Mechanical, thermal, and morphological properties of low-density polyethylene nanocomposites reinforced with montmorillonite: Fabrication and characterizations

Abstract

Nanoparticle incorporation in polymeric matrices to generate polymer nanocomposite with the intention of maximizing the “nano-effect” derived from the nanoparticles and minimizing the drawbacks of the polymer is an emerging field of research. In this study, low-density polyethylene (LDPE) was mixed with varying concentrations of montmorillonite (MMT) nanoclays to create a polymer nanocomposite with desirable characteristics. Composite sheets with nanoclays contents of (0, 1, 2, 3, and 4 wt%) were prepared for hardness, tensile-fractography, thermal conductivity, and tensile testing (elongation and stress-at-break). The results showed that, according to scanning electron microscope (SEM) study, LDPE has a low flexibility temperature and is prone to corrosion. For larger MMT filler loadings (>3% wt), tensile-fractography showed nanoclays particle micro-aggregation. The pure LDPE sample fracture’s tensile-fractography showed plastic deformation. MMT/LDPE samples with 3% wt hard MMT filler have brittle fractures without appreciable plastic deformation. Thermal conductivity test results show that LDPE/MMT composite thermal conductivity decreased with increasing clay concentration. The thermal conductivity values were reduced from a value of 0.13 W/m.K to a value of 0.039 W/m.K when reinforced with 0% wt to 3% wt filler loading, respectively. With 4% wt filler loading, LDPE/MMT composites had the highest shore hardness at 47.4. Yet, tensile tests indicated that increasing clay content improved the composite’s characteristics. At modest loading percentages (1–2 wt%), tensile results were excellent. Furthermore, the elongation at break of the unadulterated LDPE was reduced by 20% and 30% after introducing 1% wt and 2% wt of MMT as reinforcement additives, respectively. Hence, MMT clay can improve the mechanical properties and thermal insulation of LDPE polymer matrix.

Keywords:

• Low-density polyethylene
• montmorillonite
• nanocomposite
• mechanical properties
• thermal conductivity

1. Introduction

During the last decades, polymers have gained a great deal of global interest in industries and for academic use due to the wide applications and properties of polymers, in addition to the lightweight and ease of manufacturing of polymers (Al Rashid et al., 2021; Alkaron et al., 2023; Kudva et al., 2022). Low-density polyethylene (LDPE) is among the most promising plastics due to its beneficial properties, such as lightweight, flexibility, processability, and low cost (El-Bagory et al., 2021; Goswami & Mangaraj, 2011). However, polymers like LDPE have common mechanical and thermal properties compared to other materials, such as metals (Chi et al., 2017; Jordan et al., 2005; Rezgar et al., 2019). Improvement of thermal and mechanical properties is required to increase the demand for LDPE in different applications (Chaurasia et al., 2019; Verma, Kumar, et al., 2019; Verma, Parashar, et al., 2019).

The best way to enhance polymer’s properties is by adding reinforcement materials (Alhazmi et al., 2021; Althahban et al., 2023; Hiremath et al., 2021). Although many works and studies were carried out on polymer reinforcement, the mechanical and thermal properties of polymers are not sufficiently developed. Clay is one of the most frequently used reinforcement materials due to its availability, and it also causes remarkable reinforcement to the polymers (G. -M. Kim et al., 2001; Liu et al., 2015; McNally et al., 2003). Adding modest amounts of clay (natural or modified) to the polymers is highly interesting as they offer improved polymer matrix properties, such as strength, toughness, and thermal. Natural clay is an inorganic layered silicate and belongs to the smectite group of minerals. Smectite is mainly used as a group of layered silicate nanoclays in polymer nanocomposites due to its possibility of intercalating process of polymers between the clay layers (Ray & Okamoto, 2003). Even though there are various types of smectite, for example, sodium montmorillonite, hectorite, and saponite, montmorillonite (MMT) still remains to be the most popular in comparison to others because of its acceptability of employing in polymers. However, the reason behind this is the high surface area which Na⁺ MMT has, aspect ratio, and the availability of MMT.

