Endothermic–Exothermic Hybrid Foaming of Recycled PET Blends

Abstract

Over the past decades, the use of polyethylene terephthalate (PET) has seen significant growth, particularly in the packaging industry. However, its long decomposition time poses serious environmental challenges. The aim of this research was to develop a process for the foaming of large quantities of recycled PET (rPET) using endothermic and exothermic foaming agents. Various formulations with different ratios of endothermic and exothermic foaming agents were prepared, as well as their mixtures. The study found that the endothermic–exothermic hybrid foaming process resulted in a finer cell-size distribution and enhanced mechanical properties, making the foams highly suitable for widespread applications. The results support the potential use of exothermic foaming agents as nucleating agents in a hybrid foaming system. In particular, the ratio of 3% endothermic and 1% exothermic foaming agents proved optimal in terms of achieving a balance between porosity and mechanical strength, thereby enabling broad industrial applicability.

1. Introduction

Over the past decades, the use of plastics, particularly polyethylene terephthalate (PET), has surged across various industries [1]. Since the 1970s, PET bottle production has grown exponentially, a trend that has continued. In 2022, global plastic production reached 400.3 million metric tonnes, marking a 1.6% increase from the previous year [2]. PET is widely used in the packaging industry due to its ease of manufacture, low density, excellent carbon barrier properties, and resistance to impact and abrasion, making it ideal for consumer goods packaging. However, PET’s biodegradation time of over 500 years raises significant environmental concerns [3]. Despite efforts, plastic recycling rates from 2006 to 2017 showed little improvement, underscoring the urgent need for more effective recycling strategies [4]. The increasing volume of plastic waste, particularly PET, continues to contribute to global environmental challenges, including threats to marine ecosystems and the potential dangers to human health [5]. Therefore, greater emphasis must be placed on reducing plastic waste and improving recycling practices. Polymer foams are materials in which gas bubbles are suspended in a polymer matrix. During the chemical foaming process, gases are generated by chemical foaming agents that decompose during processing, creating bubbles in the cooling polymeric slurry. The expansion of these bubbles results in a closed-cell structure with a cell density that gradually increases towards the core, creating a strong structural foam. The principles and processes of polymer foaming allow the material to be used in a variety of industrial applications, as foamed polymers have excellent mechanical properties and a low density [6,7,8,9].

Chain breakage during PET recycling is a major challenge in the chemical foaming process because the fluidity of the material increases, which hinders foam formation. In their research, Bocz et al. successfully reduced chain slippage using CESA Extend, a chain extender. The chain-extender additive optimised the length of the molecular chains, thus promoting foam formation [10]. In previous research, the aim was to increase the mechanical strength of a foam structure. A 10% impact modifier was added to the mix, which improved the Charpy impact strength by 18.48%. The results showed that the chemical foaming process could effectively be used to produce recycled PET materials with a closed-cell foam structure. Overcoming chain distortion and improving the mechanical properties of the foam structure during the process are essential for the industrial application of the technology, as these factors have a significant impact on the quality and usability of the final product [11,12].

Endothermic–exothermic hybrid foaming is a pioneering technique that offers significant advantages in the development of foam materials and opens new possibilities for different industries. It enables a finer cell-size distribution, an increased cell number, faster foaming, and improved mechanical properties, which make hybrid foams widely applicable in thermosetting, packaging, building materials, and automotive applications. In hybrid foaming, the exothermic component generates heat during decomposition, while the endothermic component absorbs heat. This temperature variation affects the foam formation process, resulting in a smaller cell-size distribution, an increased cell number, and improved mechanical properties. The finer cell size increases the foam’s strength and stiffness and improves its thermal conductivity, while the increased cell number enhances its thermal insulation properties. In addition, the hybrid foaming process allows for faster foaming, resulting in shorter production times and lower costs, making the process more economical [13,14].

The nucleating efficiency of endothermic chemical foaming agents exceeds that of so-called inert nucleators such as talc. Commonly used types such as sodium bicarbonate and citric acid are three to five times more effective as clot formers than talc and are less expensive. They are preferably used in the production of finer cellular foam products. The choice of the decomposition temperature range of the foaming agent is of paramount importance to match the processing temperature range of the matrix polymer. This ensures that the chemical reaction is fully carried out during processing. It is important to note that residual additive components such as undegraded foaming agents and carriers can affect the properties of the polymer matrix to be processed and the operation of the processing equipment. Exothermic chemical foaming agents, particularly azodicarbonamide, are mainly used for the insulation of direct gas wires and cables because their residual material after decomposition has minimal effects on the electrical properties. To achieve optimal results, it is essential to select the correct foaming agents and additives that are harmonised with the processing parameters and the targeted applications [15].

