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of PLA in the market. In this process, PLA is derived from

lactic acid existing in two stereochemical conﬁgurations

(i.e., L or D lactic acid), which is catalytically converted

under low pressure into LL-lactide, meso or LD-lactide,

and DD-lactide, and then it is ring open polymerized into

different structures of poly(L-lactic acid, D-lactic acid, or

LD-lactic acid). In the direct condensation and azeotropic

dehydration condensation polymerization, PLA is derived

from L and D-LA, directly. So, high-molecular poly(L,D, or

LD LA) can also be produced (2).

PLA resins between 92% L-LA and 98% L-LA, and

stereocomplex PLA (50/50 blend of PLLA and PDLA) can

be obtained commercially. These resin grades can be

processed by extrusion/thermoforming, injection molding,

biaxially oriented ﬁlms, injection stretch blow molded

bottles, ﬁber melt spinning, and spunbond items. All these

resins are mainly targeted for particular application;

therefore, their degree of L-and D-lactide varies according

to the applications (4). The latest developed stereocomplex

resins are mainly used for ﬁbers, ﬁlms, and thermoform-

ing articles.

PROPERTIES

Structure

The principal peaks of PLA absorption in the infrared

range are summarized in Table 1. The strong infrared (IR)

band at 2997, 2946, and 2877 cm

1

are assigned to the CH

stretching region, n

, and nCH modes, re-

spectively. The CQO stretching region is observed by a

large band at 1748 cm

1

. The region between 1500 and

1360 cm

1

is characterized by the 1456 cm

1

bend.

The CH deformation and asymmetric bend appear at 1382

and 1365 cm

1

, respectively. The bands that appear at

1315 and 1300 cm

1

are caused by the CH bend modes. In

the region from 1300 to 1000 cm

1

, the C–O stretching

modes of the ester groups at 1225 cm

1

and the -O-C

asymmetric mode at 1090 cm

1

are presented. Between

1000 and 800 cm

1

, we can observe peaks at 956.36 and

921.94 cm

1

, which can be attributed to the characteristic

vibrations of the helical backbone, mixed with the CH

rocking modes. Two bands related to the crystalline and

amorphous phases of PLA are found at 871 and 756 cm

1

The peak at 871 cm

1

can be assigned to the amorphous

phase, and the one at 756 cm

1

to the crystalline phase.

The peaks below 300 cm

1

correspond mainly to CH

torsion modes and to the skeletal C-C torsions.

Semicrystalline PLAs have superior physical and

structural performance compared with amorphous PLAs.

Researchers have postulated that the improvements in

mechanical and barrier properties of semicrystalline poly-

mers are related directly to complex crystalline structures

and their distribution in the amorphous matrix (8–10).

The amorphous polymer region cannot be in the crystal

lattice of the semicrystalline polymer, and so it must

reside between the lamellar stacks (interlamellar) or in

the interﬁbrillar region within the spherulites (inter-

spherulitic). The variation in size and shape of the crystal-

lites, their spatial distribution, the crystalline morphology

(which depends on molecular weight, polydispersity index,

and processing conditions), the degree of crystallinity, and

the space and distribution available for the amorphous

region will all dictate the ﬁnal physical, mechanical, and

barrier properties of PLA.

Depending on the preparation conditions, poly(L-lactic

acid) crystallizes in three forms (a-, b-, and g-) (2). The

stable a-form exhibits a well-deﬁned diffraction pattern

with a space group orthorhombic P2

, with a unit cell

containing two antiparallel chains. The chain conforma-

tion is a 2-fold (15*2/7) helix distorted periodically form of

the regular s(3*10/7 helix). The lattice parameters are

a = 10.66 A

; b = 6.16 A

, and c (chain axis) = 28.88 A

, with a

crystal density around 1.26 g/cm

. The b-form of PLA is

generally prepared at a high draw ratio and high drawing

temperature. The chain conformation is a left-handed

3-fold helix. The b-form of PLA has an orthorhombic

unit cell (containing 6 chains) that contains a 3 l (3 A

rise/1 monomeric unit) polymeric helix. The lattice para-

meters of the unit cell are a = 10.31 A

, b = 18.21 A

, and c =

9.0 A

. The a-structure is more stable than the b-structure,

with a melting point of 1851C compared with 1751C for the

Scheme 1. Synthesis methods for obtain-

ing high-molecular-weight polylactide and

poly(lactic acid); scheme is from Auras et

al. (2) by permission of Wiley-VCH Verlag

GmbH & Co. KGaA

968 POLY(LACTIC ACID)

b-structure. Chain packing in the b-form has recently

been termed a frustrated structure, where the crystal

structure comprises a frustrated packing of three 3-fold

helices in a trigonal unit cell of parameters a = b = 10.52 A

c = 8.8 A

, and space group P3

(11). The g-form is formed by

epitaxial crystallization and contains two antiparallel

s(3/2) helices in the pseudoorthorhombic unit cell

a = 9.95 A

, b = 6.25 A

, and c = 8.8 A

, and it assumes the

known 3-fold helix of poly(lactic acid) (2).

