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. Increased proportion of trans isomers, which cer-

tainly has implications under current labeling re-

quirements for trans fats.

Although storage temperatures are always considered,

the changes described above are much more pronounced

at temperatures used in processing than at typical trans-

portation and storage temperatures. Even relatively small

increases in processing temperatures can dramatically

shorten shelf life, as can be seen in the decreased induc-

tion period in oils [from Kaya et al. (4)]:

Induction Period of Fresh Oils (h)

Sunﬂower oil

Olive oil

1001C

9.0

12.4

1101C

4.4

6.2

1201C

2.2

2.8

Thus, control of temperature effects on lipid oxidation

must be addressed at every stage of production as well as

storage.

Water

No discussion of lipid oxidation is complete without dis-

cussion of moisture effects. As can be seen in the classical

graph of moisture and water activity effects (Figure 5),

lipid oxidation has very different reactivity patterns than

other degradation reactions in foods. Unlike browning,

enzyme reactions, nutrient decomposition, and microbial

growth, which can all be stopped by sufﬁcient removal or

binding of water, there are no conditions under which lipid

oxidation is not active. As would be expected, lipid oxida-

tion is rapid at high moistures and water activity, as are

all reactions (D), but surprisingly, lipid oxidation is just as

rapid in very dry systems (A). The region of lowest lipid

oxidation (B) corresponds to conditions in only a mono-

layer of water molecules is present on molecules in foods.

Moving between regions, water appears to have both pro-

and antioxidant effects on lipid oxidation. This biphasic

action may be explained as follows:

A Dry systems—high oxidation:

Molecular sites of oxidation are bare, pro-

viding ready access of oxygen to lipids.

A-B

B-C

C-D

Metals are unhydrated and reactive.

Hydroperoxides are uncomplexed and de-

composed readily.

The food matrix is open and porous, allow-

ing free ﬂow of oxygen

Water binding to molecules provides surface

protection—decreasing oxidation:

Water hydrogen bonds to LOOH, retards

decomposition.

Water hydrates metals, inhibits

electron transfers, and shifts redox

potentials.

Water binding to surfaces of the macromo-

lecular matrix retards oxygen diffusion.

Water attaches to potentially reactive sites,

excluding oxygen.

Monolayer value—lowest oxidation:

Conceptual single water molecules on

molecular surface protects reactive sites

from oxygen and catalysts but is insufﬁ-

cient for mobilization.

Intermediate moisture region—increasing

oxidation:

Hydration of molecules increases molecular

mobility and reactivity.

Multilayers of water accumulate on mole-

cular surfaces, dissolve catalysts, and in-

crease their diffusion and interaction with

lipids.

Metals mobilized and activated.

Fluid water region—high but decreasing

oxidation:

Water dilutes metals and other catalysts

and reactants.

Emulsion formation separates lipid phases

from most catalysts.

High moisture promotes nonenzymatic

browning

products

are

antioxidants.

It is clear from the graph that the moisture content

and water activity (a

) of a product dictate an innate

propensity for oxidation of products with unsaturated

fatty acids, over and above effects of metals, heat, light,

Relative reaction rate

0.2 0.4

Water activity (a

)

0.6 0.8 1.0

Lipid oxidation

Monolayer

Non-enzymatic browning

Enzymatic activity

Mold growth

Bacteria growth

Yeast growth

Moisture content (%)

Figure 5. Effects of moisture content and water activity

on rates of lipid oxidation in food products. [Modiﬁed from

Labuza et al. (5). Used with permission.]

668 LIPID OXIDATION: CHEMICAL STABILIZATION

and other factors. While few foods or other products move

from one extreme to the other during storage, drying out

and moisture absorption to moderate extents are quite

common and can occur within relatively short times,

leading to rapid changing in lipid oxidation kinetics and

products. A dry high-lipid product stabilized at the mono-

layer a

can undergo explosive oxidation when it either

loses or gains moisture and changes a

. Similarly, inter-

mediate moisture foods are moderately reactive, but they

can be stabilized by losing moisture or have shelf-life

severely shortened by gaining moisture and increasing

. Thus, moisture content and degree of water binding by

system molecules (a

) must be carefully controlled by

formulation and packaging in order to stabilize foods

and other materials against lipid oxidation.

FACTORS AFFECTING LIPID OXIDATION—

ANTIOXIDANTS

Type 1. Agents that Prevent Free Radical Production

These are metals chelators (the most important of which is

EDTA, ethylenediamine tetraacetic acid, shown below),

metal complexers (e.g., citric acid, polyphosphates, and to

a lesser extent ascorbic acid), and singlet oxygen scaven-

gers, particularly carotenoids. The conjugated double

bonds of carotenoids provide many sites that preferen-

tially react with

and spare fatty acids.

