chemical degradation of crosslinked ethylene-propylene-diene rubber in an acidic environment. part...

Polymer Degradation and Stability 91 (2006) 81e93

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Chemical degradation of crosslinked ethylene-propylene-dienerubber in an acidic environment. Part II. Effect of peroxide

crosslinking in the presence of a coagent

Susanta Mitra a,*, Afshin Ghanbari-Siahkali a, Peter Kingshott a,Helle Kem Rehmeier b, Hans Abildgaard c, Kristoffer Almdal a

a The Danish Polymer Centre, Risø National Laboratory, Building 124, PO Box 49, Frederiksborgvej 399, DK-4000 Roskilde, Denmarkb Research Department, Grundfos Management A/S, Poul Due Jensens Vej 7, DK-8850 Bjerringbro, Denmark

c AVK Gummi A/S, Gl. Arhusvej 1, DK-8670 Lasby, Denmark

Received 8 February 2005; received in revised form 15 March 2005; accepted 28 April 2005

Available online 24 June 2005

Abstract

An investigation on the time-dependent chemical degradation of ethylene-propylene diene rubber containing 5-ethylidene-

2-norbornene as diene cured by peroxide crosslinking in the presence of a coagent in an acidic environment (20% Cr/H2SO4) hasbeen made. Two types of rubber, with comparable monomer composition, but having significant differences in molar mass andlevels of long chain branching were tested. Dicumyl peroxide and triallylcyanurate under similar conditions were used for curing the

rubbers. The molecular mechanisms of chemical degradation at the surface were studied using X-ray photoelectron spectroscopyand attenuated total reflectance Fourier transform infrared spectroscopy, which demonstrate that several oxygenated species evolveduring exposure. The primary process of degradation is hydrolytic attack on the crosslink sites, which is manifested by a decrease in

crosslink density. The surface degradation is found to be strong enough to alter the bulk mechanical properties as observed by thechange in retention in tensile strength, elongation at break, modulus at 50% elongation and, the change in micro-hardness.Retention in modulus at 50% elongation is found to follow a negative linear correlation with decrease in crosslink density. Withhigher molar mass and level of long chain branching more crosslinking occurs and thus comparatively more hydrolytic attack

ensues. Scanning electron microscopy shows that the surface topography is significantly altered upon exposure and supports thenotion of the dependence of degradation on the crosslinking density of the samples. Importantly, the coagent used in this study isshown to enhance the chemical degradation through formation of weaker sites for hydrolysis. The results also show that upon

prolonged exposure the resulting oxygenated species tend to combine with each other.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Chemical degradation; ENBeEPDM; Rubber; XPS; ATR-FTIR; Crosslink density; Mechanical properties; SEM

1. Introduction

EPDM (Ethylene (E) propylene (P) diene (D)) rubberis one of the most useful synthetic rubbers for heat, light,ozone, and UV resistant applications. EPDM with ENB

* Corresponding author. Tel.: C45 46 77 5482; fax: C45 46 77 4791.

E-mail address: [email protected] (S. Mitra).

0141-3910/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymdegradstab.2005.04.031

(5-ethylidene-2-norbornene) as diene can be cured byboth accelerated sulphur and peroxide curing agents [1].However, several applications require more stringentparameters (heat resistant, compression set etc.), whichcan only be achieved through peroxide crosslinking [1].Curing by using peroxide alone often leads to unwantedchain scission due to the presence of tertiary hydrogen inthe main EPDM backbone (propylene unit) and com-petes with the desired crosslinking resulting in poor

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82 S. Mitra et al. / Polymer Degradation and Stability 91 (2006) 81e93

crosslinking efficiency [2]. Thus, the use of a coagent hasbecome increasingly popular in the peroxide curing ofrubber particularly for those containing considerableamounts of tertiary hydrogen in the main chain. Thecoagent is believed to participate directly in the cross-linking reaction through its reactive double bonds andreacts with the scission products to result in bettercrosslinking efficiency [1,3]. The participation of a co-agent suppresses the chain scission and forms crosslinkscontaining the domains of coagents [3]. Many types ofcoagents are commercially available, for example,triallylcyanurate (TAC), zinc diacrylate and dimetha-crylate, diallyl benzene, trimethylolpropane trimetha-crylate (TMPTMA), high vinyl polybutadiene and lowmolecular weight polybutadiene [4]. Research on the roleof the different coagents has emphasized mainly onimproving the curing efficiency vis-a-vis the mechanicalproperties and comparisons of the different moleculesused [4]. Several reports on the photo-, thermal andnatural ageing of the coagent cured EPDM in presenceor absence of peroxide are available [5e7]. Unfortunately,no studies on the use of the coagent from the viewpointof chemical degradation are available in the literature.So far, only one review exists where the behaviour of thecoagent in peroxide cured EPDM under geothermalenvironmental conditions was published [8].

