Reinforcing effects of ground tyre rubber on physical,mechanical,and thermal properties of polyethylene terephthalate

Abstract

This study examines the use of waste and recyclable polyethylene terephthalate (rPET) along with micro-sized ground tire rubber (GTR) to produce a polymer matrix composite (PMC) material. The PMC materials are made in a Twin-screw extruder, with different ratios of rPET to GTR ranging from 90:10 to 50:50. In order to assess the performance of the newly created rPET-GTR matrix composite materials, the physical, mechanical properties are empirically observed. The thermal properties and crystallinity (X_c) of PMC materials are analysed using DSC. SEM and microscopic imaging are used to determine the characteristics of rPET-GTR composite materials, including elemental compositions like carbon, oxygen, and silicon. The functional group and chemical structure are confirmed through IR spectra analysis using FTIR techniques. The inclusion of micro-sized GTR has shown a significant effect on the physical (density = 1.306 - 1.208 g/cm³, water absorption = 1.76%–10.74%, and melt flow index = 8.12 - 12.79 gm/10 min), mechanical (tensile strength = 52.78 – 73.58 MPa, impact strength = 110.64 - 138.44 J/m, hardness = 56.14 - 76.37, and flexural modulus = 66.23 - 106.32 MPa), and thermal (melting temperature = 220.25°C–192.57°C, X_c = 20.30 to 30.58%) properties of rPET. These studies all work together to improve the performance of waste rPET and GTR materials by repurposing them. It helps to promote sustainability, cost-effectiveness, and expands the range of applications (automotive parts, building materials, sports equipment, electrical insulation and prosthetics) for rPET-based composites with potential uses in the polymer industries.

Keywords

Polyethylene terephthalate ground tire rubber polymer matrix composite waste plastic rubber

Introduction

PET, a commonly used plastic material in industries such as packaging, clothing, and carpets, is produced in massive quantities worldwide, with over 82 million metric tons produced each year. However, the disposal of PET waste in landfills and oceans, where it takes hundreds of years to degrade, has become a global concern.^1,2 To combat this issue, PET recycling is being recognized as a crucial solution to reduce plastic waste and promote sustainability. One of the future trends in PET recycling is for brands to collect and recycle their own products, reducing the need for external recycling facilities and ensuring higher-quality recycled polyethylene terephthalate (rPET) feedstock. Governments around the world are also implementing regulations to address plastic pollution, which will drive the demand for rPET.³ Advancements in PET recycling technology are highlighting the potential for chemical recycling to reduce the environmental impact and resource usage. Enzyme-based PET recycling, in particular, shows promise in improving upon conventional, fossil-based methods of PET production across various aspects, including energy, carbon emissions, and socioeconomic impacts.⁴ The global closed-loop PET bottle recycling market is projected to grow at a compound annual growth rate of 5.2% between 2023 and 2033, indicating significant potential for PET recycling expansion. The increasing demand for rPET, coupled with technological innovations and market growth opportunities, is driving a sustainable revolution in PET recycling, which is crucial for addressing the plastic pollution crisis.⁵

PET-GTR composites have become increasingly popular in recent years due to their potential benefits in terms of sustainability and performance. This literature review examines several studies on PET-GTR composites and their properties. One study by Cosnita et al.⁶ investigates new composites made from PET, rubber, high-density polyethylene, and wood waste. The goal of the study is to understand how the amount of waste PET affects the durability and mechanical properties of the composites. This research is important because of the growing amount of polymeric waste, such as tyre rubber and PET bottles. The findings of this study could contribute to the development of sustainable composite materials and waste management strategies. Cazan et al.⁷ conducted a study that explores the impact of PET functionalization on rubber-PET-HDPE composites. The research investigates how the functionalization process changes the PET surface, affects the composite interfaces, and modifies the transition temperatures. It also emphasizes the need to optimize the composition and technical parameters, such as the percentage of PET. The study suggests that PET functionalization is a practical and effective method for improving the interface between rubber and fillers in the composite. Comparisons are made between chemical and photochemical functionalization on the mechanical properties of PET fabric, highlighting the potential for enhancing fiber-rubber adhesion through surface modification. The research emphasizes the importance of surface modification in improving the bonding between PET fibers and the rubber matrix, with a particular focus on chemical methods. Results indicate that adding organic groups to the PET fiber surface can enhance its bonding to rubbers. Overall, this study provides valuable insights into the potential applications of PET-reinforced composites and the significance of surface modification in optimizing their properties. The research conducted by Ameli et al.^8,9 explores the effects of PET, GTR, and anti-stripping agents (ASAs) on binders and stone matrix asphalt (SMA) mixtures. PET has been found to enhance the durability and resistance of asphalt pavements to rutting and fatigue cracking. In addition, the presence of PET in SMA mixtures greatly impacts their water sensitivity, suggesting that PET could be used as an alternative to traditional binders. GTR, on the other hand, improves the performance of asphalt pavements by increasing their resistance to rutting, fatigue cracking, and other types of damage. Its impact on the water sensitivity of SMA mixtures is also studied, showing enhanced performance in terms of rutting and fatigue resistance. ASAs are effective in improving resistance to stripping, a common issue in asphalt pavements, particularly in warm climates. ASAs have also been demonstrated to enhance resistance to rutting and fatigue cracking. Generally, this research demonstrates that the use of GTR, PET, and ASA can significantly improve the performance of binders and SMA mixtures. As well, it highlights the potential of recycled GTR and waste PET as sustainable and cost-effective materials for asphalt pavements.

