Degradation and aging of polymer blends I. Thermomechanical and thermal degradation

Degradation and aging of polymer blendsI. Thermomechanical and thermal degradation

Jan PospõÂ sÏ il a,*, ZdeneÏ k HoraÂ k a, ZdeneÏ k KrulisÏ a,Stanislav NesÏ puÊ rek a, Shin-ichi Kuroda b

aInstitute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, HeyrovskyÂ Sq. 2, 162 06 Prague 6, Czech RepublicbDepartment of Chemistry, Gunma University, Kiryu, Gunma 376, Japan

Received 11 December 1998; accepted 27 January 1999

Abstract

The commercial importance of polymer blends implies interest in knowledge of their degradation behaviour under environmentalstresses. Processes accompanying thermomechanical and thermal degradation of blends of commodity polymers are reviewed.Thermomechanical degradation, characteristic of melt processing, includes reactions between component macromolecules and

relevant macroradicals. Cross-reactions giving grafted copolymers occur in some blends. Thermal degradation in an inert environ-ment includes cross-reactions between macromolecules and low-molecular weight molecules or free radicals migrating over phaseboundaries. As a consequence, thermal stability of the component polymers is either reduced or increased, according to the reac-tivity of involved species. # 1999 Elsevier Science Ltd. All rights reserved.

1. Introduction

It has been generally accepted that new polymers arenot always necessary to meet needs for new materials.Blending of existing commodity or engineering poly-mers often can be implemented more rapidly and is aless expensive alternative than realization of new poly-mer chemistry including development of monomersynthesis and polymerization technology [1].Blending of commodity and high-tech polymers

(electroconducting, photoactive, biodegradable) is adeveloping area. Moreover, blending of virgin and usedpolymers is a promising approach to ecological andeconomical exploitation of waste plastics. Polymerblending has a developed scienti®c base [2,3] and theimpressive increase in commercialization of blends isone of the most prominent and rapidly growing featuresof the contemporary polymer industry.Long-term properties of polymer blends under envir-

onmental stresses are strongly in¯uenced by co-reactiv-ity between individual component polymers. The ®nale�ect on the lifetime of blends is di�cult to predict on

the basis of known behaviour of individual polymers.The results obtained in degradation studies with poly-blends are summarized. Some of the results serve for abetter understanding of the long-term behavior of vir-gin/recycled plastics blends. This paper (including itssecond part) deals with blends of commodity plasticsand elastomer-modi®ed plastics. Basic aspects of poly-mer blends and degradation processes occurring in themare brie¯y outlined.

2. Basic aspects of polymer blends

Polymer blends are intimate mixtures of di�erentcommercially available polymers with no covalentbonds between individual component polymers [4].They are formed by melt-mixing or solution blendingand/or by coprecipitation or co-coagulation of systemsarising by polymerization of a monomer in a latexmedium. Properties of the resulting materials may betailored to meet requirements of customers or expecta-tions of speci®c new applications with satisfactory bal-ance of a wide range of material properties and costs[5]. In most cases, each component polymer contributesin a speci®c manner to the overall property pro®le. The®nal materials are mostly characterized by combinations

0141-3910/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.

PI I : S0141-3910(99 )00029-4

Polymer Degradation and Stability 65 (1999) 405±414

* Corresponding author. Tel.: +42-2-20403251; fax: +42-2-367

981.

E-mail address: [email protected] (J. PospõÂ sÏ il)

of the useful properties of the components of thepolymer blend, superior to those of single polymers.A kind of synergism in material properties is obtained[1,3,5].Blending substantially improves the properties of

commodity plastics, such as polyole®ns, styrenics orpoly(vinyl chloride) (PVC). Encouraging commercialresults were obtained with blends containing engineer-ing and speciality plastics, such as polyamides (PA),polycarbonates (PC), poly(phenylene oxide) (PPO) orelectroconducting polymers, like polyaniline, poly-thiophene, polypyrrole or polyacetylene. The main goalof blending is modi®cation of mechanical properties,improvement of impact strength at low temperatures, inparticular abrasion resistance and, last but not least,improvement of processability [6]. Moreover, blending ofrecycled and virgin plastics is a promising technique inplastics waste management.However, mutual in¯uences of polymer components,

re¯ected in changes in long-term resistance of blendsagainst environmental stresses and potential deteriora-tion of bene®cial mechanical properties arising fromblending, must be anticipated and cannot be foreseen.Miscibility and compatibility of component polymersare important material properties. Polymer blends maybe homogeneous or heterogeneous on a microscopicscale,but should not exhibit any obvious inhomogeneityon the macroscopic scale [7]. The terms ``compatible'' or`ìncompatible'' refer to the degree of intimacy inblends. A blend that is heterogeneous on the macro-scopic level and exhibits symptoms of polymer segrega-tion is considered incompatible. Heterogeneous blendsappear in a variety of morphologies. These include mostfrequently a dispersion of one polymer in the matrix ofthe other and/or a co-continuous two-phase morphol-ogy [1]. The blend morphology and character of inter-faces in¯uence co-reactivity in degrading blends.A low degree of compatibility is a serious problem

encountered in polymer blends. Consequently, there is astrong e�ort to enhance the compatibility by usingproper additives (``compatibilizers''). They reduce thedimensions of dispersed particles and prevent undesir-able processes such as separation of phases, delamina-tion, agglomeration or skinning and ultimate physicalfailure [1,8]. For the blends discussed in this paper, non-reactive compatibilizers based on block or graft copo-lymers are of particular importance. These are suitablefor both virgin and recycled plastics. Free-radical initi-ated, in situ grafting, arising during melt processing andcalled reactive compatibilization, is of relevance fortopics discussed in this paper as well.In spite of excellent contribution of compatibilizers

to the material properties of polymer blends, their roleas an `ìnherent impurity'' having a potential negativein¯uence on the lifetime of blends has been ratherunderestimated. These in¯uences are reported in the

part dealing with thermal and photo-oxidation ofblends.

