Microstructure and Corrosion Resistance of Composite Based on Ultra-High Molecular Weight Polyethylene in Acidic Media

Abstract

In this work, the effect of an acidic environment on the structure of composite samples based on ultra-high molecular weight polyethylene (UHMWPE) modified with mineral filler in the form of diabase (DB) is studied. The stability of samples was investigated in solutions of sulfuric (H₂SO₄) and hydrochloric (HCl) acids with concentrations of 10 vol% and 20 vol% at room temperature for 16 weeks. It was found that the introduction of 10 wt% DB into the UHMWPE matrix significantly increases the resistance of the composite sample to aggressive media, which is confirmed by the minimum degree of swelling compared to pure UHMWPE and composites with higher filler content. Scanning electron microscopy (SEM) demonstrated a uniform distribution of DB in the sample structure and the absence of defects such as agglomeration and cracks. The methods of infrared spectroscopy (IRS) and X-ray structural analysis (XRD) revealed a decrease in the degree of crystallinity of the samples after acid exposure, but no significant changes in the chemical structure of the materials were recorded, which confirms their resistance to chemical degradation. The best chemical resistance was demonstrated by composites containing 10 wt% DB, which is associated with the formation of a barrier structure preventing the diffusion of acids. The obtained results indicate the promising application of UHMWPE with DB filler to create samples resistant to aggressive media.

1. Introduction

The development and research of composite materials with improved performance characteristics, including corrosion resistance, represent one of the key tasks of modern science and technology. One of the promising directions in this context is the creation of polymer matrices modified with solid inorganic fillers. Such composites demonstrate improved mechanical properties, increased wear resistance and resistance to aggressive chemical media.

UHMWPE has unique properties, including low friction coefficient, high wear resistance and chemical inertness [1,2,3], leading to its widespread use in protective coatings, including anti-corrosion coatings for carbon steels used in the oil and gas and petrochemical industries [4,5,6]. However, in real operating conditions, such coatings are subjected to destruction under the influence of aggressive environmental factors and radiation [7,8]. This necessitates the development of materials with improved mechanical characteristics and increased resistance to corrosion.

The analysis of modern studies shows that composites based on UHMWPE modified with various fillers are of considerable interest for application in aggressive environments. For example, the study [9] demonstrated an increase in the corrosion resistance of UHMWPE composites reinforced with graphene nanosheets (GNs) on stainless steel substrates. The use of filler in concentrations of 0.15–1 wt% significantly reduced the corrosion effects of 3.5% NaCl solution. It was shown in [10] that the reinforcement of UHMWPE with graphene nanocomposites provides a 29% and 36% reduction in wear and friction, respectively, as well as an increase in the corrosion resistance of alumina-magnesium alloys under the influence of chloride solutions. These results confirm the efficiency of fillers application to increase the functional characteristics of UHMWPE-based composites.

According to the literature analysis [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16], UHMWPE is one of the most promising polymers due to its outstanding mechanical and chemical properties.

At the same time, the use of mineral fillers, such as DB, opens up new opportunities for improving the performance characteristics of composites. DB, as a natural mineral, has high mechanical strength, resistance to chemical attack, affordable cost and environmental friendliness [17]. Its chemical composition, including silicon, calcium, magnesium, and aluminum oxides [18], improves the corrosion and wear resistance of the material, which makes it an attractive candidate for reinforcing polymer matrices (see

Table 1. Characteristics of UHMWPE fillers.

Characteristics	Diabase (DB)	Graphene Nanosheets	Other Fillers (Glass Fibers, Carbon Fibers)
Chemical stability	Excellent	Excellent	Medium (environment dependent)
Optimal concentration	10%–20%	0.5%–2%	15%–30%
Cost	Low	High	Medium (glass fiber), High (carbon fiber)
Environmental friendliness	High	Moderate	Moderate

).

Despite the promising use of DB as a filler, the effect of its addition on the structure and corrosion properties of UHMWPE coatings remains insufficiently studied. The present work is aimed at filling this gap. The main attention is paid to the study of the microstructure of UHMWPE composites with different DB content and their behavior under the influence of an acidic environment.

The purpose of this work is to study the effect of the acidic environment on the structure and properties of UHMWPE-based composites with the addition of mineral filler in the form of DB.