The effects of different aqueous acidic solutions ontwo different kinds of pure uncrosslinked EPDM (calledE-1 and E-2) having significantly different molar massand levels of long chain branching particularly inpresence and in absence of an oxidising agent havebeen investigated [9,10]. The study revealed that 20%Cr/H2SO4 (CSA) had significant effect on the chemicaldegradation independent of the molar mass and level oflong chain branching of the pure rubber. Cr (VI),present in CSA participates in the degradation of pureEPDM rubber and attack on the allylic CeH bonds[9,10]. Furthermore, the aqueous acid solution attacksthe C]C of ENB and result in formation of severaloxygenated species on the surface [9,10]. We continuedto investigate further the effect of chemical degradationon cured EPDM using the same E-1 and E-2 on thebasis of the knowledge developed from these previousstudies [9,10]. In Part I of this study we tested thedegradation of E-1 and E-2 crosslinked with a sulphur/accelerator curing system. The results revealed that thehydrolysis of crosslinks was the primary process ofaqueous acid induced degradation with a noticeableeffect of molar mass and long chain branching [11]. Theaim of this part of the study is to investigate the stabilityof the peroxide crosslinking system in the presence ofa coagent under an identical degradation environmentalso with emphasis on the role of molar mass and thelevel of long chain branching.

An aqueous acidic environment i.e., CSA for differentperiods of time (1e12 weeks) was used. Triallylcyanurate

[2,4,6-tris(allyloxy)-s-triazine] (TAC) was used as a co-agent and dicumyl peroxide (DCP) as a peroxide curingagent. The structures of these two chemicals are shownin Scheme 1. X-ray photoelectron spectroscopy (XPS),and attenuated total reflectance Fourier transforminfrared (ATR-FTIR) spectroscopy were used forsurface analysis and to elucidate the interfacial chemicaldegradation mechanisms. Changes in the surface topog-raphy owing to chemical degradation were monitoredusing scanning electron microscopy (SEM). The effectof chemical degradation on the mechanical properties ofthe EPDM rubber was also monitored as a measureof the change in bulk properties. In addition, the effectof exposure media on the crosslinking was determinedthrough measuring crosslink density as a function ofexposure time.

2. Experimental

2.1. Materials

Two EPDM rubbers (E-1 and E-2) were used as purerubbers in this study (Part I). The details of the pureEPDM rubber characterisation are described elsewhere [9].

Compounding of both E-1 and E-2 was done usingrubber grade chemicals as per the recipe given in Table 1.Carbon black, anti-degradants and other necessaryingredients were deliberately withdrawn from the finalformulation to avoid complexity of the rubber modelstudied in this work. The compound was then cured for10 min at 170 �C using compression moulding to formw2 mm thick rubber sheet. Postcuring was done for 6 hmaintaining a temperature of 150 �C. The rubber sheetswere die cut to dumbbell shape tensile test specimenswith dimensions as per ISO 37:1994, which were thenexposed to the media under study.

Analytical grade Cr/H2SO4 (Merck e containingchromium (VI)-oxide 1e10% and sulphuric acidO50%) were volumetrically mixed with Millipore waterto make 20% solution (CSA) as an exposure media.

2.2. Ageing and characterisation methods

E-1P and E-2P samples were exposed to a solution ofCSA for various periods from 1 to 12 weeks. Table 2describes the nomenclature of different samples beforeand after exposure for different period of time. The agedsamples were taken out at appropriate time andsubsequently tested for mechanical properties viz.,tensile strength, % elongation at break, modulus at50% elongation and micro-hardness (IRHD).

The change in surface chemistry due to change inchemical functionalities of the samples was monitoredusing XPS and ATR-FTIR. XPS analysis was

83S. Mitra et al. / Polymer Degradation and Stability 91 (2006) 81e93

C6H5 C6H5C6H5 C6H5

C6H5

C O O C

CH3

CH3

CH3

CH2

CH2 CH2 CH2

CH2

CH2CH2

CH2CH2

CH2

CH2

CH2

CH2

CH2

CH2 CH2

CH2

CH3

CH3

CH3CH3 CH3

CH3

CH3 CH3.

CH3

C O. RHC OH + R.

C

O

+RH C

H

R. C.

R. + CH N CHN

CH

N

O

OO

RH + CH CH N CHN

CH

N

O

OO

. R.

