Photo- and thermo-oxidative degradation of photocrosslinkedethylene–propylene–diene terpolymer§

Weizhi Wang, Baojun Qu*

State Key Laboratory of Fire Science and Department of Polymer Science and Engineering,

University of Science and Technology of China, 230026 Hefei, Anhui, PR China

Received 6 March 2003; accepted 11 April 2003

Abstract

Photocrosslinking of ethylene–propylene–diene terpolymer (EPDM), and photo- and thermo-oxidative degradation of photo-crosslinked EPDM have been studied by photoacoustic Fourier transform infrared spectroscopy (PAS-FTIR), scanning electronmicroscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The relative ratios of degradation to crosslinking for photo-

crosslinked EPDM by swelling measurement were estimated by Charlesby’s random crosslinking theory to be 0.06 for EPDM 4770and 0.1 for EPDM 4045. The PAS-FTIR and XPS data gave the evidence that the surface photo- and thermo-oxidation degrada-tion of photocrosslinked EPDM samples with a given UV irradiation time apparently increase with aging time. The main photo-

oxidation products were identified as hydroperoxides and various carbonyl compounds. The SEM measurements show that thephoto-oxidative degradation of photocrosslinked EPDM has a crucial effect compared with thermo-oxidative degradation.# 2003 Elsevier Ltd. All rights reserved.

Keywords: Photo-oxidation; Thermo-oxidation; Degradation; Photocrosslinking; EPDM

1. Introduction

Ethylene–propylene–diene terpolymers (EPDM) asthe fastest growing elastomers on the markets are widelyused in outdoor applications [1,2], because they aremore stable than other conventional elastomers, suchas butadiene and isoprene rubbers. EPDM is com-posed of three different types of monomers, i.e., eth-ylene, propylene or some other higher alpha-olefin, anda non-conjugated diene, such as 2-ethylidene-5-norbor-nene, 1,4-hexadiene or dicyclopentadiene. In EPDMterpolymer, ethylene and propylene monomers providea saturated backbone interrupted by the incorporationof non-conjugated diene monomers, which provideunsaturated groups in EPDM. The unsaturated siteson the side chain of EPDM make the cross-linkingprocess easily. The crosslinked EPDM materials can

be produced by three processes, high energy irradiation(60Co g-ray or accelerated electron beam) [3], thermo-initiated chemical process (peroxide method) [4–6], andUV irradiation [7,8]. In these processes, the crosslinkingof polymers is always accompanied with degradation indifferent degrees. Therefore, the stabilization of poly-mers is important for applications.Many researchers have reported EPDM degradation

induced by sunlight at ambient temperature [9–11]. Inour previous article, we described the preparation, themorphological structures, and related properties ofdynamically photocrosslinked PP/EPDM blends [12].The results show that controlling the degradation of PPand EPDM can enhance the mechanical properties ofphotocrosslinked PP/EPDM blends. In order to solvethe above problems, we have studied the mechanism ofphoto- and thermo-oxidative degradation in the photo-crosslinking of EPDM. To simplify the crosslinkingprocess, we apply the static method to study thedegradation behaviour of EPDM. The photo- andthermo-oxidative degradation mechanism of dynami-cally photocrosslinked PP/EPDM will be discussed inlater papers.

0141-3910/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S0141-3910(03)00154-X

Polymer Degradation and Stability 81 (2003) 531–537

www.elsevier.com/locate/polydegstab

§ Supported by the China NKBRSF project (No. 2001 CB409600)

and the National Nature Science Foundation of China (No.

50073022).

* Corresponding author. Fax: +86-551-360-7245.

E-mail address: qubj@ustc.edu.cn (B. Qu).

In the present work, the photo-and thermo-oxidationdegradation of photocrosslinked EPDM were studiedby photo-acoustic Fourier transform infrared spectro-scopy (PAS-FTIR), X-ray photoelectron spectroscopy(XPS), and scanning electron microscopy (SEM). Therelative ratio of degradation to crosslinking ofEPDM with a suitable photoinitiator was estimatedby Charlesby’s random crosslinking theory.

2. Experimental

2.1. Materials

Two types of EPDM used in the present work wereEPDM 4045 (Mitsui Petrochemical Industries Co. Ltd.,Japan) and EPDM 4770 (DuPont Dow Elastomer Co.Ltd., USA). The ethylene and diene contents are 52 and3 wt.% for EPDM 4045, and 70 and 5 wt.% for EPDM4770, respectively. 2-Ethylidene-5-norbornene was usedas the diene. Benzil dimethyl ketal (BDK), used asphotoinitiator, and hindered amine Tinuvin 144, used asphotostabilizer, were obtained from Ciba SpecialtyChemicals, Switzerland. Trimethylolpropane triacrylate(TMPTA, UCB, Belgium) was used as a crosslinkingagent.