Optical, Thermo-Physical, Rheological, Mechanical, and

Barrier

The optical, physical, mechanical, and barrier properties

of PLA compared with PS and polyethylene terephtalate

(PET) are shown in Table 2. PLA has similar density than

PET, and a similar semycrystalline structure can be

obtained during processing. Because the glass transition

temperature (T

) of PLA and its heat deﬂection tem-

peratures (HDT) are very close, PLA suffers heat deforma-

tion when it is exposed to temperatures higher than 501C.

The tensile strength values obtained for PLA are within

the range of values reported in the literature for PS under

the same testing conditions. However, when compared

with PET, PLA ﬁlms showed lower values. Table 2 also

shows the water, oxygen, carbon dioxide, and aroma

barrier properties of PLA and these values are compared

with those of PS and PET when data were available. PLA

has a better barrier for carbon dioxide, oxygen, and water

vapor permeability when compared with PS but a lower

barrier when compared with PET. PLA has an aroma

barrier similar to PET, so it makes a good polymer for the

storage of products with high ﬂavor. Theoretical predic-

tions based on the solubility parameters indicate that PLA

will interact with nitrogen compounds, anhydrides, and

some alcohol but will not interact with aromatic hydro-

carbons, ketones, esters, sulfur compounds, and water.

PROCESSING

PLA suffers hydrolysis, which is a reduction in molecular

weight caused by the presence of water at temperatures

near the glass transition temperature (T

B551C), so for

processing it must be dried below 250 ppm (12). Additional

drying should be conducted for processes that have long

residence time. Drying of semicrystalline PLA should be

conducted in the range of 80 to 1001C and below T

for

amorphous PLA (4). Because PLA degrades at elevated

temperatures and relative humidity (RH), the resins

should be protected from hot and humid environments

during storage. Henton et al. (12) reported that amor-

phous PLA can dramatically reduce its molecular weight

) in less than a month when exposed to 601C and 80%

RH. More information about the drying condition of PLA

can be found elsewhere (4).

The main way to process PLA is through extrusion and

injection molding, blow molding, ﬁlm blowing, and melt

spinning. To ensure that all the crystalline PLA phases

are melted and to achieve an optimal melt viscosity for

processing, PLA is generally extruded around 160 to

1951C. In general, commercial-grade PLA resins can

typically be processed using a conventional extruder.

Cast and ﬁlm sheet of PLA are produced with a screw

diameter of 19.3 mm, compression ratio of 2:1, and length/

diameter ratio of 25. The recommended compression ratio

for PLA processing is in the range of 2:3 (13). High roller

temperatures (i.e., 25501C) are generally used to cast

PLA avoiding the building up of lactide around the die.

General equipment to process PET and PS can be used

and/or modiﬁed to process PLA. The physical properties of

PLA as for other polymers can be enhanced by orientation.

Resins with a higher D-lactide content allow a higher

drawing ratio. A blown ﬁlm of virgin PLA resin is difﬁcult

to obtain. So, blown PLA ﬁlms contain additives, such as

viscosity enhancers, are to strengthen PLA’s melting

strength, and slip additives are used to reduce the coefﬁ-

cient of friction (4).

Thermoforming of PLA is normally conducted to pro-

duce PLA clamshells and containers. The properties of

thermoformed PLA containers have been compared with

PS and PET showing that PLA could out perform PET and

PS at lower temperatures (14, 15).

PLA bottles have been produced based on injection

stretch blow molding (ISBM) techniques. The ISBM pro-

cess produces biaxial-orientated PLA bottles and has

better properties than injection molding of amorphous

PLA (4). Most of these bottles are currently being used

for noncarbonated drinks.

PLA foaming has been gaining attention because of its

potential to reduce weight and reduce thermal conductiv-

ity. Foaming of PLA has been a niche application in tissue

engineering and medical implant applications. As the

price of PLA is dropping, PLA foam can ﬁnd new applica-

tions in packaging, such as cups, bowls, and loose-ﬁll

packaging materials (4). Most of the challenge in foaming

PLA is controlling the bubble size of the foam structure

(16). Another special category of growing interest for PLA

applications is ﬁber spinning and electrospinning of ultra-

ﬁne ﬁbers. The former application has mainly reached

market applications in textile production, and it is having

great acceptability because of lower water-barrier proper-

ties. However the electrospinng PLA ultraﬁne ﬁbers is

mainly being developed for tissue engineering and biome-

dical applications (4).

Table 1. Infrared spectroscopy data: Peak band

assignments for PLA infrared spectra

Assignment

Peak Position,

1

–OH stretch (free) 3571

–CH- stretch 2997 (asym),

2946(sym), 2877

–CQO carbonyl stretch 1748

–CH

bend 1456

–CH– deformation including symmetric

and asymmetric bend

1382, 1365

–CQO bend 1225

–C–O– stretch 1194, 1130, 1093

–OH bend 1047

–C–C– stretch 926, 868

POLY(LACTIC ACID) 969

Table 2. Optical, physical, mechanical, and barrier properties of PLA compared with PS and PET

PLA PS PET

Optical

Refractice index

1.35–1.45 1.591 1.54

Clarity Clear—yellow Clear Clear

Thermo-Physical

Density amorphous,

,kg.m

3

1250 1030 1330

Density 100% crystalline,

,kg.m

3

1490 1150 1465

Van der Waals volume (V

,cm

.mol

1

34.45 62.9 94.2

Molar volume of glassy amorphous (Vg),

,cm

.mol

1

55.12 99.0 144.5

Molar volume of semycrystalline polymer (Vc),

,cm

.mol

1

49.44 92.0 130.0

Solubility parameter (dp), 251C,

0.5

1.5

19–20.5 19.4 22.9

Glass transition temperature (T

)