CH CH N CHN

CH

N

O

OO

Rand so forth

N N

N

O

OO

EPDM crosslink with TAC domain

Triallyl Cyanurate (TAC)

EPDM backbone withteriary hydrogen

EPDM macro radical

Dicumyl peroxide (DCP)Cumyl acohol

Acetophenone

CH CHCH3C

Scheme 1. The crosslinking reaction of EPDM rubber results in crosslinks with TAC domains.

performed on all samples using a Sage 100 (SPECS,Berlin, Germany) with unmonochromated Mg Ka X-raysource operated at a pressure of !10�7 Torr. Detailsof the techniques and experimental parameters aredescribed elsewhere [9]. High-resolution XPS analysiswas performed on the carbon 1s region (C1s) and thespectra were deconvoluted by curve fitting usinga 100% Gaussian component and linear backgroundsubtraction that relied on least squares minimizationroutine in the software. The full width at half

maximum (FWHM) for all peaks was constrained to2.0 eV. The binding energies were corrected byreferencing to the hydrocarbon component (CeC/CeHx) at 285.0 eV [9,12].

A Perkin Elmer Instruments (Spectrum One) wasused for performing the ATR-FTIR measurements. Theexperiments were conducted with 32 scans and a resolu-tion of 4 cm�1.

The crosslink density was evaluated as per the FloryeRehner equation [13]. The details of the experimental


techniques and parameters are described in the previouspaper and equally hold well for this work [11].

Mechanical testing were conducted on the Instronuniversal test machine (Model 4303) as per ISO 37:1994.AWallacemicro indentation tester was used to determinemicro-hardness on the surface as per ISO 48:1994.

Topographical changes on the surface before andafter exposure were examined by an SEM (Zeiss DSM-960). The samples were coated with thin layer of gold toincrease the image contrast and to reduce the chargingeffect prior to SEM imaging.

Control samples (i.e., unexposed) of E-1P and E-2Pwere also characterised simultaneously under similarconditions for comparison purpose.

3. Results

3.1. XPS

XPS furnishes quantitative elemental and chemicalinformation of the outer 10 nm of the surface. The

Table 1

Compounding recipe of the EPDM rubbers used for the present study

Compounding

ingredients

Chemical

name

Phr

(parts per

hundred parts

of rubber)

Function

E-1/E-2 EPDM 100.0 Rubber

DCP Dicumyl peroxide

(40% active content)

6.0 Curing agent

TAC Triallylcyanurate

(70% active content)

2.0 Coagent

ZnO Zinc oxide 3.0 Curing activator

St. A Stearic acid 0.5 Curing activator

Total 111.5

oxygen to carbon ratio (O/C) for the E-1P and E-2Psamples as a function of exposure times is shown inFig. 1. The presence of oxygen in controlled samples(w8.1e8.6%) is probably due to the curing reactionsbetween EPDM, DCP and TAC in presence of ZnO andSt. A (Scheme 1 and Table 1). The change in O/C ratiofollows a similar trend with exposure time for both E-2P/CSA and E-1P/CSA exposed samples. It increases upto 7 weeks and reaches a maximum for both samples,which is three to four times higher than the control (upto 0.27 and 0.35, respectively, compared to 0.09). At 12weeks exposure, the O/C ratio decreases significantlyand attains a value similar to the 1 week exposedsamples that is; the ratios for E-1P/CSA/12 and E-2P/CSA/12 are 0.18 and 0.21, respectively. This suggests thepossibility of combination reactions of the oxygenatedspecies generated out of chemical degradation uponprolonged exposure.

Fig. 1 also shows relatively more changes in O/C forE-2P (high molar mass and higher level of long chainbranching) in comparison to that of the cured low molarmass EPDM (i.e., E-1P). But our previous study on thesame acid induced chemical degradation of the un-crosslinked pure E-2 and E-1 revealed the almostidentical behaviour of O/C as a function of exposuretime upon exposure to CSA irrespective of molar massand level of long chain branching [10]. Thus thedifferences in O/C with exposure time for differentrubbers observed in the present study is attributed to thedifferent extent of crosslinking reactions. High molarmass rubber increases crosslinking efficiency in compar-ison to the low molar mass material despite same recipeand identical curing conditions [13]. Information onperoxide crosslinking of EPDM in the presence of TACshows that most of the crosslinks forms through TACdomains (Scheme 1) [3]. Hydrolysis of crosslinks by

Table 2

Nomenclature of the EPDM rubbers for the exposure media and for different time of exposure

EPDM

type

Crosslinked

by peroxide/

coagent

Name of

exposure

media

Abbreviated

form for

exposure

media

Abbreviated

form for

sample/

exposure

media

Exposure

time

(weeks)

Abbreviation for the

ageing samples after

a particular

exposure time

E-1 P Unexposed

(controlled)

E-1P 0 E-1P

E-1 P 20% Chromo

sulphuric

acid (Cr (VI)/

H2SO4)

CSA E-1P/CSA 12 E-1P/CSA/12

E-2 P Unexposed

(controlled)

E-2P 0 E-2P

E-2 P 20% Chromo

sulphuric

acid (Cr (VI)/

H2SO4)

CSA E-2P/CSA 12 E-2P/CSA/12


CSA associated with TAC could lead to formation ofoxygenated species at the surface as observed for theincrease in O/C ratio with exposure time, especially upto 7 weeks. It is plausible to suggest that more hydrolysisreactions at the surface occur because of the higheramount of available crosslinks. Thus, the E-2P showsa higher O/C in comparison to E-1P under the identicalconditions.