The obtained results can be used for development of new composites intended for application in chemical, oil and gas, shipbuilding and other industries, where high requirements to corrosion resistance of materials are imposed.

2. Materials and Methods of Research

2.1. Materials and Method of Production of UHMWPE-Based Composite

Commercial UHMWPE powder (Yangzhou Guotai Fiberglass Co., Ltd., Yangzhou, China), density 930 kg/m³, with molecular weight 2 × 10⁶ mol⁻¹, T_melt = 135–150 °C, bulk density > 0.4 g/cm³, was chosen as the main material for coating. The filler is dispersed DB powder containing 10, 20, 30 and 40 wt%, and the average size of each DB particle is 14 μm. The mixing of UHMWPE particles and DB filler particles was carried out in a ball mill at 1500 rpm, and the mixing time was 5 min according to [19].

The coatings were obtained by the gas-flame method accordingly to [20]. Standard aluminum foil was used as a substrate. For subsequent tests and investigations, the coatings were mechanically separated from the substrate. The obtained samples have a square shape with dimensions of 50 mm × 50 mm × 1 mm.

2.2. Test Methods

Resistance to Acidic Environment

The resistance to the acidic environment of the UHMWPE sample with DB filler was investigated in accordance with the recommendations of GOST 12020-2018 [21] (ISO 175:2010). The study was carried out at room temperature, 23 °C. The mass of the UHMWPE and DB-filled samples was measured on analytical “Crystal” scales with accuracy up to 0.0001 g. In the process of testing, test liquids with concentrations of 10 vol% and 20 vol% (H₂SO₄ and HCl) for each of the acids were used. The samples to be measured were placed in chemical beakers with a capacity of 500 cm³ into which 200 cm³ of the test liquid was poured. The soaking time of the samples in the test liquid was 16 weeks. Control measurements were carried out at intervals of seven days. The specimens were removed from the medium, washed with running water and then distilled water, and dried thoroughly with filter paper or lint-free cloth. In addition, the test fluid was replaced every week with a volume equal to the original volume.

The resistance to acidic environment ($\alpha$—degree of swelling) was calculated according to the known formula:

$$\alpha ={\displaystyle \frac{m_t-m_0}{m_t}}\times 100\%$$

(1)

where $m_t$ is the swollen mass over a period of time, g; and $m_0$ is the initial mass, g.

2.3. Research Methods

2.3.1. IR Analysis

UHMWPE and composites based on it and after exposure to aggressive media were studied using a Fourier transform infrared spectrometer (FTIR-801 Simex, Novosibirsk, Russia) in the laboratory of Amanzholov University with a resolution of 1 cm⁻¹ in the range of 600–3500 cm⁻¹ according to the standard method and using auxiliary equipment to measure the attenuated total reflection (ATR) and specular diffuse reflectance (SDR) [18].

2.3.2. X-Ray Diffraction Analysis

X-ray diffraction analysis of the powders and composite polymer was performed on an Xpert PRO PANalytical diffractometer (Panalytical, Almelo, The Netherlands). During the study, a voltage of 40 kV and a current of 30 mA were applied to the anode copper tube, Cu-Kα radiation (λ = 1.541 Å), the imaging step was 0.02°, and the counting time was 0.5 s/step. Phase analysis on the basis of the obtained diffractogram lines was carried out using HighScore Plus software(version 4.5) complexes. Sample preparation, the selection of imaging modes, and the calculation of diffractograms were performed according to the methods described in [22,23].

The degree of crystallinity (χ) was determined by the XRD method described in [24], within the range of diffraction angles 2θ 10–60°, which characterizes the ratio of crystalline and amorphous phases in the polymer according to the following formula:

$$\mathsf{\chi }\left(\%\right)={\displaystyle \frac{S_c}{S_c+S_a}}\times 100\%$$

(2)

where S_c is the area of the crystalline part (above the halo); S_a is the area of the amorphous part (below the halo).

2.3.3. SEM Analysis

The microstructure of UHMWPE without and with the DB filler before and after exposure to an aggressive environment was investigated by scanning electron microscopy CrossBeam XB 540 (Carl Zeiss, Jena, Germany) with an INCA energy-dispersive microanalysis system. The working distance between the sample surface and the bottom of the objective lens was 4.7 mm, and the accelerating voltage was 20 kV. Before SEM examination, the surfaces of the samples were coated with a conductive Au film.

