Investigation of the Mechanical Properties of Thermosetting Polymers Reinforced with Carbon Particles ^†

Abstract

In this work, the mechanical properties of composites with a polymer matrix and reinforced with carbon particles have been studied. It has been established that the obtained engineering materials have increased elastic and plastic characteristics. The thermosetting polymers used are epoxy, polyester, and vinylester resins. The carbon particles are carbon nanotubes and waste carbon from the plasma decomposition of methane in the production of green hydrogen. The carbon particles used are in an amount of 1 wt% and 2 wt% of the weight of the composite, and they are not subjected to pre-treatment (modification). The studied composites are used in shipping, automotive, and aviation technology, and the presence of carbon particles in them is a prerequisite for improving their anti-radar properties.

1. Introduction

Composites with a polymer matrix reinforced with carbon fibers (CFs) are promising materials due to their low weight, high specific strength, and high stiffness with applications in the automotive, aerospace, and military industries, as well as in wind energy generation [1]. A major problem in the production of this type of composites is related to the fact that carbon fibers (CFs) are an unordered graphitic structure with a smooth, chemically inert surface, and low surface energy, which is the basis of poor interfacial bonding with the resin [2,3]. To solve this problem, a process of modification of the carbon particles is carried out in order to introduce sufficiently polar groups on their surface, usually carboxyl, hydroxyl [4], epoxy [5], and amino groups [6], which not only increases their wettability, but also leads to chemical reactions with the resins.

In recent years, there has been a trend towards the development of polymer matrix composites incorporating carbon nanotubes (CNTs). Carbon nanotubes (CNTs) have good mechanical, electrical, and thermal properties, which make them extremely suitable for reinforcing polymer composites [7,8]. The incorporation of CNTs into polymer matrices significantly improves the conductivity, strength, and elasticity of the resulting composites, i.e., obtaining new materials with improved characteristics [9,10,11,12,13]. The development of polymer composites with CNTs has led to their application in electronic components, microbatteries, circuits, electromagnetic shielding, electromagnetic wave shielding, heat spreaders, and high-strength structures [12,14].

In recent years, hydrogen has been established as a promising energy carrier because it is light and environmentally friendly, offering a sustainable alternative that reduces greenhouse gas emissions [15,16]. Various methods are used to produce hydrogen [17,18,19,20,21,22], but the most environmentally friendly method is the catalytic decomposition of methane, which produces pure hydrogen without carbon dioxide emissions and produces residual carbon. At present, due to the morphology of waste carbon and the average diameter of its particles, it is mainly used in the processing and vulcanization of rubber compounds.

The development of innovative composite materials with a polymer matrix and carbon particles embedded in it, which have enhanced mechanical and operational characteristics, including those intended for the construction of systems and devices for detecting and countering low-flying unmanned aerial vehicles [23,24,25], is a current engineering topic.

In this work, the mechanical properties of composites with a polymer matrix and embedded carbon nanotubes (CNTs) and waste carbon obtained from green hydrogen production are investigated. The experimental results are compared and the possibility of using waste carbon as a component in innovative polymer composites is discussed.

2. Materials and Methods

To conduct the planned experiments, epoxy, polyester, and vinylester resins were used. The epoxy system “LETOXIT” is based on epoxy resin for lamination “LETOXIT” PR 227 and hardener “LETOXIT” EM 315. Due to its low viscosity and low surface tension, the used resin system wets the laminating fabrics well and is therefore used for the production of laminates with glass fibers, carbon or aramid fibers. The manufacturer’s prescription was followed for the preparation of the resin system, namely 100:38 mass proportions when mixing (resin:hardener). The polyester resin used is “DISTITRON” 429 BSX25Q, the hardener is “MEKR50” in an amount of 2 wt%, and the amount of accelerator “MEKR” used is also 2 wt%. The vinyl ester resin is “DISTITRON” VE 100, the hardener is “MEKR50”, and again the amount used is 2 wt%.

To obtain the composites, multi-walled carbon nanotubes (CNTs) and waste carbon from the production of green hydrogen by plasma decomposition of methane in an amount of 1 wt% and 2 wt% of the polymer mass were used. The multi-walled carbon nanotubes have a length of ≥2 µm, a diameter of 10–40 nm, a number of walls of 10–20 and a specific surface area of 55 m²/g. In the conducted studies of the waste carbon, it was found that the carbon (C) content was 99.1 at% and the oxygen (O) content was 0.9 at% in it (Figure 1), and its structure is shown in Figure 2.

