IntroductionBiodegradable plastic has gained attention in recent years due to concerns about waste disposal and environmental conservation.

Polylactic acid (PLA) is a biodegradable plastic derived from plants and continues to be widely used in packing, fibers and medical materials. Crystallinity is an important consideration for the strength, impact resistance, and transparency requirements of these products and also influences biodegradability. Furthermore, lactic acid, the PLA monomer, has asymmetrical carbon and thus optical isomers. The isomer ratio and molecular weight of polymers influence crystallinity and heat resistance, so they are factors in the molding process.

In this paper, the crystallinity and heat resistance of polylactic acid are evaluated using DSC and TG. Three samples had roughly the same molecular weight but different optical isomer L-form/D-form ratios and two samples had the same L-form/D-form ratio but different molecular weights.

Thermal characterization of Polylactic Acid by Thermal AnalysisYoshikazu.Nishiyama, Yuichi Kasai, Nobuaki.Okubo and Yutaka.IchimuraAnalytical Application Engineering Department, SII NanoTechnology Inc.

ConclusionCrystallinity and heat resistance are important factors in PLA plastic molding. This study shows that DSC can be used to learn about

crystallinity and the validity of crystallization conditions. Furthermore, thermal decomposition measurements TGA were able to evaluate heat resistance at molding temperatures.

ExperimentSample 1:

Polylactic acid ; a, b, c and c’L-form ratio ; a < b < c = c’Molecular weight ; a = b = c > c’

Instrument :DSC6220 and TG/DTA6200(SII NanoTechnology Inc.)

Measurement Condition :DSC measurementsSamples were heated to 200℃ and then cooled atvarious rates (quench, 10, 5, 1, 0.5 and 0.1℃/min).The samples were then heated from 20℃ to 200℃at 10℃/min in N2 atmosphere.TG measurementsSamples were prepared without any thermal treatments,but simply heated from room temperature to 400℃at rates of 10, 5, 2 or 1℃/ min.

C

H

COOH

H3C OH

L-lactic acid

10.9136c’21.6155c15.4144b

Lifetime*［hr］ΔE［kJ/mol］Sample

Log

B/ ℃

/min

1.0

0.8

0.6

0.4

0.2

0.0

-0.2

1.2

1/T / 1000/K1.50 1.55 1.60 1.65 1.70 1.75 1.80

Arrhenius plot for sample c

*:230℃, Reaction fraction 10%0.7080.6250.015－

0.5

0.2730.1640.005－

1

0.0330.009

0－

5

0.0070.003

0－

10

－－a

0.0061c’0.0010.981c

00.303b

quench0.1cooling rate

[℃/min]Sample

After 10℃/min coolingAfter quench

After 5℃/min coolingAfter 1℃/min coolingAfter 0.5℃/min coolingAfter 0.1℃/min cooling

Comparison of the relative crystallinity*

* : Sample c’ melting heat ( after 0.1℃/min cooling ) is set as 1.

DSC curves after 0.1℃/min coolingTemperature / ℃

180160140120100806040

DS

C /

mW

0

-5

-10

-15

-20

-25

-30

60.1℃

16.2mJ/mg62.0℃

167.5℃

167.5℃

62.1℃

53.4mJ/mg

b

c

c’62.0℃

a

52.4mJ/mg147.7℃

DSC curves after quench

167.3℃

Temperature / ℃180160140120100806040

DS

C /

mW

0

-2

-4

-6

-8

-10

-12167.3℃

137.1℃4.71mJ/mg

-4.41mJ/mg

1.85mJ/mg-1.79mJ/mg

138.5℃

57.3℃

53.9℃

56.3℃

57.2℃b

c

c’

a

Calculation results of Activation Energy

Temperature / ℃180160140120100806040

DS

C /

mW

10

5

0

-5

-10

-15

-20

55.3℃

56.8℃

57.7℃

53.9℃

60.1℃

55.1℃

DSC curves for sample a

1.85mJ/mg

Temperature / ℃180160140120100806040

DS

C /

mW

0

-5

-10

-15

-20

-25

-30

168.3℃

13.2mJ/mg-4.42mJ/mg

59.7℃ 128.1℃

167.3℃

57.2℃

-1.79mJ/mg

138.5℃

167.5℃

62.1℃ 52.4mJ/mg

168.5℃

33.4mJ/mg60.9℃

167.8℃-2.60mJ/mg3.09mJ/mg58.8℃ 133.7℃

58.6℃168.2℃

138.1℃

-1.82mJ/mg

1.96mJ/mg

DSC curves for sample c

Melting peak is not observed

DSC curves for sample bTemperature / ℃

180160140120100806040

DS

C /

mW

10

5

0

-5

-10

-15

-20

62.0℃

147.7℃

16.2mJ/mg

57.6℃

59.0℃

151.1℃0.29mJ/mg

60.0℃

151.2℃0.79mJ/mg

56.3℃

57.2℃

18.7mJ/mg

168.9℃

Temperature / ℃180160140120100806040

DS

C /

mW

0

-5

-10

-15

-20

-25

-30167.5℃

57.3℃ 4.71mJ/mg137.1℃

-4.41mJ/mg

60.2℃ 128.8℃

-4.12mJ/mg

169.8℃

60.2℃ 37.8mJ/mg

167.9℃

58.4℃ 135.8℃

-5.67mJ/mg7.44mJ/mg

168.3℃

58.1℃

-4.16mJ/mg4.56mJ/mg136.3℃

167.3℃

53.4mJ/mg62.0℃

DSC curves for sample c’

Tendency to increase crystallinityat cooling rate less

than 1℃/min

Smaller molecular weight c’ has the higher

crystallinity

Temperature / ℃400350300250200

TG /

%

20

0

-20

-40

-60

-80

-100

-120

2℃/min1℃/min

5℃/min10℃/min

b c

TG curves for sample b and c

Small difference in the thermal decomposition

on TG curves

Smaller molecular weight c’ reacts

quickly

Smaller L-form ratio b reacts

quickly

No difference in crystallinity caused

by the different molecular weight ot

sample c and c’

Compared with c, c’ likely has

the higher crystallinitybecause of the higher

melting heat

The higher theL-form ratio is, so is

the crystallinity.

Cooling rate dependent

TG measurements

DSC measurements after thermal treatment

19th MRS-J Academic Symposium @ Yokohama