At 12 weeks of exposure, the oxygenated species mostlikely combine. This process is expected to be preferen-tially higher in E-2P/CSA as the rate of combinationreaction is a function of both molar mass andconcentration of the oxygenated species. This could bethe reason for a relatively more visible downward trendin O/C for E-2P/CSA compared to E-1P/CSA after 7weeks of exposure. This postulate was tested by plottingthe nitrogen to oxygen ratio (N/O) as a function ofexposure time for both rubbers and is shown in Fig. 2.With a given %N, the %O increases due to acid inducedhydrolysis of crosslinks and thus N/O decreases withexposure time as shown in Fig. 2. A significant decreaseis noticed for both the rubbers after 1 week of exposurefollowed by a minor decrease upon prolonged exposure.A similar feature is also noticed for O/C as a function ofexposure time (Fig. 1).

High-resolution XPS analysis was performed toobtain more detailed information about the chemicalchanges occurring in the E-1P/CSA and E-2P/CSAsamples. Deconvolution of both control and exposedsamples for the C1s region was undertaken by curvefitting. The curve fitting helps to determine precisely thechanges in the chemical environment of the carbonatoms. Normalized C1s spectra of E-1P and E-2P forboth control and the 12 weeks exposed samples areshown in Fig. 3. Table 3 shows a summary of the resultsof the curve fitting. The C1s spectrum for the E-1P andE-2P control samples can be fitted to 3 componentscorresponding to the CeC/CeH (285.0 eV), CeO/CeN(286.2e286.3 eV) and NeC]O/C]O (288.3 eV), re-spectively [12]. The assignment of nitrogen was included

0 2 4 6 8 10 120,080,100,120,140,160,180,200,220,240,260,280,300,320,340,360,38

E-1P/CSA E-2P/CSA

Exposure time, week(s)

O/C

Rat

io

Fig. 1. Change in O/C ratio with time for E-1P and E-2P samples

before and after exposure to 20% Cr (VI)/H2SO4.

considering a small contribution because of the presenceof N (w1.6e2.5%). The amount of CeO/CeN (w15%)and NeC]O/C]O (w4e5%) in the control samples isattributed to the crosslinking reactions of E-1 and E-2with TAC and DCP in the presence of ZnO and St. A(Scheme 1). Table 3 shows a significant change in thechemical functionality surrounding the carbon atomsafter exposure to CSA. The most significant changes arethe evolution of the C]O (287.7e287.9 eV) and the OeC]O (289.2e289.4 eV) [12] components after 12 weeks.These are associated with hydrolysis of crosslinks [11]and/or attack of aqueous acidic medium on the allylicCeH and/or C]C of ENB [9e11]. This is consistentwith the changes in the atomic ratios (Figs. 1 and 2).

3.2. ATR-FTIR

ATR-FTIR spectra for the control E-1P and E-2Pare almost identical as shown in Fig. 4. The bands at2925 and 2855 cm�1 are assigned to asymmetric and

0 2 4 6 8 10 12

0,08

0,10

0,12

0,14

0,16

0,18

0,20

0,22

0,24

0,26

0,28

0,30

E-1P/CSAE-2P/CSA

N/O

Rat

io


Fig. 2. Change in N/O ratio with time for E-1P and E-2P samples

before and after exposure to 20% Cr (VI)/H2SO4.

0,0

0,2

0,4

0,6

0,8

1,0

E-2P E-2P/CSA/12

292 290 288 286 284 282

292 290 288 286 284 282

0,0

0,2

0,4

0,6

0,8

1,0

N-C=OO-C=O

C=O

C-O/C-N

C-C/C-H

Binding Energy, eV

Nor

mal

ized

Int

ensi

ty (a

rbit

rary

uni

t)

E-1P/CSA E-1P/CSA/12

Fig. 3. Normalized high-resolution C1s spectra of E-1P and E-2P

samples both control and 12 weeks exposed to 20% Cr (VI)/H2SO4.