EDTA

Carotenoids

HO O C -CH

N-CH

-N

-COOH

Initiation inhibitors are commonly overlooked as

antioxidants but serve perhaps the most critical role

because they control the radical load that must be over-

come by free radical scavengers. The radical load is

the cumulative total of radicals from all sources that

must be quenched to seriously limit progression of lipid

oxidation. The radical load determines the type, amount,

and reactivity of antioxidant necessary to stabilize a given

system. If the radical load is too high, no amount of radical

chain stopper—what is usually thought of as antioxi-

dants—can prevent lipid oxidation. At the same time,

initiation inhibitors cannot work alone. Some radicals

from various sources are always produced, and these

must be quenched to prevent establishment of cycling

chain reactions. Thus, Type 1 antioxidants are usually

combined with Type 2 antioxidants for improved all-round

efﬁciency.

Type 2. Free Radical Chain Stoppers

These are radical quenchers that donate hydrogens to

LOO



or LO



faster than lipid chains:

LOO



þ AH ! LOOH þ A



ð17aÞ

LOO



þ AH

! LOOH þAH



! A þRH ð17bÞ

For a compound to be an antioxidant, its radical A



must

be stable and unreactive or yield nonradical products, so it

does not propagate radical chains. The dominant antiox-

idant structures with H-transferring activity are phenolic

compounds in which the A



reactivity is reduced by

resonance delocalization of the free electron over the

aromatic ring.

The standard synthetic antioxidants BHA, BHT, and

propyl gallate are all phenolic compounds, as are natural

antioxidant tocopherols, ﬂavonoids in fruits, and many

herb and spice components, to mention just a few. Struc-

tures of some common phenolic antioxidants are shown in

Figure 6.

Some antioxidants have more than one phenolic group

per ring. In general, antioxidant activity increases with

the number of phenolic groups on a single ring and per

molecule in complex structures. Phenols with multiple –

OH groups next to each other on the aromatic ring also

complex metals, so they have multiple mechanisms for

inhibiting lipid oxidation.

Type 3: Environmental Factors that Limit Oxidation

Environmental factors are not initiators or catalysts

of lipid oxidation in themselves, but they provide condi-

tions under which oxidation reactions occur, and thus they

play critical roles in controlling the rate and direction

of lipid oxidation as well as the resulting product

mix during storage. Indeed, if environmental factors are

ignored, no amount of primary antioxidant will be sufﬁ-

cient to stabilize a food or other material against lipid

oxidation.

Environmental factors that can inﬂuence lipid oxida-

tion encompass a wide variety of conditions to which

a product is exposed in moving from formulation through

processing to storage. Formulation factors include

LIPID OXIDATION: CHEMICAL STABILIZATION 669

moisture levels and pro-oxidant enzymes such as lipoxy-

genase and lipase that require blanching to inactivate.

Processing environment factors that are important in-

clude everything from potential metal contamination

from equipment and processing water to temperature,

light, and oxygen exposure during processing. Packaging

and storage environments include similar factors: pO

inside the package and surrounding it, vacuum versus

inert atmosphere versus air headspace, moisture inside

the package and migrating into or out of it, temperature

and light exposures, and so on. Control of environmental

factors to limit or prevent lipid oxidation during storage

must be a major focus for packaging of foods.

WHAT ROLE DOES PACKAGING PLAY IN

CONTROLLING LIPID OXIDATION?

Successful strategies for stabilizing any product against

lipid oxidation start with product components and formu-

lations that ﬁrst minimize endogenous catalysts, then

incorporate chelators, antioxidants, and

quenchers

to deactivate those that cannot be eliminated. Then add

processing that protects at every step (e.g., by use of

lowest feasible temperatures and light exposure), as well

as limitation of oxygen via nitrogen blankets or vacuum.

Packaging is the critical ﬁnishing step that maintains

product quality during storage. Packaging cannot cure

deﬁciencies of formulation and processing, but the storage

environment it provides largely determines what reac-

tions occur and how long products are stable during

storage.

Packaging barrier properties and delivery modes con-

tribute to control of lipid oxidation at all three stages:

initiation, propagation and chain branching, and termina-

tion. Different reactions dominate and different factors

affect each stage, so it is not likely that a single protective

approach will meet the demands of all stages. Here, the

concept of radical load must be highlighted. To be success-

ful, any antioxidant or protective function must deal with

the total lipid radical load generated from all sources in

formulation, processing, and packaging. If the total radi-

cal load is extremely high, no single approach can elim-

inate the oxidation. Thus, two general guidelines should

be followed to efﬁciently limit lipid oxidation:

1. When designing antioxidant strategies, emphasize

limitation of initiation. Radicals not generated do

not propagate chains or need to be quenched.

2. Use combinations of antioxidant approaches that

address action at each stage of lipid oxidation and

that act on multiple aspects of lipid oxidation

simultaneously.

