Polyethylene of Raised Temperature Resistance (PE-RT) Nanocomposites Reinforced with Graphene Oxide for Application in Flexible Pipelines

Abstract

Polyethylene of Raised Temperature Resistance (PE-RT) is a specially designed high-density polyethylene utilizing bimodal resin technology. It is suitable for oil and gas pipeline applications, exhibiting excellent long-term creep resistance at elevated temperatures. Flexible risers require security against potential offshore oil leaks, prompting the reinforcement of PE-RT with graphene oxide (GO) at concentrations of 0.5, 1.0, and 2.0 wt.%. The nanocomposites were processed in a twin-screw mini extruder at 180°C, with a screw speed of 100 rpm and a residence time of 6 min. The results indicated an increase in the thermal stability of the nanocomposite by 14°C. XRD and DSC analyses revealed a higher crystallinity index for the sample with the lowest GO content. The water contact angle values increased from 98° in the pure matrix to 102° in the nanocomposite, suggesting a slight increase in the material's hydrophobicity due to nanoparticle incorporation. SEM images demonstrated a good interface between GO nanoparticles and PE-RT without agglomeration. The Shore D hardness increased from 50 to 59 with 2.0 wt.% GO. Although the 0.5 wt.% GO composition exhibited a higher degree of crystallinity, other properties such as thermal resistance, Shore D hardness, contact angle, dispersion, and homogeneity tended to improve with increasing nanoparticle content. Consequently, it can be concluded that the optimal concentration of GO in the PE-RT matrix is approximately 1.0 wt.%. Incorporating 1.0 wt.% GO resulted in an increase in Young's modulus from 335 ± 37 MPa to 409 ± 42 MPa, indicating enhanced mechanical strength of PE-RT. Finally, aging tests showed that GO reduced the swelling degree of the final material, suggesting that PE-RT/GO is a promising candidate for pipeline applications.

Keywords:
Polyethylene of Raised Temperature Resistance (PE-RT); graphene oxide; nanocomposites; nanotechnology

1. Introduction

Flexible pipelines, also called risers, are used to transport oil from the well to production systems, as well as to transfer production to processing units¹. They are made of on-adherent polymeric and metallic layers, each responsible for a specific structural function. Sheet metal can suffer from a failure mechanism known as fatigue corrosion, caused by seawater entry into the pipeline's annulus, initiated by any damage to the outer layer and/or the transport of condensed water and contaminants received through the pipeline². Crude oil contains various corrosive contaminants such as CO₂ and H₂S, which, together with displacement caused by current and cyclic waves in the flexible riser, can lead to resistance fatigue, with cracks and collapses³. In flexible pipelines, the polymeric barrier layer must be leakproof, which is an essential requirement. Any damage to the barrier that permits oil to leak into the external environment can result in serious environmental disasters, most of which are irreparable⁴. Non-adherent flexible pipelines are designed following the specification of the main requirement to be met by the barrier layer, which is to maintain the tightness capacity throughout the useful life of the pipeline, generally corresponding to 25 years⁵. High-density polyethylene (HDPE) is one of the most widely used polymeric materials as a component of petroleum risers, especially employed in low-pressure and temperature conditions due to its moderate thermal stability, excellent chemical/oxidation resistance, and hydrophobicity⁶,⁷.

According to the literature⁵-⁷, it is plausible to anticipate that the polymeric layer of pipelines may experience swelling due to exposure to the transported materials. This swelling can potentially lead to the complete collapse of the tube, primarily because the polymeric layer may be unable to expand freely due to external constraints imposed by the surrounding steel layers. Swelling can cause a severe decrease in the mechanical properties of polymeric materials, weakening the structural stability of the entire pipeline.

The major limitation of polyethylenes, in general, is that when in contact with non-polar solvents, they undergo swelling, partial dissolution, modification of their original color, and, over time, loss of properties occurs, resulting in a reduction in the mechanical resistance of the material, due to the effect of surface tension cracking⁸. Kong and collaborators⁹ investigated a collapsed tube, concluding through shore D hardness tests and thermogravimetric analysis that the collapse occurred due to the combination of swelling and exposure to high working temperature (90 °C), proving that these two factors are mainly responsible for the limitation of polyethylenes as a polymeric barrier layer material.

Polyethylene of Raised Temperature Resistance (PE-RT), a special type of HDPE, can be classified into two types: I and II. Type II has a higher density and offers better resistance to high temperature and pressure compared to Type I⁹. The two primary processes for producing PE-RT are solution polymerization, developed by Dow Chemical Company, and bulk polymerization, industrialized by Basell Company. It is designed with bimodal resin technology. It can involve either a single reactor containing two different catalysts in the solution process or two reactors in series in the gas-phase process. This technology produces PE-RT resin with two very different molecular mass fractions, achieving excellent performance at elevated temperatures¹⁰. It contains a greater fraction of high molar mass chains compared to other polyethylenes, resulting in enhanced properties and durability¹¹. In addition, the long-term strength, toughness, ductility, resistance to fatigue, and the ability for PE-RT pipe to be cold-bent in the field, eliminating many elbow connections, make this material stand out in a different light¹.

PE-RT is a copolymer of ethylene and α-olefin, where the α-olefins commonly used are C3 (propylene), C4 (1-butene), C6 (1-hexene) and C8 (1-octene)². Ethylene-α-olefin copolymers are important products with favorable properties and are good alternatives to ethylene homopolymers. Incorporating relatively low amounts of α-olefin comonomer in the polyethylene changes the crystalline structure of the final product due to the addition of short chains pendant in the polymer backbone³. An improvement in resistance to environmental stress cracking (ESCR) is observed by the increased concentration of tie molecules since imperfections are created in the crystalline structure of the polymer by the short branching, excluding long chains from the lamellae⁴,¹². Therefore, lamellar crystalline structures are interconnected through amorphous tie molecule segments. The likelihood of forming these binding chains increases as the tie molecule's length becomes greater⁶,¹². As it forms a tie between the crystals, it greatly increases their thermal resistance. In fact, these tie molecules increase the material's useful life due to the connection between several crystals, exhibiting extensibility and mobility, absorbing and dissipating energy, increasing toughness and long-term fracture resistance⁷.

Therefore, PE-RT is widely used in hot water piping systems because of its high-temperature resistance, long-term high-pressure resistance, and excellent hot melt properties⁸. It has two advantages: stiffness (high modulus) and excellent long-term high-temperature creep resistance⁹. Furthermore, these unique structures can improve mechanical strength, creep resistance against external stresses, long-term hydrostatic strength, resistance to slow crack growth (SCG), and rapid crack propagation (RCP)⁸,⁹.