3. Results and Discussion

3.1. SEM Analysis of Powders

The morphology of the initial UHMWPE is a white powder, as shown in Figure 1a, with an average particle size of 150 μm and a spherical shape. Expectedly, only the carbon spectrum is present in the energy-dispersive analysis.

The structure of the DB powder is mixed (see Figure 1b), with lamellar, needle and spherical particles with an average size of 14 microns. According to the results of EDS analysis using SEM, DB powders contain chemically active elements in their composition (%): Si 24.06, O 41.40, Fe 13.61, Ca 10.82, Mg 8.03, Al 2.08.

3.2. Results of Determination of Composite Samples Stability in Acidic Environment

Figure 2a presents the results showing that UHMWPE samples exhibit chemical resistance when exposed to acids (H₂SO₄ and HCl) for a long time. However, the degree of swelling and mass change in the samples increases with the increasing acid content and duration of exposure, which leads to the diffusion of acid molecules in the polymer composition and partial degradation of the material. Also, in the crystal structure, the bonds between atoms are stronger, which increases the resistance of the material to destruction under the influence of an aggressive environment. Diffusion in such materials requires more energy than in amorphous materials, which additionally slows down the process.

The chemical resistance of samples decreases with the content of containing acids (

Table 2. Average value of swelling degree of UHMWPE samples in acids.

	10% HCl	20% HCl	10% H2SO4	20% H2SO4
Degree of swelling	0.008536	0.026258	0.02119	0.035776

). For 10% solutions, the degree of swelling of the sample remains moderately low (0.0085–0.0212), indicating partial acid penetration into the polymer matrix. At 20% concentrations, more intense swelling occurs (0.0263–0.0358), which is associated with variations in intermolecular interactions and chemical degradation of the material.

Sulfuric acid has a more aggressive effect on samples due to its high oxidizing power, increased ionic strength and two-step dissociation. This results in more significant swelling and weight determination of the samples. Hydrochloric acid has a less intense effect due to its lower oxidizing power and increased ionic strength. However, in long-term tests, there are signs of saturation of the polymer matrix with HCl, which may limit further mass changes.

The swelling of UHMWPE samples is important for preservation in long-term testing. After the first few weeks, mass changes decrease, indicating the acid saturation of the polymer structure. This indicates that UHMWPE samples are able to retain their basic characteristics in moderately aggressive environments. UHMWPE samples have sufficient chemical resistance for use in environments with low and medium acid concentrations, which makes them promising for use in moderate chemical exposure.

Figure 2b shows that UHMWPE composite samples with the addition of DB demonstrate increased chemical resistance in hydrochloric acid compared to pure UHMWPE samples. The addition of filler reduces the degree of swelling of the sample by creating a barrier structure that prevents the penetration of acid molecules. However, when the filler content increases, the chemical resistance deteriorates; this may be due to the integrity of the polymer matrix being compromised.

At a DB content of 10 wt%, the sample shows the best resistance (swelling degree is 0.00598), which is due to the uniform distribution of the filler and enhanced barrier properties (

Table 3. Average value of swelling degree of UHMWPE composite samples with different DB content in hydrochloric acid.

	UHMWPE	UHMWPE + DB10	UHMWPE + DB20	UHMWPE + DB30	UHMWPE + DB40
Degree of swelling	0.02119	0.00598	0.01564	0.0329	0.058

). With increasing DB content up to 20 wt%, the chemical resistance of the sample remains high, but the degree of swelling increases to 0.01564 due to the initial disruption of intermolecular bonds in the matrix. At 30 wt% and 40 wt% DB content, the chemical resistance decreases dramatically (swelling degree is 0.0329 and 0.058, respectively). It can be assumed that this is due to the excessive amount of filler, which creates defects in the coating structure and a reduction in the interfacial interaction between the matrix and filler, increasing porosity and allowing the acid to penetrate deeper.

DB, being an inert inorganic material, at low content (10%–20%) increases the chemical resistance of the sample due to the creation of a physical barrier that hinders the penetration of acid. At high content (30%–40%), DB destroys the polymer matrix, which leads to increased swelling of the sample and reduced resistance to acid.

