Understanding polymers – Plastic wrap.

Here is an interesting little experiment you can do at home with the kids. Take some plastic food wrap, and tear off a rectangle section thats around 5cm (2″) wide and as long as the box. Try to get an even tear.

Tearing a strip of plastic wrap.

Then cut off another section that is a square, rotate it 90 degrees and place it on the same tear strip. Then tear it to the same size as the first bit. Keep note which is which.

The square strip of plastic wrap ready to be torn, rotated and torn again.

You now have two strips of plastic wrap of roughly the same size but from different orientations on the role.

The torn strips with arrows showing the direction they came off the roll.

Now take one of the strips and hold onto the ends and gently try to stretch it apart. If you do it slowly it will stretch and then at some point break. Do the same for the other strip.

What you will notice is that one strip of plastic wrap has stretched more than twice its length while while the other just breaks after very little extension.

The stretched plastic. The strip cut in normal on the left and the rotated strip on the right.

So why do these two strips of different orientations which look the same have such different properties? Well the following is my best guess.

The plastic wrap I have started out as LDPE nurdles, which are melted and then extruded into a large balloon like structure. During the extrusion process the longer chained polymers all become aligned in one direction. Just as a coiled garden hose will straighten if you drag it behind you, the friction between the long polymers and the extruder die will cause them to align in one direction.

A rough sketch of how the polymers are aligned for each cut section.

So the strip of plastic wrap cut in the direction that the roll of plastic unwinds has all the polymer chains aligned in one direction. This makes it stronger, but at the expense of ductility. The strip cut in the direction of the box however has far fewer bonds (branching between chains and secondary bonds) and so is much weaker. When its stretched through, these branched bonds are able to break and reform at new locations, thus allowing the plastic to stretch by incredible amounts.

Another good example is wood. The wood is much stronger with the grain, than against it for similar reasons.

How does all this relate to repraps? I suspect the same effect takes place in extruded filaments and so may play a part in warping. If not, then its just an interesting side note. Something to think about all the same.

Here is a short video on the manufacturing process if your interested.

About Richard

I am a Materials Engineering working in the field of Magnetic Materials in Melbourne, Australia. This blog covers my personal interest in all things CNC.
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2 Responses to Understanding polymers – Plastic wrap.

  1. For cling wrap, being stiff and brittle in one direction is a big help – it lets you tear it more cleanly and easily from the roll.

    Also, regarding the effect of plastic extrusion on the mechanical properties – according to my materials science textbook: “For an amorphous polymer that is drawn at an elevated temperature, the oriented molecular structure is retained only when the material is quickly cooled to the ambient; this procedure gives rise to the strengthening and stiffening effects described. On the other hand, if, after stretching, the polymer is held at the temperature of drawing, molecular chains relax and assume random conformations characteristic of the predeformed state; as a consequence, drawing will have no effect on the mechanical characteristics of the material.”

    So I guess it depends a lot on the cooling.

    • Richard says:

      Thats a good point.

      “the oriented molecular structure is retained only when the material is quickly cooled to the ambient” I wonder if this rate of cooling, and thus relaxing of the polymers, differs greatly between ABS, PLA and HDPE? Further more, how much does this effect contribute to warping as the outside of a printed object cools quicker than the inside?

      Further research warranted =)

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