Rubber Compounding - Chemistry and Applications Part 11 ppsx

Antioxidants react with oxygen to prevent oxidation of vulcanizedrubber and react with free radicals that degrade vulcanized rubber.. The alkyl radicals reactrapidly wth atmospheric oxyg

Antioxidants and Other

Protectant Systems

Sung W Hong

Crompton Corporation, Uniroyal Chemical,

Naugatuck, Connecticut, U.S.A

I INTRODUCTION

To extend the service life of vulcanized rubber goods, it is very important toprotect them from oxygen, ozone, light, heat, and flex fatigue Most naturaland synthetic rubbers containing unsaturated backbones—natural rubber(NR), styrene butadiene rubber (SBR), polybutadiene rubber (BR), andnitrile rubber (NBR), for example—must be protected against oxygen andozone Usually, internal components that are not exposed to atmosphererequire only antioxidants However, external components that are exposed

to the environment require both antiozonants and antioxidants

Antioxidants react with oxygen to prevent oxidation of vulcanizedrubber and react with free radicals that degrade vulcanized rubber

There are four principal theories on the mechanisms of antiozonantprotection of vulcanized rubber The first is the scavenger theory, whichpostulates that the antiozonant competes with the rubber for ozone Thesecond theory is that the ozonized antiozonant forms a protective film on thesurface of the vulcanized rubber, preventing further attack The thirdmechanism postulated is that the antiozonants react with elastomer ozonidefragments, relinking them and essentially restoring the polymer chain Thefourth theorized mechanism suggests that Criege zwitterions are formed fromthe ozonide produced

Paraffinic and microcrystalline waxes are often added to the rubber asprotective agents Usually, waxes have poor solubility in rubbers, so they

migrate to the surface of the vulcanized rubber and form a protective film orbarrier that prevents ozone attack During dynamic flexing, these barriers can

be broken, exposing the rubber to ozone attack Therefore, waxes can protectagainst ozone only under static conditions Antiozonants alone do notprevent ozone cracking at the initial stage, because their migration rate ismuch slower than that of waxes owing to their better solubility One theorysupporting the combination of antiozonant with wax is that the wax wouldaccelerate migration of the antiozonant to the surface for protection againstozone Therefore most exterior rubber goods contain both antiozonant andwax for both static and dynamic protection from ozone cracking

Commercially available antioxidants usually protect vulcanized rubber

at temperature below 120jC Above this temperature, special polymers ortypes of cross-link systems provide better protection For example, a peroxidecure system would provide carbon–carbon cross-links whose bonding energy

is the strongest Also, a monosulfuric cross-link has much higher energy than

a polysulfuric bond The lower the bonding energy, the more easily cross-linksbreak, which would deteriorate the vulcanized rubber’s physical propertiesafter heat aging Cure systems with a higher level of accelerators and lowersulfur levels are known as semiefficient cure systems (semi EV cure) Such asystem provides better heat aging properties than the conventional curesystem, which has a smaller amount of accelerator and a higher level ofsulfur In this chapter, the protection of mechanical rubber goods throughselection of antioxidants, antiozonants, waxes, and the design of the vulcan-ization system will be discussed

Oxidative degradation of the polymers is a free radical process Thisoxidation process, known as auto-oxidation, consists of three steps: initiation,propagation, and termination, as depicted inFigure 1

Free radicals are formed during initiation reactions Energy from heat,mechanical shearing, or high energy radiation can dissociate the chemicalbonds in the polymers (RH) resulting in the formation of free radicals (R
)[reaction (1)] In auto-oxidation mechanisms of polymers, the molecularreaction of oxygen with the polymers by thermal energy has been suggested

for the initiation of the first free radicals in the polymer (1–4) [reaction (2)].Hydroperoxide concentration builds up as the auto-oxidation proceeds, andconsequently decomposition of the hydroperoxide eventually becomes thedominant initiation process [reactions (3) and (4)] This is usually proceded by

a short period of induction

The alkyl (R
) and alkylperoxyl (ROO
) radicals resulting from theinitiation reactions are the chain-propagating species The alkyl radicals reactrapidly wth atmospheric oxygen to form alkylperoxyl radicals [reaction (5)].The alkylperoxyl radicals abstract labile hydrogens on the polymer, regener-ate new alkyl radicals, and yield hydroperoxides (ROOH) as the primaryoxidation product [reaction (6)] Reactions (5) and (6) form a cycle in thepropagation step As the hydroperoxide concentration builds up, more alk-oxyl and alkylperoxyl radicals are formed via decomposition of the hydro-peroxide to start new cycles