3. General aspects of polymer degradation

In a broader sense, the term ``degradation of poly-mers'' includes all changes in chemical structure andphysical properties of polymers due to external chemicalor physical stresses leading to materials with character-istics di�erent from those of the starting material [4].Usually, degradation means worsened properties. Accor-ding to their chemical structure, organic polymers arevulnerable by harmful e�ects of the environment. Thisincludes attack by chemical deteriogens (oxygen, itsactive forms, humidity, harmful anthropogenic emis-sions and atmospheric pollutants such as NOx) andphysical stresses (heat, mechanical forces, radiation).According to the lifetime stages of polymers, the rele-vant processes are classi®ed as melt degradation, long-term heat aging and weathering. Based on the mechan-isms involved, thermomechanical, thermal, catalytic andradiation-induced oxidations and environmental biode-gradation are involved [9].In addition to e�ects of the regular polymer structure,

di�erences in sensitivity to individual degradation pro-cesses arise from the e�ects of low amounts of structuralpolymeric inhomogeneities (e.g. unsaturation, oxyge-nated structures) and non-polymeric impurities such asdi�erent metallic contaminants or photoactive pigments[10]. The concentration of active impurities (catalysts orsensitizers) increases during the polymer lifetime. Theknowledge of degradation mechanisms of homo-polymers and copolymers is helpful only to some extentin elucidation of degradation of polymer blends. How-ever, individual components of a blend may behaverather di�erently from their behaviour as isolated poly-mers [5,11]. This may in¯uence the degradation resis-tance of the blend in a positive or negative manner.Consequently, the degradation behavior of blends ishardly predictable without experiments [5]. It is due tothe co-reaction phenomena on interfaces of blendedpolymers controlled by the morphology of the blend.The processes are complicated by reactivity of compati-bilizers [12]. The heterogeneous character of the system,reactions in the bulk and at the boundaries of individualphases and involvement of macromolecular and low-molecular-weight (LMW) degradation products increasethe complexity of reactions in blends [4,5,12±14]. Thus,improvement of mechanical properties by blending aresometimes at the expense of stability [12]. Structuralchanges accounting for aging produce an associatede�ect on various physical properties. The practical ser-vice lifetime of blends in general and blends containingrecyclates in particular is considerably a�ected by theirresistance to degradation. Accordingly, performance

406 J. PospõÂsÏil et al. / Polymer Degradation and Stability 65 (1999) 405±414

characteristics of blends are of major commercialimportance.

4. Thermomechanical degradation of polymer blends

Degradation due to mechanically induced thermalprocesses takes place during melt processing of polymersat high temperatures in oxygen-de®cient atmosphere andaccounts for chemical changes in the polymer structure.Consequently, it modi®es mechanical properties andweathering resistance of the ®nal material [5,10,15].Melt (processing) degradation is a short-term processproceeding under severe microenvironmental attacks.The e�ect of the mechanical stress increases in meltswith high viscosity, or in processes using high mixingspeeds, because high mechanical forces have to beapplied to attain mixing performance [4,16]. Mechanicalstress has a dominating e�ect over thermal e�ects.Polyole®ns, rather nonreactive polymers, are a goodexample. They undergo thermomechanical degradationin the range of temperatures where they are practicallyuna�ected by thermal treatment alone. Under industrialconditions, trace amounts of oxygen are present in pro-cessing equipment and thermal oxidation contributes tosome extent to degradation. The ®nal process has amechanochemical character.Basic features of the processing degradation of indivi-

dual polyole®ns were described in detail [15] and are veryuseful in elucidation of processes taking place in blends.The initiation step accounts for macroradicals P�

formed from a polyole®n PH by shear/thermal treat-ment [Eq. (1)]

PH�; shearÿÿÿÿÿÿÿÿÿ! P� �1�

Macroradicals P� are formed via statistical breaking ofCÿC bonds (this accounts for terminal C-radicals, e.g.1) or CÿH bonds (generation of in-chain radicals, e.g.2)

The breaking of CÿC bonds results in chain scission,a process characteristic of polypropylene (PP) or poly-styrene (PS).Macroalkyls are transformed (termination steps) by

disproportionation (this accounts for unsaturation ofone of the reactant, species 3 and 4 arise from radicals 1and 2, respectively) and recombination accounting forbranching, (e.g. 5) and crosslink 6 formation.

Branching and crosslinking are characteristic of meltdegradation of polyethylene (PE). All macroalkyls P�react very e�ectively with trace amounts of oxygen pre-sent in the melt. Alkylperoxy radicals arise in the ®rststep and are transformed by a chain mechanism intoalkylhydroperoxides and products of their thermolysis,alkoxy radicals and carbonyl compounds [Eq. (2)] [17].

