Page 141 - Biodegradable Polyesters

5.4 Crystal Growth Rate 119

PLA
100 + 1% talc
+ 1% talc + 5% ATC
+ 1% talc + 5% PEG
80
Half-time of crystallization (min) 40
60

10

5

0

60 80 100 120 140
Crystallization temperature (°C)

Figure 5.9 Half-time of crystallization of PLA, PLA with 1% talc, PLA with 1% talc and 5%
ATC, and PLA with 1% talc and 5% PEG. Adapted from Ref. [57], Copyright (2007), with per-
mission from Elsevier.

Although addition of plasticizers is primarily intended to increase the tough-
ness, ductility, and drawability of PLA, it also affects the crystallization behavior
because it is connected with a decrease of the glass transition temperature, widen-
ing the temperature window for crystallization. Plasticizers for PLA are glycerol,
PEG, or citrate esters such as acetyl triethyl citrate (ATC). Figure 5.9 is a plot
of half-times of crystallization of unmodified PLA, PLA with 1% talc, PLA with
1% talc and 5% ATC, and PLA with 1% talc and 5% PEG. Addition of 1% talc
reduces the minimum half-time of crystallization from about 40 to less than 2 min,
while the addition of plasticizers in particular affects the half-time of crystalliza-
∘
tion at high supercooling, at temperatures lower than 100 C, due to the decrease
of T .
g
5.4
Crystal Growth Rate

The kinetics of melt-crystallization of PLA has been analyzed by a number of
research groups [14, 35, 39, 71–75]. Isothermal bulk crystallization rates were
∘
determined in a wide temperature range from 70 to 165 C [71, 72]. The maximum
∘
crystallization rate is observed around 100 C, and the most peculiar behavior
∘
is a discontinuity in the phase change kinetics around 110–120 C, an example
of which is shown in Figure 5.10. Figure 5.10a reports the half-time of crystal-
lization of PLA as function of the isothermal crystallization temperature. The
∘
data set shows a broad minimum around 108 C and a step/discontinuity around
∘
116–118 C, indicated by the arrow. The sudden variation in crystallization rate