In their research, Maryam Valipour et al. [16] studied the reactive extrusion foaming of polylactide (PLA). The aim was to fine-tune the properties of the foam using different chemical foaming agents and chain extenders at different extrusion temperature profiles. The experiments were carried out using a shoulder-surface methodology. The results showed that exothermic foaming agents produced the highest void fraction (0.55) at higher temperatures, while endothermic foaming agents were less effective (void fraction < 0.05). The type of foaming agent had the greatest effect on the foam properties.

Irina Sophia Emel and co-workers [17] investigated the pre-salting of foamed polypropylene (PP) in the presence of an alicyclic carboxylate crystallisation nucleation agent and a foaming agent. Various alicyclic carboxylates such as diester, diacetate, and diamide were tested. The results showed that the properties of foamed PP were significantly affected by the type and amount of nucleating agents. Exothermic foaming agents resulted in a higher gas release, an improved foam structure, and a lower density, while endothermic foaming agents were less effective. Optimum foaming was achieved with less than 0.05 weight percentage alicyclic carboxylate and 0.25–5 weight percentage foaming agent.

Exothermic chemical foaming is a relatively under-researched area in polymer processing. Studies on extrusion have produced several results, but there are significant gaps in the field of injection moulding. Research in the literature has shown that endothermic foaming agents are particularly suitable for gel formation because, although they operate at lower gas yields than exothermic foaming agents, they still promote gel formation. In our tests, two different types of foaming agents were sub-analysed. One was the endothermic compound Tracel IM 7200 (Tramaco, Tornesch, Germany), which had a gas expansion of 120 mL/g, a blowing agent content of 70% and a decomposition temperature of 220 °C. The other was Tracel IM 3170 MS exothermic foaming agent, with a 50 mL/g gas evolution and a 170 °C decomposition temperature. In the experiments, both foaming agents were tested as clot formers to determine the effectiveness and interactions of the different foaming agents. Based on the higher gas yield of the endothermic foaming agent and the faster reaction of the exothermic foaming agent, different samples were prepared to thoroughly analyse their properties and effectiveness in clot formation.

The primary goal of this research was to explore if the efficiency of the foaming process could be enhanced by utilising a combination of endothermic and exothermic foaming agents, with a particular focus on the application to recycled PET (rPET) materials. Although a reduction in porosity was the main objective, a significant emphasis was placed on achieving a more uniform distribution of the cell structure. Enhancing the mechanical properties of the final foam was also important. These factors are essential to ensure the quality and industrial applicability of foamed materials. A critical aspect of the investigation was to determine how varying the weight ratios of the foaming agents influenced key performance outcomes. Specifically, the research sought to understand if the gas yield, processing temperature window, or nature of the endothermic and exothermic reactions played a more decisive role in dictating which agent would act as a nucleating agent in the system. This was important for the identification of the most effective conditions for foam formation because the nucleating agent influences the size, distribution, and stability of the cells within a foam.

Moreover, the research aimed to assess if the simultaneous use of both endothermic and exothermic foaming agents could produce synergistic effects resulting in superior porosity control, a more homogeneous cell structure, and improved mechanical properties. In addition to these technical objectives, a broader goal of the research was to contribute to the development of more sustainable foaming processes for the recycling of PET. By improving the quality and consistency of foamed rPET products, the aim was to make this material more suitable for large-scale industrial applications, which could help with plastic waste reduction and promote more efficient recycling technologies. Ultimately, the research aimed to advance the state of knowledge of hybrid foaming methods while supporting the broader adoption of rPET in industries where lower-density plastics are required.

2. Materials and Methods

In the following section, the materials and the methodology of the research are detailed.

