A whole new spin on adhesives

Dave Wilson went to visit Loctite in Munich where he saw a new market for adhesives open up before him.

To the layman, the advent of the Digital Video (or Digital Versatile) Disk may not appear to have a lot in common with the adhesive industry. But to manufacture one of these new DVDs or optical disks, two layers comprising the halves of a disk need to be sandwiched together with around 0.5g of adhesive. That doesn’t sound like a lot, but when you consider that DVDs can be manufactured in their billions, that’s a lot of adhesive.

Traditional audio CDs have a storage capacity of about 650MBytes. DVDs, however, hold a whole lot more. That’s not just because of a reduced track pitch and optical pit size, but because some DVD formats use two information layers that are sandwiched together during production. The result is a new medium that can store Gigabytes of information on the same physical format as an audio CD. That’s enough for a movie, several soundtracks in a variety of European languages, as well as games that can run at the same time as the movie and even interact with it.

But there isn’t just one DVD format – to complicate matters, there are three: the DVD 5 (storing 5GBytes) the DVD 10 (10GBytes), and the DVD 9 (9Gbytes). The difference between them lies in the manufacturing process: the DVD 5 comprises just one layer of information, while the other two formats comprise two. But, while the user needs to physically remove a DVD10 disk from a player and turn it over to play the other side, the DVD 9 format allows both information layers to be read from one side of the disk.

The DVD 9 looks set to become the most popular DVD format due to its capacity and ease of use. But because the disk comprises two information layers bonded together during manufacture, it’s also the toughest to produce. Looking at the disk formats in detail explains why.

The DVD 5 disk has a blank polycarbonate layer and a totally reflective layer of aluminium. A laser in the DVD player reads data directly from the polycarbonate layer. DVD 10 disks are essentially two DVD 5 disks bonded together in a sandwich: hence, twice the data. The DVD 9, on the other hand, comprises disks of two different types bonded together: one with a polycarbonate layer and a semi-transparent layer made of silicon, silicon carbide or gold, and the second with a totally reflective layer made of aluminium and another layer of polycarbonate. To read data off the disk, the laser read head is focused on the first layer then the second. Between the silicon carbide and the aluminium layer is a layer of adhesive that must be applied to hold the two halves of the disk together.

To build such a complex disk, DVD equipment manufacturers such as Munich-based Krauss-Maffei have built relationships with suppliers like Loctite to optimise quality and the speed of disk production. The operation of such a DVD production machine is reasonably straightforward: in the Kraus-Maffei machine, for example, the digital information is first pressed into polycarbonate by an injection moulding machine with a clamping force of 18tonnes. Then, it’s brought into a sputtering machine, where an aluminium, silicon, or silicon carbide layer is sputtered onto the disk at a thickness of about 50nm, depending on which half of the disk is to be manufactured.

Next, the disk is spun to ensure that, when it is applied a constant layer of adhesive is spread across the disk. During the spinning process, the second disk is brought into contact with the first. Rotating both disks up to 4000rpm, and keeping them apart at a defined distance, ensures that the adhesive is spread very accurately across the surface. After both halves are brought together, the adhesive is cured by UV light in less than 2sec. Finally, the DVD product is inspected to ensure that the layer thickness is constant.

One of the challenges that Krauss-Maffei and Loctite had to address was to ensure that no adhesive was spun out of the disk during manufacture. This meant that the system developers didn’t need to edge cure the adhesive on the outside of the disk. They did it by employing a volumetric dispenser in the machine, so that just enough adhesive was used to cover the whole bonding area.

Initially, Loctite experimented with a standard adhesive product and adapted the viscosity so that that it would flow correctly across the disk. But perhaps the most challenging task was to formulate an adhesive that could adhere to all of the materials that comprise the semi-transparent substrate layer, be it made of gold, aluminium or silicon carbide. It was also important to ensure that, once cured, the optical quality of the adhesive would not interfere with the light returned to the pickup from the laser: in the DVD 9 disk, the laser must read through both the semi-transparent layer and through the adhesive layer too.

Of course, there’s some secret sauce in the application of the adhesive too. Aside from the adhesive formulation, the adhesive had to make the optimum contact angle with the disk upon its initial application. The angle had to be high enough so that air bubbles were not trapped in the disk during the spinning process, but low enough to ensure that during the spinning process, the entire surface of the disk was covered.

Defining the accuracy of the process was also very important to minimise the errors that occur in the manufacturing process. In fact, two sorts of errors must be measured in DVD production: geometrical errors and data errors. Geometrical errors are highly dependent on the moulding machine, temperature and UV curing, while data errors are influenced by factors such as moisture in the environment.

Geometrical errors are measured by the system itself during production, while data errors, like drop out, are performed offline after the DVDs have been made. The geometrical error measurement is performed by a Contec scanner – a machine that uses a laser to measure the dimensions of the disk on-line while the disks are being produced. Offline equipment reads the data on the disk in order to calculate the number of drop outs on the disk. This function can only be performed off line because of the large amount of data on the disk.

At the moment, traditional CD lines stamp out a new CD every 3sec. In the DVD arena, things aren’t quite as snappy just yet: most machines sport cycle times between 4 and 10sec, primarily because of the additional steps needed to produce and then bond two disks together. Nevertheless, both Krauss-Maffei and Loctite feel that, given time, they can reduce this cycle time further until it becomes competitive with that of existing CD processes.