The investigated composites based on the thermosetting polymers and carbon particles used were produced by mechanical stirring at a working temperature of 25 °C (according to the manufacturers’ instructions for working with the epoxy resin system). Test specimens were gravity cast for tensile testing and impact toughness testing. Before conducting the mechanical tests, the samples based on epoxy resin were subjected to secondary polymerization at a temperature of 55 °C for 12 h (resin manufacturer’s instructions).

The uniaxial tensile test was performed on a Zwick Roell Z050 testing machine, the impact toughness test was performed with an Izod GT-7045-HM device under the following conditions: Izod Impact Test, 1J (10.20 kg·cm), Method—reversed notch, Speed 3.46 m/s, and the hardness of the composites was measured using the Shore method (scale D—HSD). This method for determining hardness is elastic-dynamic, i.e., the tip is lowered from a certain height and the height of the rebound is used to determine the hardness of the material, using a table showing the value of the hardness versus the rebound value. The measurement was performed using an EQUOTIP hardness tester.

3. Results

The results of the mechanical tests conducted on the epoxy resin-based composites reinforced with both types of carbon particles are shown in

Table 1. Results of mechanical tests of an epoxy resin-based composite.

A Type of Polymer	Carbon Particles [wt%]	σm [MPa]	ε [%]	HSD	KCV1/10,2 [J/m2]
“Letoxit” PR 227	without	49	0.8	76.7	1797.8
“Letoxit” PR 227	CNT—1 wt%	40	1.0	81.2	2581.3
“Letoxit” PR 227	CNT—2 wt%	31.1	1.5	80.4	3790.2
“Letoxit” PR 227	Carbon—1 wt%	42.2	1.0	83.8	1400.1
“Letoxit” PR 227	Carbon—2 wt%	21.7	-	85.4	491

.

The results obtained show that the introduction of both types of carbon particles into the epoxy resin system does not lead to an increase in the tensile strength of the composite. When using carbon nanotubes, an increase in the elongation, hardness, and impact toughness of the resulting composite was recorded. The use of carbon as reinforcing particles leads only to an increase in the hardness of the composite, but the other studied characteristics deteriorate. In this particular case, the use of waste carbon from the production of green hydrogen as a reinforcing component in the studied composite is not appropriate; the advantage is on the side of carbon nanotubes.

Table 2. Results of mechanical tests of a polyester resin-based composite.

A Type of Polymer	Carbon Particles [wt%]	σm [MPa]	ε [%]	HSD	KCV1/10,2 [J/m2]
“Distitron” 429 BSX25Q	without	47.8	-	79.8	299.5
“Distitron” 429 BSX25Q	CNT—1 wt%	27	0.8	87.8	314.6
“Distitron” 429 BSX25Q	CNT—2 wt%	27	1.2	82	281.13
“Distitron” 429 BSX25Q	Carbon—1 wt%	27.1	1.7	87.7	405.58
“Distitron” 429 BSX25Q	Carbon—2 wt%	22.6	1.2	84.7	300.16

shows the results of the mechanical tests conducted on polyester resin reinforced with carbon particles.

The use of carbon nanotubes in an amount of 1 wt% and 2 wt% to obtain a composite based on polyester resin “DISTITRON” 429 BSX25Q leads to a decrease in the tensile strength—but unlike the pure resin in which no elongation was registered, at the two used CNT concentrations, elongation was registered. When introducing 1 wt% CNTs, the material has higher values of hardness and impact toughness compared to the pure resin and the composite with 2 wt% CNTs. In the composites with embedded carbon particles (C), a decrease in the values of tensile strength compared to the tensile strength of the pure polymer is again observed, but the obtained values are identical to those of the composites with CNTs. The results show that the addition of waste carbon to the composites leads to an increase in the values of elongation, with 2 wt% C having the highest measured values. The hardness of the composite as well as the impact toughness were increased, and when using 1 wt% C, the highest impact toughness value of the presented composites with a polymer matrix from the polyester resin used was registered. The use of waste carbon for the development of a composite with a matrix based on polyester resin “DISTITRON” 429 BSX25Q is appropriate; the mechanical properties of the obtained material are not inferior to those of the composite with embedded CNTs.

The results of the mechanical tests conducted on the vinylester resin-based composites reinforced with both types of carbon particles are shown in

Table 3. Results of mechanical tests of a vinylester resin-based composite.

A Type of Polymer	Carbon Particles [wt%]	σm [MPa]	ε [%]	HSD	KCV1/10,2 [J/m2]
Distitron VE 100	without	20.5	-	75.7	389.3
Distitron VE 100	CNT—1 wt%	40	-	80.9	524.8
Distitron VE 100	CNT—2 wt%	20.6	0.5	82.3	282.4
Distitron VE 100	Carbon—1 wt%	21.4	-	81	410.7
Distitron VE 100	Carbon—2 wt%	32.7	0.6	78	482.9

.