P � O2ÿÿÿÿ! POO � PHÿÿÿÿ! POOH�ÿÿÿÿ! PO�; > CO

�2�

In systems with low steric hindrance in the chains, suchas high density polyethylene (HDPE), addition of mac-roalkyls to unsaturation > C�C < is possible [Eq. (3)]and accounts for another carbon-centered free radical.

(3)

All the mentioned reactions are possible in thermo-mechanical degradation of blends. The process is, how-ever, more complicated and includes reactions in thebulk of individual components and, potentially, cross-reactions between macroradicals arising from di�erentpolymers, accounting for in situ formation of graftedcopolymers with the potential function of compati-bilizers [4,5,11]. It is expected that the graft copolymersare formed on interfaces between phases consisting ofindividual polymers.In blends with negligible interactions between macro-

molecules, a phenomenon of `àdditivity'' in degrada-tion can be expected, depending on the relative contentof individual components [5]. This additivity e�ect wasreported for PE/PP blends [18]. Cross-recombination insuch immiscible blends is improbable without the use ofan e�cient free-radical initiator.As mentioned earlier, di�erent hydrocarbon polymers

forming a blend show various responses to thermo-

J. PospõÂsÏil et al. / Polymer Degradation and Stability 65 (1999) 405±414 407

mechanical stress. PP degrades via chain scission.HDPE undergoes mostly branching and crosslinking, inaddition to chain scission (with a speci®c impact of dif-ferences in behavior between Ziegler- and Phillips-typeHDPE) [15]. This results in viscosity increase in the PEphase and, vice versa, in viscosity drop in the PP phase.At a particular PP/PE blend composition, both e�ectsmight be compensated to some extent, and an apparent``no-degradation e�ect'' can be observed by viscosi-metric measurements [4,5].The composition of the blend and mixing conditions

during processing (25 and 200 rpm, temperatures 180and 280�C were compared) have a strong in¯uence onprocesses in the blend [18]. Rheological measurementsindicated a synergistic e�ect between thermal andmechanical degradation. For blends containing morethan 20% PP, the viscosity decreases at 180�C withincreasing mixing speed (the chain scission pre-dominates over crosslinking) (Fig. 1). The viscosity ofblends with high contents of HDPE increases con-tinuously with rotational speed due to the long-chainbranching and crosslinking. At a given rotational speed,an increase in the processing temperature resulted in amore severe degradation accounting for HDPE chainscission even in HDPE-rich blends [18].Rheological and mechanical measurements of melt-

processed blends of polyisobutylene (PIB) with PSre¯ected di�erent responses of the component polymersto rotational speed during mixing [4]. Chain scission ischaracteristic of thermomechanical degradation of PSand accounts at 160�C for a rapid increase of the melt¯ow. However, in elucidation of thermomechanical

degradation of PIB/PS blends, the melt ¯ow indexincreased only moderately for PS-rich blends. It seemsthat the degradation of the blend proceeds at theexpense of the PIB component. Some intermolecularreactions between free macroradicals were considered aswell. Elongation-at-break measurements indicated animproved compatibility. Electron-microscopic inspec-tion revealed that the dispersed particles had smalldimensions and formation of a kind of compatibiliz-ing agent was observed [4]. This indicates a higherdegree of intermolecular reactivity between macro-radicals on the phase boundaries in comparison withPP/PE blends.Similar cross-reactions occur in blends of PVC with

low-density polyethylene (LDPE) [19]. LDPE alonedegrades via branching and crosslinking of relatedmacroradicals. PVC su�ers from easy thermal dehydro-chlorination at processing temperatures [20]. Isolateddouble bonds and systems of conjugated double bondsare formed stepwise in degrading PVC [Eq. (4)]. Mac-roalkyls may arise after splitting o� of chlorine radicalsCl� in the initiating step [21].

(4)

Chemical transformations characteristic of formationof graft copolymers proceed during processing ofLDPE/PVC blends [19]. The amount of PVC graftedonto LDPE after ®ve min mixing at 180�C increaseswith the PVC content in the blend (Fig. 2). In LDPE/PVC 80:20 blends, about 30% of the present PVC waslinked to PE as LDPE-g-PVC graft copolymer, as aresult of cross-recombination of macroradicals arisingfrom both component polymers.The chance of a thermomechanical cross-recombina-

tion (formation of graft copolymers) in immisciblepolymers might be increased by simultaneous use offree-radical initiators. Organic peroxides, such as di-tert-butyl peroxide or dicumyl peroxide are the usualcrosslinking agents for polyole®ns [1]. The process isconsidered as reactive processing resulting in in situformation of a compatibilizing species. However, prac-tical exploitation does not seem so far e�ective [22] inspite of numerous studies performed with PE/PP, PE/PSor PE/poly(ethylene-co-vinyl acetate) (EVA). With theexception of LDPE/PP processed in the presence of0.5% dicumyl peroxide [23], where a signi®cantimprovement of mechanical properties was reported,usually only a small improvement of mechanical prop-erties was achieved.

Fig. 1. Dimensionless viscosity (Z), obtained by dividing viscosity of

the molten blend at any rotational speed by viscosity of the non-

degraded material) as a function of the mixing speed (rpm) at 180�C.Symbols with a line protruding from the top refer to samples treated at

280�C. (Reprinted from Polymer Degradation and Stability, Vol. 13,

F.P. LaMantia, A. Valenza, and D. Acierno: Thermomechanical

degradation of blends of isotactic polypropylene and high density

polyethylene, pp. 1±9, 1985, with permission from Elsevier Science.)


