Re: How strong is Plastic Wrap?

Interesting question because I think that you have the wrong impression
about the strength of plastic wrap. It isn't "stronger" when tight. 
However, I understand why it might appear this way since stretching it 
across the mouth of a bowl makes it seal better and trying to puncture it
does make it stretch until it is very tight before it breaks.

As to how plastic wraps are so thin - this is related to the type of 
material that is used in their construction. Plastic wraps are made from
polymers which are long chains of carbon atoms linked together. They look
like: -C-C-C-C-C-C-C-C- where each dash represents a covalent bond. And
covalent bonds are very strong. It takes a lot of energy to break a
polymer.

But the plastic wrap is made up of a lot of polymer molecules, not just
one. They overlap and interact through inter-molecular forces. These are
very weak forces compared to covalent bonds and normally, they only
loosely hold atoms and molecules together. (A covalent bond is on the
order of several hundred kJ/mol while the inter-molecular forces are
on the order of 10 kJ/mol.)

I said "normally" because in most molecules there are very few of these
inter-molecular interactions. With polymers, there are millions of them 
and while each one is weak, the total is incredibly strong. Its a bit like 
picking up a heavy object - one person might not be able to but given 
enough people, anything can be moved.

In the case of plastic wrap (and plastics in general), the internal 
molecular bonding keeps the polymer together. The many millions of 
inter-molecular interactions keep the plastic sheet together and allow it
to stretch by allowing felxibility between the polymers themselves. But 
when the force applied is enough to overcome these inter-molecular forces, 
then the plastic wrap breaks - regardless of whether or not it is 
"stretched tight".

Hope this answers your question.