On the mechanical and morphological properties of highly performant composite laminates based on epoxy resin and oxidized ultrahigh-molecular-weight polyethylene fibers

Abstract

In this study, new high-performance composite laminates were prepared from epoxy resin and surface modified ultrahigh-molecular-weight polyethylene (UHMWPE) fibers. The UHMWPE fibers underwent two types of chemical modifications, namely through chromic acid and potassium permanganate oxidations. The adopted chemical procedure aimed the grafting of polar groups on the outer surface of fibers for an improved chemical and physical compatibility with the polymeric matrix. The efficiency of the grafting methodology was confirmed by vibrational, thermal, and morphological analyses, and the grafting mechanism was thoroughly discussed. Furthermore, composite laminates were prepared to study the effects of chemical treatments on the mechanical and morphological properties of the resulting composites. The grafting techniques allowed consequent improvements in the tensile and bending properties, up to 34% and 23% for the tensile and flexural strengths, respectively. The study of the fractured surfaces confirmed the exceptional compatibility between the fillers and the polymeric matrix and further corroborated the mechanical findings. Finally, the adopted modification techniques can be regarded as cost-effective and highly suitable for the manufacturing of structural composites for advanced applications.

Keywords

UHMWPE epoxy surface modifications polymer–matrix composites mechanical properties

Introduction

Ultrahigh-molecular-weight polyethylene (UHMWPE) fibers, referred to as one of the strongest polymeric fibers, exhibit an outstanding combination of interesting features. For instance, their extremely lightweight is combined with high-performance mechanical and dielectric properties, which allow them to be used in extremely exigent applications such as aerospace and aircraft industries.^1,2 The UHMWPE fibers also display high impact resistance properties quite similar to those obtained with para-aramid fibers.^3

–6 Interestingly, the recent advances in the field of radiation protection preconize the use of such fibers as an efficient neutrons shielding material due to the high contents of hydrogen atoms in their polymeric structure.^7

–10 On the other hand, the UHMWPE fibers have low thermal stability, which greatly affects their combination with advanced polymeric matrices. In addition, these fibers are chemically inactive and have an extremely low surface free energy because of the extended regular chains with the absence of polar chemical functions and the high degree of crystallinity.¹¹ Such behavior leads to poor interfacial adhesion between the UHWPE fibers and the matrix, hindering the exploitation of their full potentials. Thus, it becomes evident that the UHMWPE fibers must first be treated before use in order to increase the surface free energy and to provide the fibers with necessary chemical functionalities for better adhesion with the polymeric matrices.

The state-of-the-art reports indicate three possible and efficient ways to treat the UHMWPE fibers including physical techniques, surface covering techniques, and chemical processes. The chemical surface treatment of the fibers involves grafting polar chemical functions on the surface of the fibers using coupling agents or through an acid etching solution. As a result, the chemical process can enhance the adhesion of UHMWPE fibers to the polymer matrix by providing strong chemical bonds between both phases. Various physical treatments such as corona treatment, modification by oxygen-plasma, and modification by argon-plasma irradiation treatment have also been employed to treat the UHMWPE fibers. Other modification techniques include the fiber sizing which consists of applying a coating on the external surface of the fibers. The coating itself can be either organic or inorganic providing great flexibility in tailoring the properties of the fibers.

