Thermo-oxidative degradation studies of ternary blends of polyethylenes

Thermo-Oxidative DegradationStudies of Ternary Blends ofPolyethylenes

DEEPAK SRIVASTAVADepartment of Plastic Technology, H. B. Technological Institute, Kanpur 208 002, India

PRAMOD KUMARDepartment of Oil and Paint Technology, H. B. Technological Institute, Kanpur 208 002, India

G. N. MATHURDefence Materials and Store Research and Development Establishment, Kanpur 208 012, India

Received: September 21, 2000Accepted: March 25, 2002

ABSTRACT: Films of low-density polyethylene (LDPE) and high-densitypolyethylene (HDPE) each containing 25 wt % of linear low-density polyethylene(LLDPE) were extruded by melt blending in a single screw extruder (L/D ratio =20:1). The samples were aged at 55, 70, 85, and 100◦C for different time periods upto 600 h. The change in molecular structure and formation of oxygenated andunsaturated groups during thermo-oxidative degradation were investigated byinfrared spectroscopy. Viscosity-average molecular weight (M̄v) was found todecrease at slower rate in the initial hours of ageing, whereas on prolongedageing, it decreased at a faster rate. In some blend samples, the tensile strengthand elongation at break initially increased up to certain hours of ageing, and thendecreased while in some blends both tensile strength and elongation-at-breakvaried differently. Increase in percent gel content supports the formation ofcross-links between the molecular chains. Thermal stability of the samples alsodecreased with ageing time and temperature. C© 2004 Wiley Periodicals, Inc.Adv Polym Techn 23: 59–70, 2004; Published online in Wiley InterScience(www.interscience.wiley.com). DOI 10.1002/adv.10067

Correspondence to: Deepak Srivastava; e-mail: deepak [email protected].

Advances in Polymer Technology, Vol. 23, No. 1, 59–70 (2004)C© 2004 Wiley Periodicals, Inc.

THERMO-OXIDATIVE DEGRADATION STUDIES OF TERNARY BLENDS OF POLYETHYLENES

KEY WORDS: Blends, Degradation, Films, Gel content, High-density polyethy-lene, Linear low-density polyethylene, Low-density polyethylene, Miscibility,Percentage elongation-at-break, Tensile strength, TGA, Thermal stability

Introduction

I n the past, binary blends of LDPE/HDPE,HDPE/LLDPE, LDPE/LLDPE etc. have been ex-

tensively investigated by various workers.1−8 Thesebinary blends have improved stiffness and impactproperties, high environmental-stress cracking resis-tance, improved shrink stability, high weld strengthetc., and are used for various applications such asextrusion coatings, moldings, wire insulation, strongweld bags, transparent films, packaging etc.

The ternary blends of these polyethylenes havehardly been studied for their mechanical, rheologi-cal, thermal, and ageing characteristics. In this study,we have prepared a blended film system consist-ing of these polyethylenes, and studied their ageingcharacteristics.

Experimental

The films of different ratios of low-densitypolyethylene (LDPE) (IPCL, Vadodara, MFI = 2.0gm/10 min, density = 0.930 gm/cm3) and high-density polyethylene (HDPE) (PIL, Mumbai, MFI =3.0 gm/10 min, density = 0.950 gm/cm3) with25 wt % linear low-density polyethylene (LLDPE)(RIL, Hazira, MFI = 1.0 gm/10 min, density =0.920 gm/cm3) were prepared by blown film extru-sion technique in a single screw extruder (L/D ratio20:1 and compression ratio = 3.2). The temperaturesof feed, compression, and metering zones were setin the range of 186–200◦C. A three roll type take-offequipment operating at a take-up velocity corre-sponding to a particular draw ratio with attachedwinder was used. The extruded film was allowedto cool at room temperature through a water bath.The blended films of uniform thickness were cut intopieces of required size according to ASTM D 638.

