Surface degradation of HDPE-100 pipe: Effects of some aggressive environments (solvents)

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Surface degradation of HDPE-100 pipe: Effects of some aggressive environments (solvents)

WafiaGhabeche¹, LatifaAlimi¹, Kamel Chaoui²

1) Welding and NDT Research Centre (CSC). BP 64 Cheraga - Algeria.

2) LR3MI, Mechanical Engineering Dept., Faculty of Engineering, Badji Mokhtar University, BP 12, Annaba, 23000, Algeria

Corresponding author: [email protected]

Abstract:

In this study, external and internal surfaces of a high density polyethylene (HDPE-100) pipe are characterized in terms of roughness and hardness in order to understand surface variances in the as-received material and to appreciate surface quality for electrofusion welding or resistance to given aggressive environments. The effects resulting from the action of 3 environmental stress cracking agents that might be in contact with pipe surfaces are studied separately following a specific protocol. In the initial state (as-received pipe), it was found that the outer surface is rougher and harder than the inner surface. In contact with distilled water, the external surface roughness increased slightly by 6% while hardness decreased by 14%

although the roughness of the internal face of the tube revealed a small decrease (<5%). The effect of hydrochloric acid was examined with varying concentrations and it was observed that roughness augmented with concentration while hardness remained relatively constant after a major drop by 40% and 32% for outer and inner surfaces respectively. Finally, in the cases of oxidizing agents, dichloromethane (CH2Cl2) and (50:50) mixture of toluene and methanol, a significant disturbance of surface quality is observed and led to a hardness decrease of both outer and inner surfaces of by 20% and 16% respectively. The mechanical properties are also affected as revealed in literature studies.

Crystallinitymeasurements confirm the gap between outer (51.55%) and inner (61.31%) surfacesindicating that degradation has taken place at the structural level when HDPE wasin contact with those aggressive agents. After exposure to these environments, results indicate that crystallinity fell approximately one third compared to as-received material, therefore reducing resistance to fracture and pipe lifetime.

Key words: polyethylene pipe; surface roughness;

hardness; degradation; aggressive environments;

crystallinity.

1. Introduction

Surface phenomena and surface properties pertaining to polymer processing and polymer products invoke various complex aspects such as surface tension, friction, wear, adhesion, tribology

and triboelectricity. It is accepted that these properties are not completely understood and depend heavily on experimental and material conditions especially for composites, nanocomposites and semi-crystalline polymers [1-3]. Although high density polyethylene (HDPE) pipes and fittings are widely used in pressurized distribution and transmission networks to supply water or natural gas for domestic and industrial customers, these properties remain unclearly specified. Of course, a range of advantages in terms of service life, maintenance operations, installation costs and resistance to corrosion, made HDPE pipes ready to win a significant advance on other metallic materials, i.e. the best substitutes for older steel and nodular cast iron underground pipes [3-7]. However, HDPE vulnerability to UV rays, Environmental Stress Cracking (ESC), heat, fluctuating pressure loads and bacterial attacks, impose various additives to enhance mechanical, thermal and chemical resistance to those degrading external agents [8-12]. At the same time, some studies continue to treat mechanical and surface properties in the light of newer HDPE applications calling on resistance aspects especially in adverse environments and its implication on buried pipe lifetime estimation [5,9].

For external surface studies, at least, two new further aspects are considered and they concern (i) PE pipe surface physico-chemical preparation for electrofusion welding [13] and (ii) surface alteration during pollutant-plastic debris interaction while in the marine environment [14]. In order to get an approved weld joint, scraping of a 0.2 mm outer layer of PE pipe is needed to eliminate oxidized and altered surface material. Different pipe preparation and conditioning were considered and it was found that cleaning with either ethanol or heptane solutions could not replace the effectiveness of the scraping operation [13]. Alternatively, it was shown that PE surface properties; for instance, surface area, surface topography, functional groups and acid-base behavior are significant factors which influence sorption and the state of the formed altered external surface controlled by seawater pH [14].

