Performance Properties and Finite Element Modelling of Forest-Based Bionanomaterials/Activated Carbon Composite Film for Sustainable Future

Abstract

Thanks to its highly crystalline structure and excellent thermal, optical, electrical and mechanical properties, carbon and its derivatives are considered the preferred reinforcement material in composites used in many industrial applications, especially in the forest and forest products sector, including oil, gas and aviation. Since hydroxyethyl cellulose (HEC) is a biopolymer, it has poor mechanical and thermal properties. These properties need to be strengthened with various additives. This study aims to improve the thermal and mechanical properties of hydroxyethyl cellulose by preparing hydroxyethyl cellulose/activated carbon (HEC/AC) composite materials. With this study, composites were obtained for the first time and their mechanical properties were examined using a 3D numerical modeling technique. The thermal stability of the prepared composite materials was investigated via thermal gravimetric analysis (TGA). The samples were heated from 30 °C to 750 °C with a heating rate of 10 °C/min under a nitrogen atmosphere and their masses were measured subsequently. The mechanical properties of the composites were investigated via the tensile test. The viscoelastic properties of the composite films were determined with dynamic mechanical thermal analyses (DMTA) and their morphologies were examined with scanning electron microscopy (SEM) images. According to the results, the best F3 sample (films containing 3 wt.% activated carbon) had an elastic modulus of 168.3 MPa, a thermal conductivity value of 0.068 W/mK, the maximum mass loss was at 328.20 °C and the initial storage modulus at 30 °C was 206.13 MPa. It was determined that the hydroxyethyl cellulose composite films containing 3 wt.% activated carbon revealed the optimum results in terms of both thermal conductivity and viscoelastic response and showed that the obtained composite films could be used in industrial applications where thermal conductivity was required.

1. Introduction

Due to the increasing need for non-fossil energy resources globally in recent years, alternative innovative solutions have begun to be sought in the sustainable forest and forest products sector against the negative effects of plastic production in terms of human and environmental health. Efforts to reduce these problems, especially based on national and international environmentally friendly economic policies, have been accelerated and cost-effective, environmentally friendly, green forest product materials have begun to replace petroleum-derived polymers. Although many synthetic polymers are used in industry, the need to integrate them into different industrial applications is an inevitable reality in terms of sustainability. It is seen that biodegradable polymers are especially preferred in obtaining biopolymers from sustainable forest products and secondary natural resources. Many studies have been conducted on composite materials due to their superior properties [1,2,3]. A significant part of these studies consists of polymer composite materials [4,5]. There are thermal, mechanical, electrical, etc., properties of polymers used as matrix materials in polymer composite materials. Many reinforcing additives are used to improve its properties [6,7].

Biocomposite materials attract a lot of attention since they are eco-friendly and renewable. Biopolymers are frequently used in the preparation phase of biocomposite materials [8]. Hydroxyethyl cellulose (HEC), being a cellulose ether derivative, is a biopolymer obtained by the nucleophilic ring-opening reaction between ethylene oxide and hydroxyl groups located on the anhydroglucose unit of cellulose. Since HEC is a biopolymer, it has poor mechanical and thermal properties. These properties need to be strengthened with various additives [9]. Due to its various advantages, hydroxyethyl cellulose is used as a thickener, binder, emulsifier, stabilizer and film former in the forest products, pharmaceuticals, cosmetics, food, textile and construction sectors. It is easily dispersed in the mixture, simply affixed to the surface to be applied, prevents the material from flowing off the surface by increasing the viscosity of the mixture, and exhibits typical properties such as those of a colloid and stabilizer [10,11].

Carbon and its derivatives are used as reinforcement additives in composite materials used in forest-based industrial sectors due to their highly crystalline structure and good thermal, optical, electrical and mechanical properties [12]. Activated carbon is obtained by the carbonization and activation of carbon-containing structures of plant origin [13]. Activated carbon has a high internal surface area, high surface reactivity, proper pore distribution and high mechanical strength. Owing to these properties, activated carbon is preferred in a wide range of application areas such as food, pharmacy, chemistry, petroleum, mining, nuclear, automobile, waste gas and water purification [14].

