AIP Conference Proceedings 2123, 020006 (2019); https://doi.org/10.1063/1.5116933 2123, 020006
© 2019 Author(s).
Experimental study of chemically aged HDPE pipe material in toluene-methanol mixture and distilled water
Cite as: AIP Conference Proceedings 2123, 020006 (2019); https://doi.org/10.1063/1.5116933 Published Online: 17 July 2019
Latifa Alimi, Kamel Chaoui, Nacira Hamlaoui, and Khouloud Bedoud
ARTICLES YOU MAY BE INTERESTED IN
Study of optical and morphological TiO2 nano-films properties deposited by magnetron sputtering on glass substrate
AIP Conference Proceedings 2123, 020005 (2019); https://doi.org/10.1063/1.5116932 Preparation of ceramic foam from porcelanite by using simple direct foaming method AIP Conference Proceedings 2123, 020007 (2019); https://doi.org/10.1063/1.5116934 Smart grid and photovoltaic system to mitigate blackouts and load shedding AIP Conference Proceedings 2123, 020001 (2019); https://doi.org/10.1063/1.5116928
Experimental study of chemically aged HDPE pipe material in toluene-methanol mixture and distilled water
Latifa Alimi
1,2,a), KamelChaoui
2), Nacira Hamlaoui
2,3), Khouloud Bedoud
1)1
Research Center in Industrial Technologies (CRTI), P.O.Box 64, Cheraga, 16014 Algiers, Algeria.
2
LR3MI, Mechanical Engineering Dept., Badji Mokhtar University, P.O. Box 12, 23052 Annaba, Algeria.
3
LR3MI, Mechanical Engineering Dept., May 8
th1945 Guelma University, P.O. Box 401, 24054 Guelma, Algeria
a)
Corresponding author:
latifaalimi@yahoo.frAbstract: Studying the aging phenomenon of plastic pipes presents simultaneously an economic achievement and a technical challenge for water and natural gas transportation systems. Very often, they are exposed to aggressive environmental agents such as UV rays, ambient oxygen, acids, bases and some other solvents, altering the material microstructure, its physical and chemical properties. The high density polyethylene (HDPE) material degradation and loss of performance are usually the consequence of unwanted changes in mechanical behaviors leading to lower resistance. In this study, we examine the effects of distilled water (DW) and a mixture of toluene-methanol (TM) in contact with an HDPE pipe. Morphological properties such as crystallinity and oxidation induction time (OIT) are investigated using DSC method. Tensile tests and thermal analysis show that the TM mixture is much more absorbed by the resin as compared to DW. An increase in crystallinity is observed as established from literature for other organic solutions. Finally, the study gives an idea about property variances and their evolution as a function of the pipe thickness which can be used as an estimation of the structural heterogeneity of the product.
Keywords: Pipe; HDPE; Aging; Distilled Water; Toluene-Methanol Mixture; Crystallinity; OIT;
INTRODUCTION
Mechanical properties of HDPE (High Density Polyethylene) tubular structures are the subject of numerous studies in relation to physical and mechanical properties variation, molecular structure changes, mechanisms of fracture and aggressive environments effects during service life [1-4]. The resistance of polymeric materials to chemical agents depends on the nature of polymer as well as process additives utilized during extrusion [5,6].
Mendes et al. [7] noted that in the case of HDPE natural aging, significant changes in properties at diverse scales of structural and mechanical characterization may occur especially, in the absence of different stabilizers or when combined with an environmental stress cracking (ESC) effect. For instance, after 74 days of HDPE weathering, it was found from IR spectroscopy that a massif appeared between 1715 and 1740 cm-1 because of the carbonyl groups and a reduction by 50% of the weight-average molecular weight (from GPC analysis). In addition, degradation was illustrated with a 6% increase in crystallinity rate (from DSC analysis), a 50%
decrease in impact resistance (from Charpy tests) and a 90% decrease in elongation at break (from tensile tests).
It was concluded that a non-stabilized PE is “mechanically” out of service after three months of exposure to an urban environment. The weakening would be mainly caused by chain scissions and enhanced by combined thermo- and photo-oxidation phenomena [4,6,7].
In this work, it is sought to establish the effects of a commercial toluene–methanol mixture on mechanical and morphological properties of HDPE pipe and subsequently, investigate the extent of such effect across the
EXPERIMENTAL APPROACH
The material used in this study is HDPE-80 pipe with a standard dimension ratio (SDR) of 11 and a minimum required strength of 8 MPa. It is pigmented in black and it is intended for the distribution of natural gas. The operating pressure is 4 bars and it is designed for pressures as high as 6 to 10 gauge bars. It is extruded by STPM CHIALI Co. (Sidi Bel-Abbes, Algeria). The HDPE resin is produced by polymerization of ethylene monomers using the Ziegler-Natta catalyst. Typical properties include density and Young’s modulus respectively in the ranges 0.95–0.98 g/cm3 and 0.55–1.00 GPa. The glass transition temperature is about 300 K and the tensile strength varies from 20 up to 30 MPa. Usually, such material exhibits an OIT higher than 20 minutes and water absorption is less than 0.01%. The resistance to cracking in surfactant environment (ESCR) is under 15 mm/day. The external diameter of the pipe and its wall thickness are 125 mm and 12.4 mm respectively. As shown in FIGURE 1, the pipe wall is divided into 5 layers and standard testing specimens were directly machined from both pipe sides according to ISO 527 in order to preserve the thermo-mechanical history.