Chi-Yuan Huang et al. reported the effects of the physical surface treatment of UHMWPE fibers by argon plasma on the overall performances of their related polyurethane-based composites.¹² Similarly, Runqin He et al. studied the mechanical behavior of plasma surface-treated UHMWPE fiber-reinforced polyimide and polyethylene composites.^13,14 The findings confirmed that the applied treatment increases the chemical functionalities on the surface of the fibers, allowing the preparation of composites with an improved roughness. R Oosterom et al. investigated the influence of various surface treatment methods such as ultraviolet (UV)/ozone modification, mechanical abrasion, corona treatment, and glow discharge modification on the adhesion properties of UHMWPE fiber-reinforced polymethyl methacrylate and methyl methacrylate composites.^15,16 Another interesting report was published by Wang et al. in which UHMWPE fibers were surface treated by UV-initiated grafting technique.¹⁷ Other researchers, such as Lin et al. also employed the plasma modification technique to study the surface performances of the treated UHMWPE fibers.¹⁸ Subsequently, Weiwei Li treated the UHMWPE short fibers with chromic acid and evaluated the impact of fiber surface modification on the composite performances.¹⁹ Finally, Dariush Firouzi et al. used the nylon coating method to enhance the mechanical performances of UHMWPE fiber-reinforced composite.²⁰ To the best of our knowledge, no study has reported the effects of acid etching treatment on the mechanical and morphological behavior of UHMWPE fabrics-reinforced multilayered composite plates. Additionally, the proposed modification technique is also cost-effective and allows the generation of a sufficient number of polar groups on the outer surfaces of the UHMWPE fibers for an efficient compatibility with the polymeric matrix.

In the present study, two kinds of acid etching modifications based on chromic acid and potassium permanganate were applied to raw, commercially available, UHMWPE fibers. Aiming to study the effect of the nature of chemical treatment on UHMWPE fibers, multilayered composite plates based on epoxy resin reinforced with surface-treated and native UHMWPE fabrics were prepared by a hand lay-up technique followed by a hot-press curing process. The impact of the fabric treatment was investigated by evaluating the structural, morphological, and mechanical properties of the laminated composite plates.

Experiment

Materials

All chemicals used in this work were reagent grade and were directly used without any further purification. Potassium dichromate (with purity 99%), potassium permanganate (with purity 99%), nitric acid, and concentrated sulfuric acid were purchased from Sigma-Aldrich (France). The typical bisphenol A-based epoxy resin (Araldite GY6010) and the cycloaliphatic amine hardener (Aradur 22) were acquired from Huntsman Corporation (France). The UHMWPE fabrics (Spectra 1000) used in this study were obtained from Dacheng Advanced Material Co., Ltd (Ningbo, China) having an areal density of about 200 g cm⁻².

Surface modification of UHMWPE fibers

The surface modification of UHMWPE fibers is begun with the preparation of acid etching solutions. The potassium permanganate solution was prepared by mixing 5 g of potassium permanganate with 150 g concentrated nitric acid. The chromic acid was obtained by mixing deionized water, potassium dichromate, and concentrated sulfuric acid in a weight ratio of 12:7:150. The native UHMWPE fabrics were initially cut in accordance with the mold dimension specifications (250 × 250 mm²) and were then placed in the etching solution at ambient temperature (potassium permanganate solution for approximately 1 min and chromic acid for approximately 5 min). To eliminate any acid sticking on the surface of fabrics, the modified UHMWPE fabrics were respectively washed with deionized water for 5 times. Finally, the obtained fabrics were dried under vacuum at ambient temperature overnight.

Preparations of multilayered composite plates

To fabricate the laminated composite plates with a dimension of 250 × 250 × 2.5 mm³, a hand lay-up technique was employed using a steel compression mold with an applied pressure of about 60 MPa. To avoid the adhesion of the laminated plates to the mold, a demolding agent (Teflon release agent) has been placed on the entire mold surface. The UHMWPE fabrics were initially impregnated in the epoxy resin with a 1:1 mass ratio of fabrics to polymer matrix (mixture of 78% epoxy resin and 22% hardener) and were then manually placed into the mold. The steel compression mold was subsequently closed and remained for nearly 2 days at ambient temperature. Tensile and bending standardized samples were afforded by cutting the afforded composite plates by a water jet cutter. The scanning electron microscopic (SEM) image of the fabricated specimens is shown in Figure 1.

Figure 1.

SEM image of the prepared samples. SEM: scanning electron microscope.