Om Prakash et al. [15] synthesized activated carbon from arhar fiber biomass by pyrolysis and chemical activation. As a result, activated carbon prepared at 800 °C was found to be effective, revealing a 504.6 m²/g surface area and a 0.245569 cm³/g pore volume, and micropores smaller than 20 Å in diameter. Additionally, it has been reported that the tensile strength of the resulting composite materials prepared with activated carbon epoxy was increased by the addition of activated carbon. Zor et al. [16] aimed to improve the properties of PVA by aiming to recycle the carbonized materials they obtained by pyrolysis of waste tires instead of disposing of them. They prepared PVA biocomposite films containing different proportions of waste carbonized rubber. They examined the thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), morphological properties, XPM, and electrical and rheological measurements of these films. They demonstrated their usability in the electronic industry by increasing the electrical conductivity and hardness of PVA biocomposite films by adding carbonized waste rubber material. In the paper by Kocatürk et al. [17], waste tires produced by pyrolysis under high pressure were reinforced with nanocellulose at levels of 0.10%, 0.25%, 0.5% and 1% by weight. Their thermal properties were examined by the scanning electron microscopy (SEM), TGA, DSC and dynamic mechanical thermal (DMTA) methods. Additionally, the percentage of gel content of the produced nanocomposites was determined. Thermal analysis revealed that the sample containing 1% carbonized waste rubber showed the highest thermal stability, and the ash yield at 750 °C increased by up to 25% compared to nanocellulose. All results indicate that green nanocellulose-based nanocomposites can be used for future applications in industry. Zor et al. [18] aimed to use the pyrolysis oil of waste tires as a reinforcing element in nanocellulose-based nanocomposite films by recycling tires produced in the automotive industry. Pyrolysis oil was added to the nanocomposites at 5%, 10% and 20% by weight. Thermal (TGA, DSC), thermomechanical (DMTA) and morphological (SEM) characterizations of the produced nanocomposite films were carried out. As a result, the superior thermal properties and structural compatibility of the produced nanocomposite films were observed, suggesting that they can be used especially in the pharmacy, coating, and packaging industries.

The aim of this study was to prepare and characterize the hydroxyethyl cellulose/activated carbon composite materials to improve the mechanical and thermal properties as well as the thermal stability of hydroxyethyl cellulose, a derivative of cellulose, which is the most widely used biopolymer of forest and forest products. For this purpose, hydroxyethyl cellulose/activated carbon composite films containing different ratios of activated carbon were prepared. The thermal stabilities of the obtained films were investigated by TGA/DTG, while their mechanical properties were examined via tensile-break tests. The thermal conductivities, viscoelastic properties and morphologies of the samples were also determined. Additionally, a finite element-based numerical homogenization was presented to predict the mechanical properties of the composites.

2. Materials and Methods

2.1. Materials

Hydroxyethyl cellulose (Mv~90.000 g/mol) was procured from Sigma-Aldrich, St. Louis, MO, USA. Activated carbon, obtained from waste rubber as 0–15 µm powder particles by the pyrolysis method under high pressure, was procured from a local company.

2.2. Preparation of Hydroxyethyl Cellulose/Activated Carbon Composite Films

A total of 3 g of hydroxyethyl cellulose was dissolved in 50 mL of distilled water and stirred until a clear solution was obtained. The resulting solution was poured into a petri dish and dried in an oven at 50 °C for 48 h. At the end of this period, the biofilm was obtained from the petri dish. Similarly, biocomposite films were produced by adding an activated carbon amount of 1 wt.% (F2), 3 wt.% (F3) and 5 wt.% (F4) of hydroxyethyl cellulose to the hydroxyethyl cellulose solution.

2.3. Measurements and Characterization

The mechanical properties of the obtained composite films were determined by the tensile test using a Zwick Roel Tester (Ulm, Germany). The average dimensions of the specimens were 25 mm × 4.3 mm × 0.37 mm (length × width × thickness). During the tests, the crossbar speed of the tester was maintained at 1 mm/min. Five tests were performed for each sample. To compare the experimental results, a simulation was performed with a 3D numerical modeling technique. The thermal conductivity of the composite films was measured using a Decagon KD2-Pro device (Pullman, Washington, USA) at 32 °C. The thermal properties of the samples were determined via TGA/DTG analyses using a thermogravimetric analyzer TGA/DSC3+ (METTLER TOLEDO, Toledo, Ohio, USA). The samples were heated from 30 °C to 750 °C at a heating rate of 10 °C/min under nitrogen atmosphere and their masses were measured subsequently. DMTA was used to determine viscoelastic properties. Analysis was performed on the HITACHI instrument. Thermomechanical properties were determined by measurements (between 30 and 200 °C, 1 Hz frequency and 5 °C/min heating rate). The morphological structure of the prepared composite films was examined by SEM (TESCAN, Brno, Czech Republic). The samples were coated with platinum prior to the analyses.