FIGURE 1. Specimen preparation: (a) Positions of extracted layers (from 1 to 5), (b) Turning and (c) Boring operations.
Prepared specimens (Type IV, ASTM D638) are identified with the corresponding layers and positions and then are immersed in previously set aging environments (tight glass jar, 22±1°C for 168 hr). Two environments are considered: (1) 50-50 % organic mixture toluene–methanol solvent (TM), (2) distilled water (DW). The reference specimens are those exposed to laboratory environment (ambient air). Mechanical and DSC analyses are performed by means of a Zwick-1120 universal testing machine and a NETZSCH DSC200-PC/E differential scanning calorimeter. Crystallinity is evaluated using the following equation:
(1)
With ΔHf, the specific fusion enthalpy of the polymer under consideration; ΔHf0 is the fusion enthalpy of 100%
crystalline polymer which is taken as 290 J/g for PE [8-10].
RESULTS AND DISCUSSION
FIGURE 2 illustrates stress-strain curves at different conditions and alternate positions within the pipe wall.
These curves display a characteristic behavior of semi–crystalline HDPE material showing an initial linear elastic region followed by a cold drawn region, consisting of a propagating neck at constant stress level, and finally, an over–stretching section featuring ultimate tearing. FIGURE 2a is a global depiction of the stress- strain behavior for all the specimens machined across the pipe wall, i.e. from outer to inner layers. It can be affirmed that the inner layers have better mechanical properties than external ones. One explanation can be
o f
f
C
H
X H
(b) (c)
deduced from the post-manufacturing state of internal stresses and the resulting mostly-crystalline morphology of the inner layers which have been subjected to slow cooling subsequent to extrusion process [10].
In FIGURES 2b, 2c, 2d, the effects of DW and TM are illustrated for 3 representative pipe layers: internal (FIGURE 2b, layer 1), intermediate (FIGURE 2c, layer 3) and external (FIGURE 2d, layer 5). It is observed that TM mixture is the most absorbed by HDPE (absorption rate ≈ 3.8% for inner layer). Globally, the ambient air case displayed the highest mechanical resistance in comparison to TM and DW environments. Since it is the most absorbed, TM seems to have the highest effect on mechanical properties. For example, the yield stress decreased as low as 17 MPa.
When it comes to properties across pipe wall, layers (3) and (5) are the most affected by TM (FIGURES 2c, 2d). For DW environment, layer (5) is the most affected (FIGURE 2d). It should be noted that in case (Layer 5, outermost one), the 3 environments are closer to each other meaning that differences are relatively small compared to the other cases. Young's modulus (E) and the yield stress (σy) have completely collapsed in the TM mixture and relatively less in contact with DW. The attenuation of E and σy is also observed for all pipe layers in TM and DW environments. These 2 properties adopt the same evolution through the pipe wall: they increase from the outer to the inner side. Compared to ambient air case; the drops in E values after exposure to TM mixture, for the external, intermediate and internal layers are 31%, 41% and 33%respectively.
FIGURE 2. Stress-strain curves for HDPE pipe resin: (a) in air, across pipe wall from inner to outer layers. Comparison with TM and DW environments for 3 positions: (b) internal (Layer 1), (c) intermediate (Layer 3) and (d) external (Layer 5).
On one hand, it is alleged that properties fluctuations are owing to morphological parameters illustrating a structural hierarchy which is governed by crystallinity during the constant volume flow process and definitely enhanced by internal stresses. On the other hand, the presence of aggressive chemical environment in direct
of a contact between a non-polar polymer and a hydrocarbon-based solvent. In addition, HDPE is known to slightly swell in aliphatic solvents and more in aromatic ones in which it partially can dissolves if the temperature is sufficiently increased [11-13].
Differential Scanning Calorimetry (DSC) method is used to determine the degree of crystallinity (c) on virgin and aged PE samples for a period of 7 days. The thermographs are shown in FIGURE 3. The effect of TM mixture is significant in comparison to DW results. For ambient air specimens, melting temperature (Tm) reached an upper limit value of 134°C while aged samples went up to 139°C.