Termination occurs when two free radicals, alkyl and/or alkylperoxylradicals, react to form the stable nonradical products The termination reactio

in the solid polymers, where oxygen is limited, usually involves two alkylradicals (R
), which undergo recombination to form RURor disproportion-

ation to form saturated and unsaturated products On the other hand, in thepresence of sufficient oxygen such as in liquid hydrocarbons, the hydroperoxylFigure 1 Mechanisms of polymer degradation by oxidation under thermal energy

radical (ROO
) concentration is much greater than the alkyl radical tration (R
) Chain termination occurs predominantly by reaction betweentwo hydroperoxyl radicals.

concen-Antioxidants are used to stabilize organic polymers concen-Antioxidantsinhibit auto-oxidation by reducing the rate of auto-oxidation during process-ing, storage, and service There are two major groups of antioxidants,commonly known as primary and secondary antioxidants Primary antiox-idants act as chain terminators, and secondary antioxidants act as hydroper-oxide decomposers The primary antioxidants remove the chain-carryingspecies (R
and ROO
), while the secondary antioxidants convert the hydro-peroxides to nonradical species

A schematic chain-termination mechanism is shown in Figure 2 Inreaction (10), the alkylperoxyl radical abstracts the reactive hydrogen fromthe antioxidant (AH) The resulting antioxidant radicals (A
) are stabilizedvia electron delocalization Consequently, the antioxidant radicals (A
) donot readily continue the radical chain either via hydrogen abstracting fromthe substrate [reaction (11a)] or via reaction with oxygen [reaction (11b)] Theresonance structures for various antioxidant radicals derived from typicalantioxidants will be illustrated in the next section Transformation productsderived from typical antioxidants will also be discussed to elucidate theantioxidant mechanism Hindered phenolics and secondary aromatic aminesare the two most commonly used primary antioxidants

Examples of secondary antioxidants that act as hydroperoxide posers include phosphite esters such as I and sulfur-containing compoundssuch as thioester II As the hydroperoxides are removed from the organicsubstrates, fewer free radicals are produced via the decomposition of thehydroperoxides Consequently, the rate of the auto-oxidation is reduced Themechanism of converting hydroperoxides to nonradical species will bediscussed in Section II.B.3

Figure 2 Mechanisms of reaction of peroxyl radical with antioxidant andexplanation of the stabilization of antioxidant radical by electron delocation, withoutcontinuing formation of radicals

B Mechanisms of Antioxidants

Commercially available antioxidants can be divided into three categories:phenolic antioxidants, aromatic amine antioxidants, and hydroperoxide-decomposing antioxidants Each antioxidant performs in a specific way toprotect polymers and rubber compounds from oxidation The performance ofeach antioxidant is related to its chemical reactivity, its rate of staining ormigration to the surface of vulcanized rubber or polymers, and its volatility.Therefore, it is very important to understand the mechanisms of variousantioxidants before applying them for experiments

1 Phenolic Antioxidants

Hindered phenolics, which act as chain terminators, are excellent idants, Phenolic antioxidants are in general nonstaining and nondiscoloring.Many of them are approved for use in food packaging

antiox-A simplified mechanism commonly used to show the process of chaintermination by a hindered phenolic is shown in Figure 3 The alkylperoxylradicals (ROO
) abstract the reactive hydrogen from the phenolics Theresulting phenoxy radical 1 is stabilized through electron delocalization asindicated by the resonance structures 1 and 1a Reaction of the alkylperoxylradical with 1a produces the nonradical product 2

More detailed information about the antioxidant mechanism was madepossible with the identification of the transformation products The mecha-

