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Mots-cls : Mise en forme des polymres ; Dgradation ; Aptitude la mise en forme ; Aptitude au recyclage

C. R. Chimie 9 (2001. Introduction

One of the main sources of problems, in polymerprocessing, comes from the high melt viscosity of thepolymers. The consumption of mechanical energy andthe cost of tools (for instance injection moulds), couldbe considerably lowered and the productivity would beincreased if the viscosity were decreased by one (or

several) order(s) of magnitude. Most of the industrialthermoplastic polymers are processed in the 400650 K temperature interval. It is clear that processingin the 8001000 K interval, where viscosities areexpected to be typically 101000 times lower, wouldminimize many important technological and econom-ical constraints. Unfortunately, this is impossible,because polymers are thermally instable at these tem-Received 3 June 2005; accepted after revision 29 June 2006Available online 10 August 2006

Abstract

This paper deals with the degradation processes occurring during polymer processing. Some general aspects of polymer pro-cessing are first recalled. Then, oxidation mechanisms and kinetics are evoked and the main processing methods are comparedfrom this point of view. Temperaturemolar mass maps allow to define a processability window and to envisage ways to widenthis window. The final chapter is devoted to a case study: the PET processing, which is characterized by an especially complexcombination of degradation processes. To cite this article: X. Colin and J. Verdu, C. R. Chimie 9 (2006). 2006 Acadmie des sciences. Published by Elsevier Masson SAS. All rights reserved.

Rsum

Cet article traite des processus de dgradation se produisant pendant la mise en forme des polymres. Tout dabord, certainsaspects gnraux de la mise en forme des polymres sont rappels. Ensuite, les mcanismes et la cintique doxydation sontvoqus et les principales mthodes de mise en forme sont compares de ce point de vue. Les cartes tempraturemasse molairepermettent de dfinir une fentre dans laquelle la mise en forme est possible et denvisager diffrentes solutions pour largir cettefentre. Le dernier chapitre est consacr une tude de cas : la mise en forme du PET, qui est caractrise par une combinaisonspcialement complexe de processus de dgradation. Pour citer cet article : X. Colin et J. Verdu, C. R. Chimie 9 (2006). 2006 Acadmie des sciences. Published by Elsevier Masson SAS. All rights reserved.

Keywords: Polymer processing; Degradation; Processability; RecyclabilityRevue

Polymer degradatio

Xavier Colin*

ENSAM/LTVP, 151, boulevard* Corresponding author.E-mail address: xavier.colin@paris.ensam.fr (X. Colin).

1631-0748/$ - see front matter 2006 Acadmie des sciences. Published bdoi:10.1016/j.crci.2006.06.004count

during processing

cques Verdu

Hpital, 75013 Paris, France

http://france.elsevier.com/direct/CRAS2C/

6) 13801395peratures. Processing operations are always performedin temperature domains where the melt viscosity is rela-tively high, just below what is called the thermal stabi-lity ceiling (TSC). But this latter is a diffuse boundary:

y Elsevier Masson SAS. All rights reserved.

cessing, starting from the basic principles of polymerphysics and degradation kinetics.

NB: Here, the term degradation is taken in itswidest sense: it covers any process leading to a changeof use properties.

Then, the case of PET extrusion will be examined indetails to illustrate the complexity of the problem.

2. Thermal ageing during processing. Some generalaspects

2.1. Isothermal ageing characteristics

Let us consider a use property P and its threshold

X. Colin, J. Verdu / C. R. Chimie 9 (2006) 13801395 1381there is no discrete threshold for degradation processes.Thus, the optimum processing conditions involve gen-erally a small sacrifice of the structural integrity ofmacromolecules, as schematized in Fig. 1.

At low temperature (Tb), thermal degradation is neg-ligible, but defects due to high melt viscosity areresponsible for low part quality.

NB: Mechanochemical degradation processes canalso occur in this region.

At high temperature (Ta), the part quality is essen-tially limited by thermal ageing. Thermal degradationcan affect processing properties through, essentially,molecular mass changes, but these effects are generallyneglected in process modeling. The correspondingstructural changes can however affect the long-termproperties (Fig. 2).

Fig. 1. Schematization of the effect of processing temperature (theprocessing time is supposed constant) on part property.(a) Effect of physical parameters. (b) Effect of thermal degradation.The aim of this lecture is to try to make a generalanalysis of polymer degradation processes during pro-

Fig. 2. Main types of structural changes durinvalue PF required for the application under considera-tion. When the polymer undergoes isothermal ageing atthe temperature T, P varies and one can define a con-version ratio x for this ageing process:

x Pt PFP0 PF (1)

where P0 and Pt are the property values at, respectively,the beginning of exposure and after the time t of expo-sure.

It is thus possible to establish the ageing function atthe temperature T:

x f t (2)

and the lifetime tF such that:

tF f 10 (3)

where f1 is the reciprocal function of f.Eq. (3) means that P PF i.e. x 0 when t tF.g processing, their main consequences.

tion c

X. Colin, J. Verdu / C. R. Chim1382The lifetime tF is a decreasing function of tempera-ture (Fig. 3).

The history of a given sample, used in given condi-tions, can be represented by a point in the timetem-perature space. Its position relatively to the curve tF f t gives immediately an information on its probablefurther evolution: if the point (for instance A in Fig. 3(right)) is below the curve, this means that the materialdisposes of a residual lifetime. If, in contrast, the pointis above the curve (for instance B), this means that theprobability of failure is close to unity. Since the 1940s[1], it has been generally assumed that the TSC can berepresented by an Arrhenius equation:

tF tF0 expH

R T

(4)

There are, however, many reasons to suppose thatArrhenius equation is, in many cases, inadequate torepresent lifetime variations in large temperature inter-vals. Very powerful models, directly derived frommechanistic schemes and free of simplifying hypoth-eses, are now available thanks to the existence of effi-cient numeric computing tools, as shown for instance in

Fig. 3. Left: principle of determination of tF in kinetic degradathe case of radical oxidation of hydrocarbon polymers[2]. Even the problem of polymers stabilized by syner-getic combinations of antioxidants is not insuperable[3].