Theoretically, the yield of macroalkyl radicals neces-sary for cross-recombination, increases due to hydrogenabstraction from PH by alkoxy radicals [Eq. (6)] arisingin homolysis of peroxides ROOR [Eq. (5)] at processingtemperatures.

ROOR ÿÿÿÿ!�2 RO� �5�

PH�RO� ÿÿÿÿ! P � �ROH �6�

It was assumed in studies performed with PE/PP blends[1,24] that P� arising from both PE and PP recombineto a grafted copolymer PE-g-PP. It is mostly claimedthat short PP chains formed during PP chain scission onCÿC bonds are linked to long PE chains. Consequently,macroradicals developed from PE should be protectedfrom crosslinking. In any case, this grafting (branching)process triggered by RO� radicals forms only a part ofthe integral degradation process and competes withundesired scission of PP and branching/crosslinking ofPE [24]. The mechanism of reactive processing is,therefore, a mechanistic analogue of mechanicallyinduced degradation.The free-radical initiated process su�ers from a lack

of chemical selectivity similar to the thermomechanicaldegradation. Moreover, in two-phase PE/PP blends, thecomponent polymers can react only on the interfacesbetween phase domains. As a consequence, the graftingis strongly hindered [24]. In this connection, the infor-mation on the phase structure of molten PE/PP blendsis very important. The larger the interface (the ®nerphase structure) the easier is the cross-recombination to

a graft copolymer due to a more intimate molecularcontact. PE/PP melts form a two-phase system. Recentmelt experiments were performed [24] in a kneader(Brabender Plastigraph, 30 ml) for ®ve min at 180�C at60 rpm in the presence of 3 or 10% 2,5-di(tert-butyl-peroxy)-2,5-dimethylhexane (initiator) under di�erentexperimental arrangements, and in a solution of a PP±PE blend in trichlorobenzene (total concentration ofpolyole®ns was 10%, the concentration of peroxide was10%). It was con®rmed that a chance of forming PE-g-PP graft copolymer exists only in presumably homo-geneous one-phase systems, i.e. in solution. In contrast,the two polymers reacted in the melt fairly indepen-dently, and grafting was only the minor reaction. Thepredominant processes were PP scission and PE cross-linking. In PP-rich blends, the crosslinked PE formsdisperse phase domains in the matrix of degraded PP. Inblends with PE as the main component, the crosslinkedPE network takes over the matrix and the whole blendbehaves like a network, the clusters of crosslinked PEbeing interconnected.The alkoxy radical-triggered degradation may be

combined with addition of a reactive multifunctionalLMW reagent or unsaturated oligomer presumablyforming links to macromolecular chains. Resultingbranched or grafted copolymers function as degrada-tion-induced compatibilizers. Hydroquinone, triallylisocyanurate or LMW unsaturated rubber [1,25] wereused as coreagents. In spite of lack of selectivity, thee�ciency of the peroxide/unsaturated co-reagent systemis very promising. Recent results [25] with binary blendsLDPE/PP (3±4:1) and ternary blends LDPE/HDPE/PP,prepared by reactive processing in a Brabender Plasti-corder at 190�C, 60 rph, in the presence of 0.2% di-tert-butyl peroxide and 2.5% of liquid polybutadiene (l-PB,Mw 3100, Mn 2900) indicated a signi®cant improvementof tensile impact strength and considerable changes inthe blend morphology. The increase in the impactstrength was apparently due to the formation of ane�ective linkage between the component polymers withthe assistance of the reactive l-PB chains. Good adhe-sion at the interfaces was created.It was postulated [25] that besides autoreactions of

carbon-centered macroradicals P� generated from PE orPP, a radical addition to C�C bonds of l-PB takesplace. The process is controlled by steric factors: lesshindered free radicals arising from PE are more proneto addition to l-PB. As a consequence, new free radicalsare generated in this three-component system accordingto Eq. (3), and are capable of recombining with anotherP� or disproportionating. l-PB surrounding the PP par-ticles dispersed in the PE matrix is the most reactivespecies in the PE/PP blend. Branched structures con-sisting of the all components present in the blend arepotentially formed. At the used concentrations, practi-cally all the added l-PB was bound in the branched

Fig. 2. E�ect of increasing PVC concentration in LDPE/PVC blends

processed at 180�C for 5 min in a closed mixer on the formation of

LDPE-g-PVC graft copolymers. 1 graft copolymer, 2 vinylidene

unsaturation (890 cmÿ1) by IR spectroscopy. (Reprinted from Poly-

mer Degradation and Stability, Vol. 3, A. Gha�ar, C. Sadrmoha-

ghegh, and G. Scott: Polymer blends, Part III. The interaction

between polyethylene and poly(vinyl chloride) during melt blending

and photooxidation, pp. 341±7, 1980±1981, with permission from

Elsevier Science.)


structures. Transmission electron microscopy measure-ments con®rmed that l-PB does not form a separatephase in the ®nal blend.The free-radical initiated reactive processing of poly-

mer blends performed in the presence of a reactive co-additive has a commercial potential in upgrading ofmixed plastics recyclates [25,26]. A promising result wasobtained with a commingled waste material consistingof (wt%) LDPE (45)/HDPE (15)/PP (7.5)/high-impactpolystyrene (HIPS, 7.5)/PVC (15)/poly(ethylene ter-ephthalate) (PET, 5) reprocessed in the presence of 1wt% dicumyl peroxide [26]. We suggest that the HIPScomponent plays a role as an unsaturated co-additive. Theunnotched Izod, stress-at-break and elongation at yieldwere dramatically improved in the reprocessed-blend.