Figure 3 Chain termination by hindered phenolics

nism depicted in Figure 4 illustrates the formation of transformation productsusing 2,6-di-t-butylhydroxytoluene (BHT) as an example The alkylperoxylradical attacks the reactive hydrogen from the BHT, yielding the phenoxyradical 3, which is stabilized via delocalization to the carbon-centered radical3a Bimolecular reaction between 3 and 3a would produce 4 and 5 Reaction

of 3a with the alkylperoxyl radical forms 6, which thermally decomposes tothe p-quinone 7 The phenoxy radical 3a would also be the precursor for theformation of compounds 9 and 10 (6,8)

Figure 4 Antioxidation mechanism of BHT (illustrated with some transformationproducts)

2 Aromatic Amine Antioxidants

Amine antioxidants in general are better antioxidants than phenolic idants However, most amine antioxidants are discoloring and staining andhave limited approval for food contact use The mechanism of chaintermination by secondary aromatic amines is shown in Figure 5, with N,NV-dialkyldiphenylamines as an example (5,6) The alkylperoxyl radicals abstractthe reactive hydrogen (NUH) from the N,NV-dialkyldiphenylamines The

antiox-Figure 5 The chain termination mechanism by N,NV-dialkyldiphenylamines

resulting aminyl radical 11 is stabilized through electron delocalization asindicated by the resonance structures 11, 11a, and 11b Reaction of thealkylperoxyl radical with 11b produces a nonradical product 12 Reaction of

11 with the primary alkylperoxyl radicals leads to a stable nitroxyl radical 13that is capable of trapping the free alkyl radical and producing the stableproduct, alkoxyamine 14 Reactions of the aminyl radical 11 with secondary

Figure 6 Antioxidation mechanism illustrated with the resonance structures andthe transformation products from TMQ

and tertiary alkylperoxyl radicals produce the hydroxylamine 15 and thenitroxyl radical 13, respectively (17–19).

Another commonly used secondary amine is polymerized 2,2,4-trimethylquinoline (TMQ) A mechanistic illustration with the reso-nance structures and the transformation products is shown inFigure 6 Thealkylperoxyl radicals abstract the reactive hydrogen (NUH) from TMQ Theresulting aminyl radical 16 is stabilized through electron delocalization asindicated by the resonance structures 16a, 16b, and 16c However, thetransformed radical 16a would be capable of trapping alkylperoxyl radicals,leading to the nonradical product 17 The aminyl radical 16 would also trapalkylperoxyl radicals to form the nitroxyl radical 18, which in turn traps alkylradicals to form the alkoxyamine 19

1,2-dihydro-N,NV-Dialkylated p-phenylenediamines are excellent antioxidants Amechanism that illustrates chain termination by the N,NV-diphenyl-p-phenyl-enediamine 20 is shown in Figure 7 (7,8) Two alkylperoxyl radicals attack thetwo reactive hydrogens from N-phenyl-NV-alkyl-p-phenylenediamine, yield-ing the quinonediamine 21, which is stabilized through electron delocalization

as shown by the resonance structure 21a Further reaction of the aminyldiradical 21a with two alkylperoxyl radicals produces the dinitroxyl radical

22, which is converted via electron delocalization to the dinitrone 22a (12,15)

Figure 7 Chain termination by N,NV-diphenyl-p-phenylenediamine

3 Hydroperoxide-Decomposing Antioxidants

Hydroperoxide-decomposing antioxidants reduce the rate of chain tion by converting hydroperoxide, ROOH, into nonradical products Twomajor classes of the hydroperoxide-decomposing antioxidants are organicphosphite esters and sulfides During the reaction with hydroperoxide, thehydroperoxide is reduced to alcohol (ROH) and the phosphite and sulfideantioxidants are oxidized to phosphates and sulfoxides, respectively (Fig 8).Further transformation of the sulfoxides has been reported (9) In general,phosphites decompose hydroperoxides at substantially lower temperaturesthan the sulfides The sulfide antioxidants are active at temperatures exceed-ing 100jC but are not active at ambient temperatures (10)

initia-An example of commonly used phosphite antioxidants is phenol) phosphite 23 Its hydroperoxide-decomposing mechanism is depicted

tris(nonyl-in Figure 9 A stable trialkyl phosphoric acid ester 24 and an alkyl alcohol areformed

