Light reading

The new generation of DVDs - which because of their extra data-holding capacity must be more durable - represents a production challenge.

Although

technology, developed by a Sony-led consortium; HD-DVD, from Toshiba and its partners; and DVD-R DL (dual layer) discs are the same size as conventional DVDs, they can hold around five times as much information. It is because of this that disc manufacturers are demanding that they are more durable.

US chemicals giant

is at the forefront of developing the adhesives and coatings that hold the new-generation discs together and protect the data they hold.

The products were developed in conjunction with the manufacturers of the machinery that makes the DVDs, said Huntsman Advanced Materials' global DVD marketing manager Mark de Heer, and had to take into account both the different construction of the discs, and the more stringent demands their data-holding capacity places on their construction materials.

One of the keys to durability is the coating that protects the data-holding layer which both keeps moisture and other potentially damaging compounds away from the information surface, and forms the final element of the pathway for the laser which reads or writes on to the disc.

It therefore has to be highly transparent, so the light isn't distorted on its way on to and away from the metal surface; for the same reason, it must be scratch-resistant; and it must form an effective barrier against the atmosphere.

The current generation of DVDs has a twin-layer structure, explained de Heer. 'If you look at the edge of a DVD, you can see two layers of polycarbonate bonded together,' he said. 'On the first layer, the information is printed on to polycarbonate, and on top of that is a reflective layer of aluminium. On the bottom side of the layer, there is also information, and that's covered with a layer of gold or silver alloy. All the layers have to be bonded together.'

New-generation DVDs, however, have only one layer, which leads to a very different method of making the discs. 'The information is much, much more dense on a Blu-Ray or similar DVD,' said de Heer. 'The information is printed on top of the polycarbonate, but it isn't covered with another layer. Instead, we use a coating which is spun on top of the polycarbonate, and that needs to be exactly 100

thick. It's quite a challenge.'

As the name suggests, spin coatings are applied by spinning the disc. An excess amount of the coating is applied to the surface, which is then rotated at high speed, using centrifugal force to spread the fluid in an even layer to the desired thickness.

The coating is only the first part of the final product. Huntsman has developed a second component for extra protection - a scratch-resistant hard coat.

'If you have a scratch on a normal DVD, the bit structure is such that the software can calculate what kind of signal you should get. But with new-generation discs, the information is so much more dense that the effect of the scratch is 10 times greater. The hard coat prevents you leaving fingerprints and it won't scratch. It's a completely new way of making an optical disc.'

Both coats are based on UV-curing acrylic chemistry, said de Heer, but they have quite different properties. 'The spin coat is the biggest part, because it's a fairly cheap product. The hard coat is there to protect it, but it's rather an expensive material - this is because the cross-linking density of the hard coat is much higher.

'We also have to be very careful with the way the coating is applied. When it cures, the cross-linking density is so high that it can shrink a bit, but because of the very tight specifications you can't have something that shrinks, as it might make the polycarbonate warp or peel.'

The combination coating provides greater than 98 per cent light transmission. Moreover, de Heer said, during a 100-hour heat and humidity test at 80

C and 85 per cent relative humidity, the coatings provided an impermeable moisture barrier and were unaffacted by the heat.

'The Araldite adhesive team at Huntsman helped pioneer the use of structural adhesives in the DVD market,' he said. 'As the DVD industry has evolved, so have our products.'