2.2. Non isothermal, static or dynamic ageing

A processing machine can be considered as a chemi-cal reactor displaying non-uniform temperature andshear fields. A first complication appears immediately:degradation occurs in non-isothermal regime. The dis-tribution of residence times in the machine [4], espe-cially the existence of stagnation zones, must be takeninto account. Another complication can come from thepossible role of shearing. First, it can play a role similarto stirring in (molecular) liquid reactors, favoring thusthe homogenization of the reaction medium, that can beimportant, for instance, in the case of stabilizers withlow diffusivity. The difference between static (noshear) and dynamic thermal ageing are well documen-ted in the literature of the 1960s and 1970s. Shearingcan also induce mechanochemical chain scissions ofwhich the main characteristic is to be disfavored by atemperature increase [5]. In the following, it will beconsidered that ageing tests are performed in conditionsrepresentative of processing ones.

Let us now return to the problem of non-isothermalageing. Our reasoning will be based on the use ofgraphs in which the polymer history, during a givenprocessing operation, is represented by a single tem-perature value TP that can be defined as follows. Theprocess under consideration is characterized by the timetP during which the polymer stays in liquid state. It isalso characterized by the final degradation index xP ofthe polymer. TP is defined as the temperature of an iso-thermal test that would lead to the degradation index xPafter the exposure time tP. In other words, TP is thetemperature of isothermal exposure equivalent to the

urves. Right: shape of the temperature dependence of lifetime.

ie 9 (2006) 13801395process history for the chosen processing conditions.The determination of TP can be illustrated by the sam-ple example of zero-order process obeying Arrheniuslaw:

dxdt r; r r0 exp HR T

; T f t (5)

So that:

xP tW0 r dt r0 tW0 exp

H

R f t

dt r0 FtW (6)

where tW is the whole processing time and F(tW) theprimitive function of exp HR f t

h i.

According to the chosen definition of TP:

xP r0 exp HR TP

tP

i:e: TP HR ln

FtW=tP

(7)

Consider a temperature history T f t for a givenprocessing operation. It is characterized by its wholeduration tW and by the time tP spent in the liquid state(Fig. 4).

2.3. The problem of small conversionsof the degradation process

Let us consider three of the main practical conse-quences of degradation during processing: a change ofrheological properties, embrittlement in solid state andcolor change (for instance yellowing). Rheologicalproperty changes can essentially result from molecularweight changes. In the case of random chain scissions,the number n of chain scissions is:

X. Colin, J. Verdu / C. R. Chimie 9 (2006) 13801395 1383Fig. 4. Above: Temperature history: the whole processing time is thetime spent above ambient temperature (Tamb).tP is the time elapsed in liquid state (Tm = melting point,TC = crystallization temperature, for a semi-crystalline polymer.Otherwise, Tm and TC are replaced by the glass-transition temperatureTg for an amorphous polymer).TP is defined as the temperature of an isothermal test that would leadto the same degradation level as the processing operation after an

exposure of duration tP.conjugated polyenes (vinyl polymers) or oxidation pro-ducts of aromatic rings such as quinones or polyphenols(aromatic polymers). The eye sensitivity to such struc-tural changes depends on many parameters: initialcolor, presence or not of pigments, mode of sampleillumination, sample thickness, etc.

But it is often relatively high, because two character-istics are combined:

these chromophores have often a high absorptivity,for instance > 104 l mol1 s1 for most of the abovecited species, or a high emissivity when colorchanges are linked to luminescence as in the caseof aromatic species;

at least for initially white samples, very small spec-tral changes can be visually detected.

In most of the cases, chromophore concentrationsn 2 102 mol kg1Indeed, noticeable changes of the rheological beha-

vior will appear far from the gelation point.Yellowing results generally from the formation ofn 1MW0

i:e: in general :n 102 mol kg1.Crosslinking affects also rheological properties:

gelation occurs when the number of crosslinks becomesequal to the number of weight average chains:n pMW

p0MW0

p0MW0

p

p0

MW0MW

1

1M n0

p

p0

MW0MW

1

where p and p0 are the polydispersity index values,respectively, after and before ageing.

The Newtonian viscosity change is linked to themolar mass change by a well-known power law:

0 MW0

MW

3:4so that n 1

Mn0

p

p0

0

0:294 1

!

At low conversion, p/p0 is in the order of unity.Thus, the number of chain scission events in order todivide the Newtonian viscosity by a factor 2, forinstance, is in the order of:

n 0:2M n0

i:e: for most of industrial polymers :less than 103 mol kg1, almost undetectable by IR or

heme

X. Colin, J. Verdu / C. R. Chimie 9 (2006) 138013951384NMR, can be the source of noticeable industrial trou-bles.

Hydroperoxides resulting from thermal oxidationduring processing play a key initiating role in furtherthermal or photochemical ageing in use conditionsand can thus significantly affect the materials durabil-ity. In the case of thermal oxidation, in the absence ofantioxidants, a critical value of the hydroperoxide con-centration could be POOHC k1u=k1b, where k1u andk1b are the respective rate constants of uni- and bimo-lecular POOH decomposition. In PE, for instance,POOHC 2:1 102 mol kg1 at 500 K [6]. In thepresence of antioxidants, for which the usual concentra-tions are generally in the 103102 mol kg1 range, itis usually assumed that each hydroperoxide decomposi-tion event leads to the consumption of one or two anti-oxidant molecules.