Table 3

Summary of the XPS results from the curve fitting of the high resolution C1s spectra of the E-1P and E-2P samples before and after exposure to 20%

Cr (VI)/H2SO4 solutions for 12 weeks

Sample Peak

A (eV)

Structure Peak B

(eV)

Structure Peak C

(eV)

Structure Peak D

(eV)

Structure

E-1P 285.0 CeH/CeC 286.2 CeO/CeN 288.3 C]O/NeC]O e e(81.6%)a (14.9%) (3.5%) e e

E-1P/CSA/12 285.0 286.2 CeO/CeN 287.7 C]O 289.2 OeC]O

(78.4%)a (14.7%) (4.7%) (2.2%)

E-2P 285.0 CeH/CeC 286.3 CeO/CeN 288.3 C]O/NeC]O e e(80.2%) (15.2%) (4.6%) e e

E-2P/CSA/12 285.0 286.4 CeO/CeN 287.9 C]O 289.4 OeC]O

(77.7%) (12.5%) (5.0%) (4.8%)

a Values within parentheses indicate the respective area percentages under the assigned peak.

symmetric stretching vibrations of CH2 present in theEPDM backbone [9,10,14]. Bands at 1740 and1717 cm�1 are either due to Ne(C]O)eN from cross-linking associated with TAC domains [14], or due to theformation of acetophenone by dissociation of DCPduring curing reactions (Scheme 1). Similarly, formationof cumyl alcohol through dissociation of DCP can alsoaccount for the very broad yet weak band at 3450 cm�1

(Scheme 1). The band at 1590 cm�1 is attributed to thequadrant stretching of TAC present as a part of theEPDM crosslinked domain [14]. Bands around 1463 and1378 cm�1 are assigned to eCH2e scissoring vibrationsand symmetric CeH stretching vibrations of eCH3

from the propylene unit, respectively [10,11,14]. Thebands at 1167, 1091 and 875 cm�1 are due to CeNeC,NeCH2 and C]C (of allylic moiety) stretchingvibrations, respectively, and arise from the TACdomains associated with crosslinks [14] (Scheme 1).The band at 722 cm�1 is typical of e(CH2)ne where,nR 5, for eCH2e rocking vibrations due to presence ofsequences of ethylene in the EPDM backbone [10,11,14].

Fig. 5 represents the difference spectra, in absoluteabsorbance scale (2000e700 cm�1), for the 12 weeksexposed E-1P and E-2P samples, after subtracting thecontrol spectra. A group of several new bands emerge in

0,01

0,02

0,03

0,04

0,05

1740

1091 875

722

1717

1590

1378 1167

1463

3450

2855

2925E-2P

0,06

0,010,020,030,040,050,06 E-1P

30004000 10002000

Abs

orba

nce

(Arb

itra

ry u

nit)

Wave numbers (cm-1)

Fig. 4. ATR-FTIR spectra (3800e685 cm�1) of E-1P and E-2P control

samples.

the spectra (Fig. 5) as a consequence of chemicaldegradation that appear at 3250 cm�1 (hydroxyl, OHand/or amide NH2 stretching) [not shown in the Fig. 5],1735 cm�1 (carbonyl, C]O), 1714 cm�1 (nonbondedamide and/or imides C]O), 1624 cm�1 (amide, NH2

deformation), 1550 cm�1 (carboxylate, COO� and/oramide, CeN stretching/CeNeH in plane bending),1363 cm�1 (tertiary alcohol OeH bending), 1203 cm�1

(CeOeC from ethers and esters), 1114 cm�1 (CeOeCfrom ethers and esters as well as eNH2 rocking), and1071/1020 cm�1 (CeOeC from ethers and esters andalso due to COO�) [14]. However, the decrease inintensity of 1463 cm�1 is not due to any changeassociated with eCH2e present in EPDM backbone,rather due to the change in R3N (where, R is the allylicgroup) associated with TAC [14]. Similarly, the decreasein intensity of 875 cm�1 is associated with the change inallylic moiety of the TAC attached to the crosslink sitesthrough rubber chains. All the above changes canplausibly be depicted as the initiation of the attack byaqueous acid taking place at the NeC(]O)eN linkages

1114

1735

10001500

1463

1071

875

1020 1203

13631550

1624

1714

700

(a)

(b)

2000

Abs

orba

nce

(Arb

itra

ry U

nit)

Wave numbers (cm-1)

Fig. 5. ATR-FTIR difference spectra (2000e700 cm�1) of (a) E-1P/

CSA/12 and (b) E-2P/CSA/12.


present in the crosslinks with TAC domain. Thissubsequently forms primary amides with a detachmentof a rubber chain containing carbonyl end groups asa consequence of hydrolysis. This reaction continuesfurther in a similar manner upon prolonged exposure.This postulation on hydrolysis can account for theformation of several characteristic primary amidegroups and carbonyl groups along with the decrease ofthe C]C and R3N in TAC moiety present in thecrosslinks. The changes in chemical functionalities inATR-FTIR support our XPS observations.

In addition to complementing the XPS results, theATR-FTIR observations also demonstrate that hydro-lysis of crosslinks associated with TAC, together withattack of the C]C bonds of ENB by CSA, result inseveral oxygenated species formed at the surface, whichundergo combination reaction upon prolonged exposure.

3.3. Crosslinking density measurements

The crosslink densities (1/2MC, mol/g) for the E-1Pand E-2P samples are plotted as a function of exposuretime before and after exposure to 20% CSA (Fig. 6).The control sample of E-2P is 67% higher than E-1Pdespite the curing being accomplished under identicalconditions. This is due to the effect of higher molar mass[13,15,16] and long chain branching on curing [16] asdescribed in the previous paper [11].