Table 2 shows the multiple functions that packaging

provides to limit lipid oxidation during storage and the

various approaches that can be used and combined to

achieve them. Limitation of initiation is especially critical

for products requiring signiﬁcantly extended shelf lives, so

most emphasis is placed on this stage. The light and

oxygen barriers of traditional packaging materials such

as glass, tin cans, and some polymers provide an essential

primary line of defense against lipid oxidation, preventing

or severely limiting levels of L



and LOO



produced during

3,5-Di-tert-butyl-4-hydroxytoluene

(BHT)

3-t-butyl-4-hydroxyanisole

(BHA)

Propyl gallate

(PG)

α-Tocopherol

(highest biological activity)

δ-Tocopherol

(highest antioxidant activity)

C(CH

)

(CH

)

OCH

Curcumin (turmeric) Gingerol (ginger)

Epigallocatechin

(

reen tea)

Carnesol

(rosemary)

Sesamol

(sesame seed)

Eugenol

(cloves)

OCH

CH=CH

C(CH

)

Figure 6. Structures of some common

antioxidants. The top row shows synthetic

antioxidants, whereas the other rows are

antioxidants from natural materials.

670 LIPID OXIDATION: CHEMICAL STABILIZATION

Table 2. Packaging Actions and Approaches for

Controlling Initiation, Propagation, and Termina

tion of Lipid Oxidation

Oxidation Stage and Action

Necessary Packaging Action

Control Factor

Packaging Approach

Examples

Initiation

Oxygen

Provide and maintain inert atmosphere Oxygen

Remove or replace oxygen Hermetic packaging with

vacuum,

deaeration, nitrogen headspace

Oxygen barrier, prevent LOO



formation

Impermeable materials Glass, EVOH, polya

mide, PVDC

Laminants with barrier

layers

Metal foils and oxides, hydrocarbon

ﬁlms, whey protein isolates,

Starch ﬁlms, hydroxy-functional

hyperbranched polyester ﬁlms

Barrier coatings

Hydrocarbon ﬁlms on PET

Polymer multilayers w/wo

additives

Montmorillonite clay nanoparticles in

PET, PVDC/PE

scavenger in package or ﬁlm, remove

residual oxygen

Oxygen sorbent sachets,

labels, lid liners

Elemental or ferrous iron, cobalt-fatty

acid salts in oleﬁn plastics

Oxygen sorbent packaging Fe

, ascorbic acid, glucose-glucose

oxidase, Co(II) acetate,

PP in food contact layer

Light

Block light access

Light

Light-impermeable materials Colored glass, cans

Impermeable laminant

layers

Metal laminant layers

Light ﬁlter

Plastics or coatings with UV

blockers

Hydroxyaromatic compounds,

hindered amines, salicylates

Metals

Remove metals from package contents Chelators

Controlled release packaging EDTA, citric acid

, amino acids

Metal-bonding layers

Chitosan, N-carboxymethyl chitosan

Radicals

Deliver radical quenching antioxidants Antioxidant

levels

Controlled release packaging Tocopherol, sesamol, BHT

, BHA,

TBHQ

Propagation

H abstraction

Deliver radical quenching antioxidants

over time

Antioxidants

Controlled release packaging Tocopherol, ses

amol, BHT, BHA,

TBHQ

LOOH decomposition

Light barrier (UV and visible):

Light

See light in Initiation

prevent decomposition of LOOH

branching

(Continued

)

671

Table 2. Continued

Oxidation Stage and Action

Necessary Packaging Action

Control Factor

Packaging Approach

Examples

Moisture barrier:

Moisture

Impermeable materials Glass, oriented

PP, metallized

polyester ﬁlms

Prevent dehydration and exposure of

reactive sites

Laminants with barrier

layers

Polyamide layers, zein, starch, soy

protein/wheat gluten,

Slow molecular migration

Propylene glycol midlayer between

PVA sheets

Stabilize LOOH

Impregnated or coated

papers

Polyoleﬁns, waxes, starch acetates,

acrylic-based latex,

Limit scission reactions that release

volatiles

Polyvinyl alcohol

Maintain low oxidation rates

Moisture sorbers

Silica gel, clays, encapsulated polyvinyl

alcohol

Temperature maintenance, prevent

LOOH decomposition

Heat

Insulation

Biofoams

Regeneration of LOO



Oxygen barrier, limit L



-LOO



Oxygen

See oxygen in Initiation

Termination

Radical recombination Oxygen barrier

, limit oxydimer and

polymer formation

Oxygen

See oxygen in Initiation

Intercept radical reactions

Antioxidant levels

Controlled release packaging Tocopherol, sesamol,

BHT, BHA,

TBHQ



scission

Moisture barrier: prevent moisture

sorption

Moisture

See moisture in Propagation

Limit LO



scission

Inhibit generation of volatile off-ﬂavors

and odor compounds

Secondary oxidation

reactions

Oxygen barrier, limit oxidation to a few

major products

Oxygen

See oxygen in Initiation

672

storage. This, in turn, allows fewer oxidation chains to be

generated and fewer radicals to be propagated, greatly

reducing the amount of antioxidant needed to quench the

radicals present. Innovative packaging that can deliver

active compounds such as UV absorbers, singlet oxygen

scavengers, metal chelators, or phenolic antioxidants sup-

plements traditional packaging and offers new opportu-

nities for creative combinations of approaches that

increase overall antioxidant effectiveness.