5. Thermal degradation

The processes described in this part proceeded inpolymer blends stressed in an inert atmosphere by asingle deteriogen Ð heat. In particular, blends based onPVC, poly(vinyl acetate) (PVA), PS and poly(methylmethacrylate) (PMMA) were studied [5,11,13,14,21].Most of the experimental data were obtained bydynamic or isothermal di�erential scanning calorimetry(DSC) and thermogravimetry (TG) or thermal volatili-zation analysis (TVA). Infrared (IR) and nuclear mag-netic resonance (NMR) spectrometry, chromatographicmethods and conductometric measurements were usedfor analysis of volatile and condensable products ofthermolysis. The results revealed interactions betweencomponent polymers in the polymer bulk with LMWmolecular or free-radical products, arising by thermo-lysis of any of the component polymers and migratingacross the phase boundaries from one polymer toanother [14,21]. Ultimately, the products of thermolysiseither trigger degradation of the blend (destabilizinge�ect) or act as stabilizing species for any of the com-ponent polymers [11,21]. The ®nal e�ect may depend onthe ratio of components or the temperature. Systemswhere the ultimate degradation rates are reduced or thedecomposition temperatures of all component polymersare shifted to higher values have the optimum beha-viour. The thermal behaviour of polymer blends showssome similarities with graft copolymers, but di�ers fromthose of random copolymers [13].Experimental data were obtained at rather high tem-

peratures and cannot be compared with those obtainedin thermomechanical or processing degradation; see thepreceding part, or in thermal oxidation, see the secondpart of this paper.Great attention has been paid to blends containing

PVC. Blending with other polymers improves propertiesof this commodity polymer and enhances its applicationpossibilities [27]. A knowledge of the thermal behaviour

of di�erent blends containing PVC is of commercialimportance.Thermal dehydrochlorination of unstabilized PVC

occurs at about 100�C and is also an undesired processin stabilized PVC at processing temperatures (180/200�C) (Ref. 20). Dehydrochlorination accounting forformation of conjugated double bonds [Eq. (4)] leads toallylic activation in degrading PVC. Hydrogen atoms ofthe methylene group in the allylic moiety 7 are able toform hydrogen bonds with functional groups (e.g.> C�O) of the component polymer [28]. Sequences ofconjugated double bonds may participate in PVCcrosslinking [21].

� CH2 ÿ CH � CH �7

The low thermal stability of PVC requires e�ectivestabilization [20]. It was realized that the performanceof tin stabilizers was enhanced in some blends of PVC incomparison with pure PVC [27].A detailed elucidation of dehydrochlorination rates of

PVC blends with HIPS containing 16% nongrafted PS,poly(styrene-co-acrylonitrile) (SAN) and acrylonitrile-butadiene-styrene polymer (ABS) containing 27% non-grafted SAN in inert atmosphere at 180�C revealed [29]accelerated degradation of the PVC component. Thedestabilizing in¯uence of HIPS on PVC was ascribed tounsaturation of the polybutadiene (PB) phase. Theincreased content of acrylonitrile (AN) in SAN enhancesPVC dehydrochlorination. The improved miscibility ofthermally treated PVC/SAN blends was ascribed to for-mation of bonds between hydrogen atom of the allylicmethylene group and the nitrile group in SAN [28].A destabilizing e�ect manifested by increased dehy-

drochlorination rate of PVC was observed in blendsPVC/chlorinated rubber [14].Degradation products of PVC (HCl, Cl�) sensitize

thermal degradation of LDPE and catalyze depolymer-ization and chemical modi®cation of PMMA [5,21].Processes in PMMA proceed in two steps and are char-acterized by lowering the temperature of PMMA depo-lymerization due to catalysis by chlorine radicals Cl�and hydrolysis and cyclization of ester groups by HClaccounting for the built-in anhydride rings 8 [Eq. (7)].

(7)

The ring structures arising in extensive PMMAdegradation reduce its depolymerization [21]. Thisaccounts for a reduced rate of monomeric methylmethacrylate formation, or in other words, for enhancedthermoresistance of PMMA in this degradation phase.


An analogous cyclization mechanism stabilizing thePMMA component was mentioned for thermodegradedPMMA/polychloroprene blend [21]. The mechanism ofinteractions in other blends containing PMMA andchlorinated polymers may be controlled by other fac-tors. For example, blending with chlorinated rubberdestabilized PMMA [5,14].A systematic investigation of the e�ect of various

poly(alkyl methacrylate)s on HCl evolution and thelength of formed polyene sequences [29,30] at 180�Crevealed that the resistance of organotin-stabilized PVCwas increased in the presence of high concentrations(75%) of poly(alkyl methacrylate)s. The butyl ester wasshown to be more e�ective in this sense than the methylester. Some destabilization of PVC was observed in thepresence of the methyl ester used in concentrationslower than 50%.Some costabilization activity between dibutyltinbis(i-

sooctyl thioglycolate) (heat stabilizer in the PVC phase)and PMMA was reported [30]. This indicates that theinherent stability of PVC may a�ect the thermal beha-vior of both components of the PVC/methacrylate blend.Investigation of the e�ect of acetic acid arising in