Das et al.¹⁰ explore the idea of using waste plastic in road construction to address environmental pollution. They suggest modifying bitumen with waste plastic, such as PET and polypropylene, as a replacement for traditional bitumen. This approach can contribute to a circular economy and promote sustainability by reducing waste. Waste plastic-modified bitumen confirm improved properties compared to unmodified bitumen, such as reduced penetration, softening point, and flash point. It also encourages recycling and waste management practices, particularly in developing countries. However, the adoption of waste plastic-based road construction materials requires supportive policies and guidelines from waste management agencies and road engineering firms. Overall, this study illustrates the potential of waste polymer in road construction as an environmentally friendly and sustainable alternative to traditional bitumen. Similarly, Karahrodi et al.¹¹ explore the impact of waste PET/GTR blends on the thermal and rheological properties of bitumen. Their study involves analyzing the composition and content of the waste PET/GTR blends and how they can modify the properties of bitumen. The research suggests that this approach can satisfactorily modify the rheological and thermal properties of bitumen. These findings provide insights into the potential use of waste materials to enhance the characteristics of bitumen, which is significant for various applications in the construction and infrastructure sectors. Furthermore, Deriszadeh et al.¹² investigate the effect of adding waste blends of PET and GTR on the mechanical properties of epoxy/GO nano-composite adhesives. Their study found that the addition of waste blends of PET/GTR improved the overall shear strength of the composite-metal joint. The stiffness of GO, along with its proper adhesion to epoxy/(WPET/GTR), was responsible for this improvement. The research suggests that using waste blends of PET/GTR in epoxy/GO nano-composite adhesives can lead to improved mechanical properties.

One of the main problems with rPET is its brittleness, which is often attributed to the presence of impurities and degradation of the polymer chains during the recycling process. However, there are several methods to overcome this issue and improve the mechanical properties of rPET. Blending rPET with materials like polycarbonate can enhance its mechanical and thermal properties, reducing brittleness. Reactive tougheners, such as modifying rPET chemically by adding coupling agents, can also improve its properties. These methods aim to increase the toughness and tensile strength of rPET, making it more suitable for a wide range of applications.^13–15 Lastly, Surface modification of rPET with plasma treatments, coating, and other chemical methods are also being explored to improve the mechanical properties of rPET. These methods increase the surface area, reduce surface roughness and introduce functional groups, leading to enhanced adhesion with other polymers and fillers.

Some of the key objective, novelties and main contributions of this research are enhancing the mechanical properties of rPET by adding GTR that acts as a reinforcing agent, enhancing the tensile strength, flexural strength, and impact resistance of PET-based composites. This improvement in mechanical properties makes the material more durable. Increase in thermal Stability as the incorporation of GTR into rPET has been found to enhance the thermal stability of the composite material. GTR acts as a heat-absorbing filler, reducing the degradation rate of rPET and increasing its thermal resistance. This makes rPET-GTR composites suitable for application in high-temperature environments. The utilization of GTR in rPET composites provides a sustainable solution for the recycling and reusing of waste tyres. By converting GTR into a valuable material, the research contributes to reducing environmental pollution caused by discarded tyres and promotes the circular economy approach. The research on rPET-GTR composites has identified a wide range of potential applications. These composites demonstrate improved properties suitable for various industries, including automotive, construction, packaging, and textiles. The versatility of rPET-GTR composites opens doors for innovative and sustainable solutions in different sectors.

Materials and properties

M/S Saha Enterprise, a company that specializes in grinding and granulating plastic scrap, donated shredded waste PET plastic materials for this research. The materials have a thickness ranging from 0.3 to 0.5 mm and are highly recyclable. Also, GTR materials are collected from the RRR (Reduce, Reuse, Recycle) Centre, Central Zone Agartala Municipal Corporation, Tripura West. GTR materials are then washed, dried and sieved to the grain sizes of 150 to 300 µm (Table 1).

Table 1.

Typical properties of materials^16–19 and characteristics.²⁰

Property		Test method	Value (unit)
Property		Test method	PET	GTR
Physical properties	Density	Hydrostatic weighing technique	1.37–1.41 g/cm³	0.93 g/cm³
	Water absorption	ASTM D570 (24 h)	0.3%–0.5%	10%–30%
	Melt flow index	ASTM D1238 (PET = 250°C, 2.16 kg. GTR = 190°C, 2.16 kg)	20–30 g/10 min	1.52 -1.85 g/10 min
Mechanical properties	Tensile strength	ASTM D638	47 ± 7 - 50 MPa	22 ± 7 MPa
Mechanical properties	Impact strength	ASTM D256	90 J/m	-
Thermal properties	Melting point	DSC	255°C–265°C	190°C
Thermal properties	Crystallization temperature	DSC	127°C–130°C	+10 to −40°C
Characteristics
PET		Concentrations	Weight percentages
		Terephthalic acid	70%
		Ethylene glycol	30%
GTR		Rubber	54%
		Carbon black	29%
		Textile	1%
		Oxidize zinc	1%
		Sulphur	1%
		Additives	13%

Methods

Fabrication of rPET-GTR composite materials

The rPET and micro-sized GTR are carefully mixed according to the blend compositions of the rPET-GTR matrix, and the mixture is then poured through the hopper into a twin-screw extruder (Pearls Plastic, Aurangabad, India). The mixture is heated through the barrel, and the materials pass through at a constant temperature and a screw speed of 250 r/min. This process, called thermal blending, is usually used to fabricate polymer matrix composites.²¹ Now after blending, the newly blended hot rPET-GTR composite material is collected on a 304 stainless steel chamber to avoid adhesion among the composite material and the collection chamber. The 304 stainless steel material is non-adhesive to soft or molten plastics and their composites. Finally, the rPET-GTR composite material is allowed to cool and solidify at standard room temperature and pressure before being removed. The resulting stock of rPET-GTR polymer matrix materials are then cut and polished to the desired dimensions for further experiments.

Characterization

The ZEISS Sigma 300 (Germany), a scanning electron microscope (SEM) is used to examine and analyze the morphology of rPET-GTR matrix composite material. Before conducting the experiment, a thin layer of gold (Au) is applied to the sample materials to make them electrically conductive. The morphological analyses are carried out using a 10KX magnifying lens and a microscopic length of 1 µm. Moreover, elemental analysis is performed using energy dispersive spectroscopy (EDS) to determine the composition of composite materials made of an rPET-GTR matrix. EDS can examine the distribution of elements in a material through techniques such as elemental mapping, compositional analysis, and morphological characterization. Additionally, The Bruker Alpha II (Germany), a Fourier-transform infrared spectroscopy (FTIR) is used to analyze the IR spectra of rPET-GTR matrix composite material. This spectrometer is commonly used in the polymer industry for examining raw materials, assessing finished products, and researching coated surfaces. One of its main advantages is that it is a non-destructive technique and it requires very little or no preparation of sample before analysis.