It can be concluded from this brief survey that rela-tively small structural changes are sufficient to inducesignificant variations of use properties such as rheolo-gical, mechanical, optical or durability ones. Studies inthis field are thus confronted to the analytical difficul-ties relative to the measurement of small concentrations.It is well known, for instance, that PET hydrolysis can-not be monitored by routine IR or NMR measurements.PP oxidation leads to embrittlement before any detect-able change of the IR spectrum, despite the relativehigh sensitivity of the IR titration of carbonyl groups[7]. In the same way, color can appear in PVC farbefore structural changes are detectable by IR or NMR.

Sc3. Oxidation

3.1. Mechanistic and kinetic aspects a brief survey

Oxidation displays two very important features:

it is a radical chain process propagated by H abstrac-tion: (II) P + O2 PO2 (k2 108 109 l mol1 s1[8]); (III) PO2 + PH PO2H + P(k3 1:5 1010 exp73000=RT l mol1 s1for PE [9]).

the hydroperoxides resulting from propagation areunstable. The dissociation energy of the OO bondis about 150 kJ mol1 against more than250 kJ mol1 for most of the polymer bonds. Thedecomposition of hydroperoxides generates radicals: (Iu) POOH PO + OH (G 100 kJ mol1at 100 C);

(Ib) POOH + POOH PO + PO2 + H2O(G 30 kJ mol1 at 100 C).

These free-energy values can be compared to the CCscission one: (IP) PP 2P (G 280 kJ mol1 at100 C for a polymethylenic sequence).

All these features explain well the main characteris-tics of oxidation processes:

since they are radical chain processes, it is possibleto envisage stabilizing ways by reducing the initia-tion rate or increasing the termination rate;

the propagation rate depends essentially on thestrength of the broken CH bond (Scheme 1).

This order corresponds well to the observed hierarchyof polymer stabilities: polymers containing only aro-matic or methyl groups (polyimides, polysulfones,polycarbonate, polydimethylsiloxane) are more stablethan PE, whereas polymers containing tertiary CH(PP), allylic CH (polydienes) or CH in of electro-negative atoms (PA, POM, etc.) are less stable than PE.

1. Due to its closed-loop character, oxidation beginsat low rate (which allows us to envisage efficientstabilization by antioxidants in low concentration),but displays an autoaccelerated behavior, linked toPOOH accumulation.

In the case of pure thermal ageing, hydroperoxidedecomposition (Iu or Ib) is expected to largely pre-dominate over polymer decomposition (IP) at lowtemperature. The Arrhenius plot of lifetime isexpected to have the shape of Fig. 5.

Below a certain temperature TA depending on thepolymer nature and sample thickness, oxidation isalways faster than decomposition in neutral atmo-sphere and displays always a lower activationenergy.Let us recall that fast and intense shearing can alsoinduce mechanochemical processes, i.e. mechani-

considered as good processing antioxidants, are notefficient against mechanochemical reactions.

Let us return to propagation processes (II) and (III).Since k3 CC. It is charac-terized by the fact that oxidation rate is independent) and termination rates without (b) and with (c) reaction (IP).

of oxygen concentration and reactions involving Pradicals are negligible;

regime L (low oxygen concentrations) for C < CC.It is characterized by the fact that the oxidation rateis concentration-dependent and reactions involvingP radicals are not negligible.

The limiting (equilibrium) oxygen concentration inthe polymer CS is linked to the oxygen pressure p:

CS S p, where S is the O2 solubility, whichranges, for most of the polymers, between 105 and104 mol m3 Pa1.

Thus, in air, at atmospheric pressure, CS takes avalue (almost temperature-independent) in the order of103 mol l1. This value is to be compared with thesubstrate concentration, for instance PH 60 mol l1in PE, 20 mol l1 in PP or 14 mol l1 in PET.

Two polymer families can be distinguished, depend-ing on their behavior in atmospheric air(p 0:02 MPa): those for which CS > CC (for instancePE) [10] and those for which CS < CC (for instancePP) [11]. The regime E will be only observable, in

Oxidation processes lead generally to a complexmixture of reaction products among which chain scis-sions (s) and crosslinks (x) are especially interesting forus, because they are susceptible to modify the mechan-ical and rheological behavior at low conversions.

The most general mechanically active chemicalevents are the following:

scissions of alkoxy radicals (Scheme 2). Alkoxyradicals coming from POOH decomposition or fromnon-terminating PO2 bimolecular combinations canpropagate radical chains by H abstraction, but theycan also rearrange by scission and this latter isoften (but not always) a chain scission;

scissions of P radicals (Scheme 3).This process is favored in polymers having weakmonomermonomer bonds (PMMA, POM, PP,etc.). It leads to the polymerization radical, which

e dep

Scheme 2.

X. Colin, J. Verdu / C. R. Chimie 9 (2006) 138013951386these conditions, for the polymers of the first familyand only in thickness layers close to the airpolymerinterface since oxidation is expected to be diffusioncontrolled (Fig. 7).

Fig. 7. Above: oxygen concentration C profile in a bulk sample (x is thconcentration of oxidation products).