1C 55–65 105 73–80

Melting temperature (T

,1C 130–180 Atactic 245–265

Initial decomposition temperature (T

d,0

)

1C 335 326 378

Half decomposition temperature (T

d,1/2

, 1C 395 363 450

Average energy of activation (E

act

, kJ.mol

1

205–297 230 —

Enthalpy (DHm),

, 100%, J.g

1

93, 135 N/A 135.8

Crystallinity,

, % 0–40 N/A 10–40

Heat deﬂection temperature,

, 1C 55–65 88 70

Vicat penetration temperature,

, 1C 59 101 76

Thermal conductivity 10

4

, cal.cm

1

2.9 4.3 5.7

Heat capacity,

cal.g

1

.1C

1

0.39 0.54 0.44

Thermal expansion coefﬁcient 10

6

, 1C

1

70 70 70

Surface tension,

, dyn.cm

1

42.0 40.7 44.6

Oxygen index (OI),

— 0.185 0.21

Friction Coefﬁcient,

0.37 0.38 0.25

Melt Flow Index,

, g. min

1

0.85 0.28 7.2

Electrical, Magnetic, and Acoustic

Dielectric constant,

, e N/A 2.25 2.9/3.2

Magnetic susceptibility, (c),

, 10

6

,cm

1

N/A 0.705 0.505

Sound velocity, m

, m.s

1

N/A 2400 N/A

Sound velocity of propagation of transverse waves, m

, m.s

1

N/A 1150 N/A

Rheological

Intrinsic viscosity, n,

, dl.g

1

— — 0.72270.029

Mark-Houwink constants

K 10

4

, DL.g

1

8.5

— 7.44

0.66

— 0.648

Mechanical

Tensile strength @ yield,

, MPa 0.88 36–51.7 48–72

Elastic modulus,

, GPa 8.6 2.3–3.3 2.8–4.1

Elongation at break,

, % 3–30 1.2–2.5 30–300

Flexural strength,

, MPa 70 83 70

Flexural Modulus,

,GPa 3.8 — 2.4–3.1

Unnotech Izod Impact,

,J.m

1

106 — —

Notched Izod Impact,

,J.m

1

26 16 101

Rockwell hardness

, 88 — 106

Impact strength Poor Poor Good

Barrier

Oxygen 10

18

, kg.m.m

2

1

.Pa

1

@251C 1.2170.07 27 0.19

Oxygen activation energy,

, kJ.mol

1

[25–451C] 41.4373.5 – 30.30

Carbon dioxide 10

17

, kg.m.m

2

1

.Pa

1

@251C 2.7770.05 15.5 0.17

Carbon dioxide activation energy,

, kJ.mol

1

15.6570.63 — —

Nitrogen 10

19

, kg.m.m

2

1

.Pa

1

468 — 0.68

Nitrogen activation energy,

, kJ.mol

1

———

Water 10

14

, kg.m.m

2

1

.Pa

1

@251C 1.7570.05 0.67 0.11

Water activation energy,

, kJ.mol

1

9.7370.27 0–8.0 —

d-limonene 10

19

, kg.m.m

2

1

.Pa

1

o1.0 — 9

d-limonene activation energy,

, kJ.mol

1

———

Ethyl acetate 10

19

, kg.m.m

2

1

.Pa

1

5.34 — —

Ethyl acetate activation energy,

, kJ.mol

1

N/A — —

970 POLY(LACTIC ACID)

Blends of PLA with other polymers have been re-

searched to improve PLA ﬁlms bursting strength, elonga-

tion and tear strength, and impact resistance properties.

Some main materials that have been blended with PLA are

thermoplastic starch, poly(ethylene glycol), triacetin, tri-

butyl citrate, poly(hydroalkanoates), and poly(3-hydroxy-

loctanoate) (4). Cost reduction is achieved when the

blending counterpart has a lower cost. Besides PLA blend-

ing, compounding of PLA with inorganic or organic parti-

cles has investigated to enhance the barrier, thermal, and

mechanical properties of the PLA composite. Special cate-

gories of compounding are nanocomposites which have

been reinforced into PLA. Among the main nanocompo-

sites used are layered silicates, carbon nanotube, hydro-

xyapaite, layered titanate, aluminum hydroxide, and

cellulose nanowiskers (4). Layered silicate clays such as

montmorillonite MMT have been researched extensively in

the industry to produce PLA with higher mechanical and

ﬂexural properties, elevated heat distortion temperature,

enhanced barrier properties, and accelerated biodegrada-

tion. Additional promise has been created with cellulose

nanowiskers to produce PLA microwavable packages (4).

End-of-life Scenario

PLA can be recycled and has been certiﬁed as a compo-

stable polymer according to ASTM D6400 and EN13432

by the main international certifying entities (7, 17). The

composting of PLA is enhanced by hydrolysis because of

the absorption of water resulting in the hydrolysis of the

ester linkages. The rate of degradation is highly affected

by temperature, moisture, and pH conditions of the

(Table footnote Continued)

N/A: Not available

Refractive index values for PLA were calculated by Gladstone and Dale, Vogel, and Lloyenga methods according to Ref. (20). PS and PET values are

experimental values obtained from Ref. (20).