Barrier properties of packaging also limit progression

of lipid oxidation, particularly by excluding light that

decomposes hydroperoxides and by controlling moisture

loss that destabilizes LOOH; also, moisture gain mobilizes

catalysts. Radical propagations are particularly active

during this period, so exclusion of oxygen remains very

important, and here the advent of oxygen scavenging ﬁlms

and sachets have added exciting new opportunities to

remove residual oxygen that remains in packages after

processing and fuels oxidation during storage. Caution is

warranted, however, when using iron-based oxygen sca-

vengers: Any leaching of molecular or solid iron into the

package can catalyze rapid lipid oxidation. New con-

trolled-release packaging is being developed to deliver

either volatile antioxidants such as BHT (synthetic) or

sesamol (natural) or nonvolatile antioxidants such as

mixed tocopherols slowly over time as endogenous anti-

oxidants are consumed. Combining controlled release with

high barrier packaging greatly increases the effectiveness

of packaging, protecting food quality and signiﬁcantly

extending shelf life.

Although usually overlooked, barrier properties of

packaging also play important roles in directing the types

of products formed during lipid oxidation. Oxygen and

moisture, in particular, favor formation of volatile break-

down products that are responsible for the off-odors and

ﬂavors associated with rancidity; these volatile products

are immediately noticeable when the package is opened

and can lead to consumer rejection. Packaging delivery of

radical scavengers in this stage, where antioxidants natu-

rally present in the food or added in the formulation have

already been consumed, can have a major impact on

limiting secondary oxidations and polymerizations that

markedly degrade product ﬂavor and aroma, color, and

texture, and nutrition.

Unfortunately, some questionable practices are also

developing. Because the off-odors of lipid oxidation are

one of the ﬁrst signs of trouble when a package is opened,

and a bad whiff can lead a consumer to dump the product

directly, there is currently intense interest in incorporat-

ing carbonyl scavengers in packaging to bind and remove

volatile odor products of lipid oxidation. However, there

are serious legal issues with this practice because FDA

regulations prohibit any practices that cover up faulty

practices or decomposing foods. Rancid off-odors deﬁnitely

indicate product deterioration and the possible presence of

toxic products. Removing them eliminates consumers’ ﬁrst

indicator of trouble and is tantamount to covering up

spoiled food. Both short-term and long-term toxicity issues

with oxidized lipids remain to be clariﬁed, but all evidence

points to the need to avoid ingestion of oxidized lipids

through foods, nutraceuticals, or cosmetic lotions.

SUMMARY

Lipid oxidation is indeed a complicated reaction that plays

havoc with stability of any product or material containing

lipids. The multiple pathways and products can sometimes

seem overwhelming and impossible to control. Neverthe-

less, there is also logic and organization in lipid oxidation

that, with careful consideration, make it possible to moni-

tor, understand, and control in a wide range of products.

Packaging alone cannot prevent lipid oxidation, but no

product can be stabilized against lipid oxidation without

appropriate and effective packaging. By providing barriers

against oxygen, light, and moisture, packaging works in

concert with endogenous antioxidants to limit the total

radical load in foods, cosmetics, and other products so

that primary antioxidants can be more effective at lower

concentrations; these barriers also provide an environment

that greatly slows down radical reaction chains that do

become established. New active packaging that delivers

radical scavengers, metal chelators, and singlet oxygen

quenchers offer unique opportunities for tailoring packa-

ging to individual product compositions and increasing

antioxidant effectiveness. Combining barrier and delivery

properties of packaging provides ﬂexible options for protect-

ing product quality and signiﬁcantly extending shelf life.

BIBLIOGRAPHY

1. T. P. Labuza, ‘‘Kinetics of Lipid Oxidation in Foods,’’ CRC Crit.

Rev. Food Sci. Nutr. 2, 355–405 (1971).

2. K. M. Schaich, ‘‘Lipid Oxidation in Specialty Oils’’ in F.

Shahidi, ed., Nutraceutical and Specialty Oils, Marcel Dekker,

New York, 2005, pp. 401–448.

3. J. L. Bolland, Quaert. Rev. 3, 1–21 (1949).

4. Kaya et al., Lebensm. Wiss. Technol. 26 , 464–468 (1993).

5. T. P. Labuza et al., Food Technol. 24, 543–544, 546–548, 550

(1970).

Selected References for General Background on Lipid

Oxidation Processes

W. O. Lundberg, ed., Autoxidation and Antioxidants. Vol. 1 and 2,

Interscience, New York, 1961.

G. Scott, Atmospheric Oxidation and Antioxidants, Elsevier,

London, 1965.

H. W.-S. Chan, Autoxidation of Unsaturated Lipids, Academic

Press, London, 1987.

E. N. Frankel, ‘‘Volatile Lipid Oxidation Products,’’ Prog. Lipid

Res. 22, 1–33 (1982).

E. N. Frankel, ‘‘Lipid Oxidation: Mechanisms, Products, and Biolo-

gical Signiﬁcance, JAm.OilChem.Soc.61, 1908–1917 (1984).