Polymer-based nanocomposites have been explored extensively for the numerous innovative and remarkable features that nanoparticles can provide to polymers⁹. Graphene has attracted attention for its advanced thermal, mechanical, and optical properties, and one of its most important derivatives is graphene oxide (GO). Oxidation of graphite powder to GO is a well-established approach to generating graphene-based materials. The large surface areas of graphene make its nanocomposites more advanced materials⁹,¹⁰. Graphite oxide is prepared by intercalating and oxidizing graphite with mixtures of strong acids, introducing abundant oxygen-containing functional groups to the graphite layers by expanding the layer spacing¹⁰.

Further exfoliation of graphite oxide by ultrasound in polar solvents produces GO, which can be considered an insulator and disordered analog of highly conductive crystalline graphene¹³. Recent studies have shown that GO is a promising material due to its high surface area and because it presents different functional groups of oxygen, such as carboxyl, carbonyl, epoxy, and hydroxyl groups, that allow its functionalization to become more effective when added to polymeric matrices¹⁴. GO is one of the most important graphene derivatives with a high negative charge density that can be prepared from graphite, offering an impressive price advantage over carbon nanotubes¹⁵. A recent study investigated the properties of PE-RT nanocomposites with 5.22 wt.% expanded graphene, and the mechanical strength showed an increase of 42%, from 16.84 MPa to 21.20 MPa⁹. Other research has reported an increase in Shore hardness of 12.5-14.5% in nanocomposites containing 0.01-0.1% by weight of carbon nanotubes functionalized with titanium stearate⁶. Finally, adding a small amount of graphene nanoplates (GNPs) to the PE-RT matrix showed that the GNPs acted as a nucleating agent, increasing the degree of crystallinity⁹.

The present study focuses on assessing the impact of incorporating graphene oxide (GO) nanoparticles into PE-RT for potential application in oil industry risers. The current literature lacks comprehensive studies evaluating the effectiveness of nanocomposites based on the PE-RT polymer matrix with GO. Therefore, the primary objective of this work is to investigate how the incorporation of GO nanoparticles influences the properties of PE-RT. Specifically, this research aims to address the prevalent issue of swelling in PE-RT, a significant limitation that affects its suitability for various applications. The produced nanocomposites were characterized by thermal, mechanical and morphological properties. The resulting properties of PE-RT nanocomposites in fixed fractions (0.5, 1.0, and 2.0 wt.%) of GO loads obtained by exfoliation/chemical reduction of graphite oxide were compared. PE-RT/GO nanocomposites were prepared using the melt blending method. It is worth noting that although several studies in the literature explore polyolefins with GO, this is the first to examine the impact of GO on bimodal polyethylene technology, which has become a trend, particularly in barrier layers for flexible petroleum pipeline.

2. Experimental Procedure

2.1. Materials

Natural graphite flakes were supplied by Nacional de Graphite Ltda., Brazil, with carbon contents of 87-99% (Graflake 99550). Polyethylene of raised temperature resistance (PE-RT) INTREPID with a density of 0.950 g/cm³ was supplied by Dow Chemical, USA. Sulfuric acid (H₂SO₄), nitric acid (HNO₃), sodium nitrite (NaNO₂), potassium permanganate (KMnO₄), and hydrogen peroxide (H₂O₂) were purchased from Sigma-Aldrich Brazil.

2.2. Graphene production by the modified Hummers method

The synthesis of oxidized graphene was carried out according to the literature¹⁶. A total of 5 g of graphite from Nacional de Grafite 99550 with an average particle size +50 mesh (> 300 μm) was used. The graphite flakes were transferred to a 1 L Erlenmeyer together with 4.5 g of sodium nitrite (NaNO₂) and 169 ml of sulfuric acid (H₂SO₄) under magnetic stirring for 3 h in an ice bath, initiating the graphite intercalation. In the intercalation step, the oxidation process started by slowly adding 22.5 g of potassium permanganate over 2 h to the reagent’s container. After this period, the ice bath was removed, and stirring was continued for 7 days. Then, the exfoliation step began, dripping 605 ml of 5% H₂SO₄ for 3 h with magnetic stirring. Subsequently, the viscous solution was washed with a solution of 3% H₂SO₄ and 0.5% H₂O₂ (15.8 ml of H₂SO₄, 7.8 ml of H₂SO₄, and 476.4 ml of distilled water) with magnetic stirring for 12 h. Twelve washes were performed with a solution of 3% H₂SO₄ and 0.5% H₂O₂ using a centrifuge. Then, the material was centrifuged and washed with distilled water until pH 7. This process generated oxidized graphite. Then, the material was dried in an oven at 100 °C until constant mass, resuspended in an Erlenmeyer with 70% ethanol, and placed in an ultrasonic bath (37-45 kHz) for 24 h in order to exfoliate the oxidized graphite to obtain oxidized graphene (GO).

2.3. Preparation of PE-RT/GO nanocomposites

The polymer pellets and GO particles were previously dried at 80°C. The nanocomposites were obtained using an extruder (Mini Lab II Haake composer) with double interpenetrating screws. Unfilled PE-RT was also processed under the same conditions for comparison. The following processing conditions were adopted: 180 ºC, screw rotation speed of 100 rpm for 6 min of residence. Four formulations were prepared in the extruder, exemplified in Table 1.

Table 1
Formulations of the obtained nanocomposites.

Codes	Description
PE-RT	neat PE-RT, processed
IGO0.5	PE-RT + 0.5 wt.% GO
IGO1.0	PE-RT + 1.0 wt.% GO
IGO2.0	PE-RT + 2.0 wt.% GO

2.4. Characterization of samples

2.4.1. Thermogravimetric analysis (TGA)

Thermal stability was measured using thermogravimetric analysis (TGA) on a Q500 instrument, TA (USA), for PE-RT and the obtained GO nanocomposites at triplicate measurements carried out under a nitrogen atmosphere. All samples (about 8 mg) were analyzed in a TA Instruments Q 500 analyzer (USA). Samples were heated at 10 °C/min up to 700 °C. Measurements were carried out in duplicate under a nitrogen atmosphere. In the thermograms obtained, T_onset (initial degradation temperature), T_max (temperature of the maximum degradation rate) and residue at 700 °C were determined.

2.4.2. X-ray diffraction (XRD)

The samples' crystalline arrangement was evaluated using X-ray diffraction (XRD) on a Rigaku Miniflex (Japan). The samples' crystallinity index (CI) was calculated through deconvolution using the ratio between the area corresponding to the crystalline regions and the area of the amorphous halo according to Equation 1¹⁷.