Density of the amorphous PLA was obtained from Ref. (21), PS and PET values are from Ref. (20).

Density of 100% PLA, PET, and PS was calculated according to the group contribution method (20).

PLA, PET, and PS values were calculated using the group contribution method (20).

PLA, PET, and PS values were obtained from Ref. (2).

and T

of PLA, PS, and PET were obtained from Ref. (1).

Td,0 and Td,1/2 of PLA was obtained from Ref. (18); PS and PET values were obtained from Ref. (20).

PLA and PET values obtained from Ref. (22).

PLA, PS, and PET values were obtained from Ref. (23).

PLA, PS, and PET values were obtained from Ref. (4).

PLA value was obtained from Ref. (2); PS and PET values were obtained from Ref. (20).

PLA value was obtained from Ref , PS and PET values were obtained from Ref (20).

PLA, PET, and PS values were obtained from Ref. (2).

PLA and PS, PET values were measured at 2001C and 5 kg according ASTM D1238.

PS and PET values were obtained from Ref. (20).

PLA and PET, PS values were measured according to ASTM D445 and D446 (PLA values were determined in Tetrahydrofuran at 30 1C, PET values were

determined in phenol and 1, 1, 2, 2-tetrachloroethane (60:40 by volume) at 25 1C.

PLA, PET, and PS values were obtained from Ref. (2).

PLA, PS, and PET values were obtained from Ref. (23).

PLA, PET, and PS values were obtained from Ref. (1, 22); oxygen activation energy are reported for temperatures between 25 and 451C.

PLA, PET, and PS values were obtained from (1); carbon dioxide activation energy is reported for temperatures between 25 and 451C.

PLA values were obtained from Ref. (24); PET and PS values were obtained from Ref. (20).

PLA, PS, and PET values were obtained from Refs. (1, 22); water activation energy is reported for temperatures between 101C and 37.81C.

PLA, PS, and PET values were obtained from Refs. (25, 26) (et hyl acetate values of PLA and PET are at 3030 PA, and 301C and 9435 Pa and 301C,

respectively; d-limonene of PLA and PET are at 245 Pa and 451C and 45 Pa and 231C, respectively).

010203040506070

Time (days)

%Mineralization

PLA

PET

Figure 1. Percentage of mineralization of

PLA, PET, and corn starch at 58721C and

approximately 5575% RH in yard waste com-

post (reproduced from Kijchavengkul et al.

(27)) by permission from Elsevier B.V.

POLY(LACTIC ACID) 971

compost (18, 19). PLA polymers with lower L-lactic

acid content degrade faster in a compost environment.

Figure 1 shows the mineralization of PLA bottles com-

pared with PET bottles as a negative control in yard waste

compost.

Figure 2 shows the energy used and amount of green-

house gases emitted in CO

equivalents, from producing

1 kg of polymer resin (cradle-to-gate boundary). According

to Figure 2, in general, production of these biodegradable

polymers, except poly(caprolactone) (PCL), uses less en-

ergy than production of many nonbiodegradable plastics,

such as PS, polypropylene (PP), PET, and polyethylene

(PE). The production of PLA and PHA (polyhydrox-

yalkanoate) produces greenhouse gases in similar

amounts as those of PS, PP, and PET. More research is

necessary regarding the end-of-life scenario of these new

biopolymers.

BIBLIOGRAPHY

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Poly(lactide) Polymers vs. Existing PET and Oriented PS for

Fresh Food Service Containers,’’ Packag. Technol. Sci. 18,

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PLA

Energy used (MJ/kg polymer)

100

PHA PH(3B) PCL

Starch+PCL

Polymer

PET PE

0.0

0.5

1.0

1.5

2.0

GHG emission (kg CO

eq./kg polymer)

2.5

3.0

Energy

GHG

3.5

Figure 2. Energy used and amount of greenhouse gases (GHG) emission in CO

equivalent from producing 1 kg of polymer resin at the

resin production plants (reproduced from Kijchavengkul and Auras (7) by permission from Wiley-VCH Verlag GmbH & Co. KGaA.)

972 POLY(LACTIC ACID)

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POLYCARBONATE

Updated by Staff

INTRODUCTION

Polycarbonate (PC) is an amorphous resin that does not

require orientation to achieve its full property proﬁle. The

molten resin can be extruded by the blown or cast pro-

cesses (see Extrusion), injection-molded (see Injection

molding), or blow-molded by the extrusion-blow or injec-

tion-blow techniques (see Blow molding). As polycarbo-

nate cools into ﬁlm, sheet, or containers, it exhibits

excellent dimensional stability, rigidity, impact resistance,

and transparency over a wide range of temperatures and

loading rates (see Table 1). Because PC is amorphous, its

wide softening range and added strength in thermoform-

ing (see Thermoforming) operations provides deep-draw

capabilities.

Polycarbonate is now the second largest engineering

plastic produced, behind crystalline nylon. Leading world

producers are BASF, General Electric, and The Dow

Chemical Company (1). Trade names include Lexan

(GE), Makrolon (Bayer), Calibre (Dow), Panlite (Teijin),

and Iuplon (Mitsubishi).

