THE MISSING INK !

[Anonymous]

The missing ink
March
2008  The instant photo is dead, long live the instant photo. After 45
years, Polaroid says it will no longer make the iconic film for the
press-click-develop cameras, blaming the rise of digital photography
for a fall-off in sales. But its replacement represents a startling
jump in printing technology: a pocket-sized printer that produces
full-colour photos without using ink.Developed by Zink, formerly part
of Polaroid’s research and development operation but spun off as an
independent company in 2005, the technology has two components: a
thermal printer, and a new type of photographic paper.Within this paper
is the key to the photos — three layers of a dye material which is
transparent and colourless in a solid crystalline form but which
becomes highly coloured when melted.’The image-forming technology is
embedded in the paper,’ said Steve Herchen, Zink’s chief technology
officer. ‘There’s never been anything like this before. It’s the first
entirely new colour printing system for over 20 years.’

At the
heart of the Zink system is the crystalline dye, which the company
refers to as an ‘amorphochromic’ material. Development of the material
started around 2000, said Herchen, when Zink was part of Polaroid. ‘Our
chemists came up with the molecular design for them, designed
syntheses, and worked with other companies to develop the processes to
make them at a larger scale,’ he said.

Like all organic dyes,
the compounds are coloured because their molecular structure contains a
chain of carbon atoms that are joined together in such a way that
electrons can zip up and down the chain freely. This requires energy,
and because electrons obey the arcane laws of quantum mechanics, they
can only absorb specific amounts of energy, corresponding to specific
wavelengths of electromagnetic radiation.

The quirk of this type
of structure is that the amount of energy the electrons absorb
corresponds to the wavelengths of visible light. Different-length
chains absorb different wavelengths and the colour you see is known as
the complementary colour to the wavelengths that have been absorbed.
For example, if a substance absorbs indigo light, it appears yellow; if
it absorbs green light, it will appear red.

Zink’s
amorphochromic dyes add a literal extra twist to this molecular
racetrack. Organic molecules are flexible and, when they crystallise,
they flex into a shape — known as a conformation — that allows them to
pack together into a geometrical lattice structure.In the case of the
Zink dyes, they can only form a lattice structure if they twist into a
shape that does not allow the electrons to zip up and down the carbon
chain. That means they cannot absorb visible light, and therefore
appear colourless.’When they melt, they’re released from the rigid
conformation that they’re held in by the crystal lattice,’ explained
Herchen. ‘They’re then free to be in many conformations, and that
enables the resonance within the chemical structure that leads to the
colour.’

Having developed this class of molecules, which Herchen
said is unique to the company and covered by patents, the Zink chemists
had to tune them in two ways.
First, they had to make three types of
compound, each giving a specific tone when melted: one magenta, one
yellow and one cyan. Second, they had to ensure that the crystalline
form of each dye melts — and therefore became coloured — at distinct
and widely- separated temperatures. The yellow dye melts at 200°C, the
magenta at 150°C, and the cyan at 100°C.

To make the photo
printing paper, the crystals are processed to a size around 2µm and
dispersed in water, along with some additives that help to control the
way the colour develops and stabilise it once the photo is printed.A
multi-layer thin-film coating process sprays several layers
simultaneously onto the backing, a white plastic film. Between each dye
layer is a thin layer that helps regulate the way heat flows through
the structure, and over the top is a UV-protective layer to prevent the
colours fading in light, then a clear polymer which protects the
crystals from the hot print head and also makes the photos glossy and
waterproof.The whole structure is then dried to leave the multi-layer
structure with the cyan-forming crystals nearest the backing, the
magenta layer in the middle and the yellow on the top.This arrangement
is designed to work with the type of thermal print head the Zink system
uses. ‘Tens of millions of thermal printers are sold every year, it’s a
well-established technology,’ Herchen said. ‘But the way the head is
driven is unique to Zink.’

Thermal print heads contain a line of
tiny resistors that heat up when a current is passed through them. The
Zink print head is 2ins long and has 600 resistors, each corresponding
to a colour pixel on the printed photo. ‘What’s crucial to Zink is that
we had to be able to control them independently, both in terms of how
hot they get and how long a thermal pulse lasts,’ said Herchen.To
obtain a yellow pigment on a single pixel, the resistor element
delivers a hot but short pulse, hot enough to take the crystals below
it to 200°C but short enough that the heat does not have time to
percolate through the structure and melt the layers underneath. A
cooler, but longer, pulse will have time to reach the middle layer and
be hot enough to melt the crystals there, but will not melt the top
layer or penetrate to the bottom, resulting in a magenta pixel.Cooler
and longer still, and the heat reaches the bottom to produce a cyan dot
but leaves the upper layers transparent. Precise control of the exact
temperature and duration dictate the amount of crystals melted, and
therefore the richness of the tone.For each pixel, the resistor must
pulse up to three times, to develop the exact proportions of yellow,
magenta and cyan which, when mixed together, produce the colour
required.

The dyes are transparent, so looking through the
layers against the pure white background gives clean, pure colours.
‘There are a number of printing technologies which use a separate black
along with the cyan, magenta and yellow, but we’ve found that we can
get very acceptable neutral blacks and greys with good density by
turning on all three colour formers,’ said Herchen.Once the colours are
melted, the other additives come into play to keep the colours stable.
‘The melted, coloured material is a highly viscous, amorphous glass,’
Herchen said. ‘If the softening temperature of the glass is kept high
enough, the molecules can’t move around much, so they can’t reorganise
into a crystal lattice.’

The printing process takes about 30
seconds to print a 2in x 3in picture, during which time there are some
200 million heat pulses.
The small size of the prints has attracted
some scepticism but it is designed to capture a specific market,
Herchen explained. ‘There isn’t any real limit to the size of print but
we wanted to capitalise on the compactness of the technology — when you
do away with ink cartridges and ribbons and so forth, you can eliminate
a lot of volume. We thought there would be a great benefit for a mobile
or pocket-size printer. We picked this size because it’s the size of a
credit card or a photo that will fit in your wallet.’

Also, each
picture has a peelable sheet on the back revealing a sticky layer,
allowing the pictures to be pasted into reports, for school and college
reports, as well as for users such as estate agents, insurance
assessors and police, who may need to produce instant pictures for
documentation.The trend for camera phones was also an important factor.
‘In 2008, the projections for multi-megapixel camera phones say that
the number of those that will be sold, just in this year, is more than
the total number of cameras of any type that have ever existed,’
Herchen said.’It’s staggering. And these camera phones will capture
billions of images, and it’s awkward, inconvenient and sometimes nearly
impossible to print them. We didn’t want to enter the market with a
me-too product, like a home photo printer — people already have options
to do that. We wanted to fill a need which had no way of being
fulfilled. So we thought of making a printer that was very small,
portable, battery powered, and can print wirelessly via Bluetooth or
via a USB cable. There’s no other product or technology that’s capable
of doing that.’

Zink’s old parent, Polaroid, will be the first
to release products using the technology, with both a mobile printer
and a digital camera incorporating a printer in the shops for next
Christmas.The company has several other licensees who plan to
incorporate the printer into their own equipment. ‘We’re very proud of
the technology,’ Herchen said. ‘And all image printing systems, from
silver halide on to ink jets and lasers, have got better over their
lifetime. This is just our first year.’

[Author]

April 4, 2008 at 3:06 PM