It is known [25] that the presence of pores and microdefects contributes to the accelerated penetration of aggressive media molecules into the depth of the material, accelerating the following processes: chemical degradation, sorption of components by the aggressive medium, and desorption of various additives from the polymer material. Chemical degradation proceeds with the breakage of chemical bonds and is accompanied by a change in the molecular weight of the polymer; the cohesion is disturbed. The degree of material destruction also depends on the porosity value, the presence of microdefects, and the intensity of the chemical interaction of fillers with the medium, primarily at the filler–matrix interface. It is also known that the rate of penetration of aggressive medium and the rate of chemical reaction have a significant effect on the process of material destruction [26].

The graph shows that samples with low DB content (10% and 20%) maintain stable swelling rates during 16 weeks of testing, indicating their long-term chemical resistance. Thus, UHMWPE samples with DB content in the range of 10–20 wt% can be successfully applied in moderately aggressive environments.

Since the composite specimen with 10 wt% DB addition showed the best results in preliminary studies, the composite specimen 10 wt% DB content as well as UHMWPE without fillers were selected for further experiments.

3.3. Results of IR Analysis

Figure 3 shows the results of investigation of the surface of the UHMWPE-based composite sample by IR spectroscopy. The IR spectra show the strain fluctuation of C-H groups at 2915 cm⁻¹, 2844 cm⁻¹ and valence co-variation in the C-H group at 1560 cm⁻¹. We can also observe the carbonyl functional group at 1248 cm⁻¹ and a peak at 721 cm⁻¹ reflecting the C=C double bond. The above-mentioned bands are characteristic of the absorption bands of the UHMWPE, as shown in [27,28].

The absorption band noted at 1476 cm⁻¹ represents the absorption peak of C-C bonds that characterize the structure of UHMWPE. Absorption peaks located in the range of 1248 cm⁻¹ may reflect stretching vibrations of C=O groups in accordance with [29].

The appearance of the absorption band at 1036 cm⁻¹ in the spectrum after chemical treatment is due to the formation of bonds characteristic of the C-O group (carbonyl group). This band appears as a result of interaction between the surface of the material (UHMWPE) and the reagents used in the treatment process, which leads to the appearance of functional groups containing the C-O bond. The appearance of the peak at 797 cm⁻¹ corresponding to C-Cl bonds may be a consequence of the reaction between UHMWPE and chlorine-containing medium, which appears on the spectrum after chemical treatment of the sample with HCl solution.

According to the obtained IR spectra of UHMWPE samples and composites based on it, it can be concluded that after chemical exposure in aggressive media, insignificant changes in the spectra are observed. However, the main coupling between the components of the samples remained unchanged, which is explained by the absence of displacements of the main peaks. This indicates that UHMWPE and composites based on it were not subjected to noticeable degradation as a result of exposure to an aggressive environment.

3.4. Results of X-Ray Diffraction Analysis

Figure 4 shows the X-ray diffraction pattern of samples made of pure UHMWPE and a UHMWPE composite with DB filler containing 10 wt% before and after interaction with an acidic environment. The X-ray diffraction patterns show two intense reflections (110) at 2θ = 21.76° and (200) at 24.15°, which characterize the crystalline part of the UHMWPE structure. UHMWPE has an orthorhombic lattice in accordance with [30]. DB peaks have low intensity due to their complex chemical composition. The most intense reflections of DB (004), (420) and (111) are observed at diffraction angles of 12.49°, 25.11° and 27.91°, respectively.

It can be seen that the intensity of the diffraction lines of the sample slightly decreases after chemical attack.

From the analysis of X-ray diffraction data, it was found that the degree of crystallinity of the pure UHMWPE sample before chemical attack is 67% (Figure 5), and for the UHMWPE-based composite with the addition of a diabase filler in the amount of 10 wt%, the degree of crystallinity is 60%. After chemical attack, the percentage of crystallinity of the UHMWPE samples and the UHMWPE-based composite is suppressed, reducing to 52 and 49%, respectively. Acids can cause the deposition of ions on the UHMWPE surface, which can lead to the formation of surface films or deposits that can reduce the degree of crystallinity of the material. It is known [31,32] that the degree of crystallinity of the polymer can be significantly reduced as a result of radiation and chemical effects.

3.5. Results of SEM Analysis

Morphological differences between the samples of UHMWPE-based composite samples containing 10 wt% DB in the initial state and after exposure to an aggressive environment are presented in SEM images (Figure 6). As follows from our previous studies [18], there are no signs of agglomeration of filler particles for composite samples containing 10 wt% diabase.