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On the other side, the inner pipe surface is largely influenced by the nature of the fluid being transported as shown in many studies. [15-17].

Unfortunately, these studies did not present the initial existing differences between outer and inner pipe surfaces and did not assess them in terms of roughness, hardness and crystallinity. This is an imperative issue as structure heterogeneities and mechanical properties are distributed across pipe wall as it was shown in a recent study [18].

The objective of this study is to compare as-received roughness and hardness values between outer and inner surfaces of an extruded HDPE-100 pipe. In a subsequent step, the effects on both surfaces once exposed to water, hydrochloric acid and oxidative agents for a given period are evaluated in terms of mechanical and surface properties.

2. Experimental approach 2.1 Specimen preparation

HDPE-100 polyethylene pipe used in this study is kindly supplied by Sonelgaz (Annaba, Algeria) and fabricated by Chiali Company. It was checked to be free from any surface alteration due to mishandling or unwanted contacts with other objects during transportation (Fig. 1a). Both standard dimension ratio (SDR) and outside diameter are respectively 11 and 113 mm. Some properties of the as-received material are summarized in Table 1.

TAB. 1: Material properties (according to manufacturer).

Property Value

Density

Melt Flow Index (MFI)

% black carbon Young's Modulus Yield Stress

Strain at failure Shore Hardness (HS) Toughness (K_IC)

Oxidation Induction Time (OIT)

Environmental Stress Cracking Resistance (ESCR)

≥ 930kg/m ³ 0.2-1.4 g/10 min 2-2.5 % 0.55 - 1 GPa 20-30 MPa

350%

61-67 2-5 MPa.m^½

 20 min



 15 mm/day

ESC test specimens were carefully cut using a lathe and then a small milling machine at very low speeds. Each prepared ring is used to produce 5 specimens. Each specimen is 50 mm high and has a curved length of 50 mm as sketched in Figure 1b.

Each testing condition should contain not less than 5 valid specimens. During measurements, any deviating value should be repeated elsewhere in the same specimen and can be disregarded in case of detected surface defect. As-received specimens are conditioned in dry tight plastic containers until

measuring date. ESC specimens are immersed in respective environments for determined periods with tight covers. The environments considered in this study are: (1) distilled water (DW), (2) dichloromethane (DCM or CH2Cl2) and (3) a commercial (50:50) mixture of toluene-methanol (TM).

FIG. 1: (a) typical surface alterations due to mishandling, (b) Geometry of specimens used measure roughness.

The data presented in this study is relative to an exposure time of 43 months. Before each measurement, the sample is rinsed with distilled water, allowed to dry freely in open air and identified according to a written protocol. Specimens and containers are stored at laboratory temperature in relatively thermally isolated space.

2.2 Characterization techniques

Roughness is followed on the outer and inner faces of each sample. Three roughness criteria are measured: (1) arithmetic average deviation of the profile (Ra), (2) standard deviation of the profile, (Rq or RMS) and (3) average height of profile or the sum of the highest heights and deepest lows (Rz) as defined by DIN-4768 (1990). Mathematically, they are expressed by the following equations:

(1c) 5 Y

Y 1 5 R 1

(1b) N Y

R 1

(1a) N Y

R 1

5

1 i

v 5

1 i

p Z

1 i

2 i q

1 i

i a

i

i

























N N

With N: Number of events (depth or peak), Yi: deviations from a mean line, Ypi: highest profile peak, Yvi: depth of the lowest profile.

These values are automatically measured with a Surftest-201 type (SJ-201M) Mitutoyo roughness meter consisting of a diamond stylus with a mechanism for automatic removal and storage of measured data. The diameter of the tip is 5 microns and moves linearly over a length of 4 mm analysis with a width of 0.8 mm. It has a measuring range up

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to 350 µm (-200 µm to 150 µm) and can have a surface texture using different roughness parameters.

These conditions are well adapted to measurements of the roughness of HDPE pipes.

Roughness measurements were carefully made on each of the dry specimens. Each value is the average of 6 measurements at different locations and the estimated uncertainty is approximately 0.01 µm.