The composite with vinylester resin matrix “DISTITRON” VE 100 and 1 wt% CNTs has the highest values of tensile strength and impact toughness; in it, as in the case of pure resin, no elongation was registered, and an increase in the hardness value was found. With an increase in the amount of CNTs to 2 wt%, elongation and an increase in the hardness of the composite were registered, but the impact toughness decreased to values lower than those of pure resin, and the tensile strength of the composite was identical to that of the base (“DISTITRON” VE 100). When using 1 wt%, a slight increase in the tensile strength was observed, as well as an increase in the values of hardness and impact toughness of the composite compared to the pure polymer. As with the use of 1 wt% CNTs, no elongation was registered. With an increasing amount of carbon particles (2 wt% C), elongation was recorded with increasing tensile strength and impact toughness of the composite, while hardness decreased.

4. Discussion

When using two types of carbon particles (CNTs and C) to obtain a composite based on epoxy resin for lamination “LETOXIT” PR 227, a structural material with reduced strength and increased elongation and hardness is obtained compared to the pure polymer. The use of different CNT concentrations (1 wt% and 2 wt%) leads to an increase in the impact toughness of the material, while the use of the same amounts of waste carbon worsens this characteristic of the composite. Apparently, the geometric nature of the particles used does not allow them to absorb the stresses that arise during tensile testing and rather they play the role of stress concentrators, as a result of which the mechanical strength of the composite is reduced. Studies have shown that waste carbon particles in the amounts used (1 wt% and 2 wt%) embrittle the composite, which is a prerequisite for conducting additional experiments with the aim of optimizing the amount of carbon particles to obtain a competitive material. The successful introduction of waste carbon obtained from the plasma decomposition of methane to produce green hydrogen will be the basis of an environmentally friendly, waste-free technology for producing alternative fuel.

The results of the research conducted to develop a composite based on the polyester resin “DISTITRON” 429 BSX25Q with carbon particles embedded in it are positive. Again, a decrease in the strength of the resulting composites compared to the base polymer was observed, but elasticity was given to a brittle material (polyester resin “DISTITRON” 429 BSX25Q), and the hardness and impact toughness of the composite were increased. It is noteworthy that the composites in which 1 wt% CNTs and C were used have better mechanical properties compared to the composites in which the amount of carbon particles used was increased to 2 wt%. The best mechanical properties are possessed by the composite polyester resin “DISTITRON“ 429 BSX25Q and 1 wt% waste carbon particles embedded in it. The positive results of the mechanical tests of the composite with the used polymer and 1 wt% C give us reason to assume that a waste product can adequately replace high-tech CNTs in this particular case. This is a prerequisite for future studies of the performance properties of the presented composite.

The introduction of 1 wt% CNTs into vinylester resin “DISTITRON” VE 100 leads to the production of a composite that has higher values of tensile strength, hardness, and impact toughness compared to the pure polymer. By increasing the amount of CNTs to 2 wt%, the tensile strength of the composite is comparable to that of the base polymer, a minimal elongation of the composite and increased hardness are registered, but the impact toughness value is reduced. The developed composites on the basis of thermosetting resin “DISTITRON” VE 100 and waste carbon particles embedded in it in an amount of 1 wt% and 2 wt% have mechanical properties higher than those of the base resin. Unlike the composite on the same basis and CNTs, when using carbon in a larger amount, the mechanical properties of the final product increase.

In thermosetting polymers with higher mechanical strength (“LETOXIT” PR 227 and “DISTITRON“ 429 BSX25Q), the introduction of the used CNTs and waste carbon particles leads to a decrease in the strength of the composite. Most likely, this is due to the small linear length of the particles, which does not allow them to absorb the stresses that have arisen in the composite, and these particles in this case have the role of stress concentrators. Another reason that can be noted is that the particles are not modified, which worsens the adhesion between them and the base polymer. In a resin with not particularly high mechanical strength (“DISTITRON” VE 100), the two types of used carbon particles in certain concentrations have a positive effect on the tensile strength of the created composites. In most cases, the used particles have a positive effect on the elasticity of the created composites, as a result of which higher values of hardness and impact toughness have been registered.

5. Conclusions

The possibility of expanding the spectrum of composite materials with a polymer matrix by using thermosetting polymers and carbon particles has been considered. The results of the mechanical tests conducted show that in certain cases, composites with reinforcing waste carbon particles have properties competing with composites with CNTs. Obtaining a machine-building material using a waste product, in addition to engineering, has an ecological and economic aspect. Implementing the waste obtained from an innovative technology for obtaining green hydrogen into a construction material makes this technology completely waste-free, i.e., not having a negative impact on the environment.