$CI=\frac{A_c}{\left(A_c+A_a\right)}$ (1)

Where CI is the crystallinity index, A_a is the amorphous halo area, and A_c is the area of crystalline peaks. The diffractometer operated with 40 kV and an electrical source of 20 mA. A scan was performed at 2θ in the 2 to 40° range, with a goniometer speed of 0.05°/min. The radiation used was CuKα of 1.5418 Å. The interplanar distance between layers (d) and mean crystallite size (L_c) values were calculated using Bragg's law and Scherrer's Equation, Equations 2 and 3¹⁷, respectively.

$n\lambda =2d\sin \left(\theta \right)$ (2)

$L_c=\frac{K\lambda }{\beta \cos ()}$ (3)

Where n is the reflection order, the wavelength of the X-rays; d is the interplanar distance between the layers; θ is the Bragg diffraction angle; Lc is the average crystallite size; K is a form factor (0.9), and β is the full width at half maximum (FWHM).

2.4.3. Differential scanning calorimetry (DSC)

DSC differential scanning calorimetry tests on a TA Instruments Q1000 (USA) were used to determine the melting temperature (T_m), crystallization temperature (T_c), and degree of crystallinity (X_c) of pure HDPE and nanocomposites. DSC tests used 4 to 6 mg samples in an airtight aluminum container. Two heating steps were performed. The first run was applied to erase the thermal history of the polymer. Then, the sample was cooled to 25 °C at a rate of 10° C/min with nitrogen flow and then heated again at 10 °C/min to 250 °C. The T_m and T_c temperatures were measured from the second heating curve. The degree of crystallinity was calculated from the enthalpy of fusion (ΔH_m) value according to Equation 4. For PE-RT, the theoretical melting value associated with the 100% crystalline polymer of 293 J/g was considered¹⁷.

$$X_c=\Delta H_m/\left(\Delta H_m{}^{100}\times ƒ\right)\times 100$$ (4)

Where X_c is the degree of crystallinity, ΔH_m is the heat of the enthalpy obtained by analyzing the sample, ΔH_m¹⁰⁰ is the theoretical melting enthalpy of the 100% crystalline material, and f is the polymer fraction in the sample.

2.4.4. Contact angle

The level of hydrophilicity of the samples was measured from the angle formed between the surface of the samples and one of the tangents of the water drop surface. Measurements were performed using a Ramé-Hart Model 250 room temperature goniometer (USA). Five drops of deionized water were deposited per sample, and the angle to the left and right sides of the drop was measured. Each measurement was performed every 0.2 s, totaling 100. The drops contained approximately 5.0 µL in volume. The images were captured by a computer connected to the system, which automatically calculated the angle value. The samples (0.1 mm thick film) were prepared in a hydraulic press at 180 °C for 6 min. Subsequently, they were cooled in a circulating bath at 30 °C.

2.4.5. Scanning electronic microscopy (SEM)

SEM analysis was used to observe the dimensions of the obtained GO and evaluate the cryofractured surface of the nanocomposites. The specimens were prepared in a hydraulic press at 180 °C for 6 min at a pressure of 69 MPa. Then, they were cooled in a circulating bath at 30 °C. Scanning electron microscopy (SEM) analysis of freeze-fractured (N₂ at -196 °C) surfaces previously coated with gold was performed using a Quanta FEG250/FEI (USA).

2.4.6. Shore D hardness

Analyzes were performed on a Shore type D hardness tester by Shore Instruments & MFG (Japan). Samples were prepared as ASTM D2240¹⁵. The dimensions of the samples were 5 × 5 mm with a minimum thickness of 6.4 mm, manufactured in a hydraulic press at a temperature of 180 °C. To reach the minimum thickness, five samples were stacked. Five measurements were taken for each sample, and the average value was considered according to the standard used.

2.4.7. Tension test

The mechanical tensile test was carried out according to the international standard ASTM D638 (Standard Test Method for Tensile Properties of Plastic)¹³, with a type V specimen in a universal testing machine Emic model L 3000 with a 30 KN (USA) load cell with 10 mm speed/min, manufactured in a hydraulic press at 180 °C. Twelve specimens of each composition were analyzed. The results concerning the mean and standard deviation of valid specimens were presented.

2.4.8. Aging test

The aging test was carried out using test specimens manufactured following the international standard ASTM D638 type V (Standard Test Method for Tensile Properties of Plastic)¹³, manufactured in a hydraulic press at a temperature of 180 °C. Samples with 1.0 wt.% graphene oxide were submerged in synthetic petroleum (70% heptane, 20% cyclohexane and 10% toluene by volume), according to ISO 23936-1¹⁴, inside steel autoclaves with a total volume of 2 L at a constant temperature of 80 ˚C and a sample was removed every month of aging. The initial (standard) mass of the specimens was measured before being submerged in the synthetic oil for monthly comparisons and calculation of the swelling degree. Subsequently, the samples were cleaned and stored in a dry environment for 3 days before being weighed to evaluate the swelling degree. After each withdrawal, the samples were dried. The variation in mass concerning time in the specimens was monitored by weighing, and the swelling degree (α) was determined from Equation (5)⁹.

$$\alpha =\Delta m/m_i\times 100$$ (5)

Where ∆𝑚 represents the variance in sample mass after immersion in synthetic oil in relation to the initial mass of the sample, and 𝑚i is the pre-test mass in grams.

3. Results and Discussion

3.1. Thermogravimetric analysis (TGA)

Monolayer graphene is a material that does not suffer degradation up to 700°C due to its high thermal stability¹⁸. Figure 1 displays the thermogravimetric curve (TG) and its derivative (DTG) of GO.

Figure 1
TG and DTG curves obtained from the GO sample.

As evident from the thermogram, oxidized graphene (GO) exhibits an initial degradation stage occurring at temperatures below 125°C. This initial stage is associated with the release of moisture, attributed to the adsorption of H₂O molecules by the GO material. The second stage, between 125°C and 300°C, is related to the loss of unstable oxygenated groups that degrade at lower temperatures¹⁹. This is due to the pyrolysis of hydroxyl, carbonyl and carboxylic acid groups into CO and CO₂²⁰. Finally, the third stage of degradation occurs continuously from 300°C to 700°C, and this is due to the presence of chemically linked oxygenated groups in graphene with greater stability than the molecules degraded in the second stage¹⁹,²⁰. Oxygenated groups such as epoxides and hydroxyls, found in the graphene network domain that do not represent defects, are relatively easy to remove. In contrast, those at the edges representing significant defects are difficult to remove completely²⁰. For more information about the graphene oxide produced, see the supplementary file.

