Tom Shelley reports on a fast way of
breaking down adhesives and polymers and perhaps fast bonding
them as well
Car windscreens will in future be de-bonded for replacement by
intense flashes of white light.
Ordinary light turns out to be the fastest,
cleanest and most efficient way of breaking down polymer bonds
when they are not wanted.
As well as easing the removal of windshields, the technique shows
great potential for speeding robotic surgery, medical
sterilisation, lithography, bonding and removing graffiti.
The present development comes out of research at University
College, Swansea, and subsequent product design work by PDD in
Hammersmith. It began when Belron International, who own
Autoglass, Carglass and other leading players in automotive
windscreen replacement approached Swansea with a view to finding
a method of doing away with the need to cut away windscreen seals
with knives or cheese wire. Whereas car windscreens used to be
held mechanically in rubber seals, they are now firmly bonded to
the body of the car. The main reason is the need to provide
sufficient strength to withstand the impact of a car driver or
passenger striking an airbag in turn supported by the windscreen.
Removing a bonded windscreen by manual cutting is hard and heavy
work, with the ever present risk of accidents to operators or the
paint work of the vehicle. The task set to the researchers was
therefore to find the best way of quickly breaking down the
polyurethane adhesive bond between the black fritted outer area
of the windscreen glass and the polyurethane seal.
There are many possible ways of doing this. The study undertaken
by Swansea and Belron's development company, Automotive Glass
Solutions International, has come to the conclusion that the
fastest and most efficient method is to use single flashes of
white light from a xenon gas filled flash tube. The inspiration
was, apparently, the light flash process used to remove human
body hair.
When low-pressure xenon gas breaks down under the action of
4,000V in the flash tubes used, it is momentarily nearly as hot
as the surface of the sun. 4000J of input energy is converted
into 2,400J of light energy in the 350 to 480nm wavelength band
(green to blue). The light is concentrated by a linear parabolic
reflector onto the windscreen, just above the black frit region
round the edge.
The black frit is baked on during the manufacturing process in
order to protect the polyurethane bond and sealing bead from
degradation by sunlight. Here it serves to absorb the input light
energy with maximum efficiency and convert it to heat. Even after
a 25% loss caused by reflection at the upper surface of the
glass, 1,800J is still absorbed by the frit.
PDD then became involved in taking the idea and turning it into a
safe to use product.
Operators have to wear goggles. In addition, the lamp cannot be
activated unless it is first pressed down on the glass surface.
PDD says that before finalising the design, they used their
behavioural research method in order to study how windscreen
fitters work, and ask them what form they would like the product
to take. The final design has the flash tube hand held, but
equipped with a little wheel so that it can be pulled around the
edge of the windscreen and flashed from time to time.
A first batch of 30 prototypes has been made for workshop use. The next stage is the development of portable battery operated units for roadside use. In order to provide sufficiently high storage capacity and discharge rates, PDD has plumped for nickel hydride technology. Lithium, the only viable alternative, is considered too un-green in this day of increasing environmental concerns. The battery pack and electronics are mounted on a two wheel trolley unit.
As well as reducing the time required to replace a car windscreen
and the risk of accidents or damage to the car, the units have
appeal for other purposes. In the removal of windscreens, they
are proving to be of interest to: those expecting to be
responsible for dismantling cars for recycling, the car repair
industry, and fire brigades responsible for removing victims from
cars involved in road accidents. In trains, there is interest in
both using the flash tubes to de-bond windshields for
replacement, and also, possibly, to accelerate the cure process
when bonding them in. Light activated adhesive curing is becoming
of increasing interest in a wide range of industries,
particularly in electronics, and there is always a market for
something capable of speeding up the process.
If the flash tube is flashed on a newspaper page, it is
sufficiently strong to be able to bleach the ink, now that
printers have turned to polymer based inks which do not rub off,
instead of traditional mixtures based on oil and carbon powder.
This has given rise to the idea of using the flash tubes to
remove graffiti by breaking down the polymer based vehicles for
the pigments. The only practicable technique available at the
moment is to use a combination of environmentally evil chemicals
and scrubbing.
Following on from the hair removal technique, flash lamps are
being seriously considered as a means of treating human scar
tissue and burning out tumours. Flash lamps offer an almost
instantaneous means of sterilising surgical instruments and
devotees of science fiction films will recall the flash lamps
used to burn off superficial skin tissue from humans entering
hypothetical, high security, biological research laboratories of
sinister purpose. Fiction apart, flash lamps are a faster and
more efficient way of inputting light energy than alternatives
for lithography, an area of increasing interest in moves beyond
silicon chip processing to the mass manufacture of micro and nano
mechanisms and machines. More information may be found at www.pdd.co.uk/magic .
Design Pointers
Xenon flash lamps are the most efficient and cost effective means
of quickly inputting large amounts of non-contact energy into a
work piece
Able to lead to the breaking down of polymer bonds, they have
great potential both for breaking down polymers and also
accelerating light induced curing
Other applications range from rapid surgery to the removal of
grafitti
Other ways of non-contact machining and breaking things
up
One of the first methods to be studied in the present
investigation was the possible use of lasers. Large lasers are
certainly good for delivering energy into a very small region,
right on target, but they are an order of magnitude less energy
efficient than flash lamps and too dangerous to use hand held.
Ultrasonics are good for shattering kidney stones and welding
plastics, but require good mechanical contact to deliver their
energy. Microwaves are good for injecting heat into foodstuffs
and work pieces for heat treatment, but insufficiently localised
to be used as a precise machining tool. Hand-held, powered
mechanical tools, used on a car windscreen seal, are likely to
lead to even more operator and inadvertent vehicle damage than
the knives and cheese wire used at present.
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