Physical properties testing

Density test

Based on ASTM D-792,²² the Archimedes method of hydrostatic weighing technique is used to determine the density of rPET-GTR PMC materials. The sample materials (5 ± 0.1 to 0.5 gm) have a theoretical density (ρ_t) greater than one. Therefore, water is often used to measure density. The following equation (1) is used to calculate the ρ_t of various rPET-GTR blends.

ρ_{t} = (wt . % \times ρ_{PET}) + (wt . % \times ρ_{GTR}) \cdot gm / {cm}^{3}

(1)

Here, wt. % = weight percentage of materials in compositional ratio, ρ = density of rPET, and GTR.

To determine the actual density (ρ_a) of rPET-GTR PMC materials, we follow a specific procedure. First, weigh the object in the air to determine its mass (m_a). Then, submerge the object in a container filled with liquid (water) and weigh it again (m_l). The difference between m_a and m_l represents the mass of the liquid that is displaced by the rPET-GTR sample. By using the known densities of water, we can convert the mass of the displaced liquid into volume. Finally, the ρ_a of each rPET-GTR PMC material is calculated by dividing m_a by m_l. The subsequent formula is used to find the ρ_a:

ρ_{a} = (m_{a} / m_{l}) \times ρ_{liquid} \cdot gm / {cm}^{3}

(2)

Here, ρ_liquid = density of waterat 23 to 25°C.

The percentage of error between the ρ_t and ρ_a is determined by the next equation.

Error % = [(ρ_{t} - ρ_{a}) / (ρ_{t})] \times 100

(3)

Water absorption test

The ASTM D570²³ is a standard test method that measures the amount of water absorbed by a plastic material when it is immersed in water. This test involves submerging a small plastic sample in water for a set amount of time and then calculating the percentage increase in weight. The samples are placed in distilled water at a temperature of 23 ± 1°C for 24 h. After 24 h, the samples are taken out of the water, any surface water is wiped off, and they are weighed accurately to the nearest 0.001 gm.

Melt flow index (MFI) test

The MFI value is a measurement of the amount of polymer that is extruded through a specific orifice. It is expressed in grams per 10 min. An auto cutter MFI tester, manufactured by Ideal Lab Engineers LLP in Mumbai, India, is used to determine the MFI values of rPET-GTR PMC materials. This tester has a working temperature range of up to 400°C and a timer that can be set from 0 to 999 s. It operates on 220V and 50 Hz power supply. The MFI test is commonly used for polyethylene and requires specific temperatures and weights. In this experiment, the MFI test is conducted on rPET-GTR PMC materials using the ASTM D1238²⁴ standard. The test is performed at a temperature of 250°C and with a weight of 2.16 kg for PET. 5 g of each PMC material sample are taken for the test. During the test, the samples are extruded for duration of 2 minutes. After extrusion, the samples are weighed to determine the MFI of the respective materials.

Mechanical properties testing

Tests are conducted at the Central Institute of Petrochemicals Engineering & Technology (CIPET) in Agartala, India to determine the mechanical properties of composite materials made from rPET-GTR matrix. In accordance with ASTM²⁴ standards, five specimens are tested for each composition of rPET-GTR to find the optimal value. ASTM offers standards for testing the mechanical properties of polymer materials used in various industries. These standards guarantee that the materials meet the necessary specifications for their intended purposes. The tests are carried out under standard laboratory conditions of 23 ± 2°C and 50 ± 10% relative humidity. Figure 4 displays the actual samples used in the experiments.

Tensile properties test

The most commonly used testing standard for determining the tensile properties of rPET-GTR PMC materials is ASTM D638. In this experiment, a sample of type V (63.5 × 9.53 × 3.40 mm), which is known as a dog bone specimen, is used. The experimental parameters for this sample include a speed (mm/min) of 1 (0.05) ± 25% and a nominal strain rate of 0.1 at the start of the test. The tensile strength, tensile modulus, and elongation at break are determined using a universal testing machine manufactured by HEM TECHSYS (HT-U2510) Vadodara, India. This machine has a specific load capacity of 100 KN, a maximum speed of 250 mm/min, a minimum speed of 0.01 mm/min, and a constant rate of crosshead movement. The following formula is used to calculate the ultimate tensile strength:

σ_{\max} = F_{\max} / A . MPa

(4)

Here, F_max = maximum load, A = original cross-sectional area.

Impact strength test

The ASTM D256 standard method is utilized to measure the impact resistance of rPET-GTR composites. This involves breaking a notched sample using a swinging hammer to determine its toughness. The test can also assess the quality of the product by determining the amount of energy lost at the notch. To conduct this test, a motorized notch cutter is used to create a notch on a sample measuring 63.5 × 12.7 × 3.2 mm. The experiments are performed using a Tinius Olsen (IT 504), USA impact tester with a force of 2.7 joules. The impact strength is calculated using the following formula:

= Breaking energy / (Width \times Thickness) . J / m

(5)

Hardness test

The ASTM D785 standard test method is utilized to determine the Rockwell hardness of rPET-GTR composites materials. This method involves measuring the depth of an indentation created by applying a large load compared to a smaller load. To measure the hardness (Shore D) of specimens that are 25 × 25 × 6 mm in size, a digital Rockwell hardness tester from HDNS KELLY (MRD-600TS), Shanghai, China is employed. The tester applies a minor load of 10 kg.

