Effect of climatic ageing on extra long-term fracturemechanical properties of polyethylene

Andras Nemeth*, Janos Marosfalvi

Gepszerkezettani Intezet, Gepelemek Tanszek, Budapesti Muszaki es Gazdasagtudomanyi Egyetem, H-1111 Budapest Muegyetem rkp. 3., Hungary

Received 29 June 2000; received in revised form 21 February 2001; accepted 25 February 2001

Abstract

The change of fracture resistance of medium density polyethylene (MDPE) has been studied over a wide time interval using SEN-

T specimens. Employing first-level sampling (FLS) method, time dependence of the fracture toughness (KQ) was revealed in 2.6�105

h (�30 years) interval. Based on the measured data, time-related fracture mechanical properties were determined, that characterisethe long-term fracture behaviour of polyethylene. Using infrared spectroscopy (IR), a relationship was detected between the changeof fracture mechanical properties and the oxidative degradation of polyethylene. The results are applicable for long-term fracture

mechanical design of polyethylene. # 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Fracture toughness; Long-term; First-level sampling; Oxidation; Polyethylene; Infrared spectroscopy; SEN-T; Fracture degradation rate

1. Introduction

Change of material characteristics over time can oftencause problems in long-term applications of polymers.These changes can usually be traced back to physical(creep, relaxation, crack condition) and chemical (degra-dation) causes. Change in material properties may affectadversely the mechanical behavior of polymers in manycases. Damage of polymer structural elements can becaused by exceeding the critical strain state or fracture[1,2]. It was proved that the different mechanical para-meters show changes in the application period for mostpolymers [3,4,9,10]. This makes it necessary to introducethe concept of lifetime in the field of applications ofstructural polymers.Some research has been done on time relations of

fracture characteristics of polymers. Based on experi-ments [5,10], fracture-mechanical characteristics alsotend to show changes in time, that cannot be neglected inmany cases. In this article time related fracture-mechan-ical material properties were defined based on extra long-term studies of a medium density polyethylene (MDPE)material. The aim of this investigation was to find thefracture lifetime for designing of polyethylene.

2. Experimental

2.1. Sampling methods

The long-term, time-dependent parameters can bedetermined by two methods. By first level sampling(FLS), the specimens produced at different times aretested approximately at the same time [Fig. 1(a)]. Morethan hundred-thousand-hour test data can be deter-mined by this method during a fraction of the time ofthe real time interval. By second level sampling (SLS)[5,10] [Fig. 1(b)] specimens manufactured at the sametime are tested at given intervals until the end of thetesting period. The advantage of this method is that theconsistency of the base materials and loads can easily becontrolled at each time. The disadvantage of SLS is thatit requires too much time; therefore, the time span islimited to a few years.

2.2. Materials, specimens

The tested materials were ME 500-05 and ME 610-05moderately stabilized MDPE (TIPELIN TVK, Hun-gary) using single edge notched tensile (SEN-T)(110�30�4 mm) specimens (Fig. 2) [6]. The sample ser-ies were produced by FLS in 2.6�105 h period. Thespecimens were cut from polyethylene storage pots of

0141-3910/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.

PI I : S0141-3910(01 )00068-4

Polymer Degradation and Stability 73 (2001) 245–249

www.elsevier.nl/locate/polydegstab

* Corresponding author. Fax: +36-92-550-016.

E-mail address: nemeth.andras@gszi.bme.hu (A. Nemeth).

serial production, from the same place and the sameorientation (Fig. 3). Within each material series, the sto-rage pots were used under Central-European climaticconditions. The series were chosen based on the records ofthe manufacturer (PANNON TARA, Hungary), in orderto have the same material at any time of the past few years.

2.3. Tests

The fracture mechanical tests were done at roomtemperature (20�C) with a ZWICK 1445 tensile-testing

machine at 10 mm/min testing rate. Testing exposuretime data were defined based on the production date ofthe base material of the specimens (application period)and the maximum force value of load-displacementrecord (proportional value of fracture toughness). Ten-sile-test diagram was drawn up for the whole section ofcrack propagation.The samples for the ifrared tests were made from dif-

ferent layers of the material of SEN-T specimens (Fig. 4).The final thickness (�60 mm) from cold sliced (�100 mm)samples was adjusted by heat pressing (150�C). Theabsorbance curves were obtained on a Perkin-Elmer 1600instrument in 1600–2000 cm�1 wave number range.

3. Results and discussion

3.1. Fracture mechanical analysis

The primary aim of the test is to determine (approxi-mately) the time to the development of brittle fracture, atendency related to fracture damages in practice.Therefore, the standard fracture toughness (KQ) [7,8]

Fig. 1. Determination of long-term mechanical properties: (a) first level sampling (FLS); (b) second level sampling (SLS).