Left: polymer of PE type. Right: polymer of PP type.th from the airpolymer interface). Below: profile of oxidation rate (or

families as a function of their predominant behavior in

X. Colin, J. Verdu / C. R. Chimie 9 (2006) 13801395 1387regime L: PP, PMMA and POM, for instance, undergopredominant chain scission because alkyl radicals rear-range easily by scission. In contrast, PE, PET andpresumably polymers containing polymethylenicsequences undergo predominant crosslinking becauseP radicals react mostly by coupling. PVC, in whichthe weakest bond is CCl, is a particular case owingto the importance of HCl zip elimination processesand the probable occurrence of crosslinking reactionsinvolving conjugated polyenes. Since these latter areeasily oxidizable, one can distinguish regime E (lowcolor, predominant chain scission) from regime L(high color, predominant crosslinking).

3.2. Stabilization against oxidation

There are many excellent books and reviews onpolymer thermal stabilization [12]. An exhaustivereview of the main stabilization ways would be out ofthe scope of this lecture. Here, we will focus only onthe main antioxidant families (not specific of the poly-mer structure), e.g. radical-chain breaking antioxidants(phenols, amines) and hydroperoxide decomposers (sul-fides, phosphites).increase.

It is thus possible to distinguish various polymercan itself rearrange (depolymerization), abstracthydrogens or terminate;

coupling of P radicals:(IVC) P + P PP (k4c).This process is often in competition with dispropor-tionation:(IVd) P + P PH + F (k4d) (F = double bond).Both reactions can occur at processing temperatures,disproportionation being favored by a temperature

Scheme 3.The chain breaking antioxidants scavenge PO2 radi-cals. Their stabilization mechanism can be schemati-cally represented by an unique reaction competitive toH abstraction:

(III) PO2 + PH POOH + P (k3) ; (S) PO2 + InH POOH + Q (kS);where Q is a species unable to propagate the radicalchain.

The efficiency of the stabilizer can be linked, in afirst approach, to the rate ratio:

S rSr3 kSInHk3PH

S must be in the order of unity (or higher) for anefficient stabilizer. Since, for economical and technicalreasons:

InHPH 10

3:

This means that, to be efficient, a stabilizer must becharacterized typically by:

kSk3

103

But kS and k3 have distinct activation energies, forinstance in PE: ES 20 kJ mol1 [13] and E3 73 kJ mol1 [9]. In other words, kS=k3 is a decreasingfunction of temperature. These stabilizers are thusexpected to be generally less efficient in the processingthan in the used-temperature range.

The hydroperoxide decomposers destroy hydroper-oxides by non-radical mechanisms:

(D) POOH + Dec inactive products (kD).This reaction is competitive with the initiation one,

for instance, for a predominant unimolecular POOHdecomposition:

(Iu) POOH 2P (k1u).Here also, the stabilizer is expected to be efficient if

the rate ratio is in the order of unity or higher:

rDr1u

kDDeck1u

1

Unfortunately, there are no literature data, to ourknowledge, on activation energies of kD. What is wellknown by practitioners is that sulfides are appropriateto use (low-temperature) conditions, whereas phos-phites are rather appropriate to processing conditions[12].

There are also extensive reviews on transport proper-ties of antioxidants, at least in polyolefin matrices [14].But most of the published data concerns solid-stateproperties. Both the solubility S and the diffusivity Dare increasing functions of the temperature but for bothquantities, the Arrhenius parameters, compiled from lit-

erature, display a compensation effect [15], with a com-

pensation temperature always close to 80100 C. Wehave thus the following alternative:

this effect is a reality. In this case, the hierarchy ofsolubility and diffusivity values at high temperature(in the molten state) would be opposite to low tem-perature (solid-state) one. Such behavior would beparadoxical: it is difficult to imagine, for instance,

optimal viscosity range corresponding to melt-flowindex values ranging from less than 0.1 (pipe extrusion)to more than 10 (paper coating). Schematically, themelt flow index is proportional to the reciprocal of theviscosity, this latter being a sharply increasing functionof the molar mass (at low shear rates: MFI K M3:4Wwhere K is an increasing function of the temperature). Itappears thus convenient to study the processability con-ditions in temperaturemolar mass maps. Two impor-tant boundaries can be defined in this map:

rocessing time Oxygenation Additive losshort ( 1 min) Limited Disfavoredhort (a few minutes) Limited Disfavorededium Full Favoredong Full Favoredhort Full Disfavored

X. Colin, J. Verdu / C. R. Chimie 9 (2006) 138013951388that the diffusivity of molecules increases withtheir size;

we are in presence of an artifact, linked, for instance,to the non-Arrhenian temperature dependences of Sand D. This hypothesis, which seems more reason-able than the preceding ones, suggests that the deter-mination of transport properties of additives inmolten-state polymers is a totally open research pro-blem, needing probably novel experimentalapproaches.

3.3. Oxidation in processing conditions

As far as oxidation is concerned, a given processingoperation is mainly characterized by two main factors:the residence time tP, as defined in Section 1.1, and themode of polymer oxygenation. The main processingoperations can be schematically described by Table 1.

In the case of extrusion and injection molding, thepolymer is almost totally confined; oxygenation is lim-ited to the polymerair interface in the feeder zone.Antioxidant loss by evaporation is disfavored, exceptpossibly in the case of film-extrusion blowing. The pro-file of oxygen concentration is expected to have theshape of Fig. 8.

The depth zE of the oxygenated zone will depend onthree variables: oxygen diffusivity in the melt, rate ofpolymer transport in the machine and temperature pro-file (i.e. rate of oxygen consumption). Indeed, this par-tial confinement favors clearly the regime L.