Moreover, there is a possibility to hydrate interlayer cations in aqueous solution that increases the gallery while facilitating the process intercalation (Reddy, 2011). MMT belongs to the phyllosilicates family with a multi-layered stack of nano-sized platelets held together by interlayer cations (Reddy, 2011). Each layer has one octahedral layer linked to two tetrahedral layers in a 2:1 layer structure. Intercalation can be achieved with an organic surfactant, which can increase the interlayer space and a reduction in surface energy, which allows the polymer particles to intercalate between the nanoclay layers (Slaný et al., 2019).

Montmorillonite (MMT) can be exfoliated and homogeneously distributed throughout the polymer matrix to enhance the thermo-mechanical properties of polymer nanocomposites (Dadfar et al., 2011; Durmuş et al., 2007). However, the most crucial challenge to improving polymers is the compatibility between polymer matrix (e.g., LDPE) and MMT clay. Thus, surface modification of MMT can be helpful, and a high-performance LDPE/MMT composite can be obtained. LDPE polymer matrix was filled with various amounts of MMT nanofiller to enhance the thermo-mechanical properties of LDPE. MMT was modified using a coupling agent: Soltrol. In addition, calcium carbonate (CaCO₃) was used to enhance the dispersion of MMT within the matrix of polymers and improve the properties of the resultant composite (Mohammad Mehdipour et al., 2019). The resultant composite was investigated and tested using tensile, hardness, and thermal conductivity tests.

It is thus motivational that these aims can be achieved at such low clay loading as below 5%. Previous studies reinforcing polymer matrices using MMT fillers have shown that higher clay loading has a negative influence on the properties of the polymer composites due to the challenges of the dispersion of clay particles within polymer matrices (Abulyazied & Ene, 2021; Merah et al., 2022; Zazoum et al., 2020). Hence, it is wise to avoid using high clay loadings.

To determine the thermal conductivity of these LDPE, it is essential to know the thermal rates of reactions and the degree to which polyethylene conducts thermal heat (Li et al., 2019). Similarly, before arriving at the mechanical properties, it is essential to know what makes up these LDPEs. Therefore, materials such as fillers, polymer matrices, and coupling agents have been used, as seen in Table 1, to contribute to the success of the research objective.

Table 1. Material specifications

Polymer Matrix	Filling Materials
LDPE	MMT	Calcium Carbonate (CaCO3)
Low-density polyethylene with a density (0.918–0.938 gm/cm) and melting temperature (110 °C) used as a polymer matrix.	MMT is a hydrous aluminosilicate mineral used as a filler in form of powder in different weight ratios (1–4%).	Used as a dispersion enhancer in the amount of 1% wt to the composite at every MMT loading.

In recent decades, the incorporation of clay into polymers has attracted a growing amount of interest. The compatibility between MMT and polymer matrix has been improved through MMT surface modification (Merinska et al., 2012; Mittal, 2013). In order to provide outstanding properties of polymer nanocomposites, commercial clays must be exfoliated and uniformly distributed in the polymer matrix (Dadfar et al., 2011; Durmuş et al., 2007).

This research paper aimed to enhance the properties of low-density polyethylene to meet the needs of end-user applications. Thus, MMT clay at different loadings was added to LDPE polymer matrix to achieve this aim with the expectation of improving the thermal conductivity and mechanical properties of the resultant composites. Composite samples with nanoclays contents of (0, 1, 2, 3, and 4 wt%) were evaluated for hardness, tensile-fractography, thermal conductivity, and tensile testing (elongation and stress-at-break). This study’s null hypothesis is that MMT clay with different concentrations would significantly affect the properties of the MMT/LDPE composite.

2. Experimental

2.1. Materials

Technical specifications of the materials used in this study are described in Table 1. LDPE is a soft, thermoplastic, short branched polymer with low melting temperature, supplied by the State Company for Petrochemical Industries in Basra with a grade: SPCILENE 23,220 (208). Natural mineral MMT used in this study is obtained from Fluorochem Ltd, UK. The MMT powder is hydrophilic in nature with platelet-shaped particles. The chemical formula of the MMT is [Al_l.67 Mg_0.33 (Na_0.33)] Si₄O_l0 (OH)₂.