Flexural modulus test

ASTM D790 is a testing method used to determine the flexural modulus of different materials, including plastics and its composites. This test measures the material’s ability to resist bending when a load is applied. In the case of polymeric materials, the data is usually calculated at 5% deformation. The test also evaluates the material’s resistance to cracking or breaking under bending stress. A universal testing machine, which is the same machine used for conducting tensile tests, is commonly used to perform this test. To calculate the flexural strength (σ) for a three-point test with specimen dimensions of 127 × 12.7 × 3.2 mm and testing parameters of strain rate = 0.01 mm/mm/min and speed = 2 mm/min, the following equation can be used:

σ = 3 FL / {2 wd}^{2} . MPa

Here, F means the maximum force applied, L is the length of the sample, w is the width of the sample and d is the depth of the sample.

Thermal analysis

Differential Scanning Calorimetry (DSC)^6,7 is a technique used to measure the heat released or absorbed by a sample as its temperature changes. It is commonly used in industries like pharmaceuticals, polymers, nanomaterials, and food products. DSC provides important information about the thermal properties of materials and has many applications in various fields. Linseis Instruments, based in Germany, offers a DSC instrument that can reach temperatures up to 800°C is used to determine the melting temperature (T_m), crystallization temperature (T_c), melting enthalpy (ΔH_m), and crystallinity (X_c) of rPET-GTR polymer matrix compositions. The experiments are conducted in a nitrogen (N₂) atmosphere using an aluminum pan, with a heating rate of 10°C/min. The X_c²⁵ is calculated according to the following equation:

Xc = [{Δ H}_{m} / (W_{PET} \times {Δ H}_{m}^{0})] \times 100 .

ΔH_m is the melting point enthalpy, W_PET = weight percentage of PET in polymer blends, and ΔH⁰_m = 140 J/gm for PET (Figure 1).

Results and discussion

Characterization of rPET-GTR composite material

Figure 2 displays the SEM and microscopic images used to study the interfacial area in rPET-GTR blends. These images provide detailed information about the microstructure of the rPET-GTR composites, including the distribution of GTR, the quality of the bonding between the matrix and GTR, and the presence of defects like cracks and voids. The SEM analysis reveals that GTR can be used to enhance traditional polymer-reinforced composite materials, increasing their strength by transferring stress from the matrix to the particles. Recent advancements in electron microscopy have expanded the capabilities for observing and characterizing the hierarchical structures of polymers.

Table 2 provides valuable insights into the distribution of elements within the rPET-GTR matrix composite material. GTR reinforced polymer matrix composites, which consist of carbon, oxygen, sodium, silicon, and mercury, have an impact on the physical and structural properties of the composites. Figure 3 shows the distribution of elements in polymer-GTR matrix composites, providing details about the composition in terms of weight and atomic percentages of different elements. This information is valuable for analyzing and understanding the elemental composition of the materials.

Table 2.

Composition of rPET-GTR matrix composite material.

SL no	Element	Weight %	Atomic %
1	C K	67.23	82.58
2	O K	14.06	12.97
3	NaK	2.96	1.90
4	SiK	3.05	1.60
5	AuL	12.19	0.91
6	HgL	0.50	0.04

The data table displays the weights and atomic percentages of various elements: carbon (C), oxygen (O), sodium (Na), silicon (Si), and mercury (Hg). Carbon (C) has the highest weight and atomic percentage in the matrix, which contributes to the strength and stiffness of the composites. This makes it suitable for applications in aerospace and automotive industries. Oxygen (O) helps in forming chemical bonds between polymer chains, which enhances the mechanical properties and resistance to environmental factors. The presence of silicon (Si) provides good thermal stability, making these polymer matrix composites suitable for high temperature applications. C, O, and Si are important elements in polymer matrix composites as they contribute to their strength, lightweight properties, and thermal stability. On the other hand, elements like sodium (Na) and mercury (Hg) have lower weights and atomic percentages in the polymer matrix composition. Na can enhance the flexibility and toughness of the polymer matrix, as well as improve its processability. Despite its low atomic percentage, Hg is a heavy element with a high atomic weight, making it useful in polymer composites for radiation shielding applications. Mercury-containing composites have been studied for their potential use as efficient gamma-radiation shields.

FTIR

The FTIR spectrum provides information about the types of chemical bonds present in a molecule, which can help identify the compound. Figure 4 shows the FTIR spectra bands of rPET-GTR polymer matrix composite, which are significant because they correspond to specific vibration modes of functional groups in a molecule. The intensity of an absorption band is influenced by the polarity of the bond. Bonds with higher polarity exhibit more intense absorption bands. These bands provide valuable information about the chemical bonds and functional groups present in the compound being analyzed. This information is crucial for identifying and characterizing the compound’s structure. Here is a brief explanation of the significance of each band: spectra bands of 1708.94 cm⁻¹ is typically associated with carbonyl (C = O) stretching vibrations, which are found in compounds containing carbonyl groups such as esters, ketones, and aldehydes. Band 1409.58 cm⁻¹is often related to aromatic C = C stretching vibrations, which are found in compounds containing aromatic rings such as benzene and its derivatives. 1236.66 cm⁻¹is characteristically related with C-O stretching vibrations, which are found in compounds containing ether groups (e.g., -O-C-O-). Wavenumber 1084.11 cm⁻¹is associated with the presence of C-O stretching in an ether group or C-N stretching in an amine group. This band is often related to C-O stretching vibrations in alcohols (e.g., -OH). 1008.85 cm⁻¹ indicates the presence of C-H in-plane bending in aromatic compounds or C-N stretching in aliphatic amines. This band is typically associated with C-N stretching vibrations, which are found in compounds containing amide groups (e.g., -C=N-C-). Band 866.17 cm⁻¹ is often related to C-H out-of-plane bending vibrations, which are found in alkanes and other hydrocarbons. 715.15 cm⁻¹ is linked with C-H out-of-plane bending vibrations in aromatic rings.

