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> CD's are actually very scratch resistant even though they often get lot's of small scratches. This is a bit of an oxymoronic sentence. How can something be b

> CD's are actually very scratch resistant even though they often get lot's of small scratches. This is a bit of an oxymoronic sentence. How can something be both scratch resistant, while accumulating a large number of scratches? The scratches a CD accumulates won't typically affect the ability to read from them, but that's due to the focal length of the reader, not the material. https://en.wikipedia.org/wiki/Compact_disc#Integrity https://en.wikipedia.org/wiki/Compact_disc#Integrity

Retric 9y ago
Resistance is not immunity. Bullet proof glass for example gets damaged by bullets. The trade off is always in how much damage and how quickly. CD's will easily survive with even moderate levels of care. What people forget is the covering is really designed to protect the data layer not the surface. And in that context the clear plastic is actually better protection than the backside.
- jdmichal 9y ago
  > What people forget is the covering is really designed to protect the data layer not the surface. Right. But that this started explicitly about calling the polycarbonate -- that clear layer -- as scratch resistant. And this is just flat out wrong: "Polycarbonate is a durable material. Although it has high impact-resistance, it has low scratch-resistance." https://en.wikipedia.org/wiki/Polycarbonate#Properties_and_processing https://en.wikipedia.org/wiki/Polycarbonate#Properties_and_p... I agree that it serves its purpose of protecting the data layer of the CD. But again, CDs and their readers were designed in a way that typical surface scratches and other imperfections would not inhibit reading. And so polycarbonate was an acceptable choice. The same approach applies to eye glass lenses; a scratch on a lens is typically not in focus and therefore has little to no effect on vision. So, TL;DR: Polycarbonate the material is not scratch resistant. CDs are scratch resistant, but that is due to their optical design and in spite of the polycarbonate.
  - Retric 9y ago
    I don't directly disagree with what your saying, my point is there is more than one type of scratch resistance. Because not all scratches are created equal. If you look at say :https://omnexus.specialchem.com/polymer-properties/properties/hardness-shore-d https://omnexus.specialchem.com/polymer-properties/propertie... You find it's good relative to other plastics at preventing penetration which is also why it's good at preventing deep scratches. So, yes if you want a crystal clear optical element then it's not scratch resistant and look for something high on the Mos scale. But, if you want a coating that protects from deeper scratches it's quite good. In that context it's good material to protect CD's from scratches that matter. Which again is why I find the word resistance tricky because it's always resistant in some context.
    - jdmichal 9y ago
      Ah, I understand your point now. Thanks for clarifying for me.

Retric 9y ago

Resistance is not immunity. Bullet proof glass for example gets damaged by bullets. The trade off is always in how much damage and how quickly. CD's will easily survive with even moderate levels of care. What people forget is the covering is really designed to protect the data layer not the surface. And in that context the clear plastic is actually better protection than the backside.

jdmichal 9y ago
> What people forget is the covering is really designed to protect the data layer not the surface. Right. But that this started explicitly about calling the polycarbonate -- that clear layer -- as scratch resistant. And this is just flat out wrong: "Polycarbonate is a durable material. Although it has high impact-resistance, it has low scratch-resistance." https://en.wikipedia.org/wiki/Polycarbonate#Properties_and_processing https://en.wikipedia.org/wiki/Polycarbonate#Properties_and_p... I agree that it serves its purpose of protecting the data layer of the CD. But again, CDs and their readers were designed in a way that typical surface scratches and other imperfections would not inhibit reading. And so polycarbonate was an acceptable choice. The same approach applies to eye glass lenses; a scratch on a lens is typically not in focus and therefore has little to no effect on vision. So, TL;DR: Polycarbonate the material is not scratch resistant. CDs are scratch resistant, but that is due to their optical design and in spite of the polycarbonate.
- Retric 9y ago
  I don't directly disagree with what your saying, my point is there is more than one type of scratch resistance. Because not all scratches are created equal. If you look at say :https://omnexus.specialchem.com/polymer-properties/properties/hardness-shore-d https://omnexus.specialchem.com/polymer-properties/propertie... You find it's good relative to other plastics at preventing penetration which is also why it's good at preventing deep scratches. So, yes if you want a crystal clear optical element then it's not scratch resistant and look for something high on the Mos scale. But, if you want a coating that protects from deeper scratches it's quite good. In that context it's good material to protect CD's from scratches that matter. Which again is why I find the word resistance tricky because it's always resistant in some context.
  - jdmichal 9y ago
    Ah, I understand your point now. Thanks for clarifying for me.