2.2. Preparation of LDPE/MMT composites

The LDPE/MMT composites were obtained by adding four different concentrations of MMT (1, 2, 3, and 4 wt %). LDPE was mechanically ground before mixing with the MMT at a speed of 50 rpm. Prior to the compounding process, the MMT was heated in an oven at a temperature of 100°C for 24 h to remove moisture. The MMT is then treated with Soltrol, acting as a coupling agent. Calcium carbonate (CaCO₃) was added in an amount of 1% wt to the composite materials at ambient temperature. After completion of the mixing process, the mixture was then placed in a hydraulic press which was set to 180°C and 15 tons. The mixture, placed in a square mold, is preheated before applying temperature and partially compressed before applying the molding pressure. Then, square sheets (100 mm wide, 100 mm length, and 2 mm thick) were finally obtained after applying the consolidation pressure for 10 min at a temperature of 180°C. Samples are rapidly cooled (quenched) to room temperature from a mold temperature of 180°C by water circulation.

2.3. Thermal conductivity test

The thermal conductivity test was performed, in the Department of Materials Engineering, University of Basrah, accordance with ASTM-E1225-04 (A. Standard, 2009) based on the standard Lee^’s Disk method by the absolute plane parallel plate technique. It consists of two plates made of bronze, standard plate, electrical heater, DC power supply, and infrared thermometer. The test specimen is placed between the bronze plates, before the clam screw is tightened to hold the discs together, the heater is, then, switched on, and the thermal conductivity K can be calculated as per the following equation (Zarr, 2001):

$\mathit{K}=\mathit{K}{ }^{\mathrm{\prime }}\frac{\mathit{d}({\mathrm{T}}_2-{\mathrm{T}}_0)({{\mathrm{T}}_1}^{\mathrm{\prime }}-{{\mathrm{T}}_2}^{\mathrm{\prime }})\mathrm{A}{ }^{\mathrm{\prime }}}{\mathit{d}{ }^{\mathrm{\prime }}({{\mathrm{T}}_2}^{\mathrm{\prime }}-{{\mathrm{T}}_0}^{\mathrm{\prime }})({\mathrm{T}}_1-{\mathrm{T}}_2)\mathrm{A}}$

where:

K is the thermal conductivity of the studied specimens.

K' is the thermal conductivity of the standard specimen (0.23 W/m.K).

d is the thickness of studied specimens which is equal to 3 mm.

d' is the thickness of standard specimens which is equal to 6 mm.

T₀ is the ambient temperature of the studied specimens = 22°C.

T₀' is the ambient temperature of the standard specimens = 25°C.

T₁, T₂ are the inlet and outlet of the studied specimen.

T₁', T₂' are the inlet and outlet of the standard specimen, which are equal to 100°C and 60°C, respectively.

A is the surface area of the studied specimen.

K' is the surface area of the studied specimen.

2.4. Hardness Test

Hardness measurements of LDPE and composites were conducted using Shore hardness tester type (D) at a maximum test load of 50 N according to ASTM (D2240–15) (A. Standard, 2015) in the laboratories of State Company of Petrochemical Industries in Basra.

2.5. Tensile test

Tensile tests were carried out on dog bone-shaped samples using PC-aided machine WP300 Universal Material Tester, GUNT Hamburg, with a crosshead speed of 5 mm/min. Different composite specimens (LDPE/MMT) containing various contents of MMT and a pure specimen (LDPE) were made and tested at 24°C. For standard preparation of the tensile test specimens, ASTM DD638–14 (A. Standard, 2014) standard specification was used. The test specimens were cut using a hand-pressed mould die sample cutting machine. Specimen dimensions are overall width of 19 mm, 2 mm thickness, and an overall length of 115 mm. Figure 1 shows the standard tensile test specimen geometry. In order to transfer the tension load to the test specimen, four 3-mm-thick steel tabs with 40 × 40 mm dimensions were placed onto both sides of the specimen ends. A 6-mm diameter hole was drilled through the centre of each of the tabs to provide a load attachment point. The roughness of tabs surface has been increased using 240-grit sandpaper to increase the friction with the specimen ends. The tabs were fitted on the specimen ends using four 4-mm diameter bolts and tightened with a nut to prevent relative slipping. Tensile test data were collected and plotted to make a comparison between LDPE and LDPE composite results. Figure 2 shows a ductile neck was formed in a specimen, and the permanent distortion was localized in this region which slowly extends to the whole length gauge. Tensile testing was used to measure tensile properties such as tensile strength, stress, and elongation at break.

