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Review
. 2023 Apr 25;15(9):2047.
doi: 10.3390/polym15092047.

Review on the Degradation of Poly(lactic acid) during Melt Processing

Ineke Velghe  1 , Bart Buffel  1 , Veerle Vandeginste  2 , Wim Thielemans  3 , Frederik Desplentere  1
Affiliations
Review

Review on the Degradation of Poly(lactic acid) during Melt Processing

Ineke Velghe et al. Polymers (Basel). .

Abstract

This review paper presents an overview of the state of the art on process-induced degradation of poly(lactic acid) (PLA) and the relative importance of different processing variables. The sensitivity of PLA to degradation, especially during melt processing, is considered a significant challenge as it may result in deterioration of its properties. The focus of this review is on degradation during melt processing techniques such as injection molding and extrusion, and therefore it does not deal with biodegradation. Firstly, the general processing and fundamental variables that determine the degradation are discussed. Secondly, the material properties (for example rheological, thermal, and mechanical) are presented that can be used to monitor and quantify the degradation. Thirdly, the effects of different processing variables on the extent of degradation are reviewed. Fourthly, additives are discussed for melt stabilization of PLA. Although current literature reports the degradation reactions and clearly indicates the effect of degradation on PLA's properties, there are still knowledge gaps in how to select and predict the processing conditions that minimize process-induced degradation to save raw materials and time during production.

Keywords: PLA; degradation; melt processing; poly(lactic acid).