Figure 2 displays the TG and DTG curves for pure PE-RT and its GO nanocomposites, and Table 2 provides the corresponding results.

Figure 2
Comparison of TG (a) and DTG (b) curves between PE-RT and graphene oxide-based nanocomposites.

Table 2
Results obtained by thermogravimetric analysis.

Sample	T10% (°C)	Tonset (°C)	Tmax (°C)	Residue (%)
PE-RT	431	446	474	0.07
IGO0.5	438	455	475	0.29
IGO1.0	442	458	477	1.02
IGO2.0	445	458	479	0.69

Based on the findings, the matrix remains stable from room temperature up to 400 °C. Above 400 °C, its thermal decomposition accelerates significantly, with almost 95% mass loss occurring in a single event within the 450–500 °C temperature range. The highest mass loss rate was recorded at 480 °C, and at 500 °C, the remaining mass was slightly less than 3%. Additionally, the DTG graph shows the percentage of mass loss per unit of time as the temperature increases at a specific heating rate. This behavior has been observed in previous studies¹⁸-²¹. As expected, the degradation temperature is well above the working temperature. The purpose of the analysis was to verify whether oxygenated groups could favor degradation, not to achieve drastic improvements. Oxygenated groups could potentially act as fuel, enhancing degradation and significantly damaging the material. However, as observed in the TGA analysis, there was even a slight increase in thermal resistance, which is not always expected with the use of this type of particle. The TGA successfully evaluated the impact of GO on the thermal resistance of PE-RT, considering both theories mentioned. The good dispersion of GO, which has excellent thermal conductivity, likely helped with heat dissipation, thereby contributing favorably to the improvement of the thermal properties in question. Table 2 presents the results, including the temperature corresponding to 10% mass loss (T_10%), the onset temperature of the degradation (T_onset), the temperature of the DTG degradation peak (T_max, representing the point at which the material degrades at the maximum rate), and the residue at 700 °C obtained from the thermograms. The incorporation of GO tended to elevate the degradation temperature of the materials, indicating that the nanoparticles enhance both the stability and thermal resistance of PE-RT.

With the incorporation of 2 wt.% GO, the T_10%/T_onset increased from 431/446 °C to 445/458 °C, respectively. This indicates a notable enhancement in thermal stability, as evidenced by a 12 °C increase in the T_onset and a 14 °C rise in the T_10% for the composite with this specific composition. The incorporation occurs due to the impermeable nature of graphene oxide nanosheets. As a result, the pyrolytic products of volatile gas manipulation must diffuse around the nanoparticle/polymer interface instead of traveling straight through the polymer chain²¹.

Thus, the tortuous gas diffusion paths through polymeric nanocomposites are increased, leading to a delay in the material degradation process, proving homogeneous distribution and effective interaction between PE-RT and GO²². Thermal degradation of PE-RT starts with C-C scission into radicals and then into the gas phase. The homogeneous distribution of GO in PE-RT and the strong interactions between them increase thermal conductivity, reducing the accumulation of heat in the polymer, thus not generating heat concentrating points that initiate the degradation of the material; that is, it delays degradation by distributing the heat better²³. Moreover, it promotes the formation of a protective char layer, shown by the increase of residue content at 700 °C, similar to a network-state structure in preventing the exchange process of heat and degradation²⁴. The results obtained by TGA indicate that the use of GO-based nanocomposites can be an economically promising strategy for the oil and gas industry, as the final product exhibits greater thermal stability compared to the pure polymer matrix used in the barrier layer, indicating a possible longer useful life of the riser.

3.2. X-ray diffraction (XRD)

Figure 3 shows the diffraction profiles of pure PE-RT and the GO nanocomposites produced. It is possible to observe a decrease in the intensity of the main polyethylene peak. It is worth mentioning that the diffractograms were not normalized according to their maximum intensity. Additionally, the intensity observed can be closely associated with the sample quantity and the film thickness in the nanocomposites. The increase in crystallinity is accompanied by a reduction in the width of the crystalline peaks, demonstrating a significant reduction in the amorphousness of the material. This occurs because the amorphous regions scatter the X-rays more widely, without ordering, giving rise to the extended amorphous halo. Another contribution to the improvement of crystallinity is the increase in the intensity of crystallographic planes at smaller angles. Thus, when making the relationship between the crystalline areas in relation to the total, the constructive effects due to the insertion of GO generate a larger crystallinity index (CI). When graphite is transformed into graphite oxide, this peak is generally shifted to 11˚, and the peak at 26.5° has its intensity reduced and broadened due to the exfoliation of graphite into graphene oxide. The peak around 26.5° of graphite corresponds to a basal reflection (002) with d spacing of 3.363 Å, which is compatible with the literature data (JCPDS 75-2078, 3.347 Å)²⁵,²⁶. When graphite is oxidized and exfoliated using the Hummers method, the resulting nanolayers become intercalated, leading to network distortion, as documented in the literature (ICSD 1543272, representing a monoclinic symmetry and a lattice parameter of a = 12.42 Å, b = 25.15 Å, c = 6.28 Å, and β = 97.8°)²⁵,²⁶. Based on data from the literature and the results obtained from TGA, Raman and FTIR (Figure S1 and S2 of Supplementary material), it can be concluded that the graphite was effectively oxidized. Still, its structure was not completely compromised as the peak did not completely shift by 26.5°. Furthermore, a drastic reduction in intensity can be observed and, together with the broadening of the peaks and the SEM and Raman results (Figure S2 of supplementary material), it can be concluded that the GO was drastically exfoliated. The same diffractogram was recorded when GO was incorporated into the polymer matrix without any obvious peak at 11˚. This may indicate that graphite oxide was exfoliated into graphene oxide during the process and was very well mixed into the PE-RT matrix across all compositions²⁷.

Figure 3
XRD profiles of graphite and GO (a) and PE-RT and nanocomposites incorporated with GO. M = monoclinic, O = Orthorhombic (b).

The XRD pattern also shows two striking and distinct reflection peaks (110) and (200) appearing at 2θ = 21.5˚ and 2θ = 23.9˚, which correspond to interplanar spacings of 4.1 and 3.7 nm, respectively, typical of an orthorhombic unit cell of a polyethylene crystalline structure. Orthorhombic is the most stable phase at room temperature but also exists a small peak at 2θ = 19.5˚, which corresponds to the (001)M plane of the monoclinic phase (M), and two little evident peaks at 2θ = 23.2˚ and 2θ = 25.1˚, referring to the (200)M and (201)M planes²⁸.