2.4. Numerical Modeling

We attempted to predict the mechanical properties of the HEC/AC composites based on the mechanical properties and volume fractions of the constituents (i.e., hydroxyethyl cellulose and activated carbon) via a finite element-based (FE-based) numerical homogenization technique [19,20]. For this purpose, a cubic representative volume element (RVE), which is large enough to represent statistically homogenous material properties, was generated for each composite film (F2, F3 and F4) using the Material Designer module of Ansys 2020. In the RVE model, the activated carbon (AC) constituent was modeled as 15 µm diameter spherical particles randomly distributed in the hydroxyethyl cellulose (HEC) matrix domain. Periodic boundary conditions were applied to the RVE faces to ensure the continuity of strain and stress fields on the boundaries.

The volume fraction ${\varphi }_f$ of the filler phase (i.e., AC particles) in each composite film was determined based on the measured densities of the composite films [21]:

$${\varphi }_f={\displaystyle \frac{{\rho }_c}{{\rho }_f}}{\psi }_f$$

(1)

where ${\rho }_c$ is the density of the composite, ${\rho }_f$ is the density of the filler (i.e., activated carbon) and ${\psi }_f$ is the weight fraction of the filler. The density of AC particles used in the present study was taken as 0.5 g/cm³ [22]. Filler volume fractions of the F2, F3 and F4 composite films were calculated to be 0.03, 0.08 and 0.13, respectively.

The density and modulus of elasticity of the neat HEC film were measured to be 1.067 g/cm³ and 222.5 MPa, respectively. The constituent properties used in the RVE models are listed in

Table 1. Properties of HEC and AC used in the RVE models.

Sample	Density, ρ (g/cm3)	Modulus of Elasticity, E (GPa)	Poisson’s Ratio, ν
HEC	1.067	0.2225	0.35
AC	0.5	10	0.3

.

Random particle periodic RVE models developed for the composite films F2 (1 wt.%), F3 (3 wt.%) and F4 (5 wt.%) are depicted in Figure 1, Figure 2 and Figure 3. In Ansys Material Designer, each RVE mesh was subjected to six different load cases (tensile and shear loadings) and the homogenized elastic properties (moduli of elasticity, shear moduli and Poisson’s ratios) of the composites were computed as listed in

Table 2. Mechanical properties of the composite films predicted from FE-based numerical homogenization.

	Modulus of Elasticity, E (MPa)	Poisson’s Ratio, ν
F2 (1 wt.%)	237.93	0.347
F3 (3 wt.%)	263.30	0.343
F4 (5 wt.%)	292.63	0.335

.

The predicted mechanical properties of the HEC/AC composite films were subsequently used in tensile test simulations on Ansys Mechanical and the results were compared with those obtained from physical experiments, as discussed in the following section.

3. Results and Discussion

3.1. Numerical Modelling and Mechanical Properties of the Composite Films

The tensile test results of the neat HEC and HEC/AC composite film specimens are listed in

Table 3. Mechanical properties of neat HEC and HEC/AC composite films.

Sample	Activated Carbon Content (wt.%)	Tensile Strength (MPa)	Modulus of Elasticity (MPa)	Elongation at Break (%)
F1	0	12.39 ± 0.17	232.5 ± 5.4	13.28 ± 0.03
F2	1	6.73 ± 0.07	123.1 ± 3.2	11.91 ± 0.06
F3	3	7.33 ± 0.05	168.3 ± 4.8	10.63 ± 0.09
F4	5	7.94 ± 0.07	131.2 ± 3.1	14.33 ± 0.05

. The tensile strength and modulus of elasticity of the HEC/AC composite specimens are observed to be lower than those of the neat HEC film (F1). The reduction in strength and elastic modulus with the addition of activated carbon is an indication of poor adhesion between the HEC polymer and activated carbon (AC) particles. Although the reduction in tensile strength can be attributed to poor bonding between the AC inclusions and the HEC matrix, a marked reduction in the modulus of elasticity of the HEC/AC composites is rather unexpected. The addition of AC fillers to HEC is expected to hinder the movement of biopolymer chains and thus to increase the stiffness [23]. In addition to being an excellent adsorbent, the AC has been reported to improve the mechanical properties of composites of various polymers including epoxy, polyvinyl alcohol (PVA), polyethylene and rubber [15,24,25]. Mohmad et al. [26] prepared composite materials containing palm kernel activated carbon and epoxy. As the activated carbon content of the obtained composites increased, the tensile strength values also increased initially—however, a decrease was reported as the activated carbon content increased further. On the other hand, the tensile strength of the HEC/graphite composite films was found to decrease gradually with an increasing amount of graphite [27].