Characterization techniques

Fourier transform infrared (FTIR) spectra were recorded for both native and surface-modified UHMWPE fibers using a PerkinElmer (Perkin Elmer, USA) Spectrum 100 spectrometer in the range of 4000–500 cm⁻¹, which was equipped with a deuterated triglycine sulfate detector and potassium bromide (KBr) optics. Transmission spectra were obtained at a resolution of 4 cm⁻¹ after averaging 10 scans by casting a thin film on a KBr plate. Differential scanning calorimetry (DSC) analysis was performed on a PerkinElmer 8000 DSC using 3–5 mg of the sample under a constant flow of nitrogen at 20 mL min⁻¹. The specimens were heated from 50°C to 200°C at a constant heating rate of 10°C min⁻¹. A high-purity indium standard was used to calibrate the instrument, and the reference material was aluminum. The tensile and bending tests were recorded on the LLYOD EZ 20 (AMETEK Lloyd Instruments, UK) universal material testing machine following the American Society for Testing and Materials D3039 M and D790 standards, respectively. For the tensile tests, the D3039 M standard was used with a sample dimension of 25 × × 250 × 2.5 mm³. For the bending tests, the D790 standard was used with a sample dimension of 12.7 × 125 × 2.5 mm³. The fracture surface morphology of the composites was investigated using SEM (Zeiss, model EVO MA, Germany)

Results and discussion

IR analysis

To assess the efficiency of adopted oxidation techniques, FTIR analyses were performed for the native fibers as well as those modified with chromic acid and potassium permanganate. The obtained spectra (Figure 2) revealed consequent changes in the spectra of the oxidized fibers with respect to the spectrum of the native ones. Both modification techniques allowed the grafting of oxygen-containing functionalities, namely hydroxyl (around 3377 cm⁻¹) and carbonyl (1640 cm⁻¹) groups. These polar groups are targeted for improving the compatibility with the polymeric matrix. The presence of these two new bands confirmed the efficiency of adopted grafting procedure. Meanwhile, the relative intensity of these groups can provide a basis for comparison between the two modification techniques. In fact, it has been confirmed that the chromic acid can provide slightly more polar groups than the potassium permanganate. Thus, it can be predicted that the laminates prepared from the latter could display lower performance. Overall, both modification techniques can be regarded as cost-effective and highly effective for the creation of the targeted polar groups.

Figure 2.

FTIR spectra of the native and the surface-modified UHMWPE fibers. FTIR: Fourier transform infrared spectroscopy; UHMWPE: ultrahigh-molecular-weight polyethylene.

Grafting mechanism

The mechanism of grafting the O-containing groups is scrutinized in Figure 3. In both cases, the acid-catalyzed oxidation happens through two consecutive oxidation steps: the first oxidation step results in the creation of hydroxyl groups, while the second oxidation step allows the formation of the carbonyl groups. Considering that the reaction time is quite rapid, the latter should be carefully chosen to avoid damaging the fibers and therefore lowering their mechanical properties. Thus, the main difference between the two chosen procedures is the force of the acid species and their ability to create polar groups within the allowed time frame.

Figure 3.

Grafting mechanism of polar groups on the outer surface of UHMWPE fibers. UHMWPE: ultrahigh-molecular-weight polyethylene.

The mechanism by which the treated fibers can react with the chosen polymeric matrix is shown in Figure 4. First, the available hydroxyl groups react with the epoxide groups and result in their opening. Then, the opening of the epoxy groups leads to the formation of hydroxyl groups in the polymeric network. Finally, hydrogen bonds are formed between the hydroxyl and the carbonyl groups. Evidently, the first reactions occur in parallel with those between the hardener and the epoxy monomers. Overall, the final composite network will contain chemically and physically linked species, and the problem of compatibility between the fibers and the polymeric matrix is hence lifted off. It should be pointed out that for woven-like UHMWPE fibers used for the preparation of structural composites, the presented modification techniques can be chosen as the most suitable, mainly because the problem of fibers agglomeration does not exist. On the other hand, for short or chopped UHMWPE fibers, other techniques should be envisaged because of the self-agglomeration of these fibers due to the formation of hydrogen bonds from the grafted polar groups. The efficiency of the grafting procedure was also visually observed by SEM, as shown in Figure 5. It can be clearly seen in this figure that the outer surfaces of the fibers have been markedly affected by the acid etching procedure. In fact, the treated fibers appeared rougher with long scratches further confirming the success of the grafting procedure.