Thioesters are often used in combination with phenolics and are morewidely used in thermoplastics, where sulfur will not interfere in the vulcani-zation process (11) A mechanistic explanation of a thioester as a hydroper-oxide decomposer is shown inFigure 10with distearyl thiopropionate 25 as

an example A stable sulfoxide 26 of the thioester and an alkyl alcohol areformed Thermal decomposition of the sulfoxide 26 produces sulfenic acid 27and stearyl acrylate Oxidation of sulfenic acid 27 by hydroperoxide yields thesulfinic acid 28 The sulfenic acid 27 could also undergo a bimolecularreaction to produce thiosulfinate ester 29 (24)

Figure 8 Oxidation of phosphites and sulfides by hydroperoxide

Figure 9 Tris(nonylphenol)phosphite as hydroperoxide decomposer

III EXPERIMENTS

A Phenolic Antioxidants

Usually, phenolic antioxidants are used in white sidewall compounds due totheir nonstaining and nondiscoloring properties They are also used as thestabilization system for nonstaining SBRor other polymers along withphosphite antioxidants, which are hydroperoxide-decomposing antioxidants.Two examples are introduced in this section

1 White Sidewall Compound

One master batch for a white sidewall without an antioxidant and curatives(accelerators and sulfur) was prepared This master batch was finalized byadding only curatives (A-1-1) and a hindered bisphenol antioxidant withcuratives (A-1-2) respectively, which are presented inTable 1 Their Mooneyviscosity at 100jC, Mooney scorch at 132jC, unaged and aged physicalproperties, DeMattia flexing, and weatherometer tests were measured Theresults indicated that slight improvements in flex fatigue, color retention, andretention of physical properties was achieved with the addition of 1.5 phr of ahindered bisphenol antioxidant (Table 2) Therefore, tire manufacturers useFigure 10 Distearyl thiopropionate as a hydrogen peroxide decomposer shownwith certain transformation products

Table 2 Physical Properties of White Sidewall Compounds A-1-1 and A-1-2a

Weatherometer for 1 week, color Slightly yellow Very slightly yellow

a

See Table 1

Table 1 Recipe for White Sidewall Compounds

EPDM, ethylene propylene diene terpolymer; CIIR, chlorobutyl rubber; SP-1068,

alkyl phenol formaldehyde resin; TBBS, N-t-butyl-2-benzothiazole sulfenamide.

1–2.0 phr of hindered bisphenol in white sidewall and cover strip compounds

to ensure improved performance

2 Stabilization System for SBR

Styrene butadiene rubber latex (type 1502) was prepared for evaluatingphosphite in comparison with a blend of phosphite and hindered bisphenol.Antioxidants were added as an emulsion and coagulated by addition oflatex to Al2(SO4)3/H2SO4, whose pH was controlled to pH 3 for 30 min Thecrumb was washed twice at 50jC and dewatered on a two-roll mill The milledsheet was dried at 40–50jC The sample was molded to form a 1 cm thickplaque and oven aged at 70jC The Mooney viscosity at 100jC was measuredover a period of 4 weeks of aging, with the results as shown in Table 3 Theseresults indicated a slight improvement with the blend system However,longer aging is necessary to differentiate these two antioxidant systems Theblend system used phosphite and hindered bisphenol antioxidants in a 5:1ratio The phosphite antioxidant was tris(monononylphenol)phosphite(TNPP), and the hindered bisphenol antioxidant was 2,2-methylenebis(4-methyl-6-nonylphenol)

B Aromatic Amine Antioxidants*

Secondary aromatic amines such as TMQ, a high temperature reactionproduct of diphenylamine and acetone (BLE), and 4,4V-bis(a,aV-dimethyl-benzyl)diphenylamine (AO 445), produced by Crompton Corporation, arecommonly used as antioxidants in tire compounds These three antioxidantsnonetheless provide different antifatigue efficiencies for rubber compounds

Table 3 Aging test—Delta Mooney Viscosity at 100jC

AdditiveaTime

0.7 phrTNPP

0.7 phrTNPP/AO

This section reports the evaluation of these three antioxidants in carcasscompounds to explain the differences in performance.