Physical properties (density, water absorption and MFI) test

With the proportion of GTR in the rPET-GTR PMC materials increases, the ρ_t and ρ_a decrease. The column for ρ_t represents the calculated density of the PMC material based on the composition of rPET and GTR. The values range from 1.37 to 1.25 gm/cm³, indicating a decrease in ρ_t with an increase in the proportion of GTR. The mass of the sample in both air and liquid provides information for further calculations and analysis. The resulted data in Table 3 have also suggests a trend of decreasing ρ_a with increasing GTR content in the rPET-GTR PMC materials. This column shows the ρ_a of the PMC material, determined by comparing the mass in air and liquid. The values range from 1.306 to 1.208 gm/cm³, indicating the measured density of the material. The percentage of error in the density measurement ranges from 0.59% to 4.67% that is relatively a low percentage of error in the density measurements, suggesting that the measurements are accurate and reliable. The density of a PMC material is an important factor in determining its mechanical properties and overall performance. A higher density generally indicates a stronger and more rigid material. PMC materials with higher density are often used in applications where strength and structural integrity are critical, such as in construction, automotive parts, or aerospace components. On the other hand, PMC materials with lower density are lighter and can be used in applications where weight reduction is important, such as in sports equipment or lightweight structures. Moreover, the water absorption percentage tends to increase with higher GTR content. The values range from 1.76% to 10.74%, suggesting the material’s ability to absorb water, which may affect its properties and performance in certain applications. The MFI values also show an increasing trend with higher GTR content. This indicates that as the GTR content increases, the flowability of the material also increases. This value of a PMC material provides information about its flow behaviour during the melting process. The values range from 8.12 to 12.79 gm/10 min, indicating the flowability or viscosity of the material during the melting process. It measures the ease with which the material can be processed and formed into different shapes. A higher MFI value indicates a higher melt flow rate, meaning the material has better flowability and can be easily molded or extruded. PMC materials with higher MFI values are often preferred in applications that require complex shapes or intricate details, such as in injection molding or 3D printing.²⁶ On the other hand, PMC materials with lower MFI values may have more limited flowability and are suitable for applications where dimensional stability and precision are important, such as in electrical components or precision parts. From the data Table 3, the material composition that would be most suitable for applications where low water absorption, high MFI values, and good processability are required is the composition of 80% rPET and 20% GTR. In this composition, the water absorption percentage is relatively low at 3.06%, indicating that the material has low water absorption properties. As well, the MFI value is 9.48 gm/10 min, which is relatively high compared to the other compositions,²⁶ indicating good processability. Therefore, the 80% rPET and 20% GTR composition would be the most suitable choice for applications that prioritize low water absorption, high MFI values, and good processability.

Table 3.

The information includes data on the theoretical density (ρt), the mass of the sample in both air (m_a) and liquid (m_l), the actual density (ρa), the percentage of error, the water absorption percentage, and the MFI values for various compositional rPET-GTR PMC materials.

rPET: GTR	ρ_t	m_a (gm)	m_l (gm)	ρ_a	Error %	Water absorption (%)	MFI/10 min
90:10	1.37	5.013	1.176	1.306	4.67	1.76 (±0.16)	8.12
80:20	1.34	4.861	1.212	1.332	0.59	3.06 (±0.23)	9.48
70:30	1.31	4.935	1.114	1.291	1.45	4.38 (±0.26)	9.64
60:40	1.28	4.876	1.027	1.266	1.08	8.11 (±0.44)	12.68
50:50	1.25	4.792	0.828	1.208	3.36	10.74 (±0.65)	12.79

Overall, the resulted data suggests that the composition of rPET-GTR PMC materials significantly influences their physical properties. The Table 3 provides insights into the density, water absorption, and flowability properties of different compositional rPET-GTR PMC materials. These properties are crucial in determining the suitability of the PMC materials for specific applications where strength, durability, and other performance factors are important considerations.

Mechanical properties (tensile, impact strength, hardness and flexural strength) test

When the rPET content decreases and the GTR content increases in rPET-GTR composite materials, there are noticeable trends in the mechanical properties (Table 4). Adding GTR to recycled rPET has been found to greatly improve both tensile strength and tensile modulus. However, this addition can also have mixed effects on the material’s tensile properties. The improvement in mechanical properties is due to several important mechanisms. Firstly, adding highly reinforcing material GTR with a high aspect ratio to rPET creates a more varied material, with rubber particles spread throughout the polymer matrix. This dispersion enhances the adhesion between the GTR particles and the polymer matrix, leading to better load transfer and improved mechanical properties. The stronger adhesion strengthens the composite material, resulting in higher tensile performance. The increased tensile strength and modulus are a result of the GTR reinforcing the rPET matrix, making the composites more resistant to bending and tensile forces. Additionally, GTR particles can absorb and dissipate energy during deformation, reducing stress concentration and preventing crack propagation. This leads to improved resistance to deformation and higher tensile strength. The addition of GTR also enhances compatibility between components, promotes chain entanglement, and forms a branched structure through reactions between the end hydroxyl group of rPET and GTR, further increasing tensile strength and modulus of elasticity. Higher crystallinity (Table 5), as achieved with the addition of GTR, results in improved tensile strength and modulus of elasticity. On the other hand, the GTR can decrease the elongation at break of rPET, due to the relatively low elongation of GTR particles. This means that rPET-GTR composites may be more brittle than pure PET, and may fracture more easily under stress indicating reduced ductility. In general, the tensile properties of rPET-GTR composites are influenced by the amount of GTR incorporated. Increasing the weight percentage of GTR usually leads to an increase in tensile strength and tensile modulus, up to a certain point. Beyond this point, the tensile properties may start to decline due to factors such as poor dispersion of GTR particles or reduced polymer-filler interaction. The optimal weight percentage range of GTR in rPET for achieving optimal tensile properties can vary depending on several factors, including the specific application and desired properties. This allows them to identify the weight percentage range that provides the desired balance between strength, modulus, and elongation at break.

Table 4.