Ageing of films was done in an air circulatoryoven having digital controls at four different temper-atures of 55, 70, 85, and 100◦C for time periods up to750 h. The aged samples were taken out at room tem-perature and kept in sealed polyethylene bags with

dry and inert atmosphere. These bags were placedin a dark chamber.

The infrared (IR) spectra of the samples wererecorded with a Perkin-Elmer (Model 599-B) infraredspectrophotometer in the wavelength range of 4000–200 cm−1 to study the formation of oxygenated andunsaturated groups. Molecular weight (M̄v) was de-termined viscometrically with xylene at 85◦C in anoil bath. The Mark-Houwink equation was used withK and α values as 115 × 10−5 dl/gm and 0.61, re-spectively. The values of K and α were determinedexperimentally with standard technique.9 Changein tensile strength and percentage elongation-at-break were measured in an automatic tensile tester(M/s Prolific Pvt. Ltd., Noida). Also, the forma-tion of insoluble material (gel) was studied fromthe solution prepared for solution viscometric anal-ysis as per methods cited in literature.10,11 The ther-mal stability of the film samples was determinedby using a thermogravimetric analyser at a heat-ing rate of 10◦C/min in a nitrogen atmosphere. Themiscibility behaviour of unaged and aged samplesof LDPE/LLDPE/HDPE blends were assessed bymelting endotherms in the scans from differentialscanning calorimeter (DSC) of TA instruments Inc.,USA (Model 2910 DSC equipped with a micropro-cessor TA 2000). These scans were taken at a heat-ing rate of 10◦C/min. The morphology of selectedcompositions were investigated by Phillips 151 scan-ning electron microscope (SEM) system. SEM micro-graphs confirmed the mixing behaviour of the blend.Variations of LDPE and HDPE with 25 wt % LLDPEare designated in Table I.

Results and Discussion

SPECTROSCOPIC ANALYSIS

The IR spectra of unaged and aged film sam-ples selected from set A-500 (refer to Table I) areshown in Fig. 1. There appeared a band near 2800–3000 cm−1, in the unaged samples and aged sam-ples at 70◦C for different time periods up to 600 h.(Fig. 1), which might correspond to the methyl andmethylene groups, and both have asymmetric and

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TABLE IDesignation for Unaged and Aged Samplesa

Ageing Conditions Ageing Hours A-1 A-2 A-3 A-4 A-5 A-6

Unaged A-100 A-200 A-300 A-400 A-500 A-600

Aged at 55◦C 150 A-151 A-251 A-351 A-451 A-551 A-651300 A-153 A-253 A-353 A-453 A-553 A-653450 A-154 A-254 A-354 A-454 A-554 A-654600 A-156 A-256 A-356 A-456 A-556 A-656




aA-1: 70% LDPE, 5% HDPE; A-2: 65% LDPE, 10% HDPE; A-3: 60% LDPE, 15% HDPE; A-4: 55% LDPE, 20% HDPE; A-5: 50% LDPE,25% HDPE; and A-6: 40% LDPE, 35% HDPE.

FIGURE 1. Change in the infrared spectra of LDPE/LLDPE/HDPE blended film from set A-5 with ageing time (hours) at70◦C.

ADVANCES IN POLYMER TECHNOLOGY 61


symmetric C H stretching vibration modes.12,13 Thepresence of these groups, due to C H asymmetric,and symmetric bending vibrations of CH3 presentin the LDPE, LLDPE and HDPE, could also be con-firmed by the band appearance near 1450 cm−1, and“umbrella-like” band near 1370 cm−1. Also, the scis-soring vibrations of CH2 group might overlap in thisregion. The peak appearance near 720 cm−1 and asmall broad peak near 1300 cm−1 confirmed the pres-ence of CH2 rocking and in-plane C H bending vi-brations. These observations have been found to bein good agreement with the values of LDPE, LLDPE,or HDPE mentioned in literature.14