Calendaring and, especially, rotomoulding are char-acterized by the length of the stage during which thehot (liquid) polymer is exposed to air (several dozensof minutes in the case of rotomoulding, which is, no

Table 1Characteristics of the main processing operations for thermoplastics

Operation Polymer state PInjection molding Liquid SExtrusion Liquid SCalendaring Liquid MRotomoulding Liquid LThermoforming Solid (rubbery) S

Welding Liquid Shortdoubt, the most critical process from the point of viewof polymer degradation). Thermoforming is expected tobe less degrading, owing to the relatively low maxi-mum temperature. In contrast, degradation at noticeableextent can be observed in certain cases of welding (byhot air streams) were the shortness of the hot stage can-not compensate the (sometimes very high) maximumtemperature.

4. Polymer processability

4.1. Temperaturemolar mass maps

A given processing method is characterized by an

Fig. 8. Schematization of the oxygen concentration C and temperatureprofiles in the case of extrusion or injection molding.Full Disfavored

the solid-rubbery/liquid boundary, i.e. the glass-transition temperature (Tg) for amorphous polymersor the melting temperature (Tm) for semi-crystallinepolymers. Both temperatures are increasing func-tions of the molar mass in the low molar massrange (where MC is the entanglement thresholdwhose structureproperty relationships are relativelywell known [16]). Tg continues to increase with Mnabove MC according to the FoxFlory law:

Tg Tg KFFM nwhere KFF is in the order of 10100 K kg mol

1, sothat Tg tends to be almost independent of molar masswhen this latter is higher than 10100 kg mol1. KFFis an increasing function of Tg (which depends

one obtains:

f TL gMmaxand TL f 1 gMmax

F M

Thus, for a given molar mass M, this function repre-sents the boundary TL FM such that for T < TL,the polymer would have a viscosity too high to be pro-cessable, whereas for T > TL, it would be, in principle,processable. In the case of a Newtonian viscosity obey-ing to Arrhenius law, one would have (for MW > MC):

max K0 expHVRT

M 3:4W i:e: TL

HVR ln max

K0M3:4W

r M)crysta

X. Colin, J. Verdu / C. R. Chimie 9 (2006) 13801395 1389Fig. 9. Shape of the temperaturemolar mass maps (logarithmic scale foG: glassy domain; R: rubbery domain; L: liquid domain; CG: semi- the rubberliquid boundary TL, which can be moreor less arbitrarily defined as the end of the rubberyplateau, the GG crossover (G and G being thecomponents of the complex shear modulus), theterminal relaxation time [17], etc. For us, this bound-ary can be defined as follows: the viscosity, for agiven shear rate o, can be expressed as the productof two functions:

f T gMLet us consider the high viscosity limit for the pro-cess under consideration:

max f TL gMcryessentially on the chain stiffness).Tm tends to decrease very slowly with the chainlength far above MC because entanglements tend toperturb crystallization;stalline polymer with the amorphous phase in rubbery state.corresponding to the onset of the entanglement regime.Schematically, for an amorphous polymer:

TL T g for M MC

and TL > Tg for M MCTL Tg is an increasingfunction of M.

For a semi-crystalline polymer, there is a molarvalue MF higher than MC such that:

TL Tm for M MFMF is the molar mass of the polymer for which thelength of the rubbery plateau corresponds to the dis-tance between Tm and Tg.

Most of the thermoplastic processing operations(except thermoforming or machining) involve at leastone elementary step in liquid state, i.e. above TL, but,

for an amorphous polymer (left) and a semi-crystalline polymer (right).lline polymer with the amorphous phase in glassy state; CR: semi-The temperaturemolar mass map would thus havethe shape of Fig. 9.

In all the cases, there is a critical molar mass MC

no way to process this polymer in the entangled regime(see Fig. 12).

PVC and PP, for instance, belong to this family ofmaterials: there is no way to process these polymerswithout an efficient system of stabilization, whichexplains the amount of literature on their thermal stabi-lization. PVC is a very interesting but complicated casebecause in this polymer, certain stabilizers, for instancemetal soaps, can act as lubricants, whereas others, forinstance alkyltin thioglycolates, display a non-negligible plasticizing effect. A complete understandingof these stabilization processes needs, no doubt, thedetailed knowledge of both chemical and physicalaspects. An interesting peculiarity of PP, is its veryhigh critical molar mass M C 200 kg mol1: embrittle-ment can occur at an extremely low conversion of theoxidation process, practically undetectable by chemical

X. Colin, J. Verdu / C. R. Chimie 9 (2006) 138013951390indeed, thermal degradation limits accessible tempera-ture range.

4.2. The TSC

The temperature at the degradation threshold can bedefined from the quantities and relationships estab-lished in Section 1.2. If xF is the endlife criterion forthe conversion ratio of the degradation process, it ispossible to define an equivalent isothermal tempera-ture TD such that: for TP TD, xP xD.

In other words, degradation would just reach theendlife criterion at the end of the processing operationif TP TD. According to Eqs. (5)(7):

F tW xDr0so that TP HR lnxD=r0 tP.

4.3. The brittle limit

All the polymers are brittle when their molar mass islower than a critical value M C. In most of the cases(amorphous polymers and semi-crystalline polymersexcept apolar ones), M C 2 3MC [18]. Ductilityand toughness are essentially linked to the existence ofan entanglement network.

In apolar polymers (PE, PP, PTFE), M C 20 30 MC [19]. This peculiarity seems to be linkedto the specific role of the chains interconnecting crystal-lites.