FIGURE 3. DSC thermograms at 5 different positions in the pipe wall for the 3 environments:
(a) Ambient air (as-received pipe), (b) Aged in TM mixture, and (c) Aged in DW.
The crystallinity rates are calculated according to equation (1) shown above, for HDPE samples aged during 7 days (in TM and DW environments). TABLE 1 illustrates the differences in melting temperatures and crystallinity measurements. It is clear that these two properties are not constant across the pipe wall and globally, they increase from the inner towards the outer layers. The smallest values, namely c=53 % and Tm=130 °C belong to the 5th layer, i.e. the outermost layer. On the other hand, c=77 % with Tm=134 °C, are values measured at the inner layer. Again, these differences in morphological properties are due to the extrusion process requiring abrupt cooling at the wall outer side. Such results are also confirmed in a recent study which compares unwelded and welded polyethylene pipes [14]. The authors calculated the degree of crystallinity on samples extracted directly from the wall of the non-welded pipes and also in the welding zone. Three layers were chosen:
(1) OL (outer side the pipe), (2) ML (middle layer) and finally (3) IL (inner side the pipe).These results indicate that the effect of the supplied heat and melting material contributed in creating a more ordered structure as deduced from crystallinity data. Certainly, the crystallinity increased by 15 % for the OL layer. In the intermediate layer (ML), calculated values show very slight variation in c between welded and unwelded specimens (< 1%).This might have a relation with the interposition of this layer which is protected by the other
two limit layers and, thus confining the effects of external disturbances in terms of unwanted material rapid cooling or heat loss by slow natural convection or structure variances like chain entanglements [14].
TABLE1. Comparison of Tm and c across pipe wall in air, TM and DW.
Parameter Ambient Air Toluene-Methanol Distilled Water
Melting Temperature, Tm (° C) 130-134 132-139 131-137
Crystallinity, c (%) 53-77 58-81 55-73
In FIGURES 4a and 4b, we compiled the results obtained from DSC analysis, particularly the melting temperature and the degree of crystallinity. It is clear that these properties are not constant across the wall. In the as-received case, we found that c varies in the range 5377 %, and that the melting temperature fluctuates within 4 °C interval. The smallest values, namely c=53 % and Tm=130 °C belong to the 5th layer (the outermost layer). On the other hand, c=77 % with Tm=134 °C, are values recorded at the inner layer.
FIGURE 4. Evolutions of crystallinity and fusion temperature in TM and DW environments.
The cooling rate of the molten polymers has a preponderant influence on crystallinity rate which will be the lower the higher the cooling rate. This property is related to the kinetics of crystallization. The growth rate varies with temperature and when the product is rapidly cooled, the crystal structure does not have time to grow (freezing state). During an extrusion process, the various local cooling conditions can lead to different morphologies across pipe thickness. In the zones of rapid cooling, we obtain rather a microstructure dominated by spherulites of very small sizes. In addition, a non-spherulitic material with a low rate of crystallinity is present as quenching from the molten state usually reduces crystallinity. Conversely, for slow cooling zones, the microstructure comprises larger spherulites with the presence of voids and crystallinity rate turns to a more uniform profile [15].
For TM medium, mechanical properties decreased, while crystallinity is augmented indicating an extent of degradation. A value, as high as 81 % in crystallinity, is recorded at the inner layer which is in accordance with the effects extrusion enhancement. For DW environment, c changed relatively little compared to the air case.
The smallest values of c and Tm, respectively 55 % and 131 °C, belong to the 5th layer, while the largest values are recorded for the 1st layer. According of their structural state, semi-crystalline polymers are intrinsically heterogeneous in nature and the crystalline phases are usually impermeable to aggressive agents as well as to microorganisms. However, only the amorphous phases are sensitive to aging phenomena because of relatively lower apparent strength. Such selectivity is then attributed to the disorder in the amorphous zones allowing the access of foreign species to macromolecular chains. It is also stated that degradation initiates first at amorphous parts of a polymer. Subsequently, this selective degradation gives rise to an increase in crystallinity rate of the
induction time (OIT) decreases, the resin is aged and becomes vulnerable to external attacks, resulting from the consumption and loss of antioxidants.
FIGURE 5.Evolution of OIT through pipe wall thickness of HDPE-80
FIGURE 5 shows the evolution of OIT across the wall for as-received samples and others as aged in TM and DW environments. It is concluded that the effect of the organic solvent is remarkable since OIT is decreased to less than 20 minutes especially at the inner layer. The TM mixture presents a rather real danger for HDPE pipes as values less than 15 min are recorded close to the inner layer (significant decrease of ~ 50% in comparison with air case). Through the wall, the outer layer appears to be the most secure with the largest OIT. This is explained by the fact that during the differential cooling required by the process, most of the antioxidant is frozen in the outer layer, which makes it more resistant [17].