Figure 1. Standard tensile test specimen geometry (A. Standard, 2014).

Figure 2. A standard LDPE specimen showed a ductile neck through the tensile test.

3. Results and discussions

3.1. Morphological analysis

The morphological analysis was done by scanning electron microscopy used to obtain the general observation of MMT powder. Typically, this analysis concerns examining various entities to offer a possible resolution to complex and unquantified problems that involve multiple factors. SEM investigations were carried out at the Polymer Research Centre in Basra. SEM imaging was carried out under low accelerating voltage (under 2kV) to avoid sample charging and damage.

The SEM micrographic investigations of the MMT at low (350×) and high (5000×) magnifications are illustrated in Figure 3a,b, respectively. Scanning electron microscopy is a technique that uses a focused beam consisting of high-energy electrons that generates vast signals at the surface of the specimen (Yas et al., 2020). Essentially, this technique was preferred due to its high-resolution power and because it is relatively easier to operate with comparably user-friendly interfaces. However, the signals derived from the electron sample in SEM were used to reveal critical information about the sample that included the external morphology, orientation, and chemical composition of LDPE. An aggregated sheet of montmorillonite clay provides a quasi-smooth texture. Montmorillonite clay may have a morphology that leads to irregular-shaped particulates with large surficial contact-area and a significant number of active site pores for binding adhesion.

Figure 3. SEM micrographs of MMT powder (a) low magnification image (350X) and 1kV accelerating voltage and (b) high magnification image (5000X) and 2kV as accelerating voltage.

The backscattered and secondary electrons in the SEM were used for imaging the LDPE to determine its morphology and structure for precise results on the mechanical properties as stated in the objectives. As noted in Figure 3, the MMT platelets were joined together to form micron-sized particles with irregular shapes. The micro-aggregation of the MMT, observed in Figure 3a, is due to the high surface energy of the MMT clay which belongs to the smectite clay group and the difference between the MMT and LDPE in the surface polarity. As the MMT is a hydrophilic clay in its nature, it weakly interacted with the hydrophobic polymer-like LDPE (Almansoori et al., 2017). Thus, the MMT tends to form large aggregated particles, which is clearly seen in the high magnification SEM image in Figure 3b. Similar findings were observed and discussed in previous studies (Almansoori et al., 2019).

3.2. Thermal Conductivity

From the experiment, the thermal conductivity of the selected compound (LDPE) was done, and the results were obtained as per the derived formula equation. By definition, this is the heat transfer rate by conduction through a cross-sectional area of an object with a perpendicular existence of a temperature gradient (Venkatesan & Bhaskar, 2021). Figure 4 presents the variation of the thermal conductivity k with the filler content, for LDPE, and the LDPE/MMT composite. It was clearly observed that the thermal conductivity of LDPE/MMT nanocomposites decreased as the MMT filler loading increased, which enhanced the thermal insulation of the composites. This may be attributed to the thermal energy transferred from molecule to molecule by a rather slow diffusion excitation process. As Figure 4 illustrates, an increase in the percentage of the filler led to a decrease of thermal conductivity. However, the thermal conductivity of LDPE was affected by various factors, such as the nature of the material that was used, temperature difference, and the length at which the thermal heat has to pass through.

Figure 4. Thermal conductivity versus the filler content for pure and LDPE/MMT nanocomposite.

Thermal excitation created quantization of elastic waves or scattering of phonons at lattice imperfection (Khan et al., 2022). The tightness of molecular structure promoted thermal conductivity (B. Kim et al., 2004). On the other hand, the addition of low-conductive materials reduces heat transport through the composites and decreases the thermal conductivity (K) (Khan et al., 2022). The reduction in K occurred where filler particles of MMT in nano size are dispersed in LDPE matrix, which resulting in strong interaction between particles of the filler and polymer molecules (Khan et al., 2022). Also, long contact surfaces obstacle the mobility of polymer-chain movement with the increasing filler matrix thermal contact resistance (Zhang et al., 2019; Šupová et al., 2011).