Table 3 displays the crystallinity index (CI), interplanar distance (d₁₁₀) and average crystallite size (L_c) values for the nanocomposites produced. The incorporation of GO tended to increase the CI of the matrix while also increasing the crystallite size, especially for the content of 0.5 wt.%. The IGO0.5% sample showed a shift of all diffraction peaks to smaller 2θ angles. It is believed that this concentration is still not enough to fill the polymer volume equally, presenting regions with lower GO contents or without the presence of such nanoparticles. Therefore, the GO nanolamellae tend to be located in regions with greater free volume in the polymer matrix, which is generally the amorphous region. However, there is also the possibility of more exfoliated nanosheets slightly increasing the interplanar distance of the crystallites. Two main theories could corroborate this phenomenon. The polymer crystallites could nucleate and grow on the surface of the corner of the most exfoliated GO nanosheets so that the planes are slightly further apart due to the separation at the edges of the crystal. The second theory would be that the oxygenated groups of GO would be responsible for the slight distortion. However, as observed in the diffractogram and by calculating d_110, such variations are very small. They are considered to have a less significant influence on the final properties of the materials than the increase in CI. Therefore, the nanoparticles promote increased crystal growth, as evidenced by the rise in CI and L_c. Compared to neat PE-RT, the L_c of the nanocomposites with 0.5 wt.% GO increased by approximately 5 nm. It implied that due to the addition of a low amount of GO nanoparticles, the lateral growth of the crystals along the (110) plane was promoted. At the same time, there is a sharp decrease in the intensity of the reflection and their half-widths in the 2θ region of 21.5˚, which indicates a change in their defectiveness²⁹. The incorporation of GO into the matrix likely helped to reduce defects at the edges of the crystalline structures, resulting in narrower signals in the diffractogram, indicative of a more ordered material. Additionally, the narrow diffraction peak suggests a smaller discrepancy in the distribution of crystal sizes and structures with different interplanar distances, leading to a higher degree of ordering of the crystallites relative to the (110)O plane in the nanocomposite.

Table 3
Crystallinity index (CI), interplanar distance (d₁₁₀), and the average size of crystals (Lc) in PE-RT and formulations based on graphene.

SAMPLE	2θ (°)	d110 (nm)	Lc (nm)	CI (%)
PE-RT	21.57	0.41	17.6	45
IGO0.5	21.19	0.42	22.8	55
IGO1.0	21.72	0.41	17.8	42
IGO2.0	21.61	0.41	18.4	43

Increasing the GO content above 0.5 wt.% caused a slight reduction in the CI, which shows that the nanoparticles seem to decrease the mobility of the polymeric chains, thus reducing their organization and decreasing the crystallinity, which overcomes the nucleating effect of the nanofiller. The excess energy present at the interface between the polymer and GO likely serves as a driving force for crystal growth, as indicated by the observed increase in L_c. It suggests that the compatibility between the materials is somewhat good. In addition, as observed in the non-variation of the interplanar distance (d₁₁₀), it can be suggested that the nanoparticles did not deform or distort the polymer crystals concerning the plane (110), and probably the nanolaminates are lodged in the amorphous regions of the material. That is, the nanocomposites present crystals with similar perfection and quality.

In addition, the crystallographic plane that generates the signal around 16.8° has a wider interplanar distance than the planes with higher angles. As observed in the diffractogram, the incorporation of the GO in the polymer matrix has a significant effect on the organization of this plane. It is believed that for the same reason that nanolaminates are allocated in amorphous regions of greater free volume, they also orient the material in a way that favors crystallographic planes with greater interplanar distances. Namely, the phenomenon of GO intercalation in organized regions with greater spacing between them cannot be excluded either. It can also be speculated that nanoparticles prefer regions with greater free volume, and they end up ordering these structures, generating a certain organization, as observed by the increase in CI. At this point mentioned, it is only possible to observe very weak signals that may be related to the orientation of the nanolaminates due to the flow created by the extrusion, generating, in a way, signals related to the organization of the parallel laminae.

3.3. Differential scanning calorimetry (DSC)

The intrinsic structure of a semicrystalline polymer matrix such as PE-RT can significantly influence some properties of its nanocomposites, especially the mechanical performance and gas permeability of graphene-based materials. In this work, DSC studied the influence of GO nanoparticles on PE-RT non-isothermal crystallization processes and determined its thermal parameters. Figure 4 shows typical exothermic melting and crystallization curves of PE-RT samples and their nanocomposites.

Figure 4
DSC curve for a sample of PE-RT and GO nanocomposites (a) melting; (b) crystallization.

The incorporation of GO does not significantly alter the T_c and T_m values of the nanocomposites compared to the polymeric matrix. That demonstrates that GO does not compromise the thermal transitions of the material, including the maximum working temperature. However, a noticeable increase is observed in the ∆H_m and X_c values of the samples containing 0.5 wt.% GO, indicating the obtaining of a material with greater organization and crystallinity. Samples with higher levels also showed a slight increase in ∆H_m. This demonstrates the enhancement of crystallinity due to the incorporation of GO. Table 4 displays the values of T_c (°C), T_m (°C), ∆H_m (J/g) and X_c (%) obtained.

Table 4
Results obtained by DSC analysis for the sample of PE-RT and its nanocomposites with GO.

Sample	Tc (°C)	Tm (°C)	∆Hm (J/g)	Xc (%)
PE-RT	116.1	131	140	48
IGO0.5	116.2	131	184	63
IGO1.0	116.7	131	141	49
IGO2.0	116.2	131	149	52

The enhancement in X_c is attributed to the nucleation effect of the GO nanoparticles, which facilitates more efficient packing of the crystalline structure³⁰,³¹. The uniform distribution of GO nanoparticles in the PE-RT matrix provides more nucleating surfaces, facilitating the involvement of numerous polyethylene molecules in the growth of the initial polymer nuclei with chain folding. This, in turn, further increases the crystallinity. The divergence between these results and the crystallinity index calculated by XRD can be elucidated by the DSC method, which reveals a self-organization effect during heating, leading to an augmentation in the melting enthalpy and, consequently, in the degree of crystallinity. In other words, in DSC, thermally active crystals are observed, while in XRD, crystals with long-range organization are detected by diffraction. Above 0.5 wt.% GO, it can be seen that the improvement in crystallinity is not as significant. Likely, the well-dispersed GO sheets within the matrix inhibit crystal growth by confining the structure between the nanolaminate barriers.

3.4. Contact angle

Table 5 shows the mean contact angles obtained from PE-RT samples and their GO-based nanocomposites. The analysis of the contact angle in polymers is used to measure the water wettability in the sample. The investigated PE-RT has a contact angle of 98º, slightly above the value reported in the literature, 95.6º³⁰-³².