Fig. 2. SEN-T specimen.

Fig. 3. Sampling: (a) ME 500-05; (b) ME 610-05. Fig. 4. Sample preparation for IR test (outer side=outer layer of pots).

246 A. Nemeth, J. Marosfalvi / Polymer Degradation and Stability 73 (2001) 245–249

was chosen as a fracture-mechanical characteristic. Thefracture parameter can be determined by expressions (1)and (2).

KQ ¼FQ

ffiffiffia

p

B Wf aW

� �ð1Þ

f aW

� �¼ 1:99� 0:41

a

Wþ 18:7

a

W

� �2�38:48

a

W

� �3

þ 53:85a

W

� �4 ð2Þ

In the expressions a, B and W are the geometricaldata of the SEN-T specimen (Fig. 2). FQ is the max-imum force on the tensile-test diagram.As Figs. 5 and 6 show in the case of both materials,

there is a relationship between fracture toughness andthe ‘‘age’’ of the material. In general, this relationship isdescribed well with Eq. (3), where parameters can be

defined by combined linear regression. In the equationKiQ is the initial fracture toughness, K0 is the scalar ofthe first-order part, and mD is the fracture degradationrate characterizing the change of fracture toughness.Regression data of the investigated materials are shownin Table 1.

KQ tð Þ ¼KiQ t < tTK0 �mD t t5tT

�ð3Þ

Fig. 5. Time-dependence of fracture toughness (ME 500-05, T=

20�C).

Fig. 6. Time-dependence of fracture toughness (ME6 10-05, T=

20�C).

Fig. 7. Absorbance per unit thickness spectrum (ME 500-05).

Table 1

Long-term fracture-mechanical properties

Material

Parameter ME 500-05 ME 610-05

KiQ MPaffiffiffiffim

p� �4.44 4.56

K0 MPaffiffiffiffim

p� �4.72 4.80

mD MPaffiffiffiffim

p=year

� �0.108 0.373

kDa MPa

ffiffiffiffim

p� ��0.54 �0.73

tT (year) 2.6 0.6

sb MPaffiffiffiffim

p� �0.54 0.39

a Slope values of relative integral-absorbance (Figs. 11 and 12).b Deviation (t>tT).

Fig. 8. Absorbance per unit thickness spectrum (ME 610-05).

Fig. 9. Interpretation of relative integral-absorbance.

A. Nemeth, J. Marosfalvi / Polymer Degradation and Stability 73 (2001) 245–249 247

3.2. Infrared spectroscopy

To compare the degradation levels of different agedpolyethylene the absorbance per unit thickness (4)spectrum was plotted (Figs. 7 and 8). In the figures itcan be seen that the heights of oxidative degradationpeaks are increasing proportionally to the exposuretime. Probably the increase in the concentration of theoxygen dissolved in the polymer correlates with theintensities corresponding to the carbonyl (1718 cm�1)and ester (1746 cm�1) groups.

A� ¼A

d¼

logI0I

dð4Þ

For the quantitative evaluation of time-dependence ofoxidative degradation, the relative integral-absorbance(Fig. 9) was determined. Considering the natural ageing

circumstances of the specimens, the extra long exposuretime as well as the moderate stabilized MDPE material,an approximately homogeneous oxidation level wasexpected through the entire cross section [10]. Fig. 10shows the distribution of degradation level through thespecimen thickness.

Fig. 10. Distribution of degradation level after long time exposure: (a) ME 500-05, (b); ME 610-05.

Fig. 11. Time-dependence of relative integral-absorbance (ME 500-05,

1680–1726 cm�1, outer side).

Fig. 12. Time-dependence of relative integral-absorbance (ME 610-05,

1680–1726 cm�1, outer side).

Fig. 13. Effect of oxidative degradation on the fracture toughness of

polyethylene.

248 A. Nemeth, J. Marosfalvi / Polymer Degradation and Stability 73 (2001) 245–249

The slopes of the regression lines (Figs. 11 and 12) areproportional to the mD fracture degradation rates(Table 1). The relationship between the fracture tough-ness and the relative integral-absorbance (Fig. 13)shows that the embrittlement of polyethylene dependson the degradation level.

4. Conclusion

The time-dependence of fracture toughness of thepolyethylene was tested over a long time interval. Arelationship was shown between fracture toughness KQ

and application period. The regression coefficients ofthe analytical approximation characterize well the long-term fracture behavior of the tested polymer. The con-nection of the fracture parameter and the degradationlevel was proved by IR spectroscopy.

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