1 Tire Casings and Other Internal Components

Tire body or carcass rubber compounds must form strong and durable bonds

to the coated fabric with resorcinol formaldehyde latex dipped solution.Usually, polyester, rayon, or nylon fibers are used to make cords for tirecasings, except for steel monoply truck tires Tire casing strength anddurability should be sufficient to insulate the tire cords and hold them inplace A tire casing compound must, however, be soft enough to permit aslight change in cord angles when the tire is flexed The body rubber serves asinsulation between the fabric plies Outstanding fatigue resistance and heat

Table 4 Tire Ply Compound

Source: Ref 12.

aging resistance are required of the carcass compounds in order to withstandcyclic deformation.

Currently, most tire companies use TMQ for protecting carcass pounds In this experiment, BLE, TMQ, AO 445, and a BLE/TMQ blend wereevaluated in an NR/BR/SBR carcass compound, which is presented inTable 4

com-To minimize experimental variations, one master batch without oxidants and curatives was prepared in a 10 L internal mixer Using the samemaster batch, antioxidants and curatives were added in a 1 L Banbury mixer.Mooney viscosity at 100jC, Mooney scorch, and Curometer at 177jC of allfive compounds were measured (Table 5) No significant differences wereobtained The results indicated that BLE, the BLE/TMQ blend, and AO 445would provide better flex fatigue properties, and TMQ and AO 445 the best

anti-Table 5 Physical Properties of Five Blendsa

DeMattia flex

Monsanto flex fatigue

Kilocycles to failure unaged Not tested 68.2 110.0 79.0 98.0

a Blend recipes given in Table 4 ML, minimum torque; MH, maximum torque.

Source: Ref 12.

heat protection The blend of TMQ and BLE is not only better in heat agingthan BLE alone but also improves flex fatigue over TMQ itself We would like

to explore the difference in the reaction mechanisms for the antifatigue andantioxidation A rationale is also proposed to explain why these three aminesperform differently under various conditions The chemical structures ofTMQ, BLE, and AO 445 are shown inFigures 11–13

The fatigue phenomenon of elastomers at room temperature is adegradation process caused by the shear of repeated mechanical stress underlimited access to oxygen The mechanical shear generates macroalkyl radicals(R
) A small fraction of the macroalkyl radicals react with oxygen to formalkylperoxyl radicals, with a high concentration of macroalkyl radicalsremaining Consequently, removal of the macroalkyl radicals in a catalyticprocess is the prevailing antifatigue process (Ref 1)

On the other hand, the macroalkyl radicals are rapidly converted toalkylperoxyl radicals by aging in air at oven temperatures The auto-oxida-tion propagated by the alkylperoxyl radicals thus dominates the degradationFigure 11 Structure of TMQ (polymerized 1,2-dihydro-2,2,4-trimethylquinoline)

Figure 12 Structures for major components of BLE (a high-temperature reactionproduct of (a) acetone and (b) diphenylamine)

process Therefore, removal of the alkylperoxyl radicals becomes the primaryfunction of an antioxidant.

It has been shown that the diarylamines (Fig 14, II) are good antifatigueagents and that diarylamine nitroxyl radicals (Fig 14, I) are even moreeffective than the parent amines (13) The antifatigue mechanism of the amineantidegradants shown inFigure 15has been proposed, with the formation ofthe intermediate nitroxyl radicals playing an active role (13) Generation ofthe nitroxyl radicals I from the free amines II is depicted in Figure 14 In thefatiguing process (Fig 15), macroalkyl radicals are generated [reaction (1)].Removal of the macroalkyl radicals by the nitroxyl radicals is shown inreactions (2) and (3) The resulting hydroxylamine III can be reoxidized byalkylperoxyl to regenerate the nitroxyl radicals in an auto-oxidation chain-breaking process [reaction (4)] The nitroxyl radicals I can be partiallyconverted back to the free diarylamine II during vulcanization through thereductive action of thiyl radicals of thiols [reaction (5)] The free diarylamine