2.1. Materials

The blue crystallised recycled PET (rPET) used for the research was provided by Fehérvári Group Zrt. (Budapest, Hungary) (inherent viscosity (IV) of 0.8 dL/g). The chain-extender additive used was CESA Extend NCA0025531-ZA, supplied by Clariant (Muttenz, Switzerland), which contains the Joncryl ADR 4368 epoxy-based styrenerol-acrylic multi-functional oligomeric reagent. Dupont Elovaloy PTW was used as an impact modifier to improve the mechanical resistance of the samples. One of the foaming agents used was the Tracel IM 7200 endothermic compound, supplied by Tramaco (Germany), with a gas expansion of 120 mL/g, a blowing agent content of 70%, and a decomposition temperature of 220 °C. The other was the exothermic foaming agent Tracel IM 3170 MS (Tramaco, Germany), with a 50 mL/g gas evolution and a decomposition temperature of 170 °C. The mixtures were dried at 120 °C for 5 h and mixed with the additives according to

Table 1. Composition of the tested samples (unit: parts per hundred resin (phr)).

	Ref	N40X0	N35X05	N30X10	N20X20	N10X30	N05X35	N0X40
rPET	100	100	100	100	100	100	100	100
Calriant CESA Extend	2	2	2	2	2	2	2	2
Du-Pont Elovaloy PTW	10	10	10	10	10	10	10	10
Tracel IM 7200	0	4	3.5	3	2	1	0.5	0
Tracel IM 3170 MS	0	0	0.5	1	2	3	3.5	4

. The material composition was designed in this manner to explore the interaction between endothermic and exothermic foaming agents and assess how their combined effects influenced the foaming process. Instead of optimising a specific material performance, the focus was on investigating how varying the proportions of these agents could impact the mechanical and structural properties of the samples. By systematically adjusting the ratios, potential synergistic or antagonistic interactions could be observed, offering insights into the complex dynamics between the two types of foaming agents and their influence on the final properties of the foamed materials.

2.2. Methods

Dumb-bell specimens were prepared using an Arburg Allrounder Advance 420C Golden Edition (Arburg, Loßburg, Germany) injection moulding machine. The manufacturing was carried out using the injection moulding parameters listed in

Table 2. Injection moulding parameters.

Description	Unit	Value
Clamping Force	kN	150
Nozzle Temperature	°C	260
Injection Pressure	bar	650
Injection Speed	cm³/s	30
Holding Pressure	bar	150–50–20
Holding-Pressure Time	s	2–1
Intermediate Mould Opening	mm	0.5
Residual Cooling after Mould Opening	s	20
Mould Temperature	°C	35

. To achieve a suitable foaming level, pressure differences can be controlled using the breathing mould technique [18]. During this process, the polymer mixed with the foaming agent is injected into the mould cavity, where a compact surface layer of the part is formed during cooling. Subsequently, if the mould is minimally opened, the space increases while the pressure decreases. This can result in the material remaining in its fluid state inside the foam. During the cooling cycle of our research, the foaming of the specimens was enabled by opening the mould, leaving a 0.5 mm gap between the movable and stationary parts.

The internal structure of the samples was examined using industrial CT equipment. The resolution was 0.027 mm, the tube voltage was 200 kV, and the tube current was 0.1 mA. A flat detector without a filter with an integration time of 700 ms was used. A total of 1440 projected images were acquired for the reconstruction. To determine the porosity and diameter of the cells, VGStudio MAX 2.2 was used. An industrial CT (computed tomography) scan determines the density of samples by accurately calculating the volume of the material while accounting for internal voids or cells present in the structure. The CT scan generates a detailed 3D image of the sample, allowing for the identification and measurement of the cell structures within. By subtracting the volume of these cells from the total volume of the sample, the remaining solid material’s volume is obtained. The sample’s density is then calculated by dividing the mass of the material by this adjusted volume, providing a precise assessment that reflects the presence of internal porosity. Tensile strength tests according to [19] were carried out using an INSTRON 5582 (Norwood, MA, USA) universal testing machine with a grip length of 100 mm and a test speed of 1 mm/min for the determination of the elastic modulus and 5 mm/min for the determination of the tensile strength. Utilising the [20] standard and the same machine, flexural tests were carried out with a cross-head speed of 5 mm/min and a 64 mm support span. The impact resistance was measured using a CEAST 65–45.000 impact tester with a 2 J hammer and a 62 mm span length according to [21].

3. Results and Discussion

In the next chapter, the summarised results of the experiments are detailed and evaluated, from which the conclusion of the research are presented.

