irradiación de polímeros crosslinking vs scission.pdf

7/28/2019 Irradiacin de polmeros Crosslinking Vs Scission.pdf

1/19

Radiation Effects on PolymericSystems

Crosslinking and Scission Both crosslinking and scission occur on irradiationof polymers; however, their relative importancevaries from polymer to polymer


2/19

Predominant Processes in IrradiatedPolymersCrosslinkina

PolyethylenePolypropylenePolystyrenePolyacrylatesPolyamidesPolyestersRubbersPolysiloxanesPolyacroleinPolymethyleneChlorinated polyethylenePolyacrylonitrilePolyethylene oxide

ScissionPolyisobutylenePoly-a-methylstyrenePolymethacrylatesPolyvinylidene chlorideCelluloseCellulose acetatePolytetrafluoroethylenePolytrifluorochloroethylenePoly-a-methacrylonitrilePolyethylene terephthalate


3/19

Polymer StructureCrosslinking vs Scission

As the hydrogen in the backbone of an organicpolymer is replaced with heavier substituents, itstendency to crosslink decreases and its tendencyto scission increases

H X XI I I I I I I I I-c -c-c- - c- c- c- - c- c- c-I I I I I I I I IH H Y

(a) (b) (e)X and Y, heavier than H, e.g., CH a, CICrosslinking a>b>c; Scission a


4/19

Yields ofGaseous Products from IrradiatedPolymersa(1- or electron lrrad, room temp)

PolymerHigh-density PEPolypropylenePolyisobutylenePoly(vinyl chloride)Poly(vinyl acetate)Poly(methyl

methacrylate)PolystyrenePOly-a-methylstyrene

Products G (product)(molecules/100 ev)H2 -3; CH 4 -0.002H2 -2.5; CH 4 -0.1H2 -1.5; CH 4 -0.5HCI -2.7; H2 -0.15; CH 4 -0.002H2 -0.6; CH4 -0.3; CO -0.28; CO 2 -0.06CH 4 - 0.6; CO -0.5; CO 2 -0.4; H2 -0.2H2 -0.03; CH 4 - 1x10-5H2 -0.04; CH 4 -0.003

a Woods and Pikaev (1994)


5/19

Free Radicals Formed onIrradiation ofPolyethylene-CH 2-CH 2-(CH2-CH 2)n- ...JV\N'+ -CH 2-CH-(CH2-CH 2)nalkyl radical

-CH 2-CH 2-(CH2-CH 2)n- ...JV\N'+ -CH-CH=CH-(CH2-CH 2)n_1-allyl radical- At very high doses-CH2-CH 2-(CH2-CH 2)n- ...JV\N'+-CH 2-CH-(CH=CH)x-(CH2-CH)n_xpolyenyl radical


6/19

Radiation Effect on Polystyrene

~ -n Ions and radicals formed on irradiation Shows thermoluminescence on irradiationat 196C and subsequent warming G(free radicals) low, -0.2

1Radiation stable ininert atmosphere

and- CH 2 - CH - CH 2 I~~ H 2H 2

Radiation-inducedoxidation in air


7/19

Radiation Effect on Polypropylene On irradiation, long-lived free radicals formedLow temperature (-196"C)

-CH2 - C - CH2ICH3(1)

Room temperature-CH-CH-C =CH-CH-CH2I I ICH CH3 CH3 .(3)

-CH2-C-CH =C-CH2-6H3 bH3(4)

Overall effect of irradiation: loss of mechanicalstrengthDole (1973); Bradley (1984)


8/19

Chemical Basis ofOxidativeDegradation Initiation

RH - 'VV\! ' . R- + H-H- + RH R- + H2R- + O2 R02-

CrosslinkingR- + R- RR(not favoured in air/oxygen)

DegradationR02- , ROOH -----...Chain scission, degradation,alcohols, ketones


9/19

Propagation Chain Reaction andPost-IrradIationDegradation ofPolymers

Oxidative degradation of polymers reduces strengthand flexibi li ty, causes cracking, increases moistureuptake and degrades electrical insulation propertiesR02 + RH .. R + ROOHHeatROOH ----.... RO + OH

LightRO + RH .. R + ROHOH + RH .. R + H20

Oxidative degradation usually continues for monthsafter irradiation, e.g., initiated by the reactions ofROOH


10/19

Radiation Effects on Polyethylene Known products on irradiation in air include H20,CO, CP2' alcoholsbketO!1es, }1ydroperoxides,peroxides and car oxyhc aCids Free radicals (alkyl a.nd allyl) usually consideredprecursors of crosshnks Free radical migration form crystal line toamorphous regions U n s a ~ u r a t i o n in PE participates in crosslinkingreactions Oxygen and additives excluded from the crystal lineregions Roles of excited states and ionic reactions not wellunderstood