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Chemical structure of (a) L-lactic acid and (b) D-lactic acid.
Figure 2
Figure 2
Working principle of the single-screw extrusion process.
Figure 3
Figure 3
Geometry of a basic screw with (a) flight depth, (b) feed zone, (c) compression zone, (d) metering zone, and (e) outer diameter D.
Figure 4
Figure 4
Working principle of the injection molding process.
Figure 5
Figure 5
GPC curves for unprocessed PLA and PLA that was extruded once or twice. Reprinted with permission from Ref. [55]. 2023, John Wiley and Sons.
Figure 6
Figure 6
Rheological dynamic frequency sweep measurements for neat PLA and PLA samples collected at sampling port 4 (SP4), sampling port 8 (SP8), and at die exit (DE). Reprinted with permission from Ref. [60]. 2023, Elsevier.
Figure 7
Figure 7
Melt flow index (MFI) for PLA, rPLA, and chain extended rPLA blends [70].
Figure 8
Figure 8
Time evolution of complex viscosity (η*) for degraded samples under different degradation conditions, (A) nitrogen atmosphere, (B) air atmosphere, and (C) oxygen atmosphere. Reprinted with permission from Ref. [32]. 2023, John Wiley and Sons.
Figure 9
Figure 9
DSC curves corresponding to the second heating cycle for virgin PLA and selected degraded samples subjected to different degradation conditions (“T”, “TO”, and “TM” correspond to thermal, thermo-oxidative, and thermomechanical degradation, respectively). Reprinted with permission from Ref. [33]. 2023, Elsevier.
Figure 10
Figure 10
Stress-strain curves of PLA that was extruded one to ten times, with P0 a sample produced from original PLA and P10 a sample from PLA that was extruded ten times. Reprinted with permission from Ref. [69]. 2023, Elsevier.
Figure 11
Figure 11
Stress at break (σb) as a function number of injection molding cycles. Reprinted with permission from Ref. [53]. 2023, Elsevier.
Figure 12
Figure 12
Strain at break (εb) as a function number of injection molding cycles. Reprinted with permission from Ref. [53]. 2023, Elsevier.
Figure 13
Figure 13
Impact strength (IS) as a function of the extrusion number. Reprinted with permission from Ref. [69]. 2023, Elsevier.
Figure 14
Figure 14
Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) spectra for virgin PLA and selected degraded samples subjected to different degradation conditions (“T”, “TO”, and “TM” correspond to thermal, thermo-oxidative, and thermomechanical degradation, respectively). Reprinted with permission from Ref. [33]. 2023, Elsevier.
Figure 15
Figure 15
Overview on how degradation affects different properties and a selection of the properties (presented at the bottom row) that are eligible to monitor the process-induced degradation. Mn- ↓ [5,12,17,20,28,38,43,47,48,51,52,53,54,55,56,57,58,59]. Mw- ↓ [12,17,20,28,29,30,38,43,48,49,50,51,52,53,55,56,58,59]. Đ = [5,30,48,50,51,53], Đ ↓ [12,17,28,54,55] or Đ ↑ [49,52]. η* and η0 ↓ [17,20,30,33,50,59,60,61,62,63,64,65]. ηapp ↓ [64,66]. [η] and Mv ↓ [18,20,38,58,61,65,67]. MFI ↑ [12,26,40,55,68,69,70]. Tg = [26,50,52,54,59,61,62,69,70,71] and Tg ↓ [33,53,68,74]. Tm = [17,26,52,53,59,65,68,70,77] or Tm ↓ [54,57]. Tcc ↓ [17,26,33,40,50,61,62,65,68,69,70,74,76]. Tc ↑ [52,53]. Xc ↑ [17,20,26,52,53,59,61,62,70] or Xc 0 [33,50,68,76]. No ∆Hc and ∆Hcc ↑ [26,50,69,74] or ∆Hc ↑ and ∆Hcc ↓ [52,53,59]. E = [26,31,59,69], E ↓ [50,52,53] or E ↑ [55,62]. σt = [59,62] or σt ↓ [55,69]. σb ↓ [26,50,53,69,74]. εb = [62,69,74] or εb ↓ [26,52,53,55,59]. IS ↓ [26,50,55,69,74]. Hardness = [26] or hardness ↓ [31,53,78]. L* = [79] or ↓ [26]. a* = [26,79]. b* ↑ [26,30,79]. Change in peaks at 650-750 cm−1 [33], 921 cm−1 [80], 1085 cm−1 [33], 1183 cm−1 [33], 1293 cm−1 [80] or 1750 cm−1 [33]. UV-vis [47,81,82]. NIR [80,84]. Raman [32,80,84]. Ultrasound [83,85]. Vertical force [18].
Figure 16
Figure 16
Averaged number-molecular weight (Mn-) as function of processing conditions. White: 20 rpm; grey: 120 rpm. Reprinted with permission from Ref. [89]. 2023, John Wiley and Sons.
Figure 17
Figure 17
Molecular weights as a function of processing temperature and rotational speed for four PLA 2500HP, PLA 4032D, PLA 3100HP, and PLA 3260HP [79].
Figure 18
Figure 18
Weight-average molecular weight (Mw-) and residence time for PLA processed using quad-screw extrusion (QSE) at a screw speed of 400 rpm and with screw configuration 2 (no kneading blocks). The boxed values represent reductions in molecular weight with respect to the molecular weight of the virgin PLA [90].
Figure 19
Figure 19
Complex viscosity (η*) of virgin and processed samples at different temperatures. Reprinted with permission from Ref. [92]. 2023, Elsevier.
Figure 20
Figure 20
The complex viscosity (η*) (a) and the storage modulus (G′) (b) angular frequency dependence at 180 °C for neat and modified PLA with chain extender after reaching the equilibrium state. Reprinted with permission from Ref. [20]. 2023, Elsevier.
Figure 21
Figure 21
Molecular weights for PLA, PLAT (tropolone) and PLAQ (quinone) as a function of mixing time. Reprinted with permission from Ref. [53]. 2023, Elsevier.
Figure 22
Figure 22
GPC curves of PLA pellets, extruded PLA and extruded PLA compounds with 0.1 wt%, 1 wt%, and 3 wt% of different carbon fillers LSAG, HSAG, and CB [56].
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