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Enthalpy

Stretched Polymer

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Hello dear friends!

Stretching stiffens polymers a lot, making wonder fibres of banal bulk materials. It brings LCP from 10GPa to 170GPa. Highly stretched polyethylene makes wonder ropes of Dyneema, Spectra and competitors.

Stretching *3, easy with a polymer, strengthens much a stripe from a polyethylene shopping bag.

Companies that stretch metal (for piano wire and others) could adapt to thicker polymer too. Or polymer manufacturers themselves could stretch or extrude the material cold or lukewarm, so mechanical engineers have stiff strong bulk polymers, lighter and easier to machine without fibre reinforcement.

The transverse properties may drop. Rolling a polymer in two directions strengthens both, as polyester (Mylar) films show. This would improve plates.

Sometimes the azimuthal stiffness too matters for rods. Schrägwalzen (I ignore the English word, check the drawings)
de.wikipedia
would improve the azimuthal direction, easy with a polymer. Combine with stretching or extrusion.

Marc Schaefer, aka Enthalpy

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At what type of temperatures would the strength and stiffness start reducing back toward "normal" and not recover?

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Not very hot, but my guess is that the Vicat temperature is a first approximation. That is, hardened PE and PP would be usable at hot weather, little more.

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PTFE too, and other weak polymers, are candidates for strengthening and stiffening. Making supermaterials is one goal, improving bad ones is one other. Creeping, flowing, low modulus all hinder the use of PTFE despite its other properties are good. Fibres exist already, hardened plates and rods would be nice.

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"Schrägwalzen" is "skew rolling" in English, while "transverse rolling" includes other interesting possibilities. Common at tube production, it serves for rods too.

Marc Schaefer, aka Enthalpy

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Stretching might stiffen and strengthen polyketone too. Initial E=1.3GPa and sigma~58MPa aren't brilliant, but polyketone is potentially very cheap, it has a good operating temperature range, low water absorption and high vibration damping. Maybe stretching brings a nice combination of properties.

Polymers used as super-fibres should also improve as bulk materials. PA and PET, PEEK too. Machining would become easier too, useful for PA.

Marc Schaefer, aka Enthalpy

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Some polymer fibres have steel's strength and stiffness thanks to stretching and keep a plastic's density. Stretched polymers could hence excel as fast-spinning parts, especially as impellers of gas compressors and vacuum pumps, as turbines too where the temperature fits.

These parts are uneasily made of fibres in a matrix. Intricate shapes hamper automatic production, thin sections held at thicker ones aren't quite natural to fibres. It's done at individual fan blades of turbofans, not at cheap small integral compressor impellers.

I suggested to deform polymer raw material. Here strength is needed in the radial direction, by squeezing a disk, and in the azimuthal too, by a torsion. Then the part could be machined by usual methods, accurate and automated - if everything works as hoped.

Or just inject the part. For instance LCP is known to harden much from small shear at injection. That would be perfect to harden the blades or thin disk of an impeller: Inject the polymer at the centre so shear is strong at the thin blades and disk periphery. Heat the resin and mould a bit less, compensate with more injection pressure.

The blades and disk periphery of impellers and turbine rotors hold at a massive ring or disk section near the axis, where the material needs azimuthal strength too. This would be achieved by a rotating part of the mould, or maybe a separate construction used as the unmoulded part is still warm. Azimuthal shear could occur in the polymer between concentric tools, an outer one (optionally a mould part) that holds the impeller at its blades and a rotating inner one where the shaft will be. Other arrangement are possible, this one limits the deformations of the impeller.

StretchRotor.png.d5360f8e2cc6fa4ec4d6da2863a88b06.png

PA too hardens by deformation. Simple injection and rotation could make very strong and cheap impellers.

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When extruding tubes, the kernel could rotate to give azimuthal shear. This can combine with the axial shear given by the extrusion.

Marc Schaefer, aka Enthalpy

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Kit (sports) cars were all the rage in the 19060s/70/80s.

At University I lost a friend to a serious motorway accident in such a car.

The plastic body had shown one of the failings of plastic v steel and the body looked more like a draped curtain than a car body over the mounting points.

Steel has superior creep properties to plastic.

 

My brother lost a sailboat to another failing of plastic.

Swelling in salt water.


The early pvc insulation for house wirting hardened and became brittle after 15 years or so in service.

 

So it is not all one way with super plastics, although some designer or other resurrects them every decade, having forgotten the lesson of the past.

 

Are these newer plastics aging proof?

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Hi Studiot, thanks for your interest!

Every materials has its limits. Plastics, or rather some plastics, should not be used in some applications.

For the tank of a gasoline lorry, I'd trust only steel of certain compositions, not the stronger lighter carbon-epoxy composites.

Some polymers withstand shocks and deformation extremely well. Polyurethane outperforms steel. Polyamides can be excellent.

PE, PP... have zero water absorption.

Polyimide doesn't age, even in space, with direct exposure to solar UV and to ionizing particles.

So while there is no universally applicable polymer, whatever new, they are very much usable, if the designer knows their limits and chooses them with judgement.

My first intention with stretched LCP, PP and others is the body of woodwind instruments. Thick material, tiny forces, no strong shock, negligible UV. Only water, which LCP and PP don't absorb. In a former job, where I designed hardware for crash-test, stiffer and stronger polymers would have been highly welcome, and neither UV nor water was present.

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I agree we have yet to discover the perfect universal material, and probably never will.

Some things I would add to your list of good and bad.

The pvc wiring covering is basically hard brittle plastic.
So a plasticiser was added to make the material softer and flexible.
But in time this plasticiser evaporates out, leaving the hardened pvc.

There are also aesthetic / comfort requirements that lead to many early uses of plastic being abandoned.
For example static and the nylon shirt.

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