When the film sample was aged at 70◦C for 150 h(sample A-571), changes were mainly in the appear-ance of the absorption band in the region of 1550–1900 cm−1. A peak at 1720 cm−1, in particular, isattributed to the ketonic carbonyl group.15,16 Theincrease in the intensity of carbonyl group absorp-tion after ageing of the film samples symbolizes thechemical effect of prolonged exposure up to 600 hof blended polyethylene film samples to oxidativedegradation. In addition, the formation of doublebonds with absorption at 1640 cm−1 and 950 cm−1

can also be noted in Fig. 1. A small peak near3580 cm−1 for aged sample A-571, due to OH-group,appeared due to the formation of peroxy and hy-droperoxy groups as a result of thermo-oxidativedegradation and whose intensity increased withageing time up to 600 h (Fig. 1). The presence of

FIGURE 2. Variation of M̄v with ageing time (hours) at 70◦C for different sets of blended film samples.

OH group could also be confirmed by the peak ap-pearing at 1300 cm−1 due to OH-deformation.

The mechanism of degradation of such blends isnot clear up to this stage as the interaction effect ofeach component is not reported elsewhere. Never-theless, on the basis of IR spectra one can propose asimilar mechanism, slightly complicated, as given byReich and Stivala [17]. However, we are quite opti-mistic that we will be able to predict the exact mech-anism of degradation of such blends in near future.

VISCOSITY-AVERAGE MOLECULARWEIGHT

The variation of viscosity-average molecularweight (M̄v) with ageing time at 70◦C for differentblend compositions (Set A-1 to Set A-6) are shownin Fig. 2. It is clear from the figure that in all blendcompositions, the molecular weight decreases withageing hours at 70◦C. The molecular weight remainsunchanged up to 450 and 300 h at 55 and 70◦C, re-spectively, for film sample of set A-5 (Fig. 3), whereasit decreased slowly when aged at 55, 70, 85, and100◦C up to 600 h. The slow decrease in molecu-lar weight may be attributed to slow chain-scissionin the initial ageing hours and temperatures. Fur-ther, this behaviour could also be attributed to theformation of long stable chains as a result of com-bination of free radicals, generated as a result ofdegradation of LDPE, with the chains from LLDPE

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FIGURE 3. Variation of M̄v with ageing time (hours) and temperature (◦C) for set A-5.

or HDPE, preferably LLDPE.15,16,18 As the ageingtime and temperatures were increased, these longstable chains formed peroxy and hydroperoxy radi-cals, due to oxygen attack at free radical sites. Owingto this, the molecular weight decreased at a fasterrate thereafter, due to faster rate of chain-scission ofC C, C H bonds, etc.19 present in the blends, andsubsequently by β-scission.20,21

As a matter of fact, the steady slow drop in molec-ular weight in the initial ageing hours represents theinduction period. During this period, the degrada-tion is activated in the molecules of the polyethylenescomprising the blends. Once the degradation is ac-tivated in the induction period, the propagation ofdegradation reaction proceeds with a faster rate withrapid drop in molecular weight and this occurs dur-

ing the autocatalytic stage of the degradation reac-tion. Further, Fig. 2 does not show a trend of increas-ing HDPE on molecular weight drop with ageinghours but rather a random behavior. This behaviormight be inherent within the system or perhaps ofnonuniformity or inhomogenity of the blends. Thesame is confirmed by the appearance of twin peaksin Fig. 12.

TENSILE STRENGTH,ELONGATION-AT-BREAK,AND GEL CONTENT

The variation of tensile strength and elongation-at-break with ageing hours for different blend com-positions at 70◦C has been shown in Figs. 4 and 5,



FIGURE 4. Variation of tensile strength (kg/cm2) with ageing time (hours) at 70◦C for different sets of blended filmsamples.

respectively. From the figures it is clear that thetensile strength of film sample from set A-1 in-creases slowly up to 300 h of ageing and then de-creases, almost linearly, with ageing hours up to600 h, whereas an almost reverse trend has beenobserved for elongation-at-break. The elongation-at-break first varies linearly up to 300 h of ageing andthen decreases exponentially thereafter. Film sam-ple from set A-2 showed a linear decrease of ten-sile strength with ageing hours up to 600 h, whereas