Figure 4.

Possible reaction route between the treated UHMWPE fibers and the epoxy. UHMWPE: ultrahigh-molecular-weight polyethylene.

Figure 5.

SEM images of the (a) native, (b) potassium permanganate-treated, and (c) chromic acid-treated UHMWPE fibers under different magnifications (×250, ×500, ×1000, and ×2000). SEM: scanning electron microscope; UHMWPE: ultrahigh-molecular-weight polyethylene.

DSC analysis

DSC analysis was performed to determine the melting point of native and surface modified UHMWPE fibers. The DSC curves are shown in Figure 6. As clearly seen in this figure, the crystalline melting point was affected by the adopted chemical modification. The melting point of the untreated fibers was measured at 147.5°C. The chromic acid and potassium permanganate-based treatment allowed higher melting temperatures. For instance, the fibers treated by chromic acid displayed a melting point of 148.8°C. Therefore, the grafting reaction affected the crystalline structure of the fiber. This behavior was also observed by Fan et al. in their study about horseradish peroxidase-catalyzed modification of UHMWPE fibers.²¹ Such increase in the melting temperature of UHMWPE fibers can enable higher processing temperatures and improved fiber wettability.

Figure 6.

DSC curves of the native and the surface modified UHMWPE fibers. DSC: differential scanning calorimetry; UHMWPE: ultrahigh-molecular-weight polyethylene.

Mechanical performances

In the field of continuous reinforcements, the fibers play the major role in the tensile load-carrying capacity of the composite structure. On the other hand, the role of the polymeric matrix includes keeping the fibers together, transferring the stress, and protecting fibers from the harsh environmental conditions and from the mechanical degradations. In terms of mechanical properties of the composite, the polymeric matrix has the major influence over the compressive, flexural, and shears properties.^22,23 Thus, in this research, the tensile and bending tests were evaluated to effectively assess the mechanical performances of the studied composites.

The bending test was performed to study the capability of composite plates to resist a flexural load before reaching the breaking point. To demonstrate the effects of the nature of chemical modification on the bending behavior, the flexural tests on 10-layered samples of laminated composite reinforced with surface-treated and untreated UHMWPE fibers were carried out. The obtained bending stress–strain curves are displayed in Figure 7. Table 1 lists the bending parameters, namely the flexural strength, strain-to-failure, and modulus. In this regard, it is worthy to highlight that the untreated UHMWPE fiber-reinforced composites showed inferior flexural strength and modulus (about 104 MPa and 3.38 GPa, respectively), comparative to those surface-treated fiber-reinforced composites. The best bending performances were reached with the chromic acid-based modification with flexural strength and modulus of about 127 MPa and 7 GPa, respectively. In terms of strain-to-failure, an opposite behavior was noticed further corroborating the assessment of an improved compatibility between the fibers and the matrix. Indeed, when the compatibility is enhanced through chemical modification of the fibers, the composites are more likely to display lower strain-to-failure. Overall, the bending tests clearly revealed the positive effects of the UHMWPE fibers oxidation in improving the flexural properties of the epoxy/UHMWPE composite. Meanwhile, the extent of the improvement also depends on the intensity of the polar groups present on the outer surface of the fibers.

Figure 7.

Flexural stress–strain curves of the composites based on native and surface modified UHMWPE fibers. UHMWPE: ultrahigh-molecular-weight polyethylene.

Table 1.

Bending parameters for the studied composites.