This critical molar mass value constitutes an impor-tant vertical boundary in the temperaturemolar massmaps: polymers are easily processable below M C, butthey cannot be used, in practice, owing to their veryhigh brittleness. Typically, M C is in the order of1020 kg mol1 for many usual amorphous (PC) orsemi-crystalline (PET, PA11, etc.) polymers and inthe order of 100200 kg mol1 for PE, PP or PTFE.

4.4. The processability window

It is then possible to build the temperaturemolarmass map for a given polymer. It must have, in themost common cases, the shape of Fig. 10.

Let us consider the intersection point D between thethermal degradation ceiling TD and the rheologicalthreshold TL. It corresponds to a molar mass MD thatcan be defined as follows: for M > MD, processing (forthe given temperaturetime history) is impossible

because the material cannot be maintained in liquidstate for the desired time without a catastrophic thermaldegradation. Each polymer is thus characterized by amolar mass interval M CMD in which the processingoperation under consideration can be used to obtainparts having acceptable use properties.

4.5. Usual ways to widen the processability window

The processability window appears as a trigonaldomain of which one boundary: M C is a material prop-erty that cannot, in principle, be shifted. There are thustwo ways to widen the processability window, corre-sponding to the two remaining boundaries (seeFig. 11).

There are cases where the TSC of the unstabilizedpolymer is so low as MD < M C. In other words, there is

Fig. 10. Temperaturemolar mass map. The dashed zone correspondsto the processability window.or spectrochemical titrations [20]. This characteristic can

ions o

X. Colin, J. Verdu / C. R. Chimie 9 (2006) 13801395 1391Fig. 11. Shifting the boundaries of the processing window by modificatexplain the relative sensitivity of this polymer to recy-cling. Indeed, in the (very frequent) case where the poly-mer perishes by oxidation, it is possible to envisage asignificant widening of the processability window byinerting the zones of polymer oxidation especially bynitrogen. This effect could be schematized by Figs. 11(left) or 12, in which TD is the thermal degradation ceil-ing in air, whereas TD is the TSC in neutral atmosphere.

5. A case study: PET processing

5.1. PET characteristics

PET is a semi-crystalline polymer with a meltingpoint at Tm 255 C. Indeed, the processing tempera-

etc.). Right: use of thermal stabilizers. The dashed line corresponds to the s

Fig. 12. Case of unprocessable unstabilized polymers: in the absenceof stabilization (TD), MD < M C, there is no way to obtain a ductile/tough polymer by the processing method. With a stabilizer (TD), MDis shifted above MC, there is a processability window (dashed area).ture cannot be lower than Tm. It is usually close to280 C. The embrittlement molar mass of PET M C isclose to 15 kg mol1. Since, for industrial grades, theinitial molar mass is not higher than 60 kg mol1, thismeans that the number of chain scissions to reachembrittlement is such that:

s 115 160 0.05 mol kg1, a value to be com-pared, for instance, to the monomer concentration[m] = 5.2 mol kg1.

PET displays two very important features: the exis-tence of ester groups extremely reactive with water at280 C and the existence of a dimethylene sequencehighly reactive with oxygen at this temperature. Thus,three main processes are expected to occur in the tem-perature range of extrusion/injection:

the hydrolysis/polycondensation process;

f the composition. Left: use of processing aids (plasticizers, lubricants,tarting polymer and the arrow indicates the sense of the modification. the anaerobic thermal degradation process; the oxidation process.

5.2. Hydrolysiscondensation

The mechanism can be written as follows:

EWkRkR

Al Ac

where E is the ester group, W is the water molecule, Aland Ac the chain end alcoholic and acid groups result-ing from hydrolysis. These latter are supposed to be inequal concentrations: Al0 Ac0 A0 in the initialstate (before processing).

Hydrolysis is expected to predominate if the initialwater concentration is higher than the equilibrium con-centration:

W kRkH

A20E0

would thus lead to an increase of b0 in a pseudo-zero-order process:

b0 b rT t

where rT is the rate of chain scission for this mechan-ism.

In the frame of a single processing operation, thisprocess is expected to have unsignificant consequences.But the corresponding structural defects accumulateirreversibly, contrarily to hydrolysis ones. Thus,after N recycling operations, one could reachb0 > 1=15 mol kg

1. Then, the molar mass would belower than M C, even in dry equilibrium state, and thepolymer would be permanently brittle.

PET chains can also contain weak structures, forinstance diethylene oxide groups, which degrade moreeasily than normal esters. They can be responsible for afast but limited degradation step, obeying to first-order

X. Colin, J. Verdu / C. R. Chimie 9 (2006) 138013951392The number of chain scissions at equilibrium is thus:

s W0 W W0 kRkH

A20E0

In fact, in polyesters, kR=kH is relatively low andA20=E0 104 so that s W0. The number ofchain scissions is almost equal to the initial number ofwater molecules. In other words, [W]0 must be signifi-cantly lower than 0.05 mol kg1, e.g., 900 ppm, toavoid embrittlement. All these characteristics are wellknown by practitioners: PET granules must be carefullydried prior to processing. On the other hand, heating indry state (with W0 < W, thus favoring polyconden-sation), allows us to restore high molar mass values inhydrolytically degraded samples [21]. The effects of thehydrolysiscondensation process on the polymer struc-ture can be considered reversible in the long term sinceone has always the possibility to repair the chain scis-sions. Indeed, they can be also catastrophic in the shortterm if the initial water concentration is in the order of900 ppm, e.g., less than 10 times lower than the equili-brium concentration in the amorphous phase at ambienttemperature.