3.1. CT Scans

Figure 1 shows the porosity (in %) of the samples as a function of the cell size (in mm). The effects of the endothermic (EN) foaming agent and the exothermic (EX) foaming agent were investigated using different samples.

Samples with a higher percentage by weight of the endothermic foaming agent achieved a higher porosity than those dominated by the exothermic foaming agent. This was because the endothermic foaming agent produced a higher gas yield, which produced a higher porosity. This is particularly important in the foaming process because the degree of porosity directly affects the quality of the sample. The higher the porosity, the greater the density reduction; this permits a wide range of applications.

In the literature, endothermic foaming agents are primarily used as nucleators in exothermic hybrid foaming [13]. This is because the foaming agents with the highest gas yields are exotherms. However, these foaming agents are difficult to obtain in accordance with European regulations. The present results show that in this system, an exothermic foaming agent can take up the role of a nucleator, which means that it helps foam formation. The exothermic foaming agent promotes the formation of nodules in a sample, given that it starts to decompose at lower sediment temperatures. Subsequently, the endothermic foaming agent takes advantage of the resulting nodules and can increase the gas bubbles while creating its own small cells. By including the exothermic foam in our experiment, it was possible to achieve increased porosity at significant rates and samples portrayed over two times the porosity when compared with samples containing only the endothermic foaming agent.

After the analysis of the samples where the exothermic foaming agent was predominant, the porosity of the samples was well below the other group of samples due to the lower gas yield. It was interesting to note that in cases where the foaming agents were equally distributed, the porosity of the sample exceeded 14%. It was assumed that, once again, the exothermic foam was the clot former, given its lower decomposition temperature.

In the figure below, it can be observed that the cell sizes were approximately the same for all samples when the standard deviation fields were taken into consideration. The primary reason for this was the low volume of the specimens, which resulted in a small and approximately the same amount of foaming. At the same time, the samples prepared mainly with endothermic foaming agents had a smaller range of cell sizes, whereas the samples dominated by exothermic foaming agents had a wider range of cell sizes. This indicated that the samples containing mainly endothermic foaming agents had a more homogeneous cell structure. Due to the higher porosity level, the need to compare the samples containing mainly the endothermic foaming agents arose. By examining the CT scans of the samples, the differences in the cell density between the samples could be observed (Figure 2).

The CT images were analysed to investigate the effect of different combinations of foaming agents on the structure of the samples. It was primarily concluded that all foaming agent mixtures were capable of forming a closed-cell integral foam structure. The mixing of endothermic and exothermic foaming agents did not compromise the solid structures separating the cells, resulting in the absence of large, continuous voids in the samples.

In the CT image of sample N40X0, the cell density was relatively evenly distributed throughout the sample volume, suggesting that the endothermic foaming agent itself was stable, but it produced a less homogeneous cell structure compared with the other samples. The porosity rate was the lowest here, indicating a stable sample structure. However, the aim of the study was to achieve the highest possible density reduction.

In the N35X05 sample, significant differences in the cell distribution and size were observed. The presence of the exothermic foaming agent contributed less to gas formation, but the cells formed during the clot formation process were more evenly distributed. The distribution of the cells was more homogeneous, resulting in an improvement in the sample structure.

For N30X10, the image clearly showed that the cell density was the highest. This combination resulted in higher gas formation and porosity, which increased the mass loss and insulating capacity of the sample, but also potentially reduced the mechanical strength.

The use of endothermic foaming agents alone resulted in less homogeneous samples with reduced porosity. The exothermic foaming agent provided a more uniform cell distribution. For example, by adding as low as 0.5%, the uniformity of the cell distribution improved; by adding at least 1% not only did the uniformity improve, but also porosity. However, increasing the proportion of the exothermic foaming agent decreased the porosity of the samples. Based on these results, fine-tuning the ratio of hybrid exothermic foaming agents should provide an opportunity to optimise the desired properties of a sample, which is essential during foaming.

3.2. Mechanical Tests

In a comprehensive analysis, the mechanical properties of the samples prepared with different combinations of foaming agents were investigated based on the data presented in the figures. The aim was to gain a deeper understanding of the behaviour of the materials under mechanical stress, with particular emphasis on the effect of the ratio of endothermic-to-exothermic foaming agents.

