A new strategy for recycling and preparation of poly(L-lactic acid): hydrolysis in the melt.
Poly(L-lactide) [i.e., poly(L-lactic acid) (PLLA)] was hydrolyzed in the melt in high-temperature and high-pressure water at the temperature range of 180-350 degrees C for a period of 30 min, and formation, racemization, and decomposition of lactic acids and molecular weight change of PLLA were investigated. The highest maximum yield of l-lactic acid, ca. 90%, was attained at 250 degrees C in the hydrolysis periods of 10-20 min. Too-high hydrolysis temperatures such as 350 degrees C induce the dramatic racemization and decomposition of formed lactic acids, resulting in decreased maximum yield of L-lactic acid. The hydrolysis of PLLA proceeds homogeneously and randomly via a bulk erosion mechanism. The molecular weight of PLLA decreased exponentially without formation of low-molecular-weight specific peaks originating from crystalline residues. The activation energy for the hydrolysis (deltaE(h)) of PLLA in the melt (180-250 degrees C) was 12.2 kcal x mol(-1), which is lower than 20.0 kcal x mol(-1) for PLLA and 19.9 kcal x mol(-1) for poly(dl-lactide) [i.e., poly(DL-lactic acid)] as a solid in the temperature range below the glass-transition temperature (21-45 degrees C). This study reveals that hydrolysis of PLLA in the melt is an effective and simple method to obtain l-lactic acid and to prepare PLLA having different molecular weights without containing the specific low-molecular-weight chains, because of the removal of the effect caused by crystalline residues.
Department of Ecological Engineering, Faculty of Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan. tsuji@eco.tut.ac.jp
Hideto
H
Tsuji
Hiroyuki
H
Daimon
Koichi
K
Fujie
D006868:Hydrolysis; D019344:Lactic Acid / Q000138:chemical synthesis* / Q000378:metabolism*; D011091:Polyesters; D011108:Polymers / Q000138:chemical synthesis* / Q000378:metabolism*
D011091:Polyesters; D011108:Polymers; D019344:Lactic Acid; C033616:poly(lactide)