11/19

Radical Buildup and Decay in GammaIrradiated Polypropylene in Air

........Radical Buildup-e-Radical Decay

,...IC ) 4.0tn.= 3.2c.en 2.4co,...o 1.6,..>< 0.8,.....,

~ o . o L - . - - . - L , . - - ~ ~ ~ ~ ~ ~o 40 80 120 2 4 6 8 10 12 14 16Dose (kGy) Decay Time (Days)Kashiwabara and Seguchi, 1992


12/19

Radiation-Induced Oxidation ofPolymersG-Value of O x y ~ n Consumption at Room Temperatureunder - 70 kPa xygen (Kashiwabara and Seguchi, 1992)Polymer Dose Dose Rate G(-02)(kGy) (kGy/h)Low-densitypolyethylene 500-1000 10 14High-density 500-1000 10 18olyethyleneMedium-density 500-1000 10 18olyethylene

E t h ~ l e n e propylene 100-500 2 8u berIsotacticpolypropylene 100-200 10 50Polyvinyl chloride 100-200 10 29Polyvinyl chloride 100-200 10 11stabillzed)


13/19

Polymer Degradation by Irradiation Radiation degradation can be used to reduce themolecular weight of commercial polymers, e.g.,polypropylene and polyethylene oxide PP-vis-breaking, a patented process

Teflon: scrap or waste Teflon is converted intouseful powder and low MW products byirradiation (500 kGy), e.g., for producinglubricants and coated non-stick pans


14/19

Effect ofAdditivesThere are four main ty'pes of additives whose effect onirradiation of polymeric systems needs to be considered1. Crosslinking agents: these are typically multifunctionalmonomers (such as di-and triacrylates) that reduce the

dose required for crosslinking2. Degradation agents: primaril oxygen; it degradespolYmers via tile formation 0 peroxy radicals3. Protective agents: basically, there are two types of

r:rotectors- the anti-rads, which act as energy acceptorssuch as pyrene and other aromatic hydrocarbons), andree radical scavengers or anti-oxidants, e.g., phenols,which protect the polymer via peroxy or carbon centredradicaf reactions

4. Neutral additives: frequently, mineral powders or fibresare added to polymers; many of these have no effect onirradiation of pofymers


15/19

Chemical Formulae of Some Antioxidants(Kashiwabara and Seguchi, 1992)ymbol

OPPO

Irganox 1010

Name and formulaN,N'-Oiphenyl-p-phenylenediamineI ~ ,J--I N H ~ " ,>-1 NH ~ G > ITetrakis[methylene-3(3,S-di-t-butyl-4hydroxyphenyl) propionate]methane

It-au ~I--=-[H-OI ~ ' = = ' , > [CH,CH,COOCH,].C I

Use of antioxidants, and mobilizers (e.g. mineral oils),reduces radiation-induced damage to most polymers


16/19

Multifunctional MonomersTrimethylolpropane triacrylate (TMPTA)o

IIo CH2-O-C-CH =CH 2II ICH2= CH-C-Q-CH2-C-CH2CH 3ICH2-Q-C-CH = CH2IIQTripropylene glycol diacrylate (TPGDA)

Q QII IICH2=CH-C-Q-CH2-CH-Q-CH2-CH-Q-CH2-CH-Q-C-CH=CH2I I ICH 3 CH 3 CH 3 These enhance crosslinking and grafting reactions


17/19

Effect ofElectron and Gamma Irradiationon the Colour ofPolycarbonate Resin

Dose(kGy)o1040

ElectronYellownessIndex-1.01.6

GammaYellownessIndex1.28.426.4

The level of colour developed varies from polymer topolymer, and also depends on the stabilizers used


18/19

Effect ofRadical Scavenger and MobilizingAgent on Polypropylene

R-O kGySIB 30 kGyS&-50 kGySR-70 kGyS

14

14

12

1210

2

en 100m 80 I$: : : : : : : - - - tr - -.......r---- .-- ....1l.~ 60"C- 40)-~ 20


19/19

"'1k-2000 Gy/hrJ i 44 Gy/hr"""ID-4.5 G Ih r

1 10 100Dose (kGy) In general, the mechanical properties of polymers are lessadverselY aff.ecJed on electron irradiation, as compared togamma Irradiation ~ a t i ~ f a ~ t o r y s t a b i l i ~ e d polymers for both types ofIrradiations are available

irradiación de polímeros crosslinking vs scission.pdf

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