Table 5
Contact angle of PE-RT and its GO-based nanocomposites.

SAMPLE	CONTACT ANGLE (degree)	STANDARD DEVIATION
PE-RT	98	0.27
IGO0.5	99	0.26
IGO1.0	100	0.26
IGO2.0	102	0.27

Adding higher GO contents in the nanocomposites gradually increased the contact angle. The addition of 2.0 wt.% GO resulted in a 4° increase in the contact angle compared to the pure polymer, indicating a slight enhancement in the material's hydrophobic character. The material used in the riser barrier layer must be resistant to water absorption, so greater hydrophobicity is expected to contribute to a better performance of PE-RT when used in flexible pipelines. A probable hypothesis is that the desirable contact angle for polymers used in risers is approximately 90°. If the material is too hydrophilic, it absorbs water, which facilitates PE-RT swelling and increases the permeability to water vapor, compromising the metal layers above the barrier layer and the integrity of the entire riser. Conversely, values significantly higher than 90° indicate a greater hydrophobic character and affinity for organic groups, which are unsuitable for risers as they can contribute to the formation of organic compound crusts and consequent clogging, impairing the flow of crude oil. The slight increase in the contact angle with the increase in GO filler content suggests good dispersion, the absence of nanoparticle agglomerates, and a surface with less roughness and high uniformity³³-⁴⁰.

3.5. Scanning electronic microscopy (SEM)

Figure 5 shows the SEM image of graphene oxide using the method described previously.

Figure 5
SEM images obtained for GO.

The micrograph was taken at 30.0 kx magnification and shows that graphene oxide appears as aggregates of different dimensions. It is possible to observe that the morphology of these aggregates is formed by superimposed sheets or layers of GO. The layers do not have a regular organization, and the size is also heterogeneous. A similar morphology for GO has been reported in the literature, demonstrating that the Top-Down method of obtaining graphene oxide from the physical and chemical exfoliation of graphite generates heterogeneous structures with a variable lateral size distribution⁴¹,⁴². In the present work, it can be stipulated that the stacking of GO particles has a lateral size between 1-5 micrometers. Furthermore, as observed in microscopy, even the dry GO agglomerates, which are already expected, are much less than 0.1 micrometers thick (resolution limit of the scale used). Higher resolutions were limited due to the equipment available for use in the present study. It is universally known that when drying nanoparticles produced by liquid phase methods, the nanoparticles are agglomerated and reorganized when the solvent is removed. Therefore, when inserting such particles into the extruder, it is observed that such coarse agglomerates are broken down, enabling good dispersion in the polymeric matrix⁴³. The process of extrusion of lamellar nanoparticles into polymeric frameworks can also inherently assist in the natural exfoliation of these structures, making them even more nanometric. Figure 6 shows the images of the cryogenic fracture surfaces of the polymeric matrix and the PE-RT/GO nanocomposites obtained by SEM with a magnification of 10,000x. The objective was to investigate whether the dispersion step was successful and the existence of clusters.

Figure 6
Images of the fracture surfaces of PE-RT and nanocomposites PE-RT/GO obtained in SEM: (a) PE-RT; (b) IGO0.5; (c) IGO1.0 and (d) IGO2.0.

At this high magnification, it is possible to verify that the surface of the original sample (PE-RT) is rough, consisting of two phases, a dispersed one occluding a more continuous phase. It is, therefore, a heterophasic polyolefin, which can be explained by the presence of two main fractions with distinct molecular mass, as it is a bimodal polymer⁴⁴. The surface of the cryofracture appears highly rough, indicative of irregular plastic deformation. Additionally, it can be observed that one phase fractured at a higher level than the other. The lower phase in the morphology shown in Figure 6a exhibits a smoother surface, characteristic of a fracture with minimal plastic deformation, while the higher region displays significant plastic deformation. This highlights the difference in mechanical properties between the phases of the bimodal polymer.

In this study was not possible to observe agglomerates of GO, and it was even difficult to identify the presence of this nanoparticle in cryofracture. This demonstrates the good dispersion of GO in the PE-RT matrix. The composite with 0.5 wt.% GO showed much smoother morphology, indicating the effect of filler reinforcement, even at low content. Furthermore, it can be suggested that GO helps distribute stress between the bimodal phases of the polymer, reducing the exaggerated plastic deformation of one phase in relation to the other. Even in nanocomposites with 2% GO by weight, it was difficult to identify the filler particles on the cryofracture surface. This suggests that the extrusion process helped very well in the dispersion of the fillers and yet that the reinforcement phase presents good adhesion to the matrix⁴⁵. Furthermore, it is worth mentioning that GO is a much better thermal conductor than the PE-RT matrix. Thus, the insertion of GO into the composites below the percolation threshold also favors better heat transfer in the nanocomposites. Above the percolation threshold, this effective transport effect may be questioned due to the continuous path of the load acting as an agglomeration and stress concentrator⁴³,⁴⁶. In nanocomposites with 2% GO, it is possible to observe that there is an indication of a process of creating roughness on the cryofracture surface, similar to the pure polymer. However, the surface does not present as drastic height variations as pure PE-RT. It can be suggested that from this concentration of GO (2.0 wt.%), load transfer starts to become less effective, generating heterogeneity due to excess filler. Finally, it is also believed that the extrusion process of GO nanoparticles in the PE-RT matrix can also help in the exfoliation of these lamellar structures, making them even more nanometric, corroborating the difficulty of identifying GO on the cryofracture surface⁴³. The image of IGO1.0 suggests that the incorporation of 1.0 wt.% GO is a satisfactory balance between effort transfer, heat transfer and adequate filler concentration so as not to saturate any of the mechanisms that could harm the material's performance. The images show that the addition of this filler at low concentrations resulted in lower roughness of the material, suppressing the heterogeneous effect of the PE-RT bimodal composition. A more uniform surface was achieved up to 1.0% of GO, characterized by fewer imperfections or holes. This uniformity was due to improved dispersion of GO within the material, thereby contributing to enhancements in mechanical properties such as increased elastic modulus. Additionally, the uniform surface played a role in elevating thermal properties, leading to an increase in the material's ability to withstand higher temperatures.

3.6. Shore D hardness

The Shore D hardness test was conducted to evaluate possible changes concerning this characteristic regarding the matrix and the nanocomposites. The values obtained from pure PE-RT and nanocomposites are shown in Table 6.

Table 6
Shore D hardness values for PE-RT and nanocomposites.