Polycarbonate is prepared by the Schotten–Baumann

condensation reaction of phosgene and an aromatic diol

using an amine catalyst or a based-catalyzed transester-

iﬁcation of bisphenol and a monomeric carbonate (2).

PROPERTIES

Polycarbonate provides an excellent combination of tensile

strength and ﬂexural modulus at high temperatures (see

Table 2). Its heat resistance, combined with superior

impact resistance at both high and low temperatures,

makes it an excellent structural layer in coextruded or

laminated packaging for hot ﬁll at 180–2101F (82–991C),

retorting at 2501F (1211C), autoclaving at 270–2801F

(132–1381C), and frozen-food packaging. In addition, PC

can be sterilized with both gamma and electron-beam

irradiation with good stability.

Polycarbonate has light-transmittance values of 88–

91% as compared with 92% for clear plate glass. It has a

haze factor of less than 1%, and it maintains these values

throughout the temperature scale. Its high gloss and easy

colorability and printing contribute to distinctive package

design. Polycarbonate has high resistance to staining by

tea, coffee, fruit juices, and tomato sauces, as well as

lipstick, ink, soaps, detergents, and many other common

household materials. Its relatively dense composition

makes it immune to odors, and its hard, smooth surface

facilitates easy removal of foodstuffs.

Some of the most signiﬁcant attributes of polycarbo-

nate stem from its very low water absorption. Added

weight increase after 24-h immersion at room tempera-

ture is only 0.15%. This low absorption level helps account

for the resin’s excellent dimensional stability and stain

resistance. It also indicates that the resin, itself tasteless

and odorless, is unlikely to pick up food odors. PC is

available in grades that meet FDA and USDA regulations

and is recognized as safe for food-contact applications.

PACKAGING APPLICATIONS

Reﬁllable Bottles

Polycarbonate is the material of choice for use in reusable

bottles, particularly 5-gal (19-L) water bottles, which

represent the resin’s chief packaging application. These

bottles take advantage of polycarbonate’s toughness (to

resist breakage) and clarity (to see the contents). The fact

that PC is much lighter than glass provides fuel savings as

well as productivity improvements, since several bottles

can be carried at once. Systems have been developed to

wash polycarbonate and provide clean bottles for reuse

with minimum impact on trippage. Lexan (now sold under

SABIC Innovative Plastics) can be washed and ﬁlled 50

times while maintaining excellent taste, neutrality, and a

transparent glasslike appearance (3).

POLYCARBONATE 973

Medical-Device Packaging

Polycarbonate meets many requirements of medical-

device packaging (see Healthcare packaging). It is clear

and tough, and it can be sterilized by commercial sterili-

zation techniques: ethylene oxide (ETO), radiation,

and autoclave sterilization (see Radiation, effects of).

The development of coextrusion technology has afforded

opportunities in all sterilization systems. Special grades

have been developed to maintain clarity and resistance to

yellowing upon gamma radiation (2). In thin ﬁlms,

PC can be coextruded with polyoleﬁn heat-seal layers

(see Coextrusions for ﬂexible packaging; Coextrusions

for semirigid packaging) to produce a cost-effective

alternative to laminations based on oriented ﬁlms (see

Films, plastic). Because it is amorphous, heat sealing

does not shrink or embrittle the ﬁlm (see Sealing, heat).

This virtually eliminates puckering, which can lead to

hairline cracks and shattering upon opening. A soft blister

package with good puncture resistance can be produced

by thermoforming heavier-gauge ﬁlm. For increased

stability in the autoclave, a polycarbonate copolymer,

poly(phthalate carbonate), can be incorporated into the

structure. Properties of monolayer PC ﬁlms are listed in

Table 3.

Food Packaging

Coextrusions of polycarbonate are being evaluated for use

in several segments of the food-packaging market. The

snack-food industry, along with other users of ﬂexible-

packaging materials, is discovering ways to use PC in

coextrusions to replace laminated ﬁlms in a portion of a

structure or to replace the entire structure (see Multilayer

ﬂexible packaging). It is polycarbonate’s toughness with-

out orientation that makes it a good candidate for coex-

trusion. PC is used for food containers and baby bottles (4).

For frozen foods, the resin’s low-temperature impact

strength adds durability to dual-‘‘ovenable’’ trays. In

coextrusions, it can add toughness to crystallized polye-

ster or polyetherimide trays. In cases where the wall

thickness is determined by impact-strength requirements,

gauge reductions up to 33% are possible.

In high-barrier multilayer containers, polycarbonate

offers the rare combination of dimensional stability at

retort and hot-ﬁll temperatures, along with crystal clarity

(see Retortable ﬂexible and semirigid packages; Multi-

layer plastic bottles). To produce a 1% distortion in PC

requires 3000 psi (20.7 MPa) stress. A similar distortion in

polypropylene occurs at less than 500 psi (3.4 MPa) stress.