The table displays the mechanical properties of rPET-GTR composite materials with varying ratios of rPET and GTR. The tensile properties (tensile strength, tensile modulus, and elongation at break) and impact strength (notched) of the composites have been evaluated along with their hardness, flexural strength.

rPET: GTR	Tensile properties			Impact strength (notched) (J/m)	Hardness (shore D)	Flexural strength (MPa)
rPET: GTR	Tensile strength (MPa)	Tensile modulus (MPa)	Elongation at break (%)	Impact strength (notched) (J/m)	Hardness (shore D)	Flexural strength (MPa)
90:10	52.78	1394.39	46.87	138.44	76.37	66.23
80:20	56.46	1419.11	45.04	135.27	71.16	72.66
70:30	68.47	1788.29	37.18	126.78	68.38	92.57
60:40	72.14	1953.12	31.21	112.48	62.82	102.58
50:50	73.58	1976.34	27.44	110.64	56.14	106.32

Table 5.

Thermal properties of rPET-GTR polymer matrix composite materials.

rPET: GTR	T_m (°C)	T_c (°C)	ΔH_m (J/g)	X_c (%)
90: 10	220.25	122.14	25.58	20.30
80: 20	211.06	115.87	25.45	22.72
70: 30	207.25	103.87	23.45	23.92
60: 40	197.28	83.36	23.42	27.88
50: 50	192.57	75.87	21.41	30.58

When it comes to the effect of an increase in weight percentage of GTR in rPET matrix composites on impact strength, it is generally observed that the impact strength tends to improve initially with the addition of GTR but may start to decline beyond a certain threshold. At lower weight percentages of GTR, the rubber particles act as energy absorbers, effectively dissipating the energy generated during impact events. This absorption of energy helps to reduce stress concentration and potential crack propagation, leading to an enhancement in impact strength. The impact strength of the composites decreases as the GTR content increases. This means that composites with higher GTR content become more susceptible to notch-induced fractures. On the other hand, composites with higher PET content have better impact strength. The highest impact strength is observed for the 90:10 rPET: GTR ratio, while the lowest value is seen for the 50:50 ratios. The optimal GTR content and dispersion are crucial for achieving the desired enhancement in impact strength. However, as the weight percentage of GTR increases beyond an optimal point, several factors come into play. The excessive presence of GTR particles can lead to poor dispersion, agglomeration, or weak interfacial bonding between the rubber particles and the PET matrix. This can result in stress concentration, reduced load transfer, and compromised mechanical properties, including impact strength.

Hardness is a measure of the material’s resistance to indentation or scratching that can give an indication of the material’s surface resistance. Generally, as the weight percentage of GTR in the composite increases, the hardness values of rPET-GTR composites tend to decrease. This is because the inclusion of GTR in the composite may introduce more flexibility and elasticity to the material, leading to a softer surface. This indicates a decrease in rigidity of the composites and it can be attributed to the inherent flexibility of GTR. These properties are influenced by factors such as the composition and structure of the composite, the bonding between the matrix and reinforcement, and the processing conditions. Composites with higher PET content have higher hardness.

In terms of flexural strength, an increasing trend is observed as the GTR content increases. Increased flexural strength benefits materials by allowing them to maintain their shape, endure bending stresses, and prevent deformation when under load. This ultimately contributes to the stability and durability of a structure. The highest flexural strength is observed for the 50:50 (rPET: GTR) ratio. Overall, the result data suggests that rPET: GTR composites with higher GTR content exhibit improved flexural strength and tensile strength, but at the cost of reduced impact strength, hardness, stiffness. On the other hand, increasing the PET content improves impact strength and hardness but may result in lower tensile and flexural strength. Therefore, the effect of increasing the weight percentage of GTR on the flexural strength of PET can be influenced by the specific composition of the composite, the processing techniques used, and the desired properties of the final product.

The choice of rPET: GTR ratio should be based on the specific requirements of the application. For applications where strength and rigidity are crucial, a higher GTR content may be preferred. For applications where impact resistance and hardness are important, a higher PET content may be more suitable. The inclusion of GTR in the composition can improve the impact resistance, flexibility, and fatigue resistance of the material. It is important to carefully consider the desired performance characteristics and trade-offs when selecting the rPET: GTR ratio for a particular application. Furthermore, it’s important to note that the mechanical properties of polymer composites are influenced by a combination of factors, including the type and amount of reinforcement, processing conditions, and the overall composite formulation. The rubber particles in GTR act as fillers, reinforcing the matrix and enhancing its overall mechanical properties. Therefore, a comprehensive understanding of the specific requirements and performance criteria of the application is necessary to select the most suitable PMC material. The composition of 50% rPET and 50% GTR can offer certain advantages in terms of strength and durability compared to other compositions. It is worth noting that the specific weight percentage range for optimal tensile properties may differ depending on the specific GTR particle distribution as well as the processing conditions and PET matrix properties. The results of this study can provide valuable insights for the design and development of rPET-GTR composite materials for various applications.

Thermal properties test

Based on the result data Table 5 provided, it can be observed that the thermal properties of rPET-GTR polymer matrix composite materials vary with the ratio of rPET and GTR. As the proportion of GTR in the composite material increases, the melting temperature (Tm) and crystallization temperature (Tc) decrease, while the enthalpy of melting (ΔHm) and crystallinity (Xc) also decrease. For instance, the composite material with a ratio of 90:10 (rPET: GTR) has the highest Tm and Tc values of 220.25°C and 122.14°C, respectively, while the composite material with a ratio of 50:50 (rPET: GTR) has the lowest Tm and Tc values of 192.57°C and 75.87°C, respectively. Similarly, the composite material with a higher proportion of rPET has a higher ΔHm and Xc. For instance, the composite material with a ratio of 90:10 (rPET: GTR) has the highest ΔHm and Xc values of 25.58 J/g and 20.30%, respectively, while the composite material with a ratio of 50:50 (rPET: GTR) has the lowest ΔHm and Xc values of 21.41 J/g and 30.58%, respectively.