SAMPLE	HARDNESS	STANDARD DEVIATION
PE-RT	50	0.2
IGO0.5	53	0.2
IGO1.0	55	0.3
IGO2.0	59	0.1

Hardness measurement methods are the most common non-destructive tests for evaluating the mechanical properties of materials⁴⁷. This is often used to study the mechanical properties of polymers. Hardness is a characteristic that expresses a material's resistance to deformation when subjected to a force concentrated on its surface⁴⁸.

It is possible to verify that, as expected, the addition of GO reinforcement increased the penetration resistance of the material. The nanocomposites showed an average increase of more than 3 Shore D hardness units, reaching 9 units more than pure PE-RT. That is because graphene oxide is a material of high hardness, transferring part of its characteristic to the polymeric matrix⁴⁹. The results show that the incorporation of GO increases the hardness of the polymeric mixture, indicating good dispersion and homogeneity of the filler, leading to resistance to penetration by the needle of the instrument used, corroborating what is observed in the literature⁵⁰. The increase in hardness of nanocomposites is attributed to the two-dimensional structure of GO, which is responsible for a very efficient transfer of forces during the indentation process, in which the applied force is predominantly transferred from the polymeric chains to the higher hardness structure of GO, preventing a deeper indentation on the surface of the nanocomposite⁵¹.

3.7. Tension test

From the already mentioned investigations of nanocomposites, the good dispersion of GO in the PE-RT matrix can result in a potential reinforcement of the mechanical properties. The results were presented in a statistical format, with Table 7 specifically detailing the data in terms of the mean ± 95% confidence interval. It is possible to verify the tensile properties of the pure polymer compared with the nanocomposite with 1.0 wt.% of GO. The concentration of 1.0 wt.% GO was chosen because it improved the previously studied properties without compromising the material or causing surface roughness, as observed in the SEM analysis of the cryofracture surface. In contrast, the material with 2.0 wt.% GO showed surface irregularities due to excess GO. It is also believed that this concentration is already above the percolation threshold, which tends to compromise the performance of the nanocomposites under operation by requiring a set of thermal, mechanical and morphological properties. On the other hand, IGO0.5% exhibited lower improvements compared to IGO1.0%. Therefore, the material containing 1.0 wt.% GO was deemed the most promising for further study. Consequently, the tensile and aging tests were conducted exclusively on the IGO1.0% nanocomposites.

Table 7
Statistical overview of tensile properties of composites with mean values and 95% confidence intervals.

Sample	Young modulus (MPa)	εy (%)	σy (MPa)	εr (%)	σr (MPa)
PE-RT	335 ± 37	13.3 ± 0.6	21.6 ± 1.4	496.0 ± 46.9	21.1 ± 1.5
IGO1.0%	409 ± 42	9.7 ± 0.5	22.1 ± 1.0	354.2 ± 59.8	18.4 ± 2.7

• εy=elongation at the yield point; σy= stress at the yield point; εr = elongation at break; σr= stress at break

In addition, in Figure 7, the profile of the stress vs. strain curve of the PE-RT/GO nanocomposite with 1.0 wt.% of GO compared to the pure PE-RT can be observed.

Figure 7
Stress vs. strain curve PE-RT pure and GO nanocomposite.

It is worth noting that the graph was plotted based on the curve that represents the specimen with median properties in relation to the population. Thus, as can be seen, some property values differ from the averages shown in Table 7. As the specimens are subjected to tension, the material initially responds with linear deformation, allowing for the calculation of Young's modulus at 0.2% strain. The stress-strain curve then reaches a maximum value, characteristic of the necking phenomenon, where the polymer chains are stretched, disrupting their conventional packing. Upon reaching the elastic limit, the polymer begins to deform plastically. During this phase, stress peaks are followed by a decline, resulting in a region of constant or slightly decreasing stress. This behavior indicates the onset of necking, which is characterized by a localized reduction in the cross-sectional area of the material. As deformation continues, this necking region elongates and propagates throughout the sample. After neck formation, deformation primarily occurs in the narrowed region, while the rest of the material remains relatively intact. Post-necking, the specimens reach a minimum in elongation, allowing for the reorganization of polyethylene chains. The stress level remains approximately constant up to 200-250% strain. During this phase, the material undergoes crystallization under tension, where the aligned chains reorganize into a crystalline structure. This enhances their resistance to further deformation, causing the stress-strain curve to rise until it reaches the strength limit of the chains, leading to failure.

Incorporating GO in the polymeric matrix made the material more resistant when verifying an increase in Young's modulus from 335 ± 37 MPa to 409 ± 42 MPa (22% increase). It means that higher stress is required for the material to deform. That is interesting for the application of the material since, under operation, it is not desired that the material changes its dimensions when subjected to tensile stresses. If the material undergoes variations along the force application axis, it would be possible for it to momentarily increase the free volume, which would reduce its barrier property to gases or even to swelling by liquids, possibly compromising the stability of the pipeline structure⁵²-⁵⁴. Furthermore, the reinforcing property added by GO can also be seen in the slight increase in stress at the yield point (σy). It demonstrates that the material in the elastic region tends to withstand greater tensions and, in the same way, a greater force would be required to deform it⁵⁵-⁵⁸. In addition, Figure 7 shows that the strain profile concerning the stress after the yield point tends to remain constant until a certain elongation. Then, the phenomenon of crystallization by stretching is observed⁵³. It is characteristic of materials that, when stretched, end up with an alignment of the chains that promote compaction and crystallization, adding mechanical resistance to the material⁵⁴. Another interesting detail is that in the plastic region, there was an increase in the material's mechanical strength due to the incorporation of nanoparticles. It is evidenced by the higher level after the yield point; the material requires greater stress to deform even with permanent deformation⁵⁵. The reduction in elongation at break was already expected since the nanoparticles insert interfaces in the material, which present weaker interconnections in relation to polymer-polymer interactions⁵⁶. It is common in nanocomposites because polymers generally have better compatibility with themselves than with other molecules due to the similarity of polarity and affinity of their chairs or tangles⁵⁷. These interfaces also act as stress concentrators that behave as crack nuclei after extensive plastic deformation. In this context, the interface substantially retains the load subjected to the material, concentrating the energy mainly at specific points of the nanoparticle⁵⁸. Thus, the plastic exceeds the activation energy required at a point of the nanocomposite, causing the phenomenon of crack propagation and the material fractures in smaller deformations⁵⁹. This phenomenon of reduction of elongation also occurs in the elastic region when observing the reduction of elongation at the yield point (Ɛy). Graphene oxide nanosheets are generally lodged in the amorphous regions of the polymer because they have a greater free volume⁶⁰. Thus, when the material is subjected to traction, it tends first to elongate those regions that present greater mobility caused by the free volume since the crystalline regions are more rigid⁶¹. Another fact is that the lamellar particles also cause spatial imprisonment in the chains, restricting their movement⁶².

