Why is packing tape nearly impossible to tear when intact, but easily shreds if you cut the slightest nick into it?
In: 6079
It’s to do the availability of electrons with valence matrix. Once you have disturbed the matrix structure it results in a weakness in the inelastic substructure caused by subvalent bond alignment differentials.
Packing tape is usually made of BOPP – Biaxially oriented polypropylene
This means the long polymer chains are aligned both in the up/down and left/right directions
This is opposed to the usual way it is done which is that the polymer chains are aligned only in up/down direction and not left/right.
When oriented just up/down, the plastic is very hard to tear across the chain (left/right), but it rips apart quite easily along the chains (up/down)
When oriented biaxially, it is relatively harder to tear in both directions, but when a tear is formed, it propagates easily.
Why does it propagate easily? Because of three reasons. First because of the orientation of the chains, the elongation of the tape is very low. It can’t stretch in either direction much before tearing. Second, to keep the tape clear, no anti fibrillation additives are used as that gives it a milky colour. Of course, this doesn’t apply if it’s not a clear tape. Third, the tape is usually quite thin, so it’s not as strong.
Let me know if you have any other questions 😅
It’s the mechanics of crack propagation. It’s applicable to most materials actually.
Basically, it’s pretty difficult to create a crack, but once a crack exists, it’s much easier to make it bigger.
You can think of the cut as a small crack, and all you’re doing is making the crack bigger until the tape fully tears.
To go into a bit more detail, when you pull on the initial crack, there is a tiny little inside corner at the end of the cut, and this tiny little radius at the end of the cut is a weak point that will allow the crack to continue. You could actually punch a bigger hole at the tip of the crack to prevent it from becoming bigger. Basically when there is a tiny radius at the end of the crack, it’s easy to make the crack propagate. When the radius at the tip of the crack gets bigger, it gets more difficult to make the crack propagate.
Pro tip: if you have a crack in something, drill a hole at the tip of the crack to prevent it from getting bigger
Tension. The tape doesn’t have a great tension strength in an individual strand. The *moment* of collapse/failure comes easy with any knick or imperfection in the tape.
It’s a material property known as fracture toughness, the ability of a material with an existing defect to resist failure. Polymers have low values (approx 1-2) whilst materials that have to perform with defects present like aerospace metals have a much higher value (40-100).
Source: I’m a materials engineer!
The same reason that airplane windows have rounded corners. Sharp angles are [stress concentrators.](https://en.wikipedia.org/wiki/Fracture_mechanics)
If you have a piece of paper in the shape of an L, and you pull on both ends, all that force is concentrated on the inside corner of the L and none of that force will be felt on the outside corner of the L (you’ll even see the paper bulge on the outside corner). The stress concentrates on that one inside corner and the paper will easily rip there first.
The same thing effectively happens with a small cut. The cut is essentially an inside corner.
This is also why hairline fractures in bones, metal beams, concrete and other load-bearing materials are so dangerous.
It’s because when you nick it, there’s a breaking point it can follow. That breaking point is where it splits, and since there isn’t any guide on an uncut piece of tape, it’s trying to evenly distribute that breaking point across the whole material because there’s nothing to show it where to break.
Imagine hanging onto a pullup bar with a full grip, if your pinky finger starts to slip, the ring finger has to hold harder as it takes more load, then it starts to slip and then it the next finger starts to slip, and so on until you can’t hold and your grip breaks.
The slipping is like plasticity, you have a little bit of slip or stretch before you completely let go, but that finger isn’t holding on as well anymore and the next finger has to make up for it.
In the case of the inelastic tape, imagine you’re holding onto a tiny ledge with your fingertips. There’s almost no transition between slipping and just letting go. So even if it’s a gorilla holding on and they’re really strong, if you push their pinky off the ledge, they’re going to have to let go all at once because they can’t slip and redistribute their grip.
A trash bag can plastically deform and compensate for losing some “grip” or material holding it up. The tape is stronger, but has little ability to deform and compensate, causing it to let go all at once.
Not an answer to your question but 3M makes a tearable packing tape. I don’t know if it will still splinter but when you tear it across it’s in a nice, clean, straight line. Also no longer needs a dispenser w/ serrated cutting edge. Total game changer.
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