3 ms·

This effect also applies to polymers! Perhaps even more so. Take a polyethylene bag (LDPE) and stretch the material in one direction. You might notice the mater

This effect also applies to polymers! Perhaps even more so. Take a polyethylene bag (LDPE) and stretch the material in one direction. You might notice the material becomes thinner but also stronger. This is due to the polymer chains becoming aligned. Eventually you get "drawn fibers" where the molecular strands are aligned with the fibers for optimum tensile strength.

01HNNWZ0MV43FF 2y ago
That makes sense, if I don't rip open a bag on the first try it'll just stretch and never open
- myself248 2y ago
  Stretch it in one direction, then grab in the middle of the stretched area and pull sideways, and it pops open like nothing.
  - ahartmetz 2y ago
    Cool, need to try that! Now I wonder if that is how these "magic cut" wasabi and ginger sachets work.
kragen 2y ago
it varies a lot with polymers, and it's a different effect. steel is entirely crystalline; ldpe is mostly amorphous. a big part of what's happening in the strain hardening of ldpe, aside from making it uniaxially oriented, is that it's crystallizing; the crystalline domains become larger, greatly reducing the amorphous volume fraction. (there are also other ways of achieving this effect, such as annealing, which you will notice softens steel rather than hardening it.) ldpe's strength isn't determined by crystal dislocation density in the same way as steel's, and of course steel doesn't have polymer chains to align
wizardforhire 2y ago
This is exactly what dyneema is only with hdpe.
- kragen 2y ago
  no, dyneema is not hdpe; it's uhmwpe, and it isn't just strain-hardened, it's gelspun
  - libria 2y ago
    Assuming these terms are all correctly spelled, this has to be the shortest sentence I've understood the least of on HN. Guess I've got some googling to do.
    - kragen 2y ago
      they are! sorry to be telegraphic
    - NavinF 2y ago
      UHMW = ultra high molecular weight. Each molecule is literally heavy because they have a lot of atoms. PE = polyethylene. The most popular plastic on this planet. HDPE = high-density polyethylene. One of the most common plastics. Milk jugs, glue bottles, etc. Stick some UHMW tape on anything that needs to slide easier. Its surface is quite slippery
      - jessebro 2y ago
        HDPE is also what Nalgenes and the like are made of (or used to be anyway), and is very popular for storing chemicals as well. It is very nearly completely inert.
        kragen 2y ago
        you cunninghammed me: nalgenes are polycarbonate, which is a lot less inert all the polyethylenes are relatively inert, because polyethylenes are in some sense just heavy paraffins. paraffin is germanized latin for 'relatively inert'

01HNNWZ0MV43FF 2y ago

That makes sense, if I don't rip open a bag on the first try it'll just stretch and never open

myself248 2y ago
Stretch it in one direction, then grab in the middle of the stretched area and pull sideways, and it pops open like nothing.
- ahartmetz 2y ago
  Cool, need to try that! Now I wonder if that is how these "magic cut" wasabi and ginger sachets work.

kragen 2y ago

it varies a lot with polymers, and it's a different effect. steel is entirely crystalline; ldpe is mostly amorphous. a big part of what's happening in the strain hardening of ldpe, aside from making it uniaxially oriented, is that it's crystallizing; the crystalline domains become larger, greatly reducing the amorphous volume fraction. (there are also other ways of achieving this effect, such as annealing, which you will notice softens steel rather than hardening it.) ldpe's strength isn't determined by crystal dislocation density in the same way as steel's, and of course steel doesn't have polymer chains to align

wizardforhire 2y ago

This is exactly what dyneema is only with hdpe.

kragen 2y ago
no, dyneema is not hdpe; it's uhmwpe, and it isn't just strain-hardened, it's gelspun
- libria 2y ago
  Assuming these terms are all correctly spelled, this has to be the shortest sentence I've understood the least of on HN. Guess I've got some googling to do.
  - kragen 2y ago
    they are! sorry to be telegraphic
  - NavinF 2y ago
    UHMW = ultra high molecular weight. Each molecule is literally heavy because they have a lot of atoms. PE = polyethylene. The most popular plastic on this planet. HDPE = high-density polyethylene. One of the most common plastics. Milk jugs, glue bottles, etc. Stick some UHMW tape on anything that needs to slide easier. Its surface is quite slippery
    - jessebro 2y ago
      HDPE is also what Nalgenes and the like are made of (or used to be anyway), and is very popular for storing chemicals as well. It is very nearly completely inert.
      - kragen 2y ago
        you cunninghammed me: nalgenes are polycarbonate, which is a lot less inert all the polyethylenes are relatively inert, because polyethylenes are in some sense just heavy paraffins. paraffin is germanized latin for 'relatively inert'