E. N. Frankel, ‘‘Chemistry of Free Radical and Singlet Oxidation

of Lipids,’’ Prog. Lipid Res. 23, 197–221 (1985).

N. A. Porter, ‘‘Mechanisms for the Autoxidation of Polyunsatu-

rated Lipids,’’ Acct. Chem. Res. 19, 262–268 (1986).

E. N. Frankel, ‘‘Secondary Products of Lipid Oxidation,’’ Chem.

Phys. Lipids 44, 73–85 (1987).

J. P. Cosgrove, D. F. Church and W. A. Pryor, ‘‘The Kinetics of the

Autoxidation of Polyunsaturated Fatty Acids,’’ Lipids 22, 299–

304 (1987).

LIPID OXIDATION: CHEMICAL STABILIZATION 673

J. C. Allen and R. J. Hamilton, Rancidity in Foods, 2nd edition,

Elsevier Applied Science, London, 1989.

H. W. Gardner, Oxygen radical chemistry of polyunsaturated

fatty acids, Free Radic. Biol. Med. 7, 65–86 (1989).

N. A. Porter, S. E. Caldwell, and K. A. Mills, ‘‘Mechanisms of Free

Radical Oxidation of Unsaturated Lipids,’’ Lipids 30, 277–290

(1995).

K. M. Schaich, ‘‘Free Radical Initiation in Proteins and Amino

Acids by Ionizing and Ultraviolet Radiations and Lipid Oxida-

tion—Part III: Free Radical Transfer from Oxidizing Lipids,’’

CRC Crit. Rev. Food Sci. Nutr. 13, 189–244 (1980).

K. M. Schaich, ‘‘Metals and Lipid Oxidation: Contemporary

Issues,’’ Lipids 27, 209–218 (1992).

K. M. Schaich, ‘‘Lipid Oxidation in Fats and Oils: An Integrated

View’’ in F. Shahidi, ed., Bailey’s Industrial Fats and Oils, 6th

edition, John Wiley & Sons, New York, 2005, pp. 2681–2767.

E. N. Frankel, Lipid Oxidation, 2nd edition, The Oily Press, PJ

Barnes & Associates, Bridgewater, UK, 2005.

References for Specialty Packaging Approaches

M. L. Rooney, Active Food Packaging; Blackie Academic & Profes-

sional, New York, 1995, p. 260.

A. L. Brody, E. R. Strupinsky, and L. R. Kline, Active Packaging for

Food Applications, Technomic Publishing: Lancaster, PA, 2001.

LIQUID-CRYSTALLINE POLYMERS,

THERMOTROPIC

ROBERT R. LUISE

Hytem Consultants, Inc.

Boothwyn, Pennsylvania

Main-chain thermotropic or melt-processible liquid-crystal-

line polymers (TLCPs) are a class of high-performance

condensation polymers represented mainly by para-ex-

tended aromatic polyesters (1). Although these polymers

were invented in U.S. industrial research laboratories in

the 1970s, development was inhibited until the 1980s by

high monomer costs and the lack of attractive markets.

With the emergence of a market niche in molded electronics

packaging and enhanced prospects for lower monomer

costs, rapid 20–25% annual growth of the TLCP market

currently estimated at 10–15 million lb is projected for the

remaining 1990s (2).

TLCP property advantages include high thermal, elec-

trical, and solvent resistance combined with exceptional ﬂow

(low-melt viscosity) in the 3001C (5721F) range, precision

moldability, and dimensional stability; these features are

ideal for injection- molded, close-tolerance applications such

as thin-walled electrical connectors that constitute about

90% of the present market. In ﬁlm form, the dense uniaxial

LC morphology affords excellent barrier properties, but

development in packaging applications has been inhibited

by high resin costs and the need for special technology to

prepare balanced ﬁlm. With lower resin costs and recent

progress in balanced and coextruded ﬁlm technology, most

notably using counter-rotating dies (3), medical and food-

packaging applications are expected to emerge rapidly. In

addition, the unique post-heat-strengthening capability of

these materials by solid-phase polymerization of the ﬁnal

part results, for example, in high-strength ﬁbers with the

mechanical properties of Kevlar aramid ﬁber (4).

FUNDAMENTALS

TLCP chemistry is relatively simple and involves reaction

of an acetylated aromatic diol with an aromatic dicar-

boxylic acid, as illustrated in Figure 1. The diol is usually

acetylated in situ with acetic anhydride at 150–2001C

(300–3921F) under nitrogen in the presence of the diacid,

and the melt polymerization ﬁnished under vacuum at

higher temperatures (3001C, or 5721F range) over a period

of several hours. Acetic acid is the principal reaction

byproduct. Principal monomers used in commerical mate-

rials shown in Figure 2 are para-oriented, phenyl or

naphthyl rings preferred, but a certain amount (r40

mol%) of non-para or ‘‘soft’’ comonomers (e.g., isophthalic

acid, ethylene glycol) can be tolerated without destroying

liquid crystallinity. Other linkages or colinkages are also

feasible, e.g., ester–amide, ester–imide, carbonate, and

azomethine, but the predominant linkage is ester.