The presence of GTR can alter the thermal conductivity, thermal expansion coefficient, and heat resistance of the PET composite. The thermal conductivity of the composite may decrease with the addition of GTR, as rubber generally has lower thermal conductivity compared to PET. This can result in improved insulation properties of the composite material. Secondly, the increase in GTR content can affect the coefficient of thermal expansion (CTE) of the composite. Rubber has a higher coefficient of thermal expansion compared to PET, so the addition of GTR may increase the overall thermal expansion of the composite material. As GTR has a different CTE compared to PET, the overall CTE of the composite may change. This can impact the dimensional stability of the material when exposed to temperature changes. Furthermore, GTR can enhance the heat resistance of the PET matrix composite. Rubber has good thermal stability, and its incorporation can improve the resistance of the composite to thermal degradation and high temperatures. It is important to note that the specific effects on the thermal properties may vary depending on the percentage of GTR added, and the overall composition of the composite. The specific range of weight percentage for GTR reinforcement that improves the thermal stability of rPET in a composite can vary depending on several factors, including the desired properties and the specific application. However, in general, the results of this study have shown that adding GTR to an rPET matrix can enhance the thermal stability of the composite. The addition of GTR can act as filler and improve the heat resistance and thermal conductivity of the material. Typically, weight percentages of GTR in the range of 10 to 30% have shown improvements in thermal stability, mechanical properties, and other characteristics (Figure 5).

Application of the obtained results and proposed new material

The results of this research would provide insights into the potential enhancement of rPET properties through the addition of GTR, offering valuable information for industries looking to improve the performance of rPET-based materials by utilizing GTR as a reinforcing component. This proposed new material could have several implications with some of the potential applications could include:

• Automotive components: rPET-GTR composites can be used in automotive components such as body panels, structural parts, and interior trims. The high tensile strength, stiffness and durability of these composites make them capable for withstanding mechanical stresses, impacts and providing structural integrity.

• Construction materials: The composition can be utilized in the manufacturing of construction materials like roofing tiles, pipes, and panels. Its durability can withstand harsh weather conditions and its strength can provide structural stability. The high tensile strength and modulus of these composites make them suitable for load-bearing applications, providing strength and longevity.

• Sporting goods: Products like skateboard decks, bicycle frames, kayak bodies and protective gear can benefit from the strength and durability of the rPET-GTR composition. It can provide the necessary rigidity and impact resistance required in these applications.

• Packaging materials: The composition can be used in the production of packaging materials such as rigid containers, trays, and pallets. Its durability ensures the protection of goods during transportation and its strength allows for stacking and handling without deformation.

• Furniture: The composition can be suitable for manufacturing durable furniture components like chair frames, table legs, and cabinet panels. Its strength can support weight and frequent use, while its durability ensures a long lifespan.

• rPET-GTR composites can also be used in electrical and electronic applications, including insulating materials, electrical enclosures, and connectors. The high tensile strength of these composites ensures the protection and reliability of electrical components.

• In the context of prosthetics, rPET-GTR composite materials can be used due to it’s high strength-to-weight ratio, which is important for the durability and comfort of the prosthetic socket. The flexibility of the material mimics the natural movements of human limbs, allowing for a more natural gait and improved overall functionality. Additionally, the composite material offers improved resistance against wear and tear, making it suitable for long-term use in demanding conditions. It can withstand impact forces, which is particularly beneficial for active individuals or athletes who require prosthetics for sports and recreational activities. Furthermore, the composite material can be easily customized and molded to fit the specific needs of the user. This flexibility allows for personalized designs that accommodate various shapes, sizes, and functional requirements, ensuring a comfortable and secure fit.

It’s important to note that the specific application and design requirements will dictate the suitability of rPET-GTR composites. Factors such as environmental conditions, cost considerations, and regulatory standards should also be taken into account when selecting materials for a particular application.

Conclusion

The contributions of this research have emphasized the importance of understanding how GTR reinforcement affects the physical, mechanical, and thermal properties of rPET. GTR has been found to greatly improve the properties of rPET, offering a viable alternative to conventional reinforcement materials. Using GTR from discarded tires in rPET made from plastic bottles shows promise in creating sustainable materials from waste. By incorporating GTR, sustainability is promoted through the use of recycled materials that would otherwise contribute to environmental waste. This method not only reduces the environmental impact of waste tire rubber but also offers a cost-effective way to produce high-performance rPET-based materials. This lays the groundwork for future studies on polymer composites, specifically focusing on using waste materials as strengthening agents. The results of the study emphasize the potential of this environmentally friendly method in creating advanced materials with improved characteristics. Some key findings and areas of investigation include: that GTR has the potential to impact the physical properties of the composites, acting as an impact modifier. However, it’s important to note that there is an optimal amount of GTR that should be added to rPET to achieve the desired physical properties. Adding too much GTR can lead to a decrease in certain properties or even result in the formation of agglomerates, which can negatively affect the overall performance of the material. Therefore, it is crucial to carefully determine the appropriate amount of GTR to achieve the desired balance of properties in rPET composites. The size and concentration of GTR particles influence the mechanical properties showcasing enhanced tensile and flexural properties of blends made from rPET and GTR. Generally, as the amount of GTR increases, the mechanical properties of rPET, tends to improve. The improved properties of rPET can be attributed to the strong interfacial bonding between the GTR and rPET matrix. Moreover, research has been conducted on composites made entirely from rPET and GTR, with varying compositions, to understand their durability and mechanical properties. Thermal characterization studies have also been carried out to understand how these materials behave under different temperatures. Overall, these studies contribute to our understanding of the potential benefits of using GTR in polymeric materials. These improvements have the potential to broaden the range of applications for PET, particularly in the manufacturing, where the reinforced PET can be used for the production of lightweight and durable component and construction industries. Besides, GTR can have an impact on the biodegradation of rPET, because GTR contains various additives and contaminants that can potentially lead to degradation of the rPET matrix over time. This degradation can impact the long-term mechanical properties and stability of the composite. Moreover, the presence of rubber particles can increase the porosity of the composite, allowing for improved microbial colonization and enhanced biodegradation. The rubber particles can serve as sites for microbial attachment and provide a source of carbon for microbial growth. The specific effects of GTR reinforcement on biodegradation can vary depending on factors such as the type of polymer, GTR content and environmental conditions. Finally, the findings from these studies suggest that ground tire rubber can significantly improve the physical, mechanical, and thermal properties of polyethylene terephthalate, paving the way for innovative solutions in material science and engineering. Also, the significance of ongoing research in the field of composite materials, particularly in exploring the reinforcement potential of recycled materials for improving the properties of rPET and other thermoplastics, contributing to the development of more sustainable and high-performing materials.

