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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.
Ilọsíwájú
1–2 days
Ìlànà
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.
Àwọn ohun èlò fún ìgbésẹ̀ yìí:
Aṣídì Lactic (ti ìmọ̀-ẹ̀rọ)1 ẹyọÀwọn irinṣẹ́ tí a nílò:
Ìgò abẹ́-róbótó
Ìyípo ẹ̀rọ ìtutù
Pátákó Gbígbóná
Ìwẹ̀ Epo
Ẹ̀rọ Ìwọ̀n Ooru Yàrá Ìwádìí2
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).
Àwọn irinṣẹ́ tí a nílò:
Ẹ̀rọ ìfàfẹ́fẹ́jáde
Ẹ̀rọ ìwọ̀n àìsí afẹ́fẹ́
Ìdẹkùn òtútùÌwẹ̀ EpoẸ̀rọ Ìwọ̀n Ooru Yàrá Ìwádìí3
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.
Àwọn irinṣẹ́ tí a nílò:
Ẹ̀rọ ìfàfẹ́fẹ́jádeẸ̀rọ ìwọ̀n àìsí afẹ́fẹ́Ìyípo ẹ̀rọ ìtutùÌwẹ̀ EpoẸ̀rọ Ìwọ̀n Ooru Yàrá Ìwádìí
Ìbọ̀wọ́ Tí Ń Fara Da Ooru4
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.
Àwọn irinṣẹ́ tí a nílò:
Ethyl Acetate
Ago Gílásì Tí Ń Kojú OoruPátákó Gbígbóná
Funnel Büchner (porcelain)
Bébà Ìsẹ́
Gílásì Ààbò Fún Ìfọ́n Kẹ́míkà5
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.
Àwọn irinṣẹ́ tí a nílò:
Ọ̀pá dígí tín-ínrínẸ̀rọ Ìwọ̀n Ooru Yàrá ÌwádìíÌwẹ̀ EpoÀwọn ohun-èlò
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