The rodlike character of the extended chain molecules

results in a preference for parallel packing in the melt,

Figure 1. Melt polymerization.

674 LIQUID-CRYSTALLINE POLYMERS, THERMOTROPIC

driven by entropy, much like ‘‘logs in a stream.’’ Packing

occurs in the form of optical-size birefringent domains that

contain uniaxially oriented polymer chains, which are

randomly oriented with respect to one another. At ﬂow

conditions, the domains easily orient with respect to each

other, which frequently results in anisotropic properties in

the ﬁnal part. This result can be an advantage in some

structural materials (e.g., ﬁbers), but a disadvantage in

others that require balanced properties (e.g., ﬁlms). Differ-

ences in ﬂow behavior of liquid crystalline and conventional

or ﬂexible chain polymers are illustrated in Figure 3. This

type of simple uniaxial ordering is referred to as a ‘‘ne-

matic’’ mesophase in LC terminology and is predominant

for TLCPs. The use of optically active comonomers can also

lead to chiral stacking of domains and the ‘‘cholesteric’’

mesophase in the melt (5).

COMMERCIAL SOURCES

Major commercial TLCPs include Vectra (Hoechst-

Celanese), Xydar (Amoco), and Zenite (DuPont). All are

high-temperature aromatic polyesters with heat-distor-

tion temperatures in the 240–2701C (464–5181F) range

and are glass-ﬁlled, geared mainly to injection-molded

applications in electronics. Vectra is a 73/27 copolymer of

p-hydroxybenzoic and hydroxynaphthoic acids, Xydar a

50/25/25 copolymer of p-hydroxybenzoic acid, 4,4u-biphe-

nol and terephthalic acid (and optionally a small amount

of isophthalic acid) developed initially by Carborundum in

the 1970s and Zenite, an all-aromatic polyester. Vectran is

a high-performance TLCP ﬁber based on the Vectra

composition manufactured by Kuraray in Japan under

license from Hoechst-Celanese. Worldwide capacity of

these materials is estimated at 15–20 million lb;

Hoechst-Celanese/Polyplastics and DuPont have recently

announced new plants in Japan and Tennessee, respec-

tively, onstream in 1995. Rodrun, a lower-temperature

TLCP based on PET (20–40%) and p-hydroxybenzoic acid

developed initially by Eastman in the 1970s, is also

available in limited quantities from Unitika–Japan.

TLCP cost is presently high: $5–$11/lb for glass- or

mineral-ﬁlled molded grades and $12–$22/lb for extrusion

grades; resin cost, however, is expected to approach the

$3–$5/lb range over the next several years (2), which

results lower prices projected for key monomers such as

2,6-naphthoic acid, also a monomer for PEN and now

available from Amoco at $1.50/lb, and economies-to-scale

price reductions expected for p-hydroxybenzoic acid (to

$1.00/lb range from $2.50/lb present) and 4,4u-biphenol (to

$2.00/lb from $5–$7/lb).

FILMS

Processing. TLCP ease-of-ﬂow orientation results in

unbalanced ﬁlms with poor lateral properties by conven-

tional ﬁlm extrusion that are generally not poststretchable.

Kuraray has reported reasonably balanced Vectra-type

ﬁlms by conventional ﬁlm blowing at high shear rates (6),

but the best results have been obtained by Superex, which

uses a combination of ﬁlm-blowing and counter-rotating

extrusion dies (3). In the latter case the annular die consists

of counter-rotating inner and outer mandrels that impart

an opposing biaxial or ‘‘crisscross’’ orientation to the respec-

tive inner and outer ﬁlm surfaces. Stretch is controlled by

gas blown through the center and attenuation/takeup

Figure 2. TLCP monomers.

Figure 3. Flow behavior of TLCPs versus conventional resins.

LIQUID-CRYSTALLINE POLYMERS, THERMOTROPIC 675

speed. The latter technology, also applicable to tubing, is

available from Superex by special licensing. Another ap-

proach reported in patents by Nippon Oil involves conven-

tional extrusion of cholesteric TLCP ﬁlm with improved

property balance through the use of chiral (optically active)

comonomers that results in a biaxial twist or chirality to the

morphology in the ﬁlm plane (7).

Properties. TLCP ﬁlms generally possess exellent per-

meability resistance to gases (e.g., oxygen, water vapor,

and carbon dioxide), which is attributed to the highly

compact uniaxial morphology. A comparison of permeabil-

ity of various polymer ﬁlms in Figure 4 illustrates the

unique barrier resistance of TLCPs to both oxygen and

water vapor, which far exceeds other packaging resins.

