【Popular Science on Polymers and Materials】--009 Why does tying things with plastic rope become looser over time? +jlszhoujidai

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This post was last edited by ljtjbx on 2015-11-9 08:06. In daily life, we often use plastic ropes to tie things together. You’ll find that the more tightly we try to tie something with a plastic rope, the sooner it seems to get longer and looser. When we try to tie it tighter again, it still becomes loose after a while. Why is this? Here, we need to mention a basic concept—mechanical relaxation. What is mechanical relaxation? Stress relaxation refers to the phenomenon in polymer materials where, under constant total strain, the elastic strain component gradually decreases over time due to the increasing viscous strain within the sample, which in turn leads to a gradual reduction in the elastic stress over time. The plastic ropes used in our daily lives (referred to as chemical ropes in some places) are made of linear polyethylene or polypropylene. These polymers are typical non-crosslinked linear polymers; when tightened, the linear polymer chains are stretched, giving the rope a tight appearance. However, over time, these linear chains slip, and this slipping is irreversible. Once the chains slip, the plastic rope becomes longer due to stretching and can no longer be tightened effectively. If attempts are made to tighten it further at this point, the linear chains will continue to slip. So when using plastic rope to tie things, the tighter it is tied, the looser it will become in the end, with significant slackness occurring. Therefore, when experienced people use plastic ropes to tie things, they should not tie them too tightly, in order to prevent severe stress relaxation of the linear polymer chains. So how can we avoid this phenomenon? Cross-linked polymer materials should be used; these materials transform linear polymer chains into a network structure through cross-linking agents. Even after being stretched and deformed, the polymer network chains can still return to their original shape with considerable strength. If a rubber rope is used, it will **improve this phenomenon; using an elastic band will make things much better. If something with excellent cross-linking is used, such as the rubber found in bicycle tires, then there will be basically no slackness – it will stay tightly secured. Want to give it a try?