How strong is the plastic I can make from milk?

[dzero]

Can I actually use it for anything other than a geeky party trick?

The basic concept is that you can extract casein from milk. When you force it into a mold and let it dry, it forms a solid structure, just like any other plastic. Here is how you do it.

As fascinating as this sounds, my guess is that you can’t really do much with plastic created solely from casein. However it’s been a while since I’ve run any plastic samples through the GCMS in my basement lab so I don’t really know (the mice that run the lab and they say I don’t pay them enough - pfttt).

I need some straight dope and I need it stat! (OK, not really, but I love saying ‘stat’).

[constanze]

Well, I don’t know how good your home-made process is going to be - but before petro-based plastics in the 1950s became popular, commercial plastics were made from Kasein, likeGalalith.

Marketed in the form of boards, pipes and rods, in 1913 thirty million litres (eight million US gallons) of milk were used to produce Galalith in Germany alone.[1] In 1914, Syrolit Ltd gained the license for manufacture in the United Kingdom. Renaming itself Erinoid Ltd, it started manufacture in the Lightpill former woollen mill in Dudbridge, Stroud, Gloucestershire.[3]

Galalith could produce gemstone imitations that looked strikingly real.

snip

By the 1930s, Galalith was also used for pens, umbrella handles, white piano keys (replacing natural ivory), and electrical goods,[4] with world production at that time reached 10,000 tons.

snip

Although Galalith was historically cheap, the fact it could not be moulded led to its demise by commercial end users. Production slowed as the restrictions of World War II led to a need for milk as a food, and niched due to new oil-derived wartime plastic developments. Production continued in Brazil until the 1960s.

Although today formaldehyde is now recognized for its carcinogenic properties, Galalith made from milk and vinegar can easily be produced by home crafters.

(I read about this because a store that specializes in old methods of production has some products made from it.)

[constanze]

This is the translation of the usage instruction for Casein knitting needles:

Legen Sie die Stricknadeln vor dem ersten Gebrauch für maximal 30 Sekunden in klares, kaltes Wasser, so daß sie etwas Feuchtigkeit aufnehmen können.

Lay the needles into cold clear water for maximum of 30 seconds before first use so that they can absorb some humidity.

Während des Strickens werden die Nadeln über Ihren Handschweiß mit der notwendigen Feuchtigkeit versorgt.

While you are knitting, the needles will be supplied with the necessary humditiy through your hand sweat.

Lagern Sie die Stricknadeln bei Nichtgebrauch in der Originalverpackung.

Store the needles if not in use in the original packaging.

Bewahren Sie die Stricknadeln gemeinsam mit der Strickarbeit in einem verschließbaren, transparenten Kunststoffbeutel auf.

Store the needles together with the work in progress in a sealable, transparent plastic bag.

Halten Sie die Stricknadeln von Wärmequellen (Heizung, direkter Sonneneinstrahlung) fern, da sie sonst austrocknen und brüchig werden.

Keep the needles away from heat sources (direct sunlight, heating), because they can dry out and become brittle.

Behandeln oder reinigen Sie die Stricknadeln keinesfalls, da hierdurch die Materialeigenschaften verloren gehen. Entfernen Sie Verunreinigungen ausschließlich mit klarem, kalten Wasser und einem weichen Tuch.

Don’t treat or clean the knitting needles, because the material qualities will get lost. Remove dirt only with clear cold water and a soft towel.

And this are the special material qualities

Die glasharte und glatte, antistatische Oberfläche ermöglicht ein leichtgängiges und nahezu geräuschloses Stricken.

The hard as glass and smooth, anti-static surface enable a smooth-running and almost noiseless knitting

So you need to beware of dryness.

This how-to-do says

"As for the final product’s strength and whatnot, I would classify it as “okay.” If you roll it thin (as I did in this instructable), it can easily be snapped in half, though it will probably survive a small drop on to carpet. Thicker pieces seem to be more resilient against average abuses - no problem dropping on to carpet, and if it’s thick enough, you wouldn’t be able to snap it in half. A blow from a hammer or other such object would quickly shatter it, though. "

[Snarky_Kong]

Make some and test it. That’s a cool little science project and it’d give you a better answer because the strength is likely to depend highly on your skill in the process.