2.2. Preparation of photocrosslinked EPDM

The EPDM sample was mixed with 1 wt.% BDK and1 wt.% TMPTA for 10 min at 150 �C using a rheomixerXSS-300. After mixing, the samples were hot-pressed tosheets of 3 mm thickness for 5 min at 150 �C using aCarver press. The sheet thickness was controlled usingframes of different standard thickness. The hot-pressedsamples were photocrosslinked in the UV-CURE device(Philips HPM 15, 2 kW) built in our laboratory with adistance of 10 cm from the lamp at 25 �C under anitrogen flow of 1 l min�1.

2.3. Gel content

After being irradiated, the samples were cut into thinslices and put into a basket made of 200 mesh stainlesssteel net. The gel contents of samples were determinedby extracting the irradiated samples (w1) in the basketfor 48 h with boiling cyclohexane stabilized by 0.2 wt.%of Tinuvin 144 and with N2 bubbling to prevent oxida-tion. The solvent was renewed after the first 24 h ofextraction. After the extraction, the basket was washedwith acetone. After being dried in a vacuum desiccatorat about 70 �C to constant weight, the insoluble residue(w2) was weighed. The average gel content (wt.%) in thetest was calculated as 100 (w2/w1). Usually, three sam-ples were analyzed to determine the average gel contentfor a given set of irradiation conditions.

2.4. Swelling test

The photocrosslinked samples (10�10�3 mm) wereextracted and placed in cyclohexane (25 �C) for swellingfor 7 days [7,8]. The swollen gel was picked up andtransferred quickly to a weighing bottle with a cover, andweighed. The datum for a sample was recorded as theaverage of five measurements. The mass of crosslinkedsample was determined by weighing after drying at about50 �C in a vacuum desiccator overnight.

2.5. Photo- and thermo-oxidation of photocrosslinkedEPDM

The photo-oxidation of photocrosslinked EPDMsamples was performed in the UV-CURE device fordifferent times (Philips HPM 15, 2 kW) with a distanceof 10 cm from the lamp at 25 �C in air. The thermo-oxidation of photocrosslinked EPDM samples was per-formed in an oven (type XG-CN, made in Nantong,China) at 120�0.2 �C for 7 days.

2.6. Analysis of samples

The PAS-FTIR spectra were recorded with aMAGNA-IR 750 spectrometer (Nicolet, USA). TheXPS spectra were recorded with a VG ESCALAB MKII spectrometer (VG company, UK) using Al Ka exci-tation radiation (h�=1253.6 eV). The SEM micro-graphs were observed by a Hitachi X650 scanningelectron microscope (Hitachi, Japan). The samples werecoated with a conductive gold layer.

3. Results and discussion

3.1. The relative ratio of crosslinking to degradationduring the photocrosslinking of EPDM

As is well known, the crosslinking of a polymer isalways accompanied by some degree of degradationwhatever crosslinking method is used. Therefore, the sta-bilization of polymers becomes very important for appli-cations. Charlesby’s random crosslinking theory dealswith the relative ratio between crosslinking and degrada-tion of a polymer induced by irradiation. In the case ofphotocrosslinking, the Charlesby formula can be expres-sed as the following equation according to literature [13],

1=� ¼ P0=Q0 þ A0=�

where P0 and Q0 are the numbers of the degraded andcrosslinked units, respectively, induced by unit dose ofirradiation, �=1/(S+S1/2) is the ‘‘crosslinking index’’,S is the sol content, equal to 1-gel content, A0 is aconstant, � is the effective crosslinking density (mol

532 W. Wang, B. Qu /Polymer Degradation and Stability 81 (2003) 531–537

cm�3) in the whole sample, equal to �*/ (1+S), whereas�* is the effective crosslinking density in the dry gel. Thevalue of �* can be calculated from the swelling datausing the Flory-Rehner theory:

�� ¼ � ln 1� q�1� �

þ q�1 þ � q�2� �

= v1 q�1=3 � 0:5q�1� �� �

where q is the degree of swelling; v1 is the partial molarvolume of the swelling solvent; and � is the Flory-Huggins

interaction parameter. For cyclohexane at 25 �C in thisstudy, v1=108ml mol

�1 and �=0.321 [7,8]. The degree ofswelling (q) was calculated according to the expression:

q ¼ weight of swollen gel=weight of dry gel

The data from the gel content, degree of swelling, andcrosslinking density of different photocrosslinkedEPDM samples are listed in Table 1.

