved lumen maintenance.
Silicon-based technology can provide
wafers, molds, sealants, driver protection, optics and thermal interface
materials for LED applications. These
include silicone gels, elastomer
encapsulants, gap fillers and thermally
conductive adhesives. They also can
be designed to cure as hard, durable
solids for fabricating discrete LED lenses.
The versatility of silicones allows package designers to obtain the optimum
hardness, while maintaining transparency and protecting sensitive electronic
components.
Processing ease is another critical element for material selection. Protective
materials based on silicone chemistry are
typically applied to LED packages and
modules with conventional dispensing
and casting techniques. Silicones are
well-adapted to precision molding and
capable of unique lens-surface features
for directing light output and minimizing
heat buildup – allowing fabrication of
lenses that can deliver specific light
intensities and distribution patterns.
Liquid injection molding processes can
help manufacturers achieve excellent
production yields and resulting costefficiencies.
rapidly evolving into adaptive systems
that shift directions during turns. These
AFLS concepts employ sensor input
for small motors that can position
the lamps according to yaw, speed,
acceleration and steering angle. Such
systems are ideal candidates for using
dry-film Molykote® brand specialty
lubricants from Dow Corning to ensure
“lifetime” reliability.
Then, too, future intelligent lighting systems may include infrared LEDs for night
vision systems, as well as headlamps
integrated with GPS and lane-departure
systems for advanced driver assistance
capabilities. Techniques for changing the
direction of light beams could change.
Shutter mechanisms may come into
play and, when high-power LEDs for
headlamps become less expensive,
chips angled to the side could be turned
on. Even further down the road, a single
light source may be employed, with a
network of fiber optics channeling light
for different automotive lighting needs.
With experience, ingenuity and problemsolving collaboration, the silicone
experts at Dow Corning will continue
to provide advanced automotive lighting
system solutions … from highly reliable
lamp-assembly packaging and highbrightness LED lighting options to
many more bright ideas beyond.
CONTACT
To learn more, visit dowcorning.com/auto
or email automotive@dowcorning.com.
FUTURE ADVANCES IN
AUTOMOTIVE LIGHTING
As brighter headlights have become
more common, systems that keep
them directed properly have gained
popularity. The headlight-leveling
systems that keep bright lamps from
blinding oncoming drivers are
Dow Corning and Molykote are registered trademarks of
Dow Corning Corporation.
OCT
03
2013
Form No. 80-3888-01