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scratched cd

Discussion in 'General' started by dannyqball, 20 Jan 2006.

  1. hitman012

    hitman012 Minimodder

    6 May 2005
    Likes Received:
    The polycarbonate layer is there only to give the disc rigidity; the information is stored on the aluminium or Al-die part on the top of the disk. As long as you can fill the scratch with an appropriate material, it won't matter - data CDs have extensive ECC frames built into their encoding format (although not as many as DVDs) specifically designed to stop data loss from scratches.

    The worst thing you can do to a CD is to scratch it tangentially to the spiral, which maximises data loss and subsequently minimises your chance of recovering it. A simple scratch parallel to its radius will do almost nothing.

    There are different kinds of plastic; thermoset and thermoplastic, and they differ greatly in their properties.

    Thermoplastic polycarbonates are usually (I think) polycarbonates of bisphenol A, and they are able to be warmed up and reshaped over and over again. This reason for this is that there's no cross-linking between their polymer chains, so it's only dipole-induced dipole and other weak molecular forces that actually hold them together, which is why they can be melted and reformed. They can be soluble if hydrogen bonds are formed in plastics such as soluble polyvinyl alcohol. You might recognise them from the dissolving sachets you put in your washing machine.

    Thermosets, like bakelite, have covalent cross-links between their chains, which holds them together in a giant covalent structure - it won't soften when you heat it, but it'll burn if it gets too hot. They're strong, hard, rigid and insoluble.

    As far as I know (and I'm not terribly sure), CDs use a thermoplastic polycarbonate, which means they they're going to expand slightly with heat and are capable of being melted. It makes sense, since if CDs really were hard and rigid, we'd see a lot more broken ones than we do now. Ever tried to snap a CD in two? It's surprisingly difficult.

    Sorry if this is too much info, but you did want it from a chemical context ;)
    Last edited: 29 Jan 2006
  2. 731|\|37

    731|\|37 ESD Engineer in Training

    5 Sep 2004
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    nono, I did and I hear whats said. I knew the first part aobut data layer and scratches.

    my mom asked me the other day when it was about 15 F in the car and she started her CD player. Wanted to know how bad it was for the CDs to have such a high RPM and a low temp.

    I should take a look at the novus stuff. you might be able to spend 5$ on that and a microfiber rag to do the same thing the disk doctor does

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