PVA thermolysis on PMMA con®rms that PMMA inblends su�ers from acid degradation products [5,14,21].Because of the higher thermostability of PVA in com-parison with PVC, the destabilizing e�ect of PVA onPMMA is less drastic.Destabilization of both components forming the

blend arising due to their mutual reactivity wasobserved for PVC/poly(acrylonitrile) (PAN) blends,PVC/PVA blends and PVC/EVA blends [5,14,21]. Theworse properties of PVC/PAN blends were explained byenhanced PVC dehydrochlorination due to the e�ect ofammonia arising from the PAN phase and by somelowering of the stability of the PAN component by HCl.However, the volatile ammonia arising during thermo-lysis of the PAN component may have a di�erent e�ectwhen blending PAN with another polymer. For exam-ple, it was reported that ammonia enhances stability ofPMMA in PAN/PMMA blends.A concerted thermolysis of PVC and PVA in PVC/

PVA blends accounts for formation of hydrochloric andacetic acids [5,14,21]. The acids migrate over phaseboundaries into adjacent phases of the immiscible blendand cross-catalyze the dehydrochlorination and deace-tylation [Eq. (8)]. Miscible PVC/PVA blends are moreprone to catalytic degradation because the acids neednot di�use between phases.

(8)

A similar explanation was used for thermal instabilityof PVC/EVA blends, particularly those containing morethan 45% vinyl acetate [5].Elimination of the catalytic e�ect of HCl in PVC

blends with a reactive component polymer improvesthermal stability of PVC. This was exempli®ed byblending PVC with epoxidized styrene-butadiene-styr-ene (SBS)F thermoplastic elastomers. Two kinds ofepoxidized polymers were used: linear thermoplasticelastomer (EB/S) containing respectively 0.07, 0.15, 0.25and 0.35 moles of epoxy groups in 100 g (denoted asEB/S 7, 15, 25 and 35, respectively) and radial blockcopolymer [E(B/S)n] containing 0.05±0.40 moles ofepoxy groups in 100 g [denoted as E(B/S)n 5 to E(B/S)n40] (Ref. [31]). For comparison, blends with non-func-tionalized SBS were tested. The time to onset of dehy-drochlorination at 170�C and 180�C increased in thesequence PVC<PVC-SBS<PVC-EB/S<PVC-E(B/S)n.The epoxidized E(B/S)n provided the best stabilizingperformance. However, in both types of epoxidizedSBS, the stability increased with increasing content ofthe epoxy moiety (Fig. 3). HCl migrating from the con-tinuous phase (PVC) into the dispersed phase (epox-idized elastomer) is able to convert the epoxy moieties[Eq. (9)] and add to C�C double bonds [Eq. (10)].

(9)

(10)

Fig. 3. The degree of PVC dehydrochlorination (x1) versus time of

heating (min) blends of PVC-10% copolymer at 170�C, VC is content

of vinyl chloride units in the blend: (a) PVC (1), PVC-BS (2), PVC-EB/S

7 (3), PVC-EB/S 15 (4), PVC-EB/S 25 (5) and PVC-EB/S 35 (6); (b)

PVC (1), PVC-(BS)n (2), PVC-E(B/S)n 5 (3), PVC-E(B/S)n 20 (4),

PVC-E(B/S)n 27 (5) and PVC-E(B/S)n 40 (6). (Reprinted from Poly-

mer Degradation and Stability, Vol. 60, W. Meissner and D.

Zuchowska: Thermal dehydrochlorination of PVC/epoxidized SBS

blends, p. 415±24, 1998, with permission from Elsevier Science.)


A bene®cial co-reactivity was observed in blends ofPVC with PS [21,32]. The resistance of PVC to dehy-drochlorination and of PS to thermolysis were higherthan those in neat PVC and PS. It was assumed that thedehydrochlorination is retarded because part of thereleased Cl� does not participate in catalysis of thechain dehydrochlorination of the PVC phase, butmigrates into the PS phase and generates PS-macro-radicals by abstraction hydrogen atoms from PS. PS-macroradicals undergo chain scission without depoly-merization (formation of LMW volatiles is suppressed).As a consequence, a more rapid decrease in molecularweight of PS takes place in the blend in comparisonwith pure PS [21].The co-reactivity of PS in blends with PVC indicates

that formation and reactivity of PS-macroradicals isalso important in blends with other polymers. Thermo-lysis of neat PS starts at 180�C and is very rapid attemperatures above 250�C (Refs. 33 and 34). In PSsynthesised by anionic polymerization, the degradationstarts at terminal benzyl groups [35]. Location of theprimary radical formed by thermolysis of PS preparedby free-radical polymerization is not exactly de®ned dueto structural inhomogeneities of the polymers [35].Generally, two types of carbon-centered macroradicalsare formed in PS: in-chain (9, 10) and terminal (11)radicals. Unsaturation and LMW volatile products areformed in subsequent reactions [10,23].