For both E-1P/CSA and E-2P/CSA, the crosslinkdensities decrease up to 7 weeks of exposure, and thenincrease at 12 weeks exposure. The estimated reductionin crosslink density E-1P and E-2P at 7 weeks is about22% and 24%, respectively. ‘‘De-crosslinking’’ throughhydrolysis of the crosslink sites is the primary process ofthis particular chemical degradation. At 12 weeks,however, the increase in crosslink density results from

combination reaction of the oxygenated species. Theobservations of de-crosslinking up to 7 weeks andcombination thereafter at 12 weeks are fully supportedby XPS and ATR-FTIR results.

3.4. Mechanical properties

The changes in tensile strength, modulus at 50%elongation and elongation at break were estimated asthe (%) retentions with respect to the control samples.Retention in tensile strength, modulus at 50% elonga-tion and % elongation at break are plotted as a functionof exposure time in Fig. 7(aec), respectively. The tensilestrength is a complex function of the nature and type ofcrosslinks, crosslink densities, the chemical structure ofthe rubber, and the changes associated with degradation[17]. The peroxide crosslinking results in a heat andcompression set resistant crosslinks at the consumptionof tensile properties. Usage of coagent in the peroxidecrosslink increases the curing efficiency to a significantextent and thus modulus increases considerably andelongation at break decreases considerably with little orno effect on the tensile strength [4]. Upon de-cross-linking, the tensile strength would be expected toincrease and thus we have observed increase in tensilestrength with exposure time for both E-1P/CSA andE-2P/CSA. At 7 weeks of exposure, the increase in tensilestrength for E-2P/CSA (w23%) is higher than that ofE-1P/CSA (w16%), which could be attributed to thehigher extent of hydrolysis reactions because of thepresence of relatively more crosslinks in E-2P incomparison to E-1P. The modulus at lower elongationof a crosslinked rubber is directly proportional to thecrosslinked density. Thus the modulus at 50% elonga-tion was found to decrease for both E-2P/CSA andE-1P/CSA with an increase in exposure time, as a

120 2 4 6 8 101,5x10-4

1,6x10-4

1,7x10-4

1,8x10-4

1,9x10-4

2,0x10-4

2,1x10-4

2,2x10-4

E-1P/CSA

Cro

ss-l

ink

dens

ity,

1/2

MC

(m

ol /g

)


2,6x10-4

2,8x10-4

3,0x10-4

3,2x10-4

3,4x10-4

3,6x10-4

E-2P/CSA

Cross-link density, 1/2M

C (m

ol/g)

Fig. 6. Change in crosslink density of E-1P and E-2P samples before and after exposure to 20% Cr (VI)/H2SO4.


manifestation of hydrolysis of crosslinks especially up to7 weeks of exposure. The decrease for both samples isquite substantial (30e31%) after 7 weeks. However, at12 weeks, the modulus retains 31e32% of the 7 weeksvalue owing to the combination reaction of oxygenatedspecies. The extent of retention in E-2P/CSA is higherthan that of E-1P/CSA as more oxygenated species aregenerated through more de-crosslinking and thus resultsin relatively more probable combination reactions.

0 2 4 6 8 10 12

98100102104106108110112114116118120122124126

E-1P/CSAE-2P/CSA




0 2 4 6 8 10 12

0 2 4 6 8 10 12

65

70

75

80

85

90

95

100

E-1P/CSAE-2P/CSA

E-1P/CSAE-2P/CSA

98100102104106108110112114116118120122124126128130132134

Ret

enti

on o

f

el

onga

tion

at

brea

k

Ret

enti

on o

f T

ensi

le s

tren

gth

R

eten

tion

of

Mod

ulus

at

50

el

onga

tion

(a)

(b)

(c)

Fig. 7. (a) % Retention of tensile strength of E-1P and E-2P samples

exposed to 20% Cr (VI)/H2SO4 at various exposure times, (b) %

retention of modulus at 50% elongation of E-1P and E-2P samples

exposed to 20% Cr (VI)/H2SO4 at various exposure times, (c) %

retention of % elongation at break of E-1P and E-2P samples exposed

to 20% Cr (VI)/H2SO4 at various exposure times.