FIGURE 5. Variation of tensile strength (kg/cm2) with ageing time (hours) and temperature (◦C) for set A-5.

percentage elongation-at-break initially increasedup to 150 h of ageing, then decreased exponen-tially up to 300 h of ageing followed by almost alinear decrease when aged beyond 300 h. As theamount of HDPE in the blend was increased to15 wt % (set A-3), the tensile strength showed a sim-ilar trend as that in sample from set A-1, while in theelongation-at-break a very slow increase has beenobserved up to 300 h of ageing. It decreased verysharply when the number of ageing hours was

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FIGURE 6. Variation of elongation-at-break (%) with ageing time (hours) at 70◦C for different sets of blended filmsamples.

increased from 300 to 450 h and thereafter it remainsalmost unchanged up to 600 h of ageing (Figs. 4and 5).

For film sample from set A-4, the tensile strengthfirst increased very slowly, almost linear, up to 300 hof ageing and then decreased slowly up to 450 h ofageing. Finally, as the sample was aged up to 600 hat 70◦C, it sharply decreased with ageing time. Theelongation-at-break varied in a very different man-ner as that of tensile strength with ageing hours

FIGURE 7. Variation of elongation-at-break (%) with ageing time (hours) and temperature (◦C) for set A-5.

of set A-4 sample. For set A-5, the tensile strength de-creased continuously up to 600 h of ageing whereaspercentage elongation-at-break varied in a similarway as that in the case of set A-4 sample. At 85 and100◦C, the tensile strength first increased to about21 and 44%, respectively, when aged for 150 h andthen decreased up to 600 h (Fig. 5). The changeof elongation-at-break with ageing time and tem-perature is shown in Fig. 7. The variation of theelongation-at-break for set A-6 at all ageing hours



FIGURE 8. Variation of gel content (%) with ageing time (hours) and temperature (◦C) for set A-5.

appeared to be parallel with the tensile strength ofthe blends.

The increase in tensile strength during ageingmight be due to higher ordering in amorphousregion22,23 and recombination of alkyl radicals, i.e.,enhanced cage effect24 from LDPE, LLDPE, or HDPE.The formation of gel confirmed the same, as thevalue of gel content increased with ageing timeand temperature (Figs. 8 and 9). The recombina-

FIGURE 9. Variation of gel content (%) with ageing time (hours) at 70◦C for different sets of blended film samples.

tion reactions might be restricted to certain level ofageing time and temperature, and further formationof free radicals decreased the value of tensile strengthdue to chain-scission. A similar effect has been re-ported by Srivastava and Mathur20 while studyingthe degradation of LDPE binary blend. The decreaseof elongation-at-break in the initial ageing hours andtemperature might be due to the shortening of amor-phous region owing to increased crystallinity16,20 in

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FIGURE 10. Variation of initial degradation temperature (IDT) (◦C) with ageing time (hours) at 70◦C for set A-5.

the presence of LLDPE, or to the role of the bound-ary phase that might contain a greater abundanceof comonomer containing segments of LLDPE. Thecomonomer might cause separation of the molec-ular chains and thus weaken their intermolecularforces, hence, facilitating the failure. This separa-tion of the molecular chains increased with age-ing time and temperature. This has been indicatedby decrease of elongation-at-break in the prolongedageing up to 600 h (Fig. 7). However, the tensilestrength, elongation-at-break, and gel content donotshow a defined trend of increasing HDPE, ratherthey show a random behaviour with ageing hours.

FIGURE 11. Variation of initial degradation temperature (IDT) (◦C) with ageing temperature (◦C) for set A-5 aged up to300 h.

This behavior might be inherent within the system,or perhaps of nonuniformity, or inhomogenity of theblends.