DB particles are uniformly distributed in the UHMWPE matrix, fully integrated into its structure and reliably retained in the samples. These morphological features indicate a high level of interaction between the filler and polymer matrix, which contributes to the formation of a strong structure resistant to chemical influences.

It is important to note that DB particles are not the centers of nucleation of cracks or defects. This indicates good adhesion between the composite components, which plays a key role in improving the durability of the sample in aggressive environments. In comparison, polymer samples with unevenly distributed fillers are often subject to localized stresses that can lead to cracking or delamination of the samples.

After exposure to aggressive media, insignificant changes are observed on the surface of UHMWPE samples with DB filler. The continuous structure of the polymer matrix is preserved, which confirms its morphological stability. The preservation of the structure can be explained by the chemical inertness and high molecular weight of UHMWPE, which prevent the diffusion of aggressive agents. These results are in agreement with the data presented in the literature [33], where a high resistance of UHMWPE to chemical degradation is indicated.

X-ray diffraction (XRD) and infrared spectroscopy (IR) results showed minimal changes in the degree of crystallinity and chemical structure of the composite after exposure to an acidic environment. This indicates that there are no significant changes in the molecular organization of the material, which once again emphasizes its chemical resistance.

Comparative analysis with similar materials based on other polymers, such as high-density polyethylene (HDPE) or polypropylene (PP), shows that UHMWPE samples outperform them in terms of acid resistance due to their higher wear resistance and low reactivity. For example, in studies on PP composites, the appearance of microcracks and a decrease in mechanical properties after two weeks of exposure to acidic environment have been observed [34,35].

Additionally, the influence of filler size and concentration on the composite properties can be emphasized. According to [36,37,38], reducing the size of filler particles to the nanometer range or increasing their content up to 15% can increase the density and strength of the sample. However, an excessive amount of filler leads to agglomeration, which reduces the homogeneity of the material and increases the probability of cracking.

UHMWPE-based composites with diabase filler have high scalability and practicality in industry due to the following:

• Availability and low cost of raw materials.
• Ease of integration into existing manufacturing processes.
• Versatility of application in various industries.
• Durability and resistance to chemical and mechanical stresses.

These materials represent an economically favorable and environmentally sustainable solution for mass industrial production, which makes them competitive in modern conditions.

Composites based on UHMWPE have high resistance to aggressive environments, which makes them in demand in such industries as chemical, oil and gas, shipbuilding and other industries. The addition of diabase filler increases their efficiency, especially when creating anti-corrosion coatings capable of working in extreme conditions. These materials are universal and justified for mining, marine, transport and construction industries. Further research should be directed toward studying the influence of composite characteristics under normal conditions as well as changing their composition to optimize chemical resistance.

4. Conclusions

Thus, the influence of acidic environment on the microstructure of UHMWPE and composite samples based on it with mineral filler in the form of DB was studied using a number of experimental methods. It was found that samples made of pure UHMWPE and UHMWPE composite with the content of DB 10 wt% have the best resistance to acidic environments. The obtained results show that UHMWPE and composites based on it do not decompose under the influence of an aggressive environment and indicate the low reactivity of the investigated material to acids. However, with increasing DB content, the resistance of the UHMWPE composite to acid media slightly decreases. Apparently, it is connected with the decrease in the share of UHMWPE matrix material in the composite.

It was also found that after exposure to an aggressive environment, the crystallinity of the pure UHMWPE sample decreases from 67 to 52%, and that of the UHMWPE composite 10 wt% DB filler decreases from 60 to 49%. IR spectroscopy of the samples of UHMWPE and composites based on it did not reveal significant changes or shifts of the main peaks as a result of the chemical influence of an aggressive environment. This indicates that UHMWPE and composites based on it were not subjected to noticeable degradation as a result of a chemical environment. The absence of visible defects on the sample surface also indicates that the microstructure of the composite material is not subject to significant chemical degradation or corrosion when exposed to an acidic environment. In our opinion, due to high moderate resistance, structural changes in UHMWPE and its composites with DB filler occur to a lesser extent than in other polymers, which makes it more attractive for use in aggressive environments.

5. Future Research Area

In the future, we plan to investigate the effect of temperature, aggressive and abrasive environment on the structure and physical and mechanical properties of UHMWPE-based composite material.