Considering only the results presented in this work, it can be suggested that the oxygenated groups of GO tend to favor a satisfactory interaction with the hydrogens of PE-RT, mainly in terms of nucleation and growth of crystals. The incorporation of GO tends to limit the stretching of PE-RT, restricting its deformation by confining the crystalline regions between the GO sheets dispersed in the matrix. Therefore, greater tensions are required to deform the polymer. Furthermore, the material will rupture at stress concentration points that will be at the interfaces of the GO nanosheets with the PE-RT. This is already expected to happen, as PE-RT has a stronger interaction with itself than with the nanoparticle. However, the reinforcement effects in the elastic region are those desired for the application, and it is in this region that the insertion of GO proves to be most advantageous (increase in Young’s modulus and σy). Furthermore, it can be mentioned that, as observed in DSC, there is a slight increase in Xc, corroborating the reinforcement effect. The formation of a three-dimensional PE-RT network is an interesting suggestion, which goes hand in hand with the confinement of the crystallites by the GO nanosheets, thus favoring the mechanical properties mainly in the elastic region and also in a stress-strain curve above the pure polymer. It is important to note that excessive deformation beyond the elastic region during use is undesirable, as it can jeopardize the dimensional stability of the structure⁶³. This property reduction is insignificant and does not compromise its application. The results of the mechanical tensile test indicated that the incorporation of GO enhanced the material's tensile properties.

3.8. Aging test

The aging of PE-RT occurs through a physical interaction where the synthetic oil penetrates the free spaces of the polymeric matrix, altering the material's structure⁶⁴. Polymer degradation can be identified by swelling, increasing the geometry of the material and reducing its service life⁶⁵. Pure samples of PE-RT and GO-based nanocomposites were carefully analyzed after being removed from immersion in synthetic oil, and no change in color and/or volume was observed.

Understanding the aging modes is important to prevent, control, and obtain a prognosis of when and how the material will fail. The swelling is generated by diffusion, that is, by installing the solvent in the polyethylene's free volumes (amorphous region)⁶⁴,⁶⁵. After the test period, it was verified that there was a greater mass increase in the unfilled samples compared to the nanocomposites, indicating that the addition of GO can effectively control the swelling of the PE-RT. Figure 8 compares the swelling degree of pure PE-RT samples and GO-based nanocomposites.

Figure 8
Swelling degree (α) of PE-RT and PE-RT/GO nanocomposite.

The swelling of polymeric materials is a phenomenon measured through the variation in mass related to the density or volume of the material due to the absorption of petroleum⁴⁶,⁶⁶. In this study, it was decided to compare the values of the initial mass of the samples before submersion in synthetic oil with values of the mass measured after removing each submerged sample. Likewise, it was also decided to conduct the aging test only with samples based on 1 wt.% GO due to the tendency for the formation of filler agglomerates from higher levels and because we believe that the lower content of GO used in this study is insufficient for possible conclusions. Finally, the values were calculated to obtain the swelling degree. Graphically, it is possible to observe that after the first two months of testing, the PE-RT samples exhibited a slight increase in mass and from the third month on, there was a marked increase in the swelling degree of the samples of 7.9; 8.2; 9.4 and 11.2% concerning the initial mass. Significant oil absorption occurs because polyethylene is a nonpolar polymer with a high affinity for hydrocarbons⁴⁶,⁶⁷. Nanocomposites provided better results than pure PE-RT, wherein the first three months of the test exhibited an increase in the swelling degree of only 1.4% relative to the initial mass (before immersion in synthetic oil). After a six-month immersion period, the samples containing GO exhibited a 4.5% increase in the swelling degree, which was significantly lower than observed in samples without additives. These findings suggest that GO demonstrates the potential to extend the lifespan of PE-RT by diminishing the void spaces between polymeric chains. This impediment to oil absorption and polymer swelling highlights the effectiveness of GO in enhancing PE-RT durability.

4. Conclusion

The results obtained in this work showed that the initial objectives were achieved. The incorporation of low amounts of graphene oxide (GO) into the polyethylene of raised temperature resistance (PE-RT) exhibited notable effects across various material properties. An increase in thermal degradation temperature was observed, indicating enhanced thermal stability. Concurrently, the Shore D hardness results revealed a proportional rise in sample hardness corresponding to higher filler contents. The GO nanoparticles notably influenced the crystallinity index and crystallite size, particularly evident at a 0.5 wt.% content. The contact angle values indicated a slight enhancement in hydrophobicity, especially at a 2.0 wt.% addition of GO. Although the 0.5 wt.% GO composition exhibits a higher degree of crystallinity, other properties such as thermal resistance, Shore D hardness, contact angle, dispersion, and homogeneity tend to improve with increasing nanoparticle content. Therefore, it can be concluded that the optimal concentration of GO in the PE-RT matrix is approximately 1.0 wt.%. Moreover, the concentration of 1.0 wt.% of GO increased Young's modulus of PE-RT, signifying improved mechanical strength. SEM imaging further depicted a remarkably smoother surface in the composite with 1.0 wt.% GO compared to the pure polymeric matrix. Remarkably, the nanocomposites displayed reduced swelling degrees than pure PE-RT, indicating the inhibitory effect of GO on material oil swelling. Overall, the findings underscore the superior thermal and mechanical properties of composites containing 0.5% and 1.0 wt.% of GO, presenting promising potential for application as flexible duct barriers.

Despite the high cost of GO as a nanofiller for PE-RT, it is important to note that the quantity required to produce the PE-RT/GO nanocomposite is very small. Even in this low quantity, significant improvements have been observed, especially for the barrier properties, the thermal degradation temperature, tensile strength, hardness and the material swelling degree. Therefore, the use of GO to produce the nanocomposite with this polymer matrix is justified, considering that it is cost-effective compared to other polymers used in barrier layers for risers.

Supplementary material

The following online material is available for this article:

Figure S1 FTIR Spectrum of graphite and graphene oxide.

Figure S2 Raman spectra of graphite and GO.

5. Acknowledgments

This work was financially supported by CNPq, CAPES, and FAPERJ (Brazil). We also thank the CENPES laboratories for the equipment available for analysis.

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Publication Dates

• Publication in this collection
  09 Aug 2024
• Date of issue
  2024

History

• Received
  18 Mar 2024
• Reviewed
  04 June 2024
• Accepted
  16 July 2024