Table 1

Gel content, degree of swelling and crosslinking density of photo-

crosslinked EPDM (in cyclohexane at 25 �C)

Irradiation

time (s)

Gel content

(%)

Degree of swelling

(q)

v* � 104

(g mol�1)

EPDM

EPDM EPDM EPDM EPDM EPDM

4045

4770 4045 4770 4045 4770

30

70.3 80.1 7.938 7.432 0.76 0.86

50

78.0 88.5 7.632 6.159 0.82 1.27

70

84.0 91.6 6.805 6.019 1.03 1.33

90

87.6 94.7 6.697 4.899 1.07 2.06

120

90.2 94.8 5.559 4.752 1.57 2.20

Fig. 1. Relationship between 1/� and 1/� for photocrosslinking of the

samples: (A) EPDM-4770, (B) EPDM 4045.

Fig. 2. PAS-FTIR spectra of photocrosslinked EPDM 4770 samples

photo-oxidized for different times: (A) hydroxyl region; (B) carbonyl

region; (C) oxygen–carbon region.

W. Wang, B. Qu /Polymer Degradation and Stability 81 (2003) 531–537 533

Fig. 1(A) and (B) show the linear relationshipbetween 1/� and 1/� for photocrosslinking of EPDM4770 and 4045 respectively. By extrapolation to 1/�=0, the values of P0/Q0 can be obtained as 0.06 and0.1 for the photocrosslinked EPDM 4770 and 4045,respectively. It means that the degradation effect inphotocrosslinked EPDM 4770 is much less than thatin EPDM 4045 due to its high ethylene content.Furthermore, the photocrosslinking rate of EPDMwill decrease rapidly with increasing UV irradiationtime due to the consumption of photoinitiators,whereas the rate of degradation increases withincreasing irradiation time. As a result, the polymerdegradation will become dominant. Thus, minimizingthe irradiation time is important not only for thephotocrosslinking efficiency and for saving energy,but also for the physico-chemical properties of finalcrosslinked products. We selected EPDM 4770 sam-ple UV-irradiated for 90 s as a model to study thephoto- and thermo-oxidative degradation in the fol-lowing parts.

3.2. Photo-oxidation of photocrosslinked EPDM

Fig. 2 shows the PAS-FTIR spectra of photo-crosslinked EPDM 4770 samples photo-oxidized fordifferent times. It can be seen that the intensity of thewide absorption band at 3300–3600 cm�1 due to thephoto-oxidation products, such as hydrogen-bondedhydroxyl groups (alcohols, hydroperoxide and car-bonyl), increase with increasing photo-oxidation agingtime, as shown in Fig. 2(A). The absorption intensity at1721 cm�1 due to ketone groups of oxidation productsincreases rapidly with the irradiation time, as shown inFig. 2(B). Furthermore, the absorption peak of ketonegroups shifts to 1716 cm�1 after irradiation for over 3

min because of overlapping with the growing peak at1710 cm�1 for carboxylic acid groups [14]. Fig. 2(C)shows the similar changes of the deformation vibrationof C–O bonds around 1150 and 1040 cm�1 with theincrease of photo-oxidation aging time.Fig. 3 shows the C1s XPS spectra of photocrosslinked

EPDM 4770 samples photo-oxidized for different times.The strong peak A at 285.0 eV can be attributed to the

Fig. 3. C1S XPS spectra of the photocrosslinked EPDM 4770 samples

photo-oxidized for different times. (A) C¼C, C–H; (B) CH2–O; (C)–

(C¼O)–O–.

Fig. 4. PAS-FTIR spectra of photocrosslinked EPDM 4770 samples

thermo-aged at 120 �C for different times: (A) hydroxyl region; (B)

carbonyl region; (C) oxygen–carbon region.

534 W. Wang, B. Qu /Polymer Degradation and Stability 81 (2003) 531–537

C1s electron from C–C and C–H bonds of EPDM. Theintensities of small peaks B and C, obtained by thecomputer separating of peaks of C1S bands, at 286.3 and289.1 eV increase with increasing photo-oxidation time,which indicates the formation of surface photo-oxidation

products, such as �CH2�O�, �Cð¼OÞ�, and �Cð¼OÞ�

O�groups.

3.3. Thermo-oxidation of photocrosslinked EPDM

The PAS-FTIR spectra of photocrosslinked EPDM4770 samples after thermo-aging for different times areshown in Fig. 4. It can be seen from the Fig. 4(A) thatthe intensity of the hydroxyl peak in the region of 3300–3600 cm�1 shows not much change for the samplesbefore and after thermo-oxidative aging. Fig. 4(B)shows the rapid increase of the intensity at the band of1700–1800 cm�1 with increasing aging time. The peaksat 1723, 1740 and 1778 cm�1 have been assigned toketone, lactone, and aldehyde compounds, respectively[14]. The wide bands at 1173 and 1242 cm�1 assigned tothe deformation vibration of C–O bonds, are shown inFig. 4(C). The above results indicate that the main pro-ducts formed from the thermo-oxidation of photo-crosslinked EPDM are ketone, lactone, and aldehyde.The C1S XPS spectra for the photocrosslinked sam-

ples before and after thermo-oxidation for 72 and 168 h

Fig. 5. C1S XPS spectra of the photocrosslinked EPDM 4770 thermo-

oxidized for different times at 120 �C. (A) C¼C, C–H; (B) CH2–O;

(C)–(C¼O)–O–.