3.3. Hardness Test

The test sample while carrying out the hardness test was free from any impurities or oily substances. The shore D hardness for LDPE was determined using the hardness tester according to the set standards. It is a standardized test that consists of measuring the depth of penetration of a particular indenter where a more penetrated substance is termed as less hard while that which is less penetrated is said to be harder. The shore hardness test is mostly used when the sample material is too soft to be measured by a Rockwell test. However, the resulting depth obtained is dependent upon the viscoelasticity and shape of the sample material and the duration the test is set to take. The hardness test was basically to ascertain the hardness value of LDPE which was determined by the penetration of the durometer into the sample although the measures were found to be dimensionless with a higher number indicating the hardest material. The effect of filler loading on the hardness of (LDPE/MMT) nanocomposites is shown in Figure 5. The filler content was seen to have an effect on the hardness where an increase in the content was observed to increase the LDPE hardness. It can be noticed that the resultant composite had improved strength properties and increased hardness with increasing filler loading because the addition of MMT nanofiller to the polymer matrix enhanced the interactions and cross-linking between molecular chains and increased the ability of the composite to resist plastic deformation (M. -K. Chang, 2015; M. K. Chang et al., 2011). As seen from the graphical representation, the hardness value remained approximately constant for three consecutive percentages and then reached the maximum at 4% wt MMT filler loading.

Figure 5. Shore hardness for pure and LDPE/MMT composite.

3.4. Tensile test

The experimental results of the tensile test for pure LDPE and LDPE with different amounts of MMT as a filler are given in Figure 6. The results showed a general comparison between the tested samples. The tensile test was done to measure the tensile strength of LDPE which can be defined as a material’s resistance to breaking under tension. Theoretically, LDPE has a low tensile strength due to its highly branched polymer chains. The branching, however, prevents the chains from stacking correctly beside one another, thus reducing the intermolecular forces of attraction between them, which reduces the resistance ability. From Figure 6, it can be seen that the stress–strain curve rises with an increase in the elongation percentage and the different amounts of MMT to an optimal point where the tensile strength levels. Continued addition of the elongation percentage beyond the threshold to LDPE decreases the tensile strength.

Figure 6. Stress-Elongation percentage relationship for composites of (LDPE) and different amounts of (MMT).

As shown in Figure 6, each test pattern exhibits an elastic region beyond the yielding point. Figures 7-9 illustrate the variation of elongation at break, stress at break, and tensile strength with different contents of MMT as wt%. It was seen, from Figures 8-9, that the tensile strength and stress at the break of LDPE increased as a result of an increase in the nano-filler loading in the range of 1–3 wt% although the situation differs when the filler concentration exceeds 3% wt. This can be attributed to the micro-aggregation of the nanoclay particles at higher MMT filler contents (>3% wt). It is because the MMT particles tend to be held together to form microsized particles which may act as defects leading to a reduction in the tensile strength as shown in Figure 10a. Previous studies on morphology has indicated that the presence of micron-sized agglomerates of MMT-based clays in part cross sections resulted in a reduction of mechanical properties (Almansoori et al., 2017; Jain et al., 2010). As seen in Figure 6, the test samples were relatively accurate, and the break distance was clearly measured following the standard. The LDPE sample broke near the middle which proved the correctness of the tests. The self-similarity for the set of experiments is maintained in the direction that leads to the tension. In some cases, despite the tensile strength values, it was possible to observe significant differences between the strain at tensile strength and break.

Figure 7. Elongation percentage at break to MMT concentration percentage.

Figure 8. Stress at break to MMT concentration percentage.

Figure 9. Tensile strength to MMT concentration percentage.

Figure 10. Low and high magnification SEM micrographs of fractured surfaces. (a) high magnification imaging of aggregated nanoclays on fractured surface of 3% MMT/LDPE. (b) and (c) fractured surfaces of pure LDPE and 3% MMT/LDPE composite respectively, and (d) high magnification SEM of pure LDPE fracture.