II thus regenerated would repeat the steps shown in Figure 14 to form morenitroxyl radicals I

Reactivity of the nitroxyl radicals is affected by delocalization, stearichindrance, and substitutions (14) Delocalization of the unpaired electronFigure 13 Structure of AO 445 (4,4V-bis(a,aV-dimethylbenzyl)diphenylamine)

Figure 14 Formation of nitroxyl radical

into the aromatic ring increases the number of reactive sites and decreases itsstability Ring substitution at the para position reduces the side reaction of thenitroxyl radicals Stearic hindrance would obviously reduce the reactivity ofthe nitroxyl radicals as an antifatigue agent Thus, the stearic hindrance inthe TMQ-nitroxyl radical (TQM, polymerized 1,2-dihydro-2,2,4-trimethyl-quinone) (IVFig 16), reduces its reactivity in intercepting the macroalkylradicals (Example Eq 3); consequently, the TMQ-nitroxyl radical IV is lessefficient as an antifatigue agent than the less hindered diarylamine nitroxylradical I Thus the free amine precursors of the nitroxyl radical I would bemore efficient antifatigue agents than the free amine precursors of the nitroxylradical IV shown in Figure 16 Therefore, the diarylamines in general, such asBLE and AO 445, are more efficient antifatigue agents than TMQ Thisargument is in agreement with the data given in a previous report (13).Figure 15 Antifatigue mechanism.

Generation of the alkylperoxyl radicals is rapid during air–oven heataging of rubbers, in contrast to the fatiguing process The alkylperoxyl rad-icals propagate an auto-oxidation degradation process The auto-oxidationmechanism is depicted in Figure 1 Removal of the alkylperoxyl radicalsbecomes the primary function of an antioxidant (Fig 17).

Heat aging is conducted at elevated temperatures; volatility of theantifatigue agents often plays an important role in the heat aging process,

in addition to the reaction mechanism discussed above The molecular weight

of BLE (i.e., Mw390 and Mn230) is much lower than that of TMQ (i.e., Mw

820 and Mn560) (15) Apparently, BLE would be more volatile than TMQ.This is confirmed by a TGA (thermal gravimetric analysis) study, which givespercent weight loss for BLE at 45.3% compared to TMQ at 7.5% in 60 min at177jC A separate TGA study concluded that TMQ is more volatile thanN,NV-diphenyl-p-phenylenediamine (DAPD), which in turn is more volatilethan AO 445 (16) DAPD is an N,NV-diphenyl-p-phenylenediamine marketed

by Crompton Corporation Thus, the volatility of the antioxidants, indescending order, is BLE > TMQ > DAPD > AO 445 The loss due tovolatility would explain why after heat aging the antifatigue efficiency of BLEbecame only slightly better than that of TMQ even though BLE wassignificantly better than TMQ when unaged In addition, BLE may not be

as effective an antioxidant as TMQ for heat aging owing at least partly to thevolatility

Figure 17 Mechanism of antioxidant action (a simplified form)

Figure 16 Nitroxyl radicals The less sterically hindered nitroxyl radical I is morereactive than the more sterically hindered IV

Based on the antifatigue mechanism and the volatility, we came to thefollowing conclusion For fatigue protection of unaged rubber compounds,

AO 445 would be better than TMQ After heat aging the efficiency of the AO

445 would become much better than that of TMQ On the other hand, theantifatigue efficiency of the BLE would be better than that of TMQ forunaged rubber compounds, but the difference in the efficiency between BLEand TMQ would be reduced after heat aging

Therefore, the experimental results are correlated well with the posed mechanisms, such as types of molecular structures (less or morehindered nitroxyl radicals) and molecular weights The less hindered nitroxylradicals and lower molecular weight, such as BLE, would provide the best flexfatigue property, while the higher molecular weight AO 445 and TMQ wouldprovide better heat aging property

pro-2 Bead Filler

To confirm our proposed mechanisms for DAPD and NV-phenyl-p-phenylenediamine (6PPD), they were evaluated in bead fillercompound along with TMQ and BLE