CONCLUSION
This study presented an experimental work which allowed investigating the variability of properties across a HDPE pipe wall. The main properties considered are mechanical behavior, crystallinity and OIT in both air and aggressive environments. The stress-strain behavior of HDPE is governed by 3 distinctive regions indicating elasticity, cold drawing process and plastic hardening. This state is prevailing even across pipe wall with an established trend from outer towards inner layers. In contact with TM mixture, HDPE properties degradation is pronounced and can be distinguished within a 7 days period. The effect of distilled water (DW) is minor compared to that of TM. Crystallinity increased under the effect of chemical environments indicating the attained extend of degradation.
ACKNOWLEDGEMENTS
Part of this work was performed at LR3MI of UBM Annaba (Algeria). The financial support through CNEPRU Project (2015-2018), Code # A11N01UN23012014122 sponsored by the MESRS is highly appreciated.
REFERENCES
1. Plastics Pipe Institute, PPI, Handbook of Polyethylene Pipe, 2nd Edition, (2008).
2. R.B. Arieby, K. Mrabet, O.A. Terfas, C. Laurent, R. Rahouadj, “Anisotropic mechanical behavior of semi- crystalline polymers: Characterization and modeling of non-monotonic loading including damage”, Int. J.
Appl. Polym. Sci. 133 (2016), pp. 1-11.
3. R. Schouwenaars, V.H. Jacobo, E. Ramos, A. Ortiz, “Slow crack growth and failure induced by manufacturing defects in HDPE tubes”, Eng. Failure Analysis, 14 (6), (2007), pp. 1124-1134.
4. B.H. Choi, Z. Zhou, A. Chudnovsky, S. Stivala, K. Sehanobish, C.P. Bosnyak, “Fracture initiation associated with chemical degradation: observation and modeling”, Int. J. Solid. & Struct., 42 (2005), pp.
681–695.
5. J. Cazenave, R. Séguéla, B. Sixou, Y. Germain, “Short-term mechanical and structural approaches for the evaluation of polyethylene stress crack resistance”, Polymer, 47(2006), pp. 3904-3914.
6. Zhang J., “Experimental Study of Stress Cracking in High Density Polyethylene Pipes”, Ph.D. Thesis, Drexel University, 2005.
7. L.C. Mendes, E.S. Rufino, F.O.C. De Paula, A.C. Torres Jr, “Mechanical, thermal and microstructure evaluation of HDPE after weathering in Rio de Janeiro City”, Polym.Degrad. & Stability, 79(2003), pp.
371-383.PII: S0141-3910(02)00337-3
8. L. Alimi, K. Chaoui, W. Ghabeche, W. Chaoui, “Short-term HDPE pipe degradationuponexposure to aggressiveenvironments” , Matériaux& Techniques, 101(2013), 701.
https://doi.org/10.1051/mattech/2013083
9. N. Hamlaoui, S. Azzouz, K. Chaoui, Z. Azari, M.-A.Yallese, “Machining of Tough Polyethylene Pipe Material: Surface Roughness and Cutting Temperature Optimization”, Int. J. Adv. Manuf. Technol., 92 (5- 8), (2017), pp. 2231-2245, https://doi.org/10.1007/s00170-017-0275-4
10. L. Alimi, “Comportement Mécanique de nouvelles résines HDPE dans des milieux agressifs”, Ph.D.
Thesis, Badji Mokhtar Annaba University, 2016.
11. M. Munaro, L. Akcelrud, “Polyethylene blends: A correlation study between morphology and environmental resistance”, Polym. Degrad. & Stability, 93 (1), (2008), pp.43-49.
12. R. Dossogne, Polyéthylène haute densité, PE-HD, Techniques de l’Ingénieur, Traité Plastiques et Composites, A 3315, (2003), pp.1-15.
13. B. Fayolle, X. Colin, L. Audouin, J. Verdu, “Mechanism of degradation induced embrittlement in polyethylene”, Polym.Degrad. & Stability, 92 (2007), pp. 231-238.
14. S. Niou, K. Chaoui, S. Azzouz, N. Hamlaoui, L. Alimi, “A method for mechanical property assessment across butt fusion weldedpolyethylene pipes”, Int.J.Adv.Manuf.Technol., 97 (1-4), (2018), pp. 543- 561.https://doi.org/10.1007/s00170-018-1908-y
15. L. Hubert et al., “Molecular architecture-property relationships of high density polyethylene for pipes”, Polymeric Mat. Sci. & Eng., 86(2002), pp. 287-288.
16. C. Eldsäteret al., “The biodegradation of amorphous and crystalline regions in film-blown poly (epsilon- caprolactone)”, Polymer, 41(4), (2000), pp. 1297-1304.
17. W. Yuet al., “Antioxidant consumption in squalane and polyethylene exposed to chlorinated aqueous media”, Polym. Degrad. & Stability, 97(2012), pp. 2370-2377.