As a consequence, the % elongation at break increasesconsiderably with increasing exposure time for bothE-1P/CSA and E-2P/CSA samples up to 7 weeks wherefor E-1P/CSA it is w21% and for E-2P/CSA it is 36%.Since the modulus at 50% is retained upon combina-tion, the elongation at break for E-2P/CSA/12 andE-1P/CSA/12 decrease significantly (by 13% and 8%,respectively) compared to the values at 7 weeks. Fig. 8represents a negative linear correlation between decreasein crosslink density and retention of modulus at 50%elongation for both E-1P/CSA (R2Z 0.8551) and E-2P/CSA (R2Z 0.8833). This indicates directly that de-crosslinking and/or combination reactions are the majorgoverning factors in altering the mechanical propertiesas a function of exposure time in the studied environ-ment. Micro-hardness measurements verify further ournotion on the chemical degradation [18]. A considerabledecrease in micro-hardness is observed at 7 weeks forboth E-1P/CSA and E-2P/CSA due to hydrolysis ofcrosslinks. However, at 12 weeks a significant increasedue to combination reaction of oxygenated speciesresulting in new crosslinking is also noticeable (Fig. 9).The chemical degradation at the surface is thus found tobe quite significant and has a considerable effect on thebulk properties of the material.

3.5. SEM

Fig. 10 shows the SE micrographs for the samplesbefore and after 12 weeks exposure to CSA. Interest-ingly, it can be observed from the micrographs of thecontrol samples of E-1P and E-2P (Fig. 10(a) and (c))that the surface structures are different, and this isattributed to the fact that E-2P has considerably highercrosslinking density (w67%) than E-1P.

The apparently regular and smooth surfaces becomevery irregular due to surface damage at 12 weeks

12 220 2 4 6 8 10 14 16 18 20 24

70

75

80

85

90

95

100

E-1P/CSA (R2=0.8551)

E-2P/CSA (R2=0,8833)

Decrease in Cross-link density

R

eten

tion

in M

odul

us a

t 50

E

long

atio

n

Fig. 8. Study on linear correlation between % retention of modulus at

50% elongation and % decrease in crosslink density for E-1P/CSA and

E-2P/CSA samples at different exposure time.


exposure to CSA (Fig. 10(b) and (d)). For E-1P/CSA/12the damage can be seen as sporadic presence of holes(Fig. 10(b)). However, more profound surface damage, asevidenced by crack formation coupled with small holesis noticeable in Fig. 10(d) occurs to the E-2P/CSA/12.

0 2 4 6 8 10 12-7

-6

-5

-4

-3

-2

-1

0

1

2

3

4

E-1P/CSAE-2P/CSA


Cha

nge

in m

icro

har

dnes

s (I

RH

D)

Fig. 9. Change in micro-hardness (IRHD) of E-1P and E-2P samples

before and after exposure to 20% Cr (VI)/H2SO4 at various exposure

times.

This supports our postulate that hydrolysis of crosslinksleading to de-crosslinking is the predominant degrada-tion mechanism. So, the higher the crosslinking density,the higher is the extent of degradation and thus E-2P/CSA/12 shows relatively more noticeable changescompared to E-1P/CSA/12.

The changes in the chemical functionality at thesurface owing to degradation and that affect the bulkproperties, are therefore attributed to the severe physicalsurface damages as observed in the SEM study.

3.6. Reaction mechanisms for degradation process

The extent and nature of crosslinking of EPDM byDCP in the presence of TAC clearly plays a role in thedegradation and needs to be discussed to appreciate themechanisms behind the degradation. Direct participa-tion of TAC results in crosslinks with the TAC domainsas shown in Scheme 1. In addition to these types ofcrosslinks, the crosslinking through EPDM macroradical and C]C bonds of ENB can also lead to CeCcrosslinks as shown in Scheme 2. The plausible

Fig. 10. SEM images of (a) control E-1P, (b) E-1P/CSA/12, (c) control E-2P, and (d) E-2P/CSA/12.


mechanisms of aqueous acid induced chemical degrada-tion of EPDM cured by DCP and TAC are shown inSchemes 3 and 4. The crosslinking of EPDM by TACand DCP results in crosslinks with TAC domains andCeC linkages [8]. The degradation in an aqueous acidicenvironment initiates at the weakest links [Ne(C]O)eN]and finally leads to de-crosslinking via hydrolysis ofTAC sites [8]. This often results in primary amide groupformation coupled with macro functionalities contain-ing carbonyl and/or carboxyl groups (Scheme 3). Inaddition, the presence of C]C of ENB, which are notfully consumed in the crosslinking reactions [2,3,15,16]also provides a site for attack by CSA and results inoxygenated species in the form of OH, C]O andCOOH [9,10]. Upon prolonged exposure, macro oxy-genated species combine with each other and forms newcrosslinkages either in the form of ester or ether linkages(Scheme 4). In fact, the de-crosslinking and combinationreaction could compete with each other through out thedegradation process [19].

The effect of molar mass and level of long chainbranching of the rubber is quite significant in thecrosslinking process and thus emerges as one of thegoverning factors of the chemical degradation. Morethe number of crosslinks more would be the hydrolysisand higher is the extent of chemical degradation.