Type of treatment	Strength (MPa)	Modulus (GPa)	Strain-to- failure (%)
Untreated	104	3.4	7.7
Potassium permanganate	111	4.8	6.7
Chromic acid	127	6.9	6.2

In a similar way, the tensile tests were recorded for composites made of 10 layers of native and chemically modified UHMWPE fibers. The tensile stress–strain curves are shown in Figure 8, and Table 2 summarizes all the tensile properties. In addition, the toughness, which represents the amount of energy absorbed at break, was also calculated following the measurements of the surface under the strain–stress curves for each sample. The epoxy/native-UHMWPE composites displayed the lowest tensile performances with the tensile strength and modulus of about 303 MPa and 6.2 GPa, respectively. These values were remarkably improved after the treatment of the fibers reaching 404 MPa and 10.4 GPa, respectively, for the composite based on the chromic acid-treated fibers. Once again, the improved degree of compatibility between the fillers and the matrix by both physical and chemical links allowed an efficient stress transfer within the material, hence improving the overall tensile performances. Although the fiber oxidation caused reduction in the values of strain-to-failure, the toughness value measurements confirmed that the fillers treatment allowed the preparation of tougher composites.

Figure 8.

Tensile stress–strain curves of the composites based on native and surface modified UHMWPE fibers. UHMWPE: ultrahigh-molecular-weight polyethylene.

Table 2.

Tensile parameters for the studied composites.

Type of treatment	Strength (MPa)	Modulus (GPa)	Strain-to- failure (%)	Toughness (MJ m⁻³)
Untreated	302	6.27	4.9	7.3
Potassium permanganate	343	7.9	4.4	7.5
Chromic acid	403	10.4	3.9	7.9

Fractured surface morphology

To further confirm the findings of mechanical properties, the tensile fractured surfaces were analyzed by SEM to understand the fracture mechanism. The SEM images of the fractured surface of the composites prepared from the native fibers are shown in Figure 9, while in Figure 10 shows the SEM micrographs of those resulting from the fracture of the composites made from chromic acid-oxidized fibers. The untreated fibers resulted in poor adhesion with the polymeric matrix by the accentuated delamination is clearly shown in Figure 9(a) to (c). In contrast with the oxidized fibers, the native ones were distinguishable by longer elongation at break (Figure 9(d)), which corroborated the relatively higher strain-to-failure values for their subsequent composites. On the other hand, the composites issued from the treated fibers showed an excellent state of adhesion confirmed by a simultaneous rupture and the fewer delaminations. Therefore, the SEM images further confirmed the mechanical findings and clearly showed the fracture mechanism.

Figure 9.

SEM images of the fractured surface of the composites based on native UHMWPE fibers with different magnifications (a) 20×, (b) 50×, (c) 75×, and (d) 500×. SEM: scanning electron microscope; UHMWPE: ultrahigh-molecular-weight polyethylene.

Figure 10.

Fractured surface of the composites based on surface modified UHMWPE fibers with different magnifications (a) 20×, (b) 50×, (c) 75×, and (d) 500×. UHMWPE: ultrahigh-molecular-weight polyethylene.

Conclusion

The present study reported on cost-effective and highly efficient UHMWPE fiber surface modification techniques. The adopted chemical procedure aimed the grafting of polar groups on the outer surface of fibers for an improved chemical and physical compatibility with the polymeric matrix. The efficiency of the grafting methodology was confirmed by vibrational, thermal, and morphological analyses, and the grafting mechanism was thoroughly discussed. Composite laminates were manufactured from native and surface-modified UHMWPE fibers, and their mechanical performances were studied. The bending and tensile results revealed that the chromic acid treatment allowed the highest improvements in the strength and modulus. For instance, the bending and tensile strength values were about 22% and 33% higher than those of the native fiber-based composites. The study of the fractured surfaces showed that the composites issued from the treated fibers showed an ameliorated state of adhesion with the polymeric matrix. Finally, the adopted modification techniques can be regarded as cost-effective and highly suitable for the manufacturing of structural composites for advanced applications.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Mehdi Derradji

Djamel Belmehdi

Oussama Mehelli

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