THERMAL STABILITY

The initial degradation temperature (IDT) of un-aged LDPE/LLDPE/HDPE films was found to havedecreased when the films were aged at 70◦C for 150,300, 450, and 600 h (Fig. 10). The effect of ageing timeand temperature for sample set A-5 on IDT can alsobe seen in Fig. 11. The final degradation tempera-ture (FDT) and the temperature at which 50% weight



FIGURE 12. DSC scans of unaged (A-500) and aged(A-573) LDPE/LLDPE/HDPE blend samples.

FIGURE 13. SEM of unaged (A-500) and aged (A-573) LDPE/LLDPE/HDPE blend samples.

loss occurred for unaged film (A-500) were found tobe 515.3 and 436.7◦C, respectively. As the film wasaged at 70◦C for 150, 300, 450, and 600 h, the FDTswere found to be 553.1, 535.1, 530.4, and 518.4◦C, re-spectively. When the samples were aged at 55, 70,85, and 100◦C for 300 h, the values were found tobe 462, 535.1, 567.1, and 472.4◦C, respectively. Thetemperatures at which 50% weight loss occurred,for A-571, A-573, A-574, and A-576, were found tobe 441.8, 427.1, 403, and 380.5◦C, respectively. Thesetemperatures decreased as the ageing temperatureswere varied from 55 to 100◦C for 300 h. The percentchar yield increased as the hours of ageing were in-creased at 70◦C. Its value was found to be 0.3% forsample A-571 which increased to 0.53, 0.91 and 2.1%for samples A-573, A-574, and A-576, respectively.

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The char yields for samples A-500, A-553, and A-583 were found to be absent. The thermal stabilityhas been found to decrease with ageing time andtemperature and which might be due to the forma-tion of several low molecular weight species duringthermo-oxidative degradation.17,18 The formation ofvarious radicals, as observed in IR spectra (Fig. 1),can confirm the decrease of thermal stability.

DIFFERENTIAL SCANNINGCALORIMETRIC ANALYSIS

The DSC scans of unaged (A-500) and aged (A-573) samples have been shown in Fig. 12. It is clearfrom the scan that there appeared two melting en-dotherms as the sample A-500 were aged at 70◦C for300 h (A-573). The shifting of melting endotherm onlower side of scale in the scan from 113.7◦C (unaged)to 111.9◦C (aged) and 127.6◦C (unaged) to 126.8◦C(aged) on the higher side of the scan has been ob-served. The melting endotherm on the lower sidemight be expected due to melting of LDPE chainmolecules,4,25–27 whereas on the higher side of thescan it might be due to melting of cocrystallized crys-tals of LLDPE and HDPE chain molecules.4,28 Thescanning electron microscopic analysis of unaged(A-500) and aged (A-573) samples also confirmedthe same, as there appeared a clear two-phase mor-phology (Fig. 13).

Conclusion

From the preceding results, it can be inferred thatthe change in molecular structure and formation ofoxygenated and unsaturated groups during thermo-oxidative degradation of ternary blends of LDPE andHDPE with 25 wt % LLDPE supports the occurrenceof some modicum of degradation. This is further ev-idenced by a slow decrease in molecular weight withageing time and temperature. However, the mecha-nism of thermo-oxidative degradation is not explic-itly understood up to this stage, as evidenced by IRspectra.

The change in tensile strength and percentageelongation-at-break, in most blend samples, showedentirely different behaviour with ageing time andtemperatures. In some samples, both showed a par-allel behaviour when considered in terms of blendcompositions. The tensile strength, for the sample

(set A-5) aged for different ageing temperatures, ini-tially increased to a certain extent at higher ageingtemperatures and then decreased, whereas it contin-uously decreased at lower ageing temperatures. Theelongation-at-break, aged for different temperatures,initially increased and then decreased for sample setA-5. The increase of tensile strength is supported byan increase in percent gel content. The thermal stabil-ity of these blends during thermo-oxidative degra-dation also decreased with ageing time and temper-ature. The blend gave two melting peaks in the DSCscans that suggest a two-phase morphology.

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Thermo-oxidative degradation studies of ternary blends of polyethylenes