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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.
高级
1–2 days
说明
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.
此步骤所需材料：
乳酸 (工业级)1 个所需工具：
圆底烧瓶
冷凝器盘管
电热板
油浴
实验室温度计2
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).
所需工具：
真空泵
真空表
冷阱油浴实验室温度计3
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.
所需工具：
真空泵真空表冷凝器盘管油浴实验室温度计
耐热手套4
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.
所需工具：
乙酸乙酯
耐热玻璃烧杯电热板
布氏漏斗（瓷制）
滤纸
化学防溅护目镜5
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.
所需工具：
玻璃毛细管实验室温度计油浴此蓝图制作
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