Blending of PS with PVC, poly(vinylidene chloride)(PVDC), PVA, PAN, polyisoprene (PI) [the dataobtained with PB and PAN are important for theknowledge of processes accompanying degradation ofcommercially important blends of HIPS, SAN, ABS orAES (acrylonitrile-EPDM-styrene polymer; EPDMstands for ethylene-propylene-5-ethylidene-2-norbor-nene rubber)] and some engineering polymers not dis-cussed in this paper, increases degradation temperatureof PS [5,12,14,21,32]. This enhanced stability of PS isexplained by deactivation of PS-macroradicals by inter-molecular reactions with products or structure elementsof the second polymer.Stabilization of the PS phase in PS/PVA blends was

ascribed to the reaction of the PS-macroradical withunsaturation arising in degrading PVA [5,14]. An ana-logous explanation of the increased thermostability ofthe PS component was also used for blends with otherpolymers forming unsaturation in thermolysis, i.e. PVC,PVDC, PAN or component polymers such as HIPS or

ABS containing PB or PI unsaturated units. However,the reactivity with C�C unsaturation is not the onlystabilization process. A detailed mechanistic study per-formed with PS/PB blends revealed [36] that the PBphase degrades in the blend earlier than the PS phaseand, in the temperature range above 350�C, generatesvolatile LMW products such as 1,3-butadiene, 4-vinyl-cyclohexene 12 and aromatic fragments from cyclizedPB blocks, such as toluene together with methane andhydrogen [37]. Compound 12 di�uses into the PS phaseand was reported as a species stabilizing (saturating) thePS-macroradicals by hydrogen atom transfer [36]. Thestabilizing performance of the migrating 4-vinylcyclo-hexene 12 is in¯uenced by the degree of dispersion ofone polymer in the other. Using model measurementswith PS, PB, PS/PB (1:1), styrene-butadiene diblockcopolymers (SB), a similarity in behavior of PS/PBand SB was shown [26,37]. The optimum conditionsfor the stabilizing performance of 12 exist in SB diblockcopolymers.

An analogous explanation of the enhanced thermalstability of PS was reported for PS/PI blends [21]. Inthis case, the deactivation of the PS macroradical wasattributed to formation and reactivity of 4-isopropenyl-1-methylcyclohexene (dipentene) 13.The PB phase has a detrimental e�ect on thermal and

photo-oxidation of the ABS multiphase system. Toreduce its sensitivity to oxidation, PB was e�ectivelyreplaced by EPDM rubber. The formed multiphase AESpolymer consists of dispersed EPDM rubber in glassymatrix of SAN, and of EPDM-g-SAN graft copolymerphase acting as a compatibilizer. Investigation of ther-mal behavior of AES [consisting of 60% SAN (with24% AN), 9.5% EPDM (60% ethylene, 9% 5-ethyli-dene-2-norbornene), 29.5% EPDM-g-SAN and 1%insoluble gel) up to 460�C (by DSC) and 600�C (by TVA)re¯ects interactions between degrading components ofthe blend resulting in stabilization [38]. The process iscontrolled by the level of dispersion between incompa-tible SAN and EPDM polymers. The strongest interac-tions were found in the EPDM-g-SAN phase with bothcomponents molecularly mixed. As a consequence, thevolatilization temperature was higher and the amountof volatile LMW products lower in comparison withequivalent physical mixtures of SAN and EPDM.Thermostable component polymers that do not

have a tendency to react with PS or PS-macroradicals


thermolyze in blends with PS at lower temperaturesthan the relevant pure polymers. This was observed forPS/PE or PS/PP blends [5] and may be explained bysensitized degradation of both polyole®ns due to thechain transfer of PS-based free radicals to polyole®nsassociated with enhanced formation of polyole®n-basedradicals and their further reactions.Component polymers that degrade more rapidly than

PS may induce formation of PS macroradicals andaccelerate chain scission of PS. This mechanism wasproposed for thermolysis of PS/poly(a-methylstyrene)-blends. It is believed that migrating LMW free radicalsarising from poly(a-methylstyrene) induce a prode-gradant e�ect by a mechanism similar to that of Cl�radicals arising in PVC/PS blends [5,14,21]. As a con-sequence, the production of monomeric styrene increa-ses. Thermal stability of PS was also reduced byproducts of thermolysis of PMMA in blends containingup to 10 wt% PMMA.

6. Conclusions

Degradation of polymer blends is in¯uenced bydegradation conditions, structures of components ofpolymer blends and by potential co-reactivity betweencomponent polymers and/or their degradation productsthat may lead to new chemical species (e.g. graftedcopolymers) and/or in¯uence either in positive or nega-tive sense the ®nal stability of the blend.In mechanically induced thermal degradation, char-

acteristic of melt processing, interactions between mac-romolecules and macroradicals generated from thecomponent polymers are predominant [4,5]. Bulk reac-tions within a phase are more probable than interac-tions at phase boundaries. Additivity of e�ectspredominates in blends of polyole®ns. Cross-recombi-nation leading to graft copolymers in polyole®n blendsmay be enhanced probably only in the presence of aperoxidic initiator and an unsaturated co-additive. Theprocess is of potential importance for reprocessing ofrecycled plastics. Some features of formation of graftcopolymers triggered by melt degradation wereobserved in blends of PVC with HDPE or PS with PIB.Reactions between macromolecules or macroradicals

with small molecules and small radicals di�using acrossthe phase boundaries are characteristic of thermaldegradation of polymer blends in an inert atmosphere[13,14,21]. The thermal behavior of polymer blendsshows some similarities with graft copolymers.Depending on the reactivity of macromolecules andLMW fragments, the resistance of component polymersis either increased or reduced in comparison with neat,unmixed polymers. Direct interactions between thetwo component polymers were not observed in high-temperature degradation.

