Why is packing tape nearly impossible to tear when intact, but easily shreds if you cut the slightest nick into it?
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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 😅
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