5.3. Anaerobic thermal degradation

Transesterification is expected to occur at high tem-perature in PET. It can modify the molar mass distribu-tion until an equilibrium distribution is reached. It canalso generate macrocycles as observed by MALDI TOFexperiments [22]. But, in the absence of oxygen, PETdegrades essentially by a non-radical mechanism invol-ving a rearrangement of the ethylene ester (Scheme 4).

This process has been investigated by many authors[23]. It can be considered, in a first approach, irrever-sible. It is very slow in usual processing conditions andpredominates only in the long term when the hydroly-siscondensation process has reached its equilibrium(see Fig. 13).

Let us return to the effects of the hydrolysisconden-sation phenomenon: the equilibrium molar mass is:Scheme 4.M n 1b0 a

where a is the equilibrium acid (or alcohol) concentra-tion:

a kHkR EW 1=2

see preceding section

and b0 is the half-concentration of non-condensablechain ends.

The above described thermal degradation process

Fig. 13. Shape of melt viscosity changes during an isothermalexposure at 280 C in dry state. The curve displays a maximum linkedto the existence of two components (dashed lines): the polycondensa-tion (a) and the thermal degradation (b).kinetics:

b1 b10 exp K1t

where K1 is the first-order rate constant for this process.

5.4. Oxidation

According to well-established structurepropertyrelationships, oxidation is expected to attack preferen-tially the methylene groups in PET. The reactivity ofthese latter can be more or less influenced by the estergroups to which they are linked, but globally, it is notvery different from the reactivity of methylenes in PE,the main difference being essentially in the substrateconcentration four to five times lower in PET than inPE. A very important characteristic of PE is that oxida-

Initiation products. Since POOH decomposition gen-erates PO radicals, the latter are expected to abstracthydrogens to give the alcohol (I) or to undergo scission to give the chain end anhydride (II)(Schemes 5 et 6).

Termination in regions of oxygen excess is expectedto give caged PO radicals leading essentially to thealcohol (I) and the corresponding anhydride (III)(Scheme 7).

Termination in regions of low oxygen concentration isexpected to give essentially long branches (IV) anddouble bonds (V) resulting of disproportionation of Pradicals (Schemes 8 et 9).

Indeed, some of these structures are relatively reac-tive at 280 C: anhydrides are highly sensitive to hydro-

Scheme 5.

Scheme 6.

X. Colin, J. Verdu / C. R. Chimie 9 (2006) 13801395 1393tion leads to a predominant chain scission at high oxy-gen concentrations and to crosslinking at low oxygenconcentrations. Both regimes necessarily coexist in thecase of PET extrusion or injection molding as a resultof oxygen concentration gradients characteristic ofthese processes, as schematized in Fig. 8. Their exis-tence has been demonstrated from rheometric experi-ments, as illustrated by the example of Fig. 14.

Finally, oxidative ageing during extrusion isexpected to lead to the following structures.

Propagation PO2 + PH POOH + P (k3).Hydroperoxide groups resulting from propagationevents are very unstable at 280 C, where theirdecomposition first-order rate constant is expectedto be in the order of k1 4:1 102 s1. Their sta-tionary concentration will be thus very low.

Fig. 14. Newtonian viscosity of PET at 280 C in neutral atmosphere.Air is admitted during a short duration at times tA and tB. Theviscosity decreases partly as a result of oxidation in oxygen excess. Itreincreases when oxygen is almost totally consumed and becomeshigher than its initial value as a result of crosslinking. This latterphenomenon can be also detected by the increase of the imaginarycomponent of the complex viscosity *: before crosslinking,

* = ; after crosslinking, * > (* = (2 + i 2)1/2).Scheme 7.

Scheme 8.Scheme 9.

X. Colin, J. Verdu / C. R. Chimie 9 (2006) 138013951394lysis and they can possibly react in transesterificationprocesses, leading thus to an excess of acid groups.The latter can possibly condensate with tertiary alco-hols (I) to give branching. But, branching is expectedto result essentially from terminations of P radicals, arare event. Thus, in usual extrusion conditions, wherepolymer oxygenation is disfavored, structural changeswill be relatively limited and will not affect, in the shortterm, the use properties. But, what happens in the longterm, after several recycling operations? Long branchesare expected to accumulate in the polymer, modifyingthus its rheological and mechanical behavior. Theyaccumulate, indeed, very slowly, but they can modifysignificantly the polymer properties, since one canrecall that the branch concentration at the gel point is M1W0, i.e. a very low value.

6. Conclusion

Processing in liquid state cannot be considered, rig-orously speaking, as a purely physical operation,because the optimum processing conditions are gener-ally just below the TSC of the polymer. A processabil-ity window can be defined in temperaturemolar massgraphs. It is limited by three boundaries: the ductilebrittle critical molar mass M C, the TSC TD and a mini-mum fluidity temperature TL sharply dependent on themolar mass. Processing aids and thermal stabilizers canshift these boundaries and open the processability win-dow.

When thermal stability is defined on the basis ofmechanical (change of rheological properties, embrittle-ment in the solid state) or optical (color changes) cri-teria, the TSC corresponds to very low conversions ofthe degradation processes, that can carry strong analy-tical problems.