Shore hardness (HS) measurements were performed using a Mitutoyo durometer Hardmatic (HH-401) based on a statistical protocol for both specimen surfaces. This test is normally the rebound of a spherical calibrated carbide projectile at a rate determined on the material to be tested, the harder the material the higher the rebound. When the projectile strikes the surface, the loss of kinetic energy that results from surface deformation is calculated using velocity measurements. The device measures the ratio of rebound speed compared to the impact speed.

Hardness reading is electronically displayed on a screen after recording corresponding signal voltages because of projectile movements. It should be noted that because of specimen curvature, inner surface hardness measurements should be carefully carried out by ensuring the true complete contact of the probe with a flat plane. Crystallinity was measured using an X'pertPro (PANalytical) X-ray diffractometer. A scan rate of 2°/min at 2000 cycles using CuKradiation of wavelength 1.596 Å was applied. A radial scan of Bragg angle versus intensity was obtained with an accuracy of ± 0.25° at the location of the peak. The processing of diffraction patterns was performed using software based on the ASTM data and crystallinity (Xc) is determined using equation (2):

(2)

with: Icr, Iam: scattering intensity of crystalline and amorphous structures respectively.

3.Results and Discussion 3.1 As-received pipe material

The as-received pipe showed higher roughness and hardness values at the outer surface. In the case of roughness, the difference is almost two folds (43.7%) while for hardness it was nearly a quarter higher (~ 19.3%) than inner surface (Fig. 2 and 3: initial state). Processing conditions are responsible for these discrepancies caused by a state of compressive internal stresses resulting from water cooling of the outer pipe layers [19]. The DRX analysis carried out on both surfaces showed 2 peaks at 21.5 and 23.9 which are characteristics of (110) and (220) lattice planes, respectively[20].The main observation in the

present work indicated a modest change in peaks’

position throughout the scan range and no significant changes of the inter-planar distances(dinner= 0.408 nm;

douter= 0.415 nm). Crystallinity measurements

indicated a difference of 15.9 % in favor of inner surface layers; this is one worthy and plausible explanation for the better mechanical properties at this pipe side (Table 2).

TAB. 3: Crystallinity, Melting and crystallization temperatures, of the two surfaces, internal and external, from the XRD spectra and DSC measurements.

Environment XCint

(%)

XCext

(%) Tfint

(°C)

Tfext

(°C) Initial State 61.31 51.55 140.5 137.6 Distilled Water 44.71 42.24 139.3 137.7

CH₂Cl₂ 46.62 38.79 - -

Toluene-methanol 44.37 39.90 137.9 136.2

3.2 Effects of some environments 3.2.1 Distilled water

Figure 2 illustrates the effect of distilled water on pipe surface roughness. For the inner side, the absolute roughness (Ra) decreased by 4.9% while for the outer surface, it increased by 5.8%. The two folds difference that existed before exposure to distilled water (Figure 2: initial state) is roughly maintained after contact (49.5%).

(a)

(b)

FIG.2: Effect of distilled water on the roughness of (a) inner surface and (b) outer surface.

Polyethylene material surface is usually hydrophobic and therefore resists to water while under wet conditions and in the presence of

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pollutants, a gradual loss of hydrophobicity occurs.

The swelling phenomenon of PE has been identified and can be worsened by other parameters such as external pressure. Kriston et al. [21] noted that for early immersion in distilled water, the weight of the specimens increased by 0.50% in 5 months but gradually declined until stabilizing around 0.19 % in 12 months. This weight loss may be related to the migration of antioxidants as described by Hoang et al.

[22]. They concluded that the rate of antioxidant consumption during immersion in water is larger on the inner surface and the loss of antioxidants makes it first predisposed to degradation. This loss is suggested in other studies as a reliable indicator for measuring polymer degradation [23]. In this study, it was found that, the weight of the specimens decreased of 0.40% of initial weight just after four weeks, which strengthens the hypothesis of migration. On the other side, hardness analysis showed a clear decrease after water contact indicating a sort of material softening and the changes are 18%

and 14% for outer and inner surfaces respectively (Figure 3). It is suggested that the process of crystallization has a direct influence on hardness of thick products as it occurs inside the solidifying material (from the melt) at elevated temperature.

