Flexplay - Chemistry

Chemistry

The reactive layer contains a polymer resin that acts as a carrier for a suitable leuco dye, which, upon contact with atmospheric oxygen, oxidizes to form an opaque or semi-opaque material.

Some of the dyes used are methylene blue, prussian blue, brilliant cresyl blue, Toluidine Blue O, Basic Blue 3, Methylene Green, Taylor's Blue, Janus Green B, Meldola's Blue, Thionin, Nile Blue, and Celestine Blue. The leuco dye is prepared by chemical reduction using a suitable reducing agent. The method commercially used by Flexplay and SpectraDisc employs methylene blue reduced by sodium dithionite.

In order to prevent the oxidation being triggered during manufacture of the discs, and in order to avoid the need for operating in inert atmosphere, the leuco dyes are further chemically modified to their blocked forms. Such blocked leuco dyes are the leuco dye precursors that form the leuco dye by a slow, controlled chemical reaction. Leucomethylene blue can be reacted with triisopropylsilyl trifluoromethanesulfonate, forming triisopropylsilyloxycarbonylleucomethylene blue (TIPSOCLMB), which is stable in presence of air oxygen. In presence of nucleophile reagents, e.g. water, the blocked leuco dye compound undergoes hydrolysis, releasing the unblocked leuco dye. This reaction is slow (several days to a week), and the hydrolytic compound is supplied either as atmospheric moisture, or in the formulation of the resin.

The rate of the deblocked leuco dye oxidation is dependent on the pH of the resin polymer matrix. By adding basic compounds (e.g. 1,4-diazabicyclo octane (DABCO), or other amines), the pH is increased, and the reaction rate increases correspondingly. Correspondingly, by adding strong protic acids (e.g. camphorsulfonic acid), the reaction rate can be decreased and the lifetime of the disc prolonged.

The residual oxygen can be removed from the substrates used for disc manufacture by storing them under vacuum or oxygen free atmosphere (e.g. pure nitrogen) for a period of time (hours to days). Alternatively, an oxygen scavenger (e.g. iron(II) salts, tin(II) salts, or organometallic compounds) can be added to the polymer formulation, reacting with the oxygen present in the polymer matrix in shorter timeframe than the hydrolysis of the blocked leuco dye takes.

The reaction timing can be further controlled by addition of other substances. As the diffusion rate of oxygen through the polymer layer is more or less constant, a supply of suitable antioxidants (e.g. organometallic compounds) which react with the oxygen preferentially to the leuco dye leads to gradual depletion of the antioxidant compound. Only after the antioxidant is consumed, the leuco dye starts being oxidized, achieving the period of delay of the reaction onset, followed by a rapid reflectivity degradation. Stannous ethylhexanoate can be used here as the antioxidant organometallic. Various resin-soluble tin(II) and iron(II) compounds can be used, e.g. chelates and fatty acid salts. Other usable compounds are e.g. hydroquinones, alkylhydroxylamines, dithionates, reducing saccharides (e.g. glucose), alpha-hydroxyketones (acetol), substituted boron hydrides and silicon hydrides.

The oxidized dyes in the expired discs absorb primarily at the wavelength of the current diode lasers (red, 650 nm) used in the DVD players. However, the new generation of DVDs is designed to use blue lasers at 450-460 or even 405 nm, for which the methylene blue is essentially transparent. While SpectraDisc did not take this in account, the Flexplay discs incorporate a layer of a color filter, blocking the blue lasers from reading the disc, expired or not. Acridine Yellow can be used for this purpose, together with 9,10-bis(phenylethynyl)anthracene, and a scale of different azo dyes, aromatic hydrocarbons, and other dyes. The dyes can be added directly to the polycarbonate resin the discs are made of.

Additionally, other methods of the disc degradation are proposed to be deployed together with the primary one; the concern of the technology developers here is that while the dye oxidation mechanism prevents the disc from being read by current consumer technologies, the information on the disc, recorded in the pattern of the pits in the reflective layer, remains intact and could be recovered. One of the methods is to make the two reflective layers in the DVD-9 disc of different metals (e.g. aluminium and silver), and separate them with an ionic conductor separator. The atmospheric oxygen then undergoes electrochemical reaction in such crude fuel cell with the silver as cathode and aluminum as anode, leading to growth of dendritic silver through the dielectric layer. When the silver reaches the aluminium, an electric short circuit forms, and galvanic corrosion then rapidly degrades the reflective layers. Many other mechanisms are proposed for this slower, less controlled "backup" method of data destruction, ranging from corrosion of the reflective layers to degradation of the polymer matrix of the disc itself.