In Figure 3, the results of the tensile tests are presented, where the data are also plotted as a function of the amount of different foaming agents. The findings were carefully analysed to highlight the influence of the varying agent ratios on the material’s tensile properties.

Based on the results of the tensile strength tests, it was concluded that the N05X35 specimen showed the lowest modulus of elasticity value. The reason for this was that the cell dimensions were the largest. This was supported by the CT analysis, which revealed that the N05X35 sample had one of the largest cell diameters at 0.30 mm, coupled with a relatively low porosity of 7.1%. These large cells acted as stress concentrators, which led to a lower tensile strength. Thus, it was concluded that although the specimen had low porosity, the larger cells acted as a stress collection point. The sample quickly broke and was brittle instead of elastic. A higher proportion of endothermic foaming agents generally resulted in more stable and stronger samples, as confirmed by the tensile strength values. This also confirmed that a higher porosity does not necessarily imply a lower resistance to mechanical stress. More important are the cell dimensions and cell distribution to obtain a stable sample.

When analysing the elastic modulus of the endotherm-dominant samples, the values of N40X0 and N30X10 were similar and only slightly lower than the unfoamed reference sample. As expected, the reference sample had the highest tensile strength. The N40X0 sample had the highest tensile strength among the endothermic-foaming-agent-dominant samples. It was observed that as the proportion of exothermic foaming agent increased, there was a small steady decrease in tensile strength. This could be paralleled with the porosity value. The N30X10 sample, which had a significantly higher porosity of 16.6% compared with N40X0, demonstrated this trend; the increased porosity and smaller cell diameter of 0.22 mm contributed to its lower tensile strength. This suggested that a lower proportion of the exothermic foaming agent affected the tensile strength, but the dominance of the endothermic component still maintained mechanical stability.

In Figure 4, the results of the flexural tests are presented, where the data are also plotted as a function of the amount of different foaming agents. The findings were carefully analysed to highlight the influence of the varying agent ratios on the material’s flexural properties.

The flexural strength results measured in the bending tests showed a mixed picture. The N30X10 specimen achieved the highest flexural strength value, indicating that the material’s resistance to bending stress remained high in the middle cross-section, despite having a porosity value of 16.62%. Interestingly, despite the relatively high porosity of N30X10, the smaller cell size of 0.22 mm (as revealed by the CT scans) contributed to its improved flexural strength because smaller cells provide greater structural integrity under bending stress. In terms of maximum stress, a higher proportion of the endothermic foaming agent generally resulted in better flexural strength. This was due to a more stable cell structure. Looking separately at the results of the flexural strength tests of the endothermic-foaming-agent-dominant samples, the N40X0 sample had a relatively high modulus of elasticity value, thus resisting bending loads well. However, the EN sample had a slightly higher value, suggesting that even a small presence of exothermic foaming agent contributed to the decrease in the elastic modulus. The N40X0 sample—with its larger cell diameter of 0.31 mm, but lower porosity of 8.0%—illustrated this well, as its lower porosity offset the effect of larger cells, leading to improved bending resistance. The flexural strength of N30X10 was the highest of the endothermic-foaming-agent-dominated samples. This suggested that the sample had optimal elastic properties due to a stable cell structure, despite nearly 17% porosity. The higher porosity in N30X10 was compensated for by the finer cell structure (0.22 mm) because the finer cells distributed the stress more evenly during flexural loading. The unfoamed units between the cell layers could absorb the forces of the bending load and responded elastically without rapid fractures in the sample.

The highest maximum stress value measured in the flexural strength tests was obtained for N40X0 among the endothermic-foaming-agent-dominant samples. This result suggested that the dominant use of the endothermic foaming agent resulted in a high flexural strength, considering that the foamed test-bars were analysed. The N35X05 sample and the N30X10 sample also portrayed high values, only slightly lower than the N40X0 sample. The N35X05 sample’s porosity of 9% and relatively smaller cell diameter of 0.24 mm compared with N40X0 explained why it also performed well in the flexural strength tests and maintained mechanical integrity, despite an increase in the exothermic agent proportion.

The results of the Charpy test are presented in

Table 3. Charpy test results of samples prepared with different foaming agents.