Oxidation lowers always the TSC. It leads to distin-guish between processing methods in which the poly-mer is well oxygenated (for instance rotomoulding) andthose where oxygenation is restricted to the feeder zone(extrusion, injection molding). But, even in these lattercases, it cannot be neglected a priori. An important andgeneral characteristic of extrusion is that it can lead tochain scissions at high oxygen concentrations andcrosslinking at low oxygen concentrations. Chain scis-sion can lead to embrittlement. Branching can modifyseriously the rheological behavior and become cata-strophic when it reaches the gel point (which occursat low conversions, especially for high molar masspolymers). Fortunately, efficient inhibitors exist for

polymer oxidation.The case of PET extrusion illustrates the complexityof structural changes occurring during processing sincethe following processes occur simultaneously:

hydrolysis and condensation (the predominating pro-cess depends on the water content);

transesterification, cycle formation; thermal degradation by ethylene ester rearrange-ment;

thermal degradation by decomposition of weakpoints (diethylene glycol);

chain scission resulting of oxidation at high oxygenconcentration;

crosslinking resulting of oxidation at low oxygenconcentration.

Oxidation and hydrolysis can predominate in thefeeder region, whereas all the other thermal processesare expected to predominate in the zones far from thefeeder where oxygen and water are not present, becausethey have been consumed in the upper part of themachine.

Modeling the polymer degradation during proces-sing needs first a kinetic scheme for degradation pro-cesses in isothermal conditions with the correspondingelementary rate constants and their activation energy,and second a model for the history of a polymer particlewithin the processing machine, taking into account thedistribution of residence times, temperature gradients,oxygen concentration gradients, etc. Such modelshave been elaborated for reactive processing andcould be probably adapted to degradation with minormodifications to take into account oxygen transport.

References

[1] D.W. Dakin, AIEE Trans 67 (1948) 113.[2] L. Rincon-Rubio, B. Fayolle, L. Audouin, J. Verdu, Polym.

Deg. Stab. 74 (2001) 177.[3] J. Verdu, J. Rychly, L. Audouin, Polym. Deg. Stab. 79 (2003)

177.[4] G. Pinto, Z. Tadmor, Polym. Eng. Sci. 10 (1970) 279.[5] A. Casale, S. Porter, Polymer Stress Reactions, (vol. 1), Aca-

demic Press, New York, 1978.[6] X. Colin, B. Fayolle, L. Audouin, J. Verdu, Polym. Deg. Stab.

80 (1) (2003) 67.[7] B. Fayolle, L. Audouin, J. Verdu, Polym. Deg. Stab. 70 (2000)

333.[8] Y. Kamiya, E. Niki, in: H.H.G. Jellinek (Ed.), Aspect of degra-

dation and stabilisation of polymers, Elsevier, New York, 1978(chap. 3, p. 86).

[9] S. Korcek, J.H.B. Chenier, J.A. Howard, K.U. Ingold, Can. J.

Chem. 50 (1972) 2285.

X. Colin, J. Verdu / C. R. Chimie 9 (2006) 13801395 1395[10] X. Colin, L. Audouin, J. Verdu, Polym. Deg. Stab 86 (2004)309.

[11] E. Richaud, F. Farcas, P. Bartolomeo, B. Fayolle, L. Audouin,J. Verdu, in: Proc. Modest3, 29 August.2 September 2004.

[12] H. Zweifel (Ed.), Plastics Additives Handbook, fifth ed, HanserPublishers, Munich, Germany, 2001.

[13] E.T. Denisov, in: S. Halim Hamid (Ed.), Handbook of PolymerDegradation, second ed, Marcel Dekker Inc., New York, 2000,p. 383.

[14] N.C. Billingham, P.D. Calvert, in: N. Grassie (Ed.), Develop-ments in Polymer Degradation, Applied Science Publishers,London, 1980, p. 139.

[15] X. Colin, B. Fayolle, L. Audouin, J. Verdu, Matriaux andTechniques 11/12 (2002) 3 and 1/2 (2003) 9.

[16] L.J. Fetters, D.L. Lohse, W.W. Gaessley, J. Appl. Polym. Sci.,Part B: Polym. Phys. 37 (1999) 1023.

[17] W.W. Graessley, in: J.E. Mark (Ed.), Physical Properties ofPolymers, Am. Chem. Soc., Washington, 1984 (Chap. 3, p.97).

[18] H.H. Kausch, N. Heyman, C.J. Plummer, P. Decroly, Matriauxpolymres. Proprits mcaniques et physiques. Principes demise en uvre, Presses polytechniques et universitairesromandes, Lausanne, Switzerland, 2001, p. 374.

[19] B. Fayolle, L. Audouin, J. Verdu, Polymers 44 (2003) 2733.[20] B. Fayolle, L. Audouin, J. Verdu, Polymers 45 (2004) 4323.[21] M. Lamba, J. Druz, A. Bouilloux, in: E. Martuscelli, C. March-

etta (Eds.), New Polymeric Materials, Proc. Int. Seminar, VNUSci. Press, Utrecht, 1986, p. 33.

[22] Y. Ma, U.S. Aggarwal, D.J. Sikkema, P.J. Lemstra, Polymers44 (2003) 4085.

[23] H. Zimmermann, in: N. Grassie (Ed.), Developments in Poly-mer Degradation, vol. 7, Applied Science Publishers, London,1987, p. 35.

Polymer degradation during processingIntroductionThermal ageing during processing. Some general aspectsIsothermal ageing characteristicsNon isothermal, static or dynamic ageingThe problem of small conversions of the degradation process

OxidationMechanistic and kinetic aspects - a brief surveyStabilization against oxidationOxidation in processing conditions

Polymer processabilityTemperature-molar mass mapsThe TSCThe brittle limitThe processability windowUsual ways to widen the processability window

A case study: PET processingPET characteristicsHydrolysis-condensationAnaerobic thermal degradationOxidation

ConclusionReferences