Polycarbonate is about three times as expensive, but

dimensional stability can be translated into value by

making lighter containers with thinner walls by faster

and more reliable closing a hot-ﬁll temperatures, higher

Table 2. Tensile Strength and Modulus of Polycarbonate

over a Temperature Range

Temperature, 1F(1C) Tensile Strength,

psi (MPa)

Flexural Modulus,

psi (MPa)

73 (23) 10,000 (68.9) 320,000 (2,206)

212 (100) 5,800 (40) 233,600 (1,610)

270 (132) 4,000 (27.6) 211,200 (1,456)

Table 1. Typical Property Values for Polycarbonate

Melt Flow Indexes

Property

ASTM Test

Method 22 16 10 6 PC Copolymer

Speciﬁc gravity D792 1.20 1.20 1.20 1.20 1.20

Light transmittance, 0.125 in.

(3.2 mm), %

D1003 89 89 89 89 85

Haze, 0.125 in. (3.2 mm), % D1003 1 1 1 1 1–2

Deﬂection temperature at 264

psi (1.8 MPa) 1F(1C)

D648 260 (127) 265 (129) 270 (132) 270 (132) 325 (163)

Flammability rating,

UL 94, at

0.060 in. (1.5 mm)

V-2 V-2 V-2 V-2 HB

Tensile strength, yield, psi (MPa) D638 9,000 (62) 9,000 (62) 9,000 (62) 9,000 (62) 9,500 (65.5)

Tensile strength, ultimate,

psi (MPa)

D638 9,500 (65.5) 10,000 (68.9) 10,000 (68.9) 10,500 (72.4) 11,300 (77.9)

Elongation, rupture, % D638 120 125 130 135 78

Flexural strength, psi (MPa) D790 13,500 (93.1) 14,000 (96.5) 14,000 (96.5) 14,200 (97.9) 14,100 (97.2)

Flexural modulus, psi (MPa) D790 335,000

(2,310)

340,000

(2,340)

340,000

(2,340)

340,000

(2,340)

338,000

(2,330)

Izod impact strength, notched,

in: (3.2 mm) thick, ft lbf/in.

(kJ/m)

D256 12 (0.64) 13 (0.69) 15 (0.80) 17 (0.94) 10 (0.53)

Tensile impact, ft lbf/in.

(J/cm

) D1822 180 (37.8) 225 (47.3) 275 (57.8) 300 (63.0) 275 (57.8)

Properties shown are average values that can be expected from typical manufacturing lots and are not intended for speciﬁcation purposes. These values are

for natural color only. Addition of pigments and other additives may alter some of the properties.

This rating is not intended to reﬂect hazards of this or any other material under actual ﬁre conditions.

974 POLYCARBONATE

retort temperatures with less critical overpressure con-

trol, and enhanced container-design ﬂexibility.

An unexpected beneﬁt from the use of PC in retort

applications is an increase in the effectiveness of the

barrier material EVOH (see Ethylene–vinyl alcohol). In

a retort, moisture is driven into all layers of a plastic

container. When the outside skin layer is polypropylene,

the polypropylene prevents moisture from entering the

structure, but it also prevents moisture trapped in the

EVOH layer from escaping. Because polycarbonate has

poorer moisture-barrier properties than polypropylene, it

allows much more rapid drying of the EVOH layer and

longer shelf life.

BIBLIOGRAPHY

J. M. Mihalich and L. E. Baccaro, ‘‘Polycarbonate’’ in The Wiley

Encyclopedia of Packaging Technology, 1st edition, John

Wiley & Sons, New York, pp. 510–511; in EPT, 2nd edition,

pp. 740–741.

Cited Publications

1. K. L. Ring, H. Janshekar, and G. Toki, ‘‘Polycarbonate Resins’’

in Chemical Economics Handbook, SRI Consulting, Menlo

Park, CA, 2005.

2. D. J. Brunelle, ‘‘Polycarbonates’’ in Kirk–Othmer Encyclopedia

of Chemical Technology, Wiley, Hoboken, NJ, 2006.

3. ‘‘Lexan’’ www.geplastics.com, accessed April 2008.

4. O. G. Piringer and A. L. Baner, Plastic Packaging, 2nd edition,

Wiley-VCH, Weinheim, 2008.

POLYESTERS, THERMOPLASTIC

Updated by Staff

INTRODUCTION

Poly(ethylene terephthalate) (PET) was ﬁrst developed

by a British company, Calico Printers, in 1941 for use

in synthetic ﬁbers. The patent rights were then acquired

by DuPont and Imperial Chemical Industries (ICI),

which in turn sold regional rights to many other compa-

nies. Polyester ﬁbers have since made a considerable

impact on the textile industry. The second principal

application of PET was ﬁlm. In 1966, PET became avail-

able for the manufacture of injection-molded and extruded

parts.

The amazing growth of PET in beverage packaging

began in the early 1970s with the technical development

of biaxially oriented PET bottles (see Blow molding) and

with the introduction of the ﬁrst 2-L PET beverage bottle

in 1976. The total world market for PET botttle resin is

still growing at a rapid rate. The total devoted to carbo-

nated soft drink or PET bottles is very large and the

United States is the largest consumer, with Europe next.

China is now advancing with a potential of 25–30 billion

PET beer bottles per annum. The annual output of all

plastic packaging materials including PET bottle resins

was 9.5 million tons in 2005 and is expected to reach 13.7

Table 3. Polycarbonate Film Properties, 1 mil (25.4 lm)

Property ASTM Test Value

Speciﬁc gravity D792 1.20

Yield, in.