Film studies attribute low permeability to low solubility

indicative of a highly ordered substrate (8); moderate

reductions in oxygen permeability were observed (9)

with orientation (45%) and solid-phase polymerization

(10–20%). Larger reductions in oxygen permeability

(200–300%) have been observed by Kuraray in studies of

aliphatic—aromatic TLCPs based on PET and PEN copo-

lymers with HBA and HNA (10), whereby permeability is

found to decrease with increasing naphthyl (vs phenyl)

content, probably due to decreased solubility. These par-

tially aliphatic materials also were found to be post-

stretchable biaxially and, in some cases, transparent.

Wholly aromatic TLCP ﬁlms (e.g., Vectra), in contrast,

are not poststretchable and are opaque, except in the case

of ultra-high-oriented (‘‘monodomain’’) ﬁlm prepared from

uniaxial layups of TLCP ﬁber (11).

Applications. A comparison of the general properties of

balanced TLCP Vectra ﬁlm with other high-performance

polyester (PET, PEN) and polyimide (Kapton, Upilex)

ﬁlms is shown in Table 1. In contrast to polyimide,

TLCP appears to offer similar high-temperature and

electrical properties with lower moisture absorption and

lower cost, for electrical applications in ﬂexible-ﬁlm sub-

strates. Compared to polyester, TLCP offers higher ther-

mal, mechanical, and barrier properties for medical and

food-packaging applications that require sterilizing, re-

torting and long shelf life. In the latter case, even at a

resin cost of $20/lb, TLCP offers about ﬁve times better

cost-barrier performance (oxygen, water vapor) vs PET

and PEN, assuming resin costs of $1 and $4/lb for the

latter polymers (12).

Also noted is the high-use temperature capability of

TLCP ﬁlm [B2001C(B3921F)] despite relatively low glass

temperature [110–1251C(230–2551F)], attributed to the

crystalline uniaxial morphology. TLCPs with glass tem-

peratures in the 2001C(3921F) range that may have future

application in packaging have recently been disclosed (13).

A TLCP drawback has been the lack of availability of

balanced ﬁlm in the marketplace, but this may change

very soon. Superex has recently announced a number of

consortium activities targeted for the marketplace, which

include development of a copper-clad balanced ﬁlm for

electronics multilayer circuit boards and multichip mod-

ules (14), and coextruded TLCP and PET ﬁlm and tubing

for electronics, medical, and food applications that require

high temperature [2051C (4001 F)] resistance (15). The

latter features a tie layer, probably of a PET/TLCP blend,

to improve heat-sealing and formability. A one million-

pound production level is expected by 1997 and cost will be

competitive with EVOH and PVDC. A possible application

is a shelf-stable, retortable food tray. Superex also has

Figure 4. Permeability of selected polymer ﬁlms. (Courtesy of

Superex Polymer, Inc.)

Table 1. Properties of Balanced Films

Vectra TLCP PET(ICI) MD/TD Kaladex PEN Kapton PI Upilex S PI

Tensile strength, MPa 345 165/228 220 90 413

Tensile modulus, GPa 8.3 4.1/4.8 6.0 2.6 8.9

, 1C 110–125/280 72/269 120/262 385/none W500/none

Upper use temperature, 1C 180–200 130 150 200 260

CTE, ppm/C 5 to + 9 23/18 20 22 2.6

Moisture absorption, % 0.02 o0.1 2.1 1.0

Solvent resistance Excellent Good Good Good Excellent

Relative permeation, O

O 1/1 100/100 25/30

Dielectric constant at 1 kHz 3.3 3.2 3.4 3.5

Dissipation factor at 1 kHz 0.003 0.005 .002 0.0013

Resin/ﬁlm price, $/lb 12–22/44 1/2–6 4/10–19 –/55 –/60

Source: Courtes y of The PST Group.

676 LIQUID-CRYSTALLINE POLYMERS, THERMOTROPIC

announced consortium development of a PET/TLCP alloy

with twice the oxygen- and moisture-barrier resistance of

PET at 10% TLCP loading (16). Flexible-to-rigid mono-

layer or coextruded packaging that is both recyclable and

reusable should be feasible. Projected price is $3/lb with

up to 50% package downgauging possible. A key to the

PET alloy technology is the preservation of the TLCP

barrier phase using special compatibilizers to control ester

interchange. In addition to ﬁlm packaging, Superex TLCP

biaxial tubing is now being manufactured under license by

ACT Medical for endoscopic surgical instruments (17).

Advantages include ﬁve times higher stiffness vs conven-

tional tubes and 50% cost savings over ﬁber-reinforced

composite tubes. Superior electrical properties and broad

sterilization capabilities are also exhibited. Superex’ biax-

ial TLCP tubing was a 1995 Award Winner selected by

R&D Magazine as one of the top 100 technologically

signiﬁcant products of the year.

Neste Oy (18) has also announced development of

coextruded LDPE/TLCP barrier ﬁlm based on a new low-

melting TLCP [210–2601C (410–5001F)] with heat-distor-

tion temperature of 1481C(3001F). The blend includes a

compatibilizer to improve heat sealing and processibility.