The moduli of elasticity of the composite films predicted by the FE-based numerical homogenization study are listed in Table 2. The modulus of elasticity of the composite film is determined to increase with increasing AC content. Accordingly, the elastic moduli of the composite films with 1, 3 and 5 wt.% AC content are determined to increase by 7, 18 and 32% compared to the modulus of elasticity of the neat HEC. It can be argued that the addition of AC particles to the HEC polymer restrains the movement of its chains and thus increases the stiffness of the resulting HEC/AC composite. Although the results of the numerical model corroborate this argument, the physical experiments revealed an opposite behavior with regards to the effect of AC inclusions on the modulus of elasticity of the composites (see Table 3). Regardless of their AC contents, all specimens subjected to physical tensile tests exhibit a significant reduction in their modulus of elasticity compared to that of pristine HEC. The modulus of elasticity of the specimens with 1, 3 and 5 wt.% AC reveals a 49, 29 and 50% reduction, respectively. The modulus of elasticity increases as the AC content is increased from 1 wt.% to 3 wt.%, but this can hardly be attributed to the presence of AC fillers considering the overall drop observed in the modulus of HEC/AC composites.

In the FE-based numerical model, we assume that the AC particles are of uniform spherical shape and they are randomly distributed in the HEC domain. While the latter assumption may not require any justification, the former simplifies the model. A platelet or a lamellar shape would be more realistic to represent the shape of AC inclusions. The numerical model employs a “perfect bonding” assumption at the interface of the AC inclusions and HEC matrix. The assumption of perfect bonding has been used extensively in classical homogenization approaches [28]. The significant reduction observed in both the elastic moduli and tensile strength of the HEC/AC composite specimens is an indication of poor adhesion at the interface of the AC fillers and the HEC matrix. Therefore, it seems that the perfect interface assumption used in the FE-based numerical homogenization scheme needs to be relaxed by incorporating more elaborate interface models (which accounts for displacement and/or traction jumps) at the expense of computational cost and added complexity in modeling [29,30,31].

Stress vs. strain plots obtained from the tensile tests of the neat HEC (F1) and HEC/AC composite films (F2, F3, F4) are given in Figure 4. Additionally, plotted in Figure 4 are the stress–strain curves of composite films obtained from finite element simulations on Ansys Mechanical. Comparison of the stress–strain curves obtained from the FE simulations and the experiments reveals a notable difference. At 4% strain, the FE simulations yield stress values 2.5, 2.1 and 2.6 times the corresponding experimental data at 1, 3 and 5 wt.% AC content, respectively. The numerical simulations are based on a linear elastic constitutive model and, therefore, they are not expected to provide any prediction beyond the initial linear elastic range on the stress vs. strain graphs.

The mechanical properties (listed in Table 2) predicted from FE-based numerical RVE homogenization were subsequently used in the finite element simulations. A solid model of the tensile specimens was prepared in Ansys Space Claim and the predicted mechanical properties were used as inputs. The model was discretized into finite elements using 20-node quadratic hexahedral elements. A convergence in the results was achieved with a total of 43,000 elements. A fixed boundary condition was applied one end, while the other end was subjected to an axial displacement boundary condition of 1 mm, which corresponds to a 4% normal strain. To account for large deformations, simulations were performed with the switching on of geometric nonlinearity. Figure 5 shows the axial normal stress distribution on the composite film (F4, 5 wt.%) at 4% strain.

3.2. Thermal Conductivity of the Composite Films

The thermal conductivity of the obtained polymeric films was measured at a temperature of 32 °C. An examination of the results showed that thermal conductivity clearly increases as the amount of activated carbon in the samples increases, as indicated in Figure 6. However, it was observed that adding more than 1% activated carbon did not cause a significant increase in thermal conductivity. Consequently, the thermal conductivity of hydroxyethyl cellulose has been improved by the utilization of activated carbon with high thermal conductivity (

Table 4. Thermal conductivity of the composite films.

Samples	Thermal Conductivity (W/mK)
F1	0.051
F2	0.650
F3	0.681
F4	0.683

). Verma et al. [28] reported that the thermal conductivity of activated carbon varies between 0.3 W/mK and 0.63 W/mK. The relatively high thermal conductivity of activated carbon increased the thermal conductivity of hydroxyethyl cellulose—yet adding more than 1 wt.% of activated carbon did not influence thermal conductivity.

3.3. Thermal Stability of the Composite Films

The retrieved data from TGA/DTG of the obtained composite materials are shown in

Table 5. Thermal stability of the composite films.

