Stabilization.pdf

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STABILIZATION

Introduction

The performance of most polymer artifacts is adversely affected during the vari-

ous stages of their lifecycle: manufacture, storage, processing/fabrication, and the

service environment. Molecular oxygen is the main cause of irreversible polymer

deterioration leading to loss of useful properties and ultimate mechanical failure.

Its deleterious effect is accelerated by several other factors: temperature, sun-

light, ozone, atmospheric pollutants, water, mechanical stress, adventitious metal,

and metal ion contaminants. Most organic polymers, therefore, require protection

against degradation, which can be achieved by the use of oxidation inhibitors re-

ferred to hereafter as antioxidants or stabilizers . Indeed the commercialization

of some of the high tonnage polymers such as polypropylene (PP), and their use

for outdoor applications, would not have been possible without the successful de-

velopment of antioxidants. The use of very efﬁcient stabilizers is also in great

demand for applications involving second-life polymers, with reprocessing and re-

cycling of thermoplastics as means of conservation of materials. Apart from cost

and customer speciﬁcations, the amount of protection offered by antioxidants and

stabilizers can vary enormously depending on the chemical structure of polymers,

their physical and morphological characteristics, the manufacturing process, and

service conditions of end-use articles (1–7).

The development of antioxidants and stabilizers started in the early part

of the twentieth century as an empirical science (8–11). Progress in stabilization

technology was made possible through a basic understanding of the underlying

mechanisms of polymer oxidation. The terminologies used to describe antioxi-

dants are wide ranging and reﬂect the polymer matrix. For example, in rubber

technology, the terms antidegradants, antifatigue agents, and antiozonants are

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1%)

they are key ingredients in the compounding of polymers. The continuous growth

of polymer markets has ensured a parallel growth in demand for antioxidants and

stabilizers, especially in the large tonnage polymers, with principal applications

in packaging, building, automotive, electrical, and electronics systems.

This article aims to present a coherent treatment of the fundamental aspects

of the science of polymer stabilization technology. The state of the art is discussed,

with particular emphasis on recent developments and progress. A brief introduc-

tion to certain aspects of oxidation of polymers is given ﬁrst before discussing the

role of antioxidants, with examples to illustrate their basic mechanisms of action.

Oxidation of Polymers

Early works on free-radical chain theory for autoxidation of hydrocarbons (12–16)

have shown that oxidation of hydrocarbons occurs autocatalytically, that is, the

reaction starts slowly, possibly with a short induction period, followed by gradual

increase in the rate (ascribed to the buildup of hydroperoxides) which will eventu-

ally subside giving rise to a sigmoidal curve as a function of time. The length of the

short induction period is reduced or removed by the presence of initiators (eg, per-

oxides, azo compounds) and extended by antioxidants and stabilizers (Fig. 1). The

basic autoxidation theory of hydrocarbons, involving a complex set of elementary

reaction steps, was subsequently extended to oxidation of polymers (17–21). How-

ever, studies of oxidation in polymers, both in the solid phase (eg, under service

conditions in presence of heat or light) and in the viscous melt during processing,

are more complex than in liquid hydrocarbons because of the problems of oxygen

diffusion, solubility, and effects of polymer crystallinity and morphology (4,22).

The role of hydroperoxides in autoxidation of polymers cannot be overemphasized

as they are associated with changes in molar mass of polymers and its properties

leading ultimately to catastrophic failure (see Fig. 2).

Scheme 1 describes the free-radical process of polymer oxidation in terms

of three basic steps: initiation, propagation, and termination (18). Although the

origin of the ﬁrst initiating macroalkyl radical is still controversial, many factors,

eg, heat, mechanical stress, light, and transition metal impurities contribute to

its formation (reaction (1a)). However, the direct reaction of polymers with oxygen

(reaction (1b)) is generally not favored because of thermodynamic and kinetic con-

siderations. It can only occur in polymers with very labile C H bonds to give a rel-

atively stable alkyl radicals that terminate, rather than propagate, the oxidation

chain reaction. Oxygen can easily penetrate the amorphous regions of polymers;

for example, in semicrystalline polymers these regions are highly oxidizable.

Propagation reactions involve the very fast reaction of oxygen with polymer

alkyl radicals. Rate constants for reactions of most alkyl radicals with oxygen

is of the order 10 7 –10 9

used, whereas in the plastics industry they are invariably referred to as melt (or

processing) antioxidants, and heat and light stabilizers (or photoantioxidants).

The terms primary and secondary antioxidants are also used, but the distinction

is rather arbitrary and does not relate to their mechanisms of action. Although an-

tioxidants and stabilizers are normally used at low concentrations (generally

s − 1 (23). These reactions lead to the formation of

macroalkylperoxyl radicals (Scheme 1, reaction 2), followed by abstraction of a

mol − 1

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STABILIZATION 181

Fig. 1. Generalized scheme for changes in autoxidation curve, caused by presence of

initiators and antioxidants.