/(lb mil) (m

/(kg mm) 23,100 (1,294)

Haze, % D1003 0.5

Optical clarity D1746 86–88

Tensile strength, psi (MPa) MD 10,735 (74)

XD 10,009 (69)

Elongation, % D882 MD 91

XD 92

Secant modulus, psi (MPa) MD 185,000 (1,275)

XD 196,000 (1,351)

Tear strength, gf/mil (N/mm)

Initial

D1004 454 (175)

Propagating

D1922 16 (6.32)

Tensile impact, S type, ft lbf/in.

(J/cm

) D1822 225–300 (47.3–63.0)

Bursting strength, 1 mil, psi (kPa) D774 27.4 (189)

Water absorption at 24 h and 731F (231C), % D570 0.15

Folding endurance

, cycles D2176 11,000

Heat-seal temperature at 40 psi (276 kPa) for 3 s, 1F(1C) 400–420 (204–216)

Oxygen permeability at 771F (231C) and 0% rh, cm

mil/

(100 in.

day atm) [cm

mm/(m

day kPa)]

D1434 240 (933)

Water-vapor transmission rate at 1001F (381C) and 90% rh,

g mil/(100 in.

day) [g mm/m

day)]

E96-66 6.5 (2.6)

Coefﬁcient of friction

Static D1894 0.570

Kinetic 0.542

Graves.

Elmendorf.

Double folds.

POLYESTERS, THERMOPLASTIC 975

million tons in 2008. PET bottle demand seems to be

assured because of its balance of properties and the ability

to be recycled. There is a strong demand for packaging

mineral waters in Europe and there is a large hot-ﬁll

market in Japan. A market survey of the United States

beverage container market for 2002–2007 shows that the

market for plastic bottles (largely PET) is still growing,

while glass and metal metal cans are static or in slight

decline (see Table 1) (1).

PET has the following formula:

HOCH

COCH

where n=100–200.

MANUFACTURE OF PET

Raw Materials

The raw materials for PET (see Figure 1) are derived from

crude oil. Para-xylene, one of the two starting materials, is

part of the naphtha feedstock that used to be fully avail-

able for chemicals because it was a byproduct of limited

value to the oil reﬁner. Now it has become the source of

additives that replace lead in unleaded gasoline in many

countries. Of the mixed xylenes, because they come from

the reformer, only p-xylene is suitable for building

straight polymer chains, and the straight conﬁguration

is necessary to give the polymer its ﬁber- and ﬁlm-forming

(or bottle-wall-forming) characteristics and ultimately its

high tensile strength. The other raw material for polyester

is ethylene, which is contained in the crude oil’s gas

fraction. It is converted to ethylene glycol by oxidation

and hydrolysis. All of these raw materials are now derived

from oil, but it is technically feasible to produce them from

coal-tar distillation.

Intermediate Products

Two different routes are used to manufacture PET: One

route is by way of dimethyl terephthalate (DMT), and the

other is by way of terephthalic acid (TPA). Both are dibasic

acids. Figure 1 shows both routes. The plants for inter-

mediate and end products are shown together, but they

are normally physically separated. The process of making

DMT is relatively simple: One end of p-xylene is ﬁrst

oxidized with air and esteriﬁed with methanol to yield a

half-ester that is subsequently converted at the other end.

The resulting DMT is puriﬁed by distillation and repeated

crystallization to remove isomers and other impurities.

The TPA route is similar. TPA can be produced by oxida-

tion of p -xylene in solution and puriﬁed by solvent

extraction.

Melt Polycondensation

Both batch- and continuous-polycondensation processes

are used. The continuous process inherently allows better

product uniformity; the batch process is preferred for

small quantities of specialty resins. On the DMT route,

DMT and ethylene glycol (EG) are continuously metered

into the ester interchanger, where the methyl end groups

of the DMT are replaced by ethyl end groups to form

diethylene glycol terephthalate (DGT), the monomer of

PET. In this step, EG is consumed while methanol is

evaporated and collected to be returned to the DMT plant.

On the other route, TPA is esteriﬁed with EG to DGT, and

water is removed as a byproduct. Subsequent polyconden-

sation is the same for both routes. In EG takeoff, excess

EG is removed and sent to the distillation plant for

recovery. Polycondensation takes place in vacuum reac-

tors designed to evaporate EG, the condensation bypro-

duct, thereby shifting the equilibrium toward long

polymer chains. The ﬁnal step includes extrusion of the

melt as strands or ribbon, quenching in water, and cutting

to the desired chip size.

Table 1. U.S. Beverage Container Market Survey a

Material

1997,

Billion Units

2002,

Billion Units

2007,

Billion Units

Plastic 35 52 70

Metal 99 100 102

Glass 28 26 30

Source: Reference 2.

Figure 1. Manufacture of PET.

976 POLYESTERS, THERMOPLASTIC

Solid-State Polycondensation

Melt polycondensation produces amorphous PET as used

in most ﬁber and ﬁlm applications. Unfortunately, this

product is not suitable for the injection molding of food

containers because the inherently high acetaldehyde level

would affect the taste of some foods. A ﬁnal solid-phase

polymerization before stretch molding must be performed

to remove the acetaldehyde (1). There are other impurities

that might promote degradation during the injection-

molding process; and amorphous resin tends to fuse and

form lumps in the drying hopper. The polymer must be

upgraded by the solid-state polycondensation process.

