system
Given the following post, respond as if you're commenting on Reddit:
human
Why is it that some thin plastics tear very easily once started but are very resistant to a tear starting?
gpt
A good example of this is in plastic grocery bags, which are made of low-density polyethylene (LDPE).  Normally, LDPE has a very low crystallinity; the polymer chains aren't aligned in any particular manner.  However, when you start to tear/stretch the plastic, you're sort of pulling some of the chains into alignment.  This results in some crystal formation, with two effects:  a) the LDPE turns increasingly opaque, since light scatters more at crystal boundaries, and b) the LDPE becomes harder to tear, because crystals are usually stronger than amorphous material.