Minimal change in

desirable properties

Rapid property change

High [ROOH]

Change in molecular weight, MW

Change in MW distribution, MWD

Mechanical failure

Loss of other desirable properties

Low [ROOH]

Time

Fig. 2. Generalized scheme relating hydroperoxide concentration and loss of useful prop-

erties.

hydrogen from another polymer molecule (reaction 3). This latter reaction in-

volves the breaking of a C H bond and, therefore, a higher activation energy as

compared to reaction 2. The rate of this reaction, which in most polymers deter-

mines the overall oxidation rate (rate determining step of autoxidation, RDS), is a

function of both the C H bond strength (allyl

benzyl and tertiary

secondary

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Scheme 1. Free-radical chain reaction involved in polymer oxidation.

primary) and the stability of the macroalkyl radical formed (21,24–26). Reac-

tion 3 gives macrohydroperoxides, the ﬁrst molecular product of the chain oxida-

tion process. These can undergo homolysis (bond dissociation energy of RO OH

is about 175 kJ

mol − 1 ) under the effect of heat, light, or metal ions (from poly-

merization or pigments), giving rise to alkoxyl and hydroxyl macroradicals (reac-

tions 4a–4c). Both these radicals can abstract a hydrogen from another polymer

molecule, leading to new macroalkyl radicals (reactions 5a and 6), which continue

the chain reaction. Alkoxyl radicals can undergo further reactions, eg,

-scission

(see reaction 5b), which lead to cleavage of the polymer backbone and generation

of further radicals.

The oxidative process terminates eventually through combination or dis-

proportionation reactions involving the various propagating radicals. The exact

nature of the terminating step, however, depends on the structure of the substrate

as well as on oxygen concentration. Since reaction 3 is rate determining, alkylper-

oxyl radicals are the predominant reactive species under normal oxygen pressure

(ie, [POO . ]

[P . ]) and termination occurs primarily through reaction 7, leading

to diperoxides, carbonyl compounds, and alcohols (27–31). In the presence of lim-

ited amount of oxygen, eg, during polymer extrusion, alkyl radicals predominate

(ie, [P . ]

[POO . ]) and termination reactions 8–10 assume greater signiﬁcance.

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STABILIZATION 183

These bimolecular termination reactions can lead to cross-linking with an increase

in molar mass and/or disproportionation which takes place without changes in

molar mass of the polymer (reactions 9 and 10). Other polymer radical reactions,

including fragmentation and addition to double bonds to give rise to further free

radicals, can also take place depending on the reaction conditions. These molecu-

lar changes lead ultimately to loss of mechanical properties (eg, impact strength,

tensile strength, elongation) and premature failure, modiﬁcation of surface ap-

pearance (eg, crack formation, loss of gloss, “chalking”), and, in many cases, to

discoloration and yellowing.

Effect of Temperature and Water. Thermal oxidation of plastics and

rubbers can occur at all stages of their lifecycle but is most pronounced during

melt processing. The combined effects of temperature (180–330 ◦ C) and shear lead

to mechanical rupture of polymer chains and formation of reactive mechano-alkyl

radical (R . ). The thermal history of a polymer, therefore, has major consequences

on its subsequent in-service performance. Table 1 summarizes the factors affecting

polymer degradation at different stages of its lifecycle.

The nature of the polymer matrix exerts a profound effect on the mechano-

chemistry of the system. For example, thermal processing of poly(vinyl chloride)

(PVC) results primarily in elimination/dehydrochlorination and discoloration

rather than a change in molar mass. The formation of PVC macroradicals is

Table 1. Factors Affecting Degradation of Polymers at Different Stages of Their

Lifecycle

Factors governing

polymer degradation

Processing a

Service lifetime b

Physical state

Melt

Solid

Temperatures

180–320 ◦ C

−

30 to 150 ◦ C

Oxygen concentration

Oxygen deﬁcient conditions

Normal oxygen level

Exposure time

Minutes

Hours to Years

Radical concentrations

]

[ROO

]

[ROO

]

Hydroperoxide, ROOH,

Low

Higher

concentration

Rate of ROOH

Fast

Moderate, depending on

decomposition

temperature

Factors accelerating

Heat

Heat (lower temp.)

the oxidation

Shear

Uv light

Oxygen (Limited)

Oxygen, ozone

Impurities and defects

Oxygenated moieties

(from polymer manufacture,

(from processing,

see Table 2)

Atmospheric pollutants

(PNA, c S and N oxides)

Water, solvents

Metal ions, eg, copper

a For example, extrusion.

b For example, heat, uv radiation.

c PNA

polynuclear aromatic compounds.

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