Acknowledgements

Financial support of this work by grant KONTAKTNo. 184-1998 from the Ministry of Education, Youthand Sport of the Czech Republic and grant No. 106/96/1377 from the Grant Agency of the Czech Republic aregratefully appreciated. The authors thank Mrs. D.DundrovaÂ for technical co-operation in preparation ofthe manuscript.

References

[1] Koning C, van Duin M, Pagnoulle C, Jerome R. Prog Polym Sci

1998;23:707±57.

[2] Folkes MJ, Hope PS, editors. Polymer blends and alloys. London:

Chapman and Hall, 1993.

[3] Utracki LA, editor. Polymer blends handbook. Toronto: Chem-

Tec Publ, 1997.

[4] LaMantia FP. Angew Makromol Chem 1994;216:45±66.

[5] LaMantia FP. In: Halim Hamid S, Amin MB, Maadhah AG,

editors. Handbook of polymer degradation. New York: M.

Dekker, 1992. p. 95±126.

[6] Billingham NC, Hoad OJ, Chenard F, Whiteman DJ. Macromol

Symp 1997;115:203±14.

[7] Fox DW, Allen RB. In: Kroschwitz JI, editor. High performance

polymers and composites. New York: J. Wiley, 1991. p. 65±82.

[8] Bonner JG, Hope PS. In: Folkes MJ, Hope PS, editors. Polymer

blends and alloys. London: Chapman and Hall, 1993, p. 65±82.

[9] PospõÂ sÏ il J, Klemchuk PP. In: PospõÂ sÏ il J, Klemchuk PP, editors.

Oxidation inhibition in organic materials, vol. I. Boca Raton:

CRC Press, 1990. p. 1±10.

[10] PospõÂ sÏ il J, HoraÂ k Z, KrulisÏ Z, NesÏ puÊ rek S. Macromol Symp

1998;135:247±63.

[11] Wypych J. Polym Networks Blends 1992;2:53±64.

[12] Chiantore O, Trossarelli L, Lazzari M. Polymer 1998;39:2777±81.

[13] McNeill IC. In: Grassie N, editor. Developments in polymer

degradation, vol. 1. London: Applied Science Publishers, 1977. p.

171±204.

[14] McNeill IC, Grassie N, Samson JNR, Jamieson A, Straiton T. J

Macromol Sci Part A 1978;12:503±29.

[15] Hinsken H, Moss S, Pauquet J-R, Zweifel H. Polym Degrad Stab

1991;34:68±71.

[16] PospõÂ sÏ il J, HoraÂ k Z, NesÏ puÊ rek S. Plasty Kauc 1997;34:68±71.

[17] Gugumus F. In: PospõÂ sÏ il J, Klemchuk PP, editors. Oxidation

inhibition in organic materials, vol. I. Boca Raton: CRC Press,

1990. p. 61±172.

[18] LaMantia FP, Valenza A, Acierno D. Polym Degrad Stab

1985;13:1±9.

[19] Gha�ar A, Sadrmohaghegh C, Scott G. Polym Degrad Stab

1980±1981;3:341±7.

[20] Braun D. Pure Appl Chem 1981;53:549±66.

[21] McNeill IC. In: Eastmond GC, Ledwith A, Russo S, Sigwalt P,

editors. Comprehensive polymer science, vol. 6. Oxford: Perga-

mon Press, 1989. p. 451±500.

[22] Hope PS, Bonner JG, Miles AF. Plast Rubber, Compos Process

Appl 1994;22:147±58.

[23] Andreopoulos AG, Tarantili PA, Annastassakis P. J Macromol

Sci, Pure Appl Chem 1998;35:751±61.

[24] Braun D, Richter S, Hellmann GP, RaÈ tzch M. J Appl Polym Sci

1998;68:2019±28.

[25] KrulisÏ Z, HoraÂ k Z, LednickyÂ F, PospõÂ sÏ il J, SufcÏ aÂ k M. Angew

Makromol Chem 1998;258:63±8.

[26] PospõÂ sÏ il J, NesÏ puÊ rek S, Pfaendner R, Zweifel H. Trends Polym

Sci 1997;5:294±300.


[27] Braun D, BoÈ hringer B. Makromol Chem Macromol Symp

1989;29:73±93.

[28] Lizymol PP, Thomas S, Jayabalan M. Polym Int 1997;44:23±9.

[29] Braun D, BoÈ hringer B, Fischer NEM. Angew Makromol Chem

1994;216:1±19.

[30] Braun D, BoÈ hringer B, Knoll W, Eidam N, Mao W. Angew

Makromol Chem 1990;181:23±39.

[31] Meissner W, Zuchowska D. Polym Degrad Stab 1998;60:415±

24.

[32] Braun D, KroÈ mmeling S. Angew Makromol Chem 1992;195:205±

11.

[33] Chiantore O, Guaita M, Grassie N. Polym Degrad Stab

1985;12:141±8.

[34] McNeill IC, Zul®qar M, Kousar T. Polym Degrad Stab

1990;28:131±51.

[35] Guyot A. Polym Degrad Stab 1986;15:205±18.

[36] McNeill IC, Stevenson WTK. Polym Degrad Stab 1985;10:247±

65.

[37] McNeill IC, Stevenson WTK. Polym Degrad Stab 1985;10:319±

34.

[38] Chiantore O, Lazzari M, Ravanetti GP, Nocci RJ. J Appl Polym

Sci, Appl Polym Symp 1992;51:249±62.


Documents

Degradation and aging of polymer blends I. Thermomechanical and thermal degradation