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. 2024 Sep 16;16(18):2621.
doi: 10.3390/polym16182621.

Opacification Kinetics of PLA during Liquid Water Sorption

Sara Liparoti  1 , Roberto Pantani  1
Affiliations

Opacification Kinetics of PLA during Liquid Water Sorption

Sara Liparoti et al. Polymers (Basel). .

Abstract

When in contact with water, poly(lactic acid), PLA, undergoes several physical changes. A very evident one is opacification, namely the change from the typical transparent appearance to a white opaque color. This phenomenon is particularly significant for many applications, including packaging, since opacity hinders the possibility of a clear look of the packed goods and also worsens the consumers' perceptions. In this work, we report an analysis of the time evolution of the phenomenon in different conditions of temperature and water concentration. The results allow us to define a time-scale of the phenomenon and to put it in relationship with the temperature and water content inside the material. In particular, opacification proceeds from the outer surface of the specimens toward the center. Both craze formation due to hydrolysis and crystallization contribute to the opacification phenomenon. Opacification becomes faster as temperature increases, whereas the increase in the solution density has the opposite effect. A model for describing the evolution of opacification was proposed and found to be consistent with the experimental data.

Keywords: PLA; diffusion; opacification; water sorption.

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Conflict of interest statement

The authors declare no conflicts of interest.

Figures

Figure 1
Figure 1
Optical image of the PLA sample (a) before and (b) after the test at 58 °C in water.
Figure 2
Figure 2
Schematic view of the system adopted for water sorption, composed of two glass slides, each 1 mm thick, and the PLA disc that is 0.4 mm thick, with a diameter of 15 mm. (a) 3D view, (b) side view. The green arrows show the direction of water sorption.
Figure 3
Figure 3
Schematic view of the experiment setup.
Figure 4
Figure 4
Micrographs of the PLA samples were taken at different times during the test at 58 °C (test A). Left column, optical images; central column, polarized optical images; right column, results of simulation with a color scale (results from Equation (5)): red is transparent, fuchsia is fully opaque, green demarcates the thickness of the opaque ring.
Figure 4
Figure 4
Micrographs of the PLA samples were taken at different times during the test at 58 °C (test A). Left column, optical images; central column, polarized optical images; right column, results of simulation with a color scale (results from Equation (5)): red is transparent, fuchsia is fully opaque, green demarcates the thickness of the opaque ring.
Figure 5
Figure 5
Profile plots of opacity versus the normalized radius (0 = center, 1 = surface in contact with water) at selected times. The threshold level of 50% is reported as a reference.
Figure 6
Figure 6
Time evolution of opacity at some radial positions (0 = center, 1 = surface in contact with water) during test A.
Figure 7
Figure 7
Thickness, d, of the opaque ring, normalized with respect to the radius of the sample, versus time. The radius of the sample is 6.5 mm.
Figure 8
Figure 8
Time evolution of opacification during time for test B, carried out in water at 53 °C, left column, and test C, carried out in water at 48 °C, right column.
Figure 9
Figure 9
Thickness d/R of the opaque ring, normalized with respect to the radius of the sample, versus time for tests carried out in distilled water at different temperatures.
Figure 10
Figure 10
Time evolution of opacification during test D, carried out in a solution with a density of 1.1 g/cm3 at 58 °C, left column, and test E, carried out in a solution with a density of 1.15 g/cm3 at 58 °C, right column.
Figure 11
Figure 11
Thickness, d, of the opaque ring normalized with respect to the radius of the sample, versus time for tests carried out in water solutions at different densities at a temperature of 58 °C.
Figure 12
Figure 12
(a) Master curve of all the results of thickness of the opaque ring versus time, scaled by a shift factor dependent on temperature (b) and equilibrium water concentration (c).
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