Multilayer coextruded and paper-coated constructions

with LDPE have been prepared by conventional proces-

sing. Special biaxial processing is not required, apparently

because ﬁlms are less anisotropic because of the higher

ﬂexibility of the TLCP (perhaps due to aliphatic linkages).

Three- and ﬁve-layer constructions with LDPE far exceed

oxygen-barrier cost performance of similar PET construc-

tions and are competitive with those of EVOH, with the

added advantage of recyclability. The LDPE/TLCP materi-

als are intended as EVOH and aluminum-foil replacement,

particularly in aseptic packaging.

SUMMARY

TLCPs are a class of high-performance engineering poly-

mers represented mainly by aromatic copolyesters with

exceptional processibility and high thermal, electrical, and

barrier properties. About 90% of the present TLCP market

of 10–15 million lb consists of injection-molded electronics

connectors, but rapid development of biaxial ﬁlm and tubing

technology (Superex) and coextruded ﬁlms and blends with

conventional packaging resins (e.g., PET, PEN and LDPE)

should expand opportunities near-term to applications in

electronics, medical, and food packaging. With resin costs

expected to drop to the $3–$5/lb range over the next several

years because of reduced monomer costs, rapid 20–25%

annual growth is forecast for the remaining 1990s to a

40–50-million-lb market by the year 2000. It is expected

that food and medical packaging will constitute a signiﬁcant

share of this market driven by improved thermal stability,

longer shelf life, and ease of recycling afforded by TLCPs .

BIBLIOGRAPHY

1. W. J. Jackson, Jr., Br. Polym. J. 154–162 (1980).

2. D. J. Williams, ‘‘Applications and Markets for Thermotropic

Liquid Crystalline Polyesters (TLCPs),’’ paper presented at

Conference on High Temperature Polymers, Executive Con-

ference Management, Clearwater Beach, FL, Feb. 20–21,

1995; in Applications of High Temperature Polymers,R.R.

Luise, ed., CRC Press, Boca Raton, FL, 1996.

3. Plast. Technol., 35 (1991).

4. R. R. Luise, ‘‘Advances in Heat-Strengthened Thermotropic

Liquid Crystalline Polymer Materials,’’ in Polymeric Materi-

als Encyclopedia, J. C. Salamone, ed., CRC Press, Boca Raton,

FL, 1996.

5. E. Chiellini and G. Gallo, ‘‘Chiral Thermotropic Liquid Crys-

tal Polymers,’’ in Recent Advances in Liquid Crystalline

Polymers, L. Chapoy, ed., Elsevier, London, U.K. 1985.

6. EP Patent Appl. 346 926 (Dec. 20, 1989), T. Ishii and M. Sato

(to Kuraray).

7. U.S. Patent 4,698,397 (Oct. 6, 1987) and U.S. Patent 4,891,418

(Jan. 2, 1990) T. Toya et al. (to Nippon Oil).

8. J. S. Chiou and D. R. Paul, J. Polym. Sci., B, Polym. Phys. 25,

1699 (1987).

9. D. H. Weinkauf and D. R. Paul, J. Polym. Sci., B. Polym. Phys.

30, 817 (1992).

10. EP Patent Appl. 466 085 (Jan. 15, 1992), T. Kashimura and

M. Matsumoto (to Kuraray).

11. R. R. Luise, ‘‘Liquid Crystalline Condensation Polymers,’’ in

Integration of Fundamental Polymer Science and Technology,

Vol. 5, P. J. Lemstra and L. A. Kleintjens, eds., Elsevier,

London, U.K. 1990.

12. M. A. Kirsch and D. J. Williams, ‘‘Understanding the Thermo-

plastic Polyester Business,’’ Chemtech, 40 (1994).

13. WO Patent Appl. 94/28069 (Dec. 8, 1994), R. R. Luise (to

DuPont).

14. T. Knoll, ‘‘Controlled Thermal Expansion Printed Wiring

Boards on TLCP Dielectrics,’’ Technical Conference Proceed-

ings, IPC, Boston MA, April 1994.

15. Plast. Technol., 104 (1994).

16. Modern Plast., 25 (1995).

17. R. W. Lusignea, ‘‘Fabrication of LCP Tubing for Medical

Applications,’’ paper presented at Specialty Polyesters ’95,

Schotland Business Research, Brussels, Belgium, June

27–28, 1995.

18. T. Heiskanen and E. Suokas, ‘‘A New Low Processing Tem-

perature LCP with High Mechanical Properties and Suit-

ability for Blending,’’ paper presented at Specialty Polyesters

’95, Schotland Business Research, Brussels, Belgium, June

27–28, 1995.

LOGISTICAL/DISTRIBUTION PACKAGING

DIANA TWEDE

School of Packaging, Michigan

State University, East Lansing,

Michigan

The two key drivers of logistics management are the goals

of reducing the total cost of logistics activities and improv-

ing customer service. It aims to integrate the supply chain

to eliminate waste and optimize service.

Logistics Management is deﬁned as ‘‘that part of supply

chain management that plans, implements, and controls

LOGISTICAL/DISTRIBUTION PACKAGING 677