Samples	T10% (°C)	Max. Weight Loss (°C)	Char Yield (%)
F1	171.65	346.56	14.21
F2	266.52	335.35	19.02
F3	266.15	328.20	20.69
F4	269.31	331.52	21.33

and their thermograms are shown in Figure 7. According to these results, as the amount of activated carbon in the samples increases, the initial degradation temperatures of the films increase significantly. This is due to the high thermal resistance properties of activated carbon. The main decomposition temperature of the samples is approximately 335 °C. The char yield of the samples at 750 °C increases clearly from 14% to 21% as the amount of activated carbon increases. The char yield prevents the flame from reaching the interiors of the material, consequently increasing the flame resistance of the material [32]. As a result, although activated carbon did not cause any change in the main decomposition temperature, it contributed to thermal resistance by clearly increasing the char yield.

3.4. Viscoelastic Properties of the Composite Films

The viscoelastic properties of the composite films were examined using the dynamic mechanical analysis (DMA) technique. The storage moduli, loss moduli and tan δ values of the composite films are shown in Figure 8. Examination of the obtained results reveals a limited increase in the storage moduli of the composite films with increasing AC content. This increase reached the highest value in the F3 sample containing 3 wt.% activated carbon. When more than 3 wt.% activated carbon is added to composite films, there is a decrease in the storage modulus. Similar results are observed for all increasing temperatures. At the initial temperature (30 °C), the storage modulus of the F3 sample is 206.13 MPa, while the storage modulus of the F4 sample is 184.24 MPa. These results show that the optimum activated carbon contribution is 3 wt.%. The peak of the tan δ curve corresponds to the glass transition temperature of the composite. When the values are examined, it is seen that the glass transition temperature decreases as the activated carbon content increases (see

Table 6. Viscoelastic properties of the composite films.

Samples	Initial Storage Modulus (MPa) 30 °C	E′ (MPa) 50 °C	E′ (MPa) 100 °C	E′ (MPa) 150 °C	Tg (°C)
F1	23.83	7.59	3.41	3.35	144.71
F2	16.89	7.99	4.85	4.15	136.37
F3	206.13	40.70	19.37	5.03	108.12
F4	184.24	31.51	16.80	2.82	110.87

). Abdul Khalil et al. [33] prepared composites by adding activated carbon obtained from coconut shell to epoxy resin in different proportions. They reported the storage modulus of epoxy resin without activated carbon as 1450 MPa, and the storage modulus of epoxy resin containing 10% activated carbon as 1659 MPa.

3.5. Morphological Properties of the Composite Films

SEM images of all samples at both ×5000 and ×10,000 resolutions are shown in Figure 9. It can be seen from the images that the activated carbon contribution is distributed homogeneously within the HEC. Thommy et al. [34] prepared hydroxyethyl cellulose/mangiferin edible films and reported that the resulting films appeared wavy and smooth.

4. Conclusions

This study aims to improve the thermal and mechanical properties of hydroxyethyl cellulose by using activated carbon as a reinforcing agent. It has been possible to prepare the hydroxyethyl cellulose composite films successfully. The following conclusions could be drawn from the results of the present study:

• It was observed that tensile strength decreased and elongation at break increased as the amount of activated carbon increased in the composite formulations. Thus, the mechanical properties of HEC are improved by obtaining a more flexible material with the addition of activated carbon.
• The thermal conductivity of the composites reached its highest value with the use of 1 wt.% activated carbon. No significant change was observed in thermal conductivity values by adding more activated carbon.
• It was observed that the char yield of the composites increased together with the increase in the activated carbon ratio in the composites. The increase in char yield prevents the flame from passing into the inner parts of the material and increases the resistance of the material to burning. In this respect, activated carbon has improved the thermal properties.
• The dynamic mechanical analysis results showed that as the activated carbon content increased, the storage modules increased, the glass transition temperature decreased and the optimum activated carbon content was 3%.
• SEM images show that the activated carbon additive is distributed homogeneously in the resulting composite films.

All obtained results showed that hydroxyethyl cellulose films containing 3 wt.% activated carbon were the optimum sample in terms of both thermal conductivity and viscoelastic response. It showed that the resulting composite films can be used in other industrial applications where thermal conductivity is needed for a sustainable future. Wood-based biomaterials and waste products especially have tremendous applications in bioenergy and environmental remediation, and as sustainable innovative materials in industry due to their special characteristics. National and international policies and their implementation at global level will increase the potential of using sustainable techniques in climate change.