Usually, such conditions are unfavorable for strength and toughness of polymers [24]. The outer surface is probably more willing to absorb water; this should have a relationship with the nature of the morphology of extruded HDPE [22].

FIG.3:Effect of distilled water on the hardness in internal and external surface of the tube.

3.3.3 Dichloromethane

Both volatile organics and mineral oils present in the ground water, or in the pipe backfill, can permeate plastic piping. The occurrence of contamination is generally identified by customers and designated by an unusual taste and odor in water distribution systems. For highly toxic substances including dichloromethaneand benzene, the taste and odor threshold should be well below the drinking water Maximum Contaminant Level (MCL) [25].

Dichloromethaneisa solvent which is known for its

volatility and its high oxidizing power, usually used in chemical resistance testing for determining the quality of polymer tubes [26,27]. Roughness results are illustrated in Figure 4. Dichloromethane effect seems reduced on outer surface ( 1% increase compared to initial value) while a razing phenomenon is observed at the inner surface (roughness drop by 10%).

(a)

(b)

FIG. 4: Effect of dichloromethane and toluene-methanol on the roughness of (a) inner surface and (b) outer surface.

Regarding hardness, a state of softening of both surfaces is observed as shown in Figure 5. The decrease is ~ 31% for inner surface and 19% for outer surface. Such observation was also made by L. Baena et al. for HDPE samples exposed in gasoline and a 10% decrease was found. This change is probably owed to gas absorption that might act as a plasticizer in such material [28].These conditions could enhance a dangerous situation for pressurized pipes which can undergo uncontrolled gas condensation or leakage of flammable fluids into the ground.

FIG. 5: Effect of two hydrocarbons on the shore

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hardness in internal and external surface of the tube.

3.3.4 Toluene-methanol mixture

For applications involving chemical substances, explicit data is rarely published for HDPE pipes.

Usually a rough indication is given for an association polymer-environment indicating whether resistance is moderate or excellent [29]. The effect of the mixture of toluene-methanol on both roughness and hardness is depicted in Figures 4 and 5. Usually a roughness increase is observed for both surfaces (21% and 11% increase for inner and outer surfaces respectively) although outer surface roughness is always higher. Literature advances that methanol is likely to reduce surface energy making the material more vulnerable to ESC and becomes more ductile [30]. In the same trend as for DCM, hardness decreasedwith exposure to toluene-methanol mixture and the changes are less than 20% for both sides.

3.4 Crystallinity evolution

In terms of material structure, crystallinity (XC) is measured for the previously discussed environments.

Two basic results are deduced: (1) inner side crystallinity is always higher compared to outer surface and (2) exposure to DW, DCM and TM reduces crystallinity up to 30%. The measured cristallinity values are shown in Table 2 for all cases together fusion temperature when possible. These findings are confirmed from literature review which indicates that usually crystallinity is evolving across the pipe wall from the outer to the inner surface [31-33]. Similar trend is also confirmed even when PE is exposed to aggressive environments which can lead to cracking [15].

4. Conclusions

This study allowed drawing the following conclusions:

1. A clear difference is quantified between outer and inner pipe wall surfaces in terms of roughness and hardness. Usually, the outer surface is rougher and harder compared to the inner one probably because of die processing.

2. Exposure to aggressive environments contributed to increase roughness and lower hardness on both sides while keeping the tendency in the same direction.

3. crystallinity is found to be lower on the outer layers and the trend is preserved after exposure to aggressive environments which is also confirmed from literature.

Acknowledgements: The authors would like to thank SONELGAZ Co., STMP CHIALI Co. and TUBOGAZ Co.

for providing pipe samples. Also, they would like to express their gratitude to the LMS laboratory of Guelma University and to the Physics Dept. of M’Sila University. Fruitful discussions with LR3MI of UBM Annaba members are highly appreciated.

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