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Lactide: Lactic Acid Condensed and Cracked under Vacuum, Recrystallised
Lactide is the ring-shaped double molecule of lactic acid, the monomer from which PLA is made. Schneider's du Pont patent (US 2,703,316, 1955) drives the water out of lactic acid, condenses it under vacuum to a syrupy low polymer, then heats that to 200–250 °C at 8–10 mm so that lactide distils off. For a tough plastic it must be very pure: recrystallise it from ethyl acetate until it melts at 120 °C or higher (best 123–125 °C).
The picture: our diagram.
Hejuru
1–2 days
Amabwiriza
1
1
Drive off the water
Drive off the water
Distil the lactic acid at atmospheric pressure at 120–130 °C until nearly all the water of dilution has gone.
Ibikoresho by'iyi ntambwe:
Aside laktike (iyo mu nganda)1 igiceIbikoresho bikenewe:
Icupa rifite igituza kizengurutse
Umuzingo w'ikondanseri
Urubaho rushyushya
Ubwogero bw'amavuta
Igipima Ubushyuhe cy'Ubushakashatsi2
2
Condense under vacuum
Condense under vacuum
Continue distilling under 17–35 mm of mercury. Water of esterification comes off and the acid becomes a syrupy low polymer (polylactic acid).
Ibikoresho bikenewe:
Pompe ikuramo umwuka
Igipima icyuho cy'umwuka
Umutego w’ubukonjeUbwogero bw'amavutaIgipima Ubushyuhe cy'Ubushakashatsi3
3
Crack to lactide
Crack to lactide
Lower the pressure to 8–10 mm and raise the temperature to 200–250 °C. Lactide distils at 130–140 °C (10–13 mm) and sets to a soft crystalline solid.
Ibikoresho bikenewe:
Pompe ikuramo umwukaIgipima icyuho cy'umwukaUmuzingo w'ikondanseriUbwogero bw'amavutaIgipima Ubushyuhe cy'Ubushakashatsi
Uturindantoki twihanganira ubushyuhe4
4
Recrystallise from ethyl acetate
Recrystallise from ethyl acetate
Recrystallise the crude lactide from ethyl acetate, filter and dry. Repeat: in the patent the melting point rose from 114–115 °C after the first crystallisation to 118, 123–124 and 124–125 °C after the fourth.
Ibikoresho bikenewe:
Etili asetati
Igikombe cy'Ikirahure Kirwanya UbushyuheUrubaho rushyushya
Ifonelo ya Büchner (ya porcelaine)
Urupapuro rwo Kuyungurura
Amadarubindi yo Kwirinda Ibinyabutabire5
5
Check the melting point
Check the melting point
Put a few crystals in a capillary tube and warm slowly. Lactide melting at 120 °C or above is fit for making PLA; below 120 °C it gives a brittle polymer.
Ibikoresho bikenewe:
Umuyoboro w'ikirahure muto cyaneIgipima Ubushyuhe cy'UbushakashatsiUbwogero bw'amavutaIbikoresho
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