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 iD

Archisman Dasgupta

References

Benyathiar

Kumar

Carpenter

, et al. Polyethylene terephthalate (PET) bottle-to-bottle recycling for the beverage industry: a Review. Polymers 2022; 14(12): 2366.

Sharifian

Asasian-Kolur

. Polyethylene terephthalate (PET) waste to carbon materials: theory, methods and applications. J Anal Appl Pyrol 2022; 163: 105496.

Darko

Yung

PWS

Chen

, et al. Review and recommendations for sustainable pathways of recycling commodity plastic waste across different economic regions. Resources, Environment and Sustainability. 2023; 14: 100134.

Samak

Jia

Sharshar

, et al. Recent advances in biocatalysts engineering for polyethylene terephthalate plastic waste green recycling. Environ Int 2020; 145: 106144.

Gracida-Alvarez

Benavides

, et al. Circular economy sustainability analysis framework for plastics: application for poly (ethylene terephthalate) (PET). ACS Sustainable Chem Eng 2023; 11(2): 514–524.

Cosnita

Cazan

Duta

. Effect of waste polyethylene terephthalate content on the durability and mechanical properties of composites with tire rubber matrix. J Compos Mater 2017; 51(3): 357–372.

Cazan

Cosnita

Duta

. Effect of PET functionalization in composites of rubber–PET–HDPE type. Arab J Chem 2017; 10(3): 300–312.

Ameli

Maher

Mosavi

, et al. Performance evaluation of binders and stone matrix asphalt (SMA) mixtures modified by ground tire rubber (GTR), waste polyethylene terephthalate (PET) and anti stripping agents (ASAs). Construct Build Mater 2020; 251: 118932.

Ameli

Babagoli

. Evaluation of the influence of antistripping agents on water sensitivity of the stone matrix asphalt mixture modified by recycled ground tire rubber and waste polyethylene terephthalate. Adv Mater Sci Eng 2021; 2021: 1–18.

10.

Das

. Road construction by waste polymer: a solution to environmental pollution. Novel Polymeric Materials for Environmental Applications. Singapore: World Scientific Publishing Company, 2023, pp. 341–382.

11.

Karahrodi

Jazani

Paran

SMR

, et al. Modification of thermal and rheological characteristics of bitumen by waste PET/GTR blends. Construct Build Mater 2017; 134: 157–166.

12.

Deriszadeh

Shahraki

Kalhori

, et al. Epoxy/GO nanocomposite adhesives reinforced with waste blends of PET/GTR: evaluation of mechanical properties. Prog Org Coating 2019; 136: 105292.

13.

Khoramnejadian

. Enhance mechanical and thermal properties of recycled Poly ethylene terephetalat (PET) from used bottle. Adv Environ Biol 2011; 5(13): 3826–3829.

14.

Viora

Combeau

Pucci

, et al. A comparative study on crystallisation for virgin and recycled polyethylene terephthalate (PET): multiscale effects on physico-mechanical properties. Polymers 2023; 15(23): 4613.

15.

Celik

Shamsuyeva

Endres

. Thermal and mechanical properties of the recycled and virgin PET-Part I. Polymers 2022; 14(7): 1326.

16.

Cazan

Duta

. Recycled rubber: composite matrix. Hauppauge, New York: Nova science publishers, 2011.

17.

Ben Zair

Jakarni

Muniandy

, et al. A brief review: application of recycled polyethylene terephthalate in asphalt pavement reinforcement. Sustainability 2021; 13(3): 1303.

18.

Sudakov

Chudyk

Sudakova

Dziubyk

. Innovative technology for insulating the borehole absorbing horizons with thermoplastic materials. E3S Web Conf. 2019; 123(01033).

19.

Elamri

Zdiri

Harzallah

, et al. Progress in polyethylene terephthalate recycling. Hauppauge, New York: Nova science publishers, 2017.

20.

Bekhiti

Trouzine

Asroun

. Properties of waste tire rubber powder. Eng Technol Appl Sci Res 2014; 4(4): 669–672.

21.

Formela

. Strategies for compatibilization of polymer/waste tire rubber systems prepared via melt-blending. Advanced Industrial and Engineering Polymer Research. 2023, In press.

22.

Jabir

Hadi

Al-Zubiedy

AAA

. Rheological and mechanical investigation of PET, LDPE and HDPE wastes blended for re-use in bio-pipes. IOP Conf Ser Mater Sci Eng 2020; 987(1): 012006.

23.

Dan-mallam

Hong

Abdul Majid

. Mechanical characterization and water absorption behaviour of interwoven kenaf/PET fibre reinforced epoxy hybrid composite. International Journal of Polymer Science 2015; 2015: 371958.

24.

McKeen

. Introduction to the physical, mechanical, and thermal properties of plastics and elastomers. In The Effect of Sterilization Methods on Plastics and Elastomers. Amsterdam the Netherlands: Elsevier, 2012, pp. 57–84.

25.

Reinaldo

JDS

Damasceno

Ueki

, et al. A microrheological study of poly (methyl methacrylate) elastomer/poly (ethylene terephthalate) (PMMA elast/PET) blends. Mater Res 2017; 20: 694–700.

26.

Singh

Kumar

Farina

, et al. Multi-material additive manufacturing of sustainable innovative materials and structures. Polymers 2019; 11(1): 62.