Fig. 6. Photos of the photocrosslinked EPDM 4770 photo-oxidized for different times: (A) before photo-oxidation; and photo-oxidation for (B) 1

min; (C) 3 min; (D) 5 min.

W. Wang, B. Qu /Polymer Degradation and Stability 81 (2003) 531–537 535

at 120 �C are shown in Fig. 5. The peaks B and Cmarked by dotted lines obtained by computer separat-ing peaks of C1S bands appear at 286.5 and 289.0 eV,and are assigned to the �CH2�O� and �Cð¼OÞ�O�groups, respectively. It can be found that the intensitiesof B and C peaks after thermal aging for oxidation

products increase with increasing aging time. These datagive positive evidence that the degree of thermo-oxida-tion increases with increasing aging time.

3.4. Morphological structures of photocrosslinkedEPDM

Fig. 6 presents the SEM photos of photocrosslinkedEPDM 4770 samples photo-oxidized for different times.Apparently, the surface of the control sample a withoutphoto-oxidation is relatively smooth, compared with theother photo-oxidized samples b, c, and d. The surface ofsample b photo-aged for 1 min appears rough and hassmall voids, but no cracks are observed. Dense voidsand small cracks on the surface of sample c photo-agedfor 3 min can be observed, whereas big cracks appearobviously on sample d photo-aged for 5 min. Themorphological structure changes of these photo-agedEPDM samples are consistent with the results from theabove PAS-FTIR measurements.Fig. 7 shows the SEM photos of photocrosslinked

EPDM 4770 samples thermo-aged for different times in

Fig. 7. Photos of photocrosslinked EPDM 4770 thermo-aged for different times: (A) before thermo-oxidation, and after thermo-oxidation for (B) 72

h, (C) 120 h, (D) 168 h.

Fig. 8. Formation mechanism of various oxidation products during

the photo- and thermo-oxidation of photocrosslinked EPDM 4770.

536 W. Wang, B. Qu /Polymer Degradation and Stability 81 (2003) 531–537

an oven. The changes on the surface morphologicalstructures are similar to those from the above photo-oxidation. The thermo-oxidation is much less thanphoto-oxidation although the thermal aging time ismuch longer. The surface degradation degrees of agedsamples obviously increase with increasing thermalaging time. The sample b thermo-aged for 72 h appearsslightly rough. The sample c aged for 120 h shows somecracks in the surface. The sample d aged for 168 h showsmany more cracks than the sample c. The earlier resultsillustrate that the photo-oxidative degradation of photo-crosslinked EPDM has a crucial effect in the same timeperiod.

3.5. Surface photo- and thermo-oxidation mechanism ofthe photocrosslinked EPDM

Based on the above experimental results, the surfacephoto- and thermo-oxidation mechanisms of photo-crosslinked EPDM can be suggested as the following.Hydroperoxides are formed first when the photo-crosslinked EPDM samples were photo- or thermo-oxi-datively aged, then immediately decomposed into Hand RO radicals, following which the R radicals pro-duced in the process, can easily react with oxygenabsorbed on the EPDM surface to produce polymericperoxy (ROO ) radicals. These ROO radicals reactfurther with EPDM to form hydroperoxide andhydroxyl compounds. These reactions will form a cyclicprocess, as shown in Fig. 8(A).The hydroperoxides are the most important inter-

mediates in the photo- and thermo-oxidation process ofEPDM. These hydroperoxides formed decomposequickly to form RO radicals which can produce variousoxidation products, such as ketone, carboxylic, lactone,and aldehydes, etc., as shown in Fig. 8(B).

4. Conclusions

The relative ratios of degradation to crosslinkingare extrapolated to be 0.06 for EPDM-4770 and 0.1for EPDM-4045, according to Charlesby’s randomcrosslinking theory. The PAS-FTIR, XPS and SEMmeasurements show that the surface photo-oxidationand the thermo-oxidative degradation of photo-crosslinked EPDM increase with increasing aging time.

Hydroperoxides and various carbonyl groups are formedas the main degradation products during the photo-oxi-dation and thermo-degradation processes. The resultsobtained in this study give the evidence that the photo-oxidation of photocrosslinked EPDM has a crucialeffect compared to the thermo-oxidative degradation.

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