The entire plausible mechanism can be summarised asfollows:

1. Hydrolysis of crosslinks with TAC domains, wherethe evidence comes from a decrease in crosslinkdensity, XPS results of N/O and deconvolution ofC1s spectra, ATR-FTIR study and the correlationstudy between decrease in crosslink density andretention of modulus at 50% elongation.

2. Attack by aqueous acid solutions on C]C bonds ofENB present in EPDM as diene, where the evidencecomes from the XPS study, and the presence ofhydroxyl, carbonyl and carboxylic derivatives asshown in the ATR-FTIR study.

3. Combination reaction of oxygenated species uponprolonged exposure results in new crosslinking asevident from the XPS (O/C), ATR-FTIR studies(presence of CeOeC and normalized absorbancearea) and change in the trend in mechanicalproperties after 12 weeks of exposure together withincrease in crosslink density at 12 weeks.

The comparison between Part I [11] and Part IIreveals that peroxide crosslinking in presence of a co-agent is not always better than accelerated sulphurcrosslinking. It is therefore, extremely important tochoose the right coagent for the peroxide crosslinking toobtain more hydrolytically stable CeC linkages. Theright choice of pure rubber grade and proper dosages ofaccelerated sulphur crosslinking sometimes yield muchbetter results in terms of its ability to resist chemical

C

CH3

CH3

CH3

CH3

CH3

CH2.

CH3

CH3

CH3 CH3

CH2

CH3

H

C.

C

H

C

C

H

C

H

.

Crosslinked EPDM through ENB

Scheme 2. The crosslinking reactions of EPDM macro radical that form crosslinks with C]C of ENB.


N N

N

O

OO

CH2

CH2

CH2 CH2

CH2

CH2

CH2CH2

CH2 CH2

CHCH

H3O +N NH

NH

O

O

CHCH

N NH

N

O

O

CH

N NH

NH2

NH2

NH2

O

O

CHO +

CO

OH

+N

O

O

+ CHO

Cr(VI)

Scheme 3. The de-crosslinking reactions take place at the TAC crosslink site of the EPDM rubber.

degradation compared to peroxide crosslinked material.Using a coagent for peroxide curing, which leavesweaker sites for comparatively easier hydrolytic attack isnot desirable for chemical resistance purposes. It is alsoequally important to optimise the crosslinking density ofa rubber vulcanizate for chemical application.

4. Conclusion

Aqueous acid induced chemical degradation ofperoxide cured EPDM in the presence of TAC ascoagent has been studied for different periods of time.The conclusion of the present work is as follows:

1. EPDM cured by peroxide and TAC undergoessubstantial chemical degradation upon exposure to20% Cr (VI)/H2SO4 solution. The degradationinitiates at the crosslinks with TAC domains andproceeds via de-crosslinking through hydrolysis ofthe cured site. In addition, attack by aqueous acid atthe C]C of ENB present in the EPDM is equallyplausible. The resulting oxygenated species on thesurface combine upon prolong exposure. The extentof surface degradation is found sufficiently strong toaffect the bulk properties. EPDM backbone is quite

stable under the studied chemical degradationenvironment.

2. Higher molar mass and higher level of long chainbranching leads to higher extent of chemicaldegradation as it yields more crosslinking underidentical situation in comparison to lower molarmass EPDM with lower level of and/or no branch-ing. Thus selection of grade of pure rubber is animportant criterion for the chemical degradation.The choice of coagent in peroxide crosslinkingsystem is also one of the most important criteriafor obtaining the hydrolytic stability in the chemicaldegradation environment.

Acknowledgement

The authors wish to acknowledge the financialsupport from MONEPOL (Danish Centre for theStudy of Polymer Degradation and Stability) a DanishResearch Agency funded centre contract (J.nr. 2000-603/4001-51). The authors are grateful to Dr. KeldWest at the Danish Polymer Centre, Risø NationalLaboratory, Denmark, for his generous help withSEM analysis. We also acknowledge the help from


OH

C

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3 CH3

CH3

CH3

CH3

CH3

CH3

CH3CH3 CH3

C

H

CH2

CH2

CH2

CH2CH2

CH2

CH2

CH2

CH2

CH2

CH2

C

C

H

O

C

C

H

C

C

H

O

C

H

C

C

O

CO

OH

Attack on C=C

Attack on C=C

Cr(VI)/H3O+

Cr(VI)/H3O+

OH

C

C

H

OH

C

C

H

OH

O

Scheme 4. The reactions of aqueous acid with C]C of ENB of the EPDM rubber and combination reactions upon prolonged exposure.

Anders G. Christensen for preparing the rubbersamples. The authors would also like to convey theirsincere thanks to AVK Gummi A/S, Denmark, forproviding with the rubber samples and the ResearchDepartment, Grundfos Management A/S, Denmark,for conducting the necessary tests and allowing us topublish the results.

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chemical degradation of crosslinked ethylene-propylene-diene rubber in an acidic environment. part...

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