N-1,3-dimethylbutyl-In both passenger vehicle and truck radial ply tires, a stiff lower sidewallconstruction is very important for handling performance The stiffnesscontrols the tire’s movement at elevated speeds to provide improved handlingand cornering The tire manufacturers continue to develop higher hardnessbead filler compounds for radial tires The current bead filler compounds arehighly filled with carbon black with increased cross-link density The highlyfilled bead filler compound will cause problems in mixing and extrusionbecause of its high Mooney viscosity Several resin manufacturers havedeveloped oil-modified phenol formaldehyde two-step resin (SP–6700) tomeet the tire manufacturers’ requirements such as lower Mooney viscosityand higher hardness The typical bead filler compound consists of 100%natural rubber, which requires protection against heat and flexing withantioxidants

Five batches were prepared using TMQ, BLE, DAPD, and 6PPD alongwith a blank compound Ten parts per hundred rubber (10 phr) of SP6700resin was added to 100% NRcompound as listed in Table 6 Mooneyviscosity at 100jC and Mooney scorch value at 132jC were determined.Curometer at 177jC and unaged and aged physical properties were deter-mined, and unaged and aged DeMattia testing was run The compounds werecured 10 min at 177jC, which simulated vulcanization of passenger car radialtires There were no significant differences in Mooney viscosity, Mooneyscorch, and unaged physical properties (Table 7) However, significantimprovement of unaged DeMattia flex was obtained by the addition of

BLE, DAPD, or 6PPD (Table 7, and heat aging properties were improvedwith less volatile antidegradants such as DAPD, 6PPD, or TMQ Theseresults also agree with our proposed mechanisms of antioxidants.

3 Hydroperoxide-Decomposing Antioxidants

Phosphite antioxidants are usually used for nonstaining polymer stabilizationsystems along with phenolic antioxidants and sulfides because these antiox-idants are reduced from ROOH to RH or ROH, which is more stable CibaGeigy developed 2,4-bis[(octylthio)methyl]-o-cresol (CG 1520) for a polymerstabilization system (17) In this section phosphite, a phosphite-phenolicblend, and CG 1520 are evaluated in cis-BRand SBR, respectively

Stabilization System for cis-BR Butadiene cement was diluted 1:1 byvolume in n-hexane and coagulated with water at 80jC by adding emulsifiedstabilizers It was dewatered and dried at 40jC The dried crumb was thencompression molded at 85jC for 10 min Then the pressed samples were aged

at 80jC until the formation of gel reached 2% or higher

Table 6 Bead Filler Compounda

SP-6700, oil-modified phenol formaldehyde

two-step resin; SP-1068, alkylphenol

form-aldehyde resin; M3P, 1-aza-5-methylol-3,

The results indicated that the blends of phosphite and phenolic oxidants or CG 1520, which has both phenolic and sulfide antioxidantfunctions, are much better than phenolic antioxidants (shown inFig 18).Stabilization System for SBR The same procedure as described inSection A-2 was adopted for evaluating phosphite, blends of phosphite–phenol antioxidants, and CG 1520 The results are shown inTable 8 The

anti-Table 7 Physical Properties

DeMattia flex test

Source: Ref 12.

Figure 18 Stabilization of low-cis polybutadiene and the influence of antioxidants

on oven aging performance CG 1520, 2,4-bis[(octylthio)methyl]-o-cresol; BHT, butyl-4-methylphenol; TNPP, tris(mono- and dinonylphenyl)phosphite Scale at left

di-t-is for days aging at 80jC until a 2% gel di-t-is formed (From Ref 18.)

Table 8 Aging Test—Change in Mooney Viscosity at 100jC

AdditiveTime

0.7 phrTNPP

0.7 phrTNPP/AO

results indicated that CG 1520 is superior to both tris(mono- anddinonylphenyl) phosphite (TNPP) and its blend with 2,6-di-t-butyl-4-methylphenol (BHT).

The mechanism for both phenolic and sulfidic activity is shown in Figure

19 First a sulfoxide is formed by reaction with ROOH to reduce to ROH,which is more stable Sulfoxide is a strong antioxidant that reacts with ROOH

Figure 19 Mechanism of hydroperoxide decomposition by CG 1520