From these figures, there is a noticeable change in the elongation percentage at break and stress at break with the increasing in wt% of the filler content. The elongation at break for the LDPE as observed by Figure 7 composites was lower than the corresponding values for the LDPE considering the variation in results. The elongation percentage for all specimens decreases with increasing wt% filler content. Thus, it appears that the tensile ductility decreases marginally upon the addition of 1 to 4 wt% MMT filler. This reduction indicates that the composites have become brittle as a result of the filler loading. However, the decrease in elongation at break, as seen in Figure 7 with an increase in MMT concentration, could be due to the presence of MMT that restrained the slippage movement of chains during deformation, eventually resulting in a decrease in the value of elongation at break. It was observed, from Figure 8, that the stress at break of LDPE increased with an increase in the nano-filler loading in the range of 1–3 wt% although it is not a similar case when the filler concentration is higher than 3% wt.

Figure 9 shows the effect of the filler loading on the tensile strength. The results indicated that the tensile strength increases with the increase in the filler concentration from 1 to 3 wt%, but when the filler concentration is higher than 3% wt, the tensile strength of the composites decreased slightly. Similar findings have shown a decline in the tensile properties above 3% clay contents when added to polyurethane as demonstrated by Lee & Lin (2006). This is probably attributed to the increase in filler loading below 3% leads to adhesion between LDPE and the MMT content better than above 3% wt (Shnean, 2005).

3.5. Tensile fractography

The micro-aggregation of nanoclay particles at higher MMT filler loading (>3% wt) has been reported in Figure 10a. The MMT particles tend to be held together to form microsized particles which may act as a defect leading to a reduction in the tensile strength as shown in Figure 10a (B. Kim et al., 2004; Khan et al., 2022; Venkatesan & Bhaskar, 2021). Figure 10b shows a plastic deformation on the sample fracture surface of the pure LDPE occurred prior to failure (M. -K. Chang, 2015; Zhang et al., 2019; Šupová et al., 2011). However, 3% wt MMT/LDPE samples exhibit a brittle fracture without a significant plastic deformation due to the addition of hard MMT fillers as can be seen in the SEM image in Figure 10c (Jain et al., 2010; Lee & Lin, 2006; M. K. Chang et al., 2011). Higher magnification SEM image is shown in Figure 10d clearly demonstrating features of ductile fracture.

The overall trend indicates that LDPE nanocomposites reinforced with MMT have the potential for practical applications as a result of their enhanced mechanical and thermal properties in comparison to unadulterated LDPE. Incorporating MMT as a filler material into LDPE matrix creates a composite material with enhanced strength, hardness, and thermal resistance, as demonstrated by the experimental findings of this study.

4. Conclusions and future outlook

This experimental study evaluated the morphological, thermal, and mechanical properties of LDPE nanocomposites reinforced with an additive of MMT. Data analysis leads to the following conclusions. The presence of nano-additives improves the thermal insulation property and the hardness of LDPE/MMT composites in comparison with pure LDPE. Greater thermal insulation and hardness of the nanocomposites are achieved at 4% wt MMT filler loading. Incorporating MMT into LDPE in range of 1–2 wt% improves tensile strength, while higher filler concentration (more than 3% wt) decreases it slightly. With an increase in the percentage of clay content, there is a corresponding increase in the tendency of micro-aggregation of nanoclay particles. This results in the formation of microsized particles that may act as a defect, ultimately leading to a reduction in the tensile strength. Elongation at break decreases with increasing the concentration of filler. The composite with 4% wt MMT nanoparticles has the lowest elongation at break compared to pure LDPE. Moreover, it appears that fracture ductility decreases as MMT content increases. The 3% wt MMT/LDPE samples exhibit a brittle fracture without a significant plastic deformation due to the addition of hard MMT filler.

This research can contribute to a better comprehension of the properties and behaviour of MMT/LDPE nanocomposites, which have potential applications in numerous industries. In addition, it can aid in identifying the main factors that influence the performance of these materials, such as the type and concentration of nanofillers.

However, chemical, topographic-mapping analysis, elemental-mapping, and structural analyses can be added in future studies, for instance, in a future study, XRD, FTIR, and TGA can be helpful investigations in which the chemical changes can be studied in detail.

Disclosure statement

No potential conflict of interest was reported by the authors.