Various agitation devices and polymer chip treatments

have been described to prevent sticking (1). The chips are

crystallized to avoid later sticking and then dried to

reduce hydrolysis at high processing temperatures. So-

lid-state polycondensation takes place in the reactor,

where the chips are subjected to high temperatures under

vacuum (batch process) or in a nitrogen or dry-air stream

(continuous process). The product’s intrinsic viscosity (IV)

is normally between 0.70 and 1.0. (Intrinsic viscosity is a

method for the characterization of the average length of

the molecule chains in PET.) High-viscosity resin is rela-

tively expensive because its production is lengthy. High-

viscosity PET (i.e., having longer molecule chains), offers

better mechanical properties than the average-viscosity

resin. These properties compensate for certain deﬁciencies

of molded articles; for example, excessive volume expan-

sion of beverage bottles can be limited by higher-viscosity

resins that creep less under load. The ﬁnal step is cooling,

since the resin should not be exposed to moist air while it

is hot.

HOMOPOLYMERS AND COPOLYMERS

PET is a homopolymer made from one part dibasic acid

(i.e., TPA or DMT), and one part EG. A copolyester

(copolymer) is made from more than one dibasic acid

and/or glycol. Copolymers remove processing limitations

and provide increased physical properties at elevated

temperatures. In addition to DMT or TPA, isophthalic

acid (IPA) can be used as a comonomer to reduce the

rate and degree of crystallization to an extent that de-

pends on its dosage. This broadens the processing para-

meters of food-container manufacturing machines.

Glycols offer several opportunities for modiﬁcation. Dur-

ing polycondensation, EG reacts with itself to some extent

to form diethylene glycol (DEG). Higher amounts of DEG

affect many polymer properties. There are other glycols

available as partial substitutes for EG (e.g., neopentyl

glycol, cyclohexane dimethanol). All these modiﬁcations

lead to desired polymer property changes (i.e., reduction

of the crystallization rate, melting point, etc.). Cyclohexane

dimethanol can react with a mixture of terephthalic and

isophthalic acids in order to increase the melt strength of

the polymer for extrusion processes (3).

On the other hand, some injection-molding (see Injec-

tion molding) and thermoforming (see Thermoforming)

applications require accelerated crystallization rates in

order to set up crystallization in the article, which prevents

physical deformation at elevated temperatures. This objec-

tive can be achieved by nucleation, which involves the

addition of other ingredients to the polymer. Inert, non-

soluble substances (e.g., mica, talc), organic substances

(e.g., aromatic alcohols), and certain polymers (e.g., PP,

PE) can be used as nucleation ingredients to increase

crystallization rates.

The use of PET and optional added substances for food-

packaging applications is governed by FDA Regulation

No. 177-1630 of March 16, 1977 (revised April 1, 2007).

Homopolymers and copolymers and additives must con-

form to this regulation.

PACKAGING APPLICATIONS

Homopolymers

By strict deﬁnition, most PET resins are modiﬁed homo-

polymers. These homopolymers are used to manufacture

containers (i.e., bottles), by injection blow molding or

injection-stretch blow molding (see Blow molding). Con-

sumption of bottles for carbonated soft drinks runs into

the billions of units in the United States alone. A main

factor for bottle use is the permeability of the bottle wall to

oxygen and carbon dioxide. Carbon dioxide pressure must

remain during storage and oxygen from the air must not

diffuse in. PET is semipermeable. Some early PET bottles

had coatings of impermeable polymers, but the process

involves extra steps and the bottles are not readily

recyclable (1). The most marketable is plain PET with

reduced permeability. PET bottles are also used for liquor,

wine, food, toiletries, and pharmaceuticals, as well as for

beer in some countries. Homopolymers cannot be pro-

cessed by extrusion blow molding because of insufﬁcient

melt strength. For bottles containing beer and wine, the

demands are far more severe than for carbonated soft

drinks. Here it is important to keep oxygen out of the

bottle. Oxygen permeability is a problem with PET. For

high-barrier one-trip bottles, people are looking to coated

or multilayer bottles to reduce oxygen permeability. There

is much activity in Europe at the moment for plastic beer

bottles. Ten percent of beer produced in Europe comes in

PET bottles. In the United States, Anheuser Busch and

Coors have started supplying their beers in 16-oz multi-

layer bottles (1).

Biaxially oriented PET ﬁlm (see Film, oriented polye-

ster) is usually manufactured by polycondensation and

subsequent continuous casting of the ﬁlm plus direct

biaxial orientation; that is, molecule chains of the resin

become biaxially oriented. Nonoriented PET ﬁlm and

sheet are manufactured by melting PET resin in an

extruder and casting the melt through a ﬂat die with

subsequent calendering.

PET is also used for ‘‘ovenable’’ trays for frozen food and

prepared meals. These trays are thermoformed from cast

PET ﬁlm and crystallized. Crystallization heat-sets the

article

to prevent deformation

during cooking and serving.

The main advantages of PET for this application include

suitability for both conventional and microwave ovens,

POLYESTERS, THERMOPLASTIC 977