Charpy Impact Test (kJ/m2)
N40X0	N35X05	N30X10	N20X20	N10X30	N05X35	N0X40
11.65 ± 2.98	3.63 ± 1.16	3.51 ± 0.46	5.98 ± 0.91	20.42 ± 1.38	21.31 ± 2.18	3.89 ± 0.31

. The Charpy test results exhibited significant variations across the different samples, with values ranging from as low as 3.51 ± 0.46 for the N30X10 specimen to as high as 21.31 ± 2.18 for the N05X35 specimen, indicating substantial differences in the impact resistance of the samples depending on the composition and ratio of the foaming agents used. Based on the impact resistance values measured in the Charpy impact test, the N05X35 sample showed the highest impact resistance, which was attributed to the high presence of the exothermic foaming agent. There were significant differences between the endothermic-foaming-agent- and the exothermic-foaming-agent-dominant samples. The higher impact resistance of the N05X35 sample could be explained by its relatively low porosity of 7.1% and larger cell diameter of 0.30 mm, which resulted in a better distribution of mechanical stress during impact. The impact resistance of the specimens with a lower porosity were much higher, whereas the impact resistance of the specimens with large foam sizes were negligible. During the evaluation of the measurement results, a high degree of standard deviation was observed. This suggested that the measured values were strongly influenced by the cell distribution of the central cross-section. The CT scans also confirmed that variations in the cell size and distribution were significant contributors to the standard deviation, as samples with more uniform cell structures tended to show more consistent mechanical properties.

4. Conclusions

The present research aimed to develop the foaming of large quantities of rPET. The effectiveness of endothermic and exothermic foaming agents was investigated, with a focus on overcoming chain breakage and improving the mechanical properties of the foam structure. Endothermic–exothermic hybrid foaming resulted in a finer cell-size distribution and improved mechanical properties, which make the foams suitable for a wide range of applications.

The samples with a higher percentage by weight of the endothermic foaming agent achieved higher porosity than the samples dominated by the exothermic foaming agent because the endothermic foaming agent resulted in higher gas yields. This is particularly important during the foaming process as porosity directly affects the quality of a sample. Endothermic foaming agents are often used as cluster-formers in exothermic foaming because they contribute to the formation of cluster points and create gas bubbles. The present study supported the hypothesis that exothermic foaming agents can also be used as nucleation agents. The analysis of the CT scans showed that the N40X0 sample had a stable, but less homogeneous, cell structure, whereas the N35X05 and N30X10 samples had a more uniform cell distribution. The N30X10 sample showed the highest cell density and porosity. The exothermic foaming agent provided a more uniform cell distribution. For example, by adding as low as 0.5%, the uniformity of the cell distribution improved; by adding at least 1%, not only the uniformity, but also the porosity values improved. Overall, the porosity more than doubled with a 1% addition of the exothermic foaming agent compared with the original endothermic foam results.

Mechanical tests showed that N30X10 was stable and had good mechanical properties. The elastic modulus values of the N30X10 sample were close to those of the unfoamed reference sample. This indicated that the N30X10 sample had significant flexural strength, despite its 16.62% porosity. In the Charpy impact test, the nearly 17% porosity of N30X10 resulted in low values, but its mechanical stability and uniform cell structure made it an excellent choice.

Overall, a stable cell structure and proper cell distribution are key to optimising mechanical properties. This research found that the gas yield was the main factor affecting the efficiency of the foaming agent. In hybrid systems, it is advisable to choose a lower gas-yield foaming agent with a lower decomposition temperature than the main foaming agent. Fine-tuning the ratio of hybrid foaming agents allowed the optimisation of the mechanical and structural properties of the samples, which is essential for the industrial application of foaming technology. Overall, for the foaming agents tested, the use of the N30X10 blend was particularly noteworthy as it provided an optimal combination of porosity, mechanical strength, and insulating properties, which could potentially permit a wide range of industrial applications.

In forthcoming research, the focus will be on integrating mixed plastic waste into hybrid foaming processes, with the aim of evaluating how the inclusion of polymers with varying thermal properties impacts foaming efficiency and the resulting mechanical characteristics of the composite materials. A key aspect of this investigation will be to analyse the interactions between foaming agents with different decomposition temperature ranges and plastics with distinct melting points. This will allow for a better understanding of the synergistic or antagonistic effects that occur during the foaming process, particularly in relation to the gas yield and cell structure formation.