REVIEW pubs.acs.org/Biomac
From Lactic Acid to Poly(lactic acid) (PLA): Characterization and Analysis of PLA and Its Precursors ,† Saara Inkinen,† Minna Hakkarainen,‡ Ann-Christine Albertsson,‡ and Anders S€odergard* †
Laboratory of Polymer Technology, Center for Functional Materials (FUNMAT), Åbo Akademi University, Piispankatu 8, 20100 Turku, Finland ‡ The Royal Institute of Technology (KTH), Department of Fibre and Polymer Technology, Teknikringen 56-58, 10044 Stockholm, Sweden ABSTRACT: The quality of the monomers lactic acid and lactide as well as the chemical changes induced during polymerization and processing are crucial parameters for controlling the properties of the resulting poly(lactic acid) (PLA) products. This review presents the most important analysis and characterization methods for quality assessment of PLA and its precursors. The impurities typically present in lactic acid or lactide monomers and their possible origins and effects on resulting PLA products are discussed. The significance of the analyses for the different polymer production stages is considered, and special applications of the methods for studying features specific for PLA-based materials are highlighted.
’ INTRODUCTION Poly(lactic acid) (PLA) polymers have been known for a relatively long time, but the interest in these materials is only accelerating. A lot of research effort is currently concentrated on the development of different polylactide modifications to make the material suitable for a wider range of products. In many cases the modification of PLA by copolymerization,1-3 surface treatment,4 stereocomplexation,5-7 or blending8,9 changes the properties and degradation pattern of the inherent polymer drastically and consequently affects its suitability for different end-use applications. Proper analysis and characterization of PLA and its precursors during different production stages is vital for controlling the properties and durability of the resulting product. The present industrial production of lactic acid is based on microbial carbohydrate fermentation because it is chemically and economically more feasible compared with the chemical route and enables the production of optically pure lactic acid.10 The optical purity of the reagent lactic acid is crucial during PLA production because small amounts of enantiomeric impurities drastically change properties such as crystallinity or biodegradation rate of the polymer. Because the fermentation broth includes a complex mixture of impurities, nutrients, and cell debris, the downstream processing of lactic acid plays a decisive role in the overall production route of PLA. The detection and removal of impurities is essential because they can strongly deteriorate the properties of the produced polymer.11 Lactide, the ring-formed dimer of lactic acid, is used in the production of high molar mass PLA in the ring-opening polymerization (ROP) route and is therefore an important intermediate in the industrial production of PLA. Because of the chiral nature of lactic acid, lactide exists in three different forms: L,L-lactide, D, r 2011 American Chemical Society
D-lactide, and D,L-lactide. A 50:50 mixture of L,L- and D,D-lactide is
referred to as racemic lactide. Lactide is the end-product of a depolymerization process, in which a low molar mass LA oligomer produced by step-growth polymerization is thermally degraded by the so-called backbiting reaction. The properties of the crude lactide therefore depend on the quality of the polymer used as a reagent as well as on the depolymerization reaction parameters. In addition to lactide, the crude product often also contains water, lactic acid, lactic acid oligomers, and residual catalyst, in case used for the polymerization of the oligomer. Careful control of the production line starting from the monomers lactic acid and lactide to the polymerization process itself, whether it is step-growth polymerization or ROP, is necessary for the production of optically pure high molar mass PLA. Figure 1 presents the simplified production routes of PLA and summarizes the most important properties of PLA and its precursors with regard to polymer production and quality. This Review presents relevant and up-to-date information on characterization of lactic acid-based polymers and their precursors lactic acid or lactide. The importance of monomer quality and different characterization and quality control steps for the properties of the final product are highlighted, and the significance of different analyses for the polymer production process are explained. Even though some special applications are included, the Review mainly concentrates on methods applied on pure PLA and its precursors and excludes material tests such as mechanical tests. Received: October 31, 2010 Revised: December 23, 2010 Published: February 18, 2011 523
dx.doi.org/10.1021/bm101302t | Biomacromolecules 2011, 12, 523–532
Biomacromolecules
REVIEW
Figure 1. Simplified production routes of PLA and a summary of the most important properties of PLA and its precursors with regard to polymer production and end-use possibilities.
’ ANALYSIS OF PRECURSORS OF POLY(LACTIC ACID)
including arsenic (
